Radiophysics and Quantum Electronics最新文献

We analyze the turn-on delay of quantum cascade lasers (QCLs) under pumping with a trapezoidal current pulse. The simulation is based on a system of rate equations for a three-level laser system. We identify five QCL regimes, for which appropriate analytical formulas for the turn-on delay estimation are obtained. The analytical results are in good qualitative agreement with numerical simulations. It is shown that the turn-on delay depends nonlinearly on the rise time of the pump-pulse leading edge.

We propose a method for synthesizing ultra-wideband pulses using a vector network analyzer and a wideband transceiving log-periodic antenna. The transfer characteristic of the antenna-feeder transmission line of the system is described by the model of a two-port network whose S-matrix elements are calibrated for at least two heights of the antenna above the reflecting surface (metal plate). The proposed method of calibrating the transfer characteristic of a log-periodic antenna allows one to minimize the amplitude- and phase-frequency distortions, which are introduced to the sensing pulse by the antenna. Employing the developed method, we experimentally demonstrate a possibility of synthesizing an ultra-wideband pulse with a duration of 0.46 ns at the level of half amplitude of the envelope (when the pulse contains several field oscillations) using a log-periodic antenna with a passband of 1.36 to 4.88 GHz (at the level −10 dB). This method is specially developed for creating miniature radar systems using portable vector network analyzers and log-periodic antennas for applications to remote sensing of an underlying surface by ultra-wideband pulses from small-size unmanned aerial vehicles.

We present the results of experimental study of the harmonics of Josephson radiation from a terahertz (THz) source based on a long Josephson junction (LJJ). The source consists of an integrated circuit including the LJJ, a matched transmitting antenna, and a harmonic mixer used for phase locking of the output signal. The LJJ and the harmonic mixer are both made of three-layer Nb-AlOx-Nb superconductor–insulator–superconductor (SIS) structures and operate at a temperature of 4.2 K. The antenna is located in the base superconducting Nb electrode of the microcircuit. To study the spectral composition and characteristics of the output radiation, two different methods were employed, using a THz Fourier transform spectrometer based on a broadband semiconductor detector and using a high-resolution THz spectrometer based on a heterodyne SIS receiver. The spectral composition of the radiation demonstrates the presence of both the fundamental harmonic of the Josephson effect and the higher harmonics. The spectral characteristics of the fundamental (at frequencies of 300–335 GHz) and the second (at frequencies of 600–670 GHz) harmonics, respectively, with a spectral resolution of about 0.1 MHz were studied using a SIS detector.

We theoretically study a fairly universal effect of screening the quantum states of an optical field from interaction with the system as a result of excitation of the modes initially being in the ground (vacuum) state. The cases of two-level and harmonic perturbing subsystems are considered. In the case of a sufficiently large number of “screening” modes, the formation of the corresponding dark state occurs with a relatively weak modification of the initial state. Therefore, an ensemble of resonators with the fields in the vacuum state is able to screen an arbitrary (including entangled) quantum state of a relatively small group of excited resonators from interaction with a dissipative subsystem.

Within the framework of the worst-case reception scenario, we have constructed an adaptive mode MUSIC algorithm intended to localize sources in an acoustic waveguide with inaccurately known parameters in the presence of spatially correlated dynamic noise. The algorithm is based on an iterative procedure that allows one to estimate unknown elements of the noise covariance matrix and eventually determine the desired positions of the sources. The results of comparing the efficiency of the proposed estimation technique (which takes into account the difference between the true and expected mode composition) with the conventional maximum likelihood and element-space MUSIC methods are presented.

The paper proposes an ingenious method for global ionospheric total electron content (TEC) mapping based on a phase-difference approach to the analysis of phase measurements of GNSS signals at a pair of coherent frequencies on a distributed network of ground-based receivers of the global IGS network. The proposed approach uses the representation of the ionosphere as a thin layer with the TEC distribution given by a truncated expansion into a series of spherical harmonics in the Sun-synchronous geomagnetic coordinate system. The expansion coefficients are determined by the least squares technique with a TEC positivity constraint, which is implemented by solving the corresponding linear complementarity problem. The proposed method does not require estimation of the differential code biases of both satellites and receivers, which makes it possible to combine data from various GNSS, such as GPS, GLONASS, and Galileo, within a single algorithm. The results of testing the proposed method on synthesized observation data using the real geometry of GNSS satellites, IGS receivers, and the ionosphere given by the NeQuick2 model, are presented. The results of comparing the real global ionospheric maps obtained by the proposed method and the maps of the CODE center are reported.

We have performed theoretical analysis and a numerical study of stationary longitudinal elastic waves in a rod, which is made of a microinhomogeneous solid body with quadratic-bimodular nonlinearity, with account taken of the geometric dispersion of the phase velocity of the waves. Nonlinear equations are obtained, and their numerical solutions are analyzed for stationary solitary and periodic waves propagating without variations in their form. Graphical analysis of the profiles of stationary waves is performed.

We consider the influence of the navigation data on the radar-image quality in a multiposition synthetic aperture radar system. We propose algorithms for adaptive synthesis with additional focusing, which provide both refinement of the navigation data using the primary radar information and the synthesis of images with improved quality. The proposed algorithms are efficient for processing the radar information obtained as a result of both the multiposition and monostatic survey. Using an example of processing the radio holograms, which were obtained from an airborne synthetic aperture radar with inaccurate navigation information, we show the efficiency of the method of a posteriori focusing, determine its efficiency conditions, and conclude on the necessity of adaptation (a priori focusing) in the case of insufficient accuracy of the navigation data. We take into account that the laws of motion of a couple of carriers can formally be reduced to the law of motion of an equivalent monostatic carrier.

We study the possibilities of tomographic observation of small-sized bodies in refractive oceanic waveguides within the framework of the ray description of the acoustic field. To increase the spatial resolution and sensitivity in solving the inverse observation problem, the waveguide-adapted focusing of the radiated and received pulses to the area of the assumed location of the observed body is used. To reduce the effect of the interference structure of the fields in the waveguide, we use the selection of individual ray tomographic projections followed by incoherent accumulation of partial solutions. A simulation model of the detection system using well-known physical models of the propagation and scattering of high-frequency acoustic waves is constructed.

We analyze the coherence degree of the radar signals reflected from the passive-interference sources with different spectral characteristics determined by different wind velocities. The entropy of the distribution of eigenvalues of the interference correlation matrix is used as a quantitative measure of the coherence degree of interference. It is shown that the entropy value can be used for estimating the maximum coefficient of the passive-interference suppression. The dependences of the coefficient of the passive-interference suppression on the rms width of the interference spectrum for various values of the process sampling length and the wavelength are given. The coherence of an additive mixture of the signals, which are reflected from the interference and the target, is estimated. It is shown that the coherence measure of radar signals in the form of the entropy of the distribution of the signal over eigensubspaces of its correlation matrix allows one to estimate the maximum subinterference visibility.