Much of the fiber used in premises applications for computer networks and closed-circuit security cameras is multimode fiber. Multimode fiber has special application and installation quirks you need to know to install and use it separately. How familiar are you with multimode? Answers and explanations are in red.

1. Multimode fiber is called “multimode” because it transmits many different modes or ________ of light.

A. Colors

B. Wavelengths

C. Photons

D. Rays

Mode is the technical term for rays of light in fiber.

2. The core of ________ multimode fiber is designed to enhance bandwidth.

A. Step-index

B. Graded-index

C. Polymer optical fiber (POF)

D. Plastic or hard-clad silica (PCS/HCS)

Graded-index fiber has an index of refraction profile designed to minimize modal dispersion.

3. The core of most multimode fibers is about the same size as ________.

A. The wavelength of light

B. A glass molecule

C. A human hair

D. A 26-gauge copper wire

Typical multimode fibers have core sizes about the same as a human hair.

4. Multimode fiber is usually terminated by ________.

A. Installing connectors on the fibers in the field

B. Splicing pigtails on the fiber

C. Fusion splicing

D. Mechanical splicing

The larger core and less sensitivity to reflectance makes field termination simpler.

5. Multimode connectors have smaller holes for the fiber and tighter tolerances, so they cost more than single-mode connectors.

True

False

Just the opposite is the case.

6. For slower speed systems under 200 megabits per second, a ________ source is often used with multimode fiber.

A. Light-emitting diode (LED)

B. Laser

C. Vertical-cavity surface-emitting laser (VCSEL)

D. Lamp

LEDs are commonly used in slower multimode systems, while lasers, usually VCSELs are used for faster links.

7. The latest standardized version of laser-optimized multimode fiber is called ________.

A. OM1

B. OM2

C. OM3

D. OM4

OM4 fiber with extremely high bandwidth for laser sources has just been approved by the TIA standards committee.

8. For short premises high-speed systems, ________ are used with high bandwidth OM3 or OM4 fibers.

A. VCSELs

B. LEDs

C. Distributed feedback (DFB) lasers

D. Fabry Perot (FP) lasers

VCSELs or vertical cavity surface-emitting lasers at 850 nm wavelength are the common transmitter for high speed multimode systems.

9. Testing loss in multimode fibers requires careful control of ________ to get reproducible results.

A. Wavelength

B. Test source power level

C. Mode power distribution

D. Source coupling

The loss measured in multimode fibers is highly dependent on the mode power distribution in the fiber.

10. Some fiber optic standards call for using a ________ when testing multimode fiber to control modal power.

A. Mandrel wrap

B. Source filter

C. Laser

D. Optical time domain reflectometer (OTDR)

A mandrel wrap and LED source is the most commonly specified test source setup for multimode fibers.

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