FIBEROPTICS BY JIM HAYES
Optical power is the “voltage” of fiber
optic measurements. Just like voltage is
the basic measurement of electricity and
the basis of the measurements of practically all electrical parameters, optical
power is the equivalent for fiber optics.
Optical power is used when measuring the output of a transmitter or input
of a receiver to test a fiber optic transmission system. That is a measurement
of absolute power, generally expressed in
decibels referenced to a milliwatt of optical power (dBm). When measuring loss,
we measure power before and after the
component being tested and that is relative power measured in decibels (dB).
Decibels are a logarithmic conversion of power in watts. Log scales such as
decibels are used when very large ranges
are involved, such as in fiber optics
where power levels can go from nearly
100 milliwatts down to 1 microwatt. It’s
easier to use + 20 dBm to – 30 dBm than
trying to use watts, milliwatts and microwatts without making mistakes.
What are the errors when measuring
power? For absolute power, calibration is the biggest source of errors. The
sensors used in optical power meters
are very sensitive to light wavelengths.
Power meters are usually calibrated at
850 nanometers (nm), 1,300 nm and
1,550 nm, the three most common light
wavelengths used in fiber optic systems.
The difference between the sensitivity of
a power meter detector at 850 nm and
1,300 nm can be more than 3 dB, so it is
important to check the wavelength set on
the power meter to ensure it is the same
as the light being measured.
Even if you have chosen the correct
calibration wavelength, the calibra-
tion uncertainty can be as much as ± 5
percent or ±0.2 dB because of errors
transferring calibration from a stan-
dards lab to the manufacturer of the
power meter and then to individual
meters. Calibration in the United States
is traceable from a National Institute
of Standards and Technology transfer
standard to the lab of the power meter
manufacturer and then to each meter
they manufacture. Each step adds some
uncertainty to the calibration.
Meters also should be recalibrated
frequently, generally every one or two
years, to ensure the meter has not
drifted off calibration. Calibration is
also done at several power levels to
check the power meter’s linearity. Since
meters are also used for measuring loss
that requires measuring and comparing
power at two different power levels, it is
important to calibrate power at different levels to ensure loss measurements
will be more accurate.
Remember last month when I wrote
about random and systematic errors?
In power measurements, calibration
causes systematic errors. A meter that
is not properly calibrated will measure
all powers in error. When making power
measurements, random errors must be
considered, and many are a result of
If you are using a patchcord to mea-
sure optical power from a source, the
coupling of light from a source to the
detector of a power meter will depend
on the quality of the connectors on the
patchcord. If the connectors are bad—
In some cases, the connection to the
source and power meter may be a vari-
able. If the adapters on the source and
meter are not precise, you may get a
different measurement every time you
connect up to make a measurement. Try
that yourself on your instruments to see
how repeatable they are.
Remember how putting stress on fiber
optic cables causes loss? That’s another
source of power measurement errors. If
the cable attached to the source or meter
is bent too tightly, especially near the connectors, the loss caused by the stress will
reduce the power measured, adding to
the random errors.
Knowing how to make accurate optical power measurements is important
even if you never test an actual communications system, because every fiber
optic technician tests loss. Testing the
loss of a cable or cable plant involves
making two optical power measurements—one at each end of the cable
being tested—doubling the importance
of reducing errors.
Next month’s column will cover measuring loss.
Testing, accuracy and optical power measurements
LAST MONTH, I wrote about measurement uncertainty and metrology, the science of
measurements. This month, I get more specific and cover the uncertainty of some
basic fiber optic measurements, starting with optical power.
HAYES is a VDV writer and educator and the president of the Fiber Optic Association. Find
JimHayes.com. I S T