lobaz wrote: ↑Sat Jul 21, 2018 6:05 pm
By the way, I wonder you still call it Bragg selectivity. I always thought that Bragg must be volumetric.
Yes, it is a misnomer. Strictly speaking, it shouldn't be called "Bragg selectivity". The reconstruction of deep amplitude surface holograms becomes more and more selective for wavelengths and angles, the range over which diffraction occurs becomes smaller and smaller, due to the vector nature of the diffraction.. With Raman Nath diffraction, all light is diffracted, no matter what angle and wavelength. But, even Raman Nath may be a misnomer, because Raman Nath diffraction is a model of diffraction by acousto-optic crystals. Low frequency acoustic waves cause the light launched into the crystal to diffract by the Raman Nath model.
I think holography had to borrow a lot of words from previous diffraction studies because it came late into the game. By the time the theories of holography were being created in the 60's, diffraction theories were a century old. So, the new holography theories simply borrowed the words from previous theories of diffraction. Consider that Bragg diffraction
is associated with narrow band reconstruction of volume holograms. it's not
equivalent to Bragg diffraction, which assumes a crystalline structure of repeating molecular positions. The coupled wave theory explains narrow band reconstruction of volume holograms by means of a repeating set of planes. You can associate the repeating planes of coupled wave theory with the repeating molecules of a crystal structure, and so call the reconstruction of volume holograms as "Bragg" reconstruction. But, the original Bragg reconstruction is not reconstructing anything - the crystals were already there, no one 'recorded' the crystals.
However, strictly speaking, if you associate Bragg diffraction with the reconstruction of a volume hologram, then, a piece of quartz is a hologram, even if the display holographers will insist that a piece of quartz is "not a hologram".
My feeling is that language is a form of communication of ideas or concepts, whose meaning is extracted from a cultural milieu or bias. It should not be strictly enforced like a law governing where you can park your car. if both of us agree to call this thing "an apple", both of us know what you mean when you say, "I'm going to eat an apple". If both of us agree that this thing is 'a hologram' - whether it's a set of interference lines on a medium, or simply a 3D image, then both of us know what you mean when you say, "I'm going to see a hologram"