PSI - Issue 37

Grzegorz Wójcik et al. / Procedia Structural Integrity 37 (2022) 179–186 Grzegorz Wo´ jcik / Structural Integrity Procedia 00 (2022) 000–000

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to be notably higher for the sensors with poor quality of their fiber optic end-faces, although the reproducibility was significantly lower. The sensitivities of fabricated sensors have been calculated according to the Equation 2.

δ k norm δ RI

S =

(2)

3.3. The influence of ambient temperature

Climatic tests were performed using the same sensor as for the previous experiments. The sensor was placed inside a climatic chamber to evaluate its performance under extreme operating temperatures (Fig. 6). Driver of the sensor remained in the room temperature in order to not introduce any additional errors caused by the temperature variation. Additionaly, a second 2-meters long optical fibre was added for a reference. Thanks to that, generated photocurrent was measured for a raw plastic optic fiber cable, as well as for the fabricated sensor. This experiment was conducted twice for di ff erent mediums. For the first attempt, the sensor was exposed to the air. Second attempt had the sensor immersed in distilled water. Charts presented in Fig. 6 show the variation of the normalized transmitted signal over wide temperature range. For the normalization, the measurements at the room temperature were used. Measurements shown in Fig. 6a were obtained for the sensor exposed to the air, which temperature was controlled in the range of -40°C up to 90°C. Tests did not exceed 90°C in order to not damage the sensor. Fig. 6b shows measurements for the sensor exposed to distilled water. In this case, minimum reached temperature was -35°C, and the maximum reached temperature was 65°C. In both cases, the signal transmitted through the raw plastic optic fiber (placed for a reference) was not significantly influenced by the temperature change. Maximum signal change was measured to be approximately 2%. The temperature of the medium influences the sensor response significantly. For the air, maximum measured variation is approximately 13%. For the distilled water, it is almost 25%.

(a) Medium - air

(b) Medium - distilled water

Fig. 6: Results of the climatic chamber tests for the sensor exposed to di ff erent medium.

4. Conclusions and future work

A low cost, side-polished evanescent wave absorption plastic optical fiber RI sensor has been fabricated based on the literature review. The sensor was calibrated with a liquid of known RI (distilled water) and was characterized mul tiple times during the experiments. The inconsistent optical coupling between POF and connectors was identified as one of the sources for the need of such calibration. Results of the first experiment indicate that there is almost no cor relation between increasing the transmitter’s forward current and the performance of the fabricated sensor -provided that the light source intensity is stable and does not fluctuate. Designed driver utilizes current-sources based on tran simpedance amplifiers, which provide stable forward current for the transmitting LED. Obtained results indicate that