<title>New approach to robust optics for HEL systems</title>

Abstract

A new material for optics is being developed that promises to be far more robust than alternative materials. It is a photo-thermo-refractive (PTR) glass in which Bragg gratings (holograms) can be written in the interior (not the surface) of the glass. The gratings are permanent as they are not removed by illuminating them with light at other wavelengths or by heating unless the temperature exceeds 400 degree(s)C. This technology can be used to make diffractive elements such as spatial filters, attenuators, switches, modulators, beam splitters, beam samplers, beam deflectors, selectors of particular wavelengths (notch filters, add/drop elements), spectral shape formers (gain equalizers), spectral sensors, angular sensors, Bragg spectrometers, and transverse and longitudinal mode selectors in a laser resonator. The PTR Bragg grating has been exposed to a 100 W, 1096 nm beam focused to 100 kW/cm<SUP>2</SUP> spot for 10 minutes without exhibiting any temperature rise. The pulsed laser damage threshold has been measured to be within 30% of that of the best silica glass used in high power 1064 nm systems. The useful spectral range of this glass is from 350 nm to 2.8 microns.