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Abstract
A surface plasmon resonance (SPR) temperature sensor on the basis of depressed double cladding fiber (DDCF) is theoretically proposed and experimentally demonstrated for the first time. Simulation analysis implies that the SPR fiber optic structure consisting of a multimode fiber (MMF) inserted into an 8 mm long DDCF is highly sensitive to the refractive index (RI) of the surrounding environment, owing to their mismatched cores, large discrepancy in cladding diameters, and the depressed inner cladding in DDCF. The experimental results further verify that the highest RI sensitivity is 7002 nm/RIU established with a 50nm Au coated DDCF-SPR sensor. Additionally, the temperature sensitivity reaches up to −2.27 nm/°C within a wide working temperature range of −30 to 330 °C by combining polydimethylsiloxane (PDMS) film as the temperature sensitive material with DDCF-Au architecture. The integrated PDMS, Au and DDCF temperature sensor possesses high performance in terms of sensing capability and physical construction, opening a route to their potential applications in other types of sensors.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Paralleling with the rapid development of social informatization, artificial intelligence and network, higher prerequisites are put forward on the performance of sensors for temperature monitoring [1,2]. In many fields, such as asphalt highway and voltage station atmospheric temperature detection, fire detection, soil desertification evaluation, etc., temperature sensors are required to have excellent chemical inertness, high accuracy, as well as can be used in high and low temperature environments. Compared with traditional temperature sensors with metal or semiconductor as the main material, optical fiber temperature sensors are characterized by electrical insulation, corrosion resistance, and nonelectric conductivity [3–5]. The latter greatly expand expected potentials of temperature sensors, especially when measuring temperature in chemically corrosive solutions or liquids stored at circulating temperatures.
In the past few years, a variety of principles and strategies exploiting optical fibers for temperature monitoring have been proposed and extensively studied. Fiber Bragg gratings (FBGs) have played an important role in temperature monitoring due to their ability to have quasi-distributed sensing networks [6,7]. Nevertheless, the conventional FBGs cannot operate at high temperature on account of the tendency of being erased. The FBGs inscribed by the point-by-point (PbP) or line-by-line (LbL) technique with femtosecond laser exhibit the properties of high temperature resistance and stability [8,9]. But cross-sensitivity problem between strain and temperature and low temperature sensitivity of grating itself have yet to be solved. Although optical fiber configurations implemented with interferometers, involving Fabry-Perot interferometers (FPIs) [10,11] and Mach-Zehnder interferometers (MZIs) [12,13], exhibit attractive alternatives to high temperature sensing applications, they suffer from relatively low sensitivities (at pm/$^\circ$C level). Several other schemes like whispering gallery modes (WGMs) [14] or Microfiber knot ring (MKR) [15] have character of ultrahigh sensitivity, but their architectures lack stability and reproducibility, and their working range is limited. In addition, two or more cascaded fiber structures based on Vernier effect [16] can enormously improve sensor sensitivity. However, they are susceptible to multiplex fluctuation, with restricted detection ranges as well.
Surface plasmon resonance (SPR) is an emerging field that offers tremendous potential for fiber optic temperature sensors, combing the advantages of high sensitivity, fast response and real-time monitoring. Some novel nanomaterials such as WS$_{2}$, Indium tin oxide (ITO) and MoS$_{2}$ provide a broader development prospect for SPR based sensors with high sensitivity and detection accuracy [17–20]. The SPR fiber optic can be converted from refractive index (RI) sensing to temperature sensing via synergistically integration of temperature sensitive materials and sensing fiber optic. Various assembly techniques have been developed to facilitate the realization of temperature sensing on the SPR fiber optic sensors [21–25]. Recently, a silver film coated optical fiber encapsulated in a capillary tube filled with alcohol was reported, achieving a combination of the alcohol with high thermal coefficient and SPR fiber [21]. The sensor can monitor temperature with a high sensitivity of 1.575 nm/$^\circ$C in temperature range of 35-70 $^\circ$C. Whereas, the easy-to-flow alcohol increases the difficulty of liquid packaging during the manufacturing process of the device. And the device cannot work at high temperature which will cause the alcohol to reach the boiling point (78 $^\circ$C) and evaporate, resulting in the rupture of the glass tube. Subsequently, another solution was proposed using the air hole in cladding of a photonic crystal fiber (PCF) as the sensing channel filled with the liquid mixture of aqueous silver nanowire colloids and chloroform [22]. The sensitivity of sensor is −2.08 nm/$^\circ$C in the temperature range from 25 $^\circ$C to 60 $^\circ$C. Similarly, this proposal is hard to promote and employ in commercial field since there are technical difficulties in liquid immobilization and encapsulation. Accordingly, optical fibers coated with curable temperature sensitive materials such as Polydimethylsiloxane (PDMS) open up a more prospective direction to realize temperature sensing in virtue of its simple operation and stability [23,24]. Most researches focused on unilaterally enhancing the temperature sensitivity of SPR fiber sensors, but few of them have investigated the temperature working range to the best of our knowledge. How to get a robust, simple, highly sensitive, and wide working range SPR fiber temperature sensor still remains unanswered.
To address the above challenges, herein we propose and demonstrate an SPR sensor for highly sensitive monitoring the ambient temperature in a wide working temperature range using a depressed double cladding fiber (DDCF) coated with gold-PDMS film as temperature probe. The sensing structure that is simply fabricated by thermal fusion splicing of optical fibers offers the SPR sensor compactness and stability. The SPR principle in DDCF is qualitatively analyzed and explained by both beam propagation method and finite element method. The highest sensitivity of −2.27 nm/$^\circ$C and lowest sensitivity of −0.26 nm/$^\circ$C is experimentally reached in the temperature range of −30-330 $^\circ$C. As far as we know, this is the first report on realization of temperature range above 360 $^\circ$C measurement for SPR sensor with nm/$^\circ$C level sensitivity.
2. Fabrication and characterization
2.1 Structure of MMF-DDCF-MMF
The proposed SPR sensor is comprised of a multimode fiber in which a segment of DDCF coated with gold film is inserted, as depicted in Fig. 1. Figure 1(a) shows the cross-section diagram of DDCF which has a depressed inner cladding. The DDCF is manufactured by the plasma chemical vapor deposition (PCVD) technique and its core, inner cladding, and outer cladding are made of germanium (Ge) and fluorine (F) co-doped silica, F-doped silica, and pure silica, respectively. It is characterized in that the refractive index of the inner cladding is smaller than that of the core and the outer cladding because of the F-doped inner cladding. Owing to this depressed inner cladding profile, the core mode will leak out to the outer cladding and propagate as a cladding mode. Its diameter of core, inner cladding and outer cladding are 7 $\mu$m, 17 $\mu$m and 80 $\mu$m, respectively. These parameters were obtained from a scanning electron microscope (SEM) image in Fig. 1(b). One end of the DDCF was firstly stripped of the coating using a stripping plier, rinsed thoroughly with absolute ethanol, and then cleaved into a neat end face by a high precision cleaver (CT-38, Fujikura). A section of MMF treated with the same above steps was stably spliced with the DDCF through a fiber fusion splicer (FSM-60S, Fujikura). Then, we performed a cleaver to cut out an 8 mm long DDCF, and finally, spliced the MMF-DDCF with the other same MMF. The schematic of the fabricated MMF-DDCF-MMF structure was plotted in Fig. 1(c). Two same MMFs with core diameters of 62.5 $\mu$m serve as transmission waveguides to transmit and gather the remaining light, respectively. The outer diameter of the MMFs are 125 $\mu$m larger than that of the DDCF. During fusion splicing process, two naturally fused taper regions were formed at between two MMFs and DDCF, corresponding to two mode coupling zones. Taking advantage of the two zones, the cladding modes of small core DDCF could be excited by the incident mode field from large core MMF coupled thereto such that an evanescent field in the DDCF reaching its fiber surface, and are eventually coupled back to the other MMF core. The fabricated hetero-core fiber structure was placed under an optical microscope and the MMF-DDCF splicing region was observed in Fig. 1(d). The length of tapered coupling zone between the MMF and the DDCF was estimated to be 75 $\mu$m in Fig. 1(e). In order to achieve SPR sensing, the well fabricated fiber structure was anchored perpendicular to a glass holder which allows to horizontal rotate and guarantee the sensing area exposing to air. Then, the whole holder with fiber was fixed on the bearing plate and put in vacuum chamber for coating. Next, a 5 nm thin chromium (Cr) film was uniformly deposited on the surface of fiber cladding to enhance the adhesion between bare fibers and gold by vacuum thermal evaporation technique. A continuous and uniform gold (Au) coating layer as a plasmonic material is then obtained on the surface of fiber.
Fig. 1. (a) Structure of the depressed double cladding fiber (DDCF). (b) SEM cross-sectional image of a DDCF. (c) Schematic of the fabricated MMF-DDCF-MMF structure. (d) Microscope photograph of the MMF-DDCF splicing area. (e) Zoom around the tapered coupling zone.
2.2 Theory and Simulation
We evaluated the transmission performance of DDCF in the construction of multimode interferometer through comparing the optical energy distribution in different hetero-core fiber structures obtained by RSOFT based on beam propagation method. The simulated results are shown in Fig. 2. Figures 2(a)-(c) demonstrate the energy distributions of the optical field using standard single-mode fiber (SMF), thin cladding single-mode fiber (TCSMF) and DDCF, respectively. The SMF, TCSMF and DDCF are all 8 mm long and the lengths of MMFs at both ends are the same of 1 mm. The core diameters of SMF and TCSMF are both 8 $\mu$m and their cladding diameters are 125 $\mu$m and 80 $\mu$m respectively. The wavelength of incident light is 0.85 $\mu$m and the surrounding medium is set to water with RI of 1.333. In these three hetero-core fiber structures, the beams from left MMF cores are coupled into these small core fibers through respective splicing region, and totally reflected at the cladding-dielectric interfaces, which are finally coupled back to the right MMF cores. However, it can be found that both near the cladding surfaces of TCSMF and DDCF, the optical energy densities are greatly increased due to their smaller cladding diameters than that of SMF, which could enhance the sensitivity of external refractive index sensing. Figure 2(d) presents the sum of the transverse optical field energy of small core optical fibers in Z direction, which indicates that in the DDCF, the excitation light from MMF distributes and transmits more proportionally to its cladding-dielectric interface than SMF and TCSMF. This is because of not only the cladding diameter reduction of optical fiber, but the depressed inner cladding with lowest RI providing a strong light filed leakage to the outer cladding according to the principle of total internal reflection. Consequently, the DDCF has a significant influence on the sensor property, which can be well applicable in the following refractive index sensing experiment.
Fig. 2. Optical field energy distribution in hetero-core fiber structures using different types of fibers: (a) 8/125 $\mu$m single-mode fiber (SMF); (b) 8/80 $\mu$m thin cladding single-mode fiber (TCSMF); (c) 7/17/80 $\mu$m DDCF. (d) Longitudinal sum of the transverse optical field energy in the SMF of (a), the TCSMF of (b) and the DDCF of (c).
In this work, the principle of surface plasmon resonance phenomenon and excitation condition of SPR in MMF-DDCF-MMF structure are described. There exists numerous incident polarized light from MMF in the DDCF region as schematically described in Fig. 2(c). As the phenomenon of total internal reflection occurs on the cladding of DDCF, the formation of evanescent waves in the interface of metal-dielectric intrigues strong collective oscillations of free charge, which is known as surface plasma waves (SPW). SPR phenomenon happens when SPW and evanescent wave attains phase-matching condition, and based on Maxwell equations the propagation constant of SPW along the interface is :
Thus, the RI of the measured object can be measured by detecting $\theta _{r}$ or $\lambda _{r}$ (resonance wavelength), which forms the theoretical basis of various SPR refractive index sensors. Since the effective RI of the plasmonic mode increases with the increase of the RI of the sample to be measured, but does not affect that of waveguide mode, the higher RI of analyte will lead to the two modes attaining phase matching at a longer wavelength.
The finite element method was employed to investigate the mode characteristics and SPR sensing effects of the DDCF and TCSMF, as shown in Fig. 3. Considering the ideal circular symmetry of the double cladding fiber, only one quarter of its cross-section (Fig. 3(a)) is chosen to save computer memory space and improve computational efficiency. Figure 3(b) illustrates the FEM gridding schematic here PML, PMC and PEC are boundary conditions. The thickness of gold layer is set to be 50nm and its relative dielectric constant from visible to near infrared is demonstrated by Drude model [26]. The guided modes which may excite surface plasmon (SP) modes and resonate with them in the optical fiber were analyzed. After the mode analysis and calculation, the electric field distribution drawings with the three different modes of plasmonic, cladding in DDCF and cladding in TCSMF were solved and shown in Figs. 3(c)-(e). We can find that the energy of the cladding mode in DDCF is more concentrated in the vicinity of the gold film where plasmon oscillation is expected, while that of TCSMF is more concentrated in a position near the core. The depressed inner cladding as a low refractive index layer enables mode guidance in the outer cladding of fiber, which strongly enhances the coupling strength between the cladding mode and plasmon. After the parametric scanning of light wavelength, we obtained by DDCF-SPR sensor a series of transmission spectra relying on the variation of measured analyte (Fig. 4(a)). The resonance wavelengths have an evident red-shifts changing from 568 nm to 957 nm when the RI of measured analyte increases in the wide range of 1.333-1.410. Figure 4(b) renders the relationship between resonant wavelength and refractive index obtained by the DDCF and TCSMF. It indicates that the proposed DDCF based SPR sensor has a high sensitivity of 2667-10400 nm/RIU in the RI range of 1.333-1.410, which is better than the TCSMF based SPR sensor whose sensitivity is 2400-8000 nm/RIU in the same RI range. Therefore, the DDCF can be a promising candidate for assembling SPR sensor with high sensitivity.
Fig. 3. (a) Cross Section diagram of DDCF-SPR Sensor. (b) FEM gridding and boundary condition setting. Electric field intensity distributions: (c) surface plasmon (SP) mode; cladding mode in (d) DDCF and (e) TCSMF. PML: perfectly matched layer. PMC: perfectly magnetic conductor. PEC: perfectly electric conductor. The white arrows indicate the electric field direction of each mode.
Fig. 4. (a) Simulated SPR spectra obtained by the DDCF with gold film thicknesses of 50 nm at different refractive indies ranging from 1.333 to 1.410. (b) The fitting curves of the resonance wavelength as a function of RI.
3. Experiment results and discussion
3.1 Refractive index detection
The experimental setup for refractive index measurement utilizing DDCF-SPR sensor are shown schematically in Fig. 5. The transmission spectra were measured via coupling the broadband light source (HL-2000-LL, Ocean Optics) to the MMF-DDCF-MMF structure based SPR sensor and recording the received light with a spectrometer (USB4000, Ocean Optics). Figure 5(a) gives the schematic of gold coated fiber structure. Both ends of the sensing fiber were aligned and spliced between two multimode fiber optic jumpers with the same specification. The light from the tungsten halogen lamp was inputted through the fiber jumper into the left MMF, propagated forward and transmitted to the sensing fiber structure where SPW was excited by the evanescent waves at certain wavelengths. Afterwards, the residual transmission light with a loss dip was collected by a fiber spectrometer. At last, the spectral data were recorded on-line by a spectroscopy software on computer. The integration time of spectrometer is 15ms. To avoid the influence of jitter during manual operation on experimental test results, an Aluminum (Al) plate equipped with micro-channels fabricated by laser marking technology served to embed the SPR sensor and infuse sample solution. Once the solutions with different refractive indies were injected in turn into the microchannel and contacted with the gold layer on the surface of sensing area, the resonance wavelength of transmission spectrum will have obvious shifts due to the high sensitivity of SPR to the surrounding medium RI. The sensing property of DDCF-SPR sensor can be characterized through the relative shift of wavelength. Moreover, the research states that the gold film thickness plays a crucial role in tuning working wavelength range and affect the fiber sensing performance. After numerical calculation, the gold layer with a thickness of 50 nm reaches a wide RI detection range and high sensitivity within the operating wavelength range of the spectrometer (345-1040 nm). The Au-SiO$_{2}$ cross section SEM image of sensing fiber in Fig. 5(b) indicates the thickness of Au overlay is 50 nm, which is the consistent with the theoretical optimal value.
Fig. 5. Experimental setup for RI sensing measurement. (a) Schematic diagram of gold-plated fiber sensing structure. (b) SEM image of the edge of DDCF.
To analyze and estimate the performance of our proposed sensor, solutions with different refractive indices were prepared for the RI measurement experiment. Different weight ratios of anhydrous glucose and deionized water were mixed to obtain a series of glucose solutions having different solubility. An Abbe was employed to determine their refractive indices at room temperature (20 $^\circ$C). Nine solutions with RIs ranging from 1.333 to 1.410 were sequentially injected into microchannel and thoroughly reacted with the SPR fiber optic for spectral measurement experiment. Unlike the simulated SPR spectra, the experimental spectra were obtained by dividing the test spectra acquired in solution by the reference spectrum in air. The transmitted spectra as RI varies are obtained in Fig. 6(a). Notably, the spectra become slightly broader and shift towards longer wavelength with the RI increasing from 1.333 to 1.410. The maximum full width at half maxima (FWHM) of 239 nm in high RI of 1.410 is observed. To more intuitively reflect the sensing performance of sensor, the quadratic fitting curve with the dependent coefficient R$^{2}$ = 0.998 for the resonant wavelength versus RI are shown in Fig. 6(b). The DDCF-SPR sensor is demonstrated to display a high wavelength sensitivity of 1060-7002 nm/RIU in the analyte RI range of 1.333-1.410. Compared with the simulation analysis result in Fig. 4(b), the experimental sensitivities are slightly lower than simulated sensitivities, but their resonance wavelength range are roughly the same. The difference between the experimental and the simulated sensitivities would attribute to that the simulation is carried out under ideal conditions, but ineluctable errors of optical fiber parameters exist in its actual manufacture and measurement. Apart from the sensitivity, the figure of merit (FOM) defined as the ratio between sensitivity and FWHM, is also an important parameter to characterize the sensing performance of the sensor. As a result, the FOMs of DDCF in the RI range of 1.333 to 1.410 were calculated to be between 9.1 RIU$^{-1}$ and 29.3 RIU$^{-1}$, indicating the sensor can detect the resonance wavelength with high accuracy.
Fig. 6. (a) Experimental transmission spectra obtained by the DDCF-SPR sensor. (b) The fitting curves of the resonance wavelength as a function of RI. The RI of the sensing solutions ranges from 1.333 to 1.410.
3.2 Temperature monitoring
On the basis of high RI sensitivity and wide RI measurement range of the DDCF-SPR sensor, we developed a temperature probe using PDMS covered RI-sensitive sensing area for highly sensitive to monitor temperature in a wide tuning range. The experimental setup for temperature testing was constructed and shown in Fig. 7. All the test devices (including the light source, spectrometer and computer) were used and connected in the same way, except a constant temperature and humidity testing machine used to place the PDMS coated sensing probe. This testing machine has a programmable controller (TEMI, South Korea) which can precisely regulate the working temperature in range of −40-100 $^\circ$C with a temperature precision of 0.1 $^\circ$C. To achieve the temperature measurement application based on SPR RI sensor, we coated a layer of PDMS at the 50 nm gold coated sensing fiber as the temperature-RI conversion medium. The RI of PDMS with high thermal-expansion coefficient of −4.66 $\times$ 10$^{-4}$ can decreases regularly in the rise of ambient temperature. Thanks to this advantage, the SPR spectra obtained can be shifted by varying the temperature parameter in virtue of the relationship between resonant wavelength and RI. At first, we mixed the liquid PDMS with the curing agent in a ratio of 10:1. The mixture was then stirred in a vacuum agitator till all air bubbles are completely removed. After that, the optical fiber RI sensing area was covered with a small drop of colloidal PDMS. And we finally heated them in an oven at 60 $^\circ$C for 2 h to obtain the cured PDMS. The cured PDMS film acts as the thermal sensitive material with higher thermal-expansion coefficient than that of silica [27]. The thickness of PDMS is about tens of micrometers, and the RI of PDMS measured by Abbe refractometer is 1.41 at room temperature. After coated with PDMS, the whole sensing fiber was steadily located inside the temperature chamber for the following temperature testing experiments.
An experimental research on the temperature sensing ability of our SPR sensors was invested and plotted in Fig. 8. The first temperature experiment was conducted under the condition of constant temperature testing machine with working temperature range of −30-80 $^\circ$C. The measurement integration time of spectrometer is less than 20 ms, and to ensure the stability of the temperature and the reliability of the measurement, the temperature measured each time remains unchanged for about 2 minutes. The transmitted spectra for the SPR sensor relying on the variation of ambient temperature was measured and shown in Fig. 8(a). When the tested temperature raises gradually from −30 $^\circ$C to 80 $^\circ$C in increments of 10 $^\circ$C, the SPR transmission spectra obtained by the gold-PDMS coated SPR sensor appear blue-shifts. This can be attributed to the expansion. In Fig. 8(b), there is a good linear relationship (R$^{2}$ = 0.992) between the resonant wavelength and temperature, which can be expressed as $\lambda$ = −2.27 $\times$ T + 899.74, where $\lambda$ refers to the resonant wavelength in micrometer, and T represents the ambient temperature. This sensor offers a high temperature sensitivity of −2.27 nm/$^\circ$C in range of −30-80 $^\circ$C. The resolution of the sensor is calculated to be around 0.01 $^\circ$C limited by the spectrometer resolution which is 0.03 nm.
Fig. 8. (a) SPR spectra of the DDCF-SPR temperature sensor at T of −30-80 $^\circ$C. (b) variation of shift in the resonance wavelength with RI when T changes from −30 $^\circ$C to 80 $^\circ$C. (c) SPR spectra of the DDCF-SPR temperature sensor at T of 90-330 $^\circ$C. (d) variation of shift in the resonance wavelength with RI when T changes from 90 $^\circ$C to 330 $^\circ$C.
The discussion hereinbefore proclaims that our sensor is capable of executing temperature measurement experiments covering both low and high temperature domains. Yet, in most practical situations, such as industry and aeronautics, the demand for higher temperature monitoring with high sensitivity is equally high on the agenda. Thus, a high temperature sensing experiment based on the same SPR temperature probe were also carried out. Due to the limitation of operating temperature range of the constant temperature machine, we replaced it with a high temperature constant temperature heating table with a working temperature between 30 $^\circ$C and 330 $^\circ$C, whereas the other test equipment remained unchanged. Figure 8(c) issues the experiment measured SPR transmission spectra results. Obviously, as the temperature increases from 90 to 330 $^\circ$C, the resonance wavelength of the SPR transmission spectra is observed to be further blue-shifted. This reveals that the RI of PDMS can still be decreased with the increase of temperature up to over 300 $^\circ$C. The corresponding quadratic fitting curve with the dependent coefficient R$^{2}$ = 0.998 with regard to the resonance wavelength and temperature of the SPR sensor was calculated and drawn in Fig. 8(d). We found that the slope of the curve (−1.03 nm/$^\circ$C to −0.26 nm/$^\circ$C) representing the sensing sensitivity in different temperature ranges decreases with the improved temperature (90 $^\circ$C to 330 $^\circ$C). This is consistent with the fact that the DDCF-SPR sensor has a lower refractive index sensitivity in the low RI range than that in the high RI range. Compared with many reported SPR optical fiber temperature sensors, the proposed sensor extends the temperature measurement range of SPR sensor using high thermo-optic coefficient liquid in Ref. [28] and [29], and provides a higher detection accuracy than Ref. [30].
Summarizing the above two sets of temperature experiment results, the coating of PDMS brought a capability of highly sensitive temperature monitoring for DDCF-SPR RI sensor. For the first time, the optical fiber sensor based on the SPR principle has attained a wide working temperature range of −30-330 $^\circ$C. Especially, it maintains high sensitivity and structural stability in low temperature test condition. However, for the temperature sensor proposed in this paper, its maximum operating range will be limited by the material characteristics of PDMS. As expected, the DDCF based SPR sensor has theoretically high potential for temperature sensing applications, enabling a wider temperature measurement range and higher sensitivity in case of choosing appropriate refractive index temperature-sensitive materials. Moreover, by intruding materials having different sensitive characteristics clapping on the gold film, it is expected to design various sensors such as pressure sensor, humidity sensor, etc.
4. Conclusion
In this study, we have demonstrated a fiber optic temperature sensor based on a high sensitivity multi-mode interferometric structure by using a DDCF combined with SPR and PDMS coating technology. The double-clad fiber with a depressed inner cladding, which is spliced between two multimode fibers, enhances the effect of multimode interference and increases the refractive index sensitivity of the SPR sensor. The Au-PDMS temperature sensor displays a strong mechanical stability, and its temperature sensitivity achieves up to −2.27 nm/$^\circ$C at a wide temperature of −30-330 $^\circ$C. The proposed SPR sensor provide a versatile and promising approach to yield further improvements in sensitivity and measurement range of temperature sensors, and the architecture can be extended to the all-fiber integration and multiplexing to enable the mass production of micro-devices in commercial field.
Funding
National Natural Science Foundation of China (61775065).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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