Cascaded Raman lasing in a multimode diode-pumped graded-index fiber

Abstract

Graded-index fibers enable efficient Raman conversion of multimode radiation into a Stokes beam with improved beam quality offering new approach for high-brightness diode-pumped all-fiber tunable lasers. Here we study an opportunity to broaden the operating wavelength range via cascaded Raman lasing in such scheme. Highly-multimode 940-nm laserdiode radiation is serially converted into the 1st(976nm), 2nd(1019nm) and 3rd(1065 nm) order Stokes beam. At the conversion the beam quality is greatly improved approaching diffraction limit (M²<1.4). Linear and half-open cavities are compared showing that the conversion efficiency is higher for half-open cavity, whereas the threshold is lower for linear cavity that was used for the 3rd -order generation. Generation of 3rd Stokes order is accompanied by unstable pulsations with higher (4th and 5th) orders involved in the lasing with total power of ~4 W, while its beam quality worsens to M²~2.