| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 98 === This thesis presents recent approaches in the development of a Si-based waveguides system for submicrometer devices. We focus on theoretical derivation and electromagnetic wave field analysis of the polarization-independent grating couplers to obtain the optimal design. Without additional steps in the fabrication of these devices, the grating couplers can also be designed to exhibit auxiliary functions such as focusing and splitting of the coupled light. The multifunctional capability of grating couplers may reduce or eliminate the need for additional optical waveguide components. An important limitation of these grating couplers is that they are designed solely for a specified polarization of the incident light. This may be a serious drawback that the polarization of the input light from an optical fiber is in most cases unknown and may even vary with time. Because the grating coupler is generally highly sensitive to the polarization, it will not work properly when input light having a polarization different from that given in the design specification, and thus it is impractical in a real optical fiber system.
In this thesis we present the couplers designed by simultaneous optimization for two orthogonal polarization states of the light incident on the two-dimensional subwavelength gratings (SWGs). The grating structure is designed where the diffractive grating is formed in the longitudinal direction, whereas the nondiffractive SWG structure in the lateral direction acts as effective medium of the grating. Such a polarization independent grating coupler can be achieved by superimposing the grating couplers which were originally designed for coupling solely the TE and the TM modes. The results show that the polarization response is highly flexible as the longitudinal and lateral grating structures vary. Polarization independence can be achieved when the coupler is designed to have the same response for coupling the two orthogonal polarizations of the incident light into the TE and TM modes, respectively, of the waveguide.
We use the software include R-Soft and MathCAD. First, MathCAD is used to calculate the theoretical derivation of the formulae. The entire structure is then analyzed numerically by using the finite-difference time-domain method (FDTD) and a fundamental mode of the optical fiber nearly incident toward the grating coupler and finally coupled into the SOI waveguide. Finally, we also used the micro genetic algorithm to optimize the coupling efficiency.
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