四能级增益介质光子晶体激光器的时域有限差分模型与模拟

基于Maxwell-Bloch方程和速率方程相结合的模型,模拟包括四能级增益介质光子晶体激光器的光学特性.根据FDTD法的基本原理,给出方程的Yee氏离散方程.通过计算电场的时域特性,模拟光子晶体激光器的频谱特性、阈值特性、动力学演化等过程,模拟结果和理论结论一致,检验了该模型的有效性.

Test study on rupture process and permeability properties of coal-rock in simulating mining stress effect

A test system was designed by using a set of self-made experimental devicesof coupled coal-gas in simulating mining stress effect, combining the equipment withRMT-150B rock mechanical experimental system, monitoring the rupture process ofcoal-rock with an acoustic emission (AE) device and collecting gas-flow rate andgas-stress data in real-time automatically with a gas flow-meter and gas pressure sensor.The fracture process and permeability properties test of the coal-rock in mining stress effect was carried out. Test results indicate that AE events and variation of stresses have thesame variance tendency and the rupture process of coal-rock can be monitored by AE.The relation curves among stresses, parameters of AE and permeability properties demonstrate that the permeability of coal-rock decreases gradually at quasi-elastic stage, increases gradually at plastic damage micro-fracture stage, rises suddenly near the peakpoint and has multi-variation at post-peak slip stage. From the results, such conclusioncan be drawn that the variation of permeability can be monitored by AE parameters orstress change.

Analysis of stimulated Brillouin scattering in multi-mode fiber by numerical solution

Stimulated Brillouin scattering in optical fibers is described by a theoretical model and numerical analysis. The results showed that, for an optical fiber pumped by a laser beam with ns-order-pulse width and kW-order peak-power, SBS reflectivity tends to saturate when the fiber length exceeds a limit, named 'effective fiber length'. Using small core-diameter and long enough fiber, the SBS reflectivity level could be raised but is limited by optical damage of the entrance surface of the fiber. Therefore, just a small dynamic range can be obtained.

Effects of Isospin on Pre-scission Particle Multiplicity of Heavy Systems and ItsExcitation Energy Dependence

Isospin effects on particle emission of fissioning isobaric sources and isotopic sources , and its dependence on the excitation energy are studied via Smoluchowski equations. It is shown that with increasing the isospin of fissioning systems, charged-particle emission is not sensitive to the strength of nuclear dissipation. In addition, we have found that increasing the excitation energy not only increases the influence of nuclear dissipation on particle emission but also greatly enhances the sensitivity of the emission of pre-scission neutrons or charged particles to the isospin of the system. Therefore, in order to extract dissipation strength more accurately by taking light particle multiplicities it is important to choose both a highly excited compound nucleus and a proper kind of particles for systems with different isospins.

Model and Simulation on Far Field Common Mode Radiation from a Flyback Power Supply

It is well known that a SMPS （switched-mode power supply） is easy to produce strong EMI （electromagnetic interference） and fails in EMC （electromagnetic compatibility） test for its far field radiation exceeds the limits between 30-200 MHz. Based on asymmetry line antenna theory, a novel far field CM （common mode） radiation model, including an equivalent driving source, radiation structure and some key influence factors, is identified and built up for a small flyback power supply. Radiation characteristics of this model are predicted by using Ansoft HFSS software and the model effectiveness is verified by experiment. In the end, the radiation role of some key factors, such as the length of output cable, common mode impedance of AC grid, layout of cable and reflected ground, are studied using simulation in detail.

Slicing-response model for ablation mass removal of polyformaldehyde irradiated by pulsed CO_2 laser in vacuum

The interaction between CO2 laser and polyformaldehyde(POM)is quite important in the research of laser irradiation effects and mechanisms.At this time,the accuracy of the existing mass-ablation models for POM irradiated by CO2 laser is poor compared with the experimental data.Based on the energy distribution deposited in the POM target,the active area excited by laser is divided into four slices,the ablation slice(the temperature-rising slice,the perturbation slice,and the undisturbed slice),and a slicing response model for the mass ablation of POM induced by pulsed CO2 laser irradiation in vacuum is developed.A formula is deduced to predict the ablated mass areal density from the model and is verified with data from several studies and our own experiments.The results show that our model fits the experimental data quite well before the shielding effect of ablation products becomes notable.The applicability of the model to other materials and the mass ablation in atmosphere are also briefly explored.

Proton Beam Generated by Multi-Lasers Interaction with Rear-Holed Target

Multi-lasers are proposed to enhance the proton acceleration in laser plasma interaction. A rear-holed target is illuminated by three lasers from different directions. The scheme is demonstrated by two-dimensional particlein-cell simulations. The electron cloud shape is controlled well and the electron density is improved significantly. The electrons accelerated by the three lasers induce an enhanced target normal sheath acceleration(TNSA) which suppresses the proton beam divergence and improves the maximum proton energy. The maximum proton energy is 22.9 Me V, which increased significantly than that of a single-laser target interaction. Meanwhile, the average divergence angle(22.3?) is reduced. The dependence of the proton beam on the length of sidewall is investigated in detail and the optimal length is obtained.

Large eddy simulation of a round jet into a counterflow

A round jet into a counterflow under different jet-to-current velocity ratios was investigated using large eddy simulation.The results agree well with experimental measurements from laser-Doppler anemometry and laser-induced fluorescence that include velocity and mean concentrations along the centerline and radial direction.Vortex rings appear in the region near the jet exit and large-scale vortex structures still occur near the stagnation point.The flow becomes more chaotic and three-dimensional with the presence of these structures.In particular,their presence near the stagnation point results in large velocity fluctuations that enhance the mixing process and dilution.These fluctuations are described by probability density functions that deviate from Gaussian distribution.The three-dimensional streamlines indicate that the jet not only oscillates in three directions but also rotates about the jet axis and around the vortex.The second and third moments of the velocity or scalar fluctuations identify that the mixing processes are greater in the region before the stagnation point.

A Model for Extreme Ultraviolet Radiation Conversion Efficiency From Laser Produced Mass-Limited Tin-Based Droplet Target Plasmas

Simple arguments are used to construct a model to explain the extreme ultraviolet radiation conversion efficiency(EUV-CE) of a tin-based droplet target laser produced plasmas by calculating the laser absorption efficiency,radiation efficiency,and spectral efficiency.The dependence of drive laser pulse duration and laser intensity on EUV-CE is investigated.The results show that at some appropriate laser intensity,where the sum energy of the thermal conduction,out-off band radiation and plasma plume kinetic losses is at a minimum,the EUV-CE should reach a maximum.The EUV-CE predicted by the present simple model is also compared with the available experimental and simulation data and a fair agreement between them is found.

Modeling Refractive Index Change in Writing Long-Period Fiber Gratings Using Mid-Infrared Laser Radiation

The research on the use of fiber sensors based on long-period fiber gratings inscribed by CO2 laser mid-infrared radiation has increased in the last years. In this paper, a set of analytical expressions are used to model the interaction between laser radiation and an optical fiber and to determine the resulting refractive index change. Thermal and residual stress analysis is exemplified for a standard single mode fiber, demonstrating the capability of these models to point out the necessary parameters to achieve proper optical fiber devices based on long period fiber gratings. Experimental results are also presented.