# Multiplexing Perfect Optical Vortex for Holographic Data Storage
## 1 Introduction
By assigning different topological charges to individual data pages, multiple pages can be stored within a single hologram, effectively increasing the storage density [13; 14]. Furthermore, the orthogonality of OAM modes allows for selectively addressing and retrieving specific data pages [15; 16]. By using reading beams with matching OAM modes, the desired data page can be reconstructed, while other pages remain undisturbed. This selective nature of optical vortices enhances the security and confidentiality of holographic data storage. In addition to increased storage capacity and improved security, optical vortices also offer advantages in terms of data transfer rates and system simplicity. The use of optical vortices enables parallel readout and parallel processing, allowing for faster data access compared to traditional serial readout methods. Moreover, the compact nature of optical-vortex-based holographic systems simplifies the optical setup and reduces the complexity of the storage architecture. When the waist of the fundamental mode beam is fixed, the beam size (cross-sectional spot diameter) of the OV is dependent on the topological charge (\(\ell\)) [17], which, naturally, affects the capacity of the holographic data storage. A larger absolute value of the topological charge corresponds to a larger spot diameter and a larger hollow region [18]. Hence, the utilization of OV holography for high multiplexing density encounters a fundamental sampling criterion that poses a challenge [19; 20]. In order to mitigate strong multiplexing crosstalk, it is essential to adhere to the requirement that the sampling distance should not be less than the diameter of the largest addressable OV mode. Unfortunately, this criterion presents a significant hurdle in enhancing the resolution and capacity of OV holography. The potential of perfect optical vortex (POV) to address this challenge has received comparatively less attention. Unlike conventional optical vortices, the cross-sectional spot diameter of a perfect optical vortex (POV) remains constant regardless of its topological charge [21; 22]. In traditional optical vortices, the spot diameter increases as the topological charge increases, leading to larger beam sizes and reduced spatial resolution. However, in the case of POV beams, the spot diameter remains unchanged regardless of the value of the topological charge. This unique property of POV beams is advantageous for applications where a consistent beam size is required in holographic data storage. In this paper, we propose a multiplexing perfect optical vortex (POV) for holographic data storage. In this method, the robust selectivity of POV holography, characterized by the topological charge (\(\ell\)), is considered as a distinctive label for each data page. In this approach, not only the spatial positions of the points within the data page but also their corresponding intensity levels serve as information carriers for data storage. Subsequently, the data page is reconstructed by illuminating the POV-recording hologram with a specialized POV beam possessing the opposite topological charge (\(-\ell\)), commonly referred to as the reading beam. By employing this reading beam, the data encoded within the hologram can be retrieved. The information can be read out by implementing distinct different intensity thresholds. To further enhance the storage capacity, a multiplexing technique is employed, whereby various POV-recording holograms are combined. This process involves coding different data pages into a POV-multiplexing hologram utilizing different topological charges. Moreover, the shorter sampling distance in the single data page is used to improve the storage density. The higher density and the multiplexing data pages are demonstrated in a proof-of-principle experiment to show the feasibility of the proposed method. Compared to our previous approach [12], the proposed new method offers two main advantages. Firstly, the storage density is improved.