Distortion of an optical signal transmitted over a fibre optic link is characterized by a receiver including a photodetector for converting optical radiation into an electrical signal. A delay line delays the electrical signal. At least two delay line taps drive a register that stores plural values corresponding with different delayed replicas of the electrical signal. Signals commensurate with the values are externally provided to a signal distortion analyser for analysis of the values to determine the optical signal distortion.