Abstract
We propose and develop a comprehensive model for estimating the refractive index (RI) response over three potential sensing zones in a multimode fiber. The model has been developed based on a combined ray optics, Gaussian beam, and wave optics analysis coupled to the consideration of the injected interrogating lightwave characteristics and validated experimentally through the realization of three sensors with different lengths of stripped cladding sections as the sensing region. The experimental results highly corroborate and validate the simulation output from the model for the three RI sensing zones. The sensors can be employed over a very wide dynamic RI range from 1.316 to over 1.608 at a wavelength of 1550 nm, with the best resolution of 2.2406 × 10−5 RI unit (RIU) obtained in Zone II for a 1-cm sensor length.
Highlights
We propose and develop a comprehensive model for estimating the refractive index (RI) response over three potential sensing zones in a multimode fiber
In Zone II, where the induced RI variation is situated between the cladding RI and the core RI, i.e. greater than ncl but less than nco, we have demonstrated that there are two concurrent optical phenomena occurring caused by evanescent wave absorption (EWA) and the reduction of the number of propagation modes due to the modification of the critical angle as a consequence of the RI change in the sensing or external medium
The laser beam is propagated in an optical fiber by total internal reflection (TIR) which occurs as the beam injected into the fiber core is incident at the core-cladding interface at an angle higher than the critical angle
Summary
We propose and develop a comprehensive model for estimating the refractive index (RI) response over three potential sensing zones in a multimode fiber. The aftermath of a successful trial experiment on low-loss optical fiber debuting in the 1970’s has since motivated researchers to exploit fiber optics for numerous sensing and measurement techniques This motivation can in large part, be attributed to certain advantages of the optical fiber over more conventional electro-mechanical sensor technologies, such as its immunity to electromagnetic interference, small size and light weight, electrical passivity at the sensing probe or head, multiplexing potential, and remote sensing capability[1,2,3]. The advent of the sol-gel technique has facilitated the synthesis of bespoke sensitized coatings that can be deposited on the surface of an optical fiber as a thin film in replacement of the cladding[11,12,13] This approach can allow the fiber-based refractometer to selectively measure specific organo-chemical species. The cryptophane-based molecular traps will absorb or entrap the CH4 molecules, and reversibly produce a bond in the bulk polymeric material that will induce variation in the RI of the sensitized region as a function of CH4 concentration
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