Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 457

to let all waves arrive in unison and greatly increase the
received intensity (power).

Notice in Fig. 15-9 how each mode is bent (and
slowed) in proportion to its entry point in the optical
fiber, keeping them in phase. When the rays arrive in
phase their powers add. This technique provides
maximum signal strength over the greatest distance
without regeneration because out-of-phase modes
subtract from the total power.

15.4.2 Characteristics of Typical Fibers

Table 15-1 gives the characteristics of typical fiber optic
cable.

Dispersion. Dispersion is the spreading of a light pulse
as it travels down the length of an optical fiber. Disper-
sion limits the bandwidth or information-carrying

capacity of a fiber. In a digital modulated system, this

causes the received pulse to be spread out in time. No

power is actually lost due to dispersion, but the peak

power is reduced as shown in Fig. 15-10. Dispersion

can be canceled to zero in single-mode fibers but with

multimode it often imposes the system design limit. The

units for dispersion are generally given in ns/km.

Loose Tube and Tight Buffer Fiber Jackets. There

are basically two types of fiber jacket protection called

loose tube and tight buffer, Fig. 15-11.

The loose tube is constructed to contain the fiber in a

plastic tube that has an inner diameter much larger than

the fiber itself. The plastic loose tube is then filled with

a gel substance. This allows the fiber to have less stress

from the exterior mechanical forces due to the running

or pulling of the cable. In multiple fiber loose tube or

single fiber loose tube extra strength members are added

to keep the fibers free of stress and to help minimize

elongation and contraction. Thus, varying the amount of

fibers inside the loose tube, the degree of shrinkage can

be controlled due to temperature change. This allows

for more consistent attenuation over temperature.

The second type, tight buffer, protects the fiber by a

direct extrusion of plastic over the basic fiber coating.

These tight buffer cables can withstand much greater

crush and impact forces without fiber breakage. While the

tight buffer has better crush capabilities and is more flex-

ible, it lacks the better attenuation figure of the loose tube

due to temperature variations which cause microbending

due to sharp bends and twisting of the cable.

Figure 15-9. Multimode graded index fiber.

Table 15-1. Characteristics of Typical Cables

Type Core

Dia.

(“m)

Clad-

ding

Dia.

(“m)

Buffer

Dia.

(“m)

NA Band-

width

MHz-km

Attenu-

ation

dB/km

Single mode 8 125 250 6 ps/km* 0.5

at 1300 nm 5 125 250 4 ps/km* 0.4

Graded index 50 125 250 0.20 400 3

at 850nm 62.5 125 250 0.275 150 3

85 125 250 0.26 200 3

100 140 250 0.30 150 4

Step index 200 380 600 0.27 25 6

at 850 nm 300 440 650 0.27 20 6

PCS† 200 350 — 0.30 20 10

at 790 nm 400 450 — 0.30 15 10

600 900 — 0.40 20 6

Plastic — 750 — 0.50 20 400

at 650 nm — 1000 — 0.50 20 400

*Dispersion per nanometer of source width.

†PCS (Plastic-clad silica: plastic cladding and glass core).

(Courtesy AMP Incorporated)

Input pulse

Light source

Modes Cladding n 2

Core n 1

Dispersion

Output

pulse

Figure 15-10. Dispersion in an optical fiber.

Figure 15-11. Loose tube and tight buffer fiber jackets.

A. Input B. Fiber C. Output

Buffer layers applied

directly over fiber coating

Coated optical fiber

A. Loose tube. B. Tight buffer.