Electronic distortion compensation in the mitigation of optical transmission impairments: the view of joint project on mitigation of optical transmission impairments by electronic means ePhoton/ONe1 project

The key design approaches and results in the field of compensation of optical impairment distortion by electronic means, as an outcome of the studies and research innovations developed within the joint project on mitigation of optical transmission impairments by electronic means work group of the ePhoton/ONet European project are presented. The research topics addressed are related to chromatic and polarisation mode dispersion, with particular reference to feed-forward/decision-feedback equaliser and maximum-likelihood sequence estimation-based equalisers as well as pre-distortion schemes. Additionally, the use of electronic compensation in metro/access applications is examined with reference to studies related to the performance enhancement of directly modulated laser transmitters, the compensation of the square-law characteristics of receivers and the equalisation of multi-level format schemes.     

Waveband converters based on four-wave mixing in SOAs

Four wave mixing (FWM) is distinguished from other wavelength conversion techniques by its ability to simultaneously convert a number of input wavelength channels. In this case, optical signal-to-noise ratio (OSNR) is insufficient to describe the performance of the device as many effects are involved. A multiwavelength FWM model is used here to simulate a waveband converter (WBC). The numerical model predicts the waveform of the FWM product. Based on that output, the Q factor of the signal and the power penalty induced to the signal can be calculated to evaluate the performance of such a device. Meanwhile, an analytical model is used for the calculation of the signal power levels and the standard deviation of the fluctuation; hence, it describes the constituent effects-namely, the extinction ratio (ER) degradation, the OSNR degradation, the gain modulation (GM) related crosstalk, and interference. The model's validity is tested against the numerical results. To the best of the authors' knowledge, this is the first time that a numerical model and an analytical model are used to systematically investigate a WBC and to identify the specific effects and derive the design rules. These rules are tested in the experiment. Finally, a tunable WBC (TWBC) based on the dual-pump configuration is described and implemented experimentally.     

OPSnet: design and demonstration of an asynchronous high-speed optical packet switch

This paper presents the results of the optical packet switched network (OPSnet) project, which investigated the design of an asynchronous optical packet switch suitable for the core of an optical transport network (OTN). The requirements for the switch were to control and route variable-length packets transmitted at bit rates beyond 100 Gbit/s. The subsystems and techniques used are analyzed and presented. Fast header encoding and passive decoding is based on the differential phase-shift keying (DPSK) method. The dual-pump four-wave mixing (d-p FWM) wavelength-conversion technique, in combination with an arrayed waveguide grating (AWG), is utilized for packet switching. An advanced and fully controllable mechanism for the packet-switch control is presented, which is implemented on field programmable gate array (FPGA) technology. The control wavelength is generated using a tunable laser, the actual wavelength and new header values are provided utilizing fast header recognition and look-up tables. The integration of the subsystems is discussed, and the results of a four-output port asynchronous packet-switch demonstrator operating at 40 Gbit/s are presented. Finally, the switch limitations are examined and design issues are discussed.     

Electronic distortion equalisation by using decision-feedback/feed-forward equaliser for transient and adiabatic chirped directly modulated lasers at 2.5 and 10 Gb/s

A thorough study on the beneficial use of electronic distortion equalisation (EDE) in the transmission performance enhancement of conventional low-cost directly modulated lasers (DMLs) is provided. The studies focus first on the compensation of the chromatic dispersion effect and then on the compensation of cascaded filtering effects of DML generated transmission signals in a system, by means of the feed-forward and decision-feedback equalisers (FFE/DFE). The reported studies consider DMLs with different chirp characteristics operating at 2.5 and 10 Gb/s, using parameters that have been extracted from real lasers, previously verified by simulations and experiments. The performance efficiency of EDE is evaluated in terms of eye-opening penalty degradation associated with the signal transmission over a standard single mode fibre. Extensive simulations reveal significant performance improvement for all the examined cases by using the FFE/DFE. Furthermore, the results show that the equalisation efficiency strongly depends on the chirp characteristics of DMLs.     

Concatenation performance of optical packet switches that incorporate wavelength converters based on four-wave mixing in semiconductor optical amplifiers

Four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) has been acknowledged as a useful wavelength conversion technique especially due to its ability to convert sequences of ultra fast pulses but also simultaneously a number of wavelengths. An analysis is used to investigate the concatenation performance of optical packet switch that incorporate wavelength or waveband converters based on FWM in SOAs.     

Dynamic behavior of wavelength converters based on FWM in SOAs

As wavelength converters based on four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) attract more attention, dynamic effects and wavelength dependent performance become key aspects to be investigated. Such issues are particularly important, as complex configurations are likely to be used to overcome challenges like tunability and polarization dependence. In this paper a numerical model is used to predict the dynamic performance of three FWM configurations and an analytical model is used to derive design rules. First, the wavelength dependent behavior of a wavelength converter is investigated and the requirement for a widely tunable converter is identified. Secondly, a configuration for extinction ratio (ER) improvement is studied and novel design rules are obtained analytically, tested experimentally and explained by the numerical model; experimental results with ER improvement at 10 Gb/s were achieved for the first time. The third configuration studied is a dual-pump arrangement enabling wide tunability. Fixed input/tunable output and tunable input/fixed output configurations are discussed in terms of optical signal-to-noise ratio and tunability. Design rules are extracted and verified for all three configurations that are likely to be deployed: simple wavelength converters, regenerating converters and tunable wavelength converters.     

Cascadability Performance Evaluation of a New NRZ SOA-MZI Wavelength Converter

We evaluate the cascadability performance of a new semiconductor optical amplifier (SOA) Mach-Zehnder interferometer-based nonreturn-to-zero wavelength converter in a loop experiment. We use the bidirectional data injection control scheme with an additional continuous-wave signal to optimize the gains and phases imparted by the SOAs. The scheme has been shown to be capable of eight cascaded, error-free wavelength conversions at 10 Gb/s.     

Cascaded Operation of a 2R Burst-Mode Regenerator for Optical Burst Switching Network Transmission

We demonstrate the cascaded operation of an all-optical 2R burst-mode regenerator consisting of a single Mach-Zehnder interferometer, using a loop configuration that introduces a 6-dB power variation on each recirculation of the data. The device is shown to regenerate error-free 10-Gb/s data streams for up to six successive loop recirculations.     

Demonstration of an agile hybrid IP-optical packet construction mechanism in wavelength routed optical packet switched networks

This paper presents a novel architecture for an ingress edge OPS router and demonstrates a mechanism that maps Internet traffic onto optical packets. The architecture utilises a high-speed reconfigurable hardware platform and a fast tunable laser and also supports the user network interface (UNI) functionality by fully interacting with the physical and higher network layers. The main design issues, including wavelength agility, traffic aggregation based on the class of service and variable length optical packet construction and transmission are also discussed.     

A Low-Cost Inband FEC Scheme for SONET/SDH Optical Metro Networks

A novel inband forward-error correction (FEC) method, devised for the electronic-mitigation of physical optical transmission impairments, is implemented and experimentally demonstrated. The proposed inband-FEC scheme, Free-Of-Charge Uniform Shield (FOCUS), is an inherently low-cost approach, applicable to standard synchronous digital hierarchy/synchronous optical network rates, allowing optically transparent signal propagation over longer distances. The performance/efficiency of the FOCUS method is evaluated in terms of tolerance against optical signal-to-noise ratio degradation and chromatic-dispersion, associated with the signal transmission over standard single-mode fiber. Bit-error-rate measurements and coding gain calculations at the standard STM-64 rate (9.953 Gb/s) reveal significant coding-gain improvement for all cases examined