Editing Optical time-domain reflectometer (section)

{{Short description|Optoelectronic instrument}}
{{redirect-distinguish-for|OTDR|On-Train Data Recorder|the Z80 CPU instruction|Zilog Z80}}
[[File:OTDR - Yokogawa AQ7270 - 1.jpg|thumb|An OTDR]]

An '''optical time-domain reflectometer''' ('''OTDR''') is an [[optoelectronic]] instrument used to characterize an [[optical fiber]]. 
It is the optical equivalent of an electronic [[time domain reflectometer]] which measures the [[Electrical impedance|impedance]] of the [[electrical cable|cable]] or [[transmission line]] under test. 
An OTDR injects a series of optical pulses into the fiber under test and extracts, from the same end of the fiber, [[light]] that is scattered ([[Rayleigh scattering|Rayleigh backscatter]]) or reflected back from points along the fiber. The scattered or reflected light that is gathered back is used to characterize the optical fiber. The strength of the return pulses is measured and integrated as a function of [[time]], and plotted as a function of length of the fiber.