Tracking Length and Differential-Wavefront-Sensing Signals from Quadrant Photodiodes in Heterodyne Interferometers with Digital Phase-Locked-Loop Readout

We propose a new method to track signals from quadrant photodiodes (QPD) in heterodyne interferometers that employ digital phase-locked loops for phase readout. Instead of separately tracking the four segments from the QPD and then combing 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 DWS signals. Benefits are increased Signal-to-Noise Ratio in the loops and the possibility to adapt the loop bandwidths to the different dynamical behaviour of length and DWS signals.