The concept of dissipative solitons has provided new insight into the complex pulse dynamics in mode-locked lasers and stimulated novel laser cavity designs. However, most of these studies are restricted to qualitative regimes, because it is difficult to quantify dissipative effects in a mode-locked laser. Meanwhile, the quantification of dissipative effects is a general problem that can be also encountered in other dissipative systems. In this paper, we demonstrate a method for quantifying dissipative effects in a mode-locked laser based on analyzing the soliton dynamics traced by time-stretch dispersive Fourier transform. As a result, we are able to quantitatively reproduce the evolution of the pulse that seeds mode-locking through simulations and gain a deeper understanding of the whole process. The obtained physical picture of mode-locking allows us to propose a simple method to quantify the energy threshold for mode-locking buildup and the stability of mode-locked states. A parameter is introduced to evaluate mode-locking conditions, which can serve as a criterion for designing mode-locked lasers. This work opens up new possibilities in the diagnosis and improvement of mode-locked lasers and studies of soliton physics.
Gain dispersion impedes the generation of ultrashort dissipative solitons, especially in mode-locked lasers, by limiting the transform-limited pulse width and causing instability near zero dispersion. We have found a hyper-surface in the parameter space of a mode-locked laser for the best suppression of the gain dispersion effects. This is achieved by analyzing the proximity of a dissipative soliton to the stationary solution of the complex cubic-quintic Ginzburg–Landau equation in the absence of gain dispersion with the method of moments. Theoretical and experimental investigations show that the combinations of system parameters in a specific region near the hyper-surface allow for a dissipative soliton width that is very close to the minimum value, as well as a large stable mode-locking region at anomalous group delay dispersion. These findings provide new insights into ultrashort dissipative soliton generation and help to optimize mode-locked lasers with dispersion, nonlinearity, loss, and gain all taken into account simultaneously.
By studying the nonlinear absorption of ultrafast laser pulses in fused silica, we examine, both with experiments and numerical simulations, the different polarization dependence of multiphoton ionization and avalanche ionization. Results show multiphoton ionization and avalanche ionization play different roles in femtosecond and picosecond laser micromachining, and the contribution via avalanche ionization increases with pulse duration. Meanwhile, the spatial distribution of the free carriers generated by circularly polarized pulses is more concentrated than those generated by linear polarization for picosecond laser pulses. These properties make the circular polarized ultrafast laser a possible way to improve the ultrafast laser micromachining efficiency and spatial quality, and can help to reduce some problematic nonlinear effects in ultrafast laser micromachining of low energy band materials.
We experimentally confirm the polarization dependent of multiphoton ionization and the polarization independent of avalanche ionization in the intensity regime of 10 TWcm -2 . By modeling the ultrafast laser interaction with bulk fused silica, we obtain the multiphoton ionization cross section for linear and circular polarization. Then we verify, both with experiments and numerical simulations, that circularly polarized picosecond pulses can be employed to improve the micromachining precision while keeping the machining efficiency.
A static grounding analysis model of non-pneumatic tire (NPT) was built and presented in this paper. The proposed NPT analysis model considers the non-linearity of the spoke stiffness and is suitable for the performance exploration of various structures of the NPT. First, the shear band, rigid rim, and spoke structure of the NPT were simplified, the main structural parameters and mechanical parameters were extracted, and an analysis model was established. The model can describe the deformation of the NPT when it is subjected to external forces. On this basis, the different stiffness of the spokes during tension and compression was considered, an iterative method was used to compensate for the difference in the deformation caused by the difference in radial stiffness of the spokes, and an analysis model of NPT with nonlinear spokes was established. Then, the contact between the tire and the road surface was introduced to iteratively compensate for the reaction force of the road surface, and the deformation of the NPT with nonlinear spokes on the road surface was obtained. Finally, the finite element software ABAQUS was used to verify the accuracy of the model. This model contains more comprehensive structural parameters and material parameters, which can more realistically simulate the structural characteristics and static grounding behavior of the NPT.
In this paper, we demonstrate a simple and cost-effective fiber chirped pulse amplification (CPA) laser system, where a nonlinear amplifier is employed to generate broadband seeding pulses. The nonlinear amplifier can generate stable pulses with 50 nm spectral bandwidth and linear chirp. With such a seeding configuration being adapted into a fiber CPA laser system, the output bandwidth can be expanded from 7 nm to 20 nm, with only minor changes to a standard industrial fiber CPA system. The increased bandwidth allows for pulse durations of less than 100 fs, which is significantly shorter than the original configuration's 250 fs. When combined with a Fourier pulse shaper, such a fiber laser system is expected to produce pulses with energy exceeding 100 µJ and duration shorter than 100 fs.
Temperature field of stainless steel for dieless forming is studied by using FEM in this paper.Different technical parameter affe cting temperature field is analysed,such as velocity of heating and cooling device,distance between heating an d cooling device,and deformed degre e.The results of analysis represent regular pattern during the process o f different technical parameter inf luencing temperature field.
The cellular Potts model (CPM) is a robust, cell-level methodology for simulation of biological tissues and morphogenesis. Both tissue physiology and morphogenesis depend on diffusion of chemical morphogens in the extra-cellular fluid or matrix (ECM). Standard diffusion solvers applied to the cellular potts model use finite difference methods on the underlying CPM lattice. However, these methods produce a diffusing field tied to the underlying lattice, which is inaccurate in many biological situations in which cell or ECM movement causes advection rapid compared to diffusion. Finite difference schemes suffer numerical instabilities solving the resulting advection-diffusion equations. To circumvent these problems we simulate advection diffusion within the framework of the CPM using off-lattice finite-difference methods. We define a set of generalized fluid particles which detach advection and diffusion from the lattice. Diffusion occurs between neighboring fluid particles by local averaging rules which approximate the Laplacian. Directed spin flips in the CPM handle the advective movement of the fluid particles. A constraint on relative velocities in the fluid explicitly accounts for fluid viscosity. We use the CPM to solve various diffusion examples including multiple instantaneous sources, continuous sources, moving sources, and different boundary geometries and conditions to validate our approximation against analytical and established numerical solutions. We also verify the CPM results for Poiseuille flow and Taylor-Aris dispersion.
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