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The optical loss of a fiber optic cable plant is the most common measurement installers make. Also called insertion loss, it is measured by using a test source, a power meter, reference test cables and connector-mating adapters. The insertion loss test mimics the actual usage of the cable plant with a transmitter and receiver, so it is the best indication of the condition of the cable plant. Another loss test, using an optical time-domain reflectometer (OTDR), is generally not allowed for acceptance testing because it uses a different measurement technique that may not correlate with actual system link loss. We’ll discuss OTDR testing in future articles.
The test source for loss measurements should be compatible to the fiber type and wavelengths of systems that may use the cable plant. Most test sources for multimode fiber use LED sources at 850 and 1,300 nm wavelengths, although many systems today only require testing at 850 nm and use a 850 nm vertical cavity surface emitting laser (VCSEL), a low-cost laser. Single-mode test sources are generally lasers at 1,310 and 1,550 nm. Short links can be tested at just 1,310 nm, but long links require testing at 1,550 nm. Testing single-mode at 1,550 nm also allows one to look for stress on the cable, as single-mode fiber is more sensitive to stress losses at 1,550 nm.
In order to get accurate loss measurements, certain precautions must be taken. Multimode sources must be “mode conditioned,” meaning the light they launch into the fiber needs to be manipulated into a standard test condition. Since most test sources are light-emitting diodes that overfill the fiber, a mandrel wrap mode filter on the launch cable is used to condition the signal. It’s important to remember this, because the mode conditioning can make a big difference in the measured loss, and the loss will be lower. In a typical multimode link of several hundred meters with two or three connections, the difference between losses without and with mode filtering can be 0.2 dB or more, a large percentage error when the total loss is only about 2 dB.
To accurately measure loss on long single-mode links, you need to know the wavelength of the source. The attenuation of all fibers varies with wavelength and is lower at longer wavelengths except in the areas of water absorption at 1,244 and 1,383 nm. Lasers within the range of 1,290 to 1,330 nm are considered acceptable for “1,310 nm” test sources. That small difference in wavelength can cause a variation in loss of around 0.4 dB in a 10 km cable plant with a total loss of only 4–5 dB. Test equipment manufacturers generally test and specify the wavelength of lasers used in their sources so you can quickly tell if you need to be concerned. Always document the source wavelength on test results in case the measurements are ever questioned.
Some instruments include both the source and power meter and are called an optical loss test set (OLTS). An OLTS is a more expensive, complicated instrument because it generally has two sources coupled into a single test port, which allows tests to be made of one fiber at both wavelengths of interest. Some even can measure a fiber in both directions. Obviously, these instruments can save a lot of time when testing many fibers, which can justify their much higher cost. Do be aware that a multimode OLTS has fiber inside the instrument and may be set up for either 50/125 or 62.5/125 fiber so it may not be as versatile as a separate light source and power meter. It also may not be calibrated as a power meter, limiting its use to testing loss.
Another problem common to an OLTS is setting pass/fail criteria. As default settings, many use the TIA-568 standards, which allow 0.75 dB loss per connection and 0.3 dB loss per splice. That is OK for premises cabling that uses some types of connectors, but is completely inappropriate for single-mode fiber, where connectors are typically 0.2 dB and splices under 0.1 dB. Check your setup before accepting pass/fail from the instrument.
Of course, the uncertainty of loss measurements depends on the instruments but also on the reference test cables and adapters and the setup. Care taken in making loss measurements is rewarded by reproducible measurements that will satisfy the customer for the installed cable plant. Next month, we’ll cover that topic.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.