This paper introduces a new embedded nonintrusive fiber-monitoring technique for time-division-multiplexing optical networks. It allows an optical transmitter to characterize the fiber plant from reflections caused by data bursts transmitted across the network instead of dedicated test signals. The probing is performed with minimal burden on data traffic so that many measurements can be averaged to improve accuracy. The method is very suitable for embedded optical time-domain reflectometers (OTDR), which reuse a network node's optical data transmitter for OTDR excitations and embed a reflectometer inside the fiber endpoint. This paper models the OTDR with Laplace transforms, an approach previously unpursued, after which it is explained how reflections from multiple data bursts with arbitrary width can be converted into one normalized format. This new class of OTDR excites the fiber with a negative step of light instead of the conventional short pulse. The signal-to-noise ratio (SNR) for backscatter and Fresnel reflections caused by the negative step and pulse are compared theoretically. It is shown that negative-step OTDR breaks the tradeoff between excitation pulsewidth and distance resolution, has a natural separation between fiber backscatter and Fresnel reflectors, and improves the SNR of nonreflective events.
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