[Laser] AW:Scintillation and Adaptive Optics
Dieter Palme
dieter_palme at online.de
Wed Aug 22 05:09:24 EDT 2007
Hi all,
Some comments and assumptions.
I assume:
1. The spot of the tx on the detector is small.
That means a small portion of the background light falls on the active
area of the diode. The modulation depth and the SNR is therefore good.
The disandvantage is, small deviations of the beam direction move the
spot away from the detector.
2. The beam has a divergence.
Keep in mind, the beam goes parallel to the earth surface and therefore
the rising bubbles of warm air are like gradient lenses and the straight
line of the beam will be crooked.
The radiation goes into the input aperture but does not arrive the
detector.
And now works a adaptive optic.
The system detect the deviation and an adaptive lens compensates these
deviation. This is possible if the beam diameter is bigger then the
aperture of the lens.
A comment on star-twickle. If you look to the zenith or away from the
horizon the air bubbles are risig parallel to the beam and the influence
is smaller. Away from the earth surface the difference in temperature of
the bubbles decrease and so the deviation is smaller.
Excuse my poor english
73 Dieter dl7udp
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> Today's Topics:
>
> 1. Scintillation and Adaptive Optics (James Whitfield)
> 2. Re: 21 mile daytime laser contact durin ARRL 10GHz &
> Upcontest (James Whitfield)
> 3. RE: Scintillation and Adaptive Optics (Terry Morris W5TDM)
> 4. Photons? (KD7JYK)
> 5. RE: Photons? (Terry Morris W5TDM)
> 6. RE: Photons? (Terry Morris W5TDM)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 21 Aug 2007 22:09:14 -0500
> From: "James Whitfield" <n5gui at cox.net>
> Subject: [Laser] Scintillation and Adaptive Optics
> To: "Free Space LASER Communications" <laser at mailman.qth.net>
> Message-ID: <002c01c7e469$d2362010$6401a8c0 at Sony2G4>
> Content-Type: text/plain; charset="iso-8859-1"
>
> I am puzzled by recent comments by Dieter, dl7udp, and Terry,
> W5TDM, about the possiblility of adaptive optics being able
> to reduce scintillation. I know that my concept of optics is
> more geometry and Newtonian than waves with diffraction and
> phase characteristics, but I just don't "see" a mechanism how
> adaptive optics can help, much less be affordable for the
> experimenter or practical communications.
>
> Since I usually ramble a bit trying to explain myself, I will
> try to put forth the three scenarios that I have come up
> with, then try to explain two of them. The first, and I
> believe the most responsible for scintillation, is a change
> in the photon flow into the receiver aperture. For this I
> see absolutely no mechanism for adaptive optics to work. The
> second, I will call "image dance", does have a mechanism that
> I believe would cause amplitude variance. The problem with
> it is that it should not affect a practical optical receiver
> system and if it did, the effects should not be enough to
> account for the scintillation that is observed. The third is
> a random phase distortion of the wavefront that results in
> time varying cancelation/reinforcment of the intensity on the
> sensor. This third scenario is so fuzzy in my head I will
> not even try to describe it, much less suggest what adaptive
> optics might do about it.
>
>
>
> Now for the ramblings: In layman's terms the twinkling of
> the stars is
> scintillation, just at a speed that we can see rather than
> hear. Stars are so far away that we can treat them as point
> sources. To model the flow of light to the eye, imagine a
> long isoceles triangle with the star at the "point" ( the
> angle between the two long equal sides ) and the "base" ( the
> short side ) equal to the diameter of the iris. In this
> model we can represent the flow of photons from the source as
> an arc moving with time away from the source. The photons
> that are inside the triangle can be seen, those outside the
> triangle cannot. Now to model the atmosphere we place a
> transparent object so that at least some of the object is
> inside and some is outside the triangle. This object changes
> randomly with time in shape and / or density so that the
> photons that pass through it change direction. Some of the
> photons will change direction enough that they will exit the
> triangle if they were previously in it or will enter it if
> they were previously outside. The more dense the atmosphere,
> the longer the path through the atmosphere, or the more
> turbulent it is, the more likely the photons will shift into
> or out of the triangle. ( Stars twinkle less on mountain tops
> than at sea level, more close to the horizon than overhead.
> ) If you
> increase the aperture ( the base of the triangle ) in this
> model you will increase the number of photons received. (
> The model is two dimensional instead of three so it would
> only increase in proportion to the aperture with no change in
> the number of in/out photons. In the real world the capture
> area is proportional to the square of the diameter and the
> in/out photons would increase proportional to the perimeter,
> that is linearly. ) The end result is that the twinkle
> becomes a smaller fraction of the average light received. (
> For this reason children and the elderly probably see more
> twinkling of the stars. Children have smaller eyes. The
> ability of the eye to dilate decreases with age. )
>
> >From this model, which I believe accounts for most of the
> scintillation
> effect, I cannot see how adaptive optics would have any
> effect on the number of photons entering the aperture of an
> optical instrument.
>
>
> The second scenario presumes that the distortion from the
> atmosphere does not change the photon flux reaching the
> instrument apeture, but rather changes the direction that the
> image enters the instrument. If you could watch the image of
> the source, it would change the location where it falls on
> the instrument's focal surface ( in a camera it would be the
> focal plane ). I think of it as the spot on the photo sensor
> as "dancing" arround. Now for our purposes the optical
> sensor ( a one pixel camera ) does not need to be, and I
> suggest there are valid reasons that it should not be, at the
> focal surface of the instrument. Further the image of the
> source we are trying to detect can be a very fuzzy patch,
> though it would be nice if it did fall entirely on the
> sensor. In this scenario the varying amplitude detected
> could be caused by the fuzzy patch dancing to, and partly
> over, the edge of the sensor. The fraction of the light
> within the overshoot of the spot would be the amount of lost signal.
>
> I can readily see how adaptive optics would be able to
> stabilize the dance of the spot, which in an imaging camera
> would reduce the distortion caused by turbulence, but I do
> not see it as having useful value on real scintillation.
>
> To carry the analogy further, in the experiment by KD0IF and
> N6IZW the range was 21 miles or 1.33 million inches with a
> transmit aperture of four inches. The source would then be
> three microradians. I forget what the receive instrument
> what, but assume that it had a 1000 mm focal length and a 1 mm
> sensor, yielding a field of view of 1000 microradians. That
> is an awful
> lot of dancefloor for the received spot. I do not see
> adaptive optics being of any real benefit for communication.
>
>
>
> James
> n5gui
>
>
>
>
>
>
>
>
>
>
>
>
>
> ------------------------------
>
> Message: 2
> Date: Tue, 21 Aug 2007 22:28:55 -0500
> From: "James Whitfield" <n5gui at cox.net>
> Subject: Re: [Laser] 21 mile daytime laser contact durin ARRL 10GHz &
> Upcontest
> To: "Free Space LASER Communications" <laser at mailman.qth.net>
> Message-ID: <003001c7e46c$91c29d80$6401a8c0 at Sony2G4>
> Content-Type: text/plain; charset="Windows-1252"
>
> 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_stabilizatio
> n 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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>
> ------------------------------
>
> Message: 3
> Date: Tue, 21 Aug 2007 23:15:46 -0700
> From: "Terry Morris W5TDM" <w5tdm at hotmail.com>
> Subject: RE: [Laser] Scintillation and Adaptive Optics
> To: laser at mailman.qth.net
> Message-ID: <BAY137-F302F22E9EDEE8EA28B1DE4F0D50 at phx.gbl>
> Content-Type: text/plain; format=flowed
>
> Hi James,
>
> I think the first problem with your concept is that
> scintillation is cause
> by a change in photon flow into the receiver aperture. Light
> does not travel
> through space or other material as a photon, but as a wave.
> Light is said to
> be dualistic, meaning it is emitted and absorbed as a
> particle (photon), but
> it travels through space/air/glass etc as a wave. So it must
> obey the laws
> of wave propagation and wave optics.
>
> When light leaves a star at "infinity" it forms a spherical
> wavefront, but
> being at infinity it arrives here very much a plane wave.
> This appears to
> the eye as a point source. The same can be said for a Laser,
> it is a point
> source which produces a plane wavefront with all points on
> the wavefront in
> phase. As this plane wavefront from star or Laser propagates
> throught the
> turbelent atmosphear it is distorted by the changing density
> which causes
> phase distortions. The phase distortions result in both
> constructive and
> destructive interference. This causes increased amplitude
> distortions in
> addition to the ampltude distortions caused by absorption by
> the atmosphere.
> Large phase distortions also result in diffraction or beam
> steering. What
> you are calling dancing of the beam. I will concure that a
> larger receiver
> aperture will collect more light, however if that light
> consists of out of
> phase wavefronts you will have considerable destructive
> interferance at the
> focal plane of the lens. If you provide a way to correct the
> wavefront with
> adaptive optics you will reduce that destructive
> interferance. The wavefront
> correction will also reduce fringe patterns which will cause
> AM modulation
> as they move across the detector due to the "dancing". By
> using the correct
> modulation/demodulation this AM noise is eliminated. The
> signal that has
> already been lost between transmitter and receiver by absorption and
> interference is gone for good.
>
> I did state in my original post that adaptive optics were not
> in our budget.
> However the DOD does make use of adaptive optics for Laser
> communications to
> overcome the same problems we encounter. They do have the
> advantage that
> they generally only have to deal with one atmosphere
> thickness, where we are
> trying to deal with several atmospheric thickness. Kerry and
> Lee's 21 mile
> link was just about one atmosphere thick. The one case I know
> of where the
> DOD has to deal with several atmosphric thickness is with the
> ABL and THEL
> programs.
>
> If you are interested, I can suggest some good books on wave optics.
>
> 73
> Terry W5TDM
>
>
> >From: "James Whitfield" <n5gui at cox.net>
> >Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
> >To: "Free Space LASER Communications" <laser at mailman.qth.net>
> >Subject: [Laser] Scintillation and Adaptive Optics
> >Date: Tue, 21 Aug 2007 22:09:14 -0500
> >
> >I am puzzled by recent comments by Dieter, dl7udp, and Terry, W5TDM,
> >about the possiblility of adaptive optics being able to reduce
> >scintillation. I know that my concept of optics is more
> geometry and
> >Newtonian than waves with diffraction and phase
> characteristics, but I
> >just don't "see" a mechanism how adaptive optics can help,
> much less be
> >affordable for the experimenter or practical communications.
> >
> >Since I usually ramble a bit trying to explain myself, I will try to
> >put forth the three scenarios that I have come up with, then try to
> >explain two of them. The first, and I believe the most
> responsible for
> >scintillation, is a change in the photon flow into the receiver
> >aperture. For this I see absolutely no mechanism for
> adaptive optics
> >to work. The second, I will call "image dance", does have a
> mechanism
> >that I believe would cause amplitude variance. The problem
> with it is
> >that it should not affect a practical optical receiver
> system and if it
> >did, the effects should not be enough to account for the
> scintillation
> >that is observed. The third is a random phase distortion of the
> >wavefront that results in time varying
> cancelation/reinforcment of the
> >intensity on the sensor. This third scenario is so fuzzy in
> my head I
> >will not even try to describe it, much less suggest what adaptive
> >optics might do about it.
> >
> >
> >
> >Now for the ramblings: In layman's terms the twinkling of
> the stars is
> >scintillation, just at a speed that we can see rather than
> hear. Stars
> >are so far away that we can treat them as point sources. To
> model the
> >flow of light to the eye, imagine a long isoceles triangle with the
> >star at the "point" ( the angle between the two long equal
> sides ) and
> >the "base" ( the short side ) equal to the diameter of the iris. In
> >this model we can represent the flow of photons from the
> source as an
> >arc moving with time away from the source. The photons that
> are inside
> >the triangle can be seen, those outside the triangle cannot. Now to
> >model the atmosphere we place a transparent object so that at least
> >some of the object is inside and some is
> >outside the triangle. This object changes randomly with
> time in shape and
> >/
> >or density so that the photons that pass through it change
> direction. Some
> >of the photons will change direction enough that they will exit the
> >triangle
> >if they were previously in it or will enter it if they were
> previously
> >outside. The more dense the atmosphere, the longer the path
> through the
> >atmosphere, or the more turbulent it is, the more likely the
> photons will
> >shift into or out of the triangle. ( Stars twinkle less on
> mountain tops
> >than at sea level, more close to the horizon than overhead.
> ) If you
> >increase the aperture ( the base of the triangle ) in this
> model you will
> >increase the number of photons received. ( The model is two
> dimensional
> >instead of three so it would only increase in proportion to
> the aperture
> >with no change in the number of in/out photons. In the real
> world the
> >capture area is proportional to the square of the diameter
> and the in/out
> >photons would increase proportional to the perimeter, that
> is linearly. )
> >The end result is that the twinkle becomes a smaller fraction of the
> >average
> >light received. ( For this reason children and the elderly
> probably see
> >more twinkling of the stars. Children have smaller eyes.
> The ability of
> >the eye to dilate decreases with age. )
> >
> > >From this model, which I believe accounts for most of the
> > >scintillation
> >effect, I cannot see how adaptive optics would have any effect on the
> >number
> >of photons entering the aperture of an optical instrument.
> >
> >
> >The second scenario presumes that the distortion from the atmosphere
> >does not change the photon flux reaching the instrument apeture, but
> >rather changes the direction that the image enters the
> instrument. If
> >you could watch the image of the source, it would change the
> location
> >where it falls on the instrument's focal surface ( in a
> camera it would
> >be the focal plane ). I think of it as the spot on the
> photo sensor as
> >"dancing" arround. Now for our purposes the optical sensor ( a one
> >pixel camera ) does not need to be, and I suggest there are valid
> >reasons that it should not be, at the focal surface of the
> instrument.
> >Further the image of the source we are trying to detect can
> be a very
> >fuzzy patch, though it would be nice if it did fall entirely on the
> >sensor. In this scenario the varying amplitude detected could be
> >caused by the fuzzy patch dancing to, and partly
> >over, the edge of the sensor. The fraction of the light within the
> >overshoot of the spot would be the amount of lost signal.
> >
> >I can readily see how adaptive optics would be able to stabilize the
> >dance of the spot, which in an imaging camera would reduce the
> >distortion caused by turbulence, but I do not see it as
> having useful
> >value on real scintillation.
> >
> >To carry the analogy further, in the experiment by KD0IF and
> N6IZW the
> >range
> >was 21 miles or 1.33 million inches with a transmit aperture of four
> >inches.
> >The source would then be three microradians. I forget what
> the receive
> >instrument what, but assume that it had a 1000 mm focal
> length and a 1 mm
> >sensor, yielding a field of view of 1000 microradians.
> That is an awful
> >lot of dancefloor for the received spot. I do not see
> adaptive optics
> >being
> >of any real benefit for communication.
> >
> >
> >
> >James
> > n5gui
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >_______________________________________________
> >Laser mailing list
> >Laser at mailman.qth.net http://mailman.qth.net/mailman/listinfo/laser
>
> _________________________________________________________________
> See what you’re getting into
before you go there
> http://newlivehotmail.com/?ocid=TXT_TAGHM_migration_HM_viral_p
> review_0507
>
>
>
> ------------------------------
>
> Message: 4
> Date: Wed, 22 Aug 2007 00:00:45 -0700
> From: "KD7JYK" <kd7jyk at earthlink.net>
> Subject: [Laser] Photons?
> To: "Free Space LASER Communications" <laser at mailman.qth.net>
> Message-ID: <001d01c7e48a$2a42f740$a563f504 at ELNkk7rc>
> Content-Type: text/plain; charset="iso-8859-1"
>
> "Light is said to be dualistic, meaning it is emitted and
> absorbed as a particle (photon), but it travels through
> space/air/glass etc as a wave."
>
> Not to be funny here, but what about RF? RF is a wave, same
> as light, but lower frequency. Light has photons, what about
> "radio waves". Radons? Sounds silly, but seriously.
>
> Kurt
>
>
>
> ------------------------------
>
> Message: 5
> Date: Wed, 22 Aug 2007 00:09:21 -0700
> From: "Terry Morris W5TDM" <w5tdm at hotmail.com>
> Subject: RE: [Laser] Photons?
> To: laser at mailman.qth.net
> Message-ID: <BAY137-F39A6A39AD367D7E4368435F0D50 at phx.gbl>
> Content-Type: text/plain; format=flowed
>
> I think RF is due to electrons, seriously!
> Terry
>
>
> >From: "KD7JYK" <kd7jyk at earthlink.net>
> >Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
> >To: "Free Space LASER Communications" <laser at mailman.qth.net>
> >Subject: [Laser] Photons?
> >Date: Wed, 22 Aug 2007 00:00:45 -0700
> >
> >"Light is said to be dualistic, meaning it is emitted and
> absorbed as a
> >particle (photon), but it travels through space/air/glass etc as a
> >wave."
> >
> >Not to be funny here, but what about RF? RF is a wave, same
> as light,
> >but lower frequency. Light has photons, what about "radio waves".
> >Radons? Sounds silly, but seriously.
> >
> >Kurt
> >
> >_______________________________________________
> >Laser mailing list
> >Laser at mailman.qth.net http://mailman.qth.net/mailman/listinfo/laser
>
> _________________________________________________________________
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>
>
> ------------------------------
>
> Message: 6
> Date: Wed, 22 Aug 2007 00:15:49 -0700
> From: "Terry Morris W5TDM" <w5tdm at hotmail.com>
> Subject: RE: [Laser] Photons?
> To: laser at mailman.qth.net
> Message-ID: <BAY137-F25F26BBA1F541E0D9D1AE6F0D50 at phx.gbl>
> Content-Type: text/plain; format=flowed
>
> Oh, you did say seriously, RF is also a photon.
> Terry
>
>
> >From: "Terry Morris W5TDM" <w5tdm at hotmail.com>
> >Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
> >To: laser at mailman.qth.net
> >Subject: RE: [Laser] Photons?
> >Date: Wed, 22 Aug 2007 00:09:21 -0700
> >
> >I think RF is due to electrons, seriously!
> >Terry
> >
> >
> >>From: "KD7JYK" <kd7jyk at earthlink.net>
> >>Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
> >>To: "Free Space LASER Communications" <laser at mailman.qth.net>
> >>Subject: [Laser] Photons?
> >>Date: Wed, 22 Aug 2007 00:00:45 -0700
> >>
> >>"Light is said to be dualistic, meaning it is emitted and
> absorbed as
> >>a particle (photon), but it travels through space/air/glass
> etc as a
> >>wave."
> >>
> >>Not to be funny here, but what about RF? RF is a wave,
> same as light,
> >>but lower frequency. Light has photons, what about "radio waves".
> >>Radons? Sounds silly, but seriously.
> >>
> >>Kurt
> >>
> >>_______________________________________________
> >>Laser mailing list
> >>Laser at mailman.qth.net http://mailman.qth.net/mailman/listinfo/laser
> >
> >_________________________________________________________________
> >Puzzles, trivia teasers, word scrambles and more. Play for
> your chance
> >to
> >win! http://club.live.com/home.aspx?icid=CLUB_hotmailtextlink
> >
> >_______________________________________________
> >Laser mailing list
> >Laser at mailman.qth.net http://mailman.qth.net/mailman/listinfo/laser
>
> _________________________________________________________________
> Now you can see trouble
before he arrives
> http://newlivehotmail.com/?ocid=TXT_TAGHM_migration_HM_viral_p
rotection_0507
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