Abstract

Coherent laser beam combining is a potentially attractive way to increase the combined beam brightness beyond the technological limits to single-mode fiber lasers. Passive phase locking (PPL) does not need external management and leads to strong simplification of the system. A specific feature of fiber amplifiers and lasers is that they possess optical path differences of the magnitude of many wavelengths. The involved problem in the PPL approach is to specify an ultimate limit to the phase-locked laser-array size. Earlier studies confirm the stabilizing role of gain saturation on beam-combining efficiency. The purpose of our study is to decipher the desired effect of nonlinearity on the combining efficiency in two architectures of a globally coupled fiber-laser array: (I) an array of amplifiers in ring resonator configuration with spatially filtered feedback; (II) an array of lasers also with external feedback. The external-cavity feedback in both systems results in global coupling, i.e., each element is coherently coupled to all the others. A semi-analytical approach based on the probability theory is developed to calculate the probability density for the efficiency as a function of system parameters in both ensembles. Comparison between (I) and (II) arrays indicates that the II-type arrays demonstrate better characteristics while scaling the array size.

© 2014 Optical Society of America

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