Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 459

operation. Making the best use of the low-loss proper-

ties of the fiber requires that the source emit light in the

low-loss regions of the fiber. Plastic fibers are best oper-

ated in the visible-light area around 650 nm.

One important feature of attenuation in an optical

fiber is that it is constant at all modulation frequencies

within the bandwidth. In a copper cable, attenuation

increases with the signal’s frequency. The higher the

frequency, the greater the attenuation. A 30 MHz signal

will be attenuated in a copper cable more than a

15 MHz signal. As a result, signal frequency limits the

distance a signal can be sent before a repeater is needed

to regenerate the signal. In an optical fiber, both signals

will be attenuated the same.

Attenuation in a fiber has three main causes:

1. Scattering.


2. Absorption.


3. Bending (Microbending).

Scattering. Scattering is the loss of optical energy due
to imperfections in the fiber and from the basic structure
of the fiber. Scattering does just what the term implies:
it scatters the light in all directions. The light is no
longer directional.
Rayleigh scattering is the same phenomenon that
causes a red sky at sunset. The shorter blue wavelengths
are scattered and absorbed while the longer red wave-
lengths suffer less scattering and reach our eyes, so we
see a red sunset. Rayleigh scattering comes from density

and compositional variations in a fiber that are natural

byproducts of manufacturing. Ideally, pure glass has a

perfect molecular structure and, therefore, uniform

density throughout. In real glass, the density of the glass

is not perfectly uniform. The result is scattering.

Since scattering is inversely proportional to the

fourth power of the wavelength (1/O)^4 , it decreases

rapidly at longer wavelengths. Scattering represents the

theoretical lower limits of attenuation, which are as

follows:

• 2.5 dB at 820 nm


• 0.24 dB at 1300 nm


• 0.012 dB at 1550 nm

Absorption. Absorption is the process by which impu-

rities in the fiber absorb optical energy and dissipate it

as a small amount of heat. The light becomes dimmer.

The high-loss regions of a fiber result from water bands,

(where hydroxyl molecules significantly absorb light).

Other impurities causing absorption include ions of

iron, copper, cobalt, vanadium, and chromium. To

maintain low losses, manufacturers must hold these ions

to less than one part per billion. Fortunately, modern

manufacturing techniques, including making fibers in a

very clean environment, permit control of impurities to

the point that absorption is not nearly as significant as it

was a few years ago.

Microbend Loss. Microbend loss is that loss resulting

from microbends, which are small variations or bumps

in the core to cladding interface. As shown in Fig.

15-14, microbends can cause high-order modes to

reflect at angles that will not allow further reflection.

The light is lost.

Microbends can occur during the manufacture of the

fiber, or they can be caused by the cable. Manufacturing

and cabling techniques have advanced to minimize

microbends and their effects.

New Reduced Bend Radius Fibers. Fiber optic cable

manufacturers have now significantly reduced the bend

radius of the fiber. The reduced bend radius allows for

more flexibility allowing installers to bend the fiber

Figure 15-12. Multimode fiber spectral attenuation.

Figure 15-13. Single-mode fiber attenuation.

8

6

4

2

0

800 1000 1200 1400 1600

Wavelength–nm

Attenuation–dB/km

4

3

2

1

0

800 1000 1200 1400 1600

Wavelength–nm

Attenuation–dB/km

Transition region

Figure 15-14. Microbend loss.

Microbend