Humidity Sensor Based on Etched Optical Fibers Coated With Graphene Composite

In this research, optical humidity sensors based on etched-optical bers coated with graphene Oxide (GO), silica gel (Sg) and a silica gel modied with GO (GSg) was studied. Their humidity sensing behavior was investigated by variation of relative differentiation of attenuation (RDA) in the presence of relative humidity (RH). As the results showed, etched bers coated with Sg and GSg are highly capable of humidity sensing. However, a Sg- coated sample is not useful as humidity sensor related to sample coated by GSg because its RDA lacks a one-to-one correspondence with RH. As it was also found, the sensitivity of a GSg-coated sample is higher when the RH is below 40% and its repeatability is considerable.


Introduction
Optical bers have a vital role in telecommunications and serve as a substitute for copper cables. This is owing to their great advantages such as high speed, insensitive to electromagnetic interference, being anti-explosion, the small size of cables, durability, and chemical inertness (1). In recent years, optical bers have been commonly used as sensors, and they have developed rapidly for biomedical and home security purposes (2). The function of these sensors is very dependent on the changes in the refractive index. In this regard, one may refer to LPG gas sensors (3) and U-shaped bending-induced interference sensors (4). Optical ber sensors can be used to measure different physical properties, such as RI, strain (5), temperature (6, 7) and humidity (6-12). So far, various types of humidity-sensing bers, such as sidepolished bers, long-period ber gratings (LPFG) and hollow-core ber sensors, have been reported (13)(14)(15)(16). Despite their advantages, the methods of fabricating these sensors are very complicated.
Graphene has hydrophobic properties, but GO is hydrophilic because of its oxygen-containing functional groups, which makes it possible to adsorb polar molecules such as water molecules (13,29,30).
High sensitivity, extensive range, fast response and short recovery time are the important industry requirements to meet. To achieve these features, it is necessary to coat optical bers with other materials and make certain physical changes on the structure of those bers.
In this study, some optical sensors are developed individually based on GO and silica gel (Sg) as a common humidity absorbent. Also, for the rst time, a combination of GO and Sg (GSg) is used to improve the relative humidity (RH) sensing of etched optical bers. These products have the bene ts of non-complicated fabrication and short processing time.

Materials And Methods
At rst, 2D layers of GO, Sg and GSg were synthesized and quali ed. To achieve a desirable evanescence eld by reducing the diameter of the bers, a part of the single-mode bers (SMFs) was corroded with hydro uoric acid (HF) for 60 minutes. Then, the synthesized materials were coated on the surface of the etched bers by the dip coating method. At the end, the optical loss test set (OLTS) as a tool of analysis, the optical power source (OPS) and the optical power meter (OPM) were used to investigate the optical humidity sensing of the bers.
A. Synthesis The UV/Visible absorption of the GO solutions was measured and plotted in Fig. 1E. As the gure shows, the transparency of GO is obvious, and wavelength peak of ~ 310 nm corresponds to the n − n * transitions of carbonyl group (39,40).
Sg was produced by the heating of silicic acid (pH ~ 3) at70 o C for two hours to form a gel. Figure 2A and Then, the mixture was heated at 70 o C to form the gel. Figure 2C illustrates the SEM image of the GSg that is coated on silicon oxide wafer.

B. The Setup of the humidity sensing experiment
In order to make the ber sensor, 3 cm of the protective layer of a ber was stripped. Then, it was wiped up with alcohol and corroded with HF for about 60 minutes to achieve a desirable evanescence eld. The achieved diameter of the etched-ber was 32.5 µm. The SEM image of this etched-ber is presented in Fig. 3A. Basically, etching a ber leads to the reduction of its clad and the creation of an evanescence eld. After etching, the clad mode is signi cantly increased, and the wavelength of incident light shifts to a longer wavelength (i.e. it becomes red-shifted) (18, 42). Figure 3B shows the etched SMF coated with GSg.
The produced sensor was placed in a humidity control box. An OPS and an OPM (OLTS) were attached to the ends of the ber. Figure 3C shows the employed temperature controller (set on 25°C), standard humidity sensor (to compare to our humidity sensor) and humidi er (to increase the relative humidity of the environment within the range of 20-90% RH).

Results And Discussion
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 rst look, it seems that the etched bers coated with Sg are good humidity sensors due to the good absorption and release of humidity by Sg. However, as in the gure, the bers have a high RDA value which has no one-to-one correspondence with RH. This makes the bers 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 bers 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 bers over the Sg-coated ones is the one-to-one correspondence of their RDA with RH (Fig. 4).
The sensing mechanism for the etched bers 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 ber 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 bers coated with GSg have a non-zero incremental behavior, and their RDA has a one-to-one correspondence with RH. These bers 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 bers 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 tted by MATLAB (Fig. 5B and Fig. 5D).
Another key parameter of sensors is sensitivity. It has been de ned 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 ndings prove the higher accuracy of the GSg-coated sensor for low-humidity conditions (under 50%) and at the 1550-nm wavelength.   RDA as a function of RH at both wavelengths for GO, Sg and GSg.