Lateral Interferometry-Its Characteristics, Technology and Applications

Abstract

This article is a survey of the characteristics, technology and applications of lateral interferometry, LI. Some of the early uses of LI in laser Doppler velocimetryl have been greatly expanded to cover new important applications in industrial instrumentations and precision, noncontact measurements. In general, LI uses a high precision optical interference pattern that is projected onto a volume in which the measurement takes place. The measurement technique involves the use of a low power laser beam (milliwatts) which is split into two beams of slightly differing frequency using a Bragg cell. When the beams are brought back together, they form an accurate fringe pattern in space. This fringe pattern is then used as a precision optical scale against which-items to be measured are compared. The fringe pattern can be adjusted to have a spatial period from submicron spacing up to several hundred microns and wider. Receiver optics and the electronics that follow detect the relationship of the fringe pattern to the item being measured. The technique is sensi-tive to the angular orientation of the item, so small angles or variation in angular orientation of an object with high resolution can be measured without making physical contact. In addition, the accurate fringe pattern can be used to measure the optical fidelity (transfer function) of lenses and optical imaging systems. One of the newest developments highlights important applications in the areas of rapid, non-contact monitoring of submicroscopic surface defects on high quality surfaces, such as semiconductor blanks that are used in IC component manufacturing, high quality optical surfaces and the like.