Hollow-core fiber (HCF) cable

Abstract

An optical fiber cable comprises a sensing fiber and a hollow-core fiber (HCF). Communication signals are propagated through the HCF. The sensing fiber permits detection of a potential fault or potential damage to the optical fiber cable.

Description

HOLLOW-CORE FIBER (HCF) CABLECROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional patent application serial number 63 / 560,935, filed 2024-March-04, having the title "Damage Prevention and Fault Detection in Hollow Core Fiber Cable," with first-named inventor DiGiovanni, which is incorporated by reference in its entirety as if expressly set forth herein.BACKGROUNDFIELD OF THE DISCLOSURE

[0002] The present disclosure relates generally to optical fiber cables and, more particularly, to hollow-core fiber (HCF) cables.DESCRIPTION OF RELATED ART

[0003] After installation, physical damage to optical fiber cables can result in intrusion of water or other elements, which can detrimentally affect performance of optical fibers near the location of the damage.SUMMARY

[0004] The present disclosure teaches detection of a potential fault or potential damage to optical fiber cables with hollow-core fibers (HCFs).

[0005] Briefly described, in one embodiment, an optical fiber cable comprises a sensing fiber and a hollow-core fiber (HCF). Communication signals are propagated through the HCF. The sensing fiber permits detection of a potential fault or potential damage to the optical fiber cable.

[0006] Other systems, devices, methods, features, and advantages will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure.Moreover, in the drawings, like reference numerals designate corresponding pans throughout the several views.

[0008] FIG. 1 is a drawing that shows an axial cross-section of an embodiment of a fault-detecting hollow-core fiber (HCF) cable.

[0009] FIG. 2 is a drawing that shows an axial cross-section of an embodiment of a fault-detecting HCF assembly.

[0010] FIG. 3 is a drawing that shows an axial cross-section of another embodiment of a fault-detecting HCF cable.

[0011] FIG. 4 is a drawing that shows an axial cross-section of another embodiment of a fault-detecting HCF cable.

[0012] FIG. 5 is a drawing that shows an axial cross-section of another embodiment of a fault-detecting HCF cable.

[0013] FIG. 6 is a drawing that shows a perspective view of components in an embodiment of a fault-detecting HCF cable.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0014] Unlike solid-core fibers (SCFs), hollow-core fibers (HCFs) include hollow regions that guide light along the HCFs. To take advantage of HCFs lower latency (as compared to SCFs), cable installations often avoid or limit the amount of slack or excess lengths of HCFs. This is because an excess length adds to the transit time for an optical signal to reach its destination, which negates to some extent the transit time saved from the lower latency.

[0015] When there is damage (such as a break or a cut) to an optical fiber cable with HCFs, exposed ends of the HCF become susceptible to intrusion of water, air, or other contaminants. Because of the hollow regions, intruding contaminants can migrate farther along the length of the HCF (as compared to a SCF). Thus, until the damaged portion of the cable is located and repaired, the intruding contaminants can increasingly degrade opticalperformance of the HCF over time. Previous experiments have shown that moisture can ingress tens of meters into the HCF from exposed ends. Additionally, as temperatures fluctuate, the moisture contamination can take the form of liquid or vapor (at higher temperatures) or ice (at below-freezing temperatures). Consequently, it is desirable to either detect imminent cable damage (thereby preventing cuts or breaks) or to quickly locate cable breaks (to minimize contaminant intrusion). Insofar as HCFs exhibit low backscatter, the HCF itself is not well suited to detecting breaks (for example, by using optical time-domain reflectometry (OTDR)).

[0016] This disclosure teaches a HCF cable with fault-detection capabilities. For some embodiments, the optical fiber cable comprises both a HCF and a sensing fiber. The HCF propagates the optical signal, while the sensing fiber detects perturbations in the vicinity of the optical fiber cable. Because cuts or breaks in an optical fiber cable are frequently preceded by soil movement (such as excavation), tampering, or other strain on the cable, the detection of such strains via the sensing fiber provides a reasonable indicator for potentially avoidable damage.

[0017] Having provided a broad technical solution to a technical problem, reference is now made in detail to the description of the embodiments as illustrated in the drawings. While several embodiments are described in connection with these drawings, there is no intent to limit the disclosure to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents.

[0018] Before describing the embodiments with reference to the drawings, this disclosure teaches the inventive nature of combining a HCF with a sensing fiber.Specifically, those with ordinary skill in the art can appreciate the inventive nature of combining a HCF with a sensing fiber because the desirable features of HCF conflict with the desirable features of a sensing fiber. Consequently, combining a HCF with a sensing fiber proceeds contrary to conventional wisdom.

[0019] For example, first, sensing fibers require transduction of an acoustic disturbance from an environment, through the protective layers of a cable, and ultimately to the sensing fiber within the cable to impart a strain on the sensing fiber (which measures the strain). In other words, strain is necessary for sensing fibers to detect disturbances. To the contrary, strain in HCFs causes micro-bending losses and undesirable multi-path interference(MPI). Because HCFs are sensitive to strain, those having skill in the art seek mitigate against strain in HCFs. Consequently, those having skill in the art would not be motivated to combine a sensing fiber (which requires strain) with a HCF (which is adversely affected by strain) within the same cable.

[0020] Second, the dominant application of HCFs is carrying large amounts of data between data centers, which are typically separated by more than a hundred kilometers(> 100km). Because of the desirability to transmit large amounts of data (e.g., for training of artificial intelligence (Al) models and other Al applications), those having skill in the art seek to maximize the bandwidth of an optical cable, which corresponds to maximizing the number of HCFs within the cable. Consequently, replacing one of the HCFs in an optical cable with a sensing fiber reduces the available bandwidth of the cable. In other words, in the context of an HCF cable (which has the purpose of high-bandwidth data transmission), one having ordinary skill would be hesitant to remove an HCF and replace it with a sensing fiber (which proceeds contrary to the purpose of maximizing bandwidth).

[0021] Third, enhanced sensing fibers (such as those described with reference to FIGS. 1 through 6, below) are an emerging technology and with limited availability currently for longer transmission cables (e.g., >100km cables). As such, enhanced sensing fibers (or enhanced backscattering fibers) are expensive and, thus, used for short lengths (e.g., down- hole sensing applications). Conversely, HCFs are used within > 100km cables. In other words, combining fibers for long-transmission-length applications (e.g., HCFs) with fibers for short-transmission-length applications (e.g., enhanced sensing fibers) within the same optical cable requires balancing of many factors.

[0022] Ultimately, sensing fibers must be perturbed for their operation, while HCFs must be protected from perturbation to enhance their operation. Consequently, those having skill in the art would not normally consider combining both types of fibers within a single optical cable. With this in mind, attention is turned to FIGS. 1 through 6 for a more detailed explanation of the various embodiments.

[0023] FIG. 1 is a drawing that shows an axial cross-section of an embodiment of a fault-detecting hollow-core fiber (HCF) cable 100. To provide a frame of reference, the cable 100 carries optical signals along a transmission axis (as would be understood by those having ordinary skill in the art). Specifically, FIG. 1 shows a four-core cable with eachoptical fiber being encased in a separate tube. For clarity, only non-redundant components are designated with reference numerals. As shown in FIG, 1, the cable 100 comprises a central strength member 105, which runs along the direction of the transmission axis. Surrounding the central member 105 are loose tubes 115a, 115b (collectively 115), also running along the transmission axis. A sensing fiber 110 is located within its own loose tube 115a. Similarly, a HCF 120 located within its own loose tube 115b. The tubes 115 are surrounded by binder tape 130, which improves water-blocking performance as well as adding strength to the cable 100. An outer jacket 135 surrounds the binder tape 130.

[0024] The sensing fiber 110 in proximity to the HCF 120 permits monitoring of acoustic events (e.g., noise, vibration, etc.) surrounding the cable 100 through known processes, such as, for example, distributed acoustic sensing (DAS). Various well-known modalities can be interrogated, including elastic scattering, Brillouin scattering, Raman scattering, etc. By way of example, an interrogator (known in the art) launches an optical pulse train into the sensing fiber 110 and measures dynamic strain along the sensing fiber 110 using Rayleigh backscatter. Insofar as these acoustic events precede potential cable damage, the strain measured by the sensing fiber .110 provides a reasonable indication of corresponding strains that the HCF 120 experiences. Insofar as sensing fibers, such as enhanced backscattering fibers (e.g., as taught in U.S. Patent Number US9766396B2), are known in the art, only a truncated discussion of sensing fibers (e.g., enhanced backscattering fibers) is provided herein.

[0025] Although the embodiment of FIG. 1 shows separate tubes 115 for the HCF 120 and the sensing fiber 110, other embodiments, such as that shown in FIG. 2, contemplate an assembly 200 in which the sensing fiber 210 and the HCF 220 are located together within the same tube 215. By providing an assembly 200 in which multiple sensing fibers 210 and multiple HCFs 220 exist in a single tube 215, one can increase the number of HCFs 220 in an optical fiber cable by simply adding another assembly 200.

[0026] FIG. 3 shows an axial cross-section of a fault-detecting HCF cable 300 that comprises two (2) of the assemblies 200 from FIG. 2. The two (2) assemblies 200 are surrounded by water-blocking material 310, such as aramid yam (which is well known in the art and requires no further explanation). For the embodiment of FIG. 3, a jacket 335 surrounds he assemblies 200 and the water-blocking material 310.

[0027] FIG. 4 is a drawing that shows an axial cross-section of another embodiment of a fault-detecting HCF cable 400. In the embodiment of FIG. 4, the cable 400 comprises an assembly 200 (similar to that shown in FIG. 2) surrounded by a water-blocking material 410 (e.g., aramid yam), which in turn is surrounded by an inner jacket 430. An outer jacket 435 surrounds the inner jacket 430.

[0028] To demonstrate the modular capabilities, FIG. 5 shows an axial cross-section of another embodiment of a fault-detecting HCF cable 500, while FIG. 6 shows a perspective view of the components in a fault-detecting HCF cable 600. As shown in FIGS. 5 and 6, the cable 500, 600 comprises a central strength member 505, 605 (which is a component that is known in the art and requires no further explanation). For some embodiments, the central strength member 505 has an optional central member jacket 510.

[0029] Surrounding the central strength member 505, 605 are multiple assemblies 200. For some embodiment, each assembly comprises a tube 215 that houses multiple sensing fibers 210 and multiple HCFs 220. For other embodiments, each sensing fiber 610 is located within its own tube 615a, while each HCF 620 is located within its own tube 615b. Here, FIG. 5 shows six (6) assemblies 200 surrounding the central strength member 505 in a hexagonal arrangement. Water blocking material 515 occupies the interstitial spaces between the assemblies 200, with a dielectric strength member 530, 630 surrounding both the water-blocking material 515 and the assemblies 200. A jacket 535, 635 surrounds the dielectric strength element 530, 630. Some embodiments include a ripcord 525, 625, which is designed to rip through the jacket 535, 635.

[0030] It is preferable to have no strain on the HCFs 120, 220, 620. Thus, for some embodiments, a helical tube lay is used to eliminate or reduce strain on the HCFs 120, 220, 620, with excess fiber lengths held in tight tolerance during manufacturing of the cable 100, 300, 400, 500, 600. Acoustic coupling to the sensing fibers 110, 210, 610 (e.g., DAS fiber) is increased by inducing strain on the sensing fibers 110, 210, 610, which can be accomplished with either longer or shorter lay lengths for the sensing fibers 110, 210, 610.

[0031] As shown from FIGS. 1 through 6 and explained above, the cost of including a sensing fiber 110, 210, 610 is much lower than the cost to replace large sections of a HCF cable 100, 300, 400, 500, 600. Comparing the replacement of solid-core fiber (SCF) segments and HCF segments reinforces the significance of introducing a sensing fiber 110,210, 610 to a HCF cable 100, 300, 400, 500, 600. When breaks occur in SCFs, approximately fifteen meters (~15m) of the broken fiber on one side of the break and ~15m of the broken fiber on the other side of the break (for a total of ~30m) are replaced with new sections of SCFs being spliced to replace the damaged regions. In other words, these are relatively small-scale replacements for SCF breaks. By comparison, when breaks occur in HCFs, the hollow regions of the fiber permit contamination to migrate farther into the HCF (much farther than for SCFs, which do not have similar hollow regions). Consequently, local small-scale repair of HCF cables becomes risky or impractical, thereby prompting replacement of longer cable sections (e.g., between two nearest manholes) that can span distances as large as ~300m (which is larger than SCF replacement by an order of magnitude). As such, replacement of HCF sections becomes a much larger and more expensive repair than SCF sections (especially given the cost of replacement hollow core cabling). This unusually large scope of repair enhances the value of sensing and intervention. Consequently, because the sensing fiber 110, 210, 610 allows for earlier detection of potential breaks or cuts in the cable 100, 300, 400, 500, 600, the teachings herein provide a more cost-efficient approach and better protection by either detecting faults before they occur, or detecting faults early enough to minimize (or reduce) the lengths of fibers that need to be replaced to mitigate for the fault or damage.

[0032] In an example embodiment, the optical fiber cable 100, 300, 400, 500, 600 has an operating temperature that is between approximately negative forty degrees Celsius (-40°C) to approximately +85°C. Preferably, the optical fiber cable 100, 300, 400, 500, 600 has a minimum bend radius that is greater than approximately 130 millimeters (~130mm).

[0033] In some embodiments, the sensing fiber 110, 210, 610 is a vibration sensing optical fiber with optimal performance for distributed acoustic sensing (DAS) systems, such as the optical fiber sold under the trademark AcoustiSens® for a wideband single-mode optical fiber by OFS Fitel, LLC. Thus, one embodiment of the sensing fiber 110, 210, 610 exhibits an enhanced backscatter that is greater than ten decibels (>10dB) over native Rayleigh backscattering. Insofar as those having ordinary skill in the art understand what is meant by native Rayleigh backscatering, no further explanation of Rayleigh backscattering is provided herein. Furthermore, for some embodiments, the sensing fiber 110, 210, 610 has an operating center wavelength (λ) between approximately 1530 nanometers (~1530nm) and~1560nm, with a maximum attenuation of less than approximately 0.7dB per kilometer (~0.7dB / km). Preferably, the sensing fiber 110, 210, 610 comprises an inner cladding with an outer diameter of approximately 125 micrometers (~ 125 μm), which is surrounded by a coating with an outer diameter that is greater than 200pm.

[0034] Although a specific example of the sensing fiber 110, 210, 610 is provided herein, other optical fibers that are generally compliant with ITU-T G.657 (which is an industry standard published by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), well known to those having ordinary skill in the art), ITU-T G.652, or other known standards can be used for the sensing fiber 110, 210, 610.

[0035] For some embodiments, the HCF 120, 220, 620 is a low-latency photonic bandgap hollow-core optical fiber, such as the optical fiber sold under the trademark AccuCore HCF® for optical transmission in the C-Band (conventional band) by OFS Fitel, LLC. Thus, one embodiment of the HCF 120, 220, 620 has an operating λ. between ~1540nm and ~1560nm. Such a fiber, at λ, exhibits a maximum attenuation of ~6dB / km, a minimum bend radius of ~10mm, a multi-path interference of less than -20dB in 1km, and a chromatic dispersion of less than 200 picoseconds-per-nanometer-per-kilometer (<200ps / nm / km).

[0036] For other embodiments, the HCF 120, 220, 620 is an anti-resonant hollowcore optical fiber (ARHCF) with wide operating X range. Such an ARHCF preferably exhibits an attenuation of less than 0.7dB / km over a greater-than 100nm spectrum (and more preferably <0.2dB / km over the same > 100nm spectrum).

[0037] Although exemplary embodiments have been shown and described, it will be clear to those of ordinary skill in the art that a number of changes, modifications, or alterations to the disclosure as described may be made. All such changes, modifications, and alterations should therefore be seen as within the scope of the disclosure.

Claims

What is claimed is:

1. An optical fiber cable, comprising: an operating temperature from approximately negative forty degrees Celsius (-40°C) to approximately +85°C; a minimum bend radius that is greater than approximately 130 millimeters (~130mm); a sensing fiber comprising: an enhanced backscatter that is greater than ten decibels (>10dB) over native Rayleigh backscattering; an operating center wavelength (λ) between approximately 1530 nanometers (~1530nm) and ~1560nm; a maximum attenuation of less than approximately 0.7dB per kilometer (~0.7dB / km): an inner cladding with an outer diameter of approximately 125 micrometers(~ 125 μm); and a coating surrounding the inner cladding, the coating having an outer diameter that is greater than 200μm; and a hollow-core fiber (HCF) comprising: a maximum attenuation of ~0.7dB / km; and an operating A range that is greater than one hundred nanometers (>100nm).

2. An optical fiber cable, comprising: a tube; a sensing fiber located within the tube; and a hollow-core fiber (HCF) located within the tube.

3. The optical fiber cable of claim 2, wherein the sensing fiber is an enhanced backscattering fiber.

4. The optical fiber cable of claim 2, comprising a cable length that is greater than onehundred kilometers (> 100km).

5. The optical fiber cable of claim 2, wherein the HCF is an anti-resonant hollow-core fiber (ARHCF).

6. The optical fiber cable of claim 5, wherein the ARHCF comprises: an operating wavelength (λ) range that is greater than one hundred nanometers (>100nm); and a maximum attenuation of approximately 0.7 decibels per kilometer (~0.7dB / km) within the λ range.

7. The optical fiber cable of claim 2, further comprising an operating temperature from approximately negative forty degrees Celsius (~40°C) to approximately +85 °C.

8. The optical fiber cable of claim 2, further comprising a minimum bend radius that is greater than approximately 130 millimeters (~130mm).

9. The optical fiber cable of claim 2, wherein the sensing fiber comprises an enhanced backscatter that is greater than ten decibels (>10dB) over native Rayleigh backscattering.

10. The optical fiber cable of claim 2, wherein the sensing fiber comprises: an inner cladding with an outer diameter of approximately 125 micrometers (- 125 μm); and a coating surrounding the inner cladding, the coating having an outer diameter that is greater than 200μm.

11. An optical fiber cable with a signal transmission axis, the cable comprising: a sensing fiber positioned along the signal transmission axis; and a hollow-core fiber (HCF) positioned along the signal transmission axis.

12. The cable of claim 11 , wherein the sensing fiber is an enhanced backscattering fiber.

13. The cable of claim 11 , comprising a cable length that is greater than one hundred kilometers (> 100km).

14. The cable of claim 11 , wherein the HCF is an anti-resonant hollow-core fiber (ARHCF).

15. The cable of claim 14, wherein the ARHCF comprises: an operating wavelength (λ) range that is greater than one hundred nanometers (>100nm); and a maximum attenuation of approximately 0.7 decibels per kilometer (~0.7dB / km) within the λ range.

16. The cable of claim 11, further comprising a minimum bend radius that is greater than approximately 130 millimeters (~ 130mm).

18. The cable of claim 11 , wherein the sensing fiber comprises an enhanced backscatter that is greater than ten decibels (> 10dB) over native Rayleigh backscattering.

19. The cable of claim 11, wherein the sensing fiber comprises a tube positioned along the signal transmission axis, the tube comprising: the sensing fiber; and the HCF.

20. The cable of claim 11, further comprising: a first tube positioned along the signal transmission axis, the sensing fiber being located within the first tube; and a second tube positioned along the signal transmission axis, the HCF being located within the second tube.

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