Fiber Amplifiers – a Technology for Many Applications, Part 1: Introduction

Fiber amplifiers are based on “active” fibers, having a fiber core which is doped with laser-active ions such as Er³⁺, Nd³⁺ or Yb³⁺. Normally a fiber coupler is used to introduce some “pump light” in addition to the input signal light. This pump light is absorbed by the laser-active ions, transferring them into excited electronic states and thus allowing the amplification of light at other wavelengths via stimulated emission. Fig. 1 shows schematically the setup of a simple fiber amplifier.

If the amplified signal becomes intense, it reduces the excitation density of the ions, so that the amplification is saturated. Fiber amplifiers are often operated in the strongly saturated regime, which also allows for the highest output power. Depending on the type of amplifier, the output power can be up to ∼80% of the pump power.

Excited ions can also spontaneously decay into their ground state and emit fluorescence light in random directions. Most of the fluorescence leaves the fiber in transverse directions. A small part, however, is guided along the fiber core (in both directions) and can also be amplified. If the amplifier gain is high (e.g. >35 dB), this amplified spontaneous emission (ASE) can extract a substantial power (far more than the transverse fluorescence) and thus limit the achievable gain.

The fiber core is often single-moded, i.e., it supports only a single guided mode per polarization direction. This leads to an excellent beam quality of the amplified light, even in cases where strong thermal effects occur in the fiber. However, efficient direct injection of pump light into the core then requires that this pump light also has an excellent beam quality. This excludes the use of high-power laser diodes as pump sources.

That problem has been solved by the development of double-clad fibers (Fig. 2). Here, the pump light is not directly injected into the fiber core, but into a pump cladding surrounding the core. The pump light can not leave the pump core toward the outer side, as it is guided by an outer cladding with lower refractive index. On the other hand, the pump light has a certain overlap with the doped core, where it can be absorbed. As the pump cladding has many guided modes, it is possible to utilize high-power pump diodes with poor beam quality. Coupling to the pump cladding is comparatively easy.

Telecom fiber amplifiers are usually doped with erbium (Er³⁺), which can be pumped e.g. at 980 nm and can amplify light around 1550 nm. The latter wavelength is ideal for telecommunications, as telecom fibers have extremely low absorption and scattering losses in this spectral region. Also, such wavelengths are relatively eye-safe, since that light is absorbed in the eye's lens before reaching the sensitive retina. By optimizing the chemical composition of the fiber core, one can tailor the gain spectrum such that a nearly constant gain over a wide spectral region is achieved. However, erbium amplifiers are not ideally suited for the highest powers and a high efficiency.

For the 1-μm spectral region, fiber amplifiers based on neodymium (Nd³⁺) were originally developed. The amplification process is in principle similar to that in the well-known Nd:YAG laser crystals, but the fiber geometry and the glass material introduce substantial differences. The possible wavelength ranges for pump and signal light are much broader, making a temperature stabilization of the pump diodes obsolete and allowing e.g. the amplification of ultrashort pulses. The waveguiding allows for a long interaction length without beam expansion or spatial distortions, guaranteeing a high beam quality of the output. Due to the small mode area, the gain efficiency (measured in dB/mW) is high, despite the reduced transition cross sections. This allows to obtain a large gain even for small pump powers.

Later, fiber amplifiers based on ytterbium (Yb³⁺) became even more popular for the 1-μm spectral region, as they have a still higher efficiency and high-power potential, and also offer a large amplification bandwidth. On the other hand, the quasi-three-level behavior of ytterbium introduces some special aspects and limitations, and a good quantitative understanding of such issues is required to realize the full performance potential.

Thulium-doped fibers can be used for longer wavelengths such as 1.9 to 2 μm. Despite the much shorter pump wavelength around 0.8 μm, which implies a large quantum defect, silica fibers with optimized doping density can be surprisingly efficient. This is because of a special energy transfer process, where an excited thulium ion shares its energy with a nearby ion. In that way, one obtains two ions in the upper laser level for one pump photon.

There are other laser-active ions in fibers, and other amplifier transitions, but these are more exotic and not discussed further here.

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[1]	Part 2 – Various Technical Issues
[2]	Part 3 – Examples for Fiber Amplifier Designs

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