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Amplifier Chains

Definition: amplifiers consisting of several stages

German: Verstärkerketten, Verstärkerkaskaden

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An optical amplifier chain is a sequence of several optical amplifiers, which can subsequently amplify some signal. The first stage is often called the preamplifier and the last stage a power amplifier. Such schemes are common in electronics, but also in laser technology (as considered here) because the concept of an amplifier chain can have advantages compared with an approach where the full performance in expected from a single amplifier stage:

There are certain trade-offs, for example between obtaining a high saturated output power and a high gain. For example, a high saturated output power calls for a large mode area in the last section of the amplifier, while the preamplifier can have a better gain efficiency when the effective mode area is chosen small. The overall performance can then be improved by splitting the amplifier into different amplifier stages which can be independently optimized. In ultrashort pulse amplifiers, the power amplifier may also need increased mode areas for reducing nonlinear effects.
It is also possible to combine different kinds of amplifiers, e.g. a regenerative amplifier as a high-gain preamplifier and a multipass amplifier as a power amplifier with moderate gain, or an optical parametric amplifier with a laser amplifier.
Sometimes it is necessary to insert certain optical elements between different stages of an amplifier chain. Some examples: – In many high-gain amplifiers, an optical isolator between the stages reduces the sensitivity to ASE and back-reflections. – A bandpass filter can strongly reduce the total ASE power for both propagation directions. – In amplifiers for ultrashort pulse amplification, an optical modulator (e.g. an acousto-optic modulator) between the stages may be opened only for some short time interval during which a pulse is transmitted to the next stage.
The use of amplifier chains allows for a more modular approach where different performance numbers can be obtained simply by differently combining existing amplifier stages (modules).

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