Guidelines For Testing And Troubleshooting Fiber Optic Installations

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

standards.

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.

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 FOTP-

8/59/60/61/78, 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)

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:

http://www.thefoa.org/tech/ref/1pstandards/

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

FTTH.)

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.

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 I 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

microscope.)

3.2.5. Testing And Troubleshooting Patch cords

FOA Standard FOA-2: Testing Loss of Fiber Optic Cables, Single Ended, (Insertion

Loss, TIA FOTP-171, OFSTP-7, , ISO/IEC 14763)

Patch cords 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.

3.2.5.1. Likely Problems

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.

3.2.5.2. 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 loss.

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

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 excessive.

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 non-laser- optimized fiber (usually 62.5/125 FDDI grade fiber) may suffer nonlinear 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 all.

vii. Single mode 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

problems.

References

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

FOA.

The FOA Reference Guide to Outside Plant Fiber Optics, by Jim Hayes, published

by the FOA.

FOA Online Reference Guide, FOA website, www.thefoa.org

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.

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.