Аннотация:We investigate the shapes, phases and weights of the found Schmidt modes for the generated
parametric down conversion light in dependence on the separation of crystals and pump
characteristics. It is found that for parameters correspondent to most experiments with two crystals the usually used two-photon amplitude function can be significantly simplified. For high distances between crystals analytical expressions of the detected Schmidt modes are found, that are based on the mode structure of one crystal of the same size. It is shown that for the case when the output beam has zero intensity in the center, many radial modes can be obtained in addition to the large number of azimuthal modes. The separability
between radial and azimuthal variables is examined. Correlations of photons in the near field and far-field regions are analyzed. Methods to provide a selective amplification of a certain individual Schmidt mode based on the seeding by conventional Laguerre-Gauusian mode with rather simple spatial structure are suggested.