Troubleshooting fiber optic cable plants and communications systems

The Fiber Optic Association, Inc.
1119 S. Mission Road #355, Fallbrook, CA 92028
1-760-451-3655 Fax 1-781-207-2421
Technical Bulletin
Guidelines For Testing And Troubleshooting
Fiber Optic Installations
This is intended as an overview and installation checklist for all managers, engineers
and installers on the overall process of testing and troubleshooting a fiber optic
communications system. This document is based on the FOA books The FOA
Reference Guide to Fiber Optics (RGFO) and The FOA Reference Guide to Premises
Cabling (RGPC) and the FOA Online Reference Guide. You should also download a
copy of the NECA/FOA 301 fiber optic installation standard as a reference.
1. Once a fiber optic cable plant, network, system or link is installed, it needs to be
tested for four reasons:
a. to insure the fiber optic cable plant was properly installed to specified industry
b. to insure the equipment intended for use on the cable plant will operate properly
on the cabling
c. to insure the communications equipment is working to specifications
d. to document the cable plant and network for reference in case of future problems
2. Tools and Test Equipment Needed
The following tools are needed to test and troubleshoot the fiber optic cable plant,
system or link properly.
a. Optical Loss Test Set or power meter and test source with optical ratings
matching the specifications of the installed system (fiber type and transmitter
wavelength and type) and proper connector adapters. An OLTS that merely tests
cable plant loss may not include a calibrated power meter needed for testing
transmitter and receiver power, so a calibrated power meter and source are a
better choice for link or system testing.
b. Reference test cables with proper sized fiber and connectors and compatible
mating adapters of known good quality. These do not generally need to be
“reference quality” but only in good condition, generally defined as having
connector losses of less than 0.5 dB.
c. Visual fiber tracer and/or visual fault locator (VFL)
d. Connector inspection microscope with magnification of 100-200X and fixturing for
proper connectors. Video microscopes are recommended.
e. Cleaning supplies intended specifically for the cleaning of fiber optic connectors.
©2014, The FOA Inc.
f. Optional: OTDR with long launch and receive cables (100 m for Multimode, 1 km
or more for singlemode)
3. Testing And Troubleshooting The Installed Cable Plant
All fiber cable plants require certain basic tests to insure they were installed correctly
and meet expected performance values. These are guidelines for testing and
troubleshooting the cable plant itself. The most valuable data one can have for
troubleshooting is the installation documentation.
Note - Cleaning: Before any testing, connectors should be cleaned carefully to ensure
that no dirt is present on the end face of the connector ferrule as this will cause high
loss and reflectance. Protective caps on connectors, often called “dust caps” – some
say that’s because they usually contain dust – do not necessarily keep connectors
clean. Use cleaning supplies intended for cleaning fiber optic connecotrs only as other
materials my leave residue or cause harm to the connectors.
3.1. What Can Go Wrong?
There are a number of possible problems with fiber optic cable installations that are
caused by installation practice. These include:
a. Damage to the cable during installation caused by improper pulling techniques
(such as not pulling the fiber cable by the strength member,) excess tension,
tight bends under tension, kinking or even too many bends. Most of these
problems will be seen on all fibers in the cable.
b. Damage to the fibers in the cable during cable preparation for splicing or
termination. Fibers may be broken or cracked during cable jacket or buffer tube
removal or fiber stripping. This may affect all fibers in the cable or buffer tube or
just one fiber.
c. High loss splices caused by improper splicing procedures, especially poor
cleaving on mechanical splices or improper programming of fusion splicers. Most
fusion splicers give feedback on most problems if the operator is properly trained.
Individual fibers can be damaged when being placed in splice trays or tubes of
fibers damaged during placement in splice closures.
d. High loss connectors may be caused by bad processes or damage after
termination. Adhesive/polish connectors may have poor end finishes or cracks in
the fiber at the end of the ferrule or internally. Prepolished/splice connectors are
generally high loss due to poor mechanical splicing processes during termination
causing high internal loss.
3.2. Testing And Troubleshooting Steps For Installed Cable Plants
FOA Standard FOA-1: Testing Loss of Installed Fiber Optic Cable Plant, (Insertion Loss,
TIA OFSTP-14, OFSTP-7, ISO/IEC 61280, ISO/IEC 14763, etc.)
FOA Standard FOA-4: OTDR Testing of Fiber Optic Cable Plant (TIA FOTP8/59/60/61/78, ISO/IEC 14763, etc.)
FOA Standard FOA-2: Testing Loss of Fiber Optic Cables, Single Ended, (Insertion
Loss, TIA FOTP-171, OFSTP-7, , ISO/IEC 14763)
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3.2.1. Before installation, it is advisable to test all cable as received on the reel for
continuity using a visual tracer or fault locator. Cables showing signs of damage in
shipment may need OTDR testing to determine if the cable itself is damaged.
Obviously, no cable showing damage should be installed.
3.2.2. Test insertion loss after installation
a. After installation, splicing (if applicable) and termination, all cables should be
tested for insertion loss using a source and meter or OLTS (optical loss test set)
according to standards OFSTP-14 for multimode fiber, OFSTP-7 for singlemode
fiber. See FOA Standards for simplified explanations of these standards:
b. Generally cables are tested individually (connector to connector for each
terminated section of cable and then a complete concatenated cable plant is
tested “end-to-end”, excluding the patchcords that will be used to connect the
communications equipment which are tested separately.
c. It is the concatenated cable test that is used to compare to the link power budget
and communications equipment power budget to insure proper operation.
d. Insertion loss testing should be done at the wavelength of intended operation if
known or at two wavelengths with appropriate sources (850/1300 nm with LEDs
for multimode fiber, 1310/1550 nm with lasers for singlemode fiber, 1490 for
e. Unless standards call for bi-directional testing, double-ended testing with both
launch and receive cables (OFSTP-7/14) is adequate.
f. Data on insertion loss of each fiber should be kept for future comparisons if
problems arise or restoration becomes necessary. Recording data on a label
inside the patchpanel or enclosure is common practice.
g. Long cables with splices may be tested with an OTDR to confirm splice quality
and detect any problems caused during installation, but insertion loss testing with
an OLTS (light source and power meter) is still required to confirm end-to-end
loss. Cables with insertion loss near expected values do not also need OTDR
testing. Cables tested with an OTDR should have the data kept on file for future
needs in restoration.
3.2.3. Troubleshooting
a. First determine if the problem is with one or all the fibers in the cable. If all fibers
are a problem, there is a likelihood of a severe cable installation problem. If all
fibers are broken or have higher than expected loss, an OTDR will show the
location of the problem on longer cables but premises cables may be too short
and need physical inspection of the cable run. If the problem is caused by kinking
or too tight a bend, the cable will have to be repaired or replaced. Generally OSP
cables will be spliced as in a restoration and if the cable is a short OSP cable or
a premises cable, replaced.
b. High loss fibers have several potential causes, but bad splices or terminations
are the most likely cause for field terminated cables. In some cases, using
improper termination practices will result in high loss for all fibers, just as in
kinking or bending losses, not just one fiber.
c. Testing for high loss fibers should start with microscope inspection of
terminations for proper polish, dirt, scratches or damage.
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d. If dirt appears to be the problem, clean the connectors and retest.
e. If other connector damage is found on visual inspection, retermination will
probably be necessary. Sometimes scratches can be polished out with diamond
film by an experienced technician.
f. Prepolished splice connectors with internal splices will generally look OK when
inspected with a microscope unless damaged after installation. The most likely
cause of loss with these connectors is high splice loss in the internal splice. They
can be tested with a visual fault locator coupled into the fiber at the far end. High
light loss will be seen as an illumination of the connector ferrule. Some
connectors have translucent backshells and can be tested with a VFL coupled
directly into the connector.
g. If the reason for high loss is not obvious and the connectors are adhesive/polish
style, the problem may be a fiber break in the back of the connector. A VFL may
help in finding fiber breaks, depending on the connector style and the opacity of
the cable jacket.
h. Cables with a fiber or fibers showing very high loss or no light transmission at all
should be tested for obvious breaks in the pigtail fiber or cable, generally at the
splice or connector, with a visual fault locator or high resolution OTDR if the
cable is of sufficient length
i. Splice loss problems can be pinpointed during OTDR testing. Confirmation with a
VFL should be done if the length from the end of the cable is short enough (~2-3
km) where a VFL is usable. The VFL can find high loss splices or cracks in fibers
caused by handling problems in the splice tray.
j. High loss links where the excessive loss is only a few dB can be tested like a
patchcord with a single-ended test with a source and power meter. When tested
in this manner, a high loss connector will show high loss when connected to the
launch cable connector but not when connected directly to the power meter
detector which picks up all the light from the fiber.
3.2.4. Hints for troubleshooting
a. Having access to design specifications and installation documentation and
specifications will greatly assist troubleshooting.
b. If possible, interview the installer to help uncover processes that may lead to
issues in installation, such as pulling methods, lubrication, intermediate pulls,
splicing or termination methods (like improper field termination of singlemode
which can lead to high loss and reflection even when connectors look OK in a
3.2.5. Testing And Troubleshooting Patchcords
FOA Standard FOA-2: Testing Loss of Fiber Optic Cables, Single Ended, (Insertion
Loss, TIA FOTP-171, OFSTP-7, , ISO/IEC 14763)
Patchcords are short factory-terminated cables usually with standard heat-cured
epoxy/polish connectors on each end. They are used to connect equipment to the cable
plant and as reference cables for testing insertion loss. Likely Problems
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Most patchcord problems are connector problems, caused by damage due to handling
or numerous matings when used as reference cables for testing other cables.
Connectors may also be damaged by breaking fibers at the back of the connector due
to excess stress during handling or by placing other equipment on top of them in
enclosures or patch panels. Testing And Troubleshooting Steps
a. All patchcords, especially those used as reference cables for insertion loss
testing, should be tested for insertion loss.
b. Patchcords should be tested with an optical loss test set (optical power meter
and source) using single-ended FOTP-171 methods with one reference cable
used as a launch cable.
c. This will test the connector mated to the reference cable and the fiber in the
patchcord, which is short enough it should have no measurable loss.
d. Since the connector connected to the power meter will not be connected to fiber
but presented directly to the detector of the power meter, it effectively has no
e. After testing in one direction, reverse the patchcord and test the other end.
f. In both directions, factory-made patchcords should have a loss of less than 0.5
or whatever performance the user has specified with patchcord vendors.
g. High loss connectors should be inspected with a microscope for dirt or damage.
h. If other connector damage is found on visual inspection, retermination will
probably be necessary but may not be cost effective, so the patchcord should be
replaced. Sometimes scratches can be polished out with diamond film by an
experienced technician.
i. Some optical loss test sets include fiber interfaces on both source and meter
ports, so all testing is done double-ended, even if the cable under test is directly
connected to an input port. A test set such as this makes reverse testing less
effective since reversing test direction may not have any significant effect. Test
ports on an OLTS like this should be kept covered when not in use and cleaned
periodically. Damaged fibers inside an OLTS will require factory repair.
4. Testing And Troubleshooting Communications Equipment
FOA Standard FOA-5 Fiber Optic Datalinks
FOA Standard FOA-3: Measuring Optical Power (Transmitter and Receiver Power,
FOTP-95, Numerous ISO/IEC standards)
After the cable plant has been tested, the communications equipment should be
properly connected using matching known-good patchcords. If the cable plant loss is
within the loss budget of the equipment (including the loss of the patchcords), the
communications link should work properly. If the link does not work, most likely potential
problems are the following.
a. Improper connections
b. Cable plant problems
c. Malfunctions of communications equipment
4.1. Testing And Troubleshooting Steps For Communications Equipment
©2014, The FOA Inc. TestTroubleshoot.doc 2/11/14 p5
a. Improper connections. The system requires a transmitter be connected to a
receiver, of course, so it is important to verify this connection for each link. Even
if the cable plant is properly documented, fibers may have been crossed at
intermediate connections, so using a visual tracer or visual fault locator will allow
quick confirmation of the connection.
b. The functioning of the communications equipment:
i. If it is connected to the cable plant but not operating properly, begin by
checking the power at the receiver on one end of the link.
ii. Disconnect the cable at the receiver input and measure power with an optical
power meter. Make sure the equipment is trying to transmit a signal. Some
equipment has a testing mode to force transmission of a test signal or the
equipment may simply keep transmitting to try to complete a connection.
iii. If the receiver power is within specifications, the receiver or electronics
beyond the link may be the problem. Use equipment diagnostics or consult
the manufacturer for assistance.
iv. If the receiver power is too high, it may be overloading the receiver and an
optical attenuator should be inserted at the receiver end to reduce the power
to the proper level.
v. If the receiver power is lower than required by operating specifications, the
cause is either low transmitter power or too much loss in the cable plant.
vi. To test transmitter power, disconnect the patchcord connecting the
transmitter to the cable plant and measure the optical power. If the power is
low, there is a problem with the transmitter or patchcord.
vii. To determine which is the problem, try testing the transmitter with a known
good patchcord. If the power is then within spec, replace the bad patchcord
and test the link again.
viii. If the transmitter power is low with a known good patchcord, the equipment
may need maintenance (cleaning) of the output port or replacement.
ix. If the transmitter tests as good but receiver power is low, the problem is
probably in the cable plant. First try to switch the communications link to
spare fibers to see if that solves the problem. Next test the loss of the suspect
fibers in the cable plant with an OLTS to determine if the cable plant loss is
c. Cable Plant Problems
i. High loss in the cable plant can be caused by damage after installation and
testing. Use a visual tracer or visual fault locator to confirm continuity and an
OLTS to test loss. See directions above on testing the loss of the cable plant.
ii. If the cable plant is long enough (>100m), it can be tested with an OTDR to
pinpoint problems.
iii. If the cable plant loss is not the problem, there are other possible issues
related to the bandwidth of the cable plant.
iv. Multimode cable plants operating at 1300 nm with LED sources may have
bandwidth problems caused by the total dispersion due to both chromatic and
modal dispersion.
v. Multimode cable plants operating at 850 nm with VCSEL sources on nonlaser-optimized fiber (usually 62.5/125 FDDI grade fiber) may suffer nonlinear
©2014, The FOA Inc. TestTroubleshoot.doc 2/11/14 p6
modal dispersion that can produce distorted pulses that will cause data
transfer problems.
vi. Multimode cable plants operating at 1300 nm with laser sources may have an
improperly installed mode-conditioning patch cord (offset-launch) or none at
vii. Singlemode links may suffer from problems caused by reflections at
connectors or mechanical splices.
viii. Reflections in singlemode terminations or splices near the source may cause
nonlinearities in the laser transmitter which distort pulse shapes, causing high
bit error rates (BER).
ix. Reflections near the receiver or at both ends can cause multiple reflections in
the cable that create “optical noise” that causes BER.
x. Reflections can be tested, if the cable plant is long enough (>100m), with an
OTDR to pinpoint problems.
xi. Reflections can be reduced by introducing an index-matching gel or fluid in
the joint (Vaseline or mineral oil works, but is messy to clean up) to see if that
solves the problem.
xii. Highly reflective connectors or splices should be replaced as soon as
possible. Remember most singlemode terminations are made by fusion
splicing factory-terminated pigtails onto installed cabling.
5. Update Documentation
After completing tests, troubleshooting and repairs, update documentation to reflect the
necessary procedures and any changes to the network. If the fix is to switch to spare
fibers and suspect fibers are not fixed, not that on documentation to prevent future
©2014, The FOA Inc. TestTroubleshoot.doc 2/11/14 p7
There are other FOA Technical Bulletins that should be used as references for the
design and planning of the network. These documents can be downloaded from the
FOA Tech Topics website. In addition to those, we recommend:
The FOA Reference Guide to Fiber Optics, by Jim Hayes, published by the FOA.
The FOA Reference Guide to Premises Cabling, by Jim Hayes, published by the
The FOA Reference Guide to Outside Plant Fiber Optics, by Jim Hayes, published
by the FOA.
FOA Online Reference Guide, FOA website,
NECA/FOA-301 Standard For Installing And Testing Fiber Optic Cables
(NECA/FOA-301), NECA Codes and Standards, 3 Bethesda Metro Center, Bethesda,
MD 20814 Download from FOA website
FOA Tech Bulletins (Printable Reference Documents)
Designing and manufacturing fiber optic communications products for manufacturers of
products using fiber optics . (PDF, 0.2 Mb)
Choosing, installing and using fiber optic products for communications network users.
(PDF, 0.1 Mb) (this document)
Designing Fiber Optic Networks - for contractors, designers, installers and users and the
reference for the FOA CFOS/D Design Certification (PDF, 1.3 MB).
Installing Fiber Optic Cable Plants. (PDF, 0.2 Mb)
Troubleshooting fiber optic cable plants and communications systems. (PDF, 0.1 Mb)
Fiber Optic Restoration - how to plan ahead and restore networks quickly. (PDF, 0.1
©2014, The FOA Inc. TestTroubleshoot.doc 2/11/14 p8
Note: This information is provided by The Fiber Optic Association, Inc. as a benefit to
those interested in designing, manufacturing, selling, installing or using fiber optic
communications systems or networks. It is intended to be used as a overview and
guideline and in no way should be considered to be complete or comprehensive. These
guidelines are strictly the opinion of the FOA and the reader is expected to use
them as a basis for creating their own documentation, specifications, etc. The FOA
assumes no liability for their use.
Do you have comments on this technical bulletin, corrections or information to
add to it to make it more complete. Please send them to the FOA at
The Fiber Optic Association, the professional society of fiber optics, has available
on its website,, guides for end users on fiber optic network design and
installation. The FOA has certified 24,000 technicians through over 200 approved
schools to create a pool of trained, experienced and certified techs who can install and
restore networks. You can search for techs or contractors with appropriate experience
throughout the world using the FOA’s free online database on its website.
The Fiber Optic Association, Inc.
1119 S. Mission Road #355, Fallbrook, California 92028 USA
1-760-451-3655 Fax 1-781-207-2421
©2014, The FOA Inc. TestTroubleshoot.doc 2/11/14 p9
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