Bragg gratings have been produced in germanosilicate optical fibers by exposing the core, through the side of the cladding, to a coherent UV two-beam interference pattern with a wavelength selected to lie in the oxygen-vacancy defect band of germania, near 244 nm. Fractional index perturbations of approximately 3 x 10(-5) have been written in a 4.4-mm length of the core with a 5-min exposure. The Bragg filters formed by this new technique had reflectivities of 50-55% and spectral widths, at half-maximum, of 42 GHz.

value, because the loss of the up-leading sensors will make it difficult to measure the reflectivity of the FBGs in the rear positions. However, by utilizing the SDM, more than one optical fiber can be connected to the interrogator. The configuration is demonstrated in Figure 1. An additional merit of this technique is that FBG sensors can be incorporated into this system and interrogated at the same time with a resolution of 1 pm. The intensity-type sensors and FBG sensors can be hybrid-interrogated if only the wavelengths of FBGs on one fiber are different. This merit would be useful when the two kinds of sensors are necessary in one measurement system. CONCLUSIONIn conclusion, a novel multiplexing method for intensity-type fiber-optic sensors based on a hybrid WDM/SDM approach is proposed and demonstrated. The sensor head is composed of two FBGs with different wavelengths and one intensity-type sensor. The loss induced by the measurand is interrogated by measuring the reflectivity of the two FBGs and by calculating the ratio. The experimental results show that the sensor in the up-leading position has no influence on the sensor in the rear position. It is anticipated that such an intensity-type of sensor could be used for a wide range of applications, in particular, as a hydrogen sensor, evanescent sensor, and chemical sensor. [5,6]. In this article, an alternative novel design of small microstripline-fed slot antenna with dual-band operations is proposed. The size of the proposed antenna can be greatly reduced by implanting grounded crossed strips in the slot of a conventional printed square slot antenna (PSSA). Although the resulting antenna size is not as small as those presented in Refs. ACKNOWLEDGMENTS This work was supported by the Program for New Century Excellent Talents in the University (NCET) of China and Chinese 863 Project (2008AA04Z406[5] and [6], the design procedure presented here is relatively simple and systematic. The grounded strips can excite two resonant modes having different electric current distributions in the metal region (consisting of both the ground plane and the strips) so as to form two disjointed resonant bands. One resonant band is lower than that of the fundamental resonant mode of the corresponding conventional PSSA, and the other is higher. The center-frequency ratio of the two resonant bands can be easily adjusted by changing the dimensions of the strips. A large range of dual-frequency ratios (1.8 -2.6) can easily be obtained. An antenna prototype has been successfully designed and fabricated for 2.45/5.25 GHz dual-band (2400 -2484 MHz and 5150 -5350 MHz) WLAN applications. Figure 1 shows the geometry of the proposed microstripline-fed PSSA, where the square slot with a side length of L ϭ 22 mm is etched from the conductor of a grounded square substrate with a side length of 40 mm, a thickness of h ϭ 0.7 mm, a dielectric constant of r ϭ 4.2, and a loss tangent of tan ␦ ϭ 0.02. Connected to the two upper inner corners of the ground are two 1-mm-wide conducting strips tha...

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