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8 September 2010 Understanding the mode coupling process in a strong long period grating
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Proceedings Volume 7653, Fourth European Workshop on Optical Fibre Sensors; 76530U (2010)
Event: (EWOFS'10) Fourth European Workshop on Optical Fibre Sensors, 2010, Porto, Portugal
A long period grating (LPG) is a longitudinal periodic optical structure that drives couplings from the fundamental core mode into phase-matched co-propagating cladding modes of an optical fiber and a series of attenuation dips are formed in the transmission spectrum [1]. LPGs have been applied as photonic sensors to detect external perturbations including temperature, strain, bending and surrounding refractive index, by monitoring the spectral shifts of the resonant dips [2]. LPGs are conventionally fabricated by UV-light exposure to induce periodic refractive-index variation of 10-5 ~ 10-4 in the fiber core. Such an LPG is regarded as weak perturbation to the fiber and the mode coupling process has been described by the wellknown coupled mode theory (CMT) [3]. In addition to the UV-inscription technique, stronger LPGs can also be formed by introducing refractive index/geometry modulation by use of CO2-laser irradiation, arc discharge, and periodic tapering [4-6]. Photonic crystal fibers (PCFs), which contain a two-dimensional array of air holes in their claddings, provide an extra-dimension for LPG-inscription through periodic deformation of the air-holes in the cladding [7]. However, the conventional CMT may not provide accurate description to these strong LPGs because of the significant modification of the mode fields and refractive indexes over the modulated regions. In this paper, the mode coupling process in a strong LPG inscribed in a PCF is quantitatively analyzed based on the coupled local-mode theory. The analysis offers a physical insight and a better understanding over the energy transfers in the LPGs. Based on the theory, a general phase-matching condition for LPG is presented, which accurately determines the resonant wavelengths λres.
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Long Jin, Wei Jin, and Jian Ju "Understanding the mode coupling process in a strong long period grating", Proc. SPIE 7653, Fourth European Workshop on Optical Fibre Sensors, 76530U (8 September 2010);

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