IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 3, MARCH 2005 717 A Novel Centrally Controlled Protection Scheme for Traffic Restoration in WDM Passive Optical Networks Zhaoxin Wang, Xiaofeng Sun, Chinlon Lin, Chun-Kit Chan, and Lian-Kuan Chen Abstract—We propose and experimentally demonstrate a novel centrally controlled protection scheme for arrayed waveguide grating-based bidirectional wavelength-division-multiplexing passive optical networks which can provide automatic traffic restoration against fiber cut between remote node and optical network unit. Compared with the previous structures, its re- quirements on the network resources are greatly reduced with negligible protection performance degradation. Index Terms—Passive optical network (PON), protection and restoration, wavelength-division multiplexing (WDM). I. INTRODUCTION T HE wavelength-division-multiplexing passive optical net- work (WDM-PON) [1]–[4] is a promising access tech- nology to deliver high capacity data to the subscribers. However, when fiber link failure occurs between the remote node (RN) and one of the optical network units (ONUs), the affected ONU will become unreachable from the optical line terminal (OLT), leading to loss in data. Therefore, fault management is one of the crucial aspects in network management to enhance the net- work reliability. Several methods have been proposed to achieve network protection. In [5], duplicated network resources such as fiber links or ONUs are implemented to provide network redun- dancy, and automatic protection switching is used to reroute the affected data traffic into those alternate protection paths. In [6] and [7], the periodic and cyclic properties of arrayed waveguide gratings (AWGs) are used to enable each distribution fiber to carry data traffic for more than one ONU. Thus, any two ad- jacent ONUs can form a group to protect each other. Protec- tion switches are incorporated in each ONU for data rerouting between the ONUs in the same group. In this letter, a novel network architecture based on a centrally controlled protection scheme is proposed and demonstrated. Compared with [6] and [7], only one set of protection equipment is located at the OLT end instead of distributed at each ONU, thus named as “centrally controlled.” Such architecture not only simplifies the ONU de- sign, but also significantly reduces the amount of required net- work resource. Manuscript received October 18, 2004; revised November 21, 2004. This work was partially supported by a research grant from Research Grants Council, Hong Kong SAR (Project CUHK4126/03E). The authors are with the Department of Information Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong (e-mail: zxwang3@ie.cuhk.edu.hk). Digital Object Identifier 10.1109/LPT.2004.842378 Fig. 1. Proposed network architecture with eight ONUs. II. PROPOSED NETWORK ARCHITECTURE AND PROTECTION PRINCIPLE Fig. 1 shows our proposed network structure with (in mul- tiple orders of two) ONUs. Without loss of generality, eight ONUs are considered here as an example to facili- tate the illustration. At the OLT end, two optical isolators with opposite directions and a 2 2 switch are inserted before the two feeder fibers. In normal operation, the switch is set to the bar state to make sure that feeder Fibers I and II are used for the downstream and the upstream traffic, respectively. A deci- sion circuit is also incorporated in the OLT to detect the fiber link failure and to activate the wavelength rerouting mechanism for protection. At the RN end, feeder Fibers I and II are con- nected to the two input ports of the 2 8 AWG, respectively, and the spectral transmission peaks of the two ports are spaced by half of the free-spectral range (FSR) of the AWG. At the ONU end, two adjacent ONUs are assigned to a group and each of them is connected to a specified AWG output port through a distribution fiber, as shown in Fig. 1. Here, we assume that the two fibers from the RN to and are diversely run. The fiber connection pattern is attributed to a proposed wave- length assignment plan, as illustrated in Fig. 2. Wavebands 1041-1135/$20.00 © 2005 IEEE