DESC:TECHNICAL FIELD
The present disclosure relates to the field of optical communication technology, and more particularly, relates to a gas leak proof corrugated sheath design for reducing friction in optical fiber cables. The present application is based on, and claims priority from an Indian Application Number 202011046806 filed on 27th October 2020, the disclosure of which is incorporated herein.

BACKGROUND
Optical fiber cables play a vital role in today’s networking infrastructure and long-haul communication. The optical fiber cables are designed to have a maximum number of optical fibers to meet the demands, of end-users, related to data, video, audio, or the like transmissions. Further, the optical fiber cables are designed to make them easy to install and maintain, easy to access, and easy to tear and cut whenever required. Generally, the optical fiber cables are installed by laying and blowing or are aerially deployed. During blowing process, an optical fiber cable is installed in a pre-installed duct. Blowing of the optical fiber cable is dependent on weight, friction, stiffness and drag force on the optical fiber cable. It is known that, friction plays an important role during blowing. The optical fiber cable with lower weight and higher stiffness as well as with lower coefficient of friction of a sheath blows to longer distances.
Once blowing is done, another step in installation is to seal the optical fiber cable in a joint closure box. The joint closure box is used as a branch out splicing point in optical fiber access networks. A typical application of the joint closure box is to splice optical fibers from a distribution cable towards drop fiber connections to end users. The joint closure box is generally kept under-ground and is at a risk of getting flooded with water or other debris. In order to prevent unwanted elements from entering, the joint closure box is filled with pressurised air. It becomes important that there should not be any leak point in the joint closure box. One such potential leak point is the sealing between a joint closure seal and the sheath (jacket) of the optical fiber cable. Thus, the optical fiber cables are required to seal perfectly.
Typically, the joint closure seal has a round inner cross-section. Thus, one can visualise that a smooth round optical fiber cable will seal well in the joint closure box. However, deviation begins when one starts to provide ribs on the sheath of the optical fiber cable, thus making the sheath irregular. While these irregularities help in blowing, they cause problems in sealing.
Accordingly, the present disclosure seeks to ameliorate one or more of the aforementioned disadvantages by providing a gas leak proof corrugated sheath design for reducing friction in the optical fiber cables.

OBJECT OF THE DISCLOSURE
A primary object of the present disclosure is to provide a gas leak proof corrugated sheath design for reducing friction in an optical fiber cable.
Another objective of the present disclosure is to reduce a number of contact points between the optical fiber cable and a duct to further reduce a coefficient of friction between a sheath of the optical fiber cable and an inner surface of the duct.

SUMMARY
The present disclosure provides a gas leak proof corrugated sheath design for reducing friction in an optical fiber cable and method of manufacturing the same. The optical fiber cable includes a plurality of ribbons, a plurality of ribbon bundles, one or more water swellable yarns, a first layer, one or more ripcords, one or more strength members and a second layer. The plurality of ribbons includes a plurality of fibers. The plurality of fibers are intermittently bonded thus forms one or more intermittently bonded ribbons. The plurality of ribbons are bundled to form the plurality of ribbon bundles. The first layer surrounds the plurality of ribbon bundles having the plurality of ribbons, which is further enclosed by the second layer, which is a corrugated sheath, having a plurality of ribs and a plurality of grooves to reduce number of contact points between the optical fiber cable and a duct. The plurality of ribs and the plurality of grooves reduce a coefficient of friction between the second layer and an inner surface of the duct, thus make the optical fiber cable suitable for efficient blowing while complying with a gas leak proof requirement. In other words, the second layer, which is a sheath, has a corrugated surface and is optimised to meet requirements of blowing as well as gas leak proof sealing in a joint closure box. Further, the second layer has the one or more strength members embedded into it to provide tensile strength and anti-buckling properties to the optical fiber cable. Furthermore, the one or more ripcords is provided for easy stripping of the optical fiber cable and the water swellable yarn is provided to prevent water ingression in the optical fiber cable. The plurality of ribs is defined by corrugation ratio up to 11% and a corrugation ratio is defined as (N_g*W*H*100)/(p(OD^2-ID^2 )*0.25) where, Ng = number of grooves, W = width of groove, H = depth of groove, OD = outer diameter of the corrugated sheath (i.e., OD is measured from top of a rib to top of diametrically opposite rib) and ID = Inner diameter of the corrugated sheath. The corrugated sheath ensures no gas leak from a joint closure box filled with air or any other suitable gas with a gauge pressure of 0.3±0.03 bar. A temperature inside a gas sealing system is in a range of 15° to 25°C, less or more. A tracer gas is used for a gas leakage testing. The testing of tracer gas involves one or more gas sensors. The tracer gas may be 5% Hydrogen and 95% Nitrogen or any other suitable tracer gas. The plurality of ribs has a height up to 0.3mm and distance between successive ribs of the plurality of ribs is 0.2 to 2.5mm. The number of ribs per unit outer diameter of optical fiber cable is 1.2 to 6.2. All ribs of the plurality of ribs are of same height. An inner surface of the corrugated sheath is smooth.
These and other aspects herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the invention herein without departing from the spirit thereof.

BRIEF DESCRIPTION OF FIGURES
The invention is illustrated in the accompanying drawings, throughout which like reference letters indicate corresponding parts in the figures. The invention herein will be better understood from the following description with reference to the drawings, in which:
FIG. 1 illustrates a corrugated sheath design for an optical fiber cable.
FIG. 2 is a representation of the optical fiber cable having the corrugated sheath inside a seal.
FIG. 3 is a representation of seal conforming to the corrugated sheath design.
FIG. 4 illustrates a corrugated sheath design for an optical fiber cable having a central strength member.

DETAILED DESCRIPTION
In the following detailed description of the invention, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be obvious to a person skilled in the art that the invention may be practiced with or without these specific details. In other instances, well known methods, procedures and components have not been described in details so as not to unnecessarily obscure aspects of the invention.
Furthermore, it will be clear that the invention is not limited to these alternatives only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art, without parting from the scope of the invention.
The accompanying drawings are used to help easily understand various technical features and it should be understood that the alternatives presented herein are not limited by the accompanying drawings. As such, the present disclosure should be construed to extend to any alterations, equivalents and substitutes in addition to those which are particularly set out in the accompanying drawings. Although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are generally only used to distinguish one element from another.
The key idea of the present disclosure is to provide corrugation on a sheath of an optical fiber cable to reduce coefficient of friction while still complying with a gas leak proof requirement. Unlike conventional sheath designs, with corrugated sheath design, a number of contact points between the optical fiber cable and a duct can be reduced that further reduces a coefficient of friction between the sheath and an inner surface of the duct.
FIG. 1 illustrates a corrugated sheath design for an optical fiber cable (100). FIG. 2 is a representation of the optical fiber cable (100) having the corrugated sheath inside a seal (120). FIG. 3 is a representation of the seal (120) conforming to the corrugated sheath design.
The optical fiber cable (100) is an air blown optical fiber cable that includes a plurality of ribbons (102), a plurality of ribbon bundles (104), a first layer (106), one or more ripcords (108), one or more water swellable yarns (110), one or more strength members (112) and a second layer (114).
The plurality of ribbons (102) includes a plurality of fibers. The plurality of fibers are intermittently bonded thus forms one or more intermittently bonded ribbons. Alternatively, the plurality of fibers are continuously bonded. Alternatively, the plurality of fibers may be loose fibers in-housed in tubes or sleeves. In an implementation, each of the plurality of ribbons (102) may have 12 optical fibers. Alternatively, each of the plurality of ribbons (102) may have less than 12 optical fibers. Alternatively, each of the plurality of ribbons (102) may have more than 12 optical fibers. The plurality of fibers may have a diameter of 250µm. Alternatively, the plurality of fibers may have other suitable diameter. Generally, an optical fiber refers to a medium associated with signal transmission over long distances in the form of light pulses. The optical fiber uses light to transmit voice and data communications over long distances when encapsulated in a jacket. The plurality of optical fibers may be single-mode optical fibers or multi-mode optical fibers. The plurality of optical fibers may be of ITU.T G.657A2 category. Alternatively, the plurality of optical fibers may be of ITU.T G.657A1 or G.657B3 or G.652D or other category. The ITU.T, stands for International Telecommunication Union-Telecommunication Standardization Sector, is one of the three sectors of the ITU. The ITU is the United Nations specialized agency in the field of telecommunications. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The plurality of optical fibers may be coloured fibers. The plurality of optical fibers may have maximum individual fiber polarization mode dispersion (PMD) = 0.2 ps/vkm. The polarization mode dispersion (PMD) is a form of modal dispersion where two different polarizations of light in a waveguide, which normally travel at same speed, travel at different speeds due to random imperfections and asymmetries, causing random spreading of optical pulses. Further, the plurality of optical fibers may have a PMD link design value (PMDQ) = 0.1 ps/vkm. Alternatively, value of the polarization mode dispersion (PMD) and PMDQ may vary.
The plurality of ribbons (102) may have a pitch of 250µm. Alternatively, the pitch may vary. The plurality of ribbons (102) may use colour coded ribbon matrix, band stripe printing or the like for ribbon identification.
The plurality of ribbons (102) are bundled to form the plurality of ribbon bundles (104). The plurality of ribbons (102) may be bundled using binder yarns. Alternatively, the plurality of ribbons (102) may be bundled using any other suitable means. The binder yarns may be coloured binder yarns. The binder yarns may be of same or of different colour. Alternatively, the binder yarns may be made of any suitable material. In an implementation, each of the plurality of ribbon bundles (104) may include 6 ribbons. Alternatively, each of the plurality of ribbon bundles (104) may include less than 6 ribbons. Alternatively, each of the plurality of ribbon bundles (104) may include more than 6 ribbons.
Accordingly, in an example, a total number of optical fibers may be 432 (i.e., 12*6*6F) or less than 432 or more than 432 in the optical fiber cable (100).
The first layer (106) surrounds the plurality of ribbon bundles (104). The first layer (106) is a tape layer. The tape layer may be a water blocking tape that prevents ingression of water inside a core of the optical fiber cable (100). The tape layer may be composed of polyester, polyacrylate swelling powder, along with corrosion inhibitor. The tape layer may be made of any suitable material. Alternatively, the first layer (106) may be made of any suitable material to enclose the plurality of ribbon bundles (104) and to prevent water ingression inside the core of the optical fiber cable (100).
The first layer (106) is enclosed by the second layer (114). The second layer (114) is a sheath layer (or sheath) or an outer jacket. The second layer (114) provides safety to the optical fiber cable (100) from external stresses and environmental conditions. The second layer (114) is made of an ultra-violet (UV) proof black polyethylene material. Alternatively, the second layer (114) is made of thermoplastic material. Alternatively, the second layer (114) is made of low smoke zero halogen material. The low smoke zero halogen is a material classification typically used for cable jacketing in wire and cable industry that is composed of thermoplastic or thermoset compounds that emit limited smoke and no halogen when exposed to high sources of heat. Alternatively, the second layer (114) is made of polyethylene material. Alternatively, the second layer (114) is made of any suitable polymeric material. In an example, the second layer (114) is made of High Density Poly Ethylene (HDPE). In another example, the second layer (114) is made of Ultra-Violet (UV) proof High Density Poly Ethylene (HDPE). Alternatively, the second layer (114) is made of medium-density polyethylene (MDPE), low-density-polyethylene (LDPE), low smoke zero halogen (LSZH), polypropylene or any other suitable material. Further, the second layer (114) may have a thickness in a range of 1.6mm to 3mm depending on fiber count.
The second layer (114) is a corrugated sheath that includes a plurality of ribs (116) and a plurality of grooves (118). The second layer has a surface with an alternate depressed region and raised regions that covers the periphery of the second layer (114). The raised region is the plurality of ribs (116) and the depressed region is the plurality of grooves (118). In an example, the number of plurality of ribs (116) is equal to the number of plurality of grooves (118). The plurality of ribs (116) is longitudinal protrusions on an external or outer surface of the corrugated sheath and is parallel to an axis of the optical fiber cable (100). Further, the second layer (114) has a non-corrugated inner surface. The formation of the plurality of ribs (116) and the plurality of grooves (118) reduces number of contact points between the optical fiber cable (100) and the duct and thus reduces the coefficient of friction between the second layer (114) and the inner surface of the duct, which provides increased blowing capacity to the optical fiber cable. Each of the plurality of ribs (116) may have equal height and width. Alternatively, each of the plurality of ribs (116) may have unequal height and width. In an example, the plurality of ribs (116) may have the height up to 0.3mm that helps maintaining required mechanical strength of the cable. Further, a distance between successive ribs of the plurality of ribs (116) may be in a range of 0.2-2.5mm. The distance between successive ribs cannot be below 0.2mm as manufacturing becomes difficult at very small size. Similarly, each of the plurality of grooves (118) may have equal depth and width. Alternatively, each of the plurality of grooves (118) may have unequal depth and width. The plurality of ribs (116) may have a density (number of ribs/outer diameter of cable) ranging between 1.2 to 6.2, where reducing the density below 1.2 can lead to gas leakage and density higher than 6.2 can be difficult to manufacture as the plurality of ribs (116) become too small. The distance between successive ribs of the plurality of ribs (116) maximum at 2.5 mm and with the density of plurality of ribs (116) 1.2 contributes to produce the sheath that is corrugated with an outer diameter of the optical fiber cable of 70 mm and also adhering to gas leak compliance.
In an implementation, the number of the plurality of ribs (116) may range between 40-50 for the optical fiber cable (100) having an outer diameter of 12.5mm. The plurality of ribs (116) and the plurality of grooves (118) may be of a rectangular shape with rounded edges, a pointy triangle shape, a curve-type shape, a rectangular shape, a triangular shape, a trapezoidal shape, an arc-shape or any other suitable shape. The plurality of ribs (116) and the plurality of grooves (118) may be made thin or thick as per requirement.
The second layer (114) having the corrugated surface is optimised to meet requirements of blowing as well as gas leak proof sealing in a joint closure box. In an implementation, a ratio of the number of the plurality of ribs (116) and the outer diameter (or diameter) of the optical fiber cable (100) may be below 6.2. The plurality of ribs (116) and the plurality of grooves (118) may be fabricated in the second layer (114) with respect to a corrugation ratio. The corrugation ratio corresponds to an amount of a void space created by the plurality of grooves (118) in a smooth sheath. In other words, the corrugation ratio defines the amount of material removed from the smooth sheath to produce corrugation as shown in FIG. 2. The corrugation ratio may go up to 11% and is defined along with a density range of ribs (1.2–6.2 ribs/OD). The corrugation ratio may be determined by: (N_g*W*H*100)/(p(OD^2-ID^2 )*0.25), where Ng is number of grooves, W is width of groove, H is depth of groove, OD represented as (122) in Fig. 2 (i.e., D1 = OD is measured from top of a rib to top of diametrically opposite rib) is outer diameter of the sheath and ID (D2) represented as (124) in Fig. 2 is inner diameter of the sheath. The width of groove (W) may be determined by:
W = 0.5(width of groove at base + width of groove at top).
That is, an area of the plurality of grooves (118) may be up to 11% of p*(OD2-ID2)*0.25. The area of the plurality of grooves (118) being up to11% of p*(OD2-ID2)*0.25 makes the optical fiber cable (100) to have a lower coefficient of friction as compared to a conventional smooth or without corrugation optical fiber cable and to be compliant with gas blockage test i.e., no gas leakage between the second layer (114) (sheath) and a joint closure seal when air or any other suitable gas is filled in excess of 0.3+0.03 bar gauge pressure inside the joint closure box. The gas that is used for testing may be a tracer gas, a non-limiting example of which is 5% Hydrogen, 95% Nitrogen. Typically, the joint closure box is used to store connections between two optical fiber cables and is kept underground in manholes. Therefore, a tight sealing (120) (as shown in FIG. 2 and FIG. 3) is required to prevent entry of dust particles, water etc. in the joint closure box, which may damage the optical fibers. To obtain this, the optical fiber cable is inserted in the joint closure box and then sealed with the gas inside. The sheath i.e., the second layer (114) and the joint closure box maintains the tight sealing (120) such that the gas does not leak outside. In an implementation, an open end of the cable is placed into a pressure chamber at a temperature ranging from 15 to 25°C with a sealing system on outside. Alternatively, the temperature may vary.
The second layer (114) has the one or more strength members (112) embedded into it. The one or more strength members (112) may be made of fiber reinforced plastic (FRP) or aramid reinforced plastic (ARP). The one or more strength members (112) may be made of any other suitable material. The one or more strength members (112) may be of circular shape. Alternatively, the one or more strength members (112) may be of any other suitable shape. In an implementation, the one or more strength members (112) may be coated. Alternatively, the one or more strength members (112) may not be coated. The one or more strength members (112) provides tensile strength and anti-buckling properties to the optical fiber cable (100). Alternatively, the optical fiber cable (100) may use a central strength member (CSM) (120) design with an optical fiber retaining element (104a) such as loose tube, buffer tube or the like as shown in FIG. 4 that includes one or more loose optical fibers, optical fiber ribbons etc. In this scenario, the sheath without embedded strength members can be used.
The optical fiber cable (100) includes the one or more ripcords (108) for easy stripping of the optical fiber cable (100). The one or more ripcords (108) are twisted yarns. The one or more ripcords (108) may be made of nylon, aramid, polyester and combination thereof. The one or more ripcords (108) may be two in numbers. Alternatively, the number of the one or more ripcords (108) may vary. The one or more ripcords (108) may be placed between the first layer (106) and the second layer (114). Alternatively, the one or more ripcords(108) may be placed at suitable location inside the optical fiber cable (100). Further, the optical fiber cable (100) includes the water swellable yarn (110) acting as a water blocking element. The water swellable yarn (110) prevents water ingression in the optical fiber cable (100). The water swellable yarn (110) may be located in the core of the optical fiber cable (100).
The optical fiber cable (100) may be defined along a longitudinal axis (not shown) passing through a geometrical center (not shown) of the optical fiber cable (100). The longitudinal axis is an imaginary axis along lengthwise direction of the optical fiber cable (100). In general, the geometrical center is a central point of the optical fiber cable (100). The optical fiber cable (100) may have maximum tensile strength of 1500N at 0.6% fiber strain. The optical fiber cable (100) may have a short term bend diameter as 12D and a long term bend diameter as 20D, where D is the diameter of the optical fiber cable (100). Further, the optical fiber cable (100) may have a crush resistance as 1000N/10cm and an impact load as 10Nm. Furthermore, the optical fiber cable (100) may have a torsion as +1800 at 100N. Alternatively, the optical fiber cable (100) may have other suitable values of tensile strength, bend diameter, crush resistance, impact load and torsion.
The optical fiber cable (100) may have temperature performance ranging between -10ºC to +85ºC. Further, the optical fiber cable (100) may have a diameter upto 13 mm with an ovality of 5% and a weight as 80+10% kg/km. Furthermore, the optical fiber cable (100) may have a length as 2Km+5%. Alternatively, the optical fiber cable (100) may have other suitable diameter, weight and length. The optical fiber cable (100) is compliant with IEC Standard and BT CW 1854.
Additionally, the present disclosure proposes a method to fabricate a corrugated optical fiber cable (100) to ease installation via blowing while being in compliant with gas leak requirements, wherein the plurality of ribs (116) and the plurality of grooves (118) are fabricated in the second layer (114) with the corrugation ratio up to 11% that is determined by: (N_g*W*H*100)/(p(OD^2-ID^2 )*0.25), where Ng is number of grooves, W is width of groove, H is depth of groove, OD (i.e., OD (D1) represented as (122) in Fig. 2 is measured from top of a rib to top of diametrically opposite rib) is outer diameter of the sheath and ID (D2) represented as (124) in Fig. 2 is inner diameter of the sheath and wherein the width of groove (W) may be determined by: W = 0.5(width of groove at base + width of groove at top).
It will be apparent to those skilled in the art that other alternatives of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific aspect, method, and examples herein. The invention should therefore not be limited by the above described alternative, method, and examples, but by all aspects and methods within the scope of the invention. It is intended that the specification and examples be considered as exemplary, with the true scope of the invention being indicated by the claims.
Conditional language used herein, such as, among others, "can," "may," "might," "may," “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain alternatives include, while other alternatives do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more alternatives or that one or more alternatives necessarily include logic for deciding, with or without other input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular alternative. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list.
Disjunctive language such as the phrase “at least one of X, Y, Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to present that an item, term, etc., may be either X, Y, or Z, or any combination thereof (e.g., X, Y, and/or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain alternatives require at least one of X, at least one of Y, or at least one of Z to each be present.
While the detailed description has shown, described, and pointed out novel features as applied to various alternatives, it can be understood that various omissions, substitutions, and changes in the form and details of the devices or algorithms illustrated can be made without departing from the scope of the disclosure. As can be recognized, certain alternatives described herein can be embodied within a form that does not provide all of the features and benefits set forth herein, as some features can be used or practiced separately from others.

CLAIMS:CLAIMS
We Claim:
A corrugated sheath (114) for an optical fiber cable (100), comprising:
a plurality of ribs (116) on an external surface of the corrugated sheath (114), wherein the plurality of ribs (116) is defined by corrugation ratio up to 11%.
The corrugated sheath (114) as claimed in claim 1, wherein the corrugation ratio is defined as (N_g*W*H*100)/(p(OD^2-ID^2 )*0.25) where,
Ng = number of grooves
W = width of groove
H = depth of groove
OD = outer diameter of the corrugated sheath
ID = Inner diameter of the corrugated sheath
The corrugated sheath (114) as claimed in claim 1, wherein the corrugated sheath (114) ensures no gas leak from a joint closure box filled with air or any other suitable gas with a gauge pressure of 0.3±0.03 bar.
The corrugated sheath (114) as claimed in claim 1, wherein temperature inside a gas sealing system is in a range of 15° to 25°C.
The corrugated sheath (114) as claimed in claim 1, wherein a tracer gas used for testing is 5% Hydrogen and 95% Nitrogen or any other suitable tracer gas.
The corrugated sheath (114) as claimed in claim 1, wherein the plurality of ribs (116) has a height up to 0.3mm.
The corrugated sheath (114) as claimed in claim 1, wherein a distance between successive ribs of the plurality of ribs (116) is 0.2 to 2.5mm.
The corrugated sheath (114) as claimed in claim 1, wherein number of ribs per unit outer diameter of optical fiber cable (100) is 1.2 to 6.2.
The corrugated sheath (114) as claimed in claim 1 has one or more strength members (112) embedded into the corrugated sheath (114).
The corrugated sheath (114) as claimed in claim 1 is used in the optical fiber cable (100), wherein the optical fiber cable (100) has a central strength member (120).
The corrugated sheath (114) as claimed in claim 1, wherein all ribs of the plurality of ribs (116) are of same height and all grooves of a plurality of grooves (118) are of same depth.
The corrugated sheath (114) as claimed in claim 1, wherein an inner surface of the corrugated sheath (114) is smooth or non-corrugated.