Thursday, February 20, 2020

How OTDRs work

Unlike the sources and power meters that measure the loss of the fiber optic cable network directly, the OTDR works indirectly. The source and the meter duplicate the transmitter and receiver of the transmission link of an optical fiber, so the measurement correlates correctly with the actual loss of the system.

The OTDR, however, uses backscattered fiber light to detect losses; It works as a RADAR (radio distance detection and measurement), sending a pulse of high-power laser light through the fiber and looking for return signals of the backscattered light on the fiber or the reflected light of the connector or splice joints. The amount of backscattered light is very small, so the OTDR sends many pulses and averages for results.

At any time, the light that the OTDR sees is the scattered light of the pulse that passes through a region of the fiber. Only a small amount of light is scattered back to the OTDR, but with wider test pulses, sensitive receivers and average signal, it is possible to make measurements over relatively long distances. Since it is possible to calibrate the pulse velocity as it passes through the fiber, the OTDR can measure time, calculate the pulse position in the fiber and correlate what it sees in backscattered light with a real location in the fiber. Therefore, you can create an instant image of the fiber,
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Since the pulse is attenuated in the fiber as it passes along it and suffers losses in the connectors and splices, the amount of power in the test pulse decreases as it passes along the fiber in the network of cables under test. Therefore, the portion of the light that is backscattered will be reduced accordingly, which will produce an image of the actual loss that occurs in the fiber. Some calculations are necessary to convert this information to display it on the screen, since the process occurs twice, once when it leaves the OTDR and once in the return path of the dispersion in the test pulse.

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