Long-wavelength optical absorption edge of photo-thermo-refractive glass

The IR absorption spectrum of high purity photo-thermo-refractive (PTR) glasses melted under atmosphere containing different concentrations of water is studied. The absorption spectra of the water and glass matrix are separated and deconvoluted to elementary Gaussian bands. The water absorption spectrum includes 11 bands with a maxima at 4320, 3560, 3053, 2897, 2815, 2765, 2525, 2315, 2219, 2150, and 1908 nm. The current technology provides a decrease of the decimal absorption coefficient at 2800 nm from 0.4 to 0.04 cm^-1. The PTR glass matrix absorption spectrum includes three bands at 4855, 3950, and 3250 nm (2060, 2532, and 3077 cm^-1). The long wavelength absorption edge for the PTR glass matrix is 2900 nm for the decimal absorption coefficient 0.01 cm^-1, 3600 nm for 0.1 cm^-1, and 4200 nm for 1 cm^-1.