A new publication from Opto-Electronic Science; DOI 10.29026/oes.2022.220004 considers high frame-rate orbital-angular-momentum multiplexing holography.
The development of mobile Internet, especially 5G technology, can provide users with more excellent experience. At the same time, larger and faster data flow also leads to higher requirements for the information processing capacity of the Internet. Due to the saturation of frequency resources, scientists recently focus on the orbital angular momentum (OAM) degree of freedom of light (electromagnetic wave). Because it is independent of other degrees of freedom, such as wavelength, polarization, frequency, etc., forming a new dimension for information transmission. The light propagating forward in a spiral attitude carries orbital angular momentum, and its spiral degree determines the value of the orbital angular momentum. Theoretically, the amount of the helix degree (OAM value) of photons is infinite. Each helix degree can be used to encode information, which boosts infinite data capacity. Introducing the OAM of light as the information carrier in holography has formed OAM holography. Different from the previous holography in which one hologram records only one picture, the OAM holography can encode and record many different pictures with different OAM values. Consequently, tens or hundreds of pictures can be recorded in a single OAM-multiplexing hologram.
Based on larger information capacity of the OAM holography, dynamic holographic display and holographic video can be realized by quick readout of multiple recorded pictures in the reconstruction process. In the process of the holographic display (decoding), in order to retrieve multiple pictures in sequence without crosstalk, it is necessary to provide a password to the illumination beam corresponding to each encoded OAM value. Only the passwords match the encoded OAM value can the picture be decoded(reconstructed), just like the dynamic password for withdrawing money from a bank. However, the dynamic password (or decoding key) in the traditional holographic decoding process is based on phase modulation, which leads to a long switching time (generally no more 30Hz). The switching rate is physically determined by the switching speed of the readout beam state. Currently, it is limited by the refresh rate of the phase-modulated spatial light modulator, which limits the future development of ultra-high-speed OAM-addressable dynamic holography.
Recently, the authors of this article proposed a new decoding method based on cross convolution for OAM holography, which greatly break through the upper limit of the information extraction rate of OAM holography. Starting from the spatial frequency domain, they designed an amplitude-modulated pattern called Amplitude Decoding Key (ADK) as a decoder (or password mentioned above) for information extraction in OAM holography. The convolution between a series of spatial frequency components of the amplitude decoding key and the OAM-multiplexing hologram leads to a cross-convolution result. A new cross-convolution theorem was developed for the success of information extraction from OAM-multiplexing holograms. The corresponding OAM-encoded holograms can be decoded independently or parallelly by a spectral component or multiple spectral components of the ADK when satisfying the cross-convolution theorem. The amplitude-modulation-based decoding method can be implemented by using a digital micromirror device (DMD) and can be operated at a frame rate of several kilohertz, thus a two-orders of magnitude increase of the information extraction rate in OAM holography can be realized in principle.
Experimentally, they demonstrated high-speed information transition based on high frame-rate OAM holography at a speed of 100 Hz. As shown in Figure 1, the OAM-multiplexing hologram in the Fourier space (FTH) is encoded with ten Arabic numerals ranging from 0 to 9 using different OAM charges (information channels) ranging from -50 to 50 with an OAM interval of 10, respectively. It is loaded on a spatial light modulator. Amplitude decoding keys are loaded on the digital micro-mirror device (DMD) to retrieve the information in these information channels. The lens near the DMD realizes a Fourier transform for the product of the hologram and the ADK, leading to a cross convolution between the spatial frequency of the OAM-multiplexing hologram and the spatial frequencies of the ADK and reproduce the image. By rapidly switching patterns (ADKs) according to the time sequence listed on the lower right inset (color represents the corresponding information channel similarly), the imaging plane will dynamically display a sequence of Arabic numerals. Thus, an information extraction frame rate of 100 Hz is demonstrated for the transmission of the first 100 significant digits of the value . They also demonstrated the holographic display of a three-dimensional image based on this method. In three-dimensional OAM holography, only when the cross convolution theorem is satisfied, the images at different positions (z coordinate) of the three-dimensional object will be reconstructed one by one according to the corresponding positions, so as to finally reconstruct the shape of the three-dimensional object.
In addition to the high information extraction speed, the scheme can further support the information extraction at any desired position corresponding to a certain spatial spectrum component and parallel reproduction of images at different positions by employing multiple spatial spectral components simultaneously. Therefore, optical based multi-channel information transmission can be realized using the scheme. On these bases, the scheme can greatly expand the information processing capability of OAM holography and has attractive application potentials in high-speed holographic display and wireless optical communication in the future.
Article reference Meng WJ, Hua YL, Cheng K, Li BL, Liu TT et al. 100 Hertz frame-rate switching three-dimensional orbital angular momentum multiplexing holography via cross convolution. Opto-Electron Sci 1, 220004 (2022). doi: 10.29026/oes.2022.220004
Keywords: orbital-angular-momentum holography / multiplexing / high frame rate / switching / cross convolution
