A fiber temperature sensor based on double FP parallel structure

Abstract

The application belongs to the technical field of temperature sensing, and particularly relates to a fiber temperature sensor based on a double-FP parallel structure, characterized in that the fiber temperature sensor is composed of a reference interferometer and a sensing interferometer in parallel, the reference interferometer is composed of a first single-mode optical fiber, a hollow-core optical fiber and a second single-mode optical fiber, and the sensing interferometer is composed of a third single-mode optical fiber, a PDMS cavity and a fourth single-mode optical fiber; wherein the optical path lengths of the PDMS cavity and the hollow-core optical fiber are controlled to be similar but not equal, the free spectral range of the sensing interferometer is similar but not equal to that of the reference interferometer, the vernier effect of the output spectrum is excited, and an envelope formed by fine stripes is formed. The fiber temperature sensor has the advantages of high sensitivity, high linearity, large detection range, good repeatability and good reversibility, and meanwhile, the size of the fiber sensing probe is in the order of microns, and the fiber temperature sensor is suitable for temperature detection in a narrow space.

Description

Technical Field

[0001] This application belongs to the field of temperature sensing technology, and in particular relates to an optical fiber temperature sensor based on a dual FP parallel structure. Background Technology

[0002] Temperature sensors, as key components in modern measurement and control systems, are widely used in industries such as industry, medicine, civil engineering, aerospace, and geological exploration, providing accurate and reliable temperature monitoring and control methods for various sectors. Fiber optic sensing technology utilizes optical fibers as both the sensing element and the information transmission medium. Due to its advantages such as small size, high sensitivity, resistance to electromagnetic interference, and fast response speed, it has been widely applied to the measurement of physical parameters such as temperature, pressure, and refractive index. Among them, the Fabry-Perot (FP) interferometer (FPI) is a sensing structure that achieves high-precision measurement of physical quantities such as temperature by causing interference fringe drift through changes in cavity length. It features simple structure, easy signal demodulation, and easy packaging.

[0003] A basic Fabry-Perot cavity (FPI) consists of a cavity formed between two reflecting surfaces. Light undergoes multiple reflections between the two planes, creating sharp interference fringes. Changes in external parameters such as pressure and temperature can alter the cavity length or the refractive index (RI) of the internal medium, both of which can adjust the optical path difference of the beam within the cavity, ultimately reflected in the sensor's interference spectrum. By precisely demodulating the interference spectrum and analyzing changes in peak positions, intensity distribution, and other characteristics, accurate detection and measurement of external parameters can be achieved. Based on the principle of multi-beam interference, FPIs can be broadly classified into intrinsic Fabry-Perot interferometers (IFPIs) and extrinsic Fabry-Perot interferometers (EFPIs). Typically, IFPIs are used for temperature measurement. To further enhance sensing sensitivity, a vernier effect can be introduced. By constructing a dual-cavity or multi-cavity interference structure, two interference spectra with slightly different free spectral ranges can be superimposed to form an amplified envelope, thereby multiplying the amplification of minute wavelength changes.

[0004] Existing technologies disclose Fabry-Perot interferometer temperature sensors based on photosensitive polymer materials. These sensors solidify photosensitive resin incorporating PDMS onto the end face of a single-mode fiber to form a microscale polymer cavity, achieving a compact, low-cost, and easily fabricated FPI structure. The temperature response of this device primarily originates from the significant thermal expansion effect of the photosensitive polymer and the incorporation of PDMS, and the sensing sensitivity is improved by adjusting the concentration of PDMS. Experimental results show that when the PDMS concentration is 30%, the sensor exhibits a maximum temperature sensitivity of only -1.18 nm / ℃ within the 20-110 °C range. This indicates that FPI fiber optic temperature sensors have significant room for improvement in temperature sensitivity. Summary of the Invention

[0005] In view of this, the purpose of this application is to provide a fiber optic temperature sensor based on a dual FP parallel structure, which has the advantages of high sensitivity, high linearity, large detection range, good repeatability and good reversibility. At the same time, the fiber optic sensing probe is in the micrometer range, making it suitable for temperature detection in confined spaces.

[0006] This application provides an optical fiber temperature sensor based on a dual FP parallel structure. The optical fiber temperature sensor is composed of a reference interferometer and a sensing interferometer connected in parallel. The reference interferometer is composed of a first single-mode fiber, a hollow fiber, and a second single-mode fiber. The sensing interferometer is composed of a third single-mode fiber, a PDMS cavity, and a fourth single-mode fiber.

[0007] In this process, by controlling the optical path lengths of the PDMS cavity and the hollow fiber to be similar but not equal, the free spectral range of the sensing interferometer is similar but not equal to the free spectral range of the reference interferometer, thereby exciting the vernier effect of the output spectrum to form an envelope composed of fine fringes.

[0008] Furthermore, the optical path lengths of the PDMS cavity and the hollow fiber are controlled to be similar but not equal in the following ways:

[0009] A reference interferometer was prepared based on the pre-defined length of the hollow fiber, and the optical path length of the hollow fiber was calculated based on the air refractive index.

[0010] The reference value for the length of the PDMS cavity was calculated based on the optical path length of the hollow fiber and the refractive index of the PDMS near the wavelength of 1550nm.

[0011] The third and fourth single-mode fibers are aligned on a glass slide with an air cavity of a specific length. The alignment of the end faces is observed and confirmed under a microscope, and optical calibration is performed. The specific length of the air cavity is similar to but not equal to the calculated reference value of the PDMS cavity length.

[0012] Connect one side of the circulator to a dual FP parallel structure, and connect the two lines on the other side to the spectrometer and the supercontinuum light source respectively. After the vernier effect occurs in the spectrum appearing on the spectrometer, calculate the free spectral range around 1550 nm.

[0013] UV adhesive was applied to the third and fourth single-mode optical fibers respectively, and cured with a UV lamp to fix the two single-mode optical fibers to the glass slide.

[0014] The bubble-free PDMS solution was dropped between two single-mode optical fibers, then placed in an 80°C drying oven and heated for 40 minutes before being removed. The actual length of the PDMS cavity was then measured and confirmed under a microscope.

[0015] Furthermore, the step of fabricating the reference interferometer according to the pre-set length of the hollow optical fiber includes:

[0016] Use a high-precision fiber optic cleaver to neatly cut the end faces of the first single-mode fiber, hollow fiber, and second single-mode fiber, ensuring that the end faces are perpendicular, smooth, and burr-free, in order to reduce splice loss.

[0017] The cut first single-mode fiber and hollow fiber are placed into the fiber fusion splicer and aligned with the axis. They are then fused together with low discharge intensity and short discharge time to prevent the hollow fiber from collapsing.

[0018] After inspecting and confirming the flatness of the fusion splice end with a microscope, the hollow fiber is cut to the predetermined length, and then a second single-mode fiber is fused to the other end of the hollow fiber in the same way to form a reflective plane.

[0019] Furthermore, the PDMS solution is prepared as follows:

[0020] Dow Corning DC184 PDMS raw material was selected. A certain amount of PDMS raw material and auxiliary curing agent were drawn with a syringe and mixed in a new centrifuge tube at a mass ratio of 10:1. The tube was then placed in a centrifuge for 30 to 60 minutes until the bubbles in the solution disappeared, thus obtaining a PDMS solution.

[0021] Furthermore, before fabricating the reference interferometer and the sensing interferometer, the coating layer of each single-mode fiber is removed using a fiber stripper, the coating layer of the hollow fiber is removed using a flame, and the end face is cleaned with anhydrous ethanol to ensure that the end face of each fiber is clean.

[0022] The fiber optic temperature sensor based on a dual-FP parallel structure provided in this application has the advantages of high sensitivity, high linearity, large detection range, good repeatability, and good reversibility. At the same time, the fiber optic sensing probe is on the order of micrometers, making it suitable for temperature detection in confined spaces. Attached Figure Description

[0023] Figure 1 A schematic diagram of a fiber optic temperature sensor based on a dual-FP parallel structure provided in an embodiment of this application is shown.

[0024] Figure 2 A schematic diagram of the fabrication process of the reference interferometer provided in an embodiment of this application is shown;

[0025] Figure 3 A schematic diagram of the fabrication process of the sensing interferometer provided in the embodiments of this application is shown;

[0026] Figure 4 A schematic diagram of the single-FPI temperature sensing experimental system provided in an embodiment of this application is shown;

[0027] Figure 5 (a) illustrates an embodiment provided in this application. Interference spectrum as a function of temperature;

[0028] Figure 5 (b) illustrates an embodiment provided in this application. Linear fitting plot of sensitivity;

[0029] Figure 6 (a) illustrates an embodiment provided in this application. Interference spectrum as a function of temperature;

[0030] Figure 6 (b) illustrates an embodiment provided in this application. Linear fitting plot of sensitivity;

[0031] Figure 7 A schematic diagram of a temperature detection system based on the vernier effect provided in an embodiment of this application is shown;

[0032] Figure 8 (a) illustrates an embodiment provided in this application. The spectrum of the vernier effect formed after parallel connection;

[0033] Figure 8 (b) illustrates an embodiment provided in this application. The parallel connection forms an FFT transform graph;

[0034] Figure 8 (c) An embodiment of this application is shown. Linear fitting diagram of the peaks and troughs of the envelope wave formed after parallel connection;

[0035] Figure 9 (a) shows a linear fitting diagram of the temperature sensitivity of the peak and trough during the heating process provided in the embodiments of this application;

[0036] Figure 9 (b) shows a linear fitting diagram of the peak and trough temperature sensitivity during the cooling process provided in the embodiments of this application. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this technical solution clearer, the following detailed description, in conjunction with specific embodiments, further illustrates this technical solution. It should be understood that these descriptions are merely exemplary and not intended to limit the scope of this technical solution.

[0038] Example 1:

[0039] First, please refer to... Figure 1The diagram shows a fiber optic temperature sensor based on a dual-FP parallel structure. In the diagram, SMF represents single-mode fiber, SMF1, SMF2, SMF3, and SMF4 correspond to the first, second, third, and fourth single-mode fibers, respectively. FPI1 represents the reference interferometer, and FPI2 represents the sensing interferometer. I 0 represents the intensity of the incident light. L 1 indicates the length of the hollow fiber. R 1 represents the reflectivity of the right end face of the first single-mode fiber. This indicates the reflectivity of the right end face of the third single-mode fiber. R 2 represents the reflectivity of the left end face of the second single-mode fiber. This indicates the reflectivity of the left end face of the fourth single-mode fiber. I R This indicates that FPI1 returns light intensity. This indicates that a single FPI2 returns light intensity. L 2 indicates the length of the PDMS cavity. For example... Figure 1 As shown, the fiber optic temperature sensor is composed of a reference interferometer and a sensing interferometer connected in parallel. The reference interferometer is composed of a first single-mode fiber, a hollow fiber, and a second single-mode fiber. The sensing interferometer is composed of a third single-mode fiber, a PDMS cavity, and a fourth single-mode fiber.

[0040] In this process, by controlling the optical path lengths of the PDMS cavity and the hollow fiber to be similar but not equal, the free spectral range of the sensing interferometer is similar but not equal to the free spectral range of the reference interferometer, thereby exciting the vernier effect of the output spectrum to form an envelope composed of fine fringes.

[0041] Furthermore, the fiber optic temperature sensor is connected to the supercontinuum light source and the spectrometer via an optical coupler for experimental temperature measurements.

[0042] Next, we will introduce the sensing principle of the fiber optic temperature sensor with a dual FP parallel structure:

[0043] The reflection spectra of the two FPIs in a parallel dual-FP structure are formed by typical two-beam interference. According to the FPI principle, the interference intensities of FPI1 and FPI2 can be expressed as follows:

[0044] ; (1)

[0045] ; (2)

[0046] In the formula, These are the refractive indices of hollow fiber (air) and PDMS, respectively. and They are respectively The length of each sensing cavity, The wavelength of light in free space, They are respectively The light intensities of the two coherent beams in the middle, They are respectively The intensity of the two coherent beams in the middle.

[0047] According to interference theory, the tilt angle wavelengths of the reflection spectra of FPI1 and FPI2 are as follows:

[0048] ; (3)

[0049] ; (4)

[0050] In the formula, m The order of the interference fringes.

[0051] The free spectral ranges (FSRs) are as follows:

[0052] ; (5)

[0053] (6)

[0054] In FPI2 temperature sensing, PDMS is a thermosensitive material. When the temperature changes, PDMS expands, and its refractive index changes. The temperature sensitivity of FPI2 can be expressed as follows:

[0055] ; (7)

[0056] In the formula, T Indicates the ambient temperature to be measured. α The coefficient of thermal expansion of PDMS is approximately 9.6 × 10⁻⁶. -4 / ℃, β The thermo-optic coefficient of PDMS is approximately -4.6 × 10⁻⁶. -4 / ℃, refractive index of PDMS near 1550nm wavelength n 2 is approximately 1.41.

[0057] In this application, the PDMS adhesive used is a mixture of an elastic polymer and a curing agent in a ratio of 10:1, and it has a stable coefficient of thermal expansion and thermo-optic coefficient in a temperature range of 25 to 100 °C.

[0058] Therefore, according to the ordinary vernier effect, if Similar but not equal, when they are connected in parallel, the output spectrum of the parallel structure is The superposition of spectra, specifically two sets of interference fringes, creates periodic fringes that fluctuate in intensity. The outer periphery of these fine fringes forms an envelope. When the sensor is used for temperature measurement, its sensitivity can also be amplified. Therefore, according to relevant interference theory, the tilt angle wavelength of the spectral envelope of a dual-FP parallel fiber optic temperature sensor can be determined. and free spectral range They can be calculated separately as follows:

[0059] ; (8)

[0060] ; (9)

[0061] At a wavelength around 1550 nm, Its temperature sensitivity is very low, theoretically around 9.41 pm / ℃. Therefore, it can be used as a reference interferometer. PDMS, on the other hand, is a thermosensitive material and is sensitive to temperature. It can be used as a sensing interferometer. The reference unit is not placed in the measurement environment. When the temperature changes... All parameters are amplified by a factor of several due to the vernier effect, with the envelope wavelength shift, amplification factor, and envelope sensitivity being:

[0062] ; (10)

[0063] ; (11)

[0064] ;(12)

[0065] According to equations (10) and (11), it can be calculated that, theoretically, after connecting the reference interferometer and the sensing interferometer in parallel, the wavelength... At that time, the temperature sensitivity was 23.2 nm / ℃, and the amplification factor was 19.3 times.

[0066] Next, the fabrication methods of the reference interferometer and the sensing interferometer will be introduced:

[0067] To generate the vernier effect, the two FP interferometers must have two similar free spectral ranges (FSRs), and the magnitude of the FSR is directly related to the length of the sensing cavity. Therefore, precise instruments must be used to prepare the FPI. The experimental apparatus used mainly consists of two xyz axis precision fine-tuning displacement platforms, a fiber optic cleaver (Furukawa-S326A), an optical microscope (Caikon-djm300c), a fiber optic fusion splicer (Fitel-S179C), and a lifting platform.

[0068] Before fabricating the reference interferometer and the sensing interferometer, the coating of each single-mode fiber was removed using a fiber stripper to expose the cladding; the coating of the fragile hollow fiber was removed using a flame to avoid damaging its hollow structure; and the end faces were cleaned with anhydrous ethanol to ensure that the end faces of each fiber were clean.

[0069] Next, please refer to... Figure 2 The diagram shows the fabrication process of the reference interferometer. Figure 2 As shown, a reference interferometer is fabricated based on a pre-defined length of hollow optical fiber using the following method:

[0070] Using a high-precision fiber cleaver, the end faces of the first single-mode fiber, hollow fiber, and second single-mode fiber are neatly cut to ensure that the end faces are perpendicular, smooth, and burr-free, thereby reducing splicing loss. The cut first single-mode fiber and hollow fiber are placed in a fiber optic fusion splicer and aligned with the axis. They are then spliced ​​using low discharge intensity and short discharge time to prevent the hollow fiber from collapsing. After inspecting and confirming the flatness of the spliced ​​end face with a microscope, the hollow fiber is cut to the pre-set length. The second single-mode fiber is then spliced ​​to the other end of the hollow fiber in the same way to form a reflective plane.

[0071] As an example, the length of the hollow fiber in this application is set to 218.4 µm. Of course, it can be set to other values ​​according to actual conditions, and this application does not impose any limitations here.

[0072] Next, the optical path length of the hollow fiber is calculated based on the air refractive index; the reference length of the PDMS cavity is calculated based on the optical path length of the hollow fiber and the refractive index of the PDMS near the 1550nm wavelength.

[0073] Here, the hollow fiber serves as the sensing cavity of the reference interferometer. Its optical path length (optical path length = geometric length × refractive index) can be calculated from its length, and this optical path length determines the free spectral range of the reference interferometer. Furthermore, to make the sensing interferometer and the reference interferometer have similar but unequal free spectral ranges, according to equations (5) and (6) above, the following steps are taken: The goal is to make them similar but not equal, meaning the optical path lengths of the PDMS cavity and the hollow fiber are similar but not equal. Based on this, we can... When the lengths are equal, a reference value for the length of the PDMS cavity is calculated so that the length of the PDMS cavity is similar to, but not equal to, the reference value when fabricating the sensing interferometer.

[0074] Next, please refer to... Figure 3 The diagram shows the fabrication process of the sensing interferometer. Figure 3 As shown, after fabricating the reference interferometer, the sensing interferometer is fabricated using the following method:

[0075] The third and fourth single-mode fibers were aligned on a glass slide with an air cavity of a specific length. The alignment of the end faces was observed and confirmed under a microscope, and optical calibration was performed. The specific length of the air cavity was similar to but not equal to the calculated reference length of the PDMS cavity. One side of the circulator was connected to a dual-FP parallel structure, and the two lines on the other side were connected to a spectrometer and a supercontinuum light source, respectively. After the vernier effect was generated in the spectrum appearing on the spectrometer, the free spectral range around 1550 nm was calculated. UV glue was dropped onto the third and fourth single-mode fibers respectively, and cured with a UV lamp to fix the two single-mode fibers to the glass slide. A PDMS solution with air bubbles removed was dropped between the two single-mode fibers, and then placed in an 80°C drying oven for 40 minutes. After that, the actual length of the PDMS cavity was measured and confirmed under a microscope.

[0076] As an example, the actual length of the PDMS cavity in this application is 146.8µm. It should be noted that the final actual length of the PDMS cavity may differ slightly from the specific length of the reserved air cavity. This is because the PDMS expands laterally after being heated, resulting in a slight change in distance.

[0077] Furthermore, the PDMS solution is prepared as follows:

[0078] Dow Corning DC184 PDMS raw material was selected. A certain amount of PDMS raw material and auxiliary curing agent were drawn with a syringe and mixed in a new centrifuge tube at a mass ratio of 10:1. The tube was then placed in a centrifuge for 30 to 60 minutes until the bubbles in the solution disappeared, thus obtaining a PDMS solution.

[0079] Here, since the curing time of PDMS solution at room temperature is relatively long, about 24~48h, prolonged placement may have a certain impact on the experimental results. Therefore, it is necessary to shorten the curing time of PDMS by heating, that is, to place the sensor interferometer structure filled with PDMS solution in an 80℃ drying oven and heat it for 40 minutes.

[0080] Example 2, Verification Experiment:

[0081] The following experiments used The length of the hollow fiber is 218.4 μm. The length of the PDMS cavity is 146.8 μm.

[0082] (1) Temperature sensing experiment with a single FPI:

[0083] Please see as follows Figure 4 The diagram shows a single-FPI temperature sensing experimental system. Figure 4As shown, the system consists of a supercontinuum light source with a wavelength of 1200~1700nm, a spectrometer with a minimum resolution of 0.02nm, a constant temperature and humidity chamber, a circulator, and two separate FPI interferometers. The two FPIs are placed in the constant temperature and humidity chamber to conduct temperature sensing experiments to explore the temperature sensing characteristics of the two interferometers when they work alone.

[0084] First of all (Composed of a PDMS cavity) Connected separately to the temperature sensing system, the temperature is raised from 25.2℃ to 29.2℃. Interference spectra are collected from the spectrometer every 0.8℃ increase. After each temperature increase, the temperature is stabilized for 15-20 minutes, and the changed interference spectra are recorded, thus obtaining... Figure 5 (a) Interference spectra as a function of temperature and such Figure 5 (b) shown Linear fitting plot of sensitivity. Figure 5 As shown in (a), the wavelength of the trough in the spectrum undergoes a redshift, around 1550 nm. The free spectral range of the interferometer is . Figure 5 As shown in (b), The sensitivity is 1.29 nm / ℃ and the linearity is 99.99%.

[0085] Next, investigate Whether it is suitable as a reference interferometer will also be The interferometer was placed separately in a constant temperature and humidity chamber, and the temperature was raised from 25.2℃ to 29.2℃. Interference spectra were collected from the spectrometer every 0.8℃ increase, thus obtaining... Figure 6 (a) Interference spectra as a function of temperature and such Figure 6 (b) Linear fitting plot of sensitivity. From Figure 6 It is easy to see from the enlarged view in (a) that when Under the same temperature conditions, The wavelength of the spectral trough remained almost unchanged, around 1550 nm. The free spectral range of the interferometer is , Figure 6 As shown in (b), The temperature sensitivity is 0.0137 nm / ℃, and the linearity is 99.99%.

[0086] This further confirms that the temperature sensitivity of all-fiber temperature sensors is relatively low. Temperature sensitivity is relatively On average, it's 2-3 orders of magnitude lower, so It can be used as a reference interferometer.

[0087] (2) Temperature sensing experiment based on vernier effect:

[0088] PDMS, as a temperature-sensitive material, makes... The response to temperature changes relative to More sensitive, therefore in this experiment, As a reference interferometer As a sensing interferometer, the PDMS expands and its refractive index changes due to temperature changes, as detailed in formula (7). Temperature changes can cause spectral shifts.

[0089] Please see as follows Figure 7 The diagram shows a temperature detection system based on the vernier effect. Figure 7 As shown, incident light is emitted from a 1200-1700 nm supercontinuum light source (YSL-SC5), and a spectrometer (OSA, YOKOGAWA, AQ6370D) with a maximum resolution of 0.02 nm is used to receive the incident light beam after it passes through the sensor. The temperature environment is a constant temperature and humidity chamber with a resolution of 0.01 °C.

[0090] To induce the vernier effect, control The FSRs are close but not equal, and then respectively... Figure 5 Performing a Fast Fourier Transform (FFT) on (a) and 6(a) yields the following results: The main peak frequency is These two positions correspond to respectively The highest point of the amplitude after the Fast Fourier Transform (FFT) is calculated using the following formula:

[0091] (13)

[0092] Please see as follows Figure 8 (a) The spectrum of the vernier effect formed after parallel connection, such as Figure 8 (b) shown The FFT transform graph formed after parallel connection and such Figure 8 (c) shown A linear fit plot of the envelope peaks and troughs formed after parallel connection. From Figure 8 As can be seen, two FPI interferometers with similar but unequal FSRs form spectra with periodically varying amplitudes, which is the vernier effect. Figure 8 The actual measured FSR of the envelope in (a) is 105.2 nm, which is very close to the theoretical FSR (106.3 nm) calculated by formula (9). Figure 8 (b) is The FFT spectrum after parallel connection, where the two main peaks correspond to the main peak frequencies of the two FP interferometers respectively. Figure 8 (c) is a linear fit plot of the envelope peaks and troughs of the vernier effect interference spectrum. It can be seen from the plot that the amplified sensitivity is... Calculations showed that the actual amplification factor was 16.7 times, which is close to the theoretical amplification factor. The difference may be due to improper setting of the discharge intensity during the fabrication process or slight misalignment of the optical fiber.

[0093] (3) Reversibility and repeatability of the sensor:

[0094] The reversibility and repeatability of fiber optic interferometers are not negligible, especially for those combined with polymers. For an explanation of the reversibility of parallel interferometers, please refer to [reference needed]. Figure 9 (a) shows the linear fitting plot of the temperature sensitivity at the peaks and troughs during the heating process, and as shown in Figure 1. Figure 9 (b) shows a linear fitting plot of the temperature sensitivity at the peaks and troughs during the cooling process. (See diagram for example.) Figure 9 As shown, the temperature sensitivity of the parallel interferometer during heating and cooling from 25.2℃ to 29.2℃ is basically equal. The temperature sensitivity for peak heating and cooling is 21.54nm / ℃ and 21.18nm / ℃, respectively, while the temperature sensitivity for trough heating and cooling is 18.66nm / ℃ and 18.52nm / ℃, respectively. The linear coefficients are all above 99%, indicating good linearity. This shows that the spectra of the parallel interferometer structure are highly overlapping when heated and cooled at the same temperature, indicating good reversibility of the interferometer and verifying the good repeatability of the sensing probe.

[0095] In summary, the fiber optic temperature sensor based on a dual-FP parallel structure provided in this application consists of a reference interferometer and a sensing interferometer connected in parallel. Experimentally, the temperature sensitivity of the sensing interferometer was measured to be 1.29 nm / ℃; the temperature sensitivity of the reference interferometer was only 0.0137 nm / ℃, which is 2-3 orders of magnitude lower than that of the sensing interferometer, thus it can be used as a reference interferometer. The free spectral ranges of the two cavities are controlled to be similar but not equal, thereby forming a significant vernier envelope in the reflection spectrum, significantly enhancing the spectral drift caused by temperature perturbations. Demodulation of the single-FPI spectrum and the parallel spectrum using Fourier transform and envelope extraction methods resulted in an enhanced sensitivity of 21.54 nm / ℃, nearly 17 times that of the single-FPI, effectively amplifying the temperature response of the PDMS cavity. This all-fiber FP interferometric temperature sensor has advantages such as high sensitivity, high linearity, large detection range, good repeatability, and good reversibility. Furthermore, the fiber optic sensing probe size is on the micrometer scale, making it very suitable for temperature detection in confined spaces.

[0096] The above content is only a preferred embodiment of the present invention. For those skilled in the art, many changes can be made in the specific implementation and application scope based on the ideas of the present invention. As long as these changes do not depart from the concept of the present invention, they all fall within the protection scope of the present invention.

Claims

1. A fiber optic temperature sensor based on a dual-FP parallel structure, characterized in that, The fiber optic temperature sensor is composed of a reference interferometer and a sensing interferometer connected in parallel. The reference interferometer is composed of a first single-mode fiber, a hollow fiber, and a second single-mode fiber. The sensing interferometer is composed of a third single-mode fiber, a PDMS cavity, and a fourth single-mode fiber. In this process, by controlling the optical path lengths of the PDMS cavity and the hollow fiber to be similar but not equal, the free spectral range of the sensing interferometer is similar but not equal to the free spectral range of the reference interferometer, thereby exciting the vernier effect of the output spectrum to form an envelope composed of fine fringes. The optical path lengths of the PDMS cavity and the hollow fiber are controlled to be similar but not equal in the following ways: A reference interferometer was prepared based on the predetermined length of the hollow fiber, and the optical path length of the hollow fiber was calculated based on the air refractive index. The reference value for the length of the PDMS cavity was calculated based on the optical path length of the hollow fiber and the refractive index of the PDMS near the wavelength of 1550nm. The third and fourth single-mode fibers are aligned on a glass slide with an air cavity of a specific length. The alignment of the end faces is observed and confirmed under a microscope, and optical calibration is performed. The specific length of the air cavity is similar to but not equal to the calculated reference value of the PDMS cavity length. Connect one side of the circulator to a dual FP parallel structure, and connect the two lines on the other side to the spectrometer and the supercontinuum light source respectively. After the vernier effect occurs in the spectrum appearing on the spectrometer, calculate the free spectral range around 1550 nm. UV adhesive was applied to the third and fourth single-mode optical fibers respectively, and cured with a UV lamp to fix the two single-mode optical fibers to the glass slide. The bubble-free PDMS solution was dropped between two single-mode optical fibers, then placed in an 80°C drying oven and heated for 40 minutes before being removed. The actual length of the PDMS cavity was then measured and confirmed under a microscope.

2. The fiber optic temperature sensor based on a dual FP parallel structure as described in claim 1, characterized in that, The process of fabricating a reference interferometer based on a pre-defined length of hollow optical fiber includes: Use a high-precision fiber optic cleaver to neatly cut the end faces of the first single-mode fiber, hollow fiber, and second single-mode fiber, ensuring that the end faces are perpendicular, smooth, and burr-free, in order to reduce splice loss. The cut first single-mode fiber and hollow fiber are placed into the fiber fusion splicer and aligned with the axis. They are then fused together with low discharge intensity and short discharge time to prevent the hollow fiber from collapsing. After inspecting and confirming the flatness of the fusion splice end with a microscope, the hollow fiber is cut to the predetermined length, and then a second single-mode fiber is fused to the other end of the hollow fiber in the same way to form a reflective plane.

3. The fiber optic temperature sensor based on a dual FP parallel structure as described in claim 1, characterized in that, Prepare the PDMS solution as follows: Dow Corning DC184 PDMS raw material was selected. A certain amount of PDMS raw material and auxiliary curing agent were drawn with a syringe and mixed in a new centrifuge tube at a mass ratio of 10:

1. The tube was then placed in a centrifuge for 30 to 60 minutes until the bubbles in the solution disappeared, thus obtaining a PDMS solution.

4. The fiber optic temperature sensor based on a dual-FP parallel structure as described in claim 1, characterized in that, Before fabricating the reference interferometer and the sensing interferometer, the coating layer of each single-mode fiber was removed using a fiber stripper, the coating layer of the hollow fiber was removed using a flame, and the end face was cleaned with anhydrous ethanol to ensure that the end face of each fiber was clean.

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