Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses

TLDR

A self‑scanned 1024‑element photodiode array and minicomputer measure wavefront phase to λ/100 by sampling a 32×32 interference pattern while piezo‑varying the reference arm, then use Fourier‑series and FFT analysis to compute and display wavefront, PSF, and OTF in contour and perspective plots within one minute. The system accurately determines and can automatically subtract interferometer wavefront errors, achieving higher‑than‑usual accuracy for measuring individual elements and overall performance of diffraction‑limited optical systems, and its rapid few‑minute loop time makes it a practical alternative to conventional test glasses. Paper WE16 presented at the OSA Annual Meeting, October 1972.

Abstract

A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.