Optical fiber cable monitoring method and system

Abstract

A method and system for determining the condition of a cable are disclosed. The method includes transmitting a query signal to a sensing optical fiber extending along the length of the cable, and receiving a backscattered signal from the sensing optical fiber in response to the query signal. The method further includes processing the backscattered signal to determine variations in strain or strain rate along the sensing optical fiber, and determining a time history of strain or strain rate to obtain accumulated strain data indicating the condition of the cable at points along the cable.

Description

Technical Field

[0001] Aspects of the present disclosure generally relate to methods and / or systems for monitoring optical fiber cables, and more particularly to methods and / or related systems for monitoring the state of optical fiber cables.

Background Art

[0002] Any reference to prior art herein does not admit or imply that this prior art forms part of the common general knowledge in any jurisdiction, or that this prior art is understood by a person skilled in the art, considered relevant, and / or reasonably foreseeable to be combined with other prior art.

[0003] Cable assets such as power cables and telecommunication cables currently need to be robustly designed to withstand various types of external man-made and natural hazards (e.g., ships, anchors, trawl nets, excavators, large marine animals, falling rocks, water currents, natural wear and corrosion, high pressure, extreme temperatures, underwater volcanoes, tsunamis, and earthquakes). Safety requirements for protecting cables include using steel armor, plastic insulators, and gels as ways to protect the cable from the surrounding damaging environment. For example, FIG. 1 shows a typical double-armored submarine optical fiber cable 100. The fiber 102 is in the center and is protected from the surrounding environment by several core layers of plastic such as polycarbonate or aluminum strands 106, steel strands 108, and nylon 104, and has an outer sheath such as PE. Such cables are also usually buried.

[0004] Generally, submarine optical cables are deployed and used until they fail or their value decreases over a specified lifespan of 20 to 25 years. During their lifespan, optical cables are subjected to tension and compression due to the presence of external hazards (e.g., earthquakes) that can cause permanent deformation of the cable and affect the performance of the fiber.

Summary of the Invention

[0005] A first aspect of the present disclosure provides a method for determining the state of a cable. The method includes transmitting a query signal to a sensing optical fiber extending along the length of the cable, and receiving a backscattered signal from the sensing optical fiber in response to the query signal. The method further includes processing the backscattered signal to determine variations in strain or strain rate along the sensing optical fiber, and determining a time history of strain or strain rate to obtain accumulated strain data indicating the state of the cable at points along the cable.

[0006] In the embodiment, the cable is a submarine optical fiber cable having at least one primary optical fiber communication, or a sensing cable that also serves as a sensing optical fiber.

[0007] In this embodiment, the cable is a power cable, and the sensing optical fiber is either embedded inside the power cable or bundled with a separate cable located nearby.

[0008] In the embodiment, processing the backscatter signal includes determining the dynamic strain velocity at adjacent locations along the length of the fiber to obtain dynamic strain velocity data, and determining the time history of the strain velocity includes integrating the dynamic strain velocity data over a predetermined period.

[0009] In embodiments, this method further includes comparing strain data accumulated based on cable specification data with a threshold, and generating an alert condition if the threshold is exceeded at least one location along the cable.

[0010] In embodiments, this method further includes determining cable risk and generating an alert state if a high risk is identified and exceeds a threshold.

[0011] In the embodiment, the risk is based on natural and / or man-made risks or hazards in the vicinity of at least one location.

[0012] In the embodiment, a dispersed optical fiber sensing (DFOS) is used to detect at least one of Rayleigh, Brillouin, and Raman scattering in order to evaluate the variation in strain or strain rate.

[0013] In embodiments, this method further includes mapping the time history of strain or strain rate data along the length of the cable in order to present a visual representation of the cable condition.

[0014] In embodiments, this method further includes mapping the locations of natural and / or man-made risks or hazards in the vicinity of the cable.

[0015] In the embodiment, in order to acquire dynamic strain rate data, determining the dynamic strain rate at adjacent positions along the length of the fiber involves converting the raw data from the backscatter signal into strain rate equivalent units using the following equation: ε(x,t)=λ / 4πngξdφ(t), where λ is the wavelength of light, n is the refractive index, g is the gauge length and is defined as the distance between two points on the fiber where the measurement is performed, and ξ=0.7869.

[0016] A second aspect of the present disclosure provides a distributed optical fiber sensing (DFOS) system for determining the state of a cable. The system includes a light source for transmitting a query signal to a sensing optical fiber extending along the length of the cable; a photodetector for receiving a backscattered signal from the sensing optical fiber in response to the query signal; and a processing unit configured to process the backscattered signal to determine a time history of strain or strain rate in order to determine a variation in strain or strain rate along the sensing optical fiber and to obtain accumulated strain data indicating the state of the cable at points along the cable.

[0017] In this embodiment, the processing unit is further configured to determine cable risk, identify high risk, and generate an alert state if a threshold is exceeded.

[0018] In the embodiment, the processing unit is configured to detect at least one of Rayleigh, Brillouin, and Raman scattering in order to evaluate the strain or the variation in strain rate.

[0019] In the embodiment, the processing unit is further configured to map the time history of strain or strain rate data along the length of the cable to present a visual representation of the cable's condition.

[0020] In some embodiments, the system further includes mapping the locations of natural and / or man-made risks or hazards in the vicinity of the cable.

[0021] In the embodiment, in order to acquire dynamic strain rate data, determining the dynamic strain rate at adjacent positions along the length of the fiber involves converting the raw data from the backscatter signal into strain rate equivalent units using the following equation: ε(x,t)=λ / 4πngξ dφ(t), where λ is the wavelength of light, n is the refractive index, g is the gauge length and is defined as the distance between two points on the fiber where the measurement is performed, and ξ=0.7869.

[0022] Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, which is given by example and with reference to the accompanying drawings. [Brief explanation of the drawing]

[0023] [Figure 1] This is an illustrative diagram of a typical double-armored submarine fiber optic cable. [Figure 2A] An example of a system for distributed acoustic sensing is shown. [Figure 2B] Figure 2A shows an example of an electrical signal generated over time by the system. [Figure 3] This is a schematic diagram of a disclosed method of dispersed acoustic sensing for detecting dynamic strain velocity, according to some aspects of the present disclosure. [Figure 4A]Schematically shows the transmission sequence of interrogation optical signals at multiple time points and the corresponding sequence of observation windows. [Figure 4B] Schematically shows an example of an amplitude-versus-distance plot provided by the system of the present disclosure. [Figure 5] Schematic diagram showing the process of processing data according to the disclosed method of distributed acoustic sensing of the present invention. [Figure 6] Exemplary cable strain map of a submarine optical fiber cable using distributed fiber optic sensing (DFOS) across the port of Wellington. [Figure 7] Exemplary cable strain map of a submarine optical fiber cable using DFOS across the port of Wellington. [Figure 8A] Exemplary strain data plot in a single channel over time of a submarine optical fiber cable using DFOS. [Figure 8B] Exemplary strain data plot in a single channel over time of a submarine optical fiber cable using DFOS.

MODE FOR CARRYING OUT THE INVENTION

[0024] Commercially available submarine cables can generally be designed with a single armor layer of steel to provide strain resistance. For example, a cable with a total (outer) diameter of 31 mm and a fiber and cable breaking load of 396 kN is not stronger than a steel wire of the same diameter and strength corresponding to a maximum strain limit of 2 mm.

[0025] Particularly in areas with a high likelihood of external hazards (e.g., high seismic or man-made risk areas), it may be desirable to monitor the state of the cable, especially during the precursor period before failure. Up-to-date information regarding the state of the cable may enable the cable to be replaced or repaired before it fails.

[0026] There are currently two common techniques used to assess and monitor the integrity of installed submarine cables. One technique involves periodic campaign-style surveys of cable assets. This may include scuba divers or remotely operated vehicles visually inspecting the cable's path for exposed points or damage. As can be understood, this method is expensive and sporadic. Furthermore, cables can accumulate tension / be damaged during the period between two surveys, which may only be detected in the next survey. Often, such damage is invisible and therefore not easily detected.

[0027] Another technique involves the use of multibeam sonar or airborne LiDAR systems to map seabed or land and cable assets. Multibeam sonar surveys typically use source-receiver arrays mounted directly on the ship's hull, while LiDAR systems use laser scanners mounted on helicopters. These methods for surveying cable assets are highly capital-intensive and time-consuming. Furthermore, in logistics, these types of surveys cannot be conducted routinely or even annually to provide the data necessary to alert cable asset owners and operators about the condition of the cables. In addition, such surveys cannot be effectively used to monitor the condition or changes in cable condition caused by cable strain, which is often not visible.

[0028] Therefore, currently there is no way to obtain data on the accumulated tension and / or compression that a cable experiences over time. There is no precise method to determine the state of stress experienced by the cable and its effect on the accumulated strain of the fiber over time. The in-situ time history of strain generated in an optical fiber cable may be useful to provide important and novel data on the cable's performance against its design specifications, and to enable monitoring of its performance and condition over time to detect or predict impending cable failure / remaining expected lifespan.

[0029] Aspects of the present invention arise from the inventors' recognition that cables can be monitored using distributed optical fiber sensing (DFOS) data to provide a time history of strain accumulated by the cable. In other words, the methods (and associated systems) disclosed in aspects of this disclosure measure the accumulation of tension and compression in a cable by measuring the channel-by-channel dynamic strain rate of the optical fibers associated with the cable. In particular, the disclosed systems and methods estimate the tension accumulated in a cable by measuring the dynamic strain rate along the length of the optical fiber cable using DFOS on an existing optical fiber cable that is embedded inside a telecommunications cable or a power cable, or bundled nearby but in a separate power cable. In some embodiments, the methods and / or systems use Rayleigh scattering and / or Brillouin scattering in the optical fiber to evaluate the strain along the optical fiber. In particular, aspects of the present invention use DFOS to detect Rayleigh and / or Brillouin and / or Raman scattering to evaluate the dynamic strain rate over a short segment or gauge length. Cables subjected to hazardous environmental phenomena (e.g., earthquakes, rockfalls, storms, tsunamis, etc.) or other types of external hazards are presumed to accumulate tension over time, causing permanent strain in the cable. These differences or fluctuations in strain may be used by the systems and methods of this disclosure to determine whether a portion of the cable is degraded or permanently damaged, and to determine the exact location of such damage.

[0030] The strain rate is defined as the change in the strain of an optical fiber over time (e.g., deformation due to the accumulation of tension and compression). In this disclosure, the optical fiber functions as a series of independent linear strain (strain rate) sensors. In other words, the strain increases linearly with increasing stress (e.g., due to environmental hazards such as earthquakes).

[0031] In one example, DFOS data is recorded from the shore side of single-mode optical fibers that are either integrated into or separated from a submarine cable wet plant but located near submarine cable assets. In another example, DFOS data is recorded from pre-installed optical fiber cables that are part of an installed optical fiber communication network or separated from the network. DFOS data is collected and used to estimate dynamic strain rate. In particular, strain history measured per channel can be used to monitor specific potential fault locations in the cable over months to years, or to capture the overall impact of a specific single event, such as a major earthquake or a near-miss cable strike. The maximum spatial resolution of this technique is approximately 10 meters.

[0032] Next, based on the DFOS data (and by comparing the DFOS data with the cable design specifications), it is determined whether any portion of the monitored cable may be degraded or damaged. If it is determined that one or more portions of the monitored cable may be degraded or damaged, a cable strain map is generated. The cable strain map can show the actual geographical location of potential areas of fiber under permanent or regular strain, and the risk level associated with the exposure. In some embodiments, the cable strain map may include contextual information such as water depth above possible cable exposure points, how often a vessel's course crosses a possible cable exposure point, the frequency of high-risk AIS navigation alerts, forecasts of storm and high wave activity, history of cable strikes in the area, and / or the risk tolerance of the cable owner. Finally, based on the cable strain map, alerts can be issued to the cable owner, cable operator, or cable infrastructure user to consider mitigation strategies such as preventative maintenance, replacement of the cable or any part thereof, measurement of the impact of a specific repair event, or avoidance of potential damaged areas. Optionally, alerts can be automatically communicated, for example, via an Automatic Identification System (AIS), to warn potential vessels in the area about any location where the cable may have been damaged.

[0033] Therefore, the system and method of the present invention can result in better design engineering of submarine cables, as well as retrofit behavior in which cable sections are replaced in-situ when they begin to show signs of excessive fatigue, thereby avoiding the consequences of frequently catastrophic failures.

[0034] These and other aspects of the Disclosure are described in detail in the following sections.

[0035] Exemplary DFOS system Generally speaking, a DFOS system includes a query unit and a sensing cable. The query unit continuously injects short pulses of optical signals into the sensing cable. As the light passes through the fiber core of the sensing cable, the incident light is scattered in different directions due to spatial variations in the refractive index of the fiber core, generating different types of backscattered light (e.g., Rayleigh, Raman, and Brillouin). When the optical fiber is disturbed and subjected to strain and / or vibration, the properties of the scattered light (wavelength, light intensity, frequency, phase, etc.) change. By analyzing the specific properties of the returned scattered light, changes in various physical parameters (e.g., axial strain, strain rate, vibration damping) can be revealed. Furthermore, by analyzing the phase information of the Rayleigh scattered light, a measurement of the dynamic strain rate can be obtained.

[0036] As an example, a system 200 for use in Distributed Fiber Optic Sensing (DFOS) is shown in Figure 2A. The DFOS system 200 includes a coherent optical time-domain reflectometer (C-OTDR) 202. The C-OTDR 202 includes a light source 204 for emitting an optical query signal 207 in the form of short optical pulses to be transmitted to one or more of the optical fibers 205A, 205B, and 205C. Otherwise, existing optical fiber submarine communication cables can be used for DFOS sensing. The fibers can be laid in any orientation to introduce two or more components of motion / deformation to the length of each gauge of subsequent DFOS measurements, or they can even be wound spirally around a central cylinder. Furthermore, existing optical fibers laid for different purposes can also be used for DFOS measurements. Multiple fibers can be joined in series and used for DFOS in a single instrument, or multiple DFOS channels (analyzed by the same or separate DFOS instruments) can be used to record DFOS data within the same perimeter.

[0037] The light source 204 is a laser source. The laser source can be used with or without an external cavity. The optical query signal 206 can be a simple pulse, a chirp pulse, or a continuous wave. Furthermore, the optical query signal 206 can be within the infrared or near-infrared frequency range. In one embodiment, the signal is pulsed laser light having a wavelength of 1550 nm in the C band, which is suitable for Rayleigh backscattering.

[0038] The C-OTDR202 includes a photodetector 208 configured to detect Rayleigh scattering from the incident wavelength by one or more optical fibers 205A, 205B, and 205C, or backscattered light 210 resulting from a Brillouin or Raman transition. Rayleigh scattering is a form of elastic scattering of light that preserves the kinetic energy of the accompanying particles in the optical fiber where the scattering occurs; that is, in this type of scattering, the scattered photon has the same energy as the incident photon. Generally, Rayleigh scattering occurs in optical fibers due to density heterogeneity that is frozen during manufacturing. Raman scattering is a form of inelastic scattering of light that does not preserve the kinetic energy of the incident particle. Brillouin scattering is a “photon-phonon” interaction, as the annihilation of the incident photon simultaneously produces a Stokes photon and a phonon. Both Raman and Brillouin scattering produce components called the Stokes component and the anti-Stokes component.

[0039] The return light 210 is dispersed and backscattered within the optical fiber 205. In some embodiments, the photodetector 208 directly passes the detected optical signal 210 to the processing unit 214. In other embodiments, the photodetector 208 may convert the optical signal 210 into an electrical signal, such that the amplitude of the electrical signal 212 is proportional to the reflected light intensity resolved over time. The time scale can be converted to a distance scale relative to the photodetector 208. Figure 2B shows a schematic plot of the amplitude of the optical signal against distance at a particular moment in time.

[0040] Returning to Figure 2A, the DAS system 200 also includes a processing unit 214, either inside or separately from the C-OTDR 202. The processing unit 214 can be configured to process the optical signal returned from the optical fiber and store the processed data as DFOS data. In some embodiments, the processing unit 214 may be an optical processor that optically analyzes the received optical signal. In other embodiments, the processing unit 214 may be a digital processor that converts the electrical signal received from the photodetector 208 into digital DFOS data and then stores this data. As referred to herein, DFOS data includes data on the distortion of the optical fiber for a specific time sample at all sensor positions along the fiber.

[0041] In a particular embodiment, the processing unit 214 uses DFOS data to determine the dynamic strain rate field along the optical fiber.

[0042] In certain examples, the input pulse may be transmitted continuously through the optical fiber, the DFOS data may be continuously updated, and the dynamic strain rate field may also be continuously calculated from the DFOS data. In other examples, the input pulse may be transmitted periodically (e.g., every few hours or days), and the DFOS data may be updated at the same frequency. The dynamic strain rate field along the optical fiber can be calculated at the same frequency or at different frequencies (e.g., every few days). In other embodiments, the strain values ​​can be calculated sporadically (e.g., in response to known activity, e.g., storms, earthquakes, cyclones, etc.).

[0043] In addition to determining strain values, the processing unit 214 may also be configured to determine whether any part of the cable is degraded or damaged (for example, by comparing strain data with design specifications), determine the geographical location of such possible damage, and generate a cable strain map. The processing unit 214 may also be configured to automatically generate alerts based on accumulated strain or strain rate.

[0044] DFOS data or records may be stored in the storage unit 215. The storage unit 215 may include volatile memory, such as random access memory (RAM), for the processing unit 214 to execute instructions, compute, calculate, or process data. The storage unit 215 may include non-volatile memory, such as a hard disk drive, for the processing unit 214 to store data for pre- and post- and / or post-processing retrieval. The processing unit 214 and the storage unit 215 may be distributed across a number of physical units and may include remote storage such as cloud storage, in which case the processing unit 214 and the storage unit 215 may be more generally defined as a cloud computing service.

[0045] Finally, the system 200 also includes a communication interface 217 that communicates with the processing unit 214, which can be used to communicate cable strain maps to cable operators and / or send alert signals to vessels near possible cable strain / damage locations. The communication interface may also be configured to receive requests for cable strain maps from one or more remote mobile terminals or fixed terminals 217A, 217B, and 217C. Upon receiving a request for a cable strain map, the processing unit 214 may be configured to generate or retrieve the requested strain map from the storage unit 215 (if it has already been generated and stored).

[0046] Figure 3 is a flowchart illustrating an exemplary method 300 for determining the dynamic strain rate of a cable, according to some aspects of the present disclosure.

[0047] Method 300 begins in step 302, in which the query optical signal 206 is transmitted by the light source 204 to one or more optical fibers 205 integrated with one or more cable assets. The optical fibers may be, for example, single-mode fibers used in typical submarine cables. Such fibers are generally designed and manufactured to conform to ITU G.654, which are single-mode fibers optimized for use in the 1500-1600 nm wavelength range, with a zero-dispersion wavelength of about 1300 nm and cutoff shift and loss minimized at about 1550 nm. G.654 fibers for submarine applications typically offer improved attenuation specifications in the range of 0.15 dB / km to 0.17 dB / km.

[0048] The query signal may be transmitted in multiple instances. Figure 4A shows an exemplary query light pulse transmitted by the light source in this process. In particular, Figure 4A shows three time points 252A, 252B, and 252C in which the query light signal 206 is transmitted within the optical fiber.

[0049] Once the signal is transmitted, the method proceeds to step 304, where the backscattered signal 210 is received from one or more optical fibers 205.

[0050] Figure 4A also shows the return signal or backscatter signal. These return signals 210 are received during observation periods 254A, 254B, and 254C between times 252A, 252B, and 252C. The time at which the return signal is received during the observation period indicates the position of the optical fiber from which the optical signal was backscattered. The later the return signal is received during the observation period, the further away the signal is along the backscattered optical fiber. The characteristics of the returned optical signal (e.g., wavelength, light intensity, frequency, phase, etc.) may vary depending on the type of strain the section of the optical fiber experiences. Therefore, by analyzing the characteristics and timing of the return signal, the processing unit can determine the strain occurring at a known position along the fiber in a particular section of the optical fiber 205.

[0051] In particular, the location of the fiber channel can be estimated from the time of flight of the return signal 210 (due to Rayleigh, Brillouin, or Raman scattering), using the known refractive index n of the optical fiber, with z = ct / 2n. For example, a typical single-mode fiber (at 1550 nm, with attenuation = 0.15 dB / km) has approximately 1,000 potential Rayleigh scattering locations per meter.

[0052] The backscatter signal 210 from the fiber channel is mixed with a reference signal in the photodetector to form an interference intensity signal.

number

[0053] It will be understood that there are different options for selecting the reference signal, including a second pulse delayed by a known time, a Michelson or Mach-Zehnder reference signal, or a heterodyne local oscillator.

[0054] Figure 4B shows the return optical signal 210 from Figure 4A, which has been converted into an electrical signal 212 by the photodetector 208. Each electrical signal 212 has a fluctuating peak 216. The different fluctuating peaks 216 shown in Figure 4B are due to different return optical signals 210 generated by different types of distortion occurring in one or more sections of the optical fiber.

[0055] The electrical or optical signals received from the optical fiber 205 are digitized and converted into DFOS data, which may be stored in the storage unit 215 by the processing unit 214 in this step. The storage unit 215 contains values ​​relating to the state of the optical fiber at a specific time sample at all sensor locations in the fiber. It will be understood that steps 302 and 304 may be repeated continuously, periodically, or performed at some other frequency. In other cases, these steps may be performed based on one or more trigger conditions—for example, the detection of one or more activities (e.g., storm, earthquake, etc.) in the vicinity of the cable in response to a request for a cable strain map.

[0056] The frequency bandwidth of DFOS data is 0.001–500 Hz, and it will be understood that this covers a wide frequency range from static or nearly static timescales to the Nyquist sampling frequency of the techniques disclosed herein. In the case of Rayleigh scattering methods such as DFOS, the Nyquist frequency can be 1000 Hz for a submarine cable of 50 km in length. Thus, the DFOS measurement bandwidth covers multiple available environmental phenomena or signals generated by active sources in the environment.

[0057] For example, DFOS technology can detect disturbances with a wide range of frequencies. These can include, for example, ocean noise (sound waves traveling through a water column), Scholte waves (interface waves traveling between a water column and the seabed), ocean loading (coupled ocean surface gravity waves), or seismic waves (elastic waves traveling through the solid Earth). The DFOS technology described herein can also detect passing vessels by changing the frequency of the input pulse to be within the frequency range of the vessel based on their dimensions and speed.

[0058] In step 306, the collected DFOS raw data is processed by the processing unit 214 to extract the time history of the strain that occurred in the optical fiber at the site. The secondary processes for processing the DFOS data are shown in Figure 5 and discussed below.

[0059] Moving to Figure 5, this method begins in step 502, when the DFOS raw data is read by the processor 214 and converted to strain rate equivalent units (step 504). In one example, the following formula can be used for this conversion:

number

[0060] This parameter is measured between consecutive pulses within the same channel, or between consecutive channels of the same pulse. The selection of a photonic embodiment is determined by whether the raw DAS dataset has units equivalent to the dynamic strain rate given by the above formula.

[0061] Alternatively, the conversion step 504 can be performed by measuring the strain rate from the strain data using the original sampling rate as the time constant of the derivative, and then converting it to strain units using equation (2).

[0062] Assuming that the refractive index and temperature are constant or vary within the boundary range, it will be understood that the above equation allows for the measurement of axial strain (ε = dL / L). This is because, under these assumptions, optical phase changes are not recorded, and therefore the strain is linearly related to the gauge length.

[0063] Furthermore, it is important to note that strain rate measurements may be affected by long-range laser frequency noise and periodically increasing optical phase noise resulting from self-homodyne and self-heterodyne interferometry. As a result, optical noise makes a significant noise floor contribution to DFOS measurements at low frequencies f < 10 Hz. However, DFOS phase measurements can be stabilized against this noise source by providing an external cavity or similar device that provides a reference path to directly remove the optical noise.

[0064] In step 506, a high-pass filter is applied to remove any laser noise if the external cavity is not used.

[0065] In step 508, the integral operator is applied over a time frame of a predetermined size (t2-t1). For example, the following formula can be used.

number

[0066] By integrating over time, it is possible to calculate the change in strain generated in the fiber before and after an event occurs (e.g., an earthquake). This represents the dynamic strain rate, which takes into account the accumulation of strain in the fiber (and therefore the strain history).

[0067] After integration step 508, data processing is complete.

[0068] Returning to Figure 3, once the processing step (step 306) is complete, the time history of the strain generated (accumulated) in the optical fiber is monitored in a distributed manner, and therefore across multiple channels of the fiber (step 308). This dynamic strain rate measurement allows for an understanding of how the cable is mechanically performing in each channel in relation to its design specifications.

[0069] In step 310, if a particularly significant event (e.g., earthquake, storm, tsunami) is identified, for example, by thresholding the change in dynamic strain rate from one sample to the next, the cable user or owner can alert fishermen and / or marine or land-based operators that an incident has been detected. For example, after an earthquake, system 100 of the present invention enables measurement of the change in strain that occurred in the fiber before and after the earthquake. If system 100 records strain that approaches or exceeds the design specifications, an alert is generated. Alerts may be issued in stages depending on the degree to which the specifications are approximated or exceeded.

[0070] In step 312, it is determined whether the time history based on accumulated strain rates recorded over time in each channel or section of the fiber has a strain score higher than a threshold score (for example, in some embodiments, the design specification is just below the threshold score, and in other embodiments, the design specification is equal to or above the threshold score, for example, by +5%). If none of the accumulated strains have a score higher than the threshold score, method 300 terminates. Alternatively, if any one of the strains has a score higher than the threshold score, the method proceeds to step 314, in which processing unit 214 may be configured to issue a warning or alert to the cable owner or cable operator or cable infrastructure user to consider mitigation strategies. Optionally, processing unit 214 may also communicate warnings via AIS to alert potential vessels in the area about possible locations of sections or channels of fiber that are generating permanent strain (or high dynamic strain rates). In some examples, alerts may be communicated via communication interface 217. The alert may include information about the geographical location of cables showing permanent strain, and can warn fishing vessels and trawlers to be vigilant in that area.

[0071] In an alternative embodiment, the measured strain rate itself can be compared to a threshold strain rate, and an alert can be generated even if the accumulated strain does not exceed the threshold. In a further embodiment, both the accumulated strain and the estimated strain can have a combined threshold that triggers an alert.

[0072] In another embodiment, the processing unit 214 can provide a cable strain map to a cable operator, who, upon receiving the cable strain map, can flag the strained locations with live AIS beacons to warn potential vessels in the area, thereby informing fishermen and operators of maritime vessels of the increased risk. Alternatively, other devices can be implemented to flag the coordinates of the location of a section or channel of fiber that is experiencing permanent strain. For example, the location coordinates may be transmitted via the Internet through an Emergency Position Indicating Radio Beacon (EPIRB) alert or other quantitative information device such as a satellite device (i.e., a GPS position tracker).

[0073] In the method 300 described above, baseline measurements may be required to determine the location of a fiber section or channel experiencing permanent strain. For this purpose, when the cable is first laid, an inquiry signal may be transmitted within the sensing optical fiber, and a return signal may be received and processed to determine the baseline strain of the optical cable (when first deployed). Alternatively, design specifications from the manufacturer can be used as baseline strain data. The baseline strain can be compared with hysteresis strain data accumulated by the optical cable to determine the difference from the baseline and identify potential points where the fiber is exhibiting permanent tension. For example, if the signal strain increases at a particular location over a long period of time from the baseline level, the processing system may determine that the location is a point where permanent damage or degradation of the fiber is possible.

[0074] Baseline readings can also be used to identify intrinsic low-distortion or high-distortion bands (e.g., ground bands or shore wave bands) which can later be used to label statistical plots and mask those bands.

[0075] Figure 6 shows a map of strain data from a submarine fiber optic cable 610 recorded by system 200 recording the entire Harbour of Wellington, New Zealand. This method was recently demonstrated for a single large earthquake using a submarine cable segment off the coast of New Zealand's North Island.

[0076] Each circle in Figure 6 represents a channel of the fiber array (e.g., channels 620A, 620M) during a day in which a significant strain event (in this case, an earthquake) occurred, showing how the submarine cable deformed over time. The grayscale color of the circles (representing channels, e.g., 620A, 620M) represents the output of the dynamic strain rate when the proposed methodology is applied. From the scale bar 650, it can be seen that the strain amplitude exceeded 40 microstrain in some channels, but was negligible in others. The cable shows variation on a 100m scale. The total length of the cable across the shore is approximately 5km.

[0077] Figure 7 is an exemplary plot of results obtained from the recording of the submarine optical fiber DFOS at Wellington Harbour, New Zealand, shown in Figure 6. The time axis on the left, 701, shows how the channels of the optical fiber cable deform at one time. In particular, Figure 7 shows that minute strains in the cable are first recorded at 701 (corresponding to the date September 23, 2022, with the first arrow indicating the first earthquake). These channel-by-channel time histories show the added deformation at the time of the second significant event, 702 (corresponding to the date October 13, 2022, with the second arrow indicating an aftershock). This is more clearly shown at 703, which shows the change in strain. The numbers from 5000 to 5800 are the channel indices (720) along the optical fiber cable 710 with a channel spacing of 6.38 m.

[0078] Figures 8A and 8B show an exemplary single-channel time history during major strain accumulation. As can be seen, the strain is interrupted. In Figure 8A, the strain during the M5.8 earthquake indicates that this section 810 of the cable entered compression (negative strain). Amplitudes 805A and 805B are shown in optical radians or equivalent strain. In Figure 8B, the strain during the subsequent large aftershock (M5.7) indicates that this section 820 of the cable entered compression by an amount equivalent to approximately 80% of the permanent strain from the major shock.

[0079] Mapping cable strain risks as described herein offers several advantages. A key advantage is that, after identifying permanent strain in a section of fiber (as shown in Figures 6 and 7), cable operators can flag this location to prevent or minimize aggressive risks to the cable there.

[0080] Furthermore, the above-described system and method can record data using the DFOS telecommunications receiver statistics itself, without requiring dedicated equipment at one end, to extract information about the state of the fiber, such as polarization characteristics or time of flight, from one end to the other. This can convey information about the rate of change over time of the fiber length, or the stress or strain state of the fiber at a point or along its length.

[0081] It will be understood that the present invention, as disclosed and defined herein, extends to all alternative combinations of two or more individual features referred to or evident from the text or drawings. All of these different combinations constitute various alternative embodiments of the present invention.

[0082] As used herein, unless the context requires otherwise, the term “comprise” and its variations such as “comprising,” “comprises,” and “comprised” are not intended to exclude any further additives, ingredients, elements, or processes.

Claims

1. A method for determining the condition of a cable, wherein the method is Transmitting an inquiry signal to a sensing optical fiber extending along the length of the cable; receiving a backscattered signal from the sensing optical fiber in response to the inquiry signal; The process of the backscattered signal to determine the strain or strain rate variation along the sensing optical fiber, and A method comprising determining the time history of the strain or strain rate in order to obtain accumulated strain data indicating the state of the cable at points along the cable.

2. The method according to claim 1, wherein the cable is a submarine optical fiber cable having at least one primary optical fiber communication, or a sensing cable that also serves as the sensing optical fiber.

3. The method according to claim 1, wherein the cable is a power cable, and the sensing optical fiber is embedded inside the power cable or bundled with a separate cable located nearby.

4. The method according to any one of claims 1 to 3, wherein processing the backscattered signal includes determining the dynamic strain rate at adjacent positions along the length of the fiber to obtain dynamic strain rate data, and determining the time history of the strain rate includes integrating the dynamic strain rate data over a predetermined period.

5. The method according to any one of claims 1 to 4, further comprising comparing the accumulated strain data with a threshold based on cable specification data, and generating an alert state if the threshold is exceeded at least one location along the cable.

6. The method according to claim 5, further comprising determining cable risk, and generating an alert state when a high risk is identified and exceeds a threshold.

7. The method according to claim 6, wherein the risk is based on natural and / or man-made risks or dangers in the vicinity of the at least one location.

8. The method according to any one of claims 1 to 7, wherein DFOS is used to detect at least one of Rayleigh, Brillouin, and Raman scattering in order to evaluate the strain or the variation in strain rate.

9. The method according to any one of claims 1 to 8, further comprising mapping the time history of strain or strain rate data along the length of the cable to present a visual representation of the state of the cable.

10. The method according to claim 9, further comprising mapping the location of natural and / or man-made risks or hazards in the vicinity of the cable.

11. To acquire dynamic strain rate data, determining the dynamic strain rate at adjacent positions along the length of the fiber includes converting the raw data from the backscatter signal into strain rate equivalent units using the following formula: ε(x,t)=λ / 4πngξdφ(t) The method according to claim 4, wherein λ is the wavelength of light, n is the refractive index, g is the gauge length, defined as the distance between two points in the fiber on which the measurement is performed, and ξ = 0.7869.

12. A distributed optical fiber sensing (DFOS) system for determining the condition of a cable, wherein the system is A light source for transmitting an inquiry signal to a sensing optical fiber extending along the length of the cable, A photodetector for receiving a backscattered signal from the e-sensing optical fiber in response to the aforementioned inquiry signal, The backscattered signal is processed to determine the strain or strain rate variation along the sensing optical fiber. A system including a processing unit configured to determine the time history of the strain or strain rate in order to acquire accumulated strain data indicating the state of the cable at points along the cable.

13. The system according to claim 12, wherein the cable is a submarine telecommunications or power cable, and the sensing optical fiber is embedded in the cable or bundled in a nearby but separate cable.

14. To process the backscattered signal, the processing unit is further configured to determine the dynamic strain rate at adjacent positions along the length of the fiber and to acquire dynamic strain rate data. The system according to any one of claims 12 to 13, wherein the processing unit is configured to integrate the dynamic strain rate data over a predetermined period in order to determine the time history of the strain rate.

15. The aforementioned processing unit is Based on the cable specification data, the accumulated strain data is compared with a threshold, The system according to any one of claims 12 to 14, further configured to generate an alert state when the threshold is exceeded at least one location along the cable.

16. The aforementioned processing unit Determine cable risk, The system according to claim 15, further configured to generate an alert state when a high risk is identified and exceeds the threshold.

17. The system according to any one of claims 12 to 16, wherein the processing unit is configured to detect at least one of Rayleigh, Brillouin, and Raman scattering in order to evaluate the strain or the variation in strain rate.

18. The system according to any one of claims 12 to 17, wherein the processing unit is further configured to map the time history of strain or strain rate data along the length of the cable to present a visual representation of the state of the cable.

19. The system according to claim 18, comprising mapping the location of natural and / or man-made risks or hazards in the vicinity of the cable.

20. To acquire dynamic strain rate data, determining the dynamic strain rate at adjacent positions along the length of the fiber includes converting the raw data from the backscatter signal into strain rate equivalent units using the following formula: ε(x,t)=λ / 4πngξdφ(t) The system according to claim 14, wherein λ is the wavelength of light, n is the refractive index, g is the length of the gauge, defined as the distance between two points in the fiber on which the measurement is performed, and ξ = 0.7869.

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