Improvement of the efficiency of parametric conversion of radiation frequency and multipass cells

The example of generation of the sum frequency in the case of a two-photon resonance of pump radiation is used to show that an increase in the efficiency of parametric frequency conversion by more than an order of magnitude is possible in multipass cells if the total phase advance of the interaction in one pass is a multiple of 2π. Then under optimal conditions the efficiency will be close to the theoretical limit.