Interaction Between Femtosecond Laser And Ferroelectric Crystals, And The Properties Of Waveguiding Coupling

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.