April 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Signaling with light goes back to
ancient times when militaries and towns communicated with simple encryption methods. Paul
Revere relied on lamp light signals (i.e.,
one if by land, two if by sea). Sailors use(d)
signal lamps relay
simple messages. Even the Warning Beacons of Gondor were a form of light beam communications (albeit
1 bit -- lit or not lit). To date, the farthest distance over which reliable (at useful data
rates) terrestrial communications has been accomplished on a light beam is about 2 miles
(3 km). Atmospheric contamination and Earth curvature (line-of-sight with a tiny bit
of refraction assistance) are the primary limiting factors. By contrast, millions of miles
have separated successful space communications, but the baud rates are typically measured
in the hundreds or thousands of bytes per second. This 1972 article from Popular Electronics
magazine reports on experiments being carried out with lasers. Their terabit data rate
predictions are being realized today.
Twisted Light Beams a Greeting over Record Distance of 143 km.
How Invisible Laser Beams Can Carry Trillions of Bits of Information Every
By David L. Heiserman
The idea of using a light beam to send messages from one
place to another is far from new. In fact, it is probably as old as civilization itself. Since
its inception, the basic method of communicating by this medium has been to interrupt the
light beam according to a prearranged code system. The principle has not changed much since
the first man realized that a light beam could be used to communicate over distances too long
for his voice to carry.
A laser diode (in the upper unit) voice communicator developed by Holobeam,
Inc. for ship-to-ship speech.
Beginning in the 19th century and coming right up to the late 1950's, sporadic attempts
were made to perfect light-beam communication equipment that would carry voice messages or
high-speed codes over moderately long distances. But, even after pouring all their efforts
into the project, scientists failed to perfect light-beam communication. Their designs in
this century, when compared with existing radio communication systems, suffered from severely
limited operating ranges, low information handling capacities, poor fidelity, and interference
from bright ambient light. Little wonder, then, that some of the sincerest of scientific efforts
at designing a practical light-beam communicator ended up as toys, construction projects in
magazines, science fair projects, and just plain curiosities.
One popular white-light communicator which resulted from scientific efforts employed a
microphone or telegraph key to operate a small loudspeaker fitted with a silvered diaphragm.
Electrical signals fed through the amplifier made the diaphragm vibrate. A beam of white light
focused on the diaphragm reflected out of the transmitter, carrying along the vibration as
changes in light intensity. In another simple arrangement, the key merely turned on and off
a light bulb, or the output of a microphone amplifier controlled the brightness of the light.
At the receiver end of the communication link, a sensitive photodetector picked up the
light beam and translated the light intensity fluctuations into voltage changes. The voltage
from the photodetector then drove an amplifier/speaker (or earphone) arrangement, reproducing
the vibrations introduced at the transmitter by the sender of the message.
Short operating ranges and poor fidelity notwithstanding, these simple light-beam communicators
gave experimenters their first inexpensive walkie-talkies. However, when the Citizens Band
came into vogue, light-beam communication was driven into a limbo which lasted until 1969
- the year light-emitting diodes finally became available at low cost.
LED Communicators. Light-emitting diodes, or LED's, are semiconductor diodes which emit
relatively pure infrared light when a current is passed through them. Light from an ordinary
incandescent lamp contains just about every color in the spectrum. Light energy from an LED,
on the other hand, is concentrated in a very narrow band of the light spectrum, making it
possible to select a photo detector that responds only to that band. With the photodetector
carefully matched to the LED emissions, ambient light has very little effect upon the communication
link. Without interference from other light sources, it becomes possible to attain communication
ranges of several miles even in broad daylight.
A helium-neon gas laser (upper left) provides beam for experimental optical
communications setup at Bell Labs.
Since the amount of light from an LED varies with the amount of current through the diode,
electrical signals from a microphone amplifier or other type of input device can directly
modulate the intensity of the beam. Hence, the transmitter is simpler, more reliable, and
has better fidelity than is possible with white-light communication systems. (Light-emitting
diodes can respond to frequencies in the megahertz range where no microphone can compete.)
The new LED communication schemes are working out so well that several companies now produce
them for industrial and commercial use.
Laser Diode Communicators. While LED's generate a narrow spectrum of infrared light, compared
to high-quality laser light, their light is highly contaminated with a number of different
phases and wavelengths. Because the intensity of an LED light beam falls off with the square
of the distance it travels, telescopic attachments can compensate for some of the losses.
But there is a point where the scheme becomes impractical. So, future application of LED communicators
will probably be restricted to low-cost portable voice communicators and short-range links.
The real future of light-beam communication rests with laser light which is so bright initially
that it retains much of its original intensity over longer transmission distances. Just as
relatively pure LED light gives light-emitting diode communicators greater range than comparable
white-light systems, the coherent nature of laser light multiplies the operating range beyond
that feasible with LED's.
Optical communication researchers are now working with three different kinds of laser sources:
laser diodes, solid-state lasers, and gas lasers. Of the laser communication systems presently
under development, those using laser diodes show the greatest promise for immediate applications.
Laser diodes, properly called injection lasers, operate on the same general principle as
LED's. The former, however, are capable of producing much greater output power in addition
to generating true coherent laser light. Unfortunately, the tendency for laser diode devices
to overheat has still to be overcome. Typically, 10 amperes of current must be pumped through
the laser diode to generate one or more useful watts of laser light. Commercially available
laser diodes are incapable of withstanding such high currents over long periods of time without
overheating. Hence, most laser diode communicators presently in use are operated in pulse
Using bigger circuits similar to those used in strobe lighting systems, a laser diode transmitter
fires its diode with large doses of current for about 0.1 μs at a time.
By permitting the diode to cool for about 100 μs between firings, a pulse operating
frequency of about 10,000 Hz can be achieved.
Simple light-beam communicator such as this one made by Infrared Industries,
Inc. used infrared filters over white light source to make the beam invisible and reduce stray
Hughes Aircraft's Santa Barbara Research Center is marketing a portable laser diode voice
communicator that uses pulse triggering. It produces 2 watts of peak power, sufficient to
establish a communication link over a distance of 6 miles in good weather. This basic equipment
range can be considerably extended by adding elaborate telescopic attachments. The trigger
pulses for the laser diode are frequency modulated. With a carrier frequency of 6000 Hz, the
system can carry a single channel of voice information with frequencies up to 2300 Hz. Although
current pulses through the laser diode may reach 40 amperes, the transmitter circuit drains
only about 10 mA average current from a 12-volt battery pack.
Another portable laser diode communicator, built by Holobeam for the Navy, also uses short-pulse
firing. The carrier is pulse-position modulated; so, voice information fed into the transmitter
varies the position of each pulse with respect to some standard "no-signal" position. Conservative
military specifications list the maximum operating range at 1.5 miles, but the transmitter's
8-watt peak output power could easily multiply this figure.
Several major companies, among them Bell Laboratories and Texas Instruments, have under
development more efficient laser diodes which are not restricted to the pulse mode. Such diodes
promise to combine the high-frequency and continuous-wave characteristics of LED's with the
high power and coherent light of modern laser diodes.
YAG Communicators. Bell Labs is working on a portable laser communicator which uses an
yttrium-aluminum-garnet (YAG) solid-state laser source. Closely related to the artificial
ruby, the YAG laser has served laser technology almost from the beginning.
Unlike laser diodes, YAG lasers are light amplifiers which convert a flash of light energy
into a more powerful beam of laser light. The problem is to find a way to fire the light that
excites the YAG crystal.
Researchers at Bell Labs believe that laser diodes make a suitable source of stimulation
for a YAG laser. By firing the YAG crystal with a laser diode flash, they hope to produce
a medium-power YAG communicator as portable as laser diode communicators. The YAG devices
will operate in a fast pulse mode as do laser diodes, but the former will generate much more
high-quality laser light.
Gas laser Communication links. All the light-beam communication work using LED's, laser
diodes, and YAG lasers is aimed at providing portable short-range communicators capable of
carrying only a few channels of information at a time. But the time is coming when the laser
links will have to take over where the already overcrowded radio, TV, and telephone channels
leave off. A single laser beam will then have to carry millions of bits of information every
second from point to point. Most researchers developing long-range, high-capacity laser links
agree that gas lasers are the most suitable sources of light for their purposes.
Gas lasers use a mixture of at least two gases. In a helium-neon (He-Ne) laser, for example,
passing a current through the tube makes the neon produce ultraviolet radiation which stimulates
laser emission from the helium. As long as current flows, the gases continue to do their work.
Laser diode chip from Bell Labs (on a penny) can operate continuously at
room temperature and is for possible use in small, low-cost communicators.
It is possible to impress information onto a gas laser beam by varying the amount of current
through the tube. A far more effective modulation technique uses special external filters
which change their planes of polarization in only one direction. Rotating a polarized filter
in the beam path changes the amount of light (intensity) passing through. The special voltage-sensitive
filters, made of a crystal such as lithium tantalate, rotate the plane of polarization according
to an applied signal voltage.
A continuous laser beam can be modulated at frequencies in the GHz range - of which no
known electronic circuits can take full advantage. The best electronics technology can do
today to take advantage of the bandwidth is to use a number of circuits to drive an equal
number of polarization filters. By passing a single light beam through all the filters, all
the electronic inputs become impressed on it. It is believed that it is possible to construct
such a communication link to carry 200,000,000,000,000 bits of information. Such a system
could be capable of handling all voice, TV, facsimile, computer, and commercial radio information
entering and leaving a city as large as New York.
Laser beams travel in straight lines; so, future long-distance communication systems will
have to use a series of mirrors or repeaters to make the beam follow the earth's curvature.
Another scheme calls for using mirrors on orbiting satellites to reflect the beam from one
point to another thousands of miles apart on the earth's surface.
An entirely different transmission technique will employ fiber optics to get the beam around.
A modified version of this will use evacuated pipes, outfitted with reflectors, to carry the
beam. These two methods have the special advantages of being immune to atmospheric disturbances.
Gas laser communications is progressing at a rather slow rate compared to the progress
being made in LED and laser diode schemes. The reason is that there is presently no real need
for communication links which have such staggering information handling capacities. When the
time is right, high-performance laser communication systems now operating in experimental
labs will be ready to open new communication channels which have virtually no limit with reference
to operating range and information-handling capacity.
Posted July 14, 2017