FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

technique is also shown in Fig. 3.6. The fiber is
lightly scored and then pulled. Care must be taken so
as not to bend the fiber. If the fiber bends a lip
tends to form when it breaks and a smooth endface does
A)STRIPJACKET
B)SCORE
~FILE
t
FIBER
6 ?
Fig. 3.4
o 0.1 0.2 0.3 0.4 0:5
TRANSVERSE DISPLACEMENT (d/D)
Connector power loss due to transverse
(lateral) displacement of the cores of two
step-index optical fiber butt-joined enda.
P
C) RESULTS OFFENDING
v
&d
h
D) RESULTS OF PULLING
—~.
misalignment is shown in Fig. 3.5. It can occur when
the fibers are not lined up axially or are not cleaved
exactly at right angles to the core axes. This effect
is also a function of NA, increasing as NA increases.
As little as a 5° angular misalignment produces approximately
a 0.4 dB loss in the connection between two
fibers each with NA = 0.15.
Fig. 3.6
A method of cleaning an optical fiber and
the results obtained.
not necessarily result. An alternative approach to
amooth cleaving consists of polishing the ends to provide
a flat surface. This can be rather costly and”
take a great deal of time. Once a smooth end has been
formed, the fiber can be inserted into a connector or
spliced to another fiber that alao haa a smooth end.
A simple snug-fit connector is shown in Flz.
3.7. A hole in- the c&nector is provided so that tie
o 1“ 4
~(DEGR&
Fig. 3.5
Connector power loss due to axial angular
misalignment of the cores of two step-index
optical fiber butt-joined ends.
As indicated above, an important criterion
for the success of either a connector or a splice is
the proper end-preparation of the fibers themselves.
Optical fibers used in sensors usually consist of a
glass core surrounded by glass cladding that is in turn
jacketed by a buffer material used to protect the surface.
One type of jacket is made of acrylic material
that can be removed with acetone and a swab. Another
type of jacketing material consists of a IOO-micronthick
layer of silicone rubber surrounded by another
100- or 200-micron-thick layer of a harder plastic,
such as Hytrelc. This may be removed with a razor
blade. In order to prevent scratching of the fiber,
the blade must be held at a very shallow angle with respect
to the axis of the fiber. Furthermore, a blade
should be used only once. After the jacket has been
removed the fiber can be cleaved in any of several ways.
One of these is shown in Fig. 3.6. Another technique
consists of laying the fiber on a curved surface with
approximately a 5 cm radius and applying a tension of
about 1/4 pound (120 grams). The fiber is then scribed
with a file causing a smooth cleave to occur. Another
Fig. 3.7 A simple snug-fit connector with hole for
index-matching fluid.
index-matching fluid can squeeze out as the fiber ends
are inserted. This is not an easily remountable type
of connector. A fairly good splice can be formed in
such a manner by using an index-matching epoxy in place
of the index-matching liquid. If the splice is snug
enough to hold the fiber fixed, an index-matching liquid
can be used. The difficulty with such remountable connectors
is that in order to be mounted and demounted
many times it is necessary to maintain a radial clearance
of at least several microns. This technique is
not practical for singlemode fibers because no more
than an 0.5 urn lateral displacement is allowed In order
to avoid excesaive power losa.
A connector, recently marketed by TRW Incorporated
is shown in Fig. 3.8 (see Ref. 1 in Subsection
3.1.1). Four relatively large diameter glass rods are