Heterodyne detection of coherent optical radiation and its application in instantaneous velocity measurements

Abstract

This paper introduces a novel technique for making rapid, highly accurate instantaneous velocity measurements. Among its other advantages over conventional velocity measurement techniques, this method offers even modestly-equipped laboratories the unprecedented capability to measure speeds down to the order of 1 micron/second in near "real-time" over extended distances. The principles involved are analogous to those used in doppler radars, sonars and velocimeters—namely, the calculation of instantaneous target velocities from the doppler shift of waves projected to and reflected back from a moving object (henceforth referred to as the target). What distinguishes this new method from the other devices mentioned above are its use of a laser beam as the transmitted wave and the theoretically elegant heterodyning process used to detect the doppler shift. In what follows, the theory behind the technique is first explained in depth. This is then followed by a detailed analysis of the workings of an actual experimental set-up built by the authors to test the theory. Sample data from these tests are then presented and results evaluated against theoretical predictions.