Handbook for Sound Engineers

456 Chapter 15

are usually not included in practical fiber analysis. The
exact characteristics of light propagation depend on the
fiber size, construction, and composition, and on the
nature of the light source injected.
Fiber performance and light propagation can be
reasonably approximated by considering light as rays.
However, more exact analysis must deal in field theory
and solutions to Maxwell’s electromagnetic equations.
Maxwell’s equations show that light does not travel
randomly through a fiber; it is channeled into modes,
which represent allowed solutions to electromagnetic
field equations. In simple terms, a mode is a possible
path for a light traveling down a fiber.
The characteristics of the glass fiber, in an extreme
sense, can be compared to light as seen through crystal
clear water, turbid water, and water containing foreign
objects. These conditions are characteristics of water
and have quite different effects on light traveling (prop-
agating) through them. The glass fibers are no different,
splices, breaks, boundary distortion, bubbles, core
out-of-round, etc., all influence the amount of light that
reaches the distant end. The main objective is to receive
maximum intensity with little or no distortion.

15.4 Fiber Optics

15.4.1 Types of Fiber

Optical fibers are usually classified by their refractive
index profiles and their core size. There are three main
types of fibers:

1. Single mode.


2. Multimode stepped index.


3. Multimode graded index.

Single Mode Fiber. Single mode fiber contains a core
diameter of 8 to 10 microns, depending on the manufac-
turer. A highly concentrated source such as a laser or
high-efficient LED must be used to produce a single
mode for radiation into the fiber. The index of refraction
in single mode fiber is very low because the highly con-
centrated beam and extremely small core prevent blos-
soming (officially referred to as scattering) of the ray.
The small core tends to prevent the entry of extra-
neous modes into the fiber, as illustrated in Fig. 15-7.
Loss in a single mode fiber is very low and permits the
economy of longer repeater (telephone amplifier)
spacing. This optical fiber has the capability of propa-
gating 1310 nm and 1550 nm wavelengths. It is well
suited for intracity and intercity applications where long
repeater spacing is desired.

Multimode Step Index Fiber. The production of opti-

cal fiber includes layer deposition of core glass inside a

started tube. If the glass core layers exhibit the same

optical properties the fiber is classed a step index fiber.

The core layers contain uniform transmission character-

istics. The fanout of the rays and their refraction at the

core-clad boundary give them the appearance of step-

ping through the glass, Fig. 15-8. Notice also that as the

individual rays step their way through, some travel far-

ther and take longer to reach the far end; the reason for

the rounded output pulse shown. This optical fiber

requires repeaters-regenerators located at short intervals.

All rays, or modes, arriving in unison will produce

the most exact and strongest replica of the input; this is

the objective. For an optical fiber to be most useful in

communications, the modes must be channeled through

the core in a controlled manner so they all arrive at

nearly the same instant.

Multimode Graded Index Fiber. The process of man-

ufacturing graded index fiber involves depositing differ-

ent grades of glass in the starting tube to provide a core

with various transmission characteristics; the outer por-

tion does not impede the passage of modes as much as

the center.

In graded index fiber, the core axis contains a higher-

density glass of slow wave (ray, mode) propagation in

this path for coordination with arrival of the waves in

the longest path. The grades of core glass deposited

from axis to perimeter are progressively less impeding

Figure 15-7. Single mode fiber.

Figure 15-8. Multimode step index fiber.

Light source

Input pulse

(Laser or high

efficient LED)

Core

Cladding n 2 Outputpulse

n 1

Single

mode

Input pulse

Light source

Modes Cladding n 2

Core n 1

Output

pulse