Fiber optic installers make some common, avoidable mistakes that cause problems for them and their customers. Do you know about these mistakes and how to avoid them? Correct answers and explanations are in red.
1. Outdoor cables should not be used indoors primarily because ______.
A. They are not flame-retardant
B. Water-blocking gel may leak out of the cables
C. Metallic armor requires careful grounding
D. All of the above
Not all outdoor cables are armored or contain water-blocking gel, but they are not rated for indoor use because of their lack of flammability rating.
2. Indoor cables should not be used outdoors because indoor cables ______ and ______. (Choose two)
A. Are generally not water-blocked
B. Contain too few fibers for outdoor use
C. Are never armored
D. Are not rugged enough for outside environments
Indoor cables are not designed for water-blocking or the rugged nature of outdoor installation.
3. Optical fiber should not be cleaned with medical-grade isopropyl (rubbing) alcohol because it ______.
A. May attack the cables’ plastics
B. May attack the connectors’ adhesives
C. Contains water that may harm fibers
D. Does not clean grease well
Isopropyl rubbing alcohol contains up to 30 percent water, which may harm the fibers or make adhesives in connectors weak or hard to set. Pure isopropyl alcohol should be used for fiber optics.
4. When installing adhesive/polish connectors, the adhesive must be ______.
A. Approved for use with glass or plastic
B. Fast-curing to speed up the process
C. Tested and approved for fiber optic connectors
Users sometimes try unapproved adhesives for fiber optic connectors but the connector may fail.
5. When polishing fiber optic connectors, using higher pressure and more polishing time will generally result in ______.
A. Higher losses
B. Lower losses
C. Fewer scratches
D. A PC finish
Higher losses wear down the fiber improperly. The proper pressure can be determined using a postal scale.
6. Do not confuse “dB” and “dBm” when testing with an optical power meter because “dB” must be used for loss measurements and “dBm” must be used for power
measurements at transmitters and receivers.
Using the wrong scale can result in big errors since one never knows where the dB reference is set.
7. Measuring optical power with the wrong wavelength can result in erroneous measurements because ______.
A. Connector loss is wavelength-sensitive
B. Splice loss is wavelength-sensitive
C. Fiber loss is wavelength-sensitive
D. Power meter measurements are not wavelength-sensitive
Only fiber loss is affected by wavelength.
8. Testing cables or connecting a cable plant with jumpers that use the wrong size fiber will result in ______.
A. Higher loss going from the larger fiber to the smaller fiber
B. Higher loss going from the smaller fiber to the larger fiber
C. The same loss in either direction
Going from a larger multimode fiber (62.5/125) to a smaller fiber (50/125) can result in up to 4 dB of loss but no excess loss in the other direction. Mixing singlemode and multimode fiber can result in up to 20 dB loss!
9. When testing the cable plant, either optical time-domain reflectometer (OTDR) testing or optical loss test set (OLTS) may be used because both instruments work the same way and will give the same results.
OTDRs use an indirect method to estimate the loss while an OLTS uses a direct method called insertion loss to test the cable plant as it will be used by a transmitter and receiver of a communications system.
10. When connecting fiber optic cables with incompatible connectors for testing or connecting equipment to the cable plant, one can use ______ or ______. (Choose two)
A. Hybrid cables with a different connector on each end
B. Hybrid cables with a different fiber size
C. Hybrid mating adapters if the connector ferrules are the same size
D. Hybrid mating adapters
If the connectors have different ferrule sizes (e.g. SC with a 2.5-mm ferrule and LC with a 1.25-mm ferrule), a hybrid mating adapter will not provide sufficiently accurate alignment for test purposes so hybrid cables should be used.
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.jimhayes.com.