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Liao Jingjing, Zhu Lianqing, Song Yanming, Xin Jingtao, Lv Zheng. Fiber Bragg grating temperature insensitive filter based on bimetal structure[J]. Infrared and Laser Engineering, 2023, 52(3): 20220505. doi: 10.3788/IRLA20220505
Citation: Liao Jingjing, Zhu Lianqing, Song Yanming, Xin Jingtao, Lv Zheng. Fiber Bragg grating temperature insensitive filter based on bimetal structure[J]. Infrared and Laser Engineering, 2023, 52(3): 20220505. doi: 10.3788/IRLA20220505

Fiber Bragg grating temperature insensitive filter based on bimetal structure

doi: 10.3788/IRLA20220505
Funds:  The Research Project of Beijing Municipal Natural Science Foundation & Beijing Education Committee (KZ201911232044)
  • Received Date: 2022-07-21
  • Rev Recd Date: 2022-09-07
  • Publish Date: 2023-03-25
  •   Objective   Fiber optic gyroscopes (FOG) are a new class of instrument capable of accurately determining the orientation of moving objects, which have been widely used in tactical missile guidance, land traffic navigation and aerospace attitude adjustment because of their long service life, easy integration and small size, etc. However, the changing operating temperature in these application fields seriously reduces the average wavelength stability of the broad spectrum light source in the fiber optic gyroscope, thus hindering the further application of the fiber optic gyroscope. To improve the average wavelength stability of the fiber optic gyro wide spectrum light source, this study proposes a method to make a temperature-insensitive filter with a bandwidth of 11.77 nm using a 60 μm ultrashort fiber grating.  Methods   The bimetallic temperature compensation structure (Fig.1) is designed based on the thermal expansion coefficient difference between two metal materials. This structure will compress/stretch the fiber grating with the increase/decrease of temperature and effectively compensate for the wavelength change of fiber grating caused by the thermo-optic effect. The filter is fabricated by combining metal material with fiber grating in two-point packaging, and the effects of the thermal expansion coefficient of the material and the geometric parameters of the filter on the temperature sensitivity of the filter are systematically studied.  Results and Discussions   The results show that the temperature sensitivity of the filter is mainly affected by the thermal expansion coefficient and length of the substrate and the strain transfer beam. The largest adjustable range of L1/L2 (the ratio of the length between two fixed points on the substrate and the length of the filter fiber grating) is achieved as the thermal expansion coefficient difference is −5.8. Besides, the temperature sensitivity of the filter exhibits a negative linear relationship with the value of L1/L2. According to the comparison of the control variable method, it is found that the adjustable range of L1 is larger, and the value range of L1 is 7 times that of L2, which is more conducive to the packaging operation. The base of the bimetal structure is made of brass with a high coefficient of thermal expansion, and the transfer beam is made of aluminum with a low coefficient of thermal expansion. At last, the temperature-insensitive filter with a size of 74 mm×6 mm×4 mm was prepared (Fig.4). When the L1/L2 is 8.39, the base length is 67.1 mm and the filter grating length is 8 mm, the temperature sensitivity coefficient of the fiber grating is 0.15 pm/℃ and the wavelength change is only 4.5 pm in the range of 30-60 ℃, which is more than 60 times lower than that before compensation (Tab.6).   Conclusions   In this study, a bimetallic temperature compensation structure has been successfully designed, and a temperature-insensitive filter with a bandwidth of 11.77 nm has been fabricated using a 60 μm ultrashort fiber grating (Fig.3). The temperature insensitivity filter can effectively improve the temperature insensitivity of the fiber grating, which can be used as a light source filter to improve the average wavelength stability of the light source and is expected to be used in high-precision fiber optic gyroscopes.
  • [1] Zhang Chunmei, Liu Xiaoqing. Research of bias temperature error compensation method in fiber optic gyroscopes [J]. Flight Control & Detection, 2020, 3(2): 70-74. (in Chinese)
    [2] 季阳. 光源设计及其对光纤陀螺性能影响的研究[D]. 哈尔滨工程大学. 2017.

    Ji Yang. Design of light source and research on its impact on fiber optic gyroscope [D]. Harbin: Harbin Engineering University, 2017. (in Chinese)
    [3] Wu Yanji, Huang Xianlin. Study on temperature stability of scale-factor in closed-loop fiber optical gyroscope [J]. Transactions of Beijing Institute of Technology, 2007, 27(7): 618-620. (in Chinese) doi:  10.3969/j.issn.1001-0645.2007.07.013
    [4] Zatta P Z, Hall D C. Ultra-high-stability two-stage superfluorescent fibre sources for fibre optic gyroscopes [J]. Electron Lett, 2002, 38: 406-408. doi:  10.1049/el:20020278
    [5] 李凯, 辛璟焘, 何巍, 等. 基于宽谱信号光注入的超荧光光纤光源[J]. 工具技术, 2018, 52(7): 5. doi:  10.3969/j.issn.1000-7008.2018.07.047
    [6] Wu Xu, Ruan Shuangchen, Liu Chengxiang. High-stability erbium-doped photonic crystal fiber source [J]. Appl Opt, 2012, 51(13): 2277-2281.
    [7] Xin Jingtao, Zhang Haitao, Hao He, et al. Experiment study of high mean wavelength and output power stability erbium-doped photonic crystal fiber superfluorescent source [J]. Chinese Journal of Lasers, 2015, 42(4): 0405002. (in Chinese)
    [8] Patrick H J, Kersey A D, Burns W K, et al. Erbium-doped superfluorescent fibre source with long period fibre grating wavelength stabilisation [J]. Electron Lett, 1997, 33(24): 2061-2063. doi:  10.1049/el:19971376
    [9] Wang A, Ou P, Feng L S, et al. High-stability Er-doped superfluorescent fiber source incorporating photonic bandgap fiber [J]. IEEE Photon Technol Lett, 2009, 21: 1843-1845. doi:  10.1109/LPT.2009.2034621
    [10] Ou P, Cao B, Zhang C X, et al. Er-doped superfluorescent fibre source with enhanced mean-wavelength stability using chirped fibre grating [J]. Electron Lett, 2008, 44(3): 187-189. doi:  10.1049/el:20082948
    [11] Wang Aimin. High stability Er-doped superfluorescent fiber source improved by incorporating bandpass filter [J]. IEEE Photon Technol Lett, 2011, 23(4): 227-229.
    [12] Yi Benshun, Hu Ruimin, Zhu Zibi, et al. Temperature compensation techniques for fibre Bragg gratings tuned by magnetostrictive transducers [J]. Chinese Journal of Lasers, 2002, 29(12): 1085-1088. (in Chinese) doi:  10.3321/j.issn:0258-7025.2002.12.008
    [13] Yoffe G W, Krug P A, Ouellette F, et al. Passive temperature-compensating package for optical fiber gratings [J]. Applied Optics, 1995, 34(30): 6859-6861.
    [14] Arya V, Sherrer D W, Wang A, et al. Application of thin-film optical filters to the temperature compensation of optical fiber grating-based devices [J]. IEEE Transactions on Instrumentation and Measurement, 1997, 46(5): 1173-1177. doi:  10.1109/19.676734
    [15] Yu Gang, He Sailing. A new package technique for fiber gratings [J]. Acta Photonica Sinica, 2004, 33(3): 291-293. (in Chinese)
    [16] Cao Bin, Ou Pan, Jia Ming, et al. An innovative temperature-compensate package for fiber Bragg grating [J]. Chinese Journal of Lasers, 2008, 35(12): 1959-1961. (in Chinese) doi:  10.3321/j.issn:0258-7025.2008.12.020
    [17] Liu Yonghong, Huang Dexiu, Ruan Yinglan, et al. A temperature insensitive fiber grating [J]. Chinese Journal of Lasers, 1997, 24(10): 895-898. (in Chinese) doi:  10.3321/j.issn:0258-7025.1997.10.008
    [18] 吴昊, 张洋, 王帅, 等. 超短FBG的高灵敏度温度传感器[J]. 激光与红外, 2021.

    Wu Hao, Zhang Yang, Wang Shuai, et al. High sensitivity temperature sensor of ultra-short FBG [J]. Laser & Infrared, 2021, 51(8): 1057-1064. (in Chinese)
    [19] Li Jiang, Xin Jingtao, Wu Hao, et al. Research on the inscription and apodization method of ultra short fiber Bragg grating [J]. Laser & Infrared, 2021, 51(3): 316-320. (in Chinese) doi:  10.3969/j.issn.1001-5078.2021.03.010
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Fiber Bragg grating temperature insensitive filter based on bimetal structure

doi: 10.3788/IRLA20220505
  • 1. Key Laboratory of the Ministry of Education for Optoelectronic Measurement Technology and Instrument, Beijing Information Science & Technology University, Beijing 100192, China
  • 2. Beijing Laboratory of Optical Fiber Sensing and System, Beijing Information Science & Technology University, Beijing 100016, China
  • 3. Beijing Key Laboratory of Optoelectronic Measurement Technology, Beijing Information Science & Technology University, Beijing 100192, China
Fund Project:  The Research Project of Beijing Municipal Natural Science Foundation & Beijing Education Committee (KZ201911232044)

Abstract:   Objective   Fiber optic gyroscopes (FOG) are a new class of instrument capable of accurately determining the orientation of moving objects, which have been widely used in tactical missile guidance, land traffic navigation and aerospace attitude adjustment because of their long service life, easy integration and small size, etc. However, the changing operating temperature in these application fields seriously reduces the average wavelength stability of the broad spectrum light source in the fiber optic gyroscope, thus hindering the further application of the fiber optic gyroscope. To improve the average wavelength stability of the fiber optic gyro wide spectrum light source, this study proposes a method to make a temperature-insensitive filter with a bandwidth of 11.77 nm using a 60 μm ultrashort fiber grating.  Methods   The bimetallic temperature compensation structure (Fig.1) is designed based on the thermal expansion coefficient difference between two metal materials. This structure will compress/stretch the fiber grating with the increase/decrease of temperature and effectively compensate for the wavelength change of fiber grating caused by the thermo-optic effect. The filter is fabricated by combining metal material with fiber grating in two-point packaging, and the effects of the thermal expansion coefficient of the material and the geometric parameters of the filter on the temperature sensitivity of the filter are systematically studied.  Results and Discussions   The results show that the temperature sensitivity of the filter is mainly affected by the thermal expansion coefficient and length of the substrate and the strain transfer beam. The largest adjustable range of L1/L2 (the ratio of the length between two fixed points on the substrate and the length of the filter fiber grating) is achieved as the thermal expansion coefficient difference is −5.8. Besides, the temperature sensitivity of the filter exhibits a negative linear relationship with the value of L1/L2. According to the comparison of the control variable method, it is found that the adjustable range of L1 is larger, and the value range of L1 is 7 times that of L2, which is more conducive to the packaging operation. The base of the bimetal structure is made of brass with a high coefficient of thermal expansion, and the transfer beam is made of aluminum with a low coefficient of thermal expansion. At last, the temperature-insensitive filter with a size of 74 mm×6 mm×4 mm was prepared (Fig.4). When the L1/L2 is 8.39, the base length is 67.1 mm and the filter grating length is 8 mm, the temperature sensitivity coefficient of the fiber grating is 0.15 pm/℃ and the wavelength change is only 4.5 pm in the range of 30-60 ℃, which is more than 60 times lower than that before compensation (Tab.6).   Conclusions   In this study, a bimetallic temperature compensation structure has been successfully designed, and a temperature-insensitive filter with a bandwidth of 11.77 nm has been fabricated using a 60 μm ultrashort fiber grating (Fig.3). The temperature insensitivity filter can effectively improve the temperature insensitivity of the fiber grating, which can be used as a light source filter to improve the average wavelength stability of the light source and is expected to be used in high-precision fiber optic gyroscopes.

    • 光纤陀螺仪(FOG)在战术导弹制导、陆地交通导航、航天姿态调整等领域应用广泛[1]。高精度光纤陀螺仪光源对其平均波长稳定性具有较高的要求。如标度因数稳定性优于5 ppm (1 ppm=10–6)的惯导级光纤陀螺,在–45~70 ℃的温度范围内,要求光源平均波长稳定性优于1 pm/℃[2]。因此,大温度范围下减小温度对光源的影响是提高光源平均波长稳定性的关键技术之一,对提升光纤陀螺性能具有重要的研究意义[3]

      目前,提高平均波长稳定性的方法主要包括优化光源结构[4-5]、使用新型光纤[6-7]及在光源中加入滤波器[8-11]。光源中加入的滤波器主要有长周期光纤光栅[8]、啁啾光纤光栅[10]、高斯滤波片[11]等。

      对光纤光栅滤波器进行温度补偿可以有效抑制光源平均波长的变化,提升光源输出波长的稳定性、可靠性,实现光源系统的全光纤化。

      近几年,国内外学者提出了许多光纤光栅温度补偿方法[12-17],主要采用设计适当的结构封装[12,15-16]和利用负热膨胀材料[13]封装。武汉大学易本顺[12]等人利用磁场和磁致伸缩效应对光纤光栅的温度进行补偿,在22~80 ℃范围内光栅的波长变化为1.77 pm/℃。南开大学黄勇林等人采用负热膨胀系数材料[13]对光纤光栅进行温度补偿,在–20~44 ℃温度范围内光纤光栅的波长变化为1.25 pm/℃。华中理工大学刘永红[17]等人设计了一种温度补偿组件,在−20~40 ℃温度范围内,光纤光栅经温度补偿后波长变化率为2 pm/℃。对比惯导级光纤陀螺光源对平均波长稳定性要求,目前使用的温度补偿方法所实现的平均波长稳定性还不足以满足应用需求[11]。因此,对光纤光栅温度补偿方法进行进一步研究具有重要的研究意义。

      为提高高精度光纤陀螺宽谱光源的平均波长稳定性,文中设计了一种双金属温度补偿结构,该结构能够随温度对光纤光栅进行压缩或拉伸,有效地补偿了光纤光栅由热光效应引起的波长变化,提高了光纤光栅滤波器的温度不敏感性。此外,根据该结构系统地研究了材料热膨胀系数、滤波器几何参数等对滤波器灵敏度的影响,制作了尺寸为74 mm×6 mm×4 mm的温度不敏感滤波器。实验结果表明,在30~60 ℃温度范围内,滤波器的温度灵敏度系数为0.15 pm/℃,较未补偿前降低了近两个数量级。该滤波器可用于较大尺寸的光纤陀螺光源,通过对滤波器尺寸的进一步优化将适用于小尺寸的光纤陀螺光源。

    • 温度不敏感滤波器温度补偿原理是利用双金属结构的热膨胀系数差补偿光纤光栅由热光效应引起的波长漂移[14]。当温度变化时,由于高热膨胀系数材料热胀冷缩的物理特性,使基底与应变传递梁的长度均发生变化。相同条件下,基底和应变传递梁长度变化量的差值传递给光纤光栅,与光纤光栅由于热光效应引起的波长漂移量抵消。因此,降低光纤光栅的温度灵敏度系数可以通过选用适当的双金属材料组合及调整基底和应变传递梁的长度实现[18]

      图1为双金属结构示意图,由基底和两个应变传递梁组成。L1为基底上的两个固定点之间的长度,L2为滤波光纤光栅的长度。

      Figure 1.  Schematic diagram of bimetal structure

      $ {a_1} $$ {a_2} $分别为基底和应变传递梁的热膨胀系数,当温度变化$ \Delta T $时,基底长度变化量为$ {a_1}{L_1}\Delta T $,应变传递梁长度变化量为$ {a_2}({L_1} - {L_2}) \cdot \Delta T $,则光纤光栅长度的变化量为$ \Delta L = (({a_1} - {a_2}){L_1} + {a_2}{L_2}) \cdot \Delta T $。采用两点式封装法将光纤光栅封装于应变传递梁上。当温度变化时,光纤光栅的波长变化由热光效应和结构热膨胀导致的弹性应变组成,可以用公式(1)表示:

      式中:参数Pe$ \xi $分别为光纤的弹光系数与热光系数;$ {\lambda _B} $为光纤光栅的中心波长。

      由于光纤光栅的波长漂移量受$ \xi $Pe变化影响,而弹光系数大小与光栅的拉伸和压缩有关,因此可以通过压缩栅格距离补偿光纤光栅的红移。

      假设由机械结构引起的应变无损耗地传递到光纤上,由温度变化引起的光纤光栅应变及相应的光纤光栅温度灵敏度系数可用公式(2)、(3)表示:

      由公式(1)~(3),双金属结构的温度灵敏度系数可表示为:

      由公式(4)可见,基底和应变传递梁的热膨胀系数和长度影响着滤波器的温度灵敏系数。当滤波器的几何参数固定,即L1L2的值不变时,双金属结构的温度灵敏度系数与双金属材料的热膨胀系数差有关。当材料确定后,光纤光栅温度灵敏度系数是与材料长度相关的常数。从理论上保证了采用光纤光栅制作温度不敏感滤波器,可以得到很好的线性输出。为了更加直观地分析温度灵敏度系数与双金属材料的热膨胀系数和几何参数的关系,设定几种材料进行关系曲线绘制。20 ℃时,常见几种材料的热膨胀系数如表1所示。

      MaterialAluminumBrassIronInvarQuartzGlass
      Coefficient of thermal expansion/℃23.9×10−619.00×10−612.20×10−61.00×10−60.55×10−64.00×10−6

      Table 1.  Thermal expansion coefficient of common materials

      当基底选用低热膨胀系数的材料、应变传递梁选用高热膨胀系数的材料时,光纤光栅中心波长为1533 nm,滤波器的温度灵敏度系数与L1/L2的曲线关系如图2所示。

      Figure 2.  Temperature sensitivity coefficient versus L1/L2

      温度升高时,由于金属材料的物理特性导致双金属结构对光纤光栅处于压缩状态,而光纤光栅由于热光效应的影响处于膨胀拉伸状态。当L1/L2的值较大时,双金属结构对光纤光栅压缩处于主导地位,因此温度灵敏度系数为负[18],相反情况下温度灵敏度系数为正。必存在一个L1/L2,使双金属材料压缩量和光纤光栅拉伸量抵消,使滤波器的温度灵敏度系数为零。选取双金属结构材料分别为黄铜/铝、铁/黄铜、铁/铝、殷钢/铝,殷钢/黄铜、殷钢/铁,当L1/L2=7.766、5.172、3.798、2.896、2.585、2.438时,双金属结构的温度灵敏度系数近乎为零。表2为温度灵敏度系数在小范围区间变化时,不同金属材料组合后L1/L2的变化范围,其中x表示L1/L2,∆Y为双金属结构温度灵敏度系数变化量。

      Bimetallic materialRelationship between L1/L2 and temperature sensitivity coefficientWhen ∆Y=0.1 pm/℃, the change of x/mm
      Brass/AluminumY= –5.85913x+45.50557±0.01707
      Iron/BrassY= –8.13103x+41.91835±0.01229
      Iron/AluminumY= –13.99016x+53.6366±0.00715
      Invar/AluminumY= –27.38245x+67.02889±0.00366
      Invar/BrassY= –21.52332x+55.31064±0.00465
      Invar/IronY= –13.39229x+39.04858±0.00747

      Table 2.  Variation range of L1/L2 with different combinations of metal materials

      表2可知,在相同温灵敏度系数变化范围,斜率的绝对值越小,L1/L2变化范围越大。此时,封装操作的可控制范围更大,更利于滤波器的封装。因此,黄铜/铝的组合适用于温度不敏感滤波器的封装。温度灵敏度系数约为0时,L1/L2约为7.766。

      表3为理想状态下,定量L1 (或L2)对应变量L2 (或L1)的取值范围。此时,温度灵敏度系数变化量为0.1 pm/℃,L1/L2的取值范围为7.766±0.017。

      Ration/mmThe range of variables/mm
      L1=70L2=9.013±0.0197
      L2=8L1=62.128±0.136

      Table 3.  Value range of quantitative corresponding variables

      对比变量的取值范围,就封装操作的可调范围而言,固定L2L1的取值范围是固定L1L2取值范围的7倍。因此,固定L2后实验操作的可调范围更大。表4为固定L2=8 mm后,理论计算出的L1/L2与温度灵敏度系数的关系。

      L1/L2The length of L1/mmTemperature sensitivity
      coefficient/pm·℃−1
      7.78062.240–0.07846
      7.77562.200–0.04916
      7.76662.1280.00005
      7.75062.0000.09731

      Table 4.  Relationship between L1/L2 and sensitivity coefficient

      表4可知,理想情况下制作温度不敏感滤波器应设计L1/L2的值为7.766,L2为8 mm,L1为62.128 mm。此时可实现温度灵敏度系数为0.00005 pm/℃,在–45~70 ℃的温度范围内,光纤光栅的波长变化为0.00575 pm。

    • 在已有的准分子激光刻写系统的基础上,通过在掩模板前放置狭缝和调节光纤与掩模板之间的距离来控制光纤上的有效曝光长度,采用单缝衍射的方法控制光束强度的分布,从而对光栅进行切趾,实现超短光纤光栅的制作[19]实验中使用具有较强光敏性由Fibercore公司生产的PS1250光纤,在距离掩模板前1 mm处放置宽度为230 μm的狭缝,光纤与掩模板的距离为6 mm。实现了光栅长度为60 μm、3 dB带宽为11.77 nm的超短光栅刻写,图3为刻写的超短光纤光栅光谱图。

      Figure 3.  Spectrum of ultra-short fiber grating

    • 图4为温度不敏感光纤光栅滤波器的装配及实物图。该滤波器主要由基底、应变传递梁、光纤光栅等组成。胶粘剂将光纤光栅固定于两传递梁中心位置,以保证滤波器所受应变和温度变化有效传递到光纤光栅上。

      Figure 4.  (a) Schematic diagram and (b) physical drawing of temperature insensitive filter

      温度补偿机构的基底采用热膨胀系数较高的黄铜,传递梁采用的是热膨胀系数较低的铝。应变传递梁两端设有两个控制端,将右侧传递量固定,左侧固定端与应变传递梁为活动连接,可以随时更改接入的应变传递长度。制作了L1长度为69 mm,L2长度为8 mm、67.1 mm、65.35 mm及62.25 mm的温度不敏感滤波器。将滤波器放置在DB-1数显不锈钢电热板上,使用EA1C-1V胶水进行两点式封装。封装前,首先用酒精对传递梁进行内外清洗,达到去污效果,保证传递梁与环氧树脂有良好的粘结型。然后将基底与传递梁组合后置于精密光纤调整架中央平台,小心地将光纤光栅穿入铝管中,调整光纤调整架的左右高度,使光栅位于两个铝管中央。然后调整光纤架使光纤光栅施加少量的张力,在此过程中监测光谱仪观察光纤光栅中心波长,直到中心波长移动0.5~1 nm[13]。封装时,先将光纤光栅的中心对准滤波器缺口的中心位置,使光纤光栅悬空放置,以保证光纤光栅被拉伸时内部应力均匀。用注射器将调配好的环氧树脂注入两根铝管端点处,等待其完全固化。滤波器的尺寸参数如表5所示。

      ParameterParameter nameNumerical value
      L1/mmLength between fixed points on the base69, 67.1, 65.35, 62.25
      L2/mmFilter fiber grating length8
      $ {\alpha }_{1} $/℃−1Coefficient of thermal expansion of brass substrate19×10−6
      $ {\alpha }_{2} $/℃−1Strain transfer beam aluminum thermal expansion coefficient23.9×10−6

      Table 5.  Filter size parameters

    • 搭建如图5所示的实验装置,研究温度对光纤光栅滤波器的影响。实验器材包括宽谱光源、光谱仪、FLUKE7381深井台式恒温槽等。将封装好的滤波器置于FLUKE7381深井台式恒温槽中,由宽带光源发出的光信号经单模光纤引入滤波器中,然后对滤波后反射回光谱分析仪(OSA)的光信号进行分析。

      Figure 5.  Experimental setup

      ASE宽谱光源的输出波长范围为1530~1565 nm,滤波光栅的中心波长为1550 nm,光谱仪型号为AQ6370D。实验中,调节恒温槽温度分别为30、40、50、60 ℃,每个温度节点静置10 min,然后记录滤波器的中心波长。

      由理论分析可知,滤波器的温度灵敏度与L1/L2呈线性关系,因此必定存在一个L1/L2,使滤波器温度灵敏度系数接近于0。实验结果表明,L1=67.1 mm,L2=8 mm,即L1/L2约为8.39时温度灵敏度系数最接近于0。表6为不同L1/L2对应的温度灵敏度系数。

      L1/L2The length of L1/mmTemperature sensitivity
      coefficient/pm·℃−1
      8.6369−2.23
      8.3967.10.15
      8.1765.351.41
      7.7862.252.4

      Table 6.  Temperature sensitivity coefficient corresponding to different L1/L2

      实验结果表明,光纤光栅滤波器的温度灵敏度系数随L1/L2的增大而降低,达到了预期效果。由图6可知,当恒温槽的温度从30 ℃加热到40 ℃时,中心波长的漂移量较大,可能是由于铝制悬臂梁传热速度较慢。前期测试实验中温度不敏感滤波器经温度跳变后,光纤光栅中心波长波动较大。经长时间观察及多次实验后发现预紧力释放实验有助于提高滤波器的稳定性。预紧力释放实验如下:将封装好的滤波器置于高温(100 ℃)与室温(约23 ℃)两个温度点间切换,约15个循环。目的是释放封装前对光纤光栅施加的部分应变力,最终光纤光栅滤波器的中心波长稳定性较好。

      Figure 6.  Fitting curve of the center wavelength of the tested fiber grating filter with temperature

      对比表4表6,发现实验测得的L1/L2的值和理论推理的有差异,可能存在以下几种原因。(1) 封装过程中胶的杨氏模量、光纤预紧力大小、胶的热固性等存在误差。(2) 环氧树脂和应变传递梁还未完全粘黏固化,导致封装后的光纤光栅的应变传递率降低。(3) 金属材料不纯,例如铝的热膨胀系数为23.2×10−6/℃,而纯铝的热膨胀系数为23×10−6/℃。(4) 应变传递梁的应变传递系数与光纤性能参数、粘接层杨氏模量和粘接两端距离有关。当光纤性能参数确定后,平均应变传递率随着粘接层材料的杨氏模量增大而增大,因此当环氧树脂胶的杨氏模量较小时,应变传递效率将低于95%。平均应变传递率随着胶厚度的增大而减小,其中理想粘接时滤波器的平均应变传递率大于98%,因此对于两点式封装应尽量减小粘接层厚度,适当地增大胶结层宽度和滤波器的粘贴长度即增大两个点胶位置与缺口长度。因此,滤波器的温度灵敏度系数可能和两个点胶位置与缺口长度的关系、使用胶的杨氏模量等有关,使得应变传递梁的传递效率达不到100%。实验结果表明,在30~60 ℃温度范围内,L1/L2=8.39,即L1=67.1 mm、L2=8 mm时光纤光栅滤波器的灵敏度系数最低为0.15 pm/℃,波长变化量仅4.5 pm,满足高精度光纤陀螺光源的应用需求。

    • 文中成功设计并制备了包含黄铜基底和铝应变传递梁的双金属温度补偿结构,因为两种金属材料的热膨胀系数不同,可以补偿光纤光栅由热光效应引起的波长漂移。分析双金属结构模型,系统研究了材料热膨胀系数、滤波器几何参数等对滤波器灵敏度的影响。研究表明,基底材料金属上两个固定点之间的长度L1和滤波光纤光栅的长度L2的比值与温度灵敏度系数呈线性关系。通过优化L1/L2,使得金属材料压缩量和光纤光栅拉伸量抵消,使滤波器的温度灵敏度系数为零。进一步通过控制变量法对比发现,控制L2调节L1时,封装操作的可调节范围更大,更利于后期滤波器的封装操作。基于双金属结构制备了尺寸为74 mm×6 mm×4 mm的温度不敏感滤波器。实验结果表明,在30~60 ℃的温度范围内,L1/L2为8.39、基底长度为67.1 mm、滤波光栅长度为8 mm时,滤波器中心波长随温度变化漂移量达到了0.15 pm/℃。该滤波器适用于较大尺寸的光纤陀螺光源,通过对滤波器尺寸的进一步优化,将适用于较小尺寸的光纤陀螺光源。

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