[Laser] a free space optical network

[Tim Toast]

Hi All,

The recent post by Les Rayburn last month (the IR SMS messengers) 
got me thinking about an optical network that could serve to expand 
the range and coverage of optical repeaters by linking them together.
Back in March 2006, Kerry Banke N6IZW put up an optical beacon on San
Miguel Mt. in San Diego. This eventually evolved into a full fledged
optical repeater by June 2007 when a first two way contact was made:

-----
"On June, 23, 2007 a two way contact was made between members 
of the San Diego Microwave Group: Kerry Banke N6IZW and Lee 
Scheppmann KD0IF, over a 14km path using a recently installed 
optical repeater on San Miguel peak near San Diego. PSK31 was 
used along with 910 nm laser diodes and PGP style detectors. 
The optical repeater was assembled from two LED array beacons, 
(the type normally used on communications towers), and driven 
by a prototype detector circuit. The PSK31 base-tone was at 
750 Hz and the received signal was greater than 43 dB above 
the noise floor."
 - Lee Scheppmann KD0IF
-----

Having an optical repeater to serve an area is great. By itself 
it's a good project for any ham radio club. Think of it as the old
telephone system "party line". When used with baseband optical
transceivers, everyone can hear everyone else - as long as no one 
signal saturates the repeater receiver(s). Having a local optical
repeater could also interest other hams in the area to participate 
that otherwise might not.

To better cover an area, you could have several local repeaters and 
link them all together with dedicated optical links. A signal 
directed at any one repeater would then be relayed to the others 
and retransmitted as if it had originated locally. Having multiple
repeaters would help finding an access point due to trees and other
obstructions in an area. These redundant dedicated links between
repeaters would be the backbone of a local area network. Each 
repeater would be linked directly with one or more others where 
possible ideally, but st least one link is required to participate 
in the network. I am assuming the repeaters are located far enough 
apart so that they don't interact much via their normal 
input/outputs. I was thinking they might start oscillating if not 
or have a reverb?? 

Eventually more distant repeaters could be linked. The locally 
linked repeaters would then act as "nodes" for a larger network. A 
long term goal could be to link nearby cities together with one or 
more long haul links. This would be a good project for ham clubs 
in adjacent cities to cooperate on. Even if some distant links go 
down frequently due to weather, they are still worth having for the
times that they DO work. As more cities are linked in this way,
eventually you are going to have a free space optical internet of 
sorts. One that constantly evolves due to weather and day/night 
periods - not unlike as with RF and the ionosphere. Very long path 
repeater linked signals would be rare (and noisy). Closer ones would 
be more common. With a large network, the "band" would be open to
somewhere pretty much constantly in good weather but it would be 
ever changing as some nodes fade out and other fade in. 

To give any repeater more capacity, you could have users use digital
modes or subcarriers on frequencies above 3 khz or so, up to whatever
the highest modulation frequency is for the repeater. There could 
still be baseband users while this is going on presumably. Even a
limited bandwidth baseband-only repeater can be packed with hundreds 
of narrowband digital modes like the popular PSK31.

At any rate, it could start with optically linking two or more 
optical repeaters together in a simple "party line" mode, either 
in two nearby cities or just one local area. Hopefully some of 
this is usefull or will spark some other ideas about optical 
comms.

-toast