Typical fiber optic cable plants are composed of a backbone cable connecting patch panels and several short jumper cables which connect the equipment onto the cable plant. Building and campus systems look like the photo to the right, where the backbone fiber is terminated in wiring closets and short jumpers connect wall outlets or directly to the equipment. These installations often have no splices at all, since distances are short.
Telco cable plants look similar, but the cable runs may be long, requiring splices every 2-4 km. In addition, the fibers are not terminated directly, but high quality factory made pigtails are spliced onto the backbone cable. The process of testing any fiber optic cable plant during and after installation includes all the procedures covered so far.
To thoroughly test the cable plant, one needs to test it three times, a continuity test of the fiber before installation, then insertion loss of each installed segment and complete end to end loss. One should test the cable on the reel for continuity before installing it, to insure no damage was done in shipment from the manufacturer to the job site. Since the cost of installation usually is high, often higher than the cost of materials, it only makes sense to insure that one does not install bad cable. It is generally sufficient to just test continuity, since most fiber is installed without connectors and then terminated in place, and connectors are the most likely problem to be uncovered by testing for loss. However, if any damage is visible on cable reels, OTDR testing may be needed to verify the cable is still good.
After installation and termination, each segment of the cable plant should be tested individually as it is installed, to insure each connector and cable is good. Finally each end to end run (from equipment placed on the cable plant to equipment) should be tested as a final check. Measured loss should be compared to the calculated loss budget for the cable plant to determine if the measured loss is acceptable.
Testing the complete cable plant is done per standard test procedures, e.g. FOA Standard FOA-1, TIA OFSTP-14 for multimode fiber or OFSTP-7 for singlemode, which use the same procedures or ISO/IEC 61280, ISO/IEC 14763, etc. These standards offers 3 different ways to set a "0 dB" reference and cover the peculiarities of multimode fiber in detail. Information for multimode cables covers the problems of controlling mode power distribution, but the same procedures apply for singlemode fiber, less the concerns expressed for mode power distribution errors.
For multimode fibers, testing is now usually done at 850 and sometimes also at 1300 nm, using LED sources with some control over mode power distribution. This will prove the performance of the cable for every datacom system, including FDDI and ESCON, and meet the requirements of all network vendors.
OFSTP-7 was written for singlemode fiber cables. Testing is usually done at 1300 nm, but 1550 or 1625 nm is sometimes required also. The1550 nm testing will show that the cable can support wavelength division multiplexing (WDM) at 1300 and 1550 nm for future service expansion. In addition, 1550 and 1625 nm testing can show microbending losses that will not be obvious at 1300 nm, since the fibers are much more sensitive to bending losses at the longer wavelengths.
If cable plant end to end loss exceeds total allowable loss, the best solution is to retest each segment of the cable plant separately, checking suspect cables each way, since the most likely problem is a single bad connector or splice. If the cable plant is long enough, an OTDR may be used to find the problem. Bad connectors must then be repolished or replaced to get the loss within acceptable ranges.
Once upon a time, OTDRs were used for all testing of installed cable plants. In fact, printouts or pictures of the OTDR traces were kept on record for every fiber in every cable. The power meter and source (or OLTS) have replaced the OTDR for most final qualification testing since the direct loss test gives a more reliable test of the end to end loss than does an OTDR. While international standards offer both OLTS and OTDR testing as options, all generally require OLTS testing for end-to-end loss testing for cable plant acceptance.
However, the OTDR may need to be used to find bad splices or optical return loss problems in connectors and splices in singlemode cable plant. Only with an OTDR can ORL problems be located for correction. Typical back reflection test sets only give a total amount of backscatter or return loss, not the effects of individual components, which is necessary to locate and fix the problem.
The OTDR can also be used to find bad connectors or splices in a high loss cable plant, if the OTDR has high enough resolution to see short, individual cable assemblies. However , if the cables are too short or the splices too near the end of the fiber (as is often the case in pigtails spliced onto singlemode fiber cables), the only way to localize the problem is to use a visual fault locator, preferable a high-power HeNe laser type, which can shine through the jacket of typical yellow or orange PVC-jacketed single fiber cables. This method of fault location is easiest if single fiber cables use yellow or orange jackets which are more translucent to thelaser light.