High power tunable thulium fiber laser with volume Bragg grating spectral control

A volume Bragg grating is used in two different configurations to control the output spectrum of a thulium doped silica fiber laser. When used in a direct feedback configuration on the end of a bidirectionally pumped resonator, a power of up to 159 W with 54% slope efficiency is produced with a narrow output spectrum centered at 2052.5 nm with <1.5 nm full-width at 10 dB down from spectral peak. Maximum laser linewidth is limited by the VBG reflectivity width. The VBG based laser is compared to a laser resonator based on a standard HR mirror and is able to maintain stable spectrally narrow operation while the HR mirror laser has a wide and varied spectral output over 20-30 nm. Both lasers have similar slope efficiency, threshold and power performance with any difference attributed to lack of AR coatings on the VBG. In a second cavity, the VBG is used in a tunable configuration by rotating the VBG away from normal incidence. Tuning range was found to be >100 nm from 1947 nm to 2052.5 nm with output powers as high as 48 W and up to 52% slope efficiency. Tuning range is determined by VBG center wavelength on the long wavelength end and by the VBG aperture size on the short end. In both system configurations, M2 is maintained at less than 1.2 at all power levels and long term operating stability at full power is demonstrated.