An optical fiber sensor for simultaneously measuring pressure and temperature

Abstract

The present invention discloses an optical fiber sensor for simultaneously measuring pressure and temperature, which relates to the technical field of optical fiber sensing. The present invention includes a broadband unit for transmitting optical signals; a sensing unit for simultaneously detecting pressure and temperature; a circulator, the first port of which is connected to the broadband unit for unidirectionally transmitting optical signals; a transmission coupling optical fiber connected to the second port of the circulator; a first pressure sensing sheet disposed in the sensing unit; a second pressure sensing sheet parallel to the first pressure sensing sheet; a first hollow optical fiber fixedly disposed between the first pressure sensing sheet and the second pressure sensing sheet and perpendicular to the transmission coupling optical fiber; a second hollow optical fiber parallel to the first hollow optical fiber; a spectrometer, the input port of which is connected to the third port of the circulator for performing spectral analysis; and a data processing unit, the input port of which is connected to the output port of the spectrometer for performing demodulation operations on the data analyzed by the spectrometer.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensing, and particularly to an optical fiber sensor capable of simultaneously measuring pressure and temperature. Background Art

[0002] Pressure sensors are the most common type of sensors and are widely used in various industrial automation environments, including many industries such as water conservancy and hydropower, railway transportation, intelligent buildings, production automation, aerospace, military, petrochemical, oil wells, electric power, ships, machine tools, pipelines, etc.

[0003] Traditional pressure sensors are large in size and heavy in weight and cannot provide electrical output; semiconductor pressure sensors are small in size, light in weight, and high in accuracy, but their manufacturing process is complex, temperature crosstalk is large, and they are vulnerable to electromagnetic interference; among existing optical fiber pressure sensors, fiber Bragg grating pressure sensors are easily affected by temperature, the length of short cavity FP cavities is difficult to control, repeatability is poor, the birefringent type has a complex structure, is not easy to manufacture, and has large temperature crosstalk. Therefore, it is necessary to distinguish temperature crosstalk to further improve the measurement accuracy of pressure sensors. Summary of the Invention

[0004] In view of the problems existing in the existing solutions, the present invention provides an optical fiber sensor for simultaneously measuring pressure and temperature, which specifically includes:

[0005] A broadband unit for transmitting optical signals.

[0006] A sensing unit for simultaneously detecting pressure and temperature.

[0007] A circulator, whose first port is connected to the broadband unit and is used for unidirectional transmission of optical signals.

[0008] A transmission coupling optical fiber disposed in the sensing unit and connected to the second port of the circulator.

[0009] A first pressure sensing sheet disposed in the sensing unit.

[0010] A second pressure sensing sheet disposed in the sensing unit and parallel to the first pressure sensing sheet.

[0011] A first hollow optical fiber fixedly disposed between the first pressure sensing sheet and the second pressure sensing sheet and perpendicular to the transmission coupling optical fiber.

[0012] A second hollow optical fiber fixedly disposed between the first pressure sensing sheet and the second pressure sensing sheet and parallel to the first hollow optical fiber.

[0013] A spectrometer, whose input port is connected to the third port of the circulator and is used for spectral analysis.

[0014] A data processing unit, whose input port is connected to the output port of the spectrometer, and is used for performing demodulation operations on the data analyzed by the spectrometer.

[0015] Furthermore, the demodulation process of the data processing unit is: difference operation, Fourier transform, inverse Fourier transform, tracking of two main frequency valley wavelengths, and sensitivity matrix operation.

[0016] Furthermore, the inner diameters of the first hollow optical fiber and the second hollow optical fiber are both 50 - 250 μm, and the outer diameters are both 145 - 360 μm.

[0017] Furthermore, the outer diameter of the transmission coupling optical fiber is 20 - 50 μm smaller than the outer diameters of the first hollow optical fiber and the second hollow optical fiber.

[0018] Furthermore, the transmission coupling optical fiber is a multimode optical fiber.

[0019] Furthermore, both the first pressure sensing sheet and the second pressure sensing sheet are stainless steel material sensing sheets.

[0020] Furthermore, the thicknesses of both the first pressure sensing sheet and the second pressure sensing sheet are 0.5 mm - 1.2 mm, the lengths are both 1 - 4 cm, and the widths are both 1 - 4 cm.

[0021] Furthermore, the wavelength resolution of the spectrometer is 0.02 nm - 0.5 nm.

[0022] Compared with the prior art, the present invention provides an optical fiber sensor for simultaneously measuring pressure and temperature, and its beneficial effects are:

[0023] The optical fiber sensor for simultaneously measuring pressure and temperature provided by the present invention applies pressure to the first and second pressure sensing sheets, and the pressure is conducted to the first hollow optical fiber, causing the first hollow optical fiber to deform. The air cavity of the first hollow optical fiber is utilized to sense the change in pressure, while the change in the cavity length of the air cavity is extremely small when the temperature changes; when the temperature changes, since the side wall of the first hollow optical fiber is made of silica material and has a relatively high thermo - optic coefficient, the side wall is sensitive to temperature, while the change in the side wall thickness is extremely small when pressure acts. By simultaneously measuring the change in the air cavity length and the change in the side wall thickness, the sensor can simultaneously sense the changes in pressure and temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a system block diagram of an optical fiber sensor for simultaneously measuring pressure and temperature provided by the present invention;

[0025] Figure 2 It is a structural schematic diagram of an optical fiber sensor for simultaneously measuring pressure and temperature provided by the present invention;

[0026] Figure 3 is a schematic structural diagram of the sensing unit;

[0027] Figure 4 is a schematic diagram of light propagation in the sensing unit.

[0028] In the figure: 1. broadband unit, 2. circulator, 3. sensing unit, 4. spectrometer, 5. data processing unit, 6. transmission coupling optical fiber, 7. first pressure sensing sheet, 8. second pressure sensing sheet, 9. first hollow optical fiber, 10. second hollow optical fiber. Specific embodiments

[0029] The following combines the attached Figures 1 to 4 , and further describes the specific embodiments of the present invention. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, and cannot be used to limit the protection scope of the present invention.

[0030] Embodiment 1: As Figure 1 and Figure 2 shown, a fiber optic sensor for simultaneously measuring pressure and temperature provided by the present invention specifically includes: a broadband unit 1 for transmitting optical signals; a sensing unit 3 for simultaneously detecting pressure and temperature; a circulator 2, whose first port is connected to the broadband unit 1 and is used for unidirectional transmission of optical signals; a transmission coupling optical fiber 6 disposed in the sensing unit 3 and connected to the second port of the circulator 2; a first pressure sensing sheet 7 disposed in the sensing unit 3; a second pressure sensing sheet 8 disposed in the sensing unit 3 and parallel to the first pressure sensing sheet 7; a first hollow optical fiber 9 fixedly disposed between the first pressure sensing sheet 7 and the second pressure sensing sheet 8 and perpendicular to the transmission coupling optical fiber 6; a second hollow optical fiber 10 fixedly disposed between the first pressure sensing sheet 7 and the second pressure sensing sheet 8 and parallel to the first hollow optical fiber 9; a spectrometer 4, whose input port is connected to the third port of the circulator 2 and is used for spectral analysis; a data processing unit 5, whose input port is connected to the output port of the spectrometer 4 and is used for demodulation operation on the data analyzed by the spectrometer 4; the demodulation process of the data processing unit 5 is: difference operation, Fourier transform, inverse Fourier transform, tracking of two main frequency valley wavelengths, and sensitivity matrix operation.

[0031] An optical fiber sensor for simultaneously measuring pressure and temperature provided by the present invention fixes a first hollow optical fiber 9 and a second hollow optical fiber 10 in parallel at a certain interval on a first pressure sensing sheet 7 through ultraviolet glue. A transmission coupling optical fiber 6 is vertically aligned with the middle part of the first hollow optical fiber 9 and fixed on the first pressure sensing sheet 7 with ultraviolet glue. A second pressure sensing sheet 8 is parallel to and aligned with the first pressure sensing sheet 7, and the second pressure sensing sheet 8 and the first pressure sensing sheet 7 sandwich the transmission coupling optical fiber 6, the first hollow optical fiber 9 and the second hollow optical fiber 10 in the middle. The first hollow optical fiber 9 and the second hollow optical fiber 10 jointly support the first pressure sensing sheet 7 and the second pressure sensing sheet 8 to prevent them from tilting. The first hollow optical fiber 9 and the second hollow optical fiber 10 are also fixed to the second pressure sensing sheet 8 with ultraviolet glue. The above-mentioned devices together constitute a sensing unit 3.

[0032] As Figure 3 shown, for an optical fiber sensor for simultaneously measuring pressure and temperature provided by the present invention, the light output by the broadband unit 1 enters the transmission coupling optical fiber 6 through the circulator 2, and then is transmitted to the first hollow optical fiber 9 through the transmission coupling optical fiber 6. The light is reflected at the front and back four surfaces of the first hollow optical fiber 9 as Figure 4 shown. The reflected light is recoupled to the transmission coupling optical fiber 6 and then transmitted to the spectrometer 4 through the circulator 2. The spectrometer 4 receives the signal for spectral analysis, and after analyzing the data, it is transmitted to the data processing unit 5 for the final demodulation operation.

[0033] In this process, multiple Fabry-Perot interferometers are formed by using the first hollow optical fiber 9 to sense the signal. As Figure 4 shown, the interference spectrum formed by the four reflected light beams on the surface of the first hollow optical fiber 9 can be described by the following formula:

[0034]

[0035] where A is the DC component, B, C, D, and F are the modulation amplitudes of the corresponding interference fringes respectively, l1 and l2 are the wall thickness and inner diameter of the first hollow optical fiber 9 respectively, λ is the light source wavelength, and n1 and n0 are the refractive indices of the first hollow optical fiber and air respectively. It can be seen from formula 1 that the interference spectrum has four frequency components, among which the frequency is only related to n1l1, the frequency is only related to n0l2. When the temperature changes, d(n1l1) / dT >> d(n0l2) / dT; when pressure acts on the first pressure sensing sheet 7 and the second pressure sensing sheet 8, it squeezes the first hollow optical fiber 9, causing it to deform. At this time, d(n0l2) / dp >> d(n1l1) / dp. Therefore, by simultaneously detecting the phase changes of these two frequencies, the simultaneous measurement of pressure and temperature can be achieved.

[0036] The operation process of the data processing unit 5 is as follows: First, perform interpolation operation on the output signal of the spectrometer so that the data of I(1 / λ) is equally spaced sampled data I'(1 / λ); then perform Fourier transform on I'(1 / λ) to extract and two main frequency signals; then perform inverse Fourier transform on the two extracted frequency signals to restore the interference spectra of the two main frequency signals; then track the change of the valley wavelength of the two main frequency interference spectra; finally, calculate the corresponding pressure and temperature changes through matrix calculation. The calculation formula is as follows:

[0037]

[0038] where p and T respectively represent the magnitudes of the pressure and temperature acting on the sensing unit, S1 and S3 are the sensitivities of n1l1 and n0l2 to pressure respectively; S2 and S4 are the sensitivities of n1l1 and n0l2 to temperature respectively; Δλ dip_l1 and Δλ dip_l2 are respectively and the magnitudes of the valley wavelength drifts corresponding to the signals.

[0039] It has been proved by experiments that when the inner diameter of the first hollow optical fiber 9 is 250 μm and the outer diameter is 356 μm; the thickness of the first pressure sensing sheet 7 and the second pressure sensing sheet 8 is 1 mm, the length is 2 cm, and the width is 2 cm, the pressure sensitivity is 1 nm / kPa, the temperature sensitivity is 50 pm / ℃, the pressure resolution is 1 Pa; the temperature resolution sensitivity is 0.05℃.

[0040] Embodiment 2: The inner diameters of the first hollow optical fiber 9 and the second hollow optical fiber 10 are both 50 - 250 μm, and the outer diameters are both 145 - 360 μm; the larger the inner diameter and the smaller the outer diameter of the first hollow optical fiber 9 and the second hollow optical fiber 10, the greater the pressure sensitivity of the sensor, but the worse the pressure resistance value. Within this range, both a relatively high sensitivity and a certain pressure resistance value of the sensor can be ensured.

[0041] The outer diameter of the transmission coupling optical fiber 6 is 20 - 50 μm smaller than the outer diameters of the first hollow optical fiber 9 and the second hollow optical fiber 10; when under pressure, the hollow optical fiber is more likely to deform, so that when the sensing unit is under pressure, the main deformation occurs on the hollow optical fiber, improving the sensitivity of the sensor. However, the difference cannot be too large, otherwise it is very difficult to ensure that the transmission coupling optical fiber is perpendicular to the first hollow optical fiber 9 during installation, affecting the optical coupling efficiency.

[0042] The transmission coupling optical fiber 6 is a multimode optical fiber; the multimode optical fiber has a large numerical aperture, which can improve the coupling efficiency of the reflected light and increase the fringe contrast and stability of the Fabry - Perot interferometer.

[0043] Both the first pressure sensing sheet 7 and the second pressure sensing sheet 8 are made of stainless steel; the stainless steel material can ensure that the sensor has high thermal conductivity, improve the temperature response speed of the sensor; and can also ensure that the sensor has certain corrosion resistance and compressive characteristics.

[0044] The thickness of both the first pressure sensing sheet 7 and the second pressure sensing sheet 8 is 0.5 mm to 1.2 mm, the length is 1 to 4 cm, and the width is 1 to 4 cm; the larger the area of the first pressure sensing sheet 7 and the second pressure sensing sheet 8, the easier it is to conduct the pressure to the first hollow optical fiber 9, thereby improving the pressure sensitivity of the sensor, but the size of the sensor is also larger. Considering comprehensively, the above dimensions are appropriate.

[0045] The wavelength resolution of the spectrometer 4 is 0.02 nm to 0.5 nm; the higher the accuracy of the spectrometer, the higher the measurement accuracy, but the cost of the sensor is also higher.

[0046] Compared with the prior art, the present invention provides an optical fiber sensor for simultaneously measuring pressure and temperature, having the following beneficial effects:

[0047] (1) For the optical fiber sensor for simultaneously measuring pressure and temperature provided by the present invention, the pressure acts on the first and second pressure sensing sheets, and the pressure is conducted to the first hollow optical fiber, causing the first hollow optical fiber to deform. The air cavity of the first hollow optical fiber is used to sense the change of pressure, and when the temperature changes, the change of the cavity length of the air cavity is extremely small; when the temperature changes, since the side wall of the first hollow optical fiber is made of silica material and has a high thermo-optic coefficient, the side wall is sensitive to temperature, and when the pressure acts, the change of the side wall thickness is extremely small. By simultaneously measuring the change of the air cavity length and the change of the side wall thickness, the sensor can simultaneously sense the change of pressure and temperature.

[0048] (2) For the optical fiber sensor for simultaneously measuring pressure and temperature provided by the present invention, the frequencies corresponding to the two cavity lengths are found by performing Fourier transform on the spectrum, and then the phase changes of the frequencies corresponding to the two cavity lengths are extracted by performing inverse Fourier transform. The change of the air cavity length of the first hollow optical fiber and the change of the side wall thickness are calculated through the phase change, and finally the magnitudes of the pressure and temperature changes are demodulated through matrix operation, having higher measurement accuracy compared with the prior art.

[0049] (3) For the optical fiber sensor for simultaneously measuring pressure and temperature provided by the present invention, the detected pressure and temperature signals are converted into optical signals for propagation through the sensing unit. Compared with the mechanical or semiconductor pressure detectors in the prior art, it has the characteristics of small propagation loss, easy long-distance transmission, convenient multiplexing, and especially strong anti-electromagnetic interference ability.

[0050] (4) The manufacturing process of the fiber optic sensor for simultaneously measuring pressure and temperature provided by the present invention is extremely simple, does not involve any complex processes, has low costs, and has good consistency.

[0051] The above-described embodiments are only preferred specific embodiments of the present invention, and the protection scope of the present invention is not limited thereto. Any simple changes or equivalent replacements of technical solutions that can be obviously obtained by those skilled in the art within the technical scope disclosed by the present invention all belong to the protection scope of the present invention.

Claims

1. An optical fiber sensor for simultaneously measuring pressure and temperature, characterized in that, Including: A broadband unit (1) for transmitting optical signals; A circulator (2) whose first port is connected to the broadband unit (1); A first pressure sensing sheet (7) and a second pressure sensing sheet (8) arranged in parallel; A first hollow optical fiber (9) and a second hollow optical fiber (10) fixedly arranged in parallel between the first pressure sensing sheet (7) and the second pressure sensing sheet (8); A transmission coupling optical fiber (6) whose one end is connected to the second port of the circulator (2) and whose other end is perpendicular to the first hollow optical fiber (9); A spectrometer (4) whose input port is connected to the third port of the circulator (2) for spectral analysis; A data processing unit (5) whose input port is connected to the output port of the spectrometer (4) for performing Fourier transform on the output signal of the spectrometer (4) to extract the frequency signals corresponding to the side wall thickness and inner diameter of the first hollow optical fiber (9), performing inverse Fourier transform on the frequency signals to extract the phase changes corresponding to the frequencies of the side wall thickness and inner diameter, calculating the changes in the inner diameter and side wall thickness of the first hollow optical fiber (9) through the phase changes, and demodulating the magnitudes of the pressure and temperature changes through matrix operations based on the changes in the inner diameter and side wall thickness.

2. A fiber optic sensor for simultaneously measuring pressure and temperature according to claim 1, characterized in that: The data processing unit (5) is specifically used for: sequentially obtaining the pressure and temperature through difference operation, Fourier transform, inverse Fourier transform, tracking of two main frequency valley wavelengths, and sensitivity matrix operation.

3. A fiber optic sensor for simultaneously measuring pressure and temperature according to claim 1, characterized in that: The inner diameters of the first hollow optical fiber (9) and the second hollow optical fiber (10) are both 50 - 250 μm, and the outer diameters are both 145 - 360 μm.

4. A fiber optic sensor for simultaneously measuring pressure and temperature according to claim 1, characterized in that: The outer diameter of the transmission coupling optical fiber (6) is 20 - 50 μm smaller than the outer diameters of the first hollow optical fiber (9) and the second hollow optical fiber (10).

5. A fiber optic sensor for simultaneously measuring pressure and temperature according to claim 1, characterized in that: The transmission coupling optical fiber (6) is a multimode optical fiber.

6. A fiber optic sensor for simultaneously measuring pressure and temperature according to claim 1, characterized in that: Both the first pressure sensing sheet (7) and the second pressure sensing sheet (8) are stainless steel material sensing sheets.

7. A fiber optic sensor for simultaneously measuring pressure and temperature according to claim 1, characterized in that: The thicknesses of both the first pressure sensing sheet (7) and the second pressure sensing sheet (8) are 0.5 mm - 1.2 mm, the lengths are both 1 - 4 cm, and the widths are both 1 - 4 cm.

8. A fiber optic sensor for simultaneously measuring pressure and temperature according to claim 1, characterized in that: The wavelength resolution of the spectrometer (4) is 0.02 nm - 0.5 nm.

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