H2 stimulated Raman scattering in a multi-pass cell

Stimulated Raman Scattering (SRS) is an effective means of laser wavelength conversion. Hydrogen is an excellent Raman medium for its high stimulated Raman gain coefficient and good flowability which can rapidly dissipate the heat generated by SRS process. In this paper we reported the H2 SRS in multiple-pass cell pumped by the fundamental frequency output of a Q-switched Nd: YAG laser. Two concave reflection mirrors (with 1000 mm curvature radius and 50 mm diameter) were used in our experiment, both mirrors with a hole near the edge and were positioned to form co-center cavity, therefore the laser could repeatedly pass and refocus in the Raman cell to achieve a high SRS conversion efficiency and reduce SRS threshold for pump laser. By changing the pass number (1～17) of optical path in the Raman cell and the pump power(0～2.5MW), the Stokes conversion efficiency is optimized. Experimental results indicated that the Raman threshold was 0.178MW and the highest photon conversion efficiency was 50 %.