Design of an orbital angular momentum demultiplexing system based on off-axis cascaded metasurfaces

Optical orbital angular momentum (OAM) has garnered widespread attention in the fields of high-speed optical communication and quantum communication due to its characteristic of spatial orthogonality. The beam carrying OAM has a spiral phase structure exp(ilθ) in the spatial phase distribution, where the topological charge l can take any integer, which can theoretically provide an infinite number of channels, thus greatly enhancing the capacity of the communication system. As an array of miniature planar structures composed of sub-wavelength structures, the metasurface, with its unique planar structure and precise control of light waves, shows great application potential in promoting miniaturization and integration of space OAM communication systems. When designing metasurface systems, factors such as electromagnetic coupling between metasurface unit structures and insufficient processing precision can lead to the actual efficiency of the metasurface are much different from the theoretical design efficiency. This can often result in stray light due to insufficient modulation efficiency of the metasurface, and the influence of these stray light as background noise in the cascade metasurfaces system will be magnified step by step, affecting the system function. In order to improve the modulation efficiency of metasurface, researchers often use genetic algorithms, topology optimization, and machine learning to design more efficient metasurface structures or use higher precision machining processes to fabricate metasurface. However, these methods are often accompanied by complex computing processes and high manufacturing price costs, which largely limit the widespread use of metasurface in practical optical systems. To reduce the impact of metasurface with limited efficiency on system performance and to decrease the demand for fabricating high-efficiency metasurface structures in high signal-to-noise ratio metasurface systems, based on the coordinate transformation method proposed by Berkhout et al, this paper proposes a design method for an orbital angular momentum demultiplexing system based on off-axis cascaded metasurfaces. By introducing an off-axis design, this work effectively separates the stray light produced in cascaded metasurface systems due to reduced metasurface efficiency. Compared to traditional methods of improving system performance by optimizing the modulation efficiency of metasurface structure, the approach presented in this paper avoids complex structural optimization and the fabrication process of high-efficiency metasurfaces, while also significantly improving optical performance. This design method can be extended to the cascade of multi-level metasurfaces to solve the problem of limiting the number of layers of cascaded super-surfaces, which is of potential application value and significance for the miniaturization and integration of optical systems.