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.
Use of annular core fiber (ACF) for chemical trace analysis is novel approach in the field of evanescent wave optical sensors. It allows extending the sensitivity of conventional absorbance spectroscopy by extending optical interaction path length of evanescent wave. ACF is waveguide, typically consisting of small diameter tubes capable of guiding light through a sample by evanescent wave. AFC waveguides are capillaries that contain a liquid or gas sample for spectroscopic analysis. The core of doubly clad step index optical fiber is a dielectric annulus lying between two different claddings. Benefit of this solution is possibility to use of small volume samples, long path lengths by constraining light propagation within a liquid or gas medium by evanescent wave. Preform preparation by MCVD process and fiber drawing and coating of this special capillary structure have been realized in the workplace of IREE AS CR. Primary shielding of silica capillary is ultraviolet radiation activated acrylic copolymer. Basic optical characteristic were measured - attenuation, refractive index (RI) profile, loss spectral distribution, near field intensity distribution.
Liquid core optical fiber waveguides are capillaries that contain a liquid core -- liquid sample for spectroscopic analysis. The sensitivity of absorbance spectroscopy can be improved by extending optical pathlength. Liquid core waveguides provide for long optical pathlengths by constraining light propagation within a liquid medium which has a higher refractive index than the surrounding solid tubing. We present a simple optic method that allows to extend the sensitivity of conventional spectroscopic measurement. For long pathlength absorbance spectroscopy measurement the Teflon AF waveguide capillary cell for low refractive index liquids was designed. For ultraviolet, visible and near infrared region this type of fiber optic sensor with liquid core was designed. Benefit of this solution is possibility to use of small volume samples, long pathlengths by constraining light propagation within a liquid medium which has a higher refractive index than the Teflon AF tubing, sensitivity increase of conventional absorbance spectroscopy by two or more orders of magnitude. No preconcentation is required in chemical analysis. The analytical procedures employed in long pathlength absorbance spectroscopy are amenable to miniaturization and autonomous operation.