Few-longitudinal-mode botdr flowing sand dune displacement rate monitoring system and method

Abstract

The application provides a few-longitudinal-mode BOTDR flowing dune displacement rate monitoring system and method, and belongs to the technical field of geographic observation and simulation. The system comprises a distributed sensing monitoring system and a groove fixing body for optical fiber laying. The few-mode sensing optical fiber in the distributed sensing monitoring system is fixedly laid in the to-be-measured dune area by the groove fixing body, so that the flowing dune displacement direction is longitudinal. The few-mode sensing optical fiber is first laid in the to-be-measured dune area in longitudinal equal intervals and then laid in the upper part of the longitudinal laying part in transverse equal intervals. The distributed sensing monitoring system is used for signal analysis by using the backscattering signal and reference light of the optical fiber, obtaining the Brillouin scattering spectrum distributed along the few-mode single-core sensing optical fiber, establishing the distribution rule of strain on the whole sensing optical fiber according to the linear relationship between BFS and strain, and further detecting the transverse and longitudinal flowing dune displacement rate of the optical fiber.

Description

Technical Field

[0001] This invention relates to the field of geographic observation and simulation technology, and in particular to a system and method for monitoring the displacement rate of flowing dunes based on a few longitudinal modes BOTDR. Background Technology

[0002] This invention relates to a distributed optical fiber sensing system for measuring the displacement rate of mobile dunes (crescent-shaped dunes), providing a high-resolution, interference-resistant method for detecting mobile dunes in deserts. By employing a few longitudinal mode stimulated Brillouin threshold superposition technique and a combined longitudinal and transverse fiber laying method, the frequency and spatial resolution of the Brillouin fiber distributed sensing system are improved, enabling long- and short-period monitoring of mobile dunes. By establishing the linear relationship between the Brillouin frequency shift spectrum and strain, the strain distribution law along the entire sensing fiber is established, thereby calculating the displacement rate of the mobile dune. This system can achieve measurement accuracy down to the centimeter level, filling the gap in mobile dune measurement at this resolution and providing an efficient technical solution and data support for desert research and desert control.

[0003] Traditional methods for monitoring mobile sand dunes mostly rely on ground marking, GPS positioning, or image remote sensing technology, which have some limitations. For example, ground marking requires regular manual measurements, which is inefficient and makes continuous monitoring difficult; GPS positioning accuracy is affected by the complex terrain of the sand dune surface, making it difficult to achieve centimeter-level accuracy; although image remote sensing technology can cover large areas, its spatial resolution is limited and it is easily affected by weather and lighting conditions, making it difficult to achieve high-precision, long-term, and short-term continuous monitoring in complex desert environments. Summary of the Invention

[0004] In view of this, the present invention provides a system and method for monitoring the displacement rate of flowing dunes based on a few longitudinal modes BOTDR, which is used for continuous segmented monitoring of the displacement rate of flowing dunes, and realizes high-precision, long-cycle and short-cycle continuous monitoring in complex desert environments.

[0005] The technical solution adopted by the embodiments of the present invention to solve its technical problem is as follows:

[0006] A flow dune displacement rate monitoring system based on few longitudinal mode BOTDR includes a distributed sensing monitoring system and a grooved fixing body for fiber optic deployment, wherein:

[0007] The few-mode sensing fiber under test in the distributed sensing monitoring system is fixedly laid in the dune area under test by the groove fixing body. The dune area under test includes the windward slope, the top of the dune, the sandfall slope, and the left and right wings in sequence according to the direction of the displacement of the sand dune. Let the direction of the displacement of the sand dune be longitudinal. The laying method of the few-mode sensing fiber under test is as follows: the fiber enters from the initial windward slope, is first laid longitudinally at equal intervals, and then is laid laterally at equal intervals starting from the windward slope. Both the longitudinal and lateral wiring parts cover the dune area under test.

[0008] The distributed sensing and monitoring system is used to: split a narrow-linewidth laser source with few longitudinal modes into beams; after pulse modulation, amplification, and spontaneous emission noise filtering in the upper branch, the beams enter the few-mode sensing fiber under test; receive the backscattered light signal from the few-mode sensing fiber under test for coupling; after fixed-drive frequency modulation and passive polarization noise filtering in the lower branch, the beams are used for coupling; finally, the two beams are coupled into a single beam, which is then converted into an IF signal for photoelectric signal acquisition and transmitted to the host computer for signal analysis to detect the transverse and longitudinal displacement rates of sand dunes in the optical fiber.

[0009] The distributed sensing and monitoring system has M modes. The host computer obtains the Brillouin scattering spectrum distributed along the few-mode single-core sensing fiber based on the photoelectric signal, and calculates the frequency shift of the Brillouin Gain Spectrum (BGS). Then, based on the linear relationship between the Brillouin Frequency Shift (BFS) spectrum and strain, it establishes the distribution law of strain along the entire sensing fiber. The sensing point of the sensing fiber under test corresponds to the spatial position of the monitored flowing sand dune. The position of the strain distribution is determined using the principle of optical time-domain reflectometry.

[0010] The longitudinal long-period flow dune displacement rate V1 of the optical fiber is V1=δ / Δt V1 , Δt V1 The time taken to measure strain at adjacent sensing nodes in the longitudinal wiring section is δ, where δ is the spatial resolution.

[0011] The transverse short-period flow dune displacement rate of the optical fiber is expressed as {V 2i}, i∈[1,I], V 2i =SR / Δt i Where I is the number of lateral partitions in the dune region to be measured, and Δt i SR is the time taken to measure the strain between any two adjacent fiber segments in the i-th partition, and SR is the fiber segment spacing in the lateral cabling section.

[0012] Preferably, the distributed sensing and monitoring system consists of a few-mode FP laser (1), a first fiber polarization-maintaining coupler (2), a first electro-optic modulator (3), an arbitrary waveform function generator (4), an erbium-doped fiber amplifier (5), a first circulator (6), a BFG filter (7), a second circulator (8), a few-mode sensing fiber under test (9), a second electro-optic modulator (10), a radio frequency microwave frequency shifter (11), a passive polarization noise cancellation subsystem (16), a second fiber polarization-maintaining coupler (17), a balanced photodetector (18), a multi-channel acquisition card (19), and a host computer (20).

[0013] The few-longitudinal-mode FP laser (1) outputs a few-longitudinal-mode narrow-linewidth laser source, which is then split into upper and lower outputs via the first fiber polarization-maintaining coupler (2):

[0014] The continuous light from the upper branch is fed into the first electro-optic modulator (3), which modulates the continuous light pulse from the upper branch into pulsed light based on the arbitrary waveform function modulated by the arbitrary waveform function generator (4). The pulsed light is amplified by the erbium-doped fiber amplifier (5), enters the BFG filter (7) through the first circulator (6) to filter out spontaneous emission noise (ASE), and then enters the few-mode test sensing fiber (9) through the second circulator (8). The second circulator (8) receives the backscattered light signal from the few-mode test sensing fiber (9) and inputs it into the second fiber polarization-maintaining coupler (17).

[0015] The continuous light from the lower branch is fed into the second electro-optic modulator (10), which modulates the continuous light from the lower branch into frequency-modulated light based on the fixed driving frequency provided by the microwave frequency shifter (11) at the radio frequency end; the frequency-modulated light becomes reference light after being processed by the passive polarization noise cancellation subsystem (16) to suppress polarization noise, and the reference light is fed into the second fiber polarization-maintaining coupler (17).

[0016] The second fiber polarization maintaining coupler (17) couples the two input optical paths into a single beam of light and sends it to the balanced photodetector (18). The single beam of light is converted into an intermediate frequency IF signal by the balanced photodetector (18) and then sent to the multi-channel acquisition card (19) for acquisition. The acquired electrical signal is transmitted to the host computer (20) for signal analysis.

[0017] Preferably, the passive polarization noise cancellation subsystem (16) consists of a polarization beam splitter (12), a polarization-maintaining fiber (14), a polarization controller (15), and a polarization combiner (13) connected in sequence. The polarization beam splitter (12) splits the polarization beams of the mutually orthogonal frequency-modulated light into two paths. One path is directly transmitted to the polarization combiner (13), and the other path is processed by the polarization-maintaining fiber (14) and the polarization controller (15) in sequence before being transmitted to the polarization combiner (13). The polarization combiner (13) combines the two input paths and outputs them to the second fiber polarization-maintaining coupler (17). The polarization-maintaining fiber (14) serves as a delay fiber.

[0018] Preferably, the longitudinal wiring section of the few-mode sensor fiber (9) is laid in a transverse serpentine pattern, starting from the windward slope; the total length L of the longitudinal wiring section of the fiber is... ver For L ver = (a+b0)×m, where a is the width of the coverage area, b0 is the vertical cabling spacing, and m is the number of vertical cabling segments;

[0019] The lateral wiring section of the few-mode sensing fiber (9) is laid in a longitudinal reciprocating serpentine pattern, starting from the windward slope; the total length L of the lateral wiring section is... hor For L hor =L*n, where L is the width of the coverage area and n is the number of horizontal cabling segments.

[0020] Preferably, the spatial resolution δ is calculated as δ = TC / 2n eff Where T is the pulse duration, C is the speed of light, and n eff The refractive index of the optical fiber is given.

[0021] Preferably, the wavelength of the few longitudinal mode FP laser (1) outputting the few longitudinal mode narrow linewidth laser source is located in the C-band of optical fiber communication, and is narrow linewidth;

[0022] The upper limit threshold of the source output power value of the few longitudinal mode FP laser (1) The expression is:

[0023]

[0024] Z = L ver +L hor

[0025] In the formula, γ is the polarization factor, and A eff L represents the effective area of ​​the optical fiber. eff n is the effective fiber length. eff For the effective refractive index, P 12λ is the optical elastic coefficient, c is the speed of light in the optical fiber, λ is the wavelength, ρ is the density of the medium, and v is the optical elastic coefficient. a For the speed of sound, denoted as Brillouin gain superposition linewidth, Z as the total length of the fiber under test, and α as the fiber loss coefficient.

[0026] Preferably, the process for determining the number of modes M is as follows:

[0027] The frequency resolution δν of the distributed sensing and monitoring system B The expression is:

[0028]

[0029] In the formula, Let SNR be the superimposed linewidth of the Brillouin gain, and SNR be the system signal-to-noise ratio. The expression is:

[0030]

[0031] In the formula, Δν B0 It is a pure quartz Brillouin linewidth; These are the maximum and minimum values ​​of the Brillouin divergence radio frequency shift for different modes coupled into the few-mode sensing fiber, respectively; i det (t) represents the coherent detection output photocurrent of the photodetector; and These represent thermal noise and shot noise, respectively; R is the photodetector sensitivity; k is the Boltzmann constant; T is the thermodynamic temperature of the material; and P... Lq For the lower sideband of the local oscillator light of the q-th longitudinal mode, P Sq (t) represents the power of the Stokes light generated by the q-th longitudinal mode at time t; Δf is the balanced photodetector bandwidth, R L q is the load resistance; I is the charge quantity. d For dark current, ω Bq ω is the Brillouin frequency shift frequency generated by the longitudinal mode q. L φ is the angular frequency of the microwave modulation signal applied to the EOM RF terminal. q (t) represents the phase difference between the longitudinal mode q of the local oscillator and the Brillouin scattered light generated by the oscillator itself;

[0032] Optimal frequency resolution max(δν) B The corresponding number of modes is the number of modes M.

[0033] Preferably, the grooved fixing body is composed of a base support (21), a grooved base (22), and a grooved fiber optic track (26); the base support (21) has a raised square grid array structure (24) at the bottom, and the base support (21) is fixed on the sand dune by ultra-long fine rivets (27); the upper surface of the base support (21) is welded to the lower surface of the grooved base (22) to form an integral structure, the inner surface of the groove of the grooved base (22) is connected to the outer surface of the grooved fiber optic track (26) by hot melt adhesive tape, the inner surface of the groove of the grooved fiber optic track (26) is a black pressure-sensitive adhesive tape surface for fixing the fiber, and the few-mode test sensing fiber (9) is fixed and supported by a set of grooved fiber optic tracks (26).

[0034] Preferably, the host computer is used to establish a time-strain relationship curve through Brillouin scattering spectrum, and further perform data analysis on the relationship curve to obtain the strain changes of different groups of optical fiber segments corresponding to the monitoring area, thereby obtaining the local and overall morphological structure changes within the monitoring area.

[0035] This invention provides a method for monitoring the displacement rate of flowing dunes based on a few longitudinal modes BOTDR, the implementing entity being the aforementioned system for monitoring the displacement rate of flowing dunes based on a few longitudinal modes BOTDR, and the steps including:

[0036] Step S1: Deploy the BOTDR flow dune displacement rate monitoring system based on few longitudinal modes, divide the dune area to be measured into I partitions laterally, and determine the number of modes M of the distributed sensing monitoring system.

[0037] In step S2, the distributed sensing and monitoring system splits a few-mode narrow-linewidth laser source into beams. The upper branch performs pulse modulation, amplification, and spontaneous emission noise filtering before inputting the beams into the few-mode sensing fiber located at the dune to be measured for measurement. The lower branch performs fixed drive frequency modulation and passive polarization noise filtering. Finally, the two beams are coupled into a single beam, which is then converted into an IF signal for photoelectric signal acquisition and transmission to the host computer.

[0038] Step S3: The host computer plots the Brillouin frequency shift spectrum based on the photoelectric signal to detect the transverse and longitudinal displacement rates of the sand dunes flowing through the optical fiber.

[0039] The longitudinal long-period flow dune displacement rate V1 of the optical fiber is V1=δ / Δt V1 , Δt V1 The time taken to measure strain at adjacent sensing points in the longitudinal wiring section, where δ is the spatial resolution;

[0040] The transverse short-period flow dune displacement rate of the optical fiber is expressed as {V 2i}, i∈[1,I], V2i =SR / Δt i , where Δt i SR is the time consumed for measuring the strain between any two adjacent fiber segments in the i-th partition, and SR is the segment spacing of the fiber in the transverse cabling section.

[0041] In step S4, the host computer establishes a time-strain relationship curve based on the Brillouin scattering spectrum, and further analyzes the relationship curve to obtain the strain changes of different groups of optical fiber segments corresponding to the monitoring area, thereby obtaining the local and overall morphological structure changes within the monitoring area.

[0042] As can be seen from the above technical solution, the Brillouin scattering (BFS) system and method for monitoring the displacement rate of flowing dunes based on few-mode BOTDR provided in this embodiment of the invention includes a distributed sensing system and a grooved fixing body for fiber optic deployment. In the distributed sensing system, the few-mode sensing fiber under test is fixedly laid in the dune region under test by the grooved fixing body, making the displacement direction of the flowing dune longitudinal. The few-mode sensing fiber under test is first laid longitudinally at equal intervals to cover the dune region under test, and then laid laterally at equal intervals above the longitudinally laid portion. The distributed sensing system is used to perform signal analysis using the backscattered signal of the fiber and the reference light to obtain the Brillouin scattering spectrum distributed along the few-mode single-core sensing fiber. Based on the linear relationship between BFS and strain, the strain distribution law on the entire sensing fiber is established, and the transverse and longitudinal displacement rates of the flowing dunes are further detected. This invention introduces a few longitudinal mode stimulated Brillouin threshold superposition method, combined with an optimized optical fiber laying scheme, to achieve continuous segmented monitoring of the displacement rate of moving sand dunes. This makes the distributed sensing method proposed in this invention applicable to complex and harsh desert environments, and allows for more accurate and reliable measurement of the displacement rate of moving sand dunes. Attached Figure Description

[0043] Figure 1 This is a map showing the spatial displacement rate monitoring of the system's flowing sand dunes;

[0044] Figure 2 It is a system-wide monitoring map of mobile sand dunes.

[0045] Figure 3 This is a block diagram of the distributed optical fiber sensing system used in this invention;

[0046] Figure 4 This is a schematic diagram of the longitudinal laying of the optical fiber to be tested;

[0047] Figure 5 This is a schematic diagram of the horizontal laying of the optical fiber to be tested;

[0048] Figure 6 This is a schematic diagram of the Brillouin scattering spectrum of the optical fiber under test.

[0049] Figure 7 This is a schematic diagram of the overall optical fiber fixing groove.

[0050] Figure 8 yes Figure 7 Schematic diagram of the optical fiber support structure.

[0051] Figure 9 yes Figure 7 Schematic diagram of the grooved fiber optic track body and ultra-long fine rivet structure.

[0052] Figure 10 yes Figure 7 Schematic diagram of the fixed groove body.

[0053] In the figure: 1. Few-mode FP laser; 2. First fiber polarization-maintaining coupler; 3. First electro-optic modulator; 4. Arbitrary waveform function generator; 5. Erbium-doped fiber amplifier; 6. First circulator; 7. BFG filter; 8. Second circulator; 9. Few-mode sensor fiber under test; 10. Second electro-optic modulator; 11. RF end microwave frequency shifter; 12. Polarization beam splitter; 13. Polarization combiner; 14. Polarization-maintaining fiber; 15. Polarization controller; 16. Passive polarization noise cancellation subsystem; 17. Second fiber polarization-maintaining coupler; 18. Balanced photodetector; 19. Multi-channel acquisition card; 20. Host computer; 21. Substrate support base; 22. Grooved base; 23. Rivet body connection hole; 24. Raised square array structure; 25. Black pressure-sensitive adhesive tape surface; 26. Grooved fiber track body; 27. Extra-long fine rivet; 28. Rivet gun head body; 29. ​​Rivet top cap; 30. Groove fixing; 31. Fiber fixing groove assembly. Detailed Implementation

[0054] This invention addresses the shortcomings of existing technologies by proposing a mobile dune displacement rate monitoring system based on few-mode stimulated Brillouin threshold superposition (BOTDR). This system achieves high-precision, long-period, and short-period continuous monitoring by optimizing the fiber optic deployment scheme and combining few-mode fiber mode division multiplexing technology. The system employs a combined longitudinal and transverse fiber deployment method, with longitudinal fibers used for long-period monitoring and transverse fibers for short-period monitoring. In this way, the system can accurately measure the displacement rate of mobile dunes at different time scales, while breaking through the spatial resolution limitations of traditional technologies to achieve centimeter-level or even higher measurement accuracy. The system also includes a fiber optic fixing groove structure for stable fiber deployment in complex desert environments, ensuring the reliability and durability of the monitoring system.

[0055] The technical solution and effects of the present invention will be further described in detail below with reference to the accompanying drawings.

[0056] This invention provides a BOTDR-based system for monitoring the displacement rate of flowing dunes with few longitudinal modes, used for measuring the displacement rate of flowing dunes (crescent-shaped dunes), with reference to... Figure 1As shown, it includes a distributed sensing and monitoring system and a grooved fixing body for fiber optic deployment, wherein:

[0057] refer to Figure 3 As shown, the distributed sensing and monitoring system consists of a few-mode FP laser (1), a first fiber polarization-maintaining coupler (2), a first electro-optic modulator (3), an arbitrary waveform function generator (4), an erbium-doped fiber amplifier (5), a first circulator (6), a BFG filter (7), a second circulator (8), a few-mode sensing fiber under test (9), a second electro-optic modulator (10), a radio frequency microwave frequency shifter (11), a passive polarization noise cancellation subsystem (16), a second fiber polarization-maintaining coupler (17), a balanced photodetector (18), a multi-channel acquisition card (19), and a host computer (20).

[0058] The few-longitudinal-mode FP laser (1) outputs a few-longitudinal-mode narrow-linewidth laser source, which is split into upper and lower outputs via a first fiber polarization-maintaining coupler (2):

[0059] The continuous light from the upper branch is fed into the first electro-optic modulator (3). The first electro-optic modulator (3) modulates the continuous light pulse from the upper branch into pulsed light based on the arbitrary waveform function modulated by the arbitrary waveform function generator (4). The pulsed light is amplified by the erbium-doped fiber amplifier (5), enters the BFG filter (7) through the first circulator (6) to filter out the spontaneous emission noise (ASE), and then enters the few-mode test sensing fiber (9) through the second circulator (8). The second circulator (8) receives the backscattered light signal from the few-mode test sensing fiber (9) and inputs it into the second fiber polarization-maintaining coupler (17).

[0060] The continuous light from the lower branch is fed into the second electro-optic modulator (10), which modulates the continuous light from the lower branch into frequency-modulated light based on the fixed driving frequency provided by the microwave frequency shifter (11) at the radio frequency end. The frequency-modulated light becomes the reference light after being processed by the passive polarization noise cancellation subsystem (16) to suppress polarization noise. The reference light is fed into the second fiber polarization-maintaining coupler (17). The passive polarization noise cancellation subsystem (16) consists of a polarization beam splitter (12) and a polarization-maintaining fiber (13) connected in sequence by the optical path. 4) The polarization controller (15) and polarization combiner (13) are composed of polarization controller (12) and polarization combiner (13). The polarization splitter (12) splits the polarization beams of the mutually orthogonal frequency-modulated light into two paths. One path is directly transmitted to polarization combiner (13), and the other path is processed by polarization-maintaining fiber (14) and polarization controller (15) in sequence before being transmitted to polarization combiner (13). Polarization combiner (13) combines the two input beams and outputs them to the second fiber polarization-maintaining coupler (17). Polarization-maintaining fiber (14) is used as delay fiber.

[0061] The second fiber polarization maintaining coupler (17) couples the two input optical paths into a single beam and sends it to the balanced photodetector (18). The single beam is converted into an intermediate frequency IF signal by the balanced photodetector (18) and then sent to the multi-channel acquisition card (19) for acquisition. The acquired electrical signal is transmitted to the host computer (20) for signal analysis.

[0062] In a distributed sensing and monitoring system, the few-mode sensing fiber under test is fixed and laid in the sand dune area to be measured by a grooved fixing body. (Reference) Figure 2 As shown, the dune area to be measured includes, in order of direction of dune displacement, the windward slope, the dune crest, the sandfall slope, and the left and right wings, as referenced. Figure 4 and Figure 5 As shown, with the displacement direction of the flowing sand dune as longitudinal, the laying method of the few-mode sensing fiber under test is as follows: the fiber enters from the initial windward slope, is first laid longitudinally at equal intervals, and then laid laterally at equal intervals starting from the windward slope. Both the longitudinal and lateral wiring sections cover the area of ​​the sand dune under test; the longitudinal wiring section of the few-mode sensing fiber under test (9) is laid in a lateral reciprocating serpentine route starting from the windward slope; the total length L of the longitudinal wiring section of the fiber is... ver For L ver = (a + b0) × m, where a is the width of the coverage area, b0 is the longitudinal cabling spacing, and m is the number of longitudinal cabling segments; (Since the longitudinal cabling spacing is relatively wide, the total length L is calculated as follows) ver The spacing b0 needs to be considered; the horizontal cabling of the few-mode sensor fiber (9) is laid from the windward slope in a longitudinal reciprocating serpentine route; the total length L of the horizontal cabling is... hor For L hor =L*n, where L is the width of the coverage area and n is the number of horizontal cabling segments (since the horizontal cabling is relatively dense and the spacing is very small, the total length L is calculated). hor (Spacing does not need to be considered at this time).

[0063] The distributed sensing and monitoring system is used to: split a narrow-linewidth laser source with few longitudinal modes into beams; after pulse modulation, amplification, and spontaneous emission noise filtering in the upper branch, the beams enter the fiber under test with few modes and are used for coupling; after backscattered light signals from the fiber under test with few modes are received in the lower branch, the beams are used for coupling after fixed drive frequency modulation and passive polarization noise filtering in the lower branch; finally, the two beams are coupled into a single beam, which is then converted into an IF signal for photoelectric signal acquisition and transmission to the host computer for signal analysis, thereby detecting the transverse and longitudinal displacement rates of sand dunes in the fiber.

[0064] The distributed sensing and monitoring system has M modes. The Brillouin scattering spectrum distributed along a few-mode single-core sensing fiber is measured (reference). Figure 6(as shown), and calculate the frequency shift of BGS. Then, based on the linear relationship between BFS and strain, establish the distribution law of strain on the entire sensing fiber. The sensing point of the sensing fiber under test corresponds to the spatial position of the monitored flowing sand dune. The position of strain distribution is determined using the principle of optical time-domain reflection.

[0065] The host computer is also used to establish the relationship curve between time and strain through Brillouin scattering spectrum, and further analyze the data of the relationship curve to obtain the strain change of different groups of optical fiber segments corresponding to the monitoring area (the strain change includes the amount and rate of change with time, which can reflect the change of the aggregation height and width of each monitoring area with time). Thus, the local and overall morphological structure changes in the monitoring area (e.g., the amount and rate of change of aggregation width and height with time) and the future changes based on existing data (aggregation width and height at future times) are obtained.

[0066] The longitudinal long-period flow dune displacement rate V1 of the optical fiber is V1=δ / Δt V1 , Δt V1 δ represents the time taken for adjacent sensing nodes in the longitudinal wiring section to measure strain, and δ is the spatial resolution; (where spatial resolution is used to define the spacing between sensing points per unit length).

[0067] refer to Figure 2 and Figure 5 As shown, the direction of displacement of the flowing sand dune is taken as the standard direction, and the direction perpendicular to it is the horizontal direction of the fiber optic cable. The length unit of the monitored fiber optic cable is centimeters. The initial sensing point of the horizontal detection of FMF-MCF is defined as X0, the width of the flowing sand dune is L, the arc length formed by the sandfall slope of the flowing sand dune is C, and the distance between the first segment of sensing fiber and the second segment of sensing fiber is SR. The horizontal sensing fiber is laid out at equal intervals. The horizontal sensing fiber is divided into I segments to be monitored as X0+L, 2(X0+L), ..., i(X0+L), ..., (I-1)(X0+L), I(X0+L). The segments to be monitored in the horizontally laid sensing fiber optic cable are grouped into a group of sensing fiber optic cables with y segments. There are a total of I = n / y monitoring groups. The monitored BFS corresponds one-to-one with the number of transversely deployed sensing fibers. The strain values ​​of the flowing dunes corresponding to the Brillouin frequency shift spectrum are divided into I monitoring regions, each monitoring region -i corresponding to monitoring group -i; monitoring region -1: X0-(X0+yL), monitoring region -2: (X0+yL)-(X0+2yL), ..., monitoring region -i: (X0+(i-1)yL)-(X0+iyL), ..., monitoring region -(I-1): (X0+(I-2)yL)-(X0+(I-1)yL), monitoring region -I: (X0+(I-1)yL)-(X0+IyL); let V 2iLet $V$ represent the transverse short-period flow dune displacement rate of partition $i$. Then, the transverse short-period flow dune displacement rate of the optical fiber in the entire monitoring area is expressed as $V$. 2i}, i∈[1,I], V 2i =SR / Δt i Where I is the number of lateral partitions in the dune region to be measured, and Δt i Let SR be the time taken to measure the strain between any two adjacent fiber segments in the i-th partition, and SR be the fiber segment spacing in the lateral cabling section. (The purpose of partitioning is to simplify the calculation process. Without partitioning, it would be necessary to read the time taken to measure the strain between all adjacent fiber segments and perform rate calculations. By partitioning and selecting data from any adjacent ends within a region for a single calculation, i.e., each region is calculated only once, the computational load can be effectively reduced and the analysis efficiency improved.)

[0068] In this invention, the spatial resolution δ of the system is calculated as δ = TC / 2n eff Where T is the pulse duration, C is the speed of light, and n eff The refractive index of the optical fiber is given.

[0069] The wavelength of the few longitudinal mode FP laser (1) outputs a few longitudinal mode narrow linewidth laser source located in the C-band of optical fiber communication, which is narrow linewidth;

[0070] When the longitudinal mode spacing of the light source output is much larger than the natural linewidth of Brillouin and the coherence length is less than the effective interaction length of SBS, the fiber SBS threshold is proportional to the number of longitudinal modes, and the upper limit threshold of the light source output power value of the few longitudinal mode FP laser (1) is... The expression is:

[0071]

[0072] Z = L ver +L hor (4)

[0073] In the formula, γ is the polarization factor, and A eff L represents the effective area of ​​the optical fiber. eff n is the effective fiber length. eff For the effective refractive index, P 12 λ is the optical elastic coefficient, c is the speed of light in the optical fiber, λ is the wavelength (1550 nm), ρ is the density of the medium, and v is the optical elastic coefficient. a For the speed of sound, denoted as Brillouin gain superposition linewidth, Z as the total length of the fiber under test, and α as the fiber loss coefficient.

[0074] In the system, the process of determining the number of modes M corresponding to the optimal frequency resolution is as follows:

[0075] Frequency resolution δν of distributed sensing and monitoring system BThe expression is:

[0076]

[0077] Because the incident angles of light differ in different modes of FMF, each mode generates its own Brillouin gain spectrum (BGS) as it propagates in the optical fiber. The interaction between these modes causes their BGS to broaden, resulting in the superposition of the Brillouin gain linewidth. The expression is:

[0078]

[0079] In the formula, Δν B0 It is a pure quartz Brillouin linewidth; These are the maximum and minimum values ​​of the Brillouin divergence radio frequency shift coupled into different modes of FMF, respectively;

[0080] Assuming that the power of each longitudinal mode of the light source is approximately equal, and neglecting the nonlinear noise effects generated by SPM, XPM, and FWM when considering system noise, the system signal-to-noise ratio (SNR) expression is as follows:

[0081]

[0082] In the formula, and These represent thermal noise and shot noise, respectively; R is the photodetector sensitivity; k is the Boltzmann constant; T is the thermodynamic temperature of the material; and P... Lq For the lower sideband of the local oscillator light of the q-th longitudinal mode, P Sq (t) represents the power of the Stokes light generated by the q-th longitudinal mode at time t; Δf is the balanced photodetector bandwidth, R L q is the load resistance; I is the charge quantity. d It is dark current;

[0083] Photodetector coherent detection output photocurrent i det The expression for (t) is:

[0084]

[0085] In the formula, ω Bq ω is the Brillouin frequency shift frequency generated by the longitudinal mode q. L φ is the angular frequency of the microwave modulation signal applied to the EOM RF terminal. q (t) represents the phase difference between the longitudinal mode q of the local oscillator and the Brillouin scattered light generated by the oscillator itself;

[0086] Optimal frequency resolution max(δν) B The corresponding number of patterns is the number of patterns M.

[0087] Reference Figures 7-10The grooved fixing body consists of a base support (21), a grooved base (22), and a grooved fiber optic track (26). The base support (21) has a raised grid array structure (24) at its bottom and is fixed to the sand dune by ultra-long fine rivets (27). The upper surface of the base support (21) is welded to the lower surface of the grooved base (22) to form an integral structure. The inner surface of the groove of the grooved base (22) is connected to the outer surface of the grooved fiber optic track (26) by hot melt adhesive tape. The inner surface of the groove of the grooved fiber optic track (26) is a black pressure-sensitive adhesive tape surface used to fix the fiber. The few-mode sensor fiber (9) is fixed and supported by a set of grooved fiber optic tracks (26). The black pressure-sensitive adhesive tape is a type of plastic tape used for pasting and fixing. The length of the ultra-long fine rivets used to assist in the fiber optic deployment should be much greater than the length of the base support, and its length is generally 0.5-1 meter, which is more suitable for soft and complex desert geographical environments.

[0088] Furthermore, based on the aforementioned BOTDR-based flow dune displacement rate monitoring system with few longitudinal modes, the present invention provides a real-time BOTDR-based flow dune displacement rate monitoring method, comprising the following steps:

[0089] Step S1: Deploy a BOTDR-based flow dune displacement rate monitoring system with few longitudinal modes, divide the dune area to be measured into I horizontal partitions, and determine the number of modes M of the distributed sensing monitoring system.

[0090] In step S2, the distributed sensing and monitoring system splits the few longitudinal mode narrow linewidth laser source into beams. The upper branch performs pulse modulation, amplification, and spontaneous emission noise filtering before inputting the beam into the few mode sensing fiber located at the dune to be measured for measurement. The lower branch performs fixed drive frequency modulation and passive polarization noise filtering. Finally, the two beams are coupled into a single beam, which is then converted into an IF signal for photoelectric signal acquisition and transmission to the host computer.

[0091] Step S3: The host computer plots the Brillouin frequency shift spectrum based on the photoelectric signal to detect the transverse and longitudinal displacement rates of the sand dunes flowing through the optical fiber.

[0092] The longitudinal long-period flow dune displacement rate V1 of the optical fiber is V1=δ / Δt V1 , Δt V1 δ represents the time taken to measure strain at adjacent sensing points in the longitudinal wiring section, where δ is the spatial resolution.

[0093] The transverse short-period flow dune displacement rate of the optical fiber is expressed as {V 2i}, i∈[1,I], V 2i =SR / Δt i , where Δt iSR is the time taken to measure the strain between any two adjacent fiber segments in the i-th partition, and SR is the fiber segment spacing in the lateral cabling section.

[0094] In step S4, the host computer establishes a time-strain relationship curve based on the Brillouin scattering spectrum, and further analyzes the data of the relationship curve to obtain the strain changes of different groups of optical fiber segments corresponding to the monitoring area, thereby obtaining the local and overall morphological structure changes within the monitoring area.

[0095] This invention employs few-mode fiber (FMF) for deployment. Compared to traditional single-mode fiber, FMF offers a larger mode field area, extremely low intermodal dispersion, and a wider selection of transmission spectra. FMF can also significantly improve the stimulated Brillouin threshold through mode division multiplexing (MDM), thereby enhancing the overall system performance. MDM allows the simultaneous transmission of multiple modes of optical signals within the same fiber; by rationally designing mode distribution and power allocation, the sensing accuracy of the fiber can be effectively improved. A distributed fiber optic sensing system combining Brillouin optical time-domain reflectometry (BOTDR) with few-mode fiber (FMF-MCF) shows great potential. The advantages of FMF in distributed sensing make it an ideal choice for solving the displacement rate monitoring problem of moving dunes. The mode division multiplexing technology of FMF can significantly improve the stimulated Brillouin threshold, thereby enhancing the system's anti-interference capability and measurement accuracy. The low intermodal dispersion characteristics of FMF can reduce signal distortion and improve the system's spatial resolution. By rationally designing fiber optic deployment schemes, high-precision monitoring of the displacement rate of shifting sand dunes can be achieved. However, applying few-mode fiber technology to shifting sand dune monitoring requires addressing several practical challenges. For example, the desert environment is complex, with soft and unstable dune surfaces; fiber optic deployment necessitates considering fixation and protection measures to prevent damage from dune movement. Furthermore, monitoring the displacement rate of shifting sand dunes requires simultaneous long-term (e.g., monthly, yearly) and short-term (e.g., daily, weekly) monitoring, which places higher demands on the system's frequency and spatial resolution.

[0096] This invention's system achieves high-precision, long-period, and short-period continuous monitoring by optimizing the fiber optic laying scheme and combining it with few-mode fiber mode division multiplexing technology. The system employs a combined longitudinal and transverse fiber laying method, with longitudinal fibers used for long-period monitoring and transverse fibers for short-period monitoring. In this way, the system can accurately measure the displacement rate of shifting sand dunes at different time scales, while breaking through the spatial resolution limitations of traditional technologies to achieve centimeter-level or even higher measurement accuracy. The system also includes a fiber optic fixing groove structure for stable fiber laying in complex desert environments, ensuring the reliability and durability of the monitoring system.

[0097] This invention combines a few-mode single-core optical fiber with a cross-fiber laying method, enabling the system to achieve centimeter-level measurement accuracy. It can be used to monitor the displacement rate of shifting sand dunes, achieving high-precision, long-period, and short-period continuous monitoring in complex desert environments. This provides a new technical means for the scientific monitoring of shifting sand dunes and desertification control. Compared with previous shifting sand dune displacement rate monitoring systems, it has the following advantages:

[0098] 1. The BOTDR-based mobile dune displacement rate monitoring system based on few longitudinal modes and stimulated Brillouin threshold superposition, as described in this invention, offers advantages over traditional mobile dune displacement rate monitoring systems and image remote sensing technology. These advantages include non-destructive distributed and long-distance monitoring capabilities, resistance to electromagnetic interference, corrosion resistance, durability, and ease of installation. Combined with the horizontal and vertical joint deployment method proposed in this invention, it enables long-term monitoring in years or months, and short-term monitoring in weeks or days. This allows the system to achieve spatial resolution exceeding meters or sub-meter levels, reaching centimeter-level measurement accuracy. Furthermore, it enables continuous monitoring of mobile dune displacement rates.

[0099] 2. The DOFS (Displacement Rate of Fiber) system based on few-mode stimulated Brillouin threshold superposition (SBR) using BOTDR is described in this invention. From the perspective of the DOFS system itself, it employs a BOTDR combined with an optical and electrical system and an FMF-MCF (Fiber-Mechanical-Fluidized Type Fiber) as the sensing and monitoring system. Furthermore, by combining FMF-MCF with mode division multiplexing (MMD) technology, the increased number of modes in the few-mode fiber leads to a more than tenfold increase in the stimulated Brillouin threshold compared to single-mode fiber, thereby improving the overall system performance. Through MDD, the sensing accuracy at each point on the fiber optic link can be improved, and the sensing length can reach tens of kilometers, realizing distributed fiber optic sensing. Continuous segmented monitoring of multiple sets of data is applied to the monitoring of displacement rates of floating dunes, giving the DOFS system strong robustness and generalization capabilities.

[0100] The embodiments disclosed in this specification are merely illustrative of one aspect of the invention, and the scope of protection of the invention is not limited to these embodiments. Any other functionally equivalent embodiments fall within the scope of protection of the invention. Those skilled in the art can make various other corresponding changes and modifications based on the technical solutions and concepts described above, and all such changes and modifications should fall within the scope of protection of the claims of this invention.

Claims

1. A system for monitoring the displacement rate of flowing sand dunes based on a few longitudinal modes BOTDR, characterized in that, Includes a distributed sensing and monitoring system and a grooved fixture for fiber optic deployment, wherein: The few-mode sensing fiber under test in the distributed sensing monitoring system is fixedly laid in the dune area under test by the groove fixing body. The dune area under test includes the windward slope, the top of the dune, the sandfall slope, and the left and right wings in sequence according to the direction of the displacement of the sand dune. Let the direction of the displacement of the sand dune be longitudinal. The laying method of the few-mode sensing fiber under test is as follows: the fiber enters from the initial windward slope, is first laid longitudinally at equal intervals, and then is laid laterally at equal intervals starting from the windward slope. Both the longitudinal and lateral wiring parts cover the dune area under test. The distributed sensing and monitoring system is used to: split a narrow-linewidth laser source with few longitudinal modes into beams; after pulse modulation, amplification, and spontaneous emission noise filtering in the upper branch, the beams enter the few-mode sensing fiber under test; receive the backscattered light signal from the few-mode sensing fiber under test for coupling; after fixed-drive frequency modulation and passive polarization noise filtering in the lower branch, the beams are used for coupling; finally, the two beams are coupled into a single beam, which is then converted into an IF signal for photoelectric signal acquisition and transmitted to the host computer for signal analysis to detect the transverse and longitudinal displacement rates of sand dunes in the optical fiber. The distributed sensing and monitoring system has M modes. The host computer obtains the Brillouin scattering spectrum distributed along the few-mode single-core sensing fiber based on the photoelectric signal, calculates the frequency shift of the BGS, and then establishes the distribution law of strain along the entire sensing fiber based on the linear relationship between the BFS and strain. The sensing point of the sensing fiber under test corresponds to the spatial position of the monitored flowing sand dune. The position of the strain distribution is determined using the principle of optical time-domain reflectometry. The longitudinal long-period flow dune displacement rate V1 of the optical fiber is V1=δ / Δt V1 , Δt V1 The time taken to measure strain at adjacent sensing nodes in the longitudinal wiring section is δ, where δ is the spatial resolution. The transverse short-period flow dune displacement rate of the optical fiber is expressed as {V 2i }, i∈[1,I], V 2i =SR / Δt i Where I is the number of lateral partitions in the dune region to be measured, and Δt i SR is the time taken to measure the strain between any two adjacent fiber segments in the i-th partition, and SR is the fiber segment spacing in the lateral cabling section.

2. The BOTDR-based flow dune displacement rate monitoring system as described in claim 1, characterized in that, The distributed sensing and monitoring system consists of a few-mode FP laser (1), a first fiber polarization-maintaining coupler (2), a first electro-optic modulator (3), an arbitrary waveform function generator (4), an erbium-doped fiber amplifier (5), a first circulator (6), a BFG filter (7), a second circulator (8), a few-mode sensing fiber under test (9), a second electro-optic modulator (10), a radio frequency microwave frequency shifter (11), a passive polarization noise cancellation subsystem (16), a second fiber polarization-maintaining coupler (17), a balanced photodetector (18), a multi-channel acquisition card (19), and a host computer (20). The few-longitudinal-mode FP laser (1) outputs a few-longitudinal-mode narrow-linewidth laser source, which is then split into upper and lower outputs via the first fiber polarization-maintaining coupler (2): The continuous light from the upper branch is fed into the first electro-optic modulator (3), which modulates the continuous light pulse from the upper branch into pulsed light based on the arbitrary waveform function modulated by the arbitrary waveform function generator (4). The pulsed light is amplified by the erbium-doped fiber amplifier (5), enters the BFG filter (7) through the first circulator (6) to filter out spontaneous emission noise (ASE), and then enters the few-mode test sensing fiber (9) through the second circulator (8). The second circulator (8) receives the backscattered light signal from the few-mode test sensing fiber (9) and inputs it into the second fiber polarization-maintaining coupler (17). The continuous light from the lower branch is fed into the second electro-optic modulator (10), which modulates the continuous light from the lower branch into frequency-modulated light based on the fixed driving frequency provided by the microwave frequency shifter (11) at the radio frequency end; the frequency-modulated light becomes reference light after being processed by the passive polarization noise cancellation subsystem (16) to suppress polarization noise, and the reference light is fed into the second fiber polarization-maintaining coupler (17). The second fiber polarization maintaining coupler (17) couples the two input optical paths into a single beam of light and sends it to the balanced photodetector (18). The single beam of light is converted into an intermediate frequency IF signal by the balanced photodetector (18) and then sent to the multi-channel acquisition card (19) for acquisition. The acquired electrical signal is transmitted to the host computer (20) for signal analysis.

3. The BOTDR-based flow dune displacement rate monitoring system as described in claim 2, characterized in that, The passive polarization noise cancellation subsystem (16) consists of a polarization beam splitter (12), a polarization-maintaining fiber (14), a polarization controller (15), and a polarization combiner (13) connected in sequence. The polarization beam splitter (12) splits the polarization beams of the frequency-modulated light into two paths. One path is directly transmitted to the polarization combiner (13), and the other path is processed by the polarization-maintaining fiber (14) and the polarization controller (15) in sequence before being transmitted to the polarization combiner (13). The polarization combiner (13) combines the two input paths and outputs them to the second fiber polarization-maintaining coupler (17). The polarization-maintaining fiber (14) serves as a delay fiber.

4. The BOTDR-based flow dune displacement rate monitoring system as described in claim 3, characterized in that: The longitudinal cabling of the few-mode sensing fiber (9) is laid in a serpentine pattern, starting from the windward slope and proceeding along a transverse reciprocating path; the total length L of the longitudinal cabling is... ver For L ver = (a+b0)×m, where a is the width of the coverage area, b0 is the vertical cabling spacing, and m is the number of vertical cabling segments; The lateral wiring section of the few-mode sensing fiber (9) is laid in a longitudinal reciprocating serpentine pattern, starting from the windward slope; the total length L of the lateral wiring section is... hor For L hor =L*n, where L is the width of the coverage area and n is the number of horizontal cabling segments.

5. The BOTDR-based flow dune displacement rate monitoring system as described in claim 4, characterized in that: The spatial resolution δ is calculated as δ = TC / 2n eff Where T is the pulse duration, C is the speed of light, and n eff The refractive index of the optical fiber is denoted as .

6. The BOTDR-based flow dune displacement rate monitoring system as described in claim 5, characterized in that, The wavelength of the few longitudinal mode FP laser (1) outputting the few longitudinal mode narrow linewidth laser source is located in the C-band of optical fiber communication, and is narrow linewidth; The upper limit threshold of the source output power value of the few longitudinal mode FP laser (1) The expression is: Z=L ver +L hor In the formula, γ is the polarization factor, and A eff L represents the effective area of ​​the optical fiber. eff For the effective fiber length, P 12 λ is the optical elastic coefficient, c is the speed of light in the optical fiber, λ is the wavelength, ρ is the density of the medium, and v is the optical elastic coefficient. a For the speed of sound, denoted as Brillouin gain superposition linewidth, Z as the total length of the fiber under test, and α as the fiber loss coefficient.

7. The BOTDR-based flow dune displacement rate monitoring system as described in claim 6, characterized in that, The process of determining the number of modes M is as follows: The frequency resolution δν of the distributed sensing and monitoring system B The expression is: In the formula, Let SNR be the superimposed linewidth of the Brillouin gain, and SNR be the system signal-to-noise ratio. The expression is: In the formula, Δν B0 It is a pure quartz Brillouin linewidth; These are the maximum and minimum values ​​of the Brillouin divergence radio frequency shift for different modes coupled into the FMF few-mode sensing fiber; i det (t) represents the coherent detection output photocurrent of the photodetector; and These represent thermal noise and shot noise, respectively; R is the photodetector sensitivity; k is the Boltzmann constant; T is the thermodynamic temperature of the material; and P... Lq For the lower sideband of the local oscillator light of the q-th longitudinal mode, P Sq (t) represents the power of the Stokes light generated by the q-th longitudinal mode at time t; Δf is the balanced photodetector bandwidth, R L q is the load resistance; I is the charge quantity. d For dark current, ω Bq ω is the Brillouin frequency shift frequency generated by the longitudinal mode q. L φ is the angular frequency of the microwave modulation signal applied to the EOM RF terminal. q (t) represents the phase difference between the longitudinal mode q of the local oscillator and the Brillouin scattered light generated by the oscillator itself; Optimal frequency resolution max(δν) B The corresponding number of modes is the number of modes M.

8. The BOTDR-based flow dune displacement rate monitoring system as described in claim 7, characterized in that, The grooved fixing body consists of a base support (21), a grooved base (22), and a grooved fiber optic track (26). The base support (21) has a raised square grid array structure (24) at its bottom and is fixed to the sand dune by ultra-long fine rivets (27). The upper surface of the base support (21) is welded to the lower surface of the grooved base (22) to form an integral structure. The inner surface of the groove of the grooved base (22) is connected to the outer surface of the grooved fiber optic track (26) by hot melt adhesive tape. The inner surface of the groove of the grooved fiber optic track (26) is a black pressure-sensitive adhesive tape surface for fixing the fiber. The few-mode test sensing fiber (9) is fixed and supported by a set of grooved fiber optic tracks (26).

9. The BOTDR-based flow dune displacement rate monitoring system as described in claim 8, characterized in that, The host computer is used to establish a time-strain relationship curve through Brillouin scattering spectrum, and further analyze the relationship curve to obtain the strain changes of different groups of optical fiber segments corresponding to the monitoring area, thereby obtaining the local and overall morphological structure changes within the monitoring area.

10. A method for monitoring the displacement rate of flowing dunes based on a few longitudinal modulus BOTDR, characterized in that, The implementing entity is the BOTDR flow dune displacement rate monitoring system based on few longitudinal modes as described in any one of claims 1-8, and the steps include: Step S1: Deploy the BOTDR flow dune displacement rate monitoring system based on few longitudinal modes, divide the dune area to be measured into I partitions, and determine the number of modes M of the distributed sensing monitoring system. In step S2, the distributed sensing and monitoring system splits a few-mode narrow-linewidth laser source into beams. The upper branch performs pulse modulation, amplification, and spontaneous emission noise filtering before inputting the beams into the few-mode sensing fiber located at the dune to be measured for measurement. The lower branch performs fixed drive frequency modulation and passive polarization noise filtering. Finally, the two beams are coupled into a single beam, which is then converted into an IF signal for photoelectric signal acquisition and transmission to the host computer. Step S3: The host computer plots the Brillouin frequency shift spectrum based on the photoelectric signal to detect the transverse and longitudinal displacement rates of the sand dunes flowing through the optical fiber. The longitudinal long-period flow dune displacement rate V1 of the optical fiber is V1=δ / Δt V1 , Δt V1 The time taken to measure strain at adjacent sensing points in the longitudinal wiring section, where δ is the spatial resolution; The transverse short-period flow dune displacement rate of the optical fiber is expressed as {V 2i }, i∈[1,I], V 2i =SR / Δt i , where Δt i SR is the time consumed for measuring the strain between any two adjacent fiber segments in the i-th partition, and SR is the segment spacing of the fiber in the transverse cabling section. In step S4, the host computer establishes a time-strain relationship curve based on the Brillouin scattering spectrum, and further analyzes the relationship curve to obtain the strain changes of different groups of optical fiber segments corresponding to the monitoring area, thereby obtaining the local and overall morphological structure changes within the monitoring area.

Images