Unique features of radiation field focusing in multimode waveguide channels

Dargeiko, M. M.; Kravtsov, Yu. A.; Петников, В. Г.; Petrosyan, Artyom; Samoilenko, Yu. I.; Slavinskii, M. M.

doi:10.1007/bf01040090

Cited by 10 publications

(7 citation statements)

References 1 publication

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“…For example, it may be required that the initial field on the aperture of the radiating antenna should be chosen in such a way that the radiation is focused into the vicinity of a given observation point [1] or the maximum (path-average) density of sound energy is reached inside a fixed interval of depths [2].…”

Section: Statement Of the Problemmentioning

confidence: 99%

Wave beam with minimum divergence in a smoothly inhomogeneous medium

Virovlyansky

Nefedova

2009

Radiophys Quantum El

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We consider acoustic wave beams propagating in a smoothly inhomogeneous medium along a given reference ray. It is shown that in the aberration-free approximation, a Gaussian beam diverges with distance (on the average) less than any other beam with the same initial width. This result has been obtained by solving a variational problem that is similar to the well-known quantummechanical problem of seeking the quantum state with minimum uncertainty (coherent state). An example of the beam with minimum divergence in the realistic model of a deep-ocean acoustic waveguide is considered. An approximate analytical estimate for the amplitudes of normal modes forming the beam is obtained. STATEMENT OF THE PROBLEMDuring acoustic monitoring of the underwater sound-channel inhomogeneities, the problem of selected illumination of separate parts of the water bulk often arises. For example, it may be required that the initial field on the aperture of the radiating antenna should be chosen in such a way that the radiation is focused into the vicinity of a given observation point [1] or the maximum (path-average) density of sound energy is reached inside a fixed interval of depths [2].In this paper, we consider the problem of choosing the initial field of the antenna for the formation of a narrow wave beam, which can propagate over large distances along a given reference ray without notable spreading. Actually, we discuss the possibility of creating an underwater searchlight illuminating the vicinity of the chosen beam up to distances of the order of hundreds of kilometers. Although we are speaking of field formation in an underwater sound channel, the expression for the initial field of a minimum-divergence beam, which is the main result of this paper, is general and applies not only for waveguides.Neglecting the horizontal refraction of waves, which is fairly weak in a deep sea, we consider the two-dimensional model of an underwater sound waveguide with constant sound density and velocity field c(x, z), where x is the horizontal distance and z is the depth. The z axis is oriented vertically upward, and the free water surface is located at the level z = 0. Focusing only on waves trapped by the waveguide, we assume that the sea depth is formally infinite. Moreover, for simplicity, we will consider the beams which propagate in the water bulk and do not reach the water surface. Thus, the surface effect actually will not be taken into account in our analysis.We assume that a monochromatic sound field with complex amplitude u(x, z) is radiated by a vertical antenna located along the x = 0 line. Our purpose is the optimal choice of the initial field on the antenna u(0, z) = u 0 (z)( 1 )

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Section: Statement Of the Problemmentioning

confidence: 99%

Wave beam with minimum divergence in a smoothly inhomogeneous medium

Virovlyansky

Nefedova

2009

Radiophys Quantum El

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“…BY VERTICAL ARRAYS According to the calculations, the most efficient method of the selective excitation and reception of few mode probing signals in UW tomography of a shallow water sea is based on the spatial filtering matched to the waveguide with the use of vertical arrays [9,11]. In practice, the arrays have finite sizes and deviate from vertical, these effects restricting the efficiency of mode selection [8,9,11,12].…”

Section: Experiments On Excitation and Reception Of Few Mode Signalsmentioning

confidence: 99%

“…Theoretical and experimental studies show that, in systems for long range acoustic monitoring of shallow water shelf regions of the ocean, pulsed signals produced by a small number of modes with frequencies of 100-400 Hz propagate well. The approach based on such a method can be called few mode pulsed tomography (FMPT) [9][10][11][12][13][14][15][16][17][18][19]. Efficiency in selectively exiting and receiving few mode signals can be achieved by optimizing the com plex factors along the apertures of vertically extended transmitting and receiving arrays [30][31][32][33][34][35].…”

mentioning

confidence: 99%

“…Solving such a problem involves the optimal selection of the positions and sizes of antenna arrays and adaptive tuning of their aperture distributions to the waveguide characteris tics,which vary in time; reverberation disturbances and noise; and variations in the shape of the transmit ting and receiving arrays under the influence of under water currents [9][10][11][12][13][14][15][16]. Theoretical and experimental studies show that, in systems for long range acoustic monitoring of shallow water shelf regions of the ocean, pulsed signals produced by a small number of modes with frequencies of 100-400 Hz propagate well.…”

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confidence: 99%

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Experimental study of mode selection in shallow-water sea

Grinyuk¹,

Burdukovskaya

Зверев

et al. 2012

Acoust. Phys.

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on how well the transmission and reception of probing signals are matched to the variability of the underwater (UW) waveguide [1][2][3][4][5][6][7][8]. Solving such a problem involves the optimal selection of the positions and sizes of antenna arrays and adaptive tuning of their aperture distributions to the waveguide characteris tics,which vary in time; reverberation disturbances and noise; and variations in the shape of the transmit ting and receiving arrays under the influence of under water currents [9][10][11][12][13][14][15][16]. Theoretical and experimental studies show that, in systems for long range acoustic monitoring of shallow water shelf regions of the ocean, pulsed signals produced by a small number of modes with frequencies of 100-400 Hz propagate well. The approach based on such a method can be called few mode pulsed tomography (FMPT) [9][10][11][12][13][14][15][16][17][18][19]. Efficiency in selectively exiting and receiving few mode signals can be achieved by optimizing the com plex factors along the apertures of vertically extended transmitting and receiving arrays [30][31][32][33][34][35]. According to theoretical estimates, successful matching of UW signals in FMPT requires transmission of highly coherent pulses, which cannot diffuse due to geomet rical dispersion and strongly attenuate in their propa gation. This makes it possible to use relatively weak, environmentally friendly signals, while maintaining high tomographic measurement accuracy. It is known that signals in shallow water UW waveguides suffer from a substantial geometrical dispersion that leads to low frequency underwater (LFUW) signals with a cumbersome interference structure, which results from coherent summation of a large number of indi vidual waveguide modes [38, 39]. The resulting speck lelike interference structure of the pulse, which ham pers sensing of inhomogeneities, can change signifi cantly as the phases of frequency components of individual modes vary. Thus, the pulse structure depends both on the variability of the medium and on variations in parameters of the transmitting and receiving systems; such a situation can lead to a decreased accuracy in tomographic reconstruction of inhomogeneities of the medium. In addition, the high number modes that form the specklelike struc ture of the UW pulse rapidly attenuate, and a substan tial part of the transmitted energy is absorbed by the bottom. To dampen the aforementioned interference noise, one should select individual modes of the waveguide or excite groups of modes, which form weakly dispersing wave beams in the waveguide [38, 39]. Here, we attempt to choose the optimal parame ters of the LFUW pulses basing on an analysis of cal Abstract-The possibilities of matching low frequency underwater sound pulses to the parameters of an oce anic waveguide are considered. The objective is to optimize the system of few mode tomographic observation in a shallow water sea. Experimental data are analyzed for two methods of selecting the low frequency few mode pulses propagating i...

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“…For decreasing the requirements to a volume of needed a priori information on the waveguide structure when dealing with a similar problem of shallow-water observation with the use of low-frequency signals, waveguide-matched low-mode pulsed signals are employed [7,17]. Such signals are formed with the help of optimally submerged and vertically phased radiating and receiving arrays, which ensure the selective excitation and reception of separate modes [19][20][21][22][23][24][25][26][27][28][29]. The observation results obtained for each partial mode tomographic projection accumulate, which permits one to weaken the influence of interference noise and make the algorithm of solution of the inverse problem of shallow-water observation more stable [30].…”

Section: Introductionmentioning

confidence: 99%

Tomographic observation of shallow-water inhomogeneities in the case of probing by a focused high-frequency acoustic field. I. Structure of simulation model

Smirnov

Khil’ko

2009

Radiophys Quantum El

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UDC 534.26We study the possibilities of acoustic tomography observation of small-size bodies in shallow-water layered oceanic waveguides within the framework of geometric acoustics. For increasing the spatial resolution and sensitivity in the considered inverse problem, we used the waveguide-matched focusing of the received and radiated fields into the region of expected location of the observed body. The algorithm of observation in this case consists of search for a global extremum of decision statistics, which is determined by the difference between the measured data and the hypothetic body location. Such hypotheses are formed on the basis of a priori information in the form of the waveguide model and the data on the structure of the observation system. For decreasing the influence of noise associated with the interference of fields in the waveguide, selection of separate tomographic ray projections with the subsequent incoherent accumulation of partial images is used. A tomographic shallow-water observation model based on such principles is analyzed.

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Unique features of radiation field focusing in multimode waveguide channels

Cited by 10 publications

References 1 publication

Wave beam with minimum divergence in a smoothly inhomogeneous medium

Wave beam with minimum divergence in a smoothly inhomogeneous medium

Experimental study of mode selection in shallow-water sea

Tomographic observation of shallow-water inhomogeneities in the case of probing by a focused high-frequency acoustic field. I. Structure of simulation model

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