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.
Second harmonic generation (SHG) of a random laser operating at 1308 nm via Rayleigh backscattering in a
phosphosilicate fiber with Raman gain has been studied. SHG power of < 100 mW at 654 nm has been obtained in PPLN
pumped by ~7 W laser power. The SHG beam is stable, herewith its spectrum has no mode structure within ~0.5 nm
bandwidth that is different from conventional Raman fiber laser with linear cavity. Direct comparison of SHG in two
laser configurations demonstrates better parameters of the random laser having great potential for imaging and other
applications requiring low-coherent visible light.
A random lasing based on Rayleigh scattering in a passive fiber directly pumped by a high-power laser diode (LD) has been demonstrated. Owing to the random distributed feedback (RDFB) the low-quality LD beam (938 nm) is converted into the high-quality laser output (980 nm). Because of the relatively low excess above the threshold with the available LD, the RDFB laser output is limited in power on 0.5 W level. In the used gradient-index (GRIN) fiber the output beam has 4.5 lower divergence as compared with the pump beam thus demonstrating a new approach for high-power fiber lasers with high-quality output.