[Laser] Strobe Light PSK and other modes

[Tim Toast]

Here's an idea i had about using strobe lights 
to communicate. It's probably not completely new 
since these things have been around so long. I 
know several on the list have mentioned using 
PSK via laser over the last year and how it lends 
itself well to square waves.

"Tubes vs. Semiconductors"

Last summer i was struck by how loud those "pops" 
can be from strobe lights on towers and emergency 
vehicles etc.. reflecting off haze and clouds in 
the area or even clear skies. They are usually 
well above the noise at a distance of two miles 
from me and are not even focused light sources. 
In fact, in the case of the tower lighting, 
they're omnidirectional.. beings they are meant 
to be seen from any direction so pilots can see 
them clearly. Near the tower, they don't appear 
any brighter than your average disco strobe. My 
idea was to use small strobe lamps, the xenon gas 
discharge type, to produce a train of light pulses 
based on PSK type modulation. 

Specifically to gate out the strobe trigger pulses 
from a steady train of two opposing phases of 
them, at say 25 Hz or some other low frequency 
(xtal controlled), to encode the 180 degree phase 
transitions used in BPSK modulation. The processed 
trigger pulses are in turn used to fire the strobe 
tube(s) at the precise times needed. The strobes 
may need to be operated at a lower energy setting 
to avoid overheating things at "fast" pulse rates 
(fast compared to typical disco strobes), but they 
would still deliver hundreds or thousands of watts 
of power in the short pulses. 

That's something you're not going to get from 
normal laser diodes, even the older pulse type 
diodes. Strobe tubes also have much longer pulse 
widths (10's or 100's of microseconds) so the 
flashes are easily audible in most any kind of 
light receiver. And they are more forgiving of 
slight over-voltages and spikes than their solid-
state "rivals". I may be off here, but even narrow 
band filtered light (say 10 ~nm) from a strobe tube 
compares reasonably to the laser diodes in power 
output although it is omnidirectional and at a 
longer pulse and spectral width.

So in effect, with some suitable software, you 
would have a PSK type system based on pulses 
instead of sine waves. A rather broadband and 
wasteful way to do an inherently narrow band mode, 
but it could be done with low cost parts and may 
also have some advantages over current lasers. 
Filtering the strobe lamps with infrared filters 
would avoid any trouble with aircraft and curious 
people without reducing the power output seriously. 
I had read, that in long-pulse mode, strobetubes put 
out most of their energy in the longer wavelengths 
anyway. And of course actually focusing the light 
in a specific direction would greatly increase the 
power levels and range. 

A few side notes to all this; (besides using an 
actual triggered pulse laser if you have one), is 
to simply gate out strings of pulses to represent 
dots and dashes manually with a code key and copy 
it by ear. 
Also you could try increasing the pulse repetition 
rate up to some higher audio frequency, then 
modulate that or use it somehow, morse, qrss etc.. 
The faster you try to pulse the strobes however, 
the lower must be the power output to compensate.. 
but maybe frequencies up into the 1000 Hz range 
wouldn't be a major problem. 
I'm assuming most existing PSK software would NOT 
be compatible with slow (25Hz) pulsed operation, 
But if the strobe speed can be increased to a usable 
audio frequency where software can handle it as a 
"normal" tone, maybe it wont matter. 
I'm not sure what the lowest frequencies are for 
normal PSK operation really.. just a few 10's of 
hertz? 50-60 Hz is bad choice.. maybe 90 to 100 Hz 
or so is about the minimum basic strobe flash rate 
that would be usable with existing PSK software.

Another thing is using a range of pulse frequencies 
to encode the type of modulation that the program 
"LaserScatter" uses. It would definatly be easier to 
get those flash rates from a strobe and the higher 
powers it would allow. thought i'd add that to the 
list as well. 

I may try to build a test setup for this sometime 
and see what kind of range it might have for NLOS. 
And if anyone experiments with this i'd be 
interested in hearing about it and the results.




=====
Tim Toast
http://www.aladal.net/toast/


		
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