Research On Tunable Lithium Niobate Waveguide Directional Coupler Assisted By Side-wall Long Periodic Grating

Wavelength Division Multiplexing?WDM?technology with reconfigurable channels,bandwidth,and network topology is expected to meet the overall bandwidth and low latency requirements of both civilian and military applications.Tunable filter can realize the dynamic selection of center wavelength by external adjustment,which is one of the key devices to realize the dynamic WDM technology.The tunable filter based on optical waveguide is mainly realized by thermal and electro-optical tuning.The tuning rate of thermal optical devices is generally ms level,and the response speed is slow,which is not suitable for high-speed data exchange of optical network nodes.The response speed of electro-optical devices can reach ns level or even faster,so they are widely used in high-speed modulation.At present,lithium niobate optical waveguide devices have been in a relatively mature stage,lithium niobate has a large electro-optical coefficient,can use a small voltage to achieve a large wavelength tuning range,so it is the preferred material for making adjustable filter.Grating-assisted directional coupler?GADC?can be used as a filter by coupling a particular wavelength of light from an input waveguide to an output from an adjacent waveguide.Combined with the advantages of materials and structures,the following research work is done in this dissertation on the side-wall long period grating assisted tunable lithium niobate waveguide directional coupler:?.Basic principle of side-wall LPGA lithium niobate waveguide directional couplerThis dissertation introduces the electro-optical effect of lithium niobate crystal,introduces the working principle of grating assisted directional coupler by coupling mode theory,and analyzes the factors affecting the performance of the device.?.Structural design of side-wall LPGA lithium niobate waveguide directional couplerBy analyzing and calculating the modes of annealed proton exchange lithium niobate waveguide,the corresponding waveguide width range of single mode and dual mode transmission is obtained.Using the forming mechanism of lithium niobate waveguide,grating is formed directly by opening the window on the side wall of the wide waveguide.After the parameters of the device are determined by experiments and simulation,the transmission characteristics of the device are simulated,and the influence of grating period and grating depth on the spectral characteristics of the device is analyzed.In order to improve the tuning efficiency,the electrode adopts push-pull structure.?.Fabrication and testing of devicesThe device mask was designed,and then the device was fabricated by the annealed proton exchange process,and three sets of devices?S₁,S₂,S₃?were tested,including:pass light test,spectral response test and electrical tuning test.The test results of S₁ and S₂ did not meet the design requirements.Before the electrode production of S₃ device,the extinction ratio of the straight end at 1550 nm was 28 dB,the 3 dB bandwidth was only6 nm,and the side mode suppression ratio of the cross end was about 6 dB.After making the electrode,the center wavelength slightly shifted to 1553 nm,the extinction ratio was slightly reduced to 23 dB,the 3 dB bandwidth was 8 nm,the side mode suppression ratio at the cross end was about 4 dB,the tuning range of±30 V voltage was about 15 nm,and the electric tuning efficiency was 0.182 nm/V.