Computational holographic CGH is introduced in detail

First, the basic principle of computational holography

The basic principle of computational holography is to use a computer to solve the phase or amplitude of Light, generate a digital hologram, and then modulate the phase or amplitude of light through optical modulators such as Spatial Light Modulator (SLM), and finally use coherent light to irradiate SLM. A refreshing light field is generated to form a dynamic holographic 3D image.

Different from traditional hologram generation, computational holography does not require two beams of light to be physically coherent, thus simplifying the hologram generation process. However, the high-precision generation of computational holograms still faces many challenges, such as the large amount of computation, high computing power requirements, and the resolution and size limitations of spatial light modulators.

Second, the key technology of computational holography

Hologram optimization algorithm

The high-precision generation of computational holograms depends on optimization algorithms. Since hologram optimization is essentially a ill-conditioned inverse problem, it is usually solved with the help of non-convex optimization algorithms. The selection and parameter setting of optimization algorithm will directly affect the quality and computational efficiency of hologram generation.

Common optimization frameworks include alternate projection method and gradient descent method. Alternate projection method finds the optimal solution satisfying the constraints of two closed sets by alternate projection between two closed sets. The gradient descent method determines the direction of loss function decline through gradient calculation, so as to find the optimal solution satisfying the constraint conditions.

Spatial light modulator

Spatial light modulator is a key device in computational holography, which can convert digitized holograms into light field modulation. Currently, most computational holographic systems rely on projection devices such as SLM or Digital Micromirror Device (DMD). However, these devices have inherent limitations in display performance, such as too small field of view Angle and multi-order diffraction.

To address these issues, researchers are exploring metasurface based holography. Metasurface can introduce mutations in the basic properties of electromagnetic wave, such as amplitude and phase, and achieve many modulation effects that are difficult to achieve in traditional modulation devices. Metasurface based holography has made great progress in large field of view, color-free imaging, color display, information capacity expansion, multi-dimensional multiplexing and so on.

Dynamic holographic display

Dynamic holographic display is an important application field of computational holography. The traditional holographic display system often has the problems of large computation and low display frame rate, which limits its application in advanced display such as advanced human-computer interaction. In order to realize dynamic holographic display with high fluency, researchers are exploring efficient computational hologram generation methods and display techniques.

For example, a team from the Wuhan National Research Center for Optoelectronics at Huazhong University of Science and Technology has proposed a dynamic interbit metasurface holography (Bit-MH) technology with high computational and display frame rates. The technique achieves efficient dynamic refresh and real-time interaction by dividing the display function of the metasurface into different spatial regions (i.e. spatial channels), and projecting a reconstructed sub-holographic pattern into each channel.

3. Application fields of computational holography

Three-dimensional display

Computational holography has a wide application prospect in the field of 3D display. With computer-generated holograms, high-precision wavefront modulation can be achieved to generate three-dimensional scenes with a continuous sense of depth. This technology can not only be used in the field of entertainment and games, but also in education, training, medical and other fields to provide a more realistic and intuitive three-dimensional visual experience.

Optical information storage and processing

Computational holography can also be used for optical information storage and processing. By generating digital holograms, information can be stored in the medium in the form of light field to achieve high density and high speed information storage and reading. In addition, computational holography can also be used in fields such as optical encryption and anti-counterfeiting to improve the security and reliability of information.

Augmented reality and virtual reality

Computational holography also has potential applications in the field of augmented reality (AR) and virtual reality (VR). By generating realistic three-dimensional holographic images, natural interaction and immersive experiences in AR and VR systems can be achieved. For example, in AR systems, computational holography technology allows users to naturally focus on the displayed content across multiple depths of the plane, solving the visual convergence conflict adjustment problem (VAC), and improving user comfort.

Laser machining and metasurface design

Computational holography can also be used in fields such as laser processing and metasurface design. By generating high-precision holograms, precise control of the laser beam can be achieved, and high-precision laser processing and micro-nano manufacturing can be achieved. In addition, computational holography can also be used for the design and optimization of metasurfaces to achieve more complex and efficient electromagnetic wave modulation effects.

Fourth, the development trend and challenge of computational holography

With the continuous development of computer technology and the continuous innovation of optical devices, computational holography technology is constantly making new progress and breakthroughs. However, computational holography still faces many challenges and problems, such as large computation amount, high computing power requirement, resolution and size limitation of spatial light modulator. To solve these problems, researchers are exploring new algorithms and techniques, such as deep learning-based hologram generation methods, metasurface based holography, etc.

In the future, computational holography technology is expected to be applied and popularized in more fields. For example, in the vehicle HUD display system, computational holographic technology can realize more realistic and intuitive 3D navigation and information display; In the medical field, computational holographic technology can be used in fields such as surgical navigation and telemedicine to improve medical level and efficiency.

In short, computational holography, as a technology with transformative potential, is constantly promoting the development of optics and information science. With the continuous progress of technology and the continuous expansion of application fields, computational holography is expected to achieve breakthroughs and innovations in more fields, bringing more convenience and surprises to mankind.