Enhancing refractive index sensitivity using micro-tapered long-period fiber grating inscribed in biconical tapered fiber

A novel micro-tapered long-period fiber grating refractive index sensor is demonstrated in this study. Its transmission characteristics are investigated numerically and experimentally. The sensor is fabricated by inscribing periodical micro-tapers in a biconical single mode optical fiber (SMF-28) using a low cost electric arc heating technique. The periodical tapering in SMF enables the fundamental core mode to be coupled into the cladding modes resulting a resonant wavelength in the transmission spectrum. The resonant wavelength shifts linearly with the variation of surrounding refractive index. The refractive index sensitivity of 8188 nm/RIU has been experimentally achieved which is much higher than the previously reported refractive index sensitivity values for the sensors based on long period fiber grating. The effects of varying the number of periodical tapers, amount of grating period and waist diameter on the sensitivity have been analysed both numerically and experimentally. Our presented sensor features a low cost, a simple structure and a high sensitivity. The results presented in this study would be useful in both chemical and bio-sensing applications.