Efficient multi-wavelength Raman conversion in methane pumped by pulsed laser at 355 nm

Multi-order Stokes generation in pure CH4 and its He/Ar mixture pumped by a pulsed Nd:YAG laser at 355 nm were investigated. Maximum energy conversion efficiencies as high as 71% and 38% (i.e. quantum efficiency of 79% and 48%), much higher than those reported in previous literatures, were attained under 2.5 MPa and 1 MPa for the first and second Stokes respectively. Under 0.5 MPa CH4, significant multi-wavelength outputs could be obtained with energy distribution of 3.6% at 322 nm, 24.5% at 355 nm, 24.3% at 396 nm, 22.3% at 448 nm, and 9.3% at 515 nm. High-pressure CH4 was good for highly efficient first Stokes conversion, while low-pressure CH4 was suitable for higher order Stokes generation. The experimental results were analyzed in detail in terms of cascade stimulated Raman scattering