A distributed weak grating array demodulation method for large amplitude signals with adjustable sensitivity

Abstract

The present invention discloses a distributed weak grating array demodulation method for large-amplitude signals with adjustable sensitivity, including: fusing the generated double-pulse light and introducing it into a distributed weak grating array to measure the disturbance signal; using IQ demodulation technology to phase-demodulate the returned reflected light signal and scattered light signal to obtain the demodulation result of the reflected light and the demodulation result of the scattered light; selecting one of the scattered light signal points, and performing a difference between the demodulation results of the scattered light signal point and the demodulation result of the reflected light signal point; performing a waveform comparison between the difference result of the scattered light signal point and the demodulation result of the reflected light, and if the waveforms of the two are consistent, outputting the demodulation result of the reflected light as the disturbance signal measurement result, otherwise, outputting the difference result as the disturbance signal measurement result. The present invention breaks through the limitation of the sampling rate on the strain measurement range by combining reflected light and scattered light for sensing, and realizes the measurement of large-amplitude signals with adjustable sensitivity.

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 demodulation method and system for large-amplitude signals with adjustable sensitivity. Background Art

[0002] Fiber optic sensing technology is an emerging discipline, and its development began in the 1970s. When light propagates in an optical fiber, external environmental factors will change the characteristic parameters of the transmitted light. By detecting the changes in optical parameters, it is possible to detect or perceive external disturbances. Distributed fiber optic sensing technology has the advantages of high sensitivity, strong anti-interference performance, and a wide range of measurement objects. It can be applied to many fields such as aerospace, national defense, military, and biomedicine.

[0003] The sensing principle of phase-sensitive optical time-domain reflectometry is: Rayleigh scattering originates from the uneven fluctuation of the refractive index in the optical fiber, and these fluctuations form "scattering points" with random positions in the optical fiber. When the detection pulse is transmitted along the optical fiber, it will generate Rayleigh scattered light in all directions after encountering random scattering points. Among them, only the backscattered light that satisfies total reflection can return to the incident end. These scattered lights are called backscattered Rayleigh light. Phase-sensitive optical time-domain reflectometry technology senses external disturbance information by detecting the phase change of backscattered Rayleigh light in the sensing optical fiber.

[0004] The sensing principle of distributed weak grating array sensing technology is to use the embedded extremely weak reflection grating as a reflector to generate a stable, intensity-controllable reflected light signal to replace the spontaneous Rayleigh scattering in the optical fiber, and then use the interference between the reflected light from the front and rear gratings to achieve vibration measurement.

[0005] However, both phase-sensitive optical time-domain reflectometry and distributed weak grating array sensing technology perform sensing based on changes in phase information. However, the phase unwrapping algorithm has limitations. Without sacrificing the frequency response range, only small amplitude signals can be correctly sensed. However, in practical applications, sensing of large amplitude signals is essential, so a large amplitude signal sensing system and demodulation method are needed. Summary of the invention

[0006] Technical problem solved: Phase-based distributed sensing technology cannot avoid phase unwrapping during phase demodulation. However, the traditional phase unwrapping algorithm requires that the actual phase jump value should not be too large, which results in large amplitude signals being unable to be correctly sensed without sacrificing the frequency response range.

[0007] Technical solution:

[0008] A large-amplitude signal distributed weak grating array demodulation method with adjustable sensitivity, the large-amplitude signal distributed weak grating array demodulation method comprising the following steps:

[0009] S1, generate double pulse light with different modulation frequencies, the same pulse width and the same peak value, merge the generated double pulse light and introduce it into the distributed weak grating array to measure the disturbance signal; wherein, on each grating, the reflected light of the two pulses is superimposed to generate interference, and the reflected light signal is returned; at the same time, when the double pulse light is transmitted along the optical fiber, the back Rayleigh scattered light is randomly generated, and the scattered light signal is returned;

[0010] S2, using IQ demodulation technology to perform phase demodulation on the returned reflected light signal and scattered light signal to obtain a demodulation result of the reflected light and a demodulation result of the scattered light;

[0011] S3, select one of the scattered light signal points, and perform a subtraction between the demodulation results of the scattered light signal point and the reflected light signal point to obtain the subtraction result of the scattered light signal point; the phase jump value of the subtraction result is between;

[0012] S4, compare the waveform of the difference result of the scattered light signal point with the demodulation result of the reflected light. If the two waveforms are consistent, output the demodulation result of the reflected light as the disturbance signal measurement result, otherwise, output the difference result as the disturbance signal measurement result.

[0013] Furthermore, in step S1, the distance between the two pulse lights satisfies:

[0014]

[0015] Wherein, c is the propagation speed of light in vacuum, n is the equivalent refractive index of optical fiber, and L is the spacing between adjacent gratings in the distributed weak grating array.

[0016] Furthermore, in step S3, the difference result of the scattered light signal point is calculated according to the following formula:

[0017]

[0018] in, is the demodulation result of the scattering point, k1 is the sensitivity coefficient of the scattering point to the external vibration signal, is the demodulation result of the reflection point, k2 is the sensitivity coefficient of the reflection point to the external vibration signal, Δε is the external vibration signal; the k1 value is related to the position of the selected scattered light signal point.

[0019] Further, in step S3, the demodulation difference result of the selected scattered light signal point and the reflected light signal point is assumed to be A, and the demodulation difference results of the scattered light signal points and the reflected light signal points after the selected scattered light signal point are N1, N2, N3, ..., N n ; N1, N2, N3, ..., Nn The waveform errors of A and A are both less than the preset error threshold.

[0020] Furthermore, the large amplitude signal distributed weak grating array demodulation method further comprises the following steps:

[0021] By selecting scattered light signal points with different sensitivity coefficients, the measurement sensitivity of the disturbance signal is adjusted.

[0022] The present invention also mentions a large amplitude signal distributed weak grating array demodulation system with adjustable sensitivity, the large amplitude signal distributed weak grating array demodulation system comprising a narrow line width laser, a first coupler, a second coupler, a first acousto-optic modulator, a second acousto-optic modulator, an erbium-doped fiber amplifier, a circulator, a sensing fiber fused with a weak grating array, a photodetector, a data acquisition card and a computer;

[0023] The continuous light emitted by the narrow linewidth laser is divided into two paths by the first coupler, one path of the optical signal is modulated into a first pulse light by the first acousto-optic modulator, and the other path of the optical signal is made into a second pulse light by the second acousto-optic modulator. The first pulse light and the second pulse light are synthesized into one pulse light by the second coupler, and the synthesized pulse light is amplified by an erbium-doped fiber amplifier and then enters the sensing fiber fused with a weak grating array through a circulator, so that the reflected light of the double pulses is superimposed and interfered in the grating, and the corresponding reflected light signal and scattered light signal are returned; the photoelectric detector detects the returned reflected light signal and scattered light signal, which are synchronously collected by a data acquisition card and sent to a computer;

[0024] The computer measures the disturbance signal using the large amplitude signal distributed weak grating array demodulation method as described above.

[0025] Furthermore, the coupling ratio between the first coupler and the second coupler is 50:50.

[0026] Beneficial effects:

[0027] First, the distributed weak grating array demodulation method for large-amplitude signals with adjustable sensitivity of the present invention uses dual-pulse technology in a weak grating array sensing system, and utilizes IQ demodulation to demodulate the reflected light signal and the back-scattered Rayleigh light signal to obtain phase signals respectively, and achieves accurate recovery of large strains by subtracting the demodulation results of the reflected light and the scattered light.

[0028] Second, the distributed weak grating array demodulation method for large-amplitude signals with adjustable sensitivity of the present invention utilizes the property that the reflected light signal completely crosses the vibration area, while the scattered light signal gradually crosses the vibration area. By selecting appropriate scattered light signal points, the sensitivity can be adjusted.

[0029] Third, the distributed weak grating array demodulation method for large-amplitude signals with adjustable sensitivity of the present invention breaks through the limitation of sampling rate on strain measurement range by combining reflected light and scattered light for sensing, and realizes large-amplitude signal measurement with adjustable sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a structural diagram of a distributed weak grating array demodulation system for large-amplitude signals with adjustable sensitivity according to an embodiment of the present invention.

[0031] Figure 2 It is a schematic diagram of the sensing principle of the large-amplitude signal distributed weak grating array system according to an embodiment of the present invention.

[0032] Figure 3 It is a schematic diagram of the demodulation principle of the large-amplitude signal distributed weak grating array system according to an embodiment of the present invention.

[0033] Figure 4 It is a demodulation flow chart of an embodiment of the present invention.

[0034] Figure 5 It is a graph of the pulse reflection signal generated by the grating array and the scattered signal generated by the optical fiber.

[0035] Figure 6 It is a schematic diagram of the demodulation results of the traditional unwinding method.

[0036] Figure 7 It is a schematic diagram of demodulation results of a distributed weak grating array demodulation method for large-amplitude signals with adjustable sensitivity according to an embodiment of the present invention.

[0037] Figure 8 is a schematic diagram of the fitting results after sensitivity adjustment; Figure 8 Part (a) of the figure is a curve diagram of the phase peak-to-peak value and the exciter current obtained by subtracting the rightmost scattered point signal from the reflected point signal; Figure 8 Part (b) of the figure is a schematic diagram of the curve between the phase peak-to-peak value and the exciter current obtained by subtracting the scattered point signal and the reflected point signal at the far left. DETAILED DESCRIPTION

[0038] 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.

[0039] This embodiment discloses a large-amplitude signal distributed weak grating array demodulation method with adjustable sensitivity, and the large-amplitude signal distributed weak grating array demodulation method comprises the following steps:

[0040] S1, generates double pulse light with different modulation frequencies, the same pulse width and the same peak value, merges the generated double pulse light and introduces it into the distributed weak grating array to measure the disturbance signal; wherein, on each grating, the reflected light of the two pulses is superimposed to produce interference, and the reflected light signal is returned; at the same time, when the double pulse light is transmitted along the optical fiber, the back Rayleigh scattered light is randomly generated, and the scattered light signal is returned.

[0041] S2, using IQ demodulation technology to perform phase demodulation on the returned reflected light signal and scattered light signal to obtain a demodulation result of the reflected light and a demodulation result of the scattered light.

[0042] S3, select one of the scattered light signal points, and perform a subtraction between the demodulation results of the scattered light signal point and the reflected light signal point to obtain the subtraction result of the scattered light signal point; the phase jump value of the subtraction result is between.

[0043] S4, compare the waveform of the difference result of the scattered light signal point with the demodulation result of the reflected light. If the two waveforms are consistent, output the demodulation result of the reflected light as the disturbance signal measurement result, otherwise, output the difference result as the disturbance signal measurement result.

[0044] This embodiment also mentions a large amplitude signal distributed weak grating array demodulation system adapted to the above demodulation method, see Figure 1 The large amplitude signal distributed weak grating array demodulation system includes a narrow line width laser, a first coupler, a second coupler, a first acousto-optic modulator AOM1, a second acousto-optic modulator AOM2, an erbium-doped fiber amplifier EDFA, a circulator, a sensing fiber fused with a weak grating array, a photodetector, a data acquisition card and a computer. The following is a description of the functions of each device:

[0045] Laser: This embodiment selects a narrow line width laser to generate continuous light.

[0046] The first coupler: splits the continuous light generated by the laser into two paths and outputs them to the acousto-optic modulator.

[0047] Acousto-optic modulator: By modulating the continuous light emitted by the laser, pulse light with a specific period and specific width is generated. For example, the modulation frequency difference between the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2 is greater than 50MHz, and the pulse width modulated by AOM1 is equal to the pulse width modulated by AOM2, thereby generating double pulse light with different frequencies and the same pulse width. By controlling the power of the first pulse light and the second pulse light, the peak values ​​of the two pulses are kept consistent.

[0048] The second coupler combines the two pulses generated by the first AOM1 and the second AOM2 into one beam and then introduces it into the erbium-doped fiber amplifier. The spacing between the two pulses in the pulsed light output by the second coupler 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.

[0049] Erbium-doped fiber amplifier: amplifies the power of amplitude-modulated double-pulse light.

[0050] 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.

[0051] Sensing fiber fused with weak grating array (distributed weak grating array): The reflected light of two pulses on each grating is superimposed to produce interference, sensing disturbance events in the sensing system.

[0052] Photodetector: used to convert optical signals into electrical signals and output them to the acquisition card.

[0053] Acquisition card: used to digitize the acquired analog signals and output them to the processor.

[0054] Computer: Analyze and process the collected data to locate and measure disturbances along the sensing optical fiber fused with the weak grating array.

[0055] The continuous light emitted by the narrow linewidth laser is divided into two paths by the first coupler, one of which is modulated into a first pulse light by the first acousto-optic modulator AOM1, and the other is modulated into a second pulse light by the second acousto-optic modulator AOM2. The first pulse light and the second pulse light are combined into one pulse light by the second coupler, which is amplified by the erbium-doped fiber amplifier EDFA and then enters the grating array through the circulator. The reflected light of the double pulses is superimposed and interfered in the grating. After the reflected light and the scattered light return, they are detected by the photoelectric detector, and the data acquisition card performs synchronous acquisition, and finally sent to the computer for processing.

[0056] In this embodiment, if Figure 1As shown in the figure, the narrow line width continuous light output by the laser is divided into two paths by a coupler with a splitting ratio of 50:50. One optical signal is modulated into a first pulse light of 300ns by 200M AOM1, and the other optical signal is modulated into a second pulse light of 300ns by 150M AOM2. The pulse interval between the two pulse lights is 500ns. The peak values ​​of the two pulses are kept consistent by power control of the first pulse light and the second pulse light. The first pulse light and the second pulse light are combined into one pulse light by the second coupler, and after being amplified by EDFA, they pass through a circulator and enter a grating array with a grating interval of 50m. The reflected light of the double pulses is superimposed and interfered in the grating. After the reflected light and scattered light return, they are detected by a photoelectric detector, and synchronously collected by a data acquisition card, and finally sent to a computer for processing.

[0057] like Figure 2 The figure shows the sensing principle of the large amplitude signal distributed weak grating array system with adjustable sensitivity. The backscattered Rayleigh light E superimposed on the half pulse width of the previous pulse at time t1 Ray1 Transmit backwards, At this moment, the backscattered Rayleigh light E superimposed with the half pulse width of the post pulse Ray2 Superposition interference occurs, forming a beat frequency. At the same time, the reflected light E of the front pulse at the front grating at time t2 Ref1 Transmit backwards, At this moment, the reflected light E of the rear pulse at the rear grating Ref2 Superposition interference occurs, forming a beat frequency. It can be seen that since the position of the grating is fixed, the reflected light will be modulated by the complete vibration; while the scattered light exists everywhere on the optical fiber, so the scattered light gradually crosses the vibration area, and the area modulated by the vibration changes from small to large and then to small. Because of this property, the sensitivity of the reflected light to the vibration is fixed, while the sensitivity of the scattered light to the vibration first increases and then decreases.

[0058] like Figure 3 As shown in the figure, it is the demodulation principle of the large amplitude signal distributed weak grating array system with adjustable sensitivity. Since the sensitivity of the reflected light signal to vibration is fixed, while the sensitivity of the scattered light to vibration is variable, by selecting the appropriate scattered light signal point, it can be ensured that the sensitivity of the two to vibration is different. Figure 3 As shown in part (a) of Figure 1, when a small amplitude vibration acts on the optical fiber, since it does not exceed the limit of the phase unwrapping algorithm, both the reflected light signal and the scattered light signal can sense the vibration, and due to their sensitivity differences, the vibration can still be sensed after the difference. Figure 3As shown in part (b) of the figure, when a large-amplitude vibration acts on the optical fiber, neither the reflected light signal nor the scattered light signal can sense the vibration because it exceeds the limit of the phase unwrapping algorithm. However, due to their sensitivity differences, the phase jump can be compressed by selecting appropriate scattered light signal points and making a difference, thereby realizing the sensing of large-amplitude vibration.

[0059] Figure 4 This is the signal demodulation flow chart. Figure 4 As shown in part (a) of , when a small amplitude vibration acts on the optical fiber, since the signal-to-noise ratio of the reflected light signal is higher than that of the scattered light signal, we directly select the demodulation result of the reflected light signal as the final output. Figure 4 As shown in part (b) of the figure, when a large amplitude vibration acts on the optical fiber, the demodulation results of the reflected light signal and the demodulation results of the scattered light will be wrong. However, by selecting the appropriate scattered light signal point, the difference between the two demodulation results is output as the final result.

[0060] The above demodulation principle can be expressed by the following formula:

[0061]

[0062] in, is the demodulation result of the scattering point, k1 is the sensitivity of the scattering point to vibration, is the demodulation result of the reflection point, k2 is the sensitivity of the reflection point to vibration, and Δε is the external vibration. Since the k1 value is optional, by selecting an appropriate k1 value, the phase jump value after subtraction can be guaranteed to be within Between, sensing of large amplitude vibrations is achieved.

[0063] Suppose the demodulation difference between the selected scattered light signal point and the reflected light signal point is A, and the demodulation difference between the scattered light signal points and the reflected light signal points after the selected point is N1, N2, N3, ..., N n The selection of scattered light signals needs to ensure that N1, N2, N3, ..., N n The waveform of A is basically the same as that of A, thus ensuring sufficient phase compression of the difference result. After the selected scattered light signal point, different scattered light signal points N1, N2, N3, ..., N n By subtracting the demodulation result from the reflected light signal point, the sensitivity can be adjusted.

[0064] like Figure 5 The following figure shows the pulse reflection signal generated by the grating array and the scattered signal generated by the optical fiber. The small figure in the upper right corner is an enlarged view of the vibration area. Select a reflection point signal between 2500-2600 and output as follows: Figure 6The demodulation result shown by the solid line shows that the unwrapping error occurs. Select a suitable scattering point signal between 2360 and 2420, and the output is as follows: Figure 6 The demodulation result shown by the dotted line shows that the unwrapping error occurs. Then, the difference between the two demodulation results is calculated, as shown in Figure 7 As shown, large amplitude signals can be correctly restored. Figure 8 is the fitting result of sensitivity adjustment. The rightmost scattering point signal is selected to make a difference with the reflection point signal. The curve between the peak-to-peak phase and the exciter current obtained by fitting (such as Figure 8 The sensitivity is 1.593. The leftmost scattering point signal is selected to make a difference with the reflection point signal, and the curve between the peak-to-peak phase and the exciter current is fitted. The sensitivity is 1.955 (as shown in the figure in part (a) of the figure). Figure 8 As shown in the (b) part of the picture); that is, the sensitivity adjustment range of this system is 1.593~1.955.

[0065] Therefore, compared with the traditional solution, the system and method of this solution have the following advantages:

[0066] First, IQ demodulation is used to demodulate the reflected light signal and the backscattered Rayleigh light signal to obtain the phase signal. By subtracting the demodulation results of the reflected light and the scattered light, the limitation of the sampling rate on the strain measurement range is broken through, and the accurate recovery of large strain is achieved. Second, the reflected light signal completely crosses the vibration area, while the scattered light signal gradually crosses the vibration area. By selecting the appropriate scattered light signal point, the sensitivity can be adjusted.

[0067] 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 demodulation method for large amplitude signals with adjustable sensitivity, characterized in that: The large amplitude signal distributed weak grating array demodulation method comprises the following steps: S1, generate double pulse light with different modulation frequencies, the same pulse width and the same peak value, merge the generated double pulse light and introduce it into the distributed weak grating array to measure the disturbance signal; wherein, on each grating, the reflected light of the two pulses is superimposed to generate interference, and the reflected light signal is returned; at the same time, when the double pulse light is transmitted along the optical fiber, the back Rayleigh scattered light is randomly generated, and the scattered light signal is returned; S2, using IQ demodulation technology to perform phase demodulation on the returned reflected light signal and scattered light signal to obtain a demodulation result of the reflected light and a demodulation result of the scattered light; S3, select one of the scattered light signal points, and perform a subtraction between the demodulation results of the scattered light signal point and the reflected light signal point to obtain the subtraction result of the scattered light signal point; the phase jump value of the subtraction result is between; S4, performing waveform comparison between the difference result of the scattered light signal point and the demodulation result of the reflected light, if the two waveforms are consistent, outputting the demodulation result of the reflected light as the disturbance signal measurement result, otherwise, outputting the difference result as the disturbance signal measurement result; By selecting scattered light signal points with different sensitivity coefficients, the measurement sensitivity of the disturbance signal is adjusted.

2. The distributed weak grating array demodulation method for large amplitude signals with adjustable sensitivity according to claim 1, characterized in that: In step S1, the distance between the two pulse lights satisfies: Wherein, c is the propagation speed of light in vacuum, n is the equivalent refractive index of optical fiber, and L is the spacing between adjacent gratings in the distributed weak grating array.

3. The distributed weak grating array demodulation method for large amplitude signals with adjustable sensitivity according to claim 1, characterized in that: In step S3, the difference result of the scattered light signal point is calculated according to the following formula: in, is the demodulation result of the scattering point, k1 is the sensitivity coefficient of the scattering point to the external vibration signal, is the demodulation result of the reflection point, k2 is the sensitivity coefficient of the reflection point to the external vibration signal, Δε is the external vibration signal; the k1 value is related to the position of the selected scattered light signal point.

4. The distributed weak grating array demodulation method for large amplitude signals with adjustable sensitivity according to claim 1, characterized in that: In step S3, the demodulation difference result of the selected scattered light signal point and the reflected light signal point is assumed to be A, and the demodulation difference results of the scattered light signal points and the reflected light signal points after the selected scattered light signal point are N1, N2, N3, ..., N n ; N1, N2, N3, ..., N n The waveform errors of A and A are both less than the preset error threshold.

5. A large amplitude signal distributed weak grating array demodulation system with adjustable sensitivity, characterized in that: The large amplitude signal distributed weak grating array demodulation system comprises a narrow line width laser, a first coupler, a second coupler, a first acousto-optic modulator, a second acousto-optic modulator, an erbium-doped fiber amplifier, a circulator, a sensing fiber fused with a weak grating array, a photodetector, a data acquisition card and a computer; The continuous light emitted by the narrow linewidth laser is divided into two paths by the first coupler, one path of the optical signal is modulated into a first pulse light by the first acousto-optic modulator, and the other path of the optical signal is made into a second pulse light by the second acousto-optic modulator. The first pulse light and the second pulse light are synthesized into one pulse light by the second coupler, and the synthesized pulse light is amplified by an erbium-doped fiber amplifier and then enters the sensing fiber fused with a weak grating array through a circulator, so that the reflected light of the double pulses is superimposed and interfered in the grating, and the corresponding reflected light signal and scattered light signal are returned; the photoelectric detector detects the returned reflected light signal and scattered light signal, which are synchronously collected by a data acquisition card and sent to a computer; The computer measures the disturbance signal using the large amplitude signal distributed weak grating array demodulation method as described in any one of claims 1 to 4.

6. The large-amplitude signal distributed weak grating array demodulation system with adjustable sensitivity according to claim 5, characterized in that: The coupling ratio between the first coupler and the second coupler is 50:50.

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