Single-mode fiber is becoming more common in newer local area networks (LANs) and data centers, used in both traditional networks and passive optical LANs (POLs). You likely will see a lot more single-mode fiber in the future. How familiar are you with it? Correct answers and explanations are in red.
1. The advantage of single-mode fiber is that it has greater _________ than multimode fiber.
C. Resistance to bending
The reason single-mode is being chosen in place of multimode is the network speeds are getting so high that multimode fiber bandwidth is limiting link distances.
2. Sometimes backbone cables that include single-mode and multimode fiber are installed. They are called _________ cables.
Here, in the United States, we call a cable that has single-mode and multimode fibers a “hybrid” cable, but some international standards are considering changing to calling it a “composite” cable. In the United States, that’s a cable with both fiber and power conductors.
3. Single-mode fiber is usually field-terminated by _________.
A. Adhesive/polish connectors
B. Prepolished/splice connectors
C. Mechanical splicing
D. Fusion-splicing on factory-made pigtails
It’s hard to field polish single-mode, so most terminations are made by fusion-splicing on factor-made pigtails. However, advances in prepolished/splice connectors may make that a valid option, too. But many networks are now being designed for factory prefab cables that require no field termination.
4. Single-mode fiber can be purchased in most of the cable types used with multimode fiber for premises applications.
Most cable types can be ordered with either single-mode or multimode fiber or both. See question 2.
5. Short lengths of single-mode fiber require careful installation and testing to prevent problems caused by _________.
A. High loss
B. Reflectance or return loss
C. Spectral attenuation
D. Polarization or chromatic dispersion
Due to its low loss, reflectance can cause signals to reflect back and forth in a fiber causing noise that affects transmission.
6. Connectors with a _________ polished end finish are often used with short single-mode fiber links to prevent reflectance problems. These connectors are color-coded _________.
A. PC, beige
B. PC, black
C. APC, green
D. APC, black
Angled physical-contact (APC) connectors effectively stop reflectance. They are standardized as green connectors.
7. The same connectors can be used on both single-mode and multimode fiber and installed in exactly the same manner.
Single-mode connectors have smaller holes with tighter tolerances. Using multimode connectors on single-mode fiber will cause higher loss. Trying to use single-mode connectors on multimode fiber may be frustrating as some fibers may not fit the hole in the ferrule.
8. Bend-insensitive single-mode fiber is generally only found in cables used for _________.
A. Buried backbones
B. Aerial installation
C. Underwater installation
D. Patchcords or premises applications
Single-mode bend-insensitive fiber is not needed in most long cable links so it is mostly used in patchcords and for fitting in tight spaces in premises installations.
9. Single-mode fiber is always tested at _________ nm and often also at _________ nm, especially if it will be used for wavelength-division multiplexing.
A. 650, 850
B. 850, 1,300
C. 850, 1,310
D. 1,310, 1,550
Most short single-mode links operate at 1,310 nm and longer ones at 1,550 nm, but even short links are often used with wavelength-division multiplexing and need testing at both wavelengths.
10. Single-mode fiber cables should be tested for insertion loss with a _________ for acceptance testing and certification.
B. Optical loss test set
C. Visual fault locator
D. Chromatic dispersion test set
Optical loss test sets measure the insertion loss that is similar to how transmission equipment uses the link. OTDR traces are a snapshot of the link and are often taken if the link is long enough and recorded for future troubleshooting. Visual fault locators can be used to trace fibers and find local faults like too tight bends in a patchcord, patch panel or splice tray.
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.jimhayes.com.