Re: Generation of Ultrasound in the Megahertz region

On Mar 30, 11:06 pm, "Anthony Fremont" <spam-...@xxxxxxxxxxx> wrote:
John Larkin wrote:
I've always wondered how much a regular quartz crystal would scatter a
laser beam when driven hard. Some crystals have a quartz disc with
plated electrodes in the center and a clear rim, so you could un-can
one, shoot the laser through the clear part, and build a nasty
oscillator that drives it insanely hard. These crystals mostly run
shear mode.

This is a bit OT, but hey it's Friday

Many moons ago when I was a programmer, I worked in St Louis for a stock
brokerage company. They had these bad-ass laser printers that printed on
continuous forms paper at a rate of 30"/second. No matter what size fonts
or lines/inch, the paper always moved at the same speed. Amazing machine
(Siemens 0777 Laserdrucken (sp?)), especially the stacker. At any rate,
there was some documentation laying around and it included a theory of
operation. Well how could anyone resist looking at that. ;-)

Now to the point. The laser went thru a "lens" (might have been quartz, I
don't recall). The lens was somehow vibrated/modulated by several audio
range frequencys. This had the effect of splitting the beam into multiples
that, IIRC, came out parallel. By turning a tone on or off, the beam would
reciprocate at that position. This allowed the laser to scan the drum 6
times as fast since it was drawing with multiple beams. Pretty smart IMO.
It's been 20 years so please forgive my rusty memory on the details. :-)

The fuser/stacker was a seperate module so the paper travelled several feet
in the open with the print on the paper only being held in place by static
electricity. People couldn't resist not touching it when the door was open,
but don't touch the edge. Not many places can you get a paper cut that
results in stitches. I wish I had video of it in operation. The stacker
was just incredible. Paddle wheels, flappers and air puffing jets to coax
the continuous form paper into fan-folding back up. The key to getting the
stacker to work was to feed the first two pages thru the fuser pre-folded.
That way you could tell how it had to fold back up. After the fuser
"ironed" the perforations, you couldn't tell which way it originally went.
But the paper knew. ;-) It had just enough memory that if you didn't start
it correctly it would mess up every 100 pages or so.

Thank you all for the responses. This demonstration would not be
carried out in air. For precisely the reasons you cited. Sound does
travel about 5 times faster in water, about 1500 meters/sec. In fact,
I was thinking of using water, just because it is so abundant, and
behaves non linearly with light and sound together.

Yes, I know about Massa Corp. they are about 10 miles from where I
live. They build sonar and sonar imaging arrays, mostly for the Navy.

About quartz crystal. It is also a non linear medium. When a laser
hits it, depending on what angle it is to the crystal axis, it
exhibits birefringence (ie, 2 different indices of refraction), so you
will see 2 beams emerge. It's really cool to watch. 1 beam in , 2 come
out. They are polarized oppositely also, if they enter the crystal
unpolarized. Otherwise, they can become circularly polarized, or
elliptically polarized. When you modulate it at audio frequencies, all
kinds of cool stuff can happen. I never saw one get split into 6 beams
tho, but it wouldn't surprise me.

Fish finders usually use 2 frequencies, around 50Khz and 160Khz, but
that is not high enough. They would be waterproof at least.

The equation for the Bragg scattering has only the angle of
diffraction, the speed of sound, and the frequency in it. I don't
think amplitude makes much difference.

Maybe I will order some quartz crystals and take them apart. What's to
lose? They are cheap enough.

I think I'll mosey on over to the alt.sci.lasers forum. Maybe Sam or
someone over there has an idea.

Thanks again for the responses,

Joe


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