Extended-Reach 10 Gb/s RSOA-Based WDM-PON using Partial Response Equalization Qi Guo, An V. Tran, Chang-Joon Chae Victoria Research Laboratory, NICTA Ltd., Level 2, Building 193, Department of Electrical and Electronic Engineering, University of Melbourne, email: q.guo@pgrad.unimelb.edu.au Abstract — We propose an extended reach 10 Gb/s WDM- PON system using low-bandwidth RSOA and partial response equalization. The system is demonstrated experimentally and via simulation over 50 km fiber link with good performance. 1. Introduction As the demand for high bandwidth services grows, wavelength-division multiplexing passive optical network (WDM-PON) has attracted a lot of attention as a solution for future broadband access networks thanks to its high capacity and scalability [1-2]. In order to reduce the cost, which is a critical issue in access networks, the implementation of upstream transmission by colorless optical network unit (ONU) is required. The use of reflective semiconductor optical amplifier (RSOA) at the ONU, performing the functions of modulator and amplifier is one of the most attractive solutions. However, the data rate of the upstream signal is limited by the electrical bandwidth of the RSOA, which is around 1~2 GHz. To overcome this problem, electrical equalization techniques such as conventional decision feedback equalization (DFE) and maximum- likelihood sequence estimation (MLSE) are employed and studied [1], [2]. However, the conventional DFE method suffers from limited performance and can not extend the fiber reach beyond 20 km by itself, while the cost and complexity of MLSE increases significantly with its order. In this paper, we propose the use of adaptive partial response equalization (PRE) technique to combat the effect of limited bandwidth of RSOA and chromatic dispersion (CD) of the transmission fiber for the realization of extended reach and high capacity WDM-PON. This advanced electrical equalization technique performs equalization in two steps unlike the conventional equalizers that try to eliminate the intersymbol interference (ISI) completely in one stage [3]. The PRE, at first, recovers the received distorted signal to an appropriate selected partial response signal with controlled ISI. Thus the combined channel for the original channel and the PRE is a perfectly known ISI channel with shorter memory. The ISI is then fully removed in the second step by a binary converter. The PRE technique has much lower computation complexity than MLSE and achieves much better results than conventional DFE and linear equalizer. The proposed WDM-PON system with PRE is demonstrated experimentally and via simulation to achieve good performance over 50 km fiber link. 2. Experimental Setup In this experiment, we only consider the single-direction upstream transmission. Fig. 1 shows the experimental configuration. This system represents the upstream link of an RSOA-based ONU in WDM-PON. At the optical line termination (OLT), a continuous optical signal at 1550 nm, serving as a seeding light source for the RSOA, is generated by a DFB laser and transmitted through a circulator. The CW seeding light is then injected into the RSOA at the ONU. The RSOA is biased with a DC current of 80 mA and modulated with a binary 2 11 -1 pseudo-random bit sequence (PRBS) at 10 Gb/s. The upstream signal is directly modulated through this RSOA, which only has an electrical bandwidth of 1.2 GHz, and launched back into the fiber. After propagating through a variable length of standard single mode fiber (SSMF), the received signal at OLT is sampled by a real time digital oscilloscope and processed offline. Fig. 1. Experimental setup In offline processing, the channel is at first equalized to a target impulse response (TIR). TIR is a selected partial response signal that matches with the output of the receiver. In our experiment, we use a 3 GHz low pass filter in front of the equalizer to suppress the high frequency components of the received signal to make the spectrum closer to the desired multilevel signal. Then we use the adaptive partial-response equalizer at the OLT to recover the distorted signal. The captured signal is equalized by a DFE that performs the function of PRE to match the received signal to a multilevel correlative signal. The DFE is composed of a 17-tap sample spaced feed-forward filter and a 3-tap feed-backward filter. In the second step, the equalized signal with residual ISI is detected and converted into binary signal by a binary converter. This converter is realized by multiple binary slicers and DC gates [4]. 3. Results and Discussion When a binary signal is transmitted through a channel with a bandwidth smaller than the signal bandwidth, the neighbouring transmitted symbols will interact with each other and produce ISI. This ISI results in a multilevel correlative signal or partial response signal at the receiver end. The ISI-induced correlation between symbols has many possibilities, depending on the characteristics of the channel. Many classes of partial response signal have been studied. Our technique is based on [4], where a multilevel correlative sequence is the sum of the present digit of binary sequence b n 345 978-1-4244-5369-6/10/$26.00 ©2010 IEEE