SNR-enhanced temperature-insensitive microfiber humidity sensor based on upconversion nanoparticles and cellulose liquid crystal coating

Journal Pre-proof SNR-enhanced temperature-insensitive microfiber humidity sensor based on upconversion nanoparticles and cellulose liquid crystal coating Siqi Hu, Shengnan Wu, Chenxi Li, Runze Chen, Erik Forsberg, Sailing He

PII:

S0925-4005(19)31716-2

DOI:

https://doi.org/10.1016/j.snb.2019.127517

Reference:

SNB 127517

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

11 June 2019

Revised Date:

22 November 2019

Accepted Date:

1 December 2019

Please cite this article as: Hu S, Wu S, Li C, Chen R, Forsberg E, He S, SNR-enhanced temperature-insensitive microfiber humidity sensor based on upconversion nanoparticles and cellulose liquid crystal coating, Sensors and Actuators: B. Chemical (2019), doi: https://doi.org/10.1016/j.snb.2019.127517

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SNR-enhanced temperature-insensitive microfiber humidity sensor based on upconversion nanoparticles and cellulose liquid crystal coating

Siqi Hua,1, Shengnan Wua,1, Chenxi Lia, Runze Chena, Erik Forsberga and Sailing Hea,*

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Centre for Optical and Electromagnetic Research, State Key Laboratory of Modern Optical Instrumentation, Zhejiang Provincial Key Laboratory for Sensing Technologies, Zhejiang University, Hangzhou 310058, China

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(* Corresponding author.TEL.: +86 13305810875, Email address: [email protected] 1 These authors contributed equally to this work.)

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Highlights：

1, A signal-to-noise ratio (SNR) enhanced and temperature-insensitive microfiber sensor based on some upconversion nanoparticles and cellulose liquid crystal (CLC) film coating is proposed.

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2, Due to the photonics band gap effect of CLC, the SNR is enhanced by 12 𝑑𝐵 compared to the structure without the CLC film coating.

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3, The proposed sensor has a much higher sensitivity of 284.75 𝑝𝑚/%𝑅𝐻 and negligible temperature cross-sensitivity, with easy fabrication process and low cost.

Abstract

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Combining upconversion nanoparticles (UCNPs) and cellulose liquid crystal (CLC) film, a microfiber relative humidity (RH) sensor with enhanced signal-to-noise ratio (SNR) is proposed. The sensor head consist of a microfiber successively coated with a UCNP fluorescent layer and a hydrophilic CLC film layer. The UCNP fluorescence spectra is located in the photonic band edge of the CLC film, resulting in mirrorless low threshold lasing emission. The addition of the CLC film enhances the SNR by 12 𝑑𝐵 . The RH response and the temperature-insensitivity are both experimentally verified. The sensitivity of the proposed sensor is 284.75 𝑝𝑚/%𝑅𝐻 . It is a promising structure for highly sensitive and accurate RH monitoring.

Keyword: signal-to-noise ratio enhancement; temperature-insensitive; microfiber sensor; upconversion nanoparticles; cellulose liquid crystal; relative humidity 1 / 17

1. Introduction

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Recent years have seen the development of a wide array of electrical and chemiresistive sensors combined with novel functional materials for a broad range of applications such as e.g. point-ofcare health diagnostics, environmental monitoring, food safety inspection [1-16]. Optical fiber sensors are advantageous for specific applications as they can enable remote real-time sensing in a wide range, free from electromagnetic interference [17-19]. Fiber laser sensors have further advantages such as high signal-to-noise ratio (SNR), narrow linewidth permitting high resolution, and excellent compatibility with fiber sensing systems. To further increase SNR performance and enable additional types of applications of fiber laser sensors, new active materials with high luminescence efficiency and low background noise are desirable. Upconversion nanoparticles (UCNPs) have been identified as promising alternatives to organic fluorophores or quantum dots in areas such as medical imaging, optical sensing and detection [20-23]. UCNPs achieve two-photon or multiphoton luminescence through near-infrared (NIR) excitation and have important applications in biological imaging due to deep penetration in high-scattering tissues, low autoluminescence and good sectioning ability. High-quality UCNPs have high optical and thermal stability and can be routinely synthesized with precise control of solubility, particle size, crystallographic phase, optical properties and shape [24, 25]. In the work presented in this paper, UCNPs are utilized as the fluorescent material in fiber lasers to achieve to high SNR performance for sensing applications. Furthermore, to achieve high resolution in practical applications, it advantageous to utilize a single longitudinal mode laser sensor. Single longitudinal mode laser sensors have been demonstrated utilizing a multitude of designs, such as intracavity saturable absorbers [26], fiber grating etalon mode filters [27], distributed feedback (DFB) structures [28–30], and distributed Bragg reflector (DBR) structures [31, 32]. Such structures are however complicated and bulky. Thus, in order to minimize the sensor head size and reduce cost, we propose to utilize a cellulose liquid crystal (CLC) film synthesized from acid hydrolysis of natural cotton [33, 34] to induce mirrorless laser emission in the visible band [35-38]. The CLC is of chiral nematic phase that is characterized by long-range orientational order of the nanorods, which form a one-dimensional photonic band gap (PBG) structure. In the liquid crystal state, the cholesteric pitch of the helical modulation is typically in the range of tens of micrometers however, upon drying, it can be reduced to sub-micrometer values, enabling Bragg reflection of visible light by dried films [39]. In addition, the CLC film also provides an excellent porous structure [40-41]. While several porous materials have been developed for sensing [14-16], the CLC film is relatively cheap and easy to synthesize. In summary then, the CLC film is a low-cost, easily accessible, bio-safe and porous material in which Bragg reflection for visible light is achievable, and thus it is a very promising material for optical fiber sensing applications. Accurate detection of relative humidity (RH) is of great importance in several areas such as e.g. civil engineering, food storage, chemical manufacturing, and air-conditioning control [18]. Various optical fiber RH sensing configurations have been proposed in the past few decades, based on techniques such as fiber Bragg gratings [42], long period fiber gratings [43], surface plasma resonance [44], fiber interferometers [45-47] and fiber laser-based sensors [48]. However, RH detectors based on these suffer from temperature crosstalk that limits their usability in practical applications. In contrast, the RH sensor presented in this paper that is based on a microfiber that is 2 / 17

successively coated by a UCNP-diffused ultraviolet (UV) gel and hydrophilic CLC film, displays excellent SNR enhancement and temperature-insensitivity. A coating system was developed inhouse for this study. Lasing emission is demonstrated and the SNR enhancement measured. Compared to other types of fiber RH sensors, the structure demonstrated in this paper is easy to fabricate at low cost, has a relative high sensitivity of 284.75 𝑝𝑚/%𝑅𝐻 and negligible temperature cross sensitivity. Table 1 compares critical parameters and performance of several typical optical fiber RH sensors as well as the sensor presented in this work.

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2. Sensing structure and fabrication process

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Fig. 1. (a) The schematic diagram of the proposed sensor head. (b) - (e) steps of the sensor fabrication process.

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The schematic diagram of the proposed sensor structure is shown in Fig. 1(a). The structure is an optical fiber sensor of reflective type, made by a tapered fiber successively coated by an UCNP fluorescent layer and a CLC film. In order to achieve a uniform fluorescent layer on the tapered fiber surface, a UV glue was chosen as the gel matrix to act as the host medium for the UCNPs. This as the UCNP was initially synthesized to a powder state, which is difficult to uniformly coat on the tapered fiber. The CLC film is used to enhance the UCNP fluorescence field and act as a porous material allowing water vapor to permeate from the surroundings. Figs. 1 (b) ~ (e) outline the sensor fabrication steps. Prior to coating, the fiber taper is made from a standard single mode fiber (SMF-28, Corning, NY, USA) using a conventional flame-heated taperdrawing technique [49], as shown in Fig. 1(b). The overall tapered region is approximate 20 𝑚𝑚 and the diameter of the thinnest region is about 2 𝜇𝑚. The UCNPs (𝑁𝑎𝑌𝐹4 : 𝐸𝑟, average diameter of 50 𝑛𝑚) were synthesized by a coprecipitation method [25]. To be brief, 0.4 𝑚𝑚𝑜𝑙 lanthanide triacetate composed of 69.5% 𝑌(𝐶𝐻3𝐶𝑂2 )3 , 30% 𝑌𝑏(𝐶𝐻3 𝐶𝑂2 )3 , 0.5% 𝐸𝑟(𝐶𝐻3 𝐶𝑂2 )3 were dissolved in 2 𝑚𝐿 deionized water. The solution was transferred to a three-necked flask, placed in a magnetic stirring heating mantle. 3 𝑚𝐿 oleic acid and 7 𝑚𝐿 1-Octadecene were slowly added to the above solution. Then the mixture was heated to 150 ℃ and kept for 90 min. After the solution was cooled to room temperature, 5 𝑚𝐿 methanol containing 1 𝑚𝑚𝑜𝑙 𝑁𝑎𝑂𝐻 and 1.6 𝑚𝑚𝑜𝑙 𝑁𝐻4 𝐹 was added and the mixture was heated to 50 ℃ and kept for 30 min. Subsequently the solution was heated to 100 ℃ to evaporate the methanol and further heated to 280 ℃ and kept for 120 min under the protection of 𝑁2 . Then 6 𝑚𝐿 ethanol was added after the solution was cooled 3 / 17

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to room temperature again. The solution was centrifuged at 13500 𝑟𝑝𝑚 for 15 min at room temperature to collect the 𝑁𝑎𝑌𝐹4 : 𝐸𝑟 nanoparticles. The nanoparticles were washed twice with 4 𝑚𝐿 ethanol and 4 𝑚𝐿 methanol. Finally, the UCNPs were dispersed in cyclohexane and stored in a refrigerator. To prepare the fluorescent layer, the UCNPs are diffused into the UV glue (NOA 61) through ultrasound dispersion. Fig. 2(a) shows a transmission electron microscope (TEM, JEM1200EX, JEOL, Japan) image of the UCNPs and Fig 2(b) shows the fluorescing UCNP-UV gel sample when excited by a 980 nm laser source in dark background.

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Fig. 2. (a) The TEM picture of the UCNPs. (b) The dark image of the fluorescing UCNP-UV gel sample excited by 980 nm laser. SEM images of (c) the cleaved CLC film and (d) the side view.

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The UCNP-UV gel was coated onto the fiber tapered zone (as illustrated on Fig. 1 (c)), using a program-controlled coating system developed in-house, as illustrated in Fig. 3. The fiber to be coated is firstly fixed parallel to a linear motor. The tip of the needle loaded with a drop of UCNPUV gel is then made to precisely touch the fiber with the help of a microscope. By then moving the tip at controlled speed along the region to be coated, a uniform and desired coating is achieved. In our experiment, the coating velocity was set at 0.2 𝑚𝑚/𝑠, and the coating region was approximate 20 𝑚𝑚. Immediately following the coating procedure, the UCNP-UV gel film is cured by UV exposure for 5 𝑚𝑖𝑛. The measured diameters of the microfiber with and without UCNP-UV gel were approximately 2.4 𝜇𝑚 and 1.8 𝜇𝑚, respectively. Thus, the thickness of the UCNP-UV gel layer was approximate 0.3 𝜇𝑚.

Fig. 3. Schematic diagram of the in-house developed program-controlled coating system used for coating. 4 / 17

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In order to enhance the fluorescence field and achieve lasing emission, a CLC film is added as an additional layer to provide Bragg reflection. Both ends of the tapered fiber were fixed on a clean quartz slide leaving the tapered area suspended in midair. A drop of prepared CLC solution was dipcoated on the tapered region, as illustrated on Fig. 1(d). Through drying process at 25 ℃ for 2 hours, the CLC film layer covered on the tapered fiber was finally formed, as shown in Fig. 1(e). The cellulose nanocrystals (CNCs) used here were formed through sulphuric acid hydrolysis of natural cotton (details of this synthesis processes are published elsewhere [33]). Colloidal suspension of CNCs in water forms a chiral cholesteric (or nematic) liquid crystalline phase. A polarizing optical microscope (POM) image of the CNC bulk suspension exhibit nematic and chiral nematic phases with typical Schlieren and fingerprint texture, which verifies the LC state after fabrication. In this study, a solid CLC film is formed by drying the CNC suspension. After drying process, the solid CLC film that has a left-handed helical structure displays the optical properties of a chiral nematic liquid crystal, however it no longer has a phase change. The center reflected wavelength of solid CLC films is determined by the chiral nematic pitch. Fig 2(c) and (d) shows scanning electron microscope (SEM, Ultra 55, Carl Zeiss, Germany) images of the cleaved solid CLC film coated on the slide, from which we can clearly see that it has a rough and porous surface but a periodic internal structure providing Bragg reflection. The CLC film can be treated as a one-dimensional (1D) PBG structure and lasing based 1D PBG has already been predicted and verified from the perspective of photonics density of state (DOS) [35-38]. The DOS is used to describe the number of eigenstates available at each energy level that can be occupied and is defined as the inverse slope of the dispersion relation [49]: 𝜌 = |𝑑𝑅𝑒(𝑘)⁄𝑑𝜔| (1) where 𝜔 and 𝑘 are the frequency and the wave vector of light, respectively. By solving the one-dimensional wave equation for periodic Bloch eigenfunctions, the eigenvalue equation yields the form 𝑘 = 𝑘(𝜔) as an implicit dispersion relation [38]. This equation has no real solution in the range of frequencies 𝜔 corresponding to the PBG, i.e. the frequencies for which the CLC film forms a stop band. However, the dispersion relation diverges at the band edge of a 1D PBG structure, which causes the DOS to dramatically increase and the photon group velocity 𝑣𝑔

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to approach zero. Assuming that the active medium has a linear intrinsic gain per unit length of g, the multiples of the enhancement can be expressed as: 𝑀 = g𝑒𝑓𝑓 ⁄𝑔 = 𝑙𝑒𝑓𝑓 ⁄𝑙 = 𝑐𝑛⁄𝑣g (2) where g𝑒𝑓𝑓 is the effective gain per unit length caused by the PBG effect; 𝑙𝑒𝑓𝑓 is the effective

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optical path length; 𝑐 is the velocity of light in vacuum and 𝑛 is the refractive index of the active medium. It can be seen that, in theory, when 𝑣g → 0, g𝑒𝑓𝑓 → ∞. This implies that if the florescence

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spectrum of the active medium overlaps with the band edge of the CLC film, then the effective optical path length increase near the photonic band edge can lead to a significant gain enhancement. In this study, pump light guided by a lead-in SMF can excite spontaneous fluorescence emission of the UCNP (active medium). Due to the PBG effect of CLC film, fluorescent light of the resonance wavelength will undergo multiple reflections in the CLC layer that forms a compact mirrorless laser resonance cavity resulting in enhanced stimulated radiation. As a result, the received Rayleigh backward scattering from the laser emission can achieve highly improved SNR and sensing resolution. It worth noting that the emission wavelength will change with the shift of the band edge of the CLC, namely, the shift of reflected spectrum of CLC film. Fig. 4 (a) shows a schematic diagram of 5 / 17

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the incident light reflected by the CLC film. The reflectance peak λ𝑚𝑎𝑥 is dependent on the average refractive index and the helical pitch that can be increased or decreased via moisture absorption or desorption due to the hydrophilicity of CLC film.

Fig. 4. Schematic diagrams of (a) incident light reflected by the CLC film, and (b) reversible size increases and decreases due to absorption or desorption of moisture.

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3. Experimental results and discussion

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Fig. 5. Sensor spectrum recording setup.

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The experimental setup to record the spectrum is shown in Fig. 5. A CW 980 nm laser diode (DRIVER-D-2A-M, Suzhou Rugkuta Optoelectronics, China) with tunable output power serves as the pump source to excite the UCNP-UV glue layer. The reflected signal coupled is fed back through a coupler to an optical spectrum analyzer (OSA, Ocean Optics USB2000+, USA) that has a resolution of 0.45 𝑛𝑚. Data storage and analysis is done by a computer. To evaluate the enhanced SNR performance, the output spectra with and without the CLC film at different pump power were recorded. The whole experimental process was carried out at 38 % RH level and 25 ℃. Fig. 6(a) displays the evolution of the fluorescence spectra as the pump power increased when the tapered fiber was covered only by the UCNP gel. The inset in Fig 6(a) shows the change of the peak intensity at 582 𝑛𝑚. Fig. 6(b) displays the corresponding spectra for when the tapered fiber was covered by both the UCNP gel and the CLC film and the inset shows the change of the peak intensity at 579 𝑛𝑚. We can clearly see that the sensor output signal is significantly stronger in the case where the CLC film is present. Even though the UCNP itself has a relatively high SNR compared to traditional fluorescent materials due to its low auto-luminescence, there is no obvious fluorescent output signal under low pump power without the CLC film. However, 6 / 17

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a narrowed lasing signal with considerable intensity could be detected at the same pump power when the CLC film was present, which we attribute to the photonic band edge enhancement effect of the CLC film.

Fig. 6. The evolution of the output spectra with increasing pump power, with the fiber coated by (a) only UCNP gel and (b) both the UCNP gel and the CLC film. The insets show the changing peak intensities at 582 𝑛𝑚 and 579 𝑛𝑚 respectively.

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In order to further demonstrate the enhanced SNR performance, the reflective spectrum of the obtained CLC film was recorded by a POM (Olympus BX53M) at 38 % RH level and 25 ℃, which is shown as the black line in Fig. 7. The reflective spectrum is located in the visible band, with a peak wavelength at 550 𝑛𝑚 and full width at half maximum (FWHM) of 124 𝑛𝑚. In addition, we recorded the spectra at the same pump power of 9.4 𝑚𝑊 for when the microfiber was coated by the UCNP layer with (red line) and without (blue line) the CLC film layer present. The SNR of the signal was improved by 12 𝑑𝐵 with the CLC film present as compared to without. Furthermore, the FWHM of the output lasing emission (1.9 𝑛𝑚) is 10.0 𝑛𝑚 narrower than the original UCNP fluorescent signal (11.9 𝑛𝑚), which helps to achieve much higher sensing precision and resolution. As mentioned above, here the mirrorless low-threshold CLC based lasing emission is due to the periodicity of the refractive index inside the CLC film, which induces PBG effect. The emission of the UCNPs at the stop band of the CLC-PBG is significantly suppressed whereas the emission near the band edges is greatly enhanced. Thus, sharp lasing emission can be obtained even at a low pump power.

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Fig. 7. Black line: The reflective spectrum of the planar CLC film. Blue line: The spectrum of the tapered fiber coated with UCNP-UV gel only. Red line: The spectrum of the tapered fiber coated successively with the UCNP-UV gel and the CLC film.

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As mentioned above, the hydrophilic CLC film will absorb or release water molecules to reach an equilibrium with the ambient atmospheric water vapor, resulting in changes of the cholesteric pitch in the CLC film, i.e. changing the PBG and consequently the wavelength of the laser emission. Fig. 8(a) shows micrographs of the planar CLC film at different RH levels (magnification of × 20), which visually verifies the swelling effect induced by the increasing RH with its color changing from green to red. Fig. 8(b) shows the schematic diagram of the experimental setup for RH testing. The fabricated sensing probe fixed on a quartz slide was put into an airtight chamber that was fabricated in-house. The RH in the chamber can easily be altered by adjusting the flow rates of dry nitrogen gas and water vapor. A commercial hygrometer (SMART SENSOR AS847) with a RH resolution of 0.1 % RH and sampling frequency of 2.5 𝐻𝑧 was used to calibrate the RH. During the RH experiment, the temperature in the airtight chamber was kept at 25 ℃ and the pump power was kept constant at 300 𝑚𝑊. The RH was then successively set to 38 %, 54 %, 68 %, 80 %, and 98 %, each for about 30 minutes after which a stable reflective spectrum was recorded. These are shown in Fig. 8(c). Fig. 8(d) shows the reflected laser wavelength shift against different RH levels, which is in accordance to the analysis above. It can be seen that the lasing emission wavelength is approximately linearly redshifted with increasing RH level. The sensitivity was calculated to be 284.75 𝑝𝑚/%𝑅𝐻 with the R-squared value of 0.99, which is relatively higher than the previously reported optical fiber RH sensors [42-46]. It should be noted that the output intensity varied and seemed irregular for different lasing wavelengths, which may be attributed by different lasing thresholds in turn dependent by multiple parameters, such as the fluorescent efficiency, the effective optical length, the reflectivity of the CLC film and the loss of resonant cavity at the specific wavelength. However, the uncertainty of the lasing intensity has no influence on RH sensing because of the wavelength demodulation scheme.

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Fig. 8. (a) × 20 micrographs of the planar CLC film at different RH levels. (b) Schematic diagram of the experimental setup for RH testing. (c) and (d) show the output spectra and the laser emission wavelengths at RH levels of 38 %, 54 %, 68 %, 80 %, and 98 %.

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As temperature cross-sensitivity always affects RH sensing performance, the temperature response of the proposed structure was also examined. The sensor head was placed in an airtight oven and the temperature adjusted from 30 ℃ to 40 ℃ in steps of 1 ℃, while the RH level was kept constant at 65 % throughout. Fig. 9(a) shows the micrographs of the planar CLC film at different temperatures (magnification of × 20). As the temperature increases, the color of the CLC film appears almost unchanged, which qualitatively verifies its temperature-insensitivity. Fig. 9(b) displays the evolution of the output spectra at varying temperatures. As the temperature increase, the lasing emission wavelength remains stable while the peak intensity fluctuates slightly with no obvious regularity. In addition to the superior optical and thermal stability of the UCNPs [24, 25], this phenomenon can potentially be attributed to the coactions of thermal expansion effect and moisture sorption capacity at different temperatures. At higher temperature the CLC film has stronger capacity of moisture desorption, i.e. the CLC film can absorb less moisture at high temperature than at low temperature when the ambient RH is constant, which induces a slight blueshift of the reflected spectrum. At the same time, thermal expansion effect of the CLC film induces a slight redshift of the reflected spectrum. The combination of these two effects thus keeps the reflected spectrum of CLC stable within a certain temperature range when the RH is constant. A more rigorous study of the mechanisms involved and derivation of a detailed temperature response merits further investigation. Compared with previous studies [42-48], the presented sensor has no temperature cross-sensitivity, or it is too low to be measured by the OSA used, which indicates that it has outstanding reliability and precision for RH measurements.

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Fig. 9. (a) × 20 micrographs of the planar CLC film at different temperatures. (b) output spectra at varying temperatures displaying strong temperature stability. 4. Conclusion

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In summary, a simple robust sensor structure based on a tapered fiber coated with a UCNP fluorescent layer and a CLC film layer has been proposed and fabricated, and we found it to have a SNR and sensing precision that are significantly improved. Due to the PBG effect of the CLC film, the SNR is enhanced by 12 𝑑𝐵 and the FWHM of the output emission is effectively reduced. Experimental results show that the sensor has a linear RH response with a high sensitivity of 284.75 𝑝𝑚/%𝑅𝐻 and the temperature cross-sensitivity is negligible for when using this sensor for typical RH sensing applications. Compared to most of the existing fiber-optic RH sensors, this novel CLC based optical fiber sensor has the advantages of low cost, high SNR, high RH sensitivity and negligible temperature crosstalk. Furthermore, because of the porous structure, by combining this sensor with selected functional materials it could be used for a wide range of chemical or biological detector applications.

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Credit Author Statement：

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Sailing He proposed the sensing structure and the construction of the measurement setup. Sailing He is one of the founders of this research, being responsible for evaluating the results of the tests, reviewing and editing the manuscript. Siqi Hu and Shengnan Wu completed the experiment, analyzed the test results, cataloged references, and prepared the manuscript. Siqi Hu and Shengnan Wu contributed equally to this work. Shengnan Wu also part-funded to this research. Erik Forsberg analyzed the experimental results and improved the theoretical derivations. He also part-funded to this research. Runze Chen prepared the upconversion nanoparticles samples. Chenxi Li prepared the cellulose liquid crystal samples. 10 / 17

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 91833303 and 61774131), the National Key Research and Development Program of China (No. 2018YFC1407503), the China Postdoctoral Science Foundation (No. 2018M642423), and the Fundamental Research Funds for the Central Universities (No. 2019FZA5002). References

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Author Biographies

Siqi Hu received the B.S. degree from South China University of Technology, Guangzhou, China, in 2016 and she is currently completing her doctor's degree in major of optical engineering in Zhejiang University, Hangzhou, China.

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Shengnan Wu received the Ph.D. degree from the Center for Optical and Electromagnetic Research (COER), Zhejiang University, Hangzhou, China, in 2018. He is now a postdoctor at the College of Optical Science and Engineering, Zhejiang University. His research interests mainly focus on novel fiber-optic machining, fiber grating devices and distributed fiber sensing technology.

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Chenxi Li received the B.S. degree from Xi Dian University, Xi'an China, in 2016 and she is currently completing her doctor's degree in major of optical engineering in Zhejiang University, Hangzhou, China.

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Runze Chen received the B.S. degree from Changchun University of Science and Technology, Changchun, China, in 2018 and she is currently completing her master's degree in major of optical engineering in Zhejiang University, Hangzhou, China.

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Erik Forsberg received his M.Sc. degree in engineering physics and Ph.D. degree in photonics in 1996 and 2003 respectively, both from the Royal Institute of Technology (KTH) in Sweden. He also studied business and economics at the Stockholm School of Economics in Sweden. During 2000 he was a visiting scientist at Hokkaido University in Japan and a postdoctoral fellow at KTH in 2003. He was a faculty member at Zhejiang University (ZJU) in China between 2004 and 2008. From 2009 until 2012, Dr. Forsberg was the Founding Graduate Dean (assoc.) at the Higher Colleges of Technology (HCT) in the United Arab Emirates. In 2013 he rejoined ZJU as an Assoc. Professor. His current research interests include nano-lasers, plasmonics and optical fiber devices. Sailing He received the Licentiate of Technology and the Ph.D. degree from the Royal Institute of Technology (KTH), Stockholm, Sweden, in 1991 and 1992, respectively. Since 1992, he has worked at KTH as an assistant professor, an associate professor, and a full professor. He also serves as the director for JORCEP (a Sino-Swedish joint research center of 15 / 17

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photonics) and a professor at Zhejiang University (China). His current research interests include subwavelength photonics and their applications. He has authored/co-authored about 600 papers in refereed international journals. He is a Fellow of IEEE, OSA, SPIE, and the Electromagnetics Academy.

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Table 1 Comparison of the typical optical fiber RH sensors. Year

Configuration

Sensitive material

Range

Sensitivity

Temperature crosstalk

[42]

2015

Fiber Bragg grating

PI1

11-97%

13.6 𝑝𝑚 /%𝑅𝐻

24.5 𝑝𝑚/℃

[43]

2015

Long period fiber grating

PAH2 and PAA3

20-80%

63 𝑝𝑚 /%𝑅𝐻

411 𝑝𝑚/℃

[44]

2018

Fiber surface plasmon resonance

PVA4

40-90%

1.01 𝑛𝑚 /%𝑅𝐻

0.20 𝑛𝑚/℃

[45]

2019

Fabry-Perot interferometer

PVDF5

20-80%

32.54 𝑝𝑚 /%𝑅𝐻

−15.2 𝑝𝑚/℃

[46]

2018

Photonic crystal fiber

GQDs6 and PVA4

30-90%

[47]

2016

Fiber sagnac interferometer

No coating

30-90%

[48]

2019

Fiber ring laser sensor

PI1

20-80%

Tapered fiber laser

UCNPs and CLC

38-98%

1

Polymide Poly(allylamine hydrochloride) 3 Poly(acrylic acid, sodium salt) 4 Polyvinyl Alcohol 5 Polyvinylidene fluoride 6 Graphene quantum dots

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−0.0901 𝑛𝑚 0.2356 𝑛𝑚/℃ /%𝑅𝐻 422 𝑝𝑚 /%𝑅𝐻

0.066 𝑛𝑚/℃

3.6 𝑝𝑚 /%𝑅𝐻

12.15 𝑝𝑚/℃

𝟐𝟖𝟒. 𝟕𝟓 𝒑𝒎 /%𝑹𝑯

None in 𝟑𝟎 ℃~𝟒𝟎 ℃

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This work

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Reference