Influence of transform-lens focal length on spectral beam combining in an external cavity with a microlens array

An experimental system of spectral beam combining of 3 large-mode-area double-clad fiber lasers has been built on the basis of an external cavity with a microlens array. The analysis models of coupling and combining efficiency, as well as beam quality were established. According to the models, the influences of focal length of transform lens on the coupling, combining efficiency, as well as beam quality were analyzed. The results show that the longer focal length of transform lens increases the number of combined emitters, while hardly affects the combining efficiency. In the tunable experiment of individual fiber laser, the laser can be tuned in a range of 40.62 nm for using transform lens with focal length of 50 mm, slightly wider than that for another one with focal length of 100 mm (40.36 nm). In the combining experiment of three fiber lasers, the measured combining efficiency for the transform lens with focal length of 50 mm was about 77.0%, with the output power of about 1.01 W, and the beam quality factor (Mx2) of 1.318. For another one with focal length of 100 mm, the measured combining efficiency was about 83.3%, with the output power of about 1.09 W, and the beam quality factor (Mx2) of 1.312.