Squeezed Light at 2128 nm for future Gravitational-Wave Detectors

Abstract

Since the first detection of gravitational waves in 2015, the field has been developing to gravitational-wave astronomy with multi-messenger detections and event statistics. Noise sources need to be reduced further to increase the detection range and to get more information about the detected events. In current observatories, coating Brownian noise is the dominant noise source around 100 Hz. To avoid this thermal noise source, the test masses in future gravitational-wave observatories (GWOs) should be cooled to cryogenic temperatures. The mechanical properties of the currently used fused silica worsen at lower temperatures, and therefore the use of crystalline silicon as test mass material is considered, together with amorphous silicon-based coatings. However, this requires a change of the laser wavelength from 1064 nm to around 2 µm. Then, the only remaining noise source in the mid-range and at higher frequencies is quantum noise. Therefore, all technologies for the new wavelength are investigated at a high priority.

This thesis demonstrates a squeezed light source at 1064 nm via wavelength-doubling, where a Nd:YAG nonplanar ring oscillator (NPRO) laser at 1064 nm is used to provide ultra-stable laser light for interferometry. An external conversion efficiency of (87.1±0.4)% (internal 93 %) could be achieved at a pump power of 52 mW. Adapting the conversion scheme to higher laser power might avoid the need of 2 µm laser amplifiers. The NPRO lasers were highly optimized over decades to reach the technological requirements of power stability, amplitude and phase noise. Available lasers around 2 µm do not yet reach the required performance level, but are currently under research. Using squeezed light, the uncertainty of the light field is reduced to (7.2±0.2)dB below shot noise, mainly limited by the quantum efficiency of (92±3)dB of available photodiodes. As part of the technology development two digital control systems were developed which are used for stabilization of cavities (NQontrol) and crystal temperatures (Raspitemp). NQontrol is an open source digital control system that can control eight channels simultaneously, providing complex loop shaping abilities and high flexibility to adapt the system for different needs. Raspitemp is a modular digital temperature control system which can control up to 20 temperatures combined with a high accuracy of <10 mK.

To reach the aimed squeezing level of 10 dB for the next-generation GWOs, the overall optical losses in the interferometer and the detection have to be below 10 %, requiring photodiodes with a quantum efficiency of 99 %. The novel approach of combining optical-parametric down-conversion with the creation of squeezed states is considered as a candidate for next-generation GWOs such as LIGO Voyager and the low-frequency part of the Einstein Telescope.