Sawtooth phase modulating interferometry for the measurement of reflectivity and phase profiles of CO2laser graded reflectivity mirrors

The knowledge of the reflectivity and phase profiles of graded reflectivity mirrors (GRMs) is important for studying the performance of lasers equipped with optical unstable resonators that use a GRM as output coupler. This paper deals with a low cost home-made interferometer which permits simultaneous measurement of GRM reflectivity and phase profiles at CO2 wavelengths. The instrument works without any beam expander and infrared camera. The reflectivity and phase profiles are measured by performing a discrete number of local measurements along the sample diameter. To minimize the data acquisition time a fringe scanning interferometric technique based on the serrodyne technique is developed. According to this technique, the reference wave is phase modulated with a sawtooth-vibrating mirror. Reflectivity and phase are directly measured with a two phase lock-in amplifier. The RMS repeatibility for reflectivity and phase profile measurements is 0.8% and lambda /200, respectively.