This article introduces the principle of beam combining, the method to improve the brightness of beam combining, and the comparison between coherent beam combining and spectral beam combining.

A method to achieve power adjustment of a laser source by stacking the outputs of multiple devices

Various different laser architectures have facilitated the development of high-power laser light sources with high beam quality. However, these methods have their limitations. However, there are some envisaged laser applications requiring high power and high brightness that all existing technologies cannot meet. Another problem is that very few ultra-high-power laser systems have been built, which makes these ultra-high-power laser systems very expensive.

A feasible solution to these problems is to use beam combining, which essentially combines the outputs of multiple laser sources into a single output beam. Even though the power of each single laser is not adjustable, this scalable beam combining technology makes the power of the combined light source tunable.

The purpose of beam combining is not only to simply multiply the output power, but also to maintain the beam quality of the output beam and increase the brightness (almost completely) synchronously while increasing the power. Therefore, it is not enough to simply combine the incoherent beams side by side, because this will increase the area of the beam but not increase the beam divergence, which means that the beam parameter product becomes larger, or the beam quality becomes worse.

There are many ways to increase brightness while combining beams, but they all fall into one of two categories:

1. Use coherent combining of coherent beams. Coherent polarization beam combining belongs to this method, and other methods include side-by-side beam combining and splicing beam combining. The simplest case is to combine monochromatic light with the same optical frequency. Of course, if the multi-frequency components possessed by a single light emitter are all of the same frequency, the outputs of the transmitters of these multi-frequency components can also be coherently combined. This method has also been used to generate broadband ultrashort pulses.
2. Spectral combining (also known as wavelength combining or incoherent combining). In this case, coherence between the individual beams is not required, but the spectra of the individual beams are not required to coincide. The individual beams are fed into a wavelength-sensitive beam combining device such as a prism, diffraction grating, dichroic mirror, volume Bragg grating.

The above two methods are described and explained in more detail in the related entries. These two methods are used in a variety of different laser sources, such as laser diodes, fiber amplifiers, high-power solid-state lasers, and cavity surface-emitting lasers (VECSELs).

1. The advantage of spectral combining over coherent combining is that it does not require temporal coherence between beams. This avoids some major technical difficulties and also makes it easier to obtain a stable composite beam at high power. This also makes theoretically there is no requirement for the stability of the beam polarization, although in practical applications such as diffraction gratings there is still a certain requirement for the stability of polarization
2. In fiber amplifiers, some characteristics related to coherent combining of single frequency operation make it difficult to obtain very high power, which is due to some nonlinear effects such as stimulated Brillouin scattering.
3. Spectral combining inevitably has different spectral components in the output, which also means that for a certain frequency component, the output brightness of spectral combining is lower than that of a single emitter.
4. Compared to side-by-side coherent combining, spectral combining has the advantage that it is easier to maintain the quality of the beam.
5. Spectral combining has advantages over coherent combining during aging. Because the power decay of a certain transmitter in spectral combining will only affect the corresponding spectral components, but for coherent combining, the power drop of one transmitter will greatly affect the quality of the output beam and thus greatly reduce the brightness of the output beam.

In conclusion, even in some applications, due to the constraints of some spectral conditions, we can only use coherent beam combining, the above methods of beam combining will play a great role and will continue to develop.

Applications Combined laser systems will be developed to power levels of several hundreds of watts in the near future. This also brings some new application possibilities, especially in the military field, such as: anti-missile weapons and some other laser weapons, of course in long-distance free space optical communication and laser-based manufacturing.