WO2022099828A1 - 一种基于弹性体的螺旋型光纤分布式声场方向判断方法 - Google Patents

一种基于弹性体的螺旋型光纤分布式声场方向判断方法 Download PDF

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WO2022099828A1
WO2022099828A1 PCT/CN2020/133833 CN2020133833W WO2022099828A1 WO 2022099828 A1 WO2022099828 A1 WO 2022099828A1 CN 2020133833 W CN2020133833 W CN 2020133833W WO 2022099828 A1 WO2022099828 A1 WO 2022099828A1
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optical fiber
fiber
elastic body
acoustic wave
refractive index
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PCT/CN2020/133833
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French (fr)
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饶云江
关宏健
傅芸
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之江实验室
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
    • G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
    • G01H9/004—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors

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  • the invention belongs to the field of optical fiber distributed sensing, and in particular relates to a method for judging the direction of a helical optical fiber distributed sound field based on an elastic body.
  • Optical fiber distributed acoustic sensing (DAS) technology based on phase-sensitive optical time domain reflectometer (Phase-sensitive Optical Time Domain Reflectometer, ⁇ -OTDR)
  • DAS Phase-sensitive optical time domain reflectometer
  • ⁇ -OTDR Phase-sensitive Optical Time Domain Reflectometer
  • DAS digital-senor
  • the technology can also realize long-distance, distributed real-time quantitative detection of dynamic strain (vibration, sound wave) along the optical fiber, which has a wide range of applications in the fields of military, national defense, people's livelihood, scientific research, etc., especially in the acoustic characteristics of underwater targets. Monitoring and exploration of oil and other resources have an irreplaceable position and are still widely studied and concerned today.
  • the principle of DAS technology is: due to the uneven refractive index of the optical fiber medium, elastic scattering occurs when light is transmitted in the optical fiber, resulting in scattered light with the same frequency as the original incident light.
  • Rayleigh scattering is a kind of elastic scattering caused by the uneven refractive index caused by the uneven distribution of the fiber material, and it has the largest scattered light intensity in the backscattered light in the fiber.
  • the optical fiber Under the action of the acoustic wave, the optical fiber is slightly strained. Due to the elastic-optic effect, the refractive index of the optical fiber will change accordingly, causing the phase difference between two adjacent points along the optical fiber of the backward Rayleigh scattered light of the coherent pulse to change.
  • the phase information can be extracted, and the external sound wave signal can be restored to realize the distributed sensing of the sound wave.
  • Sensitivity and then it can separate the components in the three directions of X, Y, and Z through the corresponding numerical calculation.
  • the above methods all obtain the force direction by separating the components in the three directions of X, Y and Z, but these methods often require the use of multiple optical fibers, and because the optical fibers need to be less stressed in a fixed direction. It is sensitive and has high requirements on the optical fiber winding process. When only one optical fiber is used to make the sensing unit, the requirements for the optical fiber winding process are also very high because it needs to be wound into a special shape.
  • the method of the present invention adopts a special arrangement method of the sensing unit, which is constructed by spirally winding the optical fiber on a cylindrical or tubular elastic body to form an optical fiber acoustic wave sensing unit, wherein the elastic body uses a smaller Young's modulus. It is made of materials such as rubber.
  • the elastic body uses a smaller Young's modulus. It is made of materials such as rubber.
  • the purpose of the present invention is to provide a method for judging the direction of a helical optical fiber distributed sound field based on an elastic body, aiming at the deficiencies of the prior art.
  • the invention solves the problem that the distributed acoustic wave sensing system is insensitive to the direction of force when the traditional straight optical fiber is used as the sensing unit.
  • the object of the present invention is to be achieved through the following technical solutions: a method for judging the direction of a helical optical fiber distributed sound field based on an elastic body, comprising the following steps:
  • optical fiber is spirally wound on the elastic body and bonded to form an optical fiber sensing unit, which is arranged in the acoustic wave measurement space;
  • E 1 is the Young's modulus of the fiber core
  • r is the outer radius of the elastomer
  • d is the inner radius of the elastomer
  • A is the transfer coefficient from the surface stress of the elastomer to the inside of the fiber
  • n eff is The effective refractive index of the fiber
  • P 11 and P 12 are the elastic-optic coefficients of the fiber core material
  • v is the Poisson's ratio of the fiber core material
  • h is the winding pitch of the optical fiber
  • R is the radius of the optical fiber
  • the optical fiber sampling point with zero refractive index change constitutes the neutral layer plane of the optical fiber sensing unit, and the reference axis It is located on the neutral layer and parallel to the end face of the elastomer, with the center of the elastomer cross section as the origin, and the angle parameter i(r) is the angle between the direction from the origin
  • the direction of the acoustic wave radial component force Fr is from the optical fiber sampling point whose refractive index changes to negative vertically to the neutral layer;
  • E 2 is the equivalent Young's modulus of the fiber
  • the direction of the acoustic wave axial component F z is judged: if the non-zero value ⁇ n z is always negative, it means that F z The direction is from the fixed end to the free end, and if it is constant, it means that F z points from the free end to the fixed end;
  • the shape of the elastic body is cylindrical or cylindrical.
  • the Young's modulus of the elastomer does not exceed 300Mpa.
  • the material of the elastomer is rubber.
  • the winding pitch h of the optical fiber helically wound on the elastomer in the step (1) is greater than the spatial resolution of the optical fiber distributed sensing system.
  • one end of the optical fiber sensing unit is fixed, and the remaining part is not limited in the measurement space.
  • the optical fiber distributed acoustic wave sensing system includes a distributed optical fiber acoustic wave sensing system host and an optical fiber sensing unit connected in sequence;
  • the distributed optical fiber acoustic wave sensing system host includes a narrow linewidth laser, a pulse modulator , signal generator, low noise optical amplifier, circulator, photodetector, signal processing module and optical fiber connection module; among them, narrow linewidth laser, pulse modulator, low noise optical amplifier, circulator and optical fiber connection module are connected in sequence , the pulse modulator is connected with the signal generator, the optical fiber connection module is connected with the optical fiber of the optical fiber sensing unit, and the circulator is also connected with the photodetector and the signal processing module in sequence.
  • sampling interval of the refractive index change distribution curve ⁇ n(l) along the optical fiber is the gauge length of the optical fiber distributed acoustic wave sensing system.
  • the value range of the angle parameter i(r) is [0, 2 ⁇ ].
  • the optical fiber is spirally wound on a cylindrical or tubular elastic body to form an optical fiber sensing unit, and the single end of the sensing unit is fixedly arranged in the sensing space, and the deformation of the optical fiber is driven by the deformation of the elastic body,
  • the optical fiber is helically wound on the elastic body, so that the accumulated phase change per unit measurement length is larger, and the sensitivity is greatly improved compared with the traditional method of using straight optical fiber as the sensing unit.
  • the present invention uses a low Young's modulus material to make the elastic body, so that the elastic body will also be deformed due to the force.
  • a low Young's modulus material to make the elastic body, so that the elastic body will also be deformed due to the force.
  • Fig. 1 is the flow chart of the method for judging the direction of the helical optical fiber distributed sound field based on the elastic body of the present invention
  • Fig. 2 is a partial block diagram of the optical fiber distributed acoustic wave sensing system of the present invention
  • FIG. 3 is a partial block diagram of the mainframe of the optical fiber distributed acoustic wave sensing system of the present invention.
  • FIG. 4 is a schematic diagram of an application scenario of the elastic body-based helical optical fiber distributed sound field direction judgment method of the present invention
  • FIG. 5 is a schematic diagram of an optical fiber sensing unit composed of an optical fiber spirally wound on an elastic body according to the present invention; wherein, (a) is a schematic diagram of a three-dimensional structure; (b) is a schematic diagram of a cross-section;
  • Fig. 6 is the simulation diagram of the optical fiber refractive index change obtained when detecting the sound field when the optical fiber sensing unit formed by winding one optical fiber is used; wherein, (a) is the actual optical fiber distributed acoustic wave sensing system host detected by the mainframe.
  • Fig. 7 is the schematic diagram that the method of the present invention is applied in oil well
  • FIG. 8 is a schematic diagram of an optical fiber sensing unit composed of two optical fibers spirally wound on an elastomer according to the present invention; wherein (a) is a schematic diagram of a three-dimensional structure; (b) is a schematic diagram of a cross-section;
  • Fig. 9 is the optical fiber refractive index change simulation diagram obtained when the optical fiber sensing unit formed by using 2 optical fibers wound to detect the sound field of the present invention; wherein, (a) is the actual optical fiber 1 detected by the mainframe of the optical fiber distributed acoustic wave sensing system. The simulation diagram of the refractive index change of the optical fiber; (b) is the simulation diagram of the actual optical fiber refractive index change of the fiber 2 detected by the mainframe of the optical fiber distributed acoustic wave sensing system; (c) is obtained after data processing.
  • the simulation diagram of the refractive index change of the fiber caused by the optical fiber (d) is the simulation diagram of the refractive index change of the fiber 1 caused by the radial component of the acoustic wave only; (e) is the refractive index change of the fiber 2 caused by the radial component of the acoustic wave only. Simulation diagram.
  • the invention is based on the elastic body-based helical optical fiber distributed sound field direction judgment method, which aims to solve the technical problem that the distributed optical fiber acoustic wave sensing system is not sensitive to the force direction when using traditional straight optical fibers.
  • the main steps of the technical means adopted in the present invention are shown in Figure 1, including the following steps:
  • Step 1 Use adhesive to wind the optical fiber helically on the cylindrical elastomer to form the optical fiber sensing unit and arrange it in the measurement space.
  • the elastomer needs to use a material with a smaller Young's modulus, such as rubber, and the shape is cylindrical or cylindrical;
  • Step 2 Real-time demodulation of the refractive index change distribution ⁇ n(l) along the fiber through the optical fiber distributed acoustic wave sensing system; the sampling interval of the refractive index distribution change curve ⁇ n(l) along the fiber length is the optical fiber distribution.
  • Step 3 Obtain the size and direction of the radial component force F r of the acoustic wave and the axial component force F z of the acoustic wave respectively through ⁇ n(l), including the following sub-steps:
  • Step 3.1 Find any point in the region where the refractive index change is positive and convex in the obtained refractive index change distribution curve ⁇ n(l) along the optical fiber to obtain the magnitude of the radial component force F r of the acoustic wave, specifically: :
  • the total refractive index change ⁇ n of the fiber can be expressed as:
  • the curve ⁇ n(l) will have a zero point every half pitch of the fiber length, and the interval L of the zero point of the curve is:
  • R is the radius of the fiber; h is the winding pitch of the fiber.
  • the winding pitch h of the optical fiber helically wound on the elastomer needs to be much larger than the spatial resolution of the optical fiber distributed sensing system and can be adjusted. The longer the elastomer is required; and the same sensing unit can contain different pitch values h.
  • the fiber helically wound on the elastomer uses an adhesive to bond the two together, so that the entire fiber wound on the elastomer will deform with the deformation of the elastomer; if the fiber follows the trajectory of the helix equation
  • the force on the surface of the elastomer will be transmitted to the inside of the fiber, causing the refractive index of the fiber to change. Due to the large winding pitch, the normal stress direction can be considered to be the same as the axial direction of the fiber. According to the elastic-optic effect:
  • B is the dielectric impermeability tensor, and its component is the reciprocal of the corresponding component of the dielectric tensor ⁇ ; P is the elastic-optic coefficient; S is the strain tensor.
  • each component of the dielectric impermeability tensor B is the reciprocal of the dielectric tensor, and the fiber core material is a non-ferromagnetic substance with a magnetic permeability of 1, the relationship between ⁇ n and ⁇ B can be obtained as:
  • P 11 and P 12 are the elastic optical coefficients of the fiber core material; v is the Poisson's ratio of the core material; n eff is the effective refractive index of the fiber; E 1 is the Young's modulus of the core material.
  • the spatial positions of the optical fibers corresponding to all zero points of the refractive index change distribution curve ⁇ n(l) of the optical fiber are in the same plane, and this plane is the neutral layer of the sensing unit; the diameter direction of the elastomer located on the neutral layer is The direction of the reference axis, the reference axis is parallel to the end face of the elastic body, and the center position of the elastic body section is the origin of the reference axis;
  • i(r) is the angle parameter, which is the angle between the reference axis and the reference axis between the direction of the center of the reference axis and the fiber sampling point.
  • A is the elastic body
  • the transfer coefficient of the surface stress to the inside of the optical fiber is related to the parameters of the elastomer and the optical fiber and the bonding method of the two, which can be obtained by testing;
  • l represents the length of the optical fiber between the sampling point and the starting section of the optical fiber; l 1 and l 2 are the length of the unstressed part of the optical fiber wound by the elastic body from the fixed end to the front end of the free end and the length of the optical fiber wound by the stressed part, respectively.
  • Step 3.2 Obtain the direction of the sound wave radial component F r , specifically:
  • the direction of the radial component force F r of the acoustic wave is from the sampling point of the optical fiber whose refractive index change is negative to the neutral layer;
  • Step 3.3 Calculate ⁇ n(l) DC bias component ⁇ n z (l) using the obtained F r , specifically:
  • the component ⁇ n r (l) with the winding pitch as the period can be obtained from F r :
  • Step 3.4 Use the non-zero value ⁇ n z in the DC bias component ⁇ n z (l) to calculate the magnitude of the acoustic wave axial component F z :
  • E 2 is the equivalent Young's modulus of the fiber.
  • Step 3.5 Use the non-zero value ⁇ n z in the DC bias component ⁇ n z (l) to calculate the direction of the acoustic wave axial component force F z , specifically:
  • the pitch of the fiber is large and the fiber is not sensitive to the force in the radial direction, it can be considered that the curve ⁇ n z (l) is only caused by F z , and the effect range of the sound field can be seen from the curve n z (l), And the non-zero value in n z (l) is always negative, which means that the direction of F z is the same as the direction of the positive stress caused by F r on the direct force surface, which is from the fixed end to the free end, and the non-zero value in ⁇ n z is always positive. It shows that the direction of F z is opposite to the direction of the normal stress caused by F r on the direct bearing surface, and the direction is from the free end to the fixed end.
  • Step 4 The magnitude and direction of the resultant force can be obtained from the magnitude and direction of F r and F z , which is the incident direction of the sound field, and the angle between the incident direction of the sound field and the reference axis
  • the optical fiber distributed acoustic wave sensing system used in the present invention as shown in FIG. 2 can be divided into two parts, the first part is the host of the optical fiber distributed acoustic wave sensing system, and the second part is the optical fiber sensing unit.
  • the main components of the optical fiber distributed acoustic wave sensing system are shown in Figure 3, including narrow linewidth laser, pulse modulator, signal generator, low noise optical amplifier, circulator, photodetector, signal processing module and optical fiber connection module.
  • the optical fiber connection module is connected with the optical fiber of the optical fiber sensing unit.
  • the common application scenarios of the optical fiber distributed acoustic wave sensing system are shown in Figure 4 and Figure 7.
  • the main engine of the optical fiber distributed acoustic wave sensing system is placed on a ship or on land, and the optical fiber sensing unit is arranged in the space that needs to be detected.
  • One end of the fiber optic sensing unit is fixed and the rest is free to move.
  • the elastic body is a cylindrical length of 50m
  • the radius r is 50mm
  • the radius R of the fiber is 0.5mm
  • the winding pitch is 10m
  • the Young's modulus of the core E1 is 80GPa
  • the equivalent Young's modulus of the fiber E2 is 80MPa
  • the effective refractive index n eff is 1.465
  • the core Poisson's ratio v is 0.17
  • the elastic-optic coefficient P 11 of the core material is 0.27
  • P 12 is 0.15
  • the transmission coefficient A is 0.01.
  • FIG. 4 shows the underwater application of the method for judging the direction of the sound field of the helical fiber optic distributed acoustic wave sensing system.
  • one end of an optical fiber sensing unit composed of a helically wound elastic body is fixed on the ship and arranged underwater.
  • the ship carries the main engine of the optical fiber distributed acoustic wave sensing system for measurement.
  • FIG. 5( a ) is a three-dimensional schematic diagram of the optical fiber sensing unit
  • FIG. 5( b ) is a top view of the optical fiber sensing unit.
  • the structure of the optical fiber sensing unit is more suitable for measuring the situation where the sound wave acts from the fixed end.
  • the refractive index of the optical fiber will change.
  • the optical fiber distributed acoustic wave sensing system host placed on the ship can detect the refractive index change of the optical fiber.
  • Figure 6(a) shows the refractive index change curve ⁇ n(l) along the length of the optical fiber detected by the optical fiber distributed sensing system. It can be obtained from the figure that the optical fiber is subjected to stress in the length range of 0-40 m, and two zero points are found. For the part where the refractive index change is positive, such as the 5-10m part, find the position of 7.5m.
  • Fig. 7 shows the application of the method for judging the direction of the sound field of the helical fiber optic distributed acoustic wave sensing system in the oil well.
  • an optical fiber sensing unit composed of two optical fibers that are spirally wound on an elastic body symmetrically at a winding angle of 180° is used as the sensing element.
  • Such a structure always has optical fibers directly affected by the sound field. Therefore, Such a structure can be used when the sound field acts at any length and position.
  • seismic waves are transmitted and act on the optical fiber sensing unit, the refractive index of the optical fiber will change.
  • the optical fiber distributed acoustic wave sensing system host placed on the ground can detect the refractive index change of the optical fiber.
  • Fig. 9(a) shows the refractive index change curve ⁇ n 1 (l) along the fiber 1 detected by the distributed optical fiber system
  • Fig. 9(b) shows the refractive index change curve ⁇ n 2 along the fiber 2 detected by the fiber distribution system (1)
  • the calculation of the size and direction of F can be performed according to the steps in Embodiment 1, and there is also a more convenient way: since the two optical fibers are wound at a winding angle of 180°, they are symmetrically spirally wound on the elastic On the body, the sin(i(r)) corresponding to the two fibers are always opposite to each other, that is, the refractive index change curves ⁇ n 1r (l) and ⁇ n 2r (l) of the two fibers caused by F r should be mutually On the contrary, the variation curve ⁇ n z (l) of the optical fiber refractive index caused by F z alone as shown in Fig. 9(c) can be expressed as:

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Abstract

一种基于弹性体的螺旋型光纤分布式声场方向判断方法，该方法将光纤螺旋缠绕在圆柱形或圆管形的弹性体上以构造成为一种光纤声波传感单元，其中弹性体使用杨氏模量较小的材料例如橡胶制成，通过对弹性体受力和光纤折射率变化的数学分析，仅使用一根光纤就能实现对声场方向的判断，可解决光纤分布式声波传感系统长期存在的使用直光纤作为传感单元时无法判断声场方向的难题。

Description

一种基于弹性体的螺旋型光纤分布式声场方向判断方法 技术领域
本发明属于光纤分布式传感领域，尤其涉及一种基于弹性体的螺旋型光纤分布式声场方向判断方法。
背景技术
基于相位解调的相位敏感型光时域反射仪(Phase-sensitive Optical Time Domain Reflectometer,Φ-OTDR)的光纤分布式声波传感(distributed fiber acoustic sensing,DAS)技术,是一种利用光纤后向瑞利散射干涉效应实现声波信号连续分布式探测的新型传感技术。除了具有普通光纤传感系统的优点，如灵敏度和精度高、固有的安全性好、抗电磁干扰、高绝缘强度、耐腐蚀、集传感与传输于一体、能与数字通信系统兼容等，DAS技术还可以实现对光纤沿线动态应变(振动、声波)的长距离、分布式的实时定量检测，在军事、国防、民生、科学研究等领域有着广泛的应用，特别在对水下目标声学特性的监测、对石油等资源的勘探等方面有着无可替代的地位，在如今仍被广泛研究和关注。
DAS技术的原理为：由于光纤介质折射率不均匀，光在光纤中传输时会发生弹性散射，产生与原入射光频率相同的散射光。其中，瑞利散射就是由于光纤材料分布不均匀导致折射率不均匀而产生的一种弹性散射，它在光纤中的后向散射光中具有最大的散射光强。在声波作用下，光纤产生微小应变，由于弹光效应，光纤折射率会发生相应改变，造成相干脉冲的后向瑞利散射光沿光纤的相邻两点之间的相位差发生变化。通过相位解调和信号处理，提取相位信息，即可还原外界声波信号，实现对声波的分布式传感。
然而，由于光纤周围的材料对光纤的径向变形有明显的阻尼作用等因素，传统的直光纤DAS系统主要对沿光纤轴向的声波敏感。同时，声波作用在光纤上时，无论声波是从径向的360°方位角中的哪一个方向入射来的，光纤都会拉伸或收缩，因此传统的传感方式无法检测出入射声场的方向。
研究人员为了解决DAS系统在使用传统直光纤时对受力方向不敏感的问题已经提出了一些方法，如Den Boer等人利用在DAS中光纤径向受力不敏感的特性，将多条光纤分别置于正交平面上，使某些光纤对X,Y,Z其中的特定方向的受力不敏感，然后通过相应的数值计算分别得到X,Y,Z方向上的受力大小，通过合并求得总的受力的方向。Hartog等人也类似的，利用在DAS中光纤径向受力不敏感的特性，使用至少一条光纤并使用特殊的缠绕方式，使某些光纤对X,Y,Z其中的特定方向的受力不敏感，然后通过相应的数值计算使其能够分离出 X,Y,Z三个方向上的分量。上述方法都是通过分离出X,Y,Z三个方向上的分量的方式得到的受力方向，但这些方法往往需要使用到多根光纤，且由于需要光纤对某个固定方向的受力不敏感，对光纤绕制工艺的要求很高。在仅使用一根光纤制作传感单元时，由于需要绕制成特殊的形状，对光纤绕制工艺的要求也很高。
本发明方法采用了特殊的传感单元布设方式，通过将光纤螺旋缠绕在圆柱形或圆管形的弹性体上以构造成为一种光纤声波传感单元，其中弹性体使用杨氏模量较小的材料例如橡胶制成，通过对弹性体受力和光纤折射率变化的数学分析，仅使用一根光纤就能实现对声场方向的判断，可解决光纤分布式声波传感系统长期存在的使用直光纤作为传感单元时无法判断声场方向的难题，进一步扩展了DAS系统的应用领域。
发明内容
本发明的目的在于针对现有技术的不足，提供一种基于弹性体的螺旋型光纤分布式声场方向判断方法。本发明解决了分布式声波传感系统在使用传统直光纤作为传感单元时对受力方向不敏感的问题。
本发明的目的是通过以下技术方案来实现的：一种基于弹性体的螺旋型光纤分布式声场方向判断方法，包括以下步骤：
(1)将光纤螺旋缠绕在弹性体上并粘合构成光纤传感单元，布置于声波测量空间中；
(2)声波使弹性体变形，通过光纤分布式声波传感系统实时解调出沿着光纤的折射率变化分布曲线Δn(l)；其中，l代表采样点与光纤起始端之间的光纤长度；
(3)在步骤(1)得到的沿着光纤的折射率变化分布曲线Δn(l)中，找到折射率变化为正的区域内任意一点得到Δn，再通过下式得到声波径向分力F r的大小：
Figure PCTCN2020133833-appb-000001
式中，E 1是光纤纤芯的杨氏模量；r是弹性体的外圆半径；d是弹性体的内圆半径；A为弹性体表面应力传递到光纤内部的传递系数；n eff是光纤的有效折射率；P 11和P 12是光纤纤芯材料的弹光系数；v是光纤纤芯材料的泊松比；l 1、l 2分别是弹性体从固定端到自由端的前端未受力部分缠绕的光纤长度和受力部分缠绕的光纤长度；h为光纤的缠绕螺距；R是光纤的半径； 折射率变化为零的光纤采样点构成光纤传感单元的中性层平面，参考轴位于中性层上且平行于弹性体端面，以弹性体横截面的圆心为原点，角度参数i(r)是原点指向光纤采样点的方向与参考轴的夹角；
(4)声波径向分力F r的方向声波径向分力F r的方向为由折射率变化为负的光纤采样点垂直指向中性层；
(5)根据步骤(3)得到的声波径向分力F r的大小算出Δn(l)的直流偏置分量Δn z(l)：
Figure PCTCN2020133833-appb-000002
(6)根据步骤(5)得到的直流偏置分量Δn z(l)中的非零值Δn z得到声波轴向分力F z的大小：
Figure PCTCN2020133833-appb-000003
式中，E 2为光纤的等效杨氏模量；
(7)根据步骤(5)得到的直流偏置分量Δn z(l)中的非零值Δn z判断声波轴向分力F z的方向：非零值Δn z恒为负则说明F z的方向为从固定端指向自由端，恒为正则说明F z从自由端指向固定端；
(8)根据步骤(3)、(4)、(6)、(7)得到的F r和F z的大小和方向计算得到声场的入射方向与轴向的夹角θ：
Figure PCTCN2020133833-appb-000004
进一步地，所述弹性体的形状为圆柱形或圆管形。
进一步地，弹性体的杨氏模量不超过300Mpa。
进一步地，所述弹性体的材料为橡胶。
进一步地，所述步骤(1)中螺旋缠绕在弹性体上的光纤的缠绕螺距h大于光纤分布式传感系统的空间分辨率。
进一步地，所述步骤(1)中所述光纤传感单元一端固定，其余部分在测量空间不受限制。
进一步地，所述光纤分布式声波传感系统包括顺次连接的分布式光纤声波传感系统主机和光纤传感单元；所述分布式光纤声波传感系统主机包括窄线宽激光器、脉冲调制器、信号 发生器、低噪声光放大器、环形器、光电探测器、信号处理模块和光纤连接模块；其中，窄线宽激光器、脉冲调制器、低噪声光放大器、环形器和光纤连接模块顺次连接，脉冲调制器和信号发生器相连，光纤连接模块和光纤传感单元的光纤相连，环形器还顺次连接光电探测器和信号处理模块。
进一步地，所述沿着光纤的折射率变化分布曲线Δn(l)的采样间隔为光纤分布式声波传感系统的标距长度。
进一步地，当光纤在弹性体上螺旋缠绕一圈时，所述角度参数i(r)的取值范围为[0,2π]。
本发明的有益效果是：
1.本发明将光纤螺旋缠绕在圆柱形或圆管形弹性体上构成了光纤传感单元，并将该传感单元单端固定地布置在传感空间内，通过弹性体变形带动光纤变形，使用光纤分布式声波传感系统主机测量光纤的折射率变化曲线，并通过分析该曲线实现对入射声场作用力方向的检测，解决分布式光纤声波传感系统在使用传统直光纤时对受力方向不敏感的问题。
2.本发明将光纤螺旋缠绕在弹性体上，使得单位测量长度积累的相位变化量更大，相比于传统的使用直光纤作为传感单元的方法，灵敏度有了大幅提升。
3.本发明使用了低杨氏模量材料来制作弹性体，使得弹性体也会由于受力而变形，通过对弹性体受力和光纤折射率变化的分析，仅使用一根光纤就能实现对受力方向的检测；另外，光纤只需要普通地螺旋缠绕在弹性体上，不需要特殊的绕制方式，降低了加工难度和成本。
附图说明
为了更清楚地说明本发明实施例的技术方案，下面将对实施例中所需要使用的附图作简单地介绍，应当理解，以下附图仅示出了本发明的某些实施例，因此不应被看作是对范围的限定，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其他相关的附图。
图1为本发明的基于弹性体的螺旋型光纤分布式声场方向判断方法的流程图；
图2为本发明的光纤分布式声波传感系统的部分组成框图；
图3为本发明的光纤分布式声波传感系统主机的部分组成框图；
图4为本发明的基于弹性体的螺旋型光纤分布式声场方向判断方法的应用场景示意图；
图5为本发明的由1根光纤螺旋缠绕在弹性体上构成的光纤传感单元的示意图；其中，(a)为三维结构示意图；(b)为横截面示意图；
图6为本发明的使用1根光纤缠绕构成的光纤传感单元时去检测声场时得到的光纤折射率变化仿真图；其中，(a)为光纤分布式声波传感系统主机检测到的实际的光纤折射率变化仿真图；(b)为仅由声波径向分力造成的光纤折射率变化仿真图；(c)为仅由声波轴向分力 造成的光纤折射率变化仿真图；
图7为本发明方法在油井中应用的示意图；
图8为本发明的由2根光纤螺旋缠绕在弹性体上构成的光纤传感单元的示意图；其中，(a)为三维结构示意图；(b)为横截面示意图；
图9为本发明的使用2根光纤缠绕构成的光纤传感单元去检测声场时得到的光纤折射率变化仿真图；其中，(a)为光纤分布式声波传感系统主机检测到的光纤1实际的光纤折射率变化仿真图；(b)为光纤分布式声波传感系统主机检测到的光纤2实际的光纤折射率变化仿真图；(c)为经过数据处理得到的仅由声波轴向分力造成的光纤折射率变化仿真图；(d)为光纤1仅由声波径向分力造成的光纤折射率变化仿真图；(e)为光纤2仅由声波径向分力造成的光纤折射率变化仿真图。
具体实施方式
为了使本发明的目的、技术方案及优点更加清楚明白，以下结合附图及实施例，对本发明进行进一步详细说明。应当理解，此处所描述的具体实施例仅用以解释本发明，并不用于限定本发明，即所描述的实施例仅仅是本发明一部分实施例，而不是全部的实施例。通常在此处附图中描述和示出的本发明实施例的组件可以以各种不同的配置来布置和设计。
因此，以下对在附图中提供的本发明的实施例的详细描述并非旨在限制要求保护的本发明的范围，而是仅仅表示本发明的选定实施例。基于本发明的实施例，本领域技术人员在没有做出创造性劳动的前提下所获得的所有其他实施例，都属于本发明保护的范围。
需要说明的是，术语“第一”和“第二”等之类的关系术语仅仅用来将一个实体或者操作与另一个实体或操作区分开来，而不一定要求或者暗示这些实体或操作之间存在任何这种实际的关系或者顺序。而且，术语“包括”、“包含”或者其任何其他变体意在涵盖非排他性的包含，从而使得包括一系列要素的过程、方法、物品或者设备不仅包括那些要素，而且还包括没有明确列出的其他要素，或者是还包括为这种过程、方法、物品或者设备所固有的要素。在没有更多限制的情况下，由语句“包括一个……”限定的要素，并不排除在包括所述要素的过程、方法、物品或者设备中还存在另外的相同要素。
本发明基于弹性体的螺旋型光纤分布式声场方向判断方法，目的在于解决分布式光纤声波传感系统在使用传统直光纤时对受力方向不敏感的技术问题。本发明采用的技术手段的主要步骤如图1所示，包括以下步骤：
步骤1：使用粘合剂将光纤螺旋缠绕在圆柱形弹性体上构成光纤传感单元并将其布置于测量空间内，布置时使光纤传感单元单端固定，其余部分可以在空间自由移动；所述弹性体需使用杨氏模量较小的材料，例如橡胶，形状为圆柱形或圆管形；
步骤2：通过光纤分布式声波传感系统实时解调出沿着光纤的折射率变化分布Δn(l)；该沿着光纤长度的折射率分布变化曲线Δn(l)的采样间隔为光纤分布式声波传感系统的标距长度(gauge length)，l代表采样点与光纤起始端之间的光纤长度；
步骤3：通过Δn(l)分别得到声波径向分力F r和声波轴向分力F z的大小和方向，包括以下子步骤：
步骤3.1：在得到的沿着光纤的折射率变化分布曲线Δn(l)中找到折射率变化为正且上凸的区域内的任意一个点求出声波径向分力F r的大小，具体为：
对于一个螺旋缠绕在柔性弹性体上的光纤，在一个特定方向的声场作用下，光纤的总折射率变化Δn可以表示为：
Δn＝Δn r+Δn z
该曲线Δn(l)每隔半个螺距的光纤长度会有一个零点，曲线零点的间隔L为：
Figure PCTCN2020133833-appb-000005
式中，R为光纤的半径；h为光纤的缠绕螺距。螺旋缠绕在弹性体上的光纤的缠绕螺距h需远大于光纤分布式传感系统的空间分辨率且可调，螺距越大则对光纤分布式声波传感系统的空间分辨率要求越低，但需要的弹性体越长；并且同一个传感单元可以包含不同的螺距值h。
螺旋缠绕在弹性体上的光纤使用粘合剂将二者粘合在一起的方式使缠绕在弹性体上的整条光纤都会随弹性体的变形而变形；若是将光纤沿着螺旋线方程的轨迹缠绕在弹性体上，弹性体表面的力会传递到光纤内部导致光纤的折射率发生变化，由于缠绕螺距较大，该正应力方向可认为与光纤轴向方向相同，根据弹光效应：
ΔB＝PS
式中，B为介电不渗透性张量，其分量为介电张量ε相应分量的倒数；P为弹光系数；S为应变张量。对于各项同性介质且忽略切应力的情况下有：
Figure PCTCN2020133833-appb-000006
因为介电不渗透性张量B各分量为介电张量的倒数，且光纤纤芯材料属于非铁磁性物质，磁导率为1，则可以得到Δn与ΔB的关系为：
Figure PCTCN2020133833-appb-000007
因为使用的光纤为单模光纤，由晶体光学中波法线椭球可以知道，纤芯受力后影响光纤 中正交偏振态的折射率变化为Δn 1和Δn 2，且Δn 1＝Δn 2，则造成的折射率变化Δn r的大小为：
Figure PCTCN2020133833-appb-000008
式中，P 11和P 12是光纤纤芯材料的弹光系数；v是纤芯材料的泊松比；n eff是光纤的有效折射率；E 1是纤芯材料的杨氏模量。
当有声波作用在该单端固定的弹性体上时，弹性体的一半圆柱会拉伸，另一半会收缩，拉伸和压缩的分界处既不拉伸也不压缩，称为中性层。光纤的折射率变化分布曲线Δn(l)的所有零点所对应的光纤的空间位置处于同一个平面，这个平面就是传感单元的中性层；位于中性层上的弹性体的直径方向即为参考轴的方向，参考轴平行于弹性体端面，以弹性体截面中心位置为参考轴的原点；
因为弹性体的截面为圆环，其表面各点正应力的计算公式为：
Figure PCTCN2020133833-appb-000009
式中，i(r)是角度参数，是参考轴圆心指向光纤采样点的方向与参考轴的夹角，当光纤螺旋缠绕一圈时其取值范围为[0,2π]；A是弹性体表面应力传递到光纤内部的传递系数，与弹性体与光纤的参数、二者的粘合方式有关，可以通过测试得到；
对于单端固定的结构来说，M z为弹性体的弯矩(z＝1、2)：
Figure PCTCN2020133833-appb-000010
式中，l代表采样点与光纤起始段之间的光纤长度；l 1、l 2分别是弹性体从固定端到自由端的前端未受力部分缠绕的光纤长度和受力部分缠绕的光纤长度。
对于圆环截面来说，横截面的惯性矩I z为：
Figure PCTCN2020133833-appb-000011
式中，r是弹性体的外圆半径；d是弹性体的内圆半径。
在得到的沿着光纤的折射率变化分布曲线Δn(l)中找到折射率变化为正且趋势为上凸的区域内的一个点，在这个点光纤仅受到F r的作用，根据上述讨论，则可得到F r的计算公式可以表示为：
Figure PCTCN2020133833-appb-000012
步骤3.2：得出声波径向分力F r的方向，具体为：
声波径向分力F r的方向为由折射率变化为负的光纤采样点垂直指向中性层；
步骤3.3：利用得到的F r算出Δn(l)直流偏置分量Δn z(l)，具体为：
由F r可以得到以缠绕螺距为周期的分量Δn r(l)：
Figure PCTCN2020133833-appb-000013
根据Δn(l)和Δn r(l)可以得到直流偏置分量Δn z(l)：
Δn z(l)＝Δn(l)-Δn r(l)
步骤3.4：利用直流偏置分量Δn z(l)中的非零值Δn z算出声波轴向分力F z的大小：
由可求出F z的大小和方向：
Figure PCTCN2020133833-appb-000014
式中，E 2是光纤的等效杨氏模量。
步骤3.5：利用直流偏置分量Δn z(l)中的非零值Δn z算出声波轴向分力F z的方向，具体：
由于光纤的螺距很大且光纤对径向方向的受力不敏感，可以认为曲线Δn z(l)是仅由F z造成的，由曲线n z(l)可以看出声场的作用范围，且n z(l)中非零值恒为负说明F z的方向与F r在直接受力面造成的正应力方向相同，为从固定端指向自由端，Δn z中非零值恒为正则说明F z的方向与F r在直接受力面造成的正应力方向相反，方向为从自由端指向固定端。
步骤4：通过F r和F z的大小和方向可以得到它们合力的大小和方向，该方向即为声场的入射方向，声场的入射方向与参考轴的夹角
Figure PCTCN2020133833-appb-000015
如图2所示的本发明中使用的光纤分布式声波传感系统可以分为两个部分，第一部分为光纤分布式声波传感系统主机，第二部分为光纤传感单元。光纤分布式声波传感系统主机的组成部分如图3所示包括窄线宽激光器、脉冲调制器、信号发生器、低噪声光放大器、环形器、光电探测器、信号处理模块和光纤连接模块，光纤连接模块和光纤传感单元的光纤相连。光纤分布式声波传感系统常见的应用场景如图4和图7所示，光纤分布式声波传感系统主机放置于船上或陆地上，光纤传感单元则布置在需要进行探测的空间内，并使光纤传感单元的一端固定且其余部分可以自由移动。
以下结合实施例对本发明的特征和性能作进一步的详细描述。
设弹性体为圆柱形长度为50m，半径r为50mm，光纤的半径R为0.5mm，缠绕螺距为10m，纤芯杨氏模量E 1为80GPa，光纤的等效杨氏模量E 2为80MPa，有效折射率n eff为1.465，纤芯泊松比v为0.17，纤芯材料的弹光系数P 11为0.27，P 12为0.15，传递系数A为0.01。
实施例1
图4所示为螺旋型光纤分布式声波传感系统声场方向判断的方法在水下的应用。将由一根螺旋缠绕在弹性体上构成的光纤传感单元如图4所示一端固定在船上并布置于水下，船上载有用于测量的光纤分布式声波传感系统主机。图5(a)为该光纤传感单元的三维示意图，图5(b)为该光纤传感单元的俯视图。由于这种结构在缠绕有光纤的背面有不能检测到声场轴向分量的盲区，该光纤传感单元的结构更适合用于测量从固定端开始就受到声波作用的情况。当水下目标产生声波作用在光纤传感单元上时，光纤的折射率会发生变化，通过放置在船上的光纤分布式声波传感系统主机可以对光纤的折射率变化进行检测。
得到的仿真结果如图6所示。图6(a)表示光纤分布式传感系统检测到的沿光纤长度的折射率变化曲线Δn(l)，由图可以得到，光纤在0-40m的长度范围内受到应力作用，找到两个零点间折射率变化为正的部分，例如5-10m部分，再找到7.5m的位置，在这个位置，F r的计算公式中sin(i(r))＝1，结合光纤和弹性体的各项参数可以求出F r＝1Pa，且方向为7.5m处的光纤所在的空间位置垂直指向5m和10m处的光纤所在的空间位置的连线。
得到F r的大小和方向后，结合F r造成的光纤折射率的变化公式可以得到如图6(b)表示的单独由F r造成的光纤折射率的变化曲线Δn r(l)。图6(c)表示通过用Δn(l)-Δn r(l)得到的单独由F z造成的光纤折射率的变化曲线Δn z(l)，由该曲线可以求出F z＝1Pa，并且该曲线的非零值恒为负，则可以知道本仿真中F z的方向为从固定端指向自由端。根据得到的声波的F r和F z的大小和方向，可以还原出F的大小和方向。
实施例2
图7为本发明的螺旋型光纤分布式声波传感系统声场方向判断的方法在油井中的应用。 使用如图8所示的由两根光纤成180°的缠绕角度对称地螺旋缠绕在弹性体上构成的光纤传感单元作为传感元件，这样的结构一直都有直接受到声场作用的光纤，因此这样的结构可以用于声场作用在任意长度和位置的情况。当地震波等传输过来作用在光纤传感单元上时，光纤的折射率会发生变化，通过放置在地面的光纤分布式声波传感系统主机可以对光纤的折射率变化进行检测。
得到的仿真结果如图9所示。图9(a)表示分布式光纤系统检测到的沿光纤1的折射率变化曲线Δn 1(l)，图9(b)表示光纤分布式系统检测到的沿光纤2的折射率变化曲线Δn 2(l)，这样的结构中对F的大小和方向的计算可以按照实施例1中的步骤进行，也有一种更便捷的方式：由于两根光纤是180°的缠绕角度对称地螺旋缠绕在弹性体上的，则两根光纤对应的sin(i(r))一直互为相反数，即两根光纤由F r造成的折射率变化曲线Δn 1r(l)和Δn 2r(l)应互为相反数，则图9(c)所表示的单独由F z造成的光纤折射率的变化曲线Δn z(l)可以表示为：
Δn z(l)＝Δn 1(l)+Δn 2(l)
由曲线Δn z(l)可以得到，曲线为非零值的地方为声波的作用范围，如图9(c)中的30-40m部分的光纤，则光纤的前端未受力部分l 1＝30m，光纤的受力部分光纤的前端未受力部分l 2＝10m。由图中数据可以计算得到F z＝1Pa，且由该曲线的非零值恒为负，可以知道本仿真中F z的方向为从固定端指向自由端。
综合Δn 1(l)、Δn 2(l)和Δn z(l)以及光纤中的单独由F z造成的光纤折射率的变化总是出现在Δn 1(l)和Δn 2(l)的负半轴的特征，可以找到缠绕在前端未受力弹性体上这一部分光纤选取合适的点来直接计算出F r的大小，如7.5m处的光纤，此时F r的计算应取0＜l＜l 1时的公式进行，且sin(i(r))＝1，可以计算出F r＝1Pa；也可以通过得到如图9(d)和图9(e)表示的Δn 1r(l)和Δn 2r(l)的变化曲线，任选一条曲线，找到缠绕在直接受力的弹性体上的这一部分光纤，并找到两个零点间折射率变化为正的部分，如图9(d)所示的光纤1的35-40m部分，再找到37.5m的位置，在这个位置，F r的计算应取l 1≤l＜l 2时的公式进行，且sin(i(r))＝1，结合光纤和弹性体的各项参数可以求出F r＝1Pa，且方向为7.5m处的光纤所在的空间位置垂直指向5m和10m处的光纤所在的空间位置的连线。根据得到的F r和F z的大小和方向，可以还原出F的大小和方向。
以上所述仅为本发明的较佳实施例而已，并不用以限制本发明，凡在本发明的精神和原则之内所作的任何修改、等同替换和改进等，均应包含在本发明的保护范围之内。

Claims (9)

  1. 一种基于弹性体的螺旋型光纤分布式声场方向判断方法，其特征在于，包括以下步骤：
    (1)将光纤螺旋缠绕在弹性体上并粘合构成光纤传感单元，布置于声波测量空间中；
    (2)声波使弹性体变形，通过光纤分布式声波传感系统实时解调出沿着光纤的折射率变化分布曲线Δn(l)；其中，l代表采样点与仪器连接端之间的光纤长度；
    (3)在步骤(1)得到的沿着光纤的折射率变化分布曲线Δn(l)中，找到折射率变化为正的区域内任意一点得到Δn，再通过下式得到声波径向分力F r的大小：
    Figure PCTCN2020133833-appb-100001
    式中，E 1是光纤纤芯的杨氏模量；r是弹性体的外圆半径；d是弹性体的内圆半径；A为弹性体表面应力传递到光纤内部的传递系数；n eff是光纤的有效折射率；P 11和P 12是光纤纤芯材料的弹光系数；v是光纤纤芯材料的泊松比；l 1、l 2分别是弹性体从固定端到自由端的前端未受力部分缠绕的光纤长度和受力部分缠绕的光纤长度；h为光纤的缠绕螺距；R是光纤的半径；折射率变化为零的光纤采样点构成光纤传感单元的中性层平面，参考轴位于中性层上且平行于弹性体端面，以弹性体横截面的圆心为原点，角度参数i(r)是原点指向光纤采样点的方向与参考轴的夹角；
    (4)声波径向分力F r的方向为由折射率变化为负的光纤采样点垂直指向中性层；
    (5)根据步骤(3)得到的声波径向分力F r的大小算出Δn(l)的直流偏置分量Δn z(l)：
    Figure PCTCN2020133833-appb-100002
    (6)根据步骤(5)得到的直流偏置分量Δn z(l)中的非零值Δn z得到声波轴向分力F z的大小：
    Figure PCTCN2020133833-appb-100003
    式中，E 2为光纤的等效杨氏模量；
    (7)根据步骤(5)得到的直流偏置分量Δn z(l)中的非零值Δn z判断声波轴向分力F z的方向：非零值Δn z恒为负则说明F z的方向为从固定端指向自由端，恒为正则说明F z从自由端指向固定端；
    (8)根据步骤(3)、(4)、(6)、(7)得到的F r和F z的大小和方向计算得到声场的入射方向与轴向的夹角θ：
    Figure PCTCN2020133833-appb-100004
  2. 根据权利要求1所述基于弹性体的螺旋型光纤分布式声场方向判断方法，其特征在于，所述弹性体的形状为圆柱形或圆管形。
  3. 根据权利要求1所述基于弹性体的螺旋型光纤分布式声场方向判断方法，其特征在于，弹性体的杨氏模量不超过300Mpa。
  4. 根据权利要求1所述基于弹性体的螺旋型光纤分布式声场方向判断方法，其特征在于，所述弹性体的材料为橡胶。
  5. 根据权利要求1所述基于弹性体的螺旋型光纤分布式声场方向判断方法，其特征在于，所述步骤(1)中螺旋缠绕在弹性体上的光纤的缠绕螺距h大于光纤分布式传感系统的空间分辨率。
  6. 根据权利要求1所述基于弹性体的螺旋型光纤分布式声场方向判断方法，其特征在于，所述步骤(1)中所述光纤传感单元一端固定，其余部分在测量空间不受限制。
  7. 根据权利要求1所述基于弹性体的螺旋型光纤分布式声场方向判断方法，其特征在于，所述光纤分布式声波传感系统包括顺次连接的分布式光纤声波传感系统主机和光纤传感单元；所述分布式光纤声波传感系统主机包括窄线宽激光器、脉冲调制器、信号发生器、低噪声光放大器、环形器、光电探测器、信号处理模块和光纤连接模块；其中，窄线宽激光器、脉冲调制器、低噪声光放大器、环形器和光纤连接模块顺次连接，脉冲调制器和信号发生器相连，光纤连接模块和光纤传感单元的光纤相连，环形器还顺次连接光电探测器和信号处理模块。
  8. 根据权利要求1所述基于弹性体的螺旋型光纤分布式声场方向判断方法，其特征在于，所述沿着光纤的折射率变化分布曲线Δn(l)的采样间隔为光纤分布式声波传感系统的标距长度。
  9. 根据权利要求1所述基于弹性体的螺旋型光纤分布式声场方向判断方法，其特征在于，当光纤在弹性体上螺旋缠绕一圈时，所述角度参数i(r)的取值范围为[0,2π]。
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