Polarization Beam Combining
Incoherent Polarization Combining
The simpler variant is incoherent combining. For example, the output beams of two broad area laser diodes, one being vertically polarized and the other one horizontally polarized, can be sent onto a thin-film polarizer such that one of the beams is reflected, the other one transmitted, and both beams then propagate in the same direction. As a result, one obtains an unpolarized beam having the combined optical power of the input beams (neglecting some parasitic losses) and the same beam quality. Accordingly, the brightness is nearly doubled.
This technique is often used e.g. for end pumping of a solid-state laser with an increased power. It works only if the laser crystal can similarly well absorb pump radiation with both polarization directions. This is the case e.g. for Nd:YAG, but not for Nd:YVO4.
Incoherent polarization beam combining can not be considered as a method for power scaling, as it is not repeatable: it requires polarized input beams and leads to an unpolarized output beam.
Coherent Polarization Combining
If two mutually coherent beams are polarization combined, it is possible to obtain a linear polarization state for the output. Assuming equal input powers for the two ports, the output polarization will be rotated by 45° with respect to the polarization direction of any of the input beams. The method is one variant of coherent beam combining.
Due to the linear output polarization, coherent polarization combining can be repeated many times. It is thus a technique suitable for power scaling.
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