High precision measurement of optical absorption in ultra-low-OH fused silica at wavelengths near 2 µm

Abstract

The first detection of Gravitational Waves from a Binary Black Hole (BBH) inspiral in September 2015 heralded the beginning of a new age in Gravitational Wave astronomy. This has been further enhanced by the detection of a binary neutron inspiral in mid 2017 and has opened up a new era of multi-messenger astronomy. To further increase detection rate in third generation gravitational wave interferometers numerous upgrades have been determined including a move to cryogenically cooled silicon test masses to reduce thermal noise and an increase in intra-cavity laser power in the interferometric optical cavities, which should reduce the shot-noise in the interferometer. This move to silicon test masses requires a shift to wavelengths longer than 1:3 µm as determined by the transparency of silicon. It has been suggested that the compensation plates, beam-splitter and other optics be composed of Fused Silica. Small, but finite, absorption of optical power in the interferometer will result in thermal gradients within the optics of the detector leading to wavefront distortion of the cavity eigenmode. Therefore accurate measurement of the optical absorption is required to better determine the suitability of fused silica optics at these wavelengths. In this thesis, I describe a method determine the wavefront distortion and hence calculate the amount of optical absorption in fused silica at 2 µm. I shall describe a Hartmann Wavefront Sensor that can measure wavefront distortion such as that cased by substrate absorption with extremely high precision and accuracy.