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, M-2 is maintained at less than 1.2 at all power levels and long term operating stability at full power is demonstrated.