Shearing Interferometry: A Flexible Technique For Wavefront Measurement

Abstract

Shearing interferometry has been under development at Itek for well over a decade for a variety of applications. Principal among these has been the high speed measurement of optical beams transmitted through atmospheric turbulence. The technique has also been applied successfully to measurement of complex highly aspheric optical surfaces, control or camera systems, and diagnostic measurement of laser beams. Shearing interferometry has several features that make it highly advantageous for these applications. First, it is self referencing and so can be used to measure wavefronts from remote sources without the usual interferometric requirement of a separate high quality coherent reference beam. Second, it can be configured to operate with broadband or 'white light' sources. Third, provided the source has some spatial coherence, it can operate with extended sources. Fourthly, it can provide linear wavefront measurement over as large a dynamic range as required (hence its value in measuring steep aspherics), and finally, it can give highly accurate wavefront measurement with the bare minimum of available light. These features make shearing interferometry a flexible and valuable technique. Itek has developed several different implementations of shearing interferometers. A particularly successful one for atmospheric turbulence measurement has been the rotating grating shearing interferometer. However in recent years, applications involving pulsed sources have led to the development of some DC (direct current) techniques, including the use of spatial carrier frequency gratings, and the use of polarization. In this paper, we describe the technique and principals of shearing interferometry and give examples of the various implementations, as well as providing results and data on their performance.