Detecting a disturbance in the phase of light propagating in an optical waveguide

A partially coherent Optical Time Domain Reflectometry (OTDR) apparatus has a light source comprising a directly modulated semiconductor Distributed FeedBack (DFB) laser diode for transmitting partially coherent light pulses along a monomode optical fibre. Light Rayleigh backscattered from the light pulses as they travel along the optical fibre is output from the end of the fibre into which the light pulses are transmitted to a Fibre Bragg Grating (FBG) filter. The FBG filter reduces the supectral width of light received at a photodetector. In one embodiment, the supectral width of the FBG filter is around one fifth of the supectral width of the light pulse after it has travelled around 1 km along the optical fibre. As a consequence of reducing the supectral width of the light received at the photodetector, the FBG filter increases the temporal coherence of the light. So, the FBG filter can ensure that the detected light is sufficiently coherent that a temporal supeckle pattern can be detected at the photodetector. At the same time, the light traveling in the optical fibre can be relatively supectrally broad so that non-linear effects in the optical fibre, such as Brillouin scattering, can be reduced.