Measurement of current and voltage in high voltage systems employing optical fibre sensors is becoming increasingly popular as the problem of insulation is practically eliminated. Vibrations, physical and acoustic, perturb the polarisation-preserving sensing fibre and cause added noise much in excess of the signal. The present paper discusses a method for reconstructing such a signal. The method, namely, signal-averaging digital phase sensitive detection (DPSD) recovers the signal of any arbitrary wave shape buried in noise. The only requirements are that the signal should be periodic and a local reference should be made available, conditions easily met in HV systems. The efficacy of the scheme is demonstrated by experimental results on a fibre optic current sensor with non-sinusoidal current. © 1997 Taylor & Francis Group, LLC.