High-gain 87 cm^-1 Raman line of KYW and its impact on continuous-wave Raman laser operation

We report a quasi-continuous-wave external cavity Raman laser based on potassium yttrium tungstate (KYW). Laser output efficiency and spectrum are severely affected by the presence of high gain Raman modes of low frequency (< 250 cm^-1) that are characteristic of this crystal class. Output spectra contained frequency combs spaced by the low frequency modes but with the overall pump-to-Stokes conversion efficiency at least an order of magnitude lower than that typically obtained in other crystal Raman lasers. We elucidate the primary factors affecting laser performance by measuring the Raman gain coefficients of the low energy modes and numerically modeling the cascading dynamics. For a pump polarization aligned to the N_g crystallo-optic axis, the 87 cm^-1 Raman mode has a gain coefficient of 9.2 cm/GW at 1064 nm and a dephasing time T₂ = 9.6 ps, which are both notably higher than for the 765 cm^-1 mode usually considered to be the prominent Raman mode of KYW. The implications for continuous-wave Raman laser design and the possible advantages for applications are discussed.