POF Measurement: Transmission Power
Power measurement is one of the most important testing procedures because it shows how effectively a fiber optic communication system is working, and whether it is meeting its minimum specifications.
Optical power is measured either using linear units of watts (W), milliwatts (mW), microwatts (μW), or nanowatts (nW), or the absolute logarithmic unit, decibel-milliwatts (dBm). Power losses and gains are measured using the relative logarithmic unit, decibels (dB).
The formulas for converting between mW and dBm are as follows.
Based on these, positive dBm values indicate the measured power is greater than the reference value of 1 mW, while negative values are less than 1 mW. Further manipulation of these equations will show that a 50% reduction in absolute power is corresponds to a 3 dB loss.
Typical power levels for POF transmission lines are between -2 dBm (0.63 mW) and -26 dBm (0.0025 mW). Test devices, which consist of a laser or LED transmitter and a photodiode receiver, can measure power levels with accuracy up to 0.01 dB.
Power Testing Methods
Single-ended cable assembly testing: FOTP-171
FOTP-171 is a test that was developed for testing the fiber and one connector of a connectorized POF cable.
A 0 dB reference is created by connecting the power meter to the output of the reference launch cable. It is then used to connect a power source to the cable under test, which is then connected to a power meter. This causes the insertion loss of the cable under test, plus the connector mating it and the reference cable, to be measured.
Double-ended cable assembly testing: OFSTP-14
Similarly, it is possible to perform a double-ended test for cable assemblies, OFSTP-14, which measures the combined loss of the connectors on both ends and the fiber itself. Between one and three reference cables are used for this test, for launch (from the power source) and receive (to the power meter), with the cable under test is connected between them. Each of these variations involves a different method of setting the 0 dB reference.
Method A: One Reference Cable (Launch)
0 dB reference is set by measuring output of launch cable directly, omitting all connector loss from measurements.
When the cable under test is measured, the insertion loss of the complete assembly is included.
Method B: Two Cables (Launch and Receive)
0 dB reference is set by measuring the output of the launch cable through the receive cable, so the insertion loss of one connector is included.
When the cable under test is measured, the insertion loss of the complete assembly is included, minus the loss of one of the mated connectors.
Method C: Three Cables (Launch, Receive, and “Golden” Reference Cable)
0 dB reference is set by measuring the output of the launch cable through the receive cable and a “golden” reference cable, so the insertion loss of two mated connectors and the fiber in the third cable are included.
When the cable under test is measured, the insertion loss of the complete assembly is included, minus the loss of the two mated connectors that were included in the reference.
References
(2014) The FOA Reference For Fiber Optics – Testing Fiber Optic Patchcords and Cables [Online]. Available: http://www.thefoa.org/tech/ref/testing/test/FOTP-171.html
(2014) The FOA Reference For Fiber Optics – Testing the Installed Fiber Optic Cable Plant [Online]. Available: http://www.thefoa.org/tech/ref/testing/test/OFSTP-14.html
Fiber Optic Technician’s Manual, 3rd edition, Clifton Park, NY, 2005.