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Sensitivity enhancement of optical fiber vibration sensor through encapsulation of acoustic Helmholtz resonator
Optik - International Journal for Light and Electron Optics 169 (2018) 33–40
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Sensitivity enhancement of optical fiber vibration sensor through encapsulation of acoustic Helmholtz resonator ⁎
T
⁎
Renxi Gaoa, , Hongri Wanga, Dezhi Zhua, Guanghua Fana, , Weiyan Jiaoa, Chunyan Lianga, Yi Liua, Yingying Wanga, Wenjun Liua, Xiongwei Hea, Jiameng Liua, Guannan Weia, Lizhong Songb, Zhanfeng Zhaob a b
Department of Optoelectronic Science, Harbin Institute of Technology at Weihai, Weihai 264209, China School of Information and Electrical Engineering, Harbin Institute of Technology at Weihai, Weihai 264209, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Acoustic Helmholtz resonators Optical fiber vibration sensor Sensitivity enhancement Encapsulation of sensor
Optical fiber sensors should be encapsulated to prevent the possible damage in practical applications. Moreover, the encapsulation should be optimized to increase the sensitivity of the sensor as much as possible. Herein three different Helmholtz resonators are designed to encapsulate the singlemode-multimode-singlemode (SMS) fiber vibration sensor. Finite element analyses exhibit the acoustic pressure distributions in the resonators. The SMS fiber structures are fixed in the resonators with different schemes. Experiments demonstrate that the encapsulations increase the sensitivity of the sensors, which are determined by the fixing schemes, the acoustic pressure distributions, and the structures of the resonators. The combined resonator with two coaxially attached cavities renders the sensor a response range from 20 to 1000 Hz, which possesses a higher sensitivity and wider frequency response range than the sensor with a single cavity of resonator. The sensitivity of the encapsulated sensor with the combined resonator is 2.11 times of the sensitivity of the un-encapsulated sensor at 120 Hz. The SMS sensor encapsulated by the combined resonator exhibits a higher sensitivity than a commercial microphone, which indicates a favorable practicability of the Helmholtz resonator to the encapsulation of various kinds of fiber vibration sensors.
1. Introduction Optical fiber possesses the merits of low loss of signal in the long distance transmission, high stability, anti-electromagnetic interference, anti-chemical corrosion, and good adaptation for integrations in many circumstances such as communication, industry, civil engineering, and military affairs. Due to these merits, the constructions of novel and practical fiber sensors are important works for the scientific and industrial communities, which are always pursued by researchers. Recently, fiber vibration sensor attracts increasing attentions in the field of acoustic vibration sensing due to its high sensitivity and wide frequency response [1]. Specific tasks in the research of fiber vibration sensor include the seeking of sensitive and effective sensing structures, and the encapsulation of the fiber sensor to prevent the possible damages in practical applications. Note that the encapsulation of the fiber sensor should increase the sensitivity of the sensor system as much as possible. Up to now, the optical fiber sensing structures that are applied to perform the acoustic vibration sensing include the fiber Bragg grating [2], the Fabry-Perot interferometer [3], the Mach-Zehnder interferometer [4], the tapered fiber structure [5,6], the singlemode-multimode-singlemode (SMS) fiber structure [7] and so on.
⁎
Corresponding authors. E-mail addresses: [email protected] (R. Gao), [email protected], [email protected] (G. Fan).
https://doi.org/10.1016/j.ijleo.2018.05.018 Received 6 February 2018; Received in revised form 2 May 2018; Accepted 3 May 2018 0030-4026/ © 2018 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 169 (2018) 33–40
R. Gao et al.
Fig. 1. (a) The schematic diagram of the SMS fiber. (b) The simulation of beam propagation in the SMS fiber, (c) The schematic diagram of the SMS fiber after the acid etch.
For the encapsulation of the fiber vibration sensor, there are some relevant works. S. Foster and co-workers packed a fiber vibration sensor into a capillary tube, which constructed a hydrophone that possessed a response range from 30 to 7000 Hz [8]. F.-X. Launay and co-workers packed a fiber vibration sensor into a tube to construct a hydrophone that possessed a response range from several to 8000 Hz, which exhibited an acoustic pressure sensitivity of 105 dB·Hz/Pa [9]. B. Tan and J. Huang proposed an encapsulation based on a Helmholtz resonator, the encapsulated sensor responded from 20 to 800 Hz with an acoustic pressure sensitivity of -131 dB·Hz/Pa [10]. J. Wang et al. proposed an encapsulation based on the fiber Bragg grating vibration sensor that responded from 5 to 100 Hz with an acceleration sensitivity of 450 pm/g [11]. Among these encapsulation structures, the acoustic Helmholtz resonator is an efficient structure and is promising in the field of acoustic vibration sensing. It is significant to explore these applications. On the other hand, in recent years, the SMS fiber sensor attracts interests for its high sensitivity [12–15]. The SMS fiber structure is constructed by connecting a multimode fiber (MMF) with a singlemode fiber (SMF) on one end and connecting another SMF on the other end. Special attentions should be paid to design and fabricate the SMS structure. For example, hydrofluoric acid etch is used to increase the sensitivity of the SMS sensor [16,17], however this process decreases the firmness of SMS structure. So the encapsulation of the SMS fiber structure is highly necessary to prevent the possible damage in practical applications. In this paper, three different Helmholtz resonators are produced to encapsulate the SMS structure and enhance the sensitivity of a SMS structure. The sensitivity of the SMS sensor with the optimized Helmholtz resonator increases 2.11 times comparing to that of the unencapsulated SMS sensor. The optimally encapsulated sensor possesses a higher sensitivity than that of a commercial microphone.
2. The SMS structure and Helmholtz resonators The SMS structure consists of a MMF and two SMFs, as shown in Fig. 1(a). As the light in a SMF propagates into a MMF, a series of eigenmodes appear and interferences among these eigenmodes arouse. The intensity of the transmitting light in the MMF is periodic along the fiber, where the positions of the local maximum intensity are the self-imaging points (SIPs), as shown in Fig. 1(b). When the light in the MMF propagates to the output SMF, the maximum coupling efficiency can be obtained when the SMF and MMF connect at a SIP, which accordingly guarantees a high sensitivity of the SMS sensor. However, in the SMS fiber structure, when the MMF connects with the output SMF, there is always a certain deviation from the SIPs because an accurate connection is difficult to be achieved in practice, which greatly reduces the optical coupling efficiency and consequently leads to a dramatic decrease of the sensitivity. Recently, we proposed a method to avoid the accurate connection between the SMF and MMF, through which the SIP can be accurately adjusted to the connection of the MMF and the output SMF by hydrofluoric acid etch [16,17]. The acid etched taper on the MMF facilitates the vibration sensing and therefore increases the sensitivity, as shown in Fig.1(c). However the taper of the MMF may be damaged easily for its fragility. So in this work the acid etched SMS fiber sensor is encapsulated by the Helmholtz resonator. An ideal Helmholtz resonator is theoretically equivalent to a vibration system [18,19]. When the acoustic wavelength is much longer than each dimension of the Helmholtz resonator, the air in cavity vibrates with the air near the holes, which forms a vibration system. Fig. 2 shows a typical structure of a Helmholtz resonator with four holes on its surface, the resonance frequency of the 34
Optik - International Journal for Light and Electron Optics 169 (2018) 33–40
R. Gao et al.
Fig. 2. The schematic diagram of a typical Helmholtz resonator.
resonator can be expressed as [20]
f=
c 2π
1 V
4
s
∑ ⎛ li ⎞ ⎜
i=1
⎟
⎝ i⎠
(1)
where c is the speed of sound, V is the volume of the Helmholtz cavity, li (i = 1,2,3,4) is the length of a hole, and si ≈ π(di/2)2 (i = 1,2,3,4) is the cross area of a hole. Since the cylindrical cavity possesses a good adaptation for the optical fiber [8–11], so in this work, three different cylindrical Helmholtz resonators are designed and fabricated. These resonators are used to contain the SMS structure with three different schemes, which form three SMS fiber vibration sensing systems. Fig. 3(a)–(c) show the three Helmholtz resonators, which are labeled as the resonator A, B, and C, respectively. The external diameter and height of the resonator A is 20 and 50 mm, respectively. Two circular holes align symmetrically at one end face of the resonator A. The diameter of the hole is 2 mm, and the distance from the hole's center to the resonator's axis is 3 mm. The external diameter and height of the resonator B is 20 and 80 mm, respectively. Four circular holes align linearly at the side wall of the resonator B. For these holes, the distance from one end face to the hole's center is 20, 30, 50, and 60 mm, respectively. The resonator C is a combined Helmholtz resonator that is constituted by a large and a small cavity. The external diameter and height of the large cylindrical cavity is 33 and 100 mm, and the external diameter and height of the small cylindrical cavity is 9.3 and 25 mm, respectively. The both end faces of the small cavity are removed. And on one end face of the large cavity, there is a circular hole with diameter of 9.3 mm, which is coaxial with the large cavity. The large cylindrical cavity combines the small cylindrical cavity through this circular hole. For all cavities, the material is aluminum and the thickness of cavity is 1 mm. The COMSOL Multiphysics software is applied to perform the finite element analysis, which gives out the eigen frequency and acoustic pressure distribution in the resonator A, B, and C, as shown in Fig. 3(d)–(f), respectively. Fig. 3(d) shows that the first order intrinsic frequency of the resonator A is 1129 Hz A part of divided contour surfaces are also shown, which locate at the height of 15.363, 26.843, 34.988, and 41.684 mm, respectively. The corresponding acoustic pressure is 1.32, 1.17, 1.03, and 0.90 Pa, respectively. As can be seen that the contour surfaces with the maximum acoustic pressure locates at the height of 15.363 mm. Fig. 3(e) shows that the first order intrinsic frequency of the resonator B is 1406.7 Hz. It also shows three relatively flat contour surfaces inside the Helmholtz resonator B. The acoustic contour surface with the largest area is vertical as a whole. Fig. 3(f) shows that the first order intrinsic frequency of the resonator C is 280.36 Hz, the contour surface with the maximum acoustic pressure locates at the height of 61.87 mm. According to these analysis and parameters, three Helmholtz resonators that correspond to the resonator A, B, and C are fabricated, as shown in Fig. 3(g)–(i), respectively. In this work, the MMF and SMF is MM-S-105/125-22 A (Nufern Corp.) and SMF-28e (Corning Corp.), respectively. The length of the MMF is 20 mm, and the claddings of the MMF is removed. Each end of the MMF is connected to a SMF through the arc fusion splice, which produces the SMS structure. A neck is etched by using hydrofluoric acid at the MMF section of the SMS structure to turn the SIPs and to enhance the sensitivity of the SMS structure, these details can be seen in reference 16 and 17. The diameter of the etched neck is smaller than 125 u m, which is friable. In order to package the SMS fiber into the Helmholtz resonator and protect the SMS structure from damage, a preprocessing of SMS fiber is implemented as follows. As shown in Fig. 4(a), an aluminum brass strip with length of 100 mm, width of 3 mm, and thickness of 2 mm is used. On this strip, a groove with length of 100 mm, width of 130 μm, and thickness of 130 μm is excavated along the axle wire of the strip. Moreover, at the middle of the strip, the depth and width of the groove is further excavated to be 1 and 3 mm, respectively. The length of the further excavated groove is 30 mm. The SMS fiber is placed in the groove, where the SMS structure is located right above the deeper groove symmetrically. At two ends of the 35
Optik - International Journal for Light and Electron Optics 169 (2018) 33–40
R. Gao et al.
Fig. 3. Models of Helmholtz resonator for the (a) resonator A, (b) resonator B, and (c) resonator C. Simulated acoustic contour surface for the (d) resonator A, (e) resonator B, and (f) resonator C. The SMS fiber vibration sensing system encapsulated by the (g) resonator A, (h) resonator B, and (i) resonator C.
deeper groove, the SMS fiber is fixed to the strip with adhesive, which forms a structure of mechanical beam that responses to the vibration easily. To combine the SMS fiber to the Helmholtz resonator, the resonator is further processed. For the resonator A, each end face of the resonator is perforated with a rectangle hole with the length and width is 3 and 2 mm, respectively. The centre of the square hole locates at the axis of the resonator. The strip which loads the SMS fiber is inserted through the two rectangle holes, where the SMS structure is located at the middle of the resonator, as shown in Fig. 3(g). For the resonator B, each end face of the resonator is also perforated with a rectangle hole with the length and width is 3 and 2 mm, respectively. The centre of the rectangle hole locates at the axis of the resonator. The strip which loads the SMS fiber is also inserted through the two rectangle holes, where the SMS structure is located at the middle of the resonator, as shown in Fig. 3(h). For the resonator C, two rectangle holes (length of 3 mm and width of 2 mm) are perforated along the diameter of the cylindrical resonator. The distance from the end face of the resonator to the diameter that the two rectangle holes locate is 61.5 mm. The strip which loads the SMS fiber is inserted through the two rectangle holes, where the SMS structure is located at the middle of the resonator, as shown in Fig. 3(i). 3. Experiments and results The scheme setup for the acoustic frequency vibration sensing of the SMS structure is shown in Fig. 4(b). An electrical speaker which is used as an acoustic source to generate single frequency sound of 65 dBA is placed 3 m away from the SMS structure. Light source at wavelength of 1550 nm is used, which emits laser with a power of 4.5 mW. The transmitted light through the SMS structure is detected by a photodiode detector, which converts the light intensity into voltage signal that can be acquainted by a data acquisition card. The data acquisition card is connected to a personal computer which records the experimental datum. The amplitude response is evaluated under different single frequency from 20 to 1000 Hz at an interval of 10 Hz. Before being inserted into the Helmholtz resonator, the SMS structure that fixed on the strip is evaluated (in absence of any 36
Optik - International Journal for Light and Electron Optics 169 (2018) 33–40
R. Gao et al.
Fig. 4. (a) The SMS fiber is placed in the groove of a strip. (b) The schematic setup for the acoustic frequency vibration sensing of the SMS structure. Where laser is the light source, PD is the photodiode detector, DAB is the data acquisition card, and PC is personal computer.
resonator) to show the property of vibration sensing, the acoustic frequency response curve is depicted in Fig. 5(a). It is shown that there are two response peaks under this circumstance, which locates at 120 and 300 Hz, respectively. As mentioned above, the SMS fiber is fixed in the groove that carved on the strip to form a simply supported beam. Comparing to the rigid strip, the supple SMS fiber is far more likely be vibrated by the acoustic wave, so the peaks at 120 and 300 Hz can be attributed to the vibration of the SMS fiber rather than the vibration of the strip. The maximum amplitude occurs at 120 Hz with an intensity of 115.3. And then, the SMS
Fig. 5. The acoustic frequency response of (a) the un-encapsulated SMS structure fixed on the strip, (b) the SMS structure encapsulated in the resonator A, (c) the SMS structure encapsulated in the resonator B, and (d) the SMS structure encapsulated in the resonator C. 37
Optik - International Journal for Light and Electron Optics 169 (2018) 33–40
R. Gao et al.
fiber on the strip is inserted into the resonator A, which forms a SMS fiber sensing system. The property of the sensing system is evaluated, the acoustic frequency response curve is depicted in Fig. 5(b). It is shown that there are three response peaks under this circumstance, which locate at 120, 310, and 440 Hz, respectively. The maximum amplitude occurs at 120 Hz with a intensity of 145.9. The peaks at 120, 310, and 440 Hz are also due to the vibrations of the SMS fiber. Comparing to the SMS fiber that is not encapsulated by the resonator A, the peak at 300 Hz shifts to 310 Hz. This shift can be attributed to the combination of the strip and the resonator A, which forms a vibration system that leads to a more pronounced vibration at 310 Hz rather than at 300 Hz. And the peak at 440 Hz is also attributed to the combination of the strip and the resonator A, which leads to this new vibration frequency that can be responded by the SMS fiber. And then, the SMS fiber on the strip is inserted into the resonator B, the vibration sensing property is evaluated, the acoustic frequency response curve is depicted in Fig. 5 (c). It is shown that there are six response peaks under this circumstance, which locate at 100, 120, 310, 350, 450, and 490 Hz, respectively. It can be seen that the combination of the strip and the resonator B brings more peaks than that of the resonator A. The maximum amplitude occurs at 120 Hz with a intensity of 183.2. At last, the SMS fiber on the strip is inserted into the resonator C, the vibration sensing property is evaluated, the acoustic frequency response curve is depicted in Fig. 5(d). It is shown that there are 14 response peaks under this circumstance. It can be seen that the combination of the strip and the resonator C brings more peaks than that of the resonator B. The maximum amplitude occurs at 120 Hz with a intensity of 243.8. For the three encapsulated SMS fiber structures, according to Fig. 5(b)–(d), it can be seen that the sensing system encapsulated by the resonator A possesses the lowest sensitivity and the least response range. The sensing system encapsulated by the resonator B possesses the moderate sensitivity and the moderate response range. And the sensing system encapsulated by the resonator C possesses the highest sensitivity and the widest response range. Comparing to the responsivity of the un-encapsulated SMS structure (only fixed on the strip), the maximum responsivity of the sensing system encapsulated by the resonator A, B, and C increases 1.26, 1.5, and 2.11 times at 120 Hz, respectively. Moreover, the response range of the encapsulated sensing system is wider than the response range of the un-encapsulated SMS structure. All these indicate that the Helmholtz resonator can increase the sensitivity and response range of the SMS fiber structure effectively. Comparing to the responsivity of the sensing systems encapsulated by the resonator A, the responsivity of the sensing systems encapsulated by the resonator B increases to 1.25 times. A reason should be that the resonator B provides a contour surface with a large area, and the SMS fiber is placed across this contour surface, which can excite a more remarkable acoustic vibration of the SMS fiber. The other reason should be that the SMS structure that encapsulated by the resonator A stretches across several contour surfaces, which cannot excite a synchronous and remarkable acoustic vibration of the SMS fiber effectively. Comparing to the responsivity of the sensing systems encapsulated by the resonator B, the responsivity of the sensing systems encapsulated by the resonator C increases to 1.33 times. A reason is that, in the resonator B, the contour surface where the SMS fiber is placed across is an irregular surface rather than a plane, which cannot excite a more remarkable acoustic vibration of the SMS fiber comparing to that of a plan. The other reason is that, in the resonator C, the SMS fiber is placed on the contour surface which possesses a large acoustic pressure, which can excite a more remarkable acoustic vibration of the SMS fiber accordingly. To evaluate the responsivity to the excitation of acoustic signal, the SMS structure encapsulated by the resonator C is exposed to the time-varying acoustic wave at 280 Hz under the sound pressure ranging from 6.325 to 63.246 m Pa (50–70 dBA). With increase of the sound pressure, the output voltage signal of the photodiode detector increases accordingly, as shown in Fig. 6(a). The peak value of the output voltage signal versus the sound pressure is shown in Fig. 6(b), which exhibits a quite linear relation between the output voltage and the sound pressure. The linear correlation index is 0.999 and the sound pressure sensitivity is 1.325 mV/mPa. Moreover, the acoustic frequency response of a commercial microphone (OV-M369, Ovann Industrial Co., as shown in Fig. 7(a)) is also evaluated. In this evaluation, the voltage signal of the microphone is acquainted by the abovementioned data acquisition card. And the microphone is placed under the same condition with the abovementioned experiments. The frequency response curve is depicted in Fig. 7(b), where the range is from 20 to 500 Hz, the response amplitude in the range from 500 to 1000 Hz is very small and not presented here. It can be seen that the maximum value of the response is 69.2, which occurs at 220 Hz. Comparing to the response value of the SMS structure encapsulated by the resonator C, obviously, the response value of the commercial microphone is
Fig. 6. (a) The time domain responses under different sound pressure and (b) the output voltage versus the sound pressure at 280 Hz for the SMS structure encapsulated by the resonator C. 38
Optik - International Journal for Light and Electron Optics 169 (2018) 33–40
R. Gao et al.
Fig. 7. (a) A commercial microphone. (b) The acoustic frequency response of the microphone.
smaller as whole. These results and the above mentioned results indicate that the SMS sensing system encapsulated by the resonator C is the optimized acoustic vibration sensing system. 4. Conclusions In this work, three kinds of Helmholtz resonators are designed and fabricated. The finite element method is used to analyze the acoustic pressure distributions in these resonators. A SMS fiber is fixed in these resonators with different schemes under the guide of finite element analyses, which forms three SMS fiber vibration sensing systems, respectively. Experiments reveal that the sensing system constituted by the resonator C is the optimized sensing system that exhibits the highest sensitivity. The reason is that the SMS fiber structure is fixed on the complanate acoustic contour surface of the resonator, where the acoustic pressure is maximal. The sensing system encapsulated by the resonator B possesses the moderate sensitivity, the reason is that the irregular acoustic contour surface cannot excite a more remarkable acoustic vibration of the SMS fiber comparing to that of a complanate acoustic contour surface. And the sensing system encapsulated by the resonator A exhibits the lowest sensitivity because the SMS fiber structure stretches across several acoustic contour surfaces. Each sensing system encapsulated in the resonator exhibits a higher sensitivity than the SMS structure without a resonator. Specially, the SMS fiber vibration sensor encapsulated by the resonator C exhibits a higher sensitivity than that of a commercial microphone. This work indicates that the Helmholtz resonator can increase the sensitivity of fiber vibration sensor effectively, which is prospective in the monitoring of environmental noise, running state of engineering machinery, structural health of building, and so on. Acknowledgements The authors acknowledge the financial support of Science and Technology Development Plan of Weihai (2012DXGJ06, 2015DXGJUS002). We are also thankful to the financial support of National Science Foundation of China (NSFC) (11504070, 11574064). This work is also partly supported by the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (Project HIT. NSRIF. 2014140), and the Project of Shandong Province Higher Educational Science and Technology Program (Project J14LJ54). Zhanfeng Zhao thanks the financial support from the science and technology bureau of Weihai city. References [1] Y.R. Garcia, J.M. Correspond, J. Goicoechea, Vibration detection using optical fiber sensors, J. Sens. (2010) 936487. [2] L. Mohanty, L.M. Koh, S.C. Tjin, Fiber Bragg grating microphone system, Appl. Phys. Lett. 89 (2006) 161109. [3] M. Li, Y. Liu, X. Zhao, R. Gao, Y. Li, S. Qu, High sensitivity fiber acoustic sensor tip working at 1550nm fabricated by two-photon polymerization technique, Sensor Actuat. A: Phys. 260 (2017) 29–34. [4] N.K. Chen, T.H. Yang, Z.Z. Feng, Y.N. Chen, C. Lin, Cellular-dimension picoliter-volume index microsensing using micro-abrupt-tapered fiber Mach-Zehnder interferometers, IEEE Photon. Technol. Lett. 24 (2012) 842–844. [5] Y. Li, X. Wang, X. Bao, Sensitive acoustic vibration sensor using single-mode fiber tapers, Appl. Opt. 50 (2011) 1873–1878. [6] B. Xu, Y. Li, M. Sun, Z.-W. Zhang, X.-Y. Dong, Z.-X. Zhang, et al., Acoustic vibration sensor based on nonadiabatic tapered fibers, Opt Lett 37 (2012) 4768–4770. [7] Yong Zhao, Xue-gang Li, Fan-chao Meng, Zhao Zhao, A vibration-sensing system based on SMS fiber structure, Sensor Actuat. A: Phys. 214 (2014) 163–167. [8] S. Goodman, S. Foster, J.V. Velzen, et al., Field demonstration of a DFB fiber laser hydrophone seabed array in Jervis Bay, SPIE 7503 (2009) 75034L. [9] F.X. Launay, R. Bouffaron, R. Lardat, et al., Acoustic antenna based on fiber laser hydrophones, SPIE 9157 (2014) 91570Y. [10] B. Tan, J.B. HUANG, Design about a novel encapsulation structure of DFB fiber laser, SPIE 9233 (2014) 92330X. [11] J.F. Wang, Y. Yu, Y. Chen, et al., Research of a double fiber Bragg gratings vibration sensor with temperature and cross axis insensitive, Optik 126 (2015) 749–753. [12] R.X. Gao, W.J. Liu, Y.Y. Wang, et al., Design and fabrication of SMS fiber refractometer for liquid, Sens. Actuators A: Phys. 179 (2012) 5–9. [13] A. Kumar, R.K. Varshney, R. Kumar, SMS fiber optic microbend sensor structures: effect of the modal interference, Opt. Commun. 232 (2004) 239–244. [14] A. Sun, Y. Semenova, G. Farrell, A novel highly sensitive optical fiber microphone based on single mode-multimode-single mode structure, Microw. Opt. Technol. Lett. 53 (2011) 442–445. [15] Q. Wu, M.W. Yang, J.H. Yuan, The use of a bend singlemode-multimode singlemode (SMS) fiber structure for vibration sensing, Opt. Laser Technol. 63 (2014) 29–33.
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[16] R.X. Gao, Q. Wang, F. Zhao, S.L. Qu, Optimal design and fabrication of SMSfiber temperature sensor for liquid, Opt. Commun. 283 (2010) 3149–3152. [17] Renxi Gao, Hongri Wang, Dezhi Zhu, Guanghua Fan, Hongdan Yan, Peng Wang, Yi Liu, Yingying Wang, Wenjun Liu, Lizhong Song, Chaoqun Wang, Chen Yuwen, Acoustic frequency vibration sensor based on tapered SMS fiber structure, Sensor Actuat. A: Phys. 271 (2018) 243–250. [18] R.L. Panton, J.M. Miller, Resonant frequencies of cylindrical Helmholtz resonators, J. Acoustical Soc. Am. 57 (1975) 1533–1535. [19] M. Alster, Improved calculation of resonant frequencies of Helmholtz resonators, J. Sound Vib. 24 (1972) 63–85. [20] Z. Wang, Y. Hu, Z. Meng, M. Ni, Fiber-optic hydrophone using a cylindrical Helmholtz resonator as a mechanical anti-aliasing filter, Opt Lett 33 (2008) 37–39.
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Sensitivity enhancement of optical fiber vibration sensor through encapsulation of acoustic Helmholtz resonator ⁎
T
⁎
Renxi Gaoa, , Hongri Wanga, Dezhi Zhua, Guanghua Fana, , Weiyan Jiaoa, Chunyan Lianga, Yi Liua, Yingying Wanga, Wenjun Liua, Xiongwei Hea, Jiameng Liua, Guannan Weia, Lizhong Songb, Zhanfeng Zhaob a b
Department of Optoelectronic Science, Harbin Institute of Technology at Weihai, Weihai 264209, China School of Information and Electrical Engineering, Harbin Institute of Technology at Weihai, Weihai 264209, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Acoustic Helmholtz resonators Optical fiber vibration sensor Sensitivity enhancement Encapsulation of sensor
Optical fiber sensors should be encapsulated to prevent the possible damage in practical applications. Moreover, the encapsulation should be optimized to increase the sensitivity of the sensor as much as possible. Herein three different Helmholtz resonators are designed to encapsulate the singlemode-multimode-singlemode (SMS) fiber vibration sensor. Finite element analyses exhibit the acoustic pressure distributions in the resonators. The SMS fiber structures are fixed in the resonators with different schemes. Experiments demonstrate that the encapsulations increase the sensitivity of the sensors, which are determined by the fixing schemes, the acoustic pressure distributions, and the structures of the resonators. The combined resonator with two coaxially attached cavities renders the sensor a response range from 20 to 1000 Hz, which possesses a higher sensitivity and wider frequency response range than the sensor with a single cavity of resonator. The sensitivity of the encapsulated sensor with the combined resonator is 2.11 times of the sensitivity of the un-encapsulated sensor at 120 Hz. The SMS sensor encapsulated by the combined resonator exhibits a higher sensitivity than a commercial microphone, which indicates a favorable practicability of the Helmholtz resonator to the encapsulation of various kinds of fiber vibration sensors.
1. Introduction Optical fiber possesses the merits of low loss of signal in the long distance transmission, high stability, anti-electromagnetic interference, anti-chemical corrosion, and good adaptation for integrations in many circumstances such as communication, industry, civil engineering, and military affairs. Due to these merits, the constructions of novel and practical fiber sensors are important works for the scientific and industrial communities, which are always pursued by researchers. Recently, fiber vibration sensor attracts increasing attentions in the field of acoustic vibration sensing due to its high sensitivity and wide frequency response [1]. Specific tasks in the research of fiber vibration sensor include the seeking of sensitive and effective sensing structures, and the encapsulation of the fiber sensor to prevent the possible damages in practical applications. Note that the encapsulation of the fiber sensor should increase the sensitivity of the sensor system as much as possible. Up to now, the optical fiber sensing structures that are applied to perform the acoustic vibration sensing include the fiber Bragg grating [2], the Fabry-Perot interferometer [3], the Mach-Zehnder interferometer [4], the tapered fiber structure [5,6], the singlemode-multimode-singlemode (SMS) fiber structure [7] and so on.
⁎
Corresponding authors. E-mail addresses: [email protected] (R. Gao), [email protected], [email protected] (G. Fan).
https://doi.org/10.1016/j.ijleo.2018.05.018 Received 6 February 2018; Received in revised form 2 May 2018; Accepted 3 May 2018 0030-4026/ © 2018 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 169 (2018) 33–40
R. Gao et al.
Fig. 1. (a) The schematic diagram of the SMS fiber. (b) The simulation of beam propagation in the SMS fiber, (c) The schematic diagram of the SMS fiber after the acid etch.
For the encapsulation of the fiber vibration sensor, there are some relevant works. S. Foster and co-workers packed a fiber vibration sensor into a capillary tube, which constructed a hydrophone that possessed a response range from 30 to 7000 Hz [8]. F.-X. Launay and co-workers packed a fiber vibration sensor into a tube to construct a hydrophone that possessed a response range from several to 8000 Hz, which exhibited an acoustic pressure sensitivity of 105 dB·Hz/Pa [9]. B. Tan and J. Huang proposed an encapsulation based on a Helmholtz resonator, the encapsulated sensor responded from 20 to 800 Hz with an acoustic pressure sensitivity of -131 dB·Hz/Pa [10]. J. Wang et al. proposed an encapsulation based on the fiber Bragg grating vibration sensor that responded from 5 to 100 Hz with an acceleration sensitivity of 450 pm/g [11]. Among these encapsulation structures, the acoustic Helmholtz resonator is an efficient structure and is promising in the field of acoustic vibration sensing. It is significant to explore these applications. On the other hand, in recent years, the SMS fiber sensor attracts interests for its high sensitivity [12–15]. The SMS fiber structure is constructed by connecting a multimode fiber (MMF) with a singlemode fiber (SMF) on one end and connecting another SMF on the other end. Special attentions should be paid to design and fabricate the SMS structure. For example, hydrofluoric acid etch is used to increase the sensitivity of the SMS sensor [16,17], however this process decreases the firmness of SMS structure. So the encapsulation of the SMS fiber structure is highly necessary to prevent the possible damage in practical applications. In this paper, three different Helmholtz resonators are produced to encapsulate the SMS structure and enhance the sensitivity of a SMS structure. The sensitivity of the SMS sensor with the optimized Helmholtz resonator increases 2.11 times comparing to that of the unencapsulated SMS sensor. The optimally encapsulated sensor possesses a higher sensitivity than that of a commercial microphone.
2. The SMS structure and Helmholtz resonators The SMS structure consists of a MMF and two SMFs, as shown in Fig. 1(a). As the light in a SMF propagates into a MMF, a series of eigenmodes appear and interferences among these eigenmodes arouse. The intensity of the transmitting light in the MMF is periodic along the fiber, where the positions of the local maximum intensity are the self-imaging points (SIPs), as shown in Fig. 1(b). When the light in the MMF propagates to the output SMF, the maximum coupling efficiency can be obtained when the SMF and MMF connect at a SIP, which accordingly guarantees a high sensitivity of the SMS sensor. However, in the SMS fiber structure, when the MMF connects with the output SMF, there is always a certain deviation from the SIPs because an accurate connection is difficult to be achieved in practice, which greatly reduces the optical coupling efficiency and consequently leads to a dramatic decrease of the sensitivity. Recently, we proposed a method to avoid the accurate connection between the SMF and MMF, through which the SIP can be accurately adjusted to the connection of the MMF and the output SMF by hydrofluoric acid etch [16,17]. The acid etched taper on the MMF facilitates the vibration sensing and therefore increases the sensitivity, as shown in Fig.1(c). However the taper of the MMF may be damaged easily for its fragility. So in this work the acid etched SMS fiber sensor is encapsulated by the Helmholtz resonator. An ideal Helmholtz resonator is theoretically equivalent to a vibration system [18,19]. When the acoustic wavelength is much longer than each dimension of the Helmholtz resonator, the air in cavity vibrates with the air near the holes, which forms a vibration system. Fig. 2 shows a typical structure of a Helmholtz resonator with four holes on its surface, the resonance frequency of the 34
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Fig. 2. The schematic diagram of a typical Helmholtz resonator.
resonator can be expressed as [20]
f=
c 2π
1 V
4
s
∑ ⎛ li ⎞ ⎜
i=1
⎟
⎝ i⎠
(1)
where c is the speed of sound, V is the volume of the Helmholtz cavity, li (i = 1,2,3,4) is the length of a hole, and si ≈ π(di/2)2 (i = 1,2,3,4) is the cross area of a hole. Since the cylindrical cavity possesses a good adaptation for the optical fiber [8–11], so in this work, three different cylindrical Helmholtz resonators are designed and fabricated. These resonators are used to contain the SMS structure with three different schemes, which form three SMS fiber vibration sensing systems. Fig. 3(a)–(c) show the three Helmholtz resonators, which are labeled as the resonator A, B, and C, respectively. The external diameter and height of the resonator A is 20 and 50 mm, respectively. Two circular holes align symmetrically at one end face of the resonator A. The diameter of the hole is 2 mm, and the distance from the hole's center to the resonator's axis is 3 mm. The external diameter and height of the resonator B is 20 and 80 mm, respectively. Four circular holes align linearly at the side wall of the resonator B. For these holes, the distance from one end face to the hole's center is 20, 30, 50, and 60 mm, respectively. The resonator C is a combined Helmholtz resonator that is constituted by a large and a small cavity. The external diameter and height of the large cylindrical cavity is 33 and 100 mm, and the external diameter and height of the small cylindrical cavity is 9.3 and 25 mm, respectively. The both end faces of the small cavity are removed. And on one end face of the large cavity, there is a circular hole with diameter of 9.3 mm, which is coaxial with the large cavity. The large cylindrical cavity combines the small cylindrical cavity through this circular hole. For all cavities, the material is aluminum and the thickness of cavity is 1 mm. The COMSOL Multiphysics software is applied to perform the finite element analysis, which gives out the eigen frequency and acoustic pressure distribution in the resonator A, B, and C, as shown in Fig. 3(d)–(f), respectively. Fig. 3(d) shows that the first order intrinsic frequency of the resonator A is 1129 Hz A part of divided contour surfaces are also shown, which locate at the height of 15.363, 26.843, 34.988, and 41.684 mm, respectively. The corresponding acoustic pressure is 1.32, 1.17, 1.03, and 0.90 Pa, respectively. As can be seen that the contour surfaces with the maximum acoustic pressure locates at the height of 15.363 mm. Fig. 3(e) shows that the first order intrinsic frequency of the resonator B is 1406.7 Hz. It also shows three relatively flat contour surfaces inside the Helmholtz resonator B. The acoustic contour surface with the largest area is vertical as a whole. Fig. 3(f) shows that the first order intrinsic frequency of the resonator C is 280.36 Hz, the contour surface with the maximum acoustic pressure locates at the height of 61.87 mm. According to these analysis and parameters, three Helmholtz resonators that correspond to the resonator A, B, and C are fabricated, as shown in Fig. 3(g)–(i), respectively. In this work, the MMF and SMF is MM-S-105/125-22 A (Nufern Corp.) and SMF-28e (Corning Corp.), respectively. The length of the MMF is 20 mm, and the claddings of the MMF is removed. Each end of the MMF is connected to a SMF through the arc fusion splice, which produces the SMS structure. A neck is etched by using hydrofluoric acid at the MMF section of the SMS structure to turn the SIPs and to enhance the sensitivity of the SMS structure, these details can be seen in reference 16 and 17. The diameter of the etched neck is smaller than 125 u m, which is friable. In order to package the SMS fiber into the Helmholtz resonator and protect the SMS structure from damage, a preprocessing of SMS fiber is implemented as follows. As shown in Fig. 4(a), an aluminum brass strip with length of 100 mm, width of 3 mm, and thickness of 2 mm is used. On this strip, a groove with length of 100 mm, width of 130 μm, and thickness of 130 μm is excavated along the axle wire of the strip. Moreover, at the middle of the strip, the depth and width of the groove is further excavated to be 1 and 3 mm, respectively. The length of the further excavated groove is 30 mm. The SMS fiber is placed in the groove, where the SMS structure is located right above the deeper groove symmetrically. At two ends of the 35
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Fig. 3. Models of Helmholtz resonator for the (a) resonator A, (b) resonator B, and (c) resonator C. Simulated acoustic contour surface for the (d) resonator A, (e) resonator B, and (f) resonator C. The SMS fiber vibration sensing system encapsulated by the (g) resonator A, (h) resonator B, and (i) resonator C.
deeper groove, the SMS fiber is fixed to the strip with adhesive, which forms a structure of mechanical beam that responses to the vibration easily. To combine the SMS fiber to the Helmholtz resonator, the resonator is further processed. For the resonator A, each end face of the resonator is perforated with a rectangle hole with the length and width is 3 and 2 mm, respectively. The centre of the square hole locates at the axis of the resonator. The strip which loads the SMS fiber is inserted through the two rectangle holes, where the SMS structure is located at the middle of the resonator, as shown in Fig. 3(g). For the resonator B, each end face of the resonator is also perforated with a rectangle hole with the length and width is 3 and 2 mm, respectively. The centre of the rectangle hole locates at the axis of the resonator. The strip which loads the SMS fiber is also inserted through the two rectangle holes, where the SMS structure is located at the middle of the resonator, as shown in Fig. 3(h). For the resonator C, two rectangle holes (length of 3 mm and width of 2 mm) are perforated along the diameter of the cylindrical resonator. The distance from the end face of the resonator to the diameter that the two rectangle holes locate is 61.5 mm. The strip which loads the SMS fiber is inserted through the two rectangle holes, where the SMS structure is located at the middle of the resonator, as shown in Fig. 3(i). 3. Experiments and results The scheme setup for the acoustic frequency vibration sensing of the SMS structure is shown in Fig. 4(b). An electrical speaker which is used as an acoustic source to generate single frequency sound of 65 dBA is placed 3 m away from the SMS structure. Light source at wavelength of 1550 nm is used, which emits laser with a power of 4.5 mW. The transmitted light through the SMS structure is detected by a photodiode detector, which converts the light intensity into voltage signal that can be acquainted by a data acquisition card. The data acquisition card is connected to a personal computer which records the experimental datum. The amplitude response is evaluated under different single frequency from 20 to 1000 Hz at an interval of 10 Hz. Before being inserted into the Helmholtz resonator, the SMS structure that fixed on the strip is evaluated (in absence of any 36
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Fig. 4. (a) The SMS fiber is placed in the groove of a strip. (b) The schematic setup for the acoustic frequency vibration sensing of the SMS structure. Where laser is the light source, PD is the photodiode detector, DAB is the data acquisition card, and PC is personal computer.
resonator) to show the property of vibration sensing, the acoustic frequency response curve is depicted in Fig. 5(a). It is shown that there are two response peaks under this circumstance, which locates at 120 and 300 Hz, respectively. As mentioned above, the SMS fiber is fixed in the groove that carved on the strip to form a simply supported beam. Comparing to the rigid strip, the supple SMS fiber is far more likely be vibrated by the acoustic wave, so the peaks at 120 and 300 Hz can be attributed to the vibration of the SMS fiber rather than the vibration of the strip. The maximum amplitude occurs at 120 Hz with an intensity of 115.3. And then, the SMS
Fig. 5. The acoustic frequency response of (a) the un-encapsulated SMS structure fixed on the strip, (b) the SMS structure encapsulated in the resonator A, (c) the SMS structure encapsulated in the resonator B, and (d) the SMS structure encapsulated in the resonator C. 37
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fiber on the strip is inserted into the resonator A, which forms a SMS fiber sensing system. The property of the sensing system is evaluated, the acoustic frequency response curve is depicted in Fig. 5(b). It is shown that there are three response peaks under this circumstance, which locate at 120, 310, and 440 Hz, respectively. The maximum amplitude occurs at 120 Hz with a intensity of 145.9. The peaks at 120, 310, and 440 Hz are also due to the vibrations of the SMS fiber. Comparing to the SMS fiber that is not encapsulated by the resonator A, the peak at 300 Hz shifts to 310 Hz. This shift can be attributed to the combination of the strip and the resonator A, which forms a vibration system that leads to a more pronounced vibration at 310 Hz rather than at 300 Hz. And the peak at 440 Hz is also attributed to the combination of the strip and the resonator A, which leads to this new vibration frequency that can be responded by the SMS fiber. And then, the SMS fiber on the strip is inserted into the resonator B, the vibration sensing property is evaluated, the acoustic frequency response curve is depicted in Fig. 5 (c). It is shown that there are six response peaks under this circumstance, which locate at 100, 120, 310, 350, 450, and 490 Hz, respectively. It can be seen that the combination of the strip and the resonator B brings more peaks than that of the resonator A. The maximum amplitude occurs at 120 Hz with a intensity of 183.2. At last, the SMS fiber on the strip is inserted into the resonator C, the vibration sensing property is evaluated, the acoustic frequency response curve is depicted in Fig. 5(d). It is shown that there are 14 response peaks under this circumstance. It can be seen that the combination of the strip and the resonator C brings more peaks than that of the resonator B. The maximum amplitude occurs at 120 Hz with a intensity of 243.8. For the three encapsulated SMS fiber structures, according to Fig. 5(b)–(d), it can be seen that the sensing system encapsulated by the resonator A possesses the lowest sensitivity and the least response range. The sensing system encapsulated by the resonator B possesses the moderate sensitivity and the moderate response range. And the sensing system encapsulated by the resonator C possesses the highest sensitivity and the widest response range. Comparing to the responsivity of the un-encapsulated SMS structure (only fixed on the strip), the maximum responsivity of the sensing system encapsulated by the resonator A, B, and C increases 1.26, 1.5, and 2.11 times at 120 Hz, respectively. Moreover, the response range of the encapsulated sensing system is wider than the response range of the un-encapsulated SMS structure. All these indicate that the Helmholtz resonator can increase the sensitivity and response range of the SMS fiber structure effectively. Comparing to the responsivity of the sensing systems encapsulated by the resonator A, the responsivity of the sensing systems encapsulated by the resonator B increases to 1.25 times. A reason should be that the resonator B provides a contour surface with a large area, and the SMS fiber is placed across this contour surface, which can excite a more remarkable acoustic vibration of the SMS fiber. The other reason should be that the SMS structure that encapsulated by the resonator A stretches across several contour surfaces, which cannot excite a synchronous and remarkable acoustic vibration of the SMS fiber effectively. Comparing to the responsivity of the sensing systems encapsulated by the resonator B, the responsivity of the sensing systems encapsulated by the resonator C increases to 1.33 times. A reason is that, in the resonator B, the contour surface where the SMS fiber is placed across is an irregular surface rather than a plane, which cannot excite a more remarkable acoustic vibration of the SMS fiber comparing to that of a plan. The other reason is that, in the resonator C, the SMS fiber is placed on the contour surface which possesses a large acoustic pressure, which can excite a more remarkable acoustic vibration of the SMS fiber accordingly. To evaluate the responsivity to the excitation of acoustic signal, the SMS structure encapsulated by the resonator C is exposed to the time-varying acoustic wave at 280 Hz under the sound pressure ranging from 6.325 to 63.246 m Pa (50–70 dBA). With increase of the sound pressure, the output voltage signal of the photodiode detector increases accordingly, as shown in Fig. 6(a). The peak value of the output voltage signal versus the sound pressure is shown in Fig. 6(b), which exhibits a quite linear relation between the output voltage and the sound pressure. The linear correlation index is 0.999 and the sound pressure sensitivity is 1.325 mV/mPa. Moreover, the acoustic frequency response of a commercial microphone (OV-M369, Ovann Industrial Co., as shown in Fig. 7(a)) is also evaluated. In this evaluation, the voltage signal of the microphone is acquainted by the abovementioned data acquisition card. And the microphone is placed under the same condition with the abovementioned experiments. The frequency response curve is depicted in Fig. 7(b), where the range is from 20 to 500 Hz, the response amplitude in the range from 500 to 1000 Hz is very small and not presented here. It can be seen that the maximum value of the response is 69.2, which occurs at 220 Hz. Comparing to the response value of the SMS structure encapsulated by the resonator C, obviously, the response value of the commercial microphone is
Fig. 6. (a) The time domain responses under different sound pressure and (b) the output voltage versus the sound pressure at 280 Hz for the SMS structure encapsulated by the resonator C. 38
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Fig. 7. (a) A commercial microphone. (b) The acoustic frequency response of the microphone.
smaller as whole. These results and the above mentioned results indicate that the SMS sensing system encapsulated by the resonator C is the optimized acoustic vibration sensing system. 4. Conclusions In this work, three kinds of Helmholtz resonators are designed and fabricated. The finite element method is used to analyze the acoustic pressure distributions in these resonators. A SMS fiber is fixed in these resonators with different schemes under the guide of finite element analyses, which forms three SMS fiber vibration sensing systems, respectively. Experiments reveal that the sensing system constituted by the resonator C is the optimized sensing system that exhibits the highest sensitivity. The reason is that the SMS fiber structure is fixed on the complanate acoustic contour surface of the resonator, where the acoustic pressure is maximal. The sensing system encapsulated by the resonator B possesses the moderate sensitivity, the reason is that the irregular acoustic contour surface cannot excite a more remarkable acoustic vibration of the SMS fiber comparing to that of a complanate acoustic contour surface. And the sensing system encapsulated by the resonator A exhibits the lowest sensitivity because the SMS fiber structure stretches across several acoustic contour surfaces. Each sensing system encapsulated in the resonator exhibits a higher sensitivity than the SMS structure without a resonator. Specially, the SMS fiber vibration sensor encapsulated by the resonator C exhibits a higher sensitivity than that of a commercial microphone. This work indicates that the Helmholtz resonator can increase the sensitivity of fiber vibration sensor effectively, which is prospective in the monitoring of environmental noise, running state of engineering machinery, structural health of building, and so on. Acknowledgements The authors acknowledge the financial support of Science and Technology Development Plan of Weihai (2012DXGJ06, 2015DXGJUS002). We are also thankful to the financial support of National Science Foundation of China (NSFC) (11504070, 11574064). This work is also partly supported by the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (Project HIT. NSRIF. 2014140), and the Project of Shandong Province Higher Educational Science and Technology Program (Project J14LJ54). Zhanfeng Zhao thanks the financial support from the science and technology bureau of Weihai city. References [1] Y.R. Garcia, J.M. Correspond, J. Goicoechea, Vibration detection using optical fiber sensors, J. Sens. (2010) 936487. [2] L. Mohanty, L.M. Koh, S.C. Tjin, Fiber Bragg grating microphone system, Appl. Phys. Lett. 89 (2006) 161109. [3] M. Li, Y. Liu, X. Zhao, R. Gao, Y. Li, S. Qu, High sensitivity fiber acoustic sensor tip working at 1550nm fabricated by two-photon polymerization technique, Sensor Actuat. A: Phys. 260 (2017) 29–34. [4] N.K. Chen, T.H. Yang, Z.Z. Feng, Y.N. Chen, C. Lin, Cellular-dimension picoliter-volume index microsensing using micro-abrupt-tapered fiber Mach-Zehnder interferometers, IEEE Photon. Technol. Lett. 24 (2012) 842–844. [5] Y. Li, X. Wang, X. Bao, Sensitive acoustic vibration sensor using single-mode fiber tapers, Appl. Opt. 50 (2011) 1873–1878. [6] B. Xu, Y. Li, M. Sun, Z.-W. Zhang, X.-Y. Dong, Z.-X. Zhang, et al., Acoustic vibration sensor based on nonadiabatic tapered fibers, Opt Lett 37 (2012) 4768–4770. [7] Yong Zhao, Xue-gang Li, Fan-chao Meng, Zhao Zhao, A vibration-sensing system based on SMS fiber structure, Sensor Actuat. A: Phys. 214 (2014) 163–167. [8] S. Goodman, S. Foster, J.V. Velzen, et al., Field demonstration of a DFB fiber laser hydrophone seabed array in Jervis Bay, SPIE 7503 (2009) 75034L. [9] F.X. Launay, R. Bouffaron, R. Lardat, et al., Acoustic antenna based on fiber laser hydrophones, SPIE 9157 (2014) 91570Y. [10] B. Tan, J.B. HUANG, Design about a novel encapsulation structure of DFB fiber laser, SPIE 9233 (2014) 92330X. [11] J.F. Wang, Y. Yu, Y. Chen, et al., Research of a double fiber Bragg gratings vibration sensor with temperature and cross axis insensitive, Optik 126 (2015) 749–753. [12] R.X. Gao, W.J. Liu, Y.Y. Wang, et al., Design and fabrication of SMS fiber refractometer for liquid, Sens. Actuators A: Phys. 179 (2012) 5–9. [13] A. Kumar, R.K. Varshney, R. Kumar, SMS fiber optic microbend sensor structures: effect of the modal interference, Opt. Commun. 232 (2004) 239–244. [14] A. Sun, Y. Semenova, G. Farrell, A novel highly sensitive optical fiber microphone based on single mode-multimode-single mode structure, Microw. Opt. Technol. Lett. 53 (2011) 442–445. [15] Q. Wu, M.W. Yang, J.H. Yuan, The use of a bend singlemode-multimode singlemode (SMS) fiber structure for vibration sensing, Opt. Laser Technol. 63 (2014) 29–33.
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