Sum-frequency generation in a multimode laser field

A theoretical analysis is made of sum-frequency generation in a multimode laser field allowing for aperture effects and diffraction. The problem is solved numerically using the method of successive approximations and allowance is made for a change in the fundamental-frequency field as a result of nonlinear interaction. The sum-frequency conversion efficiency (expressed in terms of the number of quanta) does not exceed 50% due to spatial modulation of the multimode radiation. A comparison is made with experimental results obtained previously.