208-W average power and 6.3-mJ pulse energy from four spectrally combined fiber amplified Q-switched nanosecond laser sources using low-cost interference filter

In this contribution we introduce a simple scheme to spectrally combine four single beams using three low-cost dielectric interference filters as combining elements. 25 ns pulses from four independent and actively Q-switched fiber seed-sources are amplified in a single stage fiber-amplifier. Temporally and spatially combined 208 W of average power and 6.3 mJ of pulse energy are obtained out of a fiber-based laser system. A detailed observation of beam quality as well as the thermal behavior of the combining elements have been carried out and reveal mutual dependency. The deterioration of beam quality can be led back to thermal induced wave-front distortions on the part of the interference filters. This effect as well as other influences on M2 will be discussed and compared to the competing combining approach with dielectric gratings.