Physics of Wave Phenomena最新文献

The possibility of performing quantitative absorption measurements of particle concentrations using frequency-tunable lasers is investigated. At fast frequency scanning, when the recording time of spectrum is shorter or comparable with its formation time, well-known time-dependent interference interactions between the radiation incident on an absorbing medium and the radiation induced in it are observed. Under these conditions steady-state absorption spectra are distorted, and the classical relations lying in the basis of absorption measurements are violated. The character of the distortions depends on the type and density of particles, their absorption state, the mechanisms of spectra formation, and the laser beam power and geometry. In this paper, we report the results of studying the manifestations of Doppler profile narrowing caused by the Dicke effect in time-dependent spectra and their influence on the results of measuring the concentrations of absorbing particles. It is shown that the static spectrum can be reconstructed and quantitative measurements by integrated absorption spectroscopy can be performed under these conditions.

Nonplanar two-dimensional (2D) spherical dust acoustic solitary waves (DASWs) in unmagnetized, collisionless, Boltzmann distributed electrons, negatively charged dust fluid and trapped ions following vortex-like ion distribution, in a dusty plasma were investigated theoretically. Using standard reductive perturbation technique, which is valid for a small but finite amplitude limit condition, nonlinear spherical modified Korteweg–de Vries (K-dV) equation was achieved. Two motions are observed in the radial and angular directions, with transverse perturbations in the angular direction. It is found that the properties of the DASWs in a 2D spherical geometry differ from 1D spherical geometry where transverse perturbations and unidirectional waves are observed for 2D spherical geometry. The effects of dusty plasma parameters and vortex-like ion distribution on the properties (such as amplitude and width) of spherical DASWs were theoretically investigated. These numerical investigations show that under such specific conditions, only compressive DASWs can exist.

Propagation of the Airy–Gauss beam is analyzed in the paraxial approximation for the case where its waist associated with the Gaussian exponential is in an arbitrarily located plane perpendicular to the propagation direction. It is shown that from the viewpoint of the quality criteria for the approximation of the Airy function, the characteristic length of diffraction-free propagation, and the beam intensity and its derivatives with respect to transverse coordinates at different points of space, the Airy–Gauss beams are superior to the widely used Airy beams. The results of the analysis of how the Airy–Gauss beam characteristics affect the longitudinal and transverse density components of the orbital and spin constituents of the momentum and angular momentum, which is necessary information for problems of optical manipulation of micro- and nanoparticles, allows these beams to be considered as more promising for solving these problems.

The characteristics of spatial relaxation of the average electron energy in pure helium and in helium containing an admixture of water vapor in a constant electric field have been investigated by Monte Carlo simulation. The conditions under which the spatial relaxation in helium has a form of damped oscillations are considered. It is shown that even a small (0.1%) additive of water vapor leads to a significant decrease in the relaxation length, and the spatial oscillations almost disappear at 3% H₂O. Calculations were also performed for a mixture He : (H₂O : H₂ : O₂ : H), where the composition of atoms and molecules in the parentheses correspond to the composition formed from the initial water vapor in discharge plasma due to the dissociation of water molecules and subsequent plasma-chemical reactions.

This research involves a novel liquid concentration measurement of helical long-period fiber grating (H-LPFG) by dual-wavelength difference. The grating with a period of 782 μm is spirally processed via the commercial welding machine. The resonant peak appears around 1520 nm. Coupled mode theory is used to study the transmission strength as a function of liquid concentration. Theoretically, the transmission intensity depends only on the liquid concentration. Then, we experimentally researched the relationship between the transmission intensity and the concentration as a function of the wavelength at 1495 and 1518 nm. Transmission intensity at these two places is e-exponential with respect to the liquid concentration. Due to the transmission intensity measurement concentration is influenced by the fluctuation of the light source, since the fluctuation of the light source will affect the transmission intensity. Finally, a demodulation system for H-LPFG is proposed using only filters and light detector. This allows a new way to develop a high-precision, low-cost, high-potential liquid concentration sensor.

The maximum range of detection of an autonomous underwater vehicle (AUV) in the high-frequency region has been estimated range applying holographic processing. The estimation is performed based on numerical simulation of the sound field attenuation with distance and the experimental spectral characteristics of the underwater vehicle noise emission and the background noise of the water area.

The robustness of the holographic processing of hydroacoustic broadband signals with respect to the spatial and temporal inhomogeneities of the ocean medium has been experimentally studied. The experiment was performed in shallow water against the background of intense internal waves (IIWs). The sound source was a towed pneumatic emitter; a single hydrophone played the role of a receiver. The interference patterns and holograms of sound pressure for different instants, when a perturbation of the medium caused horizontal refraction or coupling of acoustic field modes, are presented. It is shown that, in the presence of IIWs (which perturb the source field), holographic processing makes it possible to separate the spectral densities of unperturbed and perturbed fields. This circumstance allows one to reconstruct the hologram and interference pattern of unperturbed source field, thus demonstrating the robustness of holographic processing of hydroacoustic signals in the presence of hydrodynamic perturbations.

The method of passive thermal compensation of the arrayed waveguide grating (AWG) demultiplexer based on a cut in the input planar waveguide and a controlled angle of rotation of a part of this waveguide is investigated. A possibility of performing this thermal compensation is shown, restrictions on the choice of the demultiplexer parameters to be observed for functioning of the demultiplexer are obtained, and relevant recommendations are given. A simple algebraic expression for the derivative of the compensation angle with respect to temperature is obtained, which is needed for development of the demultiplexer. In the given numerical example, this derivative is 3.5 × 10^–5 rad/K.

The design of resonant planar metal-insulator-metal (MIM) structures offers promising applications in the optical image processing field, especially for developing efficient and ultrafast systems for optical computing and edge detection. The present work extends approaches based on electromagnetic theory and coupled-mode theory to describe nature and characteristics of the resonances in absorptive interference structures. Obtained analytical expressions based on Fano representation relate the resonance properties of the interference structures with their geometrical and optical parameters. This approach was efficiently employed to describe optical properties of the MIM structures and optimize their geometrical parameters for applications in optical filtration and image processing. The review of recent developments for all-optical edge detection both in reflection and transmission highlights various challenges encountered.

We study analytically and numerically the possibility of implementing a simple mesoscopic demultiplexer based on bound states in the continuum (BICs), induced transparency, and Fano resonances. The demultiplexer is made of a Y-shaped waveguide with an input line and two output lines where each output line contains two stubs grafted on two different sites in a U-shape far from the input line. The BICs are obtained for specific values of the lengths of the stubs of U-shaped structures and the segment separating them. By detuning the two stubs slightly in an appropriate way, BIC transforms into Fano or electromagnetic induced transparency (EIT) resonances. We give closed-form expressions of the geometrical parameters that allow a selective transfer of the given state in the first waveguide without perturbing the second waveguide. The effect of temperature on the transmission resonances is also examined using Landauer–Buttiker formula.