Coherent combination of fiber laser beams

Coherent combination of free-space beams from continuous-wave, 1547 nm, narrow-band, single-mode fiber-lasers was experimentally investigated. Beams from fiber lasers (in a master oscillator/power amplifier arrangement) were collimated. The beams were then overlapped on a video camera to form an interference pattern. The interference pattern drifted slowly under lab conditions. The drift was observed to mainly be due to: a) wavelength drift of the master oscillator combined with an optical path difference and b) small thermal fluxuations in the optical fibers that, in turn, cause wavefront phase changes. Images from the video camera were acquired by computer and analyzed in real-time. A phase control signal was fed back to a fiber stretcher to achieve a stable interference pattern. The interference pattern peak could also be steered within the beam overlap region to a desired location.