Abstract—Electronic equalizers are more and more applied in optical transmission systems to compensate for static as well as time varying fiber distortions. In this paper we investigate the performance of 4-states and 16-states MLSE equalizers in 10 Gbit/s field trials over buried single mode fibers. Our results show that the MLSE equalizer enables the joint compensation of chromatic dispersion up to 4480 ps/nm and polarization mode dispersion up to 92 ps. This will increase the uncompensated transmission length and enables a more robust and flexible optical link design. Keywords-dispersion compensation; equalization; MLSE; optical networks I. INTRODUCTION Optical network design is more and more influenced by the rapidly advancing electronic signal processing in optical communications. Modern digital optical transmission systems take advantage of i.e. advanced forward error correction codes, coded modulation formats or digital electronic equalizers at the receiver. The main driver for the application of these techniques is to overcome the transmission distance limitations due to chromatic dispersion (CD), polarization mode dispersion (PMD) and nonlinear effects like self-phase modulation (SPM), some of the main sources of inter-symbol interference (ISI) in optical fiber communications today. Electronic equalizers at the receiver side are of particular interest, as they enable the adaptive compensation of these distortion effects. This is not only necessary for the compensation of time varying ISI sources like PMD, but also enables a much easier link design, as the residual CD can be compensated independently of the actual link length. In contrast, the usual inline dispersion compensation based on dispersion compensating fiber (DCF) modules has to be installed and adjusted depending on the link properties like amplifier spacing, total length and wavelength. Thus the adaptive compensation is very attractive from a network carrier’s point of view as it enables to discard the DCF modules and concentrate the link management at the transmitter or receiver stations. Individual WDM channels can then be dropped and added at any OADM node without taking care of residual dispersion or dispersion management. This enables a much more flexible and robust network design. Furthermore electronic equalizers are able to compensate for multiple distortions simultaneously. This is especially important for uncompensated transmission over old fibers with high PMD values or for link operation at different fiber input powers due to varying channel load. Compared to other equalization techniques like feed forward and decision feedback equalizer (FFE/DFE) structures, the maximum likelihood sequence estimator (MLSE) equalizer uses the Viterbi detector [1] and has evolved as the most effective commercially available electronic equalizer today. Although it has the highest complexity it also shows the best performance. In this paper we give an overview of the current MSLE technology performance in direct detection optical transmission systems and its impact on optical link design. The evolution of the MLSE from concepts and simulations to lab experiments and finally to field trials is shown. The main part of the paper is focused on field trials at 10 Gbit/s over buried fibers in the network of Deutsche Telekom AG. It shows the performance of the 4-states and the 16-states MLSE, the latter being the most powerful MLSE realization at these data rates reported so far. II. MLSE TECHNOLOGY IN OPTICAL COMMUNICATIONS A. History Early experiments investigated the MLSE performance in optical systems using a digital storage oscilloscope and post processing on a computer [2]. However, these experiments did not address the implementation issues of a real time MLSE equalizer for 10 Gbit/s data rates. The first physical realization of a 10 Gbit/s MLSE receiver with a 4-states Viterbi detector was reported in [3]. First experiments with this kind of optical receiver were focused on the general performance for chromatic dispersion compensation in non-return-to-zero (NRZ) systems followed by the investigation of other Experimental Investigation of Real Time 10 Gbit/s MLSE Equalizer Using 4-states and 16-states Viterbi Detector Daniel Fritzsche 1 , Dirk Breuer 2 , Lars Schürer 3 , Armin Ehrhardt 3 , Hamdi Oeruen 4 and Christian G. Schäffer 5 1 European Center for Information and Communication Technologies – EICT GmbH, Berlin, Germany 2 Deutsche Telekom AG, Laboratories, Berlin, Germany 3 Deutsche Telekom Netzproduktion GmbH, Zentrum TE, Berlin, Germany 4 CoreOptics Inc., Nuremberg, Germany 5 University of the Federal Armed Forces Hamburg, Germany Email: daniel.fritzsche@eict.de This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings. 978-1-4244-4148-8/09/$25.00 ©2009