A frequency difference selection method for slope-assisted Brillouin optical fiber sensing

Abstract

A frequency difference selection method for slope-assisted Brillouin optical fiber sensing, comprising the following steps: Step 1, input the amplitude of the external vibration signal, the strain coefficient of the optical fiber, the minimum detectable power of the detector, and the Brillouin gain spectrum of each position point of the optical fiber; Step 2, for the first position point, calculate the set of allowable frequency differences between the pump light and the probe light at this position point; Step 3, repeat Step 2 until the calculations are completed for all position points to obtain a set of sets; Step 4, for the set obtained in Step 3, find the common intersection V; Step 5, arbitrarily select an element from V, which is the frequency difference between the pump light and the probe light. If V is an empty set, it means that under the conditions given in Step 1, there is no suitable frequency difference available for selection. The present invention performs a full scan of the selectable range of the frequency difference between the pump light and the probe light, avoiding the problems of low error tolerance and difficulty in realizing distributed vibration measurement caused by setting the frequency difference between the pump light and the probe light to half of the full width at half maximum of the BGS; using a single value as the frequency difference between the pump light and the probe light, avoiding the problems of increased frequency sweep points and long acquisition time caused by the multi-slope assistance method.

Description

Technical Field

[0001] The present invention relates to a distributed optical fiber Brillouin strain and temperature sensor, belonging to the technical field of distributed optical fiber sensing, and particularly relates to a frequency selection method for slope-assisted Brillouin optical fiber vibration sensing. Background Art

[0002] Distributed optical fiber sensing is a new type of sensing method. In this method, the optical fiber is used as the sensing medium and is laid on the surface or inside of an object, and the strain and temperature distribution on the surface or inside of the object can be measured. Compared with traditional monitoring means, the distributed optical fiber sensing technology has the advantages of distributed measurement, accurate positioning, low unit cost, good maintainability, and strong anti-electromagnetic interference ability, and is applicable to explosion-proof, radiation, high-temperature, and dangerous places such as coal mines, oil fields, power plants, oil refineries, and steelmaking furnaces.

[0003] The principle of distributed optical fiber strain sensing is as follows: Two beams of light are input to both ends of the optical fiber, and the scattered signals returned in the optical fiber are resolved into strain and temperature changes. When the pump light and the probe light meet in the optical fiber and the frequency difference is within the Brillouin spectrum, the Brillouin scattering effect will occur, and the intensity of the probe light is changed by the pump light. If the frequency of the probe light is swept, the Brillouin gain spectrum characteristics at each position point in the optical fiber can be measured. The Brillouin frequency shift can be extracted from the Brillouin gain spectrum. Since the Brillouin frequency shift has a linear relationship with the stress and temperature of the optical fiber within a certain range, the strain and temperature distribution at each position point of the optical fiber can be deduced by measuring the Brillouin gain spectrum (Brillouin Gain Spectrum, abbreviated as BGS).

[0004] Since distributed optical fiber sensing requires a time-consuming averaging and frequency sweeping process, it is generally applicable to static strain measurement. In order to improve the measurement speed of distributed optical fiber sensing and achieve distributed vibration sensing, in 2009, Bernini et al. utilized one side slope of the BGS to realize the dynamic measurement of temperature and strain, that is, distributed vibration sensing. The principle is as follows: When the BGS is known in advance, the frequency of the probe light is fixed, and the power distribution at each position point at this frequency is measured. When an external vibration signal acts on the optical fiber, the strain on the optical fiber will change, resulting in the movement of the BGS. The movement of the BGS causes a change in the optical power. By fixing the frequency difference between the probe light and the pump light and recording the change in the optical power, the external vibration signal can be restored. This is also called the slope-assisted Brillouin optical fiber sensing technology.

[0005] The frequency difference between the pump and probe light significantly impacts the measurement results of slope-assisted Brillouin fiber sensing. Traditional methods typically set the frequency difference between the pump and probe light at half the full-width at half-maximum (FWHM) of the BGS. This approach is suitable for a single point, but the BGS may vary for every point along the fiber, resulting in low error tolerance and difficulty in implementing distributed vibration measurement. Some researchers have also introduced multi-slope assisted methods, but this can result in increased frequency sweep points and prolonged acquisition time, hindering the implementation of distributed vibration sensing. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art and to provide a frequency difference selection method for slope-assisted Brillouin optical fiber sensing.

[0007] The purpose of the present invention is to be achieved through the following technical solutions:

[0008] A Brillouin frequency shift extraction method based on a proximity algorithm, characterized by comprising the following steps:

[0009] Step 1: Input the amplitude A of the external vibration signal, the gauge coefficient C of the optical fiber, the minimum detectable power P0 of the detector, and the Brillouin gain spectrum of each position point of the optical fiber, denoted as P(z,f), where position point z = [z1,z2,z3,…,z n ], frequency f=[f1,f2,f3,…,f m ], n and m are positive integers.

[0010] Step 2: When i=1, for position z i , that is, the position point z1, the set V is calculated according to the following formula i , that is, V1:

[0011]

[0012] where f c is the frequency difference between the pump light and the probe light allowed at this point, and its value is within the frequency range of f. min(x,y) means the minimum value of x and y.

[0013] Step 3: Repeat step 2 until all the locations are calculated and a set of V1, V2, V3, ..., V is obtained. n .

[0014] Step 4. For the sets obtained in step 3, find the common intersection V:

[0015]

[0016] Step 5: Arbitrarily select an element from V, which is the frequency difference between the pump light and the probe light. If V is an empty set, it means that under the conditions given in Step 1, there is no appropriate frequency difference available for selection.

[0017] Preferably, in the said Step 1, the Brillouin gain spectrum is selected from one of the following: Lorentzian line shape, Gaussian line shape, Pseudo-Voigt line shape.

[0018] Preferably, in the said Step 2, the specific calculation process of the set V1 is as follows: f c Take values from f1, f2, f3, …, f m in sequence. For each value of f c , calculate whether the following judgment condition is satisfied

[0019]

[0020] If the judgment condition is satisfied, put the current value of f c into the set V1; otherwise, do not put the current value of f c into the set V1.

[0021] Preferably, in the said Step 2, for the first-order, second-order, and third-order partial differential terms in the formula, if the BGS has an analytical solution, use the analytical form of the first derivative of the BGS to replace it; if the BGS does not have or is difficult to obtain an analytical solution, use the difference term to replace it, that is, use the first-order, second-order, and third-order differences to replace the first-order, second-order, and third-order partial differential terms. The difference form can be forward difference, backward difference, central difference, etc.; taking the backward difference as an example, the difference of the first-order partial differential term is:

[0022]

[0023] where Δf is the step size, and P fc+Δf and P fc represent the BGS power at frequencies f c +Δf and f c respectively;

[0024] The difference of the nth-order partial differential term can be expressed as

[0025]

[0026] In the above formula, the (n - 1)th-order partial differential term can be replaced by the (n - 1)th-order difference term. Therefore, the second-order difference can be expressed by the first-order difference, and the first-order difference can be expressed by the second-order difference.

[0027] The beneficial effects of the present invention are as follows: The frequency difference between the pump light and the probe light is scanned comprehensively within the selectable range, avoiding the problems of low error tolerance and difficulty in realizing distributed vibration measurement caused by setting the frequency difference between the pump light and the probe light to half of the full width at half maximum of the BGS. Using a single value as the frequency difference between the pump light and the probe light avoids the problems of increased frequency sweep points and long acquisition time caused by the multi-ramp assist method. Description of the Drawings

[0028] Figure 1 is a flowchart of the method of the present invention. Detailed Embodiments

[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0030] As Figure 1 shown, a method for selecting the frequency difference for ramp-assisted Brillouin optical fiber sensing includes the following steps:

[0031] Step 1, input the amplitude A of the external vibration signal, the strain coefficient C of the optical fiber, the minimum detectable power P0 of the detector, and the BGS at each position point of the optical fiber, denoted as P(z, f), where z = [z1, z2, z3,..., z n .

[0032] For Brillouin optical fiber sensing, set the external amplitude to 200 με, the strain coefficient of the optical fiber to 20 με / MHz, the minimum detectable power of the detector to 0.01 mW, and the BGS at each position point of the optical fiber satisfies the Lorentz curve distribution, that is:

[0033]

[0034] where f B is the Brillouin gain bandwidth, generally 50 MHz can be taken, f0 is the Brillouin frequency shift, here taken as 10850 MHz. For simplicity, it is assumed that the BGS at each position point adopts the same distribution.

[0035] Step 2, for the position point z1, calculate the set V1 according to the following formula:

[0036]

[0037] For the solution of the above formula, f c can be discretized first, starting from 10600 MHz, taking values at a step size of 0.1 MHz until 11000 MHz.

[0038] For the first-order, second-order, and third-order partial derivative terms in the above formula, if the BGS has an analytical solution, the analytical form of the first derivative of the BGS can be used to replace them. For example, the first derivative form of the Lorentz curve distribution is:

[0039]

[0040] The analytical solutions of the second-order and third-order derivatives can be obtained similarly.

[0041] Substitute the value of f c into f in the above formula, and the terms of the formula described in step 2 can be obtained.

[0042] If the BGS does not have or is difficult to obtain an analytical solution, difference terms can be used to replace them, that is, first-order, second-order, and third-order differences are used to replace the first-order, second-order, and third-order partial derivative terms. The difference form can be forward difference, backward difference, central difference, etc. Taking the backward difference as an example, the difference of the first-order partial derivative term is:

[0043]

[0044] where Δf is the step size, which can be taken as 0.1 MHz, and P fc+Δf and P fc represent the BGS power at frequencies f c +Δf and f c respectively.

[0045] The difference of the nth-order partial derivative term can be expressed as

[0046]

[0047] In the above formula, the (n - 1)th-order partial derivative term can be replaced by the (n - 1)th-order difference term. Therefore, the second-order difference can be represented by the first-order difference, and the third-order difference can be represented by the second-order difference.

[0048] Substitute the value of f c into each order of difference terms, and the terms of the formula described in step 2 can be obtained.

[0049] For the solution of step 2, we can get:

[0050] V i =[10680 MHz, 10840 MHz] ∪ [10860 MHz, 11020 MHz] (8)

[0051] Step 3: Repeat step 2 until all position points are calculated, and a set of V1, V2, V3,..., V n is obtained:

[0052] Since it is assumed here that the BGS distributions at each position point are the same, the set obtained in step 3 is as follows:

[0053] V i =[10680MHz, 10840MHz] ∪ [10860MHz, 11020MHz] (8)

[0054] where i = 1, 2,..., n.

[0055] Step 4, for the set obtained in step 3, find the common intersection V

[0056]

[0057] At this time, it is:

[0058] V = [10680MHz, 10840MHz] ∪ [10860MHz, 11020MHz] (9)

[0059] For the case where the BGS distributions are different, the sets obtained in step 3 are also different, and the intersection at this time is different.

[0060] Step 5, select an element from V, which is the frequency difference between the pump light and the probe light. If V is an empty set, it means that under the conditions given in step 1, there is no suitable frequency difference to choose from.

[0061] Since the set V is non-empty, an element can be arbitrarily selected as the frequency difference between the pump light and the probe light. For example, 10800MHz can be used.

[0062] The present invention performs a full scan of the selectable range of the frequency difference between the pump light and the probe light, avoiding the problems of low error tolerance and difficulty in realizing distributed vibration measurement caused by setting the frequency difference between the pump light and the probe light to half of the full width at half maximum of the BGS; using a single value as the frequency difference between the pump light and the probe light, avoiding the problems of increased frequency sweep points and long acquisition time caused by the multi-ramp assisted method.

[0063] The above embodiments merely illustrate the principles and effects of the present invention, rather than limiting the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical idea disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A frequency difference selection method for slope-assisted Brillouin optical fiber sensing, characterized in that, It includes the following steps: Step 1, input the amplitude A of the external vibration signal, the strain coefficient C of the optical fiber, the minimum detectable power P0 of the detector, and the Brillouin gain spectrum of each position point of the optical fiber, denoted as P(z,f), where the position point z = [z1, z2, z3, …, z n , the frequency f = [f1, f2, f3, …, f m , and n and m are positive integers; Step 2, when i = 1, for position point z i , that is, position point z1, calculate the set V according to the following formula i , that is, V1 where f c is the allowable frequency difference between the pump light and the probe light at this position point, and its value is within the range of the frequency f. min(x, y) represents taking the minimum value of x and y; Step 3, repeat Step 2 until all position points are calculated, obtaining a set of V1, V2, V3, …, V n ; Step 4: For the set obtained in Step 3, find the common intersection V: Step 5: Arbitrarily select an element from V, which is the frequency difference between the pump light and the probe light. If V is an empty set, it means that under the conditions given in Step 1, there is no appropriate frequency difference available for selection.

2. The frequency difference selection method for slope-assisted Brillouin optical fiber sensing according to claim 1, characterized in that In the said Step 1, the Brillouin gain spectrum selects one of the following: Lorentzian line shape, Gaussian line shape, Pseudo-Voigt line shape.

3. A frequency difference selection method for slope-assisted Brillouin optical fiber sensing according to claim 1, characterized in that In step 2, the specific calculation process of set V1 is: f c Take values from f1, f2, f3, …, f m in sequence. For each value of f c , calculate whether the following judgment condition is satisfied If the determination condition is satisfied, the current value of f c is placed into the set V1; otherwise, the current value of f c is not placed into the set V1.

4. The frequency difference selection method for slope-assisted Brillouin optical fiber sensing according to claim 1, wherein In the said Step 2, for the first-order, second-order, and third-order partial differential terms in the formula, if the BGS has an analytical solution, use the analytical form of the first derivative of the BGS to replace it; if the BGS does not have or is difficult to obtain an analytical solution, use the difference term to replace it, that is, use the first-order, second-order, and third-order differences to replace the first-order, second-order, and third-order partial differential terms. The difference of the first-order partial differential term is: where Δf is the step size, and represent the BGS power at frequencies f c +Δf and f c respectively; The difference of the nth-order partial differential term is expressed as In the above formula, the (n - 1)th-order partial differential term is replaced by the (n - 1)th-order difference term. Therefore, the second-order difference is represented by the first-order difference, and the third-order difference is represented by the second-order difference.

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