  A photonics-based leakage cancellation and echo signal de-chirping approach
for frequency-modulated continuous-wave radar systems is proposed based on a
dual-drive Mach-Zehnder modulator (DD-MZM), with its performance evaluated by
the radar measurement and imaging. The de-chirp reference signal and the
leakage cancellation reference signal are combined and applied to the upper arm
of the DD-MZM, while the received signal including the leakage signal and echo
signals is applied to the lower arm of the DD-MZM. When the amplitudes and
delays of the leakage cancellation reference signal and the leakage signal are
precisely matched and the DD-MZM is biased at the minimum transmission point,
the leakage signal is canceled in the optical domain. The de-chirped signals
are obtained after the leakage-free optical signal is detected in a
photodetector. An experiment is performed. The cancellation depth of the
de-chirped leakage signal is around 23 dB when the center frequency and
bandwidth of the linearly frequency-modulated signal are 11.5 and 2 GHz. The
leakage cancellation scheme is used in a radar system. When the leakage
cancellation is not employed, the leakage signal will seriously affect the
imaging results and distance measurement accuracy of the radar system. When the
leakage cancellation is applied, the imaging results of multiple targets can be
clearly distinguished, and the error of the distance measurement results is
significantly reduced to 10 cm.
