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Abstract
The sensors with a wide gas pressure detection range are urgently demanded in many industrial applications. Here, we propose a gas pressure sensor based on an all-solid open Fabry–Pérot interferometer, which is prepared by using optical contact bonding to ensure high structural strength and high-quality factor of 8.8 × 105. The applied pressure induces a change in the refractive index of the air, leading to the shift of the resonant spectrum. The pressure is detected by calibrating this shift. The sensor exhibits a pressure sensitivity of 4.20 ± 0.01 nm/MPa in a pressure range of 0 to 10 MPa and has a minimum pressure resolution of 0.005 MPa. Additionally, it shows a lower temperature cross-sensitivity of -0.25 kPa/°C. These findings affirm that the sensor achieves high-sensitivity pressure sensing across a wide detection range. Moreover, owing to its exceptional mechanical strength, it holds great promise for applications in harsh environments, such as high temperature and high pressure.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fiber optical gas pressure sensors offer significant advantages [1–4], including excellent stability, high sensitivity, fast response time, and immunity to electromagnetic interference. These qualities make them highly promising for applications in these fields such as aerospace [5,6], energy industry [7,8], and environmental monitoring [9,10]. In the past, various designs of gas pressure sensors had been proposed, including fiber Bragg gratings (FBGs) [2,11], Mach-Zehnder interferometers (MZIs) [12,13], and Fabry–Pérot interferometers (FPIs) [9,14–16]. Among them, pressure sensors based on FPIs have emerged as particularly promising due to their simple structure, robust measurement performance, and cost-effectiveness. Consequently, they have become a key focus of research in the field of optical pressure sensing [17,18].
Optical F-P gas pressure sensors can be categorized based on their construction into two types: thin-diaphragm closed structure and diaphragm-free open-cavity structure [3,6,17]. The former detects pressure by measuring changes in cavity length, while the latter senses pressure by detecting changes in air refractive index. Thin-diaphragm pressure sensors are typically composed of diaphragms made from various materials within a closed F-P micro-cavity. These sensors can achieve sensitivity ranging from several to several tens of nm/MPa. However, their pressure measurement range is limited due to the mechanical strength constraints of the diaphragm [2,6,19,20]. In 2017, Zhang et al. [21] prepared a novel fiber-optic micro-cavity pressure sensor using a thin polyvinyl chloride (PVC) diaphragm. This sensor exhibited an ultra-high sensitivity of 65.5 nm/MPa. However, its usability was limited to the pressure range of 0-0.06 MPa. In 2020, Wei et al. [22] utilized femtosecond laser three-dimensional printing to fabricate a polymer thin film pressure sensor on the fiber end face with a maximum pressure sensitivity of 4.10 nm/MPa. Nonetheless, it was constrained by the thin film and could only be used in pressure environments below 1.1 MPa. In 2022, Fu et al. [23] developed an F-P pressure sensor by injecting polydimethylsiloxane (PDMS) into a capillary tube in segments. This approach expanded the pressure measurement range to 2 MPa and had maintaining a sensitivity of 20.63 nm/MPa. To achieve a larger pressure measurement range, researchers have turned their attention to fiber optic open-cavity pressure sensors, which have emerged as an ideal research direction.
Open-cavity gas pressure sensors are typically designed with a solid structure that does not include a diaphragm. This construction allows for a wider pressure measurement range and reduces vulnerability to damage [5,24,25]. In the open-cavity structure, the F-P cavity is in direct contact with the surrounding air. The gas pressure is detected by monitoring the shift in the resonance spectrum, which occurs as a result of the refractive index change caused by the varying pressure. In 2016, Hou et al. [26] achieved pressure measurement in the range of 0-2 MPa with a sensitivity of 3.59 nm/MPa using a femtosecond laser to create an air cavity on a hollow fiber. In 2019, He et al. [2] developed a pressure sensor by splicing two sections of silica capillary tubes (SCTs) with different inner diameters to a single-mode fiber, enabling pressure measurements up to 2.7 MPa with a sensitivity of 4.24 nm/MPa. In 2022, Rana et al. [27] proposed a new F-P gas pressure sensor prepared by photonic crystal fiber (PCF), which extended the pressure measurement range to 5 MPa, but its sensitivity is only 0.5 nm/MPa. Although these developments have expanded the pressure detection range of fiber optic gas pressure sensors, there is still a scarcity of reports on high-sensitivity gas pressure sensing capable of operating in high-pressure environments of up to 10 MPa.
In order to solve the issue that the sensor is easily damaged in a large pressure environment and achieve high-sensitivity pressure sensing, we propose a fiber optic pressure sensor based on an all-solid open FPI. The F-P cavity is prepared by the optical contact bonding to obtain high mechanical strength, and its bonding surfaces are observed by scanning electron microscopy (SEM). The two surfaces of the open F-P cavity are coated with a high-reflectivity SiO2 films, to enhance the finesse contrast to achieve higher sensitivity. The self-designed sensor detects pressure by converting pressure change into a shift in resonance peak. To verify the performance of the sensor, we have built a high-pressure experimental system to test the pressure detection range and sensitivity of the sensors. Furthermore, we conducted high-temperature experiment for the sensor and discussed its temperature dependence in depth, hoping to understand the effect of temperature change on the results of the pressure experiment.
2. Materials and methods
2.1 Introduction to sensor
Figure 1(a) illustrates the configuration of the proposed gas pressure sensor. The sensor comprises an F-P gas pressure sensing unit, two glass capillary tubes, and two optical fiber collimators. The sensing unit is prepared from ultra-low-expansion (ULE) glass, highly reflective films S1 and S2, and anti-reflective films S3 and S4 through an optical contact technology, which will be described specifically in the next subsection. The optical fiber collimators are connected to single-mode fibers and has a beam diameter of 300 µm. The glass capillary tubes are installed at the periphery of the collimators and connected to the sensing unit by ceramic adhesive. The incident light has an interference between S1 and S2 to generate the resonance peaks. When the applied gas pressure change the molecular density in the air cavity, it will lead to a change in the refractive index of the air, which in turn cause a shift in the resonance spectrum, as shown in Fig. 1(b).
2.2 Preparation of sensors
We use optical contact bonding to prepare the F-P cavity with high Q-factor. The optical contact bonding is an optical processing technology based on intermolecular attraction. When the smoothness of the surface exceeds a certain limit, the intermolecular distance between two adjacent surfaces is very small, and the molecules will attract each other due to electromagnetic interaction. Two smooth-surfaced components are held together by the intermolecular attraction.
The preparation process of the self-designed F-P cavity is illustrated in Fig. 2. Firstly, ULE glass (${\alpha _l} = 0 \pm 0.02 \cdot {10^{ - 6}}/\textrm{K}$) from Corning Incorporated is cut into pieces of corresponding size and the long holes are made in the intermediate sheet (Fig. 2(a)). Then, the inner and outer surfaces of the upper sheet and lower sheet are coated with a highly reflective SiO2 film (thickness of 100 ± 10 nm, reflectivity > 99%, incident angle of 0°) and an anti-reflective SiO2 film (thickness of 100 ± 10 nm, reflectivity < 0.2%, incident angle of 0°), respectively (Fig. 2(b-c)). Furthermore, three pieces of glass are aligned and stacked together to achieve the initial bonding and cut into complete strips (Fig. 2(d)). Subsequently, the three layers of sheets are laminated using an optical contact bonding, and are slipped to separate each whole from each other (Fig. 2(e)). The sheets are then cleaned and tested for lamination strength after the initial optical contact process and cut them into standard size F-P cavity chunks (Fig. 2(f)). Once the bonding surface is confirmed to be fine, a second optical contact process is performed to further enhance the bonding (Fig. 2(g)). Finally, after the final cleaning and testing, a complete F-P cavity is obtained (Fig. 2(h)).
In Fig. 3, we perform scanning electron microscopy (SEM) for test the bonding interface of the F-P cavity. When the F-P cavity is magnified 13X, it is observed to have a well-integrated structure with a flat surface and no collapsed edges, as shown in Fig. 3(a). Subsequently, the optical contact interface is magnified 4.6KX in Fig. 3(b), revealing a tightly combined bonding surface without bubbles or holes. These results indicate that the F-P gas pressure probe exhibits high bonding strength to ensure the reliability of the sensing structure.
Fig. 3. The surface diagram of F-P cavity is observed under SEM. (a) The surface diagram of F-P cavity on 13X magnification. (b) The bonding interface on 4.6KX magnification.
After checking that the F-P cavity is qualified, we prepared the sensor by a platform of automatic alignment. Firstly, the optical fiber collimators are aligned using a six-dimensional automatic alignment platform to minimize optical power loss. Next, the F-P cavity is placed on the middle fixation, and the angle of the displacement stage is manually adjusted until the interference spectrum reaches the desired state. Finally, all individual components are bonded and integrated into an F-P gas pressure sensor using a high-temperature ceramic adhesive. The actual sensor is depicted in Fig. 4(a). The core of the sensor is placed in a designed ceramic package to ensure that it is not easily damaged in harsh environments, as shown in Fig. 4(b). After completing the preparation of the sensor, we calibrate the linewidth of the sensor's resonance peak to 0.00176 nm using a tunable narrow linewidth laser (laser center wavelength of 1550.12 nm) and a high-precision oscilloscope (Tektronix, MSO64). The Q-factor of the sensor is obtained as 8.8 × 105 [25], as shown in Fig. 4(c).
Fig. 4. (a) The substance of the gas pressure sensor. (b) Actual sensor with ceramic package. (c) Calibration of the Q-factor.
2.3 Principal analysis
As shown in Fig. 1, the two parallel planes S1 and S2 constitute the two reflective surfaces of the F-P cavity, which are coated with a SiO2 film with a reflectivity of 99%, and the distance between them is 2 mm. Consequently, the incident light with a constant phase difference will undergo multi-beam interference within the F-P cavity, and the total intensity of transmitted light at the output can be expressed as follows:
where T and R are the transmittance and reflectance of the high-reflectivity plane, respectively, $\delta $ is the phase difference between two adjacent beams, ${\lambda _0}$ and ${\theta _\textrm{t}}$ are the wavelength and the refraction angle of the initial beam in vacuum, n and L are the refractive index of the air and distance between the two planes.When different pressures are exerted on the central open sensor, it will induce variations in the molecular density within the air cavities, to change the refractive index of the air. Because the sensor is prepared using ULE glass, which has a very low expansion coefficient, the change in cavity length due to material expansion is so small as to be negligible in the experiment. According to the updated Edlen equation [2] and Rüeger equation [28], the refractive index of air and the wavelength of light in a standard atmospheric pressure environment have the following relationship:
According to Eq. (4), the pressure and refractive index have a linear relationship. Due to a pressure change of 1 Pa at room temperature (20 °C) resulting in a refractive index change of 2.6511 × 10−9, we analyze the effect of different pressures on the resonant spectrum using Eq. (1), as shown in Fig. 5(a). It can be observed from Fig. 5(a) that the resonant spectrum shifts towards longer wavelengths with increasing pressure. Figure 5(b) illustrates a linear relationship between pressure variation and the peak wavelength of the resonant spectrum. The analytical pressure sensitivity of the sensor is determined to be 4.11 nm/MPa. The correlation between temperature and the change in the refractive index of air is depicted in Fig. 5(c) by Eq. (4), indicating a decreasing change rate of refractive index with increasing temperature. Figure 5(d) displays the shift of the resonance spectrum towards shorter wavelengths with the increasing of temperature, accompanied by a gradual decrease in the amplitude of the shift.
Fig. 5. (a) The analytical model results of resonant spectra under different pressures. (b) The analytical model results of pressure sensitivity. (c) The relationship between temperature and the change in refractive index. (d) The shift of the resonant spectrum at different temperatures.
3. Results and discussion
The experimental setup is constructed to evaluate the performance of the fiber-optic F-P gas pressure sensor in a high-pressure environment, as shown in Fig. 6. The F-P pressure sensor is positioned within a gas pressure tank, where the gas pressure can be adjusted using an air pump (ConST162). The pressure is measured and calibrated using a standard manometer (ConST211) with an accuracy of 0.05% full scale. The pressure tank is made of steel and can withstand air pressures up to 20 MPa. Light from an amplified spontaneous emission (ASE) light source (TOP photonics ASE-C + L, wavelength range: 1520-1600 nm, maximum optical power output: 18 mW) is directly emitted into the sensor's input. The transmitted light through the sensor is collected by an optical spectrum analyzer (OSA, YOKOGAWA, AQ6370D). The collected spectral data are processed by a computer for further analysis of the experimental results. In gas pressure experiments, we select the wavelength of a certain peak point in the spectrum as the tracking object. We observe the shift of the peak center wavelength with pressure change and establish the relationship between them. The measured pressure value is obtained by demodulating the shift of the calibrated wavelength. The gas pressure experiment is conducted at a room temperature of 20 °C.
The pressure in the tank is incrementally increased from 0 to 10 MPa with a step size of 1 MPa, and then decreased back to 0 MPa using the same step size. Each pressure level is maintained for 5 minutes, and the corresponding wavelength value of the peak point in the spectrum is recorded. The experiment is repeated three times, and the results are presented in Fig. 7(a). The peak wavelength of the sensor's spectrum gradually shifts towards longer wavelengths as the pressure increase. This shift remains consistent during both pressure rise and reduction. And the sensor exhibits excellent linearity in all three repeatability experiments. Next, the pressure sensitivity of the sensor is measured in 0.5 MPa increments in the pressure range of 0-10 MPa, as shown in Fig. 7(b). The sensitivity of the sensor is determined through linear fitting. The sensitivities during the three repetitive pressure cycle tests are listed in Table 1. The average sensitivity is 4.20 ± 0.01 nm/MPa in the pressure range of 0-10 MPa. There is minimal disparity between the measured sensitivity and the theoretical value of 4.11 nm/MPa in Fig. 5(b), thereby validating the accuracy of the air pressure sensing.
Fig. 7. (a) The variations of spectral peak wavelength versus gas pressure of 0-10 MPa. (b) The pressure sensitivity of the sensor during the 0-10 MPa test.
Table 1. Comparison of sensitivity in several experiments
The test performance in low pressure is a crucial indicator for pressure sensors. The theoretical pressure detection limit of the sensor is 0.42 kPa, which can be obtained from the sensor's Q-factor and pressure sensitivity. In the experiment, the OSA has a minimum wavelength resolution of 0.02 nm, the corresponding minimum pressure resolution is approximately 0.005 MPa. We conduct the test in 0.005 MPa increments across a pressure range of 0-0.05 MPa. Figures 8(a) and (b) depict the spectral shifts during pressure increase and decrease, respectively, exhibiting consistent spectral changes. The interference spectrum displays a notable red shift with pressure increment, while it shifts in the opposite wavelength direction with pressure reduction. Figure 8(c) exhibits a linear fit between the resonant peak's wavelength of the sensor and the pressure variation from 0 to 0.05 MPa, a sensitivity of 4.08 nm/MPa is obtained for low-pressure detection. This sensitivity closely aligns with the sensitivity for high-pressure detection, affirming the sensor's capability for high-sensitivity pressure sensing throughout the entire pressure range.
Fig. 8. (a) The red shift of the peak wavelength of the spectrum with the increase of air pressure from 0 to 0.05 MPa. (b) The blue shift of the peak wavelength of the spectrum with the reduction of air pressure from 0.05 to 0 MPa. (c) The sensitivity of the sensor in low pressure. (d) Test stability of the sensor at different pressures over 90 minutes.
To investigate the stability of the sensor under different pressures, the peak wavelength variation of the sensor is examined at pressures of 0.1, 5, and 10 MPa over a duration of 90 minutes. The wavelength values are recorded at 10-minute intervals. The results in Fig. 8(d) show that the sensor maintains stable measurements at various pressures over an extended period, and has greater stability in low-pressure measurements. Specifically, the maximum change in the peak wavelength of the sensor's spectrum is 0.002 nm at 0.02 MPa, 0.008 nm at 5 MPa, and a slightly larger change of 0.012 nm at 10 MPa. This discrepancy is attributed to the pressure instability in the pressure tank and pressurization unit at higher pressures.
In order to evaluate the temperature tolerance of the sensor, we place it in a tube furnace with a temperature accuracy of ±1 °C. Figure 9(a) illustrates the peak wavelength of the spectrum with the temperatures of 20-160 °C. The red circles represent the wavelength values during temperature increase, while the blue square correspond to the wavelength values during temperature decrease. It is evident that the peak wavelength of the sensor's spectrum continuously drifts towards shorter wavelengths with the increase of temperature from 20 to 160 °C. Conversely, the spectrum drifts towards longer wavelengths with the decrease of temperature, which is consistent with the theoretical calculation. The observed wavelength values during the cycles of temperature increase and decrease exhibit significant overlap, indicating the sensor's strong consistency at different temperatures. In Fig. 9(b), a linear regression is conducted on the data points ranging from 20 to 70 °C. The resulting temperature sensitivity of the sensor in this linear region is determined to be -1.06 pm/°C. Consequently, the pressure sensor's low-temperature cross-sensitivity can be calculated as -0.25 kPa/°C. This means that the sensor is very little influence by temperature when sensing pressure. The change in refractive index due to a pressure change of 10 MPa is 3.4 × 104 times that of 1 °C. Therefore, the effect of temperature change on the test results at room temperature is very small. As a result, the experimental error caused by temperature can be negligible for large-range pressure detection. Thus, the gas pressure sensor can reliably operate in temperature environments of up to 160 °C. Exploring alternative coupling methods may enable the application of the sensor at elevated temperatures.
Fig. 9. (a) The peak wavelength of the spectrum with the temperatures of 20-160 °C. (b) The linear fit of the peak wavelength in the range of 20-70 °C
Compared to the reported gas pressure sensors listed in Table 2, although the gas pressure sensitivity of the proposed sensor is slightly lower than that of diaphragm sensors [3,21,23] and Vernier effect-based sensors [5,8,14], but our sensor exhibits a broader measurement range (0–10 MPa) and lower temperature cross-sensitivity (-0.25 kPa/°C). As a result, the sensor is less influenced by temperature and offers enhanced practicality for large-range gas pressure measurement. It can be enhanced to match or even surpass them by adjusting the open cavity structure or incorporating a cascaded Fabry-Pérot etalon to establish the Vernier effect. In contrast to sensors based on hollow-core optical fiber structures, the sensors provide superior measurement accuracy and structural robustness, making them suitable for operation in challenging environments.
Table 2. Comparison of different optical gas pressure sensors
4. Conclusion
In summary, we have proposed and experimentally demonstrated a gas pressure sensor based on an all-solid open F-P cavity. The F-P cavity with a high-quality factor is prepared using the optical contact bonding. The compact structure and high mechanical strength of the sensor ensure the suitability for application in high-pressure environments. The sensor exhibits a sensitivity of 4.20 ± 0.01 nm/MPa in the range of 0-10 MPa. Due to the limitations of the wavelength resolution of OSA, the minimum pressure resolution is 0.005 MPa, which can be further enhanced by employing a higher-precision test system. The high-temperature experiments reveal a temperature sensitivity of -1.06 pm/°C for the sensor, corresponding to an extremely low temperature cross-sensitivity of -0.25 kPa/°C. Thin-diaphragm sensors have a high sensitivity and can be used in many applications that require a fast response, such as medical pressure detection. The self-designed sensor may not work well in similar fields. However, our sensors have a wider pressure detection range and lower temperature cross-sensitivity than thin-film pressure sensors and other open sensors. This means that our sensor is less likely to be damaged in harsh environments and is very promising for applications in many industrial fields, such as aerospace, petroleum metallurgy, environmental monitoring.
Funding
National Natural Science Foundation of China (62131018, 12104417, 62205308); Fundamental Research Program of Shanxi Province (202103021222012, 20210302124161, 202103021223202); Shanxi “1331 Project” Key Subject Construction (1331KSC).
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 may be obtained from the authors upon reasonable request.
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