A fiber optic microsensor is described which utilizes the surface plasmon resonance (SPR) effect to detect the chemical environment surrounding the fiber. The sensor is a multimode step index optic fiber, which is constructed by removing the fiber cladding layer with hot sulfuric acid and coated with gold film on fiber core and on distal end. The changes in the light reflectivity are recorded as SPR spectra, which are highly sensitive to the optical properties of the samples adjacent to the

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... or surface. The incident light is guided through a splitter to excite and record SPR in both visible and near infrared (NIR) regions. The NIR spectrum has a larger and sharper resonant peak than visible one. It thus provides a more sensitive mechanism to probe the vicinity of interface for biochip applications.