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SUMMARY
Now that you have completed this chapter, let's review some of the new terms, concepts,
and ideas that you have learned. You should have a thorough understanding of these
principles before moving on to chapter 6.
END USERS (equipment manufacturers, shipbuilders, maintenance personnel, test
personnel, and so on) should measure some component parameters upon receipt before
installing the component into the fiber optic data link. In addition, they should measure
some component parameters after installing or repairing fiber optic components in the
field.
LABORATORY MEASUREMENTS of the optical fiber and optical connections performed by
end users in the laboratory include attenuation, cutoff wavelength (single mode),
bandwidth (multimode), chromatic dispersion, fiber geometry, core diameter, numerical
aperture (multimode), mode field diameter (single mode), insertion loss, and reflectance
and return loss.
ATTENUATION is the loss of optical power as light travels along the fiber. It is a
result of absorption, scattering, bending, and other loss mechanisms.
The LAUNCH SPOT SIZE and the ANGULAR DISTRIBUTION may affect multimode fiber
attenuation measurement results by affecting modal distributions.
An UNDERFILLED launch results when the launch spot size and angular distribution
are smaller than that of the fiber core.
An OVERFILLED launch condition occurs when the launch spot size and angular
distribution are larger than that of the fiber core.
A CLADDING-MODE STRIPPER is a device that removes any cladding mode power from
the fiber.
A MODE FILTER is a device that attenuates a specific mode or modes propagating
in the core of an optical fiber.
The CUTOFF WAVELENGTH of a single mode fiber is the wavelength above which the
fiber propagates only the fundamental mode. The cutoff wavelength of a single mode fiber
varies according to the fiber's radius of curvature and length. The fiber cutoff
wavelength (λcf) will generally be higher than the cable cutoff
wavelength (λcc).
PULSE DISTORTION is the spreading of the light pulse as it travels along the fiber
caused by dispersion. It reduces the bandwidth, or information-carrying capacity, of an
optical fiber.
Two BASIC TECHNIQUES are used for measuring the modal bandwidth of an optical
fiber. The first characterizes dispersion by measuring the IMPULSE RESPONSE h(t) of
the fiber in the time domain. The second characterizes modal dispersion by measuring the BASEBAND
Frequency response H(f) of the fiber in the frequency domain.
The LOWEST FREQUENCY at which the magnitude of the fiber Frequency response has
decreased to one half its zero-frequency value is the -3 decibel (dB) optical power
frequency ( f3dB).
CHROMATIC DISPERSION occurs because different colors of light travel through the
fiber at different speeds. Since the different colors of light have different velocities,
some colors arrive at the fiber end before others.
The DIFFERENTIAL GROUP DELAY τ(λ) is the variation in
propagation delay that occurs because of the different group velocities of each wavelength
in an optical fiber.
The RANGE OF WAVELENGTHS over which meaningful chromatic dispersion data is
obtained depends on the wavelength range of optical source(s) used.
FIBER GEOMETRY MEASUREMENTS are performed by end users to reduce system attenuation
and coupling loss resulting from poor fiber fabrication.
The CLADDING DIAMETER is the average diameter of the cladding.
CLADDING NONCIRCULARITY, or ellipticity, is the difference between the smallest
radius of the fiber (Rgmin) and the largest radius of the fiber (Rgmax)
divided by the average cladding radius (Rg).
The CORE-CLADDING CONCENTRICITY ERROR for multimode fibers is the distance
between the core and cladding centers divided by the core diameter. The core-cladding
concentricity error for single mode fibers is defined as the distance between the core and
cladding centers.
CORE NONCIRCULARITY is the difference between the smallest radius of the core
(Rcmin)
and the largest radius of the core (Rcmax) divided by the core radius
(Rc).
The NEAR-FIELD POWER DISTRIBUTION is defined as the emitted power per unit area
(radiance) for each position in the plane of the emitting surface.
The NEAR-FIELD REGION is the region close to the fiber-end face.
The CORE DIAMETER is derived from the normalized output near-field radiation
pattern. The core diameter (D) is defined as the diameter at the 2.5 percent (0.025)
level.
The NUMERICAL APERTURE (NA) is a measurement of the ability of a multimode
optical fiber to capture light.
The FAR-FIELD POWER DISTRIBUTION describes the emitted power per unit area as a
function of angle Θ some distance away from the fiber-end face.
The FAR-FIELD REGION is the region far from the fiber-end face.
Single mode fibers with large MODE FIELD DIAMETERS are more sensitive to fiber
bending. Single mode fibers with small mode field diameters show higher coupling losses at
connections.
INSERTION LOSS is composed of the connection coupling loss and additional fiber
losses in the fiber following the connection.
REFLECTANCE is a measure of the portion of incident light that is reflected back
into the source fiber at the point of connection.
RETURN LOSS is the amount of loss of the reflected light compared with the power of
the incident beam at the interface.
OPTICAL FIBER AND OPTICAL CONNECTION FIELD MEASUREMENTS measure only the
transmission properties affected by component or system installation or repair.
OPTICAL TIME-DOMAIN REFLECTOMETRY is recommended for conducting field measurements
on installed optical fibers or cable plants of 50 meters or more in length.
An OPTICAL LOSS TEST SET (OLTS) combines the power meter and source functions
into one physical unit.
An OPTICAL TIME-DOMAIN REFLECTOMETER (OTDR) measures the fraction of light that
is reflected back because of Rayleigh scattering and Fresnel reflection.
A FIBER OPTIC CABLE PLANT consists of optical fiber cables, connectors, splices,
mounting panels, jumper cables, and other passive components. A cable plant does not
include active components such as optical transmitters or receivers.
A POINT DEFECT is a temporary or permanent local deviation of the OTDR signal in
the upward or downward direction. Point defects are caused by connectors, splices, or
fiber breaks. Point defects, or faults, can be reflective or nonreflective.
A DEAD-ZONE fiber is placed between the test fiber and OTDR to reduce the
influence of the initial pulse resulting from Fresnel reflection at the OTDR connection.
The AMOUNT OF OPTICAL POWER BACKSCATTERED because of Rayleigh scattering at one
point depends on the forward optical power and the fibers backscatter capture coefficient.
The EFFECTS OF BACKSCATTER VARIATIONS can be eliminated by test personnel by
performing the OTDR attenuation measurement in each direction along the test fiber and
averaging (bidirectional readings).
A POINT DEFECT may exhibit apparent gain because the backscatter coefficient of
the fiber present before the point defect is higher than that of the fiber present after.
To MEASURE FIBER ATTENUATION and <emphasis type="b.GIF">TRANSMISSION
LOSS</emphasis> in the field, test personnel use an optical power meter and
stabilized light source.
OPTICAL POWER METER MEASUREMENTS are recommended when the length of an installed
optical fiber cable or cable plant is less than 50 meters.
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