Long-haul Fiber-optic Systems Research Articles

On Numerical Simulations of Ultra-Wideband Long-Haul Optical Communication Systems

This paper discusses the key issues in reliable modeling and numerical simulation of ultra-wideband (UWB) long-haul fiber-optic systems, such as those spanning the C+L band. We analyze the complexity of the split-step Fourier method (SSFM), including modern scenarios based on graphical processing units (GPU), and its numerical setup for accurate simulations. We explain why using the Manakov equation in the nonlinear step of the SSFM remains a good solution also in the UWB scenario, even though the underlying assumptions may be violated. We next propose a technique, called virtual channel grouping, to speed up SSFM simulations in the UWB regime. Importance sampling-based techniques are proposed to speed up the computation of the enhanced Gaussian noise (EGN) model, with computational times of the order of seconds. With such novel solutions, accurate simulations are feasible even for UWB systems.

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

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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A narrow bandpass microstrip filter for high-speed fiber optic systems

Transmission systems employing passively retimed regenerators require a bandpass timing filter to extract the timing wave from the data stream. Typically, filters with Q exceeding 100 are necessary in long-haul fiber-optic systems. The fabrication of a microstrip filter with a loaded Q of over 500 and an insertion loss of 8 dB, is reported. The midband return loss in 4.8 dB. The filter is fabricated on a 1-in*1-in*1-in Ba/sub 2/Ti/sub 9/O/sub 20/ substrate. This material has a dielectric constant of about 39 and can be formulated with temperature coefficients of resonant frequency from -2 to +6 p.p.m./ degrees C. The fabricated device has a temperature coefficient of +2 p.p.m./ degrees C. This combination of material properties facilitates high Q and small filter size and has the potential to compensate for temperature-induced electronic phase shifts between signal and clock recovery paths to the bit-decision circuit. >