[time-nuts] Laser Temperature System
Mike Monett
XDE-L2G3 at myamail.com
Fri Aug 22 21:12:57 UTC 2008
WB6BNQ <wb6bnq at cox.net> wrote:
> WAS Re: time-nuts Restoring GR 1120-AB Frequency Standard
> Mike,
> I fail to see how you would apply the laser interferometer system
> to the measurement of temperature. At least as a viable means of
> temperature control to any reasonable degree.
> At the Navy's Primary Standards Lab, here in San Diego, there is a
> very complete laser interferometer measurement system. It is
> housed in its own room that is "completely" controlled for
> environmental conditions, i.e., temperature, humidity, air
> filtration, etc. The actual laser platform is a huge, highly
> machined micro-flat granite table of considerable weight mounted
> to a fine grain concrete pillar that goes down to the bedrock,
> some 30 feet below, and is completely separate from the rest of
> the building's foundation.
> When the operator enters this laser room, it takes a little over
> 30 minutes for his body heat to be stabilized into the matrix of
> the rooms environment before any real measurements can be made.
> Even though this granite table is, like in the neighborhood of,
> around 2000 lb., laying a standard US Navy glass coffee cup
> (empty) on the very, very corner of the granite will deform the
> light beam quite visually.
> My reason for the above description is to point out the scale of
> the installation, the sensitivities and the problematic issues one
> would have in trying to control temperature sans everything else
> changing. I suppose it can be reduced in size, but then doing so
> also reduces the sensitivity of the system. Did I mention that the
> granite table is 3 feet square ? That allows for folded beams to
> increase the length for the needed sensitivity.
> Now equate all of the above to a small oven in an instrument. Not
> likely possible.
> Bill. WB6BNQ
Hi Bill,
Thanks for the info. I am quite familiar with using laser
interferometers on granite slabs. I used them for precise
positioning to write servo tracks for hard disk drives, but mine
were a bit bigger than 3ft. I am also familiar with the original
Michleson-Morley interferometer. It was in the basement of a
building at MIT for many years. It used folded beams to increase the
patch length to measure motion through the ether. No motion was
found.
A laser interferometer doesn't need folded beams to increase the
sensitivity. It simply compares two light beams in a static
assembly. One beam is the reference, and the other is reflected from
the object.
Since the wavelength of red light is about 0.63 micron, the assembly
could be quite small, and could be on the order of a few
micrometers.
The original HP 5525A Laser Interferometer was introduced in 1971,
and gave a resolution of 1 um. The interferometer head that
contained the laser and electronics was 5" high, 7" wide, and 20.7"
long:
http://www.n4mw.com/hp5526/hp71.pdf
I recall setting it up to measure the length of a very thick I-beam
about 3 ft long. You could put a quarter in the middle and see the
bar shrink in length due to the droop from the extra weight. So
everything is made of rubber at micrometer dimensions, and you
really want to get the package as small as possible to minimize
errors due to stress and strain.
The Aerotech's LZR series laser interferometers are about the same
size as the original HP 5525A, but they can give a resolution of 0.3
nm. You can see a picture of how it is used for precise positioning
here:
http://www.aerotech.com/products/pdf/fg2000.pdf
Clearly, some care is needed to maintain constant temperature and
minimize vibration. But a room such as you describe is not needed.
Regarding miniaturization, the early Cesium Clocks were large enough
to fill a room. The HP HP5071A was quite a bit smaller, but still
fairly large:
http://tycho.usno.navy.mil/cesium.html
Now, NIST has shrunk a cesium clock to the size of a grain of rice:
http://tf.nist.gov/ofm/smallclock/index.htm
Since both technologies contain a somewhat similar modulated light
source, optics, and electronics, there is no reason to believe an
interferometer could not be made as small.
And it would make an outstanding temperature sensor!
Regards,
Mike Monett
More information about the Time-nuts_lists.febo.com
mailing list