Frequency doubling of graded-index fiber Raman lasers with multimode diode pumping

Graded-index fibers with special in-fiber Bragg gratings are shown to convert efficiently highly multimode (M2 ~30) laser diode pump radiation in all-fiber Raman laser configuration into a high-quality Stokes beam at 954 and 976 nm (M2 =1.6-2.6 in different configurations). The beam quality improvement is provided by well-known Raman beam cleanup effect defined by Raman gain profile, and additional impact of transverse mode-selective properties of cavity feedback provided by special femtosecond-pulse inscribed fiber Bragg gratings and/or random Rayleigh backscattering distributed along the fiber. Here we study frequency doubling of the diode-pumped Raman fiber laser in a simple singlepass scheme with 5-mm PPLN crystal demonstrating efficient second harmonics generation at 477 and 488 nm with high beam quality after elimination of interfering effects.