Characterization and evasion of backscattered light in the squeezed-light enhanced gravitational wave interferometer GEO 600

authored by
Fabio Bergamin, James Lough, Emil Schreiber, Hartmut Grote, Moritz Mehmet, Henning Vahlbruch, Christoph Affeldt, Tomislav Andric, Aparna Bisht, Marc Brinkmann, Volker Kringel, Harald Lück, Nikhil Mukund, Severin Nadji, Borja Sorazu, Kenneth Strain, Michael Weinert, Karsten Danzmann

Squeezed light is injected into the dark port of gravitational wave interferometers, in order to reduce the quantum noise. A fraction of the interferometer output light can reach the OPO due to sub-optimal isolation of the squeezing injection path. This backscattered light interacts with squeezed light generation process, introducing additional measurement noise. We present a theoretical description of the noise coupling mechanism and we prove the model with experimental results. We propose a control scheme to achieve a de-amplification of the backscattered light inside the OPO with a consequent reduction of the noise caused by it. The scheme was implemented at the GEO 600 detector and has proven to be crucial in maintaining a good level of quantum noise reduction of the interferometer for high parametric gain of the OPO. In particular, the mitigation of the backscattered light noise helped in reaching 6 dB of quantum noise reduction [Phys. Rev. Lett. 126, 041102 (2021)]. We show that the impact of backscattered-light-induced noise on the squeezing performance is phenomenologically equivalent to increased phase noise of the squeezing angle control. The results discussed in this paper provide a way for a more accurate estimation of the residual phase noise of the squeezed light field. Finally, the knowledge of the backscattered light noise coupling mechanism is a useful tool to inform the design of the squeezing injection path in terms of path stability and optical isolation.

Institute of Gravitation Physics
External Organisation(s)
Max Planck Institute for Gravitational Physics (Albert Einstein Institute)
Cardiff University
University of Glasgow
Optics express
No. of pages
Publication date
Publication status
Peer reviewed
ASJC Scopus subject areas
Atomic and Molecular Physics, and Optics
Electronic version(s) (Access: Open)