Comparison of Risk from Orbital Debris and Meteoroid Environment Models on the Extravehicular Mobility Unit (EMU)

A well-known hazard associated with exposure to the space environment is the risk of failure from an impact from a meteoroid and orbital debris (MMOD) particle. An extravehicular mobility unit (EMU) spacesuit impact during a US extravehicular activity (EVA) is of great concern as a large leak could prevent an astronaut from safely reaching the airlock in time resulting in a loss of life. A risk assessment is provided to the EVA office at the Johnson Space Center (JSC) by the Hypervelocity Impact Technology (HVIT) group prior to certification of readiness for each US EVA. Need to understand the effect of updated meteoroid and orbital debris environment models to EMU risk

High single-mode power conversion efficiency vertical-cavity top-surface-emitting lasers

Includes bibliographical references.We report advances in the power conversion (wall-plug) efficiency of vertical-cavity top-surface-emitting lasers. The devices were fabricated from molecular beam epitaxial layers using deep proton implants to define gain-guided lasers. The epitaxial structure included low resistance, piecewise linearly graded n-type and p-type mirrors, a triple In0.2Ga0.8As quantum-well active region, and a delta-doped contact layer. Power conversion efficiencies as high as 12.7% for continuous-wave single-mode operation were measured after several hours of device operation.This work was supported by the U.S. Department of Energy under Contract DE-AC04-76DP00789

Uniparabolic mirror grading for vertical cavity surface emitting lasers

Includes bibliographical references (page 607).We report details of mirror grading profiles for high efficiency vertical cavity surface emitting lasers. The mirrors provide low vertical resistance in conjunction with improvements in optical reflectivity, thermal conductivity, and lateral electrical conductivity in comparison to earlier grading profiles. The enhancement of these properties is verified by a comparison of thermal resistance and total electrical resistance for lasers of varying size.This work was supported by the United States Department of Energy under Contract No. DE-AC04-94AL85000

Data pattern dependence of VCSEL far-field distributions

Includes bibliographical references.The far field divergence angle distribution of vertical-cavity surface-emitting lasers is found to exhibit dependence on the data pattern driving the lasers. Two 50% duty cycle 1.25-Gb/s data patterns chosen to cause the same thermal conditions but with frequency content differing by a factor of 16 resulted in changes in the beam profile distributions with up to a 30% power variation in the central on-axis portion of the beam. Examination of the temporal waveforms as a function of far field angle revealed overshoot in the on-axis power and undershoot in the off-axis portion of the beam

Dielectric characteristics of spin-coated dielectric films using on-wafer parallel-plate capacitors at microwave frequencies

Includes bibliographical references.Dielectric properties of spin-coated dielectric insulators suitable for high-speed device fabrication are investigated. Complex dielectric permittivities and tangential losses of two polyimides, bisbenzocyclobutene (BCB), and a spin-on-glass (SOG) were extracted from the measured microwave reflection coefficient, S11, of parallel-plate capacitors over a frequency range of 50 MHz to 40 GHz. A model for the dielectric permittivity as a function of frequency is developed based on measured data with a minimum square error of less than 10-4 between measured and modeled microwave reflection coefficients. A circuit model for the pad capacitance is obtained based on geometrical and physical considerations. The relationship between the dielectric loss and its thickness is considered. Experimental results are fitted to Debye and Cole-Cole models.This work has been supported in part by the Defense Advanced Research Projects Agency under contract number DAAD19-03-1-0059 and by Yarmouk University in Irbid, Jordan

Different mirror, A

Includes bibliographical references.Mirrors grown in the crystalline structure ease manufacture of vertical-cavity lasers, which emit collimated circular beams and can form large two-dimensional arrays. The authors discuss the fabrication of the surface emitting laser mirrors. By means of techniques such as molecular beam epitaxy and metal-organic vapor phase epitaxy, hundreds of layers of semiconductor materials can be grown one on top of the other. By mixing and matching the materials to create "designer" alloys, it is possible to grow a crystalline structure with all the electrical and optical properties desired for its various parts. This method of tailoring semiconductor structures is called bandgap engineering. The principles of the mirrors and their applications are discussed.This work was sponsored by the U.S. Department of Energy under contract number DE-AC04-94AL85000

Squeezed light generated by a microcavity laser

Includes bibliographical references (page 3326).Photon-number fluctuations 1.3 dB below the semiclassical shot-noise limit are observed in the output of a semiconductor microcavity laser. Although the laser oscillates in a single longitudinal mode, photon-number squeezed light is realized through nonclassical correlations between two orthogonally polarized, transverse laser modes

Resonant-tunneling diodes with emitter prewells

Includes bibliographical references (page 1304).Resonant-tunneling diodes (RTDs) incorporating an emitter prewell structure are studied both theoretically and experimentally in order to investigate the utility of the emitter region as a device design parameter. The experiments show a tendency for peak bias, current, and the peak-to-valley ratio to increase for wider prewells, behavior likewise seen in both very simple and detailed calculations. Both the simple and more complete models point to interactions between states associated with the prewell and the main quantum well as the reasons for the increase in peak current. These results suggest design guidelines to affect peak bias, current, or the peak-to-valley ratio of RTDs

Differential carrier lifetime in oxide-confined vertical cavity lasers obtained from electrical impedance measurements

Includes bibliographical references (page 901).Differential carrier lifetime measurements were performed on index-guided oxide-confined vertical cavity surface emitting lasers operating at 980 nm. Lifetimes were extracted from laser impedance measurements at subthreshold currents, with device size as a parameter, using a simple small-signal model. The carrier lifetimes ranged from 21 ns at 9 µA, to about 1 ns at a bias close to threshold. For a 6 × 6 µm2 oxide aperture device the threshold carrier density was nth ~ 2 × 1018cm-3. The effect of carrier diffusion was also considered. An ambipolar diffusion coefficient of D ~ 11 cm2s-1 was obtained.Work at Texas Tech is supported by BMDO (monitored by Lou Lome), DARPA, and the J. F. Maddox Foundation

Buried refractive microlenses formed by selective oxidation of AlGaAs

Includes bibliographical references (page 1408).The authors demonstrate a novel method of fabricating buried refractive microlenses formed by selective oxidation of AlGaAs epitaxial layers on a GaAs substrate. By appropriate tailoring of the Al mole fraction in the vertical direction, a lens-shaped oxidation shape was achieved. Performance of the microlenses formed in this way was experimentally evaluated at 980nm, and modelled theoretically