Optical
(through-the-air)
communications
 
Left:  A 3-watt red Luxeon LED at a distance of 14.91 miles (23.85 km) with downtown Salt Lake City in the foreground.
Right:  Transmitting with a 50+ watt LED with the light from the 95 mile (152km) distant end being visible at the terminus of the red beam.
Click on an image for a larger version.

Luxeon 3 watt LED at a distance of 15
                            miles; Downtown Salt Lake City is in the
                            foreground The south end of a 95 mile (152km)
                            optical path near Salt Lake City

Figure 1:
A video showing the effects of scintillation on small-diameter beam from a laser pointer and that from a collimated LED at a distance of approximately 24km.

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.


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:
Constructed equipment:
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.

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.

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...

Miscellaneous information:

A few other pages at this site that directly or indirectly relate to optical communications.



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.

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.

Yet more interesting links, in no particular order:
Note that some of these may be academic, while others may be commercial in nature.



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 (117.6km) 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.

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:
Pages specific to the "how to" aspects of lightbeam communications include:


Historical references:


Other "How-to" information:


Comments:

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: 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 2007-2014 by Clint Turner, KA7OEI.  Last update:  20140204
Count since January, 2010: