Duobinary signaling is combined with a proposed electrical pre-equalization scheme to extend the reach of 10-Gb/s signals that are transmitted over standard single-mode fiber. The proposed scheme is based on predistorting the duobinary signal using two T/2-spaced finite-impulese response (FIR) filters. The outputs of the FIR filters then modulate two optical carriers that are in phase quadrature. Simulation results show that distances in excess of 400 km at bit-error rates less than 10^-15 are possible. Incorporating a forward-error correction scheme can extend the reach to distances in excess of 800 km. The reach limitation arises not from chromatic dispersion but from fiber nonlinearity, relative intensity noise due to phase-modulation-to-amplitude-modulation noise conversion, and optical amplifier noise accumulation. To demonstrate the feasibility of implementing the proposed scheme, a test chip is implemented in a 0.5-µm SiGe BiCMOS technology. The chip incorporates two 10-tap T/2-spaced FIR filters, which are sufficient to equalize a 10-Gb/s duobinary signal that is transmitted over distances in excess of 400 km. The pre-equalization capabilities of the chip are tested by postprocessing the measured chip output to mimic the effects of the optical channel.
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