Laser beam combining is used is various areas of physics, engineering, amongst other scientific fields of study to multiply the output power and also preserve the quality of the beam. There are experiments ongoing with various institutes to create the best results and they are incorporating several techniques in doing so. These techniques can be broken down into three main categories.
This is a relatively simply technique of combining two linearly polarized laser beams. This means that when you have one horizontally polarized beam diode and one vertically polarized beam diode and send the output beams into a thin-film polarizer, one is reflected and the other is transmitted. They propagate in the same direction and one obtains an unpolarized beam that has the same beam quality with twice the brightness.
This technique of laser beam combining is used for such things as end pumping a solid-state laser, but it only works if the laser crystal absorbs the pump radiation well with both directions of polarization. This is not one of the methods for power scaling because the process cannot be repeated. It needs to have polarized input beams and creates an unpolarized output beam.
This is a class of technique that fits into the group of power scaling techniques. This method includes combining several laser beams to make one single beam to obtain more power and brightness without reducing the quality. This technique also ensures that the bandwidth stays the same.
The techniques included in this class can be subdivided into two groups called the side-by-side combining and filled-aperture techniques. The first creates a larger beam with a decreased divergence and the latter has the same beam size and divergence using a grating splitter. In both of these cases, coherence of the involved beam is essential for success. These techniques were actually inspired by radio frequencies and microwave transmitters and receivers, but have been modified for other uses.
This is a class of laser beaming combining that also falls into the category of power scaling which combines several beams into one beam with a higher strength and brightness, while keeping the same quality. The principle of the combining is different, however.
These beams are from overlapping optical spectra and are combined with a wavelength-sensitive beam combiner. These beam combiners come in such forms as prisms and diffraction gratings, which deflect the incident beams only according to the actual wavelength. To combine the outputs of several different lasers of the same type, each laser has to have the same emission bandwidth, but with a largely reduced amount of gain bandwidth or the quality will suffer greatly. Also, the beam combiner that is used has to have a strong dispersion capability and the wavelengths of the lasers must also be stable.