IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 18, SEPTEMBER 15, 2008 1533 10-Gb/s Operation of RSOA for WDM PON K. Y. Cho, Y. Takushima, and Y. C.Chung, Fellow, IEEE Abstract—We report on the 10-Gb/s operation of the reflective semiconductor optical amplifier (RSOA) for the next-generation wavelength-division-multiplexed passive optical network (WDM PON). The bandwidth of the RSOA used in this experiment is merely 2.2 GHz. Nevertheless, a clear eye opening is obtained at 10 Gb/s by using the electronic equalizer processed offline. We investigate the impacts of the network’s operating conditions (such as the injection power to the RSOA and the fiber length) on the performances of these equalizers. The results show that the RSOA-based WDM PON is operable at 10 Gb/s and the maximum reach can be extended to 20 km with the help of the forward error correction codes. Index Terms—Passive optical network (PON), semiconductor optical amplifier (SOA), wavelength-division multiplexing (WDM). I. INTRODUCTION T HE wavelength-division-multiplexed passive optical net- work (WDM PON) has long been considered as an ulti- mate solution for the next-generation broadband access network capable of providing 10-Gb s data to each subscriber. There have been numerous attempts to develop practical WDM PONs by using colorless light sources such as reflective semiconductor optical amplifiers (RSOAs) and ASE-injected Fabry–Perot laser diodes (FP-LDs) [1]–[5]. However, until now, these networks have been implemented to operate at the moderate speed in the range of 155 Mb/s–5 Gb/s due to the limited bandwidths of the directly modulated colorless light sources [3], [5]. A simple method to overcome this limitation and increase the transmis- sion speed to 10 Gb s would be the use of high-speed ex- ternal modulators at the optical network units (ONUs) [6], [7]. However, since this method requires an additional 10-Gb/s mod- ulator at every ONU, it is not realistic for the use in cost-sensi- tive access networks. It has also been proposed to overcome this limitation by using the FP-LD which is injection-locked to the seed light provided from the central office (CO) [8]. While this laser could be directly modulated at 10 Gb/s, the colorless oper- ation was not possible due to its extremely narrow lock-in range 0.02 nm . In this letter, we demonstrate the feasibility of implementing a 10-Gb/s WDM PON by using directly modulated RSOAs to- gether with electronic equalizers. The modulation bandwidth of the RSOA, limited by carrier lifetime, is only about 2 GHz [3], [5]. Thus, it is nearly impossible to modulate such a device at 10 Gb/s. However, unlike semiconductor lasers, we note that the frequency response of the RSOA has a smooth rolloff with no relaxation oscillation peak, while its modulation has a good linearity similar to the laser diode. These properties are almost Manuscript received April 23, 2008; revised June 23, 2008. The authors are with the Department of Electrical Engineering, KAIST, Dae- jeon 305-701, Korea (e-mail: ychung@ee.kaist.ac.kr). 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.2008.928834 Fig. 1. Measured frequency response of RSOA. ideal for the electronic equalization using the decision feedback equalizer (DFE) consisting of feedforward and feedback filters [9], [10]. Thus, despite its extremely limited bandwidth, it is possible to operate the RSOA at 10 Gb/s by using the electronic equalization technique. For a demonstration, we show that the power penalty caused by the limited bandwidth of the RSOA can be suppressed to 2.5 dB by using electronic equalizers. We evaluate the impacts of the optical signal-to-noise ratio (OSNR) and chromatic dispersion (CD) in the RSOA-based WDM PON operating at 10 Gb/s using electronic equalizers. These results show that the maximum reach of this network can be extended to 20 km with the help of the forward error correction (FEC) technique. II. ELECTRONIC EQUALIZATION OF LIMITED MODULATION BANDWIDTH OF RSOA We first evaluated the frequency response of an RSOA packaged in a transistor outline metal-can (TO-CAN) shown in the inset of Fig. 1 [3]. We injected continuouse-wave (CW) seed light (1550 nm) into the RSOA and measured the small signal response by directly modulating its injection current. The optical power incident on the RSOA was -12 dBm. The bias current of the RSOA was 80 mA. Under these conditions, the gain and output power of the RSOA were measured to be 17 and 5 dBm, respectively. As shown in Fig. 1, the 3-dB bandwidth of the RSOA was measured to be only 2.2 GHz. However, the rolloff characteristics were smooth (i.e., there was no peak or zero within the Nyquist bandwidth for 10-Gb/s signals). When we attempted to operate this RSOA at 10 Gb/s, a severe penalty was observed due to the limited bandwidth. For example, Fig. 2(a) shows the measured eye diagram when the RSOA was directly modulated with a 9.95-Gb/s NRZ signal. There was no eye opening at the decision point. However, we could still observe the transient responses and the degradation was not catastrophic. We evaluated the possibility of overcoming this bandwidth limitation of the RSOA by using electronic equalizers. The signal from the RSOA was first filtered by an optical bandpass filter which has Gaussian-shape passband (1 nm) and detected 1041-1135/$25.00 © 2008 IEEE