Opto-Mechanical Inertial Sensors (OMIS) for High Temporal Resolution Gravimetry

Gravity field measurement by free-falling atoms has the potential for very high stability
over time as the measurement exposes a direct, fundamental relationship between mass
and acceleration. However, the measurement rate of the current state-of-the-art limits
the performance at short timescales (greater than 1 Hz). Classical inertial sensors operate
at much faster response times and are thus natural companions for free-falling atom
sensors. Such a hybrid device would gain the ultra-high stability of the free-falling atom
sensor while greatly extending the bandwidth to higher frequency using the classical
sensor. This requires the stable bandwidth of both devices to overlap sufficiently. We
have developed opto-mechanical inertial sensors (OMIS) with good long term stability for
just this purpose. The sensors are made of highly stable fused silica material, feature a
monolithic optical cavity for displacement readout, and utilize a laser diode stabilized to
a molecular reference. With no temperature control and only the thermal shielding
provided by the vacuum chamber, this device is stable down to 0.1 Hz which overlaps
with the bandwidth of free-falling atom sensors. The OMIS are self-calibrating by
converting the fundamental resonances of a molecular gas into length using the
free-spectral range of the optical cavity,  FSR = c/2nL,  and then sampling the OMIS
mechanical damping rate and resonance frequency using a nearby piezo. This
acceleration calibration is potentially transferable to a companion free-falling atom
sensor. Readout is performed by modulating the cavity length of the OMIS with one
cavity mirror being the OMIS itself and the other being a high frequency resonator. The
high frequency resonator is driven by a nearby piezo well above the response rate of the
OMIS and acts like an ultrastable quartz clock. The resulting highly stable tone is
demodulated by the readout electronics. For the low finesse optical cavity used here, this
yields a displacement resolution of 2x10^-13 m/√Hz and a high frequency acceleration
resolution of 400 ng /√Hz. At 0.1 Hz the acceleration resolution is 1.5 μg /√Hz limited by
the stability of our vibration isolation stage. The OMIS dimensions are about 30 mm x 30
mm x 5 mm and can be fiber coupled to enable co-location with other sensors or as
standalone devices for future gravimetry both on Earth and in space