Strain-modified fiber optic cable

Abstract

The strain-compensating optical cable includes a strength member extending substantially along the length of the optical cable. The optical cable has a first buffer tube and a second buffer tube extending along the length of the optical cable. A strain-measuring single-mode fiber (SMF) is disposed within the first buffer tube. A hollow-core fiber (HCF) is disposed within the second buffer tube. The SMF is used as a means for measuring strain, thereby allowing relaxation of strain experienced by the HCF. A stranding material extends substantially along the length of the optical cable and strands the first buffer tube and the second buffer tube. An outer jacket surrounds the stranding material and extends substantially along the length of the optical cable.

Description

[Background technology]

[0001] FIELD OF THE DISCLOSURE The present disclosure relates generally to optical fibers and, more particularly, to fiber optic cables.

[0002] Fiber optic cables (also called optical cables) are typically designed to protect the optical fibers present within the cable. In some cases, the performance of the optical fibers is affected by the strain they are subjected to. Summary of the Invention

[0003] The present disclosure provides systems and processes related to strain-compensated fiber optic cables (or optical cables).

[0004] Briefly, in an architecture, one embodiment of an optical cable includes a strength member extending substantially along the length of the optical cable. The optical cable has a first buffer tube and a second buffer tube extending along the length of the optical cable. A strain-measuring single-mode fiber (SMF) is disposed within the first buffer tube. A hollow-core fiber (HCF) is disposed within the second buffer tube. Stranding material extends substantially along the length of the optical cable and strands the first and second buffer tubes. An outer jacket surrounds the stranding material and extends substantially along the length of the optical cable. The strain-measuring SMF enables strain measurement during manufacturing of the optical cable. Strain measurement can control strain at various points in the cable manufacturing process, thereby reducing strain in the HCF.

[0005] Other systems, devices, methods, features, and advantages will become apparent to one with skill in the art upon examination of the following figures 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 this disclosure, and be protected by the accompanying claims.

[0006] Many aspects of the present disclosure can be better understood by 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 indicate corresponding parts throughout the several views. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a cross-sectional view of an embodiment of an optical cable. [Figure 2] 1 is a flow chart illustrating one embodiment of a process for manufacturing a strain sensitive optical cable. [Figure 3] An example of an optical spectrum analyzer (OSA) trace of a conventional hollow-core fiber (HCF) manufactured without distortion compensation at various stages of the manufacturing process. [Figure 4] 1 is an example optical spectrum analyzer (OSA) trace for an embodiment of an HCF fabricated using an embodiment of the distortion compensation taught herein. [Figure 5] 4 is a graph showing strain (microstrain (με)) versus distance (meters (m)) for the conventional HCF shown in FIG. 3. [Figure 6A] 1 is a graph illustrating strain (microstrain (με)) versus distance (meters (m)) for one embodiment of an HCF fabricated using strain compensation as taught herein. [Figure 6B] 1 is a graph showing strain (microstrain (με)) versus distance (meters (m)) for another embodiment (i.e., from a different lot) of an HCF manufactured using strain compensation as taught herein. [Figure 6C] 1 is a graph showing strain (microstrain (με)) versus distance (meters (m)) for yet another embodiment (i.e., from yet a different lot) of an HCF manufactured using strain compensation as taught herein. DETAILED DESCRIPTION OF THE INVENTION

[0008] Fiber optic cables (also called optical cables) are typically designed to protect the optical fiber residing within the cable. The performance of some optical fibers is affected by the strain they are subjected to. For example, strain in hollow-core fibers (HCFs) increases attenuation as well as unwanted coupling between modes within the core of the HCF. In some cases, unwanted strain is induced in the optical fiber during the cabling process, and if the unwanted strain becomes high enough, the product will no longer meet the quality requirements for fiber optic cables.

[0009] Both strain and fiber properties contribute to the attenuation effect, so it is difficult to separate the two effects when considering only attenuation values measured after the optical cable is fully manufactured.

[0010] Finally, strain-induced attenuation can be both direct and indirect: for example, optical fibers can exhibit wavelength-dependent attenuation due to indirect strain, whereby wavelengths are spectrally shifted by strain.

[0011] To separate attenuation from cable processing distortions and from fiber properties, the present disclosure teaches optical cables and manufacturing processes that measure and appropriately compensate for distortions during the cable manufacturing process. Specifically, in some embodiments, distortions are measured at different points during the cable manufacturing process, thereby enabling distortion compensation during processing steps subsequent to the distortion measurement.

[0012] In general, one embodiment of an optical cable includes a strength member extending substantially along the length of the optical cable. The optical cable includes a first buffer tube and a second buffer tube extending along the length of the optical cable. The first buffer tube includes a strain-measuring single-mode fiber (SMF), and the second buffer tube includes a hollow-core fiber (HCF). The strain-measuring SMF allows for strain measurements at various points in the optical cable manufacturing process, thereby enabling adjustment of manufacturing parameters in subsequent steps, thereby allowing compensation for process-induced strain.

[0013] Having provided a broad range of technical solutions to technical problems, reference will now be made in detail to the description of the embodiments illustrated in the drawings. While several embodiments will be described in connection with these drawings, there is no intent to limit the disclosure to the embodiments disclosed herein. On the contrary, the intent of this specification is to cover all alternatives, modifications, and equivalents.

[0014] Referring to the drawings, Figure 1 illustrates one embodiment of an optical cable 100 that compensates for strain during cabling. As shown in Figure 1, the optical cable 100 includes strength members 105 that extend substantially along the length of the optical cable 100.

[0015] Optical cable 100 further includes a first buffer tube 110 that extends substantially along the length of optical cable 100. First buffer tube 110 extends substantially along strength member 105. For future reference, first buffer tube includes a first length and a first tube inner diameter (ID).

[0016] Disposed within the first buffer tube 110 is a first strain-measuring single-mode fiber (SMF) 115 that extends substantially along the length of the optical cable 100. The first strain-measuring SMF 115 includes a first SMF fiber length that is longer than the first tube length, thereby providing excess fiber length (EFL) within the first buffer tube 110. The first strain-measuring SMF 115 also has a first SMF fiber outer diameter (OD) that is smaller than the first tube ID, which provides space for the unit to expand and contract during manufacturing without applying undue stress or strain to the first strain-measuring SMF 115. In the finished optical cable 100, the strain of the first strain-measuring SMF 115 is measured between +100 microstrain (με) and -100 με (strain is a dimensionless relative quantity, so με is 10 -6 and is specified without formal units).

[0017] Optical cable 100 further includes a second buffer tube 120 having a second strain-measuring SMF 125. Like first buffer tube 110, second buffer tube 120 extends substantially along the length of optical cable 100 and along strength member 105. Second buffer tube 120 has a second length and a second ID.

[0018] Within the second buffer tube 120 is a second strain-measuring SMF 125 that extends substantially along the length of the optical cable 105. Again, to provide sufficient space for expansion and contraction during manufacturing, the second strain-measuring SMF 125 includes a length of second SMF fiber that is longer than the length of the second tube, thereby providing an EFL within the second buffer tube 120. The second strain-measuring SMF 125 also has a second SMF fiber OD that is smaller than the second tube ID, allowing the unit to expand and contract during manufacturing without applying excessive stress or strain to the second strain-measuring SMF 125. On the completed optical cable 100, the strain on the second strain-measuring SMF 125 is measured between +100 με and -100 με.

[0019] Optical cable 100 provides at least two means for measuring strain within optical cable 100 at various points during the manufacturing process, between first strain-measuring SMF 115 and second strain-measuring SMF 125 .

[0020] Additionally, optical cable 100 further includes a third buffer tube 130 that extends substantially along the length of optical cable 100 and substantially along strength member 105. Third buffer tube 130 includes a third tube length and a third tube ID.

[0021] A first hollow-core fiber (HCF) 135 is disposed within the third buffer tube 130. The first HCF 135 extends substantially along the length of the optical cable 100 and includes a first HCF fiber length that is longer than the third tube length, thereby providing an EFL within the third buffer tube 130. The first HCF 135 also includes a first HCF fiber OD that is smaller than the third tube ID. The EFL and the space between the fiber OD and the tube ID allow the unit to be stretched or contracted during manufacturing without inducing substantial stress or strain on the first HCF 135.

[0022] Optical cable 100 further includes a fourth buffer tube 140 extending along the length of optical cable 100. Like the other buffer tubes 110, 120, and 130, fourth buffer tube 140 extends substantially along strength member 105. Fourth buffer tube 140 has a fourth tube length and a fourth tube ID.

[0023] A second HCF 145, disposed within the fourth buffer tube 140, extends substantially along the length of the optical cable 100. The second HCF includes an HCF fiber length longer than the fourth tube length and an HCF fiber OD smaller than the fourth tube ID. Again, the EFL and the space between the fiber OD and the tube ID allow for expansion and contraction of the unit without inducing substantial stress or strain in the second HCF 145. Those skilled in the art will appreciate that jackets (not shown) can optionally be applied to the buffer tubes 110, 120, 130, and 140, each of which may include one or more aramid strands or other strength members to improve structural strength.

[0024] Due to the geometric characteristics of the buffer tubes 110, 120, 130, and 140 and the strength members 105, gaps 150 exist between the buffer tubes 110, 120, 130, and 140. The optical cable 100 then includes stranding material 155 that surrounds the first buffer tubes 110, 120, 130, and 140 and the gaps 150. As known to those skilled in the art, the stranding material 155 adds strength to the optical cable 100 and maintains the organization of the buffer tubes 110, 120, 130, and 140 relative to the strength members 105.

[0025] Finally, optical cable 100 includes an outer jacket 160 that surrounds stranding material 155 and extends substantially along the length of optical cable 100. As will be appreciated by those skilled in the art, some embodiments include aramid strands or other strength members disposed between stranding material 155 and outer jacket 160, thereby improving the strength characteristics of optical cable 100.

[0026] Finally, the optical cable 100 includes strain measurement SMFs 115, 125 for troubleshooting and maintenance during the cable manufacturing process, the installation process, or both. If the strains experienced by the strain measurement SMFs 115, 125 during cable manufacturing are indicative of the strains experienced by the HCFs 135, 145 during cable manufacturing, much of the manufacturing process-induced strain on the HCFs 135, 145 can be estimated from the measured strains on the strain measurement SMFs 115, 125. This allows for adjustments at various points in the cable manufacturing process, resulting in an optical cable having fibers 115, 125, 135, 145 exhibiting between +100 με and -100 με when the optical cable manufacturing process is complete.

[0027] In other words, by measuring the process-induced strain on the strain measurement SMFs 115, 125 and adjusting the manufacturing process in response to the measured strain, the final product (i.e., the manufactured and shippable optical cable 100) preferably has an HCF 135, 145 that exhibits a strain within ±100 με.

[0028] Although two strain measurement SMFs 115, 125 and two HCFs 135, 145 are shown in Figure 1, it should be understood that any number of strain measurement SMFs (including a single strain measurement SMF) and any number of HCFs can be used without adversely affecting the performance of optical cable 100. As long as the assumption is valid (all fibers experience substantially similar manufacturing process-related strains), a single strain measurement SMF should adequately reflect the strain imparted to all fibers during the manufacturing process. The two SMF embodiment (as shown in Figure 1) provides redundancy.

[0029] Turning now to one embodiment of an optical cable manufacturing process, FIG. 2 shows a flow chart of one embodiment of a manufacturing process involving strain measurements at various points during the manufacturing process.

[0030] As shown in Figure 2, one embodiment of the process begins with extruding 205 a first buffer tube around a single mode fiber (SMF). In some embodiments, the extrusion step 205 is conventional and can be performed by many different prior art methods.

[0031] Upon extruding 205 the first buffer around the SMF, the process measures 210 a first strain (ε1) experienced by the SMF. In some embodiments, ε1 is measured 210 using known techniques such as Brillouin Optical Time Domain Reflectometry (BOTDR) or Brillouin Optical Time Domain Analysis (BOTDA). It should be understood that other known techniques can be used to measure 210 ε1. Preferably, ε1 is between +100 με and −100 με (also referred to as between ±100 με).

[0032] Based on ε1, the process determines the inner diameter (ID) of the second buffer tube (215). In other words, the ID of the second buffer tube is a function of the measured ε1. Because ε1 reflects the strain experienced by the SMF as a result of the extrusion step (205), the ID determination (215) is responsive to and follows the extrusion step (205). Specifically, if ε1 on the SMF is determined to be higher than expected or acceptable, the ID is increased (e.g., 0.5 mm to 0.6 mm) to provide more space between the fiber and the inner diameter of the buffer tube to reduce the effects of strain due to expansion or contraction during the manufacturing process.

[0033] Instead of determining the appropriate ID (215), the process can determine the appropriate amount of excess fiber length (EFL). For example, if the process determines that there is too much tensile strain, a longer EFL is provided to compensate for the additional tensile strain. Conversely, if the process determines that there is too much compressive strain, a shorter amount of EFL may be sufficient.

[0034] Next, the process unwinds (220) the HCF and extrudes (225) a second buffer tube around the unwinded HCF at the determined (215) ID (or appropriate amount of EFL). As described above, because the appropriate ID (and / or EFL) has been determined (215) from the measured 210 ε1, there is at least partial compensation for the distortion when the HCF is unwinded (220) and the second buffer tube is extruded (225) around the HCF.

[0035] Continuing with FIG. 2, the first and second buffer tubes are positioned for stranding (230), after which a second strain (ε2) is measured (235) using the SMF. From the measured ε2, the process determines a strand tension (240). Preferably, the determined (240) strand tension strikes a balance between a tension high enough to maintain the structural integrity of the optical cable and a tension low enough to avoid both manufacturing process-induced strain on the HCF and inadvertent reduction of the EFL. Once the appropriate strand tension is determined (240), a conventional stranding process is applied (245) using that strand tension.

[0036] During stranding, a third strain (ε3) is measured (250), and an allowable deformation is determined (255) in response to the measured (250) ε3. The measured (250) ε3 reflects the cumulative strain on the cable subunit throughout the applied (245) stranding process, thereby providing an indication of the amount of strain that is allowable for the remainder of the cabling process. Typically, ε3 is within ±100 με, and therefore the remaining steps of the cabling process should preferably maintain rather than adversely affect the strain value.

[0037] With this in mind, the jacket material is selected based on how much strain is tolerable 260. Those skilled in the art will appreciate that since ε3 should reflect the tolerable level of strain, an important consideration in the final jacketing process is to avoid adding strain.

[0038] Once the appropriate jacket material is selected (260), the process applies the jacket material (265), thereby completing the jacketing step. In some embodiments, a fourth strain (ε4) is measured (270), which reflects the final manufacturing process-induced strain experienced by the optical cable. Preferably, each of the process-induced strains ε1, ε2, ε3, and ε4 is in the range of ±100 με.

[0039] Also, similar to the method for measuring ε using BOTDR / BOTDA, other strain measurements (for ε, ε, ε) can also be measured using BOTDR / BOTDA. As one skilled in the art will appreciate, any known method for measuring strain using SMF can be used in combination with or in place of the disclosed strain measurement process.

[0040] In some embodiments, not all of the steps shown in FIG. 2 are required for every cable manufactured. Preferably, the process of FIG. 2 is performed multiple times on different cables until strain levels within ±100 με are consistently observed. Once a substantially consistent result of ±100 με is reached, the cable manufacturing parameters that achieved that substantially consistent result are used as the manufacturing parameters. Thus, for subsequently manufactured optical cables, the process does not require measuring strain values at every processing step; instead, strain can be measured intermittently at various points in the process as a spot check or quality control assessment.

[0041] With the cable 100 of Figure 1 and the process of Figure 2 in mind, and attention turned to Figures 3, 4, 5, 6A, 6B, and 6C, a comparison is shown of: (a) an HCF manufactured without strain compensation at various points in the cable manufacturing process (also referred to as a conventional cable), and (b) an HCF that takes into account process-induced strain at various points in the cable manufacturing process (also referred to as a strain-compensated cable).

[0042] As shown in Figures 3 and 5, sample optical spectrum analyzer (OSA) traces and strain loss profiles for conventionally manufactured optical cables indicate that the strain values appear to be varied and asymmetric (i.e., the strain is not symmetric around 0, but is offset from 0). Furthermore, from the measured strain over distance, conventionally manufactured cables exhibit relatively high strain values, i.e., between +500 με and +1500 με. That is, conventionally manufactured optical cables exhibit both higher strain values (above +500 με) and a larger strain range (1000 με (+500 to +1500)).

[0043] Next, Figure 4 shows an example of a sample OSA trace of an HCF fabricated using one embodiment of the strain compensation taught herein, and Figures 6A, 6B, and 6C (collectively shown in Figure 6) show the strain loss profiles of at least three physically different optical cables fabricated using one embodiment of the strain compensation process shown in Figure 2.

[0044] Comparing Figures 4 and 6 with Figures 3 and 5, the strain-compensated cable appears to exhibit more symmetrical strain (i.e., strain is substantially centered around zero) and a much smaller range of strain values. Specifically, as shown in Figure 6, strain values are within ±100 με. In some embodiments, strains within +50 με were also exhibited. In other words, the strain-compensated optical cable exhibited a five-fold improvement over conventional optical cables that were not strain-compensated during the manufacturing process.

[0045] As can be seen from Figures 1-6, by measuring strain at various points during the optical cable manufacturing process, the performance impact due to cable-induced strain can be more easily separated from that due to the fiber alone. Separating these effects can reduce unwanted strain induced in the optical fiber from the manufacturing process. For optical fibers whose optical performance is significantly affected by the strain the fiber experiences during its operational life (e.g., temperature-sensing fiber, polarization-maintaining fiber, hollow-core fiber, etc.), reducing residual strain from the cable manufacturing process can result in higher quality optical cables. Furthermore, other strain-related effects that degrade optical properties can be mitigated to some extent by compensating for manufacturing-process-related strain.

[0046] The process descriptions or blocks in the flowcharts should be understood as representing modules, segments, or portions of code that contain one or more executable instructions for implementing particular logical functions or steps in the process; alternative implementations are included within the scope of the preferred embodiments of the present disclosure, and functions may be performed in a different order than that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as will be understood by those skilled in the art of the present disclosure.

[0047] While exemplary embodiments have been shown and described, it will be apparent to those skilled in the art that many changes, modifications, or variations may be made to the described disclosure, and all such changes, modifications, and variations should therefore be considered to be within the scope of the present disclosure.

Claims

1. a strength member extending substantially along the length of the optical cable; a first buffer tube extending along the length of the optical cable and further extending substantially along the strength member; a first tube length; a first tube inner diameter (ID); the first buffer tube having a first strain-measuring single-mode fiber (SMF) extending substantially along the length of the optical cable and disposed within the first buffer tube; a first SMF fiber length longer than the first tube length; a first SMF fiber outer diameter (OD) smaller than the first tube ID; a first measurable strain between +100 με and −100 με; the first distortion measuring SMF having a second buffer tube extending along the length of the optical cable and further extending substantially along the strength member; a second tube length; a second tube ID; and the second buffer tube having a second strain-measuring SMF extending substantially along the length of the optical cable and disposed within the second buffer tube, a second SMF fiber length longer than the second tube length; a second SMF fiber OD smaller than the second tube ID; a second measurable strain between +100 με and −100 με; the second strain measurement SMF having a third buffer tube extending along the length of the optical cable and further extending substantially along the strength member; a third tube length; and a third tube ID; and the third buffer tube having a first hollow-core fiber (HCF) extending substantially along the length of the optical cable and disposed within the third buffer tube, a first HCF fiber length longer than the third tube length; a first HCF fiber OD smaller than the third tube ID; the first HCF having a fourth buffer tube extending along the length of the optical cable and further extending substantially along the strength member; a fourth tube length; and a fourth tube ID; and the fourth buffer tube having a second HCF extending substantially along the length of the optical cable and disposed within the fourth buffer tube, a length of HCF fiber greater than the length of the fourth tube; a fourth HCF fiber OD smaller than the tube ID; the second HCF having stranding material extending substantially along the length of the optical cable, the stranding material surrounding the first buffer tube, the second buffer tube, the third buffer tube, and the fourth buffer tube; an outer jacket extending substantially along the length of the optical cable, the outer jacket surrounding the stranding material; An optical cable comprising:

2. A method for manufacturing an optical cable, comprising: extruding a first buffer tube around a single mode fiber (SMF); measuring a first strain (ε1) on the SMF, the ε1 being dependent on the extruded first buffer tube; determining an inner diameter (ID) or an excess fiber length (EFL) of a second buffer tube according to the measured ε1; Paying out a hollow-core fiber (HCF); extruding the second buffer tube around the dispensed HCF at the determined ID or the determined EFL; positioning the first buffer tube and the second buffer tube in preparation for stranding; measuring a second strain (ε) on the SMF; determining strand tension in response to the measured ε2; applying a stranding process at said determined strand tension.

3. measuring a third strain (ε3) on the SMF; determining an allowable deformation amount in accordance with the measured ε3; selecting a jacket material having material properties corresponding to the determined allowable deformation amount; performing jacket processing using the selected jacket material; measuring a fourth strain (ε4) on the SMF; 3. The method of claim 2 further comprising:

4. measuring ε includes measuring ε using Brillouin Optical Time Domain Reflectometry (BOTDR) or Brillouin Optical Time Domain Analysis (BOTDA); The method of claim 3 , wherein the step of measuring ε4 includes measuring ε2 using the BOTDR or the BOTDA.

5. The step of measuring ε1 includes measuring ε1 using Brillouin Optical Time Domain Reflectometry (BOTDR) or Brillouin Optical Time Domain Analysis (BOTDA); The method of claim 2 , wherein measuring the ε2 comprises measuring the ε2 using a BOTDR or a BOTDA.

6. The method of claim 2 , wherein determining the EFL comprises determining an EFL that imparts a strain to the HCF within ±100 με during extrusion of the second buffer tube.

7. 3. The method of claim 2, wherein determining the ID of the second buffer tube comprises determining an ID that imparts a strain to within ±100 με to the HCF during extrusion of the second buffer tube.

8. 3. The method of claim 2, wherein determining the strand tension comprises determining the strand tension that imparts a strain to the HCF within ±100 με during application of the stranding process.

9. The method of claim 2 , wherein determining the allowable deformation comprises determining a deformation that maintains the strain of the HCF within ±100 με during the jacketing process.

10. a strength member extending substantially along the length of the optical cable; a first buffer tube extending along the length of the optical cable; a strain-measuring single-mode fiber (SMF) disposed within the first buffer tube; a second buffer tube extending along the length of the optical cable; a hollow-core fiber (HCF) disposed within the second buffer tube; stranding material extending substantially along the length of the optical cable; an outer jacket extending substantially along the length of the optical cable; An optical cable comprising:

11. The first buffer tube comprises: a first tube length; a first tube inner diameter (ID); The distortion measurement SMF is a first SMF fiber length longer than the first tube length; a first SMF fiber outer diameter (OD) smaller than the first tube ID; a first measurable strain between +100 microstrain (με) and −100 με; The optical cable according to claim 10.

12. The second buffer tube comprises: a second tube length; a second tube ID; The HCF is a length of HCF fiber greater than the second tube length; an HCF fiber OD smaller than the second tube ID; and an HCF strain between +100 με and −100 με; The optical cable according to claim 11.

13. the HCF is a first HCF, and the optical cable further comprises a third buffer tube; The third buffer tube comprises: a third buffer tube length; and a third buffer tube inner diameter (ID); and a second HCF disposed within the third buffer tube; The optical cable according to claim 10.

14. The second HCF is a second HCF fiber length longer than the third tube length; a second HCF fiber outer diameter (OD) smaller than the third buffer tube ID; a second HCF strain between +100 με and −100 με; 14. The optical cable according to claim 13.

15. the strain measurement SMF is a first strain measurement SMF, and the optical cable further includes a fourth buffer tube; The fourth buffer tube comprises: a fourth buffer tube length; and a fourth buffer tube ID; a second strain-measuring SMF disposed within the fourth buffer tube; 14. The optical cable according to claim 13.

16. The second distortion measurement SMF comprises: a second SMF fiber outer diameter (OD) smaller than the second tube ID; a second measurable strain between +100 με and −100 με; 16. The optical cable according to claim 15.

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