Testing Loss

Loss of a cable is the difference between the power coupled into the cable at the transmitter end and what comes out at the receiver end. Testing for loss (also called "insertion loss") requires measuring the optical power lost in a cable (including fiber attenuation, connector loss and splice loss) with a fiber optic light source and power meter (LSPM) or optical loss test set (OLTS.) Loss testing is done at wavelengths appropriate for the fiber and its usage. Generally multimode fiber is tested at 850 nm and optionally at 1300 nm with LED sources. Singlemode fiber is tested at 1310 nm and optionally at 1550 nm with laser sources.

The insertion loss measurement is made by mating the cable being tested to known good reference cables with a calibrated launch power that becomes the "0 dB" loss reference. Why do you need reference cables to measure loss? Why can't you just plug the cable to test into a source and power meter and measure the power? There are several reasons:
1)You need a cable to measure the output power of the source for calibration of "0 dB" loss.
2) In order to measure the loss of the connectors you must mate them to a similar, known good, connector.
This is an important point often not fully explained. When we say connector loss, we really mean "connection" loss - the loss of a mated pair of connectors. Thus, testing connectors requires mating them to reference connector which must be high quality connectors themselves to not adversely affect the measured loss when mated to an unknown connector.
3)Testing with reference cables on each end simulates a cable plant with patchcords connecting to transmission equipment.

December 20, 2011

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