Femtosecond laser-based phase-shifting interferometry for optical surface measurement

This paper demonstrates an unequal-path phase-shifting interferometer for precise optical surface measurement using a femtosecond laser. According to the periodic low temporal coherence of the femtosecond laser, the relative time delay between pulses from the reference and target surfaces is scanned by sweeping the repetition frequency for phase shifting when the optical path length difference is set to integer times of the pulse interval, which removes mechanical scanning devices in the interferometer. In particular, we employ an iterative least-squares fitting algorithm to derive the phase. With this method, a glass slide surface is reconstructed that agrees well with the surface measured using a commercial Fizeau interferometer. The comparison results show that the difference in the peak-to-valley value is 0.050 μm.