  A photonics-based short-time Fourier transform (STFT) system is proposed and
experimentally demonstrated based on stimulated Brillouin scattering (SBS)
without using high-frequency electronic devices and equipment. The wavelength
of a distributed feedback laser diode is periodically swept by using a
low-speed periodic sawtooth/triangular driving current. The periodic
frequency-sweep optical signal is modulated by the signal under test (SUT) and
then injected into a section of SBS medium. The optical signal from another
laser diode as the pump wave is reversely injected into the SBS medium. After
simply detecting the forward transmission optical signals in a low-speed
photodetector, the STFT of the SUT can be implemented. The system is
characterized by the absence of any high-frequency electronic devices or
equipment. An experiment is performed. The STFT of a variety of RF signals is
carried out in a 4-GHz bandwidth. The dynamic frequency resolution is
demonstrated to be around 60 MHz.
