Re: [ATM] Dumb Question about interferometers

[Mark Suchting <masuch@cia.com.au>]

Scott

You can have 4%/ 96%  ( uncoated ) splitter in the Ceravalo...you'll just 
see a dimmer image but it wont affect the contrast of the fringes. My first 
unit used an uncoated beamsplitter with two prisms glued together.   In my 
final unit , a polarising filter is used at the eye to adjust fringe 
contrast to isolate a discrete laser mode.  When contrast is adjusted , 
brightness seems about right. I would stick to the 50/50 design if you 
don't want to muck around later.

There is a vibrant Yahoo Group  `Interferometry' which would be a good 
place to get technical advice on these issues.

`Mark






At 02:52 PM 4/24/2004 -0700, Scott Rychnovsky wrote:
>I am starting to plan out an interferometer based on one of Peter 
>Ceravolo's reference elements and the recommended laser, etc.  My question 
>is about the beamsplitter.  Normally one uses a 50/50 beamsplitter, but is 
>this important?  Could it be 60/40 or 10/90? Part of my confusion is that 
>I don't really understand how it all works.
>
>In the recommended setup, the laser goes through a spreading lens (GRIN) 
>and then hits the beam splitter.  Half of the beam reflects and is 
>discarded.  The other half (50%?, 40%?, 90%?) travels through the 
>beamsplitter and them goes through the reference element.  4% of this beam 
>is reflected back along the axis.  the transmitted beam (96%) hits the 
>uncoated test optic and another 4% is reflected back along the path.  If I 
>understand it, these reflected beams are the only important ones in the 
>interferometer.  They hit the beamsplitter and are partially reflected out 
>of the interferometer and to the detector.  Interference between the 
>reflected reference element beam and the reflected test optic beam leads 
>to the fringe pattern, and all the other beams are just discarded.
>
>OK, here is the question:
>
>The intensity of the reference beam reflection is given by refbeam = BS-T 
>X 4% ref-R X BS-R.
>
>The intensity of the test optic reflection is given by testbeam = BS-T X 
>96% X test-R X 96% X BS-R.
>
>I have not thrown in every transmission number in the system, but did add 
>another 96%  (X2) to for the test beam because it goes through the 
>reference front surface twice.
>
>At the end all I need is for the two beams to have similar intensities so 
>they interfere effectively.
>
>The beamsplitter transmission and reflection each show up once for both 
>beams.  For a 50/50 BS that adds a factor of 0.25 to the intensity.  For a 
>33/67 beamsplitter it adds a factor of 0.22 to the intensity.  For an 8/92 
>beamsplitter it adds a factor of 0.074 to the intensity.  Certainly a BS 
>value close to 50/50 would be desirable, but it does not look at all 
>critical.  My conclusion is that a good quality beamsplitter is more 
>important than the precise R/T numbers.
>
>What do you think of this argument?  Am I missing anything?
>
>Here is my feeble attempt at ACSII artwork:
>
>               detector
>                  ||
>                  ||
>                 \||    ref beam  |               |
>                  \<--------------|<__test beam___|
>Laser ---->-------\------------->|-------------->|
>                    \             |               |
>                     \            |               |
>                   BS             ref          test
>
>
>Scott
>--
>Scott Rychnovsky
>srychnov@chem.ps.uci.edu
>ATM Page: http://tlepage.home.mindspring.com/
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