Item

Abstract

We propose a method to track signals from quadrant photodiodes (QPD) in
heterodyne laser interferometers that employ digital phase-locked loops for
phase readout. Instead of separately tracking the four segments from the QPD
and then combining the results into length and Differential Wavefront Sensing
(DWS) signals, this method employs a set of coupled tracking loops that operate
directly on the combined length and angular signals. Benefits are increased
signal-to-noise ratio in the loops and the possibility to adapt the loop
bandwidths to the different dynamical behavior of the signals being tracked,
which now correspond to physically meaningful observables. We demonstrate an
improvement of up to 6 dB over single-segment tracking, which makes this scheme
an attractive solution for applications in precision inter-satellite laser
interferometry in ultra-low-light conditions.