Horst Kiemle

Phase holograms in photographic emulsions for digital data storage

Holography offers a way to develop permanent computer memories with large capacities and short access times. We analysed the influence of imaging properties and statistical light scattering of phase holograms on their storage capacity and optical signal-to-noise ratio at readout. We found that above a certain value of reconstruction efficiency to scattering efficiency ratio the hologram store performs as if it were free from scattering. We measured this ratio for holograms bleached with mercuric chloride and potassium bichromate. In the first case it is below, and in the second above, the threshold. This shows that photographic emulsions allow production of holograms which fulfil the most important requirements of optical data storage.

Experiments on technology and performance of Lippmann-Bragg phase holograms

For many technical applications, Lippmann-Bragg phase holograms are interesting because of their wavelength selectivity and the theoretical possibility of obtaining reconstruction efficiencies near 100%. We performed experiments on such holograms using photographic emulsions. With commercially available plates having 7μm emulsion thickness we obtained reproducible efficiencies above 10% and very low light scattering, optimum exposure conditions to achieve this are described. The dependence of the reflection factor on wavelength and emulsion thickness is in agreement with theory. With our results an efficiency of 50~ can be predicted for the same emulsion but with a thickness of 20~m. This indicates that despite their high spatial frequency requirements, Lippmann-Bragg holograms can be superior to the more conventional transmission holograms.