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Manipulation of period-doubled soliton molecules in a passive mode-locked fiber laser
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DOI：10.7498/aps.75.20251609.png)
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Abstract
En 中文
Passively mode-locked fiber lasers have provided an ideal experimental platform for exploring nonlinear dynamical phenomena, owing to their ability to generate stable optical solitons. The period-doubled solitons, as one of the characteristic indicators of a nonlinear system transitioning from stability to chaos, have attracted considerable research interest. In the period-doubled regime, solitons still circulate at the fundamental cavity round-trip time, while pulse parameters such as pulse energy or peak intensity alternate between two adjacent round trips. So that the pulse state repeats itself only after two cavity round trips. In this work, we experimentally and numerically investigated the manipulation and properties of period-doubled soliton molecules (PDSMs). In the experiment, stable soliton molecules could be obtained when the pump power was set to 18 mW. When the pump power was set to 22 mW, PDSMs with separations of 7.5, 15, and 19.5 ps could be obtained by adjusting the polarization controller. By further increasing the pump power to 26 mW and adjusting the polarization controller, period-doubled triple-pulse soliton molecules with equal separations of 13 ps were achieved. These results indicate that pump power and the adjustment of the polarization controller play an important role in the formation of period-doubled soliton molecules. Meanwhile, the dispersive Fourier transform technique was used to observe the real-time evolution of the PDSMs mentioned above in the experiment. It was found that the odd and even pulse energies exhibit a stable intensity difference, while their separations remain consistent. Meanwhile, the phase difference within the soliton molecules was also found to remain unchanged during the period-doubling process, indicating a stable internal phase relationship. The numerical simulation was carried out using a pulse tracing model based on the coupled nonlinear Schr & ouml;dinger equations, which successfully reproduced the PDSMs phenomena observed in the experiment. The key characteristics, including the oscillation of odd and even pulse energies, the constant separation, and the stable phase-difference evolution, were in good agreement with the experimental results. Both experimental and numerical results indicate that the formation of period-doubled soliton molecules is dominated by the self-phase modulation effect, under the combined action of gain, loss, Kerr nonlinearity, and saturable absorption, leading to a self-consistent dynamical evolution inside the laser cavity. This work helps to reveal the internal dynamics of soliton molecules in mode-locked fiber lasers and the physical mechanisms of period-doubling bifurcations in nonlinear systems.
Keywords:
fiber laser
soliton molecules
period-doubled
dispersive Fourier transform
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