[time-nuts] Laser Temperature System

Mike Monett XDE-L2G3 at myamail.com
Fri Aug 22 17:12:57 EDT 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



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