Fiber Amplifiers

Author: the photonics expert Dr. Rüdiger Paschotta

Fiber amplifiers are optical amplifiers based on optical fibers as laser 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).

Operation Principles

The basic operation principle of a fiber amplifier – just that of other laser amplifiers – is stimulated emission of radiation. When a signal photon meets an excited laser-active ion, it can stimulate that to emit another photon at the same wavelength and propagating in the same direction, effectively amplifying that signal light.

During operation of a fiber amplifier, a substantial fraction of the laser-active ions contained in the fiber core are excited into a metastable state as they are exposed to pump light, typically at a shorter optical wavelength. In most cases, that pump light is injected through one or two fiber ends, and a substantial fraction of its power is absorbed in the fiber. To maintain the amplifier gain, a steady input of pump light is needed to replenish the reservoir of excited ions which is emptied both by stimulated emission processes (when a signal is amplified) and by spontaneous emission. Effectively, pump input power is converted into additional signal power, and that process can be fairly efficient.

Different types of laser-active ions are available for fiber amplifiers. By far most common are certain rare earth ions, particularly Er³⁺ (erbium), Yb³⁺ (ytterbium), Nd³⁺ (neodymium) and Tm³⁺ (thulium). Less common examples are Pr³⁺ (praseodymium) and Ho³⁺ (holmium). Each ion type is suitable for certain ranges of signal and pump wavelengths, which also to some extend depends on the used host glass, as it influences the characteristics of the laser-active ions.

Further, one exploits the waveguiding effect of an optical fiber, in particular for the signal light, and normally also for the pump light.

Applications of Fiber Amplifiers

The originally dominating application of fiber amplifiers was in optical fiber communications over large distances, where signals need to be periodically amplified. Still, one typically uses erbium-doped fiber amplifiers with signals of moderate optical power (output power e.g. a few hundred milliwatts) in the 1.5-μm spectral regions. Often, many different wavelength channels are simultaneously amplified in a single fiber amplifier (→ wavelength division multiplexing).

Other important application areas of fiber amplifiers have been developed later. In particular, high-power fiber amplifiers have been developed which can produce output powers of hundreds of watts or even many kilowatts. Such amplifier are now widely used in laser material processing, replacing solid-state bulk lasers and CO₂ lasers, for example. They are mostly based on ytterbium-doped double-clad fibers for signals in the spectral region of 1.03–1.1 μm.

It is often very convenient (e.g. in optical fiber communications) that light from a fiber can easily be sent into a fiber amplifier, and the amplified light can be sent into further transmission fiber. Besides the active (amplifying) fiber, some additional components may be used, e.g. dichroic fiber couplers and Faraday isolators (see Figure 1).

Case Studies

Case Study: Erbium-doped Fiber Amplifier for a Long-wavelength Signal

Amplified spontaneous emission (ASE) turns out to be a limiting factor, requiring a dual-stage amplifier design.

Case Studies

Case Study: Erbium-doped Fiber Amplifier for Multiple Signals

We optimize an amplifier for equal output powers of signals spanning a substantial wavelength range. There is a trade-off between power efficiency and noise performance.

Case Studies

Case Study: Designing a Double-clad Fiber Amplifier

We develop a double-clad fiber amplifier with high gain, where we have to care about limiting losses by ASE.

Gain and Output Power

Due to the possible small mode area and long length of an optical fiber, a high gain of e.g. 30 or 40 dB can be achieved with a moderate pump power, i.e., the gain efficiency can be very high (particularly for low-power amplifiers). 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 (sometimes several kilowatts) with close to diffraction-limited beam quality, 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.

Amplification of Light Pulses

Fiber amplifiers cannot only amplify continuous signals, but also short or even ultrashort light pulses. Here, one can utilize that such an amplifier stores a substantial amount of energy, which can then be released within a very short while through an intense pulse. Further, rather short pulse durations (e.g. 100 fs or less) are possible due to the substantial gain bandwidth. However, there are limitations for pulse amplification in fiber amplifiers:

• The saturation energy and thus the possible pulse energy is substantially lower than for a bulk amplifiers based on similar materials, essentially because the mode area is much smaller (even when using large mode area fibers).
• The high peak powers of short pulses (even for moderate pulse energies) in conjunction with the smaller mode area can lead to strong nonlinear effects, which can be detrimental in various ways.

Case Studies

Case Study: Erbium-doped Fiber Amplifier for Rectangular Nanosecond Pulses

We deal with deformations of the pulse shape due to gain saturation. These can be minimized by pre-distorting the input pulses.

Case Studies

Case Study: Parabolic Pulses in a Fiber Amplifier

We explore the regime of parabolic pulse amplification in an Yb-doped single-mode fiber. We find suitable system parameters and investigate limiting effects.

Case Studies

Case Study: Soliton Pulses in a Fiber Amplifier

We investigate to which extent soliton pulses could be amplified in a fiber amplifier, preserving the soliton shape and compressing the pulses temporally.

Saturation Characteristics

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. At the same time, the upper-state lifetime is rather long – e.g. around 8 ms for erbium-doped amplifiers. 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 of the order of the saturation energy or higher, pulse distortions through saturation become significant. For amplifying the output of a mode-locked laser, where the pulse energy is quite low, the gain saturation is normally the same as for a continuous-wave laser with the same average power.

These saturation characteristics are also important for optical fiber communications because they prevent any intersymbol interference as can occur with semiconductor optical amplifiers.

In terms of average power, fiber amplifiers are often operated in the strongly saturated regime. This allows for the highest average output power and power conversion efficiency, 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 determines the fundamental limitation of the amplifier noise properties. While the excess noise of a loss-less four-level amplifier can approach the theoretical limit (the standard quantum limit), corresponding e.g. to a noise figure of 3 dB in the case of high gain, the excess noise can be substantially stronger for the usual quasi-three-level laser 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 act as 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. A superluminescent source has to contain little more than a high-gain fiber amplifier.

Case Studies

Case Study: ASE in Ytterbium-doped Fibers

We study various aspects of amplified spontaneous emission (ASE) in ytterbium-doped fibers – for example, why it is different in forward and backward direction, how the fiber length can have a crucial impact, and how the fiber core diameter matters.

Case Studies

Case Study: Erbium-doped Fiber Amplifier for a Long-wavelength Signal

Amplified spontaneous emission (ASE) turns out to be a limiting factor, requiring a dual-stage amplifier design.

Modeling and Simulation of Fiber Amplifiers

It is possible to model (→ laser modeling and simulation) the essential performance aspects of fiber amplifiers in various ways, normally using suitable fiber simulation software. This is often essential e.g. for the efficient development of fiber amplifier systems. Note that a purely experimental approach is typically inefficient for various reasons:

• Usually, only the generated output light is accessible for measurements; the internal workings are hard to reveal, lacking practical opportunities for measurements inside the fibers. (Using some fiber couplers for extracting some diagnostic outputs can mitigate that problem only to some extent.)
• Even well accessible output beams are not easy to completely characterize, even with sophisticated equipment.
• A simulation model gives complete insight, e.g. concerning the optical powers and excitation densities at any location in the fiber and (for pulse systems) at any moment of time. That can be vital for understanding problems and fixing them efficiently.
• Design changes are easy, quick and cheap to test in a simulation model, but often cumbersome, time-consuming and expensive in experiments. For example, imagine the resources required for ordering different versions of active fibers, building the amplifier system with all those, and testing them all just to find out which ones works best.

Different kinds of physical models can be utilized in simulation software, depending on what effects are of interest:

Designing Fiber Amplifiers

For designing a fiber amplifier system, a suitable simulator is essential to have. Only with that, you get complete insight into how it works and how it can be optimized. The RP Fiber Power software is an ideal tool for such work.

• Basically always, one requires a model for the behavior of laser-active ions and their interactions with the optical fields. Typically, one uses a set of rate equations, with which the population densities of the relevant ionic states can be calculated, for example in the steady state for given signal and pump intensities, or for dynamic simulations.
• Such a rate equation model may be incorporated in a more comprehensive model which then calculates the optical powers along the fiber.
• Frequently, one requires properties of the guided modes of a fiber. For that, one requires a mode solver. In many cases, LP modes are a suitable description, and an LP mode solver can be far more efficient than a more general 2D mode solver. However, more sophisticated mode solvers are required for photonic crystal fibers, for example.
• For devices involving ultrashort pulses, more or less sophisticated pulse propagation modeling is required, as explained in detail in that article.

Applications of amplifier simulations are manifold. Some typical examples:

• One can find out what would be the ideal performance of an amplifier with certain parameters. Deviations can give clues concerning deficiencies or deviations of parts from specifications.
• One can investigate the impact of various detrimental effects on the amplifier performance. In a model, these effects can easily be switched on or off for comparing the results.
• One can quickly check the effects of various design changes, and thus find an optimized fiber amplifier design far more efficiently.
• Often, the detailed insight produced by a simulation model triggers new ideas for elegant solutions and further improvements.

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. See the article on fiber simulation software for more details.

Even in simple cases, relatively complex behavior can result, which would be very hard to anticipate without computer simulations. 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.

ASE in forward and backward direction keep the ytterbium excitation lower at both ends. Therefore, the pump absorption varies substantially. The simulation has been done with the software RP Fiber Power.

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. For cases with erbium–ytterbium energy transfers, double-clad fibers, pulsed pumping, pulse 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 [4].

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. High-power versions based on double-clad fibers also exist, although they are significantly more limited in performance than ytterbium amplifiers. See the article on erbium-doped fiber amplifiers for more details.

Case Studies

Case Study: Erbium-doped Fiber Amplifier for a Long-wavelength Signal

Amplified spontaneous emission (ASE) turns out to be a limiting factor, requiring a dual-stage amplifier design.

Case Studies

Case Study: Erbium-doped Fiber Amplifier for Multiple Signals

We optimize an amplifier for equal output powers of signals spanning a substantial wavelength range. There is a trade-off between power efficiency and noise performance.

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

Forward and Backward Pumping

Fiber amplifiers can be pumped in forward direction (i.e. 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.

Quasi-three-level Gain Media

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 signal attenuation caused by the active ions; only when a certain excitation level is exceeded, actual amplification takes 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

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 that can lead to concentration quenching.

Chirped-pulse Amplification

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. 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.

Multi-stage Amplifiers

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.

Polarization Issues

Most fiber amplifiers are not made of polarization-maintaining fibers, so they do not preserve the polarization state of the input. However, a wide range of polarization-maintaining active fibers is also available.

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 OEM fiber amplifier modules which can be convenient for system integrators. Input and output are then often done through 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 optical 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.

Case Studies

Case Study: Raman Scattering in a Fiber Amplifier

We investigate the effects of stimulated Raman scattering in an ytterbium-doped fiber amplifier for ultrashort pulses, considering three very different input pulse duration regimes. Surprisingly, the effect of Raman scattering always gets substantial only on the last meter, although the input peak powers vary by two orders of magnitude.

More to Learn

Tutorials:

• Tutorial on Fiber Amplifiers
• Tutorial on Modeling and Simulation of Fiber Amplifiers and Lasers

Case studies:

• Case Study: Erbium-doped Fiber Amplifier for a Long-wavelength Signal
• Case Study: Erbium-doped Fiber Amplifier for Multiple Signals
• Case Study: Designing a Double-clad Fiber Amplifier
• Case Study: Erbium-doped Fiber Amplifier for Rectangular Nanosecond Pulses
• Case Study: Parabolic Pulses in a Fiber Amplifier
• Case Study: Soliton Pulses in a Fiber Amplifier
• Case Study: ASE in Ytterbium-doped Fibers
• Case Study: Raman Scattering in a Fiber Amplifier

Encyclopedia articles:

• erbium-doped fiber amplifiers
• high-power fiber lasers and amplifiers
• rare-earth-doped fibers
• fibers
• double-clad fibers