Phase Gratings Research Articles

POSSIBILITIES OF USING OPTICAL RADIATION DIFFRACTION MANAGEMENT METHODS ON REFLECTIVE COATINGS OF ARMAMENT AND MILITARY EQUIPMENT FOR ITS PROTECTION AGAINST AMMUNITION WITH SEMI-ACTIVE LASER GUIDANCE SYSTEMS

The article substantiates the need to improve the existent methods of combat defense and combat equipment (DAM) against bombs (guided combat munitions from UAVs, stray combat munitions, etc.)with semi-active laser targeting systems (LSA) on the basis of controlled reflective coatings. Methods of management the diffraction of optical radiation on reflecting coatings are based on applications in coating materials. At the same time, the reflecting properties can be purposefully changed by applying acousto-optical, electro-optical and thermos-optical effects. The acousto-optic method of diffraction control involves the spatial modulation of the beating coefficient in the medium, which is carried out by ultrasonic (acoustic) waves. During the distribution of ultrasound in the reflective coverage (areas of compression and discharge of the material of the waves distribution) appears in the reflective coverage. A purposeful change of descriptions of the acoustic wave leads to the possibility of forming the reflecting phase grating with a given parameter on which it will diffract a radiation. The electro-optical method of management the diffraction of laser radiation is based on the phenomenon of induced anisotropy of media under the influence of a field (electro-optical effect). However, the creation of reflective diffractive elements (coatings)using the electro-optical effect is limited by a number of disadvantages of this method. The thermo-optical method of management light diffraction consists in changing the optical constants of reflective coatings under the influence of temperature. On the basis of the multilayer interference systems (MLIS) with tapes, peak and phase diffraction gratings can be implemented. The use of MLIS coatings with ribbons in the display allows you to implement the functions of selecting and unlocking coatings in the infrared range. The last two methods are of the greatest practical interest since, unlike, the electron-beam method they allow heating a fairly large surface area of the coating. The implementation of diffraction control of optical radiation on the reflective coatings of DAM samples will allow to remove the ammunition control system with semi-active LSA from the working state as a result of the aiming point fluctuation.

All-in-One Collimating Splitter Based on a Meta-Fiber Platform

The use of array generators has become ubiquitous in various applications such as laser fabrication, face identification, and motion sensing. The Dammann grating, a diffractive optical element, is the mainstream approach for generating uniform spot arrays. However, its limited capability and the contradiction between the performance and the complexity of fabrication hinder its application. To address this issue, an all-in-one collimating splitter based on metasurfaces is theoretically proposed by synthesizing the phase of an inverse-optimized Dammann grating and a collimating lens. Leveraging both the diffraction effect of Dammann grating and the Fourier transformation of the collimating lens, the number of spot arrays can be largely increased with a single lenslet. The proposed design shows a large field of view of 62° × 62° and a high uniformity of 1.29% in generating a spot array of 3 × 3 on a single-fiber platform, confirmed by both the scalar and full-wave simulation. Further, a larger spot array up to 15 × 15 is also derived in the far field by integrating the proposed metasurface on a 5 × 5 fiber array platform, confirmed by the scalar simulation. Our design may be transplanted to the vertical-cavity surface-emitting laser platform, and shows great potential in various applications including face identification and motion sensing.

Neutron interferometry using a single modulated phase grating.

Neutron grating interferometry provides information on phase and small-angle scatter in addition to attenuation. Previously, phase grating moiré interferometers (PGMI) with two or three phase gratings have been developed. These phase-grating systems use the moiré far-field technique to avoid the need for high-aspect absorption gratings used in Talbot-Lau interferometers (TLI) that reduce the neutron flux reaching the detector. We first demonstrate, through theory and simulations, a novel phase grating interferometer system for cold neutrons that requires a single modulated phase grating (MPG) for phase-contrast imaging, as opposed to the two or three phase gratings in previously employed PGMI systems. The theory shows the dual modulation of MPG with a large period and a smaller carrier pitch P, resulting in large fringes at the detector. The theory was compared to the full Sommerfeld-Rayleigh diffraction integral simulator. Then, we proceeded to compare the MPG system to experiments in the literature that use a two-phase-grating-based PGMI with best-case visibility of around 39%. The simulations of the MPG system show improved visibility in comparison to that of the two-phase-grating-based PGMI. An MPG with a modulation period of 300 µm, the pitch of 2 µm, and grating heights with a phase modulation of (π,0, illuminated by a monochromatic beam produces visibility of 94.2% with a comparable source-to-detector distance (SDD) as the two-phase-grating-based PGMI. Phase sensitivity, another important performance metric of the grating interferometer, was compared to values available in the literature, viz. the conventional TLI with the phase sensitivity of 4.5 × 103 for an SDD of 3.5m and a beam wavelength of 0.44nm. For a range of modulation periods, the MPG system provides comparable or greater theoretical maximum phase sensitivity of 4.1 × 103 to 10.0 × 103 for SDDs of up to 3.5m. This proposed MPG system appears capable of providing high-performance PGMI that obviates the need for the alignment of two phase gratings.

Toward Arbitrary Spin‐Orbit Flat Optics Via Structured Geometric Phase Gratings

AbstractReciprocal spin‐orbit coupling (SOC) via geometric phase with flat optics provides a promising platform for shaping and controlling paraxial structured light. Current devices, from the pioneering q‐plates to the recent J‐plates, provide only spin‐dependent wavefront modulation without amplitude control. However, achieving control over all the spatial dimensions of paraxial SOC states requires spin‐dependent control of corresponding complex amplitude, which remains challenging for flat optics. Here, to address this issue, a new type of flat‐optics elements termed structured geometric phase gratings is presented, that is capable of conjugated complex‐amplitude control for orthogonal input circular polarizations. By using a microstructured liquid crystal photoalignment technique, a series of flat‐optics elements is engineered and their excellent precision in arbitrary SOC control is shown. This principle unlocks the full‐field control of paraxial structured light via flat optics, providing a promising way to develop an information exchange and processing units for general photonic SOC states, as well as extra‐/intracavity mode convertors for high‐precision laser beam shaping.

System design and error correction of simultaneous phase-shifting point-diffraction interferometer for dynamic wavefront detection with 400 mm aperture.

In this paper, a simultaneous phase-shifting point-diffraction interferometer (SPS-PDI) at632.8nm is designed with the assistance of an off-axis parabolic mirror (OAPM), through which the dynamic wavefront with 400mm aperture can be detected. In the system, a polarization point-diffraction plate (P-PDP) is developed to modulate the polarization states of the reference light and the test light through a simultaneous phase-shifting system based on a chessboard phase grating and a retarder array, and four phase-shifting interferograms can be acquired to realize dynamic detection. Furthermore, the circular carrier squeezing interferometry (CCSI) is proposed to suppress the phase errors generated by position mismatch, intensity distortion, and phase-shift error. The detection result of the SPS-PDI is consistent with the 4D PhaseCam6000 dynamic interferometer. The difference of the peak-to-valley (PV) and root-mean-square (RMS) values are only 0.04λ and 0.008λ. Additionally, the capacity to detect dynamic wavefront is good.

Viscoplastic Modeling of Surface Relief Grating Growth on Isotropic and Preoriented Azopolymer Films.

We report on solving of two intriguing issues concerning the inscription of surface relief gratings within azopolymer thin films under irradiation with SS, PP and RL interference patterns. For this, we utilize the orientation approach and viscoplastic modeling in combination with experimental results, where the change in surface topography is acquired in situ during irradiation with modulated light. First, the initial orientation state of polymer backbones is proved to be responsible for the contradictory experimental reports on the efficiency of the SS interference pattern. Different orientation states can influence not only the phase of SS grating but also its height, which is experimentally confirmed by using special pretreatments. Second, the faster growth of gratings inscribed by the RL interference pattern is shown to be promoted by a weak photosoftening effect. Overall, the modeled results are in good agreement with the order of relative growth efficiency: RL-PP-SS.

Phase Off-Axis Modulation Metasurface for Orbital Angular Momentum Mode Multiplexing/Demultiplexing

Orbital angular momentum (OAM) mode multiplexing provides a promising solution for enlarging communication capacity density. Although various OAM mode multiplexing technologies have been investigated, (de)multiplexers possessing a wide spectral response and polarization insensitivity to combine wavelength division multiplexing (WDM) and polarization division multiplexing (PDM) remain elusive. Herein, we propose a phase off-axis modulation strategy for OAM mode (de)multiplexing by encoding the binary Dammann optical vortex grating (DOVG) phase into a geometric phase metasurface that consists of metallic nanoantennas with spatially varying orientations on a dielectric-metal substrate. We show that four OAM modes (−2, −1, 1, 2) were successfully multiplexed and demultiplexed by virtue of the multilevel diffraction of DOVG and its reverse process. The working wavelength of this metasurface ranges from 1260 nm to 1675 nm due to the dispersion-free geometric phase, moreover, the mode symmetry of DOVG and the conjugate modulation of the geometric phase yield polarization insensitivity. As a proof of concept, an OAM mode multiplexing communication system combining WDM and PDM was constructed, and 1.56 Tbit/s QPSK signals were transmitted with the bit-error-rate approaching 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−6</sup> . These results demonstrate that this phase off-axis modulation metasurface can effectively (de)multiplex OAM modes and shows good compatibility with WDM and PDM, which may promote the practical application of OAM communication and all-optical interconnection.

Retrieving the phase of diffraction orders generated with tailored gratings.

A technique is performed to quantitatively evaluate the intensity and phase of the diffraction orders generated by tailored phase gratings displayed onto a liquid-crystal spatial light modulator (LC-SLM). The SLM displays the grating together with a lens to obtain the Fourier transform. The setup is converted into a polarization common-path interferometer by simply rotating a polarizer. This configuration allows a phase-shifting interferometry algorithm to be applied to retrieve the phase of the diffraction orders. The quadratic phase arising in the system, which must be subtracted, is calibrated using triplicator gratings of varying periods. Various tailored designs with controlled phase shift between diffraction orders are experimentally tested to prove the advantage and simplicity of the technique.

Fringe visibility in X-ray interferometer using dual triangular phase gratings

In recent years, the X-ray interferometer using dual phase gratings has been extensively studied. The large periodic fringes produced by the X-ray interferometer using dual phase gratings can be directly detected by ordinary detectors. At the same time, the X-ray interferometer using dual phase gratings can reduce the radiation dose of the sample without using absorption gratings. Meanwhile, a high fringe visibility is always preferred to achieve a high signal-to-noise ratio for X-ray grating interferometry. However, recent studies have reported that experimental fringe visibility in X-ray interferometer using dual rectangular phase gratings is relatively low. Therefore, it is necessary to further increase the fringe visibility in X-ray interferometry using dual phase gratings. This work focuses on the analysis of fringe visibility in X-ray interferometer using dual triangular phase gratings. Based on the fringe intensity distribution formula of X-ray dual phase grating interferometer, the fringe visibility of the dual triangular phase grating interferometer is investigated as a function of the grating spacing under monochromatic and polychromatic illumination, respectively. For comparison, the fringe visibility of the dual rectangular phase grating interferometer is also studied under the same condition. The results show that the maximum fringe visibility of the dual triangular phase grating interferometer increases with the phase shift increasing regardless of monochromatic or polychromatic illumination. Under monochromatic illumination, the maximum fringe visibility of dual 5π/2 triangular phase gratings is about 21% higher than that of dual rectangular phase gratings. Under polychromatic illumination, the fringe visibility of dual 5π/2 triangular phase gratings is at least 23% higher than that of dual rectangular phase gratings. Under polychromatic illumination, the greater the deviation of X-ray average energy from the grating design energy, the greater the decrease of maximum fringe visibility of the dual phase grating interferometer is. In addition, with the increase of the focal size of X-ray source, the maximum fringe visibility of the dual phase grating interferometer decreases, under polychromatic illumination. We hope that those results can be used as guidelines for designing and optimizing X-ray interferometer using dual triangular phase gratings.

Simulation Study of Sub-Period Super-Resolution X-ray Phase Imaging with Triangular Phase Grating

X-ray phase contrast image has higher sensitivity to materials with light elements than the conventional absorption contrast. Phase image measurement technique using diffraction gratings can be used in the laboratory. Furthermore, X-ray phase microscope using the diffraction gratings has been also developed. However, phase measurement with diffraction gratings has a drawback that the spatial resolution is limited to the grating period. To overcome this problem, a scanning system using a triangular phase grating has recently been proposed, which is expected to archive a large field of view and a high spatial resolution. This study aims to investigate the feasibility of the system and a deep understanding of the optical system by simulation.

Optical vortex beam controlling based on fork grating stored in a dye-doped liquid crystal cell.

In this paper, we investigate the generation and controlling of the optical vortex beam using a dye-doped liquid crystal (DDLC) cell. The spatial distribution of the quasi-sinusoidal orientation of the liquid crystal molecules creates a quasi-sinusoidal phase grating (PG) in the DDLC cell. Depending on the incident light pattern, Trans to Cis photoisomerization of the dye molecules affects the orientation of the liquid crystal molecules. To do so, an amplitude fork grating (FG) is used as a mask, and its pattern is stored in the cell by a pattern printing method as the PG. One of the particular features of the stored grating in the cell is its capability in the diffraction efficiency controlled by the applied electric field. The results show, based on the central defect in the FG pattern, the diffracted probe beam in different orders is optical vortices. As a new technique, this type of stored pattern acts like an amplitude grating but according to the results, its structure is in fact a PG. This technique leads to the vortex beam switching capability by applying an electric field to the cell. The results show that by applying 22V, all the diffraction orders vanish. Meanwhile, the vortex beams reappear by removing the applied voltage. The diffraction efficiency of the vortex beams as well as its generation dependency on the polarization of the incident beam studied. The maximum efficiency of the first diffraction order for linear polarized incident beam was obtained at 0V, about 8%. Based on the presented theory, a simulation has been done which shows the Cis form of the dye molecules has been able to change the angle of LC molecules on average about 12.7°. The study of diffracted beam profiles proves that they are electrically controllable vortex beams.

Generation of phase-only holograms with high-diffraction-order reconstruction by a U-Net-based neural network: A phase grating perspective

Implicit periodic structure in phase-only holograms will result in many diffraction orders in the diffraction field. We analyzed the diffraction pattern from a phase gratings point of view and proved that the diffraction orders were jointly influenced by the phase factor, the single-beam diffraction factor, and the multibeam interference factor. According to the analysis, we proposed the high-diffraction-order angular spectrum method (HDO-ASM) for the numerical reconstruction of high diffraction orders. Different from the conventional methods of removing high diffraction orders, we chose to reconstruct target images in high diffraction orders with HDO-ASM and a U-Net-based neural network. Finally, the 4 K phase-only holograms with high-diffraction-order reconstruction were generated in 0.09s and had a mean reconstruction quality of 34.3 dB (PSNR) in the DIV2K valid dataset. Theoretical and experimental results demonstrated that there are few speckle noises and fringes in the reconstructed images of holograms generated by the proposed method.

1D spatially chirped periodic structures: managing their spatial spectrum and investigating their near-field diffraction.

This work introduces a class of 1D spatial-frequency-modulated structures with transmittance T(x), in which the period changes along the x axis so that the corresponding spatial frequency f(x) sinusoidally alternates between two values. It is shown that T(x) generally is an almost-periodic function and has an impulsive spatial spectrum. However, we find the condition under which T(x) is a periodic function and its spatial spectrum form a lattice of impulses. When the periodicity condition is fulfilled, we call these structures as 1D spatially chirped periodic structures. These structures are characterized by two natural numbers, named as n c and n a v , and a real parameter named as frequency modulation strength (FMS). As an important special case, we define a 1D spatially chirped amplitude sinusoidal grating (SCASG) based on the transmission function of a conventional amplitude sinusoidal grating, in which the phase of conventional amplitude sinusoidal grating is replaced by desired chirped phase. Then the spatial spectrum of a 1D SCASG is investigated in detail, and it is shown that the spatial spectrum can be managed by changing the value of FMS. In other words, the grating's spectrum can be manipulated by adjusting the value of FMS. This feature might find applications in optical sharing of the incident power among different diffraction orders. Moreover, near-field diffraction from 1D SCASGs is studied by using the so-called angular (spatial) spectrum method, and Talbot distances for these gratings are determined and verified experimentally. It is shown that the intensity profiles at quartet- and octant-Talbot distances strongly depend on the values of the parameters n c and n a v . In comparison with the conventional gratings, we see some new and interesting aspects in the diffraction from 1D SCASGs. For instance, unlike the conventional gratings, in some propagation distances, the diffraction patterns possess sharp and smooth intensity bars at which the intensity is several times of the incident light beam's intensity. It is shown that the maximum intensity of these bright bars over the diffraction patterns depends on the characteristic parameters of the grating, including n c , n a v , and FMS of the grating. These intensity bars might find applications for trapping and aggregation of particles along straight lines.

POLARIZATION FEATURES OF ANISOTROPIC BRAGG LIGHT DIFFRACTION ON TRANSMISSIVE HOLOGRAPHIC PHASE GRATINGS

We established that for anisotropic Bragg light diffraction on holographic phase gratings for significant perturbation amplitudes of the refractive index of the medium, in the transmitted diffraction orders there is a significant rotation of the polarization plane of the diffracted wave, which is absent in the reflected diffraction orders. It is shown that periodic changes in the transmission coefficients of diffracted waves with a change in the azimuth of polarization of incident light are due to a significant difference in the Fresnel reflection coefficients at the layer boundaries for s- and p-components of incident light; these periodic changes are absent in the reflected diffraction orders.

Characterization of terahertz wavefront aberrations using computational shear-interferometry

We propose a new solution for sensing a terahertz (THz) wavefront based on a THz reference-less shear interferometer. The key component of the experimental configuration of the proposed interferometer is a THz Ronchi phase grating (RPG). The RPG is custom designed and fabricated for a 0.28 THz source using mechanical milling on a block of high-density polyethylene with a computer numerical control machine. It acts as a shearing element that generates two diffraction orders, thereby, creating two laterally shifted copies of the investigated wavefront in the sensor plane where a THz camera is placed. The direction of the shear can be varied by rotating the grating. Since the grating is a phase grating, the diffraction efficiency is very high. The approach is verified experimentally by demonstrating interferograms of a spherical wave and wavefront reconstruction from five different shears using a gradient-based iterative process.

Extending the degree of polarization concept to higher-order and orbital angular momentum Poincaré spheres

In this work, the density matrix formalism that describes any standard polarization state (fully or partially polarized) is applied to describe vector beams and spatial modes with orbital angular momentum (OAM). Within this framework, we provide a comprehensive description of the mapping between the corresponding Poincaré spheres (PSs); namely: the polarization PS, the higher-order PS (HOPS) and the orbital angular momentum PS (OAMPS). Whereas previous works focus on states located on the surface of these spheres, here we study vector and scalar modes lying inside the corresponding PS. We show that they can be obtained as the incoherent superposition of two orthogonal vector (or scalar) modes lying on the corresponding sphere surface. The degree of polarization (DoP) of a classical polarization state is thus extended to vector beams and OAM modes. Experimental results validate the theoretical physical interpretation, where we used a q-plate to map any state in the polarization PS onto the HOPS, and a linear polarizer to finally project onto the OAMPS. Three input states to such q-plate-polarizer system are considered: totally unpolarized, partially polarized, and fully polarized light. For that purpose, we design a new polarization state generator, based on two geometric phase gratings and a randomly polarized laser, which generates partially polarized light in an efficient and controlled way. We believe that the extension of the DoP concept to vector and OAM beams introduces a degree of freedom to describe spatially polarization and phase variant light beams.

Sound diffusion with spatiotemporally modulated acoustic metasurfaces

Traditional sound diffusers are quasi-random phase gratings attached to reflecting surfaces to introduce spatiotemporal incoherence into the backscattered acoustic field. Early designs consisted of periodically tiled diffuser grating unit cells to cover large surfaces. However, spatial periodicity leads to coherent constructive and destructive interference, which is undesirable for achieving acoustic diffusivity. This problem was partially addressed by using aperiodic tiling of unit cells based on pseudorandom sequences. While an aperiodic diffuser spacing can overcome the problems introduced by spatial periodicity, the improvements in performance come at the expense of increased thickness. In this work, we investigate spatiotemporal modulation of the surface acoustic admittance of a metasurface diffuser to improve sound diffusion. Using semi-analytical and finite element models, we demonstrate that the effects of the spatial periodicity can be mitigated without introducing an aperiodic spatial spacing, thus simultaneously minimizing diffuser thickness and improving diffusivity of the backscattered field. We develop a semi-analytical model that employs Fourier series expansion to determine the scattered sound field from a surface admittance consisting of a quadratic residue diffuser whose individual well admittances are modulated in a traveling wave fashion with modulation frequency, ωm, amplitude, Ym, and a wavenumber that matches the unit cell length, Λ. We observe significant improvement in diffusion due to the fact that the spatiotemporal modulation scatters sound into additional frequency-wavenumber pairs associated with harmonics of ωm and their diffraction orders. The semi-analytical model results are verified using a time-domain finite element model and compared with periodic and aperiodic diffuser designs.

The U-Net-based phase-only CGH using the two-dimensional phase grating.

In this paper, the phase-only holograms with clear first diffraction orders have been generated based on the U-Net and the two-dimensional phase grating. Firstly, we proved the modulation effect of two-dimensional phase grating on diffraction field, and came to a conclusion that it could move the diffraction pattern of the hologram to the odd-numbered diffraction orders' center of that. Then we changed the generation process of phase-only holograms and the training strategy for U-Net according to this conclusion, which converted the optimization target of the U-Net from the zeroth diffraction order in the center of diffraction field to the first diffraction order in the edge of that. And we also used a method called "phase recombination" to improve the structure of U-Net for less memory footprint and faster generating speed. Finally, the holograms with the 4K resolution have been generated in 0.05s, and the average peak signal to noise ratio (PSNR) of the reconstructed images is about 37.2 dB in DIV2K-valid-HR dataset.

Electromagnetically Induced Grating of Surface Polaritons via Coherent Population Oscillation

We propose a scheme to study the electromagnetically induced grating (EIG) of surface polaritons (SPs) in a negative index metamaterial/rare-earth-ion-doped crystal interface waveguide system, based on coherent population oscillation (CPO) modulating by a standing wave control field. Absorption grating can be formed via the large absorption modulation induced by the linear susceptibility of the system; the diffraction of SPs can be realized but with a very small first-order diffraction efficiency and the phase modulation in this case, is negligible. However, when the giant Kerr nonlinearity is taken into account, the phase modulation can be significantly enhanced and accompanied by high transmission at the same time, thus, a phase grating, which effectively diffracts SPs into a high-order direction, can be induced. For both the absorption and phase grating, the dependencies of the first-order diffraction efficiency on the Rabi frequency of the standing wave control field, optical detuning, and interaction length are discussed. The results obtained here have certain theoretical significance for spectral enhancements and precision measurements at the micro–nanoscales.

Annular phase grating-assisted recording of an ultrahigh-order optical orbital angular momentum.

Ultrahigh-order optical orbital angular momentum (OAM) states of the identification over ±270 orders are implemented by annular phase grating (APG) and Gaussian beams with different wavelengths. Particularly, the far-field diffraction intensity patterns feature the spiral stripes instead of Hermitian-Gaussian (HG)-like fringes. It's worth noting that the spiral stripes present uniform distribution, thus the order of OAM states can be intuitively acquired. More specifically, the OAM states can be confirmed from the total amount and rotating direction of the spiral stripes. Compared with traditional methods, the propose scheme contributes to the perfect-distributed and sharper spiral stripes. Moreover, it also makes an easier observation of the patterns in the CCD camera with limited imaging targets. In our experimental setup, the optical filter is removed and the APG parameters are not strictly required. Therefore, the propose optical transmission system is equipped with the advantages of efficiency, robustness and low cost, which paves a promising way for the communication capacity enhancement.