Modern industries increasingly demand large-aperture, high numerical aperture (NA) optical systems, such as high-throughput lithography objectives, astronomical imaging systems, and inertial confinement fusion setups. Large-aperture optical components are widely used in these applications. For high-precision optical surface testing, point diffraction interferometers use microscale structures to produce near-ideal diffracted spherical waves, making them more suitable than Fizeau or Twyman-Green interferometers. Two common methods, dual-fiber point diffraction and reflective pinhole point diffraction, are used to increase NA. However, the dual-fiber method is limited by the NA of the fiber, while the reflective pinhole method achieves higher NA but requires separation of the test and reference beams, which restricts the aperture and NA of the test mirror. In addition, reflective pinhole interferometers place both the imaging path and the test mirror outside the main interferometer body, necessitating the alignment of both components, which increases the complexity and time required for setup. For large-aperture component testing, longer interferometric cavities are more susceptible to environmental disturbances such as air turbulence, affecting the reliability of conventional temporal phase-shifting methods (e.g., piezoelectric or wavelength shifting). In this paper, we propose a point diffraction transient interferometer for large-aperture optical surface testing, designed to increase the interferometer’s NA, enable precise large-aperture measurements, and mitigate environmental disturbances associated with long interferometric cavities. This approach aims to support the development of new large-aperture point diffraction interferometers.

In this paper, we integrate dual-fiber and pinhole point diffraction techniques, utilizing a 45° pinhole plate to generate reference and test beams with an NA of 0.5 from different directions of the same pinhole. Synchronous phase-shifting using spatial polarization is employed, where a polarization camera array captures four phase-shifted interference images in a single acquisition, enabling direct surface measurement. The telecentric imaging path design allows the interferometer to gather mid-to-low frequency surface information effectively. To address convex surface testing, the Hindle sphere method is used to convert convex surface measurement into concave surface measurement, simplifying the testing of large-aperture convex surfaces. A beam-splitting prism is also incorporated for alignment, ensuring precise positioning of the test mirror.

Using the same pinhole to generate independent reference and test beams, the design achieves a wavefront error root mean square (RMS) below 0.01λ within 500 μm of the pinhole at NA=0.5. Comparisons between circular and elliptical pinholes show that the wavefront error for elliptical pinholes is 2?3 times higher. Further analysis indicates the optimal minor axis length for elliptical pinholes is 0.6 μm, with minimal sensitivity to deformation and angle-of-incidence deviations, supporting the feasibility of the elliptical pinhole approach (Fig. 3). The polarization camera array, comprising four polarizers oriented at 0°, 45°, 90°, and 135°, enables single-shot four-step phase-shifting, reducing environmental effects such as air turbulence and vibrations. The telecentric imaging path, with a F-number of 0.0625 and a magnification of 1.75, achieves wavefront error RMS values of 0.0003λ at the center and 0.0012λ at the periphery of the field of view. This design, with its large depth of field and low distortion, achieves a resolution frequency of 16 mm^-1 for mid-frequency measurements and a negligible distortion rate of 0.0045% (Fig. 5). The Hindle sphere is used to measure a convex spherical surface with a diameter of 200 mm and a radius of curvature of 600 mm. The high-order aspheric, with an aperture of 409.8 mm, demonstrates a wave aberration RMS below 0.001λ, validating the potential of the proposed interferometer for large-aperture convex surface testing (Fig. 6). Furthermore, the alignment path, featuring switchable low- and high-magnification objectives, provides both coarse and fine adjustment capabilities. A field of view of 1.82 mm and an alignment accuracy better than 0.7 μm are achieved, ensuring practical and precise adjustment of the test mirror.

To address the requirements of large-aperture surface testing, we combine dual-fiber and pinhole point diffraction methods, generating NA=0.5 reference and test beams from a single pinhole positioned at a 45° angle. Using the spatial polarization synchronous phase-shifting method, the system captures four phase-shifted interference images in a single acquisition, enabling accurate surface measurements while mitigating environmental disturbances. The telecentric imaging path enhances mid-to-low frequency measurement capabilities, achieving a resolution of 16 mm^-1 and a distortion rate of 0.0045%, ensuring precise measurements. The Hindle sphere method transforms convex surface measurement into concave surface measurement, allowing point diffraction interferometers to effectively measure convex surfaces. In addition, an alignment path with a field of view of 1.82 mm and an accuracy of 0.7 μm enhances the precision of test mirror alignment. Consequently, the proposed point diffraction interferometer design fulfills diverse large-aperture surface measurement requirements, delivers accurate mid-to-low frequency data, and streamlines operation.