A fiber-optic dual-parameter sensor based on quadratic vernier effect

Abstract

The application discloses a kind of optical fiber double parameter sensors based on secondary vernier effect, belong to optical fiber sensor technical field.SLD wide spectrum light source and first 3dB coupler are split, and Solc-Sagnac interferometer (containing 90° fusion two sections of thin-diameter polarization-maintaining optical fiber) and Sagnac interferometer (containing single section of thin-diameter polarization-maintaining optical fiber) are connected in parallel, two light signals are modulated and are synthesized by fourth 3dB coupler, and spectrometer displays the final output spectrum.Solc-Sagnac interferometer occurs first vernier effect, the free spectral range of Sagnac interferometer and Solc-Sagnac interferometer is matched in similar range, second vernier effect occurs, and temperature and stress sensitivity are further amplified, when temperature change / stress is applied, envelope valley displacement is doubled, the optical fiber double parameter sensor of the application has the characteristics of small size, simple structure, double parameter measurement, high sensitivity and the like.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, and more specifically, relates to a fiber optic dual-parameter sensor based on the quadratic vernier effect. Background Technology

[0002] With the continuous expansion of application fields and the increasing complexity of environments, fiber optic sensors have many advantages over traditional electrical sensing technologies: First, optical fibers have excellent anti-interference capabilities, effectively resisting external electromagnetic and radiation interference; simultaneously, their mechanical properties are also excellent, with small size, flexible texture, and ultra-light weight; furthermore, the electrical properties of optical fibers are exceptionally good, possessing not only insulation effects but also being unaffected by induction; moreover, the chemical properties of optical fibers are outstanding, being waterproof, high-temperature resistant, and corrosion-resistant, making them suitable for parameter measurement and safety monitoring in various harsh environments. In oil and gas and mineral resource exploration, biomedicine, bridge structural safety measurement, and other scenarios, fiber optic sensors are widely used to measure temperature and stress, and higher sensitivity can be obtained by utilizing the vernier effect. However, vernier-based work can only improve the sensitivity of single-parameter detection. Therefore, achieving high-sensitivity dual-parameter measurement using the vernier effect is a noteworthy issue, making high-sensitivity measurement with fiber optic dual-parameter sensors a research hotspot both domestically and internationally. Summary of the Invention

[0003] To address the difficulty of achieving high sensitivity simultaneously in current mainstream fiber optic dual-parameter sensor research schemes, this invention proposes a fiber optic dual-parameter sensor based on the quadratic vernier effect. This sensor amplifies the sensitivity to temperature and stress through the quadratic vernier effect.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] A fiber optic dual-parameter sensor based on the quadratic vernier effect includes an SLD broadband light source, a first 3dB coupler, a fourth 3dB coupler, a spectrometer 11, and a Solc-Sagnac interferometer and a Sagnac interferometer connected in parallel with similar free spectral widths; the Solc-Sagnac interferometer includes a second 3dB coupler, a first narrow-diameter polarization-maintaining fiber, a second narrow-diameter polarization-maintaining fiber, and a first polarization controller; the Sagnac interferometer includes a third 3dB coupler, a third narrow-diameter polarization-maintaining fiber, and a second polarization controller.

[0006] The SLD broadband light source is connected to one port of the first 3dB coupler. The second port of the first 3dB coupler is connected to one port of the second 3dB coupler, which is used to split the input optical signal into a Solc-Sagnac interferometer. The optical signal passes sequentially through the second port of the second 3dB coupler, the first narrow-diameter polarization-maintaining fiber, the second narrow-diameter polarization-maintaining fiber, the first polarization controller, the third port of the second 3dB coupler, and the fourth port of the second 3dB coupler within the Solc-Sagnac interferometer. The third port of the first 3dB coupler is connected to one port of the third 3dB coupler, which is used to split the input optical signal into a Sagnac interferometer. The optical signal passes sequentially through the third port of the third 3dB coupler, the third narrow-diameter polarization-maintaining fiber, the second polarization controller, the second port of the third 3dB coupler, and the fourth port of the third 3dB coupler within the Sagnac interferometer.

[0007] The four ports of the second 3dB coupler are connected to the one port of the fourth 3dB coupler, and the four ports of the third 3dB coupler are connected to the two ports of the fourth 3dB coupler, respectively transmitting the output optical signals of the Solc-Sagnac interferometer and the Sagnac interferometer to the fourth 3dB coupler. The three ports of the fourth 3dB coupler are connected to the spectrometer.

[0008] As a preferred embodiment of the present invention, the Solc-Sagnac interferometer and the Sagnac interferometer are connected in parallel to generate a secondary vernier effect, the specific implementation process of which includes:

[0009] The Solc-Sagnac interferometer exhibits a first vernier effect, resulting in an envelope shift in the interference spectrum and amplifying the dual-parameter sensitivity. When the free spectral widths of the Sagnac interferometer and the Solc-Sagnac interferometer are matched within a similar range, a second vernier effect occurs, further amplifying the temperature and stress sensitivity. By adjusting the lengths of the first, second, and third narrow-diameter polarization-maintaining fibers, the free spectral widths are made to meet the matching conditions.

[0010] As a preferred embodiment of the present invention, the first, second, and third narrow-diameter polarization-maintaining optical fibers are connected to the single-mode optical fiber by discharge fusion splicing.

[0011] As a preferred embodiment of the present invention, the first and second narrow-diameter polarization-maintaining fibers are directly fused together at a fast axis angle of 90°.

[0012] As a preferred embodiment of the present invention, the envelope displacement caused by applying a temperature change or stress change to the second-diameter polarization-maintaining fiber is expressed as:

[0013]

[0014] Where, Δλ EΔB1 represents the wavelength shift of the envelope under temperature or strain changes, ΔL1 represents the change in birefringence of the second narrow-diameter polarization-maintaining fiber after temperature or stress is applied, λ represents the wavelength, L1 and B1 represent the length and birefringence of the first narrow-diameter polarization-maintaining fiber, L2 and B2 represent the length and birefringence of the second narrow-diameter polarization-maintaining fiber, and L3 and B3 represent the length and birefringence of the third narrow-diameter polarization-maintaining fiber.

[0015] As a preferred embodiment of the present invention, the temperature sensing process includes:

[0016] A temperature change is applied to the second-diameter polarization-maintaining fiber, gradually heating it from the lowest value to the highest value within a preset temperature range, with a step size of 1℃.

[0017] The output spectra at different temperatures were obtained using a spectrometer. The trough wavelengths of the output spectra shifted toward shorter wavelengths as the temperature increased.

[0018] Linear fitting of the relationship curve between trough wavelength and temperature.

[0019] As a preferred embodiment of the present invention, the stress sensing process includes:

[0020] Different stresses are applied to the second-diameter polarization-maintaining fiber, gradually increasing from the lowest value to the highest value within a preset stress range, with a step size of 100 με.

[0021] The output spectra under different stresses were obtained using a spectrometer. The trough wavelengths of the output spectra shifted towards longer wavelengths as the stress increased.

[0022] The relationship between the trough wavelength and stress was obtained by linear fitting.

[0023] As a preferred embodiment of the present invention, the core diameter and outer diameter of the fine-diameter polarization-maintaining fiber are 6μm and 80μm, respectively, and the length of the first fine-diameter polarization-maintaining fiber 4 is 20cm, the length of the second fine-diameter polarization-maintaining fiber 5 is 30cm, and the length of the third fine-diameter polarization-maintaining fiber 8 is 9.1cm.

[0024] The beneficial effects of this invention are:

[0025] 1. By using discharge fusion splicing between narrow polarization-maintaining fiber and single-mode fiber, the sensor's response speed to temperature can be improved.

[0026] 2. The thin-diameter polarization-maintaining fiber 1 and the thin-diameter polarization-maintaining fiber 2 are fused together at a 90° angle to the fast axis and connected to the 3dB coupler 2 and the polarization controller 1 to form a Solc-Sagnac interferometer, which causes the first vernier effect and realizes the first amplification of the dual-parameter sensitivity.

[0027] 3. The Solc-Sagnac interferometer and the Sagnac interferometer are connected in parallel with similar free-range spectral widths to generate a second vernier effect, thereby achieving a second amplification of the sensitivity of the dual parameters. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure of the fiber optic dual-parameter sensor based on the quadratic vernier effect according to an embodiment of the present invention.

[0029] Figure 2 The images show the output spectra of the fiber optic dual-parameter sensor based on the quadratic vernier effect at different temperatures, according to an embodiment of the present invention.

[0030] Figure 3 The temperature response curve of the fiber optic dual-parameter sensor based on the quadratic vernier effect is shown in the embodiment of the present invention.

[0031] Figure 4 The images show the output spectra of the fiber optic dual-parameter sensor based on the quadratic vernier effect under different stresses, according to an embodiment of the present invention.

[0032] Figure 5 The stress response curve of the fiber optic dual-parameter sensor based on the quadratic vernier effect is shown in the embodiment of the present invention. Detailed Implementation

[0033] The following provides a detailed description of the structure and working principle of this invention.

[0034] This invention proposes a fiber optic dual-parameter sensor based on the quadratic vernier effect, comprising an SLD broadband light source 1, a first 3dB coupler 2, a second 3dB coupler 3, a first narrow-diameter polarization-maintaining fiber 4, a second narrow-diameter polarization-maintaining fiber 5, a first polarization controller 6, a third 3dB coupler 7, a third narrow-diameter polarization-maintaining fiber 8, a second polarization controller 9, a fourth 3dB coupler 10, and a spectrometer 11; the diameters of the first narrow-diameter polarization-maintaining fiber 4, the second narrow-diameter polarization-maintaining fiber 5, and the third narrow-diameter polarization-maintaining fiber 8 are smaller than the diameter of the single-mode fiber used to connect the various components.

[0035] The SLD broadband light source 1 is connected to port 201 of the first 3dB coupler. Port 202 of the first 3dB coupler is connected to port 301 of the second 3dB coupler. Port 302 of the second 3dB coupler is connected to one end of the first narrow-diameter polarization-maintaining fiber 4. The other end of the first narrow-diameter polarization-maintaining fiber 4 is fused to one end of the second narrow-diameter polarization-maintaining fiber 5 at a 90° fast axis angle. The other end of the second narrow-diameter polarization-maintaining fiber 5 is connected to one end of the first polarization controller 6. The other end of the first polarization controller 6 is connected to port 303 of the second 3dB coupler. Port 304 of the second 3dB coupler is connected to the first polarization controller 6. 4 is connected to port 1001 of the fourth 3dB coupler. Port 203 of the first 3dB coupler is connected to port 701 of the third 3dB coupler. Port 703 of the third 3dB coupler is connected to one end of the third narrow-diameter polarization-maintaining fiber 8. The other end of the third narrow-diameter polarization-maintaining fiber 8 is connected to one end of the second polarization controller 9. The other end of the second polarization controller 9 is connected to port 702 of the third 3dB coupler. Port 704 of the third 3dB coupler is connected to port 1002 of the fourth 3dB coupler. Port 1003 of the fourth 3dB coupler is connected to the spectrometer 11.

[0036] The working principle of the above-mentioned fiber optic dual-parameter sensor based on the quadratic vernier effect is as follows:

[0037] The broadband light output from the first SLD broadband light source 1, with wavelengths from 1250nm to 1650nm, enters the first 3dB coupler through port 201 and is split into two beams, A and B, where:

[0038] Beam A is output from port 202 of the first 3dB coupler, enters the second 3dB coupler through port 301, and is split into two beams A1 and A2. Beam A1 is output from port 302 of the second 3dB coupler and is transmitted sequentially to the first narrow-diameter polarization-maintaining fiber 4, the second narrow-diameter polarization-maintaining fiber 5, and the first polarization controller 6, and finally returns to the second 3dB coupler 3 from port 303. Beam A2 is output from port 303 of the second 3dB coupler and is transmitted sequentially to the first polarization controller 6, the second narrow-diameter polarization-maintaining fiber 5, and the first narrow-diameter polarization-maintaining fiber 4, and finally returns to the second 3dB coupler 3 from port 302. The two beams A1 and A2 returning to the second 3dB coupler 3 undergo the first vernier effect at port 304 of the second 3dB coupler to produce interference fringes, and then enter the fourth 3dB coupler 10 from port 1001 of the fourth 3dB coupler.

[0039] Meanwhile, beam B is output from port 203 of the first 3dB coupler, passes through port 701 of the third 3dB coupler, and enters the third 3dB coupler 7, where it is split into two beams, B1 and B2. Beam B1 is output from port 702 of the third 3dB coupler and is transmitted sequentially to the second polarization controller and the third narrow-diameter polarization-maintaining fiber, and finally returns from port 703 of the third 3dB coupler to the third 3dB coupler 7. Beam B2 is output from port 703 of the third 3dB coupler and is transmitted sequentially to the third narrow-diameter polarization-maintaining fiber 8 and the second polarization controller 9, and finally returns from port 702 of the third 3dB coupler to the third 3dB coupler 7. The two beams B1 and B2 returning to the third 3dB coupler 7 generate interference fringes through port 704 of the third 3dB coupler, and then enter the fourth 3dB coupler 10 from port 1002 of the fourth 3dB coupler.

[0040] The two interference fringes entering the fourth 3dB coupler 10 are coupled at the three ports 1003 of the fourth 3dB coupler and a second vernier effect occurs to form interference fringes. Finally, the light formed is transmitted to the spectrometer 11.

[0041] In one specific embodiment of the invention, the core diameter and outer diameter of the fine-diameter polarization-maintaining fiber are 6μm and 80μm, respectively, wherein the length of the first fine-diameter polarization-maintaining fiber 4 is 20cm, the length of the second fine-diameter polarization-maintaining fiber 5 is 30cm, and the length of the third fine-diameter polarization-maintaining fiber 8 is 9.1cm.

[0042] In this invention, the second-diameter polarization-maintaining fiber 5 is used for temperature and stress measurement.

[0043] The transmission spectrum output formula based on the Solc-Sagnac interferometer is:

[0044]

[0045] Where θ2 represents the angle between the fast axes of the first narrow-diameter polarization-maintaining fiber 4 and the second narrow-diameter polarization-maintaining fiber 5, θ1 represents the polarization rotation angle introduced from the second 3dB coupler's two ports 302 to the splice of the first narrow-diameter polarization-maintaining fiber 4, and θ3 represents the polarization rotation angle introduced from the splice of the second narrow-diameter polarization-maintaining fiber 5 to one end of the first polarization controller 6. λ represents the wavelength, L1 and B1 represent the length and birefringence of the first narrow-diameter polarization-maintaining fiber 4, and L2 and B2 represent the length and birefringence of the second narrow-diameter polarization-maintaining fiber 5.

[0046] Of the parameters mentioned above, the sum of θ1 and θ3 can be adjusted by the first polarization controller 6. Specifically, during the experiment, θ1 + θ3 can be controlled to be π / 4. When θ2 is 90°, the final transmission spectrum only contains a low-frequency envelope with respect to sinθ2cos(θ1 + θ3)cos[(πB1L1 - πB2L2) / λ]. This structure realizes the first vernier effect, thus performing the first amplification of sensitivity.

[0047] The transmission spectrum output formula based on the Sagnac interferometer is:

[0048]

[0049] Wherein, L3 and B3 represent the length and birefringence of the third narrow-diameter polarization-maintaining fiber 8.

[0050] Therefore, the transmission spectrum output formula of the fiber optic dual-parameter sensor based on the quadratic vernier effect is:

[0051] T = T SSI +T SI

[0052] In one example of the invention, to generate the secondary vernier effect, the free spectral range of the Sagnac interferometer needs to be set near the free spectral range of the Solc-Sagnac interferometer, as mentioned above regarding the length parameters of the narrow-diameter polarization-maintaining fiber, with the structure as follows: Figure 1 As shown.

[0053] During the sensing process, only a temperature change or stress change needs to be applied to the second narrow polarization-maintaining fiber 5, and its transmission spectrum will shift. The relevant temperature or stress information can be obtained by solving the shift information of the transmission spectrum.

[0054] In a fiber optic dual-parameter sensor based on the quadratic vernier effect, the envelope displacement caused by applying a temperature or stress change to the second-diameter polarization-maintaining fiber 5 can be expressed as:

[0055]

[0056] Where, Δλ E The values ​​represent the wavelength shift of the envelope under temperature or strain changes. ΔB1 represents the change in birefringence of the second-diameter polarization-maintaining fiber 5 after temperature or stress is applied, ΔL1 represents the change in length of the second-diameter polarization-maintaining fiber 5 after temperature or stress is applied, and L3 and B3 represent the length and birefringence of the third-diameter polarization-maintaining fiber. When the length difference (B1L1-B2L2-B3L3) remains constant, the sensitivity increases with increasing L1. When L1 is constant, the sensitivity is inversely proportional to the length difference (B1L1-B2L2-B3L3). Simultaneously, it is directly proportional to the wavelength.

[0057] To verify the actual performance of the sensor, temperature and strain measurement experiments were conducted. The second-diameter polarization-maintaining fiber 5 was placed in a temperature chamber and heated from 29℃ to 35℃ in 1℃ increments. Spectra at different temperatures were obtained, and the relationship between temperature and trough displacement was shown below. Figure 2 As shown in the figure, in the experiment, as the temperature increases, the wavelength corresponding to the trough of the output spectrum (specifically the global minimum point of the envelope curve) shifts towards shorter wavelengths. Figure 3 The temperature response curve is shown. Through linear fitting, the temperature sensitivity is found to be -61.88℃ / nm.

[0058] The experiment also verified the stress sensing performance of the sensor. The two ends of the second-diameter polarization-maintaining fiber 5 were fixed to two triaxial displacement stages, each with an adjustable minimum step size of 30 μm. A stress of 100 με was applied, and the spectra under different stresses were obtained. The relationship between stress and trough displacement is shown in the figure below. Figure 4 As shown in the figure, in the experiment, as the stress increases, the wavelength corresponding to the trough of the output spectrum (specifically the global minimum point of the envelope curve) shifts towards longer wavelengths. Figure 5 The stress response curve was obtained through linear fitting, yielding a stress sensitivity of -0.78 nm / με. Based on the experimentally obtained stress and temperature sensitivities, dual-parameter measurement of stress and temperature can be achieved.

[0059] This invention achieves a secondary vernier effect by using a parallel connection of a Solc-Sagnac interferometer and a Sagnac interferometer, which amplifies the sensitivity to temperature and stress a second time. Compared with existing fiber optic dual-parameter sensors, it has higher sensitivity, making it more widely applicable, and the system structure is simple and easy to integrate. This structure can monitor temperature and stress more accurately.

[0060] For the purposes of illustration and description, the foregoing illustrative examples relating to the invention are provided. This is not intended to be an exhaustive description of the invention or to limit it to the precise forms described; modifications and variations can be made based on the foregoing description. The embodiments were chosen and described to explain the principles of the invention and as practical applications thereof, enabling those skilled in the art to use the invention in various embodiments and to make various modifications for specific purposes. All variations that can be directly derived or conceived by those skilled in the art from the disclosure of this invention should be considered within the scope of protection of this invention.

Claims

1. A fiber-optic dual-parameter sensor based on the quadratic Vernier effect, characterized in that, It includes an SLD broadband light source, a first 3dB coupler, a fourth 3dB coupler, a spectrometer (11), and a Solc-Sagnac interferometer and a Sagnac interferometer connected in parallel with similar free spectral widths; the Solc-Sagnac interferometer includes a second 3dB coupler, a first narrow-diameter polarization-maintaining fiber, a second narrow-diameter polarization-maintaining fiber, and a first polarization controller; the Sagnac interferometer includes a third 3dB coupler, a third narrow-diameter polarization-maintaining fiber, and a second polarization controller; The SLD broadband light source is connected to one port of the first 3dB coupler. The second port of the first 3dB coupler is connected to one port of the second 3dB coupler, which is used to split the input optical signal into a Solc-Sagnac interferometer. The optical signal passes sequentially through the second port of the second 3dB coupler, the first narrow-diameter polarization-maintaining fiber, the second narrow-diameter polarization-maintaining fiber, the first polarization controller, the third port of the second 3dB coupler, and the fourth port of the second 3dB coupler within the Solc-Sagnac interferometer. The third port of the first 3dB coupler is connected to one port of the third 3dB coupler, which is used to split the input optical signal into a Sagnac interferometer. The optical signal passes sequentially through the third port of the third 3dB coupler, the third narrow-diameter polarization-maintaining fiber, the second polarization controller, the second port of the third 3dB coupler, and the fourth port of the third 3dB coupler within the Sagnac interferometer. The four ports of the second 3dB coupler are connected to the one port of the fourth 3dB coupler, and the four ports of the third 3dB coupler are connected to the two ports of the fourth 3dB coupler, respectively transmitting the output optical signals of the Solc-Sagnac interferometer and the Sagnac interferometer to the fourth 3dB coupler. The three ports of the fourth 3dB coupler are connected to the spectrometer.

2. The optical fiber bi-parameters sensor of secondary F-P effect according to claim 1, characterized in that, Solc-Sagnac interferometers and Sagnac interferometers, when connected in parallel, exhibit a secondary vernier effect. The specific implementation process includes: The Solc-Sagnac interferometer exhibits a first vernier effect, resulting in an envelope shift in the interference spectrum and amplifying the dual-parameter sensitivity. When the free spectral widths of the Sagnac interferometer and the Solc-Sagnac interferometer are matched within a similar range, a second vernier effect occurs, further amplifying the temperature and stress sensitivity. By adjusting the lengths of the first, second, and third narrow-diameter polarization-maintaining fibers, the free spectral widths are made to meet the matching conditions.

3. The optical fiber bi-parameters sensor with secondary Vernier effect according to claim 1, characterized in that, The first, second, and third narrow-diameter polarization-maintaining fibers are connected to the single-mode fiber via discharge fusion splicing.

4. The optical fiber bi-parameters sensor of secondary Vernier effect according to claim 3, characterized in that, The first and second narrow-diameter polarization-maintaining fibers are directly fused together at a 90° angle between their fast axes.

5. The optical fiber bi-parameters sensor with secondary Vernier effect according to claim 1, characterized in that, When a temperature or stress change is applied to the second-diameter polarization-maintaining fiber, the resulting envelope displacement is expressed as: Where, Δλ E ΔB1 represents the wavelength shift of the envelope under temperature or strain changes, ΔL1 represents the change in birefringence of the second narrow-diameter polarization-maintaining fiber after temperature or stress is applied, λ represents the wavelength, L1 and B1 represent the length and birefringence of the first narrow-diameter polarization-maintaining fiber, L2 and B2 represent the length and birefringence of the second narrow-diameter polarization-maintaining fiber, and L3 and B3 represent the length and birefringence of the third narrow-diameter polarization-maintaining fiber.

6. The optical fiber bi-parameters sensor with secondary Vernier effect according to claim 1, characterized in that, The implementation process of temperature sensing includes: A temperature change is applied to the second-diameter polarization-maintaining fiber, gradually heating it from the lowest value to the highest value within a preset temperature range, with a step size of 1℃. The output spectra at different temperatures were obtained using a spectrometer. The trough wavelengths of the output spectra shifted toward shorter wavelengths as the temperature increased. Linear fitting of the relationship curve between trough wavelength and temperature.

7. The fiber optic dual-parameter sensor with secondary vernier effect according to claim 1, characterized in that, The process of stress sensing includes: Different stresses are applied to the second-diameter polarization-maintaining fiber, gradually increasing from the lowest value to the highest value within a preset stress range, with a step size of 100 με. The output spectra under different stresses were obtained using a spectrometer. The trough wavelengths of the output spectra shifted towards longer wavelengths as the stress increased. The relationship between the trough wavelength and stress was obtained by linear fitting.

8. The optical fiber bi-parameters sensor with secondary Vernier effect according to claim 1, characterized in that, The core diameter and outer diameter of the fine-diameter polarization-maintaining fiber are 6μm and 80μm, respectively. The length of the first fine-diameter polarization-maintaining fiber (4) is 20cm, the length of the second fine-diameter polarization-maintaining fiber (5) is 30cm, and the length of the third fine-diameter polarization-maintaining fiber (8) is 9.1cm.

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