Cassegrainian/Gregorian-type null correctors for surface measurements of radio telescope reflectors

The (sub)millimeter wavelength radio observatory of the next generation will probably be an interferometer array of some 50 telescopes with parabolic reflectors 10-15 m in diameter. At this scale of mass production it is convenient to have at hand for workshop assembly a reflector surface measurement technique that is precise and easy to operate. We discuss the possibility of reflector measurements based on 10.6-mum CO(2) laser interferometry using Cassegrainian/Gregorian-type null correctors.     

High solar energy concentration with a Fresnel lens

The high solar energy density achieved in our simple and cheap Fresnel installation has been used for several surface modifications of metallic materials. This equipment is a very useful tool to apply concentrated solar energy in the field of high and very high temperatures (1500–2000 K). These temperatures are achieved in a few seconds and usually the materials treatments are completed in minutes. Fresnel lens installation is a serious alternative to the conventional equipment for material treatment and even to the large solar installations. In this work we review the surface modifications produced by concentration of solar energy with a Fresnel lens.     

可用作分光元件的二元菲涅耳透镜

根据菲涅耳衍射理论,采用微光学元件制作技术,通过三次套刻制作出八位相台阶的二元离轴菲涅耳透镜。这种微型光学元件同时具有色散分光和聚焦功能,用来作为分光元件时,具有较高的光谱分辨本领和衍射效率。实验测出其衍射效率大于68%。     

用于OCT动态聚焦的液晶菲涅尔波带透镜研究

提出了利用液晶菲涅尔波带透镜实现光学相干层析成像(OCT)动态聚焦的方法.根据扭曲相液晶空间光调制器(TN-LCSLM)的光学特性,设计了适用于OCT动态聚焦的菲涅耳波带透镜.利用TN-LCSLM型菲涅耳波带透镜进行了变焦控制实验,焦距实测结果与设计值比较吻合.此外,本文讨论了采用TN-LCSLM型菲涅耳波带透镜实施动态聚焦涉及到的一些问题.     

All-optical switchable holographic Fresnel lens based on azo-dye-doped polymer-dispersed liquid crystals

Fabrication of an all-optical switchable holographic liquid crystal (LC) Fresnel lens based on azo-dye-doped polymer-dispersed LCs is reported using a Michelson interferometer. It is found that, upon circularly polarized photoirradiation, the diffraction efficiency of the fabricated Fresnel lens was increased significantly in a reversible manner. We believe this is due to the anisotropy induced by reorientation of the LC molecules coupled with azo-dye molecule orientation due to trans-cis-trans photoisomerization, which modulates the refractive index of the LC-rich regions. We also studied the effect of azo dye on the polarization dependency of the fabricated lens.     

Multilevel-type multilayer X-ray lens (Fresnel zone plate) by sputter deposition

We have developed a multilevel-type multilayer X-ray lens (Fresnel zone plate, FZP) using sputter deposition. Such a multilevel-type FZP can attain high diffraction efficiency which results in the reduction of radiation damage to biological specimens and the simplification of the X-ray optical system because unwanted diffraction orders are considerably suppressed. Previously, we have reported the effectiveness (i.e., realizing high diffraction efficiency) of a multilevel multilayer FZP with 4-step structure. This paper presents two experimental results on the FZP: (1) the focusing test of a multilayer FZP with 6-step structure in order to realize higher efficiency and (2) the change over time of the focusing characteristic of the FZP with 4-step structure. For (1), the diffraction efficiency of the 1st order focus and the focused beam size measured by knife-edge scanning were found to be more than 50% and 0.8 μm, respectively, at 41.3 keV, and superior to the previous results. For (2), no significant differences have been observed: it can be said that the multilevel FZP is stable at least for 9 months.     

Electrically switchable holographic liquid crystal/polymer Fresnel lens using a Michelson interferometer

A holographic technique for fabricating an electrically switchable liquid crystal/polymer composite Fresnel lens is reported. A Michelson interferometer is used to produce the required Fresnel pattern, by placing a convex lens into one path of the interferometer. Simplicity of the method and the possibility of fabricating different focal length lenses in a single arrangement are advantages of the method. The performance of the fabricated lens was demonstrated and its electro-optical properties were investigated for its primary focal length.     

Experimental realization of the devil's vortex Fresnel lens with a programmable spatial light modulator

We present a unique method for experimentally generating multiple vortices by way of a devil's vortex lens combined with a Fresnel lens using a spatial light modulator. These lenses have the multifocal properties of fractal zone plates combined with the orbital angular momentum of a spiral phase plate and can be tailored to fit within a small space on an optical bench. Results are presented alongside numerical simulations, demonstrating the robust nature of both the experimental method and the predictive power of the Huygens-Fresnel wavelet theory.     

Design and evaluation of a null lens for testing the optical performance of silicone intraocular lenses

A two-element null lens, optimized for a 20-diopter silicone intraocular lens (IOL), was designed and tested. The performance of the null lens was examined for silicone IOL's with in situ powers from 16 to 24 diopters. Improvements in resolution, resolution efficiency, and modulation transfer function were obtained over the tested power range.     

Hierarchical-genetic-algorithm-based design of a large scale Fresnel lens for a reading light system with multiple LED sources

A conventional Fresnel lens is suitable to be used in the reading light system due to its features of directing and collecting light rays, as well as its properties of being essentially flat, plastic, lightweight, and cost efficient. However, it is not suitable for a reading light system with multiple light sources. To a reading light system with multiple light sources, a Fresnel lens with suitably designed groove angles can be used to improve the performance of that system in both illuminance and uniformity. Nevertheless, suitable groove angles are rather difficult to find if a Fresnel lens consists of a lot of groove angles and each angle covers a wide range of degrees. We develop a hierarchical genetic algorithm (HGA) to search for a set of optimal groove angles to design a Fresnel lens for a reading light system with multiple light sources from an enormous searching space. In addition, the groove angles of the Fresnel lens are directly derived from a conventional Fresnel lens database. The design goal is to maximize the illuminance and simultaneously maintain the uniformity of light rays incident to a specified reading surface. As a result, we can demonstrate that a HGA really works better than a genetic algorithm and the optimally designed Fresnel lens, indeed, offers a better light-guiding performance than a conventional Fresnel lens for a multiple-LED reading light system.     

Computation of Fresnel Integrals

This paper describes a method for spreadsheet computations of Fresnel integrals to six significant figures, based on successive improvements of known rational approximations which are accurate to only three figures. Outside the range of validity of the improved approximations, known series expansions are used to obtain the Fresnel integrals to six figures.     

Investigation of gaseous light-beam wavefront correctors as adaptive optics elements

The possibility of increasing the rate of variation of the optical parameters of gaseous focusing systems by directing a light beam perpendicular to the gas flow direction is investigated experimentally. It is shown that the rate of vartation of the focal length can reach considerable values close to those of adaptive optics. Academic Scientific Complex “A. V. Luikov Institute of Heat and Mass Transfer of the Academy of Sciences of Belarus”, Minsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 69, No. 4, pp. 568–572, July–August, 1996.     

Ray tracing of Fresnel systems

In this study based on an original program of aberration calculations and automated optical lens design we try to increase ray tracing of a Fresnel system so that the original program not only contains the initial functions but also satisfies the calculations of a Fresnel or mixed system (both general and Fresnel lenses).     

Demonstration of a Fresnel axicon

We design and manufacture a Fresnel axicon (fraxicon) that generates a quasi-diffraction-free/Bessel beam with a large depth of field. The novel optical element is characterized with both coherent and incoherent light, and its behavior is compared with that of a classical axicon. While the fraxicon exhibits a strong interference pattern in the on-axis intensity distribution, it can be smoothed out when using broadband light, partial spatial coherence light, or by period randomization. As inexpensive, compact/lightweight, and low-absorption elements, fraxicons may find applications in imaging, illumination, and situations where low absorption and dispersion are important.     

Fresnel diffraction at an opaque strip expressed by means of asymptotic representations of Fresnel integrals

The paper presents an asymptotic expression of relative intensity distribution in a Fresnel diffraction pattern at an opaque straight strip illuminated with a spherical wave. The asymptotic expression is used in an analysis showing an area of validity where the asymptotic expression reduces to an asymptotic expression of relative intensity distribution in a Fresnel diffraction at a half plane. The area of validity is defined through width of the geometrical shadow in a Fresnel diffraction pattern at an opaque straight strip and distance of a point under study to the center of the Fresnel diffraction pattern. Within this area, relative intensity in the Fresnel diffraction pattern at an opaque straight strip shows sinusoidal behavior, which can be used in easy location of maxima or minima of the relative intensity. The result of the analysis is supported by experiments realized in the area of validity and outside it.