JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 20, OCTOBER 15, 2012 3211 Ring-Based WDM Access Network Providing Both Rayleigh Backscattering Noise Mitigation and Fiber-Fault Protection C. H. Yeh, Member, IEEE, C. W. Chow, Member, IEEE, S. P. Huang, J. Y. Sung, Y. L. Liu, and C. L. Pan, Fellow, IEEE, OSA Abstract—In this paper, we propose and investigate a ring-based wavelength-division-multiplexing passive optical network (WDM-PON) providing both Rayleigh backscattering (RB) noise mitigation and fiber-fault protection. The proposed ring-based WDM-PON has a dual-ring architecture to protect and restore any fiber fault. Besides, the ring architecture also can mitigate the RB beat noise, since the RB and the upstream wavelengths are propagating in opposite directions. Here, we discuss and analyze the characteristics of the downstream signals using 10 Gb/s ON–OFF keying (OOK), 10 Gb/s differential phase-shift keying, and 10 Gb/s orthogonal frequency-division multiplexing (OFDM) modulations, respectively. Moreover, we also investigate the upstream traffic using the 2.5 Gb/s OOK and 10 Gb/s OFDM generated by the directly modulated reflective semiconductor optical amplifier-based optical network unit, respectively. Index Terms—Differential phase-shift keying (DPSK), fiber-fault protection, ON–OFF keying (OOK), orthogonal fre- quency-division multiplexing (OFDM), ring access, wavelength- division-multiplexing passive optical network (WDM-PON). I. INTRODUCTION B ECAUSE of the rapid growth in the demand of broad- band multiservices, passive optical network (PON) is a promising solution for the future fiber access systems [1]–[4]. Furthermore, wavelength-division-multiplexed passive optical networks (WDM-PONs), using directly modulated laser (DML) at the central office (CO) [5], [6] and reflective semiconductor optical amplifier (RSOA) for signal reuse and remodulation at each optical network unit (ONU) [7], [8] are desirable system architectures for the future high-speed and high-capacity PON [9]. However, in colorless WDM-PON system, the Rayleigh backscattering (RB) interferometric beat noises will be gener- ated by the downstream signal and result in impairment of net- Manuscript received June 18, 2012; revised August 01, 2012, August 14, 2012; accepted August 16, 2012. Date of publication August 24, 2012; date of current version October 03, 2012. C. H. Yeh and Y. L. Liu are with the Information and Communications Re- search Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan. C. W. Chow, S. P. Huang, and J. Y. Sung are with the Department of Pho- tonics and the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan (e-mail: cwchow@faculty.nctu.edu.tw). C. L. Pan is with the Department of Physics and the Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 30013, Taiwan. 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.2012.2214374 work performance [10], [11]. Hence, in order to reduce the RB interferometric beat noise, different RB mitigation techniques have been proposed and discussed, such as using the phase and bias-current dithering, utilizing wavelength-shifting technique, employing advanced modulation formats, using double laser bands source, etc. [11]–[15]. However, these proposed methods would increase the complexity and the cost of PON. In addition, these proposed techniques were mainly employed in tree-based WDM-PONs. Furthermore, to enhance the spectral efficiency and reduce the cost of high-speed transmitters (Tx) and receivers (Rx), optical orthogonal frequency-division multiplexing quadrature amplitude modulation (OFDM-QAM) has been proposed and reported [16]–[20]. And this modulation format could be a promising candidate for the future WDM-PONs. Next-generation WDM-PON must provide high quality of service and reliability. When the fiber between the CO and each ONU is broken in WDM-PONs, the data traffic cannot reach the affected ONU, leading to data loss. Hence, the protection man- agement of fiber fault is one of the critical issues in PON access [21]–[24]. Furthermore, the ring-based TDM- and WDM-PON has been actively studied [2], [25]–[28] and is regarded as one of the important architectures for next-generation PON. In this paper, we propose and experimentally demonstrate the ring-based WDM-PON system with the RB noise mitigation and fiber-fault protection. Here, we propose a new design in each RSOA-based ONU to produce the downstream and up- stream traffic propagating in opposite directions. Consequently, advanced modulation techniques are not required for mitigation of the RB noise. In the proposed architecture, dual fiber ring configuration is used to protect the occurrence of fiber fault. Besides, each ONU can select the fiber path automatically to transmit the data traffic. In this paper, the downstream 10 Gb/s differential phase-shift keying (DPSK), ON–OFF keying (OOK) and OFDM signals, and the upstream 2.5 Gb/s OOK and 10 Gb/s OFDM upstream signals by using RSOA-based ONU have been achieved and analyzed, respectively. II. FIBER-FAULT PROTECTION First, a ring-based WDM-PON architecture with fiber-fault protection and RB noise mitigation is proposed, as illustrated in Fig. 1. In the CO, the downstream and CW signals are com- bined via a blue/red-band filter (BRF) and transmitted through a WDM multiplexer and a 1 2 optical coupler and then into each ONU in counterclockwise (CCW) direction. Here, each 0733-8724/$31.00 © 2012 IEEE