端面薄膜法布里-珀罗腔光纤动态压力传感器仿真与实验研究【增强内容出版】
Shock wave is a kind of compression wave in which the wavefront propagates in the form of a synoptic surface in an elastic medium. Its typical feature is the discontinuous abrupt changes of state parameters of the medium on the abrupt surface, such as pressure, density, and temperature. As the study of shock waves progresses, it has been found that shock wave technology has great civilian value, so the measurement of shock wave signals has become increasingly important. The formation and propagation of shock waves are accompanied by overpressure and rapid changes in pressure. The response speed and reliability of the corresponding pressure sensors have more demanding requirements. Traditional electrical shock wave pressure sensors are susceptible to electromagnetic interference, temperature range tolerance, rise time, and other issues, which limit the application of such sensors. Fiber-optic Fabry-Perot (F-P) pressure sensors, as an important branch of fiber-optic sensors, provide new possibilities for dynamic pressure measurement of shock waves due to their advantages of fast response speed, high sensitivity, small size, and high resistance to electromagnetic interference. To achieve the dynamic pressure measurement of shock waves, a thin-film fiber-optic F-P pressure sensor with a fiber-tip coating is studied.
The basic structure of the thin-film fiber-optic F-P sensor studied in this paper mainly consists of two gold films with different thicknesses, a layer of parylene film serving as the F-P cavity, and a single-mode optical fiber for optical field coupling. When the shock wave pressure was applied to the end surface of the sensor, the parylene film was subjected to pressure, and deformation was produced, causing a change in the F-P cavity length. This change in length then affected the interference of reflected light produced by the two gold films on the front and back surfaces of the F-P cavity. Before the sensor was fabricated, the optical and mechanical aspects of the sensor were simulated using finite element simulation software, and the performance of the sensor under different parameters was calculated by combining theoretical formulas. In addition, the parameters of the sensor were determined. After the sensor was fabricated, the static and dynamic pressure measurement system was designed and constructed, and the experimental results were analyzed.
In the pressure range of 0-60 MPa, a static pressure measurement experiment is conducted on a thin-film fiber-optic F-P pressure sensor using a bench-top oil pressure pump. The reflected spectrum signal of the sensor is obtained and processed to calculate the cavity length of the F-P cavities of different pressure sensors. From the reflectance spectrum curves (Fig. 12) of the wavelength and corresponding light intensity under different pressures, it can be seen that with increasing pressure, the overall reflectance spectrum of the sensor drifts to the left. Based on the wave valley values at different pressures, the length information of the sensor cavity corresponding to the pressure is calculated (Fig. 13), yielding wavelength sensitivity and cavity length sensitivity of the sensor of 0.0809 nm/MPa and 0.3200 nm/MPa, respectively, which are consistent with the simulation results. In the dynamic pressure measurement experiments, the sensor successfully captures the shock wave signal with a peak pressure of 7.47 MPa and a rise time of 75 ns (Fig. 15).
For measuring shock wave signals, we propose a thin-film fiber-optic F-P pressure sensor. The effective structure of the sensor is a three-layer structure consisting of gold film, polymer film, and gold film. By utilizing the change of the peak position of the sensor's reflected spectral wave, the sensor causes a change of spectral intensity, so as to realize the measurement of the signal pressure. In the pressure measurement range of 0-60 MPa, the wavelength sensitivity is 0.0809 nm/MPa, and the cavity length sensitivity is 0.3200 nm/MPa. Within the range of dynamic pressure measurement, the sensor can measure the dynamic signals with a pressure rise time of 75 ns and a pressure rise amplitude of 7.41 MPa. The experimental results show that the sensor has a large range of pressure measurement ability and high sensitivity, and it has a small size, light weight, and anti-electromagnetic interference. Therefore, the sensor has greater application prospects in the field of shock wave pressure measurement.
1 引言
冲击波是一种波面以突跃面形式在弹性介质中传播的压缩波,其典型特征是,在突跃面上,介质的压强、密度、温度等状态参数发生不连续的突跃变化。冲击波的形成与传播具有持续时间短、压力幅值变化大、上升时间快的特点[1]。弹药爆炸、子弹或战斗机以超音速运动,均会产生冲击波。冲击波对弹药的毁伤效果、子弹的飞行状态、战斗机的飞行安全等有重要影响,因此被深入研究[2]。随着对冲击波研究的深入,人们发现冲击波技术还有广泛的民用价值,已被用于石油开采、环境清洁、生物医学和材料加工等领域[3-6],因此对冲击波信号的测量也变得日益重要。
冲击波的形成与传播均伴随超压以及压强的快速变化过程,因此,对相应压力传感器的响应速度与可靠性有较为苛刻的要求。传统的冲击波压力传感器多采用电学压阻或压电式动态压力传感器[7]。这些传感器压力测量范围量程可达数十兆帕到百兆帕量级,其上升时间多在微秒到亚微秒水平,可以满足一般炸药爆炸或小口径枪炮冲击波压力的测试需求。不过,电学类冲击波压力传感器存在易受电磁干扰、耐受温度范围有限、上升时间不够短等问题,这限制了此类传感器的应用范围。光纤法布里-珀罗(F-P)压力传感器作为光纤传感器的一个重要分支,因其响应速度快、灵敏度高、尺寸小、抗电磁干扰能力强等优点,受到了研究者与技术人员的广泛关注,为冲击波的动态压力测量提供了新的可能性[8]。
光纤F-P压力传感器依赖于F-P腔的形变量实现对外界压力的测量。目前,光纤F-P压力传感器的制作工艺有微机电系统(MEMS)技术、电弧放电、化学腐蚀、化学气相沉积等[9-11]。光纤F-P压力传感器敏感元件有金属薄膜、石英膜片、有机物薄膜等[12-14]。Beard等[15]将聚合物薄膜材料作为敏感元件,以金属铝作为反射薄膜材料,覆盖在单模光纤的端面处,完成低精细度的光纤F-P传感器的制作,用于工业超声、医学超声的测量。Zou等[16]制作了二氧化硅膜片厚度为3 μm、谐振频率为4.11 MHz的光纤F-P压力传感器用于多介质中冲击波压力的测量。Cranch等[17]采用光纤端面镀膜的方式,研究了固体介质中冲击波的传播规律,确定了冲击波信号的特性。王俊杰等[18]制作了膜厚为2.9 μm的超微型全石英光纤压力传感器用于水下冲击波压力的测量。王昭等[19]以厚度为35 μm、半径为8 mm的不锈钢薄膜为膜片设计薄膜式的光纤压力传感器,用于测量冲击波反射超压峰值,所测压力信号的幅值为0.994 MPa,压力信号的上升时间为2.5 μs。这里以及下文所说的压力都表示传感器端面单位面积所受到的压力,因此单位用MPa表示。
面向冲击波信号的动态压力测量,本文研制了一种基于光纤端面镀膜的薄膜式光纤F-P压力传感器。通过磁控溅射[20]和化学气相沉积法,将不同厚度的金膜和派瑞林有机薄膜依次镀制在单模光纤端面处,制作了微型化、快响应、高精细度的薄膜型光纤F-P压力传感。派瑞林是一种高聚合物有机材料,机械性能优异、高频性能突出,并且杨氏模量较低,在同等压力作用下,会产生较大的形变量,腔长明显变化,灵敏度高。因此选用派瑞林作为传感器的压力敏感元件即F-P腔的腔体,可以实现对外界压力信号的快速响应与较高灵敏度,实现冲击波压力信号的动态测量。
2 传感器的结构与制作
所研究的薄膜式光纤F-P传感器的基本结构如
图 1. 薄膜式光纤F-P压力传感器的结构示意图
Fig. 1. Structure diagram of thin-film fiber-optic F-P pressure sensor
在传感器的三层膜结构中,金膜通过磁控溅射的方式制备,派瑞林薄膜通过化学气相沉积的方式制备。其制作步骤为:1)对一批同规格的单模光纤进行处理,除去涂覆层2~3 mm,用切割刀切割光纤端面,尽可能保证光纤端面垂直于光纤轴线;2)采用磁控溅射法在处理好的光纤端面上镀一层金膜,金膜厚度约为13 nm;3)采用化学气相沉积法在金膜外镀一层数微米厚的派瑞林有机薄膜,作为传感器的F-P腔;4)采用磁控溅射法在派瑞林薄膜外再镀一层金膜,金膜厚度约为50 nm;5)采用化学气相沉积法在金膜外再镀一层厚度约为2 μm的派瑞林薄膜作为传感器的保护膜。
采用以上制备步骤,完成了薄膜式光纤F-P传感器传感探头的制作,所制备的传感器如
图 2. 薄膜式光纤F-P压力传感器。(a)传感探头光学显微图(侧视);(b)薄膜式光纤F-P压力传感器实物图
Fig. 2. Thin-film fiber-optic F-P pressure sensor. (a) Optical micrograph of sensor head (side view); (b) physical photograph of thin-film fiber-optic F-P pressure sensor
3 传感器测量原理与仿真
3.1 测量原理
当光波由单模光纤(SMF)耦合至多层介质膜传感器时,一部分光从SMF端面处的金膜反射,另一部分光入射到F-P腔,经第二层金膜反射后再进入到SMF中。因此,双光束干涉中的反射光强
式中:
不同的频率范围,传感器的响应灵敏度有所不同[21]。在低频范围内,传感器的静压灵敏度为
式中:n为折射率;E为杨氏模量;μ为泊松比;d为聚合物F-P腔的长度;λ为激光光源的波长。
在高频范围内,传感器的压力灵敏度可以表现为派瑞林F-P腔层厚度的变化。本文所提出的薄膜式光纤F-P压力传感器的介质层可以被视为一种线性、弹性、层状介质。因此,当压力波传递至传感器时,实际的三维模型可以等效为二维模型。
根据波动方程和欧拉方程,派瑞林F-P腔层的压力
式中:A和B为每层介质中正向波和后向波振幅;k表示相关介质传输过程中声波的波数;c表示相关介质下的声波传递速度。
由
式中:
3.2 传感器仿真
利用有限元仿真软件COMSOL的波动光学模块和结构力学模块对薄膜式光纤F-P压力传感器进行仿真分析。仿真分析的基本步骤为:1)构建传感器模型,定义模型材料参数;2)调整物理场参数,添加物理场条件;3)网格划分;4)仿真计算。仿真流程如
利用波动光学模块对光纤F-P压力传感器的电磁场能量分布进行仿真,分析不同频率下传感器纤芯中的能量场分布以及传感器的反射率。为减少模型的计算量,选取SMF的纤芯直径为模型直径进行结构绘制。对所构建的传感器结构模型进行仿真,输入光谱范围设置为1400~1700 nm,从仿真结果中分别选取了1600、1543、1535.6 nm的电磁能量分布图,如
图 5. 不同波长下传感器的电磁能量分布图。(a)波长为1600 nm;(b)波长为1543 nm;(c)波长为1535.6 nm
Fig. 5. Electromagnetic energy distributions of sensor at different wavelengths. (a) At wavelength of 1600 nm; (b) at wavelength of 1543 nm; (c) at wavelength of 1535.6 nm
利用软件的后处理功能采集传感器的反射率,并利用Origin软件进行绘图,其结果如
利用结构力学模块分析薄膜式光纤F-P压力传感器受到静态压力后的薄膜形变量,计算传感器的F-P腔形变量。对建模的传感器结构模型施加边界载荷,并根据实际情况在模型的底面施加固定约束,保证计算时的稳定。传感器结构受压后的形变图如
图 7. 薄膜式光纤F-P压力传感器结构模型受压仿真图。(a)传感器结构受压形变图;(b)传感器受压三维观察图
Fig. 7. Simulation diagram of thin-film fiber-optic F-P pressure sensor structure model under pressure. (a) Pressure deformation diagram of sensor structure; (b) three-dimensional observation diagram of sensor under pressure
改变边界压力载荷和薄膜厚度,边界压力载荷从0 MPa至60 MPa,每次增加5 MPa;薄膜厚度从5 μm到15 μm,每次增加1 μm,对传感器模型进行仿真分析。对薄膜上下表面中心点处的形变量进行差值分析,计算传感器F-P腔受到压力后的形变量。仿真结果如
图 8. 不同压力载荷作用下传感器F-P腔腔长的变化量
Fig. 8. F-P cavity length variations of sensors under different pressure loads
如
传感器所能捕捉到的信号频率与传感器的谐振频率紧密相关,利用仿真软件对传感器模型进行谐振频率研究,仿真得到不同薄膜厚度的谐振频率,其结果如
从
4 实验与分析
4.1 静压压力测量实验
首先,设计并搭建了如
在1400~1700 nm波长范围内,测量未加压的薄膜式光纤F-P压力传感器的反射光谱,如
图 11. 未加载压力条件下薄膜式光纤F-P压力传感器的反射光谱
Fig. 11. Reflection spectrum of thin-film fiber-optic F-P pressure sensor without pressure loading
因为传感器结构中的金膜是通过磁控溅射工艺进行镀制的,该工艺无法精确地控制镀制金膜的沉积速率,可能会导致镀制出的金膜表面出现较大的晶粒,粗糙不平整的薄膜表面影响了光的折射与透射,进而影响干涉光谱致使实际光谱与仿真光谱存在一定差距。考虑加工工艺对F-P腔干涉的影响,将传感器光谱与第2节利用有限元仿真软件得到的反射光谱进行比较。结果发现,传感器的反射光谱与仿真得到的光谱具有一致性。
在0~60 MPa的压力范围内,利用台式油压泵对薄膜式光纤F-P压力传感器进行静态压力测量实验,获得传感器的反射光谱信号。对传感器的反射光谱信号进行处理,计算不同压力传感器F-P腔的腔长值。
不同压力下传感器的波长和对应光强的反射光谱曲线如
图 12. 不同压力条件下薄膜式光纤F-P压力传感器的反射光谱
Fig. 12. Reflection spectra of thin-film fiber-optic F-P pressure sensor under different pressures
根据实验中采集到的反射光谱信号,构造一个与反射光谱相似的模板函数,对模板函数与反射光谱信号进行互相关运算计算出传感器对应压力下的腔长的信息,如下式所示:
式中:
图 13. 薄膜式光纤F-P压力传感器的压力与波长的关系
Fig. 13. Relationship between pressure and wavelength of thin film fiber-optic F-P pressure sensor
如
如
静态压力测量实验说明传感器可以承受60 MPa的压力。对传感器的加压实验过程中,当压力从0 MPa增加至60.04 MPa时,传感器的腔长减小18.8 nm,反射光谱漂移了4.77 nm。即压力每增加1 MPa,腔长减小0.3200 nm,波长变化0.0809 nm。
在第2节结构力学的有限元仿真中,当压力从0 MPa增加至60.04 MPa时,传感器的F-P腔腔长值减小了15.9 nm。即压力每增加1 MPa,传感器的腔长减小0.2650 nm。仿真结果与实验结果进行比较,在每兆帕压力下,腔长减小量两者相差0.0550 nm。考虑外层派瑞林保护层,以及制作工艺的不确定性,派瑞林整体保护层的厚度要大于腔长值。那么,该传感器在60 MPa压力作用下,腔长减小范围应在15.9~21.3 nm。计算得到的18.8 nm处于仿真范围内,因此实验结果与仿真结果保持一致。
4.2 动态压力测量实验
在动态压力测量实验中,利用激光诱导等离子体技术将灯泵YAG激光器出射的短脉冲高峰值激光通过透镜聚焦在合金靶材上。激光使靶材产生电离并产生微小的爆炸生成冲击波信号。实验采用强度法对传感器采集信号进行解调,实验装置原理图如
图 14. 动态压力测量实验装置原理图
Fig. 14. Schematic diagram of dynamic pressure measuring experimental setup
传感器在0.622 μs时候被触发,并在示波器上产生一个脉冲信号。在时间为0.639 μs时,信号幅值为最大幅值的10%,在时间为0.714 μs时,信号幅值为最大幅值的90%。信号变化过程中,电压值变化了0.32 V。根据强度法,通过示波器采集的两路电压值相除可以得到传感器的反射率,进而可通过
5 结论
针对冲击波信号的测量,本文提出了一种薄膜式光纤F-P压力传感器,该传感器的有效结构由金膜-聚合物薄膜-金膜三层膜结构组成,利用传感器反射光谱波谷位置的变化,引起光谱强度的变化实现对信号压力的测量。在0~60 MPa压力测量范围内,传感器的波长灵敏度为0.0809 nm/MPa,腔长灵敏度为0.3200 nm/MPa。在动态压力测量范围内,传感器可以实现对压力上升时间为75 ns、压力上升幅值为7.41 MPa的动态信号的测量。
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