Preface:Table of Contents, Introduction, Why through-the-air communications?

LIGHT THEORY:The Spectrum, Human Eye Response, Silicon Detector Response >>

PREFACE

About the author:

David A. Johnson, P.E. is consulting electronics engineer with a broad spectrum of experience that

includes product research, design and development; electronic circuit design; design, building and

testing prototypes; electro-optics; and custom test instruments. Doing business for more than 17

years as David Johnson and Associates, Dave has established himself as an electronics engineer

who can provide a variety of services.

His proficiency is based on "hands-on" experience in general engineering, electronics and electro-

optics. Mr. Johnson is licensed by the State of Colorado as a Professional Engineer; he is a

graduate of University of Idaho and is a member of IEEE. Holds three patents and has four more

pending.

He remains well informed of the latest scientific and engineering advancements through

independent studies. Dave is a published author with articles and designs in EDN, Electric Design,

Midnight Engineering and Popular Electronics.

He may be reach via email at dajpe@aol.com.

I became interested in optical through-the-air communications around 1980. At that time I was

doing research in high-speed fiber optic computer data networks for a large aerospace company. My

research assignment was to produce a report that made recommendations for the best ways of using

the latest optical fiber technologies to satisfy the increased demands for fast data transmission in the

aerospace industry. My research involved pouring through mountains of technical papers, scientific

journals, patents and manufacturer's application notes.

As my research progressed I began to notice that nearly all the optical communications systems

described used optical fibers. Little was being written on the subject of through-the-atmosphere

communications. It seemed logical to me that many of the techniques being used in fiber optic

communications could also be applied in through-the-air communications. I was puzzled by the

technical hole that seemed to exist. This lack of information started my personal crusade to learn

more about communicating through-the-air using light.

During my studies I reviewed many of the light communications construction projects that were

published in some electronics magazines. I was often disappointed with the lack of sophistication

they offered and usually found their performance lacking in many ways. Many of the circuits were

only able to transmit a signal a few feet. I thought that with a few changes they could go miles. I

was determined to see how far the technology could be pushed without becoming impractical. So, I

took many of the published circuits and made them work better. I discovered better ways to process

the weak light signals and methods to get more light from some common light emitters. I found

ways to reduce the influence ambient light had on the sensitive light detector circuits and I

developed techniques to increase the practical distance between a light transmitter and receiver. I

also experimented with many common light sources such as fluorescent lamps and xenon camera

flash tubes to see if they too could be used to send information. To my delight they were indeed

found to be very useful.

Today, my crusade continues. I am still discovering ways to apply what I have learned and I'm still

making improvements. However, after having devoted some 20 years of work toward advancing the

technology I felt it was time to collect what I have learned and pass some of the information on to

others. Thus, this book was conceived.

This handbook may be found at http://www.imagineeringezine.com/air-bk2.html.

Page 2 of 68

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

TABLE OF CONTTENTS

Preface

....................................................................................... 1

Table of Contents

...................................................................... 3

Introduction:

............................................................................... 5

Brief History

......................................................................

Why Optical Communications? ....................................................

Why through-the-air communications? ..............................................

What are some of the limitations of through-the-air communications? ........

How can these light-beam techniques be used? .........................................

Possible uses for optical through-the-air communications

................

Chapter One LIGHT THEORY ...................................................... 10

The Spectrum, Human Eye Response

............................................ 10

Silicon Detector Response ..............................................................

Units of Light

.......................................................................

Light Power and Intensity ..............................................................

Miscellaneous Stuff .......................................................................

Chapter Two LIGHT DETECTORS

...........................................

What Does a Light Detector Do? ...............................................................

The Silicon PIN Photodiode

............................................................

InGaAs PIN Diode ...............................................................................

Typical PIN Diode Specifications ............................................................

Package

..............................................................................

Active Area ..............................................................................

Response Time

.....................................................................

Capacitance

.....................................................................

Dark Current

.....................................................................

Noise Figure ..............................................................................

.......................................................................

Other Light Detectors

Photo Transistor

......................................................................

Avalanche Photodiode

.............................................................

Photo Multiplier Tube

.............................................................

Optical Heterodyning

..............................................................

Future Detectors

......................................................................

Detector Noise

......................................................................

Minimum Detectable Light Levels ..............................................................

Page 3 of 68

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

Chapter Three LIGHT EMITTERS

............................................. 23

Introduction to Light Emitters

.............................................................. 23

Light Emitting Diodes (LEDs)

.............................................................. 23

GaAlAs IR LED ................................................................................ 23

GaAs IR LED

......................................................................... 24

GaAsP Visible Red LEDs

.............................................................. 25

Solid State Semiconductor Lasers .............................................................. 25

GaAs (Hetrojunction) Lasers .............................................................. 25

GaAlAs (CW) Lasers ........................................................................ 26

Surface Emitting Lasers

.............................................................. 27

Externally Excited Solid State Lasers

..................................................... 27

Gas Lasers ......................................................................................... 27

Fluorescent Light Sources ....................................................................... 29

Fluorescent Lamps

....................................................................... 29

Cathode Ray Tubes (CRT)

.............................................................. 29

Gas Discharge Sources

....................................................................... 30

Xenon Gas Discharge Tubes .............................................................. 30

Nitrogen Gas (air) Sparks

.............................................................. 31

Other Gas Discharge Sources .............................................................. 31

External Light Modulators ....................................................................... 32

Chapter Four LIGHT SYSTEMS CONFIGURATIONS

.................

Opposed Configuration

.......................................................................

Diffuse Reflective Configuration ..............................................................

Retro Reflective Configuration

..............................................................

Chapter Five LIGHT PROCESSING THEORY

.......................... 37

Lenses as Antennas ................................................................................ 37

Mirrors and Lenses ................................................................................ 37

Types of Lenses

................................................................................. 37

Divergence Angle ................................................................................ 38

Acceptance Angle ................................................................................ 38

Light Collimators and Collectors .............................................................. 38

Multiple Lenses, Multiple Sources .............................................................. 39

Optical Filters

................................................................................ 39

Make your own optical low-pass filter

..................................................... 41

Inverse Square Law ................................................................................ 41

Range Equation

................................................................................ 42

Chapter Six - OPTICAL RECEIVER CIRCUITS

..........................

Light Collector

................................................................................

Light Detector

................................................................................

Stray Light Filters ................................................................................

Current to Voltage Converter Circuits

.....................................................

High Impedance Detector Circuit .....................................................

Transimpedance Amplifier Detector Circuit

with resistor feedback

............................................................

Page 4 of 68

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

Transimpedance Amplifier Detector Circuit

with inductor feedback

.............................................................. 46

Transimpedance Amplifier Detector Circuit

with limited Q feedback

.............................................................. 47

Post Signal Amplifiers

....................................................................... 48

Signal Pulse Discriminators ....................................................................... 49

Frequency to Voltage Converters .............................................................. 49

Modulation Frequency Filters

.............................................................. 49

Audio Power Amplifiers

....................................................................... 49

Light Receiver Noise Considerations

..................................................... 50

Other Receiver Circuits

....................................................................... 50

Sample of Receiver Circuits

......................................................... 52 - 58

Chapter Seven - OPTICAL TRANSMITTER CIRCUITS

................. 59

Audio Amplifier with Filters

.............................................................. 59

Voltage to Frequency Converters .............................................................. 59

Pulsed Light Emitters

....................................................................... 60

Light Collimators ................................................................................ 60

Multiple Light Sources for Extended Range

............................................ 61

Wide Area Light Transmitters

.............................................................. 63

Wide Area Information Broadcasting

..................................................... 63

Samples of Transmitter Circuits ........................................................... 65-66

Page 5 of 68

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

INTRODUCTION

Brief History

Communications using light is not a new science. Old Roman records indicate that polished metal

plates were sometimes used as mirrors to reflect sunlight for long range signaling. The U.S. military

used similar sunlight powered devices to send telegraph information from mountain top to mountain

top in the early 1800s. For centuries the navies of the world have been using and still use blinking

lights to send messages from one ship to another. Back in 1880, Alexander Graham Bell

experimented with his "Photophone" that used sunlight reflected off a vibrating mirror and a

selenium photo cell to send telephone like signals over a range of 600 feet. During both world wars

some lightwave communications experiments were conducted, but radio and radar had more success

and took the spotlight. It wasn't until the invention of the laser, some new semiconductor devices

and optical fibers in the 1960s that optical communications finally began getting some real

attention.

During the last thirty years great strides have been made in electro-optics. Lightbeam

communications devices are now finding their way into many common appliances, telephone

equipment and computer systems. On-going defense research programs may lead to some major

breakthroughs in long range optical communications. Ground-station to orbiting satellite optical

links have already been demonstrated, as well as very long range satellite to satellite

communications. Today, with the recent drop in price of some critical components, practical

through-the-air communications systems are now within the grasp of the average experimenter. You

can now construct a system to transmit and receive audio, television or even high speed computer

data over long distances using rather inexpensive components.

Why Optical Communications?

Since the invention of radio more and more of the electro-magnetic frequency spectrum has been

gobbled up for business, the military, entertainment broadcasting and telephone communications.

Like some of our cities and highways, the airwaves are becoming severely overcrowded. Businesses

looking for ways to improve their communications systems and hobbyist wishing to experiment are

frustrated by all the restrictions and regulations governing the transmission of information by radio.

There is simply little room left in the radio frequency spectrum to add more information

transmitting channels. For this reason, many companies and individuals are looking toward light as

a way to provide the needed room for communications expansion. By using modulated light as a

carrier instead of radio, an almost limitless, and so far unregulated, spectrum becomes available.

Let me give you an example of how much information an optical system could transmit. Imagine a

single laser light source. Let's say it is a semiconductor laser that emits a narrow wavelength (color)

of light. Such devices have already been developed that can be modulated at a rate in excess of 60

gigahertz (60,000MHz). If modulated at a modest 10GHz rate, such a single laser source could

transmit in one second: 900 high density floppy disks, 650,000 pages of text, 1000 novels, two 30-

volume encyclopedias, 200 minutes of high quality music or 10,000 TV pictures. In less than 12

hours, a single light source could transmit the entire contents of the library of congress. Such a

Page 6 of 68

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

modulation rate has the capacity to provide virtually all of the typical radio, TV and business

communications needs of a large metropolitan area. However, with the addition of more light

sources, each at a different wavelength (colors), even more information channels could be added to

the communications system without interference. Color channels could be added until they

numbered in the thousands. Such an enormous information capacity would be impossible to

duplicate with radio.

Why through-the-air communications?

One of the first large scale users for optical communications were the telephone companies. They

replaced less efficient copper cables with glass fibers (fiber optics) in some complex long distance

systems. A single optical fiber could carry the equivalent information that would require tens of

thousands of copper wires. The fibers could also carry the information over much longer distances

than the copper cables they replaced. However, complex fiber optic networks that could bring such

improvements directly to the small business or home, are still many years away. The phone

companies don't want to spend the money to connect each home with optical fibers. Until fiber optic

networks become available, through-the-air communications could help bridge the gap. The term

"the last mile" is often used to describe the communications bottleneck between the neighborhood

telephone switching network and the home or office.

Although light can be efficiently injected into tiny glass fibers (fiber optics) and used like copper

cables to route the light information where it might be needed, there are many applications where

only the space between the light information transmitter and the receiver is needed. This "freespace"

technique requires only a clear line-of-sight path between the transmitter and the distant receiver to

form an information link. No cables need to be buried, no complex network of switches and

amplifiers are needed and no right-of-way agreements need to be made with landowners. Also, like

fiber optic communications, an optical through-the-air technique has a very large information

handling capacity. Very high data rates are possible from multiple color light sources. In addition,

systems could be designed to provide wide area communications, stretching out to perhaps ten to

twenty miles in all directions. Such systems could furnish a city with badly needed information

broadcasting systems at a fraction of the cost of microwave or radio systems, and all without any

FCC licenses required.

What are some of the limitations of through-the-air communications?

The main factor that can influence the ability of an optical communications system to send

information through the air is weather. "Pea soup" fog, heavy rain and snow can be severe enough

to block the light path and interrupt communications. Fortunately, our eyes are poor judges of how

far a signal can go. Some infrared wavelengths, used by many of the light transmitters in this book,

are able to penetrate poor weather much better than visible light. Also, if the distances are not too

great (less than 5 miles), systems can be designed with sufficient power to punch through most

weather conditions. Unfortunately, little useful information exists on the true effects weather has on

long-range optical systems. But, this should not be a hindrance to the development of a through-the-

air system, because there are many areas of the world where bad weather seldom occurs. In

addition, it would be a shame to completely reject an optical communications system as a viable

alternate to radio solely due to a few short interruptions each year. Even with present day systems,

TV, radio and cable systems are frequently interrupted by electrical storms. How may times has

your cable or TV service been interrupted due to bad weather? I think the advantages that through-

the-air communications can provide outweigh the disadvantages from weather.

Page 7 of 68

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

Another limitation of light beam communications is that since light can't penetrate trees, hills or

buildings. A clear line-of-sight path must exist between the light transmitter and the receiver. This

means that you will have to position some installations so their light processing hardware would be

in more favorable line-of-sight locations.

A third limitation, one that is often overlooked, is the position of the sun relative to the light

transmitter and receiver. Some systems may violate a "forbidden alignment" rule that places the

light receiver or transmitter in a position that would allow sunlight to be focused directly onto the

light detector or emitter during certain times of the year. Such a condition would certainly damage

some components and must be avoided. Many installations try to maintain a north/south alignment

to lessen the chance for sun blindness.

How can these light-beam techniques be used?

I believe that optical through-the-air or "Freespace" communications will play a significant role in

this century. Many of you are already using some of this new technology without even being aware

of it. Most remote control devices for TVs, VCRs and stereo systems rely on pulses of light instead

of radio. Many commercially available wireless stereo headphones are using optical techniques to

send high quality audio within a room, giving the user freedom of movement. In addition, research

is on going to test the feasibility of using optical communications in a variety of other applications.

Some military research companies are examining ways to send data from one satellite to another

using optical approaches. One such experiment sent data between two satellites that were separated

by over 18,000 miles. Space agencies are also exploring optical techniques to improve

communications to very distant space probes. Some college campuses and large business complexes

are experimenting with optical through-the-air techniques for high-speed computer networks that

can form communications links between multiple buildings. Some military bases, banks and

government centers are using point-to-point optical communications to provide high speed

computer data links that are difficult to tap into or interfere with. But, don't become overwhelmed,

there are many simple and practical applications for you experimenters. Several such applications

will be covered in this handbook. Below are some examples of existing and possible future uses for

light-beam communications.

POSSIBLE USES FOR OPTICAL THROUGH-THE-AIR COMMUNICATIONS

Short Range Applications

Industrial controls and monitors

Museum audio; walking tours, talking homes

Garage door openers

Lighting controls

Driveway annunciators

Intrusion alarms

Weather monitors; fog, snow, rain using light back-scatter

Traffic counting and monitoring

Animal controls and monitors; cattle guards, electronic scarecrow

Medical monitors; remote EKG, blood pressure, respiration

Page 8 of 68

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

Long Range Applications

Deep space probe communications; distances measured in light-years

Building to building computer data links; very high data rates.

Ship to ship communications; high data rates with complete security.

Telemetry transmitters from remote monitors; weather, geophysical.

Electronic distance measurements; hand held units out to 1000 ft.

Optical radar; shape, speed, direction and range.

Remote telephone links; cheaper than microwave

Wide Area Applications

Campus wide computer networks

City-wide information broadcasting

Inter-office data links

Computer to printer links

Office or store pagers

Systems for the hearing impaired; schools, churches, movies

Cloud bounce broadcasting

Page 9 of 68

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

Table of Contents: