Applications of computer-generated holograms in optical testing

Abstract

Optical testing often requires a measurement of the phase difference between light from two different optical systems. One system is a master or reference system, and the other is a sample or test system. In the optical shop, the reference may be a precision optical surface and the test system may be a newly fabricated surface. A computer generated hologram is a geometric pattern that can be used as a precise reference in an optical test. Computer-generated holograms can be used to make reference systems that would be very difficult to make by other methods. Various encoding methods for making computer-generated holograms are discussed, and a new method is presented that can easily be used on image recorders intended for image processing applications. This general encoding method has many characteristics in common with earlier computer-generated holograms. Examples are given to demonstrate the properties of synthetic holograms and the differences among different encoding techniques. Geometric ray tracing is an essential part of the process of developing holograms for optical systems. A computer ray-trace code was developed to model the optical performance of equipment used in optical testing. This program was used to obtain numeric coefficients that describe the optical properties (optical path) needed to define a reference wavefront. A review of interferometer design leads to a discussion of how the hologram functions as a part of the interferometer and of the limitations to the computer-generated hologram. The diffraction pattern from the hologram, observed in the focal plane of a lens, is the key to understanding the use and limitations of the hologram in an interferometer. A detailed prescription is given for making a computer-generated hologram for a commercial interferometer designed for use with holograms. Problems of finding the proper focal point, the correct hologram size, and preparation of the final hologram image are discussed. An example of an actual test is included. Finally, an analysis of various errors encountered and the limitations of the methods used is presented. Within these limitations, computer-generated holograms can easily and routinely be used to test aspheric optical components.