기하학적 위상 소자 기반 실시간 밀림 간섭계 연구

Abstract

Interferometry has been an indispensable tool in various fields of industry and academia because of high precision, well-establishing measurement algorithms, and careful considerations to minimize measurement errors. In optical testing, hence, the measurement by interferometry is also a fundamental approach and manifold research works have been reported. In this research, geometric phase components are introduced, relatively not familiar with optical metrology, and their applications are provided as shearing interferometers. Their attractive and interesting features have the capability to significantly improve the performances of interferometric metrology from the point of technical view such as snapshot measurement, immunity to environmental conditions, and minimization of the system. The geometric phase, i.e. Pancharatnam phase or Berry phase, is distinguished from the dynamic phase caused by the optical path length and can be generated by the polarization state change of the wave during its propagation. It can be easily observed in a polarizing optic system to obtain the phase shift by rotating a polarizer. Recently, the generation of the geometric phase is realized by a birefringent material like a liquid crystal, and 3D structures as meta-surfaces are fabricated on a thin glass substrate. Therefore, the geometric phase components have multi-functionalities of polarizing beam split, phase retardation, and diffraction. Among them, a geometric phase lens and a polarization grating are interesting for the implementation of shearing interferometers. In a geometric phase lens, the liquid crystal is arranged in the form of a phase zone plate, and one of the incident beams is converged (focused) and the other is diverged according to the polarization status. In the polarization grating, two diffracted beams are angularly separated with orthogonal polarization states each other. When the beams go through these components twice, a geometric phase lens can produce two radially sheared beams, and a polarization grating can generate two laterally sheared beams. Moreover, the polarization states of the output beam through each component are circularly orthogonal. The optical configuration of the radial shearing interferometer in this investigation can generate two radially-sheared wavefronts with a geometric phase lens pair, and immediately measure the wavefronts with the aid of a polarization camera, where a polarizer array with 0°, 45°, 90°, and 135° transmission axes, to obtain four phase-shifted interferograms from a single image. When a linearly polarized beam is incident to the geometric phase lens pair, two radially sheared beams are generated with the two orthogonal circular polarizations. Then, two circularly polarized beams are combined by the polarization array inside of the polarization camera, which can generate four phase-shifted interferograms. Then, the phase map is instantaneously calculated, and the original wavefront is obtained by the reconstruction algorithm. On the other hand, the proposed lateral shearing interferometer consists of a polarization grating, a mirror, and a polarization camera. The beam reflected off from the specimen is incident to the lateral shearing device, where the incident beam is split into two beams by the polarization grating, and the returning beams are laterally shifted after reflecting off the flat mirror and passing through the polarization grating again. These two beams are not only laterally shifted, but also their polarization states are orthogonal to each other as circular polarizations. Then, the polarization camera is put on the imaging sensor, can obtain four phase-shifted interferograms at once to calculate the phase map based on the spatial phase shifting technique. With a single image obtained by the polarization camera, the proposed lateral shearing interferometer can obtain the phase map corresponding to the x-sheared interferogram, and the other phase map can be calculated from another single image obtained by the 90° rotation of the shearing device. As the applications of proposed shearing interferometers based on geometric phase components, a dynamic wavefront sensor is proposed and experimentally verified. A deformable mirror was operated to generate various wavefronts, and they were measured by the radial shearing interferometer. For the measurement result comparison, a commercial Shack-Hartmann wavefront sensor was used. The surface figure measruement is also one of good application candidates for shearing interferometers. Similar to wavefront measurements, the radial shearing interferometer is capable of measuring surface figures of optical components, and even the measurement area can be extended by using various incident wavefronts. Furthermore, aspheric or freeform surfaces can be measured by the proposed lateral shearing interferometer. In the lateral shearing interferomter, two slopes of the wavefronts along x- and y-directions are obtained, and then the original wavefront corresponding to the surface figure of the specimen can be reconstructed via the wavefront reconstruction algorithms without any constraints such as an axial symmetry. In the experiment, the surface figures of several mirrors were measured and compared with other commercial devices. Furthermore, a compact snapshot optical roughness tester to estimate surface roughness by measuring its lateral surface gradient is proposed. From the wavefront corresponding to the measurement surface, the lateral shearing interferometric technique using a polarization grating can instantaneously obtain the lateral gradient without a reference surface and is configured as a roughness measuring sensor. In addition, an advanced roughness evaluation techniques with the partial integration method is applied to extract roughness parameters. To summarize, we proposed and experimentally verified shearing interferometers based on geometric phase components in this investigation. By the polarization nature of the geometric phase components, a polarization camera can directly obtain the phase map with two orthogonally polarized wavefronts. As the applications of the proposed interferometers, a dynamic wavefront sensor, a surface measurement tool and simple roughness tester were provided and experimentally verified to confirm the performances.