Abstract:
A wide band dispersion-controlled fiber which comprises a core forming an optical signal transmission path and having a peak refractive index, and a cladding surrounding the core and having a peak refractive index lower than the peak refractive index of the core. The wide band dispersion-controlled fiber further comprises at least one dispersion control layer arranged between the core and the cladding and having a refractive index profile such that its refractive index increases from an inner periphery to an outer periphery. The minimum refractive index of the dispersion control layer is less than the peak refractive indices of the core and cladding.

Description:
PRIORITY 
   This application claims priority to an application entitled “WIDE BAND DISPERSION-CONTROLLED FIBER”, filed in the Korean Industrial Property Office on Nov. 30, 2001 and assigned Serial No. 2001-75152, 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. More particularly, the present invention relates to a dispersion-controlled fiber. 
   2. Description of the Related Art 
   In general, the dispersion characteristics of an optical fiber can be effectively controlled by positioning a region of a depressed refractive index between a core and a cladding of the fiber. This is disclosed in U.S. Pat. No. 4,715,679 (title: “LOW DISPERSION, LOW-LOSS SINGLE-MODE OPTICAL WAVEGUIDE”) invented by and issued to Venkata A. Bhagavatula, the contents of which are incorporated by reference as background material. 
     FIG. 1  is a graph illustrating prior art dispersion characteristics of a single-mode fiber (SMF). In this illustration, a dispersion curve  110  for the SMF is shown. The SMF has a step-index profile because there is no region having a depressed refractive index. As seen from the dispersion curve  110 , the SMF has a unit dispersion value of about 17 ps/nm/km at a wavelength of 1550 nm. If the SMF is used for a long distance transmission, an accumulated dispersion of an optical signal received through the SMF is increased and, as a result, a distortion of the optical signal becomes more severe. There are various dispersion compensation techniques in the prior art for minimizing the accumulated dispersion occurring during the long distance transmission of the optical signal. Generally, a method of using a dispersion-controlled fiber has been widely employed to minimize the accumulated dispersion. 
   Dispersion-controlled fiber has a high negative dispersion value because of a depressed refractive index region surrounding its core. Further, the dispersion-controlled fiber can be connected to one end of the SMF to compensate for the accumulated dispersion of the SMF. The dispersion-controlled fiber has a high negative unit dispersion value at a wavelength of 1550 nm and its length may be adjusted to offset the accumulated dispersion of the SMF, so that the total dispersion becomes zero. 
   However, if the dispersion-controlled fiber is adapted for dispersion compensation of the SMF, a sum of an accumulated dispersion of the dispersion-controlled fiber and the accumulated dispersion of the SMF may not be zero at wavelengths other than 1550 nm. In this regard, there is a problem in which it is not appropriate to apply the dispersion-controlled fiber to a wavelength division multiplexing system. 
   In order to overcome the above problem, research has recently been done to provide a fiber capable of compensating for both a dispersion and a dispersion slope together. To compensate for both the dispersion and dispersion slope, it is required to let a dispersion value and dispersion slope of the SMF be D SMF  and DS SMF  and those of the dispersion-controlled fiber be D DCF  and DS DCF , respectively, such that the D DCF  and DS DCF  satisfy the following equation 1.
 
D SMF :DS SMF ≅D DCF :DS DCF   [Equation 1]
 
   If the dispersion and dispersion slope (D DCF  and DS DCF ) of the dispersion-controlled fiber satisfy equation 1, compensation for the accumulated dispersion of the SMF occurs not only at a wavelength of 1550 nm, but also at wavelengths other than 1550 nm. However, there is a great deal of difficulty implementing a fiber that perfectly satisfies equation 1 over the entire wavelength range. For this reason, the current state of the art simply compensates for the dispersion and dispersion slope at C-band wavelengths of 1530-1570 nm. In a wide band wavelength division multiplexing system, there is a need to perform the dispersion and dispersion slope compensations at any wavelength in a range of wavelengths including an S-band of 1450-1530 nm and L-band of 1570-1610 nm as well as the C-band. 
   SUMMARY OF THE INVENTION 
   Therefore, the present invention provides a dispersion-controlled fiber applicable to a wide band wavelength division multiplexing system, with such a wide band wavelength being heretofore unknown in the art. 
   In accordance with the present invention, the above and other objects can be accomplished by providing a wide band dispersion-controlled fiber comprising a core forming an optical signal transmission path and having a peak refractive index, and a clad surrounding the core and having a peak refractive index lower than the peak refractive index of the core, further comprising at least one dispersion control layer arranged between the core and the cladding and having a refractive index profile such that its refractive index is increased from an inner periphery of the dispersion control layer having a minimum refractive index lower than the peak refractive indices of the core and cladding to its outer periphery. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, 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 graph illustrating conventional dispersion characteristics of a single-mode fiber; 
       FIG. 2  is a view showing a structure and refractive index profile of a wide band dispersion-controlled fiber in accordance with a first embodiment of the present invention; 
       FIG. 3  is a view showing a structure and refractive index profile of a wide band dispersion-controlled fiber in accordance with a second embodiment of the present invention; 
       FIG. 4  is a view showing a structure and refractive index profile of a wide band dispersion-controlled fiber in accordance with a third embodiment of the present invention; 
       FIG. 5  is a view illustrating a function of the wide band dispersion-controlled fiber in  FIG. 2 ; 
       FIG. 6  is a graph illustrating dispersion characteristics of the wide band dispersion-controlled fiber in  FIG. 2 ; 
       FIG. 7  is a graph illustrating an example of compensating for a dispersion of a single-mode fiber using the wide band dispersion-controlled fiber in  FIG. 2 ; and 
       FIG. 8  is a view illustrating a process of manufacturing a preform of the wide band dispersion-controlled fiber in FIG.  2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Now, preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. In the following description, a variety of specific elements such as constituent elements are described. The description of such elements has been made only for a better understanding of the present invention. Those skilled in the art will appreciate that various modifications, additions, and substitutions to the specific elements are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 
     FIG. 2  illustrates a structure and a respective refractive index profile of a wide band dispersion-controlled fiber in accordance with a first embodiment of the present invention. As shown in this drawing, the wide band dispersion-controlled fiber  200  has a core  210 , a dispersion-controlled layer  220  and cladding  230 . 
   The core  210  is arranged in the center of the wide band dispersion-controlled fiber  200  and has a radius of A 1  and a refractive index of N 1 . The core  210  is bar-shaped and has a dispersion profile is set to a constant value N 1 . A general formula for the refractive index profile is expressed as in the following equation 2. 
               N   ⁡     (   R   )       =         N   1     ⁡     [     1   -     2   ⁢         Δ   1     ⁡     (     R   A     )         α   1           ]         1   /   2               [     Equation   ⁢           ⁢   2     ]             
         where, R(≦A) is a diametrical distance, A(≦A 1 ) a diametrical distance to a certain point within the core  210 , N(R) a refractive index according to the R, N 1  a peak refractive index of the core  210 , Δ 1  a first refractive index difference and α 1 (0&lt;α 1 ≦∞) a first shape index determining a shape of the refractive index profile. Further, the first refractive index difference can be expressed as in the following equation 3. 
               Δ   1     =             N   1   2     -     N   2   2       )       2   ⁢     N   1   2         ≈       (       N   1     -     N   2       )       N   1                 [     Equation   ⁢           ⁢   3     ]             
   where, N 2  is a peak refractive index of the cladding  230 .       

   If necessary, the N 2  in the equation 3 can be substituted for any value less than the peak refractive index N 1  of the core  210  and more than a minimum refractive index N 4  of the dispersion-controlled layer  220 . 
   The dispersion-controlled layer  220  is arranged between the core  210  and cladding  230  and has an inner radius A 1 , an outer radius A 3 , peak refractive index N 3  and the minimum refractive index N 4 . The dispersion-controlled layer  220  further is tubeshaped and has a refractive index that increases linearly from its inner periphery to its outer periphery. A refractive index profile of the dispersion-controlled layer  220  can be expressed as the following equation 4. 
               N   ⁡     (   R   )       =         N   4     ⁡     [     1   -     2   ⁢         Δ   2     ⁡     (     R   A     )         α   2           ]         1   /   2               [     Equation   ⁢           ⁢   4     ]             
         where, the A(A 1 ≦A≦A 2 ) is a diametrical distance to any point in the dispersion-controlled layer  220 , R(A 1 ≦R≦A) a diametrical distance, N 4  the minimum refractive index of the dispersion-controlled layer  220 , Δ 2  a second refractive index difference, α 2 (0&lt;α 2 ≦∞) a second shape index determining a shape of the refractive index profile. Further, the second refractive index difference can be expressed by the following equation 5. 
               Δ   2     =         (       N   4   2     -     N   3   2       )       2   ⁢     N   4   2         ≈       (       N   4     -     N   3       )       N   4                 [     Equation   ⁢           ⁢   5     ]             
   where, N 3  is a peak refractive index of the dispersion-controlled layer  220 .       

   The cladding  230  is arranged outside of the wide band dispersion-controlled fiber  200  and has a radius of A 3  and refractive index of N 2 . 
   If necessary, the dispersion-controlled layer, according to the present invention, can be implemented in various shapes. This variety of the implemented shapes will be described below with second and third embodiments of the present invention. 
     FIG. 3  illustrates a structure and a respective refractive index profile of a wide band dispersion-controlled fiber in accordance with the second embodiment of the present invention. As shown in this drawing, the wide band dispersion-controlled fiber  300  has a core  310 , dispersion-controlled layer  320  and cladding  330 . 
   The core  310  is arranged in the center of the wide band dispersion-controlled fiber  300  and has a radius of A 1  and a refractive index of N 1 . The core  310  is bar-shaped and has a dispersion profile that is set to a constant value N 1 . 
   The dispersion-controlled layer  320  is arranged between the core  310  and cladding  330  and has an inner radius A 1 , outer radius A 3 , peak refractive index N 3  and minimum refractive index N 4 . The dispersion-controlled layer  320  further has a tube shape and its refractive index increases curvilinearly from the inner radius to the outer radius. 
   The cladding  330  is arranged outside of the wide band dispersion-controlled fiber  300  and has a radius of A 3  and refractive index of N 2 . 
     FIG. 4  illustrates a structure and a respective refractive index profile of a wide band dispersion-controlled fiber in accordance with the third embodiment of the present invention. As shown in this drawing, the wide band dispersion-controlled fiber  400  has a core  410 , dispersion-controlled layer  420  and cladding  330 . 
   The core  410  is arranged in the center of the wide band dispersion-controlled fiber  400  and has a radius of A 1  and a refractive index of N 1 . The core  410  further is bar-shaped and its dispersion profile is set to a constant value N 1 . 
   The dispersion-controlled layer  420  is arranged between the core  410  and cladding  430  and has an inner radius A 1 , an outer radius A 3 , a peak refractive index N 3  and a minimum refractive index N 4 . The dispersion-controlled layer  420  further has a tube shape and its refractive index increases step-wise from its inner periphery to its outer periphery. 
   The cladding  430  is arranged outside of the wide band dispersion-controlled fiber  400  and has a radius of A 3  and a refractive index of N 2 . 
     FIG. 5  illustrates a function of the wide band dispersion-controlled fiber  200  shown in FIG.  2 . This drawing shows intensity curves  510  and  520  for optical signals of shorter and longer wavelengths, which travel through the dispersion-controlled fiber  200 . Namely, the curves  510  and  520  represent optical signal intensity profiles corresponding to a certain cross section of the wide band dispersion-controlled fiber  200 . 
   As seen from the intensity curve  510  for the shorter wavelength optical signal, a peak intensity point of the curve  510  is almost identical to the center of the core  210  and the intensity profile is concentrated at a core position. In other words, where the shorter wavelength optical signal travels through the wide band dispersion-controlled fiber  200 , the amount of this optical signal which penetrates into the dispersion-controlled layer  220  is relatively small and most of the optical signal travels in the core  210 . As a result, the dispersion-controlled layer  220  has a relatively small effect on the shorter wavelength optical signal, in connection with dispersion. 
   As seen from the intensity curve  520  for the longer wavelength optical signal, a peak intensity point of the curve  510  is almost identical to the center of the core  210  and the intensity profile is dispersed over positions of the core  210  and dispersion-controlled layer  220 . In other words, the longer wavelength optical signal penetrates into the dispersion-controlled layer  220  in a relatively great amount as it travels through the wide band dispersion-controlled fiber  200  and a considerable part of the optical signal travels through the dispersion-controlled layer  220 . As a result, the dispersion-controlled layer  220  has a relatively great effect on the longer wavelength optical signal, in connection with dispersion. 
   As a dispersion-characteristic control for the longer wavelength optical signal is made possible, it is possible to control the dispersion curves, according to wavelengths, for the wide band dispersion-controlled fiber  200 . This control process will be described step by step below. 
   Firstly, a dispersion curve by wavelengths of a longer wavelength band is set through controlling respective refractive index profiles of the core  210  and dispersion control layer  220  under the condition that a refractive index profile of the cladding  230  is set to a constant value. 
   Secondly, a dispersion curve by wavelengths of a shorter wavelength band is set through controlling a slope of a refractive index profile of the dispersion control layer  220 . 
     FIG. 6  is a graph illustrating dispersion characteristics of the wide band dispersion-controlled fiber in FIG.  2 . This drawing shows a first dispersion curve  610  when the difference between the peak refractive index N 3  and the minimum refractive index N 4  is zero, a second dispersion curve  620  when the difference is 0.0005, a third dispersion curve  630  when the difference is 0.001 and a fourth dispersion curve  640  when the difference is 0.0015. 
   The first to fourth dispersion curves  610 , 620 , 630  and  640  are so similar to each other that it is difficult to distinguish any one of them from the others in a shorter wavelength band. On the other hand, there is an apparent difference between those dispersion curves in a longer wavelength band, or at wavelengths of 1500 nm or more. 
   Referring to  FIG. 7 , a description will be given regarding a method for compensating for a dispersion and a dispersion slope of a single-mode fiber by controlling respective refractive indexes of the core  210  and dispersion control layer  220  of the wide band dispersion-controlled fiber  200  shown in FIG.  2 .  FIG. 7  shows a dispersion curve  710  of the single-mode fiber, a dispersion curve  720  of the wide band dispersion-controlled fiber  200  whose dispersion control layer  220  is controlled to adjust its dispersion slope, and a dispersion curve  730  representative of the total dispersion when the single-mode fiber and wide band dispersion-controlled fiber  200  are interconnected at a length ratio of 1:1. As seen from the total dispersion curve  730 , the dispersion compensation can be accomplished for a wavelength region including an S-band and L-band as well as a C-band using the wide band dispersion-controlled fiber  200 . 
   As shown in  FIGS. 6 and 7 , by adjusting the dispersion slope of the dispersion control layer  220 , the dispersion and dispersion slope of the dispersion-controlled fiber  200  are adjusted such that the dispersion-controlled fiber  200  has a negative dispersion value, thereby being capable of compensating for the dispersion of the single-mode fiber with the negative dispersion value over a wide band including the S-band, C-band and L-band. 
   With reference to  FIG. 8 , a description will be given regarding a method for manufacturing a pre-form of the wide band dispersion-controlled fiber in FIG.  2 . The fiber pre-form manufacturing method may be MCVD (Modified Chemical Vapor Deposition), VAD (Vapor Phase Axial Deposition), OVD (Outside Vapor Phase Deposition), or so forth. Here, a method for manufacturing the fiber pre-form using the MCVD is described. Because the MCVD is a known art, only condensing and collapsing processes are described. 
   A pre-form manufacturing apparatus comprises a raw material gas supplier  820 , a shelf  850  and an oxygen/hydrogen burner  860 . 
   The raw material gas supplier  820  acts to mix oxygen and a plurality of additives and supplies oxygen and raw material gas, such as SiCl 4 , GeCl 4 , POCl 3 , CF 4 , SiF 4  and so forth, to an inner part of a tube  810 . The GeCl 4  and POCl 3  are used for raising a refractive index of a deposition region and the CF 4 , and SiF 4  for reducing the refractive index of the deposition region. The raw material gas supplier  820  appropriately adjusts amounts of oxygen and raw material gas flowing to the tube  810  to obtain the refractive index profile as shown in FIG.  2 . For example, in the case where the dispersion control layer  220  is deposited, as the deposition process is repeatedly performed, the raw material gas supplier  820  adjusts the ratio of CF 4  or SiF 4 , supplied to the deposition tube  810 , to the mixture of oxygen, SiCl 4 , GeCl 4 , and POCl 3  to generate a desired slope of the refractive index. In the case where the core  210  is deposited, as the deposition process is repeatedly performed, the raw material gas supplier  820  adjusts the ratio of GeCl 4 , supplied to the deposition tube  810 , to the mixture of oxygen and SiCl 4  to generate a change in the refractive index. 
   The shelf  850  has a pair of chucks  832  and  836  and a guide  840 . The deposition tube  840  is rotatably fixed between the pair of chucks  832  and  836 . The guide  840  is movably mounted onto the oxygen/hydrogen burner  860 . 
   The oxygen/hydrogen burner  860  is supplied with oxygen and hydrogen to apply heat to a periphery of the deposition tube  840  while moving along the guide  840  at a constant rate. As a result, a high temperature region is formed at the inner part of the deposition tube  840  and the formed raw material gas passes through the high temperature region to generate a reactant. An associated reaction formula may be expressed by, for example, SiCl 4 +O 2 →SiO 2 +2Cl 2  and GeCl 4 +O 2 →GeO 2 +2Cl 2 . By means of a thermophoretic mechanism, the reactant moves to an inner wall of the deposition tube  810 , which is at a relatively low temperature, and is then deposited on the inner wall of the deposition tube  810 . 
   Although one dispersion control layer is provided in the dispersion-controlled fiber in the preferred embodiments of the present invention, multiple dispersion control layers can be arranged between the core and the cladding of the dispersion-controlled fiber if necessary. An intensity profile dispersion of an optical fiber varies with a wavelength from a shorter wavelength to a longer wavelength. In this regard, the multiple dispersion control layers can be employed when there is a need for a finer control of dispersion characteristic-by-wavelength of the wide band dispersion-controlled fiber. 
   As apparent from the above description, it is possible to control dispersion characteristics of the wide band dispersion-controlled fiber according to the present invention for a longer wavelength band using the refractive index profile of the dispersion control layer thereof. As a result, the wide band dispersion-controlled fiber according to the present invention has an advantage in that it is applicable to a wide band wavelength division multiplexing system. 
   Although the preferred embodiments of the present invention have been disclosed 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.