Definition – What is The Optical Power Meter?
Once you install and terminate fiber optic cables, it’s time to test them. A test should be conducted for each fiber optic cable plant for three main areas: continuity, loss, and power. To do this, you’ll need a fiber optic power meter. An optical power meter (OPM) is a device used to measure the power in an optical signal. The term usually refers to a device for testing average power in fiber optic systems. Other generaly purpose light power measuring devices are usually called radiometers, photometers, laser power meters, fiber optic meters or lux meters.
A typical optical power meter consists of a calibrated sensor, measuring amplifier and display. The sensor primarily consists of a photodiode selected for the appropriate range of wavelengths and power levels. On the display unit, the measured optical power and set wavelength is displayed. Power meters are calibrated using a traceable calibration standard such as a NIST standard.
A traditional optical power meter responds to a broad spectrum of light, however the calibration is wavelength dependent. This is not normally an issue, since the test wavelength is usually known, however it has a couple of drawbacks. Firstly, the user must set the meter to the correct test wavelength, and secondly if there are other spurious wavelengths present, then wrong readings will result.
Before Using a Power Meter
There are a few things you should do to prepare before using a fiber optic power meter. First, read the manual and run some practice tests with your power meter before going onto the job site. Next, prepare documents showing how and where all your cables are installed. Don’t depend on memory because you can easily lose track of your cables, especially if it’s a large project. Also, create a worksheet to document your testing data. Some power meters have a memory feature for keeping your data. These come in handy for large projects as well.
Use Safety Precautions
It’s important to follow safety precautions while testing fiber optic cables with a power meter. Wear eye protection when working with high power cables. Even with low-power layouts, it’s wise to check the connectors with your power meter before looking.
Using a Power Meter for Power Measurement
When measuring fiber optic power with a power meter, attach the meter to the cable. Turn on the source of power (transmitter), and view the meter’s measurement. Compare the meter measurement with the specified correct power for that particular system to be sure it doesn’t have too much or too little power. The reason correct power measurement is so important with fiber optic cables is because the system works similar to electric circuit voltage, and the power must be just the right amount to work properly.
Testing Loss with a Power Meter
A power meter is used to test loss, but a test source is needed as well. The meter will measure the optical power that is lost in every part of the cable. The cable is mated with a working reference cable. Two methods used in loss testing are single-ended loss and double-ended loss. With single-ended loss testing, only the launch cable is used. With double-ended loss testing, a receive cable is also attached to the meter.
There are certain loss guidelines to follow for connectors, splices, multimode fibers, and singlemode fibers. You should understand these guidelines as well as the standard formula for calculating loss before getting started. Other tools that may be required for the processes above include optical loss test kits, matching reference test cables, mating adapters with hybrids, a visual fault locator or fiber tracer, cleaning items, and an ODTR (for outdoor networks). Fiber optic cables can be of great value to companies if they work properly. Use a fiber optic power meter and other useful tools to ensure that your fiber optic system will operate smoothly around the clock.
Three types of equipment can be used to measure optical power loss:
1. Component equipment – Optical power meters and Stabilized Light Sources are packaged separately, but when used together they can provide a measurement of end-to-end optical attenuation over an optical path. Such component equipment can also be used for other measurements.
2. Integrated test set – When an stabilized light source and optical power meter are packaged in one unit, it is called an integrated test set. Traditionally, an integrated test set is usually called an OLTS. Generic Requirements for Hand-Held Stabilized Light Sources, Optical Power Meters, Reflectance Meters, and Optical Loss Test Sets, discusses OLTS equipment in depth.
3. An Optical Time Domain Reflectometer (OTDR) can be used to measure optical link loss if its markers are set at the terminus points for which the fiber loss is desired. The accuracy of such a measurement can be increased if the measurement is made as a bidirectional average of the fiber.
