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Refractive index measurement based on fiber Bragg grating connected with a multimode fiber core
Optics Communications 351 (2015) 70–74
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Refractive index measurement based on fiber Bragg grating connected with a multimode fiber core Min Shao a,n, Xueguang Qiao b, Zhenan Jiasurname a, Haiwei Fusurname a, Yinggang Liu a, Huidong Li a, Xue Zhao a a b
Ministry of Education Key Laboratory on Photoelectric Oil-Gas Logging and Detecting, School of Science, Xi'an Shiyou University, Xi'an 710065, China School of Physics, Northwest University, Xi'an 710069, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 January 2015 Received in revised form 23 March 2015 Accepted 8 April 2015 Available online 9 April 2015
A novel fiber refractive index sensor based on a fiber-Bragg grating (FBG) connected with a section of multimode fiber core (MMFC) is proposed and demonstrated. The MMFC excites high-order modes to form modal interference, and the core mode reflected by the FBG is sensitive to the surrounding refractive index (SRI) for the power of the core mode within MMFC is dependent on SRI. Measuring the reflective power variation of the core mode could realize the refractive index (RI) detection. Experimental results show that the core mode of FBG has a linear response to RI with enhanced sensitivity of 193.55 dB/RIU in the RI range of 1.3350–1.4042 RIU. The temperature effect of the sensor is also discussed. & 2015 Elsevier B.V. All rights reserved.
Keywords: Fiber Bragg grating Multimode fiber core Refractive index sensing
1. Introduction Fiber optical refractive index (RI) sensors [1–3] have been exploited extensively for the advantages of compact size, remotesensing, capability of multiplexing, could work under the severe poisonous and electromagnetic environment. Fiber Bragg grating (FBG) based RI sensors have attracted more attention in recent years. As known, FBG is not sensitive to the surrounding refractive index (SRI), which is different from high RI sensitivity of long period fiber gratings (LPFG) based on the high-order mode coupling principle. To solve this problem, several schemes are reported and mainly focused on three methods. One method is using chemical etching or side polishing to remove the fiber cladding and exposure the core mode to the SRI medium [4]. The second is imprinting FBG in special fibers or imprinting special FBG in single-mode fiber, such as multimode fiber Bragg grating, polarization-maintaining FBG, microfiber Bragg grating and tilted FBG (TFBG). By utilizing the coupling of the core mode and high-order modes [5,6], the RI sensitivity of the sensor is improved. The third is combination of FBG with other fiber structures to excite and couple the high-order mode [7,8]. In comparison with the former two techniques, FBG assisted with fiber structures have attracted attention for improving RI sensitivity without reducing the FBG strength and durability. Moreover, compared to the special FBGs, n
Corresponding author. E-mail address: [email protected] (M. Shao).
http://dx.doi.org/10.1016/j.optcom.2015.04.028 0030-4018/& 2015 Elsevier B.V. All rights reserved.
FBG assisted with fiber structures don’t need more complicate fabrication and the cost is inexpensive. In previous study, FBG assisted with fiber structures, such as taper, LPFG, thinned-core fiber (TCF), or multimode fiber (MMF) [9–11] were designed and obtained good results. This shows that the FBG assisted with fiber structures based RI sensor is a potential technology, need further investigations in the purpose of improving sensitivity and reducing the cross sensitivity of temperature. Recently, the multimode fiber core (MMFC) based on refractive index sensor [12–14] has aroused the researchers' great interest for its high RI sensitivity. The operating principle of MMFC based sensors is modal interference by core diameter mismatching, and the sensor is easily affected by the SRI for the SRI severs as the cladding of MMFC. In this letter, we demonstrate a novel and high sensitivity RI sensor based on a FBG connected with a section of MMFC. Using of the MMFC, could not only excite high-order modes of FBG, but also experience the SRI changing to improve the RI sensitivity of the sensor. The advantages of this sensor are easy fabrication, compact structure, and cost-effective. Theoretical analysis of the optical field distribution within the MMFC is undertaken through using beam propagation method (BPM).
2. Principle and fabrication of the sensor The schematic diagram of the proposed sensor structure is shown in Fig. 1. The sensor is composed of a single-mode fiber
M. Shao et al. / Optics Communications 351 (2015) 70–74
71
Fig. 1. Schematic diagram of the sensor.
(SMF) fusion spliced to a section of MMFC followed by an FBG inscribed in SMF. When the light propagates into the MMFC from an SMF, assuming the fusion splicing of the SMF and MMFC is ideal, only linear polarized radial modes LP0m modes will be excited and propagate in the MMFC due to the circular symmetry [15,16]. The mode field of LP0m mode can be written as
Em (r) = cm J0 (um r)exp (−iβm z) where
J0
is
the
Bessel
(1) function
of
the
zero
order;
1/2
(
)
2 is the normalized transverse propagation um = k02 nco − βm2 constant of the fiber core; βm is the longitudinal propagation constant; a is the radius of the MMFC; k0 = 2π /λ is free-space wave number; nco is the refractive index of the MMFC; cm is field excitation coefficient from the LP01 mode of SMF to the LP0m mode of MMFC, which can be expressed as
∞
cm =
∫0 Em (r) Es (r) r dr ∞ ⎡ ∞ ⎤1/2 2 2 ⎣ ∫0 Em (r) r dr ∫0 Es (r) r dr ⎦
(2)
where Es (r) is the mode field of LP01 mode in the SMF, which can be approximated as a Gaussian beam. The excited modes will transmit along in the MMFC and interfere at the MMFC–SMF region. The output electric field distribution can be expressed as N
E (r) =
∑
cm J0 (um r)exp ( − iβm L)
m=1
(3)
where L is the length of MMFC. Thus, the output power of the modal interference is N
I (r) = E (r) E ⁎ (r) =
N
∑ ∑ cm cn⁎ ψm (r) ψn⁎ (r)exp ⎡⎣ − i (βm − βn ) L⎤⎦ m= 1 n= 1
(4)
As SRI changes, the effective refractive indices of high-order modes changes, then βm and Em (r)changes, this leads to the field excitation coefficients of LP0m modes change. Consequently, the output power to the lead-out SMF varies and the interference fringes shift. To illustrate the output power response of the SMF–MMFC–SMF fiber structure to the SRI, the optical field distribution along the SMF–MMFC–SMF fiber structure is simulated as shown in Fig. 2. In the simulation, the lengths of the two sections SMF and MMFC are 1mm, 1mm and 35mm, respectively. The SMF has a core and cladding diameter of 9 μm and 125 μm, and the refractive indices of the core and the cladding are 1.4681 and 1.4628, respectively. The MMFC has a diameter of 85 μm, and the refractive indices of the core and the cladding (surrounding air) are 1.4667 and 1.0, respectively. The free space wavelength is set as 1550 nm, and the input power is normalized as 1.0. Fig. 2 shows that when the light enters MMFC from SMF, multiple high-order modes are excited and resulting in the intermodal coupling, which makes the optical field distribution changing in the propagation distance. It can be found that the reimaging distance in the MMFC is clear at 28 mm.
Fig. 2. Beam propagation in the SMF–MMFC–SMF fiber structure.
The calculated output power for the LP01 mode of the SMS fiber structure is shown in Fig. 3. Fig. 3 shows that the output power of LP01 mode changes with SRI at a specific wavelength. When choosing the proper parameters of MMFC, the output power variation of the LP01 mode could be intensified. When the LP01 mode and high-order modes arrive at the FBG, they will be partly reflected by the FBG and recoupled to the MMFC, and then come into the SMF. As known, the core mode (LP01) of FBG is insensitive to SRI, so the Bragg wavelength will keep unchanged with SRI changing. Nevertheless, the output power of the LP01 mode in the SMF– MMFC–SMF fiber structure changes with SRI. Therefore, the power of the LP01 mode in the FBG is modulated by the SMF–MMFC–SMF fiber structure. Through monitoring the reflective power variation of the LP01 mode for the FBG, the SRI can be determined. The sensor was fabricated by a commercial fusion splicer (Furukawa, S177B). The core/high-order diameters of the SMF and MMF used in our experiment were 9.1 μm/125 μm and 105 μm/ 125 μm, respectively. The MMFC was prepared by putting a piece of MMF into the 40% concentrated hydrofluoric (HF) acid solution to remove the fiber cladding. The length and the core diameter of the MMFC were 35 mm and 85 μm, respectively. The FBG was inscribed in an SMF-28 with grating length of 10 mm and reflectivity of 85%. The fibers were fusion spliced by a commercial fusion splicer (Furukawa, S177B). In order to obtain high coupling efficiency and excite more high-order modes, the fiber ends must be flat and choose the no-biased core fusion. Fig. 4 is the photograph of MMFC and SMF fusion splicing region. The distance between the MMFC and the FBG was 3 mm. The experimental setup is shown in Fig. 5. Light from a
Fig. 3. Calculated normalized output power for the LP01 mode of the SMF–MMFC– SMF fiber structure with different MMFC parameters.
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M. Shao et al. / Optics Communications 351 (2015) 70–74
Fig. 4. Photograph of MMFC and SMF region after fusion splicing.
Fig. 5. Scheme of the experimental setup.
broadband source (BBS) was emitted into the sensor through a couple, and the reflective spectrum was recorded by an optical spectrum analyzer (Anritsu, MS9740A). The sensor is fixed on a plexiglass. The transmission spectrum of the SMF–MMFC–SMF fiber is shown in Fig. 6(a). Fig. 6(a) shows that the spectrum has a transmission peak approximately 1550 nm. The core Bragg wavelength of the FBG is 1553.48 nm, and the reflective spectrum of the FBG is shown in Fig. 6(b). As shown in Fig. 6(c), the reflective spectrum of the FBG connected with MMFC has several peaks at the shorter wavelength band in addition to the core mode reflection. This is due to the fact that multiple high-order modes excited by the MMFC are reflected by the FBG and recoupled to the MMFC. The MMFC used in the experiment is a “key” to the SRI of the sensor, for it can not only experience the SRI variation, but also transmit the LP0m mode of FBG. While the FBG acts as a filter and mirror to remain and reflect the core mode and some special highorder mode.
3. Experiment and discussion The fabricated sensor was fixed on a plexiglass to keep straight. Sucrose solutions of different concentration of 0%, 9.1%, 16.6%, 23.1%, 28.5% 33.4%, 37.5%, and 41.1% were prepared as the SRI liquid samples for the measurement. The RIs of the samples were certified by an Abbe refractometer. The temperature of the experimental environment was kept at 18.3 °C. In experiment, the RI liquid was dropped on the sensor head, and the response reflective spectrum of the sensor was monitored. After each measurement of RI liquid sample, the sensor head was repeatedly cleaned by water or alcohol and dried in air. Fig. 7 shows the reflective spectrum responses of the core mode to different SRI. It can be observed that
Fig. 6. (a) Transmission spectrum of the SMF–MMFC–SMF fiber structure, (b) reflective spectrum of the FBG, and (c) reflective spectrum of the FBG connected with MMFC.
as the SRI increases, the power of the core mode increases, which is in accord with the theoretical analysis. The power variation of the core mode with SRI is shown in Fig. 8. The dots in Fig. 8 are measured values. The straight line is the linear fit, which has an R2 value of 0.996. When the SRI
M. Shao et al. / Optics Communications 351 (2015) 70–74
Fig. 7. Reflective spectral responses of the sensor to SRI.
73
Fig. 9. Temperature sensitivity of the sensor. Inset shows the spectra response.
the fact that the transmission spectrum of SMF–MMFC–SMF fiber structure is slightly changes with temperature increasing. The corresponding maximum RI measurement error is about 1.72 10 5 RIU/°C. This confirms that the RI measurement of the sensor is reliable, and temperature can’t have significant effects on the SRI measurement if the temperature variation is less than 1 °C, which implies that the sensor has a potential for temperatureimmune RI detection. For comparison, sensors with different MMFC parameters were fabricated and the SRI responses were tested as shown in Fig. 10. Sensitivities of –100.65 dB/RIU and 187.44 dB/RIU were achieved while the MMFC length was 30 mm and 41.3 mm, with diameter of 96 μm and 95 μm, respectively. The experimental results indicate that the RI sensitivity could be optimized by controlling MMFC parameters. It should be noted that the Bragg wavelength of the FBG and the splicing quality also have effects on the RI sensitivity, which need further optimization.
Fig. 8. The power of the core mode as a function of the SRI.
changes from 1.3350 to 1.4042RIU, the power of the core mode shifts about 12.81 dB, and the RI sensitivity is 193.55 dB/RIU, which is higher than TCF structure with FBG (133.26 dB/RIU) [11]. This is different from the MMFþ FBG fiber structure [8] in that the core mode keeps still with SRI changing. The reason for this is that we use MMFC here. In this paper, the SRI liquid acts as the cladding layer of the MMFC, which makes the effective indices of the core mode and high-order modes change more severely with the SRI than those in MMF. Thus, the transmission spectrum of SMF– MMFC–SMF fiber structure will strongly modulate the core mode of FBG. When the power resolution of the OSA is 0.01 dB, the corresponding resolution of RI measurement is 5.16 10 5RIU in the RI ranges of 1.3350–1.4042 RIU. The isolation of the FBG used in the paper is over 15 dB, and the extinction ratio of the SMF– MMFC–SMF fiber structure is 15 dB, therefore the maximum dynamic range of this sensor is 15 dB. Temperature information is important in RI measurement because thermo-optic effect will also lead in the spectrum variation and induce the cross sensitivity. In order to discuss the temperature response, the sensor head was put into a water bath with increasing of temperature from 20 to 80 °C, the wavelength and power of the core mode were recorded as shown in Fig. 9. The temperature sensitivity is 0.008 nm/°C with a linear fitting of 0.999. From Fig. 9, it can be seen that the peak power fluctuation caused by increasing temperature 10°C is less than 0.2 dB due to
4. Conclusion In conclusion, we have proposed and demonstrated a new type of fiber RI sensor based on FBG connected with MMFC. The SMF– MMFC–SMF fiber structure is sensitive to SRI, and its output power variation with SRI could modulate the core mode of FBG. This
Fig. 10. SRI responses of sensors with different MMFC parameters.
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M. Shao et al. / Optics Communications 351 (2015) 70–74
transforms the SRI information into the power variation of the FBG core mode to further realize the SRI detection. In the experiment, an enhanced linear sensitivity of 193.55 dB/RIU is achieved in the RI range of 1.3350–1.4042 RIU. Temperature experimental results show that the RI measurement error caused by temperature is less than 1.72 10 5 RIU/°C. Moreover, the sensor has merits of compact structure, easy fabrication and reflective detection. This make it is a good candidate for RI sensor in chemical and biological applications.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Acknowledgment This work is supported by the National Science Foundation of China under Grants 61275088 and 61077060, and Science Research Plan Projects of Shaanxi Education Department under Grant 14JK1580.
[10] [11] [12] [13] [14] [15] [16]
Z.L. Ran, Y.J. Rao, W.J. Liu, X. Liao, K.S. Chiang, Opt. Express 3 (16) (2008) 2252. A. Bueno, C. Caucheteur, D. Kinet, P. Megret, Proc. SPIE 8794 (2013) 87940Y-1. L.C. Li, X. Li, Z.H. Xie, Z.L. Liao, F. Tu, D.M. Liu, Opt. Commun. 285 (2012) 3945. A.N. Chryssis, S.S. Saini, S.M. Lee, H. Yi, W.E. Bentley, M. Dagenais, IEEE J. Quantum Electr. 11 (4) (2005) 864. Y. Zhang, B. Lin, S.C. Tjin, H. Zhang, G. Wang, P. Shum, X. Zhang, Opt. Express 18 (25) (2010) 26345. T. Guo, H.Y. Tam, P.A. Krug, J. Albert, Opt. Express 17 (7) (2009) 5736. K. Zhou, Z.J. Yan, L. Zhang, I. Bennion, Opt. Express 19 (12) (2011) 11769. Q. Wu, Y. Semenova, B. Yan, Y. Ma, P. Wang, C. Yu, G. Farrell, Opt. Lett. 36 (2) (2011) 2197. L.Y. Shao, A.P. Zhang, W.S. Liu, H.Y. Fu, S.L. He, IEEE Photon. Technol. Lett. 19 (1) (2007) 30. M. Han, F. Guo, Y. Lu, Opt. Lett. 35 (3) (2010) 399. Y. Ma, X.G. Qiao, T. Guo, R.H. Wang, J. Zhang, Y.Y. Weng, Q.Z. Rong, M.L. Hu, Z. Y. Feng, Opt. Lett. 37 (3) (2012) 323. Y. Zhao, L. Cai, X.G. Li, F.C. Meng, Sens. Actuators B 196 (2014) 518. Q. Wu, Y. Semenova, P.F. Wang, G. Farrell, Opt. Express 19 (9) (2011) 7937. L.L. Xue, L. Yang, J. Lightwave Technol. 30 (10) (2012) 1463. W.S. Mohammed, A. Mehta, E.G. Johnson, J. Lightwave Technol. 22 (2) (2004) 469. E. Li, IEEE Photon. Technol. Lett. 19 (10) (2007) 1266.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Refractive index measurement based on fiber Bragg grating connected with a multimode fiber core Min Shao a,n, Xueguang Qiao b, Zhenan Jiasurname a, Haiwei Fusurname a, Yinggang Liu a, Huidong Li a, Xue Zhao a a b
Ministry of Education Key Laboratory on Photoelectric Oil-Gas Logging and Detecting, School of Science, Xi'an Shiyou University, Xi'an 710065, China School of Physics, Northwest University, Xi'an 710069, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 January 2015 Received in revised form 23 March 2015 Accepted 8 April 2015 Available online 9 April 2015
A novel fiber refractive index sensor based on a fiber-Bragg grating (FBG) connected with a section of multimode fiber core (MMFC) is proposed and demonstrated. The MMFC excites high-order modes to form modal interference, and the core mode reflected by the FBG is sensitive to the surrounding refractive index (SRI) for the power of the core mode within MMFC is dependent on SRI. Measuring the reflective power variation of the core mode could realize the refractive index (RI) detection. Experimental results show that the core mode of FBG has a linear response to RI with enhanced sensitivity of 193.55 dB/RIU in the RI range of 1.3350–1.4042 RIU. The temperature effect of the sensor is also discussed. & 2015 Elsevier B.V. All rights reserved.
Keywords: Fiber Bragg grating Multimode fiber core Refractive index sensing
1. Introduction Fiber optical refractive index (RI) sensors [1–3] have been exploited extensively for the advantages of compact size, remotesensing, capability of multiplexing, could work under the severe poisonous and electromagnetic environment. Fiber Bragg grating (FBG) based RI sensors have attracted more attention in recent years. As known, FBG is not sensitive to the surrounding refractive index (SRI), which is different from high RI sensitivity of long period fiber gratings (LPFG) based on the high-order mode coupling principle. To solve this problem, several schemes are reported and mainly focused on three methods. One method is using chemical etching or side polishing to remove the fiber cladding and exposure the core mode to the SRI medium [4]. The second is imprinting FBG in special fibers or imprinting special FBG in single-mode fiber, such as multimode fiber Bragg grating, polarization-maintaining FBG, microfiber Bragg grating and tilted FBG (TFBG). By utilizing the coupling of the core mode and high-order modes [5,6], the RI sensitivity of the sensor is improved. The third is combination of FBG with other fiber structures to excite and couple the high-order mode [7,8]. In comparison with the former two techniques, FBG assisted with fiber structures have attracted attention for improving RI sensitivity without reducing the FBG strength and durability. Moreover, compared to the special FBGs, n
Corresponding author. E-mail address: [email protected] (M. Shao).
http://dx.doi.org/10.1016/j.optcom.2015.04.028 0030-4018/& 2015 Elsevier B.V. All rights reserved.
FBG assisted with fiber structures don’t need more complicate fabrication and the cost is inexpensive. In previous study, FBG assisted with fiber structures, such as taper, LPFG, thinned-core fiber (TCF), or multimode fiber (MMF) [9–11] were designed and obtained good results. This shows that the FBG assisted with fiber structures based RI sensor is a potential technology, need further investigations in the purpose of improving sensitivity and reducing the cross sensitivity of temperature. Recently, the multimode fiber core (MMFC) based on refractive index sensor [12–14] has aroused the researchers' great interest for its high RI sensitivity. The operating principle of MMFC based sensors is modal interference by core diameter mismatching, and the sensor is easily affected by the SRI for the SRI severs as the cladding of MMFC. In this letter, we demonstrate a novel and high sensitivity RI sensor based on a FBG connected with a section of MMFC. Using of the MMFC, could not only excite high-order modes of FBG, but also experience the SRI changing to improve the RI sensitivity of the sensor. The advantages of this sensor are easy fabrication, compact structure, and cost-effective. Theoretical analysis of the optical field distribution within the MMFC is undertaken through using beam propagation method (BPM).
2. Principle and fabrication of the sensor The schematic diagram of the proposed sensor structure is shown in Fig. 1. The sensor is composed of a single-mode fiber
M. Shao et al. / Optics Communications 351 (2015) 70–74
71
Fig. 1. Schematic diagram of the sensor.
(SMF) fusion spliced to a section of MMFC followed by an FBG inscribed in SMF. When the light propagates into the MMFC from an SMF, assuming the fusion splicing of the SMF and MMFC is ideal, only linear polarized radial modes LP0m modes will be excited and propagate in the MMFC due to the circular symmetry [15,16]. The mode field of LP0m mode can be written as
Em (r) = cm J0 (um r)exp (−iβm z) where
J0
is
the
Bessel
(1) function
of
the
zero
order;
1/2
(
)
2 is the normalized transverse propagation um = k02 nco − βm2 constant of the fiber core; βm is the longitudinal propagation constant; a is the radius of the MMFC; k0 = 2π /λ is free-space wave number; nco is the refractive index of the MMFC; cm is field excitation coefficient from the LP01 mode of SMF to the LP0m mode of MMFC, which can be expressed as
∞
cm =
∫0 Em (r) Es (r) r dr ∞ ⎡ ∞ ⎤1/2 2 2 ⎣ ∫0 Em (r) r dr ∫0 Es (r) r dr ⎦
(2)
where Es (r) is the mode field of LP01 mode in the SMF, which can be approximated as a Gaussian beam. The excited modes will transmit along in the MMFC and interfere at the MMFC–SMF region. The output electric field distribution can be expressed as N
E (r) =
∑
cm J0 (um r)exp ( − iβm L)
m=1
(3)
where L is the length of MMFC. Thus, the output power of the modal interference is N
I (r) = E (r) E ⁎ (r) =
N
∑ ∑ cm cn⁎ ψm (r) ψn⁎ (r)exp ⎡⎣ − i (βm − βn ) L⎤⎦ m= 1 n= 1
(4)
As SRI changes, the effective refractive indices of high-order modes changes, then βm and Em (r)changes, this leads to the field excitation coefficients of LP0m modes change. Consequently, the output power to the lead-out SMF varies and the interference fringes shift. To illustrate the output power response of the SMF–MMFC–SMF fiber structure to the SRI, the optical field distribution along the SMF–MMFC–SMF fiber structure is simulated as shown in Fig. 2. In the simulation, the lengths of the two sections SMF and MMFC are 1mm, 1mm and 35mm, respectively. The SMF has a core and cladding diameter of 9 μm and 125 μm, and the refractive indices of the core and the cladding are 1.4681 and 1.4628, respectively. The MMFC has a diameter of 85 μm, and the refractive indices of the core and the cladding (surrounding air) are 1.4667 and 1.0, respectively. The free space wavelength is set as 1550 nm, and the input power is normalized as 1.0. Fig. 2 shows that when the light enters MMFC from SMF, multiple high-order modes are excited and resulting in the intermodal coupling, which makes the optical field distribution changing in the propagation distance. It can be found that the reimaging distance in the MMFC is clear at 28 mm.
Fig. 2. Beam propagation in the SMF–MMFC–SMF fiber structure.
The calculated output power for the LP01 mode of the SMS fiber structure is shown in Fig. 3. Fig. 3 shows that the output power of LP01 mode changes with SRI at a specific wavelength. When choosing the proper parameters of MMFC, the output power variation of the LP01 mode could be intensified. When the LP01 mode and high-order modes arrive at the FBG, they will be partly reflected by the FBG and recoupled to the MMFC, and then come into the SMF. As known, the core mode (LP01) of FBG is insensitive to SRI, so the Bragg wavelength will keep unchanged with SRI changing. Nevertheless, the output power of the LP01 mode in the SMF– MMFC–SMF fiber structure changes with SRI. Therefore, the power of the LP01 mode in the FBG is modulated by the SMF–MMFC–SMF fiber structure. Through monitoring the reflective power variation of the LP01 mode for the FBG, the SRI can be determined. The sensor was fabricated by a commercial fusion splicer (Furukawa, S177B). The core/high-order diameters of the SMF and MMF used in our experiment were 9.1 μm/125 μm and 105 μm/ 125 μm, respectively. The MMFC was prepared by putting a piece of MMF into the 40% concentrated hydrofluoric (HF) acid solution to remove the fiber cladding. The length and the core diameter of the MMFC were 35 mm and 85 μm, respectively. The FBG was inscribed in an SMF-28 with grating length of 10 mm and reflectivity of 85%. The fibers were fusion spliced by a commercial fusion splicer (Furukawa, S177B). In order to obtain high coupling efficiency and excite more high-order modes, the fiber ends must be flat and choose the no-biased core fusion. Fig. 4 is the photograph of MMFC and SMF fusion splicing region. The distance between the MMFC and the FBG was 3 mm. The experimental setup is shown in Fig. 5. Light from a
Fig. 3. Calculated normalized output power for the LP01 mode of the SMF–MMFC– SMF fiber structure with different MMFC parameters.
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M. Shao et al. / Optics Communications 351 (2015) 70–74
Fig. 4. Photograph of MMFC and SMF region after fusion splicing.
Fig. 5. Scheme of the experimental setup.
broadband source (BBS) was emitted into the sensor through a couple, and the reflective spectrum was recorded by an optical spectrum analyzer (Anritsu, MS9740A). The sensor is fixed on a plexiglass. The transmission spectrum of the SMF–MMFC–SMF fiber is shown in Fig. 6(a). Fig. 6(a) shows that the spectrum has a transmission peak approximately 1550 nm. The core Bragg wavelength of the FBG is 1553.48 nm, and the reflective spectrum of the FBG is shown in Fig. 6(b). As shown in Fig. 6(c), the reflective spectrum of the FBG connected with MMFC has several peaks at the shorter wavelength band in addition to the core mode reflection. This is due to the fact that multiple high-order modes excited by the MMFC are reflected by the FBG and recoupled to the MMFC. The MMFC used in the experiment is a “key” to the SRI of the sensor, for it can not only experience the SRI variation, but also transmit the LP0m mode of FBG. While the FBG acts as a filter and mirror to remain and reflect the core mode and some special highorder mode.
3. Experiment and discussion The fabricated sensor was fixed on a plexiglass to keep straight. Sucrose solutions of different concentration of 0%, 9.1%, 16.6%, 23.1%, 28.5% 33.4%, 37.5%, and 41.1% were prepared as the SRI liquid samples for the measurement. The RIs of the samples were certified by an Abbe refractometer. The temperature of the experimental environment was kept at 18.3 °C. In experiment, the RI liquid was dropped on the sensor head, and the response reflective spectrum of the sensor was monitored. After each measurement of RI liquid sample, the sensor head was repeatedly cleaned by water or alcohol and dried in air. Fig. 7 shows the reflective spectrum responses of the core mode to different SRI. It can be observed that
Fig. 6. (a) Transmission spectrum of the SMF–MMFC–SMF fiber structure, (b) reflective spectrum of the FBG, and (c) reflective spectrum of the FBG connected with MMFC.
as the SRI increases, the power of the core mode increases, which is in accord with the theoretical analysis. The power variation of the core mode with SRI is shown in Fig. 8. The dots in Fig. 8 are measured values. The straight line is the linear fit, which has an R2 value of 0.996. When the SRI
M. Shao et al. / Optics Communications 351 (2015) 70–74
Fig. 7. Reflective spectral responses of the sensor to SRI.
73
Fig. 9. Temperature sensitivity of the sensor. Inset shows the spectra response.
the fact that the transmission spectrum of SMF–MMFC–SMF fiber structure is slightly changes with temperature increasing. The corresponding maximum RI measurement error is about 1.72 10 5 RIU/°C. This confirms that the RI measurement of the sensor is reliable, and temperature can’t have significant effects on the SRI measurement if the temperature variation is less than 1 °C, which implies that the sensor has a potential for temperatureimmune RI detection. For comparison, sensors with different MMFC parameters were fabricated and the SRI responses were tested as shown in Fig. 10. Sensitivities of –100.65 dB/RIU and 187.44 dB/RIU were achieved while the MMFC length was 30 mm and 41.3 mm, with diameter of 96 μm and 95 μm, respectively. The experimental results indicate that the RI sensitivity could be optimized by controlling MMFC parameters. It should be noted that the Bragg wavelength of the FBG and the splicing quality also have effects on the RI sensitivity, which need further optimization.
Fig. 8. The power of the core mode as a function of the SRI.
changes from 1.3350 to 1.4042RIU, the power of the core mode shifts about 12.81 dB, and the RI sensitivity is 193.55 dB/RIU, which is higher than TCF structure with FBG (133.26 dB/RIU) [11]. This is different from the MMFþ FBG fiber structure [8] in that the core mode keeps still with SRI changing. The reason for this is that we use MMFC here. In this paper, the SRI liquid acts as the cladding layer of the MMFC, which makes the effective indices of the core mode and high-order modes change more severely with the SRI than those in MMF. Thus, the transmission spectrum of SMF– MMFC–SMF fiber structure will strongly modulate the core mode of FBG. When the power resolution of the OSA is 0.01 dB, the corresponding resolution of RI measurement is 5.16 10 5RIU in the RI ranges of 1.3350–1.4042 RIU. The isolation of the FBG used in the paper is over 15 dB, and the extinction ratio of the SMF– MMFC–SMF fiber structure is 15 dB, therefore the maximum dynamic range of this sensor is 15 dB. Temperature information is important in RI measurement because thermo-optic effect will also lead in the spectrum variation and induce the cross sensitivity. In order to discuss the temperature response, the sensor head was put into a water bath with increasing of temperature from 20 to 80 °C, the wavelength and power of the core mode were recorded as shown in Fig. 9. The temperature sensitivity is 0.008 nm/°C with a linear fitting of 0.999. From Fig. 9, it can be seen that the peak power fluctuation caused by increasing temperature 10°C is less than 0.2 dB due to
4. Conclusion In conclusion, we have proposed and demonstrated a new type of fiber RI sensor based on FBG connected with MMFC. The SMF– MMFC–SMF fiber structure is sensitive to SRI, and its output power variation with SRI could modulate the core mode of FBG. This
Fig. 10. SRI responses of sensors with different MMFC parameters.
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transforms the SRI information into the power variation of the FBG core mode to further realize the SRI detection. In the experiment, an enhanced linear sensitivity of 193.55 dB/RIU is achieved in the RI range of 1.3350–1.4042 RIU. Temperature experimental results show that the RI measurement error caused by temperature is less than 1.72 10 5 RIU/°C. Moreover, the sensor has merits of compact structure, easy fabrication and reflective detection. This make it is a good candidate for RI sensor in chemical and biological applications.
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Acknowledgment This work is supported by the National Science Foundation of China under Grants 61275088 and 61077060, and Science Research Plan Projects of Shaanxi Education Department under Grant 14JK1580.
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