Wavelength calibration with dual constraints for dual-wavelength interference microscope

The measurement accuracy of a dual-wavelength interference microscope (DWIM) is ensured by the two optical wavelengths, which must be calibrated to be traceable in length. A calibration method was proposed for DWIMs that can determine both the relationships between two wavelengths and between the wavelengths and the length reference. First, the relationship between the reconstructed surface and wavelengths was analyzed theoretically, where the surface jump and height place sufficient constraints on the two wavelengths. Then, based on the analysis, a dual-constraint calibration method was proposed to determine the wavelengths. Together with this method, by measuring a flat and a height standard step, two constraints on the wavelength ratio and arithmetic value were established. With the constraints, the optical wavelengths can be calculated by solving equations. Simulations demonstrate a wavelength calibration accuracy of 0.1 nm with the proposed method. A DWIM calibrated with the method achieves measurement accuracy up to 0.5 nm by measuring the depth of a micro-groove. The calibration only needs a height step standard. The proposed method provides an accurate, but simple, calibration for a DWIM and can be applied to dual-wavelength digital holography and three-wavelength interferometry as well. © 2021, Science Press. All right reserved.