Fig. 2
Demonstration of time-to-frequency mapping in FSLs. a in dark blue, temporal evolution of the output waveforms after heterodyning with the CW laser. The intensity of the input modulation signal (here, a square-like or flat-top signal) is plotted in light blue. b selected individual output waveforms after heterodyning with the CW laser. c instantaneous power spectrum of the output waveforms after heterodyning (in green). The output individual waveforms are numerically extracted from the recorded time trace, and Fourier transformed. The frequency axis represents the baseband frequency (i.e., the relative optical frequency with respect to f0). Notice that the input signal was imprinted onto the CW laser by means of an AOM driven at 80 MHz, which explains the shift of the lowest output frequencies from DC to 80 MHz. For any time lower than 2 µs, only the leading edge of the modulation signal has entered the FSL, resulting in a truncation of the (flat-top) optical output spectrum. At later times, the instantaneous spectrum shifts by 1/τc (=9.482 MHz) every 1/fs (=105.5 ns)
