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Sensitivity-enhanced temperature sensor with cascaded fiber optic Sagnac interferometers based on Vernier-effect
Optics Communications 336 (2015) 73–76
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Sensitivity-enhanced temperature sensor with cascaded fiber optic Sagnac interferometers based on Vernier-effect Li-Yang Shao n, Yuan Luo, Zhiyong Zhang, Xihua Zou, Bin Luo, Wei Pan, Lianshan Yan n Center for Information Photonics & Communications, School of Information Science and Technology, Southwest Jiaotong University, Chengdu 610031, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 July 2014 Received in revised form 27 September 2014 Accepted 29 September 2014 Available online 7 October 2014
A novel fiber optic temperature sensor has been proposed and experimentally demonstrated with 9 times sensitivity enhancement by using two cascaded Sagnac interferometers. These two Sagnac interferometers consist of the same type of polarization maintaining fibers with slightly different lengths. The working principle is analogous to a Vernier scale. One interferometer acts as filter, while the other is for temperature sensing. The envelope of the cascaded sensor shifts much more than single one with a certain enhancement factor, which related to the free space range difference between the filter and sensor interferometers. Experimental results show that the temperature sensitivity is enhanced from 1.46 nm/°C based on single Sagnac configuration to 13.36 nm/°C. & Elsevier B.V. All rights reserved.
Keywords: Fiber optic sensor Sagnac interferometer Vernier effect Temperature measurement
1. Introduction The fiber loop mirror or Sagnac interferometer based on a ring configuration is one of the important devices in communications and sensors [1]. Normally, a section of birefrigent fiber is inserted in the loop and can introduce optical path difference of the two counter-propagating waves and cause an interference spectrum, which is used as a sensing element. Recent years, considerable research efforts have been made to develop Sagnac interferometer sensors for various applications, such as temperature sensors, torsion sensors, strain sensors, and biochemical sensors [2–5]. Among them, temperature sensors based on single ring configuration with conventional PANDA fiber have a sensitivity ( 1.90 nm/°C) [6]. Instead of conventional PANDA fiber, Qian has proposed a temperature sensor using a section of alcohol-filled HiBi-PCF and the sensitivity is improved to 6.6 nm/°C [7]. However, alcohol-filled HiBi-PCF is difficult to produce and cost much more than conventional PANDA fiber. Thereafter, a few schemes have been proposed to enhance the sensitivity of fiber optic temperature sensor. Dong proposed a sensitivity enhanced temperature sensor based on the cladding mode interference by splicing a mode mis-matched dispersion compensation fiber between single mode fibers [8]. Geng demonstrates a high temperature sensor with higher sensitivity by courtesy of photonic bandgap fiber mode interference [9]. Similarly, cladding mode n
Corresponding authors. E-mail addresses: [email protected] (L.-Y. Shao), [email protected] (L. Yan). http://dx.doi.org/10.1016/j.optcom.2014.09.075 0030-4018/& Elsevier B.V. All rights reserved.
associated interference has been employed to improve the temperature sensitivity, including a multimode fiber-single mode fiber-multimode fiber structure [10], a high germanium doped PCF interferometer [11], and PCF mode interferometer surrounded by a material overlay with a high thermo-optic coefficient [12]. However, limited to the inherent temperature coefficient of fiber materials or sensor's structure, the temperature sensitivities obtained among these works (from 50 pm/°C to 0.92 nm/°C) are not high as the one of Qian's sensor. In this paper, we proposed a temperature sensor based on two cascaded Sagnac interferometers for high sensitivity temperature sensing. Other than exploring the fiber material or optimizing the mode interference, the same PM-fibers have been used in our proposed sensor which not only eases the fabrication and also realizes a low cost sensing system. The sensitivity enhances obviously based on Vernier effect in the transmission spectra of cascaded Sagnac interferometers. In our experiment, the results show the cascaded Sagnac sensor with Vernier effect enhancing the temperature sensitivity around 9 times ( 13.36 nm/°C) comparing to that of the single individual Sagnac sensor ( 1.46 nm/°C).
2. Theory and simulation The schematic diagram of the proposed temperature sensor is shown in Fig. 1. It includes two conventional 3 dB single-mode fiber couplers and two pieces of polarization maintain fibers (PMFs). In a Sagnac interferometer, the transmission is decided by the birefringence induced phase shift (ƞ ) between the two
74
L.-Y. Shao et al. / Optics Communications 336 (2015) 73–76
Fig. 1. Configuration of the proposed cascaded Sagnac loop sensors based on the Vernier effect.
principal polarization modes, given as
T = (1 ℬ cos ƞ)/2
(1)
where ƞ = 2ưBL/ƥ is the phase shift between two polarization modes, B is the birefringence of the PMF, L is the length of the PMF, ƥ is the wavelength. When the temperature changed, the wavelength will gain an extra component ʁ ƥ = S ʁ T . S is the temperature sensitivity and ʁ T is the change of temperature. Thus the real phase shift can be written as
ƞ=
2ưBL ƥ ℬ Sʁ T
(2)
We can simulate the output of the single Sagnac interferometer sensor and obtain the result shown in Fig. 2a (fsrsensor ¼ 1.8 nm). The transmission is just like a scale ruler and the scale of the ruler is as wide as the free spectrum range (fsr) which can be written as
fsr =
ƥ2 BL
(3)
Thus we can improve the sensitivity by constructing a Vernierscale with two cascaded Sagnac interferometers. The Vernier-scale is an efficient method to enhance the accuracy of measurement instruments, which consists of two scales with different periods. It is widely used in calipers and barometers, and it has also been applied in photonic devices [13,14]. The overlap between two scale lines is employed to perform the measurement. Fig. 1 shows how the Vernierscale concept is applied to Sagnac loops. The interference signal of the filter Sagnac loop serves as input for the sensor one. The total transmission spectrum of the cascaded Sagnac loops is the product of the individual ones, which exhibits peaks at wavelengths where two interference peaks of the respective Sagnac loops partially overlap, and the height of each of these peaks will be determined by the amount of overlap. In practice, we can manage the length of the PMF to get the ideal fsr. The first Sagnac loop, filter Sagnac loop is well shielded from environment temperature or strain change, which acts as the fixed part of the Vernier-scale. The second sensor Sagnac is more like the sliding part of the Vernier-scale, as the temperature changes will cause a shift of the interference wavelengths. The envelope period is given by
fsrsensor ¬ fsr filter fsrsensor ℬ fsr filter
(4)
The highest peak of the total transmission occurs when the peaks of the two individual loops are at the same wavelength. If one loop shifted with a difference value of the two fsr, the highest peak will hop to the adjacent peak. When the temperature changed, the transmission of the single Sagnac loop sensor will shift. Then the shift of cascaded Sagnac envelope is magnified by a certain factor. And the enhanced factor is decided by
fsr filter fsrsensor ℬ fsr filter
(5)
The simulation result is shown in Fig. 2b. fsrfilter is set to be 2.0 nm. When the shift of single Sagnac is 0.2 nm, the shift in the envelope is magnified to 2.0 nm. The sensitivity is enhanced by 10 times which is consistent with the theoretical anylasis. 3. Experiments In the experiment, a broadband superluminescent light-emitting diode (SLED) source with 100 nm wavelength range is used as an input light source. The transmission spectra of the Sagnac interferometers are measured by an optical spectrum analyzer with a wavelength resolution of 0.01 nm. Fig. 3 illustrates the spectra of the filter Sagnac interferometer and cascaded Sagnac interferometers (20 °C). Here, the PMF is Panda fiber from Yangtze Optical Fiber and Cable Company Ltd. (YOFC) (B¼ 3 10-4). The
Fig. 2. Simulation results of the transmission spectrum shifts of (a) single Sagnac interferometer and (b) cascaded Sagnac interferometers.
Fig. 3. Measured spectra of two individual and cascaded Sagnac interferometers.
L.-Y. Shao et al. / Optics Communications 336 (2015) 73–76
Fig. 4. Spectra Spectral shift at the temperature of 31.0 °C and 32.0 °C of a single Sagnac interferometer.
75
Fig. 6 illustrates the temperature sensivity of two kinds of sensors, respectively. Obviously, the cascaded Sagnac sensor has a much higher temperature sensitivity (13.36 nm/ °C) compared to the individual one ( 1.46 nm/ °C). The R-squares for linear fitting of single Sagnac sensor is 0.998, and the one for cascaded sensor is 0.995. So the experimental enhancement factor is 9.15, which is slightly bigger than the theoretical one (7.83). This is caused by the determination of the envelope position in the spectrum. The sensitivity can be enhanced much more according to the calculated enhancement factor (see as Eq. (5)) by choosing two interferometers with smaller difference in fsr. But in practice, there is a compromise between the sensitivity and measurement accuracy. Because the smaller fsr difference will increase the fsr of envelope and reduce the number of envelopes in a certain wavelength window, which introduces the difficulty in tracking the shift of the envelope peak.
4. Conclusion We proposed and experimentally demonstrated a cascaded Sagnac interferometer sensor with enhanced temperature sensitivity by Vernier effect. The experimental temperature sensitivity is as high as 13.36 nm/ °C which is 9.15 times higher than that of the single individual Sagnac sensor. This technique can be explored to measure other measurands which need high sensitivity.
Acknowledgment Fig. 5. Spectral shift at the temperature of 31.0 °C and 32.0 °C of cascaded Sagnac sensor.
This work was supported by the International Science and Technology Cooperation Program of China (2014DFA11170), the Fundamental Research Fund for the Central Universities (2682014RC22), the National Natural Science Foundation of China (61475128 and 61325023), the Key Grant Project of Chinese Ministry of Education (No. 313049) and the Key Project of Sichuan Province of China (2011GZ0239).
References
Fig. 6. The measured temperature responses of two kinds of sensors.
lengths of PMFs in sensor and filter Sagnac interferometers are measured to be 2.05 m and 1.78 m. fsrsensor and fsrfilter are measured to be 3.87 nm and 4.43 nm, therefore the calculated enhancement factor is 7.91. The temperature characteristics of the individual and cascaded Sagnac interferometers are tested by placing the sensors in a temperature controlled furnace with the temperature range from 30 °C to 40 °C. For a clear comparison of two kinds of sensors, Figs. 4 and 5 show the transmission spectra of the individual Sagnac sensor and cascaded Sagnac sensor at two temperatures of 31.0 °C and 32.0 °C. Both of two sensors have a blue wavelength shift with the increase of temperature. The wavelength shift of cascaded Sagnac sensor is drawn from the envelope shift.
[1] D.B. Mortimore, Fiber loop reflectors, IEEE/OSA J. Lightwave Technol.LT-6 (1998) 1217–1224. [2] E. De la Rosa, L.A. Zenteno, A.N. Starodumov, D. Monzon, All-fiber absolute temperature sensor using an unbalanced high-birefringence Sagnac loop, Opt. Lett. 22 (1997) 481–483. [3] B.H. Kim, S.H. Lee, A. Lin, C.L. Lee, J. Lee, W.T. Han, Large temperature sensitivity of Sagnac loop interferometer based on the birefringent holey fiber filled with metal indium, Opt. Express 17 (3) (2009) 1789–1794. [4] H.-M. Kim, T.-H. Kim, K. Bongkyun, C. Youngjoo, Temperature-insensitive torsion sensor with enhanced sensitivity by use of a highly birefringent photonic crystal fiber, IEEE Photonics Technol. Lett. 22 (2010) 1539–1541. [5] O. Frazão, B.V. Marques, P. Jorge, J.M. Bapitista, J.L. Santos, High birefringence D-type fibre loop mirror used as refractometer, Sens. Actuators B: Chem. 135 (2008) 108–111. [6] Y. Liu, B. Liu, X. Feng, W. Zhang, G. Zhou, S. Yuan, G. Kai, X. Dong, Highbirefringence fiber loop mirrors and their applications as sensors, Appl. Opt. 44 (2005) 2382–2390. [7] W. Qian, C.-L. Zhao, S. He, X. Dong, S. Zhang, Z. Zhang, S. Jin, J. GuoNakatani, B. Journet, High-sensitivity temperature sensor based on an alcohol-filled photonic crystal fiber loop mirror, Opt. Lett. 36 (2011) 1548–1550. [8] B. Dong, L. Wei, D.-P. Zhou, Miniature high-sensitivity high-temperature fiber sensor with a dispersion compensation fiber-based interferometer, Appl. Opt. 48 (2009) 6466–6469. [9] Y. Geng, X. Li, X. Tan, Y. Deng, Y. Yu, Sensitivity-enhanced high-temperature sensing using all-solid photonic bandgap fiber modal interference, Appl. Opt. 50 (2011) 468–472. [10] J. Wo, Q. Sun, H. Liu, X. Li, J. Zhang, D. Liu, P.P. Shum, Sensitivity-enhanced fiber optic temperature sensor with strain response suppression, Opt. Fiber Technol. 19 (2013) 289–292. [11] F.C. Favero, R. Spittel, F. Just, J. Kobelke, M. Rothhardt, H. Bartelt, A miniature temperature high germanium doped PCF interferometer sensor, Opt. Express 21 (2013) 30266–30274.
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L.-Y. Shao et al. / Optics Communications 336 (2015) 73–76
[12] J.-M. Hsu, C.-L. Lee, P.-J. Huang, C.-H. Hung, P.-Y. Tai, Temperature sensor with enhanced sensitivity based on photonic crystal fiber interferometer with material overlay, IEEE Photonics Technol. Lett. 24 (2012) 1761–1764. [13] M. La Notte, V.M.N. Passaro, Ultra high sensitivity chemical photonic sensing by Mach–Zehnder interferometer enhanced Vernier-effect, Sens. Actuators B: Chem. 176 (2013) 994–1007.
[14] T. Cales, W. Bogaerts, P. Bienstman, Experimental characterization of a silicon photonic biosensor consisting of two cascaded ring resonators based on the Vernier-effect and introduction of a curve fitting method for an improved detection limit, Opt. Express 18 (2010) 22747–22761.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Sensitivity-enhanced temperature sensor with cascaded fiber optic Sagnac interferometers based on Vernier-effect Li-Yang Shao n, Yuan Luo, Zhiyong Zhang, Xihua Zou, Bin Luo, Wei Pan, Lianshan Yan n Center for Information Photonics & Communications, School of Information Science and Technology, Southwest Jiaotong University, Chengdu 610031, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 July 2014 Received in revised form 27 September 2014 Accepted 29 September 2014 Available online 7 October 2014
A novel fiber optic temperature sensor has been proposed and experimentally demonstrated with 9 times sensitivity enhancement by using two cascaded Sagnac interferometers. These two Sagnac interferometers consist of the same type of polarization maintaining fibers with slightly different lengths. The working principle is analogous to a Vernier scale. One interferometer acts as filter, while the other is for temperature sensing. The envelope of the cascaded sensor shifts much more than single one with a certain enhancement factor, which related to the free space range difference between the filter and sensor interferometers. Experimental results show that the temperature sensitivity is enhanced from 1.46 nm/°C based on single Sagnac configuration to 13.36 nm/°C. & Elsevier B.V. All rights reserved.
Keywords: Fiber optic sensor Sagnac interferometer Vernier effect Temperature measurement
1. Introduction The fiber loop mirror or Sagnac interferometer based on a ring configuration is one of the important devices in communications and sensors [1]. Normally, a section of birefrigent fiber is inserted in the loop and can introduce optical path difference of the two counter-propagating waves and cause an interference spectrum, which is used as a sensing element. Recent years, considerable research efforts have been made to develop Sagnac interferometer sensors for various applications, such as temperature sensors, torsion sensors, strain sensors, and biochemical sensors [2–5]. Among them, temperature sensors based on single ring configuration with conventional PANDA fiber have a sensitivity ( 1.90 nm/°C) [6]. Instead of conventional PANDA fiber, Qian has proposed a temperature sensor using a section of alcohol-filled HiBi-PCF and the sensitivity is improved to 6.6 nm/°C [7]. However, alcohol-filled HiBi-PCF is difficult to produce and cost much more than conventional PANDA fiber. Thereafter, a few schemes have been proposed to enhance the sensitivity of fiber optic temperature sensor. Dong proposed a sensitivity enhanced temperature sensor based on the cladding mode interference by splicing a mode mis-matched dispersion compensation fiber between single mode fibers [8]. Geng demonstrates a high temperature sensor with higher sensitivity by courtesy of photonic bandgap fiber mode interference [9]. Similarly, cladding mode n
Corresponding authors. E-mail addresses: [email protected] (L.-Y. Shao), [email protected] (L. Yan). http://dx.doi.org/10.1016/j.optcom.2014.09.075 0030-4018/& Elsevier B.V. All rights reserved.
associated interference has been employed to improve the temperature sensitivity, including a multimode fiber-single mode fiber-multimode fiber structure [10], a high germanium doped PCF interferometer [11], and PCF mode interferometer surrounded by a material overlay with a high thermo-optic coefficient [12]. However, limited to the inherent temperature coefficient of fiber materials or sensor's structure, the temperature sensitivities obtained among these works (from 50 pm/°C to 0.92 nm/°C) are not high as the one of Qian's sensor. In this paper, we proposed a temperature sensor based on two cascaded Sagnac interferometers for high sensitivity temperature sensing. Other than exploring the fiber material or optimizing the mode interference, the same PM-fibers have been used in our proposed sensor which not only eases the fabrication and also realizes a low cost sensing system. The sensitivity enhances obviously based on Vernier effect in the transmission spectra of cascaded Sagnac interferometers. In our experiment, the results show the cascaded Sagnac sensor with Vernier effect enhancing the temperature sensitivity around 9 times ( 13.36 nm/°C) comparing to that of the single individual Sagnac sensor ( 1.46 nm/°C).
2. Theory and simulation The schematic diagram of the proposed temperature sensor is shown in Fig. 1. It includes two conventional 3 dB single-mode fiber couplers and two pieces of polarization maintain fibers (PMFs). In a Sagnac interferometer, the transmission is decided by the birefringence induced phase shift (ƞ ) between the two
74
L.-Y. Shao et al. / Optics Communications 336 (2015) 73–76
Fig. 1. Configuration of the proposed cascaded Sagnac loop sensors based on the Vernier effect.
principal polarization modes, given as
T = (1 ℬ cos ƞ)/2
(1)
where ƞ = 2ưBL/ƥ is the phase shift between two polarization modes, B is the birefringence of the PMF, L is the length of the PMF, ƥ is the wavelength. When the temperature changed, the wavelength will gain an extra component ʁ ƥ = S ʁ T . S is the temperature sensitivity and ʁ T is the change of temperature. Thus the real phase shift can be written as
ƞ=
2ưBL ƥ ℬ Sʁ T
(2)
We can simulate the output of the single Sagnac interferometer sensor and obtain the result shown in Fig. 2a (fsrsensor ¼ 1.8 nm). The transmission is just like a scale ruler and the scale of the ruler is as wide as the free spectrum range (fsr) which can be written as
fsr =
ƥ2 BL
(3)
Thus we can improve the sensitivity by constructing a Vernierscale with two cascaded Sagnac interferometers. The Vernier-scale is an efficient method to enhance the accuracy of measurement instruments, which consists of two scales with different periods. It is widely used in calipers and barometers, and it has also been applied in photonic devices [13,14]. The overlap between two scale lines is employed to perform the measurement. Fig. 1 shows how the Vernierscale concept is applied to Sagnac loops. The interference signal of the filter Sagnac loop serves as input for the sensor one. The total transmission spectrum of the cascaded Sagnac loops is the product of the individual ones, which exhibits peaks at wavelengths where two interference peaks of the respective Sagnac loops partially overlap, and the height of each of these peaks will be determined by the amount of overlap. In practice, we can manage the length of the PMF to get the ideal fsr. The first Sagnac loop, filter Sagnac loop is well shielded from environment temperature or strain change, which acts as the fixed part of the Vernier-scale. The second sensor Sagnac is more like the sliding part of the Vernier-scale, as the temperature changes will cause a shift of the interference wavelengths. The envelope period is given by
fsrsensor ¬ fsr filter fsrsensor ℬ fsr filter
(4)
The highest peak of the total transmission occurs when the peaks of the two individual loops are at the same wavelength. If one loop shifted with a difference value of the two fsr, the highest peak will hop to the adjacent peak. When the temperature changed, the transmission of the single Sagnac loop sensor will shift. Then the shift of cascaded Sagnac envelope is magnified by a certain factor. And the enhanced factor is decided by
fsr filter fsrsensor ℬ fsr filter
(5)
The simulation result is shown in Fig. 2b. fsrfilter is set to be 2.0 nm. When the shift of single Sagnac is 0.2 nm, the shift in the envelope is magnified to 2.0 nm. The sensitivity is enhanced by 10 times which is consistent with the theoretical anylasis. 3. Experiments In the experiment, a broadband superluminescent light-emitting diode (SLED) source with 100 nm wavelength range is used as an input light source. The transmission spectra of the Sagnac interferometers are measured by an optical spectrum analyzer with a wavelength resolution of 0.01 nm. Fig. 3 illustrates the spectra of the filter Sagnac interferometer and cascaded Sagnac interferometers (20 °C). Here, the PMF is Panda fiber from Yangtze Optical Fiber and Cable Company Ltd. (YOFC) (B¼ 3 10-4). The
Fig. 2. Simulation results of the transmission spectrum shifts of (a) single Sagnac interferometer and (b) cascaded Sagnac interferometers.
Fig. 3. Measured spectra of two individual and cascaded Sagnac interferometers.
L.-Y. Shao et al. / Optics Communications 336 (2015) 73–76
Fig. 4. Spectra Spectral shift at the temperature of 31.0 °C and 32.0 °C of a single Sagnac interferometer.
75
Fig. 6 illustrates the temperature sensivity of two kinds of sensors, respectively. Obviously, the cascaded Sagnac sensor has a much higher temperature sensitivity (13.36 nm/ °C) compared to the individual one ( 1.46 nm/ °C). The R-squares for linear fitting of single Sagnac sensor is 0.998, and the one for cascaded sensor is 0.995. So the experimental enhancement factor is 9.15, which is slightly bigger than the theoretical one (7.83). This is caused by the determination of the envelope position in the spectrum. The sensitivity can be enhanced much more according to the calculated enhancement factor (see as Eq. (5)) by choosing two interferometers with smaller difference in fsr. But in practice, there is a compromise between the sensitivity and measurement accuracy. Because the smaller fsr difference will increase the fsr of envelope and reduce the number of envelopes in a certain wavelength window, which introduces the difficulty in tracking the shift of the envelope peak.
4. Conclusion We proposed and experimentally demonstrated a cascaded Sagnac interferometer sensor with enhanced temperature sensitivity by Vernier effect. The experimental temperature sensitivity is as high as 13.36 nm/ °C which is 9.15 times higher than that of the single individual Sagnac sensor. This technique can be explored to measure other measurands which need high sensitivity.
Acknowledgment Fig. 5. Spectral shift at the temperature of 31.0 °C and 32.0 °C of cascaded Sagnac sensor.
This work was supported by the International Science and Technology Cooperation Program of China (2014DFA11170), the Fundamental Research Fund for the Central Universities (2682014RC22), the National Natural Science Foundation of China (61475128 and 61325023), the Key Grant Project of Chinese Ministry of Education (No. 313049) and the Key Project of Sichuan Province of China (2011GZ0239).
References
Fig. 6. The measured temperature responses of two kinds of sensors.
lengths of PMFs in sensor and filter Sagnac interferometers are measured to be 2.05 m and 1.78 m. fsrsensor and fsrfilter are measured to be 3.87 nm and 4.43 nm, therefore the calculated enhancement factor is 7.91. The temperature characteristics of the individual and cascaded Sagnac interferometers are tested by placing the sensors in a temperature controlled furnace with the temperature range from 30 °C to 40 °C. For a clear comparison of two kinds of sensors, Figs. 4 and 5 show the transmission spectra of the individual Sagnac sensor and cascaded Sagnac sensor at two temperatures of 31.0 °C and 32.0 °C. Both of two sensors have a blue wavelength shift with the increase of temperature. The wavelength shift of cascaded Sagnac sensor is drawn from the envelope shift.
[1] D.B. Mortimore, Fiber loop reflectors, IEEE/OSA J. Lightwave Technol.LT-6 (1998) 1217–1224. [2] E. De la Rosa, L.A. Zenteno, A.N. Starodumov, D. Monzon, All-fiber absolute temperature sensor using an unbalanced high-birefringence Sagnac loop, Opt. Lett. 22 (1997) 481–483. [3] B.H. Kim, S.H. Lee, A. Lin, C.L. Lee, J. Lee, W.T. Han, Large temperature sensitivity of Sagnac loop interferometer based on the birefringent holey fiber filled with metal indium, Opt. Express 17 (3) (2009) 1789–1794. [4] H.-M. Kim, T.-H. Kim, K. Bongkyun, C. Youngjoo, Temperature-insensitive torsion sensor with enhanced sensitivity by use of a highly birefringent photonic crystal fiber, IEEE Photonics Technol. Lett. 22 (2010) 1539–1541. [5] O. Frazão, B.V. Marques, P. Jorge, J.M. Bapitista, J.L. Santos, High birefringence D-type fibre loop mirror used as refractometer, Sens. Actuators B: Chem. 135 (2008) 108–111. [6] Y. Liu, B. Liu, X. Feng, W. Zhang, G. Zhou, S. Yuan, G. Kai, X. Dong, Highbirefringence fiber loop mirrors and their applications as sensors, Appl. Opt. 44 (2005) 2382–2390. [7] W. Qian, C.-L. Zhao, S. He, X. Dong, S. Zhang, Z. Zhang, S. Jin, J. GuoNakatani, B. Journet, High-sensitivity temperature sensor based on an alcohol-filled photonic crystal fiber loop mirror, Opt. Lett. 36 (2011) 1548–1550. [8] B. Dong, L. Wei, D.-P. Zhou, Miniature high-sensitivity high-temperature fiber sensor with a dispersion compensation fiber-based interferometer, Appl. Opt. 48 (2009) 6466–6469. [9] Y. Geng, X. Li, X. Tan, Y. Deng, Y. Yu, Sensitivity-enhanced high-temperature sensing using all-solid photonic bandgap fiber modal interference, Appl. Opt. 50 (2011) 468–472. [10] J. Wo, Q. Sun, H. Liu, X. Li, J. Zhang, D. Liu, P.P. Shum, Sensitivity-enhanced fiber optic temperature sensor with strain response suppression, Opt. Fiber Technol. 19 (2013) 289–292. [11] F.C. Favero, R. Spittel, F. Just, J. Kobelke, M. Rothhardt, H. Bartelt, A miniature temperature high germanium doped PCF interferometer sensor, Opt. Express 21 (2013) 30266–30274.
76
L.-Y. Shao et al. / Optics Communications 336 (2015) 73–76
[12] J.-M. Hsu, C.-L. Lee, P.-J. Huang, C.-H. Hung, P.-Y. Tai, Temperature sensor with enhanced sensitivity based on photonic crystal fiber interferometer with material overlay, IEEE Photonics Technol. Lett. 24 (2012) 1761–1764. [13] M. La Notte, V.M.N. Passaro, Ultra high sensitivity chemical photonic sensing by Mach–Zehnder interferometer enhanced Vernier-effect, Sens. Actuators B: Chem. 176 (2013) 994–1007.
[14] T. Cales, W. Bogaerts, P. Bienstman, Experimental characterization of a silicon photonic biosensor consisting of two cascaded ring resonators based on the Vernier-effect and introduction of a curve fitting method for an improved detection limit, Opt. Express 18 (2010) 22747–22761.