The cost of installation for most fiber optic cable exceeds the cost of the cable itself, so ensuring the cable is good before installation is important. The first test the installer must do with a spool of cable is visually inspect it. The spool of cable should show no visible damage, indicating it has been properly handled during shipment.
Even if the spool shows no damage, it only takes a few minutes to check a few fibers for continuity. Continuity tests can be done with either a visible fiber tracer, essentially a flashlight with a fixture to couple it to an optical fiber cable, or a visual fault locator, a similar device that uses a higher powered laser source. These testers are generally designed to attach to a fiber optic connector, but the spool of cable is likely to not yet have been terminated, so you will need to test bare fibers.
Testing bare fibers is not a problem; you just need a bare fiber adapter (BFA) or a plain, unused fiber optic connector. A BFA is a connector with a clamp on the backshell to hold the fiber, a gadget many installers make themselves with an unterminated fiber optic connector and masking tape. The cable should be stripped back a few inches to expose the fiber to be checked, the fiber is cleaved and inserted into the BFA and connected to the fiber tracer or fault locator. Then, strip the far end of the cable to expose the fibers, find the fiber being tested using color codes and visually check to see if light is coming out of the fiber, indicating continuity.
If the spool is visibly damaged, at a minimum, several fibers should be checked for continuity and perhaps even tested for loss. How do you test bare fibers for loss? You can use a cutback test, a variation of the terminated cable insertion loss test or an optical time--domain reflectometer (OTDR) test.
A cutback test is the most accurate. You need your lighting source, power meter and two bare fiber adapters. Strip off about 1 meter of fiber on each end of the cable. One BFA is used to connect one end of the fiber to the source, and, the other is used to connect the fiber to the meter. Read the power in decibels, and if possible, set this as your “0 dB reference.” Remove the fiber from the meter, but leave the source alone. Cut and cleave the fiber attached to the source, leaving about 1 meter of fiber, then cleave it and insert into the BFA at the meter. Read the power out of the short length of fiber. The difference between this and the reference measurement is the loss of the fiber.
Since you are making the measurement backward—with the reference at the far end, not at the source—the meter reading will be a positive number for loss, not negative. Compare this loss to the loss expected for the fiber length being measured and decide if the fiber is damaged.
You can also test the fiber like you would a patchcord (FOTP-171 test), attaching a 1 meter fiber or a pigtail to the source and the meter with BFAs, and measure a reference power that way. Use a mechanical splice to attach the source fiber to the fiber you want to test, and measure the loss at the far end with the meter. This is a less accurate measurement, as it includes a mechanical splice loss, which is dependent on the quality of the fiber cleaves.
Finally, you can use the same method with an OTDR. Use a long fiber pigtail as a launch cable and connect to the fiber under test with a mechanical splice. If you have an OTDR on site, this is the best way to test bare fibers, since it will show the location of any problems in the spool of fiber. If you don’t have an OTDR, it can be a very expensive solution if you rent the OTDR or hire a contractor with one.
Note: My September 2009 column, “Up to the Test,” was about equipping the installer with testing equipment. I recommend you reread this article at www.ECmag.com. Search for “Hayes” or “Fiber Optics.”
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.