by Din » Thu Jun 15, 2017 9:12 am
Petr, yes, some idea.
In a Bragg reflection hologram, the efficiency depends on the modulation, which depends on the ratio, and the hardness of the emulsion, assuming proper exposure and development. But, the planes are sinusoidal. This means that if it's overexposed, or overdeveloped, then the sinusoidal nature becomes more rectangular-wave. The exposure/development is non-linear. This gives rise to additional Fourier components. This means the output bandwidth broadens and flattens, since the output waveform is a Fourier transform of the plane distribution. So, I think of the area under the output curve as a measure of efficiency. The broader the bandwidth, and the flatter the output curve, the less efficient overall. This is a good thing for display holograms, since the broader the bandwidth, the brighter is the hologram (The brightness is the integral of the bandwidth multiplied by the photopic function, or, broadly, the area under the output curve). But, for HOEs, it wastes energy into less desirable wavelengths. So, for display, I try and make it broadband, and for HOEs, I try to make it narrowband.
The same holds for transmission holograms, but transmission holograms have one advantage over reflection holograms. They are dispersive. That means if, for some exposure and development, you get orders that diffract in different directions, then the fringes are non-linear. Energy is going into directions that you don't want. So, for a particular exposure/development scheme, I look for orders. Sometimes, in a resist hologram, I develop too aggressively, and I see third orders, and even dim fourth orders. I know I've developed too aggressively, and I dilute the etch.
In both cases, I try and think of the fringe/plane structure. I've found that almost everyone thinks of the hologram in terms of the output, be it display or HOE. I try and think of the Bragg planes (for a volume hologram) or the surface profiles (for a surface hologram) inside the emulsion and the photons interacting with the structure. I then try and think of boundaries. When the light gets into the emulsion, it starts electrons oscillating. If there is a sharp distinction between an interference bright area and an interference dark area, then you get a sharp change in the density of the emulsion and so a sharp change in the number of electrons at the barrier. I think this means that polarisation effects are beginning to happen, especially if the change is rapid and large. So, if these sharp boundaries border small changes in density, ie the amplitude of the rectangular wave is small, then I think there are no polarisation effects. But, if the sharp changes are high, ie the amplitude of the rectangular is high, then I think polarisation effects begin to happen. I think this is the basis of the Moharam and Gaylord paper.
Petr, yes, some idea.
In a Bragg reflection hologram, the efficiency depends on the modulation, which depends on the ratio, and the hardness of the emulsion, assuming proper exposure and development. But, the planes are sinusoidal. This means that if it's overexposed, or overdeveloped, then the sinusoidal nature becomes more rectangular-wave. The exposure/development is non-linear. This gives rise to additional Fourier components. This means the output bandwidth broadens and flattens, since the output waveform is a Fourier transform of the plane distribution. So, I think of the area under the output curve as a measure of efficiency. The broader the bandwidth, and the flatter the output curve, the less efficient overall. This is a good thing for display holograms, since the broader the bandwidth, the brighter is the hologram (The brightness is the integral of the bandwidth multiplied by the photopic function, or, broadly, the area under the output curve). But, for HOEs, it wastes energy into less desirable wavelengths. So, for display, I try and make it broadband, and for HOEs, I try to make it narrowband.
The same holds for transmission holograms, but transmission holograms have one advantage over reflection holograms. They are dispersive. That means if, for some exposure and development, you get orders that diffract in different directions, then the fringes are non-linear. Energy is going into directions that you don't want. So, for a particular exposure/development scheme, I look for orders. Sometimes, in a resist hologram, I develop too aggressively, and I see third orders, and even dim fourth orders. I know I've developed too aggressively, and I dilute the etch.
In both cases, I try and think of the fringe/plane structure. I've found that almost everyone thinks of the hologram in terms of the output, be it display or HOE. I try and think of the Bragg planes (for a volume hologram) or the surface profiles (for a surface hologram) inside the emulsion and the photons interacting with the structure. I then try and think of boundaries. When the light gets into the emulsion, it starts electrons oscillating. If there is a sharp distinction between an interference bright area and an interference dark area, then you get a sharp change in the density of the emulsion and so a sharp change in the number of electrons at the barrier. I think this means that polarisation effects are beginning to happen, especially if the change is rapid and large. So, if these sharp boundaries border small changes in density, ie the amplitude of the rectangular wave is small, then I think there are no polarisation effects. But, if the sharp changes are high, ie the amplitude of the rectangular is high, then I think polarisation effects begin to happen. I think this is the basis of the Moharam and Gaylord paper.