Research On The Propagation And Focusing Properties Of Broadband Laser Pulses

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.