Compact low-frequency optomechanical triaxial inertial sensor

George D(1), Hernandez Rivero JD(1), Mehmet M(1)(2), Miranda Espino R(1), Guo X(1)(3), Nelson A(1)(4), Sanjuan J(1), Guzman F(1). Triaxial optomechanical accelerometers offer a compact, ground-testable complement to free-fall electrostatic accelerometers used in satellite geodesy, alongside applications in seismometry and precision metrology. We demonstrate a compact triaxial inertial sensor using monolithic fused-silica resonators with dual heterodyne interferometric readout. The X axis reaches a 0.3 pm/√(Hz) readout noise floor, measured with the test mass mechanically anchored, which translates to 60 pico-g/√(Hz) in acceleration at 1 Hz, and the in-plane axes resolve ambient seismic ground motion in agreement with a commercial seismometer mounted on the same platform, from 4 mHz to 7 Hz. The Z axis exhibits higher noise under 1g loading due to gravity-induced geometric stiffening (44.45 Hz versus 11.07 Hz in a horizontal configuration equivalent to 0g), with its readout noise floor, rescaled to the 0g-equivalent resonance, projecting substantially improved in-orbit performance, though short of parity with the in-plane axes. The in-plane results support optomechanical accelerometry for ground-based seismometry, while full triaxial operation for compact, secondary spacecraft accelerometry will require further Z-axis resonator development.