Evanescent wave absorption sensing and spectroscopy are the widely used techniques for the detection of unknown chemicals. Tapered fibres are used in evanescent wave absorption sensing applications, due to the large fraction of modal power extended into the external environment as evanescent wave. The core-mode cut-off play a major role in the evanescent field absorption based tapered fibre sensors. The core-mode cut-off can be calculated by solving the eigen value equations of a tapered fibre considering as a weakly guiding three layer optical waveguide. Single mode tapered fibre has been fabricated to make a precision sensor element suitable for chemical detection. Finally, the sensor element has been used to detect absorption peak of ethylenediamine (EDA). The results are presented in which the absorption peak at 1540nm is observed. In the last chapter a fibre loop cavity resonator has been theoretically analyzed. The resonance conditions and steady state response of fibre loop cavity resonator has been analyzed. The fibre loop cavity resonator can be used for chemical sensing applications.