Objective Because of their high application potential in long-distance optical communications and ultrafast laser physics, optical solitons, which are localized structures in nonlinear systems, have piqued the interest of researchers. During long-distance propagation, optical solitons can interact with each other, resulting in a variety of bound-soliton states known as “soliton molecules”. Therefore, mode-locked lasers that can support the long-distance propagation of multiple solitons within their cavities are widely regarded as ideal platforms for studying soliton interactions and dynamics. However, the studies of the complex interactions of several optical solitons are difficult in traditional passive mode-locked lasers because fast drifts and the frequent collisions of solitons caused due to intense soliton interactions can degrade the stability of the laser mode-locking operation. In this paper, we use a high-repetition-rate optomechanically mode-locked fiber laser to successfully study the complex interactions of many optical solitons. The strong optomechanical effect in a short length of solid-core photonic crystal fiber (PCF) allows forming a robust optomechanical lattice in the laser cavity, and multiple solitons can be stably trapped within each cycle of the optomechanical lattice. Experimental results reveal that complex soliton interactions can be observed and partially controlled in this optomechanically mode-locked fiber laser, highlighting the significant potential of this unique optomechanical fiber laser system for studying complex soliton dynamics.

Methods To investigate the complex phenomena of multi-pulse interactions, we created an optomechanically mode-locked fiber laser with a short solid-core PCF length as the harmonically mode-locking element. Because of the strong coupling between optical and acoustic waves in the PCF, a robust optomechanical lattice formed in the laser cavity, dividing the laser cavity into two halves. Multiple solitonic pulses can be trapped within each of these time-slots, working as an optical-soliton “reactor.” The stability of the optomechanical lattice is largely enhanced by the strong optomechanical interactions in the PCF core. In contrast, the multiple solitons trapped in each lattice cycle were observed to interact intensely with each other.

In the experiments (Fig. 1), an erbium-doped fiber amplifier was used with two 980 nm laser diodes as the pump sources. Two polarization controllers working together with an optical polarizer acted as an artificial saturable absorber through nonlinear polarization rotation (NPR). A time-stretch dispersion Fourier transform (TS-DFT) setup was built using a 5-km-long single-mode fiber (SMF) as the stretching element to demonstrate the detailed information of the multi-soliton interaction processes. Two 45-GHz bandwidth detectors and a 33-GHz bandwidth oscilloscope were used to record the laser output pulses' time-domain trace and DFT signal. Furthermore, the laser output spectrum was recorded using an optical spectrum analyzer with a resolution of 0.02 nm. Since most of the laser cavity was made from SMF, the cavity dispersion was strongly anomalous with a calculated average value of -23.8 ps²/km, leading to soliton operation of the laser with hyperbolic pulse shape.

Results and Discussions By carefully adjusting the intra-cavity polarizer controllers, stable harmonic mode-locking at 1.89 GHz resonance frequency of the acoustic core resonance in the PCF could be realized when both of the two pump diodes have pump powers of approximately 380 mW at 980 nm. The laser output spectrum, as well as the time-domain pulse sequence, were captured. When only one soliton is trapped in each cycle of the optomechanical lattice, the stable acousto-optic mode-locking state could be obtained with a 3 dB spectral bandwidth of 2.46 nm [Fig. 2(a) and Fig. 2(d)]. At this state, a strong Kelly-sideband observed on the pulse spectrum indicates that the laser was operating in the soliton regime. Through the experiments, we found that by adjusting the intra-cavity polarizer controllers, the laser output spectrum could be varied gradually from 2.46 nm to less than 0.6 nm [Figs. 2(b)--(f)], while the optomechanical lattice remained to be stable and the average output power of the laser was kept almost constantly at approximately 70 mW, giving rise to a series of quasi-stable acousto-optic mode-locking states. In those states, multiple soliton pulses were trapped within each cycle of the optomechanical lattice. The DFT signal unveiled that intense and complex interactions between the trapped solitons occurred within each cycle of the lattice (Figs. 3--5). As an entire, this optomechanically mode-locked fiber laser system permits several solitons to coexist in its cavity, and complex interactions between these solitons in one cycle could be studied in the future using this unique platform.

Conclusions In the experiments, we obtained a large number of quasi-stable states in an optomechanically mode-locked fiber laser. We discovered that each isolated cycle of the optomechanical lattice could function as robust optical-soliton “reactors,” allowing us to study complex and intense soliton interactions. We could partially adjust the number of pulses trapped in each cycle of the optomechanical lattice and, thus, the total number of solitons generated in the laser cavity by adjusting the working point of the NPR effect in the laser cavity. In this way, we could control to some extent the multi-soliton interaction processes. Compared with a traditional passive mode-locked laser, this system's stability, flexibility, and high-repetition-rate features make it an ideal experimental platform for studying complex multi-soliton interactions, providing some useful insights on soliton dynamics.