Next-generation long-haul, fiber-optic communications systems are being designed to operate at 100 gigabits per second over distances of 1,000 kilometers or more. Coherent optical receivers have been proposed as an alternative to conventional direct detection receivers for high-speed, fiber-optic systems because, among other reasons, they recover the phase of optical electric fields. When in-phase (I) and quadrature (Q) components of an optical signal are known, exact equalization of linear channel impairments is possible in principle and the effects of nonlinear impairments may be reduced.
Frequency-domain adaptive equalizers provide optimal linear channel compensation. The frequency taps of such an equalizer may be updated according to feedback from a slicer that makes symbol identification decisions. The difference between the slicer's output and input is used as an error signal to adjust equalizer taps. In quasi steady-state operation, an adaptive equalizer can run indefinitely with its taps being adjusted by small amounts to compensate for slowly changing channel conditions.
Starting an adaptive equalizer “blind” (i.e. with no channel knowledge), however, is problematic. The equalizer may be slow to converge to an optimal compensation estimate or it may not converge at all. It can get hung up on singularities. Therefore what is needed is a coherent optical receiver that has an adaptive equalizer initialization system. Such a system should allow a blind, adaptive equalizer to converge rapidly so that a high-speed fiber-optic link can be started or re-started in just a few milliseconds.