Random lasing in multimode diode-pumped graded-index fiber based on artificial Rayleigh scattering in fs-inscribed random structure

Abstract

We study an opportunity to obtain low-threshold random Raman lasing in multimode diode-pumped graded-index (GRIN) fiber with enhanced backscattering in artificial fs-inscribed random structure. Proof-of-principle experiments have been performed with a 1-km multimode GRIN fiber of 100-µm core directly pumped by three fiber-combined high-power 940- nm LDs. The laser has half-open all-fiber cavity consisting of highly-reflective FBG which is UV-inscribed in GRIN fiber near the LD input and artificial random reflector near its output end. Different types of random structures were formed by the point-by-point femtosecond-pulse technique in the near-axis area of GRIN fiber core (different pitch, length and transverse profile). Comparison of their lasing properties in the multimode diode-pumped GRIN fiber has been performed. The lowest threshold of ~140 W and the highest output power of 6.2 W (at ~180 W pumping) with the beam quality M²~3.3 is achieved for 2D structure. It is shown that the quality is improved to M²~2.4 for 1D structure at the expense of slight reduction in output power (5.5 W).