Fiber-based phase camera for laser interferometers: a proof-of-concept study

For the operation of gravitational wave laser interferometers, high precision sensing is key for alignment and control. Differential wavefront sensors utilizing heterodyne detection can be used to monitor phase and amplitude of individual wavefronts of the main carrier and of sidebands frequencies. In this paper, we introduce the concept of a fiber-based phase camera, where a multimode fiber array is used as static image sensor. With this camera, we can observe the carrier and all sidebands in parallel with a stable spatial phase relation between pixels at high frame rates up to 400 Hz. A core part of this phase camera is the CTC ASIC digitiser providing a sampling rate of ≥ 500MSa/s, which was originally developed for gamma-ray cameras of the Cherenkov Telescope Array observatory (CTAO). A proof-of-concept version is presented here utilizing 56 pixel of a 1D fiber array. The influence of the fiber-array on the phase is fully characterised by a constant shift 30.6 (9) mradm/MHZ depending on the demodulation frequency. With an amplitude noise of 1mV over the dynamical range, sidebands up to 60 dB smaller than the reference can be monitored. Due to the static nature of the image sensor, the phase resolution of the camera is only limited by integration time and signal strength. The commissioning of the phase camera prototype at the ET pathfinder with its first results are presented here. These motivate the future development of a full-scale 4096-pixel 2D version of the camera.