CLAIM OF PRIORITY 
     This application claims priority to an application entitled “SMALL, LIGHTWEIGHT OPTICAL CABLE”, filed in the Korean Industrial Property Office on Mar. 4, 2002 and assigned Serial No. 2002-11282, 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 cable, and more particularly to a high-density optical cable. 
     2. Description of the Related Art 
     Currently, the demands for compact, light weight, and high-density optical cables are growing as there is a shortage of installation spaces for the new cables due to the already occupied optical cables. To address this, an attempt to reduce the outer diameter of the optical cables has been made to make possible installation of such cables in the existing narrow ducts. 
       FIG. 1  is a sectional view illustrating a conventional compact optical cable. The compact optical cable comprises multiple cores of optical fibers  110 , a tube  120  surrounding the optical core members, two pairs of core strength members  140 , and a sheath  130 . The tube  120  has the shape of a hollow cylinder and includes the multiple cores of optical fibers  110  mounted therein. The tube  120  has a thickness of about 1 mm. The sheath  130  with a predetermined thickness is formed by an extrusion process and surrounds the tube  120 . The sheath constitutes the outermost layer of the compact optical cable to protect its interior components against the external environment. 
     The two pairs of the core strength members  140  complement the mechanical weakness of the optical cables to a certain extent. However, if the external stress is severe, the strength members tend to deteriorate the mechanical properties of the optical cables. 
       FIGS. 2 and 3  are illustrates the shape of the compact optical cables shown in  FIG. 1  when an excessive stress is applied. As shown in  FIG. 2 , the external stress  150  is applied to the outer sheath layer of the cable, then, as shown in  FIG. 3 , the optical cable gets severely deformed by the stress applied thereto. The strength members  140 , which have been originally arranged inside the sheath  130 , penetrate into inside the sheath and press the tube  120  when subject to stress. Note that the tube  120  has a thickness of about 1 mm, but the strength members  140  have a higher degree of hardness than those of the tube  120  or sheath  130 . As result, the optical fibers  110  mounted inside the tube  120  are subject to severe stress due to the movement of the strength members  140 . 
     As described above, the conventional compact optical cable is provided with a plurality of strength members to increase the mechanical strength of the cable. However, the strength members tend to have opposite effect if excessive external stress is applied to the cable. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-described problems, and provides additional advantages, by providing a small, lightweight optical cable, fabricated to prevent the strength members from penetrating into the inside a sheath, thereby making it possible to minimize the deterioration of its mechanical properties upon application of excessive external stress. 
     According to one aspect of the invention, the inventive lightweight optical cable includes: a tube having a shape of a hollow cylinder including multiple cores of optical fibers mounted therein; a first sheath formed by an extrusion process to surround the tube at a predetermined thickness; a second sheath formed by an extrusion process to surround the first sheath at a predetermined thickness; and, a plurality of strength members arranged inside the second sheath. 
     According to another aspect of the present invention, the small, lightweight optical cable includes a tube having a shape of a hollow cylinder and including multiple cores of optical fibers mounted therein; a sheath formed by an extrusion process to surround the tube at a predetermined thickness; a plurality of strength members arranged inside the sheath; and, a plurality of pads intervening between the tube and the strength members, each pad being separately formed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a sectional view illustrating the configuration of a conventional small, lightweight optical cable; 
         FIGS. 2 and 3  are views for showing the small, lightweight optical cable, illustrated in  FIG. 1 , upon an application of excessive stress; 
         FIG. 4  is a view illustrating the configuration of a small, lightweight optical cable in accordance with a preferred embodiment of the present invention; and, 
         FIG. 5  is a view illustrating the configuration of a small, lightweight optical cable in accordance with an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted as it may make the subject matter of the present invention rather unclear. 
       FIG. 4  shows the configuration of a small, lightweight optical cable in accordance with a preferred embodiment of the present invention. The inventive optical cable includes a plurality of cores  210 , a tube  220  surrounding the cores, a first sheath  250 , a second sheath  230  and two pairs of core strength members  240  at both sides. The tube  220  has a shape of a hollow cylinder and has a thickness of 1 mm and below. The first sheath  250  with a predetermined thickness is formed by an extrusion process and surrounds the tube  220 , leaving a circumferential gap  235 , between the exterior surface of the tube  220  and the interior surface of the first sheath  250 . The first sheath  250  performs the function of blocking the penetration by the strength members  240  into the second sheath  230  when an external stress applied thereto. As a material for the first sheath  250 , a high-density polyethylene (HDPE) maybe used. 
     The second sheath  230  with a predetermined thickness is formed by an extrusion process to surround the first sheath  250 . The second sheath  230  constitutes the outermost layer of the inventive optical cable and protects its interior components from external environments. As a material for the second sheath  230 , a polyvinyl chloride (PVC) or polyethylene (PE) may be used. 
     The two pairs of the core strength members  240  are arranged inside the second sheath  230 , and each pair is arranged symmetrically around the tube  220 . Each of the strength members  240  has a long thread shape like the optical fibers  210 . The strength members  240  perform the function of enhancing the mechanical properties of the inventive optical cable. As a material for the strength members  240 , a fiberglass reinforced plastic (FRP) may be used. For forming the first and second sheaths  250  and  230 , a dual extrusion process may be used. 
       FIG. 5  shows the configuration of a small, lightweight optical cable in accordance with another preferred embodiment of the present invention. The inventive optical cable includes a tube  320  surrounding a plurality of optical cores  310 , two pads  350 , a sheath  330 , and two pairs of the core strength members  340 . The tube  320  has the shape of a hollow cylinder and has a thickness of 1 mm and below. It can be seen from the Figure that there is a circumferential gap  335  arranged between the outer surface of the tube  320  and the inner surface of the sheath  330 . 
     The two pads  350  are arranged symmetrically within a solid area of the sheath  330  and occupy a portion of the sheath  330  between the tube  320  and the pairs of the strength members  340 . The pads  350  perform the function of blocking penetration of the strength members  340  into the sheath  330  when an external stress is applied. As shown in  FIG. 5  the pads  350  intervene between the rube and the strength members without affecting the area of the circumferential gap. As a material for the pads  350 , a fiberglass reinforced plastic (FRP) may be used. 
     The sheath  330  with a certain thickness is formed by an extrusion process and surrounds the two pads  350  and the tube  320 . The sheath  330  constitutes the outermost layer of the inventive optical cable and protects its interior components against external environments. As a material for the sheath  330 , a polyvinyl chloride (PVC) or polyethylene (PE) may be used. 
     Two pairs of the strength members  340  are arranged inside the sheath  330 , each pair of the strength members being arranged symmetrically around the tube  320 . Each of the strength members  340  has a long thread shape similar to the optical fibers  310 . The strength members  340  serve to enhance the mechanical properties of the inventive optical cable. As a material for the strength members  340 , a fiberglass reinforced plastic (FRP) may be used. For forming the sheath  330 , a dual extrusion process may be used. 
     While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt to a particular situation and the teaching of the present invention without departing from the central scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out the present invention, but that the present invention include all embodiments falling within the scope of the appended claims.
A compact optical cable comprises a tube having the shape of a hollow cylinder and including multiple cores of optical fibers mounted therein; a sheath formed by an extrusion process to surround the tube at a certain thickness; a plurality of strength members arranged inside the sheath; and, a plurality of pads arranged inside the sheath to ensure that each pad intervenes between the tube and the strength members.