Large brightness enhancement for quasi-continuous beams by diamond Raman laser conversion

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 M=2.3-7.3, efficient pump-limited conversion to an M=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 M=20 or more, such as thin-disk and slab lasers and fiber lasers operating in a mode instability regime.

High Power Diamond Raman Lasers

“…In these experiments, output beam quality and slope efficiency (81% and 40% for the first-and second-Stokes lasers, respectively) remained high for the entire range of investigated pump M 2 beam factors provided that the pump beam was focused into the cavity fundamental mode. Under such conditions, the threshold for the first-Stokes laser increases by 30% for an M 2 increase from 2.3 to 7.3 [113]. Gain calculations as a function of pump beam quality supported these observations and predict only minor increases in threshold for pumps of M 2 = 20 and beyond [113].…”

“…Under such conditions, the threshold for the first-Stokes laser increases by 30% for an M 2 increase from 2.3 to 7.3 [113]. Gain calculations as a function of pump beam quality supported these observations and predict only minor increases in threshold for pumps of M 2 = 20 and beyond [113]. These results not only indicate that CW brightness enhancement in diamond is possible, but that large-scale brightness enhancement can be achieved using very poor beam quality pumps.…”

“…A DRL threshold intensity of 2.9 MW/cm 2 (230 W) was obtained for the highest M 2 beam by focusing the pump to a small waist (50 μm) in the diamond. With this arrangement, 1240 nm first-Stokes output at 180 W output power was generated with an M 2 = 1.1, corresponding to an output brightness 12.7× higher than the 1064 nm pump [113]. More recently, with cavity mirrors designed for second-Stokes (1485 nm) operation, 302 W of output power was obtained in a diffraction-limited beam, 6.0× brighter than the pump of power 823 W and M 2 = 6.4.…”

Diamond Raman laser: a promising high-beam-quality and low-thermal-effect laser

“…The research progress on beam quality optimization of DRLs is briefly reviewed in Table 2. [37,83,84] : (a) pump spot distribution; (b) corresponding Stokes spot distribution.…”

“…For the second-order Stokes beam (1485 nm), the beam quality factor M 2 = 1.17, was 2.7 times lower than the pump beam quality factor. Bai et al reported a method to improve the brightness of high-power CW beams by using medium-beam-quality pumping light in 2018 [83] . The 1064 nm pulse pump light was generated by a quasi-CW diode-pumped Nd:YAG laser.…”