The impact of nonequilibrium gain in a spectral laser diode model

We describe the inclusion of nonequilibrium gain into a self-consistent 2.5D CW spectral laser diode model and report on the use of this model to investigate the origin of gain compression in a 975 nm high-brightness tapered QW laser diode. Nonequilibrium gain is calculated using a dynamic gain model, which simulates the dynamic relaxation of the quantum well carrier energy distributions under the influence of steady-state electrical and optical excitation. Calculated gain and spontaneous emission spectra are included in the laser model via parameterised look up tables. Both simulated and experimentally measured intracavity spontaneous emission spectra show an increased carrier density and a blue-shift of the gain maximum with increasing bias caused by carrier heating and spectral hole burning. The accurate incorporation of nonequilibrium gain compression is therefore vital for the accurate prediction of the operating characteristics of these devices and for the experimental determination of the active region temperature.     

Electron-impact fragmentation of aliphatic polynitro compounds

The mass spectra of two series of aliphatic polynitro compounds are reported and discussed. The fragmentation patterns of aliphatic nitro and polynitro compounds are similar in that no appreciable molecular ion current is observed; however, there are several other features in the fragmentation of aliphatic polynitro compounds which differ from that of nitroalkane spectra. Both series of compounds studied-C(NO₂)_x(CH₃)_4?x, where x = 4 to 0 and C₂(NO₂)_x(CH₃)_6?x, where x = 6,4,2-show a decrease in the number and intensity of alkylions with an increase in the NO⁺ and NO₂⁺ ion current as x increases. The main ions resulting from the more nitrated compounds are [NO]⁺, [NO₂]⁺, [CO₂]^+. and [CH₃CO]⁺, whose noncharged counterparts are the principal species produced in the detonation of these compounds. This similarity of the products of the two processes suggests the use of mass spectroscopy for the investigation of the initial explosive processes. The principal fragmentation pathways of the polynitroalkanes have been elucidated by exact mass measurements and the observation of metastable ion transitions.     

InGaAs/GaAs multiple-quantum-well modulators and switches

The design, fabrication and characterization of electrooptical modulators and switches based on pseudomorphic InGaAs/GaAs multiple-quantum-well (MQW) structures is presented. The absorption and refractive index changes (, n) of In_0.2Ga_0.8As/GaAs MQW structures due to the quantum-confined Stark effect are examined in detail. The figures of merit /₀ and n/₀ give information on the design of modulation and switching devices. Based on these results, we develop two types of efficient and high-speed modulators, vertical and waveguide modulators, and for the first time an InGaAs/GaAs intersectional X-type switch. Recent experimental results for each device are presented.     

Atomic line profile measurements on hollow-cathode and electrodeless discharge lamps using a high-resolution echelle monochromator

Commercial r.f.-excited electrodeless discharge lamps (EDLs) and hollow-cathode lamps (HCLs) of the elements As, Bi, Cd, Sb, Se, Sn, Te and Zn were studied using a 2.54-m scanning echelle monochromator. The variation of the widths of the resonance lines from these lamps was investigated for a range of power and current levels. In general, the tendency to self-absorption and self-reversal is greater for the more volatile elements, although the strong self-reversal of the Pb 217.0 nm line from a HCl is an exception. EDLs for the less volatile elements, which are incorporated as iodides, show relatively little variation in line width except at the highest powers.The extent of the overlap between the Bi and I atomic lines at 206.2 nm is demonstrated and, for As, a hyperfine structure splitting constant (A-value) of 0.016 cm⁻¹ was determined for the 5s⁴P energy level.     

Photoionisation cross-section calculations for satellite processes: Relaxed Hartree—Fock theory with application to helium n = 2 lines

The calculation of photoionisation cross-sections for atomic diagram and satellite transitions in a relaxed-orbital Hartree—Fock framework within the dipole approximation is considered, with specific numerical application to the helium system. The length, velocity and acceleration forms of the dipole operator are investigated. A reformulation of the dipole-operator expression is required for conjugate shake-up processes to ensure that the ratio of the various dipole forms is independent of photon energy at the Hartree—Fock level. The most suitable treatment produces cross-sections of accuracy comparable to experimental precision for the diagram transition at photon energies down to threshold, and for satellite transitions down to 15 eV above threshold. On the basis of the present findings, electron correlation contributions other than relaxation effects may not be as important as previously thought in contributing to satellite intensities in singly ionised systems, except in the threshold region.     

Differential inhibition of postnatal brain,spinal cord and body growth by a growth hormone antagonist

Background  

Growth hormone (GH) plays an incompletely understood role in the development of the central nervous system (CNS). In this study, we use transgenic mice expressing a growth hormone antagonist (GHA) to explore the role of GH in regulating postnatal brain, spinal cord and body growth into adulthood. The GHA transgene encodes a protein that inhibits the binding of GH to its receptor, specifically antagonizing the trophic effects of endogenous GH.     

A theoretical study of the photoelectron spectrum of atomic lithium

The 1s photoelectron spectrum of atomic lithium has been calculated for incident photon energies of 151 and 1487 eV. For this open-shell system there are four possible final-state manifolds for the residual ion, with symmetries ³S, ¹S, ³P^o and ¹P^o. The line energies for the first four members of each manifold have been calculated using relaxed Hartree—Fock or configuration interaction wavefunctions as required. The agreement with available optical data is excellent.The intensity expressions neglecting interchannel coupling have been evaluated to allow a comparison of line intensities between final-state manifolds and hence the synthesis of theoretical spectra. The variation of the spectral profile with photon energy is clearly demonstrated. Conjugate shake-up, as well as shake-up processes are predicted to be important at high photon energy.     

A configuration interaction study of the valence shell photoelectron spectrum of argon

The detailed satellite structure of the photoelectron spectrum of the valence shell of Ar is investigated theoretically with correlated wavefunctions calculated by a configuration interaction technique using near Hartree-Fock numerical orbitals as a basis set. It is shown that, while the 3s²3p⁴ns,d ²S satellites gain intensity principally from mixing with the 3s3p⁶²S state, the 3s²3p⁴np²p^o satellites gain intensity principally from relaxation. The remaining structure can largely be explained in terms of the 3s²3p⁴ns,d ²D states, which gain intensity by initial state configuration interaction.     

Optical signal quality metric based on statistical moments and Laguerre expansion

A new and straightforward optical signal quality metric, which is based on the approximation of the analytical probability density function by its Laguerre expansion, is proposed. This expansion uses statistical moments to approximate the true statistics, but, unlike the well-known Gaussian approximation, higher order moments can be used as well. The quality of the proposed metric is discussed and the metric is compared with the Gaussian approximation. It is shown that the new metric provides not only a good estimation of the bit error rate in the range of practical interest but, contrary to the Gaussian approximation, correctly estimates optimum decision threshold and accurately pictures the shape of the noise distribution.