1,739 research outputs found

    Concept for advanced satellite communications and required technologies

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    The advanced communications technology satellite (ACTS) program of NASA is aimed at the development of high risk technologies that will enable exploiting higher frequency bands and techniques for improving frequency reuse. The technologies under development include multiple beam spacecraft antennas, on-board switching and processing, RF devices and components and advanced Earth stations. The program focus is on the Ka-band (30/20 GHz) as the implementing frequency since it has five times the bandwidth of either the C- or Ku-bands. However, the technology being developed is applicable to other frequency bands as well and will support a wide range of future communications systems required by NASA, other Government agencies and the commercial sector. An overview is presented of an operational 30/20 GHz satellite system that may evolve. How the system addresses service requirements is discussed, and the technology required and being developed is considered

    Lateral coupled cavity semiconductor laser

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    We report the fabrication and operation of a lateral coupled cavity semiconductor laser that consists of two phase-locked parallel lasers of different lengths and with separate electrical contacts. Mode selectivity that results from the interaction between the two supermodes is investigated experimentally. Frequency selectivity and tunability are obtained by controlling the current to each laser separately. Highly stable single mode operation is also demonstrated

    Frequency selectivity in laterally coupled semiconductor laser arrays

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    A longitudinal-mode analysis of a system of laterally coupled waveguided resonators is presented in the coupled-mode approximation. It is shown that variations in the mirror reflectivity of the individual channels result in coupling between the supermodes of the structure. This may lead to mode suppression by modulation of the threshold gain of different Fabry-Perot modes

    Tilted-mirror semiconductor lasers

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    Broad-area GaAs heterostructure lasers with a tilted mirror were demonstrated for the first time, with the tilted mirror fabricated by etching. These lasers operate in a smooth and stable single lateral mode with a high degree of spatial coherence. The suppression of filamentation manifests itself in a high degree of reproducibility in the near-field pattern

    Confocal unstable-resonator semiconductor laser

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    GaAs/GaAlAs heterostructure lasers with a monolithic confocal unstable resonator were demonstrated. The curved mirrors satisfying the confocal condition were fabricated by etching. Close to threshold, the lasers operate in a single lateral mode with a nearly collimated output beam. A single-lobe far-field intensity distribution as narrow as 1.90 full width at half maximum was measured

    Study of Apollo water impact. Volume 6 - User's manual - Interaction Final report

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    Computer program for hydroelastic responses of flexible shells of revolution during axially symmetric impact into incompressible fluids as in Apollo water impac

    Telecommunications forecast for ITU Region 2 to the year 1995

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    Telecommunications activity was studied. The primary objective was to forecast the need for fixed service satellites (FSS) by countries within ITU Region 2 excluding the United States and Greenland. Forecasts of telecommunications equipment needs were developed as a yardstick of the relative level of telecommunications activity among developing countries within the region. A likely scenario for the implementation of domestic and regional communications satellites is forecasted to provide services to and among countries in ITU Region 2. By 1995, it is forecast that 15 fixed service satellites will be implemented. A forecast of the countries requirements indicates that, with the possible exception of Canada, this constellation of satellites will meet these countries' needs to beyond the year 2000

    Unstable resonator cavity semiconductor lasers

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    GaAs heterostructure lasers with unstable resonator cavities were demonstrated for the first time with both curved mirrors fabricated by etching. Typical output powers of 0.35 W were observed in a stable, highly coherent lateral mode. The laser operated stably in a single longitudinal mode over a large range of injection currents. The external quantum efficiency was 70% of that of a similar laser with both mirror facets cleaved implying good output coupling of the energy from the entire region

    Modal properties of unstable resonator semiconductor lasers with a lateral waveguide

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    The modal properties of unstable resonator lasers with a lateral waveguide have been analyzed, and an unstable resonator semiconductor laser with a real index lateral waveguide has been demonstrated. Output powers in excess of 400 mW were observed with a stable, highly coherent lateral field distribution. The incorporation of a lateral real index waveguide with the unstable resonator configuration results in an increase in the external quantum efficiency and the appearance of ripples in the lateral field distribution

    A Policy Maker’s Guide to Designing Payments for Ecosystem Services

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    Over the past five years, there has been increasing interest around the globe in payment schemes for the provision of ecosystem services, such as water purification, carbon sequestration, flood control, etc. Written for an Asian Development Bank project in China, this report provides a user-friendly guide to designing payments for the provision of ecosystem services. Part I explains the different types of ecosystem services, different ways of assessing their value, and why they are traditionally under-protected by law and policy. This is followed by an analysis of when payments for services are a preferable approach to other policy instruments. Part II explains the design issues underlying payments for services. These include identification of the service as well as potential buyers and sellers, the level of service needed, payment timing, payment type, and risk allocation. Part II contains a detailed analysis of the different types of payment mechanisms, ranging from general subsidy and certification to mitigation and offset payments. Part III explores the challenges to designing a payment scheme. These include the ability to monitor service provision, secure property rights, perverse incentives, supporting institutions, and poverty alleviation
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