The gain medium of an optical amplifier can achieve only a limited amount of gain. A method for obtaining a much higher gain is to geometrically arrange multiple passes of light through an amplifier, which is then called a multipass amplifier. The simplest case is that of a double-pass amplifier, where a beam passes the crystal just two times, usually with exactly or nearly opposite propagation directions.
Many multipass amplifiers are based on a laser crystal, which may be end pumped or side pumped, and contain a number of laser mirrors which fold the signal beam path such that it does multiple passes through the crystal. As the beams corresponding to the different passes need to remain well separated, they usually have slightly different angular directions, although they can in principle also be parallel if they have a certain spatial offset. The different beams may strongly overlap in the crystal if the crystal is relatively thin – in the extreme case, part of a thin-disk laser head.
If the case of strong overlap of different passes through the crystal, the overall gain in decibels can be estimated as the number of passes times the single-pass gain. On the other hand, the effective saturation energy is reduced – in the case of small overall gain, it is reduced by the number of passes.
For arranging a large number of passes, one is often forced to use beam directions which do not all lie in one plane. The design and alignment of such an amplifier system can be a relatively sophisticated matter.
Apart from being amplified, the signal beam may also experience other effects, such as thermal lensing or nonlinear effects in the crystal. Particularly thermal lensing can strongly affect the transverse beam profile; gain guiding may do that in addition. Such effects (and also the natural beam divergence) may be counteracted by focusing optical elements (usually curved laser mirrors) in the setup. Note that thermal lensing can also deflect beams, if they do not exactly travel through the center of the thermal lens. Optimum alignment may then depend on the applied pump power and also possibly on the signal power.
A regenerative amplifier may be considered as a special kind of multipass amplifier. Here, the multiple passes are arranged not via the geometrical beam path, but with an optical switch. This works well for ultrashort pulses, the duration of which is far below the round-trip time. It is then possible to inject a pulse, let it circulate many times, and then eject it. This principle is convenient for obtaining a very high overall gain in many passes – more than may be feasible in geometrically arranged multiple passes.
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