We numerically study on the key factors and their influences which affected the generation and interaction of solitons in the figure-9 fiber laser based on nonlinear amplifying loop mirror (NALM) mode-locking mechanism. Our simulation used a more authentic approach, which tracing the pulse propagation within the loop cavity. It could act like a saturable absorber which exactly followed the working principle of the NALM. For this reason, the study on the self-starting of mode-locked operation could be carried out systematacially. The simulation results showed that the phase shift difference of the non-reciprocal phase shifter, the splitting ratio of the coupler in the Sagnac loop, and the strength of the pump power all played a role in enabling the mode-locking establishment. Further, multipulse generation were also studied by changing the pump power, and the final steady state presented that all solitons had the same pulse properties. Soliton interactions including attraction, repelling, collision and annihilation could also be observed. In addition, we explored the pump hysteresis phenomena in our simulations. The numerical results verified that the parameters affecting the self-starting of mode-locked operation, the mechanism of multisoliton formation and the interactions between solitons in figure-9 fiber laser, which would enlighten more various experiments and study of solitons dynamics.
We demonstrate the optical part of the optical frequency comb based on a figure-of-9 erbium-doped fiber laser. In this experiment, we embedded an integrated nonreciprocal phase shifter in an all-polarization-maintain fiber oscillator, in order to reduce the mode locking threshold and facilitate self-starting. The repetition rate/ 3 dB spectral bandwidth/ pulse width of the output pulse is 46 MHz/ 15.8 nm/ 527.73 fs, respectively. The pulse trains are injected into the high nonlinear fiber after a two-stage erbium-doped fiber amplifier and a piece of single-mode fiber for compressing the pulse width, then an octave-spanning supercontinuum can be obtained. Finally, the signal of the carrier-envelope offset frequency(fceo) with a signal-to-noise ratio of 35 dB is detected by using a collinear self-referenced f-2f interferometer. Constructing a compact system structure with high signal-to-noise ratio fceo signal is critical for long-term stability of optical frequency combs. Our study has laid a foundation for future development of combs applied in space.
We experimentally demonstrate a special architecture for robust erbium-doped mode-locked fiber laser using a nonlinear amplifying loop mirror with all polarization-maintaining fiber. In order to obtain high repetition rate, the pigtails of components were precisely controlled while the wavelength division multiplexer was placed outside the cavity. The total cavity length was reduced to ~2 m, corresponding to a repetition rate of 101 MHz. In addition, a numerical simulation based on Ginzburg-Landau equation was built to follow the real pulse evolution process in the cavity, instead of introducing an ideal saturable absorption model. The results of the simulation were consistent with the experiment, which verify the guidance of this model for experimental designs. We believe that this fiber laser with high repletion rate and robust structure has great potential as an oscillator of optical frequency comb and could also be an outstanding ultrafast seed laser for industrial applications.
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