This chapter aims to study related to biological sensing with plastic optical fiber fibers as a proposal for the detection of microorganisms with low cost and fast response time. One of the optical characteristics used in the biological sensor under study is the attenuation of the output power in the optical fiber by total internal reflection. The chapter discusses the study and analysis of the characteristics of the biological sensor using a curved optical fiber with a fixed angle of 180°, aiming at the analysis of the relation between the output powers as a function of the index of refraction of the external medium to the fiber. It introduces the step-by-step development of a biosensor based on polymer optical fiber, from a sensor for measuring and monitoring the refractive index associated with the mentioned technologies and phenomena, for the detection of Escherichia coli.