Improving squeezing performance in GEO600
Understanding, modelling, and mitigating noise from light scattered back into the gravitational-wave detector’s squeezed-light source
Current gravitational-wave detectors are limited in sensitivity by quantum mechanical noise over the most part of their frequency range. The German-British detector GEO600 close to Hannover, Germany, uses a squeezed-light source to reduce this quantum noise by up to 6 dB at high frequencies. The squeezed states are injected into the output port of the interferometer. If the optical isolation in this injection path is imperfect, a small part of the output light can reach the squeezed-light source. There, it can leak into the optical parametric oscillator (OPO), which generates the squeezed light to be injected into the detector, and cause additional measurement noise. A team of GEO600 researchers has now developed a theoretical description of this noise generation process, and additionally a new method to reduce the amount of light backscattered into the OPO to mitigate the resulting noise. The implementation of this scheme was crucial in reaching and maintaining the high level of squeezing in GEO600. The results also imply that some requirements for squeezed-light sources in future gravitational wave detectors, such as the Einstein Telescope and Cosmic Explorer, might need to be more stringent than assumed so far.
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.