Optical Communications
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You may be asking yourself, "Self, what is 'Mountain-Bounce'?"
If you've read other pages at this web site, you already know that a group of us has managed to fascinate ourselves by launching red photons at each other, only to catch a very small number of them - calling the entire effort "Optical Communications."
Having had good success in past experiments with optical ("lightbeam") communications, managing to have traversed distances ranging up to 173 miles and using various types of light sources, we decided to try something else for the hell of it: Bouncing light off a mountain.
Choosing the mountain:
This part was easy: Ron, K7RJ, simply needed to step out onto his back patio and look up (see Figure 1) to see Grandeur Peak, a relatively prominent feature along the Wasatch Front. Being only about 2 miles from Ron's doorstep, he had a rather commanding view of this peak!
My house, being across the valley from Ron's, meant that my view of Grandeur peak had less "grandeur" about it, but it had the distinct advantage that I simply needed to step out onto my back deck and look toward the east-northeast to see it - about 12.4 miles in the distance.
The point of this was that we both had a common geographical feature at which we could blast our red photons and then gather up as many of them as possible.
Would this work? Based on our own experiences and of others having done "cloudbounce" experiments, we figured that we had a reasonably good shot of at least achieving a one-way (Clint transmitting to Ron) communications. Because my path crosses the majority of the Salt Lake Valley, however, I knew that optical signals reaching my receiver would be significantly "diluted" by the scattered, upward light from the city: Ron, being quite close to the mountain and at the edge of the valley had a far "less-polluted" shot!
First attempt - sometime in early-mid January:
It didn't work.
Actually, we weren't too surprised about this. We chose to try, on this occasion, at least in part because both of us had some time to make the attempt. Mother Nature was conspiring against us, however, as there was some fog/smog in the valley trapped by a temperature inversion. While Ron could see Grandeur Peak in the dark, I could not - but we persisted anyway to, if nothing else, become more familiar with the gear and computer programs for when we were able to re-try under better conditions.
For a time, it seemed as though Ron was able to see my signal on the computer despite the conditions, but just as we were being amazed by this it suddenly occurred to Ron that it he might be seeing a signal from the modulator - which had been disabled, but not turned off - that was making its way into the extremely sensitive receiver electronics. He decided to power-down the modulator completely: He did, and it was. Drat! Lesson learned.
Another time is the charm:
On the evening of January 17th, Ron and I happened to get together, along with his wife, Elaine (N7BDZ) and other friends, for dinner. On the way back home I noticed the crystal clarity of the air in the valley - a marked contrast to the past week or two of temperature-inversion smog that had been suffocating the area. Already being fairly late on a Saturday night there was nothing else on the schedule so we dragged the gear out of our respective houses and tried it again.
For this experiment, we used the same optical gear that we had used during our previous attempt - which was also the same gear that we'd used for the 2007 and 2008 ARRL "10 GHz and up" contests. This gear consists of a high-power LED transmitter and a sensitive optical receiver, both using large, plastic Fresnel lenses. This gear had already been proven to perform well, having spanned across 173 miles of Utah desert under less-than-ideal conditions.
Half-Duplex
Also, for this experiment, we used half-duplex mode - that is, we made no attempt to transmit and listen at the same time, even though the optical transceivers were perfectly capable of such. The reason for this is that we didn't wish to "de-sensitize" our receivers with light from our own transmitters. At both of our ends there were some local objects that were at least partially lit-up by or slightly blocked the beam: On Ron's end, he was shooting through some bare tree branches and overhead wires while my path skimmed along the roof of a neighbor's garage and furnace vent stack: The reflected light from these objects caused us to receive some of our own signal. Not only this, there was both Rayleigh and dust scattering to contend with and each of these effects could also "mask" a weak signal from the other end were we to attempt to transmit while trying to receive!
Generating and detecting signals
One of the features of the optical modulator is a built-in tone generator: Programmed into this generator are a number of precise, discrete tones - none of which bear any harmonic relationship to the 60 Hz AC mains frequency used here. For this test, we chose the "Middle-C" tone (about 262 Hz) - a frequency that was high enough to be heard, but low enough in frequency to be in the "more-sensitive" range of the optical receiver's frequency response.
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For detection, we used narrowband techniques utilizing the Spectran program. Feeding the audio output from the optical receivers into the sound card input of our laptop computers, we could use this configuration to detect extremely weak signals. For our tests, we chose an 8kHz sampling rate and a 0.031 Hz detection bandwidth.
The high-power LED transmitter is strong enough that when operated in the dark, one can see the beam emerging from the transmitter's lens via Rayleigh and dust scattering - a fact that aids in aiming the antenna as one simply points the red shaft of light at the object of interest - in this case, Grandeur Peak. Once aimed, the receiver - which is mechanically coupled to the transmitter - is also precisely aimed at the same object and with both of us having aimed at Grandeur Peak, Ron transmitted first while I "listened."
Using such a narrow detection bandwidth, it takes about a minute before any detected signal appears on the computer's "waterfall" display, and after several minutes of Ron's transmitting, I didn't see anything appear. At this point Ron re-aimed his transmitter and we tried again: To my surprised, I soon saw a weak - but distinct - signal appear on my waterfall display. To verify, he shut his transmitter off, and the signal subsequently disappeared see the top image of Figure 2.
Quickly, we turned the link around: After resolving a minor configuration problem on his end, Ron soon saw a very distinct signal appear on his computer's waterfall display (the bottom image of Figure 2) and it, too, appeared and disappeared as I turned my transmitter on and off. Note that due to the sampling rate on Ron's computer being slightly off, he received my 262 Hz signal at "249" Hz.
At this point, we decided to try to "peak" our respective signals by slightly re-aiming our gear. Even when we weren't changing the aiming of our gear, we were somewhat puzzled by the rapidly-varying signal levels which seemed to bounce from "as good as before" to "completely gone" when we noticed that weather was starting to move in and the mountain was being intermittently obscured by clouds. The final straw occurred when a light rain started to fall across the Salt Lake Valley, wiping out the optical path completely - so we gathered up our things and went back inside.
More experiments:
The evening of February 5, 2009 was breezy, but clear in the Salt Lake Valley and I could see Grandeur Peak in the moonlight.
Ideally, one would do "mountainbounce" tests sans moon, but we really didn't have any say in the matter that night - and we were wondering if the extra illumination was likely to cause a significant amount of "desense" - that is, additional thermal noise from the moonlight. Since a lunar eclipse hadn't been scheduled for that evening, we really couldn't make an "A/B" comparison, so we plowed ahead.
With our prior experience, we knew more-or-less where to aim our transceivers: At Grandeur Peak itself - even though recent warm-ish weather had stripped the mountain of some of its white snow coat. Dragging our gear outside again - Ron under his back patio and I onto my deck - we began to alternately squirt red photons at the mountains while the other "listened."
Again, we chose the 262-ish Hz "middle C" tone, with it being some distance away (frequency-wise) from 60/120 Hz powerline harmonics from the city's' lighting. Almost immediately, we detected each others' tones using Spectran and made minor adjustments to peak our signals: Since the optical transceivers have parallel transmit and receive "beams", peaking one naturally peaks the other, and the red shaft emerging from the transmitter simplified the aiming greatly! At my end, I wielded a night-vision scope that made it a bit easier to spot the red shaft of light in the clear air and where it seemed to be aimed.
On this occasion, I was surprised to be able to very easily detect Ron's signals. Switching from the narrow 0.031 Hz bandwidth to progressively wider ones, I could still discern his signal on the waterfall display even with a >1Hz detection bandwidth! Switching back to 0.47Hz bandwidth offered a good compromise between sensitivity and update speed: The narrower bandwidths offer better effective sensitivity by virtue of selectivity, but being narrower, any changes in signal strength are reported more-slowly and with a greater delay: At 0.47 Hz, I could make a change and see the results, having only to wait 3-4 seconds rather than 30 seconds or more!
Sending Callsigns:
With "good" signals visible at a 0.47 Hz bandwidth, Ron decided to send his callsign using "QRSS" - that is, very slow Morse in which a "dit" lasted 5 seconds (hence the designation "QRSS5) and a "dah" lasted 15 seconds.
As it turns out, even though Ron's callsign is shorter than mine, trying to maintain concentration in sending so slowly is quite a challenge. At higher speeds, one automatically forms the letters without thinking about how many dots or dashes there are, but at this speed, that's no longer the case! Not only is it necessary to count, timing the length of the dits and dahs in seconds, but one must remember where, in the sequence, one is, as it is very easy to get lost! Being with Ron on the telephone at this time, I decided to simply set the phone down and avoid any temptation to distract him!
About 4 minutes later, he finished and I had his complete callsign displayed on the screen, as can be seen in the top image of Figure 3. On this image, one can see, on the horizontal waterfall display on the lower half of the image, the dots and dashes spelling out "de K7RJ" ("de" meaning "from.") You can also see a pair of bright lines near the top and bottom of the waterfall display: Comparing the frequency scale on the right side of the waterfall with the "pips" on the frequency display on the top of the image, one notes that the frequencies of these lines are 240 Hz and 360 Hz - with an additional "pip" (not shown on the waterfall) at 480 Hz - all of these being harmonics of the powerline-related 120Hz frequency, with this energy coming from city lighting. The smaller pip, at 262 Hz, is Ron's signal. During this test, I used the built-in audio filtering of Spectran to see if I could hear his signal via ear: While I might have been able to discern his "key-down" periods, the results were inconclusive and I doubt that I could have accurately detected them.
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After successfully completing this transmission, we decided to double-check the peaking of each others' systems to assure the best-possible alignment. While Ron was doing this, he suddenly lost my signal completely and, conversely, I could no longer see his. After several minutes of thrashing about, trying narrower bandwidths and re-checking the clarity of the air across the valley to the peak, I finally noticed that my transceiver was off-point: This probably happened either from my bumping it as I went in and out through the door between my kitchen and deck, or from a gust of wind that nudged it very slightly: It had only been off-pointed by a degree or two, but that was enough! After re-aligning our ends again, we both acquired each other's signals - albeit seemingly weaker than before - and I decided to send my callsign via QRSS.
Rather than trying to maintain the concentration required to send my callsign at a QRSS5 rate - something that would have taken about 6 minutes - I quickly prepared, using a sound editing program, a .WAV file with the dits and dahs generated via computer, sending "de KA7OEI". The tones from the playback of this file would be inputted to the optical transmitter, while I could stand back in the warmth of my house - and while Ron continued to stay outside, in the cold... With Ron's setting his bandwidth to 0.12 Hz, I let the file play while Ron saw my callsign slowly worm its way across his waterfall display, as seen in the bottom portion of Figure 3. For whatever reason, his signals from me weren't as good as they had been earlier in the evening, but as can be seen from the picture, the callsign is readily discernible. Again, note that the sound card in Ron's computer was running about 6% high, causing his reported frequency to be low by the same amount.
It was noticed that when sending via the computer, my optical transceiver was more fully-modulated than it was with its built-in tone generator. The reason for this was that I was running my audio too "hot" into the modulator's input, causing it to distort to a clipped square-wave. As it turns out, this increased the amount of spectral energy at the modulated frequency somewhat, improving Ron's "copy" of me slightly! If you look carefully at the bottom picture of Figure 3, at the far-right edge, you'll notice a slight downward frequency shift indicated by the horizontal line moving down as well, just after the completion of the callsign. This was due to the QRSS keying in my audio file being generated at exactly 262.0 Hz while the modulator's internal tone generator was set to the more musically-correct 261.6 Hz. It also points out, by virtue of the "brightness" of the lower-frequency trace, that this signal was, in fact, slightly weaker!
A few other experiments:
Having completed our callsign exchange, we were curious about how different pitches of tones might be affected by the amount of thermal noise that we were both experiencing from urban lighting. We had originally chosen 262 Hz (middle-C) because it was a reasonable compromise: It was high enough to be audible, but not so high that it might drop into the higher-frequency roll-off frequency of the receiver. Because our noise floor was limited by the ambient light - from both the city and the moon - absolute receiver sensitivity wasn't likely to be a problem!
In addition to the 262 Hz, we also tried 31 (a B), 41.1 (an E), and 1318.5 (also an E) to see if there was an appreciable difference in the received signal-noise ratio. The results were inconclusive - that is to say, one frequency range didn't seem to have any obvious advantage of any other, although there seemed to be a perception that the low frequencies (31 and 41 Hz) fared slightly better than the higher (262 and 1318 Hz) tones.
A few comments about these experiments:
"Optical Noise Floor" degradation:
At this point there are a few things that can be said about these experiment and their results.
As can be seen from Figure 2 the signals were fairly weak. Also, as expected, the signal-to-noise ratio that Ron received from my signal was significantly better than that which I received from his signal. While some of this (2dB or so) is due to my optical transmitter's having better optical flux, most of it has to do with the fact that I was receiving Ron's signal across a population center that was radiating a significant "glow" of light! When setting up the receiver, I observed that my the noise floor of my optical receiver was about 10dB higher when pointed across the valley than it was when I pointed it down to the ground - this being due to the scattered light in the atmosphere.
This added noise was mostly in the form of a white-noise "hiss" - although there was a strong component of 120 Hz energy and its components, with one of the strongest peaks being at 360 Hz. While our "Middle-C" tone frequency was sufficiently far away from any of the 120 Hz harmonics, there was absolutely no escape of the pervading "hiss!"
As mentioned before, Ron lives fairly close to the mountain, near the edge of town and had relatively little inhabited area between him and the peak. Nevertheless, he also was detecting a significant amount of 120Hz energy and its harmonics - but this was mostly from the fact that the snow-covered mountain was being bathed in city lights!
It should be noted that the transmitters did not have anywhere near enough power to impart even even a hint of a red cast to the mountain, but it could accurately be said that anyone who was on the mountain would, without being prompted, immediately notice either red light source! Past experience has shown that even amongst a sea of city lights, these LED transmitters are conspicuous not only because they are red, but also because they are bright, as compared with other urban light sources.
Mismatch of "spot size":
Another factor that caused a bit of degradation of the signals was likely due to the difference in the "spot size" on the mountain. Consider that our transmitters and receivers have about the same beamwidth - about 1/4 of a degree. Clearly, with Ron being at about 1/6th the distance, the "spot" that he was projecting onto the mountain was much smaller than the spot that I would have been. The converse would also be true: My transmitter would have made a much larger red spot on the mountain than his!
This is also true of the receiver: Much of the "large" spot produced by my transmitter was being completely missed by the relatively "small" spot of his receiver - and the converse would be true of my receiver and his transmitter! What this means is that he was missing most of the light that I was shining on the mountain, while my receiver was being "diluted" by being able to see that "extra" part of the mountain that his transmitter didn't illuminate! (Note: An advantage of a small spot size is that less stray light from the city and moon would be intercepted - but this has the disadvantage of being more difficult to aim!)
One way to "solve" this problem would have been to pick a different mountain - one that was equidistant to us both: Our respective "spot" sizes would have been similar in that case! The reason why we did not do this was mentioned before: We simply chose the mountain that both of us could conveniently see from our back yards! Another possibility would be to modify the receivers and transmitters to modify their respective beamwidths to match the specific situation - but that requires a bit more planning - not to mention flexibility of the optical gear - and our gear wasn't designed for that.
The "Aspect" of the mountain:
As can be seen from the top picture in Figure 1, Ron could see only the top portion of the Grandeur Peak mass from his location while I could see pretty much all of it. It is possible that if he were to choose a different location where he could see more of the upper portion, we might have experienced better signals. Also, it might have been possible that if he'd aimed at the lower portion that both of us could see, that, too, might have also yielded better results! At some point, we hope to try this again and find out!
Using Spectran:
As has been mentioned, we used the Spectran program, useful utility written, in part, by Alberto, I2PHD. This program includes not only the spectrum and waterfall displays, but other features such as audio bandpass and notch filters, noise reduction, and the capability of manually and automatically capturing screen displays.
One thing that Ron noted immediately was that the 120 Hz harmonics of the received light were "off" frequency, according to the display. This was a result of either inaccuracies of the sampling rate of the sound card in the computer that he was using, or some operating-system induced offsets caused by internal sample-rate conversion. Using the measured-versus-actual frequencies of the 120 Hz harmonics, he was quickly able to determine a "correction factor" to determine at what frequency our 262-ish Hz tone would appear - and this is why his tone frequency is "off" in Figures 2 and 3.
Remember: If you plan to use Spectran - or any other program - for precise frequency measurement, make sure that you measure a known-accurate frequency so that you can discover any offsets and correct for them!
Final comments:
This was, by no means, spectacular DX, nor did we really tried to optimize our results. It was, simply, a fun experiment to do and it required almost no effort on our part to do this, aside from lugging gear just outside our houses!
I must confess, however, that Ron's hardship was the greater one: While I set up my optical gear on my back deck, my audio cables easily reached into my housewhere I operated the computer from the inside - only needing to step outside briefly to make adjustments! Ron, on the other hand, had to stay outside in the cold!
I'd like to thank those that helped, including:
- Ron, K7RJ, at the other end.
- Elaine, N7BDZ, Ron's better half, who took the photos at Ron's end.
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Path statistics:
- Distance between Ron's house and Grandeur Peak: About 2.11 miles (3.40 km)
- Distance between Clint's house and Grandeur Peak: About
12.38 miles (19.93km)
- Total path distance: About 14.49 miles (23.33km)
- Actual distance between our two houses: 10.54 miles (16.96 km)
- Since we weren't actually bouncing light off the peak - but
rather the sides of the mountain below the peak, the actual
distances were probably slightly different.
Equipment common to both sides of the QSO:
- The LED was amplitude modulated with a current-linear modulator
with a resting current of 1.1 amps. Details of the
modulator are here: LED_linear_modulator.html
- The transmit LED in both cases was a Red Luxeon III emitter module (Lumileds M/N: LXHL-PD09) epoxied to a heat sink.
- The optical receivers were my "version 3" design, described here: optical_rx1.html#ka7oei_rx_ver3 with both receivers using BPW34 photodiodes.
- Audio interface units, incorporating audio amplifiers, audio
recorder interface, audible S-meter, and a few other features were used
- details are here: optical_comm_audio_interface_device.html
- Both transceivers have separate and identical TX and RX lenses mounted side-by-side.
- This enclosure is described in detail here: Optical_enclosure_first_version.html
- Lens size: Unmounted, the Fresnel Lenses are 250mm x 318mm and have a focal length of 318mm. The mounting frames vignette the lenses by about 10mm in each dimension, so the available lens area is about 240mm x 308mm. Each lens is protected by a sheet of Plexiglas and the front surface has been coated with a protective polymer to prevent scratching and moisture accumulation.
- For optimal far-field optical flux density, a glass PCX (Plano-ConveX) lens is used in front of the LED to appropriately illuminate the Fresnel, the LED-Lens distance being set empirically for best output.
LED-based Optical transceiver used at Clint's end:
- The enclosure is described
here: Optical_enclosure_foldable_version.html
- Lens size: Unmounted, the Fresnel lenses are 404mm x 430mm and have a focal length of 229 mm. The mounting frames vignette the lens by about 10mm in each dimension, so the available lens area is about 394mm x 420mm. Each lens is protected by a sheet of Plexiglas and the front surface has been coated with a protective polymer to prevent scratching and moisture accumulation.
- For optimal far-field optical flux density, an optical acrylic DCX (Double-ConveX) lens was reground to an aspherical shape to provide optimal illumination of the Fresnel. This turned out to be necessary owing to the very short focal length of the lens that made it difficult to efficiently illuminate the lens. After adjustment, this LED/Lens combination produces about 25% higher far-field flux than the other assembly, with an almost identical half-power beamwidth.
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Keywords: Lightbeam communications, light beam, lightbeam, laser beam, modulated light, optical communications, through-the-air optical communications, FSO communications, Free-Space Optical communications, 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
2009. Last update: 20091222