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Open Access Publications from the University of California

An electronic equalization technique for nonlinear dispersion compensation in optical fiber communications links

  • Author(s): Metzger, Andre G.
  • et al.

Gigabit optical fiber communications channels typically exhibit linear and nonlinear distortion as a result of non -ideal transmitter, fiber, receiver, and optical amplifier components. Transmitted data bits begin to spread and overlap as a result of chromatic and polarization mode dispersion in the fiber, which is exacerbated by the finite spectral width of the modulated optical source. The pulse spreading is pattern-dependent and thus can be characterized as intersymbol interference, with associated jitter influenced by the bit rate, component choices, and link length. Any uncompensated link has a maximum data rate and/or maximum transmission length beyond which irresolvable bit errors will be incurred; compensation is required to further increase data rates and/or extend transmission length. Optical correction methods are effective for chromatic dispersion compensation but are difficult to implement for polarization mode dispersion, which can vary over time and atmospheric conditions. Electronic equalization on the receiver side of the link is an attractive low cost conceptual solution, but is complicated by the nonlinearities introduced by the magnitude-squared response of the optoelectronic receiver. This work describes the development, analysis, and implementation of a technique for purely electronic compensation of both linear and nonlinear distortion mechanisms. The proposed circuit is composed of one or more Analog-to-Digital Converters (ADCs) and a Look Up Table (LUT) memory. Samples of photodetector output signal data taken at various points in time are fed to the LUT which links received vectors to an estimate of the transmitted pattern. The size and complexity of the design can be scaled and its configuration can be optimized for the compensation of a particular fiber channel. A high- speed electronic equalizer, composed of 3 independent and bypassable 2 bit ADC's and a 64 bit LUT, has been demonstrated in this work using InGaP/GaAs HBT technology. The circuit includes a novel memory cell design to implement the LUT. With optimized settings, the equalizer is able to distinguish between transmitted 1's and 0's up to 5GHz with dispersion over 3 bit periods, using select distortion examples for which a standard decision circuit fails. A method to approximate the Bit Error Rate (BER) of the equalized system is applied to analysis of the equalization technique using extensive fiber simulations and a numerical equalizer model. The results indicate that the best results can be obtained when the fiber communications channel is tuned to optimize transmission. In these conditions, system simulations indicate that the system with the equalizer can outperform systems with a standard decision circuit by extending transmission lengths in some cases as much as 60% at 10Gbps

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