Method and apparatus for controlling mode coupling in high power laser system

Abstract

A high power fiber laser is configured with a multimode active fiber and input and output single mode passive fibers butt-spliced to respective opposite ends of the active fiber. If the input passive and active fibers do not have substantially matched diameters, a SM radiation coupled into the active fiber may excite fundamental and high order modes which, while interfering with one another, create a non-uniform distribution of refractive index in each of forward and backward light propagation directions along the resonator of the laser. The variable longitudinal perturbation components of the refractive index in respective forward and backward directions along an optical path in the active fiber are distributed in accordance with respective cosine functions. The length of the optical path is set so that the cosine functions of the respective perturbation components are shifted in a counter-phase position in which a cross-coupling coefficient between fundamental and high-order modes is substantially minimized. The optimal length of the optical path is maintained by controlling by either an ambient temperature or an electric field of piezo-element coupled to the MM active fiber. As a consequence, the disclosed high power fiber laser emits radiation in a fundamental mode having minimum power losses.