Propagation Of Optical Radiation Research Articles

Імітаційне моделювання процесів міграції фотонів в біологічному середовищі

The article developed an improved simulation model of the propagation of photons in scattering biotissue based on the Monte Carlo method, which allows comprehensive consideration of all the main optical processes that occur during the propagation of optical radiation through BT. On the basis of the developed statistical model, simulation modeling of photon propagation processes in the skin and its separate layers was carried out, which allowed to establish the scattering and transmission characteristics for each of the skin layers with model parameters taking into account the angles of incidence of radiation in the visible and near-IR ranges, the thickness of the skin layers. Also, the simulation made it possible to determine the relative intensity of scattered photons depending on localization.

Inverse scattering transform algorithm for the Manakov system

A numerical algorithm is described for solving the inverse spectral scattering problem associated with the Manakov model of the vector nonlinear Schrödinger equation. This model of wave processes simultaneously considers dispersion, nonlinearity and polarization effects. It is in demand in nonlinear physical optics and is especially perspective for describing optical radiation propagation through the fiber communication lines. In the presented algorithm, the solution to the inverse scattering problem based on the inversion of a set of nested matrices of the discretized system of Gelfand–Levitan–Marchenko integral equations, using a block version of the Levinson-type Toeplitz bordering algorithm. Numerical tests carried out by comparing calculations with known exact analytical solutions confirm the stability and second order of accuracy of the proposed algorithm. We also give an example of the algorithm application to simulate the collision of a differently polarized pair of Manakov optical vector solitons.

Sampling volume assessment for wearable multimodal optical diagnostic device.

The process and results of numerical Monte Carlo simulation of optical radiation propagation in laser Doppler flowmetry (LDF) and fluorescence spectroscopy (FS) channels of a wearable diagnostic multimodal device are described in this paper. To achieve the goal, a multilayer skin model with different parameters of blood and melanin content and different distances between sources and radiation receivers was designed. The changes in the sampling (diagnostic) volume depending on the anatomical features of the biological tissues, as well as on the technical parameters of the device were shown. Depending on the scattering media optical properties and the source-detector configuration of the device, the diagnostic volume can range from 2 to 7 mm3 . The obtained results allow the formation of specialized medical and technical requirements for wearable multimodal devices implementing LDF and FS channels.

Experimental study of the compensation method for atmospheric attenuation of probing laser radiation in active electro-optical systems that provide an increase of target image contrast

The article deals with the experimental study of the compensation method for atmospheric attenuation of laser radiation in active electro-optical remote sensing systems with dynamic spectral processing of optical signals that provides an increase of target image contrast. The compensation method for atmospheric attenuation of laser radiation in active electro-optical systems consists in forming the spectral intensity of the probing radiation not only on the basis of a priori data on the spectral features of the reflecting surfaces of the target and the background, but also taking into account the spectral transmittance of the optical radiation propagation medium. The experimental setup has been developed, in which the source of radiation in the transmitting part of the electro- optical system is three semiconductor lasers operating in the ranges of the red, green and blue parts of the spectrum. Absorption light filters were used in the experiment as elements simulating the spectral properties of the reflecting surfaces of the target and background. The cuvettes with a liquid absorbing optical radiation were used as elements simulating atmospheric attenuation.

Compensation method for atmospheric attenuation of laser radiation in active electro-optical systems with dynamic spectral processing of optical signals

This paper describes the compensation method for atmospheric attenuation of laser radiation, which can be implemented in active electro-optical systems with pre-detector dynamic spectral processing of optical signals. In these electro-optical systems, the spectral flux of sensing radiation is formed using multispectral laser signals based on a priori information about the spectral reflectance of the target and background. The sensing signal formed in this way ensures maximum suppression of the background signal at the output of system with minimal attenuation of the target signal. The influence of atmospheric radiation attenuation on operation of an active electro-optical system with dynamic spectral processing has been analyzed. It has been shown that the laser radiation attenuation in the atmosphere significantly affects the efficiency of dynamic spectral processing of optical signals (leads to a decrease in the target image contrast). The developed compensation method for atmospheric attenuation of radiation in active electro-optical systems with dynamic spectral processing is based on the fact that the spectral intensity of the sensing radiation is formed not only on the basis of a priori data on spectral characteristics of the target and background, but also takes into account the spectral transmittance of the optical radiation propagation medium.

Radiation shift from triple to quadruple frequency caused by the interaction of terahertz pulses with a nonlinear Kerr medium

High-intensity optical radiation propagation in a transparent dielectric medium causes the phenomena of pulse self-action and radiation generation at triple frequencies due to the cubic nonlinearity of the medium. However, quadratic nonlinear effects usually outshine the cubic ones in anisotropic nonlinear crystals. In this work, we demonstrate that for certain experimental parameters the nonlinear effect of the third order can be stronger than the second order one in the MgO:hbox {LiNbO}_3 crystal for terahertz frequency range. We experimentally and theoretically show that this effect can lead to the significant modification of the classical phenomenon of radiation generation at triple frequencies in the case when the pulse represents only one complete oscillation of the optical field. The experiment demonstrated that the phenomenon of generation of radiation at triple frequencies with respect to the frequency of the maximum spectral density in a nonlinear medium of the pulse disappears, and it is replaced by the generation of radiation at quadruple frequencies. The analysis confirms that this effect is based on the asymmetry and large width of the initial spectrum of such extremely short pulses in terms of the number of oscillations.

A three-axis optical radiation positioning device

On the basis of the conducted analysis, an optical radiation positioning device is developed, the working body of which is a 3-axis differential stepper motor that allows increasing the accuracy of positioning the direction of optical radiation propagation and expanding its functionality by introducing additional rotors designed in the form of spheres placed at an angle of 90º relative to each other in mutually perpendicular planes, passing through the centers of the spheres with a possibility of angular rotation around the three axes of the rectangular coordinate system. This device is a component of optical telecommunication networks and fiber-optic communication lines and can be used as a three-axis optical radiation positioning device. In the proposed device at the value of angular rotation steps of the rotors ranging from 4.0μm to 0.2mm the accuracy of rotor positioning in a step mode of operation is 1.4÷2.1μm.

Rationale for the Choice of the Ellipsoidal Reflector Parameters for Biomedical Photometers

Biomedical photometersʼ information-measuring systems with ellipsoidal reflectors have acceptable results in determining of biological tissues optical properties in the visible and near-infrared spectral range. These photometers make it possible to study the optical radiation propagation in turbid media for direct and inverse problems of light-scattering optics. The purpose of this work is to study the influence of the ellipsoidal reflectors design parameters on the results of biomedical photometry when simulating the optical radiation propagation in a system of biological tissue and reflectors in transmitted and reflected light.The paper substantiates the choice of the ellipsoidal reflectors’ focal parameter for efficient registration of forward and backscattered light. The methodology of the process is illustrated by the results of a model experiment using the Monte Carlo simulation for samples of human brain white and gray matter at the visible range of 405 nm, 532 nm, and 650 nm. The total transmittance, diffuse reflectance, and absorption graphs depending on the sample thickness were obtained. Based on the introduced concepts of the ellipsoidal reflector efficiency index and its efficiency factor, the expediency of choosing the ellipsoidal reflectors focal parameter is analyzed to ensure the registration of the maximum amount of scattered light. The graphs of efficiency index in reflected and transmitted light for different thickness samples of white and gray matter and efficiency factors depending on the sample thickness were obtained.The influence of the reflectors ellipticity on the illuminance of various zones of photometric images using the example of an absorbing biological medium – pig liver tissue – at wavelength of 405 nm with a Monte Carlo simulation was analyzed.The optical properties of biological media (scattering and absorption coefficients, scattering anisotropy factor, refractive index) and the samples’ geometric dimensions, particularly the thickness, are predetermined when choosing the ellipsoidal reflectors parameters for registration of the scattered light. Coordinates of the output of photons and their statistical weight obtained in the Monte Carlo simulation of light propagation in biological tissue have a physical effect on a characteristic scattering spot formation in the receiving plane of a biomedical photometer with ellipsoidal reflectors.

The study of the birefrigence modulator based on lithium niobate

This work investigates a lithium niobate X-cut birefringence modulator with a titanium diffusion channel waveguide. The wave voltage of the modulator depends on the growth and processing conditions of the lithium niobate crystal and on the technology of waveguide formation. The tolerance for determining the length of the electrodes and the gap between them exceeds 1 %. In this regard, the calculated values of the wave voltage can differ significantly, and an experimental measurement of the wave voltage is required for practical use. The authors present an experimental refinement for the value of the wave voltage of the modulator and perform a comparison of the value with the theoretical one. In the experiment, the wave voltage was determined using a scanning Michelson interferometer. It is shown that the experimentally measured value of the wave voltage diverges from the calculated one by more than 26 %. This difference is based on the assumption that the vector of the electric field inside the crystal is directed perpendicular to the axis of propagation of optical radiation, and the magnitude of the electric field does not change over the depth of the crystal. In this case, the overlap integrals of the ordinary and extraordinary waves are equal. In real modulators with a channel waveguide formed by titanium diffusion technology, these assumptions are not fulfilled. The refractive index of lithium niobate and the electro-optical coefficient may vary for different crystal samples, depending on the conditions of their growth, processing and waveguide formation technology. The results of the work can find application in the field of interferometric measuring devices, in which a birefringence modulator is used, since the value of the wave voltage is necessary for the design of control electronics.

MONTE CARLO SIMULATION OF LIGHT SCATTERING IN HUMAN SKIN LAYERS BY SPATIAL PHOTOMETRY METHODS

The ability to register and analyze the spatial distribution of light scattered within the full solid angle is the basis for the development and improvement of information-measuring systems and software and hardware complexes for problems of optical biomedical diagnostics. The greatest contribution to light scattering at non-invasive methods of biomedical research are made by the layers of human skin, affecting the depth of probing and the resolution of diagnostic systems. The significant individual variability of the optical properties of biological tissues does not allow practically (clinically) assessing their effect on the light scattering characteristics; therefore, the use of methods for modeling the optical radiation propagation in media in the measuring tools functioning context makes it possible to provide such a prognostic analysis. The goal of this work is a comparative evaluation of the results of the light propagation in human skin layers by Monte Carlo simulation using information-measuring systems of a biomedical photometer with ellipsoidal reflectors and a goniophotometer.
 The Monte Carlo simulation results of light scattering in dermis and epidermis at a wavelength of 632.8 nm using spatial photometry methods and the "BT_Mod" software, as well as coordinates, direction, and statistical weight of photons, allows the ray-tracing in a biomedical photometer with ellipsoidal reflectors are presented in this work.
 As a result of modeling, graphs of the dependence of optical coefficients (transmission T, diffuse reflection Rd, and absorption A) for the studied tissues of various thicknesses on the value of the scattering anisotropy factor were obtained, as well as photometric images of the second focal plane of ellipsoidal reflectors when receiving a scattering spot in reflected and transmitted light. Diagrams of the averaged scattering indicatrix at three thicknesses of the epidermis and dermis were obtained for a set of biophysically significant values of the scattering anisotropy factor, based on which the integral distribution of the photons statistical weight in diffuse scattered light was analyzed. A quantitative assessment of the illuminance level of images is carried out according to the zone analysis principles in photometry by ellipsoidal reflectors. The resulting graphs of the illuminance dependence the external and middle rings of photometric images in reflected and transmitted light.
 The results of the research make it possible to analyze the spatial distribution of light scattered by the human skin layers (epidermis and dermis) within the full solid angle, which can be used in problems of optical dosimetry and medical imaging in diagnostic, endoscopic, and therapeutic methods of biophotonics.

Research of multisensor characteristics based on optical fiber

The research results of multisensors based on optical fiber, the principle of which is to change the conditions of propagation of optical radiation in the optical fiber in the places where macro-bends are formed at the points of impact, are presented in the paper. The formation of macro-bends leads to an additional attenuation of the power of optical radiation propagating through the optical fiber. A single-mode optical fiber was used with the parameters, which are supported by numerous manufacturers and comply with the recommendations of ITU-T G.655. The measurements were carried out for four wavelengths of optical radiation (1310, 1490, 1550, 1625 nm), corresponding to the transparency windows of the optical loss spectrum of the optical fiber. Using optical reflectometry methods, it was determined that the amount of attenuation of optical radiation of each macro-bend formed at the point of action of the multisensor does not depend on the number of simultaneously formed macro-bends and also does not depend on the location of the point of action along the length of the multisensor. The dependences of the attenuation of the optical radiation power introduced by the macro-bends of the optical fiber on the radius, length, or angle of the macro-bends formed at the multisensory impact points are determined experimentally. The obtained dependences also allow one to determine the optimal parameters of the formed macro-bends of the multisensor to obtain the maximum range of attenuation change for each value of the wavelength. The values of the minimum distance between the impact points, the maximum number of impact points and the optimal values of the radius and angle of the optical fiber macro-bend at the impact points are determined. The results obtained provide opportunities to continue the development of multisensors that allow us to receive information about parameters from several impact points, that are located on a single optical fiber, simultaneously.

Study of the dependence of the spatial structure of the optical response of a microsatellite on its speed and orbit height

The paper considers the problem of optical radiation propagation in a passive laser reflector satellite orbiting the Earth, along with reflected signal formation at the earth surface. In these conditions, the optical system will be significantly affected by moving media optics, which may interfere with the possibility of receiving the signal reflected by the satellite. The paper aims to develop a mathematical simulation taking into account the effects of moving media optics and track the spatiotemporal structure of the signal as a function of the optical system velocity.

Imitation of optical coherence tomography images by wave Monte Carlo-based approach implemented with the Leontovich–Fock equation

We present a computational modeling approach for imitation of the time-domain optical coherence tomography (OCT) images of biotissues. The developed modeling technique is based on the implementation of the Leontovich–Fock equation into the wave Monte Carlo (MC) method. We discuss the benefits of the developed computational model in comparison to the conventional MC method based on the modeling of OCT images of a nevus. The developed model takes into account diffraction on bulk-absorbing microstructures and allows consideration of the influence of the amplitude–phase profile of the wave beam on the quality of the OCT images. The selection of optical parameters of modeling medium, used for simulation of optical radiation propagation in biotissues, is based on the results obtained experimentally by OCT. The developed computational model can be used for imitation of the light waves propagation both in time-domain and spectral-domain OCT approaches.

The Structure of Oxide Film on the Porous Silicon Surface

A prolonged stay of porous silicon in the air environment gives rise to structural changes in its surface layer, and the standard single-layer model is not sufficiently accurate to describe them. In this work, the structure of the near-surface layer in porous silicon is studied using the polygonal ellipsometry method. A combined approach is proposed to analyze the angular ellipsometry data for the parameters ф and Δ. It consists in the application of the multilayer medium model and the matrix method, while simulating the propagation of optical radiation in this medium in order to obtain the theoretical angular dependences of tan ф and cosΔ. In this method, the dependence of the sought optical profile on the specimen depth is an additional condition imposed on the multilayer model. Evolutionary numerical methods are used for finding the global minimum of the mean squared error (MSE) between the corresponding theoretical and experimental dependences, and the parameters of an optical profile are determined. A model in which the inner non-oxidized layer of porous silicon is homogeneous, whereas the refractive index in the outer oxidized layer has a linear profile, is analyzed. It is shown that the linear and two-step models for the refractive index of an oxidized film provided the best agreement with the experimental ellipsometric functions. The adequacy of the theoretical model is also confirmed by determining the color coordinates of the specimen.

Mathematical model of the optical radiation propagation in the microsatellite moving in near-earth orbit

The paper considers the problem of the laser radiation propagation from the Earth surface to a Luneburg lens moving at a speed V along a geosynchronous orbit, and the registration by a ground-based observer of radiation reflected by a lens. Preliminary calculations show that the beams reflected by the moving lens deviate at different angles, which leads to the mixing of the beams on the Earth’s surface in the region of the radiation recorded. That, in turn, affects the appearance of the interference pattern and the process of recording the radiation reflected by the lens. Therefore, the purpose of this work is to calculate the formed optical response in the reception area of the radiation, taking into account the optical effects of the moving media: the Fizeau effect, the Doppler effect, violation of the Snell’s law, the kinematic effect of the beam points displacement on the optical surface caused by the final velocity of radiation in the lens.

Методика расчета видимости при пожаре

Цель. Разработать методику расчета видимости при пожаре, основанную на математической модели распространения оптического излучения через рассеивающую среду и дополненную учетом цветовой гаммы интерьера помещений, доли сгоревшего материала и количественными характеристиками пожарной нагрузки. Методы. Моделирование динамики задымления помещения в условиях пожара. Результаты. Разработана методика расчета видимости пожара, основанная на математической модели распространения оптического излучения в рассеивающей среде и дополненная учетом цвета внутренней части помещений, комплексной оценкой дымообразующей способности пожарной нагрузки и долей ее сгорания. Описана методика определения значений и применения параметров для моделирования видимости при пожаре. Область применения исследований. Результаты исследования могут быть применены при моделировании пожаров.

Coherence of a Bessel beam and a conic wave in turbulent atmosphere

Coherent properties of diffraction-free Bessel-like optical beams propagating in the turbulent atmosphere are theoretically studied. An analytical solution of the equation for the second-order transverse function of mutual coherence of the optical radiation field, which was obtained from paraxial approximation of the scalar wave equation, is analyzed. The behavior of the degree of coherence, coherence radius, and integral scale of the degree of coherence of the Bessel–Gaussian beam and conic wave obtained through conic focusing of a Gaussian beam by an axicon is analyzed for different beam parameters and characteristics of the turbulent atmosphere. Significant qualitative and quantitative differences are discovered between the studied coherence characteristics of the Bessel–Gaussian beam and the conic wave. In general, under identical propagation conditions in the turbulent atmosphere, the coherence of the conic wave is higher than that of the Bessel–Gaussian beam. The comparison of two spatial scales of the degree of coherence of optical beams reveals that the integral scale for diffraction-free beams is a more representative characteristic than the coherence radius. The integral scale of the degree of coherence of diffraction-free beams is more unequivocally connected with the conditions of propagation of optical radiation in the turbulent atmosphere.

Simulation of white light generation and near light bullets using a novel numerical technique

An accurate and efficient simulation has been devised, employing a new numerical technique to simulate the derivative generalised non-linear Schrödinger equation in all three spatial dimensions and time. The simulation models all pertinent effects such as self-steepening and plasma for the non-linear propagation of ultrafast optical radiation in bulk material. Simulation results are compared to published experimental spectral data of an example ytterbium aluminum garnet system at 3.1µm radiation and fits to within a factor of 5. The simulation shows that there is a stability point near the end of the 2mm crystal where a quasi-light bullet (spatial temporal soliton) is present. Within this region, the pulse is collimated at a reduced diameter (factor of ∼2) and there exists a near temporal soliton at the spatial center. The temporal intensity within this stable region is compressed by a factor of ∼4 compared to the input. This study shows that the simulation highlights new physical phenomena based on the interplay of various linear, non-linear and plasma effects that go beyond the experiment and is thus integral to achieving accurate designs of white light generation systems for optical applications. An adaptive error reduction algorithm tailor made for this simulation will also be presented in appendix.

Spatial scales of coherence of diffraction-free beams in a turbulent atmosphere

Coherent properties of diffraction-free optical beams propagating in a turbulent atmosphere are studied. The analysis is based on the solution of the equation for the second-order mutual coherence function of an optical radiation field. The behavior of the degree of coherence of the diffraction-free (cosine and fundamental Bessel) optical beams depending on the beam parameters and characteristics of the turbulent atmosphere is investigated. It turns out that the oscillating character of the degree of coherence of these beams is a fundamental property of diffraction-free beams, which is shown under weak fluctuations in a turbulent atmosphere. At high levels of fluctuations in a turbulent atmosphere, the degree of coherence of a diffraction-free cosine beam becomes closer to that of a plane wave, and of a diffraction-free fundamental Bessel beam, to a spherical wave. The analysis of two spatial scales of the degree of coherence of optical beams has shown that the integral scale of the degree of coherence for diffraction-free beams is a more representative characteristic than the coherence length; the former definitely correlates with optical radiation propagation conditions in a turbulent atmosphere.

Soliton Formation and Superluminality Effect due to Nonlinear Absorption of Femtosecond Laser Pulse Energy by the Medium Containing Nanorods

We investigate a femtosecond pulse propagation in a medium, containing nanorods, with taking into account the dependence of multi-photon absorption from the aspect ratio of nanorods. Nanorods melting due to the laser energy absorption leads to the non-stationary interaction of laser pulse with the medium and time-dependent nanorod aspect ratio changing. Under certain conditions, we found out the soliton-like mode of a laser pulse propagation and the superluminality effect: acceleration of light (fast light) in comparison with light propagation in a linear medium. We discuss a physical mechanism of superluminality effect for considering laser pulse propagation. Using spatio-temporal analogy, one can see the similarity between the pulse centre evolution along longitudinal coordinate and the beam centre evolution under the infrared optical radiation propagation in a cloud, or fog, which moves across the beam, with taking into account its thermal blooming.