How to Test Fiber Optic Cables by OTDR

OTDR uses backscattered light of the fiber to imply loss, which is an indirect measurement of the fiber. OTDR works by sending a high power laser light source pulse down the fiber and looking for return signals from backscattered light in the fiber itself or reflected light from connectors or splice interface. OTDR testing requires a launch cable for the instrument to settle down after reflections from the high powered test pulse overloads the instrument. OTDRs can either use one launch cable or a launch cable with a receive cable, the tester result of each is also different.

Test With Launch Cable Only
A long lauch cable allows the OTDR to settle down after the initial pulse and provides a reference cable for testing the first connector on the cable. When testing with an OTDR using only the launch cable, the trace will show the launch cable, the connection to the cable under test with a peak from the reflectance from the connection, the under testing cable and likely a reflection from the far end if it is terminated or cleaved. Most terminations will show reflectance that helps identify the ends of the cable.

By this method, it can not test the connector on the far end of the under testing cable since it is not connected to another connector, and connection to a reference connector is necessary to make a connection loss measurement.

Test With Launch And Receive Cable
By placing a receive cable at the far end of the under testing cable, the OTDR can measure the loss of all factors along the cable plant no matter the connector, the fiber of cables, and other connections or splices in the cable under test. Most OTDRs have a least squares test method that can substract out the cable included in the measurement of every single connector, but keep in mind, this may not workable when the tested cable is with two end.

During the process you should always keep in mind to start with the OTDR set for the shortest pulse width for best resolution and a range at least twice the length of the cable you are testing. Make an initial trace and see how you need to change the parameters to get better results.

OTDRs can used to detect almost any problems in the cable plant caused during the installation. If the fiber of the cable is broken, or if any excessive stress is placed on the cable, it will show up the end of the fire much shorter than the cable or a high loss splice at the problem locations.

Except OTDR testing, the source and optical power meter method is another measurement which will test the loss of the fiber optic cable plant directly, The source and meter duplicate the transmitter and receiver of the fiber optic transmission link, so the measurement correlates well with actual system loss.

Related Articles - optical time-domain reflectometer, laser light source, optical power meter,

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