Ultra low cross talk in crossed strip waveguides with the assistance of a photonic crystal cavity Rami A. Wahsheh*, Zhaolin Lu, Mustafa A. G. Abushagur, and Stefan F. Preble Microsystems Engineering, Kate Gleason College of Engineering, Rochester Institute of Technology, Rochester, New York, USA 14623 ABSTRACT In this paper, ultra low cross talk is achieved by using a resonant cavity at the intersection between two strip waveguides formed in a square lattice photonic crystal structure (PhC). Two PhC structures are studied: one consists of cylindrical rods and another consists of cubic rods. The Q-Factor of the cavity is changed by increasing the number of rods that form the cavity and by decreasing the spacing between the waveguide and the cavity. Our two dimensional simulation results show that the latter method resulted in cross talk reduction of more than 21 dB for both structures. The overall cross talk was -90.50 dB for the cylindrical rods structure and -105.0 dB for the cubic rods structure. The optimized PhC structures were fabricated on a silicon-on-insulator platform. The rods were buried in silicon oxide in order to maximize the photonic band gap and provide index guiding in the vertical direction. Keywords: Integrated optics devices, waveguides, photonic integrated circuits. 1. INTRODUCTION When waveguides are crossed guided waves suddenly expand due to the lack of confinement in the lateral direction. This results in coupling into the intersecting waveguides in addition to radiation and scattering losses. Ultra low cross talk between intersecting waveguides is required in optical integrated circuits in order to minimize the required area to produce multiple optical devices on the same chip. Low cross talk is also beneficial for improving bit rate in optical communications systems. Recent work has shown that cross talk between photonic devices can be reduced to a much smaller degree than that between their electronic counterparts [1]. However, the low cross talk essentially relies on designing innovative photonic structures. More recently, a number of structures have been proposed and investigated to eliminate cross talk [2-6]. One method that attracts great attention is based on cavity coupling that can achieve low cross talk over a wide spectrum [3-6]. The key idea is to excite modes orthogonal to each other at the intersection area. Johnson, et al. [3] proposed a resonant cavity that supported two orthogonal modes at the intersection area of two line- defect waveguides in a two dimensional (2D) square lattice photonic crystal (PhC) structure, which was composed of periodic cylindrical rods in air. In the work of Johnson, et al. [3], as the quality factor (Q-factor) of the cavity increased by adding more rods next to the defect rod, cross talk could be reduced. As a result of the Q-factor change, both the output bandwidth spectrum and cross talk are controlled. Based on a similar structure, Liu, et al. [4] reported cross talk reduction by using two single mode coupled resonator optical waveguides that had nonoverlapping photonic band gap. Their results are very attractive and promising. Furthermore, all-optical transistors can potentially be achieved based on the PhC cross-waveguide geometry [7]. However, in both works of Johnson et al. [3] and Liu et al. [4] the structures had an infinite thickness and light was guided in air, or void photonic band gap (PBG) waveguides, instead of dielectric waveguides. As a result, the structures are only ideal 2D models that cannot be experimentally realized. In order to experimentally demonstrate the structure proposed by Johnson, et al. [3], Roh, et al. [5] used two aluminum metal plates to insure confinement in the out of plane direction. One plate was placed on the top, and the other was placed at the bottom of the cylindrical alumina rods in air. Cross talk reduction as large as −30 dB was experimentally achieved at the resonant frequency. However, this metallic-cladding structure can not be scaled down for telecom wavelengths. To work for the telecom wavelengths, Teo et al. [6] fabricated a structure that was composed of 13μm high silicon rods in air, which are too high to provide effective out-of-plane field confinement. For practical applications, a widely used way is to convert a 2D structure into a planar structure. Unfortunately, in a planar structure light cannot be confined in void PBG waveguides without out-of-plane confinement on light propagation. *raw7949@rit.edu Photonic Fiber and Crystal Devices: Advances in Materials and Innovations in Device Applications II, edited by Shizhuo Yin, Ruyan Guo, Proc. of SPIE Vol. 7056, 70560E, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.795250 Proc. of SPIE Vol. 7056 70560E-1 2008 SPIE Digital Library -- Subscriber Archive Copy