Experimental observation of stimulated Raman scattering in a fluoride fiber

Stimulated Raman scattering is an attractive nonlinear optical process that produces the power transfer from a given frequency to one or several down-shifted Stokes beams [1-4]. It is becoming an exciting issue because of the wide application, ranging from sensors, slow light generation to Raman amplifiers and lasers [5-11]. Ever since Stolen et al. first reported it in 1972[3], SRS has been successfully demonstrated based on various fiber structures and fiber materials [12-24]. Moreover, in optical fibers with sufficiently high Raman gain and pump power, multiple-order cascaded Raman shift can be generated, which has already been observed in silica, tellurite and chalcogenide optical fibers [18, 25-28]. However, relatively few researches concerning this have been carried out on fluoride fibers, although they have high transparency from the ultraviolet (UV) region to the mid-infrared (MIR) region up to ~8 μm. In this work, we demonstrated fourth-order cascaded Raman shift in a birefringence ZrF4‒BaF2‒LaF3‒AlF3‒NaF (ZBLAN) fluoride fiber. To the best of our knowledge, this is the first demonstration of fourth-order cascaded Raman shift in fluoride fibers. The pump source was a picosecond laser with the center wavelength of 1064 nm. At the average pump power of ~7W, fourth-order cascaded Raman shift was observed if the pump pulse was polarized to the slow axis, and only third-order cascaded Raman shift was obtained if the pump pulse was polarized to the fast axis. Furthermore, the generalized nonlinear Schrödinger equation (GNLSE) was used to simulate the cascaded Raman shift generation process, and the results were in good agreement with the experiments.