Having covered how to design and install fiber optic networks in recent columns, we are ready to start a new series, “Testing the Fiber Optic Cable Plant.”
After fiber optic cables are installed, spliced and terminated, they must be tested. Fiber optic testing verifies that the installation has been done properly and provides the end-user with proof-of-completion documentation defined by the installation contract.
Testing should begin before installation, when cable delivered to the work site will be checked to ensure no damage occurred during shipment. If there is no visible damage to the cable, testing may involve a simple continuity check on a few fibers in a cable. Cable that you suspect may have been damaged or shows visible signs of damage should be tested for loss. This ensures you do not install damaged cable.
Once you have installed the fiber optic cable plant, you need to test for continuity and polarity, end-to-end insertion loss and then troubleshoot any problems. If it is a long outside plant cable with intermediate splices, you will probably want to verify the individual splices with an optical time-domain reflectometer (OTDR) test also, since that is the only way to make sure that each splice is good. New patchcords may not need to be checked, unless they are part of a batch where bad cables are suspected, but they can generally just be cleaned and installed.
Test results must be used to determine if an installation is good or bad. That decision is made after comparing the test results to the expected results, which should be calculated during the design phase when a loss budget is created. When loss budgets are not available, the personnel testing the cable plant must calculate expected losses, so they have data to compare to their test results.
If you are installing the communications equipment or are the network user, you may also need to test transmitter and receiver power, as power is the measurement that ultimately tells you whether the system is operating properly over the installed cable plant. Sometimes, the power is too high, so you must install attenuators to reduce the power to an acceptable level.
Prefabricated cable plants, those made in factories to a building design and already terminated, are easy to install since they require no termination, but they still require testing. Here, the process can be tricky, as it means your field-tests will be compared to the controlled conditions used in factory tests.
All testing must be done according to industry standards. In fact, testing is the subject of the majority of industry standards for fiber optics, detailing both how the tests must be performed and what test conditions must be met. Testing standards are needed to verify component and system specifications in a consistent manner so everyone—manufacturers, installers and users—can agree on the performance of the components. Standard test procedures exist for testing individual components, factory manufactured cable assemblies and field-installed cabling.
Test conditions are carefully specified by the standards so that results of tests from several sources are accurate and comparable. The most widely quoted test conditions apply to measurements of insertion loss in multimode cable plants where the results of loss tests are highly dependent on how the source couples power into the fiber. Long-haul single-mode cable plant testing requirements primarily cover source wavelength, especially in dense wavelength-division multiplexing (DWDM) systems, but may also include chromatic and polarization-dependent dispersion highly complex tests.
An extremely important part of fiber optic testing is having the proper testing equipment and knowing how to use it. No matter which test is being performed, the accuracy of the results is always going to be dependent on using the right equipment, which has been properly calibrated; knowing the correct test setup; and interpreting the results properly.
Three factors—test equipment, test setup and interpretation of the results—are the key to every fiber optic testing situation. But, if you have read this far, you may feel a bit overwhelmed by the extent of knowledge required to test fiber optics properly. That is understandable, since it’s a complicated subject. However, over the next few months, I’ll break fiber optic testing down into smaller topics and explain each with clarity so you will hopefully end up with the knowledge required to test any fiber optic system properly.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.