498 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 7, APRIL 1, 2010 Development of Hierarchical Optical Cross-Connect System for ROADM-Ring Connection Using PLC Technologies Kiyo Ishii, Student Member, IEEE, Osamu Moriwaki, Member, IEEE, Hiroshi Hasegawa, Member, IEEE, Ken-ichi Sato, Fellow, IEEE, Yoshiteru Jinnouchi, Masayuki Okuno, Member, IEEE, and Hiroshi Takahashi, Member, IEEE Abstract—An efficient reconfigurable optical add–drop multi- plexer ring connecting node architecture is proposed that utilizes waveband routing; it achieves a small footprint and excellent cost- effectiveness. The key component devices are implemented using planar lightwave circuit technologies and their performances are verified in experiments. Index Terms—Hierarchical systems, metropolitan area net- works, photonic switching systems. I. INTRODUCTION T HE large-scale deployment of reconfigurable optical add–drop multiplexer (ROADM)-based ring networks has recently started in North America and Japan. At present, ring interconnection is done in an electrical layer with op- tical-to-electrical/electrical-to-optical (OE/EO) conversion and electrical switches. Removing the costly and power consuming electrical stage can be realized by exploiting optical path routing using optical cross-connects (OXC). OXC architectures that utilize multiple large-scale wavelength selective switches (WSS) or optical matrix switches have been investigated, however, the higher costs needed to add the OXCs prevented their introduction. Waveband (WB) paths, bundles of wavelength paths, are be- coming better known [1], and the hierarchical OXC (HOXC) ar- chitecture has been shown to substantially reduce OXC switch scale. This is true for WSS-based [2] and matrix switch-based [3] OXC architectures. We have proposed a two-ring connecting node architecture that partially applies WB routing [4]. It was shown that the node switch scale can be greatly reduced; more than 80% when inter-ring traffic is 60% [4]. The requirements for the reduction are as follows: 1) wavelengths should be pre- viously arranged into two groups that are used within a ring and Manuscript received July 06, 2009; revised September 09, 2009; accepted January 07, 2010. First published February 02, 2010; current version published March 10, 2010. This work was supported in part by JST. K. Ishii, H. Hasegawa, and K. Sato are with Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan (e-mail: k_isii@echo.nuee. nagoya-u.ac.jp; hasegawa@nuee.nagoya-u.ac.jp; sato@nuee.nagoya-u.ac.jp). O. Moriwaki and H. Takahashi are with NTT Photonics Laborato- ries, NTT Corporation, Atsugi, Kanagawa, 243-0198, Japan (e-mail: moriwaki@aecl.ntt.co.jp; hiroshi@aecl.ntt.co.jp). Y. Jinnouchi and M. Okuno are with NTT Electronics, NTT Corporation, Naka, Ibaraki, 311-0122, Japan (e-mail: jinnouti@photo.nel.co.jp; okuno@hqs. nel.co.jp). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2010.2040977 between rings, 2) inter-ring traffic should be routed as a WB at the ring connecting node, and 3) traffic within each ring is routed as wavelength paths. This scheme can be extended to cover mul- tiple-ring interconnection; however, the increase in the number of concatenated rings degrades the degree of switch scale re- duction because of the more limited use of WB routing and the necessity of grouping wavelengths that are used for inner- and inter-ring in advance. We have recently succeeded in developing a new efficient op- tical path demand accommodation algorithm [5] that imposes no constraint on wavelength assignment regarding inner- and inter-ring traffic. WB routing is applied to all traffic at the ring connecting node; wavelength level grooming can be done if necessary. Accordingly, the algorithm can be extended to mul- tiple-ring connections without degrading the degree of switch scale reduction. The algorithm attains almost the same (in the case of two-ring connection) or larger (more than three rings are connected) switch scale reduction than the previous one de- scribed in [4]. The optical path demand accommodation effi- ciency offset was proven to be marginal [5] compared to single- layer optical path rings. Based on this achievement, we imple- ment the key components of the ring-connecting HOXC system using planar lightwave circuit (PLC) technologies in this letter. The preliminary work will be presented in [6]. Section II shows the proposed HOXC node architecture. This switch architecture is shown to achieve 60% switch scale reduction compared to a single layer architecture [3]. Section III describes the perfor- mances of the key component devices. Section IV shows that the optical performance of the proposed HOXC prototype node is sufficient for practical applications. II. PROPOSED HOXC ARCHITECTURE Fig. 1 depicts two- and three-ring connecting nodes. Other nodes than the ring connecting node are conventional ROADM nodes which may be WSS-based, matrix-switch-based, or wavelength-blocker-based, etc., that perform wavelength level routing. Fig. 2 shows that the proposed HOXC node switch architecture consists of WB multi/demultiplexers (MUX/DEMUX), WB path switches (SWs), wavelength MUX/DEMUXs, and wavelength path switches. When wave- length path level routing or termination is necessary for some wavelength paths in WB paths, the WB paths are sent to the wavelength cross-connect (WXC) layer where they are groomed or terminated at the wavelength path level [7]. The node-degree of the fabricated system is 6, so the node prototype can connect up to three rings; each ring consists of two bidirectional fibers. Each input fiber carries 40 wavelengths 1041-1135/$26.00 © 2010 IEEE