Spatial filtering for high power laser beam by volume Bragg grating

Abstract

Nowadays, the most widespread used space filters are pinhole filters, consisted of a lens with a pinhole in the focal plane, requiring for matching spatially a focused laser beam to a small hole. Experimentally, the initial alignment of spatial filters is difficult, if the pinhole position changes, a laser can damage the pinhole when the power is increased or even permanent damage due to heating. In contrast, non-spatial filtering, a holographic filter element, which made of volume Bragg grating, is inserted in the laser beam path to selectively diffract light propagating at a particular angle, without a lens or a pinhole. A volume Bragg grating is operated directly on the laser beam propagation angle without focus, made use of the grating's angular selectivity, which alignment is easier than pinhole filter, and endures a high-power laser. In this thesis, a volume Bragg grating was fabricated in a 40μm-thick photopolymer, with period of 911.3nm, preparing for a low-pass non-spatial filtering. It achieves an angular selectivity of 35mrad; diffraction efficiency about 95%. Nevertheless, the results of the experiment can be verified with the theory, but not suitable for high-power application. In that case, the photopolymer's grating should be replaced with a photo-thermo-refractive glass.