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Volume 44 Issue 5
Sep.  2020
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Design of doped double-core photonic crystal fiber sound pressure sensor with high sensitivity

  • Corresponding author: ZHANG Kefei, zhangkefeijike@163.com
  • Received Date: 2019-09-09
    Accepted Date: 2019-11-07
  • In order to break the limitation of high Young's modulus of quartz material on the sensitivity improvement of fiber optic sensor in uniform radial direction, a doped double-core photonic crystal fiber sensing structure was proposed. The circular air holes were arranged on the fiber cladding to form hexagonal lattice. Polymethyl methacrylate was doped into the base-material area surrounded by the air holes on one side of the double-core photonic crystal fiber. The influence of cross-section parameters on sound pressure sensitivity under uniform stress was analyzed by COMSOL and the optimal structure was obtained. At the kPa level, the free spectral width is about 13nm. At the MPa level, the free spectral width is about 2.5465nm. The results show that the sensitivity of x polarized sound pressure is 0.15942nm/kPa at 1550μm. Compared with the Sagnac PCF pressure sensor, the size of this sensor is smaller, and the sensitivity at uniform radial direction is increased about 46.6 times. This work contributes to the design of the next generation underwater sound pressure sensor.
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    ZHU H X, YE T, ZHANG K F. Study on high sensitivity pressure sensing characteristics of photonic crystal fiber[J]. Laser Technology, 2019, 43(4): 511-516(in Chinese).
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    HAN B, GAO P. Research progress of photonic crystal fiber sensing technology based on Mach-Zehnder Interference[J]. Transducer World, 2018, 24(2):7-11(in Chinese).
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    LIANG H Q. Research and application of special optical fiber biosensor[D]. Nanchang: Nanchang Aviation University, 2019: 17-56(in Chinese).
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    LIU J R, QIU X F, SHEN H P. An intrusion detection system based on fiber hydrophone[J]. Proceedings of the SPIE, 2017, 10464:1046415.
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    TONG K, DANG P, WANG M T, et al. Modeling and analysis of sensitivity of surface plasmon resonance biosensor based on TiO2 thin film enhanced photonic crystal fiber[J]. Chinese Journal of Lasers, 2018, 45(6): 0610002(in Chinese). doi: 10.3788/CJL201845.0610002
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    PENG R R, LIU B, CHEN J. Research on SPR refractive index sensing characteristics of single-core photonic crystal fiber[J]. Laser Technology, 2018, 42(5):713-717(in Chinese).
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    LI D M, HU J F. Design on a novel birefringent photonic crystal fiber with polycarbonate-ellipse core for pressure sensing[J]. Ocean Acoustics, 2016(4): 23-27.
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    LIN M, JIN L, WANG Y, et al. Highly sensitive hydrostatic pressure sensor based on a selectively filled photonic crystal fiber[J]. Proceedings of the SPIE, 2013, 8924: 892405. doi: 10.1117/12.2031311
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    HOU M, WANG Y, LIU S, et al. Multi-components interferometer based on partially filled dual-core photonic crystal fiber for temperature and strain sensing[J]. IEEE Sensors Journal, 2016, 16(16):6192-6196. doi: 10.1109/JSEN.2016.2581302
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    JIANG M, LI K, ZHAO Z, et al. Selectively filled dual-core photonic crystal fiber sensors interrogated by low coherence interferometer for temperature measurement[C]// 2017 Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR). New York, USA: IEEE, 2017: 134-137.
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    SHI F Q. Research on sensing properties of filled photonic crystal fiber[D]. Jinhua: Zhejiang Normal University, 2018: 56-77(in Chinese).
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    MOUTUSI D, VINOD K S. Multi-purpose photonic crystal fiber having advanced optical properties and long sensing range[J]. Photonics and Nanostructures, 2019, 36(1): 7-13.
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    PAN R. Research on design and sensing characteristics of interference sensor based on photonic crystal fiber[D]. Harbin: Harbin University of Science and Technology, 2019: 107-123(in Chinese).
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    HUO W H. Dual-parameter sensing and performance analysis based on polarization-maintaining photonic crystal fiber grating[D]. Jinhua: Zhejiang Normal University, 2018: 23-27(in Chinese).
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    KANG Z X, SUN J, MA L, et al. Temperature and stress decoupling dual measurement sensing system based on double-core fiber cascading Bragg fiber grating[J]. Acta Optica Sinica, 2015, 35(5):0506004(in Chinese). doi: 10.3788/AOS201535.0506004
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Design of doped double-core photonic crystal fiber sound pressure sensor with high sensitivity

    Corresponding author: ZHANG Kefei, zhangkefeijike@163.com
  • 1. School of Science, Southwest University of Science and Technology, Mianyang 621010, China
  • 2. School of Computer Science and Technology, Southwest University of Science and Technology, Mianyang 621010, China

Abstract: In order to break the limitation of high Young's modulus of quartz material on the sensitivity improvement of fiber optic sensor in uniform radial direction, a doped double-core photonic crystal fiber sensing structure was proposed. The circular air holes were arranged on the fiber cladding to form hexagonal lattice. Polymethyl methacrylate was doped into the base-material area surrounded by the air holes on one side of the double-core photonic crystal fiber. The influence of cross-section parameters on sound pressure sensitivity under uniform stress was analyzed by COMSOL and the optimal structure was obtained. At the kPa level, the free spectral width is about 13nm. At the MPa level, the free spectral width is about 2.5465nm. The results show that the sensitivity of x polarized sound pressure is 0.15942nm/kPa at 1550μm. Compared with the Sagnac PCF pressure sensor, the size of this sensor is smaller, and the sensitivity at uniform radial direction is increased about 46.6 times. This work contributes to the design of the next generation underwater sound pressure sensor.

引言
  • 光纤式水下声压传感装置在声压灵敏度、插损、波分复用能力方面受到结构及材料的限制,阻碍了水下传感网探测能力的进一步提升。光子晶体光纤(photonic crystal fiber, PCF)的多孔微结构为光导模式与物质作用提供了增敏的可能性[1-6],利用掺杂还可大幅提升传感性能,进一步提高压力灵敏度。

    2014年,LI等人[7]用聚碳酸酯填充PCF椭圆纤芯的结构掺杂设计,在1.55μm成功实现了5.84×10-3的高双折射,但其纤芯十分微小,制作成本高。2015年, 暨南大学LIN等人[8]提出对PCF选择性地填充高折射率液体,利用石英与液体之间的力学差异性并优化熔接方法后压力敏感度达-32pm/MPa。2016年, HOU等人[9]做了部分填充的双芯光子晶体光纤(double-core photonic crystal fiber, DC-PCF)的多分量干涉仪,理论和实验说明了双芯模式之间的干扰情况。2017年,JIANG等人[10]提出去离子水填充双芯光子晶体的干涉传感装置,从系统设计的角度证实了DC-PCF实现多路复用和大范围测量的可能。2018年,SHI[11]提出一种基于乙醇非对称填充光子晶体光纤的新型Sagnac光纤传感器,实现了对温度与应力的测量,利用此可搭建温度与应力检测系统,误差较小。2019年,MOUTUSI等人[12]提出一种内部包层区域有两个大的椭圆形气孔的六边形光子晶体光纤,具有高达3.93×10-3的双折射率,对于x偏振模式,它具有高达49.42%的相对灵敏度。同年,PAN[13]制备了具有游标效应的光子晶体光纤传感器,进一步优化单模光纤、石英管以及光子晶体光纤的无塌陷熔接制备,将气压灵敏度提升至-29.99nm/MPa。

    DC-PCF对声压十分敏感,通过掺杂从一定程度上可避免温度与声压交叉敏感的情况。作者基于马赫-曾德尔干涉传感原理,设计了一种DC-PCF声压传感器,用聚碳酸酯、环氧树脂等光弹材料掺杂于DC-PCF,通过研究掺杂关系与传感灵敏度的作用关系,提出了一种掺杂提敏的设计方案。

1.   掺杂型DC-PCF声压传感原理
  • 水下目标发出或反射声波,在传感器周围形成声场变化,声场的变化引起水压的变化,光纤声压传感器通过感知水压变化来获取水声信号。将DC-PCF传感器沉浸在一定深度的海水中,四周环绕的海水会对光纤产生均匀径向压力,此时不存在切向应力[14],如图 1所示。

    Figure 1.  Uniform pressure on the optical fiber

    非耦合型DC-PCF传感器类似于全光纤马赫-曾德尔干涉仪[15],双干涉臂在一根光纤中,单根光纤实现传输与干涉。因两纤芯的传播常数不同,导致出射端双芯有明显相位差,使得光纤出射端干涉透射谱波谷波长发生平移。

    自由光谱宽度(free spectral range,FSR)是声压传感器主要性能参量之一,即干涉透射谱的周期,代表着梳状透射谱在一定谱宽下的疏密程度,适当的自由光谱宽度可便于传感信息的读取[16]

    由上式可知,器件的自由光谱宽度RFSR与波长λ的平方成正比,与双芯传感臂长度L和双芯有效折射率差Δneff呈反比。通过调整双芯光纤长度和双芯结构参量,改变有效折射率差的变化,最终获得合适的FSR。

    利用COMSOL模拟声压加载DC-PCF的受压形变情况,计算DC-PCF存在应力下折射率的改变量,在电磁波频域模块中求得DC-PCF对应有效折射率。在DC-PCF长度已知的情况下,结合均匀径向压力作用下的有效折射率差,可计算双芯传感部分的相位改变量,代入干涉透射谱表达式中计算特定压力下的传输透射谱,再用波谷的波长移动量与光纤所加载的静压力的比值求得DC-PCF的压力传感灵敏度。

    式中,Δλp为某一波谷在压力p作用后的移动量,λ0为波谷移动前的位置,λp为波谷移动后的位置。ΔRFSR为自由光谱宽度在压力p作用后的改变量, Δλ为波长的改变量。

    在施加应变后,根据弹性力学原理,分别求取DC-PCF的有效折射率差和长度的改变量,可导出波谷移动量与应力及光纤材料的本质作用关系:

    式中,δλ为应变作用后波长移动量,ε为应变大小, Peff, 1Peff, 2分别为两个纤芯的基模有效折射率弹光系数,neff, 1neff, 2分别为两个纤芯的有效折射率。由此可知,若双芯结构和材料确定情况下,Peff为定值。波长移动量与应变呈线性关系。

2.   仿真实验
  • 以丹麦Crystal-Fiber公司生产的高双折射光子晶体光纤PM-1550-01为声压传感的基础结构。用石英材料作为光子晶体光纤的背景材料,建立如图 2所示的由6层空气孔排列的六边形基本光子晶体光纤结构,引入两个稍大于包层空气孔的圆形纤芯结构,对称分列于圆心两侧。

    Figure 2.  Sound pressure sensing model diagram of PM-1550-01

    图 2中,D为DC-PCF的光纤直径,d1为纤芯材料直径,d2为包层空气孔材料直径,Λ1为纤芯距离圆心的距离,Λ2是包层空气孔的水平相距间隔,d3为掺杂纤芯直径。

  • 挑选了两种低杨氏模量的光弹性材料:聚甲基丙烯酸甲酯(polymethyl methacrylate, PMMA)和聚碳酸酯(polycarbonate, PC),均是大气窗口透过性好的光学塑料,弹性性能较好。

    在COMSOL中建立仿真模型,左侧纤芯中引入PMMA或PC压敏材料。初始参量λ=1.55μm; d3= 2.2μm; 两芯间距6×4.4μm; d1=4.5μm; d2=2.2μm; Λ1=3×4.4μm; Λ2=4.4μm; 空气孔层数N=5,D=46μm,给测试模型施加x方向均匀压力,每间隔30kPa测定一组对应有效折射率。有效折射率随压力不同而改变,两纤芯的同偏振模有效折射率差近乎相同,均为0.018左右。

    用有效折射率差计算相位差并绘制干涉透射谱如图 3所示。在外力作用下,透射光谱发生明显红移,即声压增大了DC-PCF等效干涉臂的传输相位差。图 4a为掺杂PMMA后对波谷波长做线性拟合求取的不同偏振下声压灵敏度的线性拟合结果。x偏振和y偏振的声压灵敏度分别为8.61nm/MPa和7.83nm/MPa。图 4b中以同样的方法绘制掺杂PC的声压灵敏度拟合曲线。x偏振和y偏振的声压灵敏度分别为0.292nm/MPa和0.194nm/MPa。

    Figure 3.  Simulation of interference transmission spectra at different sound pressures

    Figure 4.  Sound pressure sensitivity fitting of doped core

  • 先讨论双芯间距的影响。采用掺杂纤芯与非掺杂纤芯对称分列于光纤圆心两侧的方式改变双芯间距,以减小熔接过程偏移对准难度。如图 5所示,双芯间距增加,声压灵敏度先出现了骤降,之后平缓下降,x偏振与y偏振均保持这样的变化趋势。

    Figure 5.  Effect of double core spacing on sensitivity under uniform radial stress

    由于掺杂纤芯向右侧空气孔靠近,即掺杂纤芯周围的等效折射率被空气孔稀释,致初始有效折射率降低,而声压灵敏度是最终有效折射率差与初始有效折射率差比值的函数。在最终有效折射率差相近的情况下,初始有效折射率越低,相应透射谱波谷波长的移动量越大,声压灵敏度也就越大。

    再讨论掺杂直径的影响。该模型具有光子晶体光纤的六轴对称性,对xy方向应力表现相同的敏感性,两者偏振声压灵敏度近乎相同。如图 6所示,随着掺杂半径的增加,0MPa的初始有效折射率差快速提升,80MPa下的有效折射率差增幅不小,但有效折射率差的比值一直在减小,使声压灵敏度逐渐减小。

    Figure 6.  Changes of sound pressure sensitivity with doping radius under uniform radial stress

  • 先讨论空气孔间距的影响。从图 7可知,空气孔间距从2.5μm提升到5.0μm,x偏振和y偏振声压灵敏度几乎相同,且保持着相同的增速,体现了模型的对称性,最大灵敏度为18.45847nm/MPa。

    Figure 7.  Change of sensitivity of x polarized and y polarized sound pressure with air hole spacing under uniform radial stress

    再讨论空气孔层数的影响。将空气孔设置为4层和5层(掺杂纤芯设置在第3层,未掺杂区域需要空气孔构成纤芯,所以最少空气孔需要4层),6层空气孔与4层和5层的外层直径分别为54μm和45μm。

    图 8可知,随着空气孔层数的增加,x偏振和y偏振的声压灵敏度均有所提升,两者声压灵敏度相近,在5层~6层中声压灵敏度增速放缓。

    Figure 8.  Changes of sensitivity of x polarized and y polarized sound pressure with the number of air hole layers under uniform radial stress

    再讨论传输波长的影响。在初始模型下,考虑光纤SiO2和掺杂材料的色散问题,采用的模型均添加COMSOL的内置材料:SiO2(Malitson)和PMMA resists(Microchem 495)。在干涉透射谱中仅取与传输波长非常接近的波谷波长来计算波长在声压作用下的移动量,以确保数据的可靠性。

    计算并拟合了如图 9所示的初始模型的声压灵敏度。可知,传输波长的增加会因色散效应对有效折射率产生一定影响,最终影响到声压灵敏度。随着波长的提升,声压灵敏度有小幅的上升。

    Figure 9.  Sound pressure sensitivity fitting for different transmission wavelengths

3.   分析与讨论
  • 水下声压传感器的工作情况符合均匀径向应力的施加,在均匀径向力下,光纤直径取45μm~125μm中间值; 在不耦合的情况下,双芯间距从器件的熔接使用角度来说,越小越好; 掺杂纤芯直径保持在1.5μm~4μm中间; 空气孔直径保持在3μm左右; 空气孔间距向3μm以上提升; 包层空气孔层数大于5层; 光纤长度6cm; 输出波长保持1550nm,增加普适性。

    由干涉传感原理可知,双芯光纤声压灵敏度正比于应力加载后有效折射率差与加载前有效折射率差的比值。结合DC-PCF结构参量对声压灵敏度的影响,设计了如图 10所示结构的掺杂型DC-PCF声压传感器。

    Figure 10.  Radial doped DC-PCF sound pressure sensor with high sensitivity

    图 10中的结构上施加均匀径向声压,DC-PCF 6cm的情况下模拟干涉透射谱,得到图 11。可知,随着声压增大,透射谱波谷波长出现了明显的红移,移动波长间隔与所施加的均匀径向声压基本呈线性变化。

    Figure 11.  Interference transmission spectrum shift of x polarization under di-fferent sound pressures

    由1550nm附近波谷的移动做直线拟合可得声压灵敏度,如图 12所示。其拟合线性度达0.9999,xy偏振声压灵敏度相近,x偏振声压灵敏度为0.15942nm/kPa,在1550nm波段,相对灵敏度为0.1029/MPa,是现有水平的近100倍[17]。相比于58.4cm Sagnac PCF压力传感器[18],灵敏度提高46.6倍。在kPa量级的声压作用下,自由光谱宽度13.0392nm; 40MPa左右的声压作用下,6cm的DC-PCF自由光谱宽度约2.5465nm。

    Figure 12.  The fitted sound pressure sensitivity of x polarization and y polarization

    该结构在不同的声压作用下表现出了差异性大的自由光谱宽度,应用到不同应力水平的领域中,可做适当的长度调整,降低检测系统的部署难度; 另一方面,也可通过调整长度使之适用于多种波分复用系统。

4.   结论
  • 提出了一种掺杂PMMA的DC-PCF声压传感结构,传感长度达6cm,在1550nm波段,相对灵敏度为0.1029/MPa,x偏振方向声压灵敏度达0.15942nm/kPa。在千帕量级的声压作用下,自由光谱宽度约13nm; 兆帕量级的声压作用下,自由光谱宽度约2.5465nm。在测量范围内,线性度高,插入损耗低,实际应用中,通过调整光纤长度,可实现波分复用式的阵列传感网。从一定程度上解决了传统光纤声压传感器的低灵敏度和复用性低的缺陷。

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