基于拉锥七芯光纤的湿度传感器研究 
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Objective The monitoring of relative humidity is very important in the fields of agriculture, medical treatment and biochemical research, which urges scholars to develop various humidity sensors. Among many humidity sensors, optical fiber humidity sensor has become a research hotspot owing to their unique advantages such as high sensitivity, anti-electromagnetic interference, compact structure and other unique advantages. There are many kinds of optical fiber humidity sensors, and the interferometric humidity sensor has attracted wide attention because of its advantages of simple preparation and high sensitivity, among which the humidity sensor based on Mach-Zehnder interferometer (MZI) is widely used. However, the sensitivity and stability of these sensors still need to be further improved. In order to improve the sensitivity of the sensor and maintain its stability, a humidity sensor based on seven core tapered fiber is proposed and demonstrated. The sensor consists of a short section of seven core fiber between two single-mode fibers, in which the seven core fiber is fused and tapered by a hydrogen oxygen flame to form a tapered structure. Mach-Zehnder interferometer is formed by interference between the base mode of the cladding and the higher mode excited after the fiber is tapered. The effective refractive index of the cladding mode is easily affected by the external environment parameters. Therefore, the structure is very sensitive to the changes of the external environment parameters. The proposed structure is particularly suitable for the situations where high measurement sensitivity and high stability are required.
Methods Firstly, the seven core fiber with a length of about 1 cm is fused between two single-mode fibers. Then, the seven core fiber in the sensing structure is fused and tapered by a fiber taper machine (Kaipule Co. Ltd. AFBT-8000MX-H). Finally, GO film is coated on the surface of the tapered area by photothermal method. The coating process is as follows: the GO prepared by the improved Hummers method is mixed into a solution with the concentration of 1 mg/mL, and the GO solution is dropped on the surface of the fiber. In the coating process, the SLED broadband light source is used to transmit the light in the sensing structure. When the exciting light passes through the tapered fiber, a part of the light enters the cladding and generates a lot of heat in the cladding. GO molecules can be firmly adsorbed on the fiber surface by using the photothermal effect of laser, and thus the film is uniform and firm.
Results and Discussions Firstly, the wavelength scanning function of Rsoft software is used to calculate the sensor uncoated with diameter of 10 μm. The refractive index sensitivity of sensor is about 1200 nm/RIU (Fig. 4), which proves the feasibility of the experiment and provide a theoretical basis for humidity measurement. In order to provide experimental basis for subsequent humidity test, the refractive index response of samples with different diameters of uncoated GO was tested. The refractive index sensitivities of the samples with the diameter of 15 μm (s-1), 12 μm (s-2) and 10 μm (s-3) were 685 nm/RIU, 753 nm/RIU and 1123 nm/RIU, respectively (Fig. 7). Thus, the refractive index sensitivity of the sample can be greatly improved by increasing the stretching length to reduce the tapered diameter of the seven core fiber. Then, the s-4 was prepared under the same parameters as the sample with the highest sensitivity. And the GO film was coated on the surface of s-4 to prepare a humidity sensor. The experimental results show that the humidity sensitivity of sample s-4 is the highest at 1533 nm, and the maximum sensitivity is -0.0535 nm/(%RH) (Fig. 10). And the humidity sensitivity of the sensor with diameter of 14 μm (s-5) is -0.0173 nm/(%RH) (Fig. 11). Thus, the humidity sensitivity can be increased by reducing the sample diameter. In addition, we also evaluated the stability of s-4. When the relative humidity is 34.8%RH, 45.0%RH and 60.3%RH, the maximum error of the sensor wavelength is 0.03 nm, 0.04 nm and 0.04 nm, respectively, which indicates that the proposed sensor has good stability (Fig. 12).
Conclusions In summary, we propose and demonstrate a humidity sensor based on tapered seven core fiber, and the seven core fiber is fused and tapered by a hydrogen oxygen flame to form a tapered structure. The experimental results show that the refractive index sensitivity is up to 1123 nm/RIU for the sensor uncoated with taper waist diameter of 10 μm, which is consistent with the simulation results. Then the humidity sensor was fabricated by coating a layer of GO film on the surface of fiber. The maximum humidity sensitivity of the sensor was -0.0535 nm/(%RH), and the linearity was 98.5%. The sensor has the advantages of high sensitivity, simple preparation and good stability, which can be used in the field of humidity sensing.
1 引言
相对湿度的监测在农业、医疗、生物化学研究等领域至关重要,故各种测试湿度的传感器已被研制出来,如传统的机械湿度计、基于电容和电阻的湿度传感器、光纤湿度传感器等[1-5]。在这些湿度传感器中,光纤湿度传感器因其具有灵敏度高、抗电磁干扰、结构紧凑等独特优势成为研究热点[6-9]。
光纤湿度传感器的种类繁多,常见的有光栅型光纤湿度传感器、环形谐振腔湿度传感器、干涉型湿度传感器等[10-12]。其中:光栅型湿度传感器结构稳定,但其传感原理的客观因素导致其灵敏度普遍较低;环形谐振腔湿度传感器一般是基于微光纤来制备的,制备过程复杂且不利于湿敏材料的涂覆,从而限制了其灵敏度的提升[11,13-14];而干涉型湿度传感器因其具有制备简单、灵敏度高等优势,引起了研究者的广泛关注。干涉型湿度传感器按照干涉仪结构可以分为法布里-珀罗干涉仪(FPI)、萨格纳克干涉仪(Sagnac)、迈克耳孙干涉仪(MI)、马赫-曾德尔干涉仪(MZI)等[15-19]。如邵敏等[19]在单模光纤(SMF)的一端熔接一段光子晶体光纤(PCF),其中熔接点为光纤粗锥,构成MI, 在30%~90%的相对湿度范围内,传感器的湿度灵敏度为-0.095 dB/(%RH)。在这些干涉型湿度传感器中,基于MZI的湿度传感器应用比较广泛,Liu等[20]将一段单模光纤与另外两段单模光纤错芯焊接并在表面涂覆氧化石墨烯(GO),制成基于MZI的湿度传感器,在相对湿度为30%RH~60%RH范围内其灵敏度为0.0272 nm/(%RH)。除此之外,对光纤进行熔融拉锥形成模间干涉是一种比较简单的制备MZI的方法。比如2016年Soltanian等[21]制备了两个基于MZI的湿度传感器,传感器由单模光纤拉锥成的两个锥形区域组成,在0%RH~90%RH的湿度范围内,锥区平均直径为4.05 μm的传感器的灵敏度为0.02 nm/(%RH),锥区平均直径为2.89 μm的传感器的灵敏度为0.01 nm/(%RH)。但是该传感器有两个直径较细的锥区部分,这给实验操作增加了难度,并且该传感器的灵敏度需要进一步提高。
本文设计制备了一种基于拉锥七芯光纤(TSCF)的湿度传感器,该传感器由单模光纤、七芯光纤、单模光纤级联熔接而成。利用光纤熔融拉锥技术对七芯光纤进行熔融拉锥,光纤拉锥后因其激发的包层基模和高阶模之间发生干涉形成MZI,模式的有效折射率容易受外界环境参量的影响,因此该结构对外界环境参量的变化十分敏感。七芯光纤由于纤芯比较多,其稳定性提升,解决了基于拉锥光纤的湿度传感器灵敏度低且稳定性差的问题。实验测试了直径不同的样品的折射率灵敏度,结果表明样品直径越小折射率灵敏度越高。最后用灵敏度最高的样品的参数制备了另一种传感结构并在其锥区表面镀GO薄膜,测试了该样品的湿度灵敏度和稳定性。该湿度传感器制备简单、灵敏度高、稳定性强,可用于环境中相对湿度的监测。
2 结构制备和理论研究
2.1 理论研究
基于拉锥七芯光纤的湿度传感器传感结构示意图如
图 1. 传感结构示意图。(a)传感器结构;(b)七芯光纤锥区显微镜图;(c)七芯光纤拉锥前横截面图
Fig. 1. Schematic diagrams of sensing structure. (a) Sensor structure; (b) microscope of the tapered area of TSCF; (c) cross section of seven core fiber untapered
所提出的传感器的光谱特性可以用双光束干涉模型来描述[22],表达式为
式中φ=2πΔneffL/λ为包层基模和高阶模的相位差,I1和I2为包层基模和高阶模的光强,λ为工作波长,L为七芯光纤锥区长度,Δneff=
当外界环境折射率发生变化时,包层基模和高阶模之间的有效折射率差发生改变,谐振波谷发生漂移,漂移为
其中δn是Δneff的变化值。当外界环境相对湿度增加时,GO膜会吸收更多的水分子,被吸收的水分子将附着在GO表面或对GO层的切片进行填充,使GO表面载流子密度增加,导致GO的电导率下降, GO的电导率σ与化学势μc之间的关系为[23-25]
其中,e,kB,T,Γ,h分别为电子的电荷、玻尔兹曼常数、环境温度、振动频率和普朗克常数。当水分子附着在GO上时,GO的表面电荷载流子密度增加,GO的费米能级增加,导致带间跃迁受阻,电导率σ降低,因此可得nGO∝σ∝
图 2. 模式有效折射率随外界环境折射率变化图。 (a)HE11; (b)HE12; (c)HE13
Fig. 2. Variation of mode effective refractive index with external refractive index. (a) HE11; (b) HE12; (c) HE13
利用Comsol Multiphysics软件,基于有限元法对拉锥后七芯光纤的模场特性进行了模拟。七芯光纤纤芯和包层的折射率分别为1.4501和1.4449,包层直径和纤芯直径等比例缩小。
为了分析光在拉锥七芯光纤中的传输特点,用Rsoft软件的Beam PROP功能对拉锥七芯光纤内光的传输行为进行模拟仿真,分析光在拉锥七芯光纤中的传输特点。单模光纤的纤芯和包层直径分别设置为9 μm和125 μm,七芯光纤的包层直径和单模光纤一样,都为125 μm,七个纤芯的直径相同,为6.4 μm,纤芯和包层折射率分别为1.4501和1.4449,七芯光纤锥区的直径为10 μm,输入光的波长为1550 nm。通过仿真计算得到光经过拉锥七芯光纤的能量分布情况,如
图 3. 直径为10 μm的七芯光纤中的能量分布
Fig. 3. Energy distribution in seven core fiber with diameter of 10 μm
为了进一步阐述拉锥七芯光纤模式干涉仪的光谱特性,利用Rsoft软件的波长扫描功能对直径为10 μm的七芯光纤在不同环境折射率条件下的归一化光功率与波长之间的关系进行模拟仿真,仿真中选取波长范围1450~1650 nm进行研究,七芯光纤直径为10 μm。仿真计算得到的干涉光谱如
图 4. 锥形光纤输出归一化光功率在不同环境折射率下随波长的变化关系
Fig. 4. Relationship between the output normalized optical power of tapered fiber and wavelength at different refractive indexes
2.2 样品结构制备
首先将长度为1 cm左右的七芯光纤熔接在两个单模光纤的中间,之后通过光纤熔融拉锥机(Kaipule Co. Ltd. AFBT-8000MX-H)对传感结构中的七芯光纤进行熔融拉锥,最后利用光压光热法在七芯光纤锥区表面涂覆GO薄膜。镀膜过程如下:将由改进的Hummers法制备的氧化石墨烯配制成质量浓度为1 mg/mL的溶液,用超声波清洗机超声30 min左右,将溶液滴在七芯光纤锥区表面,使用宽带光源(SLED)对传感结构通光。当激光通过七芯光纤锥区时,一部分光进入包层,并在包层中产生大量的热量,利用激光的这种光压光热效应,光纤表面可以牢牢地吸附住氧化石墨烯分子,因此获得的薄膜均匀牢固。
图 5. 扫描电镜图。(a)氧化石墨烯;(b)未涂覆氧化石墨烯的光纤;(c)涂覆氧化石墨烯后的光纤;(d)涂覆氧化石墨烯后的光纤局部放大图
Fig. 5. Scanning electron microscopes. (a) Graphene oxide; (b) uncoated optical fiber; (c) optical fiber coated with graphene oxide; (d) partially enlarged optical fiber coated with graphene oxide
传感器拉锥前后及涂覆GO薄膜前后的透射光谱如
图 6. 传感结构拉锥前后及涂覆GO前后的透射光谱图对比
Fig. 6. Transmission spectra of sensor structure before and after tapering and GO coating
3 实验结果和讨论
3.1 传感器镀膜前的折射率响应特性
为了对后续湿度测试提供实验基础,对不同直径、未涂覆GO的样品进行折射率响应实验测试,样品s-1、s-2、s-3的直径分别为15 μm、12 μm、10 μm,将样品放置在充满氯化钠(NaCl)液体的盒子中,通过改变液体折射率来测试样品对外界环境折射率变化的响应。样品的折射率响应实验步骤如下:先将适量的NaCl固体加入去离子水中配制成一定浓度的NaCl溶液,再将样品放入待测溶液中固定,实验过程中通过向NaCl溶液中加入一定量的去离子水改变NaCl溶液的浓度,从而使待测溶液的折射率发生改变,整个实验过程中由光谱仪(OSA)监测透射光谱并保存相应图谱。通过理论计算得到液体浓度及其对应的折射率,并通过阿贝折射仪进行折射率标定。
三个不同直径样品的折射率响应如
图 7. 不同直径样品的折射率灵敏度。(a)(b)样品s-1;(c)(d)样品s-2;(e)(f)样品s-3
Fig. 7. Refractive index sensitivity of samples with different diameters. (a)(b) Sample s-1; (c)(d) sample s-2; (e)(f) sample s-3
3.2 传感器镀膜后的湿度响应特性
由3.1节得出,七芯光纤锥区直径为10 μm的传感结构即样品s-3的折射率灵敏度最大,在与s-3同样参数情况下制备样品s-4,在其表面涂覆GO薄膜,并对所制备的湿度传感器进行湿度测量,湿度测试系统实验装置如
图 9. 样品s-4在不同相对湿度下的透射光谱
Fig. 9. Transmission spectra of sample s-4 at different relative humidity
图 10. 样品s-4不同谐振波谷随湿度变化的光谱响应。(a)(b)Dip A; (c)(d)Dip B; (e)(f)Dip C
Fig. 10. Spectral response of different resonant waves of sample s-4 varying with humidity. (a)(b) Dip A; (c)(d) Dip B; (e)(f) Dip C
为了探讨直径对湿度传感器灵敏度的影响,用同样的方法制备了直径为14 μm的样品(s-5),其湿度响应特性如
图 11. 样品s-5的湿度响应。(a)透射谱;(b)湿度灵敏度
Fig. 11. Humidity response of sample s-5. (a) Transmission; (b) sensitivity of relative humidity
稳定性是评价相对湿度传感器性能的一个重要指标。样品s-4直径最细,湿度灵敏度最高,因此我们测试了样品s-4的稳定性。在34.8%RH、45.0%RH和60.3%RH的三个固定相对湿度水平下测试了传感器的稳定性。在不同湿度环境下,样品每隔10 min记录一次光谱,记录了样品1 h内的光谱变化。
图 12. 样品s-4的在不同相对湿度下的稳定性测试
Fig. 12. Stability test of sample s-4 under different relative humidity
4 结论
设计制备了一种基于拉锥七芯光纤的湿度传感器。七芯光纤熔接在两段单模光纤中间,利用熔融拉锥技术对七芯光纤进行熔融拉锥,并对该结构进行理论分析和实验研究,实验测得当七芯光纤直径为10 μm左右时折射率灵敏度达到1123 nm/RIU,实验结果与仿真计算结果一致。此外,通过在七芯光纤锥区表面涂覆一层GO薄膜制得湿度传感器,实验测得传感器湿度灵敏度最大为-0.0535 nm/(%RH), 线性度为98.5%。本文提出的传感器,灵敏度高、制备简单、稳定性好,可应用于湿度传感领域。
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