A distributed weak grating array sensing system and method with multiple demodulation schemes

Abstract

The present invention provides a distributed weak grating array sensing system with multiple demodulation schemes, including a narrow linewidth laser, an acousto-optic modulator, a phase modulator, a dual pulse signal generator, a signal generator, an erbium-doped fiber amplifier, a circulator, a sensing fiber fused with a weak grating array, a photodetector, and a data acquisition card. The present invention can simultaneously realize multiple demodulation schemes: π / 2 phase difference demodulation, 2π / 3 phase difference demodulation, and π / 4 phase difference demodulation, to meet different application requirements, while the system structure remains unchanged; in addition, the influence of the instability of the half-wave voltage of the phase modulator is eliminated, and the phase difference stability of the photocurrent signal used for demodulation is guaranteed.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber sensing, and in particular relates to a distributed weak grating array sensing system and method with multiple demodulation schemes. Background Art

[0002] Fiber optic sensing technology originated in the 1970s and has developed rapidly with the advent of low-loss optical fiber. After nearly fifty years of research and development, fiber optic sensing technology has become increasingly mature in all aspects and has been applied in many fields such as military, security, and industry.

[0003] Fiber optic sensing technology uses optical fiber as both a transmission medium and a sensing element, and light waves as carriers of external signals. When light waves propagate in the optical fiber, when external environmental factors such as temperature, strain, and vibration change, some of its characteristic parameters (such as frequency, polarization state, phase, etc.) will also change accordingly, that is, backscattered light modulated by external information will be generated, and then the relevant parameters of the light wave can be detected to demodulate the information loaded on the light wave, and then the changes in the environment in which the optical fiber is located can be obtained.

[0004] The principle of distributed weak grating array sensing is: treat the weak grating array embedded in the optical fiber as a group of "weak reflectors" to provide stable, intensity-controllable reflected light signals at the specified position of the optical fiber, use these reflected light signals to replace the spontaneous Rayleigh scattering in the optical fiber, and finally obtain the external disturbance signal by demodulating the phase, power and other changes of the interference light. Compared with the traditional optical fiber sensing system, the reflected light signal obtained by the distributed weak grating array sensing system is more stable and has a higher signal-to-noise ratio.

[0005] There are many demodulation schemes for fiber optic sensing technology, including π / 2 phase difference demodulation, 2π / 3 phase difference demodulation, π / 4 phase difference demodulation, etc. They have their own advantages and disadvantages and are suitable for different practical scenarios. Different demodulation schemes mean different system structures. But in reality, the requirements are flexible and changeable. It is unrealistic to change the system in real time to meet the changing requirements. At present, there is no sensing system and method that can realize multiple demodulation schemes at the same time and is suitable for different scenarios.

[0006] At the same time, π / 2 phase difference demodulation, 2π / 3 phase difference demodulation, and π / 4 phase difference demodulation all have great requirements for the stability of the phase difference. However, due to the instability of the half-wave voltage of the phase modulator, ensuring the stability of the phase difference is also an issue that needs to be urgently addressed. Summary of the invention

[0007] Technical problems to be solved: The technical problems to be solved by the present invention include: (1) There is a corresponding relationship between the current system structure and the demodulation scheme, and the same system structure is difficult to flexibly meet the needs of different scenarios; (2) Due to the instability of the half-wave voltage of the phase modulator, it is difficult to ensure the stability of the phase difference.

[0008] Technical solution:

[0009] A distributed weak grating array sensing system with multiple demodulation schemes, the distributed weak grating array sensing system comprising a narrow linewidth laser, an acousto-optic modulator, a phase modulator, a dual-pulse signal generator, a signal generator, an erbium-doped fiber amplifier, a circulator, a sensing fiber fused with a weak grating array, a photodetector and a data acquisition card;

[0010] The continuous light emitted by the narrow linewidth laser is output to the acousto-optic modulator through a coupler, and the dual-pulse signal generator sends a synchronization signal as a trigger signal of the signal generator, so that the acousto-optic modulator modulates the continuous light under the control of the signal generator to generate a dual-pulse light with a specified pulse width and pulse spacing; the phase modulator phase-modulates the front pulse light of the dual-pulse light so that the phase of a partial area increases linearly;

[0011] The erbium-doped fiber amplifier performs power amplification processing on the double pulse light after phase modulation processing and then outputs it to the circulator, and enters the sensing fiber through the circulator, so that the reflected light of the double pulse light is superimposed and interfered in the grating, and the generated interference light returns to the circulator; the photoelectric detector converts the returned interference light signal into a corresponding electrical signal and outputs it to the data acquisition card, and the data acquisition card performs digital processing on the collected electrical signal and then outputs it to the computer;

[0012] The computer analyzes the time domain signals collected multiple times, locates the disturbance by observing the fluctuation of the reflection peak signal, and obtains the change amount of the position corresponding to N same phase periods from the photocurrent signal at the disturbance position. For the photocurrent signal with a phase difference of Φ, the photocurrent signal with a sampling point difference of ΦN / 2π is selected for demodulation.

[0013] Furthermore, for π / 2 phase difference demodulation, two photocurrent signals with a phase difference of 1 / 4 phase period are selected for demodulation; for 2π / 3 phase difference demodulation, three photocurrent signals with a phase difference of 1 / 3 phase period are selected for demodulation; for π / 4 phase difference demodulation, four photocurrent signals with a phase difference of 1 / 8 phase period are selected for demodulation; N is a positive integer greater than 1.

[0014] Furthermore, for disturbances whose strain value is greater than a preset strain value threshold, 2π / 3 phase difference demodulation is selected, otherwise π / 4 phase difference demodulation is adopted; for disturbances whose required demodulation speed is greater than a preset demodulation speed threshold, π / 2 phase difference demodulation is selected.

[0015] Furthermore, the pulse widths of the two pulses of the double pulse light are both t1, and the pulse spacing between the two pulses satisfies: Where c is the propagation speed of light in vacuum, n is the equivalent refractive index of the optical fiber, and L is the spacing between adjacent gratings in the grating array.

[0016] Furthermore, the period of the control electrical signal emitted by the signal generator is consistent with the period of the electrical signal of the double pulse light, and its voltage range is 0V to 2V. π , V π is the half-wave voltage of the phase modulator.

[0017] Furthermore, assuming that the light field distribution of the reflected light of the front pulse of the double pulse light is E1, the angular frequency of the pulse light is ω, and the initial phase is The electric field amplitude is E r1 ; The light field distribution of the reflected light of the pulse is E2, the angular frequency of the pulse light is ω, and the initial phase is The electric field amplitude is E r2 ; is the phase change caused by the external disturbance at time t, ψ(x) is the phase change introduced by the phase modulator;

[0018] The electric field of the reflected light of the front pulse is expressed as:

[0019]

[0020] The electric field of the reflected light of the post-pulse is expressed as:

[0021]

[0022] The reflected light of the front pulse and the reflected light of the rear pulse are superimposed and interfere with each other. The optical signal of the interference light is expressed as:

[0023]

[0024] In the formula, Ignoring the initial phase difference and DC term of the two beams, the optical signal of the interference light is expressed as:

[0025]

[0026] Wherein, ψ(x) is a function whose value increases linearly with the increase of position x, and there is a period of 2π between the value of the optical signal of the interference light and ψ(x).

[0027] The present invention proposes a distributed weak light grating array sensing method with multiple demodulation schemes, wherein the distributed weak light grating array sensing method demodulates the optical signal at the disturbance position based on the above-mentioned distributed weak light grating array sensing system;

[0028] The distributed weak light array sensing method comprises the following steps:

[0029] S1, outputting the continuous light generated by the narrow linewidth laser to the acousto-optic modulator through a coupler, modulating the continuous light into a first pulse light and a second pulse light, wherein the pulse width of the first pulse light is equal to the pulse width of the second pulse light;

[0030] S2, performing linear phase modulation processing on the first pulse light, and the phase of a part of the area increases linearly;

[0031] S3, after the second pulse light and the first pulse light after phase modulation are compensated by an erbium-doped fiber amplifier, they enter the sensing fiber of the fused grating array through a circulator; a photoelectric detector is used to detect the optical signal of the superposition and interference of the reflected light on adjacent gratings, and the collected electrical signal is digitized by a data acquisition card to collect the time domain signal of the interference light;

[0032] S4, analyzing the time domain signals collected multiple times, and realizing disturbance location by observing the fluctuation size of the reflection peak signal;

[0033] S5, obtaining the number of sampling points N corresponding to one phase cycle from the photocurrent signal at the disturbance position;

[0034] S6, determine the phase difference demodulation mode according to actual needs. For disturbances with a strain value greater than a preset strain value threshold, select 2π / 3 phase difference demodulation, otherwise use π / 4 phase difference demodulation; for disturbances with a required demodulation speed greater than a preset demodulation speed threshold, select π / 2 phase difference demodulation;

[0035] If π / 2 phase difference demodulation is to be performed, two photocurrent signals with a phase difference of 1 / 4 period are selected for demodulation; if 2π / 3 phase difference demodulation is to be performed, three photocurrent signals with a phase difference of 1 / 3 period are selected for demodulation; if π / 4 phase difference demodulation is to be performed, four photocurrent signals with a phase difference of 1 / 8 period are selected for demodulation;

[0036] S7, obtaining multiple groups of demodulation results from the photocurrent signal at the disturbance position, and averaging them to obtain a final demodulation result.

[0037] Beneficial effects:

[0038] First, the distributed weak grating array sensing system and method with multiple demodulation schemes of the present invention can simultaneously implement multiple demodulation schemes: π / 2 phase difference demodulation, 2π / 3 phase difference demodulation, and π / 4 phase difference demodulation without changing the system structure, so as to meet different application requirements.

[0039] Second, the distributed weak grating array sensing system and method of the multi-demodulation scheme of the present invention eliminates the influence of the half-wave voltage instability of the phase modulator and ensures the phase difference stability of the photocurrent signal used for demodulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a structural schematic diagram of a distributed weak grating array sensing system of a multi-demodulation scheme according to an embodiment of the present invention.

[0041] Figure 2 Schematic diagram of the pulse reflection signal of the vibrating position grating.

[0042] Figure 3 This is a schematic diagram of the linear phase-modulated photocurrent signal obtained at the vibrating position grating (3D perspective).

[0043] Figure 4 This is a schematic diagram of the linear phase-modulated photocurrent signal obtained at the vibrating position grating (2D perspective).

[0044] Figure 5 This is a schematic diagram of two extracted photocurrent signals with a phase difference of π / 2.

[0045] Figure 6 This is a schematic diagram of the demodulation results using π / 2 phase difference demodulation.

[0046] Figure 7 Schematic diagram of three extracted photocurrent signals with a phase difference of 2π / 3.

[0047] Figure 8 This is a schematic diagram of the demodulation results using 2π / 3 phase difference demodulation.

[0048] Fig. 9 This is a schematic diagram of the four extracted photocurrent signals with a phase difference of π / 4.

[0049] Fig.10 This is a schematic diagram of the demodulation results using π / 4 phase difference demodulation. DETAILED DESCRIPTION

[0050] The following examples will enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0051] Embodiment 1

[0052] Figure 1 Schematic diagram of the structure of the distributed weak grating array sensing system of the multi-demodulation scheme of the embodiment of the present invention. Figure 1The distributed weak grating array sensing system includes a narrow linewidth laser, an acousto-optic modulator, a phase modulator, a dual pulse signal generator, a signal generator, an erbium-doped fiber amplifier, a circulator, a sensing fiber fused with a weak grating array, a photodetector and a data acquisition card. The functions of each functional component are as follows:

[0053] Laser: Select a narrow linewidth laser and generate continuous light which is output to the acousto-optic modulator through a coupler.

[0054] Acousto-optic modulator: By modulating the continuous light emitted by the laser, double pulse light with a specific period, specific width and specific pulse spacing is generated.

[0055] Phase modulator: By linearly phase modulating the front pulse light in the double pulse light emitted by the acousto-optic modulator, a linear increase in phase in some areas is obtained.

[0056] Double pulse signal generator: Generates double pulse electrical signals with specific period, specific width, specific pulse spacing and specific peak-to-peak value.

[0057] Signal generator: Generates a linearly increasing electrical signal with a specific period, specific peak value, and specific rise time.

[0058] Erbium-doped fiber amplifier: amplifies the power of amplitude-modulated and phase-modulated double-pulse light.

[0059] Circulator: It is used to guide the pulse light after power amplification into the sensing fiber fused with weak grating array, and transmit the interference light on each grating to the photodetector.

[0060] Sensing fiber fused with weak grating array: The reflected light of two pulses on each grating is superimposed to produce interference, sensing disturbance events in the system.

[0061] Photodetector: used to convert optical signals into electrical signals and output them to the acquisition card.

[0062] Acquisition card: used to digitize the acquired analog signals and output them to the processor.

[0063] Computer: Analyze and process the collected data to locate and measure disturbances along the sensing optical fiber fused with the weak grating array.

[0064] An acousto-optic modulator (AOM) is used to modulate the amplitude of the light wave output by the laser to form a double pulse light with a suitable pulse width and pulse spacing; then a phase modulator (PM) is used to linearly phase-modulate the front pulse light in the double pulse light; the double pulse light after amplitude and phase modulation is amplified by an erbium-doped fiber amplifier (EDFA) and enters the grating array through a circulator; the double pulse light generates interference after the reflected light on the adjacent gratings is superimposed, and then the interference light enters the photodetector through the circulator and collects data through the acquisition card; for the collected signal, the disturbance is located through the fluctuation of the reflected signal; from the photocurrent signal at the disturbance position, the number of sampling points corresponding to a phase period (2π) is accurately obtained; according to the needs, π / 2 phase difference demodulation, 2π / 3 phase difference demodulation, and π / 4 phase difference demodulation can be selected to achieve.

[0065] The period, pulse width, pulse spacing and specific peak-to-peak value of the double pulse electrical signal emitted by the double pulse signal generator can be adjusted so that the power of the two pulse signals emitted is equal, the pulse width of the two pulses is t1, and the pulse spacing of the two pulses satisfies: Where c is the propagation speed of light in vacuum, n is the equivalent refractive index of the optical fiber, and L is the spacing between adjacent gratings in the grating array.

[0066] The dual pulse signal generator sends out a synchronization signal as the trigger signal of the signal generator, and then the signal generator sends out a linearly increasing electrical signal to PM to achieve phase modulation. The period, peak value and rise time of the linearly increasing electrical signal sent by the signal generator can be adjusted so that the period of the sent electrical signal is consistent with the period of the dual pulse electrical signal, and the peak value rises from 0V to more than 2V. π , the rise time is slightly less than the pulse width. V π is the half-wave voltage of PM.

[0067] The dual pulse signal generator provides a synchronization signal as a trigger signal of the signal generator.

[0068] Assuming that the double pulse is modulated by an acousto-optic modulator and a phase modulator, the light field distribution of the front pulse reflected light is E1, the angular frequency of the pulse light is ω, and the initial phase is The electric field amplitude is E r1 ; The light field distribution of the reflected light of the pulse is E2, the angular frequency of the pulse light is ω, and the initial phase is The electric field amplitude is E r2 ; is the phase change caused by external disturbance, and ψ(x) is the phase change introduced by the phase modulator.

[0069] The electric field of the reflected light of the front pulse is expressed as:

[0070]

[0071] The electric field of the reflected light of the post-pulse is expressed as:

[0072]

[0073] The reflected light of the front pulse and the reflected light of the rear pulse are superimposed and interfere with each other. The optical signal can be expressed as:

[0074]

[0075] in, Ignoring the initial phase difference and DC term between the two beams, the above formula can be expressed as:

[0076]

[0077] Because we apply linear phase modulation, ψ(x) is a function whose value increases linearly as the position X increases. As for the photocurrent signal I, due to its trigonometric form, there is a 2π period between its value and ψ(x), so we can obtain the change in position X (number of sampling points) corresponding to the 2π phase modulation based on the value of the photocurrent signal at different positions. Based on this standard, we can extract photocurrent signals with various phase differences from the photocurrent signals at different positions, thereby realizing a variety of demodulation schemes.

[0078] In addition, since we obtain the change in position X (number of sampling points) corresponding to the 2π phase modulation in real time, the problem caused by the instability of the half-wave voltage of the phase modulator is solved.

[0079] Use Figure 1 The experimental device shown in the figure is that a narrow linewidth laser emits a continuous laser which enters an acousto-optic modulator with a modulation frequency of 150MHz, and modulates to obtain two pulses of light with a pulse width of 300ns and a pulse interval of 500ns; a dual-pulse signal generator gives a dual-pulse electrical signal to the acousto-optic modulator to achieve amplitude modulation; the amplitude-modulated dual pulses enter the phase modulator, and the modulation obtains a linear increase in phase in some areas; the dual-pulse signal generator sends a synchronization signal to the signal generator as a trigger signal, and the signal generator gives a linearly increasing electrical signal with a rise time of 200ns and a peak value rising from 0V to 10V to the phase modulator to achieve phase modulation; the phase-modulated dual pulses enter the erbium-doped fiber amplifier for amplification and are sent to the grating array by a circulator for sensing; the returned interference light is received by the detector through the circulator, collected by the data acquisition card, and sent to the computer for disturbance positioning and measurement.

[0080] The vibration source at 400 meters near the grating applies a sinusoidal signal with a frequency of 100Hz and a voltage of 10V. By observing the amplitude fluctuation of the reflection peak, the position of the disturbance can be located, such as Figure 2 As shown, the amplitude fluctuation of the reflection peak at the grating at the disturbance position is significantly greater than that in other areas.

[0081] like Figure 3 , 4 As shown in the figure, it is the photocurrent signal at different positions of the disturbance position grating. From its 3D and 2D perspectives, we can see that the photocurrent signal changes periodically with the change of position, and its periodicity corresponds to the phase modulation of 2π. From the 2D graph, we can calibrate that the number of points corresponding to the phase modulation of 2π is 38.

[0082] Based on this standard, we can get: the number of points corresponding to the photocurrent signal with a phase difference of π / 2 is 19, the number of points corresponding to the photocurrent signal with a phase difference of 2π / 3 is about 25, and the number of points corresponding to the photocurrent signal with a phase difference of π / 4 is about 9. Thus, we can get Figure 5 , Figure 7 , Fig. 9 The two photocurrent signals shown have a phase difference of π / 2, three photocurrent signals have a phase difference of 2π / 3, and four photocurrent signals have a phase difference of π / 4. After using π / 2 phase difference demodulation, 2π / 3 phase difference demodulation, and π / 4 phase difference demodulation on these photocurrent signals, we can get the following: Figure 6 , Figure 8 , Fig.10 The demodulation results are shown in Figure 1. A total of 0.050 seconds of signal was collected, and the frequency of the externally applied signal was 100 Hz, which is consistent with the actual applied frequency information in the experiment.

[0083] Since 2π / 3 phase difference demodulation does not require phase unwrapping, it can be used to measure larger strains, but its signal-to-noise ratio is low due to the operations of integration and differentiation; while π / 2 phase difference demodulation and π / 4 phase difference demodulation have higher signal-to-noise ratios, but a smaller strain measurement range. π / 4 phase difference demodulation uses more photocurrent and is more stable than π / 2 phase difference demodulation. Therefore, on the whole, if you need to sense a large strain, it is recommended to use 2π / 3 phase difference demodulation, and if you need to sense a normal strain with a high signal-to-noise ratio, it is recommended to use π / 4 phase difference demodulation.

[0084] Embodiment 2

[0085] In another embodiment, a distributed weak grating array sensing method based on direct detection is provided, comprising the following steps:

[0086] Step 1: Generate continuous light.

[0087] Step 2: modulate the continuous light into a first pulse light and a second pulse light, wherein the pulse width of the first pulse light is equal to the pulse width of the second pulse light.

[0088] Step three, performing linear phase modulation processing on one of the first pulse light and the second pulse light, and the phase of a partial area increases linearly.

[0089] Step 4: After the first pulse light and the second pulse light are compensated by EDFA, they enter the sensing optical fiber of the fused grating array through the circulator. The photodetector detects the optical signal of the superposition and interference of the reflected light on adjacent gratings. The data acquisition card digitizes the collected electrical signal and outputs it to the computer.

[0090] Step 5: Display the time domain signals collected multiple times together and locate the disturbance by observing the fluctuation of the reflection peak signal.

[0091] Step 7: From the photocurrent signal at the disturbance position, accurately obtain the number of sampling points corresponding to a phase period (2π). Assume that the number of sampling points corresponding to a phase period (2π) obtained in step 7 is N. Then, when selecting the photocurrent signal with a phase difference of Φ, it is necessary to select the photocurrent signal with a sampling point difference of ΦN / 2π.

[0092] Step 8: The demodulation scheme in step 8 can be selected according to the actual needs: if a large strain needs to be sensed, 2π / 3 phase difference demodulation is usually used; if a normal strain needs to be sensed with a high signal-to-noise ratio, π / 4 phase difference demodulation is usually used; if the demodulation speed needs to be increased, π / 2 phase difference demodulation is usually used. If π / 2 phase difference demodulation is to be achieved, two photocurrent signals with a phase difference of π / 2 are selected for demodulation; if 2π / 3 phase difference demodulation is to be achieved, three photocurrent signals with a phase difference of 2π / 3 are selected for demodulation; if π / 4 phase difference demodulation is to be achieved, four photocurrent signals with a phase difference of π / 4 are selected for demodulation.

[0093] Step nine, obtaining multiple groups of demodulation results from the photocurrent signal at the disturbance position, and averaging them to obtain the final demodulation result.

[0094] Preferably, the spacing of the grating arrays in the sensing optical fiber fused with the grating array remains substantially consistent.

[0095] The distributed weak grating array sensing system and method with multiple demodulation schemes of the present embodiment can simultaneously implement multiple demodulation schemes: π / 2 phase difference demodulation, 2π / 3 phase difference demodulation, and π / 4 phase difference demodulation, without changing the system structure, to meet different application requirements; at the same time, it eliminates the influence of the half-wave voltage instability of the phase modulator, and ensures the phase difference stability of the photocurrent signal used for demodulation.

[0096] The above embodiments are only for illustrating the technical idea of ​​the present invention, and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. A distributed weak grating array sensing system with multiple demodulation schemes, characterized in that: The distributed weak grating array sensing system includes a narrow line width laser, an acousto-optic modulator, a phase modulator, a dual pulse signal generator, a signal generator, an erbium-doped fiber amplifier, a circulator, a sensing fiber fused with a weak grating array, a photodetector and a data acquisition card; The continuous light emitted by the narrow linewidth laser is output to the acousto-optic modulator through a coupler, and the dual-pulse signal generator sends a synchronization signal as a trigger signal of the signal generator, so that the acousto-optic modulator modulates the continuous light under the control of the signal generator to generate a dual-pulse light with a specified pulse width and pulse spacing; the phase modulator phase-modulates the front pulse light of the dual-pulse light so that the phase of a partial area increases linearly; The erbium-doped fiber amplifier performs power amplification processing on the double pulse light after phase modulation processing and outputs it to the circulator, and enters the sensing fiber through the circulator, so that the reflected light of the double pulse light is superimposed and interfered in the grating, and the generated interference light returns to the circulator; The photoelectric detector converts the returned interference light signal into a corresponding electrical signal and outputs it to the data acquisition card, and the data acquisition card digitally processes the collected electrical signal and outputs it to the computer; The computer analyzes the time domain signals collected multiple times, locates the disturbance by observing the fluctuation of the reflection peak signal, and obtains the change amount of the position corresponding to N same phase periods from the photocurrent signal at the disturbance position, and for the photocurrent signal with a phase difference of Φ, selects the photocurrent signal with a sampling point difference of ΦN / 2π for demodulation; N is a positive integer greater than 1.

2. The distributed weak grating array sensing system with multiple demodulation schemes according to claim 1 is characterized in that: For π / 2 phase difference demodulation, two photocurrent signals with a phase difference of 1 / 4 period are selected for demodulation; for 2π / 3 phase difference demodulation, three photocurrent signals with a phase difference of 1 / 3 period are selected for demodulation; For π / 4 phase difference demodulation, four photocurrent signals with a phase difference of 1 / 8 period are selected for demodulation.

3. The distributed weak grating array sensing system with multiple demodulation schemes according to claim 2 is characterized in that: For disturbances whose strain is greater than the preset strain threshold, 2π / 3 phase difference demodulation is selected, otherwise π / 4 phase difference demodulation is used; for disturbances whose required demodulation speed is greater than the preset demodulation speed threshold, π / 2 phase difference demodulation is selected.

4. The distributed weak grating array sensing system with multiple demodulation schemes according to claim 1, characterized in that: The pulse widths of the two pulses of the double pulse light are both t1, and the pulse spacing between the two pulses satisfies: Where c is the propagation speed of light in vacuum, n is the equivalent refractive index of the optical fiber, and L is the spacing between adjacent gratings in the grating array.

5. The distributed weak grating array sensing system with multiple demodulation schemes according to claim 1, characterized in that: The period of the control electric signal emitted by the signal generator is consistent with the period of the electric signal of the double pulse light, and its voltage range is 0V to 2V π , V π is the half-wave voltage of the phase modulator.

6. The distributed weak grating array sensing system with multiple demodulation schemes according to claim 1, characterized in that: Assume that the light field distribution of the reflected light of the front pulse of the double pulse light is E1, the angular frequency of the pulse light is ω, and the initial phase is The electric field amplitude is E r1 ; The light field distribution of the reflected light of the pulse is E2, the angular frequency of the pulse light is ω, and the initial phase is The electric field amplitude is E r2 ; is the phase change caused by the external disturbance at time t, ψ(x) is the phase change introduced by the phase modulator; The electric field of the reflected light of the front pulse is expressed as: The electric field of the reflected light of the post-pulse is expressed as: The reflected light of the front pulse and the reflected light of the rear pulse are superimposed and interfere with each other. The optical signal of the interference light is expressed as: In the formula, Ignoring the initial phase difference and DC term of the two beams, the optical signal of the interference light is expressed as: Wherein, ψ(x) is a function whose value increases linearly with the increase of position x, and there is a period of 2π between the value of the optical signal of the interference light and ψ(x).

7. A distributed weak grating array sensing method with multiple demodulation schemes, characterized in that: The distributed weak light grating array sensing method demodulates the optical signal at the disturbance position based on the distributed weak light grating array sensing system described in any one of claims 1 to 6; The distributed weak light array sensing method comprises the following steps: S1, outputting the continuous light generated by the narrow linewidth laser to the acousto-optic modulator through a coupler, modulating the continuous light into a first pulse light and a second pulse light, wherein the pulse width of the first pulse light is equal to the pulse width of the second pulse light; S2, performing linear phase modulation processing on the first pulse light, and the phase of a part of the area increases linearly; S3, after the second pulse light and the first pulse light after phase modulation are compensated by an erbium-doped fiber amplifier, they enter the sensing fiber of the fused grating array through a circulator; a photoelectric detector is used to detect the optical signal of the superposition and interference of the reflected light on adjacent gratings, and the collected electrical signal is digitized by a data acquisition card to collect the time domain signal of the interference light; S4, analyzing the time domain signals collected multiple times, and realizing disturbance location by observing the fluctuation size of the reflection peak signal; S5, obtaining the number of sampling points N corresponding to one phase cycle from the photocurrent signal at the disturbance position; S6, determine the phase difference demodulation mode according to actual needs. For disturbances with a strain value greater than a preset strain value threshold, select 2π / 3 phase difference demodulation, otherwise use π / 4 phase difference demodulation; for disturbances with a required demodulation speed greater than a preset demodulation speed threshold, select π / 2 phase difference demodulation; If π / 2 phase difference demodulation is to be performed, two photocurrent signals with a phase difference of 1 / 4 period are selected for demodulation; If 2π / 3 phase difference demodulation is to be performed, three photocurrent signals with a phase difference of 1 / 3 period are selected for demodulation; if π / 4 phase difference demodulation is to be performed, four photocurrent signals with a phase difference of 1 / 8 period are selected for demodulation; S7, obtaining multiple groups of demodulation results from the photocurrent signal at the disturbance position, and averaging them to obtain a final demodulation result.

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