You get what you pay for

Every fiber optic cable needs to be tested for loss with an optical loss test set (OLTS). The OLTS simulates the actual network hardware that will run on the cable so the test results will show if the network will work properly on the cable. It consists of a test source, which simulates the transmitter of a fiber optic link, and a power meter, which measures the loss. Choosing an OLTS involves two major issues: finding an OLTS appropriate for the cables to be tested and deciding what OLTS options are desired.

The OLTS must match the cables to be tested. If the fiber is multimode, you need a LED source at 850 and 1300nm, the typical sources for multimode fibers. If users ask you to test multimode fiber for Gigabit Ethernet, you may also need an 850nm VCSEL laser source to match the network transmitter.

Singlemode fibers need a laser source for testing. If the fibers are short ones, such as those in building or campus networks, a 1,310nm laser may be all you need, although testing at 1,550nm is required for longer outside plant runs.

A fiber optic power meter calibrated at 850, 1,300 and 1,550nm will work for all types of fibers, singlemode or multimode.

There are three types of optical loss test sets available: the one-piece OLTS with the source and meter in a single instrument, test kits with separate sources and meters and adapters for copper cable testers. If you buy a one-piece OLTS, you can only test cables when you have access to both ends of the cable in one location, such as on a reel before installation, when it’s probably not terminated. In the real world, you will need two of these instruments, one for each end of the cable. This means you can test two fibers at once, but you have to invest twice as much money in instruments! The one-piece OLTS usually offers automated reference and loss measurements and test data storage, making it more efficient once you learn how to use it.

Test kits with separate sources and meters are more versatile and less expensive. You are only buying one each of a source and meter, so it costs less. If you need both a LED and laser source, they share one power meter, further reducing costs. Since the instruments are separate, you can test fibers one at a time, no matter where the ends are. The individual power meter can also be used to measure transmitter or receiver power, not usually possible with other types of OLTS. And general purpose power meters come in many varieties, from inexpensive manual units which require calculating loss and writing down measurements to automated units which measure power, calculate loss and store data for reports. You can choose a test kit to fit both your needs and your budget.

Most manufacturers of cable testers offer adapters that convert their Category 5/5e/6 tester to an OLTS. This allows you to share the automation built into the cable testers and usually the data reporting software. The negative aspect of this approach is the adapters are expensive and tie up an even more expensive instrument that could be needed for testing copper cables elsewhere on the job.

Good reference cables are the key to accurate measurements. They must be cables with low-loss connectors and they must be kept clean or they will adversely affect measurements. You do not need special high-grade cables, just regular cables with low connector loss, preferably factory-made with epoxy-polish connectors. Store the cables where they are not under stress and clean the connector ends before each use. Use your instruments to test them for loss and inspect the cables under a microscope periodically. Sometimes you can repolish cables that get worn or scratched, but they must be replaced fairly often.

Unlike copper cabling, all fiber optic cables do not have the same type of connector. If you choose an OLTS with a fixed fiber optic connector interface, you will need to carry special hybrid reference test cables for every type of connector you have to test. Most fiber optic power meters offer several types of screw-on adapters to convert the test interface to the connector type being tested, simplifying reference cable stocking.

Choose your instruments wisely, buy them from a reliable source and learn how to use them well before taking them on a job. Job sites are not the place to try to figure out how a new tester works.

Interested in learning how to test fiber with an OLTS? See Lennie Lightwave’s Guide to Fiber Optics for a virtual hands-on tutorial on fiber optic testing at www.LennieLightwave.com. EC

HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.