In this work a Q-switched mode-locked (QML) noise-like pulse (NLP) emission from a thulium-doped all-fiber laser with passive saturable absorption action through the nonlinear polarization rotation effect is demonstrated. QML-NLP pulses were obtained at 812 kHz repetition rate within a 11 μs QML envelope and pulse energy of 460 nJ. Central modelocked NLP sub-pulses were measured with 8.71 ns of width and 4.17 of peak power. The evolution of QML-NLP is experimentally investigated, and the results demonstrate the feasibility of an unconventional pulse operation regime of NLPs and provide insights into the dynamics of 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.
We report a passive mode-locked fiber laser (PMLFL) in a novel configuration to generate a single soliton with ultra-low repetition rate. The configuration includes a Faraday mirror after the first half of the cavity length to counteract the nonlinear polarization rotation effects. The total cavity length is 428 m including a 400-m SMF-28 fiber which was twisted to cancel the linear birefringence. The strict polarization control establishes a relation between the regimes of generation and the polarization state of the pulses propagating in the cavity. By properly adjusting the initial polarization state, we observed three different emission regimes, the single soliton regime (SR), conventional noise-like pulses (NLP) and noise-like square-waveform pulse (NLSWP). In the SR, we obtain a 2.9 ps pulse duration centered at 1558.7 nm with a 467.2 kHz repetition rate.
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