FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

fibers are cleaned carefully after removing the jacket.
Then they are twisted together and while remaining
twisted they are etched to remove most of the cladding,
leaving approximately one or two microns of cladding
around the core. The diameters of the fibers after
etching are less than 10% of their initial diameters,
therefore the resulting sections of fibers are quite
fragile. The joint IS held fixed (ruggedized) by either
potting in an indexmatching material or by fusing under
an axial tension that prevents the fiber from sagging
due to gravity and at the same time stretches the fiber
slightly, thus forming a biconical taper. This is shown
in Fig. 3.20 on the right. Index matching silicone
liquids have been proven to be highly temperature dependent.
Better results are obtained with index-matching
silicone rubber. However, there is still a temperaturedependence
problem. A great deal of success has been
achieved recently using a gel glass material (see Ref.
2 in Subsection 3.2.1) that is initially in liquid form.
This material consists of metal oxides dissolved in an
organic material. When heated the organic material is
driven off and the metal oxides form a glass. The refractive
index of the resulting glasa, and therefore
the degree of coupling, can be controlled by adjusting
the temperature and the length of time utilized to cure
or form the gel glass.
Temperature dependence can be minimized if
the fibers are actually fused. The biconical taper
arrangement mentioned above is shown in Fig. 3.21. To
aturized bulk optical components similar to the laboratory
devices shown in Figs 3.16, 3.17, and 3.18. These
bulk components and fibers may be connected by various
means, such as GRIN rods.
Using similar techniques star couplers have
been fabricated that allow the light from one fiber to
be coupled equally into as many as 32 other fibers.
Such devices would be useful for permitting a single
optical source to simultaneously furnish optical power
to a number of sensors.
In summary, satisfactory techniques now exist
for forming low-loss singlemode splices and remountable
connectors and for fabricating low insertion loss
singlemode couplers. Recently, commercially available
remountable connectors have appeared on the market.
Singlemode biconical tapered couplera are also becoming
available. Several other types of couplers are under
development by a number of groups and thus can be expected
to appear on the market in the near future.
3.2.1 References
1.
2.
3.
S. Sheem, Appl. Phys. Lett ~, 869 (1980).
D. Tran, K. Koo, and S. Sheem, J. Quant. Electron.
QE-17, 988 (1981).
R. Ulrich, S. Rashleigh, ‘Beam-to-Fiber Coupling
with Low Standing-Wave Rtutio”, Appl. Opt. ~, 2453
(1980).
END VIEW
OF FIBER
4.
G.B. Hocker, “Unidirectional Star Coupler for Single-Fiber
Distribution System”, Opt. Lett. ~, 124
(1977).
5.
S. K. Sheem, T. G. Giallorenzi, “Single-Mode Fiber
Optical Power Divider: Encapsulated Etching”, Opt.
Lett. ~, 29 (1979).
6.
J. G. Ackerhusen, “Microlenses to Improve LED-to-
Fiber Optical Coupling”, Appl. Opt. ~, 3694
(1979).
Fig. 3.21
CROSS SECTION
VIEW
OF FUSED COUPLER
A biconical (tapered)
fiberoptic coupler.
7.
8.
M. Saruwatari, K. Nawata, “Semiconductor Laaer to
Single-Mode Fiber Coupler”, APP1. Opt. ~, 1847
(1979).
H. Kuwahara, N. Tokoyo, M. Sasaki, “Efficient
Coupling from Semiconductor Lasers into Single-
Mode Fibers with Tapered Heimspherical Ends”,
Appl. Opt. g, 2578 (1980).
make this coupler a portion of the cladding is removed
from each of ‘the fibers. These are then twisted together
and etched until the thickness of the remaining
cladding is about half the core diameter. The twisted
pair is then fused under some axial tension causing a
decrease in diameter, especially in the region of contact,
i.e., the interaction region. The original core
dimensions indicated in the upper right of Fig. 3.21
are reduced as shown in the lower center of Fig. 3.21.
The result is that optical energy that initially was
confined close to the core tends to spread into the
cladding in the region where the core diameter has
been decreased. This results in a stronger overlap and
therefore a higher coupling ratio. The Electrooptics
Product Division of ITT has recently developed a specialized
optical fiber that allows them to produce
fused copulers by this method without the necesaity of
etching. These couplers are available for sale. An
alternate to these fiber couplers is the use of mini-
3.3 FIBEROPTIC CABLES
3.3.1 General
Just as with conventional wire interconnections
and communication links, there is a need for fiberoptic
cables to accomplish a number of different
purposea. In many ways, the individual fibers in an
optical cable are treated very much like varnished or
plastic insulated copper wires. The individual fibera
consist of 100- to 200-micron-OD core-cladding waveguide
elements having an outer coating that may be as
thin as several microns of lacquer or as thick as 200
to 400 microns of plastic, such as nylon, teflon, or
polypropylene. Cabling serves the purpose of spacedivision
multiplexing, combining anywhere from a few to
many individual fibers into a single conveniently-pack-
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