Forward:

Normally, one would do optical communications testing at night - and for some pretty obvious (and not-as-obvious) reasons:

Figure 1:
Top:  A zoomed-in view of the distant transmitter located at the QTH of N0KGM at a distance of about 21.3km (13.25 miles.)
Bottom:  A wide-angle view of the above picture.  Both images have been contrast-enhanced to overcome some of the effects of haze.
Click on either picture for a larger version.

• Competition from the sun.  Compared to anything man-made, the sun is really bright!  The light from the sun will "dilute" other light from such sources as an optical transmitter, be it a Laser or LED as it is reflected from the landscape surrounding the distant transmitting station - not to mention additional loss of contrast due to sunlit haze in the air.
  

• Risk of equipment damage.  Optical communications - for both receive and transmit - requires fairly large lenses to be used.  Extreme care must be taken to avoid accidental exposure to the sun or strong reflections of it.  If the transmitter or receiver is sun-directed for even an instant, the object at the focus of the lens - a detector or emitter device - may be instantly burned by the intense, focused sunlight.  Don't forget that even a Fresnel Lens the size of a "Letter" or A4 sheet of paper can vitrify sand - let alone incinerate a piece of silicon and epoxy!

• Complication in aiming.  This is a two-edged sword:  At night, aiming of optical gear is a bit complicated by the fact that familiar landmarks tend to disappear, causing disorientation, but this can be accommodated by a bit of planning beforehand but the obvious plus is that when put "on-point" the distant light source from the other end can be easily spotted once aiming is "pretty close."  In the daytime, however, aiming can be more difficult as the distant light source may not be readily discernible from the background.  If there is direct sun, however, this can be mitigated through the use of signal mirrors as an aid in alignment.
  

With all of these in mind, it is not surprising that daytime optical communications is a bit "trickier" than that done at night.  It is also to be expected that the ultimate range would be reduced compared to that obtainable at night.

A few techniques:

In addition to the techniques applied for "normal" nighttime optical communications, there are a few other things that can help when daylight optical communications are attempted:

• Maximize transmitted energy.  An obvious tactic would be to emit as much light as possible in the direction of the receiving station to maximize the ratio of the desired signal to the sunlit terrain.

• Minimize transmitted beamwidth.  Perhaps more important than the "brightness" of the emitter is the directivity of the transmit system as a whole.  Remember that halving the beamwidth of the transmitter would, other things being equal, cause a fourfold increase in the amount of light reaching the receiver.

• Detector selectivity.  Because the optical detectors are radiometric - that is, they respond just to the amount of light hitting them pretty much regardless of its wavelength - it would make sense to limit the spectrum hitting the detector to match, as much as possible, that of the emitter.  For this, colored gels and infrared-stopping filters (e.g. "hot mirrors"), various types of optical bandpass filters or some combination of the above may be used limit the amount of energy from wavelengths other than those of the emitter as much as possible to minimize "dilution" of the desired signal.
  

• Minimized detector beamwidth.  For a given optical system that is optimally-focused, the effective "beamwidth" is based on two main factors:  The size of the detector and the quality of the lens.  The larger the "active area" of the detector, the wider an area it will "see" which would not only include the transmitter itself, but the sunlit area surrounding it:  Clearly, a smaller detector (or a larger detector with a small hole to mask it) would reduce the effective beamwidth.  The quality of the optics used to focus onto the detector and how small of a spot can be focused will also play a part in determining the minimum size of the detector/mask.

• Aiming aids.  As noted above, having a good "feel" for the lay of the land and knowing where to look is extremely helpful.  In daylight, a useful aiming aid can be a signal mirror, the utility of which is obvious in helping the other end know where to look and aim!  Also helpful is the use of a modulated tone or other aspect of the signal that can indicate to the person aiming the receiver when the transmitted signal is being intercepted.

June 12, 2011 daytime optical experimentation:

It was during the "June VHF Contest" (50 MHz and "up") that we decided to try our hand at daytime optical communications.  At that time, the mountains near Salt Lake City were still full of snow and many of the higher-altitudes roads were still closed, so we settled for a fairly short "across the valley" shot of about 21.3km (13.25 miles) between the QTH of Robb, N0KGM and a location west of my house in West Jordan, Utah.  Because the geography is that of a "bowl" and since each of us were partway up our respective sides, we had a clear, line-of-sight shot, traversing across some heavily-populated portions of the Salt Lake valley.

For this contact, Ron went over to Robb's house with the optical gear and set it up on a west-facing deck and after a bit of fussing around trying to figure out where each other was located (I'd forgotten to bring a mirror!) we finally spotted each others' lights and completed the contact as can be heard in the audio clip below:

• Initial contact during the daytime, June 12, 2011:  (MP3 file, 0:27)  Initial exchange between Clint, KA7OEI and Robb, N0KGM over a daylight optical link over a distance of approximately 21.3km (13.25 miles.)  In the left channel can be heard the local, transmitted audio while the right channel contains audio from the distant end via the optical receiver.

Figure 2:
A brief YouTube video clip showing what the distant light looked like from about 21.3 km (13.25 miles) distant.  Sorry about the rather poor video quality!

As is typical across the Salt Lake Valley, there was a bit of a thermal layer which caused a bit of scintillation (flickering) of the distant light and this can be heard in the above audio clip as slight, rapid fading.  Perhaps most dominant in the audio clip is the "hiss" - which is what the sun sounds like at optical frequencies!  It is this "hiss", caused by the extraneous reflected sunlight falling on everything within the field-of-view of the receiver and from the haze in the air that is the limiting factor for daylight operation.

The receivers used at each end were unmodified "Version 3" optical detectors, in front of each was placed a piece of red gel of the sort used in theatrical lighting.  In other testing it was determined that the use of this red gel improved the signal-noise ratio by about 6dB but it is expected that significant benefit would be obtained from more-selective filtering!  Testing was also done with my APD-based optical receivers and despite a bit of distortion caused by the amount of light driving the receiver's electronics into a nonlinear range, good results were obtained.

How did it sound?

As can be heard from the clips, signals are reasonably good!  A "current extinction" test was also carried out in which random words were said as the LED current was gradually reduced and it was noted that audio was still fairly easily copyable even when the LED current was reduced to less than 10% of the original value - a reduction of 20dB - indicating that we could more than double our distance with the current configuration (e.g. no receiver improvements):  If Morse were used, "naked ear" copy down to just 2-3% of the original LED current would have been possible and a "sound card" mode such as QRSS or one of the modes in the WSJT suite would have permitted, perhaps, another 10-15dB in reduction.  Interestingly, it was also at around the 10% level that the distant light became difficult to spot with the naked eye.

Although it is difficult to tell from the pictures, the light from the distant end when operating a maximum current was very obvious to the casual observer and of a striking, brilliant red color reminiscent of a strong specular reflection from, say, a mirror - except, of course, that it was red!

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What's next?

Having gotten our feet wet with daytime optical communications, we went away having learned a number of things - namely those mentioned above - and we hope to try for even longer distances in the near future.

Return to the KA7OEI Optical communications Index page.

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