We present a detailed investigation into the sensitivity of long-period fiber gratings (LPFGs) as a function of temperature, strain, and surrounding refractive index, with particular attention to the higher order cladding modes and the possibilities for ultrasensitive sensors. From a general theoretical analysis, we identify a general sensitivity factor which offers new physical insight into LPFG behavior and represents a useful design aid in conjunction with a set of measurand-specific sensitivity factors. Our analysis reveals the existence of turning points in the mode dispersion characteristics at which ultrasensitive operation may be obtained. In an extensive set of coordinated experiments, we verify the theoretical predictions with close agreement and provide demonstrations of the device behavior close to the turning points. Alternative sensor schemes for temperature, strain, and refractive index based,respectively, on measurement of the dual resonance characteristic of the modes and on the transmission characteristics close to the turning points, utilizing higher order modes of the LPFG, are presented. For two variables at least,we record the highest LPFG sensitivities yet reported.
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