[Laser] 5mw kit - scintillation compensator

[Tim Toast]

Hi Clint

All i can say is WOW! the scintillation compensator is amazing how 
well it works. Thank you very much for the detailed info on it. A 
Very interesting project. I must have glossed over it in the past 
not realising what it was.. because i know i have read most of the 
pages at modulatedlight.org. Something like that will go a long way
towards working out the last remaining bugs in laser coms. As 
tempted as i would be to include it in a kit, i guess i would not 
for cost reasons. It would make an excellent add-on though or for 
later more deluxe versions.

thanks again
tim toast


> 
> ------------------------------
> 
> Message: 2
> Date: Wed, 22 Sep 2010 10:54:46 -0600
> From: "C. Turner" <turner at ussc.com>
> Subject: Re: [Laser] 5mw laser tranceiver kit
> To: laser at mailman.qth.net
> Message-ID: <4C9A34D6.4060403 at ussc.com>
> Content-Type: text/plain; charset=ISO-8859-1;
> format=flowed
> 
 * * *
> 
> A method of "compensating" for scintillation was suggested
> and, as Chris 
> mentioned, a method has been implemented as seen (and
> heard!) here:
> 
> http://modulatedlight.org/optical_comms/optical_comm_audio_interface_device.html#scintillation_compensator
> 
> In  this system, a 4 kHz "pilot" tone is sent along
> with the rest of the 
> audio at a level that is 25% (or 12dB down) with respect to
> "peak" 
> (100%) modulation.  At the receive end, this tone is
> extracted from the 
> audio and used to drive a very fast AGC and is also
> notch-filtered to 
> prevent the user from being driven insane.
> 
> There is also an audio clip:
> 
> http://modulatedlight.com/optical_comms/scint_comp_test2.mp3
> 
> (The "original" audio clip - pilot tone removed for most of
> it - can be 
> heard here:  
> http://ka7oei.com/optical_comms/Laser_LED_scintillation_comparisons_pilot_removed.mp3
> 
> )
> 
> This demonstrates the same audio clip with and without
> compensation.  
> This signal was transmitted using a laser and an 8"
> diameter reflector 
> telescope and the optical path - about 23km - crossed a
> strong thermal 
> boundary layer that capped the Salt Lake Valley - and was
> about as bad 
> as an experience with scintillation that we have
> experienced - save for 
> the use of a laser pointer on that same path during that
> evening.  
> Uncompressed .WAV recordings containing the original 4 kHz
> pilot were 
> made and later played back for this demonstration clip,
> notching out the 
> 4 kHz tone in each case.
> 
> As can be heard, the compensation is extremely effective,
> minimizing the 
> effects of the scintillation and its distortion on the
> speech and 
> music.  Of course, as Chris points out, it cannot
> recover what was lost 
> in the noise.
> 
> When designing the system, there was some concern about
> whether or not 
> to remove energy in the area of 4 kHz from the transmit
> audio in the 
> fear that it might confuse the AGC on the receive
> end.  Using recordings 
> made in the field with actual scintillation (some from
> Laser pointers, 
> even!) numerous "simulations" were run and it was
> determined that 
> practically speaking, incident audio caused very little
> problem and 
> whatever effects there might be were very transient in
> nature:  In other 
> words, unless you were looking for an interaction, you'd
> never spot it!  
> The field recordings also provided a testbed for typical
> worst-case 
> scintillation to be expected and the AGC system was
> designed to 
> accommodate as they were representative of the sorts of
> effects that one 
> might encounter - and far easier and accurate to make than
> simulations!
> 
> A pilot frequency of 4 kHz was chosen because of the
> (intentionally) 
> limited bandwidth of our optical receivers and the fact
> that a 4 kHz 
> tone can be easily synthesized using DDS techniques: 
> The fact that our 
> modulators include a PIC-based tone generator system made
> this a 
> "no-brainer."  A simple notch filter at 4 kHz doesn't
> materially affect 
> the way the audio "sounds" as can be observed from
> listening to the 
> clip.  Being that the pilot tone is consuming 25% of
> the audio power, 
> its presence degrades the peak power only minimally. 
> The way the 
> modulator we use is designed, it preserves the AGC's margin
> when 
> achieving 100% modulation of the LED.
> 
> Interestingly, we've had relatively little need for the
> scintillation 
> compensator as the problem of severe scintillation
> manifests itself to 
> such a degree ONLY with the use of lasers.  Since we
> don't often use 
> lasers, the scintillation hasn't been too bad.
> 
> Another observation is that if there is a noticeable amount
> of dust or 
> haze in the air, scintillation can go nearly to zero. 
> While the path 
> attenuation will tend to skyrocket, we've observed that
> moderate amounts 
> of optical dispersion tends to more-rapidly break down
> coherence along 
> the path.
> 
> This effect has been seen on several of the occasions that
> we have 
> spanned one of our "standard" 107 mile (173km) paths. 
> Having done this 
> path with crystal clear air, extremely hazy air (that is,
> the opposite 
> end of the path could not be seen with the naked eye) and
> somewhere 
> in-between, we've noted that the worst case is the "crystal
> clear" air 
> path.  In the other two instances, the scintillation
> was quite low - 
> even when we've used just Laser Pointers for 2-way
> communications on 
> that same path.  (Again, the scintillation on the
> LED-based systems - 
> which we use to coordinate or Laser activities - was
> comparatively low 
> in any case.)
> 
> Now, were I designing a kit, would I include a
> scintillation 
> compensator?  No.
> 
> * * *
> 
> We've done some testing with diversity transmission and
> detection as well.