Tapered optical fiber-based interferometric sensors have potential applications in some fields for the measurements of solution concentration, refractive index, biomolecules, etc. By coating specific functional film onto the surface of a tapered optical fiber, it is possible to further extend the detection range and enhance the measurement sensitivity. In this paper, a tapered optical fiber sensor based on Mach-Zehnder modal interference is studied. An atactic polystyrene thin film was used as a functional layer to coat onto the surface of the tapered region. Detection of aqueous glucose of different concentrations was carried out using the tapered fiber sensor with or without the coating. The experimental results show that the measurement sensitivity can be significantly improved by coating the atactic polystyrene thin film onto the tapered fiber with suitable waist parameter.
A single mode optical fiber with a rectangular hole is proposed. Power coupling characteristics of the optical fiber are analyzed and discussed using numerical method. Firstly, the coupling intensity from the fiber core to the rectangular hole is investigated in different distances between the fiber core and the rectangular hole and different refractive index of the rectangular hole. Secondly, the power complete conversion and transmission characteristics are also investigated in different distances between the fiber core and the rectangular hole and different refractive index of the rectangular hole. The results show that the coupling efficiency of the proposed optical fiber from the fiber core to the rectangular hole can nearly reach to 50%. The high coupling efficiency from the fiber core to the rectangular hole can be used to develop the high efficiency mode converters. Moreover, the results also show that the coupling efficiency is higher when the refractive index of the rectangular hole is bigger than the refractive index of the fiber core. So, the proposed optical fiber has the potential application value in high refractive index sensing.
An eccentric core optical fiber (ECOF) for distributed evanescent field sensing is proposed. The finite element method (FEM) is utilized to analyze characteristics of the ECOF, including the fractional power of coating and core. A strong evanescent field exits in the coating of the ECOF due to a small distance between the core and the coating. The sensitivity or resolution of this kind sensing fiber is analyzed. The sensitivity is ~0.25dB/RIU or resolution of 4×10<sup>−3</sup> in the range of 1.4-1.42. The sensing length is in the range of 16m-160m.
Modal fields, propagation constant and power distribution of single-mode eccentric core optical fiber are calculated
based on the weakly wave-guiding approximation. The calculation of the propagation constant and power distribution is
in terms of the change of the coating refractive index and eccentric distance. It is found that the evanescent field power
depends on the coating refractive index and eccentric distance. In eccentric core side coating, the higher coating
refractive index, the stronger evanescent field power and the larger eccentric distance, the weaker evanescent field
power. The present results will be useful for deciding the geometric structure of the fiber to achieve maximum power of
the evanescent field in eccentric core side coating to enhance the sensing sensitivity.