Volume Holographic Gratings Research Articles

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.

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.

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.

3D Measurements and Characterizations of Refractive Index Distributions of Volume Holographic Gratings Using Low‐Coherence Holotomography

AbstractHolographic Optical Elements (HOEs) employ refractive index (RI) modulation to manipulate light, establishing their widespread application in augmented and virtual reality technologies. Despite significant strides in HOEs' fabrication, a comprehensive 3D examination and measurement of these elements remain largely unexplored. This study targets this void by conducting an in‐depth exploration of the 3D RI distributions of volume holographic gratings (VHGs), a quintessential representative of HOEs. Direct tomographic measurements of the 3D RI distributions within millimeter‐scale VHGs are performed at a spatial resolution of 156 nm × 156 nm × 1.2 µm and an RI precision of 2×10−4. This level of precision facilitates a systematic high‐resolution characterization of VHGs. The findings present a substantial evolution from traditional measurement techniques and promise to considerably advance the evaluation of intricate HOE structures and refine the corresponding fabrication processes.

Genetic-algorithm-based waveguide display system with a multiplexed volume holographic grating.

Most of the current holographic waveguide display systems are designed based on the center beam. When the incident beam consists of rays with different angles, the field of view and optical efficiency would greatly reduce. The heavy angular dependence of the volume holographic grating (VHG) and the back-coupling loss are two main reasons. This paper proposes a design method of the waveguide display system with multiplexed VHG, which is based on a genetic algorithm to optimize and calculate the parameters both of the VHG and the waveguide. The simulation results show that the diagonal field of view of the holographic waveguide system is increased to 28°, and its optical efficiency is improved by 30%. The design method of the waveguide system with the multiplexed grating proposed in this paper can effectively expand the field of view and improve the optical efficiency.

Common-path digital holographic microscopy based on a volume holographic grating for quantitative phase imaging.

Digital holographic microscopy (DHM) is a powerful quantitative phase imaging (QPI) technique that is capable of recording sample's phase information to enhance image contrast. In off-axis DHM, high-quality QPI images can be generated within a single recorded hologram, and the system stability can be enhanced by common-path configuration. Diffraction gratings are widely used components in common-path DHM systems; however, the presence of multiple diffraction beams leads to system power loss. Here, we propose and demonstrate implementation of a volume holographic grating (VHG) in common-path DHM, which provides single diffraction order. VHG in common-path DHM (i.e., VHG-DHM) helps in improving signal-to-noise ratio as compared to the conventional DHM. In addition, VHG, with inherently high angular selectivity, reduces image noise caused by stray light. With a simple fabrication process, it is convenient to utilize VHG to control the beam separation angle of DHM. Further, by using Bragg-matched wavelength degeneracy to avoid potential cell damaging effect in blue light, the VHG is designed for recording at a maximum sensitive wavelength of ∼488 nm, while our VHG-DHM is operated at the longer wavelength of red 632.8 nm for cell observation. Experimental results, measured by the VHG-DHM, show the measurement of target thickness ranging from 100 nm to 350 nm. In addition, stability of the system is quantitatively measured. High-contrast QPI images of human lung cancer cells are demonstrated.

Volume holographic illuminator for Airy light-sheet microscopy.

Airy light sheets combined with the deconvolution approach can provide multiple benefits, including large field of view (FOV), thin optical sectioning, and high axial resolution. The efficient design of an Airy light-sheet fluorescence microscope requires a compact illumination system. Here, we show that an Airy light sheet can be conveniently implemented in microscopy using a volume holographic grating (VHG). To verify the FOV and the axial resolution of the proposed VHG-based Airy light-sheet fluorescence microscope, ex-vivo fluorescently labeled Caenorhabditis elegans (C. elegans) embryos were imaged, and the Richardson-Lucy deconvolution method was used to improve the image contrast. Optimized parameters for deconvolution were compared with different methods. The experimental results show that the FOV and the axial resolution were 196 µm and 3 µm, respectively. The proposed method of using a compact VHG to replace the common spatial light modulator provides a direct solution to construct a compact light-sheet fluorescence microscope.

Study on Two-Dimensional Exit Pupil Expansion for Diffractive Waveguide Based on Holographic Volume Grating

Diffraction gratings are becoming a preferred option for waveguide head-mounted in–out coupling devices due to their flexible optical properties and small size and light weight. At present, diffraction waveguide coupling devices for AR head-mounted displays (HMD) have difficulties such as a long development cycle and complicated processing. In this paper, we first establish a set of two-dimensional (2D) grating ray tracing models, based on which we determine the initial architecture of the dual-region two-dimensional exit pupil expansion (2D-EPE) AR-HMD holographic waveguide diffraction system. Second, we propose a honeycomb coupled grating array and optimize the optical energy utilization and brightness uniformity of the holographic waveguide and use a custom dynamic linked library (DLL) function to implement the ray tracing of the 2D grating and add a probabilistic splitting function to the DLL, which reduces the single simulation time from 11.853 min to 1.77 min. We also propose a holographic lithography device composed of holographic optical elements (HOEs) and a method for preparing HOEs. Finally, in order to obtain the diffraction efficiency preoptimized by the above DLL for the uniformity of the exit pupil brightness and light energy utilization, we inverse design with the preparation process parameters as the optimization variables and develop the adaptable electromagnetic calculation program Holo-RCWA. Using Holo-RCWA with nondominated sorting genetic algorithm II (NSGA-II), we inverse design to obtain the process parameters satisfying the diffraction efficiency distribution, and the optimization time of the whole system is reduced from 2–3 days to 10 h. This work is of great significance for AR/VR applications.

Laser Thomson scattering system for anisotropic electron temperature measurement in NUMBER

A laser Thomson scattering system is developed to obtain anisotropic electron temperature in a pulse operated electron cyclotron resonance plasma device. Injecting a laser with an oblique angle to the external magnetic field and detecting scattering photon from a line of sight along another oblique angle enable us to obtain parallel and perpendicular components of electron temperature. Backward (165°) scattering spectrum corresponds to quasi perpendicular velocity distribution, while that for forward (15°) scattering; quasi parallel. Collecting lenses are set in vacuum to maximize solid angle in a limited space of port, where the laser path and collecting optics share an ICF152 flange. Rayleigh scattering intensity is measured as a function of argon gas pressure. An initial result on residual stray light intensity is equivalent to the argon Rayleigh scattering intensity under ≤ 1 kPa of filled pressure. In order to obtain Thomson scattering spectra, a notch filter type stray light rejection optics, which utilize a reflective type volume holographic grating, is evaluated. Notch width (0.2 nm) and flat transmittance (≥ 90%) outside the notch measured for a prototype show that it is an effective optics.

High Refractive Index Monomers for Improving the Holographic Recording Performance of Two-Stage Photopolymers.

Photopolymers hold great promise for the preparation of transparent volume holographic gratings (VHG), which are core optical elements in many application fields. To improve the holographic recording property of a two-stage photopolymer, four new (meth)acrylate monomers (CTA, CTMA, CTBA, CTBMA) with high refractive indices (1.59-1.63) are designed and synthesized in this study. Using them as one writing monomer, a series of photopolymer samples with different formulations and thicknesses are fabricated for holographic recording. Among them, a formulation containing 9 wt % CTMA shows the best performance. Using it as a recording medium, a VHG with high resolution and diffraction efficiency is constructed. Its refractive index modulation reaches 0.046. Moreover, its total transmittance within 400-800 nm achieves 96.62% after photobleaching. The results indicate that the CTMA-based formulation has great application potential in developing high-performance transparent VHG.

The Interplay of Processing-Related Influences on the Formation of Volume Holographic Gratings in a Free-Surface Epoxy-Based Recording Material

Understanding the formation processes of holographic gratings in polymers as a function of material composition and processing is important for the development of new materials for holography and its associated applications. Among the processing-related factors that affect grating formation in volume holographic recording material, pre-exposure, prebaking and dark storage, as well as the associated variations in layer thickness and composition, are usually underestimated. This study highlights the influence and interaction of these factors and shows that they should not be neglected. This is of particular importance for samples with a free surface. Here, one such epoxy-based free-surface material is investigated. To determine the influence of prebaking on the holographic grating formation, as well as on the achieved refractive index contrast, angular resolved analysis of volume holographic phase gratings is applied through point-by-point scanning of the local material response. Grating characteristics are determined by comparison with simulations based on rigorous coupled wave theory. Thus, the optimal dose for prebaking can be determined, as well as the optimal exposure time, depending on the dose. The influence of dark storage on the material response is investigated over a period of 12 weeks and shows a strong dependence on the deposited energy density.

P‐2.13: Design of Volume Holographic Grating based on Genetic Algorithm

High-performance volume holographic grating (VHG) is an important coupling element of holographic waveguide. Small angular bandwidth and low average diffraction efficiency are important disadvantages restricting the applications of VHG. This paper presents a VHG design method based on genetic algorithm. Genetic algorithm is a method of searching for the optimal solution by simulating the natural evolution process.When using genetic algorithm to design VHG, it is necessary to establish a suitable evaluation function, in which angular bandwidth and diffraction efficiency are calculated on the basis of Kogelnik coupled wave theory. The simulation results show that the diffraction efficiency of the designed VHG can reach 96.5% after the optimized design, and the angular bandwidth is 14.1°. Compared with the traditional VHG design method, it is simpler and easier to find the optimal preparation parameters, which provides a theoretical basis for the analysis and preparation of VHG.

P‐2.6: A method for accurately measuring the refractive index modulation of volume holographic grating

In this paper, we propose a method to accurately measure the refractive index modulation of VHG. By adjusting the position of the photon detector in real time, we can accurately measure the variation of the diffracted light intensity with the angle of the incident light, and combining the Rigorous Coupled Wave Analysis (RCWA) fitting, the refractive index modulation of the grating can be measured.

Holographic waveguide display with large field of view based on volume holographic grating

AbstractThe field of view (FOV) of waveguide display systems based on volume holographic grating (VHG) is primarily constrained by the diffraction response bandwidth, which is currently insufficient to meet the need for AR immersive displays. Through the composite diffraction response bandwidths, red‐responsive and green‐responsive double‐layer VHG structures are proposed to expand the FOV under varied image sources. The influence of several input spectral bandwidths on FOV improvement is also considered, and OLED micro‐display and LCOS micro‐display are selected as image sources. As a consequence, the horizontal FOV of the red‐responsive double‐layer VHG waveguide based on the LCOS micro‐display is 19.5°, and that of the green‐responsive double‐layer VHG waveguide based on the OLED micro‐display is 33.4°. Through the simulation and experiment, we effectively illustrate the effect of the double‐layer VHG and the input spectral bandwidth on the FOV.

Formation and properties of volume and relief holographic gratings in photopolymer materials

Formation and properties of volume and relief holographic gratings in photopolymer materials

Full-color eye-box expansion via holographic volume gratings recorded in photo-thermo-refractive glass.

Conventional head-up displays (HUDs) suffer from a limited exit pupil and a lack of compactness mainly due to the use of bulky optics. HUDs need a high-quality image with a large field of view (FOV) in small packaging to gain commercial acceptability. Holographic HUDs are phase-only devices that allow vision correction and focus adjustment while having a wide FOV. However, the limited bandwidth of a spatial light modulator (SLM) imposes a trade-off between the FOV and eye-box size. Combining a holographic system with an image-replicating element eliminates such a tradeoff. For image replication, we designed and fabricated a compact 2D diffractive beam splitter formed from two perpendicular volume gratings operating in the Raman-Nath regime. The gratings were recorded holographically in photo-thermo-refractive (PTR) glass, with optimized index modulation, thickness, and period to provide uniform intensity distribution across all desired orders for the fundamental red, green and blue (RGB) colors. We demonstrated a full-color holographic projection with an eye-box expanded by the designed 2D diffractive beam splitters.

Expanding the angular bandwidth of augmented reality coupling element volume holographic grating by multiplexing equal-period and variable-inclination-angle interference fringes

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

On the Use of Haloalkane/Acrylate-Based Holographic Gratings as Compression and Rotation Sensors.

In this work, we test the effectiveness of using highly transparent holographic phase reflection and transmission volume gratings based on multifunctional acrylates as linear compression and rotation sensors. The gratings are recorded in a holographic mixture based on multi-reticulated acrylate and haloalkanes. To activate the photo-polymerization process, we used a mixture of 6-oxocamphore and rhodamine 6G. The mixture is a simplified version of the mixture used in previous works and shows some interesting features mainly in connection with the different roles played by the rhodamine 6G dye at different writing wavelengths λ = 532 nm and λ = 460 nm. Regarding reflection gratings, the maximum achieved diffraction efficiency is ≈50% and their use as linear compression sensors produces a shift in the reflection peak of 2 nm. Following the removal of compression, the grating slowly returns to the initial state. Regarding transmission gratings, the maximum achieved diffraction efficiency is ≈45% and they demonstrate very high sensitivity to even small rotations in a free-standing configuration.