We consider implementation of coupled nonlinear optical processes in aperiodically poled nonlinear crystals which
can be produced by the method of the nonlinearity modulation superposition. The wave interaction consisting
of three three-frequency processes with multiple frequencies, is analyzed in detail. It is shown that under certain
conditions the superposition of waves, generated in this case, can form the light pulses with subfemtosecond
Paper contains experimental results and theoretical estimations of quasi-phase matched second harmonic generation
in PPLN, where the nonlinear grating vector is noncollinear to wave vectors of fundamental radiation and its second harmonic.
Accuracy of some approximate ultra-fast algorithms, specially developed for real-time solution of direct and inverse problems of diffusion optical tomography of extra-large (with size about 1000 scattering lengths and more) objects, will be considered.
We introduce the results of the study of quasi-phase matched processes of self-frequency conversion in periodically poled active nonlinear crystals. The processes of self-frequency doubling and self-frequency summing have been experimentally realized and studied in periodically poled Nd:Mg:LiNbO<sub>3</sub> crystals in CW and Q-switched regimes. The theory of self-frequency conversion has developed. The results of theory are in good agreement with experiments.
A new ultra-fast (increasing the calculation rate about 9 orders and more) calculation scheme, enabling one to solve small-angle multi-scattering problems for large (size about 100 scattering lengths and more) objects is described.
Fast approximate statistical nonlinear algorithm of the diffusion optical tomography inverse problem solution has been described. Results of this algorithm approbation by the example of reconstruction of a rather complicated internal structure of strongly scattering and weekly absorbing large- size model objects have been presented. CW radiation of low- power diode lasers of near IR region has been used in experimental realizations.