Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 479

misalignment between the connector and the diode

package.

15.12 Fiber Optic Considerations

The professional audio engineer, technician or person-
nel is now facing many new challenges of distributing
audio signals. The use of fiber optics is becoming eas-
ier, more efficient and cost effective over its copper
counterpart. The many breakthroughs in fiber optic
technology are leading the way into the future. Glass
optical fiber cables are more robust and cost effective
enough to use for longer runs exceeding 2 km. Plastic
fibers (POFs) are very good at shorter distances (25 ft or
less), but they do not meet the fire codes for most build-
ing structures. Jitter still seems to be problematic with
POFs even at 15 feet in some cases. However, plastic
fiber is improving by mixing combinations of glass and
plastic which is referred to as plastic-clad silica (PCS)
(plastic cladding and glass core). These PCS are being
used in industrial applications as well as some telecom-
munication areas. There are many types of fiber optic
system link designs. Usually, the designer is far better
off designing and buying components from one or two
vendors, which took the guess work out of system com-
patibility. The advancements of tools for connecting and
splicing optical fibers has now become simple and time
efficient enough to easily integrate in any audio system.
As bandwidths keep increasing, the only thing that will
keep up with it is fiber optics. The integrity of the audio
signals will not be altered, while keeping the quality at
high levels. We are now experiencing fiber to the home
FTTH and, to coin a phrase, fiber to the studio FTTS.
The audio community is seeing many technological
breakthroughs and these fiber optic cables, connectors,
and opto-chips are becoming an integral part of pro-
audio systems.

15.13 Glossary of Fiber Optic Terms

Absorption: Together with scattering, absorption forms

the principal cause of the attenuation of an optical

waveguide. It results from unwanted impurities in the

waveguide material and has an effect only at certain

wavelengths.

Angle of Incidence: The angle between an incident ray

and the normal to a reflecting surface.

Attenuation: The reduction of average optical power in
an optical waveguide, expressed in dB. The main causes

are scattering and absorption, as well as optical losses in

connectors and splices. Attenuation or loss is expressed by

Attenuator: An optical element that reduces intensity

of a optical signal passing through it (i.e., attenuates it).

Example: AT&T makes attenuators built into connec-

tors that incorporate a biconic sleeve consisting of a car-

bon-coated mylar filter. They come in steps of 6 dB,

12 dB, 16 dB, and 22 dB values.

Avalanche Photodiode (APD): A photodiode designed

to take advantage of avalanche multiplication of photo-

current. As the reverse-bias voltage approaches the

breakdown voltage, hole-electron pairs created by

absorbed photons acquire sufficient energy to create

additional hole-electron pairs when they collide with

ions; thus, a multiplication or signal gain is achieved.

Axial Ray: A light ray that travels along the axis of an

optical fiber.

Backscattering: A small fraction of light that is deflected

out of the original direction of propagation by scattering

suffers a reversal of direction. In other words, it propa-

gates in the optical waveguide towards the transmitter.

Bandwidth: The lowest frequency at which the magni-

tude of the waveguide transfer function decreases to

3 dB (optical power) below its zero frequency value. The

bandwidth is a function of the length of the waveguide,

but may not be directly proportional to the length.

Bandwidth Distance Product (BDP): The bandwidth

distance product is a figure of merit that is normalized

for a distance of 1 km and is equal to the product of the

optical fiber’s length and the 3 dB bandwidth of the

optical signal. The bandwidth distance product is usu-

ally expressed in megahertz kilometer (MHz km) or

gigahertz kilometer (GHz km). For example, a com-

mon multimode fiber with bandwidth-distance product

of 500 MHz km could carry a 500 MHz signal for

1 km. Therefore, a 1000 MHz or 1 GHz signal for

0.5 km. Thus, as the distance increases, for 2 km, the

BDP would be 250 MHz etc.

Beamsplitter: A device used to divide or split an opti-

cal beam into two or more separate beams.

Beamwidth: The distance between two diametrically

opposed points at which the irradiance is a specified

fraction of the beam’s peak irradiance; Beamwidth is

most often applied to beams that are circular in cross

section.

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