Optical Coupler Based On The Photorefractive Long-Period Waveguide Grating

Optical coupler based on the waveguide gratings structure is playing a key role in the new generation of high-speed communication system. It is the important device to achieve high capacity, high integration and high speed optical network communication. Optical coupler based on photorefractive long-period waveguide gratings (LPWGs) of lithium niobate (LiNbO3) has been researched in this thesis.Firstly, a novel LPWGs filter based on photorefractive grating in a Ti-diffused strip LiNbO3waveguide embedded in a Ti-diffused planar LiNbO3waveguide is designed and theoretically analyzed. In the condition of initial thickness of the Ti strip for the first Ti-diffusionτ=30nm, diffused depth d=73μm, initial thickness τ=80nm and width w=4μm of the Ti strip for the second Ti-diffusion, diffused depth in X direction and Y direction, the single-mode waveguide and the cladding structure with only lth order mode can be fabricated. The TM mode effective indices of the planar and the strip waveguide have been calculated in the case of proper initial condition, which are2.13781and2.14169, respectively. In this case, the grating pitch and the included angle of the two coherent light beams have also been calculated, they are400μm and0.076°. According to the coupled-mode theory, we have got the coupling coefficient κ=112.23m-1and the100%coupling length L=1.4cm, which is acceptable for the miniaturization and integration of optical devices. At the same time, the spectra in the100%coupling situation has also been calculated. The FWHM is about25nm and the side lobes are very small, which can also be acceptable for the CWDM system. The grating pitch can be tuned by change the included angle of the two coherent light beams in order to fit the different input wavelengths.Secondly, the LPWGs filter based on photorefractive grating in a Ti: LiNbO3waveguide is experimentally researched. The plans of the gratings fabrication and the filtering detection have been designed. The photorefractive volume holographic Bragg grating was written into the LiNbO3crystal using two-beam interference. The filtering effect at the wavelength of1548nm was obtained, the diffracted efficiency was10%and the full wave at half maximum was4nm. It offers experimental bases for the next work of the LPWGs filter based on photorefractive grating in a Ti: LiNbO3waveguide. Finally, a novel LPWGs coupler based on photorefractive grating in a Ti-diffused strip LiNbO3waveguide embedded in a Ti-diffused planar LiNbO3waveguide is designed and theoretically analyzed. The evanescent field coupling effect can be ignored when the distance of the two strip waveguide greater than7μm. The LPWGs coupling equation has been solved in the case of two LPWGs are perfectly aligned. Then the normalized power in both of the strip waveguides and the planar waveguide can be obtained. And the minimum100%coupling length Lmin=1.98cm. After that, the transmission spectrum at the minimum100%coupling length in launching waveguide and tapping waveguide, and the coupling dynamics of the resonant wavelength can be obtained in the condition of composite signal light is launched in one waveguide.