Fast demodulation method based on improved space phase shift

Abstract

The invention relates to the technical field of optical fiber sensing, in particular to a rapid demodulation method based on improved space phase shift, which comprises the following steps: firstly, designing a space phase shift angle, then constructing a space phase shift matrix, and finally demodulating the amplitude or phase of a signal through a demodulation formula. According to the method, the intermediate frequency matrix is directly utilized to construct the orthogonal matrix, the orthogonal space phase shift angle is designed and introduced, the first space phase shift matrix and the second space phase shift matrix are accurately designed, the relationship among the space phase shift angle, the sampling frequency and the acousto-optic modulator frequency shift is fully utilized, the calculation efficiency of system demodulation is effectively improved, and the system demodulation accuracy is improved. And complex frequency mixing filtering operation and additional hardware equipment required by a traditional demodulation method are reduced, and the system demodulation cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensing, and in particular to a fast demodulation method based on improved spatial phase shift. Background Art

[0002] Large-scale infrastructure construction (such as bridges, tunnels, slopes, and railways) is crucial for national security and social development. Disasters (such as bridge and tunnel collapse, slope and rail damage) can lead to significant loss of life and property. Disasters often occur at random and scattered locations. Traditional disaster monitoring methods, such as video surveillance, suffer from poor real-time positioning, low reliability, and an inability to achieve continuous, strip-like monitoring. However, phase-sensitive optical time-domain reflectometry (φ-OTDR), utilizing coherent detection principles, offers high sensitivity and enables strip-like monitoring, capable of locating vibration events over hundreds of kilometers of sensing fiber. Consequently, φ-OTDR is attracting increasing attention in a variety of fields, including disaster monitoring and intrusion detection.

[0003] φ-OTDRs typically locate events by measuring the changes between successive Rayleigh backscatter traces on a sensing fiber to detect the presence of vibrations. However, due to the long distances of infrastructure such as bridges, tunnels, and railways, the amount of data a φ-OTDR must collect and process in real time increases significantly. Traditional demodulation methods, such as quadrature demodulation, Hilbert transform demodulation, and three-port coupler demodulation, require mixing and filtering the electrical signals output by the balanced detector, or incur additional hardware costs, to demodulate the amplitude or phase of Rayleigh backscatter at each spatial sampling point on the sensing fiber. As the amount of data processed increases, the time required for traditional φ-OTDR demodulation increases exponentially, significantly increasing the φ-OTDR's real-time response time. In the event of an incident (such as a bridge or tunnel collapse or railway track damage), this long response time prevents a quick and accurate alarm from being issued, resulting in significant losses and potentially life-threatening consequences. Summary of the Invention

[0004] The purpose of the present invention is to provide a fast demodulation method based on improved spatial phase shift, introduce orthogonal spatial phase shift angles, and then construct a spatial phase shift matrix to complete amplitude and phase demodulation, thereby solving the problem that traditional demodulation algorithms require additional hardware costs and a long time to demodulate data.

[0005] To achieve the above object, the present invention provides a fast demodulation method based on improved spatial phase shift, comprising the following steps:

[0006] Step 1: Reorganize the continuous one-dimensional intermediate frequency vector Y output by the balanced detector in the phase-sensitive time domain reflectometer φ-OTDR into an intermediate frequency matrix D[M,N] based on the number of sensing fiber vibration sampling points N and the number of pulses M of collected backscattered Rayleigh light;

[0007] Step 2: Calculate the spatial phase shift angle θ using the sampling frequency of the acquisition card and the frequency shift of the AOM;

[0008] Step 3: Design the first orthogonal spatial phase shift angle ω1 and the second orthogonal spatial phase shift angle ω2;

[0009] Step 4: Construct the first spatial phase shift matrix;

[0010] Step 5: Construct the second spatial phase shift matrix;

[0011] Step 6: Calculate the backscattered Rayleigh signal amplitude matrix using the first spatial phase shift matrix and the second spatial phase shift matrix according to the amplitude demodulation formula;

[0012] Step 7: According to the phase demodulation formula, the backscattered Rayleigh scattering signal phase matrix is ​​calculated using the first spatial phase shift matrix and the second spatial phase shift matrix.

[0013] Optionally, the intermediate frequency matrix D[M,N] in step 1 is expressed as follows:

[0014]

[0015] Among them, d m,n It represents the intensity of the nth sampling point on the sensing fiber under the mth sampling pulse, where m = 1, 2, …, M is the number of detection pulses; n = 1, 2, …, N, where N is the number of vibration sampling points on the sensing fiber, and N = L / δ, where L is the fiber length and δ is the sampling interval.

[0016] Optionally, the expression of the spatial phase shift angle θ in step 2 is as follows:

[0017] θ=2πf b / f s

[0018] Among them, f b is the frequency shift of the acousto-optic modulator, f s is the sampling frequency of the acquisition card, f s ≥2f b .

[0019] Optionally, the first orthogonal spatial phase shift angle ω1 designed in step 3 is:

[0020] ω1=1-cosθ

[0021] The second orthogonal spatial phase shift angle ω2 is:

[0022] ω2=sinθ。

[0023] Optionally, the execution process of step 4 is specifically to perform a difference operation on the n-1th and n+1th columns in the intermediate frequency matrix D[M,N] to generate a first intermediate frequency difference matrix, and multiply it by the designed spatial phase shift angle to generate a first spatial phase shift matrix;

[0024] The element in the i-th row and j-th column of the first intermediate frequency difference matrix, that is, the first intermediate frequency difference matrix Δd1(i,j) at the j-th fiber sampling point at the i-th pulse sampling time, is:

[0025] Δd1(i,j)=d m，n-1 -d m,n+1

[0026] The corresponding first spatial phase shift matrix for:

[0027]

[0028] Among them, i=1,2,…,M, j=3,4,…,N-1.

[0029] Optionally, the execution process of step 5 is specifically to perform a difference operation on the n-1th column, the nth column, and the n+1th column in the intermediate frequency matrix D[M,N] to generate a second intermediate frequency difference matrix, and multiply it by the designed spatial phase shift angle to generate a second spatial phase shift matrix;

[0030] The element in the i-th row and j-th column of the second intermediate frequency matrix, that is, the second intermediate frequency matrix Δd2(i,j) at the j-th fiber sampling point at the i-th pulse sampling time, is:

[0031] Δd2(i,j)=2×d m,n -d m,n-1 -d m,n+1

[0032] The second spatial phase shift matrix for:

[0033]

[0034] Among them, i=1,2,…,M, j=3,4,…,N-1.

[0035] Optionally, in step 6, the squares of the first and second spatial phase shift matrices are added and square rooted to generate a backscattered Rayleigh scattering signal amplitude matrix;

[0036] The element in the i-th row and j-th column of the backscattered Rayleigh signal amplitude matrix is ​​the backscattered Rayleigh signal amplitude matrix F at the j-th fiber sampling point at the i-th pulse sampling time.k (i,j) is:

[0037]

[0038] Optionally, in step 7, each element of the first spatial phase shift matrix and the second spatial phase shift matrix are divided correspondingly, and their arc tangent results are calculated to generate a backscattered Rayleigh scattering signal phase matrix;

[0039] The element in the i-th row and j-th column of the backscattered signal phase matrix is ​​the backscattered signal phase matrix P at the j-th fiber sampling point at the i-th pulse sampling time. k (i,j) is:

[0040]

[0041] Here, k is an integer determined by the unwrapping function.

[0042] This invention provides a fast demodulation method based on improved spatial phase shifting. The method first designs the spatial phase shift angle, then constructs a spatial phase shift matrix, and finally uses a demodulation formula to demodulate the signal's amplitude or phase. This method directly constructs an orthogonal matrix using an intermediate frequency matrix, introduces an orthogonal spatial phase shift angle, and accurately designs the first and second spatial phase shift matrices. This method fully utilizes the relationship between the spatial phase shift angle, sampling frequency, and the frequency shift of an acousto-optic modulator (AOM). This method effectively improves the computational efficiency of system demodulation and reduces the complex mixing and filtering operations and additional hardware required by traditional demodulation methods, thereby reducing system demodulation costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0044] Figure 1 The present invention is a flowchart of a fast demodulation method based on improved spatial phase shift.

[0045] Figure 2 It is a schematic diagram of a fast demodulation system based on improved spatial phase shift of the present invention.

[0046] Figure 3 These are the amplitude demodulation positioning result diagrams of the present invention, (a) traditional demodulation method, and (b) improved spatial phase shift demodulation method.

[0047] Figure 4These are the phase demodulation result diagrams of the present invention, (a) improved spatial phase shift demodulation method, (b) phase recovery at 2000m position using the improved spatial phase shift demodulation method, (c) traditional demodulation method, and (d) phase recovery at 2000m position using the traditional demodulation method. DETAILED DESCRIPTION

[0048] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and are not to be construed as limiting the present invention.

[0049] The present invention provides a fast demodulation method based on improved spatial phase shift, comprising the following steps:

[0050] Step 1: Reorganize the continuous one-dimensional intermediate frequency vector Y output by the balanced detector in the phase-sensitive time domain reflectometer φ-OTDR into an intermediate frequency matrix D[M,N] based on the number of sensing fiber vibration sampling points N and the number of pulses M of collected backscattered Rayleigh light;

[0051] Step 2: Calculate the spatial phase shift angle θ using the sampling frequency of the acquisition card and the frequency shift of the AOM;

[0052] Step 3: Design the first orthogonal spatial phase shift angle ω1 and the second orthogonal spatial phase shift angle ω2;

[0053] Step 4: Construct the first spatial phase shift matrix;

[0054] Step 5: Construct the second spatial phase shift matrix;

[0055] Step 6: Calculate the backscattered Rayleigh signal amplitude matrix using the first spatial phase shift matrix and the second spatial phase shift matrix according to the amplitude demodulation formula;

[0056] Step 7: According to the phase demodulation formula, the backscattered Rayleigh scattering signal phase matrix is ​​calculated using the first spatial phase shift matrix and the second spatial phase shift matrix.

[0057] The following is further explained with reference to specific embodiments and execution steps:

[0058] See also Figure 2The phase-sensitive time-domain reflectometer φ-OTDR used in this embodiment consists of a laser, a first coupler, an acousto-optic modulator, a signal generator, a circulator, a sensing fiber, a polarization modulator, a second coupler, a balanced detector, and a host computer. The output of the laser is connected to the input of the first coupler, one output of the first coupler is connected to one input of the acousto-optic modulator, and the other output of the first coupler is connected to one input of the second coupler. The output of the signal generator is connected to the other input of the acousto-optic modulator, the output of the acousto-optic modulator is connected to the first port of the circulator, the second port of the circulator is connected to the sensing fiber, and the third port of the circulator is connected to the other input of the second coupler. The output of the second coupler is connected to the input of the balanced detector, and the output of the balanced detector is connected to the host computer.

[0059] The laser is a high-power, narrow-linewidth laser with a spectral linewidth of 3kHz, a central wavelength of 1550.15nm, a maximum output power of 100mW, and a power instability of less than 1%. The first and second couplers have different splitting ratios: the first coupler has a 90:10 splitting ratio, while the second coupler has a 50:50 splitting ratio. The acousto-optic modulator has a frequency shift of 200MHz. The balanced detector has a bandwidth of 200MHz and a detection responsivity of 0.9A / W.

[0060] The specific steps are as follows:

[0061] Step 1: Reorganize the continuous one-dimensional intermediate frequency vector Y output by the balanced detector in the phase-sensitive time domain reflectometer φ-OTDR into an intermediate frequency matrix D[M,N] according to the number of sensing fiber vibration sampling points N and the number of pulses M of the collected backscattered Rayleigh light.

[0062]

[0063] Among them, d m,n represents the intensity at the nth sampling point on the sensing fiber under the mth sampling pulse, where m = 1, 2, ..., M is the number of detection pulses. n = 1, 2, ..., N, where N is the number of vibration sampling points on the sensing fiber. In this example, the RBS amplitude signals of 200 detection pulses were collected, i.e., M = 200; the spatial sampling interval of the data acquisition card is δ = 0.2 m, and the total length of the sensing fiber is L = 2.8 km, i.e., N = L / δ = 14000.

[0064] Step 2: Calculate the spatial phase shift angle using the sampling frequency of the acquisition card and the frequency shift of the acousto-optic modulator.

[0065] The spatial phase shift angle θ is:

[0066] θ=2πf b / f s

[0067] Among them, f b is the frequency shift of the acousto-optic modulator, f s is the sampling frequency of the acquisition card, f s ≥2f b In this embodiment, the frequency shift of the AOM itself is 200 MHz, and the sampling frequency of the acquisition card is 500 MHz.

[0068] Step 3: Design the first orthogonal spatial phase shift angle ω1 and the second orthogonal spatial phase shift angle ω2.

[0069] The first orthogonal spatial phase shift angle ω1 is:

[0070] ω1=1-cosθ

[0071] The second orthogonal spatial phase shift angle ω2 is:

[0072] ω2=sinθ

[0073] Step 4: Construct the first spatial phase shift matrix. Perform a differential operation on the n-1th and n+1th columns of the intermediate frequency matrix D[M,N] to generate the first intermediate frequency difference matrix. Multiply this matrix by the designed spatial phase shift angle to generate the first spatial phase shift matrix. Based on the first spatial phase shift matrix, the first spatial phase shift intensity value at each optical fiber sampling point can be determined.

[0074] The element in the i-th row and j-th column of the first intermediate frequency difference matrix, that is, the first intermediate frequency difference matrix Δd1(i,j) at the j-th fiber sampling point at the i-th pulse sampling time, is:

[0075] Δd1(i,j)=d m，n-1 -d m,n+1

[0076] Design the first spatial phase shift matrix for:

[0077]

[0078] Among them, i=1,2,…,M, j=3,4,…,N-1.

[0079] Step 5: Construct a second spatial phase shift matrix. Perform a differential operation on the n-1th, nth, and n+1th columns of the intermediate frequency matrix D[M,N] to generate a second intermediate frequency difference matrix. Multiply this matrix by the designed spatial phase shift angle to generate a second spatial phase shift matrix. Based on the second spatial phase shift matrix, the second spatial phase shift intensity value at each optical fiber sampling point can be determined.

[0080] The element in the i-th row and j-th column of the second intermediate frequency matrix, that is, the second intermediate frequency matrix Δd2(i,j) at the j-th fiber sampling point at the i-th pulse sampling time, is:

[0081] Δd2(i,j)=2×d m,n -d m,n-1 -d m,n+1

[0082] Design the second spatial phase shift matrix for:

[0083]

[0084] Among them, i=1,2,…,M, j=3,4,…,N-1.

[0085] Step 6: Calculate the backscattered signal amplitude matrix using the first spatial phase shift matrix and the second spatial phase shift matrix according to the amplitude demodulation formula. Add the square root of each corresponding element of the first spatial phase shift matrix and the second spatial phase shift matrix to generate the backscattered signal amplitude matrix.

[0086] The element in the i-th row and j-th column of the backscattered Rayleigh signal amplitude matrix is ​​the backscattered Rayleigh signal amplitude matrix F at the j-th fiber sampling point at the i-th pulse sampling time. k (i,j) is:

[0087]

[0088] Step 7: Calculate the backscattered signal phase matrix using the first spatial phase shift matrix and the second spatial phase shift matrix according to the phase demodulation formula. Divide each element of the first spatial phase shift matrix and the second spatial phase shift matrix accordingly, and calculate the inverse tangent of the result to generate the backscattered signal phase matrix.

[0089] The element in the i-th row and j-th column of the backscattered signal phase matrix is ​​the backscattered signal phase matrix P at the j-th fiber sampling point at the i-th pulse sampling time. k (i,j) is:

[0090]

[0091] Here, k is an integer determined by the unwrapping function.

[0092] The above disclosure is merely one or more preferred embodiments of the present invention, and certainly cannot be used to limit the scope of the present invention. A person skilled in the art can understand that all or part of the processes of the above embodiments and equivalent changes made in accordance with the claims of the present invention still fall within the scope of the invention.

Claims

1. A fast demodulation method based on improved spatial phase shift, characterized in that: The following steps are involved: Step 1: Reorganize the continuous one-dimensional intermediate frequency vector Y output by the balanced detector in the phase-sensitive time domain reflectometer φ-OTDR into an intermediate frequency matrix D[M,N] based on the number of sensing fiber vibration sampling points N and the number of pulses M of collected backscattered Rayleigh light; Step 2: Calculate the spatial phase shift angle θ using the sampling frequency of the acquisition card and the frequency shift of the AOM; Step 3: Design the first orthogonal spatial phase shift angle ω1 and the second orthogonal spatial phase shift angle ω2; Step 4: Construct the first spatial phase shift matrix; Step 5: Construct the second spatial phase shift matrix; Step 6: Calculate the backscattered Rayleigh signal amplitude matrix using the first spatial phase shift matrix and the second spatial phase shift matrix according to the amplitude demodulation formula; Step 7: According to the phase demodulation formula, the backscattered Rayleigh scattering signal phase matrix is ​​calculated using the first spatial phase shift matrix and the second spatial phase shift matrix.

2. The fast demodulation method based on improved spatial phase shift according to claim 1, wherein: The intermediate frequency matrix D[M,N] in step 1 is expressed as follows: Among them, d m,n It represents the intensity of the nth sampling point on the sensing fiber under the mth sampling pulse, where m = 1, 2, ..., M is the number of detection pulses; and n = 1, 2, ..., N, where N is the number of vibration sampling points on the sensing fiber.

3. The fast demodulation method based on improved spatial phase shift according to claim 2, wherein: The expression for the spatial phase shift angle θ in step 2 is as follows: θ=2πf b / f s Among them, f b is the frequency shift of the acousto-optic modulator, f s is the sampling frequency of the acquisition card, f s ≥2f b .

4. The fast demodulation method based on improved spatial phase shift according to claim 3, wherein: The first orthogonal spatial phase shift angle ω1 designed in step 3 is: ω1=1-cosθ The second orthogonal spatial phase shift angle ω2 is: ω2=sinθ。 5. The fast demodulation method based on improved spatial phase shift according to claim 4, characterized in that: The execution process of step 4 is specifically to perform a difference operation on the n-1th and n+1th columns in the intermediate frequency matrix D[M,N] to generate a first intermediate frequency difference matrix, and multiply it by the designed spatial phase shift angle to generate a first spatial phase shift matrix; The element in the i-th row and j-th column of the first intermediate frequency difference matrix, that is, the first intermediate frequency difference matrix Δd1(i,j) at the j-th fiber sampling point at the i-th pulse sampling time, is: Δd1(i,j)=d m，n-1 -d m,n+1 The corresponding first spatial phase shift matrix for: Among them, i=1,2,…,M, j=3,4,…,N-1.

6. The fast demodulation method based on improved spatial phase shift according to claim 5, characterized in that: The execution process of step 5 is specifically to perform a difference operation on the n-1th column, the nth column, and the n+1th column in the intermediate frequency matrix D[M,N] to generate a second intermediate frequency difference matrix, and multiply it by the designed spatial phase shift angle to generate a second spatial phase shift matrix; The element in the i-th row and j-th column of the second intermediate frequency matrix, that is, the second intermediate frequency matrix Δd2(i,j) at the j-th fiber sampling point at the i-th pulse sampling time, is: Δd2(i,j)=2×d m,n -d m,n-1 -d m,n+1 The second spatial phase shift matrix for: Among them, i=1,2,…,M, j=3,4,…,N-1.

7. The fast demodulation method based on improved spatial phase shift according to claim 6, characterized in that: In step 6, the squares of the first and second spatial phase shift matrices are added and square rooted to generate a backscattered Rayleigh signal amplitude matrix. The element in the i-th row and j-th column of the backscattered Rayleigh signal amplitude matrix is ​​the backscattered Rayleigh signal amplitude matrix F at the j-th fiber sampling point at the i-th pulse sampling time. k (i,j) is:

8. The fast demodulation method based on improved spatial phase shift according to claim 7, characterized in that: In step 7, each element of the first spatial phase shift matrix and the second spatial phase shift matrix is ​​divided correspondingly, and the arc tangent result is calculated to generate the back Rayleigh scattering signal phase matrix; The element in the i-th row and j-th column of the backscattered signal phase matrix is ​​the backscattered signal phase matrix P at the j-th fiber sampling point at the i-th pulse sampling time. k (i,j) is: Here, k is an integer determined by the unwrapping function.

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