Study On The Near-Field Diffraction Of A Small Aperture And Its Application

The light density has been directly explained the square of an absolute value in the traditional scalar diffraction theory, which has big localization . In fact, the traditional definition of the light intensity is only suit to paraxial-scalar diffraction light field, but is not well used for the non-paraxial scalar diffraction light field or vector diffraction light field. By taking the vectorial property of the time-average energy flow density into account, a new extensive definition of the light intensity is proposed. It is expressed as the time average of the amount of energy which crosses in a unit area and a unit time inside , so it can be related to the measurement of the light intensity.In this thesis the theory of the angular spectrum representation has been applied and the diffraction field of the plane aperture has been calculated and expressed the homogeneous and evanescent waves; Through integral of the energy flux density vector, the transmission coefficient of the plane wave and Gaussian beam of the small-aperture has been calculated; The energy transmission of near-axis diffraction has been approximately superseded by the energy flux density vector on the axis; As the use of the diffraction theory in the transmission field ,the accurate formula of the light intensity on the transverse plane perpendicular to the propagation axis has been applied and the propagation law of the second intensity moment is given for axially symmetric non- paraxial scalar beam. Base on it, the bean waist radius, the far-field divergence angle and the beam quality factor are obtained. When comparing to the tradition definition of the light intensity , weget some new results. For the diffraction of a small aperture, the scalar diffraction has been modified by the vector diffraction theory.