# Intensity Loss of ZnO Coated on Fiber Optic

Authors: Noor Azie Azura Mohd Arif; Sahbudin Shaari; Abang Annuar Ehsan
Journal: Journal of Nanomaterials
 (2023)

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## Abstract

Macrobends are optical fiber structures suitable for detecting motion changes. This structure has been developed using single-mode fibers and a combination of single-mode and multimode fibers called hetero-core. In this study, a new macrobending structure was designed and developed by adding a nano-ZnO element to the fiber optic core based on Revolution 4.0. The addition of nanomaterial elements involves an etching process that uses harmful chemicals or high-cost laser technology. Therefore, hetero-core was applied in this study to replace the etching process. The ZnO-coated fiber optics with 10 (ZnO1), 20 (ZnO2), and 30 (ZnO3) times of dip coating were developed using the dip-coating method and characterized using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. Sensitivity measurement was conducted with glued optical fiber in the form of bending using a tape with a bending dimension of 2.5 cm × 1.5 cm and a wavelength of 1,550 nm. Morphological characterization using SEM proves that nanoparticles are attached to the optical fiber, and the EDX characterization confirms that the nanoparticles are ZnO elements. Optical fiber sensor sensitivity using core sizes 9, 50–9–50, 50–9–50 (ZnO1), 50–9–50 (ZnO2), and 50–9–50 (ZnO3) achieved sensitivity values of 0.91, 1.61, 2.98, 3.34, and 3.51, respectively. This study successfully produced ZnO-coated optical fiber sensors with a hetero-core structure without performing the etching process and successfully increased the sensitivity of the sensors.


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## Body

## 1. Introduction

The studies on respiratory detection using smart watches known as Fitbit have evolved since 2013 [1]. In 2006, researchers in Japan started producing respiratory detection systems using optical fibers affixed to elastic textiles [2]. This study has attracted significant attention because of its importance for human health. Several studies have been conducted on smart textiles. Researchers have studied smart textiles using Bragg grating structures, microbending, and macrobending. The production of smart textiles using Bragg grating-structured optical fibers [3–7] has been developed; however, the operation process is complex and involves high-cost usage [8]. Simpler and more effective smart textiles are designed based on macro- and microbending structures. Krebber et al. [9] conducted studies using a single-mode and bending numbers of more than seven. Alemdar et al. [10] and Koyama et al. [11] developed macrobending and hetero-core-structured sensors to improve the performance of sensor sensitivity [10–12]. The operation of hetero-core optical fiber sensors (62.5–50–62.5 μm) typically involves evanescent field mechanisms to improve sensor sensitivity. This study proves that the sensitivity increases by using only a bending number of seven with the influence of the bending radius [13]. Dhia et al. [13] and Purnamaningsih et al. [14] developed sensors using a single fiber optic mode with a wavelength of 1,550 nm. They successfully produced a bending number of seven without a hetero-core structure. Based on a recent study, macrostructured optical fiber sensors have been successfully used by researchers to detect respiratory system movements. However, the large number of bends makes it difficult for users to wear freely, and most likely, the optical fiber is easily broken when multiple bending is adopted.The sensitivity generated from an optical fiber sensor can be improved by changing the design of the optical fiber structure. The sensitivity of the optical fiber sensor increases if the fiber coating refractive index is higher than the fiber core refractive index. The addition of nanomaterial elements to the optical fiber in line with Industry Revolution 4.0 has attracted significant attention because these elements can improve the performance of the optical fiber sensor in terms of the sensor’s sensitivity. Macrobending sensors with a small number of macrobending can be developed. However, the addition of nanomaterial elements to the optical fiber involves the removal of the optical fiber coating layer. Nano-coated optical fiber sensors have been developed; however, this process is difficult because it involves an etching process [15–21]. Wet etching can be performed using a cheap method for removing optical fiber silica coated with harmful chemicals, such as hydrofluoric acid [22–25]. Dry etching and inexpensive processes can be conducted mechanically using sandpaper; however, this method is only suitable for polymer fiber optics [26]. Dry etching can be applied to silica fibers using plasma or laser, but it involves high costs [27]. The fragility of equipment and technology for the production process, cost savings, and risk reduction in this research has sparked the idea of developing optical fiber sensors without involving the exposure process. Moreover, the use of hetero-core structures can replace the etching process. This structure helps create the evanescent field phenomenon and helps the light move easily to the fiber coating. This study aimed to develop sensors through the optical fiber inhibition process and the ZnO coating process of nanomaterial on the core optical fiber based on a design optimized using the Taguchi method with a bending dimension of 2.5 cm × 1.5 cm and analyzing the light intensity output on the sensor designed.


## 2. Methodology

### 2.1. Preparation of ZnO-Coated Optical Fiber

ZnO solution was produced by dissolving ZnO powder nanomaterial (544906-50G Sigma Aldrich) into 99.5% ethanol, acting as a solvent. Stirring was conducted for 1 hr with a magnetic stirrer (SH-2). During the stirring process, the solution temperature was set to 78°C, and the process was performed for 30 min. The coating method was used to produce ZnO-coated fiber optic sensors. This method was selected to obtain a uniform coating thickness of ZnO nanoparticles. Optical fiber modification is unnecessary because optical fiber connectivity involves the use of different core sizes but retains the same size of cladding fiber optic. ZnO nanoparticles were coated using a dip coating technique with a dipping movement of 0.0195 cm s−1. The motion controller was placed on the top of the oven (Constance GCH series) to facilitate the coating, evaporation, and annealing processes without involving any movement on the optical fiber (Figure 1). After the coating process, the optical fiber was dried in an oven for 1 min at 78°C to ensure full evaporation. The coating and heating processes were repeated to form several layers of ZnO nanoparticles on the fiber optic surface. ZnO samples of nanomaterials were characterized using SEM and EDX. Furthermore, characterization was performed to confirm that the resulting sample is a ZnO nanomaterial, and morphological structure was observed to determine the thickness and size of the nanomaterial coated on optical fiber.Figure 1
Dip-coating process of ZnO nanoparticles.


### 2.2. Output Intensity Measurement

Figure2 shows a simple and easy method for measuring the intensity output produced by optical fiber sensors. Optical fibers are glued to the paper in the form of bending using a tape. Optimal parameters with bending dimensions of 2.5 cm × 1.5 cm and wavelength of 1,550 nm are used [28].Figure 2
Optical fibers are formed in accordance with the U-shaped printed on thick paper.


## 2.1. Preparation of ZnO-Coated Optical Fiber

ZnO solution was produced by dissolving ZnO powder nanomaterial (544906-50G Sigma Aldrich) into 99.5% ethanol, acting as a solvent. Stirring was conducted for 1 hr with a magnetic stirrer (SH-2). During the stirring process, the solution temperature was set to 78°C, and the process was performed for 30 min. The coating method was used to produce ZnO-coated fiber optic sensors. This method was selected to obtain a uniform coating thickness of ZnO nanoparticles. Optical fiber modification is unnecessary because optical fiber connectivity involves the use of different core sizes but retains the same size of cladding fiber optic. ZnO nanoparticles were coated using a dip coating technique with a dipping movement of 0.0195 cm s−1. The motion controller was placed on the top of the oven (Constance GCH series) to facilitate the coating, evaporation, and annealing processes without involving any movement on the optical fiber (Figure 1). After the coating process, the optical fiber was dried in an oven for 1 min at 78°C to ensure full evaporation. The coating and heating processes were repeated to form several layers of ZnO nanoparticles on the fiber optic surface. ZnO samples of nanomaterials were characterized using SEM and EDX. Furthermore, characterization was performed to confirm that the resulting sample is a ZnO nanomaterial, and morphological structure was observed to determine the thickness and size of the nanomaterial coated on optical fiber.Figure 1
Dip-coating process of ZnO nanoparticles.


## 2.2. Output Intensity Measurement

Figure2 shows a simple and easy method for measuring the intensity output produced by optical fiber sensors. Optical fibers are glued to the paper in the form of bending using a tape. Optimal parameters with bending dimensions of 2.5 cm × 1.5 cm and wavelength of 1,550 nm are used [28].Figure 2
Optical fibers are formed in accordance with the U-shaped printed on thick paper.


## 3. Results and Discussion

### 3.1. Characterization of ZnO Nanoparticles

ZnO1, ZnO2, and ZnO3 samples were produced with a total of 10, 20, and 30 times, respectively. The characteristics discussed are morphological structure, film thickness, and film composition. Figure3 shows the SEM images of ZnO film-coated optical fibers with different thicknesses. It is worth noting that ZnO is successfully coated on the fiber optic surface, and the thickness of ZnO nanoparticles is higher after the fibers had a dip coating of 30 times. The layer of ZnO nanoparticles has thicknesses of 300, 600, and 800 nm with dip coating of 10, 20, and 30 times, respectively. The SEM image clearly shows that the spherical part of the ZnO layer is more attached to the part when the optical fiber is dipped 30 times. High surface balance is visually visible. Additionally, the layer of ZnO nanoparticles approaches a good level of uniformity where mostly the entire surface of the optical fiber is coated with ZnO nanoparticles.Figure 3
SEM image cross-section sample. (a) ZnO1, (b) ZnO2, and (C) ZnO3 on the fiber optic sphere with a size of 500x magnification, image thickness of ZnO nanoparticles coating scanned on spherical section, and image layer of ZnO nanoparticles on optical fiber surface scanned from the optical fiber surface.
(a)(b)(c)Figure4 shows that Zn and O are elements detected in silica (Si) compounds. The presence of Si compounds is due to the platform, which is a Si optical fiber. The data reinforce the presence of ZnO nanoparticles formed on the fiber optic layer. Table 1 shows the percentage of composition in three different samples. The Zn values are higher in the ZnO3 sample. This finding is consistent with SEM imaging, indicating that ZnO3 sample has more ZnO nanoparticles on optical fibers dip coated 30 times.Figure 4
Zn and O composition detected by EDX characterization on samples: (a) ZnO1; (b) ZnO2; (c) ZnO3 optical fibers.
(a)(b)(c)Table 1
Percentage of atoms measured in ZnO1, ZnO2, and ZnO3 samples through EDX analysis.
ElementLinePercentage of atoms (%)ZnO1ZnO2ZnO3OK series47.9347.2346.57SiK series17.0014.727.06ZnL series35.0638.0546.36Total100.00100.00100.00


### 3.2. Analysis of the Design of Optical Fiber Sensor

The resulting output intensity by comparing 9, 50–9–50, and 50–9–50μm with ZnO nanoparticles was discussed. Bending numbers 1–3 were used to view the intensity output with bending dimensions of 2.5 cm × 1.5 cm operating at a wavelength of 1,550 nm. This study used optimized parameters with the Taguchi method. Figure 5 shows the resulting variation. This figure shows that the combination of hetero-core fiber structures with the addition of ZnO layers can increase the sensitivity in terms of bending loss produced by the optical fiber sensor. This sensitivity is observed through the output value generated at the optical fiber output terminal with increasing bending numbers. ZnO1, ZnO2, and ZnO3 labels indicate that optical fibers are coated with a frequency of 10, 20, and 30 times.Figure 5
Variation in output intensity against bending numbers with the use of 9, 50–9–50, and 50–9–50μm ZnO nanocoating with different times of dip coating.The gradient on the graph is determined by Origin software, and the resulting data are summarized in Table2 to determine the specific sensitivity of each combination. All five combinations showed an increase in loss with an increase in the number of bends. Table 2 clearly shows that the presence of ZnO elements can enhance the sensitivity of the designed fiber optic sensors. Optical fiber sensor sensitivity using core sizes 9, 50–9–50, 50–9–50 (ZnO1), 50–9–50 (ZnO2), and 50–9–50 (ZnO3) provided a sensitivity value of 0.91, 1.61, 2.98, 3.34, and 3.51, respectively.Table 2
Analytical data for optical fiber combinations with core sizes 9, 50–9–50, 50–9–50 (ZnO1), 50–9–50 (ZnO2), and 50–9–50 (ZnO3)μm using Origin.
Fiber optic combination (μm)Slope (dB)Intercept (dB)Pearson value,R90.91−0.240.9750–9–501.61−0.010.9950–9–50 (ZnO1)2.98−0.650.9850–9–50 (ZnO2)3.34−0.201.0050–9–50 (ZnO3)3.510.071.00The sensitivity of these sensors can be attributed to the index differences in the optical fiber structure. The index of the fiber optic coating ranges from 1.45 to 1.55 [29]. Optical fiber coating uses polymer material to protect the optical fiber from fracture. In this study, optical fiber coating was removed using a cleaver and replaced with a ZnO nanoparticle coated with a refractive index of 1.7–2.5 [30–35]. In addition to the nanoparticles produced on optical fibers, the ZnO refractive index plays an essential role in the light propagation throughout the designed optical fiber. This condition is linked to a study by Michel et al. [36]. They stated that greater loss occurs when the coating refractive index is high from the optical fiber cladding refractive index. This study reinforces the findings of Michel et al. [36], where the refractive index is the dominant parameter that determines the output yield at the optical fiber output terminal. The concept highlighted in this study is to perform custom fiber optic sensor coating. Light will be provided at one end of the optical fiber input, and light transmission will be received at the end of the optical fiber output. When the modification is performed on the optical fiber, some of the light that propagated along the optical fiber is lost through the coating modification area. Changes in output intensity are associated with interactions between optical fiber modification surfaces with light leaks [37].The light propagation situation is described as where light moves from the dense medium to the less dense and causes the rays to bend away from the normal line. With the addition of ZnO to the hetero-core, light is more likely to go out and cause loss. This condition causes the output of the optical fiber output terminal to decrease compared with that without the ZnO. This change is associated with the light scattering (nZnO > ncoating) condition. In terms of the light scattering mechanism, the light propagates in the fibers because the ZnO refractive index is higher than the cladding refractive index.This situation has also been investigated by Apriyanto et al. [38]. Here, when the coating has a low refractive index of the cladding, the mode propagates in the fiber core called full reflection, where the evanescent wave is generated throughout the core–cladding interface. However, when the coating has a refractive index between the core and cladding of the fiber optic core, two phenomena affect the loss of optical power, which reduces the number of propagation modes due to the critical angle influenced by the coating refractive index and the absorption of evanescent waves that occur on the coating–coating interface. Meanwhile, when the coating refractive index is larger than the cladding and core refractive index, the propagation leading to full reflection is no longer met. The transmission beam is not reflected back into the core fiber optic. However, some parts of the power are reflected back into the fiber core with an external reflection mechanism [38]. Figure 5 shows that higher loss is experienced by propagating light inside the optical fiber with an increase in the number of dips. This condition is consistent with the Beer–Lambert law, where the higher the sample thickness, the higher the loss experienced by the input light [39].


## 3.1. Characterization of ZnO Nanoparticles

ZnO1, ZnO2, and ZnO3 samples were produced with a total of 10, 20, and 30 times, respectively. The characteristics discussed are morphological structure, film thickness, and film composition. Figure3 shows the SEM images of ZnO film-coated optical fibers with different thicknesses. It is worth noting that ZnO is successfully coated on the fiber optic surface, and the thickness of ZnO nanoparticles is higher after the fibers had a dip coating of 30 times. The layer of ZnO nanoparticles has thicknesses of 300, 600, and 800 nm with dip coating of 10, 20, and 30 times, respectively. The SEM image clearly shows that the spherical part of the ZnO layer is more attached to the part when the optical fiber is dipped 30 times. High surface balance is visually visible. Additionally, the layer of ZnO nanoparticles approaches a good level of uniformity where mostly the entire surface of the optical fiber is coated with ZnO nanoparticles.Figure 3
SEM image cross-section sample. (a) ZnO1, (b) ZnO2, and (C) ZnO3 on the fiber optic sphere with a size of 500x magnification, image thickness of ZnO nanoparticles coating scanned on spherical section, and image layer of ZnO nanoparticles on optical fiber surface scanned from the optical fiber surface.
(a)(b)(c)Figure4 shows that Zn and O are elements detected in silica (Si) compounds. The presence of Si compounds is due to the platform, which is a Si optical fiber. The data reinforce the presence of ZnO nanoparticles formed on the fiber optic layer. Table 1 shows the percentage of composition in three different samples. The Zn values are higher in the ZnO3 sample. This finding is consistent with SEM imaging, indicating that ZnO3 sample has more ZnO nanoparticles on optical fibers dip coated 30 times.Figure 4
Zn and O composition detected by EDX characterization on samples: (a) ZnO1; (b) ZnO2; (c) ZnO3 optical fibers.
(a)(b)(c)Table 1
Percentage of atoms measured in ZnO1, ZnO2, and ZnO3 samples through EDX analysis.
ElementLinePercentage of atoms (%)ZnO1ZnO2ZnO3OK series47.9347.2346.57SiK series17.0014.727.06ZnL series35.0638.0546.36Total100.00100.00100.00


## 3.2. Analysis of the Design of Optical Fiber Sensor

The resulting output intensity by comparing 9, 50–9–50, and 50–9–50μm with ZnO nanoparticles was discussed. Bending numbers 1–3 were used to view the intensity output with bending dimensions of 2.5 cm × 1.5 cm operating at a wavelength of 1,550 nm. This study used optimized parameters with the Taguchi method. Figure 5 shows the resulting variation. This figure shows that the combination of hetero-core fiber structures with the addition of ZnO layers can increase the sensitivity in terms of bending loss produced by the optical fiber sensor. This sensitivity is observed through the output value generated at the optical fiber output terminal with increasing bending numbers. ZnO1, ZnO2, and ZnO3 labels indicate that optical fibers are coated with a frequency of 10, 20, and 30 times.Figure 5
Variation in output intensity against bending numbers with the use of 9, 50–9–50, and 50–9–50μm ZnO nanocoating with different times of dip coating.The gradient on the graph is determined by Origin software, and the resulting data are summarized in Table2 to determine the specific sensitivity of each combination. All five combinations showed an increase in loss with an increase in the number of bends. Table 2 clearly shows that the presence of ZnO elements can enhance the sensitivity of the designed fiber optic sensors. Optical fiber sensor sensitivity using core sizes 9, 50–9–50, 50–9–50 (ZnO1), 50–9–50 (ZnO2), and 50–9–50 (ZnO3) provided a sensitivity value of 0.91, 1.61, 2.98, 3.34, and 3.51, respectively.Table 2
Analytical data for optical fiber combinations with core sizes 9, 50–9–50, 50–9–50 (ZnO1), 50–9–50 (ZnO2), and 50–9–50 (ZnO3)μm using Origin.
Fiber optic combination (μm)Slope (dB)Intercept (dB)Pearson value,R90.91−0.240.9750–9–501.61−0.010.9950–9–50 (ZnO1)2.98−0.650.9850–9–50 (ZnO2)3.34−0.201.0050–9–50 (ZnO3)3.510.071.00The sensitivity of these sensors can be attributed to the index differences in the optical fiber structure. The index of the fiber optic coating ranges from 1.45 to 1.55 [29]. Optical fiber coating uses polymer material to protect the optical fiber from fracture. In this study, optical fiber coating was removed using a cleaver and replaced with a ZnO nanoparticle coated with a refractive index of 1.7–2.5 [30–35]. In addition to the nanoparticles produced on optical fibers, the ZnO refractive index plays an essential role in the light propagation throughout the designed optical fiber. This condition is linked to a study by Michel et al. [36]. They stated that greater loss occurs when the coating refractive index is high from the optical fiber cladding refractive index. This study reinforces the findings of Michel et al. [36], where the refractive index is the dominant parameter that determines the output yield at the optical fiber output terminal. The concept highlighted in this study is to perform custom fiber optic sensor coating. Light will be provided at one end of the optical fiber input, and light transmission will be received at the end of the optical fiber output. When the modification is performed on the optical fiber, some of the light that propagated along the optical fiber is lost through the coating modification area. Changes in output intensity are associated with interactions between optical fiber modification surfaces with light leaks [37].The light propagation situation is described as where light moves from the dense medium to the less dense and causes the rays to bend away from the normal line. With the addition of ZnO to the hetero-core, light is more likely to go out and cause loss. This condition causes the output of the optical fiber output terminal to decrease compared with that without the ZnO. This change is associated with the light scattering (nZnO > ncoating) condition. In terms of the light scattering mechanism, the light propagates in the fibers because the ZnO refractive index is higher than the cladding refractive index.This situation has also been investigated by Apriyanto et al. [38]. Here, when the coating has a low refractive index of the cladding, the mode propagates in the fiber core called full reflection, where the evanescent wave is generated throughout the core–cladding interface. However, when the coating has a refractive index between the core and cladding of the fiber optic core, two phenomena affect the loss of optical power, which reduces the number of propagation modes due to the critical angle influenced by the coating refractive index and the absorption of evanescent waves that occur on the coating–coating interface. Meanwhile, when the coating refractive index is larger than the cladding and core refractive index, the propagation leading to full reflection is no longer met. The transmission beam is not reflected back into the core fiber optic. However, some parts of the power are reflected back into the fiber core with an external reflection mechanism [38]. Figure 5 shows that higher loss is experienced by propagating light inside the optical fiber with an increase in the number of dips. This condition is consistent with the Beer–Lambert law, where the higher the sample thickness, the higher the loss experienced by the input light [39].


## 4. Conclusion

This study successfully demonstrated the influence of the addition of ZnO nanoparticle elements on optical fibers on the performance of hetero-core macrostructured optical sensor efficiency. The morphological characterization using SEM proves that nanoparticles are attached to the optical fiber, and the EDX characterization confirms that the nanocrystal is ZnO. Comparison is conducted based on the sensitivity of the optical fiber sensor coated with the ZnO nanocrystal with different coating numbers and without the ZnO nanoparticles through the plotted graph on Origin software. Optical fiber sensor sensitivity using core sizes 9, 50–9–50, 50–9–50 (ZnO1), 50–9–50 (ZnO2), and 50–9–50 (ZnO3) provided a sensitivity value of 0.91, 1.61, 2.98, 3.34, and 3.51, respectively. The addition of nanomaterial elements according to Industry Revolution 4.0 can increase the sensitivity of the designed sensors and benefit from research on manufacturing motion detection sensors.



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*Source: 9645272-2023-02-13.xml*