Optical Fiber Sensing System, Method and Apparatus for Simultaneously Measuring Temperature, Strain, and Pressure

Abstract

An optical fiber sensing system, method and apparatus for simultaneously measuring temperature, strain, and pressure are provided and belong to the field of optical fiber sensors. A distributed optical fiber temperature sensor is configured to monitor the temperature, and transmit the monitored temperature to a fiber grating strain and pressure sensor; the fiber grating strain and pressure sensor performs self temperature compensation based on received temperature; and the fiber grating strain and pressure sensor monitors the strain and the pressure. The distributed optical fiber temperature sensor is used to replace a temperature compensation function of the fiber grating strain sensor, and sense temperature distribution of each point along a route. Further, the fiber grating strain and pressure sensor is simplified inside, temperature demodulation is no longer required and speed of obtaining values of the strain and the pressure has been accelerated.

Claims

1. An optical fiber sensing system for simultaneously measuring temperature, strain, and pressure, comprising a distributed optical fiber temperature sensor and a fiber grating strain and pressure sensor; wherein the distributed optical fiber temperature sensor is configured to monitor the temperature, and transmit the monitored temperature to the fiber grating strain and pressure sensor; the fiber grating strain and pressure sensor is configured to perform self temperature compensation based on received temperature; and the fiber grating strain and pressure sensor is further configured to monitor the strain and the pressure.

2. The optical fiber sensing system for simultaneously measuring the temperature, the strain, and the pressure according to claim 1, wherein an internal structure of the fiber grating strain and pressure sensor comprises a circular metal diaphragm and two gratings, the two gratings are a pressure grating and a strain grating.

3. A method for simultaneously measuring temperature, strain, and pressure, comprising: S1. monitoring the temperature by using a distributed optical fiber temperature sensor, and transmitting the monitored temperature to a fiber grating strain and pressure sensor; S2. performing, by the fiber grating strain and pressure sensor, self temperature compensation based on received temperature; and S3. monitoring the strain and the pressure by the fiber grating strain and pressure sensor.

4. An apparatus for simultaneously measuring temperature, strain, and pressure, comprising a distributed optical fiber temperature sensor and a fiber grating strain and pressure sensor; wherein the distributed optical fiber temperature sensor is configured to monitor the temperature, and transmit the monitored temperature to the fiber grating strain and pressure sensor; the fiber grating strain and pressure sensor is configured to perform self temperature compensation based on received temperature; and the fiber grating strain and pressure sensor is further configured to monitor the strain and the pressure.

5. The apparatus for simultaneously measuring the temperature, the strain, and the pressure according to claim 4, wherein an internal structure of the fiber grating strain and pressure sensor comprises a circular metal diaphragm and two gratings, the two gratings are a pressure grating and a strain grating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] To describe the technical solutions in the embodiments of the disclosure or in the conventional art more clearly, the following briefly describes the accompanying drawings required for the description of the embodiments or the conventional art. Obviously, a person of ordinary skill in the art may still derive other accompanying drawings from these accompanying drawings without creative efforts.

[0026] FIG. 1 is a system flowchart of an embodiment of the disclosure;

[0027] FIG. 2 is a schematic diagram of an internal structure of an fiber grating strain and pressure sensor according to an embodiment of the disclosure; and

[0028] FIG. 3 is an overall schematic diagram of an embodiment of the disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] The following clearly and completely describes the technical solutions in the embodiments of the disclosure with reference to accompanying drawings in the embodiments of the disclosure. Obviously, the described embodiments are merely a part rather than all of the embodiments of the disclosure. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the disclosure without creative efforts shall fall within the scope of the present disclosure.

[0030] A fiber grating strain and pressure sensor is a cylindrical object as a whole, with a housing to wrap and protect the internal structure, a circular diaphragm at a bottom to increase sensitivity, and two gratings inside corresponding to pressure and strain. A tie rod structure is adopted for the pressure. A high-elastic metal diaphragm at the bottom is used for increasing sensitivity. Received pressure is applied to the metal diaphragm to cause a slight displacement, and then is transmitted to the tie rod structure, so as to change a reflection wavelength of the grating. The grating is fixed in the internal structure for the strain. Deformation caused by force is acted on the grating, so as to change a period and refractive index of the grating, as shown in FIG. 2.

[0031] As shown in FIG. 1, according to the disclosure, the temperature is first measured by using a distributed optical fiber temperature sensor, then a temperature value is returned to the fiber grating strain and pressure sensor for temperature compensation, afterwards the strain and the pressure are measured, and finally the temperature, the strain, and the pressure are sensed. A temperature grating is required inside a previous fiber grating strain and pressure sensor to realize a function of the temperature compensation, so it is troublesome to measure the pressure and the strain simultaneously. The distributed optical fiber temperature sensor is used to replace the temperature grating to realize the function of the temperature compensation, and sense temperature distribution of each point along a route. Further, the fiber grating strain and pressure sensor is simplified inside, temperature demodulation is no longer required, and speed of obtaining values of the strain and the pressure has been accelerated.

[0032] Further, as shown in FIG. 3, the distributed optical fiber temperature sensor is a functional optical fiber sensor, that is, an entire optical fiber cable realizes two functions of transmission and sensing. A transmitted signal is finally demodulated into a temperature value by using a demodulator. A structure of a measuring part of the distributed optical fiber temperature sensor is an optical cable. The above-mentioned fiber grating strain and pressure sensor is of a single-point measurement type, and can only measure a parameter of a part at which the sensor is located. However, a plurality of fiber grating strain and pressure sensors can be connected in series by using a multiplexing technology, for example, three sensors below shown in FIG. 3. An overall structure is that the plurality of fiber grating strain and pressure sensors are connected, and then fixed on the temperature sensor to realize common measurement.

[0033] According to a principle of the distributed optical fiber temperature sensor, a Raman scattered light signal is more sensitive to the temperature, the temperature is sensed by collecting a Stokes Raman scattered light signal and an anti-Stokes Raman scattered light signal.

[0034] A defect in an optical fiber (caused in a manufacturing process, at an interconnection of different sections, or the like) may affect uniformity of a refractive index. When light passes through the optical fiber, photons collide inelastically with optical phonons of the optical fiber, that is, a Raman effect. In a scattered spectrum, a part with a wavelength smaller than that of an incident light is anti-Stokes light, and a part with a wavelength greater than that of the incident light is Stokes light. The Anti-Stokes signal is relatively sensitive to a change of the temperature, and therefore usually used as a signal channel, and the Stokes signal is used as a reference channel At any temperature T, a luminous flux ratio of the Anti-Stokes and the Stokes is:

[00001]$\begin{array}{cc}\frac{{\Phi }_{\mathrm{AS}}\left(T\right)}{{\Phi }_S\left(T\right)}=\frac{K_{\mathrm{AS}}}{K_S}.Math.{\left(\frac{v_{\mathrm{AS}}}{v_S}\right)}^4.Math.\frac{R_{\mathrm{AS}}\left(T\right)}{R_S\left(T\right)}.Math.\exp \left[\left({\alpha }_S-{\alpha }_{\mathrm{AS}}\right).Math.L\right]& \left(1\right)\end{array}$

[0035] R.sub.AS and R.sub.S are temperature modulation functions of the Anti-Stokes and the Stokes, and a relationship is:

[00002]$\begin{array}{cc}{R}_{\mathrm{AS}}\left(T\right)={\left[\exp \left(h\Delta v/\mathrm{kT}\right)-1\right]}^{-1}& \left(2\right)\end{array}$$\begin{array}{cc}{R}_S\left(T\right)={\left[1-\exp \left(-h\Delta v/\mathrm{kT}\right)\right]}^{-1}& \left(3\right)\end{array}$$\begin{array}{cc}\frac{{\Phi }_{\mathrm{AS}}\left(T\right)}{{\Phi }_S\left(T\right)}=\frac{K_{\mathrm{AS}}}{K_S}.Math.{\left(\frac{v_{\mathrm{AS}}}{v_S}\right)}^4.Math.\exp \left[-\left(h\Delta v/\mathrm{kT}\right)\right].Math.\exp \left[\left({\alpha }_S-{\alpha }_{\mathrm{AS}}\right).Math.L\right]& \left(4\right)\end{array}$

[0036] Assuming that a reference temperature is T.sub.0, the ratio of the luminous fluxes of the Anti-Stokes and the Stokes at T.sub.0 is:

[00003]$\begin{array}{cc}\frac{{\Phi }_{\mathrm{AS}}\left(T_0\right)}{{\Phi }_S\left(T_0\right)}=\frac{K_{\mathrm{AS}}}{K_S}.Math.{\left(\frac{v_{\mathrm{AS}}}{v_S}\right)}^4.Math.\exp \left[-\left(h\Delta v/{\mathrm{kT}}_0\right)\right].Math.\exp \left[\left({\alpha }_S-{\alpha }_{\mathrm{AS}}\right).Math.L\right]& \left(5\right)\end{array}$$\begin{array}{cc}\frac{{\Phi }_{\mathrm{AS}}\left(T\right)/{\Phi }_S\left(T\right)}{{\Phi }_{\mathrm{AS}}\left(T_0\right)/{\Phi }_S\left(T_0\right)}=\exp \left[\frac{h\Delta v}{k}\left(\frac{1}{T_0}-\frac{1}{T}\right)\right]& \left(6\right)\end{array}$

[0037] Final temperature value is:

[00004]$\begin{array}{cc}T=\frac{h\Delta {\mathrm{vT}}_0}{h\Delta v-{\mathrm{kT}}_0\ln \frac{{\Phi }_{\mathrm{AS}}\left(T\right)/{\Phi }_S\left(T\right)}{{\Phi }_{\mathrm{AS}}\left(T_0\right)/{\Phi }_S\left(T_0\right)}}& \left(7\right)\end{array}$

[0038] Where, Φ.sub.AS and Φ.sub.S are the luminous fluxes of the Anti-Stokes and the Stokes at temperature T; K.sub.AS and K.sub.S are section coefficients of the Anti-Stokes and the Stokes; v.sub.AS and v.sub.S are frequencies of photons of the Anti-Stokes and the Stokes photons; α.sub.AS and α.sub.S are losses of Anti-Stokes light and Stokes light transmitted through the optical fiber; L is a position of scattered light in the optical fiber; h is a Planck constant, and a value thereof is 6.626×10.sup.−34 J.Math.s; Δv is an optical phonon frequency of the optical fiber, and a value thereof is 1.32×10.sup.13 Hz; and k is a Boltzmann constant , and a value thereof is 1.38×10.sup.−23 J.Math.K.

[0039] For measurements of the strain and the pressure in the fiber grating strain and pressure sensor, a wavelength shift is affected by the period and the refractive index:

Δλ.sub.B=2Λ.Math.Δn.sub.e+2n.sub.e.Math.ΔΛ  (8)

[0040] Under an action of axial strain ε.sub.z, the following can be obtained:

[00005]$\begin{array}{cc}\Delta \left(\frac{1}{n_e^2}\right)=\left(p_{11}+p_{12}\right){\epsilon }_x+p_{12}{\epsilon }_{𝓏}& \left(9\right)\end{array}$

[0041] Transverse strain ε.sub.x can be expressed as: ε.sub.x=−με.sub.z.

[0042] A relationship between a change of a grating period and the axial strain in an elastic range is:

[00006]$\frac{\mathrm{\Delta \Lambda }}{\Lambda }={\epsilon }_{𝓏}$

[0043] An effective elastic-optical coefficient is set as p.sub.e, and expressed as

[00007]$p_e=\frac{n_e^2}{2}\left[p_{12}-\mu \left(p_{11}+p_{12}\right)\right]$

[0044] Therefore, a wavelength shift caused by the strain is:

[00008]$\begin{array}{cc}\frac{{\mathrm{\Delta \lambda }}_B}{{\lambda }_B}=\left(1-p_e\right){\epsilon }_{𝓏}& \left(10\right)\end{array}$

[0045] Through the metal diaphragm, the strain can be related to the pressure. When the pressure is set as P, the axial strain under the pressure is expressed as:

ε.sub.z=−P.Math.(1−2μ)|E  (11)

[0046] A relationship between the grating period and the pressure is: ΔΛ=Λ.Math.ε.sub.z=−Λ.Math.P.Math.(1−2μ)|E.

[0047] According to an elastic-optical effect of a material, the following can be obtained:

[00009]$\begin{array}{cc}\Delta n_e=\frac{1}{2}.Math.n_e^3.Math.\left[p_{12}-v.Math.\left(p_{11}+p_{12}\right)\right].Math.\left(1-2\mu \right).Math.\left(P/E\right)& \left(12\right)\end{array}$

[0048] After the effective elastic-optical coefficient is substituted into the formula, the wavelength shift caused by the pressure is:

[00010]$\begin{array}{cc}\frac{{\mathrm{\Delta \lambda }}_B}{{\lambda }_B}=\frac{\left(1-2\mu \right).Math.\left(p_e-1\right)}{E}.Math.P& \left(13\right)\end{array}$

[0049] A wavelength shift caused by the temperature is:

[00011]$\begin{array}{cc}\frac{{\mathrm{\Delta \lambda }}_B}{{\lambda }_B}=\left(\alpha +\xi \right).Math.T& \left(14\right)\end{array}$

[0050] When three parameters act simultaneously, influence of the temperature needs to be considered, that is:

[00012]$\begin{array}{cc}\frac{{\mathrm{\Delta \lambda }}_B}{{\lambda }_B}=\left(\alpha +\xi \right).Math.T+\frac{\left(1-2\mu \right).Math.\left(p_e-1\right)}{E}.Math.P+\left(1-p_e\right).Math.{\epsilon }_{𝓏}& \left(15\right)\end{array}$

[0051] Where, the strain grating and the pressure grating are separated, so individual grating is only affected by the temperature and a corresponding parameter. If K.sub.T=(α+ξ) λ.sub.B expresses a temperature coefficient, K.sub.p=(1−2v)(p.sub.e−1)λB/E expresses a pressure coefficient, and K.sub.ε=(α+ξ)λ.sub.B expresses a strain coefficient, and when the temperature T is known, the following can be obtained:

Δλ.sub.B1=K.sub.T1.Math.T+K.sub.ε.Math.ε.sub.z  (16)

Δλ.sub.B2=K.sub.T2.Math.T+K.sub.p.Math.P  (17)

[0052] The strain and the pressure obtained after the temperature compensation can be expressed as:

[00013]$\begin{array}{cc}{\epsilon }_{𝓏}=\frac{{\mathrm{\Delta \lambda }}_{B1}-K_{T1}.Math.T}{K_{\epsilon }}& \left(18\right)\end{array}$$\begin{array}{cc}P=\frac{{\mathrm{\Delta \lambda }}_{B2}-K_{T2}.Math.T}{K_p}& \left(19\right)\end{array}$

[0053] Where, n.sub.e is an effective refractive index of the optical fiber, Λ is the period of the grating, α is a thermal expansion coefficient of an optical fiber material, ξ is a thermo-optical coefficient of the optical fiber material, μ is a Poisson's ratio of the optical fiber material, E is a Young's modulus of the optical fiber material, and p.sub.11 and p.sub.12 are elastic-optical coefficients and values thereof depend on a material used.

[0054] In the description of this specification, the description of the terms “one embodiment”, “example”, “specific example” or the like means that specific features, structures, materials or characteristics described with reference to the embodiment(s) or example(s) are included in at least one embodiment or example of the disclosure. In the specification, the schematic description of the above terms is unnecessarily against the same embodiment or example. Moreover, the particular features, structures, materials or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0055] The basic principles, main features, and advantages of the disclosure are shown and described above. It should be understood by those skilled in the art that, the disclosure is not limited by the aforementioned embodiments. The aforementioned embodiments and the description only illustrate the principle of the disclosure. Various changes and modifications may be made to the disclosure without departing from the spirit and scope of the disclosure. Such changes and modifications all fall within the claimed scope of the disclosure.