Compact optomechanical accelerometers for space geodesy

GRACE has fundamentally improved our understanding of the surface anomalies in Earth's gravitational field and its dynamics. However, GRACE has shown that certain technologies, such as low-frequency high-sensitivity accelerometers, require improvements in both sensitivity and reliability to ensure future high quality science outputs. Significant progress in the development of optomechanical sensing technologies has been achieved by the scientific community over the past decade. Building upon these advancements, we target the development of compact optomechanical low-frequency acceleration sensors with sensitivity levels that enable mass change and geodesy observations. Monolithic inertially-sensitive optomechanical sensors yield high mechanical quality factors and, therefore, high acceleration sensitivities. Readout of these sensors is performed by laser interferometry. Careful selection of materials is necessary to ensure low internal mechanical losses. Furthermore, these materials are non-magnetic resulting in a measurement system that is less sensitive to external electromagnetic fields that typically affect electrostatically readout sensors. Material selections include low-loss glass ceramics – such as fused-silica – and crystalline silicon or silicon-nitride, among others. The materials used to fabricate the mechanical oscillators and the built-in optical components – which constitute the compact test mass sensing interferometers – are inherently compatible with vacuum operations and show low susceptibility to radiation and magnetic effects. Moreover, the materials selected typically exhibit very low coefficients of thermal expansion (CTE), in the order of 10^-7 K^-1. The design of mechanical oscillators and laser interferometer topologies are designed such that the impact of thermal effects and temperature fluctuations can be minimal. Laboratory optical sensor prototypes have demonstrated displacement sensitivities of the order of 10^-13 – 10^-15 m/rtHz over measurement frequencies of 2 mHz up to 100 Hz, respectively. Also, micro-fabricated oscillators with natural frequencies around 10 Hz and below demonstrated mechanical quality factors above 270,000; indicating acceleration noise floors at levels below 10^-10 m s^-2/√Hz, within the observation bandwidth of interest for mass change. The Mass Change Designated Observable has identified strategic value in this technology and has funded a Category 3 effort to advance it to a level where it can be more vigorously supported by a NASA technology development program. Furthermore, it is worth mentioning the value in the realization of a US-sourced low-SWaP low-frequency accelerometer that enables Earth science applications. Presently, no similar US-sourced alternatives exist. The Laboratory of Space Systems and Optomechanics (LASSO) research group at Texas A&M University, led by Dr. F. Guzman, will develop this instrument. LASSO will collaborate with Dr. Christopher McCullough at JPL, who will assist the research program by providing scientific expertise to help steer the technology development in a direction that maximizes scientific output, as well as by conducting gravity field recovery simulations using our experimentally determined instrument performance as determined at progressive stages in the development process.

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