Optical waveguides for surface spectroscopy: FePO4 thin-film/K+-doped glass composite optical waveguide systems having tapered velocity couplers

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Abstract
Basic characteristics were examined for optical waveguides (OWGs) composed of a thin‐film OWG deposited onto a diffused OWG in order to construct OWG systems suitable for biochemical sensors and for monitoring surface chemical reactions. Conditions were examined experimentally in detail for adiabatic and nonadiabatic transition of the guided light from the K+‐doped glass OWG to the FePO4 thin film by changing the length of the tapered velocity couplers made at the ends of the films. A miniband structure of the FePO4 film, 1 mm wide and having 0.5‐mm slopes at the ends, was constructed for optical sensing. It exhibited high sensitivity despite the fact that the FePO4 OWGs were found to have a large attenuation coefficient, 60–80 dB/cm.