A torsion balance as a weak-force testbed for novel optical inertial sensors

Torsion balances (TBs) are versatile instruments known for their ability to measure tiny forces and accelerations with high precision. We are currently commissioning a new TB facility to support the development and testing of novel optical inertial sensor units for future gravity-related space missions. Here, we report on the status of our apparatus and present first sensitivity curves that demonstrate acceleration and torque sensitivities of 5 ⋅ 10 − 11 m s − 2 and 1 ⋅ 10 − 12 N m H z − 1 at frequencies around 4 m H z , respectively. Capacitive sensors and optical levers measure the dynamics of the system with a displacement sensitivity of down to 9 ⋅ 10 − 10 m H z − 1 for the former and 2 ⋅ 10 − 11 m H z − 1 for the latter. Combining the readout of the suspended inertial member (IM) with environmental sensor signals, the system is characterized, and limiting noise sources are identified. We find that, in particular, the coupling of ambient seismic motion is limiting over a broad frequency range and show that due to its high susceptibility to ground motion, our TB is also a promising platform for exploring ground motion sensing in multiple degrees of freedom. Future upgrades will focus on mitigating seismic noise by controlling the torsion fiber suspension point using piezoelectric actuators and the integration of precision interferometric readout of the IM. These improvements will further increase the sensitivity towards the thermal noise limit which constrains the performance to 1 ⋅ 10 − 13 m s − 2 H z − 1 at 4 m H z .