[Laser] Misc. optical happenings...

[C. Turner]

Since there's been only a little bit of activity here, recently, I 
thought that I'd at least start the digest's February archive.

First, with the help of Barry, G8AGN, I've produced a comb filter that 
will remove mains-induced hum from the audio from optical receivers.  It 
is described here:

http://modulatedlight.org/optical_comms/comb_filter.html

I appreciate the feedback that Barry has provided on a number of  topics 
in our recent email exchanges and I congratulate him and Gordon, G0EWN 
on what is believed to be a new UK optical DX record of 87km!  (A link 
to Barry's may be found on previous postings here or at the bottom of 
the above-linked page.)

***

Although no new DX was achieved, I dragged out the optical gear and 
braved the cold temperatures during the Saturday Night of the January 
VHF (and up) contest.  After working on the "DC" bands (50, 144, 222, 
432, 1296, and both 10 and 24 gig SSB) we had some extra time and went 
around inconveniencing large numbers of photons.

We did our first-ever "daylight" optical tests.  The distance wasn't 
great (1.13 miles, IIRC) but even in the daylight, the distant 3-watt 
LED was still annoyingly bright, requiring that the current be backed 
down to a fraction of full power to avoid complete receiver saturation - 
which was bad enough already from the ambient light!

A brief clip of that is here:

http://www.youtube.com/watch?v=PvAF0nlyhpQ

This isn't a very good video at all since I was both operator and 
cameraman.  Between the numerous interruptions and the fact that, by the 
time I did this video, I could no longer feel my fingers (it was 20-25F 
at that location) I didn't get as video much as I wanted.  However, the 
audio in the background and my "open-mic" communications to the far end 
was all via the full-duplex optical link.  By the time this clip was 
shot it was nearly sunset, so the receiver's desense level due to to 
ambient light was rapidly decreasing.

***

After this we went back down to the warmer (28F or so) valley and did 
some "bent light" communications with both of us illuminating a patch of 
snow-covered mountain that we could both see.  For this, Ron, K7RJ, was 
at his QTH about 0.85 miles from the school parking lot in which we'd 
set up and the total optical path was a bit over 2 miles.  This wouldn't 
be an NLOS DX record for us (we'd done >14 mile 1-way voice, 2-way CW 
last year over a similar and decidedly non-optimal NLOS path across the 
Salt Lake Valley) but it would be fun, nonetheless!

For these sorts of indirect paths it's extremely convenient that we can 
aim our "radios" simply by looking for the "RF" - in this case, we could 
easily see each others' "spot" on the mountain with the naked eye.  As 
you might guess, 2-way voice was a piece of cake.

A video clip of this may be found here:

http://www.youtube.com/watch?v=-cSqAkF5dRo

After using Red for a while, I switched to Green - mostly because its so 
cool to watch!  Since the human eye is much more sensitive to Green, the 
beam that shoots out due to Rayleigh scattering is truly impressive - 
looking almost solid enough to walk on.  The added bonus was that 
full-duplex operation was a much easier using green as I had installed a 
red filter Gel in front of my detector, making it "green blind" which 
meant that I wasn't getting feedback from my own (very bright) spot and 
could dispense with the need for a headset.  The downside, of course, is 
that the sensitivity of silicon detectors is noticeably worse at green 
than red, but with the margins involved on this easy path, there was no 
problem at all.

When we were both running red we were using half-duplex - to save 
battery and also to avoid feedback.  It struck me as very odd that I 
could "see" the signal with my eyes:  A red spot would appear on the 
mountain and I'd hear audio - and then with a "click" it would disappear 
at the end of transmissions giving one a vague idea what the world would 
look like were we actually able to "see" RF from our own ham 
transmissions!  (On second thought, it's probably just as well that we 
can't see RF!)

At this point I'd like to make the usual disclaimer:

- Even though the "ERP" is estimated to be about a megawatt or so (e.g. 
gain-beamwidth multiplied by power and expected losses) the power 
density across the beam is quite low - less than one would get from the 
high beams of the car of an inconsiderate driver as it passes, and at 
least an order of magnitude less than that of a cheap, red Class 2 laser 
pointer.  Of course, we avoided pointing anything anywhere near passing 
aircraft, vehicles or even residences:  From our location we were quite 
well-shielded from any vehicles or houses and any aircraft corridors 
were behind us.

***

Although it's hard to tell from the second video, the red "spot" on the 
mountain from the other end (the video camera picked up the green far 
better than the red) - while "square" looking to Ron, was more of an 
ellipse-shaped from my perspective:  Likewise, my "square" beam had the 
same sort of elliptical shape from his location - but slanting the 
"other" way, of course!

A time exposure picture taken from Ron's end may be found here:

http://modulatedlight.org/optical_comms/Red_and_green_from_Rons.JPG

And an even longer time-exposure from my end:

http://modulatedlight.org/optical_comms/SDIM4408.jpg

(I was some distance away from the transmitter, but you can just see the 
Rayleigh scattering coming in from the right-hand side of the picture.)

This clearly illustrates one of the difficulties with non-line-of-sight 
communications and "cloudbounce" in particular.  Unlike line-of-sight, 
the beam changes form something that is well-defined to more of a 
"smear" - and since true "cloud-bounce" would be more like the 
illuminating of fog rather than the illumination of cumulo-granite, it's 
rather less-well defined.

To each other, the beam width of the other person's signal was much 
wider than originally transmitted - and this also meant that there was 
some degree of inefficiency in the receiver's too:  With careful 
adjustment and masking, the effective beamwidth of our receivers is 
comparable to that of the transmit beams - perhaps a bit less.  Ideally, 
one would have an adjustable mask at the focal plane that could be made 
to be of a shape similar to that of the received beam:  For LOS, this 
would be just a circle, but for NLOS, this would be more of a line or 
ellipse.  Exactly how this would be done - particularly, on-the-fly and 
in the field - is a decidedly non-trivial task!

Of course, one of the difficulties in NLOS work - particularly 
"cloudbounce" is the fact that it's difficult to know exactly where to 
aim either end as it can be challenging to know precisely where the 
reflecting cloud layer is in terms of altitude - not to mention its 
(less than optimal) reflective properties - but doing this sort of 
"ground-based" work can certainly hone one's techniques.

At some point - when the weather warms up - we'll drag the gear out some 
more and do even more experiments.

73,

Clint
KA7OEI