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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 11, JUNE 1, 2014
Refractive Index and Temperature Sensor Based on Double-Pass M–Z Interferometer With an FBG Yanfang Lu, Changyu Shen, Chuan Zhong, Debao Chen, Xinyong Dong, and Jinhui Cai
Abstract— In order to measure the refractive index (RI) and temperature simultaneously, a double-pass in-line Mach–Zehnder interferometer (MZI) is proposed. The in-line MZI was formed by inscribing a pair of long period gratings in a single mode fiber. A fiber Bragg grating (FBG) was inscribed next to the output port of the MZI. Part of the MZI interference light was reflected by the FBG and the fiber end, and then a double MZI interference pattern was formed. A sensitivity of −21.07 nm/RIU is obtained in the RI range of 1.33–1.43. In addition, the FBG is also used as a temperature measuring and compensating section. This kind of RI and temperature simultaneous measurement optical fiber sensor could be used in chemical or biochemical sensing fields. Index Terms— Refractive index sensor, double-pass in-line Mach–Zehnder interferometer, fiber Bragg grating, long period grating.
I. I NTRODUCTION
I
N-LINE fiber interferometers have attracted considerable attentions because of their compact size and high sensitivity. Recently, several new types of in-line Mach–Zehnder interferometer (MZI) based fiber sensors have been reported [1]–[18]. They have been successfully applied as refractive index (RI) [4], [5], temperature [6], curvature [7], [8], displacement [9], strain [10] and bend [11] sensors. The traditional way to improve the sensitivity of an in-line MZI sensor is by increasing its cavity length [12]. However, long cavity makes the sensor more difficult to package. Fan [13] proposed a highly sensitive RI sensor based on two cascaded long period gratings (LPGs). The LPGs were inscribed with a rotary RI modulation approach. The sensitivity is 3.5 times higher than that of a MZI formed by two normal LPGs. Wu [14] proposed a RI sensor based on a MZI formed by three cascaded SMF tapers. The obtained sensitivity was 4 times higher than that of the normal two cascaded taper based MZI. Li [15] proposed a double-pass in-line fiber taper MZI based RI sensor. The double-pass configuration is achieved by connecting a reflecting mirror to one port of the MZI and the RI sensitivity was Manuscript received March 5, 2014; revised March 20, 2014; accepted March 29, 2014. Date of publication April 4, 2014; date of current version May 1, 2014. This work was supported in part by the National Natural Science Foundation of China under Grant 51374188 and in part by the Zhejiang Provincial Natural Science Foundation for Distinguished Young Scientists under Grant LR13E040001. (Corresponding author: J. Cai.) Y. Lu, C. Shen, D. Chen, X. Dong, and J. Cai are with the College of Optoelectronic of China, Jiliang University, Hangzhou 310018, China (e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]). C. Zhong is with the School of Physics, Trinity College Dublin, Dublin, Ireland (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2315804
improved by 1.87 times compare to the single-pass sensor. Yang [16] proposed a single “S”-like fiber taper MZI based fiber sensor. A high RI sensitivity of 1590 nm/RIU has been achieved which was 30 times higher than that of the normal two-taper-based MZI sensors. A high sensitivity of taperbased MZI embedded in a thinned optical fiber for RI sensing was proposed, which is 10 times higher than that of normal taper based MZI sensors [17]. These methods mentioned above can improve the sensitivity of the fiber in-line MZI based sensor effectively. However, there are still some aspects need to improve. For example, the LPG with a rotary RI modulation was difficult to fabricate and the tapered fiber resulted in weak sensor strength. Therefore, the new method to improve the sensitivity of the in-line MZI based sensor without complicated fabrication process was desired. In this letter, a simple double-pass in-line fiber MZI structure is proposed. The MZI was formed by a normal LPG pair. A fiber Bragg grating (FBG) was inscribed next to the output port of the MZI, which can reflect a narrowband of light into the MZI. The achieved interference pattern of the MZI shifts with the ambient RI and temperature variation, while the Bragg wavelength of the FBG is only sensitive to temperature. By measuring the resonance wavelength shifts of the MZI and the FBG, the changes of the RI and temperature can be detected simultaneously. II. E XPERIMENTS The experimental setup of the proposed reflective optical refractometer is shown in Fig. 1(a). An amplified spontaneous emission (ASE) source of 1450 to 1650nm wavelength range is used as the light source. Light from the ASE source was launched into the MZI through a circulator. The reflected light from the double-pass MZI would be detected with an optical spectrum analyzer (OSA, AQ6370, Advantest, Japan). The maximum resolution of the OSA is 0.02nm. The fiber holder was used to keep the sensor head free from any strain and bending. Glycerol solutions with different concentrations were used as the test samples. The corresponding RI was measured by an Abbe refractometer and the RI values for the samples were changed from 1.33 to 1.43. For different concentration solution samples, before each measurement, the sensing head was cleaned carefully by alcohol and then air-dried for 1 minute. The schematic diagram of the proposed reflective fiber refractometer is shown in Fig. 1(b). An FBG and a pair of LPGs (LPG1 and LPG2) were inscribed into the same SMF.
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LU et al.: REFRACTIVE INDEX AND TEMPERATURE SENSOR
Fig. 1. (a) Schematic diagram of the experimental setup for the RI measurement. (b) Schematic diagram of the proposed reflective fiber refractometer.
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cladding modes. The excited cladding modes were re-coupled to the core after propagating through the LPG2. Therefore, the two beams coming from the fiber core and cladding interfered at LPG2. Without the FBG, the fiber end will reflect part of interference light back to the MZI, and then a double MZI interference pattern was formed as shown in Fig. 2(c). If an FBG is inscribed next to the LPG2 as shown in Fig. 1(b), a double MZI interference pattern with an FBG reflected peak is formed as shown in Fig. 2(d). The reflectivity of FBG is higher than that of the fiber end. So, the energy reflected by the FBG is higher than that of the interference pattern at the FBG’s Bragg wavelength. It is known that after the light single-pass through the inline fiber MZI, the phase difference m between the core and the cladding modes can be described as [14], m = 2πn m e f f L/λ
(1)
m ef f
In Eq.(1), is the effective RI difference between the fiber core and the mth cladding modes, λ is the center wavelength of the input light, L is the length between the centers of LPG1 and LPG2. The intensity Is of the singlepass interference patterns is Is (λ) = I1 + I2 + 2 I1 I2 cos(m ) (2) where I1 and I2 are the intensities of the light propagating along the fiber core and cladding, respectively. After the interference light was reflected back to the MZI by the FBG and fiber end, the optical path of the interference light is double than that of the single-pass interference light, the phase difference m of double-pass MZI between the core and the cladding modes can be described as Fig. 2. (a) Transmission spectrum of a single LPG. (b) Reflection spectrum of the FBG. (c), (d) Interference spectra of the fiber in-line double-pass MZI without and with an FBG.
The LPG pair forms a single-pass MZI. After the MZI interference light passed through the FBG, the Bragg wavelength of the FBG was reflected. And after that, the fiber end will reflect part of interference light back to the MZI and a fiber in-line double-pass MZI structure was formed. The length between the center of the LPG1 and LPG2 is 2.4 cm and the coating of the fiber between the LPG pair is stripped. The grating pitch of the LPG1 and LPG2 is about 559 μm and the grating length is 22.36 mm (40 grating periods). The LPG writing laser is a high-frequency pulsed CO2 laser (HANS LASER-H10, China) with a maximum output power of 10 W. By properly controlling the writing time, LPG1 and LPG2 can be obtained with almost same parameters as shown in Fig. 2(a). The FBG was fabricated by exposing a deeply hydrogen-loaded (one month under room temperature) single-mode fiber to a 244 nm UV laser beam (ATLEX-300M, Germany) through a phase mask. The exposure time and intensity are 2 minutes and 50mW respectively. As shown in Fig. 2(b), the center wavelength of the FBG is 1550.02 nm with a reflectivity of 90%. The initial interference patterns of the double-pass MZI without and with an FBG are shown in Fig. 2(c) and Fig. 2(d). When the light travels along the fiber, the LPG1 will couple part of the core mode energy into the cladding and excite some
m = 4πn m e f f L/λ
(3)
The resonant dip wavelength satisfies the equation of m = (2k + 1)π, where k is a random integer. Therefore, the resonant dip wavelength λ D of the double-pass MZI can be described as, λ D = 4m e f f L/(2k + 1)
(4)
It is expected that the evanescent field will be coupled out from the cladding modes as the light propagates in the fiber cladding. Consequently, the effective RI of the cladding mode will change with the variation of the surrounding RI between LPG1 and LPG2, while the effective RI of the core mode stays almost constant because the core mode has a relativity small mode diameter. Therefore, the m e f f will decrease with the increasing of the surrounding RI. From Eq. (4), the shifts of the resonant dip wavelength λ D has the following form, δλ D = 4δm e f f l/(2k + 1)
(5)
where l is the interaction length of the sensor. We can measure the RI of an unknown sample or determine the concentration of a known solution sample by measuring the δλ. And it can be seen that without increasing the interaction length, the sensitivity of the double-pass MZI will improve by 2 times than that of the single-pass MZI theoretically. In addition, during the RI detecting process,
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 11, JUNE 1, 2014
Fig. 3. Interference fringes variations of the double-pass MZI with different surrounding RI.
Fig. 5. atures.
Fig. 4. Wavelength shift of the double-pass MZI as a function of the surrounding RI variation. The inset shows that the Bragg wavelength of the FBG has no shift.
Fig. 6. Wavelength shift of the proposed sensor as a function of the temperature.
the RI variation is only occurred between the two LPGs. So the FBG’s reflected peak can be used as temperature measurement and cross-sensitivity compensation.
of temperature on the proposed RI sensor was also studied. The total MZI part was placed into a temperature controlled container. The temperature of the controlled container was set to increase from 20 to 100 °C with a step of 10 °C. As shown in Fig. 5, the resonant dip wavelengths of the MZI and FBG shows red shifts as the temperature increased. Fig. 6 shows the linear relationship of the proposed sensor against temperature. The temperature sensitivities of 30.3 pm/°C and 9.7 pm/°C for the double-pass MZI and FBG were obtained respectively. Based on these characteristics, the RI and temperature can be measured simultaneously by using the following matrix equation, 1 k M Z I n −k F BGn T = n k F BGT k M Z I n − k F BGn k M Z I T −k M Z I T k F BGT λ F BG × (6) λ M Z I
III. R ESULTS AND D ISCUSSIONS Fig. 3 shows the resonant wavelength of the interference patterns varied with the increasing of the surrounding RI. The experiment was carried out at room temperature (20 °C). As shown in Fig. 3, the interference resonant dip wavelength of the in-line double-pass MZI has a blue shift as the surrounding RI varied from 1.33 to 1.43, which is satisfied well with the theoretical expectation. Fig. 4 shows the wavelength shift of the double-pass MZI as a function of the surrounding RI variation. It can be seen that the RI sensitivity of the doublepass MZI is −21.07 nm/RIU, which is 2 times higher than that of single-pass MZI. In addition, the Bragg wavelength of FBG almost remains unchanged with the variation of surrounding RI as shown in the inset of Fig. 4. The RI experimental measurement was performed in a temperature-controlled environment and the temperature variation was less than 0.1 °C. But in practical applications, the surrounding temperature is not invariable. The influence
Interference patterns of the proposed sensor under different temper-
In Eq. (6), n is the variation of the surrounding RI, T is the variation of the temperature, λ F BG and λ M Z I are the wavelength shifts corresponding to the FBG and double-pass MZI, respectively. k F BGn and k F BGT are the RI and temperature coefficients for the FBG, respectively.
LU et al.: REFRACTIVE INDEX AND TEMPERATURE SENSOR
k M Z I n and k M Z I T are the RI and temperature coefficients for the double-pass MZI, respectively. The RI and temperature coefficients of the FBG and the double pass-double MZI are obtained by linear fit of the measured data shown in Fig. 4 and Fig. 6, respectively. Therefore, Eq. (6) can be rewritten as follows, 1 −21.0714 0 λ F BG T (7) = λ M Z I n −0.2044 −0.0303 0.0097 Based on the above matrix, the RI and temperature variations can be calculated by measuring the resonant wavelength shifts of the double-pass in-line MZI and the FBG. Moreover, the resolution of the proposed sensor is calculated as 9.49 × 10−4 at the limit resolution of the OSA of 0.02 nm. IV. C ONCLUSION In conclusion, a compact fiber RI sensor based on double-pass in-line MZI has been reported. Benefited from using double-pass in-line MZI structure with an FBG and fiber end reflection, the RI changes and temperature variations can be measured simultaneously. The obtained RI sensitivity of 21.07 nm/RIU is 2 times higher than that of single-pass MZI. The simple fabrication process shows that the proposed sensor has a great potential for many sensing applications. R EFERENCES [1] S. K. Abi Kaed Bey, T. Sun, and K. T. Grattan, “Optimization of a long-period grating-based Mach–Zehnder interferometer for temperature measurement,” Opt. Commun., vol. 272, no. 1, pp. 15–21, Apr. 2007. [2] H. Y. Choi, M. J. Kim, and B. H. Lee, “All-fiber Mach–Zehnder type interferometers formed in photonic crystal fiber,” Opt. Express, vol. 15, no. 9, pp. 5711–5720, Apr. 2007. [3] L. Li, L. Xia, Z. Xie, and D. Liu, “All-fiber Mach–Zehnder interferometers for sensing applications,” Opt. Express, vol. 20, no. 10, pp. 11109–11120, May 2012. [4] P. Lu, L. Men, K. Sooley, and Q. Chen, “Tapered fiber Mach–Zehnder interferometer for simultaneous measurement of refractive index and temperature,” Appl. Phys. Lett., vol. 94, no. 13, pp. 131110-1–131110-3, Mar. 2009.
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[5] Y. Wang, M. Yang, D. N. Wang, S. Liu, and P. Lu, “Fiber in-line Mach–Zehnder interferometer fabricated by femtosecond laser micromachining for refractive index measurement with high sensitivity,” J. Opt. Soc. Amer. B, vol. 27, no. 3, pp. 370–374, Mar. 2010. [6] L. Jiang, J. Yang, S. Wang, B. Li, and M. Wang, “Fiber Mach–Zehnder interferometer based on microcavities for high-temperature sensing with high sensitivity,” Opt. Lett., vol. 36, no. 19, pp. 3753–3755, Oct. 2011. [7] C. Y. Shen et al., “Polarization-dependent curvature sensor based on an in-fiber Mach–Zehnder interferometer with a difference arithmetic demodulation method,” Opt. Express, vol. 20, no. 14, pp. 15406–15417, Jul. 2012. [8] Y. Zhou et al., “Simultaneous measurement of curvature and temperature based on PCF-based interferometer and fiber Bragg grating,” Opt. Commun., vol. 284, no. 24, pp. 5669–5672, Dec. 2011. [9] C. Zhong et al., “Temperature-insensitive optical fiber two-dimensional micrometric displacement sensor based on an-inline Mach–Zehnder interferometer,” J. Opt. Soc. Amer. B, vol. 29, no. 29, pp. 1136–1140, May 2012. [10] B. Dong, J. Z. Hao, C. Y. Liaw, B. Lin, and S. C. Tjin, “Simultaneous strain and temperature measurement using a compact photonic crystal fiber inter-modal interferometer and a fiber Bragg grating,” Appl. Opt., vol. 49, no. 32, pp. 6232–6235, Nov. 2010. [11] M. Deng, C. P. Tang, T. Zhu, and Y. J. Rao, “Highly sensitive bend sensor based on Mach–Zehnder interferometer using photonic crystal fiber,” Opt. Commun., vol. 284, no. 12, pp. 2849–2853, Jun. 2011. [12] Z. B. Tian and S. H. Yam, “In-line abrupt taper optical fiber Mach–Zehnder interferometric strain sensor,” IEEE Photon. Technol. Lett., vol. 21, no. 3, pp. 161–163, Feb. 1, 2009. [13] Y. E. Fan, T. Zhu, L. L. Shi, and Y. J. Rao, “Highly sensitive refractive index sensor based on two cascaded special long-period fiber gratings with rotary refractive index modulation,” Appl. Opt., vol. 50, no. 23, pp. 4604–4610, Aug. 2011. [14] D. Wu et al., “Refractive index sensing based on Mach–Zehnder interferometer formed by three cascaded single-mode fiber tapers,” Appl. Opt., vol. 50, no. 11, pp. 1548–1553, Apr. 2011. [15] Y. Li, L. Chen, E. Harris, and X. Y. Bao, “Double-pass in-line fiber taper Mach–Zehnder interferometer sensor,” IEEE Photon. Technol. Lett., vol. 22, no. 23, pp. 1750–1752, Dec. 1, 2010. [16] R. Yang, Y. S. Yu, Y. Xue, C. Chen, Q. D. Chen, and H. B. Sun, “Single S-tapered fiber Mach–Zehnder interferometers,” Opt. Lett., vol. 36, no. 23, pp. 4482–4484, Dec. 2011. [17] J. Yang et al., “High sensitivity of taper-based Mach–Zehnder interferometer embedded in a thinned optical fiber for refractive index sensing,” Appl. Opt., vol. 50, no. 28, pp. 5503–5507, Oct. 2011. [18] C. R. Liao, Y. Wang, D. N. Wang, and M. W. Yang, “Fiber in-line Mach–Zehnder interferometer embedded in FBG for simultaneous refractive index and temperature measurement,” IEEE Photon. Technol. Lett., vol. 22, no. 22, pp. 1686–1688, Nov. 15, 2010.
