There are two methods that are used to measure the loss

There are two methods that are used to measure the loss by insertion with a light source and power meter, a "connection cable test", also called "single-end loss", according to the TIA FOTP-171 standard, and a "cable network test installed" or "loss of two ends", according to the TIA OFSTP-14 (multimode) and OFSTP-7 (single mode) standard. The difference between the two tests is that the single loss test end uses only a launch cable and tests only the connector attached to the launch cable plus the fiber and any other component in the cable. The single-end test is mainly used to test connection cables or short cables since you can test each connector individually.

The two-end loss check uses a launch cable and a receive cable attached to the meter and measures the loss of the connectors at both ends of the cable under test.

The single-end check is generally used in connection cables to be able to test the connectors at each end of an individual short cable, in order to ensure that both are good and to find which connector could be defective in case of any problem. . Double-ended testing is used with a cable network installed to ensure that the cable network has been properly installed and to compare the test results with the estimated optical losses.

The loss of a single end is measured by joining the cable you want to test to the reference launch cable and measuring the power that comes out of the end with the meter. By doing this, you only measure the loss of the connector attached to the launch cable and the loss of any fiber, splice or another connector on the cable you are testing. Since you are pointing the connector at the end of the cable to a detector in the power meter instead of attaching it to another connector, it does not have any loss, so it is not included in the measurement. This method is described in FOTP-171 and is shown in the drawing. An advantage of this test is that you can locate cable problems and find a faulty connector since you can reverse the cable to test the connectors at each end individually. When the loss is high, the defective connector is attached to the reference cable.

In a two-end loss check, you connect the cable under test between two reference wires, one connected to the source and the other to the meter. In this way, you measure the losses of the connectors at each end, plus the loss of the entire cable or all cables, including the connectors and splices, which are in the middle. This is the method specified in OFSTP-14 (multimode, the single-mode test is OFSTP-7), the standard test for loss in an installed cable network.

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Set the reference "0 dB" to prove the loss

In order to measure the loss, it is first necessary to establish a reference launch power for the loss that becomes 0 dB. Properly setting the 0 dB reference power is essential to make good loss measurements.

For single-end tests, the reference power for 0 dB is set to the end of the reference cable. Simply connect the power meter to the end of the cable, measure the output power and, on most meters, set that power as the reference for loss measurements. The meter will then mark the loss of each cable measured directly.

There are three methods to establish the reference for a two-ended test, either using one, two or three reference wires, and the chosen method will affect the measured loss. Why are there three methods? All three methods were developed due to variations in connector styles and the way the test equipment is made.