![](/api/v2/epubs/9780123751645/files/bg79.png)
Thus, a graded index preform is created. The exact
profile is much more difficult to achieve than a silica
preform by chemical vapor deposition.
3.2.3.1.3 Fiber drawing
The drawability of a POF preform is very sensitive to its
glass transition temperature, which is about 110
C, and
its average molecular weight, which should be less than
80,000. In the drawing process, the tip of the preform is
slowly fed into the furnace under computer control at
a temperature between 28 and 290
C, as shown in
Fig. 3.2.8. The preform method allows us to manufacture
a variety of POFs, for example, single mode and multi-
mode fiber, twin-core fiber, dye-doped fiber, inorganic-
doped fiber (e.g., erbium), and electrooptic fiber. The
disadvantage of the preform method is that only a finite
length of the preform is made each time, which means
that a limited amount of fiber is produced. This is not
commercially attractive.
3.2.3.2 Extrusion method
With this method, the starting material is not monom er
but polymer in the form of powder or pebbles that can be
bought directly from commercial suppliers. Fig. 3.2.9
shows the extrusion setup [6,7] . The core powder is in-
troduced through feeder 1 while the cladding powder is
fed through feeder 2. Both powders are pushed toward
the output die at the exit end of the diffusion zone by a set
of feed screws. There is a temperature gradient through-
out the diffusion zone, reaching a melt temperature of
about 280
C. This serves to help the creat ion of graded-
index fiber. The extrusion rate varies between 93 and
245 g/hr. The length of the diffusion zone is about 6.5 cm.
The die has two concentric nostrils serving to create the
core and cladding of the fiber. The diameters of the nostrils
determine the dimensions of the fiber core and cladding.
The advantage of the extrusion method is its ability to
produce a very long length of fiber, which is commercially
attractive. However, the fiber purity cannot be controlled
but depends on the purity of the starting material.
3.2.4 Comparison between silica
fiber and polymer fiber
3.2.4.1 Difference in diameters
The standard multimode silica fiber for communication
has a core diameter of 62.5 mm and an overall fiber di-
ameter of 125 mm. In the case of polymer fiber, the
standard core diameter is 900 mm and the overall fiber
diameter, 1000 mm. Thus, the cross-section of polymer
fiber is much larger than that of silica fiber. Consequently,
it is easier to join two polymer fibers with low loss than
the silica fiber. This is significant because it leads to more
economical installation of optical fiber systems.
The recent low-loss POF development [8] leads to
a smaller core diameter in the order of 120 mm and
a cladding diameter of 250 mm. The reason for such
a reduction is the cost of the polymer, esp ecially the
perfluorinated polym er. However, such a reduction
removes the advantage of the ease of joining. To keep the
advantage, the fiber is jacketed with a sleeve so that the
overall diameter is still about 1 mm. The multilayer
structure of the fiber may introduce an undesirable effect
such as the nonconcentricity of the core, leading to
excess joint loss [9].
3.2.4.2 Minimum bend radius
Polymer fiber has a much smaller Young’s modulus than
silica, about 30 times less. It is therefore easier to nego-
tiate bends without breakage. Hence, the minimum bend
furnace
preform
drawing motor
feeding motor
drivers & electronics
PC control
temperature
control
bearing
V
1
V
2
Fig. 3.2.8 POF drawing machine.
Take-up
Drum
Diffusion Zone
Feeder 2
(for cladding)
Feeder 1
(for core)
Fig. 3.2.9 Extrusion of POF.
114
SECTION THREE Optical Fibers
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