【作者】 陈洪云；

【导师】 陈险峰；

【作者基本信息】 上海交通大学 ， 光学， 2008， 博士

【摘要】 飞秒激光与透明介质的相互作用是近年来研究的热点领域。将飞秒激光聚焦到透明材料的表面,当照射激光能量高于材料损伤阈值时,会造成材料表面的物理损伤。另一方面,通过显微物镜将飞秒激光聚焦到介质材料内部,当激光能量低于材料破坏阈值时,通过飞秒激光与介质之间的非线性作用,致使介质被作用的区域发生折射率变化,利用飞秒激光的这种特性可以加工出光波导,这已成为制作光波导的一种新方法。首先本文研究了飞秒激光与铁电晶体铌酸锂、钽酸锂的表面烧蚀。由于铌酸锂、钽酸锂是两种重要的非线性光学晶体材料,研究飞秒激光与铁电晶体之间的相互作用,不但可以帮助人们加深理解飞秒激光与此类晶体相互作用的物理规律,而且还对飞秒激光在铁电晶体中的微加工应用具有重要的参考价值。我们利用烧蚀面积与激光能量的对数关系,确定这两种晶体在单脉冲及多脉冲作用下的烧蚀阈值。研究发现,由于累积效应,当作用铌酸锂晶体上的飞秒脉冲个数大于80的时候,多个脉冲的破坏阈值Ft h(∞)趋近一常数。我们还研究了飞秒激光作用下,钽酸锂晶体中导带电子光致电离速率和电场强度之间的关系,通过导带电子数密度的演化和数值计算,得到了钽酸锂的碰撞系数约为1.01。飞秒激光能简单快速制作二维波导阵列的优点,使得在研究二维波导阵列中的离散衍射,离散孤子等线性和非线性现象时更为方便地提供实验样品。论文接着在理论上研究了二维波导阵列中的衍射情况,在不同的衍射情况下,数值模拟出二维波导阵列中的耦合输出特性。实验上采用飞秒写入的办法,在石英玻璃介质中制作出一个2 2的波导阵列,并观测到其耦合输出情况。由于波导阵列之间的耦合系数在实际应用中具有重要的意义,论文中提出了一种测量耦合系数的方法:利用耦合波理论,根据相关波导输出功率之比,计算出波导的耦合系数,最后得到了波导阵列水平方向、垂直方向的耦合系数分别为飞秒激光在铁电晶体材料中诱导的各种波导结构在光通信系统中具有重要的应用价值。本论文最后利用飞秒激光在z方向切割的铌酸锂中制备了一个1×4的波导阵列。在波导耦合实验中观测到了铌酸锂波导阵列的偏振相关耦合特性,结果表明波导阵列对TM模式的限制要比对TE模式限制更强。同时,对两种偏振输入光所对应的波导耦合系数也进行了测量计算。更多还原

【Abstract】 The interaction between femtoseocnd laser and transparent materials has been studied intensively in recent years. When the femtosecond laser was focused onto the surface of the transparent materials, if the laser fluence applied to the sample exceeds the material’s fluence threshold, ablation occurs. On the other hand, by focusing ultrashort laser pulses inside optical transparent materials through a microscope objective, if the laser fluence applied to the sample is less than the material’s fluence threshold, a localized and permanent increase of the refractive index can be achieved based on nonlinear absorption around the focal volume of femtosecond laser pulses, and it becomes a new way to fabricate optical waveguide through femtosecond laser writing.In this dissertation firstly we study the surface ablation of lithium niobate, lithium tantalate by femtosecond laser. The study not only can help us understand better the physical rules about the interaction between femtosecond laser and ferroelectric crystals, but also it has importantreference on the microfabrication in such materials by femtosecond laser.The ablation thresholds of these two materials were calculated using thelogarithm relation of the ablation area and the laser fluence underirradiation by single and multiple femtoseocnd laser pulses. It was foundthat the threshold fluence for an infinite number of pulses convergeto a common value ofth F (∞)0.52±0.06 J/cm2 for . We also study therelationship of the photoionization rate and electric field, and achieve theavalanche coefficientN > 80βto be 1.01 by calculating the evolution ofthe conduction-band electron density in lithium tantalate.The two-dimensional waveguide array can be produced easily andfastly using femtosecond laser, thus it can conveniently provide us thesamples to study the discrete diffraction, discrete solitons or other linearand nonlinear properties. So next we have studied the discrete diffractionbehavior of two-dimensional waveguide arrays in theory. Thefundamental peculiarities of discrete diffraction in the two-dimensionalwaveguide arrays were studied. In experiment, we fabricatea waveguide array by femtosecond laser pulses in fused silica, anddemonstrate the coupling of such array. Moreover, the coupling constantbetween waveguides is essential for practical applications of waveguides,so we develop a method to measure the coupling constant: the couplingconstants of the waveguide array can be obtained by measuring the ratio of output power of each waveguide by the coupled-mode theory. The coupling constants for the horizontal and vertical directions are ch = 0.853cm?1, cv = 0.877cm?1 respectively.Many different waveguide structures induced by femtosecond laser in ferroelectric material are essential for practical applications of waveguides in optical communication. In this dissertation lastly we demonstrate a 1×4 waveguide array produced by a femtosecond laser in z-cut lithium niobate. The polarization dependence of light coupling in such waveguide structure is investigated experimentally, and it is found that the TM mode will be more confined than that of TE mode in such waveguide array. At the same time, the coupling constants of the waveguide array are obtained for extraordinary rays and ordinary rays.更多还原