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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-04-29

Definition: devices for amplifying the power of light beams

An optical amplifier is a device which receives some input signal and generates an output signal with higher optical power. Typically, inputs and outputs are laser beams, either propagating as Gaussian beams in free space or in a fiber. The amplification occurs in a so-called gain medium, which has to be "pumped" (i.e., provided with some energy) from an external source. Most optical amplifiers are either optically or electrically pumped.

Laser Amplifiers versus Amplifiers Based on Optical Nonlinearities

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 (→ semiconductor optical amplifiers), which can be electrically or optically pumped. 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). 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.

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.

Multipass Arrangements, Regenerative Amplifiers, and Amplifier Chains

A bulk-optical 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, or by using several amplifiers in a sequence (→ amplifier chains).

Figure 1: Setup of a multipass femtosecond amplifier.

Multipass operation (Figure 1) can be achieved with combinations of mirrors (for several passes with slightly different angular directions), or (mostly for ultrashort pulses) with regenerative amplifiers (Figure 2).

Figure 2: Setup of a regenerative amplifier for picosecond pulses.

For very large amplification factors, multi-stage amplifiers (amplifier chains) are often better suited. For example, a regenerative amplifier may amplify pulses to an energy of a few millijoules, and a multipass amplifier further boosts the pulse energy to hundreds of millijoules. Between the amplifier stages, the pulses can be spatially or spectrally filtered in various ways, helping to achieve a high beam quality and/or a shorter pulse duration.

Gain Saturation

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, laser amplifiers (particularly those based on solid-state gain media) do store some amount of energy in the gain medium, and this energy can be extracted within a very short time. Therefore, the output power can exceed the pump power by many orders of magnitude during some short time interval.

Detrimental Effects

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.

A related effect is that amplifiers also add some excess noise to the output. 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 amplification of ultrashort pulses often involves very high optical intensities, which can cause detrimental nonlinear effects such as spectral broadening and pulse distortion. Techniques to mitigate such effects are chirped-pulse amplification and divided-pulse amplification.

Important Parameters of an Optical Amplifier

Important parameters of an optical amplifier include:

• the maximum gain, specified as an amplification factor or in decibel (dB)
• the saturation power, which is related to the gain efficiency
• the saturated output power (for a given pump power)
• the saturation energy
• the time of energy storage (→ upper-state lifetime)
• the gain bandwidth (and possibly smoothness of gain spectrum)
• the power efficiency and pump power requirements
• the noise figure and possibly more detailed noise specifications
• the sensitivity to back-reflections

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.

Applications

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.

See also: gain saturation, amplifier noise, amplified spontaneous emission, amplification factor, gain clamping, fiber amplifiers, Raman amplifiers, semiconductor optical amplifiers, optical parametric amplifiers, regenerative amplifiers, gain equalization, chirped-pulse amplification, divided-pulse amplification

Category: amplifiers