中国激光, 2023, 50 (18): 1813013, 网络出版: 2023-08-28  

基于超表面的阵列光场纵向维度信息编解码 下载: 699次亮点文章特邀研究论文

Longitudinally Encoding and Decoding Information in Light Field Arrays Based on Metasurface
作者单位
光场调控和信息感知工业和信息化部重点实验室,陕西省光信息技术重点实验室,西北工业大学物理科学与技术学院,陕西 西安 710129
摘要
随着大数据时代的到来,空间光通信已被广泛应用于各种通信系统中,但随之而来的是容量瓶颈的挑战。基于光场频率、时间、偏振、横向空间模式等维度调控的信息编解码方法在解决容量问题方面展现了优异性能,但光场的纵向维度却未被应用于信息编解码。针对此问题,本文提出了一种基于电介质超表面的光场纵向维度信息编解码新方法,基于四原子结构的几何相位和传输相位联合调控,实现了透射场自旋相关的复振幅调控。同时,利用光学冻结波原理产生了轨道角动量模式叠加态的纵向可控变化,并验证了光场模态的纵向调控能够以指数级提升信道中的模态容量。纵向维度作为一个全新的编码自由度,有望进一步提高自由空间光通信性能。
Abstract
Objective

Free-space optical communication, which uses a laser as an information carrier, is a wireless communication technology that is widely used in various communication systems. Owing to sufficient parametric dimensions, a series of new technologies have been developed to enhance optical communication capacity based on light field modulation. However, in recent years, with the exhaustion of these traditional dimensional resources and the advent of the big data era, optical communication is once again facing the challenge of a capacity crisis. In response, spatial structures (modes) of light fields are gradually being developed to address the growing capacity bottlenecks. For example, vortex beams carrying orbital angular momentum (OAM) have become an important breakthrough for sustainable capacity expansion of optical communications because their spatial modes are a new degree of freedom in constructing a high-dimensional Hilbert space. Consequently, various methods have emerged to encode information using the azimuthal order of OAM modes. Besides the azimuthal order multiplexing of OAM modes, the radial index and the vector modes are also used to achieve further information capacity enhancement. Although the spatial modes obtained based on the transverse modulation of light fields have fully demonstrated their feasibility in classical and quantum communications, another important spatial dimension of the light field, that is, the longitudinal dimension, has not been exploited for information encoding and decoding. Herein, we propose a dielectric metasurface for longitudinally modulating the superposition state of OAM modes and demonstrate the capability of these modulated light fields in information-encoding scenarios. The metasurface consists of tetratomic macropixels, which enable spin-dependent complex amplitude modulation of the transmitted field because of polarization-dependent interference and can produce a 5×5 light beam array with each beam channel presenting longitudinally variant superposition states of 0 to 15 orders. By introducing longitudinal modulation, the mode capacity of individual channels can be increased exponentially, depending on the number of longitudinal segments in the superposition state of the OAM modes.

Methods

The longitudinal encoding and decoding principles are shown in Fig. 1(a). The information from Bob at the transmitter is compiled into multiple superposition states composed of two OAM modes with topological charges of l1 and l2 according to the American Standard Code for Information Interchange (ASCII). As the superposition state presents the intensity distribution as the shape of |l1-l2| petals, it is characterized by the azimuthal order |l1-l2|. These superposition states are then loaded into light beams with longitudinal on-demand transformations for spatial transmission. Alice at the receiver side can obtain the correct information by measuring the superposition modes in different transmission planes, such as z1, z2, and z3, and by decoding the correct sequence of operations. Figure 1(b) shows the codec table corresponding to the ASCII codes and the superposition states with azimuthal orders varying from 0 to 15. Moreover, a spatial array structure can be introduced to increase the parallel transmission rate of information.

An optical frozen wave was used to realize the longitudinal modulation of the OAM modes, and a tetratomic macro-pixel dielectric metasurface was designed to experimentally generate such a light field array with encoded information. The tetratomic macro-pixel is shown in Fig. 2(a), which considers the intrinsic relationship and interaction between the optical responses of rectangular meta-atoms and can achieve complex amplitude modulation because of polarization-dependent interference. According to this principle, the transmission phases φ0A and φ0B and rotation angles θA and θB of the meta-atoms A and B in a tetratomic pixel can be obtained by combining equations (3) and (8). The transmission fields of rectangular nanopillars with different lengths and widths were scanned under the conditions of height H0=590 nm and period P0=400 nm to find 17 different geometries, of which the transmitted phases φ0 linearly increased as shown in Fig. 2(b).

Results and Discussions

As an example, the message Northwestern Polytechnical University is used to demonstrate the longitudinally encoding capability of this specially structured light field. According to the ASCII hexadecimal code elements, the letters and spaces in the message are transformed into 74 superposition states, which are then divided into a 5×5 beam array. The longitudinal variation of each beam channel contains three segments, which are located at propagation distances of z=0-0.4 mm, 0.4-0.8 mm, and 0.8-1.2 mm, respectively. Because the superposition state in each beam channel can change three times along the longitudinal dimension, the total mode capacity of the beam array is expanded to 163. The simulated and measured intensity distributions of beam arrays in z1=0.1 mm, z2=0.5 mm, and z3=0.9 mm planes are shown in Fig. 3. The experimental measurements are consistent with that of the simulations. Therefore, the correct message can be obtained by decoding two hexadecimal digits in a Z-shaped sequence starting from the first line in z1. It should be noted that this method requires the measurement of the light field modes in three different planes for decoding. In this study, a longitudinal scanning method is used. However, to improve the decoding efficiency, a split-plane imaging approach can also be used to obtain the light-field distribution in three longitudinal planes simultaneously by splitting the light twice and then imaging different longitudinal planes. In addition, according to the propagation characteristics of light waves, if the complex amplitude information of the light field is measured in a single plane, the complex amplitude distributions of other planes can be obtained by numerical calculations, and the light field patterns of multiple longitudinal planes can then be obtained.

Conclusions

In this study, a dielectric metasurface that can independently control the amplitude and phase of two light fields is proposed. Flexible modulation of the OAM modes of the beam array in the longitudinal dimension is achieved using the spectral modulation principle of frozen waves. Using these longitudinally modulated light fields, exponential expansion of the mode capacity is experimentally realized to enhance the mode capacity of free-space optical communication. The longitudinal dimensional coding and decoding functionality verified in this study is expected to be a breakthrough in the capacity improvement of free-space optical communication.

樊鑫豪, 武炫光, 周亮, 刘圣, 赵建林, 李鹏. 基于超表面的阵列光场纵向维度信息编解码[J]. 中国激光, 2023, 50(18): 1813013. Xinhao Fan, Xuanguang Wu, Liang Zhou, Sheng Liu, Jianlin Zhao, Peng Li. Longitudinally Encoding and Decoding Information in Light Field Arrays Based on Metasurface[J]. Chinese Journal of Lasers, 2023, 50(18): 1813013.

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