Optical Cavity Developments for GRACE-like Architectures

The Laser Ranging Interferometer (LRI) on GRACE FO provided orders of magnitude improved precision satellite-to-satellite ranging measurements over the microwave ranging instrument. The LRI successes have resulted in the next evolution of GRACE mission architectures (GRACE C and NGGM) to baseline the LRI as the main science instrument. The optical cavity assembly was critical to the success of the LRI, as it provides the laser frequency stability required for precision measurement of nanometer-scale range changes between two spacecraft. This optical cavity has been developed over the last 20 years at BAE Systems SMS (formerly Ball Aerospace) under internal and NASA/JPL funding. The optical cavity design successfully flew and operated on GRACE FO for the duration of LRI measurements. The locked laser frequency noise performance is 30 Hz/sqrt(Hz) down to mHz frequencies.

For GRACE C, several key improvements were made to the cavity to improve its manufacturability, reliability, and compatibility with GRACE C LRI architecture (accommodation of the Scale Factor Unit). The cavity is based on a Fabry-Perot etalon at center wavelength 1064nm, with free spectral range of 1.93 GHz and Finesse of ~50,000. The cavity has integrated photodiodes for sensing the transmitted and reflected beams from the etalon, and a photodiode pre-amplifier which provides the feedback signal to the Laser Ranging Processor for performing the Pound-Drever-Hall Lock for laser stabilization. BAE Systems SMS has delivered 3 tested and qualified optical cavities to JPL for integration into the LRI instrument for the GRACE C mission, anticipated to launch in 2028. A very similar optical cavity is baselined for the LISA mission, and BAE Systems SMS’s cavity design can be adapted to other space platform with laser stability needs.