optical communications in the middle of the day...
Normally, one would do optical communications testing at night -
and for some pretty obvious (and not-as-obvious) reasons:
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:
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.
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.
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.
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
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:
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.
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
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!
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.
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free to contact me using the information at this URL. Keywords:
beam, modulated light, optical communications,
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daylight optical communications, FSO communications,
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