Fig. 1. (a) Schematic diagram of the widely tunable mode-locked fiber laser. (b) The measured nonlinear transmission of SWCNT. (c) Reflection spectrum of filters. The inset shows the overlap between two filters: by applying mechanical stress to filter 2, the ultra-narrow bandwidth tunable filter is constructed, and the wavelength interval of the two filters can be adjusted from 32 to 16 pm.

Fig. 2. Characterization of the laser output with a filter bandwidth of 32 pm. (a) Optical spectrum. (b) Oscilloscope trace. (c) Pulse duration. (d) The RF spectrum with a 0.8 MHz span and a 100 Hz resolution bandwidth. The inset shows the RF spectrum in a 50-MHz range with 1-kHz resolution bandwidth.

Fig. 3. Pulse width tunability of mode-locked fiber lasers. (a) Optical spectrum: the laser wavelength shifts from 1545.330 to 1545.352 nm by changing the center wavelength of filter 2. The FWHM spectral bandwidths are about 15, 10, 6, and 4 pm at filter bandwidths of 32, 26, 18, and 16 pm, respectively. (b) Pulse duration: the pulse duration can be tuned without pulse breakup over the ns range (481 ps to 1.38 ns). (c), (d) Filter bandwidth dependence of the laser parameters: (c) pump power (purple curve) and output power (orange curve); (d) SNR (black curve) and peak power (blue curve).

Fig. 4. Characterization of the laser output with filter bandwidth of 16 pm. (a) Optical spectrum with ultra-bandwidth of 4 pm (502 MHz). The inset shows the normalized spectral contour (gray line) and Gaussian fitting (pink-dashed line). Note that a few longitudinal mode-locked laser has a narrow spectrum with a Gaussian shape. (b) Oscilloscope trace. (c) Pulse duration. (d) The RF spectrum with a 0.8 MHz span and a 100 Hz resolution bandwidth. The inset shows the RF spectrum in a 50-MHz range with 1-kHz resolution bandwidth.

Fig. 5. Output pulse parameters of the laser obtained from the simulation: (a) spectral bandwidth and (b) pulse duration; numerically simulated results of pulse evolution with 588 round trips: (c) spectral bandwidth and (d) pulse duration.

Fig. 6. Intracavity evolution of the ns pulse for a cavity fiber length of 228 m: (a) frequency domain and (b) time domain. Such pulse evolution behaves unusually similarly to a quasi-CW with an extremely low breathing ratio, which is in striking contrast to conventional mode-locked lasers. The inset shows the evolution during 206 to 226 m: the time-frequency evolution changes only weakly when the pulse passes through filter 1, breaking the equivalent ultra-narrow bandwidth filter.