Physics of Wave Phenomena最新文献

The optical breakdown in water, caused by irradiation with nanosecond and picosecond laser pulses at a wavelength of 1064 nm, has been studied. It is proven experimentally that optical breakdown centers are both individual nanobubbles of dissolved gas and clusters formed by these nanobubbles. It is found that the first stage of optical breakdown on nanobubble clusters implies induced optical coalescence (collapse) of such a cluster and formation of a macroscopic vapor bubble. The obtained experimental results for nanosecond and picosecond pulses are in good agreement with the predictions of the previously developed theoretical model.

We report on single and dual-wavelength laser performance of a diode-side-pumped neodymium-doped YAG crystal in the range of 1.3–1.44 µm with switchable spectral output due to an intracavity Pellin–Broca prism for the first time, to the best of our knowledge. Polarized laser pulses with an energy of 402 mJ at a wavelength of 1338 nm were obtained under an optical pumping energy of 3.6 J. Oscillations of six spectral components at wavelengths of 1319, 1338, 1356, 1415, 1432, and 1444 nm with a slope efficiency of 19.1, 20.1, 19.7, 4.65, 4.82, and 8.19%, respectively, were obtained. The dual-wavelength oscillations at 1319 + 1338, 1338 + 1356, 1415 + 1432, and 1432 + 1444 nm were obtained for an intermediate position of the output coupler with a slope efficiency of 17.6, 21.7, 2.65 and 6.3%, correspondingly. Our results showed that the Nd:YAG laser with an intracavity Pellin–Broca prism is promising for producing high-energy laser pulses with single and dual-wavelength output in the spectral range of 1.3–1.44 µm.

In this paper, we give an analytical demonstration of the possibility to realize a simple photonic multichannel tunable filter based on electromagnetically induced transparency (EIT) resonances. On one hand, we present a simple photonic device consisting of multiple grafted resonators at the same site; the resonator lengths depend on each other following the Fibonacci sequence. We have proposed this simple device to obtain EIT type resonances that are situated between two transmission zeros because each resonator induces its own transmission zero. The quality factor of these EIT resonances depends on the difference between the resonator lengths. On the other hand, we investigate the propagation of electromagnetic waves in one-dimensional perfect star waveguides (SWGs) structure composed of the periodicity of segments and grafted in its extremity by a finite number of symmetric resonators. This perfect star waveguide structure presents passbands separated by large bandgaps, these gaps become wider when the number of resonators increases. The insertion of five defect resonators of lengths d_02i = d₀₂ (i = 1–5) located in the same site introduce transmission peaks (defect mode) in the transmission and phase spectrums and in the phase with a high-quality factor (very narrow defect modes) that reach Q = 1 348 700 and therefore our proposed case behaves as a very narrow filter which can select one frequency. To realize a multichannel filter with high performance, we introduce five defect resonators of different lengths d_02i (i = 1–5) according to the Fibonacci sequence in the perfect SWGs. These defect systems can create a large number of defect modes which reach seven defect modes with an important transmission rate and very high-quality factor. With an appropriate choice of the geometrical parameters (the lengths d_02i of the defect resonators), our proposed system can filter a very large number of frequencies (multichannel filter) which can reach ninety-nine frequencies with very high performances.

A new adaptive method is proposed for holographic processing of broadband hydroacoustic signals using vector-scalar receivers. The method allows underwater sources to be detected and localized by their noise field when information on the transfer function of the propagation environment is unavailable. It is simpler in implementation and more disturbance immune compared to earlier proposed adaptive algorithms. The results are presented that have been obtained in the numerical experiment on the stability of the method in the presence of intensive internal waves causing interaction of sound field modes of the noise source.

Mid-IR lasing of a Tb³⁺-doped chalcogenide fiber has been demonstrated. Pumping was performed by a CW Er:ZBLAN laser, emitting at a wavelength of 2.795 µm. Fiber length optimization has made it possible to obtain output power up to 2 mW in the wavelength range of 5.2–5.3 µm at 1.8% slope efficiency and room temperature.

A highly transient stimulated Raman scattering in a SrMoO₄ crystal with not only the long (888cm^–1), but also the short (327cm^–1) Raman shift under ultrafast double-pulse pumping with a controllable chirp by the orthogonally polarized pump pulses at 1030 nm with a different delay between them was investigated. In the scheme with the 5.7-ps delay of the ordinary-wave pump pulse relative to the extraordinary-wave pump pulse, at the pump pulses stretched from 0.25 ps up to 6 ps by negative chirping, the highest Raman conversion efficiency of up to 44% into the long-shifted Stokes component at 1134 nm was achieved, that is 1.57 times higher than for the single-pulse pumping scheme with the same input pump energy (40 μJ). In the scheme without the delay between the input ordinary- and extraordinary-wave pump pulses, efficient and simultaneous Raman generation with both the short and long Raman shifts at the wavelengths of 1066 and 1134 nm was achieved in a wide range of the chirped pump pulse durations. In particular, the short-shifted Raman conversion into 1066 nm was efficient (10.5%) even for the pump pulse durations shorter than 1 ps corresponding to strong nonlinear phase modulation suppressing Raman conversion in the single-pulse pumping scheme.

All-fiber autocollimation schemes of the laser Doppler velocimeter (LDV) are presented that enable measuring velocities of hard-to-access objects in the range from several cm/s to several km/s. The autocollimation scheme covered in this work allows distant (several tens and hundreds of meters away) objects with the only working channel less than 5 mm in diameter to be investigated using a narrow laser beam. An acousto-optic modulator is used to modify Mach–Zehnder interferometer scheme to extend the range of measured velocities and allow distinguishing the motion direction. Air guns with the 4.5-mm-diameter bore have been used for test measurements. Velocities ranging from 0.1 to 330 m/s have been successfully measured.

Holographic processing of hydroacoustic noise signals in randomly inhomogeneous ocean environment is presented. A method of hologram purification from a perturbed field is described. An algorithm of resolving signals using a single detector is presented. Focusing of signal spectral density on a hologram is considered. Criteria of signal resolution are proposed. Results of numerical simulation of the resolution of two hydroacoustic signals against the background of intense internal waves, conditioning coupling of sound field modes, are shown.

Specific features of the angular distribution of scattering intensity in a two-component turbid medium consisting of spherical particles have been investigated using numerical simulation. The algorithm for calculating multiple scattering in a turbid medium was based on the Monte Carlo method. Laws of change in the shape of light-scattering phase function in dependence of the size and number of particles per unit volume of the medium are revealed. It is shown that, in comparison with the case of single-component medium, addition of a fraction of particles with sizes larger than the main-fraction particles by an order of magnitude leads to deviation of intensity at forward scattering angles (from 0° to ~40°) to smaller values. Simulation was performed for intermediate-size particles of the main and impurity components of the turbid medium, which approximately correspond to the fat phase of milk and cellular impurities, respectively.

In many problems of practical importance the interference structure of broadband-signal intensity field in shallow water is determined by the close-to-unity value of the waveguide invariant (WI) β (which is often referred to as the Chuprov invariant). This circumstance is used in ranging, when estimating the distance to the source or its relative velocity. However, the characteristics of the β invariant in deep sea have been studied insufficiently. The WI properties in the near-field acoustic-illumination zone (NFAIZ) of deep sea are investigated below. Its value is shown to be unstable: WI changes in a wide range and actually is not an invariant. Another value—phase–energy invariant (PEI) β_ef—proved to be more promising in deep sea. In the NFAIZ of deep sea, at real depths of sources and PEI detectors, it is equal to unity with a high accuracy (except for the interference minima zones). It is also found that coherent addition of Fourier components in the complex plane can be implemented in the NFAIZ, provided that a correction to phase variation is introduced when summing spectral densities along ridges. To this end, one must take into account that the Fourier-component phase on a ridge changes almost linearly with an increase or decrease in frequency. In principle, consideration of these characteristics of signals makes it possible to solve more efficiently various applied problems of acoustics. However, to implement this possibility, it is necessary to develop a fairly complex algorithm of signal power accumulation in the frequency–space domain.