The potential for electrical signal processing to mitigate the effect of intersymbol interference in long-haul fiber-optic systems is discussed. Intersymbol interference can severely degrade performance and consequently limit both the maximum distance and data rate of the system. Several techniques for reducing intersymbol interference in single-mode fiber systems with single-frequency lasers are presented, and those techniques which are appropriate at high data rates in direct coherent detection systems are identified. The performances of linear equalization (tapped delay lines), nonlinear cancellation (variable threshold detection), maximum-likelihood detection, coding, and multilevel signaling are analyzed. The results for a simulated binary 8-Gb/s system show that simple techniques can be used to reduce intersymbol interference substantially, thereby increasing the system margin by several decibels. A six-tap linear equalizer increases the dispersion-limited distance (due to chromatic or polarization dispersion) by 20% (or reduces the optical power penalty by as much as a factor of two) in direct detection systems, even when the distortion is nonlinear. A nonlinear cancellation technique (adjusting the decision threshold in the detector based on previously detected bits) can more than double the dispersion-limited distance and/or data rate. >

Electrical signal processing techniques in long-haul fiber-optic systems

A comprehensive treatment of digital backward propagation (DBP) accounting for the vectorial nature of optical transmission is presented. Experimental results show that self-phase and cross-phase modulation are the major sources of nonlinear impairments, even for small channel spacings and for transmission in low dispersion fibers. It is verified that compensating only the incoherent nonlinear impairments not only has the advantage of requiring lower computational load but also removes the necessity of using phase-locked carriers for the signal or phase-locked local oscillators. Simulation results show that polarization-mode dispersion has to be taken into account for practical wavelength division multiplexing systems for DBP to work properly. It is found that to compensate interchannel nonlinear impairments, the changes in the polarization states of channels have to be followed at every span.

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Nonlinear Impairment Compensation for Polarization-Division Multiplexed WDM Transmission Using Digital Backward Propagation

The major impairments in lightwave systems and the signal-processing techniques that can be used to reduce these impairments are discussed. Guidelines for the use of these techniques are presented. These guidelines are used to determine the techniques with the most potential for providing improved performance in long-haul systems, local-area networks, and local loop systems. Although many of the techniques have been previously considered for these applications, few have been implemented in commercial products. Some of the most promising techniques include dispersion-compensating fibers, polarization control, nonlinear cancellation (NLC), and coding for reducing the effect of distortion in long-haul single-frequency-laser direct-detection systems. >

Reducing the effects of transmission impairments in digital fiber optic systems

The performance of a fractionally spaced analog tapped delay line equalizer that has the advantages of being adaptive, being capable of equalizing other linear distortions such as polarization dispersion and nonideal receiver response, and eliminating chromatic dispersion over any distance if a sufficient number of taps are used is discussed. It is shown how this equalizer can be implemented at intermediate frequency and at baseband (for homodyne detection). Results show that an N-tap equalizer increases the maximum bit rate distance approximately (N-1)/twofold (e.g. a threefold increase in distance with a seven-tap equalizer). >

Equalization in coherent lightwave systems using a fractionally spaced equalizer

Chromatic dispersion compensation techniques in coherent transmission systems are reviewed and discussed for potential feasibility. The key compensation device is the wideband delay equalizer. It is shown that stripline-type delay equalizers have the potential for compensating distortion up to 10 Gb/s using a conventional 1.3- mu m zero-dispersion single-mode fiber at 1.5 mu m. Chromatic dispersion is successfully compensated with a stripline delay for CPFSK transmission at 4 and 6 Gb/s over 200 km of 1.3- mu m zero-dispersion single-mode fiber at 1.55 mu m. The bandwidth requirement of the compensation techniques for heterodyne detection is more than 10 GHz. However, it is difficult to realize such broadband receivers. Therefore, phase diversity detection with dispersion compensation is a promising scheme. >

Chromatic dispersion compensation in coherent optical communications

Compensation of fibre chromatic dispersion in coherent optical fibre transmission is demonstrated. The chromatic dispersion of a 70km single-mode fibre with 1.3μm zero dispersion wavelength is compensated for using a microstrip line equaliser in the intermediate frequency band. Amplitude distortion due to fibre chromatic dispersion at 1.55μm wavelength is reduced to below 2.5% with the equaliser.

K. Iwashita

Papers

Compensation of fibre chromatic dispersion in optical heterodyne detection