Search results for "Spectral"

A spectral mapping theorem for perturbed strongly continuous semigroups

1991

SOME SPECTRAL PROPERTIES OF MULTIPLIERS ON SEMI-PRIME BANACH ALGEBRAS

1995

Abstract We extend to arbitrary semi-prime Banach algebras some results of spectral theory and Fredholm theory obtained in [1] and [2] for multipliers defined in commutative semi-simple Banach algebras.

Comparison between the shifted-Laplacian preconditioning and the controllability methods for computational acoustics

2010

Processes that can be modelled with numerical calculations of acoustic pressure fields include medical and industrial ultrasound, echo sounding, and environmental noise. We present two methods for making these calculations based on Helmholtz equation. The first method is based directly on the complex-valued Helmholtz equation and an algebraic multigrid approximation of the discretized shifted-Laplacian operator; i.e. the damped Helmholtz operator as a preconditioner. The second approach returns to a transient wave equation, and finds the time-periodic solution using a controllability technique. We concentrate on acoustic problems, but our methods can be used for other types of Helmholtz pro…

Thermostimulated processes in statically and dynamically disordered luminescent solids

1993

Abstract Trap spectra depending on the concentration of defects in crystalline KBr-T1, LiF-Mg, Ti, A1 2 O 3 and glassy Na 2 O·SiO 2 are investigated. The nature of inhomogeneous broadening and continuous distribution of traps over activation energies at sufficiently high defect concentrations is considered. Impurity hopping controlled recombination of electronic excitations is observed. The presence of enlarged values of process activation energies and frequency factors is discussed in terms of the concept of dynamic disorder, taking into account the fluctuational changes in both the perfectpart and the structure of defects in the solid at high temperatures.

Control of signal coherence in parametric frequency mixing with incoherent pumps: Narrowband mid-infrared light generation by downconversion of broad…

2012

International audience; We study, with numerical simulations using the generalized nonlinear envelope equation, the processes of optical parametric and difference- and sum-frequency generation (SFG) with incoherent pumps in optical media with both quadratic and third-order nonlinearity, such as periodically poled lithium niobate. With ultrabroadband amplified spontaneous emission pumps or continua (spectral widths > 10 THz), group-velocity matching of a near-IR pump and a short-wavelength mid-IR (MIR) idler in optical parametric generation may lead to more than 15-fold relative spectral narrowing of the generated MIR signal. Moreover, the SFG process may also lead to 6-fold signal coherence…

ASE narrow‐band noise pulsing in erbium‐doped fiber amplifier and its effect on self‐phase modulation

2019

In this paper, we report a study of the features of polarized and unpolarized narrow-band amplified spontaneous emission (ASE) in a low-doped erbium fiber at 976-nm pumping. We demonstrate that ASE noise can be treated as a train of Gaussian-like pulses with random magnitudes, widths, and inter-pulse intervals. ASE noise can also provide a statistical analysis of these three parameters. We also present the data that reveal ASE noise’s role in optical spectrum broadening through self-phase modulation of light propagating in a communication fiber. In particular, the data show that the ASE noise derivative defines the broadening’s spectral shape.

Fourier integral operators and inhomogeneous Gevrey classes

1988

Fourier integral operators with inhomogeneous amplitude and phase junction are studied in the frame of Gevrey classes. Applications are given to propagation of singularities for a pseudodifferential equation.

Identification of a specific type of PD from acoustic emission frequency spectra

2001

The paper presents an attempt to apply spectral analysis tools in processing acoustic emission (AE) pulses generated by partial discharge (FD). The experimental part of the paper describes spark gaps generating four types of PD and specifies parameters of measured acoustic signals and recalls the system used for measurement and analysis of the frequency spectra. Also, a spectral analysis procedure is presented, and frequency-domain descriptors characterizing AE pulses are defined. The results of the analysis are given both as time plots and amplitude and energy density spectra, related to values of the associated descriptors. The spectral analysis results cover AE pulses generated in system…

Nonresonant dielectric hole burning spectroscopy of supercooled liquids

1997

The nonexponential response of propylene carbonate and glycerol near their glass transitions could be selectively altered using nonresonant spectral hole burning (NSHB) experiments. This observation provides evidence of the existence of a distribution of relaxation times in these supercooled liquids. NSHB is based on a pump, wait, and probe scheme and uses low-frequency large amplitude electrical fields to modify the dielectric relaxation. The temporal evolution of the polarization of the sample is then measured subsequent to a small voltage step. By variation of a recovery time inserted between pump and probe, the refilling of the spectral features could be monitored and was found to take …

Evaluation and elimination of the blank bias error using the H-point standard additions method (HPSAM) in the simultaneous spectrophotometric determi…

1997

Abstract The aim of this report is to demonstrate the usefulness of the HPSAM in all instances where the determination of two analytes requires the use of an absorbent reagent, particularly when it does not present a constant analytical signal because of its consumption. This problem can be treated as a ternary mixture by the HPSAM method and then, the fundamentals of the method for resolving these kind of mixtures can be applied. The method can calculate the concentration of one species from spectral data at two wavelengths where the two other species present the same absorbance relationship. These wavelength pairs are easily found, and can be selected to give the most precise results. The…