Forced air cooling of volume Bragg gratings for spectral beam combination

Abstract

ABSTRACT Volume Bragg gratings have been successfully used in spectr al beam combining of high po wer fiber lasers with narrow channel separationand in four channel passive coherent beam combining of fiber lasers. Future application of beam combining with kilowatt level lasers requires a more detailed understanding of how to cool the gratings without hurting beam quality. Forced air cooling blown across both surfaces of the grating is both easy and cheap, but has been avoided in the past due to concerns of how the air density fluctuations will hurt beam quality. It is now shown that forced air cooling has no adverse effect on the M 2 parameter due to density fluctuations in the air, and can efficiently cool VBG’s such that no degradation in beam quality is seen due to thermal distortions. Volume Bragg gratings are routinely made with low absorption on the level of 10 -4 cm -1 . To model grating operation for kilowatt level lasers, absorption of the studied grating was artif icially increased by 140 times. Thus the use of 80 W fiber lasers enabled imitation of 11kW beams, and the effects of forced air cooling on the beam quality were measured. Without cooling, the M