- Email: [email protected]
Temperature insensitive refractive index sensor based on a combination interference structure
Optik 126 (2015) 697–700
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Temperature insensitive refractive index sensor based on a combination interference structure Yong Zhao a,b,∗ , Peng Gao a,c , Hai-feng Hu a a
Northeastern University, College of Information Science and Engineering, Shenyang 110004, China Stat Key Laboratory of Integrated Automation of Process Industry Technology and Research Center of National Metallurgical Automation, Shenyang 110819, China c College of Physics Science and Technology, Shenyang Normal University, Shenyang 110034, China b
a r t i c l e
i n f o
Article history: Received 11 February 2014 Accepted 19 February 2015 Keywords: Liquid refractive index sensor Optical fiber sensor Photonic crystal fiber Fiber loop mirror
a b s t r a c t A sensing head consisting of fiber loop mirror inscribed with a highly birefringence photonic crystal fiber (HiBi-PCF) was proposed and experimentally demonstrated. The HiBi-PCF was completely collapsed near the splicing points, thus the cladding mode in the HiBi-PCF would be excited, which was sensitive to the refractive index (RI) of surrounding medium. Owing to the low thermo-optic and thermo-expansion coefficient of the HiBi-PCF, the sensing head in our design was temperature insensitive. High sensitivity of 306.6 ± 0.2 nm/RIU (refractive index unit) and a resolution of 6.5 × 10−5 RIU have been achieved for the proposed liquid refractive index sensor. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction Optical fiber sensors hold numeral advantages over conventional electrical-based sensors. The former are inexpensive, compact, light weight, immune to electromagnetic interference. This resulted in a great demand for fiber sensors in biochemical sensing applications. In recent years, optical fiber refractive index (RI) sensors have attracted lots of attention, because they are useful in many kinds of areas ranging from environmental monitoring to biomedical sensing. Both fiber Bragg gratings (FBGs) and longperiod gratings (LPGs) have been used in RI fiber sensors. For an FBG-based RI sensor, the FBG is often etched or polished to gain the access of the evanescent field of the guided mode to the surrounded material to be measured [1–3]. Compared with RI sensors of FBG type, sensors based on LPGs [4–7] are more applicable because of their intrinsical coupling mechanism. A RI sensor based on Fabry–Perot (F–P) interferometers has been also reported [8]. Such a sensing structure makes use of the variation of the maximum contrast of the interference fringes to measure RI. Frazão [9] proposed a refractometer based on high birefringence etched Dtype fiber loop mirror. The evanescent field for RI measurement was increased by using the etched D-type structure fiber. Optical refractometers based on photonic crystal fibers were also proposed
[10]. However, the measurement principles of those refractometers are using the wavelength shifts to detect the external RI variation, which was faced with a big problem of temperature cross sensitivity. Thus, a temperature compensation part was necessary. Meanwhile, the obtained sensitivity was not high enough in the RI sensing structure which was reported previously. Therefore, fiber refractometer which can realize high sensitivity without the needing of an additional temperature compensation part was desirable in practical applications. In this work, a sensing head consisting of fiber loop mirror inscribed with a highly birefringence photonic crystal fiber (HiBiPCF) was proposed and experimentally demonstrated. The HiBi-PCF was completely collapsed near the splicing points, thus the cladding mode in the HiBi-PCF would be excited, which was sensitive to the refractive index (RI) of surrounding medium. Owing to the low thermo-optic and thermo-expansion coefficient of the HiBiPCF, the sensing head in our design was temperature insensitive. The collapsed HiBi-PCF worked just like a Mach–Zehnder interferometer (MZI), considering the FLM structure, a combination interferometer was formed, which result in a high sensitivity. The RI value of the liquid can be measured by detecting the interference fringes shift corresponding to the FLM and MZI. 2. Sensor fabrication and operation principle
∗ Corresponding author at: Northeastern University, College of Information Science and Engineering, Shenyang 110004, China. Tel.: +86 13998812362. E-mail address: [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.ijleo.2015.02.046 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
The schematic diagram of the proposed sensor head was shown in Fig. 1. An amplified spontaneous emission (ASE) source of
698
Y. Zhao et al. / Optik 126 (2015) 697–700
Fig. 1. (a) Schematic diagram of the proposed RI sensor; (b) SEM of the used HiBi-PCF and (c) the partial enlarged drawing of the sensing head.
1450–1650 nm wavelength range is connected to the input of the FLM, and the output spectrum is detected with an optical spectrum analyzer (OSA, AQ6370, Advantest, Japan). The maximum resolution of the OSA is 20 pm. The FLM is formed by splicing (using a commercial fusion splicer (Fujikura FSM-40S)) a sections of HiBiPCF (LMA-10) to the arms of a 2 × 2.3 dB coupler. The HiBi-PCF with the length of 5 cm and both ends of it was collapsed fusion splicing between two identical single mode fibers (SMF-28) with was in the arms of the coupler. The inset of Fig. 1 shows the cross sectional view of the HiBi-PCF, which has a solid core of diameter of 11 m surrounded by four layers of air-holes with an outer diameter of 125 m. Splicing of the PCF to SMF was carried out by using a commercial fusion splicer (Fujikura FSM-40S). Strong electronic arc discharges caused localized heating on the PCF, leading to collapse of the airholes in the cladding area of the heated PCF section. It can be seen from the micrograph of the fabricated splicing point, where the airhole structure of the PCF was collapsed near the splicing point over a short length of ∼140 m. The PCF was no longer single mode since the fiber had no core–cladding structure any more at the region of the collapsed area. When the fundamental mode of the lead-in SMF propagates into the collapsed region of PCF, its mode field diameter would be broadened due to the diffraction, allowing the excitation of core and cladding modes in the intact PCF section [11,12]. Then, the excited core and cladding modes are further diffracted and recoupled back to the core mode of the lead-out SMF at the second splicing point. Therefore, it worked just like a kind of MZI. The input light from the ASE is equally splits into two counterpropagating light by the 3 dB coupler, and subsequently they recombine at the coupler after clockwise and counterclockwise light beams propagating around the loop. A polarization controller (PC) is used to adjust the polarization states of the two lights. The counter-propagating light beams introduced a relative phase difference due to the birefringence property of the inserted HiBi-PCF. So interferences generate when they recombine at the coupler. The transmission optical intensity It in terms of the phase difference can be described as, 1 − cos It = 2
(1)
with =
2LB
(2)
where is the phase difference. is the center wavelength of the light source. L is the length of the HiBi-PCF. B = ns − nf is the birefringence index of the HiBi-PCF. ns and nf are the effective refractive index of the slow and fast axis, respectively. The resonant dip wavelength satisfies the equation of = 2k, where k is a random integer.
Fig. 2. Initial interference spectrum of the RI sensor.
Fig. 3. Interference spectrum variation of the FLM was subject to liquid with different RI.
Therefore, the resonant dip wavelength can be described as, =
BL k
(3)
The interference fringes corresponding to the FLM and MZI with different resonant dip wavelengths which result in a spectrum overlap as shown in Fig. 2. 3. Experiment and discussions To test the HiBi-PCF-FLM based liquid RI sensor, the collapse fusion spliced HiBi-PCF was immersed in the sample of liquid combine with different percentages of glycerin. The liquid samples were calibrated through an Abbe refractometer with a nominal accuracy of ±0.0001. We can get Eq. (2), =
L · (B + B) k
(4)
where is the wavelength shift as put PCF in liquid with varying refractive index. ≡ is the change of the birefringence index caused by the effective refractive index variation of the fast and slow axis corresponding to PCF. From Eq. (4), it can be seen that is directly proportional to ≡. External RI variation can be detected by measuring the wavelength shift of the interference spectrum. As the HiBi-PCF was immersed in liquid with RI value variation in a range of 1.41–1.43 at a room temperature (25 ◦ C), the variations of the interference spectrum for the FLM were shown in Fig. 3. It can be seen that the resonant dip wavelength 2 of the interference fringes were shifted from 1556.11 nm to 1549.66 nm. The resonant dip wavelength shifts of interference spectrum of the
Y. Zhao et al. / Optik 126 (2015) 697–700
699
Fig. 6. Temperature errors graph of the proposed sensor.
Fig. 4. Wavelength shifts of the FLM as function of external RI variation.
thermo-optic and thermo-expansion coefficient of the HiBi-PCF, and the birefringence will be little influenced by the temperature variation. The temperature errors graph of the proposed sensor was shown in Fig. 6. We selected the interference pattern corresponding to = 1537 nm, 1552 nm and 1575 nm as the experimental parameters. As the surrounding temperature increased from 10 ◦ C to 100 ◦ C, the maximum temperature errors were 0.52 nm, 0.49 nm and 0.48 nm at the wavelength of 1537 nm, 1552 nm and 1575 nm, respectively. Compare to the wavelength shift range of 6.45 nm, the temperature variation induced measurement error was so small which can be neglected in practical application. 4. Conclusion Fig. 5. Temperature response of the proposed RI sensor.
FLM as a function of RI changing is shown in Fig. 4. It satisfied well with the theoretical derivation. As shown in Fig. 4, as expected, the wavelength shift of the FLM has a linear relationship with the external RI variation. Owing to the use of a combination interference structure, high sensitivity of 306.6 ± 0.2 nm/RIU was achieved, which was about 5 times higher than the previous report [13]. The rms deviation value (SD) between the linear fit and the measured data was 0.2, which is equal to the uncertainty of our calibration measurement using the Abbe refractometer. The resolution of the proposed RI sensor is 6.5 × 10−5 RIU which is limited by resolution of the OSA. Large range of the wavelength shift will lead to an overlap of the fringes. Therefore, the measurement range was limited by fringe separation of interference spectrum of the FLM. The wavelength spacing S between the adjacent interference spectrum can be expressed as 2 S= BL
(5)
In practical applications, the measurement range of the proposed sensor can be adjusted by changing the birefringence of the HiBiPCF. The RI variation experimental measurement was performed in a temperature-controlled environment, and the temperature variation was less than ±0.1 ◦ C. So the error of displacement measurement induced by temperature can be neglected. But in practical applications, the surrounding temperature was not invariable. The influence of temperature on the proposed RI sensor was also studied. The total FLM part was placed into a temperature controlled container. The temperature of the controlled container was set to increase from 10 ◦ C to 100 ◦ C with a step of 10 ◦ C. As shown in Fig. 5, the resonant dip wavelength of the FLM has a little shift as the temperature increasing. This results owing to that the low
We have described a sensing head consisting of fiber loop mirror inscribed with a highly birefringence photonic crystal fiber (HiBi-PCF). Owing to the low thermo-optic and thermo-expansion coefficient of the HiBi-PCF, the sensing head in our design was temperature insensitive. High sensitivities of (306.6 ± 0.2) nm/RIU (refractive index unit) and a resolution of 6.5 × 10−5 RIU have been achieved for the proposed liquid refractive index sensor. Compared with other refractive index sensor, the proposed sensor can obtain a high sensitivity by using a combination interference structure. Owing to the low thermo-optic and thermo-expansion coefficient of the HiBi-PCF, the sensing head in our design was temperature insensitive without the needing of an additional temperature compensation part. Acknowledgments This work was supported in part by the National Natural Science Foundation of China under grant 61203206 and 61273059, and IAPI Fundamental Research Funds under grant 2013ZCX02-05, and Liaoning S&T Project under grant 2010220012. References [1] K. Schroeder, W. Ecke, R. Mueller, R. Willsch, A. Andreev, A fibre Bragg grating refractometer, Meas. Sci. Technol. 12 (2001) 757–764. [2] W. Liang, Y.Y. Huang, Y. Xu, R.K. Lee, A. Yariv, Highly sensitive fiber Bragg grating refractive index sensors, Appl. Phys. Lett. 86 (2005) 15112. [3] A. Iadicicco, A. Cusano, A. Cutolo, R. Bernini, M. Giordano, Thinned fiber Bragg gratings as high sensitivity refractive index sensor, IEEE Photonics Technol. Lett. 16 (2004) 1149–1151. [4] A.M. Rios, D.M. Hernandez, I.T. Gomez, Highly sensitive cladding-etched arcinduced long-period fiber gratings for refractive index sensing, Opt. Commun. 283 (2010) 958–962. [5] I.D. Villar, I.R. Matias, F.J. Arregui, Enhancement of sensitivity in long-period fiber gratings with deposition of low-refractive-index materials, Opt. Lett. 30 (2005) 2363–2365.
700
Y. Zhao et al. / Optik 126 (2015) 697–700
[6] T. Zhu, Y.J. Rao, Q.J. Mo, Simultaneous measurement of refractive index and temperature using a single ultra-long-period fiber grating, IEEE Photonics Technol. Lett. 17 (2005) 2700–2702. [7] A. Iadicicco, S. Campopiano, M. Giordano, A. Cusano, Spectral behavior in thinned long-period gratings: effects of fiber diameter on refractive index sensitivity, Appl. Opt. 46 (2007) 6945–6952. [8] Z.L. Ran, Y.J. Rao, W.J. Liu, X. Liao, K.S. Chiang, Laser-micromachined Fabry–Perot optical fiber tip sensor for high-resolution temperature-independent measurement of refractive index, Opt. Express 16 (2008) 2252–2263. [9] O. Frazão, B.V. Marques, P. Jorge, J.M. Baptista, J.L. Santos, High birefringence D-type fiber loop mirror used as refractometer, Sens. Actuators B: Chem. 135 (2008) 5108–5111.
[10] S. Silva, J.L. Santos, F.X. Malcata, J. Kobelke, K. Schuster, O. Frazão, Optical refractometer based on large-core air-clad photonic crystal fibers, Opt. Lett. 366 (2011) 852–854. [11] J. Villatoro, V.P. Minkovich, V. Pruneri, G. Badenes, Simple all-microstructuredoptical-fiber interferometer built via fusion splicing, Opt. Express 15 (4) (2007) 1491–1496. [12] G.A. Cárdenas-Sevilla, V. Finazzi, J. Villatoro, V. Pruneri, Photonic crystal fiber sensor array based on modes overlapping, Opt. Express 19 (8) (2011) 7596–7602. [13] C. Zhong, C.Y. Shen, Y. You, J.L. Chu, X. Zhou, X.Y. Dong, Y.X. Jin, J.F. Wang, A polarization-maintaining fiber loop mirror based sensor for liquid refractive index absolute measurement, Sens. Actuators: B 168 (2012) 360–364.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Temperature insensitive refractive index sensor based on a combination interference structure Yong Zhao a,b,∗ , Peng Gao a,c , Hai-feng Hu a a
Northeastern University, College of Information Science and Engineering, Shenyang 110004, China Stat Key Laboratory of Integrated Automation of Process Industry Technology and Research Center of National Metallurgical Automation, Shenyang 110819, China c College of Physics Science and Technology, Shenyang Normal University, Shenyang 110034, China b
a r t i c l e
i n f o
Article history: Received 11 February 2014 Accepted 19 February 2015 Keywords: Liquid refractive index sensor Optical fiber sensor Photonic crystal fiber Fiber loop mirror
a b s t r a c t A sensing head consisting of fiber loop mirror inscribed with a highly birefringence photonic crystal fiber (HiBi-PCF) was proposed and experimentally demonstrated. The HiBi-PCF was completely collapsed near the splicing points, thus the cladding mode in the HiBi-PCF would be excited, which was sensitive to the refractive index (RI) of surrounding medium. Owing to the low thermo-optic and thermo-expansion coefficient of the HiBi-PCF, the sensing head in our design was temperature insensitive. High sensitivity of 306.6 ± 0.2 nm/RIU (refractive index unit) and a resolution of 6.5 × 10−5 RIU have been achieved for the proposed liquid refractive index sensor. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction Optical fiber sensors hold numeral advantages over conventional electrical-based sensors. The former are inexpensive, compact, light weight, immune to electromagnetic interference. This resulted in a great demand for fiber sensors in biochemical sensing applications. In recent years, optical fiber refractive index (RI) sensors have attracted lots of attention, because they are useful in many kinds of areas ranging from environmental monitoring to biomedical sensing. Both fiber Bragg gratings (FBGs) and longperiod gratings (LPGs) have been used in RI fiber sensors. For an FBG-based RI sensor, the FBG is often etched or polished to gain the access of the evanescent field of the guided mode to the surrounded material to be measured [1–3]. Compared with RI sensors of FBG type, sensors based on LPGs [4–7] are more applicable because of their intrinsical coupling mechanism. A RI sensor based on Fabry–Perot (F–P) interferometers has been also reported [8]. Such a sensing structure makes use of the variation of the maximum contrast of the interference fringes to measure RI. Frazão [9] proposed a refractometer based on high birefringence etched Dtype fiber loop mirror. The evanescent field for RI measurement was increased by using the etched D-type structure fiber. Optical refractometers based on photonic crystal fibers were also proposed
[10]. However, the measurement principles of those refractometers are using the wavelength shifts to detect the external RI variation, which was faced with a big problem of temperature cross sensitivity. Thus, a temperature compensation part was necessary. Meanwhile, the obtained sensitivity was not high enough in the RI sensing structure which was reported previously. Therefore, fiber refractometer which can realize high sensitivity without the needing of an additional temperature compensation part was desirable in practical applications. In this work, a sensing head consisting of fiber loop mirror inscribed with a highly birefringence photonic crystal fiber (HiBiPCF) was proposed and experimentally demonstrated. The HiBi-PCF was completely collapsed near the splicing points, thus the cladding mode in the HiBi-PCF would be excited, which was sensitive to the refractive index (RI) of surrounding medium. Owing to the low thermo-optic and thermo-expansion coefficient of the HiBiPCF, the sensing head in our design was temperature insensitive. The collapsed HiBi-PCF worked just like a Mach–Zehnder interferometer (MZI), considering the FLM structure, a combination interferometer was formed, which result in a high sensitivity. The RI value of the liquid can be measured by detecting the interference fringes shift corresponding to the FLM and MZI. 2. Sensor fabrication and operation principle
∗ Corresponding author at: Northeastern University, College of Information Science and Engineering, Shenyang 110004, China. Tel.: +86 13998812362. E-mail address: [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.ijleo.2015.02.046 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
The schematic diagram of the proposed sensor head was shown in Fig. 1. An amplified spontaneous emission (ASE) source of
698
Y. Zhao et al. / Optik 126 (2015) 697–700
Fig. 1. (a) Schematic diagram of the proposed RI sensor; (b) SEM of the used HiBi-PCF and (c) the partial enlarged drawing of the sensing head.
1450–1650 nm wavelength range is connected to the input of the FLM, and the output spectrum is detected with an optical spectrum analyzer (OSA, AQ6370, Advantest, Japan). The maximum resolution of the OSA is 20 pm. The FLM is formed by splicing (using a commercial fusion splicer (Fujikura FSM-40S)) a sections of HiBiPCF (LMA-10) to the arms of a 2 × 2.3 dB coupler. The HiBi-PCF with the length of 5 cm and both ends of it was collapsed fusion splicing between two identical single mode fibers (SMF-28) with was in the arms of the coupler. The inset of Fig. 1 shows the cross sectional view of the HiBi-PCF, which has a solid core of diameter of 11 m surrounded by four layers of air-holes with an outer diameter of 125 m. Splicing of the PCF to SMF was carried out by using a commercial fusion splicer (Fujikura FSM-40S). Strong electronic arc discharges caused localized heating on the PCF, leading to collapse of the airholes in the cladding area of the heated PCF section. It can be seen from the micrograph of the fabricated splicing point, where the airhole structure of the PCF was collapsed near the splicing point over a short length of ∼140 m. The PCF was no longer single mode since the fiber had no core–cladding structure any more at the region of the collapsed area. When the fundamental mode of the lead-in SMF propagates into the collapsed region of PCF, its mode field diameter would be broadened due to the diffraction, allowing the excitation of core and cladding modes in the intact PCF section [11,12]. Then, the excited core and cladding modes are further diffracted and recoupled back to the core mode of the lead-out SMF at the second splicing point. Therefore, it worked just like a kind of MZI. The input light from the ASE is equally splits into two counterpropagating light by the 3 dB coupler, and subsequently they recombine at the coupler after clockwise and counterclockwise light beams propagating around the loop. A polarization controller (PC) is used to adjust the polarization states of the two lights. The counter-propagating light beams introduced a relative phase difference due to the birefringence property of the inserted HiBi-PCF. So interferences generate when they recombine at the coupler. The transmission optical intensity It in terms of the phase difference can be described as, 1 − cos It = 2
(1)
with =
2LB
(2)
where is the phase difference. is the center wavelength of the light source. L is the length of the HiBi-PCF. B = ns − nf is the birefringence index of the HiBi-PCF. ns and nf are the effective refractive index of the slow and fast axis, respectively. The resonant dip wavelength satisfies the equation of = 2k, where k is a random integer.
Fig. 2. Initial interference spectrum of the RI sensor.
Fig. 3. Interference spectrum variation of the FLM was subject to liquid with different RI.
Therefore, the resonant dip wavelength can be described as, =
BL k
(3)
The interference fringes corresponding to the FLM and MZI with different resonant dip wavelengths which result in a spectrum overlap as shown in Fig. 2. 3. Experiment and discussions To test the HiBi-PCF-FLM based liquid RI sensor, the collapse fusion spliced HiBi-PCF was immersed in the sample of liquid combine with different percentages of glycerin. The liquid samples were calibrated through an Abbe refractometer with a nominal accuracy of ±0.0001. We can get Eq. (2), =
L · (B + B) k
(4)
where is the wavelength shift as put PCF in liquid with varying refractive index. ≡ is the change of the birefringence index caused by the effective refractive index variation of the fast and slow axis corresponding to PCF. From Eq. (4), it can be seen that is directly proportional to ≡. External RI variation can be detected by measuring the wavelength shift of the interference spectrum. As the HiBi-PCF was immersed in liquid with RI value variation in a range of 1.41–1.43 at a room temperature (25 ◦ C), the variations of the interference spectrum for the FLM were shown in Fig. 3. It can be seen that the resonant dip wavelength 2 of the interference fringes were shifted from 1556.11 nm to 1549.66 nm. The resonant dip wavelength shifts of interference spectrum of the
Y. Zhao et al. / Optik 126 (2015) 697–700
699
Fig. 6. Temperature errors graph of the proposed sensor.
Fig. 4. Wavelength shifts of the FLM as function of external RI variation.
thermo-optic and thermo-expansion coefficient of the HiBi-PCF, and the birefringence will be little influenced by the temperature variation. The temperature errors graph of the proposed sensor was shown in Fig. 6. We selected the interference pattern corresponding to = 1537 nm, 1552 nm and 1575 nm as the experimental parameters. As the surrounding temperature increased from 10 ◦ C to 100 ◦ C, the maximum temperature errors were 0.52 nm, 0.49 nm and 0.48 nm at the wavelength of 1537 nm, 1552 nm and 1575 nm, respectively. Compare to the wavelength shift range of 6.45 nm, the temperature variation induced measurement error was so small which can be neglected in practical application. 4. Conclusion Fig. 5. Temperature response of the proposed RI sensor.
FLM as a function of RI changing is shown in Fig. 4. It satisfied well with the theoretical derivation. As shown in Fig. 4, as expected, the wavelength shift of the FLM has a linear relationship with the external RI variation. Owing to the use of a combination interference structure, high sensitivity of 306.6 ± 0.2 nm/RIU was achieved, which was about 5 times higher than the previous report [13]. The rms deviation value (SD) between the linear fit and the measured data was 0.2, which is equal to the uncertainty of our calibration measurement using the Abbe refractometer. The resolution of the proposed RI sensor is 6.5 × 10−5 RIU which is limited by resolution of the OSA. Large range of the wavelength shift will lead to an overlap of the fringes. Therefore, the measurement range was limited by fringe separation of interference spectrum of the FLM. The wavelength spacing S between the adjacent interference spectrum can be expressed as 2 S= BL
(5)
In practical applications, the measurement range of the proposed sensor can be adjusted by changing the birefringence of the HiBiPCF. The RI variation experimental measurement was performed in a temperature-controlled environment, and the temperature variation was less than ±0.1 ◦ C. So the error of displacement measurement induced by temperature can be neglected. But in practical applications, the surrounding temperature was not invariable. The influence of temperature on the proposed RI sensor was also studied. The total FLM part was placed into a temperature controlled container. The temperature of the controlled container was set to increase from 10 ◦ C to 100 ◦ C with a step of 10 ◦ C. As shown in Fig. 5, the resonant dip wavelength of the FLM has a little shift as the temperature increasing. This results owing to that the low
We have described a sensing head consisting of fiber loop mirror inscribed with a highly birefringence photonic crystal fiber (HiBi-PCF). Owing to the low thermo-optic and thermo-expansion coefficient of the HiBi-PCF, the sensing head in our design was temperature insensitive. High sensitivities of (306.6 ± 0.2) nm/RIU (refractive index unit) and a resolution of 6.5 × 10−5 RIU have been achieved for the proposed liquid refractive index sensor. Compared with other refractive index sensor, the proposed sensor can obtain a high sensitivity by using a combination interference structure. Owing to the low thermo-optic and thermo-expansion coefficient of the HiBi-PCF, the sensing head in our design was temperature insensitive without the needing of an additional temperature compensation part. Acknowledgments This work was supported in part by the National Natural Science Foundation of China under grant 61203206 and 61273059, and IAPI Fundamental Research Funds under grant 2013ZCX02-05, and Liaoning S&T Project under grant 2010220012. References [1] K. Schroeder, W. Ecke, R. Mueller, R. Willsch, A. Andreev, A fibre Bragg grating refractometer, Meas. Sci. Technol. 12 (2001) 757–764. [2] W. Liang, Y.Y. Huang, Y. Xu, R.K. Lee, A. Yariv, Highly sensitive fiber Bragg grating refractive index sensors, Appl. Phys. Lett. 86 (2005) 15112. [3] A. Iadicicco, A. Cusano, A. Cutolo, R. Bernini, M. Giordano, Thinned fiber Bragg gratings as high sensitivity refractive index sensor, IEEE Photonics Technol. Lett. 16 (2004) 1149–1151. [4] A.M. Rios, D.M. Hernandez, I.T. Gomez, Highly sensitive cladding-etched arcinduced long-period fiber gratings for refractive index sensing, Opt. Commun. 283 (2010) 958–962. [5] I.D. Villar, I.R. Matias, F.J. Arregui, Enhancement of sensitivity in long-period fiber gratings with deposition of low-refractive-index materials, Opt. Lett. 30 (2005) 2363–2365.
700
Y. Zhao et al. / Optik 126 (2015) 697–700
[6] T. Zhu, Y.J. Rao, Q.J. Mo, Simultaneous measurement of refractive index and temperature using a single ultra-long-period fiber grating, IEEE Photonics Technol. Lett. 17 (2005) 2700–2702. [7] A. Iadicicco, S. Campopiano, M. Giordano, A. Cusano, Spectral behavior in thinned long-period gratings: effects of fiber diameter on refractive index sensitivity, Appl. Opt. 46 (2007) 6945–6952. [8] Z.L. Ran, Y.J. Rao, W.J. Liu, X. Liao, K.S. Chiang, Laser-micromachined Fabry–Perot optical fiber tip sensor for high-resolution temperature-independent measurement of refractive index, Opt. Express 16 (2008) 2252–2263. [9] O. Frazão, B.V. Marques, P. Jorge, J.M. Baptista, J.L. Santos, High birefringence D-type fiber loop mirror used as refractometer, Sens. Actuators B: Chem. 135 (2008) 5108–5111.
[10] S. Silva, J.L. Santos, F.X. Malcata, J. Kobelke, K. Schuster, O. Frazão, Optical refractometer based on large-core air-clad photonic crystal fibers, Opt. Lett. 366 (2011) 852–854. [11] J. Villatoro, V.P. Minkovich, V. Pruneri, G. Badenes, Simple all-microstructuredoptical-fiber interferometer built via fusion splicing, Opt. Express 15 (4) (2007) 1491–1496. [12] G.A. Cárdenas-Sevilla, V. Finazzi, J. Villatoro, V. Pruneri, Photonic crystal fiber sensor array based on modes overlapping, Opt. Express 19 (8) (2011) 7596–7602. [13] C. Zhong, C.Y. Shen, Y. You, J.L. Chu, X. Zhou, X.Y. Dong, Y.X. Jin, J.F. Wang, A polarization-maintaining fiber loop mirror based sensor for liquid refractive index absolute measurement, Sens. Actuators: B 168 (2012) 360–364.