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29 November 2011Cladding-mode obtained by core-offset structure and applied in fiber Bragg grating sensor
Comparing to core-modes of optical fibers, some cladding-modes are more sensitive to the surroundings which are very
valuable to sensing application; recently, a novel type of FBG sensor with core-offset structure attracts more and more
interests. Normally, the forward core-mode is not only reflected and coupled to the backward core mode by the Fiber
Bragg Grating in the step-type photosensitive single mode fiber, but also coupled to the backward cladding-modes and
the radiation modes, eventually they will leak or be absorbed by the high refraction index coating layer. These backward
cladding-modes can also be used for sensing analysis. In this paper, we propose and develop a core-offset structure to
obtain the backward core-mode and backward cladding-modes by using the wavelength shift of the backward core-mode
and the power of the backward cladding-modes in Fiber Bragg Grating sensor, and the power of the backward
cladding-modes are independent from temperature variation. We develop a mode coupling sensor model between the
forward core-mode and the backward cladding-modes, and demonstrate two coupling methods in the core-offset
structure experimentally. The sensor is fabricated and demonstrated for refractive index monitoring. Some specific works
are under investigation now, more analysis and fabrication will be done to improve this cladding-mode based sensor
design for applicable sensing technology.