Active vibration suppression in a suspended Fabry-Perot cavity

This paper is concerned with active vibration suppression in a suspended Fabry-Perot cavity, employed as a force sensor in a thrust-stand, called a nanobalance. The nanobalance aims to exploit sensitivity of in-vacuum Fabry-Perot interferometers to sub-nanometric displacements in order to measure micro-thrusts with micronewton accuracy. The instrument has been conceived around an in-vacuum optical cavity embraced by two pendulums suspended to an athermic spacer. The objective of the paper is to suppress beat motion around the pendulum natural frequency (>10 Hz) without affecting the measurement bandwidth (<2 Hz) where thrust has to be measured. Beat motion arises because of small pendulum imbalances excited by ground noise. Relevant digital control strategies and experimental results are presented and discussed.