How Much Do You Know Fiber Optic Testing?

Fiber OpticFor every fiber optic cable plant, you need to test for continuity and polarity, end-to-end insertion loss, etc. If there were a problem, it must be fixed to keep the fiber optic cable plant working p…

For every fiber optic cable plant, you need to test for continuity and polarity, end-to-end insertion loss, etc. If there were a problem, it must be fixed to keep the fiber optic cable plant working properly and ensure the communications equipment operate well.


Testing Tools

Fiber optic cable testing needs special tools and instruments. And they must be appropriate for the components or cable plants being tested. The following five kinds of fiber testing tools are needed for the testing work.

OLTS—Optical loss test set (OLTS) with optical ratings matching the specifications of the installed system (fiber type and transmitter wavelength and type) and proper connector adapters. Power meter and source are also needed for testing transmitter and receiver power for the system testing.
Reference test cable—This cable should be with proper sized fiber and connectors and compatible mating adapters of known good quality. And the connector loss is less than 0.5 dB.
VFL—Visual fiber tracer or visual fault locator (VFL)
Microscope—Connector inspection microscope with magnification of 100-200X, video microscopes recommended.
Cleaning Materials—Cleaning materials intended specifically for the cleaning of fiber optic connectors, such as dry cleaning kits or lint free cleaning wipes and pure alcohol.

Notes Before Testing Cleaning Issue

Before testing, it’s very important to keep connector clean so that there is no dirt present on the end face of the connector ferrule as the dirt will cause high loss and reflectance. For example, the dust caps which is used to keep connectors clean usually contain dust. So it may leave residue or cause harm to the connectors to use cleaning tools with dirt.
Eye Protection

Connector inspection microscopes focus all the light into the eye and can increase the danger. Some DWDM and CATV systems have very high power and they could be harmful. Though fiber optic testing sources are too low in power to cause eye damage, it’s still suggested to check connectors with a power meter before looking it. As most fiber optic sources are at infrared wavelengths that are invisible to the eye, making them more dangerous. So better protect your eyes from these potential harms.

Loss Budget

Before testing, you should clearly know the loss budget as reference loss values for the cable plant to be tested. Here are some guidelines:

For connectors, 0.3-0.5 dB loss; for adhesive/polish connectors, 0.75 dB loss; for prepolished/splice connectors (0.75 max from TIA-568)
For single-mode fiber, 0.5 dB/km for 1300 nm, 0.4 dB/km for 1550 nm. It means a loss of 0.1 dB per 600 feet for 1300 nm, 0.1 dB per 750 feet for 1550 nm.
For each splice, 0.2 dB
For multimode fiber, the loss is about 3 dB/km for 850 nm, 1 dB/km for 1300 nm. It means a loss of 0.1 dB per 100 feet for 850 nm, 0.1 dB per 300 feet for 1300 nm.

So for the loss of a cable plant will calculated as (0.5 dB X # connectors) + (0.2 dB x # splices) + fiber loss on the total length of cable.
Fiber Optic Loss Testing

Before installation, it’s necessary to inspect all cables as received on the reel for continuity using a visual tracer or fault locator. An OTDR is needed to test if cables are damaged during the shipment. Any cable showing damage should not be installed.

After installation, all cables should be tested for insertion loss using a meter of OLTS according to standards OFSTP-14 for multimode fiber and OFSTP-7 for single-mode fiber. Usually 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 patch cords that will be used to connect the communications equipment which are tested separately. Insertion loss testing should be done at the wavelengths of 850/1300 nm with LEDs for multimode fiber, 1310/1550 nm with lasers for single-mode fiber, 1490 for FTTH. Keep the data on insertion loss for future comparisons if problems arise or restoration becomes necessary. 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.
Testing Results and Methods

If the cable plant loss is tested within the loss budget, the communication link should work properly.

If the loss is higher than the loss budget, first you need to test in the opposite direction using the single-ended method. Since this method can only test the connector on one end, you can isolate a bad connector. If the tested losses are the same on both directions, you need to test each segment separately to isolate the bad segment or use an OTDR if it is long enough.

If there is no light through the cable and only darkness when tested with your visual tracerArticle Submission, there must be very high loss. Then you need to cut the connector on one end (maybe the wrong one) by your decision.