[Laser] 21 mile daytime laser contact durin ARRL 10GHz & Upcontest

Tue Aug 21 23:28:55 EDT 2007

I concur that increasing beam and /or receiver aperture is the most obvious,
and perhaps the easiest, way to deal with scintillation.

Consider that at least some of the benefits of increased aperture can be
obtained by spacial separation.  A much more complex arrangement, and
perhaps not cost effective or worth the effort.

For example, two light sources separated by an adjustable distance.  It
might be impractical for lasers since they would need to be alligned to the
same target ( the receiver ).  If I were going to experiment, I would
probably use LEDs.  Not as narrow beam and easier to control power linearly.

For receive you might be able to use mirrors. Sort of like the optical range
finders,  The two images would not need to be aligned to the same spot, just
as long as the two spots both fall on the sensor.

James
 n5gui

----- Original Message -----
From: "Jim Moss" <n9jim-6 at pacbell.net>
To: "Free Space LASER Communications" <laser at mailman.qth.net>
Sent: Monday, August 20, 2007 11:35 PM
Subject: Fw: [Laser] 21 mile daytime laser contact durin ARRL 10GHz &
Upcontest

WTG Kerry..

Scintillation is not improved by BW filtering.
Increasing the beam size to greater than the turbulence cell, and the RX
lens size will improve it.
Some studies suggest typical turbulence cells are about 6".

Jim
N9JIM

----- Forwarded Message ----
From: Terry Morris W5TDM <w5tdm at hotmail.com>
To: laser at mailman.qth.net
Sent: Monday, August 20, 2007 2:56:51 PM
Subject: RE: [Laser] 21 mile daytime laser contact durin ARRL 10GHz & Up
contest

Congratulations on your 21 mile Laser contact.

As to the narrow band filter reducing the scintillation, I must agree with
Dieter DI7UDP, NO! The filter only reduced the receiver bandwidth which
results in increased SNR as shown by your 50 dB above noise floor
measurment. The scintillation is totally due to atmospheric turbulence which
distorts the transmitted beam in both amplitude and phase. The phase
distortion adds additional amplitude distortion and in severe phase
distortion causes beam steering. I think there is little that can be done on
the receiver end short of adaptive optics.

http://en.wikipedia.org/wiki/Adaptive_optics#Beam_stabilization  This is
probably well outside the budget for amateur experimenters. The only
reasonable cost adaptive optical method I have see is the use of two optical
wedges in rotary stages controlled by output from a quadrant detector. This
is used to look at scatter from a probe beam that has a shorter wavelength
than the main link beam.

The MITRE org has some useful information at :
http://www.mitre.org/news/events/tech06/briefings/2146.pdf

73
Terry W5TDM

>From: Kerry Banke Reply-To: Free Space LASER Communications To:
>laser at mailman.qth.net Subject: [Laser] 21 mile daytime laser contact durin
>ARRL 10GHz & Up contest Date: Sun, 19 Aug 2007 15:43:13 -0700 (PDT)
>
>  Yesterday Lee, KD0IF and I made a mid day laser contact over a distance
>of 21 miles near San Diego  as part of the ARRL National  10 GHz and Up
>contest. Last year we did a 7 mile daytime contact but experienced very
>heavy scintillation which we have found to be typical for warm weather in
>the San diego area.  This year we added narrow wavelength optical bandpass
>filters ahead of our optical receivers and are using a fast AGC provided in
>the Spectrum Lab software.  This was our first two way  daytime
>communications  tests with the filters. The scintillation on the raw signal
>appears to be very minimal compared to our contact last year. We don't know
>if the narrow optical filters have provided this improvement or if we just
>had unusually  good conditions so we need to do more daylight tests with
>and without filters for comparison. The filters are at 920 nm with a 30 nm
>half power bandwidth. When operating at high power with  1W  collimated to
>a 4"  dia beam, the laser  transmitters delivered  signals that  were a
>stable 50 dB above the noise in a 1 Hz BW at 21 miles.   Communications was
>done using PSK31 with a center frequency of 755 Hz.   Does anyone know if
>adding the  narrow optical filters should reduce scintillation? Thanks also
>to Greg, K6QPV for assisting with the contact.    - Kerry N6IZW -
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