Tuesday, September 18, 2012

Throwing one's voice 95 miles on a lightbeam

For more information about long-distance optical communications, go to the modulatedlight.org web page  (link)


This past weekend (September 15-16, 2012) was the 2nd part of the annual ARRL 10 GHz and up contest and we decided to use one of the highest-available amateur bands - the one known in the FCC rules as "275 GHz and up."  Actually, this covers a lot of territory including submillimeter radio frequencies and far infrared wavelengths, but the part that we are more interested in is that for which most of us are equipped to detect directly - light.

We've done this before, managing to have spanned 107 miles (173 km) on several occasions and even 173 miles (278 km) (read about those efforts here - link) so we weren't going to break any of our own DX (distance) records, but it's fun to do this, anyway - and it gave us an excuse (as if we really needed one...) to go out and test some new gear that had not yet been tested over anything but relatively short (20 km or so) paths.

The two locations for the stations were about 96 miles (154 km) apart with Ron and Elaine Jones (K7RJ and N7BDZ) being at the far end at an elevation of about 5600 ft (1700 m) ASL near Park Valley, Utah in the extreme northwestern corner of Utah, a few miles from where the U.S. Transcontinental Railroad was joined for the first time in 1869 and only a few hundred meters away from the historic stagecoach route that paralleled part of that later railroad.  Along with friends Gordon (K7HFV) and Gary (AB1IP), I  was closer to home at about 9300 feet (2830 m) near a minor protuberance known as "Bountiful Peak" about 10 miles north of Salt Lake City.  As it turns out, the path is a grazing one and were it not for the slight refraction of the Earth's atmosphere, it may not even quite be "line of sight."

We'd tried this same path during the first weekend of the 10 GHz and up contest but the thick veil of smoke from wild fires elsewhere in the western U.S. prevented a successful contact - although our light beam was occasionally just visible to the binocular-aided eye in Park Valley.  This time, however, the air was reasonably clear, only somewhat hazy from the still-burning fires:  Since we "almost" made contact a month ago we were confident that this time we would have no problems.
Figure 1:
The high-power red LED shining to the north-northwest.
The lights of Layton, Utah and surrounding communities
may be seen way below, in the background! The dot at the terminus
of the red shaft of light is the light from Ron's end of the path.
Click on the image for a larger version.


Soon after we arrived on site Ron shone a 500,000 candlepower halogen spotlight in our direction and immediately we noted a lone, flickering, yellow-red dot in the blackness "above" the last ribbon of visible lights from the populated areas of Layton and Ogden about a mile below us.  Using this as a visual reference I swung my high-power LED in his direction, using the Rayleigh-scattered shaft of red light as a guide, and immediately Ron reported that it was easily the brightest light visible:  Considering that there were only a small handful of lights visible from his dark, rural location, anyway, this wasn't saying much, but if anyone where to have dropped by and looked in that direction they would have seen the bright, red light and asked, unprompted, "What's that?!?"

Using our light as a guide Ron immediately fine-tuned his pointing and soon, a very obvious red light appeared in the darkness.  Initially starting out with the lower power 3-watt LED he soon switched to the much higher-powered 20-30-ish watt LED and the red dot in the distance was even more striking than before.  The dot at the end of the red shaft of light in the above picture was from Ron's LED.

Soon after we brought our transmitters up to full power we reduced them again to 1/4-1/15th as each other's signals were strong enough that there was noticeable distortion in the received audio - and it also allowed us to run full-duplex (e.g. both sides being able to send and receive simultaneously) without intercepting as much of our own, scattered transmit light and causing acoustic feedback between our speaker and microphone.

This was the first actual "long distance" test of the Phlatlight-based optical transmitter - these using CBT-54 LEDs and permitting a 20dB improvement on the audio received at the far end.  This also was the first test of some APD (Avalanche PhotoDiode) based optical receivers that I'd built some time ago (see the link at the bottom of the page) so we set about reduce each other's LED currents to do a sort of "limbo" dance - that's to say we wanted to answer the question "How low can we go?"

It immediately became apparent that even though we could read the Phlatlight modulators' current with a resolution of 0.1 amp, this was still too coarse when we got down to the lowest readable current and were still able to hear each other, so Ron switched to the older 3 watt Luxeon on which the LED current could be measured and adjusted down to the single digits of milliamps.  As it turned out, speech was copyable - with some difficulty - down to the 40-50 milliamp range with the old receivers but the APD receivers extended this down to around 20 milliamps - an approximately 6-10dB improvement, a number that agreed reasonably well with what had been calculated using similar measurements done at home on my "Photon Range" using a very dim LED and test receivers.

Practically speaking this meant that at full power with the Phlatlight LEDs we had about 50dB of  excess signal at the output of the receivers as compared to the minimum possible signal level using baseband speech and the "naked" ear.  Switching to MCW (tone-modulated Morse code) we could extend this by another 6-10dB and the the use of narrowband digital signalling techniques (such as WSPR or QRSS CW - very slow Morse) could have extended this by even another 20 dB or so.  The implication of this is that, in theory, we could communicate over that distance with only a milliamp or two of LED current!
Figure 2:
This time, a high-power green LED!
Click on the image for a larger version.

Satisfied with our tests I switched to a green CBT-54.  Interestingly - but not too surprisingly - Ron reported that subjectively the green LED wasn't really any brighter than the red had been.  On previous tests at much shorter distances, the green far outshone the red owing to the fact that the human eye is at least 5 times as sensitive to green than the wavelength of red LED that we were using.  For these distances, however, the atmospheric attenuation was sapping the vast majority of our light since the shorter (green) wavelengths are attenuated at a far higher rate than the longer ones, a fact that explains red sunsets and that we observed, at the beginning of our testing, that his white, halogen spotlight appeared to us as distinctly yellow/red in color.

The silicon photodetector didn't fare any better since it had far less sensitivity at green than red, the two factors (atmospheric and the Si sensitivity) adding up to between 20 and 30dB in degradation - assuming that the subjective measurement of "equal" brightness between red and green was correct.  As it turns out the degradation was probably far greater than that as the APD-based receiver could hardly detect voice at all, but this may have been also due, in part, to the fact that the gain of a standard APD drops off precipitously with shorter wavelengths and that it was likely not focused properly for green light due to chromatic aberration of the Fresnel lens!  In retrospect we should have switched to a receiver with a larger, "non-APD" detector - and thus less sensitive to misfocusing due to chromatic aberration.

In addition to using high-power LEDs, we also exchanged 2-way communications using plain, ordinary, cheap low-power red LED laser pointers.  The signals were far weaker - mostly owing to the lower optical power of the laser pointer - but each other's lasers were visible to the naked eye over the distance.  Because of the combination of the laser's (relatively) coherent light and its small exit aperture (small beam diameter) the scintillation (fading) on the laser-based link was terrible while on the LED-based link it was only just noticeable.  Some of the methods and techniques to communicate using laser pointers may be found in the September 5th entry of this blog.

After several hours of standing around in the dark on the mountain, we decided that it was getting early (approaching 2 AM!) and packed things up and made our way down the mountain.

Overall, it was a fun little jaunt giving us a healthy dose of nerdiness... enough to last for a few weeks, anyway!

Afterward:
While we run these tests, we'll often play something from portable audio players so that we have a continuous source of sound.  In this case, one of the audio sources that I used was from a Soldersmoke podcast.
For the heck of it, I emailed Bill, N2CQR, who produces this podcast and he put it on his blog page (link) as well as commenting on it in his next Soldersmoke podcast (link)!  This may have had something to do with this post appearing on Hack-A-Day (link)!


Links from the "Modulated Light" (link) web site:
Be sure to check out the "ModulatedLight.org" web site's other pages as well!

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