A real-time adaptive phase-shifting interferometry

A two-path adaptive phase-shifting interferometry, which is insensitive to the disturbance of constant term of phase perturbations induced by environmental vibration and air turbulence, is proposed. The beam sent from the source is divided into two parts by polarization beam splitter and the two parts enter a phase-shifting interference system, and then a two-path adaptive phase-shifting interference system is formed. The beam intensity of the one path is modulated in high frequency with an acousto-optic modulator, and phase lock-in as well as the closed loop control techniques is used in this path. Thus, the wobble of interference fringes can be compensated, and sequentially the fringes will be locked at arbitrarily chosen phase for phase-shifting testing. Meantime, surface profile of an optical element is measured in another path with five-step phase-shifting interferometry algorithm. Comparing with other similar techniques, here the depth of signal modulation is not limited by the optical path difference and the contrast of the fringe used to evaluate the surface profile is not reduced, hence a high signal-to-noise ratio can be obtained. In order to verify the feasibility of this interferometry, a two-path adaptive phase-shifting interference system is set up and the experiments of phase-locking and surface profile testing have been carried out. After phase-locking being realized, the residual wobble amplitude of optical phase difference is 2.65°RMS. Comparing with ZYGO interferometer, the result of surface profile testing is the same but the measurement repeatability is better.