An Application Study Of Filtering And Sensing Based On Long-Period Grating In Ti:LiNbO₃ Waveguide

Long-Period Waveguide Grating(LPWG)has advantages over the long-period fiber grating(LPFG)in material choice,grating fabrication method,as well as geometry and type of both waveguide and grating.Over the past few years,a number of LPWG devices based on various materials have been reported.Among them,the device based on the electro-optic effect of lithium niobate has wide application prospects in high-speed optical filtering and optical intensity modulation.The author of the present dissertation has successfully developed the electro-optic modulation type LPWG device based on titanium-diffused lithium niobate waveguide and further explored the new application of the device in dual-wavelength filtering and refractive index sensing.The main work of this thesis includes following three aspects:1.Accomplish the fabrication of electro-optic modulation type LPWG device based on titanium-diffused lithium niobate waveguide as well as the characterization of its guided wave properties and filtering performance.During the fabrication process,a silicon dioxide buffer layer with stronger adhesive force(to waveguide surface)was prepared by using the method of clamping rod,enabling to improve the probability of succeeding in device fabrication.The experimental results of the characterization of guided wave properties show that the titanium-diffused lithium niobate strip waveguide of LPWG supports single-mode propagation in 1.5 ?m wavelength region with a coupling loss of ?2 dB and a propagation loss of ?1 dB/cm.An experimental system used for filtering performance characterization was setup,and the performance of the LPWG was characterized.The results show that as the driving voltage is increased from0 to 324 V,the dip(stopband)of LPWG redshifts nearly linearly by ?80 nm,the amplitude of the stopband shows a non-monotonic variation feature,and the maximum rejection amplitude is ?28 dB.2.Explore the application of the developed electro-optic type LPWG device in temperature sensing.The temperature characteristics of the dips that appeared in the transmission spectrum of the LPWG device were studied experimentally.The results show that as the temperature is increased from 23.2 ?(room temperature),the amplitude of stopband gradually decreases and disappears at 27.8 ?.As the temperature was further increased,two new dips appear and their attenuation amplitudes keep increasing to 24 dB.However,the locations of the two dips remain unchanged.Further experimental results verify that the phenomenon was caused by thermally induced multimode interference.Present experimental results verify the feasibility of using the temperature-varying characteristics of the electro-optic type LPWG transmission spectrum to realize a dual-wavelength band-rejection filter.3.Explore the application of the electro-optic type LPWG device in sensing of refractive index.Based on the characteristics of the electro-optical type LPWG device,present dissertation proposes a novel method to implement refractive index sensing.The principle of the method is using a matching liquid to change successively the coupling efficiency between the input fiber and the strip waveguide of the LPWG device,the light intensity in the strip waveguide,refractive index induced by photorefractive effect,the effective refractive index of the mode guided in the strip waveguide,and hence the resonance wavelength.Accordingly,refractive index sensing on the basis of the electro-optic type LPWG will be realized.