Sensitivity and control of a six-axis fused-silica seismometer

We present a pair of seismometers capable of measurement in all six axes of rigid motion. The vacuum-compatible devices implement compact interferometric displacement sensors to surpass the sensitivity of typical electrical-readout schemes. Together with the capability to subtract the sensitivity-limiting coupling of ground tilt into horizontal motion, our seismometers can widen the sensing band toward megahertz frequencies. This has notable applications across a range of fields requiring access to low-frequency signals, such as seismology and climate research. We particularly highlight their potential application in gravitational-wave observatories (the Laser Interferometer Gravitational-Wave Observatory, LIGO) in improving their observation capability of intermediate-mass black holes (approximately <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mn>1000</a:mn><a:msub><a:mi>M</a:mi><a:mo>⊙</a:mo></a:msub></a:math>). The sensors are based on a near-monolithic fused-silica design consisting of a fused-silica mass and fiber, showing improved stability and robustness to tilt drifts, alignment, and control compared to all-metal or mixed metal-silica designs. We demonstrate tilt sensitivity that surpasses the best commercial alternatives in a significantly reduced footprint compared to our previous iterations of these sensors.