Classical and non-classical laser sources for current and future gravitational wave detectors

Abstract

Current and future gravitational wave detectors (GWDs) place high demands on their subsystems to reach their sensitivity target. Therefore, the stabilized laser systems and squeezed light sources have to fulfill the highest requirements to allow for the anticipated sensitivity. Currently, second-generation GWDs use lasers at a wavelength of 1064 nm to measure differential arm length changes in their Michelson interferometers and since 2015 they are detecting gravitational waves. In this thesis, Nd:YVO4 solid-state laser amplifiers with output powers of up to 114 W and a very high spatial purity down to 2.9% higher order mode content were set up, tested, and integrated into a GWD laser stabilization environment. The amplifiers allowed for low noise and highly reliable operation, such that they were integrated into the laser systems of currently operating GWDs. Future ground-based third-generation GWDs, like the Einstein Telescope or Cosmic Explorer, are supposed to increase their sensitivity by more than one order of magnitude compared to the current generation. One foreseen improvement is to lower the mirrors' thermal noise by installing cryogenically-cooled silicon mirrors in some of their interferometers. Due to the required transparency of silicon, a change of the laser wavelength to either 1550 nm or 2 µm is necessary. A detailed characterization of laser sources and amplifiers at 1550 nm is presented in this thesis to select a suitable configuration for a GWD laser system at this wavelength. High-bandwidth frequency and power stabilization schemes were designed for the selected laser system, which were tailored for the needs of GWDs. These laser stabilizations were operated simultaneously and characterized by out-of-loop sensors. Independent measurements proved a shot noise limited operation of the power stabilization, below a relative power noise of 10^{-8} Hz^{-1/2} between 100 Hz to 100 kHz, and a frequency noise down to 400 mHzHz^{-1/2}, achieved with an active frequency stabilization with a unity-gain frequency above 2 MHz. The generation of strongly squeezed vacuum states of light is a key technology for current and future ground-based GWDs to reach sensitivities beyond their classical quantum noise limit. By employing the stabilized laser system in a newly designed squeezed light source, the direct measurement of up to 11.5 dB squeezing at 1550 nm wavelength over the entire detection bandwidth of future ground-based GWDs ranging from 10 kHz down to below 1 Hz was demonstrated, for the first time in literature. Furthermore, the direct observation of a quantum shot-noise reduction of up to 13.5 +/- 0.1 dB at MHz frequencies allowed to derive a precise constraint on the absolute quantum efficiency of the photodiodes used for balanced homodyne detection. All these results provide important knowledge regarding laser systems and squeezed light sources for future GWDs, as well as for the whole field of high precision metrology or cryptography, where ultra-low noise laser systems and non-classical states of light are of great interest.