Frequency tuning of long-wavelength VCSELs

Tuning properties of long-wavelength VCSELs have been studied experimentally, for the first time to our knowledge. Injection current and temperature tuning rates of two VCSELs operating near 1,512 and 1,577 nm have been measured using a Fabry-Perot etalon with free spectral range 0.056 cm(-1). A 100-Hz saw-tooth modulation with depths of modulation of approximately 10% or less was superimposed on a direct injection current (dc bias) to tune lasers in narrow spectral intervals (0.3-1.2 cm(-1)) around a central frequency set by the dc bias. The lasers have been found to be capable of being tuned faster at higher levels of dc bias. The enhancement factors were up to approximately 2 and approximately 3 for the 1,512- and 1,577-nm lasers, respectively, as compared with their tuning rates measured at the levels of the dc bias close to the threshold of lasing. A linear dependence between injection current tuning rates and the levels of dc bias has been observed. Temperature tuning coefficients have been proved to be independent of the laser heat sink temperature and of the dc bias. Frequency tuning curves were approximated with a second-order polynomial. The frequencies of more than 40 absorption lines of CO, CO(2), H(2)O and NH(3) known from spectral databases were compared with the calculated frequencies. The accuracy of the approximation was found to be within 0.2 cm(-1) for spectral intervals up to 38 cm(-1). The dependence of current tuning rates of the VCSELs on dc bias was shown to be taken into account for accurate analysis of absorption line profiles. The results obtained can be used for precise spectroscopic measurements with long-wavelength VCSELs.     

Comments on the spectra of Pisot numbers

For q∈(1,2), Erd?s, Joó and Komornik studied the spectra of q, defined as

     

Cross-correlation markers in stochastic dynamics of complex systems

The neuromagnetic activity (magnetoencephalogram, MEG) from healthy human brain and from an epileptic patient against chromatic flickering stimuli has been earlier analyzed on the basis of a memory functions formalism (MFF). Information measures of memory as well as relaxation parameters revealed high individuality and unique features in the neuromagnetic brain responses of each subject. The current paper demonstrates new capabilities of MFF by studying cross-correlations between MEG signals obtained from multiple and distant brain regions. It is shown that the MEG signals of healthy subjects are characterized by well-defined effects of frequency synchronization and at the same time by the domination of low-frequency processes. On the contrary, the MEG of a patient is characterized by a sharp abnormality of frequency synchronization, and also by prevalence of high-frequency quasi-periodic processes. Modification of synchronization effects and dynamics of cross-correlations offer a promising method of detecting pathological abnormalities in brain responses.     

Fast unsteady flow computations with a Jacobian-free Newton–Krylov algorithm

Despite the advances in computer power and numerical algorithms over the last decades, solutions to unsteady flow problems remain computing time intensive. Especially for large Reynolds number flows, nonlinear multigrid, which is commonly used to solve the nonlinear systems of equations, converges slowly. The stiffness induced by the large aspect ratio cells and turbulence is not tackled well by this solution method.     

High order finite volume methods on wavelet-adapted grids with local time-stepping on multicore architectures for the simulation of shock-bubble interactions

We present a space–time adaptive solver for single- and multi-phase compressible flows that couples average interpolating wavelets with high-order finite volume schemes. The solver introduces the concept of wavelet blocks, handles large jumps in resolution and employs local time-stepping for efficient time integration. We demonstrate that the inherently sequential wavelet-based adaptivity can be implemented efficiently in multicore computer architectures using task-based parallelism and introducing the concept of wavelet blocks. We validate our computational method on a number of benchmark problems and we present simulations of shock-bubble interaction at different Mach numbers, demonstrating the accuracy and computational performance of the method.     

Numerical simulation of flow and heat transfer in a random packed bed

Random packed beds have more complex interior structure than structured beds and are widely used in industry and engineering.CFD simulation was carried out to investigate and analyze the local flow and heat transfer in a 120-sphere random packed bed.3D Navier-Stokes equation was solved with a finite volume formulation based on the Chimera meshing technique.Investigation was focused on low Reynolds number flow(Re=4.6-56.2),which typically occurs in packed bed reactors in bio-chemical fields.Detailed temperature field information was obtained.Inhomogeneity of flow and heat transfer due to the non-uniform distribution of void fraction was discussed and analyzed.     

The Influence of Skewed Measurement Distribution Commonly Found in Analytical Geochemistry

Abstract

Transformation of data with skew distributions into symmetrie ones is an important aid by evaluation of histograms and accentuates class groupings in multivariate principal components analysis.     

M(H) shape reconstruction using magnetic spectroscopy

Knowledge about the magnetization behavior M(H) is crucial for the use of magnetic materials in engineering applications. To date many systems exist that are able to measure the magnetization behavior, e.g. VSM, VCM, MOKE. In addition to their huge costs, complex and space-consuming measurement setup, large amount of preparatory work and restricted surface measurements are handicaps which restrict their field of application. Furthermore, the influence of additional physical quantities such as temperature, strain or pressure can only be investigated with great efforts. These influences are, however, of major importance in the development of magnetic sensor systems that are based on the change in magnetic properties.In this paper, a new measurement principle based on a frequency mixing technique is introduced for investigation of the shape of the magnetization curve of soft non-hysteretic magnetic materials. Based on the Taylor expansion of the magnetization curve and the spectral investigation of the inductively detected signal, a mathematical model for the reconstruction of M(H) is proposed. The model is both numerically and experimentally verified. It is shown that the magnetization curve of a nanocrystalline soft magnetic material used in this study can be reconstructed very accurately and the influence of an additional parameter, i.e. strain, can be investigated in detail as well.     

Influence of the AC voltage frequency on the oscillation trajectory and path length of particles inside a planar–type electric curtain

Experimental investigations of the average path length of oscillating dust particles inside a planar –type electric curtain (PTEC) are presented as a function of the frequency of the AC voltage. The frequency was adjusted within the range of 10–300 Hz. The oscillation paths of feldspar particles of diameter 40–60 μm inside a small cloud were recorded photographically. The main purpose of this investigation was to study the changes in average path length as a function of the supply voltage frequency. These results can be used to improve the precipitation and separation processes for charged dust particles inside a PTEC.