Phase shifting interferometry for large-sized surface measurements by sweeping the repetition rate of femtosecond light pulses

Abstract

We report the use of a femtosecond pulse laser as an interferometric low-coherent light source for precision surface-profile metrology. Unequal-path non-symmetric low-coherence interferometer is configured to measure large-sized optics with a small reference mirror; such measurement is feasible only with ultra-short mode-locked pulses with periodic temporal low-coherence and high spatial coherence. The temporal delay between pulses from the reference and the target optics is precisely scanned for phase shifting interferometry by tuning the repetition rate of femtosecond pulses referenced to the Rb atomic clock of a time standard. This method enables us to perform high-precision surface measurements without parasitic fringes being produced by stray reflection and removes all mechanical scanning parts from the interferometer.