Optical Amplifiers

Most optical amplifiers are laser amplifiers, where the amplification is based on stimulated emission. Here, the gain medium contains some atoms, ions or molecules in an excited state, which can be stimulated by the signal light to emit more light into the same radiation modes. Such gain media are either insulators doped with some laser-active ions, or semiconductors (see below). Doped insulators for laser amplification are laser crystals and glasses used in bulk form, or some types of waveguides, such as optical fibers (→ fiber amplifiers, see Figure 1). The laser-active ions are usually either rare earth ions or (less frequently) transition-metal ions. A particularly important type of laser amplifier is the erbium-doped fiber amplifier, which is used mostly for optical fiber communications.

Semiconductor optical amplifiers can be electrically or optically pumped. In most cases, electrical pumping is used, which is highly convenient and allows for very compact amplifiers. While ordinary semiconductor optical amplifiers are quite limited in output power, substantially higher powers (up to several watts) can be obtained from tapered amplifiers.

Two laser diodes (LDs) provide the pump power for the erbium-doped fiber, allowing it to amplify light with wavelength around 1550 nm. Two pig-tailed Faraday isolators strongly reduce the sensitivity of the device to back-reflections.

In addition to stimulated emission, there also exist other physical mechanisms for optical amplification, which are based on various types of optical nonlinearities. Optical parametric amplifiers are usually based on a medium with ($\chi^{(2)}$) nonlinearity, but there are also parametric fiber devices using the ($\chi^{(3)}$) nonlinearity of a fiber. Other types of nonlinear amplifiers are Raman amplifiers and Brillouin amplifiers, exploiting the delayed nonlinear response of a medium.

An important difference between laser amplifiers and amplifiers based on nonlinearities is that laser amplifiers can store some amount of energy, whereas nonlinear amplifiers provide gain only as long as the pump light is present.

Some types of optical amplifiers are intrinsically multimode devices. For example, an amplifier based on bulk laser crystal can amplify multimode beams and can also handle beams with different angular positions and directions in certain ranges, which makes it possible to construct a multipass amplifier (see above).

Other optical amplifiers are single-mode devices. For example, many fiber amplifiers and semiconductor optical amplifiers are based on a single-mode fiber or waveguide. In such cases, only a single mode can be amplified, and that has to be mode-matched to the amplifier.

When a single-mode laser amplifier provides a high gain within some bandwidth, it inevitably generates a substantial level of amplified spontaneous emission (ASE). In a multimode amplifier, this effect is correspondingly stronger. Even if ASE is negligible (for lower gain), a multimode amplifier typically exhibits more power losses by spontaneous emission (and thus has a lower gain efficiency) because a larger amount of laser-active material has to be kept in the excited state.

For high values of the input light intensity or fluence, the amplification factor of a gain medium saturates, i.e., is reduced (→ gain saturation). This is a natural consequence of the fact that an amplifier cannot add arbitrary levels of energy or power to an input signal. However, as laser amplifiers (particularly those based on solid-state gain media) store some amount of energy in the gain medium, this energy can be extracted within a very short time. Therefore, during some short time interval the output power can exceed the pump power by many orders of magnitude.

For high gain, weak parasitic reflections can cause parasitic lasing, i.e., oscillation without an input signal, or additional output components not caused by the input signal. This effect then limits the achievable gain. Even without any parasitic reflections, amplified spontaneous emission may extract a significant power from an amplifier.

Generally, amplifiers do not only amplify any intensity or phase noise of the input, but also add some excess noise. This applies not only to laser amplifiers, where excess noise can partly be explained as the effect of spontaneous emission, but also to nonlinear amplifiers. The noise figure e.g. of a fiber amplifier is a measure of how much excess noise occurs. Quantum optics dictates a minimum amount of excess intensity and phase noise for phase-insensitive amplifiers.

Important parameters of an optical amplifier include:

• the maximum gain, specified as an amplification factor or in decibels (dB)
• the saturation power, which is related to the gain efficiency
• the saturated output power (for a given pump power)
• the power efficiency and pump power requirements
• the saturation energy
• the time of energy storage (→ upper-state lifetime)
• the gain bandwidth (and possibly smoothness of gain spectrum)
• the noise figure and possibly more detailed noise specifications
• the sensitivity to back-reflections
• the number of modes it can amplify (see above, multimode and single-mode amplifiers)

Different kinds of amplifiers differ substantially e.g. in terms of saturation properties. For example, rare-earth-doped laser gain media can store substantial amounts of energy, whereas optical parametric amplifiers provide amplification only as long as the pump beam is present. As another example, semiconductor optical amplifiers store much less energy than fiber amplifiers, and this has important implications for optical fiber communications.

In the context of optical fiber communications, optical amplifiers are often categorized by their specific function in a transmission link, which dictates their primary design constraints:

• Preamplifiers are placed just before the receiver. Their job is to boost a very weak signal to a level suitable for the photodetector. For preamplifiers, the noise figure is the dominating performance metric, as it directly impacts the achievable bit error rate, while the maximum output power is less critical.
• In-line amplifiers are placed at intermediate points along a long fiber link to compensate for fiber propagation losses. They need a balance of reasonably high output power and low noise figure to avoid degrading the signal-to-noise ratio (SNR) excessively over multiple stages.
• Booster amplifiers (or power amplifiers) are placed directly after the transmitter. Their main purpose is to launch the highest possible power into the fiber link. Therefore, a high saturation power is the most critical parameter, while the noise figure is less important since the input signal level is high.