Abstract

The edge technique is a powerful new method for the measurement of small frequency shifts. When applied to the measurement of Doppler shifts, the edge technique can be used to obtain high accuracy remote measurements of instantaneous velocity. In the edge technique the laser frequency is located on the edge of the spectral response function of a high resolution optical filter. Because of the steep slope of the edge, very small frequency shifts cause large changes in filter transmission. The Doppler shift is determined from a differential measurement of the frequency of the unshifted laser beam and the Doppler-shifted frequency of the laser energy scattered from the target. The differential nature of the measurement makes the Doppler shift determination essentially insensitive to the initial location of the laser on the edge and also to frequency jitter and drift. The high sensitivity and accuracy of the edge technique and the insensitivity to frequency jitter and drift have been demonstrated in recent laboratory experiments. The velocity of a calibrated target was measured using a He-Ne laser and the edge technique. A Fabry-Perot etalon with a spectral resolving power of 8.9 × 10⁶ (Airy width of 54 MHz) was used as the edge filter. Velocity accuracy varied from 0.19 to 0.3 m/sec depending on the initial location of the laser on the edge. This corresponds to a Doppler shift measurement accuracy of 600 kHz which is ~1 % of the spectral bandwidth (combined laser and filter spectral widths) of the measurement.

© 1991 Optical Society of America