moving wall demo, laser interferometer

[William Beaty <billb@ESKIMO.COM>]

Forgot to send this earlier private response to PHYS-L

On Tue, 27 Jun 2000, kyle forinash wrote:
> Can anyone give me a reference to a demonstration I saw a few years back?
> It was something along the lines of gluing a tiny mirror to a wall,
> shining a laser off the mirror, collecting the laser light with a
> sensor of some kind and turning the beam into sound (or some kind of

"Laser Audio Interferometer"  I tried this and it's simple.  The mirror
reflects the laser beam back down the bore of the HeNe tube, forming a
second optical cavity external to the laser.  A small photocell collects
some of the spill-light from the beam, and the photocell output is sent to
an audio amp and loudspeaker.  If you move the wall (or the laser), then
as the "cavity" changes length, it sweeps across hundreds of resonant
lengths located every 1/2 wavelength of the light.  The light intensity
sequentially grows dimmer and brighter, generating a sinewave signal.  The
frequency is proportional to the velocity of the wall.  Connect the
photocell to the microphone input of an audio system, and when the wall is
pushed, you hear moans and squeals.  It's like raking your fingernail
along the teeth of a comb, but with the teeth spaced every 300 nanometers!

Rather than using a fancy mirror with X-Y positioner, I just put the laser
on a cart, aimed its spot on the wall, then slapped a chip from a broken
mirror up on the wall with double-stick foam tape.  I then could move the
cart around until the reflected spot fell upon the laser's exit aperture.
My photocell was a tiny (3mm) square silicon from my junkbox, but I
suppose that any silicon photovoltaic cell would work.  I taped the
photocell to the front of the laser so it was very near the aperature and
facing outwards. The mirror chip wasn't perfectly clean, so lots of
scattered light surrounded the return beam, and the photocell received a
strong signal.

If you have a large photocell with a glass-smooth surface, you might
consider mounting it on the wall instead of the mirror.  Use the photocell
as a crude mirror, and bounce the reflected beam back into the laser's
aperature.  I found it more convenient to use a mirror, just because the
laser and audio amp could be placed on the same lab-cart.

I found that a concrete wall didn't respond (too stiff).  Wood or
sheetrock worked well.  I couldn't even approach the wheeled cart with the
laser, since the flexing of the wood floor was enough to generate all
kinds of squeals.  If you have only a concrete wall, then put the mirror
on a separate table, so that delicately touching the table will create all
kinds of squealing from the system.

I used a HeNe laser, and I don't know if a diode laser will work in this
application. You need a long coherence length.  The demonstration worked
well even with the mirror separated by several meters from the laser.  If
you only have laser pointers available, perhaps the demo will work if the
mirror is positioned within a few cm of the laser.

Here's a mystery.  I reasoned that the photocell might be seeing a MOVING
PATTERN of light, and not just a changing intensity.  I mounted a white
card on the front of the laser, with the beam shining through a small
hole.  Sure enough, when I bounced the beam from the distant mirror back
into the laser, I saw a bullseye-shaped interference pattern in the
scattered light coming back from the dirty mirror. Tiny motions of the
laser's cart made this pattern expand and contract. I've always wondered
about the origins of this pattern, and how the spacing between nodes might
be calculated.

PS, if your photocell is too large, it might "wash out" the signal by
receiving several fringes of that bullseye pattern.  Experiment by masking
down your photocell until it's only a couple of mm wide.  In hindsight I
see that I also could have put a narroband red filter on my photocell in
order to reject the 120Hz noise from flourescent lights (the AC noise
forced me to demonstrate the effect in a darkened room.)


((((((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))))
William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  http://www.amasci.com
EE/programmer/sci-exhibits          science projects, tesla, weird science
Seattle, WA   206-781-3320          freenrg-L taoshum-L vortex-L webhead-L