All-fiber continuous-wave Raman fiber oscillator operating at 2118 nm

An all-fiber Raman fiber oscillator operating at 2118 nm pumped by homemade thulium-doped fiber laser with continuous output power of 4.2 W has been demonstrated in this paper. By means of 26 m ultrahigh-numerical-aperture fibers, the average slope efficiency can be up to 23.6%. Because of the Raman spectrum broadening of the first Stokes light, the intensity of high order Raman Stokes has been suppressed. The mechanism of Raman spectrum broadening has been analyzed by group-velocity dispersion and Four-Wave Mixing frequency theories in detail. Two mode field adaptors have been used to reduce the intracavity loss and thermal deposition, which indicates that the proposed architecture is robust and has significant power scaling potential.