An interferometric sensor for measuring small oscillations of torsional oscillators

An interferometric sensor for measuring the rotation angle of the plate of a mechanical torsional oscillator is described. The sensor is based on a Michelson interferometer, is compact, compensates for frequency and amplitude fluctuations of an optical-radiation source, and has a high sensitivity. The sensor is mainly designed to investigate fluctuations of the rotation angle in torsional oscillators. A sensitivity of ∼5 × 10−11 rad/√Hz that is limited by seismic noise, which acts on the torsional oscillator, is demonstrated at frequencies close to 60 Hz. The potential sensitivity of the sensor limited by photodetector noise is ∼10−11 rad/√Hz. The sensor can also be used in devices where it is necessary to measure small differential displacements of mirrors.