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Advanced QUadrature sensitive Interferometer REadout for gravitational wave detectors

Periodic Reporting for period 1 - AQUIRE (Advanced QUadrature sensitive Interferometer REadout for gravitational wave detectors)

Reporting period: 2015-11-01 to 2017-10-31

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.
At the beginning of the project stood the creation of a test-setup for investigating various noise couplings in balanced homodyne detection (WP1). For this, some elements of a precursor experiment (see PhysRevD.92.072009) could be reused. The setup was extended to include a spatial-light modulator for the creation of higher-order spatial modes, aimed at investigating the influence of higher-order mode contents on the sensitivity of BHD (WP3). While this experiment was not completed except for early, it was accompanied by a thorough theoretical investigation of mode-coupling and beam-jitter coupling, led by T. Zhang and S. Danilishin, to which S. Steinlechner significantly contributed. This work was disseminated in PhysRevD.95.062001.
For reasons of time constraints in the Speedmeter Project, the construction of a first suspended BHD detector (WP4) for use with this project was started already before a full investigation of the noise couplings could be completed. Fortunately, the results from our theoretical investigation showed that the design considerations that we had foreseen so far would be sufficient for the Speedmeter Project. Since the BHD needs to be suspended in vacuum to reduce unwanted pathlength noise from seismic and acoustic vibrations, significant effort was invested to design a quasi-monolithic platform containing all the required optics and photo diodes, to be suspended as a double-pendulum via thin stainless-steel wires. This was then transformed into a mechanical drawing by the Institute for Gravitational Research’s technical engineer, R. Jones. Optical elements were procured and parts sent out for machining. In addition, a new electronics front-end for a low-noise, low-frequency readout, compatible with the Speedmeter Project’s data acquisition system, were designed. By the end of this project, all required components had arrived and were handed over to the Speedmeter team for assembly and testing.
Even though this project did not run over the full intended period, it contributed significantly to advancing the state of the art. For the first time, a thorough framework for the analysis of noise couplings due to static and dynamic higher-order modes in BHD was created, for which this project made a large contribution. The project also led to the first design of a compact, suspended BHD setup for use with in-vacuum, suspended interferometers such as gravitational wave detectors. Already in this early stage, the project received a lot of attention within the LIGO Scientific Collaboration (LSC) of gravitational-wave detectors, with the aim of implementing BHD in the large-scale detectors as well.
Mechanical Drawing of BHD suspension platform