Diffraction of light bv two parallel superposed supersonic waves,one being the second harmonic,the other the third harmonic of the same fundamental. Description of two methods leading to approximate solutions in finite form

The diffraction of light by superposed parallel ultrasonic waves, with frequency ratio 2:3, is solved by the NOA-method (N?th order approximation method). Explicit calculations of the intensities are given forN = 2. The same problem is also treated with the SA-method (method of successive approximations). The latter results are improved by iteration. In both methods the general symmetry property of the diffraction pattern is verified, namely, that symmetry with respect to the zero order occurs when the phase-difference of the supersonic waves δ = (2k + 1) π/4.