Figure 10 Measured peak gain of the proposed antenna REFERENCES 1. R.K. Mongia and P. Bhartia, Dielectric resonator antennas A review and general design relations for resonant frequency and bandwidth, Int J Microwave Millimeter-Wave Comput Aided Eng 4 (1994), 230 247. 2. A.A. Kishk, B. Ahn, and D. Kajfez, Broadband stacked dielectric resonator antennas, Electron Lett 25 (1989), 1232 1233. 3. A.G. Walsh, S.D. Young, and S.A. Long, An investigation of stacked and embedded cylindrical dielectric resonator antennas, IEEE Antennas Wireless Propag Lett 5 (2006), 130 133. 4. Y. Coulibaly, T.A. Denidni, and L. Talbi, Wideband impedance bandwidth hybrid dielectric resonator antenna for X-band applications, In: Proc IEEE Antennas Propag Soc, 2006, pp. 2429 2432. 5. A. Rashidian and D.M. Klymyshyn, On the two segmented and high aspect ratio rectangular dielectric resonator antennas for bandwidth enhancement and miniaturization, IEEE Trans Antennas Propag 57 (2009), 2775 2780. 6. Q. Rao, T.A. Denidni, and A.R. Sebak, A new dual-frequency hybrid resonator antenna, IEEE Antennas Wireless Propag Lett 4 (2005), 308 311. 7. Q. Rao, T.A. Denidni, A.R. Sebak, and R.H. Johnston, Compact independent dual-band hybrid resonator antenna with multifunctional beams, IEEE Antennas Wireless Propag Lett 5 (2006), 239 242. 8. K.P. Esselle and T.S. Bird, A hybrid-resonator antenna: Experimental results, IEEE Trans Antennas Propag 53 (2005), 870 871. 9. J. Janapsatya, K.P. Esselle, and T.S. Bird, Compact wideband dielectric-resonator on patch antenna, Electron Lett 42 (2006), 1071 1072. 10. F. Yang, X. Zhang, X. Ye, and Y. Rahmat-Samii, Wide-band E- shaped patch antennas for wireless communications, IEEE Trans Antennas Propag 49 (2001), 1094 1100. 11. A. Petosa, Dielectric resonator, In: Antennas handbook, Artech House Inc., Norwood, MA, 2007, pp. 51 55. The measured peak gains of the proposed antenna across the operating frequency band are depicted in Figure 10, showing variations of about 3.47 7.13 dbi. Our numerical and experimental results indicate that the proposed DRoP antenna with the E-shaped patch can provide a larger impedance bandwidth than the referential antenna in Ref. 9. Meanwhile, only two parallel slots are inserted in the planar patch to realize this bandwidth enhancement. The volume of the proposed antenna is the same as the referential one, without increasing structure dimensions. 4. CONCLUSIONS In this article, a compact DRoP antenna with bandwidth enhancement is presented. Compared with the original compact DRoP antenna, larger impedance bandwidth is achieved by adopting an E-shaped patch. The wideband mechanism is explored by investigating the behavior of the currents on the patch and the electric field distributed in the near field. In addition, a parametric investigation is carried out to optimize the proposed design. To show the validity of the design concept, a prototype is fabricated and tested. The proposed antenna can provide a bandwidth of 58% (4.38 7.96 GHz) without increasing the antenna volume. Such compact wideband DRoP antenna may be useful for broadband wireless devices. ACKNOWLEDGMENTS This work is supported in part by National Basic Research Program of China 2010CB327400 and by the Tsinghua-QUALCOMM Associated Research Plan. VC 2011 Wiley Periodicals, Inc. FABRICATION AND SENSING CHARACTERISTICS OF THE CHEMICAL COMPOSITION GRATING SENSOR AT HIGH TEMPERATURES Guoyu Li, 1 Mingsheng Liu, 1 Yan Li, 1 and Bai-ou Guan 2 1 Institute of Information Engineering, Handan College, Handan 056005, China; Corresponding author: guoyu_li@yahoo.cn 2 Institute of Photonics Technology, Jinan University, Guangzhou 510632, China Received 7 April 2011 ABSTRACT: The fabrication of the chemical composition gratings at high temperatures has been described. In the process of chemical composition, gratings formation, annealing, and development are necessary steps. Furthermore, the sensing characteristics of chemical composition grating sensor have been studied. Experimental results show that the temperature response is nonlinear over a temperature range from room temperature to 900 C, strain response of chemical composition grating sensor is linear at each temperature, and the strain sensitivity of the chemical composition grating sensor is about 1.10 pm le 1 over a temperature range from room temperature to 900 C. So, the chemical composition grating sensors will be an attractive solution for sensing application at high-temperature environments. VC 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:71 75, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26454 Key words: fiber Bragg gratings; chemical composition grating sensor; strain response 1. INTRODUCTION Fiber Bragg gratings (FBGs) are one of the most significant developments in the field of optical engineering over the last decades. They are formed by modulating the refractive-index of the core of an optical fiber. FBGs have been widely used to monitor temperatures and strain in engineering structure. But traditional FBGs have limited to work at high temperatures, typically in the range of room temperature to 500 C. Chemical composition gratings (CCGs) are the FBGs that the refractiveindex modulation is caused by a spatially varying concentration of dopants along the fiber core [1 3]. As compared with the conventional FBGs, the CCGs are used as fiber grating sensors at high temperatures that exceed 1000 C. So the CCG sensors DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 1 January 2012 71
Figure 1 Experimental setup for CCG developing and sensing Figure 4 The reflection spectrum of the initial FBG decay and CCG development at 1000 C. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] Figure 2 The reflection spectrum of initial FBG at room temperature are becoming a promising technology for a wide range of optical sensing techniques, applications may range from health monitoring of components in power plants, engines, or aircraft to temperature or pressure monitoring of down-hole oil wells [4, 5]. In this article, the annealing and development of CCGs and the corresponding experimental results are presented during the CCGs fabrication. Furthermore, the temperature and strain response of CCG sensor over a temperature range from room temperature to 900 C have been studied. Finally, the experimental results are also discussed. 2. EXPERIMENTAL SETUP The experimental setup is shown schematically in Figure 1. The fiber containing initial FBG or CCG was passed through Figure 5 The reflection spectrum of the CCG during thermal stability process at 1000 C. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] a tube oven, with both sides bonded onto two translation stages with epoxy glue. A thermocouple was placed near the initial FBG or CCG for the measurement of the temperature. Figure 3 The reflection spectrum of the initial FBG during annealing at 700 C. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] Figure 6 The reflection spectrum of CCG naturally cooled to room temperature 72 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 1 January 2012 DOI 10.1002/mop
Bragg wavelength was monitored by a multiline wavemeter (EXFO WA-7600). Figure 7 The temperature response of CCG over a temperature range from room temperature to 1173 K. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] The temperature of the oven was controlled by a controller. The light from a broad band source was incident on the initial FBG or CCG via a 3 db coupler, and the reflected 3. CCG DEVELOPING AND FABRICATION The initial FBG and CCG used for this study were fabricated in our laboratory. An erbium-doped fiber had a core diameter of 8.5 lm and a refractive-index step of Dn 4.8 10 3. The fiber was hydrogen loaded at 100 atm and 70 C for several days before UV exposure. The periodic UV exposure was performed with an ArF laser at a 193 nm wavelength in a two-beam interferometric setup. The initial FBG was 1.5-cm long with a Gaussian-apodized profile at a Bragg wavelength of 1545.13 nm as shown in Figure 2. The fabrication of the CCG was divided into two stages: annealing and development. Annealing was carried out at a lower temperature (500 900 C). The initial FBG was placed into a tube oven and kept unstrained. And the temperature was increased up to 700 C for initial FBG annealing before CCG development. The temperature was kept constant, and the duration was about half an hour. The reflection spectrum during the annealing process is shown in the Figure 3. The solid peak represents the initial FBG annealing, when the temperature has just reached 700 C, and the dashed peak shows the initial FBG Figure 8 The strain response of CCG over a range of temperatures:(a) 24 C, (b) 100 C, (c) 200 C, (d) 300 C, (e) 400 C, (f) 500 C, (g) 600 C, (h) 700 C, (i) 800 C, (j) 900 C. The strain response are (a) 1.14 6 0.004 pm le 1 ; (b) 1.13 6 0.004 pm le 1 ; (c) 1.11 6 0.004 pm le 1 ; (d) 1.10 6 0.003 pm le 1 ; (e) 1.10 6 0.004 pm le 1 ; (f) 1.09 6 0.005 pm le 1 ; (g) 1.09 6 0.004 pm le 1 ; (h) 1.08 6 0.004 pm le 1 ; (i) 1.16 6 0.008 pm le 1 ; (j) 1.20 6 0.011 pm le 1. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 1 January 2012 73
Figure 8 Continued annealing after half an hour at 700 C. From Figure 3, it can be seen that the thermally induced shift of the grating wavelength tends to the short wavelength a little, when the temperature keeps constant for half an hour. In the process of CCG formation, the annealing procedure is very crucial. Otherwise in some cases, the CCG is not observed without annealing process, because the refractive-index modulation of the CCG is highly depend on the annealing time and annealing temperature before development. The optimum annealing temperature is near 600 700 C [6]. After the annealing process, the temperature was then increased up to 1000 C, the initial FBG then decayed 74 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 1 January 2012 DOI 10.1002/mop
quickly, and the new grating (CCG) was developing quickly. Figure 4 shows the initial FBG decay and the CCG developing at a certain time. The dashed peak represents the initial FBG decay, and dash dotted peak shows the CCG developing. The peak wavelength difference of initial FBG and CCG is 0.4 nm. To increase the thermal stability of the CCG, the temperature was kept constant at 1000 C for 30 min. The spectrum of CCG at the saturated state and after 30 min decay is shown in Figure 5. The dashed peak represents the CCG at the saturated state ( 13 db), and the dashed peak shows the CCG after 30 min decay. It is clear that the thermally induced shift of CCG wavelength tends to long wavelength after 30 min thermal treatment at 1000 C. The traditional, Type I grating is limited to work at high temperatures, so the increasing in temperature results in the decay of the Type I grating, which is consequently erased. Furthermore, the thermal treatment leads to a periodic redistribution of dopants in the fiber core by chemical reactions, and the modulation is created, and the diffusion process has reached equilibrium. Because concentration of the dopants affects the refractive-index, the final result will be a periodic refractive-index structure, so the second grating (CCG) grows quickly [5]. Finally, the CCG was naturally cooled to room temperature. The reflection spectrum of CCG at room temperature is shown in Figure 6 with peak wavelength of 1545 nm. As compared to the initial FBG in Figure 2, the reflectivity of the CCG decreases owing to the thermal decay. 4. TEMPERATURE AND STRAIN RESPONSE OF CCG SENSOR The CCGs can be used as temperature and strain sensors in high-temperature environments. We first measured the temperature response of the CCG sensor over a temperature range from room temperature to 900 C, and the wavelength of the CCG was monitored with the multiwavemeter when the CCG sensor was heated. In the temperature measurement, the CCG sensor was kept unstrained. We then investigated the strain response of the CCG sensor. To observe the effect of high temperature on the strain response of the CCG sensor, strain measurements were conducted in two steps. First, the temperature in the oven was kept constant for several hours, and the average wavelength of the CCG was recorded by the multiline wavemeter. Second, strain was loaded in steps of 100 le to a maximum of 1000 le at constant temperature. The strain response of the CCG sensor was first measured at room temperature. The strain was loaded in steps of 100 le to a maximum of 1000 le by adjusting the translation stage, while the wavelength of the CCG was monitored with the multiline wavemeter. The process was repeated over a temperature range from 100 to 900 C in steps of 100 C. At each temperature, the strain response of the CCG sensor with strain loading was measured. 4.1. Temperature Response of CCG Sensor The temperature response of a CCG sensor with a peak wavelength of 1545.13 nm is shown in Figure 7. The squares represent the peak wavelength data measured by multiline wavemeter at different temperatures, and the solid line is a third-order polynomial curve of CCG peak wavelength versus temperature. The polynomial is kðtþ ¼1541:904 þ 9:448 10 3 T þ 3:308 10 6 T 2 þ 4:6 10 10 T 3 (1) where k is in nanometers and T is in kelvins. The polynomial for the temperature coefficient is K T ðtþ ¼ 1 k B dkðtþ dt ¼ 6:115 10 6 þ 4:282 10 9 T þ 8:932 10 13 T 2 It is clear that the temperature response of CCG sensor is nonlinear over the whole temperature range from room temperature to 1173 K. The wavelength of grating is highly dependent on the thermal optical coefficient and the thermal expansion coefficient [7]. Over a large temperature range, the changes in the thermal optical coefficient and the thermal expansion coefficient as a function of temperature are not ignored, and hence the temperature sensitivity coefficient increases with increasing temperature. So the temperature response curve is not linear over the whole temperature range. 4.2. Strain Response of CCG Sensor The strain response of the CCG sensor at a range of temperatures from 24 to 900 C is plotted in Figures 8(a) 8(j). The solid squares represent the loading strain data, and the lines are linear fit curves of the loading strain data at different temperatures. 5. CONCLUSIONS The fabrication of the CCGs at high temperatures has been described. Annealing and development are necessary steps during the process of CCGs formation. Furthermore, the sensing characteristics of CCG sensor have been studied, Experimental results show that the temperature response is nonlinear over a temperature range from room temperature to 900 C, strain response of CCG sensor is linear at each temperature and the strain sensitivity of the CCGs is about 1.10 pm le 1 over a temperature range from room temperature to 900 C. So, the CCG sensors will be an attractive solution for sensing application at high-temperature environments. ACKNOWLEDGMENTS This work was supported by the project of National Natural Science Foundation of China (No. 60807033) and the Doctoral Research Fund of Handan College (No. 2010001). REFERENCES 1. M. Fokine, Thermal stability of chemical composition gratings in fluorine germanium-doped silica fibers, Opt Lett 27 (2002), 1016 1018. 2. M. Fokine, Formation of thermally stable chemical composition gratings in optical fibers, J Opt Soc Am B 19 (2002), 1759 1765. 3. B. Zhang and M. Kahrizi, High-temperature resistance fiber Bragg grating temperature sensor fabrication, IEEE Sens J 7 (2007), 586 591. 4. M. Fokine, Thermal stability of oxygen-modulated chemical-composition gratings in stand telecommunication fiber, Opt Lett 29 (2004), 1185 1187. 5. M. Fokine, Underlying mechanisms, application, and limitations of chemical composition gratings in silica based fibers, J Non-Cryst Solids 349 (2004), 98 104. 6. M. Fokine, Growth dynamics of chemical composition gratings in fluorine-doped silica optical fibers, Opt Lett 27 (2002), 1974 1976. 7. R.R.J. Maier, W.N. MacPherson, and J.S. Barton, Temperature dependence of the stress response of fiber Bragg gratings, Meas Sci Technol 15 (2004), 1601 1606. VC 2011 Wiley Periodicals, Inc. (2) DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 1 January 2012 75