定义：
一种部分透射的激光反射镜，用来将激光谐振腔中的输出光束提取出来。

输出耦合器是一种用在激光器谐振腔中的半透明的二色性反射镜。它的功能是将腔内的光功率部分的透射出去，得到激光器的有用输出。 
在稳定的光学谐振腔情况下，输出耦合器的反射镜通常具有空间一致的输出耦合透射率。不稳定谐振腔中的输出耦合器则非常不同：它们具有横向可变的反射率，例如高斯型或者超高斯型，有时甚至在有些半径情况下发生全反射，而该半径外则是全透射。后面只考虑稳定谐振腔中的输出耦合器。 

输出耦合器的透射率 
输出耦合器的一个重要的性质是透射程度。输出耦合器透射率低对应的泵浦功率阈值低，并且如果输出耦合损耗并不比激光器谐振腔中其它的损耗高的情况下，激光器效率也很低（输出耦合效率低）。输出耦合器的透射率通常选为使输出功率最大化，尽管为了某些设计考虑，其峰值可能更低或者更高，例如，设计时考虑使腔内强度最小或者抑制无源锁模激光器中的Q开关不稳定性。 

平的或者弯曲的输出耦合器 
输出耦合器反射镜通常具有平的表面，因为这样很容易在输出端得到准直光束。（对于线性的谐振腔，谐振腔模式的波片需要与反射镜表面温和，因此在该平面上平的输出耦合器会产生一个焦点。）同时，除了输出耦合器，其它任何光学元件未对准时只会引起输出光束有一个平移，但是不改变方向（参阅波束指向波动）。若输出耦合器具有曲面，这时需要考虑到反射镜衬底会对输出光束有聚焦或者散焦效应。 

背向反射 
需要注意的是，输出耦合器反射镜玻璃沉底的背面也存在一定的反射，会影响激光器的性能。即使背面涂覆抗反射涂层，其残余反射也会引起干涉效应（如果表面平滑和平行），会改变器件的有效透射率，与波长有关，并且会影响激光器带宽。在锁模激光器中，这种效应会引起不稳定。避免这种问题的一个方法是采用稍微倾斜的输出耦合器，这样背向反射光与激光器的模式不会干涉。

Definition: partially transparent laser mirrors, used for extracting output beams from laser resonators

Alternative term: output coupling mirrors

More general term: laser mirrors

An output coupler is a semi-transparent dielectric mirror used in a laser resonator. Its function is to transmit part of the circulating intracavity optical power in order to generate a useful output of the laser.

As for basically all laser mirrors, the reflecting coating of an output coupler mirror is on the inner side, i.e., on the side of the mirror substrate which is oriented towards the laser resonator. This is because otherwise one would have a serious impact of losses by parasitic reflections on that inner side, which is exposed to the full circulating optical power.

Output Coupler Transmissivity

An important property of an output coupler is its transmissivity (the degree of its transmission of optical power). A low output coupler transmissivity leads to a low threshold pump power, but also possibly to a poor laser efficiency if the losses due to output coupling do not dominate over other (parasitic) losses in the laser resonator (low output coupling efficiency). The output coupler transmissivity is often chosen to maximize the output power, although its optimum value may be lower or higher if there are other design goals, e.g. minimizing the intracavity intensities or suppressing Q-switching instabilities in a passively mode-locked laser.

Flat and Curved Output Couplers

Output coupler mirrors are often mirrors with a flat surface, since it is often convenient to obtain a collimated beam at the output. (For a linear resonator, the wavefronts of the resonator modes must match the mirror surface, so that a flat output coupler results in a focus at that place.) Also, misalignment of any optical element other than the output coupler itself will then lead only to a parallel shift of the output beam, but not to a change in direction (→ beam pointing fluctuations). In the case of an output coupler with a curved (e.g. concave) surface, one also has to take into account that the mirror substrate will usually have a focusing or defocusing effect on the output beam.

Reflections from the Back Side

Note also that the back side of the glass substrate of an output coupler mirror has some reflectivity, which may have an effect on the laser performance. Even the residual reflectivity of an anti-reflection-coated back side may create an interference (etalon) effect (if the surfaces are flat and parallel), which modulates the effective transmission of the device as a function of wavelength and can thus influence the laser bandwidth. In a mode-locked laser, such effects (even at a very low level) can cause instabilities. A way to avoid such problems is to use a slightly wedged output coupler, so that the reflected light from the back side does not interfere with the laser modes.

Variable Reflectivity Mirrors

In the case of stable optical resonators, the output coupler mirror usually has a spatially constant output coupler transmission (see below). Output couplers of unstable resonators can be very different: they are often variable reflectivity mirrors, having a transversely variable reflectance e.g. with a Gaussian or super-Gaussian profile, possibly even with total reflection within some radius and total transmission outside that radius.