High precision surface-profile metrology by scanning the repetition rate of femtosecond pulses

We report high-precision surface-profile metrology using a femtosecond pulse laser as a low-coherence interferometric light source. Unequal-path non-symmetric interferometer is configured to test a large-sized optics with a small reference mirror, which is only feasible through ultra-short mode-locked pulses with repeated periodic temporal coherence and high spatial coherence. The temporal delay between pulses from the reference and the target optics is precisely scanned by tuning the repetition rate of femtosecond pulses. This method enables us to perform high precision surface metrology without parasitic effects by stray reflections and remove all the moving parts in an interferometer.Ab