The Fiber Optic Association
- Tech Topics
Fiber Optic Testing
- We get questions all the time,
so we have started putting those we feel have general interest
on this web page. As it grows larger, we'll index it for convenience.
Here's more general FAQs too.
- Looking for more information?
Try the FOA Online Reference Guide on Testing or Tech
- Topics covered in this FAQ:
- FO Power Measurements
- Fiber and Cable
- Reflectance/Optical Return
- Fiber Optic
More on power measurements.
- Optical Power Measurements
- What are the measurement
units for power?
Optical power is measured in linear units of milliwatts (mW),
microwatts (uW - really the greek letter "mu"W), nanowatts
(nW) and decibels (dB).
- What is the difference between
"dBm" and "dB"?
dB is a ratio of two powers, for example the loss in a fiber
optic cable. When power is measured in linear units (mW, uW or
nW), dB is calculated on a log scale using this formula:
- power (dB) = 10 log (power1/power2)
- If we are measuring absolute
power levels, the measurement is generally referenced to 1 milliwatt
(mW), is expressed as "dBm" and the calculation becomes:
- power (dBm) = 10 log (power/1
- Thus 1 mW = 0 dBm, 1 uW = -30
dBm, 1 nW = -60 dBm and two equal powers compared are 0dB (eg.
power being the same, there is no loss.)
- What power level should a
It depends on the type of source. When coupled into a good test
cable, the source output power will be in these ranges:
LED: -10 to -25 dBm into 62.5/125 fiber
Telcom/LAN laser: 0 to -13 dBm into singlemode fiber, to +20
dBm with DWDM and fiber amplifier systems
CATV Laser : +16 to 0 dBm into singlemode fiber
- What power level should a
It depends on the network and type of source. When measured at
the end of the network cable, the source output power will usually
be in these ranges:
LAN/LED: -20 to -35 dBm into 62.5/125 fiber
Telcom/LAN laser: -20 to -45 dBm into singlemode fiber
CATV Laser : 0 to -10 dBm into singlemode fiber
- How do you calculate a loss budget?
- The loss budget is a calculation
of how much attenuation a link should have. You compare that
loss to the dynamic range of the networking equipment to see
if the range and link loss are compatible.
- How accurate are fiber optic
All optical power meters which are calibrated to NIST (the US
standards body) or any national standards lab will measure optical
power to an uncertainty of about +/- 0.2 dB or 5%. Therefore,
since every power meter has an uncertainty of +/- 0.2 dB, any
two meters can differ by 0.4 dB in the worst case (one at +0.2
dB and one at -0.2 dB) even if both are within their specification!
More information on calibration uncertainty.
- Are more complex or higher
priced FO power meters more accurate?
The high priced meters offer better dynamic range and more features,
but not better absolute measurement uncertainty.
- Why is the measurement uncertainty
so high? That is because there are three to four calibration
transfers from the NIST absolute optical power standard before
the meter is delivered to the customer. The NIST standard has
an uncertainty of about 1% itself and every transfer adds errors
of about 1%. More information on calibration
- Why do most meters only offer
calibrations at a few wavelengths?
NIST only offers calibrations at 850, 1300 and 1550 nm, so those
meters that have calibrations at other wavelengths have to extrapolate
to those values, increasing the measurement uncertainty at those
- If my source is at a slightly
different wavelength from the standard calibration wavelength,
doesn't that add to measurement error?
Perhaps, but the wavelength of most sources is not known by the
person making the measurement. If everyone uses meters calibrated
at only a few specific wavelengths, everyone can be testing to
the same standard and will get more closely correlated mesurements
on sources of unknown wavelengths .
- Fiber Optic Cable Loss
Standard: FOTP-171 for cable assemblies
Standard: OFSTP-14 for the installed multimode cable plant, OFSTP-7 for the installed singlemode cable plant
of 2011, OFSTP-14 has been replaced by an international standard that
as of mid-2011 is very controversial. First of all, it allows the use
of either insertion loss testing with a light source and power meter or
OTDR testing. This was predicated on comparisons of OTDR tests on cable
plants for 10GbE of less than 2 dB loss. For other more typical
multimode links of 5-10 dB, the two methods will generally give
divergent results. Since insertion loss is designed to test according
to how the link will be used, it should be the test used for longer
links. The new standard also includes a new metric for measuring mode
power distribution in multimode fiber called Encircled Flux (EF). It is also controversial, but international documents say the older source with mandrel wrap meets their requirements. Here is a more detailed explanation of all the options in cable testing.
exception is long distance networks which need testing for chromatic
dispersion (CD) and polarization mode dispersion (PMD). Here is more on CD and PMD.
- Why do you use a launch cable
on the source?
You use a launch cable to set the proper test conditions for
testing another cable. The launch cable should match the fiber
size and connector type of the cable you want to test, and be
tested to insure it is a low loss connector.
- Why can't I just attach my
cable directly to the source?
Sources have a great deal of variety in how they launch light
into the cable, which can cause undesirable variations in loss
measurements. Furthermore, the coupled power can vary considerably
with each insertion, depending on the alignment of the connector
ferrule in the source output connector.
- What makes a launch or receive
A good launch or receive cable will have low loss - less than
0.5 dB loss when tested in a single-ended FOTP-171 test.
- Do I always need a laser
source to test singlemode?
No, you can use a LED source for short SM patchcords or cables
up to about 5 km long. Longer SM cables will show higher loss
with an LED due to the spectral width of the LED causing higher
loss at the upper and lower ends of its spectral output. FOTP-171
actualy calls for LED sources to test SM patchcords to prevent
problems with the interference caused by the coherent light of
- What is a receive cable?
The receive cable is used in a double ended test to measure the
connector loss on both ends of the cable.
- When doing a double-ended
loss test, why don't you set the reference with both launch and
receive cables connected together?
There are several reasons why you use the same reference method
for single-ended and double-ended tests.
If you want to measure the loss of connectors on both ends of
the cable being tested, you need a reference with the launch
- A two cable reference removes
one connector loss from the measurement, since you include it
in the reference. You can also reference with three cables, where
you simply replace the middle cable with the cable under test,
but the loss removes two connectors from the test value, as they
are included in the reference. However all three methods are
approved standard methods and the three cable reference is sometimes
the only way you can test cable plants with connectors like the
MT-RJ that cannot be directly connected to test instruments. Here is a more detailed explanation of all the options in cable testing.
What happens if the launch and/or receive cables have bad connectors
with say 3 dB loss.
- When the zero reference is set, it will include
the loss of two bad connectors. When you attach a cable between
them to test, you will measure erroneously high loss for one
or both connectors on the cable being tested, invalidating the
measurement, so it is very important to keep all test connectors
- How do I test cables with
different connector types?
- Use a two or three cable reference
as necessary. More.
- What should my cable loss
be? Are my measurements correct?
A basic guideline for loss measurements is to estimate the loss
using these approximate values:
Connector loss: 0.5 dB per mated pair
Splice loss: 0.2 dB per splice
Multimode: 3 dB/km @ 850 nm, 1 dB/km @ 1300 nm
Singlemode: 0.4 dB/km @ 1300 nm, 0.3 dB/km @ 1550 nm
- For example: a 1.5 km multimode
link with 3 connections @ 850 nm:
Loss = (1.5 km X 3 dB/km) + (3 conn X 0.5 dB)
Loss = 4.5 dB + 1.5 dB = 6 dB
- You should also read this
more detailed explanation and this page about "loss
- How Long a Fiber Can My Meter
and Source Test?
- That depends on the output power
of the source and the sensitivity of the meter. For example,
one of our LED sources will have a maximum output into 62.5/125
fiber of about -15dBm. Your meter should be used at power levels
above about 10 dB higher than its minimum spec. A meter can easily
read to -45 dBm (min spec is -55 dBm), giving us a range of 30
dB (-45 dBm from -15 dBm gives us 30 dB). At 850 nm and a loss
of 3 dB/km, that's 10 km of fiber, less our connector and splice
loss, and at 1300 nm,a loss of 1 dB/km, it's 30 km less connector
and splice loss, both lots longer than any networks operating
on multimode fiber.
- For singlemode testing, lasers
can give you 0 to -10 dBm output, giving a range of 35 to 55
dB, corresponding to over 100 km of fiber, even approacing 200
km at 1550 nm!
- Optical Time Domain Reflectometers (OTDRs)
- Reference: Understanding
- Can't I use an OTDR to test
Well, Yes and No. The OTDR will measure the loss in the cable
plant, but using a technique based on backscatter signals that
indirectly measure loss, unlike a source and power meter that
measure loss directly. The OTDR measurement technique doesn't
correllate well with the source and meter. Since the source and
meter tests loss just like the transmission link, all standard
cable plant tests specify using a source and meter to measure
loss. You must always use them to test the cable plant loss.
Even the outside plant singlemode test standard, OFSTP-7, says
you should accept only the source/meter results.
- Why do I use an OTDR?
Use the OTDR for troubleshooting. If you have a cable break,
especially in the outside plant, the OTDR is the best way to
find it. You can also use it to verify splice loss (but test
both ways and average to get a reliable measurement) or find
problems with back reflection (optical return loss).
- How do I see close features
with an OTDR?
The blind spot of an OTDR caused by crosstalk from the test pulse
can be overcome by using a "pulse suppressor", a long
(1 km is normal) length of cable to allow the OTDR to settle
down after the initial pulse.
- Do I need to test bandwidth?
Generally no. Most systems are specified for use with a minimum
bandwidth fiber and most fiber is much better than minimum specification.
Besides test equipment is not cost effective or readily available
for field use. Manufacturers of fiber and cables have the expensive
lab equipment to reliably test bandwidth (or actually dispersion),
but there are no good field testers. If you need bandwidth data
for an unusual application, ask the manufacturer of the fiber
or cable to assist you or use a simulation program, which are
available from some manufacturers of fiber.
More on Reflectance Testing
- Reflectance and Optical Return Loss
What is Reflectance and Optical Return Loss?
is the light reflected back from a connector or splice. Optical Return
Loss (ORL) is generally used to combine the reflectance from connectors
or splices with the backscatter from the fiber, so the term is
primarily used for longer cable runs. Reflectance was once called back
reflection, but that term, which is really redundant, has lost favor.
- When do I need to test optical
Reflectance or optical return loss mostly affects very high bitrate
digital or analog singlemode systems. None of today's multimode systems
are very sensitive to reflectance or ORL although high amounts can
create background noise in short links adversely affecting BER or data
transfer. For affecting laser sources, ORL is only important in the
first few connectors in the cable nearest the laser transmitter. But
some short SM cable plants used in premises systems are too short to
attenuate the reflected power from connectors so undergoes multiple
reflections until it builds up background noise that cab affect
- How do I test optical return
Use an OTDR on cable plants, OCWR on patchcords. ORL testing
with what people call a ORL tester ( or what Telcordia/Bellcore
calls an OCWR or optical continuous wave reflectomenter) is only
applicable to short patchcords. If you try using them on an installed
cable plant, the integrated backscatter from the length of the
fiber will overwhelm the connector back reflection. Twenty km
of fiber gives about the same backscatter as a connector with
20 dB ORL and you cannot tell where the backscatter comes from!
It's better to use an OTDR to find ORL problems on installed
- How accurate are ORL measurements?
The measurement uncertainty of ORL is very high, about +/-1 dB for
singlemode and +/-5dB for multimode, according to round robin results
from standards committees. This is a function of the difficulty in
creating a reference for the measurement, the reflection is very small
compared to the test signal causing noise problems and the dependence
of the measurement on the connector on the test apparatus is very high.
To minimize uncertainty, keep the connections extremely clean and
inspect the connectors continuously with a microscope and repolish when
needed. Making measurements to a 0.01 dB resolution is ridiculous;
remember this is a +/- 1 dB measurement.
- Do I need special instruments
to test ORL?
No, the special ORL tester is unnecessary. A good laser source
and power meter, along with a coupler that costs a few hundred
dollars will make a very good tester.
Most instructions for using OCWRs suggest using a mandrel wrap to
reduce the reflectance from the connector on the end of the
cable. If the cable has bend-insensitive fiber, as do many patchcords,
this method does not work. Instead dip the end connector in index
matching gel or fluid (vaseline, alcohol or mineral oil works in a
- Fiber Optic Cleaning Procedures
More on Cleaning
- How do you clean connectors
- With fiber optics, our tolerance
to dirt is near zero. Airborne particles are about the size of
the core of SM fiber and are ususlly silica based- they may scratch
PC connectors if not removed! Test equipment that has fiber-bulkhead
outputs need periodic cleaning, since they may have hundreds
of insertions of test cables in short time frames. Here's a summary
of what we have learned.
- 1. Always keep dust caps on
connectors, bulkhead splices, patch panels or anything else that
is going to have a connection made with it.
Use any of the commercial cleaning kits to clean connectors and mating
adapters. Alternatively, use lint free pads and isoproply alcohol to
clean the connectors. Some solvents MIGHT attack epoxy, so only pure
alcohol should be used. Cotton swabs and cloth leave threads behind.
Some optical cleaners leave residues. Residues usually attract dirt and
make it stick.
- 3. All "canned air"
has a liquid propellant. Years ago, you could buy a can of plain
dry nitrogen to blow things out with, but it's long gone. Today's
aerosol cleaners use non-CFC propellant and will leave a residue
unless you 1. hold them perfectly level when spraying and 2.
spray for 3-5 seconds before using to insure that any liquid
propellant is expelled from the nozzle. These cans can be used
to blow dust out of bulkheads with a connector in the other side
or an active device mount (xmit/rcvr). NEVER use compressed air
from a hose (they emit a fine spray of oil from the compressor!)
or blow on them (you breath is full of moisture , not to mention
all those yukky germs!)
- 4. A better way to clean these
bulkheads is to remove both connectors and clean with Alco Pads,
then use a swab made of the same material with alcohol on it
to clean out the bulkhead.
- 5. Detectors on FO power meters
should also be cleaned with the AlcoPads occasionally to remove
dirt. Take the connector adapter off and wipe the surface, then
- 6. Ferrules on the connectors/cables
used for testing will get dirty by scraping off the material
of the alignment sleeve in the splice bushing. Some of these
sleeves are molded glass-filled thermoplastic and sold for multimode
applications. These will give you a dirty connector ferrule in
10 insertions! You can see the front edge of the connector ferrule
getting black! The alignment sleeve will build up an internal
ledge and create a gap between the mating ferrules - viola:
a 1-2 dB attenuator! Use the metal or ceramic alignment sleeve
bulkheads only if you are expecting repeated insertions. Cleaning
the above reguires agressive scrubbing on the ferrules with the
AlcoPad and tossing the bulkhead away.
- 7. You can buy a cleaning kit
for fiber optics. They are good solutions but perhaps not as
cost effective as making your own to meet your needs.
(C) 2002-11, The Fiber Optic Association,
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