Low-Power Phase Tracking in Inter-Spacecraft Laser Interferometry

Abstract

Inter-spacecraft laser interferometers perform measurements that are impossible to make from Earth. The Gravity Recovery and Climate Experiment Follow-On mission (GRACE-FO), the first such mission, monitors gravitational anomalies in the Earth's field, inferring critical information about global ice mass, groundwater movement, and sea-level rise. Future missions, such as the Laser Interferometer Space Antenna (LISA), will be the first gravitational-wave observatories capable of probing the millihertz frequency band for gravitational phenomena. These missions provide critical insights into our planet and universe.

As light propagates the vast distances in space between satellites, the majority is lost (-70 dB for GRACE-FO), requiring the interferometer to perform measurement with the small amount of light captured. The minimum received optical power with which inter-spacecraft laser links can robustly function is a significant factor in their design and operation.

This thesis presents research exploring the low optical power limits of inter-spacecraft optical links. This work focuses on analysing the phasemeter, the digital device within the inter-spacecraft interferometric detection chain where measurement is performed. The work extensively explores phasemeter performance and optimisation for operation with low optical power signals, resulting in robust phasemeter measurement of femtowatt-level optical signals.

First, the thesis presents analytical models, numerical simulations, and an optical experiment that explores phasemeter performance in tracking low-power optical interference signals. The experimental system enabled phasemeter tracking of a signal with controllable amplitudes, relative additive noise, and laser frequency noise characteristics. Phasemeter tracking error and cycle slip rate measurements are compared to analytical phase-locked loop models and hardware-in-loop numerical simulations. The findings are used to optimise phasemeter tuning for the low-power regime, resulting in a 90-minute robust measurement of a femtowatt-level optical field.

Next, a second experiment is described, performed in collaboration with the NASA Jet Propulsion Laboratory, to determine the optimal phasemeter tuning for low-power tracking of the GRACE-FO flight laser. The results demonstrate robust tracking at an optical power level of 200~femtowatts --- $10$~dB below the tracking requirement of the GRACE-FO mission. This work supports the exploration of future inter-spacecraft interferometric missions operating with relaxed optical power requirements.

Following this, the optical power requirements of passive retroreflector inter-spacecraft interferometric architectures are explored using hardware-in-loop numerical simulations and analytical modelling. Observed phasemeter measurement errors are compared between different inter-spacecraft optical configurations, received optical power levels and phasemeter parameter tunings. The work reveals that passive retroreflector architectures can robustly operate with thousands of times less light than their transponder counterparts.

Next, an alternate technology for phase tracking DC-centred carrier frequencies is proposed, named here the dual quadrature phasemeter. This approach leverages coherent optical detection to adjust the internal structure of the phasemeter, enabling operation with larger bandwidths and unconstrained carrier frequency (down to and through DC). This technology enables a new class of inter-spacecraft laser link that operates with a simplified optical topology and at a lower cost than traditional transponder methods.

The thesis concludes by combining the low-power phasemeter tracking findings with the dual quadrature phasemeter to present a novel optical architecture for a simplified passive-retroreflector mission design. The design's advantages and challenges are outlined, as is the future work required for verification.