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Design of an orbital angular momentum demultiplexing system based on off-axis cascaded metasurfaces
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摘要:
设计超表面系统时，实际制作的超表面效率与理论设计效率相差较大，常会存在超表面调制效率不足产生的杂散光，这些杂散光作为背景噪声在级联超表面系统中的影响会逐级放大，影响系统功能。为了降低有限效率的超表面对系统性能的影响，本文提出了一种基于离轴级联超表面的轨道角动量解复用系统设计方法。这一工作通过加入离轴相位设计，有效消除了级联超表面系统中因超表面效率降低所产生的杂散光。利用FDTD (时域有限差分)仿真软件进行计算和验证，结果表明，离轴级联超表面系统能有效减少因调制效率不足而产生的杂散光，与同轴系统相比，实现了最大串扰降低4.15 dB以及平均80%的杂散光消除，展现出较大的性能优势。
Abstract:When designing metasurface systems, the actual efficiency of the metasurface is much different from the theoretical design efficiency. This can lead to stray light caused by insufficient modulation efficiency of the metasurfaces, which acts as background noise and is magnified in cascaded metasurface systems step by step, affecting system functionality. To reduce the impact of metasurfaces with limited efficiency on system performance, this paper proposes a design method for an orbital angular momentum demultiplexing system based on off-axis cascaded metasurfaces. By incorporating an off-axis phase design, the stray light generated by the reduced efficiency of the metasurface in a cascaded metasurface system is effectively eliminated. Using FDTD (finite difference time domain) simulation software for calculation and validation, the results demonstrate that the off-axis cascaded metasurface system can effectively reduce stray light caused by insufficient modulation efficiency. Compared to the on-axis system, it achieves a maximum reduction in crosstalk of 4.15 dB and an average of 80% stray light elimination, showing a significant performance advantage.
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Key words:
- metasurface /
- off-axis /
- cascade /
- orbital angular momentum
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Overview: 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.
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图 1 (a)同轴与(b)离轴级联超表面轨道角动量解复用系统示意图
Figure 1. Schematic of (a) on-axis and (b) off-axis cascade metasurface OAM demultiplexing system
图 2 (a)单元结构偏振转化率随波长变化曲线；(b) 单元结构偏振转化率随离轴角度变化曲线
Figure 2. (a) Polarization conversion efficiency of a unitary structure as a function of wavelength; (b) Polarization conversion efficiency of a unitary structure as a function of off-angle
图 3 (a)同轴级联超表面轨道角动量解复用系统中信号光与杂散光示意图；(b)解复用系统信噪比随超表面偏振转化率变化曲线
Figure 3. (a) Schematic of signal/noise in OAM demultiplexing system based on on-axis cascaded metasurface; (b) SNR of demultiplexing system as a function of polarization conversion efficiency
图 4 (a)离轴级联超表面轨道角动量解复用系统中信号光与杂散光示意图；(b)解复用系统信噪比随超表面偏振转换率以及离轴角度的变化曲线图
Figure 4. (a) Schematic of signal/noise in OAM demultiplexing system based on off-axis cascaded metasurface; (b) SNR of demultiplexing system as a function of polarization conversion efficiency and off-angle
图 5 (a)同轴与(b)离轴系统中不同OAM状态(l = ±1, ±3)对应的的出射光强分布图；(c) 同轴与离轴系统中不同OAM状态(l = ±1, ±3)出射场范围内归一化杂散光强度
Figure 5. (a) On-axis and (b) off-axis , intensity profiles corresponding to different OAM states (l = ±1, ±3); (c) Normalized noise intensities in the range of outgoing field for different OAM states (l = ±1, ±3) in on-axis and off-axis system
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