CLIAMS:no claims ,TagSPECI:FIELD OF THE INVENTION
[0001] The present invention relates to optical fiber cables. More particularly, the present invention relates to a direct-buried cable.

DESCRIPTION OF THE RELATED ART
[0002] Optical fiber cables are finding increasing importance in modern communication systems and are being vastly used in telecommunication, broadband communication, communication over passive optical networks (PONs), and the like. Telecommunication companies use optical fiber cables to transmit telephone and television signals, and also facilitate internet communication.

[0003] An optical fiber cable includes a sleeve that encloses one or more optical fibers in a sheath. The sheath of the sleeve protects the optical fibers from physical damage. The sleeves are of mainly two types: tight and loose sleeves, based on the proximity of the optical fibers to the sheath. Tight sleeves enclose the optical fibers tightly in the sheath and are mainly used for indoor applications. Loose sleeves enclose the optical fibers loosely in the sheath and are suitable for outdoor applications.

[0004] Optical fiber cables are relatively delicate compared to traditional cables used in communication systems. An optical fiber cable can be installed aerially or buried underground for outdoor applications. When the optical fiber cable is used for outdoor applications, the outdoor environment subjects the optical fiber cable to extreme environmental conditions, such as fluctuating temperatures, thermal shock, extreme winds, moisture, and lightning. The optical fiber cable needs to be protected so that optical properties of the optical fibers enclosed therein are preserved. The optical fiber cables that are installed aerially have to be non-metallic as the metal tape or rod may damage the cable during lightings. The optical fiber cables that are buried underground can be damaged by rodents or from crushing forces due to sharp impacts.

[0005] Snapping of optical fiber cables which are buried underground by rodents is a common problem which leads to frequent line maintenance and results in high maintenance costs. To provide protection from rodents, a steel armor layer in a conventional optical fiber cable provides required hardness to the optical fiber cable in order to protect from the rodent bites. However, the steel armor layer may get exposed to outside environment (such as humid air) and rust when the outer sheath is snapped by the rodents, thereby damaging the optical fiber cable. The cable cannot be used as direct-buried cable due to its little resistance to the crushing and impact forces.

[0006] In light of the foregoing, there exists a need for an optical fiber cable which overcomes the above cited drawbacks of conventionally known optical fiber cables and is particularly suitable for outdoor applications that requires high crush resistance, protection from rodent bites, and can be used as a direct-buried cable.

OBJECTS OF THE INVENTION
[0007] An object of the present invention is to provide an optical fiber cable that is resistant to rodent bites.

[0008] Another object of the present invention is to provide an optical fiber cable that acts as resistant to crushing forces.

[0009] Yet another object of the present invention is to provide an optical fiber cable that can be used as a direct-buried cable.

[0010] These and other objects of the present invention will be apparent from more detailed description which follows.

SUMMARY
[0011] An embodiment of the present invention provides an improved optical fiber cable. The optical fiber cable includes sleeves which are stranded in “S-Z” fashion. The sleeves are loose sleeves and enclose optical fibers. The optical fiber cable further includes water swellable yarns, which are disposed in between the sleeves to prevent the ingression of water inside the optical fiber cable. A layer of high-strengthening yarns such as aramid yarns is wound helically on the “S-Z” stranded sleeves. A water blocking tape is wrapped linearly on the high-strengthening yarns to prevent ingression of the water inside the optical fiber cable. An inner sheath encloses the sleeves, the water swellable yarns, the high-strengthening yarns and the water blocking tape. The inner sheath includes strength members which are embedded longitudinally therein and are positioned diametrically opposite to each other. Additionally, rip cords are positioned between the water blocking tape and the inner sheath for facilitating easy stripping or opening of the inner sheath.

[0012] A plurality of strengthening members is helically wound around the inner sheath to provide additional strength to the optical fiber cable. The entire arrangement of sleeves, the water swellable yarns, the high-strengthening yarns, the water blocking tape, the inner sheath and the strengthening members is housed in an outer sheath. The arrangement of inner sheath, the strengthening members and the outer sheath together allow the optical fiber cable to be used in applications where rodent protection and high crush resistance are required. The arrangement also allows the optical fiber cables to use them as direct-buried cables.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Embodiments of the present invention will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the scope, wherein like designations denote like elements, and in which:

[0014] FIG. 1 is a cross-sectional view of an optical fiber cable, in accordance with an embodiment of the present invention; and

[0015] FIG. 2 is a perspective view of the optical fiber cable of FIG. 1, in accordance with an embodiment of the present invention.

[0016] It should be noted that drawings are intended to provide a clear understanding of the present invention may not be necessarily drawn to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

[0017] The invention is described in detail below in connection with the Figures for purposes of illustration, only. Terminology used throughout the specification herein is given their ordinary meanings.

[0018] As used in the specification, the singular forms “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “an article” may include a plurality of articles unless the context clearly dictates otherwise.

[0019] Those with ordinary skill in the art will appreciate that the elements in the Figures are illustrated for simplicity and clarity and are not necessarily drawn to scale. For example, the dimensions of some of the elements in the Figures may be exaggerated, relative to other elements, in order to improve the understanding of the present invention.

[0020] There may be additional components described in the foregoing application that are not depicted on one of the described drawings. In the event such a component is described, but not depicted in a drawing, the absence of such a drawing should not be considered as an omission of such design from the specification.

[0021] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting but rather to provide an understandable description of the invention.

[0022] While the specification defines the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawings, in which like reference numerals are carried forward.
Construction:
[0023] Reference will now be made in detail to selected embodiments of the present invention in conjunction with the accompanying figures. Referring together to FIGS. 1 and 2, cross-sectional and perspective views of an optical fiber cable 100 (interchangeably referred to as YOGALITE™ optical fiber cable 100, which is a registered trademark of Sterlite Technologies Limited), in accordance with an embodiment of the present invention, respectively, are shown. It is apparent from FIG. 2 that the optical fiber cable 100 extends longitudinally to form a cylindrical tube structure.

[0024] The optical fiber cable 100 includes a plurality of sleeves 102a-102l (hereinafter, collectively referred to as sleeves 102). It should be noted that in FIG. 1, twelve sleeves 102 are shown for illustrative purpose only and any suitable number of sleeves 102 may be included without departing from scope and spirit of the present invention. Each sleeve 102 encloses a plurality of optical fibers 104 (hereinafter, collectively referred to as optical fibers 104). The number of optical fibers 104 in each sleeve 102 may vary depending on the cable requirements. Each sleeve 102 encapsulates at least one optical fiber 104. In a preferred embodiment of the present invention, each sleeve 102 carries twelve optical fibers 104, and the optical fiber cable 100 includes twelve sleeves 102, thereby making a total of 144 optical fibers 104 in the optical fiber cable 100.

[0025] The sleeves 102 are typically stranded about each other to form a stranded core. Such stranding may be either helical or “S-Z” fashion in order to provide a lay length, which is typically less than circumference of the barrel diameter of a drum on which the optical fiber cable 100 is wound. As used herein, the term “lay length” refers to the longitudinal distance along an optical fiber cable 100 that is required for one sleeve 102 to go all the way around the optical fiber cable 100. As used herein, the term “S-Z” fashion refers to a form of stranding where the sleeves 102 are stranded around the axis of the optical fiber cable 100 in sections with a first direction of stranding in S-shape alternating with sections with an opposite direction of stranding in Z-shape. The “S-Z” stranding allows uniform distribution of the stress across all the sleeves 102 in the optical fiber cable 100.

[0026] Water swellable yarns 106a-106c (hereinafter, collectively referred to as water swellable yarns 106) are dispersed between the sleeves 102 to aid moisture absorbance of the optical fiber cable 100. The water swellable yarns 106 further prevent water ingression, if any, inside the optical fiber cable 100.

[0027] The optical fiber cable 100 further includes a layer of high-strengthening yarns 108 which acts as binding element for the “S-Z” stranded sleeves 102. The high-strengthening yarns 108 are wound helically around the “S-Z” stranded sleeves 102 to provide tensile strength to the sleeves 102. Thus, the high-strengthening yarns 108 reduce the stress on the optical fibers 104 during tensile loads. The “S-Z” stranded sleeves 102 tend to open up after a certain number of turns, typically after 3 to 5 turns, due to reverse oscillation. In order to prevent the opening up of the stranded sleeves 102, the high-strengthening yarns 108 are used as the binding elements to bind the “S-Z” stranded sleeves 102. Therefore, the sleeves 102 retain the lay length and stay unaffected by reverse oscillation.

[0028] A water blocking tape 110 is wrapped linearly around the high-strengthening yarns 108. The water blocking tape 110 provides a moisture barrier for the optical fiber cable 100, i.e., the water blocking tape 110 does not allow outside moisture to enter the optical fiber cable 100, thereby preventing exposure of the sleeves 102 and the optical fibers 104 therein to moisture and other contaminants such as Hydroxyl ions (OH- ions).

[0029] An inner sheath 112 surrounds and encloses the sleeves 102, the water swellable yarns 106, the high-strengthening yarns 108, and the water blocking tape 110. The inner sheath 112 includes two strength members 114a-114b (hereinafter, collectively referred to as strength members 114) embedded therein. The strength members 114 are embedded diagonally opposite to each other in the inner sheath 112. The strength members 114 are circular in cross-section and extend longitudinally in the inner sheath 112. The embedded strength members 114 provide robustness and tensile strength to optical fiber cable 100. The shrinkage of inner sheath 112 during negative temperatures results in increase in the length of the sleeves 102 inside the optical fiber cable 100. The increase in length of the sleeves 102 results in micro and macro bends inside the optical fibers 104, which further results in higher attenuation or power loss in the optical fiber cable 100. Therefore, the embedded strength members 114 act as anti-buckling or anti-shrink elements by preventing the inner sheath 112 to shrink back during negative temperatures.

[0030] The water blocking tape 110 is preferably placed underneath the inner sheath 112. The arrangement of water blocking tape 110 between the high-strengthening yarns 108 and the inner sheath 112 prevents risk of sticking of the high-strengthening yarns 108 to the inner sheath 112 during sheathing process. Additionally, rip cords 116a-116b (herein after, collectively referred to as ripcords 116) are positioned diagonally opposite to each other between the water blocking tape 110 and the inner sheath 112. The rip cords 116 extend longitudinally between the water blocking tape 110 and the inner sheath 112, for facilitating easy stripping or opening of the inner sheath 112. Such rip cords 116 are fairly well known in the art and further description is omitted for the sake of brevity.

[0031] A plurality of strengthening members 118a-118h (hereinafter, collectively referred to as strengthening members 118), are helically wound around the inner sheath 112 to provide additional protection and hardness to the optical fiber cable 100. The number of strengthening members 118 may vary depending on the cable requirements. The strengthening members 118 provide high crush resistance to the optical fiber cable 100. An outer sheath 120 surrounds and encloses the sleeves 102, the water swellable yarns 106, the high-strengthening yarns 108, the water blocking tape 110, the inner sheath 112 and the strengthening members 118. Each strengthening member 118 may include either round rods or flats that are positioned between the inner and outer sheaths 112 and 120.

Materials and dimensions:
[0032] The following description lists the materials used for the various components of the YOGALITE™ optical fiber cable 100 of the present invention, along with their dimensions. It should be noted that the materials and dimensions included herein are for exemplary purpose only and they do not restrict the scope of the invention in any way. Various alternative materials known in the art can be used and the dimensions may vary depending on the design parameters and constraints.

[0033] In the exemplary embodiment illustrated in FIG. 1, the sleeves 102 that enclose the optical fibers 104 are made of Thermoplastic co-polyester elastomer, TPE (however, based on requirements and suitability, other materials can also be used). In an embodiment, each sleeve 102 has an outer diameter of about 1.35 millimeters and an inner diameter of about 1.05 millimeters. In another embodiment, the optical fiber cable 100 has one sleeve 102.

[0034] The optical fibers 104 are standard ITU-T G.652D silica optical fibers. Each sleeve 102 includes 12 optical fibers 104. In an embodiment, the optical fiber cable 100 has total of 12 optical fibers 104.

[0035] In an embodiment, the diameter of the stranded core is about 2.3 millimeters and free space for sleeves 102 access in the optical fiber cable 100 is about 0.7 millimeters.

[0036] The water swellable yarns 106 include super absorbent polymer (SAP) coated polyester threads. In an embodiment, the optical fiber cable 100 has two water swellable yarns 106.

[0037] The high-strengthening yarns 108 may include aramid yarns. The water blocking tape 110 is a super absorbent polymer (SAP) tape. In an embodiment, the thickness of the layer of the water blocking tape 110 is about 0.3 millimeters.

[0038] The inner sheath 112 is made of high density poly-ethylene, HDPE (however, based on requirements and suitability, other materials such medium density poly-ethylene (MDPE) can also be used). In an embodiment, the inner sheath 112 has a thickness of about 1.8 millimeters.

[0039] The embedded strength members 114 are made of a fiber reinforced plastic (FRP) (however, based on requirements and suitability, other materials can also be used). The optical fiber cable 100 has two embedded strength members 114. In an embodiment, the diameter of the each strength member 114 is about 1.2 millimeters. The embedded strength members 114 are coated with EAA (Ethyl Acrylic Acetate) in order to provide robust bonding with inner sheath 112. The rip cords 116 are made of polyester and have circular cross-sections. In an embodiment, the diameter of the inner cable core is about 6.2 millimeters.

[0040] In an embodiment, the strengthening members 118 are made up of fiber reinforced plastic (FRP) (however, based on requirements and suitability, other materials can also be used). Each strengthening member 118 may be either of flat FRPs or round FRPs which are helically applied around the inner sheath 112. In an embodiment, the optical fiber cable 100 includes six strengthening members 118. In yet another embodiment, the breadth of each strengthening member 118 such as flat FRP is about three millimeters and the thickness of each strengthening member 118 such as flat FRP is about 0.7 millimeters.

[0041] The outer sheath 120 is made of high density poly-ethylene, HDPE (however, based on requirements and suitability, other materials such medium density poly-ethylene (MDPE) can also be used). In an embodiment, the outer sheath 120 has a thickness of about 1.5 millimeters.

[0042] In an embodiment, the optical fiber cable 100 has an overall diameter of about 10.2 millimeters.

[0043] In another exemplary embodiment illustrated in FIG. 1, each sleeve 102 has an outer diameter of about 1.35 millimeters and an inner diameter of about 1.05 millimeters. The optical fiber cable 100 has 12 sleeves 102. Each sleeve 102 includes 12 optical fibers 104. The optical fiber cable 100 has total of 144 optical fibers 104. The diameter of the stranded core is about 6.1 millimeters and free space for sleeves 102 access in the optical fiber cable 100 is about 0.7 millimeters. The optical fiber cable 100 has two water swellable yarns 106. The thickness of the layer of the water blocking tape 110 is about 0.3 millimeters. The inner sheath 112 has a thickness of about 1.8 millimeters. The optical fiber cable 100 has two embedded strength members 114. The diameter of the each strength member 114 is about 1.2 millimeters. The diameter of the inner cable core is about 10 millimeters. The optical fiber cable 100 includes eight strengthening members 118. The breadth of each strengthening member 118 such as flat FRP is about three millimeters and the thickness of each strengthening member 118 such as flat FRP is about 0.7 millimeters. The outer sheath 120 has a thickness of about 1.5 millimeters. The optical fiber cable 100 has an overall diameter of about 14.4 millimeters.

[0044] The YOGALITE™ optical fiber cable 100 of the present invention offers a number of advantages over the conventional polybutylene terephthalate (PBT) cables. Firstly, the optical fiber cable 100 is easier to handle during the installation process and does not require special tools to peel off the sleeve 102. The sleeve 102 can be peeled off using fingers. The sleeves 102 require less amount of gel which reduces messiness and clean up time during the installation process. Therefore, the optical fiber cable 100 requires less time for installation. Further, mid-span operation of the optical fiber cable 100 is relatively easier and consumes approximately half the time as that required by the conventional cables.

[0045] The sleeves 102 are kink free and they can be bent to approximately five millimeters diameter for short term. For the optical fiber cable 100, the arrangement of the high-strengthening yarns 108, the water blocking tape 110 and the inner sheath 112 prevents the risk of sticking of the high-strengthening yarns 108 and the inner sheath 112, thereby resulting in easy installation of the optical fiber cable 100.

[0046] The YOGALITE™ optical fiber cable 100 is suitable for outdoor applications. More specifically, the optical fiber cable 100 is used for applications where rodent protection and high crush resistance are required. The strengthening members 118 between the inner and outer sheaths 112 and 120 provide additional hardness to the optical fiber cable 100. The strengthening members 118 resist the rodent bites as the FRP rods repel the rodents from biting or chewing the optical fiber cable 100. In an event where rodents manage to tear the outer sheath 120, the inner sheath 112 along with the strengthening members 118 serves as additional protection for the sleeves 102 and protects the sleeves 102 from further damage. The embedded strength members 114 allow the optical fiber cable 100 to withstand high crushing forces. The embedded strength members 114 provide additional protection by absorbing the impact forces and thereby preventing the transfer of forces to the sleeves 102. Also, the strengthening members 118 further improve the crush resistance of the optical fiber cable 100.

[0047] The YOGALITE™ optical fiber cable 100 can be used as a direct-buried cable. The direct-buried cables are especially designed to be buried under the ground without any kind of extra covering, sheathing, or piping to protect them. The optical fiber cable 100 is stiffened by the strengthening members 118 and the embedded strength members 114. The strengthening members 118 along with the inner and outer sheaths 112 and 120 allow the optical fiber cable 100 to be tolerant to specific environmental conditions such as heat, moisture, and soil acidity.

[0048] The present invention has been described and illustrated with the help of accompanying drawings which are not intended to limit the scope of the present invention. It is obvious to the persons skilled in the art that it is possible to modify the disclosed optical fiber cables without deviating from the scope of the present invention. Accordingly, such modifications are intended to be included in the scope of the present invention.