The Optical Communications Reference

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