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

Gerhard Heinzel,Juan José Esteban Delgado,Alvise Pizzella,Nils Brause,Miguel Dovale Álvarez

doi:10.1103/physrevapplied.14.054013

Gerhard Heinzel, Juan José Esteban Delgado + Show 3 more

Open Access

https://doi.org/10.1103/physrevapplied.14.054013

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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.

Highlights

Laser interferometry is a powerful method to measure tiny distance variations as changes of optical path lengths
The interference pattern is recorded with a photodiode, which produces a photocurrent with a sinusoidal component at the heterodyne frequency
Differential changes of optical path length are converted into phase changes of that sinusoidal beat note, which are measured with a phasemeter instrument

Summary

INTRODUCTION

Laser interferometry is a powerful method to measure tiny distance variations as changes of optical path lengths. E.g., over interspacecraft optical links such as the laser interferometer space antenna (LISA) mission [13,14] or the Laser Ranging Interferometer (LRI) on GRACE Follow-On [15,16]—the usual technique is to employ a digital-phase-locked-loop (DPLL) [7,8,9,10,11,12,17] architecture to track the amplitude, frequency, and phase of the beat note, even when the received beam is sensed at the subpicowatt power level due to the beam divergence over the large propagation distances typical of intersatellite laser links.

PHASE MEASUREMENT WITH DIGITAL PHASE-LOCKED LOOPS

DIFFERENTIAL WAVEFRONT SENSING WITH DPLL PHASE METERS

ALTERNATIVE ARCHITECTURE FOR TRACKING LENGTH AND DWS SIGNALS WITH

NOISE INVESTIGATIONS

CONCLUSION