定义：
在增益介质中具有多个通道的放大器

光学放大器的增益介质只能实现有限的增益。一种方法通过在几何上设置使光通过放大器时通过多个通道来得到更高的增益，称为多通放大器。最简单的是双通放大器，这时光束通过晶体两次，通常具有完全或者接近相反的传播方向。 
许多多通放大器是采用的激光晶体，被端泵浦或者边泵浦，然后包含一些激光反射镜从而使光束多次通过晶体。由于不同通道的光束需要很高的被分开，具有不同的角方向，尽管原理上采用特定的空间偏移可以可以使它们传播方向平行。如果晶体相对较薄，不同的光束在晶体内部会有很强的交叠，极限情况就是薄盘激光器。 

图1：多通放大器装置示意图。 
当不同通道的光在晶体中有很强交叠时，总体的增益大约等于通道数与单通道增益的乘积。另外，有效饱和能量也会减小，对于总体增益比较小时，它会随着通道减小而减小。 
如果设置很多通道的放大器时，通常使光束的方向不在一个平面上。这种放大器的设计和排列会是非常复杂的问题。 
除了被放大，信号光束还会经历其它的一些效应，例如热透镜或者晶体中的非线性效应。特别是热透镜会极大的影响横向的光束形状；增益导引也同样会导致这种结果。这种效应（自然的光束发散）可以通过将聚焦光学成分（通常采用弯曲的激光反射镜）来抵消。热透镜还可以使光束偏斜，如果它们没有沿着热透镜中心传播。最优的排列依赖于泵浦功率和信号光功率。 
正反馈放大器被看做一种特殊的多通放大器。这里不是通过设置光束的几何路程来得到多个通道，而是采用光学开关。这适用于超短脉冲，脉冲的长度远小于往返时间。所以可以注入一个脉冲，让其循环许多次，然后输出它。这对于得到非常高的整体增益非常方便，比几何设置的多通放大器更可行。

Definition: an amplifier with multiple passes through the gain medium

More general term: optical amplifiers

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.

Figure 1: Setup of a multipass amplifier.

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 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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