Volume Holographic Optical Elements Research Articles

P‐230: Late‐News Poster: Enhancement of the color uniformity of a VHOE‐waveguide‐based AR eyewear display through drive signal management scheme

In this paper, we present an enhanced approach‐a drive signal management scheme employed on a micro‐display device of optical engine to retune the color uniformity of an Augmented Reality (AR) eyewear display with a the Volume Holographic Optical Elements (VHOEs)‐based waveguide. Our method streamlines multiplexing complexity, necessitating just one optical waveguide and three RGB gratings to attain a full‐color eyewear display with nearly a 16° horizontal field of view (FOV) and less than 3% ΔELab color non‐uniformity.

Illumination system contributing zooming function to lensless digital holographic microscope by using lightguide incorporated with volume holographic optical elements

Illumination systems play a crucial role in determining the image quality of lens- less images. In this paper, we propose an illumination system that uses a light guide with a volume holographic optical element (VHOE) and apply it to a lensless digital holographic microscope (DHM). The light guide has an in-coupler that is maturely developed for AR/MR applications. A VHOE serves as an in-coupler and directs the laser light into the guiding mode. Unlike the conventional light guide in the AR/MR application, the out-coupler is changed to a functional VHOE to diffract a divergent spherical wave. Through clever arrangement, the interference fringe caught by the CMOS image sensor can be used to retrieve the tested image. By controlling the focusing point of the diffracted spherical wave, the field of view (FOV) and the resolution can be adjusted. We have demonstrated the use of different VHOEs to perform different FOV and image resolutions. Thus, a zoomable-compact lensless DHM with holographic light guide technology is achievable.

Review of recent advances in photosensitive polymer materials and requirements for transmission diffractive optical elements for LED light sources

Scattered light from outdoor LED sources is a major contributor to blue-spectrum light pollution which negatively impacts ecology and public health. Volume holographic diffractive optical elements (DOEs), made with low-cost photosensitive polymers, are proposed as a technological solution that can minimise this scattering. This review examines requirements needed for efficient operation of transmission-format DOEs with LEDs and implications for material choices. For such DOEs to have a single diffraction order and high efficiency there is a corresponding range of acceptable Δn. A survey of materials also found that many current materials satisfy the Δn requirements for single-element DOEs. It is proposed that future efforts focus on robustness, environmental sustainability, scalability, multiplexing, optical quality, and reduced cost.

Wide-spectrum laser beam shaping for full-color volume holographic optical element recording.

For homogeneous diffraction efficiency of the recorded volume holographic optical element (vHOE), a recording beam of uniform intensity is required. A multicolor vHOE is recorded by an RGB laser source with Gaussian intensity distribution; during equal exposure time, recording beams of different intensities would result in different diffraction efficiencies in different recording areas. In this paper, we present a wide-spectrum laser beam shaping system design method, by which the incident RGB laser beam can be controlled into uniform intensity distribution with a spherical wavefront. This beam shaping system can be added to any recording system to obtain uniform intensity distribution without altering the beam shaping effect of the original recording system. The proposed beam shaping system is composed of two aspherical lens groups, and the design method with an initial point design and optimization design method is given. An example is built to demonstrate the feasibility of the proposed beam shaping system.

Volume Holographic Optical Element Used for Light Coupling in XR's Near-Eye Glass

Volume Holographic Optical Element Used for Light Coupling in XR's Near-Eye Glass

Modelling and Design of Holographic Optical Elements for Beam-Coupling Applications for a Range of Incident Beam Angles

Theoretical modelling has been used to calculate the holographic recording beam angles required in air (at any recording wavelength) to produce a Volume Holographic Optical Element (VHOE) for any defined input and output beam angles. The approach is used to facilitate the design and fabrication of diffractive coupling elements through a holographic process that avoids the use of coupling prisms during recording and will help in the design of recording arrangements that better suit the mass production of low-cost elements, especially those designed for non-normal incidence. In this study, the recording angles needed for a range of recording wavelengths were explored for VHOE couplers designed for input angles (in air) ranging from 0° to −55°. Then, in order to validate the model, holographic recording in Bayfol HX 200 photopolymer at 532 nm was used to fabricate photopolymer VHOE couplers for 633 nm light (−45° input angle in air). Bragg curves obtained experimentally for different probe wavelengths (403 nm, 532 nm and 633 nm) confirm the recording of the desired grating structures to a precision of ±1°, and coupling is demonstrated at 633 nm with a diffraction efficiency of up to 72%. Furthermore, the model is used to identify the origins of some weaker spurious gratings observed alongside the expected ones.

Improving the Holographic Recording Characteristics of a Water-Resistant Photosensitive Sol–Gel for Use in Volume Holographic Optical Elements

Continual improvements to holographic recording materials make the development of volume holographic optical elements increasingly more attainable for applications where highly efficient, lightweight diffractive optical elements can replace conventional optics. A fast-curing, water resistant photosensitive sol–gel capable of volume holographic recording has recently drawn attention for its extreme environmental and physical robustness, in particular its water/moisture and scratch resistance. However, to date, the refractive index modulation has been limited. While water-resistant properties are invaluable in the face of the weathering which many practical systems for outdoor applications will endure, high refractive index modulation is also important in order to facilitate high diffraction efficiency holograms recorded in relatively thin layers. Lower grating thickness ensures a large angular and wavelength range of operation-properties that are critical for many applications of holographic optical elements such as solar light harvesting, optical displays and illumination management. For any application where low-cost mass production is envisaged, sensitivity/writing speed is also a crucial factor. In this research, we studied the recording properties of these water-resistant photosensitive sol–gel layers at two different recording wavelengths (532 and 476 nm) and investigated methods for improving these properties. We report more than two-fold improvement of the refractive index modulation from 1.4×10−3 to 3.3×10−3 in layers of thickness ranging from 40–100 μm and more than an order of magnitude increase in photosensitivity/recording speed through better matching between recording wavelength and layer absorption, chemical alterations and thermal post-processing techniques.

Computational see-through screen camera based on a holographic waveguide device.

This study proposes a novel computational imaging system that integrates a see-through screen (STS) with volume holographic optical elements (vHOEs) and a digital camera unit. Because of the unique features of the vHOE, the STS can function as a holographic waveguide device (HWD) and enable the camera to capture the frontal image when the user gazes at the screen. This system not only provides an innovative solution to a high-quality video communication system by realizing eye-contact but also contributes to other visual applications due to its refined structure. However, there is a dilemma in the proposed imaging system: for a wider field of view, a larger vHOE is necessary. If the size of the vHOE is larger, the light rays from the same object point are diffracted at different Bragg conditions and reflect a different number of times, which causes blurring of the captured image. The system imaging process is analyzed by ray tracing, and a digital image reconstruction method was employed to obtain a clear picture in this study. Optical experiments confirmed the effectiveness of the proposed HWD-STS camera.

Multiplexed Holographic Combiner with Extended Eye Box Fabricated by Wave Front Printing

We present an array-based volume holographic optical element (vHOE) recorded as an optical combiner for novel display applications such as smart glasses. The vHOE performs multiple, complex optical functions in the form of large off-axis to on-axis wave front transformations and an extended eye box implemented in the form of two distinct vertex points with red and green chromatic functions. The holographic combiner is fabricated by our extended immersion-based wave front printing setup, which provides extensive prototyping capabilities due to independent wave front modulation and large possible off-axis recording angles, enabling vHOEs in reflection with a wide range of different recording configurations. The presented vHOE is build up as an array of sub-holograms, where each element is recorded with individual optical functions. We introduce a design and fabrication method to combine two angular and two spectral functions in the volume grating of individual sub-holograms, demonstrating complex holographic elements with four multiplexed optical functions comprised in a single layer of photopolymer film. The introduced design and fabrication process allows the precise tuning of the vHOE’s diffractive properties to achieve well-balanced diffraction efficiencies and angular distributions between individual multiplexed functions.

Portable autostereoscopic display based on multi-directional backlight.

A multi-directional backlight autostereoscopic display system with high resolution, low crosstalk, and motion parallax is developed in this paper. The proposed multi-directional backlight system is based on the Bragg mismatched reconstruction of volume holographic optical element (VHOE), and includes a set of light sources which are uniformly arrayed along one direction. Each light source produces its corresponding directional lighting to follow the human eye position detected by an eye tracker. Two scenarios are presented to build the multi-directional backlight system. The prism-type backlight system which guides the incident beam with a prism is relatively simple and easy to implement. The waveguide-type one which employs a transflective film to expand the incident light beam within the waveguide and modulate the intensity of the incident beam, is relatively thin and is applicable to large-area display. Two prototypes are built and the effectiveness of the proposed autostereoscopic display system is verified by the experimental results.

Lateral spectrum splitting system with perovskite photovoltaic cells

We examine the potential of a multijunction spectrum-splitting photovoltaic (PV) solar energy system with perovskite PV cells. Spectrum splitting allows combinations of different energy band gap PV cells that are laterally separated and avoids the complications of fabricating tandem stack architectures. Volume holographic optical elements have been shown to be effective for the spectrum-splitting operation and can be incorporated into compact module packages. However, one of the remaining issues for spectrum splitting systems is the availability of low-cost wide band gap and intermediate band gap cells that are required for realizing high overall conversion efficiency. Perovskite PV cells have been fabricated with a wide range of band gap energies that potentially satisfy the requirements for multijunction spectrum-splitting systems. A spectrum-splitting system is evaluated for a combination of perovskite PV cells with energy band gaps of 2.30, 1.63, and 1.25 eV and with conversion efficiencies of 10.4%, 21.6%, and 20.4%, respectively, which have been demonstrated experimentally in the literature. First, the design of a cascaded volume holographic lens for spectral separation in three spectral bands is presented. Second, a rigorous coupled wave model is developed for computing the diffraction efficiency of a cascaded hologram. The model accounts for cross-coupling between higher diffraction orders in the upper and lower holograms, which previous models have not accounted for but is included here with the experimental verification. Lastly, the optical losses in the system are analyzed and the hypothetical power conversion efficiency is calculated to be 26.7%.

Application of volume hologram optical elements. Part 1

This paper is devoted to an overview of practical applications of volume holographic optical elements in devices developed at the Siberian State Geodetic Academy (SSGA) and at the CKB «Tochpribor» (Novosibirsk). In the first part of the work, examples of the use of volume holographic optical elements in a holographic level, in a holographic sight and in a holographic spectrum indicator are considered. It is shown that the use of volume holographic optical elements opens up the possibility of developing and creating fundamentally new design solutions for optical and spectral devices. The paper notes both the advantages and some disadvantages of using volume holographic optical elements in the developed holographic devices. It is indicated how the emerging shortcomings of these devices can be corrected with their further improvement.

Pixelated volume holographic optical element for augmented reality 3D display.

Augmented reality (AR) three-dimensional (3D) display is the hardware entrance of metaverse and attracts great interest. The fusion of physical world with 3D virtual images is non-trivial. In this paper, we proposed an AR 3D display based on a pixelated volume holographic optical element (P-VHOE). The see-through combiner is prepared by spatial multiplexing. A prototype of AR 3D display with high diffraction efficiency (78.59%), high transmission (>80%) and non-repeating views is realized. Virtual 3D objects with high fidelity in depth is reconstructed by P-VHOE, with a complex wavelet structural similarity (CW-SSIM) value of 0.9882. The proposed prototype provides an efficient solution for a compact glasses-free AR 3D display. Potential applications include window display, exhibition, education, teleconference.

A Multi-Plane Augmented Reality Head-Up Display System Based on Volume Holographic Optical Elements With Large Area

The traditional head-up display (HUD) system has the disadvantages of a small area and a single display plane, here we propose and design an augmented reality (AR) HUD system with multi-plane, large area, high diffraction efficiency and a single picture generation unit (PGU) based on holographic optical elements (HOEs). Since volume HOEs have excellent angle selectivity and wavelength selectivity, HOEs of different wavelengths can be designed to display images in different planes. Experimental and simulated results verify the feasibility of this method. Experimental results show that the diffraction efficiencies of the red, green and blue HOEs are 75.2%, 73.1% and 67.5%. And the size of HOEs is 20 cm × 15 cm. Moreover, the three HOEs of red, green and blue display images at different depths of 150 cm, 500 cm and 1000 cm, respectively. In addition, the field of view (FOV) and eye-box (EB) of the system are 12° × 10° and 9.5 cm × 11.2 cm. Furthermore, the light transmittance of the system has reached 60%. It is believed that this technique can be applied to the augmented reality navigation display of vehicles and aviation.

Hologram aberrations resulting from the violation of Bragg’s law and variations in the depth and shape of the profile of their surface gratings

Hologram aberrations resulting from the violation of Bragg’s law and variations in the depth and shape of the profile of their surface gratings are investigated. This study is conducted for specific cases of reflecting relief-phase holograms and lossless transmitting volume holograms. The commonly used and previously published assumption that the imaging properties of the hologram, including its aberrations, are fully determined by the pattern of the strata exposure on the hologram surface was shown to be generally true for reflecting relief-phase holograms and only partially true for transmitting volume holograms. For the latter, the deviations from Bragg’s law both in incidence angle and wavelength of the reconstructing beam can lead to significant aberrations in the ray beams transformed by the holographic optical elements. This study unambiguously demonstrates that the modules for calculating the aberrations caused by the deviations from Bragg’s law should be added to the software used to design optical systems with volume holographic optical elements.

Design of a waveguide eye-tracking system operating in near-infrared with holographic optical elements

Recent research in augmented reality (AR) eyewear has prompted interest in using volume holographic optical elements for this application. However, many sensing operations in AR systems require the use of wavelengths in the near-infrared (NIR) (750 to 900 nm). These wavelengths typically exceed the sensitivity range of available commercial holographic recording materials (450 to 650 nm), which complicates the design of optical elements with power since significant aberrations result when the reconstruction wavelength differs from the construction wavelength. Several methodologies for designing a waveguide hologram imaging system in NIR are reviewed and evaluated. The design approach presented in our work integrates the most effective practices such as fabrication point source location optimization and aberration analysis to realize effective holographic waveguide couplers formed with visible wavelength light and reconstructed in the NIR. The technique is demonstrated by designing and fabricating an input waveguide hologram in conjunction with a multiplexed output coupling hologram. The resulting input/output waveguide holograms can achieve an image resolution of (∼3 lp / mm) with a 0.6-mm-thick glass substrate that has a refractive index of 1.8.

Design of a high-resolution holographic waveguide eye-tracking system operating in near-infrared with conventional optical elements.

The eye-tracking system plays an essential role in the augmented reality (AR) eyewear. Waveguide volume holographic optical elements (HOE) that can be made with high efficiency, thin form, and lightweight are well-suited for this application. Traditional holographic lenses formed with spherical wavefronts at visible wavelengths and used for near-infrared (NIR) eye-tracking systems suffer from significant image aberrations, image tilt, and ghost images. This work describes a complete holographic lens design methodology that systematically addresses these issues and details the specifications of conventional optical elements that provide the optimized wavefronts for the hologram construction beams. The resulting waveguide HOE NIR eye-tracking system has an image resolution of ∼10 lp/mm when the designed holographic lens is deposited on a waveguide with a refractive index of 1.8 and thickness of 0.5 mm.

Development and Testing of a Dual-Wavelength Sensitive Photopolymer Layer for Applications in Stacking of HOE Lenses

Diffractive optical elements (DOEs) have been in development for many years and are an exciting technology with the capability to re-direct light, using diffraction rather than refraction. Holographic Optical Elements (HOEs) are a subset of diffractive optical elements for which the photonic structure is created holographically, i.e., by recording a specific interference pattern in a suitable, photosensitive optical material. Volume HOEs are of particular interest for some applications because of their very high diffraction efficiency and single diffracted order; however, high dispersion and angular wavelength selectivity still present significant challenges. This paper explores a method for producing a compound DOE useful for situations where elements designed for two separate target wavelengths can be advantageously combined to achieve a highly efficient HOE with reduced dispersion. A photopolymer material consisting of two independently sensitized laminated layers is prepared and used in sequential holographic recording at two different wavelengths. The photonic structures recorded are investigated through examination of their diffraction peaks and comparison with the structure predicted by modeling. Finally, the device is illuminated with an expanded diverging beam at both target wavelengths and with white light, and a strong diffracted beam is observed.

Unmixing of the background components in an off-axis holographic-mirror-based imaging system using spectral image processing.

An imaging system employing a volume holographic optical element (vHOE) has a unique feature by which the image of an object placed in front of a transparent screen can be captured. The system obtains an image of the light diffracted by the vHOE; however, unwanted background components, including direct reflection or transmitted components, are present in the captured image. In this study, we propose a method to eliminate such background components from the captured images via multispectral image processing. The image components obtained from the diffraction by vHOE can be successfully extracted utilizing the facts that it diffracts light with a narrow spectral range and the reflection spectrum of a natural object is mostly smooth in the wavelength axis. Computer simulations and experiments confirm the effectiveness of the proposed method, along with video applications that demonstrate its real-time capability.

Volume holographic optical element for light sheet fluorescence microscopy.

Three-dimensional (3D) imaging of living organisms requires fine optical sectioning and high-speed image acquisition, which can be achieved by light sheet fluorescence microscopy (LSFM). However, orthogonal illumination and detection arms in the LSFM system make it bulky. Here, we propose and demonstrate the application of a volume holographic optical element (photopolymer-based volume holographic grating) for designing a compact LSFM system, called a volume holographic LSFM (VHLSFM). Using the VHLSFM, we performed in vivo imaging of Caenorhabditis elegans (C. elegans) and observed high-contrast optically sectioned fluorescence images of the oocytes and embryonic development in real time for 3D imaging.