The first detection of gravitational waves from a binary black hole inspiral by LIGO in September 2015 heralded the beginning of a new age in gravitational wave astronomy. The detection of a binary neutron inspiral in August 2017 and has now opened up a new era of multi-messenger astronomy. To increase the sensitivity of future gravitational wave detectors, a change to cryogenic silicon test masses and an increase in laser power may be required. Silicon is a compelling choice as it has high thermal conductivity at cryo- genic temperatures, which reduces temperature gradients generated by optical absorption. Additionally, at 123 K, its thermal expansion coefficient crosses zero. Thus, near this temperature, thermo-elastic distortion of the mirror surface should be drastically reduced, as would the effect of thermo-elastic noise due to thermodynamic temperature fluctuations. However, the adoption of silicon for the optical substrates would necessitate a shift of operating wavelength from 1064 nm to >1.3 μm where silicon is transparent. While potential wavelengths include ca. 1.55 μm and 2.0 μm, the longer wavelengths may be preferred due to lower scattering loss and coating absorption.
High levels of mode matching are required for optimal performance in interferometric gravitational wave detectors that use squeezed light injection. We propose a technique for measuring the magnitude and direction of mode mismatch by inducing a radio frequency waist size and position modulation and demodulating the reflected field using a single element photodiode.
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