Information Technology Reference
In-Depth Information
metal construction, and must meet the same plenum requirements as cables. Corners
and bends will require the use of a cable pulley when cables are initially installed.
Fiber Terminations
The physical termination procedure for optical fibers varies enormously with the par-
ticular type and brand of fiber connector you are using. Accordingly, we will not cover
all of the options here. However, there are some common concepts that we can cover
briefly, so that you will understand what is required for this complex operation.
All fiber termination requires the preparation of a bare fiber for assembly
within an appropriate connector. The outer jacket and fibrous plastic strength mem-
bers must be removed if you are using a jacketed cable. In this type of cable, the
outer jacket is slit around its circumference and slipped off the end of the cable.
Next, the Kevlar fibers are combed to one side and cut off at the edge of the remain-
ing jacket. The fibers are extremely fine, and a high-quality cross-cutter must be
used to trim them back.
The buffer-coated fiber (or fibers) must now be prepared. The tight buffer is
removed with a tool that is essentially a wire stripper. The buffer-stripping tool is often
a fixed-opening tool that is specifically designed for the diameter of the bare fiber (core
and cladding), so that the fiber is not nicked when the buffer is removed. Any nick will
severely compromise the flexural strength of the silica fiber and may result in sponta-
neous breakage. The rule of thumb is that such a break will always occur at the last
possible moment, so that the installer will have to start over totally. So, the lesson is
that no nick is a good nick, or nothing can truly be done in the nick of time.
The fiber is now trimmed to the proper length, usually with a tool that pur-
posely scores (yes, nicks) the fiber, and then breaks the unwanted portion away from
the remaining fiber. The breaking process is called
cleaving
the fiber, and it should
leave an orthogonal break at the fiber end. This brings some basic rules of optics
into play. First, if this break is not precisely 90°, the light will exit the fiber at a slight
angle. Second, the break is never exactly 90°. The cure in a quick-termination con-
nector is to place the fiber end into a matching fluid, so that the off-axis break has
virtually no effect. In a so-called field termination (slow-termination) connector, the
fiber end is inserted through the connector ferrule so that it protrudes slightly from
the end of the ferrule. That fiber end is then polished down until it is exactly flush
with the end of the ferrule, and consequently precisely square with the fiber.
Final assembly is done by inserting the fiber into the connector and securing the
fiber mechanically (Fig. 11.11). Older connector types, such as the SMA and the ST,
generally require the application and curing of epoxy to secure the fiber to the con-
nector. Newer quick-termination connectors of this generation use mechanical crimp-
