Research On The Design And Fabrication Of Lithium Niobate Thin Film Second Order Nonlinear Optical Waveguide

Lithium niobate crystal is the type of optical material,which has the advantage of favourable second-order nonlinear coefficient,wide low-loss transparent window,and high stability of chemical and physical properties.In addition,lithium niobate is a ferroelectric material,the phase mismatch in the nonlinear process can be compensated by quasi-phase matching technique after periodic polarization reversal at high voltage.These make lithium niobate become a hot studied material in the field of second-order nonlinear optical effects.However,the relative refractive index difference of the lithium niobate waveguide fabricated by titanium diffusion technology or proton exchange technology is small,which leads to insufficient light field limitation and low nonlinear coupling efficiency.Therefore,it is necessary to increase the interaction length to make up for this deficiency.As a result,the size of the optical devices fabricated with lithium niobate is usually too large to realize large-scale integration.With the appearance and the mature of waveguide fabrication technology of lithium niobate thin film,the problem of traditional bulk lithium niobate in the study of second-order nonlinear effect has been overcome successfully.The optical waveguide devices based on the new-style lithium niobate thin film have many advantages,such as small cross-sectional area,large relative refractive index difference and strong light limiting ability,which greatly improves the nonlinear coupling efficiency.However,there are still some problems to be solved in the fabrication of lithium niobate thin film waveguides,the high quality of periodic polarization reversal and the application of second-order nonlinear effect.In this thesis,the second-order nonlinear effect in the optical waveguide fabricated on lithium niobate thin film is studied.The work contents are as follows:1.Second-order nonlinear coupled mode theory of periodically polarized lithium niobate thin film optical waveguideThe mechanism of second-order nonlinear effect in lithium niobate is introduced.The coupled mode theory of periodically polarized lithium niobate thin film waveguide is analyzed by perturbation theory.Emphatically,the second harmonic generation,the sum frequency generation and the difference frequency generation are introduced.2.Structural design of lithium niobate thin film optical waveguide and polarization electrodeThe effective index method and the finite element method are used to calculate and analyze the modes of lithium niobate thin film ridged waveguide.The effective refractive indices of the TE₀ modes of the second harmonic light and pump light in the second harmonic generation are obtained.The period of the grid electrode is acquired under the condition of quasi-phase matching.The impact of the waveguide parameters on the electrode period and the normalized conversion efficiency are analyzed.Finally,the electric field distribution of the electrode is simulated.The simulation parameters of the polarization voltage source are determined.3.Manufacture and test of the devicesThe fabrication of the devices is divided into three parts.In the first step,the ridged waveguide is obtained by proton exchange and inductively coupled plasma etching.The second step is to evaporate the aluminum electrode.The third step is to carry out the polarization reversal work under the high voltage short pulse.The spot test and the spectrum test of the devices is carried out.4.A neoteric optical filter based on periodically polarized lithium niobate thin film optical waveguideA notch filter based on the sum frequency generation of this device is proposed.The characteristics of the tunability and the multi-channel filtering is analyzed by simulation.It can be seen from the simulation that the central wavelength of each filtering central wavelength can be tuned by controlling the wavelength of the pump light,and the number of the filtering channel is determined by the number of the pump light.The effects of various physical parameters on the extinction ratio and 3 dB bandwidth of the filter are also analyzed.