The cascade effect produced by polariton in CAES spectra of crystals without center of inversion is discussed. An expression for total third-order susceptibility is derived. CARS and GSRS spectra of LiNbO3 and BNN crystals are measured with the non-collinear polarization configuration at some pumping wavelengths. By filting the experimental results with theoretical expression two-photon absorption coefficients of LiNbO3 crystal are obtained.