Orbital angular momentum (OAM) is implemented as an independent information channel in multiplexed holography. Vortex beams carrying OAM are characterized by helical wavefront. By theoretically infinite orthogonal modes, OAM can be used as an additional degree of freedom (DOF) to increase the information capacity of holography. However, the helical phase pattern can be encoded into only one hologram, that is, a topological charge (TC) channel can be associated with only one target image, which limits the multiplexing capability of OAM holography. In this paper, elliptic perfect optical vortex multiplexed holography with fractional-order TC is proposed. The modulated parameters of FEPOV, namely TC, ellipticity, and gap number, can be used as independent DOFs to encode and decode holograms. Using these three parameters as encryption keys, single-parameter and multi-parameter encrypted holograms are designed to enhance information capacity and security. The method demonstrates great potential in the field of information storage and encryption.
KEYWORDS: Holography, Multiplexing, Holograms, 3D image reconstruction, Angular momentum, Signal to noise ratio, Design and modelling, Spatial light modulators, Photonics, Phase distribution
Optical orbital angular momentum (OAM) multiplexed holography has been implemented as an effective method for information encryption and storage. Multiramp helicoconical-OAM multiplexed holography is proposed and experimentally implemented. The mode selectivity of the multiramp mixed screw-edge dislocations, constant parameter K, and normalized factor are investigated, respectively, which demonstrates that those parameters can be used as additional coding degrees of freedom for holographic multiplexing. The combination of the topological charge and the other three parameters can provide a four-dimensional multiplexed holography and can enhance information capacity.
Due to its self-healing property, the Ring Airy Gaussian Vortex Beams (RAiGV) have potential applications in the fields of particle capture and handling, laser medical treatment, and super-resolution imaging. This paper proposes a cotangent phase modulated RAiGV, and carries out a numerical simulation study on its self-healing property. The results show that the beam will split into multiple lobes during the transmission process. Each lobe consists of a main lobe and multiple side lobes, and the lobes shrink, rotate and converge continuously; when one of the lobes is completely blocked or when blocking all the side lobes of the beam, the beam does not have the self-healing ability; when all the main lobes of the beam are blocked, blocking the innermost ring of the beam, or blocking all the outer rings of the beam, the beam has the self-healing ability. After a certain distance of transmission, the beam field is basically the same as the selffocusing beam field.
Volume Bragg gratings based on PTR glass are widely applied in high-power laser systems because of its good Bragg selection, high diffraction efficiency and high laser damage threshold. In this work, the photo-thermo induced nucleation mechanism of PTR glass was studied by comparing the absorption spectra of PTR glass before and after UV irradiation and nucleation heat treatment with Gaussian decomposition method. The results show that: the Ag0 atom formed by UV irradiation tends to be saturated at the irradiation dose of 1J/cm2 ; under the same irradiation dose, the irradiation power has no obvious effect on the number of Ag0 atoms, but it will change the the number of free electrons in glass matrix; after UV irradiation and nucleated at 460°C for 2h, the photosensitivity of PTR glass will reappear and AgBr clusters will be dominant in the silver micelles.
The realization of high-resolution imaging of images through scattering media has always been an important problem to be solved in the field. In this paper, our purpose here is to create a new framework that can realize the imaging through long-range scattering media. To do so, we establish a long-range scattering medium model, and use the model to generate simulated speckle pattern. In particular, we are designing a new neural network that is able to learn the statistical information found in the pattern of speckle intensity. The simulated speckle data were used as train sets for the neural network, and the learning rate of the SGD was 0.001, so that the model converged, which had good effects in the aspects of recovery time, imaging quality, mobility, convergence rate and so on. The peak signal to noise ratio (PSNR), Pearson correlation coefficient (PCC), structural similarity (SSIM) and other indexes were used to evaluate the performance of the convolution neural network in restoring images. Our neural network has achieved good results under this evaluation index from those results. PSNR value is 16.939, SSIM value is 0.842, and PCC value is 0.884, indicating that our new neural network model can realize long-range scattering media imaging and improve the imaging quality of scattering imaging.
There are various scattering media in nature. How to achieve rapid imaging through scattering media is an important issue in the field of optical imaging. In order to improve the speed of scattering imaging, this paper adopts a new type of residual neural network called XRNet, which mainly uses the advantages of easy optimization of residual neural network(ResNet) and combines U-Net structure and depthwise separable convolution instead of standard convolution to reduce the complexity of training model, thereby improving recovery speed of imaging. The image restoration speed and imaging quality under this model are simulated and calculated, and the convolutional neural network (CNN), ResNet and XRNet are analyzed by Pearson Correlation Coefficient (PCC), Peak Signal to Noise Ratio (PSNR) and Structural Similarity Index Measure (SSIM). The results show that XRNet has the fastest imaging speed when the imaging quality is basically unchanged. Finally, the imaging quality and recovery speed of XRNet at different depths are evaluated, and the best values are obtained at a depth of 31 layers.
The ability to remove the black crust from stone substrates has great research significance in Cultural Heritage. Laser cleaning has achieved good cleaning results in stone heritage in recent years. However, the extremely high light absorption and low melting point of biotite in the mineral composition of granite can easily cause damage during the cleaning process. In this study, the thermal stress generated by the laser action in the black crust was used to peel off the black crust. And a model of three-dimensional thermal stress was established under the movement of the laser heat source based on the experimental phenomenon of warping to peel off the black crust. The model agrees with the experimental results that the black crust is peeled off by tensile and shear stresses.
Surface defects of fused silica glass have become the main reason that limits the resistance of fused silica components to laser damage. When the laser enters the scratch, the scratch modulates the incident light field to increase the local light intensity, increase the probability of damage to the optical element, and reduce the damage threshold. In order to study the modulation effect of scratches on the light field, this paper considers radial triangular scratches and uses the finite difference time domain (FDTD) method to establish a laser incident triangular scratch model. As a result, with the increase of the scratch width w, the maximum light intensity enhancement factor (LIEF) on the front surface of the scratch is 4.6, and the maximum LIEF on the rear surface can reach 9.2. With the increase of the scratch depth d, the LIEF on the front surface of the scratch is 7.5, and the maximum LIEF on the rear surface can reach 12.2. When the scratch is located on the rear surface, the incident laser occurs totally reflection, the interference effect is obvious, the LIEF becomes larger, and the rear surface is more prone to damage.
Photo-acoustic detection is carried out on the laser-induced damage of fused silica glass. The Nd: YAG laser with the wavelength of 1064 nm, pulse duration of 10 ns and the maximum output energy of 7.0 J is used in the experimental. Results showed that: photo-acoustic could not be detected when damage has not occurred. Photo-acoustic signals containing high frequency components can be received after damage occurs. Compared with the sensor attached to the end surface and placed in the air, attaching the sensor to the side has the characteristics of high signal-to-noise ratio and does not affect the optical path. Above work provide a new technical idea for the photo-acoustic method in the damage detection.
The light field distribution of the vortex beam propagating through the cube-corner prism is studied, considering the existence of the dihedral angle error and the flatness error in the process of the cube-corner prism, based on vector form of reflection-refraction law. The reflection-refraction mode of the cube-corner prism is established, and the phase distributions of the emitted light field and the diffraction light field are given when the dihedral angle error and the flatness error are considered. As for the cube-corner prism with circular section, the diffraction distribution of the vortex beam on the dihedral angle error and the flatness error is calculated. Meanwhile, the effects of diffraction distribution are analyzed. Simulation results show that, the divergence of diffraction intensity distribution increases with dihedral angle error increases. Also, the output diffraction pattern of the cube-corner prism with the three equal dihedral angle processing errors has six symmetrically distributed symmetrical petal structures. The quality of the output diffraction mode is significantly reduced at the existence of flatness error.
Based on the generalized Huygens-Fresnel principle, the quadratic approximation and integral transformation techniques of the Rytov phase structure function are used to derive the expression of the turbulence distance and the M2 -factor of array Gaussian-Schell mode beam propagating through oceanic turbulence. The simulation methods of beam superposition are cross-spectral density function superposition and intensity superposition. The simulation results show that the turbulence distance and M2 -factor of the linear array Gaussian-Schell mode beam in oceanic turbulence are related to the coherence length, the number of array beam and the separation distance. The turbulence distance and the M2 -factor for superimposition of the cross-spectral density function are always smaller than for the superposition of intensity. For superimposition of the cross-spectral density function, when the propagation distance is less than 30 m and beam number is 3, the M2 -factor is the smallest curve. Also, the propagation distance is less than 700 m and beam number is 1, the M2 -factor is the minimum curve. When the propagating distance is more than 700 m and beam number is 3, the M2 -factor is the minimum curve. The optimization of beam parameters and superposition methods can obtain the better beam quality.
The problem of the imaging through turbid medium such as fog, haze and human tissue has always been the focus of attention. The turbid medium destroys the wavefront of the incident beam, so the key to the recovery of the image is the reconstruction process of the wavefront. The spatial light modulator (SLM) is employed to reconstructs the wavefront digitally. In order to realize imaging through the turbid medium, a prior image is restored at the target location in the simulation, and the phase modulation of the SLM is obtained by the advanced genetic algorithm. Then the priori image is replaced with the new image to be restored. According to the optical memory effect, the new image can be imaging at the target location. The optical memory effect was tested using a series of circular spots of different sizes, and the results showed that the recovery ability gradually decreased with the increased image size. Finally, the curve of the cost function with the number of iterations in 7 different population is analyzed. The results show that the convergence value increases first and then stops with the increase of the group, and the convergence rate is not positively correlated with the group size. So choosing the right group size makes a lot of sense for image restoration.
Based on the extended Huygens–Fresnel principle and the second-order moments of the Wigner distribution function, propagation properties of radially polarized partially coherent beam in the oceanic turbulence are investigated. An analytical formula for the propagation factors (M2 -factors) of partially coherent one-dimensional radially polarized beams in turbulent ocean is derived. The intensity distribution, degree of polarization and beam characteristic parameters of radially polarized partially coherent beam in turbulent ocean are numerically described in detail. It is shown that the oceanic turbulence parameters and the initial coherence length have significant influence on the degree of polarization and coherence for radially polarized partially coherent beams, and such beam is depolarized on propagation. As the value of the oceanic turbulence parameters (χT or w) increases and the coherence length of the beam decreases, the normalized M2 -factor increases more rapidly. The results will be useful in long-distance free-space optical communications.
The ability to control the propagation of light through scattering media has great research significance in biomedical treatment and laser material processing. How to achieve effective focusing by modulating the laser beam passing through the highly scattering media is extremely important. In this paper, based on the traditional continuous sequence algorithm, improved scanning methods are used to modulate the phase of incident light wave-front, thus achieving effective focusing of the laser beam through the scattering media and increasing the convergency speed of the algorithm to nearly 1.5 times. Similarly, using continuous sequences and their optimization algorithms can focus the light into the desired pattern and the performance of three different scanning methods of continuous sequence algorithm are discussed.
In order to study the spatial wavelength division diffraction effect, the Chirped Volume Bragg Gratings (CVBG) based on the Photo-thermo-refractive glass on the oblique incident light was analyzed by the fundamental matrix method. When diffracted by grating, the complex beam would be separated into multiple beams in space by wavelength. The diffraction efficiency of the separated optical wave can reach up to 90%. Furthermore, the diffraction efficiency, increases as the refractive index modulation depth increases and decreases as the chirp rate increases. The diffraction spectrum has the characteristics of flat-top band pass. To increase the spatial separation distance, multiple schemes of CVBGs in parallel combination are designed. By combining a self-focusing lens array, the proposed system can realize wavelength division multiplexing with bandwidth less than 0.4nm.
A collimating system is designed by two orthogonal aspheric cylindrical lenses, which is used to collimate the the output beam of the Laser Diode (LD). The theory of light transmission is used to analyze the optical collimating system. The theoretical expressions are derived and the numerical results are obtained. The nonlinear Levenberg-Marquardt (LM) fitting algorithm based on the trust-region rule is applied to the surface shape evaluation. Residual analysis is used to test the rationality of the model hypothesis, and a simple analysis is made on the source of the error and the data optimization process. And the optical simulation of model optimization results is established. Simulation results show that the semidiverging angles of 20° and 9° for the fast and slow axes of semiconductor laser beam are both reduced to 0.05 mrad after passing the collimation system which is much better than that of a normal cylindrical lens collimating system 3 mrad. A uniform circular spot can be formed in the far field by adjusting the distance between the two aspheric lenses.
The intensity distribution of an initially plane wave incident on ceria in subsurface layer is calculated numerically with Finite-different time-domain (FDTD) solutions.The results show that the light intensity enhancement is caused by lens effect due to the high refractive index of ceria, and the surface damage characteristics of fused silica is very sensitive to location of ceria, ceria size and incident wavelength. The ceria located on the exit surface of fused silica generates electric field enhancements that are stronger than those on the entrance surface. The increasing of ceria size can lead to higher light intensity enhancement factor (LIEF) and the LIEFs can reach two orders of magnitude when the diameter of ceria is three times that of the incident wavelength. The light intensity enhancement caused by ceria with the same location and diameter decreases with the increasing of wavelength. As ceria on polished surfaces is randomly oriented, the probaility for large intensity enhancements to occur is high. The model may provide effective support for research on laser-induced damage and improvement of processing technology for fused silica.
The slit spatial filter can be used in high-power laser system. The performance of two-lens slit spatial filter is described and discussed. The laser intensity at the slit edge and the peak intensity of the focal line with different F-numbers and the cut-off frequencies are compared with that of the traditional spatial filter. Simulation results show that the laser intensity at the slit edge and the peak intensity of the focal line are less two orders of magnitude than that of the traditional spatial filter. Besides, the vacuum degree required in the slit spatial filters is about 10-1Torr, which is less two orders of magnitude than that of the traditional spatial filter.
In this paper, the laser cleaning soil rust layer on the surface of ceramic artifacts by the way of ablation and thermal stress with infrared high-repetition pulse laser is carried out. A cleaning effect can be achieved with laser scanning 10 times at the speed of 480 mm/s and fluence of 795.7747 J/m2 near the ablation threshold. However, the external force is required to make the soil rust layer fallen off. In contrast, a better cleaning effect that the soil rust layer is directly peeled off under the effect of thermal stress without ablation at the contact surface and external force can be observed with the fluence of 1591.5494 J/m2 and laser scanning at 1 time. Furthermore, a two-layer structure model is built to analyze the mechanism of cleaning by thermal stress based on the heat conduction and thermal stress equation. The maximum peeling thermal stress at the contact surface is 2.854×107 N/m2, which is greater than the adhesion stress of 2.050×107 N/m2. This is in agreement with experiments.
Paint removal based thermal stress with high repetition pulse is considered in this paper. The temperature distribution of aluminum substrate and paint under laser irradiation is simulated and the thermal stress generated by thermal expansion is calculated. The adhesion force between the paint and the substrate was calculated according to adhesion formula. The conditions for the paint removal can be obtained by comparing the force of thermal stress and adhesion. At the scan speed of 5000 mm/s, the fiber laser with wavelength of 1064 nm, pulse width of 240 ns and frequency of 100 kHz is used to strip red paint from aluminum without any damage. And the stripping effect at the different output power is also taken into account.
The processing parameters of soda-lime glass cutting with several nanosecond 532 nm pulsed fiber laser are studied in order to obtain sufficiently large ablation rate and better processing quality. The influences of laser processing parameters on effective cutting speed and cutting quality of 1~2 mm thick soda-lime glass are studied. The experimental results show that larger laser pulse energy will lead to higher effective cutting speed and larger maximum edge collapse of the front side of the glass samples. Compared with that of 1.1 mm thick glass samples, the 2.0 mm thick glass samples is more difficult to cut. With the pulse energy of 51.2 μJ, the maximum edge collapse is more than 200 μm for the 2.0 mm thick glass samples. In order to achieve the high effective cutting speed and good cutting quality at the same time, the dual energy overlapping method is used to obtain the better cutting performance for the 2.0 mm thick glass samples, and the cutting speed of 194 mm/s and the maximum edge collapse of less than 132 μm are realized.
An experimental method is used to measure the optical parameters of a GRIN lens. The intrinsic properties of the lens are well characterized by measuring the intercept values of the different GRIN lenses. Firstly, the intercept equation is derived from the transmission matrix of the GRIN lens, and the measurement method is described in detail. Secondly, we design and make the optical path measuring system. Finally, the error analysis of the experimental results is presented, which shows the feasibility of the working principle and the experiment operation. The principle and equipment of this measuring method are relatively effective, which affords great practical significance for the measurement of the GRIN lens.
In this paper, we have fabricated the tellurite glass (70TeO2-20ZnO-5Al2O3-5La2O3, mol%, TZAL) by using high temperature melting method. Considering the material dispersion, the dispersion properties of TZAL glass photonic crystal fiber (PCFs) for various structures are analyzed and precisely described based on the vector finite element method (FEM). A novel structure with three-ring TZAL Glass PCF is proposed by introducing large elliptical holes in the inner ring. The simulation results show that ZDW decreases from 1.586μm to1.485μm when the numbers of elliptical holes increases from two to six. ZDW of horizontally disposed ellipses of PCF is a litter lower than that of vertically positioned ellipses. Furthermore, with optimizing parameters of pitch period and diameter of air hole, ZDW is reduced to 1.396μm. The dispersion characteristics can be flexibly designed and adjusted. The approach and results can be guidance for design, manufacture the photonic crystal fibers.
The traditional measurement for Verdet constant of magneto-optic glass is to measure the Faraday rotation angle indirectly by using optical power meter, which is based on the Faraday rotation effect. There is a relatively large measurement error, because the method neglects the influence of the extinction ratio of polarizer and the axis angle. In our research, a method based on Jones matrix and simulation is studied and used to improve the measurement accuracy. The extinction ratio of polarizer and the axis angle are discussed during the measurement. The experiment system is built for measuring the Verdet constants of ZF7 glass and Ce3+/Tb3+ co-doped glass, by using the He-Ne laser light source, and different polarizers. The results show that the changes of the polarizer’s extinction ratio mainly affected the Verdet constant measurement. The lower extinction ratio, the larger measurement error. The extinction ratio more than 1:1000 for the polarizer is necessary for an accurate measurement, which has important value in terms of reference and guidance for the accurate measurement of Verdet constant of magneto optic glass.
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