Bispectral analysis techniques provide us with both the theoretical and experimental insight into nonlinear wave process which can not be available from ordinary linear spectral analysis. Bicoherence spectra isolate the nonlinear phase coupling between the triads of Fourier modes. The degree of mode-mode coupling for period doubling sequence route to chaos of van der Pol-Duffing oscillator will be studied using both bispectrum and bicoherence.