NETWORKS-ON-CHIP (NoC) plays an important role in

3736 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 23, DECEMBER 1, 2012 A Universal Method for Constructing N-Port Nonblocking Optical Router for Photonic Networks-On-Chip Rui Min, Ruiqiang Ji, Qiaoshan Chen, Lei Zhang, Member, IEEE, Member, OSA, and LinYang, Member, IEEE Abstract We propose a universal method for constructing N-port nonblocking optical routers based on microring resonators. The topologies for five-, six-, seven-, and eight-port nonblocking optical routers are presented. As a case study, we compare the five-port optical router constructed by our method with the previously reported ones. The simulation results show that the mesh photonic network constructed by the proposed five-port optical routers has low insertion loss and high optical signal-to-noise ratio. Moreover, high-radix optical routers can be easily constructed by the proposed method, which can support more complicated and efficient network. Index Terms Silicon photonics, optical router, microring, networks-on-chip. Fig. 1. (a) Four-port and (b) five-port reported nonblocking optical routers. I. INTRODUCTION NETWORKS-ON-CHIP (NoC) plays an important role in the intrachip multiprocessor-core interconnection. However, as the number of the processor core and the local clock frequency increase in the chip multiprocessor (CMP), larger communication bandwidth of the NoC is required to realize the communication among the processor cores. With the increasingly required communication bandwidth, power consumption and transmission latency are becoming the bottleneck of the traditional electronic NoC. Photonic NoC, which has larger communication bandwidth, lower transmission latency, and lower power consumption, has been proposed recently [1] [4], aiming at addressing the problems that the traditional metallic interconnection encounters. High-performance devices required for photonic NoC have been reported, such as Ge waveguide-integrated avalanche photodiode [5], silicon-based CMOS compatible high-speed modulator [6], compact silicon comb switch [7], making photonic NoC a feasible scheme for future intrachip interconnect for the CMP. The topology of photonic NoC is diverse, such as torus [1], fat tree [8], mesh, c-mesh, and clos [9]. Optical router is a key Manuscript received July 23, 2012; revised September 23, 2012, October 27, 2012; accepted October 30, 2012. Date of publication November 15, 2012; date of current version December 11, 2012. This work was supported by the National Natural Science Foundation of China under Grant 60977037, and by the National High Technology Research and Development Program of China under Grant 2009AA03Z416. The authors are with the State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China (e-mail: minrui@semi.ac.cn; jiruiqiang@semi.ac.cn; chenqs@semi.ac.cn; zhanglei@semi.ac.cn; oip@semi.ac.cn). 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.2012.2227945 component for these photonic NoCs. Several nonblocking optical routers have been reported, including four-port and fiveport optical routers [10] [15] (see Fig. 1). Some optical routers are based on microring resonators (MRR) [10], [12] [15], and others are based on Mach Zehnder interferometer [11]. However, none of them is scalable. Only four- and five-port optical routers are far from enough to construct all photonic NoCs. High-radix nonblocking optical routers have potential applications in the future intrachip optical interconnections, such as c-mesh and clos network. Here, we propose a universal method for constructing N-port nonblocking optical router and presents the comparison of the five-port optical router constructed by this method with the reported ones. This paper is organized as follows. In Section II, we explain how to construct N-port nonblocking optical router. In Section III, we compare the five-port nonblocking optical router constructed by our method with the reported ones. In Section IV, we make some conclusions on the universal constructing method. II. CONSTRUCTING N-PORT NONBLOCKING OPTICAL ROUTER Fig. 2 illustrates a 4 4 mesh photonic network and a fiveport nonblocking optical router. The five-port optical router has five ports which are Center, North, East, South,andWest ports. The Center port is connected to the processor core. Each port is a bidirectional port containing an input and an output. Light injected into the input of one port can be guided to the output of any other ports. For example, light injected into the input of the West port can be guided to any output of the Center, North, East, and South ports, by shifting the resonance wavelengths of the MRRs. However, light is not required to be guided to the output of the West port. Suppose that light is injected into the input of 0733-8724/$31.00 2012 IEEE

MIN et al.: UNIVERSAL METHOD FOR CONSTRUCTING N-PORT 3737 Fig. 4. Structure of one bus waveguide whose drop points lie before add points. Fig. 2. Mesh network and a five-port nonblocking optical router. Fig. 3. Simple demonstration of a blocking four-port optical router; (a) routing normally; (b) blocking occurs, the red and green links hit at the red waveguide zone. the North port and guided to the output of the South port; this physical link would never block the possible links between the remaining inputs and outputs. Mesh NoC requires the five-port nonblocking optical routers, whereas other NoCs may demand for high-radix nonblocking optical routers. However, the properties described previously are similar except that higher radix optical router has more ports. We can conclude that N-port nonblocking optical router must satisfy the following properties. 1) Light injected into the input of any port can be guided to the output of any other ports. 2) Light injected into the input of any port should not be guided to the output of the same port (named no U turn). 3) Any physical link between an input and an output would never block the possible links between the remaining inputs and outputs. In order to explore the universal method for constructing N-port nonblocking optical router, we first consider a simple four-port optical router with two pairs of orthogonal straight waveguides and eight MRRs, as shown in Fig. 3. The optical router works at single wavelength and all MRRs have the same resonant wavelength. It is notable that light injected into each input of the four ports can be guided to the outputs of the other three ports and no U turn exists. The optical router can normally route in some cases [see Fig. 3(a)]. However, blocking happens in other cases [see Fig. 3(b)]. In Fig. 3(b), light injected into the input of the West port is expected to be directed to the output of the South port through R4, and the signal from the input of the North port is directed to the output of the West port through R6 at the same time, which means that R4 and R6 are simultaneously resonant at the same wavelength. Then, light from the input of the West port will be downloaded to the output of the West port rather than the output of the South port. The similar case happens to the light injected into the input of the North port, which is supposed to be guided to the output of the West port, and is actually downloaded to the output of the East port by R4. MRRs coupled with waveguide crossings are arranged along the bus waveguide (black line), which has an input port and an output port (see Fig. 4). Light injected into the input port of the bus waveguide can be downloaded to other waveguides through the MRRs at the drop points. Light from other waveguides can be uploaded to the bus waveguide through the MRRs at the add points. Blocking only occurs when a drop point lies behind an add point. In Fig. 3(b), if we choose the waveguide connecting the input of North port and output of South port as a bus waveguide, then R4 and the crossing form an add point, and R6 and the crossing form a drop point. Moreover, the drop point lies behind the add point, so the blocking happens. If we rearrange the add points and drop points along the bus waveguide, a nonblocking optical router can be achieved. In Fig. 4, all the drop points are arranged before the add points. When light is injected into the input port, it either goes through the waveguide and reaches the output port, or is downloaded by one of the drop points. When the transmission path of light is determined, only one MRR at the specific drop point is resonant at most, so the drop points will not interrupt with each other. The similar situation happens to the add points when light is uploaded to the bus waveguide. For an N-port nonblocking optical router, light injected into the input of one port must be able to be guided to any output of the remaining ( ) ports. Therefore, there must be ( ) optical links for an input. As shown in Fig. 4, one optical link is established by bus waveguide directly, and the left ( ) optical links are established by the ( )droppoints.asan N-port nonblocking optical router has N inputs and N outputs, N bus waveguides as shown in Fig. 4 are required to construct an N-port nonblocking optical router. Different arrangement of the N waveguides in 2-D plane makes the performance of the optical router different, which includes average insertion loss and crosstalk per optical link. Here, we propose an arrangement, in which the drop points and the add points are implemented by MRRs coupled with waveguide crossings. Fig. 5 shows the connections between the add points and the drop points.ifwenamethe th drop point and the th add point

3738 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 23, DECEMBER 1, 2012 Fig. 5. Connections between the add points and the drop points of N-port nonblocking optical router. in th waveguide and, respectively, is connected to %. The operator % means modulo operation. The connections in Fig. 5 seem to be complicated. Fortunately, an MRR coupled with a waveguide crossing is a drop point to one waveguide of the crossing, while it is a add point to the other waveguide of the crossing. So a connection between an add point and a drop point is actually a waveguide crossing and an MRR. So the practical topology of the N-port nonblocking optical router constructed by our method is much simpler than that shown in Fig. 5. Fig. 6 shows the topologies of the nonblocking five-, six-, seven-, and eight-port optical routers constructed with the above method. We concentrate on the blue waveguide, in which the green MRRs belong to the drop points and the yellow MRRs belong to the add points. It satisfies the aforementioned rules. Because all the constructed optical routers are centrally symmetric, each waveguide in the optical routers satisfies the rules mentioned above. There are totally crossings, microrings, and N waveguides for the N-port nonblocking optical router. For each optical link, only one MRR is resonant at most, which can reduce the power consumption of the optical router, and the details can be found in Table I. III. CASE STUDY Insertion loss and crosstalk are two critical figures of merit for an optical router, which determine the feasibility and scalability of the NoC, as well as the power consumption of E/O interface to generate and detect the optical signal. Both waveguide crossings and MRRs coupled with waveguides can introduce insertion loss and crosstalk to the optical router. More details have been analyzed in [16] and [17]. The insertion loss of the waveguide crossing is about 0.2 db, and the MRR pass loss is 0.5 db. Waveguide also has propagation loss, which is mainly due to the roughness of the sidewall (about 1.5 db/cm) and is related to the footprint of the optical router. We only concerns about the topology of the optical router and do not consider this kind of loss. The crosstalk of the waveguide crossing is db and the crosstalk introduced by the resonant MRR is db. We compare the five-port optical router constructed with our method with the previously reported ones (see Fig. 7), including

MIN et al.: UNIVERSAL METHOD FOR CONSTRUCTING N-PORT 3739 Fig. 6. N-port nonblocking optical routers. (a),(b),(c),and(d). TABLE I MERITSOFTHECONSTRUCTED N-PORT NONBLOCKING OPTICAL ROUTER Ji s router [13], Cygnus router [14], and Poon s router [15]. We analyze the performance of different optical routers in a 16 16 photonic mesh network. To construct a 16 16 mesh network, 3840 MRRs are required for our optical router, 4096 MRRs are required for Ji s router and Cygnus router, and 5120 MRRs are required for Poon s router. Less MRRs means smaller footprint, lower propagation loss, and lower power consumption. Therefore, our optical router has the smallest footprint, lowest propagation loss, and power consumption. Our simulation is based on, an object-oriented modular discrete event network simulation framework. First, we model all kinds of devices used in the photonic network. Then, we configure the topology of the 16 16 photonic mesh network using NED language. Finally, we record the insertion loss and crosstalk of each link established during the simulation. Note that the worst insertion loss and optical signal to noise ratio (OSNR) determine the lowest requirements of the photonic network on the optical devices. And average insertion loss and OSNR can give the average requirements of the photonic network on the optical devices, which is also important. We analyze both the worst and average insertion losses and OSNRs of the 16 16 mesh networks constructed by different optical routers. Fig. 7. Four five-port nonblocking optical routers. Green arrows show the ports connected to the processor cores. (a) Poon s router, (b) Ji s router, (c) Cygnus router, and (d) our router (we redraw the waveguide bend to right angle). As shown in Fig. 8, the 16 16 Mesh networks constructed by our router and Cygnus router have the smallest average insertion loss of about 14.5 db. The 16 16 Mesh network constructed by Poon s router has the largest average insertion loss of about 22 db. OSNR decides the reliability of the communication between two processor cores. Higher OSNR means higher reliability between two processor cores, which can reduce the cost of error correction and retransmission. Our router gains the largest average OSNR of about 16 db. Poon s router gets

3740 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 23, DECEMBER 1, 2012 distribution, and their distribution are a little narrower than Ji s router. Overall, our nonblocking five-port optical router is comparable with Cygnus and both of them behave better than Ji s router and Poon s router in a 16 16 Mesh network. Fig. 8. Simulation results of different optical routers in a 16 16 mesh network. (a) Average insertion loss and (b) average OSNR. Fig. 9. Simulation results of different optical routers in a 16 16 mesh network. (a) Maximum insertion loss and (b) worst OSNR. IV. CONCLUSION In summary, by analyzing the properties of the N-port nonblocking optical router, we get some rules for constructing an N-port nonblocking optical router and propose a universal method for constructing an N-port nonblocking optical router based on MRR. The cases for and 8 are presented. As a case study, we compare the five-port nonblocking optical router constructed by the proposed universal method with the reported ones. Our five-port nonblocking optical router has fewer MRRs, fewer waveguides, and fewer average crossings per link, which means lower crosstalk, lower insertion loss, and lower power consumption. Further simulation shows that the 16 16 mesh network constructed by our optical router has the smallest average insertion loss and the largest worst case OSNR. Moreover, high-radix nonblocking optical routers can be constructed by our method easily, which can support more complicated and efficient network such as c-mesh and clos network. Fig. 10. Insertion loss distributions of the 16 16 mesh networks constructed by different optical routers. the smallest average OSNR of about 12.8 db. The OSNRs of Cygnus router and Ji s router are 15 and 13 db, respectively. Fig. 9 shows the worst case insertion loss and OSNR of each optical router. The 16 16 mesh network constructed by Cygnus router has the smallest maximum insertion loss of about 37 db. The maximum insertion loss of 16 16 mesh network constructed by our router is about 38.5 db, which is slightly larger than Cygnus router. The smallest OSNR is about db for our router, db for Poon s router, db for Cygnus router, and db for Ji s router. Since the maximum insertion loss is quite large and the worst OSNR is quite small (see Fig. 9), the 16 16 mesh network constructed by these routers possibly cannot work normally. However, using these two figures of merit to evaluate the optical routers is still acceptable. The distribution of the insertion loss is another significant figure of merit. As shown in Fig. 10, Poon s router has the widest distribution, Cygnus router and our router almost have the same REFERENCES [1] A. Shacham, K. Bergman, and L. P. Carloni, Photonic networks-on-chip for future generations of chip multiprocessors, IEEE Trans. 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MIN et al.: UNIVERSAL METHOD FOR CONSTRUCTING N-PORT 3741 [14] H. X. Gu, K. H. Mo, J. Xu, and W. Zhang, A low-power low-cost optical router for optical networks-on-chip in multiprocessor systems-onchip, in Proc. IEEE Comput. Soc. Annu. Symp., 2009, pp. 19 24. [15] A. W. Poon, X. Luo, F. Xu, and H. Chen, Cascaded microresonatorbased matrix switch for silicon on-chip optical interconnection, Proc. IEEE, vol. 97, no. 7, pp. 1216 1238, Jul. 2009. [16] A. Bianco, D. Cuda, R. Gaudino, G. Gavilanes, F. Neri, and M. Petracca, Scalability of optical interconnects based on microring resonators, IEEE Photon. Technol. Lett., vol. 22, no. 15, pp. 1081 1083, Aug. 2010. [17] J. Chan, A. Biberman, B. G. Lee, and K. Bergman, Insertion loss analysis in a photonic interconnection network for on-chip and off-chip communications, in Proc. Annu. Meet. IEEE Lasers Electro-Opt. Soc., 2008, pp. 300 301. Rui Min was born in 1988. He received the B.S. degree in solid-state electronics from the Huazhong University of Science and Technology, Wuhan, China, in 2010. He is currently working toward the M.S. degree in microelectronics and solid-state electronics at the Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China. His research interest is silicon-based photonic devices for optical interconnect, such as optical routers and modulators. Ruiqiang Ji was born in 1985. He received the B.S. degree in optical information science and technology from the Beijing University of Posts and Telecommunications, Beijing, China, in 2008. He is currently working toward the Ph.D. degree in microelectronics and solid-state electronics at the Institute of Semiconductors, Chinese Academy of Sciences, Beijing. His research interest is include modeling, design, and characterization of nanophotonic components, and especially based on submicrometer silicon waveguides in silicon-on-insulator platform. Qiaoshan Chen was born in 1990. She received the B.S. degree in solid-state electronics from Nankai University, Tianjin, China, in 2012. She is currently working toward the M.S. degree in microelectronics and solid-state electronics at the Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China. Her research interest includes on-chip photonic networks-on-chip and optical routers. Lei Zhang (S 08 M 11) received the B.S. degree from the Beijing University of Posts and Telecommunications, Beijing, China, in 2006, and the Ph.D. degree in microelectronics and solid-state electronics from Institute of Semiconductors, Chinese Academy of Sciences, Beijing, in 2011. He is currently an Assistant Professor with the State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences. He is the author or coauthor of more than 20 journal papers. His research interests include silicon photonics, microwave photonics, on-chip optical interconnect, and information processing. Dr. Zhang is a member of the IEEE Photonics Society, the IEEE Communications Society, and the Optical Society of America. Lin Yang (M 11) received the Ph.D. degree in microelectronics and solid-state electronics from the Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China, in 2003. From 2003 to 2007, he was a Postdoctoral Fellow of Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo, Japan. He is currently a Professor in State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing. He is the author or coauthor of more than 50 refereed scientific journal papers. His current research interests include silicon-based photonic devices for optical interconnect, optical computing, and optical communication.