Some Recent Developments in Laser Interferometry

Abstract

An account is given of some new interferometric developments which can be used to improve the performance of the laser interferometer as a position sensing device. The first of these developments, termed the double pass attachment, is an optical accessory which doubles the resolution of the Hewlett-Packard linear and plane mirror interferometers. Unlike previous attempts to extend resolution optically, this device does not fold one of the two interfering beams and hence does not introduce error due to its own motion. The simple addition of a quarter wave plate to the attachment can be used to give a differential version of the plane mirror interferometer. Various configurations of this differential interferometer, and their application to machine tool and integrated circuit lithographic and inspection equipment, are discussed. A new “wavelength tracking” device, based on differential interferometry, is described. This device directly and precisely monitors changes in laser wavelength inside a highly stable mechanical cavity. It can be easily incorporated, as an additional interferometer axis, into precision machines to give superior compensation for changes in laser wavelength.