Phase holograms

Phase Materials. Phase holograms can be recorded in a large variety of materials, the most popular of which are dichromated gelatin, photopolymers, thermoplastic materials, and photorefractive crystals. Dichromated gelatin and some photopolymers require wet processing, and thermoplastic materials require heat processing. Photorefractive crystals are unique in that they are considered to be real-time materials and require no after-exposure processing. 

AH attempts to develop photopolymers or photothermoplasts suitable for fast and reversible recording and read-out of volume-phase holograms, however, have not gained commercial appHcation. The most important characteristics of materials for holographic information recording are Hsted in Table 4 (158). 

For unslated thick phase hologram gratings (6), the strength of the grating is usually described by a parameter v, defined by... 

TABLE 2. Refractive Index Modulations Required to Achieve 100% Scattering Efficiencies in Phase Holograms of Various Thicknesses, and the Corresponding Approximate Angular and Spectral Bandwidths of the Holograms. The Angular Bandwidths are Measured in the Air Outside the Hologram. 

In the holographic scanner a series of phase holograms is recorded in a photopolymer film on a glass cylinder (Fig. 7). When the cylinder is rotated, the holograms cause an input beam to scan in the same way as a multifaceted prism would. However, because of the cylindrical symmetry of the device, it does not distort, even at high rotation rates. In addition, the holograms serve as lenses to focus the beam. The output beam quality is sufficient to resolve 1917 spots per scanning line at a rate of 7.76 x 10 spots per second (11). 

From eqs. 2 and 12 one can easily show that to record a phase hologram with 100% efficiency (v = ir/2) in a photochromlc material, one must convert an average of X cos0/2no molecules per unit area of the sample. Since at least one photon is required for each molecule converted, this means that for light at X = 500 nm, a material with np = 5 x 10 23 cm would require an exposure of at least 200 mJ/cm. In practice the quantum yields for pho-tochromic reactions are less than unity, and not all of the incident photons are absorbed by the active species, so the required exposures are likely to be several times this value. Materials with larger n0 s will require correspondingly smaller exposures, and vice versa. 

Fig. 1.7 CGH generated by thresholding the phase of FT (target). Left, binary phase hologram, right is the replay field generated from it...

H. Dammann, K. Gdrtler, High-efificiency in-line multiple imaging by means of multiple phase holograms. Opt. Commun. 3, 312-315 (1971)... 

Fig. 1. a - dependence of diffraction efficiency (q) on phase modulation amplitude (rpi) for volume phase transmission (curve 1) and reflection (curve 2) holograms amplitude-phase transmission hologram with absorption index yo = yi = 0.1 (curve 3). b,c - intensity distribution in diffracted (solid lines) and zero (dotted lines) beams at deviation from Bragg conditions ( ) at reconstruction of transmission phase hologram (b) and transmission amplitude-phase hologram (c) at yi = Yo = 0.1 with phase modulation 1 - q>i = 0.25n, 2 -cpi = 0.75n, 3 - q>i = 1.25n, 4 - qu = 1.75n. 

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