Abstract:
The invention relates to a dispersion flattened fiber (DFF) with high negative dispersion and a manufacturing method thereof. The dispersion flattened fiber comprises a central core; ring-type cores and low refractive regions alternately formed outside the central core; a cladding surrounding outside the ring-type cores and low refractive regions; and a coating outside the cladding. Since the dispersion flattened fiber has the dispersion of −20 to −60, it has a wide range of application and can be used for various purposes in the field of optical telecommunication.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 09/776,762, filed Feb. 6, 2001 now U.S. Pat. No. 6,650,813. The contents of this prior application is incorporated herein in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a dispersion flattened fiber (DFF) with high negative dispersion and a method for the manufacture thereof; and, more particularly, to a dispersion flattened fiber with high negative dispersion to be utilized for a dispersion compensation in a conventional single mode fiber (SMF) or a non-zero dispersion shifted fiber (NZDSF) by setting up a dispersion thereof to be high negative, i.e., ranging, e.g., from −20 to −60, not zero, at a wavelength band of 1.55 μm. 
     DESCRIPTION OF THE PRIOR ART 
     In the field of optical communications, dispersion is defined as a pulse-spreading phenomenon caused due to the fact that the wave velocity of an optical signal passing through an optical fiber varies depending on the wavelength thereof. 
     As a conventional optical fiber for transmission, there exist an SMF optimized for a 1.31 wavelength band and an NZDSF with a small dispersion for 1.55 μm wavelength band, and the like. 
     However, when the conventional SMF or NZDSF is used, a maximum transmission distance is limited as a transmission speed increases. Generally, the relationship between the transmission speed B [Gb/s] and the maximum transmission distance L is shown as follows:              L   =     104000       B   2     ×   D               Eq   .              1                                
     wherein D represents a dispersion. 
     When the SMF (whose dispersion is about 17 ps/nm/km at a wavelength of 1.55 μm) is used to transmit an optical signal at a speed of 2.5 Gb/s, a maximum transmission distance is 979 km according to Eq. 1, but when the transmission speed increases to 10 Gb/s, the maximum transmission distance is diminished to just about 60 km. When the NZDSF (a dispersion thereof is about 2 to 7 ps/nm/km) is used to transmit the optical signal with the transmission speed of 10 Gb/s, the maximum transmission distance is limited to about 148 km. In the case of adopting a wavelength division multiplexing (WDM) transmission method that features high speed and big capacity, a dispersion slope as well as the dispersion must be taken into consideration in order to estimate a maximum transmission distance. 
     Accordingly, in order to increase the maximum transmission distance at a predetermined wavelength band, it is essential to compensate not only the dispersion but also the dispersion slope. As a solution to this assignment, a dispersion compensation fiber (DCF) has been developed. Although the DCF compensates for both the dispersion and the dispersion slope simultaneously, the manufacturing process thereof is too complicated. 
     Up to now, researches in the DCF have been mainly focused on a method for flattening the dispersion to be nearly zero at a wavelength band of 1.55 μm. 
     When the SMF is employed to transmit an optical signal at a transmission speed of more than 10 Gbps, both the dispersion and dispersion slope, which limit directly the maximum transmission distance, must be compensated. This can be achieved by employing a DCF that compensates both the dispersion and the dispersion slope at the same time. However, a manufacture of the DCF has not been easy. 
     A variety of methods for compensating a dispersion of an optical fiber by using a dispersion compensation module, which comprises DCFs, have been developed. Since it is not easy to produce a DCF capable of simultaneously compensating both the dispersion and the dispersion slope, an alternative method using two separate DCFs for exact dispersion compensation has also been developed as follows: 
     
       
           L   DCF1   ×D   DCF1   +L   DCF2   ×D   DCF2   +L   SMF   ×D   SMF =0  Eq.2 
       
     
     
       
         
           
             
               
                 
                   
                     
                       S 
                       SMF 
                     
                     
                       D 
                       SMF 
                     
                   
                   = 
                   
                     
                       
                         
                           L 
                           DCF1 
                         
                         × 
                         
                           S 
                           DCF1 
                         
                       
                       + 
                       
                         
                           L 
                           DCF2 
                         
                         × 
                         
                           S 
                           DCF2 
                         
                       
                     
                     
                       
                         
                           L 
                           DCF1 
                         
                         × 
                         
                           D 
                           DCF1 
                         
                       
                       + 
                       
                         
                           L 
                           DCF2 
                         
                         × 
                         
                           D 
                           DCF2 
                         
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                    
                   3 
                 
               
             
           
         
                 
         
             
         
      
     
     wherein L DCF1 , L DCF2  and L SMF  represent the maximum transmission distance of a first DCF, a second DCF and a SMF, respectively; D DCF1 , D DCF2  and D SMF  stand for the dispersion of the first DCF, the second DCF and the SMF, respectively; and S DCF1 , S DCF2  and S SMF  represent the dispersion slope of the first DCF, the second DCF and the SMF, respectively. 
     In the case of using two different DCFs, it is required to combine the dispersions and the dispersion slopes of the two DCFs, so that the exact compensation for the dispersion becomes more difficult. 
     SUMMARY OF THE INVENTION 
     It is, therefore, the object of the present invention to provide a dispersion flattened fiber having high negative dispersion as well as flat dispersion characteristic at a transmission wavelength band so as to compensate the dispersion with advanced facility and exactness, and also provide a manufacturing method of such dispersion flattened fiber. 
     In accordance with a preferred embodiment of the present invention, there is provided a dispersion flattened fiber with high negative dispersion comprising: 
     a central core; 
     ring-type cores and low refractive regions alternately formed outside the central core; 
     a cladding formed surrounding the ring-type cores and the low refractive regions; and 
     a coating formed outside the cladding so as to protect the central core, the ring-typed cores, the low refractive regions and the cladding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given with reference to the accompanying drawings in which: 
     FIG. 1 represents a cross sectional view of a dispersion flattened fiber with high negative dispersion in accordance with a first embodiment of the present invention; 
     FIG. 2 is a schematic drawing showing a refractive index of the optical fiber of FIG. 1 along its radius; 
     FIG. 3 is a graph showing a C-band characteristic of the optical fiber in FIG. 1; 
     FIG. 4 presents a graph illustrating an L-band characteristic of the optical fiber as shown in FIG. 1; 
     FIG. 5 depicts a table describing a design and characteristics of the optical fiber shown in FIG. 1; and 
     FIG. 6 sets forth a flow chart of a manufacturing method for the dispersion flattened fiber with high negative dispersion in accordance with the first embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a cross sectional view for showing a structure of a dispersion flattened fiber with high negative dispersion in accordance with a first embodiment of the present invention. The dispersion flattened fiber comprises a cladding  15 , a first and a second ring-type core  14  and  12 , a first and a second low refractive region  13  and  11 , and a central core  10 . 
     At the outmost region of the dispersion flattened fiber, there is formed a polymer coating (not shown) to protect the dispersion flattened fiber. There is located the cladding  15  inside the polymer coating and the first ring-type core  14  in accordance with the present invention within the cladding  15 . The first low refractive region  13  is formed at the inner region of the first ring-type core  14 . Inside the first low refractive region  13  there is located the second ring-type core  12  and, the second low refractive region  11  is formed within the second ring-type core  12 . Finally, at the central area within the second low refractive region  11 , there is formed the central core  10 . 
     The refractive indexes of the central core  10  and the second ring-type core  12  are higher than those of the other regions. The refractive index of the cladding  15  is equal to that of pure silica. The first and the second low refractive region  13  and  11  have lower refractive indexes than the cladding  15 . The refractive index of the second ring-type core  12  is the same as that of the first ring-type core  14 . The second low refractive region  11  has the same refractive index as the first low refractive region  13 . Ge or P may be added to increase the refractive index of the central core  10  and the first and the second ring-type core  12  and  14 . 
     FIG. 2 is a schematic drawing for showing a refractive index profile along the radius of the fiber, in which the central core  10  has the highest refractive index and the first and the second ring-type core  12  and  14  have lower refractive indexes than the central core  10 . Although a step-type refractive index profile has been used in FIG. 2, a hill-type or curved refractive index profile can be included as well. 
     FIG. 5 is a table for showing design data and characteristics of the optical fiber shown in FIG. 1, wherein variation of the refractive index along the radius of the optical fiber is shown. FIG. 3 shows the C band (1.55 μm wavelength band) dispersion characteristic of the optical fiber having the features described in FIG.  5 . 
     Small changes in the diameters of the first low refractive region  13  and the first ring-type core  14  do not influence much on the dispersion and the dispersion flattened characteristics of the optical fiber. Unlike most of the conventional optical fibers, the dispersion flattened fiber of the present invention has a much better bend loss characteristic, e.g., about 0.0001102 dB/km. Further, the dispersion slope of the present invention is flatter than that of the conventional dispersion flattened fibers. 
     FIG. 4 is a graph presenting an L-band (1570 to 1620 nm) dispersion characteristic of the optical fiber as shown in FIG.  1 . The L-band is required for the high-density wavelength division multiplexing mode. 
     FIG. 6 is a flow chart illustrating a step-by-step process for manufacturing the dispersion flattened fiber with high negative dispersion through a modified chemical vapor deposition in accordance with the first embodiment of the present invention. 
     First, in step S 2  silica tubes are arranged exactly on a MCVD board. 
     The silica tubes are heated in step S 4  by an oxygen/hydrogen burner at a temperature of 1900° C. to get rid of any impurities inside and outside the silica tubes. 
     In step S 6 , the cladding  15  is formed to prevent an invasion of OH radicals by using SiCl 4  to make the refractive index of the cladding  15  identical with that of the silica tubes. 
     In step S 8 , GeCl 4  or POCl 3  is used together with SiCl 4  to form the first ring-type core  14  whose refractive index is higher than that of the silica tubes within the cladding  15 . 
     In step S 10 , the first low refractive region  13  whose refractive index is lower than that of the silica tubes is formed inside the first ring-type core  14  by keeping fluorine source, e.g., C 2 F 6  or SiF 4 , flowing into the silica tubes together with SiCl 4 . 
     The second ring-type core  12  having a higher refractive index than that of the silica tube is formed within the first low refractive region  13  by using GeCl 4  or POCl 3  gas together with SiCl 4  in step S 12 . 
     In step S 14 , the second low refractive region  11  having a lower refractive index than that of the silica tube is formed within the second ring-type core  12  by having fluorine gas C 2 F 6  or SiF 4  together with SiCl 4  flow into the silica tube. 
     The central core  10  with the highest refractive index is formed within the second low refractive region  11  by providing both SiCl 4  and GeCl 4  into the silica tube and heating them by the burner in step S 16 . 
     A preform of the optical fiber having the refractive index profile given in accordance with the present invention is manufactured in step S 18  by heating the silica tube using an oxygen/hydrogen burner under high temperature of 2000° C. or beyond to completely infill remaining holes within the silica tube. 
     Over-cladding or jacketing process can be carried out in step S 20  if required, where a silica tube is jacketed on the preform of the optical fiber. 
     From the preform of the optical fiber manufactured as recited above, optical fiber of 125 μm in diameter may be extracted with an optical fiber take-out apparatus. During this process, the optical fiber goes through a first and a second coating, and finally gets the optical fiber of 250 μm in diameter in step S 22 . 
     In view of the foregoing, the dispersion flattened fiber of the present invention has a negative dispersion ranging from −20 to −60 at the wavelength band of about 1.55 μm and also has a dispersion slope much flatter than those of conventional dispersion flattened fibers. In addition, the dispersion flattened fiber can be easily manufactured because of its high flexibility on the diameter. 
     While the present invention has been described with respect to the particular preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.