ABSTRACT
We report a high power quasi-continuous-wave (QCW) 620 nm laser from an external cavity diamond Raman laser utilizing intracavity frequency doubling in lithium triborate. Output power of 30 W for durations of 0.25 ms at 15% conversion efficiency was achieved with a beam quality factor M2 = 1.1 from a free-running Nd:YAG pump laser of M2 = 1.5. The critical design parameters that affect conversion efficiency and power were analysed with the aid of an analytical model. By adaptation to other pump technologies, the diamond approach provides a novel pathway towards high brightness CW beam generation in the visible and ultraviolet regions.
ABSTRACT
High average power lasers with high beam quality are critical for emerging applications in industry and research for defense, materials processing, and space applications. However, overcoming thermal effects in the gain medium remains the key challenge for increasing laser brightness at high powers. Here we report a means for increasing the beam brightness of high-power continuous-wave (CW) beams based on external cavity Raman lasers using diamond, a material with thermal properties far superior to any other laser material. With pump beam quality in the range M2=2.3-7.3, efficient pump-limited conversion to an M2=1.1 Stokes beam is achieved in all cases, with increases in brightness from the pump by factors as high as 12.7. The influence of pump beam quality on laser threshold and slope efficiency is analyzed. This Letter foreshadows an alternative approach for scaling the brightness of CW lasers using high-power, moderate beam quality pumps up to M2=20 or more, such as thin-disk and slab lasers and fiber lasers operating in a mode instability regime.
ABSTRACT
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 Ng 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.