5.4 Summary
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3. The radiance of the light source, which is the optical power radiated into a solid
angle per emitting area (measured in watts per square centimeter per steradian).
4. The spatial radiation pattern of the source. The incompatibility between the
wide beam divergence of LEDs and the narrow acceptance cone of the fiber is a
major contributor to coupling loss. This holds to a lesser extent for laser diodes.
In practice, many suppliers offer optical sources that have a short length of optical
fiber (nominally 1–2 m) already attached in an optimum power-coupling configuration. This fiber, which is referred to as a flylead or a pigtail, makes it easier for
the user to couple the source to a system fiber. The power-launching problem now
becomes a simpler one of coupling optical power from one fiber into another. To
achieve a low coupling loss, the fiber flylead should be connected to a system fiber
having a nominally identical numerical aperture and core diameter.
Fiber-to-fiber joints can exist between the source flylead and the system fiber,
at the photodetector, at intermediate points in a link where two cable sections are
interconnected, or at the fiber to component junctions in a communication link. The
two principal types of joints are splices, which are permanent bonds between two
fibers, and optical connectors, which are used when an easily demountable connection
between fibers or between a fiber and an optical component is desired.
Each jointing technique is subject to certain conditions, which can cause varying
degrees of optical power loss at the joint. These parameters depend on factors such
as the following:
1. The geometrical characteristics of the fibers. For example, optical power will be
lost because of area mismatches if an emitting fiber has a larger core diameter
than the receiving fiber.
2. The waveguide characteristics of the fibers. For example, if an emitting fiber
has a larger numerical aperture than the receiving fiber, all optical power falling
outside of the acceptance cone of the receiving fiber is lost.
3. The various mechanical misalignments between the two fiber ends at the joint.
These misalignments include longitudinal separation, angular misalignment,
and axial (or lateral) displacement. The most common misalignment occurring
in practice, which also causes the greatest power loss, is axial displacement.
4. The input power distribution to the joint. If all the modes of an emitting fiber
are equally excited, there must be perfect mechanical alignment between the
two optical waveguides, and their geometric and waveguide characteristics must
match precisely in order for no optical power loss to occur at the joint. On the
other hand, if steady-state modal equilibrium has been established in the emitting
fiber (which happens in long fiber lengths), most of the energy is concentrated
in the lower-order fiber modes. In this case, slight mechanical misalignments
of two joined fibers and small variations in their geometric and waveguide
characteristics do not contribute significantly to joint loss.
5. The fiber end face quality. One criterion for low-low joints is that the fiber end
faces be clean and smooth. End preparation techniques include sawing, grinding
and polishing, and controlled fracture.
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3. The radiance of the light source, which is the optical power radiated into a solid
angle per emitting area (measured in watts per square centimeter per steradian).
4. The spatial radiation pattern of the source. The incompatibility between the
wide beam divergence of LEDs and the narrow acceptance cone of the fiber is a
major contributor to coupling loss. This holds to a lesser extent for laser diodes.
In practice, many suppliers offer optical sources that have a short length of optical
fiber (nominally 1–2 m) already attached in an optimum power-coupling configuration. This fiber, which is referred to as a flylead or a pigtail, makes it easier for
the user to couple the source to a system fiber. The power-launching problem now
becomes a simpler one of coupling optical power from one fiber into another. To
achieve a low coupling loss, the fiber flylead should be connected to a system fiber
having a nominally identical numerical aperture and core diameter.
Fiber-to-fiber joints can exist between the source flylead and the system fiber,
at the photodetector, at intermediate points in a link where two cable sections are
interconnected, or at the fiber to component junctions in a communication link. The
two principal types of joints are splices, which are permanent bonds between two
fibers, and optical connectors, which are used when an easily demountable connection
between fibers or between a fiber and an optical component is desired.
Each jointing technique is subject to certain conditions, which can cause varying
degrees of optical power loss at the joint. These parameters depend on factors such
as the following:
1. The geometrical characteristics of the fibers. For example, optical power will be
lost because of area mismatches if an emitting fiber has a larger core diameter
than the receiving fiber.
2. The waveguide characteristics of the fibers. For example, if an emitting fiber
has a larger numerical aperture than the receiving fiber, all optical power falling
outside of the acceptance cone of the receiving fiber is lost.
3. The various mechanical misalignments between the two fiber ends at the joint.
These misalignments include longitudinal separation, angular misalignment,
and axial (or lateral) displacement. The most common misalignment occurring
in practice, which also causes the greatest power loss, is axial displacement.
4. The input power distribution to the joint. If all the modes of an emitting fiber
are equally excited, there must be perfect mechanical alignment between the
two optical waveguides, and their geometric and waveguide characteristics must
match precisely in order for no optical power loss to occur at the joint. On the
other hand, if steady-state modal equilibrium has been established in the emitting
fiber (which happens in long fiber lengths), most of the energy is concentrated
in the lower-order fiber modes. In this case, slight mechanical misalignments
of two joined fibers and small variations in their geometric and waveguide
characteristics do not contribute significantly to joint loss.
5. The fiber end face quality. One criterion for low-low joints is that the fiber end
faces be clean and smooth. End preparation techniques include sawing, grinding
and polishing, and controlled fracture.