基于法布里-珀罗干涉和反共振效应的光纤温湿度传感器
The temperature and relative humidity (RH) measurement plays a crucial role in various fields such as food processing, environmental monitoring, and biomedical applications. Fiber optic sensors have been extensively studied due to their prominent advantages of low cost, small size, strong immunity to electromagnetic interference, and high sensitivity compared to traditional electronic hygrometers. Fiber optic sensors based on the Fabry-Perot interferometer (FPI) structure feature simple fabrication process and stable performance. However, previously proposed fiber optic temperature and humidity sensors exhibit low sensitivity or are limited to single-parameter measurements. Therefore, the development of temperature and humidity dual-parameter sensors with stable performance and high sensitivity holds practical significance. The cascaded structure allows for the connection of multiple fiber optic sensing units, enabling simultaneous multi-parameter measurement. The vernier effect is utilized to amplify the humidity measurement sensitivity, while the anti-resonance (AR) effect of polymer-coated no-core fiber (NCF) provides high temperature measurement sensitivity. We propose and fabricate an FPI cascaded AR temperature and humidity dual-parameter fiber optic sensor, which further enhances the temperature and humidity measurement sensitivity and eliminates the need for complex FFT demodulation processes.
The sensor is composed of cascading an Fabry-Perot (FP) cavity and an NCF with a polymer coating. Meanwhile, the single mode fiber (SMF) and the NCF with polyimide (PI) cured on its end-face are placed into a non-enclosed silicon tube. The non-enclosed silicon allows the humidity-sensitive material PI to have sufficient contact with water molecules in the air. The air cavity is selected as the sensing cavity, while the air-PI mixed cavity serves as the reference cavity. The superimposition of the spectra from the air cavity and the air-PI mixed cavity produces a vernier effect, enabling easy and highly sensitive RH measurement by tracking the spectral envelope of the sensor. The light transmitted through the FP structure further transmits into the NCF. In the NCF segment coated with acrylic resin, the refractive index of the coating is higher than that of the cladding, which causes partial reflection of the light at the interface between the coating and NCF, while the rest is refracted into the coating and reflected at the coating-air interface, which creates MPI inducing the AR effect in the coated NCF. When the external temperature changes, the refractive index of the coating alters, resulting in a wavelength shift of the non-transmitted light for temperature measurement. Real-time temperature and RH can be calculated by adopting a decoupling equation system. The characteristic wavelengths of the reflected and transmitted spectra are measured, and the temperature and RH are simultaneously changed to validate the accuracy of the calculation formula. Additionally, error analysis is performed on the experimental results based on the set standard values, with RH and temperature relative errors of 0.74% and 0.19% respectively, which indicates that the sensor has a certain level of practicality.
Under the temperature of 29.5 ℃, the sensor's spectral drift characteristics are tested as the RH increases from 10% to 80% (Fig. 5). As the humidity grows, the resonance wavelength of the FPI envelope shifts towards longer wavelengths, with the RH sensitivity of 510.25 pm/%. The characteristic wavelength of the AR spectrum shows a red shift in the range of 10% to 60% and a blue shift in the range of 60% to 80%. With the humidity kept at 33%, the temperature is increased from 26 ℃ to 35 ℃, and the interference spectrum wavelength is recorded every 1 ℃ (Fig. 6). The envelope wavelength of FPI interference spectra remains unchanged with the rising temperature. Due to the high thermo-optic coefficient of the acrylic resin, the temperature change alters the refractive index of the coating, affecting the non-transmitted wavelength. The characteristic wavelength of the AR spectrum shows a blue shift with increasing temperature and exhibits amplitude variations with a slope of -4.48 nm/℃.
We present a high-sensitivity cascade sensor based on FPI and AR effects for temperature and RH measurement. The sensor incorporates an SMF with a PI film-coated NCF that is inserted into an open-ended silicon tube. By superimposing two FPI spectra with similar optical paths, a vernier effect is generated to amplify the sensor's low sensitivity. The high-sensitivity RH measurement is achieved by detecting the envelope movement. The AR effect is formed by the optical coupling of the NCF high refractive index acrylic resin coating and the low refractive index cladding, which produces a shift in the non-transmitted wavelength due to the refractive index change of the coating caused by temperature. The experimental results show that within the range of 10% to 80%, the RH sensitivity is 510.25 pm/%, which amplifies the original sensitivity by approximately five times. In the range of 26 ℃ to 35 ℃, the temperature sensitivity is -4.48 nm/℃. In summary, the proposed sensor features simple fabrication and high sensitivity and holds potential practical significance in such fields as health monitoring and biomedical applications.
1 引言
相对湿度(RH)的精确测量在食品加工、环境监测和生物医学等领域具有十分重要的作用和意义[1-2]。相较于传统的电子湿度计,光纤传感器因其成本低、体积小、电磁抗干扰性强、灵敏度高等突出优点而得到深入研究[3-4]。其中,基于法布里-珀罗干涉仪(FPI)[5-6]结构的光纤传感器具有工艺简单、性能稳定等优点,尤其对引起法布里-珀罗(FP)腔长度或者折射率变化的物理量非常敏感,被广泛应用于高灵敏度高精度的湿度测量[7-8]。温度作为重要的物理参量,其变化会直接影响相对湿度的平衡状态[9],实际应用中往往需要对温度、相对湿度进行双参数的同步测量。在测量相对湿度的FPI传感器中引入只对温度敏感而对湿度不敏感的光纤布拉格光栅(FBG)[10-12]成为实现温湿度双参数测量的重要技术手段。2018年,Wang等[13]报道了一种FBG级联聚酰亚胺(PI)填充中空毛细管尖端的FPI光纤传感器,实验测得的相对湿度和温度灵敏度分别为22.07 pm/%和9.98 pm/℃。然而,FBG传感器受到光纤材料自身低热膨胀系数的影响,其温度灵敏度的进一步提高受到限制。2020年,Tong等[14]提出一种马赫-曾德尔干涉仪(MZI)[15-16]和FPI结合的温湿度双参数光纤传感器,将一段单模光纤(SMF)拼接在两个具有大芯偏移的SMF之间,用相对湿度敏感材料石墨烯量子点和聚乙烯醇(GQDs-PVA)包覆,通过监测FPI和MZI的光谱实现温湿度的同时检测,其相对湿度灵敏度和温度灵敏度分别提高到132 pm/%和370 pm/℃。由于该传感器基于单独的湿敏材料实现温湿度测量,因此其温度敏感特性相对较弱。2022年,Li等[17]提出一种基于级联C型光纤的FPI温湿度传感器,在SMF之间拼接两段C型光纤,两段C型光纤内分别填充了对温度和湿度具有不同敏感性的聚二甲基硅氧烷(PDMS)和聚乙烯醇(PVA)材料,温度和相对湿度灵敏度分别达到-722 pm/℃和-128 pm/%。虽然该传感器在结构上采用不同的敏感材料来提升温度灵敏度,但与前述研究相比,其相对湿度灵敏度偏低,并且在解调过程中需要额外进行滤波以处理光谱数据。
本文提出并制备了一种基于FPI和反共振(AR)效应[18-19]的级联光纤温湿度传感器。该传感器由空气、PI腔的FP结构和一段带丙烯酸树脂涂层的无芯光纤(NCF)级联而成,利用空气腔和空气-PI混合腔的干涉光谱叠加产生游标效应[20],提升相对湿度检测灵敏度;NCF涂层和包层的光耦合产生AR效应,利用涂层材料对温度的敏感特性,实现温度的高灵敏度测量。此外,该传感器通过分别监测FPI反射光谱和AR透射光谱实现温湿度的同时测量,极大地简化了光谱数据处理和系统的复杂度。所提出的传感器具有结构简单、成本低、灵敏度高等显著优势,在高精度的温湿度监测方面具有重要应用。
2 传感器的设计与制备
2.1 传感器的结构及传感原理
根据三光束干涉分析传感原理,FPI的总强度[24]可表示为
式中:A1、A2、A3为反射光振幅;λ为真空中的波长;φ1、φ2分别为空气腔、PI腔的相移。
式中:LAir和LPI分别为空气腔、PI腔的长度;nAir和nPI分别表示空气腔和PI腔的折射率。3个FP腔干涉条纹的FSR表示为
在FPI结构中,具有相似光程的两个FP腔的光谱干涉会产生较大的游标效应包络。随着外界湿度的升高,PI腔体积迅速膨胀,空气腔FP1的腔长减小,干涉谱发生偏移。因此选择FP1作为传感腔,FP3作为参考腔,包络的FSR[25](dFSR,e)为
FPI的放大系数M对应于周期的放大倍数[26],表示为
因此可以通过监测包络线的位移来获得相对湿度灵敏度,包络灵敏度
式中:
在NCF部分,对于裸NCF段,MMI起主导作用,MMI器件的峰值波长[27]为
式中:neff=nCladding sin θn为NCF的有效RI;DNCF和L1分别为NCF包层的直径和长度;p为自像数。基于MMI效应的NCF透射强度[27]可表示为
式中:η0,n表示通过SMF的基模与NCF中L1P0,n模之间的能量耦合系数。对于涂层的NCF截面,可以认为它是由低折射率二氧化硅被高折射率聚合物包围而形成的波导。处于AR状态的光由于相消干涉在包层中传输,表现为光谱的高透射区。满足共振条件的波长将沿着聚合物涂层向前传播并泄漏出去,非透射波长[28]可写为
式中:d为涂层的厚度;nCoating和nCladding分别为NCF涂层和包层的RI;N为共振阶数。忽略温度对二氧化硅包层的影响,AR结构的温度灵敏度可表示为
由此可见,传感器的温度敏感性主要取决于NCF聚合物涂层的性质。由AR效应引起的涂层NCF的透射强度[29]表示为
式中:
式中:A和B分别定义为AR和MMI效应的强度系数,且A+B=1。
2.2 传感器的制备
游标效应参考信号与传感信号的FSR差值越小,包络的FSR越大,但dFSR,1与dFSR,3过于接近,包络线就会太大而超出仪器的检测范围。NCF越长,AR的干涉谷越尖锐,但也更容易受到应力的影响。综合考虑后选择长度为136 μm的空气腔、16 μm的PI腔以及12.5 mm的NCF。光纤传感器的制作过程如
图 2. 级联传感器的制作流程图。(a)HF溶液腐蚀硅管图;(b)SMF和带涂层的NCF熔接图;(c)NCF端面涂覆PI图;(d)光纤插入硅管图;(e)FPI级联AR结构图
Fig. 2. Manufacturing process diagrams of cascaded sensors. (a) Diagram of silicon tube corroded by HF solution; (b) diagram of fusion between SMF and NCF with coating; (c) diagram of PI coated NCF end face; (d) diagram of fiber insertion into silicon tube; (e) diagram of FPI and AR structure cascade
3 实验结果与分析
传感器的实物图和性能测试的实验装置如
图 3. 传感器显微图和测试装置示意图。(a)传感器显微图;(b)实验测试装置图
Fig. 3. Sensor micrograph and test setup diagram. (a) Micrograph of sensor; (b) experimental test setup diagram
图 4. 游标效应。(a)反射光谱;(b)干涉谱的FFT结果
Fig. 4. Vernier effect. (a) Reflectance spectrum; (b) FFT result of interference spectrum
3.1 湿度传感实验
在温度为29.5 ℃的实验条件下,相对湿度从10%逐步上升至80%,每隔10%记录一次特征波长,并对这些数据进行拟合,传感器对相对湿度的响应特性如
图 5. 相对湿度响应特性。(a)干涉谱包络;(b)包络位移曲线拟合;(c)透射光谱;(d)非透射波长位移曲线拟合
Fig. 5. Relative humidity response characteristics. (a) Envelope of the interference spectra; (b) fitting of envelope displacement curve; (c) transmittance spectra; (d) fitting of non-transmission wavelength displacement curve
3.2 温度传感实验
在相对湿度为33%的实验条件下,温度从26 ℃升高至35 ℃,每隔1 ℃记录一次特征波长,并对这些数据进行拟合,传感器对温度的响应特性如
图 6. 温度响应特性。(a)干涉谱包络;(b)包络位移曲线拟合;(c)透射光谱;(d)非透射波长位移曲线拟合
Fig. 6. Temperature response characteristics. (a) Envelope of interference spectra; (b) fitting of envelope displacement curve; (c) transmittance spectra; (d) fitting of non-transmission wavelength displacement curve
3.3 实验分析
当温度为29.5 ℃时,相对湿度与FPI包络共振波长和AR非透射波长的函数关系为
当相对湿度为33%时,温度对FPI包络共振波长没有影响,和AR非透射波长的函数关系为
以温度为29.5 ℃时的测量值为基准,根据上述实验结果和数据拟合,FPI包络共振波长和非透射波长在光谱中的位置为
利用光谱仪读出反射谱的λFPI和透射谱的λAR,将实验结果代入
为了验证计算公式的准确性,同时改变温度和相对湿度,通过测量反射谱和透射谱的特征波长来获得温度和相对湿度。在温度为27.5 ℃、湿度为30%的初始条件下,将温度升高至32 ℃,湿度升高至55%,实验结果如
图 7. 计算公式测试结果。(a)反射谱;(b)透射谱
Fig. 7. Calculation test results. (a) Reflection spectra; (b) transmission spectra
为了对传感器的稳定性进行测试,将传感器放在温度为30 ℃、相对湿度为30%的环境中,60 min内每15 min记录一次特征波长的位置,结果如
图 8. 稳定性测试结果。(a)(c)特征光谱变化;(b)(d)波长位移
Fig. 8. Stability test results. (a)(c) Characteristic spectral changes; (b)(d) shift of wavelength position
为了评估传感器的重复性,首先在29.5 ℃下,在湿度从10%增加到90%的过程中进行3次测试。FPI包络共振波长和AR非透射波长的位置如
图 9. 重复性测试结果。(a)(b)湿度重复性试验;(c)(d)温度重复性试验
Fig. 9. Reproducibility test results. (a)(b) Humidity repeatability test results; (c)(d) temperature repeatability test results
将实验结果与其他的光纤温湿度传感器性能进行比较,如
表 1. 温湿度同时测量的相关光纤传感器性能比较
Table 1. Performance comparison of fiber optic sensors for simultaneous measurement of temperature and humidity
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4 结论
提出一种基于FPI和AR效应的高灵敏度级联传感器,并用于温度和相对湿度测量。该传感器把SMF和端面固化了PI膜的带涂层NCF插入非封闭的硅管中,叠加两个相似光程的FPI光谱以产生游标效应,对传感器的低灵敏度进行放大,通过检测包络的移动来实现相对湿度的高灵敏度测量。NCF高折射率丙烯酸树脂涂层和低折射率包层的光耦合形成AR效应,利用温度引起涂层折射率的改变导致AR非透射波长产生漂移,实现对温度的高灵敏度测量。实验结果表明:在10%~80%的相对湿度范围内,相对湿度灵敏度为510.25 pm/%,相较于原来的灵敏度放大了约5倍;在26~35 ℃的温度范围内,温度灵敏度为-4.48 nm/℃。所提的传感器具有制作简单、灵敏度高等优点,在健康监测、生物医学等领域具有重要的实用价值。
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