Abstract: An optical time domain reflectometry apparatus has a laser and light modulator for producing coherent light pulses, each having two sections of higher intensity separated by a gap of lower or substantially zero intensity. As the light pulses propagate along the optical fibre, light is continuously Rayleigh backscattered by inhomogeneities of the optical fibre. A photodetector generates backscatter signals representing the intensity of light Rayleigh backscattered in the optical fibre as each light pulse travels along the optical fibre. The PC uses these backscatter signals to derive a difference signal representing a change dI in intensity between signals generated from two successive pulses. The PC then calculates the Root Mean Square (RMS) of the difference signal averaged over the interval between the two sections of the light pulses.