Raman shifter optimized for lidar at a 1.5 μm wavelength

A Raman shifter is optimized for generating high-energy laser pulses at a 1.54 m wavelength. A forward-scattering design is described, including details of the multiple pass and nonfocused optical design, Stokes injection seeding, and internal gas recirculation. First-Stokes conversion efficiencies up to 43% - equivalent to 62% photon conversion efficiency - were measured. Experimental results show output average power in excess of 17.5 W, pulse energies of 350 mJ at 50 Hz, with good beam quality (M² < 6). Narrow bandwidth and tunable output is produced when pumping with a single longitudinal mode Nd:YAG laser and seeding the process with a Stokes wavelength narrowband laser diode.