定义：
一种通过叠加多个设备的输出从而实现激光源功率调整的方法。

各种不同的激光架构都促进了具有高光束质量的高功率激光光源的发展。然而这些方法都有其局限性。而有一些设想中的激光应用所需要的高功率和高亮度是现有的所有技术都无法满足的。另一个问题是，当前已经建成的超高功率激光系统很少，这也就使得这些超高功率的激光系统非常昂贵。 
对于这些问题一个可行的解决方案就是利用合束，其原理本质上就是将多个激光源的输出合成为一个单一的输出光束。即便每个单一的激光器的功率不可调，但这种可扩展的合束技术使得合成后的光源的功率变成可调。 
合束的目的不仅仅是单纯的成倍的增加输出功率，还要使得输出光束保持光束质量，并且在增加功率的同时（几乎完全）同步的增加亮度。因此仅仅并排的将非相干的合束在一起是不够的，因为这将增加光束的面积但却没有增加光束发散度，也就意味着光束参数乘积变大了，或者说光束质量变差了。 
在合束的同时增加亮度的方法有很多，但是这些方法都可以归为以下两类中的一种： 

1. 利用相干光束的相干合束。相干偏振合束就属于这一种方法，其他方法还包括并排合束和拼接合束。最简单的情况就是将具有相同光学频率的单色光合成在一起。当然如果单个发光器拥有的多频率分量都是相同的频率，那也可以将这些多频率分量的发射器的输出进行相干合束。这种方法也被用来产生宽带超短脉冲。 
2. 光谱合束（也称为波长合束或非相干合束）。这种情况下并不要求各个光束之间的相干性，但是要求各个光束的光谱不重合。单个光束都被送入一个对波长敏感的合束器件中，如棱镜、衍射光栅、双色镜、体布拉格光栅。 

在相关词条中对以上两种方法有着更详细的描述和解释。这两种方法被应用于各种不同的激光光源中，例如，激光二极管、光纤放大器、高功率固体激光器和腔面发射激光器（VECSELs）。 
相干合束和光谱合束的对比： 

1. 光谱合束相对于相干合束的优势在于其不需要光束之间的时域相干性。这就避免了一些重大的技术难题，并且也更容易在高功率下获得稳定的合成光束。这也使得在理论上来说对于光束偏振的稳定性并没有要求，虽然在诸如衍射光栅的实际应用中还是对偏振的稳定度有一定的要求。
2. 在光纤放大器中，单频工作一些与相干合成相关的特性使得很难得到很高的功率，这是由于一些例如受激布里渊散射等的非线性效应的影响。 
3. 光谱合束不可避免地在输出中有不同的频谱分量，同时这也意味着对于某一频率分量，相比于单发射器的输出，光谱合束的输出亮度更低。 
4. 相比于并排的相干合束，光谱合束有一个优势，也就是其更容易保持光束的质量。 
5. 光谱合束在老化过程中相比于相干合束有着优势。因为光谱合束中某一个发射器的功率衰退只会影响相应的光谱分量，但是对于相干合束，一个发射器功率下降会很大的影响输出光束质量从而大大的降低输出光束的亮度。 

总之，即使在某些应用中，由于一些光谱条件的约束使得我们只能利用相干合束，以上各种合束的方法都会发挥很大的作用，并且会不断发展。 
应用 合束的激光系统在不久的将来将会发展到数百瓦的功率水平。这也就带来了一些新的应用可能，特别是军事领域，如：反导弹武器和一些其他的激光武器，当然在远距离的自由空间光通信和基于激光的制造业。

Definition: a class of techniques for power scaling of laser sources by combining the outputs of multiple devices

More specific terms: coherent beam combining, spectral beam combining, polarization beam combining

Various laser architectures allow the development of high-power laser sources with high beam quality, i.e., high radiance (brightness). However, all of these approaches have their limitations, and some envisaged laser applications would require a higher laser power and brightness than appears to be feasible with any known laser technology. Another problem is that very high-power laser systems are developed and built only in small numbers, making the devices fairly expensive.

A viable solution to these challenges could be the principle of beam combining, which essentially means combining the outputs of multiple laser sources (often in the form of laser arrays) so as to obtain a single output beam. The use of a scalable beam-combining technology leads to a power-scalable laser source, even if the single lasers are not scalable.

The goal of beam combining is generally not only to multiply the output power, but also to preserve the beam quality, so that the radiance is increased (nearly) as much as the output power. Therefore, it is generally not sufficient, e.g., to combine mutually incoherent beams side by side, because this would increase the beam area while not decreasing the beam divergence, consequently increasing the beam parameter product and reducing the beam quality.

There are many different approaches for beam combining with increased brightness, but these can all be grouped into one of two categories:

• Coherent beam combining works with beams which are mutually coherent. Coherent polarization beam combining is one variant; other techniques are side-by-side combining and filled-aperture combining. In the conceptually simplest case, monochromatic beams with the same optical frequency are combined. However, some schemes of coherent beam combination work with emission on multiple frequencies, with the emission spectra of all emitters being the same. That technique can also be applied to broadband ultrashort pulses.
• Spectral beam combining (also called wavelength beam combining or incoherent beam combining) does not require mutual coherence, but rather uses emitters with non-overlapping optical spectra. The single beams are then fed into a wavelength-sensitive beam combiner, such as a prism, a diffraction grating, a dichroic mirror, or a volume Bragg grating.

These techniques are discussed in more detail in the corresponding articles. They can be applied to various laser sources, e.g., based on laser diodes (particularly diode bars) and fiber amplifiers, but also to high-power solid-state bulk lasers and VECSELs.

Comparison of Coherent and Spectral Beam Combining

Coherent and spectral beam combining techniques are substantially different in various respects:

• Spectral beam combining has the great advantage of not requiring mutual temporal coherence of the combined beams. This eliminates some important technical challenges and makes it much easier to obtain stable operation at high power levels. It may also be beneficial that a stable polarization is not required in principle, although e.g. the use of diffraction gratings often reintroduces this requirement.
• If fiber amplifiers are used, the requirement of single-frequency operation, which is associated with some techniques of coherent combining, makes it more difficult to reach high power levels, because it favors nonlinear effects such as stimulated Brillouin scattering.
• Spectral beam combining inevitably generates output beams with several (or many) spectral components, thus spanning a significant optical bandwidth. This means that the spectral brightness is even reduced compared with that of the single emitters. This does not matter for some applications while excluding others, where a narrow-bandwidth output is required.
• At least compared with tiled-aperture techniques of coherent beam combining, spectral beam combining makes it easier to combine the beams without any significant loss of beam quality.
• Spectral beam combining is also superior in terms of more graceful degradation, because the failure of one emitter simply reduces the output power accordingly, whereas in the case of tiled-aperture coherent beam combining it also affects the output beam quality and thus reduces the output brightness even more than the power.

In conclusion, it is to be expected that methods of spectral beam combining will find more widespread application, although coherent methods will in some cases be required, e.g. due to constraints concerning the optical spectrum.

Applications

It is expected that beam-combined laser systems will in the near future reach output power levels of tens to hundreds of kilowatts. Possible applications of such systems are often in the military sector, e.g., in the context of anti-missile and other directed energy laser weapons. There are also possibilities for long-distance free-space optical communications [1] and laser-based manufacturing.

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