| Optical (through-the-air) communications |
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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.
Why LEDs instead of lasers?
As it turns out, the coherent light emitted by lasers has several undesirable properties when propagated over long distances through the atmosphere, namely those related to scintillation and phase cancellation effects. It is also desirable that as large an aperture (e.g. lens) as possible be used to minimize these and other effects and the use of non-coherent light sources such as LEDs allows very large and inexpensive lenses (e.g. not diffraction limited, as would be required were lasers used) to be used instead. For a graphic example of the effects of scintillation, see Figure 1, to the right.
By using large lenses one can improve performance by virtue of the large aperture (e.g. reduction of "local coherence" and the application of "aperture averaging"). Additionally, a beam with a large cross section also implies a greatly reduced energy density, a factor that improves the safety of a system using high-power LEDs versus even medium-power lasers!
For a more detailed article about optical communications using noncoherent light, see the page:
"Optical Communications using Coherent and Non-Coherent light - an overview." - link
Don't forget to visit these other pages at this site:
- The
Modulated
Light DX pages - This page details the fine work
done in Australia over the past several decades.
- Operation Red Line - The historic 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.
- Operation Red Line photo gallery - Pictures and detailed descriptions of the events and equipment used during the May, 1963 experiment. This and the above page was produced with help of some of the participants of that historical event.
Below are links to pages on this site that describe some of the equipment that I have been building and testing as well as a few informational articles. This list will grow as I have time to add the information.
- Optical enclosure - first version - If you are going to do anything with optics, the various pieces need to be held in precise alignment and 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 - What good is building an optical transceiver if it's the only one? Using "foam core" posterboard, cheap "full-page" Fresnel magnifiers and picture frames, one can construct a reasonable facsimile of an enclosure inexpensively.
- Optical
enclosure
- foldable version - After testing with
the "cheap" optical enclosure, another high-performance
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.
- A Highly-sensitive optical receiver optimized for speech bandwidth - This page describes an implementation of the "Version 3" receiver described in the above link. It also includes a link to a circuit board layout of this receiver.
- 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-power 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.
- A "Simpler" Pulse-Width Modulator for LEDs, Lasers and whatnot and a simpler foam-core enclosure - This is a pulse-width modulator designed to be used with high-power LEDs, but it could also be used for laser pointers and "normal" LEDs as well. It is "simpler" than the one described above, but it still has features like an audio AGC and tone generator. This page also gives a few details a simple foam-core enclosure was made using inexpensive, rigid Fresnel lenses.
- NEW
- An updated (and slightly simpler) Pulse-Width
Modulator for LEDs and Lasers - This is an
update of the "Simpler" PIC-based Pulse Width Modulator
circuit, above, showing a slightly simplified circuit.
The newer firmware also allows for completely manual gain
control via a potentiometer and selectable tone (and audio
gain) settings via settings of logic levels on the PIC's
pins with an even simpler circuit!
- Using Laser pointers for free-space optical communications - If you really want to try your hand at using Laser pointers, this page tells you a bit about what is required. The techniques described on this page are based on several years of experience - including success and failures. Please be sure to note and heed the warnings about laser safety and responsible use!
- Fresnel Lens Comparison - On this page, a number of different Fresnel lenses are tested and the results compared. Based on these results it is possible to get an idea of the appropriate sizes of detectors or emitters that one might use in the design and construction of Fresnel-based optical receivers and transmitters. This page also compares the size of a len's "blur circle" to the predicted size of an airy disc for a hypothetical "perfect" lens of the same type.
- Two comb filter projects:
- A Comb filter to combat mains-induced hum from urban lighting, Version 1 - In optical communications we sometimes pick a path that is a bit too close to some of the city lights and have to deal with the resulting interference. If - even after you've exhausted other ways of reducing this hum - it still bothers your link, this small, low-power PIC-based DSP comb filter may help. This is the original version that filters at 100 or 120 Hz comb spacing.
- Updated version of the
Comb Filter (Version
2) - This is an updated version that uses a
newer processor, allowing a much higher sample rate (approx.
32 kHz) and also has the ability to filter at 50/60
Hz comb spacing in addition to the 100/120 Hz comb spacing
which means that it can also be used for removing power
line noise in VLF/ULF "Natural Radio" receivers. It
also has a few additional functions.
- Modulator for very high-power LEDs - This was designed to modulate very high-power LEDs, such as the Luminus "Phlatlight" (tm) devices, some of which can dissipate many 10's of watts. This modulator with its integrated high-efficiency switching voltage converter was specifically designed to drive CBT-54/PT-54 LEDs to a peak current of over 20 amps - but it could be easily adapted for lower or higher-power devices.
- APD-based
optical receiver for speech bandwidth communications
- This receiver uses an APD (Avalanche Photo Diodes) to gain
even more sensitivity from a weak-signal optical
communications circuit.
Experiments - in chronological order:
These are but a few of the experiments that have been carried out over the years dealing with through-the-air optical communications.
- Operation Red Line 50th Anniversary! - The historic 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. This page was produced with help of some of the participants of that historical event.
- Operation Red Line photo gallery - Photos and detailed explanations from this project.
- KA7OEI's blog entry commemorating the 50th anniversary of Project Red Line.
- From the "Hack-A-Day" web site, a
"Retrotechtacular" about Project Red Line.
- 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 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 above except that it has more in-depth comparisons and analyses of the results of comparisons between 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 during the 2007 ARRL 10
GHz and Up contest - and here are the results! This QSO was mentioned on page
80 of the March, 2008 issue of QST - Read
about the results of the 2007 ARRL 10 GHz and Up contest
online.
- 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 where 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! This is believed to be the current DX
record for "Above 300 GHz" amateur radio operation.
- 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, one of the members of the 1963 Operation Red Line team. While we were there we managed to get time to complete a 2-way optical QSO across Yucca valley using both LEDs and lasers.
- Microwave and Optical QSOs for the 2008 ARRL "10 GHz and up" contest - For this occasion, K7RJ, KA7OEI and others made various microwave and "Red-Band" QSOs. Also, see the Soapbox for the 2008 ARRL 10GHz and up contest where, amongst others, K7RJ and KA7OEI contributed, describing our efforts to make 10 GHz, 24 GHz, and "Red-Band" QSOs made on 16 August, 2008. Please note that you may go to the second page to find the soapbox entries for K7RJ and KA7OEI. This QSO was mentioned on Page 87 of the March, 2009 QST. Read about the results of the 2008 ARRL 10 GHz and Up contest online.
- "Mountain-Bounce" optical communications - Near the end of January, 2009, Ron, K7RJ and Clint, KA7OEI spared no effort to set up gear (in our back yards) to span a huge distance (tens of millions of millimeters) across the Salt Lake Valley. The object? To see if we could shine our red lights on a mountain and detect each others' signals! We did, and we did!
- Daylight optical
experimentation - Doing optical communications in the
middle of the day is a bit more challenging for a number of
reasons, namely the sun! Here are some results from an
experiment in which we spanned about 23km (13 miles) during
the middle of the day.
Sources of electronic and optical components:
Are you wondering where to get things like LEDs, lenses and other optical/electronic components? The page below is a place to start...
- 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 information:
A few other pages at this site that directly or indirectly relate to optical communications.
- "Optical Communications
using Coherent and Non-Coherent light - an overview."
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This page gives an overview of LED versus Laser (coherent)
optical communications as well as some of the difficulties
experienced with long-distance, through-the-air optical
communications.
- Photographs from atop Mount Ellen, Utah - Pictures from an expedition to Mount Ellen, Utah - the site of one end of the 1894 long-distance heliographic communications.
- Swasey Peak
Register - A few pictures and a documentation of
the contents of the hiking register found atop Swasey Peak
in the mailbox. This peak - in the House range of
mountains west of Delta, Utah, was near one end of the 173
mile optical communications done in 2007 - see "A 173 mile optical QSO"
above.
- 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 paper, in .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.
- Comments on optical
subcarrier communications - A few comments on
the reasons why and how subcarrier systems are used on
optical communications systems.
Archive of files from the "UK Nanowave" Yahoo Group:
As of October, 28, 2019 Yahoo disabled the uploading of files to their Yahoo Groups and as of December 14, 2019, this content is to be deleted. The page linked below contains the files from the Yahoo Group, captured after the October 28, 2019 "freeze" date.
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Changes to ARRL "Rule 1.12"
regarding contests above 50 MHz
On July 16, 2010 during ARRL Board of Directors meeting, changes were made to the General Rules for ARRL Contests Above 50 MHz. Previously, rule 1.12 read: "Above 300 GHz contacts are permitted for contest credit only between licensed amateurs using coherent radiation on transmission (for example, LASER) and employing at least one stage of electronic detection on receive."The rule now reads: "Above 300 GHz, contacts are permitted for contest credit only between licensed amateurs using monochromatic signal sources (for example, LASER and LED) and employing at least one stage of electronic detection on receive. LASER usage is restricted to ANSI Z136 Class I, II, IIa, and IIIa (i.e., output power is less than 5mW)."Note: The current FCC rules allow operation - with certain restrictions - an all frequencies above 275 GHz even though many references and contest rules refer to the older "...above 300 GHz" allocations. As noted, the changes allow sources of light other than lasers to be used - including LEDs. This wording could also be taken to also allow various types of gas-discharge lamps - perhaps even filtered multi-line emitters. It does seem to disallow direct use of broadband "thermal" sources such as tungsten lamps, however. Allowing the use of noncoherent light sources such as LEDs allows other types optics to be utilized (e.g. non diffraction-limited) which can be used to greatly increase the exit aperture of a transmitter, decreasing the power density and thus the potential hazards to the operators and others that might encounter the beam while simultaneously increasing the total amount of power one can radiate safely. Keep in mind that the above rules apply ONLY to ARRL contests and shouldn't limit one's imagination: If you wish to conduct experiments using techniques or gear that doesn't fall within the scope of the rule, feel free to do so safely! It is with such experimentation that new techniques are developed and perhaps, one day, may be included in the rules! Comment: As of the time of writing this the rules covering VUCC contacts still specify the use of coherent light. |
Other (possibly) relevant links:
The links below generally relate to optical communications by others throughout the world.
- REAST Optical
page (from archive.org)- An archived set of articles from the website
of the Radio and Electronics Association
of Southern Tasmania, optical
experimentation - including Non-Line-of-Sight (NLOS)
operations. Note: You may have
to carefully look at this page to find their most recent
work as the reports aren't necessarily listed in
chronological order.
- 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:
This page may not render properly on some Mozilla or
Netscape-type browsers.
- The F1AVY
Experiment Laser Corner - (This new web site
location is currently available in French only - Click
here for a Google translation of the main page
into English.) 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 - Rado, OM2ZZ, has been doing some optical experimentation (via Google Translate)
- RONJA (Reasonable Optical Near Joint Access) is a system developed, at least in part, by Twibright Labs in the Czech republic. This system uses high-brightness LEDs and reasonably-sized optics and is rated to provide reliable 10 megabit links at distances of up to 1.4 kilometers (almost a mile.) Because this is a purely binary system (on/off) it is immune to the effects of scintillation - provided that the minimum amplitude of the scintillatory troughs is above the receiver's threshold.
- G8AGN's Laser
and LED
pages - Barry, G8AGN and his friends have been doing
optical communications experiments for several years now - first,
with lasers, and more recently with
high-power LEDs. A video clip from
one end of the January, 2011 87km 2-way contact may
be seen here. On 6 April, 2011 Barry
was on one end of a 2-way optical communications of over
111km with good signals between the North York Moors and
Edgemont near Sheffield - a U.K. record, for a time...
Click
here for a YouTube video documenting this event.
- Laser ATV in Germany - Transmission of video via Laser in the Hannover area. This web page is in German: Click HERE for a Google translation to English.
- LA6NCA's
web site (via Google Translate) shows how he has refurbished and
operated some WW2-vintage German 'Lichtsprecher' sets that
operate using modulated light - read
about that work here and view
the YouTube video of its use. The
Lightsprecher set used a tungsten lamp for the light source,
but modulated the light in a rather interesting way - The
method is discussed in brief near the beginning of Chris
Long's article, Optical
Communications
for the Radio Amateur.
- N9JIM's Laser Communications page (from archive.org) - More experimentation.
- Max Carter's Laser Pointer Modulator/Demodulator - This page describes one of the better-designed FM subcarrier-based laser communications systems that can be found on the web. Be sure to scroll to the bottom of the page and take a look at some of his other laser/light beam related pages.
Email/Internet groups related to optical communications:
These are groups that have as their main theme something to do with optical communications. In all cases, membership in the group is required to be able to participate in the discussions and view online pictures and files, but most (if not all) allow non-members to read messages.
- The UK Nanowaves Yahoo group - This group, based in the UK, discusses "Nano-Wave" communications (generally visible light and near-Infrared) with a particular emphasis on gear intended to facilitate contacts in a manner similar to those done on the "traditional" microwave bands.
- The Optical DX Yahoo group - This is the Australian Optical DX group - although it is open to anyone with such interests.
- 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. This link points
to the mailing list archive from January, 2002 to the most
recent.
- Archive for the Laser list for March 1998 - December 2001
- Archive for the Laser list for March 1996 - October 1997 These archives are in plain text format with multiple messages and threads lumped together making reading slightly awkward. There appears to be a gap in the archive between Oct. '97 and Mar. '98.
- The Carolina Flashers Yahoo group - This U.S. based group also deals with optical communications.
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 (from archive.org) - 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.
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Misc. News
items/Chronology:
May 3, 1963: One-way voice communications using a 125 microwatt HeNe Laser tube over a distance of greater than 119 miles (191km) by W6POP, W6QYY and many others - See the web pages, Operation Red Line and the accompanying Photo Gallery. Also see the Blog entry that I wrote about this event for its 50th anniversary and the Hack-A-Day post that commemorated this anniversary as well. June 8, 1991: MCW over a distance of nearly 154 miles (248km) using a HeCd (Violet) laser by KY7B, WA7CJO and WA7LYI in Arizona as noted on this page. February 19, 2005: Two-way voice using high-power LEDs on the island of Tasmania over a distance of 104 miles (167km) by Mike, VK7MJ and Chris Long, now VK3AML - Read about those efforts here. April 4, 2007: One-way video was transmitted via laser by DL9OBD and DJ1WF near Hannover over a distance of about 52 miles (83.3km) - Read more here. August 18, 2007: Two way voice using high-power LEDs in Utah over a distance of over 107 miles (172km) using high-power LEDs by K7RJ and KA7OEI despite very poor seeing conditions due to smoke from wildfires. See: A 107+ Mile optical QSO. A few weeks later (September 3) a one-way voice contact was made using a laser pointer over the same path. October 3, 2007: Two way MCW and one-way voice over a path greater than 173 miles (278km) across the Utah desert under somewhat poor seeing conditions by K7RJ and KA7OEI. See: A 173 Mile optical QSO. August 20, 2008: A two-way optical QSO over the same 107 mile path as on August 18, 2007, but this time we also spanned the distance using cheap, red laser pointers! See: Microwave and Optical QSOs for the 2008 ARRL "10 GHz and up" contest. April 6, 2011: Barry, G8AGN and Gordon, G0EWN spanned a distance of about 69 miles (over 111km) setting a new UK record - click here for details. This record did not stand for long, however, as G8CYW and M0DTS pushed this out to about 73 miles (approx. 117.6) on April 12, 2011 over a path that was largely over the North Sea. And so it continues! The above list is, by
no means, complete. If there is something
that you feel that is incorrect or missing, feel
free to send an email with the details using the
link near the bottom of this page.
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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:
- Modulated
Light DX page - This is an excellent
resource based largely on the work and experiences of
Australian Optical DX enthusiasts. This page
hasn't been updated much since 2006, but work is in
progress to bring it up-to-date.
Pages specific to the "how to" aspects of lightbeam communications include:
- Photophones Revisited - by Dr. Mike Groth, VK7MJ, this 1987 article describes, in some technical detail, various aspects of the design of through-the-air optical ("lightbeam") communications techniques.
- Optical Communications for the Amateur - by Chris Long, (now VK3AML.) This 1979 article, annotated and updated in 2005, gives a historical background and practical tips for through-the-air ("lightbeam") communications techniques.
- Handbook of Optical Through the Air Communications - by David Johnson. This is a fairly comprehensive (but slightly dated) downloadable reference book covering many aspects of optical, through-the air ("lightbeam") communications.
- Amateur Lightwave Communication... Practical and Affordable (from the Internet Archive) - by Steve Noll, WA6EJO. This is an online version of an article that appeared in the 1994 Microwave Update Proceedings. Now somewhat dated, it describes the state-of-the-art at that time.
Historical references:
- Wireless Telephony, in Theory and Practice by Ernst W. Ruhmer (PDF, 14.7 Meg) - This version, a translation from the original 1908 German text, is a treatise on the state of the art concerning wireless communications of voice in a largely pre-vacuum tube (electronic amplification) era. Not only are details concerning the light-based "Photophone" described, along with details of experiments and then-recent advancements, but other electro-mechanical means of generating modulated RF as well including alternator and arc are covered.
- This book, from the Internet Archive (link), bears no copyright notice and is believed to be in the public domain.
- PLEASE NOTE: It has been
observed that some browser-based readers may indicate that
this file is corrupted/damaged. If this happens to
you, simply right-click on the book's link,
save it to your machine, and then read from the local
copy. It is unknown why some browser-based .PDF
viewers complain while others don't...
Other "How-to" information:
- Using Laser
pointers for free-space optical communications - If
you really want to try your hand at using
Laser pointers, this page tells you a bit about what is
required. The techniques described on this page are
based on several years of experience - including success and
failures. Please be sure to note and heed the
warnings about laser safety and responsible use!
Comments:
- You may note that there is not a "how-to" page on this web site about setting up contacts using gear based on LEDs... 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.
- It is your responsibility to determine the
safety and legality of any of the technologies and/or
methods described here and their uses. Before
engaging in any experiments, testing or activities that
involve electricity, light or other energy, you should
familiarize yourself with the hazards involved and take
precautions to minimize these hazards to yourself and
others! If you intend to use lasers in any of your
experiments, please read this Wikipedia
page on Laser Safety and the pages linked from
it. Although much of this information does not
directly apply to the use of high-power LEDs, it still
contains practical advice as to recommended precautions
that should be taken when working with any intense light
source.
Go to the modulatedlight.org main page, or go to the ka7oei.com page.
Keywords: Keywords: Lightbeam communications, light beam, lightbeam, laser beam, modulated light, optical communications, through-the-air optical communications, FSO communications, Free-Space Optical communications, lightbeam communicator, 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 2007-2019 by Clint Turner, KA7OEI. Last
update: 20190507