Abstract: Based on the method of wavefront phase- factors and considering of the influence of limited aperture of grating, we analyze the diffraction fields by a circular cosine-grating which contain three components, i. e. , the plane diffraction wave, the divergence spherical diffraction wave and the convergence spherical diffraction wave, they are all limited by the circular aperture. The corresponding three integral expressions of the diffraction fields are given. Three formulae of the amplitude distribution of diffraction fields along axis are obtained and corresponding a series of curves are shown. Finally the transverse line-width and the axial line-width of that real focal-spot are obtained. This study may further provide a theoretical foundation to analyze the technique of the diffraction focalization and image formation by a circular cosine-grating.

Key words: cosine-grating, wavefront phase-factor, Fresnel diffraction, Fraunhofer diffraction