452 Chapter 15
The practical use of fiber optics for communications
began in the mid- and late 1970s with test trials.
However, the popularization of fiber optics wasn’t until
the 1980 Winter Olympics at Lake Placid, New York,
when the joint effort of New York Telephone, AT&T,
Western Electric, and Bell Labs installed a fiber optic
system. Its purpose was to transform the Lake Placid
telephone facility into a professional communications
center capable of handling a wide range of telecommu-
nications services necessary to support the Olympic
events. Today fiber optics is an established technology.
15.2 Advantages of Using Fiber Optics for Audio
There are at least four advantages in using fiber over
hardwired systems. One is the superb performance in
transmission, allowing extremely large bandwidths and
low loss which minimizes the need for preamplifying a
signal for long haul applications. Digital data can be
easily transmitted with rates of 100 Mb/s or higher
showing more information handling capability and
greater efficiency. Since the optical fiber is nonmetallic
(made of glass, plastic, etc.), it is immune to problems
caused by electromagnetic interference (EMI) and radio
frequency interference (RFI). Also the problem of
crosstalk is eliminated—a quality advantage.
With optical fiber one no longer needs to worry
about impedance matching, electrical grounding or
shorting problems, or no ground loops. Safety is an
important feature of fiber optics because a broken cable
will not spark, possibly causing shock or an explosion
in a dangerous environment.
Another plus is fiber optic cable weighs about
9 lbs/1000 ft and takes up less space than wire, useful
especially when running in conduits. Cost is now less
than or comparable to copper. And finally an optical
fiber system cannot be easily tapped, which allows for
better security.
15.2.1 Applications for Audio
Telephone companies have many fiber links which can
connect Japan and Europe to the United States. Think of
the many possibilities of doing a multitrack recording
from many different places all over the world over a
fiber optic cable without worrying about SNR, interfer-
ence, distortion, etc. Top-of-the-line compact disc and
DAT players already provide an optical fiber link out-
put. Also, there are companies like Klotz Digital of Ger-
many and Wadia Digital Corporation of the United
States who are manufacturing fiber optic digital audio
links, which employ an AES/EBU input and output at
each end.
Many recording studios are located in high rise
apartment buildings. A perfect application of a digital
audio fiber optic link is to connect, for instance, studio
A which is located on the 21st floor, to studio B which
is located on the 24th floor. This is ideal because the
user doesn’t have to worry about noise and interference
caused by fluorescent lighting and elevator motors, to
name a few. Another perfect use is to connect MIDI
stations together.
Another recent advance is a recording studio can
record in real time by using DWDM (dense wavelength
division multiplexing) lasers and erbium doped optical
fibers to send the AES3 audio channels over the
Atlantic or Pacific Ocean and then to the appropriate
recording studio. The Internet is also being used to
establish a fiber optic end-to-end recording session.15.3 Physics of LightBefore discussing optical fiber, we must understand the
physics on light.Light. Light is electromagnetic energy, as are radio
waves, x-rays, television, radar, and electronic digital
pulses. The frequencies of light used in fiber optic data
transmission are around 200 THz–400 THz
(400 × 10^12 ), several orders of magnitude higher on the
electromagnetic energy spectrum than the highest radio
waves, see Fig. 15-2. Wavelength, a more common way
of describing light waves, are correspondingly shorter
than radio wavelengths. Visible light, with wavelengths
from about 400 nm for deep violet to 750 nm for deep
red, is only a small portion of the light spectrum. While
fiber optic data transmission sometimes uses visible
light in the 600 nm to 700 nm range, the near infrared
region extending from 750 nm to 1550 nm is of greater
interest because fibers propagate the light of these
wavelengths more efficiently.
The main distinction between different waves lies in
their frequency or wavelength. Frequency, of course,
defines the number of sine-wave cycles per second and
is expressed in hertz (Hz). Wavelength is the distance
between the same points on two consecutive waves (or
it is the distance a wave travels in a single cycle).
Wavelength and frequency are related. The wavelength
(O) equalsO v (15-1)
f=--