A pair of luminescent organic dyes, Coumarin 540A and Rhodamine 6G, that functions as donor and acceptor of excitation energy were doped in core and cladding of the fiber, respectively. By taking this structure, stress applied to the fiber could be detected as an increase in cladding intensity. This happens because emission of core dye leaks to the cladding due to the deformation, and it excites the cladding dye.
Using the fabricated POF, change in near field pattern upon different fiber bending diameter was detected. Lasers with three different wavelengths were used; namely, 406 nm, 532 nm, and 635 nm. 406 nm is the optimum wavelength for exciting the Coumarin 540A of the core efficiently. 532 nm excites Rhodamin 6G well, but does not excite Coumarin 540A well. 635 nm excites neither dyes. Pigtail end was coupled to the test fiber with direct contact to the core center of the test fiber in which no cladding excitation was expected. Fibers were stressed in various magnitudes by bending them in different loop diameters, namely, 1.5 cm, 3 cm, and 6 cm. Single loop was created in the middle point of the whole length. Relaxed condition, which the fiber was naturally straightened with no loop, was also investigated. Output beam from the test fiber was analyzed using Near Field Pattern measuring system (Hamamatsu C9664-01G02).
In addition, dye-doped POF with lower numerical aperture had shown enhancement in sensitivity. Therefore, it was suggested that sensitivity of the sensor is controllable by further optimization of the waveguide parameters. Therefore, the proposed results suggests a potential for an enhanced fiber-optic stress detection with simpler and inexpensive accessory components.