Long Term Laser Frequency Readout for Space Based Interferometry

Abstract

Future space based laser ranging interferometric missions such as the GRACE Continuity (GRACE-C) mission are expected to require absolute laser frequency knowledge. The GRACE Follow-On (GRACE-FO) mission, launched in 2018, included a Laser Ranging Instrument (LRI) as a technology demonstrator. The success of the LRI has led to its selection as the primary instrument for the next mission (GRACE-C), and it will likely also be the primary instrument for other similar future missions.

The Gravity Recovery and Climate Experiment (GRACE) missions perform key environmental and climate measurements, particularly monitoring the movement of water around the Earth. The data generated by the GRACE and GRACE-FO missions contribute to approximately one quarter of the fifty-five 'Essential Climate Variables' tracked as part of the Global Climate Observing System (GCOS).

The use of a LRI as the primary instrument for future missions requires the development of a new technique to provide absolute laser frequency knowledge, which will allow the science data to be compared over longer timescales of months and years. This thesis presents a proposed technique to achieve this using a dual frequency modulation, and details the experimental demonstration of the technique as well as the risk mitigation activities undertaken. In 2022, the National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory (JPL) Mission Concept Review recommended this technique for inclusion on the GRACE-C mission, and it now forms part of the mission baseline.

The proposed technique adds additional modulation tones to the existing laser stabilisation scheme, which relies on Pound-Drever-Hall (PDH) locking. The additional tones are used to phase modulate the laser light, and can be readout using a similar approach to the PDH technique. By measuring changes to the optical cavity Free Spectral Range (FSR), the readout is sensitive to changes in the optical cavity length, which can be related back to the absolute laser frequency through the relationship dL/L = dv(Laser)/v(Laser) = dv(FSR)/v(FSR).

This thesis describes the testing and development of such a dual frequency modulation technique suitable for use on GRACE-C. A proof-of-concept experiment demonstrated the viability of this technique, achieving performance exceeding the expected requirement. A prototype unit was subsequently designed and built in collaboration with CEA Technologies, and has been tested in a flight-like testbed at the ANU and also in the flight-hardware testbed at NASA JPL. This testing identified a number of limitations of the technique, including highlighting the impact of spurious backreflections. The design requirements, interfaces and testing requirements for the prototype unit are all described in detail, along with performance tests undertaken to demonstrate the prototype could achieve performance requirements under expected mission conditions.

A high fidelity simulation was developed to model the expected behaviour of the system and has been compared against measured performance, highlighting parameters that must be tightly controlled and where requirements may be relaxed. Finally, an analysis of the limitations of the technique are presented, including a discussion of noise sources and how they compare with typical PDH systems, an assessment of the contributions of spurious backreflections, as well as experimental demonstrations of risk mitigation activities. Show more