New schemes of Raman fiber lasers with random distributed feedback

Fiber lasers have been actively developed in the last decade demonstrating unique efficiency and performance. It is known that the laser generation is possible not only in active fibers, but also in passive fibers owing to the effect of stimulated Raman scattering (SRS). In the SRS process, pump radiation induces amplification of the Stokes-shifted scattered light (by ~13 THz and ~40 THz for Si0 2 /Ge02 and P 2O5 , respectively), see [1] for a review. The Raman gain spectrum in silica-based fibers is rather broad (>10 THz). Therefore, Raman fiber lasers (RFLs) are treated as perspective laser sources in spectral bands, which are not available from rare-earth (RE) doped fiber lasers [2]. The RFL cavity is usually formed by a set of fiber Bragg gratings (FBGs) which reflect Stokes waves of the first and higher orders. As a result, a cascaded generation with stepwise tuning in a broad spectral range is possible, which may be combined with continuous tuning within the Raman gain spectrum of individual Stokes components. However, a relatively low Raman gain requires rather long (0.1-1 km) passive fibers with high-power pumping into the single-mode fiber core, e.g. by 1-μm single-mode Yb-doped fiber lasers (YDFLs) securing cascaded Raman generation in 1-1.5 μm range [1].