Bonded mounts for small cryogenic optics

摘要

Adhesive bonded mounting of small optics for use at cryogenic temperatures provides improved heat transfer, low optical surface distortion, and reduced cost in comparison with conventional flexural mounts. A design methodology based on the thermo-elastic properties of the adhesive and its interaction with the mounted optic is presented. Key factors in the selection of the appropriate adhesive are high thermal conductivity, a low elastic modulus, a low glass-transition temperature, good adhesion characteristics to optic and substrate, and low outgassing. A design example of 17-mm diameter, 2-mm thick circular polycrystalline germanium window used at a temperature of below 100 K is discussed. During cooling at a rate of more than 3 K/sec the temperature at the center of the wondow mounted in this way lags behind the mount by no more than 20 K at any instant, and reaches equilibrium with the mount in about 50 sec. Maximum optical surface deformation of the mounted optic is less than 0.031 waves RMS differential (1 wave = 633 nm) for a temperature change of 300 K to 102 K. Predicted peak tensile stress is les sthan 17 MPa. The adhesive bonded mount is also simple and economical in comparison with the complex flexural mounts often used for cryogenic optics.