Raman fiber lasers (RFLs) represent an important class of fiber lasers with all benefits of fiber design, in which a passive fiber with pump-induced Raman gain is used as an active medium. Due to the broad Raman gain spectrum and efficient cascaded generation of higher Stokes orders, they may operate at almost any wavelength in transmission window (1-2 µm) of conventional silica fibers with an appropriate pumping . Most of RFL schemes utilize single-mode fibers pumped into the core by another single-mode fiber laser such as Yb-doped fiber laser (YDFL) operating near 1 µm, which in its turn is cladding-pumped by multimode laser diodes (LDs) at 915 or 976 nm. Direct LD pumping is now one of the main challenges for RFL development. Two key opportunities have been explored recently: widely used telecom multimode graded-index fiber (GIF) or specially designed double-clad passive fibers which exhibit relatively good characteristics at weakly-multimode fiber laser pumping but degrade in efficiency at highly-multimode LD pumping, see  and citation therein.
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