In August, we looked at the historical development of unshielded twisted pair (UTP) cable. This month, let’s look at the much shorter—but no less important—history of multimode optical fiber and examine today’s choices. Answers and explanations are highlighted in red.

1. The first optical fiber, which was widely used for the first telephone systems but then faded into obscurity, has become popular again. What was that fiber?
A. Single-mode
B. Multimode, 50/125 core/cladding diameter
C. Multimode, 62.5/125 core/cladding diameter
D. Multimode, 100/140 core/cladding diameter

Until singlemode fiber was perfected around 1983, 50/125 fiber was used in all telephone fiber optic networks.

2. Why has that fiber regained its popularity?
A. It offers better performance with laser sources in high-speed links.
B. It has significantly lower loss than other fiber types.
C. It’s much easier to terminate.
D. It’s half the price of other fibers.

Gigabit Ethernet was too fast for LEDs, so it used low cost vertical cavity surface-emitting lasers (VCSELs), which worked better on 50/125 fiber.

3. When fiber began being used in premises applications, such as LANs and datalinks, that same fiber was not as popular because _________.
A. It was not easy to terminate
B. It coupled much less light from LED sources
C. It was too expensive
D. All of the above

50/125 fiber has a smaller diameter and lower numerical aperture than 62.5/125 or 100/140 fiber, so it coupled less light from the typical LED source, limiting transmission distances.

4. A replacement fiber design offered for 100/140 fiber in datalinks and LANs was an 85/125 fiber that never gained significant popularity.
True
False

The 85/125 fiber was optimized for the LEDs of the mid-1980s but was not significantly better than the 62.5/125 fiber already in wide use.

5. Of all the multimode fibers, 100/140 coupled the most power from LED sources but lost favor because _________.
A. It was too sensitive to bending losses
B. It was difficult to polish with the available connectors
C. It cost twice as much as
other fibers
D. It was incompatible with most connectors that had ferrule holes for 125 micron fiber

Connector manufacturers were not pleased to have to make special connectors for just one fiber – 100/140.

6. FDDI, the first computer network designed exclusively for optical fiber, standardized on one fiber, which was known for years as “FDDI Grade” fiber. What fiber was it?
A. Single-mode
B. Multimode, 50/125 core/cladding diameter
C. Multimode, 62.5/125 core/cladding diameter
D. Multimode, 100/140 core/cladding diameter

FDDI standardized on 62.5/125 multimode fiber, the most popular fiber at the time, lending it a new name: FDDI grade fiber

7. Gigabit Ethernet caused the revival in the older fiber size because of its greater compatibility with _________.
A. VCSEL laser sources used in gigabit Ethernet
B. Small form factor connectors used in gigabit transceivers
C. LEDs sources at 1,310 nm
D. Wavelength division multiplexing

See 2 above

8. Further development of that same fiber created what we now call _________ or _________ fiber.
A. OM1, laser-rated
B. OM2, laser-standard
C. OM3, laser-optimized
D. OM4, laser-graded

International standards now call laser-optimized 50/125 OM3 fiber. Regular 50/125 is called OM2 fiber and 62.5/125 is OM1 fiber.

9. Development of this same fiber continues in an attempt to provide longer distances for networks operating at 1, 10, 40 and even 100 gigabits per second.
True
False

OM3 fiber with even higher bandwidth capability is being developed, perhaps to be called OM4 or enhanced OM3.

10. While this fiber development continues, networks are being created that work at higher speeds using _________.
A. New types of faster lasers
B. Multiple wavelengths on a single fiber
C. Bidirectional signals like on UTP Cat 6A
D. Digital signal processing copied from 10Gbase-T

Using wavelength-division multiplexing allows sending several signals over the same fiber simultaneously, allowing it to carry even more data.

HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.