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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 13, JULY 1, 2012
Long-Period Grating Inscribed on Concatenated Double-Clad and Single-Clad Fiber for Simultaneous Measurement of Temperature and Refractive Index Qun Han, Xinwei Lan, Jie Huang, Amardeep Kaur, Tao Wei, Zhan Gao, and Hai Xiao, Senior Member, IEEE
Abstract— In this letter, a kind of long-period fiber grating (LPFG) that is capable of simultaneous measurement of temperature and surrounding refractive index (RI) is fabricated and demonstrated. The compact LPFG is written on a piece of spliced standard single-mode fiber (SMF) and double-clad fiber (DCF) with the CO2 laser point-by-point irradiation technique. The LPFG section in the DCF is solely sensitive to temperature, while the section of the LPFG in the SMF is sensitive to both temperature and surrounding RI. After temperature and RI calibration, the LPFG has been used to measure the RI change of ethylene glycol with the change of temperature to demonstrate its capability for dual parameter simultaneous measurement. Index Terms— Chemical sensors, fiber gratings, fiber sensors.
I. I NTRODUCTION
L
ONG-PERIOD fiber gratings (LPFGs) have attracted considerate attention as optical sensors to measure various parameters such as temperature, strain, and refractive index (RI) [1], [2]. Due to the RI-sensitive capability, LPFGs can potentially be used as chemical sensors to measure the concentration of chemicals in a solution [3], [4]. However, because of the large inherent thermal and RI cross sensitivity, conventional LPFGs in general are not ideal chemical sensors for RI-encoded concentration measurement. A temperature compensation mechanism must be introduced for a precise measurement since the RI of a chemical solution also varies with temperature. So it is vital for such applications that the temperature be simultaneously measured with the RI.
Manuscript received February 29, 2012; revised March 27, 2012; accepted March 28, 2012. Date of publication April 25, 2012; date of current version May 25, 2012. This work was supported in part by the National Natural Science Foundation of China under Grant 61107035 and in part by the U.S. Department of Energy under Grant DE-FE0001127. Q. Han is with the Department of Electrical and Computer Engineering, Missouri University of Science and Technology, Rolla, MO 65409 USA, and also with the College of Precision Instrument and Opto-Electronics Engineering, and the Key Laboratory of Opto-Electronics Information Technology of the Ministry of Education, Tianjin University, Tianjin 300072, China (e-mail:
[email protected]). X. Lan, J. Huang, A. Kaur, T. Wei, Z. Gao, and H. Xiao are with the Department of Electrical and Computer Engineering, Missouri University of Science and Technology, Rolla, MO 65409 USA (e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected];
[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.2012.2196428
Inner cladding SMF
DCF
Buffer
Fig. 1.
Cladding SMF
Outer cladding
LPFG
Core
Schematic of the LPFG.
Various methods have been proposed in the literature for the simultaneous measurement of temperature and RI using LPFGs, such as cascaded LPFGs with different periods [5], hybrid LPFG and fiber Bragg grating (FBG) [6], hybrid LPFG and intrinsic/extrinsic Fabry-Perot interferometer [7], [8]. In this letter, a LPFG is point-by-point inscribed into a piece of spliced standard single-mode fiber (SMF) and signalmode double-clad fiber(DCF) with a CO2 laser. The section of the LPFG in the SMF is both sensitive to temperature and surrounding RI, while the LPFG section in the DCF is solely sensitive to temperature. After proper calibration, such LPFGs can potentially be used as real-time chemsensors for the precise measurement of chemical concentration even the temperature of the solution is varying. The fabrication is a onetime written process and no post-processing (such as etching, etc.) is needed. Compared with other previously proposed schemes, this method features easy fabrication, compactness, and high-temperature survivability. II. E XPERIMENTS AND D ISCUSSION A schematic diagram of the proposed LPFG is shown in Fig. 1. A 10 cm-long DCF (Nufern SM-9/105/125-20A) was first fusion-spliced between two pieces of SMF (Corning SMF-28). The buffer of part of the DCF and part of the SMF beside one splicing point were stripped and placed on a motorized linear translation stage with a resolution of 100 nm. Then a LPFG was inscribed point-by-point by a CO2 laser starting from the SMF side at ∼3 cm from the splicing point. During the fabrication, the power of the CO2 laser was ∼7.8 W. The laser beam was focused by a cylindrical ZnSe lens with a focal length of 50 mm, creating a narrow line of ∼2 mm in length and ∼100 μm in width perpendicular onto the fiber [9]. A broadband light source (Agilent
1041–1135/$31.00 © 2012 IEEE
HAN et al.: LONG-PERIOD GRATING INSCRIBED ON CONCATENATED DOUBLE-CLAD AND SINGLE-CLAD FIBER
Fig. 2.
Transmission loss spectrum of the fabricated LPFG.
83437A) and an optical spectrum analyzer (Ando AQ6319) were used to monitor the transmission spectrum. The grating period is 370±0.1 μm. The total length of the grating is about 8 cm with ∼3 cm in the SMF and ∼5 cm in the DCF. These lengths were determined through experiments to ensure deep resonance peaks in both kinds of fibers. Figure 2 shows the transmission loss spectrum of the fabricated LPFG in air at a room temperature of 25 °C. From Fig. 2 it can be seen that there are four resonance peaks in the whole spectrum range. Among them, the first and third peaks belong to the grating section in the DCF and the second and fourth belong to the grating section in the SMF, identified at the time of fabrication. The left two peaks and the right two peaks result from the coupling of the fundamental core-mode LP01 to the cladding modes LP04 and LP05 , respectively. The order of these modes was determined via numerical simulation with the fibers’ parameters as in [9]. The small ripples on the spectrum were caused by the weak cross-coupling of the two sections of the LPFG. The total insertion loss of the grating was about 1 dB with 0.5 dB from the two splicing points. and As shown in Fig. 2, the extinction ratios of the LPDCF 05 the LPSMF peaks are ∼24 dB and ∼29 dB, respectively. Here 05 the superscripts indicate the fiber type that the corresponding resonance peak belongs to, and the subscripts indicate the order of the corresponding cladding mode. Due to the higher sensitivity than the left two peaks, these two resonance peaks were used in the following experiments. To characterize the grating’s response to temperature change, it was placed in a silica tube and then placed in a programmable furnace. Both ends of the tube were sealed with asbestos to isolate the influence of airflow and to ensure a uniform temperature environment during the measurement. One end of the grating was fixed and the other end was attached with a small load to keep the grating straight and under a constant stress. The furnace was heated up to 200 °C and kept there for more than 2 hours. Then the wavelengthshifts were recorded with a step of 20 °C as the temperature decreasing from 200 °C to 40 °C. At each step, the spectrum was recorded after 10 minutes after the temperature reached the target value to ensure that the temperature in the cavity of the furnace was stabilized. The change of the resonance wavelengths (λ) relative to the wavelengths at 25 °C of the SMF LPDCF 05 and LP05 modes as a function of temperature (T ) are
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Fig. 3.
Temperature response of the LPFG.
Fig. 4.
RI response of the LPFG.
shown in Fig. 3. Linear-fitting curves to the experiment data are also shown in the figure. The linear fitting equations are as following λSMF = −3.2524 + 0.1301 × T, λDCF = −2.8801 + 0.1152 × T. (R 2 )
(1) (2) LPSMF 05
of the fitting for The coefficients of determination and LPDCF are also shown in the figure. From the figure it can 05 be seen that both resonance peaks have a linear response to temperature. Response of the grating to external RI was characterized by using aqueous ethylene glycol solutions with different concentrations. Eleven liquid samples with a volume percentage from 0% to 100% with a variation step of 10% were used. At 25 °C the calculated RIs at 1.55 μm are 1.3254, 1.3330, 1.3404, 1.3476, 1.3547, 1.3616, 1.3683, 1.3750, 1.3814, 1.3878, and 1.3940, respectively [10]–[12]. During the experiment, the LPFG was hung into a graduated cylinder with the upper-end fixed and the lower-end attached with the same small load. The grating was first totally immersed into distilled water or ethylene glycol solution with known concentration. Then certain volume of pure ethylene glycol was slowly injected by a burette. A slim glass rod was used to slowly stir the solution for several minutes to ensure a uniform solution. Then it was kept static for about 10 minutes before the record of the wavelength shift. Figure 4 shows the wavelength shift of mode as a function of the RI (n). The LPDCF the LPSMF 05 05 mode exhibits no measurable sensitivity to external RI as this
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 13, JULY 1, 2012
shown in the figure. The maximum absolute error between the measured and predicted RI is ∼4.41×10−4, and the standard deviation is ∼2.75×10−4 in the measurement. In our experiments, three LPFGs of this kind have been fabricated and tested. Due to the small variations of the fabrication parameters such as the laser power and the splicing loss, there is a small difference in the transmission spectrum of the resulted LPFGs, such as the resonance wavelength and depth of the peaks. However, their response (λ) to temperature and RI are the almost the same. So for LPFGs made with the same kind of fibers the calibration established for one sensor can be readily applied to other similar ones. Fig. 5. Wavelength shift of the two modes and the calculated RI as a function of temperature.
mode is actually coupled into the inner cladding of the DCF and without interaction with the surrounding environment. A nonlinear fitting of the experimental results (from n = 1.3 to 1.4) is as follow λ = 1.368/ (n − 1.459) .
(3)
Numerical simulation result is also shown in the figure. In the simulation, the theoretical model and numerical method described in [13] were implemented in MATLAB. The LPFG in the SMF was modeled as a three layer cylindrical waveguide. The RI of the third layer equals that of one of the given solutions. The material dispersion of the fiber core and cladding were calculated using the Sellmeier equation [14], while the dispersion of the ethylene glycol solutions were calculated using the method described in [11]. Then the transmission spectra under different external RI were simulated and from which the data of the theoretical curve shown in Fig. 4 was obtained. From the figure it can be seen that the experimental result agrees fairly well with the simulation result. After the temperature and RI characterization, the LPFG was used to measure the RI change of pure ethylene glycol with temperature, demonstrating its capability for dual parameter measurement. In the experiment, the LPFG was placed in the middle of a thin steel tube with one end stuffed and fixed with a small rubber stopper, which also fixed one end of the fiber. The other end of the tube was slightly bent up to prevent the fluid from flowing out. The same small load was attached to the fiber at this end to keep the grating straight and under a constant stress. Then the tube was placed in the programmable furnace and pure ethylene glycol was slowly filled into the tube. The spectra of the grating were recorded with a temperature change step of 5 °C from 40-80 °C. The wavelength shift relative to the wavelengths of the two modes at 25 °C and the calculated RI are shown in Fig. 5. Due to shifts toward shorter being immersed in ethylene glycol λSMF 05 wavelength, the initial wavelength spacing of the LPDCF and 05 LPSMF modes shrunk from ∼68.8nm as shown in Fig. 2 to 05 ∼19.5 nm as shown in Fig. 5. The temperature value in the figure was calculated from the measured λDCF using Eq.(2). Then Eq.(1) and Eq. (3) were used to calculate the RI. The calculated RI and it’s theoretical value [11] are also
III. C ONCLUSION In conclusion, a kind of compact and easy fabrication LPFG capable of simultaneously measuring temperature and RI has been proposed and experimentally demonstrated. After proper calibration, the LPFG can potentially be used as a refractometer or a chemisensor for chemical concentration measurement with a temperature self-correction capability. R EFERENCES [1] B. H. Lee, Y. Liu, S. B. Lee, S. S. Choi, and J. N. Jang, “Displacements of the resonant peaks of a long-period fiber grating induced by a change of ambient refractive index,” Opt. Lett., vol. 22, no. 23, pp. 1769–1771, 1997. [2] M. J. Kim, Y. H. Kim, and B. H. Lee, “Simultaneous measurement of temperature and strain based on double cladding fiber interferometer assisted by fiber grating pair,” IEEE Photon. Technol. Lett., vol. 20, no. 15, pp. 1290–1292, Aug. 1, 2008. [3] L. Rindorf and O. Bang, “Highly sensitive refractometer with a photoniccrystal-fiber long-period grating,” Opt. Lett., vol. 33, no. 6, pp. 563–565, 2008. [4] L. Mosquera, D. Sáez-Rodriguez, J. L. Cruz, and M. V. Andrés, “Infiber Fabry-Perot refractometer assisted by a long-period grating,” Opt. Lett., vol. 35, no. 4, pp. 613–615, 2010. [5] B. A. L. Gwandu, et al., “Simultaneous refractive index and temperature measurement using cascaded long-period grating in double-cladding fibre,” Electron. Lett., vol. 38, no. 14, pp. 695–696, Jul. 2002. [6] X. Chen, K. Zhou, L. Zhang, and I. Bennion, “Simultaneous measurement of temperature and external refractive index by use of a hybrid grating in D fiber with enhanced sensitivity by HF etching,” Appl. Opt., vol. 44, no. 2, pp. 178–182, 2005. [7] C. Lee, W. Liu, Z. Weng, and F. Hu, “Hybrid AG-FFPI/RLPFG for simultaneously sensing refractive index and temperature,” IEEE Photon. Technol. Lett., vol. 23, no. 17, pp. 1231–1233, Sep. 1, 2011. [8] D. W. Kim, F. Shen, X. Chen, and A. Wang, “Simultaneous measurement of refractive index and temperature based on a reflection-mode longperiod grating and an intrinsic Fabry-Perot interferometer sensor,” Opt. Lett., vol. 30, no. 22, pp. 3000–3002, 2005. [9] X. Lan, Q. Han, T. Wei, J. Huang, and H. Xiao, “Turn-around-point longperiod fiber gratings fabricated by CO2 laser point-by-point irradiations,” IEEE Photon. Technol. Lett., vol. 23, no. 22, pp. 1664–1666, Nov. 15, 2011. [10] E. T. Fogg, A. N. Hixson, and A. R. Thompson, “Densities and refractive indexes for ethylene glycol-water solutions,” Anal. Chem., vol. 27, no. 10, pp. 1609–1611, 1955. [11] E.-K. Hassan, “The necessary requirements imposed on polar dielectric laser dye solvents,” Phys. B, Condensed Matter, vol. 279, nos. 3–4, pp. 295–301, 2000. [12] A. H. Harvey, J. S. Gallagher, and J. M. H. L. Sengers, “Revised formulation for the refractive index of water and steam as a function of wavelength, temperature and density,” J. Phys. Chem. Ref. Data, vol. 27, no. 4, pp. 761–774, 1998. [13] E. Anemogiannis, E. N. Glytsis, and T. K. Gaylord, “Transmission characteristics of long-period fiber gratings having arbitrary azimuthal/radial refractive index variations,” J. Lightw. Technol., vol. 21, no. 1, pp. 218– 227, Jan. 2003. [14] H. Murata, Handbook of Optical Fibers and Cables, 2nd ed. New York: CRC Press, 1996.
