Many technological arenas, including sensing and directed energy, are constantly pushing the limits of what is achievable from a laser system. A recurring goal in the art is to maximize (i.e., power scale) the output power of a laser beam while maintaining beam quality.
Power scaling of a laser system faces obstacles associated with the degradation of efficiency and beam quality at high output powers due to thermal effects in the lasing medium. One solution to achieve high power is to combine a number of lower-power beams into a single beam. By operating each individual lasing element at lower power, thermal degradations to beam quality and efficiency may be avoided. Many known techniques, both coherent and incoherent, are utilized to scale the power of a laser beam by combining the outputs of multiple lasing elements.
To date, free-space phased arrays incorporating multiple emitters closely packed together have been the most popular architectures for coherent combining. However, phased arrays are characterized by a non-unity fill factor, due to the fact that the individual emitters do not overlap in the near-field. Moreover, since the far-field power produced by phased arrays is partially diverted into side-lobes, their combining efficiency is limited to approximately 40-80% of power in the central lobe because of the losses to the side lobes.
Further still, it is often accepted in the art that polarization combination is not a viable candidate for power scaling because of the inherent limit of the combination to two beams. This limitation is based on the belief that polarization combination produces a statistical mixture of two orthogonal states that is unsuitable for any further combination with additional beams.