阅读说明：本技术 一种用于隧道表面形貌测量的四纤芯光纤光栅传感器 (Four-fiber-core fiber grating sensor for measuring tunnel surface morphology ) 是由 李俊 邓军 王伟峰 翟小伟 *** 郭彦希 于 2019-11-07 设计创作，主要内容包括：本发明属于隧道结构监测技术领域,尤其为一种用于隧道表面形貌测量的四纤芯光纤光栅传感器,利用多纤芯光纤中的每芯中的光纤光栅在受力后同时产生拉伸应变的原理,可求出对应的的曲率,进而得到对应的表面位移变化情况来反应结构体的表面形貌状况,通过光纤和其他光纤传感器相互连接,可实现结构体的位移、温度、振动,等物理量的远程实时监测,大大节省了线缆的数量和敷设成本,适用于恶劣环境的监测需求。(The invention belongs to the technical field of tunnel structure monitoring, in particular to a four-fiber-core fiber grating sensor for measuring the surface appearance of a tunnel, which can calculate the corresponding curvature by utilizing the principle that the fiber grating in each core of a multi-fiber-core fiber simultaneously generates tensile strain after being stressed, further obtain the corresponding surface displacement change condition to reflect the surface appearance condition of a structural body, and can realize the remote real-time monitoring of physical quantities such as displacement, temperature, vibration and the like of the structural body by mutually connecting the fiber and other fiber sensors, thereby greatly saving the quantity and laying cost of cables and being suitable for the monitoring requirement of severe environment.)

1. A four-fiber-core fiber grating sensor for measuring the surface topography of a tunnel is characterized in that:

adopting four-core optical fiber made of fused quartz and writing grating structure in the optical fiber;

the fan-out system is used for distributing the optical signals of each core of the four-core optical fiber to the independent fiber grating adjusting channels;

and the fiber grating demodulator is used for reading out the grating strain information.

2. The four-core fiber grating sensor for tunnel topography measurement according to claim 1, wherein: the fiber cores of the four-core optical fiber are distributed on the cross section of the optical fiber in a centrosymmetric manner.

3. A method of inspection using the four-core fiber grating sensor of claim 1 for tunnel topography, comprising: the method comprises the following steps:

s1, fixing two ends of an optical fiber rod containing a four-core fiber grating, and driving the middle part of the optical fiber rod through an adjusting frame to enable the optical fiber to generate a certain bending deformation so as to generate transverse displacement;

s2, distributing the optical signal of each core of the four-core optical fiber to an independent fiber grating demodulation channel through a fan-out system;

and S3, reading the strain information of the grating by a fiber grating demodulator.

4. A method of inspection according to claim 3 using a four-core fiber grating sensor according to claim 1 for tunnel topography measurement, wherein:

in the four-core optical fiber, two core optical fibers along the stress direction respectively receive the effects of tensile strain and compressive strain, wherein the bending deformation is expressed by the following formula:

Δε＝d/R＝(d/L)Δθ (1.2)

wherein epsilon₁、ε₂Respectively the strain produced by the fiber grating, d is the distance of the two gratings, and R is the radius of curvature of the beam.

Technical Field

The invention belongs to the technical field of tunnel structure detection, and particularly relates to a four-fiber-core fiber grating sensor for measuring the surface topography of a tunnel.

Background

A fiber grating sensor (FBG) is one type of quasi-distributed fiber sensor. A Bragg grating is a grating made in an optical fiber with a periodic variation in the refractive index, which reflects light at different wavelengths with different periods. When the optical fiber with the Bragg grating is stretched or compressed and the temperature of the optical fiber changes, the period of the optical fiber changes, so that the wavelength of reflected light also changes, and the strain of the optical fiber or the temperature value of the optical fiber can be obtained by measuring the change of the wavelength of the reflected light. The principle of fiber Bragg grating measurement is shown in fig. 1. FBG sensing can measure physical strain and temperature, but cannot obtain the displacement topography of an object.

In the tunnel operation process, tunnel horizontal displacement and settlement monitoring are needed to be carried out on tunnel portal geological structure instability and accidents which easily occur, so that the structural condition of the section can be comprehensively known, and data basis is provided for safety monitoring of the high-speed rail tunnel.

The invention with the application number of CN201810820816.7 provides a tunnel roof settlement monitoring device and a tunnel settlement monitoring method. Wherein, tunnel roof settlement monitoring device includes: the fiber bragg grating height difference meters are sequentially arranged along a top plate of the tunnel, and the height of at least one fiber bragg grating height difference meter is higher than the heights of other fiber bragg grating height difference meters; the fiber bragg grating height difference meters are communicated through the connecting pipes; and the demodulation instrument is connected with the fiber grating height difference meter and can receive and process the monitoring data of the fiber grating height difference meter. The fiber grating sensor is a commercial device formed by etching gratings in communication optical fibers and is easily influenced by external disturbance such as temperature.

The optical fiber monitoring can be free from the influence of electromagnetic interference, is intrinsically safe, is easy to network and transmit in a long distance, and is suitable for the tunnel safety monitoring requirement of the railway and highway industries.

The invention of which the prior application has a publication number of CN104454007A provides a coal mine safety early warning system based on a multi-fiber-core optical fiber, which is used for simultaneously measuring underground temperature, strain and vibration. The distributed real-time measurement of the physical quantities is realized by adopting different fiber cores in one multi-fiber-core optical fiber to sense the physical quantities such as temperature, strain, vibration and the like through a multi-fiber-core beam splitter. The invention adopts one optical fiber to comprehensively evaluate the underground health condition of the coal mine in a multi-sensor fusion mode, greatly saves the laying cost of the optical cable, only occupies less bandwidth of the communication optical cable, and is suitable for the monitoring requirement of severe environment.

However, in the field of structure monitoring, a common fiber grating can only continuously monitor strain information of a structure in real time, and cannot acquire the stress of the structure and the change situation of surface morphology and displacement after the stress.

The multi-fiber core optical fiber is one of the hot spots of academic research in recent years, is applied to next-generation high-capacity optical communication, and has great significance for improving the transmission bandwidth of optical communication. The application of the four-fiber-core fiber grating sensor to optical fiber sensing is not much, and the engineering application of the multi-fiber-core optical fiber sensing in China is not common, so that the four-fiber-core fiber grating sensor is urgently needed to be used for monitoring tunnels, structural body curvature and surface morphology in real time.

Disclosure of Invention

The invention provides a four-fiber-core fiber grating sensor for measuring the surface topography of a tunnel, which aims to solve the problem that the surface topography of the traditional tunnel structure body is difficult to accurately monitor in the background technology.

In order to achieve the purpose, the invention provides the following technical scheme: a four-fiber core fiber grating sensor for measuring the surface topography of a tunnel adopts a four-core fiber grating structure made of fused quartz;

the fan-out system is used for distributing the optical signals of each core of the four-core optical fiber to the independent fiber grating adjusting channels;

and a multi-channel fiber grating demodulator for reading out the grating strain information.

Preferably, the cores of the four-core optical fiber are distributed in a central symmetry manner on the cross section of the optical fiber.

A detection method using the four-fiber-core fiber grating surface topography measurement sensor comprises the following steps:

s1, fixing two ends of an optical fiber rod containing a four-core fiber grating, and driving the middle part of the optical fiber rod through an adjusting frame to enable the optical fiber to generate a certain bending deformation so as to generate transverse displacement;

s2, distributing the optical signal of each core of the four-core optical fiber to an independent fiber grating demodulation channel through a fan-out system;

and S3, reading the strain information of the grating by a fiber grating demodulator.

Preferably, two of the four-core optical fibers along the force-receiving direction are respectively subjected to tensile strain and compressive strain, wherein the bending deformation is expressed by the following formula:

Δε＝d/R＝(d/L)Δθ (1.2)

θ is the angle at which the fiber is bent and can be expressed by the following equation:

θ＝L/R (1.3)

the overall phase change produced by the fiber can be expressed as:

for germanium doped fibers (n + dn/d ε) typically at 1.159, so there are:

it can be seen that when the wavelength and the distance between two fiber gratings in the same stress direction in the four-core fiber are fixed, the bending angle of the fiber is only related to the phase.

Wherein epsilon₁、ε₂Respectively, strain generated by the fiber grating, d is the distance between the two gratings, and R is the curvature radius of the beam;

the amount of bending of the fiber in the x, y axes is measured according to the above equation.

Compared with the prior art, the invention has the beneficial effects that:

the distributed real-time measurement of each physical quantity is realized through the multi-fiber-core beam splitter, the health condition of a structure can be comprehensively evaluated by adopting one optical fiber in a multi-sensor fusion mode, the optical cable laying cost is greatly saved, remote monitoring networking can be realized simultaneously, the detection host can be placed at a long distance from a sensor, and the monitoring requirement of severe field environment is met.

1. The monitoring of the change of the surface appearance of the tunnel is realized through the optical fiber comprising the four fiber gratings, and the whole sensor is small in size, light in weight and convenient to install after being packaged;

2. the distributed or point-type measurement of multiple parameters can be realized by adopting different fiber cores in one optical fiber, the combination is flexible and changeable according to the sensing requirement, each fiber core in one optical fiber can measure a physical quantity and position the change position of the physical quantity on the length of the optical fiber;

3. the sensing media are all in one optical fiber, and the device is formed in one step in the preparation process, so that the environmental influences such as temperature and the like are eliminated or reduced, and the system error is avoided to the maximum extent;

4. the sensing structure is compact, the integration level is high, the detection precision is improved, and the system error is reduced; finally, the multi-core optical fiber beam splitter transmits physical quantities acquired by different fiber cores of the multi-core optical fiber to different designed demodulation systems to demodulate multi-parameter information in real time and transmit the multi-parameter information to a network transmission system;

5. different fiber cores in the same optical fiber can complete distributed measurement of different monitoring quantities, and physical quantities to be monitored can be changed only by changing a light source and a demodulation system according to monitoring requirements, so that the method is flexible and convenient;

6. writing a plurality of fiber Bragg gratings in the four-core single-mode fiber to realize distributed monitoring of the appearance of the tunnel body and position the point of the change of the appearance of the tunnel;

7. the optical fiber monitoring network is formed by conveniently networking with other sensors, and remote transmission and analysis of monitoring data are realized;

8. the whole sensor adopts one optical fiber as a transmission sensing medium, can meet the monitoring requirement by adopting less communication bandwidth, reduces the cost and complexity of optical cable laying and saves the communication bandwidth.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention.

In the drawings:

FIG. 1 is a schematic diagram of a prior art fiber Bragg grating measurement;

FIG. 2 is a schematic view of the principle of monitoring surface deformation in an embodiment of the present invention;

FIG. 3 is a schematic diagram of an algorithm for measuring surface topography in an embodiment of the present invention;

FIG. 4 is a diagram of a four-core fiber grating structure made of silica rods according to an embodiment of the present invention;

FIG. 5 is a schematic diagram of a simulation test of fiber rod concatenation in an embodiment of the invention;

FIG. 6 is a graph showing the variation of the output phase of the sensor with the variation of the bending angle of the optical fiber according to the embodiment of the present invention;

FIG. 7 is a schematic diagram of experimental testing in an embodiment of the present invention.

In the drawings: 1. a fiber optic transceiver; 2. a fan-out system; 3. a fixed block; 4. and (4) an adjusting frame.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The invention provides the following technical scheme: a four-core fiber grating sensor for measuring the surface topography of a tunnel adopts a four-core fiber grating structure made of silicon dioxide and is used for distributing optical signals of each core of a four-core fiber to a fan-out system in an independent fiber grating adjusting channel; and the fiber grating demodulator is used for reading out the grating strain information.

Specifically, referring to fig. 4, the cores of the four-core optical fiber are distributed on the cross section of the optical fiber in a centrosymmetric manner.

In this embodiment, referring to fig. 7, two ends of a round rod containing a multi-core fiber grating are fixed on the fixed block 3, the middle part of the round rod is driven by an adjusting frame 4 (an electric push rod can be used) to generate a certain bending deformation amount on the optical fiber so as to generate a transverse displacement, the size of the transverse displacement can be controlled by the stepping amount of the adjusting frame, and two core fibers in the four core fibers along the stress direction respectively receive the tensile strain and the compressive strain.

In this embodiment, as shown in fig. 2 and fig. 3, two optical fibers of the four-core optical fiber along the force-receiving direction receive the tensile strain and the compressive strain, respectively, wherein the bending deformation amount can be expressed by the following formula:

Δε＝d/R＝(d/L)Δθ (1.2)

θ is the angle at which the fiber is bent and can be expressed by the following equation:

θ＝L/R (1.3)

the overall phase change produced by the fiber can be expressed as:

for germanium doped fibers (n + dn/d ε) typically at 1.159, so there are:

it can be seen that when the wavelength and the distance between two fiber gratings in the same stress direction in the four-core fiber are fixed, the bending angle of the fiber is only related to the phase.

Wherein epsilon₁、ε₂Respectively, strain generated by the fiber grating, d is the distance between the two gratings, and R is the curvature radius of the beam;

the amount of bending of the fiber in the x, y axes is measured according to the above equation.

As shown in FIG. 2, where ε₁、ε₂Respectively the strain produced by the fiber grating, d is the distance of the two gratings, and R is the radius of curvature of the beam.

Through the relational expression, if the strain quantity of the multi-core fiber four-core fiber grating is known, the bending quantity of the fiber on the x and y axes can be measured, and if the structural sensor is fixed on a tunnel, the detection of the transverse and longitudinal bending curvatures of the tunnel can be realized.

The optical signal of each core of the four-core optical fiber is distributed to an independent fiber grating demodulation channel through the fan-out system 2, and the strain information of the grating can be read through the fiber grating demodulator 1, and the specific embodiment can refer to fig. 5.

This scheme can be applied to traffic tunnel, hydraulic tunnel, municipal tunnel, mine tunnel, the realization is to tunnel structure body camber, the real-time supervision of surface morphology, it is multiplexing to establish ties the fiber grating in the multicore optic fibre, can monitor meeting an emergency, temperature, other physical quantities such as vibration, adopt an optic fibre to acquire the health status of structure body with the mode that the multisensor fuses promptly, the optical cable laying cost has been saved greatly, only occupy the less bandwidth of communication optical cable simultaneously, it is safe in nature, the monitoring requirement that is not electrified to be applicable to adverse circumstances.

Specifically, the method can be applied to the object morphology, as shown in fig. 5, a four-fiber-core optical fiber including fiber gratings is wound on the surface of a cylinder, two fiber gratings stressed in corresponding directions are respectively subjected to tensile strain and compressive strain in the same direction, the curvature of the object at the point can be calculated according to the description of the morphology measurement, and further the deformation amount distribution is obtained, the distribution condition of the surface morphology of the whole cylinder can be obtained by connecting multi-point measurement data in the optical fiber, the morphology of the cylinder surface is detected, and the track of the cylinder surface is drawn out through the detected data of the curvature and the detected position, and the morphology can be detected.

Specifically, as shown in fig. 6, the sensor output phase changes with the change in the bending angle of the optical fiber. It can be seen that when the bending angle of the optical fiber to be measured is changed, the phase of the output also changes linearly with the input.

The invention has the following advantages:

1. the monitoring of the change of the surface appearance of the tunnel is realized through the optical fiber comprising the four fiber gratings, and the whole sensor is small in size, light in weight and convenient to install after being packaged;

2. the distributed or point-type measurement of multiple parameters can be realized by adopting different fiber cores in one optical fiber, the combination is flexible and changeable according to the sensing requirement, each fiber core in one optical fiber can measure a physical quantity and position the change position of the physical quantity on the length of the optical fiber;

3. the sensing media are all in one optical fiber, the influence of the environment is small, and the system error is avoided to the maximum extent;

4. the sensing structure is compact, the integration level is high, the detection precision is improved, and the system error is reduced; finally, the multi-core optical fiber beam splitter transmits physical quantities acquired by different fiber cores of the multi-core optical fiber to different designed demodulation systems to demodulate multi-parameter information in real time and transmit the multi-parameter information to a network transmission system;

5. different fiber cores in the same optical fiber can complete distributed measurement of different monitoring quantities, and physical quantities to be monitored can be changed only by changing a light source and a demodulation system according to monitoring requirements, so that the method is flexible and convenient;

6. writing a plurality of fiber Bragg gratings in the four-core single-mode fiber to realize distributed monitoring of the appearance of the tunnel body and position the point of the change of the appearance of the tunnel;

7. the optical fiber monitoring network is formed by conveniently networking with other sensors, and remote transmission and analysis of monitoring data are realized;

8. the whole sensor adopts one optical fiber as a transmission sensing medium, can meet the monitoring requirement by adopting less communication bandwidth, reduces the cost and complexity of optical cable laying and saves the communication bandwidth.

Finally, it should be noted that: although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that changes may be made in the embodiments and/or equivalents thereof without departing from the spirit and scope of the invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.