The numerical aperture (NA) and power of diffraction wave in point-diffraction interferometer (PDI) could significantly limit the measurement range of the system. A fiber point-diffraction interferometer with high NA is proposed for the measurement of absolute three-dimensional coordinates. Based on the single-mode fiber with submicron aperture, the diffraction wave with both high NA and high power is obtained, by which the achievable measurement range of the PDI can be extended. A double-iterative method based on Levenbery-Marquardt algorithm is proposed to determine the three-dimensional coordinates under measurement. Numerical simulation and comparison experiments have been carried out to demonstrate the accuracy and feasibility of the proposed PDI system, with both high measurement precision and nice repeatability achieved.

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It is a key issue to measure the point-diffraction wavefront error, which determines the achievable accuracy of point-diffraction interferometer (PDI). A high-precision method based on shearing interferometry is proposed to measure submicron-aperture fiber point-diffraction wavefront with high numerical aperture (NA). To obtain the true shearing point-diffraction wavefront, a double-step calibration method based on three-dimensional coordinate reconstruction and symmetric lateral displacement compensation is proposed to calibrate the geometric aberration in the case of high NA and large lateral wavefront displacement. The calibration can be carried out without any prior knowledge about the system configuration parameters. With the true shearing wavefront, the differential Zernike polynomials fitting method is applied to reconstruct the point-diffraction wavefront. Numerical simulation and experiments have been carried out to demonstrate the accuracy and feasibility of the proposed measurement method, and a good measurement accuracy is achieved.

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