Abstract

We show that adaptive optical autocorrelation can be used to measure the typical grain size of a rough surface. According to second-order statistics,¹ the autocorrelation of the optical image of a surface provides a measurement of its grain size. This grain size may be found from the ratio of the peak (de) intensity to the broad background intensity in the autocorrelation. A photorefractive autocorrelator may be made by modifying a four-wave mixing correlator.² A beam bearing an image of the rough surface is presented as one input to a correlator using bismuth germanate (BGO) in the Fourier plane. After passage through the BGO, this beam is phase conjugated by a barium titanate crystal. The resulting complex conjugate of the image beam is presented automatically self-aligned as the second input. The third input is a plane wave in the Fourier plane. The output of the device is the autocorrelation of the constant amplitude, phase modulated surface image. This autocorrelation consists of a sharp (de component) surrounded by a broad low-intensity background of the cross correlation of the rough surface.³ The ratio of the de component to the cross correlation component is used to determine the surface roughness properties. Care was taken to capture all of the diffracted light from the surface to avoid finite aperture effects.⁴ The autocorrelator measurements agree with microscopic surface measurements.

© 1991 Optical Society of America