Applications of volume Bragg gratings for spectral control and beam combining of high power fiber lasers

Volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass are used in a wide range of high-power laser applications due to their unique spectral response and excellent optical and thermo-mechanical properties. Experimental results of applications of narrow-band reflecting VBGs to spectral beam combining (SBC) and wavelength control of fiber lasers are presented. Output power of 770 W from a system combining five fiber lasers with 91.7% efficiency is demonstrated with spectral separation between channels of 0.5 nm around 1064 nm and no distortions in diffracted beams. Similar system with 0.25 nm channel separation around 1550 nm is demonstrated with the same efficiency and M-2 of the spectrally-combined beam < 1.15. A novel compact monolithic multi-channel beam combiner based on stacked tilted VBGs is suggested. Absolute efficiency exceeding 90% is reported for a four-channel device with 0.7 nm spectral separation of channels. We show that a linear stack of monolithic combining elements enables compact spectrally-combined laser systems with output power of 10-100 kW. A common-cavity approach to multi-channel spectral beam combining of high-power lasers is demonstrated. In this configuration wavelengths of the sources are passively controlled by a combination of a common output coupler and intra-cavity VBGs, which also act as combining elements. Laser wavelengths are self tuned to match resonant wavelengths of respective gratings and provide maximum combining efficiency. Stable operation of a passively-controlled system combining two amplifiers with 0.4 nm spectral separation is demonstrated. Wavelengths of amplifiers are shown to follow Bragg condition of VBGs during heating of gratings.