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Abstract
This paper reports a dual-core photonic crystal fiber (PCF) based temperature sensor designed to measure the temperature of water as an analyte. The sample sneaked into the smallest air hole in cladding, which serves as analyte core, and the center of the cladding serves as silica core. Using the finite element method (FEM), the optical energy shifted from the silica core to analyte core is examined to fulfill the phase-matching condition. The wavelength sensitivity of temperature in the water sample is recorded as 25000 nm/refractive index unit (RIU) with a detection level of 0.0012 RIU. To the best of our knowledge, this is the highest sensitivity for a PCF based temperature sensor, which is obtained for analyte refractive indices from 1.3328 to 1.3375. The maximum temperature sensitivity of 818 pm/° C is achieved from this proposed structure. Moreover, the temperature level varies from 30° C to 70° C to obtain the highest sensitivity response. This structure can be applied in the detection of biomolecules, organic chemicals, and biological and biomedical analysis upon modification as well.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
PCF has a great ability to form cladding throughout the core of the fiber by using photonic crystal. A photonic crystal is formed using an array of tiny air holes in periodic order for a particular length of the fiber. For transmission spectra, the existence of air holes assists in guiding a modulated response depending on the kind of materials introduced in it [1]. A mentionable use of PCF has been seen in temperature sensing application because an enhanced level of sensitivity can be achieved by tuning the refractive index.
PCF based sensing technology is more attracting due to its optical superb performance. PCF are now using in a very diverse area of application such as chemical sensor [2], gas sensor [3], salinity sensor [4], pressure sensor [5,6], pH sensor [7], strain sensor [8,9], humidity sensor [10,11], temperature sensor [12,13], magnetic sensor [14,15], dew detection or dew sensor [16], current sensor [17], viscosity sensor [18] and many more. Among those applications, PCF based temperature sensor plays a significant role in the area of temperature sensing. Compared with conventional temperature sensor in this work a microstructured sensor has been suggest with a high degree of accuracy result. More enhancements in sensing applications, various formatted sensors like D-shaped fiber [19], tapered fiber [20], U-shaped fiber [21] and two Peanut-shaped fiber [22] have been developed. Various sensing mechanisms may be regarded as a conceptual application in the diverse field of biochemical, salinity detection, antibody detection, temperature detection and glucose detection [23–27].
The PCF based temperature sensor plays an important role due to its compact structure and design flexibility. The wavelength sensitivity of this structure is numerically obtained by calculating the confinement loss spectra in different temperature of water. For example, a microfluidic refractive index based PCF is proposed by Darran K. C. Wu et al. has directional coupler architecture and achieved a higher refractive index (RI) sensitivity [28]. In this structure, the PCF based liquid-filled analyte performs as a directional coupler which assists in light coupling from the silica core mode to the analyte core mode. Selectively filter through only one air hole among all of the liquid-penetration patterns, ultra-high sensitivity has been obtained and this process is known as a well accessible manner [29–31].
In this paper, the impacts of the gap between the water-filled analyte core and the default core are examined theoretically on the mode characteristics as well as on sensitivity. In recent years, the fiber-based temperature sensor has attracted our interests for its numerous unique characteristics. The smallest hole in this structure is designed to form analyte core and the empty region between two big air holes acts as silica core. By using this approach, the sensitivity is improved. The mode field transmitted into water infiltrated core, due to the existence of two bigger air holes around the center of the PCF assists to fulfill the condition of resonance coupling between liquid and silica modes. It can be observed from the analyses that the wavelength peaks have a blue shift and the confinement loss varied with the increase of temperature. The benefit of this proposed structure is that it has higher temperature sensitivity with good linearity.
2. Structural design
The cross-section view of the proposed PCF sensor is shown in Fig. 1 which is based on the interrelationship of analyte core with the silica-based core. The air holes are arranged in a modified quasi pattern. The lattice pitch of the air holes is p = 2 $\mu$m. In this structure, the middle hole is eliminated which performs as silica core. The diameter of the analyte core is d2=0.6 $\mu$m which is located vertically with the silica core and marked as red color. The diameter of the air holes in blue beside the silica core is d1 = 2.2 $\mu$m. The remaining air holes in gray color are d3=1.4 $\mu$m. The diameter of the outer cladding layer is 7.7 $\mu$m. The measurement of d2 and d1 is taken more carefully because the variation of those diameters will impact more on the result.
Fused silica is used as the background material of the fiber, whose refractive index is calculated by the Sellmeier equation (1) [32], as
Figure 2 shows the connection diagram of the proposed temperature sensor. For connecting the sensor there is used a broadband optical source, an isolator, a circulator, an optical spectrum analyzer (OSA) and the proposed sensor. The broadband optical source is connecting to the isolator which is connected to the circulator. On the other hand, the temperature sensor is connected to the circulator which reflects a loss peak in the OSA that has also a connection to the circulator.
3. Results analysis and discussion
3.1 Mode propagation
In this investigation, four modes are considered for the result analysis. From those, two modes act as core modes and the remaining two act as coupling mode. Among the core mode, one is analyte core mode and another is the silica core mode. All these four modes have two polarizations such as x-polarization and y-polarization. Here, Fig. 3(a) and 3(b) refer to the silica mode propagation for x-polarization and y-polarization respectively. The analyte core mode is shown in Fig. 3(c) and 3(d). Figure 3(e) and 3(f) represent the coupling mode where the light passes more tightly through silica core than analyte core. The last modes in Fig. 3(g) and 3(h) are also coupling mode in which the light passes more tightly through analyte core than silica core.
Fig. 3. Mode propagation of silica core mode for (a) x-polarization (b) y-polarization, defect core mode for (c) x-polarization (d) y-polarization, coupling between silica and defect core where light pass more tightly in silica core for (e) x-polarization(f) y-polarization and coupling and coupling between silica and defect core where light pass more tightly in defect core for (e) x-polarization(f) y-polarization and coupling.
3.2 Effective refractive index and resonance
As the melting point of silica is 1670° C. So normally in PCF-based temperature sensor silica is used as a base material. The temperature from 30° C to 70° C has applied in this structure which is comparatively lower than the melting point of silica.
The effective refractive index and confinement loss variation with respect to wavelength are plotted in Fig. 4(a). In Fig. 4(a), X-axis represents the wavelength, the left side of Y-axis represents the real effective index and the right side of the Y-axis represent the confinement loss for 60° C temperature. The blue color and black slopes are representing the real part of the silica core and liquid filled core respectively. It is shown that both slopes are downward by the increment of wavelength and at point 1.33 $\mu$m where they intersect. On the other side, the red color curve is representing the confinement loss in dB/m. The confinement loss curve is increasing by the increment of wavelength at a certain time and then it will be again decreased. The peak of the confinement loss curve is found at the wavelength where the previous two slopes are intersected and this is at 1.33 $\mu$m. The same procedure is done in Fig. 4(b) for 70° C temperature where the intersection and peak confinement loss are found at the wavelength 1.32 $\mu$m. We know that the effective refractive index value will be varied at a large portion beside the peak resonance point. The same behavior has been observed nicely from Fig. 4(b).
Fig. 4. Real part of the effective refractive index of silica core mode and liquid core mode and loss spectra with respect to wavelength variation for (a) 60° C and (b) 70° C.
3.3 Confinement loss evaluation
The confinement loss is a very important factor of a fiber. This is calculated by the following equation (3) [33,34]. Figure 5 depicts the confinement loss spectra for coupling mode between silica and analyte core. The confinement loss curve is plotted by the wavelength range of 1.31-1.39 $\mu$m at a different temperature ranging from 30° C to 70° C. The losses and their resonance point in wavelengths are increased with the decrement of temperature. The range of the peak loss is 13.2 to 18.8 dB/m where the maximum loss is 18.8 dB/m for 30° C at $\lambda$=1.38 $\mu$m. The others loss peak wavelengths at 1350, 1340, 1330 and 1320 nm for temperature 40° C, 50° C, 60° C and 70° C respectively.
Fig. 5. Confinement loss spectra of coupling mode under different temperature as a function of wavelength.
3.4 Peak wavelength vs. temperature
From the confinement loss spectrum, the peak at a different wavelength for different temperatures is found. The relationship between the peak wavelength and the temperature is shown in Fig. 6, where the range of temperature is 30° C to 70° C. It shows that for the temperature decreasing from 70° C to 40° C the peak wavelength is increasing linearly but for 30° C the peak wavelength is increased more than others. As the sensitivity is depended on peak wavelength, so here peak wavelength is very important. If the difference in peak wavelength is more, the sensitivity will be better.
3.5 Wavelength shift vs. temperature
The wavelength shift is calculated from the peak wavelength of two nearby temperatures. As the range of temperature 30° C to 70° C is taken for all measurement of the proposed sensor, the wavelength shift will be found for 40° C to 70° C. In Fig. 7, the wavelength shift spectrum is shown with respect to temperature. It is depicted that the wavelength shift increased linearly with the increment of temperature. Note that, good linearity can help us to estimate unknown values.
3.6 Sensitivity measurement
The sensitivity is calculated by S = $\Delta \lambda _{peak}$/$\Delta$n, where, $\Delta \lambda _{peak}$ is the peak wavelength and $\Delta$n is the refractive index difference [35]. The sensitivity curve is shown in Fig. 8. From the spectrum, it is noticeable that with the decrement of the temperature, the sensitivity is increased. From the above formula, it is clear that sensitivity depends on peak wavelength and the more peak wavelength ensures the high sensitivity. In Fig. 6, it is shown that peak wavelength is increased more when the temperature is shift from 40° C to 30° C than others. So in 40° C temperature, the sensitivity will be highest which is 25000 nm/RIU. The others sensitivities are 17391 nm/RIU, 14285 nm/RIU and 12765 nm/RIU for 50° C, 60° C, and 70° C respectively which is shown in Table 1. In Table 1, there is also noted the peak wavelength and the refractive index difference at different temperatures.
Table 1. Sensitivity variation with different temperature.
3.7 Effect of pitch distance on sensitivity
The investigation results are depended on the structural design. So, if the pitch distance of the air holes or diameter of the core are changed then it affects the result more. The proposed sensor is investigated using the pitch p = 2 $\mu$m as standard. The loss curve is illustrated in Fig. 9 concerning the variation of pitch distance. With the increment of the pitch, the peak wavelength is decreased more than the increment of the peak wavelength when the pitch is decreased. So, the sensitivity is increased for pitch increment and decreased for pitch decrement. In this spectrum the dotted line is defined for 30° C and the straight line is for 40° C. It is noticeable that the loss is more at 30° C compared to 40° C. On the other hand, the peak wavelength is more at 40° C than 30° C. From Table 2, it is shown that the sensitivities for pitch 1.9, 2 and 2.1 $\mu$m are 16666, 25000 and 25000 nm/RIU respectively.
Table 2. Performance of proposed structure for different pitch values.
Table 3. Sensitivity variation in different values of d2 diameters.
Table 4. Comparison of the sensitivity performance of proposed sensor with previously published sensors.
It is also seen that for the proposed model, if the diameter of pitch is decreased, the sensitivity is also decreased. But if the pitch distance is increased, the sensitivity is the same as the default pitch distance sensitivity.
3.8 Effect of the core diameter on sensitivity
Like pitch distance variation, the core diameter also varies the result in Table 3. Here, the standard value of core diameter d2 is selected as 0.6 $\mu$m as it gives better performance. Like Fig. 8, the similar analysis is done in Fig. 10 for the variation of core diameter. Here, the peak wavelength is increased with the increment of the core diameter. For the proposed sensor when the core diameter is increased the sensitivity is decreased slightly but when the diameter is lessened the sensitivity is reduced very much. The sensitivity is highest for d2=0.6 $\mu$m than other variations.
3.9 Temperature sensitivity calculation
The relationship between the wavelength of the loss peak ($\lambda$) and the temperature is shown in Fig. 11. The wavelength has a linear relationship with temperature and the temperature sensitivity is 818 pm/° C.
In Table 4, the comparison of the sensitivity performance of proposed sensor with previously published sensors has been visualized. It is clearly displayed that the proposed sensor shows better performance.
4. Conclusion
In this work, a microstructured photonic crystal fiber-based temperature sensor has been suggested. The state and art of the suggested sensor have been designed and investigated by FEM based commercially available software package COMSOL Multiphysics version 4.3b. Through the FEM based rigorous numerical investigation, sensor exposes outstanding performance, low confinement loss. In the design, the smallest hole is injected with water which is used as analyte core. Besides, the center point of the structure is served as a defect core. A comprehensive numerical investigation has been carried over from the wavelength domain $\lambda$ = 1.27 $\mu$m to 1.4 $\mu$m by ensuring the RI of the analyte departs from 1.3328 to 1.3375. From this probe, it shows an enhanced sensitivity of 25000 nm/RIU and maximum temperature sensitivity of 818 nm/° C. Our best of knowledge the attained wavelength and temperature sensitivity is better than previously reported PCF based temperature sensors. The detection limit was found to be 0.0012 RIU with the changes of the refractive index. In our proposed model, the temperature is varied from 30°C to 70°C and we obtained the highest sensitivity for temperature 30°C and 40°C. Moreover, the confinement loss spectrum is shifted due to the change in temperature and that shift corresponds to change in peak wavelength. Based on these tremendous sensing performances the proposed dual-core PCF based sensor opens a new window for further temperature sensor research.
Acknowledgments
The authors are grateful to all of the subjects who have contributed to this research.
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