Patent Publication Number: US-2005117858-A1

Title: Optical fiber cable and method of manufacturing therefor

Description:
CLAIM OF PRIORITY  
      This application claims priority to an application entitled “Optical fiber cable and method of manufacturing the same,” filed in the Korean Intellectual Property Office on Nov. 27, 2003 and assigned Serial No. 2003-85069, the contents of which are hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the invention  
      The present invention relates to an optical fiber cable, and more particularly to an optical fiber cable, installed using air pressure, and including an external sheath having a multi-layer structure.  
      2. Description of the Related Art  
      Conventional optical fibers includes a core made of glass material or plastic material having a high refractive index at a center portion thereof, a clad surrounding an outer surface of the core and having a refractive index lower than that of the core, and a coating layer coated on an outer surface of the clad in order to protect the core and the clad.  
      Such optical fibers provide an optical signal transmission medium capable of simultaneously transmitting a plurality of optical signals through one circuit. Such optical fibers can also be adapted digital data transmission. However, these optical fibers have low tensile strength due to a material characteristic thereof, so that they are easily damaged under certain external circumstances.  
      One attempt to solve the above-mentioned problems involves binding a plurality of optical fibers with tensile members. Such an optical fiber cable can be installed and can simultaneously provide a plurality of circuits. Generally, such an optical fiber cable may be installed through an air-blown installation method, which installs the optical fiber cable by blowing air into a buried duct.  
      The above-mentioned optical fiber cable is formed at an external sheath with a plurality of recesses having a crater shape in order to easily perform the air-blown installation. The above-mentioned recesses have a crater shape that are formed by spraying pressurized water towards the external sheath, which is being extruded.  
      However, it is difficult to control water pressure to be sprayed, and the recesses are irregularly formed as water flows through the external sheath. In addition, recesses having irregular depths and diameters cause deviation of air distribution applied to the optical fiber cable during the air-blown installation. This causes the optical fiber cable to be bent or damaged during the air-blown installation.  
     SUMMARY OF THE INVENTION  
      One object of the present invention is to reduce the above-mentioned problems occurring in the prior art.  
      Another object of the present invention is to provide an optical fiber cable including an external sheath capable of reducing physical impact caused by irregularly formed recesses and having a plurality of recesses with uniform depth and diameter.  
      One embodiment of the present invention is directed to an optical fiber cable including at least one optical fiber having a core, which is an optical signal transmission medium, a clad surrounding an outer peripheral surface of the core, and a coating layer surrounding an outer peripheral surface of the clad. The optical fiber cable also includes an external sheath including a first layer having a predetermined thickness and a second layer made of polymer material and formed at an outer peripheral surface of the first layer through an extruding process, a plurality of recesses having a predetermined depth and a predetermined diameter being formed on the second layer; and a waterproof member accommodated between the external sheath and the optical fibers.  
      Another embodiment of the present invention is directed to a method for fabricating an optical fiber cable having at least one optical fiber. The method includes the step of binding the optical fibers by using a waterproof member, and forming an external sheath including a first layer having a predetermined thickness and formed at an outer surface of the waterproof member, which binds the optical fibers together, and a second layer formed onto an outer surface of the first layer in such a manner that a plurality of recesses are formed in the second layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above object, features and embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a diagram showing an optical fiber cable including an external sheath having a multi-layer structure according to a first embodiment of the present invention;  
       FIG. 2  is a side sectional view of the external sheath shown in  FIG. 1 ;  
       FIG. 3  is a graph showing a pressure difference between a conventional optical fiber cable including an external sheath and an optical fiber cable of the present invention as a function of a length of a duct when air pressure is applied to the optical fiber cable to install the optical fiber cable in the duct; and  
       FIG. 4  is a diagram showing an optical fiber cable including an external sheath structure having a multi-layer structure formed with a ribbon optical fiber according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. In the following description and drawings, the same reference numerals are used to designate the same or similar components, and so repetition of the description on the same or similar components will be omitted.  
       FIG. 1  is a diagram showing an optical fiber cable  100  including an external sheath having a multi-layer structure including a second layer formed with a crater according to a first embodiment of the present invention. The optical fiber cable  100  includes at least one optical fiber  110 , an external sheath  130  binding optical fibers  110  together, and a waterproof member  120  for preventing moisture from penetrating into the optical fibers  110 .  
      Each of the optical fibers  110  includes a core  111 , which is an optical signal transmission medium, a clad  112  surrounding an outer surface of the core  111 , and a coating layer  113  coated at an outer surface of the clad  112 . In addition, each of optical fibers  110  further includes a colored layer for identifying the optical fibers by coating a specific color on the outer surface of the coating layer  113 .  
      The core  111  includes a material capable of transmitting the optical signal. The core  111  is surrounded by the clad  112 , which has a refractive index lower than that of the core  111 , so that optical signals incident on the inside of the core  111  is reflected from the clad  112  so that the optical signals proceed to the interior of the core  111 .  
       FIG. 2  is a side sectional view showing the external sheath shown in  FIG. 1 . Referring to  FIG. 2 , the external sheath  130  includes a first layer  131  having a uniform thickness and a second layer  132  formed at an outer peripheral surface of the first layer  131 . The second layer  132  may be formed through an extruding process and having a plurality of recesses  132   a  with a predetermined depth and a predetermined diameter. The external sheath  130  binds optical fibers  110  and the waterproof member  120  together.  
      The first layer  131  is formed with a uniform thickness. The second layer  132  may be made of polymer material and formed through the extruding process in such a manner that the outer peripheral surface of the first layer  131  is surrounded by the second layer  132 . The second layer  132  may include polymer material, such as foam polyethylene, polyolefin, PVC, and urethane. In addition, due to characteristic of such polymer materials, recesses  132   a  in the form of craters having a depth and a diameter in a range of 10˜500 μm are formed when performing the extruding process. Particularly, polymer can be formed with the recesses having the uniform depth and the uniform diameter by carrying out extruding process with respect to polymer under a relatively higher temperature condition, which may be in the range of 10 to 50° C. higher than a general extruding temperature which does not make the recesses in polymer.  
      The waterproof member  120  may be accommodated between the first layer  131  of the external sheath  130  and the optical fibers  110  to prevent moisture from penetrating into optical fibers  110  and to reduce physical impact. The waterproof member  120  may include yarn or similar substance.  
      Since the optical fiber cable  100  according to this embodiment of the present invention includes the external sheath  130  having the multi-layer structure (e.g., made of the polymer material). This can allow the optical fiber cable  100  to reduce the physical impact causing a deformation of the optical fiber cable  100 , such as bending, generated when installing the optical fiber cable  100 . The optical fiber cable  100  can form recesses  132   a  having the uniform depth and the uniform diameter on the external sheath  130  when extruding the external sheath  130  according to the temperature applied to polymer. One aspect of the above-mentioned recesses  132   a  allows the optical fiber cable  100  to be installed through an air-blown installation.  
       FIG. 4  is a diagram showing an optical fiber cable  200  including an external sheath structure having a multi-layer structure formed with a ribbon optical fiber according to a second embodiment of the present invention. The optical fiber cable  200  includes at least one ribbon optical fiber  210 , an external sheath  240  for binding the ribbon optical fibers  210  together, and a waterproof member  250  formed between the external sheath  240  and the ribbon optical fibers  210 .  
      Each of ribbon optical fibers  210  includes a plurality of optical fibers  230  aligned in line with each other and a ribbon coating layer  220  surrounding an outer surfaces of the optical fibers  230  in order to bind optical fibers  230  together. Also, each of the optical fibers  230  includes a core  231 , which is an optical signal transmission medium, a clad  232  surrounding the core  231 , and a coating layer  233  surrounding an outer surface of the clad  232 .  
      Predetermined colors may be coated to the ribbon coating layer  220  in order to identify each of the optical fibers  230 . The ribbon coating layer  220  includes an ultraviolet curing agent, and the ribbon optical fibers  210  can be accommodated in the optical fiber cable  200  in the form of a ribbon optical fiber bundle including a plurality of stacked optical fibers.  
      The external sheath  240  includes a first layer  241  binding the ribbon optical fibers  210  together and having a uniform thickness, and a second layer  242  made of a polymer material and formed at the outer surface of the first layer  241  through an extruding process. In addition, recesses  242   a  having uniform sizes are formed on the second layer  242 .  
      The first layer  241  has the uniform thickness. The second layer  242  may be formed on the outer surface of the first layer  241  by extruding polymer, such as foam polyethylene, polyolefin and PVC, on the outer surface of the first layer  241  under a relatively higher temperature condition, which may be in the range of 10 to 50° C. higher than a general extruding temperature, so that a plurality of recesses  242  having uniform diameter and depth are formed in the second layer  222 .  
      The general extruding temperature signifies a temperature, which does not make bubbles in polymer. Since polymer is subject to the extruding process at a temperature above the general extruding temperature, the recesses having the uniform size can be formed in the second layer  242 . The size of the recesses  242   a  formed in the second layer  242  can be adjusted by controlling the extruding temperature.  
      For instance, PVC has the general extruding temperature in a range of 150 to 190° C., polyethylene has the general extruding temperature in a range of 180˜230° C., and elastomer based material, such as nylon, PBT and polyester, has the general extruding temperature in a range of 185˜270° C.  
      Such general extruding temperatures can be varied according to materials of the external sheath, a length of the optical fiber cable to be fabricated, working environment, or working devices, such as an extruder.  
      In addition to the optical fiber cable having the above-mentioned structure, an optical fiber cable having a loose tube structure including a plurality of loose tubes for binding a plurality of optical fibers together, and a ribbon optical fiber cable including ribbon optical fibers can be used according to other embodiments of the present invention.  
       FIG. 3  is a graph showing the pressure difference between a conventional optical fiber cable including an external sheath and an optical fiber cable according to embodiments of the present invention as a function of a length of a duct when air pressure is applied to the optical fiber cable to install the optical fiber cable in the duct. Referring to  FIG. 3 , the optical fiber cable formed at an outer surface thereof with recesses having a crater shape can be installed under air pressure less than 2,000 dyne/m even if a diameter of an installation duct for installing the optical fiber cable becomes reduced. In the conventional optical fiber cable, which is not formed with recesses having the crater shape, air pressure required for installing the optical fiber cable may suddenly increase from 2,000 dyne/m to 8,000 dyne/m when a diameter of the installation duct becomes reduced to in a range of 220˜170 mm.  
      As mentioned above, the optical fiber cable according to embodiments of the present invention includes the external sheath having the layer formed at the outer surface thereof with the recesses having the crater shape and the layer having the uniform thickness, so that the air-blown installation can be more easily carried out as compared to conventional methods. This should prevent the optical fiber cable from being bent. In addition, the optical fibers or the optical fiber cables according to embodiments of the present invention can be installed with a longer installation length as compared to conventional methods when the air-blown installation for the optical fiber cables is carried out.  
      Although embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.