【作者】 游开明；

【导师】 文双春；

【作者基本信息】 武汉理工大学 ， 交通信息工程及控制， 2009， 博士

【摘要】 宽频带超短激光脉冲技术是当今重大科学前沿领域之一。同时,研究宽频带激光脉冲的传输和聚焦特性对于高功率激光系统的总体设计、性能优化和安全运行等具有重要现实意义。本论文采用解析和数值模拟方法,研究了宽频带激光束线性传输的空间与时间特性,以及在Kerr介质和负折射率介质中非线性传输的自聚焦特性,取得如下结果:第一,基于单色光的传输理论,推导出了宽带激光通过线性介质、透镜和空间滤波器的传输方程,并发展了相应的数值计算算法和程序。特别是,为提高宽带激光4维时空传输的数值模拟计算速度,开发了基于Dini级数展开的准离散Hankel变换算法,该算法不仅能直接计算对称轴上点的值,而且能高保真地进行许多次变换,对模拟计算宽带激光在圆对称系统中的传输,特别是非线性传输具有十分重要的意义。第二,研究了宽带激光束通过自由空间和空间滤波器传输的调制特性,用数值方法得出了调制深度与菲涅尔数F和衍射极限倍数M的定量关系。同时定性地得出了带宽对光束空间均匀性的影响,发现带宽越大,光束空间横截面的均匀性越好。进一步通过解析方法得到了宽带激光自由空间传输消除菲涅尔衍射的条件,发现当带宽△λ=2λ₀/F时（λ₀是光束的中心波长）,菲涅耳衍射完全消失。这些定量关系可为高功率激光系统设计参数的选择提供参考依据。第三,研究了宽带激光通过平板色散介质的空域、频域和时域特性,得到了衍射环数、高斯光束空间光腰和脉冲展宽与带宽和介质厚度的关系;导出了宽带激光通过色散楔的传输方程,发现一定的带宽对光束的空间分布有很好的平滑作用,但在不同位置它同时使脉冲的时间波形产生不同程度的失真;在考虑高阶色散的情形下,研究了透镜的聚焦特性,得到了透镜在光束中心波长对应的焦点附近的聚焦行为。最后,研究了宽带激光的非线性热像现象,发现宽带激光的带宽对非线性热像有一定程度的抑制作用,但效果不明显。考虑到双色散楔不仅对光束的空间分布有很好的平滑作用,而且还可避免脉冲波形的失真,进一步研究了在光路中加入双色散楔来抑制非线性热像形成的方案,发现随着色散楔顶角的增大,热像强度大幅度减小。更多还原

【Abstract】 The broadband ultrashort pulse technology,as one of the most important scientific frontiers,has attracted a lot of theoretical and experimental investigations in the past decades.Meanwhile,the investigation of the propagation and focusing properties of broadband laser pulses is of great practical significance for the design, performance optimization and safe operation of high-power laser systems.In this dissertation,by adopting analytical and simulation methods,the spatial and temporal properties of the linear propagation of broadband laser beams,as well as the properties of their self-focusing in Kerr medium and negative refractive-index medium,are investigated in detail.Our work and main results are presented as follows:Firstly,based on the propagation theory of monochromatic light,the propagation equation of broadband laser through linear media,lens and spatial filter is derived,and corresponding numerical simulative algorithm and program are developed.Especially,based on Dini series expansion,we have developed a quasi-discrete Hankel transform algorithm,which greatly accelerates the operation speed of the simulation of 4-dimentional spatiotemporal propagation of broadband laser.The algorithm can not only directly calculate the values on symmetry axis,but also keep very high precision after numerous transformations,this is very important for the simulation of the propagation,especially the nonlinear propagation,of broadband laser in circular symmetric systems.Secondly,the modulation characteristics of broadband laser beam through free space and spatial filters are investigated.With numerical method,the quantitative relations of modulation depth with Fresnel number（F） and the multiple of diffraction limit（M） are obtained,respectively.Meanwhile,the qualitative relation between the bandwidth of a laser beam and its spatial uniformity is demonstrated.It is found that bigger bandwidth gives rise to better beam profile uniformity.Further, with analytical approach,the condition for eliminating Fresnel diffraction of broadband laser propagation in free space is found to be△λ≥2λ₀/F（where△λandλ₀ are the bandwidth and center wavelength of the beam）.These quantitative relations provide the basis for the selection of parameters in designing high-power laser systems.Thirdly,by investigating the characteristics of broadband laser passing through a dispersive medium slab in spatial domain,frequency domain and time domain,we have obtained the relation of the number of diffraction rings,pulse broadening in time domain and the waist of Gaussian beams in frequency domain with bandwidth and the medium slab thickness.We have also precisely derived the propagation equation for the broadband laser through a dispersive wedge and found that the dispersion wedge has a good smoothing effect on beams,but leads to pulse distortion of different extent at different transverse position.Considering the high-order dispersion,we have investigated the focusing property of lens and obtained the lens’ focusing behavior in the vicinity of the focus corresponding to the center wavelength.Finally,the phenomenon of the formation of hot image in broadband beam is studied.It is found that bandwidth can suppress the formation of hot image,but the suppressing effect is not satisfying.Considering that double-dispersion-wedge not only has a good effect on beam smoothing but also can avoid distortion in the waveform of pulse,we have further investigated the scheme to suppress hot image by insert double-dispersion-wedge in optical path.It is shown that the intensity of hot image decreases dramatically as the apex angle of dispersion wedge increases.更多还原