Measurement of the magnification and distortion of high-precision imaging system using dual-fiber point diffraction interferometer

Magnification and distortion are two important parameters for high-precision imaging systems. Point diffraction interferometers (PDIs) can measure the magnification, distortion, and wavefront aberration of imaging systems with high precision. However, determining the precise pinhole alignment of the classical PDI is difficult. A new method for measurement of the magnification and distortion based on a dual-fiber point diffraction interferometer (DFPDI) is proposed. The end faces of two fibers are placed on the object plane of the optics under test and imaged to the image plane. The distance between the image points in the x and y directions are proportional to the Z2 and Z3 Zernike coefficients of the wavefront measurement result, respectively. The measurements of the image placement shift and precise alignment of the point diffraction pinhole are realized rapidly with high accuracy. The feasibility of the method is verified experimentally. The wavefront aberration, magnification, and distortion of a 5x reduction lens with numerical aperture (NA) of 0.3 is measured jointly. The measurement uncertainties (3 sigma) of the magnification in the x and y directions and distortion are 756 ppm, 793 ppm, and 0.233 mu m, respectively. Error analysis shows that the position error of the object- and imageplane stages is the main error source. An improved measurement scheme with a pinhole-pinhole pairs array in the object plane and a pinhole-window pairs array in the image plane is proposed. The influence of the position errors of the stages is eliminated with optimized measurement procedure. The DFPDI's measurement repeatability (3 sigma) of the Z2 and Z3 coefficients is 0.65 and 0.33 nm, respectively, corresponding measurement uncertainties (3 sigma) of the magnification (in the x and y directions) and distortion can reach 1.88 ppm, 1.69 ppm, and 0.812 nm, respectively. (C) 2021 Society of Photo-Optical Instrumentation Engineers (SPIE)