Kepler telescope was first invented in 1611, which has important applications in astronomy and optics. Apart from imaging in the optical field, it is one of the most classic structures of shear interferometry, which is widely used in wavefront detection, imaging, optical element quality detection, and adaptive optics because it is easy to design in a common-path setup, requires no standard reference waves and has strong system stability.

For radial-shearing interferometry, the most important thing is to divide the incident light into two beams with different sizes so as to achieve shearogram. The most widely used method is to adopt a loop structure composed of a telescope system. However, this structure often requires multiple optical devices such as mirrors, lenses and beam splitter. This makes it complicated and inconvenient to adjust when applied to different optical systems. Moreover, the difficulty in processing a large-aperture standard plane mirror and lens makes it unsuitable for large-aperture optical systems. Theon-ladder sieve is an amplitude-only diffractive lens and can be easily fabricated in large scale by current lithography.

Recently, researchers at Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, have proposed a new kind of telescope structure - Theon-Kepler bifocal telescope to realize radial-shearing interferometry in common-path setup. This research has been published in Applied Optics.

In the experiment, they chose LED as the light source, and used a multi-wavelength superposition method to replace multi-exposure recording of the monochrome beam by rotating a diffuser. When the test object was placed at the front long-focal plane of the first Theon-ladder sieve, the biplanar images were produced, and the responding lateral magnifications were -1.4142 and -0.7071, respectively. In this case, the incident wavefront was divided into two beams with different sizes by Theon-Kepler bifocal telescope, and the radial-shearing interference occurred.

They found that, although the background light affected the recording a little bit due to diffractive lens, the signal-to-noise ratio of shearogram was high enough to reconstruct the test wavefront. The simulation results consisting of a test wavefront and a peaks function under test showed that the high-precision reconstructed image could be realized from a frame of shearogram.

The proposed setup is not only a common path but also has the functionality of the cyclic radial shearing interferometry.

This work was supported by National Natural Science Foundation of China and Youth Innovation Promotion Association of the Chinese Academy of Sciences.

Fig. 1. The optical structure of Theon-Kepler bifocal telescope. (Image by SIOM)

Fig.2. Imaging experiments of bifocal telescope. (Image by SIOM)

Article website: https://doi.org/10.1364/AO.392574

Contact:
Mr. CAO Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: caoyong@siom.ac.cn