Time reversal of ultrafast waveforms by wave mixing of spectrally decomposed waves

Two different realizations of time-reversal experiments of ultrafast waveforms are carried out in real time by use of four-wave mixing arrangements of spectrally decomposed waves. The first, conventional, method is based on phase conjugation of the waveform's spectrum and achieves time reversal of real amplitude waveforms. The second arrangement of the spectrally decomposed waves spatially inverts the waveform's spectrum with respect to the optical axis of the processor and achieves true time reversal for complex-amplitude ultrafast waveforms. We compare and contrast these two real-time techniques.