Short-wavelength Part Research Articles

Chirped fiber Bragg gratings directly inscribed by a femtosecond laser in an ytterbium-doped fiber for spectrum recovery, loss compensation and effective use in a CPA scheme

Point-by-point femtosecond laser inscribed chirped Bragg gratings are good candidates for use in fiber chirped pulse amplification systems due to the high flexibility of their dispersion properties, low cost, and no length limitation. However, such gratings have two significant drawbacks: high losses and distortions of the short-wavelength part of the spectrum when operating in the normal group velocity dispersion mode. We propose a novel approach based on writing such gratings in a highly doped active fiber, which allows us to correct both shortcomings. We have also demonstrated, for the first time to our knowledge, a successful use of such gratings as stretchers in a fiber chirped pulse amplification system.

Effects of subcell bandgap matching on the performance of perovskite/Cu(In,Ga)Se2 tandem solar cells

The tandem solar cell is composed of a wide bandgap top subcell and a narrow bandgap bottom subcell, which is a promising technology to surpass the theoretical efficiency of single-junction solar cell. Since the currents are generated by photo-generated carriers and the two subcells are united in series, the absorber thickness should be adjusted with the change of the bandgaps to ensure that the two subcells have the same currents. In this study, the impacts of subcell bandgaps on perovskite/Cu(In,Ga)Se2 tandem solar cells were investigated using SCAPS-1D. For perovskite top subcells, the short-circuit current and power conversion efficiency increased with the absorber thickness increasing, and photons in short-wavelength part could be absorbed by perovskite solar cells, while photons in long-wavelength part were almost not absorbed. In tandem solar cells, the increasing bandgap of top subcell and the decreasing bandgap of bottom subcell required higher top subcell thickness to achieve the current matching condition. The top subcell bandgap needed to be wide enough and the bottom subcell bandgap needed to be narrow enough to get the same current. The efficiency increased as the top subcell bandgap increased to 1.7 eV, and started to decline with the further increase of bandgap. Moreover, the efficiency increased as the bottom subcell bandgap increased up to 1.28 eV, but decreased when the bandgap kept increasing. The optimized tandem solar cell achieved the highest efficiency of 32.71 % with the top subcell bandgap of 1.7 eV and the bottom subcell bandgap of 1.28 eV.

Detection of focused beams of surface magnetostatic waves in YIG / Pt structures

The purpose of this work is to experimentally study, using the inverse spin Hall effect (ISHE), the detection of focused beams of magnetostatic surface waves (MSSW) in integrated YIG (3.9 µm) / Pt (4 nm) thin-film microstructures, where the focusing effect was ensured by the curvilinear shape of the exciting antenna. Make a comparison with the case of detecting MSSWs excited by a rectilinear antenna. Methods. Experiments were carried out using the delay line structures based on the YIG/Pt. The amplitude-frequency characteristics of the YIG/Pt structure and the frequency dependence of the EMF (V(f)) induced in platinum were studied. Results. It was shown that at frequencies f near the long-wavelength limit of the MSSW spectrum, the magnitude of the EMF V(f) generated by a focused MSSW can be several times higher than the values of V(f) in the case of MSSW excitation by a common (straight) antenna. In this case, in the short-wavelength part of the spectrum, on the contrary, the magnitude of the EMF generated by the focused MSSW beam turns out to be noticeably smaller. This behavior is associated with chromatic aberration of the focusing antenna for the MSSW, which manifests itself in the frequency dependence of the focal length of the antenna, which is confirmed by the results of micromagnetic modeling. It is shown that the drop in the EMF signal generated by a focused MSSW beam in the short-wavelength part of the spectrum is associated with the focus reaching the area of the YIG not covered with the Pt film. In this case, the increase in V(f) in the long-wavelength region of the MSSW spectrum is explained by an increase in the linear power density of the MSSW and the formation of caustics under the Pt film. Conclusion. Obtained results can be used for the development of highly sensitive spin wave detectors and the creation of spin logic devices.

Кремниевые дифференциальные фотоприемники. Технология, характеристики, применение

A silicon-based photodetector containing two identical n+-p photodiodes is described. One of the photodiodes had a wide spectral response with high sensitivity in the ultraviolet region. The sensitivity of the second was reduced in the short-wavelength part of the spectrum by creating additional recombination centers in the near-surface region by implanting As ions. The spectral sensitivity of the differential signal obtained by subtracting photocurrents had a pronounced short-wavelength characteristic. The long-wavelength limit of the spectral range in terms of the λ0.5 level, depending on the doping dose, was in the range of 0.37 — 0.47 μm. The sensitivity maximum corresponded to λmax=0.32 — 0.37 μm. The electrical and noise characteristics of the photodetector are given. The possibility of using differential photodetectors as two-color ones is shown.

Temporal contrast improvement through cascaded second-order nonlinear processes in a thin BBO crystal.

We demonstrate temporal contrast improvement through cascaded second-order nonlinear processes in a 340-µm BBO crystal. The process was initiated by second harmonic (SH) generation, followed by difference frequency generation (DFG) between the SH and the short wavelength part of the fundamental wave (FW). The idler of DFG was selected by a spectral filter, and an output pulse energy of 573 µJ was obtained at 1 kHz with excellent spatial profile and a power fluctuation as low as 0.076% (rms) in 14 hours. The temporal contrast was improved by more than 2 orders of magnitude to approximately 1011, which could be further enhanced with different spectral filters. The excellent stability, energy scalability, and contrast enhancement ability make this simple and robust method very suitable to be integrated into the pulse cleaning system in many different ultra-intense laser facilities.

Features of Carbon and Silicon Thin Films Reflection Spectra in the Visible Region

The article is devoted to the reflection of visible light from a thin absorbing film. For a number of reasons, thin films have recently been the subject of close study. As part of the work, several non-metal films were fabricated on a glass substrate: silicon films (by resistive evaporation) and carbon films (using laser evaporation). The resulting films have an optical thickness in the range of 1 μm (that is, the actual layer thickness is in the range of 0.2-0.4 μm) and at the same time noticeably absorb the light flux. The films were processed according to the standard method in the visible region using a USB4000 spectrometer. Reflection spectra for each of the films were plotted for several different points on the surface. The spectra look similar when differences in numerical values are not considered. The contribution of the interference of rays reflected from the front and rear boundaries is significant at long wavelengths (in the red region), whereas the interference practically disappears against the background of instrument noise in the short wavelength region (violet part of the spectrum). The reason for this is that absorption during propagation of the short-wavelength part inside the film significantly reduces the intensity of the beam reflected from the rear surface of the film.

Monitoring of natural water pollution by fluorescence spectroscopy

Introduction. Environmental monitoring of the state of water bodies is an important component of environmental protection. A promising direction for assessing the quality of water bodies is to determine their generalized indicators. Fluorescence spectroscopy can serve as a source of important information about the state of water bodies and sources of their pollution. Purpose of the study - to assess the water pollution of the Moskva River and its tributaries using the fluorescence spectroscopy method. To reveal the influence of oil products on the fluorescence spectra of river water. Materials and methods. The object of the study was water from the Moskva River and its tributaries: Pakhra, Kotlovka, Setun. Water samples from the Moskva River were taken in different parts of the city, which are characterized by varying degrees of pollution. Water fluorescence spectra were measured with SM2203 spectrofluorimeter (ZAO Solar, Belarus), total organic and inorganic carbon, on a TOC-VCHP total carbon analyzer (Shimadzu, Japan). Results. The nature of the fluorescence spectra of the Moskva River and its tributaries in the absence of significant anthropogenic pollution are characterized by relative constancy. Bacterial pollution and pollution with aromatic compounds of anthropogenic origin, including oil products, leads to an increase in the fluorescence intensity in the short-wavelength part of the spectrum. The correlation coefficient between the optical density at a wavelength of 254 nm and the content of dissolved organic carbon (DOC) for the studied samples was 0.66, and between the fluorescence intensity at a wavelength of 400 nm and DOC was 0.74. Limitations. The method cannot be used for the quantitative determination of individual substances; it is intended for a semi-quantitative assessment of water pollution. Conclusion. Measurement of fluorescence spectra and total organic carbon makes it possible to give a general assessment of the contamination of a water body (which is especially important for a sample of unknown composition) and to choose the optimal scheme for its targeted analysis. High dissolved organic carbon (DOC) values and high fluorescence intensity at 320 nm are highly likely to indicate chemical contamination of water, while DOC values characteristic of a water body and high fluorescence intensity at 320 nm indicate bacterial contamination.

О некоторых возможностях многочастотного дистанционного зондирования водной поверхности

This study is aimed at expanding the number of measured parameters to analyze the features of the formation of surface waves under the influence of wind. The paper develops an original approach to obtaining information on the variability of the short-wavelength part of the wave spectrum (examples are given for wavelengths from about 50 cm to 2 cm in 6 intervals) and the long-wave component of the wave spectrum (> 1 m) in marine conditions. Retrievable information about waves will allow studying the interaction of wind simultaneously with the short-wave and long-wave components of the wave spectrum and will be in demand by scientists involved in numerical modeling of the wave climate and interested in refining the model of near-surface wind interaction with waves. In addition, new information about the short-wave part of the wave spectrum in different wavelength ranges will improve the accuracy of near-surface wind speed retrieval from remote sensing data. This work is devoted to a theoretical analysis of slope variance retrieved from reflected acoustic pulses for different radiation frequencies depending on the near-surface wind speed and swell wave height. A study was made of the measured slope variances of large-scale waves, compared to the radiation wavelength, in six intervals depending on the wind speed and swell height. It is shown that the use of difference slope variances of the large-scale wave makes it possible to get rid of the influence of swell in the case of mixed waves and obtain a better correlation with the wind speed. A method is proposed for retrieving the declination degree for the spectrum of wave heights in given intervals of radiation wavelengths. Within the framework of this method, it is possible to retrieve the boundary wave numbers for each radiation wavelength.

Postirradiation Transmission Characteristics of CYTOP Fiber Exposed by Gamma Radiation

We investigated offline the evolution of radiation-induced attenuation (RIA) in polymer perfluorinated cyclic transparent optical polymer (CYTOP) optical fiber irradiated with different gamma radiation doses of 1–50 kGy. We considered the wavelength range varying from 800 up to 1750 nm. The RIA spectra measured at different times after irradiation have qualitatively similar shapes. However, quantitatively the RIA evolution significantly depends on the spectral range: in the short-wavelength part, the transmission partly recovers, while in the infrared, at wavelengths above 1300 nm the RIA grows and then saturates becoming permanent. The effects of temperature and photobleaching on RIA dynamics are examined. The ability of the fiber transmission recovery is investigated in the 600–1750-nm range with the use of climatic chamber.

Satellite magnetic anomalies with a smooth spectral transition to long wavelengths

In most cases, the magnetic field anomalies from satellite data and models are determined by using a cut-off degree that splits the Earth's internal magnetic field (supposed to be of the core and lithospheric origin) into long-wavelength and a short-wavelength parts. This practice allows to effectively compare the signal of both components with other models and controls the signal content as well. In this study an alternative approach is followed to provide the components in long and short wavelengths without cutting the spectrum. Instead, we seek iteratively for the signal loss when the magnetic signal (from degree 1 to the maximum degree of the model) is propagated from the core-mantle boundary to satellite altitude. The problem is generally non-unique, it is required that the signal fit has to follow the slope of the geomagnetic spectrum over the lowest degrees (the degrees least affected by the magnetic lithosphere). The tool operates in the spatial domain – the spectral slope criterion is met after the procedure ends by selecting an appropriate solution. The method can be applied to the vector components individually as soon as they are given in the Earth-fixed Earth-centered frame, for which the integral equation is valid. The obtained magnetic anomalies have different spectral properties because the transition from the long wavelengths to the short wavelengths becomes smooth, although the appropriate number of iterations is questionable. It is found that the long-wavelength signal hands over the magnitude dominance to the short-wavelength part at around degrees 16 to 18.

Ozone and ultraviolet in the Earth's atmosphere

The appearance of the ozone layer (OL) in the Earth’s atmosphere and the associated processes of attenuation of the ultraviolet (UV) part of solar radiation reaching the surface are considered. It is shown that the role of OL in protecting living organisms from the destructive effects of exposure to the short-wavelength part of the insolation spectrum is immensely exaggerated. Ozone is generated by UV radiation below the ionosphere when oxygen molecules in the air are divided. When absorbed by UV rays, it is destroyed. Ozone exists for the longest time at minimum temperatures of the atmosphere. This circumstance and the increased density lead to the accumulation of O3 above the tropopause, where the OL is formed. The amount of simultaneously existing gas is extremely insignificant — about 10⁻⁶ air. It is this circumstance that makes it possible to estimate its contribution to the absorption of UV rays as vanishingly small. Compared with it, the scattering of rays by air molecules is incomparably more effective, although each single act of absorption of the middle part of the ultraviolet is several times greater than a single result of scattering. One can agree with the idea of the elimination of ozone molecules by reaction with freons and other gases containing halogens, including those of volcanic origin. It is also possible that it interacts with hydrogen, but H2 rather arising in the atmosphere, and not in the bowels of the Earth. But the processes of changing the OL, including the periodic formation of «ozone holes», do not pose any danger and are explained by natural phenomena, and not by human activity. A very expensive campaign to save the layer makes no sense. On the other hand, ground5level ozone concentration increases over time precisely under the influence of industrial emissions. In the cities of developed countries, the content of this poisonous gas already often exceeds permissible levels and it is time to take up more effective counteraction to its accumulation.

Investigation of the dynamic parameters of electroluminescence in different parts of the spectrum in local regions of the light-emitting heterostructure

A method for measuring the distribution of the differential charge carriers lifetime over energy levels in the local regions of a light-emitting heterostructure is presented. The method has been tested on commercial green InGaN-based LEDs. It has been determined that with an increase in the energy level (with decreasing wavelength), the charge carriers lifetime decreases. It is shown that the relative inhomogeneity of the distribution of the charge carriers lifetimes in local regions of the die in the long-wavelength part of the electroluminescence spectrum is higher than in the short-wavelength part of the spectrum.

Sonoluminescence and circulating light

The phenomenon of sonoluminescence is that in ordinary cold water under the action of a spherical standing acoustic wave, a brightly luminous ball with anomalous properties appears in the center of the sphere. The most unusual is the spectrum of its radiation. Estimates based on the spectral analysis show that the temperature of the luminous ball is above a hundred of thousands of degrees. There is no satisfactory explanation for the emergence of such a spectrum. We show that the shift of radiation to the short-wavelength part of the spectrum is explained by the compression of this light with a decrease in the diameter of this ball. The disappearance of the bands in the spectrum is explained by the fact that at each moment of time the observer sees a set of radiations that have arisen at different moments of time and have undergone different degrees of compression. Thus, the observed emission spectrum of the ball corresponds not to the radiation of an absolutely black body heated over a hundred thousand degrees, but to the radiation of the circulating light arising from the adiabatic compression of gases inside the ball. Attempts to use the extremely high temperature inside the ball to carry out a thermonuclear fusion reaction are known. From the above consideration, it can be concluded that such attempts are futile since there is no extremely high temperature inside the ball.

Narrowband MSM Detector of the Visible Part of the Spectrum Based on a ZnCdSe/ZnSSe/GaAs Heterostructure

The results of experimental studies of photodiodes based on interdigital MSM (metal–semiconductor–metal) Schottky barrier contacts to a ZnCdSe/ZnSSe/GaAs heterobarrier structure are presented. The detector provides a narrowband response (FWHM = 4.3 nm) at a wavelength of 460 nm, a sharp drop in the photosensitivity in the short-wavelength part of the response signal, high ampere-watt sensitivity, and low dark current.

Analysis of limitations on recovery of gravity field based on satellite gravity gradient data

Although satellite gravity gradient data plays a great role in determining short-wavelength part of static gravity field model, accuracy of the long-wavelength part of gravity field model recovered by them are poor, which leads to only a few applications in time-variable gravity investigation. The reason is that some factors limit the accuracy of the gravity field recovered using gradient data, including accuracy of the gravity gradient observations, measurement bandwidth (MBW) of gradiometer, satellite inclination, etc. This paper aims at analyzing the influence of these limitations on gravity field recovery and discusses the possibility of time-variable gravity field detection by using gravity gradient observations. Firstly, for arbitrary satellite orbit inclination, we give the frequency distributions of all the components of gravity gradients (i.e. Txx,Tyy,Tzz,Txy,TxzandTyz, ). The results show that the maximum frequency of each component of the gravity gradients is the same, i.e. l/Ts (l is degree of the gravity field model, Ts is the orbital periods), and it is not influenced by the inclination of the satellite orbits. Secondly, the paper gives a theory proof to explain why only the low orders of the coefficients are influenced by polar gaps. Big polar gaps are experimented by a numerical test with inclination of 45°. Finally, considering that the measurement bandwidth can be expanded and accuracy of gradient observations can be improved by superconducting gravity gradiometer (SGG) compared to gradiometer used in Gravity field and steady-state Ocean Circulation Explorer (GOCE), the possibility of detecting time-variable gravity using gravity gradient observations is discussed. The results show that the SGG creates errors in MBW with magnitude of 0.014 mE, which is smaller than the magnitude of the time-variable gravity gradient signals (i.e., 0.02 mE) derived from Gravity Recovery and Climate Experiment (GRACE) gravity field models. This indicates the potential of SGG in time-variable gravity detection.

Scattering of small-amplitude phonons on discrete breathers in Fermi-Pasta-Ulam-Tsingou chain

There are two traditionally discussed dispersion relations (DR) in condensed media: gapless phonon DR and DR with an energy or frequency gap. The third type of DR is of interest in various fields of condensed matter physics: states corresponding to gap effects in k-space (Gapped Momentum States - GMS). The growing interest in GMS is associated with important consequences for the dynamic and thermodynamic properties of the system (hydrodynamic turbulence, plasticity, fracture). Traditionally, GMS arise in the Maxwell-Frenkel approach as applied to the viscoelastic properties of liquids and solids, when the DR-gaps can continuously vary from energy to momentum space. This work is the first in a series of studies devoted to the analysis of dispersion effects associated with the anharmonicity of the potential and the emergence of collective breather-type modes, the so-called discrete breathers (DBs), and their influence on the macroscopic properties of nonlinear lattices, for example, on the heat conductivity. Recently, the influence of discrete breathers (DBs) on the macroscopic properties of nonlinear lattices, for example, on thermal conductivity has been investigated. When solving this problem, it is important to know how phonons interact with DBs. The scattering of phonon wave packets of small amplitude by standing DBs in the β-Fermi-Pasta-Ulam-Tsingou (β-FPUT) chain is studied numerically for different amplitudes of DBs. It is found that DBs of sufficiently large amplitudes reflect short-wavelength phonons, but remain transparent for long-wavelength phonons. An increase in the DB amplitude expands the reflection region in the short-wavelength part of the first Brillouin zone. These results suggest that DBs in the β-FPUT chain do not strongly affect the thermal conductivity, since heat is transferred mainly by long-wavelength phonons, which are weakly affected by DBs.

The effects of screen light filtering software on cognitive performance and sleep among night workers.

Background: Previous studies have reported impaired performance, sleepiness and sleep deprivation among night workers. The purpose of this study was to investigate the effect of color screen Light Filtering software on cognitive performance, alertness and sleep quality among night shift operators of a medical emergency operations center. Methods: This field trial interventional study was carried out among 30 nightshift operators of shiraz emergency control center. The baseline assessments were carried out under the existing computer screen light conditions in the week preceding the installation of f.lux software. The same measurements were repeated again 4 weeks after installing the software. The cognitive performance of the participants was measured using continuous performance test (CPT) and n-back, while their sleep quality was assessed through Pietersburg Sleep Quality Index (PSQI). Further, to assess their subjective and objective alertness, Stanford sleepiness index and go/nogo test were used, respectively. Results: The results of this study showed that Screen Light Filtering software significantly increased subjective (P<0.001) and objective alertness (P<0.05). Additionally, the performance of the working memory (P=0.008) and sleep quality (P=0.008) improved significantly after the intervention. Conclusion: The results revealed that using Screen Light Filtering software is an effective and low-cost method to improve sleep quality and cognitive performance since it filters the short wavelength part of the spectrum and helps body adaptation.

SPECTRAL APPROACH TO THE ESTIMATION OF THE TIME SERIES OF THE POWER CHANGE IN THE SUBSTATION METERING UNITS AND ITS APPLICATION IN THE CONTROL MODES OF SUBSTATION ELECTRICITY METERING SYSTEMS

In order to create models of extended surfaces in the long and short wavelength parts of the wave spectrum, it is necessary to know the dispersion characteristics of tape-like obstacles perfectly located both in free space and on an extended surface. Methods of remote studying of extended surfaces, based on an analysis of signals that are generated by reflection or radiation of electromagnetic waves passing neoplasms allows us to view the topography, characteristics, and type of the different locations, sizes of various objects. One of the effective methods of study is to solve the problem of diffraction of electromagnetic waves on an infinitely extended object in free space. The purpose of this study is characteristics of electromagnetic waves during diffraction of electromagnetic waves at objects that can be used as radar systems. The method of statistical processing of experimental data from field tests and mathematical description of electromagnetic radar situation, as well as its computer simulation were used. The result of this study can be formulated as the development of analytical and software methods for analyzing diffraction pattern of electromagnetic waves in multi-wave mode of reception on infinitely extended object in free space, as well as a software method for calculating diffraction characteristics at frequencies exceeding the basic frequency, taking into account the inhomogeneity of electromagnetic fields in these conditions. The practical significance is the research results used in the development of a universal technique for design and development of radar systems capable of working adequately in conditions of electromagnetic environment in question. The results of this research will allow a complete description of the electrodynamic picture of wave propagation and its diffraction, namely, taking into account the diffraction properties of radar systems will allow us to develop a direction for their wider production and distribution.

Wavelength-adjustable mode-locked Tm-Ho co-doped fiber laser from 1839 nm to 1876 nm

Abstract We demonstrate a passively mode-locked Tm-Ho co-doped fiber laser using GaAs saturable absorber (SA). The laser has a very broad output spectrum covering from 1700 nm to 2100 nm with output wavelengths adjustable from 1839 nm to 1876 nm. The significant broadening produced by mode-locking is due to the laser-induced self-phase modulation (SPM). Results show that the peak wavelength is red-shifted with the increase of pump power. On the other hand, with the increase of the cavity length, the peak wavelength is blue-shifted. The red-shift generated by the increasing pump power may be due to the reabsorption of the short wavelength part of the laser by Ho3+ ions, while the increase in the blue shift due to the increasing cavity length is due to the absorption loss in the long wavelength direction inside the optical fiber at the broad-band laser since the long wave part of the output has already entered the absorption band of the fiber material. As the length of the fiber increases, the longer wavelength side of the laser output spectrum is absorbed more, causing a blue-shift of the peak wavelength. Therefore, the laser peak wavelength can be adjusted by changing the pump power or cavity length.

Forced convection heat transfer from an asymmetric wavy cylinder at a subcritical Reynolds number

The present study considered the asymmetric wavy (ASW) disturbance which has been confirmed as the passive control to modify the force coefficients Yoon et al. [25]. We evaluated the effect of the asymmetric wavy disturbance on the forced convection heat transfer. For the purpose of the comparison, the smooth (CY) and symmetric wavy (SW) cylinders are considered. Finally, the LES has been carried out to solve the momentum and energy equations governing the flow and thermal fields at the subcritical Reynolds number of 3000. The validation of the present numerical methods were successively done by comparing the force coefficients and the Nusselt number with the previous results. The ASW cylinder provided the smallest mean and fluctuation of the time-and total surface-averaged Nusselt number than the CY and SW cylinders. This dependence of the Nusselt number on the cylinder shape is correlated to the force coefficients. The time-and local spanwise surface averaged Nusselt number of the SW cylinder shows the regional dependent behaviors of an invariant region and an increasing region. The ASW cylinder shows the increasing and decreasing behaviors in the short and long wavelength parts. The double wavy formation of the temperature isosurface in the wake for the ASW cylinder is consistent with the 3D vortical structures. The most significant variation of the time-averaged local Nusselt Number appears in the upstream surface of the SW and ASW cylinders which induce the spanwise dependent incoming flow.