Application Of Rotating Grating Systems In Laser Doppler Velocimetry

Abstract

In this paper recent developments in the production of a rotating radial diffraction grating are presented. A diffraction grating is in general an optical component that can be used as a wavelength dependant laser beam splitter. However, when a radial diffraction grating is rotated, the diffracted beams are in addition shifted in frequency. These properties make the rotating grating suitable for application in interferometric systems, such as laser Doppler velocimeters (LDV). A rotating radial grating, driven by a small electronic motor, is a simple and a multifunctional component with small dimensions. Therefore it has substantial advantages over other types of beamsplitting/frequencyshifting equipment, which are usually complex, relatively large in size and more expensive. Rotating gratings however can only be used for limited frequency shifts and they cause larger instabilities of the frequency shift. The application of a new type of driving motor system and other recent improvements have made the disadvantages of a rotating grating relative to other frequency shift equipment smaller. At the moment there is a tendency towards the development of smaller and simpler LDV systems for industrial applications. Especially for this kind of applications the rotating diffraction grating is well suited.