We've already done this. I'm not in the lab now (Saturday), but I'll put up pics on Monday.holomark wrote: I look forward to seeing an edge lit image planed hologram.
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.holomark wrote:I wonder about the "coverage" when edge lighting and matching the dispersion of the object beam and edge lighting...
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.BobH wrote: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?
Hadn't thought of that. Good idea.BobH wrote: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.
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.BobH wrote: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.
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 technicalitiesDinesh wrote: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.
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