  A photonics-assisted microwave pulse detection and frequency measurement
scheme is proposed. The unknown microwave pulse is converted to the optical
domain and then injected into a fiber loop for pulse replication, which makes
it easier to identify the microwave pulse with large pulse repetition interval
(PRI), whereas stimulated Brillouin scattering-based frequency-to-time mapping
(FTTM) is utilized to measure the carrier frequency of the microwave pulse. A
sweep optical carrier is generated and modulated by the unknown microwave pulse
and a continuous-wave single-frequency reference, generating two different
frequency sweep optical signals, which are combined and used as the probe wave
to detect a fixed Brillouin gain spectrum. When the optical signal is detected
in a photodetector, FTTM is realized and the frequency of the microwave pulse
can be determined. An experiment is performed. For a fiber loop containing a
210-m fiber, pulse replication and FTTM of the pulses with a PRI of 20 {\mu}s
and pulse width of 1.20, 1.00, 0.85, and 0.65 {\mu}s are realized. Under a
certain sweep frequency chirp rate of 0.978 THz/s, the measurement errors are
below {\pm}12 and {\pm}5 MHz by using one pair of pulses and multiple pairs of
pulses, respectively. The influence of the sweep frequency chirp rate and pulse
width on the measurement error has also been studied. To a certain extent, the
faster the frequency sweep, the greater the frequency measurement error. For a
specific sweep frequency chirp rate, the measurement error is almost unaffected
by the pulse width to be measured.
