PRIORITY 0001 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 0002 1. Field of the Invention 0003 The present invention relates to an optical fiber. More particularly, the present invention relates to a dispersion-controlled fiber. 0004 2. Description of the Related Art 0005 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. 0006 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. 0007 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. 0008 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. 0009 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 DSMF and DSSMF and those of the dispersion-controlled fiber be DDCF and DSDCF, respectively, such that the DDCF and DSDCF satisfy the following equation 1.

DSMF:DSSMFDDCF:DSDCFEquation 1

0010 If the dispersion and dispersion slope (DDCF and DSDCF ) 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 0011 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. 0012 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 0013 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: 0014 FIG. 1 is a graph illustrating conventional dispersion characteristics of a single-mode fiber; 0015 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; 0016 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; 0017 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; 0018 FIG. 5 is a view illustrating a function of the wide band dispersion-controlled fiber in FIG. 2; 0019 FIG. 6 is a graph illustrating dispersion characteristics of the wide band dispersion-controlled fiber in FIG. 2; 0020 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 0021 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 0022 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. 0023 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. 0024 The core 210 is arranged in the center of the wide band dispersion-controlled fiber 200 and has a radius of A1 and a refractive index of N1. The core 210 is bar-shaped and has a dispersion profile is set to a constant value N1. A general formula for the refractive index profile is expressed as in the following equation 2. 1

\begin{matrix} N (R) = {N_{1} [1 - 2 {_{1} (\frac{R}{A})}^{_{1}}]}^{1 / 2} & [Equation 2] \end{matrix}

0025 where, R(A) is a diametrical distance, A(A1) a diametrical distance to a certain point within the core 210, N(R) a refractive index according to the R, N1 a peak refractive index of the core 210, 1 a first refractive index difference and 1(0<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. 2

\begin{matrix} _{1} = \frac{N_{1}^{2} - N_{2}^{2})}{2 N_{1}^{2}} \frac{(N_{1} - N_{2})}{N_{1}} & [Equation 3] \end{matrix}

0026 where, N2 is a peak refractive index of the cladding 230. 0027 If necessary, the N2 in the equation 3 can be substituted for any value less than the peak refractive index N1 of the core 210 and more than a minimum refractive index N4 of the dispersion-controlled layer 220. 0028 The dispersion-controlled layer 220 is arranged between the core 210 and cladding 230 and has an inner radius A1, an outer radius A3, peak refractive index N3 and the minimum refractive index N4. 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. 3

\begin{matrix} N (R) = {N_{4} [1 - 2 {_{2} (\frac{R}{A})}^{_{2}}]}^{1 / 2} & [Equation 4] \end{matrix}

0029 where, the A(A1AA2) is a diametrical distance to any point in the dispersion-controlled layer 220, R(A1RA) a diametrical distance, N4 the minimum refractive index of the dispersion-controlled layer 220, 2 a second refractive index difference, 2(0<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. 4

\begin{matrix} _{2} = \frac{(N_{4}^{2} - N_{3}^{2})}{2 N_{4}^{2}} \frac{(N_{4} - N_{3})}{N_{4}} & [Equation 5] \end{matrix}

0030 where, N3 is a peak refractive index of the dispersion-controlled layer 220. 0031 The cladding 230 is arranged outside of the wide band dispersion-controlled fiber 200 and has a radius of A3 and refractive index of N2. 0032 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. 0033 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. 0034 The core 310 is arranged in the center of the wide band dispersion-controlled fiber 300 and has a radius of A1 and a refractive index of N1. The core 310 is bar-shaped and has a dispersion profile that is set to a constant value N1. 0035 The dispersion-controlled layer 320 is arranged between the core 310 and cladding 330 and has an inner radius A1, outer radius A3, peak refractive index N3 and minimum refractive index N4. The dispersion-controlled layer 320 further has a tube shape and its refractive index increases curvilinearly from the inner radius to the outer radius. 0036 The cladding 330 is arranged outside of the wide band dispersion-controlled fiber 300 and has a radius of A3 and refractive index of N2. 0037 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. 0038 The core 410 is arranged in the center of the wide band dispersion-controlled fiber 400 and has a radius of A1 and a refractive index of N1. The core 410 further is bar-shaped and its dispersion profile is set to a constant value N1. 0039 The dispersion-controlled layer 420 is arranged between the core 410 and cladding 430 and has an inner radius A1, an outer radius A3, a peak refractive index N3 and a minimum refractive index N4. 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. 0040 The cladding 430 is arranged outside of the wide band dispersion-controlled fiber 400 and has a radius of A3 and a refractive index of N2. 0041 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. 0042 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. 0043 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. 0044 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. 0045 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. 0046 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. 0047 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 N3 and the minimum refractive index N4 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. 0048 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. 0049 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. 0050 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. 0051 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. 0052 A pre-form manufacturing apparatus comprises a raw material gas supplier 820, a shelf 850 and an oxygen/hydrogen burner 860. 0053 The raw material gas supplier 820 acts to mix oxygen and a plurality of additives and supplies oxygen and raw material gas, such as SiCl4, GeCl4, POCl3, CF4, SiF4 and so forth, to an inner part of a tube 810. The GeCl4and POCl3 are used for raising a refractive index of a deposition region and the CF4, and SiF4 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 CF4or SiF4, supplied to the deposition tube 810, to the mixture of oxygen, SiCl4, GeCl4, and POCl3 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 GeCl4, supplied to the deposition tube 810, to the mixture of oxygen and SiCl4 to generate a change in the refractive index. 0054 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. 0055 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, SiCl4O2SiO22Cl2 and GeCl4O2GeO22Cl2. 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. 0056 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. 0057 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. 0058 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.