- Email: [email protected]
Polymer microfiber bridging Bi-tapered refractive index sensor based on evanescent field
Optics Communications 414 (2018) 134–139
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Polymer microfiber bridging Bi-tapered refractive index sensor based on evanescent field Ri-Qing Lv a , Qi Wang a,b, *, Bo-Tao Wang a , Yu Liu a , Lingxin Kong a a b
College of Information Science and Engineering, Northeastern University, Shenyang 110819, China State Key Laboratory of Synthetical Automation for Process Industries (Northeastern University), Shenyang 110819, China
a r t i c l e
i n f o
Keywords: Refractive index sensor Micro optical waveguide Tapered fiber PMMA PDMS High sensitivity
a b s t r a c t A PDMS/graphene enhanced PMMA micro optical waveguide sensor is reported in terms of fabrication method and optical characteristics. The micro optical waveguide with a diameter of 6 μm and a length of 800 μm is used as the sensing probe to realize refractive index (RI) measurement suspended in NaCl solutions with different concentrations. Experimental results show that the refractive index sensing sensitivity can reach 2027.97 nm/RIU within the refractive index ranging from 1.3333–1.3426. Research results show that PMMA/graphene micro optical waveguide doped with PDMS is an excellent high sensitive sensing technology in refractive index detection field. © 2018 Elsevier B.V. All rights reserved.
1. Introduction With the development of bioscience and clinical medicine, the biochemical have attracted the attention of more and more researchers at home and abroad. In the field of environmental monitoring, clinical testing, food testing and other scientific researches, refractive index is an important parameter that reflects the nature of liquid information. Therefore, refractive index sensors play an increasingly important role in the experiment research and practical application [1]. Optical fiber refractive index sensor has been widely studied due to the advantages of high sensitivity, corrosion resistance, high temperature resistance, small size and fast response. A number of fiber refractive index sensors based on different fibers have been proposed in recent years. In 2012, C. R. Liao et al. proposed a refractive index sensor based on optical fiber Fabry–Perot interferometer, the refractive index sensitivity obtained is 994 nm/RIU [2]. In 2015, Haiwei Fu et al. fabricated a novel refractive index Michelson interferometer is achieved 178.424 dB/RIU [3]. In 2016, Farid Ahmed et al. proposed a miniaturized tapered photonic crystal fiber Mach–Zehnder interferometer, whose sensitivity reached 334.03, 673.91, and 1426.70 nm/RIU within the RI range of 1.3327–1.3634, 1.3634–1.3917, and 1.3917–1.4204, respectively [4]. In 2017, Yong Zhao et al. made a spectrum online-tunable Mach– Zehnder interferometer with a sensitivity of 185.79 nm/RIU in the RI range of 1.3333–1.3673 [5]. However, these reported refractive index sensors exhibit low sensitivity and complex fabrication. In recent years,
polymer optical fibers have attracted increasing research interest [6– 8] for its advantages of easy handling, lower cost, great flexibility and have great potential application in optical sensing [9–11]. With the rapid development of material science, many excellent optical materials were found or developed for the preparation of polymer optical sensors, especially graphene [12,13] and Polydimethylsiloxane (PDMS) [14,15], etc. Graphene as a single atomic plane has superior optical, mechanical and electrical properties, such as high surface to volume ratio, high electron mobility, and stable structure a single atomic plane which could provide a large surface area for physical and chemical sensing. [16]. PDMS [17] is optically transparent, excellent biocompatibility, easy bonding with a variety of materials at room temperature and high elasticity (structural flexibility) because of the low Young’s modulus. In addition, PDMS is easy to concentrate in the air interface, and produce hydrophobic surface protective coating to improve the antifouling resistance. In 2014, Yao et al. [18] introduced a highly sensitive all-optical interferometric NH3 sensor based on a graphene/microfiber hybrid waveguide. Highly sensitive detection of NH3 concentration with a resolution of 0.3 ppm is achieved. In 2015, Liu et al. [19] introduced a compact light intensity controlled microfiber Mach–Zehnder interferometer, and experimental results indicate that the interference transmission spectra are highly sensitive to the applied excitation laser power density. An optimal ethyl orange weight percentage of 0.5 wt% has been experimentally acquired, with which the largest interference dip wavelength sensitivity reaches 0.02576 nm/(mW⋅cm−2 ).
* Corresponding author at: College of Information Science and Engineering, Northeastern University, Shenyang 110819, China.
E-mail address: [email protected] (Q. Wang). https://doi.org/10.1016/j.optcom.2017.12.063 Received 29 July 2017; Received in revised form 12 December 2017; Accepted 23 December 2017 0030-4018/© 2018 Elsevier B.V. All rights reserved.
R. Lv et al.
Optics Communications 414 (2018) 134–139
Fig. 1. The preparation process of graphene reinforced PDMS–PMMA elastomer.
In this article, we studied the fabrication process, optical characterization and applications of PDMS/graphene enhanced PMMA micro optical waveguide. Experimental results show that the refractive index sensitivity can reach 2027.97 nm/RIU within the refractive index ranging from 1.3333 to 1.3426. Predictably, the rise of new optical materials will bring a great improvement in optical fiber sensing field. 2. Preparation Bi-tapered fiber Mach–Zehnder interferometer structure [20,21] with taper diameter under 10 μm is very difficult to be prepared by the bunsen burner or fusion splicer, and can be easily broken in the experimental process due to its fragile characteristics as a quartz material. Based on the above considerations, a polymer microfiber sensor is presented in this paper. Manual stretching is the most convenient, fast and low-cost preparation method of polymer microfibers, but the polymer microfibers prepared by manual stretching are fragile due to the material properties defects of Polymethylmethacrylate (PMMA) collosol. Based on the above considerations, adding PDMS to enhance elasticity of PMMA [22] and adding graphene to improve optical sensitivity of organic microfiber [23] have been successfully implemented in this paper. The microfiber preparation steps are shown in Fig. 1: (a) PDMS collosol and graphene are mixed together at fraction ratio of 1:0.12; Adding minute quantities of cross-linking agent Tetraethyl orthosilicate (TEOS) and catalyst Dibutyltin dilaurate (DBTL), and stirring with a magnetic stirrer for 10 min, then getting Graphene–PDMS collosol. It is important to emphasize that Graphene–PDMS collosol after curing is the key materials of soft sensor; (b) Trichloromethane solution (CHCl3 ) and PMMA powder are mixed together at fraction ratio of 1.5:1; then mixing with trace Graphene–PDMS collosol and stirring with a magnetic stirrer for 15 h at constant temperature of 40 ◦ C, obtaining PDMS– PMMA collosol. (c) Using the fiber taper to rapid pull PDMS–PMMA collosol, then microfibers are got. Two fiber cones are drawn in optical fiber fusion splicer and overlapped with PDMS–PMMA microfiber by UV-lamp.
Fig. 2. The schematic diagrams of sensor probe structure and sensing principle. The barrier refer to the refractive index interface.
𝑍𝑦 =
𝑍𝑥 =
′
𝜆 1 , √ 2𝜋 sin 𝜃 2 − 𝑛 21
(3)
𝜆 tan 𝜃 . √ 𝜋 sin 𝜃 2 − 𝑛 21
(4)
As the ambient refractive index increases, the penetration depth of fundamental mode and low order modes will increase. The output interference intensity can be expressed as Eq. (5): √ 2𝜋Δ𝑛𝑒𝑓 𝑓 𝐿 𝐼 = 𝐼1 + 𝐼2 + 2 𝐼1 𝐼2 cos (5) 𝜆 where 𝐼1 and 𝐼2 are the light intensity transmit in the fiber core and fiber cladding, respectively, Δ𝑛𝑒𝑓 𝑓 is the difference between effective refractive index of fiber core and cladding, Δ𝑛𝑒𝑓 𝑓 = 𝑛𝑐𝑜𝑟𝑒 − 𝑛𝑐𝑙𝑎𝑑 .𝜑 = 𝑒𝑓 𝑓 𝑒𝑓 𝑓 2𝜋Δ𝑛𝑒𝑓 𝑓 𝐿
is phase difference. Δ𝑛𝑒𝑓 𝑓 will decrease along with ambient 𝜆 refractive index increases. So, Eq. (5) shows that output spectrum power will decrease more rapidly along with ambient refractive index increases. Therefore, optical path difference between fundamental mode and low order modes decreases leading to spectrum blue shift [28]. Sensor probe design includes two steps: fiber taper size programming and microfiber size programming. Firstly, the fiber taper with a cuttingedge cladding diameter of 15 μm and a length of 400 μm is prepared by fusion splicing mechanism. The key of fiber taper design is to ensure that fundamental mode could be excited into microfiber. The preparation process conforms to the law of Newton fluid mechanics, so the diameter
Based on Schrödinger Equation Eq. (1) [24,25], a wave function 𝜓𝑇 can go through the barrier. ℏ is Dirac constant, m is quality, d is derivative.
⎧𝐴𝑒𝑖𝑘1 𝑥 + 𝐴′ 𝑒−𝑖𝑘1 𝑥 ⎪ ′ Ψ = ⎨𝐵𝑒𝑖𝑘2 𝑥 + 𝐵 𝑒−𝑖𝑘2 𝑥 ⎪𝐶𝑒𝑖𝑘1 𝑥 + 𝐶 ′ 𝑒−𝑖𝑘1 𝑥 ⎩
is initial barrier, incident
where 𝜃 is the incident angle, 𝑛21 = 𝑛2 ∕𝑛1 . Equivalent wave distance, also known as Goos–Hä nchen Shift [27] is shown in
A PDMS–PMMA microfiber connects two tapered optical fibers, and the two connection points are cured by UV adhesive, as shown in Fig. 2.
2
√
= 𝐴 𝑒−𝑖𝑘1 𝑥 , transmission transmitted wave rapidly attenuates, and penetration depth is of the wavelength order. Equivalent reflection model is shown in Fig. 2. Penetration depth of evanescent wave [26] is shown as
3. Principle and sensor structure
2
̂ = 𝐸𝜓 ; 𝐻̂ = − ℏ 𝑑 + 𝑈 (𝑥) 𝐻𝜓 2𝑚 𝑑𝑥2 The wave function Ψ can be obtained by solving Eq. (1).
√
2𝑚(𝐸−𝑈0 ) 2𝑚𝐸 , 𝑘2 = , 𝑈0 ℏ2 ℏ2 wave: 𝜓𝐼 = 𝐴𝑒𝑖𝑘1 𝑥 , reflected wave: 𝜓𝑅 wave:𝜓𝑇 = 𝐶𝑒𝑖𝑘1 𝑥 . Eq. (2) shows that the
where 𝑘1 =
(1)
(2)
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Optics Communications 414 (2018) 134–139
dip within the wavelength range of 1520 nm–1570 nm, and in combination with the formula of free spectral range (FSR): Λ = 𝜆2 ∕Δ𝑛𝑒𝑓 𝑓 𝐿, and according to finite element software COMSOL Multiphysics, the calculated value of Δ𝑛𝑒𝑓 𝑓 was about 0.8, so we can determine that the range of microfiber length L should be less than 3 mm. The longer the fiber length, the higher the sensor sensitivity, but we can only capture 800 μm uniform microfiber in the actual process of preparation. The smaller the diameter of microfiber, the lower the highest mode order inside, the higher the sensor sensitivity is, but if the microfiber diameter is undersize, only fundamental mode (LP01 ) is allowed to exist inside the microfiber, the interference between different modes cannot be completed. We can eventually produce microfiber with 6 μm in diameter, the highest mode is LP61 in this microfiber by finite element analysis. The connecting way of fiber taper and the micro optical waveguide wire has 3 kinds: bridging, dislocation and coupling, as shown in Fig. 4. The bridging is better, but gravity occurs will causes microdisplacement between waveguide wire and fiber tapers in the progress of preparation. A waveguide wire which is 6 μm in diameter is analyzed, and the misalignment quantity are 5 μm, 10 μm, 15 μm, 20 μm and infinity separately, as shown in Fig. 5. According to the analysis results based on Finite Difference Beam Propagation Method (FD-BPM), light energy coupling from silica fiber taper into polymer micro optical waveguide wire is sufficient to use it for sensing when the displacement amount is less than 10 μm. Distance between silica fiber taper and polymer micro optical waveguide in practice is about 5 μm, therefore the spectrum of the sensor structure still has good feasibility. The simulation result shows that the effective refractive index of LP61 is 1.33 and the effective refractive index of LP01 is 1.49, and the interference will occur between LP61 and LP01 when diameter of microfiber is about 6 μm. The spectrum produces blue shift along with the increasing of ambient refractive index which fits the pattern of loworder modes interference [29].
Fig. 3. Relationship between fundamental mode field and fiber core diameter.
ratio between fiber core and cladding remains unchanged. The cuttingedge core diameter is about 1 μm. According to Matthews criterion, the relationships between fundamental mode field and fiber core diameter can be calculated, as shown in Fig. 3. It can be seen from Fig. 3 that higher order modes are excited when the fundamental mode enters into the fiber cladding at 𝑑𝑐𝑜𝑟𝑒 = 3.55. So the fiber taper with a cutting-edge core diameter of 1 μm can be suitable for this sensor. Secondly, diameter and length of microfiber are also a main factor for sensing. The length of microfiber determines the free spectral range (FSM) of the Mach–Zehnder interferometer. The light source used in the experiments is an ASE broadband light source with the wavelength range of 1520 nm–1570 nm. In order to ensure that at least one whole
Fig. 4. The schematic diagram of different overlap joints between silica fiber taper and polymer micro optical waveguide.
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Optics Communications 414 (2018) 134–139
Fig. 5. The mode field distribution of different dislocations between silica fiber taper and polymer micro optical waveguide.
The sensor based on mode interference meets the Mach–Zehnder interference formula [20], The interference spectrum period 𝑇 at 1550 nm wave band is about 𝑇 = Δ𝜆 ≈
1.552 𝜆2 ≈ = 0.01877μm. Δ𝑛𝑒𝑓 𝑓 ∗ 𝐿 (1.49 − 1.33) × 800
shows that the sensor has good repeatability. Compared with Ref. [30], the sensitivity of our proposed PDMS–PMMA microfiber is about 538 nm/RIU higher than that structure.
(6) 5. Conclusion
4. Experimental results and discussions
In this paper, a PDMS/graphene enhanced PMMA micro optical waveguide sensor based on evanescent field is reported in terms of fabrication and optical characteristics, which has been used to realize refractive index sensing. The single organic micro waveguide with a diameter of 6 μm and a length of 800 μm suspended in different concentrations NaCl solution is used as a sensing probe to realize refractive index measurement. Experimental results show that the refractive index sensitivity can reach 2027.97 nm/RIU within refractive index ranging from 1.3333–1.3426. Compared to traditional optical fibers, polymer waveguides show many amazing characteristics, such as a high fraction of evanescent fields, an ultra-small allowable bending radius and a small footprint. Polymer micro waveguides exhibit excellent mechanical flexibility and biocompatibility, as well as simple, low-cost fabrication in the fiber-sensing field.
The experimental setup and microscope photo of the sensor probe are shown in Fig. 6. The experimental setup includes an ASE broadband light source (ASE-C, CONQUER) with the wavelength range of 1520 nm1570 nm, an optical spectrum analyzer (AQ6370B, Yokogawa), a high precision micro displacement platform (NFP-x462, ZOLIX), a fluoride magnesium substrate, a video optical microscope with a charge coupled device (CCD) camera was used for observing and capturing sensor probe photos. Two fiber tapers were fixed by tunable six-axis microdisplacement stages and bridged by the PDMS–PMMA microfiber were immersed in NaCl solution with different concentrations. Sodium chloride solution: refractive index of NaCl solutions with mass concentration of 0% (pure water), 1%, 2%, 3%, 4% and 5% are 1.3333, 1.3352, 1.3371, 1.3389, 1.3407, 1.3426. The PDMS–PMMA microfiber sensor is dripped on an MgF2 substrate, and the diameter of the micro optical waveguide wire and fiber taper tip is 6 μm and 15 μm respectively. The surface of sensor was cleaned after the first experiment, but the results show a slight difference. Analysis shows that sodium chloride solution cannot be completely removed, which causes slight pollution in surface of sensor. Fig. 7 is the interference spectrum of the sensor at different NaCl solutions. The three sets experimental measurement data is given in Fig. 8. The sensitivity and fitting linearity of the twice measurements is 2025.71 nm/RIU, 2027.97 nm/RIU and 2021.49 nm/RIU, respectively, which
Acknowledgments This work was supported by the Natural Science Foundation of Liaoning Province under Grant 201602262, the Fundamental Research Funds for the Central Universities under Grant N160405001, N160408001 and N150401001, the National Science Foundation for Distinguished Young Scholars of China under Grant 61425003, the National Natural Science Foundation of China under Grant 51607028. 137
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Optics Communications 414 (2018) 134–139
Fig. 6. Experimental setup and microscope photo of the sensor probe.
Fig. 7. Interference spectrum of the sensor at different NaCl solutions. Fig. 8. The fitting curve of interference spectrum dips and ambient refractive index.
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139
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Polymer microfiber bridging Bi-tapered refractive index sensor based on evanescent field Ri-Qing Lv a , Qi Wang a,b, *, Bo-Tao Wang a , Yu Liu a , Lingxin Kong a a b
College of Information Science and Engineering, Northeastern University, Shenyang 110819, China State Key Laboratory of Synthetical Automation for Process Industries (Northeastern University), Shenyang 110819, China
a r t i c l e
i n f o
Keywords: Refractive index sensor Micro optical waveguide Tapered fiber PMMA PDMS High sensitivity
a b s t r a c t A PDMS/graphene enhanced PMMA micro optical waveguide sensor is reported in terms of fabrication method and optical characteristics. The micro optical waveguide with a diameter of 6 μm and a length of 800 μm is used as the sensing probe to realize refractive index (RI) measurement suspended in NaCl solutions with different concentrations. Experimental results show that the refractive index sensing sensitivity can reach 2027.97 nm/RIU within the refractive index ranging from 1.3333–1.3426. Research results show that PMMA/graphene micro optical waveguide doped with PDMS is an excellent high sensitive sensing technology in refractive index detection field. © 2018 Elsevier B.V. All rights reserved.
1. Introduction With the development of bioscience and clinical medicine, the biochemical have attracted the attention of more and more researchers at home and abroad. In the field of environmental monitoring, clinical testing, food testing and other scientific researches, refractive index is an important parameter that reflects the nature of liquid information. Therefore, refractive index sensors play an increasingly important role in the experiment research and practical application [1]. Optical fiber refractive index sensor has been widely studied due to the advantages of high sensitivity, corrosion resistance, high temperature resistance, small size and fast response. A number of fiber refractive index sensors based on different fibers have been proposed in recent years. In 2012, C. R. Liao et al. proposed a refractive index sensor based on optical fiber Fabry–Perot interferometer, the refractive index sensitivity obtained is 994 nm/RIU [2]. In 2015, Haiwei Fu et al. fabricated a novel refractive index Michelson interferometer is achieved 178.424 dB/RIU [3]. In 2016, Farid Ahmed et al. proposed a miniaturized tapered photonic crystal fiber Mach–Zehnder interferometer, whose sensitivity reached 334.03, 673.91, and 1426.70 nm/RIU within the RI range of 1.3327–1.3634, 1.3634–1.3917, and 1.3917–1.4204, respectively [4]. In 2017, Yong Zhao et al. made a spectrum online-tunable Mach– Zehnder interferometer with a sensitivity of 185.79 nm/RIU in the RI range of 1.3333–1.3673 [5]. However, these reported refractive index sensors exhibit low sensitivity and complex fabrication. In recent years,
polymer optical fibers have attracted increasing research interest [6– 8] for its advantages of easy handling, lower cost, great flexibility and have great potential application in optical sensing [9–11]. With the rapid development of material science, many excellent optical materials were found or developed for the preparation of polymer optical sensors, especially graphene [12,13] and Polydimethylsiloxane (PDMS) [14,15], etc. Graphene as a single atomic plane has superior optical, mechanical and electrical properties, such as high surface to volume ratio, high electron mobility, and stable structure a single atomic plane which could provide a large surface area for physical and chemical sensing. [16]. PDMS [17] is optically transparent, excellent biocompatibility, easy bonding with a variety of materials at room temperature and high elasticity (structural flexibility) because of the low Young’s modulus. In addition, PDMS is easy to concentrate in the air interface, and produce hydrophobic surface protective coating to improve the antifouling resistance. In 2014, Yao et al. [18] introduced a highly sensitive all-optical interferometric NH3 sensor based on a graphene/microfiber hybrid waveguide. Highly sensitive detection of NH3 concentration with a resolution of 0.3 ppm is achieved. In 2015, Liu et al. [19] introduced a compact light intensity controlled microfiber Mach–Zehnder interferometer, and experimental results indicate that the interference transmission spectra are highly sensitive to the applied excitation laser power density. An optimal ethyl orange weight percentage of 0.5 wt% has been experimentally acquired, with which the largest interference dip wavelength sensitivity reaches 0.02576 nm/(mW⋅cm−2 ).
* Corresponding author at: College of Information Science and Engineering, Northeastern University, Shenyang 110819, China.
E-mail address: [email protected] (Q. Wang). https://doi.org/10.1016/j.optcom.2017.12.063 Received 29 July 2017; Received in revised form 12 December 2017; Accepted 23 December 2017 0030-4018/© 2018 Elsevier B.V. All rights reserved.
R. Lv et al.
Optics Communications 414 (2018) 134–139
Fig. 1. The preparation process of graphene reinforced PDMS–PMMA elastomer.
In this article, we studied the fabrication process, optical characterization and applications of PDMS/graphene enhanced PMMA micro optical waveguide. Experimental results show that the refractive index sensitivity can reach 2027.97 nm/RIU within the refractive index ranging from 1.3333 to 1.3426. Predictably, the rise of new optical materials will bring a great improvement in optical fiber sensing field. 2. Preparation Bi-tapered fiber Mach–Zehnder interferometer structure [20,21] with taper diameter under 10 μm is very difficult to be prepared by the bunsen burner or fusion splicer, and can be easily broken in the experimental process due to its fragile characteristics as a quartz material. Based on the above considerations, a polymer microfiber sensor is presented in this paper. Manual stretching is the most convenient, fast and low-cost preparation method of polymer microfibers, but the polymer microfibers prepared by manual stretching are fragile due to the material properties defects of Polymethylmethacrylate (PMMA) collosol. Based on the above considerations, adding PDMS to enhance elasticity of PMMA [22] and adding graphene to improve optical sensitivity of organic microfiber [23] have been successfully implemented in this paper. The microfiber preparation steps are shown in Fig. 1: (a) PDMS collosol and graphene are mixed together at fraction ratio of 1:0.12; Adding minute quantities of cross-linking agent Tetraethyl orthosilicate (TEOS) and catalyst Dibutyltin dilaurate (DBTL), and stirring with a magnetic stirrer for 10 min, then getting Graphene–PDMS collosol. It is important to emphasize that Graphene–PDMS collosol after curing is the key materials of soft sensor; (b) Trichloromethane solution (CHCl3 ) and PMMA powder are mixed together at fraction ratio of 1.5:1; then mixing with trace Graphene–PDMS collosol and stirring with a magnetic stirrer for 15 h at constant temperature of 40 ◦ C, obtaining PDMS– PMMA collosol. (c) Using the fiber taper to rapid pull PDMS–PMMA collosol, then microfibers are got. Two fiber cones are drawn in optical fiber fusion splicer and overlapped with PDMS–PMMA microfiber by UV-lamp.
Fig. 2. The schematic diagrams of sensor probe structure and sensing principle. The barrier refer to the refractive index interface.
𝑍𝑦 =
𝑍𝑥 =
′
𝜆 1 , √ 2𝜋 sin 𝜃 2 − 𝑛 21
(3)
𝜆 tan 𝜃 . √ 𝜋 sin 𝜃 2 − 𝑛 21
(4)
As the ambient refractive index increases, the penetration depth of fundamental mode and low order modes will increase. The output interference intensity can be expressed as Eq. (5): √ 2𝜋Δ𝑛𝑒𝑓 𝑓 𝐿 𝐼 = 𝐼1 + 𝐼2 + 2 𝐼1 𝐼2 cos (5) 𝜆 where 𝐼1 and 𝐼2 are the light intensity transmit in the fiber core and fiber cladding, respectively, Δ𝑛𝑒𝑓 𝑓 is the difference between effective refractive index of fiber core and cladding, Δ𝑛𝑒𝑓 𝑓 = 𝑛𝑐𝑜𝑟𝑒 − 𝑛𝑐𝑙𝑎𝑑 .𝜑 = 𝑒𝑓 𝑓 𝑒𝑓 𝑓 2𝜋Δ𝑛𝑒𝑓 𝑓 𝐿
is phase difference. Δ𝑛𝑒𝑓 𝑓 will decrease along with ambient 𝜆 refractive index increases. So, Eq. (5) shows that output spectrum power will decrease more rapidly along with ambient refractive index increases. Therefore, optical path difference between fundamental mode and low order modes decreases leading to spectrum blue shift [28]. Sensor probe design includes two steps: fiber taper size programming and microfiber size programming. Firstly, the fiber taper with a cuttingedge cladding diameter of 15 μm and a length of 400 μm is prepared by fusion splicing mechanism. The key of fiber taper design is to ensure that fundamental mode could be excited into microfiber. The preparation process conforms to the law of Newton fluid mechanics, so the diameter
Based on Schrödinger Equation Eq. (1) [24,25], a wave function 𝜓𝑇 can go through the barrier. ℏ is Dirac constant, m is quality, d is derivative.
⎧𝐴𝑒𝑖𝑘1 𝑥 + 𝐴′ 𝑒−𝑖𝑘1 𝑥 ⎪ ′ Ψ = ⎨𝐵𝑒𝑖𝑘2 𝑥 + 𝐵 𝑒−𝑖𝑘2 𝑥 ⎪𝐶𝑒𝑖𝑘1 𝑥 + 𝐶 ′ 𝑒−𝑖𝑘1 𝑥 ⎩
is initial barrier, incident
where 𝜃 is the incident angle, 𝑛21 = 𝑛2 ∕𝑛1 . Equivalent wave distance, also known as Goos–Hä nchen Shift [27] is shown in
A PDMS–PMMA microfiber connects two tapered optical fibers, and the two connection points are cured by UV adhesive, as shown in Fig. 2.
2
√
= 𝐴 𝑒−𝑖𝑘1 𝑥 , transmission transmitted wave rapidly attenuates, and penetration depth is of the wavelength order. Equivalent reflection model is shown in Fig. 2. Penetration depth of evanescent wave [26] is shown as
3. Principle and sensor structure
2
̂ = 𝐸𝜓 ; 𝐻̂ = − ℏ 𝑑 + 𝑈 (𝑥) 𝐻𝜓 2𝑚 𝑑𝑥2 The wave function Ψ can be obtained by solving Eq. (1).
√
2𝑚(𝐸−𝑈0 ) 2𝑚𝐸 , 𝑘2 = , 𝑈0 ℏ2 ℏ2 wave: 𝜓𝐼 = 𝐴𝑒𝑖𝑘1 𝑥 , reflected wave: 𝜓𝑅 wave:𝜓𝑇 = 𝐶𝑒𝑖𝑘1 𝑥 . Eq. (2) shows that the
where 𝑘1 =
(1)
(2)
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dip within the wavelength range of 1520 nm–1570 nm, and in combination with the formula of free spectral range (FSR): Λ = 𝜆2 ∕Δ𝑛𝑒𝑓 𝑓 𝐿, and according to finite element software COMSOL Multiphysics, the calculated value of Δ𝑛𝑒𝑓 𝑓 was about 0.8, so we can determine that the range of microfiber length L should be less than 3 mm. The longer the fiber length, the higher the sensor sensitivity, but we can only capture 800 μm uniform microfiber in the actual process of preparation. The smaller the diameter of microfiber, the lower the highest mode order inside, the higher the sensor sensitivity is, but if the microfiber diameter is undersize, only fundamental mode (LP01 ) is allowed to exist inside the microfiber, the interference between different modes cannot be completed. We can eventually produce microfiber with 6 μm in diameter, the highest mode is LP61 in this microfiber by finite element analysis. The connecting way of fiber taper and the micro optical waveguide wire has 3 kinds: bridging, dislocation and coupling, as shown in Fig. 4. The bridging is better, but gravity occurs will causes microdisplacement between waveguide wire and fiber tapers in the progress of preparation. A waveguide wire which is 6 μm in diameter is analyzed, and the misalignment quantity are 5 μm, 10 μm, 15 μm, 20 μm and infinity separately, as shown in Fig. 5. According to the analysis results based on Finite Difference Beam Propagation Method (FD-BPM), light energy coupling from silica fiber taper into polymer micro optical waveguide wire is sufficient to use it for sensing when the displacement amount is less than 10 μm. Distance between silica fiber taper and polymer micro optical waveguide in practice is about 5 μm, therefore the spectrum of the sensor structure still has good feasibility. The simulation result shows that the effective refractive index of LP61 is 1.33 and the effective refractive index of LP01 is 1.49, and the interference will occur between LP61 and LP01 when diameter of microfiber is about 6 μm. The spectrum produces blue shift along with the increasing of ambient refractive index which fits the pattern of loworder modes interference [29].
Fig. 3. Relationship between fundamental mode field and fiber core diameter.
ratio between fiber core and cladding remains unchanged. The cuttingedge core diameter is about 1 μm. According to Matthews criterion, the relationships between fundamental mode field and fiber core diameter can be calculated, as shown in Fig. 3. It can be seen from Fig. 3 that higher order modes are excited when the fundamental mode enters into the fiber cladding at 𝑑𝑐𝑜𝑟𝑒 = 3.55. So the fiber taper with a cutting-edge core diameter of 1 μm can be suitable for this sensor. Secondly, diameter and length of microfiber are also a main factor for sensing. The length of microfiber determines the free spectral range (FSM) of the Mach–Zehnder interferometer. The light source used in the experiments is an ASE broadband light source with the wavelength range of 1520 nm–1570 nm. In order to ensure that at least one whole
Fig. 4. The schematic diagram of different overlap joints between silica fiber taper and polymer micro optical waveguide.
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Fig. 5. The mode field distribution of different dislocations between silica fiber taper and polymer micro optical waveguide.
The sensor based on mode interference meets the Mach–Zehnder interference formula [20], The interference spectrum period 𝑇 at 1550 nm wave band is about 𝑇 = Δ𝜆 ≈
1.552 𝜆2 ≈ = 0.01877μm. Δ𝑛𝑒𝑓 𝑓 ∗ 𝐿 (1.49 − 1.33) × 800
shows that the sensor has good repeatability. Compared with Ref. [30], the sensitivity of our proposed PDMS–PMMA microfiber is about 538 nm/RIU higher than that structure.
(6) 5. Conclusion
4. Experimental results and discussions
In this paper, a PDMS/graphene enhanced PMMA micro optical waveguide sensor based on evanescent field is reported in terms of fabrication and optical characteristics, which has been used to realize refractive index sensing. The single organic micro waveguide with a diameter of 6 μm and a length of 800 μm suspended in different concentrations NaCl solution is used as a sensing probe to realize refractive index measurement. Experimental results show that the refractive index sensitivity can reach 2027.97 nm/RIU within refractive index ranging from 1.3333–1.3426. Compared to traditional optical fibers, polymer waveguides show many amazing characteristics, such as a high fraction of evanescent fields, an ultra-small allowable bending radius and a small footprint. Polymer micro waveguides exhibit excellent mechanical flexibility and biocompatibility, as well as simple, low-cost fabrication in the fiber-sensing field.
The experimental setup and microscope photo of the sensor probe are shown in Fig. 6. The experimental setup includes an ASE broadband light source (ASE-C, CONQUER) with the wavelength range of 1520 nm1570 nm, an optical spectrum analyzer (AQ6370B, Yokogawa), a high precision micro displacement platform (NFP-x462, ZOLIX), a fluoride magnesium substrate, a video optical microscope with a charge coupled device (CCD) camera was used for observing and capturing sensor probe photos. Two fiber tapers were fixed by tunable six-axis microdisplacement stages and bridged by the PDMS–PMMA microfiber were immersed in NaCl solution with different concentrations. Sodium chloride solution: refractive index of NaCl solutions with mass concentration of 0% (pure water), 1%, 2%, 3%, 4% and 5% are 1.3333, 1.3352, 1.3371, 1.3389, 1.3407, 1.3426. The PDMS–PMMA microfiber sensor is dripped on an MgF2 substrate, and the diameter of the micro optical waveguide wire and fiber taper tip is 6 μm and 15 μm respectively. The surface of sensor was cleaned after the first experiment, but the results show a slight difference. Analysis shows that sodium chloride solution cannot be completely removed, which causes slight pollution in surface of sensor. Fig. 7 is the interference spectrum of the sensor at different NaCl solutions. The three sets experimental measurement data is given in Fig. 8. The sensitivity and fitting linearity of the twice measurements is 2025.71 nm/RIU, 2027.97 nm/RIU and 2021.49 nm/RIU, respectively, which
Acknowledgments This work was supported by the Natural Science Foundation of Liaoning Province under Grant 201602262, the Fundamental Research Funds for the Central Universities under Grant N160405001, N160408001 and N150401001, the National Science Foundation for Distinguished Young Scholars of China under Grant 61425003, the National Natural Science Foundation of China under Grant 51607028. 137
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Fig. 6. Experimental setup and microscope photo of the sensor probe.
Fig. 7. Interference spectrum of the sensor at different NaCl solutions. Fig. 8. The fitting curve of interference spectrum dips and ambient refractive index.
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