IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 11, JUNE 1, 2007 837 Carrier-Reuse WDM-PON Using a Shared Delay Interferometer for Separating Carriers and Subcarriers Zhaowen Xu, Yang Jing Wen, Senior Member, IEEE, Wen-De Zhong, Senior Member, IEEE, Manik Attygalle, Member, IEEE, Xiao Fei Cheng, Yixin Wang, and Chao Lu, Member, IEEE Abstract—We propose a novel optical carrier-reuse scheme for the wavelength-division-multiplexed passive optical network. The scheme is based on a single delay interferometer (DI) at the remote node and reflective semiconductor optical amplifiers (RSOAs) at optical network units as the colorless devices for upstream transmission. The downstream subcarrier signals are optically separated by the DI for baseband detection of the downstream data and the separated optical carriers are used for seeding the RSOAs. Eight-channel upstream and downstream transmission is demonstrated at 1.25 Gb/s using the proposed scheme. The impact of optical carrier-to-subcarrier ratio of downlink signal and seeding light power on performance is also investigated. Index Terms—Carrier reuse, passive optical network (PON), reflective semiconductor optical amplifier (RSOA), wavelength-division multiplexing (WDM). I. INTRODUCTION T HE wavelength-division-multiplexed passive optical net- work (WDM-PON) is very promising for access networks due to its potential of large capacity, network security, and upgradability [1], [2]. However, so far, the WDM-PON is still considered a next-generation solution because of relatively high cost. A low-cost light source, especially the upstream light source at optical network units (ONUs), is the key component for practical implementation. Among the uplink light sources considered so far, the use of centralized light sources appears most promising. In such a scheme, seeding lights from the central office (CO) are sent to ONUs and then injected into low-cost colorless uplink light sources such as Fabry–Pérot laser diodes or reflective semiconductor optical amplifiers (RSOAs) to achieve single wavelength operation [3]–[6]. How- ever, the scheme still requires two wavelengths arrangement for downlink and uplink, respectively, and costly seeding sources. Recently, a subcarrier transmission technique was introduced in WDM-PONs, where the downlink data is carried Manuscript received October 25, 2006; revised March 20, 2007. Z. Xu is with the Institute for Infocomm Research I R,A STAR, Singa- pore 119613, Singapore. He is also with the Nanyang Technological University, Singapore. Y. J. Wen, X. F. Cheng, and Y. Wang are with the Institute for Infocomm Research I R ,A STAR, Singapore 119613, Singapore (e-mail: yjwen@i2r.a- star.edu.sg). W.-D Zhong is with Nanyang Technological University, Singapore 639798, Singapore. M. Attygalle is with the University of Melbourne, Melbourne, Victoria 3010, Australia. C. Lu is with the Hong Kong Polytechnic University, Hong Kong, Kowloon, Hong Kong. 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.2007.897298 Fig. 1. Proposed scheme for carrier-reuse WDM-PON. on subcarriers of the downstream wavelength. At each ONU, the subcarriers are filtered out for baseband detection, while the optical carrier is remodulated by an external modulator for upstream transmission [7]. This letter proposes a new cost-effective scheme for carrier- reuse upstream transmission in WDM-PON based on a single delay interferometer (DI) and RSOAs. Rather than using a DI to block the optical carrier at each ONU [7], a shared DI and two multiplexers located at the remote node (RN) separate/com- bine all the downlink/uplink signals [8]. At each ONU, the op- tical carrier is remodulated and amplified by an RSOA as an upstream light source and sent back to the CO with other uplink channels via the same DI and feeder fiber. The signal amplifi- cation in RSOAs also improves the power budget of the uplink system. Eight-channel upstream and downstream transmission is demonstrated experimentally at 1.25 Gb/s using the proposed scheme. II. EXPERIMENTAL SETUP AND RESULTS Fig. 1 shows the proposed optical carrier-reuse architecture for WDM-PON. At the CO, the independent downlink data streams are modulated in amplitude-shift keying format by mixing the downlink data with the subcarrier with a frequency of . After multiplexing (MUX1), all WDM channels are transmitted over the feeder fiber via an optical circulator. At the RN, a single DI is introduced to separate the optical carriers and subcarriers. Note that this DI has a free-spectral range of FSR, twice of , and hence it can transmit all-optical carriers to one output port and all subcarriers to the other output port. The separated subcarriers are demultiplexed (MUX3) and fed to ONUs for downlink detection with baseband receivers. After demultiplexing (MUX2), each optical carrier is directly in- jected to an RSOA for upstream data modulation. All the uplink channels are combined again by MUX2 and pass through the same DI and feeder fiber back to the CO. Since the DI is shared by all the downlink and uplink channels and distribution fiber is 1041-1135/$25.00 © 2007 IEEE