Optical AC coupling power stabilization at frequencies close to the gravitational wave detection band

Optical AC coupling (OAC) has been established as a highly sensitive alternative to multi-photodiode arrays in the detection of laser power noise. Previous experiments were limited by the linewidth of the resonators used in these experiments. This Letter describes a dedicated setup, which uses an optical resonator with a linewidth of 4 kHz which is one magnitude smaller than in earlier experiments. To optimize the OAC gain, the novel approach of using a tunable impedance matching is described. An OAC coupling-based power stabilization feedback control loop is set up and characterized at frequencies overlapping with the terrestrial gravitational wave detection band. The performance of the new experiment is mainly limited by expected noise sources down to frequencies of about 1 kHz and about 7 dB better than its traditional equivalent at frequencies between 8 and 60 kHz.