Optical AC coupling in the gravitational wave detection band

Abstract

Active laser power stabilization is essential to meet the high laser power stability demands of second generation gravitational wave detectors. To achieve a sufficient shot noise limited performance high optical power needs to be detected in traditional stabilization schemes. This leads to technical complications, for example, those caused by the thermal loads induced in the sensors. Therefore, novel detection schemes for laser power noise become desirable. One of these is Optical AC coupling, which reduces the amount of optical power that needs to be detected in order to reach a certain shot noise limited sensitivity by placing the photodetector in reflection of an optical resonator. To investigate the potential for implementing Optical AC coupling in gravitational wave detectors, different experiments were performed in the 0.3Hz to 60 kHz frequency range, thereby covering the gravitational wave detection band. A dedicated laboratory experiment was designed and set up, allowing for an Optical AC coupling based power stabilization at previously unattained sensitivity. At the core of this experiment is a 1m long, high Finesse optical resonator with a corner frequency of 2 kHz. Detailed noise investigations including effects due to non perfect mode matching were crucial in obtaining the final performance. Furthermore, the potential to increase the power stability of light injected into full scale gravitational wave detectors via Optical AC coupling was explored. For the first time, measurements of the so called Optical AC coupling transfer function of the coupled cavities, formed by the power recycling mirror and the main interferometer, of the Advanced LIGO Livingston and the GEO600 detectors are presented. Experiments towards an implementation of the Optical AC coupling technique into the power stabilization feedback control scheme of GEO600 provided valuable insight into the specific challenges that arise with such an implementation at gravitational wave detectors.