| Summary: | 博士 === 臺灣大學 === 光電工程學研究所 === 98 === In this research several optical and terahertz (THz) guided-wave structures are analyzed using the Yee-mesh-based finite-difference frequency-domain (FDFD) method and beam propagation method (BPM). When solving leaky waveguide modes, the perfectly matched layer (PML) is employed as the boundary condition. Our results show that the reflection coefficient, which is one PML parameter, has a significant effect on leaky-mode analysis and its preferred magnitude is much smaller than the conventional values.
We propose a simple leaky structure for THz waveguiding, i.e., the pipe waveguide, which is a simple pipe with a large air core and a thin dielectric layer with uniform but low index. Modal characteristics of the pipe waveguide are investigated in the THz region, by calculating the modal indices and attenuation constants of the core modes for various core diameters, cladding thicknesses, and cladding refractive indices. Numerical results reveal that the guiding mechanism of the core modes is that of the antiresonant reflecting guiding. Moreover, modal patterns including modal intensity distributions and electric field vector distributions are shown for the fundamental mode and the higher order modes, and these patterns resemble those of the conventional step-index fiber.
Then, the effect of metallic coating on pipe waveguide is investigated. For the 1-D case, i.e., the slab-type pipe waveguide, numerical results indicate that the loss spectrum will shift half period for TE polarization, but will not for TM polarization. This is because the magnitudes of the electric field for TE waves must be continuous in both sides of the metal-dielectric interface; whereas for TM waves, the magnitudes can be different. Such spectral shift phenomenon is promising for the application of the THz polarization filter. For the 2-D case, i.e., the circular pipe waveguide, the spectrum of TE01 mode will shift half period and the spectrum of TM01 mode remains unmoved. While for the HE11 and HE21 modes, their period becomes one half owing to their hybrid nature (mixed TE and TM).
We also study two types of THz couplers. One is composed of two slab-type pipe waveguides and the other is composed of two subwavelength dielectric fibers. For the first type THz coupler, the odd system mode of the coupler is unstable near the antiresonant frequencies, which makes the coupler unable to be operated like the conventional fiber-optic ones. Adding an additional layer between the two slab-type pipe waveguides can solve this problem. For the second type THz coupler, when the fiber separation is small, odd-mode cutoff occurs and only the even system mode is guided. Thus the beating phenomenon usually observed in conventional fiber-optic couplers can not take place. The x-polarized even system mode has a more confined field distribution than that of the y-polarized one when the fiber separation is small, similar to the behavior of a slot waveguide, which leads to different attributes between the two polarizations. With these characteristics, the THz couplers might be applied to THz devices such as power dividers and polarization filters.
In the final part, behaviors of some optical guided-wave devices are investigated, including two different types of rib-waveguide based polarization convertors and the gain-guided and index-antiguided slab waveguide which is subject to gain saturation. The effect of gain saturation is observed from the propagation of the fundamental mode.
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