Distributed Sensing System and Method for Measuring High-Precision Large-Dynamic Absolute Strain

Abstract

The present invention provides a distributed sensing system and method for measuring high-precision large-dynamic absolute strain. A first coupler divides the laser signal provided by a laser into two paths, one path is transmitted to a measurement branch, and the other path is respectively transmitted to a φ-OTDR measurement branch and a BOTDR measurement branch through a second coupler; the measurement branch transmits the measurement light generated according to the laser signal to a sensing optical fiber, so that the sensing optical fiber reversely transmits back the backscattered light, and the backscattered light is divided into two paths and respectively transmitted to the φ-OTDR measurement branch and the BOTDR measurement branch; the φ-OTDR measurement branch performs beat frequency between the Rayleigh scattered light in the backscattered light and a first reference light generated according to the laser signal to obtain a Rayleigh scattering signal; the BOTDR measurement branch performs beat frequency between the Brillouin scattered light in the backscattered light and a second reference light generated according to the laser signal to generate a Brillouin scattering signal; a processor determines the synthetic absolute strain at each position on the sensing optical fiber according to the Rayleigh scattering signal and the Brillouin scattering signal.

Description

Technical Field

[0001] The present invention belongs to the field of strain detection, and particularly relates to a distributed sensing system and method for measuring high-precision large-dynamic absolute strain. Background Art

[0002] Compared with traditional electrical sensors, fiber optic sensing technology has the advantages of anti-electromagnetic interference and intrinsic safety. It has important application values in navigation, aerospace, national defense, industrial production, intelligent transportation, etc. In the application of distributed strain measurement in various fields, it is required that the strain sensing range of distributed strain sensing technology can be further expanded, the accuracy can be further improved, and the absolute measurement of strain can be realized, etc.

[0003] Both the distributed sensing technology based on Rayleigh scattering and the distributed sensing technology based on Brillouin scattering can realize strain measurement, and each has its own advantages. For the φ-OTDR (Phase-Sensitive Optical Time Domain Reflectometer) distributed sensing technology based on Rayleigh scattering, its phase change is linearly related to the strain change, and it has the advantages of high accuracy, high signal-to-noise ratio, high sampling rate, etc. However, the φ-OTDR distributed sensing system cannot measure the absolute strain, and can only obtain the relative strain between measurement points on the sensing fiber through continuous measurement for a short time. Contrary to the φ-OTDR distributed sensing system, for the BOTDA (Brillouin Optical Time Domain Analysis) technology based on Brillouin scattering, it can measure the absolute strain, but its strain resolution is much lower than that of the φ-OTDR distributed sensing system. It can be seen that there is a problem of low strain resolution in the current strain sensing system when measuring the absolute strain. Summary of the Invention

[0004] The present invention provides a distributed sensing system and method for measuring high-precision large-dynamic absolute strain to solve the problem of low strain resolution in the strain sensing system when measuring the absolute strain.

[0005] According to the first aspect of the embodiments of the present invention, a distributed sensing system for measuring high-precision large-dynamic absolute strain is provided, including a laser, a first coupler, a second coupler, a third coupler, a measurement branch, a sensing fiber, a φ-OTDR measurement branch, a BOTDR measurement branch, and a processor. The first coupler divides the laser signal provided by the laser into two paths, one path is transmitted to the measurement branch, and the other path is respectively transmitted to the φ-OTDR measurement branch and the BOTDR measurement branch through the second coupler; the measurement branch processes the laser signal it receives to generate measurement light, the φ-OTDR measurement branch processes the laser signal it receives to generate a first reference light, and the BOTDR measurement branch processes the laser signal it receives to generate a second reference light;

[0006] The measurement branch transmits the measurement light to the sensing optical fiber, so that the sensing optical fiber reversely transmits the backscattered light. The measurement branch divides the backscattered light into two paths through the third coupler and transmits them to the φ-OTDR measurement branch and the BOTDR measurement branch respectively;

[0007] The φ-OTDR measurement branch beats the Rayleigh scattered light in the backscattered light with the first reference light to obtain a Rayleigh scattering signal. The BOTDR measurement branch beats the Brillouin scattered light in the backscattered light with the second reference light to generate a Brillouin scattering signal. The processor determines the synthetic absolute strain at each position on the sensing optical fiber according to the Rayleigh scattering signal and the Brillouin scattering signal.

[0008] In an optional implementation, the measurement branch includes a waveform generator, an optical pulse modulator, an optical amplifier, a first band-pass optical filter, and a circulator. The input end and the control end of the optical pulse modulator are respectively connected to the first output end of the first coupler and the waveform generator, and the output end is sequentially connected to the sensing optical fiber through the optical amplifier, the first band-pass optical filter, and the circulator;

[0009] The waveform generator controls the optical pulse modulator to modulate the laser signal received from the first coupler into a periodic narrow pulse square wave. The periodic narrow pulse square wave is amplified by the optical amplifier and the noise is filtered by the first band-pass optical filter to generate measurement light. The measurement light is transmitted to the sensing optical fiber through the circulator, and the sensing optical fiber reversely transmits the backscattered light back to the circulator. The circulator divides the backscattered light into two paths through the third coupler and transmits them to the φ-OTDR measurement branch and the BOTDR measurement branch respectively.

[0010] In another optional implementation, the φ-OTDR measurement branch includes a polarization controller, a fourth coupler, and a first photodetector. The input end of the polarization controller is connected to the first output end of the second coupler, and the output end is connected to the first input end of the fourth coupler. The second input end of the fourth coupler is connected to the first output end of the third coupler, and the output end is connected to the first photodetector;

[0011] The polarization controller performs polarization processing on the received laser signal to generate a first reference light and sends it to the fourth coupler. The fourth coupler beats the Rayleigh scattered light in the backscattered light with the first reference light to obtain a first beat signal. The first photodetector converts the first beat signal into the Rayleigh scattering signal.

[0012] In another alternative implementation, the BOTDR measurement branch includes a radio frequency source, an electro-optic modulator, a second band-pass optical filter, a polarization scrambler, a fifth coupler, and a second photodetector. The input end and the control end of the electro-optic modulator are respectively connected to the second output end of the second coupler and the radio frequency source, and the output end is sequentially connected to the second photodetector through the second band-pass optical filter, the polarization scrambler, and the fifth coupler;

[0013] The radio frequency source controls the electro-optic modulator so that the electro-optic modulator cooperates with the second band-pass optical filter to shift the frequency of the laser signal received by the electro-optic modulator; the frequency-shifted laser signal is subjected to polarization scrambling processing by the polarization scrambler to generate a second reference light, which is transmitted to the fifth coupler; the fifth coupler performs beat frequency on the Brillouin scattered light in the backscattered light and the second reference light to generate a second beat frequency signal; the second photodetector converts the second beat frequency signal into the Brillouin scattered signal.

[0014] According to the second aspect of the embodiments of the present invention, there is also provided a high-precision large-dynamic absolute strain measurement method based on the above distributed sensing system. The processor determines the synthetic absolute strain at each position on the sensing optical fiber according to the Rayleigh scattered signal and the Brillouin scattered signal according to the following steps:

[0015] Step S110: Process the Rayleigh scattered signal and the Brillouin scattered signal respectively to obtain the relative strain of the sensing optical fiber B (t);

[0016] Step S120: Map the relative strain to an equivalent Brillouin frequency shift

[0017] Step S130: Determine the equivalent Brillouin spectrum g according to the equivalent Brillouin frequency shift B,eq and the average number of times N of the Brillouin spectrum of the Brillouin scattered signal, perform Lorentz fitting on the equivalent Brillouin spectrum g B,eq (v,t) to obtain the equivalent relative Brillouin frequency shift f B represents the sampling rate of the Brillouin scattered signal, represents the sampling rate of the Rayleigh scattered signal, which is equal to the pulse repetition frequency of the measurement light, and N is an integer greater than 1;

[0018] Step S140: According to the equivalent relative Brillouin frequency shift and the Brillouin frequency shift amount v of the Brillouin scattering signal B (t), fitting to obtain the bias Brillouin frequency shift amount v offset , thereby according to the bias Brillouin frequency shift amount v offset , determining the bias strain ε offset ;

[0019] Step S150, according to the relative strain and the bias strain ε offset , determining the synthetic absolute strain of the sensing optical fiber

[0020] In an optional implementation manner, the step S110 specifically includes: performing Hilbert transform and phase unwrapping on the Rayleigh scattering signal to obtain the relative strain of the sensing optical fiber Performing short-time Fourier transform and Lorentz fitting on the Brillouin scattering signal in sequence to obtain the Brillouin frequency shift amount v B (t);

[0021] Before the step S110, it further includes: processing the Rayleigh scattering signal by using a spectrum drift suppression and interference fading suppression algorithm to reduce the low-frequency drift amount of the Rayleigh scattering signal

[0022] In another optional implementation manner, the step S120 specifically includes: mapping the relative strain to an equivalent Brillouin frequency shift amount

[0023]

[0024] where C ε represents the Brillouin frequency shift sensitivity coefficient, and ε0 represents the initial strain of the sensing optical fiber

[0025] In another optional implementation manner, in the step S130, according to the equivalent Brillouin frequency shift amount and the Brillouin spectrum average number N of the Brillouin scattering signal, determining the equivalent Brillouin spectrum g B,eq (v,t) according to the following formula

[0026]

[0027] where i is an integer and less than or equal to N, v represents the frequency on the Brillouin gain spectrum frequency axis of the Brillouin scattering signal, t represents time, and Δv B represents the full width at half maximum of the Brillouin spectrum of the Brillouin scattering signal

[0028] In another alternative implementation, step S140 specifically includes:

[0029] Step S141: According to the equivalent relative Brillouin frequency shift and the Brillouin frequency shift amount v B (t) of the Brillouin scattering signal, the bias Brillouin frequency shift amount v offse is obtained by fitting according to the following formula:

[0030]

[0031] where t represents the fitting time;

[0032] Step S142: According to the bias Brillouin frequency shift amount v offset , the bias strain ε offset is determined according to the following formula: ε offse =v offset / C ε , where C ε represents the Brillouin frequency shift sensitivity coefficient;

[0033] where as the fitting time t in step S140 increases, the determination accuracy of the bias strain ε offset increases, but the low-frequency drift amount of the relative strain increases; t takes the fitting time when the low-frequency drift amount of the Rayleigh scattering signal is equal to the frequency shift uncertainty of the Brillouin scattering signal after averaging the Brillouin scattering spectrum N times.

[0034] In another alternative implementation, step S150 specifically includes: According to the relative strain and the bias strain ε offset , the composite absolute strain

[0035]

[0036] The beneficial effects of the present invention are:

[0037] 1. The present invention obtains Rayleigh scattering signals and Brillouin scattering signals based on the same measurement light. Since the strain changes obtained by demodulating the two scattering signals are in the same phase, the strain detection results obtained based on the two scattering signals can be mutually referenced. The present invention combines the two mechanisms of the distributed sensing system and the BOTDR sensing system, can synthesize the absolute strain, and at the same time makes the synthesized absolute strain have the advantages of high precision and high resolution;

[0038] 2. The synthesized absolute strain of the present invention is composed of two parts, one part is One part of the corresponding variable obtained by the phase demodulation of the distributed sensing technology is the offset strain. In the present invention, the corresponding variable is mapped to an equivalent Brillouin frequency shift, and an equivalent Brillouin spectrum is obtained according to the equivalent Brillouin frequency shift. The equivalent Brillouin spectrum is Lorentz-fitted to obtain an equivalent relative Brillouin frequency shift. Then, the offset strain is obtained by combining the equivalent relative Brillouin frequency shift with the Brillouin frequency shift in the Brillouin scattering signal. The offset strain combines the characteristics of the Rayleigh scattering signal and the Brillouin scattering signal, enabling the present invention to measure both the absolute strain and simultaneously have the high sampling rate, high relative strain measurement accuracy, and high resolution of the distributed sensing technology, as well as the high absolute strain measurement accuracy of the BOTDR technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a schematic structural diagram of an embodiment of the distributed sensing system for measuring high-precision large-dynamic absolute strain of the present invention;

[0040] Figure 2 is a schematic structural diagram of another embodiment of the distributed sensing system for measuring high-precision large-dynamic absolute strain of the present invention;

[0041] Figure 3 is a flowchart of an embodiment of the method for measuring high-precision large-dynamic absolute strain of the present invention;

[0042] Figure 4 is a flowchart of another embodiment of the method for measuring high-precision large-dynamic absolute strain of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] In order to enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention and make the above-mentioned objects, features, and advantages of the embodiments of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be further described in detail below with reference to the drawings.

[0044] In the description of the present invention, unless otherwise specified and defined, it should be noted that the term "connection" should be understood in a broad sense. For example, it can be a mechanical connection or an electrical connection, or it can be the internal communication of two components. It can be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meaning of the above terms can be understood according to specific circumstances.

[0045] See Figure 1, which is a schematic structural diagram of an embodiment of the distributed sensing system for measuring high-precision large-dynamic absolute strain in the present invention. The distributed sensing system may include a laser, a first coupler, a second coupler, a third coupler, a measurement branch, a sensing optical fiber, a φ-OTDR measurement branch, a BOTDR measurement branch, and a processor. The first coupler divides the laser signal provided by the laser into two paths, one path is transmitted to the measurement branch, and the other path is respectively transmitted to the φ-OTDR measurement branch and the BOTDR measurement branch through the second coupler; the measurement branch processes the laser signal it receives to generate measurement light, the φ-OTDR measurement branch processes the laser signal it receives to generate a first reference light, and the BOTDR measurement branch processes the laser signal it receives to generate a second reference light; the measurement branch transmits the measurement light to the sensing optical fiber so that the sensing optical fiber reversely transmits backscattered light, and the measurement branch divides the backscattered light into two paths through the third coupler and transmits them to the φ-OTDR measurement branch and the BOTDR measurement branch respectively; the φ-OTDR measurement branch beats the Rayleigh scattered light in the backscattered light with the first reference light to obtain a Rayleigh scattering signal, and the BOTDR measurement branch beats the Brillouin scattered light in the backscattered light with the second reference light to generate a Brillouin scattering signal; the processor determines the composite absolute strain at each position on the sensing optical fiber according to the Rayleigh scattering signal and the Brillouin scattering signal.

[0046] In this embodiment, the output end of the laser is connected to the input end of the first coupler, the first output end of the first coupler is connected to the measurement branch, the second output end is connected to the input end of the second coupler, the input / output end of the measurement branch is connected to the sensing optical fiber and the input end of the third coupler, the first output end of the second coupler is connected to the first input end of the φ-OTDR measurement branch, the second output end is connected to the first input end of the BOTDR measurement branch, the first output end of the third coupler is connected to the second input end of the φ-OTDR measurement branch, the second output end is connected to the second output end of the BOTDR measurement branch, and the output ends of the φ-OTDR measurement branch and the BOTDR measurement branch are connected to the processor. Among them, the laser may be a narrow linewidth laser.

[0047] As can be seen from the above embodiment, the present invention obtains a Rayleigh scattering signal and a Brillouin scattering signal based on the same measurement light. Since the strain changes obtained by demodulating the two scattering signals are in the same phase, the strain detection results obtained based on the two scattering signals can be mutually referenced. The present invention combines two mechanisms of a distributed sensing system and a BOTDR sensing system, can synthesize the absolute strain, and at the same time make the synthesized absolute strain have the advantages of high precision and high resolution.

[0048] See Figure 2 , which is a schematic structural diagram of another embodiment of the distributed sensing system for measuring high-precision large-dynamic absolute strain in the present invention. Figure 2 The difference from the Figure 1 distributed sensing system shown is that the measurement branch may include a waveform generator, an optical pulse modulator, an optical amplifier, a first band-pass optical filter, and a circulator. The input end and the control end of the optical pulse modulator are respectively connected to the first output end of the first coupler and the waveform generator, and the output end is sequentially connected to the sensing optical fiber through the optical amplifier, the first band-pass optical filter, and the circulator; the waveform generator controls the optical pulse modulator to modulate the laser signal received from the first coupler into a periodic narrow pulse square wave by the optical pulse modulator; the periodic narrow pulse square wave is amplified by the optical amplifier and filtered by the first band-pass optical filter to generate measurement light, and the measurement light is transmitted to the sensing optical fiber through the circulator, and the sensing optical fiber transmits the backscattered light back to the circulator in the reverse direction; the circulator divides the backscattered light into two paths through the third coupler and transmits them to the φ-OTDR measurement branch and the BOTDR measurement branch respectively.

[0049] Figure 2 The difference from the Figure 1 distributed sensing system shown is also that the φ-OTDR measurement branch may include a polarization controller, a fourth coupler, and a first photodetector. The input end of the polarization controller is connected to the first output end of the second coupler, and the output end is connected to the first input end of the fourth coupler. The second input end of the fourth coupler is connected to the first output end of the third coupler, and the output end is connected to the first photodetector; the polarization controller performs polarization processing on the received laser signal to generate a first reference light and sends it to the fourth coupler; the fourth coupler performs beat frequency on the Rayleigh scattered light in the backscattered light and the first reference light to obtain a first beat frequency signal; the first photodetector converts the first beat frequency signal into the Rayleigh scattered signal signal.

[0050] Figure 2 The difference from the Figure 1 distributed sensing system shown is also that the BOTDR measurement branch includes a radio frequency source, an electro-optic modulator, a second band-pass optical filter, a depolarizer, a fifth coupler, and a second photodetector. The input end and the control end of the electro-optic modulator are respectively connected to the second output end of the second coupler and the radio frequency source, and the output end is sequentially connected to the second photodetector through the second band-pass optical filter, the depolarizer, and the fifth coupler;

[0051] The radio frequency source controls the electro-optic modulator, so that the electro-optic modulator cooperates with the second band-pass optical filter to shift the frequency of the laser signal received by the electro-optic modulator; the frequency-shifted laser signal is subjected to polarization scrambling processing by the polarization scrambler to generate a second reference light, which is transmitted to the fifth coupler; the fifth coupler performs beat frequency on the Brillouin scattered light in the backscattered light and the second reference light to generate a second beat frequency signal; the second photodetector converts the second beat frequency signal into the Brillouin scattered signal BOTDR signal.

[0052] As can be seen from the above embodiments, the present invention obtains Rayleigh scattering signals and Brillouin scattering signals based on the same measurement light. Since the strain changes obtained by demodulating the two scattering signals are in the same phase, the strain detection results obtained based on the two scattering signals can be mutually referenced. The present invention combines two mechanisms of a distributed sensing system and a BOTDR sensing system, can synthesize an absolute strain, and at the same time makes the synthesized absolute strain have the advantages of high precision and high resolution.

[0053] Although both distributed sensing technology and BOTDR technology can detect strain, the distributed sensing technology detects the relative strain. For two adjacent measurement points separated by a set distance L, the change amount of the phase difference of the Rayleigh scattered light is proportional to the relative strain of the optical fiber segment between the two adjacent measurement points The change amount of the phase difference and the relative strain The relationship between them can be expressed as: where K ε is the phase sensitivity coefficient. For the BOTDR technology, the Brillouin frequency shift v B (t) of the Brillouin scattered light is proportional to the absolute strain ε B (t), and the relationship between the Brillouin frequency shift v B (t) and the absolute strain ε B (t) can be expressed as:

[0054]

[0055] where C ε represents the Brillouin frequency shift sensitivity coefficient, ε0 represents the initial strain of the sensing optical fiber, and v B0 represents the Brillouin frequency shift when the strain of the sensing optical fiber is ε0.

[0056] In BOTDR technology, in order to obtain a Brillouin spectrum with a high signal-to-noise ratio, the Brillouin spectrum is usually averaged thousands of times. After the Brillouin spectrum averaging is completed, the averaged Brillouin spectrum is subjected to Lorentz fitting to obtain the Brillouin frequency shift. Therefore, when measuring the absolute strain, what is detected by the BOTDR technology is not the true strain at a certain moment, but the new spectrum synthesized from the Brillouin spectra during this averaging time, and the strain result obtained by Lorentz fitting of this new spectrum. Since the strain obtained by the distributed sensing technology and the BOTDR technology is in the same phase, the present invention proposes a high-precision large-dynamic absolute strain measurement method based on the above-mentioned distributed sensor, as Figure 3 and Figure 4 shown. In this method, the processor determines the synthesized absolute strain at each position on the sensing optical fiber according to the Rayleigh scattering signal and the Brillouin scattering signal according to the following steps:

[0057] Step S110: Process the Rayleigh scattering signal and the Brillouin scattering signal respectively, and correspondingly obtain the relative strain of the sensing optical fiber B (t) and the Brillouin frequency shift amount v

[0058] of the Brillouin scattering signal. In this step, step S110 may specifically include: performing Hilbert transform and phase unwrapping on the Rayleigh scattering signal to obtain the relative strain of the sensing optical fiber, and performing short-time Fourier transform and Lorentz fitting on the Brillouin scattering signal in sequence to obtain the Brillouin frequency shift amount v B (t). Since there is usually a low-frequency phase drift in the Rayleigh scattering signal in the distributed sensing technology, in order to reduce the low-frequency phase drift amount, before step S110, it may further include: processing the Rayleigh scattering signal by using a spectrum drift suppression and interference fading suppression algorithm.

[0059] Step S120: Map the relative strain to an equivalent Brillouin frequency shift amount

[0060] In this step, step S120 may specifically include: mapping the relative strain to an equivalent Brillouin frequency shift amount

[0061]

[0062] where C ε represents the Brillouin frequency shift sensitivity coefficient, and ε0 represents the initial strain of the sensing optical fiber.

[0063] Step S130. According to the equivalent Brillouin frequency shift and the average number N of the Brillouin spectra of the Brillouin scattering signal (i.e., the number of times the Brillouin spectrum is averaged when obtaining a high signal-to-noise ratio Brillouin spectrum based on the Brillouin scattering signal), determine the equivalent Brillouin spectrum g B,eq (v, t), and perform Lorentz fitting on the equivalent Brillouin spectrum g B,eq (v, t) to obtain the equivalent relative Brillouin frequency shift f B represents the sampling rate of the Brillouin scattering signal, represents the sampling rate of the Rayleigh scattering signal, which is equal to the pulse repetition frequency of the measurement light, and N is an integer greater than 1.

[0064] In this step, assume that the sampling rate of the Rayleigh scattering signal is equal to the pulse repetition frequency of the measurement light, and the sampling rate of the Brillouin scattering signal After obtaining the average number N of the Brillouin spectra, in step S130, the equivalent Brillouin spectrum g can be determined according to the equivalent Brillouin frequency shift and the average number N of the Brillouin spectra of the Brillouin scattering signal according to the following formula: B,eq (v, t):

[0065]

[0066] where i is an integer and less than or equal to N, v represents the frequency on the Brillouin gain spectrum frequency axis of the Brillouin scattering signal, t represents time, and Δv B represents the full width at half maximum of the Brillouin spectrum of the Brillouin scattering signal.

[0067] Step S140. According to the equivalent relative Brillouin frequency shift and the Brillouin frequency shift v B (t) of the Brillouin scattering signal, fit to obtain the offset Brillouin frequency shift v offset , and thus determine the offset strain ε offset according to the offset Brillouin frequency shift v offset .

[0068] In this step, step S140 specifically includes:

[0069] Step S141. According to the equivalent relative Brillouin frequency shift and the Brillouin frequency shift v B (t) of the Brillouin scattering signal, fit to obtain the offset Brillouin frequency shift v offse according to the following formula:

[0070]

[0071] where t represents the fitting time;

[0072] Step S142: According to the offset Brillouin frequency shift v offset , determine the offset strain ε according to the following formula offset : ε offse = v offset / C ε , where C ε represents the Brillouin frequency shift sensitivity coefficient;

[0073] where as the fitting time t in the step S140 increases, the determination accuracy of the offset strain ε offset improves, but the low-frequency drift of the relative strain increases; t takes the fitting time when the low-frequency drift of the Rayleigh scattering signal is equal to the frequency shift uncertainty of the Brillouin scattering signal after averaging the Brillouin spectrum N times, so that the accuracy of the polarization strain ε offset and the low-frequency drift of the relative strain can both meet the requirements. The number of times N of averaging the Brillouin spectrum can be set according to this frequency shift uncertainty, as long as the frequency shift uncertainty is within the set range (for example, 1 MHz).

[0074] Step S150: According to the relative strain and the offset strain ε offset , determine the synthetic absolute strain

[0075] In this step, the step S150 specifically includes: according to the relative strain and the offset strain ε offset , determine the synthetic absolute strain

[0076] As can be seen from the above embodiments, the synthetic absolute strain of the present invention consists of two parts. One part is the relative strain obtained by phase demodulation using the distributed sensing technology, and the other part is the offset strain. The present invention maps the relative strain to an equivalent Brillouin frequency shift, obtains an equivalent Brillouin spectrum according to the equivalent Brillouin frequency shift, performs Lorentz fitting on the equivalent Brillouin spectrum to obtain an equivalent relative Brillouin frequency shift, and then combines the equivalent relative Brillouin frequency shift with the Brillouin frequency shift in the Brillouin scattering signal to obtain the offset strain. The offset strain combines the characteristics of the Rayleigh scattering signal and the Brillouin scattering signal, enabling the present invention to measure both the absolute strain and simultaneously have ​The high sampling rate, high relative strain measurement accuracy, and high resolution of distributed sensing technology, as well as the high absolute strain measurement accuracy of BOTDR technology. The present invention is particularly applicable to dynamic strain measurement in the range of 1 με to 3000 με.

[0077] It should be noted that: when the dynamic range is small, less than the measurement accuracy of BOTDR, and when BOTDR cannot detect the dynamic signal, the fitting in step S140 is meaningless, so the polarization strain ε offset does not need to be fitted, and the average value of the strain measured by BOTDR is directly used for calculation. If there is an unusual large-range dynamic strain offset within a short period of time, and BOTDR does not detect this change, it is considered that the measurement result is incorrect due to interference fading or other reasons. At this time, the strain signal measured by BOTDR is directly used to replace the measurement result.

[0078] After considering the specification and practicing the invention disclosed herein, those skilled in the art will readily conceive of other embodiments of the present invention. This application is intended to cover any variations, uses, or adaptations of the present invention that follow the general principles of the present invention and include the known common knowledge or conventional technical means in the technical field not disclosed by the present invention. The specification and examples are only regarded as exemplary, and the true scope and spirit of the present invention are pointed out by the following claims.

[0079] It should be understood that the present invention is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is only regulated by the appended claims.

Claims

1. A distributed sensing system for measuring high-precision large-dynamic absolute strain, characterized in that, It includes a laser, a first coupler, a second coupler, a third coupler, a measurement branch, a sensing optical fiber, a φ-OTDR measurement branch, a BOTDR measurement branch and a processor. The first coupler divides the laser signal provided by the laser into two paths, one path is transmitted to the measurement branch, and the other path is respectively transmitted to the φ-OTDR measurement branch and the BOTDR measurement branch through the second coupler; the measurement branch processes the laser signal it receives to generate measurement light, the φ-OTDR measurement branch processes the laser signal it receives to generate a first reference light, and the BOTDR measurement branch processes the laser signal it receives to generate a second reference light; The measurement branch transmits the measurement light to the sensing optical fiber so that the sensing optical fiber reversely transmits back the backscattered light. The measurement branch divides the backscattered light into two paths through the third coupler and transmits them to the φ-OTDR measurement branch and the BOTDR measurement branch respectively; The φ-OTDR measurement branch beats the Rayleigh scattered light in the backscattered light with the first reference light to obtain a Rayleigh scattered signal, and the BOTDR measurement branch beats the Brillouin scattered light in the backscattered light with the second reference light to generate a Brillouin scattered signal; The processor determines the synthetic absolute strain at each position on the sensing optical fiber according to the Rayleigh scattered signal and the Brillouin scattered signal.

2. The distributed sensing system for measuring high-precision large-dynamic absolute strain according to claim 1, characterized in that, The measurement branch includes a waveform generator, an optical pulse modulator, an optical amplifier, a first band-pass optical filter and a circulator. The input end and the control end of the optical pulse modulator are respectively connected to the first output end of the first coupler and the waveform generator, and the output end is sequentially connected to the sensing optical fiber through the optical amplifier, the first band-pass optical filter and the circulator; The waveform generator controls the optical pulse modulator so that the optical pulse modulator modulates the laser signal received from the first coupler into a periodic narrow pulse square wave; the periodic narrow pulse square wave is amplified by the optical amplifier and the noise is filtered by the first band-pass optical filter to generate measurement light. The measurement light is transmitted to the sensing optical fiber through the circulator, and the sensing optical fiber reversely transmits the backscattered light back to the circulator; the circulator divides the backscattered light into two paths through the third coupler and transmits them to the φ-OTDR measurement branch and the BOTDR measurement branch respectively.

3. The distributed sensing system for measuring high-precision large-dynamic absolute strain according to claim 1, characterized in that, The φ-OTDR measurement branch includes a polarization controller, a fourth coupler and a first photodetector. The input end of the polarization controller is connected to the first output end of the second coupler, the output end is connected to the first input end of the fourth coupler, the second input end of the fourth coupler is connected to the first output end of the third coupler, and the output end is connected to the first photodetector; The polarization controller performs polarization processing on the received laser signal to generate a first reference light, and sends it to the fourth coupler; the fourth coupler performs beat frequency on the Rayleigh scattered light in the backscattered light and the first reference light to obtain a first beat frequency signal; the first photodetector converts the first beat frequency signal into the Rayleigh scattered signal.

4. The distributed sensing system for measuring high-precision large-dynamic absolute strain according to claim 1, characterized in that The BOTDR measurement branch includes a radio frequency source, an electro-optic modulator, a second band-pass optical filter, a polarization scrambler, a fifth coupler, and a second photodetector. The input end and the control end of the electro-optic modulator are respectively connected to the second output end of the second coupler and the radio frequency source, and the output end is sequentially connected to the second photodetector through the second band-pass optical filter, the polarization scrambler, and the fifth coupler; The radio frequency source controls the electro-optic modulator so that the electro-optic modulator cooperates with the second band-pass optical filter to shift the frequency of the laser signal received by the electro-optic modulator; the frequency-shifted laser signal generates a second reference light after being processed by the polarization scrambler and is transmitted to the fifth coupler; the fifth coupler performs beat frequency on the Brillouin scattered light in the backscattered light and the second reference light to generate a second beat frequency signal; the second photodetector converts the second beat frequency signal into the Brillouin scattered signal.

5. A high-precision large-dynamic absolute strain measurement method for the distributed sensing system according to any one of claims 1 to 4, characterized in that The processor determines the composite absolute strain at each position on the sensing optical fiber according to the Rayleigh scattered signal and the Brillouin scattered signal according to the following steps: Step S110: Process the Rayleigh scattering signal and the Brillouin scattering signal respectively to obtain the corresponding variables of the sensing optical fiber and the Brillouin frequency shift v B (t) of the Brillouin scattering signal; Step S120: Map the corresponding variable to an equivalent Brillouin frequency shift Step S130: Determine the equivalent Brillouin spectrum g (v, t) according to the equivalent Brillouin frequency shift amount B,eq and the average number N of the Brillouin spectra of the Brillouin scattering signal, and perform Lorentz fitting on the equivalent Brillouin spectrum g B,eq (v, t) to obtain the equivalent relative Brillouin frequency shift f B where represents the sampling rate of the Brillouin scattering signal, represents the sampling rate of the Rayleigh scattering signal, which is equal to the pulse repetition frequency of the measurement light, and N is an integer greater than 1; Step S140, according to the equivalent relative Brillouin frequency shift and the Brillouin frequency shift amount v B (t) of the Brillouin scattering signal, fitting to obtain the bias Brillouin frequency shift amount v offset , and thus according to the bias Brillouin frequency shift amount v offset , determining the bias strain ε offset ; Step S150, determine the composite absolute strain variable of the sensing optical fiber according to the corresponding variable and the offset strain variable ε offset ​ 6. The high-precision large-dynamic absolute strain measurement method according to claim 5, characterized in that The specific steps of the step S110 include: performing Hilbert transform and phase unwrapping on the Rayleigh scattering signal to obtain the corresponding strain variable of the sensing optical fiber Performing short-time Fourier transform and Lorentz fitting on the Brillouin scattering signal in sequence to obtain the Brillouin frequency shift v B (t); Before the step S110, it further includes: processing the Rayleigh scattered signal by using a spectrum drift suppression and interference fading suppression algorithm to reduce the low-frequency drift amount of the Rayleigh scattered signal.

7. The high-precision large-dynamic absolute strain measurement method according to claim 5, characterized in that The specific steps of S120 include: mapping the corresponding variable to an equivalent Brillouin frequency shift Among them, C ε represents the Brillouin frequency shift sensitivity coefficient, and ε0 represents the initial strain of the sensing optical fiber.

8. The high-precision large-dynamic absolute strain measurement method according to claim 5, characterized in that In the step S130, according to the equivalent Brillouin frequency shift amount and the average number N of Brillouin spectra of the Brillouin scattering signal, the equivalent Brillouin spectrum g B,eq (v, t) is determined according to the following formula: where i is an integer and less than or equal to N, v represents the frequency on the frequency axis of the Brillouin gain spectrum of the Brillouin scattering signal, t represents time, and Δv B represents the full width at half maximum of the Brillouin spectrum of the Brillouin scattering signal.

9. The high-precision large-dynamic absolute strain measurement method according to claim 5, wherein The step S140 specifically includes: Step S141, according to the equivalent relative Brillouin frequency shift and the Brillouin frequency shift amount v of the Brillouin scattering signal B (t), the offset Brillouin frequency shift amount v is obtained by fitting according to the following formula offse : where t represents the fitting time; Step S142: According to the offset Brillouin frequency shift amount v offset , determine the offset strain ε according to the following formula offset : ε offse = v offset / C ε , where C ε represents the Brillouin frequency shift sensitivity coefficient; wherein, as the fitting time t in the step S140 increases, the determination accuracy of the bias strain ε offset is improved, but the low-frequency drift amount of the relative strain increases; t is the fitting time when the low-frequency drift amount of the Rayleigh scattering signal is equal to the frequency shift uncertainty of the Brillouin scattering signal after averaging the Brillouin scattering spectrum N times.

10. The high-precision large-dynamic absolute strain measurement method according to claim 5, characterized in that The specific steps of S150 include: according to the corresponding variable and the offset strain variable ε offset , determine the composite absolute strain variable of the sensing optical fiber according to the following formula

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