Vol 63, No 9 (2018)

The paper presents the results of theoretical and experimental study of the backscattering of light in ground tests of a space lidar operating in the continuous or quasi-continuous mode when the time gating and the elimination of the effect of backscattering of light in the atmosphere on the received signal are impossible. The small discrepancy between the theoretical and experimental data caused by cloudy weather and the presence of moisture in the atmosphere is explained. The backscattering is reduced to the magnitude of the photodetector noise by increasing the distance between the receiver and the transmitter by 25 cm.

A 2D frequency-scanned array focused in the near-field zone is analyzed. A method for the synthesis of this array ensuring perfect field focusing in two points at two frequencies is proposed. Array scanning in two planes (longitudinal and transverse) is analyzed with the use of an approximate model. The motion trajectory of the focal spot (scanning arc) and optical aberrations associated with scanning are studied. Different variants of determination of the scanning arc, using the criterion of the maximum field intensity and the criterion of the maximum aperture efficiency of the array, are considered. It is shown that application of bifocal arrays can improve main quality indices as compared to those of single-focus arrays.

Problems and methods of development of the general theory of multiple scattering of electromagnetic waves in fractal randomly inhomogeneous media based on modifications of the classical Foldy–Tversky theory are considered. The backscattering processes, which are specific for radar probing of a fractal medium or a fractal target are studied. It is shown that the fractal signature can be used to study the dependence of volume scattering on distance and space-time evolution of atmospheric formations and multiple groups of artificial and natural objects.

The truncation error of the Kravchenko–Kotelnikov series, which is a generalization of the Whittaker–Kotelnikov–Shannon series, is studied. The basis functions of the Kravchenko–Kotelnikov series are the spectra F_a(t) of the atomic function h_a(x), linearly transformed with respect to the argument. In this case, the function F_a(t) is defined by an infinite product. Two theorems on the truncation error bound for the Kravchenko–Kotelnikov series are proven. A practically important case in which the infinite product F_a(t) is replaced by a partial one is considered. A comparative analysis of the obtained formulae is carried out.

Development of a receiver of incoherent microwave radiation that can be generated by wideband generators of chaotic oscillations or reflected by objects and surfaces irradiated by such generators is considered. A structure of the device is proposed, an experimental prototype is described, and detection of incoherent sources of microwave radiation with an integral power of about 2 mW is demonstrated at a distance of greater than 100 m. Experimental results are presented to prove that the receiver can be used for distance measurement and as a motion sensor in the absence of direct visibility.

Principles and mechanisms of generation of terahertz electromagnetic oscillations by generators based on solid-state micro- and nanostructures are discussed. Parameters of experimental models of such devices are given. A theoretical model of terahertz radiation in magnetic metal junctions is presented, and the possibility of varying the frequency of a spin-injection terahertz oscillator is demonstrated. The results of development of methods for examining the spectral characteristics of radiation in the 0.15–80 THz range with a spectral resolution of up to 1.8 GHz are detailed.

In the geometry of a plane-parallel in-plane magnetized ferrite plate with dissipation, the dispersion of forward and backward magnetostatic surface waves propagating in an arbitrary direction relative to the field has been investigated. Dispersion curves as functions of the imaginary and real parts of the complex wave number has been constructed. The group velocity of both wave propagation and increase of wave attenuation have been considered. The frequency range of the experimentally observable waves has been estimated at a specified level of sensitivity of the measuring instruments.

The ferromagnetic resonance spectrum at a frequency of ~9.85 GHz for an in-plane magnetized 2D square lattice with the unit cell parameter а ≈ 15 μm consisting of orthogonal microwaveguides with a width of w ≈ 5 μm on the basis of a permalloy film with thickness of d ≈ 90 nm has been experimentally and numerically investigated. It is shown that upon variation in the angle θ between the magnetic field direction and the unit cell axis, the total spectrum of spin-wave excitations of the lattice can be presented as a superposition of the spectra of separate permalloy microstrips magnetized at angles θ and π/2–θ and regions corresponding to the lattice nodes. It has been found that, in the case of magnetization along the lattice diagonal (θ ≈ 45о), excitations localized at the lattice nodes dominate in the spectrum, whereas, at θ ≈ 0 and 90°, the main contribution is made by excitations localized mainly on the microwaveguide segments between the lattice nodes; at θ ≈ 10°–13° and 18–20°, “repulsion” of absorption lines in the spectrum is observed.

The design of a microwave oscillator based on a microstrip log-periodic antenna integrated with a field effect transistor and a waveguide built in a dielectric substrate has been developed and analyzed. The waveguide geometry provides the possibility of propagation and radiation at both the fundamental frequency of the log-periodic antenna and harmonics. Computer simulation of the oscillator design in a frequency range near resonance frequencies of the log-periodic antenna is conducted. The possibility of summation of powers of several antenna–oscillators arranged ias a linear phased array is investigated.

Conditions for fabrication of diamond-like carbon films on the surface of oxidized single-crystalline silicon using the technique of high-frequency diode sputtering of graphite target are determined. It has been found that the deposited films have amorphous structure. The Raman spectroscopy technique has been used to show the presence of carbon phases with sp²- and sp³-hybridization, the ratio between which can be controlled by the growth conditions.

A new method to estimate the phase coupling of electroencephalogram (EEG) signals in different channels is based on the calculation of difference of phase characteristics for channel signals at the ridge points (maximum magnitudes) of wavelet spectra. It is shown that the method can be used to determine phase-coupled pairs of EEG leads and distinguish such pairs from uncoupled pairs. The interchannel phase coupling of EEG is estimated for cognitive and motor tests of a normal patient and a patient with craniocerebral injury

The optimization of technological parameters for fabrication of the inclined texture [0001] in ZnO films has been conducted. It is shown that the inclination of the texture axis is determined by at least two factors: the average vector of falling the deposited particles and the intensity of bombardment of the growing film with negative ions of oxygen. Optimum displacements of the substrate position relative to the axis of the sputtering system and the distance between the planes of the target and the substrate are determined. Films of zinc oxide with optimum angles of inclination of texture axis have been obtained by the RF sputtering technique.

Main characteristics of a model of the microwave photonic receiving channel with optical heterodyning have been numerically simulated and experimentally investigated. The transducer is based on a twoarm (signal and reference) balanced circuit with a continuous wave laser, amplitude modulators, narrow-band optical filters, and a photodetector. The possibility of implementation of the receiving channel with a signal-to-noise ratio of up to 60–70 dB, a carrier frequency of up to 40 GHz and more, and a detection bandwidth of up to 1 GHz is demonstrated. It is shown that semiconductor lasers without outer stabilizing cavities can be used in an optical pumping source by means of compensation of the laser frequency noise. It has been found, that if the modulator working point corresponds to the optical carrier frequency suppression mode, the noise characteristics of the detector can be maintained without application of narrow-band optical filters.

This paper deals with the second part of the overview on analytical calculations of the characteristics of avalanche photodiodes (APDs) based predominantly on direct gap semiconductors. In the first part of the overview (JCTE 2017, vol. 62, no. 9, p. 1027), a general formulation of the problem is carried out and an approach to its solution is reported. The program for calculations of multiplication coefficientins is fulfilled. In the most typical situations, they are represented analytically. It is demonstrated that the obtained analytical results are in good quantitative agreement with previously published numerical calculations and experimental data. In the given part of the overview, the dependences between the interband tunnel current of the heterostructure with a p⁺–n junction in the “wide-gap” (wg) layer and the parameters of used semiconducting materials, layer-doping levels, and their thicknesses corresponding to avalanche breakdown voltages of a heterostructure are analyzed theoretically. It is demonstrated that, as a rule, the tunnel current depends nonmonotonically on the dopant concentration in the “high-resistance” region of a wg layer. There is an optimal concentration of the given dopant at which the tunnel current reaches its absolute minimum. Simple formula for determining the optimal concentration is derived. In addition, an analytical expression for determining the minimum tunnel current is obtained. In real cases, tunnel currents can vary by several orders of magnitude. It is found that, in many cases, an increase in the doping level of a “narrow gap” layer diminishes the tunnel current. It is shown that the tunnel current does not vanish with a decrease in the doping level of the high-resistance layers of a heterostructure but, beginning with a certain concentration, becomes independent of the doping level. An analogous effect is inherent to a homogeneous p⁺–n junction. Physical reasons of such behavior of tunnel currents, which are observed at an avalanche breakdown voltage, are discussed. A technique for the optimizing the parameters of the APD heterostructure with separate absorption and multiplication regions (SAMRs) is developed. As an example, specific calculations are carried out for a widely used InP–In_0.53Ga_0.47As–InP heterostructure. The opportunity of description of transient phenomena in p–i–n APDs is considered first of all in the case where the initial voltage V₀ is higher than the avalanche breakdown voltage V_BD. This problem is formulated because there is a need to know the explicit conditions whereby the Geiger mode is generated in APD operation. A formula describing the total time of progress in the avalanche Geiger process is derived. An analytical expression for the implementation of the Geiger mode is presented. At the end of the given paper, the advantages of avalanche heterophotodiodes with SAMRs of the low-high-low type over classical samples are demonstrated and discussed on the basis of analytical calculations. The numeration of formulas, figures, and references continues that used in Part I.

A topical problem of photoelectronics is the development of array photodetector devices of the near infrared spectral range based on the In_xGa_1–xAs/InP epitaxial layers of the megapixel format. In this paper, we present the results of studies of current–voltage characteristics of photosensitive elements in arrays of the 320 × 256 format with a step of 30 μm based on heteroepitaxial structures with an InGaAs absorbing layer on InP short-wave infrared substrates. Arrays of photosensitive elements (PSEs) are fabricated using planar, mesa, and mesa planar technologies based on nB(Al_0.48In_0.52As)p-structures. In arrays produced using the mesa planar technology based on nB(Al_0.48In_0.52As)p-structures, small dark current and ampere–watt sensitivity to IR radiation of the 1–1.7 μm range are shown to combine successfully at low bias voltages. Electrophysical parameters of functional layers of initial heteroepitaxial n-B-p structures affect efficiently the dark currents and ampere–watt sensitivity of the array elements. Based on these studies, parameters of the n-B(Al_0.48In_0.52As) functional layers of p-structures were optimized and high-efficiency photodiode arrays of the 320 × 256 format with a step of 30 μm and structures of the 640 × 512 format with a step of 15 μm were fabricated with a defectiveness not exceeding 0.5%.

Parameters of multi-row photodetectors (PDs) based on HgCdTe heteroepitaxial structures of different formats, including 288 × 4, 480 × 6, 576 × 4, and 576 × 6, with a step of 28 to 14 microns are studied. Owing to the choice of a N⁺/P-/р-architecture, PDs operate at elevated temperatures in the time delay and integration (TDI) mode with the implementation of the analog mode of TDI and the replacement of defective elements directly in the readout LSI. The PDs are capable of forming high-definition images of the 768 × 576 format at a frame rate of 50 Hz in real time. For the multi-row PDs, high photoelectric parameters were obtained: the detection capacity at the maximum of the spectral sensitivity D* ≥ 5 × 10¹² cm W^–1 Hz^1/2 at temperatures Т~ 170–200 K and the number of working channels is not less than 99.0%.