Miniaturization of high beam quality 1.543 μm Raman laser with backward stimulated Raman scattering

It is very challenging to make high peak power (big pulse energy) Raman lasers compact, due to laser induce breakdown (LIB) effect; and it is even more difficult to achieve a decent beam quality meanwhile. In this work, a pulsed 1064 nm laser was used as pump source; pressurized methane was used as Raman active medium, which had the largest ratio of backward/forward Raman gain coefficient among all gaseous media; and backward first Stokes (BS1) Raman laser of 1543 nm with good beam quality was realized. When an f=0.5 m lens was used to focus pump beam into a 0.9 m long Raman cell filled with 3.5 MPa methane, 276.1 mJ BS1 was achieved, the corresponding photon conversion efficiency was 70.8% and the peak power was 83.7 MW. BS1 beam quality was measured to be M_x²=2.54,M_y²=2.28, which was significantly better than that of pump laser. In a setup of f=0.3 m focal lens and 0.5 m long Raman cell, 197.8 mJ BS1 was achieved, with the company of serious LIB. In order to meliorate LIB and improve BS1 conversion efficiency, the focal lens was tilted by 20°, pump laser beam waist and depth of focus increased significantly, BS1 was improved to 225.0 mJ, the corresponding photon conversion efficiency was 58.5%. An even short focal lens and Raman cell with reasonable BS1 energy was possibly achieved by tilting a shorter focal lens by a larger angle. This work also demonstrated that the increment of BS1 conversion may help to reduce the effect of LIB on SRS process.