[Laser] Polarization modulation
Tim Toast
toasty256 at yahoo.com
Sun Mar 13 22:53:02 EDT 2011
Here are some other mentions of polarization as modulation or other over the years:
(from the text archives)
Date: 2 Apr 1997 18:34:18 -0800
From: "Stone Richard" <Stone_Richard at mm.rdd.lmsc.lockheed.com>
Subject: Re combining SS laser diodes,etc.
To: "laser" <laser at berlioz.nsc.com>
X-Mailer: Mail*Link SMTP-MS 3.0.2
Sender: owner-laser at berlioz.nsc.com
Precedence: bulk
John,
Optical EME is a ambitious project keep at it and I wish you success. Let
us know when you plan to try maybe some of us could pick you out of the noise
with a PMT and FFT integrating software.
As far as beam combining , I have heard of combining multiple laser beams
using polarization or dichroic beam splitters (beam splitters that reflect
some wavelengths and transmit others.) You can probably find some surplus
polarizing cubes around but the polarization scheme can only combine 2 diodes.
You will find that your laser diodes are polarized and you can rotate their
mountings to get 2 orthogonal polarizations. The problem here is to figure out
how to combine more than 2 laser diode beams.
There was 'once upon a time' a space flight qualified laser transmitter made
up that used 16ea. 100mW single frequency laser diodes all at a slightly
different wavelength to get approxamately a 1 watt beam. The beam from each one
was directed into the beam path by a dichroic that reflected its beam but passed
through all the beams form the laser diodes upstream from it. This worked but
as you can guess was an alignment nightmare. It was their backup plan if their
first choice failed(it did). The main problem that I see here is finding the
necessary beamsplitters at an affordable price and all the mechanical assembly
necessary. Also then the wavelength ( read temperature) of each diode would have
to be carefully controlled. Can you picture aligning up 50 5mW diodes in this
way to get a beam of about 2 watts?
Another solution may be to try to couple as many laser diodes into a optical
fiber as you can letting the fiber do the beam combining for you. You would
need to figure out how to position as many lasers beams as possible within the
acceptance angle of the fiber and minimize the coupling loss for each one.. So
that would be the limit of how many lasers could be coupled this way. One other
consideration would be to select the fiber core diameter such that the output
was a low order mode so you could get a relatively clean (minimum holes and side
lobes) beam out the output end of the fiber. The mechanical stability needed
for all the mounts and optics to do this is challenging but not impossible. I
have seen systems like this that sucessfully couple 10 watts into a fiber from
high power diode bars (using only 3 diode bars) These are used to pump solid
state lasers. One of these would be great but the cost could be close to your
house value or at least an expensive car!
SDL and other manufacturers also make laser diode bar packages that are
already fiber pigtailed with 10 to 15 watts (for pumping solid state lasers)
The price on these is only silghtly less than a new car as well. but maybe some
of these will show up on the surplus market some day.
I don't feel like I have helped much with this rambling. I think I have
a couple of " high power " laser diodes I'll have to find them and and there is
no promise that they work. A friend gave me some reject quasi CW laser diodes
that are supposed to produce about 1 watt peak power at a low duty cycle. They
run at about 800 nm. They would be happy running at a khz rate as long as the
pulses were short. These diodes are in "C" mounts which means that the bare
diode chip is mounted on a small ( approx. 5mmX7mm) gold plated heat sink with a
wire bond connection to the anode . This would have to be mounted on a much
larger heat sink(or preferably a temperature controled one) and protected from
the environment ( remember the bare chip is exposed). You would also need some
lenses to collect, circularize, and collimate the output. If this doesn't seem
too difficult and worth the effort to you let's talk further . I'd be willing to
let you have one for the cause if you thought you could use it for EME.
-- Richard KD6BQ
Date: Thu, 3 Apr 1997 15:11:54 -0800 (PST)
From: Walter Miller <aj6t at slip.net>
Reply-To: Walter Miller <aj6t at slip.net>
To: "Art Allen, KY1K" <aballen at colby.edu>
Cc: laser at berlioz.nsc.com
Subject: Re: stacking lasers
In-Reply-To: <199704031445.JAA19984 at host-04.colby.edu>
Message-Id: <Pine.GSO.3.95q.970403122816.9366A-100000 at slip-3.slip.net>
Mime-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: owner-laser at berlioz.nsc.com
Precedence: bulk
On Thu, 3 Apr 1997, Art Allen, KY1K wrote:
> Hi everyone...
>
> I'm not sure about stacking lasers-if we want to maintain the coherancy of
> the laser beam, then we need to make sure they are all on the exact same
> frequency.
>
> Even the HE-NE doesn't remain steady on 632.8 nm. There are some
> 'stabalized' lasers that are lab instruments, very expensive and out of our
> reach as far as cost.
>
> Melles Griot makes a line of these type of HE-NE.
>
> Obviously, if there is the slightest difference in output frequency, the
> beams will be non-coherant after a few miles at best. This is a problem.
>
> One should bear in mind that the coherancy of the laser beam is lost soon
> after it enters the atmosphere also-due to reflections, gravity and other
> atmospheric trauma. So, even a single HE-NE laser isn't coherant after a
> couple miles of atmosphere.
This really is not an issue for the type of communication work we amateurs
are doing. Our concern is basically how many photons/second are emitted
by the laser (or array of lasers), and what fraction of them is collected
on the surface of the detector. An important issue with stacking lasers
is the relative pointing of each beam...getting them aligned is not
trivial when the beamwidths are only in the milliradian range. We are
detecting only the total power incident on the detector, and the lasers
can be viewed as a source of streaming particles (photons).....kind of
like a machine gun shooting bullets. If you can get two machine guns
(lasers) with two separate trains of bullets to strike the same target,
your received "power" will go up by a factor of two (3 dB).
By the way, despite their reputation, lasers are notoriously
non-monochromatic. It is true that their fractional bandwidth (BW/F0) is
small, but the actual spectrum is almost always an array of separate lines
whose spacing is related to the size of the standing wave cavity formed by
the two laser mirrors (delta F=c/2L where L is the cavity length).
The relative amplitude and phases (and sometimes polarizations) of these
multiple lines are constantly changing as the lines compete with each
other, and as the cavity length changes.Even
"single frequency" lasers in which all modes except one are suppressed,
are not very stable with residual jitter in the kilohertz to Megahertz
range (due to effective cavity length modulation).
Since most amateur optical experiments do not involve heterodyning, most
of these considerations are not relevant. We are just interested in raw
power.
73, Walt
------------------------------------------------------------------------
Walter Miller, AJ6T Saratoga, CA USA CM87
Reply to aj6t at slip.net
Date: 3 Apr 1997 19:03:04 -0800
From: "Stone Richard" <Stone_Richard at mm.rdd.lmsc.lockheed.com>
Subject: RE: Combining Solid State Laser Diodes
To: "Neil Spokes" <spokes at mnsinc.com>
Cc: "laser" <laser at berlioz.nsc.com>
X-Mailer: Mail*Link SMTP-MS 3.0.2
Sender: owner-laser at berlioz.nsc.com
Precedence: bulk
I think that there is a slight misunderstanding here of this fundamental law of
physics. I believe that the law of conservation of brightness (related to
conservation of energy) or conservation of entendu is what you are refering to .
I believe it has to do with the conservation of brightness of single source
with optical transformations ( you can always loose energy but you cant gain it
without adding it in from somewhere) but does not specifcally prohibit the
combination of sources. I'm not sure so give me some time to look it up. Or
maybe someone else can help here. There are various ways to combine (or
separate) beams of light all based on what make them different. Different
frequency,different polarization, or different spatial properties. Maybe someone
else can help us out here. --Richard KD6BQ
Date: Fri, 04 Apr 1997 18:44:07 -0800
From: Neil Spokes <spokes at mnsinc.com>
X-Mailer: Mozilla 2.01 (Win16; U)
Mime-Version: 1.0
To: Richard Stone <Stone_Richard at mm.rdd.lmsc.lockheed.com>,
"John, K3PGP" <k3pgp at juno.com>
Cc: Laser <laser at berlioz.nsc.com>
Subject: Re: Combining Solid State Laser Diodes
References: <n1352017069.56639 at mm.rdd.lmsc.lockheed.com>
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: quoted-printable
X-Mime-Autoconverted: from 8bit to quoted-printable by news2.mnsinc.com id SAA24818
Sender: owner-laser at berlioz.nsc.com
Precedence: bulk
I can probably throw a little more info into the maelstrom - Neil, AB4YK:
`````````
Brightness e.g. of a black body, is measured in watts per [m2] of source,
per nm at the specific frequency per steradian. If the light intensity
of the original laser corresponds to an equivalent black body "temperature"
T kelvin then, unless you put an amplifier in line with the source or add
power from another frequency you are limited to the effective temperature
Tmax of the hottest laser. A dichroic mirror can be used to add power but
at another frequency, -- most of the dichroics that I have seen have a
transition band between high and low transmission of tens of nm, although
probably you can do better if you pay enough but then you would have to be
able to control your laser freq. I don't know anything about optical
transformation - sounds like black magic unless you have coherent light,
but that's not in the hopper of options.
Basically, if you try to combine light of two unpolarized lasers at the
same frequency using a 50 % reflecting mirror (or any other device) you
throw away 50 % of the light of laser 1 and 50 % of the light of laser 2
so that you have zero gain (at best). Maybe, if you could control the
polarization of a pair of lasers you could get a factor of two power
increase by combining them with a simple combiner. My understanding is
that the polarization of cheap solid state lasers is random, however.
NS.
>From laser at mailman.qth.net Sat Jan 25 11:19:23 2003
From: laser at mailman.qth.net (M0AYF)
Date: Sat, 25 Jan 2003 11:19:23 -0000
Subject: [Laser] How do you pulse laser diodes correctly?
Message-ID: <NFBBKDLAOLDOAAHCPNFFOEMDCAAA.m0ayf at thersgb.net>
I have been experimenting with pulse driving some surplus laser diodes from
range finders. I have no data on the diodes so have been "playing it by ear"
so to speak. So far I have pulsed the diodes with currents ranging from
100's of milliamps to several amps and with times ranging from a few 10's of
nS to about 1 microsecond. In all cases I have been able to detect light
being given off by the devices but have been unable to confirm it is laser
light.
I read somewhere that most laser diodes give out polarised light and in the
past have been able to confirm when C.W. IR laser diodes have reached the
lasing threshold by the presence of polarised light. I confirm the polarised
light by rotating a polarised filter in front of the laser diode and then
observe the change in light O/P. With CW diodes I use a CCTV camera/monitor
combination to observe what is happening. With the pulse diodes I have been
using a PIN photodiode (pointing at the laser) connected to an oscilloscope
and then rotating a polarised filter between the laser diode and photodiode
I look for a change in O/P signal from the photodiode. So far I have seen no
change in O/P amplitude and no evidence of laser light.
Is it possible to confirm laser light is being given off by the pulse diode
by this method or some other simple means? Not knowing the device
characteristics is it reasonable to increase the diode pulse current until I
see a definite threshold effect (as in CW diodes) in the light O/P and is
this likely to be the lasing threshold? Any advice or help would be most
welcome.
Regards and 73,s to all in the group.
Des (M0AYF)
>From TWOSIG at aol.com Mon Mar 14 01:21:20 2005
From: TWOSIG at aol.com (TWOSIG at aol.com)
Date: Mon Mar 14 01:24:21 2005
Subject: [Laser] Atmosphere turbulence bubbles
Message-ID: <1a6.33a643e2.2f668760 at aol.com>
Chris Long spoke of light beams collimated by large fresnel lens having an
advantage over small, presumably laser, beams when dealing with what he termed
"turbulence bubbles".
As I understand, the smaller the light beam, the smaller the bubble that is
likely to disrupt the communication path, and the more frequently the
disruption will occur. I think that puts a lower practical limit on the beam size
and total beam power that can be used to establish a communications channel
over a given distance with a minimum data rate. Otherwise, for a light
communication channel, you could use a large and powerful transmission beam with a
small aperture receiver, or a moderate beam with less power to a moderate
aperture receiver, or in theory a small transmit beam with low power and a
large aperture receiver. This disruption by the atmosphere will make a small
beam diameter channel un-reliable, even with increased power. ( And adding
power to a small beam, just makes it more of a hazard. ) More than just beam
size, the effect that Chris was discussing seems to put a laser at even more
of a disadvantage because its coherence leads to diffraction interference
that further disrupts the signal.
I discussed the idea with my brother, and he suggested two things. The
first was that there may also be an effect due to the polarization of a laser
beam. I am not sure what the polarization parameters of a gas laser or rod
laser, but laser diodes have (I am told) polarization that is similar to the
elliptical beam shape of the emitter. Why this would be important, I do not
understand. ( After spending the weekend with two grandsons, age 1 and 3, I am
sure that if he did explain it to me, I have forgotten. Wonderful things,
grandchildren. They remind me that it is not so important if I forget technical
stuff. )
Anyway, my brother's comments about polarization lead to the second
suggestion. That is to build a test rig to determine the effects of polarization and
spatial separation on an atmospheric optical communications channel. Start
with a laser diode emitter over a range that has channel disruption. Vary
its power and polarity to gather baseline data. Then add a second laser diode
emitter that has similar disruption characteristics for power and polarity.
Run the two of them at various separations, power settings, and
polarizations to see what the effects are. Maybe spacing two lasers 20 centimeters
apart, with 90 degree polarization differences will improve the channel
reliability. Then get creative and use four laser diodes. My brother suggested the
four equally spaced in a line. I think on the corners of a square would be
better. If somebody was really curious about the coherence effects, they could
get a four way beam splitter so the four beams could come from a common
source..........( Did I mention that my brother is a EE that used to be the test
engineer on an electronics productions line, and that I used to test ICBM and
satellite rocket motors for USAF. )
Ok, so I dream up a lot of elaborate testing. Maybe the idea of multiple
beams from a cluster of emitters is not worth the effort and cost compared to a
more powerful laser and a beam expander to get the hazard down while
compensating for atmospheric turbulence.
If somebody likes the idea, let us know how the testing turns out.
James
N5GUI
>From vocalion1928 at hotmail.com Fri Mar 18 06:52:34 2005
From: vocalion1928 at hotmail.com (Chris L)
Date: Fri Mar 18 06:56:10 2005
Subject: [Laser] References to effect of atmospheric turbulence on laser
comms.
Message-ID: <BAY18-F2690B4DE63F0E9DC7EA7B6D24A0 at phx.gbl>
Dear group,
These refs, though a little old, were assembled from practical research
undertaken in the days when atmospheric optical comms were promising far
more than they ever delivered - late 60s/early 70s. They stimulated
organisations like Bell labs to start work on optical fibres which (of
course) grew to be the major part of practical optical comms. However they
do elucidate and quantify the problems of transmitting coherent beams
through the atmosphere...
D L Fried: "Atmospheric modulation noise in an optical heterodyne receiver".
IEEE Journal of Quantum Electronics, vol QE-3, pps 213 - 221, June 1967.
"Optical heterodyne detection of an atmospherically distorted wave front".
Proceedings of IEEE, vol 55, pps 55 - 67, January 1967.
These references are also useful:
F E Goodwin: "A Review of Operational Laser Communication Systems", Proc
IEEE, vol 58, pps 1746 - 1752, October 1970. (Note that some of these
systems used polarisation modulation, a system relevant to a recent posting
on this website).
D L Begley: "Free Space Laser Communications" - SPIE Optical Engineering
Press, Bellingham, Washington USA, circa 1990.
Let me also quote from the classic early text "Laser Receivers" by Monte
Ross, one of the first textbooks in this field (John Wiley and Sons, New
York, 1966, page 125):
"...One of the practical problems of photomixing (heterodyne detection) is
that it requires a stable light source for the local oscillator and the
carrier. Except for the HeNe type and possibly the ruby laser, the present
laser or quasi-laser devices have too broad a spectrum to qualify... The
spectrum generated by a coherent GaAs diode which is not specially cooled is
about ... 3 by 10 to the 11th power Hz. Any modulation frequency less than
(this) number will be lost in the self beats and cross beats of the local
oscillator spectrum components and the carrier spectrum".
As I'm on a freemail service I'll provide the rest of this in a second
posting to follow,
With thanks,
Chris Long
>From toasty256 at yahoo.com Wed Feb 14 02:37:39 2007
From: toasty256 at yahoo.com (Tim Toast)
Date: Wed Feb 14 02:41:39 2007
Subject: [Laser] Re: fundmentals
Message-ID: <680815.89765.qm at web37907.mail.mud.yahoo.com>
hi James and Glenn
i just wanted to point out there IS another modulation
other than AM or OOK for light beam carriers. Polarization.
You can shift or rotate the polarization of any light beam,
be it coherent or not. It would use the principle of the
"polarizer-analyzer" setup:
http://plc.cwru.edu/tutorial/enhanced/files/lc/light/light.htm
One way you could do it would be:
At the transmitter you shine the light beam through a
polarizing filter. The filter is mounted so that it can be
mechanically rotated to change the polarization of the beam
through 360 degrees - actually spun via a motor at
different rates to encode the message say.. Similar to the
motor driven chopper modulations mentioned in here before.
Then at the receiver, in front of the photodetector, you
have a second polarizing filter that is stationary (the
analyzer) to 'decode' the beam. In the end, the beam is
converted into AM by the analyzer...(a nice smooth
sinewave)
at the frequency of the spinning filter of the transmitter.
http://www.aladal.net/toast/polarmod.jpg
Maybe polarization modulation has an advantage over AM in
some cases?? Good quality polarizers can nearly 100% the
beam, although some light is lost and absorbed by the
filter. Some filters are more efficient than others. While
searching through my list archive I noticed James proposed
some experiments with polarized light to see what effects
it had on a channel a couple years ago. (see march 2005)
There are also electronic ways to shift or rotate the
polarization of a beam that could work at much faster
speeds.
-toast
>From KY1K at verizon.net Wed Feb 14 09:20:19 2007
From: KY1K at verizon.net (Art)
Date: Wed Feb 14 09:24:30 2007
Subject: [Laser] Re: fundmentals
In-Reply-To: <680815.89765.qm at web37907.mail.mud.yahoo.com>
References: <680815.89765.qm at web37907.mail.mud.yahoo.com>
Message-ID: <7.0.1.0.0.20070214091545.035a7768 at verizon.net>
I'm not sure polarization is useful in free space, especially at
medium and long ranges. I think the atmosphere destroys the
polarization and shifts it wildly at random rates.
Perhaps it's more usable at shorter distances.
Maybe someone has tried this recently. There was a discussion on this
list many years ago about polarization filters, but I don't have the
full archive here.
Regards,
Art
>From aa6eg at hotmail.com Wed Feb 14 12:11:24 2007
From: aa6eg at hotmail.com (Pat Barthelow)
Date: Wed Feb 14 12:15:34 2007
Subject: [Laser] Re: fundmentals
Message-ID: <BAY103-F18F0F57C4DA69D8404175DFB970 at phx.gbl>
Re polarization rotation... It can be done electronically...
"Back in the Olden Days" , I was in school, learning how to use a
Geodimeter, Model 6.
(
http://www.gmat.unsw.edu.au/currentstudents/ug/projects/f_pall/html/e12.html
)
to precision measure long distances in Surveying. My ham radio interests
at the time, got me nominated by fellow students, to learn the electronic
innards of the Model 6. What I found was an RF transmitter, switchable, to
4 different, crystal oven stabilized frequencies, (between 10 and 30 mhz)
whose output was applied to the electrodes of a Kerr Cell modulator.
See Kerr Cell Stuff:
http://www.elec.gla.ac.uk/groups/opto/Kerr.html
http://en.wikipedia.org/wiki/Kerr_effect#AC_Kerr_effect
http://americanhistory.si.edu/collections/surveying/object.cfm?recordnumber=758694
http://home.earthlink.net/~jimlux/hv/eo.htm
http://www.star.le.ac.uk/~rw/courses/lect4313_fig57.jpg
That Model 6, used an ordinary, incandescent light bulb, and an internal
optical path that went through two polarized filters oriented at 90
degrees--NO light transmission, unless you rotate, using the Kerr cell, the
polarization 90 degrees in the transit between the two filters. The RF
applied to the Kerr Cell, a glass vial, with parallel windows, containing
liquid Nitrobenzene,
(explosive, under the right conditions) rotated the incoming polarization,
producing a sine wave amplitude out the second polarizing filter.
That sine wave amplitude modulated wave visible light beam went to and from
a reflector, up to 10 miles away, at night. (Later models use a laser, a
relatively simple upgrade) The return beam amplitude information, was
mixed with the outgoing beam, and adjustable electronic delays were
instituted, to create a phase cancellation between the incoming and outgoing
sine waves. Phase cancellation was detected, and carefully measured, using
a highly damped, center reading milliameter.
It was very tedious to do by the operator, but after about 15 readings of
the delay line, (half hour) and another half hour of paper calculations, you
came up with distance, accurate to about +-10ppm.
I remember seeing "noise" on the meter, and wondering if that was due to
random polarization changes, or simply amplitude changes due the variable
path losses.
So, I think it is probable that polarization of visible light beams is
maintained at least to a large degree during their travel through free
space.
Back to the present:
I recently took apart an early model visible laser bar code scanner device.
The laser beam passed through a small metal encased modulator (the metal can
was about 1/2" x 1/2" x 3 inches with power and a modulation signal
terminal, Two holes in the can allowed passage of a laser beam through the
modulating crystal, inside. It was a precision crystalline
device, some sort of digital shutter, or perhaps linear modulator. At the
local Silicon Valley Flea Markets, these laser scanners were plentiful, and
cheap; might be useful to ham/laser free space communications...
73, DX, de Pat AA6EG aa6eg at hotmail.com;
Skype: Sparky599
Moon or Bust!--Jamesburg Gang Rides Again!
>From KY1K at verizon.net Wed Feb 14 16:24:44 2007
From: KY1K at verizon.net (Art)
Date: Wed Feb 14 16:28:48 2007
Subject: [Laser] Re: fundmentals
In-Reply-To: <BAY103-F18F0F57C4DA69D8404175DFB970 at phx.gbl>
References: <BAY103-F18F0F57C4DA69D8404175DFB970 at phx.gbl>
Message-ID: <7.0.1.0.0.20070214155021.03283160 at verizon.net>
WOW, +/- 10 ppm is almost unbelievable.
I remember reading something similar once, possibly about the same
hardware. But, the author had a PC program and a programmable
calculator program to go with it.
At the time I wasn't sure it involved a Kerr Cell, and didn't know it
worked by polarization.
It was a long time ago and I never looked into it further. Not sure
about the nitrobenzine though-sounds a little shaky::>
Art
>From toasty256 at yahoo.com Thu Feb 15 17:55:34 2007
From: toasty256 at yahoo.com (Tim Toast)
Date: Thu Feb 15 17:59:45 2007
Subject: [Laser] polarization modulation
Message-ID: <376897.5504.qm at web37909.mail.mud.yahoo.com>
To avoid getting the other topic off too far, i made
another one if it gets any further discussion. "Coffee
grinder polarization modulation fundamentals" hi.
Maybe the atmosphere only mangles the polarization as much
as it does the amplitude, but it makes sense that it would
be effected as is phase and frequency.
A mechanical rotating polaroid filter at TX and matching
stationary analyzer at the RX is the simplest setup maybe.
But here are some electrical modulators that could be used
for pure polarization modulation experiments:
Kerr cell:
http://www.elec.gla.ac.uk/groups/opto/Kerr.html
http://en.wikipedia.org/wiki/Kerr_effect#AC_Kerr_effect
- scary toxic liquids and HV or RF required. Remove the
analyzer and use it in the receiver.
Modified Kerr cells:
- distilled water or other liquids may work at reduced
efficiency, remove analyzer from cell (or keep for AM
modulation)
Apparently most solids and liquids have a small Kerr
effect, with nitrobenzene being the most efficient. One
spin i had on the solid-block-of-glass original was; Using
a stack of clean microscope slides, like the common 1" x 3"
x 1 mm types, and use strips of aluminum foil interleaved
with them on the edges to provide electrical contacts. zap
it with HV pulses or HVDC to create polarization pulses or
dc shifts in the polarization. If this is not insulated
enough from the high voltage required, it may only spark or
'Flash' like a flash tube. Any homemade glass device would
probably require very high voltages to shift the
polarization enough. You would need 90 degrees of shift to
fully modulate the beam, but you might only get a few
degrees.. maybe not enough to be worth all the trouble.
Faraday rotation:
http://www.mmresearch.com/articles/article3/
http://en.wikipedia.org/wiki/Faraday_effect
Have any transparent magnetic materials? Lead-glass (flint)
apparently has enough metal in it to work, even though i
don't think lead is very magnetic.. Iron glass? Ferrite rod
glass? cheap plastic wrap?? who knows. :)
terbium gallium garnet (TGG) is the best optical material,
but i'm fresh out.. also it's only capable of about 45
degrees of rotation at best. How about a liquid solution of
finely powdered metal? etc.. Time to become alchemists
again.. Again as with the Kerr cell, remove the analyzer or
you will just produce AM.
Pockels effect:
http://en.wikipedia.org/wiki/Pockels_cell
QMR effect:
http://en.wikipedia.org/wiki/QMR_effect
magneto-optic effect:
http://en.wikipedia.org/wiki/Magneto-optic_effect
Zeeman effect:
http://en.wikipedia.org/wiki/Zeeman_effect
may be a form of true FM for laser beams? small shifts
though.
acousto-optic effect:
http://en.wikipedia.org/wiki/Acousto-optic_modulator
- might be modified for pure polarization modulation??
polarization in general:
http://en.wikipedia.org/wiki/Polarization
>From d29602960 at yahoo.co.uk Mon May 21 09:33:04 2007
From: d29602960 at yahoo.co.uk (Des)
Date: Mon May 21 09:41:41 2007
Subject: [Laser] RE: Beacon to test a photodetector sensitivity limit.
Message-ID: <641342.69540.qm at web27202.mail.ukl.yahoo.com>
Hi to everyone on the list.
Terry (W5TDM) wrote:
use optical methods to control intensity. Best methods
would be either very
small pin holes or ND filters, or both. Pin holes can
control down to
decade level, then fine tune level with ND filters.
**********************************
I never seem to get enough time to play with lasers
but I did get as far as building a modest laser beacon
to do some tests with. This was about 4 years ago and
used a 650nm laser pointer diode. For indoor tests
this was way to bright.
I wanted to do some general sensitivity tests within
the shack (without swamping my PGP receiver front end)
so I used a Polarizing Filter and aranged for it to be
rotatable. I think this appears in "The Laser
Cookbook" in some detail.
The front end was still "swamped" so I added a second
rotating polarizing filter, this reduced the O/P of
the 1mW laser to well below visible limits but still
detectabe with the PGP rx.
I used the polarizing filters from a couple of
obsolete LCD displays. I mounted the polarizing
filters in the center of a couple of old CD discs
(covering the spindle hole) the discs themselves are
epoxy glued to a couple of PL259 plug shells as
described in the cookbook. These screw onto matching
PL259 sockets (with the center connection drilled out
to let the light through) and this permits the
disc/filter to rotate. The large diameter of the CD
makes for easy rotation of the polarizing filters and
very small angular movements. To "stiffen" the threads
a little I fitted some soft springs between the CD
discs and the PL259 socket body.
The system was not calibrated in any way but it did
give very smooth attenuation over very wide limits.
I hope this helps someone, if not please excuse the
BW.
73,s to all on the list.
More information about the Laser
mailing list