CryogenicQ-factor measurement of optical substrates for optimization of gravitational wave detectors

Abstract

Future generations of gravitational wave interferometers are likely to be operated at cryogenic temperatures because one of the sensitivity limiting factors of the present generation is the thermal noise of end mirrors and beam splitters that occurs in the optical substrates as well as in the dielectric coatings. A possible method for minimizing thermal noise is cooling to cryogenic temperatures, maximizing the mechanical quality factor Q, and maximizing the eigenfrequencies of the substrate. We present experimental details of a new cryogenic apparatus that is suitable for the measurement of the temperature-dependent Q-factor of reflective, transmissive as well as nano-structured grating optics down to 5 K. In particular, the SQUID-based and the optical interferometric approaches to the measurement of the amplitude of vibrating test bodies are compared and the method of ring-down recording is described.

A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity

“…Also, aging-related frequency drifts commonly encountered in conventional glass materials are completely absent in crystalline materials, as demonstrated by cryogenic sapphire optical resonators operated at a temperature of 4.2 K [21,22]. An all-silicon interferometer can be made insensitive to temperature fluctuations as the coefficient of thermal expansion of silicon has a zero crossing near 124 K. In this temperature range the Q of silicon is orders of magnitude higher than in the conventional cavity glass materials such as Ultralow Expansion glass (ULE) or fused silica [23]. A similar approach has been used with relatively low-finesse optical Fabry-Perot interferometers [24] and has recently been proposed for gravitational wave detection [25].…”

High mechanical Q-factor measurements on calcium fluoride at cryogenic temperatures

“…The structure is fed by a high (up to 1600 V) AC voltage with the resonant frequency [24]. The substrate vibration is recorded by a Michelson-like interferometer [25].…”

ISEC 2005—Conference summary and comments on SQUIDs and coolers

“…Finally, the application of SQUID in gravitational wave detectors [42] has had a renaissance in many groups, e.g. [45][46][47], for resonant as well as for interferometric detectors.…”