A multi-point displacement sensing system based on optical fiber macrobending loss and its use method

Abstract

The invention discloses a multi-point displacement sensing system based on optical fiber macrobending loss, which belongs to the field of optical fiber sensing, and includes a pulse laser, a first optical fiber coupler, an optical fiber connector, a first single-mode optical fiber, a second optical fiber coupler, a third optical fiber coupler, a displacement sensor No. 1, a single-mode optical fiber No. 2, an optical fiber coupler No. 4, an optical fiber coupler No. 5, a displacement sensor No. 2, a single-mode optical fiber No. 3, a photoelectric detector, a voltage amplifier, an A / D converter, and a data collector. The change of displacement will significantly change the value of the SNS macrobending loss, thereby causing the change of the FLM reflection energy. Each FLM constitutes a displacement sensing unit, and each sensing unit only inputs 5% of the pulse light energy, which reduces the influence of the light source energy fluctuation, increases the number of reusable sensors, and each sensor can maintain a similar sensing effect. The change of temperature is difficult to cause the change of the peak energy of each FLM, which can be widely used in the field of structural health monitoring.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing, and in particular to a multi-point displacement sensing system. Background Art

[0002] In recent years, fiber optic displacement sensors have been widely studied. Compared with traditional displacement sensors, fiber optic displacement sensors have the advantages of compact structure, corrosion resistance, anti-electromagnetic interference and multiplexing. They are widely used in roadbed settlement monitoring, soil deformation monitoring and civil structure crack monitoring. With the continuous development of fiber optic displacement sensors, most of the current fiber optic displacement sensors are based on fiber Bragg gratings, long period gratings, Fabry-Perot cavities and fiber interferometers. They generally use wavelength demodulation, and most of them have the problem of cross-sensitivity between displacement and temperature, which is not conducive to their engineering applications.

[0003] In contrast, displacement sensors based on optical fiber bending loss have the advantages of simple structure, low cost, and multi-point measurement. This method can be truly applied to structural health monitoring engineering problems to achieve improved engineering economic benefits. Displacement sensors based on SMF bending losses of different structures are constantly being proposed. Since the bending loss of SMF is not obvious, and since SMF is directly used as the sensor head, the measured dynamic range is generally very low, which is greatly limited in practical engineering applications. Therefore, how to realize a multi-point optical fiber displacement sensing system that can overcome the problem of temperature and displacement cross-sensitivity, high dynamic measurement range, high stability and low cost has become an urgent problem to be solved. Summary of the invention

[0004] The present invention aims to solve the above problems existing in optical fiber sensing, and proposes a multi-point displacement sensing system based on optical fiber macrobending loss. The system consists of three parts: an optical fiber displacement sensor, an optical fiber loop reflector (FLM), and an optical time domain reflectometer (OTDR). The optical fiber displacement sensor adopts a cascaded single-mode-coreless-single-mode (SNS) optical fiber design, and the SNS is connected to the FLM composed of a 3dB coupler to improve the measurement dynamic range. The change in displacement will significantly change the value of the SNS macrobending loss, thereby causing a change in the FLM reflection energy, and finally realizing displacement measurement. Each FLM constitutes a displacement sensing unit, and a multi-point displacement sensing system is built based on this sensing unit, thereby realizing displacement temperature insensitivity, high dynamic range, wide range, low noise, high stability, and low-cost rapid sensing.

[0005] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A multi-point displacement sensing system based on optical fiber macrobending loss, comprising a connected pulse laser, a first optical fiber coupler, an optical fiber connector, a first single-mode optical fiber, a second optical fiber coupler, a third optical fiber coupler, a first displacement sensor, a second single-mode optical fiber, a fourth optical fiber coupler, a fifth optical fiber coupler, a second displacement sensor, a third single-mode optical fiber, a photoelectric detector, a voltage amplifier, an A / D converter, and a data collector; wherein:

[0007] The first optical fiber coupler includes three ports P1, P2, and P3, the P1 end is connected to the output end of the pulse laser, and the P2 and P3 ends are respectively connected to the input end of the photodetector and the input end of the optical fiber connector at the front end;

[0008] The output end of the optical fiber connector is connected to the head end of the first single-mode optical fiber, the input end of the second optical fiber coupler is connected to the tail end of the first single-mode optical fiber, the two output ends are respectively connected to the head end of the second single-mode optical fiber and the input end of the third optical fiber coupler, and the two ends of the first displacement sensor are respectively connected to the two output ends of the third optical fiber coupler;

[0009] The input end of the fourth optical fiber coupler is connected to the tail end of the second single-mode optical fiber, the two output ends are respectively connected to the head end of the third single-mode optical fiber and the input end of the fifth optical fiber coupler, and the two ends of the displacement sensor are respectively connected to the two output ends of the fifth optical fiber coupler;

[0010] The input and output ends of the voltage amplifier are respectively connected to the output end of the photoelectric detector and the input end of the A / D converter, and the output end of the A / D converter is connected to the input end of the data collector.

[0011] In order to further optimize the present invention, the following technical solutions may be preferably used:

[0012] Preferably, the pulse power of the pulse laser is 10 mW, the pulse width is 10 ns, the loss measurement accuracy is 0.01 dB, and the group refractive index ranges from 1.400000 to 1.699999.

[0013] Preferably, the splitting ratios of the first optical fiber coupler, the third optical fiber coupler and the fifth optical fiber coupler are all 50:50; and the splitting ratios of the second optical fiber coupler and the fourth optical fiber coupler are all 95:5.

[0014] Preferably, the length of the first single-mode optical fiber is 1030 m, the length of the second single-mode optical fiber is 860 m, the length of the third single-mode optical fiber is 595 m, and the attenuation coefficients are all 0.18 dB / km.

[0015] Preferably, the first displacement sensor has the same structure as the second displacement sensor, both comprising a fourth single-mode optical fiber, a coreless optical fiber, and a fifth single-mode optical fiber; both ends of the coreless optical fiber are coaxially fused with the tail end of the fourth single-mode optical fiber and the head end of the fifth single-mode optical fiber, respectively.

[0016] Preferably, the photodetector is a balanced photodetector, which converts the collected optical signal into an electrical signal, amplifies the voltage through a voltage amplifier, converts the signal into a digital signal through an A / D converter, and finally processes and displays the signal through a data collector.

[0017] Preferably, the optical paths of all devices are connected by arc discharge fusion connection through single-mode optical fibers.

[0018] A method for using a multi-point displacement sensing system comprises the following steps: the pulse light emitted by the pulse laser passes through a first optical fiber coupler, an optical fiber connector, and a first single-mode optical fiber in sequence, and finally reaches a second optical fiber coupler, 5% of the pulse light is separated and reaches an optical fiber ring mirror formed by a third optical fiber coupler, wherein a first displacement sensor is connected to the optical fiber ring mirror; the other 95% of the pulse light continues to be transmitted to the next identical sensing unit; wherein due to the macrobending loss of SNS caused by the displacement, the reflected light energy signal of the optical fiber reflector carrying the macrobending loss passes through the optical fiber coupler to a photoelectric detector, and finally restores the energy signal filtered by an edge filter after voltage amplification, A / D conversion, and data acquisition, thereby realizing the sensing of multi-point displacement.

[0019] The present invention has the following beneficial effects:

[0020] 1. The displacement sensor in this scheme can be repeatedly manufactured, and the manufactured displacement sensor is installed on the fiber ring mirror to form a displacement sensing unit. The displacement change of the displacement sensor significantly affects the value of its macro bending loss, thereby affecting the peak reflection energy of the fiber ring mirror monitored by the optical time domain reflectometer. In this scheme, the manufactured SNS is mounted on the ring mirror to build a displacement sensing system. The displacement change of the SNS affects the value of the SNS macro bending loss, which in turn affects the peak energy of the reflected light of the ring mirror. This peak is monitored by OTDR. For each fiber ring mirror, we only introduce 5% of the light source energy to reduce the sensor system's dependence on the light source and increase the number of reusable sensors. The system is stable, and each sensor can maintain a similar sensing effect. The change in temperature is unlikely to cause a change in the peak reflection light energy of the fiber ring mirror, overcoming the problem of cross-sensitivity between displacement and temperature. It has a wide range of engineering application value in structural health monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a structural schematic diagram of the multi-point displacement sensing system in the present invention;

[0022] Figure 2 A connection diagram of a displacement sensor introduced in the present invention;

[0023] Figure 3 It is a fitting diagram of the displacement and the peak energy of the reflected light of the optical fiber ring mirror obtained from the experimental data of the present invention;

[0024] Figure 4 The graph of the change of the peak energy of the reflected light of the optical fiber ring mirror with the temperature obtained from the experimental data of the present invention;

[0025] Among them, 1-pulse laser; 2-first fiber coupler; 3-fiber connector; 4-first single-mode optical fiber; 5-second fiber coupler; 6-third fiber coupler; 7-first displacement sensor; 8-second single-mode optical fiber; 9-fourth fiber coupler; 10-fifth fiber coupler; 11-second displacement sensor; 12-third single-mode optical fiber; 13-photodetector; 14-voltage amplifier; 15-A / D converter; 16-data acquisition device; 7-1-fourth single-mode optical fiber, 7-2-coreless optical fiber, 7-3-fifth single-mode optical fiber. DETAILED DESCRIPTION

[0026] In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0027] The following will be combined with the drawings in the embodiments of the invention to clearly and completely describe the technical solutions in the embodiments of the invention. Obviously, the described embodiments are only part of the embodiments of the invention, not all of the embodiments. Based on the embodiments of the invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the invention.

[0028] Embodiment 1:

[0029] like Figure 1-4: A multi-point displacement sensing system based on optical fiber macrobending loss, characterized in that it includes a connected pulse laser 1, a first optical fiber coupler 2, an optical fiber connector 3, a first single-mode optical fiber 4, a second optical fiber coupler 5, a third optical fiber coupler 6, a first displacement sensor 7, a second single-mode optical fiber 8, a fourth optical fiber coupler 9, a fifth optical fiber coupler 10, a second displacement sensor 11, a third single-mode optical fiber 12, a photodetector 13, a voltage amplifier 14, an A / D converter 15, and a data collector 16; wherein:

[0030] The first fiber coupler 2 includes three ports P1, P2 and P3, the P1 end is connected to the output end of the pulse laser 1, and the P2 and P3 ends are respectively connected to the input end of the photodetector 9 and the input end of the front fiber connector 3;

[0031] The output end of the optical fiber connector 3 is connected to the head end of the first single-mode optical fiber 4, the input end of the second optical fiber coupler 5 is connected to the tail end of the first single-mode optical fiber 4, the two output ends are respectively connected to the head end of the second single-mode optical fiber 8 and the input end of the third optical fiber coupler 6, and the two ends of the first displacement sensor 7 are respectively connected to the two output ends of the third optical fiber coupler 6;

[0032] The input end of the fourth optical fiber coupler 9 is connected to the tail end of the second single-mode optical fiber 8, and the two output ends are respectively connected to the head end of the third single-mode optical fiber 12 and the input end of the fifth optical fiber coupler 10, and the two ends of the displacement sensor 11 are respectively connected to the two output ends of the fifth optical fiber coupler 10;

[0033] The input and output ends of the voltage amplifier 14 are respectively connected to the output end of the photodetector 13 and the input end of the A / D converter 15 , and the output end of the A / D converter 15 is connected to the input end of the data collector 16 .

[0034] The pulse power of the pulse laser 1 is 10 mW, the pulse width is 10 ns, the loss measurement accuracy is 0.01 dB, and the group refractive index range is 1.400000 -1.699999.

[0035] The splitting ratios of the first optical fiber coupler 2, the third optical fiber coupler 6 and the fifth optical fiber coupler 10 are all 50:50; the splitting ratios of the second optical fiber coupler and the fourth optical fiber coupler 9 are all 95:5.

[0036] The length of the first single-mode optical fiber 4 is 1030 m, the length of the second single-mode optical fiber 8 is 860 m, the length of the third single-mode optical fiber 12 is 595 m, and the attenuation coefficients are all 0.18 dB / km.

[0037] The first displacement sensor has the same structure as the second displacement sensor, both of which include a fourth single-mode optical fiber 7-1, a coreless optical fiber 7-2, and a fifth single-mode optical fiber 5; the two ends of the coreless optical fiber 7-2 are coaxially fused with the tail end of the fourth single-mode optical fiber 7-1 and the head end of the fifth single-mode optical fiber 7-3 respectively; the length of the coreless optical fiber 7-2 is 2 mm, and the cladding diameter is 125 um.

[0038] The photodetector 13 is a balanced photodetector, which converts the collected optical signal into an electrical signal, amplifies the voltage through the voltage amplifier 14, converts it into a digital signal through the A / D converter 15, and finally processes and displays it by the data collector 16. The optical path connection of all devices is connected by arc discharge fusion through single-mode optical fiber.

[0039] A method for using a multi-point displacement sensing system comprises the following steps: pulse light emitted by a pulse laser 1 passes through a first fiber coupler 2-a fiber connector 3-a first single-mode fiber 4-in turn to reach a second fiber coupler 5, 5% of the pulse light is separated and reaches a fiber ring mirror formed by a third fiber coupler 6, wherein a first displacement sensor 7 is connected to the fiber ring mirror; the other 95% of the pulse light continues to be transmitted to the next identical sensing unit; wherein due to the macrobending loss of SNS caused by the displacement, the reflected light energy signal of the fiber reflector carrying the macrobending loss passes through the fiber coupler to a photoelectric detector, and finally restores the energy signal filtered by an edge filter after voltage amplification, A / D conversion, and data acquisition, thereby realizing multi-point displacement sensing.

[0040] The measurement principle of this system is:

[0041] The optical fiber macro bending loss coefficient is:

[0042] α c =A c r -1 / 2 exp(-Ur) (1)

[0043] Among them, α c is the macro bending loss per unit length, r is the radius of curvature of the bent fiber, A c is the fiber type constant, and U is the light source working state constant.

[0044] The macroscopic bending loss is expressed as follows

[0045] ΔI loss =I i -I o =2ΔSα c (2)

[0046] Among them, I i is the optical power input into the bent fiber, I ois the optical power output of the bent fiber, and ΔS is the length of the bent fiber. Combining equation (1) and equation (2), we can get:

[0047] ΔI loss =2ΔSA c r-1 / 2exp(-Ur) (3)

[0048] It can be seen from formula (3) that when the fiber type, light source working mode, and bent fiber length are all determined, the macrobending loss of the fiber only depends on the curvature radius of the bent fiber. Since the bending length of the SNS is fixed, we give the SNS different displacements to change the curvature radius of the SNS, thereby changing the macrobending loss value of the SNS. We mainly use reflection events that carry fiber macrobending loss information to measure displacement. The OTDR trace of the reflection event can be expressed as:

[0049] RL(z)=-10lg[P R / P i (z)] (4)

[0050] Among them, P R is the reflected light power, P i (Z) is the incident light power at point Z. Due to the insertion loss of the coupler and sensor, the reflected light power finally detected by the OTDR can be expressed as

[25] : P(z) = 10 -[2A(z)+RL(z)] / 10 P o

[0051] Where A(z) is the insertion loss of the entire sensor system, P o is the optical output power of the OTDR.

[0052] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A multi-point displacement sensing system based on optical fiber macrobending loss, characterized in that: The invention comprises a pulse laser (1), a first optical fiber coupler (2), an optical fiber connector (3), a first single-mode optical fiber (4), a second optical fiber coupler (5), a third optical fiber coupler (6), a first displacement sensor (7), a second single-mode optical fiber (8), a fourth optical fiber coupler (9), a fifth optical fiber coupler (10), a second displacement sensor (11), a third single-mode optical fiber (12), a photoelectric detector (13), a voltage amplifier (14), an A / D converter (15), and a data acquisition device (16); wherein: The first optical fiber coupler (2) comprises three ports P1, P2 and P3, the P1 end is connected to the output end of the pulse laser (1), and the P2 and P3 ends are respectively connected to the input end of the photodetector (9) and the input end of the optical fiber connector (3) at the front end; The output end of the optical fiber connector (3) is connected to the head end of the first single-mode optical fiber (4), the input end of the second optical fiber coupler (5) is connected to the tail end of the first single-mode optical fiber (4), the two output ends are respectively connected to the head end of the second single-mode optical fiber (8) and the input end of the third optical fiber coupler (6), and the two ends of the first displacement sensor (7) are respectively connected to the two output ends of the third optical fiber coupler (6); The input end of the fourth optical fiber coupler (9) is connected to the tail end of the second single-mode optical fiber (8), the two output ends are respectively connected to the head end of the third single-mode optical fiber (12) and the input end of the fifth optical fiber coupler (10), and the two ends of the displacement sensor (11) are respectively connected to the two output ends of the fifth optical fiber coupler (10); The input and output ends of the voltage amplifier (14) are respectively connected to the output end of the photodetector (13) and the input end of the A / D converter (15), and the output end of the A / D converter (15) is connected to the input end of the data collector (16).

2. A multi-point displacement sensing system based on optical fiber macrobending loss according to claim 1, characterized in that: The pulse power of the pulse laser (1) is 10 mW, the pulse width is 10 ns, the loss measurement accuracy is 0.01 dB, and the group refractive index ranges from 1.400000 to 1.699999.

3. The multi-point displacement sensing system based on optical fiber macrobending loss according to claim 1, characterized in that: The splitting ratios of the first optical fiber coupler (2), the third optical fiber coupler (6) and the fifth optical fiber coupler (10) are all 50:50; the splitting ratios of the second optical fiber coupler and the fourth optical fiber coupler (9) are all 95:

5.

4. The multi-point displacement sensing system based on optical fiber macrobending loss according to claim 1, characterized in that: The length of the first single-mode optical fiber (4) is 1030 m, the length of the second single-mode optical fiber (8) is 860 m, and the length of the third single-mode optical fiber (12) is 595 m. The attenuation coefficients are all 0.18 dB / km.

5. The multi-point displacement sensing system based on optical fiber macrobending loss according to claim 1, characterized in that: The first displacement sensor and the second displacement sensor have the same structure, both comprising a fourth single-mode optical fiber (7-1), a coreless optical fiber (7-2), and a fifth single-mode optical fiber (7-3); the two ends of the coreless optical fiber (7-2) are coaxially fused with the tail end of the fourth single-mode optical fiber (7-1) and the head end of the fifth single-mode optical fiber (7-3), respectively.

6. The multi-point displacement sensing system based on optical fiber macrobending loss according to claim 1, characterized in that: The photoelectric detector (13) is a balanced photoelectric detector, which converts the collected optical signal into an electrical signal, amplifies the voltage through a voltage amplifier (14), converts the signal into a digital signal through an A / D converter (15), and finally processes and displays the signal through a data acquisition device (16).

7. The multi-point displacement sensing system based on optical fiber macrobending loss according to claim 1, characterized in that: The optical connections of all devices are made through single-mode optical fibers using arc discharge fusion connection.

8. A method for using the multi-point displacement sensing system according to claim 1, comprising the following steps: The pulse light emitted by the pulse laser (1) passes through the first fiber coupler (2), the fiber connector (3), the first single-mode fiber (4) in sequence, and then reaches the second fiber coupler (5), wherein 5% of the pulse light is separated and reaches the fiber ring mirror formed by the third fiber coupler (6), wherein the first displacement sensor (7) is connected to the fiber ring mirror; the other 95% of the pulse light continues to be transmitted to the next identical sensing unit; wherein the macrobending loss of the SNS caused by the displacement, the reflected light energy signal of the fiber reflector carrying the macrobending loss passes through the fiber coupler to the photoelectric detector, and finally restores the energy signal after being filtered by the edge filter after voltage amplification, A / D conversion, and data acquisition, thereby realizing the sensing of multi-point displacement.

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