The LISA interferometer: impact of stray light on the phase of the heterodyne signal

TL;DR

This paper investigates how stray light affects the phase noise in the LISA gravitational wave detector's heterodyne interferometry, proposing a balanced detection scheme to mitigate phase noise caused by stray photons.

Contribution

It analyzes the impact of stray light phase variations on heterodyne signals and demonstrates that balanced detection significantly reduces this noise in LISA.

Findings

Balanced detection reduces stray light phase noise more effectively.

Stray light incoherent power must be below 10 nW/W to limit phase noise.

Incoherent stray light power impacts the sensitivity of the interferometer.

Abstract

The Laser Interferometer Space Antenna is a foreseen gravitational wave detector, which aims to detect $10^{-20}$ strains in the frequency range from 0.1 mHz to 1 Hz. It is a triangular constellation, with equal sides of $2,5 \times 10^9$ m, of three spacecraft, where heterodyne interferometry measures the spacecraft distances. The stray light from the powerful transmitted beam can overlap with the received one and interfere with the heterodyne signal. We investigated the contribution of random phase variations of the stray photons to the noise of the heterodyne signal. A balanced detection scheme more effectively mitigates this adverse effect than a separation of the frequencies of the transmitted and local radiation. In the balanced scheme, in order to limit the phase noise to picometer level, the incoherent power of the stray light must be kept below about 10 nW/W for an asymmetry of the recombination beam splitter of 1%.