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We present a new method for the calculation of the dispersion diagrams of general nonuniform waveguides. The method is based on the spatial Fourier transform (SFT) of the electromagnetic field distribution along the guiding direction. We demonstrate the validity and robustness of the SFT technique using several test cases. As an application, we apply the method to analyze the dispersion of biperiodic photonic crystal waveguides and show that the guiding bandwidths of these waveguides can be significantly enhanced by a proper choice of the two distinctive spatial periods of the biperiodic waveguides.