Quantum theory of successive nonlinear transformations

For the first time, nondegenerate processes are considered: sum harmonic generation and simultaneous generation of sum frequencies in periodically poled nonlinear crystals having quadratic nonlinearity. The results of the quantum description for long interaction lengths are obtained using the numerical method. Almost periodic oscillations of the average number of photons are detected. Strong correlation and anticorrelation between modes are found. Strong correlations can become good candidates for image restoration using computer processing of information. The effects of inversion of the mutual correlation of the modes can be found at certain interaction length.