Continuous refractive index sensing based on carbon-nanotube-deposited photonic crystal fibers

We present a carbon nanotubes (CNTs) deposited photonic crystal fiber (PCF) featuring a Mach-Zehnder interferometer configuration for refractive index (RI) sensing applications. The high RI of the CNTs deposited on the surface of the PCF not only enhances the interaction of the evanescent waves of the cladding modes with the ambient environment around the fiber, but also modifies the sensing scheme to that of intensity variations. Such a modification makes the sensor susceptible to power fluctuations from the optical source but allows the sensor to gain immunity to free spectral range limitations which is commonly found in PCF-based sensors. As such continuous and repeatable measurements can be obtained for the range of RIs being measured. The sensor registered a sensitivity of 19.4 dB/RIU within the RI range of 1.33 to 1.38 and a sensitivity of 24.2 dB/RIU within the RI range of 1.38 to 1.42. Since there is no mechanical modification of the overall structure of the sensing element, the fiber retains its mechanical strength which makes it viable for practical applications. The experimental results are found to be consistent with the modeling of the sensor's behavior. (C) 2014 Elsevier B.V. All rights reserved.