High-precision surface form measurement of meter-scale optical flat

Abstract

Optical flats used to realize pulse compression grating in ultra-intense and ultra-fast laser systems are large and have stringent face form requirements. This study employed an interferometer with an optical aperture of Φ800 mm to meet the demand for high-precision surface form measurements of meter-scale optical flats. The measurement accuracy of this interferometer improved using absolute surface form measurement. In addition, the full-aperture reflection surface form measurement of meter-scale optical flats was realized by exploiting sub-aperture stitching technology. The absolute measurement principle, based on the combination of rotational averaging and parity function, was used to calibrate the system error of the interferometer for absolute surface form measurement. The absolute surface form peak-to-valley (PV) repeatability (2σ) of the calibrated interferometer system error was 1.96 nm, and the root mean square (RMS) repeatability (2σ) was 0.44 nm. The calibration results show that the absolute surface-form measurement technique can effectively remove the interferometer system error and improve the surface-form measurement accuracy of the optical flat. The calibrated interferometer was then used to perform a two-dimensional stitching inspection of the meter-scale optical flat. The repeatability (2σ) of the PV value was 21.52 nm, whereas the repeatability (2σ) of the RMS value was 4.43 nm for a 1,680 × 1,100 × 200 mm flat. The measurement results show that the high-precision surface form measurement of meterscale optical flats was realized.