Annotation of the results obtained in 2021
An active element (AE) based on manganese chloride vapor has been developed. The active element has a container with zeolite saturated with HCl. In the absence of a hydrogen-containing additive, the GDT voltage was 10.2 kV, the GDT current was 174 A. The rise time of the voltage pulse at the level of 10%-90% was 21.1 ns. The duration of the radiation pulse at the base is 38.5 ns, at the level of 0.5·Um – 12.4 ns. Modification of the AE kinetics by adding HCl (100 ℃) led to an increase in the voltage on the GDT up to 12.3 kV. The rise time of the voltage pulse in this case was 25 ns, the duration of the radiation pulse at the base was 46 ns, and at the level of 0.5·Um it was 19.8 ns. An increase in the beam diameter was also observed. To study excitation by a barrier capacitive discharge, an AE (l=50 cm, d=3 cm) was made with the possibility of changing the capacitance of electrodes made of niobium foil. The pulse repetition frequency (PRF) of excitation was increased to 125 kHz using a TGU1-1000/25 tasitron. At a PRF of 52.5 kHz, the voltage was 6.2 kV, the current was 260 mA. Gain was recorded both in the visible region of the spectrum and in the near IR. The duration of the visible radiation pulse at the level of 0.5·Um is 9.52 ns. The duration of the IR radiation pulse is 9.05 ns. The pulse of near-IR radiation occurs earlier than the visible one by 9.55 ns at a level of 0.1·Um, unlike the experiments with conventional pumping at a PRF of 17 kHz, when the pulse of visible radiation appeared earlier than the IR by 4.2 ns. Thus, the optimal operation modes of AEs on copper bromide and manganese chloride vapors, as well as the excitation conditions for converting optical signals in the visible and near-IR ranges of the spectrum, are determined. The results were obtained for the PRF range from 20 to 150 kHz. It has been established that the use of HBr is more expedient than HCl to modify the kinetics of the active medium. Modification of kinetics by HBr is completely determined by the kinetics of H atoms and H2 molecules – details are presented in the main report. 3 laboratory models of laser active optical systems were developed, including a model of a bistatic laser active optical system for solving scientific and practical problems of high-speed diagnostics, material processing in the visible and near-IR spectral ranges. It has been experimentally shown for the first time that continuous spectrum background illumination begins to introduce distortions into the images formed at the output of the functional converter (brightness amplifier) not due to the excess of the own noise amplifier, but due to the high signal energy, which leads to the superposition of two images. This disadvantage can be eliminated by passive filtering, but it must be taken into account in the mathematical analysis of images. The limiting temporal resolution is not worse than 8 µs, the spatial resolution is not worse than 10 µm. Synchronization with external signals (processes) was carried out with a jitter of no more than 10 ns. A laboratory model of a laser active optical system has been designed, which makes it possible to change the parameters of the input signal of the functional converter in a wide range. The laboratory model was equipped with two high-speed cameras (AOS-Q-PRI, MegaSpeed 103S) for studying the processes of interaction of radiation from an ytterbium fiber laser (LS-07N) with targets made of various oxides. The size range of droplets formed during laser ablation of the Nd:Y2O3 target has a wide range (0.3÷150) µm. Most (99%) are droplets larger than 10 µm. The imaging results allow us to conclude that the formation of a liquid melt is a rather complex hydrodynamic process. In most cases, the material is “carried away” in separate jets, which are separated into separate droplets in the air. Such dynamics is well recorded when the process is image directly in the crater, when the target is located at an angle of 45 degrees to the axis of brightness amplifier. The experiments carried out made it possible to establish that with a decrease in the power and intensity of laser radiation, the number of drops decreases. At a radiation power on the target of 205 W and a fiber laser pulse duration of 1280 μs, drops did not form in half of the cases, and the melt was simply squeezed out of the crater. Obviously, this value of the radiation intensity (0.14 MW/cm2) is close to the threshold value, below which large droplets are no longer sprayed. Images were obtained with different useful input signal levels, background radiation levels, and different gains of the active medium, which was provided by changing the temperature of the containers with the working substance, as well as by varying the parameters of the impact on the target of the power laser. Images of the laser ablation process have been obtained, which make it possible to determine the minimum gain at which the “useful signal/noise” ratio in the generated images becomes insufficient for analyzing the observed process. It is shown that the imaging of such processes may cause negative effects associated with manifestations of radial inhomogeneities in the gain profile, especially when operating at higher frequencies (50 – 125 kHz). By optimizing the operating mode and excitation of the active medium on manganese chloride vapor, it was possible to achieve simultaneous imaging in the visible and near-IR ranges with a quality sufficient for mathematical analysis. Experimental results prove the prospects for the practical use of such imaging systems. It should be noted that a SWIR camera with a Russian-made InGaAs sensor (JSC “SPA “Orion”, Moscow) was used for imaging in the IR range. Thus, a system has been implemented that makes it possible to obtain brightness-enhanced images of an object shielded by broadband noise simultaneously in the visible and near-IR spectral ranges with a time resolution of at least 100 μs. A model based on an I-controller with a delay circuit has been modified for a theoretical study of the features of the operation of functional converters on metal vapors. The model is built in the Simulink package. The simulation results are in very good agreement (deviation less than 10%) with the obtained experimental results in terms of the effect of delays on the contrast of the formed images and the value of the output signal power. A model of a functional converter of optical signals is proposed, which describes the process of signal amplification, including at sufficiently low input signal levels. The limiting characteristics of high-power signal formation systems with a given intensity distribution in systems with functional optical signal converters are experimentally determined. Input signal power, intensity distribution, as well as the time delay between the excitation pulses of the functional converter and the illumination source were varied. An experimental setup of a MOPA system with an illumination source (l=50 cm, d=3.2 cm, V=402 cm3) and a brightness amplifier (l=90 cm, d=5.0 cm, V=1767 cm3) on copper bromide vapor has been implemented. Synchronization of pulse-repetition modes of active elements provides radiation power Pout=4.5 W at an input signal power of 150 mW, which corresponds to a gain G=30. Equipping the experimental setup with an optical system makes it possible to use it as a bistatic laser monitor that provides the formation of signals with a given intensity distribution (images) with high contrast. For comparison, the imaging results obtained in a monostatic scheme of a laser monitor and by the laser illumination method are also presented. The results demonstrate that the laser illumination method provides the lowest brightness and contrast of images, since it eliminates the amplification of the generated signal in the active medium due to population inversion. It is obvious that both of these indicators increase in the monostatic scheme of the laser monitor, however, the local contrast of the image is still less than 10 relative units. The level of contrast is limited by the technical noise of the brightness amplifier, caused by parasitic reflections of amplified spontaneous radiation from the lens. Technical noise leads to an increase in the brightness of the background, which negatively affects the image quality. It is possible to significantly reduce the influence of noise in the scheme of a bistatic laser monitor, since its principle of operation allows one to direct a useful signal into the active volume of the amplifier before parasitic reflections. In cases where the influence of noise is insignificant, a bistatic laser monitor still contributes to an increase in contrast, since the object of observation is additionally illuminated by laser radiation from an independent illumination source. The contrast of images obtained in a bistatic scheme of a laser monitor is an order of magnitude higher than at imaging in a monostatic scheme or using laser illumination. It has been established that the contrast of images obtained in a bistatic laser monitor largely depends on the background, which in turn is determined by the enhanced spontaneous emission, the level of technical noise, and the projection screen. Detailed investigations of the amplifying properties of active media on vapors of sodium and potassium halides have been carried out. A special cell was made and experimental studies of the amplification characteristics during pumping in the UV range were carried out, and the spectra of luminescence and amplified spontaneous emission (ASE) were recorded. The first alkali halide (AH) salt that was loaded into the cuvette was the sodium iodide salt NaJ. Starting from a cell temperature of 450 degrees C, pump radiation with λ = 212.8 nm, focused at the center of the cell, excited spontaneous emission at the known sodium resonance lines λ1 = 589.6 nm and λ2 = 589.0 nm. As the cell was heated to a temperature of 600 degrees C, the radiation of the long-wavelength sodium line with λ1 = 589.6 nm switched to the ASE mode. During the experiments, it was found that the reason for the irreversible breakdown of the ASE mode is due to the appearance of stationary absorption on the resonance lines of the sodium atom. Optical pumping of NaBr vapors by radiation with λ = 212.8 nm was accompanied only by the appearance of spontaneous emission on resonance lines. However, none of the resonant transitions succeeded in realizing the transition from the spontaneous emission mode to the ASE mode. Similar studies were carried out for potassium salts. It was possible to get ASE only on KBr. Even at a cell temperature of 560 degrees C, spontaneous emission appeared at resonant transitions of potassium with wavelengths λ1 = 770.0 nm, λ2 = 766.5 nm. In the future, it seems appropriate to change the design of the cuvettes for the preparation of vapors of alkaline salts. First of all, it means a decrease in the length of the vapor column of alkaline hydrogen salts up to the size of the constriction region.

 

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Annotation of the results obtained in 2019
The first stage of the project was focused on development and design active elements of optical signals converters in the visible and near IR ranges. In total 6 active elements were made. 4 active elements were designed to study the possibilities of the functional conversion in media on self-terminating transitions of alkali metal atoms. Two longitudinally pumped active elements have an active zone length of 8 cm. Both of them have the steel insert an outer diameter of 1,4 cm, an inner one - 1,2 cm. The metal (active substance) enters through the wick, which represents a mesh with cell (0,5x0,5). The active element with a square section (5x5 cm) is used for transverse pumping. The cooled flanges provide the introducing optical radiation - this prevents contamination of the exit windows. High pressure cell was developed and designed too. Its internal pressure can reach high 10 atm. 3 active elements were design to study of functional conversion on self-terminating transitions of copper and manganese atoms with capacitive type of excitation. One of the active elements was designed to detailed study the excitation of an active medium by a capacitive barrier discharge. The temperature operating mode was provided by special external independent PID controllers. These active elements has a length of 50 cm and diameter of 2,5 cm. The design of each active element allow to change the type of buffer gas and its pressure. The stability of the excitation pulses is ensured no worse than 2 ns. The pump source for high-frequency excitation of functional converters on self-terminating transitions in metals was developed. The pulse repetition frequency (PRF) varied up to 150 kHz, amplitude of excitation pulse reached 8 kV, consumption power - 500 W. The stability of the excitation pulses is ensured no worse than 2 ns. The switching characteristics of the commutators, their synchronous triggering, as well as the PRF, determine the efficiency of the excitation. Transistors IRZ60R037P7 are used as commutators, ultrafast IXDD609 used as drivers. The control pulse duration is 40 ns and its front duration is 10 ns. The pulse jitter is no more than 2 ns. The developed power supply was used to pump gas discharge tube (GDT) with an active zone length of 25 cm and a diameter of 1 cm. The radiation power reached 100 mW with a plane-parallel resonator at a 150 kHz pumping frequency and a 3 kV excitation voltage. The amplitude of the current through the GDT was 10 A with 340 W of power consumption. The concentration of the working substance was determined based on the maximum power radiation. The radiation was generated mainly at a wavelength of 578.2 nm (90%) with increasing PRF from 100 kHz to 150 kHz. A prototype of a bistatic laser active optical system was developed to study the transfer, amplification and frequency-energy characteristics. The system consists of illumination sources and functional converters on self-terminating transitions of copper and manganese atoms, excitation units, synchronization units and units for the formation of a given intensity distribution. The functional converters is pumped by capacitive type of excitation. Optical and electrical characteristics are detected by special equipment. MegaSpeed-103, AOS Q-PRI (10-100 μs) and SWIR cameras (NPO Orion) are used for high-speed recorded of optical images. The digital synchronization system based on STM32F03C provides a coordinated operation modes of the functional converter (brightness amplifier) ​​and the laser (illumination source). The coaxial photocells and Thorlabs DET10A/M photodiode are used to detect the signals in visible range and the InGaAs biased detector (DET10N/M) is used in near IR. A program for evaluating the performance of a functional converter on transitions of metal atoms was developed. The program determines the numerical (informative) data of the images: average brightness, entropy, arithmetic mean values ​​for the mean square error and the normalized correlation coefficient. The program can process an array of images obtained at various operating modes of the active optical system. The current image is also compared with the template. The dependence of the coefficient of one-pass amplification (amplification characteristic) and superradiance power (transfer characteristic) on the concentration of the working substance are determined. The active medium on manganese atom transitions was researched. The behavior of these parameters is preserved with increasing PRF up to 125 kHz. Despite the rather large input powers, the amplification factor decreased quite significantly even at a container temperature of 590 C. It should be noted that in the framework of this study measurements were performed for the visible range of the spectrum - it means that radiation was recorded at the lines of 534,1 and 542 nm. Obviously, a change in the concentration of the working substance and, in general, the operating mode of the active element leads to a redistribution of amplification between transitions in the IR and visible regions of the spectrum. The images of the object under various operating conditions were obtained to demonstrate the distortions of transmitted signal (image). For the first time, images in a laser monitor with a SWIR camera were detected. The sensitivity of the developed functional converters was determined in a wide frequency range (from 10 to 100 kHz). To evaluate the sensitivity of the functional converters of optical signals, it is necessary to calculate the own noise of the active element. It is obvious that media on self-terminating transitions of metal atoms are narrow-band amplifiers. Therefore, both the gain band and the background energy (noise) are very important factors. It was found that the contribution of collisional broadening is 0,5% of the total broadening - it can be explained by the low pressure of the buffer gas in the active element. The resulting gain profiles (gain band) for transitions of the copper and manganese atoms were determined by use he Voigt function. The base width of these profiles are 2,06 pm (510,5 nm), 2,34 pm (578,2 nm), 2,6 pm (534,2 nm and 542 nm), 6,26 pm (1289 nm), 6,47 pm (1332 nm); 6,62 pm (1362 nm). The noise level is about 11 pJ for “visible” lines and 0,35 pJ for IR transitions. The noise energy drops to 2 pJ for “visible” transitions and to 0,05 pJ for IR transitions with PRF increasing to up 100 kHz. The results make it possible to formulate requirements for the input signal - their duration (from 20 to 40 ns), spectral width (not more than 2 pm) and energy (not less than 22 pJ). Verification was carried out according to available experimental data. Within the framework of the stage, a zero-dimensional kinetic model of the active medium on transitions of a manganese atom with a buffer gas neon was developed. To calculate the rates of processes in a plasma, experimental data and analytical approximate formulas were used. In particular, the Meve formula was used to excite by electron impact on electric dipole transitions. The approximation proposed by Meve gives strongly overestimated reaction rates for quadrupole transitions, that is why we used a reduction coefficient for them, which was estimated from the data for copper atoms. The Lotz formula was used for ionization by electron impact. The rates of de-excitation and triple recombination processes were calculated based on the ratio of the detailed balance. The oscillator strengths of the corresponding transitions were found in open access work. The model contains equations for the concentration of plasma components (manganese atoms and neon in each of the states taken into account), equations for the density of photons for each of the considered laser transitions and an equation for the electron temperature. To evaluate the energy input, the model uses differential equations that describe the pump circuit and the discharge circuit. Model was used to study the medium at transitions of a manganese atom in a gas discharge tube 30 cm long and 1 cm in diameter with neon under a pressure of 30 torr. To pump the active medium, we used a direct capacitance discharge circuit with a value of 500 pF charged to 10 kV at a PRF of 15 kHz. The time dependences of main plasma components were calculated for the active element. The lack of direct experimental data characterizing IR transitions is one of the main difficulties in developing a model of active medium on transitions in manganese vapor. Therefore, part of the data on these transitions was obtained by comparing the experimental temporal and energy characteristics of radiation at IR transitions with the results of kinetic modeling. Thus, our model made it possible to study IR transitions on the basis of indirect data that we can measure in experiments. To study in detail the processes in the active medium at transitions of metal atoms, a special spatiotemporal kinetic model was developed. This model allows to set the parameters of the input optical signal. The spectral composition of the input signal coincided with the amplification profile of the copper vapor medium. The developed model allows to select the radiation into two components: amplified spontaneous emission (ASE) and radiation formed as a result of amplification of the input signal (AIS). The integral characteristics were determined with an input signal of such duration that it overlaps the entire inversion duration in a medium with an active zone length of 20 cm and a diameter of 1 cm. The signal was set uniform in time and space, parallel to the axis of the GDT. It is shown that the sensitivity of the output signal to small changes in the input signal decreases monotonically with increasing power of the input signal. This means that the amplification of the input signal dominates the change in the ASE. It should be noted that in spite of the highest value of differential amplification, the region of small input signals is also characterized by a very small ratio of AIS to ASE - this makes direct measurement of the output signal problematic. This problem can be partially solved by subtracting from the entire output signal USI, previously measured in the absence of an input signal. According to the results of the stage, 8 articles were prepared. 4 of them were published in the reporting period.

 

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Annotation of the results obtained in 2020
The speed (pulse repetition frequency) of active elements on self-terminating transitions in metal vapors was increased. The diagnostic gas discharge tube (GDT) was designed. The design includes current and voltage probes. The efficiency of energy transfer to the active medium under contactless excitation is estimated. Discharge images at different high voltage (from 1 to 8 kV) were recorded synchronously with waveforms. Each frame was formed in 3 us. Record amplification (emission) frequency was achieved upon excitation by a longitudinal capacitive discharge by optimizing the operating mode of active elements at the transitions of the copper and manganese atoms, as well as optimizing the excitation mode. Radiation power was 490 mW at PRF of 100 kHz, 195 mW at PRF of 125 kHz and 60 mW at a PRF of 150 kHz. The amplitude of the excitation current pulses decreased from 23.5 A to 13 A. The beam diameter decreased from 0.7 to 0.5 cm. It should be noted that excitation mode did not lead to a drop in the radiation intensity at the center of the beam, although it is typically for active elements with traditional pumping. The barrier capacitive discharge does not allow to use of a hydrogen-containing additive while increasing the PRF excitation. With a decrease in the capacitance of the electrodes, the threshold value of the pump power (for the power source) was 1600 W. The need to exceed the threshold value of the discharge voltage and specific energy input did not make it possible to obtain lasing at frequencies above 100 kHz in the presence of an HBr additive. The design of the semiconductor pumping source was modified and optimized based on model and experimental research. The switching parameters were optimized for each element. The PRF of the excitation pulse was increased to 200 kHz, the consumption power was 600 W. The voltage rise time on the switch was 4 ns. The rise time of the current at the load (GDT) was 30 ns. The experimental and theoretical research of functional converter amplifying characteristics were performed, including those for individual spectral ranges: medium at the transitions of the manganese atom – IR (1.29 um, 1.33 um) and visible (534.1 nm, 542 nm) ranges; medium at the transitions of the copper atom – visible (510.6 nm, 578.2 nm). The influence of the input signal parameters on the own noise and amplification, including when transmitting a signal with a given intensity distribution was analyzed. The signal conversion within the framework of one pulse has been investigated. The dependence of the active medium gain on the input signal level, including at each spectral range, was established. The gain of a small signal at a PRR of 100 kHz is 0.2 cm ^ -1. The sensitivity of the functional converter is (0.004 ÷ 0.027)% of the superradiance power. Such studies were performed for each spectral line of the functional converter under various conditions. Based on the developed at the previous stage zero-dimensional model of the active medium on manganese vapor, the radial (one-dimensional space-time) model of the Mn atom active medium was constructed. The experience of developing a model of an active medium based on copper vapors showed that the introduction of a radial variable into the model allows not only to study the radial profiles of plasma parameters, but also provides much better agreement with experiment for integral characteristics, especially at high frequencies for small tubes. Although diffusion processes were taken into account in the zero-dimensional model, with a change in the tube thickness and PRF, the radial concentration profiles can change significantly, which does not allow making an universal account of diffusion processes without solving the partial differential equations. The issues of the concentration of manganese ions on the GDT wall boundary conditions are considered separately, since it choice is non-trivial. In all the known works, the mathematical description of the discharge in active media on metal vapors, a homogeneous boundary condition of the first kind was used, i.e. the concentration of ions was assumed to be zero on the GDT walls. This is the approximation of a contracted discharge. In this case, we have no right to set a boundary condition of the first kind on the electron temperature on the walls of the GDT, assuming it to be equal to the temperature of the GDT walls. If there are no ions in the near-wall region, then there should be no electrons either, i.e. the electron gas does not interact with the walls of the GDT. On the basis of the performed experimental studies, the threshold pump levels were determined. At this pumping conditions, the radiation of the active medium on the transitions of the sodium atom is detected with confidence. For pumping, we used the second harmonic of a solid-state laser (manufactured by LOTIS), which was tuned using a dye. For pumping sodium, the most optimal is Rhodamine S. The pump pulse energy was 50 mJ, the pulse duration was 18 ns. As part of this stage, a working model of a bistatic laser active optical system was implemented. With the use of the bistatic laser active optical system was used for imaging of the processes hidden by the background radiation. The maximum shooting speed was 15000 frames / sec, the duration of the illumination and amplification pulse was 45 ns. The achieved results are presented in 7 scientific publications, including 3 in editions from the first quartile (Q1).

 

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