Interaction Between Femtosecond Laser And Ferroelectric Crysals, And The Properties Of Waveguide

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 (LN), lithium tantalite (LT) and MgO doped LN by femtosecond laser. This study not only can help us have a good understand about the physical rules about the interaction between femtosecond laser and ferroelectric crystals, but also it serves as a firsthand reference on the micro-fabrication in such materials by femtosecond laser. The ablation thresholds of these three materials were calculated using the logarithm relation of the ablation area and the laser fluence under irradiation by single and multiple femtoseocnd laser pulses. The threshold of lithium tantalite under single-shot is found to be 1.84J /cm 2 and the avalanche rate was determined to be 1.01 cm 2 /J by calculation. We found that avalanche dominates the ablation process, while photoionization serves as a free electron provider. As far as LN is concerned, we find that the threshold fluence for an infinite number of pulses Ft h(∞)converges to a common value of 0.52±0.06 J /cm2for N > 80. We also investigate the surface ablation of MgO doped LN, and our result shows that the threshold of MgO doped LN is Ft h= 5.26 J /cm2, which is larger than that of LN. We also find that the ablated area differs greatly as the pulse energy increases.The 1-D waveguide array can be produced easily and fast using femtosecond laser, thus it can conveniently provide us the samples to study the discrete diffraction, discrete solitons or other linear and nonlinear properties. In this dissertation we demonstrate a 1×4 waveguide array produced by a femtosecond laser in z-cut LT. 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.At the end of this dissertation, we theoretically investigate the propagation of light pulse in 1-D waveguide array. It was found that the energy distribution varies as the input power of the pulse increases from 240W to 1500W, and the shape of the pulse after propagating throw a 1cm waveguide array differs greatly with that of a single waveguide. These results may help us a lot in future study on waveguide arrays.