Optomechanical gravity gradiometer for remote sensing

We present the construction and characterization of a system of two optomechanical accelerometers used as a gravity gradiometer unit. The unit is sensitive to accelerations along a single axis, which drive a single mode of each accelerometer. The excited mode is linear and uniaxial and can be read out via a single interferometric measurement. High mechanical quality factors on the order of 5 × 10⁵ and continued development of novel high sensitivity interferometers have allowed us to achieve sensor noise floors at sub-picometer displacements, which correspond to an acceleration noise floor of 1.7 × 10^-9 ms-2/ √ Hz at 1 Hz. The mechanical accelerometers have a theoretical thermal noise floor of 1 × 10^-14 m/√ Hz at 1 Hz, and noise attributable to the optical readout has been the primary limit of our sensors. We have designed our sensors to be sensitive in the band of 10 μHz to 1 Hz with the fundamental mode of our accelerometers typically between 5-10 Hz. In this frequency band we are sensitive to several noise sources including laser frequency noise, environmental temperature and pressure fluctuations, non-linear optical path difference (OPD) fiber noise, laser random intensity noise (RIN), electronic noise, and seismic noise, among others. The seismic signal is of note, as systems agree well with commercial seismometers, showing that we are limited by seismic signal above 10 mHz in all on ground measurements with our sensors. The sections of this paper will cover the mechanical design of our gradiometer sensor head, the optical design of our interferometric readout system, and the noise contributions of several mechanisms along with mitigation/correction efforts for each effect.