A DAS is an advanced variant of an Optical Time Domain Reflectometer (OTDR) that monitors the coherent Rayleigh backscatter noise signature in a fiber optic cable as pulsed light is sent into the fiber. The coherent Rayleigh noise generates a fine structure in the backscatter signature of the fiber cable. The DAS focuses on the Raleigh component to increase its prominence in the backscatter trace.
DAS is optimized to measure small changes in the coherent Rayleigh noise structure that occurs from pulse to pulse. Since the coherent Rayleigh noise structure is generated interferometrically within the fiber by the relative locations and strengths of local scattering centers intrinsic to the structure of the glass, very small physical (acoustic or vibration) disturbances at a point in the fiber can make detectable changes in the interferometric signal.
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