2460 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 11, NOVEMBER 2005 Bidirectional WDM-PON Based on Gain-Saturated Reﬂective Semiconductor Optical Ampliﬁers Wooram Lee, Mahn Yong Park, Seung Hyun Cho, Jihyun Lee, Chulyoung Kim, Geon Jeong, and Byoung Whi Kim Abstract—We propose a bidirectional wavelength-division-mul- tiplexed passive optical network by employing gain-saturated reﬂective semiconductor optical ampliﬁers (RSOAs) for wave- length-independent optical network terminals. The fabricated RSOA module has the input saturation power less than 13 dBm and its saturated gain higher than 13 dB within the -band for arbitrary polarization states. The upstream signals are generated by remodulating the downstream signals whose modulation am- plitude is squeezed through gain-saturated RSOAs. We present experimental results on bidirectional transmission with 1.25 Gb/s for upstream and 2.5 Gb/s for downstream data rates over 20-km transmission distance. Index Terms—Reﬂective semiconductor optical ampliﬁer (RSOA), remodulation scheme, wavelength-division-multiplexed (WDM) access network, wavelength-independent optical network terminal (ONT). I. INTRODUCTION T HERE have been substantial research and development ef- forts placed on implementing ﬁber-to-the-home (FTTH) networks to accommodate upcoming demands for various mul- timedia services in the near future. Among the various solu- tions to the FTTH realization, the wavelength-division-multi- plexed passive optical network (WDM-PON) has been consid- ered as an ultimate next-generation FTTH network. For a suc- cessful deployment of the WDM-PON, the optical network ter- minal (ONT) placed in the subscriber premises should be wave- length-independent, in addition to being cost-competitive to ex- isting technologies. To this end, a remodulation scheme, where downstream optical signal is reused for upstream transmission, has been proposed based on broad-band optical gain medium for the ONT such as a Fabry–Pérot laser diode (FP-LD) [1]–[3], a semiconductor optical ampliﬁer (SOA) [4], and a reﬂective SOA (RSOA) [5]. In the case of employing FP-LD, the polarization states of downstream signals need to be adjusted for maximizing gain of FP-LD [3]. In the case of using SOAs, the downstream signals received at ONTs are preampliﬁed by an extra optical ampliﬁer. Then, the SOA operating in gain-saturation regime squeezes the modulated downstream optical signals, whereas the downstream signal injected into the SOA is remodulated with upstream data directly. This structure, however, would be costly due to the use of extra optical ampliﬁer such as an er- bium-doped ﬁber ampliﬁer. To reduce such expensive ONT cost, an RSOA operating in the linear regime has been proposed as a Manuscript received April 29, 2005; revised August 10, 2005. The authors are with the WDM-PON Technology Team, BcN Research Lab- oratory, Electronics and Telecommunications Research Institute, Daejeon 305- 350, Korea (e-mail: wooram@etri.re.kr; myparkgo@etri.re.kr; shc@etri.re.kr; jhlee@etri.re.kr; kcy0027@etri.re.kr; Geon.jeong@etri.re.kr; kbw@etri.re.kr). Digital Object Identiﬁer 10.1109/LPT.2005.858148 Fig. 1. Proposed WDM-PON structure. low-cost ONT [3]. Although an RSOA in the linear regime has the optical gain higher than that in the saturation regime, the al- lowable extinction ratio (ER) in the downstream transmission is limited to be less than around 3 dB because the high downstream ER induces the upstream transmission performance severely de- graded by the uncompressed thick “1-level” [4]. In this letter, we propose a bidirectional WDM-PON struc- ture without a polarization control and an extra optical ampli- ﬁer, which is simpler than the previous ones of the kind. The RSOA employed in the ONT was fabricated to satisfy two con- ditions: gain saturation for sufﬁciently low input power levels, and high saturation gain for the downstream signal to be ampli- ﬁed and reused for the upstream transmission. We present the wavelength-dependent gain characteristics of the RSOA module and the transmission performance for 2.5-Gb/s downstream and for 1.25-Gb/s upstream. II. WDM-PON STRUCTURE AND RESULTS A. Proposed Structure Fig. 1 shows the proposed WDM-PON structure. The downstream signals generated by directly modulating the single-mode lasers (SMLs) in the central ofﬁce (CO) are transmitted to the ONTs through the wavelength multiplexer at CO, the feeder ﬁber and demultiplexer at the remote node. The optical power received at ONT is divided into 75% to the RSOA and 25% to the receiver by an optical coupler in front of the RSOA and receiver. The splitting ratio was carefully determined by taking into account the three conditions: 1) the downstream power level at the receiver should be higher than the receiver sensitivity, 2) the power level injected to the RSOA is within the gain-saturation regime, and 3) the power ratio of upstream signal to Rayleigh backscattered noise is sufﬁciently large. It should be stressed that the RSOA chip gain should also be high enough to satisfy the last condition. 1041-1135/$20.00 © 2005 IEEE