Research On Tunable Lithium Niobate Optical Waveguide Mode Division Multiplexer

With the development of Internet technology,people's demand for transmission capacity of optical communication system is growing.Because of capacity constraints,the traditional single-mode fiber transmission technology will eventually be unable to meet people's needs.The mode division multiplexing technology,which introduces the spatial orthogonal mode as a new degree of freedom,can greatly improve the transmission capacity of optical communication systems,and attracts much attention.The mode multiplexer/demultiplexer are important part of the mode division multiplexing system.The mode multiplexer/demultiplexer based on the planar optical waveguide structure have the advantages of flexible structure,many materials,easy to realize tunable,and easy to coupling with the fiber.On the one hand,the tunable feature can adjust the coupling center wavelength of the device,and reduce the difficulty of making the device.On the other hand,the tunable feature can implement mode switching or mode routing.At present,the waveguide tuning method mainly contains the using of the thermo-optic effect to achieve the thermo-optical tuning and the using of electro-optic effect to achieve the electro-optical tuning.However,the thermo-optical tuning speed is slow,so in the case of high-speed applications,electro-optical tuning is a good choice.In order to achieve electro-optical tuning characteristics,the author chose electro-optical materials to make waveguide devices.With large electro-optical coefficient,the lithium niobate material is the best choice.At present,lithium niobate optical waveguide production process mainly contains titanium diffusion and annealing proton exchange.The annealing proton exchange process can adjust the refractive index of the waveguide by matching annealing and proton exchange parameters,and the controllable factors are more.In this thesis,a tunable lithium niobate optical waveguide multiplexer based on an asymmetric directional coupler structure is proposed by using an annealing proton exchange process.Since the device operates under the electric field,the center wavelength and the corresponding coupling efficiency can be tuned based on electro-optic effects.And the device can be used as ordinary modular multiplexer,but more importantly,can be used as tunable broadband mode switch,which greatly increased the device application flexibility.Firstly,by using the method of effective refractive index analysis and MATLAB simulation software,the author obtained the dispersion curve of lithium niobate optical waveguide,and then obtained the size range of single-mode waveguide and dual-mode waveguide,which can meet the phase matching condition.Based on the above results,the author produced lithium niobate single-mode and dual-mode optical waveguide by annealing proton exchange process,and measured the refractive index of the waveguide by prism coupling instrument.According to the measurement results,the author analyzed the influence of proton exchange and annealing on the refractive index of the waveguide,and then obtained the sizes of single-mode waveguide and dual-mode waveguide,which meet the phase matching conditions experimentally.Based on the above analysis,the author designed the general structure of the device,and then simulated the structure of the waveguide asymmetric directional coupler by BPM simulation software for getting the coupling interval,coupling length and other parameters of the device in theory.Combined with the above analysis,the author designed a reasonable device structure and the corresponding electrode structure,and then made the device and the corresponding electrode.Secondly,according to the above analysis and design parameters of the corresponding waveguide and electrode mask version,the author studied the exchange and annealing parameters on the phase matching conditions,the low-loss optical waveguide production process and high-quality electrode production process.Then,according to these process parameters,the tunable lithium niobate optical waveguide multiplexer was fabricated in this thesis.Finally,the successful device was tested and analyzed.The main test parameters include the device's transmission spectrum,near-field output and insertion loss.The test results show that the device not only implements the function of the mode division multiplexer,but also realizes the broadband mode switching function with coupling efficiency and center wavelength tunable.The sensitivity of electro-optical tuning is about 5 nm/V,and the isolation is up to 27 dB when the operating wavelength is 1560 nm and the tuning voltage is 9 V.