The first direct detection of gravitational waves has recently gained a lot of attention and marks the beginning of the area of gravitational-wave astronomy, where listening for gravitational waves will open up a completely new window to the universe.
There was already a wealth of new science coming out of the first detections by the LIGO interferometers, and work is continuing in many areas to further improve the detection sensitivity to be able to detect more signals and increase the signal-to-noise ratio. One of these areas deals with the quantum-noise of the light field, and concepts to reduce this noise by incorporating techniques from quantum optics and changing the interferometer configuration in specific ways to optimize the detector’s response in the presence of quantum noise. Many of the proposed schemes, some of which have been demonstrated already on a table-top scale, require a detection of light fields that is not only sensitive to the light’s intensity, but also to its phase. At the same time, this so-called quadrature-sensitive readout needs to allow quantum-noise limited detection efficiency. In principle, these requirements can be met by balanced homodyne detection (BHD), which is a well-established technique within table-top experiments in quantum optics. However, up to now no knowledge exists of whether BHD is compatible with the extreme stability and noise requirements of large-scale interferometers. The goal of this project was to investigate the performance of BHD in interferometric setups with suspended optics, i.e. in an environment as in gravitational-wave detectors. This included precursor experiments to find out how various noise sources, such as beam jitter, mode mismatch, and path-length noise would affect the readout. The results from these experiments were to inform the design and construction of a suspended BHD, to be tested and used within the Sagnac Speedmeter testbed at the University of Glasgow. Finally, a design study for the implementation of BHD in large-scale gravitational wave detectors was intended to finalize this project.
As the project was terminated early, not all of the mentioned goals could be achieved. Together with colleages T. Zhang and S. Danilishin, the coupling of beam jitter and mode-mismatch at the balanced homodyne detector was theoretically investigated and the results recently published in Phys. Rev. A. From these calculations we concluded that the requirements for the Sagnac Speedmeter project are reachable with a relatively simple, two-stage pendulum suspension for the BHD. An optical layout for a suspended BHD, i.e. the arrangement of beam-splitter, focusing optics and two photo diodes was designed and simulated. Together with the Institute for Gravitational Research’s technical engineer, R. Jones, a technical drawing for the suspended BHD was completed and parts procured. By the time that this project ended, the manufactured parts had just arrived for assembly and were handed over to the Speedmeter group for completion.
