Power cable external force monitoring system and method based on distributed optical fiber sensing

Abstract

A power cable external force monitoring system and method based on distributed optical fiber sensing, the monitoring system includes: a laser, a 50:50 optical fiber coupler, an optical Michelson interference system and a phase-sensitive optical time-domain reflectometer system; when the power cable to be detected vibrates, the light with vibration information is collected by the Michelson interference system and the phase-sensitive optical time-domain reflectometer system to obtain optical information; the frequency change information of the vibration information is obtained by analyzing the optical information of the Michelson interference system; the amplitude information of the optical information is obtained by analyzing the back Rayleigh scattering information received by the phase-sensitive optical time-domain reflectometer system, and the vibration position is obtained by the amplitude information. By combining the Michelson interference structure with the Michelson structure, the Michelson structure analyzes the vibration frequency characteristics and the vibration position, and the present invention can effectively reduce the complexity of data analysis and improve the calculation speed.

Description

Technical Field

[0001] The present invention belongs to the technical field of power cables, and in particular relates to a power cable external force monitoring system and method based on distributed optical fiber sensing. Background Art

[0002] Power cables are widely distributed and have a complex operating environment. External force damage to cables caused by construction machinery, theft, and modification seriously threatens the safety of cable operation, and there is a lack of effective monitoring and early warning methods. By detecting the vibration signal of the external force on the cable, the existence of external force can be determined and the force point can be located. Traditional piezoelectric sensors can only perform quasi-distributed measurements on multiple monitoring points and cannot achieve fully distributed measurements. In addition, they are large in size and cannot be deployed and used in underground, power cable pipes, etc. Compared with traditional piezoelectric sensors, distributed optical fiber sensors have the advantages of full distribution, small size, wide measurement range, and resistance to electromagnetic interference.

[0003] The existing Mach-Zehnder interferometer requires a photodetector at the end of the optical fiber and a separate signal loop to return the signal. The layout process is cumbersome and the signal is easily interfered during transmission. However, Michelson interferometer does not require a photodetector at the end of the optical fiber. The signal can be obtained through the return end of the optical fiber circulator on the laser emission side, which is convenient for layout. Under long-distance monitoring conditions, the Michelson interferometer structure is simpler and easier to arrange.

[0004] Prior art 1 (CN 102997946 A) discloses "fiber-optic distributed disturbance sensor and disturbance positioning method thereof", including: a dual Michelson interferometer, having a first Michelson interference optical path and a second Michelson interference optical path, and the reflectors therein are all Faraday rotators with a rotation angle of 45°; a preprocessing module, a spectrum analysis module and a disturbance positioning module; and a disturbance positioning method using the above-mentioned fiber-optic distributed disturbance sensor, including: S1: obtaining a first disturbed interference signal and a second disturbed interference signal respectively through the dual Michelson interferometer; S2: preprocessing the first and second disturbed interference signals respectively; S3: performing spectrum analysis on the first disturbed interference signal and the second disturbed interference signal after the preprocessing respectively; S4: performing inverse processing on the spectrum analysis results of the first and second disturbed interference signals to obtain disturbance position information. The disadvantage of prior art document 1 is that the calculation process is relatively complicated. The data processing process of the present invention is relatively simple and can effectively improve the calculation speed.

[0005] Prior art 2 (CN 111238551 B) discloses "a distributed phase-sensitive optical time-domain reflectometer sensing system and phase extraction method", the method includes: at the transmitting end, the signal light is modulated into pulse light by an acousto-optic modulator, the pulse light is injected into the sensing optical fiber through a circulator, and the Rayleigh scattered light reflected from the sensing optical fiber is output through the circulator, wherein the pulse light becomes a single sideband signal under the action of the frequency shift of the acousto-optic modulator; at the receiving end, the local oscillator light and the Rayleigh scattered light output by the circulator are mixed in the second coupler, wherein the optical signal after mixing is a single sideband signal, the optical signal after mixing is photoelectrically converted by a single photodetector, the electrical signal after photoelectric conversion is bandpass filtered and a sideband in the filtered electrical signal is taken, and the extracted sideband is digitally processed to obtain the position and phase of the optical fiber disturbance signal. The disadvantage of prior art document 2 is that only phase-sensitive optical time-domain reflectometer technology is used, and its frequency response characteristics are poor. The Michelson interference structure of the present invention has a better frequency response effect. Summary of the invention

[0006] In order to solve the deficiencies in the prior art, the purpose of the present invention is to provide a power cable external force monitoring system and method based on distributed optical fiber sensing, which can obtain spatial and temporal multi-dimensional distribution information of vibration information, and measure and monitor the spatiotemporal distribution of the vibration field in real time.

[0007] The present invention adopts the following technical solution.

[0008] Power cable external force monitoring system based on distributed fiber optic sensing, including: laser, 50:50 fiber coupler, optical Michelson interferometer system and phase sensitive optical time domain reflectometer system;

[0009] The optical Michelson interferometer system includes: a first optical fiber circulator, a one-to-two optical fiber coupler, a reference arm optical fiber, a measurement arm optical fiber, a first Faraday reflector, a second Faraday reflector, a photoelectric detector, and an acquisition card;

[0010] The phase-sensitive optical time-domain reflectometer system includes: a balanced photodetector, a 2×2 coupler, a 90:10 fiber coupler, an acousto-optic modulator, an erbium-doped fiber amplifier, a second fiber circulator, Sensing fiber;

[0011] The laser output by the laser is divided into two branches through a 50:50 fiber coupler, and the lasers of the two branches enter the optical Michelson interferometer system and the phase-sensitive optical time-domain reflectometer system respectively;

[0012] The laser entering the optical Michelson interferometer system passes through the first fiber circulator, and through the one-to-two coupler, the light beam is input into the reference arm fiber and the measuring arm fiber respectively. The output end of the reference arm fiber is connected to the first Faraday reflector, and the output end of the measuring arm fiber is connected to the second Faraday reflector. The return end of the first fiber circulator is connected to the photoelectric detector, and the optical information is converted into electrical information and sent to the acquisition card for data acquisition.

[0013] The laser entering the phase-sensitive optical time-domain reflectometer system is divided into two branches through a 90:10 coupler. The branch laser with a ratio of 90 passes through an acousto-optic modulator and an erbium-doped fiber amplifier, and the second fiber circulator enters the OTDR sensing fiber. The return end of the second fiber circulator and the branch laser with a ratio of 10 enter the 2×2 fiber coupler together, and then enter the balanced photodetector. The balanced photodetector converts optical information into electrical information and sends it to the acquisition card for data acquisition.

[0014] Laser is used to emit laser light;

[0015] The 50:50 fiber coupler is used to connect the laser, the first fiber circulator and the 90:10 fiber coupler;

[0016] The first optical fiber circulator is used to connect the 50:50 optical fiber coupler, the one-to-two optical fiber coupler and the photoelectric detector, and inject the return light of the one-to-two optical fiber coupler into the photoelectric detector;

[0017] The one-to-two coupler is used to connect the optical fiber circulator, the reference arm optical fiber and the measurement arm optical fiber;

[0018] The first Faraday reflector is used to reflect the optical information so that the optical information of the reference arm optical fiber returns along the original optical path;

[0019] The second Faraday reflector is used to reflect the light information so that the light information of the optical fiber of the measuring arm returns along the original light path;

[0020] The photodetector is used to receive the return light from the one-to-two optical fiber coupler injected by the optical fiber circulator.

[0021] Acousto-optic modulators are used to modulate optical information;

[0022] Erbium-doped fiber amplifiers are used to increase optical power;

[0023] The second fiber circulator is used to connect the erbium-doped fiber amplifier, Sensing fiber and 2×2 fiber coupler. The backscattered Rayleigh light returned from the sensing fiber is injected into the 2×2 fiber coupler;

[0024] Sensing optical fiber is used for vibration information sensing;

[0025] The balanced photodetector is used to receive optical information from the 2×2 optical fiber coupler 12 and convert the received optical information into electrical information; and perform differential amplification detection on the interference optical information;

[0026] The 90:10 coupler is used to connect the 50:50 fiber coupler, the 2×2 fiber coupler and the acousto-optic modulator, and input the laser into the acousto-optic modulator and the 2×2 fiber coupler in a ratio of 90:10 respectively;

[0027] A 2×2 fiber coupler is used to modulate the optical phase.

[0028] Preferably, the acquisition card is used to acquire information from the photodetector and the balanced photodetector.

[0029] Preferably, the sensing optical fiber is adhered to the outer surface of the power cable and fixed by an adhesive.

[0030] A method for monitoring external force of power cables based on distributed optical fiber sensing includes the following steps:

[0031] Step 1: When the power cable to be detected vibrates, light carrying vibration information is collected by a Michelson interferometer system and a phase-sensitive optical time-domain reflectometer system to obtain optical information;

[0032] Step 2, obtaining frequency change information of vibration information by analyzing the optical information of the Michelson interference system;

[0033] By analyzing the received phase-sensitive optical time-domain reflectometry system Backscattering information is used to obtain the amplitude information of light information, and the vibration position is obtained through the amplitude information.

[0034] Preferably, in step 2, the information from the balanced photodetector is demodulated, and the information is subjected to bandpass filtering, orthogonal demodulation, and low-pass filtering to calculate the amplitude information of the sampled information.

[0035] The orthogonal demodulation process is as follows:

[0036]

[0037]

[0038] In the formula,

[0039] n represents the serial number of the sampling point,

[0040] N represents the total number of sampling points.

[0041] S(n) represents sampling information,

[0042] cos(Δω n n) represents I same-frequency orthogonal information,

[0043] sin(Δω n n) represents Q same-frequency orthogonal information,

[0044] represents the sum-frequency term in the I information,

[0045] Δω n =2πΔf / f s , f s represents the sampling rate of the acquisition card, Δf represents the frequency of the modulation information,

[0046] E s (n) represents the light field intensity of the information light,

[0047] E o (n) represents the light field intensity of the local oscillator light,

[0048] represents the phase of external disturbance information,

[0049] It indicates that the information I contains the external disturbance information.

[0050] represents the sum frequency term in the Q information,

[0051] It indicates that the Q information contains the external disturbance information.

[0052] Y I Represents the I information obtained through operation,

[0053] Y Q Indicates the Q information obtained through calculation.

[0054] Preferably, the sum frequency term in the I and Q information is obtained by orthogonal demodulation and The low-pass filter is used to filter out the double frequency and obtain the information containing external disturbance. and and and The relationship is as follows:

[0055]

[0056]

[0057] In the formula,

[0058] Y I ' represents the I information after passing through the low-pass filter,

[0059] Y Q' represents the Q information after passing through the low-pass filter.

[0060] According to the low-pass filtering, Y I ' and Y Q ', the amplitude information of the sampling information is as follows:

[0061]

[0062] According to the amplitude information, the sampling point number at the vibration point where the amplitude protrudes is obtained. The sampling point number is multiplied by the sampling interval time and the speed of light, and then divided by the refractive index of the optical fiber. The calculated result is divided by 2, which is the distance between the vibration point and the transmitting end.

[0063] The beneficial effect of the present invention is that, compared with the prior art,

[0064] The Michelson interference structure is more sensitive to the spectral characteristics of vibration information and can compensate The structure has the disadvantage of poor response to spectral characteristics. It has the advantages of wide vibration frequency response range and high spatial resolution. The frequency and position of vibration information are measured. The data analysis process is complicated, but through the Michelson interferometer structure and Combined with the Michelson structure analysis of vibration frequency characteristics, Analyzing the vibration position can effectively reduce the complexity of data analysis and improve the calculation speed. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 It is a structural diagram of the power cable external force monitoring system based on distributed optical fiber sensing of the present invention;

[0066] The accompanying drawings are marked as follows:

[0067] 1-laser, 2-50:50 fiber coupler, 3-first fiber circulator, 4-one-to-two fiber coupler, 5-reference arm fiber, 6-measurement arm fiber, 7-first Faraday reflector, 8-second Faraday reflector, 9-photodetector, 10-acquisition card, 11-balanced photodetector, 12-coupler, 13-90:10 fiber coupler, 14-acoustic-optic modulator, 15-erbium-doped fiber amplifier, 16-second fiber circulator, Sensing fiber. DETAILED DESCRIPTION

[0068] The present application is further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present application.

[0069] Example 1.

[0070] Power cable external force monitoring system based on distributed optical fiber sensing, such as Figure 1 As shown, the monitoring system includes: a laser 1, a 50:50 fiber coupler 2, an optical Michelson interferometer system and a phase-sensitive optical time domain reflectometer system;

[0071] The optical Michelson interferometer system includes: a first fiber circulator 3, a one-to-two fiber coupler 4, a reference arm fiber 5, a measurement arm fiber 6, a first Faraday reflector 7, a second Faraday reflector 8, a photodetector 9, an acquisition card 10, a balanced photodetector 11, a 2×2 coupler 12,

[0072] The phase-sensitive optical time domain reflectometer system includes: a 90:10 optical fiber coupler 13, an acousto-optic modulator 14, an erbium-doped optical fiber amplifier 15, a second optical fiber circulator 16, Sensing optical fiber 17,

[0073] In this embodiment, preferably, the laser 1 is a 1550 nm laser.

[0074] The laser 1 is used for emitting laser light;

[0075] The 50:50 fiber coupler 2 is used to connect the laser 1, the first fiber circulator 3 and the 90:10 fiber coupler 13;

[0076] The laser outputted by the laser 1 is divided into two branches through the 50:50 fiber coupler 2, and the lasers of the two branches enter the optical Michelson interferometer system and the phase-sensitive optical time domain reflectometer system respectively;

[0077] The laser entering the optical Michelson interferometer system passes through the first fiber circulator 3, and through the one-to-two fiber coupler 4, the light beam is input into the reference arm fiber 5 and the measuring arm fiber 6 respectively. The output end of the reference arm fiber 5 is connected to the first Faraday reflector 7, and the output end of the measuring arm fiber 6 is connected to the second Faraday reflector 8. The return end of the first fiber circulator 3 is connected to the photodetector 9, and the optical information is converted into electrical information and sent to the acquisition card 10 for data acquisition.

[0078] The first optical fiber circulator 3 is used to connect the 50:50 optical fiber coupler 2 , the one-to-two optical fiber coupler 4 and the photodetector 9 , and inject the return light of the one-to-two optical fiber coupler 4 into the photodetector 9 .

[0079] The one-to-two optical fiber coupler 4 is used to connect the optical fiber circulator 3 , the reference arm optical fiber 5 and the measurement arm optical fiber 6 .

[0080] The first Faraday reflector 7 is used to reflect light information so that the light information of the reference arm optical fiber 5 returns along the original light path.

[0081] The second Faraday reflector 8 is used to reflect light information so that the light information of the measuring arm optical fiber 6 returns along the original light path.

[0082] The photodetector 9 is used to receive the return light injected by the optical fiber circulator 3 from the one-to-two optical fiber coupler 4 .

[0083] The laser entering the phase-sensitive optical time-domain reflectometer system is divided into two branches through a 90:10 coupler 13. The branch laser with a ratio of 90 passes through an acousto-optic modulator 14 and an erbium-doped fiber amplifier 15, and the second fiber circulator 16 enters the OTDR sensing fiber 17. The return end of the second fiber circulator 16 enters the 2×2 fiber coupler 12 together with the branch laser with a ratio of 10, and then enters the balanced photodetector 11. The balanced photodetector 11 converts optical information into electrical information and sends it to the acquisition card 10 for data acquisition.

[0084] The AOM 14 is used to modulate optical information.

[0085] The erbium-doped fiber amplifier 15 is used to increase the optical power.

[0086] The second optical fiber circulator 16 is used to connect the erbium-doped optical fiber amplifier 15, Sensing fiber 17 and 2×2 fiber coupler 12. The backward Rayleigh scattered light returned by the sensing optical fiber 17 is injected into the 2×2 optical fiber coupler 12 .

[0087] The sensing optical fiber 17 is used for vibration information sensing.

[0088] The balanced photodetector 11 is used to receive optical information from the 2×2 optical fiber coupler 12 and convert the received optical information into electrical information, and perform differential amplification detection on the interference optical information.

[0089] The 90:10 coupler 13 is used to connect the 50:50 fiber coupler 2, the 2×2 fiber coupler 12 and the acousto-optic modulator 14, and input the laser light into the acousto-optic modulator 14 and the 2×2 fiber coupler 12 in a ratio of 90:10.

[0090] The 2×2 fiber coupler 12 is used to modulate the optical phase.

[0091] The acquisition card 10 is used to acquire information from the photodetector 9 and the balanced photodetector 11 .

[0092] The sensing optical fiber 17 is adhered to the outer surface of the power cable and fixed by an adhesive.

[0093] Example 2.

[0094] A method for monitoring external force of power cables based on distributed optical fiber sensing includes the following steps:

[0095] Step 1: When the power cable to be tested vibrates, the light carrying vibration information is collected through the Michelson interferometer system and the phase-sensitive optical time-domain reflectometer system to obtain optical information, including vibration information generated by the actions of engineering equipment such as electric drills, electric hammers, shovels, and excavators.

[0096] Step 2, by analyzing the information of the Michelson interferometer system, the frequency change information of the vibration information is obtained; when the power cable to be detected vibrates, the optical path difference between the reference arm and the measuring arm in the Michelson interferometer system changes continuously due to the vibration of the power cable, causing the waveform of the interference information to change. Different external vibration information (such as electric drills, excavators, etc.) has different frequency characteristics. By obtaining and analyzing the frequency change information, the relevant vibration application source can be obtained.

[0097] By analyzing the balanced photodetector received Backscattering information is used to obtain the position information of vibration information;

[0098] When the power cable to be tested vibrates, The energy of the vibration position in the backscattered Rayleigh information changes.

[0099] The optical information is collected by the photoelectric detector, balanced photoelectric detector and acquisition card. The optical information from the photoelectric detector is subjected to spectrum analysis to obtain the frequency characteristics of the vibration information. The information from the balanced photoelectric detector is demodulated, and the information is subjected to bandpass filtering, digital orthogonal demodulation, and low-pass filtering.

[0100] Step 3, obtain the vibration position by observing the energy change.

[0101] The in-phase / quadrature demodulation process is as follows: The orthogonal I and Q information obtained by the calculation of S(n) can be expressed as:

[0102]

[0103]

[0104] In the formula,

[0105] n represents the serial number of the sampling point,

[0106] N represents the total number of sampling points.

[0107] S(n) represents sampling information,

[0108] cos(Δω n n) represents I same-frequency orthogonal information,

[0109] sin(Δω n n) represents Q same-frequency orthogonal information,

[0110] represents the sum-frequency term in the I information,

[0111] Δω n =2πΔf / f s , f s represents the sampling rate of the acquisition card, Δf represents the frequency of the modulation information,

[0112] E s (n) represents the light field intensity of the information light,

[0113] E o (n) represents the light field intensity of the local oscillator light,

[0114] The phase of external disturbance information,

[0115] It indicates that the information I contains the external disturbance information.

[0116] represents the sum frequency term in the Q information,

[0117] It indicates that the Q information contains the external disturbance information.

[0118] Y I Represents the I information obtained through operation,

[0119] Y Q represents the Q information obtained through calculation,

[0120] Sum frequency term in I and Q information and The frequency in is twice the AOM shift frequency. By filtering out the double frequency through a low-pass filter, we can get the signal containing external disturbance information. and Y I ' and Y Q 'and and The relationship is as follows:

[0121]

[0122]

[0123] In the formula,

[0124] Y I ' represents the I information after passing through the low-pass filter,

[0125] YQ ' represents the Q information after passing through the low-pass filter,

[0126] Then we can get the amplitude information of S(n), which can be expressed as:

[0127]

[0128] According to the amplitude information, the sampling point number at the vibration point where the amplitude protrudes is obtained. The sampling point number is multiplied by the sampling interval time and the speed of light, and then divided by the refractive index of the optical fiber. The calculated result is divided by 2, which is the distance between the vibration point and the transmitting end.

[0129] The beneficial effect of the present invention is that, compared with the prior art,

[0130] The Michelson interference structure is more sensitive to the spectral characteristics of vibration information and can compensate The structure has the disadvantage of poor response to spectral characteristics. It has the advantages of wide vibration frequency response range and high spatial resolution. The frequency and position of vibration information are measured. The data analysis process is complicated, but through the Michelson interferometer structure and Combined with the Michelson structure analysis of vibration frequency characteristics, Analyzing the vibration position can effectively reduce the complexity of data analysis and improve the calculation speed.

[0131] The applicant of the present invention has made a detailed explanation and description of the implementation examples of the present invention in conjunction with the drawings in the specification. However, those skilled in the art should understand that the above implementation examples are only preferred implementation schemes of the present invention, and the detailed description is only to help readers better understand the spirit of the present invention, but not to limit the scope of protection of the present invention. On the contrary, any improvements or modifications based on the inventive spirit of the present invention should fall within the scope of protection of the present invention.

Claims

1. A power cable external force monitoring system based on distributed optical fiber sensing, characterized in that: The monitoring system includes: laser, 50:50 fiber coupler, optical Michelson interferometer system and phase sensitive optical time domain reflectometer system; The optical Michelson interferometer system includes: a first optical fiber circulator, a one-to-two optical fiber coupler, a reference arm optical fiber, a measurement arm optical fiber, a first Faraday reflector, a second Faraday reflector, a photoelectric detector, and an acquisition card; The phase-sensitive optical time-domain reflectometer system includes: a balanced photodetector, a 2×2 coupler, a 90:10 fiber coupler, an acousto-optic modulator, an erbium-doped fiber amplifier, a second fiber circulator, Sensing optical fiber; the sensing optical fiber is adhered to the outer surface of the power cable and fixed by an adhesive; The laser output by the laser is divided into two branches through a 50:50 fiber coupler, and the lasers of the two branches enter the optical Michelson interferometer system and the phase-sensitive optical time-domain reflectometer system respectively; The laser entering the optical Michelson interferometer system passes through the first fiber circulator, and through the one-to-two coupler, the light beam is input into the reference arm fiber and the measuring arm fiber respectively. The output end of the reference arm fiber is connected to the first Faraday reflector, and the output end of the measuring arm fiber is connected to the second Faraday reflector. The return end of the first fiber circulator is connected to the photoelectric detector, and the optical information is converted into electrical information and sent to the acquisition card for data acquisition. The laser entering the phase-sensitive optical time-domain reflectometer system is divided into two branches through a 90:10 coupler. The branch laser with a ratio of 90 passes through an acousto-optic modulator and an erbium-doped fiber amplifier, and the second fiber circulator enters the OTDR sensing fiber. The return end of the second fiber circulator and the branch laser with a ratio of 10 enter the 2×2 fiber coupler together, and then enter the balanced photodetector. The balanced photodetector converts optical information into electrical information and sends it to the acquisition card for data acquisition.

2. The power cable external force monitoring system based on distributed optical fiber sensing according to claim 1 is characterized in that: Laser is used to emit laser light; The 50:50 fiber coupler is used to connect the laser, the first fiber circulator and the 90:10 fiber coupler; The first optical fiber circulator is used to connect the 50:50 optical fiber coupler, the one-to-two optical fiber coupler and the photoelectric detector, and inject the return light of the one-to-two optical fiber coupler into the photoelectric detector; The one-to-two coupler is used to connect the optical fiber circulator, the reference arm optical fiber and the measurement arm optical fiber; The first Faraday reflector is used to reflect the optical information so that the optical information of the reference arm optical fiber returns along the original optical path; The second Faraday reflector is used to reflect the light information so that the light information of the optical fiber of the measuring arm returns along the original light path; The photodetector is used to receive the return light from the one-to-two optical fiber coupler injected by the optical fiber circulator.

3. The power cable external force monitoring system based on distributed optical fiber sensing according to claim 1 is characterized in that: Acousto-optic modulators are used to modulate optical information; Erbium-doped fiber amplifiers are used to increase optical power; The second fiber circulator is used to connect the erbium-doped fiber amplifier, The sensing fiber and 2×2 fiber coupler will The backscattered Rayleigh light returned from the sensing fiber is injected into the 2×2 fiber coupler; Sensing optical fiber is used for vibration information sensing; The balanced photodetector is used to receive optical information from the 2×2 optical fiber coupler and convert the received optical information into electrical information; differential amplification and detection of the interference optical information; The 90:10 coupler is used to connect the 50:50 fiber coupler, the 2×2 fiber coupler and the acousto-optic modulator, and input the laser into the acousto-optic modulator and the 2×2 fiber coupler in a ratio of 90:10 respectively; A 2×2 fiber coupler is used to modulate the optical phase.

4. The power cable external force monitoring system based on distributed optical fiber sensing according to claim 1 is characterized in that: The acquisition card is used to acquire information from the photodetector and the balanced photodetector.

5. A method for monitoring external force of power cables based on distributed optical fiber sensing implemented by using the system described in any one of claims 1 to 4, characterized in that: The following steps are involved: Step 1: When the power cable to be detected vibrates, light carrying vibration information is collected by a Michelson interferometer system and a phase-sensitive optical time-domain reflectometer system to obtain optical information; Step 2, obtaining frequency change information of vibration information by analyzing the optical information of the Michelson interference system; By analyzing the received phase-sensitive optical time-domain reflectometry system Backscattering information is used to obtain the amplitude information of light information, and the vibration position is obtained through the amplitude information.

6. The method for monitoring external force of power cables based on distributed optical fiber sensing according to claim 5, characterized in that: In step 2, the information from the balanced photodetector is demodulated, and the information is subjected to bandpass filtering, orthogonal demodulation, and low-pass filtering to calculate the amplitude information of the sampled information.

7. The method for monitoring external force of power cables based on distributed optical fiber sensing according to claim 6, characterized in that: The orthogonal demodulation process is as follows: In the formula, n represents the serial number of the sampling point, N represents the total number of sampling points. S(n) represents sampling information, cos(Δω n n) represents I same-frequency orthogonal information, sin(Δω n n) represents Q same-frequency orthogonal information, represents the sum-frequency term in the I information, Δω n =2πΔf / f s , f s represents the sampling rate of the acquisition card, Δf represents the frequency of the modulation information, E s (n) represents the light field intensity of the information light, E o (n) represents the light field intensity of the local oscillator light, represents the phase of external disturbance information, It indicates that the information I contains the external disturbance information. represents the sum frequency term in the Q information, It indicates that the Q information contains the external disturbance information. Y I Represents the I information obtained through operation, Y Q Indicates the Q information obtained through calculation.

8. The method for monitoring external force of power cables based on distributed optical fiber sensing according to claim 7, characterized in that: After orthogonal demodulation, the sum frequency terms of I and Q information are obtained and The low-pass filter is used to filter out the double frequency and obtain the information containing external disturbance. and and and The relationship is as follows: In the formula, Y' I Represents the I information after passing through the low-pass filter, Y' Q Indicates the Q information after passing through the low-pass filter.

9. The method for monitoring external force of power cables based on distributed optical fiber sensing according to claim 8, characterized in that: According to the low-pass filtering, Y' I and Y' Q , the amplitude information of the sampling information is as follows: According to the amplitude information, the sampling point number at the vibration point where the amplitude protrudes is obtained. The sampling point number is multiplied by the sampling interval time and the speed of light, and then divided by the refractive index of the optical fiber. The calculated result is divided by 2, which is the distance between the vibration point and the transmitting end.

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