Carbon-nanotube-deposited long period fiber grating for continuous refractive index sensor applications

We present a carbon-nanotube-deposited long period fiber grating for refractive index sensing applications in liquid. Carbon nanotubes are deposited around the surface of a long period fiber grating to form the refractive index sensing element. The sensing mechanism relies mainly on the high refractive index properties of the carbon-nanotube thin film, which enhances the cladding mode of the long period fiber grating in order to have a significant interaction between the propagating light and the target medium. A sensitivity of 31 dB/RIU and 47 dB/RIU are obtained for the refractive index ranges of 1.33–1.38 and 1.38–1.42, respectively, which have not been demonstrated with normal long period fiber gratings as the sensing element. As the sensing mechanism is based on the change of the transmitted optical power, our proposed scheme can intrinsically solve the limitations of the free spectral range commonly seen in other reported schemes, and continuous and repeatable measurements can be obtained while only acquiring errors mainly from the power fluctuations from the light source. The fiber grating also does not require any further mechanical modification like etching or tapering, which allows the sensing element to have the advantage of mechanical strength for practical applications. The experimental results are consistent with the modeling of the sensing mechanism.