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A semiconductor-optical-amplifier-based technique to generate the conjugate of an optical signal is presented. The original probe signal and its conjugate appear at opposite ends of the semiconductor optical amplifier, improving, therefore, existing techniques. The basic concept was proposed many years ago but, to the best of our knowledge, has never been experimentally verified. An explanation is given as to why this was not possible, what modifications render the idea practical are explained and experimental results that prove its feasibility are shown.

A bs t~ act-A space efficient fully parallel stochastic architecture is described in this paper. This stochastic architecture circumvents the main drawback of stochastic implementations of neural networks -the concurrent processing of a high number of weighed input signals, leading to a simple realization of stochastic summation. An unlimited number of stochastically coded pulse sequences can be added in parallel using only very simple and space efficient digital circuitry. Any neural network, either recurrent or feedforward, can be implemented using this scheme provided that neurons take discrete values. Design criteria are deduced from the mathematical analysys of the involved stochastic operations. Simulation results are also given.

A new photosensitivity physical model for Ge-doped silica preforms based on color-center photoreactions is presented. Simulation results are in close agreement with experimental results obtained by several condensed matter physics research groups working in this field, suggesting that the photoreactions of this model may, indeed, describe the physical processes involved in Ge-doped silica preform photosensitivity. The proposed photosensitivity model is defined by two differential equations that describe the temporal evolution of a set of color-center concentrations. The first is a modification of a very fast reversible reaction previously proposed by Fujimaki et al., where the reaction precursor has a different chemical structure (it is a neutral oxygen divacancy NODV unrelated to the previously proposed germanium lone pair center GLPC). The chemical structure of this precursor defect explains the generation of nonintrinsic neutral oxygen monovacancy ðNOMV Þ color centers. These centers are transformed into GeE 0 defects by means of a second nonlinear reaction. This justifies the slow increase in the absorption peak experimentally measured at 6.3 eV, which had no satisfactory explanation.