Microwave cavity diagnostics of CO2 laser effects on the dielectric properties of select materials

A variety of dielectric samples such as Cordopreg (E293), Slip-Cast Fused Silica (SCFS), Impregnated Slip-Cast Fused Silica (ISCFS), Pyroceram (Corning 9606), Silicon Nitride (SN), and Impregnated Silicon Nitride (ISN) were individually placed in a microwave cavity resonating approximately at 6 GHz in the TE111 mode. These samples are irradiated by a cw, 10.6 micron laser beam for less than 5 sec and with a power density up to 1,000 w/sq cm. The complex permittivity of the samples was determined during and after irradiation by standard microwave cavity perturbation techniques. The technique is found to be a sensitive method to measure small and rapid changes of the loss tangent of the dielectric. The sensitivity is dependent on the quality factor of the cavity, and the rapidity of detection is limited by how fast the sweep oscillator can be swept through the resonance. Microwave absorptivity of all samples increased during laser irradiation for intensities greater than 60 w/sq cm. Cordopreg has shown considerable mechanical, electrical and physical damage both during and after laser irradiation. Considerable specular reflection of laser radiation was observed from silicon nitride and impregnated silicon nitride samples.