[en] Parametric down conversion in a strongly non-degenerate regime with output terahertz (THz) and optical beams is investigated. The theoretical approach based on the concept of independent Schmidt modes is developed and the shapes and weights of optical and THz Schmidt modes are analyzed both in a far-field and near-field regions. Analytical approximation for examined radial Schmidt modes is found. Strong azimuthal correlations between optical and THz photons are established. THz photons are shown to be involved simultaneously both in parametric down conversion and sum frequency generation processes. Photons in THz, Stocks and anti-Stocks channels are demonstrated to be strongly entangled. Methods to examine the properties of the THz radiation by measuring the correlations between generated Stocks and anti-Stocks components are suggested. (paper)