Technologies for future ground-based cryogenic gravitational-wave detectors

Abstract

The first direct observation of gravitational waves in 2015 opened a new cosmic window to phenomena that electromagnetic telescopes are not able to see. To date, the Advanced Laser Interferometry Gravitational-Wave Observatory (aLIGO) and the Advanced Virgo (AdV) observatories have detected 90 compact binary coalescences in their coordinated observing runs. Electromagnetic follow-up observations triggered by the detection of a binary neutron star merger in 2017 marked the beginning of multi-messenger astronomy.

Ground-based gravitational-wave observatories are large complex opto-mechanical systems, and their thermal environment sets a physical limit on their sensitivity. To reduce the impact of thermal noise in mirrors and suspensions, next-generation interferometric gravitational-wave observatories such as LIGO Voyager and the Einstein Telescope will operate at cryogenic temperatures. Current gravitational-wave observatories use fused silica test masses, however fused silica has high mechanical loss at low temperatures, thereby motivating a transition to crystalline silicon optics and suspensions. At cryogenic temperatures, silicon has higher thermal conductivity and the coefficient of thermal expansion crosses zero at 123 K, resulting in the vanishing of thermo-elastic noise. Using crystalline silicon optics requires the operating laser wavelength to shift from 1.064 micrometers (1.064 micron) to >1.5 micron where silicon is transparent.

This thesis focuses on prototyping technologies for future ground-based cryogenic gravitational-wave detectors at 2 micron, which will extend their astrophysical reach, allowing for detection of new and exotic astrophysical sources. The first part of this thesis addresses the need for low-noise laser sources at 2 micron wavelengths for future detectors. A 2-micron-band external cavity diode laser was designed, built, and characterised using a fibre Mach-Zehnder interferometer. A linewidth of 20 kHz at 10 ms integration time was measured without active stabilisation, with a frequency noise of 15 Hz/sqrt(Hz) between 3 kHz and 100 kHz. This performance is suitable for the generation of quantum squeezed light. Additionally, the measured intensity noise is comparable to Nd:YAG non-planar ring oscillator (NPRO) lasers used in current gravitational-wave interferometers. The laser unit has been replicated at various laboratories across the LIGO Scientific Collaboration advancing research into 2 micron photonics, material studies for optical coatings and squeezed light generation.

The second part of this thesis focuses on designing, modelling and commissioning a vacuum and cryogenic low-vibration test facility for delicate opto-mechanical assemblies and optical cavities. The cryogenic facility comprises multi-layer thermal shields enclosing an optical test cavity that is suspended from a complex multi-stage pendulum chain providing isolation from acoustic and environmental noise. A plano-concave Fabry-Perot test cavity with one mirror glued to a crystalline silicon cantilever is used to measure displacement noise spectra in the audio-frequency band between 10 Hz and 10 kHz. This 3 kg test cavity can be radiatively cooled to 123 K in 41 hours and held at that temperature over many months with a stability of +-1 mK. The facility was then used to study displacements, with the goal to characterise thermal-noise-induced components, and mechanical ringdown in different silicon cantilever samples at temperatures ranging between 100 K and 295 K. Finally, this thesis suggests ways to improve the design of silicon cantilevers for low mechanical loss and future directions for experiments in this new cryogenic facility. Show more