Principle of a novel fiber-optic strain sensor using dye-doped polymer optical fiber (POF) was demonstrated in the earlier ECS meeting[1]. This time, we demonstrate the ability to tune the sensitivity of this sensor by varying the numerical aperture of the fiber. The sensor uses radiative emission-absorption mechanism and it is capable in sensing stress (strain) by simple viewing of the large-core cross section. Conventional well-known fiber-optic strain sensors such as BOTDR or FBG types require spectral analysis to determine the strain value. Our sensor does not require expensive equipment such as spectral analyzer or well-tuned light source that are required in conventional fiber-optic strain sensors.

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[2].

POFs with three different NA were fabricated by varying the amount of high-refractive index dopant in the core[2]. Using the fabricated POF, change in near field pattern and output visible spectrum upon different fiber bending diameter were detected. 406 nm output pigtail laser was used in order to excite the donor dye efficiently. 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) and spectral analyzing system (JASCO MF-3500).

As a result, linear change in sensor sensitivity was observed as the NA increases. 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.

[1] Kamimura and Furukawa, 232nd ECS meeting.

[2] Kamimura and Furukawa,Appl. Phys. Lett. 111, 063301 (2017)