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Numerical Aperture
The numerical aperture (NA) is a measurement of the ability of an optical fiber to
capture light. The NA can be defined from the refractive index profile or the output
far-field radiation pattern. Our discussion is limited to measuring the NA from the output
far-field radiation pattern.
The NA of a multimode fiber having a near-parabolic refractive index profile is
measured using EIA/TIA-455-177. In EIA/TIA-455-177, the fiber NA is measured from the
output far-field radiation pattern. The far-field power distribution describes the emitted
power per unit area in the far-field region. The far-field region is the region far
from the fiber-end face. 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 distance between the fiber-end face and detector in the far-field
region is in the centimeters (cm) range for multimode fibers and millimeters (mm) range
for single mode fibers.
EIA/TIA-455-47 describes various procedures, or methods, for measuring the far-field
power distribution of optical waveguides. These procedures involve either an angular or
spacial scan. Figure 5-10 illustrates an angular and spacial scan for measuring the
far-field power distribution.
Figure 5-10 - Angular and spacial scan methods for measuring the far-field power
distribution.
Figure 5-10 (method A) illustrates a far-field angular scan of the fiber-end face by a
rotating detector. The fiber output radiation pattern is scanned by a rotating detector in
the far-field. The detector rotates in a spherical manner. A record of the far-field power
distribution is kept as a function of angle Θ.
Figure 5-10 (method B) illustrates a far-field spacial scan of the fiber-end face by a
movable (planar) detector. In a far-field spacial scan, lens L1 performs a
Fourier transform of the fiber output near-field pattern. A second lens, L2, is
positioned to magnify and relay the transformed image to the detector plane. The image is
scanned in a plane by a movable detector. The scan position y in
the Fourier transform plane is proportional to the far-field scan angle Θ. A
record of the far-field power distribution is kept as a function of the far-field scan
angle.
The normalized far-field pattern is plotted as a function of the far-field scan angle
Θ. Figure 5-11 shows the plot of the normalized far-field radiation pattern as a
function of scan angle.
Figure 5-11. - Normalized far-field radiation pattern.
Fiber NA is defined by the 5 percent intensity level, or the 0.05 intensity level, as
indicated in figure 5-11. The 0.05 intensity level intersects the normalized curve at scan
angles ΘA and ΘB. The fiber NA is defined as
where Θ5 is the 5 percent intensity
half angle. Θ5 is determined from ΘA and
ΘB as shown below:
Q.19 Far-field power distributions describe the emitted power per unit area as a
function of angle Θ in the far-field region. Describe the far-field region.
Mode Field Diameter
The mode field diameter (MFD) of a single mode fiber is related to the spot size of the
fundamental mode. This spot has a mode field radius w0. The mode field diameter
is equal to 2w0. The size of the mode field diameter correlates to the
performance of the single mode fiber. 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.
The mode field diameter of a single mode fiber can be measured using
EIA/TIA-455-167.
This method involves measuring the output far-field power distribution of the single mode
fiber using a set of apertures of various sizes. This far-field power distribution data is
transformed into the near-field before using complex mathematical procedures. The mode
field diameter is calculated from the transformed near field data. The mathematics behind
the transformation between the far-field and near-field is too complicated for discussion
in this chapter. Refer to EIA/TIA-455-167 for information on this transformation
procedure.
Q.20 Will fiber coupling loss generally increase or decrease if the mode field diameter
of a single mode fiber is decreased?
Insertion Loss
Insertion loss is composed of the connection coupling loss and additional fiber losses
in the fiber following the connection. In multimode fiber, fiber joints can increase fiber
attenuation following the joint by disturbing the fiber's mode power distribution
(MPD).
Fiber joints may increase fiber attenuation because disturbing the MPD may excite
radiative modes. <emphasis type="b.GIF">Radiative modes</emphasis> are
unbound modes that radiate out of the fiber contributing to joint loss. In single mode
fibers, fiber joints can cause the second-order mode to propagate in the fiber following
the joint. As long as the coupling loss of the connection is small, neither radiative
modes (multimode fiber) or the second-order mode (single mode fiber) are excited.
Insertion loss of both multimode and single mode interconnection devices is measured
using EIA/TIA-455-34. For Navy applications, an overfill launch condition is used at the
input fiber. For other applications a mandrel wrap may be used to strip out high-order
mode power. The length of fiber before the connection and after the connection may be
specified for some applications. Power measurements are made on an optical fiber or fiber
optic cable before the joint is inserted and after the joint is made. Figure 5-12
illustrates the mandrel wrap method of measuring the insertion loss of an interconnecting
device in EIA/TIA-455-34.
Figure 5-12. - Insertion loss measurement of an interconnecting device.
Initial power measurements at the detector (P0) and at the source monitoring
equipment (PM0) are taken before inserting the interconnecting device into the
test setup. The test fiber is then cut at the location specified by the end user. The cut
results in a fiber of lengths L1 and L2 before and after the
interconnection device that simulates the actual system configuration. After
interconnection, the power at the detector (P1) and at the source monitoring
equipment (PM1) is measured. The insertion loss is calculated as shown below:
If the source power is constant, then the calculation of the insertion loss is similar
to that of fiber attenuation.
Q.21 In multimode fibers, how do fiber joints increase fiber attenuation following the
joint?
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