OPGW optical cable icing monitoring system and method based on optical frequency comb BOTDR

Abstract

The invention discloses an optical frequency comb BOTDR (Brillouin Optical Time Domain Reflectometer)-based OPGW (Optical Fiber Composite Overhead Ground Wire) optical cable icing monitoring system and method. Comprising an electro-optical frequency comb, an optical isolator, a first optical coupler, a pulse signal generator, an acoustic optical modulator, a first EDFA optical amplifier, an optical circulator, an OPGW sensing optical cable, a temperature / strain sensing area, a second EDFA optical amplifier, an FBG optical fiber filter, a scrambler, a microwave signal generator, an electro-optical modulator, a variable optical attenuator, a second optical coupler and a balance detector. And a low-frequency amplifier and a data processing module. The system can perform real-time and accurate monitoring and early warning on the icing thickness of the OPGW optical cable.

Description

Technical Field

[0001] The present invention relates to a distributed optical fiber sensing system, belonging to the technical field of optical fiber sensing and sensing networks. Background Art

[0002] Optical Fiber Composite Overhead Ground Wire (OPGW) undertakes the dual functions of communication and ground wire simultaneously, and is widely used in extra-high voltage overhead transmission lines above 220 kV. Its operating state is related to the safety of high-voltage transmission lines and communication systems.

[0003] During power transmission, the OPGW optical cable passes through areas with complex terrain and severe climate such as high altitudes, heavy icing, forested mountains, etc., resulting in the OPGW transmission line being vulnerable to damages such as line icing, overload, abnormal sag, abnormal current, lightning strikes, etc. Among them, the probability of line icing is very high, especially in southern China with high humidity or easy snowfall, where the phenomenon of line icing is extremely likely to occur. The icing of the transmission line seriously threatens the safe operation of the power grid. As the amount of ice accretion increases, the vertical load on the overhead optical cable will increase correspondingly, thereby stretching and lengthening the cable and increasing the sag. When the ice accretion reaches a certain level, major accidents such as line galloping, tripping and power outage, wire breakage, and even tower collapse will occur. Therefore, real-time and accurate monitoring of the icing condition of long-distance OPGW transmission lines and timely issuing of warning signals are important measures to ensure the reliable operation of equipment, reduce disaster losses, and ensure the safe and stable operation of power and communication systems.

[0004] Early detection of transmission line icing included methods such as manual inspection, image recognition, and tensiometers. However, these methods are difficult to meet the requirements put forward by the smart grid in terms of monitoring real-time performance, line full-course monitoring, and result quantification. Distributed optical fiber sensing has the characteristics of compact structure, long measurement distance, anti-electromagnetic interference, and strong real-time performance. Particularly importantly, multiple redundant optical fibers have been preset in the OPGW line, providing convenient conditions for implementing distributed optical fiber sensing detection.

[0005] Brillouin optical time-domain reflectometry (BOTDR) based on self-published Brillouin scattering can simultaneously provide long-distance and high-precision temperature and strain detection, and is widely used in the real-time safety monitoring of large-scale facilities such as power pipelines, mine tunnels, and bridges. The Brillouin scattering time-domain reflectometry technique senses the temperature and strain borne by the object to be measured by measuring the frequency of the reflected signal (i.e., the Brillouin frequency shift). By using the BOTDR technique, the strain of the optical fiber caused by icing on the OPGW line can be measured, and thus the icing thickness can be obtained. However, how to distinguish the Brillouin frequency shifts caused by temperature and strain respectively has always been a difficult problem that the BOTDR technique needs to solve. A common solution is to select special optical fibers for sensing, such as multimode optical fibers and large effective area optical fibers. Since different modes of the optical fiber show different response sensitivities to temperature and strain, the temperature and strain can be inversely obtained by simultaneously measuring the Brillouin frequency shifts of the two modes. However, the ordinary single-mode optical fiber pre-buried in the OPGW transmission line cannot use the above method to distinguish the effects of temperature and strain on the Brillouin frequency shift respectively. Therefore, there is an urgent need for a BOTDR system measurement scheme that can quickly and accurately separate the temperature and strain of the OPGW transmission line, so as to accurately obtain the icing information. Summary of the Invention

[0006] Aiming at the problem that it is difficult to distinguish the temperature and strain information of the OPGW transmission line by the existing BOTDR technology, the present invention proposes a dual-wavelength BOTDR scheme with an optical frequency comb as the light source. On the one hand, the optical pulses with equal frequency intervals generated by the optical frequency comb are injected into the OPGW optical fiber, and mixed frequency occurs to obtain a difference frequency microwave signal. On the other hand, the optical pulses are transmitted along the OPGW optical fiber to generate a backward Brillouin scattering signal. Therefore, two signals with completely different characteristics, namely the difference frequency signal and the Brillouin scattering signal, are simultaneously measured at the detection end. The response sensitivities of these two signals to strain and temperature are different, and the strain information at the icing location of the optical fiber can be accurately extracted through signal processing. Then, the icing thickness can be obtained according to the relationship between the strain and the icing amount. In addition, the system adopts a fixed-frequency measurement method for the difference frequency signal and the Brillouin frequency shift signal, without the need to perform time-consuming frequency sweeping. Therefore, it has a better response speed compared with the conventional BOTDR scheme. The optical frequency comb BOTDR scheme disclosed by the present invention can accurately measure the temperature and strain of the OPGW line, thereby sensing the icing state of the line. The measurement has the advantages of high sensing spatial resolution and fast measurement speed, and has great application prospects in engineering applications.

[0007] The present invention is implemented by the following technical solutions:

[0008] An OPGW optical cable icing monitoring system based on an optical frequency comb BOTDR, comprising an electro-optical frequency comb, an optical isolator, a first optical coupler, a pulse signal generator, an acousto-optic modulator, a first EDFA optical amplifier, an optical circulator, an OPGW sensing optical cable, a temperature / strain sensing area, a second EDFA optical amplifier, an FBG optical fiber filter, a polarization scrambler, a microwave signal generator, an electro-optical modulator, an adjustable optical attenuator, a second optical coupler, a balanced detector, a low-frequency amplifier and a data processing module.

[0009] The pulsed laser generated by the electro-optical frequency comb enters the first optical coupler through the optical isolator.

[0010] The first output end of the first optical coupler is connected to the input end of the acousto-optic modulator, and the output waveform of the acousto-optic modulator is controlled by the pulse signal generator. The output end of the acousto-optic modulator is connected to the input end of the first EDFA optical amplifier, and the output end of the first EDFA optical amplifier is connected to the input end of the optical circulator.

[0011] The first output end of the optical circulator is connected to the OPGW sensing optical cable. The tail end of the OPGW sensing optical cable is located in the temperature / strain sensing area, and the temperature / strain sensing area is used to simulate the temperature and strain borne by the OPGW optical cable under field conditions.

[0012] The second output end of the optical circulator is connected to the input end of the FBG optical fiber filter after passing through the second EDFA optical amplifier, and the output end of the FBG optical fiber filter is connected to the first input end of the second optical coupler.

[0013] The second output end of the first optical coupler is connected to the input end of the polarization scrambler, the output end of the polarization scrambler is connected to the input end of the electro-optical modulator, and the output of the electro-optical modulator is controlled by the microwave signal generator. The output end of the electro-optical modulator is connected to the input end of the adjustable optical attenuator, the output end of the adjustable optical attenuator is connected to the second input end of the second optical coupler, the output end of the second optical coupler is connected to the input end of the balanced detector, the output end of the balanced detector is connected to the input end of the low-frequency amplifier, and the output end of the balanced detector is connected to the data processing module.

[0014] Further, the central wavelength of the electro-optical frequency comb is 1550 nm, the output frequency interval is adjustable between 1 GHz and 3 GHz, the power is 2 W, and the pulse line width is 30 ns.

[0015] Further, the coupling ratio of the first optical coupler is 90%:10%.

[0016] Further, the frequency of the balanced detector is 150 MHz.

[0017] Further, the coupling ratio of the second optical coupler is 50%:50%.

[0018] By performing signal processing on the real-time power of the difference-frequency optical signal and the Brillouin scattering optical signal output by the amplified balanced detector, the data processing module can respectively obtain the temperature and strain information of the OPGW optical cable. Furthermore, the ice-covering mass and thickness at different positions can be obtained through the fiber strain. When the ice-covering thickness exceeds the warning value, the system issues a warning.

[0019] The present invention also provides an OPGW optical cable ice-covering monitoring method for an OPGW optical cable ice-covering monitoring system based on an optical frequency comb BOTDR, including the following:

[0020] S1 An optical pulse with a wavelength of 1550 nm, a frequency interval of Δf = 1 GHz, and a pulse width of 30 ns is generated by an electro-optic frequency comb (1), and enters a first optical coupler (3) through an optical isolator (2). The isolation degree of the optical isolator (2) is higher than 35 dB, ensuring that the reflected light does not enter the electro-optic frequency comb (1);

[0021] S2 After the optical pulse exits from the first optical coupler (3), it is divided into two paths. One path of light is used as the probe light and is injected into the OPGW optical fiber (8) to generate Brillouin backscattered light; the other path is used as the reference light, and a Brillouin frequency shift signal is obtained by beating with the Brillouin scattered light generated by the first path. The power ratio of the probe light to the reference light is 90%:10%;

[0022] S3 The first path of pulsed light with a power ratio of 90% exits from the first output terminal of the first optical coupler (3), passes through an acousto-optic modulator (5). A control signal sent by a pulse signal generator (4) is loaded on the acousto-optic modulator (5). Therefore, after the pulsed light passes through the acousto-optic modulator (5), its pulse width and waveform are loaded with the modulation information of the pulse signal generator (4);

[0023] S4 After the pulsed light exits from the acousto-optic modulator (5), it enters a first EDFA optical amplifier (6) to amplify the optical power, and then is injected into the OPGW sensing optical cable (8) through an optical circulator (7), and is reflected by the temperature / strain sensing region (9). The reflected light is Brillouin scattered light encoded with temperature and strain information, including a low-frequency component (i.e., Brillouin Stokes light) and a high-frequency component (Brillouin anti-Stokes light). The central frequencies of both are Brillouin frequency shifted by approximately 11 GHz relative to the incident probe laser. The exact value of the Brillouin frequency shift depends on the change amount of the sensing parameter (temperature or strain) and the sensing sensitivity of the optical fiber;

[0024] The reflected light of S5 exits from the second output end of the optical circulator (7), passes through the second EDFA optical amplifier, and then enters the FBG optical fiber filter (11) to filter out the high-frequency Brillouin scattered light (i.e., Brillouin anti-Stokes light) and other unwanted stray light, only retaining the low-frequency Brillouin scattered light. The filtered low-frequency Brillouin scattered light enters the second optical coupler (16);

[0025] S6 The second pulsed light with a power ratio of 10% serves as the reference light and exits from the second output end of the first coupler (3). After passing through the polarization scrambler (12) to change the polarization state of the reference light, it plays a role in suppressing depolarization noise. The reference light is transmitted through the electro-optic modulator (14) controlled by the microwave signal generator (13) to generate an output light containing two frequency sidebands. The output light containing two frequency sidebands passes through the variable optical attenuator (15) to adjust the optical power to be the same as that of the Brillouin scattered light, and then is input into the second optical coupler (16);

[0026] S7 The two beams of light enter the balanced detector (17) through the second optical coupler (16). After mixing, the mixed signal of the two beams of light is detected, and its intensity I is:

[0027]

[0028] In the formula, the first term on the right side represents the difference-frequency light after the optical frequency input pulse loads the temperature and strain information, and the second term represents the Brillouin scattered light. f1 = 1 GHz and f2 ≈ 11 GHz are the eigenfrequency of the difference-frequency light and the Brillouin scattering frequency shift respectively, A and B are the light intensities of the difference-frequency light and the Brillouin scattered light respectively, are the initial phases of the difference-frequency light and the Brillouin scattered light respectively;

[0029] S8 The mixed signal detected by the balanced detector (17) is two independent Lorentzian spectral lines. The center frequency of the difference-frequency light is insensitive to temperature and strain. Therefore, when the temperature and strain of the OPGW sensing optical cable change, the difference-frequency light only undergoes an amplitude change, while the center frequency of the Brillouin scattered light is sensitive to temperature and strain. Therefore, the Brillouin signal undergoes an obvious frequency shift with the change of temperature and strain;

[0030] S9 The measurement result of the balanced detector (17) is amplified by the low-frequency amplifier (18) and then input into the data processing module (19) for data processing to obtain the temperature and strain suffered by the OPGW sensing optical cable. Assume that when (temperature, strain) changes from (T1, ε1) to (T2, ε2), the optical power change amount at the center frequency f1 of the difference-frequency light is ΔI1, and the optical power change amount of the Brillouin scattered light at the working frequency f2 is ΔI2. From the equation the temperature change amount ΔT and the strain change amount Δε can be obtained; In the formula, C T1 and Cε1 are the change coefficients of the difference-frequency optical power with respect to temperature and strain; C T2 , C ε2 are the change coefficients of the Brillouin scattering optical power with respect to temperature and strain;

[0031] S10 measures the icing mass of the OPGW sensing optical cable (8); the icing mass (m) at a certain point on the optical fiber under static conditions is obtained through , where ε max and ε(x) are the maximum strain of the optical fiber and the strain at any position, A is the tension coefficient, and g is the acceleration due to gravity;

[0032] S11 measures the icing thickness; after obtaining the icing mass m, the icing thickness is calculated through the relational expression ; where h represents the icing thickness, ρ is the density of ice, l is the length of the OPGW optical cable in the icing section, and d is the radius of the optical cable;

[0033] S12 issues an early warning when the icing thickness exceeds the warning value.

[0034] Furthermore, during measurement, only the center frequency of the difference-frequency light and the slope interruption of the Brillouin scattering light are measured at fixed frequencies in real time to improve the response speed of the system.

[0035] Furthermore, the coefficients C T1 , C ε1 , C T2 , C ε2 can be obtained by pre-applying temperature changes or strains to the OPGW sensing optical cable and measuring the corresponding power changes; the initial difference-frequency optical power I 10 and the Brillouin scattering optical power I 20 of the OPGW sensing optical cable at known temperature T0 and strain ε0 are also measured in advance.

[0036] Furthermore, in S10, for the OPGW optical cable in a dynamic state, the icing mass can be obtained by first performing finite element analysis through Ansys and then fitting and correcting the static relationship.

[0037] Advantages of the present invention:

[0038] (1) The system disclosed in the present invention uses an optical frequency comb as a light source, and the optical pulses emitted by it generate a difference-frequency signal and a Brillouin scattering signal simultaneously after passing through the OPGW optical fiber. By detecting the changes in these two signals, the temperature and strain of the OPGW optical cable can be obtained simultaneously. It solves the problem that it is difficult to separate the temperature and strain information of the OPGW optical cable.

[0039] (2) Since the formation mechanisms of the difference frequency signal and the Brillouin scattering signal are different, there are significant differences in their response sensitivities to temperature and strain, which helps to improve the accuracy of separating temperature and strain signals.

[0040] (3) The system adopts a technical solution of measuring fixed frequency points of the difference frequency signal spectrum and the Brillouin scattering signal spectrum, which greatly saves the signal detection time compared with the traditional swept-frequency BOTDR system and improves the detection efficiency.

[0041] (4) The optical pulses emitted by the dual optical frequency combs have good temporal coherence, which helps to improve the signal-to-noise ratio of the detected signal; at the same time, the small pulse width helps to improve the spatial resolution of optical fiber sensing. Description of the Drawings

[0042] Figure 1 is a schematic structural diagram of the embodiment.

[0043] Figure 2 is the optical pulse sequence output by the electro-optic frequency comb.

[0044] Figure 3 is the schematic diagram of signal detection for fixed frequency points adopted in the embodiment.

[0045] Figure 4 is the comparison diagram of the OPGW cable strain distribution measured in the embodiment and the catenary shape of the cable.

[0046] Figure 1 The meanings of the numbers in are as follows:

[0047] 1 - Electro-optic frequency comb, 2 - Optical isolator, 3 - First optical coupler, 4 - Pulse signal generator, 5 - Acousto-optic modulator, 6 - First EDFA optical amplifier, 7 - Optical circulator, 8 - OPGW sensing cable, 9 - Temperature / strain sensing area, 10 - Second EDFA optical amplifier, 11 - FBG fiber filter, 12 - Polarization scrambler, 13 - Microwave signal generator, 14 - Electro-optic modulator, 15 - Variable optical attenuator, 16 - Second optical coupler, 17 - Balanced detector, 18 - Low-frequency amplifier, 19 - Data processing module. Detailed Embodiments

[0048] The present invention will be further described below with reference to the drawings.

[0049] An OPGW cable icing monitoring system based on optical frequency comb BOTDR, as Figure 1As shown, the system includes an electro-optic frequency comb 1, an optical isolator 2, a first optical coupler 3, a pulse signal generator 4, an acousto-optic modulator 5, a first EDFA optical amplifier 6, an optical circulator 7, an OPGW sensing optical cable 8, a temperature / strain sensing region 9, a second EDFA optical amplifier 10, an FBG optical fiber filter 11, a polarization scrambler 12, a microwave signal generator 13, an electro-optic modulator 14, an optical variable attenuator 15, a second optical coupler 16, a balanced detector 17, a low-frequency amplifier 18, and a data processing module 19.

[0050] The electro-optic frequency comb 1 generates optical pulses with a wavelength of 1550 nm, a frequency interval of Δf = 1 GHz, and a pulse width of 30 ns (see Figure 2 ), which enter the first optical coupler 3 after passing through the optical isolator 2. The isolation degree of the optical isolator 2 is higher than 35 dB, ensuring that the reflected light does not enter the electro-optic frequency comb 1.

[0051] After the pulsed light exits from the first optical coupler 3, it is divided into two paths. One path of the light is used as the probe light and injected into the OPGW optical fiber to generate Brillouin backscattered light; the other path is used as the reference light, and the Brillouin frequency shift signal is obtained by beating with the Brillouin scattered light generated by the first path. The power ratio of the probe light to the reference light is 90%:10%.

[0052] The first path of pulsed light with a power ratio of 90% exits from the first output terminal of the first optical coupler 3 and is transmitted through the acousto-optic modulator 5. Since a control signal emitted by the pulse signal generator 4 is loaded on the acousto-optic modulator 5, the pulse width and waveform of the pulsed light are loaded with the modulation information of the pulse signal generator 4 after passing through the acousto-optic modulator 5. After the pulsed light exits from the acousto-optic modulator 5, it enters the first EDFA optical amplifier 6 to amplify the optical power, and then is injected into the OPGW sensing optical cable 8 through the optical circulator 7 and is reflected by the temperature / strain sensing region 9. The reflected light is Brillouin scattered light encoded with temperature and strain information, including a low-frequency component (i.e., Brillouin Stokes light) and a high-frequency component (Brillouin anti-Stokes light), and the central frequencies of both are shifted by approximately 11 GHz with respect to the incident probe laser. The exact value of the Brillouin frequency shift depends on the change in the sensing parameter (temperature or strain) and the sensing sensitivity of the optical fiber. After the reflected light exits from the second output terminal of the optical circulator 7 and passes through the second EDFA optical amplifier, it enters the FBG optical fiber filter 11 to filter out the high-frequency Brillouin scattered light (i.e., Brillouin anti-Stokes light) and other unwanted stray light, and only the low-frequency Brillouin scattered light is retained. The filtered low-frequency Brillouin scattered light enters the second optical coupler 16.

[0053] The second pulsed light with a power ratio of 10% exits from the second output end of the first coupler 3 as the reference light. After passing through the polarization scrambler 12, the polarization state of the reference light is changed, which plays a role in suppressing the depolarization noise. The reference light is transmitted through the electro-optic modulator 14 controlled by the microwave signal generator 13 to generate the output light containing two frequency sidebands. The output light containing two frequency sidebands passes through the variable optical attenuator 15 to adjust the optical power to make it have the same optical power as the Brillouin scattered light. The two beams of light enter the balanced detector 17 through the second optical coupler 16. After mixing, the mixing signal of the two beams of light is detected, and its intensity I is:

[0054]

[0055] In the formula, the first term on the right side represents the difference-frequency light after the optical frequency comb pulse loads the temperature and strain information, and the second term represents the Brillouin scattered light. In the formula, f1 = 1 GHz and f2 ≈ 11 GHz are the intrinsic frequency of the difference-frequency light and the Brillouin frequency shift respectively. A and B are the optical intensities of the difference-frequency light and the Brillouin scattered light respectively, and are the initial phases of the difference-frequency light and the Brillouin scattered light respectively.

[0056] As Figure 3 shown, the mixing signal detected by the balanced detector 17 is two independent Lorentzian spectral lines. The left side is the spectrum of the difference-frequency light, and the right side is the spectrum of the Brillouin scattered light. The center frequency of the difference-frequency light is insensitive to temperature and strain. Therefore, when the temperature and strain of the OPGW sensing optical cable change, only the amplitude of the difference-frequency light changes. However, the center frequency of the Brillouin scattered light is sensitive to temperature and strain. Therefore, the Brillouin signal undergoes an obvious frequency shift with the change of temperature and strain.

[0057] Finally, the measurement result of the balanced detector 17 is amplified by the low-frequency amplifier 18 and then input into the data processing module 19 for data processing to obtain the temperature and strain suffered by the OPGW sensing optical cable. In order to improve the response speed of the system, the system performs real-time measurement of fixed frequency points on the center frequency of the difference-frequency light and the ramp interruption of the Brillouin scattered light. Assume that when (temperature, strain) changes from (T1, ε1) to (T2, ε2), the optical power change amount at the center frequency f1 of the difference-frequency light is ΔI1, and the optical power change amount of the Brillouin scattered light at the operating frequency f2 is ΔI2. From the equation the temperature change amount (ΔT) and the strain change amount (Δε) can be obtained. In the formula, C T1 and C ε1 are the change coefficients of the difference-frequency optical power with respect to temperature and strain respectively; C T2 , C ε2They are the coefficients of the change in Brillouin scattering optical power with temperature and strain respectively. These coefficients can be obtained by pre-applying temperature changes or strain to the OPGW sensing optical cable and measuring the corresponding power changes. The initial differential frequency optical power I of the OPGW sensing optical cable at a known temperature T0 and strain ε0 10 and the Brillouin scattering optical power I 20 are also measured in advance. On the premise of knowing the above data, the real-time data of temperature and strain can be obtained by measuring the change in optical power.

[0058] The ice thickness of the OPGW sensing optical cable is closely related to the strain of the optical fiber. The ice mass (m) at a certain point on the optical fiber under static conditions is obtained by In the formula, ε max and ε(x) are the maximum strain of the optical fiber and the strain at any position respectively, A is the tension coefficient, g is the acceleration due to gravity, is the gradient operator.

[0059] As Figure 4 shown, the OPGW sensing optical cables on both sides of the connecting tower present a catenary (solid line) shape, which matches the measured strain distribution of the optical fiber. For the OPGW optical cable in the dynamic state, finite element analysis can be carried out by Ansys first, and then the static relationship can be fitted and corrected. Finally, through the obtained relationship where h represents the ice thickness, ρ is the density of ice, l is the length of the ice-covered section of the OPGW optical cable, and d is the radius of the optical cable. When the ice thickness exceeds the warning value, the system will issue a warning.

[0060] This system has the characteristics of fast response speed, accurate separation of temperature and strain, and high spatial resolution, which is beneficial to the real-time and effective monitoring and warning of the ice thickness of the OPGW.

[0061] The series of detailed descriptions listed above are only specific descriptions of the feasible implementation modes of the present invention, and they are not intended to limit the protection scope of the present invention. Any equivalent modes or changes that do not depart from the technology created by the present invention should be included in the protection scope of the present invention.

Claims

1. An OPGW optical cable icing monitoring system based on an optical frequency comb BOTDR, characterized in that, Including: An electro-optic frequency comb (1), an optical isolator (2), a first optical coupler (3), a pulse signal generator (4), an acousto-optic modulator (5), a first EDFA optical amplifier (6), an optical circulator (7), an OPGW sensing optical cable (8), a second EDFA optical amplifier (10), an FBG optical fiber filter (11), a polarization scrambler (12), a microwave signal generator (13), an electro-optic modulator (14), an adjustable optical attenuator (15), a second optical coupler (16), a balanced detector (17), a low-frequency amplifier (18), and a data processing module (19); The pulsed laser generated by the electro-optic frequency comb (1) enters the first optical coupler (3) through the optical isolator (2); The first output end of the first optical coupler (3) is connected to the input end of the acousto-optic modulator (5). The output waveform of the acousto-optic modulator (5) is controlled by the pulse signal generator (4). The output end of the acousto-optic modulator (5) is connected to the input end of the first EDFA optical amplifier (6). The output end of the first EDFA optical amplifier (6) is connected to the input end of the optical circulator (7); The first output end of the optical circulator (7) is connected to the OPGW sensing optical cable (8). The tail end of the OPGW sensing optical cable (8) is located in the temperature / strain sensing area (9), and the temperature / strain sensing area (9) is used to represent the temperature and strain that the OPGW optical cable bears under field conditions; The second output end of the optical circulator (7) is connected to the input end of the FBG optical fiber filter (11) after passing through the second EDFA optical amplifier (10). The output end of the FBG optical fiber filter (11) is connected to the first input end of the second optical coupler (16); The second output end of the first optical coupler (3) is connected to the input end of the polarization scrambler (12). The output end of the polarization scrambler (12) is connected to the input end of the electro-optic modulator (14). The output of the electro-optic modulator (14) is controlled by the microwave signal generator (13). The output end of the electro-optic modulator (14) is connected to the input end of the adjustable optical attenuator (15). The output end of the adjustable optical attenuator (15) is connected to the second input end of the second optical coupler (16). The output end of the second optical coupler (16) is connected to the input end of the balanced detector (17). The output end of the balanced detector (17) is connected to the input end of the low-frequency amplifier (18). The output end of the balanced detector (17) is connected to the data processing module (19); By performing signal processing on the real-time powers of the difference-frequency optical signal and the Brillouin scattering optical signal output by the amplified balanced detector (17), the data processing module (19) can respectively obtain the temperature and strain information of the OPGW optical cable. Furthermore, the ice-covering mass and thickness at different positions can be obtained through the fiber strain amount. When the ice-covering thickness exceeds the warning value, the system will issue a warning.

2. The OPGW optical cable icing monitoring system based on optical frequency comb BOTDR according to claim 1, wherein The central wavelength of the pulsed laser emitted by the electro-optic frequency comb (1) is 1550 nm, the output frequency interval is adjustable between 1 GHz and 3 GHz, the power is 2 W, and the pulse linewidth is 30 ns.

3. The OPGW optical cable icing monitoring system based on optical frequency comb BOTDR according to claim 1, characterized in that, The coupling ratio of the first output end and the second output end of the first optical coupler (3) is 90%:10%. The light emitted from the first output end is used as the detection light and is injected into the OPGW sensing optical cable to generate Brillouin backscattered light. The light emitted from the second output end is used as the reference light, and the Brillouin frequency shift signal is obtained by beating with the Brillouin backscattered light generated in the first path.

4. The OPGW optical cable icing monitoring system based on optical frequency comb BOTDR according to claim 1, characterized in that The frequency of the balanced detector (17) is 150 MHz.

5. The OPGW optical cable icing monitoring system based on optical frequency comb BOTDR according to claim 1, wherein The coupling ratio of the second optical coupler (16) is 50%:50%.

6. The OPGW optical cable icing monitoring system based on optical frequency comb BOTDR according to claim 1, wherein, The isolation degree of the optical isolator (2) is higher than 35 dB, ensuring that the reflected light does not enter the electro-optic frequency comb (1).

7. The OPGW optical cable icing monitoring method of the OPGW optical cable icing monitoring system based on optical frequency comb BOTDR according to claim 1, characterized in that, It includes the following: S1: Optical pulses with a wavelength of 1550 nm, a frequency interval of Δf = 1 GHz, and a pulse width of 30 ns are generated by the electro-optic frequency comb (1), pass through the optical isolator (2) and enter the first optical coupler (3). The isolation degree of the optical isolator (2) is higher than 35 dB, ensuring that the reflected light does not enter the electro-optic frequency comb (1). S2: After the optical pulses are emitted from the first optical coupler (3), they are divided into two paths. One path of light is used as the detection light and is injected into the OPGW optical fiber (8) to generate Brillouin backscattered light. The other path is used as the reference light, and the Brillouin frequency shift signal is obtained by beating with the Brillouin scattered light generated in the first path. The power ratio of the detection light and the reference light is 90%:10%. S3: The first path of pulsed light with a power ratio of 90% is emitted from the first output end of the first optical coupler (3), passes through the acousto-optic modulator (5). A control signal emitted by the pulse signal generator (4) is loaded on the acousto-optic modulator (5). Therefore, after passing through the acousto-optic modulator (5), the pulse width and waveform of the pulsed light are loaded with the modulation information of the pulse signal generator (4). S4: After the pulsed light is emitted from the acousto-optic modulator (5), it enters the first EDFA optical amplifier (6) to amplify the optical power, and then is injected into the OPGW sensing optical cable (8) through the optical circulator (7), and is reflected by the temperature / strain sensing region (9). The reflected light is Brillouin scattered light encoded with temperature and strain information, including a low-frequency component (i.e., Brillouin Stokes light) and a high-frequency component (Brillouin anti-Stokes light). The center frequencies of both are shifted by approximately 11 GHz with respect to the incident detection laser. The exact value of the Brillouin frequency shift depends on the change in the sensing parameter (temperature or strain) and the sensing sensitivity of the optical fiber. S5: After the reflected light is emitted from the second output end of the optical circulator (7), it enters the FBG optical fiber filter (11) after passing through the second EDFA optical amplifier, filtering out the high-frequency Brillouin scattered light (i.e., Brillouin anti-Stokes light) and other unwanted stray light, and only retaining the low-frequency Brillouin scattered light. The filtered low-frequency Brillouin scattered light enters the second optical coupler (16). The second pulsed light with a power ratio of 10% in S6 serves as the reference light and exits from the second output end of the first coupler (3). After passing through the polarization scrambler (12), the polarization state of the reference light is changed, which plays a role in suppressing depolarization noise. The reference light is transmitted through the electro-optic modulator (14) controlled by the microwave signal generator (13) to generate output light containing two frequency sidebands. The output light containing two frequency sidebands passes through the variable optical attenuator (15) to adjust the optical power to be the same as that of the Brillouin scattered light, and then is input into the second optical coupler (16). In S7, the two beams of light enter the balanced detector (17) through the second optical coupler (16). After mixing, the mixing signal of the two beams of light is detected, and its intensity I is: In the formula, the first term on the right side represents the difference-frequency light of the optical-frequency input pulse after loading temperature and strain information, and the second term represents the Brillouin scattering light. f1 = 1 GHz and f2 ≈ 11 GHz are the intrinsic frequency of the difference-frequency light and the Brillouin scattering frequency shift respectively, and A and B are the optical intensities of the difference-frequency light and the Brillouin scattering light respectively. They are the initial phases of the difference-frequency light and the Brillouin scattering light respectively; The mixing signal detected by the balanced detector (17) is two independent Lorentzian spectral lines. The center frequency of the difference-frequency light is insensitive to temperature and strain. Therefore, when the temperature and strain of the OPGW sensing optical cable change, only the amplitude of the difference-frequency light changes, while the center frequency of the Brillouin scattered light is sensitive to temperature and strain. Therefore, the Brillouin signal undergoes an obvious frequency shift with the change of temperature and strain. The measurement results of the S9 balance detector (17) are amplified by the low-frequency amplifier (18) and then input into the data processing module (19) for data processing to obtain the temperature and strain suffered by the OPGW sensing optical cable; assuming that when (temperature, strain) changes from (T1, ε1) to (T2, ε2), the optical power change amount at the center frequency f1 of the beat frequency light is ΔI1, and the optical power change amount of the Brillouin scattered light at the operating frequency f2 is ΔI2, from the equation the temperature change amount ΔT and the strain change amount Δε can be obtained; where C T1 and C ε1 are the change coefficients of the beat frequency optical power with respect to temperature and strain respectively; C T2 , C ε2 are the change coefficients of the Brillouin scattered optical power with respect to temperature and strain respectively; S10 Measure the ice accretion mass of the OPGW sensing optical cable (8); The ice accretion mass (m) at a certain point on the optical fiber under static conditions is obtained by where ε max and ε(x) are the maximum strain and the strain at any position of the optical fiber respectively, A is the tension coefficient, and g is the acceleration due to gravity; S11, measure the ice coating thickness; after obtaining the ice coating mass m, calculate the ice coating thickness through the relational expression ; where h represents the ice coating thickness, ρ is the density of ice, l is the length of the OPGW optical cable in the ice coating section, and d is the radius of the optical cable. In S12, when the ice coating thickness exceeds the warning value, a warning is issued.

8. The OPGW optical cable icing monitoring method according to claim 7, characterized in that, During measurement, only the center frequency of the difference-frequency light and the ramp interruption of the Brillouin scattered light are measured at fixed frequency points in real time to improve the response speed of the system.

9. The OPGW optical cable icing monitoring method according to claim 7, characterized in that Coefficient C T1 , C ε1 , C T2 , C ε2 can be obtained by pre-applying temperature changes or strains to the OPGW sensing optical cable and measuring the corresponding power changes; the initial differential frequency optical power I 10 and the Brillouin scattered optical power I 20 are also measured in advance.

10. The OPGW optical cable icing monitoring method according to claim 7, wherein In S10, for the OPGW optical cable in the dynamic state, the ice coating mass can be obtained by first performing finite element analysis through Ansys and then fitting and correcting the static relationship.

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