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 pulse (up to several watts) can be obtained from tapered amplifiers.

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 χ⁽²⁾ nonlinearity, but there are also parametric fiber devices using the χ⁽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.

Amplifiers of different kinds may also be used for amplifying ultrashort pulses. In some cases, a high repetition rate pulse train is amplified, leading to a high average power while the pulse energy remains moderate. In other cases, a much higher gain is applied to pulses at lower repetition rates, leading to high pulse energies and correspondingly huge peak powers. An essential point is the ability of a laser amplifier to store some amount of pump energy until the amplified pulse extracts led energy. For more details, see the article on ultrafast amplifiers.

A bulk-optical laser amplifier often provides only a moderate amount of gain, typically only few decibels. This applies particularly to ultrashort pulse amplifiers, since they must be based on broadband gain media, which tend to have lower emission cross sections. The effective gain may then be increased either by arranging for multiple passes of the radiation through the same amplifier medium (multipass amplifier), or by using several amplifiers in a sequence (→ amplifier chains).

Multipass operation (Figure 2) can be achieved with combinations of mirrors (for several passes with slightly different angular directions), or (mostly for ultrashort pulses) with regenerative amplifiers.

For very large amplification factors, multi-stage amplifiers (amplifier chains) are often better suited. For example, one may have a low-power but high-gain preamplifier followed by a power amplifier or booster amplifier, which delivers a modest gain but a high output power level. Between the amplifier stages, the optical signal may be spatially or spectrally filtered in various ways, helping e.g. to achieve a high beam quality and/or a shorter pulse duration. Reasons for choosing a multi-stage amplifier design may be exactly the opportunity for filtering between the stages, or the use of adapted parts for the different power levels – for example, a high-gain amplifier fiber with small effective mode area for a preamplifier and a double-clad large mode area fiber for the power amplifier.

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 for 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 very much e.g. in terms of saturation properties. For example, rare-earth-doped 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.

Typical applications of optical amplifiers are:

• An amplifier can boost the (average) power of a laser output to higher levels (→ master oscillator power amplifier = MOPA).
• It can generate extremely high peak powers, particularly in ultrashort pulses, if the stored energy is extracted within a short time.
• It can amplify weak signals before photodetection, and thus reduce the detection noise, unless the added amplifier noise is large.
• In long fiber-optic links for optical fiber communications, the optical power level has to be raised between long sections of fiber before the information is lost in the noise.

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