CN116592922A - Branch distributed positioning system based on single-core feedback interferometer - Google Patents

Abstract

The invention belongs to the technical field of optical fiber sensing, and particularly relates to a branch distributed positioning system based on a single-core feedback interferometer. The system combines two single-core feedback interferometers with different wavelengths through wavelength division multiplexing; the induction optical cable is divided into a main path and a branch path; multiple branches can extend out of the sensing optical cable; sensing that the optical cable is disturbed by the outside to cause the phase change of light in the optical fiber; demodulating interference signals with two different wavelengths generated by the same vibration to obtain two paths of phase difference signals; a group of signals with fixed time delay can be obtained through signal processing; determining whether the disturbance occurs on the main path or the branch path by using a positioning algorithm of the time delay estimation: the invention can not only distinguish whether the disturbance is applied to the main road or a certain branch road, but also accurately position the disturbance, and can reduce the complexity and the equipment cost of the sensing system.

Description

Branch Distributed Positioning System Based on Single-Core Feedback Interferometer

technical field

The invention belongs to the technical field of optical fiber sensing, in particular to a distributed optical fiber positioning system on branch roads.

Background technique

External disturbances will cause changes in the optical properties of the optical fiber, and optical fiber sensing technology is based on this characteristic. Nowadays, optical fiber sensing technology is developing rapidly. Among them, distributed optical fiber sensing technology is widely used in oil and gas pipelines, border security, bridge monitoring and other fields due to its high sensitivity, large dynamic range, and long monitoring distance.

At present, the distributed optical fiber sensing system is mainly based on optical time domain reflectometry (OTDR) and coherent interferometry. Among them, the interferometric optical fiber sensor can detect external signals by detecting the phase change of coherent light, which has higher real-time and sensitivity. In the past decades, various structures based on fiber optic interferometers have been proposed to realize distributed sensing systems, such as Michelson interferometer (MI), Mach-Zehnder interferometer (MZI) and Sagnac interferometer (SI), single-core feedback interferometer (linear Sagnac interferometer). In order to adapt to various complex actual engineering environments and improve sensor performance, some new interferometer structures and positioning algorithms have been proposed.

However, in many practical application scenarios, such as monitoring important optical communication networks or oil and gas pipelines, the directions of communication lines and pipelines may have multi-branches and multi-branches such as star and tree. In a multi-branch scenario, if you want to detect and locate the disturbance on the branch, using the existing technical solutions, you need to build and run multiple sensing systems at the same time, which will greatly increase the complexity and stability of the system. and incur higher costs.

The present invention proposes a branch positioning system based on a single-core feedback interferometer, which realizes the monitoring of branch and bifurcated lines with only one set of sensing systems, and can detect and locate disturbances on the main road and the branch , it can be distinguished whether the disturbance is on the main road or the branch road, and the specific location of the disturbance can be calculated.

Contents of the invention

Aiming at a monitoring line with multiple branches, the present invention proposes a sensing optical fiber line structure capable of extending multiple branches, that is, a branch distributed positioning system based on a single-core feedback interferometer.

The branch distributed positioning system based on the single-core feedback interferometer provided by the present invention combines two single-core feedback interferometers with different wavelengths through wavelength division multiplexing. The system structure is shown in Figure 1 . Among them, the induction optical cable is divided into main road and branch road; the main road uses one core in the optical cable, and the branch road uses two cores in the same optical cable; At the end far away from the main road, the two optical fiber tails of the branch road are connected to each other. Through this connection method, multiple branches can be extended from the sensing optical cable, generally 3-6 branches. The connection mode of the main road and the branch road is shown in Figure 2.

Based on the above structure, the sensing optical cable is disturbed by the outside, causing the phase change of the light in the optical fiber. The interference signals with two different wavelengths generated by the same vibration are demodulated to obtain two phase difference signals. Through further signal processing, a set of signals with fixed time delay can be obtained. Use the positioning algorithm of time delay estimation to determine whether the disturbance occurs on the main road or the branch road:

(1) When the disturbance acts on the main road, use the positioning algorithm of time delay estimation to directly solve the location where the disturbance occurs;

(2) When the disturbance acts on the branch, first use the positioning algorithm of time delay estimation to determine the branch of the disturbance, and then use the notch point positioning method of the frequency spectrum to locate. In the process of using the notch point for positioning in the branch, since the notch point of a certain phase difference signal is directly used for positioning, there will be a plurality of disturbing notch points, which will affect the determination of the position. Therefore, the present invention first The obtained two phase difference signals are subtracted, and then the spectrum analysis is performed on the subtracted signal, and the disturbance position on the branch is determined through the notch point.

Specifically, the branch distributed positioning system based on the single-core feedback interferometer provided by the present invention, as shown in FIG. 1 , includes three parts: a front-end module, a sensing optical cable and a tail-end module. The front-end module is composed of a beam splitting and combining unit 1, a splitter 2, and a first delay fiber 3 composed of one or several optical fiber components; the beam splitting and combining unit 1 includes a 3×3 coupler, which has two ports 1b1, 1b2, the splitter 2 has two input ports 2b1, 2b2, and one output port 2a; the light input from the beam splitting unit 1 is divided into two beams, which are respectively output through ports 1b1, 1b2; the light returned from the optical fiber interference path Enter ports 1b1 and 1b2 respectively, meet in the beam splitting and combining unit 1, and interfere. The port 1b1 is connected to the input port 2b1 of the splitter 2 via the first delay fiber 3, the port 1b2 is directly connected to the input port 2b2 of the splitter 2, and the output port 2a of the splitter 2 is connected to one end of the induction optical cable. The induction optical cable is divided into a main optical cable 9 and a plurality of branch optical cables 10, wherein the main optical cable 9 uses one core in the optical cable, and the branch optical cables 10 use two cores in the same optical cable. At one end close to the main path 9, the two cores of the branch path 10 are respectively connected to the optical fibers of the main path, and at the end far away from the main path optical cable 9, the two optical fiber tails of the branch path optical cable 10 are connected to each other. The location of the disturbance point on the main road is 11, and the location of the disturbance point on the branch is 12. Tail-end module is made up of two wavelength division multiplexers 4 and 5, the second delay fiber 6, phase modulation device 7 and feedback device 8; Wherein, wavelength division multiplexer 4 has an input port 4a, two output ports 4b1, 4b2, the wavelength division multiplexer 5 has two input ports 5b1, 5b2 and an output port 5a; the input port 4a of the wavelength division multiplexer 4 is connected to the output end of the main road optical cable 9, and the output port 4b1 passes through the second Delay fiber 6 is connected with the input port 4b1 of the wavelength division multiplexer 5, and the output port 4b2 of the wavelength division multiplexer 4 is connected with the input port 4b2 of the wavelength division multiplexer 5; the output port 5a of the wavelength division multiplexer 5 In turn with the phase modulation device 7 and the feedback device 8 .

Assume that in the system of the present invention, the light with the wavelengths λ ₁ and λ ₂ generates two pairs of coherent light beams; then:

The interference path of light with wavelength _λ1 is:

(a) 1b1→3→2b1→2a→9→10→4a→4b1→6→5b1→5a→7→8→7→5a→5b1→6→4b1→4a→10→9→2a→2b2→1b2 ;

(b) 1b2→2b2→2a→9→10→4a→4b1→6→5b1→5a→7→8→7→5a→5b1→6→4b1→4a→10→9→2a→2b1→3→1b1 ;

Similarly, the interference path with wavelength _λ2 is:

(c) 1b1→3→2b1→2a→9→10→4a→4b2→5b2→5a→7→8→7→5a→5b2→4b2→4a→10→9→

2a→2b2→1b2;

(d) 1b2→2b2→2a→9→10→4a→4b2→5b2→5a→7→8→7→5a→5b2→4b2→4a→10→9→2a→2b1→3→1b1;

Wherein, "→" represents the path, and the label represents the device through which the light passes and the port of the device.

After the light with a wavelength of _λ1 passes through the path a and the path b, it forms stable interference in the beam splitting and combining unit 11, and is finally detected by the photodetector.

After the light with a wavelength of _λ2 passes through the paths c and d, it forms stable interference in the beam splitting and combining unit 11, and is finally detected by the photodetector.

Since the light wavelengths in the two interfering optical paths are different, no interference will occur between the two.

For interferometric sensing systems, external disturbances can cause changes in the phase of light. The output light intensity of the interferometer is a nonlinear function of the phase change. In order to obtain the phase signal, it is necessary to demodulate the detected intensity signal to obtain the external disturbance signal that causes the phase change.

In this system, assuming that a disturbance signal is applied somewhere on the branch, the interference signal with a wavelength of _λ1 is:

The interference signal whose wavelength is _λ2 is:

Among them, A ₁ , A ₂ , B ₁ , B ₂ are parameters related to the input optical power, and /> is the phase difference caused by the disturbance, φcos(ωt) is the phase signal loaded by the phase modulator, /> and /> It is the initial phase difference caused by the 3×3 coupler in the beam splitting and combining unit 1; by adjusting the amplitude of the phase-generated carrier, the maximum and minimum values of _IPD are obtained, which lays the foundation for further phase demodulation. On the other hand, the phase carrier signal can modulate the interference signal to a high frequency, realize the separation of the interference signal and the low-frequency noise signal, and provide the possibility to eliminate the low-frequency signal.

Further through the phase restoration algorithm, such as the demodulation algorithm of the inverse triangulation method, the signals can be demodulated separately:

Afterwards, the phase carrier signal φcos(ωt) is filtered out by a low-pass filter, and the phase difference caused by external disturbance can be obtained

and />

In the present invention, the system uses wavelength division multiplexing technology, and two sub-interference systems with two different wavelengths can monitor the signals propagated by the same disturbance through different paths. When the disturbance signal is applied to the main optical cable, the sensor monitors the phase difference caused by the same external disturbance signal and /> That is:

In the above formula, c is the speed of light in vacuum, n is the core refractive index, L1 is the length of the first delay coil 3, L2 is the length of the second delay coil 6, and L is the length of the optical fiber from the disturbance point 11 to the end module 8 ( The length of the second delay coil 6 is not included). The coefficient α caused by the photoelastic effect, such that

The phase difference generated for the same disturbance signal and /> To transform, let:

Signals Δφ _1m and Δφ _2m with fixed delay difference are obtained:

Fixed time delay Δτ=2nL/c. Perform cross-correlation operations on (7) and (8):

The time delay can be determined by the peak position of R(t), and the sensing distance L can be obtained by the following formula:

When the disturbance signal is applied to the branch optical cable, the sensor monitors the phase difference caused by the same external disturbance signal and /> can be written as:

L _x is the distance from the disturbance point 12 to the far end of the branch, L _z is the length of the branch, and L _b is the distance from the near end of the branch to the end module 8 .

If you continue to use the positioning algorithm on the main road, the phase difference for the same disturbance signal and To transform, let:

Signals Δφ _1b and Δφ _2b with fixed delay difference are obtained:

fixed time delay The term L _x is not included in this formula. Therefore, when disturbances are applied at different positions on the branch, the time delay method can only calculate L _b +L _z , that is, the distance from the far end of the branch to the tail end module. We cannot use this method to calculate the specific location of the disturbance point. However, in the case of multiple branches, because the length of each branch and the position on the main road are different, when disturbances are applied to different branches, this method can only calculate the distance from the far end to the tail end of the disturbed branch. Module distance, so as to distinguish which branch has received the disturbance.

After the disturbed branch is determined, in order to further determine the specific disturbing position on the branch, the notch point method of frequency spectrum is used for positioning. The spectrum notch method is commonly used in the disturbance location of Sagnac interferometer and single-core feedback interferometer. According to the principle of vibration spectrum analysis, any vibration can be decomposed into a series of superposition of single vibration frequency. Then when the disturbance with frequency ω _i is applied to the branch optical cable, the phase difference caused by and /> You can write:

in, To further simplify formula (16):

(17)

If the spectrum analysis is performed directly on the phase difference signal of one wavelength, according to formula (17), when:

at /> The AC quantity corresponding to frequency ω _i on the frequency spectrum is 0, and a notch point will appear on the frequency spectrum. At this point, there are three possible situations:

cosω _i (-t _x )＝0, cosω _i (-t _b -t _z )＝0 or

Because L _b + L _z corresponding to different branches is different, and these two lengths are determined by the actual optical path of the system, so when cosω _i (-t _b -t _z ) = 0, the notch on the frequency spectrum caused The points will disturb us, so that it is difficult to find the notch point caused by cosω _i (-t _x )=0, which hinders the location of the disturbance point on the branch.

Utilize the novel dual-wavelength notch point positioning method proposed by the present invention, first subtract the phase difference of the two wavelengths caused by the disturbance on the branch optical cable, and do further simplification to obtain:

From formula (18), it can be seen that when at /> The frequency ω _i corresponds to 0 AC on the frequency spectrum, and there are obvious notch points in the frequency spectrum. This situation can be divided into the following two possibilities:

1) When cosω _i (-t _x )=0, Where k is a natural number; at this time, the frequency of the notch point is recorded as f _n (k), then:

The location of the disturbance point corresponding to the frequency of the notch point is:

2) when Or when sinω _i (t ₂ )=0, /> or /> where k is a natural number. Since L ₁ and L ₂ can be taken very small, the corresponding first notch frequency f _n (1) will be very large. As long as the lengths of the first delay coil and the second delay coil are selected appropriately, the interference of f _n ^′ (k) on f _n (k) can be avoided.

Therefore, the phase demodulation is performed on the four interference signals of two wavelengths obtained by the photodetector, and the phase difference of the two wavelength interferometers is calculated. and /> Then subtract the two-way phase difference, and then perform Fourier transform on it to obtain the frequency domain spectrum, find the notch point from the frequency domain spectrum, and then obtain Lx according to formula (20), so that it can be determined to act on the branch road The specific location of the disturbance point.

The invention proposes a sensor optical cable layout mode that can extend multiple branches. The sensing optical cable is divided into a main road and a branch road. The main road uses one core in the optical cable, and the branch road uses two cores in the optical cable. At the junction of the branch road and the main road, the two cores in the branch road are respectively connected to the front and back of one core in the main road; at the end of the branch road, the two cores are connected to each other. Through this connection method, the light beam transmitted in the optical fiber first enters the branch circuit from the main circuit, goes back and forth in the branch cable, and returns to the main circuit from the branch circuit again.

The invention utilizes a positioning algorithm of time delay estimation to accurately sense and locate the disturbance on the main path of the sensing optical cable, and distinguish the disturbance on the branch paths of the sensing optical cable, and can distinguish which branch path is disturbed.

The invention subtracts two wavelength phase difference signals generated by the same disturbance on the optical cable, and then performs spectrum analysis on the difference signal to find notch points on the spectrum to determine the specific position of the disturbance on the branch.

The present invention provides a new set of multi-branch optical fiber sensing system for monitoring lines with multiple branches or branch roads. Using a set of systems can sense and locate disturbances on the main road and branch roads, reducing the sensor System complexity and equipment cost.

Description of drawings

Fig. 1 is a branch location system structure of the present invention.

Fig. 2 is a schematic diagram of the connection mode of the main circuit and the branch circuit on the induction optical cable.

Fig. 3 is the specific structure of the branch positioning system of the embodiment of the present invention.

Fig. 4 is the positioning result measured and calculated by the time delay algorithm in the present invention by applying artificial disturbance at 10 km on the three branch roads in the embodiment.

Fig. 5 is the positioning result calculated by the notch point algorithm in the present invention by applying artificial disturbance at 10 km on the three branch roads in the embodiment.

Numbers in the figure: 1 is the beam splitting and combining unit, 2 is the fiber splitter, 3 is the delay fiber; 1b1 and 1b2 are the output ports of the beam splitting and combining unit 1, and 2b1 and 2b2 are the two inputs of the splitter 2 port, 2a is the output port 2a of splitter 2; 4 and 5 are two wavelength division multiplexers, 4a is the input port of wavelength division multiplexer 4, 4b1, 4b2 are two wavelength division multiplexers 4 Output ports, 5b1 and 5b2 are two input ports of the wavelength division multiplexer 5, and the output port of the wavelength division multiplexer 5; 6 is a delay fiber, 7 is a phase modulation device, and 8 is a feedback device, such as a pigtail reflector; 9 is the main road optical cable, and 10 is the branch optical cable. 11 is the disturbance point on the main road, and 12 is the disturbance point on the branch road. 13 is the main road optical cable, and 14 is the branch optical cable. 15 is a superluminescent light-emitting diode (SLD), 16 is an erbium-doped fiber amplifier, 17, 18, 20, and 21 are four photodetectors respectively, 19 and 22 are two wavelength division multiplexers, and 23 is a symmetrical 3X3 fiber coupling device, 24 is a symmetrical 2X2 fiber coupler, and 25 is a first delay fiber. 26 is the optical cable for the main road, 27 is the three branch optical cables extending from the main road, 28 is the position where the disturbance is applied on the main road, and 29 is the position where the disturbance is applied on the branch road. 30 and 31 are two wavelength division multiplexers, 32 is a second delay fiber, 33 is a phase modulator, and 34 is a Faraday rotating mirror.

Detailed ways

The present invention will be further described below through specific embodiments in conjunction with the accompanying drawings.

Fig. 1 is the structure of the branch positioning system of the present invention.

Fig. 2 is a schematic diagram of the connection mode of the main circuit and the branch circuit on the induction optical cable. 13 is the main optical cable, and 14 is a branch optical cable extending from the main optical cable. One core of the optical cable is used for the main path, and two cores of the same optical cable are used for the branch path. At one end close to the main road, the two cores of the branch road are respectively connected to the optical fibers of the main road, and at the end far away from the main road 9, the two optical fiber tails of the branch road 10 are connected to each other. The thick arrow indicates the propagation path of the light beam in the induction optical cable. The light beam first enters the branch from the main road, travels back and forth in the branch optical cable, and returns to the main road from the branch again.

Figure 3 shows. 15 is a superluminescent light-emitting diode (SLD), 16 is an erbium-doped fiber amplifier, 17, 18, 20, and 21 are four photodetectors respectively, 19 and 22 are two wavelength division multiplexers, and 23 is a symmetrical 3X3 fiber coupling device, 24 is a symmetrical 2X2 fiber coupler, and 25 is a first delay fiber. The induction optical cable can be divided into a main road and a branch road, 26 is the main road, 27 is three branch roads extending from the main road, 28 is the position of the disturbance on the main road, and 29 is the position of the disturbance on the branch road. 30 and 31 are two wavelength division multiplexers, 32 is a second delay fiber, 33 is a phase modulator, and 34 is a Faraday rotating mirror.

The central wavelength of the superradiant light-emitting diode 15 is 1550nm, the bandwidth is 40nm, and its coherent length is about tens of microns. The light emitted by the light emitting diode 15 is divided into two wavelengths λ ₁ =1553nm and λ ₂ =1548nm. The optical power of the output light is adjusted by 16 and divided into two beams by the wavelength division multiplexer 19 , one beam passes through the delay fiber 25 and the 2X2 fiber coupler 24 , and the other beam directly passes through the 2X2 fiber coupler 24 . The two beams of light continue to enter the wavelength division multiplexer 30 after passing through the sensing fiber, and the wavelength division multiplexer 30 divides the light beam into two different wavelengths. Compared with the light beam with a wavelength of _λ2 , _λ1 will pass through a longer delay fiber 32, and then the wavelength division multiplexer 31 combines the two beams of light. After the light beam is modulated by the phase modulator 33, it is reflected by the Faraday rotating mirror 34 and returns along the original path, and finally returns to the 3X3 fiber coupler 23 in the front end to form stable interference. The multiplexer 22 separates the four photodetectors 17, 18, 20, 21 to collect interference signals. A/D conversion is realized by using a high-speed data acquisition card with a sampling rate of 5Mbps. Use the processing program based on Labview to realize the signal processing algorithm. In this embodiment, the length of the first delay coil 25 is selected to be 200 m, and the length of the second delay coil 32 is 4 km. The total length of the main optical cable is 4km, and three 15km-long branch roads are extended near the end.

Use a small hammer to apply a disturbance at about 10km on the three branch roads (the distance measured by OTDR is 9.98km), and knock 20 times in a row. Using the time delay method, it can be calculated that the branch where the disturbance is located is far away from the main road. At one end of the road, the length of the optical fiber from the far end of the system. Tap multiple times, and the calculated results are shown in Figure 4. The calculation result of the knocking branch 27a is about 75000 meters, the calculation result of the branch 27b is about 45000 meters, and the result of the branch 27c is about 15000 meters. It shows that the branches extending from the same position on the main road can distinguish the disturbed branch through the time delay method. After the disturbed branch is judged by the time delay method, the spectrum analysis is carried out after subtracting the two phase recovery signals, and the specific position of the disturbance on the branch can be calculated from the position of the first-order notch point. The positioning results of the disturbance at 10 km on the three branch roads after multiple knocks are shown in Figure 5. Most of the positioning results are 9.958km, which can determine the specific position of the disturbance point on the branch road, and the error with the actual position is 0.022km.

Claims (4)

1. A branch distributed positioning system based on a single-core feedback interferometer is characterized in that two single-core feedback interferometers with different wavelengths are combined together through wavelength division multiplexing; the induction optical cable is divided into a main path and a branch path; the main path uses one core in the optical cable, and the branch path uses two cores in the same optical cable; two cores of the branch are respectively connected with the optical fibers of the main road at one end close to the main road; at one end far away from the main road, two optical fiber tails of the branch road are connected with each other; by the connection mode, a plurality of branches extend out of the sensing optical cable;

based on the sensing optical cable structure, the sensing optical cable is disturbed by the outside to cause the phase change of light in the optical fiber; demodulating interference signals with two different wavelengths generated by the same vibration to obtain two paths of phase difference signals; a group of signals with fixed time delay can be obtained through signal processing; determining whether the disturbance occurs on the main path or the branch path by using a positioning algorithm of the time delay estimation:

(1) When the disturbance acts on the main road, the position where the disturbance occurs is directly solved by using a positioning algorithm of time delay estimation;

(2) When the disturbance acts on the branch, determining the branch of the disturbance by using a positioning algorithm of time delay estimation, and then positioning by using a notch point positioning method of a frequency spectrum; in the processing process of positioning the branch by adopting the notch point, the two obtained phase difference signals are subtracted, then spectrum analysis is carried out on the subtracted signals, and the disturbance position on the branch is determined through the notch point.

2. The single-core feedback interferometer-based branch distributed positioning system of claim 1, comprising three major parts, a front end module, an inductive fiber optic cable, and a tail end module; wherein:

the front end module consists of a beam splitting and combining unit (1) formed by one or a plurality of optical fiber devices, a splitter (2) and a first delay optical fiber (3); the beam splitting and combining unit (1) comprises a 3x3 coupler with two ports (1 b1, 1b 2), and the splitter (2) is provided with two input ports (2 b1, 2b 2) and an output port (2 a); light input from the beam splitting and combining unit (1) is split into two beams, and the two beams are output through two ports (1 b1, 1b 2) respectively; light returning from the optical fiber interference path enters ports (1 b1, 1b 2) respectively, and is converged in the beam splitting and combining unit (1) to generate interference; the first port (1 b 1) of the beam splitting and combining unit (1) is connected with the first input port (2 b 1) of the splitter (2) through the first delay optical fiber (3), the second port (1 b 2) of the beam splitting and combining unit (1) is directly connected with the second input port (2 b 2) of the splitter (2), and the output port (2 a) of the splitter (2) is connected with one end of the sensing optical cable;

the induction optical cable is divided into a main optical cable (9) and a plurality of branch optical cables (10), wherein one core is utilized in the main optical cable (9), and two cores in the same optical cable are utilized in the branch optical cable (10); two cores of the branch optical cable (10) are respectively connected with optical fibers of the main optical cable (9) at one end close to the main optical cable (9), and two optical fiber tails of the branch optical cable (10) are connected with each other at one end far away from the main optical cable (9); a position (11) of a disturbance point on the main optical cable, and a position (12) of a disturbance point on the branch optical cable;

the tail end module consists of two wavelength division multiplexers (4 and 5), a second delay optical fiber (6), a phase modulation device (7) and a feedback device (8); wherein the first wavelength division multiplexer (4) has one input port (4 a), two output ports (4 b1, 4b 2), and the second wavelength division multiplexer (5) has two input ports (5 b1, 5b 2) and one output port (5 a); an input port (4 a) of the first wavelength division multiplexer (4) is connected with an output end of the main optical cable (9), a first output port (4 b 1) of the first wavelength division multiplexer (4) is connected with a first input port (4 b 1) of the second wavelength division multiplexer (5) through a second delay optical fiber (6), a second output port (4 b 2) of the first wavelength division multiplexer (4) is connected with a second input port (4 b 2) of the second wavelength division multiplexer (5), and an output port (5 a) of the second wavelength division multiplexer (5) is sequentially connected with the phase modulation device (7) and the feedback device (8).

3. The single-core feedback interferometer-based branch distributed positioning system of claim 2, wherein the wavelengths are λ, respectively ₁ 、λ ₂ Generating two pairs of coherent light beams; then:

wavelength lambda ₁ The interference path of light is:

(a)1b1→3→2b1→2a→9→10→4a→4b1→6→5b1→5a→7→8→7→5a→5b1→6→4b1→4a→10→9→2a→2b2→1b2；

(b)1b2→2b2→2a→9→10→4a→4b1→6→5b1→5a→7→8→7→5a→5b1→6→4b1→4a→10→9→2a→2b1→3→1b1；

similarly, the wavelength is lambda ₂ The interference path of (a) is:

(c)1b1→3→2b1→2a→9→10→4a→4b2→5b2→5a→7→8→7→5a→5b2→4b2→4a→10→9→2a→2b2→1b2；

(d)1b2→2b2→2a→9→10→4a→4b2→5b2→5a→7→8→7→5a→5b2→4b2→4a→10→9→2a→2b1→3→1b1；

wherein "→" denotes a path, and a reference numeral denotes a device through which light passes and a port of the device;

wavelength lambda ₁ After passing through the path a and the path b, stable interference is formed in the beam splitting and combining unit 1, and finally the light is detected by the photoelectric detector;

wavelength lambda ₂ After passing through the path c and the path d, stable interference is formed in the beam splitting and combining unit 1, and finally, the light is detected by the photodetector.

4. The single-core feedback interferometer-based branch distributed positioning system of claim 3, wherein the specific process for identifying and determining the external disturbance signal is as follows:

assuming that a disturbing signal is applied somewhere on the branch, the wavelength is lambda ₁ The interference signals of (a) are:

wavelength lambda ₂ The interference signals of (a) are:

wherein ,A₁ ,A ₂ ,B ₁ ,B ₂ Is a parameter related to the magnitude of the input optical power, and />Is the phase difference caused by disturbance, phi cos (ωt) is the phase signal loaded by the phase modulator,/-> and />Is an initial phase difference caused by a 3x3 coupler in the beam splitting and combining unit (1); by adjusting the amplitude of the phase-generated carrier, I is obtained _PD Maximum and minimum of (2); on the other hand, phase carrier signalsModulating the interference signal to high frequency to realize the separation of the interference signal and the low-frequency noise signal;

the signals are demodulated by a phase recovery algorithm, namely a demodulation algorithm of an inverse trigonometry, respectively:

then filtering the phase carrier signal phi cos (omega t) by a low-pass filter to obtain the phase difference caused by external disturbance and />

Two sub-interference systems with different wavelengths in the system monitor signals of the same disturbance transmitted through different paths;

when disturbance signals are applied to the main optical cable, the sensor monitors the obtained phase difference caused by the same external disturbance signals and />The method comprises the following steps:

in the above formula, c is the vacuum light velocity, n is the refractive index of the fiber core, L1 is the length of the first delay coil (3), L2 is the length of the second delay coil (6), and L is the disturbance point (11) on the main optical cable toThe length of the optical fiber of the tail end module (8); coefficient alpha caused by photoelastic effect such that

Phase difference generated for the same disturbance signal and />And (3) performing transformation, and enabling:

obtaining a signal delta phi with a fixed time delay difference _1m and Δφ_2m ：

A fixed time delay Δτ=2nl/c; performing cross-correlation operation on the steps (7) and (8):

the time delay is determined by the peak position of R (t), and the sensing distance L is given by:

when disturbance signal is applied to the branchWhen the optical cable is on, the sensor monitors the phase difference caused by the same external disturbance signal obtained and />The writing is as follows:

L _x is the distance from the disturbance point (12) on the branch optical cable to the far end of the branch, L _z Is the length of the branch, L _b Is the distance from the proximal end of the branch to the tail end module (8);

if the positioning algorithm on the main road is continuously used, the phase difference generated for the same disturbance signal and />And (3) performing transformation, and enabling:

obtaining a signal delta phi with a fixed time delay difference _1b and Δφ_2b ：

Fixed time delay

Under the condition of multiple branches, as the length of each branch and the position of each branch on a main road are different, when disturbance is applied to different branches, the distance from the far end of the disturbed branch to the tail end module can be calculated, so that the branch which is disturbed can be distinguished;

after determining a disturbed branch, determining a specific disturbance position on the branch, and positioning by adopting a frequency spectrum notch point method; according to the vibration spectrum analysis principle, any vibration can be decomposed into a series of superposition of single vibration frequencies; then when the frequency is omega _i Phase difference caused when disturbance of (b) is applied to branch optical cable and />Writing:

wherein ,further simplifying formula (16):

the method comprises the steps of subtracting phase differences of two wavelengths caused by disturbance on a branch optical cable by adopting a double-wavelength notch point positioning method, and further simplifying to obtain:

as can be seen from formula (18), whenAt->Is spectrally at frequency omega _i The corresponding alternating current quantity is 0, and obvious notch points exist on the frequency spectrum; there are two possibilities for this:

1) When cos omega _i (-t _x ) When the value of the sum is =0,wherein k is a natural number; the notch frequency is denoted as f _n (k) Then:

the positions of the disturbance points corresponding to the notch point frequencies are as follows:

2) When (when)Or sin omega _i (t ₂ ) When=0,>or->Wherein k is a natural number; due to L ₁ and L₂ Can be made very small, then the first notch point frequency f corresponding thereto _n (1) Will be very large; by selecting the appropriate lengths of the first delay coil and the second delay coil, f can be avoided _n ^′ (k) For f _n (k) Is a part of the interference of the (c).