Analysis And Performance Of A Picosecond Dye Laser Amplifier Chain

Design considerations are discussed for a simple, easy to use and relatively efficient high gain dye laser amplifier chain for CW mode-locked dye lasers. The amplifier boosts the output of a synchronously mode-locked dye laser to obtain ≈005 mj, ≤ 1 psec pulses over a ≈ 400 Å bandwidth. These pulses are suitable for efficient Raman Shifting, frequency mixing and continuum generation to vastly extend the spectral range of the system. Our amplifier is pumped by a frequency doubled Nd:YAG oscillator only, which longitudinally pumps three identical brewster cells with the same flowing dye solution in each. Contrary to popular belief, high small signal gains (≥ 105) are easily attained in a single stage with longitudinal pumping, with better beam homogeneity and easier alignment than transverse pumping. Gain saturation measurements are presented which agree well with calculations. Factors which relax the pump timing sensitivity are examined. The importance of gain saturation for both efficient amplification and for amplitude stability is also discussed. The need for isolated amplifier stages is stressed and optimal amplifier cell areas for a given stage are calculated

On the linewidth enhancement factor alpha in semiconductor injection lasers

A simple model for the linewidth enhancement factor alpha and its frequency dependence in semiconductor lasers is presented. Calculations based on this model are in reasonable agreement with experimental results

Graded collector heterojunction bipolar transistor

A graded collector heterojunction bipolar transistor is proposed. The graded collector improves device speed performance at high current densities by reducing the influence of the Kirk effect

Effect of spin-orbit interaction on heterojunction band discontinuities

The effect of spin-orbit interaction is included in the linear combination of atomic orbitals calculation of heterojunction band discontinuities. It is found that spin-orbit interaction is not negligible when the atomic number of the constituent atoms exceeds about 40. The effect of spin-orbit interaction as well as some interesting observations and their implications are briefly discussed

Internal photoemission from quantum well heterojunction superlattices by phononless free-carrier absorption

The possibility of phononless free-carrier absorption in quantum well heterojunction superlattices was investigated. Order of magnitude calculation showed that the absorption coefficient was significantly enhanced over the phonon-assisted process. Important aspects of the enhancement in the design of infrared photodetectors are discussed

Combining WMAP and SDSS Quasar Data on Reionization Constrains Cosmological Parameters and the Star Formation Efficiency

We present constraints on cosmological and star formation parameters based on combining observations of the Wilkinson Microwave Anisotropy Probe (WMAP) and high-redshift quasars from the Sloan Digital Sky Survey (SDSS). We use a semi-analytic model for reionization (Chiu and Ostriker 2000) that takes into account a number of important physical processes both within collapsing halos and in the intergalactic medium. Assuming that the efficiency of producing UV photons per baryon is constant, we derive a constraint of the form sigma_8 Omega_0^0.5~0.33 in a flat, Lambda-dominated universe with h=0.72, n=0.99, and Omega_b h^2=0.024. However, the calculated optical depth to electron scattering of tau_es~0.06 is well below the value found by WMAP of 0.17+/-(0.04~0.07) (Spergel et al 2003). Since the WMAP constraints on tau_es are somewhat degenerate with the value of the spectral index n, we then permit the primordial spectral index n to float and fix Omega_0 h^2=0.14, while normalizing the power spectrum using WMAP. In addition, we allow the UV-efficiency to have time-dependence. Combining the WMAP constraints with the quasar transmission data, our analysis then favors a model with tau_es=0.11^{+0.02}_{-0.03}, n=0.96^{+0.02}_{-0.03}$, implying sigma_8=0.83^{+0.03}_{-0.05} (95% confidence), and an effective UV-efficiency that was at least ~10x greater at z >> 6. These results indicate that the quasar and WMAP observations are consistent. If future observations confirm an optical depth to electron scattering tau_es~0.1, then it would appear that no more "exotic" sources of UV-photons, such as mini-quasars or AGNs, are necessary; but our analysis indicates that a determination of tau_es>~0.17 would require a more radical solution.Comment: 31 pages, 7 figures, 5 table

Sintered silicon nitrode recuperator fabrication

The preliminary design and a demonstration of the feasibility of fabricating submodules of an automotive Stirling engine recuperator for waste heat recovery at 370 C are described. Sinterable silicon nitride (Sialon) tubing and plates were fabricated by extrusion and hydrostatic pressing, respectively, suitable for demonstrating a potential method of constructing ceramic recuperator-type heat exchangers. These components were fired in nitrogen atmosphere to 1800 C without significant scale formation so that they can be used in the as-fired condition. A refractory glass composition (Al2O3 x 4.5 CaO.MgO x 11SiO2) was used to join and seal component parts by a brazing technique which formed strong recuperator submodules capable of withstanding repeated thermal cycling to 1370 C. The corrosion resistance of these materials to Na2SO4 + NaCl carbon mixtures was also assessed in atmospheres of air, hydrogen and CO2-N2-H2O mixtures at both 870 C and 1370 C for times to 1000 hours. No significant reaction was observed under any of these test conditions

Two-dimensional quasineutral description of particles and fields above discrete auroral arcs

Stationary hot and cool particle distributions in the auroral magnetosphere are modelled using adiabatic assumptions of particle motion in the presence of broad-scale electrostatic potential structure. The study has identified geometrical restrictions on the type of broadscale potential structure which can be supported by a multispecies plasma having specified sources and energies. Without energization of cool thermal ionospheric electrons, a substantial parallel potential drop cannot be supported down to altitudes of 2000 km or less. Observed upward-directed field-aligned currents must be closed by return currents along field lines which support little net potential drop. In such regions the plasma density appears significantly enhanced. Model details agree well with recent broad-scale implications of satellite observations

On the structures and mapping of auroral electrostatic potentials

The mapping of magnetospheric and ionospheric electric fields in a kinetic model of magnetospheric-ionospheric electrodynamic coupling proposed for the aurora is examined. One feature is the generalization of the kinetic current-potential relationship to the return current region (identified as a region where the parallel drop from magnetosphere to ionosphere is positive); such a return current always exists unless the ionosphere is electrically charged to grossly unphysical values. A coherent phenomenological picture of both the low energy return current and the high energy precipitation of an inverted-V is given. The mapping between magnetospheric and ionospheric electric fields is phrased in terms of a Green's function which acts as a filter, emphasizing magnetospheric latitudinal spatial scales of order (when mapped to the ionosphere) 50 to 150 km. This same length, when multiplied by electric fields just above the ionosphere, sets the scale for potential drops between the ionosphere and equatorial magnetosphere

Direct measurement of the carrier leakage in an InGaAsP/InP laser

Carrier leakage over the heterobarrier in an InGaAsP/InP laser is measured directly in a laser-bipolar-transistor structure. Experimental results indicate a significant amount of carrier leakage under normal laser operating conditions