Sensitivity-Enhanced Composite Multi-Cavity FPI Temperature Sensor, Preparation Method and Application

Abstract

The present invention belongs to the field of optical technology, and discloses a sensitivity-enhanced composite multi-cavity FPI temperature sensor, a preparation method and an application thereof. In this sensor structure, a composite multi-cavity FPI is formed. When the temperature rises, different interference cavities generate opposite cavity length changes, that is, different interference cavities have the characteristic of anti-phase temperature response. By calculating the phase difference between the air cavity FPI and the air-PDMS cavity FPI, the demodulation sensitivity is improved, and it has the advantages of compact structure, high sensitivity, flexible and convenient application, etc. Compared with the sensitivity amplification method and the generation of the Vernier effect proposed by the present invention, the strict requirements on the cavity length or the optical path difference ratio of different FP cavities of the composite interferometer are reduced, greatly improving the production success rate, and making it easier to achieve sensitivity amplification and high-precision and simple signal demodulation. The sensor proposed by the present invention is a reflective probe structure, which is easy to manufacture, low in cost and very small in size, and can be used in temperature measurement occasions such as medical treatment that require a probe structure and small size.

Description

Technical Field

[0001] The present invention belongs to the technical field of optics and relates to a sensitivity-enhanced composite microcavity FPI temperature sensor based on differential phase demodulation. Background Technique

[0002] Optical fiber Fabry-Perot interferometer (FPI) sensors have the advantages of small size, light weight, low cost, high sensitivity, strong anti-interference ability, etc., and have received extensive attention from researchers in recent years. When the length of the FP cavity or the refractive index of the medium in the cavity changes, the phase difference between the interfering beams that generate FP interference will change, resulting in a change in the interference spectrum. By measuring the change in the phase difference or the spectral shift, the change in the physical quantity to be measured can be obtained. FPI has been widely used to measure various environmental parameters, such as temperature, refractive index, strain, humidity, magnetic field, etc. The fabrication of FPI has evolved from simple fusion splicing and chemical etching to femtosecond laser processing, 3D printing, etc. The fusion splicing fabrication process is the simplest, and the fabrication techniques using femtosecond laser processing and 3D printing have high precision but also high costs. With the development of fiber optic sensors, more and more researchers have introduced sensitive materials with good optical properties into the FP cavity to increase the sensitivity of the FP cavity to certain parameters. In 2019, Li Z B et al. proposed a micro FP temperature sensor that encapsulated an ultra-fine optical fiber in a section of hollow-core fiber (HCF) using polydimethylsiloxane (PDMS). PDMS has a strong thermal expansion and contraction effect under the action of temperature and changes the length of the FP cavity at different temperatures. Utilizing the thermal expansion effect of PDMS, a temperature sensing sensitivity of up to 11.86 nm / °C was achieved, which is a significant improvement compared to sensors without encapsulated materials. In 2022, Li F et al. proposed a temperature and humidity simultaneous measurement sensor based on a cascaded C-shaped FPI. PDMS and polyvinyl alcohol (PVA) were respectively filled into two sections of C-shaped optical fibers to improve the sensitivity to temperature and humidity. For this cascaded FPI, the interference spectra of the two FPIs were extracted separately by means of spectral separation. Spectral separation is a common method for processing the superimposed spectra of cascaded interferometers, but the steps are relatively complex, and the problem that the resolution of high-sensitivity sensors is limited by the resolution of spectral analysis equipment is more serious in spectral demodulation. In addition to introducing sensitive materials, some principles or effects can also be introduced into FPI to improve the measurement sensitivity. In 2021, Zhao Y et al. proposed a high-sensitivity reflective FP magnetic field sensor with a vernier effect. The magnetic flux cavity and the air cavity of the FP magnetic field sensor form a vernier effect to achieve high-sensitivity magnetic field measurement. However, generating the vernier effect has relatively strict requirements for the cavity length or optical path ratio of different FP cavities in a cascaded or composite interferometer. Usually, the optical path ratio of different interference cavities is required to be around 1:1.1 - 1.3, and the optical path ratio of different interference cavities with a vernier spectrum of the first harmonic is usually around 1:2.1 - 2.3, which makes the fabrication difficult. Therefore, there is an urgent need for an optical fiber temperature sensor with high sensitivity that can avoid being limited by the resolution of spectral analysis equipment and has less stringent preparation requirements. Summary of the Invention

[0003] To more easily achieve sensitivity enhancement and high-precision, simple signal demodulation, the present invention proposes a sensitivity-enhanced composite microcavity FPI temperature sensor based on differential phase demodulation. By calculating the phase difference between the air cavity FPI and the air-PDMS cavity FPI, the demodulation sensitivity is improved, and it has the advantages of compact structure, high sensitivity, flexible and convenient application, etc. Compared with the sensitivity amplification method and the generation of the Vernier effect proposed in the present invention, the strict requirements for the cavity length or the optical path difference ratio of different FP cavities in the composite interferometer are reduced, greatly improving the production success rate.

[0004] The technical solution of the present invention is as follows:

[0005] A sensitivity-enhanced composite multi-cavity FPI temperature sensor, which includes an air cavity and a PDMS cavity, and a composite cavity composed of the air cavity and the PDMS cavity; the air cavity is adjacent to the PDMS cavity; the air cavity is inside a hollow fiber and filled with air, one end is the fusion interface of the single-mode fiber and the hollow fiber, and the other end is the interface between the air in the hollow fiber and PDMS formed by filling PDMS in the hollow fiber; one end of the PDMS cavity is the interface between the air and PDMS in the hollow fiber of the air cavity, and the other end is the interface between PDMS and the external air; one end of the composite cavity is the fusion interface of the single-mode fiber and the hollow fiber of the air cavity, and the other end is the interface between PDMS and the external air of the PDMS cavity.

[0006] Preferably, the length of the PDMS cavity is 50-100 microns, and the length of the air cavity is 20-100 microns.

[0007] Preferably, the inner diameter of the hollow fiber is 30-75 microns.

[0008] The preparation method of the above-mentioned sensitivity-enhanced composite multi-cavity FPI temperature sensor is that the sensor is made by fusing one end of a hollow fiber with a single-mode fiber and filling the other end with PDMS (polydimethylsiloxane).

[0009] Preferably, the PDMS filling process of the hollow fiber is: the on-line observation droplet transfer filling method under a microscope. Use a section of fiber to dip a drop of PDMS, and under the microscope, use a micro-displacement adjustment platform to align the end face of the fiber dipped with PDMS with the end face of the hollow fiber to be filled. Then make them slightly contact and quickly separate to transfer part of the PDMS droplet to the end face of the hollow fiber to be filled, and observe the PDMS filling situation in real time under the microscope; at the same time, observe the superposition situation and morphology of the reflection spectrum in real time through a fiber grating demodulator. After the target state appears (when the reflection spectrum shows an obvious superposition spectrum and the spectrum morphology is good), put the filled structure into an incubator preheated to 80°C for PDMS curing.

[0010] Preferably, a method for fabricating a sensitivity-enhanced composite multi-cavity FPI temperature sensor includes the following steps:

[0011] Step 1: First, cut flat end faces of a single-mode optical fiber and a hollow-core optical fiber. Then, use an optical fiber fusion splicer to fuse the two without collapse. Finally, cut the hollow-core optical fiber to a specified length to obtain a single-mode optical fiber - hollow-core optical fiber structure.

[0012] Step 2: First, dip a section of optical fiber into a drop of PDMS. Under a microscope, use a micro-displacement adjustment platform to align the end face of the optical fiber dipped with PDMS with the end face of the hollow-core optical fiber to be filled. Then, adjust the micro-displacement platform to make the end face of the optical fiber dipped with PDMS slightly contact and then quickly leave the end face of the hollow-core optical fiber to be filled, so as to realize partial transfer of the PDMS droplet to the end face of the hollow-core optical fiber to be filled. Then, observe the PDMS filling situation in real time under the microscope. Under capillary action, the PDMS droplet flows from the end of the hollow-core optical fiber to the end connected to the single-mode optical fiber, forming an air micro-cavity between the single-mode optical fiber and the PDMS. At the same time, connect the single-mode optical fiber to an optical fiber grating demodulator and observe the change of the reflection spectrum during the filling process in real time through the optical fiber grating demodulator.

[0013] Step 3: When the target state appears in the observed reflection spectrum (when an obvious superimposed spectrum appears and the spectral form is good), put the filled structure into an incubator preheated to 80 °C and cure it for 1 hour to obtain a composite multi-cavity FPI temperature sensor. The structure of this sensor is a reflective probe structure, which is suitable for temperature measurement in extremely narrow spaces.

[0014] Preferably, before filling PDMS, mix and stir PDMS and a curing agent in a ratio of 10:1, let it stand for 30 - 40 minutes. After the bubbles in the solution are completely dissipated, transfer the PDMS solution to the end of the hollow-core optical fiber by the droplet transfer method.

[0015] For the application of the above-mentioned sensitivity-enhanced composite multi-cavity FPI temperature sensor, connect it to a temperature sensing system, use an optical fiber grating demodulator to collect the reflection spectrum at the corresponding temperature, perform a fast Fourier transform on the collected reflection spectrum of the composite multi-cavity FPI temperature sensor, record the frequencies corresponding to the air cavity FPI and the air-PDMS composite cavity FPI in the frequency spectrum diagram, calculate the phase difference between the two cavities at the corresponding frequencies, and obtain the fitting function relationship between the phase difference and the temperature.

[0016] When the temperature rises, due to the volume expansion of PDMS, the lengths of the PDMS cavity and the air-PDMS composite cavity will increase. Since the expanded PDMS will compress the air cavity, the length of the air cavity will decrease. The opposite changes in cavity length result in the phase change of the air cavity FPI being opposite to that of the PDMS cavity FPI or the air-PDMS composite cavity FPI. Therefore, the sensitivity can be improved by extracting the phase difference between the air cavity FPI and the air-PDMS composite cavity FPI.

[0017] The signal demodulation method for generating the superimposed spectrum of a composite interferometer or a cascaded interferometer is usually to directly read the wavelength values of different valleys in the superimposed spectrum, or to separate different spectral components of the superimposed spectrum and extract the corresponding spectra respectively. Since the superimposed spectrum is composed of the superposition of spectra with different free spectral ranges (spectral periods), and different spectra have different sensitivities to the measured quantity, that is, when the measured quantity changes, the wavelength shifts of different component spectra are different, which may lead to changes in the morphology of the superimposed spectrum, thus causing difficulties in reading wavelengths or resulting in wavelength value reading errors. For spectral separation, different component spectra can be extracted in the wavelength domain or separated in the frequency domain. Figure 10 (a) is a flowchart for separating the spectral signals of different components of the superimposed spectrum of a composite interferometer or a cascaded interferometer in the wavelength domain. First, it is necessary to perform a fast Fourier transform on the collected spectrum to obtain the frequency spectrum, then determine the filtering center frequency and filtering window according to the frequency spectrum, and then filter the original spectrum, so as to extract the spectra corresponding to different interferometers from the superimposed spectrum.

[0018] In the present invention, the differential phase demodulation is frequency domain separation, and its process is as Figure 10 (b) shown. By performing a fast Fourier transform on the collected spectrum to obtain the frequency spectrum, since the free spectral ranges of different component spectra of the superimposed spectrum are different, the frequencies corresponding to different component spectra are directly separated in the frequency domain. In the wavelength domain separation, the selection of the filtering center frequency and the filtering bandwidth will affect the spectral filtering result, thus affecting the sensitivity of the sensor. In addition, the sensitivity of wavelength demodulation also has a certain valley dependence, that is, the sensitivities of different valleys are different. In phase demodulation, the phases of each cavity can be directly resolved in the spatial frequency domain, and the phase values of each cavity can be directly read in the spatial frequency domain. The sensitivity calculated by the phase demodulation method is independent of the selection of the filtering center frequency or bandwidth, the valley selection, and the spectral noise, and is not limited by the wavelength resolution of the spectral analysis equipment. Therefore, it is superior to the wavelength demodulation method in terms of simplicity, speed, stability, resolution, and accuracy of sensitivity.

[0019] The beneficial effects of the present invention are:

[0020] (1) A sensitivity-enhanced composite multi-cavity FPI temperature sensor based on differential phase demodulation proposed by the present invention forms a composite multi-cavity FPI in this sensor structure. When the temperature rises, different interference cavities generate opposite cavity length changes, that is, different interference cavities have the characteristic of opposite phase response to temperature. Therefore, the amplification of the phase demodulation sensitivity can be achieved by calculating the phase difference between the two interference cavities.

[0021] (2) The sensor proposed by the present invention adopts the demodulation method of differential phase. For the signal demodulation of a composite interferometer or a cascaded interferometer, this phase demodulation method is simpler, faster to process, more convenient, has higher demodulation resolution and accuracy, and is not limited by the wavelength resolution of spectral analysis equipment. In addition, the amplification of sensitivity can be achieved through differential phase.

[0022] (3) The sensor proposed by the present invention is a reflective probe structure, which has a simple structure, is easy to fabricate, has a low cost, and is very small in size. It can be used in temperature measurement occasions such as medical applications that require a probe structure and small size. Description of the Drawings

[0023] Figure 1 It is a schematic diagram of the proposed composite multi-cavity structure.

[0024] Figure 2 It is a schematic diagram of the manufacturing process of the proposed composite multi-cavity structure.

[0025] Among them, (a) is to fuse a section of HCF with SMF; (b) is to cut the HCF with a fixed length; (c) is to fill the online monitoring system with PDMS; (d) is the temperature sensing experimental system.

[0026] Figure 3 It is a microscopic image of the sensing structures fabricated in 3 embodiments.

[0027] Among them, for embodiment 1 of (a) and (d): the length of the air cavity is 81 μm, the length of the PDMS cavity is 53 μm, the length of the air-PDMS composite cavity is 134 μm, and the maximum diameter of the sensor is 167 μm.

[0028] (b) and (e) For embodiment 2: the length of the air cavity is 38 μm, the length of the PDMS cavity is 90 μm, the length of the air-PDMS composite cavity is 128 μm, and the maximum diameter of the sensor is 318 μm.

[0029] (c) and (f) For embodiment 3: the length of the air cavity is 77 μm, the length of the PDMS cavity is 69 μm, the length of the air-PDMS composite cavity is 146 μm, and the maximum diameter of the sensor is 140 μm.

[0030] Figure 4Experimental spectra of the probe-type sensing structures of 3 embodiments at 23.960 °C;

[0031] Among them, (a) is the experimental spectrum of Embodiment 1; (b) is the experimental spectrum of Embodiment 2; (c) is the experimental spectrum of Embodiment 3.

[0032] Figure 5 Spatial spectra of 3 embodiments; among them, (a) is the amplitude spectrum; (b) is the phase spectrum.

[0033] Figure 6 Phase spectrum change of Embodiment 1 when the temperature rises from 23.960 °C to 40.158 °C.

[0034] Figure 7 Fitting relationship between the changes of phase and phase difference of different cavities and temperature during the heating test of Embodiment 1.

[0035] Figure 8 Fitting relationship between the changes of phase and phase difference of different cavities and temperature during the heating test of Embodiment 2.

[0036] Figure 9 Fitting relationship between the changes of phase and phase difference of different cavities and temperature during the heating test of Embodiment 3.

[0037] Figure 10 (a) Flow chart for separating spectral signals of different components from the superimposed spectrum of a composite interferometer or a cascaded interferometer in the wavelength domain; (b) Flow chart for demodulating differential phase into frequency domain separation in the present invention.

[0038] In the figure: 1 single-mode optical fiber; 2 hollow-core optical fiber; 3 air cavity; 4 PDMS; 5 microscope; 6 single-mode optical fiber dipped with PDMS; 7 fiber grating demodulator; 8 computer; 9 thermostat; 10 Pt resistance thermometer; 11 composite multi-cavity FPI temperature sensor probe. Specific implementation mode

[0039] To make the objectives, technical solutions and advantages of the present invention clearer, the following further elaborates in detail the specific structure, principle and performance optimization process of the present invention in combination with specific embodiments of the present invention and with reference to the drawings.

[0040] Embodiment 1

[0041] A sensitivity-enhanced composite micro-cavity FPI temperature sensor based on differential phase demodulation for realizing temperature measurement with enhanced sensitivity, including the following steps:

[0042] Step 1: Using an optical fiber fixed-length cutting and fusion splicing system composed of a lifting table, a three-dimensional micro-displacement platform, an optical fiber cutter, an optical fiber fusion splicer, a fixed pulley, a magnetic base, a fixed support rod, etc., a single-mode fiber-hollow fiber structure is prepared. First, an optical fiber cutter is used to cut a single-mode fiber 1 and a hollow fiber 2 respectively to cut out flat end faces. Then, the cut single-mode fiber and hollow fiber are fusion spliced without collapse in the fusion splicer, as shown in Figure 2 (a). After fusion splicing, the single-mode fiber-hollow fiber structure is fixed back on the three-dimensional micro-displacement table of the optical fiber fixed-length cutting and fusion splicing system, and the X-axis movement amount of the micro-displacement table is adjusted to control the distance between the fusion splicing point of the single-mode fiber-hollow fiber and the cutter, that is, the length of the hollow fiber to be cut. Then, the hollow fiber is cut again with the cutter to obtain a hollow fiber with a fixed length, as shown in Figure 2 (b).

[0043] Step 2: Prepare a PDMS solution, mix PDMS and a curing agent in a ratio of 10:1 and stir, let it stand for 30 - 40 minutes. After the bubbles in the solution are completely dissipated, using a filling process and real-time online monitoring system for generating spectra, the transfer, filling, and filling process monitoring of PDMS droplets are completed under a microscope. First, a section of single-mode fiber is used to dip a small drop of PDMS, and under the microscope, using a micro-displacement adjustment platform, the end face of the fiber dipped with PDMS is adjusted to be aligned with the end face of the hollow fiber to be filled, as shown in Figure 2 (c); then the micro-displacement platform is adjusted to make the end face of the fiber dipped with PDMS slightly contact the end face of the hollow fiber to be filled and then quickly leave, so as to realize the partial transfer of the PDMS droplet to the end face of the hollow fiber to be filled; then the filling situation of PDMS is observed in real time under the microscope. Under capillary action, the PDMS droplet flows from the end of the hollow fiber to the end connected to the single-mode fiber, and an air microcavity is formed between the single-mode fiber and PDMS. The structure is as shown in Figure 1 , and at the same time, the single-mode fiber is connected to an optical fiber grating demodulator, and the change of the reflection spectrum during the filling process is observed in real time through the optical fiber grating demodulator, as shown in Figure 2 (c).

[0044] Step 3: When it is observed that the reflection spectrum shows an obvious superimposed spectrum and the spectral form is good, the filled structure is placed in an oven preheated to 80 °C and cured for 1 hour to obtain a composite multi-cavity FPI temperature sensor. The air cavity length of this sensor is 81 μm, the PDMS cavity length is 53 μm, the length of the air-PDMS composite cavity is 134 μm, and the maximum diameter of the sensor is 167 μm, as shown in Figure 3 (a) and Figure 3 (d).

[0045] Step 4: Connect the temperature sensing system. The temperature sensing system consists of a thermostat, a Pt resistance thermometer, a fiber Bragg grating demodulator, and a computer, as shown in Figure 2 (d). The PDMS-filled composite multi-cavity FPI temperature sensor is fixed in the thermostat. The SMF end of the composite multi-cavity FPI temperature sensor is connected to the fiber Bragg grating demodulator. The broadband light emitted by the fiber Bragg grating demodulator is reflected back to the fiber Bragg grating demodulator by the composite multi-cavity FPI. The demodulator receives the reflected spectrum and displays it on the computer side. Adjust the set temperature of the thermostat to control the temperature in the thermostat to gradually increase from about 25 °C to about 65 °C. Use the Pt resistance thermometer to record the temperature when the temperature in the thermostat is constant, and at the same time use the fiber Bragg grating demodulator to collect the reflected spectrum at the corresponding temperature. Record the experimental data approximately every 1 °C. At 23.960 °C, the reflected spectrum collected by the fiber Bragg grating demodulator is as shown in Figure 4 (a).

[0046] Step 5: Perform a fast Fourier transform on the collected reflected spectrum of the composite multi-cavity FPI temperature sensor to obtain the amplitude frequency spectrum and the phase frequency spectrum, as shown in Figure 5 (a) and Figure 5 (b) respectively. There are 2 peaks in the amplitude frequency spectrum, located at frequencies f 1 = 0.075 nm 1 and f 2 = 0.1375 nm 1 respectively. Through theoretical analysis and comparison with experimental results, it is proved that these 2 frequencies correspond to the air cavity FPI and the air-PDMS composite cavity FPI respectively; when the temperature increases, the phase change directions at these 2 frequencies are opposite, as shown in Figure 6 . The phase at the frequency f 1 = 0.075 nm 1 increases with the increase of temperature, and the phase at the frequency f 2 = 0.1375 nm 1 decreases with the increase of temperature; calculate the phase difference between the two cavities at the corresponding frequencies to obtain the fitting function relationship between the phase difference and the temperature, as shown in Figure 7 . The temperature sensitivity of the air cavity FPI is about 0.076 rad / °C, the temperature sensitivity of the air-PDMS composite cavity FPI is about 0.16 rad / °C, and the temperature sensitivity of the phase difference between the air cavity FPI and the air-PDMS composite cavity FPI is about 0.236 rad / °C. Using the differential phase, the temperature sensitivity is increased to 3.11 times that of the air cavity and 1.48 times that of the air-PDMS composite cavity.

[0047] Table 1 Wavelength demodulation and phase demodulation results when the temperature in Example 1 is near 24.013 °C

[0048]

[0049] During the experiment, the temperature was controlled to vary near 24.013 °C. Since the temperature change was very small, less than 0.001 °C, the Pt resistance thermometer used for calibration only showed 24.013 °C due to the limitation of its resolution. When the temperature was near 24.013 °C, the wavelength demodulation and phase demodulation results are shown in Table 1. It can be seen from Table 1 that when wavelength demodulation using spectral separation in the wavelength domain was adopted, the wavelengths of the air cavity FPI and the air-PDMS composite cavity FPI hardly changed. This is because the resolution of wavelength demodulation is limited by the wavelength resolution (0.02 nm) of the spectral analysis equipment, resulting in a low temperature resolution of the sensor. However, when the phase demodulation method of the present invention was adopted, the phases of the air cavity FPI and the air-PDMS composite cavity FPI both changed with the temperature change. The above proves that the resolution of the phase demodulation method of the present invention is better than that of the wavelength demodulation method, and the sensitivity-enhanced composite microcavity FPI temperature sensor based on differential phase demodulation proposed by the present invention has the characteristic of high temperature resolution.

[0050] Example 2

[0051] A sensitivity-enhanced composite microcavity FPI temperature sensor based on differential phase demodulation for realizing temperature measurement with enhanced sensitivity, comprising the following steps:

[0052] Steps 1 and 2: The same as in Example 1;

[0053] Step 3: When it was observed that the reflected spectrum showed obvious superimposed spectra and the spectral pattern was good, the filled structure was placed in an incubator preheated to 80 °C and cured for 1 hour to obtain a composite multi-cavity FPI temperature sensor. The length of the air cavity of this sensor was 38 μm, the length of the PDMS cavity was 90 μm, the length of the air-PDMS composite cavity was 128 μm, and the maximum diameter of the sensor was 318 μm, as shown in Figure 3 (b) and Figure 3 (e).

[0054] Step 4: Connect the temperature sensing system. The temperature sensing system consists of an incubator, a Pt resistance thermometer, a fiber Bragg grating demodulator, and a computer, as shown in Figure 2(d). The PDMS-filled composite multi-cavity FPI temperature sensor is fixed in an incubator. The SMF end of the composite multi-cavity FPI temperature sensor is connected to a fiber Bragg grating demodulator. The broadband light emitted by the fiber Bragg grating demodulator is reflected back to the fiber Bragg grating demodulator by the composite multi-cavity FPI. The demodulator receives the reflected spectrum and displays it on the computer side. Adjust the set temperature of the incubator to control the temperature in the incubator to gradually increase from about 25 °C to about 65 °C. Use a Pt resistance thermometer to record the temperature when the temperature in the incubator is constant, and at the same time use the fiber Bragg grating demodulator to collect the reflected spectrum at the corresponding temperature, and record the experimental data about every 1 °C. At 23.960 °C, the reflected spectrum collected by the fiber Bragg grating demodulator is as shown in Figure 4 (b).

[0055] Step 5: Perform a fast Fourier transform on the collected reflected spectrum of the composite multi-cavity FPI temperature sensor to obtain the amplitude frequency spectrum and the phase frequency spectrum, as shown in Figure 5 (a) and Figure 5 (b) respectively. There are 2 peaks in the amplitude frequency spectrum, located at frequencies f 3 = 0.0375 nm 1 and f 4 = 0.15 nm 1 respectively. Through theoretical analysis and comparison with experimental results, it is proved that these 2 frequencies correspond to the air cavity FPI and the air-PDMS composite cavity FPI respectively; when the temperature increases, the phase change directions at these 2 frequencies are opposite. The phase at the frequency f 3 = 0.0375 nm 1 increases with the increase of temperature, and the phase at the frequency f 4 = 0.15 nm 1 decreases with the increase of temperature; calculate the phase difference between the two cavities at the corresponding frequencies, and obtain the fitting function relationship between the phase difference and the temperature, as shown in Figure 8 respectively. The temperature sensitivity of the air cavity FPI is about 0.232 rad / °C, the temperature sensitivity of the air-PDMS composite cavity FPI is about 0.114 rad / °C, and the temperature sensitivity of the phase difference between the air cavity FPI and the air-PDMS composite cavity FPI is about 0.346 rad / °C. Using the differential phase, the temperature sensitivity is increased to 1.49 times that of the air cavity and 3.04 times that of the air-PDMS composite cavity.

[0056] Example 3

[0057] A sensitivity-enhanced composite micro-cavity FPI temperature sensor based on differential phase demodulation for realizing temperature measurement with enhanced sensitivity, including the following steps:

[0058] Step 1, Step 2: The same as Example 1;

[0059] Step 3: When it is observed that the reflected spectrum shows an obvious superimposed spectrum and the spectral morphology is good, put the filled structure into an incubator preheated to 80 °C and cure it for 1 hour to obtain a composite multi-cavity FPI temperature sensor. The air cavity length of this sensor is 77 μm, the PDMS cavity length is 69 μm, the length of the air-PDMS composite cavity is 146 μm, and the maximum diameter of the sensor is 140 μm, as Figure 3 (c) and Figure 3 (f) shown.

[0060] Step 4: Connect the temperature sensing system. The temperature sensing system consists of an incubator, a Pt resistance thermometer, a fiber Bragg grating demodulator, and a computer, as Figure 2 (d) shown. The PDMS-filled composite multi-cavity FPI temperature sensor is fixed in the incubator. The SMF end of the composite multi-cavity FPI temperature sensor is connected to the fiber Bragg grating demodulator. The broadband light emitted by the fiber Bragg grating demodulator is reflected back to the fiber Bragg grating demodulator by the composite multi-cavity FPI. The demodulator receives the reflected spectrum and displays it on the computer side. Adjust the set temperature of the incubator, control the temperature in the incubator to gradually increase from about 25 °C to about 65 °C, use the Pt resistance thermometer to record the temperature when the temperature in the incubator is constant, and at the same time use the fiber Bragg grating demodulator to collect the reflected spectrum at the corresponding temperature, and record the experimental data about every 1 °C. At 23.960 °C, the reflected spectrum collected by the fiber Bragg grating demodulator is as Figure 4 (c) shown.

[0061] Step 5: Perform a fast Fourier transform on the collected reflected spectrum of the composite multi-cavity FPI temperature sensor to obtain the amplitude frequency spectrum and the phase frequency spectrum, as Figure 5 (a) and Figure 5 (b) shown. There are 3 peaks in the amplitude frequency spectrum, which are located at frequencies f 5 = 0.0625 nm 1 , f 6 = 0.15 nm 1 and f 7 = 0.225 nm 1 respectively. Through theoretical analysis and comparison with experimental results, it is proved that f 5 = 0.0625 nm 1 and f 6 = 0.15 nm 1 these 2 frequencies respectively correspond to the air cavity FPI and the air-PDMS composite cavity FPI; when the temperature increases, the phase change directions at these 2 frequencies are opposite. The phase at the frequency f 5 = 0.0625 nm 1 increases with the increase of temperature, and the phase at the frequency f 6 = 0.15 nm 1The phase at [specific location] decreases as the temperature increases; calculate the phase difference between the two cavities at the corresponding frequency, and obtain the fitting function relationship between the phase difference and the temperature, as Figure 9 shown. The temperature sensitivity of the air cavity FPI is approximately 0.155 rad / °C, the temperature sensitivity of the air-PDMS composite cavity FPI is approximately 0.131 rad / °C, and the temperature sensitivity of the phase difference between the air cavity FPI and the air-PDMS composite cavity FPI is approximately 0.285 rad / °C. By using the differential phase, the temperature sensitivity is increased to 1.84 times that of the air cavity and 2.18 times that of the air-PDMS composite cavity.

[0062] As mentioned above, only the preferred embodiments of the present invention are described, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes should be covered within the protection scope of the present invention.

Claims

1. Sensitivity-enhanced composite multi-cavity FPI temperature sensor, Characterized in that, The sensor includes an air cavity and a PDMS cavity, and a composite cavity composed of the air cavity and the PDMS cavity; the air cavity is adjacent to the PDMS cavity; the air cavity is inside the hollow fiber and filled with air, one end is the fusion interface of the single-mode fiber and the hollow fiber, and the other end is the interface between the air and PDMS inside the hollow fiber formed by filling PDMS in the hollow fiber; one end of the PDMS cavity is the interface between the air and PDMS inside the hollow fiber of the air cavity, and the other end is the interface between the PDMS and the outside air; One end of the composite cavity is the fusion interface of the single-mode fiber and the hollow fiber of the air cavity, and the other end is the interface between the PDMS and the outside air of the PDMS cavity; The preparation method of the sensitivity-enhanced composite multi-cavity FPI temperature sensor includes the following steps: Step 1: First, cut the single-mode fiber and the hollow fiber into flat end faces, then use a fiber fusion splicer to fuse the two without collapse, and finally cut the hollow fiber into a specified length to obtain a single-mode fiber-hollow fiber structure; Step 2: First, dip a section of fiber into a drop of PDMS, and use a micro-displacement adjustment platform under a microscope to align the end face of the fiber dipped with PDMS with the end face of the hollow fiber to be filled; then adjust the micro-displacement platform to make the end face of the fiber dipped with PDMS slightly contact and then quickly leave the end face of the hollow fiber to be filled, so as to realize the partial transfer of the PDMS droplet to the end face of the hollow fiber to be filled; then observe the PDMS filling situation in real time under the microscope. Under the capillary action, the PDMS droplet flows from the end of the hollow fiber to the end connected to the single-mode fiber, forming an air micro-cavity between the single-mode fiber and the PDMS. At the same time, connect the single-mode fiber to a fiber grating demodulator, and observe the change of the reflection spectrum during the filling process through the fiber grating demodulator in real time; Step 3: When the target state appears in the observed reflection spectrum, put the filled structure into an incubator preheated to 80 °C and cure it for 1 hour to obtain a composite multi-cavity FPI temperature sensor.

2. The sensitivity-enhanced composite multi-cavity FPI temperature sensor according to claim 1, Characterized in that, The length of the PDMS cavity is 50-100 microns, and the length of the air cavity is 20-100 microns.

3. The sensitivity-enhanced composite multi-cavity FPI temperature sensor according to claim 1, Characterized in that, The inner diameter of the hollow fiber is 30-75 microns.

4. The sensitivity-enhanced composite multi-cavity FPI temperature sensor according to claim 1, Characterized in that, Before filling the PDMS, mix and stir the PDMS and the curing agent in a ratio of 10:1, let it stand for 30-40 minutes, and after the bubbles in the solution are completely dissipated, transfer the PDMS solution to the end of the hollow fiber by the droplet transfer method.

5. The application of the sensitivity-enhanced composite multi-cavity FPI temperature sensor according to any one of claims 1-4, Characterized in that, Connect the temperature sensing system, use an optical fiber grating demodulator to collect the reflection spectrum at the corresponding temperature, perform a fast Fourier transform on the collected reflection spectrum of the composite multi-cavity FPI temperature sensor, record the corresponding frequencies of the air cavity FPI and the air-PDMS composite cavity FPI in the frequency spectrum diagram, calculate the phase difference between the two cavities at the corresponding frequencies, and obtain the fitting function relationship between the phase difference and the temperature.

Images