Readout for intersatellite laser interferometry: Measuring low frequency phase fluctuations of HF signals with microradian precision

TL;DR

This paper presents highly precise phase readout systems, called phasemeters, capable of measuring low frequency phase fluctuations of HF signals with microradian accuracy, crucial for space-based gravitational wave detection.

Contribution

The paper introduces a novel phase readout system for intersatellite laser interferometry that achieves the required precision and discusses noise reduction techniques and non-linear noise influences.

Findings

Achieved microradian precision in phase measurement.

Demonstrated performance of the LISA phasemeter prototype.

Identified key noise sources affecting measurement accuracy.

Abstract

Precision phase readout of optical beat note signals is one of the core techniques required for intersatellite laser interferometry. Future space based gravitational wave detectors like eLISA require such a readout over a wide range of MHz frequencies, due to orbit induced Doppler shifts, with a precision in the order of $\mu \textrm{rad}/\sqrt{\textrm{Hz}}$ at frequencies between $0.1\,\textrm{mHz}$ and $1\,\textrm{Hz}$. In this paper, we present phase readout systems, so-called phasemeters, that are able to achieve such precisions and we discuss various means that have been employed to reduce noise in the analogue circuit domain and during digitisation. We also discuss the influence of some non-linear noise sources in the analogue domain of such phasemeters. And finally, we present the performance that was achieved during testing of the elegant breadboard model of the LISA phasemeter, that was developed in the scope of an ESA technology development activity.