Student Project Abstracts 2005 - Pluto - University of Washington

OPTIMIZING HYBRID WAVEGUIDESpropagation constants; one when the index of the cladding layeris unshifted or without an applied voltage (for example 1.202),the other is when the index of the cladding layer is shifted due anapplied voltage (for example 1.200).Index of RefractionLooking at the effect the shift has on the propagation constantwhen the base index of refraction of the cladding layer isvaried, it is found that over the range of 1 to 1.5 the changesis relatively constant. With dimensions of height 200nm, width180nm and space 50nm, the change only varies by 6m -1 over anindex change of 0.03, as illustrated in Fig. 3. A larger range ofindexes was sampled and similar results were found.Figure 2. Top: A power flow diagram of a hybrid waveguide’s guidedmode. Bottom: Power flow diagram of a traditional waveguide guided mode.The goal for this summer’s research is to discover howchanging the dimensions of hybrid waveguides will affect thewaveguide’s propagation constant and also what dimensions willproduce the greatest change in the propagation constant due to aconstant change in the index of refraction without allowing lightto form a guided mode in the silicon blocks.To accomplish this task, a modeling program called FEM-LAB 3 was used for simulation of the structures. The dimensionsthat can be changed are: the index of the cladding layer, theheight and width of the blocks, and the space between the twoblocks (space).RESULTSAn EO polymer’s ability to alter the index of refraction canbe modeled by a simple equation: 3 (1)Where n is the original index of refraction, r 33is an electro-opticcoefficient, V is voltage applied, and d is the distancebetween electrodes. Using appropriate values, the typical Δn isaround -.002 to -.003. In this paper all shifts of index of refractionof the cladding layer will be -.002, and will be referred toas “the shift”, and “the change” or “change” will refer to howmuch the propagation constant changes due to a -.002 change inthe index of refraction of the cladding layer. All the graphs willshow the absolute change of the propagation constant. Also notethat the graphed data is derived from finding the difference of twoFigure 3. The amount the propagation constant changes with a -.002 change of the index of the cladding layer. Data observedat height: 200nm, width: 180nm, and space: 50nm.It is important to note that the larger the index of claddinglayer will result in a larger change in the index of refraction, accordingto Eq. (1). This was not taken in to account in the aboveanalysis.Height of the Silicon BlocksAdjusting the height of the silicon blocks has a much differenteffect than changing the index of the cladding. To determinethe maximum change in propagation constant, a range of heightsfrom 200nm to 390nm were sampled with a width of 200nm, aspace of 50nm, and a index of the cladding layer of 1.002 (seeFig. 4). A maximum change was found at 370nm. However, withsilicon blocks this tall, a second guided mode exists within theblocks themselves. The shorter the silicon blocks are, the morethe undesired guided mode becomes a substrate radiation mode.The question then becomes, when does the guided mode inthe silicon blocks become primarily a substrate radiation mode(see Fig. 5)? It seems that a useful range for heights will be between160nm to 270nm depending on the width of the siliconblocks. In this range the power going through the substrate willbe around half as much as the power going through the space. Itshould also be noted that since the range of effective heights is far54 CMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005