For electricians, voltage is the primary thing measured. In fact every electrical measurement is based on voltage, based on the relationship V = IR. In fiber optics, the most basic measurement is the optical power of the light at the end of a fiber. Optical power measurement is the basis for loss measurements as well as the power in an operating system at the source or receiver.
So, electricians use voltmeters. What do fiber optic technicians use? Optical power meters are the primary fiber optic measurement instrument, while optical loss test sets (OLTSs) and optical time domain reflectometers (OTDRs) also measure power in testing loss, which is similar to how an ohmmeter or ammeter measures voltage and converts it to the appropriate units.
Optical power is based on the heating power of the light, and some optical lab instruments actually measure optical power by the heat absorbed by a detector. While this may work for high power lasers, these detectors are not sensitive enough for the low power levels typical for fiber optic communication systems.
Optical power meters use semiconductor detectors, which are sensitive to light in the wavelengths and power levels common to fiber optics. Most fiber optic power meters are available with a choice of three different detectors: silicon (Si) for short wavelength systems (650 to 850 nm), and germanium (Ge) or indium-gallium-arsenide (InGaAs) for general purpose use in the range of 850 to 1,600 nm. Unless you are dealing with plastic fiber, a germanium detector is the most versatile and cost-effective.
Optical power measurements are calibrated to standards set by the U.S. National Institute of Standards and Technology or similar international standards bodies. All detectors are sensitive to the wavelength of light, so they are calibrated at the wavelengths commonly used in fiber optics: 850, 1,300 and 1,550 nm. Some meters are calibrated at other wavelengths, such as 1,310 nm, the most common wavelength for single-mode lasers, but only those three wavelengths are used for standard calibration for all systems.
The units of optical power are decibels (dB). This is a logarithmic measurement, where 10 dB equals a change in power of a factor of 10, 20 dB is a factor of 100 and 3 dB is a factor of 2. This system was adopted in fiber optics because most measurements are loss measurements, which are always expressed in decibels. Being a log scale, a gain, or an increase in power, is a positive decibel (+dB) while a loss or decrease in power is negative dB (–dB).
Absolute power, such as transmitter or receiver power, is expressed in decibels referenced to 1 milliwatt (mW) of optical power (dBm), a level near the typical value of system power. As with loss, +dBm means power greater than 1 mW and –dBm means power below 1 mW.
Optical power meters, therefore, have controls that allow you to select the wavelength you are measuring (850, 1,300 and 1,550 nm) and the units of measurement (dB or dBm). Most meters have another control, which allows you to set an arbitrary reference level (0 dB) for loss testing. This is used to calibrate the meter to the test source used in loss testing, so the meter reads loss directly.
Of course, there are still a number of different fiber optic connectors that the meter must be compatible with to be useful with today’s fiber optic systems. Power meters generally have screw-on adapters for fiber optic connectors. As a minimum, you need adapters for SC, ST and LC connectors.
When using an optical power meter, it’s important to always remember these three steps:
1. Set the meter to the wavelength you are working at, whether it is system wavelength or test source wavelength
2. Set the meter to “dBm” for measuring optical power or “dB” for measuring loss.
3. Make certain that when you use the 0 dB reference setting that you don’t change it while making loss measurements
When you buy a power meter, choose one that has the features you want. Each will measure accurately and the only differences are in features. You may, for example, like having memory to store readings for printing reports for your customers. Plus, the manufacturer should offer test sources for loss testing; purchasing a meter and source together may mean saving some cost.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.