Home – Home – Jornals – Avtometriya 2025 number 1

In this study, we consider the dynamics of interaction of near-IR electromagnetic radiation with a space charge wave (SCW) propagating in the volume of a cylindrical n - GaAs waveguide. A possibility of phase modulation of a slow electromagnetic wave (like the whispering gallery mode) with a depth of 10 to 30 radians is demonstrated. It is found that the phase-modulated radiation splits into a sequence of ultrashort pulses during further propagation in the optical waveguide. The repetition rate of the formed pulses coincides with the frequency of the SCW realized in the waveguide. The peak power of the generated pulses exceeds the average power of the radiation introduced into the waveguide approximately by three orders of magnitude and can reach values of 20-50 W. The numerical analysis shows that the optimal length at which optical pulses are formed outside the waveguide depends on the amplitude and frequency of the modulation of the concentration of free charge carriers associated with the propagating SCW.

In this study, a two-core fiber based on tellurite glass with interacting single-mode cores is fabricated, and the linear and nonlinear propagation of ultrashort pulses is investigated. In the fabricated fiber, the effect of nonlinear optical switching of ultrashort pulses at a wavelength of 1.56 μm is experimentally demonstrated, which manifests itself as energy redistribution between the cores at the fiber output with an increase in the energy of the input pulse launched into one of the fiber cores. The effect is observed at relatively low pulse energies of less than 2 nJ, which is of interest for the design of saturable absorbers for mode-locked fiber lasers.

The role of the cross-phase modulation between the pump and generation waves in a fiber laser with randomly distributed feedback due to weak Rayleigh backscattering, operating in the ultra-narrow regime, which is observed just above the threshold, is discussed. The spectrum in this case consists of narrow (less than 1 MHz) modes with a typical lifetime of 1 ms. It was demonstrated earlier that both external noises (thermal or acoustic) and the nonlinear process of four-wave interaction between the generation modes can be responsible for the mode decomposition process. In present study, a model of the nonlinear interaction of the pump wave with an ultra-narrow generation mode is constructed, and it is shown that the cross-phase modulation between them can lead to the destruction of the mode if the walk-off is sufficiently slow. It means that the narrow-band generation depends on the fiber dispersion, because the latter determines the corresponding walk-off length. A comparison with experiments confirms this conclusion: in lasers based on fibers with low dispersion, narrow-band generation is not observed, while it does exist in cases with high dispersion coefficients.

A new type of an element providing mode synchronization in a fiber laser source of ultrashort pulses is proposed. This element is a pair of coupled microfiber tapers coated with a layer of carbon nanotubes and combines the functions of a saturable absorber and an output coupler. The proposed version of manufacturing a saturable absorber is technologically simple and can be used in almost any optical laboratory. Based on the manufactured sample of the absorber-coupler and active fibers doped with erbium, a ring fiber laser is manufactured, in which mode synchronization with the generation of picosecond soliton pulses is experimentally demonstrated.

Here we present an erbium fiber laser in a configuration of a half-open cavity formed by a narrow-linewidth fiber Bragg grating and artificial Rayleigh reflector provided random distributed feedback. For additional spectral filtering, a 10-m twin-core fiber wound on a spool with a diameter of 9 cm is used. A single-frequency generation regime with the maximum output power of 4.4 mW is demonstrated, and the generation linewidth is 2 kHz.

A new method of transverse mode selection in a cylindrical optical fiber using a thin absorbing cladding is proposed. The results of simulations in the COMSOL ® Multiphysics demonstrate the possibility of selecting the HE₁₂ mode based on the losses in a multimode fiber at a wavelength of 1550 nm, when the thin absorbing cladding (with a thickness of 2.5-10 nm) is located in the vicinity of the minima of the field’s components |E_r,φ|. The cladding radius is optimized by the minimum of the linear loss α for a given mode; its value is relatively small and varies from -0.02 to -0.06 dB / cm depending on the cladding parameters, while these values for other modes are greater by a factor of tens. An estimated equation for the linear losses is derived for the analytical description. The optimal radii of the absorbing cladding from the viewpoint of minimizing the losses of the selected mode for various absorption coefficients and cladding thicknesses are investigated. It is shown that the spectral dependence of the linear loss coefficient for a given cladding thickness and absorption coefficient is weak.

A method for increasing the quantum key distribution rate in free-space quantum communication lines by simultaneously transmitting single photons with wavelengths of 780 nm and 850 nm through one quantum channel has been proposed and implemented experimentally. Our laboratory system has demonstrated an approximately twofold increase in the "sifted" quantum key distribution rate using two setups operating at wavelengths of 780 nm and 850 nm and transmitting photons through one 23-cm-long quantum channel. The "sifted" quantum key distribution rate and the quantum bit error rate for a wavelength of 780 nm are found to be equal to 11502 ± 290 bits/s and 2.4 ± 0.4%, and the corresponding values for a wavelength of 850 nm are 10627 ± 290 bits/s and 3.9 ± 0.5%.

Spontaneous symmetry breaking in glass dielectric microspheres with the Kerr nonlinearity is studied experimentally and theoretically, taking into account the linear Rayleigh scattering between counterpropagating waves under bidirectional symmetric optical pumping. It is shown that the higher the Rayleigh scattering coefficient, the higher the threshold power required to observe the effect. An increase in the Rayleigh coupling coefficient between counterpropagating waves leads to narrowing of the region of the pump frequency detuning from the exact resonance frequency where spontaneous symmetry breaking is realized and to a decrease in the asymmetry of the light states.

A method of nanostructural modification of the surface of metal samples previously subjected to heat treatment using femtosecond laser radiation is considered. The results of experiments on the study of the process and determination of technological modes of laser microtreatment (surface peening and laser grinding) are presented. An increase in surface microhardness for hardened workpieces made of tool steels and a subsequent decrease in roughness without loss of hardness are shown.

The influence of the formation of thin layers CdTe and ZnTe on the planar side on the shape of GaAs instrument plates has been clarified. In the formation of thin buffer layers of ZnTe (20-300 nm thick) and CdTe (5-7 µm thick), their bending is found to increase by more than 1 µm. A technological solution has been found to reduce the bending of GaAs instrument plates by more than 1 µm after the formation of thin layers (CdTe and ZnTe). After technological annealing at a temperature of 600 °C in high vacuum, the sample slowly cools down (at a rate of 0.5 deg/min) within 11 hours.

The results of the application of a new methodology for testing the hypothesis of the independence of random variables in the analysis of remote sensing data of anthropogenic territories are presented. The basis of the technique is a nonparametric pattern recognition algorithm corresponding to the maximum likelihood criterion. Linear and nonlinear dependences between the spectral features that characterize anthropogenic territories are determined. The results of the assessment of anthropogenic territories based on spectral remote sensing data are considered.

An algorithm for single pulse chirp signal detection at low signal/noise ratios is proposed. The proposed detection algorithm is based on the cross-correlation of the input chirp pulse and its copy subjected to a frequency shift. As a result of the proposed transformation, a gain in the signal/noise ratio is achieved, which allows the effective use of threshold detection.

The use of thermal imaging for reconstructing historical heritage sites is limited by the influence of modern plowing and the absence of archaeological sites with contrasting thermal properties. A distinctive feature of the proposed statistical analysis algorithm is the use of Haralick texture features at the stage of feature extraction. Subsequent segmentation of the image makes it possible to identify sections of the occupation layer of various capacities as areal landscape objects. The interpretation of their properties is based on the reference data of interdisciplinary research - excavations, geophysics, and soil drilling. An assessment of the trends in the relative locations of segments of different classes and the ratio of their areas allows us to put forward a hypothesis about the boundaries of an archaeological site. At the site of the medieval Kushmanskoe III settlement (Udmurtia), the analysis of the structure of the segmented thermal image reveals fundamental differences in the distribution of the humus layer in the settlement and the adjacent territory, which was not subjected to anthropogenic influence. The proposed algorithm can be considered as an effective tool for studying historical heritage sites.

The results of testing the effectiveness of using two measurement procedures: self-calibration and auto-compensation are presented. These procedures are used in laser image generators with circular scanning simultaneously to reduce the error in the synthesis structures. It is revealed that, in using such a strategy, it becomes advisable to limit the complexity of the hardware implementation of the self-calibration procedure due to the action of disregarded sources of distortion. To reduce this objectively occurring limit, it is proposed to use an optical-mechanical system configuration in these image generators, similar to what actually occurs when calibrating their angular sensors, as well as continuously monitor and take into account the manifestations of distorting factors whose contributions are unstable over time.