Holographic Gratings Research Articles

Numerical Estimation of Bending in Holographic Volume Gratings by Means of RCWA and Deep Learning

In this paper, we introduce a novel approach to model bending phenomena on holographic volume gratings based on Rigorous Coupled Wave Analysis (RCWA), in which the bending as a phase in the dielectric permittivity expansion is introduced, and the Shooting Method (SM) is employed to solve the resulting system of equations. Further validation of our model is conducted by comparing its predictions to those obtained from reference Finite-Difference Time-Domain (FDTD) simulations and Coupled Wave Theory (CWT, referring to Kubota’s model that includes the bending phenomenon). Furthermore, we propose a methodology for estimating the bending from the diffraction efficiency curves in transmission volume gratings based on deep learning models, with a subsequent study of their accuracy and applicability.

Establishment of Theoretically Guided Understanding for Magneto‐Optical Interactions through Ferromagnetic Fluids in Uniform, Static Magnetic Field

Prior magneto‐optical transmission models through ferrofluids are limited by oversimplifications which hinder accuracy and generalizability. To overcome this challenge, this study models the magnetic field effect on the magneto‐optical transmission through hematite ferrofluid using the method of invariant embedding. Optical coherence tomography (OCT) is used to extract the changes in the refractive index, enabling a discovery of a magnetodielectric effect for the hematite ferrofluid. To establish a basis for control and understanding, the influence of the magnetic field on the spatial transmission profile at different wavelengths, optical propagation lengths, and the effect of variation of each size parameter on the optical transmission have been investigated. Further parametric studies allowed finding out an analogy to nonuniform, cascade, volume holographic diffraction gratings, which is crucial for a wide range of optical and bioimaging applications. Compared to the experimental results, the presented model achieves 99% accuracy over the wavelength range (300–1100) nm under uniform static magnetic field (0–6.5) mT. Besides, the conspicuous lead of evidentiary understanding of magneto‐optical interactions with magnetic fluids, the structural milestones of the model can be further utilized to model similar challenging constructs in complex media. The innumerable applications of study extend to involve communication engineering, biomedical, optical, and security applications.

Recording Reflection Volume Holographic Gratings with Enhanced Performance by Two‐Stage Photopolymers Containing Bifunctional Monomers

AbstractA holographic optical waveguide (HOW) is an optimal solution for augmented reality (AR) displays, in which reflection volume holographic gratings are core optical elements. In this study, five high‐refractive‐index (≈1.6) bifunctional acrylate monomers (TPBSAs) are designed and synthesized. A series of two‐stage photopolymers are prepared using TPBSAs as writing monomers. Among them, one containing 9 wt.% TPBSA‐3 shows the best holographic recording performance with a sensitivity as high as 26.59 cm mJ−1. Using it as the recording medium, a reflection VHG can be obtained with high spatial frequency (5634 lines per mm), diffraction efficiency (97.14%), refractive index modulation (0.029), and good transparency within 400−800 nm after photobleaching. Furthermore, a HOW capable of achieving virtual and real image fusion is fabricated.

Exit pupil uniformity improvement of holographic waveguide displays based on a genetic algorithm

Waveguide displays based on a volume holographic grating (VHG) have the benefit of expanding the exit pupil. Exit pupil size and exit pupil uniformity are two important indicators of waveguide displays. Most current optimizations for exit pupil uniformity are based on the central viewing angle. However, this does not ensure satisfied exit pupil uniformity for other viewing angles. In this paper, a method based on a genetic algorithm to optimize the exit pupil uniformity under the whole horizontal field of view (HFOV) is proposed. First, the ray-tracing-based mathematical model to analyze the uniformity of the whole HFOV at all wavelengths is established. Based on the mathematical model, the objective function can be defined, which can optimize the refractive index modulation distribution on several regions of the out-coupling VHG. The simulation results show that for a holographic waveguide display with the exit pupil size of 16mm×12mm, the average exit pupil uniformity can reach 86.87% at a diagonal field of view (DFOV) of 30°, a 9.16% improvement over the previous method. The genetic-algorithm-based method proposed in the paper can effectively improve the exit pupil uniformity.

Characterization of two ultraviolet–blue volume-phase holographic gratings based on dichromated gelatin and photopolymer recording materials

Characterization of two ultraviolet–blue volume-phase holographic gratings based on dichromated gelatin and photopolymer recording materials

Study on the Design Method of High-Resolution Volume-Phase Holographic Gratings.

Volume-phase holographic gratings are suitable for use in greenhouse gas detection imaging spectrometers, enabling the detection instruments to achieve high spectral resolution, high signal-to-noise ratios, and high operational efficiency. However, when utilized in the infrared wavelength band with high dispersion requirements, gratings struggle to meet the demands for low polarization sensitivity due to changes in diffraction performance caused by phase delays in the incidence of light waves with distinct polarization states, and current methods for designing bulk-phase holographic gratings require a large number of calculations that complicate the balance of diffraction properties. To overcome this problem, a design method for transmissive bulk-phase holographic gratings is proposed in this study. The proposed method combines two diffraction theories (namely, Kogelnik coupled-wave theory and rigorous coupled-wave theory) and establishes a parameter optimization sequence based on the influence of design parameters on diffraction characteristics. Kogelnik coupled-wave theory is employed to establish the initial Bragg angle range, ensuring that the diffraction efficiency and phase delay of the grating thickness curve meet the requirements for incident light waves in various polarization states. Utilizing rigorous coupled-wave theory, we optimize grating settings based on criteria such as a center wavelength diffraction efficiency greater than 95%, polarization sensitivity less than 10%, maximum bandwidth, and spectral diffraction efficiency exceeding 80%. The ideal grating parameters are ultimately determined, and the manufacturing tolerances for various grating parameters are analyzed. The design results show that the grating stripe frequency is 1067 lines per millimeter, and the diffraction efficiencies of TE and TM waves are 96% and 99.89%, respectively. The diffraction efficiency of unpolarized light is more than 88% over the whole spectral range with an average efficiency of 94.49%, an effective bandwidth of 32 nm, and a polarization sensitivity of less than 7%. These characteristics meet the performance requirements for dispersive elements based on greenhouse gas detection, the spectral resolution of the detection instrument is up to 0.1 nm, and the signal-to-noise ratio and working efficiency are improved by increasing the transmittance of the instrument.

Determination of optimal directions of the wave vector of the phase holographic grating in cubic photorefractive crystal

The dependence of the change in the components of the inverse permittivity tensor of a cubic photorefractive Bi12SiO20 crystal on the direction of the wave vector of holographic grating in the crystal coordinate system has been studied. It is shown that, when recording a phase hologram, the largest change in the refractive index of Bi12SiO20 crystal is attained when the holographic grating wave vector is oriented along symmetrically equivalent 111 directions. The maximum possible modulation amplitude of the refractive index of a holographic grating with the wave vector oriented along the 110 directions is found to exceed that in the case of orientation along the 100 directions. The components of the inverse permittivity tensor of Bi12SiO20 crystal were calculated taking into account that a phase hologram is recorded under linear electro-optic, photoelastic, and inverse piezoelectric effects.

Storage Optimization of Transmission Holographic Gratings in Photohydrogels.

The development and optimization of holographic materials represent a great challenge today. These materials must be synthesized according to the characteristics that are desirable in photonic devices whose application is the object of investigation. In certain holographic sensors and biosensors, it is essential that the recording material be stable in liquid media. Furthermore, the holographic gratings stored in them must have temporal and structural stability, so that they can act as transducers of the analytical signal. Therefore, it is essential to optimize its storage in terms of the chemical composition of the material and the optical parameters of recording. This work focuses on the study of the storage optimization of unslanted transmission volume phase holograms in photohydrogels based on acrylamide and N,N'-methylenebis(acrylamide). Hydrogel matrices, also composed of acrylamide and N,N'-methylenebis(acrylamide), with different degrees of cross-linking were used and analyzed by scanning electron microscopy and UV-visible spectroscopy. The best results in terms of diffraction efficiency were reached for hydrogel matrices with an acrylamide/N,N'-methylenebis(acrylamide) molar ratio between 19.9 and 26. This relationship was also optimized in the incubator solution used to incorporate the components necessary for the formation of the holograms in the hydrogel matrices. The maximum diffraction efficiency, about 35%, was achieved when using an incubation solution with an acrylamide/N,N'-methylenebis(acrylamide) molar ratio of 4.35. The influence of the physical thickness of the hydrogel layers, the intensity, and the exposure time on the diffraction efficiency was also investigated and optimized. In addition, the behavior of the hologram was analyzed after a washing stage with PBST. A simple model that considered the effects of bending and attenuation of holographic gratings was proposed and used to obtain the optical parameters of the holograms.

Design and development of a stand-off Raman brassboard (SDU-RRS) for the spectroscopic study of planetary materials

Raman spectroscopy has emerged as a crucial mineral analysis technique in planetary surface exploration missions. Nonetheless, the inherently low Raman scattering efficiency of planetary silicate materials makes it challenging to extract enough Raman information. Theoretical and experimental studies of the remote Raman scattering properties of planetary materials are also urgent requirements for future lunar and planetary explorations. Here, Shandong University Remote Raman Spectrometer (SDU-RRS) was developed to demonstrate the feasibility of lunar remote Raman technology and conduct preliminary research on remote Raman scattering properties. SDU-RRS utilizes a pulsed 532 nm laser, a non-focal Cassegrain telescope, a volume phase holographic grating, an intensified charge-coupled device, and the time-gating technique to detect weak-signal silicate minerals. The spectral resolution obtained with atomic emission lamps was <4.91 cm−1, and the wavelength accuracy was <1 cm−1, across the spectral range of 241–2430 cm−1. SDU-RRS can detect natural augite within a feldspar-olivine-augite matrix at a concentration of 20 % at ∼1 m under ambient lighting conditions. A series of experiments were conducted to evaluate the influence of measurement conditions and physical matrix effects on acquired Raman signals, either qualitatively or quantitatively, on geological materials. The study indicates that the transmission of Raman-scattered light conforms to Lambert’s cosine law, and a linear correlation exists between Raman intensity and laser power. The study also evaluated the impact of grain size, surface roughness, porosity, and shadow-hiding effects. Reducing grain size decreases Raman intensity and broadens Raman spectra. These characteristics are essential for achieving definitive mineralogical information from granular materials by remote Raman spectroscopy in lunar and planetary explorations.

High Refractive Index, Low Birefringence Holographic Materials via the Homopolymerization of 1,2-Dithiolanes.

High refractive index, low birefringence photopolymers were created via the radical-mediated, ring opening homopolymerization of 1,2-dithiolane functionalized monomers and were subsequently evaluated as holographic recording media. This investigation systematically characterized the reaction kinetics, thermodynamics, and volume shrinkage of the 1,2-dithiolane homopolymerization as well as the optical transparency, refractive index, birefringence, and holographic performance of multifunctional 1,2-dithiolane functionalized monomers and their resultant polymers. Real-time kinetic and thermodynamic analyses of a monofunctional 1,2-dithiolane monomer, lipoic acid methyl ester (LipOMe), indicated rapid monomer conversion, exceeding 90% in 60 s, with an overall enthalpy of reaction of 18 ± 1 kJ/mol. The ring-opening polymerization resulted in low shrinkage (10.6 ± 0.3 cm3/mol dithiolane) and a significant bulk refractive index increase (0.030 ± 0.003). The resulting photopolymers exhibited high optical transparency, minimal haze, and negligible birefringence, suggesting the potential of 1,2-homopolymers as optical materials. To further explore the specific capabilities for use as high-performance holographic recording applications, several multifunctional monomers were synthesized with the ethanedithiol lipoic acid monomer (EDT-Lip2) selected for experimentation. Holographic diffraction gratings written using this monomer achieved a peak-to-mean refractive index modulation of 0.008 with minimal haze and birefringence.

Higher-Dimensional Communications Using Multimode Fibers and Compact Components to Enable a Dense Set of Communicating Channels

Higher-dimensional communications are of interest for multiple reasons, including increasing the classical transmission capacity and, more recently, the quantum state transfer through fibers using the many modes within the fiber. For quantum communications, this enables an increase in the number of bits per photon, increasing quantum fidelity, increasing error thresholds and enabling hyperentanglement transfer, among other possibilities. A high-dimensional quantum state transfer can be transported through multimode fiber using the many modes available. However, this transfer of information through multimode optical fiber is limited by attenuation and mode coupling among the various spatial and polarization modes. Here, we consider how this mode coupling impacts the transfer process. We consider the fiber’s modal properties, including orbital angular momentum, modal group numbers, and principal modes. We also investigate and propose input and output optical components, as well as fiber properties, which better mitigate the deleterious effects of mode coupling. We use the WKB approximation to the scaler wave equation as a guidance to quantify this coupling and then implement corrections to this approximation using exact solutions to the scaler wave equation. We consider methods to circumvent this mode coupling using optical fiber designs, holographic optical components and devices that are commercially available today. Some of these components, such as the holographic gratings and lenses, could be implemented using flat optics.

Holographic surface relief diffraction gratings made of hydrogels for direct label-free biosensing of IgGs

This work describes the development of a label-free (LF) biosensing platform for the direct detection of targets based on diffractive structures fabricated with acrylamide-based hydrogels biofunctionalized with proteins and antibodies. Hydrogels containing Bovine Serum Albumin protein (BSA) with different crosslinking degrees were synthesized and characterized to find the optimal conditions for the suitable fabrication of surface relief gratings (SRGs). The bioavailability of BSA-functionalized hydrogels for the specific recognition of anti-BSA antibodies was verified by fluorescence detection. After the hydrogel-based SRG fabrication, diffraction efficiency measures at two different laser wavelengths were used for the direct LF detection of anti-BSA antibodies. The limit of detection in the sub mg L−1 range was read. Additionally, SRGs were prepared with hydrogels biofunctionalized with anti-rabbit antibodies for the direct detection of IgGs from rabbit serum, obtaining similar analytical performance without the necessity of labeling or applying amplification strategies.

Optically accessible gas exposure apparatus for testing and characterization of holographic gas sensors

Holographic gas sensors are of great interest due to their widespread applicability and potential for high sensitivity, fast response, immunity to electromagnetic interference, and compact and lightweight nature. For effective design and development of holographic gas sensors, it is essential to have a reliable and safe gas exposure system allowing for optical access for testing purposes. Here, the design and operation of a custom gas exposure apparatus for the performance evaluation of holographic grating-based gas sensors within a research laboratory setting is presented. The apparatus enables the real-time measurement of analyte-induced changes in key holographic grating parameters: grating diffraction efficiency and reconstruction wavelength. A demonstration of the capabilities of the optically accessible apparatus to evaluate sensor response time, sensitivity to different volatile organic compound analytes, and response to cyclical gas exposure is presented. The AutoCAD designs, as well as the material and equipment specifications for the custom apparatus, are provided to facilitate reproduction of the gas development and gas exposure setup.

Simulation of gradient period polarization volume gratings for augmented reality displays

Augmented reality (AR) displays are gaining attention as next-generation intelligent display technologies. Diffractive waveguide technologies are progressively becoming the AR display industry's preferred option. Gradient period polarization volume holographic gratings (PVGs), which are considered to have the potential to expand the field of view (FOV) of waveguide display systems due to their wide bandwidth diffraction characteristics, have been proposed as coupling elements for diffraction waveguide systems in recent years. Here, what we believe to be a novel modeling method for gradient period PVGs is proposed by incorporating grating stacking and scattering analysis utilizing rigorous coupled-wave analysis (RCWA) theory. The diffraction efficiency and polarization response were extensively explored using this simulation model. In addition, a dual-layer full-color diffractive waveguide imaging simulation using proposed gradient period PVGs is accomplished in Zemax software using a self-compiled dynamic link library (DLL), achieving a 53° diagonal FOV at a 16:9 aspect ratio. This work furthers the development of PVGs by providing unique ideas for the field of view design of AR display.

Method to overlay an interference field over a grating using only the grating's profile symmetry.

We theoretically present and experimentally validate a method to overlay the interference field (for making straight-line holographic gratings) with a real grating and a derived method to measure the lateral shift between two real gratings. The methods rely only on the real gratings' profile symmetry, and they are robust against variations of the real gratings' profile parameters and functional form.

Easily variable and scalable terahertz pulse source based on tilted-pulse-front pumped semiconductors

A new type of terahertz source containing only two optical elements - a volume phase holographic grating, and a semiconductor nonlinear slab - is proposed. The setup does not require any microstructuring, has only one diffraction order, and can be scaled to large pump sizes without any principal limitations. Furthermore, it can be easily adapted to different pump wavelengths and THz phase-matching frequencies. The Fresnel loss at the boundary of the materials can be significant at conventional pump polarizations (s-pol), but a single-layer anti-reflection (AR) coating can reduce it. Pumping such a setup with polarization in the dispersion plane (p-pol, TM mode) can reduce the effective nonlinear polarization and consequently the terahertz generation efficiency. However, in the absence of AR coating, this reduction is overcompensated by the reduced Fresnel loss.

Design and fabrication of two-dimensional holographic grating waveguide with large field of view and high uniformity

Augmented Reality (AR) technology is a technique that integrates virtual im- ages with the real environment. With the continuous improvement of comput- ing power in mobile electronic products, mobile devices with AR capabilities are gradually becoming the next generation computing platform. When eval- uating the performance of holographic waveguides, key indicators include the field of view (FOV), eye relief, and eye box uniformity. This study proposes a method for designing holographic waveguides with a large field of view and high uniformity based on a two-dimensional holographic grating (2DHG). By lever- aging the multiplexing characteristics of photopolymer, we superimposed two incoherent one-dimensional gratings on a holographic material layer, achieving dual-channel propagation of the field of view, with each channel responsible for half of the FOV. In the end, these two channels are stitched together to form a complete FOV. This method not only enlarges the eye box but also helps im- prove the system's FOV. By controlling the transmittance of the exposure area, the uniformity of the brightness of the wide-field eye box is improved. Finally, we conducted a performance analysis of the prepared holographic waveguide, verifying the feasibility and correctness of the proposed 2DHG waveguide in two-dimensional eye box expansion and field of view extension. Through eye box expansion, the eye box reached 14 mm × 17 mm, the diagonal field of view reached 58◦, the eye relief is 15 mm, and the eye box brightness uniformity reached 79.7 %.

Initial measurement of ultrafast charge exchange recombination spectroscopy on the EAST tokamak

Ultrafast charge exchange recombination spectroscopy (UF-CXRS) has been developed on the EAST tokamak (Yingying Li et al 2019 Fusion Eng. Des. 146 522) to measure fast evolutions of ion temperature and toroidal velocity. Here, we report the preliminary diagnostic measurements after relative sensitivity calibration. The measurement results show a much higher temporal resolution compared with conventional CXRS, benefiting from the usage of a prism-coupled, high-dispersion volume-phase holographic transmission grating and a high quantum efficiency, high-gain detector array. Utilizing the UF-CXRS diagnostic, the fast evolutions of the ion temperature and rotation velocity during a set of high-frequency small-amplitude edge-localized modes (ELMs) are obtained on the EAST tokamak, which are then compared with the case of large-amplitude ELMs.

Temperature dependence of the diffusion, thermodiffusion and Soret coefficient of a dilute polystyrene/toluene solution as measured by means of a transient holographic grating technique

We have measured diffusion, thermodiffusion and Soret coefficients of a dilute solution of polystyrene in toluene by means of the transient holographic grating technique of thermal diffusion forced Rayleigh scattering over a temperature range from 10 to 50 °C. The results show a systematic deviation from optical beam deflection measurements over a narrower temperature interval of the order of ten percent but the identical temperature dependence. A comparison with previous measurements from our laboratory, new control measurements and literature data does not lead to a clear conclusion about the correct values. From the diffusion coefficient a temperature independent hydrodynamic radius is obtained. Both the Soret coefficient and the viscosity-scaled thermodiffusion coefficient ηDT, for which a universal value has been reported in the case of asymmetric systems with large solute entities, decrease with temperature.

Compensated DOE in a VHG-based waveguide display to improve uniformity

Augmented reality head-mounted displays (AR-HMDs) utilizing diffractive waveguides have emerged as a popular research focus. However, the illuminance uniformity over the fields of view (FOV) is often unsatisfactory in volume holographic grating (VHG) based waveguide displays. This paper proposes a high uniformity AR waveguide display system. Firstly, the angular uniformity of the VHG-based waveguide displays is analyzed. Subsequently, diffractive optical elements (DOEs) are seamlessly integrated onto the outer coupling surface of the waveguide substrate to improve the angular uniformity through phase compensation. To design the DOE phase, the multi-objective stochastic gradient descent (MO-SGD) algorithm is proposed. A single DOE is used to compensating various images form the image source. A hybrid loss, which includes the learned perceptual image patch similarity (LPIPS) metric, is applied to enhance the algorithm performance. Simulation results show that the proposed method effectively suppresses illumination degradation at the edge FOV in exit pupil images of the waveguide display system. In the results, the peak signal-to-noise ratio (PSNR) is improved by 5.54 dB. Optical experiments validate the effectiveness of the proposed method. The measured nonuniformity (NU) against FOVs is improved by 53.05% from 0.3749 to 0.1760.