Page 129 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
8

Connectors and Splices

Now that we have looked at the major pieces of an optic fiber link, the next
concern is how to hook them together mechanically and optically. From an opti-
cal point of view the challenges are how to launch optical power into a particu-
lar type of fiber and how to couple optical power from one fiber into another.
Launching light from a source into a fiber entails considerations such as the
numerical aperture, core size, and core-cladding refractive index differences of
the fiber, plus the size, radiance, and angular power distribution of the optical
source. Mechanical factors include highly precise alignment of fibers, low loss
and repeatability of connections, and ruggedness of fiber-to-fiber joints.
This chapter first considers the issues involved in coupling light from a source
into an optical fiber. The next topic covers the conditions that need to be taken
into account in making a fiber-to-fiber joint. Here the difficulty lies in how to
precisely align fibers that are roughly the diameter of a human hair (which is 50
to 100µm in diameter) so that only a minute fraction of light is lost across a fiber
joint. A further topic notes that no matter what alignment and coupling scheme
is used, the end faces of the fiber must be prepared properly. The final two
sections deal with connectors and splices. As will be seen in that discussion, a
splice is a permanent joint between two fibers, whereas connectors are mounted
on the ends of fiber cables so they can be plugged and unplugged easily and
often.

8.1. Source-to-Fiber Power Coupling
In practice, many source suppliers offer devices with a short length of optical
fiber (1m or less) already attached to the source in an optimum power coupling
configuration. This section of fiber commonly is referred to as a flylead or a pig-
tail. The power-launching problem from these pigtailed sources thus reduces to
a simpler one of coupling optical power from one fiber into another. So let us look
at how to achieve optimum light coupling from a source into a fiber flylead.
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