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.

What this page is about:

This page deals with ground-based optical, through-the-air communications using various light sources - primarily high-power LEDs, but the use of Lasers is touched upon as well.

Unlike more familiar fiber-based schemes, communications through the atmosphere has some unique challenges.

• The entire path is in the atmosphere,.  Compare that with a hypothetical Earth-to-Space optical communications system where there is, at most, only a few kilometers of atmosphere to traverse - and the atmosphere becomes increasingly rarefied with distance and altitude.
  
• With the entire path in the atmosphere, pollution and particulates more consistently attenuate the signal along the entire path.
• An all-atmosphere path is also far more subject to atmospheric gradients:  Differing air densities and moving air masses will affect the light's transmission, causing increased scintillation.
  
• "Light pollution" is likely to be more of a problem with an entirely ground-based optical path.
• Weather is a factor anywhere along an entirely ground-based optical patch.  For a satellite-to-Earth path, the only weather that usually has an effect is that that is fairly close to the Earth station's site.

(It need not be said that most of these simply don't apply with a "glass" path - that is, a fiber optic link.)

Being that the emphasis here is on experimental schemes that are not intended to be either highly reliable or high-speed, the commercial viability of some of the techniques described here is irrelevant:  Much of 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:

• Lasers are cool!
• Lasers are typically packaged such they produce beams that are inherently collimated and are able to maintain a good power density over long distances.
• Lasers are cool!
• Unlike most conventional light sources, Lasers can be modulated at fairly high frequencies - either directly, or with optical cells.
  
• Lasers are cool!
• Lasers, operating at stable wavelengths, lend themselves to detection via narrowband optical filters to remove most of the ambient light, improving performance in the presence of light pollution and even daylight.
• Lasers are cool!

Historically speaking, however, Lasers are relative newcomers in the optical communications field:  For many centuries, reflected sunlight or flames have been used.  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 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 any kind of laser electronically to much depth, so it is more common to use a scheme such as an FM subcarrier a digital modulation that can be done simply by turning the light on and off or effecting a step change in the brightness.  (Note:  It is a bit more awkward to electrically modulate gas lasers, but this can be done by modulating the tube current, or by modulating a magnetic field surrounding the laser tube.)

Recently, some 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 nearby.

Like traditional LEDs, high-power LEDs 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 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 lenses 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 good, historical overview of optical communications see:

•  BIBLIOGRAPHY OF OPTICAL (AUDIO) COMMUNICATIONS, 1878 - 1940 and
  
• Optical comms. before electronics: 'THE HELIOGRAPH' (written c1899) - both of which are among others on the Modulated Light DX page.

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 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, 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 likely to be able to "straddle" multiple cells of air:  While portions of this beam may be scintillated 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.

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.

It turns out that a phenomenon often referred to as "local coherence" is at play when the angular source size is very small.  This effect, noted by A.A. Michelson, is one of the causes of scintillation.  Because of the extremely narrow angle, 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.  For this reason, amongst several others, a large aperture will contribute toward the reduction of the phenomenon of local coherence.

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.

It should also be remembered that the amount of scintillation also depends on the device 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.

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 coherent 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 very much worse than a non coherent beam due to diffraction, owing to the fact that  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) as well as the same 50.27 square inch (289 sq. cm) optics (a reflector telescope) being used for transmitting in each  case.  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, 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 while noncoherent 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.

Comment about lenses, 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:

• Cost.  The price of lenses (or reflectors) seems to go up exponentially with size.
• Weight.  The weight of a solid lens seems to be roughly proportional to cost - that is, it goes up exponentially with size.
• Fragility.  Large lenses, being bulky and more precise are seemingly easier to damage than their smaller counterparts.  It should go without saying that a large, heavy glass lens assembly is seemingly more prone to damage due to handling or accident than a lighter optical assembly.
  
• Practicality.  The bigger and heavier it is, the harder it is to use a larger lens.  It should also go without saying that aiming a very precise beam is much more difficult than aiming a less-precise (more divergent) beam.
  

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.  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 out 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 - but they can easily provide acceptable results if not using coherent light sources.  When using Fresnel lenses, the quality of the collimation is ultimately limited by the quality of the lens itself, if not by the size of the light source and its effect on divergence.  Using Fresnel lenses, they can easily be made large enough (>25-30cm for visible wavelengths over a 100km path) to minimize the effects of local coherence described above.

There are disadvantages of Fresnel lenses, however:  With the increased beam dispersal, more light is lost in transmission to the far end, requiring more luminous intensity of the emitting light source, more optical gain on the receive side, or a combination of both.  For this reason, modern, high-powered LEDs become more attractive:  They can emit hundreds of milliwatts of optical power fairly efficiently and inexpensively and at least partially mitigate the disadvantages, making up for lost light output.  One saving grace of "inaccurate" optics, of course, is that with the more-dispersed beam, aiming is far less critical and atmospheric "bending" of the light (due to refraction) has much less of an influence on a more widely-dispersed beam than a very tightly-collimated beam, not to mention the fact that aiming is simplified when such optics are used.  It should be noted that due to diffraction effects, plastic Fresnel lenses are probably not good candidates for generating a collimated beam using coherent (laser) light sources - both due to the relative inaccuracy of the Fresnel lens itself (e.g. diffraction problems) and due to practical difficulties in properly illuminating many short focal-length Fresnel lenses.

Methods of signaling:

Perhaps the simplest form of signaling is on/off keying of the light - but 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 it into the realm of being processed by one's "Gray Matter DSP" (that is, the brain) which, for 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:  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 - including tungsten lamps which, by their nature, tend to resist modulation of the filament owing to "thermal inertia."

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.  Modulation of speech does, however, complicate things as it is difficult to satisfactorily modulate speech onto 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 the awkward voltage and/or current requirements of such devices.  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 - not to mention the fact that they, too, can be difficult to modulate.

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, however, 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 to 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 by voice, video or even data.

Types of 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 - but if you do that, 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, recent experiments have tended to use simple amplitude-modulated voice as well as extremely narrowband digital techniques such as WOLF or WSJT - 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 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 tend to be somewhat specialized and expensive and low-cost devices are not as easily found on the surplus market as PMTs.

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 subcarriers to convey voice information - and for some technically-sound reasons:

• The amplitude of the recovered audio is independent of the amplitude of the received signal.  It does not matter how weak or strong the signal may be, as long as it is above the minimum threshold for demodulation, the amplitude of the recovered signal will be the same.
• Intrinsic rejection of noise sources - namely light pollution.  FM, by its nature rejects many noise sources, so this property - plus the fact that the subcarrier will typically be at several 10's of kHz, meaning that the 120 Hz (or 100 Hz in those areas with a 50 Hz mains frequency) energy - plus the immediate harmonics - will be largely rejected.

There is a problem that can arise when trying to demodulate signal that are near the threshold of the typical PIN-diode optical detector:  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 be 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.  When all is said and done, the hypothetical FM-based system will likely be at least 10 dB less sensitive than one using just amplitude modulation owing to the combination of both receiver sensitivity and the required threshold for reasonable FM demodulation.


Final comments:

It seems fairly clear that the majority of the research in through-the-air optical communications has been directed to short range (under just a few kilometers) use - and for good reason:  The variability of the atmospheric conditions (not to mention daylight) simply prohibit 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.

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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:

• Optical enclosure - first version -  This page describes some of the details of the enclosure used to hold a pair of Fresnel lenses.  This assembly may be used to provide either full-duplex operation (e.g. one side being used for transmit, the other side for receive) or two transmit or receive lenses in parallel.
• Optical enclosure - cheap version -  Using "foam core" posterboard, full-sheet Fresnel magnifiers and picture frames, one can construct a reasonable facsimile of an enclosure for fairly cheaply.
• Optical enclosure - foldable version -  Another optical transceiver was constructed using fairly large Fresnel lenses.  In order to make it more convenient to transport, it folds together!  This unit also uses short (<0.6) focal length Fresnel lenses - something that caused a bit of extra complication.
  
• Optical Receivers for low-bandwidth through-the-air communications - This page describes a number of optical receivers (detectors) intended for low-bandwidth (speech frequencies or lower) operation.
• Pulse Width Modulator for high-power LEDs - A PIC-based Pulse Width Modulator (PWM) that includes audio compressor and tone generation.  This modulator also allows the continuous variation of LED current while maintaining 100% modulation any current setting.
  
• Linear Modulator for high-power LEDs - This linear modulator also provides audio compression and generation of test tones in addition to the ability to continuously vary the LED current while maintaining 100% modulation.
• Current limiting protection for high-power LEDs - This page describes a simple circuit to protect power LEDs from excessive current.
• LED AM Video link - It is possible to transmit video using a high-powered LED, but would you want to do it this way?
• Audio interface unit for optical communications - This device combines several useful features into one compact unit:  Audio amplifier, Audio recorder interface, Audible S-meter, and a Scintillation compensator.
  

Experiments:

• Operation Red Line - The May, 1963 118+ mile optical transmission by the EOS Amateur Radio club.  The remarkable feat was accomplished just months after the invention of the visible-light HeNe laser!  This page includes many pictures taken at the time of the event plus some audio clips.
  
• Our first optical QSO - On March 31, 2007, we finally dragged some gear to opposite sides of the valley and did some experimentation.
• More optical testing - After our first optical QSO (on April 25, 2007) we decided to go back into the field and do more testing - namely "Coherent versus Noncoherent light" - plus a bit of screaming at our gear...
• Comparison of coherent versus nonconherent light for transmission of audio on an atmospheric path - This page is mostly the same as the "More Optical testing" page except that it has more-detailed comparisons of the coherent/noncoherent and collimated/uncollimated light sources.
• A 107+ mile optical QSO - Even though the weather hadn't been very good earlier in the day and the air was hazy, we decided to try to make a 107+ mile optical QSO - and here are the results!  This QSO was mentioned on page 80 of the March, 2008 issue of QST.
  
• Revisiting the 107 mile optical path - Because the conditions were terrible the first time, we decided to go back and re-do this path on a day during which we had good weather and could run more tests.
• A 173 mile optical QSO - We decided to push the limits (and our luck) even further - despite the lack of cooperation from mother nature...
• A VK3/W6 optical QSO - In February of 2008, Chris (VK3AML) was visiting the USA and he and Clint, KA7OEI drove to California and visited Bob, W6QYY.  While we were there, we managed to get time to complete a 2-way optical QSO across Yucca valley with both LEDs and lasers.
  

Sources of electronic and optical components:

• Misc. Source of optical-related components - This is an (incomplete!) list of some places were you can get things like photodiodes, high-power LEDs, photomultiplier tubes, Fresnel and standard lenses, etc.

Miscellaneous:

• Photographs from atop Mount Ellen, Utah - Pictures from an expedition to Mount Ellen, Utah - site of one end of the 1894 long-distance heliographic communications.
• Dollars versus Decibels:  Long-Range atmospheric optical communications on a tight budget - In January, 2008, Chris Long, VK3AML, traveled from Melbourne, Australia to present this paper at the 2008 Photonics West conference in California.  This .PDF describes some of the techniques used to achieve long-distance communications using inexpensive optics and high-power LEDs.  This paper is made available here, by the authors, according to the terms of the copyright agreement.
  

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Other (possibly) relevant links:

• Modulated Light DX page -  This is an excellent resource based largely on the work and experiences of Australian Optical DX enthusiasts.
• K3PGP Experimenter's Corner -  This page contains much information about optical communications:  Look under the Construction, Laser, and Astronomy headings for information on optical transmission and reception.  Note that this page may not render properly on some older Mozilla or Netscape-type browsers.
  
• The F1AVY Experiment Laser Corner - Yves, F1AVY, has done quite a bit of experimentation on long-distance optical communications - some of it using rather esoteric equipment.
• OM2ZZ's Optical Experimentation page - Rado, OM2ZZ, has been doing some optical experimentation.
• Laser ATV in Germany - Transmission of video via Laser in the Hannover area.
• N9JIM's Laser Communications page - More experimentation.
  
• The Optical DX Yahoo group - This is the Australian Optical DX group - although it is open to anyone with such interests.  Membership in this Yahoo group is required for full access to pictures and files.
• Laser mailing list at qth.net - This is a mailing list that, while mostly geared toward Laser-based communications, also covers other non-Laser aspects of optical communications as well.  The link given points to the mailing list archive:  You may subscribe to the list and receive individual emails or daily digests.  Subscribing is required if you wish to participate.
  

Yet more interesting links, in no particular order:

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

• The Sizes of Stars - This paper, by J.B Calvert discusses, among other things, the effects of coherence with respect to objects of small angular diameter.
  
• Free-Space Laser Propagation:  Atmospheric effects - This paper describes, in brief, some of the practical difficulties experienced with free-space communications schemes.
• Making Free-Space Optics work - This page contains formulas and graphs showing different aspects of absorption and scintillation.
• Scintillation notes for P3k - This paper shows effects of scintillation versus wavelength and aperture for the Palomar Palm3000 telescope.
• Understanding the performance of free-space optics - This is from a presentation about the viability and performance of various high-speed optical paths under real-world conditions.
• Free-Space Optics:  LEDs versus Lasers - A brief note describing the differences between laser-based and LED-based short-range optical communications system.
  

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.