Abstract
Evanescent interaction between an SM fiber of partially removed cladding and liquid film of large thickness (T > 200 micrometers and refractive index close to that of fiber core) overlaid on the etched portion is analyzed using coupled mode theory--modeling the liquid film as an equivalent multimode cylindrical waveguide acting as mode sink. A closed-form expression is derived for the loss coefficient of half and one-third coupler in the presence of liquid. Theoretical plots of attenuation of the half-coupler as a function of remaining cladding thickness (h) for different values of interaction length (Li) are shown and verified with experimental results using liquid film as well as liquid droplets. In the experiments, the refractive index of the liquids related to their concentration is varied from that of the fiber core to cladding. The attenuation was found to be independent of film thickness in the process of fabricating half or one-third coupler, due to the presence of abrasive particles suspended in the index matching liquid during polishing. Light stripped out from the fiber by the index matching oil is immediately scattered and absorbed by the particles in suspension, and the experimental loss calibration curves are found to be same when liquid film to large thickness is deposited on the polished fiber.
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