moire patterns caused by real-time materials

[Joe Farina]

Around 1977 MacQuigg described a fringe locking method using a "secondary" hologram used behind the hologram to be recorded, to generate moire-like fringes which could then be locked. The diagram from the first page is attached below.

About a decade later Frejlich (et al.) published a series of papers which described a very similar fringe-locking arrangement. The diagrams from a couple papers (including a review by Kaveh Bazargan) are attached below. Instead of using a secondary hologram, Frejlich relied on the real-time behavior of the recording material to "substitute" for the secondary hologram. Since the real-time material was building up a fringe structure as the exposure progressed, this fringe structure is capable of producing a moire-like fringe pattern (to be used for locking), when two beams are incident on that point on the emulsion (in transmission mode, as shown in the Bazargan and Frejlich papers). The recording material needs to have some real-time behavior (like DCG, polymers, or resists, but not silver halide).

I was wondering, has anyone seen such a moire pattern when doing (say for example) a transmission grating in DCG? I assume the moire pattern would be more evident when two un-spread beams (say a few millimeters in diameter) are striking a point on the emulsion (from the same side of the plate, in transmission mode). I am also wondering what effect the angle of separation (of the two beams) would have.

[BobH]

I have, with holographic PDLC material. I don't think it would be fun to use those fringes to lock the system because they don't exist at the beginning of the exposure. At some point during the growth of the fringes, the moire fringes will have enough contrast to lock, but until then the system is floating in the breeze.

[Joe Farina]

Thank you, Bob, at least that confirms there are moire fringes to begin with. I'm also skeptical about the lack of fringes in the beginning of the exposure, yet the authors claim that "The stabilization appears to be effective almost from the beginning of the recording in spite of the fact that the fully recorded real-time-effect hologram in our samples has a transmission diffraction efficiency as low as approximately 0.008%." The sensitivity of their detectors might have something to do with this, though.

[Din]

Well, you do have a chicken-and-egg situation.

It seems a reasonable assumption that the "interaction time", ie the rise of the modulation, is parabolic. Why? Because the energy of interaction is a function of the square of the E field, while it's the E field that causes the ionisations/ionic transfers necessary to create the latent image. Thus, assuming perfect recording (no variation od phase relationships), then the fringe modulation is proportional to t^2, ie a slow beginning, getting increasingly rapid. But, if the recording were perfect, you would have no need of this moire technique. So, as the recording proceeds, if we assume that the initial phase relationships are "wandering", the bandwidth increases as t^2, ie initially slowly then rapidly increasing. In such a situation, what is the moire locking on to? The (low) initial phase mismatch, or the (better) phase match over a longer period of time. In other words, is there an independant test of the efficacy of the moire technique. If you simply get a display hologram that's "bright", it may be that the initial phase mismatch is low enough that you end up with a bandwidth a slight bit higher than a perfect recording would have produced. But, if you judge the final result by eye, you won't notice tha slight increase in bandwidth. You need to see a little falsifiability thrown into experimental technique.

By the way, I like the "homemade developer" reference! You have no idea what the bandwidth of this "homemade developer" is.

[Joe Farina]

Thanks Dinesh. I made a pdf copy of a long paper regarding the self-stabilized technique, with the intention of posting it here on the forum (in the pdf input section). But the file size appears to have just exceeded the limit of 2MB. I emailed it to you, if you are interested.