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图 1所示为基于色散光学技术的光波束形成网络原理图。一束具有等波长间隔的多波长光波由多波长激光源产生,波长分别为λ1, λ2, λ3, …, λM,波长间隔为Δλ。具有M个波长的光波输入电光调制器中被微波信号调制,被调制后的光信号通过环形器传输至色散系数可调的线性啁啾光纤布喇格光栅(linear chirped fiber Bragg grating,LCFBG)中,不同的波长在LCFBG中不同位置反射,并通过环形器输入高色散光纤阵列中。高色散光纤阵列由两个1×N光开关和N路具有不同长度的高色散光纤组成,通过1×N光开关切换使得光信号通过不同长度的高色散光纤。通过高色散光纤阵列的光信号输入波分复用器中解调为M路不同波长的光信号,M路光信号通过不同长度的单模光纤传输后至M个光电探测器,M路光信号分别在光电探测器中转换为微波信号,并通过等距离天线阵列发射到空间中,形成具有M路OTTD的光波束形成网络。
在如图 1所示的光波束形成网络中,色散系数可调的LCFBG的色散系数为D1,单位为ps/(km·nm);高色散光纤的色散系数为D2,单位为ps/nm。LCFBG的色散系数可以通过温度调节和长度拉伸等方法进行微调。N路高色散光纤的长度分别为F1, F2, F3, …, FN。因此, LCFBG与高色散光纤在波束形成网络中引入的等延时差为:
$ {\rm{\Delta }}{\mathit{\tau }_{\rm{1}}}{\rm{ = }}{\mathit{D}_{\rm{1}}}{\mathit{F}_\mathit{N}}{\rm{\Delta }}\mathit{\lambda }{\rm{ + }}{\mathit{D}_{\rm{2}}}{\rm{\Delta }}\mathit{\lambda } $
(1) 式中,第1项为LCFBG引入的等延时差,第2项为高色散光纤引入的等延时差。
多波长光信号经波分复用器解复用后传输光纤长度分别为L1, L2, L3, …, LM,各个光纤长度等延时差为Δτ2,因此如图 1所示的光波束形成网络中,M路OTTD等延时差为:
$ {\rm{\Delta }}\mathit{\tau }{\rm{ = \Delta }}{\mathit{\tau }_{\rm{1}}}{\rm{ + \Delta }}{\mathit{\tau }_{\rm{2}}} $
(2) 取天线距离为d=c/(2ƒ),其中, c为光在真空中的传播速度, ƒ为微波信号的频率。光波束形成网络的指向角为:
$ \mathit{\theta }{\rm{ = arcsin(2}}\mathit{f}{\rm{\Delta }}\mathit{\tau }{\rm{)}} $
(3) 根据(1)式~(3)式,可通过光开关实现N路高色散光纤路径切换获得大步进等延时差,进而实现波束指向的大范围和大步进调节;通过调谐线性啁啾光纤光栅色散系数连续调谐获得小步进等延时差,进而实现波束指向的小范围和小步进调节,最终实现大波束指向范围和精细步进调节的光波束形成网络。
基于色散光学的光波束形成网络
An optical beamforming network based on dispersing optics
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摘要: 为了实现大波束指向范围和精细步进调节的光波束形成网络, 采用了基于光学色散实现光真延时的方法。通过光开关实现N路高色散光纤路径切换获得大步进等延时差, 实现了波束指向的大范围和大步进调节; 通过调谐线性啁啾光纤光栅色散系数连续调谐获得小步进等延时差, 实现了波束指向的小范围和小步进调节。结果表明, 波束指向角度范围为-73.74°~+73.74°, 切换步进为0.458°, 基于光学色散原理可以实现光波束形成网络的大波束指向范围和精细步进调节。这一结果对优化相控阵天线系统设计是有帮助的。Abstract: In order to implement the large beam pointing range and the fine step adjustment of an optical beamforming network (OBFN), a method of optical dispersion-based optical true time delay (OTTD) was adopted. The path switching of N-way high dispersion fiber was realized by the optical switch, and thus the equally large-step delay difference was realized, then the beam pointing with a large range and a stride adjustment was realized. By tuning the dispersion coefficient of the linear chirped fiber Bragg grating (LCFBG), the equally small-step delay difference was realized, and then beam pointing with a small range and a small-step adjustment was realized. The theoretical analysis and simulation verification of the above method were carried out, and a beam pointing angle range of -73.74°~+73.74° and a switching step of 0.458° was obtained. The result shows that the large beam pointing range and the fine step adjustment of the OBFN were realized based on optical dispersion, which is helpful to improve the system designing of phased-array antenna.
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