[Laser] Re: Laser Digest, Vol 2, Issue 5

[Staff]

I would suggest the following:

We are manufacturing and marketing Free Space Optical communications
equipments for short and long distance.

>From my experience, I would suggest you to reduce the F.O.V.(Field of View)
on your receiver optics by:

1.  Increasing the focal length of the lens.
F# of one of our products is about 7.8( 7.8 = 39"long / 5" diameter).  That
means, your receiver is going to get long. However, we are getting away with
the length issues using folded optics(Cassegrain type telescope). This
device is working day and night.

2. And/Or Add honeycomb like structure or in front of your receiver lens.
This is the same idea as adding a tube in front of your receiver lens but
much more compact. (let say you have a honey comb like structure that is 1"
thick and has a 0.25" x 0.25" holes on it. F# for the honey comb is 4 ( 4 =
1 / 0.25 ).
Thus, this is equivalent to having a 20 inch tube in front of 5" diameter
lens except you have lengthen the receiver tube by 1" of course you will
loose some optical efficiency due to the honey comb structure.

3. Put red(if you are using red laser) piece of plastic in front of the
receiver lens and/or sensor(Little help)

4. Find (e-bay?) cheap, band pass filer for your specific laser (helps some
what)

Or, we can provide you with component(modules) (circuit boards($300/Pair),
optics(5" Cassegrain telesocpe, $700/Pair), sensors($20/pair), diode
lasers(780nM, 670nM) and hardware(Precision alignmnt mounting)) that can
work with 100Mbps Ethernet (LAN) signal. E-mail me if you want to take that
route (beyungkim "at" hotmail.com )

-----Original Message-----
From: laser-bounces at mailman.qth.net
[mailto:laser-bounces at mailman.qth.net]On Behalf Of wa4qal at ix.netcom.com
Sent: Thursday, June 10, 2004 10:23 AM
To: laser at mailman.qth.net
Subject: [Laser] Re: Laser Digest, Vol 2, Issue 5


One of the problems with most designs is that the dynamic
range is so small in the optical receiver front end that ambient
daylight saturates the front end.  And, once the front end has
been saturated, it doesn't matter how much more signal power
you pump into it, you won't get any signal out.

Obviously, the first thing to do is to design the optical part of the
system to exclude as much ambient light as possible.  This usually
means long light shades (e.g., PVC pipe spray painted black inside
to limit the acceptance angle of incoming radiation, and to minimize
reflections).  Additionally, it may mean focusing devices to further
limit the acceptance angle, and to increase the signal power to
ambient illumination.

Spectral filters can help, too.  Note that spectral filters don't have to
be exceedingly expensive.  Sure, the custom designed ones with
100 Angstrom bandwidths are worth their weight in gold (or more!).
But, something as simple as a theater lamp gel can attenuate some
ambient light without reducing the signal light excessively.  And, they're
pretty cheap, too.

After you've done all you can to minimize the ambient light in the
signal path, then it's time to start working on the electronics.  One
approach that I've used is to transformer couple the optical detector
(PIN photodiode) to the high gain, high impedance front end (JFET IC).
The transformer coupling removes the DC component of the signal
while still allowing any AC component through.  It may not be perfect,
but I've used it to communicate across a well lighted (fluorescent)
conference room.  Anyway, it's another design to think about.

Dave
WA4QAL

-----Original Message-----

Date: Wed, 09 Jun 2004 16:49:57 -0400
From: "Andrew T. Flowers, K0SM" <aflowers at frontiernet.net>
Subject: [Laser] daylight receiver
To: laser at mailman.qth.net
Message-ID: <40C777F5.5080802 at frontiernet.net>
Content-Type: text/plain; charset=us-ascii; format=flowed

Does anyone have a design for a good daylight receiver?  My K3PGP front
end won't hear much in daylight.  I'm not looking for something that is
super sensitive--just good enough for a LOS QSO on a sunny afternoon.  I
was thinking about putting the RX in a black box and using a very small
lens, thus limiting the amount of total light hitting the surface.  I
may also try using a low-value resitor on my OPT310 RX to widenthe BW
and decrease the gain.  Right now it makes a pretty good oscillator if
you hit it with the laser directly :-)

( and yes, I'm too poor to to afford narrowband filters)

Andy K0SM/2


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