Optical Power
The most basic fiber optic measurement is optical power
from
the end of a fiber. This measurement is the basis for loss measurements
as well as the power from a source or presented at a receiver.
Typically both transmitters and receivers have receptacles for fiber
optic connectors, so measuring the power of a transmitter is done by
attaching a test cable to the source and measuring the power at the
other end. For receivers, one disconnects the cable attached to the
receiver receptacle and measures the output with the meter.
While optical
power meters are the primary power measurement instrument, optical loss
test sets (OLTSs) and optical time domain reflectometers (OTDRs)
also measure power in testing loss. TIA standard test FOTP-95
covers the measurement of optical power.
Optical power is based on the heating power of the light, and
some optical lab instruments actually measure the heat when light is absorbed
in 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 (Table 1).
Optical power meters
typically use semiconductor detectors since they 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 3
different detectors, silicon (Si), Germanium (Ge), or Indium-Gallium-Arsenide
(InGaAs).
Table 3.1. Optical power levels typical of fiber optic communication
systems
|
Network Type |
Wavelength, nm |
Power Range, dBm |
Power Range, W |
|
Telecom |
1310, 1550 |
+3 to -45 dBm |
50 nW to 2mW |
|
Datacom |
650, 850, 1300 |
0 to -30 dBm |
1 to 100uW |
|
CATV, DWDM |
1310,1550 |
+20 to -6 dBm |
250 uW to 10mW |
Silicon photodiodes are sensitive to light in the range of
400 to 1000 nm and germanium and indium-gallium-arsenide photodiodes
are sensitive to light in the range of 800 to 1600 nm.
Silicon detectors are very low noise detectors sensitive to
light at approximately 400 to 1100 nm wavelength, depending on
the exact method of fabrication. Thus, they are useful for standard
datacom links using 820 nm LEDs and glass fiber or 665 nm LEDs
and plastic fiber. They can also be used with older telecom systems
that used 850 nm lasers.
Silicon detectors have inherently low noise, low leakage currents
and therefore very low noise floors when used with transimpedance
amplifiers in power meters. Typical noise floors on fiber optic
instruments using Si detectors is -70 to -90 dBm, or about 1 to
100 picowatts.
Germanium detectors are sensitive to light in the 800 to 1800
nm wavelength, making them useful for all systems using glass
fiber, including 1300 and 1550 nm single mode systems. Ge detectors
are noisier however, creating a higher noise floor for low level
measurements. This noise is proportional to detector area, so
by using a smaller detector, one obtains a lower noise floor.
However, smaller detectors require positioning the fiber end very
close to the window of the detector and centered accurately over
the detector sensitive area. The noise of a 2 mm Ge detector is
typically 10 to 50 times lower than room temperature 5 mm Ge detectors.
Some manufacturers of fiber optic power meters have chosen
to cool these large Ge detectors to reduce the noise and get lower
measurement limits. This leads to more sensitive measurements
but with a penalty of increased circuit complexity, .instrument
weight and short battery life, since one must provide up to 1
amp current to the thermoelectric cooler in the Ge detector package.
Another solution for extremely low level measurements at 1300
and 1550 nm is to utilize InGaAs detector technology, , which
has been developed for the receivers of high speed long wavelength
communication systems. InGaAs detectors have the same sensitivity
range as Ge, but are much less noisy. With InGaAs detectors, measurements
can be made to -65 dBm (less than 0.5 nW) with ease. However,
InGaAs detectors are very expensive, limiting their usage to only
the most expensive instruments.
Table 3.2. Characteristics of detectors used in Fiber Optic
Power Meters
|
Detector Type |
Wavelength Range (nm) |
Power Range (dBm) |
Comments |
|
Silicon |
400-1100 |
+10 to -70 | |
|
Germanium |
800-1600 |
+10 to -60 |
-70 with small area detectors, +30 with attenuator windows |
|
InGaAs |
800-1600 |
+10 to -70 |
Small area detectors with fiber pigtails often used |
Measurement Units: "dB" and "dBm"
Whenever
tests are performed on fiber optic networks, the results are displayed
on a meter readout in “dB.” Optical loss is measured in “dB” while
optical power is measured in “dBm.” Loss is a negative number (like
–3.2 dB) as are most power measurements. Confused? Many fiber optic
techs are too. Let’s see if we can clear up some of the confusion.
When
we make fiber optic measurements, we are measuring the power in the
light we measure. The standards we use for power measurements,
maintained by NIST (the US National Institute of Standards and
Technology,) are actually determined by the heating effect of the light
as it is absorbed in a detector. Every fiber optic power meter sold is
calibrated traceable to the NIST standard so different meters should
measure the same power, within the limits of calibration uncertainty.
Optical
power in fiber optics is similar to the heating power of a light bulb,
just at much lower power levels. While a light bulb may put out 100
watts, most fiber optic sources are in the milliwatt to microwatt range
(0.001 to 0.000001 watts), so you won’t feel the power coming out of a
fiber and it’s generally not harmful.
In the early
days of fiber optics, source output power was usually measured in
milliwatts and loss was measured in dB or deciBels. Over the years, all
measurements migrated to dB for convenience. This was when the
confusion began.
Loss measurements were generally
measured in dB since dB is a ratio of two power levels, one of which is
considered the reference value. The dB is a logarithmic scale (remember
“logs” from high school math?) where each 10 dB represents a ratio of
10 times. The actual equation used to calculate dB is
dB = 10 log (measured power / reference power).
Here is an Excel spreadsheet that calculates dB/power ratio and dBm/milliwatts.
So
10 dB is a ratio of 10 times (either 10 times as much or one-tenth as
much), 20 dB is a ratio of 100, 30 dB is a ratio of 1000, etc. When the
two optical powers compared are equal, dB = 0, a result of the log
scale used in dB but a convenient value that’s easily remembered.
If
we have loss in a fiber optic system, the measured power is less than
the reference power, so the ratio of measured power to reference power
is less than 1 and the log is negative, making dB a negative number.
When we set the reference value, the meter reads “0 dB” because the
reference value we set and the value the meter is measuring is the
same. Then when we measure loss, the power measured is less, so the
meter will read “ – 3.0 dB” for example, if the tested power is half
the reference value. Although meters measure a negative number for
loss, convention has us saying the loss is a positive number, so we say
the loss is 3.0 dB when the meter reads – 3.0 dB.
Measurements
of optical power are expressed in units of dBm. The “m” in dBm refers
to the reference power which is 1 milliwatt. Thus a source with a power
level of 0 dBm has a power of 1 milliwatt. Likewise, -10 dBm is 0.1
milliwatt and +10 dBm is 10 milliwatts.
Instruments
that measure in dB can be either optical power meters or optical loss
test sets (OLTS). The optical power meter usually reads in dBm for
power measurements or dB with respect to a user-set reference value for
loss. While most power meters have ranges of +3 to –50 dBm, most
sources are in the range of 0 to –10 dBm for lasers and –10 to –20 dBm
for LEDs. Only lasers used in CATV or long-haul telephone systems have
powers high enough to be really dangerous, up to +20 dBm – that’s 100
milliwatts or a tenth of a watt!
The OLTS or the
power meter on the dB scale measures relative power or loss with
respect to the reference level set by the user. The range they measure
will be determined by the output power of the source in the unit and
the sensitivity of the detector. For multimode fiber, an OLTS using a
LED source will usually measure over a range of 0-30 dB, more than
adequate for most multimode cable plants which are under 10 dB loss.
Singlemode networks use lasers and may have loss ranges of up to 50 dB
for long-haul telecom systems, but campus cabling using singlemode may
only have 1-3 dB loss. Thus a singlemode OLTS may be different for
short and long systems.
If you remember that dB is
for measuring loss, dBm is for measuring power and the more negative a
number is, the higher the loss, it’s hard to go wrong. Set your zero
before measuring loss and check it occasionally while making
measurements.
Here is an Excel spreadsheet that calculates dB/power ratio and dBm/milliwatts.
Calibration
Calibrating fiber optic power measurement equipment requires
setting up a reference standard traceable to a national standards like like the US National Institute
of Standards and Technology (NIST, Boulder, CO) for comparison purposes while calibrating
every power meter or other instrument. The NIST standard for all
power measurements is an ECPR, or electrically calibrated pyroelectric
radiometer, which measures optical power by comparing the heating
power of the light to the well-known heating power of a resistor.
Calibration is done at 850, 1300 and 1550 nm. Sometimes, 1310
nm is used as the calibrated wavelength on a power meter, a holdover from the early 1980s when the telcos and
AT&T used 1310 nm as a standard, but the standard for power meter calibration is
1300 nm. To conveniently transfer this measurement to fiber optic
power meter manufacturers calibration laboratories, NIST currently
uses a laboratory optical power meter which is sent around to labs to use as a transfer standard.
To transfer from this transfer standard to production instruments,
power meter manufacturers use calibrated detectors or power meters
which are regularly checked against one another to detect any
one detector's variability, and all are periodically recalibrated
to NIST's transfer standards.
In order to transfer the calibration, one needs a source of
known characteristics. Typically laser sources at 850, 1300 and
1550 nm pigtailed to single mode fibers are used . The laser sources
have very narrow spectral width to allow accurate wavelength calibration,
and the single mode fiber controls the output beam presented to
the detector of the instrument. Each of these sources is checked
for wavelength regularly to insure that no drift has occurred.
The output power of these lasers is precisely controlled by an
optical feedback circuit to insure stability. Even the temperature
of the laser is often controlled precisely to insure no drift
in output power or wavelength occurs during the calibration process.
Using the sources described above, one measures the output
of one of the lasers on a transfer standard meter or detector and record
the value. The instrument under test is then adjusted to read
the same value as the transfer standard detector and a single point calibration
is done.
For all power meters, especially those with autoranging, one
must calibrate on every range, double checking to insure that
the meters have a smooth transition between ranges to prevent
calibration discontinuities. Calibration is therefore checked
at several points near the top and bottom of the range for every
meter.
Meters calibrated in this manner have an uncertainty of calibration
of about +/- 5%, compared to the NIST primary standards. Limitations
in the uncertainty are the inherent inconsistencies in optical
coupling, about 1% at every transfer, and slight variations in
wavelength calibration. NIST is working continuously with instrument
manufacturers and private calibration labs to try to reduce the
uncertainty of these calibrations.
Recalibration of instruments should be done annually, however
experience has shown that the accuracy of meters rarely changes
significantly during that period, as long as the electronics of
the meter do not fail. Unfortunately, the calibration of fiber
optic power meters requires considerable investment in capital
equipment and continual updating of the transfer standards, so
very few private calibration labs exist today. Most meters must
be returned to the original manufacturer for calibration.
Understanding FO power meter measurement uncertainty
Much attention has been paid to developing transfer standards
for fiber optic power measurements. The US NIST in Boulder, Colorado
and standards organizations of most other countries have worked
to provide good standards to work from. We can now assure traceability
for our calibrations, but even so the errors involved in making
measurements are not ignorable. Understanding those errors and their
probable causes will insure a realistic viewpoint on fiber optic
power measurements.
The first source of error is optical coupling. Light from the
fiber is expanding in a cone. It is important that the detector
to fiber geometry be such that all the light from the fiber hits
the detector, otherwise the measurement will be lower than the
actual value. But every time light passes through a glass to air
interface, such as the window on the detector, a small amount
of the light is reflected. Some is lost, but some can be re-reflected
by the polished end surface of the connector back into the detector
, the amount dependent on the type of connector and the quality
of its polished surface. And although detectors have an antireflection
coating, some light is reflected from the detector surface, which
can be re-reflected from the window, connector, etc. Finally,
the cleanliness of the optical surfaces involved can cause absorption
and scattering. The sum total of these potential errors will be
dependent on the connector type, wavelength, fiber size and NA.
Beyond the coupling errors, one has errors associated with
the wavelength calibration. Semiconductor detectors used in fiber
optic instruments (and systems too) have a sensitivity that is
wavelength dependent. Since the actual source wavelength is rarely
known, there is an error associated with the spectral sensitivity
of the detector. By industry convention, the three cardinal wavelengths
(850, 1300 and 1550 nm) are used for all power measurements, not
the exact source wavelength. The source has a finite spectral
width, very narrow for lasers, quite broad for a LED. In order
to accurately measure the power of the source, one needs to know
the spectral power distribution of the actual source being measured,
the sensitivity of the detector and perform a complicated integration
of the two.
Another source of error exists for high and low level measurements.
At high levels, the optical power may overload and saturate the
detector, causing the measurement to be in error on the low side.
Consistent overload may even permanently damage the detector,
especially with small area detectors. This is particularly a problem
with CATV systems, where the transmitter power is extremely high
to get good signal to noise performance at the receiver. CATV
power levels are high enough to damage the detector in many power
meters, especially those with small area InGaAs detectors. Specialized
CATV meters exist where the detector window has been replaced
by a calibrated attenuator of approximately 20 dB. Thus they can
make measurements at high power levels, up to +20 or +30 dBm,
but sacrifice low level power measurements.
At low levels, the inherent detector noise adds to the signal
and becomes an error. If the signal is 10 dB above the noise floor
(10 time the noise), the offset error is 10% or 0.4 dB. Fotec
has always specified the measurement range of its fiber optic
power meters as 10 dB above the noise floor, but at least one
manufacturer specifies it as only 3 dB, which can cause an error
of 50% !
Even when two fiber optic power meters are calibrated within specifications, the
uncertainty may be +/- 5% (about 0.2 dB) on each meter. A worst
case scenario could have two meters deviating from nominal in opposite directions, leading to a
potential 10% (0.4 dB) error when measuring the same source and
fiber combination. A similar error can occur in a FOPM when amplifiers
autorange, unless the manufacturer includes a balance adjustment
for calibration of the meter.
When one considers why fiber optic power is measured (determining source
output or receiver power to determine if a system in within margin
or measuring loss), the impact of errors becomes apparent. But
without knowing the system source spectral output, system detector
spectral sensitivity and the spectral attenuation characteristics
of the fiber, one cannot accurately predict system performance
anyway.
How does one cope with all this uncertainty. On short systems,
design the system with adequate margin. On long systems, specify
system and test source wavelength and test the cable at that wavelength
(or correct for variations in system sources and test source wavelengths.)
And remember that the error in optical power measurement may be
small to the unknown variations in system components.
Fiber optic components are sensitive to physical stress which
can induce loss. One can see the effects of physical movement
of fiber optic cables and connectors on fiber optic assemblies.
A simple bend in singlemode fiber cable can induce several dB
loss. All connectors are very sensitive to forces acting on the
cable as it exits the backshell. Just handling fibers to make
measurements can cause readings to vary by several tenths of dB.
Instrument Resolution vs. Measurement Uncertainty
Considering the uncertainty of most fiber optic measurements,
instrument manufacturers have provided power and loss meters with
a measurement resolution that is usually much greater than needed.
The uncertainty of optical power measurements is about 0.2 dB
(5%), loss measurements are more likely to have uncertainties
of 0.2-0.5 dB or more, and optical return loss measurements have a
1 dB uncertainty. Instruments which have readouts with a resolution
of 0.01 dB are generally only appropriate for laboratory measurements
of very low losses such as connectors or splices under 1 dB or
for monitoring small changes in loss or power over environmental
changes Within the laboratory, a resolution of 0.01 dB can be
extremely useful, since one often measures the loss of connectors
or splices that are under 0.10 dB or changes in loss under environmental
stress that are under 0.1 dB. Stability of sources and physical
stress on cables limits measurement uncertainty to about 0.02
to 0.05 dB per day, but 0.01 dB resolution can be helpful in determining
small changes in component performance.
Field instruments are better when the instrument resolution
is limited to 0.1dB, since the readings will be less likely to
be unstable when being read and more indicative of the measurement
uncertainty and especially important since field personnel are
usually not as well trained in the nuances of measurement uncertainty
Non-Intrusive Power Measurements
Since one can induce loss in the fiber or cable by
bending
it, this lost power can be measured. By using a clip-on detector,
such as used in fiber identifiers or fusion splicer LID (local
injection and detection) systems, the induced loss can be measured.
However, the uncertainty of the measurement is very high, due
to the uncertain percentage of the power in the fiber that will
be coupled out of the core by the induced stress, the amount of
power that will be transmitted through the buffer of the fiber
(especially with colored buffers) and the jacket of the fiber.
Thus this type of measurement is only used as an qualitative indicator
of systems power presence, not quantitative measure of system
power.
