CLIAMS:What is claimed is:
1. An optical fiber cable comprising:
a central strength member lying along a longitudinal axis of said optical fiber cable, said central strength member being made of a fiber reinforced plastic;
a plurality of compact fiber units being laid around said central strength member, each of said compact fiber units including one or more optical fibers,
wherein each of said plurality of compact fiber units further comprises a first covering layer made of thermoplastic material and a second covering layer made of acrylic material,
said thermoplastic material having coefficient of thermal expansion ≤ 2.0×10-4, /°C
said first covering layer surrounding the second covering layer,
said second covering layer further surrounding one or more optical fibers, and neither the first covering layer nor the second covering layer is foamed; and
an outer jacket surrounding said plurality of compact fiber units and said central strength member,
wherein said outer jacket further comprises a first layer made of thermoplastic polyurethane, and a second layer made of aramid yarns.

2. An optical fiber cable as claimed in claim 1, wherein said thermoplastic material of the first covering layer is either Polypropylene or Nylon.
3. An optical fiber cable as claimed in claim 1, wherein said outer jacket further comprises a third layer made of thermoplastic polyurethane, said third layer being positioned such that the second layer lies between said first layer and said third layer.
4. An optical fiber cable as claimed in claim 1, wherein said second covering layer further surrounds a layer of thixotropic gel.
5. An optical fiber cable as claimed in claim 1, wherein each of the said plurality of compact fiber units is similar in structure and dimensions.
6. An optical fiber cable as claimed in claim 5, wherein periphery of cross-section of each of said plurality of compact fiber units is circular in shape.
7. An optical fiber cable as claimed in claim 6, wherein optical fiber packing density of each of said plurality of compact fiber units is greater than 90%.
8. An optical fiber cable as claimed in claim 1, wherein said one or more optical fibers are selected from a group of following optical fiber categories:
i. ITU-T G.652B optical fiber
ii. ITU-T G.652D optical fiber
iii. ITU-T G.657A1 optical fiber
iv. ITU-T G.657A2 optical fiber
v. ITU-T G.657B2 optical fiber
vi. ITU-T G.657B3 optical fiber
vii. ITU-T G.655C optical fiber
viii. ITU-T G.655D optical fiber
ix. ITU-T G.655E optical fiber.
9. An optical fiber cable as claimed in claim 1, wherein outer diameter of said one or more optical fibers is ≤ 262 microns.
10. An optical fiber cable as claimed in claim 1, wherein said one or more optical fibers is a single mode optical fiber.
11. An optical fiber cable as claimed in claim 1, wherein said one or more optical fibers is a multimode optical fiber.
12. An optical fiber cable as claimed in claim 1, wherein said one or more optical fibers is a coloured ink coated optical fiber.

Dated: 20th Day of November, 2014 Signature
Arun Kishore Narasani Patent Agent ,TagSPECI:FORM 2
The Patent Act 1970
(39 of 1970)
&
The Patent Rules, 2005

COMPLETE SPECIFICATION
(SEE SECTION 10 AND RULE 13)

TITLE OF THE INVENTION

“Optical Fiber Cable”

APPLICANTS:

Name : Sterlite Technologies Limited.
Nationality : Indian
Address : E-1, E-2, E-3 MIDC Waluj, Aurangabad,
Maharashtra - -431136
The following specification particularly describes and ascertains the nature of this invention and the manner in which it is to be performed:
FIELD OF INVENTION
[0001] The present invention relates to the field of optical fiber cables and, in particular, relates to robust optical fiber cable which would be suitable for being deployed in the field of underground natural resource explorations such as in the field of oil and gas explorations.
BACKGROUND
[0002] Optical fiber cables are backbone of modern communication infrastructure and systems. Our reliance on optical fibers and optical fiber cables for telecommunications, broadband communication, communication over passive optical networks, sensor applications, medicinal and surgical applications, and the like is growing day by day. In the field of underground natural resource explorations, say, oil and gas explorations, apart from a medium to transmit optical communication signals, optical fibers and cables have also been used as sensors for monitoring seismic and other underground field exploration parameters. In another application in same field, optical fiber cables are coupled with sensors to enable optical transmission of sensor signals. A conventional optical fiber cable for application in the field of oil and gas exploration would typically include multiple buffer tubes which are wound around a central strength member. The central strength member provides strength and integrity to the optical fiber cable. The buffer tubes include one or more optical fibers. Available free space within each buffer tube permits, to an extent, free lateral movement of optical fibers. If required, the buffer tubes may further include a gel-filling. The gel-filling inside the buffer tubes blocks moisture and may facilitate smooth movement of optical fibers within the available free space of respective buffer tubes. These buffer tubes along with the central strength member are surrounded by an outer jacket to protect them (and optical fibers included in them) from wear and tear, crushing forces, heat, shock, etc.
[0003] Currently known optical fiber cables have some major drawbacks which make them unsuitable for delivering efficient, reliable and durable performance under testing terrains and extreme temperature conditions in the field of underground natural resource explorations. For example, conventional optical fiber cables for outdoor applications use an outer jacket made of Polyethylene which offers resistance against mechanical and thermal shocks, but fails to offer reasonable flexibility to the cable. In difficult terrains, lack of sufficient flexibility may lead to permanent damage to the cable, or may lead to induced stresses in the optical fibers included within the cable. This results in macro bending losses and transmission losses over long distances. In addition, conventional optical fiber cables for outdoor applications lack sufficient resistance against kinking. As an example, in most optical fiber cables for outdoor applications, the value of kinking radius is provided by formula 15×D. ‘D’ being diameter of the cable. However, for applications in the field of natural resource explorations, it is desired that the value of kink radius of the cable be lowered.
[0004] Furthermore, some applications of an optical fiber cable deployed in the field of underground natural resource explorations requires optical fiber/s to be connected to sensors which are placed in sensor tray/s. For receiving accurate sensor signals, it is required that that free movement of optical fibers within buffer tubes of a deployed cable be restricted. Since conventional optical fiber cable fail to meet this requirement, they are prone to transmit inaccurate or attenuated readings. Furthermore, since buffer tubes of conventional optical fiber cable also lack sufficient flexibility, their reliable deployment within sensor tray/s without kinking remains a challenge.
[0005] Also, since most conventional optical fiber cables are not necessarily equipped with provisions for delivering reliable performance at extreme temperature conditions. Most conventional optical fiber cables may not perform reliably on a broad range of temperature. Additionally, most conventional optical fiber cables do not support higher packing density of optical fibers included in their buffer tubes. In fact, optical fibers of most conventional cables are prone to stress induced attenuation if they are packed with higher packing densities along with other optical fibers within their respective buffer tubes.
[0006] The above cited drawbacks of conventional optical fiber cables such as inappropriate flexibility, free movement of fiber movement inside the buffer tubes, unreliable performance over a wide range of temperatures, larger kinking radius, and lower support for increased optical fiber packing densities make these cables unsuitable for applications in the field of underground natural resource explorations such as oil and gas explorations.
[0007] Additionally, apart from the above cited drawbacks, most conventional cables are also not designed for re-deployment. Since suitability for re-deployment of same optical fiber cable at different sites is a desired feature for deployable optical fiber cables in the field of underground natural resource explorations, most conventional cables are not fit for being used at sites of underground natural resource explorations, such as oil and gas explorations.
Accordingly, there is an acute need for a robust, reliable and durable optical fiber cable that overcomes the above stated drawbacks and is also suitable for being re-deployed at different locations in the field of underground natural resource explorations, such as oil and gas explorations. The optical fiber cable should deliver reliable performance under at difficult operating conditions at the sites of underground natural resource explorations (such as difficult terrain, different temperature conditions, deployment at routes with bends, deployment under stresses, etc.) and should also support high optical fiber packing density
OBJECT OF THE INVENTION
[0008] A primary object of the present invention is to provide an optical fiber cable which would be suitable for being deployed in the field of underground natural resource explorations, such as oil and gas explorations.
[0009] Another object of the present invention is to provide an optical fiber cable that would facilitate restricted movement of fibers placed in their corresponding enclosure within the cable.
[0010] Yet another object of the present invention is to provide an optical fiber cable that would support higher optical fiber packing density and would still perform under difficult testing conditions of underground natural resource explorations.
[0011] Yet another object of the present invention is to provide the optical fiber cable which would have sufficient flexibility for being deployed in the field of oil and gas explorations.
[0012] Yet another object of the present invention is to provide the optical fiber cable which when deployed at site of underground natural resource explorations would have better immunity towards attenuation in transmitted optical signals.
[0013] Yet another object of the present invention to provide an optical fiber cable having improved crush resistance.
[0014] Yet another object of the present invention to provide an optical fiber cable having improved kink resistance.
[0015] Yet another object of the present invention to provide an optical fiber cable which would perform satisfactorily over a wide range of temperatures.
[0016] It is also an object of the present invention to provide an optical fiber cable which is suitable for re-deployment.
[0017] These and other objects of the invention will become readily apparent upon reference to the specification claims and drawings herein.
SUMMARY
[0018] Accordingly, the invention provides an optical fiber cable which overcomes the drawbacks of conventional optical fiber cables as described above and achieves objects of the invention. The present invention provides a robust optical fiber cable which would be suitable for applications in the field of underground natural resource explorations such as in the field oil and gas explorations. The optical fiber cable as provided by the present invention delivers reliable performance under at difficult operating conditions at the sites of underground natural resource explorations (such as difficult terrain, different temperature conditions, deployment at routes with bends, deployment under stresses, etc.) and also supports high optical fiber packing density. An optical fiber cable, in accordance with the present invention, includes a central strength member lying along a longitudinal axis of the optical fiber cable, a plurality of compact fiber units, said plurality of compact fiber units being laid around the central strength member, and an outer jacket surrounding said plurality of compact fiber units and said central strength member. The central strength member is made of a fiber reinforced plastic (also referred as ‘FRP’ hereinafter). FRP is a composite material made of a polymer matrix reinforced with fibers such as glass, aramid, etc. The outer jacket further comprises a first layer made of thermoplastic polyurethane, and a second layer made of aramid yarns. The first layer surrounds the second layer. First layer of Thermoplastic polyurethane serves an important purpose. Apart from providing protection to the cable, it also enhances flexibility of the cable. The outer jacket surrounds plurality of compact fiber units and said central strength member. Each of the plurality of compact fiber units includes one or more optical fibers, and further comprises a first covering layer and a second covering layer. The first covering layer is made of a thermoplastic material (such as polypropylene or nylon) having a coefficient of thermal expansion (CTE)≤ 2.0×10-4/°C. Such thermoplastic materials exhibit low shrinkage and are capable of providing low thickness layers. Hence, such thermoplastic materials contribute in achieving objects of the present invention. The second covering layer is made of an acrylic material. Neither the first covering layer nor the second covering layer is foamed. Since both first covering layer and the second covering layer are not foamed, the compact fiber unit achieves better robustness and kink resistance. The first covering layer and the second covering layer collectively form a corresponding compact fiber unit. The second covering layer surrounds the optical fibers/s included within the compact fiber unit.
In an embodiment of the optical fiber cable made in accordance with the present invention, the first layer forms the outermost covering of the cable. The first layer also surrounds the second layer. Further, in this embodiment, each of the plurality of compact fiber units is helically wound around the central strength member.
In another embodiment of the optical fiber cable made in accordance with the present invention, the outer jacket further comprises a third layer made of thermoplastic polyurethane. In this embodiment, first layer forms outermost covering of the cable and surrounds the second layer. The second layer surrounds the third layer and is positioned between the first layer and the third layer. The third layer surrounds the plurality of compact fiber units and the central strength member. Further, each of the plurality of compact fiber units is helically wound around the central strength member.
[0019] In yet another embodiment of the optical fiber cable made in accordance with the present invention, a layer of thixotropic gel is applied on the inner surface of the second covering layer of each of the plurality of compact fiber units. Presence of a layer of thixotropic gel on the inner surface of the second covering layer of a compact fiber unit prevents sticking of one or more optical fibers included within the compact fiber unit with the inner surface of the second covering layer of the compact fiber unit.
[0020] The optical fiber cable as provided by the present invention achieves the objects of the invention as mentioned herein above, delivers reliable performance under at difficult operating conditions at the sites of underground natural resource explorations and is suitable for re-deployment.

BRIEF DESCRIPTION OF FIGURES
[0021] FIG. 1 illustrates an optical fiber cable in accordance with a first embodiment of the present invention;
[0022] FIG. 2 illustrates a cross-sectional view of the optical fiber cable in accordance with the first embodiment of the present invention.
[0023] FIG. 3 illustrates an optical fiber cable in accordance with a second embodiment of the present invention;
[0024] FIG. 4 illustrates a cross-sectional view of the optical fiber cable in accordance with the second embodiment of the present invention.
[0025] FIG. 5 illustrates a cross-sectional view of an optical fiber cable in accordance with a third embodiment of the present invention.
[0026] It should be noted that the accompanying figures are intended to provide a better understanding of exemplary embodiments of the present invention. These figures do not limit the scope of the present invention. It should also be noted that accompanying figures are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE INVENTION
[0027] Reference will now be made in detail to selected embodiments of the present invention in conjunction with accompanying figures. The embodiments described herein are not intended to limit the scope of the invention, and the present invention should not be construed as limited to the embodiments described. This invention maybe embodied in different forms without departing from the scope and spirit of the invention. It should be understood that the accompanying figures are intended and provided to illustrate embodiments of the invention described below and are not necessarily drawn to scale. In the drawings, depiction of thicknesses, and size of some components may be exaggerated for providing better clarity and ease of understanding.
[0028] FIG. 1 illustrates perspective view of an optical fiber cable 100 in accordance with a first embodiment of the present invention. Across-sectional view of the optical fiber cable in accordance with the first embodiment of the invention is illustrated in FIG. 2. Cross-sectional view of the optical fiber cable 100 as shown in FIG. 2 is taken along a plane which is perpendicular to a longitudinal axis 102 (shown in FIG. 1) of cable 100. Optical fiber cable 100 is suitable for being used in the field of underground natural resource explorations such as oil and gas explorations. In a possible application, the optical fiber cable 100 can be operationally coupled with sensor arrays to sense and/or monitor various underground operational parameters pertaining to the oil and gas explorations. In this application, the optical fiber cable 100 may be referred to as seismic sensor array optical fiber cable. In another possible application, the optical fiber cable 100 may simply be used to transmit optical signals (which may carry sensor data or communication data) to a receiver from the site of oil and gas explorations.
[0029] As shown in FIG. 1, optical fiber cable 100 includes a longitudinal axis 102, a central strength member 104, five hollow compact fiber units 106 and an outer jacket 108. Central strength member 104 has a circular cross-section and it lies symmetrically around the longitudinal axis 102. Further, the central strength member 104 is made of fiber reinforced plastic (hereinafter referred as ‘FRP’). FRP is a composite material made of a polymer matrix reinforced with glass fibers. The central strength member 104 provides physical strength to the cable 100, resists over bending of the cable 100 due bending and/or crushing forces, and acts as an anti-buckling agent. Each of the plurality of compact fiber units 106 are wound helically around the central strength member 104. Further, in this embodiment, each of the compact fiber units 106 includes twelve optical fibers 110. However, for simplicity of presentation in FIG. 1 (and in FIG. 2), only four optical fibers 110 are explicitly shown within of each of the compact fiber units 106. Hence, total count of fibers within cable 100 as provided by the first embodiment of the invention is (12×5) =60. Additionally, in this embodiment, each of the optical fibers 110 are single mode ITU-T G.652D compliant polymer coated silica fibers and each of the optical fibers 110 have similar shape, structure, dimensions and optical characteristics.
[0030] The outer jacket 108 surrounds each of the compact fiber units 106 and the central strength member 104. The outer jacket 108 further comprises a first layer 112 made of thermoplastic polyurethane and a second layer 114 made of aramid yarns. Apart from providing protection to the cable, the first layer 112 of thermoplastic polyurethane also enhances flexibility of the cable. The first layer 112 surrounds the second layer 114 and forms outermost covering of the cable 100. The second layer 114 surrounds the plurality of compact fiber units 106 and the central strength member 104. The second layer 114 packs the helically wound compact fiber units 106 such that their helical winding pattern over the central strength member 104 is maintained. It is to be noted that in this embodiment, the first layer 112 and the second layer 114 collectively form the outer jacket 108. Each of the plurality of compact fiber units 106 further comprises two layers, namely, a first covering layer 116 and a second covering layer 118. The first covering layer 116 is made of polypropylene (a thermoplastic material having a coefficient of thermal expansion ≤ 2.0×10-4/°C). Polypropylene exhibits low shrinkage and is capable of providing low thickness layers. Hence, polypropylene contributes in achieving objects of the present invention. The first covering layer 116 surrounds the second covering layer 118. The second covering layer 118 is made of polymethyl methacrylate (an acrylic material). It is to be noted that both the first covering layer 116 and the second covering layer 118 collectively form the compact fiber unit 106, and neither the first covering layer 116 nor the second covering layer 118 is foamed. Since both first covering layer 116 and the second covering layer 118 are not foamed, the compact fiber unit 106 achieves better robustness and kink resistance. The second covering layer 118 of each of the compact fiber units 106 directly surrounds void 120 and optical fibers 110 included within the compact fiber unit 106. Unoccupied free space of the cable surrounded by outer jacket 108 (i.e. free space surrounded by the outer jacket 108) and which is not occupied by either compact fiber units 106 or central strength member 104 or optical fibers 110 or void 120, is represented by void 122.
[0031] Dimensional and compositional of details components of cable 100, as prepared in accordance with the first embodiment of the present invention, is provided in the TABLE 1 below.
TABLE 1
S. No. Component Material/Compositional details Dimensional details
1 First layer 112 Thermoplastic Polyurethane(TPU)
Thickness = 1.1 mm± 0.2mm
Outer diameter (same as outer diameter of the cable) = 7.4 mm± 0.3 mm
2 Second layer 114 Aramid yarns Thickness = 0.4 mm
3 First covering layer 116 Polypropylene
• Having Young's modulus = 1.50 GPa, and
• Having Coefficient of Thermal Expansion (CTE) = 0.00020/°C Thickness = 0.225mm ± 0.05mm
Outer diameter (same as outer diameter of the compact fiber unit106) = 1.8 mm ± 0.05 mm
4 Second covering layer 118 Acrylic (polymethyl methacrylate)
• Having Young's modulus = 0.96 GPa, and
• Having Coefficient of Thermal Expansion (CTE) = 0.00008/°C Thickness = 0.2 mm ± 0.05mm
5 Central core 104 Fiber reinforced plastic (FRP) Outer diameter = 1.2 mm ± 0.05
6 Each of the optical fibers 110 Silica optical fiber coated with polymer coatings Outer diameter = 250 µm

[0032] A kilometer length of optical fiber cable 100 prepared in accordance with the first embodiment of the invention and, having structure and dimensions as described above, weighted 50 (± 5) Kilograms. Performance of cable 100 prepared in accordance with the first embodiment of the invention (with structural details as provided in TABLE 1) was evaluated by exposing cable 100 to various performance tests. Details of performance tests and test results are provided in TABLE 2 below. Test results prove that the first embodiment of the optical fiber cable as described has improved resistance to kinking.
TABLE 2
S. No. Test Details Acceptance requirements Test Results
1 Standard Repeated Bending test : This test was performed in accordance with standard test procedure laid by IEC 60794-1-2-E6
The test was performed at 23°C with following specification details:
• Bending radius: 37.5 mm
• Number of turns: 10
• Number of cycles: 10000
• Mass of the weight: 5Kgs • Change in attenuation should be ≤ 0.05 db
• Cable should not undergo any physical damage
• No kink should occur on the cable • Change in attenuation = 0.015 db
• No physical damage caused to the cable
• No kinking occurred

Cable passed Standard Repeated Bending test in accordance with standard test procedure laid by IEC 60794-1-2-E6
2 Crush test: This test was performed in accordance with standard test procedure laid by IEC 60794-1-2-E3
The test was performed at 23°C with no cable rotation and following specification details:
• Total applied force: 2000N
• Duration of application of the force: 60 Secs
• Number of tests: 5
• Spacing between test places: 100mm • Change in attenuation should be ≤ 0.05 db
• Cable should not undergo any physical damage
• No kink should occur on the cable • Change in attenuation = 0.013 db
• No physical damage caused to the cable
• No kinking occurred
Cable passed Standard Repeated Bending test in accordance with standard test procedure laid by IEC 60794-1-2-E3
3 Kink resistance test: the cable was wound on a mandrel of 25 mm • No kink should occur on the cable • No kink occurred on the cable
Cable has substantial kink resistance

[0033] Reference will now be made to a second embodiment of the present invention. FIG. 3 illustrates an optical fiber cable 200 in accordance with a second embodiment of the present invention. A cross-sectional view of the optical fiber cable 200 in accordance with the second embodiment of the invention is illustrated in FIG. 4. Cross-sectional view of the optical fiber cable 200 as shown in FIG. 4 is taken along a plane which is perpendicular to a longitudinal axis 202 (shown in FIG. 1) of cable 200. The optical fiber cable 200 is suitable for being used in the field of underground natural resource explorations such as oil and gas explorations. In a possible application, the optical fiber cable 200 can be operationally coupled with sensor arrays to sense and/or monitor various underground operational parameters pertaining to the oil and gas explorations. In this application, the optical fiber cable 200 may be referred to as seismic sensor array optical fiber cable. In another possible application, the optical fiber cable 200 may simply be used to transmit optical signals (which may carry sensor data or communication data) to a receiver from the site of oil and gas explorations.
[0034] As shown in FIG. 3, optical fiber cable 200 includes a longitudinal axis 202, a central strength member 204, five hollow compact fiber units 206 and an outer jacket 208. The central strength member 204 has a circular cross-section and it lies symmetrically around the longitudinal axis 202. Further, the central strength member 204 is made of flexible fiber reinforced plastic (hereinafter referred as ‘FRP’). FRP is a composite material made of a polymer matrix reinforced with glass fibers. The central strength member 204 provides physical strength to the cable 200, resists over bending of the cable 200 due to bending and/or crushing forces, and acts as an anti-buckling agent. Each of the plurality of compact fiber units 206 are wound helically around the central strength member 204. Further, in this embodiment, each of the compact fiber units 206 includes twelve optical fibers 210. However, for simplicity of presentation in FIG. 3 (and in FIG. 4), only four optical fibers 110 are explicitly shown within of each of the compact fiber units 206. Hence, total count of fibers within cable 200 as provided by the first embodiment is (12×5) =60. Additionally, in this embodiment, each of the optical fibers 210 are single mode ITU-T G.652D compliant polymer coated silica fibers and each of the optical fibers 210 have similar shape, structure, dimensions and characteristics.
[0035] The outer jacket 208 surrounds each of the compact fiber units 206 and the central strength member 204. The outer jacket 208 further comprises a first layer 212 made of thermoplastic polyurethane, a second layer 214 made of aramid yarns, and a third layer 216 made of 85A polyether type thermoplastic polyurethane. Apart from providing protection to the cable, the first layer 212 of thermoplastic polyurethane also enhances flexibility of the cable. The first layer 212 surrounds the second layer 214 and forms outer most covering of the cable 200. The second layer 214 surrounds the third layer 216 and is positioned between the first layer 212 and the third layer 216. The third layer 216 surrounds the plurality of compact fiber units 206 and the central strength member 204. It is to be noted that in this embodiment, the first layer 212, the second layer 214 and the third layer 216 collectively form the outer jacket 208. The third layer 216 packs the helically wound compact fiber units 206 such that their helical winding pattern over the central strength member 204 is maintained. Each of the plurality of compact fiber units 206 further comprises two layers, namely, a first covering layer 218 and a second covering layer 220. The first covering layer 218 is made of polypropylene (a thermoplastic material, having a coefficient of thermal expansion ≤ 2.0×10-4/°C) and it surrounds the second covering layer 220. Polypropylene exhibits low shrinkage and can be made in to low thickness layers. Hence, usage of polypropylene contributes in achieving objects of the present invention. The second covering layer 220 is made of polymethyl methacrylate (an acrylic material). It is to be noted that both the first covering layer 218 and the second covering layer 220 collectively form the compact fiber unit 206. Neither the first covering layer 218 nor the second covering layer 220 is foamed. Since both first covering layer 218 and the second covering layer 220 are not foamed, the compact fiber unit 206 achieves better robustness and kink resistance. The second covering layer 220 of each of the compact fiber units 206 directly surrounds void 222 and optical fibers 210 included within the compact fiber unit 206. Unoccupied free space surrounded by outer jacket 208 (i.e. available space within outer jacket 208) and which is not occupied by compact fiber units 206 or central strength member 204 or optical fibers 210 or void 222, is represented by void 224.
[0036] Dimensional and compositional of details components of cable 200, as prepared in accordance with the second embodiment of the present invention, is provided in the TABLE 3 below.
TABLE 3
S. No. Component Material/Compositional details Dimensional details
1 First layer 212 Thermoplastic Polyurethane(TPU)
• CTE, if any=0.000135 /C Thickness = 0.7 mm ±0.2mm
Outer diameter (same as outer diameter of the cable) = 7.4 mm ± 0.3 mm
2 Second layer 214 Aramid yarns Thickness = 0.4 mm ± 0.1 mm
3 Third layer 216 85A polyether type Thermoplastic polyurethane(TPU) Thickness =0.4 ± 0.1 mm
4 First covering layer 218 Polypropylene
• Having Young's modulus = 1.50 GPa, and
• Having Coefficient of Thermal Expansion (CTE) = 0.00020 Thickness = 0.225mm ± 0.05mm
Outer diameter (same as outer diameter of the compact fiber unit206) = 1.8 mm ± 0.05 mm
5 Second covering layer 220 Acrylic (polymethyl methacrylate)
• Having Young's modulus = 0.96 GPa, and
• Having Coefficient of Thermal Expansion (CTE) = 0.00008 Thickness = 0.2 mm ± 0.05mm
6 Central core 204 Fiber reinforced plastic (FRP) Outer diameter = 1.2 mm ± 0.05
7 Each of the optical fibers 210 Glass fiber coated with polymer coatings Outer diameter = 250 µm

[0037] A kilometer length of optical fiber cable 200 prepared in accordance with the second embodiment of the invention and, having structure and dimensions as described above, weighted 50(± 5) Kilograms. Performance of cable 200 prepared in accordance with the second embodiment of the invention (with structural details as provided in TABLE 3 above) was evaluated by exposing cable 200 to various performance tests. Details of performance tests and test results are provided in TABLE4 below. Test results prove that the second embodiment of the optical fiber cable as described has improved resistance to kinking.
TABLE 4
S. No. Test Details Acceptance requirements Test Results
1 Standard Repeated Bending test : This test was performed in accordance with standard test procedure laid by IEC 60794-1-2-E6
The test was performed at 23°C with following specification details:
• Bending radius: 37.5 mm
• Number of turns: 10
• Number of cycles: 10000
• Mass of the weight: 5Kgs • Change in attenuation ≤ 0.05 db
• Cable should not undergo any physical damage
• No kink should occur on the cable • Change in attenuation = 0.015 db
• No physical damage caused to the cable
• No kinking occurred

Cable passed Standard Repeated Bending test in accordance with standard test procedure laid by IEC 60794-1-2-E6
2 Crush test: This test was performed in accordance with standard test procedure laid by IEC 60794-1-2-E3
The test was performed at 23°C with following specification details:
• Total applied force: 2000N
• Duration of application of the force: 60 Secs
• Number of tests: 5
• Spacing between test places: 100mm • Change in attenuation ≤ 0.05 db
• Cable should not undergo any physical damage
• No kink should occur on the cable • Change in attenuation = 0.013 db
• No physical damage caused to the cable
• No kinking occurred
Cable passed Standard Repeated Bending test in accordance with standard test procedure laid by IEC 60794-1-2-E3

3 Kink resistance test: the cable was wound on a mandrel of 25 mm No kink should occur on the cable • No kink occurred on the cable
Cable has substantial kink resistance

[0038] Reference will now be made to a third embodiment of the present invention, a cross-sectional view of which is provided in Fig.5. This embodiment is similar in structure to the first embodiment of the invention described above, except that it includes an additional feature which prevents sticking of optical fiber/s of the cable with the inner walls of their respective compact fiber units.
[0039] Cross-sectional view of the optical fiber cable 300 as shown in FIG. 5 is taken along a plane which is perpendicular to a longitudinal axis 302 (not shown in FIG. 5). Cable 300 includes a central strength member 304, five hollow compact fiber units 306 and an outer jacket 308. Central strength member 304 has a circular cross-section and it lies symmetrically around the longitudinal axis 302. Further, the central strength member 304 is made of flexible fiber reinforced plastic (hereinafter referred as ‘FRP’). FRP is a composite material made of a polymer matrix reinforced with glass fibers. The central strength member 304 provides physical strength to the cable 300 and resists over bending of the cable 300 due bending and/or crushing forces. Each of the plurality of compact fiber units 306 are wound helically around the central strength member 304 and each of the plurality of compact fiber units 306 further includes twelve optical fibers 310. In this embodiment, it should be noted that, each of the compact fiber units 306 includes twelve optical fibers 310. It should be noted that, in this embodiment, each of the compact fiber units 306 includes twelve optical fibers 310. However, for simplicity of presentation in FIG. 5, only four optical fibers 310 are explicitly shown within of each of the compact fiber units 306. Hence, total count of fibers within cable 300 as provided by the third embodiment is (12×5) =60. Additionally, in this embodiment, each of the optical fibers 310 are single mode ITU-T G.652D compliant polymer coated silica fibers and each of the optical fibers 310 have similar shape, structure, dimensions and characteristics.
[0040] An outer jacket 308 surrounds each of the compact fiber units 306 and the central strength member 304. The outer jacket 308 further comprises a first layer 312 made of thermoplastic polyurethane and a second layer 314 made of aramid yarns. Apart from providing protection to the cable, the first layer 312 of thermoplastic polyurethane also enhances flexibility of the cable. The first layer 312 surrounds the second layer 314 and forms outermost covering of the cable 300. The second layer 316 surrounds the plurality of compact fiber units 306 and the central strength member 304. The second layer 314 packs the helically wound compact fiber units 306 such that their helical winding pattern over the central strength member 304 is maintained. It is to be noted that the combination of the first layer 312 and the second layer 314 collectively form the outer jacket 308. The outer jacket 308 surrounds plurality of compact fiber units 306 which are helically wound around central strength member 304. Each of the plurality of compact fiber units 306 are made of two layers, namely, a first covering layer 316 and a second covering layer 318. The first covering layer 316 and the second covering layer 318 collectively form a corresponding compact fiber unit 306. The first covering layer 316 is made of polypropylene (a thermoplastic material, having a coefficient of thermal expansion ≤ 2.0×10-4/°C) and it surrounds the second covering layer 318. The second covering layer 318 is made of polymethyl methacrylate. It is to be noted that both, the first covering layer 316 and the second covering layer 318 collectively form the compact fiber unit 306. Neither the first covering layer 316 nor the second covering layer 318 is foamed. Since both first covering layer 316 and the second covering layer 318 are not foamed, the compact fiber unit 306 achieves better robustness and kink resistance. In this embodiment, instead of directly surrounding the optical fibers included within a compact fiber unit, the second covering layer 318 further surrounds a layer of thixotropic gel 320. The layer of thixotropic gel 320 is applied on the inner surface of the second covering layer 318. The layer of thixotropic gel 320 directly surrounds void 322 and optical fibers 310 included within the compact fiber unit 306. Application of layer of thixotropic gel 320 prevents sticking of optical fibers 310 with the inner surface of the second covering layer 318. It should be noted ‘inner surface’ of second covering layer 318, as mentioned above, represents the surface of second covering layer 318 which lies in contact with the layer of thixotropic gel 320. Since sticking of optical fibers 310 with the inner surface of second covering layer 318 is prevented due to application of the layer of thixotropic gel 320, stresses induced on the optical fiber 310 due to sticking with the inner surface of the second covering layer 318 are also prevented. Unoccupied free space of the cable surrounded by outer jacket 308 (i.e. available space within outer jacket 308, which is not occupied by compact fiber units 306 or central strength member 304 or optical fibers 310 or void 322), is represented by void 324.
[0041] Dimensional details and material composition details of components of cable 300 were similar to the first embodiment of the invention as described above. Since thixotropic gel is a non-solid component (or semi-solid component), the thickness of layer thixotropic gel 320 within each of the compact fiber units 306 was variable throughout.
[0042] Performance of cable 300 prepared in accordance with the third embodiment of the invention (with same structural and material details of cable 100 as provided in TABLE1 above) was evaluated by exposing cable 300 to various performance tests as mentioned in TABLE 2 above. On all the mentioned tests, performance of cable 300 was at par with the performance of cable 100 provided by the first embodiment of the invention. Additionally however, due to presence of layer thixotropic gel 320, cable 300 has an additional advantage. It is designed to be more immune towards losses induced due to sticking of optical fibers with internal walls of the respective compact fiber units 306.
It should be noted that the embodiments of the present invention as described above are not intended to limit the scope of the present invention. The scope of the invention is also not limited by shape and the physical dimensions of the components of embodiments of optical fiber cable described above. In other words, all possible variations in cross-sectional shape and dimensions of components of the embodiments of optical fiber cable described above are fully covered within the scope of the invention.
Other embodiments of the optical fiber cable, which are in accordance with the present invention and which are fully covered within its scope, may include variations in parameters such as count of compact fiber units included within the cable, and/or types of optical fibers included in one or more compact fiber units, and/or count of optical fibers included within each compact fiber unit, and/or packing density of optical fibers included within each compact fiber unit, and/or dimensional values of one or more components of the cable, and/or colour of any component (including optical fiber/s) included within the optical fiber cable.
Embodiments of the invention may include optical fibers selected from various ITU-T recommended categories of optical fibers (such as ITU-T G.652category, ITU-T G.657category, ITU-T G.655category, and ITU-T G.656category) with all respective sub-categories such as G.652B , G.652D, G.657A1, G.657A2, G.657B2, G.657B3, G.655C, G.655D, G.655E, etc., included. Embodiments of the invention may also include optical fibers selected from various categories of optical fibers such as multi-core optical fibers, multi-mode optical fibers, plastic/polymer optical fibers, nano structured optical fibers (including photonic crystal fibers and holey optical fibers), and specialty optical fibers. As an example, a possible embodiment of the optical fiber cable, which is in accordance to the present invention and which is fully covered within its scope, would be similar to first embodiment described above except that each of the compact fiber units would include optical fibers of different ITU-T recommended categories in accordance with the intended use or application of the cable (say, one compact fiber unit would include specialty optical fibers which would be good for being used as sensors, another compact fiber unit would include low attenuation optical fibers which may be used to transmit optical signals with reduced attenuation, and another compact fiber unit would include bend-insensitive optical fibers (ITU-T G.657 recommended category which would be useful for transmitting optical signals on cable path which includes sharp bends, and the like). Another possible embodiment of the optical fiber cable, which is in accordance to the present invention and fully covered within its scope, would be similar to first embodiment described above except that each of the compact fiber units would include optical fibers selected from either or both of ITU-T G.652 recommended category or ITU-T G.657 recommended category. In another possible embodiment of the present invention, the optical fiber cable may have coloured optical fibers included in one or more compact fiber units. In another possible embodiment of the present invention, the optical fiber cable may have multimode optical fibers included in one or more compact fiber units. In another possible embodiment of the present invention, the optical fiber cable may have multicore optical fibers included in one or more compact fiber units. In another possible embodiment of the present invention, the optical fiber cable may have specialty optical fibers included in one or more compact fiber units. In yet another possible embodiment of the invention, the optical fiber cable may have holey or photonic crystal optical fibers included in one or more compact fiber units. In yet another possible embodiment of the invention, the optical fiber cable may have optical fibers, which are suitable for sensor applications, included in one or more compact fiber units.
Scope of the invention also does not get limited by the count of compact fiber units within the cable. Scope of the invention also does not get limited by the count of optical fibers used within each compact fiber unit of the cable. An embodiment of optical fiber cable, which is in accordance and fully covered within the scope of the present invention, may have different counts of optical fibers among one or more compact fiber units.
In another embodiment of the optical fiber cable, in accordance to the present invention and which is a modification over the first embodiment of the invention described above, each of the compact fiber units would have optical fiber packing density of more than 90%. Such a compact packing of the optical fibers within the compact fiber units, reduces free movement of the optical fibers within the compact fiber units. Deployment of optical fiber cable provided by this embodiment for sensor applications has principal advantage. When an optical fiber cable provided by this embodiment is coupled to a sensor tray, restricted free movement of optical fibers within a compact fiber unit reduces induced stress on the optical fibers. Hence, losses induced because of stress caused due to free movement of optical fibers in sensor trays are reduced.
Optical fiber packing density, within cross-section of a compact fiber unit as described above, is defined as a ratio of A1/A2 , wherein
A1 represents diameter (in millimeters) of a circle of smallest area within which cross-sections of all optical fibers included in the compact fiber unit would simultaneously fit-in without an overlap. Mathematically, for a compact fiber unit which includes optical fibers of same diameter, A1 can be calculated as follows:
A1= (1.155) × (d) × √n
Where:
‘d’ is diameter (in millimeters) of an optical fiber included in the compact fiber unit
‘n’ represents total count of optical fibers contained within the compact fiber unit.
A2 represents inner diameter (in millimeters) of the second covering layer of the compact fiber unit. As an example, in the first embodiment of the invention as described above, A2 would represent inner diameter of second covering layer 118 (which is same as diameter of interface of second covering layer 118 and void 120). Similarly, in the third embodiment of the invention as described above, A2 would represent inner diameter of second covering layer 318 (which is same as the diameter of interface of second covering layer 318 and the outer diameter of layer of thixotropic gel 320).
Another embodiment of the optical fiber cable, which is in accordance with the present invention, is a variation of the second embodiment of the invention described above. In this embodiment, as a variation in the second embodiment of the invention, the second layer of the outer jacket of the cable may further include two sub-layers of aramid yarns namely, the first sub-layer and the second sub-layer. The first sub-layer comprises of aramid yarns wound around the third layer of the outer jacket in a clockwise direction, and the and the second sub-layer comprises of aramid yarns wound around the first sub-layer in anti-clockwise direction (i.e. in a direction which is reverse of the direction of winding of the first sub-layer). Provision of two layers of aramid yarns wound in clockwise and anticlockwise manner enhances robustness of the cable against twisting. Presence of feature of robustness against twisting makes the optical fiber cable of the present embodiment well suited for redeployment.
Still another embodiment of the optical fiber cable, which is in accordance with the present invention, is another variation of the first embodiment of the invention described above. In this embodiment, as a variation in the first embodiment on the invention, the central strength member of the cable is made of FRP and has a hollow tubular structure (i.e. instead of the central strength member being a solid rod of circular cross-section, the central strength member is in the form of a hollow FRP tube). It should be noted that the scope of the present invention is also not limited by the cross-sectional shape and dimensions of the central strength member.
Scope of the present invention is also not limited by the manner in which compact fiber units are wound over the central strength member. Embodiments of the invention in which compact fiber units are wound over the central strength member in reverse oscillation manner (i.e. in a manner in which the direction of helical winding of compact fiber units is reversed after fixed spans of cable length) are also fully covered within the scope of the invention.
It should be noted that in some embodiments of the present invention, binding threads (such as aramid yarn threads or any other suitable thread or binding component) may be wound over the assembly of compact fiber units to keep them in place around the central strength member. This is to maintain compaction and keep the compact fiber units in place over the central strength member (for example, if compact fiber units are helically wound over the central strength member, binding threads over the compact fiber units would keep winding pattern of compact fiber units over the central strength member intact).
It would be is obvious and apparent to those skilled in the art that many modifications and variations are possible in the exemplary embodiments of the invention described above. For example, in a modified version of the first embodiment of the present invention the first covering layer 116 of each of the plurality of compact fiber units 106 are colour coded for easy identification of respective compact fiber units. This embodiment with colour coded compact fiber units is fully covered under scope of the present invention.
[0043] The foregoing is description and illustration of the present invention is not to be construed as limiting thereof. Although exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many more modifications are possible in the exemplary embodiments without departing from the scope, spirit, teachings and advantages of the invention. In other words, while the invention has been presented with respect to certain specific embodiments, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the scope, spirit teachings and advantages of the invention. 
STATEMENT OF CLAIMS
What is claimed is:
1. An optical fiber cable comprising:
a central strength member lying along a longitudinal axis of said optical fiber cable, said central strength member being made of a fiber reinforced plastic;
a plurality of compact fiber units being laid around said central strength member, each of said compact fiber units including one or more optical fibers,
wherein each of said plurality of compact fiber units further comprises a first covering layer made of thermoplastic material and a second covering layer made of acrylic material,
said thermoplastic material having coefficient of thermal expansion ≤ 2.0×10-4, /°C
said first covering layer surrounding the second covering layer,
said second covering layer further surrounding one or more optical fibers, and neither the first covering layer nor the second covering layer is foamed; and
an outer jacket surrounding said plurality of compact fiber units and said central strength member,
wherein said outer jacket further comprises a first layer made of thermoplastic polyurethane, and a second layer made of aramid yarns.

2. An optical fiber cable as claimed in claim 1, wherein said thermoplastic material of the first covering layer is either Polypropylene or Nylon.
3. An optical fiber cable as claimed in claim 1, wherein said outer jacket further comprises a third layer made of thermoplastic polyurethane, said third layer being positioned such that the second layer lies between said first layer and said third layer.
4. An optical fiber cable as claimed in claim 1, wherein said second covering layer further surrounds a layer of thixotropic gel.
5. An optical fiber cable as claimed in claim 1, wherein each of the said plurality of compact fiber units is similar in structure and dimensions.
6. An optical fiber cable as claimed in claim 5, wherein periphery of cross-section of each of said plurality of compact fiber units is circular in shape.
7. An optical fiber cable as claimed in claim 6, wherein optical fiber packing density of each of said plurality of compact fiber units is greater than 90%.
8. An optical fiber cable as claimed in claim 1, wherein said one or more optical fibers are selected from a group of following optical fiber categories:
i. ITU-T G.652B optical fiber
ii. ITU-T G.652D optical fiber
iii. ITU-T G.657A1 optical fiber
iv. ITU-T G.657A2 optical fiber
v. ITU-T G.657B2 optical fiber
vi. ITU-T G.657B3 optical fiber
vii. ITU-T G.655C optical fiber
viii. ITU-T G.655D optical fiber
ix. ITU-T G.655E optical fiber.
9. An optical fiber cable as claimed in claim 1, wherein outer diameter of said one or more optical fibers is ≤ 262 microns.
10. An optical fiber cable as claimed in claim 1, wherein said one or more optical fibers is a single mode optical fiber.
11. An optical fiber cable as claimed in claim 1, wherein said one or more optical fibers is a multimode optical fiber.
12. An optical fiber cable as claimed in claim 1, wherein said one or more optical fibers is a coloured ink coated optical fiber.

Dated: 20th Day of November, 2014 Signature
Arun Kishore Narasani Patent Agent
 
ABSTRACT
Disclosed optical fiber cable includes a central strength member, a plurality of compact fiber units (CFUs) and an outer jacket. The central strength member is made of fiber reinforced plastic (FRP) and lies along a longitudinal axis of the cable. The CFUs lie around the central strength member. Each CFU includes one or more optical fibers and further comprises a first covering layer and a second covering layer. The first covering layer is made of thermoplastic material having coefficient of thermal expansion ≤ 2.0×10-4/°C and the second covering layer is made of acrylic material. The first covering layer surrounds the second covering layer, and the second covering layer surrounds the optical fiber/s. Neither the first covering layer nor the second covering layer is foamed. The outer jacket further comprises a first layer of thermoplastic polyurethane (TPU) and a second layer of aramid yarns. The first layer surrounds the second layer.