IR laser power transmission through silver halide crystals and polycrystalline fibers

Laser-induced breakdown (LIB) thresholds in AgCl_xBr _1-x crystals and fibers were studied under CO₂ laser pulses and CW excitation. The value of LIB threshold P_c of the bulk crystals is about 7.2·10⁸ W/cm² for AgCl and 4·10⁹ W/cm² for AgBr under 60 ns TEA laser excitation. The LIB threshold in fibers is much smaller; about 2·10 ⁸ W/cm². The absorption of the crystals at 10.6 μm changes from 8·10^-5 cm^-1 for AgCl to 2·10^-5 cm^-1 for AgBr; for fibers with the same composition absorption is much greater (3·10^-4 cm^-1 for AgCl; 2·10^-4 cm^-1 for AgBr). The dependence of the LIB and IR absorption on composition, and mechanical and temperature treatments suggests that the LIB in silver halide crystals and fibers is due to the avalanche electrons in a high electric field. The initial free electrons for this process are supplied by ionization of the cation vacancy - charged dislocation complexes. The relatively lower optical stability of the fibers is due to the increased concentration of defects formed in the hot extrusion of the crystal. A simple thermal annealing method for reduction of the IR absorption is proposed.