Opto-it. Рубрика в журнале - Компьютерная оптика
Статья научная
Based on the use of methods for solving the inverse problem of heat conduction, we developed an algorithm for calculating the power density distribution of the laser beam to create a desired thermal effect on technological objects. It was shown that the redistribution of power density of moving distributed surface heat sources can adjust the temperature distribution in the treated zone. The results of thermal processes calculation show the ability of the developed algorithm to create a more uniform temperature field across the width of the heat affected zone. Equalization of maximum temperature values is achieved in the center and on the periphery of the heat affected zone with an increase in the width of the regions, where required temperature is reached. The application of diffractive optical elements gives an opportunity to obtain the required properties of treated materials in the heat affected zone. The research performed has enabled parameters of the temperature field in chrome-nickel-molybdenum steel to be adjusted for laser heat treatment. In addition to achieving uniform temperature conditions across the width of the heat affected zone, the proposed approach allows the increase of the width of the isotherms of the temperature fields; this provides an opportunity to process a larger area per unit time at the same laser beam power.
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Asymmetric apodization for the comma aberrated point spread function
Статья научная
This paper deals with the study of light flux distributions in the point spread function formed by an optical system with a one-dimensional aperture under the influence of the coma aberration. The traditional design of an asymmetric optical filter improves the resolution of a diffraction-limited optical imaging system. In this approach we explore the control of monochromatic aberrations through pupil engineering with asymmetric apodization. This technique employs the amplitude and phase apodization for the mitigation of the effects of third-order aberrations on the diffracted image. On introducing the coma wave aberration effect, the central peak intensity in the field of diffraction is a function of the edge strips width and the amplitude apodization parameter of a one-dimensional pupil filter, whereas the magnitude of the reduction of optical side-lobes is a function of the degree of phase apodization at the periphery of the aperture. The analytically computed results are illustrated graphically in terms of point spread function curves under various considerations of the coma aberrations and a different degree of amplitude and phase apodization. Hence, for the optimum values of apodization, the axial resolution has been analyzed using well-defined quality criteria.
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Conditions of a single-mode rib channel waveguide based on dielectric TiO2/SiO2
Статья научная
In this paper, we propose conditions for the design of a single-mode rib channel waveguide based on dielectric materials such as titanium dioxide (TiO2) and silicon dioxide (SiO2) for the 0.633-µm visible light. We also design Y-splitter structures, which show high-degree optical confinement and low bend losses at various radii of curvatures. Small radii of curvatures are extremely desirable in integrated photonics as they permit decreasing the dimensions but can also potentially reduce power consumption in the active devices.
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Design, simulation, and fabrication of silicon-on-insulator MEMS vibratory decoupled gyroscope
Статья научная
This paper reports the design and fabrication of a 2-degree-of-freedom (DOF) decoupled vibratory gyroscope based on a silicon-on-insulator (SOI) MEMS process. The 2-DOF capacitive comb structure is deliberately designed to have a decoupled drive and sense mode oscillation to prevent the unstable operation due to mechanical coupling, resulting in a low zero rate out-put drift. It is well known that the closer are the drive and sense resonances, the higher is the angular rate resolution of the gyroscope. Generally, this is achieved by using symmetric suspensions, but it results in a reduced bandwidth. The proposed design has been configured to achieve a bandwidth of about 150 Hz, while ensuring the decoupled operation of the drive and sense modes. An analytical method has been employed to study the steady state response of the 2-DOF structure. FEM analysis has been carried out in CoventorWare® MEMS Design software and the simulation results show that the drive resonance occurs at 21.48 kHz and sense resonance at 21.63 kHz, which are in close agreement with the theoretical results. The structure is designed with a 15 µm thick device layer. Fabrication of the design is proposed using a two mask process based on Deep reactive-ion etching (DRIE) and sacrificial wet release etching on a SOI wafer. DRIE etching with an aspect ratio of 1:5 has been successfully carried out as desired and the results have been presented.
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Статья научная
We have studied the nanostructuring and colorizing of the copper surface by scanning with a femtosecond laser beam with a near-Gaussian beam profile. The experimental studies were conducted using a femtosecond laser comprising a Ti:Sapphire oscillator and a multi-pass amplifier with the maximum pulse energy of 0.7 mJ, pulse frequency of 1 kHz, and pulse duration <30 fs. It is shown that the use of a short-pulsed femtosecond laser leads to the formation of wavelength scale periodic surface structures and eventually increases the brightness of the color of the copper surface. It is revealed that via reciprocally scanning the copper surface by multiple ultrashort laser pulses with a weakly asymmetric spatial energy density distribution and an energy density below the material ablation threshold, it is possible to create a combined nanostructure composed of low-spatial-frequency laser-induced periodic surface structures coated with nanoscale roughness. It is shown that relatively minor changes in the nanostructures obtained by scanning the copper surface by multiple ultrashort laser pulses can lead to a significant change in the color during surface colorizing.
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Focal-plane field when lighting double-ring phase elements
Статья научная
The focal-plane field amplitude is calculated when lighting double-ring phase elements by flat and Gaussian beams. Emerging conditions in the minimum or maximum centers, including flat-top maxima, are given. For the field amplitude, we obtain equations that define the radius of the first zero-intensity ring based on the deduced expressions. The root values are listed for several parameters of optical elements and incident beams due to the lack of analytical solutions. Numerical simulation results are given for flat incident beams; they are fully consistent with the theoretical calculations.
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High performance 2D simulations for the problem of optical breakdown
Статья научная
Methods of numerical simulation of two-dimensional short laser pulse nonlinear dynamics are discussed. In this work parallel processing methods for modern CPU (central processing units) architectures supporting non-uniform memory access are considered. The method of adaptive mesh subdivision is proposed to reduce non-uniform load of each CPU during processing of nonlinearity. The results of the tests performed on the Intel Nehalem based a workstation with eight cores are presented.
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Integrated fiber-based transverse mode converter
Статья научная
A transverse mode converter based on a binary microrelief implemented directly on the end-face of a few-mode fiber was numerically investigated. The results of numerical simulation demonstrated the converter to form LP-11 and LP-21 modes with high efficiency, providing a more-than 92 % mode purity. Transformations of modes excited by a fiber microbending were also numerically investigated. The excited beams were shown to save their mode purity even in a strong bending as the arising parasitical modes were mostly unguided by the fiber. The resulting beam power and mode content were also demonstrated to depend on the beam and bending mutual orientation for beams with strong rotational symmetry.
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Interferometric testing of steep cylindrical surfaces with on-axis CGHs
Статья научная
We present a new approach for testing cylindrical optical surfaces using a Null-test. We suggest using a Co mp uter Ge nerated Hologra m (CG H) in co mbi natio n w ith a Trans mission Sp here. It is shown that in such an optical layout the period of the diffractive structure is larger than in the case of a conventional scheme using a collimated beam. Therefore, this kind of hologram enables the test of cylinder surfaces with higher numerical apertures.
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Статья научная
The relaxation of a three-level atom interacting with a photon heat bath and an external stochastic field is investigated. For the reduced density matrix, a master equation averaged over stochastic process realizations is derived. An exact solution is obtained and the radiation line shapes are calculated.
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Статья научная
We report a design for creating multilayer dielectric optical filters based on TiO2 and SiO2/MgF2 alternating layers. We have selected Titanium dioxide (TiO2) for high refractive index (2.5), Silicon dioxide (SiO2) and Magnesium fluoride (MgF2) as a low refractive index layer (1.45 and 1.37) respectively. Miniaturized visible spectrometers are useful for quick and mobile characterization of biological samples. Such devices can be fabricated by using Fabry-Perot (FP) filters consisting of two highly reflecting mirrors with a central cavity in between. Distributed Bragg Re-flectors (DBRs) consisting of alternating high and low refractive index material pairs are the most commonly used mirrors in FP filters, due to their high reflectivity. However, DBRs have high re-flectivity for a selected range of wavelengths known as the stopband of the DBR. This range is usually much smaller than the sensitivity range of the spectrometer. Therefore, bandpass filters are required to restrict the wavelength outside the stopband of the FP DBRs. The proposed filter shows high quality with an average transmission of 97 % within the passbands and the transmission outside the passband is around 3 %. Special attention has been given to keep the thickness of the filters within the economic limits. It can be suggested that these filters are exceptionally promising for florescence imaging and narrow-band imaging endoscopy.
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Nanophotonic structure formation by dry e-beam etching of the resist: resolution limitation origins
Статья научная
A wide range of structures for nanophotonics and optoelectronics can be formed by dry e-beam etching of the resist (DEBER). High resist sensitivity due to chain depolymerization reaction provides efficient etching with high throughput of the method. The structures obtained by the DEBER in this research are well-rounded diffraction gratings, binary gratings and staircase profiles. The major disadvantage of DEBER is poor lateral resolution, which may be caused by different physical mechanisms. Four groups of possible mechanisms leading to the resolution limitation are determined and the influence of some mechanisms is estimated.
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Optical properties of lowest-energy carbon allotropes from first-principles calculations
Статья научная
We study optical properties of lowest-energy carbon allotropes in the infrared, visible and ultraviolet spectral ranges in the general gradient approximation of the density functional theory. In our calculations we use an all-electron approach as well as a pseudo-potential approximation. In the infrared range, complex dielectric functions, infrared and Raman spectra have been calculated using a CRYSTAL14 program. Electronic properties and energy-dependent dielectric functions in the visible and ultraviolet spectral ranges are calculated using a VASP program. We describe with good accuracy the experimentally known optical properties of a cubic diamond crystal. Using the obtained set of relevant calculation parameters, we predict the optical constants, dielectric functions and Raman spectra of the lowest-energy hypothetical carbon allotropes and lonsdaleite.
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Статья научная
Scattering potentials of hollow particles with ellipsoid-, cylinder- and parallelepiped-like shapes and adjustable edge sharpness are introduced as a difference of two 3D multi-Gaussian functions with suitable parameters. The far-zone intensity distributions generated on weak scattering from such potentials are shown to depend on the scatterer’s boundary thickness, edge softness as well as on its size relative to the wavelength. Possible extension to potentials formed by nested shells of the same or different types and potentials with semi-hollow center is outlined.
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Simulation of a long-haul fiber optic link with a two-mode optical fiber
Статья научная
Modern telecommunication networks approach the capacity crunch, which is associated with the so-called nonlinear Shannon limit. So, the passage to fiber optic links with few-mode optical fibers is considered as an alternative solution of the described problem concerned with high nonlinearity of conventional commercial single-mode optical fibers. Presently, various designs of few-mode optical fibers have been known, with the recently published works presenting experimental results demonstrating their potentialities for long-haul fiber optic links. A lot of models of long-haul fiber optic links with few-mode optical fibers have been developed based on which features of a few-mode optical fiber transport network were numerically simulated. This work presents the results of simulation of a 6000-km long-haul fiber optic link with a two-mode optical fiber and 100-km-per-span Erbium doped fiber optic amplifiers system under 100 Gbps DP-DQPSK data transmission. We studied the use of particular linearly polarized modes and optical vortices for signal transmission. The computation results were compared with the simulation of the same fiber optic link with a single-mode optical fiber under the identical conditions.
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Статья научная
Features of laser-assisted warm forming of dual phase steel DP1000 are determined. Simulation of forming processes with local heating is performed. In the simulation procedure, the forming parameters of three dimensional forming were adapted to keep them within tolerable limits even in critical areas as well as identifying the localization and type of critical stresses. The capabilities of Abaqus were extended by use of the Python language to independently evaluate selected element strains, the position of deformed elements within the forming limit diagram and user-defined failure criteria. The simulation led to an adapted forming process permitting a significantly increased bulge forming depth by local laser heating of the forming zones. The developed simulation model shows a satisfactory conformity with experiments, performed using a fibre-coupled laser with a wavelength of 1070 nm and a maximum output power of 1500 W, and a servo bending press TRUMPF TruBend 7018. The required distribution of the laser beam energy can be obtained by using diffractive optical elements. The use of the model for technological operations opens up possibilities not only for the solution of the presented specific objective of laser assisted warm forming, but also for others applications.
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Статья научная
In this work a solitary surface plasmon-polariton was obtained by using a frequency-dependent finite difference time-domain method for the TM- and radially polarized light at 532 nm, which was propagated through silver nano-elements (a nano-strip and a nano-ring), placed in an aqueous medium. The device's height and width were equal to 20 nm and 215 nm respectively. The intensity of surface plasmon-polariton was four times higher than that of the incident radiation. The full width at half maximum of the nanojet was 138 nm and 158 nm for the case of using a nano-strip and a nano-ring respectively. The results can be used to design devices that allow capturing and moving particles in water or other biofluids.
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Tightly focused laser light with azimuthal polarization and singular phase
Статья научная
Using simplified Richards-Wolf formulas we show that laser light with azimuthal polarization and singular phase can produce a smaller focal spot than that from a laser beam with radial polarization, other conditions remaining the same. It is numerically shown that when focusing an azimuthally polarized laser beam with phase singularity using a zone plate a 1.3 times smaller focal spot can be attained than when an aplanatic lens is used. A spiral phase plate can be replaced with a phase step with a π phase shift. In this case the subwavelength focal spot from a laser beam with azimuthal polarization, which is formed near the zone plate surface, loses circular symmetry, while becoming smaller and acquiring an elliptical form with radiuses of 0.273λ and 0.314λ (NA=1).
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Статья научная
A tunable diffraction grating based on an electrooptic X-cut lithium niobate crystal has been manufactured and experimentally analyzed. The period of electrodes is 290 μm, the electrode width is 117.5 μm, and the thickness of an electrode is 150 - 160 nm. The electrodes are made of a transparent conducting indium-tin oxide that serves as an antireflection coating with the aim of increasing the optical transmission. In order to prevent crystal polarization switching and electrical breakdown an optimized electrode topology with end ellipticity 1:1 and increased interelectrode gap is used. The optical diagram of the tunable grating with alternating electrode potentials for various gap voltages is analyzed. The intensity of the zero order of diffraction is shown to decrease by 40 % at a voltage of 800 V. At the same time, the origination of new diffraction orders at angles ± λ / (2 d ) is noted. The measurement of the forward-bias and reverse-bias regions of the modulation characteristic reveals the absence of hysteresis, which confirms the correctness of the electrode topology design.
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Vortex beams in turbulent media: review
Статья научная
The review covers publications concerned with propagation of laser beams through turbulent media described by the Kolmogorov theory and generalizations thereof to describe signal transmission in optical communications and detection systems. In this case, the turbulent medium is interpreted as an optical channel with random parameters. Various optical signals considered include partially coherent beams, non-uniformly polarized vector beams, as well as specifically configured spatial laser beams. Special attention is given to vortex laser beams. The latter are shown to have a number of remarkable properties that give them an advantage over conventional Gaussian beams.
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