The fiber optic power meter is a special light meter that measures how much light is coming out of the end of the fiber optic cable. The power meter needs to be able to measure the light at the proper wavelength and over the appropriate power range. Most power meters used in datacom networks are designed to work at 850nm and 1300nn. Power levels are modest, in the range of –15 to –35dBm for multimode links, 0 to –40dBm for single mode links. Power meters generally can be adapted to a variety of connector styles such as SC, ST, FC, SMA, LC, MU, etc.
Optical power is usually measured in dBm, or decibels referenced to one milliwatt. This log scale is used because of the large dynamic range of fiber optic links, a range of 1000 or more. Some power levels may be given in microwatts, which many meters measure directly.
Power meters measure average optical power, not peak power, so they are sensitive to the duty cycle of the data being transmitted. It is important to specify the test conditions for measuring the optical power of a transmitter or at a receiver in terms of the data transmitted. Most networks have a diagnostic test signal for just this purpose.
Optical Power Meter Types
Optical power meters, like Digital Multimeters, come in a variety of types. The measurement uncertainty of practically all fiber optic power meters is the same, limited by the physical constrains of transferring standards with optical connectors. Most meters have an uncertainty of +/-5% or approximately 0.2dB, no matter what the resolution of the display may be.
Lower cost field optical power meters usually have a resolution of 0.1dB, laboratory meters display 0.01dB, and a resolution of 0.001dB is available on a few specialized fiber optic power meters.
The appropriate resolution for a measurement should be chosen according to the test. Laboratory measurements of low-loss patch cables, connectors, and fiber splices can be made to 0.01dB resolution and an uncertainty of 0.05dB or less if great care is used in controlling the test conditions.
Field measurements of absolute power are no better than the absolute calibration uncertainty, but relative power measurements can be made to 0.1dB. Cable plant loss is affected by not only the meter uncertainty, but also the characteristics of the test source, and may have an uncertainty of 0.5dB or more.
Test Source for Optical Power Testing
The test source is a portable version of the source that is used in the communication network attached to the fiber. It simulates the signal in the fiber for loss testing with a power meter, so the test source should match the wavelength and source type of the system source.
Generally, multimode fiber is tested with LEDs at both 850nm and 1300nm and single mode fiber is tested with lasers at 1310nm and 1550nm. The test source will typically be a LED for multimode fiber unless the fiber is being used for Gigabit Ethernet or other high-speed networks that use laser sources. LEDs can be used to test single mode fibers less than 5000 meters long, while a laser should be used for long single mode fibers.
You should always match the test light source wavelength to the network requirements. While one refers to 850, 1300, or 1550nm routinely, actual source wavelength may vary. One long runs of fiber typical of WANs or long-distance networks, the difference becomes important.
The fiber attenuation coefficient is a function of wavelength, so source spectral characteristics can be important. When testing long lengths of fiber, you may need to make corrections to nominal source wavelength losses.