Dielectric annular core fiber for optical sensing

The paper deals with a special type of sensing optical fiber, the annular core fiber. This fiber is composed of a narrow annular low-loss glass optical core and glass optical cladding which are applied onto the inner wall of a silica capillary. In the paper transmission characteristics are determined theoretically from the solution of the Helmholtz equation as well as experimentally from measurements of the near field and angular distributions of the output power from the fiber excited by an inclined collimated beam. The sensitivity of the fiber to gaseous toluene is determined under the excitation of the fiber by an inclined beam at 670 nm, when refractive-index changes in the evanescent field are responsible for the sensitivity. These measurements are compared with those carried out with reference capillary silica fibers and PCS fibers. It is shown that the annular core fibers have the highest sensitivity in comparison with the reference fibers. A detection limit of 0.06 % can be determined from the sensitivity measurements.

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