Lithium Niobate Waveguide Written By Femtosecond Laser And Its Second-order Nonlinear Effects

Femtosecond laser micromachining has obvious advantages in micromachining fields: compared to long pulse laser processing,it has a higher precision;compared to excimer laser processing,it can be implemented at any position of materials.When femtosecond laser with ultrashort pulses and high peak power interacts with transparent materials,the laser energy could be absorbed by materials quickly,and the energy deposition will change the electron motion and absorption,which make femtosecond laser micromachining be "cold" process.Characteristics of high integration and convenience make writing waveguides become an important research field of femtosecond laser micromachining.Optical waveguide is a key element and fundamental carrier for functional integrated optical chips,in addition,this feature of a high optical influence density constrained in optical waveguide makes it useful to achieve a variety of phenomena based on nonlinear optical waveguide.This paper discusses femtosecond laser lithium niobate waveguide and its second-order nonlinear effects.Lithium niobate crystal is a nonlinear crystal with excellent properties.The nature of lithium niobate crystal and the characteristics of optical waveguide make lithium niobate waveguide be highly prized.According to refractive index profile after writing process,lithium niobate waveguides can be divided into type ? and type ? optical waveguides.Waveguide core layer of type ? lithium niobate waveguide locates around the femtosecond laser irradiation field,and the lattice structure of waveguide core layer is not destroyed,therefore type ? lithium niobate waveguide could maintain the bulk of nonlinear properties.We use femtosecond laser to write type ? lithium niobate waveguides and obtain optimized lithium niobate waveguides.At the same time,we focus on second-harmonic generation based on mode phase matching of lithium niobate thin-film waveguides and ridge waveguides.Lithium niobate thinfilm waveguides have advantaged superiority,which both possess the integrated chip features and the advantages of the optical waveguide.We study second-harmonic generation based on mode phase matching in slab waveguides and ridge waveguides.After analyzing relationship between wavelengths,waveguides' thicknesses and effective refractive indexes in thin-film waveguides and ridge waveguides,we present multiple types of mode phase matching condition of second-harmonic generation by analyzing effective refractive indexes in thin-film second-harmonic generation waveguides with different thickness less than 1 ?m,which include type ? and type ? second-harmonic generation.Furthermore,we calculate bandwidths of type ? and type ? second-harmonic generation in poled period lithium niobate crystal,and get bandwidths of type ? and type ? second-harmonic generation in different interaction lengths.