A Wideband Quasi-Distributed Fiber Optic Acoustic Wave Sensing Method and System Based on Sinusoidal Frequency Sweep

Abstract

This invention relates to the field of fiber optic sensing measurement, specifically to a high-bandwidth fiber optic acoustic wave sensing method and system based on sinusoidal frequency sweeping. The method includes the following steps: first, using a sinusoidally swept laser as the probe light; then, using coherent detection to acquire the reflection information of a weakly reflected fiber optic array (FBG); next, digitally mixing conjugate signals of different FBG reflection signals in the digital domain, and filtering out the reflection signal of the FBG using a digital filter; finally, calculating the phase of the filtered signal to obtain the optical path change from the initial position of the fiber to the FBG, thereby demodulating the disturbance experienced by the fiber. This invention addresses the technical problem that the sensing bandwidth of current quasi-distributed fiber optic sensing systems is limited by the fiber length.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing and measurement, specifically to a broadband quasi-distributed fiber optic acoustic wave sensing method and system based on sinusoidal frequency sweep. Background Technology

[0002] Fiber optic sensing systems are now widely used in geological exploration, structural health monitoring, and fiber optic hydrophones. Traditional distributed / quasi-distributed acoustic sensing systems based on Rayleigh scattering or weakly reflected FBG arrays have a natural mutual constraint between their measurement repeatability (twice the sensing bandwidth) and fiber length: f scan The value ≤c / 2nL means that, within a given receiver bandwidth, both measurement repetition rate and sensing distance cannot be simultaneously achieved. For example, when the sensing distance is 100km, the system's measurement repetition rate will not exceed 1kHz.

[0003] Currently, various multiplexing techniques are commonly used to improve measurement repetition rate, such as frequency division multiplexing (FDM) and positive / negative frequency multiplexing. FDM involves sequentially injecting several probe signals of different frequencies into the sensing fiber, achieving an N-fold increase in measurement repetition rate, which also increases the bandwidth of the scattered signal by N times. However, due to the limited bandwidth of the system receiver, the improvement factor of this method is limited. Positive / negative frequency multiplexing can double the measurement repetition rate / sensing bandwidth while increasing the receiver bandwidth, but the improvement of 2 times is still relatively limited. Summary of the Invention

[0004] Based on the above problems, this invention provides a broadband quasi-distributed fiber optic acoustic wave sensing method and system based on sinusoidal frequency sweep, which solves the technical problems of low measurement repeatability and insufficient sensing bandwidth in existing quasi-distributed acoustic wave sensing systems.

[0005] To solve the above technical problems, the present invention adopts the following technical solution: The first aspect of the present invention provides a broadband quasi-distributed fiber optic acoustic wave sensing method and system based on sinusoidal frequency sweep, the method comprising the following steps:

[0006] Step 1: The laser emits continuous sinusoidal sweeping light as the system's probe light;

[0007] Step 2: Divide the probe light into two parts. One part is injected into the weakly reflected FBG array through a circulator. The other part is used as the local oscillator light and the reflected light from the FBG array for coherent detection. The signal obtained from the coherent detection is then converted into the digital domain.

[0008] Step 3: In the digital domain, use the conjugate signal of the i-th FBG reflection signal to perform digital mixing. Only the entire energy of the i-th FBG reflection signal is transferred to the low frequency. Then, the reflection signal of the i-th FBG can be filtered out by a digital low-pass filter.

[0009] Step 4: Calculate the phase of the filtered signal to obtain the optical path change from the initial position of the fiber to the i-th FBG.

[0010] Step 5: Repeat steps 3 and 4 for all FBGs in the weak reflection FBG array to obtain the optical path change from the starting position of the fiber to all FBGs. By performing differential calculations before and after, the optical path change at different positions can be obtained, thereby realizing quasi-distributed sensing.

[0011] Furthermore, in step one, the sinusoidal sweeping continuous light is the laser whose frequency is modulated by a sinusoidal signal using a direct-modulated laser, or the laser whose frequency is modulated by an external modulator to convert a single-frequency laser into a sinusoidal sweeping laser.

[0012] Furthermore, in step two, coherent detection is achieved through a 90° optical mixer, a 2×4 coupler, or a 2×2 coupler.

[0013] Furthermore, in step three, the expression for the conjugate signal of the i-th FBG reflection signal is:

[0014]

[0015] Where ΔF is the range of the sinusoidal sweep signal, ω r τ is the angular frequency of the sinusoidal sweep. i Let be the delay of the i-th FBG.

[0016] Furthermore, in step three, the digital low-pass filter is an FIR filter or an IIR filter.

[0017] Furthermore, in step four, the process of determining the phase includes using inverse trigonometric functions to obtain the radians and the phase unwinding process.

[0018] Furthermore, in step five, the number of points for differential calculation is determined by the interval of the FBG.

[0019] A second aspect of this invention provides a broadband quasi-distributed fiber optic acoustic wave sensing system based on sinusoidal frequency sweep, applied to the aforementioned fiber optic acoustic wave sensing method based on sinusoidal frequency sweep. The system includes a laser module, a modulator module, a first coupler, an arbitrary waveform generator, a circulator, a second coupler, and a frequency shifter. The continuous laser output from the laser module is connected to the modulator for modulation. The modulated signal enters the first coupler and is divided into two parts. One part is injected into a weakly reflected FBG array through the circulator; the other part is connected to the second coupler as a local oscillator signal for coherent detection. The reflected light from the weakly reflected FBG array is connected to the second coupler through the circulator as signal light for coherent detection. The detection result is acquired by a receiver and converted into a digital signal.

[0020] Compared with the prior art, the beneficial effects of the present invention are: the present invention breaks through the inherent limitations of the measurement repetition rate / sensing bandwidth of the traditional quasi-distributed optical fiber sensing system, which is limited by the optical fiber sensing distance and receiver bandwidth, and realizes wideband quasi-distributed acoustic wave sensing. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the device structure in Embodiment 1;

[0022] Figure 2 This is a flowchart of Embodiment 1;

[0023] The attached diagram shows: laser module 1, modulator module 2, first coupler 3, arbitrary waveform generator 4, circulator 5, second coupler 6, and frequency shifter 7. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings. Embodiments of the present invention include, but are not limited to, the following examples.

[0025] Example 1

[0026] like Figure 1 As shown, this embodiment provides a broadband quasi-distributed fiber optic acoustic wave sensing system based on sinusoidal frequency sweep. The measurement system structure includes: a laser module 1, a modulator module 2, a first coupler 3, a circulator 4, a weakly inverted FBG array 5, a second coupler 6, and a receiver 7.

[0027] Specifically, in embodiment 1, the continuous laser output from laser module 1 is connected to modulator module 2 and modulated into a continuous laser with a sinusoidal frequency. The modulated signal enters the first coupler 3 and is split into two parts. One part is injected into the weakly reflected FBG array 5 through circulator 4; the other part is connected to the second coupler 6 as the local oscillator signal for coherent detection. The reflected light from the weakly reflected FBG array is connected to the second coupler 6 through circulator 4 as signal light for coherent detection. The detection result is collected by the receiver and converted into a digital signal.

[0028] Example 2

[0029] Based on the aforementioned broadband quasi-distributed fiber optic acoustic wave sensing system based on sinusoidal frequency sweep, this invention proposes a broadband quasi-distributed acoustic wave sensing method based on sinusoidal frequency sweep, the method comprising the following steps:

[0030] Step 1: After being modulated by the modulator, the laser emits a continuous sinusoidal sweeping light as the system's probe light, the expression of which is:

[0031]

[0032] Where A0 is the signal amplitude, f0 is the initial frequency, ΔF is the range of the sinusoidal sweep signal, and ω r It is the angular frequency of the sinusoidal sweep.

[0033] Furthermore, in step one, the continuous light with sinusoidal frequency sweep can be obtained by modulating the frequency of a directly modulated laser with a sinusoidal signal; or by using the frequency modulation of an external modulator to convert a single-frequency laser into a laser with sinusoidal frequency sweep.

[0034] Step 2: Divide the probe light into two parts. One part is injected into the weakly reflected FBG array through a circulator, and the other part is used as the local oscillator light and the reflected light of the FBG array for coherent detection. The signal obtained from the coherent detection is then converted into the digital domain.

[0035] Specifically, the reflected signal of the i-th FBG can be represented as:

[0036]

[0037] Among them, R i Let τ be the reflectivity of the i-th FBG. i Let be the delay of the i-th FBG.

[0038] After coherent detection with the local oscillator signal, the resulting signal expression is:

[0039]

[0040] Among them, R PD denoted as the detector's response coefficient.

[0041] It can be seen that the frequencies of the coherent detection signals obtained by the FBG at different locations have different amplitudes and phases.

[0042] Furthermore, in step two, coherent detection can be achieved through a 90° optical mixer, a 2×4 coupler, or a 2×2 coupler.

[0043] Step 3: In the digital domain, use the conjugate signal of the i-th FBG reflection signal to perform digital mixing. Only the entire energy of the i-th FBG reflection signal is transferred to the low frequency, while only a very small portion of the energy is transferred to the low frequency at other locations. Then, the reflection signal of the i-th FBG can be filtered out by a digital low-pass filter.

[0044] Additionally, the conjugate signal of the i-th FBG reflected signal is used in the digital domain:

[0045]

[0046] Where, τ i Let be the delay of the i-th FBG.

[0047] The result of digital mixing is as follows:

[0048]

[0049] It can be seen that only the i-th FBG's reflected signal has all its energy transferred to the low frequency, while only a very small portion of the energy is transferred to the low frequency at other locations. Therefore, the reflected signal of the i-th FBG can be filtered out using a digital low-pass filter.

[0050] Furthermore, the digital low-pass filter is an FIR filter or an IIR filter.

[0051] Step 4: Calculate the phase of the filtered signal to obtain the optical path change from the initial position of the fiber to the i-th FBG.

[0052] Furthermore, determining the phase of the filtered signal specifically involves using inverse trigonometric functions to obtain the radians and phase, followed by a dewinding process.

[0053] Step 5: Perform the operations of Step 3 and Step 4 on all FBGs in the weak reflection FBG array to obtain the optical path change from the starting position of the fiber to all FBGs. By performing differential calculations before and after, the optical path change at different positions can be obtained, thereby realizing quasi-distributed sensing.

[0054] Furthermore, the number of points for the difference between the preceding and following steps is determined by the interval of the FBG.

[0055] Using this method, the system's measurement repetition rate is independent of the sensing distance and depends directly on the receiver's sampling rate. High-speed acquisition cards can be used to achieve wide-bandwidth, high-capacity quasi-distributed acoustic wave sensing.

[0056] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A broadband quasi-distributed fiber optic acoustic wave sensing method based on sinusoidal frequency sweep, characterized in that, The method includes the following steps: Step 1: The laser emits continuous sinusoidal sweeping light as the system's probe light; Step 2: Divide the probe light into two parts. One part is injected into the weakly reflected FBG array through a circulator. The other part is used as the local oscillator light and the reflected light from the FBG array for coherent detection. The signal obtained from the coherent detection is then converted into the digital domain. Step 3: Perform digital mixing in the digital domain using the conjugate signal of the i-th FBG reflected signal. Only one FBG's reflected signal will have all its energy transferred to a lower frequency. Then, the reflected signal of the i-th FBG can be filtered out using a digital low-pass filter. The conjugate signal of the i-th FBG reflected signal is expressed as: Where ΔF is the range of the sinusoidal sweep signal, ω r τ is the angular frequency of the sinusoidal sweep. i Let be the delay of the i-th FBG; The result of digital mixing is as follows: In the formula, R PD Here, A0 is the detector's response coefficient, f0 is the signal amplitude, and R is the initial frequency. i Let τ be the reflectivity of the i-th FBG. j Let be the delay of the j-th FBG; Step 4: Calculate the phase of the filtered signal to obtain the optical path change from the initial position of the fiber to the i-th FBG. Step 5: Repeat steps 3 and 4 for all FBGs in the weak reflection FBG array to obtain the optical path change from the starting position of the fiber to all FBGs. By performing differential calculations before and after, the optical path change at different positions can be obtained, thereby realizing quasi-distributed sensing.

2. The fiber optic acoustic wave sensing method based on sinusoidal frequency sweep according to claim 1, characterized in that, In step one, the continuous light with sinusoidal frequency sweep is the laser whose frequency is modulated by a sinusoidal signal using a direct-modulated laser, or the laser whose frequency is modulated by an external modulator to convert a single-frequency laser into a laser with sinusoidal frequency sweep.

3. The fiber optic acoustic wave sensing method based on sinusoidal frequency sweep according to claim 1, characterized in that, In step two, coherent detection is achieved through a 90° optical mixer, a 2×4 coupler, or a 2×2 coupler.

4. The fiber optic acoustic wave sensing method based on sinusoidal frequency sweep according to claim 1, characterized in that, In step three, the digital low-pass filter is an FIR filter or an IIR filter.

5. The fiber optic acoustic wave sensing method based on sinusoidal frequency sweep according to claim 1, characterized in that, In step four, the process of determining the phase includes using inverse trigonometric functions to obtain the radians and the phase unwinding process.

6. The fiber optic acoustic wave sensing method based on sinusoidal frequency sweep according to claim 1, characterized in that, In step five, the number of points for difference between the preceding and following steps is determined by the interval of the FBG.

7. A broadband quasi-distributed fiber optic acoustic wave sensing system based on sinusoidal frequency sweep, applied to the fiber optic acoustic wave sensing method based on sinusoidal frequency sweep as described in any one of claims 1 to 6, characterized in that, It includes a laser module, a modulator module, a first coupler, an arbitrary waveform generator, a circulator, a second coupler, and a frequency shifter; the continuous laser output from the laser module is connected to the modulator for modulation, and the modulated signal enters the first coupler and is divided into two parts; one part is injected into the weakly inverted FBG array through the circulator; Another local oscillator signal is used for coherent detection by the second coupler; the reflected light from the weakly reflected FBG array is connected to the second coupler through a circulator as signal light for coherent detection, and the detection result is collected by the receiver and converted into a digital signal.

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