The cladding-modified long-period-grating (LPG) refractive-index sensors with a high-refractive-index overlay are optimized based on systematical studies on the structure-modified LPG. Dependence of the cladding-mode effective indexes on the parameters of the structure-modified LPG, such as the overlay refractive index (RI), the overlay thickness, the cladding-layer radius, and the ambient RI, as well as the order of cladding mode, are investigated in detail. An optimization procedure based on reducing the cladding radius, keeping the grating period as an adaptive parameter, and employing the HE<sub>13</sub> mode notch-wavelength shift is proposed for the LPG RI sensors. By using the proposed optimization method, it is shown that an ambient RI sensitivity as large as 5980 nm/RI can be achieved, representing a threefold sensitivity enhancement, as compared to the best result obtained from the reported structure, in which the high HE<sub>17</sub> mode resonate notch wavelength was employed in sensing.
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