四孔径衍射屏在近场形成的光强分布及拓扑荷

Intensity Distribution and Topological Charge Generated by Four-Pinhole Aperture Diffraction Screens in Near-Field Region

利用基尔霍夫衍射理论计算模拟了四圆孔径衍射屏在菲涅耳深区形成的衍射光场的强度、零值线和相位的分布,发现衍射光场亮斑关于中心呈对称分布,在距离衍射屏较近的观察面上,光强值为零的点组成光强零值线段,该线段上光强等值线的离心率都接近或等于1,其两侧的光强值变化非常剧烈。复振幅的实部和虚部零值线多为封闭的曲线,零值线交叉点的个数为偶数,并且正负相位奇异点的个数相等。特殊相位奇异点周围的相位不仅呈对称分布,而且该点的拓扑荷的值近似为零。随着光波的传播,在不同的观察面上光强零值线段逐渐变短,最终趋于一点。

The Kirchhoff diffraction theory is applied to the four-pinhole aperture diffraction screen, so that the intensity, the zero contour of the real and imaginary parts of complex amplitude and the phase distribution in deep Fresnel diffraction region are simulated, and it is found that the bright spots in diffraction field show central symmetric distribution. When the observation plane closes to the diffraction screen, the zero-value points of light intensity can form line segment, on which the eccentricities of the light intensity isoline are close or equal to 1, and the intensity changes very fast on both sides of the zero line of light intensity. The zero contours of the real and imaginary parts of complex amplitude are closed curves. The number of intersection points of the zero contour is even, and positive and negative singularities are equal. Not only the phase around special phase singularities appears symmetric distribution, but also the topological charges of special phase singularities equals zero. With the propagation of the optical wave, the line segment of zero-value intensity changes to be shorter and shorter, finally to be a point.