Interference method of accurate determination of the relative magnitude of nonlinear optical coefficients

A method for accurate determination of the relative magnitude and sign of nonlinear optical (NLO) coefficients is described. It is based on the interference between optical second harmonic (SH) signals generated in two crystals and involves measurements of the maximum and minimum in the interference by changing the spacing between two crystals. This interference method allows one to determine more accurately the relative magnitude of NLO coefficients in the case that two SH signals are closer to each other, and may be especially suitable for observation of a small change of a NLO coefficient or a small difference between NLO coefficients. We have applied this method to measure the relative magnitude of NLO coefficients d14 and d36 of KDP, which are taken to be equal by Kleinman’s symmetry relation, and found that d14/d36=1.064±0.008 at the fundamental wavelength of 1.064 μm.