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[quote="Arturo"][quote="Dinesh"]The area of the exit beam - the diffracted beam - is the same as the coverage from the original laser beam on the table. My aim in this is to launch light at the edge of the plate and have a focused beam come out of the face. In other words, to act like a lens, but without light coming in from the other face of the lens. [/quote]

So, given a plate has four edges, you might (theoretically) use three for different colors and have a curious color combiner for your R, R & B lasers... ?!?!? (not to be taken seriously, you could write a whole encyclopedia with all I ignore about optics/holography, but this seemed right, once you overcome the technicalities ;) ).[/quote]
Funny you should say this. I have a student who wants to do an extended study for extra credit. She's a Physics major with a Visual Arts minor. This is one of the suggestions I made to her. The major problem is film sensitivity. I can make a hologram in red, with a red laser, and have it reconstruct in green. This is possible with film swelling, angular tricks etc etc. In these cases, you take advantage of the Bragg selectivity. But, in an edge lit, are the "fringes" Bragg fringes, or surface fringes. Is the diffraction Bragg or Raman-Nath? I don't know (yet). Also, I can't use my usual angular tricks for altering colours, since there's only one angle I can use - the (almost) TIR angle. Yes, I can alter the material so I can change the TIR angle, but not by that much. Standard commercial pan film has a fixed emulsion thickness on standard glass, so I can't use that. Bayer is too thin, it doesn't come on glass. So, I have to have a red sensitive dcg. Some years ago, I tried to find an additive for dcg that would make it red sensitive without dyes, because dyes make the film too insensitive. I tried a whole set of materials and thought of a whole lot more (including haemoglobin!). I got some promising results using copper salts and managed to redshift the sensitivity of dcg towards green with a combination of copper salts and potassium permanganate. In the end I had to drop it because I had to do the work for the "DCG as Resist Medium" paper for the Holopak conference. Perhaps I might pick it up again, because it also occurred to me that if you made three colours on the three edges, but were able to vary the reconstruction intensity electronically, you could do real-time colour mixing by simply varying the reconstruction beam intensity. I'm now playing with programmable leds to do just that.

As for the student, I've decided that this "edge lit" approach to full colour may be a bit difficult. If you're interested, I have three projects for her:
A computational method for objectively determining the colour of a reconstruction against the colour of the original object. In other words, give me a number, a "figure of merit" for a reconstructed hologram.
A way of getting rid of cross talk in full colour laser transmission holography
Making an image plane full colour reflection hologram from a full colour laser transmission. In other words, an H1/H2 transfer with three lasers.

[quote="Dinesh"]The area of the exit beam - the diffracted beam - is the same as the coverage from the original laser beam on the table. My aim in this is to launch light at the edge of the plate and have a focused beam come out of the face. In other words, to act like a lens, but without light coming in from the other face of the lens. [/quote]

So, given a plate has four edges, you might (theoretically) use three for different colors and have a curious color combiner for your R, R & B lasers... ?!?!? (not to be taken seriously, you could write a whole encyclopedia with all I ignore about optics/holography, but this seemed right, once you overcome the technicalities ;) ).

Gregg,
Thanks. No I hadn't seen that. I knew Ryder. When I say 'knew' in the past participle I only mean that I haven't seen him in over 12 years, so no idea what he's up to now. The last I heard, he was working for Boeing in Seattle. I wonder if they're still doing edge-lit at MIT?

Hi Dinesh -

I don't follow the field of edge-lit holograms, so you probably are aware of this already or maybe it's not relevant. The Spatial Imaging folks at the Media Lab used to publish in this area. For example do you already have Ryder Nesbitt's SM thesis? [url=https://www.google.com/search?q=Edgelit+Holography%3A+Extending+Size+and+Color]https://www.google.com/search?q=Edgelit+Holography%3A+Extending+Size+and+Color[/url]

g

[quote="BobH"]If you paint the back surface of the plate black, you'll absorb rays that hit that surface and keep them from reconstructing the hologram. Efficiency wouldn't decrease if it was from the evanescent wave.[/quote]Hadn't thought of that. Good idea.

[quote="BobH"]To get an evanescent wave, don't you have to introduce light at (or very very near) the critical angle? Grazing incidence will only reflect by TIR.[/quote]Yes,from the inside. Grazing incidence will internally reflect at 100%, so there'll be no reconstruction at all. None of the light will interact with the fringes at the surface. However, in order to get FTIR, you need to have the light come in along the surface at just the right angle, which I think of as very, very near the critical angle. I think there's a small difference between critical angle and grazing incidence, a small angle difference I mean, since conceptually, the difference is quite large. I think that some of the light from the led at the edge is undergoing TIR, but I think (hope!) that some of it is also undergoing FTIR. Then again, you have the Goos-Haenchen effect that shows that near TIR, there is a shift in the light that "shifts" some of the light into FTIR if the light is coherent and polarised. Since at such shallow angles any TIR would be close to 100%, I assume there cannot be any coupling and so there cannot be any diffraction. So, observing diffraction gives me an idea that I might be progressing along the right lines. I'm hoping that some of the rays from the led are undergoing FTIR, but perhaps not all.

However, for now, the holographic parameters are pretty daunting. How do you get a ratio when one beam is all the way inside the plate? What is the correct ratio? How do you measure the beam inside the glass? All of these questions are what we're trying to solve first. Also, the edge of commercially available glass is nowhere near the lambda/5 I need to prevent scattering on the surface of the edge. To get the kind of glass I need is possible, but expensive. So, I figure I'll try to latch down the holography first, then work on the physics.

If you paint the back surface of the plate black, you'll absorb rays that hit that surface and keep them from reconstructing the hologram. Efficiency wouldn't decrease if it was from the evanescent wave.

To get an evanescent wave, don't you have to introduce light at (or very very near) the critical angle? Grazing incidence will only reflect by TIR.

[quote="BobH"]Interesting Dinesh. You seem to be describing an evanescent wave being used to reconstruct the hologram. The light source used for reconstruction appears to be diverging at all angles going in to the edge. How do you know you're not reconstructing the hologram with light trapped inside the base, at angles greater than the critical angle, all splayed out by the source?[/quote]
Bob, I'm not sure. That is, I am trying to reconstruct using an evanescent wave, but I'm not sure that I'm actually completely doing it. As you mention, the led is diverging as it goes into the edge, so I'm not completely sure that some of the reconstruction isn't coming from an extremely glancing set or rays. However, at such large angles, there'll be quite a bit of internal reflection, so the hologram should be weak, which it isn't. I also looked at the back end of the hologram, where the reflected TIR would show as a glow on the surface, and I don't see anything.

So, I'm sure there is some of the internal reflection reconstruction going on, but I'm hoping that this also shows some evanescent reconstruction going on also. I'm hoping that the evanescent reconstruction is stronger than the TIR recon, but I need to do a few more tests and to develop methods to determine that the reconstruction is evanescent (or rather, that it's not TIR). This is the reason I'm not rushing to publish. The biggest problem for me is that there's no theory. So, I don't know what efficiency I should be getting, what the beam profile should be and what sort of aberrations I can expect. If I can show clearly that the beam parameters force it to be evanescent, rather than TIR, I'll have goine a long way towards what I'm trying to do. But, it's still early days.

The next set of tests will involve shooting a laser beam into the edge from a cylindrical lens. That is, launching a sheet of light into the edge. Hopefully, I will be able to see what the light is doing. (Very!) hopefully, I won't see any internal reflection at all, but still see diffraction.

Interesting Dinesh. You seem to be describing an evanescent wave being used to reconstruct the hologram. The light source used for reconstruction appears to be diverging at all angles going in to the edge. How do you know you're not reconstructing the hologram with light trapped inside the base, at angles greater than the critical angle, all splayed out by the source?

Mark
Thanks for the comments.

[quote="holomark"] I look forward to seeing an edge lit image planed hologram.[/quote]We've already done this. I'm not in the lab now (Saturday), but I'll put up pics on Monday.

[quote="holomark"]I wonder about the "coverage" when edge lighting and matching the dispersion of the object beam and edge lighting...
[/quote] In edge lit holograms, the entire face is covered by the reconstruction beam because the light enters the entire edge and runs along the entire face. The area of the exit beam - the diffracted beam - is the same as the coverage from the original laser beam on the table. My aim in this is to launch light at the edge of the plate and have a focused beam come out of the face. In other words, to act like a lens, but without light coming in from the other face of the lens. Roughly, the way it's supposed to work is to use an FTIR of sorts. When you launch light into a slab of glass, the light has three possibilities: it can enter the glass at one face and exit out the other (refraction), it can enter the glass and bounce around inside without exiting (total internal reflection), or it can "run" along the face of the glass and so neither refracting nor totally internal reflecting. This latter is called Frustrated Total Internal Reflection (FTIR) because the angle of entry is chosen such that the light "tries" to totally internally reflect, but is "frustrated" and can only run along the face. Now, if a hologram is shot such that the "fringes" are recorded along the face only, then the reconstruction beam is launched so that it undergoes FTIR, the light "running" along the face of the glass couples with the "fringes" on the face of the glass and will diffract out of the glass. I suppose one way to look at it is: "Holographically Resolved Frustrated Internal Reflection" (HRFTIR?). So, if I record the exact same fringes I would record to make a holographic lens, except I use this HRFTIR technique, I should be able to creat a lens in which there is effectively no input beam. The problem is that it's bloody difficult to create such a fringe system, because you have to be careful not to confuse HRFTIR with an extremely high, but standard, ref beam. This was the point of this first test. We placed the led inside the sleeve so as to ensure, as best we could, that it was indeed coupling with an FTIR and not a very large ref.

It's a first test, so we still have a ways to go before we have proved anything. But, it's a promising start, I think.

Dinesh,
congratulations on your progress. I look forward to seeing an edge lit image planed hologram.
I wonder about the "coverage" when edge lighting and matching the dispersion of the object beam and edge lighting...
The images you provided appear to show an expanding white light which does not reach portions of the plate (near the light). Are you looking at ways to prevent these blind spots? Using multiple light sources at different angles or using a more columated light source/setup?
Look forward to seeing more about this.
BTW, I think this sould go under General Holography, not off Topic.
Mark