High temperature and high pressure resistant optical fiber cascade refractive index-temperature dual parameter integrated sensor

Abstract

The application belongs to the technical field of optical fiber high-temperature sensing, and discloses a high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual-parameter integrated sensor, which comprises a packaging structure, an optical fiber sensing device and a transmission optical fiber; the optical fiber sensing device comprises photonic crystal fiber, hollow Bragg fiber, connecting optical fiber and hollow capillary fiber which are connected in sequence; the hollow capillary fiber at the end of the optical fiber sensing device is connected with the transmission optical fiber; the optical fiber sensing device is arranged in the packaging structure, and an outer air hole is arranged at the position opposite to the photonic crystal fiber on the side wall of the packaging structure; the optical fiber sensing device transmits sensing signals to the outside of the packaging structure through the transmission optical fiber. The application can realize in-situ dual-parameter synchronous high-precision measurement of refractive index-temperature at 1100 DEG C, and has important application value in the fields of combustion state monitoring of a space engine combustion chamber, coolant concentration analysis of a nuclear reactor, component analysis of oil exploitation and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-temperature sensing, and particularly relates to a high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual-parameter integrated sensor. BACKGROUND

[0002] Refractive index sensing and temperature sensing have extremely important application value in many industrial fields, such as monitoring of combustion states of aerospace engine combustion chambers, analysis of nuclear reactor coolant concentrations, and analysis of oil extraction components. These industrial application scenarios are often accompanied by extreme working environments such as high temperature and high pressure, which requires the sensor to maintain good sensing performance at high temperature. In a harsh and complex environment, the changes of refractive index and temperature generally occur simultaneously, and in order to improve the accuracy of detection, it is often necessary to simultaneously detect the refractive index and the temperature and compensate for the temperature error of the refractive index.

[0003] At present, in the field of sensor technology research, there are relatively rich research results on electrical refractive index sensors and optical fiber temperature sensors, but there are still many deficiencies in the application in high-temperature environments. For example, the traditional refractive index sensor cannot work stably at high temperature, and due to the lack of temperature compensation mechanism, the measurement value is seriously affected by thermal drift; and in a high-temperature environment, refractive index measurement is affected by thermal light effect and thermal expansion effect, which easily leads to data distortion. The existing technology usually uses a discrete temperature sensor to correct the refractive index data later, but this way is difficult to realize real-time dynamic compensation and is easy to introduce additional errors. Optical fiber sensors have advantages such as anti-electromagnetic interference, corrosion resistance, small size, can withstand 1100℃ high temperature, and have stronger environmental adaptability, and become an ideal choice for high-temperature environment monitoring. However, the existing optical fiber refractive index sensor is easily affected by temperature factors in a complex environment, has a large measurement error in a high-temperature environment, and there is cross-coupling interference between temperature and refractive index, which makes it difficult to realize high-precision measurement.

[0004] Therefore, in order to realize the composite dual-parameter sensing of refractive index-temperature in a high-temperature and high-pressure harsh environment, the structure of the optical fiber sensor needs to be improved to solve the problem of high-temperature refractive index measurement in a harsh environment, which is of great significance for monitoring gas flow component, reaction state and equipment safety. SUMMARY

[0005] In order to solve the problem that the existing optical fiber sensor cannot realize synchronous in-situ accurate measurement of refractive index-temperature in a high-temperature and high-pressure harsh environment, the application provides a high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual-parameter integrated sensor, which forms a double-Fabry-Perot interference cavity structure through optical fiber cascade to realize in-situ accurate sensing of refractive index-temperature dual parameters in a high-temperature and high-pressure environment.

[0006] In order to solve the above technical problems, the technical scheme adopted by the present application is as follows: a high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual parameter integrated sensor, comprising: a packaging structure, an optical fiber sensing device and a transmission optical fiber;

[0007] The optical fiber sensing device comprises a photonic crystal fiber, a hollow Bragg fiber, a connecting optical fiber and a hollow capillary fiber connected in sequence; the hollow capillary fiber at the end of the optical fiber sensing device is connected with the transmission optical fiber;

[0008] Among them, the photonic crystal fiber, the hollow Bragg fiber and the connecting optical fiber form a first Fabry-Perot interferometer for measuring refractive index, and the connecting optical fiber, the hollow capillary fiber and the transmission optical fiber form a second Fabry-Perot interferometer for measuring temperature;

[0009] The optical fiber sensing device is arranged in the packaging structure, and the side wall of the packaging structure is provided with an air hole in a position opposite to the photonic crystal fiber; the optical fiber sensing device transmits a sensing signal to the outside of the packaging structure through the transmission optical fiber.

[0010] Further, the packaging structure comprises a front-end protective cap and an outer packaging tube;

[0011] The outer packaging tube is provided with a central through hole for accommodating the transmission optical fiber, and one end of the central through hole is provided with a cavity for accommodating the optical fiber sensing device; the front-end protective cap is fixedly connected with the outer packaging tube and closes the cavity; the front-end protective cap is provided with a plurality of air holes.

[0012] Further, the front-end protective cap is provided with an internal thread in the center, the outer packaging tube is provided with an external thread on the outer periphery, and the front-end protective cap is connected with the outer packaging tube through the thread and is reinforced by high-temperature resistant anaerobic inorganic glue.

[0013] Further, the hollow Bragg fiber is provided with a fiber cavity in the center, and the cylindrical wall of the fiber cavity has a Bragg structure with periodic refractive index modulation; the photonic crystal fiber is provided with a plurality of penetrating fiber air holes at a position corresponding to the fiber cavity.

[0014] Further, the hollow capillary fiber is provided with a central cavity in the center.

[0015] Further, the outer side end face of the photonic crystal fiber is a bevel or a rough end face to avoid the formation of multi-stage interference.

[0016] Further, the high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual parameter integrated sensor further comprises a fixed tube, the fixed tube is arranged in the central through hole, and the transmission optical fiber is arranged in the fixed tube; the fixed tube is provided with high-temperature resistant ceramic glue at one end close to the optical fiber sensing device, the high-temperature resistant ceramic glue is used for sealing and fixing the transmission optical fiber, the fixed tube and the outer packaging tube; and the fixed tube is connected and fixed with the outer packaging tube through a rubber ring at the other end away from the optical fiber sensing device.

[0017] Further, the fixed tube is made of high-temperature resistant corundum.

[0018] Further, the photonic crystal fiber, the hollow Bragg fiber, the connecting fiber, the hollow capillary fiber and the transmission optical fiber are all made of quartz material, and the packaging structure is made of zirconia ceramic material with a temperature resistance of more than 1500 DEG C.

[0019] The high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual parameter integrated sensor further comprises a continuous white light source, a fiber amplifier, a fiber circulator, a spectrometer and a data processor, broadband light generated by the continuous white light source is first subjected to optical power amplification by the fiber amplifier, and then is transmitted to the transmission optical fiber and the optical fiber sensing device through the fiber circulator, the cascade reflection spectrum generated in the optical fiber sensing device is output again through the fiber circulator and then is transmitted to the spectrometer for spectral data acquisition, and then the spectral data is transmitted to the data processor for data processing and demodulation analysis to obtain the measured refractive index and temperature; the formula for calculating the measured temperature and refractive index is as follows:

[0020] ;

[0021] ;

[0022] wherein, and respectively represent a temperature change amount and a refractive index change amount, represents a change amount of an optical path difference of the first Fabry-Perot interferometer; represents a change amount of an optical path difference of the second Fabry-Perot interferometer; respectively represent a refractive index sensitivity and a temperature sensitivity of the first Fabry-Perot interferometer, represents a change amount of an optical path difference of the second Fabry-Perot interferometer and the temperature change amount and the temperature change amount

[0023] Compared with the prior art, the application has the following beneficial effects:

[0024] 1. The application provides a high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual parameter integrated sensor, which is formed by optical fiber cascade to form a double-Fabry-Perot interference cavity structure, realizes in-situ refractive index-temperature dual parameter synchronous sensing in a high-temperature and high-pressure environment, and the two Fabry-Perot interferometer structures are independent of each other, so that the cross-sensitivity of refractive index and temperature is avoided in structure, and therefore the application can improve the detection precision of refractive index in a high-temperature environment through temperature compensation, and solves the problem of high-precision refractive index detection in a high-temperature environment. In conclusion, the application can realize in-situ refractive index-temperature dual parameter synchronous sensing and accurate sensing in a high-temperature and high-pressure environment, solves the limitation of a single physical quantity sensor of a conventional sensor, and improves the detection precision of refractive index and temperature in a harsh environment such as high temperature and high pressure.

[0025] 2. The sensor of the application is packaged by zirconium oxide ceramic, which greatly improves the protection capability of the sensor in a high-temperature and high-pressure harsh environment, and a plurality of micro external air holes are arranged on the packaging structure, which can provide a sensing environment for the first Fabry-Perot interferometer on the one hand, and can avoid the deformation of the sensing structure caused by the high-pressure environment on the other hand, greatly improving the high-pressure resistance of the sensor and the environmental adaptability. Therefore, the application can be used as an effective means for high-precision detection of gas refractive index and environmental temperature at 1100 DEG C.

[0026] 3. Moreover, the sensing structure of the application is a full-optical fiber probe structure, which has the advantages of small size and low cost, and can be arranged and applied in extremely narrow test environment according to actual application environment, which is a prominent advantage distinguishing from the larger size of traditional sensors. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 Fig. 1 is a whole structure sectional view of a high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual parameter integrated sensor according to an embodiment of the application;

[0028] Figure 2 Fig. 2 is a structure schematic view of an optical fiber sensing device in the embodiment of the application;

[0029] Figure 3 Fig. 3 is a sectional schematic view of an outer packaging tube in the embodiment of the application;

[0030] Figure 4 Fig. 4 is a sectional schematic view of a front-end protective cap in the embodiment of the application;

[0031] Figure 5 Fig. 5 is a structure schematic view of a high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual parameter integrated sensor according to an embodiment of the application;

[0032] Figure 6 Fig. 6 is a composite cascade reflection spectrum schematic view obtained in the embodiment of the application;

[0033] Figure 7 A schematic diagram of an interference spectrum corresponding to the extracted refractive index detection signal;

[0034] Figure 8 A schematic diagram of an interference spectrum corresponding to the extracted temperature detection signal;

[0035] Figure 9 A schematic diagram of the optical path difference change amount of the first Fabry-Perot interferometer under different refractive indexes;

[0036] Figure 10 A schematic diagram of the optical path difference change amount of the first Fabry-Perot interferometer under different temperatures;

[0037] Figure 11 A schematic diagram of the optical path difference change amount of the second Fabry-Perot interferometer under different temperatures;

[0038] In the figure, 1 is a front end protection cap, 2 is a fiber sensing device, 3 is a high-temperature-resistant ceramic glue, 4 is an outer packaging tube, 5 is a fixed tube, 6 is a transmission optical fiber, 7 is a rubber ring, 11 is an inner thread, 12 is an outer air hole, 21 is a photonic crystal fiber, 22 is a hollow Bragg fiber, 23 is a hollow capillary fiber, 24 is a first Fabry-Perot interferometer, 25 is a second Fabry-Perot interferometer, 26 is an optical fiber air hole, 27 is an optical fiber cavity, 28 is a central cavity, 29 is a connecting optical fiber, 41 is a central through hole, 42 is an outer thread, 43 is a cavity, 51 is a continuous white light source, 52 is a fiber amplifier, 53 is a fiber circulator, 54 is a sensing terminal, 55 is a spectrometer, 56 is a data processor, and 57 is a computer. DETAILED DESCRIPTION

[0039] To make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0040] As shown in Figures 1-4 The present application provides a high-temperature and high-pressure resistant fiber cascade refractive index-temperature dual-parameter integrated sensor, which comprises a packaging structure, a fiber sensing device 2 and a transmission optical fiber 6. The fiber sensing device 2 comprises a photonic crystal fiber 21, a hollow Bragg fiber 22, a connecting optical fiber 29 and a hollow capillary fiber 23 connected in sequence. The hollow capillary fiber 23 at the end of the fiber sensing device 2 is connected with the transmission optical fiber 6.

[0041] The photonic crystal fiber 21, the hollow Bragg fiber 22 and the connecting fiber 29 form a first Fabry-Perot interferometer 24 for measuring the refractive index, and the connecting fiber 29, the hollow capillary fiber 23 and the transmission fiber 6 form a second Fabry-Perot interferometer 25 for measuring the temperature.

[0042] The fiber sensing device 2 is arranged in the packaging structure, and the packaging structure is provided with the air holes 12 at positions opposite to the photonic crystal fiber 21; the fiber sensing device 2 transmits the sensing signal to the outside of the packaging structure through the transmission fiber 6.

[0043] Specifically, as shown in Figure 1 , 3 , the packaging structure in the embodiment includes the front-end protective cap 1 and the outer packaging tube 4; the outer packaging tube 4 is provided with a central through hole 41 for accommodating the transmission fiber 6, and one end of the central through hole 41 is provided with a cavity 43 for accommodating the fiber sensing device 2; the front-end protective cap 1 is fixedly connected to the outer packaging tube 4 and seals the cavity 43; the front-end protective cap 1 is provided with a plurality of air holes 12.

[0044] Further, in the embodiment, the front-end protective cap 1 is provided with an internal thread 11 at the center, the outer packaging tube 4 is provided with an external thread 42 at the outer periphery, the front-end protective cap 1 is connected to the outer packaging tube 4 through the threads, and the effectiveness of the packaging protection is ensured by using high-temperature-resistant anaerobic inorganic glue to reinforce the threads.

[0045] In the embodiment, the packaging structure composed of the front-end protective cap 1 and the outer packaging tube 4 can protect the fiber sensing device 2 from being damaged, and the air holes 12 arranged on the packaging structure can enable the external physical quantity to act on the fiber sensing device 2 of the sensor.

[0046] In the embodiment, the connecting fiber 29 and the transmission fiber 6 are both single-mode fibers.

[0047] Further, as shown in Figure 2 , in the embodiment, the hollow Bragg fiber 22 is provided with a fiber cavity 27 at the center, and the cylindrical wall of the fiber cavity 27 has a Bragg structure with a periodic refractive index modulation; the photonic crystal fiber 21 is provided with a plurality of penetrating fiber air holes 26 at positions corresponding to the fiber cavity 27.

[0048] Specifically, through the outer air hole 12 and the fiber air hole 26 on the photonic crystal fiber 21, it can be ensured that the gas molecules can enter the first Fabry-Perot interferometer 24 and be detected by it to change the gas refractive index. Specifically, the diameter of the outer air hole 12 should not be set too large, preferably less than 2 mm, to prevent damage that may be caused by the gas impact on the sensor sensitive unit. In this embodiment, the diameter of the outer air hole 12 is set to 1 mm. The number of outer air holes 12 can be 3-10, preferably 8.

[0049] In this embodiment, the diameter of the fiber air hole 26 on the photonic crystal fiber 21 is about 1-5 μm, and 3 μm is used in this embodiment. The fiber air hole 26 can be provided in multiple, for example, 5-20. The diameter of the fiber cavity 27 in the center of the hollow Bragg fiber 22 is 20-50 μm, and the diameter of the fiber cavity 27 used in this embodiment is 32 μm.

[0050] Specifically, as shown in Figure 2 , in this embodiment, the hollow capillary fiber 23 is provided with a central cavity 28 in the center. The hollow capillary fiber 23 and the connecting fiber 29 and the transmission fiber 6 at both ends form a closed second Fabry-Perot interferometer 25, which is not in communication with the outside world and thus is not sensitive to changes in refractive index; the hollow capillary fiber 23 will expand at high temperatures and is only sensitive to temperature; in order to avoid the effect of external pressure changes on the hollow capillary fiber 23 and cause measurement errors, the length of the hollow capillary fiber 23 should be less than 200 μm.

[0051] Further, in this embodiment, the central cavity 28 in the center of the hollow capillary fiber 23 has a diameter of 20-60 μm, which can be selected to be 50 μm or 30 μm, and increasing the thickness of the tube wall can further avoid deformation caused by pressure changes.

[0052] Further, in this embodiment, the outer side end face of the photonic crystal fiber 21, i.e. Figure 2 , the leftmost end face is beveled or roughened to avoid forming multiple levels of interference. Multiple levels of interference will result in the refractive index signal and temperature signal of the sensor being unable to be effectively extracted, which is not conducive to demodulation accuracy; specifically, when processing the outer side end face of the photonic crystal fiber 21, it is necessary to ensure that the fiber air hole 26 is not blocked.

[0053] Further, as shown in Figure 1As shown, the high-temperature and high-pressure resistant fiber cascade refractive index-temperature dual-parameter integrated sensor in the embodiment also includes a fixed tube 5 arranged in the central through hole 41, and a transmission optical fiber 6 arranged in the fixed tube 5; the fixed tube 5 is provided with a high-temperature resistant ceramic glue 3 at one end close to the optical fiber sensing device 2, and the high-temperature resistant ceramic glue 3 is used to seal and fix the transmission optical fiber 6, the fixed tube 5 and the outer packaging tube 4; the other end of the fixed tube 5 away from the optical fiber sensing device 2 is connected and fixed with the outer packaging tube 4 through a rubber ring 7.

[0054] Further, in the embodiment, the material of the fixed tube 5 is high-temperature resistant corundum. In the embodiment, the inner diameter of the fixed tube 5 is 130 μm, which is larger than the diameter of the transmission optical fiber 6, so as to ensure that the transmission optical fiber 6 can pass through; the main body of the transmission optical fiber 6 is arranged at the center of the fixed tube 5 and is used to transmit the spectrum signal, and the two axial centers are collinear; the signal transmission in the transmission optical fiber 6 is bidirectional, that is, both the input light and the reflected spectrum carrying physical quantity are transmitted, and the two will not interfere with each other.

[0055] Specifically, in the embodiment, the photonic crystal fiber 21, the hollow Bragg fiber 22, the connecting optical fiber 29 and the hollow capillary fiber 23 are all made of quartz material. In addition, the transmission optical fiber 6 is also made of quartz material, which can ensure that the sensor can withstand a high temperature of 1100℃ without coating or using a carbon coating layer. In the embodiment, the splicing between the photonic crystal fiber 21, the hollow Bragg fiber 22, the connecting optical fiber 29, the hollow capillary fiber 23 and the transmission optical fiber 6 can be completed by using an optical fiber fusion splicer through arc discharge technology. By adjusting the discharge power, discharge time and discharge interval, the fusion splicing of different optical fibers can be well completed.

[0056] In addition, in the embodiment, the outer diameters of the photonic crystal fiber 21, the hollow Bragg fiber 22, the connecting optical fiber 29, the hollow capillary fiber 23 and the transmission optical fiber 6 are all 125 μm, which has good size compatibility.

[0057] Specifically, in the embodiment, the front-end protective cap 1 and the outer packaging tube 4 in the packaging structure are made of zirconia ceramic material with a temperature resistance of more than 1500℃.

[0058] The working procedure of the high-temperature and high-pressure resistant fiber cascade refractive index-temperature dual-parameter integrated sensor in the embodiment for detecting refractive index and temperature is as follows: when broadband light is input into the optical fiber sensing device 2 through the transmission optical fiber 6, the broadband light first passes through the second Fabry-Perot interferometer 25 and then passes through the first Fabry-Perot interferometer 24. The light will undergo double-beam interference in the second Fabry-Perot interferometer 25 and the first Fabry-Perot interferometer 24. The superposition of the two will form a cascade composite reflection spectrum. After the reflection spectrum is received by the transmission optical fiber 6, the single interference spectrum carrying the refractive index and temperature information is extracted by the Fourier transform filtering method, and the refractive index and temperature signals to be measured can be obtained by demodulation.

[0059] The working principle of the refractive index detection is as follows: when the refractive index of the external environment in which the sensor is located increases, the gas molecules enter the packaging structure, enter the optical fiber air hole 26 of the photonic crystal optical fiber 21, and then enter the optical fiber cavity 27 of the hollow Bragg optical fiber 22. The increase of the gas molecules causes the increase of the air medium density and the change of the refractive index between the two interference planes of the first Fabry-Perot interferometer 24, and further causes the increase of the optical path difference of the first Fabry-Perot interferometer 24. The refractive index and the optical path difference of the first Fabry-Perot interferometer 24 are linearly related. When the external refractive index decreases, the whole process presents a reversible change, and the optical path difference of the first Fabry-Perot interferometer 24 decreases. Therefore, by using the linear relationship between the refractive index and the optical path difference of the first Fabry-Perot interferometer 24, the change of the optical path difference can be demodulated from the extracted interference spectrum carrying the refractive index information, and the value of the refractive index can be detected in real time.

[0060] Further, when the refractive index of the external environment in which the sensor is located changes, the second Fabry-Perot interferometer 25 is closed, and the gas molecules cannot enter the interior of the second Fabry-Perot interferometer 25. Therefore, the second Fabry-Perot interferometer 25 is not sensitive to the change of the refractive index.

[0061] The working principle of the temperature detection is as follows: when the temperature of the external environment in which the sensor is located increases, the length of the hollow capillary optical fiber 23 increases due to thermal expansion, that is, the physical cavity length of the second Fabry-Perot interferometer 25 increases, and further causes the increase of the optical path difference of the second Fabry-Perot interferometer 25. The temperature and the optical path difference of the second Fabry-Perot interferometer 25 are exponentially related. When the external temperature decreases, the whole process presents a reversible change, and the optical path difference of the second Fabry-Perot interferometer 25 decreases. Therefore, by using the exponential relationship between the temperature and the optical path difference of the second Fabry-Perot interferometer 25, the change of the optical path difference can be demodulated from the extracted interference spectrum carrying the temperature information, and the value of the temperature can be detected in real time.

[0062] Further, in the embodiment, the high-temperature and high-pressure resistant fiber-optic cascade refractive index-temperature dual-parameter integrated sensor can detect the refractive index and temperature simultaneously at high temperature. The change in temperature causes the expansion of gas molecules, thereby reducing the refractive index. The temperature also affects the optical path difference of the first Fabry-Perot interferometer 24. Therefore, the refractive index needs to be compensated for the refractive index error caused by the temperature at high temperature.

[0063] Specifically, in the embodiment, at high temperature, the change in the optical path difference of the first Fabry-Perot interferometer 24 caused by the combined action of the refractive index and temperature can be represented as:

[0064] ; (1)

[0065] In formula (1), represents the change in the optical path difference of the first Fabry-Perot interferometer 24, and respectively represent the change in the optical path difference of the first Fabry-Perot interferometer 24 caused by the change in the temperature and the refractive index, and respectively represent the temperature sensitivity and the refractive index sensitivity of the first Fabry-Perot interferometer 24; and respectively represent the change in the temperature and the change in the refractive index.

[0066] Further, in the embodiment, the mechanism for compensating for the refractive index test error at high temperature by using the second Fabry-Perot interferometer 25 to sense the real-time temperature is as follows:

[0067] First, the relationship between the change in the optical path difference of the first Fabry-Perot interferometer 24 caused by the change in the temperature and the change in the temperature is established through temperature calibration, that is, the temperature sensitivity of the first Fabry-Perot interferometer 24 is determined. Further, the refractive index sensitivity of the first Fabry-Perot interferometer 24 is determined through refractive index calibration. Then, the real-time change in the temperature is detected by the second Fabry-Perot interferometer 25. Finally, the change in the optical path difference of the first Fabry-Perot interferometer 24 is measured, that is, the accurate change in the refractive index can be calculated, and the calculation formula is as follows:

[0068] ; (2)

[0069] Therefore, in the embodiment, the accuracy of the refractive index sensing at high temperature can be greatly improved through the temperature compensation mechanism.

[0070] Further, as Figure 1 , 2 indicated, in the embodiment, the first Fabry-Perot interferometer 24 for refractive index detection and the second Fabry-Perot interferometer 25 for temperature detection are in series in the optical path, both can work at the same time and get real-time values of refractive index and temperature, and the temperature compensation mechanism is realized in the demodulation algorithm, and there is no time delay.

[0071] Further, for the open structure of the first Fabry-Perot interferometer 24, the detection of the refractive index is the most basic sensing mechanism, and for the coupling form of the physical quantity changing the refractive index, for example, the sound signal and the pressure signal change the refractive index of the medium in the propagation process; therefore, the structure of the embodiment can also be used for sensing the sound signal and the pressure signal in principle, and the embodiment can be expanded in the application level.

[0072] Embodiment two

[0073] As Figure 5 indicated, the embodiment two of the application provides a high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual-parameter integrated sensor, which, like the embodiment one, comprises a packaging structure, a sensing terminal 54 formed by the optical fiber sensing device 2 and the transmission optical fiber 6; in addition, the embodiment further comprises a continuous white light source 51, an optical fiber amplifier 52, an optical fiber circulator 53, an optical spectrum instrument 55, a data processor 56, and a computer 57, broadband light generated by the continuous white light source 51 is first subjected to optical power amplification by the optical fiber amplifier 52, and then transmitted to the sensing terminal 54 through the optical fiber circulator 53, the cascade reflection spectrum generated in the optical fiber sensing device 2 in the sensing terminal 54 is output again through the optical fiber circulator 53 and then transmitted to the optical spectrum instrument 55 for spectral data acquisition, and then the spectral data is transmitted to the data processor 56 for data processing and demodulation analysis to obtain actual values of the refractive index and the temperature to be measured; the demodulation result is transmitted to the computer 57 for display, and the optical spectrum instrument 55 can adopt a miniature high-precision optical spectrum instrument.

[0074] Specifically, in the embodiment, the specific method for the data processor 56 to calculate the refractive index and the temperature to be measured is as follows:

[0075] Step 1: as Figure 6 indicated, the composite cascade reflection spectrum is subjected to fast Fourier transform to convert the spectral signal from the wavelength domain to the frequency domain; then a self-defined band-pass filter is used to filter and extract the refractive index detection frequency domain signal and the temperature detection frequency domain signal from the frequency domain signal.

[0076] Step 2: Perform an inverse fast Fourier transform on the refractive index detection frequency domain signal and the temperature detection frequency domain signal to obtain the interference spectrum corresponding to the refractive index detection signal and the interference spectrum corresponding to the temperature detection signal, as shown below. Figure 7 , 8 As shown.

[0077] Step 3: Using the least mean square error comparison method, calculate the real-time optical path difference between the interference spectrum corresponding to the refractive index detection signal and the interference spectrum corresponding to the temperature detection signal obtained in Step 2. Subtract the initial optical path difference of the sensor from each, and you can obtain the change in optical path difference between the first Fabry-Perot interferometer 24 and the second Fabry-Perot interferometer 25. and .

[0078] Step 4: Calculate the temperature and refractive index to be measured. The calculation formula is as follows:

[0079] (3)

[0080] (4)

[0081] in, This represents the change in optical path difference of the second Fabry-Perot interferometer 25. With temperature change The functional relationship between the two can be used to demodulate the real-time temperature, and the refractive index can be calculated by substituting it into formula (4).

[0082] Among them, the optical path difference variation of the second Fabry-Perot interferometer 25 and temperature change Functional relationship The temperature sensitivity of the first Fabry-Perot interferometer can be calibrated in advance. and refractive index sensitivity It also needs to be calibrated in advance.

[0083] like Figure 6 As shown, this is the composite cascade reflectance spectrum collected by spectrometer 55 in this embodiment; Figure 7 The image shows a schematic diagram of the interference spectrum corresponding to the extracted refractive index detection signal; as shown... Figure 8 The figure shows a schematic diagram of the interference spectrum corresponding to the extracted temperature detection signal.

[0084] Figure 9 This is a schematic diagram showing the change in optical path difference of the first Fabry-Perot interferometer under different refractive index conditions measured in an embodiment of the present invention; from Figure 9It can be seen from the figure that the refractive index and the first Fabry-Perot interferometer 24 optical path difference change amount present a linear relationship, and the slope is determined by linear fitting, that is, the refractive index sensitivity of the first Fabry-Perot interferometer 24 . Figure 10 The figure is a schematic diagram of the first Fabry-Perot interferometer 24 optical path difference change amount measured at different temperatures in the embodiment of the present application; from Figure 10 It can be seen from the figure that the temperature and the first Fabry-Perot interferometer 24 optical path difference change amount present a linear relationship, and the slope is determined by linear fitting, that is, the temperature sensitivity of the first Fabry-Perot interferometer 24 .

[0085] Figure 11 The figure is a schematic diagram of the second Fabry-Perot interferometer 25 optical path difference change amount measured at different temperatures in the embodiment of the present application; from Figure 11 It can be seen from the figure that the temperature change amount and the second Fabry-Perot interferometer 25 optical path difference change amount present an exponential relationship, and the exponential equation is fitted by the exponential equation:

[0086] ; (5)

[0087] Wherein, a, b, c, d, m, n are fitting parameters, and e represents the base number of natural logarithm; the fitting parameters can be obtained by fitting the data measured in the calibration experiment. The relationship obtained by data fitting can be expressed as: Figure 11

[0088] ; (6)

[0089] Therefore, in the embodiment, after the fitting parameters a, b, c, d, m, n of the exponential equation are determined by experimental calibration and fitting, the second Fabry-Perot interferometer 25 optical path difference change amount is substituted into equation (5), the temperature change amount can be directly demodulated, and the refractive index change amount can be calculated by combining equation (4).

[0090] Finally, it should be pointed out that: the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.​

Claims

1. A high-temperature and high-pressure resistant optical fiber cascade refractive index-temperature dual-parameter integrated sensor, characterized in that, It comprises a packaging structure, a fiber sensitive device (2) and a transmission fiber (6); The fiber sensitive device (2) comprises a photonic crystal fiber (21), a hollow Bragg fiber (22), a connecting fiber (29) and a hollow capillary fiber (23) connected in sequence; the hollow capillary fiber (23) at the end of the fiber sensitive device (2) is connected with the transmission fiber (6); The photonic crystal fiber (21), the hollow Bragg fiber (22) and the connecting fiber (29) form a first Fabry-Perot interferometer (24) for measuring the refractive index; the connecting fiber (29), the hollow capillary fiber (23) and the transmission fiber (6) form a second Fabry-Perot interferometer (25) for measuring the temperature; The fiber sensitive device (2) is arranged in the packaging structure, and an outer air hole (12) is arranged at a position opposite to the photonic crystal fiber (21) on the side wall of the packaging structure; the fiber sensitive device (2) transmits a sensing signal to the outside of the packaging structure through the transmission fiber (6); The hollow Bragg fiber (22) is provided with a fiber cavity (27) in the center, and a cylindrical wall around the fiber cavity (27) is provided with a Bragg structure with a periodic refractive index modulation; the photonic crystal fiber (21) is provided with a plurality of penetrating fiber air holes (26) at a position corresponding to the fiber cavity (27) in the center; the packaging structure comprises a front-end protective cap (1) and an outer packaging tube (4); The outer packaging tube (4) is provided with a central through hole (41) in the center for accommodating the transmission fiber (6), and one end of the central through hole (41) is provided with a cavity (43) for accommodating the fiber sensitive device (2); the front-end protective cap (1) is fixedly connected with the outer packaging tube (4) and closes the cavity (43); a plurality of outer air holes (12) are arranged on the front-end protective cap (1); The outer side end face of the photonic crystal fiber (21) is a bevel or a rough end face, so as to avoid forming a multi-stage interference; The second Fabry-Perot interferometer (25) is also used for in-situ calibration of the refractive index measured by the first Fabry-Perot interferometer (24). 2.The high-temperature and high-pressure resistant optical fiber concatenated refractive index-temperature dual-parameter integrated sensor of claim 1, wherein, The front-end protective cap (1) is provided with an internal thread (11) in the center, the outer packaging tube (4) is provided with an external thread (42) around the outer packaging tube (4), the front-end protective cap (1) is connected with the outer packaging tube (4) through the threads, and is reinforced by high-temperature-resistant anaerobic inorganic glue. 3.The high-temperature and high-pressure resistant optical fiber concatenated refractive index-temperature dual-parameter integrated sensor of claim 1, wherein, The hollow capillary fiber (23) is provided with a central cavity (28) in the center.

4. The high-temperature and high-pressure resistant optical fiber concatenated refractive index-temperature dual-parameter integrated sensor according to claim 1, characterized in that, It also comprises a fixing tube (5) arranged in the central through hole (41), and the transmission fiber (6) is arranged in the fixing tube (5); the fixing tube (5) is provided with high-temperature-resistant ceramic glue (3) at one end close to the fiber sensitive device (2), the high-temperature-resistant ceramic glue (3) is used for sealing and fixing the transmission fiber (6), the fixing tube (5) and the outer packaging tube (4); the other end of the fixing tube (5) away from the fiber sensitive device (2) is connected and fixed with the outer packaging tube (4) through a rubber ring (7).

5. The high-temperature and high-pressure resistant optical fiber concatenated refractive index-temperature dual-parameter integrated sensor according to claim 4, characterized in that, The material of the fixing tube (5) is high-temperature-resistant corundum.

6. The high-temperature and high-pressure resistant optical fiber concatenated refractive index-temperature dual-parameter integrated sensor according to claim 1, characterized in that, The photonic crystal fiber (21), the hollow Bragg fiber (22), the connecting fiber (29), the hollow capillary fiber (23) and the transmission fiber (6) are made of quartz material, and the packaging structure is made of zirconia ceramic material with a temperature resistance of more than 1500 DEG C.

7. The high-temperature and high-pressure resistant optical fiber concatenated refractive index-temperature dual-parameter integrated sensor according to claim 1, characterized in that, The continuous white light source (51), the fiber amplifier (52), the fiber circulator (53), the optical spectrum analyzer (55) and the data processor (56) are also included. The broadband light generated by the continuous white light source (51) is first amplified in optical power by the fiber amplifier (52), and then transmitted to the transmission fiber (6) and the fiber sensitive device (2) through the fiber circulator (53). The cascade reflection spectrum generated in the fiber sensitive device (2) is output again through the fiber circulator (53) and then transmitted to the optical spectrum analyzer (55) for spectral data acquisition. Then the spectral data is transmitted to the data processor (56) for data processing and demodulation analysis to obtain the refractive index and temperature to be measured. The formula for calculating the temperature and refractive index to be measured obtained by the data processor (56) is: ； ； wherein, and respectively represent a temperature change amount and a refractive index change amount, represents a change amount of an optical path difference of the first Fabry-Perot interferometer (24); represents a change amount of an optical path difference of the second Fabry-Perot interferometer (25); respectively represent a refractive index sensitivity and a temperature sensitivity of the first Fabry-Perot interferometer (24), represents a change amount of an optical path difference of the second Fabry-Perot interferometer (25) and a temperature change amount have a functional relationship, a, b, c, d, m, n are all fitting parameters, and e represents a base number of a natural logarithm.

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