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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 1, JANUARY 1, 2007
Optical Refractive-Index Sensor Based on Dual Fiber-Bragg Gratings Interposed With a Multimode-Fiber Taper Li-Yang Shao, A. Ping Zhang, Member, IEEE, Wei-Sheng Liu, Hong-Yan Fu, and Sailing He, Senior Member, IEEE
Abstract—A new type of optical refractive-index (RI) sensor is proposed and experimentally demonstrated by using a structure of two single-mode fiber (SMF) Bragg gratings with a multimode fiber (MMF) taper in-between. The loss induced by a mismatch of waveguide structure between SMFs and MMFs is amplified by a tapering process, and is utilized for RI sensing through evanescent field. Experimental results show that the sensor possesses a tailorable sensitivity to the change of external RI and has a good linear response in the simultaneous measurement of external RI and temperature. Index Terms—Fiber Bragg grating (FBG), fiber-optic sensor, multimode fiber taper (MFT), refractive-index (RI) measurement.
I. INTRODUCTION
O
PTICAL refractive-index (RI) sensors or refractometers have been exploited extensively for a variety of industrial applications, such as chemical or biological sensors. Fiber grating-based RI sensors have become one of the most attractive schemes in recent years because of their compact size, high sensitivity, multiplexing, and remote-sensing capabilities [1]–[6]. Both long-period gratings (LPGs) and fiber Bragg gratings (FBGs) have been utilized for the realization of optical fiber RI sensors. LPG-based devices can be used directly to measure the change of external RI [1]–[3] because of the cladding modesbased coupling mechanism. However, an LPG is a broadband device working in transmission mode, and then the multiplexing of LPG sensors is not as flexible as FBG sensors. On the other side, an FBG is essentially not very sensitive to the change of external RI, though it has been proved as an excellent sensor in, e.g., strain and temperature measurement. Recently, some special FBG devices with specific postprocesses [4]–[6] were proposed to detect the external RI. Schroeder et al. proposed a side-polished FBG refractometer with RI resolutions of and for RI around 1.45 and 1.33 (the interrogation unit is with 1-pm resolution at 1550 nm) [4]. Pereira et al. utilized two
Manuscript received September 22, 2006; revised November 6, 2006. This work was supported in part by the Natural Science Foundation of China (Grant 60607011) and in part by the Hong Kong Polytechnic University-Zhejiang University under Joint Research Project G-U224. L.-Y. Shao, A. P. Zhang, W.-S. Liu, and H.-Y. Fu are with the Centre for Optical and Electromagnetic Research, Department of Optical Engineering, Zhejiang University, 310058 Hangzhou, China (e-mail:
[email protected]). S. He is with the Centre for Optical and Electromagnetic Research, Department of Optical Engineering, Zhejiang University, 310058 Hangzhou, China, and also with the Division of Electromagnetic Engineering, School of Electrical Engineering, Royal Institute of Technology, 10044 Stockholm, Sweden (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2006.889010
Fig. 1. Schematic diagram of the measurement scheme based on FBG RI sensors. Inset shows the designed sensor structure.
FBGs sensing system (one FBG is etched) with RI resolution around 1.33 [5]. A nonuniform thinned FBG sensor, of where only a part of grating is etched to the core for sensing RI, was demonstrated by Iadicicco et al. [6]. Its RI resolutions are and for RI around 1.45 and 1.33, respectively. Another flexible technique for making a fiber-based RI sensor is the fiber tapering process [2], [7]. In this letter, we combine the fiber gratings and tapering technique, and propose a new type of sensor structure, i.e., paired FBGs interposed with a multimode fiber taper (MFT). The schematic diagram of the measurement scheme and sensor structure is shown in Fig. 1. Since self-reference is important for the practical fiber-optic sensor [8], [9], two FBGs have been employed to realize a simultaneous measurement of the external RI and temperature. Compared with our previously reported fiber-taper seeded LPG pair [2], LPGs are replaced by FBGs and the single-mode fiber (SMF) taper is replaced by an MFT. As we will show, this sensor structure not only possesses the advantages of FBG sensors, but also gains a tailorable RI-sensing capability. II. OPERATION PRINCIPLE As shown in Fig. 1, two FBGs, denoted by FBG and FBG (fabricated in an SMF), with different Bragg wavelengths are utilized for signal demodulation. A symmetric taper made by multimode fiber (MMF) acts as the element sensitive to the change of external RI. When the light propagates into an MMF from an SMF, the light will be diverged due to the change of waveguide structure. If the MMF is centrally aligned with the SMF, only a few low-order modes of the MMF are excited. It is of interest if a tapering process is further utilized on this MMF section. Therefore, the pattern of light will be further altered due to a gradual change of fiber geometric parameters. The propagation of light becomes RI sensitive through the evanescent field around the MFT. Moreover, the light passed through the MFT will meet
1041-1135/$20.00 © 2006 IEEE
SHAO et al.: OPTICAL RI SENSOR BASED ON DUAL FBGs INTERPOSED WITH AN MMF TAPER
Fig. 2. (a) Measured transmission spectrum of FBG , (b) FBG and FBG , and (c) two FBGs with an MFT.
a mismatch of waveguide once again. Only a small part of the light will reflected by FBG . Therefore, one can use the intensity difference of two FBG signals to deduce the change of external RI. Meanwhile, the intensity variation induced by light source and propagation is automatically balanced because the light reflected by the FBG passes the same optical path as that reflected by FBG except the region of the MFT. Since the FBG and FBG are made by using the same photosensitive fiber, they possess the same temperature sensitivity. Thus, one can employ the average of Bragg wavelengths of the two FBGs to monitor the change of temperature around the sensor. Some evident advantages of the proposed sensor include: 1) the narrowband reflection signals from two FBGs make it easy for multiplexing, and operating in reflection-mode for single-end access; 2) the control of tapering degree allows tailoring the sensitivity of the sensor; 3) the sandwiched FBGs and MFT structure can simultaneously measure the change of external RI and temperature; 4) the intensity difference can automatically eliminate the effect of the fluctuation of light source, and the average of two Bragg wavelengths can precisely deduce the temperature at sensing region with a difference scheme. III. FABRICATION AND MEASUREMENT FBGs were fabricated by using a KrF excimer laser (TuiLaser Ltd. Germany) with a phase-mask technique. The UV beam is focused onto the fiber by using a cylindrical lens, and a metal slit of about 8 mm is put just before the phase mask to ensure only the central portion of the laser beam strikes the fiber. The fiber used for FBG fabrication is commercial SMF (Corning SMF28) after a hydrogen loading at 90 atm, 100 C for two days. We make the FBGs in the sequence of FBG and FBG . In order to achieve a high signal-to-noise ratio, the transmission dip of FBG is about 26.45 dB, which transmission spectrum is shown in Fig. 2(a). After fabricating FBG , we elongate the clamped fiber to shift the Bragg wavelength of FBG by about 2.3 nm (to longer wavelength). Then, FBG is fabricated at a position with a spacing of 2.4 cm to FBG in the same fiber. Fig. 2(b) is the transmission spectrum for light passing through
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Fig. 3. Measured spectral responses of the two FBGs interposed by an MFT with the taper waist diameter of 23.78 m.
both FBG and FBG (after releasing strain). The Bragg wavelengths of FBG and FBG under no strain are 1547.416 and 1549.716 nm, respectively. The MFT is made by using commercial MMF (Yangtze Optical Fiber and Cable Co.) with a step-index profile (core and cladding diameters are 50 and 125 m, respectively). A section of MMF with a length of 2.5 cm is spliced between two FBGs. Then, a fiber-coupler machine is used to tailor a fiber taper. The tapering processing is carried out by heating and stretching fiber with a computer control, which performs a good repeatability of taper fabrication. The stretching velocity is 0.1 mm/s, and the scanning velocity of flame is zero (for realizing a short fiber taper). Fabricated fiber tapers are characterized with a microscopy. For the fiber tapers with stretching lengths of 7.6, 8.4, and 8.8 mm, the diameters of taper waists are 31.39, 25.97, and 23.78 m, respectively. Fig. 2(c) gives a transmission spectrum of sensor with a taper-waist of 23.78 m. Three fabricated sensors with different taper-waist diameters were packaged with shallow aluminum boxes and tested by using an optical spectrum analyzer. A series of sucrose solutions is prepared as samples, whose RIs are in the range from 1.3333 to 1.4206 [2], [3]. Fig. 3 shows the measured reflection spectra of a sensor (with waist diameter of 23.78 m) as the external RI changes. One can observe that the reflection peak of FBG fluctuates up and down with the change of external RI, whereas the reflection peak of FBG has not significant change, as expected. Fig. 4 shows the responses of intensity difference (between FBG and FBG ) to the change of the external RI for three sensors with different taper waist diameters. The sensitivities of sensors are 2.969, 47.81, 91.31 (dB/R.I.U), respectively, which were observed a good repeatability in many times of measurement. One can see that the sensor becomes more sensitive when the taper-waist diameter decreases. The maximum sensitivity of sensor depends on the tapering degree, taper length, and the parameter of MMF. For a measurement with power resolution of 0.001 dB, the RI resolution is about (for RI range from 1.3333 to 1.4206). The temperature response of sensor was tested by putting the sensor into a bath of deioned water. The temperature is adjusted by a heater from 27 C to 71 C and monitored by a
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Fig. 4. Intensity differences (between FBG and FBG ) as a function of external RI for three sensors with different taper waist diameters.
Fig. 5. Wavelengths of FBG (triangle), FBG (rectangle), and the average value of two FBGs (circle) as functions of the temperature.
thermometer. The measured variations of intensity are less than 1%, which means the thermal expansion induced change of taper shape has negligible effect on the sensor. Fig. 5 shows the measured wavelength of FBG and FBG with the change of temperature. The average value of two Bragg wavelengths used to deduce the temperature of measurand is with a temperature sensitivity of 0.011 nm C.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 1, JANUARY 1, 2007
IV. DISCUSSION AND CONCLUSION From the experimental results, one can see the MFT is an effective element to introduce the dependence of light propagation on the external RI, particularly when it is interposed between two FBGs fabricated in SMF. It is notable that the selection of MMF for making the fiber-taper is extremely important to achieve good performance. Although the section of MMF is short (about 1 cm), the induced loss mechanism by a mismatch of waveguide structure will be amplified by the tapering process. Furthermore, the fiber taper introduces a considerable evanescent field, and therefore, makes the light propagating in this section become RI-dependent. In our experiments, we found that the performance of sensor is unacceptable if an SMF taper is interposed between two FBGs. In conclusion, we have proposed and experimentally demonstrated a new type of fiber-optic RI sensor based on fiber grating and tapering technology. In addition to its flexible multiplexing capability, the sensor can simultaneously measure temperature and external RI, and operates single-end access in reflectionmode, and therefore, could be used for practical biochemical sensing applications. REFERENCES [1] H. J. Patrick, A. D. Kersey, F. Bucholtz, K. J. Ewing, J. B. Judkins, and A. M. Vengsarkar, “Chemical sensor based on long-period fibre grating response to index of refraction,” in Proc. Lasers and Electro-Optics (CLEO ’97), 1997, vol. 11, pp. 420–421. [2] J. F. Ding, A. P. Zhang, L. Y. Shao, J. H. Yan, and S. L. He, “Fiber-taper seeded long-period grating pair as a highly sensitive refractive-index sensor,” IEEE Photon. Technol. Lett., vol. 17, no. 6, pp. 1247–1249, Jun. 2005. [3] A. P. Zhang, L. Y. Shao, J. F. Ding, and S. He, “Sandwiched long-period gratings for simultaneous measurement of refractive index and temperature,” IEEE Photon. Technol. Lett., vol. 17, no. 11, pp. 2397–2399, Nov. 2005. [4] K. Schroeder, W. Ecke, R. Mueller, R. Willsch, and A. Andreev, “A fiber Bragg grating refractometer,” Meas. Sci. Technol., vol. 12, pp. 757–764, 2001. [5] D. A. Pereira, O. Frazao, and J. L. Santos, “Fiber Bragg grating sensing system for simultaneous measurement of salinity and temperature,” Opt. Eng., vol. 43, no. 2, pp. 299–304, 2004. [6] A. Iadicicco, S. Campopiano, A. Cutolo, M. Giordano, and A. Cutolo, “Nonuniform thinned fiber Bragg gratings for simultaneous refractive index and temperature measurements,” IEEE Photon. Technol. Lett., vol. 17, no. 7, pp. 1495–1497, Jul. 2005. [7] J. Villatoro, D. Monzon-Hernandez, and D. Talavera, “High resolution refractive index sensing with cladded multimode tapered optical fibre,” Electron. Lett., vol. 40, pp. 106–107, 2004. [8] S. Abad, F. M. Araújo, L. A. Ferreira, J. L. Santos, and M. López-Amo, “Transparent network for hybrid multiplexing of fiber Bragg gratings and intensity-modulated fiber-optic sensors,” Appl. Opt., vol. 42, no. 25, pp. 5040–5045, 2003. [9] Y.-L. Lo, T.-Y. Yan, and C.-P. Kuo, “Self-referenced intensity-based fiber optic sensor system using fiber Bragg gratings,” Opt. Eng., vol. 41, no. 5, pp. 1087–1092, 2002.
