Branch arm filtered coherent combining of tunable fiber lasers

A branch arm filtering technique is firstly proposed and experimentally demonstrated. A tunable band pass filter is inserted into one branch arm of the Mach-Zehnder type resonator instead of into the common arm as usual, in the coherent combining of two tunable fiber lasers. The arrangement improves the efficiency of the laser without obvious spectral quality penalty. The laser can be tuned from 1530 nm to 1570 nm with little power fluctuation, which is limited by the tuning range of the filter. A novel scaling scheme is also proposed, allowing the technique to be applied to the tuning of an extremely high power laser with a low power filter. The technique is expected to be compatible with other kinds of lasers such as linearly polarized lasers, Michelson type resonator and bulk lasers as well.