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Abstract
In this study, a new processing design of an optical fiber cryogenic temperature sensor (OFCTS) is presented. The sensing unit is constituted by NaYF4:Yb3+, Er3+@NaYF4 core-shell upconversion nanocrystals-polymethyl methacrylate (UCNCs-PMMA) nanocomposites. The coupling is achieved by fiber fusion in the embodiment. The relative sensitivity of the OFCTS can reach the maximal value 13.241×10−3 K−1 at 80 K in a cryogenic environment, and stability is good with a standard deviation of 0.012. Research results show that the proposed OFCTS has good temperature responses at the cryogenic environment, and has a great potential of the superconducting application for generator, transmission line, maglev train and quantum interferometer.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, superconducting materials and cryogenic technologies such as superconducting cables, superconducting magnetic levitation and superconducting power generation have developed rapidly. They are widely used in military, aerospace, industrial engineering and many other fields [1,2]. Superconducting materials have zero resistance and linear repulsive magnetic lines under certain cryogenic environments, but the temperature of the quenching point will rise rapidly in the event of quenching the superconducting material, which will lead to major faults in the superconducting system [3–7]. In order to ensure the stability of the superconducting system, it is necessary to develop measurement technologies, which are suitable for cryogenic systems. Since superconducting systems have very high magnetic fields, the used electrical instruments need complex shielding seal [8,9]. Traditional cryogenic sensors are mainly based on changes in thermal resistances, which are not suitable to form the temperature monitoring system because they are highly sensitive to strong electromagnetic field interference [10,11]. Furthermore, in condition of large-scale spot measurement, signal transmission cable grows exponentially and leads to huge amounts of ground equipment, which brings a great inconvenience to onsite operation and testing [12]. In contrast, the optical fiber sensors have gained wider applications in different industrial fields because of their small size, high sensitivity, long-distance access, corrosion resistance, and electromagnetic interference resistance [7,13–16]. It is known to all that the perception of temperature changes by the optical fiber sensors is based on thermal expansion coefficient (TEC) and thermo-optic coefficient (TOC). However, these parameters of optical fiber materials are small in the cryogenic temperature range, which is the main reason for low temperature sensitivity. Although many pieces of research have reported various ways to enhance temperature sensitivity, for example, by means of metal re-coating of silica optical fiber Bragg gratings, these proposals still have many problems to be solved in the stability of optical fiber sensors and their manufacturing process [17–19].
Meanwhile, with the continuous development of nanoscience, many novel materials with high temperature sensitivity have been constantly reported, for example, BiPO4:Yb3+, RE3+ (RE3+ = Ho3+, Er3+ and Tm3+), NaYF4:Yb3+, RE3+(RE3+=Er3+, Nd3+) and NaAlSiO4:Cr3+ [20–22]. Feng et al. reported the low temperature sensing behavior of upconversion luminescence in Er3+/Yb3+ co-doped PLZT transparent ceramic by fluorescence intensity ratios (FIRs) (2017), the temperature sensitivity of intensity ratio is 2.18×10−3 K−1 (at 320 K) in the temperature range of 10 to 320 K [23]. Zhang et al. researched Er-doped strontium tungstate phosphors for optical temperature sensing applications (2018), the maximal relative sensitivity is 12.75×10−3 K−1 at 523 K with a broad temperature range from 83 to 563 K [24]. Kaczmarek et al. investigated Yb3+, Er3+ co-doped LaF3 nanoparticles (2019), with a remarkably high relative sensitivity up to 66.092×10−3 K−1 (at 15 K) in the cryogenic temperature region from 15 to 105 K [25]. The above research results show that such nanocrystals have excellent temperature sensitivity in the cryogenic environment. However, there are few studies on the implements of cryogenic temperature sensors with nanocrystals in the current public reports, moreover, it is far away from practical industrial applications. The development of advanced temperature sensors has been one of our research interests [26,27], and an idea of integrating nanocrystals with the optical fiber was first proposed in 2018, to realize temperature measurement. However, the obtained results did not show good performance in the cryogenic environment due to the design and process defects. Therefore, a new design on the process of the optical fiber cryogenic temperature sensor (OFCTS) is presented in this study. Here, the sensing unit is fabricated with NaYF4:Yb3+, Er3+@NaYF4 core-shell upconversion nanocrystals-polymethyl methacrylate (UCNCs-PMMA) nanocomposites which are fused in the optical fiber by an optimized fabrication process. Research results prove that this type of sensor has many advantages such as good temperature response, heat stability and flat fabrication end face standing for a strong structure.
2. Experiment
Since the performance of the OFCTS is mainly determined by its sensing unit, existing research results show that it can be fabricated based on NaYF4: Yb3+, Er3+@NaYF4 core-shell UCNCs-PMMA nanocomposites to build a high sensitivity [28–30]. UCNCs-PMMA nanocomposites are synthesized by copolymerization in our lab with NaYF4:Yb3+, Er3+@NaYF4 core-shell nanocrystals and PMMA, as shown in Figs. 1(a) and 1(b). The transmission electron microscope (TEM) measurement of the NaYF4: Yb3+, Er3+@NaYF4 core-shell UCNCs was carried out on the JEM-2100F TEM with acceleration voltage of 200 kV. The TEM image in the inside of Fig. 1(a) indicates the size of the core-shell UCNCs is around 13.8 nm. The synthesis process is as follows: (1) 15 mL methyl methacrylate (MMA, 99.0%) and 8 mg 2-methylpropionitrile (AIBN, 99.0%) are added into a beaker flask and heated to 85 °C for 30 minutes. (2) When the colloid becomes viscous and transparent, transfer it into a 20 ml centrifuge tube. (3) Add the as-synthesized NaYF4:Yb3+, Er3+@NaYF4 core-shell UCNCs into the above solution and keep stirring until it becomes a milk-like colloid.
Fig. 1. Raw materials of the nanocomposites. (a) Images of NaYF4:Yb3+, Er3+@NaYF4 core-shell UCNCs. (b) PMMA.
PMMA is liquid at room temperature in the beginning. The state can be turned from liquid to solid when it is heated up to 90 °C and kept for 30 min. Once the nanocomposites solidify, it can ensure that the nanocrystals are adhered to the optical fiber accurately and evenly. In order to ensure the quality of the fiber fusion, a prefabricated platform is designed and completed to optimize the shape of the fiber end face. As shown in Fig. 2(a), it is composed of three-axis slider, fiber clamp, microscope, and computer. The operation process is as follows: (1) A drop of the colloid is placed on the glass slide. (2) The fiber is vertically fixed through the fiber clamp to slowly approach the liquid surface. (3) The optical fiber is lifted up and fixed for solidification after touching the liquid surface. (4) Repeat steps (1)-(3) for the other fiber. As shown in Figs. 2(b)–2(e), full fabrication processes can be observed from the computer screen, which is controlled by the three-axis slider. A perfect curved surface formed by UCNCs-PMMA nanocomposites can be seen from Fig. 2(e) on the end face of the optical fiber which will be used in fusion after solidification.
Fig. 2. Prefabrication process before fusion. (a) The prefabrication platform composed of three-axis slider, fiber clamp, microscope and computer. (b) An image of the practical end-face fabrication platform. (c) An image of the fiber optic before touching the liquid surface. (d) An image of the fiber optic during touching the liquid surface. (e) An image of the fiber optic after touching the liquid surface.
Next, a fiber fusion splicer (TYPE-81C, Sumitomo Electric Industries, Japan) is used for OFCTS preparation. The welding parameters are mainly set as follows. The alignment method is outer diameter alignment, the discharge time is 3.0 s, the fiber end face angle permissible value is 5.0° and the allowable value of the fiber fusion angle is 0.1°. The used optical fiber is a multi-mode quartz fiber (GT21-FC-FC-05, Datang Storm Ltd., China) with a core diameter of 62.5 µm. The fiber end face before fusion is shown in Fig. 3(a), presenting smooth curved surfaces, which is consistent with the shape seen in the microscope. In addition, Figs. 3(b) and 3(c) are the images of the fiber after fusion in X-axis and Y-axis direction. The side faces of fusion position are very flat which proves the optimization of the fabrication method. UCNCs-PMMA nanocomposites are successfully embedded in quartz fiber through the fiber fusion technology.
Fig. 3. Fabrication process of the fusion technology. (a) An image of the fiber end face before fusion. (b) An image of the fiber end face after fusion in X-axis. (c) An image of the fiber end face after fusion in Y-axis.
The performance testing of the OFCTS is conducted on the cryogenic environment, where the testing platform mainly includes 980 nm single-mode laser (ADR-1805, Radium Photoelectric Technology Co., Ltd., China), spectrometer (BIM-6001, Brolight Technology Co., Ltd., China), and cryogenic temperature chamber (HSFE91/TP94, Linkam Scientific Instruments Ltd., UK) as shown in Fig. 4(a). Specifically, the core of the cooling system is a liquid nitrogen cooling pump, the temperature controlling accuracy is ±1.0 K through the temperature controller. A 60 mm-diameter closed chamber is used to place the sensing unit which is shown in Fig. 4(b), its maximum rise/fall rate is 90 K/min.
Fig. 4. The performance testing on the cryogenic environment. (a) An image of the cryogenic platform. (b) An image of the HSFE91 heating and cooling stage.
3. Analysis
In the previous section, the cryogenic temperature chamber is set from 80 to 150 K with a step of 10 K. The acquisition range of the spectrometer is 450-700 nm, the integration time is 4 s, and the average number of times is 20. A bandpass optical filter placed in the observation window has 97% transmission in the passband of 400-700 nm and 99% attenuation in the stopband, which can greatly reduce interference from the light source. Calculated upconversion luminescent (UCL) spectra by the difference between test and reference spectra are shown in Fig. 5. Under the 980 nm laser excitation, the NaYF4: Yb3+, Er3+@NaYF4 core-shell nanocrystals exhibit visible emission in the wavelength range of 520 to 570 nm. The upconversion process can be described as follows: firstly, the successive energy transfers from the sensitizer Yb3+ ions excite Er3+ from the ground state to the 4F7/2 energy level. And then, the nonradiative relaxations of 4F7/2→2H11/2 and 2H11/2→4S3/2 populate the 2H11/2 and the 4S3/2 energy levels of Er3+. Subsequently, the radiative transitions of 2H11/2→4H15/2 and 4S3/2→4H15/2 give off the emissions peaked at 525 nm and 545 nm, respectively [31].
Therefore, this study uses the FIRs method of 525 and 545 nm generated by the two thermally coupled energy levels of 2H11/2 and 4S3/2. Based on the Boltzmann formula, the temperature detection is achieved by the temperature-sensitive properties of the UCL spectrum. The relationship between the intensity ratio and the temperature is reflected in Eq. (1), and can be reformulated (Eq. (2)) in a logarithmic form [23].
Where, k is the Boltzmann constant, T is the absolute temperature, RHS(0) is FIR when T = 0 K, ΔE is the energy difference between energy levels.As shown in Figs. 6(a) and 6(b), the intensity ratio is exponential with the temperature, where the expression is $y = 0.6464\exp ( - 87.54/x)$ and the R-squared value of the fitting curve is 0.911. Meanwhile, the logarithm of the intensity ratio is linear with the reciprocal of the temperature, where the expression is $y ={-} 0.4617 - 84.74x$ and the R-squared value of the fitting curve is 0.9231. The fitting curves in Figs. 6(a) and 6(b) can accurately correspond to the theoretical formulas as Eq. (1) and Eq. (2), respectively. As for the relative sensitivity, the formula can be written as
Therefore, ΔE/k in Eq. (2) can be obtained as a value of 84.74 according to Fig. 6(b). With the temperature increases from 80 to 150 K, the value of the relative sensitivity decreases from 13.241×10−3 to 3.766×10−3 K−1, and is maximized at 80 K. The results show that the lower the temperature is, the higher the relative sensitivity is, which is consistent with the Boltzmann formula.Fig. 6. Temperature-dependent characteristics. (a) RHS(I525/I545) is exponential with the temperature. (b) Logarithm of RHS(I525/I545) is linear with the reciprocal of temperature.
It should be noted here that the spectral data are acquired during the temperature rising from 80 to 150 K, because of the limitations of the cooling system that cooling from high temperature is not as stable as heating from low temperature. The drift of the former is ±3 K, it leads to a larger error than that of the latter, which is about ±1 K. What’s more, it takes a long time for the cooling system to reach temperature stability, which is very detrimental to the continuous acquisition of data. To analyze the stability of the OFCTS, its repeatability responses are measured for an hour with an interval of three minutes at a fixed temperature of 150 K. FIRs over the 60 minutes span are shown in Fig. 7, with a standard deviation of 0.012. The proposed OFCTS exhibited excellent characteristics in the experimental evaluations. However, there is still much work needed to be done. For example, the cross-sensitivities to measurands other than temperature have not been verified. More researches on the stress effects by the sensor packaging process are planned to be carried out in subsequent studies.
4. Conclusion
In this study, a new OFCTS is fabricated with NaYF4: Yb3+, Er3+@NaYF4 core-shell UCNCs-PMMA nanocomposites, which is evaluated in the cryogenic environment from 80 to 150 K. The relative sensitivity of the fabricated sensor can reach the maximal value at 80 K with 13.241×10−3 K−1, and the stability is good. Results prove the potential of this measurement technology in cryogenic detection. However, some interference noise of the gathering spectrum affects the measurement accuracy. Better noise reduction algorithms can be employed to improve the signal to noise ratio. The fabrication method adhering to the nanocomposites to the head of the optical fiber is considered to be taken, so as to form the probe-type optical fiber sensor and realize the aim of temperature detection in small-space applications. These will be the direction for future research.
Funding
Fundamental Research Funds for the Central Universities (22120180299); Natural Science Foundation of Shanghai (18ZR1441900); National Natural Science Foundation of China (51872200).
Disclosures
The authors declare no conflicts of interest.
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