Highly Efficient D-type Photonic Crystal Fiber Surface Plasmon Resonance Sensor for Same Space–Time Temperature and Refractive Index Detection

In order to realize temperature and refractive index (RI) detection of the analyte in the same space–time, a dual parameter detection photonic crystal fiber (PCF) optical sensor is proposed in this paper. Meanwhile, it can effectively avoid the interference of analyte temperature with RI detection. Furthermore, a dual polished D-type structure is designed to provide mutually independent detection channels for temperature and RI, respectively. In addition, silver is used as the metal layer material, and polydimethylsiloxane (PDMS) and titanium dioxide (TiO2) are used as the temperature-sensitive materials and RI-sensitive materials, respectively. This structure design enables the two detection results to be output in parallel without interfering with each other. The proposed optical fiber sensor has a wide detection range and good detection performance. The temperature detection range can be as wide as −60 to 100°C, and the RI detection range is 1.30–1.40. The best temperature resolution can be 1.42 × 10−2 RIU, and the temperature wavelength sensitivity is 0.7 nm/°C. While the RI wavelength sensitivity is up to 1.25 × 104 nm/RIU, and the wavelength resolution is 8 × 10− 6 RIU. The detailed fabrication process of the sensor is given in this paper, as well as the practical application scenarios. Therefore, the optical fiber sensor proposed in this paper has research significance for the development of dual parameter detection optical fiber sensors.


Introduction
Temperature and RI are fundamental parameters of the properties of substances and thus are important to be monitored in fields such as oil extraction, biochemistry, and transportation [1][2][3]. In particular, during the preparation of chemical synthesis often chemical reactions accompany the process. Thus, the control of temperature and concentration of chemicals in each step of synthesis is crucial to preparation [4][5][6]. In addition, the ambient temperature interferes with the RI of the analyte during the analytical process, which affects the accuracy of the detection results. Therefore, it is important to investigate a sensor that can monitor the temperature and RI of the analyte at the same time and space.
PCF-SPR sensors which combine photonic crystal fiber with SPR technology have become popular for research [7,8]. PCF-SPR sensor not only has the advantages of high temperature and chemical resistance [9,10], but also has strong dispersion controllability [11] and good nonlinearity [12,13]. Therefore, it has important research significance and wide application prospects in the field of sensing [14]. And PCF-SPR sensor works based on the material of the dielectric layer being sensitive to RI [15]. Therefore, when the RI near the sensitive layer changes, the resonance peak also changes. With this principle, RI detection of analytes can be achieved. Meanwhile, according to this principle, the RI of 1 3 temperature-sensitive materials changes with temperature, enabling temperature detection.
There have been a large number of research results presented so far for photonic crystal fiber sensors [16][17][18]. In the design of the sensor structure, many studies have used the deposition of a sensitive material inside the fiber or filled it to achieve RI detection [19][20][21]. For example, the hollow core negative curvature fiber (HC-NCF) temperature sensor proposed by Chen et al. is filled with a thermos optical mixture of toluene and chloroform after depositing a gold layer on the inner wall of the air bore, and the temperature measurement sensitivity is 1.071 nm/°C [22]. Fang et al. proposed a RI detection PCF sensor with two annular microfluidic channels with Ag-TiO 2 sensitive layer coated on the inner wall of the channels and the sensitivity of 60,600 nm/ RIU for RI of 1.33-1.36 [23]. In addition, a large number of scholars have also investigated external sensing forms [24]. However, the fabrication process of the internal sensing structure is complicated by the coating of the inner wall of the air hole. Moreover, the tension effect when the analyte is injected into the air hole makes the experiment more difficult. The coreless photonic crystal fiber sensor proposed by Yin et al. used Ag and PDMS as sensitive materials to achieve a maximum spectral sensitivity of 5200 nm/RIU and 7.2 nm/°C for the sensor, respectively [25]. Conventional optical fiber sensors usually have only one detection channel which greatly reduces the efficiency of the sensor [26,27]. As such, Wang et al. proposed an asymmetric doublepolished D-type structure, with the maximum wavelength sensitivity of 14,000 nm/RIU for both channels when the RI is changed from 1.33 to 1.40 [28]. The problem of curved surface coating non-uniformity is solved by depositing sensitive materials on the polished surface. The asymmetric structure design of the dual side channels enables simultaneous detection of two detection channels and parallel output of detection results.
In this paper, an optical fiber sensor that enables the detection RI and temperature in the same space-time is investigated. The novelty of the research lies in the possibility of simultaneous detection of the refractive index and temperature of an analyte at a certain detection site. And it has an outstanding contribution to the chemical field, which can be used for continuous monitoring of chemical reactions. Moreover, it can achieve a wide range of detection, with a temperature detection range of −60 to 100°C and an RI detection range of 1.30 to 1.40. The best temperature resolution is up to1.42 × 10 −2 RIU, and the temperature wavelength sensitivity is 0.7 nm/°C. The refractive index wavelength sensitivity is 1.25 × 10 4 nm/ RIU, and the wavelength resolution is 0.8 × 10 −7 RIU. In addition, with the combination of multiple parameter value detection in one small sensor design, it is of great value for the development of miniaturization and integration of sensors. In conclusion, the simulations of optical fiber sensors proposed in this paper have profound implications for the exploitation of PCF sensors for multi-parameter detection.

Design of Mode
The model of the optical fiber sensor is illustrated in Fig. 1. A dual polished D-type structure is used, with polishing depths of d 1 = 2.50 μm and d 2 = 2.70 μm on both sides, and the two polishing surfaces are parallel to each other. By creating the polished surface the distance between the deposited metal and the fiber core can be made smaller, and fiber core energy is better coupled to the metal surface. The air hole in the center with a radius (r 0 ) of 0.2 μm, the small air hole radius (r 1 ) of 0.4 μm, and six outer air holes (r 2 ) having a radius of 0.6 μm are set inside the fiber. The spacing between the large air holes and the small air holes is 0.2 μm. The air holes can prevent the energy leakage of the fiber core and effectively confine the energy to the core area. Metal is the key material to excite SPR. As the metal layer material, silver has higher sensitivity and sharper resonance peaks, which is beneficial to the analysis of detection results. The thickness of the silver layer in the temperature detection channel (t 2 ) and the refractive index detection channel (t 3 ) of the proposed sensor are set to 30 nm and 50 nm, respectively. The deposition of the second layer is mainly to prevent the oxidation of the silver layer and improve the sensor performance, and the chemical stability of TiO 2 and its high refractive index effectively promote the phase matching of the core mode and plasma mode. Therefore, TiO 2 with a thickness of 15 nm is deposited outside the metal for the RI detection channel. In addition, the refractive index detection channel uses a micrograting structure to deposit metal and sensitive layers, and the micrograting structure can effectively improve the detection performance of the sensor. The number of microgratings is set to 50 with the spacing of 25 nm. And the temperature detection channel uses temperature-sensitive materials to acutely sense the ambient temperature change and achieve the purpose of temperature detection through the offset of the resonance peak. The temperature-sensitive material used in this paper is PDMS, which has stable chemical properties and is non-toxic. It is cured and coated on the outside of the silver layer with the thickness of 0.25 μm.
Metal Ag's dielectric constant is expressed by the Drude formula as Eq. (1), λ denotes a light wavelength of the incoming light, λ c = 17.614 µm, λ p = 0.14541 µm [26]: Calculation of the RI of TiO 2 can be done by the following method [24]: The equation of PDMS with temperature [25], where n 0 = 1.4204, is the refractive index at 22 °C, y is the photothermal coefficient with a value of − 4.66 × 10 -4 RIU/°C, and ΔT is the amount of temperature change.
Optical fiber sensor transmission loss equation is represented as Eq. (4), Im (n eff ) which means the imaginary part of effective RI, where λ is the incident wavelength [29]: The wavelength sensitivity (Sw) as a fundamental performance parameter of PCF sensors can be determined by the following method [27]: In which, ∆λ peak is the value of the difference between two nearby resonant wavelengths, where ∆n a is the change value of the RI. When calculating the temperature wavelength sensitivity ∆n a is the temperature change value. The amplitude sensitivity (S a ) is the basic performance parameter [29], which is given by the following equation: For Eq. (6), the transmission loss is given by α(λ, n a ) with RI is n a , ∂α(λ, n a ) denotes the transmission loss change value after the change in RI, and ∂n a is the RI change value of the analyte. While the wavelength resolution (R) is given by Eq. (7), ∆λ min is 0.1 nm [30].
When RI is 1.33 and temperature is 0°C, the effective RI of the core mode and the electric field distribution calculated by the finite element method according to COMSOL Multiphysics software are shown in Fig. 2. According to the transmission loss equation as Eq. (4), it is known that the effective refractive index imaginary part is positively correlated with the transmission loss. Indicating that the larger the effective refractive index imaginary part is, the more obvious the plasma resonance phenomenon is. Among the following modes, the effective refractive index imaginary part is the largest in the y-polarization direction couple mode, and the plasma resonance phenomenon near the temperature detection channel is the most obvious. Therefore, the y-polarization direction even mode has been chosen for the sensor performance analysis.
When the RI of the fiber sensor detects the analyte is 1.33 and the temperature is 0°C, the transmission loss curve and the dispersion relationship between the core mode and the plasma mode are illustrated in Fig. 3. In which the points (I) and (II) in the figure are the corresponding phase matching points when the effective refractive indices of the plasma mode and the core mode are identical. Two obvious resonance peaks appear in the loss curve at the phase matching point, corresponding to the temperature and RI detection channel, with resonance wavelengths of 383 nm and 603 nm, respectively. In addition, Fig. 3(a), (b) show the electric field distributions of the fiber core mode and the plasma mode, respectively. Figure 3(c) illustrates the energy distribution of the core loss mode corresponding to the resonance peak. And the more intense surface plasma resonance can be observed on the temperature detection channel. Figure 3(d), (e) show the electric field energy distribution of the core and plasma modes, respectively. Figure 3(f) shows the electric field energy distribution of the core mode at the phase matching point (II), where the strongest surface plasma resonance is observed at the RI detection channel. Beyond that, the two resonance peaks in Fig. 3 do not interfere with each other, which can fully realize the simultaneous detection of temperature and RI.

Results and Discussions
It is crucial to study the inner structural factors of optical fiber sensors since they have an impact on mode coupling. Figure 4 shows the effect of changing air hole parameters on the performance of the sensor at the temperature of 0°C and RI is 1.33. Figure 4(a) shows the transmission loss curve obtained by varying the size of the central air hole r 0 . The figure shows that as r 0 is changed from 0.1 to 0.3 μm, the loss curve is slightly red-shifted and the loss   Figure 4(b) represents the loss curve corresponding to the change of the air hole radius (r 1 ) from 0.2 to 0.6 μm. The resonance intensity gradually decreases with the increase of r 1 . When r 1 is too large, the resonance peak tends to flatten. Such condition is not conducive to the analysis of detection results. Setting r 1 to 0.4 μm is most appropriate. Figure 4(c) shows the loss curve corresponding to the increase in the radius of the outer air hole (r 2 ), and the loss curve hardly changes during the change from 0.60 to 0.80 μm. And r 2 is set to 0.7 μm. Figure 4(d) shows the effect of adjusting the spacing of the air holes (d 0 ) on the transmission loss. It can be seen from the figure that with the increase of d 0 , the transmission loss curve is slightly shifted. And d 0 is set to 2 μm. Figure 5 shows the influence of polishing depth on the loss curve of both sides. The polishing depth of the temperature detection channel gradually increases, the loss curve in the temperature detection wavelength range (320-500 nm) is slightly red shift and the resonance intensity gradually decreases. With the temperature increases, the resonance peak blue shift, and the highest temperature sensitivity is 0.6 nm/°C at d 1 = 2.5 μm. While the polishing depth of the RI detection channel (d 2 ) changes corresponding to the RI detection wavelength range (500-700 nm) resonance wavelength almost does not change, and the transmission loss gradually decreases. Therefore, d 1 and d 2 are set to 2.5 μm and 2.7 μm, respectively. Moreover, the sensitive layer parameters have an important effect on the performance of the dual parameter detection fiber sensor. Figure 6(a) shows the spectrum of the variation of the loss curve during the change of the thickness of the metal layer t 1 from 20 to 40 nm for the temperature detection channel. As t 1 increases, the corresponding resonance curve is a red shift and the resonance intensity  Table 1, it is found that the temperature sensitivity corresponding to t 1 = 30 nm is the largest with 0.6 nm/°C. Figure 6(b) displays the change curve of amplitude sensitivity, which can be seen that the amplitude sensitivity increases from 247.57 to 343.01 RIU −1 as the thickness of the silver layer increases. Comparing the performance parameters of the sensor with different thicknesses of t 1 in Table 1, it can be found that t 1 = 30 nm is considered the best performance. Figure 6(c) displays the transmission loss curve spectra of the RI detection channel silver layer at different thicknesses. From the figure, it is observed that the resonance peak in the wavelength range of RI detection undergoes a significant red shift and the resonance depth gradually decreases. Figure 6(d) shows the amplitude sensitivity curves corresponding to different thicknesses of silver layers, with the thickness of the silver layer increasing the amplitude sensitivity gradually decreases. In addition, Table 2 shows that the maximum RI wavelength sensitivity of the optical fiber sensor reaches 2700 nm/RIU when t 4 = 50 nm. Considering other performance parameters, it can be found that the performance of the sensor is relatively best at t 4 = 50 nm. TiO 2 is a sensitive layer material for the RI detection channel, and its thickness has a significance on the performance of the sensor. Figure 7(a) shows the transmission loss curve of the refractive index detection channel with the change in TiO 2 thickness. When t 3 increases from 5 to 15 nm, the resonance peak is a red shift, and the loss intensity increases. Figure 7(b) shows the amplitude sensitivity curve corresponding to the change of t 3 , and the maximum amplitude sensitivity is 669.23 RIU −1 at t 3 = 10 nm. Table 3 shows the data of RI sensitivity and amplitude sensitivity corresponding to different t 3 , and the maximum RI sensitivity is 2700 nm/RIU at t 3 = 10 nm.
The sensitive layer of the RI detection channel is deposited in the form of micrograting, and the micrograting  structure can effectively enhance the detection performance of the sensor. Figure 8(a) shows the effect of changing the micrograting spacing (d 3 ) on the transmission loss. When increasing the d 3 from 20 to 30 nm, the resonance peak is a red shift and the loss value gradually decreases. Figure 8(b) shows the amplitude sensitivity curve for varying d 3 , and the maximum amplitude sensitivity reaches 608.82 RIU −1 at d 3 = 25 nm. According to Table 4, the wavelength sensitivity and amplitude sensitivity are the highest when the micrograting spacing is 25 nm. Therefore, the optimal micrograting spacing is 25 nm. Figure 9(a) shows the transmission loss spectrum by varying the number of microgratings (m). There is a small red shift in transmission loss with increasing m and a slight  . 7 (a) The transmission loss curves corresponding to different thicknesses of TiO 2 at temperature of 0°C. (b) Amplitude sensitivity curve at different thicknesses of TiO 2     increase in resonance depth. The effect of temperaturesensitive material PDMS thickness (t 2 ) change on transmission loss is shown in Fig. 9(b). The position of the resonance peak and the resonance depth hardly change during the increase of t 2 from 0.15 to 0.35 μm. It also shows that the sensor properties in the range of 0.15 to 0.35 μm for t 2 are always favorable. Figure 10(a) shows the spectrum of the variation of the transmission loss curve of the optical fiber sensor during the increase of the temperature from −60 to 100°C at the analyte RI of 1.33. From the figure, it can be seen that as the temperature increases, the resonance peak in the temperature detection wavelength range (320 to 500 nm) undergoes a significant blue shift and the loss depth gradually decreases. While the wavelengths after 500 nm in the figure are in the RI detection wavelength range, so the resonance wave does not change. Figure 10(b) shows the fitted curve of analyte temperature and the corresponding resonance wavelength. It can be seen from the figure that the adjusted R-square is 0.99986, which indicates a good fit. The maximum temperature sensitivity calculated by the equation reaches 0.7 nm/℃ with a resolution of 1.42 × 10 −2 RIU.
The transmission loss spectrum for detection at a temperature of 0°C and the analyte RI of 1.30 to 1.40 is shown in Fig. S1 from the Supporting Information. While the transmission loss curve for the RI changed from 1.30 to 1.38 as shown in Fig. 11(a). As the RI increases, the resonance peak corresponding to the RI detection wavelength range is a significantly red shift, and the resonance depth gradually increases. The resonance peaks in the temperature detection wavelength range do not change because the detection temperature remains constant. Figure 11(b) shows the fitted curve of RI of the analyte (1.30-1.40) versus resonance wavelength. And the adjusted R-square is 0.99105, which indicates that the fit is better. Table 5 shows the performance parameters of the dual parameter detection optical fiber sensor. From the table, it can be seen the maximum wavelength sensitivity of 1.25 × 10 4 nm/RIU and the wavelength resolution of 8 × 10 −6 RIU, temperature sensitivity of up to 0.7 nm/℃, and wavelength resolution of up to 1.42 × 10 −2 RIU.
The setup of the optical fiber sensor is shown in Fig. 12. First, the light source is connected to the optical fiber to excite the surface plasma excitation phenomenon. Then, the spectrometer is used to receive the light transmitted in the fiber to obtain the detection results. In order to realize the monitoring of continuous chemical reactions in industrial production, the optical fiber sensor is applied to the  Fig. 12(a). The optical fiber sensor is placed inside the reaction chamber to monitor the temperature and RI of the chemical reaction in real time. Therefore, it is facilitated to master the state of the chemical reaction proceeding. Figure 12(b) shows the embedding of optical fibers into a microfluidic chip [31] to enable the detection of trace chemical samples and their continuous chemical reactions. Using automatic injection pumper for precise adjustment of sample flow rate and dosage. Such a testing device can reduce the harm caused by monitoring harmful substances to the experimenters, saving the consumption of precious biochemical samples. Recommended manufacturing process of the dual parameter optical fiber sensor is described in Fig. 13. The internal microstructure of the optical fiber is realized by ultrasonic [32], and the flat polished surface of the external fiber is realized by polishing technique [33]. Regarding the deposition of the sensitive material, first, a polymethyl methacrylate (PMMA) template is obtained using an electron etching technique [34] as an auxiliary template for the deposition of the refractive index sensitive material. The templates are combined with optical fibers using micromachining techniques [35]. Immediately afterwards, Ag and TiO 2 are deposited successively at the groove formed by the template and the fiber throw surface in the refractive index detection channel. Ag and PDMS are deposited in the temperature detection channel. where PDMS consists of liquid silicone oil and curing agent with a mass ratio of 10:1 [36]. Generally, curing is achieved by heating at 60°C for 2 h. For TiO 2 , it can be prepared by mixing tetrabutyl titanate with dilute hydrochloric acid [37]. Ag is obtained by silver mirror reaction [38]. After completing the deposition of the material, the PMMA template is removed using acetone. Finally, the two detection channels are cleaned with methanol and dried by passing a stream of nitrogen.

Conclusion
The photonic crystal fiber sensor proposed in this paper has a function of dual parameter detection in the same space-time. And it is possible to obtain two non-interfering detection results in the same spectrum. Therefore, it can effectively avoid the problem of temperature and RI cross-sensitivity. In addition, it has a more outstanding contribution in the biochemical field and can be used for continuous monitoring of reaction temperature and RI. Moreover, the detection performance is even better in the case of a large temperature and RI detection range. The best temperature resolution is up to 1.42 × 10 −2 RIU, and the temperature wavelength sensitivity is 0.7 nm/°C. The refractive index wavelength sensitivity is 1.25 × 10 4 nm/RIU, and the wavelength resolution is 8 × 10 −6 RIU. Furthermore, the structural design of this external detection reduces the inconvenience of fabrication and use. In conclusion, it provides an idea to investigate the optical fiber sensor for dual-parameter detection in the same space-time.  Data Availability Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.
Code Availability Not applicable.

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