The relative differentiation of attenuation (RDA) as a function of RH is illustrated in Fig. 4. The values belong to GO, Sg and GSg at both standard telecommunication wavelengths of 1310 nm and 1550 nm. All the measurements were performed by OLTS at a room temperature.
At the first look, it seems that the etched fibers coated with Sg are good humidity sensors due to the good absorption and release of humidity by Sg. However, as in the figure, the fibers have a high RDA value which has no one-to-one correspondence with RH. This makes the fibers unusable for humidity sensing. Moreover, the RDAs versus RH are the same for both 1310-nm and 1550-nm wavelengths.
Despite the well-known rapid and excellent sensing properties of GO (30), the etched fibers coated with it have a zero and low RDA value below and above 40% RH respectively. Also, the difference between the curves are obvious for both 1310-nm and 1550-nm wavelengths. The main advantage of the GO-coated fibers over the Sg-coated ones is the one-to-one correspondence of their RDA with RH (Fig. 4).
The sensing mechanism for the etched fibers coated with GO is based on their refractive index variations. As the humidity in the environment increases, more water molecules are adsorbed to the GO. This changes the gap between the GO and the fiber refractive indices. When water molecules reach near the surface of GO, they become ionized (2H2O ⇔ H3O+ + OH −) and make bonds with phenol (C − OH) or carbonyl (C = O) in the GO groups (18, 43). The adsorption of water molecules onto the GO leads to a change in its electrical energy gap and, thus, a change in its refractive index.
The etched fibers coated with GSg have a non-zero incremental behavior, and their RDA has a one-to-one correspondence with RH. These fibers are, therefore, more reasonable than the others to be used as humidity sensors. Their RDA intensity is also higher at 1550 nm, which is because of the existence of GO in their coated layer.
Since repeatability is an important quality of sensors, the measurement of RDA versus RH was instantly repeated for the etched fibers coated with GSg, and the re-test curve was obtained. Figure 5A and Fig. 5C show the good repeatability of this humidity sensor. To calculate the RH from attenuation, the reverse curves were fitted by MATLAB (Fig. 5B and Fig. 5D).
Another key parameter of sensors is sensitivity. It has been defined as the ratio of the output signal to the initial signal of a humidity sensor (44). As shown in Fig. 6a, the sensitivity of GSg at the 1550-nm wavelength was higher than that at 1310 nm. Also, GSg had higher sensitivity at a low RH index (less than 50%). So, it can be more useful in this humidity range. The variance of each test from the average value is plotted in Fig. 6b. As it can be seen, the variance for the investigated sensor was small at RH values less than 50%. In addition, the variance was smaller at the 1550-nm wavelength than at 1310 nm. These findings prove the higher accuracy of the GSg- coated sensor for low-humidity conditions (under 50%) and at the 1550-nm wavelength.
This research was conducted to evaluate the ability of etched SMFs coated with GO, Sg and GSg and used as a humidity sensor. For this purpose, RDA was analyzed in the presence of RH and at both standard wavelengths of telecommunication, namely 1310 nm and 1550 nm. As the results showed, etched SMFs coated with Sg are not useful for humidity sensing. It is due to lack of a one-to-one correspondence of their RDA versus RH although the RDA is high. Also, GO-based etched fibers have a low RDA value. Of the three samples, etched SMFs coated with GSg have proved to be the best humidity sensors. Their RDA is relatively high and has a one-to-one correspondence with RH. Besides, their repeatability and sensitivity are reasonable, and their low variance is suggestive of their accuracy. The proposed GSg-based etched SMFs have high, rapid, repeatable and accurate responses at RH indices below 40%. So, they can be used as low-humidity sensors.