Full-field feed-forward equalizer with adaptive system optimization

Abstract

We numerically investigate the combination of full-field detection and feed-forward equalizer (FFE) for adaptive chromatic dispersion compensation up to 2160 km in a 10 Gbit/s on–off keyed optical transmission system. The technique, with respect to earlier reports, incorporates several important implementation modules, including the algorithm for adaptive equalization of the gain imbalance between the two receiver chains, compensation of phase misalignment of the asymmetric Mach–Zehnder interferometer, and simplified implementation of field calculation. We also show that in addition to enabling fast adaptation and simplification of field calculation, full-field FFE exhibits enhanced tolerance to the sampling phase misalignment and reduced sampling rate when compared to the full-field implementation using a dispersive transmission line.

Introduction

Recently, the advance of high-speed microelectronics, such as 30 Gsamples/s analogue to digital converter (ADC) [1], has enabled the application of electronic dispersion compensation (EDC) in optical communication systems at 10 Gbit/s [2], [3], [4]. Compared to its optical counterpart, EDC is more flexible, easier to integrate into transmitters and receivers, and is expected to be more cost effective. Receiver-side EDC, which can adapt quickly to changes in link conditions, is of particular value for future transparent optical networks with many reconfigurable add- and drop-nodes. However, the performance of conventional EDC using direct detection (DD EDC) is limited due to loss of the phase information [2], [3]. EDC based on coherent detection achieves optimum performance theoretically, but requires additional tight-specification components such as narrow-linewidth local oscillator, and signal polarization and phase recovery [4], [5].

In contrast, direct-detection based full-field reconstruction, by extracting the intensity and instantaneous frequency information simultaneously using a single asymmetric Mach–Zehnder interferometer (AMZI) [6], [7], [8] or two AMZIs [9], [10], is a cost-effective approach. The recovered information allows for full-field chromatic dispersion (CD) compensation using electrical-domain signal processing. Full-field maximum likelihood sequence estimation [6] is optimal in compensation performance, but has exponentially increasing complexity with fiber length (or channel memory length), hindering its application to long distance transmission systems. Full-field EDC using a simple dispersive transmission line can enable 2000 km transmission [7]. However, it is based on static CD compensation. Furthermore, as will be shown in this paper, this method cannot mitigate other impairments such as sampling phase misalignment and its performance is sensitive to calculation inaccuracy in full-field reconstruction. Recently, we experimentally demonstrated the use of an adaptive feed-forward equalizer (FFE) to recover a 10 Gbit/s on–off keyed (OOK) signal after transmission over 500 km of field-installed single mode fiber (SMF) [8]. Whilst its feasibility was verified, this method has potential for longer distance transmission because the complexity is linearly scaled with distance. In this paper, we numerically investigate the performance limit of full-field FFE for adaptive dispersion compensation up to 2160 km. The capability of this technique to compensate inter-symbol interference (ISI) regardless of the impairment sources allows the simplification of field calculation structure and enhances the robustness to system parameters. We will show that full-field FFE exhibits greatly improved tolerance to the sampling phase misalignment and reduced sampling rate when compared to a full-field implementation using a fixed dispersive transmission line. Automatic controls of several EDC parameters are also enabled by the technique to further increase the potential for adaptive CD compensation.