Spectral Beam Combining of Thulium Fiber Laser Systems

Beams from three frequency stabilized master oscillator power amplifier (MOPA) thulium fiber laser systems were spectrally beam combined using a metal diffraction grating. Two of the laser oscillators were stabilized with guided mode resonances filters while the third was stabilized using a gold-coated diffraction grating. Each system was capable of producing a minimum of 40 W output powers with slope efficiencies between 50-60 %. The three lasers undergoing combination were operating at wavelengths of 1984.3, 2002.1, and 2011.9 nm with spectral linewidths between 250-400 pm. Beam combining was accomplished by spatially overlapping the spectrally separated beams on a water-cooled gold-coated diffraction grating with 600 lines/mm. Beam quality measurements were completed using M-2 measurements at multiple power levels of the combined beam. Power levels of 49 W were achieved before thermal heating of the metal diffraction grating cause degradation in beam quality. The combining grating was similar to 66% efficient for the unpolarized light corresponding to a total optical-to-optical efficiency of 33% with respect to launched pump power.