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Fading reduction in a phase optical time-domain reflectometer with multimode sensitive fiber

In the present paper we propose a novel type of a coherent phase-sensitive optical time-domain reflectometer (OTDR) that utilizes a multimode optical fiber as a sensitive element and is capable of considerable reduction of signal fading. Elimination of OTDR signal fading consequently removes randomly occurring insensitivity of the fiber regions to an external phase action. The backscattered light field at the input of OTDR sensitive multimode optical fiber is represented by a speckle-like pattern, due to a so called modal noise phenomenon. This speckle pattern randomly changes when an optical probe pulse propagates in the fiber line. The backscattered field intensity in every single speckle changes in time statistically independently from the intensity change in every other speckle remote enough from the first one. Thus, on the output of a multimode sensitive fiber, there exist several statistically independent reflectograms, and every single reflectogram contains the same information about external action. The joint independent analysis of these reflectograms can result in reduced or complete fading elimination.