How to get circular fringes?

[admin_jsfisher]

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I tried to post the relevant pages from my copy, but it seems very difficult to post papers here on the forum, and the .pdf format is not allowed anymore.
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I'm not sure what problems you were encountering, but .PDF files are definitely permitted. It may have been the 256KB file size limit, I guess. I've upped that to 2MB.

I'll continue to look, though....

[Joe Farina]

I tried just now to post a 680k .pdf file in this section (and also in the pdf-input section) and got the message "The extension pdf is not allowed".

[Din]

Just for the record, I got the same message when I tried to upload a pdf.

[admin_jsfisher]

Well, that sucks and needs to be fixed.

[jsfisher]

Ok. I think the PDF upload problem is resolved. That system admin guy really ought to be fired.

[Joe Farina]

Thanks

[lobaz]

Ok, got it. Plane wave + spherical wave.

Hi,
yes, plane wave is a special case of a spherical wave where the radius of curvature = infinity. Actually, if your collimation is not perfect (which is usually the case), you get slightly converging or slightly diverging spherical waves; their radii of curvature are high and differ in the sign.

I think problem of your original setup is that there are too many unwanted interference patterns - from front/back surfaces of the lenses and from front/back surfaces of the flat beam splitter. That is why it is the best to use just pinhole to produce a spherical wave. The setup must be also very precise - if the beams leaving the beam splitter are not perfectly parallel, the center of the circular fringes moves quickly off and it is hard to find it.

Petr

[Joe Farina]

The concepts of plane and spherical waves were things I never really considered, thanks for your help and clarification. Yes, my plane wave was really a "weak" spherical wave (raw slightly diverging laser beam). After reading Kock's description of an "abstracted" hologram consisting of a plane wave interfering with 3 spherical waves (from pinholes in different positions in space), I really liked it. I think it's a good way to get a handle on the way a hologram works. A (roughly) plane wave interfering with a huge number of spherical waves (from various points on the object).

[Din]

I think it's a good way to get a handle on the way a hologram works. A (roughly) plane wave interfering with a huge number of spherical waves (from various points on the object).

This is a slide that Joy uses to show how a hologram works. This was a part of a presentation we gave to Dorkbot about 5 years ago

[Joe Farina]

I like that drawing because it's less abstract (than Kock's example of three pinhole-points in space). It also has three points: the cat's ear, nose, and butt. Also, the reference is shown as spherical (not a "pure plane" as in Kock). As Petr suggested, the reference beam should (I think) be considered as a spherical wave, even if collimated.

One thing I like about the Kock drawing (and description) is that it shows how the "zone plates" generated from the two bottom pinholes extend upward to overlap the zone plate generated from the top pinhole, and how the holographic plate is offset (towards the top). Even though the centers of the projected zone plates from the two lower pinholes are not located on the holographic plate, their outer rings overlap onto the plate. (This is clearly suggested, but not actually shown in the drawing.) But all three pinholes are still visible in the final holographic reconstruction.