2009 Vol. 2, No. 2
SiC has favorable physical and mechanical properties and it has become an important material for the large aperture substrates of space mirrors due to its excellent synthetical performance. The technologies of fabrication, manufacturing and surface modification of SiC mirrors have acted a more and more important role in the application of space optical systems. Combined with the engineering application, the characteristics of the SiC material in common use for space mirrors are analyzed and four kinds of fabricating methods are introduced. The current status of the application and the surface modifications of SiC mirrors at home and abroad are researched and the guide lines and the technics for the surface modification coatings are discussed in detail. According to the internal current status of engineering application, it isconsidered that developing high performance SiC substrate materials in a high speed and using the large aperture PVD equipment to prepare high quality SiC coatings for the surface modifications are the development direction in the future.
In order to meet the requirements of light weights, high specific strength and high specific moduli for a space payload, the advantages and disadvantages of the engineering materials used in the supports of the space payload were analyzed, and the feasibility of the application of the carbon/epoxy composites to the suppors structures was explored. The design methods of the carbon/epoxy composites were introduced by the actual application in space fields. Finally, a carbon/epoxy composite structure was designed and fabricated to support a 80 kg weight payload and to undergo both sine and random vibration tests. Experimental results indicate that the structure can resist the acceleration of the random vibration tests in a structural basic frequency of 103.6 Hz successfully, while the mass has reduced almost 30％ as compared with that of the structure made by Al alloy materials. It is shown that the carbon/epoxy composites can be used as the support structures of the space payloads.
A deployable space telescope with an aperture bellow one meter can be realized easily by using foldable multipod parallel structures based on tape springs to support the secondary mirror of a Cassegraintype telescope and by adjusting the structures on orbit. This paper introduces the structural features of the deployable space telescopes based on tape springs, and presents the research advances in the four techniques published in recent years for the space telescope precision deployments based on tape springs, namely, quadripod, octapod, tripod, and hexapod supports. It suggests that research on the precision deployments of space telescopes based on tape springs in future should put the focuses on the modelling and optimizing of deployment structures and the measurement and correction of the deployment deviation. Moreover, making an experiment in simulated gravity environments will be conductive to better research on the structure dynamics and deployment precision. Also, the development of an asymmetric deployable structure is a good choice for an offaxis telescope. Finally, it indicates that the improvement of deployment structures for space telescopes reported here will be a promising technique for highresolution remote sensing imaging and free space optical communications.
Space membrane reflectors have became promising mirrors for space science fields in recent years, for they are favorable properties in low area density, low producing costs and the reliability easy to fold and deploy. It is proved that the space membrane reflectors can meet the requirements of ultra lightweight and super size space mirrors. In this paper, the development of membrane reflectors is introduced, including the theoretical foundation of membrane reflectors, the representative achievements of inflatable membrane reflectors and electrostatic stretch membrane reflectors and other kinds of membrane reflectors. Then, it puts the focuses on the discussion about the production status of polyimide(PI) films for membrane reflectors, and the shape control and shape tests of membrane reflectors, also the design of supporting structures of reflectors. In addition, it analyzes the technical difficulty for developing the space membrane reflectors and summarizes the development, application and current status of the membrane reflectors. From analysis and discussion reported here, it is clear that the space membrane reflectors will be widely used in space telescopes, space surveillance cameras, artificial suns and microwave antennas in the space science fields in the coming decades.
High resolved imaging and superresolved imaging technologies are the hot topics in the electronicoptical tracking, optical remote sensing and the aerospace industry. With transforming an optical imaging system into an electronicoptical imaging one, how to improve the geometric resolution of CCD has become an important problem to develop the high resolution electronicoptical imaging system. In this paper, several methods to realize high resolved constructions are introduced based on theoretical analysis and engineering practice. Then, two kinds of image construction technologies, micro scanning and subpixel, are described, and their shortcomings are discussed in detail. Furthermore, to improve the band width by reconstruction without the spectral alias in supperresolution technologies, a new coding technology combining a optical encoding and the subpixel method is proposed. With the method, the bandwidth has been amplified by three times as compared with that of traditional methods. From what has been discussed above, it comes a conclusion that an optical imaging system combined optical, electronicoptical and signal processing technologies will be an important developing trend.
The flat 3D display technology is a novel virtual reality technology, in which people can see 3D stereoscopic images from a screen directly and do not require any device to be worn. To promote the development of the flat 3D display technology, this paper introduces the imaging principle of parallax pictures and some flat 3D display methods including barrier 3D displays, microlens 3D displays, polarizer 3D displays, multiview lenticular stereoscopic displays and so on. Then, it analyzes on the advantages and disadvantages about these methods. By taking the fourview autostereoscopic displays from Sanyo in Japan, multiview 3D displays from Nanjing University and the Horizontally Doubledensity Pixels(HDDPs) 3D displays in NEC for examples, it introduces the recent research and potential development, describes technique problems, and forecasts the application prospects for the flat 3D display technology.
Iron disilicide(β-FeSi2) is a promising material due to its advantages of abundant raw materials and environmentallyfriend. This paper introduces the luminescent mechanism of β-FeSi2, and discusses several kinds of preparing methods forβ-FeSi2 thin films. Then, the development and research achievements of β-FeSi2 thin films are described in detail. On the basis of achievements above and experiences by authors, it emphasizes that the good crystallized quality and the fewest possible crystal defect are the fundament for high effective luminescence, so that it is necessary for β-FeSi2 films to explore new preparing methods and to improve traditional growth technologies. Furthermore, doping new elements appropriately into the β-FeSi2 thin films and exciting p-i-n structures effectively for β-FeSi2 thin films will be the research focuses further. Finally, this paper reviews the general developing trends and prospects for β-FeSi2 thin films.
The photoelectric shaft encoder is a sensor to measure angles based on a code disc or a metrological grating disc. Recently, to meet the requirements of miniaturization, intelligence, integration of the photoelectric shaft encoders, a lot of research work are developed to reduce the sizes of photoelectric shaft encoders in many countries, especially in research on the coding modes. In this paper, the coding modes and coding theory are introduced. Then, based on explaining the coding modes in detail, the restricted factors of miniaturizing photoelectric shaft encoders are analyzed. The results indicate that effect factors on coding modes are the sizes of luminescent and receive elements. Finally, it discusses the developing trend of the coding modes of photoelectric shaft encoders and suggests that the variety of coding modes and miniaturization of code lines will be conductive to minimize the sizes of encoders and speed up the development of photoelectric shaft encoders.
In order to improve detection methods and detection efficiency for encoders, an automatic detection system for encoder errors is brought forward based on the analysis of encoder error detection status. The working principle and the design of hardwares and softwares for the proposed system are introduced in detail. Experimental results show that the automatic detection system can realize the positioning of measuring points, data acquisition and error analysis for encoders automatically by using motion control and all closedloop control in taking a 24 bit incremental encoder as the feedback element. It has the positioning accuracy of 2″ and can be used to detect the errors of all kinds of encoders less than 18 bits. In the experiment, the detection efficiency of the automatic detection system is 6 times as high as that of the standard device, which proves the feasibility of the project.
The no reference image quality evaluation is an important method in image processing technologies when the original image can not be obtained. In this paper, the basic problems in noreference image quality evaluation methods are discussed and several methods based on block effects, image blurs and interactive neural evolution used in noreference image quality evaluation are explained and analyzed. An overall noreference image quality evaluation principle is presented in accordance with the shortnesses above mentioned, and it suggests that the evaluation principle should be a synthetical index applying fuzzy measures and fuzzy integrals, incorporating Human Vision System(HVS) characteristics and adhering to the philosophy of quantifying quality by blind distortion measurements.
To extract the original features of a image exactly, a new image fusion method based on the curvelet transform is proposed. A series of original images are decomposed by the curvelet transform， and then a region image is obtained by an inverse curvelettransform and a domain processing. For the edges of high frequency areas, the edge distribution image is obtained by the activity of every area in different images and the region image is gotten by an interpolation. Finally, the image fusion is accomplished by Gaussian distribution sums for high frequency coefficients and by mean values for low frequency coefficients in the transform domain. A image fusion experiment is undertaken, and the experiment results indicate that the proposed method can obtain a better fusion image with high contrast, clear edges and more closed to a reference image as compared with that of conventional wavelet transform and pixelbased curvelet transform methods.
The transmission properties of the 1D Fibonacci sequence photonic crystals made up of dispersive SiO2 and TiO2 thin films in visible region are investigated by an eigen matrix method, and the results are compared with that without dispersion conditions. The comparison results show that the number of band gaps increase with the increases of the sequence terms. Moreover, the number of band gaps in transmission spectra from initial media in SiO2 thin films are higher than those of from initial media in TiO2 thin films. When the total physical length keeps a constant, the width of the band gap becomes narrower as the increase of the length of SiO2thin film, but the central wavelength of the band gap is a constant. Furthermore, the central wavelength of the band gap becomes shorter and the width of the band gap becomes narrower as the increase of the incident angle. When the dispersion is ignored, both the widest band gap and the central wavelength of the widest band gap have been increased. The conclusions can contribute to the study on the effect of dispersion on photonic crystals.
Two types of concentric optical systems used in imaging spectrometers, Offner convex grating imaging spectrometers and Dyson concave grating imaging spectrometers, are introduced. The Offner convex grating imaging spectrometers are characterized by their broadly spectrum band, small influence from environments and easy to be fabricated and assembled, and the Dyson concave grating imaging spectrometers have advantages in their small volumes and easy to realize the compact systems. The design examples of the above types are given, and the results show that both the imaging spectrometers can offer high quality optical modulation transfer functions and can reach the perfect results in distortion less than 0.005% and spectrum resolution less than 3 nm. Moreover, several kind of foreoptics systems matched with the imaging spectrometers are given and the methods to eliminate the stray light and to restrain the spectrum overlaps in the optical systems are discussed.
The characteristics of an Er3+/Yb3+co-doped phosphate glass waveguide laser are investigated on the basis of the working theory of a waveguide laser. Using the overlapping factor, the ratepropagation equations of the fourlevel system of the phosphate glass waveguide laser are simplified. Without considering amplified spontaneous emission, the relations between the laser outputs and the laser parameters are researched by numerical simulations and the relation curves of Er3+/Yb3+ ion concentration, pump powers and waveguide lengths with the laser powers are obtained. The result shows that selecting right Er3+/Yb3+ ion concentration is the key for the optical waveguide laser. The best fit results can be obtained from the following parameters in Er3+ ion concentration of 2.0×1026/m3, Yb3+/Er3+ concentration ratio of 7-10, and a waveguide length about 20 cm, when the pumping power is 80 mW at a wavelength of 980 nm.
A underwater laser localization method using cooperative targets was proposed to acquire the moving trajectories of underwater moving objectives to improve the tracking data precisely. By installing cooperative targets with total reflection characteristics on a measured objective, the angles and distances between the detection system and the objective could be measured by a pulselaser. The localization method and system structure for underwater detection were introduced and a digital model and measured results for the underwater laser localization were also given. Simulated results show that the proposed method is worth applying to engineering. According to engineering application, some practical problems about underwater laser localization to be solved were analyzed.