Optical sensors use light to probe various physical quantities in a wide range of sensing applications. Optical sensors can be realized by employing different schemes, including cavities, surface plasmon resonance, waveguides, photonic crystals, and optical fibers. Among these arrangements, a combination of optical fiber and cavity at 1550nm has massive potential for developing a sensor with high sensitivity, low detection limit, compact structure, and relatively more straightforward fabrication. Various strategies have been adopted in the literature to improve fiber cavity sensors' performance. However, developing a viable fiber sensor for liquid phase applications and mainly at 1550 nm, is still an open question. Therefore, the present work presents novel theoretical formulations and experimental schemes towards developing fiber cavity sensors with high sensitivity and low detection limit for addressing liquid sample applications.

This thesis explores several aspects of fiber cavity optical sensors with tapered fibers as sensing heads. Firstly, we explore tapered fiber's thermal response in optical sensors to estimate thermally induced uncertainties in measurements theoretically. We derive an analytical expression that describes the temporospatial thermal response of the tapered fiber under the assumption of fundamental mode excitation and Gaussian heat source across the tapered fiber cross-sectional area. Secondly, we employ finite element method simulations to determine an optimum radius of tapered fiber in an active fiber cavity to maximize the sensor sensitivity. Thirdly, we propose a unique experimental configuration for conducting cavity ring down spectroscopy in an active fiber linear cavity sensing setup. Compared to previous approaches, our sensing modality allows the light to transverse the tapered fiber twice in one round trip, enhancing the sensor's sensitivity. Fourth and lastly, we propose a multi-cavity digital system based on the Vernier effect that carefully selects cavity lengths to predictably control sensor sensitivity, detection limit, and dynamic range of the sensor.

Significantly, all theoretical formulations and experimental schemes presented in this thesis are adaptable for integrated waveguide structures. Moreover, the temporospatial thermal model applies to optical sensing modalities that use tapered fibers as sensing heads. Furthermore, the current work is relevant for both liquid and gaseous phase sensing applications. We believe that the present thesis of novel single and multicavity optical fiber systems will find many sensing applications in chemical analysis, environmental monitoring, and biosensing.