Operating LISA as a Sagnac interferometer

TL;DR

This paper proposes a simplified Sagnac interferometer configuration for LISA that inherently reduces noise sensitivities and simplifies data processing, but requires improved laser stability or additional noise removal techniques for optimal performance.

Contribution

It introduces a phase-locking scheme for LISA that simplifies operation and reduces noise sensitivities without needing inter-spacecraft phase exchange or clock synchronization.

Findings

The Sagnac signal is insensitive to laser frequency noise and optical bench motion.

Orbital motion causes a 14 km path length difference affecting noise.

Current laser stability is insufficient for full sensitivity in Sagnac mode.

Abstract

A phase-locking configuration for LISA is proposed that provides a significantly simpler mode of operation. The scheme provides one Sagnac signal readout inherently insensitive to laser frequency noise and optical bench motion for a non-rotating LISA array. This Sagnac output is also insensitive to clock noise, requires no time shifting of data, nor absolute arm length knowledge. As all measurements are made at one spacecraft, neither clock synchronization nor exchange of phase information between spacecraft is required. The phase-locking configuration provides these advantages for only one Sagnac variable yet retains compatibility with the baseline approach for obtaining the other TDI variables. The orbital motion of the LISA constellation is shown to produce a 14 km path length difference between the counter-propagating beams in the Sagnac interferometer. With this length difference a laser frequency noise spectral density of 1 Hz/$\sqrt{\rm Hz}$ would consume the entire optical path noise budget of the Sagnac variables. A significant improvement of laser frequency stability (currently at 30 Hz/$\sqrt{\rm Hz}$) would be needed for full-sensitivity LISA operation in the Sagnac mode. Alternatively, an additional level of time-delay processing could be applied to remove the laser frequency noise. The new time-delayed combinations of the phase measurements are presented.