Patent Publication Number: US-6335995-B1

Title: Dispersion-shifted fiber

Description:
This is a continuation of application Ser. No. 08/998,425, filed Dec. 24, 1997 now U.S. Pat. No. 6,072,929. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a single-mode optical fiber (hereinafter referred to as S-mode optical fiber) used for transmitting light in long-haul optical communications or the like and, in particular, to a dispersion-shifted fiber suitable for wavelength-multiplexing transmission. 
     2. Related Background Art 
     Conventionally, optical communication systems employing a S-mode optical fiber as their transmission line have often utilized light in the wavelength band of 1.3 μm or 1.55 μm as their signal light for communications. Recently, in order to reduce transmission loss in the transmission line, the light in the wavelength band of 1.55 μm has been in use more and more. The S-mode optical fiber employed in such a transmission line for light in the wavelength band of 1.55 μm (hereinafter referred to as 1.55-μm S-mode optical fiber) has been designed such that its wavelength dispersion (phenomenon in which pulse wave spreads due to the fact that velocity of propagation of light changes depending on its wavelength) is nullified (namely, to yield a dispersion-shifted fiber whose zero-dispersion wavelength is 1.55 μm). For example, as such a dispersion-shifted fiber, Japanese Patent Publication No. 3-18161 discloses a dispersion-shifted fiber having a dual-shape core type refractive index profile in which a core is constituted by an inner core layer and an outer core layer having a refractive index lower than that of the inner core layer. Further, Japanese Patent Application Laid-Open No. 63-43107 and No. 2-141704 propose a dispersion-shifted fiber having a depressed cladding/dual-shape core type refractive index profile in which, in addition to the double core structure mentioned above, a cladding is constituted by an inner cladding layer and an outer cladding layer having a refractive index higher than that of the inner cladding layer. 
     On the other hand, long-haul light transmission has recently become possible with the advent of wavelength division multiplex (WDM) transmission and optical amplifiers. Under such circumstances, however, influences of nonlinear optical effects cannot be neglected. Accordingly, in order to eliminate the nonlinear optical effects, it has been proposed to deform the refractive index profiles mentioned above, thereby shifting their zero-dispersion wavelength toward the shorter or longer wavelength side of their signal wavelength band (Japanese Patent Application Laid-Open No. 7-168046 and U.S. Pat. No. 5,483,612). Here, a nonlinear optical effect is a phenomenon in which a signal light pulse is distorted in proportion to density of light intensity or the like. This phenomenon becomes a factor restricting transmission speed, as well as a relay distance in a relaying transmission system. 
     SUMMARY OF THE INVENTION 
     As a result of studies concerning the above-mentioned prior art, the inventors have discovered the following problems. Namely, in the above-mentioned dispersion-shifted fibers proposed for wavelength division multiplex transmission, the zero-dispersion wavelength is set to a level different from the wavelength level of signal wavelength band so as to restrain nonlinear optical effects from occurring, while their effective core cross-sectional area A eff  is set on the order of 55 μm 2 . Though the conventional dispersion-shifted fibers for wavelength division multiplex transmission are sufficient for the conventional applications, it may be difficult for the prior art to keep a suitable transmission quality in the conventional transmission distance in view of further advance in wavelength multiplexing which will occur as communications become more sophisticated. 
     Here, as disclosed in Japanese Patent Application Laid-Open No. 8-248251, effective core cross-sectional area A eff  is given by the following expression:          A   eff     =     2                       π        (       ∫   0   ∞            E   2        r                      r         )       2     /     (       ∫   0   ∞            E   4        r                      r         )                         
     wherein E is an electric field accompanying propagated light, and r is a radial distance from a core center. 
     It is an object of the present invention to provide a dispersion-shifted fiber which can effectively restrain the nonlinear optical effects from occurring, and is suitable for long-haul light transmission. 
     The dispersion-shifted fiber according to the present invention is a S-mode optical fiber mainly composed of silica glass, whose zero-dispersion wavelength is shifted toward the shorter or longer wavelength side of a signal light wavelength band. The object to be transmitted through the dispersion-shifted fiber according to the present invention is at least one light component whose center wavelength is within the range of 1,500 to 1,600 nm (signal light wavelength band). In this specification, light in a 1.55-μm wavelength band equals to light in the signal light wavelength band. The dispersion-shifted fiber has a zero-dispersion wavelength out of a wavelength band of 1.53 μm (1,530 nm) to 1.56 μm (1,560 nm) and has, as various characteristics at 1,550 nm, a dispersion level of 1.0 to 4.5 ps/nm/km in terms of absolute value, a dispersion slope not greater than 0.13 ps/nm 2 /km in terms of absolute value, an effective core cross-sectional area A eff  of 70 μm 2  or more, and a transmission loss not greater than 0.25 dB/km with respect to light in a wavelength band of 1.55 μm. 
     Here, when the dispersion level in terms of absolute value is smaller than 1.0 ps/nm/km, waveform distortion caused by four-wave mixing, unstable modulation, and the like cannot practically be neglected in long-haul light transmission over 20 km or more. When the dispersion level in terms of absolute value is greater than 4.5 ps/nm/km, by contrast, waveform distortion caused by wavelength dispersion and by self phase modulation cannot practically be neglected in long-haul light transmission over 20 km or more. 
     In the dispersion-shifted fiber according to the present invention, the absolute value of dispersion slope is not greater than 0.13 ps/nm 2 /km. Accordingly, it is possible to transmit-signal lights in which the variation in the amount of waveform distortion due to the dispersion wavelength in signal lights is effectively decreased. 
     The amount of nonlinear optical effects generated is in proportion to nonlinear optical effect constant (N 2 /A eff ). Accordingly, at the same propagating light condition, nonlinear optical effects are effectively restrained from occurring when the nonlinear optical effect constant (N 2 /A eff ) is made smaller. On the other hand, since nonlinear refractive index N 2  is substantially defined by a main material of the optical fiber, it is difficult for the optical fiber made of the same main material to change the nonlinear refractive index N 2  from its conventional level so as to restrain the nonlinear optical effects from occurring. 
     Therefore, in the dispersion-shifted fiber according to the present invention, the effective core cross-sectional area A eff  is increased to 70 μm 2  or greater, thereby the amount of nonlinear optical effects generated becomes smaller than that of the conventional dispersion-shifted fiber by at least 20%. 
     FIG. 1 is a graph showing a relationship between effective core cross-sectional area A eff  and nonlinear optical constant (N 2 /A eff ) in a dispersion-shifted fiber having a typical composition. From FIG. 1, it can be seen that nonlinear optical constant (N 2 /A eff ), which is 5.8×10 −10  (1/W) when effective core cross-sectional area A eff  is 55 μm 2 , becomes 4.6×10 −10  (1/W) when effective core cross-sectional area A eff  is 70 μm 2 , thus being reduced by about 20%. Accordingly, as compared with the conventional dispersion-shifted fiber, the dispersion-shifted fiber according to the present invention can increase the degree of wavelength multiplexing in signal light. 
     In general, refractive index N of a medium under strong light changes depending on light intensity. Accordingly, the minimum order of effect on refractive index N is expressed by: 
     
       
         
           N=N 
           0 
           N 
           2 
           ·E 
           2 
         
       
     
     wherein No is a linear refractive index, N 2  is a nonlinear refractive index, and E is a field amplitude. Namely, under strong light, the refractive index N of the medium is given by the sum of N o  and an increase which is in proportion to the square of field amplitude E. In particular, the constant of proportion N 2  (unit: m 2 /V 2 ) in the second term is known as nonlinear refractive index. Since the distortion in signal light pulse is mainly influenced by, of nonlinear refractive indices, the nonlinear refractive index in the second term, nonlinear refractive index in this specification mainly refers to this second-order nonlinear refractive index. 
     Also, in the dispersion-shifted fiber according to the present invention, since its incident signal light power can be increased by about 20% (about 1 dB) as compared with the conventional dispersion-shifted fiber, signal light can be transmitted over a transmission distance longer than that of the conventional fiber by 5 km when transmission loss is assumed to be 0.2 dB/km. As a result, in the case where the conventional repeater spacing is 50 km, for example, the number of repeaters can be reduced by about 10%. 
     Further, the dispersion-shifted fiber according to the present invention has a bending loss of 0.5 dB/turn or less when bent at a diameter of 32 mm. Here, the bending loss is measured in a state where a fiber to be measured is wound around a mandrel having a diameter of 32 mm, and a value thus obtained is expressed per turn. 
     In general, the greater is effective core cross-sectional area A eff , the higher becomes the density of light intensity on the outer periphery side, thus yielding a greater bending loss. An optical fiber with a greater bending loss generates a greater optical loss due to the bending inevitably generated by cable-forming step, cable-laying step, excess-length processing upon connection, and the like. The dispersion-shifted fiber according to the present invention has a bending loss of 0.5 dB/turn or less when bent at a diameter of 32 mm, thereby effectively suppressing the optical loss caused by the bending generated at the cable-forming step or the like. 
     Preferably, in the dispersion-shifted fiber according to the present invention, the absolute value of dispersion slope is 0.09 ps/nm 2 /km or more. The smaller the dispersion slope is, the less becomes the variation in the amount of waveform distortion caused by wavelength dispersion in the signal lights. On the other hand, the smaller the dispersion slope is, the more likely satisfied is a phase-matching condition for generating the four-wave mixing that is one of nonlinear optical phenomena. Therefore, in the dispersion-shifted fiber according to the present invention, the absolute value of dispersion slope is preferably at least 0.09 ps/nm 2 /km but not greater than 0.13 ps/nm 2 /km, so as to restrain not only the variation in the amount of waveform distortion caused by wavelength dispersion, but also the occurrence of the four-wave mixing, while the signal lights are transmitted. 
     In order to realize the foregoing characteristics, the dispersion-shifted fiber according to the present invention can be realized by dual-shape core type or segmented-core type refractive index profile. Both refractive index profiles have a depressed cladding structure. 
     Here, a first embodiment of the dispersion-shifted fiber according to the present invention has a depressed cladding/dual-shape core type refractive index profile. The first embodiment of the dispersion-shifted fiber comprises an inner core having a predetermined refractive index and an outside diameter of 2a; an outer core, disposed around the outer periphery of the inner core, having a refractive index lower than that of the inner core and an outside diameter of 2b; an inner cladding, disposed around the outer periphery of the outer core, having a refractive index lower than that of the outer core; and an outer cladding, disposed around the outer periphery of the inner cladding, having a refractive index higher than that of the inner cladding. 
     According to the findings obtained by the inventors as a result of studies, when effective core cross-sectional area A eff  is increased in a dispersion-shifted fiber having a simple dual-shape core type refractive index profile without a depression cladding structure, namely, non-depressed cladding/dual-shape core type refractive index profile, in a state where its absolute value of dispersion is set to 1.0 to 4.5 ps/nm/km at the wavelength of 1,550 nm, its cutoff wavelength becomes shorter, and its bending loss increases. Even when the refractive index profile is adjusted to increase the cutoff wavelength in order to reduce the bending loss, due to the restriction that the cutoff wavelength must not exceed the signal light wavelength, the bending loss can not sufficiently be ameliorated. 
     Also, according to the findings of the inventors, when the bending loss of a dispersion-shifted fiber having a depressed cladding/dual-shape core type refractive index is lower than that of a dispersion-shifted fiber having a simple dual-shape core type refractive index. Accordingly, an optical fiber (having a depressed cladding/dual-shape core type refractive index profile) employing the configuration mentioned above can favorably realize the foregoing various characteristics, and suppress the bending loss to a predetermined level or lower. 
     Preferably, the first embodiment of the dispersion-shifted fiber satisfies the following relationships: 
     
       
           a/b≦ 0.15  (1) 
       
     
     
       
         0.8%≦Δ n   1 ≦1.2%  (2) 
       
     
     
       
         0.12%≦Δ n   2 ≦0.30%  (3) 
       
     
     
       
           Δn   3   /Δn   2 ≦0.95  (4) 
       
     
     wherein Δn 1  is a relative refractive index difference of the inner core with respect to the inner cladding, Δn 2  is a relative refractive index difference of the outer core with respect to the inner cladding, and Δn 3  is a relative refractive index difference of the outer cladding with respect to the inner cladding. 
     The dispersion-shifted fiber satisfying these relationships can favorably realize the foregoing various characteristics, and allows its absolute value of dispersion slope to become at least 0.09 ps/nm 2 /km but not larger than 0.13 ps/nm 2 /km. 
     Here, the relative refractive index difference Δn 1  of the inner core with respect to the inner cladding, relative refractive index difference Δn 2  of the outer core with respect to the inner cladding, and relative refractive index difference Δn 3  of the outer cladding with respect to the inner cladding are respectively defined as follows: 
     
       
           Δn   1 =( n   1   2   −n   3   2 )/(2 n   1   2 )  (5) 
       
     
     
       
           Δn   2 =( n   2   2   −n   3   2 )/(2 n   2   2 )  (6) 
       
     
     
       
           Δn   3 =( n   4   2   −n   3   2 )/(2 n   4   2 )  (7) 
       
     
     wherein n 1  is the refractive index of the inner core, n 2  is the refractive index of the outer core, n 3  is the refractive index of the inner cladding, and n 4  is the refractive index of the outer cladding. In this specification, each relative refractive index difference is expressed in terms of percentage. 
     Preferably, the first embodiment of the dispersion-shifted fiber further satisfies the following relationship: 
     
       
         1.2≦ c/b≦ 3.5  (8) 
       
     
     wherein 2c is an outside diameter of the inner cladding. 
     The above relationship is preferable in view of the fact that, in the first embodiment of the dispersion-shifted fiber, the bending-loss-reducing effect, which is generated by the existence of the inner cladding, can not sufficiently be yielded when the inner cladding is too thin. On the other hand, when the inner cladding is too thick, it functions in a way similar to a normal cladding and fails to yield the cutoff-wavelength-shortening effect of the depressed cladding type refractive index. When the dispersion-shifted fiber satisfies the relationship of c/b≧1.2, the bending loss in the case where it is bent at a diameter of 32 mm can become 0.5 dB/turn or less. 
     On the other hand, as the first embodiment of the dispersion-shifted fiber satisfies the relationship of c/b≦3.5, its cutoff wavelength can favorably be made shorter, thereby making it easy to secure a wavelength range of signal light which allows single-mode transmission. 
     Next, a second embodiment of the dispersion-shifted fiber according to the present invention has a depressed cladding/segmented-core type refractive index profile. The second embodiment of the dispersion-shifted fiber comprises an inner core having a predetermined refractive index and an outside diameter of 2a; an intermediate core, disposed around the outer periphery of the inner core, having a refractive index lower than that of the inner core and an outside diameter of 2b; an outer core, disposed around the outer periphery of the intermediate core, having a refractive index higher than that of the intermediate core and an outside diameter of 2c; an inner cladding, disposed around the outer periphery of the outer core, having a refractive index lower than that of the outer core; and an outer cladding, disposed around the outer periphery of the inner cladding, having a refractive index higher than that of the inner cladding. 
     Preferably, the second embodiment of the dispersion-shifted fiber satisfies the following relationships: 
     
       
           a/c≦ 0.42  (9) 
       
     
     
       
           b/c≧ 0.60  (10) 
       
     
     
       
         0.5%≦ Δn   1 ≦1.1%  (11) 
       
     
     
       
         0.2%≦ Δn   3   −Δn   2 ≦0.7%  (12) 
       
     
     
       
           Δn   4   /Δn   3 ≦0.95  (13) 
       
     
     wherein Δn 1  is a relative refractive index difference of said inner core with respect to said inner cladding, Δn 2  is a relative refractive index difference of said intermediate core with respect to said inner cladding, Δn 3  is a relative refractive index difference of said outer core with respect to said inner cladding, and Δn 4  is a relative refractive index difference of said outer cladding with respect to said inner cladding. 
     The above-relationships (9) and (10) are conditions to satisfy the effective core cross-section area A eff  of 70 μm 2 . The relative refractive index difference Δn 1  of said inner core with respect to said inner cladding is preferably 0.5% or more in order to satisfy the condition that the dispersion level in terms of absolute value falls within 1.0 to 4.5 ps/nm/km. Further, when the relative index difference Δn 1  is 1.1% or less, the dispersion slope at wavelength of 1,550 nm falls within a range of 0.09 to 0.13 ps/nm 2 /km. The value (Δn 3 −Δn 2  ) should be 0.2% or more in order to satisfy the condition that the bending loss when bent at a diaimeter of 32 mm becomes 0.5 dB/turn or less, and it should be 0.7% or less in order to make cutoff wavelength at a length of 2 m set 2.2 μm or less. The relationship (13) is a condition to restrain the transmission loss with respect to light in a 1.55-μm wavelength band so as not to exceed 0.25 dB/km. 
     Further, the second embodiment of the dispersion-shifted fiber satisfies the following relationship: 
     
       
         1.2≦ d/c≦ 3.5  (14) 
       
     
     wherein 2d is an outside diameter of the inner cladding. 
     The bending loss when bent at a diameter of 32 mm becomes 0.5 dB/km or less when (d/c) is not less than 1.2, and the reducing effect of the cut off wavelength becomes saturated when (d/c) exceeds 3.5. 
     The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing an example of relationship between effective core cross-sectional area A eff  and nonlinear optical constant (N 2 /A eff ); 
     FIG. 2 is a view showing a cross-sectional configuration of a typical embodiment of the dispersion-shifted fiber according to the present invention and its refractive index profile (depressed cladding/dual-shape core type); 
     FIG. 3 is a graph showing a relationship between value (a/b) and effective core cross-sectional area A eff ; 
     FIG. 4 is a graph showing a relationship between relative refractive index difference Δn 1  and absolute value |D| of dispersion level D with respect to light having a wavelength of 1,550 nm; 
     FIG. 5 is a graph showing a relationship between relative refractive index difference Δn 1  and dispersion slope; 
     FIG. 6 is a graph showing a relationship between relative refractive index difference Δn 2  and bending loss generated when bent at a diameter of 32 mm; 
     FIG. 7 is a graph showing a relationship between relative refractive index difference Δn 2  and cutoff wavelength at a reference length of 2 m; 
     FIG. 8 is a graph showing a relationship between value (Δn 3 /Δn 2 ) and transmission loss; 
     FIG. 9 is a graph showing a relationship between value (c/b) and bending loss generated when bent at a diameter of 32 mm; 
     FIG. 10 is a graph showing a relationship between value (c/b) and cutoff wavelength at a reference length of 2 m; 
     FIG. 11 is a view showing a cross-sectional configuration of a first embodiment of the dispersion-shifted fiber according to the present invention and its refractive index profile (depressed cladding/dual-shape core type); 
     FIG. 12 is a view showing a cross-sectional configuration of a second embodiment of the dispersion-shifted fiber according to the present invention and its refractive index profile (depressed cladding/segmented-core type); 
     FIG. 13 is a view showing a first application of the depressed cladding/segmented-core type refractive index profile in the second embodiment of the dispersion-shifted fiber shown in FIG. 12; and 
     FIG. 14 is a view showing a second application of the depressed cladding/segmented-core type refractive index profile in the second embodiment of the dispersion-shifted fiber shown in FIG.  12 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, the dispersion-shifted fiber according to the present invention will be explained with reference to FIGS. 2 to  11 . In the explanation of drawings, elements identical to each other will be referred to with numerals or letters identical to each other, without their overlapping descriptions being repeated. 
     FIG. 2 is a view showing a cross-sectional configuration of a typical embodiment of the dispersion-shifted fiber according to the present invention and its refractive index profile. As shown in FIG. 2, this dispersion-shifted fiber  100  comprises an inner core  110  having a refractive index n 1  as its maximum refractive index and an outside diameter 2a; an outer core  120 , disposed around the outer periphery of the inner core  110 , having a refractive index n 2  (&lt;n 1 ) and an outside diameter 2b; an inner cladding  210 , disposed around the outer periphery of the outer core  120 , having a refractive index n 3  (&lt;n 2 ) and an outside diameter 2c; and an outer cladding  220 , disposed around the outer periphery of the inner cladding  210 , having a refractive index n 4  (&gt;n 3 ). As a result of this configuration, the dispersion-shifted fiber  100  realizes a depressed cladding/dual-shape core type refractive index profile  101 . 
     This dispersion-shifted fiber satisfies the following relationships: 
     
       
           a/b≦ 0.15  (1) 
       
     
     
       
         0.8% ≦Δn   1 ≦1.2%  (2) 
       
     
     
       
         0.12% ≦Δn   2 ≦0.30%  (3) 
       
     
     
       
           Δn   3   /Δn   2 ≦0.95  (4) 
       
     
     
       
         1.2≦ c/b ≦3.5  (8) 
       
     
     wherein Δn 1  is a relative refractive index difference of the inner core  110  with respect to the inner cladding  210 , Δn 2  is a relative refractive index difference of the outer core  120  with respect to the inner cladding  210 , and Δn 3  is a relative refractive index difference of the outer cladding  220  with respect to the inner cladding  210 . 
     FIG. 3 is a graph showing a relationship between the ratio of outside diameter 2a of the inner core  110  to outside diameter 2b of the outer core  120  and effective core cross-sectional area A eff . In FIG. 3, while Δn 1  is 1.0% and Δn 2  is 0.2%, the outside diameter 2a and the outside diameter 2b of the inner core  110  are changed so as to attain a zero-dispersion wavelength of 1,580 nm. It can be seen from FIG. 3 that A eff  becomes 70 μm 2  or more when (a/b) does not exceed 0.15. 
     FIG. 4 is a graph showing a relationship between the relative refractive index difference Δn 1  of the inner core  110  with respect to the inner cladding  210  and absolute value |D| of dispersion level D with respect to light having a wavelength of 1,550 nm. In FIG. 4, the outside diameter 2a of the inner core  110 , outside diameter 2b of the outer core  120 , relative refractive index difference Δn 1  of the inner core  110  with respect to the inner cladding  210 , and relative refractive index difference Δn 2  of the outer core  120  with respect to the inner cladding  210  are changed so as to satisfy that (a/b) is 0.13 and A eff  is 80 μm 2 . It can be seen from FIG. 4 that |D| is equal to or larger than 1.0 ps/nm/km when Δn 1  exceeds 0.8%, and that |D| is equal to or smaller than 4.5 ps/nm/km when Δn 1  is approximately 1.2%. 
     FIG. 5 is a graph showing a relationship between the relative refractive index difference Δn 1  of the inner core  110  with respect to the inner cladding  210  and dispersion slope. In FIG. 5, the outside diameter 2a of the inner core  110 , outside diameter 2b of the outer core  120 , relative refractive index difference Δn 1  of the inner core  110  with respect to the inner cladding  210 , and relative refractive index difference Δn 2  of the outer core  120  with respect to the inner cladding  210  are changed so as to satisfy 0.13 of (a/b), 80 μm 2  of A eff , 0.1 dB/turn of bending loss when bent at a diameter of 32 mm, and 1,580 nm of zero-dispersion wavelength. It can be seen from FIG. 5 that the dispersion slope becomes 0.13 ps/nm 2 /km or more when Δn 1  is equal to or greater than 1.2%. 
     FIG. 6 is a graph showing a relationship between the relative refractive index difference Δn 2  of the outer core  120  with respect to the inner cladding  210  and bending loss generated when bent at a diameter of 32 mm. In FIG. 6, the outside diameter 2a of the inner core  110  outside diameter 2b of the outer core  120  and relative refractive index difference Δn 2  of the outer core  120  with respect to the inner cladding  210  are changed so as to satisfy that relative refractive index difference Δn 1  is 1.0% in the inner core  110  with respect to the inner cladding  210 , (a/b) is 0.13, A eff  is 80 μm 2 , and zero-dispersion wavelength is 1,580 nm. It can be seen from FIG. 6 that the bending loss upon bending at a diameter of 32 mm is 0.5 dB/turn or less when Δn 2  is equal to or greater than 0.12%. Here, the bending loss is measured as a fiber to be measured is wound around a mandrel having a diameter of 32 mm by a predetermined number of turns (e.g., 100 turns), and is given by a value obtained when thus measured value is expressed per turn. 
     FIG. 7 is a graph showing a relationship between the relative refractive index difference Δn 2  of the outer core  120  with respect to the inner cladding  210  and cutoff wavelength at a reference length of 2 m. In FIG. 7, the outside diameter 2a of the inner core  110  outside diameter 2b of the outer core  120  relative refractive index difference Δn 2  of the outer core  120  with respect to the inner cladding  210 , and relative refractive index difference Δn 3  of the outer cladding  220  with respect to the inner cladding  210  are changed so as to satisfy that Δn 1  is 1.0% in the inner core  110  with respect to the inner cladding  210 , (a/b) is 0.13, A eff  is 80 μm 2 , zero-dispersion wavelength is 1,580 nm, and Δn 3 /Δn 2  is 0.8. 
     Normally, cutoff wavelength λ c  of an optical fiber is measured according to a bending process performed at a length of 2 m, which is recommended by CCITT-G.650. When a S-mode optical fiber has a length L of 2 m, the cutoff wavelength λ c , as the lowest wavelength allowing single-mode transmission, coincides with the result of the above-mentioned measurement. It has been known that, as the length L increases, the cutoff wavelength λ c  changes according to the following expression (15): 
     
       
         λ c ( L )=λ c ( L   0 &lt;2 m )−0.184×log 10 ( L/L   0 )  (15) 
       
     
     (see T. Kato et al.,  OECC&#39; 96  Technical Digest,  July 1966, Makuhari Messe, pp. 160-161). 
     On the other hand, the dispersion-shifted fiber according to the present invention is assumed to transmit signal light having a wavelength in the band of 1.55 μm over a long distance of 20 km or more. Consequently, it is necessary for the cutoff wavelength λ c (L 0 ) at a length of 2 m to satisfy the following expression (16): 
     
       
         λ c ( L )≦1.5+0.732[μ m] ≃2.2  μm   (16) 
       
     
     From FIG. 7, it can be seen that the cutoff wavelength λ c (L 0 ) is equal to or less than 2.2 μm at a length of 2 m when Δn 2 ≦0.30%. 
     FIG. 8 is a graph showing a relationship between value (Δn 3 /Δn 2 ) and transmission loss. In FIG. 8, the relative refractive index difference Δn 3  of the outer cladding  220  with respect to the inner cladding  210  is changed under the following conditions: 
     relative refractive index difference Δn 1 =1.0%; 
     relative refractive index difference Δn 2 =0.20%; and 
     (a/b)=0.13. 
     It can be seen from FIG. 8 that transmission loss drastically increases beyond 0.25 dB/km when (Δn 3 /Δn 2 ) is greater than 0.95. 
     FIG. 9 is a graph showing a relationship between value (c/b) and bending loss at a diameter of 32 mm. In FIG. 9, the outside diameter 2c of the inner cladding  210  is changed-under the following conditions: 
     relative refractive index difference Δn 1 =1.0%; 
     relative refractive index difference Δn 2 =0.20%; 
     relative refractive index difference Δn 3 =0.12%; 
     radius a=2.1 μm; and 
     radius b is 16.0 μm. 
     It can be seen from FIG. 9 that the bending loss generated when bent at a diameter of 32 mm drastically increases beyond 0.5 dB/turn when (c/b) does not exceed 1.2. 
     FIG. 10 is a graph showing a relationship between value (c/b) and cutoff wavelength at a length of 2 m. In FIG. 10, the outside diameter 2c of the inner cladding  210  is changed under the following conditions: 
     relative refractive index difference Δn 1 =1.0%; 
     relative refractive index difference Δn 2 =0.20%; 
     relative refractive index difference Δn 3 =0.12%; 
     radius a=2.1 μm; and 
     radius b=16.0 μm. 
     It can be seen from FIG. 10 that the effect on lowering cutoff wavelength is saturated when (c/b) does not exceed 3.5. 
     Namely, when the dispersion-shifted fiber according to the present invention has a depressed cladding/dual-shape core type refractive index profile, it satisfies the following relationships: 
     
       
           a/b≦ 0.15  (1) 
       
     
     
       
         0.8%≦ Δn   1 ≦1.2%  (2) 
       
     
     
       
         0.12% ≦Δn   2 ≦0.30%  (3) 
       
     
     
       
           Δn   3   /Δn   2 ≦0.95  (4) 
       
     
     
       
         1.2≦ c/b≦ 3.5  (8) 
       
     
     Accordingly, it favorably satisfies, at the wavelength of 1,550 nm, various characteristics such as a dispersion level of 1.0 to 4.5 ps/nm/km in terms of absolute value, a dispersion slope not greater than 0.13 ps/nm 2 /km in terms of absolute value, an effective core cross-sectional area A eff  of 70 μm 2  or more, a transmission loss not greater than 0.25 dB/km with respect to light in the wavelength band of 1.55 μm, and a bending loss of 0.5 dB/turn or less when bent at a diameter of 32 mm. 
     Thus, favorably realized is a dispersion-shifted fiber which can effectively restrain nonlinear optical effects from occurring and is suitable for long-haul light transmission. 
     On the other hand, without being restricted to the foregoing dispersion-shifted fiber having a depressed cladding/dual-shape core type refractive index profile, the present invention can be embodied in various manners. For example, the dispersion-shifted fiber can be applied to a dispersion-shifted fiber having a segmented-core type refractive index profile as follows. 
     The dispersion-shifted fiber having a segmented-core type refractive index profile comprises an inner core having a predetermined refractive index and an outside diameter of 2a; an intermediate core, disposed around the outer periphery of the inner core, having a refractive index lower than that of the inner core and an outside diameter of 2b; an outer core, disposed around the outer periphery of the intermediate core, having a refractive index higher than that of the intermediate core and an outside diameter of 2c; an inner cladding, disposed around the outer periphery of the outer core, having a refractive index lower than that of the outer core and an outside diameter of 2d; and an outer cladding, disposed around the outer periphery of the inner cladding, having a refractive index higher than that of the inner cladding. 
     Accordingly, the dispersion-shifted fiber having the above-mentioned various characteristics should satisfy the following relationships: 
     
       
           a/c≦ 0.42  (9) 
       
     
     
       
           b/c≧ 0.60  (10) 
       
     
     
       
         0.5%≦ Δn   1 ≦1.1%  (11) 
       
     
     
       
         0.2% ≦Δn   3   −Δn   2 ≦0.7%  (12) 
       
     
     
       
           Δn   4   /Δn   3 ≦0.95  (13) 
       
     
     
       
         1.2 ≦d/c≦ 3.5  (14) 
       
     
     wherein Δn 1  is a relative refractive index difference of said inner core with respect to said inner cladding, Δn 2  is a relative refractive index difference of said intermediate core with respect to said inner cladding, Δn 3  is a relative refractive index difference of said outer core with respect to said inner cladding, and Δn 4  is a relative refractive index difference of said outer cladding with respect to said inner cladding. wherein 2d is an outside diameter of the inner cladding. 
     Here, in the segmented-core type refractive index profile, the relative refractive index difference of each glass region with respect to the inner cladding is defined as follows: 
       Δn   x =( n   x   2   −n   cld   2 )/(2 n   x   2 ) 
     
       
           x= 1,2,3,4 
       
     
     wherein n 1  is the refractive index of the inner core, n 2  is the refractive index of the intermediate core, n 3  is the refractive index of the outer core, and n 4  is the refractive index of the outer cladding. 
     FIG. 11 is a view showing a cross-sectional configuration of a first typical example (dual-shape core type) of a dispersion-shifted fiber according to the present invention and its refractive index profile. As shown in FIG. 11, the dispersion-shifted fiber  200  has a depressed cladding/dual-shape core type refractive index profile  201  and comprises an inner core  111  having an outside diameter of 4.2 μm; an outer core  121 , disposed around the outer periphery of the inner core  111 , having an outside diameter 2b of 32 μm; an inner cladding  211 , disposed around the outer periphery of the outer core  121 , having an outside diameter 2c of 63 μm; and an outer cladding  221  disposed around the outer periphery of the inner cladding  211 . 
     Also, the first embodiment of the dispersion-shifted fiber  200  satisfies the above-mentioned relational expressions (1) to (4) and (8) as follows: 
     
       
           a/b= 0.13&lt;0.15 
       
     
     
       
         0.8%&lt; Δn   1 =0.98%&lt;1.2% 
       
     
      0.12%&lt; Δn   2 =0.20%&lt;0.30% 
     
       
           Δn   3 =0.12% 
       
     
     
       
           Δn   3   /Δn   2 =0.6 &lt;0.95 
       
     
     
       
         1.2&lt; c/b= 1.97&lt;3.5 
       
     
     Further, characteristics of the first embodiment of the dispersion-shifted fiber measured at the wavelength of 1,500 nm are as follows: 
     zero-dispersion wavelength =1,585 nm; 
     dispersion level at 1,550 nm=−3.8 ps/nm/km; 
     dispersion slope =0.111 ps/nm 2 /km; 
     effective core cross-sectional area A eff =78.2 μm; 
     cutoff wavelength =1.59 μm; 
     bending loss =0.1 dB/turn when bent at a diameter of 32 mm; and 
     transmission loss =0.21 dB/km. 
     Next, FIG. 12 is a view showing a cross-sectional configuration of a second embodiment of the dispersion-shifted fiber according to the present invention and its refractive index profile. As shown in FIG. 12, the second embodiment of the dispersion-shifted fiber  300  has a segmented-core type refractive index profile and comprises an inner core  301  having an outside diameter 2 a   of 7.0 μm; an intermediate core  302 , disposed around the outer periphery of the inner core  301 , having an outside diameter 2b of 13.4 μm; an outer core  303 , disposed around the outer periphery of the intermediate core  302 , having an outside diameter 2c of 19.2 μm; an inner cladding  304 , disposed around the outer periphery of the outer core  303 , having an outside diameter 2d of 38.4 μm; and an outer cladding  305  disposed around the outer periphery of the inner cladding  304 . 
     Also, the second embodiment of the dispersion-shifted fiber  300  satisfies the above-mentioned relational expressions (9) to (14) as follows: 
     
       
           a/c= 0.36&lt;0.42 
       
     
     
       
           b/c= 0.7&gt;0.60 
       
     
     
       
         0.5%&lt; Δn   1 =0.72%&lt;1.1% 
       
     
     
       
         0.2%&lt;( Δn   3   −Δn   2 )=0.4%&lt;0.7% 
       
     
     
       
           Δn   4   /Δn   3 =0.15&lt;0.95 
       
     
     
       
         1.2&lt; d/c= 2&lt;3.5 
       
     
     Also, the depressed cladding/segmented-core type refractive index profile can be modified in various manners as shown in FIGS. 13 and 14. For example, FIG. 13 shows a first application of the depressed cladding/segmented-core type refractive index profile at the condition that the refractive index of the intermediate core  302  is higher than that of the inner cladding  304  (Δn 2 &gt;0), and FIG. 14 shows a second application of the depressed cladding/segmented-core type refractive index profile at the condition that the refractive index of the intermediate core  302  is lower than that of the inner cladding  304  (Δn 2 &lt;0). 
     Further, characteristics of the second embodiment of the dispersion-shifted fiber measured at the wavelength of 1,550 nm are as follows: 
     zero-dispersion wavelength =1,567 nm; 
     dispersion level at 1,550 nm=−1.8 ps/nm/km; 
     dispersion slope =0.110 ps/nm 2 /km; 
     effective core cross-sectional area A eff =81.8 μm; 
     cutoff wavelength =1.74 μm; 
     bending loss =0.1 dB/turn when bent at a diameter of 32 mm; and 
     transmission loss =0.21 dB/km. 
     As explained in detail in the foregoing, in the dispersion-shifted fiber according to the present invention, its zero-dispersion wavelength does not exist at least within the wavelength range of 1.53 to 1.56 μm, while its effective core cross-sectional area is set to 70 μm 2  or greater, nonlinear optical effects are effectively restrained from occurring. Accordingly, it is possible to favorably realize a dispersion-shifted fiber, suitable for long-haul light transmission, which can effectively restrain the nonlinear optical effects from occurring. 
     Also, as the dispersion-shifted fiber according to the present invention has a configuration with a depressed cladding/dual-shape core type refractive index profile or a segmented-core type refractive index profile, it can reduce bending loss and favorably realize the aimed dispersion-shifted fiber. 
     From the invention thus described, it will be obvious that the implementation of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.