Power Measurement in Fiber Optics, How it is Done

Optical Power

The basic unit of measurement in fiber optics is the light power. Just like electric power, optic power is measured in watts.

For light, the total energy Q is given by:

Q = NQ_p

Where Q_p is the energy of a single photon and N is the number of photons, therefore:

The Power Meter: How to use it

You need a power meter to measure power in a fiber optic system; most power meters come with a screw-on-adapter that matches the connector being tested and a little aid from the network electronics to turn on the transmitter. During the measurement of power, the meter must be set to the proper range (typically dBm, at times microwatts, but never dB, a relative power range used only for testing loss) and the proper wavelengths matching the source being used.

As aforementioned, power levels can be indicated in mW or in decibels as dBm, relative to one milliwatt or as dBr, relative to a previously noted value.

To measure power, attach the meter to the cable that has the output you want to measure. This can be done at the receiver to measure receiver power or to reference test cable (i.e. tested and known to be fine) that is attached to the transmitter, acting as the ‘source’ to measure transmitter power. Turn on the transmitter/source and note the power the meter measures, compare it to the specified power for the system and ensure it has enough power but not too much or too little (note that, too much power can overload the receiver and cause errors, on the other hand too little power may cause the receiver not to be able to distinguish the signal from noise).

If the light source and power meter are to be used for power measurement, check an installation or repair on a commercial basis, the client will need the assurance that the reading of the power meter is accurate. This proof is provided by a calibration certificate from an authorized firm, for each instrument which must be renewed at intervals, normally yearly.

Measurement of Loss in Fiber Optics

Loss in this case is basically the difference between the power coupled into the cable at the transmitter and what comes out at the receiver end. To test for loss, you need to measure the optical power lost in a cable including connectors, splices, etc. with a fiber optic source and power meter by connecting the cable being tested to a known reliable reference cable.

To measure loss, a power meter along with a test source is needed. The test source should match the type of source (LED or laser) and wavelength (850, 1300, 1550 nm).

There are two techniques used for measurement of loss, i.e. single-ended loss and double-ended loss. Single-ended loss uses only the launch cable, whereas double-ended loss utilizes a receiver cable attached to the meter.

Single-ended loss is measured by mating the cable under test to the reference launch cable and measuring the power output at the far end of the meter. By performing this, total loss is being measured, i.e. loss of the connector mated to the launch cable and the loss of any fiber, splices or other connectors in the cable under test. This is illustrated in the figure below. Reverse the cable to test the connector on the other end.

For double-ended loss measurement, attach the cable to test between two reference cables, one attached to the source and one to the meter. In this manner, two ‘connectors’ losses are being measured one on each end, plus the loss of all the cable or cables in between. This is demonstrated below:

Note that, the accuracy of the measurement will depend on the quality of your reference cables. Reference cables always require to be tested by the single-ended method, to ensure they are fine before starting the test of other cables.

Also read:

• RF Power and Voltage Measurement
• Telemetry in Instrumentation Systems
• Power Measurements in DC Circuits
• Power Measurement in AC Circuits (Single-phase & Polyphase Systems)
• Fiber Optic Temperature Sensors: Principle of Operation & Applications