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全光纤电流传感器基于法拉第效应,采用光纤作为传输媒质和传感元件。法拉第效应是传感光纤中的偏振光受到电流产生的磁场作用发生偏振面旋转的现象(见图 1),偏振面旋转的角度称为法拉第旋转角。图 1中,E表示偏振光所处的偏振面,H表示磁场强度(A/m), L表示偏振光穿过介质的长度(m),θ表示法拉第旋转角(rad)。法拉第旋转角的大小和磁场强度以及磁场与光传播方向夹角的余弦值成正比,可描述为[12]:
$ \mathit{\theta }{\rm{ = }}\mathit{V}\int_{_\mathit{L}} {\mathit{\boldsymbol{H}} \cdot {\rm{d}}\mathit{l}} $
(1) 式中,V表示费尔德常数(rad/A)。
法拉第效应和自然旋光的原理都是基于菲涅耳旋光性,但是前者具有非互易性,即当偏振光被反射镜反射后在介质中往返一次,偏振面的旋转角度将会加倍,而不是像自然旋光那样转回起始位置。
根据光路结构和信号检测方式不同,AFOCS可分为偏振型和干涉型两大类。两者的区别是:偏振型AFOCS的结构中不存在相位调制器,直接通过检测输出光信号的光强关系得到法拉第旋转角,继而求得待测电流值;而干涉型AFOCS是利用调制器对光信号的相位进行调制,通过检测输出干涉光的相位差得到待测电流大小。
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偏振型AFOCS的基本结构如图 2所示。光源发出的光经过起偏器形成线偏振光进入光纤环中,出射的线偏振光经检偏器达到信号处理模块,然后对信号进行分析处理,这种结构称为单光路检测[13]。
这种结构虽然结构简单,但是无法对直流进行测量,因此德国学者PAPP和HARMS提出基于Wollaston棱镜的双光路检测方案(如图 3所示),实现了对直流的测量以及对温度的补偿,目前通常采用光纤偏振分束器(polarizing beam splitter,PBS)代替Wollaston棱镜来降低系统损耗[14]。
它的基本原理是Wollaston棱镜将出射的线偏振光分为两路正交的偏振光,然后由两个探测器分别检测两路光信号的光强,根据这两路偏振光的光强与法拉第旋转角的关系,可以计算出法拉第旋转角的数值:
$ \mathit{P}{\rm{ = }}\frac{{{\mathit{L}_\mathit{x}}{\rm{ - }}{\mathit{L}_\mathit{y}}}}{{{\mathit{L}_\mathit{x}}{\rm{ + }}{\mathit{L}_\mathit{y}}}}{\rm{ = sin(2}}\mathit{\theta }{\rm{)}} \approx {\rm{2}}\mathit{\theta } $
(2) 式中,P定义为偏振度,表示两路光强之间的关系,Lx和Ly分别表示两路正交偏振光的光强。
这种结构虽然有效地提高了系统的灵敏度,但是对外界环境的变化十分敏感[15],因此一般会采用低双折射光纤、旋转高双折射光纤作为AFOCS的传感光纤,以此提高系统的稳定性,但是这些特殊材料会使得成本增加。
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干涉型AFOCS是通过检测输出光信号相位的变化来获取被测对象的相关信息,可以从结构上分为Sagnac型和反射式两种结构。
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图 4所示是Sagnac型AFOCS的基本结构示意图[16-17]。光源发出的光信号经起偏器起偏后形成线偏振光,再由耦合器分成两路相同的信号,分别被λ/4波片转换为圆偏振光以相反的方向进入光纤环中进行循环,然后携带待测电流信息的两路光信号在起偏器处发生干涉,最终由探测器进行接收。
根据法拉第效应中的非互易性原理,这种结构测得的法拉第旋转角是偏振型AFOCS的两倍,因此它对外界的敏感性远低于偏振型结构。但是从图中可以看出,Sagnac型AFOCS使用了两个λ/4波片,由于λ/4波片对外界变化十分敏感,同时它对制造工艺的要求比较高,所以导致成本增加。
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反射式是干涉型AFOCS的另一种结构,又被称为in-line结构[18],常用的反射镜主要是正交共轭反射镜(orthogonal conjugate reflector,OCR)和法拉第旋转镜(Faraday rotation mirror,FRM)。反射式AFOCS的基本结构如图 5所示。光源发出的光经起偏器形成线偏振光,被45°熔接点分为两束相互垂直的偏振光,再由λ/4波片转化为左右圆偏振光进入光纤环中。当两束圆偏振光达到光纤环末端时被反射镜反射后以相反的方向再次通过光纤环,最后在起偏器处发生干涉,再由探测器将信息采集[12]。由于光信号在光纤环中经历了两次法拉第旋转,因此测得的法拉第旋转角是偏振型结构的4倍。
反射式AFOCS可以将互易性旋光相互抵消,大大降低了系统对温度、振动等因素的影响,同时这种结构用到的光学器件相对较少,避免了一些不必要的损耗,因此灵敏度和稳定性要远高于以上两种基本结构的AFOCS。但由于两束左右圆偏振光需要进行同时调制,而一般的相位调制器很难实现双轴调制,所以这种结构的输出信号会受到一定的影响[19]。
高灵敏度全光纤电流传感器研究进展
Research progress of high sensitivity all fiber optic current sensor
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摘要: 全光纤电流传感器作为智能电网中的重要设备之一, 具有比传统电磁式互感器更显著的优势, 在高压及超高压环境中有广阔的应用前景。首先阐明了影响全光纤电流传感器灵敏度的主要因素, 综述了近年来国内外学者提高电流传感灵敏度的解决方案和研究成果; 其次着重分析一些改进型结构的全光纤电流传感器消除温度、线性双折射等对传感灵敏度影响的工作原理, 并讨论了各自主要的优缺点; 最后, 结合高灵敏度全光纤电流传感器的研究现状, 指出了其未来发展趋势。Abstract: As one of the important devices in smart grid, all fiber optic current sensor has more significant advantages than traditional electromagnetic transformer, and has broad application prospects in high voltage and ultra-high voltage environment. Firstly, the main factors affecting the sensitivity of all fiber optic current sensor are clarified, and the solutions and research results for improving the sensitivity of current sensor by domestic and foreign scholars in recent years are summarized; Secondly, the working principle of some improved all fiber optic current sensors to eliminate the influence of temperature and linear birefringence on the sensing sensitivity is emphatically analyzed, and their main advantages and disadvantages are discussed; Finally, combined with the research status of high sensitivity all fiber current sensor, the future development trend is pointed out.
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