Realization of a miniaturized optical sensor for biomedical applications

Optical sensing techniques are considered as a powerful source of information on the biochemistry of tissues, blood and physiological fluids. Two sensing modalities are considered: (1) the direct spectroscopic study of the biological medium; (2) the spectroscopic study of a sensitive medium (reagent) in contact with the biological medium. We report the development of optical sensors based on the use of polished optic fibers. The sensing cell is a section of a single-mode fiber where part of the cladding has been removed by polishing. A technique, initially developed for the batch fabrication of monomode fiber couplers, has been used. A major interest of this technique is that very tiny probes can be realized, which can be easily inserted into hypodermic needles or catheters. Two sensing mechanisms are considered: (1) Evanescent mode sensing: the proximity of the fiber core to the biological or sensing medium allows the modal field tail to penetrate into the medium and collect the spectroscopic data. Attenuation of the excitation beam depends on both the refractive index and the optical absorption of the sensed medium. For fluorescence spectroscopy, the search for an optimum has been achieved and is discussed. (2) A resonant coupling mechanism between the sensed medium and the optical fiber: an enhanced sensitivity of the optical probe can be obtained. The realization of a miniaturized pH sensor is described as an example of the application of this polished optical fiber technology.