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We represent the double phase conjugation technique for uncorrelated complex optical signals at arbitrarily different (incommensurable) wave lengths for implementation of long-term (archive) storage, i.e. coupling and mutual associative reconstruction of such signals. The essence of the proposed approach consists in exploiting natural recording nonlinearity of a static hologram that results in formation of the combined (summation) pseudogratings corresponding to the quadratic component of the amplitude response of a static nonlinearly recorded hologram. In contrast to earlier (real-time holography) version of the double phase conjugation using photorefractive crystals, we use for the each signal wave collimated (plane) reference wave, so that the wave vectors of two reference waves are strictly opposite to each other. It is shown that under these conditions nonlinear mixing of two sets of cross-gratings results in formation of the complete set of pseudogratings constituting a second-order hologram defined, following H.J. Caulfield, as ‘a hologram between two (linear) holograms’. Being read out by any of two stored signals or its incomplete/distorted version (in absence of the reference waves, just as in a photorefractive prototype), a hologram reconstructs the phase-conjugate replica of the second signal (heteroassociative response) at the wavelength of the readout beam, with predictable on the wavelength ratio scaling and angular shift from the nominal position. Especial attention is paid to determination of the experimental conditions for providing the combination pseudogratings to be thin (by applying the Klein’s parameter), if even the partial cross-gratings are thick (volume), by proper choice of the angular conditions of the experiment. If this condition is violated, the Bragg selectivity can hinder heteroassociative reconstruction.
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