Multimode Fiber Raman Lasers Directly Pumped by Laser Diodes

Raman fiber lasers (RFLs) are usually based on single-mode fibers core-pumped by high-power rare-earth-doped fiber lasers with single transverse mode output that leads to a rather complicated design of RFLs. One of interesting possibilities to simplify the RFL design is its direct pumping by cheap and reliable high-power multimode laser diodes (LDs). It is attractive to use standard graded-index multimode passive fibers characterized by very high quality and low cost due to their wide use in telecom. In this case, one can directly couple the multimode radiation of high-power LDs with moderate brightness into the core of multimode graded-index fiber with much higher efficiency than into the core of single-mode fiber. Using commercially available multimode LDs with operating wavelengths of 915-940 nm, it is possible to obtain high-power high-beam-quality Raman lasing in wavelength range of 950-1000 nm, which is problematic for rare-earth-doped fiber lasers. Here, we review the results obtained in this direction and report on the demonstration of all-fiber LD-pumped CW Raman laser based on the graded-index fiber. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of fiber core, results in high-efficiency conversion of multimode (M²~26) pump at 915 nm into a high-quality output beam at 954 nm. Fibers with core diameter of 62.5, 85, and 100 μm are compared. With core enlargement, the conversion efficiency increases sufficiently (from 47% to 84%) at the expense of slight beam-quality parameter reduction (M² = 1.3-3). The generated spectrum remains to be rather narrow (<;0.4 nm) at output power >60 W.