Fig. 2: Demonstration of broadband joint time-frequency (TF) filtering with nanosecond resolution.
From: Agile manipulation of the time-frequency distribution of high-speed electromagnetic waves
The experimental setup is shown at the centre of the figure. a Temporal trace of the SUT, involving two linearly-chirped sinusoidal waveforms (‘S1’ and ‘S2’) and the zoomed-in traces corresponding to three different sections of the SUT. b Numerical spectrogram of the generated SUT with pre-compensation, depicting the temporal evolution of the frequency spectrum and the uneven intensity. c Measured TM-SP with several zooms around three different relevant time analysis periods. We note that the TM-SP function exhibits a significant component at the centre location of each analysis window, corresponding to the optical carrier frequency; however, this term is strongly attenuated in the representation of the TM-SP traces shown here to facilitate observation and interpretation of the obtained TF distributions of the analysed optical signals. d 2D representation of the signal joint TF distribution that is numerically rescaled from the output measured temporal trace of (c) showing a close-up of the obtained distribution near the cross point of the two chirped signals. e Three zooms of the temporal filtering pattern with each pulse width of ∼ \({t}_{s}\), reconfigured every \({T}_{r}\) = 1.5 ns. The varying time location of the filtering mask along each time slot aligns with the corresponding frequency terms of the increasing frequency-chirp component (S1) in order to filter in this component from the SUT. f The measured output after temporal modulation with the designed filtering pattern shows that the temporal pulses representing the frequency components of S2 are strongly attenuated. g The 2D representation of the TM-SP that is obtained after the temporal modulation. h Temporal trace of the processed microwave signal recovered after dispersion compensation and photo-detection, with the same zoomed-in regions as the input in (a). i Numerical spectrogram of the processed measured signal, confirming that S2 is nearly suppressed and a pure linear chirp with increasing frequency (S1) is recovered. SUT: Signal Under Test, TM-SP: Time-Mapped Spectrogram, STFT: Short-time Fourier Transform, 2D: two-dimensional, CW: Continuous-Wave laser, MZM: electro-optic Mach-Zehnder Modulator, PM: electro-optic Phase Modulator, LCFBG: Linearly Chirped Fibre Bragg Grating, PD: Photo-Diode.
