[Laser] Re: coherence and scintillation

[Chuck Hast]

James, Clint, and group:

I have been reading a bit about FSO links and indeed there is a lot more
material out there than first strikes the eye. I have also been looking at
commercial (big bux) FSO gear. One of the things I have noticed is that
the equipment designed for longer paths (1.5 to 5Km, most of them will
go much farther, but they will not publish it due to issues such as building
sway and fog/smoke/snow obstruction) and the one thing of note is that
they all have 3 diodes spaced around the active end of the transciever and
they spacing from diode to diode is between 6 and 8 inches. On the device
I am building the spacing is 6 inches from diode to diode. The toilet flange
is for a 4" pvc, by the time you add the collar and thickness of the pipe
and flange there is 6 inches between the two diodes, that appears to be
the minimum spacing that I have seen.  Here is a web page where I am
placing information, pictures and other data:
www.wchast.com/wch

I have a book on FSO at home, and the authors point out that the size
of a atmospheric cell is on the order of 6 inches or less. I am looking
forward to getting this link up and running, I will add data to the page as
I get it.

Here is a U.S. Navy web page with FSO data. They have a nice FSO
experiment on the Chesapeake bay that is testing data comms 2.5 Gb/s
down speeding to keep the link running under adverse conditions.
The link is 16Km from end to end, and 32Km round trip (they are using a
retroreflector on one end). This page is a pretty good treasure trove for all
of us, but a lot of it is inclined to doing high speed data over FSO paths.
http://www.nrl.navy.mil/fpco/publications.html

I will keep you all posted as to how things go. Our test path will be set up
between two packet switch facilites, the distance between them is about
6000 meters.

By the way the lasers are 140 mw units used for 8X DVD write. These
things can be run way below max power and still put out a bundle of
power. I am using a 74hc04 to drive the diodes. I was looking at the wave
form at 10Mhz and the diode follows the wave form very faithfully.

One of the things we are finding out is that the best modulation appears
to be Bi-Phase, we are going to test using Bi-Phase Mark. There is tons
of bandwidth so we do not have to worry about the fact that the ones bits
are signaling at twice the baud rate. Bi-Phase is just really FM where your
zero is a single bit transition and your one is a two bit transistion.


-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."