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Abstract
Low temperature sensitivity and low spectral contrast are serious but common issues for most Fabry Perot (FP) sensors with an air cavity. In this paper, a high-temperature-sensitive and spectrum-contrast-enhanced Fabry Perot interferometer (FPI) is proposed and experimentally demonstrated. The device is composed of a hollow cylindrical waveguide (HCW) filled with polydimethylsiloxane (PDMS) and a semi-elliptic PDMS end face. The semi-elliptic PDMS end face increases the spectral contrast significantly due to the focusing effect. Experimentally, the spectral contrast is 11.97 dB, which is two times higher than the sensor without semi-elliptic PDMS end face. Ultra-high temperature sensitivity of 3.1501 nm/°C was demonstrated. The proposed sensor exhibits excellent structural stability, high spectral contrast and high temperature sensitivity, showing great potential in biomedicine, industrial manufacturing, agricultural production and other applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Compared with other conventional sensors, the fiber optic sensors offer many inherent advantages such as high flexibility, high radiation resistance, compact size and low cost. Beyond that, the fiber optic sensors have been extensively used in microfluidic chips [1,2], structural health monitoring [3,4] and biochemical analysis [5–7]. Air-cavity-based fiber sensors have been widely studied for temperature sensing. However, spectral contrast is only several decibels [8–12], and the low thermo-optical coefficient of air also causes the temperature sensitivity to be smaller [13,14].
Vernier effect can considerably improve the sensing sensitivity. It has been extensively used in different types of interferometers, such as Fabry-Perot interferometer (FPI) [15,16], Mach-Zehnder interferometer (MZI) [17,18] and Sagnac interferometer (SI) [19,20]. Moreover, Vernier effect has been used in the detection of various physical quantities such as temperature [21,22], strain [23,24], refractive index [14,25], humidity [26,27] and magnetic field [28,29] sensing.
Experimentally, polydimethylsiloxane (PDMS) has been used for temperature sensing due to its good hydrophobicity, high radiation resistance, high elasticity and high thermal-optical coefficient [30]. As of now, there have been several methods for filling liquid into hollow cylindrical waveguide (HCW). Yang et al. used the capillary effect to fill the mercury into HCW. However, it generated a bubble when splicing single-mode fiber (SMF) with HCW [31]. Because the presence of a bubble, the structural stability will decrease. Cao et al. delivered liquid to a fiber micro-cavity by employing femtosecond laser induced breakdown technology [32]. However, it is expensive to manufacture, and there is still some residual air in the cavity. At present, the main method to improve the spectral contrast is to reduce the Fabry Perot (FP) cavity length [33,34]. This will make the FSR larger and the vernier spectrum formed after spectral superposition is not obvious. Another method is to plate a metal film on the end face to improve reflectivity [35,36]. However, the metal film will oxidize and the thickness is not easy to control.
Here, a novel FPI composed of a HCW filled with PDMS and a semi-elliptic PDMS end face is proposed and fabricated. A groove is created by using a fiber optic polishing machine (FOPM) and fusion splicer, and the HCW can be filled with PDMS by using the capillary effect. Comparing with the sensor reported by Nan et al. [37], our sensor has higher spectral contrast. That is due to the focusing effect of the semi-elliptic PDMS end face. The Vernier effect was used to further improve the temperature sensitivity. Experiments show that the contrast of the reflection spectrum is many times higher than the previously reported FP sensor with a cylindrical air cavity. The temperature sensitivity can achieve −3.1501 nm/°C within the temperature range of 25°C-95°C, and the linear response is 0.9848. The proposed temperature sensor has the advantages of high sensitivity, high spectral contrast, compact size, low cost and convenient manufacturing. Undoubtedly, it will benefit applications in biomedicine, industrial production, aerospace and other fields.
2. Fabrication and principle
The structural diagram of the FP sensor is shown in Fig. 1(a). A section of HCW that has an oblique gap is fusion spliced with the SMF, and it is fully filled with PDMS. Additionally, there is a semi-elliptic PDMS structure at the end face of the HCW. Thus, the FPI is constructed of two mirrors M1, M2. Figure 1(b) is the polarizing microscope image of the sensor. The length of the FP cavity is 201.63 µm, and the surface of the semi-elliptic PDMS structure is relatively smooth.
Fig. 1. (a) Schematic of the temperature sensor. (b) Polarizing microscope image of the temperature sensor.
The SMF used in the experiment is Corning SM-28e with a core diameter of 9 µm and a cladding diameter of 125 µm. The length of the HCW is 181.29 µm, and the inner diameter of the HCW is 80 µm. The thermosensitive material used in the experiment, PDMS solution, was manufactured by thoroughly stirring basic agent (Sylgard 184-a) and curing agent (Sylgard 184-B) with a mixture ratio of 10:1. Then, the mixture is centrifuged to exclude the mixed air bubbles. The refractive index of the PDMS measured by the Abbe refractometer is 1.407.
The electric field component of the two reflective surfaces M1, M2 is expressed as [38]:
The total reflected electric field component is:
The light intensity can be calculated as:
where E0 is the electric field component of the incident light. The total reflected light intensity of sensing FPI (FPIS) and reference FPI (FPIr) can be expressed as:The FSR of FPIS and FPIr can be expressed as:
The vernier spectrum can be obtained by the following formula:
Figure 2(a), (b) show the optical path in the FP cavity without semi-elliptic PDMS and the optical path in the FP cavities with semi-elliptic PDMS, respectively. In the case of the flat end face, the incident light diffused into HCW and reflected at the PDMS-air interface, causing some optical loss and interference. However, the semi-elliptic PDMS structure allows more light reflected back to the SMF. Thus, the spectral contrast (difference between the maximum and minimum spectral values) of the sensor with a semi-elliptic end face will be higher.
Fig. 2. (a)The optical path of the sensor without semi-elliptic PDMS. (b) The optical path of the sensor with semi-elliptic PDMS.
Two reflection spectra can be superimposed to form the Vernier effect, which can improve by multiple folds the sensing sensitivity. Figure 3(a), (b) are the reflection spectra calculated by Eq. (5). The two FP cavities length L1 and L2 are set 225 µm and 220 µm, respectively. The reflection loss and transmission loss are all set 0.1. The FSRs measured in Fig. 3(a), (b) are 3.78 nm and 3.49 nm, respectively. They agree well with the value (3.66 nm) calculated by Eq. (7). Figure 3(c) clearly shows the Vernier effect envelope.
Fig. 3. (a), (b) Reflection spectra of sensor with cavity lengths of 225 µm and 220 µm, respectively. (c) The Vernier effect were realized by superimposing two FP spectra from above.
The Vernier effect amplification factor (M) can be expressed as:
where FSRr is the FSR of reference FPI. FSRs is the FSR of the sensing FPI.The electric field distribution and reflection spectrum are obtained by using finite element analysis software Comsol. The refractive index of the SMF core is 1.462, the refractive index of the cladding is 1.45, the refractive index of PDMS is 1.407, the incident light wavelength is 1.55 µm, and the length of the cavity is 230 µm. Figure 4(a) shows the electric field distribution. It can see that the beam is reflected at the PDMS-air interface and reflected back to the SMF. The reflection spectrum is shown in Fig. 4(b). The FSR (3.71 nm) is almost consistent with the result calculated by Eq. (7).
Fig. 4. (a) Electric field norm obtained by finite element analysis. (b) Reflection spectrum obtained by finite element analysis.
Figure 5(a)-(f) illustrate the fabrication process of the temperature sensor. We firstly used a fiber fusion splicer (KL-280G) to fusion spliced a section of HCW (core diameter is 80 µm) with SMF. Then, the SMF-HCW structure was inserted into the fiber clamp of the FOPM, and the HCW against the sandpaper to form an oblique angle between the HCW and sandpaper. The polishing process requires just two minutes, and the air core was exposed to air. After that, the polished structure was fusion spliced with another SMF to form a groove and cut it by using the precise optical fiber cutting device. Thanks to the two grooves and capillary effect, the PDMS can automatically fill the entire air cavity. The PDMS was solidified by using a thermostatic heater, and the curing process requires 4 hours. After that, the solidified structure was vertically immersed into PDMS to form a semi-elliptic end face, and the curing process was repeated. As the above fabrication process, a PDMS cavity with high thermo-optical coefficient and a semi-elliptic PDMS end face with focusing effect have been successfully made. Thus, it can significantly improve the temperature sensitivity and the spectral contrast.
3. Experimental results and discussion
A temperature experimental setup was built to investigate the performance of this temperature sensor, as shown in Fig. 6. The incident light is derived from the supercontinuum sources (SC-5), and it can be guided into the 50:50 coupler through the circulator. The reflection spectrum is measured by the OSA (YOKOGAWA AQ6370B). The thermostatic heater (X1020TBD) is used to control environment temperature, and the two sensors FPI1 and FPI2 are connected with the 50:50 coupler. In order to make the sensor based on the Vernier effect achieves maximum sensitivity, FPI1 is put on the thermostatic heater, and FPI2 is put on the optical platform [14].
The differences between the sensor with semi-elliptic PDMS end face and the sensor without semi-elliptic PDMS end face have been investigated. As shown in Fig. 7(a), the sensor with a FP cavity length of 175.84 µm was produced. The reflection spectrum is indicated with black line, as presented in Fig. 7(c). The spectral contrast is 5.68 dB. When there is a semi-elliptic end face, the length of the FP cavity increases to 201.63 µm, as shown in Fig. 7(b). The reflection spectrum is indicated with orange line, as presented in Fig. 7(c). The spectral contrast is 11.91 dB. In summary, the change of spectral FSR is very small. The sensor with a semi-elliptic PDMS end face has higher spectral contrast, due to the focusing effect. This also helps to improve the quality factor. The higher quality factor is beneficial to reduce interference signals from irregular structures. Besides, it is more conducive to accurately finding the peak of the vernier spectrum and reducing the error.
Fig. 7. (a) Polarizing microscope image of the sensor without semi-elliptic PDMS end face. (b) Polarizing microscope image of the sensor with semi-elliptic PDMS end face. (c) The reflection spectrum of the sensor with the cavity length of 175.84µm and 201.63µm, respectively.
Figure 8(a), (b) show two reflection spectra of the sensors with different cavity lengths of 201.36 µm and 205.28 µm, and the two FSRs are 3.89 nm and 3.64 nm, respectively. The experimental results were in good agreement with the Eq. (7). We have used the spectrometer to scan the spectrum many times and the spectrum has no obvious deformation. It shows good stability. In addition, in the process of heating and cooling, the wavelength is almost unchanged at the same temperature. Therefore, the repeatability of the sensor is also good. After superimposing two FP spectra, the obvious envelope based on Vernier effect is observed, as shown in Fig. 8(c). To avoid interference from irregular structures, Fast Fourier transform (FFT) was applied to the Vernier effect envelope, as shown in Fig. 8(d). The shape of the filtered spectrum agrees well with Fig. 3(c). To observe the shift of the Vernier effect envelope more clearly, the upper envelope curve acquired by curve fitting is shown Fig. 8(d) in orange line.
Fig. 8. (a), (b) Reflection spectra of FP sensor with different cavities length of 201.36 µm and 205.28 µm, respectively. (c) The Vernier effect envelope obtained by superimposing two FP spectra. (d) FFT result corresponding to (c).
The upper envelopes curve were obtained and shifted when the ambient temperature varies. With the increase in temperature, the envelope in Fig. 9(a) experienced a blue shift. Figure 9(b) shows the relationships between the temperature and the dip wavelength for different temperature values. By using the Vernier effect and PDMS with a high negative thermo-optical coefficient, this novel FPI exhibits an excellent linear response to external temperature variation, the achieved sensitivity being up to −3.1501 nm/°C with R2 of 0.9848.
Fig. 9. (a) The shift of the upper envelope with different temperatures in the range of 25-95°C. (b) The dip wavelength shift in response to the temperature change.
The fabricated sensor is compared with other FP sensors and temperature sensors, as shown in Tables 1 and 2. It is clear that our sensor has the advantages of higher sensitivity and wider temperature detection range.
Table 1. Comparison of the proposed sensor with other Fabry Perot sensor
Table 2. Comparison of the proposed sensor with other temperature sensor using vernier effect
4. Conclusion
In conclusion, a temperature sensor with high sensitivity and high spectral contrast was proposed. Specifically, the sensor has a bullet-shaped fiber cavity filled with PDMS. The experiments demonstrate that, the contrast of reflection spectrum in such sensor can reach to 11.97 dB. By using the Vernier effect and PDMS, the sensor exhibits a high temperature sensitivity of −3.1501 nm/°C within the range of 25 to 95°C, and the linear response is 0.9848. The temperature sensitivity is several times higher than the previously reported temperature sensors. The proposed sensor has advantages of high sensitivity, high spectral contrast and simple manufacturing process, making it has great potential in temperature sensing applications.
Funding
National Natural Science Foundation of China (11874010, 61965006); Natural Science Foundation of Guangxi Province (2020GXNSFAA238040, 2021GXNSFAA075012, 2021GXNSFAA220057).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
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