ИСТИНА

One of the large technological problems for the cryogenic version of the OGRAN setup is to keep mirror’s optical characteristics invariable at low temperature under illumination of the 0.1-1 W laser power. At such power level, the thermal induced lenses in the laser spot on the mirrors can have significant influence. The mirror’s substrates should have low thermal noises to achieve high sensitivity and good thermo conductivity at low temperatures to dissipate absorbed power. At present, sitall, sapphire and calcium fluoride (CaF2) are considered as promising materials for mirror’s substrates at the wave length ~ 1 mkm. In this talk we describe experiments in measuring of evolution of the integral characteristics of FP cavities such as the finesse and contrast of output interference pattern under process of FP cavity cooling from 300 K down to 6 K. It resulted in the final recommendation in favor of fluoride calcium mirrors.