Single-mode (SM) Fiber is practically the only fiber used in outside plant (OSP) installations and is even becoming more common in premises networks. Telephone, cable television, smart grid and cellular systems are all on SM fiber. As local area networks (LANs) become faster, going up to 40 and 100 gigabits per second, backbones sometimes will include SM fiber. In addition, SM fiber makes up the new passive optical LAN (POL) architecture (as covered in ELECTRICAL CONTRACTOR, February 2010) for fiber to the home (FTTH) technology.
In many ways, SM fiber is easier to test than multimode. Since light is only carried in one mode or ray of light, the problems one has controlling mode power distribution when testing multimode fibers is not an issue with SM fiber. But SM links often cover longer distances, which require splicing of several cables to reach the full length of the link, and those splices need testing. Really long SM links may require dispersion testing, and any link intended for wavelength-division multiplexing may require attenuation testing over the full wavelength range, which will ensure proper operation with transmission systems.
Insertion-loss testing of SM fiber with a source and power meter is straightforward; it uses the same methods for setting references and testing cables as multimode fiber (ELECTRICAL CONTRACTOR, September 2010), but requires a compatible test source. For longer lengths of fiber, one should always use a laser source at the proper wavelengths, usually 1,310 and 1,550 nm. Reference test cables should be low-loss cables that are cleaned and inspected regularly to ensure dirt or scratches do not affect loss measurements.
With SM test cables, there are two things to remember. First, laser sources may launch several modes into short SM fibers, which should be removed by placing a 2–4 inch (50–100 mm) loop in the launch cable. Second, SM fibers are much more sensitive to stress than multimode, so avoid tight bends in the reference cables, especially at 1,550 nm and around connections.
Longer cables with splices will generally be tested with an optical time-domain reflectometer (OTDR), a sophisticated tester that works like radar to test the attenuation coefficients of the fiber, measure splice and connector loss, and create a snapshot of the fiber under test. OTDRs are often used to verify splices as they are made to ensure only good splices are sealed in splice closures. SM fiber systems can be very sensitive to reflectance at connectors or splices, and OTDRs are the only way to pinpoint reflectance problems. OTDRs can also detect problems with fibers damaged during installation or under stress at splice or termination points.
OTDRs can’t be used easily for short links because they have limited distance resolution and often provide confusing data on short links used in premises cabling or FTTH. OTDRs are also complicated instruments that require good training to understand how to use them properly. (Stay tuned, I’m going to be discussing OTDRs, their advantages and limitations, and how to use them properly in the next few columns.)
Many SM links are very long, some up to thousands of kilometers long. While some people think that SM fiber has infinite bandwidth, long links can suffer from dispersion too. Chromatic dispersion (CD) is caused by the spectral width of the laser sources used as transmitters. Polarization-mode dispersion (PMD) is caused by polarization of light in the fiber by the variations in the fiber’s core or stress on the cable. Both CD and PMD can be issues on long links that require testing. CD is straightforward and can even be tested with some OTDRs, but PMD is quirky. The magnitude of PMD can change as the fiber is stressed, so testing aerial cable can be tricky if it’s windy, for example, or the temperature changes. Both CD and PMD have several ways they can be measured and both require special equipment that is expensive.
The final SM test that may be required is spectral attenuation (SA), which characterizes the attenuation of the fiber over the full wavelength range of operation. Wavelength division multiplexing (WDM) can use sources from 1,260 to 1,675 nm, including around 1,383 nm where some fibers show a high loss due to water absorption. Telcos often use WDM to expand services instead of using additional fibers because it’s cheaper and more simple, so measuring SA is becoming more common.
Overall, testing SM fiber optic cabling can be more difficult than multimode, so the required testing needs to be defined before the contractor begins to ensure the data will satisfy the customer.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.JimHayes.com.