科研之友
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摘要
Objective Wavefront aberration is a crucial parameter for describing the properties of an imaging optical system. With the quick development of optical technology, the requirement for aberration quality of imaging systems is increasing. Phase-shifting point diffraction interferometer (PS/PDI) is a high-precision instrument for detecting wave aberration in an optical imaging system. A recent study from the Lawrence Berkeley National Laboratory demonstrated that the root means square (RMS) of PS/PDI detection accuracy could be better than 0.1 nm. The PS/PDI classical technique is to change the reference light's phase step by step and record the interferograms to achieve high-precision detection. However, because of its dependence on the external environment, the conventional phase-shifting measurement approach is susceptible to interference from ambient vibration and airflow. Thus, its application in high-precision detection in a complicated environment is challenging. In this paper, we report a polarization phase-shifting point diffraction interferometer that has the benefit of being insensitive to mechanical vibrations and air disturbances. We hope that our study will aid wavefront aberration detection in complicated scenarios with interference, including optical workshops and telephoto imaging systems. Methods A polarization phase-shifting point diffraction interferometer (PPS/DHPDI) was proposed in this study. The polarization synchronous phase-shifting technique was employed in double fiber point diffraction interferometry. A multi-longitudinal mode diode-pumped solid-state laser operating at 532 nm was employed. The coherence length of the laser was several centimeters. It produced two orthogonal linearly polarized lights from two beams. The two beams passed through the imaging system under test, and the object points were imaged on the image plane. The image mask comprised a measurement window and a diffraction pinhole. The measurement light was generated by one beam passing through the window, whereas the reference light was generated by the other beam passing through the diffraction pinhole. After the two beams passed through the mask, they passed through a 1/4 wave plate whose principal axis direction is 45° to the polarization direction of the two beams, and they became left and right circularly polarized light. Subsequently, they passed through a micro polarization array of the same size as a charge-coupled device (CCD) pixel unit, and the CCD collected a single interference pattern. Four phase-shifting interference images with fixed additional phase differences (0, π/2, π, and 3π/2) were achieved by sampling a combination of the single interference patterns. Two measurement modes, point diffraction, and a system error measurement mode were designed based on the interferometer. Two beams passed through the window form the point diffraction measurement mode. One beam passed through the window, and the other beam passed through the pinhole form the system error measurement mode. The measurement findings of the point diffraction measurement mode included geometrical path error of the test light and point diffraction light, system error introduced by the wave plate, and detector tilt error. These systematic errors can be calibrated quickly and conveniently using system error mode measurement. Results and Discussions A dual-fiber point diffraction interference system based on a polarization phase-shifting system was built for assessing the wavefront aberration of a 5X demagnification transmission projection objective lens. The measurement findings and experimental error were examined. With high vibration noise at low frequencies, 32 repeatability tests were conducted in a vibration environment. Among the 37 Zernike fitting coefficients, the measurement repeatability (RMS) of Z5 to Z9 was less than 0.5 nm. These findings reveal that the system has good vibration resistance and repeatability in a vibration environment. Conclusions We investigated a dual-fiber point diffraction interference approach based on polarization phase-shifting to measure the imaging system's wavefront aberration. Single-mode polarization-maintaining fiber is employed to generate ideal spherical waves whose polarization states are perpendicular to each other on the object plane. On the image plane, we employ a pinhole to produce the reference light. A 1/4 wave plate and a CCD camera with an integrated micro polarization array is employed to form and collect a single image and obtain four phase-shifting interferograms. The interferometer's antivibration performance is enhanced and realized in the high-precision real-time detection of the imaging system's wave aberration. The transmission microprojection objective lens verifies the validity of the detection technology proposed in this study.
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