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

TL;DR

This paper analyzes the backscattered light noise in the GEO 600 gravitational wave detector, proposes a control scheme to reduce it, and demonstrates its effectiveness in maintaining quantum noise reduction.

Contribution

It introduces a theoretical model of backscattered light noise coupling and implements a control scheme at GEO 600 to mitigate this noise, enhancing squeezing performance.

Findings

The control scheme effectively reduces backscattered light noise.

Implementation helped achieve 6dB quantum noise reduction.

Backscattered light noise mimics increased phase noise in squeezing.

Abstract

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. 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 6dB of quantum noise reduction [Phys. Rev. Lett. 126, 041102 (2021)]. 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.