PSI - Issue 37

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

182

4

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

Fig. 2: Optical microscopy images of the sensing area after each polishing stage.

3. Experiment description and results

A series of experiments was conducted using the fabricated sensor. During the tests, the fabricated POF sensor was connected to the transmitting and receiving connectors (SFH757V and SFH250V respectively). The transmitter’s LED was driven with constant currents (in the range of 10 mA - 20 mA). Generated photocurrents are in the range of few microamperes. In order to not distort these signals through measuring circuits, transimpedance amplifiers with high-impedance inputs were used for the current-to-voltage conversion. The performance of the fabricated sensor was evaluated by exposing the sensing region to liquids of known RI values (in the range of 1.3324 nD - 1.4167 nD). Such characterization was done using solutions of distilled water and sucrose of di ff erent weight to weight (w / w) concentrations starting from 0% (distilled water) and ending at 50% with a step of 10%. Solutions were prepared using magnetic stirrer at room temperature. Refractive indices of obtained solutions were verified using digital refractometer Kern ORF-R (measurement range: 1,3330 nD - 1,5400 nD, accuracy: 0,0005 nD) and listed in Tab. 1.

Table 1: Measured refractive indices of prepared sucrose solutions

Measured refractive index [nD]

Estimated concentration [% w / w]

1.3324 1.3480 1.3614 1.3822 1.3992 1.4167

0.00 10.0 19.0 31.0 40.0 49.0

3.1. The influence of light intensity

First set of tests investigated the influence of the transmitter’s light intensity (LED’s forward current) on the fabri cated evanescent wave absorption refractive index sensor performance. The transmitter was controlled with following constant current values for each consecutive test: 10 mA, 15 mA, 20 mA. Fig. 3a presents raw measurements, i.e. generated photocurrents. In order to make the obtained measurements more readable and easier for further data pro cessing, the measurements were normalized. The normalization ( k norm ) is made by dividing measurements ( k solution ), by a common reference measurement ( k water ), as shown by Equation 1.

k solution k water

k norm =

(1)