Multi-pulse oscillations are prevalent phenomena observed in mode-locked lasers and nonlinear microresonators, where the short- and long-range interactions between nonlinear wavepackets give rise to diverse pulse patterns such as soliton molecules, soliton crystals, and soliton bursts. However, these intricate nonlinear interactions are highly sensitive to the parameters of dissipative systems, leaving the properties of multiple pulses far from being controlled, which hampers their applications such as high-speed optical communication and material processing. In this study, we propose a universal approach for quantitatively tailoring multiple solitons in mode-locked fibre lasers through spectral phase programming, enabling the on-demand generation of soliton patterns with separations that follow from constant, geometric, or arithmetic sequences. By combining with spectral filtering, we demonstrate dual-colour soliton patterns in the same cavity, further highlighting the adaptability of soliton structures. Numerical simulations validate the experimental observations, demonstrating that the spectral phase modulates solitons to emit sub-pulses, which interact with other solitons to generate trapping potentials, thereby giving rise to diverse soliton patterns.

In ischemia-reperfusion stroke, microglia play a dual role in brain injury as well as brain repair, and promoting their switch from a pro-inflammatory M1 phenotype to an anti-inflammatory M2 phenotype is considered to be a potential therapeutic strategy. Docosahexaenoic acid (DHA) is an essential long-chain omega-3 polyunsaturated fatty acid that exhibits potent anti-inflammatory properties in the acute phase of ischemic stroke, but its effect on microglia polarization is unknown. Thus, the objective of this study was to investigate the neuroprotective effects of DHA on rat brain following ischemia-reperfusion injury, and to investigate the mechanism by which DHA regulates microglia polarization. We administered DHA 5 mg/kg intraperitoneally daily for 3 d following a transient middle cerebral artery occlusion reperfusion model in rats. The protective effects of DHA on cerebral ischemia-reperfusion injury were detected by TTC staining, HE staining, Nissler staining, and TUNEL staining. Quantitative real-time PCR, immunofluorescence, western blot, and enzyme-linked immunosorbent assay were used to detect the expression of M1 and M2 microglia-associated markers and PPARγ-mediated ERK/AKT signaling pathway proteins. We found that DHA significantly improved brain injury by decreasing the expression of the M1 phenotypic marker (iNOS, CD16) and increasing the expression of the M2 phenotypic marker (Arg-1, CD206). DHA also increased the expression of peroxisome proliferator-activated receptor gamma (PPARγ) mRNA and protein, increased the expression of the pathway protein AKT, and decreased the expression of ERK1/2. In addition, DHA promoted the expression of anti-inflammatory factor IL-10 and decreased the expression of pro-inflammatory factors TNF-α and IL-1β. However, the PPARγ antagonist GW9662 greatly blocked these beneficial effects. These results suggest that DHA may activate PPARγ to inhibit ERK and activate AKT signaling pathways to regulate microglia polarization, thereby reducing neuroinflammation and promoting neurological recovery to alleviate cerebral ischemia-reperfusion injury.

Coherent dispersive wave emission, as an important phenomenon of soliton dynamics, manifests itself in multiple platforms of nonlinear optics from fibre waveguides to integrated photonics. Limited by its resonance nature, efficient generation of coherent dispersive wave with ultra-broad bandwidth has, however, proved difficult to realize. Here, we unveil a new regime of soliton dynamics in which the dispersive wave emission process strongly couples with the splitting dynamics of the driving pulse. High-order dispersion and self-steepening effects, accumulated over soliton self-compression, break the system symmetry, giving rise to high-efficiency generation of coherent dispersive wave in the ultraviolet region. Simultaneously, asymmetric soliton splitting results in the appearance of a temporally-delayed ultrashort pulse with high intensity, overlapping and copropagating with the dispersive wave pulse. Intense cross-phase modulations lead to octave-wide broadening of the dispersive wave spectrum, covering 200–400 nm wavelengths. The highly-coherent, octave-wide ultraviolet spectrum, generated from the simple capillary fibre set-up, is in great demand for time-resolved spectroscopy, ultrafast electron microscopy and frequency metrology applications, and the critical role of the secondary pulse in this process reveals some new opportunities for all-optical control of versatile soliton dynamics.

To the best of our knowledge, we demonstrate the first time-domain measurement of µJ-level, few-fs ultraviolet dispersive-wave (DW) pulses generated in gas-filled hollow capillary fibers (HCFs) in an atmosphere environment using several chirped mirrors. The pulse temporal profiles, measured using a self-diffraction frequency-resolved optical gating setup, exhibit full width at half maximum pulse widths of 9.6 fs at 384 nm and 9.4 fs at 430 nm, close to the Fourier-transform limits. Moreover, theoretical and experimental studies reveal the strong influences of driving pulse energy and HCF length on temporal width and shape of the measured DW pulses. The ultraviolet pulses obtained in an atmosphere environment with µJ-level pulse energy, few-fs pulse width, and broadband wavelength tunability are ready to be used in many applications.

Harmonic mode-locking, realized actively or passively, is an effective technique for increasing the repetition rate of ultrafast lasers. It is critically important to understand how a harmonically mode-locked pulse train responds to external perturbations and noise, so as to make sure that it is stable and resistant to noise. Here, in a series of carefully designed experiments, we elucidate the retiming dynamics of laser pulses generated in a soliton fiber laser harmonically mode-locked at GHz frequencies to the acoustic resonance in a photonic crystal fiber (PCF) core. We characterize the self-driven optomechanical lattice, which is distributed along the PCF and provides the structure that supports harmonic mode-locking, using a homodyne setup. We reveal that, after an abrupt perturbation, each soliton in the lattice undergoes damped oscillatory retiming within its trapping potential, while the retiming is strongly coupled to soliton dissipation. In addition, we show, through statistical analysis of the intra-cavity pulse spacing, how the trapping potentials are effective for suppressing timing jitter. The measurements and the theory developed in this work lay the groundwork for studies of the general stability and noise performance of harmonically mode-locked lasers as well as providing valuable insight into generic multi-pulse phenomena in mode-locked lasers.

Dispersive waves (DWs) are ubiquitous in nonlinear optical systems that host optical solitons. In particular, DWs in soliton fiber lasers have attracted continuous interest over the past decades since they are closely related to laser soliton instabilities. Meanwhile, intra-cavity DWs also play important roles in solion interactions. While previous studies mostly focused on their spectral properties, characterizations and manipulations of the intra-cavity DWs in the time domain remain unexplored. Here, we report the experimental studies on temporal structures of intra-cavity DWs, unveiling their exponentially-decaying profiles which can be manipulated by varying cavity parameters, following a simple analytic model. We also studied the evolution dynamics of the intra-cavity DWs under abrupt cavity parameter variations. We observed that both the amplitude and the width of the DW waveform went through variations as the pump power was changed, while their evolution trajectories followed intimately that of the soliton energy. These results provide insights into the DW emission mechanism in mode-locked lasers, and the control methods of intra-cavity DWs may find potential applications in the fields of ultrafast pulse generation and soliton dynamics.