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Chapter Three

LIGHT EMITTERS

Introduction to Light Emitters

Unlike the limited number of useable light detectors, there is a wide variety of light emitters that

you can use for optical through-the-air communications. Your communications system will depend

much more on the type of light source used than on the light detector. You should choose the light

source based on the type of information that needs to be transmitted and the distance you wish cover

to reach the optical receiver. In all cases the light source must be modulated (usually turned on and

off or varied in intensity) to transmit information.

The modulation rate will determine the

maximum rate information can be

transmitted. You may have to make some

tradeoffs between the modulation rates

needed, the distance to be covered and the

amount of money you wish to spend.

Many light sources listed below are useful

for low to medium speed modulation rates

and can have ranges up to several miles. A

few others are ideal for low speed

telemetry transmission that can reach

beyond 50 miles. If you need high speed

Samples of Emitters

information transmission, there are only a

few choices, and those tend to be expensive. But, as the technology improves the prices should

come down. I have also described some of the latest devices that may become available to the

experimenter in a few years, but only demonstration devices exist today.

Light Emitting Diodes (LEDS)

For most through-the-air communications applications the infrared light emitting diode (IRLED) is

the most common choice. Although visible light emitting devices do exist, the infrared parts are

generally chosen for their higher efficiency and more favorable wavelength, especially when used

with silicon photodiode light detectors.

GaAlAs IR LED

GaAlAs (gallium, aluminum arsenic) infrared LEDs are the most widely used modulated IR light

sources. They have moderate electrical to optical efficiencies, (at low currents 4%), and produce

light that matches the common silicon PIN detector response curve (900nm). Most devices can be

pulsed at high current levels, as long as the average power does not exceed the manufacturer's

maximum power dissipation specification (typically 0.25 watts). Some devices can be pulsed up to

10 amps, if the duty cycle (ratio of on time to the time between pulses) is less than 0.2% (0.002:1

ratio). Some of the faster devices have response times that allow them to be driven with current

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pulses as short as 100 nanoseconds but

most devices require at least 900

nanoseconds. At a current level of about 6

amps a quality device can emit about 0.15

watts of infrared light. However, at higher

current levels their efficiency is generally

poor, dropping to less than 0.5% (See

Figures 3a, 3b, 3c and 3d.) Many

resemble the commonly used visible LEDs

and will typically be packaged in molded

plastic assemblies that have small 3/16"

lenses at the end. The position of the

actual LED chip within the package will

determine the divergence (spreading out)

of the exiting light. The typical T-1 3/4

Figure 3a

style device will have a half angle divergence ranging from 15 to 40 degrees. They are low cost,

medium speed (up to 1 million pulses per second) sources, with long operating lifetimes (typically

greater than 100,000 hours).

They are a good choice for short and

medium distance control links and general

communications applications. When used

with a large lens, a single device can be

used for a communications system with a

multi-mile range. Multi-device arrays can

also be constructed to transmit information

over wider areas or longer distances. They

generally cost between $0.30 to $2.00 each

and

are

available

from

many

manufacturers.

GaAs IR LED

These devices are the older and less

efficient cousin to the GaAlAs devices.

They come in all styles and shapes. The

more useful devices have smaller emitting

surfaces than GaAlAs LED's, permitting

narrow divergence angles with small

Figure 3b

lenses. Also, the small emitting areas make

them very useful for fiber optic applications. Some commercial devices have miniature lenses

cemented directly to the semiconductor chip to produce a small exiting light angle (divergence

angle). In conjunction with a small lens (typically 0.5") such devices can launch light with a narrow

divergence angle (0.5 degrees). The most important feature of the GaAs LED is its speed. They are

generally 10 times faster than GaAlAs LED's but many only produce 1/6 as much light. They are

often picked when medium speed transmission over short distances is required. Their price is

typically a little more than the GaAlAs LED's, even though they use an older technology. They will

cost between $2.00 to $25.00.

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GaAsP Visible Red LEDs

Although not as efficient as the infrared

devices some visible red LEDs (Figure

3d-1)are now available, that might find

limited use in some short range through-

the-air applications. Some so called "super

bright" LEDs boast high light output.

However, even the brightest components

will still produce only 1/3 as much light as

a quality infrared part.

Also, since their light is a visible red color,

an automatic 2:1 penalty will be paid when

the devices are used with a standard

silicon detector that has a weaker response

to red light. The visible red LEDs are

generally faster (up to 2 million pulses per

second) than IR components and can

therefore be used for medium speed

Figure 3c

applications. Also, since their light is

visible, they are much easier to align than invisible IR devices, especially when the devices are used

with lenses.

Solid State Semiconductor Lasers

GaAs (Hetrojunction) Lasers

These devices have been around since the

1960s and can produce very powerful light

pulses. Some devices are able to launch

light pulses in excess of 20 watts, which is

some 200 times more powerful than a

typical GaAlAs LED. But, these devices

can only be driven with duty cycles, less

than 0.1% (off time must be 1000 times

longer than on time). Also, their maximum

pulse width must be kept short (typically

less than 200 nanoseconds) even under

low pulse rate applications. However,

despite their limitations these devices can

be used in some voice transmitter systems

if some careful circuit designs are used.

As in most semiconductor lasers, the GaAs

laser does require a minimum current level

Figure 3d

(typically 10 to 20 amps) before it begins

emitting useable light. Such high operating currents demand more complicated drive circuits.

Despite a 10:1 sensitivity reduction, caused by the rather narrow emitted pulses (see receiver circuit

discussion), the more powerful light pulses available from GaAs lasers can increase the useful range

of a communications system by a factor of about 3, over a typical transmitter using a single LED. In

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addition, since their emitting spot sizes are

very small, they can also be focused into

very tight beams using rather small lenses.

In addition, since their spectral widths are

very narrow the matching light detector

circuit can use an optical band pass filter

to reduce the noise levels associated with

ambient light (see receiver circuit section).

For low speed and long distance

applications, the GaAs laser should be

considered. However, they do have some

disadvantages. They typically cost much

more than a GaAlAs LED (up to $75).

They have shorter lifetimes (may only last

a few hundred hours) and are sensitive to

Figure 3d-1

temperature. Therefore, they require a carefully designed transmitter circuit that can switch 20 or

more amps at high speeds and can compensate for changes in operating temperature.

GaAlAs (CW) Lasers

These are the latest in infrared light emitting semiconductor devices and are rapidly maturing. The

first wide spread application for these devices was in audio compact disk players and CD-

ROM computer disk drives. They are also

being used in some computer laser

printers, bar code readers and FAX

machines. They have very small emitting

areas, can produce peak power levels in

excess of 0.2 watts and have narrow

spectral bandwidths (see Figure 3e.) The

most important improvement over other

light sources is that they can be modulated

at frequencies measured in gigahertz.

However, as in any new technology they

are still rather expensive. Low power units

that emit less than 0.01 watts of 880nm

infrared light, sell for about $20.00. Some

of the more powerful devices can cost as

Figure 3e

much as $20,000 each. Although the use of a laser in a communications system might give a project

a high tech sound, a much cheaper IR LED will almost always out-perform a low power laser

(typical LED will be able to emit 10 times more light at 1/10 the cost) in low to medium speed

applications. But, when very high-speed modulation rates (up to 1 billion pulses per second) are

needed, these devices would be a good choice.

Although expensive now, these devices should come down in price over the next few years. They

will also most likely be available at higher power levels too. But, until then, their advantages do not

justify their expense and the more useful high power units are beyond the reach of practical

experimental designs. I suggest using these devices only when necessary.

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Surface Emitting Lasers (VCSEL)

These devices are just now beginning to appear in some catalogs. Many companies have been

experimenting with these latest semiconductor devices since about 1988. Their small size and high

efficiency make them very suitable for some applications. They are mostly used in optical fiber

communications. Instead of being grown as single chip emitters, these devices are fabricated into

large arrays of very small individual laser sources sharing a common substrate. Since the individual

laser diode emitters can be as small as one micron (1/10,000cm) as many as 100 million separate

devices could be placed into a 1cm X 1cm area.

The output efficiency (electrical power to light power) has been reported to be about 40%, with

each tiny device emitting about 0.003 watts. Although each device may emit only a small amount of

light, when used as an array, 100 million such devices could launch some 100,000 watts of IR light

from about 200,000 watts of electricity. Of course, cooling such a powerful array would be a real

challenge, if not impossible. But, perhaps smaller arrays could be placed into common

semiconductor packages for easy mounting and cooling. Maybe a 0.1-watt device would be placed

into inexpensive LED style packages. Other devices may be mounted in better heat conducting

metal packages to allow perhaps 100 watts of light to be emitted. Since their maximum modulation

rates have been measured in the multi-billion pulses per second rate, surface-emitting lasers would

be ideal for many future through-the-air communications applications. They would especially be

useful in broadcasting optical information over a citywide area, where very powerful high-speed

light sources are needed. A 10,000-watt source, emitting light in a specially shaped 360-degree

pattern, might be able to transmit information over an area covering some 500 square miles. Such a

broadcasting system might be used to transmit library type information from large centralized

databases.

Externally Excited Solid State Lasers

Some of the very first lasers made were the Ruby and YAG lasers. Most of these lasers are excited

externally using large xenon flash tubes that are positioned around the central glass laser rod. A

small portion of the light from the xenon flash excites the specially positioned rod material, forming

short coherent light pulses. Although these lasers are capable of emitting very power light pulses,

with very narrow divergence angles, they are generally much too expensive and too complicated for

the average experimenter. They would therefore find very limited use in earth-bound optical

communications. However, some scientists believe that the extremely powerful light pulses that

these devices are capable of producing, might be useful in transmitting information into very deep

space. Since some pulsed lasers have been reported to launch light pulses approaching one terawatt

(1000 billion watts), low speed communications might be possible to a range of several light years

(one light year = 6 trillion miles). Such a feat would be very difficult to accomplish with microwave

techniques.

Gas Lasers

Helium-neon, carbon dioxide and argon are the more common types of gas lasers. The light emitted

from a gas arc, inside a glass tube, is bounced back and forth through the excited gas using specially

fabricated mirrors. A portion of the light is allowed to escape through one of the mirrors and

emerges as very monochromatic (one wavelength) and highly coherent (same phase) light. Such

lasers have narrow divergence angles (typically less than 0.1 degrees) but have very low conversion

efficiencies (much less than 0.1%). They are also expensive and bulky that makes them impractical

for most optical communications applications. Some published designs that did provide

experimental optical communications using helium-neon lasers were designed to transmit voice

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

audio information over a range of only a few miles. The modulation technique was to vary the gas

arc current that then produced a light intensity modulation. However, the extra cost and relative low

power that resulted usually did not warrant the trouble. A properly designed system using a single

LED will usually out perform any short-range helium-neon laser communications system at a

fraction of the cost.

Although too expensive for the experimenter, some gas lasers have been used by the military for

many years. In particular, carbon dioxide lasers, that emit long infrared wavelengths (10,000

nanometers), have been used in some military targeting systems. The long infrared wavelength can

penetrate smoke and fog better than visible or near IR lasers. Also, the Navy has been

experimenting with some blue-green laser light to attempt to provide communications to

submarines deep under water. But, overall gas lasers fall short of the ideal for practical through-the-

air communications.

Fluorescent Light Sources

Fluorescent Lamps

Fluorescent lamps work on the principle of

"fluorescence" and because of their low

cost

have

many

through-the-air

applications. An electrical current passed

through a mercury vapor inside a glass

tube causes the gas discharge to emit

ultraviolet "UV" light. The UV light

causes a mixture of phosphors, painted on

the inside wall of the tube, to glow at a

number of visible light wavelengths (see

Figure 3f.) The electrical to optical

conversion efficiency of these light

sources is fairly good, with about 3 watts

of electricity required to produce about 1

watt of light. A cathode electrode at each

Figure 3f

end of the lamp that is heated by the discharge current, aids in maintaining the discharge efficiency,

by providing rich electron sources. By turning on and off the electrical discharge current, the light

being emitted by the phosphor, can be modulated. Also, by driving the tubes with higher than

normal currents and at low duty cycles, a fluorescent lamp can be forced to produce powerful light

pulses. However, like the pulse techniques used with LEDs, the fluorescent lamp pulsing techniques

must use short pulse widths to avoid destruction of the lamp.

To modulate a fluorescent lamp to transmit useful information, the negative resistance characteristic

of the mercury vapor discharge within the lamp must be dealt with. This requires the drive circuit to

limit the current through the tube. The two heated cathode electrodes of most lamps also require the

use of alternating polarity current pulses to avoid premature tube darkening. The typical household

fluorescent lighting uses an inductive ballast method to limit the lamp current. Although such a

method is efficient, the inductive current limiting scheme slows the rise and fall times of the

discharge current through the tube and thus produces longer then desired light pulses. To achieve a

short light pulse emission, a resistive current limiting scheme seems to work better. In addition,

there seems to be a relationship between tube length and the maximum modulation rate. Long tubes

do not respond as fast as shorter tubes. As an example, a typical 48" 40 watt lamp can be modulated

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

up to about 10,000 pulses per second, but

some miniature 2" tubes can be driven up

to 200,000 pulses per second. The main

factor that ultimately limits the modulation

speed is the response time of the phosphor

used inside the lamp. Most visible

phosphors will not allow pulsing much

faster than about 500,000 pulses per

second. The visible light emitted by the

typical "cool white" lamp is also not ideal

when used with a silicon photodiode.

However, some special infrared light

emitting phosphors could be used to

increase the relative power output from a

fluorescent lamp, which may also produce

Figure 3g

faster response times. (see Figure 3g.)

If a conventional "cool white" lamp is used, a 2:1 power penalty will be paid due to the broad

spectrum of visible light being emitted (see Figure 3f.) This results since the visible light does not

appear as bright to a silicon light detector as IR light (see section on light detectors). Also, light

detectors with built-in visible filters should not be used, since they would not be sensitive to the

large amount of visible light emitted by the lamps. Although the average fluorescent lamp is not an

ideal light source, the relative low cost and the large emitting surface area make it ideal for

communications applications requiring light to be broadcasted over a wide area. Experiments

indicate that about 20 watts of light can be launched from some small 9-watt lamps at voice

frequency pulse rates (10,000/sec). Such power levels would require about 100 IR LEDs to

duplicate. But, the large surface emitting areas of fluorescent lamps makes them impractical for

long-range applications, since the light could not be easily collected and directed into a tight beam.

(For additional information see section on fluorescent lamp transmitter/receiver circuits.)

Cathode Ray Tubes (CRT)

CRTs work somewhat like fluorescent lamps, since they too use fluorescence emission techniques.

Electrons, emitted from a heated cathode end of the cathode ray vacuum tube, are accelerated

toward the anode end by the force of a high voltage applied between the cathode and anode

electrodes. Before hitting the anode screen, the electrons are forced to pass through a phosphor

painted onto the inside of the screen. In response to the high-speed electrons, the phosphor emits

light at various wavelengths. A voltage applied to a special metal grid near the tube's cathode end is

used to modulate the electron beam and can thus produce a modulation in the emitted light. This

principle is used in most computer and TV screens. Since the electron beam can be modulated at

very high rates, the light source modulation rate is limited only by the response time of the

phosphor used. Depending on the type of phosphor, the electrical to optical efficiency can be as

high as 10%. Some specially made cathode ray tubes produce powerful broad (unfocused) electron

beams that illuminate the entire front screen of the CRT instead of a small dot. Such tubes can yield

powerful light sources, with large flat emitting areas. A variation on the usual television type CRT

design positions a curved phosphor screen at the back of the vacuum tube and places the cathode

electrode at the front or side of a clear glass screen (some portable Sony TVs use such CRTs). This

technique increases the overall efficiency, since it allows the light from the phosphor to exit from

the same side as the electron source. With the aid of external cooling, such techniques could create

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

very powerful light sources that might be able to launch tens of thousands of watts of light, pulsed

at rates exceeding tens of millions of light pulses per second. Although the typical experimenter

may not be interested in such light power levels it does raise some interesting possibilities for use in

city wide optical communications.

Gas Discharge Sources

Xenon Gas Discharge Tubes

The most common form of this class of

light source is the electronic camera flash.

These devices are some of the most

intense light sources available to the

experimenter and have many interesting

applications. The discharge lamps are

typically made from a glass tube with a

metal electrode installed at each end. They

are filled with xenon gas at about one

atmosphere of pressure (14psi). The gas

inside the tube can be made to glow with

very high intensity when an electrical

current is passed through it.

Xenon Lamps

As illustrated in Figure 3h, the xenon arc

emits light over a broad spectrum with some large peaks in the near infrared range. The electrical to

optical conversion is fairly good. A typical camera flash can produce about 2,000 watts of light

from about 10,000 watts of electrical power (20% efficiency). Some specially made discharge tubes

can generate flashes that exceed one million watts of light power. As in fluorescent lamps, the

minimum flash duration is somewhat dependent on the length of the discharge tube. A typical

camera flash tube has an electrode gap of about 15mm (0.6") and will usually produce a flash,

which lasts about one millisecond. The energy used to produce the short flash comes from

discharging a special capacitor, charged to

several hundred volts. By decreasing the

size of the capacitor (say to 6 microfarads)

and increasing the voltage (say to 300

volts) the camera flash tube can be made

to produce flashes as short as 20

microseconds. Shorter discharge flashes

are only possible by using specially made

discharge tubes with very narrow electrode

gaps (0.5mm). These narrow gap lamps

can produce flashes as short as one half

microsecond. However, the physics of the

xenon gas arc prevents flashes much

shorter.

Figure 3h

Flash rates up to 10,000 per second are

possible with the short gap lamps, but the typical camera flash tube can't be pulsed much faster than

about 100 flashes per second. Since some special high speed lamps can dissipate up to 75 watts of

average power, it is possible to design an optical voice information transmitter which could launch

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as much as 1000 watts of light with a narrow divergence. Such a transmitter would certainly have

some long-range possibilities. However, most xenon discharge lamps are more useful for low speed

and long-range applications, requiring very powerful light pulses. Many years ago, I constructed a

demonstration telemetry system that launched very powerful light pulses at a low data rate that had

a useable range of 50 miles. (See discussion on long-range telemetry transmitters using xenon flash

sources.)

Nitrogen Gas (air) Sparks

For very powerful and very short light pulse applications, a simple electrical spark in air can be

used. Some simple systems use two closely spaced (0.5mm) electrodes (usually made of

tungsten) in open air. With sufficient

voltage, the air between the electrodes can

be made to ionize briefly, forming a small

spark. Some gas barbecue grill igniters

that use piezoelectric crystals to produce

the needed high voltage, can be modified

to produce useful sparks for some

experiments. Commercially made nitrogen

spark sources claim to generate light

flashes that pack about 100,000 watts of

light power into short 5 nanosecond

pulses.

The nitrogen (air) arc emits a broad

Figure 3i

spectrum of light with large peaks in the

visible blue and invisible ultraviolet (see Figure 3i.) Such a spectrum is not ideal when used with

silicon detectors. But the small emission areas of the sparks allow simple lenses or mirrors to be

used to form very tight divergence angles. But, the air ionization (sparking) can be become very

unstable at high pulse rates, without using specially made discharge tubes and drive circuits.

Therefore, the sparks are best used for powerful, very short pulse applications that demand only low

pulse rates. Optical radar, electronic distance measurements, air turbulence monitors and wind shear

analysis are some possible uses for such a light source. You shouldn't be fooled by the seemingly

dim appearance of these light emitters. To our human eyes the tiny flashes may not seem very

bright, but to a fast detector they can be very powerful. However, to take advantage of these unique

pulses, a fast light detector and an equally fast amplifier must be used. Since few experiments have

been conducted with these unique light sources, it is a great area for the experimenter to see what

can be done.

Other Gas Discharge Sources

Glass discharge tubes filled with Cesium, Krypton or Rubidium will all produce lots of infrared

light. Krypton behaves much like Xenon and has a very similar emission output. Cesium and

Rubidium are both semi-liquids at room temperatures and can be operated under high or low

pressures in a discharge lamp. Such lamps might be constructed in a similar manner to the more

common yellow color sodium vapor street lamp. Cesium, in particular, appears to be a good

candidate for some experimentation in developing some powerful light sources with high peak

power outputs. Since kilowatt size sodium vapor street lamps are being manufactured, perhaps

similar lamps using cesium could be made. Such lamps might be able to produce multi-kilowatts of

modulated infrared light using pulse methods.

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External Light Modulators

Ferroelectric light valves, modulated mirror arrays, piezoelectric shutters, Kerr cells, Pockels cells,

Bragg cells and liquid crystals are all light modulators. They can be used to intensity modulate light

being emitted by an external source as it passes through them or reflects off them. The light can

originate from incandescent lamps, CW xenon gas arc lamps, light from a gas laser or even focused

sunlight. Although usually very expensive, some of the devices can be used to produce powerful

modulated light signals at high pulse rates.

Liquid crystal modulators are perhaps the slowest of the group. Most can't be driven much faster

than about 100 flashes per second. Ferroelectric light valves and piezoelectric shutters are a little

faster and can be pushed to perhaps 10,000 flashes per second. Kerr cells, Bragg cells and Pockel

cells, on the other hand, are known to be very fast. However, they work best when used with laser

light at a specific wavelength and at narrow angles. Some of these devices can modulate the light

from a laser at rates beyond 100 million pulses per second. But, most of these devices are very

expensive, are complicated and are therefore impractical for the average experimenter.

A new device developed by Texas

Instruments (Figure 3j) has some

interesting possibilities. The technology

was originally developed for flat panel

computer and TV displays, but the

techniques might be useful for optical

communications. TI's process fabricates a

large array of very small mirrors that can

be moved using a voltage difference

between the mirror and an area behind the

mirror. Like tiny fans, each mirror would

wave back and forth in response to the

drive voltage. Because the mirrors are very

small, the modulation rates might be

pushed to perhaps 100,000 activations per

second. If the mirrors were used to reflect

light from an intense light emitter, a nice

source of modulated light could be

Figure 3j

produced.

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