How to record and read data using holograms
(a) Holographic storage of a single data bit. The spherical wave from a single pixel (the object shown in blue) interferes with a coherent plane wave (red) in the reference beam. The resulting interference pattern (green) changes the refractive properties of the photosensitive medium (yellow). (b) The hologram is read out using the original reference beam, which is diffracted by the interference pattern stored in the medium to reconstruct the original spherical wavefront. An image of this beam can be formed on a single detector pixel, resulting in the retrieval of a single bit. (c) The hologram can also be read out by illuminating it with a counter-propagating (or "phase-conjugate") reference beam. The phase-conjugate beam returns to its point of origin, where the bit value can be read without a high-quality imaging system. (d) A third way to retrieve data involves illumination with a diverging beam called an object beam, which reconstructs the original plane-wave reference beam. This beam can be focused onto a detector and provides an optical measurement of the correlation between the stored data and the object beam. This technique allows us to search the stored data according to its content, rather than its address.