However, spacecraft navigation differs from ground tasks due to high reliability requirements but lack of large datasets. Meanwhile, deep learning has reached great success in different areas, especially in computer vision, which has also attracted the attention of space researchers. The development of on-board electronics systems has enabled the use of vision-based and LiDAR-based methods to achieve better performances. This review is believed to be helpful and educational for the progress and further development of machining of ultra-precision Fresnel lens structures for advanced optical applications.Īutonomous spacecraft relative navigation technology has been planned for and applied to many famous space missions. In addition, a discussion and analysis of the strengths and future areas of research interest of the reviewed techniques are conducted. This paper summarizes the existing fabrication techniques to generate these lens structures, excluding molding. Fresnel lenses and molds differ from each other in terms of lens design, work material and tool/workpiece geometries, and such variations will then drive the development of different machining and characterization methods in terms of tool setting, tool wear, tool path generation as well as lens measurement. Unfortunately, there is a lack of a comprehensive study to summarize and evaluate the recent advances of UPM techniques to generate these optical Fresnel lens structures. Thus, the process of molding Fresnel lenses are not substantially different from that of conventional molding processes for other optical components, still relying on the accuracies of the molds utilized. Precision forming processes, like molding and embossing, are usually used in mass production to replicate the optical features and profiles usually employ the use of lens molds generated by subtractive fabrication processes. In general, the processes to manufacture Fresnel lenses can be categorized into two distinct groups: subtractive fabrication and precision forming. These include the reduction in the lens thickness, smaller occupied volume and less material required compared to their conventional spherical and aspherical counterparts. These lenses are the most widely utilized diffractive optical element in the industry and has been extensively applied in various optical systems due to their distinct advantages. Non-linear effects might become observable by mixing synchrotron radiation with laser light.įresnel lenses are important compact optical components, which are comprised of multiple faceted concentric rings. Brightness is now sufficient to prepare spatially coherent X-ray beam with sufficient intensity for holography or the observation of coherency effects in the fluctuations of the light intensity. Synchrotron sources in the XUV region are now reaching the brightness which visible light had at the beginning of the sixties and the first soft X-ray lasers are available. fields which are only allowed by quantum theory but have no analogue in classical physics, is a more recent development. The generation of non-classical fields, i.e. By providing many photons in a single mode, non-linear effects, such as the generation of higher harmonics or sum and difference frequencies became possible. The laser offered the first possibility to observe easily interference effects between different, independent light sources. With the invention of the laser, fully coherent light became available, leading to further clarification of the coherence concept. Coherence was originally defined by the visibility of interference fringes, and the main effort of early work was to produce coherency from non-coherent, chaotic or thermal sources. The development of the concept of coherence of light and its application for experiments in the XUV range are reviewed.
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