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Abstract
The pristine fiber has a tensile strength of 5 GPa while it can be reduced to 0.4 MPa after writing FBG by using the UV laser based phase mask technology. Herein, we report four lamellar polyimide (PI)-coated FBG sensors with great increase of the tensile strength. Our results show that the average tensile strength of the lamellar PI-coated FBG sensors is 2.8 times higher than the value of the uncoated FBG sensors. More importantly, compared with the uncoated FBG sensors, the lamellar PI-coated film can effectively protect the uncoated FBG sensors from a fracture at the grating area. In addition, the lamellar PI-coated FBG sensors also possess good force sensing capabilities, which indicate that the lamellar PI-coated FBG sensors can be considered as a candidate for force sensing applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical fiber sensors have received extensive attention for their possessing lots of advantages such as multiplexing, multi-functionality, long transmission distance, and immune to electromagnetic interference [1–4]. Generally, depending on the sensing range, the optical fiber sensors can be classified as local (or point), quasi-distributed and distributed sensors [1]. Many intensity-based sensors, such as fiber optic Fabry-Perot sensors [5,6] are local sensors, which can detect physical or chemical changes at specified local points in a structure. The fiber Bragg grating (FBG) sensors [7,8], which can be easily multiplexed to measure physical changes at many locations, are a kind of typical quasi-distributed sensors. Distributed sensors [9,10] are most suitable for large structural applications, since all the segments of the optical fiber act as sensors, and thus the perturbations within various segments of the structure can be sensed. As the most widely used optical fiber sensors in engineering, the FBG sensors have always been the research hotspot in the fields of optical sensing [1–6]. Recently, much research attention has been focused on the tensile strength of the FBG sensors because weak tensile strength will cause fatal damage in the application systems [11–14]. Experimental results show that the pristine fiber has a tensile strength of 5 GPa [12,14,15] while it can be reduced to 0.4 MPa [16] after writing FBG by using the UV laser based phase mask method [17]. This is because the hydrogen loading process, the mechanical stripping step or the chemically stripping step can cause great damage to the tensile strength of optical fibers [15,16,18]. Fortunately, material coating is an effective method to improve the tensile strength of FBG sensors. For example, Kim et al. achieved a 2.1-fold increase of tensile strength by coating with a metallic layer on the FBG sensors [13]. Although the metal-coated FBG sensors possess some excellent advantages such as stability, resistance to high and low temperatures, the durability of metal packaging, determined the measurement accuracy and lifetime of the metal-coated FBG sensors, is limited [19]. In addition, due to the introduction of metal materials at the grating area, the metal-coated FBG sensors are easy to conduct current. Polyimide (PI), a high performances polymer, exhibits some excellent advantages such as temperature stability, resistance to solvents and high mechanical strength. Due to their superior properties and adhesion properties [20,21], PI is used for coating applications such as humidity measurements [22,23], salinity measurements [24,25] and temperature measurements [25]. The uniform and concentric PI coatings also can be applied with no loss of original fiber strength by using the polyimide recoater [26]. However, the price of the polyimide recoater is expensive. In addition, although the femtosecond-FBGs [12] and the draw tower gratings [27] have almost perfect tensile strength, the construction costs of building a femtosecond system and building the commercially available draw tower are extremely high. Recently, Sun et al. have mentioned that using the (very cheap) lamellar PI-coated method could improve the mechanical strength of the FBG salinity sensors [24]. However, they focused more attention on the salinity sensing characteristics and did not provided any experimental data to prove that the tensile strength of the lamellar PI-coated FBG sensors is higher than the tensile strength of the uncoated FBG sensors.
In order to illustrate that the tensile strength of the lamellar PI-coated FBG sensors is really higher than the tensile strength of the uncoated FBG sensors, the lamellar PI-coated method is used in this paper. The tensile strength results show that the average tensile strength of the lamellar PI-coated FBG sensors is 2.8 times higher than the average tensile strength of the uncoated FBG sensors. Furthermore, compared with the uncoated FBG sensors, the lamellar PI-coated film can effectively protect the uncoated FBG sensors from a fracture at the grating area. The lamellar PI-coated FBG sensors also exhibit good linear relationship between the wavelength and the applied force. These properties indicate that the lamellar PI-coated FBG sensors are superior to the uncoated FBG sensors in practical applications where high tensile strength is required.
2. Principle and fabrication of FBG sensor
2.1 Principle of the FBG
FBG is a periodic structure of refractive index along with the fiber core which is fabricated by exposing the photosensitized fiber core to ultraviolet light [17]. The structure and principle of the FBG are illustrated in Fig. 1(a). When the incident light injects into the grating, a narrowband spectral component is reflected by the grating. According to the Bragg law, the reflected light at the Bragg wavelength (${\lambda _\textrm{B}}$) can be expressed as [28]:
where ${n_{eff}}$ is the effective refractive index of grating and $\Lambda $ is the grating period. When the FBG is affected by both strain and temperature, the shift in Bragg wavelength ($\Delta \lambda$) can be expressed as [24,29]:
Fig. 1. (a)The structure and principle of the FBG and (b) the structure and the left view of the lamellar PI-coated FBG.
In this work, we use the lamellar PI-coated technology to improve the tensile strength of the FBG, as depicted in Fig. 1(b). The grating area is coated by a lamellar PI film with a size of ${10^{(x)}} \times {2^{(y)}} \times {h^{(z)}}$ mm3.We can control the h to achieve different coating thickness. In order to achieve the higher force sensitivity, the h is less than the thickness of the original fiber coating, as depicted in Fig. 1(b). Due to the lamellar PI-coated FBGs are sensitive to the ambient humidity (humidification will expand and dehumidification will shrink, resulting in the strain occurred) and the ambient temperature, in order to eliminate the humidity-induced and the temperature-induced wavelength shifts, the ambient temperature and humidity, in our experiments, remain constant at 25.0 °C and 48 %RH, respectively.
2.2 Fabrication of the uncoated FBG sensors and the lamellar PI-coated FBG sensors
Before fabricating the uncoated FBG sensors, we first ordered the SMF-28e fiber (acrylate coated) from Corning. Then, the high-pressure hydrogen loading was carried out to improve the photosensitization of the fiber. After that, the mechanical stripping (or the chemically stripping) was needed to remove of the original fiber coating (the stripping length is 8 mm). After finishing the above steps, we begin to fabricate the uncoated FBG sensors. The schematic for fabricating the uncoated FBG sensors as shown in Fig. 2. The 248-nm UV laser beam, emitted from the excimer laser source, firstly passes through the cylindrical lens and then vertically passes through the phase mask. After passing through the phase mask, the laser beam is mainly divided into three parts (+1 order, -1 order and 0 order diffraction). The +1 order and -1 order diffraction will form the interference fringes near the phase mask. The bare optical fiber is mounted in the interference regions by the fiber clamp. Then, the interference fringes directly act on the optical fiber core and periodically modulate the refractive index of the fiber core to form the FBG. The FBG spectra can be monitored in real time by the optical spectrum analyzer. Finally, the annealing process is needed to remove the excessive hydrogen from the fiber and to improve the stability of the FBG. After finishing the above steps, the uncoated FBG sensors are fabricated (the grating length is 5 mm).
Fig. 2. Schematic for fabricating the uncoated FBG sensors using the UV laser based phase mask technology.
The fabrication process of the lamellar PI-coated FBG sensor is schematically illustrated in Fig. 3. The glass substrate has consisted of three pieces of quartz glass with the required sizes of ${15^{(x)}} \times {15^{(y)}} \times {1^{(z)}}$ mm3, ${15^{(x)}} \times {15^{(y)}} \times {1^{(z)}}$ mm3 and ${95^{(x)}} \times {40^{(y)}} \times {1^{(z)}}$mm3, respectively. Two smaller pieces of quartz glass are mounted onto the surface of the biggest one by using the high temperature adhesive, as shown in Fig. 3(a). Then, the uncoated FBG sensor, fabricated via using the UV laser based phase mask technology, is mounted onto the glass substrate by using high temperature tape. The high temperature tape not only fixes the uncoated FBG sensor, but also prevents the polyamide acid (PAA) solution from flowing out the forming area of PI film, as shown in Fig. 3(b). The PAA solution is injected into the forming area of PI film by using a needle, as shown in Fig. 3(c). After PAA solution deposition, the whole equipment is put into a high temperature oven (Fig. 3(d)) to finish polyamide imidization reaction. The heating process of imidization is that 100 °C (for 1 hour) $\to$160 °C (for 1 hour) $\to$220 °C (for 2 hours). After the heating process, the PI film is formed. Subsequently, turning off the oven and it is allowed to cool naturally to the required temperature such as 150 °C. When the temperature is 150 °C, we take out the whole equipment, remove the high temperature tape, and quickly separate the PI film from the glass substrate by using an art knife, as shown in Fig. 3(e). The last step is trimming the PI film to obtain the needed shape with a size of ${10^{(x)}} \times {2^{(y)}} \times {h^{(z)}}$ mm3, as shown in Fig. 3(f). After finishing the above steps, the lamellar PI-coated FBG sensors are fabricated.
3. Results and discussion
To better understand the influence of the lamellar PI coating on the uncoated FBG sensors, the comparison of the spectra of the lamellar PI-coated FBG sensors before coating (initial state) and after coating (coated state) were carried out, as depicted in Fig. 4. One can observe that compared the initial state, the central wavelengths in the coated state are decreased (i.e., the changes of central wavelength of the lamellar PI-coated FBG1 sensor, FBG2 sensor, FBG3 sensor and FBG4 sensor are from 1544.675 nm to 1542.750 nm, from 1544.925 nm to 1543.250 nm, from 1544.350 nm to 1543.475 nm, from 1544.770 nm to 1543.775 nm, respectively). The decrease in central wavelength of the lamellar PI-coated FBG sensors can attribute to the compression of grating period induced by the lamellar PI-coated film and the temperature. Because the lamellar PI film is formed and attached to the grating at high temperature, when the temperature from the high value drops to the room temperature, the grating period will be compressed, which leads to the decrease in central wavelength of the lamellar PI-coated FBG sensors. We also notice that the spectrum of the lamellar PI-coated FBG1 sensor shows a double peak due to birefringence caused by radial (or transversal) forces [31,32]. The radial forces per fiber length lead to anisotropic refractive indices ${n_x}$ and ${n_y}$ in the fiber core (as depicted in the insert of Fig. 4(a), i.e., left view). This effects a splitting of the Bragg peak into two separate peaks. Correspondingly, the lamellar PI-coated FBG1 reflects two single-peaked spectra at two Bragg wavelength ${\lambda _x}$ and ${\lambda _y}$, resulting in the expansion of bandwidth as shown in Fig. 4(a). While due to possessing the thinner lamellar PI-coated film (the thicknesses of the lamellar PI-coated film for the FBG1 sensor, FBG2 sensor, FBG3 sensor and FBG4 sensor are 93 um, 87 um, 70 um and 57 um, respectively), the double peak is not observed in the spectrum of the lamellar PI-coated FBG2 sensor due to the weaker effects of radial forces. However, the asymmetric axial (or longitudinal) strain/forces lead to the one-side distortion of the Bragg peak in the spectra [33]. In our experiments, the lamellar PI-coated FBG2 and FBG3 sensors have the obvious asymmetric lamellar PI-coated films, as shown in the inserts of Fig. 4(b) and 4(c) (top view), which may lead to the asymmetric axial and transversal strain/forces, resulting in the one-side distortion. It is worth to notice that the transversal force load can also lead to the one-side distortion in the spectrum (see Fig. S3 in response letter). Fortunately, the spectrum of the lamellar PI-coated FBG4 sensor is perfect, because the lamellar PI-coated FBG4 sensor has the weakest force effects, due to possessing the thinnest lamellar PI-coated film and the symmetric axial lamellar PI-coated film configuration.
Fig. 4. Comparison of the spectra of the lamellar PI-coated FBG sensors before coating (initial state) and after coating (coated state): (a) the lamellar PI-coated FBG1 sensor, (b) the lamellar PI-coated FBG2 sensor, (c) the lamellar PI-coated FBG3 sensor, (d) the lamellar PI-coated FBG4 sensor. The inserts represent the left view and the top view of the lamellar PI-coated area.
Subsequently, in order to know the force sensing characteristics of the lamellar PI-coated FBG sensors, the uniaxial tensile tests on the lamellar PI-coated FBG sensors are carried out by using a computer servo control universal testing machine (RS-8010 Model) by moving the upper capstan at a constant rate of 0.01 N/s. In order to avoid the break and sliding of fiber at clamping points [11,34], The lamellar PI-coated FBG sensors are vertically mounted at two capstans (i.e., moving capstan and fixed capstan) by wrapping, as shown in Fig. 5(a). The uniaxial tension applying to the lamellar PI-coated FBG sensors by the moving capstan is controlled by a computer. According to the Eq. (2) and Eq. (3), one can obtain the relationship between the wavelength shift ($\Delta \lambda$) and the axial applied force ($F$) is $\Delta \lambda = {\lambda _B} \cdot (1 - {P_e}) \cdot F/(E \cdot A)$ in the range of elastic deformations for the optical fiber. For a specific FBG sensor, the ${\lambda _B}$, ${P_e}$, E and A are constant. Thus, $\Delta \lambda$ is proportional to F. This is the main sensing idea of the lamellar PI-coated FBG sensors. It is worth to notice that A is the cross-section area of the lamellar PI-coated optical fiber (rather than the bare optical fiber). The measured results of the lamellar PI-coated FBG sensors are shown in Figs. 5(b)–(e). We can observe that the lamellar PI-coated FBG sensors exhibit the good linear relationships between the wavelength ($y$) and the applied force value ($x$). For the lamellar PI-coated FBG1 sensor, FBG2 sensor, FBG3 sensor and FBG4 sensor, the linear relationships are $y = 0.6954x + 1541.9295$(its force sensitivity is 0.6954 nm/N), $y = 0.7244x + 1542.2339$(its force sensitivity is 0.7244 nm/N), $y = 0.9205x + 1542.2424$(its force sensitivity is 0.9205 nm/N) and $y = 1.0051x + 1542.7943$ (its force sensitivity is 1.0051 nm/N), respectively. One can see that the thinner lamellar PI-coated film leads to the higher force sensitivity. In addition, the coefficients determination R2 of the lamellar PI-coated FBG1 sensor, FBG2 sensor, FBG3 sensor and FBG4 sensor are 0.9981, 0.9943, 0.9981 and 0.9992, respectively, indicating the lamellar PI-coated FBG sensors have good force sensing capacities. Notice that for every lamellar PI-coated FBG sensor, there is a yellow region (as shown in the inserts of Fig. 5(b)–5(e)) which departed from the fitted straight line. After careful checking, we confirm that the lamellar PI-coated FBG sensors (or the gratings) are not in the stretched state in this regions, because the lamellar PI-coated FBG sensors are mounted at two capstans by manually wrapping. Thus, the lamellar PI-coated FBG sensors are not straightened at the beginning rather than in a relaxed state. The first three data points keep a stable wavelength value, which also prove that. Thus, the lamellar PI-coated FBG sensors need to calibrate by using a standard electronic tension meter before they can be used. In additions, the lamellar PI-coated FBG sensors also exist two disadvantages (see paragraph 4, page 10 in response letter).
Fig. 5. (a) Experimental setup for the tension measurement of the lamellar PI-coated FBG sensors. (b)-(e) The measured results of the lamellar PI-coated FBG sensors. The yellow regions in (b)-(e) represent the lamellar PI-coated gratings are not in the stretched state.
Finally, the tensile failure tests, i.e., the FBG sensors are loaded up to failure at a constant ramp rate of 0.01 lbs/s, are carried out by using a rotary proof tester (VYT-200-C Integrated Module Controller, Vytran). In order to achieve the average ultimate tensile strength, we select four uncoated FBG sensors and four lamellar PI-coated FBG sensors to test. Figure 6(a) and 6(b) show the ultimate tensile data (i.e., 123 kpsi and 353 kpsi, 1 kpsi = 6.895 MPa) of the uncoated FBG-C sensor and the lamellar PI-coated FBG2 sensor, respectively. One can observe that the uncoated FBG-C sensor is fractured at the grating area because the fiber coating at the grating area was stripped and the grating is very fragile. However, the lamellar PI-coated FBG2 sensor is fractured at the left side of the grating area (rather than the grating area).
Fig. 6. Experimental results of tensile strength of the uncoated FBG-C sensor (a) and the lamellar PI-coated FBG2 sensor (b) at failure. (c) Comparison of the tensile parameters and the fracture location between the uncoated FBG sensors and the lamellar PI-coated FBG sensors at failure.
In order to clearly know the comparison of the ultimate tensile strength between the uncoated FBG sensors and the lamellar PI-coated FBG sensors, we measure the ultimate tensile strength of the FBG sensors, as shown in Fig. 6(c). One can see that the average ultimate tensile strength of the uncoated FBG sensors is 834.295 MPa while the average ultimate tensile strength of the lamellar PI-coated FBG sensors is 2302.930 MPa, which is 2.8 times higher than the value of the uncoated FBG sensors. In other words, the tensile strength can be greatly improved by coating a lamellar PI film on the uncoated FBG sensors. In addition, we also observe that the uncoated FBG sensors are fractured at the grating area while the lamellar PI-coated FBG sensors are fractured at the both sides of the grating area (rather than the grating area), probably because the mechanical stripping and the wiping process (before fabricating the uncoated FBG sensors using the UV laser based phase mask technology) cause great damage to the stripping area, especially, on the both sides of the grating area where are the stripping starting point and the stripping end point. In addition, we also notice that the thinner PI-coated film leads to the smaller tensile strength, which is the opposite of the force sensitivity obtained from the Fig. 5.
4. Conclusion
In summary, we have investigated the spectral characteristics, force sensing capabilities and tensile strength properties of the lamellar PI-coated FBG sensors. Our experimental results show that firstly, the lamellar PI-coated film has an important influence on the spectra of the uncoated FBG sensors. Secondly, the lamellar PI-coated FBG sensors exhibit good linear relationship between the wavelength and the applied force, which can be used for force sensing applications. And the thinner lamellar PI-coated film leads to the higher force sensitivity. Thirdly, the tensile strength results show that the average tensile strength of the lamellar PI-coated FBG sensors is 2.8 times higher than the value of the uncoated FBG sensors, illustrating that the tensile strength of the uncoated FBG sensors can be greatly improved by coating a lamellar PI film on the grating. More importantly, the lamellar PI-coated film can effectively protect the uncoated FBG sensors from a fracture at the grating area. Finally, the thicker PI-coated film leads to the bigger tensile strength, which is the opposite of the force sensitivity.
Funding
Science and Technology Planning Project of Shenzhen Municipality (JSGG20191129110033736); National Natural Science Foundation of China (6217030813); Basic and Applied Basic Research Foundation of Guangdong Province (2021A1515010964); Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20170412111625378, JCYJ20200109105810074, SGDX20190919094803949).
Disclosures
The authors declare no conflict of interest.
Data availability
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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