We make a comparison study of linear and nonlinear diffraction by a periodically poled lithium niobate[PPLN]dual linear-nonlinear thin-plate grating with weak surface corrugation against four types of white light sour...We make a comparison study of linear and nonlinear diffraction by a periodically poled lithium niobate[PPLN]dual linear-nonlinear thin-plate grating with weak surface corrugation against four types of white light sources.They are the ordinary halogen lamp thermal radiation source[WL1],the silica photonic crystal fiber supercontinuum nanosecond laser source[WL2],and two femtosecond white lasers made by a Ti:sapphire pump laser beam passing through a fused silica plate with significant spectrum broadening[WL3]and through a cascaded silica plate and a chirped PPLN crystal with ultrabroadband second-harmonic generation[WL4].The experiments show that the coherence,peak power,spectral bandwidth,profile,and flatness of the pump white light all contribute to shaping the characteristics of linear and nonlinear optical diffraction patterns.In particular,for WL1,WL2,and WL4,the peak power is not sufficiently large;thus,only linear diffraction occurs.For incoherent WL1,the interference is absent.Remarkably,when a femtosecond white laser with sufficiently large peak power[WL3]shines upon the PPLN grating,bright and sharp linear Bragg scattering spots,dispersive colored linear diffraction patterns,and nonlinear Cherenkov radiation nonlinear diffraction patterns are observed simultaneously by naked eyes.The experiments would enrich the basic physical and optical understanding of linear and nonlinear optical diffraction characteristics of ultrabroadband white laser sources.展开更多
摆振会降低起落架的使用寿命,影响乘坐舒适性,甚至会引起机体损坏,导致飞机失事。为了抑制起落架摆振,本文采用时滞反馈非线性能量汇(nonlinear energy sink,NES)对起落架摆振进行多目标优化。以某轻型飞机起落架为研究对象,设计了基于...摆振会降低起落架的使用寿命,影响乘坐舒适性,甚至会引起机体损坏,导致飞机失事。为了抑制起落架摆振,本文采用时滞反馈非线性能量汇(nonlinear energy sink,NES)对起落架摆振进行多目标优化。以某轻型飞机起落架为研究对象,设计了基于NES的时滞反馈半主动控制减摆器,并建立了前起落架摆振系统分析动力学方程。通过线性多步法求解出时滞反馈半主动控制摆振系统的特征根,分析了减摆器可控阻尼系数对特征根最大实部的影响。以时滞量为设计变量,将振幅衰减时间和第四周期振幅的线性加权组合确定为目标函数。采用粒子群算法对该优化目标进行全局搜索,得到相应的最优时滞量,并与遗传算法得到的结果进行对比,验证了最优时滞量的可靠性,并在时域内对减摆器的减摆效果进行了验证。结果表明,与无时滞控制系统相比,采用最优时滞反馈的半主动控制系统的前起落架摆振幅值显著降低。展开更多
基金supported by the National Natural Science Foundation of China(No.12434016)the Science and Technology Project of Guangdong(No.2020B010190001)the National Funded Postdoctoral Researcher Program(No.GZB20240785)。
文摘We make a comparison study of linear and nonlinear diffraction by a periodically poled lithium niobate[PPLN]dual linear-nonlinear thin-plate grating with weak surface corrugation against four types of white light sources.They are the ordinary halogen lamp thermal radiation source[WL1],the silica photonic crystal fiber supercontinuum nanosecond laser source[WL2],and two femtosecond white lasers made by a Ti:sapphire pump laser beam passing through a fused silica plate with significant spectrum broadening[WL3]and through a cascaded silica plate and a chirped PPLN crystal with ultrabroadband second-harmonic generation[WL4].The experiments show that the coherence,peak power,spectral bandwidth,profile,and flatness of the pump white light all contribute to shaping the characteristics of linear and nonlinear optical diffraction patterns.In particular,for WL1,WL2,and WL4,the peak power is not sufficiently large;thus,only linear diffraction occurs.For incoherent WL1,the interference is absent.Remarkably,when a femtosecond white laser with sufficiently large peak power[WL3]shines upon the PPLN grating,bright and sharp linear Bragg scattering spots,dispersive colored linear diffraction patterns,and nonlinear Cherenkov radiation nonlinear diffraction patterns are observed simultaneously by naked eyes.The experiments would enrich the basic physical and optical understanding of linear and nonlinear optical diffraction characteristics of ultrabroadband white laser sources.
文摘摆振会降低起落架的使用寿命,影响乘坐舒适性,甚至会引起机体损坏,导致飞机失事。为了抑制起落架摆振,本文采用时滞反馈非线性能量汇(nonlinear energy sink,NES)对起落架摆振进行多目标优化。以某轻型飞机起落架为研究对象,设计了基于NES的时滞反馈半主动控制减摆器,并建立了前起落架摆振系统分析动力学方程。通过线性多步法求解出时滞反馈半主动控制摆振系统的特征根,分析了减摆器可控阻尼系数对特征根最大实部的影响。以时滞量为设计变量,将振幅衰减时间和第四周期振幅的线性加权组合确定为目标函数。采用粒子群算法对该优化目标进行全局搜索,得到相应的最优时滞量,并与遗传算法得到的结果进行对比,验证了最优时滞量的可靠性,并在时域内对减摆器的减摆效果进行了验证。结果表明,与无时滞控制系统相比,采用最优时滞反馈的半主动控制系统的前起落架摆振幅值显著降低。
