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Abstract
A dielectric/Ag-coated hollow fiber (HF) temperature sensor based on long-range surface plasmon resonance (LRSPR) is proposed and experimentally demonstrated. The structural parameters, including the dielectric material and layer thicknesses, are optimized through comprehensive theoretical analysis to achieve the best performance. By filling it with a high refractive index (RI) thermosensitive liquid, the GK570/Ag-coated HF temperature sensor with optimal structural parameters is fabricated. Due to the high sensitivity of the LRSPR sensor and the optimized design, the fabricated sensor achieves a temperature sensitivity of 3.6∼20.5 nm/°C, which is almost the highest among the optical fiber temperature sensors based on surface plasmon resonance reported experimentally. Moreover, the detection range of the proposed sensor can be easily tuned up to 170°C by varying the RI of the filled thermosensitive liquid, and the sensor performance remains stable. Considering that most temperature sensors using polydimethylsiloxane have a fixed detection range, this is an outstanding advantage that could expand the application field of the optical fiber temperature sensor.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Accurate temperature measurement plays a significant role in various applications such as manufacturing industry, environmental monitoring and chemical reaction. Compared with temperature sensors based on electronic systems, optical fiber temperature sensors have become more popular due to their advantages of small size, anti-electromagnetic interference, corrosion resistance and compact structure. To date, optical fiber temperature sensors based on various structure and sensing principles have been proposed and extensively studied, including fiber Bragg gratings [1], optical fiber interferometers [2–4] and whispering gallery mode [5], which exhibit attractive properties in temperature sensing applications. However, the sensors mentioned above suffer from relatively low sensitivity. Surface plasmon resonance (SPR) is a physical phenomenon excited at the interface between a metal layer and a dielectric layer. Owing to the high sensitivity to the change of surrounding refractive index (RI), SPR sensors have been widely applied in many fields such as disease diagnosis [6], food safety [7] and biochemical sensing [8–10]. By integrating with thermosensitive materials such as polydimethylsiloxane (PDMS) and liquid crystal, optical fiber SPR sensors show great potential in highly sensitive temperature measurement through monitoring the RI change induced by the ambient temperature variation [11–14]. In the past decades, several kinds of optical fibers have been applied in SPR temperature sensors, including single-mode fiber (SMF) [15,16], multi-mode fiber (MMF) [17,18], no-core fiber (NCF) [19], photonic crystal fiber (PCF) [20–22], hollow fiber (HF) [23,24] and depressed double cladding fiber (DDCF) [25]. However, the sensitivity of these sensors was not high enough and the width of the SPR dip was large, leading to a low figure of merit (FOM). Recently, some special optical fiber structures such as twin-core fiber [26], helical-core fiber [27] and multi-hole optical fiber [28] were employed to improve the sensitivity and FOM of the SPR temperature sensors. But the manufacturing process is complicated and time-consuming.
When a thin metal layer is sandwiched between two dielectric mediums that have similar RI, the surface plasmon waves on both sides of the metal layer would couple into two bound modes, namely the long-range surface plasmon polariton (LRSPP) and the short-range surface plasmon polariton (SRSPP) [29]. Correspondingly, the physical phenomenon of the optical excitation of these two modes is called long-range surface plasmon resonance (LRSPR) and short-range surface plasmon resonance (SRSPR), respectively. Due to the much lower loss, stronger surface electromagnetic field and larger penetration depth of the evanescent field, the LRSPR demonstrates itself as a narrower resonance dip with higher sensitivity to the surrounding medium than the conventional SPR and SRSPR, resulting in a better sensor performance [30,31]. Furthermore, LRSPR sensors have more options for the materials of the dielectric layer and the metal layer, which provides great room for performance improvement. However, there were few studies about the optical fiber temperature sensors based on LRSPR. In 2021, we reported an optical fiber temperature sensor which consists of a short piece of HF coated with a thin silver layer and an ethylene-vinyl acetate (EVA) dielectric layer sequentially [24]. Taking advantage of the LRSPR, the temperature sensitivity of 1.60∼5.21 nm/°C and FOM of 0.0215∼0.0453 °C-1 were achieved in the range from 20°C to 60°C. Although the above performances were better than most optical fiber SPR temperature sensors, the sensitivity and FOM of such HF LRSPR sensor could be further improved through the optimization of the structural parameters, including the dielectric material and the thickness of the metal and dielectric layers.
In this work, a comprehensive theoretical analysis about the influence of the structural parameters on the sensor performance is carried out. Among the dielectric materials that have similar RI with fused silica, GK570 (a kind of ethylene-tetrafluoroethylene copolymer) is adopted considering the feasibility of the deposition in the HF structure. The thicknesses of the silver and GK570 layers are also optimized theoretically to achieve the best performance. Compared with the EVA/Ag-coated sensor reported previously, the temperature sensitivity and FOM of the GK570/Ag-coated sensor were enhanced by approximately 2∼3 times. Moreover, the detection range of the proposed sensor can be easily tuned by varying the RI of the thermosensitive liquid filled in the HF. Due to the simple manufacturing process, high temperature sensitivity and tunable detection range, the GK570/Ag-coated HF LRSPR temperature sensor may have widespread applications in the field of biomedicine, environmental monitoring and manufacturing industry.
2. Sensor structure and theoretical analysis
2.1 Sensor structure and ray transmission model
The structure of the proposed HF LRSPR temperature sensor is shown in Fig. 1(a). A thin metal layer and a dielectric layer are sequentially coated on the inner surface of the fused silica supporting tube with an inner diameter of 700µm. Here, silver is chosen to be the metal material since it is well known as a good candidate for SPR sensors. The dielectric constant of the silver layer is given by the following Drude free electron model [32]
Fig. 1. (a) Structure of the HF LRSPR temperature sensor. (b) Lengthwise section with the ray transmission model and cross section.
Since the inner diameter of the HF is much larger than the wavelength of the input light, a ray transmission model illustrated in Fig. 1(b) is employed to theoretically calculate the transmission spectrum of the proposed sensor [31]. Here we only consider the meridional rays. The power distribution of the input light is close to Gaussian distribution with profile Pin(φ) expressed approximately as
where φ is the launching angle of the incident light, φ0 depends on the divergence angle of the light source and the coupling multi-mode fiber. The power of the output light transmitted through the sensor at a given wavelength can be expressed as2.2 Theoretical analysis and optimization
The ray transmission model presented above has been proved to be effective and accurate in predicting the performance of such kinds of HF SPR sensors in our previous work [31,34]. Therefore, it is also adopted for analyzing the performance of the proposed sensor and optimizing its structural parameters. Figure 2(a) shows the calculated transmission spectrum of the proposed HF LRSPR sensor with the structural parameters of n0 = 1.53, nd = 1.451, dd = 600 nm and dm = 50 nm, where n0, nd, dd and dm denote the RI of the filled liquid, the RI of the dielectric material, the thickness of the dielectric layer and the thickness of the silver layer, respectively. Obviously, two resonance dips appear in the spectrum, corresponding to the LRSPR and SRSPR, respectively. Since LRSPP has much smaller transmission loss than SRSPP, the LRSPR dip located at 591 nm has much narrower full width at half maximum (FWHM) than the SRSPR dip at 854 nm, which indicates that LRSPR will have much larger FOM than SRSPR. Figure 2(b) shows the dispersion curves of LRSPP and SRSPP in the GK570/Ag/SiO2 structure with same layer thicknesses as the sensor in Fig. 2(a). The two vertical dashed lines denote the resonance wavelength (RW) of the LRSPR dip (591 nm) and the SRSPR dip (854 nm) shown in Fig. 2(a). The shadow region represents the lights transmitted in the HF sensor with θ from 80° to 90°. The upper boundary of the effective RI is Neff =1.53, which represents the light with θ=90°. According to the power distribution in Eq. (2), the lights with θ>80° hold more than 99.5% of the whole power of the input lights and dominate the transmission spectrum. Thus, the light with θ=80° and Neff =1.53×sin80° is taken as the bottom boundary here. It can be seen that both intersections of the two dispersion curves and their RW lines are located in the shadow region. This agrees well with the spectrum shown in Fig. 2(a) since the SPR dips in the transmission spectra are the total results of all transmitted lights in the sensor.
Fig. 2. (a) Calculated transmission spectrum of the proposed HF LRSPR sensor. (b) Dispersion curves of the LRSPP and SRSPP. (c) Dispersion curves of LRSPP with different RI of the dielectric layer material. (d) The variations of sensitivity versus nd.
As the LRSPR has much better performance than the SPSPR, hereafter only the LRSPR dip is considered. Moreover, since the sensitivity of such wavelength interrogated optical fiber SPR sensor usually increases as the RW becomes longer, all performance comparisons are taken under the same given RWs in the analysis below.
The influence of the dielectric layer material on sensor sensitivity could be easily obtained from the dispersion curves of the LRSPP with different nd as shown in Fig. 2(c). For a given RW of 950 nm which is indicated as the vertical dashed line, the three dispersion curves intersect with the dashed line at different points. It means that the RI of the filled liquid (n0) is different to ensure the same RW of the LRSPR dip when nd is different. As shown in Fig. 2(c), when n0 changes the same value Δn0 = 0.002, the three dispersion curves show different RW shifts, which is 44.2 nm, 53.8 nm and 67.1 nm for nd = 1.48, 1.46 and 1.44, respectively. That is, the sensor with lower nd has a larger RW shift. The sensitivity could be calculated approximately by dividing the RW shift (Δλres) by the RI change (Δn0) as Sn=Δλres/Δn0. The variation of the calculated sensitivity at the RW of 950 nm along with nd in the range of 1.42∼1.50 is shown in Fig. 2(d). When nd is below 1.42 or above 1.50, the SPPs on both sides of the silver layer can hardly couple with each other since the RI difference between the dielectric layer and fused silica is too large. It can be observed that the sensitivity increases monotonically as nd decreases, which means that the lower nd, the better. There are few dielectric materials in this RI range that is available for deposition in the HF structure. EVA (RI = 1.474), GK570 (RI = 1.451) and polyvinylidene fluoride (PVDF) (RI = 1.42) are three alternatives. As shown in Fig. 2(d), the sensitivity of GK570/Ag-coated sensor is 29700 nm/RIU, 26% higher than that of EVA/Ag-coated sensor, which is 23600 nm/RIU. Although the PVDF/Ag-coated sensor shows the highest sensitivity of 45200 nm/RIU, there is no method to deposit a uniform PVDF film with thickness of several hundred nanometers in the HF until now. Considering that there are no other suitable dielectric materials with RI between 1.42 and 1.45, GK570 (ZEFFLE, DAIKIN Industries Ltd.) is the optimal choice for the dielectric layer material at present. Moreover, GK570 has a much higher heat-resistant temperature than EVA, which could significantly expand the temperature detection range of the HF LRSPR sensor.
Taking GK570 as the dielectric layer material, the thicknesses of the silver and GK570 layers may have an effect on the performance of the GK570/Ag-coated HF LRSPR sensor. Therefore, it is necessary to analyze the influence of these layer thicknesses for structural parameter optimization. The calculated transmission spectra of the sensors with GK570 layer thickness of 600 nm and different silver layer thicknesses are shown in Fig. 3(a). To compare the performance in a unified standard based on the operation wavelength range, the variation range of the RW is restricted to 600∼1000 nm. The solid curves show the transmission spectra of the three sensors with same RW at 600 nm, while the n0 are 1.4758, 1.4934 and 1.5114 for the silver layer thickness of 20 nm, 30 nm and40nm, respectively. The dashed curves show the transmission spectra when n0 decrease by 0.01 to 1.4658, 1.4834 and 1.5014. The shifts of RW are 18 nm, 78 nm and 47 nm for the sensor with dm of 20 nm, 30 nm and 40 nm, respectively. Obviously, a larger shift in RW means a higher sensitivity to the RI change. Figure 3(b) shows the entire sensitivities of the three sensors within the RW range of 600∼1000 nm. It can be observed that the sensitivity increases greatly as the silver layer thickness decreases. The sensor with dm of 20 nm shows the highest sensitivity, which is almost twice the sensitivity of the sensor with dm of 30 nm. The FOM of the sensors, which is defined as FOM = Sn/FWHM, is also calculated and shown in Fig. 3(b). Similar to the sensitivity, the FOM also increases with decreasing silver layer thickness. Thus, in order to achieve better sensor performance, the silver layer should be deposited as thin as possible. However, the silver layer in the proposed HF sensor is coated by the chemical liquid phase deposition method. Unlike the physical deposition method such as ion beam sputtering, the chemically coated silver layer will be island-shaped if it is too thin. Since it is impractical to deposit a uniform silver film thinner than 20 nm, the situation of dm below 20 nm is not presented here. Besides, it was almost impossible to obtain a uniform silver film with thickness less than 30 nm in the HF structure as reported in the previous studies [23,24,34]. In this work, we achieved a breakthrough in deposition and successfully coated a 23-nm-thick uniform silver film in the HF by precisely controlling the deposition process. The details are presented in the sensor fabrication section below. Taking 20 nm as the silver layer thickness, the influence of the GK570 layer thickness on the performance is also studied. The transmission spectra of the proposed sensors with dd of 600 nm, 700 nm and 800 nm are shown in Fig. 3(c). The FWHM of the resonance dip has small variations with the GK570 layer thickness. The sensitivities and FOMs within the RW range of 600∼1000 nm are shown in Fig. 3(d). We can find that a thickness variation of 200 nm in GK570 layer brings little change to the sensitivity and FOM, which means the proposed sensor has a large tolerance to the thickness of the GK570 layer without degrading the performance. This is an outstanding advantage that will greatly simplify the fabrication process.
Fig. 3. (a) The variations of the calculated spectra of the sensors with GK570 layer thickness of 600 nm and different silver layer thickness when n0 changes by 0.01. (b) The sensitivity and FOM comparison of the sensors in (a). (c) The calculated spectra of the sensors with silver layer thickness of 20 nm and different GK570 layer thickness. (d) The sensitivity and FOM comparison of the sensors in (c).
3. Sensor fabrication and experimental setup
A silver-coated HF was first fabricated by coating a thin silver layer on the inner surface of the fused silica capillary by the liquid phase deposition method that has been reported in details in our previous work [34]. The deposition time, temperature and flow rate of the solutions must be carefully controlled to obtain a uniform nanoscale silver layer, which is as thin as possible according to the theoretical analysis shown in Fig. 3(b). In our experiments, the temperature and flow rate of the solutions were controlled around 20°C and 50 ml/min, respectively. The deposition time was 17s, starting from the moment that the mixed solution flows into the fused silica capillary. Then, the GK570 solution (19% w/w in butyl acetate) was forced to flow through the silver-coated HF with a speed of 6.3 cm/min by a peristaltic pump, leaving a GK570 liquid film on the inner surface. After the drying process with nitrogen gas, the solvent is totally evaporated and a GK570 solid film is formed. A 5cm-long piece was cut from the fabricated Ag-coated and GK570/Ag-coated HF to estimate the thickness of the silver layer and the GK570 layer, respectively. Due to the poor conductivity of fused silica, it is difficult to obtain clear scanning electron microscope (SEM) photos at large magnifications because of the charging effect. Thus, a method of comparing the measured transmission spectrum with the theoretically calculated spectrum was adopted to estimate the thickness of the silver and GK570 layers. It has been proved in our previous study that the value of thickness obtained by this method is more effective and accurate than that estimated from the SEM images [31,34]. The measured and calculated transmission spectra of the silver-coated HF filled with polymethylphenyl silicone oil (PMPS) with RI of 1.5775 are shown in Fig. 4(a). The estimated silver layer thickness is as low as 23 nm, which is a breakthrough for the deposition of silver film using liquid phase deposition method. Similarly, the thickness of the GK570 layer was obtained by comparing the measured and calculated transmission spectra of the GK570/Ag-coated HF. As shown in Fig. 4(b), the positions of the interference dips in the calculated spectrum agree well with the measured results, where the thickness of the GK570 layer adopted in the theoretical calculation is 810 nm. The SEM photos of the cross sections of the Ag-coated and GK570/Ag-coated HFs are also shown in the insets of Fig. 4, indicating that a uniform silver layer and a GK570 layer have been coated on the inner surface of the supporting tube. Finally, a LRSPR temperature sensor was fabricated by filling the GK570/Ag-coated HF with high RI thermosensitive liquid, which is the mixture of two kinds of PMPS with RI of 1.5775 and 1.4751, respectively. Therefore, the RI of the thermosensitive liquid could be tuned in the range of 1.4751∼1.5775 by changing the mixing ratio.
Fig. 4. Comparison between the measured and calculated transmission spectra. (a) Ag-coated HF with silver layer thickness of 23 nm. (b) GK570/Ag-coated HF with GK570 layer thickness of 810 nm.
The experimental setup for measuring the transmission spectra of the HF LRSPR sensor under different temperatures is illustrated in Fig. 5. As shown in the inset, two segments of MMF with a core/cladding diameter of 200/220 µm were inserted into the HF and sealed with epoxy glue. Then, two SMA905 adaptors were installed on the ends of the lead-in and lead-out MMFs to connect the sensor with the light source and the spectrometer, respectively. The fabricated sensor was immersed in an oil bath with temperature adjustment range from room temperature to 200°C. Meanwhile, a thermometer with resolution of 0.1°C was placed near the sensor to monitor the real-time environmental temperature. The broadband light emitted from a halogen lamp was launched into the HF LRSPR temperature sensor via the lead-in MMF. Then the spectrum of the light transmitted through the sensor was recorded by a spectrometer (PG2000-pro, Ideaoptics) via the lead-out MMF.
4. Results and discussion
The RI sensing performance of the fabricated GK570/Ag-coated HF with 23-nm-thick silver layer and 810-nm-thick GK570 layer was first analyzed. The mixtures of PMPS with different RI were filled into the hollow core of the fiber and the transmission spectra were measured at 20°C as shown in Fig. 6(a). The corresponding sensed RIs are as labelled in the figure, which were measured at the wavelength of 589 nm by the Abbe refractometer just before each measurement. It can be observed that the LRSPR dip redshifts as the RI decreases. Due to the thin silver layer thickness of only 23 nm, the coupling strength of the surface plasmon waves on both sides of the silver layer is strong, resulting in a large distance between the LRSPR dip and the SRSPR dip. Since the operating wavelength range of the spectrometer is 200-1100 nm, only the LRSPR dip can be observed in the measured spectra, while the SRSPR dip is located at the infrared range and cannot be detected. The RW versus RI is measured and shown in Fig. 6(b), which could be well fitted by an exponential function with R-square of 0.9977. Calculating with the adjacent RW data points, the sensitivity increases quickly from 17334 nm/RIU to 58193 nm/RIU as the RW redshifts from 600 nm to 900 nm approximately, which is much higher than that of most optical fiber SPR sensors. Taking advantage of such high RI sensitivity, the proposed GK570/Ag-coated HF LRSPR sensor filled with thermosensitive liquid will naturally achieve high performance in temperature sensing.
Fig. 6. (a) The measured transmission spectra of the fabricated GK570/Ag-coated HF filled with liquids of different RI. (b) The RW versus RI and the exponential fitting curve.
Three HF LRSPR temperature sensors named TS20, TS70 and TS120 were fabricated in the experiments, with the detection range of 20∼65°C, 70∼115°C and 120∼170°C, respectively. The RI and thermo-optic coefficient (TOC) of the filled thermosensitive liquids are 1.4843, 1.4988, 1.5129 and -4.0514 × 10−4, -4.1247 × 10−4, -4.2082 × 10−4 RIU/°C for TS20, TS70, TS120, respectively. The TOC of each liquid was obtained by linear fitting the RIs measured by the Abbey refractometer under different temperatures as reported in our previous work [24,28]. The measured transmission spectra of each sensor under different temperatures are shown in Figs. 7(a)-7(c). Due to the negative value of the TOC, the RIs of the thermosensitive liquids decrease as the environmental temperature increases, resulting in a redshift of the LRSPR dip. The RW versus temperature is measured and shown in Fig. 7(d). It can be seen that for each sensor the RW shifts gradually from 600 nm to 1000 nm as the temperature increases. Figure 7(e) shows the temperature sensitivity which is calculated from the RW data with the equation ST=Δλres/ΔT, where ΔT is the variation of temperature. The sensitivities of TS20, TS70 and TS120 are 3.7∼19.9 nm/°C, 3.6∼20.5 nm/°C and 3.4∼19.0 nm/°C respectively, which are 8.2 nm/°C, 8.5 nm/°C and 7.8 nm/°C in average. The FOMs of each sensor are also calculated with the equation FOM = ST/FWHM and shown in Fig. 7(f), which are 0.0563∼0.0974 °C-1, 0.0536∼0.0941 °C-1 and 0.0485∼0.0904 °C-1 for TS20, TS70 and TS120, respectively. It can be seen that the three sensors have very similar sensitivity and FOM, indicating that the performance of the proposed HF LRSPR temperature sensor remains stable when the detection range changes. This is an excellent characteristic that the sensor could adapt itself to different applications by tuning the temperature detection range with guaranteed performance.
Fig. 7. The measured spectra under different temperature of the sensors with different detection range; (a) 20∼65°C, (b) 70∼115°C, (c)120∼170°C. The comparison of performance of the three sensors; (d) RW, (e) sensitivity, (f) FOM.
Temperature cycling test was conducted to investigate the repeatability and stability of the proposed sensor. With TS20 sensor immersed in the oil bath, the temperature first increased from 20°C to 65°C and then decreased to 20°C with a step of 5°C. The above process was repeated three times. The RW versus temperature was measured and shown in Fig. 8. The RW redshifts gradually from 626 nm to 997 nm as the temperature increases, and then blueshifts back to 626 nm as the temperature decreases. It can be observed that the sensing results during heating process is consistent to that of cooling process. The RW at each temperature remains basically unchanged over the three cycles. The standard deviations of the RW are calculated to be 0.29 nm, 0.37 nm, 0.33 nm, 0.42 nm, 0.49 nm, 0.53 nm, 0.72 nm, 0.93 nm, 1.13 nm and 1.23 nm for temperature from 20°C to 65°C with a step of 5°C, respectively. By dividing the RW standard deviation by the sensitivity at each temperature, the standard deviations of the temperature can be obtained, which is calculated to be less than the thermometer resolution of 0.1°C for all measured temperatures. The above results demonstrate that the proposed fiber temperature sensor exhibits excellent repeatability and stability.
Some recently reported experimental works of the optical fiber SPR/LRSPR sensors for temperature sensing with their performances are listed in Tab. 1 for comparison. The presented GK570/Ag-coated HF LRSPR sensor has almost the highest sensitivity among all competitors. The helical-core fiber sensor [27] has similar high sensitivity, however, the sensor in this work has much larger and tunable temperature detection range. Moreover, compared with the EVA/Ag-coated HF sensor reported previously [24], both sensitivity and FOM of the presented sensor are enhanced by 2∼3 times approximately, proving that the optimization and improvement of material, structure and fabrication technique in this work are very effective.
Table 1. Comparison of fiber-based SPR/LRSPR sensors for temperature sensing
5. Conclusion
In conclusion, a GK570/Ag-coated HF LRSPR temperature sensor with ultra-high sensitivity and tunable detection range has been proposed and demonstrated. By the comprehensive theoretical analysis about the influence of the structural parameters on the sensor performance, GK570 is adopted as the optimal dielectric layer material considering the feasibility of the deposition in the HF structure. The thicknesses of the silver and GK570 layers are also optimized theoretically to achieve the best performance. On this basis, the proposed sensor is fabricated by successively depositing the silver and GK570 layers with optimal thicknesses of 23 nm and 810 nm on the inner surface of the HF. The realization of the 23-nm-thick uniform silver layer is a breakthrough in the chemical liquid phase deposition method for silver deposition in HF structure, which nearly hits the thickness limit of uniform silver layer deposited by such method. The fabricated GK570/Ag-coated HF LRSPR sensor achieves the temperature sensitivity as high as 3.6∼20.5 nm/°C, which is almost the highest among all experimental optical fiber temperature sensors based on SPR technique reported previously. Moreover, the detection range of the proposed sensor can be easily tuned up to 170°C by varying the RI of the filled thermosensitive liquid and the performance of the sensor remains stable. Due to the simple manufacturing process, high temperature sensitivity and tunable detection range, the GK570/Ag-coated HF LRSPR temperature sensor may have widespread applications in the field of biomedicine, environmental monitoring and manufacturing industry.
Funding
National Natural Science Foundation of China (61975034).
Acknowledgments
The authors thank DAIKIN Industries Ltd for sample provision.
Disclosures
The authors declare no conflicts of interest.
Data availability
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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