Bandwidth Of Holograms Research Articles

Complex-amplitude metasurface-based orbital angular momentum holography in momentum space.

Digital optical holograms can achieve nanometre-scale resolution as a result of recent advances in metasurface technologies. This has raised hopes for applications in data encryption, data storage, information processing and displays. However, the hologram bandwidth has remained too low for any practical use. To overcome this limitation, information can be stored in the orbital angular momentum of light, as this degree of freedom has an unbounded set of orthogonal helical modes that could function as information channels. Thus far, orbital angular momentum holography has been achieved using phase-only metasurfaces, which, however, are marred by channel crosstalk. As a result, multiplex information from only four channels has been demonstrated. Here, we demonstrate an orbital angular momentum holography technology that is capable of multiplexing up to 200 independent orbital angular momentum channels. This has been achieved by designing a complex-amplitude metasurface in momentum space capable of complete and independent amplitude and phase manipulation. Information was then extracted by Fourier transform using different orbital angular momentum modes of light, allowing lensless reconstruction and holographic videos to be displayed. Our metasurface can be three-dimensionally printed in a polymer matrix on SiO2 for large-area fabrication.

Unitary Transforms Using Time-Frequency Warping for Digital Holograms of Deep Scenes

With the advent of ultrahigh-resolution holographic displays, viewing macroscopic deep scenes with large viewing angles becomes a possibility. These deep holograms possess different signal properties in contrast with common applications where the scene content is assumed to lie around a planar slice. Therefore, the conventional approach of refocusing at a fixed depth is ineffective. There is a need for an efficient invertible transformation that is able to account for the wide depth range of macroscopic three-dimensional scenes. To this end, we derive necessary invertibility conditions for the diffraction from nonplanar surfaces for symmetric light propagation kernels, such as Fresnel diffraction. We construct a unitary transform for modeling deep holographic scenes using a generalization of linear canonical transforms. From the symplectic properties of the time–frequency domain, we obtain invertibility conditions of the transforms depending on surface shape, hologram bandwidth, and wavelength. These transforms can be subsequently combined with other sparsifying transforms for compression. Experiments demonstrate one application in lossy coding of holograms by implementing a computationally efficient subset of the transforms for piecewise depth profiles that is combined with the JPEG 2000 codec. Results show improved reconstruction quality. A significant visual gain is observed as the depth information is well preserved under identical encoding rates in contrast to using Fresnel propagation at a fixed depth. This paper shows that it is possible to effectively represent holograms of variable-depth scenes and our local adaptive transform leads to a practical holographic compression framework.

Hologram Bandwidth Reduction by Space–Time Multiplexing*

This paper describes a method for reducing the spatial bandwidth of holograms by use of a moving dispersion structure between the object and the hologram. The theory developed in the paper includes the Haines–Brumm stationary dispersion structure as a special case. The added flexibility permits the use of simple dispersion structures that can be easily reproduced and scaled, and relatively complicated motions to produce virtual images without auxiliary optics.