An optical time domain reflectometer (OTDR) is a precision instrument used to locate events or faults along a fiber link, typically within an optical communications network. The OTDR launches a series of high speed optical pulses into the fiber to be measured. Various events on the fiber generates a Rayleigh back scatter that returns to the OTDR and the strength of the return pulses are measured and integrated as a function of time, and plotted as a function of fiber length. The horizontal axis is the distance and the vertical axis is the loss.

OTDRs are mainly used in the optical fiber installation and maintenance servicing of access networks (communications links between telephone exchanges and telephone poles) and user networks (communications links between user sites and telephone poles).

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