Design and optimization of Optical power splitter based on Multimode Interference for 1.55-μm operation. Abdulaziz M. Al-hetar 1 , Abu Sahmah Bin Mohd Supa’at, N. M. Kassim and A. B. Mohammad Photonic Technology Center, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Johor, Malaysia. 1 alhetar_aziz@yahoo.com Abstract – In this paper, we design and optimize 1X2, 1X4, 1X8, 1X16, and 1X32 optical power splitter based on Multimode Interference (MMI). A mathematical model is used to get accurate values of propagation constants and subsequently calculates the optimum value of coupler length of the MMI region, L MMI . The results are predicted by the mathematical model, which gives the optimum values of the device properties such as excess loss and imbalance. Keywords: Multimode interference (MMI), optical power splitter 1. Introduction The challenge in optical access networking is to bring optical fibers as close to the end-users as possible and that is called Fiber to the home (FTTH). One way to realize this economically is to employ the passive double star (PDS) topology [I]. Therefore, it is necessary to use plenty of passive optical power splitters in the central office for distribution purposes. Some of the important characteristics of such splitter are low loss, compactness, compatibility with optical single mode fibers, uniform distribution of the output power on the output waveguides and a low cost. A power splitter 1×2 is usually a symmetric element, which equally divides power from a straight waveguide between two output waveguides. The simplest version of a power splitter is the Y-branch, which is easy to design and relatively insensitive to fabrication tolerances. Nevertheless, the curvature radii of the two branches, as well as the junction, must be carefully designed in order to avoid power losses. Also, if the two branches are separated by tilted straight waveguides, the tilt angle must be small, typically a few degrees [2]. A different version of a power splitter is the multimode interference element (MMI). This name comes from the multimodal character of the wide waveguide region where the power split takes place. The advantage of this design is the short length of the MMI compared to that of the Y-branch. Although the dimensions of the MMI are not critical, allowing wide tolerances, this element must be designed for a particular wavelength. The two power splitters, which have been described, are symmetric, and thus 50% of the input power was carried by each output waveguide. Nevertheless, asymmetric splitters can also be designed for specific purposes. In addition, it is possible to fabricate splitters with N output waveguides, and in that case the element is called a 1 × N splitter. MMI devices are important components for photonic and optoelectronic integrated circuits due to their simple structure, low loss [3], large optical bandwidth and fabrication tolerance [4]. These structures provide power splitting or combining. The organization of the paper is as follows. First, the theory and the principle of operation are described. The design of 1 X N power splitter based on MMI is presented. Then the geometrical design parameters as well as the number of input/output ports and MMI section are discussed. Finally, a brief conclusion is given. 2. Theory Figure1 shows the top view of the MMI power splitter. The MMI power splitter consists of three parts: a single input waveguide of width w, a wide multimode waveguide section of width W M , and a section of output waveguides of width w. Here W M and L MMI are the coupler width and the coupler length of the MMI region, respectively. To launch light into and receive light from that multimode waveguide, a number of single-mode waveguides are placed at its beginning and its end. The basic principle that governs the reproduction of input images at the output waveguides is called the self-imaging principle [3]. Self-imaging is a property of multimode waveguides by which an input field profile is reproduced in single or multiple images at periodic intervals along the propagation direction of the guide. 1-4244-1435-0/07/$25.00©2007 IEEE