Point Diffraction Interferometry

Abstract

The point diffraction interferometer (PDI) employs a point-diffraction spherical wavefront as ideal measurement reference, and it overcomes the accuracy limitation of reference optics in traditional interferometers. To overcome the limitation of measurement range either with pinhole (low light transmission) or with single-mode fiber (low NA), a single-mode fiber with narrowed exit aperture has been proposed to obtain the point-diffraction wavefront with both high NA and high power. It is a key issue to analyze the point-diffraction wavefront error in PDI, which determines the achievable accuracy of the system. The FDTD method based on the vector diffraction theory provides a powerful tool for the design and optimization of the PDI system. In addition, a high-precision method based on shearing interferometry can be applied to measure point-diffraction wavefront with high NA, in which a double-step calibration including three-dimensional coordinate reconstruction and symmetric lateral displacement compensation is used to calibrate the geometric aberration. The PDI is expected to be a powerful tool for high-precision optical testing. With the PDI method, a high accuracy with RMS value better than subnanometer can be obtained in the optical surface testing and submicron in the absolute three-dimensional coordinate measurement, demonstrating the feasibility and wide application foreground of PDI.

Testing optical surface form by a precise point diffraction interferometer D7

“…Figure 2. View of the whole D7 system: interferometer D7 (2), optical table (1), SLM laser 632.8 nm (3), optical accessory lens DA2 Ø250 mm (4), 6d stage for test parts positioning carries a flat mirror Ø100 mm; S_add is the additional fiber cable to align OPD between the test and reference beams of D7 for matching coherence length of the laser.…”

“…The need for metrology instruments with sufficient sensitivity and accuracy is increasingly important. A point diffraction phase shifting interferometer with the absolute reference produced by diffraction 1 can be used in a great variety of applicationsfrom measurements in scientific laboratories for experimental and exclusive high technology purposes 1,10 to testing irregularity and flatness of common optics: spherical and aspheric lenses and mirrors 2 , plane parallel plates, prisms, windows, etc. Testing common optics is often conditioned by easiness and high speed of measurement process due to mass production and required lower expenses.…”