We review recent advancements in the digital longitudinal monitoring (DLM) of fiber-optic links. DLM visualizes physical link parameters distributed along the entire length of the link at a coherent receiver, such as longitudinal optical power profiles, locations of loss anomalies, and amplifier gains. This monitoring technique facilitates optical path provisioning and fault detection without the need for dedicated hardware devices such as OTDR. DLM is formulated as an inverse problem of the Nonlinear Schrödinger Equation (NLSE), where distributed coefficients in the NLSE are reconstructed from the boundary conditions, namely transmitted and received signals. We will present DLM from its theoretical aspect to practical applications in optical networks.
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