Holography involves recording the three-dimensional (3D) information of an object through the interference and diffraction of light on a holographic material, followed by reconstructing the 3D visual. This article discusses holographic optical elements, their applications, and recent studies.
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What are Holographic Optical Elements?
Holography emerged in 1947 when Dennis Gabor, while working to improve the electron microscope, conceptualized a two-step process: recording an interference pattern on a photographic plate and reconstructing the wavefront with visible light. In 1948, Gabor created the first hologram using a mercury arc lamp with a green filter.
Holographic optical elements (HOEs) are optical devices that utilize holography principles to manipulate light waves for specific applications. A light beam splits into a reference beam and an object beam; the object beam interacts with the object being recorded, creating interference patterns on a holographic recording medium that contains information about shape and features. When illuminated by the reference beam, the recorded hologram reconstructs the original object wavefront.
Applications of Holographic Optical Elements
A valuable application of HOEs in imaging is augmented reality (AR), which provides immersive and realistic experiences by recreating 3D images and enhancing depth perception and overall visual fidelity. AR introduces a new approach in industries such as gaming, education, and even medicine, where surgeons can utilize holographic overlays during procedures.
Similarly, by projecting vital information, such as speed, navigation, and warnings, directly onto the driver's line of sight, HOEs contribute to safer and more convenient driving experiences. The holographic nature of the displays ensures minimal distraction and allows drivers to focus on the road while accessing crucial information in real-time.
Holographic optical elements play a significant role in optical processing by offering various functionalities, making them valuable tools for manipulating light. For instance, they can precisely split and combine light beams, allowing intricate control over their paths and facilitating useful applications (such as intricate communication networks and sophisticated laser systems).
HOEs form the foundation for spatial light modulators, which enable crucial functions like beam steering, optical switching, and vital communication system development.
Recent Innovations
Applications of HOE-based AR HUD in Navigation
In a 2021 study, researchers proposed and designed an AR head-up display (HUD) system that overcomes the limitations of traditional HUDs by utilizing holographic optical elements. This system features multi-plane, large area, high diffraction efficiency, and employs a single picture generation unit (PGU).
The researchers used volume HOEs with wavelength selectivity, allowing the display of images at different depths. Experimental results demonstrated diffraction efficiencies of 75.2 %, 73.1 %, and 67.5 % for red, green, and blue HOEs, respectively, with a system field of view (FOV) and eye-box (EB) of 12 ° × 10 ° and 9.5 cm × 11.2 cm, respectively. The system's light transmittance reached 60 %, showcasing its potential application in AR navigation displays for vehicles and aviation.
Enhancing HOEs with Water-Resistant Sol-Gel
In a 2022 study, researchers have enhanced holographic optical elements using a fast-curing, water-resistant photosensitive sol–gel. This material is known for its environmental robustness, particularly against water and scratches. Despite this, sol-gel faced challenges with limited refractive index modulation. The study addressed this limitation to optimize the diffraction efficiency of holograms recorded in thin layers.
The researchers achieved a more than two-fold improvement in refractive index modulation, from 1.4 × 10-3 to 3.3 × 10-3, by exploring recording properties at different wavelengths and employing chemical alterations and thermal post-processing techniques. These advancements are crucial for applications like optical displays, illumination management, and solar light harvesting, where efficient, lightweight, diffractive optical elements are increasingly replacing traditional optics.
Optimizing Imaging with Grating Vector Fields in HOEs
In a 2020 study, researchers have advanced the design and fabrication of holographic optical elements, focusing on enhancing imaging performance and enabling new applications. The study introduces a pipeline for designing, optimizing, and fabricating complex, customized HOEs emphasizing grating vector fields.
The study explores two distinct fabrication methods: one utilizing freeform refractive elements for high optical quality and precision and the other employing a holographic printer with two wavefront-modulating arms for rapid prototyping.
The study showcases the versatility of these methods by fabricating specialized HOEs, including an aspheric lens, a head-up display lens, a lens array, and, notably, a full-color caustic projection element. This development marks a significant stride in expanding the capabilities of HOEs, particularly in applications like virtual and AR displays.
Enhancing Eye-tracking with NIR HOEs
In another study, researchers explored the application of volume holographic optical elements in AR eyewear, focusing on near-infrared (NIR) sensing operations.
The research integrated point source optimization and aberration analysis to design effective holographic waveguide couplers that function with visible wavelength light and are reconstructed in the NIR spectrum. This approach facilitated the realization of substrate-mode volume holographic lenses suitable for eye motion-sensing applications.
The study also investigated the design of output HOEs for waveguide coupling, aiming to increase the angular bandwidth for eye-tracking applications. The findings demonstrate the feasibility of the designed waveguide components, achieving an image resolution of approximately 3 line pairs per millimeter (lp/mm) with a glass substrate of 0.6 mm thickness.
Conclusion
Holographic optical elements are reshaping imaging and optical processing. Recent studies—including optimizing imaging with grating vector fields, designing AR HUD systems, and introducing innovative concepts like composite HOEs—showcase advancements in this technology. These advancements not only propel the field of optoelectronics forward but also promise transformative applications in areas such as AR navigation displays, spectrograph designs, and lightweight diffractive optical elements, marking a profound impact on various industries.
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References and Further Reading
Beléndez, A. (2015). Dennis Gabor, "Father of Holography." BBVA. Available at: https://www.bbvaopenmind.com/en/science/leading-figures/dennis-gabor-father-of-holography/ (Accessed March 1, 2024).
Jang, C., Mercier, O., Bang, K., Li, G., Zhao, Y., Lanman, D. (2020). Design and fabrication of freeform holographic optical elements. ACM Transactions on Graphics (TOG). doi.org/10.1145/3414685.3417762
Kim, N., Piao, Y. L., Wu, H. Y. (2017). Holographic optical elements and application. Holographic Materials and Optical Systems. doi.org/10.5772/67297
Lee, B., Yoo, C., Jeong, J. (2020). Holographic optical elements for augmented reality systems. Holography, Diffractive Optics, and Applications X. doi.org/10.1117/12.2573605
Lv, Z., Liu, J., Xu, L. (2021). A multi-plane augmented reality head-up display system based on volume holographic optical elements with large area. IEEE Photonics Journal. doi.org/10.1109/JPHOT.2021.3105670
Zhao, J., Chrysler, BD., Kostuk, RK. (2021). Design of a waveguide eye-tracking system operating in near-infrared with holographic optical elements. Optical Engineering. doi.org/10.1117/1.OE.60.8.085101.
Rogers, B., Mikulchyk, T., Oubaha, M., Cody, D., Martin, S., Naydenova, I. (2022). Improving the holographic recording characteristics of a water-resistant photosensitive sol–gel for use in volume holographic optical elements. Photonics. doi.org/10.3390/photonics9090636
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