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[NOC-Tutor]Fiber-Optic Performance Factors

Fiber-Optic Performance Factors

During the course of a normal fiber installation, you must be aware of a few factors that can
negatively affect performance. They are as follows:
? Attenuation
? Acceptance angle
? Numerical aperture (NA)
? Modal dispersion
? Chromatic dispersion

Attenuation
The biggest negative factor in any fiber-optic cabling installation is attenuation, or the loss or
decrease in power of a data-carrying signal (in the case of fiber, the light signal). It is measured in
decibels (dB or dB/km for a particular cable run). In real-world terms, a 3dB attenuation loss in a
fiber connection is equal to about a 50 percent loss of signal. Picture-1 graphs attenuation in
decibels versus percent signal loss. Notice that the relationship is exponential.

Picture-1 Relationship of attenuation to percent signal loss of a fiber optic transmission

The more attenuation that exists in a fiber-optic cable from transmitter to receiver, the
shorter the maximum distance between them. Attenuation negatively affects transmission
speeds and distances of all cabling systems, but fiber-optic transmissions are particularly sensitive
to attenuation.
Many different problems can cause attenuation of a light signal in an optical fiber, including
the following:
? Excessive gap between fibers in a connection
? Improperly installed connectors
? Impurities in the fiber
? Excessive bending of the cable
? Excessive stretching of the cable

Acceptance Angle
Another factor that affects the performance of a fiber-optic cabling system is the acceptance
angle of the optical-fiber core. The acceptance angle (as shown in Figure 10.16) is the angle
through which a particular (multimode) fiber can accept light as input.
The greater the acceptance angle difference between two or more signals in a multimode
fiber, the greater the effect of modal dispersion (see the section “Modal Dispersion”). The
modal-dispersion effect also has a negative effect on the total performance of a particular cable
segment.

Picture-2

Numerical Aperture (NA)
A characteristic of fiber-optic cable that is related to the acceptance angle is the numerical aperture
(NA). The NA is calculated from the acceptance angle. The result of the calculation is a decimal
number between 0 and 1 that reflects the ability of a particular optical fiber to accept light.
A value for NA of 0 indicates that the fiber accepts, or gathers, no light. A value of 1 for NA
indicates that the fiber will accept all light it’s exposed to. A higher NA value means that light
can enter and exit the fiber from a wide range of angles, including severe angles that will not
reflect inside the core, but be lost to refraction. A lower NA value means that light can enter
and exit the fiber only at shallow angles, which helps assure the light will be properly reflected
within the core. Multi-mode fibers typically have higher NA values than single-mode fibers.
This is a reason why the less focused light from LEDs can be used to transmit over multi-mode
fibers as opposed to the focused light of a laser that is required for single-mode fibers.

Modal Dispersion
Multimode cables suffer from a unique problem known as modal dispersion, which is similar in
effect to delay skew, described in Chapter 1 relative to twisted-pair cabling. Modal dispersion
causes transmission delays in multimode fibers. Here’s how it occurs. The modes (signals)
enter the multimode fiber at varying angles, so the signals will bounce differently inside the
fiber and arrive at different times (as shown in Picture-3). The more severe the difference
between the entrance angles, the greater the arrival delay between the modes. In Picture-3,
mode A will exit the fiber first because it has fewer bounces inside the core than mode B. Mode
A has fewer bounces because its entrance angle is less severe (i.e., it’s of a lower order) than that
of mode B. The difference between the time mode A and mode B exit is the modal dispersion.
Modal dispersion gets larger, or worse, as the difference between the entrance angles increases.

Chromatic Dispersion
The last fiber-optic performance factor is chromatic dispersion, which limits the bandwidth of
certain single-mode optical fibers. It occurs when the various wavelengths of light spread out
as they travel through an optical fiber. This happens because different wavelengths of light
travel different speeds through the same media. As they bounce through the fiber, the various
wavelengths will reflect off the sides of the fibers at different angles (as shown in Picture-4).
The wavelengths will spread farther and farther apart until they arrive at the destination at
completely different times.