# Tests of cryogenic Fabry-Perot cavity with mirrors on different substrates

## TL;DR

This study investigates the performance of cryogenic Fabry-Perot cavities with mirrors on different substrates, revealing calcium fluoride's stability at low temperatures for potential optical applications.

## Contribution

It provides experimental comparison of FP cavity properties on sitall, sapphire, and calcium fluoride substrates across a wide temperature range, highlighting calcium fluoride's stability.

## Key findings

- Calcium fluoride maintains stable optical properties from 300K to 10K.
- Sapphire exhibits birefringence effects at low temperatures.
- Sitall shows degradation due to loss of ULE properties during cooling.

## Abstract

Experiments were performed with Fabry-Perot optical resonators in vacuum at low temperatures. Mirrors were applied on substrates of various optical materials. The infrared laser with a wavelength of 1.064 microns was used. The pump power at the maximum could reach 450 mW. The evolution of the optical properties of the FP cavity was traced in the temperature range (300 - 10)K. The main parameters measured were the integral characteristics of the FP resonances: sharpness (finesse) and contrast of interference. Three types of substrates were tested: a sitall -- the optical glass with ultra low thermal expansion (ULE), sapphire and calcium fluoride. During cooling, the degradation of the integral characteristics of the FP cavity was observed for the sitall mirrors due to the loss of the properties of (ULE), for sapphire mirrors due to the birefringence effect. The satisfactory constancy of the integral characteristics of the FP resonator on calcium fluoride was demonstrated in the entire temperature range studied 1