Fourier analysis of optical spectra for characterization of regimes in passively mode-locked fiber lasers

We propose to use the fast Fourier transformation (FFT) to analyze the spectral data of passively mode-locked fiber lasers, with the purpose of characterizing and classifying the different pulsed regimes arising in these lasers. The results show temporal information at the sub-ps scale. Specially, in the Fourier domain, the spectra of bound solitons depict several solitons with relative phases of π/2 and close to 0, distributed into a window that extends beyond the limited span of the autocorrelator. Besides, the FFT amplitude traces can help to classify the multi-pulse regimes generated by the laser; in particular, the exponential decay of intensity in such amplitude traces can be a fingerprint of noise-like pulses. Furthermore, the amplitude traces show some modulation that we attribute to Kelly’s sidebands. The proposed technique allows examining spectra from a mode-locked fiber laser independently of the active medium or laser cavity configuration without the need to use ultrafast photo- detectors.