Refractive index sensor based on an abrupt biconical fiber taper

In this project, a novel refractive index (RI) sensor based on the abruptly tapered biconical fiber was proposed and demonstrated. The RI sensor is based on the theory of mode coupling in the tapered fiber. It has been shown that when a single mode fiber is tapered, the fundamental mode will be coupled into cladding mode and core mode in the tapered region. The transmission power spectrum of the tapered fiber has sinusoidal relation with the wavelength. The change of the external RI of the solution can be reflected in the shift of the wavelength response. The structure of the microfiber RI sensor can be customized based on the application. Different structures of microfiber sensor were tapered and each one was tested in NaCl solution of different RI. The results matched well with the theoretical analysis. The sensitivity of the microfiber RI sensor can reach as high as around 3900 nm/RIU, which is much higher than most of the current optical fiber RI sensors. The sensitivity of the RI sensor depends on several factors. A smaller waist diameter will generally give higher sensitivity because of larger evanescent field. It is observed in the transmission spectrum of the RI sensor that the dips do not shift uniformly but more at the larger wavelength, which leads to the conclusion of higher sensitivity at larger wavelength. The drawback of this type of RI sensor is that the wavelength response is periodic. It is unable identity how many periods the transmission spectrum has shifted if the refractive index change is large. The length of the waist was shortened to give longer period, but it is difficult to ensure the small diameter at the same time. Thus the sensitivity is sacrificed for longer period. The research discovered that slower tapering speed could achieve a relatively smaller diameter with limited waist length.Bachelor of Engineerin