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The Optical-Time Domain Reflectometer

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The OTDR is a device that measures distances to a reflection surface by measuring the time it takes for a lightwave pulse to reflect from the surface. Reflection surfaces include the ends of fiber cables, breaks in the fiber, splice locations, and connector locations. The ability to provide these measurements simplifies the fault location procedure for fiber systems.

The OTDR measures the scattered and reflected light returning from a pulse of transmitted light. Pulses of light are generated from the OTDR, and reflected light is displayed on the oscilloscope portion of the OTDR. Knowing the time delay of the reflected light pulse and knowing the speed light travels in the fiber determines the distance to the discontinuity of the fiber. Discontinuities or impedance changes of the fiber cable reflect light back to the OTDR. It is important to know the index of refraction of the fiber core to accurately determine distance. This setting must be correctly set on the OTDR. Keep in mind that power level is displayed on vertical axis, and distance is displayed on the horizontal axis of the screen. Abrupt changes in reflected power can represent fiber “open” conditions. The OTDR will display small losses from couplers and connectors.

The accuracy of the OTDR can be in the order of .01 dB splice seen more than 40 km (25 miles) away to within 1 meter of the problem. Loss performance of new installations is usually recorded on floppy disks and/or hard copy printouts for future
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