Highly accurate adjustment and stabilization of a fiber interferometer for displacement measurements

An interferometer used for displacement measurement is typically adjusted to the center or another appropriate point of the interferometer fringe as a working point to yield maximum detection sensitivity and linearity. The interferometer is prone to varying misalignment in the course of measurements, most noticeable due to thermal drift affecting the interferometer dimensions. We introduce an automatic correction mechanism based on a proportional/integral (PI) control loop to remove any error in the alignment of the fiber interferometer, specifically long-term drift. The method is based on introducing a harmonic displacement by driving an optical element, which is a micro-cantilever in this work. The second harmonic of the interferometric signal that is shown to be present only for a misaligned Michelson or low-finesse Fabry-Pérot interferometer is processed using a lock-in detector, phase-locked to the drive signal, where the in-phase output signal is used as the feedback signal of the PI control loop operated with zero signal setpoint. The loop output control signal is amplified and supplied to the piezo element, adjusting the optical element to yield perfect alignment. As thermal drift is a slow process, the control loop can mostly be operated with a large time constant, allowing for a highly accurate stabilization, limiting misalignment to less than 2% of the interferometer fringe.