Abstract: Apparatus for correcting distortion on an optical transmission link carrying a multiplicity of optical transmission channels, the apparatus comprising: an adjustable optical equalizer, through which a plurality of said channels pass; a field sampler that samples signals passing through said equalizer, such that a plurality of channels passing through the adjustable equalizer are separately sampled; and a controller that receives the samples, determines control parameters for the equalizer therefrom and adjusts the equalizer, responsive to said determined control parameters.