Abstract:
The present invention includes an apparatus for combining a water barrier fluid to a bundle of optical fibers including an entrance die having an orifice which is dimensioned to allow for a bundle of optical fibers to be drawn therethrough. Also, an exit die having an orifice is provided. The entrance die and the exit die, respectively, have inner sides which define a cavity. The cavity is in fluid communication with the orifice of the entrance die and the orifice of the exit die, such that a gap is formed at a meeting point between the cavity and the respective orifices of the entrance and the exit die. The gap is radially surrounded by an extension of the cavity to define a critical flow region. A plurality of baffles are formed in the exit die which are operative to inject fluid into the cavity. Also provided is a main body which supports the entrance die and the exit die. The main body includes a passageway that is in fluid communication with the plurality of baffles. A retaining ring is included which secures the entrance die and the exit die to the main body.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to the field of fiber optic cables, in particular the present invention is directed to a method and apparatus for applying water barrier gels to optical fibers or fiber bundles at high speeds. 
   2. Discussion of Related Art 
   Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at high speeds, and with extremely low signal loss as compared to standard wire or cable networks. Optical fiber has found increasingly widespread application and currently constitutes the backbone of the worldwide telecommunication network. Because of this development, there has been a growing need for better quality optical fibers with a decrease in production time and costs, while ensuring adequate material strength for continued operation in increasingly harsh conditions. An important aspect for making better optical fibers is the reduction of structural faults or impurities in the protective coatings applied to the optical fiber during manufacture. 
   In general, optical fibers are manufactured from relatively large diameter glass preforms. Fiber optic preforms are generally made with three concentric glass layers. The inner layer, or core, is made of a very high quality, high purity SiO 2  glass, which for example, may be about 5 mm in diameter. This high purity core is the portion of the optical fiber in which the optical data is transmitted. Concentrically positioned around the high purity core is a second layer of glass, or cladding, with a lower index of refraction then the inner core, and generally is less pure. The difference in refraction indices between the core and cladding allows the optical signals in the core to be continuously reflected back into the core as they travel along the fiber. The combination of the core and cladding layers is often referred to as the “primary preform.” The optical fiber is then formed by heating and softening a portion of the preform, and rapidly drawing the softened portion with specialized equipment. The length of the drawn optical fiber is typically several thousands of times the length of the primary preform. Optical fibers intended for manufacture of telecommunications cables are typically coated with one or more polymer layers. The polymers provide mechanical protection of the fiber surface, and are colored for identification purposes. The coated optical fibers, singly or in groups, are typically covered with one or more of a number of jackets that provide structural support and environmental protections. The aggregate of the optical fiber, jackets, and additional integrated mechanical supports, is typically referred to as an optical fiber cable. 
   Exposure to water or humid air causes chemical changes in the surface of the optical fiber, resulting in a degradation of its ability to carry information. The most common method used to prevent or mitigate this degradation, is to reduce or eliminate water contact on the fiber surface by substantially filling the protective housings with a water barrier compound such as a hydrophobic fluid. For a number of reasons, including cable behavior during installation and long-term stability of the cables during use, the hydrophobic fluid is typically a gel. Gels tend to flow when mechanically stressed, but tend to remain static when under a low mechanical load. 
   Known methods for applying gel to fibers include drawing the fibers through a reservoir filled with gel so that the fibers are coated. However, the use of such a method often results in an inconsistent coating on the fibers due to air entrapped air. Accordingly, gel applicators have been developed, such as the device disclosed in Griser et al. U.S. Pat. No. 5,395,557, which attempts to reduce air entrapment by using a reservoir filled with pressurized gel. This device includes a housing having a cavity through which a plurality of separated optical fibers are fed. Gel is provided to the cavity from a gel reservoir via a pump. The optical fibers are then drawn through the gel so that the fibers are coated with the gel. The gel is provided under pressure in an attempt to reduce air gaps that may form upon the fibers. However, this technique has numerous draw backs. For example, a relatively large driving pressure is placed upon the gel in the reservoir to reduce air entrainment. Rapid application of barrier gel with this method requires relatively long and narrow application regions to prevent uncontrolled ejection of fluid from application regions, due to the large pressures. 
   Consequently, an apparatus for applying gel to a plurality of optical fibers, which substantially overcomes the above-recited drawbacks is highly desirable and needed in the optical fiber industry. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to eliminating the above problems associated with the application of water barrier fluids, such as a gel, to optical fibers and optical fiber bundles. Thus, the invention improves the quality of the optical fiber cable and manufacturing process used to apply the gel. 
   The present invention addresses the above problems by providing a gel application apparatus that applies the gel with a flow having a high velocity in a direction normal to the surface of the optical fibers, as the fibers pass between an entrance die and an exit die. This creates a linear velocity great enough to overcome the kinetic energy of an air boundary layer traveling along with the fibers through the die entrance. Thus, the method and apparatus is capable of accurately and efficiently coating optical fibers while eliminating unwanted air pockets. 
   More specifically, the present invention relates to an apparatus for applying a coating of a water barrier fluid, such as a gel to an optical fiber including a die having an entrance side and exit side. An orifice is formed in the die which extends through the entrance side and exit side in a width-wise direction, and which is dimensioned to allow for an optical fiber to be drawn therethough. A cavity is formed in the die, and is in fluid communication with the orifice. A fluid insertion opening is formed in the die for injecting fluid into the cavity. When a fluid is injected into the cavity it travels through the cavity and out of a circumferential exit gap, such that it coats a portion of the optical fiber. The circumferential gap is formed at a meeting point between an inner portion of the cavity and the respective orifices of the entrance and the exit die. 
   The present invention still further provides for an apparatus for applying a coating to several optical fibers or a bundle of optical fibers, including an entrance die having an orifice which is dimensioned to allow for a bundle of optical fibers to be drawn therethrough. Also, an exit die having an orifice is provided. The entrance die and the exit die, respectively, have inner sides, which define a cavity. The cavity is in fluid communication with the orifice of the entrance die and the orifice of the exit die, such that a circumferential gap is formed at a meeting point between the cavity and the respective orifices of the entrance die and the exit die. Thus, the circumferential gap is radially surrounded by an extension of the cavity, to define a critical flow region. A plurality of baffles are formed in the exit die, which are operative to inject fluid into the cavity. Also provided is a main body, which supports the entrance die and the exit die. The main body includes a passageway which is in fluid communication with the plurality of baffles. A retaining ring is also included, which secures the entrance die and the exit die to the main body. 
   Additionally, when fluid is passed through the circumferential gap toward the plurality of fibers it travels at a velocity which is sufficient to overcome kinetic energy of an air boundary layer traveling along with the optical fibers drawn through the entrance die, prior to the fibers being drawn through the exit die. 
   Still further the invention provides for a method of applying a water barrier fluid, such as a gel to one or more optical fibers, including the steps of drawing an optical fiber through an orifice formed in a die; and injecting a fluid into a cavity formed in the die, wherein the cavity is in fluid communication with the orifice, and wherein the fluid is pressurized out of the orifice and onto the optical fiber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature and various additional features of the invention will appear more fully upon consideration of illustrative embodiments of the invention which are schematically set forth in the drawings, in which: 
       FIG. 1  is front view of an exemplary arrangement of fibers having a gel provided thereon, according to the present invention; 
       FIG. 2  is an exploded perspective view of an applicator according to the present invention; 
       FIG. 3  is a sectional view of an applicator according to the present invention being supported by a base; and 
       FIG. 4  is an enlarged sectional view of a critical flow region of the applicator. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will be explained in further detail by making reference to the accompanying drawings, which do not limit the scope of the invention in any way. 
   With reference to  FIG. 1 , a plurality of optical fibers  10  are shown in a radial arrangement forming a fiber bundle  22 . In this embodiment, twelve optical fibers  10  are shown; however, it will be appreciated that the optical fiber bundle  22  may consist of a varying arrangement and number of optical fibers  10 . The fiber bundle  22  is shown as having an outer portion  24  and an inner portion  26 . 
   According to the present invention, a water barrier fluid  28 , for example, a thixotropic gel, is disposed onto the outer  24  and inner  26  portions of the fiber bundle  22 , as described below. The gel  28  acts to prevent ingress of water to the optical fiber surface produced from direct liquid contact or exposure to humid air. Although, thixotropic gel is described, any of a broad classification of fluid polymeric materials may be used, provided that the materials meet the criteria of chemical compatibility with the optical fibers and their coatings, and that the water barrier fluid possess a chemical nature that materially limits the transport of water to the optical fiber surface. For example, other suitable materials may include Newtonian liquids, dilute solutions containing polymer molecules, and liquid slurries containing solid particles, although not limited to such materials. In addition, it is typically desired that the fluid does not leak from open ends of cable housings. This undesirable behavior would result in an eventual exposure of a length of each fiber being exposed to the cable environment. The intrinsic mechanical behavior of gels makes this class of materials most appropriate for use as a water barrier in optical fiber cables. 
     FIG. 2  and  FIG. 3  illustrate a die assembly  30  for forming the above cable. The die assembly  30  includes a retaining ring  32  which is attached to a main body die  72 . Positioned between the retaining ring  32  and the main body die  72 , is an entrance die  42  and an exit die  54 . These elements are operative to allow for optical fibers to pass through a center thereof. 
   In further detail, the retaining ring  32  has an entrance side  34  and a containing side  36 , which are in communication with each other. The containing side  36  has a recessed area for accommodating the entrance die  42 . The retaining ring  32  also has an outer portion  40 , which is threaded. 
   The entrance die  42  has an inner side  44 , and an outer side  46 . The entrance die  42  may be made from a material, such as tungsten carbide. The inner side  44  has a conical center portion  48  and is dimensioned so as to allow the entrance die  42  to be disposed within the recessed area  38  of the retaining ring  32  so that the outer side  46  of the entrance die  42  is in contact with a wall portion of the recessed area  38 . An orifice  50  is provided in the entrance die  42  and is centrally positioned in relation to the conical center portion  48 . The outer side  46  of the entrance die  42  has an inwardly tapered section which is angled towards the orifice  50 . 
   In further accordance with the present invention, the exit die  54  is provided with an inner side  56  and an outer side  58 . The exit die  54  may be made from a material, such as carbon steel. The exit die  54  also has a centrally positioned orifice  60 , which is concentrically positioned with respect to the orifice  50  of the entrance die  42 . The exit die  54  further contains a plurality of baffles or baffle holes  62 , which are disposed around the orifice  60 . 
   A cylindrically shaped spacer ring  64  is positioned between the entrance die  42  and the exit die  54 . The spacer ring  64  is operative to position the entrance die  42  and the exit die  54  at predetermined relationship with respect to each other. The spacer ring  64  is dimensioned to contact wall portions of the entrance and exit dies  42  and  54 , so as not to interfere with the plurality of baffles  62  and orifice  60  of the exit die  54 , and orifice  50  of the entrance die  42 . 
   The contiguous positioning of the entrance and exit dies  42  and  54  form a fluid cavity  66 , as shown in  FIGS. 3 and 4 . The fluid cavity  66  is defined by the conical center portion  48  of the entrance die  42  and the inner side  56  of the exit die  54 . The fluid cavity  66  extends circumferentially around, and is in communication with, the orifice  50  of the entrance die  42  and the orifice  60  of the exit die  54 , thus producing an exit gap G, having a dimension d 1 . 
   With further reference to  FIG. 4 , the exit gap G forms an integral part of a critical flow region  69 . The critical flow region  69  is further defined by dimensions d 2  and d 3 , which respectively represent the orifice diameters of the entrance die  42  and the exit die  54 . To prevent sporadic application of a barrier coating to the fibers passing through the invention, the barrier fluid must not be materially disturbed by air that is naturally accelerated toward the die by the approaching fibers. The present invention sizes the critical flow region such that the kinetic energy of the barrier fluid that passes through the exit gap G and contacts the fibers is large in comparison to the air accelerated toward the die entrance by the moving fibers. The upper limit for the dimensions of the critical flow region is chosen such that the kinetic energy of the barrier fluid is larger, for example on the order of several hundred times that, of the potentially entrained air. The lower limit for the dimensions of the critical flow region is constrained by the need to apply the barrier fluid at pressures readily supplied by inexpensive process fluid handling equipment. Also taken into consideration when determining the dimensions of the critical flow region is the desired fiber bundle geometry, as required by the cable product. An exemplary embodiment of gap dimensions which have been shown to produce favorable results include an entrance die diameter d 2  and an exit die diameter d 3  of 1.04 mm, and a gap G width d 1  of 0.5 mm. During testing, such dimensions have resulted in a kinetic power of an extrudate of 4.94 Watts. It was also found that a boundary layer of air around a bundle of 12 fibers traveling at a rate of 1000 m/min produced 0.01 Watts of power. Thus, the kinetic power of the extrudate is much larger than the boundary layer of air around the bundle, which results in a proper application of gel to the bundle without the presence of detrimental air pockets. These dimensions are given by way of example and may change depending on the size of the bundle to be coated. 
   A slip ring  70  is provided around an outer circumferential surface of both the entrance  42  and exit  54  die. The slip ring  70  forms a slip fit with the dies and is operative to aid in keeping the dies properly aligned. 
   The main die body  72  has a first recessed portion  74 , for receiving the exit die  54 , the spacer ring  64  and the entrance die  42 . The recessed portion  74  has a first diameter which is dimensioned to form a proper fit with the slip ring  70 . The main die body  72  also has a second recessed portion  75  with threads formed thereon, for engaging with the outer threaded portion  40  of the retaining ring  32 . Accordingly, when the retaining ring  32  is threadedly engaged with the main die body  72 , the entrance die  42 , the spacer ring  64 , the exit die  54  and the slip ring  70  are secured together to form the die assembly  30 . 
   The main die body  72  also has a conical side  78  which is angled in towards a center portion of the die main body  72 . It is also noted that the conical design is given by way of example, and that this side may be formed to be flat in shape. An orifice  80  is provided in the main die body  72  which is centrally positioned with respect to the conical side  78 , so as to be in communication with the orifice  60  of the exit die  54  and the orifice  50  of the entrance die  42 . In one embodiment of the present invention, an o-ring  82 , as shown in  FIG. 3 , is provided between the retaining ring  32  and the entrance die  42 . Additionally, an o-ring  84  is provided between the exit die  54  and the die main body  72 . The o-rings may be made from a material, such as nitrile rubber. 
   An injection port  86  is provided on an outer portion of the main body  72 . A cavity  87 , which may be annular, is formed to be in communication with the injection port  86 , and abuts baffle holes  62 . It will also be appreciated that the injection port  86  may be placed in the conical side  78  of the main body  72 . The injection port  86  is formed to be in communication with the baffles  62  of the exit die  54 . The injection port  86  is also connected to a pumping system, which is operative to supply the gel in a pressurized state and is capable of providing a sufficient quantity of fluid at uniform rates to produce the desired amount to be combined with the group of optical fibers, which is passed therethrough. 
   With further reference to  FIG. 3 , during an implementation of the applicator for high-speed gel buffering of optical fiber bundles according to the present invention, the bundle of optical fibers  22  are fed into the die assembly  30  through the entrance side  34  of the retaining ring  32  and into the entrance die  42 . The bundle of fibers  22  is then drawn through to the exit die  54 , while passing the critical flow region  69 . The bundle  22  is then drawn through the outer side  58  of the exit die  54 , and out of the die assembly  30 . It is noted that the present invention can also be implemented to coat an individual optical fiber, as well as the described optical fiber bundle  22 . 
   The coating of the optical fiber bundle  22  is accomplished by pressurizing gel into the injection port  86  of the main body  72  and through the cavity  87 . The pressurized gel then travels into the fluid cavity  66 , which is formed between the entrance and exit dies  42  and  54 . The shape of the fluid cavity  66  is chosen to have a section wide enough such that resistance to filling of the cavity is small, and varies smoothly, such that flow-induced shear stress on the gel is gradually increased toward the exit gap G. 
   With additional reference to  FIG. 4 , the pressurized gel is ejected into the critical flow  69  region via the exit gap G and onto the bundle of fibers  22 . The orifice  50  of the entrance die  42  has a dimension d 2  so as to slightly compress the original diameter of the bundle of fibers  22 . As discussed above, an exemplary size of d 2  is 1.04 mm and is chosen to compact the individual optical fibers  10  of the bundle  22 , towards each other such that excess air is removed from the bundle  22  and the fiber group attains the degree of compaction required by the cable manufacturing process. With additional reference to  FIG. 1 , upon the pressurizing of the gel  28  onto the bundle  22 , the gel  28  not only coats the outer portion of the bundle  24 , but is also forced into the inner portion  26  of the bundle  22 . 
   According to the present invention, the gel  28  is applied by controlling the volumetric flow rate and pressure. For example, for a flow rate of about 57,000 mm 3  per minute of gel, cavity pressure of 48,000 Pascal was measured while applying gel on a bundle of 12 fibers. The gel  28  is applied at a high flow rate or velocity in a direction normal to the surface of the optical fibers as the fibers pass between the entrance die  42  and an exit die  54 . For example, a mean gel velocity, normal to the fiber bundle in the gap, of 13,000 mm per minute may be used. This creates a linear velocity great enough to overcome the kinetic energy of an air boundary layer traveling along with the fibers toward the die entrance. This is because the kinetic power of the extruded gel is much larger than the boundary layer of air around the bundle of fibers  22 . Thus, the method and apparatus is capable of accurately and efficiently combing fluids with optical fibers while eliminating unwanted air pockets. 
   It will be appreciated by one skilled in the art that the proper application of the gel, according to the present invention, is dependent upon the proper dimensioning of the elements of the die assembly  30 . For example, such critical dimensions include the respective diameters d 2  and d 3  and concentricity of the orifices  50  and  60 , of the entrance die  42  and the exit die  54 , and the width d 1  of the exit gap G, as discussed above. 
   Although the invention describes the use of a plurality of baffles in the exit die, and an injection port in the main die body, it will be appreciated that a plurality of injection ports may be used, and that the size and shape of the baffles and the injection port may be altered depending on the type of gel used, the shape of the cavity and the rate at which the fibers are drawn through the die assembly. 
   Although the invention describes the use of a conical region formed on the entrance die for creating a particular shaped cavity, it will be appreciated that various configurations of the inner side of the entrance die, and the inner side of the exit die may be used to obtain various shaped cavities depending on the desired flow behavior of the gel. 
   It is, of course, understood that departures can be made from the preferred embodiments of the invention by those of ordinary skill in the art without departing from the spirit and scope of the invention that is limited only by the following claims.