Band tailing and the spontaneous spectra of compensated epitaxial GaAs laser junctions

The spectra of the spontaneous emission from GaAs laser diodes fabricated by vapor-phase epitaxy and containing a high degree of compensation in the p-type side of the junction are, in many respects, similar to those previously reported for the cathodoluminescence of homogeneous p-type material. The temperature dependence of the spectra of these diodes supports a model where the emission is attributed to radiative transitions between a narrow band of states near the conduction-band edge and an exponential distribution of states extending the valence band into the forbidden gap. An increase in temperature then results in an increase or a decrease in the radiative power output depending on the position of the quasi-Fermi level for electrons relative to the above narrow band of states and a distribution of nonradiative levels below it.     

Efficient power extraction at TRAPATT harmonics

Using silicon planar diodes in a coaxial cavity that closely approximates to- Evans's TRAPATT circuit, microwave power up to the fourth harmonic has been extracted efficiently with a very low subharmonic content. The efficiency at the fundamental (1 GHz) and the third harmonic (3 GHz) were almost identical at 35% and 34%, respectively.     

Active RC networks using a parallel T

A method of independent control over the positions of the poles and zeros of an active RC network is described, Secondorder transfer functions can be realised by using these circuits. The adjustments of their characteristics after the construction are very easy. A simple experiment shows good agreement with the theory.     

InGaAs-InP P-I (MQW)-N surface-normal electroabsorption modulatorsexhibiting better than 8:1 contrast ratio for 1.55-μm applicationsgrown by gas-source MBE

We report growth of In_0.53Ga_0.47 As-InP multiple quantum well (MQW) modulators operating at 1.55 μm for fiber-to-the-home applications. By employing a 200-period InGaAs-InP MQW stack in the intrinsic region of a p-i-n structure and working in reflection, we have been able to realize surface-normal modulator devices that exhibit better than an 8:1 contrast ratio. This is the highest contrast ratio reported to date for this type of device working at this wavelength     

Cut-off frequency in radially inhomogeneous single-mode fibre

The limit for single-mode operation in a graded-index fibre has been obtained by calculating the normalised cut-off frequency of the TE01 mode, The affect of diffusion at the core-cladding boundary has been estimated.     

Effect of dip in the referctive index on the cut-off frequency of a single-mode fibre

The normalised frequency for single-mode cut-off in an optical fibre has been calculated as a function of the dip in the refractive index at the centre of the core. For a particular class of profiles, dip width up to 40% have a negligible effect on the propagation characteristics.     

Theoretical analysis of a coated single-mode fiber termination

The antireflection efficiency of a single layer coating in front of an abruptly terminated monomode optical fiber is treated analytically by means of an integral equation, that is appropriate to solve the corresponding boundary value problem. Both guided and radiation modes are taken into account, when describing the field inside the optical fiber region. Then the integral equation is solved approximately by a Neumann series procedure giving highly accurate results when the weakly guidance condition holds, which is a case of practical interest in the optical communications area. The reflection coefficient and the radiation pattern of this configuration are computed. Numerical results are given for several values of the parameters of the geometry under consideration.     

High temperature superconducting transresistance amplifiers for farinfrared detectors

Far-infrared (IR) focal plane arrays are becoming increasingly important for terrestrial and space-based imaging applications. For noise reasons, it is desirable to have transresistance preamplifiers at the focal plane, and, therefore, they need to operate over wide temperature ranges. High-temperature superconducting (HTS) amplifiers that have a high gain, relatively low noise, and response times of <200 ps over at least a 10-80-K temperature range have been developed     

Optical properties of undoped n-AlGaN/GaN superlattices as affected by built-in and external-electric field and by ar-implantation-induced partial disordering

High-resolution x-ray diffraction patterns and 90 K microcathodoluminescence (MCL) spectra were taken for undoped, symmetric AlGaN/GaN superlattices (SLs) with GaN quantum-well (QW) widths of 35 Å and 80 Å. The short-period SL spectra were blue shifted by about 60 meV compared to the GaN substrate, and the magnitude of the blue shift was increased by about 20 meV by application of a reverse bias of ?3 V (electric field of about 4 · 10⁵ V/cm) to a Schottky diode prepared on this SL. A small red shift of about 40 meV compared to GaN was observed for the long-period SL. The two latter observations were interpreted as manifestations of the presence of a strong built-in piezoelectric field, giving rise to the quantum-confined Stark effect (QCSE). Partial disordering of the short-period SL was observed after Ar ion implantation (energy 150 keV, dose 8·10¹³ cm^?2 and 80 keV, 2·10¹³ cm^?2) and subsequent annealing at 1000°C for 3 h under the protective layer of Si₃N₄. However, it was observed that this partial disordering was accompanied by strain relaxation via formation of misfit dislocations or cracks.