Abstract:
An indoor optical fiber cable including a plurality of optical fiber elements, which are optical transmission media; sheath deployed in the outermost of the indoor optical fiber cable to envelop the optical fiber elements; and a first peripheral strength member (PSM) embedded in the sheath, wherein the indoor optical fiber cable does not include a central strength member (CSM) deployed in the center.

Description:
CLAIM OF PRIORITY 
   This application claims priority under 35 U.S.C. § 119 to an application entitled “Indoor Optical Fiber Cable,” filed in the Korean Intellectual Property Office on Jan. 4, 2005 and assigned Serial No. 2005-587, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to optical fiber cable and in particular, to an indoor optical fiber cable without a central strength member. 
   2. Description of the Related Art 
   As a result of development in the industry and growing demands for more information at higher speed, an era of fiber to the home (FTTH) in has come to play. Various methods have been used to lay an indoor optical fiber cable in a building. Among the methods, a method of directly pulling cable, connecting the indoor optical fiber cable to existing copper cable, and laying the indoor optical fiber cable by pulling the copper cable out have been used. 
   While the copper serves as a strength member (SM) in the copper cable, optical fibers cannot play a major role of the SM in the optical fiber cable, thus requiring an additional central strength member (CSM) or SM. 
     FIG. 1  is a sectional diagram of a conventional optical fiber cable  100  including a CSM  110 . As shown, the optical fiber cable  100  includes the CSM  110  deployed in the center, a plurality of optical fiber elements  120  wound in a spiral shape around the CSM  110 , sheath  140  deployed in the outermost of the optical fiber cable  100  to envelop the CSM  110  and the optical fiber elements  120 , and a SM  130  filled in a space inside the sheath  140  to surround the CSM  110  and the optical fiber elements  120 . 
   However, since the optical fiber cable  100  uses steel wire or fiberglass reinforced plastic (FRP) having a very high elastic modulus as the CSM  110 , it is difficult to bend the optical fiber cable  100  including the CSM  110 , thereby requiring a very high pull tension in the installation environment or disabling the installation of the optical fiber cable  100 . As such, it is common for the optical fiber cable not to include a CSM as an installation route of the optical fiber cable is complex and coarse. 
     FIGS. 2A and 2B  illustrate a conventional indoor optical fiber cable  200  without a CSM. In particular,  FIG. 2A  is a sectional diagram of the indoor optical fiber cable  200 , and  FIG. 2B  is a side view of the indoor optical fiber cable  200 . 
   Referring to  FIGS. 2A and 2B , the indoor optical fiber cable  200  includes a plurality of optical fiber elements  210 , which are optical transmission media, sheath  230  deployed in the outermost of the indoor optical fiber cable  200  to envelop the optical fiber elements  210 , and a peripheral strength member (PSM)  220  filled in a space inside the sheath  230  to surround the optical fiber elements  210 . 
   However, the sheath  230  of the indoor optical fiber cable  200  may be easily stretched when a strong pull tension is applied during the installation or when the indoor optical fiber cable  200  is stuck in an installation route. 
     FIGS. 3A and 3B  illustrate a stretched state of the indoor optical fiber cable  200 . In particular,  FIG. 3A  is a sectional diagram of the stretched indoor optical fiber cable  200 , and  FIG. 3B  is a side view of the stretched indoor optical fiber cable  200 . 
   Referring to  FIGS. 3A and 3B , when the sheath  230  is stretched, an inside diameter of the indoor optical fiber cable  200  is reduced, thus reducing a space between the optical fiber elements  210 . At this time, since the optical fiber elements  210  is compressed by the SM  220  and the sheath  230 , an increase of an optical loss may be caused, and if there exists an additional stress from the outside, the optical loss may be more increased. 
   In addition, if the sheath  230  contracts due to a drop in temperature in a state of the stretched sheath  230 , the inside diameter of the sheath  230  is reduced further, thus increasing the optical loss of the optical fiber elements  210 . 
   As described above, for the typical indoor optical fiber cable  200  without a CSM, an information transmission characteristic can be deteriorated due to the stretch effect of the sheath  230 . 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide an indoor optical fiber cable that can guarantee a stable information transmission characteristic by making installation easy due to non-use of a central strength member (CSM) inside the cable and minimizing a stretch of sheath due to the non-use of the CSM. 
   In one embodiment, there is provided an indoor optical fiber cable comprising: a plurality of optical fiber elements, each of which serves as an optical transmission medium; sheath deployed in the outermost of the indoor optical fiber cable to envelop the optical fiber elements; and a first peripheral strength member (PSM) embedded in the sheath, wherein the indoor optical fiber cable does not include a central strength member (CSM) deployed in the center. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a sectional diagram of a typical optical fiber cable including a CSM; 
       FIGS. 2A and 2B  illustrate a typical indoor optical fiber cable without a CSM; 
       FIGS. 3A and 3B  illustrate a stretched state of the indoor optical fiber cable shown in  FIGS. 2A and 2B ; and 
       FIG. 4  is a sectional diagram of an indoor optical fiber cable without a CSM according to a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention will be described herein below with reference to the accompanying drawings. For the purposes of clarity and simplicity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail. 
     FIG. 4  is a sectional diagram of an indoor optical fiber cable  300  without a central strength member (CSM) according to an embodiment of the present invention. As shown, the indoor optical fiber cable  300  includes a plurality of optical fiber elements  310 , first and second peripheral strength members (PSMs)  340  and  320 , and sheath  330 . 
   The optical fiber elements  310  are optical transmission mediums which may be an optical fiber, a buffered optical fiber, a ribbon optical fiber or a loose tube. That is, the optical fiber element  310  is a bare optical fiber which may be: a core of a typical glass material and a clad, a resin coated bare optical fiber (this type is typically called the optical fiber), a colored optical fiber for easy identification, a plastic press coated optical fiber (this is called the buffered optical fiber), one body formed by resin-coating a plurality of optical fiber (this is called the ribbon optical fiber), or obtained by installing the colored optical fiber or the ribbon optical fiber in a jelly compound filled plastic tube (this is called the loose tube). 
   The sheath  330  envelops the optical fiber elements  310  and is deployed in the outermost part of the indoor optical fiber cable  300 . The sheath  330  may be made of a plastic material, e.g., polyethylene (PE), ethylene vinyl acetate copolymer (EVA), or polyvinyl chloride. It is preferable that an oxygen index of the sheath  330  is more than 28% to guarantee a sufficient flame retardant characteristic. The oxygen index is a non-dimensional value of a limit oxygen density in which a flammable solid can catch fire, called a limit oxygen index (LOI). The sheath  330  can contain halogen compounds, aluminum hydroxide or magnesium hydroxide to increase the oxygen index. The sheath  330  can be processed to have a broken surface to decrease its coefficient of friction (i.e., to be easily installed). 
   The first PSM  340  includes a plurality of peripheral strength units  345  embedded in the sheath  330  and deployed with a predetermined gap in the sheath  330 . It is preferable that the first PSM  340  includes at least four peripheral strength units  345  symmetrically deployed based on the center of the indoor optical fiber cable  300  to provide a sufficient tensile strength. Each peripheral strength unit  345  may be peripheral strength yarn, such as aramid yarn, glass yarn, or resin coated peripheral strength yarn. 
   The second PSM  320  is deployed in a space inside the sheath  330  to surround the optical fiber elements  310 . The second PSM  320  includes a plurality of peripheral strength units and fully fills in the space inside the sheath  330 . Each peripheral strength unit may be peripheral strength yarn such as aramid yarn, glass yarn. For the second PSM  320  to have water resistance, each peripheral strength unit also may be super absorbent powder coated aramid yarn or glass yarn, water swellable yarn, or a combination of aramid yarn. 
   The first PSM  340  prevents the sheath  330  from being stretched, and the second PSM  320  provides a tensile strength to the indoor optical fiber cable  300  with the first PSM  340  together. 
   Table 1 shows comparison results obtained by performing a tensile experiment on the sheath  330  of the indoor optical fiber cable  300  and the sheath  230  of the conventional indoor optical fiber cable  200  shown in  FIGS. 2A and 2B . The tensile experiment is performed using commercialized instron. 
   
     
       
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
               Present embodiment 
               Prior art 
             
           
        
         
             
                 
                 
               Distortion 
                 
               Distortion 
             
             
                 
               Weight (kg · f) 
               factor (%) 
               Weight (kg · f) 
               factor (%) 
             
             
                 
                 
             
           
        
         
             
               Sample 1 
               43.33 
               2.73 
               8.38 
               48.22 
             
             
               Sample 2 
               67.81 
               2.74 
               9.17 
               72.00 
             
             
               Sample 3 
               45.46 
               2.73 
               8.82 
               49.85 
             
             
               Sample 4 
               53.48 
               2.73 
               9.06 
               58.16 
             
             
               Sample 5 
               40.40 
               2.74 
               9.48 
               74.78 
             
             
               Max value 
               67.81 
               2.74 
               9.48 
               74.78 
             
             
               Min value 
               40.40 
               2.73 
               8.38 
               48.22 
             
             
               Mean value 
               50.10 
               2.73 
               8.98 
               60.60 
             
             
                 
             
           
        
       
     
   
   As shown in Table 1, while the sheath  230  of the conventional indoor optical fiber cable  200  without a CSM stretches around 50% with respect to a pull tension of around 8.5 kg, the sheath  330  of the conventional indoor optical fiber cable  300  including the first PSM  340  stretches around 3% with respect to the pull tension of around 50 kg. 
   As described above, an indoor optical fiber cable without a CSM according to an embodiment of the present invention embeds a first PSM in the sheath, thereby making installation of the cable easy and minimizing a stretch of the sheath. 
   While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.