  In this paper, all-optical short-time Fourier transform (STFT) based on
stimulated Brillouin scattering (SBS) is proposed and further used for
real-time time-frequency analysis of different radio frequency (RF) signals. In
the proposed all-optical STFT system, SBS not only provides a band-pass filter
for implementing the window function in conjunction with a periodic
frequency-sweep optical signal but also obtains the frequency domain
information in different time windows through the generated waveform via
frequency-to-time mapping (FTTM). A periodic frequency-sweep optical signal is
generated and then modulated at a Mach-Zehnder modulator by the electrical
signal under test (SUT). During different sweep periods, the fixed Brillouin
gain functions as a bandpass filter to select a specific range of the spectrum,
which is equivalent to applying a sliding window function to the corresponding
section of the temporal signal with the help of the sweep optical signal. At
the same time, after the optical signal is selectively amplified by the SBS
gain and converted back to the electrical domain, SBS also implements the
real-time FTTM, which can be utilized to obtain the frequency domain
information corresponding to different time windows through the generated
waveforms via the FTTM. The frequency domain information corresponding to
different time windows is formed and spliced to analyze the time-frequency
relationship of the SUT in real-time. An experiment is performed. STFTs of a
variety of RF signals are carried out in a 12-GHz bandwidth limited only by the
equipment, and the dynamic frequency resolution is better than 60 MHz.
