[Laser] [carolinaflashers] a free space optical network

[Les Rayburn]

Tim,

Thought provoking post, and a worthwhile project for hams.

Our "optical" band has been ignored by the mainstream amateur community for 
far too long, and it is an area where we do have a lot of ability to be 
pioneers. One of the advantages to such a network of repeaters would be that 
it could get around the content restrictions currently imposed on HinterNet 
networks. For instance, you could carry commercial web traffic on such a 
link if desired. You can also provide encryption, which makes the system 
more attractive to our served agencies in EMCOMM settings.

While we mainly concern ourselves with DX, we should not overlook the very 
real uses of shorter range optical links, which can be very reliable out to 
several miles. For point to point communications, it offers a number of 
advantages over RF.

73,


Les Rayburn, N1LF
EM63nf
121 Mayfair Park
Maylene, AL 35114

6M VUCC #1712
Grid Bandit #222
Life Member Central States VHF Society


-----Original Message----- 
From: Tim Toast
Sent: Thursday, March 03, 2011 4:53 PM
To: laser mailinglist
Cc: carolinaflashers at yahoogroups.com
Subject: [carolinaflashers] a free space optical network




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




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