JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 16, AUGUST 15, 2012 2633 The Basic Building Block of -Router With Multimode-Interference Waveguide Crossings on Silicon-on-Insulator Technology Guofang Fan, Regis Orobtchouk, Bing Han, Jean-Marc Fedelin, Xinhou Liu, and Zhen Zhen Abstract—Optical properties of the -router basic building block are simulated using a matrix method. Experiments are performed for the -router basic building block with multi- mode-interference (MMI) crossings, which shows large free spectral range ( nm) and more than 24 dB on/off contrast of the drop resonance. Theoretical and experimental results reveal that the -router basic building block with MMI crossings can suppress the crosstalk (defined as the difference between the drop efficiency and the throughput attenuation at resonance), offer relatively symmetric resonance, and increase the on/off contrast of the drop resonance, compared with a -router building block using conventional crossings. Index Terms—Conventional crossing, microring resonator, multimode-interference (MMI) crossing, -router basic building block. I. INTRODUCTION O PTICAL NETWORK on CHIP (ONoC) has recently become popular as an alternative option for increasing bandwidth, decreasing latency, and reducing power in chip multiprocessors. ONoC is composed of three types of blocks: 1) transmitters; 2) a passive integrated photonic routing structure ( -router); and 3) receivers. The -router, which constitutes the core of the ONoC, is a passive, wavelength-routed optical net- work, and designed to route data. The basic building blocks in most of the -routers reported in the literature [1]–[5] are based on four-port optical switches with two microring resonators (see Fig. 1). Manuscript received March 20, 2012; revised May 16, 2012; accepted June 13, 2012. Date of publication June 19, 2012; date of current version July 18, 2012. This work was supported in part by the Young Scientists Fund of the National Natural Science Foundation of China under Grant 11104284 and the European Union through the FP7 project WADIMOS. G. Fan, X. Liu, and Z. Zhen are with the Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 100190, Beijing, China (e-mail: fan- guofang@mail.ipc.ac.cn; xhliu@mail.ipc.ac.cn; zhenzhen@mail.ipc.ac.cn). R. Orobtchouk is with the Institut des Nanotechnologies de Lyon, INSA-Lyon, Université de Lyon, F-69621 Villeurbanne, France (e-mail: regis.orobtchouk@insa-lyon.fr). B. Han is with Dalian Actech Inc., Liaoning 116600, China (e-mail: ipcx- iezhengwen@hotmail.com). J. M. Fedeli is with CEA-LETI, Minatec, CEA-Grenoble, F-38054 Grenoble cedex 9, France (e-mail: jean-marc.fedeli@cea.fr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2012.2205221 Fig. 1. -router basic building block with the (a) conventional crossing and (b) MMI crossing. In this paper, we report a -router basic building block using the MMI crossings to get request routing, because the conven- tional crossings have relatively large insertion loss and crosstalk at the crossing junction due to wavefront expansion, particularly in a high-index-contrast waveguide platform [6]. In order to in- crease the coupling between the bus waveguides and the ring waveguides in the microring resonators, we use the bend–bend coupler in the coupling region and design the bus waveguides with smaller width than the ring waveguides for phase matching in the microring resonators [7], [8]. II. MODEL Using the matrix approach, for the single-ring resonator in Fig. 2(a), we have (1) (2) (3) where , and are electric field com- ponents as shown in Fig. 2(a), and are self- and cross-coupling coefficients of the couplers, respectively, which describe the interaction intensity in the coupling region, and , and are the ring resonator losses, the propagation con- stant, and perimeter of the ring resonators, respectively. 0733-8724/$31.00 © 2012 IEEE