Cascade parametric light amplification with low-frequency pumping

Cascade parametric amplification in a regular domain structure is examined theoretically taking the variations in phase of all the interacting waves in a dissipative medium into account. Analytic expressions are obtained for the conversion efficiency of the laser energy with low-frequency pumping. The dependence of the parametric amplification efficiency on the number of layers is given. Ways of increasing the frequency conversion efficiency are discussed. The presence of a nonzero input intensity at the sum frequency is found to cause a nonlinear increase in the high-frequency signal at the output of the structure. As the losses of the interacting waves increase, both the frequency conversion efficiency and the optimum domain length decrease.     

High-resolution deconvolved beamforming for three-dimensional imaging sonars

Three-dimensional(3D) imaging sonars based on the conventional beamforming(CBF) suffers from relatively wide main-lobes and high sidelobe level.To improve the spatial resolution of 3D imaging sonars,a deconvolved beamforming method is proposed with the iterative Richardson-Lucy algorithm in this paper.At first,each distance slice can be processed to obtain images by the CBF.For the near field,the Fresnel approximation is used.Then,the deconvolution technique is applied to the CBF outputs to obtain high-resolution images and suppress the sidelobe level.The simulations demonstrate that the proposed algorithm is able to improve the spatial resolution significantly and suppress the sidelobes for 3D imaging sonars.Meanwhile,the algorithm shows similar robustness with the CBF in the case of wideband and sparse array.The priority of the proposed algorithm is also validated by the tank experiment data.The presented results indicate that the spatial resolution is increased by one time and the average sidelobe level is reduced by 20 dB.     

Standard small targets for calibrating underwater sonars

Ping-pong balls form convenient reference targets for underwater sonars. Theoretical and experimental studies have confirmed that the acoustic target strength of such a ball can be predicted to within about ±·58 dB over a wide range of frequencies, even down to frequencies at which the wavelength in water exceeds the radius of the ball. Computed and experimental results are compared.     

Calibration of a Bonner sphere extension (BSE) for high-energy neutron spectrometry

In a recent work, we constructed modular multisphere system which expands upon the design of an existing, commercially available Bonner sphere system by adding concentric shells of copper, tungsten, or lead. Our modular multisphere system is referred to as the Bonner Sphere Extension (BSE). The BSE was tested in a high energy neutron beam (thermal to 800 MeV) at Los Alamos Neutron Science Center and provided improvement in the measurement of the neutron spectrum in the energy regions above 20 MeV when compared to the standard BSS (Burgett, 2008 and Howell et al., 2009).However, when the initial test of the system was carried-out at LANSCE, the BSE had not yet been calibrated. Therefore the objective of the present study was to perform calibration measurements. These calibration measurements were carried out using monoenergetic neutron ISO 8529-1 reference beams at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany. The following monoenergetic reference beams were used for these experiments: 14.8 MeV, 1.2 MeV, 565 keV, and 144 keV. Response functions for the BSE were calculated using the Monte Carlo N-Particle Code, eXtended (MCNPX). The percent difference between the measured and calculated responses was calculated for each sphere and energy. The difference between measured and calculated responses for individual spheres ranged between 7.9 % and 16.7 % and the arithmetic mean for all spheres was (10.9 ± 1.8) %. These sphere specific correction factors will be applied for all future measurements carried-out with the BSE.     

Structure of the magnetic field perturbations of a low-frequency dipole by a metallic sphere

The potentials of the magnetic field of a low-frequency magnetic dipole reflected by a sphere are analyzed. It is shown that for short ranges between the dipole and the sphere this field can be described by one potential, where the field is equivalent to the field of a system of dipoles and charges.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 73–77, March, 1976.     

声呐测量数据中异常值的辨识方法

本文提出一种辨识声呐系统对声源的方位测量数据中异常值的新方法。假定声源短时间内作匀速直线运动。辨识异常值的方法由三步组成:第一步把测量数据按每四个分为一组,采用Robust方法从每组中剔去两点,当数据中包含的异常值数少于50%时,则至少有一组剩下的两点是\     

Multistability and chaos by parametric excitation of longwave modes in a YIG sphere

Spin-wave instabilities in spheres of yttrium iron garnet have been studied by ferromagnetic resonance within the coincidence regime of the first-order Suhl instability (1.8–3.4 GHz), i.e. both the uniform mode and pairs of spin-waves were simultaneously excited on resonance. From the characteristic behaviour above the threshold, three different regimes of resonance fields (and frequencies) could be distinguished: For low fields (640–680 Oe) and for high fields (950–1210 Oe) the amplitude of magnetization remains time independent and only at high input power becomes suddenly chaotic. In the intermediate regime (680–950 Oe) a very complex multistable behaviour occurs and a variety of oscillations and sequences of bifurcations are observed. We discuss our experimental results in terms of a multi-mode model which, beyond spin-waves, considers the excitation of longwave modes with wavelengths in the order of sample dimension. Magnetostatic modes and spin-waves can be described within a unified picture by introducing spherical spin-waves which are classified by symmetry. A new mechanism based on the indirect excitation of the magnetostatic (4,3,0) mode is introduced. Numerical simulations show that this mechanism can explain both the multistability and the complex dynamic behaviour of the system.     

A method for identifying outliers in data observed from sonars

A new method is proposed for identifying outliers in the direction-of-arrival(DOA)data of a source observed from a linear array sonar.Supposea source moves uniformly along a straight line.The method for identifyingoutliers consists of three steps.(i)Divide the data into groups,each with foursample points,and delete certain two sample points from every group by meansof a robust method pesented in this paper.When the percentage of the outliersis less than 50%,there exists at least one group in which the remaining two sam-ple points are"good".(ii)Estimate the DOA and its Change rate,(θ_0,θ_0),using the remaining two simple points of every group,and computethe objective functions of M-etsimator using the resulting estimates of allgroups respectively.A"good"estimate of(θ_0,θ_0),which minmizes the objec-tive function is then obtained.(iii)Iterate the M-estimator with the"good"esti-mate of(θ_0,θ_0)as the initial value,obtain an accurate estimate of(θ_0,θ_0),and identify outliers in the observed data usi     

On the possibility of the nondegenerate parametric amplification of optical waves at low-frequency pumping

It is shown that parametric amplification at low-frequency pumping can be implemented in coupled nonlinear optical interactions in aperiodic nonlinear photonic crystals created by the method of the superposition of the modulation of the second-order susceptibility. In this process, the intensities of waves with frequencies higher than the pump frequency increase monotonically with the interaction length as in the case of the traditional process of parametric amplification at high-frequency pumping. The dependence of the gain factor on the coupling constants of interacting waves is obtained. The process under consideration makes it possible to ensure the tuning of the generated frequency in the ultraviolet range upon pumping in the visible range. Analysis of the process is performed on the example of aperiodic nonlinear photonic crystals of lithium niobate.     

On the possibility of the nondegenerate parametric amplification of optical waves at low-frequency pumping

It is shown that parametric amplification at low-frequency pumping can be implemented in coupled nonlinear optical interactions in aperiodic nonlinear photonic crystals created by the method of the superposition of the modulation of the second-order susceptibility. In this process, the intensities of waves with frequencies higher than the pump frequency increase monotonically with the interaction length as in the case of the traditional process of parametric amplification at high-frequency pumping. The dependence of the gain factor on the coupling constants of interacting waves is obtained. The process under consideration makes it possible to ensure the tuning of the generated frequency in the ultraviolet range upon pumping in the visible range. Analysis of the process is performed on the example of aperiodic nonlinear photonic crystals of lithium niobate. Original Russian Text ? A.S. Chirkin, I.V. Shutov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 11, pp. 803–807.     

Absolute parametric instability of low-frequency waves in a 2D nonuniform anisotropic warm plasma

Using the separation method, absolute parametric instability (API) of electrostatic waves in a magnetized pumped warm plasma is investigated. In this case the effect of static strong magnetic field is considered. The problem of strong magnetic field is solved in two-dimensional (2D) nonuniform plane plasma. Equations which describe the spatial part of the electric potential are obtained. Also, the growth rates and conditions of the parametric instability for periodic and aperiodic cases are obtained. It is found that the spatial nonuniformity of the plasma exerts a stabilizing effect on the API. It is shown that the growth rates of periodic and aperiodic API in warm plasma are less when compared to that in cold plasma.     

并列式多入多出声纳宽带高分辨波束形成方法

并列式多入多出(Multiple-Input Multiple-Output,MIMO)声纳可以通过高分辨波束形成方法进一步提高方位分辨能力。本文通过对窄带方法的扩展,提出一种宽带高分辨波束形成的方法,数值仿真分析了该方法的性能,并通过湖试数据进行了验证。结果表明,该方法可以对并列式MIMO声纳进行宽带高分辨波束形成,高信噪比情况下可以得到较高的方位分辨能力,但方位分辨力随信噪比减小下降较快,且运算量远大于常规波束形成方法,在运算能力满足要求且信噪比较高时,用本方法可显著提高系统的方位分辨性能。     

Improved continuous wave frequency modulated sonars with aural displays

This paper discusses methods for reducing the effects of the reset hiatus and wavelength related variations in received signal strength on the aural displays produced by simple continuous wave frequency modulated sonars. Two techniques that have been developed for reducing the effects of signal phase and amplitude discontinuities are described. As a practical example of the improved performance afforded by one of these techniques, a novel short range sonar for examining cardiovascular structures is discussed in detail.     

Calibration chain design based on integrating sphere transfer radiometer for SI-traceable on-orbit spectral radiometric calibration and its uncertainty analysis

In order to satisfy the requirement of SI-traceable on-orbit absolute radiation calibration transfer with high accuracy for satellite remote sensors,a transfer chain consisting of a fiber coupling monochromator(FBM) and an integrating sphere transfer radiometer(ISTR) was designed in this paper.Depending on the Sun,this chain based on detectors provides precise spectral radiometric calibration and measurement to spectrometers in the reflective solar band(RSB) covering 300–2500 nm with a spectral bandwidth of 0.5–6 nm.It shortens the traditional chain based on lamp source and reduces the calibration uncertainty from 5% to 0.5% by using the cryogenic radiometer in space as a radiometric benchmark and trap detectors as secondary standard.This paper also gives a detailed uncertainty budget with reasonable distribution of each impact factor,including the weak spectral signal measurement with uncertainty of 0.28%.According to the peculiar design and comprehensive uncertainty analysis,it illustrates that the spectral radiance measurement uncertainty of the ISTR system can reach to 0.48%.The result satisfies the requirements of SI-traceable on-orbit calibration and has wider significance for expanding the application of the remote sensing data with high-quality.     

On P-representation for parametric amplifier and parametric frequency converter

The statistical properties of a parametric amplifier and a frequency converter are studied by means of quantum mechanical methods. The Schrödinger picture and the P-representation of the density matrix are used. Carrying out the Fourier transformation of the Liouville equation a partial differential equation for a generating function is obtained. The inverse Fourier transform of a solution of this equation is a time-dependent P-representationPN( ₁, ₂,t). For the parametric amplifier a relation is derived which enables us to compute the functionPA( ₁, ₂,t) = =1< ₁, ₂/ ₁> is shown thatPA is classical distribution ifPN( ₁, ₂,0) is a positive distribution, while the P-representationPN( ₁, ₂,t) need not exist as a distribution and the P-representationPN( ₁, ₂,t) for the parametric frequency converter is constant along classical trajectories.The author wishes to thank Dr. J. Peina for stimulating discussions.     

Hydroacoustic compatibility of sonars in a multistatic underwater observation system

An algorithm is proposed and developed to solve the problem of hydroacoustic compatibility of a set of similar scanning sonars which implies minimization of the interfering effect of forward fields from neighboring sonars on the operation of each sonar in the system.     

Modern methods of improving the range accuracy of CTFM sonars

In a CTFM (continuous transmission frequency modulated) sonar, the error in range measurement is usually dominated by the low resolution of the range coding frequency of the system. Modern real time spectrum analysers using 1024 point DFTs (discrete Fourier transforms) can display the range codes of the sonar at a resolution approximately equal to 0.2%. To produce a range code accurate to half of the resolution, the linearity of the CTFM sweep must, however, be kept as low as 0.01%. This requirement creates several difficulties in the design of the frequency sweep when both non-linearity and stability of the voltage-controlled oscillator (VCO) and of the controlling voltage ramp are to be considered. A practical approach for achieving the above specifications is described. Properties of electronic components and of state of the art devices and the restrictions of their operating conditions are discussed. Illustrations are given for two measuring instruments: an underwater sonar system for quantitative assessment of fish population, and a diagnostic sonar.     

Torsion-bar antenna for low-frequency gravitational-wave observations

We propose a novel type of gravitational-wave antenna, formed by two bar-shaped test masses and laser-interferometric sensors to monitor their differential angular fluctuations. This antenna has a fundamental sensitivity to low-frequency signals below 1 Hz, even with a ground-based configuration. In addition, it is possible to expand the observation band to a lower limit determined by the observation time, by using modulation and up-conversion of gravitational-wave signals by rotation of the antenna. The potential sensitivity of this antenna is superior to those of current detectors in a 1 mHz-10 Hz frequency band and is sufficient for observations of gravitational waves radiated from in-spiral and merger events of intermediate-mass black holes.