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 Spectroscopy  Zhu Yiming,
Shi Chenjun,
Wu Xu,
Peng Yan,
Abstract + Free to read the article if you have an account for login, or you need to pay. Based on the features of nonionization, noninvasiveness, high penetration, high resolution, and spectral fingerprinting of terahertz (THz) waves, terahertz spectroscopy has great potential in the biomedical field. Based on terahertz spectroscopy, combined with different analysis algorithms, different research groups have achieved qualitative and quantitative identification of mixture samples. However, actual biological mixture samples often comprise different components, including water, which results in poor spectral signal-to-noise ratio and large errors in the final spectral analysis results. For these problems, the use of noise reduction and reconstruction algorithms is effective solutions. These algorithms improve the signal-to-noise ratio of the spectrum by eliminating invalid information in the spectral data or extracting valid information. Finally, these algorithms can be combined with analysis algorithms to provide high-precision qualitative and quantitative identification of biological samples. In this paper, we discuss the main algorithms applied in terahertz spectroscopy over the past five years and summarize their advantages and disadvantages. Acta Optica Sinica, 2021, 41(1): 0130001
Geometric optics Yang Tong,
Duan Yingzhe,
Cheng Dewen,
Wang Yongtian,
Abstract + Free to read the article if you have an account for login, or you need to pay. Traditional spherical and aspherical surfaces offer limited degrees of freedom for optical system design. Freeform surface breaks the geometric constraints of rotational or translational symmetry. It can correct the aberrations in non-rotationally symmetric systems while decreasing the system size, mass, and number of elements in optical design. The system specifications, configurations, and functions which are difficult to be realized by traditional spherical or aspherical systems can be achieved by using freeform surfaces. The use of freeform surfaces not only offers great potential in the development of the optical design field, but also introduces new difficulties and challenges. This review briefly summarizes the current status of research on the freeform imaging system design. The commonly used types of freeform surface mathematical expression and the aberration theory of freeform imaging systems are demonstrated. The design methods of freeform imaging systems and the applications of freeform surface in various imaging systems are summarized. Finally, the future research directions of the freeform imaging system design are discussed and analyzed. Acta Optica Sinica, 2021, 41(1): 0108001
Instrumentation, measurement and metrology Zhu Rihong,
Sun Yue,
Shen Hua,
Abstract + Free to read the article if you have an account for login, or you need to pay. An optical freeform surface has been recognized as a revolutionary element in a modern optical system because of its powerful ability of simultaneous aberration correction and structural optimization. However, the complex shape of its surface brings enormous difficulties and challenges to the precise measurement, which limits its manufacture level and has been one of the bottlenecks of its broad applications. Currently, the main measurement methods of an optical freeform surface are mainly developed from the ideas of aspheric measurement technologies. In this paper, we reviewed the development of the optical freeform surface measurement methods, especially focused on several typical measurement methods and their characteristics, and looked forward to the development trend of freeform surface measurement in future. Acta Optica Sinica, 2021, 41(1): 0112001 Rao Ruizhong,
Abstract + Free to read the article if you have an account for login, or you need to pay. Based on published literatures about lunar laser ranging with three retro-reflectors landed on the Moon by the astronauts of Apollo 11, 14, and 15 missions in 1969—1972 and the two retro-reflectors landed on the Soviet roving vehicles in 1970—1971, some related physical and technical problems in lunar laser ranging are analyzed and discussed. These problems include: 1) the direction of the retro-reflector array and the accurate determination of its position on the lunar surface; 2) the identification of laser echo signals from the retro-reflector; 3) the intensity of receiving signals and the signal-to-noise ratio; 4) the precision of the lunar laser ranging results; 5) the difficulty of ranging in the full moon period; 6) the effect of the Earth''s atmosphere on lunar laser ranging and its precision. Some key technical details in the ranging loop have not been given in the published literature. There is a lack of direct physical evidences for the identification of signals as the laser back from retro-reflectors. The further analysis on the relationship between the lunar ranging precision claimed and the laser characteristics is needed. Acta Optica Sinica, 2021, 41(1): 0112002 |