Photonic real-time Fourier transform via optical phase conjugation

We propose a novel approach, to our knowledge, for implementing photonic real-time Fourier transform (RTFT) based on optical phase conjugation, which enables the spectral content of an input microwave signal to be mapped onto the time-domain waveform of output pulses. In the approach, the phase-conjugated signal is generated via four-wave mixing in a semiconductor optical amplifier, and frequency-to-time mapping is performed using two identical dispersive elements (DEs). This configuration effectively maps the spectrum of the input microwave signal to the output time-domain waveform, removing the requirement for DEs with opposite dispersion signs, which are typically required in conventional RTFT systems. The feasibility of this RTFT approach for microwave frequency measurement is validated through an experiment utilizing modulated pulses, phase conjugation, and two identical DEs, with the frequency resolution reaching the theoretical limit determined by the electrical bandwidth and the dispersion amount of the system.