  A simplified Doppler frequency shift measurement approach based on Serrodyne
optical frequency translation is reported. A sawtooth wave with an appropriate
amplitude is sent to one phase modulation arm of a Mach-Zehnder modulator in
conjunction with the transmitted signal to implement the Serrodyne optical
frequency transition, as well as the optical phase modulation of the
transmitted signal on the frequency-shifted optical carrier. The echo signal is
applied to the other phase modulation arm of the Mach-Zehnder modulator. The
optical signals from the two arms are combined in the Mach-Zehnder modulator,
whose lower optical sidebands are selected by an optical bandpass filter and
then detected in a photodetector. By simply measuring the frequency of the
output low-frequency signal, the value and direction of DFS can be determined
simultaneously. An experiment is performed. DFS from -100 to 100 kHz is
measured for microwave signals from 6 to 17 GHz with a measurement error of
less than 0.03 Hz and a measurement stability of 0.015 Hz in 30 minutes when a
500-kHz sawtooth wave is used as the reference.
