PSI - Issue 37

Grzegorz Wójcik et al. / Procedia Structural Integrity 37 (2022) 179–186

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Grzegorz Wo´ jcik / Structural Integrity Procedia 00 (2022) 000–000

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to 15-20 cleaves). A work done by Wang et al. (2020) thoroughly inspects the e ff ect of manufacturing parameters (curvature radius, polishing position and depth) as well as the temperature on the fabricated, double side-polished, u-shaped refractive index sensor. Authors have evaluated optimal parameters to increase the sensor sensitivity. More over, the temperature impact has been investigated, and it was reported that the the propagation loss of the refractive index sensor induced by the temperature is related not only to the thermo-optic and thermal expansion e ff ects of the sensor, but also by the thermo-optic e ff ect of the measured liquid itself. This paper investigates the influence of the light source intensity, the ambient temperature, as well as the quality of polymer optical fiber end-faces after cleaving on the transmission capacities of the refractive index evanescent wave (EW) absorption POF sensor and its performance. The sensor was fabricated using polishing methods.

2. Sensor fabrication

A low-cost, side-polished evanescent wave absorption POF RI sensor was fabricated, based on conclusions drawn by Sequeira et al. (2019). To achieve an intrinsic, intensity-based refractive index POF sensor, authors have modified the structure of the fiber (through polishing) to enhance the EW absorption e ff ect. In this paper, a refractive index sen sor was fabricated using similar techniques, although di ff erent POF (industrial grade Asahi TCN-1000-15-23N(24O)) was used. The structure of this fiber di ff ers in its construction when compared to traditional POFs. The industrial solu tion consists of a POF itself (made of a PMMA core and a fluorinated polymer cladding), an inner jacket (polyamide), and an outer jacket (soft polyamide). This makes the sensor fabrication process more di ffi cult, as it is necessary to re move the outer and inner jackets prior to reaching the clad-core structure. The POF was cut to a length of 200 cm. For the sensing area, a 3 cm log section was selected. The fabrication started with the removal of the outer jacket, which was done carefully using a utility knife. Then, a sequence of polishing procedures was performed, using sandpapers of di ff erent grit sizes in decreasing order. Smoother surfaces allow for more light being transmitted through the POF, whereas rougher surfaces lead to more scattering losses (which results in less transmitted light). According to the aforementioned research, the best sensor performance (by means of sensor sensitivity) was achieved when a balance between roughness (enhancing EW absorption e ff ect) and transmission losses (due to morphological changes) were obtained. Such refractive index sensor was prepared only once and used during all of the experiments.

Fig. 1: A 3D printed platform used for a POF sensor fabrication.

A custom polishing platform was designed and 3D printed with the Prusa i3 MK3S + (instrumental error of 0.1 mm) from polyethylene terephthalate glycol material in order to mount and fix the position of the POF (Fig. 1). The polishing sequence started with P80 ( 195 µ m grit size) that was used for removing the inner jacket. This was followed by polishing with P220 ( 65 µ m grit size), and P400 ( 38 µ m grit size). The final polishing was done using P600 ( 26 µ m grit size) sandpaper. After each polishing procedure: the sensing area was cleaned twice using distilled water; microscopy images of the sensing regions were taken; the sensor was characterized by means of refractive index sensitivity. Fig. 2 presents the optical microscopy images of the sensing region after each polishing stage and cleaning with distilled water. A decrease in the surface roughness is visible after each consecutive polishing procedure. Bottom images show the sensing region with the red light being guided through the plastic optic fiber, without any additional artificial illumination coming from the microscope.