携带轨道角动量的太赫兹贝塞尔波束在磁化等离子体中的传输和模式串扰研究

Terahertz wave has great potential in solving the “blackout” problem of reentry communication for hypersonic vehicle. The terahertz Bessel beam with orbital angular momentum (OAM), non-diffraction and self-reconstruction characteristics can further improve transmission speed and communication capacity, but its transmission mechanism in plasma is not yet clear. Based on vector angular spectrum and vector wave function expansions, the amplitude, phase, polarization and orbital angular momentum density, as well as the convergent forms and applicable conditions of beam coefficients of the high-order Bessel beam and its combined beam are investigated. Using vector wave function expansion and Fourier transform, the transmission characteristics of single / multiple terahertz Bessel beams with OAM incident into stratified magnetized plasmas are analyzed, along with the discussion about the power, transverse intensity, phase and polarization of the reflected and transmitted beams. The evolution of OAM state and modal crosstalk are studied by using of the spiral spectrum expansion of the transmitted beam. The influence and physical mechanism of incident beam's topological charge, half-cone angle, direction, frequency, plasma density, temperature, thickness, collision frequency and magnetic field intensity are revealed. The research results will provide a theoretical basis for the application of the terahertz vortices in the reentry communication.

太赫兹波在解决高超声速飞行器再入通信“黑障”问题中有很大潜力，携带轨道角动量(OAM)、具有无衍射和自重建特性的太赫兹贝塞尔波束能够进一步提高通信容量和传输速率，但其在等离子体中传输机理尚不清晰。本项目基于矢量角谱展开和矢量波函数展开，研究不同极化高阶贝塞尔波束及其合成波束的振幅、相位、极化和轨道角动量密度特性，以及波形系数收敛式和适用条件；利用矢量波函数展开和傅里叶变换，解析研究单/多个携带OAM的太赫兹贝塞尔波束入射分层磁化等离子体的传输特性，分析反射和透射波束功率、横向强度、相位和极化变化规律；运用螺旋谱展开研究透射波束的OAM态演化和模式串扰；揭示入射波束拓扑荷数、半锥角、方向和频率以及等离子体的密度、温度、厚度、碰撞频率和磁场强度等因素的影响规律和物理机理。研究成果为再入通信中太赫兹涡旋的应用提供理论依据。

等离子体中电磁波的传播特性长期以来一直是空间通信、目标探测和等离子体诊断等领域的重要研究方向之一。测控、导航和通信中的“黑障”是长期困扰世界各国高超声速飞行器发展的瓶颈之一，解决“黑障”问题的关键点之一是明确等离子体鞘套对电磁波传播的影响机制，研究携带轨道角动量的高阶贝塞尔波束在磁化等离子体中的传输机理有助于进一步探索太赫兹涡旋在解决高超声速飞行器再入通信“黑障”问题中的应用前景。本项目主要研究内容和成果如下：给出了不同极化高阶贝塞尔波束及其合成波束的电磁场的平面波角谱展开和圆柱矢量波函数展开两种表述形式；突破了研究单/多个太赫兹高阶贝塞尔波束在均匀磁化等离子体中传输的解析方法，并扩展到了非均匀分层磁化等离子体介质，导出了反射和透射场的圆柱矢量波函数展开系数的级联形式；揭示了单/多个同轴太赫兹高阶贝塞尔波束入射时，波束参数和等离子体参数对反射和透射场的横向幅度、相位、极化的影响机理；研究了单/多个太赫兹高阶贝塞尔波束经过等离子体后的OAM态（orbit angular momentum）畸变特性，明确了波束和等离子体参数对反射和透射波束的OAM态畸变和模式串扰规律及物理解释；阐明了子波束参数特别是相邻子波束的拓扑荷数差等造成反射和透射波束的OAM态畸变和模式串扰机理，提出了降低模式串扰的方法；研究了太赫兹高阶贝塞尔波束在等离子体湍流中的传输特性。本项目成果为分析复杂等离子体环境对太赫兹涡旋波传播影响提供了重要的理论依据，为太赫兹涡旋波在通信和目标探测等领域中的应用提供了支撑。