Lithium Niobate Modulator In The Fiber Optic Gyroscope System Analysis And Design

Lithium niobate optical modulator is widely applied to optical communication system, integrated interferometer fiber optical gyroscope (I-FOG) and so on. It is one of core components in those systems. Especially in I-FOG, people always expect to have the high performance component. Some military and other important applications are more eager for it in our country.This thesis performs the research how to analyze and design lithium niobate optical modulator in I-FOG. The analysis concentrates on those performance parameters of a modulator: half-wave voltage, single mode performance, quasi TE/TM polarization, loss, bandwidth and split-ratio.A basic design is first proposed that modulator is manufactured by annealing the waveguides that come into being by proton exchange on the substrate of X-cut Y propagation lithium niobate. The quasi TE/TM polarization is easily realized by the technology. From Maxwell equations, the cause of this performance is discussed.Finite element method (FEM) is a method that is efficient in electromagnetics .For the first time the thesis applied vectorial FEM to single mode performance and half-wave voltage analysis and design. As the first step, a variational function is derived from 3-D time harmonic field Maxwell equations and for X-cut Y propagation lithium niobate waveguide, the according variational function can also be gotten by a similar process. As we know, FEM is a method that transforms differential equations with boundary conditions into a variational problem, and then performs a numerical analysis to get the result. The thesis gives the realization of FEM detailedly. Important problems in FEM realization are how to deal with boundary condition and spurious solutions. In this thesis, the detail of enforcing boundary condition to the realization of FEM and eliminating spurious solution is discussed. With former people's work, an appropriate method is applied to our analysis and design: a reasonable relation between refractive index profileinmodel and parameters in the fabrication technology is proposed, and on this basis single mode performance and half wave voltage are analyzed by vectorial FEM. As a result, process and structural parameters are derived. The deficiency in the method is reviewed too.Loss in the device includes coupling loss, Y-branch loss, electrode absorbing loss and material loss. The two main elements are analyzed in detail: coupling loss and Y-branch loss. For coupling loss, the method to reduce this loss is pointed out without changing other structural parameters decided by the above requirements. Y-branch loss is analyzed by BPM (Beam Propagation Method). Because lithium niobate is a kind of anisotropic material, anisotropic FD-BPM is used. The thesis gives a simulation result of S-shape Y-branch structure with numerical computation software and it also meets the demand.For the application of I-FOG, bandwidth can easily be met. We simply discuss those which are of importance to realizftg bandwidth requirement and with an approximation formula, the bandwidth is estimated. The branch split-ratio is decided by practical fabrication process, so we haven't discussed it.The thesis also makes a comparison between some measurement results and design conclusion. It indicates that our design is basically reasonable, but some have to be proved by future experiments. At the end of the thesis, the deficiency in our analysis is summed up and some possible improvement methods are proposed for the future research.