A fiber optical sensor includes a light source, a detector and an optical resonator having a multimode optical fiber and two reflective layers respectively provided adjacent to or on ends of the optical fiber. Excitation light from the light source is coupled into the optical resonator through a transparent hole formed in one of the reflective layers. An evanescent wave extends outside the optical fiber in the active region of the optical fiber. The fluorescent light generated by a fluorescent material excited by the evanescent wave in response to the presence or concentration of the analyte in a sample to be assayed is detected by the detector. The light beam is reflected back and forth between the first and second reflective layers many times within the optical resonator whereby the strength of the fluorescent light is significantly increased thereby resulting in a significantly signal amplification.