Satellite Constellation Design for Complex Coverage

Most traditional satellite constellation design methods are associated with a simple zonal or global, continuous or discontinuous coverage connected with a visibility of points on the Earth's surface. A new geometric approach for more complex coverage of a geographic region is proposed. Full and partial coverage of regions is considered. It implies that, at any time, the region is completely or partially within the instantaneous access area of a satellite of the constellation. The key idea of the method is a two-dimensional space application for maps of the satellite constellation and coverage requirements. The space dimensions are right ascension of ascending node and argument of latitude. Visibility requirements of each region can be presented as a polygon and satellite constellation as a uniform moving grid. At any time, at least one grid vertex must belong to the polygon. The optimal configuration of the satellite constellation corresponds to the maximum sparse grid. The method is suitable for continuous and discontinuous coverage. In the last case, a vertex belonging to the polygon should be examined with a revisit time. Examples of continuous coverage for a space communication network and of the United States are considered. Examples of discontinuous coverage are also presented.

High power continuous-wave operation of self-organized In(Ga)As/GaAs quantum dot lasers

Quantum dot (QD) lasers are expected to have superior properties over conventional quantum well lasers due to a delta-function like density of states resulting from three dimensional quantum confinements. QD lasers can only be realized till significant improvements in uniformity of QDs with free of defects and increasing QD density as well in recent years. In this paper, we first briefly give a review on the techniques for preparing QDs, and emphasis on strain induced self-organized quantum dot growth. Secondly, self-organized In(Ga)As/GaAs, InAlAs/GaAlAs and InAs/InAlAs Qds grown on both GaAs and InP substrates with different orientations by using MBE and the Stranski-Krastanow (SK) growth mode at our labs are presented. Under optimizing the growth conditions such as growth temperature, V/III ratio, the amount of InAs, InxGa1-xAs, InxAl1-xAs coverage, the composition x etc., controlling the thickness of the strained layers, for example, just slightly larger than the critical thickness and choosing the substrate orientation or patterned substrates as well, the sheet density of ODs can reach as high as 10(11) cm(-2), and the dot size distribution is controlled to be less than 10% (see Fig. 1). Those are very important to obtain the lower threshold current density (J(th)) of the QD Laser. How to improve the dot lateral ordering and the dot vertical alignment for realizing lasing from the ground states of the QDs and further reducing the Jth Of the QD lasers are also described in detail. Thirdly based on the optimization of the band engineering design for QD laser and the structure geometry and growth conditions of QDs, a 1W continuous-wave (cw) laser operation of a single composite sheet or vertically coupled In(Ga)As quantum dots in a GaAs matrix (see Fig. 2) and a larger than 10W semiconductor laser module consisted nineteen QD laser diodes are demonstrated. The lifetime of the QD laser with an emitting wavelength around 960nm and 0.613W cw operation at room temperature is over than 3000 hrs, at this point the output power was only reduced to 0.83db. This is the best result as we know at moment. Finally the future trends and perspectives of the QD laser are also discussed.

Kinetic model of continuous-wave flow chemical lasers

In this paper an analysis of the kinetic theory of the continuous-wave flow chemical lasers(CWFCL) is presented with emphasis being laid on the effects of inhomogeneous broadeningon CWFCL's performance. The results obtained are applicable to the case where laser fre-quency is either coincident or incoincident with that of the eenter of the line shape. This rela-tion has been,compared with that of the rate model in common use. These two models are almostidentical as the broadening parameter η is larger than 1. The smaller the value of η, thegreater the difference between the results of these two models will be. For fixed η, the dif-ferences between fhe results of the two models increase with the increase of the frequencyshift parameter ξ. When η is about less than 0.2. the kinetic model can predict exactly the in-homogeneous broadening effects,while the rate model cannot.

High-Strength Composite Fibers: Realizing True Potential of Carbon Nanotubes in Polymer Matrix through Continuous Reticulate Architecture and Molecular Level Couplings

Carbon nanotubes have unprecedented mechanical properties as defect-free nanoscale building blocks, but their potential has not been fully realized in composite materials due to weakness at the interfaces. Here we demonstrate that through load-transfer-favored three-dimensional architecture and molecular level couplings with polymer chains, true potential of CNTs can be realized in composites as Initially envisioned. Composite fibers with reticulate nanotube architectures show order of magnitude improvement in strength compared to randomly dispersed short CNT reinforced composites reported before. The molecular level couplings between nanotubes and polymer chains results in drastic differences in the properties of thermoset and thermoplastic composite fibers, which indicate that conventional macroscopic composite theory falls to explain the overall hybrid behavior at nanoscale.

Continuous radial superresolution filters with large focal depth

A new set of continuous superresolution filters is proposed which exhibits a radial superresolution performance with an extended depth of focus in an optical system by properly choosing the design parameters. Numerical simulation results of the performance parameters of the superresolution gain, the radial central core size, the Strehl ratio, the side-lobe factor and the depth of focus with different design parameters for the optimized patterns are displayed. We also give a design example for this kind of filter characterized by a birefringent element inserted between two parallel polarizers. This kind of filter would be useful in fields such as optical data storage systems.

Transverse superresolution with the radial continuous transmittance filter

The radial continuous transmittance filter is presented to realize transverse superresolution. It consists of two parallel polarizers and a radial birefringent element sandwiched between of them. By adjusting the angle between optical axis of the radial birefringent element and the polarization direction of the polarizers, transverse superresolution can be realized. But transverse superresolution is obtained at the cost of the axial resolution and the increase of the side-lobes in strength. So we then mend such filter, with it not only enhance the transverse resolution but also suppress the influence of the side-lobes and the reduction of the axial resolution. At the same time, the Strehl ratio increases. The advantage of such a filter used in superresolution technique is that it is easy to fabricate because its fabrication does not deal with the variation of the phase. (c) 2005 Elsevier GmbH. All rights reserved.

Compact continuous-wave blue lasers by direct frequency doubling of laser diodes with periodically poled lithium niobate waveguide crystals

A compact continuous-wave blue laser has been demonstrated by direct frequency doubling of a laser diode with a periodically poled lithium niobate (PPLN) waveguide crystal. The optimum PPLN temperature is near 28 degreesC, and the dependence of waveguide crystals on crystal temperature is less sensitive than that of bulk crystals. A total of 14.8 mW of 488-nm laser power has been achieved. (C) 2005 Optical Society of America.

Diode-pumped room-temperature continuous wave Tm3+ doped Lu2SiO5 laser

We report on room temperature laser actions of a novel thulium-doped crystal Tm center dot Lu2SiO5 (LSO) under diode pumping. An optical optical conversion efficiency of 12% and a slope efficiency of 21% were obtained with the maximum continuous wave (CW) output power of 0.67 W. The emission wavelengths of Tm LSO laser were centered at 2058.4 nm with bandwidth of similar to 13.6 nm.