A novel method to simultaneously obtain phase modulation depth and residual amplitude modulation ratio in sinusoidal phase modulating interferometer

Abstract

Sinusoidal phase modulating (SPM) interferometry is a well-established technique for ultra-precise measurements. In SPM interferometry, the phase modulation depth is of crucial importance: it is either a measurand or a key parameter for accurate phase demodulation. For laser diode SPM interferometry, the problem of residual amplitude modulation (RAM) should be considered, thus the measurement of phase modulation depth becomes difficult, especially when the RAM ratio is unknown. In this paper, a novel method is proposed to simultaneously obtain phase modulation depth and RAM ratio: by solving a twelve-variable nonlinear equation set, phase modulation depth and RAM ratio can be retrieved numerically. Preliminary simulations show that this method is rarely influenced by the choice of initial values for the numerical solving process.