Characterization of the pinhole diffraction based on the waveguide effect in a point diffraction interferometer.

The nearly ideal spherical wavefront generated by pinhole diffraction is the key factor determining the achievable accuracy in point diffraction interferometers (PDIs), as it is employed as the reference wavefront. A comprehensive characterization of the diffraction of a pinhole at the operating-wavelength scale that is normally adopted in PDI is given. The incident light is coupled into the pinhole, which functions as a cylindrical waveguide, and is diffracted in the end. The field in the pinhole is analyzed based on mode theory and the diffraction wave in the far field is derived from the field equivalence principle. The diffraction wave is characterized by the light transmittance, the polarization, and the wavefront aberration, which are all determined by the properties of the mode in the pinhole. The diameter of the pinhole should not be smaller than 0.6λ to make the transmittance sufficient. With a linearly polarized incident light, the diffraction wave is elliptically polarized, and the wavefront aberration is dominated by the astigmatic component. The method explicitly reveals the physical mechanism of pinhole diffraction. The analytic solutions are fast to compute, easy to analyze, and intuitively show the diffractive properties of the pinhole. The conclusions are significant for insight into the nature of pinhole diffraction and provide theoretical reference for analysis of numerical results and the design of PDI systems.