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10-Gb/s data transmission over optical fiber is limited in reach due to optical dispersion phenomena that results in inter-symbol interference. Electronic dispersion compensators employing linear and nonlinear equalization techniques are a compact, cost-effective, and adaptive alternative to optical dispersion compensating techniques and devices that are often bulky, expensive, and not adaptive. The decision-feedback equalizer (DFE) is a type of nonlinear equalizer that can effectively compensate for the nonlinear dispersion effects exhibited by fiber channels. However, at multigigabit/second data rates, the implementation of the DFE is challenged by the first feedback-loop latency requirements and process technology speed limitations. This study demonstrates a novel analog approach and circuit architecture to perform decision-feedback equalization at multigigabit/second data rates. The analog decision-feedback equalizer (ADFE) consists of a four-tap linear analog feed-forward filter and a one-tap nonlinear analog feedback filter and is implemented in a 0.18-m CMOS process. The ADFE is completely differential and a broadband single-differential converter is designed for the ADFE front-end to interface with the single-ended output of a photodetector. The circuit is unclocked and uses current-mode logic techniques. The ADFE is used to extend the transmission distance of multimode fiber at 10-Gb/s to 300 m and of single-mode fiber to 120 km.

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