Generation and pulsating behaviors of loosely bound solitons in a passively mode-locked fiber laser

We demonstrate the experimental observation of loosely bound solitons and pulsations in a passively mode-locked fiber laser. The soliton amount inside one soliton bunch typically increases with the increment of pump power, while the average pulse-to-pulse separation in the regime of hundreds of picoseconds decreases in principle accordingly. Besides, by employing the real-time measurement based on the dispersive Fourier-transform technique, we find that some solitons in a single bunch are experiencing a simultaneous pulsating process with a period corresponding to 100 times longer than the cavity round-trip time. The experimental results can be beneficial for better understanding the formation of multiple solitons and pulsating solitons and enriching the nonlinear phenomena in complex soliton systems.

Figure 1

Schematic diagram of the experimental setup. LD: laser diode; WDM: wavelength division multiplexer; EDF: erbium-doped fiber; OC: optical coupler; PC: polarization controller; PD-ISO: polarization-dependent isolator.

Figure 2

The mode-locking operation of multiple solitons at 150-mW pump power: (a) Optical spectrum, (b) temporal pulse trains based on the 1-GHz oscilloscope, (c) the autocorrelation evolution trace, (d) rf spectrum, (e) temporal pulse trains based on the 20-GHz oscilloscope, and (f) closeups of the red dotted part in (e).

Figure 3

Spatiotemporal traces at pump powers of (a) 150 mW and (b) 450 mW. (c) Pulse numbers of the soliton bunches in the single round trip at different pump powers. (d) Dependence of the number of pulses on the pump power. (e) Average pulse separation of the soliton bunch in the single round trip at different pump powers.

Figure 4

Spectra in the multiple-pulse operation at different pump powers. (Inset) Closeups of the spectra.

Figure 5

(a) Temporal evolution of the loosely bunched solitons measured through the DFT technique. (b) Closeups of the blue dotted section in (a).

Figure 6

(a) Average temporal waveform of more than 5000 consecutive single-shot waveforms in Fig. 5. (b) Single-shot temporal waveforms at round trips A (blue) and B (red) in Fig. 5. The pump power is 150 mW. (Inset) Corresponding closeups.