High-temperature and high-pressure sensor based on MI and FPI cascade structure optical fiber interferometer and preparation method of high-temperature and high-pressure sensor

Abstract

The invention provides a high-temperature and high-pressure sensor based on an optical fiber interferometer of a cascade structure of a Michelson interferometer (MI) and a Fabry-Perot interferometer (FPI) and a preparation method of the high-temperature and high-pressure sensor. According to the sensor, the conical single-mode fiber and the coreless fiber are welded to form an MI structure, the MI structure is sealed in the quartz capillary to form an FPI cascade structure, and due to the fact that the sensor adopts an arc discharge non-adhesive packaging technology, high temperature and high pressure can be measured at the same time. Wherein temperature sensing can be realized by monitoring MI characteristic spectrum change, and pressure measurement can be realized by monitoring FPI cavity length change caused by pressure. The sensor has the advantages of strong anti-interference capability, high stability and the like, and is suitable for temperature and pressure measurement under complicated working conditions of petrochemical engineering, aerospace and the like.

Description

Technical Field

[0001] The present invention belongs to the field of optical fiber sensing technology, and specifically relates to a high-temperature and high-pressure sensor based on an optical fiber interferometer with a cascade structure of a Michelson interferometer (MI) and a Fabry-Perot interferometer (FPI), and a preparation method thereof. Technical Background

[0002] In the oil and gas extraction sector, accurate downhole temperature and pressure measurement is crucial for ensuring operational safety. Traditional electrical sensors often face numerous challenges, such as susceptibility to electromagnetic interference, insufficient high-temperature resistance, and variable long-term stability. Fiber optic sensors, with their electromagnetic interference resistance, enable stable signal transmission in strong electromagnetic environments. Their excellent high-temperature resistance enables continuous operation in extreme temperatures, and their high sensitivity ensures accurate measurement of even the smallest pressure and temperature changes. Therefore, fiber optic sensors have become an ideal alternative to traditional electrical sensors, offering significant technical advantages and broad application prospects.

[0003] Currently, fiber optic temperature and pressure sensors are primarily categorized into interferometric and fiber Bragg grating (FBG) types. Interferometric fiber optic sensors, with their high sensitivity, compact structure, and strong environmental adaptability, are suitable for temperature and pressure measurements. However, traditional single fiber interferometric sensors suffer from cross-sensitivity between temperature and pressure, which affects measurement accuracy. To overcome this limitation, researchers have recently proposed cascading two fiber optic sensors to construct dual-parameter sensors. Common cascade structures include FBG-on-FBG, FPI-on-FPI, and FPI-on-FBG. For example, Chinese invention patent application number 201310606330.0, "Fiber optic FP cavity pressure sensor with temperature self-compensation," provides a fiber optic sensor for temperature and pressure measurement. The sensor structure consists of a fiber Bragg grating (FBG) and a fiber FP cavity welded together using a CO2 laser. The Chinese utility model patent application number 202121458869.2, "A diaphragm-type, small-size fiber Bragg grating sensor for measuring high temperature and high pressure," provides a sensor for a dual-grating series structure. The sensor consists of a cylindrical frame made of titanium alloy, a cover plate, and an end cap that are sealed by laser welding to form a closed air cavity, with an integrated temperature grid and pressure grid inside. The Chinese invention patent application number 202311157784.4, "A high-temperature resistant, wide-range sapphire FP interferometer fiber pressure sensor," utilizes a three-layer sapphire structure to achieve temperature compensation through independent temperature-sensitive cavities and pressure-sensitive cavities. However, the above solutions either rely on complex structures or require special materials, resulting in high manufacturing costs and difficult processes. Therefore, it is of great value to develop a fiber optic sensor with a simple structure, low cost, and the ability to effectively eliminate temperature-pressure cross-sensitivity.

[0004] The present invention provides a high-temperature, high-pressure sensor based on a cascaded fiber interferometer (MI) and fiber-optic interferometer (FPI) structure, and its fabrication method. The sensor utilizes a tapered single-mode fiber (SMF) fused with a coreless fiber (NCF) to form the MI structure, which is then sealed in a quartz capillary tube to form the MI and FPI cascade. The sensor senses temperature by monitoring changes in the MI characteristic spectrum and measures pressure by monitoring changes in the FPI cavity length caused by pressure.

[0005] The present invention relates to a high-temperature, high-pressure sensor based on a fiber interferometer with a cascaded structure of fiber optic interferometer (MI) and fiber optic fiber interferometer (FPI), and a preparation method thereof. The preparation steps are as follows: First, a fiber fusion splicer is used to precisely control the taper parameters to draw a tapered structure from a SMF. Then, a high-precision fiber cutting platform is used to cut a certain length of SMF and fuse it with an NCF. After fusion, the NCF is cut using a fixed-length cutting platform, thereby completing the preparation of the MI structure. Subsequently, another section of SMF is taken and inserted into the ends of a quartz capillary tube along with the prepared fiber MI structure. Arc discharge technology is used to fuse the capillary wall to the SMF and the fiber MI structure, respectively, to prepare a cascaded structure of the fiber MI and FPI. During this process, a spectrometer is used to monitor spectral changes in real time to obtain an ideal spectral condition.

[0006] The quartz capillary in the high-temperature, high-pressure sensor based on the cascaded fiber interferometer (MI) and fiber-optic interferometer (FPI) structure described in this invention has a length of 20-40 mm, an outer diameter of 300-1000 μm, an inner diameter of 126-135 μm, and a distance between the two melting points of 10-30 mm. The SMF and NCF used in the MI have an outer diameter of 125 μm, an SMF core diameter range of 8-9 μm, a tapered SMF length of 500-600 μm, a tapered SMF waist diameter of 45-50 μm, and an FP cavity length of 100-1000 μm.

[0007] The working principle of the present invention is as follows: When light passes through the tapered region of the SMF, due to the reduction in fiber diameter, some light leaks from the core to the cladding. At the SMF-NCF interface, light in the SMF core and cladding enters the NCF. When the light reaches the NCF-air interface, some of the light is reflected according to the Fresnel reflection law. The reflected light from the core and cladding can couple in the tapered region, causing interference between different propagation modes. Phase difference in MI structure It can be expressed as:

[0008]

[0009] Where n co is the effective refractive index of the cladding mode; n clis the effective refractive index of the fiber core, L1 is the length of the sensing area of MI; λ is the wavelength. When m is an integer, the central wavelength of the interference peak of MI is:

[0010]

[0011] Where Δn eff is the effective refractive index difference between the core and the cladding. When the ambient temperature changes, Δn eff and L1 vary with the thermo-optical effect and thermal expansion effect. Therefore, the wavelength of the interference peak will change, and the temperature sensitivity of MI can be expressed as:

[0012]

[0013] Where S MI-T is the temperature sensitivity of the fiber MI, Δλ is the wavelength offset of the fiber MI, and ΔT is the temperature change. When the ambient pressure changes, the structural design of the MI prevents the pressure from causing the MI interference peak to shift.

[0014] For FPI, when the capillary is subjected to an external pressure, it deforms, resulting in a change in the cavity length. Using the generalized Hooke's law, the change in cavity length ΔG can be derived as:

[0015]

[0016] Where P is the ambient pressure acting on the capillary, L is the length between the two melting points, and r o With r i are the outer and inner diameters of the capillary, E is the Young's modulus of the material, and μ is the Poisson's ratio. Therefore, the pressure sensitivity of the FPI can be expressed as:

[0017]

[0018] Since the FPI cavity is filled with air, the change in the refractive index of air with temperature changes is negligible, so only the change in cavity length caused by thermal expansion of the material needs to be considered. Since the optical fiber and capillary have different thermal expansion coefficients, the cavity length will change with temperature. The temperature sensitivity of the FPI can be expressed as:

[0019]

[0020] Where L2 is the length from melting point 1 to the NCF end face, L3 is the length from melting point 2 to the reflecting fiber end face, α f and α c are the thermal expansion coefficients of the optical fiber and capillary, respectively.

[0021] The reflection spectrum processing of the sensor of the present invention realizes the precise separation of MI and FPI interference signals through Fourier transform and frequency domain filtering technology. The specific method is: first, fast Fourier transform is performed on the reflection spectrum to convert the wavelength domain signal into a frequency domain signal, and the high-frequency component corresponding to MI and the low-frequency component corresponding to FPI are used to design high- and low-frequency bandpass filters to extract independent frequency bands, and then the interference spectrum of MI and FPI is reconstructed through inverse Fourier transform. For the MI signal, the temperature is measured by tracking the wavelength drift of the interference peak; for the FPI signal, the cavity length change of the FPI is solved by combining discrete Fourier transform and least squares spectral matching algorithm, and a sensitivity matrix is established to eliminate the cross-interference between temperature and pressure, thereby realizing the simultaneous measurement of pressure and temperature.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] 1. The high-temperature and high-pressure sensor based on the MI and FPI cascade structure fiber interferometer of the present invention has a simple structure. During the preparation process, only a fusion splicer and standard optical fiber materials are required, without the need for complex processes. In addition, it has good long-term stability in high-temperature and high-pressure environments.

[0024] 2. The present invention separates the MI and FPI spectra through Fourier transform and bandpass filtering, and combines inverse Fourier transform and least squares spectrum matching algorithm to demodulate the cavity length change. The algorithm is simple and reliable and supports real-time monitoring.

[0025] 3. This invention achieves simultaneous temperature and pressure measurement by cascading a Michelson interferometer and a Fabry-Perot interferometer. The MI is sensitive to temperature, while the FPI is sensitive to both pressure and temperature. Establishing a sensitivity matrix effectively eliminates cross-interference and improves measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a flow chart for preparing a high-temperature and high-pressure sensor based on a fiber optic interferometer with a cascaded structure of MI and FPI according to the present invention;

[0027] Figure 2 This is a schematic structural diagram of a high-temperature and high-pressure sensor based on a MI and FPI cascaded fiber interferometer of the present invention;

[0028] Figure 3 It is the reflection spectrum of the sensor at room temperature and pressure, the frequency domain diagram and the interference spectrum of MI and FPI obtained by signal processing;

[0029] Figure 4 is the interference spectrum of MI at different temperatures under normal pressure and the fitting curve of interference peak wavelength and temperature;

[0030] Figure 5 is the interference spectrum of FPI at different temperatures under normal pressure and the fitting curve of FP cavity length and temperature;

[0031] Figure 6 is the interference spectrum of FPI at room temperature and different pressures and the fitting curve of FP cavity length and pressure;

[0032] Figure 7 is the fitting curve of FP cavity length and pressure at different temperatures; DETAILED DESCRIPTION

[0033] The specific implementation of the present invention will be further described in detail below with reference to the accompanying drawings.

[0034] See also Figure 1 , the present invention relates to a flow chart for preparing a high-temperature and high-pressure sensor based on an MI and FPI cascade structure fiber interferometer. The present invention relates to a method for preparing a high-temperature and high-pressure sensor based on an MI and FPI cascade structure fiber interferometer, the steps of which are as follows: first, a fiber fusion splicer is used to precisely control the tapering parameters to draw the SMF into a tapered structure, and then a high-precision fiber cutting platform is used to cut a certain length of SMF and fuse it with the NCF. After the fusion, the NCF is cut by a fixed-length cutting platform to complete the preparation of the MI structure. Afterwards, another section of SMF is taken, and it and the prepared fiber MI structure are respectively inserted into the two ends of a quartz capillary, and the capillary wall is respectively fused with the SMF and the fiber MI structure by arc discharge technology, thereby preparing a fiber MI and FPI cascade structure. During this process, a spectrometer is required to monitor the spectrum changes in real time to obtain an ideal spectrum condition. The quartz capillary in the fiber-optic high-temperature, high-pressure sensor based on the MI and FPI cascade structure described in this invention has a length of 20-40 mm, an outer diameter of 300-1000 μm, an inner diameter of 126-135 μm, and a distance between the two melting points of 10-30 mm. The SMF and NCF used in the MI have an outer diameter of 125 μm, an SMF core diameter range of 8-9 μm, a tapered SMF length of 500-600 μm, a tapered SMF waist diameter of 45-50 μm, and an FP cavity length of 100-1000 μm.

[0035] After the above preparation process, the sensor probe of the present invention can be obtained. Figure 2The figure shows a schematic diagram of the structure of the high-temperature and high-pressure sensor based on the MI and FPI cascade structure fiber interferometer of the present invention. Its working principle is: when light passes through the conical area of ​​the SMF, due to the reduction in the diameter of the optical fiber, part of the light will leak from the core to the cladding. At the interface of SMF-NCF, the light in the SMF core and cladding will enter the NCF. When the light reaches the NCF-air interface, part of the light will be reflected according to the Fresnel reflection law. The reflected light from the core and cladding can be coupled in the conical area, and interference occurs between different propagation modes to form MI. The transmitted light at the NCF-air interface continues to transmit in the air and is reflected at the air-SMF interface. The reflected light generated at the NCF-air interface interferes with the reflected light generated at the air-SMF interface to form FPI.

[0036] See also Figure 3 The present invention is to analyze the reflection spectrum of the sensor at room temperature and pressure, the frequency domain diagram and the interference spectrum of MI and FPI obtained by signal processing. Figure 3 (a)) to perform fast Fourier transform and convert the wavelength domain signal into the frequency domain signal (as shown in Figure 3 (b) ), using the characteristics of the high-frequency component corresponding to MI and the low-frequency component corresponding to FPI, high- and low-frequency band-pass filters are designed to extract independent frequency bands, and then the interference spectra of FPI and MI are reconstructed by inverse Fourier transform, as shown in Figure 3 As shown in Figures (c) and (d) above, for the MI signal, the temperature is measured by tracking the wavelength drift of the interference peak. For the FPI signal, the FPI cavity length change is calculated by combining discrete Fourier transform with the least squares spectral matching algorithm. A sensitivity matrix is then established to eliminate the cross-interference between temperature and pressure, enabling simultaneous measurement of pressure and temperature.

[0037] See also Figure 4 The interference spectrum of the sensor of the present invention at different temperatures under normal pressure and the fitting curve of the interference peak wavelength and temperature are obtained. Figure 4 (a) It can be seen that when the ambient temperature rises from 25℃ to 150℃, the MI interference peak redshifts. Figure 4 (b) It can be seen that the wavelength of the MI interference peak is linearly related to temperature, and the temperature sensitivity of MI is 42.95 pm / ℃.

[0038] See also Figure 5 , the interference spectra of FPI at different temperatures under normal pressure and the fitting curves of FP cavity length and temperature, from Figure 5 (a) It can be seen that when the ambient temperature rises from 25℃ to 150℃, the FPI interference peak red shifts. Figure 5 (b) It can be seen that the FP cavity length is linearly related to temperature, and the temperature sensitivity of FPI is 136.26nm / ℃.

[0039] See also Figure 6 The interference spectrum of FPI under different pressures at room temperature and the fitting curve of FP cavity length and pressure of the sensor of the present invention are shown in FIG. Figure 5 (a) It can be seen that when the ambient pressure increases from 0 MPa to 35 MPa, the FPI interference peak shifts to the blue. Figure 5 (b) It can be seen that the FP cavity length is linearly related to pressure, and the pressure sensitivity of FPI at room temperature is -194.97nm / MPa.

[0040] See also Figure 7 The fitting curves of the FP cavity length and pressure at different temperatures of the sensor of the present invention show that under the condition of constant temperature, the FP cavity length decreases with increasing pressure, and the fitting curves at different temperatures are approximately parallel. By taking the average of the pressure sensitivity at different temperatures, it can be obtained that the pressure sensitivity of FPI is -195.38nm / MPa.

[0041] Since FPI is sensitive to both temperature and pressure, a sensitivity matrix is established to demodulate temperature and pressure changes. The matrix expression is as follows:

[0042] Where S FPI-T With S FPI-P are the sensitivity of FPI to temperature and pressure, S MI-T and S MI-P are the sensitivities of MI to temperature and pressure, respectively. When the ambient temperature and pressure change simultaneously, the temperature and pressure values can be accurately demodulated by substituting the change in FP cavity length and the drift in MI interference peak wavelength into the above equation.

Claims

1. A high-temperature and high-pressure sensor based on a cascaded fiber interferometer structure of a Michelson interferometer (MI) and a Fabry-Perot interferometer (FPI) and a method for preparing the same are proposed, characterized by: The sensor is constructed by fusing a tapered single-mode fiber (SMF) with a coreless fiber (NCF) to form an MI structure, which is then sealed in a quartz capillary tube to form a cascaded FPI structure. The sensor can sense temperature by monitoring changes in the MI characteristic spectrum and measure pressure by monitoring changes in the FPI cavity length caused by pressure.

2. The method for preparing a high-temperature, high-pressure sensor based on a fiber interferometer with a cascaded structure of MI and FPI as described in claim 1 is as follows: first, using a fiber fusion splicer, by precisely controlling the taper parameters, the SMF is pulled into a tapered structure. Then, a certain length of SMF is cut using a high-precision fiber cutting platform and fused with the NCF. After fusion, the NCF is cut to a fixed length using a fixed-length cutting platform, thereby completing the preparation of the MI structure. Thereafter, another section of SMF is taken, and it and the prepared fiber MI structure are respectively inserted into the two ends of a quartz capillary. The capillary wall is fused to the SMF and the fiber MI structure respectively using arc discharge technology, thereby preparing the fiber MI and FPI cascade structure. During this process, a spectrometer is required to monitor the spectral changes in real time to obtain the ideal spectral conditions.

3. In the fiber-optic high-temperature, high-pressure sensor based on the MI and FPI cascade structure as described in claim 1, the quartz capillary has a length of 20-40 mm, an outer diameter of 300-1000 μm, an inner diameter of 126-135 μm, and a distance between the two melting points of 10-30 mm. The SMF and NCF used in the MI have an outer diameter of 125 μm, an SMF core diameter range of 8-9 μm, a tapered SMF length of 500-600 μm, a tapered SMF waist diameter of 45-50 μm, and an FP cavity length of 100-1000 μm.

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