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

[Terry Morris W5TDM]

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 - 
>_______________________________________________ Laser mailing list 
>Laser at mailman.qth.net http://mailman.qth.net/mailman/listinfo/laser

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