Coherent addition of orthogonally polarized fibre lasers with high combining efficiency

Abstract

We demonstrate a free-space combining of two orthogonally polarized fibre lasers by using polarization selective loss and a common adjustable output coupler. The lasers were locked coherently in phase in such a way that the combined beam experienced a minimal cavity loss. In contrast to a conventional coherent locking scheme due to interference, when combined beams are of the same polarization state, this scheme has the advantage of easy tunability in the output power, providing high combining efficiency even when the power of both fibre lasers is highly imbalanced. A high combining efficiency of the fibre laser beams was achieved by the coherent polarization locking method. In comparing the output power of both free-running fibre lasers with the output power in coherently locked state, it was observed that the combining efficiency exceeded 100%. The output power enhancement was due to the difference in optimal feedback conditions for separate free-running lasers. One laser was additionally seeded from the second laser, and the seeding originated the higher output power of coherently added lasers. The combining efficiency estimated by comparing the output of all optimized lasers was as high as 95.2%. As the resulting combined beam has linear polarization, the approach can be applied for combining a desirable number of fibre lasers. Keywords: fibre lasers, fibre optics, laser beam combining, laser resonators