Phase RMS gradient of the distorted wavefront for high power optical components

A distorted wavefront model is set up, according to the practical distorted wavefront of the optical components. In the calculation of phase RMS gradient, three different ways to handle the distorted wavefront are introduced.They are handling directly, Hanning window handling and the best one is the Hanning window handling, Fourier transformation, low-filter, inverse Fourier transformation, inverse Hanning window are accomplished in order after adding the zero-sampling points to the original distorted wavefront edges, finally a modelled distorted wavefront is cut off with the same size as the original one on the distorted wavefront gotten. RMS error of phase gradient of the wavefront can be the least. The correlations between the number of adding zero-sampling points,the size of wavefront