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Investigation of Composite Structure with Dual Fabry–Perot Cavities for Temperature and Pressure Sensing

Investigation of Composite Structure with Dual Fabry–Perot Cavities for Temperature and Pressure... hv photonics Article Investigation of Composite Structure with Dual Fabry–Perot Cavities for Temperature and Pressure Sensing 1 1 1 1 1 , 2 1 1 , 2 , 3 , Jun Wang , Long Li , Shuaicheng Liu , Diyang Wu , Wei Wang , Ming Song , Guanjun Wang * 1 , 2 , and Mengxing Huang * School of Information and Communication Engineering, Hainan University, Haikou 570228, China; 20180581310080@hainanu.edu.cn (J.W.); 20181683310167@hainanu.edu.cn (L.L.); 20181683310012@hainanu.edu.cn (S.L.); 20181683310111@hainanu.edu.cn (D.W.); 20081000110002@hainanu.edu.cn (W.W.); 20171683310058@hainanu.edu.cn (M.S.) State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, China Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China * Correspondence: wangguanjun@hainanu.edu.cn (G.W.); huangmx09@hainanu.edu.cn (M.H.) Abstract: To deeply analyze the influence of diaphragm materials on the temperature and pressure sensitivity of Fabry–Perot interferometer-based dual-parameter fiber sensors, the multiple transfer method was used to fabricate the dual Fabry–Perot cavities, respectively, consisting of the following combinations: epoxy resin AB/polydimethylsiloxane (PDMS), Ecoflex0030 silicone rubber /PDMS, and PDMS/Ecoflex0030 silicone rubber. Experimental results show that the temperature sensitivities are, respectively, 528, 540, and 1033 pm/ C in the range of 40–100 C. Within the applied pres- sure range of 100–400 kPa, the pressure sensitivities are, respectively, 16.0, 34.6, and 30.2 pm/kPa. The proposed sensors have advantages of proper sensitivity, simple fabrication, cost-effectiveness, controllable cavity length, and suitability for practical sensing applications. Citation: Wang, J.; Li, L.; Liu, S.; Wu, D.; Wang, W.; Song, M.; Wang, G.; Keywords: Fabry–Perot cavity; temperature/pressure sensing; composite structure; multiple trans- Huang, M. Investigation of fer method Composite Structure with Dual Fabry–Perot Cavities for Temperature and Pressure Sensing. Photonics 2021, 8, 138. https://doi.org/10.3390/ 1. Introduction photonics8050138 Optical fiber sensors have been widely used in many domains owing to the advan- tages of low cost, high sensitivity, fast response time, and good stability. Simultaneously, Received: 24 February 2021 different sensor structures with diverse sensitivities to various parameters can be used to Accepted: 9 April 2021 realize dual- or even multi-parameter sensing, such as temperature, pressure, refractive Published: 23 April 2021 index, etc. [1,2]. Several studies have characterized the investigations of single or dual-parameter Publisher’s Note: MDPI stays neutral monitoring based on fiber tip bubble [3], Mach–Zehnder interferometer [4], fiber Bragg with regard to jurisdictional claims in gratings [5], Sagnac interferometer [6], etc. Due to the advantages of small size, good published maps and institutional affil- measurement performance, and survivability in complicated electromagnetic environ- iations. ment [7–19], Fabry–Perot Interferometer (FPI)-based dual-parameter fiber sensors have been extensively studied for the measurement of temperature and pressure. However, FPI-based fiber sensors impose the limitation of the durability and operation stability of the thin diaphragm that is used to form the FP cavity. Consequently, series of dual FP Copyright: © 2021 by the authors. cavities sensors based on various diaphragms to improve the sensing performance are Licensee MDPI, Basel, Switzerland. utilized to monitor the temperature and pressure [7–12], which share the characteristic of This article is an open access article ultra-high sensitivity. While endlessly pursuing high sensitivity will bring several problems distributed under the terms and which should be paid more attention, only this kind of fiber sensor can be applied for conditions of the Creative Commons ultra-precise monitoring applications. Moreover, it will result in a great challenge for the Attribution (CC BY) license (https:// design of the interrogator ’s bandwidth. Another dual FPI-based fiber sensor consisting creativecommons.org/licenses/by/ of two tiny segments of hollow-core fiber located at the end of lead-in single mode fiber, 4.0/). Photonics 2021, 8, 138. https://doi.org/10.3390/photonics8050138 https://www.mdpi.com/journal/photonics Photonics 2021, 8, x FOR PEER REVIEW 2 of 14 the design of the interrogator’s bandwidth. Another dual FPI-based fiber sensor consist- ing of two tiny segments of hollow-core fiber located at the end of lead-in single mode Photonics 2021, 8, 138 2 of 14 fiber, with a misalignment fusion splicing between the two hollow-core fibers with differ- ent core diameter, is reported in [13]. The proposed sensor can be used in simultaneous measurement of pressure and temperature, but its sensitivity should be significantly en- with a misalignment fusion splicing between the two hollow-core fibers with different core hanced. Some chemical etching method-assisted FPI fabrication processes are reported in diameter, is reported in [13]. The proposed sensor can be used in simultaneous measure- [14–16], which can provide good sensing performances. Unfortunately, the corrosive ef- ment of pressure and temperature, but its sensitivity should be significantly enhanced. fects of the chemical etching and the precise etching time are difficult to control. The au- Some chemical etching method-assisted FPI fabrication processes are reported in [14–16], thors of [17,18] demonstrated easy-to-fabricate dual-FPIs. However, the sensitivity and which can provide good sensing performances. Unfortunately, the corrosive effects of the repeatability of the fabrication for the FP cavity should be improved. By filling a hollow chemical etching and the precise etching time are difficult to control. The authors of [17,18] capillary with two sections of PDMS, which are fused to the single-mode fiber, the authors demonstrated easy-to-fabricate dual-FPIs. However, the sensitivity and repeatability of the of [19] investigated a novel dual FP cavities-based fiber sensor to detect the temperature fabrication for the FP cavity should be improved. By filling a hollow capillary with two sec- and pressure. However, the fabrication of this kind of structure is costly due to the fact an tions of PDMS, which are fused to the single-mode fiber, the authors of [19] investigated a especially made capillary cone is required to inject the PDMS into the hollow capillary. novel dual FP cavities-based fiber sensor to detect the temperature and pressure. However, Although many studies reported dual-parameter monitoring based on FPI struc- the fabrication of this kind of structure is costly due to the fact an especially made capillary tures, it is still worth continuously investigating and demonstrating the design of sensor cone is required to inject the PDMS into the hollow capillary. structures, the selection of sensor materials, and the optimization of fabrication processes. Although many studies reported dual-parameter monitoring based on FPI structures, Hence, in this paper, a novel FP cavity with composite structure for fiber sensing based it is still worth continuously investigating and demonstrating the design of sensor struc- on the multiple transfer method is proposed to measure the temperature and pressure, tures, the selection of sensor materials, and the optimization of fabrication processes. Hence, which possesses the advantages of proper sensitivity, simple fabrication, cost-effective- in this paper, a novel FP cavity with composite structure for fiber sensing based on the ness, and controllable cavity length. multiple transfer method is proposed to measure the temperature and pressure, which possesses the advantages of proper sensitivity, simple fabrication, cost-effectiveness, and 2. Sensing Principle and Fabrication Process controllable cavity length. 2.1. Sensing Principle 2. Sensing Principle and Fabrication Process 2.1. Sensing Principle The composite structure with dual FP cavities studied in this paper is depicted in The composite structure with dual FP cavities studied in this paper is depicted Figure 1. The sensor consists of a single mode fiber tail and two diaphragms with different in Figure 1. The sensor consists of a single mode fiber tail and two diaphragms with materials. different materials. Figure 1. The composite structure based on dual FP cavities. Figure 1. The composite structure based on dual FP cavities. The formed structure is composed of three reflective surfaces. The interfaces of The formed structure is composed of three reflective surfaces. The interfaces of SMF/Material 1, Material 1/Material 2, Material 2/air, respectively, are reflective surfaces SMF/Material 1, Material 1/Material 2, Material 2/air, respectively, are reflective surfaces 1, 2 and 3. When light reaches the end face of the optical fiber, the incident light will be 1, 2 and 3. When light reaches the end face of the optical fiber, the incident light will be reflected by the reflective surface within a certain wavelength range. However, part of reflected by the reflective surface within a certain wavelength range. However, part of the the light will still pass through Reflective Surface 1 and be reflected by Reflective Surfaces light will still pass through Reflective Surface 1 and be reflected by Reflective Surfaces 2 2 and 3 within a certain range. The interference spectrum results from the phase delay and 3 within a certain range. The interference spectrum results from the phase delay caused by optical path difference and the different reflectivity of each reflection surface of caused by optical path difference and the different reflectivity of each reflection surface of the composite structure, as the composite structure, as 4𝜋𝑛𝐿 4pnL I =𝐼= I + 𝐼 I+𝐼+ 2+2I I𝐼 cos 𝐼 𝑐𝑜𝑠 ++𝜑j (1) (1) 1 2 1 2 0 l 𝜆 Formula (1) is the dual-beam interference model of the single FP cavity. 𝐼 and 𝐼 Formula (1) is the dual-beam interference model of the single FP cavity. I and I 1 2 represent the reflected light intensity of the two-beam interference, 𝜆 is the wavelength represent the reflected light intensity of the two-beam interference, l is the wavelength of the incident light, 𝑛 is the refractive index of the FP cavity, 𝐿 is the length between of the incident light, n is the refractive index of the FP cavity, L is the length between two two reflected surfaces, and 𝜑 is the initial phase of the inference. reflected surfaces, and j is the initial phase of the inference. In this paper, an improved three-beam interference model based on the sensor struc- ture is presented and analyzed. According to the principle of multi-beam interference, the corresponding three-beam interference intensity [20] can be described as p p 4p 4p 4p I = I + I + I 2 I I cos n L + j + 2 I I cos n L + j 2 I I cos (n L + n L ) + j (2) 1 2 3 1 2 1 1 1 2 3 2 2 2 1 2 1 1 2 2 3 l l l Photonics 2021, 8, 138 3 of 14 where I , I , and I , respectively, are the reflected light intensity at the three reflecting 1 2 3 surfaces; j , j , and j are the initial phases of the reflected light; and n and n are the 1 2 3 1 2 refractive index of Material 1 and Material 2, respectively. L = l  (3) 2(l l ) 2 1 where l , l are the wavelengths corresponding to the adjacent peaks or valleys in the temperature and pressure test interference spectrum, and L is the length of FP cavity. The optical path difference (OPD) of the reflected light l can be expressed as l = 2nL (4) The wavelength spacing between adjacent peaks or valleys of the sensor interference spectrum is the free spectral range (FSR), the FSR is expressed as l l l 1 2 0 FSR = = (5) 2nL 2nL where the l and l are two adjacent peaks or valleys of the interference spectrum, l is 1 2 0 the mean wavelength of l and l , and L corresponds to the cavity length in formula (3). 1 2 The FSR is mainly affected by the thermal expansion coefficient (which is related to the change of L) and the thermo–optic coefficient (which is related to the change of n). It can be clearly seen that the FSR of the interference spectrum decreases as the n and L increase. For the temperature/pressure measurement of the FPI-based fiber sensor, the response of the sensor can be attributed to thermal expansion effects, thermo–optic effects, elastic deformation effects and refractive index factors. During temperature measurement, the refractive index and cavity length of the FP cavity change as the temperature increases, since these are related to the thermo–optical coefficient and the thermal expansion coefficient, respectively. This results in the variations of OPD. The OPD variation is defined as [21] Dl = 2DnL + 2nDL = 2nL(d + a)DT Dn = dDTn (6) DL = aDTL where Dl, DT, Dn and DL are the variations of OPD, temperature, refractive index and FP cavity length, respectively; d and a are the thermo–optic coefficient and thermal expansion coefficient, respectively, that are closely related to the properties of diaphragms. The formula (6) indicates that the temperature-induced OPD variations can be expressed as the change of FPI cavity length and refractive index. For the pressure measurement, the change of FPI cavity length depends on the diaphragm’s elastic deformation effects, and Formula (7) shows the pressure sensing principle [22]: 2 4 3 1 m r DL =  DP (7) 16 Eh where DP, h, r, m and E, respectively, are the change of pressure of the test environment, the thickness, the effective radius, Poisson’s Ratio, Young’s modulus of the diaphragms. The theoretical interference spectrum is depicted in Figure 2, which is simulated by Matlab platform. It is obtained by comprehensively considering the relevant parameters of the diaphragms, such as thermo–optical coefficient, thermal expansion coefficient, Young’s modulus and Poisson’s ratio in the proposed formulas. Photonics 2021, 8, x FOR PEER REVIEW 4 of 14 Photonics 2021, 8, 138 4 of 14 of the diaphragms, such as thermo–optical coefficient, thermal expansion coefficient, Young’s modulus and Poisson’s ratio in the proposed formulas. (a) (b) Figure 2. (a) The theoretical spectrum of temperature (𝛥𝑇 = 10 °C) response (𝛥𝜆 ≈ 10.0 nm); and (b) the theoretical spec- Figure 2. (a) The theoretical spectrum of temperature (DT = 10 C) response ( Dl  10.0 nm); and (b) the theoretical trum of pressure (𝛥𝑃 ≈ 0.1 MPa) response (𝛥𝜆 ≈ 0.9 nm). spectrum of pressure (DP  0.1 MPa) response (Dl  0.9 nm). The The sen sensitivity sitivity (S) (S) o off t the he p prrepar epared ed sensor sensor is is defi defined ned as as the the ratio ratio of of the wa the wavelength velength shift shift over over the the corr correspondin espondinggtemperatur temperatur ee or pr or pressur essur e e change. changeSimilarly . Similarly, we , we define definthe e the ra- ratio of the minimum resolution W (W = 20 pm) of the spectrometer (OSA) over the sensor tio of the minimum resolution 𝛺 (𝛺= 20 pm ) of the spectrometer (OSA) over the sensor sensitivity S as the minimum measurement accuracy (MMA)—and the MMA is given sensitivity S as the minimum measurement accuracy ( )—and the is given by by [23] [23] MMA = (8) = (8) As Table 1 shows the optic and physical properties of the diaphragms, this paper fully demonstrates the different properties of materials that make up different FP cavities. The As Table 1 shows the optic and physical properties of the diaphragms, this paper fully thermo–optical coefficient and thermal expansion coefficient are closely related to the tem- demonstrates the different properties of materials that make up different FP cavities. The perature effect, which, respectively, affect the refractive index and the cavity length of the thermo–optical coefficient and thermal expansion coefficient are closely related to the diaphragms. Young’s modulus is the modulus of elasticity along the longitudinal direction, temperature effect, which, respectively, affect the refractive index and the cavity length of which also indicates the rigidity of the material. The lower Young’s modulus induces the the diaphragms. Young’s modulus is the modulus of elasticity along the longitudinal di- greater elastic deformation. Additionally, the tensile strength is similar to Young’s modulus. rection, which also indicates the rigidity of the material. The lower Young’s modulus in- Poisson’s ratio effectively reflects the elastic constant of material transverse deformation. duces the greater elastic deformation. Additionally, the tensile strength is similar to The pressure sensing characteristics of the sensors mainly rely on the Young’s modulus Young’s modulus. Poisson’s ratio effectively reflects the elastic constant of material trans- and Poisson’s ratio of the diaphragms. verse deformation. The pressure sensing characteristics of the sensors mainly rely on the Table Young’s mod 1. The optic ulu and s and physical Poispr son’s r operties atio o of the f the diaphrag diaphragms ms. [24– 29]. PDMS Ecoflex0030 Silicone Rubber Epoxy Resin AB Table 1. The optic and physical properties of the diaphragms [24–29]. 1 4 4 4 Thermo–optic coefficient ( C ) 5.0  10 3.1  10 1.0  10 6 4 6 Thermal expansion coefficient (m/m C) 300  10 5.9–7.9 Ecofle  10 x0030 Silicone 1.948  10 PDMS Epoxy Resin AB Refractive index (RIU) 1.418 1.41–1.53 1.45–1.52 Rubber Young’s modulus (MPa) 5 2 21,250 Thermo–optic coefficient Poisson’s ratio 0.46 0.369 0.25 −5.0 10 −3.1 10 −1.0 10 3 4 Tensile strength (psi) −1 200 1.015  10 1.044  10 (°C ) Thermal expansion coeffi- 300 10 5.9–7.9 10 1.948 10 cient (m/m°C) 2.2. Fabrication Process Refractive index (RIU) 1.418 1.41–1.53 1.45–1.52 Figure 3 illustrates the fabrication process that is divided into the five steps: (I) The Young’s modulus (MPa) 5 2 21,250 standard single mode fiber (SMF1) is well cut by a fiber cleaver; (II) PDMS, Ecoflex0030 and epoxy resin AB are prepared and stored at ratios of 10:1, 1:1, and 1:1, respectively. The Poisson’s ratio 0.46 0.369 0.25 Material 1 is transferred to SMF2 by the multiple transfers method to form the appropriate Tensile strength (psi) 1.015 10 200 1.044 10 diaphragm thickness; (III) The SMF1, SMF2 coated with Material 1 are fixed on the fiber holder. The motor is tuned, which holds SMF2, to coaxially shift the tail of SMF2 to access 2.2. Fabrication Process to the tail of SMF1, before moving it away immediately; (IV) The SMF1 is left standing or heated to make the Material 1 solidified to form the diaphragms. The Material 2 is transferred onto Material 1 by repeating steps (II) and (III); (V) The diaphragms are cured 𝑀𝑀𝐴 𝑀𝑀𝐴 𝑀𝑀𝐴 Photonics 2021, 8, x FOR PEER REVIEW 5 of 14 Figure 3 illustrates the fabrication process that is divided into the five steps: (I) The standard single mode fiber (SMF1) is well cut by a fiber cleaver; (Ⅱ) PDMS, Ecoflex0030 and epoxy resin AB are prepared and stored at ratios of 10:1, 1:1, and 1:1, respectively. The Material 1 is transferred to SMF2 by the multiple transfers method to form the appro- priate diaphragm thickness; (Ⅲ) The SMF1, SMF2 coated with Material 1 are fixed on the fiber holder. The motor is tuned, which holds SMF2, to coaxially shift the tail of SMF2 to access to the tail of SMF1, before moving it away immediately; (Ⅳ) The SMF1 is left stand- ing or heated to make the Material 1 solidified to form the diaphragms. The Material 2 is transferred onto Material 1 by repeating steps (Ⅱ) and (Ⅲ); (Ⅴ) The diaphragms are cured Photonics 2021, 8, 138 5 of 14 on SMF1 and a well-fabricated composite structure with dual FP cavities is completed. In addition, the diaphragm thickness can be controlled by increasing or reducing the transfer on SMF1 and a well-fabricated composite structure with dual FP cavities is completed. In addition, the diaphragm thickness can be controlled by increasing or reducing the transfer times. The multiple transfer method can also be used to increase the diaphragm thickness times. The multiple transfer method can also be used to increase the diaphragm thickness in batches. Consequently, we achieve a controllable cavity length of about 10–30 μm. It in batches. Consequently, we achieve a controllable cavity length of about 10–30 m. It can can be heated properly to increase the solidification of the diaphragms, which is also help- be heated properly to increase the solidification of the diaphragms, which is also helpful to control the cavity length. ful to control the cavity length. Figure 3. The preparation process of the composite structure based on dual FP cavities. Figure 3. The preparation process of the composite structure based on dual FP cavities. Figure 4a,c demonstrate the interference spectra of a single-cavity structure and a dual-cavity structure (S ) based on the three-beam interference principle. Figure 4a,c can Figure 4a,c demonstrate t 1 he interference spectra of a single-cavity structure and a only display the interference spectrum within the range of 1525–1610 nm due to the limited dual-cavity structure (𝑆 ) based on the three-beam interference principle. Figure 4a,c can bandwidth of ASE light source. The free spectrum ranges of the sensors are 53 and 27 nm, only di respectively splay th , owing e interference spectrum wi to the difference of cavity length thi and n the ra refractivenge of index. Simultaneously 1525–1610, nm due to the lim- different FP cavities have different contributions to the reflected intensity. This can be ited bandwidth of ASE light source. The free spectrum ranges of the sensors are 53 and 27 demonstrated by the fast Fourier transform (FFT) of the total reflected spectrum of the nm, respectively, owing to the difference of cavity length and refractive index. Simultane- sensor. As shown in Figure 4d, there are two particularly distinct frequency peaks, labeled 1 1 Peak 1 (0.03525 nm ) and Peak 2 (0.05875 nm ). ously, different FP cavities have different contributions to the reflected intensity. This can be demonstrated by the fast Fourier transform (FFT) of the total reflected spectrum of the sensor. As shown in Figure 4d, there are two particularly distinct frequency peaks, labeled −1 −1 Peak 1 (0.03525 nm ) and Peak 2 (0.05875 nm ). Photonics 2021, 8, x FOR PEER REVIEW 6 of 14 Photonics 2021, 8, 138 6 of 14 (a) (b) (c) (d) Figure 4. Comparison of the fabricated FP sensors: (a) the interference spectrum of a single cavity sensor; (b) the Fourier Figure 4. Comparison of the fabricated FP sensors: (a) the interference spectrum of a single cavity sensor; (b) the Fourier transform spectrum of Figure 4a; (c) the interference spectrum of a dual cavities sensor 𝑆 ; and (d) the Fourier transform transform spectrum of Figure 4a; (c) the interference spectrum of a dual cavities sensor S ; and (d) the Fourier transform spectrum of 𝑆 . spectrum of S . 3. Experimental Results and Analysis 3. Experimental Results and Analysis To ensure the reliability and accuracy of the sensors, this paper carried out a compar- at To ive ensur anaely the sis, m reliability ainly d and iscuss accuracy ing three of t the ypes of co sensors, this mposite structure, paper carried outfollow a compara- ed by 𝑆 - tive analysis, mainly discussing three types of composite structure, followed by S -Epoxy Epoxy resin AB/PDMS; 𝑆 -Ecoflex0030 silicone rubber/PDMS, 𝑆 -PDMS/Ecoflex0030 sil- resin AB/PDMS; S -Ecoflex0030 silicone rubber/PDMS, S -PDMS/Ecoflex0030 silicone icone rubber. The experimental results showed that the other three composite structures 2 3 rubber. The experimental results showed that the other three composite structures have have poor responses to the temperature/pressure. Therefore, they are not discussed in this poor responses to the temperature/pressure. Therefore, they are not discussed in this paper. paper. Table 2 displays the diaphragm thickness of the above three composite (𝑆 , 𝑆 and Table 2 displays the diaphragm thickness of the above three composite (S , S and S ) for 𝑆 ) for temperature/pressure sensing. 1 2 3 temperature/pressure sensing. Table 2. Diaphragm thickness of each composite structure with different materials. Table 2. Diaphragm thickness of each composite structure with different materials. Composite Structure Material Thickness Composite Structure Material Thickness Material 1: AB 32 μm Material 1: AB 32 m S Material 2: PDMS 11 μm Material 2: PDMS 11 m Material 1: Ecoflex0030 31 μm Material 1: Ecoflex0030 31 m Material 2: PDMS 10 m Material 2: PDMS 10 μm Material 1: PDMS 28 m S Material 1: PDMS 28 μm Material 2: Ecoflex0030 12 m Material 2: Ecoflex0030 12 μm Photonics 2021, 8, x FOR PEER REVIEW 7 of 14 Photonics 2021, 8, 138 7 of 14 3.1. Temperature Sensitivity Analysis The three samples were tested by the system shown in Figure 5. The temperature 3.1. Temperature Sensitivity Analysis increased from 40 to 150 °C with increments of 10 °C. Meanwhile, the spectrometer per- The three samples were tested by the system shown in Figure 5. The temperature sistently monitored the change of interference spectrum. The experimental results demon- increased from 40 to 150 C with increments of 10 C. Meanwhile, the spectrometer persis- strate that the measurement range of 𝑆 was about 40–120 °C, while 𝑆 and 𝑆 have a tently monitored the change of interference spectr um. The experimental results demon- better strate tempera that the tmeasur ure response ement range around 4 of S was 0–130 about °C, 40–120 which is r C, while elated to t S andhSe unique pr have a operties 1 2 3 better temperature response around 40–130 C, which is related to the unique properties of of materials at high temperature. The stable experimental results over the range of 40–100 materials at high temperature. The stable experimental results over the range of 40–100 C °C were selected for analysis. were selected for analysis. Figure 5. Temperature detection system. Figure 5. Temperature detection system. As shown in Figure 6a, this clearly illustrates that the interference peaks exhibit a significant wavelength shift as temperature increases; Figure 6b displays the linear fitting As shown in Figure 6a, this clearly illustrates that the interference peaks exhibit a analysis of temperature response. The experimental results show that the wavelength significant wavelength shift as temperature increases; Figure 6b displays the linear fitting shift is about 30.25 nm and the consistency of the red shift is excellent. The temperature analy sensitivity sis of te of mperature r the sensor ise about sponse. The 528 pm/eC. xperime As revealed ntal r in esu Figur lts show t e 7a, the htemperatur at the wave e length shift response of S has multiple interference peaks and a significant red shift from 1525 to is about 30.25 nm and the consistency of the red shift is excellent. The temperature sensi- 1610 nm. According to the fitting results shown in Figure 7b, the temperature sensitivity of tivity of the sensor is about 528 pm/°C. As revealed in Figure 7a, the temperature response S is marginally improved compared to S . The wavelength shifts reach up to 32.96 nm, 2 1 of 𝑆 has multiple interference peaks and a significant red shift from 1525 to 1610 nm. which clearly demonstrates relatively great fitness with a linear curve, and the calculated According to the fitting results shown in Figure 7b, the temperature sensitivity of 𝑆 is temperature sensitivity is approximately 540 pm/ C. Figure 8a shows the interference spectrum of S . The fitting result in Figure 8b demonstrates that the peak shift of the S is marginally improved compared to 3 𝑆 . The wavelength shifts reach up to 32.96 n 3 m, which about 61.46 nm, and the temperature sensitivity reaches up to 1033pm/ C. Almost exactly clearly demonstrates relatively great fitness with a linear curve, and the calculated tem- twice the amount S was investigated for having high sensitivity. S has the broadest 3 3 perature sensitivity is approximately 540 pm/°C. Figure 8a shows the interference spec- temperature response range and the highest temperature sensitivity in fabricated samples. trum of 𝑆 . The fitting result in Figure 8b demonstrates that the peak shift of the 𝑆 is This is due to the fact that temperature sensitivity mainly relies on the thermal expansion coefficient and thermo–optic coefficient of the diaphragms. Additionally, the spectra of the about 61.46 nm, and the temperature sensitivity reaches up to 1033pm/°C. Almost exactly latter two are slightly similar to a single FP cavity. This is caused by the little differences of twice the amount 𝑆 was investigated for having high sensitivity. 𝑆 has the broadest refractive indexes of the two diaphragms. temperature response range and the highest temperature sensitivity in fabricated samples. This is due to the fact that temperature sensitivity mainly relies on the thermal expansion coefficient and thermo–optic coefficient of the diaphragms. Additionally, the spectra of the latter two are slightly similar to a single FP cavity. This is caused by the little differ- ences of refractive indexes of the two diaphragms. Photonics 2021, 8, x FOR PEER REVIEW 8 of 14 Photonics 2021, 8, 138 8 of 14 Photonics 2021, 8, x FOR PEER REVIEW 8 of 14 (a) (b) (a) (b) Figure 6. (a) The temperature (ranging from 40 to 100 °C, in strides of 10 °C) response interference spectrum of Figure 6. (a) The temperature (ranging from 40 to 100 C, in strides of 10 C) response interference spectrum of Figure 6. (a) The temperature (ranging from 40 to 100 °C, in strides of 10 °C) response interference spectrum of 𝑆 (AB/PDMS); and (b) the temperature sensitivity of 𝑆 . S (AB/PDMS); and (b) the temperature sensitivity of S . 1 1 𝑆 (AB/PDMS); and (b) the temperature sensitivity of 𝑆 . (a) (b) (a) (b) Figure 7. (a) The temperature (ranging from 40 to 100 °C, in strides 10 °C) response interference spectrum of 𝑆 (Eco- Figure 7. (a) The temperature (ranging from 40 to 100 °C, in strides 10 °C) response interference spectrum of 𝑆 (Eco- Figure flex0030/PDMS); and ( 7. (a) The temperatur b) temperature sensiti e (ranging from vity of 40 to 𝑆 . 100 C, in strides 10 C) response interference spectrum of S flex0030/PDMS); and (b) temperature sensitivity of 𝑆 . (Ecoflex0030/PDMS); and (b) temperature sensitivity of S . Photonics 2021, 8, x FOR PEER REVIEW 9 of 14 Photonics 2021, 8, x FOR PEER REVIEW 9 of 14 Photonics 2021, 8, 138 9 of 14 (a) (b) (a) (b) Figure 8. (a) The temperature (ranging from 40 to 100 °C, in strides of 10 °C) response interference Scheme 3. (PDMS/Eco- Figure 8. (a) The temperature (ranging from 40 to 100 °C, in strides of 10 °C) response interference Scheme 3. (PDMS/Eco- Figure 8. (a) The temperature (ranging from 40 to 100 C, in strides of 10 C) response interference Scheme 3. flex0030); and (b) temperature sensitivity of 𝑆 . flex0030); and (b) temperature sensitivity of 𝑆 . (PDMS/Ecoflex0030); and (b) temperature sensitivity of S . 3.2. Pressure Sensitivity Analysis 3.2. Pressure Sensitivity Analysis 3.2. Pressure Sensitivity Analysis The schematic diagram of the pressure detection system is illustrated in Figure 9. The The schematic diagram of the pressure detection system is illustrated in Figure 9. The The schematic diagram of the pressure detection system is illustrated in Figure 9. The pressure test range is set from 100 to 400 kPa with increments of 10 kPa. The fabricated pressure test range is set from 100 to 400 kPa with increments of 10 kPa. The fabricated pressure test range is set from 100 to 400 kPa with increments of 10 kPa. The fabricated samples are placed in the air chamber and sealed with strong adhesive. samples are placed in the air chamber and sealed with strong adhesive. samples are placed in the air chamber and sealed with strong adhesive. Figure 9. Pressure detection system. Figure 9. Pressure detection system. Figure 9. Pressure detection system. Figure 10a displays that the interference spectrum of S has multiple prominent Figure 10a displays that the interference spectrum of 𝑆 has multiple prominent in- interference peaks over the detected wavelength range. The wavelength peaks increase as Figure 10a displays that the interference spectrum of 𝑆 has multiple prominent in- terference peaks over the detected wavelength range. The wavelength peaks increase as gas pressure increases. As described in Figure 10b, the peak shift is 4.78 nm and the fitting terference peaks over the detected wavelength range. The wavelength peaks increase as gas corr pressur elation e increases. coefficient AR s describe is about d in Fig 0.998. u The re 10 calculated b, the peak pre shif ssur t is 4.78 e sensitivity nm anis d t 16.0 he fi pm/kPa. tting gas pressure increases. As described in Figure 10b, the peak shift is 4.78 nm and the fitting correlation coefficient 𝑅 is about 0.998. The calculated pressure sensitivity is 16.0 The low sensitivity found in S is due to the poor elastic effects. Figure 11a also shows the correlation coefficient 𝑅 is about 0.998. The calculated pressure sensitivity is 16.0 pm/ red kPa. The shift oflow sens the spectra itivit as y foun pressur d in e incr 𝑆 is due to the p eases and the spectra oor elastic e present ffects. Figur a consiste ent 11a trend. also In pm/kPa. The low sensitivity found in 𝑆 is due to the poor elastic effects. Figure 11a also shows the Figure red 11b, shift of the the linear fitting spectra result as pres shows sure that incre the asewavelength s and the spect shift ra pre is about sent a c 10.48 onsistent nm, and shows the red shift of the spectra as pressure increases and the spectra present a consistent the pressure sensitivity reaches up to 34.6 pm/kPa. More than twice the amount of S was trend. In Figure 11b, the linear fitting result shows that the wavelength shift is about 10.48 trend. In Figure 11b, the linear fitting result shows that the wavelength shift is about 10.48 fabricated having a high sensitivity to S . Figure 12a depicts that the S has a marginally nm, and the pressure sensitivity reaches up to 34.6 pm/kPa. More than twice the amount 1 3 nm, and the pressure sensitivity reaches up to 34.6 pm/kPa. More than twice the amount lower pressure sensitivity compared to its high temperature sensitivity. According to of 𝑆 was fabricated having a high sensitivity to 𝑆 . Figure 12a depicts that the 𝑆 has a of 𝑆 was fabricated having a high sensitivity to 𝑆 . Figure 12a depicts that the 𝑆 has a Figure 12b, after linear fitting analysis, the wavelength shift and gas pressure sensitivity Photonics 2021, 8, 138 10 of 14 are calculated to be 9.09 nm and 30.2 pm/kPa. The gas pressure sensitivity of the S was the best one among the fabricated samples. The sample S , which was provided with a wide measurement range, exhibits good response under high pressure. Photonics 2021, 8, x FOR PEER REVIEW 10 of 14 Photonics 2021, 8, x FOR PEER REVIEW 10 of 14 As Table 3 shows, the performances of the proposed sensor and existing reports were concluded. The results demonstrate that the composite structures with dual FP cavities proposed in this paper possess moderate temperature and pressure sensitivity. However, we found that it is temporarily impossible to realize the simultaneous detection of the marginally lower pressure sensitivity compared to its high temperature sensitivity. Ac- marginally lower pressure sensitivity compared to its high temperature sensitivity. Ac- dual parameters due to the existing detection scheme and limited bandwidth of the used cording to Figure 12b, after linear fitting analysis, the wavelength shift and gas pressure cording to Figure 12b, after linear fitting analysis, the wavelength shift and gas pressure ASE light source. An ASE light source with a wider spectral range is needed to display sensitivity are calculated to be 9.09 nm and 30.2 pm/kPa. The gas pressure sensitivity of sensitivity are calculated to be 9.09 nm and 30.2 pm/kPa. The gas pressure sensitivity of more obvious three-beam interference peaks or valleys, and thus we can demodulate the the 𝑆 was the best one among the fabricated samples. The sample 𝑆 , which was pro- the 𝑆 was the best one among the fabricated samples. The sample 𝑆 , which was pro- high-frequency and low-frequency components to detect dual parameters simultaneously. vided with a wide measurement range, exhibits good response under high pressure. vided with a wide measurement range, exhibits good response under high pressure. (a) (b) (a) (b) Figure 10. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of 𝑆 Figure 10. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of S Figure 10. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of 𝑆 (AB/PDMS); and (b) pressure sensitivity of 𝑆 . (AB/PDMS); and (b) pressure sensitivity of 𝑆 . (AB/PDMS); and (b) pressure sensitivity of S . (a) (b) (a) (b) Figure 11. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of 𝑆 Figure 11. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of 𝑆 Figure 11. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of S (Ecoflex0030/PDMS); and (b) pressure sensitivity of 𝑆 . (Ecoflex0030/PDMS); and (b) pressure sensitivity of 𝑆 . (Ecoflex0030/PDMS); and (b) pressure sensitivity of S . Photonics 2021, 8, 138 11 of 14 Photonics 2021, 8, x FOR PEER REVIEW 11 of 14 (a) (b) Figure 12. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of 𝑆 Figure 12. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of S (PDMS/Ecoflex0030); and (b) pressure sensitivity of 𝑆 . (PDMS/Ecoflex0030); and (b) pressure sensitivity of S . As Table 3 shows, the performances of the proposed sensor and existing reports were Table 3. Comparison for the performances of the proposed sensor and existing reports. concluded. The results demonstrate that the composite structures with dual FP cavities proposed in this paper possess moderate temperature and pressure sensitivity. However, Temperature Pressure Sensor Structure Simultaneous Ref. Sensitivity Sensitivity we found that it is temporarily impossible to realize the simultaneous detection of the FBG cascade FPI 223.4 pm/ C 24.99 pm/kPa Yes 2019 [5] dual parameters due to the existing detection scheme and limited bandwidth of the used Hybrid Miniature FPI with Dual Optical Cavities 2.9 nm/ C 12.2nm/kPa Yes 2014 [8] ASE light source. An ASE light source with a wider spectral range is needed to display SMF-SMF-HCF-CF 19.8nm/ C 98pm/kPa Yes 2018 [9] more obvious three-beam interference peaks or valleys, and thus we can demodulate the Dual-cavity FPI with Cascade Hollow-core Fibers 17 nm/ C 1.336 nm/kPa No 2018 [11] Hollow-Core Fiber-Based All-Fiber FPI 9.22 pm/ C 1.05 pm/kPa Yes 2019 [13] high-frequency and low-frequency components to detect dual parameters simultane- FBG incorporated FPI 0.871 pm/ C 4.071 pm/MPa Yes 2016 [15] ously. FPI based on Pendant Polymer Droplet 249 pm/ C 1.130 pm/kPa Yes 2015 [17] FPI embedded with Microspheres 7.1 pm/ C 2.126 pm/kPa Yes 2016 [18] Table 3. Comparison for the performances of the proposed sensor and existing reports. SMF-HCF-SMF 0.584 nm/ C 3.884 pm/kPa No 2019 [30] Diaphragm-Free Fiber-Optic FPI 14.8 pm/ C 4.28 pm/kPa No 2018 [31] FPI based on In-fiber Micro-cavity and Fiber-tip 0.0108 nm/ C Tempe 4.158 ratpm/kPa ure Pressure Yes Simultane- 2018 [32] Sensor Structure  Ref. A Dual-Core Photonic Crystal Fiber Sensor 20.7 pm/ C 3.47 pm/MPa No 2011 [33] Sensitivity Sensitivity ous S : 528 pm/ C S : 16.0 pm/kPa 1 1 Composite Structure with Dual FP Cavities FBG cascaS de : 540 FPI pm/ C 223.S 4 p : 34.6 m/°pm/kPa C 24.99 pm/kP No a Yes Our work 2019 [5] 2 2 S : 1033 pm/ C S : 30.2 pm/kPa 3 3 Hybrid Miniature FPI with 2.9 nm/°C 12.2nm/kPa Yes 2014 [8] Dual Optical Cavities In addition,SMF-SMF-HCF- as FigurCF 1 es 13 and 149.8 shownm/°C , this paper98pm/kPa also set up anotherYes experiment2018 [9] to verify the repeatability and stability of the sensor. Figure 14 shows the repeatability Dual-cavity FPI with Cascade 17 nm/°C 1.336 nm/kPa No 2018 [11] and stability of the sensors by linearly fitting the peak shift in the heating and cooling Hollow-core Fibers experiment. Figure 14 displays that the peak shift error over the temperature range of Hollow-Core Fiber-Based All- 9.22 pm/°C 1.05 pm/kPa Yes 2019 [13] 60–100 C is caused by the residual temperature under the cooling process. The fitting Fiber FPI curves show similar slopes and a high degree of coincidence. It was proven that the dual FBG incorporated FPI 0.871 pm/°C 4.071 pm/MPa Yes 2016 [15] FP cavities structure has excellent recovery capability for the thermal expansion effect and FPI based on Pendant Polymer thermo–optical effect. 249 pm/°C 1.130 pm/kPa Yes 2015 [17] Droplet FPI embedded with Micro- 7.1 pm/°C 2.126 pm/kPa Yes 2016 [18] spheres SMF-HCF-SMF 0.584 nm/°C 3.884 pm/kPa No 2019 [30] Diaphragm-Free Fiber-Optic 14.8 pm/°C 4.28 pm/kPa No 2018 [31] FPI FPI based on In-fiber Micro-cav- 0.0108 nm/°C 4.158 pm/kPa Yes 2018 [32] ity and Fiber-tip Photonics 2021, 8, x FOR PEER REVIEW 12 of 14 Photonics 2021, 8, x FOR PEER REVIEW 12 of 14 A Dual-Core Photonic Crystal A Dual-Core Photonic Crystal 20.7 pm/°C −3.47 pm/MPa No 2011 [33] 20.7 pm/°C −3.47 pm/MPa No 2011 [33] Fiber Sensor Fiber Sensor 𝑆 : 528 pm/°C 𝑆 : 16.0 pm/kPa 𝑆 : 528 pm/°C 𝑆 : 16.0 pm/kPa Composite Structure with Dual Composite Structure with Dual 𝑆 : 540 pm/°C 𝑆 : 34.6 pm/kPa No Our work 𝑆 : 540 pm/°C 𝑆 : 34.6 pm/kPa No Our work FP Cavities FP Cavities 𝑆 : 1033pm/°C 𝑆 : 30.2 pm/kPa 𝑆 : 1033pm/°C 𝑆 : 30.2 pm/kPa In addition, as Figures 13 and 14 show, this paper also set up another experiment to In addition, as Figures 13 and 14 show, this paper also set up another experiment to verify the repeatability and stability of the sensor. Figure 14 shows the repeatability and verify the repeatability and stability of the sensor. Figure 14 shows the repeatability and stability of the sensors by linearly fitting the peak shift in the heating and cooling experi- stability of the sensors by linearly fitting the peak shift in the heating and cooling experi- ment. Figure 14 displays that the peak shift error over the temperature range of 60–100 °C ment. Figure 14 displays that the peak shift error over the temperature range of 60–100 °C is caused by the residual temperature under the cooling process. The fitting curves show is caused by the residual temperature under the cooling process. The fitting curves show similar slopes and a high degree of coincidence. It was proven that the dual FP cavities similar slopes and a high degree of coincidence. It was proven that the dual FP cavities Photonics 2021, 8, 138 12 of 14 structure has excellent recovery capability for the thermal expansion effect and thermo– structure has excellent recovery capability for the thermal expansion effect and thermo– optical effect. optical effect. (a) (b) (a) (b) Figure 13. The performances for the repeatability and stability of the sensor: (a) the interference Scheme 40. to 100 °C; and Figure 13. The performances for the repeatability and stability of the sensor: (a) the interference Scheme 40. to 100 C; and Figure 13. The performances for the repeatability and stability of the sensor: (a) the interference Scheme 40. to 100 °C; and (b) the interference spectrum with temperature drops from 100 to 40 °C. (b) the interference spectrum with temperature drops from 100 to 40 C. (b) the interference spectrum with temperature drops from 100 to 40 °C. Figure 14. The sensitivity of the repeatability and stability of the sensor. Figure 14. The sensitivity of the repeatability and stability of the sensor. Figure 14. The sensitivity of the repeatability and stability of the sensor. 4. Conclusions 4. Conclusions 4. Conclusions In this paper, a novel composite structure composed of dual FP cavities for fiber sens- In this paper, a novel composite structure composed of dual FP cavities for fiber In this paper, a novel composite structure composed of dual FP cavities for fiber sens- ing based on the multiple transfer method was proposed to measure the temperature and sensing based on the multiple transfer method was proposed to measure the temperature ing based on the multiple transfer method was proposed to measure the temperature and and pressure, which possesses the advantages of proper sensitivity, simple fabrication, cost- effectiveness, and controllable cavity length. It was proven that the measured temperature or pressure sensitivity was closely related to the properties and combination modes of the diaphragms. According to the experimental results, by optimizing the combinations and parameters of dual-diaphragms, this study found that the temperature or pressure sensitivity can be adjusted over a certain range within the test temperature range of 40–100 C and a pressure range of 100–400 kPa. This shows that the composite structure designed with dual FP cavities in this study has a proper sensitivity and can meet various sensitivity-demanding application scenarios. Author Contributions: Conceptualization, J.W., L.L., S.L., D.W. and G.W.; methodology, J.W. and G.W.; formal analysis, J.W. and L.L.; writing—original draft preparation, J.W., L.L. and S.L.; writing— review and editing, J.W., L.L., S.L., W.W., M.S. and G.W.; visualization, J.W., W.W. and D.W.; and super- vision, G.W. and M.H. All authors have read and agreed to the published version of the manuscript. Photonics 2021, 8, 138 13 of 14 Funding: This research was funded by Hainan Key R&D Program (ZDYF2019115), the National Natural Science Foundation of China (Nos. 61865005 and 61762033), the Open Project Program of Wuhan National Laboratory for Optoelectronics (No. 2020WNLOKF001), the Natural Science Foundation of Hainan Province (2019CXTD400 and 617079), the National Key Technology Support Program (2015BAH55F04 and 2015BAH55F01), the Major Science and Technology Project of Hainan Province (ZDKJ2016015), and the Scientific Research Staring Foundation of Hainan University (KYQD(ZR)1882). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request. Acknowledgments: We are very grateful to the relevant funds for their support of this paper. Conflicts of Interest: The authors declare no conflict of interest. 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Investigation of Composite Structure with Dual Fabry–Perot Cavities for Temperature and Pressure Sensing

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hv photonics Article Investigation of Composite Structure with Dual Fabry–Perot Cavities for Temperature and Pressure Sensing 1 1 1 1 1 , 2 1 1 , 2 , 3 , Jun Wang , Long Li , Shuaicheng Liu , Diyang Wu , Wei Wang , Ming Song , Guanjun Wang * 1 , 2 , and Mengxing Huang * School of Information and Communication Engineering, Hainan University, Haikou 570228, China; 20180581310080@hainanu.edu.cn (J.W.); 20181683310167@hainanu.edu.cn (L.L.); 20181683310012@hainanu.edu.cn (S.L.); 20181683310111@hainanu.edu.cn (D.W.); 20081000110002@hainanu.edu.cn (W.W.); 20171683310058@hainanu.edu.cn (M.S.) State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, China Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China * Correspondence: wangguanjun@hainanu.edu.cn (G.W.); huangmx09@hainanu.edu.cn (M.H.) Abstract: To deeply analyze the influence of diaphragm materials on the temperature and pressure sensitivity of Fabry–Perot interferometer-based dual-parameter fiber sensors, the multiple transfer method was used to fabricate the dual Fabry–Perot cavities, respectively, consisting of the following combinations: epoxy resin AB/polydimethylsiloxane (PDMS), Ecoflex0030 silicone rubber /PDMS, and PDMS/Ecoflex0030 silicone rubber. Experimental results show that the temperature sensitivities are, respectively, 528, 540, and 1033 pm/ C in the range of 40–100 C. Within the applied pres- sure range of 100–400 kPa, the pressure sensitivities are, respectively, 16.0, 34.6, and 30.2 pm/kPa. The proposed sensors have advantages of proper sensitivity, simple fabrication, cost-effectiveness, controllable cavity length, and suitability for practical sensing applications. Citation: Wang, J.; Li, L.; Liu, S.; Wu, D.; Wang, W.; Song, M.; Wang, G.; Keywords: Fabry–Perot cavity; temperature/pressure sensing; composite structure; multiple trans- Huang, M. Investigation of fer method Composite Structure with Dual Fabry–Perot Cavities for Temperature and Pressure Sensing. Photonics 2021, 8, 138. https://doi.org/10.3390/ 1. Introduction photonics8050138 Optical fiber sensors have been widely used in many domains owing to the advan- tages of low cost, high sensitivity, fast response time, and good stability. Simultaneously, Received: 24 February 2021 different sensor structures with diverse sensitivities to various parameters can be used to Accepted: 9 April 2021 realize dual- or even multi-parameter sensing, such as temperature, pressure, refractive Published: 23 April 2021 index, etc. [1,2]. Several studies have characterized the investigations of single or dual-parameter Publisher’s Note: MDPI stays neutral monitoring based on fiber tip bubble [3], Mach–Zehnder interferometer [4], fiber Bragg with regard to jurisdictional claims in gratings [5], Sagnac interferometer [6], etc. Due to the advantages of small size, good published maps and institutional affil- measurement performance, and survivability in complicated electromagnetic environ- iations. ment [7–19], Fabry–Perot Interferometer (FPI)-based dual-parameter fiber sensors have been extensively studied for the measurement of temperature and pressure. However, FPI-based fiber sensors impose the limitation of the durability and operation stability of the thin diaphragm that is used to form the FP cavity. Consequently, series of dual FP Copyright: © 2021 by the authors. cavities sensors based on various diaphragms to improve the sensing performance are Licensee MDPI, Basel, Switzerland. utilized to monitor the temperature and pressure [7–12], which share the characteristic of This article is an open access article ultra-high sensitivity. While endlessly pursuing high sensitivity will bring several problems distributed under the terms and which should be paid more attention, only this kind of fiber sensor can be applied for conditions of the Creative Commons ultra-precise monitoring applications. Moreover, it will result in a great challenge for the Attribution (CC BY) license (https:// design of the interrogator ’s bandwidth. Another dual FPI-based fiber sensor consisting creativecommons.org/licenses/by/ of two tiny segments of hollow-core fiber located at the end of lead-in single mode fiber, 4.0/). Photonics 2021, 8, 138. https://doi.org/10.3390/photonics8050138 https://www.mdpi.com/journal/photonics Photonics 2021, 8, x FOR PEER REVIEW 2 of 14 the design of the interrogator’s bandwidth. Another dual FPI-based fiber sensor consist- ing of two tiny segments of hollow-core fiber located at the end of lead-in single mode Photonics 2021, 8, 138 2 of 14 fiber, with a misalignment fusion splicing between the two hollow-core fibers with differ- ent core diameter, is reported in [13]. The proposed sensor can be used in simultaneous measurement of pressure and temperature, but its sensitivity should be significantly en- with a misalignment fusion splicing between the two hollow-core fibers with different core hanced. Some chemical etching method-assisted FPI fabrication processes are reported in diameter, is reported in [13]. The proposed sensor can be used in simultaneous measure- [14–16], which can provide good sensing performances. Unfortunately, the corrosive ef- ment of pressure and temperature, but its sensitivity should be significantly enhanced. fects of the chemical etching and the precise etching time are difficult to control. The au- Some chemical etching method-assisted FPI fabrication processes are reported in [14–16], thors of [17,18] demonstrated easy-to-fabricate dual-FPIs. However, the sensitivity and which can provide good sensing performances. Unfortunately, the corrosive effects of the repeatability of the fabrication for the FP cavity should be improved. By filling a hollow chemical etching and the precise etching time are difficult to control. The authors of [17,18] capillary with two sections of PDMS, which are fused to the single-mode fiber, the authors demonstrated easy-to-fabricate dual-FPIs. However, the sensitivity and repeatability of the of [19] investigated a novel dual FP cavities-based fiber sensor to detect the temperature fabrication for the FP cavity should be improved. By filling a hollow capillary with two sec- and pressure. However, the fabrication of this kind of structure is costly due to the fact an tions of PDMS, which are fused to the single-mode fiber, the authors of [19] investigated a especially made capillary cone is required to inject the PDMS into the hollow capillary. novel dual FP cavities-based fiber sensor to detect the temperature and pressure. However, Although many studies reported dual-parameter monitoring based on FPI struc- the fabrication of this kind of structure is costly due to the fact an especially made capillary tures, it is still worth continuously investigating and demonstrating the design of sensor cone is required to inject the PDMS into the hollow capillary. structures, the selection of sensor materials, and the optimization of fabrication processes. Although many studies reported dual-parameter monitoring based on FPI structures, Hence, in this paper, a novel FP cavity with composite structure for fiber sensing based it is still worth continuously investigating and demonstrating the design of sensor struc- on the multiple transfer method is proposed to measure the temperature and pressure, tures, the selection of sensor materials, and the optimization of fabrication processes. Hence, which possesses the advantages of proper sensitivity, simple fabrication, cost-effective- in this paper, a novel FP cavity with composite structure for fiber sensing based on the ness, and controllable cavity length. multiple transfer method is proposed to measure the temperature and pressure, which possesses the advantages of proper sensitivity, simple fabrication, cost-effectiveness, and 2. Sensing Principle and Fabrication Process controllable cavity length. 2.1. Sensing Principle 2. Sensing Principle and Fabrication Process 2.1. Sensing Principle The composite structure with dual FP cavities studied in this paper is depicted in The composite structure with dual FP cavities studied in this paper is depicted Figure 1. The sensor consists of a single mode fiber tail and two diaphragms with different in Figure 1. The sensor consists of a single mode fiber tail and two diaphragms with materials. different materials. Figure 1. The composite structure based on dual FP cavities. Figure 1. The composite structure based on dual FP cavities. The formed structure is composed of three reflective surfaces. The interfaces of The formed structure is composed of three reflective surfaces. The interfaces of SMF/Material 1, Material 1/Material 2, Material 2/air, respectively, are reflective surfaces SMF/Material 1, Material 1/Material 2, Material 2/air, respectively, are reflective surfaces 1, 2 and 3. When light reaches the end face of the optical fiber, the incident light will be 1, 2 and 3. When light reaches the end face of the optical fiber, the incident light will be reflected by the reflective surface within a certain wavelength range. However, part of reflected by the reflective surface within a certain wavelength range. However, part of the the light will still pass through Reflective Surface 1 and be reflected by Reflective Surfaces light will still pass through Reflective Surface 1 and be reflected by Reflective Surfaces 2 2 and 3 within a certain range. The interference spectrum results from the phase delay and 3 within a certain range. The interference spectrum results from the phase delay caused by optical path difference and the different reflectivity of each reflection surface of caused by optical path difference and the different reflectivity of each reflection surface of the composite structure, as the composite structure, as 4𝜋𝑛𝐿 4pnL I =𝐼= I + 𝐼 I+𝐼+ 2+2I I𝐼 cos 𝐼 𝑐𝑜𝑠 ++𝜑j (1) (1) 1 2 1 2 0 l 𝜆 Formula (1) is the dual-beam interference model of the single FP cavity. 𝐼 and 𝐼 Formula (1) is the dual-beam interference model of the single FP cavity. I and I 1 2 represent the reflected light intensity of the two-beam interference, 𝜆 is the wavelength represent the reflected light intensity of the two-beam interference, l is the wavelength of the incident light, 𝑛 is the refractive index of the FP cavity, 𝐿 is the length between of the incident light, n is the refractive index of the FP cavity, L is the length between two two reflected surfaces, and 𝜑 is the initial phase of the inference. reflected surfaces, and j is the initial phase of the inference. In this paper, an improved three-beam interference model based on the sensor struc- ture is presented and analyzed. According to the principle of multi-beam interference, the corresponding three-beam interference intensity [20] can be described as p p 4p 4p 4p I = I + I + I 2 I I cos n L + j + 2 I I cos n L + j 2 I I cos (n L + n L ) + j (2) 1 2 3 1 2 1 1 1 2 3 2 2 2 1 2 1 1 2 2 3 l l l Photonics 2021, 8, 138 3 of 14 where I , I , and I , respectively, are the reflected light intensity at the three reflecting 1 2 3 surfaces; j , j , and j are the initial phases of the reflected light; and n and n are the 1 2 3 1 2 refractive index of Material 1 and Material 2, respectively. L = l  (3) 2(l l ) 2 1 where l , l are the wavelengths corresponding to the adjacent peaks or valleys in the temperature and pressure test interference spectrum, and L is the length of FP cavity. The optical path difference (OPD) of the reflected light l can be expressed as l = 2nL (4) The wavelength spacing between adjacent peaks or valleys of the sensor interference spectrum is the free spectral range (FSR), the FSR is expressed as l l l 1 2 0 FSR = = (5) 2nL 2nL where the l and l are two adjacent peaks or valleys of the interference spectrum, l is 1 2 0 the mean wavelength of l and l , and L corresponds to the cavity length in formula (3). 1 2 The FSR is mainly affected by the thermal expansion coefficient (which is related to the change of L) and the thermo–optic coefficient (which is related to the change of n). It can be clearly seen that the FSR of the interference spectrum decreases as the n and L increase. For the temperature/pressure measurement of the FPI-based fiber sensor, the response of the sensor can be attributed to thermal expansion effects, thermo–optic effects, elastic deformation effects and refractive index factors. During temperature measurement, the refractive index and cavity length of the FP cavity change as the temperature increases, since these are related to the thermo–optical coefficient and the thermal expansion coefficient, respectively. This results in the variations of OPD. The OPD variation is defined as [21] Dl = 2DnL + 2nDL = 2nL(d + a)DT Dn = dDTn (6) DL = aDTL where Dl, DT, Dn and DL are the variations of OPD, temperature, refractive index and FP cavity length, respectively; d and a are the thermo–optic coefficient and thermal expansion coefficient, respectively, that are closely related to the properties of diaphragms. The formula (6) indicates that the temperature-induced OPD variations can be expressed as the change of FPI cavity length and refractive index. For the pressure measurement, the change of FPI cavity length depends on the diaphragm’s elastic deformation effects, and Formula (7) shows the pressure sensing principle [22]: 2 4 3 1 m r DL =  DP (7) 16 Eh where DP, h, r, m and E, respectively, are the change of pressure of the test environment, the thickness, the effective radius, Poisson’s Ratio, Young’s modulus of the diaphragms. The theoretical interference spectrum is depicted in Figure 2, which is simulated by Matlab platform. It is obtained by comprehensively considering the relevant parameters of the diaphragms, such as thermo–optical coefficient, thermal expansion coefficient, Young’s modulus and Poisson’s ratio in the proposed formulas. Photonics 2021, 8, x FOR PEER REVIEW 4 of 14 Photonics 2021, 8, 138 4 of 14 of the diaphragms, such as thermo–optical coefficient, thermal expansion coefficient, Young’s modulus and Poisson’s ratio in the proposed formulas. (a) (b) Figure 2. (a) The theoretical spectrum of temperature (𝛥𝑇 = 10 °C) response (𝛥𝜆 ≈ 10.0 nm); and (b) the theoretical spec- Figure 2. (a) The theoretical spectrum of temperature (DT = 10 C) response ( Dl  10.0 nm); and (b) the theoretical trum of pressure (𝛥𝑃 ≈ 0.1 MPa) response (𝛥𝜆 ≈ 0.9 nm). spectrum of pressure (DP  0.1 MPa) response (Dl  0.9 nm). The The sen sensitivity sitivity (S) (S) o off t the he p prrepar epared ed sensor sensor is is defi defined ned as as the the ratio ratio of of the wa the wavelength velength shift shift over over the the corr correspondin espondinggtemperatur temperatur ee or pr or pressur essur e e change. changeSimilarly . Similarly, we , we define definthe e the ra- ratio of the minimum resolution W (W = 20 pm) of the spectrometer (OSA) over the sensor tio of the minimum resolution 𝛺 (𝛺= 20 pm ) of the spectrometer (OSA) over the sensor sensitivity S as the minimum measurement accuracy (MMA)—and the MMA is given sensitivity S as the minimum measurement accuracy ( )—and the is given by by [23] [23] MMA = (8) = (8) As Table 1 shows the optic and physical properties of the diaphragms, this paper fully demonstrates the different properties of materials that make up different FP cavities. The As Table 1 shows the optic and physical properties of the diaphragms, this paper fully thermo–optical coefficient and thermal expansion coefficient are closely related to the tem- demonstrates the different properties of materials that make up different FP cavities. The perature effect, which, respectively, affect the refractive index and the cavity length of the thermo–optical coefficient and thermal expansion coefficient are closely related to the diaphragms. Young’s modulus is the modulus of elasticity along the longitudinal direction, temperature effect, which, respectively, affect the refractive index and the cavity length of which also indicates the rigidity of the material. The lower Young’s modulus induces the the diaphragms. Young’s modulus is the modulus of elasticity along the longitudinal di- greater elastic deformation. Additionally, the tensile strength is similar to Young’s modulus. rection, which also indicates the rigidity of the material. The lower Young’s modulus in- Poisson’s ratio effectively reflects the elastic constant of material transverse deformation. duces the greater elastic deformation. Additionally, the tensile strength is similar to The pressure sensing characteristics of the sensors mainly rely on the Young’s modulus Young’s modulus. Poisson’s ratio effectively reflects the elastic constant of material trans- and Poisson’s ratio of the diaphragms. verse deformation. The pressure sensing characteristics of the sensors mainly rely on the Table Young’s mod 1. The optic ulu and s and physical Poispr son’s r operties atio o of the f the diaphrag diaphragms ms. [24– 29]. PDMS Ecoflex0030 Silicone Rubber Epoxy Resin AB Table 1. The optic and physical properties of the diaphragms [24–29]. 1 4 4 4 Thermo–optic coefficient ( C ) 5.0  10 3.1  10 1.0  10 6 4 6 Thermal expansion coefficient (m/m C) 300  10 5.9–7.9 Ecofle  10 x0030 Silicone 1.948  10 PDMS Epoxy Resin AB Refractive index (RIU) 1.418 1.41–1.53 1.45–1.52 Rubber Young’s modulus (MPa) 5 2 21,250 Thermo–optic coefficient Poisson’s ratio 0.46 0.369 0.25 −5.0 10 −3.1 10 −1.0 10 3 4 Tensile strength (psi) −1 200 1.015  10 1.044  10 (°C ) Thermal expansion coeffi- 300 10 5.9–7.9 10 1.948 10 cient (m/m°C) 2.2. Fabrication Process Refractive index (RIU) 1.418 1.41–1.53 1.45–1.52 Figure 3 illustrates the fabrication process that is divided into the five steps: (I) The Young’s modulus (MPa) 5 2 21,250 standard single mode fiber (SMF1) is well cut by a fiber cleaver; (II) PDMS, Ecoflex0030 and epoxy resin AB are prepared and stored at ratios of 10:1, 1:1, and 1:1, respectively. The Poisson’s ratio 0.46 0.369 0.25 Material 1 is transferred to SMF2 by the multiple transfers method to form the appropriate Tensile strength (psi) 1.015 10 200 1.044 10 diaphragm thickness; (III) The SMF1, SMF2 coated with Material 1 are fixed on the fiber holder. The motor is tuned, which holds SMF2, to coaxially shift the tail of SMF2 to access 2.2. Fabrication Process to the tail of SMF1, before moving it away immediately; (IV) The SMF1 is left standing or heated to make the Material 1 solidified to form the diaphragms. The Material 2 is transferred onto Material 1 by repeating steps (II) and (III); (V) The diaphragms are cured 𝑀𝑀𝐴 𝑀𝑀𝐴 𝑀𝑀𝐴 Photonics 2021, 8, x FOR PEER REVIEW 5 of 14 Figure 3 illustrates the fabrication process that is divided into the five steps: (I) The standard single mode fiber (SMF1) is well cut by a fiber cleaver; (Ⅱ) PDMS, Ecoflex0030 and epoxy resin AB are prepared and stored at ratios of 10:1, 1:1, and 1:1, respectively. The Material 1 is transferred to SMF2 by the multiple transfers method to form the appro- priate diaphragm thickness; (Ⅲ) The SMF1, SMF2 coated with Material 1 are fixed on the fiber holder. The motor is tuned, which holds SMF2, to coaxially shift the tail of SMF2 to access to the tail of SMF1, before moving it away immediately; (Ⅳ) The SMF1 is left stand- ing or heated to make the Material 1 solidified to form the diaphragms. The Material 2 is transferred onto Material 1 by repeating steps (Ⅱ) and (Ⅲ); (Ⅴ) The diaphragms are cured Photonics 2021, 8, 138 5 of 14 on SMF1 and a well-fabricated composite structure with dual FP cavities is completed. In addition, the diaphragm thickness can be controlled by increasing or reducing the transfer on SMF1 and a well-fabricated composite structure with dual FP cavities is completed. In addition, the diaphragm thickness can be controlled by increasing or reducing the transfer times. The multiple transfer method can also be used to increase the diaphragm thickness times. The multiple transfer method can also be used to increase the diaphragm thickness in batches. Consequently, we achieve a controllable cavity length of about 10–30 μm. It in batches. Consequently, we achieve a controllable cavity length of about 10–30 m. It can can be heated properly to increase the solidification of the diaphragms, which is also help- be heated properly to increase the solidification of the diaphragms, which is also helpful to control the cavity length. ful to control the cavity length. Figure 3. The preparation process of the composite structure based on dual FP cavities. Figure 3. The preparation process of the composite structure based on dual FP cavities. Figure 4a,c demonstrate the interference spectra of a single-cavity structure and a dual-cavity structure (S ) based on the three-beam interference principle. Figure 4a,c can Figure 4a,c demonstrate t 1 he interference spectra of a single-cavity structure and a only display the interference spectrum within the range of 1525–1610 nm due to the limited dual-cavity structure (𝑆 ) based on the three-beam interference principle. Figure 4a,c can bandwidth of ASE light source. The free spectrum ranges of the sensors are 53 and 27 nm, only di respectively splay th , owing e interference spectrum wi to the difference of cavity length thi and n the ra refractivenge of index. Simultaneously 1525–1610, nm due to the lim- different FP cavities have different contributions to the reflected intensity. This can be ited bandwidth of ASE light source. The free spectrum ranges of the sensors are 53 and 27 demonstrated by the fast Fourier transform (FFT) of the total reflected spectrum of the nm, respectively, owing to the difference of cavity length and refractive index. Simultane- sensor. As shown in Figure 4d, there are two particularly distinct frequency peaks, labeled 1 1 Peak 1 (0.03525 nm ) and Peak 2 (0.05875 nm ). ously, different FP cavities have different contributions to the reflected intensity. This can be demonstrated by the fast Fourier transform (FFT) of the total reflected spectrum of the sensor. As shown in Figure 4d, there are two particularly distinct frequency peaks, labeled −1 −1 Peak 1 (0.03525 nm ) and Peak 2 (0.05875 nm ). Photonics 2021, 8, x FOR PEER REVIEW 6 of 14 Photonics 2021, 8, 138 6 of 14 (a) (b) (c) (d) Figure 4. Comparison of the fabricated FP sensors: (a) the interference spectrum of a single cavity sensor; (b) the Fourier Figure 4. Comparison of the fabricated FP sensors: (a) the interference spectrum of a single cavity sensor; (b) the Fourier transform spectrum of Figure 4a; (c) the interference spectrum of a dual cavities sensor 𝑆 ; and (d) the Fourier transform transform spectrum of Figure 4a; (c) the interference spectrum of a dual cavities sensor S ; and (d) the Fourier transform spectrum of 𝑆 . spectrum of S . 3. Experimental Results and Analysis 3. Experimental Results and Analysis To ensure the reliability and accuracy of the sensors, this paper carried out a compar- at To ive ensur anaely the sis, m reliability ainly d and iscuss accuracy ing three of t the ypes of co sensors, this mposite structure, paper carried outfollow a compara- ed by 𝑆 - tive analysis, mainly discussing three types of composite structure, followed by S -Epoxy Epoxy resin AB/PDMS; 𝑆 -Ecoflex0030 silicone rubber/PDMS, 𝑆 -PDMS/Ecoflex0030 sil- resin AB/PDMS; S -Ecoflex0030 silicone rubber/PDMS, S -PDMS/Ecoflex0030 silicone icone rubber. The experimental results showed that the other three composite structures 2 3 rubber. The experimental results showed that the other three composite structures have have poor responses to the temperature/pressure. Therefore, they are not discussed in this poor responses to the temperature/pressure. Therefore, they are not discussed in this paper. paper. Table 2 displays the diaphragm thickness of the above three composite (𝑆 , 𝑆 and Table 2 displays the diaphragm thickness of the above three composite (S , S and S ) for 𝑆 ) for temperature/pressure sensing. 1 2 3 temperature/pressure sensing. Table 2. Diaphragm thickness of each composite structure with different materials. Table 2. Diaphragm thickness of each composite structure with different materials. Composite Structure Material Thickness Composite Structure Material Thickness Material 1: AB 32 μm Material 1: AB 32 m S Material 2: PDMS 11 μm Material 2: PDMS 11 m Material 1: Ecoflex0030 31 μm Material 1: Ecoflex0030 31 m Material 2: PDMS 10 m Material 2: PDMS 10 μm Material 1: PDMS 28 m S Material 1: PDMS 28 μm Material 2: Ecoflex0030 12 m Material 2: Ecoflex0030 12 μm Photonics 2021, 8, x FOR PEER REVIEW 7 of 14 Photonics 2021, 8, 138 7 of 14 3.1. Temperature Sensitivity Analysis The three samples were tested by the system shown in Figure 5. The temperature 3.1. Temperature Sensitivity Analysis increased from 40 to 150 °C with increments of 10 °C. Meanwhile, the spectrometer per- The three samples were tested by the system shown in Figure 5. The temperature sistently monitored the change of interference spectrum. The experimental results demon- increased from 40 to 150 C with increments of 10 C. Meanwhile, the spectrometer persis- strate that the measurement range of 𝑆 was about 40–120 °C, while 𝑆 and 𝑆 have a tently monitored the change of interference spectr um. The experimental results demon- better strate tempera that the tmeasur ure response ement range around 4 of S was 0–130 about °C, 40–120 which is r C, while elated to t S andhSe unique pr have a operties 1 2 3 better temperature response around 40–130 C, which is related to the unique properties of of materials at high temperature. The stable experimental results over the range of 40–100 materials at high temperature. The stable experimental results over the range of 40–100 C °C were selected for analysis. were selected for analysis. Figure 5. Temperature detection system. Figure 5. Temperature detection system. As shown in Figure 6a, this clearly illustrates that the interference peaks exhibit a significant wavelength shift as temperature increases; Figure 6b displays the linear fitting As shown in Figure 6a, this clearly illustrates that the interference peaks exhibit a analysis of temperature response. The experimental results show that the wavelength significant wavelength shift as temperature increases; Figure 6b displays the linear fitting shift is about 30.25 nm and the consistency of the red shift is excellent. The temperature analy sensitivity sis of te of mperature r the sensor ise about sponse. The 528 pm/eC. xperime As revealed ntal r in esu Figur lts show t e 7a, the htemperatur at the wave e length shift response of S has multiple interference peaks and a significant red shift from 1525 to is about 30.25 nm and the consistency of the red shift is excellent. The temperature sensi- 1610 nm. According to the fitting results shown in Figure 7b, the temperature sensitivity of tivity of the sensor is about 528 pm/°C. As revealed in Figure 7a, the temperature response S is marginally improved compared to S . The wavelength shifts reach up to 32.96 nm, 2 1 of 𝑆 has multiple interference peaks and a significant red shift from 1525 to 1610 nm. which clearly demonstrates relatively great fitness with a linear curve, and the calculated According to the fitting results shown in Figure 7b, the temperature sensitivity of 𝑆 is temperature sensitivity is approximately 540 pm/ C. Figure 8a shows the interference spectrum of S . The fitting result in Figure 8b demonstrates that the peak shift of the S is marginally improved compared to 3 𝑆 . The wavelength shifts reach up to 32.96 n 3 m, which about 61.46 nm, and the temperature sensitivity reaches up to 1033pm/ C. Almost exactly clearly demonstrates relatively great fitness with a linear curve, and the calculated tem- twice the amount S was investigated for having high sensitivity. S has the broadest 3 3 perature sensitivity is approximately 540 pm/°C. Figure 8a shows the interference spec- temperature response range and the highest temperature sensitivity in fabricated samples. trum of 𝑆 . The fitting result in Figure 8b demonstrates that the peak shift of the 𝑆 is This is due to the fact that temperature sensitivity mainly relies on the thermal expansion coefficient and thermo–optic coefficient of the diaphragms. Additionally, the spectra of the about 61.46 nm, and the temperature sensitivity reaches up to 1033pm/°C. Almost exactly latter two are slightly similar to a single FP cavity. This is caused by the little differences of twice the amount 𝑆 was investigated for having high sensitivity. 𝑆 has the broadest refractive indexes of the two diaphragms. temperature response range and the highest temperature sensitivity in fabricated samples. This is due to the fact that temperature sensitivity mainly relies on the thermal expansion coefficient and thermo–optic coefficient of the diaphragms. Additionally, the spectra of the latter two are slightly similar to a single FP cavity. This is caused by the little differ- ences of refractive indexes of the two diaphragms. Photonics 2021, 8, x FOR PEER REVIEW 8 of 14 Photonics 2021, 8, 138 8 of 14 Photonics 2021, 8, x FOR PEER REVIEW 8 of 14 (a) (b) (a) (b) Figure 6. (a) The temperature (ranging from 40 to 100 °C, in strides of 10 °C) response interference spectrum of Figure 6. (a) The temperature (ranging from 40 to 100 C, in strides of 10 C) response interference spectrum of Figure 6. (a) The temperature (ranging from 40 to 100 °C, in strides of 10 °C) response interference spectrum of 𝑆 (AB/PDMS); and (b) the temperature sensitivity of 𝑆 . S (AB/PDMS); and (b) the temperature sensitivity of S . 1 1 𝑆 (AB/PDMS); and (b) the temperature sensitivity of 𝑆 . (a) (b) (a) (b) Figure 7. (a) The temperature (ranging from 40 to 100 °C, in strides 10 °C) response interference spectrum of 𝑆 (Eco- Figure 7. (a) The temperature (ranging from 40 to 100 °C, in strides 10 °C) response interference spectrum of 𝑆 (Eco- Figure flex0030/PDMS); and ( 7. (a) The temperatur b) temperature sensiti e (ranging from vity of 40 to 𝑆 . 100 C, in strides 10 C) response interference spectrum of S flex0030/PDMS); and (b) temperature sensitivity of 𝑆 . (Ecoflex0030/PDMS); and (b) temperature sensitivity of S . Photonics 2021, 8, x FOR PEER REVIEW 9 of 14 Photonics 2021, 8, x FOR PEER REVIEW 9 of 14 Photonics 2021, 8, 138 9 of 14 (a) (b) (a) (b) Figure 8. (a) The temperature (ranging from 40 to 100 °C, in strides of 10 °C) response interference Scheme 3. (PDMS/Eco- Figure 8. (a) The temperature (ranging from 40 to 100 °C, in strides of 10 °C) response interference Scheme 3. (PDMS/Eco- Figure 8. (a) The temperature (ranging from 40 to 100 C, in strides of 10 C) response interference Scheme 3. flex0030); and (b) temperature sensitivity of 𝑆 . flex0030); and (b) temperature sensitivity of 𝑆 . (PDMS/Ecoflex0030); and (b) temperature sensitivity of S . 3.2. Pressure Sensitivity Analysis 3.2. Pressure Sensitivity Analysis 3.2. Pressure Sensitivity Analysis The schematic diagram of the pressure detection system is illustrated in Figure 9. The The schematic diagram of the pressure detection system is illustrated in Figure 9. The The schematic diagram of the pressure detection system is illustrated in Figure 9. The pressure test range is set from 100 to 400 kPa with increments of 10 kPa. The fabricated pressure test range is set from 100 to 400 kPa with increments of 10 kPa. The fabricated pressure test range is set from 100 to 400 kPa with increments of 10 kPa. The fabricated samples are placed in the air chamber and sealed with strong adhesive. samples are placed in the air chamber and sealed with strong adhesive. samples are placed in the air chamber and sealed with strong adhesive. Figure 9. Pressure detection system. Figure 9. Pressure detection system. Figure 9. Pressure detection system. Figure 10a displays that the interference spectrum of S has multiple prominent Figure 10a displays that the interference spectrum of 𝑆 has multiple prominent in- interference peaks over the detected wavelength range. The wavelength peaks increase as Figure 10a displays that the interference spectrum of 𝑆 has multiple prominent in- terference peaks over the detected wavelength range. The wavelength peaks increase as gas pressure increases. As described in Figure 10b, the peak shift is 4.78 nm and the fitting terference peaks over the detected wavelength range. The wavelength peaks increase as gas corr pressur elation e increases. coefficient AR s describe is about d in Fig 0.998. u The re 10 calculated b, the peak pre shif ssur t is 4.78 e sensitivity nm anis d t 16.0 he fi pm/kPa. tting gas pressure increases. As described in Figure 10b, the peak shift is 4.78 nm and the fitting correlation coefficient 𝑅 is about 0.998. The calculated pressure sensitivity is 16.0 The low sensitivity found in S is due to the poor elastic effects. Figure 11a also shows the correlation coefficient 𝑅 is about 0.998. The calculated pressure sensitivity is 16.0 pm/ red kPa. The shift oflow sens the spectra itivit as y foun pressur d in e incr 𝑆 is due to the p eases and the spectra oor elastic e present ffects. Figur a consiste ent 11a trend. also In pm/kPa. The low sensitivity found in 𝑆 is due to the poor elastic effects. Figure 11a also shows the Figure red 11b, shift of the the linear fitting spectra result as pres shows sure that incre the asewavelength s and the spect shift ra pre is about sent a c 10.48 onsistent nm, and shows the red shift of the spectra as pressure increases and the spectra present a consistent the pressure sensitivity reaches up to 34.6 pm/kPa. More than twice the amount of S was trend. In Figure 11b, the linear fitting result shows that the wavelength shift is about 10.48 trend. In Figure 11b, the linear fitting result shows that the wavelength shift is about 10.48 fabricated having a high sensitivity to S . Figure 12a depicts that the S has a marginally nm, and the pressure sensitivity reaches up to 34.6 pm/kPa. More than twice the amount 1 3 nm, and the pressure sensitivity reaches up to 34.6 pm/kPa. More than twice the amount lower pressure sensitivity compared to its high temperature sensitivity. According to of 𝑆 was fabricated having a high sensitivity to 𝑆 . Figure 12a depicts that the 𝑆 has a of 𝑆 was fabricated having a high sensitivity to 𝑆 . Figure 12a depicts that the 𝑆 has a Figure 12b, after linear fitting analysis, the wavelength shift and gas pressure sensitivity Photonics 2021, 8, 138 10 of 14 are calculated to be 9.09 nm and 30.2 pm/kPa. The gas pressure sensitivity of the S was the best one among the fabricated samples. The sample S , which was provided with a wide measurement range, exhibits good response under high pressure. Photonics 2021, 8, x FOR PEER REVIEW 10 of 14 Photonics 2021, 8, x FOR PEER REVIEW 10 of 14 As Table 3 shows, the performances of the proposed sensor and existing reports were concluded. The results demonstrate that the composite structures with dual FP cavities proposed in this paper possess moderate temperature and pressure sensitivity. However, we found that it is temporarily impossible to realize the simultaneous detection of the marginally lower pressure sensitivity compared to its high temperature sensitivity. Ac- marginally lower pressure sensitivity compared to its high temperature sensitivity. Ac- dual parameters due to the existing detection scheme and limited bandwidth of the used cording to Figure 12b, after linear fitting analysis, the wavelength shift and gas pressure cording to Figure 12b, after linear fitting analysis, the wavelength shift and gas pressure ASE light source. An ASE light source with a wider spectral range is needed to display sensitivity are calculated to be 9.09 nm and 30.2 pm/kPa. The gas pressure sensitivity of sensitivity are calculated to be 9.09 nm and 30.2 pm/kPa. The gas pressure sensitivity of more obvious three-beam interference peaks or valleys, and thus we can demodulate the the 𝑆 was the best one among the fabricated samples. The sample 𝑆 , which was pro- the 𝑆 was the best one among the fabricated samples. The sample 𝑆 , which was pro- high-frequency and low-frequency components to detect dual parameters simultaneously. vided with a wide measurement range, exhibits good response under high pressure. vided with a wide measurement range, exhibits good response under high pressure. (a) (b) (a) (b) Figure 10. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of 𝑆 Figure 10. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of S Figure 10. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of 𝑆 (AB/PDMS); and (b) pressure sensitivity of 𝑆 . (AB/PDMS); and (b) pressure sensitivity of 𝑆 . (AB/PDMS); and (b) pressure sensitivity of S . (a) (b) (a) (b) Figure 11. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of 𝑆 Figure 11. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of 𝑆 Figure 11. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of S (Ecoflex0030/PDMS); and (b) pressure sensitivity of 𝑆 . (Ecoflex0030/PDMS); and (b) pressure sensitivity of 𝑆 . (Ecoflex0030/PDMS); and (b) pressure sensitivity of S . Photonics 2021, 8, 138 11 of 14 Photonics 2021, 8, x FOR PEER REVIEW 11 of 14 (a) (b) Figure 12. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of 𝑆 Figure 12. (a) The gas pressure (ranging from 100 to 400 kPa, in strides of 10 kPa) response interference spectrum of S (PDMS/Ecoflex0030); and (b) pressure sensitivity of 𝑆 . (PDMS/Ecoflex0030); and (b) pressure sensitivity of S . As Table 3 shows, the performances of the proposed sensor and existing reports were Table 3. Comparison for the performances of the proposed sensor and existing reports. concluded. The results demonstrate that the composite structures with dual FP cavities proposed in this paper possess moderate temperature and pressure sensitivity. However, Temperature Pressure Sensor Structure Simultaneous Ref. Sensitivity Sensitivity we found that it is temporarily impossible to realize the simultaneous detection of the FBG cascade FPI 223.4 pm/ C 24.99 pm/kPa Yes 2019 [5] dual parameters due to the existing detection scheme and limited bandwidth of the used Hybrid Miniature FPI with Dual Optical Cavities 2.9 nm/ C 12.2nm/kPa Yes 2014 [8] ASE light source. An ASE light source with a wider spectral range is needed to display SMF-SMF-HCF-CF 19.8nm/ C 98pm/kPa Yes 2018 [9] more obvious three-beam interference peaks or valleys, and thus we can demodulate the Dual-cavity FPI with Cascade Hollow-core Fibers 17 nm/ C 1.336 nm/kPa No 2018 [11] Hollow-Core Fiber-Based All-Fiber FPI 9.22 pm/ C 1.05 pm/kPa Yes 2019 [13] high-frequency and low-frequency components to detect dual parameters simultane- FBG incorporated FPI 0.871 pm/ C 4.071 pm/MPa Yes 2016 [15] ously. FPI based on Pendant Polymer Droplet 249 pm/ C 1.130 pm/kPa Yes 2015 [17] FPI embedded with Microspheres 7.1 pm/ C 2.126 pm/kPa Yes 2016 [18] Table 3. Comparison for the performances of the proposed sensor and existing reports. SMF-HCF-SMF 0.584 nm/ C 3.884 pm/kPa No 2019 [30] Diaphragm-Free Fiber-Optic FPI 14.8 pm/ C 4.28 pm/kPa No 2018 [31] FPI based on In-fiber Micro-cavity and Fiber-tip 0.0108 nm/ C Tempe 4.158 ratpm/kPa ure Pressure Yes Simultane- 2018 [32] Sensor Structure  Ref. A Dual-Core Photonic Crystal Fiber Sensor 20.7 pm/ C 3.47 pm/MPa No 2011 [33] Sensitivity Sensitivity ous S : 528 pm/ C S : 16.0 pm/kPa 1 1 Composite Structure with Dual FP Cavities FBG cascaS de : 540 FPI pm/ C 223.S 4 p : 34.6 m/°pm/kPa C 24.99 pm/kP No a Yes Our work 2019 [5] 2 2 S : 1033 pm/ C S : 30.2 pm/kPa 3 3 Hybrid Miniature FPI with 2.9 nm/°C 12.2nm/kPa Yes 2014 [8] Dual Optical Cavities In addition,SMF-SMF-HCF- as FigurCF 1 es 13 and 149.8 shownm/°C , this paper98pm/kPa also set up anotherYes experiment2018 [9] to verify the repeatability and stability of the sensor. Figure 14 shows the repeatability Dual-cavity FPI with Cascade 17 nm/°C 1.336 nm/kPa No 2018 [11] and stability of the sensors by linearly fitting the peak shift in the heating and cooling Hollow-core Fibers experiment. Figure 14 displays that the peak shift error over the temperature range of Hollow-Core Fiber-Based All- 9.22 pm/°C 1.05 pm/kPa Yes 2019 [13] 60–100 C is caused by the residual temperature under the cooling process. The fitting Fiber FPI curves show similar slopes and a high degree of coincidence. It was proven that the dual FBG incorporated FPI 0.871 pm/°C 4.071 pm/MPa Yes 2016 [15] FP cavities structure has excellent recovery capability for the thermal expansion effect and FPI based on Pendant Polymer thermo–optical effect. 249 pm/°C 1.130 pm/kPa Yes 2015 [17] Droplet FPI embedded with Micro- 7.1 pm/°C 2.126 pm/kPa Yes 2016 [18] spheres SMF-HCF-SMF 0.584 nm/°C 3.884 pm/kPa No 2019 [30] Diaphragm-Free Fiber-Optic 14.8 pm/°C 4.28 pm/kPa No 2018 [31] FPI FPI based on In-fiber Micro-cav- 0.0108 nm/°C 4.158 pm/kPa Yes 2018 [32] ity and Fiber-tip Photonics 2021, 8, x FOR PEER REVIEW 12 of 14 Photonics 2021, 8, x FOR PEER REVIEW 12 of 14 A Dual-Core Photonic Crystal A Dual-Core Photonic Crystal 20.7 pm/°C −3.47 pm/MPa No 2011 [33] 20.7 pm/°C −3.47 pm/MPa No 2011 [33] Fiber Sensor Fiber Sensor 𝑆 : 528 pm/°C 𝑆 : 16.0 pm/kPa 𝑆 : 528 pm/°C 𝑆 : 16.0 pm/kPa Composite Structure with Dual Composite Structure with Dual 𝑆 : 540 pm/°C 𝑆 : 34.6 pm/kPa No Our work 𝑆 : 540 pm/°C 𝑆 : 34.6 pm/kPa No Our work FP Cavities FP Cavities 𝑆 : 1033pm/°C 𝑆 : 30.2 pm/kPa 𝑆 : 1033pm/°C 𝑆 : 30.2 pm/kPa In addition, as Figures 13 and 14 show, this paper also set up another experiment to In addition, as Figures 13 and 14 show, this paper also set up another experiment to verify the repeatability and stability of the sensor. Figure 14 shows the repeatability and verify the repeatability and stability of the sensor. Figure 14 shows the repeatability and stability of the sensors by linearly fitting the peak shift in the heating and cooling experi- stability of the sensors by linearly fitting the peak shift in the heating and cooling experi- ment. Figure 14 displays that the peak shift error over the temperature range of 60–100 °C ment. Figure 14 displays that the peak shift error over the temperature range of 60–100 °C is caused by the residual temperature under the cooling process. The fitting curves show is caused by the residual temperature under the cooling process. The fitting curves show similar slopes and a high degree of coincidence. It was proven that the dual FP cavities similar slopes and a high degree of coincidence. It was proven that the dual FP cavities Photonics 2021, 8, 138 12 of 14 structure has excellent recovery capability for the thermal expansion effect and thermo– structure has excellent recovery capability for the thermal expansion effect and thermo– optical effect. optical effect. (a) (b) (a) (b) Figure 13. The performances for the repeatability and stability of the sensor: (a) the interference Scheme 40. to 100 °C; and Figure 13. The performances for the repeatability and stability of the sensor: (a) the interference Scheme 40. to 100 C; and Figure 13. The performances for the repeatability and stability of the sensor: (a) the interference Scheme 40. to 100 °C; and (b) the interference spectrum with temperature drops from 100 to 40 °C. (b) the interference spectrum with temperature drops from 100 to 40 C. (b) the interference spectrum with temperature drops from 100 to 40 °C. Figure 14. The sensitivity of the repeatability and stability of the sensor. Figure 14. The sensitivity of the repeatability and stability of the sensor. Figure 14. The sensitivity of the repeatability and stability of the sensor. 4. Conclusions 4. Conclusions 4. Conclusions In this paper, a novel composite structure composed of dual FP cavities for fiber sens- In this paper, a novel composite structure composed of dual FP cavities for fiber In this paper, a novel composite structure composed of dual FP cavities for fiber sens- ing based on the multiple transfer method was proposed to measure the temperature and sensing based on the multiple transfer method was proposed to measure the temperature ing based on the multiple transfer method was proposed to measure the temperature and and pressure, which possesses the advantages of proper sensitivity, simple fabrication, cost- effectiveness, and controllable cavity length. It was proven that the measured temperature or pressure sensitivity was closely related to the properties and combination modes of the diaphragms. According to the experimental results, by optimizing the combinations and parameters of dual-diaphragms, this study found that the temperature or pressure sensitivity can be adjusted over a certain range within the test temperature range of 40–100 C and a pressure range of 100–400 kPa. This shows that the composite structure designed with dual FP cavities in this study has a proper sensitivity and can meet various sensitivity-demanding application scenarios. Author Contributions: Conceptualization, J.W., L.L., S.L., D.W. and G.W.; methodology, J.W. and G.W.; formal analysis, J.W. and L.L.; writing—original draft preparation, J.W., L.L. and S.L.; writing— review and editing, J.W., L.L., S.L., W.W., M.S. and G.W.; visualization, J.W., W.W. and D.W.; and super- vision, G.W. and M.H. All authors have read and agreed to the published version of the manuscript. Photonics 2021, 8, 138 13 of 14 Funding: This research was funded by Hainan Key R&D Program (ZDYF2019115), the National Natural Science Foundation of China (Nos. 61865005 and 61762033), the Open Project Program of Wuhan National Laboratory for Optoelectronics (No. 2020WNLOKF001), the Natural Science Foundation of Hainan Province (2019CXTD400 and 617079), the National Key Technology Support Program (2015BAH55F04 and 2015BAH55F01), the Major Science and Technology Project of Hainan Province (ZDKJ2016015), and the Scientific Research Staring Foundation of Hainan University (KYQD(ZR)1882). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request. Acknowledgments: We are very grateful to the relevant funds for their support of this paper. Conflicts of Interest: The authors declare no conflict of interest. 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Journal

Photonics – Multidisciplinary Digital Publishing Institute

Published: Apr 23, 2021

Keywords: Fabry–Perot cavity; temperature/pressure sensing; composite structure; multiple transfer method

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