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I need help explaining a strange observation

Posted: Wed Sep 22, 2010 1:17 pm
by rzeheb
Last night I shot a transmission master in preparation for doing an H1 to H2 copy. I used a Slavich VRP-M plate (4 by 5 inches), and a JDSU microgreen laser running at about 40 mW. The reference beam was filtered, collimated and illuminated the plate from above at close to Brewster's angle. The object was illuminated from both the left and right sides. The object was non-metalic and diffused the light fairly well so light should be coming off the object randomly polarized. I have used this setup before and gotten beautiful holograms. This time, however, when I put the developed and dried plate back into the plate holder the virtual image was not as clear as I expected. It was almost as though I was viewing it through a thin film or haze on the plate. When I flipped the plate around to get the real image it got even worse. The image was almost lost in a haze of "snow". Now here's the strange part: if I pivoted the plate slightly around its central axis (imagine a nail driven down into the center top edge of the plate), about 10 or 15 degrees *in either direction* (clockwise or counterclockwise) the "snow" disappeared and the image cleared up to a nice bright, sharp, detailed image!

I am at a loss to explain this. In theory, the image should have been best when illuminated with the reference beam coming in the same way it was originally shot (at least I think so?). I'm beginning to wonder if the plates I'm using have been slowly fogging in a way that strongly scatters the light if it hits at a particular angle but not if the angle is changed slightly either way??? Is that a reasonable explanation? Does anyone have any other ideas or suggestions they can offer? The plates *are* oldish. I purchased them at a discount because they were about a year past their "expiration" and I've been using them about 8 months so they are probably more than 2 years past expiration.

I need help explaining a strange observation

Posted: Wed Sep 22, 2010 1:25 pm
by BobH
Sounds like intermodulation noise to me. I'd raise the beam ratio (more reference light). Or you could put a polarizer in front of the recording plane to remove the unwanted state from the object light. One way you can test to see if this is the problem is to index-match a glass plate to the emulsion side of your hologram. If the noise is reduced significantly, that's your problem. Intermodulation fringes are big compared to the fringes of the recording, and cause rippling of the surface. Index-matching takes that away.

I need help explaining a strange observation

Posted: Wed Sep 22, 2010 1:45 pm
by dave battin
BobH wrote:Sounds like intermodulation noise to me. I'd raise the beam ratio (more reference light).
i agree with BobH, when you stated its was brighter off axis , that is where i would test first...................

if your setup has not been changed take readings in the area where you see the bright image, then compare it to the energy that you have in the center and see if your readings agree with this.

I need help explaining a strange observation

Posted: Wed Sep 22, 2010 4:07 pm
by rzeheb
Bingo! and Eureka! Intermodulation noise it is. :dance: Thank you Bob and Dave for setting me on the right path. :clap: So much to learn, so little time......

BTW, Dave, when are you going to visit me up in the Boston area? Any movement on having your work sold at the MIT museum of holography?

I need help explaining a strange observation

Posted: Thu Sep 23, 2010 11:30 am
by dave battin
rzeheb wrote: Any movement on having your work sold at the MIT museum of holography?
That project is on temporary hold, as a little blue diode got in the way ............. :wall: