IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 20, OCTOBER 15, 2011
1511
Fiber Bragg Grating With Micro-Holes for Simultaneous and Independent Refractive Index and Temperature Sensing Minwei Yang, D. N. Wang, Member, IEEE, and C. R. Liao, Student Member, IEEE
Abstract—A compact optical fiber sensor based on a fiber Bragg grating with multiple micro-holes is effectively used for simultaneous and independent refractive index and temperature sensing. The micro-holes are drilled by use of femtosecond laser micromachining. The measurement is implemented by simple detection of the grating resonant wavelength shift and its intensity variation, respectively. The refractive index sensitivity of 29.50 dB/RIU (refractive index unit) is obtained in the refractive index range between 1.30 and 1.45, with a good linearity. The system operation is simple, convenient and reliable. Index Terms—Fiber Bragg gratings, laser ablation, optical fiber sensors, ultrafast optics.
I. INTRODUCTION
F
IBER in-line refractive index (RI) sensors are attractive for chemical, biological and environmental sensing because of their compactness, convenient operation, low cost and many other advantages provided by optical fibers. However, most of the liquids to be measured are temperature sensitive, which makes it difficult to determine their RI value accurately because of the temperature induced cross sensitivity [1]. It is therefore necessary to implement a simultaneous measurement of RI and temperature. Various types of optical fiber sensors have been developed for this purpose, including sampled fiber Bragg grating, hybrid structure of fiber Bragg grating (FBG) and long period fiber grating (LPFG), cascaded LPFGs, tilted FBG, and fiber interferometers, etc. [1]–[7]. In general, these sensors determine the two parameters by simultaneously monitoring the two characteristic wavelengths, and the demodulating systems are complex. The usually involved nonlinear RI response due to evanescent-field coupling [8] also leads to a rather complicated sensitivity matrix (e.g., different matrix coefficients exist in different RI ranges, or the matrix is expressed as the polynomials in proper order [2]), and significant difficulties in performing automatic and real-time measurement. Recently, by use of femtosecond (fs) laser micromachining, RI sensors based on a micro-hole drilled in single mode fiber
Manuscript received April 27, 2011; revised June 13, 2011; accepted July 26, 2011. Date of publication August 04, 2011; date of current version September 23, 2011. This work was supported by the Hong Kong SAR government through a GRF (general research fund) grant PolyU 5306/08E. The authors are with the Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China (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.2011.2163624
(SMF) have been demonstrated with the advantages of easy fabrication and the capability of performing temperature independent measurement [9], [10]. The RI sensitivity of the device would be further increased if more micro-holes could be drilled along the fiber length. In this letter, we present a simple structured fiber in-line sensor for simultaneous and independent RI and temperature sensing. The system is based on a number of asymmetrical micro-holes located at the FBG position, to help light coupling from the fiber core to the medium. The temperature and RI sensing can be carried out independently by detecting the FBG resonant wavelength shift and the intensity variation, respectively, which greatly simplify the detection system. II. MICRO-HOLE FABRICATION AND MORPHOLOGY In the experiment, fs laser pulses (with wavelength of 800 nm, pulse duration of 120 fs and repetition rate of 1 kHz) were focused onto the SMF cladding surface through a 20 objective lens (with numerical aperature of 0.50). The pulse energy was 10 , with irradiation time of 60 s. A type-II FBG with length of 3 mm was initially fabricated in SMF mounted on a three dimensional stage [11]. For type-II FBG, the high laser power may affect the fiber glass structure and create damage, and the periodical damage along the fiber forms the grating structure [12]. A broadband light source and an optical spectrum analyzer (OSA) with the resolution of 0.01 nm were used to observe the transmission spectrum during the fabrication. The focus point was away from the fiber core center along the y direction and on the cladding surface. 8 micro-holes were drilled, with the hole-spacing of 500 . The device here is not just a combination of micro-holes and FBG as: 1) most of the micro-holes are fabricated along the grating length, which supports a compact device; 2) the micro-holes are asymmetrical positioned a few away from the core center (being different from that reported in [9]), which makes one side of the fiber core partially exposed to the external surrounding medium without seriously damaging the periodical structure of the FBG. Fig. 1 shows the morphology of the FBG with micro-holes in different views. It can be seen from Fig. 1(d) that the main periodic structure of the FBG remains unaffected. The diameter of the micro-holes near the fiber core is estimated to be and the depth of the micro-hole is estimated to be . III. INDIVIDUAL RI AND TEMPERATURE MEASUREMENTS Two samples (S1 and S2) with 8 micro-holes were fabricated to reveal that different FBGs with similar micro-hole structure
1041-1135/$26.00 © 2011 IEEE
1512
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 20, OCTOBER 15, 2011
Fig. 3. Individual RI measurement. (a) FBG dip intensity versus RI (circles: S1; squares: S2). (b) Selected transmission spectra of S1 in different RI liquids.
Fig. 1. Morphology of the device. (a) Top view (focused at cladding surface). (b) Cross-section view. (c) Side view .(d) Top view (focused at core).
Fig. 4. Individual temperature measurement: FBG resonant wavelength of S1 versus temperature (circles: experimental results; dotted line: linear fitted).
Fig. 2. Transmission spectra of FBG and FBG with micro-holes. (a) S1. (b) S2.
exhibit similar transmission loss and hence similar RI sensitivity. Fig. 2(a) and Fig. 2(b) show the transmission spectra of the initial FBG and the FBG with micro-holes for S1 and S2. Their corresponding insertion losses are 6.94 and 7.30 dB, respectively. The initial spectra (when no micro-hole is drilled) have a pristine insertion loss of about 1 and 2.5 dB out of the resonant band, which are the insertion loss of the FBGs. In-situ RI measurement was carried by immersing the FBG with micro-holes into different RI liquids (Cargille LABS) in the range of 1.30–1.45 (25 at 589.3 nm). After each measurement, the sample was cleaned by isopropanol and left alone until its transmission spectrum matched that in air, before being immersed into another RI liquid. Fig. 3(a) shows the FBG response of the two samples to the RI change, where the intensity was measured at the FBG resonant dip, . For S1, the linear fitted RI sensitivity was 29.5 dB/RIU in the range between 1.300 and 1.450, with a linear regression value of . For S2, the linear fitted RI sensitivity was 33.7 dB/RIU in the range between 1.300 and 1.395, with . Both the samples exhibit similar RI sensitivity and linear regression values, which ensure a good linearity. Meanwhile, both the resonant dips do not shift with the increase of RI (S1 and S2 remain at 1570.85 and 1544.14 nm respectively), owing to the fact that only a small portion of the fiber core is actually exposed to external medium. This means that light energy is mainly bounded inside the fiber core and thus the effective RI of core mode hardly changes with the variation of external RI. Fig. 3(b) shows the transmission spectra of S1 in different RI liquids, and no shift of dip wavelength is observed. The irregular wavelength dependent loss
existed in Fig. 3(b) may result from: 1) the periodical asymmetrical micro-holes can couple energy from the core mode to the cladding mode, which results in small irregular transmission dips when phase matching condition is met; 2) each micro-hole can be seen as a Mach-Zehnder interferometer with small fringe visibility [13], and a number of cascaded holes can enhance such an interference effect. Individual temperature measurement was performed by placing S1 in an oven, exposed to air, and detecting the shift of when the temperature was increased from 20 to 90 . The sensitivity obtained is 10.7 , as shown in Fig. 4. The standard deviation (error bar) is calculated according to the analysis of [14], with a supposed signal to noise ratio of 50 dB. IV. SIMULTANEOUS RI AND TEMPERATURE MEASUREMENTS The simultaneous RI and temperature measurement was carried out, by immersing S1 in a RI liquid with the value of 1.340 at 25 (with the temperature coefficient of ), which is close to the RI of water. The temperature variation range was chosen as 20–80 , in which the RI liquid could stand without obvious evaporation. When the sensor was immersed into the RI liquid, the rise of temperature led to the reduction of the RI of the liquid and thus the resonant dip intensity would also decrease. The resonant dip intensity is measured at different temperatures while the measured RI is determined by and the linear fitted results shown in Fig. 3(a). The calculated RI is obtained according to its temperature coefficient. When comparing the two RI values in Fig. 5(a), a maximum RI error of 0.0034 is obtained, which may be due to the fact that the temperature coefficient of the liquid cannot remain the same over the entire temperature range. Based on the calculated RI
YANG et al.: FIBER BRAGG GRATING WITH MICRO-HOLES
1513
in an air hole, which is not suitable for a large RI range measurement as the RI liquid is difficult to be changed. In conclusion, an FBG with asymmetrical micro-hole structured sensor is fabricated by use of fs laser irradiation. By direct detection of the FBG resonant wavelength shift and its intensity variation, a simultaneous and independent temperature and RI sensing can be achieved. The two parameters can be determined by tracing only one characteristic wavelength, which can largely simplify the detection system. The RI sensitivity obtained is 29.5 dB/RIU in the RI region 1.300–1.450, with a good linearity, which can be further increased by increasing the number of micro-holes however, the price paid is the large insertion loss and reduced robustness. REFERENCES
Fig. 5. Simultaneous RI and temperature measurement of S1. (a) FBG resonant wavelength intensity versus RI (circles: measured RI, squares: calculated RI of liquid at different temperatures), inset: FBG resonant wavelength intensity variation with temperature in the RI liquid (circles: measured intensity I0; squares: calculated intensity I1). (b) FBG resonant wavelength versus temperature (circles: experiment; solid line: linear fitted).
value, the calculated intensity can be obtained according to the linear fitted results obtained in Fig. 3(a). The intensity error is defined as the difference between and , as shown in the inset of Fig. 5(a), which has the maximum value of 0.03 dB. The temperature induced wavelength shift of is shown in Fig. 5(b), where the sensitivity obtained is 10.6 , almost the same as that shown in Fig. 4. When immersed in RI liquid, a slight increase of 3-dB bandwidth can be observed due to the evanescent field coupling, which increases the standard deviation of resonant wavelength but has little effect on temperature induced resonant wavelength shift. V. DISCUSSION AND CONCLUSION In order to check the mechanical strength of the fiber with micro-holes, we measured a sample with 8 micro-holes (with total fiber length of 24 cm) and the failure strain is 0.2%. Suppose the glass fiber has a Young’s modulus of 70 GPa, this failure strain corresponds to a failure stress of 0.14 GPa (or failure tension of 1.72 N), which can be considered as a high load [15]. Another sample could be bent to a curvature radius of about 1.5 cm before being broken. In experiments, the samples used were not too fragile to be easily broken, although proper care needs to be taken. The temperature sensitivity of the sample here is not as high as that in [16], which has a narrow resonant bandwidth and thus a rather small detection limit. However, the device in [16] is essentially a single parameter (either temperature or RI) sensor. In addition, the RI analyte in [16] is enclosed
[1] X. Shu, B. A. L. Gwandu, Y. Liu, L. Zhang, and I. Bennion, “Sampled fiber Bragg grating for simultaneous refractive-index and temperature measurement,” Opt. Lett., vol. 26, pp. 774–776, Jun. 2001. [2] 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, pp. 178–182, Jan. 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] C. L. Zhao, X. Yang, M. S. Demokan, and W. Jin, “Simultaneous temperature and refractive index measurements using a 3 slanted multimode fiber Bragg grating,” IEEE J. Lightw. Technol., vol. 24, no. 2, pp. 879–883, Feb. 2006. [5] D. W. Kim, F. Shen, X. Chen, and A. Wang, “Simultaneous measurement of refractive index and temperature based on a reflectionmode long-period grating and an intrinsic Fabry-Perot interferometer sensor,” Opt. Lett., vol. 30, pp. 3000–3002, Nov. 2005. [6] 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, pp. 131110-1–131110-3, Apr. 2009. [7] K. Zhou, X. Chen, L. Zhang, and I. Bennion, “Implementation of optical chemsensors based on HF-etched fibre Bragg grating structures,” Meas. Sci. Technol., vol. 17, pp. 1140–1145, Apr. 2006. [8] T. Wei, Y. Han, Y. Li, H. Tsai, and H. Xiao, “Temperature-insensitive miniaturized fiber inline Fabry-Perot interferometer for highly sensitive refractive index measurement,” Opt. Express, vol. 16, pp. 5764–5769, Apr. 2008. [9] Y. Wang, D. N. Wang, M. Yang, W. Hong, and P. Lu, “Refractive index sensor based on a microhole in single-mode fiber created by the use of femtosecond laser micromachining,” Opt. Lett., vol. 34, pp. 3328–3330, Nov. 2009. [10] Y. Lai, K. Zhou, and I. Bennion, “Microchannels in conventional single-mode fibers,” Opt. Lett., vol. 31, pp. 2559–2561, Sep. 2006. [11] Y. Li, C. R. Liao, D. N. Wang, S. Tun, and K. T. V. Grattan, “Study of spectral and annealing properties of fiber Bragg gratings written in H2-free and H2-loaded fibers by use of femtosecond laser pulses,” Opt. Express, vol. 16, pp. 21239–21247, Dec. 2008. [12] C. W. Smelser, S. J. Mihailov, and D. Grobnic, “Formation of type I-IR and Type II-IR gratings with an ultrafast IR laser and a phase mask,” Opt. Express, vol. 13, pp. 5377–5386, Jul. 2005. [13] 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, pp. 370–374, Mar. 2010. [14] I. White and X. Fan, “On the performance of quantification of resonant refractive index sensors,” Opt. Express, vol. 16, no. 2, pp. 1020–1028, Jan. 2008. [15] B. H. Kim, Y. Park, T.-J. Ahn, D. Y. Kim, B. H. Lee, Y. Chung, U. C. Paek, and W.-T. Han, “Residual stress relaxation in the core of optical laser irradiation,” Opt. Lett., vol. 26, pp. 1657–1659, fiber by Nov. 2001. [16] D. K. C. Wu, B. T. Kuhlmey, and B. J. Eggleton, “Ultrasensitive photonic crystal fiber refractive index sensor,” Opt. Lett., vol. 34, no. 3, pp. 322–324, Feb. 2009.
