part of the Virtual Library

Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-03-18

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 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 (fed 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 breed of fiber amplifiers are Raman amplifiers (see below).

Figure 1: Schematic setup of a simple erbium-doped fiber amplifier. Two laser diodes (LDs) provide the pump power for the erbium-doped fiber. Two pig-tailed optical isolators strongly reduce the sensitivity of the device to back reflections.

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. The achievable gain 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.

Saturation Characteristics

In terms of gain saturation, fiber amplifiers are very different e.g. from semiconductor optical amplifiers (SOAs). Due to the small transition cross sections, the saturation energy is rather 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 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.

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

ASE and Noise

The achievable gain is often limited not by the available pump power, but by amplified spontaneous emission (ASE). This can become quite 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 e.g. directly affect the gain and thus the signal output power, but 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.

Erbium Fiber Amplifiers

Fiber amplifiers based on erbium-doped single-mode fibers (acronym: EDFAs) are widely used in long-range optical fiber communications 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 nm or 1400 nm can be used for amplification in the telecom S band around 1460-1530 nm, or even around 1.65μm. 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).

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; 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].

Some Design Issues

Fiber amplifiers can be pumped in forward direction (i.e., with a pump wave copropagating with the signal wave), in 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 as well as 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) are operating 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, actual amplification takes place. The quasi-three-level nature also has implications on 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 quite significant in fiber amplifiers, particularly for those amplifying ultrashort pulses. 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.

In some cases, a multi-stage amplifier, i.e., an amplifier chain, needs to be realized. This allows e.g. for ASE suppression with filters or modulators between the stages, for an optimized power efficiency and noise figure, and possibly for a modular approach which increases the flexibility for further amplifier developments.

Note that most fiber amplifiers are not made of polarization-maintaining fibers, so that 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 quite convenient for OEM system integrators. Input and output are then often attached with the usual fiber connectors. The compact case of the 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, 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 separate article on Raman amplifiers.

Amplifier Modeling

It is possible to model (→ laser modeling) the essential performance aspects of fiber amplifiers in various ways. 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. Particularly for situations where short pulses are amplified, fiber nonlinearities such as stimulated Brillouin scattering and Raman scattering may be of interest.

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.

Bibliography

See also: erbium-doped fiber amplifiers, rare-earth-doped fibers, ytterbium-doped gain media, fibers, double-clad fibers, power scaling of lasers, gain equalization, fiber lasers, amplifiers, Raman amplifiers, amplifier noise, superluminescent sources, parabolic pulses

Categories: amplifiers, communications, fibers and other waveguides, photonic devices

See also the article on "Fiber-based high-power laser systems", contributed by external authors.