Optical
(through-the-air)
communications
 
A 3-watt red Luxeon LED at a distance of 14.91 miles (23.85 km) with downtown Salt Lake City in the foreground.
Luxeon 3 watt LED at a distance of 15 miles; Downtown Salt Lake City is in the foreground

What this page is about:

This page deals with ground-based optical, through-the-air optical ("lightbeam") communications using various light sources - primarily high-power LEDs, but the use of Lasers is also covered in brief.
Unlike more familiar fiber-based schemes, communications through the atmosphere has some unique challenges.
(It need not be said that most of these simply don't apply with a "glass" path - that is, a fiber-optic link.)

Note that the emphasis here is on experimental schemes and those described here are not intended to be either highly reliable or high-speed.  Because of this, the commercial viability of some of the techniques described here is irrelevant:  Much of the goal of this experimentation is to simply try different things and to have fun while doing it - and hopefully learning something that we didn't know before in the process!

Coherent versus non-coherent optical communications

"To lase, or not to lase..."

Lasers have been the darling of optical through-the-air communications for decades - and for some fairly good reasons:
Historically speaking, Lasers are relative newcomers in the optical communications field:  For many centuries, reflected sunlight or flames have been used while more recently, electric lights (incandescent and gas discharge tubes) have been used in some manner to provide a source of modulated light - but high modulation depth with good frequency response has always been a problem.  It turns out to be very difficult to modulate most "conventional" high-intensity, non coherent (e.g. things other than a Laser) light sources with properties of both full modulation and/or usable frequency response.  While there are various schemes that can accomplish this, they often are quite complicated and/or beyond the means of the average experimenter.

Lasers, on the other hand, would seem to lend themselves quite nicely to long-distance optical communications, with gas lasers and readily-available laser diode assemblies already designed to produce collimated, intense light.  It is quite easy to electronically modulate a laser diode, and either gas or solid state lasers can be modulated mechanically at very high frequencies with an optical cell.  One sticking point with lasers is that it can be quite a challenge to linearly modulate some types of lasers electronically to much depth, so it is more common to use schemes such as FM subcarriers or digital modulation that can be accomplished simply by turning the light on and off with the modulation rather than trying to change the brightness in some linear way.  (Note:  It is a bit more awkward to electrically modulate gas lasers, but this can be done by modulating the tube current or, in some cases, by modulating a magnetic field surrounding the laser tube.)

Recently, non-coherent solid state light sources have appeared in the form of high-power LEDs:  These devices have luminous outputs higher than that of most lasers - at least ones that an experimenter is likely to find or afford!  With these recent technology advances it is practical to get fairly high conversion efficiencies from single-color, high-power LEDs, easily achieving hundreds of milliwatts of light output:  A multi-hundred milliwatt Laser would not only be quite expensive, but it could also be extremely hazardous to anyone anywhere nearby!

Like traditional LEDs, high-power LEDs can lend themselves nicely to current modulation and their upper modulation frequencies are limited, for the most part, by their intrinsic capacitance, making them practical for video or data modulation.  The downside is that these LEDs do not usually come in a form that produces collimated light so it is up to the user to add the optics necessary to collimate the beam to make it useful for long-distance use.  Another important consideration is that, unlike lasers, LEDs do not produce coherent light - but this can be a distinct advantage as we shall see.

For a historical overview of optical communications see:
Large, collimated beams are best:

Whether one is using a Laser or not, successful long-distance optical communications requires that the emitted beam be collimated - preferably into as large a diameter beam as possible.  While it may seem like a good idea to preserve the extremely intense, pencil-like beam of a laser, it is important to realize that this beam has a very small cross-sectional area.  Because air is turbulent by nature, as the beam passes through small "cells" of air with different characteristics (temperature, humidity, pressure) the path of the light beam is refracted slightly every time it does so.  A very small-diameter beam of light will, therefore, slice its way through a narrow path of air cells, accumulating more and more disruption as it passes along.  At the far end, the received beam may have been through enough of these cells of air so that the beam can be significantly disrupted and randomly vary in brightness - a phenomenon known as scintillation .

A large, collimated beam, on the other hand, has lower beam density and may appear dimmer to the naked eye near the emitter, but only because the energy has been spread out over a larger area as compared to the pupil of the eye or due to beam divergence in the collimation optics.  Because of its larger area, it is more likely to be able to "straddle" multiple cells of air:  While portions of this beam's cross-section may be degraded by air cells, other portions are likely to not have been so-effected at the same instant and because of this, the overall amount of scintillation is reduced, an effect called aperture averaging.

In part, this effect can be easily demonstrated in the night sky:  Usually, stars twinkle, but planets don't!  Why is this?  A star is, for all practical purposes to the naked eye, a point-source of light:  While the star itself may be large, the vast distances in space make its subtended angle negligible in size.  A planet, on the other hand, being very much closer, is not simply a point of light:  Even a small telescope will resolve the nearby planets (Venus, Mars, Jupiter and Saturn, in particular) as obvious disks rather than points of light.

Another contributor of the "stars twinkle, planets don't" phenomenon is a phenomenon often referred to as "local coherence" which is at play when the angular source size is very small.  This effect, noted by A.A. Michelson, is another of the causes of scintillation.  If the emission of light is from a source with extremely small apparent angle (like the "pinpoint" of a star) even noncoherent light can take on many of the properties of coherent light, notably interference.  For more information on this topic, read "The Sizes of Stars" by Calvert.  This effect is yet another reason why a large aperture will contribute toward the reduction of the phenomenon of local coherence and thus scintillation.
Figure 1
The Top trace, covering a time span of about 0.8 seconds, shows 4 kHz signal emitted by a standard, high-brightness red LED being received over a 15 mile (24km) path having been emitted from large-area (approx. 50 sq. in, or about 289 sq cm) aperture.  This shows about 17dB of scintillation at a relatively slow rate.
The Bottom trace, covering a time span of about 0.28 seconds, shows 4 kHz laser signal, over the same path, using the very same optics as above for both receive and transmit.  Close inspection of this waveform reveals at least 40dB of scintillation occurring at a much higher rate than with the LED.  The signals were both received using the same 70 sq in (452 sq cm) Fresnel lens optical receiver.
Click on either image for a larger version.
Waveform of a signal emitted from collimated lens
Waveform of signal emitted from the same aperture as the LED

It should also be remembered that the amount of scintillation also depends on the size of the aperture being used as an optical receiver:  The iris of the eye, being only millimeters in diameter, is very small when compared with the objective lens of even a small telescope.  For this reason, stars that appear to twinkle to the naked eye are far less "twinkly" when viewed with a telescope or binoculars.

Scintillation and coherent light:

One unique property of lasers is that they produce light that is, for most purposes, emitted at a single frequency:  The light leaves the laser in (mostly) phase-coherent wavefronts.  If one were to cause a portion of the light beam the light to be delayed slightly as compared with another portion, wavefront cancellation can occur - and this is precisely what happens when an interference pattern is generated:  This phenomenon can be readily demonstrated using a CD or DVD to reflect laser light and seeing the resulting pattern of dots being reflected, or inferred by that familiar "laser speckle" that one sees on a surface illuminated by laser light.

This very property has a downside in through-the-air communications:  The cells of air can offer slightly different velocities of propagation to the light in addition to refraction.  The ultimate result of this is that scintillation of a coherent light beam is usually very much worse than a non coherent beam due to diffraction, owing to the fact that random wavefront phase cancellation is occurring in addition to just refraction.

Figure 1 illustrates this point clearly.  In each case, the same 15 mile (24km) path was used, along with the same detector optics (70 sq in, or 452 sq cm Fresnel lenses) and the same 50.27 square inch (289 sq. cm) optics (a reflector telescope) being used for transmitting.  The top image shows the amplitude of the LED (noncoherent light) being affected by the scintillation to a depth of about 17dB or so.  The bottom trace shows the received Laser signal being much more strongly affected - and at a faster rate.  Note that the two images in Figure 1 have different horizontal time scales.

For an audio clip that demonstrates the difference between Coherent and Noncoherent light and their passage through the atmosphere, click here.

Another property of coherent light that should not be overlooked is that the transmissivity of the atmosphere has many narrowband peaks and nulls throughout the visible spectrum, and a narrowband light source (such as a laser) could easily fall into one of those nulls:  Concoherent light sources, by their very nature, are not likely to be as susceptible to the effects of very narrow nulls in the transmissivity.

For more information about the atmospheric effects on light beams, see the article  "OPTICAL COMMUNICATIONS FOR THE AMATEUR" on the Modulated Light DX page.   Also, refer to the links near the bottom of the Modulated Light DX page - particularly those linked to papers by Korotkova. Comments about various lenses, and coherent and non-coherent light emitters:

No matter what light source is to be used, it is advantageous to use as large an optical aperture (lens) as possible in order to minimize scintillation - not to mention to maximize "optical gain" at the receive end.  There are a few practical matters that have to be considered when choosing a large lens (or reflector) for beam transmission or reception:
When one is using a laser to provide a coherent source for a collimated beam it makes sense to use good quality optics to minimize beam divergence - but this poses a number of problems (including cost and practicality) that severely limit the size of lens that one is likely to be able to use - not to mention being able to transport the optics and associated mounting gear.  These complications aside, the optical alignment (e.g. aiming) of a very tightly collimated beam over large distances requires extreme precision and mechanical stability - both factors that complicate the logistics of experimentation while in the field.

A practical alternative to a large glass (or even plastic) lens is a molded Fresnel lens.  These lenses are flat and, if properly constructed, can have excellent optical characteristics - including absence of spherical aberration.  Affordable plastic Fresnel lenses are, however, no match for a good quality set of glass (or optical plastic) lenses in terms of performance and because of this, they cannot easily be used to collimate a coherent light source such as a laser.  Even though Fresnel lenses are not suitable for collimating coherent light, they may still be used at the distant "receiving" end because of the fact that the atmospheric path will "de-cohere" the light.

Inexpensive Fresnel lenses can provide excellent results when used as a beam collimator if not using coherent light sources - such as high-power LEDs.  When using good-quality Fresnel lenses, it has been observed that it is the size of light source itself (e.g. the LED's die) and not the quality of the lens has been the main factor in determining beam divergence and values of less than 0.3 degrees (approximately 5.2 milliradians) are easily attainable and values significantly lower than this would be possible if a high-intensity "point-source" LED was practical:  Unfortunately, such a "point-source" LED does not exist and, again, the apparent size of the LED's emitter - not the quality of the lens - is the major limitation for the reduction of beam divergence.

Even though the use of high-power LEDs implies a more-divergent beam than that obtained from a typical laser, the sheer magnitude of luminous flux available from the LED still allows a combination of respectable  far-field flux, larger transmitting aperture, and less-stringent requirements in aiming - all of which, in practical terms, allow for excellent in-the-field performance!  Another important consideration is that in using Fresnel lenses, lenses large enough (>25-30cm equivalent diameter or larger for visible wavelengths over a 100km path) to minimize the effects of the "local coherence" and disruption due to the "cells" of air described above are quite affordable and practical in their use!

Methods of signaling:

Perhaps the simplest form of signaling is on/off keying of the light - but this scheme is not well-suited for electronic detection of weak signals.  Traditionally, Morse code has been imposed on a tone-modulated light source, being detected - by ear - at the far end.  This scheme has the advantage that it moves the detected signals into the realm of one's "Gray Matter DSP" (that is, the brain) and a skilled operator can easily copy signals that would appear to be below the noise.  On/off tone modulation is also fairly easy to accomplish:  Simply interrupt a tone-modulated light source to send the Morse characters.

Simple tone modulation may be done in a number of ways and one of the oldest is the "chopper modulator."  Used by Alexander Graham Bell in his early photophone experiments, this device simply interrupts the light source - usually with a spinning, slotted disk - to impose audio modulation onto the light source and this modulated light source is, itself, interrupted to "key" the tone.  This mechanical scheme has the advantage of being intuitively obvious and it may be used to modulate practically any light source.

Amplitude modulation of plain speech is highly attractive in that it does not require that the operators be skilled in Morse in order to communicate.  The use of speech complicates things as it is difficult to satisfactorily modulate it onto many light sources, such as a slow-to-respond tungsten filament.  It is possible to modulate other high-intensity light sources such as arc or gas-discharge lamps provided that one deal with the complexity of dealing with the awkward voltage and/or current requirements of such devices and accept that fact that the depth and frequency response may be limited by the nature of the device.  Speech modulation of gas lasers can be complicated as well, owing to the nature of many laser tubes to resist modulation to a significant depth.  Both gas and semiconductor lasers can be modulated with an optical cell (such as a Kerr cell) but these pose their own complications, such as the need for a high-voltage source and/or the use of dangerous substances.

Modern laser diodes may be directly current-modulated provided that one strictly observes the ratings pertaining to minimum and maximum current and temperature.  More commonly, laser diodes are simply on/off modulated - either with an audible tone to facilitate Morse communications, or with a much higher-frequency tone that is either frequency or duty-cycle modulated.  Duty-cycle modulation (or PWM - Pulse Width Modulation) is a relatively simple method where simple on/off modulation can be varied in a way that it will synthesize linear modulation, with the advantage of not having to worry much about the linearity of the device being modulated.

Another scheme that is often used is Frequency Modulation or FM.  Like PWM, the light source (e.g. a Laser Diode) is simply turned on and off, but the varying frequency is used to convey the modulation be it voice, video or even data - but more on this later.

Types of detectors:

For reasons of practicality, all of our detection schemes have been radiometric - that is, we are simply detecting energy from the transmitter in a manner that is intrinsically frequency-insensitive:  The more light we receive from the distant transmitter, the more signal we can recover from our detectors.

Perhaps the most inexpensive type of detector is the PIN photodiode.  These devices are particularly sensitive in the red and near-infrared spectrum - which just happens to be in the same range as the optimal wavelength for through-the-air communications.  The problem with photodiodes is that in order for them to be very sensitive, they need to be very lightly loaded owing to their lack of any intrinsic self-amplification - but if you do load them lightly enough to get good sensitivity, their intrinsic capacitance (10's to 100's of pF) can seriously limit high frequency response:  If one wishes to obtain the ultimate in sensitivity from a photodiode, best sensitivity is only possible up to a few kHz, with the optimum range being below a few hundred Hz.  These frequency limitations effectively rule out using any subcarrier scheme to convey voice or high-speed data if one wishes to have, simultaneously, the best possible sensitivity.  For these reasons, our experiments have tended to use simple, amplitude-modulated voice as well as extremely narrowband digital techniques such as WOLF, WSJT or QRSS (extremely slow Morse) - all in or below the 3kHz speech range.

An obvious alternative to using PIN photodiodes is to use Photomultiplier tubes (PMT's.)  Photomultiplier tubes have the obvious advantage in that they can be extremely sensitive owing to their self-amplification properties while maintaining excellent bandwidth - but they do have some disadvantages:  They are rather expensive - especially compared with a photodiode,  they require a high voltage (about 1000 volt) supply, many types have rather poor red-wavelength sensitivity - an important factor when you consider that the longer wavelengths are preferred for through-the-atmosphere communications, and in comparison to a photodiode, they are rather fragile both mechanically and electrically.  A solid-state alternative to the PMT is the Avalanche PhotoDiode (APD) as these devices can have excellent red sensitivity - greatly exceeding that of many surplus PMTs - but APDs, like PMTs, tend to be somewhat specialized and expensive and low-cost devices are not always readily available on the surplus market..

For more background on detectors, see the page "Optical Receivers for low-bandwidth through-the-air communications" and its related links.

Why not FM subcarriers?

Many previously-published articles have used FM in the form of subcarriers to convey voice information - and for some technically-sound reasons:
There is a problem that can arise when trying to demodulate signals that are near the noise threshold of the typical PIN-diode optical detector system:  When using photodiodes, the available sensitivity decreases with increasing frequency response.  What this means is that a receiver that will receive, say, a 40 kHz subcarrier will likely need to be at least 10-20dB less sensitive than a receiver optimized to receive only speech (up to 3 kHz) bandwidth.  Another factor has to do with the fact that a skilled listener can easily copy speech with only an 8-10dB signal-noise ratio - and this happens to be approximately the same amount of signal-noise ratio that is required for an FM demodulator to work.

Final comments:

It seems fairly clear that the majority of the research in through-the-air optical communications has been directed toward short range (under just a few kilometers) use - and for good reason:  The variability of the atmospheric conditions (not to mention daylight) simply prohibits the use of such techniques for use as a full-time, highly-reliable communications system.  As mentioned above, the goal is not to attempt to create an ultra-reliable, high-speed, optical through-the-air communications system, but rather see what we can do with fairly simple and inexpensive hardware.


Local links:

These are links to pages on this site that describe some of the equipment that I have been building and testing.  This list will grow as I have time to add the information.

Constructed equipment:
Experiments - in chronological order:
Sources of electronic and optical components:
Miscellaneous:


Other (possibly) relevant links:

Yet more interesting links, in no particular order:

Note that some of these may be academic, while others may be commercial in nature.


General "How-to" information about Optical through-the-air ("lightbeam") communications:
In addition to the information contained on this very web site (in the links above) there is other information that can be found elsewhere on the web.
Note that some of the technology described on the pages below may be somewhat dated, but the basic theory and many of the techniques are still applicable:
Comment:  You may note that there is not a "how-to" page on this web site... yet.  An article of this type is a work-in-progress and it is hoped that it will appear in some form or another in the fairly near future.

If you have questions or comments concerning the contents of this page, feel free to contact me using the information at this URL.

Go to the modulatedlight.org main page, or go to the ka7oei.com  page.

Keywords:  Lightbeam communications, light beam, lightbeam, laser beam, modulated light, optical communications, through-the-air optical communications, FSO communications, Free-Space Optical communications, LED communications, laser communications, LED, laser, light-emitting diode, lens, fresnel, fresnel lens, photodiode, photomultiplier, PMT, phototransistor, laser tube, laser diode, high power LED, luxeon, cree, phlatlight, lumileds, modulator, detector
This page and contents copyright 2007-2009 by Clint Turner, KA7OEI.  Last update:  20091221