Abstract: A method is disclosed for detecting changes in an interval of time (?T) between an optical or electrical signal and an optical or electrical reference signal using a photodetector. The method may be used to synchronize an optical or electrical signal with an optical or electrical reference signal. An apparatus for carrying out the method is also disclosed. The method comprises the steps of: receiving the optical signal and the optical reference signal by means of the photodetector, outputting an electrical response signal at an output of the photodetector, the electrical response signal having a frequency spectrum which depends on the interval of time (?T), filtering a selected harmonic from the frequency spectrum of the electrical response signal which has been output, and detecting changes in the interval of time (?T) from changes in the amplitude of the selected harmonic.