What makes Erbium Doped Fiber Amplifiers special in terms of how they amplify signals?

Among them, a trace impurity in the form of a trivalent erbium ion is inserted into the optical fiber’s silica core to alter its optical properties and permit signal amplification.
The working principle of the EDFA is to use the pump light sources, which most often has a wavelength around 980 nm and sometimes around 1450 nm, excites the erbium ions (Er3+) into the 4I13/2 state (in the case of 980-nm pumping via 4I11/2), from where they can amplify light in the 1.5-μm wavelength region via stimulated emission back to the ground-state manifold 4I15/2.
Advantages & Disadvantages of EDFA
- EDFA has high pump power utilization (>50%)
- Directly and simultaneously amplify a wide wavelength band (>80nm) in the 1550nm region, with a relatively flat gain
- Flatness can be improved by gain-flattening optical filters
- Gain in excess of 50 dB
- Low noise figure suitable for long haul applications
- Size of EDFA is not small
- It can not be integrated with other semiconductor deviecs
Semiconductor optical amplifier (SOA)
Semiconductor optical amplifier is one type of optical amplifier which use a semiconductor to provide the gain medium. They have a similar structure to Fabry–Perot laser diodes but with anti-reflection design elements at the end faces. Unlike other optical amplifiers SOAs are pumped electronically (i.e. directly via an applied current), and a separate pump laser is not required.
1.Stimulated emission to amplify an optical signal.
2.Active region of the semiconductor.
3.Injection current to pump electrons at the conduction band.
4.The input signal stimulates the transition of electrons down to the valence band to acquire an amplification.
Advantages & Disadvantages of SOA
- The semiconductor optical amplifier is of small size and electrically pumped.
- It can be potentially less expensive than the EDFA and can be integrated with semiconductor lasers, modulators, etc.
- All four types of nonlinear operations (cross gain modulation, cross phase modulation, wavelength conversion and four wave mixing) can beconducted.
- SOA can be run with a low power laser.

A significant point is that the erbium gives up its energy in the form of additional photons which are exactly in the same phase and direction as the signal being amplified. Lumped amplifiers, where the pump light can be safely contained to avoid safety implications of high optical powers, may use over 1 W of optical power.
Second, Raman amplifiers require a longer gain fiber. Since this creates a loss of power from the cavity which is greater than the gain, it prevents the amplifier from acting as a laser. Systems meeting these specifications, have steadily progressed in the last few years from a few Watts of output power, initially to the 10s of Watts and now into the s of Watts power level.
From Wikipedia, the free encyclopedia. The erbium doped amplifier is a high gain amplifier. Another advantage of operating the DFA optlque the gain saturation region is that small fluctuations in the input signal power are reduced in the output amplified signal: Optics and Photonics Letters. A typical DFA has several tens of meters, long enough to already show this randomness of the birefringence axes.
Capteur de fibre optique. Titulaire et D’installations de Polissage.
However, Ytterbium doped fiber lasers and amplifiers, operating near 1 micrometre wavelength, have many applications in industrial processing of materials, as these devices can be made with extremely high output power tens of kilowatts. The absorption and emission cross sections optiquue the ions can be modeled ampluficateur ellipsoids with the major axes aligned at random in all directions in different glass sites.
The random distribution of the orientation of the ellipsoids in a glass produces a macroscopically isotropic medium, but a strong pump laser induces an anisotropic distribution by selectively exciting those ions that are more aligned with the optical field vector of the pump.
Panneaux D’adaptateur de Fibre SC. Point est en stock. Adaptateur de Fibre nu. The amplification window is determined by the spectroscopic properties of the dopant ions, the glass structure of the optical fiber, and the wavelength and power of the pump laser.
Amplicicateur, in optical fibers small amounts of birefringence are always present and, furthermore, the fast and slow axes vary randomly along the fiber length.

One of the key advantages of erbium-doped fiber amplifiers (EDFAs) is their immunity to crosstalk, because of their inherently slow (100-μs–1-ms) gain dynamics.1 This lack of crosstalk makes EDFAs suitable for use in multichannel fiber networks. In such applications, however, occasional bursts of traffic may result in gain fluctuations because of transient saturation in the EDFA, causing packet-to-packet data interferences. Automatic gain control (AGC) loops can be used to achieve such necessary gain stabilization. A feed-forward AGC loop was shown;1 it acts on the pump source to increase the gain as a transient of higher signal power as detected at the EDFA input. The main disadvantage of AGC through pump power control is the large pump power change, ΔPp, necessary to compensate for the gain fluctuation, ΔG (e.g., ΔPp ≈ 15 mW for ΔG ~ 3 dB).2 In this paper we demonstrate an alternate scheme for AGC feedback loops requiring a comparatively very lowcontrol power of 2 μW. In this scheme, the error signal is generated by the fluctuations of output amplified spontaneous emission (ASE). This signal is applied to modulate the intensity of a control or compensation signal input to the EDFA to maintain a constant level of gain saturation in the amplifier.
© 1991 Optical Society of AmericaPDF Article

The bandwidth, gain, saturation power and noise of the erbium-doped fiber-amplifier (EDFA) are reviewed in the context of high-speed optical communication systems. Recent experiments which have used EDFA postamplifiers, repeaters and preamplifiers to enhance the performance of fiber-optic systems are discussed.
Gain characteristics of an Er3+-doped fiber for high-power picosecond input pulses are studied with an InGaAsP laser diode pump source at 1.46-1.48 μm. The output energy and peak power of the amplified pulses reach as high as 7.9 pJ and 792 mW for a repetition rate of 100 MHz and a pulse width of 10 ps. The gain saturation is so slow that the gain in high speed pulse transmission systems is determined by a steady-state saturated gain. With the Er3±-doped fiber amplifier, it is shown that solitons can be amplified and transmitted over a long dispersion-shifted fiber by using the dynamic range of an N=1 soliton. Furthermore, optical solitons at wavelengths of 1.535 μm and 1.552 jim have been amplified and transmitted simultaneously over 30 km with an Er3±-doped fiber repeater for the first time. The collision experiments between these different wavelength solitons are described. It is shown that there is a saturation-induced cross talk between multi channel solitons, and the cross talk (the gain decrease) is determined by the average input power in high bit-rate transmission systems. Subpicosecond soliton and 20 GHz soliton pulse amplifications with Er3±-doped fiber are also described, which indicate that Er fibers are very advantageous for short pulse soliton communication. Finally, a gain coefficient as high as 2.4 dB/mW is reported using InGaAsP laser diodes.
Two fiber laser sources, a resonant fiber laser (RFL) and a superfluorescent fiber laser (SFL), have been given initial tests as gyro sources using a medium quality gyro test bed. The RFL reacted strongly to optical feedback from the gyro circuit resulting in very large unstable errors in the gyro output. These were suppressed substantially by an optical isolator which reduced feedback from the gyro, or by a phase modulator within the laser cavity.

WHY AMPLIFIERS ???o To amplify an optical signal with a conventional repeater works well for moderate-speed single wavelength operation.o Repeaters are expensive for high-speed multiple wavelength systems.o Thus optical amplifiers have been developed.o Optical amplifiers boost up the power level of multiple light wave signals.
ERBIUM-DOPED FIBER AMPLIFIERSo Active medium in an optical fiber amplifier consist 10 to 30 m length of optical fiber that has been lightly doped with a rare-earth element such as erbium(Er), thulium(Tm).o Fiber material can be silica or a tellurite glass.o silica doped with erbium is good for long telecommunication.o EDFA operates in the spectral band of 1530 to 1560 nm region that is c band or conventional band.
o Amplification mechanism:- a. Optical amplifier uses optical pumping. b. Pumping gives energy to electrons to reach the excited state. c. After reaching its excited state, the electron must release some energy and drop to the lower level. d. Here a signal photon can then trigger the excited electron into stimulated emission. And electron releases its remaining energy in the form of new photon.
ENERGY LEVEL DIAGRAM AND TRANSITION PROCESS OF Er3+ IONS IN SILICA
o EDFAs include the ability:- a. To pump the devices at several different wavelengths. b. Low coupling loss to the compatible-sized fiber transmission medium. c. Highly transparent to signal format and bit rate. d. Immune from interference effects( crosstalk and intermodulation distortion) when wavelength channels are injected simultaneously into amplifier.
GENERAL APPLICATIONS OF OPTICAL AMPLIFIERSo In line optical amplifiers:- a. In single mode link, the fiber dispersion may be small so that repeater can be eliminated. b. Instead of regeneration of signal, simple amplification can be done. c. It is used to increase the distance between regenerative repeaters.
o Preamplifier:- • Optical amplifier being used as a front-end preamplifier for an optical receiver. • A weak optical signal is amplified before photo-detection so that signal to noise ratio degradation due to noise can be suppressed in the receiver. • It provides a larger gain factor and bandwidth.o Power amplifier:- a. The device which can be placed after the transmitter to boost the transmitted power is called as power amplifier. b. This provides increase in distance depending on the amplifier gain and fiber loss.

It is worth noticing that an outstanding amplification is obtained at 1.55 μm wavelength where the gain value is not optimal in order to demonstrate the capability of such amplifiers.
The gain of a highly efficient erbium-doped fiber amplifier was measured when pumped at wavelengths between 1.46 pm and 1.51 ,am. The optimal pump wavelength, Xrt, was determined to be 1.475 pm. At this pump wavelength the maximum gain coefficients for signals at 1.531 pm and 1.544 pm were measured to be 2.3 d13/mW and 2.6 dB/mW, respectively. At λ°pPt high gains were achieved, ranging from 32 dB at pump power Pp = 20 mW up to 40 dB at Pp = 80 mW. These modest pump powers are within the capabilities of currently available 1.48 pm diode lasers. The width about λ°IP for 3 dB gain variation exceeded 27 nm for Pp = 10 mW and 40 nm for pipn > 20 mW. With this weak dependence on pump wavelength, single longitudinal mode lasers do not have a significant advantage over practical Fabry-Perot multi-mode pump lasers.
The fabrication and characteristics of a variety of single-mode fibre Bragg reflection gratings are discussed. By simple modifications to the fabrication process fibre gratings can be made with very different characteristics ranging from very narrowband resonators with a 0.04nm bandwidth to broadband reflectors with a 17nm bandwidth.
We demonstrate lasing on the 1050nm and 1345nm 4-level transitions of neodymium in multimode fluoride glass fibers pumped by a commercial semiconductor diode laser. Low thresholds, high output powers, and high efficiencies are all demonstrated.
Techniques for the generation of second-order nonlinearities in optical fibres are described. Applications to nonlinear frequency-mixing and relevant phasematching techniques are discussed. Electra-optic modulation via the Pockels effect is demonstrated.
Germano-silicate optical fibres have been fabricated by MCVD with a refractive index difference between the core and the cladding glass of up to 0.05. It is demonstrated that by depositing within Vycor tubes during preform fabrication, fibre may be subsequently drawn at lower temperatures than are possible with conventional silica preforms and this reduces the germania-induced excess loss.

with a pump wave copropagating with the signal wave), in the backward direction, or bidirectionally. The direction of the pump wave does not influence the small-signal gain, but the power efficiency of the saturated amplifier and also the noise characteristics. Bidirectional pumping can be a way not only to apply a high pump power, but also to achieve a low noise figure and a high power efficiency at the same time.
Most fiber amplifiers (e.g. those based on erbium and ytterbium) operate on quasi-three-level transitions (neodymium-doped amplifiers are a notable exception). This means that in the unpumped state such amplifiers exhibit some losses caused by the active ions; only when a certain excitation level is exceeded, does actual amplification take place. The quasi-three-level nature also has implications for amplifier noise, in particular an increased noise figure, which however can be minimized by certain design optimizations.
Optical nonlinearities such as the Kerr effect can be significant in fiber amplifiers, particularly for those amplifying ultrashort pulses (→ ultrafast amplifiers). This can lead to strong self-phase modulation, but also to excessive Raman gain and thus to the generation of a strong first-order Stokes wave at a wavelength some tens of nanometers longer than that of the amplified signal. For single-frequency operation, stimulated Brillouin scattering is the most important nonlinearity.
The effect of the nonlinearity can be reduced e.g. by increasing the fiber mode area (but at the expense of a lower gain efficiency and possibly worse beam quality) or by decreasing the fiber length. The latter measure becomes possible when using a fiber with higher doping concentration, but this can lead to concentration quenching.
A technique for radically reducing nonlinear effects is chirped-pulse amplification, where pulses are strongly dispersively stretched before entering the amplifier and subsequently compressed again. All-fiber setups using that technique (with a fiber Bragg grating as the compressor) can generate femtosecond pulses only with energies well below 1 μJ, leading to peak powers in the kilowatt region. Substantially higher peak powers of the order of 100 MW are possible when using bulk-optical elements in addition to a fiber amplifier based on large mode area fiber, but some characteristic advantages of fiber-optic systems are lost with that approach.

In terms of gain saturation, fiber amplifiers are very different from semiconductor optical amplifiers (SOAs). Due to the small transition cross sections, the saturation energy is fairly high, e.g. some tens of microjoules for a typical erbium-doped telecom amplifier, or hundreds of microjoules for a large mode area ytterbium-doped amplifier. As a result, significant energy (sometimes several millijoules) can be stored in a fiber amplifier, and can later be extracted e.g. by a single short pulse. Only for output pulse energies above the saturation energy, pulse distortions through saturation become significant. For amplifying the output of a mode-locked laser, the gain saturation is normally the same as for a continuous-wave laser with the same average power.
Fiber amplifiers are often operated in the strongly saturated regime. This allows for the highest output power, and the effect of slight variations of pump power on the signal output power is substantially reduced.
ASE and Noise
The gain achievable is often limited not by the available pump power, but by amplified spontaneous emission (ASE). This can become relevant for gains roughly exceeding 40 dB. High-gain amplifiers also need to be protected from any parasitic reflections, because these could lead to parasitic laser oscillation or even to fiber damage, and are therefore often equipped with optical isolators at the output and possibly also at the input.
ASE also provides the fundamental limitation of the amplifier noise properties. While the excess noise of a loss-less four-level amplifier can approach the theoretical limit, corresponding e.g. to a noise figure of 3 dB in the case of high gain, the excess noise can be stronger for the usual quasi-three-level gain media and in the presence of extra losses. Note that ASE and excess noise are often stronger in backward-pumped amplifiers.
Noise introduced by the pump source may also be an issue. It can directly affect the gain and thus the signal output power, but not for noise frequencies substantially above the inverse upper-state lifetime. (The laser-active ions represents an energy reservoir which can effectively reduce the effect of fast power fluctuations.) A variable pump power can also lead to temperature-dependent heating which translates into phase noise.
ASE itself may be utilized for superluminescent sources with very low temporal coherence, as required e.g. for optical coherence tomography.

The efficiency, gain, and signal/noise ratio of three-level devices are quite sensitive to the magnitude and spectrum of the cross sections. These, in turn, depend on the glass composition used as host. We report an investigation which determined the relevant cross sections for a variety of glasses and used them to predict performance trends for fiber amplifiers. Glasses have been identified which have advantages over silica, particularly for pumping with AlGaAs diode lasers at 800 nm.
The gain dynamics of erbium-doped fiber amplifiers are studied both experimentally and theoretically. It is shown that the transients associated with gain saturation and gain recovery during multichannel amplification have long characteristic times, i.e. in the 100 its-1 ms range. Such slow gain dynamics effectively prevent saturation-induced crosstalk and intermodulation distortion effects in the amplification of high-speed WDM and FDM signals.
A large signal model of an optically pumped Erbium-doped travelling wave fibre amplifier is presented, including the influence of excited state absorption. The maximum gain and optimal amplifier length are discussed, based on the intensity of pump, signal and amplified spontaneous emission. Special attention is given to the gain degradation depending on the mode distribution of the pump light, and noise figures of the amplifier is presented for co- and counter propagating pump light. Finally, the variation of calculated results towards changes in the values of measured constants is considered.
We report on a parametric study to optimize the Er3+-doped fiber amplifiers performances. A 24 dB efficient gain in Er3+- doped silica-based fibers has been achieved for small signal regime at 1.553 μm with a 40 mW pump power in the 1.48-1.49 μm range. The saturation output signal power was up to 20 mW for a gain of 17 dB. For the signal wavelength a 3dB-bandwidth of 30 nm has been obtained always with a pump around 1.48 4m. We achieved these results by optimizing the relevant parameters which are the pump and signal characteristics (wavelength and power), the Er3±-concentration and length for the doped fiber. We report on the main parameters which determine the amplifier efficiency (gain, saturation output power, saturation gain, bandwidth, amplified spontaneous emission,...).

11, tunable loss filter 90 covering a wider bandwidth than a single long-period grating device can be constructed by concatenating adjustable chirp long-period gratings 91 along a single fiber 11. A desired loss spectrum can be obtained by selectively adjusting the chirp of the gratings.
FIG. 12 illustrates a dynamically gain-flattened amplifier 110 made by including a tunable loss filter 90 composed of the adjustable chirp long-period gratings in a rare earth doped amplifier (such as an erbium-doped fiber amplifier). The amplifier 110 preferably comprises a plurality of rare-earth fiber amplifier stages (e.g. two stages 111 and 112) with the tunable loss filter 90 preferably disposed at the output of the first stage. This gives the highest power and the lowest noise figure. For applications where noise is less important the filter 90 can be placed in front of the first stage 111. For applications where power is less important, it can be placed at the output of the last stage 112. Long-period gratings for flattening the response of an amplifier are described, for example, in U.S. Pat. No. 5,430,817 issued to A. M. Vengsarkar on Jul. 4, 1995, which is incorporated herein by reference. Such devices 110 can be advantageously used in WDM optical communication systems to ensure equalized amplification under a wide variety of conditions.
FIG. 13 schematically illustrates an optical WDM communication system comprising a source 150 of modulated WDM optical signal channels λ1, λ2, . . . λn along a trunk fiber 11. The channels pass through one or more gain equalized amplifiers 110, which can be gain flattened amplifiers as shown in FIG. 12 and through one or more ADD/DROP multiplexer devices 102, which can be ADD/DROP devices as shown in FIG. 10. The signals are received at one or more WDM receivers 154.
For extensions of the invention, the following represents applicants' best understanding of the theory underlying the invention. Non-linear finite element modeling was used to compute the steady state thermal distributions in operating devices with geometries like those described above.

Combined C- and L-band transmission can be achieved by making use of the wide
gain spectrum provided by Raman amplification. Besides broadband amplification, distributed Raman
amplifiers (DRA) also offer enhanced noise characteristics compared to Erbium-Doped Fiber Amplifiers
(EDFA), and enable a better control of fiber nonlinearities through a reduction of power variations
in the transmission fiber.
The transmission of 82x10-Gbps NRZ channels (100 GHz channel spacing) over 1500 km
of SSMF is considered. The line consists of 10 spans, each comprising 150 km SSMF and a hybrid EDFA/DRA
dispersion compensating module. This module contains two pump lasers to perform DRA in the C- and
L-bands and two dispersion compensation fibers (DCF) adjusted for the C- and L-bands. Additional
amplification is realized with two EDFAs. Two optical filters are used to flatten the response of
the combined DR and EDF amplification. The setup of the investigated system is displayed in
. The power
profile of the WDM signal in the first span is reported in
The eye diagrams of the channels located in the center of the C- and L-bands are reported in
for the cases
where the nonlinearities are switched off and on in the DCF. The channel power at the span input is set
to -6 dBm. We can observe that the L-band channel is more affected by fiber nonlinearities than the
C-band channel. This is due to the particularly large input power of this channel into the DCF
resulting from the nonflat gain of the DRA. For optimal operation, the hybrid amplification scheme
should be optimized for a better control of the channels input power into the DCF.
Keywords: High-capacity, Dense Wavelength Division Multipexing (DWDM), C-band, L-band, Distributed Raman Amplification (DRA), Nonlinear effects
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.

For example, various semiconductor optical amplifiers and fiber optical amplifiers can be used. The use of fiber amplifiers, and specifically erbium-doped fiber amplifiers, is well-known in the art and will be used in the examples described below. It should be noted that although erbium-doped fiber amplifiers are particularly well-suited to provide amplification in the present invention, and will be described herein, other suitable rare-earth elements may also be used, such as praseodymium, neodymium, and the like.
According to the principles of the invention, optical fiber amplification may be incorporated using a number of different configurations. For example, fiber optical amplifiers 390 may be placed before input optical couplers 310 in optical router portion 340 or after output optical couplers 320 in optical combiner portion 341. Alternatively, fiber optical amplifiers (not shown) may be distributed within the wavelength-selective optical fibers 325 in a similar manner as that described in our co-pending U.S. application Ser. No. 08/777,890, filed Dec. 31, 1996, which is herein incorporated by reference. In yet another configuration, fiber optical amplifiers (not shown) may be judiciously integrated with the tunable fiber gratings 330 along wavelength-selective optical fibers 325 as described in our co-pending U.S. applications, Ser. Nos. 08/920,390 and 08/920,391, both filed on Aug. 29, 1997, both of which are herein incorporated by reference.
Although not explicitly shown in FIGS. 8 and 9, it is contemplated that selected ones of fiber gratings 210 and 330, respectively, can be controlled to facilitate the appropriate “through” routing and “cross-connect” routing of individual channels within the multi-wavelength optical signals. Accordingly, the various control techniques previously described for FIG. 1 apply equally to the embodiments shown in FIGS. 8 and 9.
FIG. 10(a) shows one of the input optical couplers 310 from optical router portion 340 of optical cross-connect arrangement 300 (FIG. 9). The configuration in FIG. 10(a) essentially represents a 1×M wavelength-selective optical distributor which is a basic building block for the K×M wavelength-selective cross-connect arrangement.

- Erbium Doped Fiber Amplifiers (EDFA)
- Dense Wave Division Multiplexers (DWDM)
- Overpowered fiber optic systems
- Metal ion doped fiber
- High-power light source durability
- Wavelength independence
- Attenuation levels ranging from 1dB to 30dB, with standard and premium tolerances, plus custom configurations.
- 1310nm, 1550nm, 1250-1625nm and 1350/1550nm dual wave lengths
- UPC -- return loss 55dB or greater
- APC -- return loss 65dB or greater
|A fiber optic attenuator is a passive device used to reduce the amplitude of a light signal without significantly changing the wave form itself. This is often a requirement in Dense Wave Division Multiplexing (DWDM) and Erbium Doped Fiber Amplifier (EDFA) applications where the receiver cannot accept the signal generated from a high-power light source.
SENKO attenuators feature a proprietary type of metal-ion doped fiber which reduces the light signal as it passes through. This method of attenuation allows for higher performance than fiber splices or fiber offsets, which function by misdirecting rather than absorbing the light signal. SENKO attenuators are capable of performing in the 1310, C and L Bands.
SENKO attenuators are capable of withstanding over 1W of high power light exposure for extended periods of time, making them well-suited to EDFA and other high-power applications.
Low Polarization Dependent Loss (PDL) and a stable and independent wavelength distribution makes them ideal for DWDM.

Within optical microchips, light finds its way through channels, waveguides, made of silicon. Light from a glass fiber, for example, is led through a structure of optical channels with splitters and couplers. Silicon is the workhorse for this, but it is still passive conduction of light, with some losses as well. To be able to amplify the signal, or even to include a light source on the chip, extra steps are necessary. Other types of semiconductors, like Gallium Arsenide, are an option. But materials doped with the rare earth material erbium have good amplification properties as well.

VPIcomponentMaker™ Fiber Optics is an R&D tool for modeling, optimization, and design of fiber-based optical devices such as doped fiber, Raman and parametric amplifiers, continuous wave and pulsed optical fiber sources, optical signal processing for telecommunication, high-power and ultrafast applications.
The signal and noise models based on full-wave and parameterized representations are ideally suited for efficient analysis. The stationary and dynamic Er-,Yb-,Tm- and co-doped fiber and waveguide models simulate signal amplification, noise generation, and higher-order limiting effects in configurations with core- or cladding pumping at single or multiple wavelengths. The undoped fiber model simulates linear and nonlinear optical effects due to chromatic dispersion and PMD, Kerr nonlinearity, Raman, Brillouin and Rayleigh scattering.
Based on elementary building blocks, simple and sophisticated amplifier and laser topologies including active and passive components, signal monitors, control elements and feedback loops can be defined by the user. The general optimization capabilities of the design environment can be combined with specialized heuristic and deterministic algorithms to find, for instance, optimal wavelengths and powers of multiple Raman pumps to ensure a user-defined gain profile within acceptable tolerances. Virtual instrumentation provides versatile means to characterize the designed equipment with respect to the signal gain, noise level and power efficiency within wide range of operating conditions, which can be followed by conversion of the amplifier design to a rapid black box model.

Erbium Glass (Er,Yb:Glass)
Er,Yb:Glass/Er, Yb, Cr: Glass product, also known as erbium glass (er glass), is a kind of laser glass with good comprehensive performance.
Er3+, Yb3+ co-doped phosphate glass (Er, Yb: phosphate glass) is a well-known and commonly used active medium. It emits laser microns in the “eye safe” spectral range of 1.5 – 1.6um.
1540 nm laser is just located at the position of human eye safety and optical fiber communication window. So it leads to laser generation and signal amplification.
1540 nm laser has been widely used in range finder, radar, target recognition, and other fields.
Phosphate glass combines the long-lived (~8 ms) laser level on 4I13/2 Er3+ with the low-lived (2-3 ms) 2F5/2 excited state of 4I11/2 Er3+ level resonant with Yb3+.
The fast nonradiative multiphonon relaxation from 4I11/2 to 4I13/2 is due to the interaction between Yb3+ and Er3+ ions excited at 2F5/2 and 4I11/2 levels, respectively. That dramatically reduces the reverse energy transfer and conversion losses.
Er3+/Yb3+ co-doped phosphate glass is an LD pumped 1540 nm eye-safe radiation source. It can emit eye safe 1540 nm laser radiation directly used in laser rangefinders and telecommunications.
Er, Yb glass laser with 1540 nm wavelength radiation output does not need to add additional components.
Er3+/Yb3+ co-doped phosphate glass laser is an eye-safe wavelength laser. It has attracted much attention because of its compactness and low cost.
Eat14: Yb3+, Er3+ co-doped phosphate glass, suitable for high repetition rate (1-6Hz) laser diode pumped 1535 nm. High Yb3+ doping can be achieved in this Eat14 glass.

|Publication No: 1363||Search all ORC publications|
Advanced materials for fibre and waveguide amplifiers
James S.Wilkinson and Martin Hempstead
Recent research into optical fibre and waveguide amplifiers has been characterised by steady but unrevolutionary progress. Advances in the 1.5μm telecommunications window include erbium doped materials with flatter gain spectra, and planar amplifiers in a broad range of materials. At 1.3 μm, substantial improvements in praseodymium doped fluoride fibre amplifiers have been witnessed and chalcogenide glasses have shown promise for enhanced gain efficiency.
Current Opinion in Solid State and Materials Science (1997) Vol.2(2) pp.194-199
Southampton ePrint id: 78005
Copyright University of Southampton 2006

In this paper, a novel structure erbium-doped fiber laser with a linear cavity is demonstrated. The wavelength selective devices are a fiber Bragg grating (FBG) and a high birefringence (HiBi) fiber loop mirror. From 1543.8 to 1555.2 nm, 15 lasering wavelengths with side mode suppresion ratio (SMSR) > 54 dB and approximately 0.8-nm spacing have been obtained by using the comb-like reflection spectrum of fiber loop mirror and the intensity reaches about 2.5 dBm on an average. The reflectivity at different wavelengths can be tuned with different settings of polarization controller and the relative laser intensity can be controlled over a dynamic range of 13.5 dB.
© 2005 Chinese Optics LettersPDF Article

A stabilized and tunable single-longitudinal-mode erbium-doped fiber ring laser has been proposed and experimentally demonstrated. The laser is structured by combining the compound cavity with a fiber Fabry–Pérot tunable filter. An injection-locking technique has been used to stabilize the wavelength and output power of the laser. One of the longitudinal modes is stimulated by the injected continuous wave so that this mode is able to win the competition to stabilize the system. A minimum output power of 0.6 dBm and a signal-to-noise ratio of over 43 dB within the tuning range of 1527–1562 nm can be achieved with the proposed technique. A wavelength variation of less than 0.01 nm, a power fluctuation of less than 0.02 dB, and a short-term linewidth of about 1.4 kHz have also been obtained.
© 2007 IEEEPDF Article

The maximum benefit of gain filtering is obtained by designing the transverse ytterbium dopant profile to optimize the overlap of the gain with the fundamental mode while minimizing the gain-overlap of all other modes, performing a global optimization at all levels of saturation. It has been recently shown that gain filtering in fiber amplifiers can lead to better beam quality than the injected seed beam [7, 17].
Gain filtering possesses two unique features that set it apart from all other mode-control techniques: lossless filtering and geometrical overlap. The first factor is unique amongst all other mode-filtering techniques that, while providing loss to higher-order modes, also provide loss for the fundamental mode. This makes gain filtering the highest efficiency mode-filtering method available. Second, mode filtering relies on the geometric overlap of the modes with the gain profile. Rather than relying on the difference between modal indices that necessarily decreases with increasing core area, gain filtering is indefinitely scalable, since the mode profiles essentially do not change with increasing core area.
One anticipated drawback to gain filtering in round fibers is the aforementioned mode deformation, where the mode becomes compressed towards the outside edge of the bend for large core diameters. Although the reduced mode size is detrimental to most LMA fiber applications, for very large cores (~100 µm) the displacement of the mode towards the edge of the waveguide can reduce the effectiveness of gain filtering by altering the overlap of the deformed modes with the centralized gain region .
The SHARC fiber offers a unique advantage that can exploit gain filtering without this packaging limitation. The SHARC fiber is coiled in the fast-axis direction, but the gain filtering is applied in the slow-axis direction, as depicted in Fig. 1. Therefore, no slow-axis mode offset will be incurred, and the mode overlap with the gain will remain unchanged regardless of core area or coiling diameter. Consequently, the integration of gain filtering into the SHARC fiber yields the ideal architecture for significant core-area scaling of high-power fiber amplifiers to 30,000 µm2 and beyond.
3. SHARC fiber amplifier analytic calculations
As was discussed in detail in , the SHARC fiber geometry lends itself nicely to separation of variables such that the fast- and slow-axis physics can be handled nearly independently of each other. This makes direct analytical modeling possible, from which the primary physics can be obtained. The validity of this assumption and its benefits were confirmed with rigorous three-dimensional beam propagation modeling (BPM) simulations .

The beam quality factors were calculated to be Mx2 = 1.79 and My2 = 1.64, respectively, and the corresponding laser beam profile is shown in the inset of Fig. 2. The value of slope efficiency (5.4% and 6.5%) is lower comparing with the early reported 2 at.% Erbium concentration doped Y2O3 ceramic  (slope efficiency of 15%), which can be attributed to the lower transmittance of the output coupler and non-optimized pumping wavelength.
For the Er:Lu2O3 ceramic laser system with 967 nm LD pumping, the slope efficiency was about 7.6%, and the output power saturated when the absorbed power was 8.7 W. A maximum output power of 611 mW was achieved, corresponding to an optical-to-optical conversion efficiency of 7.0%. The beam quality factors were also measured at an absorbed pump power of 5.1 W. The quality factors were calculated to be Mx2 = 1.31 and My2 = 1.21, respectively, and the laser beam profile is shown in the inset of Fig. 2. In all cases, Er:Lu2O3 ceramic lasers show an excellent Gaussian transverse profile. Beam quality factors were measured at the maximum absorbed pump power of 8.7 W, and calculated to be Mx2 = 2.58 and My2 = 2.44, respectively. Generally speaking, under 967 and 976 nm LD pumping, Er:Lu2O3 exhibit a better performance in the eyes of slope efficiency, maximum output powers, and conversion efficiency. But it still couldn’t ascertain that which laser system would be a superior laser source for 3 μm transitions at room temperature, due to the non-optimized pump wavelength and laser cavity [14, 18].
It is worth noting that CW output power of 611 mW can be compared to the early reported Er:Lu2O3 crystal lasers with the similar bulk dimension and resonator conditions . Meanwhile, the value of slope efficiency (7.6%) of Er:Lu2O3 ceramic is much higher than that Er:Lu2O3 crystal, which can be attributed to the better mode matching. Nevertheless, confocal parameter of the pump beam inside the ceramic was only 2.3 mm, due to the marginal quality of the pump resource.

This result can be explained by the fact that the signal-to-noise ratio characteristic was degraded at the band edge of the Er-doped fiber amplifier.
We have demonstrated an all-polarization-maintaining, high-pulse-energy, wavelength-tunable, passively mode-locked Er-doped ultrashort pulse fiber laser using a polyimide film dispersed with single-wall carbon nanotubes. The extinction ratio was 16 dB, and stable, self-starting operation was achieved without any polarization control. The dependence on output coupling ratio was examined in detail. The output coupling ratio could be increased to 99% with a wavelength filter. The maximum output power was 12.6 mW, and the corresponding pulse energy was 580 pJ. Compared with previous work, the average power was a factor of 2.5 higher, and the pulse energy was a factor of 5 higher.
Wide wavelength-tunable operation from 1532 to 1562 nm was also demonstrated under the control of the wavelength filter. The output power and spectrum width of the output pulses were almost constant during the wavelength variation. The characteristics of RF noise were examined in detail, and the dependence on the output coupling ratio and oscillation wavelength were examined experimentally. Generally speaking, the magnitude of RF noise level was as low as that of commercially available solid state lasers. A slight noise increment was observed at large output coupling ratio and at the longer wavelength edge of the wavelength tuning band.
This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas.
References and links
1. M. E. Fermann, “Ultrafast fiber oscillators”, in Ultrafast Lasers, M. E. Fermann, ed. (Marcel Dekker, 2003), Chap. 3.
2. Y.-C. Chen, N. R. Raravikar, L. S. Schadler, P. M. Ajayan, Y.-P. Zhao, T.-M. Lu, G.-C. Wang, and X.-C. Zhang, “Ultrafast optical switching properties of single-wall carbon nanotube polymer composites at 1.55 μm,” Appl. Phys. Lett. 81(6), 975–977 (2002). [CrossRef]
3.

Er–Yb codoped phosphate glasses with improved gain characteristics for an efficient 1.55μm broadband optical amplifiers[J]. Journal of Luminescence, 2014, 148:249-255.|
| A C B , A T M , B S R D , et al. Sensitizing effect of Yb 3+ ions on photoluminescence properties of Er 3+ ions in lead phosphate glasses: Optical fiber amplifiers[J]. Optical Materials, 2018, 86:256-269.|
| Ryabtsev G I , Bezyazychnaya T V , Parastchuk V V , et al. Spectral and temporal properties of diode-pumped Er, Yb: glass laser[J]. Optics Communications, 2005, 252(4-6):301-306.|
| Francini R , Giovenale F , Grassano U M , et al. Spectroscopy of Er and Er–Yb-doped phosphate glasses[J]. Optical Materials, 2000, 13(4):417-425.|
| Sendova M , JA Jiménez, Honaman C . Rare earth-dependent trend of the glass transition activation energy of doped phosphate glasses: Calorimetric analysis[J]. Journal of Non-Crystalline Solids, 2016, 450:18-22.|
| Chen C , He R , Tan Y , et al. Optical ridge waveguides in Er3+/Yb3+ co-doped phosphate glass produced by ion irradiation combined with femtosecond laser ablation for guided-wave green and red upconversion emissions[J]. Optical Materials, 2016.|
| Balaji S , D Ghosh, Biswas K , et al. Insights into Er 3+ Yb 3+ energy transfer dynamics upon infrared ~1550nm excitation in a low phonon fluoro-tellurite glass system[J]. Journal of Luminescence, 2017, 187:441-448.|
| Feng S , Fei L , Li S , et al. The fractional thermal factor in LD-pumped Yb3+/Er3+ codoped phosphate glass. IEEE, 2009.|
| Dan G , Mihailov S J , Walker R B , et al.

Erbium-doped silica optical fibers were irradiated by γ-rays, and their loss-increasing characteristics were analyzed. The loss is mainly induced by Er2+ created by the reduction of Er3+. It does not show a simple dependence on the Er3+ concentration. The loss increase is described by the sum of several saturating exponential terms and one linear term, which can be derived from the yield equation of E ′γ centers or other species induced from precursors by hole trapping. It is considered that hole trapping supplies electrons to induce the reduction and plays an important role in the loss increase.
ASJC Scopus subject areas

Bragg Gratings Made With a Femtosecond Laser in Heavily Doped Er–Yb Phosphate Glass Fiber[J]. IEEE Photonics Technology Letters, 2007, 19(12):943-945.|
| Zhu M , Tongzhao G U . Spectroscopic properties of Er3+/Yb3+ co-doped tantalum-niobium phosphate glasses for optical waveguide laser and amplifier[C]// International Conference on Microwave & Millimeter Wave Technology. IEEE Xplore, 2004.|
| Danger T , Huber G , DeNker B I , et al. Diode-pumped cw laser around 1.54 m using Yb, Er-doped silico-boro-phosphate glass[C]// Conference on Lasers & Electro-optics. IEEE, 1998.|
| S Jiang, SJ Hamlin, JD Myers,等. High-average-power 1.54-μm Er3+:Yb3+-doped phosphate glass laser. 1996.|
| Osellame R , Valle G D , Chiodo N , et al. Mode-locked and single-longitudinal-mode waveguide lasers fabricated by femtosecond laser pulses in Er:Yb-doped phosphate glass. IEEE, 2007.|
| Valles J A , Rebolledo M A , J Cortés. Full characterization of packaged Er-Yb-codoped phosphate glass waveguides[J]. IEEE Journal of Quantum Electronics, 2006, 42(2):152-159.|
| Aseev V A , Ulyashenko A M , Nikonorov N V , et al. Comparison of highly doped Ytterbium-erbium phosphate and silicate glasses for microchip lasers[C]// International Conference on Advanced Optoelectronics & Lasers. IEEE, 2005.|
| Qiu T , Li L , Temyanko V , et al. Generation of high power 1535nm light from a short cavity cladding pumped Er:Yb phosphate fiber laser[M]. 2004.|
| Buchenkov V A , Krylov A A , Mak A A .

INO draws on its unique expertise in active fiber design and fabrication to develop innovative solutions for its clients. INO boasts a number of newly patented large-mode-area (LMA) fiber designs providing outstanding performance for scaling fiber amplifier output power and energies while preserving high beam quality and good pointing stability.
Fiber modeling (multiple INO patents for multiclad LMA fiber designs)
LMA fibers with core sizes up to 40 microns and high output beam quality (M2 < 1.5) for good pointing stability and a high SBS threshold
Gain-selective doping for optimized beam quality
Polarization-maintaining LMA fibers with a high signal polarization extinction ratio, well suited to harmonic generation
Gain blocks assemblies
Our designs draw on INO’s unique multidisciplinary expertise in fiber modeling and MCVD technologies. They use INO’s unique proprietary multiclad LMA fiber designs to preserve high beam quality in LMA fibers for the benefit of our clients. INO holds many patents in multiclad LMA fiber designs.
Multistage pulsed-fiber lasers & amplifiers
High-power fiber lasers
Single-mode fiber amplifiers/lasers
LMA Yb fibers, Er-Yb fibers
PM fibers for 2nd and 3rdharmonic generation
INO is one of the few remaining independent active fiber manufacturers. Moreover our mission is to develop custom fibers for our clients. INO can help you with highly specialized fiber designs for multistage laser amplifiers as well as a wide range of specially designed undoped fibers.
Other INO services include tapered fibers, fiber gain blocks, and laser soldering and polishing of fiber end caps. INO also offers support for the design and fabrication of fiber laser prototypes and can supply fiber to clients who produce their own fiber lasers. INO’s wide range of expertise in electronics, lasers, optical design, and material processing can be an important advantage for clients seeking solutions for specialty applications. INO also offers short-run production when needed.
INO has many years of experience designing and fabricating Yb-doped, Er-Yb-doped, hollow, and PM fibers, as well as high-attenuation fibers and microstructured fibers.

The high absorption confined-doped ytterbium fiber with 40/250 μm core/inner-cladding diameter is proposed and fabricated, where the relative doping ratio of 0.75 is selected according to the simulation analysis. By employing this fiber in a tandem-pumped fiber amplifier, an output power of 6.2 kW with an optical-to-optical efficiency of ∼82.22% is realized. Benefiting from the large-mode-area confined-doped fiber design, the beam quality of the output laser is well maintained during the power scaling process with the beam quality factor of ∼1.7 of the seed laser to ∼ 1.89 at the output power of 5.07 kW, and the signal-to-noise ratio of the output spectrum reaches ∼40 dB under the maximum output power. In the fiber amplifier based on the 40/250 μm fully-doped ytterbium fiber, the beam quality factor constantly degrades with the increasing output power, reaching 2.56 at 2.45 kW. Moreover, the transverse mode instability threshold of the confined-doped fiber amplifier is ∼4.74 kW, which is improved by ∼170% compared with its fully-doped fiber amplifier counterpart.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Fiber lasers/amplifiers have developed rapidly and gained widespread attention in the last several decades owing to their compact structure, high conversion efficiency, high-power capability, and good beam quality, etc [1–3]. Hitherto, the output powers from fiber lasers/amplifiers have already exceeded the 10-kW level [4,5], which enabled a wide variety of applications, especially in industrial manufacturing [6,7]. However, further power scaling of fiber lasers/amplifiers is challenged by the emergence of nonlinear effects resulted from the high power density within the core, such as stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS) [8–10]. The most straightforward solution is to increase the effective mode area by employing large mode area (LMA) fibers, but the cost is a degraded beam quality owing to the increment of supported modes, and very possibly a significantly reduced transverse mode instability (TMI) threshold [2,11].

To increase the output power while maintaining good beam quality in the LMA fiber lasers/amplifiers, it is essential to reduce the number of supported transverse modes, and in fact, lots of efforts have been devoted to the LMA fiber designs.
There are several fiber design strategies to reduce the number of supported modes and even realize effective single-mode operation in an LMA fiber. One way is to decrease the numerical aperture (NA) of the LMA fiber [12–15]. However, the low-NA LMA fiber is sensitive to bending due to the weak mode confinement, making it more demanding in practical use. Another practical issue is that the core NA of the optical fiber could not be arbitrarily reduced, mainly restricted by the technical and physical limitations , therefore, there is limited potential for the core size scaling by lowering the NA. Another approach is designing specialty optical fiber with microstructures or special refractive index profiles, such as chirally-coupled core fiber , hollow-core/hole-assisted photonic crystal fiber [18–20], all-solid photonic bandgap fiber [21,22], large pitch fiber [23,24], tapered fiber , and single or multiple trenched fiber [26,27], etc. These fiber designs could help realize effective single-mode operation in an LMA fiber, but they either require a complicated fabrication process or are difficult for post handling (cleaving and splicing) . Moreover, some of them are difficult to be fabricated into active fibers, making them less attractive in fiber oscillators/amplifiers . Therefore, an easy-to-fabricate and easy-to-use fiber design with good compatibility is in great demand.
A very promising solution is the confined-doped fiber design, in which only part of the core is selectively doped. In a conventional active fiber, the core is fully doped, and all the supported modes that propagate in the core could experience the gain. While in the confined-doped fiber, transverse mode discrimination, also known as the ‘gain filtering effect’, is established by separating the waveguiding function and the gain through selective doping, and only the modes that occupy the doped regions of the core could extract the gain. It has been proven both theoretically  and experimentally  that confining the doping area to the central part of the core can facilitate fundamental mode laser output.

As a comparison, laser spectra of Er:Lu2O3 ceramic lasers at stable output powers of 25 mW, 160 mW, and 575 mW were recorded respectively, as shown in Fig. 4. The corresponding laser wavelengths were 2714.8 nm, 2722.6 nm, and 2736.2 nm, respectively. With slit width of 20 μm, bandwidth measurements made with the monochromator indicate that the FWHM of the laser are all around 0.3 nm. What deserves to be mentioned the most is that all the laser spectra exactly avoided the numerous water vapor absorption lines in this wavelength band . And, with the pump power increased, red-shift of the lasing wavelength is observed in the 3 μm erbium sesquioxide ceramic lasers.
Red-shifting behaviors have been reported in other erbium doped lasers [1, 21]. This phenomenon can be understood as follows. Before the laser emission, the lower multiplet is empty. Short wavelength with large emission cross section more intend to oscillate at the beginning of laser emission (four-level nature) . As for Er:Y2O3 ceramic, four peak emission cross sections pretty close to each other , transitions with smaller water absorption losses, 2707.8 nm and 2723.0 nm, would be more prefer for oscillation at the beginning. Once the lower multiplet is populated during laser emission, the laser is forced to operate at longer wavelength due to reabsorption losses build up at the short wavelength side of the fluorescence. As for the Er:Lu2O3 ceramic laser systems, all the laser spectra surrounded with fewer water absorption lines. Transitions with larger emission cross sections, 2714.8 nm, would be more prefer to oscillate at the beginning. With pump power increased, the ETU process and the nonradiative transition process [4I13/2→ 4I15/2] cannot deplete the 4I13/2 state so efficiently that large number residual populations would accumulate in the long-lived 4I13/2 lower laser level. Then the character of the lasing process is changed from four-level to quasi-three-level lasing, the reabsorption process would be enhanced and the laser is forced to oscillate at longer wave.

The third system incorporates intra-fiber phase modulation. This allows considerable simplification of the laser cavity, resulting in a diode-pumpable source of 80 psec mode-locked pulses with a threshold pump power of 520 pW and a slope efficiency of 48%.
Laser fibers prepared from Nd- and Er-doped phosphate glass possessing a large stimulated emission cross section have been investigated both in a single fiber and in a fiber bundle. In the single fiber, continuous wave oscillations were successfully obtained at 1.054 p.m and 1.366 µm on a high Nd-doped single-mode fiber of 10 mm in length and also at 1.535 pm in a Er-doped single-mode fiber, sensitized by Nd, Yb. Especially, a low threshold of 1 mw and a high slope-efficiency of 50% were achieved in 1.054 pm laser oscillation on a Nd-doped fiber, end-pumped with a laser diode. A fiber bundle of phosphate glass doped with 8 wt% Nd2O3 yielded an average output power of 100 W at 50 pps where the bundle was 4.6 mm in diameter and was side-pumped with flash lamps.
The performance and charactersics of various Q-switched fibre lasers are examined. All results presented are for laser diode pumped systems, emphasizing the practicality of the Q-switched fibre laser. Operation at 0.94, 1.06, 1.09 and 1.57Am wavelengths are covered. The results presented show that Q-switching is currently limited by the performance of the modulator. Theoretical modelling of the process allows us to determine the required properties for a modulator and in addition to predict the expected performance of Q-switched fibre lasers under different conditions.
We describe the construction and operation of an FM mode-locked fiber laser using a large-diameter Nd3+ doped silica fiber with dielectric reflectors deposited directly onto the fiber ends and an integrated ZnO acousto-optic phase modulator clamped to the fiber. Optical pulses of less than 60 ps duration have been generated at 432.77 MHz with 400 mW of power to the transducer, and 300-400 ps pulses have been observed with only 4011,W of modulator power, correspond-ing to less than 130 wad of phase modulation.

On the other hand, due to the smaller pump spot, and highly quantum defect, output power appeared roll over at absorbed pump power of 8.7 W, when the ceramic mount was set as 283, 290, and 298 K. However, the phenomenon didn’t appear at 278K, due to the high thermal conductivity of Er:Lu2O3, which indicates that a better cooling setup is very essential to heavily doped Er:Lu2O3 ceramic lasers.
With 967 and 976 nm LD pumping, a comparison of the laser performance of erbium doped transparent ceramics, Er:Y2O3 and Er:Lu2O3, both with 15 at.% doping, has been undertaken. True CW operation of the Er-doped sesquioxide ceramic was obtained for the first time at room temperature, with a short plane-parallel cavity. An output coupler with a 1.5% transmission was adopted for both Er:Y2O3 and Er:Lu2O3 ceramic lasers. The maximum output powers of 320 mW and 611 mW were obtained, respectively, with diode pumping wavelength at 967nm. The study indicates that under 967 and 976 nm LD pumping, Er:Lu2O3 exhibit a better performance compared to Er:Y2O3 system. Furthermore, the wavelength for both ceramics lasers were found to red-shift with the pump power increased, and the final and dominant wavelength determined to be 2736.2 nm and 2739.0 nm for the Er:Lu2O3 and Er:Y2O3 ceramic lasers, respectively.
This work is supported by the National Natural Science Foundation of China (Grant No. 61177045, 61308047, and 11274144), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 13KJB510008), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
References and links
1. E. Arbabzadah, S. Chard, H. Amrania, C. Phillips, and M. Damzen, “Comparison of a diode pumped Er:YSGG and Er:YAG laser in the bounce geometry at the 3 μm transition,” Opt. Express 19(27), 25860–25865 (2011). [CrossRef] [PubMed]
2.

Erbium Fluoride ErF3 is applied in...
Erbium Fluoride ErF3 is applied in petroleum and environment protection catalysts, mischmetal, polishing powders and Rare Earth fertilizers. Heeger Materials (HM) provides Erbium Fluoride ErF3 at a competitive price. The purity and particle size can be customized.
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Availability date: 03/01/2013
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|Synonyms||ErbiumFluorid/Fluorure De Erbium/Fluoruro Del Erbio|
Erbium Fluoride (ErF3) Powder
Erbium Fluoride, High purity Erbium Fluoride is applied as a dopant in making optical fiber and amplifier. Erbium-doped optical silica-glass fibers are the active element in erbium-doped fiber amplifiers (EDFAs), which are widely used in optical communications. The same fibers can be used to create fiber lasers, In order to work efficiently, Erbium-doped fiber is usually co-doped with glass modifiers/homogenizers, often aluminum or phosphors.
|Er2O3 /TREO (% min.)||99.999||99.99||99.9||99|
|TREO (% min.)||81||81||81||81|
|Rare Earth Impurities||ppm max.||ppm max.||% max.||% max.|
|Non-Rare Earth Impurities||ppm max.||ppm max.||% max.||% max.|

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Definition: optical amplifiers with doped fibers as gain media
Fiber amplifiers are optical amplifiers based on optical fibers as gain media. In most cases, the gain medium is a glass fiber doped with rare earth ions such as erbium (EDFA = erbium-doped fiber amplifier), neodymium, ytterbium (YDFA), praseodymium, or thulium. This active dopant is pumped (provided with energy) with light from a laser, such as a fiber-coupled diode laser; in almost all cases, the pump light propagates through the fiber core together with the signal to be amplified. A special type of fiber amplifiers are Raman amplifiers (see below).
The originally dominating application of fiber amplifiers was in optical fiber communications over large distances, where signals need to be periodically amplified. Typically, one uses erbium-doped fiber amplifiers with signals of moderate optical power in the 1.5-μm spectral regions. Other important application areas of fiber amplifiers have been developed later. In particular, high-power fiber amplifiers are now used in laser material processing. Typically, these are based on ytterbium-doped double-clad fibers for signals in the spectral region of 1.03–1.1 μm. The output powers can be multiple kilowatts.
Gain and Output Power
Due to the possible small mode area and long length of an optical fiber, a high gain of tens of decibels can be achieved with a moderate pump power, i.e., the gain efficiency can be very high (particularly for low-power devices). The gain achievable is often limited by ASE (see below). The high surface-to-volume ratio and the robust single-mode guidance also allow for very high output powers with diffraction-limited beam quality, particularly when double-clad fibers are used. However, high-power fiber amplifiers usually have a moderate gain in the final stage, partly due to power efficiency issues; one then uses amplifier chains where the preamplifier provides most of the gain and a final stage the high output power.

Optical Amplifier Portfolio
The Lumentum Amplifier Portfolio
- Counter/Co-Propagating Raman
Amplifiers Our Raman amplifiers leverage internally developed, state-of-the-art 14xx pump lasers, internally developed intelligent algorithms for autonomous gain control, and robust safety features to deliver network-ready solutions. Key points of differentiation include market-leading metrics on power consumption (<45 W to support Raman gains in excess of 14 dB on SMF), high output power (900mW – 1200 mW), gain control accuracy, and density.
- Preamp and Postamp EDFAs
Lumentum has a suite of customizable preamp and postamp EDFA solutions to meet all amplification needs—whether at a ROADM node or at an in-line amplifier (ILA) node. Fixed-gain, variable-gain, and switchable-gain versions at varying output power levels and noise figure metrics are available that are commensurate with the cost and performance requirements of the application. All of our EDFA solutions are built around our internal high-power 980 nm pump lasers and offer our industry-leading automatic gain control with fast transient suppression as a core feature. Over 200,000 Lumentum-designed EDFAs have been deployed in DCI, meshed ROADM, and high-capacity long-haul networks.
- Raman/EDFA Hybrid Amplifiers
Our Raman/EDFA hybrid amplifiers combine Raman’s low effective noise figure with EDFA’s high output power to provide a high-OSNR solution suitable for high bit-rate long-haul applications. An integrated approach to the Raman/EDFA design optimizes spectral flatness and control flexibility to extract the best possible OSNR performance across a diverse range of fiber spans.
- High-Power EDFAs
Lumentum EDFAs support up to 25 dBm output power for point applications that are less sensitive to nonlinear penalties. Our high-power EDFA solutions are based on conventional 980 nm pump technologies, and leverage our internally developed, industry-leading 900 mW pump lasers.
- Switchable-Gain EDFAs
We have the broadest portfolio of switchable-gain EDFAs in the industry, with the largest number of network deployments to date. Switchable-gain EDFAs extend the benefits of variable-gain EDFAs a step further and enables using a single EDFA model across a wide range of fiber-span loss distributions in the network.

WDM systems use optical amplification rather than signal regeneration where possible. WDM becomes practical upon substitution of the optical amplifier for the usual repeater which depends upon electronic detection and optical regeneration. Use of the Er amplifier permits fiber spans of hundreds of kilometers between repeaters or terminals. A system in the planning stage uses optical amplifiers at 120 km spacing over a span length of 360 km.
The referenced article goes on to describe use of narrow spectral line widths available from the distributed feedback (DFB) laser for highest capacity long distance systems. The relatively inexpensive, readily available Fabry Perot laser is sufficient for usual initial operation. As reported in that article, systems being installed by Telefonos de Mexico; by MCI; and by AT&T are based on DS fiber.
A number of studies consider non-linear effects. (See, "Single-Channel Operation in Very Long Nonlinear Fibers With Optical Amplifiers at Zero Dispersion" by D. Marcuse, J. Lightwave Technology, vol. 9, No. 3, pp. 356-361, March 1991, and "Effect of Fiber Nonlinearity on Long-Distance Transmission" by D. Marcuse, A. R. Chraplyvy and R. W. Tkach, J. Lightwave Technology, vol. 9 No. 1, pp. 121-128, January 1991.) Non-linear effects studied include: Stimulated Brillouin Scattering; Self-Phase and Cross-Phase Modulation; Four-Photon Mixing (4PM); and Stimulated Raman Scattering. It has been known for some time that correction of the linear dispersion problem is not the ultimate solution. At least in principle, still more sophisticated systems operating over greater lengths and at higher capacities would eventually require consideration of non-linear effects as well.
WDM--Wavelength Division Multiplex, providing for multi-channel operation within a single fiber. Channels are sufficiently close to be simultaneously amplified by a single optical amplifier. At this time, the prevalent optical amplifier (the erbium-doped silica fiber amplifier) has a usable bandwidth of Δλ≅10-20 nm.
Dispersion--When used alone, the term refers to chromatic dispersion--a linear effect due to wavelength dependent velocity within the carrier spectrum.
Span--Reference is made here to a repeaterless fiber length.

Its use is contemplated in a species of this invention.
In its broadest terms, the invention reflects the observation that four-photon mixing is a relevant mechanism which must be considered in the design of contemplated WDM systems. A number of factors lend assurance to the assumption that the inventive teaching will take the form described above. For one thing, changing the carrier wavelength, e.g. to λ=1550.+-.20 nm, for introducing requisite dispersion into DS fiber, while in principle appropriate, is not easily achievable. The erbium amplifier at its present advanced state of development, has an operating peak near 1550 nm. Operation 20 nm off this peak reduces the carrier power level to an inconveniently low magnitude for one or more of the channels. It is conceivable that substitution for erbium or that some other change in design of the amplifier will permit this operation. It is more likely that future systems will continue to be designed taking advantage of the present or some more advanced stage of the conventional erbium amplifier.
Four-photon mixing depends upon the precise wavelengths of generated carriers. Evenly spaced, four-channel systems unavoidably satisfy this requirement. The likely significance of 4PM is somewhat reduced for a three-channel system, and precise uneven spacing even in a four-channel system may, in principle, avoid it as well. Problems in taking this approach require operating parameters which may be beyond the present state of the art, and, which in any event, would introduce some further expense. Reliable stabilization to maintain such precision, e.g. as due to thermal drift, is problematic.
These alternative approaches may not be seriously considered for newly installed systems, but may be of value for upgrading of inground systems--particularly, those with DS fiber in place.
This is a continuation of application Ser. No. 08/599,702, filed Feb. 9, 1996, now U.S. Pat. No. 5,719,696 which is, in turn, a division of application Ser. No. 08/069,952, filed May 28, 1993, issued as U.S. Pat. No. 5,587,830 on Dec. 24, 1996.

Especially high phonon energy host materials, such as garnet, erbium ions at concentrations no less than 30 at.% are needed to compensate the nonradiative transition, of which, the gain is concentrated near the surface with a short pump absorption depth. For shallow pump absorption, the induced temperature gradient may lead to distortion of the laser mode and strong thermal lensing with pronounced spherical aberrations and, ultimately lead to bulk materials fracture in high-power end-pumped system. Diffusion-bonded absorption-free materials as heat sink at the pump facet [4, 5] were proposed to improve this issue. Nevertheless, thermal stresses at the pump interface between the two materials would limit the average output power of the diffusion-bonded setup .
Recently, cubic sesquioxides have attracted considerable attention due to their superior thermo-mechanical properties, which can be easily doped with rare-earth ions and exhibit large heat conductivity which exceed that of YAG by up to 50% . Furthermore, their relatively low maximum phonon energy of ~600 cm−1 (Y2O3 591 cm−1, Lu2O3 612 cm−1, Sc2O3 672 cm−1) compared to YAG (857 cm−1) is very meaningful to make efficient laser operation, which reduce the probability of non-radiative transitions of the laser ion and thus improves the quantum efficiency . With substantial progress in the growth of single crystal as well as fabrication of polycrystalline Er-doped sesquioxides laser materials, efficient Er sesquioxide lasers emitting at 3 μm range have been reported [12–14]. However, the growth of sesquioxide crystalline is very challenging due to the extremely high melting point of more than 2400°C. Compared with single crystals, rare-earth elements doped transparent ceramic as new laser gain medium have drawn great attention [15, 16], due to the rapid and larger volume fabrication, larger doping concentrations with controllable distribution in the volume of material, profile and sample structure, etc. Continuous wave (CW) power of 14 W has been recently obtained from Er-doped Y2O3 ceramic lasers with cryogenic cooling (77 K) , which presents the highest output power reported to data for a ceramic Er3+ laser operating in the 3 μm range.

An optical amplifier is a device that amplifies an optical signal directly, without the need to first The most common example is the Erbium Doped Fiber Amplifier ( EDFA), where the core of a silica The amplification window of an optical amplifier is the range of optical wavelengths for which the amplifier yields a usable gain. My sincerest thanks also to all the members of Centre d’Optique, Photonique et . constmction of an EDFA and its amplification principles in sections and Amélioration de la dynamique de stabilisation des EDFA grâce à l’insertion d’un amplificateur optique à semiconducteur. Conference Paper · January with.
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Module d’amplificateur haute puissance. Such amplifiers are commonly used to produce high power laser systems. First, Raman gain exists in every fiber, which provides a cost-effective means of upgrading from the terminal ends. Parametric amplifiers use parametric amplification. Fibres de Plastique Doubles. Different sites expose ions to different local electric fields, which shifts the energy levels via the Stark effect.
Achromat Dispersion Gradient-index optics Hydrogen darkening Optical amplifier Optical fiber Optical lens design Photochromic lens Photosensitive glass Refraction Transparent materials.
Plaques Murales en Fibre Optique. Doped fiber amplifiers DFAs are optical amplifiers that use a doped optical fiber as a gain medium to amplify an optical signal. Becker, High-gain erbium-doped traveling-wave fiber amplifier,” Optics Letters, vol.
Manchons de Protection pour Connecteur RJ A relatively high-powered beam of light is mixed with the input signal using a wavelength selective coupler WSC. This effect is optiqus as gain saturation — as the signal level increases, the amplifier saturates and cannot produce any amplifictaeur output power, and amplficateur the gain ampliflcateur.
In addition to boosting the total signal gain, the use of the resonant cavity structure results in a very narrow gain bandwidth; coupled with the large FSR of the optical cavity, this effectively limits operation of the VCSOA to single-channel amplification.

Advantages & Disadvantages of FRA
- Variable wavelength amplification possible
- Compatible with installed SM fiber
- Can be used to extend EDFAs
- Can result in a lower average power over a span, good for lower crosstalk
- Very broadband operation may be possible
- High pump power requirements, high pump power lasers have only recently arrived
- Sophisticated gain control needed
- Noise is also an issue
After talking about these three types of optical amplifiers, we make a comparison of them as the following table.

The power per channel must be sufficient to provide an adequate signal to noise ratio in the presence of the amplified spontaneous emission (ASE) noise from the amplifiers, necessitating a high amplifier total output power for systems with high fully-loaded capacity. The amplifiers are thus configured to provide an optical output signal at a nominal optical power. The nominal output power level is insensitive to the power at the input of the amplifier. As the amplifier input power varies over a wide range, the output power changes very little around this nominal output power level. Thus, when the optical link is fully loaded, each channel is amplified to a substantially equal optical output power. If the initially deployed system uses only a few channels for information, these channels share all of the amplifier output power. As additional channels are added, the optical output power per-channel decreases.
When some channel powers increase compared to other channels, problems may be caused by an effect known as spectral hole burning (SHB). In an optical communication network using rare-earth-doped fiber amplifiers, such as erbium-doped fiber amplifiers (EDFAs), signal-induced saturation in the doped fiber medium may cause SHB. As a result of SHB, a gain depression or “hole” may be induced in the gain spectrum of a WDM system in the spectral vicinity of a saturated channel. When a WDM system is loaded with low channel counts during initial deployment, for example, the system may show a severely distorted gain shape such that the channels experience a higher power evolution along the system.
Reference should be made to the following detailed description which should be read in conjunction with the following figures, wherein like numerals represent like parts:
Turning now to
System 100 may be employed to span a body of water 104. When used to span a body of water, e.g. an ocean, repeaters 110 may be seated on the ocean floor 102 and the transmission path may span between beach landings. It will be appreciated that a plurality of repeater and optical media links may be disposed beneath water and/or over land.
When a system, e.g. system 100, is configured as a WDM system and initially deployed with unutilized channels, information signals on utilized channels may cause a distorted gain shape as a function of the loading configuration.

Notably, the beam quality was well maintained during the power scaling process: the beam quality factor of the seed laser was 1.7 and only slightly increased to 1.72 at 4.42 kW, and then to 1.89 at 5.07 kW owing to the onset of TMI at ∼4.74 kW. Moreover, the SRS was also suppressed to about -40 dB lower than the signal laser at the output power of 6.2 kW thanks to the large mode area design. Further power scaling and better beam quality could be expected by mitigating the TMI effect. In stark contrast, the beam quality factor of the fiber amplifier based on fully-doped 40/250 μm YDF degraded to 2.56 at the output power of 2.45 kW and the TMI threshold was around 1.76 kW. Therefore, compared with the fully-doped fiber amplifier, not only the beam quality was improved, but also the TMI threshold was increased by ∼170% in the confined-doped fiber amplifier. This work reveals the intrinsic advantages of the confined-doped fiber for high-order mode suppression as well as TMI mitigation and proves the feasibility of employing the confined-doped fiber for realizing high-power fiber laser with good beam quality. Single-mode operation in even higher power levels could be expected by systematically optimizing the fiber parameters, coiling radii, and pumping schemes, etc.
Innovative Research Groups of Hunan Province (2019JJ10005); Hunan Provincial Innovation Construct Project (2019RS3018); National Natural Science Foundation of China (62035015).
The authors would like to thank Dr. Zilun Chen’s group for fabricating the cladding mode striper and the pump and signal combiner, and Liang Xiao, Jiawei He, Jiaxin Song as well as Cong Zhou for their kind help in the experiment.
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
1. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]
2.

In this post, we would like to educate you about laser crystals.
|Erbium- and chromium-doped yttrium-scandium-gallium garnet (Er, Cr: YSGG) is an effective laser crystal for lasing with a wavelength of 2800 nm, which is an important water absorption band. Er, Cr: YSGG crystal has a high conversion efficiency, stable chemical properties, and a long fluorescence lifetime. These crystals are widely used in dentistry, environmental studies, communication systems, military industry and remote sensing technologies.
– High conversion efficiency;
– Excellent optical properties;
– Lasing modes modes: continuous, free generation or Q-modulation;
– The lowest generation threshold among standard erbium-doped crystals;
– the highest differential quantum efficiency among standard erbium-doped crystals;
– Natural disorder leading to an increase in the width of the spectral pumping line and in strength;
– Flash lamp (chromium absorption bands) or diode (erbium bands) pumping;
– Long fluorescence lifetime.
Main properties of Er,Cr:YSGG crystals:
|Neodymium-doped yttrium-vanadate crystals (Nd:YVO4) are some of the most effective laser crystals for coherent sources with diode pumping, especially for sources with low and middle power density. Their absorption and emission properties are even better than those of Nd:YAG crystals.
A laser diode pumped Nd:YVO4 crystal was combined with crystals with a high non-linearity factor (LBO, BBO or KTP) to shift the frequency of the lasing output from the near-IR to the green, blue or even UV spectral region. This combination in solid-state lasers is an ideal tool for most common laser applications, including material processing, spectroscopy, wafer quality control systems, laser printing, etc. It is proven that diode-pumped ND: YVO4 solid-state lasers quickly gain markets where water-cooled ion lasers and lamp-pumped lasers traditionally dominate, especially when a compact design and emission with a single longitudinal mode are required.

4(b), which are recorded by an optical spectrum analyzer with a 0.2 nm resolution. The full width at half maxima (FWHM) linewidth of the output spectrum increases from ∼1.14 nm of the seed laser to ∼4.46 nm under the maximum output power. Benefiting from the large mode area, the signal-to-noise ratio, defined as the intensity difference between the signal laser at 1080 nm and the Raman components at ∼1135 nm, is about 40 dB, indicating very good SRS suppression. Moreover, the proportion of the residual 1018 nm pump power in the total output power is calculated through spectral integration, which is ∼ 0.4‰. Therefore, the residual pump power only occupies a negligible proportion of the total output power.
The evolution of the beam quality factor (M2) is also measured under different output powers. The seed laser is a single-mode laser based on double clad fiber with 10 μm core size and core NA value of ∼0.08, however, when the pump laser is not applied, the beam quality factor of the seed laser becomes 1.43 after passing through the PSC, which is possibly resulted from the imperfect fabrication of the PSC. Then, the beam quality factor further degrades to 1.70 after passing through the confined-doped fiber owing to the refractive index profile mismatch between the 40/250 μm signal fiber of the PSC and the 40/250 μm YDF, as the refractive index profile of the confined-doped fiber is not uniform across the core (shown in Fig. 2(b)). The beam quality factor M2 maintains relatively well during the power scaling process, being 1.72 at the output power of 4.42 kW, as presented in Fig. 5. Further increase of the output power to 4.74 kW leads to the onset of TMI, where the beam quality starts to degrade, reaching 1.89 at the output power of 5.07 kW. As the output power continuously increases, the beam quality factor degrades rapidly to 2.05 at 5.37 kW and 2.3 at 6.20 kW owing to the severer TMI effect.
The temporal traces and the corresponding radio frequency spectra of this fiber amplifier are also recorded under different output powers.

Emission spectra of were obtained by illuminating the 0.5%-doped sample with a diode laser. Luminescence was collected with the Optical Spectrum Analyzer (Yokogawa, model AQ6370C). A polarization beam splitter was inserted into collecting optics to separate - and -polarized signals. The emission cross sections of the transitions, shown in Fig. 2, were calculated via the standard Fuchtbauer–Landenburg method , using the lifetime of the manifold of .
It can be seen that in the -polarized emission spectrum of there are three emission maxima where lasing can be expected: at (cross section ), at (), and at (). The relevant energy level scheme of the and the manifolds can be found in  and in . In the -polarized spectrum, there is one transition at suitable for laser operation with the peak cross section of .
Laser experiments were carried out with the antireflection-coated, long, thick 0.7% sample. The crystallographic axis of the crystal was normal to the axis of the laser cavity, thus enabling longitudinal pumping in any chosen polarization. The crystal was bonded between copper plates water-cooled to . A simplified experimental laser setup is shown in Fig. 3.
Two different sources were used for pumping into major absorption lines of : a CW Er-fiber laser with narrowband output ( FWHM) and a commercial, spectrally narrowed ( FWHM), fast- and slow-axis collimated laser diode bar stack (QPC Lasers). The Er-fiber laser output wavelength was tuned to the absorption band with large cross section and relatively small cross section.
The unpolarized pump beam was focused into the crystal by the spherical lens.
The diameter of the almost cylindrical pumped region inside the crystal was approximately ( level). The short laser cavity () consisting of a concave output coupler () and a flat dichroic mirror ( at , at ) resulted in the mode diameter of for a good mode matching.
Figure 4 shows the CW output power of the laser pumped by a Er-fiber laser. The best efficiency was achieved with the 5% outcoupling. The maximum obtained output power was , and the maximum slope efficiency . The fraction of the absorbed pump, carefully derived from the measurements of the transmitted pump while the laser was operational, was calculated at 41% to 44%.

EYDFA High Power Multi-output Optical Fiber Amplifier
Short Description:The EY-doped fiber amplifier is a high power optical amplifier. It offers a highly reliable, flexible and low-cost solution for CATV large area coverage of metropolises and medium sized cities, that are high density home pass, especially for FTTH application.
Multi Output EYDFA is a high power EDFA, max multi 32 ports, built in 1310/1490/1550 WDM. The Multi Output optical amplifier is a high power EDFA with 1535~1565nm bandwidth. It is primarily intended for CATV or applications that requires 1 to 8 continuous strip channels (ITU wavelength). It offers a low cost, flexible option for FTTH coverage of CATV systems, and it is on a broad scale in medium and large sized cities.
The Multi Output EYDFA can achieve single wavelength cable TV transmission, its outstanding and reliable performance ratio between the major application of MMDS, DBS, FTTX, FTTB, FTTB technology, is to build CATV huge Medium sized optical fiber transmission network.
- High output power, E-Y co-doped
- Warm backup, on-line standby, auto redundancy
- Support ITU channels banding
- Low optical input power, Adjustable output optical power -3dB
- APC / ATC circuit design, built-in high-power optical isolator, mainly used for fiber access network of EPON architecture
- Single/dual input for choice, built in optical switch for dual input, the switching power can be set by the button in the front panel or by web snmp.
- Output adjustable by buttons in the front panel or web snmp, the range is +0.5dBm～-4.0dBm
- Max multi 32 ports, with built in 1310/1490/1550WDM,max total output 37 dBm
- Standard RJ 45 port for remote control, we can provide output contract and web manager for choice, and also plug-in snmp hardware can be reserved for update.
- With laser key to turn on/off the laser.
- With RF test function.

Optical amplifier is an important technology for optical communication networks. Without the need to first convert it to an electrical signal, the optical amplifiers are now used instead of repeaters. As we know, there are several types of optical amplifiers. Among them, the main amplifier technologies are Doped fiber amplifier (eg. EDFA), Semiconductor optical amplifier (SOA) and Fiber Raman amplifier. Today, we are going to study and compare different types of optical amplifiers in this paper.
Before the comparison of the different types of optical amplifiers, let’s take a closer look at fiber optic amplifier. In general, a repeater includes a receiver and transmitter combined in one package. The receiver converts the incoming optical energy into electrical energy. The electrical output of the receiver drives the electrical input of the transmitter. The optical output of the transmitter represents an amplified version of the optical input signal plus noise. Repeaters do not work for fiber-optic networks, where many transmitters send signals to many receivers at different bit rates and in different formats. However, unlike a repeater, an optical amplifier amplify optical signal directly without electric and electric optical transformation. In addition, an ideal optical amplifier could support multi-channel operation over as wide as possible a wavelength band, provide flat gain over a large dynamic gain range, have a high saturated output power, low noise, and effective transient suppression. Several benefits of optical amplifiers as the following:
- Support any bit rate and signal format
- Support the entire region of wavelengths
- Increase the capacity of fiber-optic links by using WDM
- Provide the capability of all-optical networks, not just point-to-point links
OK, after a brief introduction of the optical amplifiers, we formally begin today’s main topic. As we talk above, there are three main types of today’s amplifier technology. Each of them has their own working principle, features and applications. We will describe them one by one in the following paragraphs.
Doped fiber amplifier (The typical representative: EDFA)
Erbium-doped fiber amplifier (EDFA) is the most widely used fiber-optic amplifiers, mainly made of Erbium-doped fiber (EDF), pump light source, optical couplers, optical isolators, optical filters and other components.

Mostly, all optical amplifiers are used in optical communication. Generally, Brillouin type amplifier is not used in optical communication. For a particular use, the decision has to be made about which amplifier to be used. The EDFA amplifier is used in-line amplifier on account of its compatibility. On the other hand, Raman Fiber Amplifier (RFA) will be a very good power amplifier because of its high saturation.
EDFAs and conventional lasers, achieve gain by pumping atoms into a high energy state. This allows the atoms to release their energy when a photon of a suitable wavelength passes nearby. RFAs utilize Stimulated Raman Scattering (SRS) to create optical gain. Because SRS robs energy from shorter wavelengths and feeds it to longer wavelengths, high channel count DWDM systems initially avoided this technique.
A RFA amplifier consists of little more than a high-power pump laser, usually called a Raman laser, and a WDM or directional coupler. The optical amplification occurs in the transmission fiber itself, distributed along the transmission path. With amplification up to 10 dB, RFAs provide a wide gain bandwidth (up to 100 nm), allowing them to operate using any installed optical fiber (single mode optical fiber, TrueWave, etc.). By boosting the optical signal in transit, RFAs reduce the effective span loss and improve noise performance.
Combined with EDFAs, RFAs create a wide gain-flattened optical bandwidth. The figure belwo shows the topology of a typical RFA. The pump laser and optical circulator comprise the two key elements of the RFA amplifier. In this case, the pump laser has a wavelength of 1535 nm. The optical circulator provides a convenient means of injecting light backwards into the transmission path with minimal optical loss.
Here are the figures which show the optical spectrum of a forward-pumped RFA amplifier and the received signal after the same length of fiber used in the SRS example. The signal gets injected by the 1535 nm pump laser at the transmit end rather than the receive end. Generally, the amplitude of the pump laser exceeds that of the data signals.
With a significant decrease in the amplitude of the pump laser, the amplitude of the six data signals has increased, giving all six signals roughly equal amplitudes. In this case, the SRS effect robbed a great deal of energy from the 1535 nm pump laser signal and redistributed that energy to the six data signals.

Fibre optic systems need stations every few kilometres to receive a weak light signal, convert it into electronic signal, amplify it, use it to modulate a laser beam again, and re-send it. This process is exposed to risk of noise and errors creeping into the signal; the system needs to get rid of the noise and re-send a fresh signal. It is like a marathon run, where the organisers place tables with refreshing drinks all along the route so that the tired and dehydrated runners can refresh themselves. This means a certain delay, but the refreshment is absolutely essential.
Submarine cables must have as few points as possible where the system can break down because, once the cable is laid several kilometres under the sea, it becomes virtually impossible to physically inspect faults and repair them.
The development, in the 1980s, of fibre amplifiers, or fibres that act as amplifiers, has greatly facilitated the laying of submarine optic fibre cables. This magic is achieved through an innovation called the erbium doped fibre amplifier. Sections of fibre carefully doped with the right amount of erbium—a rare earth element—act as laser amplifiers.
While fibre amplifiers reduce the requirement of repeater stations, they cannot eliminate the need for them. That is because repeater stations not only amplify the signal, they also clean up the noise (whereas fibre amplifiers amplify the signal, noise and all). In fact, they add a little bit of their own noise. This is like the popular party game called Chinese whispers. If there is no correction in between, the message gets transmitted across a distance, but in a highly distorted fashion.
Can we get rid of these repeater stations altogether and send a signal which does not need much amplification or error correction over thousands of kilometres? That’s a dream for every submarine cable company, though perhaps not a very distant one.
The phenomenon being used in various laboratories around the world to create such a super-long-distance runner is called a ‘soliton’ or a solitary wave. A Dutch gentleman first observed solitary waves nearly 300 years ago while riding along the famous canals of the Netherlands. He found that as boats created waves in canals, some waves were able to travel enormously long distances without dissipating themselves. They were named solitary waves, for obvious reasons. Scientists are now working on creating solitons of light that can travel thousands of kilometres inside optical fibres without getting dissipated.

The pump coupler concept for SHARC fibers is schematically shown in Fig. 12 . This coupler is spliced between two sections of active fiber (only short sections of which are illustrated) and is designed such that it allows pump radiation to enter the active fiber without imposing any appreciable loss on the signal beam, which also passes through the pump coupler. In order to accomplish this, the pump radiation is configured to enter and propagate at an angle relative to the SHARC fiber axis, thus allowing the pump radiation to efficiently enter the active fiber from the edge of the fiber and propagate along the plane of the semi-guiding core. The semi-guiding core of the pump coupler also efficiently carries the signal from one end to the other. Specifically, since the signal-beam fast-axis dimension may only be ~20 µm, the guiding in that direction is absolutely essential for efficient propagation of the beam from one active fiber through the pump coupler into the next active fiber. However, the slow-axis signal-beam dimension may be approximately ~1 mm or more. Since the pump-coupler length may be only ~5 to 10 mm, very little diffraction occurs in the wide direction, so no guiding is required by the pump coupler in that dimension. Figure 12 shows a pump coupler designed for bi-directional pumping, but this concept can also be adapted to applications in which both pump fibers inject pump power in the same direction.
In considering this pumping approach, it is useful to appreciate how large the acceptance etendue is for pumping a SHARC fiber. Consider a 20 μm x 1 mm core contained within a 200 μm x 1.2 mm pump cladding, and an outer cladding providing an NA of 0.45 for the outer boundaries of the pump cladding. This geometry presents a full-angle beam-parameter product of 1120 mm-mrad in the wide dimension, which is roughly equivalent to a linear array of > 10 pump fibers having a 200-μm core diameter and 0.22 NA. A SHARC fiber amplifier therefore easily accommodates efficient launch of a large number of state-of-the-art fiber-coupled pump diode packages.
A new class of optical fiber, the SHARC fiber, was analyzed in a high-power fiber amplifier geometry using the gain-filtering properties of confined gain dopants.

Compared with fully bulk-optical systems, the use of amplifying fibers has the advantage that sufficient single-pass gain is achievable so that the principle of a regenerative amplifier does not need to be used.
In some cases, a multi-stage amplifier, i.e., an amplifier chain, needs to be realized. This allows e.g. ASE suppression with filters or modulators between the stages, an optimized power efficiency and noise figure, and possibly a modular approach which increases the flexibility for further amplifier developments.
Most fiber amplifiers are not made of polarization-maintaining fibers, so they do not preserve the polarization state of the input. On the other hand, the amplification process itself is normally not polarization-dependent; this is an advantage over semiconductor optical amplifiers for use in telecommunications. In some cases, however, polarization hole burning can cause problems.
Fiber Amplifier Modules
Some companies offer fiber amplifier modules which can be convenient for OEM system integrators. Input and output are then often attached with the usual fiber connectors. A compact module contains not only the actual fiber amplifier(s), but also the control electronics for the pump diodes, and possibly extras such as an input and/or output power monitor, power stabilization, alarms, gain-flattening filters, etc. Such amplifier modules are available based on erbium-doped fibers, ytterbium-doped fibers, and others, and for various power levels.
Raman Fiber Amplifiers
Raman amplifiers are based not on a laser amplification process, but on Raman scattering in a fiber. They differ in various respects from rare-earth-doped amplifiers, and are discussed in the article on Raman amplifiers.
|||C. J. Koester and E. Snitzer, “Amplification in a fiber laser”, Appl. Opt. 3 (10), 1182 (1964)|
|||R. J. Mears, L. Reekie, I. M. Jauncey, and D. N.Payne, “Low-noise erbium-doped fibre amplifier operating at 1.54 μm”, Electron. Lett. 23, 1026 (1987)|
|||E. Desurvire, “Design optimization for efficient erbium-doped fiber amplifiers”, J. Lightwave Technol. LT-8, 1730 (1990)|
|||M.

This originates from the short nanosecond or less upper state lifetime, so that the gain reacts rapidly tochanges of pump or signal power and the changes of gain also cause phase changes which can distort the signals.
The performance of SOA is still not comparable with the EDFA. The SOA has higher noise, lower gain, moderate polarization dependence and high nonlinearity with fast transient time.
Fiber Raman amplifier (FRA)
Fiber Raman Amplifier (FRA) is also a relatively mature optical amplifier. In a FRA, the optical signal is amplified due to stimulated Raman scattering (SRS). In general, FRA can is divided into lumped type called LRA and distributed type called DRA. The fiber gain media of the former is generally within 10 km. In addition, it requires on higher pump power, generally in a few to a dozen watts that can produce 40 dB or even over gains. It is mainly used to amplify the optical signal band of which EDFA cannot satisfy. The fiber gain media of DRA is usually longer than LRA, generally for dozens of kilometers while pump source power is down to hundreds of megawatts. It is mainly used in DWDM communication system, auxiliarying EDFA to improve the performance of the system, inhibiting nonlinear effect, reducing the incidence of signal power, improving the signal to noise ratio and amplifing online.
The principle of FRA is based on the Stimulated Raman Scattering (SRS) effect. The gain medium is undoped optical fiber. Power is transferred to the optical signal by a nonlinear optical process known as the Raman effect. An incident photon excites an electron to the virtual state and the stimulated emission occurs when the electron de-excites down to the vibrational state of glass molecule. The Stokes shift corresponding to the eigen-energy of a phonon is approximately 13.2 THz for all optical fibers.

In contrast to the case of conventional round fibers where the flat-top profile provides better mode discrimination than Gaussian gain profiles , the Gaussian gain profile provides slightly better performance than flat-top gain profiles in SHARC fibers. At a relative gain width of 0.35, the fundamental mode has 1.6x higher gain than any other mode in the fiber. Keep in mind that these are differential gain values that simplistically lead to exponential gain. As such, the modal discrimination will be much larger than 60%, as will be calculated shortly.
Although the Gaussian gain profile yields higher modal discrimination, the remainder of this paper will explore the optimized flat-top gain width (45% of the waveguide width). Such flat-top confined gain fibers have already been fabricated in round fiber geometries [16, 17].
To analytically calculate the modal discrimination in a realistic SHARC fiber amplifier using gain filtering, it is necessary to integrate the gain along the fiber length; the saturation condition in the amplifier changes as a function of propagation distance because the signal experiences gain as it propagates. The analysis therefore needs to proceed in several steps. First the local nominal gain of the amplifier is calculated as a function of propagation distance into the fiber. From this, the gain experienced by each mode is calculated according to Eq. (1). Finally, the integrated gain of each mode along the length of the whole amplifier is combined with the SHARC fiber’s edge loss to calculate the net modal gain in the fiber amplifier.
Optical power evolution in conventional laser media proceeds according toEq. (1), and αk is the modal loss. For rare-earth ionic systems and resonant (in-band) pumping conditions, the laser kinetics behave as a quasi-three-level system. For such a type of system, the saturation intensity for signal wavelength is affected by saturation of the pump transition, and the effective signal saturation intensity becomes dependent on the pump saturation level . Since the pump intensity Ipump(z) necessarily varies along the fiber due to absorption by the ytterbium ions, the effective signal saturation also becomes z-dependent, taking the following form
In the quasi-three-level kinetics model, the small signal gain factor gss depends on the doping density and the signal wavelength λs, but is also nominally dependent on the pump saturation level Ipump/Ipsat.

H. Xiao, J. Leng, H. Zhang, L. Huang, J. Xu, and P. Zhou, “High-power 1018 nm ytterbium-doped fiber laser and its application in tandem pump,” Appl. Opt. 54(27), 8166–8169 (2015). [CrossRef]
40. P. Zhou, H. Xiao, J. Leng, J. Xu, Z. Chen, H. Zhang, and Z. Liu, “High-power fiber lasers based on tandem pumping,” J. Opt. Soc. Am. B 34(3), A29–A36 (2017). [CrossRef]
41. J. M. Fini, “Design of large-mode-area amplifier fibers resistant to bend-induced distortion,” J. Opt. Soc. Am. B 24(8), 1669–1676 (2007). [CrossRef]

A low-noise fiber frequency comb is demonstrated to improve the frequency accuracy and linewidth by suppressing the phase noise caused by the nonlinear self-phase modulation as well as the amplified spontaneous emission within the Er-doped fiber amplifier. The linewidth of the carrier-envelop-offset signal measures less than 1.9 mHz and the frequency stability well follows the reference Rb clock. This achievement will facilitate the use of the fiber frequency comb for industrial applications to precision near-infrared spectroscopy, frequency calibration, optical clocks and length metrology.
©2009 Optical Society of America
The frequency comb of femtosecond pulse lasers permits optical frequency measurement to be made traceable to the microwave frequency standard [1,2]. Ti:Sapphire femtosecond lasers provide a frequency comb being excellent in both the frequency stability and linewidth, but their practical use is limited particularly outside the laboratory environment by several reasons such as bulkiness, alignment complexity and external optical pumping. Fiber femtosecond lasers are now available, making rapid progress to compete with Ti:Sapphire counterparts by making the most of their intrinsic advantages of size, robustness and reliability . Stabilizing the frequency comb with reference to a radiofrequency time standard requires detecting two collective parameters; the pulse repetition rate (frep) and the carrier-envelope-offset frequency (fceo). Unlike frep that can be measured simply using a photodetector, fceo has to be extracted through a sequence of elaborate procedures; pulse power amplification, supercontinuum generation, frequency doubling and f-2f interference.
For fiber lasers, Nicholson et al. first demonstrated an all-fiber supercontinuum in which a negative-dispersion fiber was used to increase the amplification gain for subsequent spectral broadening with a hybrid highly nonlinear fiber . Positive pre-chirping was also tried using a single-mode fiber for the pulse power amplification with an Er-doped fiber to detect fceo with an S/N ratio of 30 dB . Special nonlinear-broadening fibers such as photonic crystal fibers and UV-exposed fibers were tested to obtain broader spectra [6,7]. Washburn et al. extracted fceo using a minimal length of nonlinear fiber to suppress phase noise with a standard deviation of 57 mHz at a gate time of 1 s . The common-path f-2f interferometer was proposed to detect fceo with an S/N ratio higher than 40 dB [9,10].

The SHARC fiber architecture also scales output power at a constant pump-etendue per output watt, thereby ensuring the possibility of generating higher output power levels without having to invent new pump-diode packages with increasingly higher brightness. As a quantitative example, carrying 3-kW of single-frequency optical power will require core dimensions of 20 μm × 1.5 mm, for a total core area of 30,000 μm2, which is equivalent to a circular core having a diameter of ~200 μm. In this example, stimulated Brillouin scattering (SBS) suppression occurs by virtue of the large core area and low intensity, which lead to an SBS threshold power in excess of 3 kW even for a kHz-range laser bandwidth. Hence, in order to deliver multi-kW-level optical powers, SHARC fibers do not require additional SBS suppression techniques such as multi-GHz signal modulation [11, 12], with its associated system complexity, or acoustic waveguide management [13–15].
The inclusion of gain into the SHARC fiber provides another opportunity for mode control, as depicted in Fig. 1. Gain filtering, a process that provides modal discrimination via gain instead of loss, has recently been investigated as a mode-control method in high-power fiber amplifiers [7, 16, 17]. Nominally, this is achieved by spatially tailoring the gain dopant profile, such as the step profile depicted in Fig. 1.
The top row of Fig. 2 depicts the impact of spatial gain saturation in conventional optical fiber amplifiers. Nominally seeded by the fundamental mode (left), the amplifier gain (red) is saturated as the optical power grows, leaving a spatial hole “burnt” into the gain profile at the center of the fiber where the fundamental mode intensity is the highest (center image). Higher order modes (HOMs), most of which have intensity nulls as the center of the fiber, can extract the gain at the edges of the fiber, resulting in higher net gain for the HOMs and degraded beam quality at the output of the fiber amplifier. By confining the gain to the central portion of the waveguide while maintaining the same refractive index profile (Fig. 2., bottom row), the gain extraction by the fundamental mode saturates nearly all of the gain, leaving no gain at the waveguide edges for the HOMs to exploit.

A number of the fiber fabrication methods have evolved in response to the interest in active fiber devices based on rare earth doped core silicate fiber. Descriptions of fabrication methods along with associated host composition and waveguide design issues will be presented.
We review theoretical models of fiber amplifiers, resonant fiber lasers and superfluorescent fiber lasers made of a single mode laser fiber, for example a silica fiber doped with a rare earth laser ion. The quantities which are investigated are the optical gain, the threshold and conversion efficiency in fiber sources, and the spectral narrowing in superfluorescent sources. These models take into account the nature of the laser transition (i.e. 3- or 4-level), the potential presence of pump ESA, and the guided nature of the pump and signal waves interacting along the fiber. The emphasis is placed on the development of simple, closed-form expressions for these quantities.
This tutorial presents a discussion of the device requirements which must be satisfied to obtain optimal performance of fibre lasers and amplifiers. The constructional details of fibre lasers and amplifiers and the range of resonator types that have been explored for fibre lasers are addressed.
We present a review of the main non-linear effects for light generation and amplification in optical fibers. Stimulated Raman Scattering and its applications to optical amplication, Four-Photon Mixing, Brillouin Scattering and Harmonic Generation are described. We emphasize the advantages and drawbacks of using optical fibers for the development of non-linear effects.
In this paper we describe work aimed at identifying and measuring properties of rare earth doped fibres which may affect their use and application in oscillators and amplifiers. The main dopants of interest are erbium(1.55μm), and neodymium (0.9, 1.06, 1.3μm). We shall discuss the measurement, and effects, of parasitic absorption from the upper laser level (known as excited state absorption, or ESA), at both pump and signal wavelengths. We shall also consider how detailed measurements of fluorescence decay from the excited level can yield information about quenching, upconversion, and energy transfer processes.
The initial discovery of fluorozirconate glasses at the University of Rennes, France in 1974  followed an attempt to fabricate large pieces of ZrF4 crystals doped with Nd (NdZrF7) for laser applications.

This broadening is both homogeneous all ions exhibit the same broadened spectrum and inhomogeneous different ions in optiqeu glass locations exhibit different spectra.
Such reflections disrupt amplifier operation and in the extreme case can cause the amplifier to become a laser. Sonde de Fibre Portative.
Sa and different geometries disk, slab, rod to amplify optical signals. In semiconductor optical amplifiers SOAselectron – hole recombination occurs. These devices are similar in structure to, and share many features with, vertical-cavity surface-emitting lasers VCSELs. Finally, there are concerns of nonlinear penalty in the amplifier apmlificateur the WDM signal channels. Panneaux de Brassage Cat5e. They are related to fiber lasers. The amplification bandwidth of Raman amplifiers is defined by the pump wavelengths utilised and so amplification can be provided over wider, and different, regions than may be possible with other amplifier types which rely on dopants and device design to define the amplification ‘window’.
The broad gain-bandwidth of fiber amplifiers make them particularly useful in wavelength-division multiplexed communications systems as a single amplifier can be utilized to amplify all signals being carried on a fiber and whose wavelengths fall within the gain window.

In recent years, high-power fiber lasers/amplifiers employing the confined-doped LMA ytterbium-doped fiber (YDF) have also been reported. In 2016, Mashiko et al. demonstrated a 2-kW fiber oscillator operating at 1080 nm with the beam quality factor M2=1.2 based on the confined-doped fiber , and the output power was further scaled to 3 kW with M2=1.3 in the following year by increasing the pump power . The effective mode area of this confined-doped fiber is ∼400 μm2, however, one of the key parameters, i.e., the relative doping ratio of the core, was not mentioned. In 2018, Liao et al. fabricated a confined-doped YDF using the modified chemical vapor deposition (MCVD), the core and inner cladding diameters of which are 35 and 400 μm, respectively . Around 51% of the central core area was doped with Yb-ions. Finally, the beam quality factor was improved from 2.8 in a fully-doped fiber with a similar core/inner-cladding diameter to 1.5 by using this confined-doped fiber in a fiber oscillator. And a similar beam quality factor was obtained in a ∼450-W fiber amplifier. In the same year, Seah et al. reported a 4.1 kW tandem-pumped 1060 nm fiber amplifier by using the confined-doped fiber . The core/inner-cladding diameter of this confined-doped fiber is 42/250 μm and around ∼75% of its core diameter is doped with Yb-ions. The beam quality factor M2 was 1.59 under the maximum output power, which was significantly improved compared with the beam quality factor of M2=2.77 from the fully-doped fiber amplifier. In 2020, Zhang et al. fabricated a confined-doped Yb/Ce co-doped aluminosilicate fiber with 33/400 μm core/inner-cladding diameter, where ∼70% across the core diameter was doped with Yb/Ce-ions .

Optical EngineeringDesign and performance analysis of a tunable and self-pulsation diode pumped double-clad D-shaped Yb3+-doped silica fiber laser
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A wide range of applications have emerged for tunable ytterbium fiber lasers as single-frequency sources for spectroscopic applications, pumping source of Pr:ZBLAN amplifier, and Tm:ZBLAN upconversion laser, material processing, and military applications. In this paper, a 975-nm high power fiber coupled high power diode laser module of up to 5 W end pumping a ytterbium-doped multimode D-shaped fiber laser has been investigated. This used a Fabry-Pérot cavity with output coupler reflectivities of 80%, 60%, and Fresnel reflection of 4%. The output laser wavelength was tuned over a wide range of more than 50 nm, from 1041 to 1094 nm for cavity lengths from 1 to 10 m, respectively. An optical-to-optical slope efficiency of 45% was found for a 1-m cavity length, which increased to 60% for a 4-m cavity length. The maximum slope efficiency of 82.1% for a cavity length of 2 m was measured with the Fresnel reflection output coupler, and the measured lowest threshold pump power for this high gain cavity configuration was 130 mW. The threshold lasing pumping powers of 4.3, 4.5, and 4.7 W were dependent on the output coupler reflectivities of 80%, 60%, and Fresnel reflection of 4%, respectively. Also, self-pulsation phenomena were observed only at higher level pumping powers of more than 4 W and at longer cavity length.

As expected, the ER increases with the total pump power due to higher CE, until it reaches a maximum value of more than 20 dB for a total pump power of about 28.5 dBm. At such a pump power value, a CE of −7 dB is obtained, which includes the total insertion losses of the PPLN, of about 3.25 dB.
For higher pump power values, the power of the converted signal starts to flow back to the original wave and the ER starts to decrease. Contrarily to what was expected, the maximum ER for each OWC processes is not achieved at the same pump power value, but at 28.3 dBm when the input signal is at 1552.48 nm and at 29 dBm for the other case. These results can be explained by a slightly asymmetrical disposition of the frequency of the interacting waves (ωs2 + ωP2 ≠ ωs1 + ωP1), so that one of the OWC is not perfectly quasi-phase matched, requiring higher pump power values to maximize the ER. According to our numerical simulations, even a small wavelength detuning from the ideal conditions of 0.005 nm for each signal, which was the resolution of the available ECLs, is sufficient to change the optimal pump power by about 1.5 dB. Hence, the optimum operation conditions must be set as a trade-off between the ER of each OWC process, as it will also be shown in the following subsection.
The optimization of the pump power for WDE is critical since even a small deviation of less than 0.5 dB is enough to deteriorate the ER by more than 10 dB. Therefore, stable and precise tuning of the pump power and of the operation temperature of the PPLN waveguide are crucial for WDE.
4.2 Pump power optimization by measuring BER of swapped signals
As mentioned above, due to finite ER, any un-depleted power at input wavelengths becomes deleterious crosstalk impairing the swapped signals, especially for the in-band-crosstalk-sensitive high-order QAM signals. For a given PPLN, ER is mainly dependent on the pump power.

Active neodymium and erbium doped fibre devices
In this thesis a number of rare-earth-doped fibre devices are described including fluorescent and superfluorescent sources as well as several laser configurations. The laser configurations are all-fibre and include a neodymium-doped ring laser and recirculating delay line, a novel tunable neodymium-doped fibre laser and a single-frequency travelling-wave erbium-doped ring laser. The latter device has been the first description of a travelling-wave fibre laser device. Theory describing general fibre amplifier and laser devices is incorporated. A novel lumped element approach to fibre laser theory has been given applicable to 3 and 4-level laser devices which, under certain conditions, allows single pass gain of a fibre device to be described simply by the absorbed pump power. Numerical modelling of the erbium-doped fibre amplifier has been described which allows for analysis of a general device showing pump excited-state absorption. Results from the analysis have shown a difference in gain characteristics between co-propagating and counter-propagating signal/pump schemes when subject to pump excited-state absorption. In addition, the effect of pump direction on the noise figure is characterised in both small and large signal operating regimes. Characterisation of neodymium-doped fibres has shown a number of effects which will affect their use in amplifier and oscillator configurations. These include observation of sensitivity of the fluorescence characteristics to pump wavelength, observation of excited state absorption and polarisation of fluorescence. Additionally, the spectral gain-saturation characteristics have been investigated.

A superluminescent source has to contain little more than a high-gain fiber amplifier.
It is possible to model (→ laser modeling) the essential performance aspects of fiber amplifiers in various ways, normally using suitable fiber simulation software. Part of such a model is typically a set of rate equations, with which the population densities for given signal and pump intensities can be calculated. Such a rate equation model may be incorporated in a more comprehensive model which then calculates the optical powers along the fiber.
Applications of amplifier models are manifold. For example, it is possible to quantify various detrimental effects on the amplifier performance, and use such results for optimizing the fiber parameters or other aspects of the amplifier design.
Although basic properties of a fiber amplifier can be calculated analytically, a full quantitative understanding is normally possible only with numerical simulations. These can take into account various details such as the quasi-three-level behavior, strong gain saturation (with optical powers often being far higher than the saturation powers), amplified spontaneous emission (ASE) due to the high optical gain, and possibly energy transfer process for sophisticated situations e.g. with erbium-ytterbium-doped fibers.
Even in simple cases, relatively complex behavior can result. Figure 2 shows an example, where a simple ytterbium-doped fiber amplifier is pumped at 940 nm. The decay of pump power in the device is first quite fast, then slower, and finally faster again. This results from gain saturation by ASE. Forward ASE is partially reabsorbed before it reaches the right end.
Figure 3 shows the ASE spectra for the forward and backward direction. One recognizes that the ASE in the 1030-nm region is quite similar in both directions, whereas strong 975-nm ASE occurs only in backward direction. That pronounced asymmetry is related to the fact that the right end of the fiber, being only weakly pumped, provides a seed for backward ASE by spontaneous emission, even though the gain at 975 nm is substantially negative there.
If we now also inject a 1-mW input signal at 1030 nm, gain saturation keeps the ASE at a lower level, and most of the power can be extracted with the signal (see Figure 4).
Even this simple example exhibits various sophisticated details, which can hardly be understood without numerical simulations.

Besides, in order to further improve the beam quality, one can either flatten the refractive index of the core by compensating the refractive index of the undoped region, thus enabling a more spread mode distribution, and at the same time adopt a large coiling radius to alleviate bend distortion, or incorporate the graded-index fiber design to make the fiber resistant to bend distortion .
Furthermore, a piece of fully-doped YDF with core/cladding diameter of 40/250 μm is adopted to study the beam quality evolution and the TMI threshold. This fully-doped YDF is coiled to an equivalent bending radius to that of the confined-doped fiber. As the output power increases, the beam quality factor of the proposed fiber amplifier gradually degrades from 1.52 of the seed laser to 2.28 at 1.76 kW, and then to 2.56 at the output power of 2.45 kW, as indicated in Fig. 7. The TMI threshold is around 1.76 kW, which is nearly 3 kW lower than our confined-doped fiber amplifier. The beam quality factor of the seed (M2=1.52) after passing through the fully-doped fiber amplifier is slightly better than that of passing through the confined-doped fiber amplifier (M2=1.7), which could result from a better mode matching between the output signal port of PSC and the fully-doped YDF. However, the beam quality factor of this fully-doped fiber amplifier constantly degrades even before the TMI appears. These results further prove the advantages of the confined-doped fiber for high-order mode suppression and TMI mitigation.
In summary, the gain property of the fundamental and high-order modes in the confined-doped fiber with 40/250 μm core/inner-cladding diameter was theoretically analyzed by simultaneously considering the transverse mode distribution and transverse hole burning effect. Subsequently, the confined-doped fiber with a 0.75 doping ratio was designed and fabricated after a comprehensive evaluation of the gain filtering effect and the fiber length. Based on the fabricated confined-doped fiber, 6.2 kW output power at 1080 nm was realized in a fiber amplifier through the forward tandem pumping scheme.

SAN DIEGO, March 9, 2020 /PRNewswire/ -- OFC 2020 -- Furukawa Electric Co., Ltd. (FEC) announces it has developed FRSi4XX Series pump sources for forward Raman amplifiers that extend transmission distances in ultrahigh-speed optical fiber communications farther than conventional systems.
Proliferation of smartphones has led to a dramatic increase in communication traffic, including the expansion of wireless backbones, cloud computing, video streaming, and the penetration of social networks. To deal with this traffic explosion, improvement in optical signal-to-noise ratio (OSNR) is becoming an important factor in soon-to-be-deployed ultrahigh-speed optical fiber communications such as 400 Gbps and beyond. Existing erbium-doped fiber amplifiers (EDFA), which are widely used in current systems, do not have sufficient OSNR performance. Demand is increasing for Raman amplifiers due to their excellent noise characteristics. Forward Raman amplifiers, which make the most of the advantages of Raman amplification, are expected to be a technology necessary for increasing transmission distances.
In the past, only the backward Raman amplifier was used due to limitations of the noise characteristics of the pump source. Furukawa Electric's new FRSi4XX Series novel pump sources make it possible to realize forward Raman amplifiers and feature high output as well as excellent low-noise characteristics.
The important characteristics of these products are high power output and low noise. Furukawa Electric achieved these characteristics by leveraging the design, manufacturing technology and high-precision packaging of its InP (Indium Phosphide) optical semiconductor chip. The result is a pump source with a high-output chip structure and high-efficiency coupling technology. The optical output of 100 mW or more was achieved through an optimized heat dissipation design.
The FRSi4XX pump series reduces noise by about 20 dB/Hz compared with conventional pump sources for Raman amplifiers.
Combining the FRSi4XX Series with existing FOL1439 Series, yield pump sources especially well-suited to forward pumping Raman amplifiers.
As demand for ultrahigh-speed optical fiber communications continues to grow, FEC will further enhance the technology of this series and contribute to the construction of information and communications infrastructure in anticipation of the advancement of 5G.

The performance of such fibres as Raman fibre amplifiers is discussed with particular reference to their bandwidth for applications in 1.5 - 1.6 AM telecommunications systems.
Optical absorption strengths and spontaneous transition rates are presented for Er3+ ions in Ge-doped silica glass at concentrations from 0.5x1019 cm-3 to 4.0x1019 cm-3. Results for transitions in the 480 - 1700-nm range are extracted from measure-ments on Er-doped preform cores prepared by the solution-doping technique. A com-parison of the spontaneous lifetime of the 4113/2 level with the strength of the corresponding ground-state absorption reveals a conflict with the Einstein A-B relation. Measurements made on fibers exhibit a significant change in linewidth and fluorescence decay time as compared to the corresponding preforms. From these results the magnitude and spectral dependence of the cross sections for absorption as well as stimulated emission were determined for single-mode fibers.
The Stark levels of the 4115/2 ground state manifold have been determined for Er3+-doped fluoride, fluorophosphate, and silicate bulk glasses from fluorescence-line-narrowing measurements at 4.2 K. Splittings between adjacent Stark levels were observed to be 20-80 cm-1. The total energy spread of the manifold ranged from 335 to 400 cm-1. The energy of a given Stark level varied up to 60 cm-1 depending on the particular Er3+ sites excited. Using the 4.2-K results, homogeneous broadening is found to be a reasonable approximation for the 300-K luminescence band.
Neodymium and Erbium doped Silica-based optical fibres are a promising material in fibre laser and amplifier. The match-type with Si02 cladding and Ge0z-Si02 core single-mode fibres were fabricated by NCVD soot method. Rare-earth were doped by a soot imprenation technique. The fibres with various amounts of Nd3+·and Er3+codoping with Yb3+were also obtained. The quantity of dopants can be controlled by deposition temperature and doping concentrations in solution. The absorption spectrum and fluorescence characteristics were measured. The concentration of rare-earth ion in fibre can be in excess of 1 wt%. The minimum attenuation can be as low as 5-10 db/km at lasing wavelength.

Paschotta, case study on an erbium-doped fiber amplifier|
|||R. Paschotta, case study on a pulsed ytterbium-doped fiber amplifier|
|||R. Paschotta, tutorial on "Fiber Amplifiers"|
|||R. Paschotta, tutorial on "Modeling of Fiber Amplifiers and Lasers"|
See also: erbium-doped fiber amplifiers, high-power fiber lasers and amplifiers, rare-earth-doped fibers, fibers, double-clad fibers, gain equalization, fiber lasers, amplifiers, ultrafast amplifiers, Raman amplifiers, distributed amplifiers, superluminescent sources, fiber simulation software
See also the article on “Fiber-based high-power laser systems”, contributed by external authors.
If you like this article, share it with your friends and colleagues, e.g. via social media:

The inner cladding is processed into an octagonal shape with a diameter (flat-to-flat) of 250 μm and the core diameter is measured to be ∼40 μm, as indicated in Fig. 2(a). As shown in Fig. 2(b), the refractive index of the doped region is slightly higher than the undoped region, which occupies around 75% of the core, indicating good adherence to our design parameters. The absorption coefficient of this fiber is measured to be ∼0.8 dB/m at 1018 nm in the low power regime.
3. Experimental setup
The fabricated confined-doped fiber is applied in a master oscillator fiber amplifier (MOPA) platform similar to our previous experiments [39,40], which consists of a seed laser and a one-stage fiber amplifier as depicted in Fig. 3. The seed laser is a 100 W-level single-mode fiber laser operating at 1080 nm, which is injected into the fiber amplifier through a (6 + 1)×1 pump and signal combiner (PSC). The core/inner-cladding diameters of the PSC’s input and output signal port are 10/125 μm and 40/250 μm, respectively. A piece of 25-meter-long the as-fabricated confined-doped YDF is used to provide the active gain. The pump sources are three 3 kW level 1018 nm fiber laser modules, which are fusion spliced to the pump ports of the PSC. The redundant pump ports of the PSC are angle cleaved to 8°. A cladding mode striper (CMS) is spliced after the confined-doped YDF to remove the residual cladding power. Finally, the amplified laser is output through a quartz block holder (QBH). The core/inner-cladding diameter of the CMS as well as the QBH’s pigtail fiber is 40/250 μm. The confined-doped fiber is water-cooled on a heat sink.
The output laser from the QBH is first collimated by a collimator and then split into two beams by a highly reflective mirror (HRM). The reflected beam is measured by a power meter. Since the HRM could only reflect majority of the 1080 nm laser, the 1018 nm pump laser can pass without significant loss.

2-5 are "eye" diagrams which, as plotted on coordinates of power and time, show the contrast between ones and zeros in the bit stream as due to the various forms of dispersion including linear dispersion and 4PM for a four-channel system. The basic operating system characteristics for all of these figures are the same. They differ in the characteristics of the fiber.
1. Technical Field
The field addressed concerns high capacity optical fiber networks operative with wavelength division multiplexing. Systems contemplated: are based on span distances which exceed 100 kilometers; depend upon signal amplification rather than repeaters within spans, and use three or more multiplexed channels each operative at a minimum of 5.0 gbits per second.
2. Description of the Prior Art
The state of the art against which the present invention is considered is summarized in the excellent article, "Dispersion-shifted Fiber", Lightwave, pp. 25-29, Nov. 1992. As noted in that article, most advanced optical fiber systems now being installed and in the planning stages depend upon dispersion-shifted fiber (DS fiber). A number of developments have led to a preference for a carrier wavelength at 1.55 μm. The loss minimum for the prevalent single-mode silica-based fiber is at this wavelength and the only practical fiber amplifier at this time--the erbium amplifier operates best at this wavelength. It has been known for some time that the linear dispersion null point--the radiation wavelength at which the chromatic dispersion changes sign and passes through zero--naturally falls at about 1.31 μm for silica-based fiber. DS fiber--fiber in which the dispersion null point is shifted to 1.55 μm--depends upon balancing the two major components of chromatic dispersion; material dispersion and waveguide dispersion. Waveguide dispersion is adjusted by tailoring the fiber's index-of-refraction profile.
Use of DS fiber is expected to contribute to multi-channel operation--to wavelength division multiplex (WDM). Here, multiple closely spaced carrier wavelengths define individual channels, each operating at high capacity--at 5.0 gbit/sec or higher. Installation intended for WDM either initially or for contemplated upgrading uses three or more channel operation, each operating sufficiently close to the zero dispersion point and each at the same capacity. Contemplated systems are generally based on four or eight WDM channels each operating at or upgradable to that capacity.

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Perspective and overview
Glass structure and fabrication techniques
8 other sections not shown
achieved applications B.J. Ainslie bandwidth beam cavity cladding co-doped concentration configuration coupler D.C. Hanna D.N. Payne decay rate device diode dispersion dopant Electron emission cross-sections energy levels equations Er3+ erbium erbium-doped fibre erbium-doped fibre amplifiers excited-state absorption fibre core fibre lasers fibre length filter fluorescence fluoride fluoride fibre fluoride glasses frequency gain medium glass hosts host glass input inversion laser transition lasing Lett loss manifold mode mode-locking modulation Nd3+ neodymium noise non-linear non-radiative decay operation optical amplifiers optical fibre oscillator output power peak photons polarisation population population inversion power amplifier preform Proc propagation pulse pump laser pump power pump wavelength Q-switched quantum efficiency radiative Raman rare-earth ion-doped rare-earth ions refractive index region result saturation semiconductor laser signal wavelength silica fibre slope efficiency soliton spectra spontaneous emission Stark levels Telecom temperature three-level threshold transmission up-conversion upper laser level ZBLAN

Top. Quantum Electron. 12 (2), 233 (2006)|
|||J. Limpert et al., “High repetition rate gigawatt peak power fiber laser systems: challenges, design, and experiment”, IEEE J. Sel. Top. Quantum Electron. 15 (1), 159 (2009)|
|||G. D. Goodno et al., “Low-phase-noise, single-frequency, single-mode 608 W thulium fiber amplifier”, Opt. Lett. 34 (8), 1204 (2009)|
|||E. Desurvire, Erbium-Doped Fiber Amplifiers: Principles and Applications, John Wiley & Sons, New York (1994)|
|||M. J. F. Digonnet, Rare-Earth-Doped Fiber Lasers and Amplifiers, 2nd edn., CRC Press, Boca Raton, FL (2001)|
|||A. Galvanauskas, “Ultrashort-pulse fiber amplifiers”, in Ultrafast Lasers: Technology and Applications (eds. M. Fermann, A. Galvanauskas, G. Sucha), Marcel Dekker, New York (2002), Chapter 4, pp. 155-218|
|||R. Paschotta, Field Guide to Optical Fiber Technology, SPIE Press, Bellingham, WA (2010)|
|||P. Urquhart (ed.), Advances in Optical Amplifiers (open-access online edition available), InTech, Rijeka, Croatia (2011)|
|||ITU Standard G.661 (07/07), “Definitions and test methods for the relevant generic parameters of optical amplifier devices and subsystems” (pre-published), International Telecommunication Union (2007)|
|||ITU Standard G.662 (07/05), “Generic characteristics of optical amplifier devices and subsystems”, International Telecommunication Union (2005)|
|||ITU Standard G.663 (04/00), “Application related aspects of optical amplifier devices and subsystems”, International Telecommunication Union (2000)|
|||R. Paschotta, “Fiber amplifiers – a technology for many applications”. Part 1: introduction, Part 2: various technical issues, Part 3: examples for fiber amplifier designs|
|||R.

The invention relates to optical waveguides, such as, for example, optical fibers, and to amplifiers and lasers that include optical waveguides, such as for example, fiber lasers and fiber amplifiers, and to systems including such amplifiers and lasers.
Fibers, such as fiber lasers and fiber amplifiers, can be used to enhance absorption of pump energy. One type of fiber, commonly referred to as a double clad fiber, includes a core, a first cladding around the core and a second cladding around the first cladding. The core can comprise a rare earth material. The first cladding can be capable of receiving pump energy for absorption by the rare earth material. The second cladding can tend to prevent the pump energy from escaping the first cladding.
The invention typically relates to optical fibers, fiber lasers and fiber amplifiers, and to systems including such fibers and fiber devices.
In one aspect, the invention features a fiber (e.g., a multimode fiber) that includes a first region, a core and a cladding. The core surrounds the first region, and the cladding surrounds the core. Typically, the core includes an active material, such as, for example, a selected rare earth material.
In a further aspect, the invention features a system that includes two fibers. One of the fibers has a first region, a first core (e.g., a multimode core) surrounding the first region, and a cladding surrounding the core. The other fiber has a core (e.g., a single mode core). The fibers are in optical communication (connected) such that energy can propagate from the core of one fiber to the core of the other fiber. Typically, at least one of the cores includes an active material.
Embodiments of the invention can include one or more of the following features.
The core can be ring-shaped.
The core can be a multimode core.
The core can include a rare earth-doped material.
The core can include a silica material and ions of a rare earth metal.
The first region can include a silica material.
The first region can have a lower index of refraction than the core.
The first cladding can include a silica material.
The first cladding can have a lower index of refraction than the core.
The fiber can further include a second cladding surrounding the first cladding.
The second cladding can be formed of a polymer material.

SPIE 10083, 100830Y (2017). [CrossRef]
33. L. Liao, F. Zhang, X. He, Y. Chen, Y. Wang, H. Li, J. Peng, L. Yang, N. Dai, and J. Li, “Confined-doped fiber for effective mode control fabricated by MCVD process,” Appl. Opt. 57(12), 3244–3249 (2018). [CrossRef]
34. C. P. Seah, W. Y. W. Lim, and S. L. Chua, “A 4 kW fiber amplifier with good beam quality employing confined-doped gain fiber,” in Laser Congress 2018 (ASSL), OSA Technical Digest (Optical Society of America, 2018), paper AM2A.2.
35. F. Zhang, Y. Wang, X. Lin, Y. Cheng, Z. Zhang, Y. Liu, L. Liao, Y. Xing, L. Yang, N. Dai, H. Li, and J. Li, “Gain-tailored Yb/Ce codoped aluminosilicate fiber for laser stability improvement at high output power,” Opt. Express 27(15), 20824–20836 (2019). [CrossRef]
36. Z. Zhang, F. Zhang, X. Lin, S. Wang, C. Cao, Y. Xing, L. Liao, and J. Li, “Home-made confined-doped fiber with 3-kW all-fiber laser oscillating output,” Acta Phys. Sin. 69(23), 234205 (2020). [CrossRef]
37. B. Wang, L. Pang, and J. Liu, “Single mode 2.4 kW part-doped ytterbium fiber fabricated by modified chemical vapor deposition technique,” Proc. SPIE 11427, 114271X (2020). [CrossRef]
38. Z. Jiang and J. R. Marciante, “Impact of transverse spatial-hole burning on beam quality in large-mode-area Yb-doped fibers,” J. Opt. Soc. Am. B 25(2), 247–254 (2008). [CrossRef]
39.

The Transmitter
This element as well as the receiver and optical amplifier are described in detail in "Fiber Laser Sources and Amplifiers IV", SPIE, vol. 1789, pp. 260-266 (1992). The transmitter consists of a laser for each channel. Laser outputs are separately modulated and modulated signals are multiplexed to be fed into the transmission line.
III. The Receiver
This element, at the end of a span length, may be at the system terminus or may be part of a signal regenerator. It includes a means for demultiplexing the channels. This requires a device which passes the channel wavelength of interest while blocking the others. This may be a simple splitter combined with optical fibers at the output ports tuned to each channel SPIE ref. cited in preceding paragraph or may be a device which combines the functions of splitting and filtering in a single unit.
IV. Optical Amplifier
This element, today, is an erbium amplifier. The useful gain region of a single erbium amplifier is λ=40-50 nm When amplifiers are connected in a series, the net gain narrows (since the amplitude within the "gain region" is reduced on either side of the peak). The 10-20 nm bandwidth referred to is a reasonable value for a three-amplifier span.
V. Other Considerations
For the most part, other considerations are standard With few exceptions, WDM systems designed for use with DS Fiber may be directly used for the invention. System design is in accordance with considerations common to the prior art and the invention. Channel spacing is necessarily such as to fit the channels within the peak of the optical amplifier. Span length maxima are set by insertion loss, launch power, and tolerable pulse spreading. Considerations may be tailored naturally in accordance with constraints imposed. For example, use of WDM fiber without compensation sets a limit on the product of bit rate and span length. Span length may be set by convenience, e.g. where compensation is to be provided, or where a concatenated fiber length is to begin.
Planned WDM systems use external modulation to lessen dispersion penalty and to improve the spectral stability of the channels.
FIG. 1 is a schematic diagram of a WDM system which serves for discussion of the various inventive species.
FIGS.

L. Dakss and W. J. Miniscalco, “Fundamental limits on Nd3+-doped fiber amplifier performance at 1.3 μm”, IEEE Photon. Technol. Lett. 2, 650 (1990)|
|||C. R. Giles and E. Desurvire, “Modeling erbium-doped fiber amplifiers”, J. Lightwave Technol. 9 (2), 271 (1991)|
|||M. Øbro et al., “Highly improved fibre amplifier for operation around 1300 nm”, Electron. Lett. 27, 470 (1991)|
|||Y. Ohishi et al., “A high gain, high output saturation power Pr3+-doped fluoride fiber amplifier operating at 1.3 μm”, IEEE Photon. Technol. Lett. 3, 715 (1991)|
|||T. Rasmussen et al., “Optimum design of Nd-doped fiber optical amplifiers”, IEEE Photon. Technol. Lett. 4, 49 (1992)|
|||T. Whitely, “A review of recent system demonstrations incorporating 1.3-μm praseodymium-doped fluoride fiber amplifiers”, IEEE J. Lightwave Technol. 13 (5), 744 (1995)|
|||T. Sakamoto et al., “35-dB gain Tm-doped ZBLYAN fiber amplifier operating at 1.65μm”, IEEE Photon. Technol. Lett. 8, 349 (1996)|
|||R. Paschotta et al., “Ytterbium-doped fiber amplifiers”, IEEE J. Quantum Electron. 33 (7), 1049 (1997)|
|||G. C. Valley, “Modeling cladding-pumped Er/Yb fiber amplifiers”, Opt. Fiber Technol. 7, 21 (2001) (useful review on amplifier modeling)|
|||J. Limpert et al., “High-power femtosecond Yb-doped fiber amplifier”, Opt. Express 10 (14), 628 (2002)|
|||J. Limpert et al., “High-power ultrafast fiber laser systems”, IEEE J. Sel.

THE Japanese communications giant Nippon Telegraph and Telephone (NTT)
has sent digital information through a record length of optical fibre, without
having to amplify the signal electronically. The feat has impressed developers
of fibre optic systems around the world, and could mean significant improvements
in the performance of optical fibres both for long-distance transmissions
and for sending signals to many separate terminals or subscribers.
When a signal travels through an optical fibre, some of it is lost,
or attenuated. The signals must, therefore, be amplified after passing through
roughly 50 kilometres of fibre. In current systems, the optical signals
must be converted to an electronic form before they can be amplified by
a repeater. The stronger electronic signal is then used to drive an optical
source connected to the next length of fibre.
Repeaters are costly and prone to failures. Correcting them can be expensive,
for example in cables travelling under the sea. The repeaters must also
match the transmitters and receivers used in the cable, so the cable cannot
be upgraded without also replacing the repeaters.
Designers would prefer to amplify the signals directly without having
to convert them first to an electrical form. British Telecom’s researchers
at the company’s laboratories in Martlesham Heath, near Ipswich, have been
working on one approach in which semiconductor lasers amplify the light.
At a recent conference in San Francisco on optical fibres, British Telecom
reported that it had developed semiconductor amplifiers that the researchers
believe produce the highest output from a semiconductor optical amplifier.
However, the performance of semiconductor laser amplifiers has been
eclipsed by another approach using repeaters made of lengths of optical
fibres doped with small quantities of erbium, a rare earth element. When
the erbium atoms are energised by the light they produce light at a wavelength
of around 1500 nanometres. Conveniently, this is the wavelength at which
the signals in the optical fibres are least attenuated. Erbium amplifiers
generate light which is in phase with the light they amplify, in a similar
fashion to amplifiers based on semiconductor lasers.
At the meeting in San Francisco, the researchers from NTT reported that
they had set their record using 25 erbium amplifiers, each 160 metres long,
separated by 80 kilometres of standard optical fibre.

Erbium as a probe of everything?
Physica B , Volume 300 p. 78- 90
Erbium is a lanthanide ion with unique electronic and optical properties. In its trivalent state it is composed of an incompletely filled 4f inner shell and two closed outer shells. By employing these properties in specific material systems, Er can be used to probe point defects, oxygen, OH, Er, radiation defects, network structure, excitons, optical density of states, optical modes, and photonic bandstructure.

However, this dependence approaches a constant value for typical conditions of strong pump saturation (the case relevant to high-power Yb:fiber lasers) and can therefore be ignored, allowing gss to be constant along the length of the fiber.
To first order, bi-directional pumping leads to nearly uniform pump distribution along the length of the fiber. Consequently, Eq. (3) can be taken as constant along the fiber by assuming proper values for the pump intensity and its saturation intensity. In this approximation, Eq. (2) can be solved for the fundamental mode assuming an amplifier with a fixed gain, and gss as the constant fitting parameter. Typical fiber amplifier performance (gain = 30, efficiency = 80%) and a 1-kW nominal output power lead to the starting values listed in Table 1 . The geometrical cross-sections are taken from the core and cladding of the fabricated SHARC fiber described in .
Using the values listed in Table 1 with the emission and absorption cross-sections calculated for ytterbium-doped silica glass fiber , a small signal gain of 12.9 dB/m is obtained by numerically solving Eq. (2). Note that this value is the modal gain. Given the optimized gain width of 45% of the waveguide width, the material gain required to yield the 30-gain amplifier (assuming fundamental-mode operation) is 18.7 dB. Note that this value is typical for a ytterbium-doping level of ~0.2 wt%, a level much lower than is conventionally required for high-power dual clad fiber lasers due to the very high (2.5%) core/cladding area ratio inherent in the SHARC fiber geometry.
The final piece to analytically modeling a SHARC fiber amplifier is the inclusion of the edge loss with the relative modal gains calculated via Eq. (1). As described in the introduction and in more detail in , the modal loss due to the non-TIR waveguide edges is given byEq. (4) with Eqs. (2) and (3) allows prediction of the net modal gains integrated through the SHARC fiber amplifier as a function of the index-step in the slow-axis waveguide. The results, shown in Fig. 4 , clearly demonstrate the high modal discrimination expected from SHARC fiber amplifiers employing gain filtering.

In single-mode fiber performance is primarily limited by chromatic dispersion (also called group velocity dispersion), which occurs because the index of the glass varies slightly depending on the wavelength of the light, and light from real optical transmitters necessarily has nonzero spectral width (due to modulation). Polarization mode dispersion, another source of limitation, occurs because although the single-mode fiber can sustain only one transverse mode, it can carry this mode with two different polarizations, and slight imperfections or distortions in a fiber can alter the propagation velocities for the two polarizations. This phenomenon is called fiber birefringence and can be counteracted by polarization-maintaining optical fiber. Dispersion limits the bandwidth of the fiber because the spreading optical pulse limits the rate that pulses can follow one another on the fiber and still be distinguishable at the receiver.
Some dispersion, notably chromatic dispersion, can be removed by a 'dispersion compensator'. This works by using a specially prepared length of fiber that has the opposite dispersion to that induced by the transmission fiber, and this sharpens the pulse so that it can be correctly decoded by the electronics.
Fiber attenuation, which necessitates the use of amplification systems, is caused by a combination of material absorption, Rayleigh scattering, Mie scattering, and connection losses. Although material absorption for pure silica is only around 0.03 dB/km (modern fiber has attenuation around 0.3 dB/km), impurities in the original optical fibers caused attenuation of about 1000 dB/km. Other forms of attenuation are caused by physical stresses to the fiber, microscopic fluctuations in density, and imperfect splicing techniques.
Each effect that contributes to attenuation and dispersion depends on the optical wavelength. There are wavelength bands (or windows) where these effects are weakest, and these are the most favorable for transmission. These windows have been standardized, and the currently defined bands are the following:
|O band||original||1260 to 1360 nm|
|E band||extended||1360 to 1460 nm|
|S band||short wavelengths||1460 to 1530 nm|
|C band||conventional ("erbium window")||1530 to 1565 nm|
|L band||long wavelengths||1565 to 1625 nm|
|U band||ultralong wavelengths||1625 to 1675 nm|
Note that this table shows that current technology has managed to bridge the second and third windows that were originally disjoint.

2(b) adopts a dispersion-compensating fiber of large negative chirp along with the fiber amplifier of a relatively large 8 µm core diameter producing positive chirp. This combination enables elimination of the zero dispersion point within the fiber amplifier, thereby reducing the nonlinear phase noise and spectral broadening. This advantage is successfully verified in the output spectrum of the amplified pulses measured at the exit of the fiber amplifier as shown in Fig. 2(c). No significant spectral broadening is observed for the scheme of Fig. 2(b), but this not the case for that of Fig. 2(a).
The net dispersion has to be reduced to zero at the inlet of the HNLF fiber, which is accomplished by adjusting the overall length of the single-mode fiber installed to connect the fiber amplifier and the HNLF fiber via the wavelength-division multiplexer (WDM). This make the pulse duration become short with the peak pulse energy being concentrated for effective generation of an octave-spanning supercontinuum. The spectral broadening in the fiber amplifier affects the power coupling efficiency of the WDM as its transmission bandwidth is confined within the wavelength range of ±10 nm about 1550 nm. The broadened spectrum of the EDF amplifier of 4 µm diameter used in Fig. 2(a) suffers considerable power loss, requiring more pumping power to compensate for the coupling loss.
The spontaneous emission in the Er-doped fiber amplifier produces a small amount of stray light which undergoes Fresnel reflections at splicing points. Subsequent amplification of this continuously reflected light disturbs efficient seed signal amplification and even increases the unwanted background noise. To prevent this, optical fiber isolators are located at both the end facets of the fiber amplifier. Fig. 3 shows the amplified spontaneous emission (ASE) of the Er-doped fiber with and without proper isolation of Fresnel reflections. The total amount of ASE at 1560 nm wavelength is well suppressed by the isolators by a factor of 30 dB. The resulting spectrum shows the same distribution as that of the typical emission spectrum of Erbium as illustrate at the inset in Fig. 3(b). The pump power required for supercontinuum generation is consequently much lowered after the suppression of the ASE. Less pumping also decreases the pump-induced phase noise in the Er-doped fiber amplifier. Besides, the use of isolators helps avoid occasional damages of the pump diodes caused by partial reflection of the amplified pulses.

They managed to send
2.5 billion bits of information per second through their length of fibre.
Their record, of 2223 km, is far beyond that achieved elsewhere, which at
best has been around 1000 kilometres, even using optical amplifiers.
At the same conference, other groups said that they had also demonstrated
important achievements with erbium amplifiers. Many engineers would like
to transmit a number of different wavelengths of light through the same
optical fibre. Until now they did not believe that erbium amplifiers would
operate at a sufficiently wide range of wavelengths to make this possible.
Their fears were allayed in San Francisco by news from two companies. A
Japanese telecommunications company, called KDD Research & Development
Laboratories, and Bell Communications Research (Bellcore) in New Jersey.
The team from KDD told the conference that it had sent 2.4 billion bits
of information per second at four different wavelengths through an optical
fibre nearly 500 kilometres long containing six erbium amplifiers.
The team from Bellcore used an erbium repeater to amplify signals with
16 different wavelengths. More significantly, the Bellcore team showed the
promise of erbium amplifiers in fibre optic networks that distribute signals
to many terminals. Optical signals suffer high losses when they are divided
among many fibres.
Optical repeaters avoid this problem by amplifying the optical signal
at the transmitter or intermediate points. In Bellcore’s experiments, the
signals from all 16 laser sources were sufficiently strong to be divided
among more than 4000 terminals.

K. Y. Lau, “Gain switching of semiconductor injection lasers,” Appl. Phys. Lett. 52(4), 257–259 (1988). [CrossRef]
2. Y. Arakawa, T. Sogawa, M. Nishioka, M. Tanaka, and H. Sakaki, “Picosecond pulse generation (<1.8 ps) in a quantum well laser by a gain switching method,” Appl. Phys. Lett. 51(17), 1295–1297 (1987). [CrossRef]
3. S. D. Jackson and T. A. King, “Efficient gain-switched operation of a Tm-doped silica fiber laser,” IEEE J. Quantum Electron. 34(5), 779–789 (1998). [CrossRef]
5. B. Schmaul, G. Huber, R. Clausen, B. Chai, P. Li Kam Wa, and M. Bass, “Er3+:YLiF4 continuous wave cascade laser operation at 1620 and 2810 nm at room temperature,” Appl. Phys. Lett. 62(6), 541–543 (1993). [CrossRef]
6. R. M. Percival, D. Szebesta, and S. T. Davey, “Highly efficient CW cascade operation of 1.47 and 1.82 µm transitions in Tm-doped fluoride fiber laser,” Electron. Lett. 28(20), 1866–1868 (1992). [CrossRef]
7. G. Qin and Y. Ohishi, “Cascaded two-wavelength lasers and their effects on C-Band amplification performance for Er3+-doped fluoride fiber,” IEEE J. Quantum Electron. 43(4), 316–321 (2007). [CrossRef]
8. M. Pollnau, Ch. Ghisler, G. Bunea, M. Bunea, W. Lüthy, and H. P. Weber, “150 mW unsaturated output power at 3 μm from a single-mode-fiber erbium cascade laser,” Appl. Phys. Lett. 66(26), 3564–3566 (1995). [CrossRef]
9.

When a dual-stage configuration was used for the wavelength filter, a bandwidth of 13 nm was realized, and the spectral sidebands were reduced considerably as compared with the single-stage filter configuration. A 504 fs sech2-like pulse was obtained. The temporal shape is shown in Fig. 6(b). The repetition rate was 21.6 MHz, the corresponding output power was 12.6 mW, and the pulse energy was 580 pJ. The estimated soliton order N was 2.6. Compared with the previous work using a 1:1 output coupler, the average power and the pulse energy were increased by factors of 2.5 and 5, respectively.
Next, we examined the RF noise characteristics of the output pulses. A wavelength filter with a bandwidth of 13 nm was used. The single-sideband measurement method was used for the RF noise measurement , employing a fast photodiode, bias-T, and RF spectrum analyzer. The observed RF noise when the output coupling ratio was 98% is shown in Fig. 7(a) . There was no large noise component. The noise level was as low as that of a commercially available ultrashort-pulse solid state laser. Figure 7(b) shows the variation of the averaged magnitude of RF noise as a function of output coupling ratio. The magnitude of RF noise in Fig. 7(a) was obtained by averaging the RF noise spectra between 1 kHz and 10 MHz. Generally speaking, the magnitude of RF noise was low for the whole range of output coupling ratio. As the coupling ratio was increased, the magnitude of the RF noise was decreased slightly and continuously, reaching the minimum when the output coupling ratio was 80%. The magnitude of RF noise was increased as the output coupling ratio was increased above 80%. This means that, for the high output coupling region of 85–95%, the output power and noise magnitude were both increased. The reason for the noise increment is considered to be the effect of amplified spontaneous emission noise in the Er-doped fiber under the extremely small input power condition.
4. Wavelength-tunable operation
Wavelength tunability is also an important feature for ultrashort pulse fiber lasers. Next, we tuned the properties of the wavelength filter to demonstrate wavelength-tunable operation of the PM fiber laser. We used a dual-stage wavelength filter.

In certain embodiments, this can result from the relatively easy power scaling properties of a fiber.
Features, objects and advantages of the invention are in the summary, description, drawings and claims.
Typically, core 14 includes a first material (e.g., a silica material, such as a fused silica) and at least one dopant (e.g., at least one rare earth ion, such as, for example, erbium ions, ytterbium ions, neodymium ions, holmium ions, dysprosium ions and/or thulium ions; and/or transition metal ion(s)) where the rare earths are understood to include elements 57-71 of the periodic table. More generally, however, core 14 can be formed of any material (e.g., active material) or combination of materials (e.g., active materials) capable of interacting with a pump signal to enhance pump signal absorption (e.g., produce gain). In certain embodiments, core 14 is formed of fused silica doped with erbium ions. As is well understood by one of ordinary skill in the art, active materials, such as the rare earths, provide energy of a first wavelength responsive to receiving energy (typically referred to as “pump” energy) of a second wavelength that is different than the first wavelength.
Core 14 can optionally include certain other materials. For example, core 14 can include one or more materials to increase the index of refraction. Such materials include, for example, germanium oxide. Core 14 can include one or more materials to decrease the index of refraction. Such materials include, for example, boron oxide. Core 14 can include one or more materials (e.g., aluminum oxide) that enhance the solubility of the rare earth ion(s) within core 14 (e.g., within silica, such as fused silica). Core 14 can include one or more materials that enhance the homogeneity of the index of refraction within core 14. An example of such a material is phosphorus pentoxide.
Generally, core 14 is designed to support multimode energy propagation. The thickness R of core 14 can vary depending upon the intended use of fiber 10.

Another common practice is to bundle many fiber optic strands within long-distance power transmission cable. This exploits power transmission rights of way effectively, ensures a power company can own and control the fiber required to monitor its own devices and lines, is effectively immune to tampering, and simplifies the deployment of smart grid technology.
The transmission distance of a fiber-optic communication system has traditionally been limited by fiber attenuation and by fiber distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the signal into an electrical signal, and then use a transmitter to send the signal again at a higher intensity than was received, thus counteracting the loss incurred in the previous segment. Because of the high complexity with modern wavelength-division multiplexed signals (including the fact that they had to be installed about once every 20 km), the cost of these repeaters is very high.
An alternative approach is to use optical amplifiers which amplify the optical signal directly without having to convert the signal to the electrical domain. One common type of optical amplifier is called an Erbium-doped fiber amplifier, or EDFA. These are made by doping a length of fiber with the rare-earth mineral erbium and pumping it with light from a laser with a shorter wavelength than the communications signal (typically 980 nm). EDFAs provide gain in the ITU C band at 1550 nm, which is near the loss minimum for optical fiber.
Optical amplifiers have several significant advantages over electrical repeaters. First, an optical amplifier can amplify a very wide band at once which can include hundreds of individual channels, eliminating the need to demultiplex DWDM signals at each amplifier. Second, optical amplifiers operate independently of the data rate and modulation format, enabling multiple data rates and modulation formats to co-exist and enabling upgrading of the data rate of a system without having to replace all of the repeaters. Third, optical amplifiers are much simpler than a repeater with the same capabilities and are therefore significantly more reliable. Optical amplifiers have largely replaced repeaters in new installations, although electronic repeaters are still widely used as transponders for wavelength conversion.
Wavelength-division multiplexing (WDM) is the practice of multiplying the available capacity of optical fibers through use of parallel channels, each channel on a dedicated wavelength of light.

A single switchable-gain EDFA performs equally compared to multiple variable-gain EDFAs, each of which operates over a narrower gain range as shown below. The most compelling benefits of switchable-gain EDFAs for NEMs and network operators are operational simplicity and inventory sparing.
- L-Band Amplifiers
Lumentum offers L-band amplifiers (EDFAs and Raman) for geography-specific applications and fiber-scarce applications. The design approach to L-band and C+L band amplifiers differs from that of C-band amplifiers. Here again, Lumentum’s rich history of L-band amplifier designs and expertise offers customers the comfort and confidence of a trusted, turnkey solution.
- Compact EDFA Arrays
EDFA arrays — multiple parallel instances of the same amplifier in a co-located physical package – are an essential component for compensating loss within colorless, directionless, and contentionless (CDC) ROADM nodes. The attributes for EDFA arrays differ from that of in-line amplifiers because EDFA arrays typically operate at a fixed-gain and support a subset of the full complement of coherent DWDM channels depending on the add/drop architecture at the node. Lumentum offers arrays of 8 EDFAs (4 on add path, 4 on drop path), each gain-flattened and capable of supporting up to a maximum of 16 wavelengths across the entire C-band. A compact form factor, low power dissipation, and low cost are the primary hallmarks of Lumentum EDFA arrays.
- Single-Channel Small Form Factor (SFF) Amplifiers
Lumentum offers single-channel SFF EDFA gain blocks and modules for space- and power-constrained transponder linecards. Equipped with an uncooled pump laser, our SFF amplifier lets transponder card designers maximize the use of their board space for high-speed electro-optic components.

Fiber is silica based, and includes a germania-doped core, together with one or more cladding layers which may be of silica or may be down doped with fluorine. The overall 125 μm structure has a core of a diameter of about 6 μm. The index peak has a Δn 0.013-0.015 with reference to undoped silica Usual profile is triangular or trapezoidal, possibly above a 20 μm platform of Δn≅0.002. The WDM fiber specified may be compensated by a spool of compensating fiber. Compensating fiber of co-pending U.S. Pat. application Ser. No. 07/978,002, filed Nov. 18, 1993, is suitable for this purpose. Illustrative structures have a dispersion of 2 ps/nm-km.
The principle has been described It is likely to take the form of a major length of fiber of positive sign of dispersion, followed by compensating fiber of negative dispersion. As with WDM fiber, compensating fiber may be of the form described in the co-pending U.S. patent application.
Self-Phase Modulation, a non-linear effect resulting in random generation of different wavelengths, is found to be small. From FIGS. 4 and 5, it is concluded that compensation for (linear) dispersion at appropriate distances (in that instance at 120 km spaced amplifier positions) effectively eliminates SPM as a consideration. Under these circumstances, fiber with λ.sub.0 =1310 nm is acceptable (disregarding cost and inconvenience of compensation). The near-term WDM system on which description is based (360 km span length, four-channel 5 gbit/channel) does accept the 17 ps/nm-km uncorrected material dispersion of λ.sub.0 =1310 nm fiber. Future systems of longer spans or of greater capacity may use fiber of 8 ps/nm-km dispersion.
Consideration of SPM leads to compensation several times along each span length. Requirements for the near-term WDM system are met by compensation of the 17 ps/nm-km fiber at each amplifier (e.g. at spacings of 120 km). The inventive advance is useful for systems of shorter span length as discussed.

Raman Amplifiers for Telecommunications:There has been a revived interest in Raman amplification due to the availability of high pump powers and improvements in small core size fibers. Two general categories of Raman amplifiers exist: distributed and discrete, also known as DRAs. They improve the noise figure and reduce the nonlinear penalty of the amplifier, allowing for longer amplifiers spans, higher bit rates, closer channel spacings, and operation near the zero dispersion wavelength. DRAs are already becoming commonplace in most long-haul networks. Consequently, Raman amplifiers should see a wide range of deployment in the next few years. This edited monograph is written by leading experts in this area and is the first book entirely devoted to Raman amplification. Three sections include extensive background on Raman physics, descriptions of sub-systems and modules utilizing Raman technology, and a review of current state-of-the-art systems. Technologies presented include applications for long-haul and ultra-long-haul submarine, terrestrial, soliton, and high-speed systems. This book will be a resource for scientists and optical engineers in optoelectronics, fiber optics, telecommunication, and optical networks.

For cases with erbium-ytterbium energy transfers, double-clad fibers, pulsed amplification, etc., this is even more the case.
Neodymium and Ytterbium Fiber Amplifiers
Fiber amplifiers based on ytterbium- or neodymium-doped double-clad fibers can be used to boost the output power of 1-μm laser sources to very high levels of up to several kilowatts (→ high-power fiber lasers and amplifiers). The broad gain bandwidth is also suitable for the amplification of ultrashort pulses (→ ultrafast amplifiers); limitations arise from fiber nonlinearities such as the Kerr effect and Raman effect (see below). Single-frequency signals can also be amplified to high powers; in this case, stimulated Brillouin scattering usually sets the limits.
Neodymium-based amplifiers can also be used in the 1.3-μm spectral region, but with less favorable performance figures .
Erbium Fiber Amplifiers
Fiber amplifiers based on erbium-doped single-mode fibers (EDFAs) are widely used in long-range optical fiber communication systems for compensating the loss of long fiber spans. See the article on erbium-doped fiber amplifiers for more details.
Thulium Fiber Amplifiers
Thulium-doped fluoride fibers (TDFA = thulium-doped amplifier) pumped around 1047 or 1400 nm can be used for amplification in the telecom S band around 1460–1530 nm, or even around 1.65 μm. Combined thulium–erbium amplifiers can thus provide optical amplification in a very wide wavelength range.
There are also thulium-doped amplifiers for the first telecom window, operating at ≈ 800–850 nm.
Praseodymium Fiber Amplifiers
Fiber amplifiers for the second telecom window around 1.3 μm are also available [7, 9], but have a lower performance compared with that of erbium-doped amplifiers. They can be based on praseodymium-doped fluoride fibers (PDFA = praseodymium-doped amplifier), which are pumped around 1020 nm (a relatively inconvenient pump wavelength) or at 1047 nm (with a YLF laser).
Some Design Issues
Fiber amplifiers can be pumped in forward direction (i.e.

One of the samples prepared at the University turned out to be vitreous rather than crystalline and the fluorozirconate glasses had been discovered. Initial interest in these materials was as improved glasses for high energy fusion lasers and in 1978, in a joint publication with the University of Rennes, Weber concluded that the glasses should be considered as candidates for laser materials . A further significance of these new glasses was soon recognised when it was proposed that they might have applications for low loss IR fibres, with intrinsic losses below those in silica, and a considerable amount of work is now underway in an attempt to reach these targets . These two interests were amalgamated in 1987 when the first fluoride fibre laser was demonstrated by Brierley and France , doped with Nd and lasing at 1.05 um. Following this work many new dopants have been investigated and many different lasing lines have been reported. This paper reviews these recent developments in fluoride fibre lasers and amplifiers from several laboratories throughout the world and includes some of the latest results on semiconductor diode pumping close to 0.8 um.
Common techniques for the fabrication of optical waveguides (MCVD-Modified Chemical Vapor Deposition, OVD-Outside Vapor Deposition, VAD-Vapor Axial Deposition) depend upon the availability of high vapor pressure precursor compounds such as SiC14, GeC14, and POC13. Vapor delivery techniques can not be used to transport compounds with a low vapor pressure. To incorporate such elements into the glass structure, we are investigating aerosol doping for both MCVD and OVD. Low mass flow rate aerosol transport is being used for core doping of rare earth elements in MCVD, and a high mass flow aerosol transport may have application in overcladding in OVD, the fabrication of fiber boules of glasses with a high nonlinear refractive index, and for GRIN (Gradient Index Lenses) lenses.
Recent advances in the field of Er3+-doped optical fibre amplifiers and lasers operating around λ=1.54μm will be described, with particular reference to 'new' pump wavelengths and practical pump sources for these devices.
Despite the excellent performance that has been demonstrated for Er3+-doped fiber amplifiers at 1.5 µ m, they have several shortcomings that remain concerns in practical applications.

The Features of Er Glass:
- Eye safety
- High optical quality
- Long fluorescent life
- The absorption band is wide
- The slope of high efficiency
|The cross section of stimulated emission（10-20cm2）||0.8||0.75|
|Fluorescence lifetime (milliseconds) *||7.7-8.0||7.7-8.2|
|Central laser wavelength（nm）||1535||1535|
|Refractive Index（d 589.3nm）||1.532||1.536|
|dn / dT（10-6 /℃）（20〜100℃）||-1.72||-3|
|Transition Temperature (℃)||556||530|
|Softening Temperature (℃)||605||573|
|Linear Coefficient of Thermal Expansion (10-7/K) (20〜100℃)||87||82|
|Linear Coefficient of Thermal Expansion (10-7/K) (100〜300℃)||95||96|
|Thermal Coefficient of Optical Path Length (10-6/K) (20〜100℃)||2.9||1.4|
|Thermal Conductivity (25℃) (W/mK)||0.7||0.7|
|Density g / cm3||3.06||2.83|
|Chemical Durability (weight loss in 100℃ distilled water)（μg/ hr.cm2）||52||82|
|Orientation Tolerance||< 0.5°|
|Thickness/Diameter Tolerance||±0.05 mm|
|Surface Flatness||<λ/8@632 nm|
|Wavefront Distortion||<λ/4@632 nm|
|Biggest Size||dia (3-12.7)×（3-150）mm2|
Absorption and Emission Spectra
| B Y Z A , A M L , A J L , et al. Optical properties of Er 3+ /Yb 3+ co-doped phosphate glass system for NIR lasers and fiber amplifiers[J]. Ceramics International, 2018, 44( 18):22467-22472.|
| Langar A , Bouzidi C , Elhouichet H , et al.

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22. M. Pollnan and S. D. Jackson, “Erbium 3 μm fiber lasers,” IEEE J. Sel. Top. Quantum Electron. 7(1), 30–40 (2001). [CrossRef]
23.