Over-coupling of the resonant band of interest has been found to have significant influence on the refractive index sensitivity of long period grating (LPG) for surrounding material having refractive index higher than the cladding material. In this paper we report, for the first time to the best of our knowledge, a quantitative analysis using the concept of radiation mode coupling that elaborates the influence of initial coupling strength on the response of LPG to high refractive index surrounding. The simulated results agree well with those obtained experimentally where it could be shown that that a suitably over-coupled resonant mode of LPG can have a sensitivity ~724 dB/RIU in the range of the ambient refractive index which is slightly higher than the cladding where the conventional LPGs have almost no sensitivity.
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Biswas et al. (2014) studied this question.