Mode selection in a Raman fiber laser directly pumped by a multimode laser diode using fiber Bragg gratings

Raman fiber lasers (RFLs) are treated as perspective laser sources in spectral bands which are not available from rare-earth (RE) doped fiber lasers. Raman lasing can be easily obtained from 1.1 to 2 μm in single-mode fibers that are core-pumped by RE-doped fiber lasers generating single transverse mode. However, the RFL generation below 1, Mm is difficult because of a lack of suitable high-power single-mode pump sources in the short-wavelength range, e.g. the lower limit for the Yb-doped fiber laser is ∼980 nm, while the influence of photodarkening effect increases with the wavelength shortening [1]. A perspective solution is to develop RFLs based on a multimode graded-index (GRIN) fiber directly core-pumped by cheap and reliable high-power multimode laser diodes (LDs) operating at wavelengths below 1 μm. This approach allows one to simplify the laser scheme and eliminate the problems associated with RE-doped fiber lasers, e.g. photodarkening. Such RFLs have shown to date the ability of high power generation (Pout ∼ 150 W) by shortening the GRIN fiber length to 0.2 km [2]. Effect of beam clean-up arising at the Raman conversion of pump radiation in GRIN fibers provides much better quality of the RFL output beam in comparison with that for LDs, while the RFL cavity is formed by a broadband bulk mirror and a cleaved fiber end [2, 3]. But it is not sufficient to provide single transverse mode operation. We propose to use fiber Bragg gratings for an additional mode selection [4].