Donald K. Cohen

Automatic focus control: the astigmatic lens approach

The use of an astigmatic lens to generate a position error signal (PES) suitable for servo control is discussed. A simple computer model was written that can predict adequately the performance of an astigmatic system. The effect of various system parameters on the PES is presented.

Techniques for measuring 1-μm diam Gaussian beams

This paper reviews various techniques for measuring the diameter of Gaussian beams and, in particular, those of ~1-μm diam. A description of measurement techniques for nonideal conditions is also included. A novel ruling used for beam-size measurement is discussed.

Measurement of 1-μm diam beams

Measurements of beams of ~1 μm in diameter are presented. Three approaches, one using a Ronchi ruling, the other two based on knife-edge scan techniques are compared experimentally. Effects of Ronchi ruling accuracy and nonconstant scan velocity are discussed. Included are results of theoretical studies on the effect of imperfect scanning edges and non-Gaussian beam profiles on measurement results.

Characteristics of a chamber used for electrooptical device performance measurements in the presence of fog

A fog chamber, developed to support measurement of the performance of electrooptical devices in the presence of fog, is discussed. Rationale for the fog chamber concept is presented. Emphasis is placed on the theory of operation of the chamber, an optical method for assessing fog particle characteristics, and the optical properties of the different types of fog produced.

Effect of detector noise on the positioning accuracy of an autofocus system

An autofocus system has been developed that automatically detects the presence of fringes in a white-light interference microscope and adjusts the vertical position of the microscope objective such that the focus error is minimal. The system consists of a motorized stage, a beam splitter, an optical detector, and associated electronics. In order for the interference microscope to provide accurate measurements of the optical wavefront, the error in the focus position must be small. The goal of this paper is to calculate the rms noise produced by the detector and electronics and use the results to predict the positioning error of the autofocus system.

Dynamic Testing Of Optical Disk Profile

It is shown that focus error signal spectrum, measured while disk is spinning, represents Fourier transform of the disk profile modified by the loop gain.