Understanding, modelling, and mitigating noise from light scattered back into GEO600’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.