RELATED APPLICATIONS 0001 This is a Continuation-In-Part application of International Patent application Ser. No. PCT/JP98/05182 filed on Nov. 18, 1998, now pending.

BACKGROUND OF THE INVENTION 0002 1. Field of the Invention 0003 The present invention relates to a single-mode optical fiber usable as a transmission line in optical communications and the like; and, in particular, to a dispersion-shifted optical fiber suitable for wavelength division multiplexing (WDM) transmission. 0004 2. Related Background Art 0005 In general, a WDM transmission system using optical fiber networks is a system enabling long-distance, large-capacity optical data communications, and is constituted by a transmitter/receiver for transmitting and receiving WDM signals of a plurality of wavelengths (light signals), an optical amplifier such as an optical fiber amplifier for amplifying the WDM signals, an optical fiber which is a transmission medium, and the like. In such a WDM transmission system, the wavelength band that can optically be amplified in the optical amplifier is from 1530 nm to 1560 nm, whereas the low-loss wavelength band in the optical fiber is from 1400 nm to 1700 nm. As a consequence, the wavelength band utilizable as the WDM signals in the conventional WDM transmission system has substantially been limited to a width of about 30 nm from 1530 nm to 1560 nm. 0006 The amplification of WDM signals by the optical amplifier increases the optical power of each light signal in the optical fiber acting as the transmission medium, thereby causing nonlinear phenomena such as four-wave mixing, self phase modulation, modulation instability, and the like. In particular, the four-wave mixing causes power variations among the individual signal components, whereas the self phase modulation distorts the pulse waveform of each light signal upon an interaction with the chromatic aberration of the optical fiber (hereinafter referred to as dispersion), whereby the occurrence of such nonlinear phenomena limits the normal transmission of light signal.

SUMMARY OF THE INVENTION 0007 The inventors have studied the case where a conventional dispersion-shifted optical fiber is employed in a WDM transmission system and, as a result, have found problems as follows. 0008 Namely, for effectively suppressing the four-wave mixing, it is preferred that the wavelength of each light signal be different from the zero-dispersion wavelength of the optical fiber. For effectively suppressing the self phase modulation, on the other hand, it is preferred that the absolute value of dispersion value of the optical fiber with respect to each light signal be not so large. 0009 When the four-wave mixing and the self phase modulation are compared with each other, the distortion in pulse waveform of each light signal caused by the self phase modulation can be alleviated to a certain extent by a dispersion-compensating technique in which a dispersion-compensating optical fiber (having a dispersion characteristic with a polarity opposite to that of the dispersion value of the optical fiber acting as the transmission medium) is inserted in the optical transmission line through which each light signal propagates, so that the dispersion value of the optical transmission line as a whole becomes nearly zero. By contrast, no technique has been known for compensating for the crosstalk between individual light signals caused by the four-wave mixing. Therefore, as compared with the self phase modulation, it is more important to suppress the four-wave mixing. 0010 In view of the increase in noise components caused by the modulation instability, on the other hand, it is preferred that the zero-dispersion wavelength be set on the longer wavelength side from the wavelength band of each light signal. Further, letting N2 be the nonlinear refractive index of the optical fiber, Aeff be the effective area thereof, P be the power of light the propagating therethrough, and Leff be the effective length of the optical fiber, the amount of occurrence of nonlinear phenomena in the optical fiber is given by the following expression (1):

N2PLeff/Aeff(1)

0011 Among these parameters, the nonlinear refractive index N2 is determined by the material of the optical fiber, whereby it is necessary for the effective area Aeff of the optical fiber to increase in order to reduce the amount of occurrence of nonlinear phenomena. 0012 Here, as shown in Japanese Patent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), the above-mentioned effective area Aeff is given by the following expression (2): 1

\begin{matrix} A_{eff} = 2 {(_{0}^{} E^{2} r r)}^{2} / (_{0}^{} E^{4} r r) & (2) \end{matrix}

0013 where E is the electric field accompanying the propagating light, and r is the radial distance from the center of the core region. 0014 In the conventional WDM transmission system, as a result of the foregoing studies, it is preferred that the zero-dispersion wavelength of the optical fiber be restricted to the range of 1560 nm to 1600 nm, and that its effective area Aeff be 50 m2 or more. Further, for suppressing the increase in loss upon cabling the optical fiber, it is preferred that its bending loss be smaller, whereby its cutoff wavelength must be set to an appropriate value. 0015 In the conventional WDM transmission system, the channel spacing between the individual light signals included in the WDM signals is about 1 nm, whereby the actual multiplicity has been limited to about 30 waves. For enhancing the transmission capacity, however, it is desirable that the wavelength multiplicity be increased. In this ca se, while a method of narrowing the channel spacing and a method of enlarging the wavelength bandwidth can be considered, the latter is preferred in view of the above-mentioned suppression of four-wave mixing. 0016 On the other hand, the amplification wavelength band of the optical fiber amplifier has been expanding along with the advance in technology, thereby making it possible to amplify the WDM signals in a wider wavelength band of 1530 nm to 1610 nm (see, for example, A. Mori, et al., 1.5 m Broadband Amplification by Tellurite-Band EDFAs, OFC '97, PD1). In contrast, the zero-dispersion wavelength of the conventional dispersion-shifted optical fiber lies within the range of 1560 nm to 1600 nm as mentioned above. Therefore, in a WDM transmission system employing the optical fiber amplifier having thus expanded amplification wavelength band and the conventional dispersion-shifted optical fiber, there is a possibility that the zero-dispersion wavelength of the conventional dispersion-shifted optical fiber may lie within the wavelength band that can be amplified by the optical fiber amplifier (the signal wavelength band of WDM signals), so that the four-wave mixing may occur strongly, whereby the WDM signals may not be transmitted normally. 0017 In order to overcome the problems such as those mentioned above, it is an object of the present invention to provide a dispersion-shifted optical fiber suitable for an optical transmission line in which an optical fiber amplifier having an expanded amplification band is installed. 0018 The dispersion-shifted optical fiber according to -he present invention is a single-mode optical fiber comprising a core region extending along a predetermined axis, and a cladding region provided on the outer periphery of the core region; wherein a zero-dispersion wavelength is set within a range of 1610 nm or more but 1750 nm or less, preferably 1610 nm or more but 1670 nm or less, so that the optical fiber is employable in a WDM transmission system including an optical amplifier having an expanded amplification wavelength band. Particularly, in order to suppress the occurrence of nonlinear phenomena across a signal wavelength band by slightly generating a dispersion, the zero-dispersion wavelength is preferably set within a range of 1640 nm or more but 1750 nm or less, further preferably 1640 nm or more but 1670 nm or less. Also, the dispersion-shifted optical fiber according to the present invention has a cutoff wavelength of 1.1 m or more at a length of 2 m and has, with respect to light having a wavelength of 1550 nm, which is light in a signal wavelength band, an effective area of 45 m2 or more, preferably 50 m2 or more, further preferably 70 m2 or more, and a dispersion slope of 0.15 ps/nm2/km or less. 0019 Since the dispersion-shifted optical fiber according to the present invention has an effective area of 45 m2 or more, preferably 50 m2 or more, further preferably 70 m2 or more, it can suppress nonlinear phenomena by the same extent as that of the conventional dispersion-shifted optical fiber or more. While the absolute value of dispersion value in a typical optical fiber having a zero-dispersion wavelength near 1.3 m becomes about 20 ps/nm/km when employed in WDM transmission in a 1.5-m wavelength band, a certain degree of transmission quality can be guaranteed by utilization of a dispersion compensating technique such as that mentioned above. Therefore, the dispersion-shifted optical fiber according to the present invention has q dispersion slope of 0.15 ps/nm2/km or less with respect to light a wavelength of 1550 nm so that the absolute value of dispersion value becomes 20 ps/nm/km or less. Further, in accordance with the specification of Bellcore, it is necessary that the bending loss upon bending at a diameter of 32 mm be 0.5 dB/turn or less. Therefore, the cutoff wavelength at a reference length of 2 m (measured by a method defined by ITU international standard) in the dispersion-shifted optical fiber according to the present invention is 1.1 m or more so that the bending loss at a diameter of 32 mm is 0.5 dB/turn or less. Here, the dispersion slope refers to the gradient of a graph indicating the wavelength dependence of dispersion value (see, for example, FIG. 5A). 0020 This dispersion-shifted optical fiber is employable in a WDM transmission system together with an optical amplifier such as an optical fiber amplifier whose amplification wavelength band is expanded, for example, to the range of 1530 nm to 1610 nm. Namely, even in the case where the individual light signals included in the wavelength band of 1530 nm to 1610 nm that can be amplified by the optical fiber amplifier propagate through the dispersion-shifted optical fiber, the occurrence of nonlinear phenomena such as four-wave mixing, self phase modulation, and the like is suppressed. Also, since the dispersion-shifted optical fiber has an excellent bending characteristic, it can realize high-quality optical transmission. 0021 For yielding various characteristics such as those mentioned above, the dispersion-shifted optical fiber according to the present invention can be realized by various refractive index profiles. 0022 For example, a desirable refractive index profile can be realized when the above-mentioned core region is constituted by a center core having a predetermined refractive index and an outer core provided on the outer periphery of the center core and having a refractive index lower than that of the center core. In this configuration, the maximum value of relative refractive index difference of the center core with respect to a reference area in the cladding region is preferably 0.9% or more in view of its relationship with the zero dispersion wavelength. 0023 Also, a desirable refractive index profile can be realized when the above-mentioned core region is constituted by a center core having a predetermined refractive index; an intermediate core provided on the outer periphery of the center core and having a refractive index lower than that of the center core; and, an outer core provided on the outer periphery of the intermediate core and having a refractive index higher than that of the intermediate core. In this configuration, the maximum value of relative refractive index difference of the center core with respect to the reference area in the cladding region is preferably 0.6% or more in view of its relationship with the zero dispersion wavelength. 0024 Further, a desirable refractive index profile yielding the above-mentioned various characteristics can be realized when the above-mentioned core region is constituted by a center core having a predetermined refractive index and an outer core provided on the outer periphery of the center core and having a refractive index higher than that of the center core. In this configuration, the maximum value of relative refractive index difference of the outer core with respect to the reference area in the cladding region is preferably 0.8% or more in view of its relationship with the zero dispersion wavelength. 0025 Here, the above-mentioned refractive index profile is represented by the relative refractive index difference ni given by the following expression (3):

ni(nincd)/ncd(3)

0026 where ncd is the refractive index of the reference area (SiO2) in the cladding region, and ni is the refractive index of each part i constituting the core region. Hence, the relative refractive index difference ni is represented with reference to the average refractive index ncd of the reference area in the cladding region. In this specification, the relative refractive index difference is expressed in terms of percentage, and regions having a negative refractive index refer to those having a refractive index lower than that of the reference area. 0027 Further, in the dispersion-shifted optical fiber according to the present invention, the cladding region can comprise a depressed cladding structure constituted by an inner cladding provided on the outer periphery of the core region, and an outer cladding provided on the outer periphery of the inner cladding and having a refractive index higher than that of the inner cladding. A combination of this depressed cladding structure and any of the core region structures mentioned above can realize a desirable refractive index profile. In the case of the depressed cladding structure, the above-mentioned relative refractive index difference is given while the above-mentioned outer cladding is employed as the reference area. 0028 The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention. 0029 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 0030 FIG. 1 is a view showing a schematic structure of a cross section of a dispersion-shifted optical fiber according to the present invention; 0031 FIG. 2 is a view showing the refractive index profile in a first embodiment of the dispersion-shifted optical fiber according to the present invention; 0032 FIG. 3 is a graph showing the relationship between bending loss and cutoff wavelength in the dispersion-shifted optical fiber according to the first embodiment; 0033 FIG. 4 is a graph showing the relationship between the zero-dispersion wavelength and the relative refractive index difference of the center core in the dispersion-shifted optical fiber according to the first embodiment; 0034 FIG. 5A is a graph showing the dispersion characteristic (wavelength dependence) of an optical fiber having a zero-dispersion wavelength near 1.3 m, whereas FIG. 5B is a graph showing the dispersion characteristic (wavelength dependence) of the dispersion-shifted optical fiber according to the present invention; 0035 FIG. 6 is a view showing the refractive index profile in a second embodiment of the dispersion-shifted optical fiber according to the present invention; 0036 FIG. 7 is a view showing the refractive index profile in a third embodiment of the dispersion-shifted optical fiber according to the present invention; 0037 FIG. 8 is a view showing the refractive index profile in a fourth embodiment of the dispersion-shifted optical fiber according to the present invention; 0038 FIG. 9 is a view showing the refractive index profile in a fifth embodiment of the dispersion-shifted optical fiber according to the present invention; 0039 FIG. 10 is a view showing the refractive index profile in a sixth embodiment of the dispersion-shifted optical fiber according to the present invention; 0040 FIG. 11 is a view showing the refractive index profile in a seventh embodiment of the dispersion-shifted optical fiber according to the present invention; 0041 FIG. 12 is a view showing the refractive index profile in an eighth embodiment of the dispersion-shifted optical fiber according to the present invention; 0042 FIG. 13 is a view showing the refractive index profile in a ninth embodiment of the dispersion-shifted optical fiber according to the present invention; 0043 FIG. 14 is a view showing the refractive index profile in a tenth embodiment of the dispersion-shifted optical fiber according to the present invention; and 0044 FIG. 15 is a table listing various characteristics of the dispersion-shifted optical fibers according to the first to tenth embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 0045 In the following, individual embodiments of the dispersion-shifted optical fiber according to the present invention will be explained with reference to FIGS. 1 to 4, 5A, 5B, and 6 to 15. In the explanation of the drawings, constituents identical to each other will be referred to with numerals or letters identical to each other without repeating their overlapping descriptions. 0046 As shown in FIG. 1, a dispersion-shifted optical fiber 100 according to the present invention is a single-mode optical fiber which comprises a core region 100A extending along a predetermined axis and having an outside diameter a, and a cladding region 100B provided on the outer periphery of the core region 100A and having an outside diameter b, and has a zero-dispersion wavelength set to 1610 nm or more but 1750 nm or less. This dispersion-shifted optical fiber 100 has a cutoff wavelength of 1. 1 m or more at a reference length of 2 m (according to the ITU international standard) and has, with respect to light having a wavelength of 1550 nm, an effective area of 45 m2 or more, preferably 50 m2 or more, further preferably 70 m or more, and a dispersion slope of 0.15 ps/nm2/km or less. As shown in first to tenth embodiments which will be mentioned later, each of the above-mentioned core region 100A and cladding region 100B can be constituted by a plurality of glass areas having refractive indices different from each other. 0047 The zero-dispersion wavelength of the dispersion-shifted optical fiber 100 according to the present invention is set outside the wavelength band of 1530 nm to 1610 nm, in which each light signal can be amplified when an optical fiber amplifier having an expanded amplification wavelength band is employed in a WDM transmission system, and on the longer wavelength band side from this amplification wavelength band. Therefore, the occurrence of four-wave mixing is effectively suppressed. Also, since the upper limit of the zero-dispersion wavelength of the dispersion-shifted optical fiber 100 is 1670 nm, whereas the upper limit of its dispersion slope is 0.15 ps/nm2/km (0.09 ps/nm2/km when the upper limit of the zero-dispersion wavelength of the dispersion-shifted optical fiber 100 is 1750 nm), the absolute value of dispersion of the dispersion-shifted optical fiber 100 can be suppressed to 21 ps/nm/km or less in the amplification wavelength band of the optical fiber, 1530 nm to 1610 nm, because of the reason that will be mentioned later. As a consequence, the interaction between the self phase modulation and the dispersion of the dispersion-shifted optical fiber 100 can sufficiently be lowered by a dispersion-compensating technique. Also, since the effective area is 45 m2 or more, preferably 50 m2 or more, further preferably 70 m2 or more, the power density of each light signal propagating through the dispersion-shifted optical fiber 100 is kept low, whereby the occurrence of nonlinear phenomena is suppressed. Further, since the cutoff wavelength at 2 m is 1.1 m or more, this dispersion-shifted optical fiber has an excellent bending characteristic. Therefore, WDM transmission systems employing the dispersion-shifted optical fiber 100 according to the present invention enable high-quality optical transmissions even when an optical fiber amplifier having an expanded amplification wavelength band is employed therein. 0048 Ten embodiments of the dispersion-shifted optical fiber 100 according to the present invention will now be explained. Here, each of the dispersion-shifted optical fibers according to the first and second embodiments has a refractive index profile of a double core structure. Each of the dispersion-shifted optical fibers according to the third to fifth embodiments has a multilayer core structure. Each of the dispersion-shifted optical fibers according to the sixth and seventh embodiments has a ring core structure. The dispersion-shifted optical fiber according to the eighth embodiment has a refractive index profile in which a depressed cladding structure and a ring core structure are combined together. The dispersion-shifted optical fiber according to the ninth embodiment has a refractive index profile of a double ring core structure. The dispersion-shifted optical fiber according to the tenth embodiment has a refractive index profile of a multilayer core structure. 0049 First Embodiment 0050 FIG. 2 is a view showing the refractive index profile of the dispersion-shifted optical fiber according to the first embodiment. This refractive index profile 110 corresponds to the refractive index of each part on a line L passing the center axis O of the dispersion-shifted optical fiber 100 shown in FIG. 1. 0051 As with the dispersion-shifted optical fiber 100 shown in FIG. 1, the dispersion-shifted optical fiber according to the first embodiment comprises a core region having an outside diameter a and a cladding region having an outside diameter b. The core region (corresponding to the core region 100A in FIG. 1) has a double core structure constituted by a center core having a maximum refractive index n1 and an outer core provided so as to surround the outer periphery of the center core and having a refractive index n2 (<n1). The cladding region (corresponding to the cladding region 100B in FIG. 1), which is provided so as to surround the outer periphery of the core region, has a refractive index n3 (<n2), whereby the respective refractive indices of the individual regions have the relationship in terms of magnitude of n3<n2<n1. 0052 The outside diameter a1 of the center core is 3.3 m, the outside diameter a2 (equal to the outside diameter a of the core region) of the outer core surrounding the center core is 20.8 m, and the outside diameter b of the cladding region surrounding the outer core is 125 m. The relative refractive index difference of each region is given by expression (3) while the cladding region having the refractive index n3 is employed as the reference area, whereby the maximum value n1 of relative refractive index difference of the center core with respect to the cladding region is 1.2%, and the relative refractive index difference n2 of the outer core with respect to the cladding region is 0.12%. 0053 Upon measurement of various characteristics of the dispersion-shifted optical fiber according to the first embodiment, the zero-dispersion wavelength and the cutoff wavelength at a length of 2 m were 1645 nm and 1.46 m, respectively. Also, with respect to light having a wavelength of 1550 nm, the effective area, the dispersion slope, the dispersion and the bending loss at a diameter of 32 mm were 56 m2, 0.13 ps/nm2/km, 12.2 ps/nm/km and 0.01 dB/turn, respectively. 0054 FIG. 3 is a graph showing the relationship between the bending loss (dB/turn) at a diameter of 32 mm and the cutoff wavelength (gm) at a length of 2 m in the dispersion-shifted optical fiber according to the present invention. This graph is the result of measurement in the state where the effective area and zero-dispersion wavelength were set within the ranges of 533 m2 and 164520 nm, respectively, showing the relationship between the cutoff wavelength and bending loss when the cutoff wavelength was successively changed as the shape of refractive index profile shown in FIG. 2 was slightly altered. 0055 For example, the bending loss at a diameter of 32 mm is 0.5 dB/turn or less according to the specification proposed by Bellcore. Therefore, in order to satisfy the specification of Bellcore, it is necessary for the cutoff wavelength at a length of 2 m to be 1.1 m or more as can be seen from FIG. 3. Here, the bending loss is expressed in terms of the value per turn of the transmission loss measured when a plurality of turns of the optical fiber to be measured are wound about a mandrel having a diameter of 32 mm. 0056 In general, while the dispersion characteristic of the optical fiber is expressed by the sum of material dispersion and waveguide dispersion (structural dispersion), it has been known that, when the material dispersion is taken into account, the zero-dispersion wavelength shifts toward the longer wavelength side as a greater amount of GeO2 is included. Therefore, the zero-dispersion wavelength can easily be shifted toward the longer wavelength side if the GeO2 concentration is made higher in an area with a higher optical power (e.g., the center core in the case of a double core structure or the outer core in the case of a ring core structure), i.e., if the relative refractive index difference of this area with respect to the cladding region (reference area) is made greater. FIG. 4 is a graph showing the relationship between the zero-dispersion wavelength and the relative refractive index difference of the area having the highest refractive index (the center core in the first embodiment) in the dispersion-shifted optical fiber according to the first embodiment. This graph shows the change in zero-dispersion wavelength when the relative refractive index difference n2 of the outer core (refractive index n2) with respect to the cladding region (refractive index n3) acting as the reference area (given by expression (3)), and the ratio (a1/a2) of the outside diameter a1 of the center core to the outside diameter a2 of the outer core in the core region are held constant. However, the outside diameter a2 of the outer core is changed such that the effective area becomes 56 m2. 0057 As can also be seen fromthe graph of FIG. 4, the maximum value n1 of relative refractive index difference of the center core with respect to the cladding region is needed to be 0.9% or more in the first embodiment in order to set the zero-dispersion wavelength to 1610 nm or more. 0058 While a typical optical fiber (optical fiber for light signals in the 1.3-m band) has a zero-dispersion wavelength which is set near 1310 nm, it can also transmit light signals in the 1.5-m band by utilizing the above-mentioned dispersion-compensating technique. In this case, while a dispersion of about 17 ps/nm/km occurs with respect to light signals in the 1.5-m band in this typical optical fiber (see FIG. 5A), there will conversely be no practical problems if the absolute value of dispersion value is suppressed to this level. If this theory is applied to the dispersion-shifted optical fiber according to this embodiment, then it can be seen that, as shown in FIG. 5B, a sufficient transmission quality is secured when the dispersion slope is set to 0.15 ps/nm2/km or less, preferably 0.09 ps/nm2/km or less (about 21 ps/nm/km at the maximum in the signal wavelength band in this embodiment) even in the case where the zero-dispersion wavelength is set on the longer wavelength side from the signal wavelength band. 0059 Second Embodiment 0060 FIG. 6 is a view showing the refractive index profile of the dispersion-shifted optical fiber according to the second embodiment. This refractive index profile 120 corresponds to the refractive index of each part on the line L passing the center axis O of the dispersion-shifted optical fiber 100 shown in FIG. 1. 0061 As with the dispersion-shifted optical fiber 100 shown in FIG. 1, the dispersion-shifted optical fiber according to the second embodiment comprises a core region having an outside diameter a and a cladding region having an outside diameter b. The core region (corresponding to the core region 100A in FIG. 1) has a double core structure constituted by a center core having a maximum refractive index n1 and an outer core provided so as to surround the outer periphery of the center core and having a refractive index n2 (<n1). The cladding region (corresponding to the cladding region 100B in FIG. 1), which is provided so as to surround the outer periphery of the core region, has a refractive index n3 (<n2), whereby the respective refractive indices of the individual regions have the relationship in terms of magnitude of n3<n2<n1. 0062 The outside diameter a1 of the center core is 2.8 m, the outside diameter a2 (equal to the outside diameter a of the core region) of the outer core surrounding the center core is 23.6 m, and the outside diameter b of the cladding region surrounding the outer core is 125 m. The relative refractive index difference of each region is given by expression (3) while the cladding region having the refractive index n3 is employed as the reference area, whereby the maximum value n1 of relative refractive index difference of the center core with respect to the cladding region is 1.4%, and the relative refractive index difference n2 of the outer core with respect to the cladding region is 0.12%. 0063 Upon measurement of various characteristics of the dispersion-shifted optical fiber according to the second embodiment, the zero-dispersion wavelength and the cutoff wavelength at a length of 2 m were 1628 nm and 1.59 m, respectively. Also, with respect to light having a wavelength of 1550 nm, the effective area, the dispersion slope, the dispersion and the bending loss at a diameter of 32 mm were 71 m2, 0.14 ps/nm2/km 11.9 ps/nm/km and 0.12 dB/turn, respectively. 0064 Third Embodiment 0065 FIG. 7 is a view showing the refractive index profile of the dispersion-shifted optical fiber according to the third embodiment. This refractive index profile 130 corresponds to the refractive index of each part on the line L passing the center axis O of the dispersion-shifted optical fiber 100 shown in FIG. 1. 0066 As with the dispersion-shifted optical fiber 100 shown in FIG. 1, the dispersion-shifted optical fiber according to the third embodiment comprises a core region having an outside diameter a and a cladding region having an outside diameter b. The core region (corresponding to the core region 100A in FIG. 1) has a multilayer core structure constituted by a center core having a maximum refractive index n1; an intermediate core provided so as to surround the outer periphery of the center core and having a refractive index n2 (<n1); and an outer core provided so as to surround the outer periphery of the intermediate core and having a refractive index n3 (>n2, <n1). The cladding region (corresponding to the cladding region 100B in FIG. 1), which is provided so as to surround the outer periphery of the core region, has a refractive index n4 (n2), whereby the respective refractive indices of the individual regions have the relationship in terms of magnitude of n2n4<n3<n1. It is not always necessary for the refractive index n2 of the intermediate core and the refractive index n4 of the cladding region to coincide with each other. 0067 The outside diameter a1 of the center core is 4.2 m, the outside diameter a2 (equal to the outside diameter a of the core region) of the outer core surrounding the center core is 14.6 m, the outside diameter a3 of the intermediate core positioned between the center core and the outer core is 9.9 m, and the outside diameter b of the cladding region surrounding the outer core is 125 m. The relative refractive index difference of each region is given by expression (3) while the cladding region having the refractive index n4 is employed as the reference area, whereby the maximum value n1 of relative refractive index difference of the center core with respect to the cladding region is 1.1%, and the relative refractive index difference n3 of the outer core with respect to the cladding region is 0.26%. 0068 Upon measurement of various characteristics of the dispersion-shifted optical fiber according to the third embodiment, the zero-dispersion wavelength and the cutoff wavelength at a length of 2 m were 1654 nm and 1.35 m, respectively. Also, with respect to light having a wavelength of 1550 nm, the effective area, the dispersion slope, the dispersion and the bending loss at a diameter of 32 mm were 57 m2, 0.15 ps/nm2/km, 15.2 ps/nm/km and 0.08 dB/turn, respectively. 0069 Fourth Embodiment 0070 FIG. 8 is a view showing the refractive index profile of the dispersion-shifted optical fiber according to the fourth embodiment. This refractive index profile 140 corresponds to the refractive index of each part on the line L passing the center axis O of the dispersion-shifted optical fiber 100 shown in FIG. 1. 0071 As with the dispersion-shifted optical fiber 100 shown in FIG. 1, the dispersion-shifted optical fiber according to the fourth embodiment comprises a core region having an outside diameter a and a cladding region having an outside diameter b. The core region (corresponding to the core region 100A in FIG. 1) has a multilayer core structure constituted by a center core having a maximum refractive index n1; an intermediate core provided so as to surround the outer periphery of the center core and having a refractive index n2 (<n1); and an outer core provided so as to surround the outer periphery of the intermediate core and having a refractive index n3 (>n2, <n1). The cladding region (corresponding to the cladding region 100B in FIG. 1), which is provided so as to surround the outer periphery of the core region, has a refractive index n4 (n2), whereby the respective refractive indices of the individual regions have the relationship in terms of magnitude of n2n4<n3<n1. It is not always necessary for the refractive index n2 of the intermediate core and the refractive index n4 of the cladding region to coincide with each other. 0072 The outside diameter a1 of the center core is 3.8 m, the outside diameter a2 (equal to the outside diameter a of the core region) of the outer core surrounding the center core is 13.8 m, the outside diameter a3 of the intermediate core positioned between the center core and the outer core is 9.7 m, and the outside diameter b of the cladding region surrounding the outer core is 125 m. The relative refractive index difference of each region is given by expression (3) while the cladding region having the refractive index n4 is employed as the reference area, whereby the maximum value n1 of relative refractive index difference of the center core with respect to the cladding region is 1.2%, and the relative refractive index difference n3 of the outer core with respect to the cladding region is 0.34%. 0073 Upon measurement of various characteristics of the dispersion-shifted optical fiber according to the fourth embodiment, the zero-dispersion wavelength and the cutoff wavelength at a length of 2 m were 1624 nm and 1.45 m, respectively. Also, with respect to light having a wavelength of 1550 nm, the effective area, the dispersion slope, the dispersion and the bending loss at a diameter of 32 mm were 71 m2, 0.15 ps/nm2/km, 12.5 ps/nm/km and 0.13 dB/turn, respectively. 0074 Fifth Embodiment 0075 FIG. 9 is a view showing the refractive index profile of the dispersion-shifted optical fiber according to the fifth embodiment. This refractive index profile 150 corresponds to the refractive index of each part on the line L passing the center axis O of the dispersion-shifted optical fiber 100 shown in FIG. 1. 0076 As with the dispersion-shifted optical fiber 100 shown in FIG. 1, the dispersion-shifted optical fiber according to the fifth embodiment comprises a core region having an outside diameter a and a cladding region having an outside diameter b. The core region (corresponding to the core region 100A in FIG. 1) has a multilayer core structure constituted by a center core having a maximum refractive index n1; an intermediate core provided so as to surround the outer periphery of the center core and having a refractive index n2 (<n1); and an outer core provided so as to surround the outer periphery of the intermediate core and having a refractive index n3 (>n2, <n1). The cladding region (corresponding to the cladding region 100B in FIG. 1), which is provided so as to surround the outer periphery of the core region, has a refractive index n4 (n2), whereby the respective refractive indices of the individual regions have the relationship in terms of magnitude of n2n4<n3<n1. It is not always necessary for the refractive index n2 of the intermediate core and the refractive index n4 of the cladding region to coincide with each other. 0077 The outside diameter a1 of the center core is 5.9 m, the outside diameter a2 (equal to the outside diameter a of the core region) of the outer core surrounding the center core is 21.0 m, the outside diameter a3 of the intermediate core positioned between the center core and the outer core is 12.6 m, and the outside diameter b of the cladding region surrounding the outer core is 125 m. The relative refractive index difference of each region is given by expression (3) while the cladding region having the refractive index n4 is employed as the reference area, whereby the maximum value n1 of relative refractive index difference of the center core with respect to the cladding region is 0.8%, and the relative refractive index difference n3 of the outer core with respect to the cladding region is 0.19%. 0078 Upon measurement of various characteristics of the dispersion-shifted optical fiber according to the fifth embodiment, the zero-dispersion wavelength and the cutoff wavelength at a length of 2 m were 1653 nm and 1.68 m, respectively. Also, with respect to light having a wavelength of 1550 nm, the effective area, the dispersion slope, the dispersion and the bending loss at a diameter of 32 mm were 56 m2, 0.08 ps/nm2/km, 9.1 ps/nm/km and 0.03 dB/turn, respectively. 0079 Sixth Embodiment 0080 FIG. 10 is a view showing the refractive index profile of the dispersion-shifted optical fiber according to the sixth embodiment. This refractive index profile 160 corresponds to the refractive index of each part on the line L passing the center axis O of the dispersion-shifted optical fiber 100 shown in FIG. 1. 0081 As with the dispersion-shifted optical fiber 100 shown in FIG. 1, the dispersion-shifted optical fiber according to the sixth embodiment comprises a core region having an outside diameter a and a cladding region having an outside diameter b. The core region (corresponding to the core region 100A in FIG. 1) has a ring core structure constituted by a center core having a refractive index n1 and an outer core provided so as to surround the outer periphery of the center core and having a refractive index n2 (>n1). The cladding region (corresponding to the cladding region 100B in FIG. 1), which is provided so as to surround the outer periphery of the core region, has a refractive index n3 (>n1, <n2), whereby the respective refractive indices of the individual regions have the relationship in terms of magnitude of n1<n3<n2. 0082 The outside diameter a1 of the center core is 3.4 m, the outside diameter a2 (equal to the outside diameter a of the core region) of the outer core surrounding the center core is 5.8 m, and the outside diameter b of the cladding region surrounding the outer core is 125 m. The relative refractive index difference of each region is given by expression (3) while the cladding region having the refractive index n3 is employed as the reference area, whereby the relative refractive index difference n3 of the center core with respect to the cladding region is 0.6%, and the maximum value n4 of relative refractive index difference of the outer core with respect to the cladding region is 1.5%. 0083 Upon measurement of various characteristics of the dispersion-shifted optical fiber according to the sixth embodiment, the zero-dispersion wavelength and the cutoff wavelength at a length of 2 m were 1634 nm and 1.58 m, respectively. Also, with respect to light having a wavelength of 1550 nm, the effective area, the dispersion slope, the dispersion and the bending loss at a diameter of 32 mm were 53 m2, 0.09 ps/nm2/km, 7.7 ps/nm/km and 51010 dB/turn, respectively. 0084 Seventh Embodiment 0085 FIG. 11 is a view showing the refractive index profile of the dispersion-shifted optical fiber according to the seventh embodiment. This refractive index profile 170 corresponds to the refractive index of each part on the line L passing the center axis O of the dispersion-shifted optical fiber 100 shown in FIG. 1. 0086 As with the dispersion-shifted optical fiber 100 shown in FIG. 1, the dispersion-shifted optical fiber according to the seventh embodiment comprises a core region having an outside diameter a and a cladding region having an outside diameter b. The core region (corresponding to the core region 100A in FIG. 1) has a ring core structure constituted by a center core having a refractive index n1 and an outer core provided so as to surround the outer periphery of the center core and having a refractive index n2 (>n1). The cladding region (corresponding to the cladding region 100B in FIG. 1), which is provided so as to surround the outer periphery of the core region, has a refractive index n3 (>n1, <n2), whereby the respective refractive indices of the individual regions have the relationship in terms of magnitude of n1<n3<n2. 0087 The outside diameter a1 of the center core is 4.4 m, the outside diameter a2 (equal to the outside diameter a of the core region) of the outer core surrounding the center core is 6.6 m, and the outside diameter b of the cladding region surrounding the outer core is 125 m. The relative refractive index difference of each region is given by expression (3) while the cladding region having the refractive index n3 is employed as the reference area, whereby the relative refractive index difference n3 of the center core with respect to the cladding region is 0.6%, and the maximum value n4 of relative refractive index difference of the outer core with respect to the cladding region is 1.5%. 0088 Upon measurement of various characteristics of the dispersion-shifted optical fiber according to the seventh embodiment, the zero-dispersion wavelength and the cutoff wavelength at a length of 2 m were 1658 nm and 1.60 m, respectively. Also, with respect to light having a wavelength of 1550 nm, the effective area, the dispersion slope, the dispersion and the bending loss at a diameter of 32 mm were 73 m2, 0.09 ps/nm2/km, 10.4 ps/nm/km and 3105 dB/turn, respectively. 0089 Eighth Embodiment 0090 FIG. 12 is a view showing the refractive index profile of the dispersion-shifted optical fiber according to the eighth embodiment. This refractive index profile 180 corresponds to the refractive index of each part on the line L passing the center axis O of the dispersion-shifted optical fiber 100 shown in FIG. 1. 0091 As with the dispersion-shifted optical fiber 100 shown in FIG. 1, the dispersion-shifted optical fiber according to the eighth embodiment comprises a core region having an outside diameter a and a cladding region having an outside diameter b. The core region (corresponding to the core region 100A in FIG. 1) has a ring core structure constituted by a center core having a refractive index n1 and an outer core provided so as to surround the outer periphery of the center core and having a refractive index n2 (>n1). The cladding region (corresponding to the cladding region 100B in FIG. 1), which is provided so as to surround the outer periphery of the core region, has a depressed cladding structure constituted by an inner cladding provided so as to surround the outer periphery of the outer core and having a refractive index n3 (n1), and an outer cladding provided so as to surround the outer periphery of the inner cladding and having a refractive index n4 (>n3, <n2). As a consequence, the respective refractive indices of the individual regions have the relationship in terms of magnitude of n1n3<n4<n2. It is not always necessary for the refractive index n1 of the center core and the refractive index n3 of the inner cladding to coincide with each other. 0092 The outside diameter a1 of the center core is 4.6 m, whereas the outside diameter a2 (equal to the outside diameter a of the core region) of the outer core surrounding the center core is 7.5 m. On the other hand, the outside diameter b1 of the inner cladding surrounding the outer core is 11.2 m, whereas the outside diameter b of the whole cladding region (equal to the outside diameter of the outer cladding) is 125 m. The relative refractive index difference of each region is given by expression (3) while the outer cladding having the refractive index n4 is employed as the reference area, whereby the relative refractive index difference n5 of the center core with respect to the outer cladding is 0.4%, the maximum value n6 of relative refractive index difference of the outer core with respect to the outer cladding region is 1.2%, and the relative refractive index difference n7 of the inner cladding with respect to the outer cladding is 0.4%. 0093 Upon measurement of various characteristics of the dispersion-shifted optical fiber according to the eighth embodiment, the zero-dispersion wavelength and the cutoff wavelength at a length of 2 m were 1663 nm and 1.52 m, respectively. Also, with respect to light having a wavelength of 1550 nm, the effective area, the dispersion slope, the dispersion and the bending loss at a diameter of 32 mm were 70 m2, 0.08 ps/nm2/km, 9.0 ps/nm/km and 4104 dB/turn, respectively. 0094 Ninth Embodiment 0095 FIG. 13 is a view showing the refractive index profile of the dispersion-shifted optical fiber according to the ninth embodiment. This refractive index profile 190 corresponds to the refractive index of each part on the line L passing the center axis O of the dispersion-shifted optical fiber 100 shown in FIG. 1. 0096 As with the dispersion-shifted optical fiber 100 shown in FIG. 1, the dispersion-shifted optical fiber according to the ninth embodiment comprises a core region having an outside diameter a and a cladding region having an outside diameter b. The core region (corresponding to the core region 100A in FIG. 1) has a ring core structure constituted by a center core having a refractive index n1; a first intermediate core provided so as to surround the outer periphery of the center core and having a refractive index n2 (>n1), a second intermediate core provided so as to surround the outer periphery of the first intermediate core and having a refractive index n3 (n1), and an outer core provided so as to surround the outer periphery of the second intermediate core and having a refractive index n4 (>n3,<n2). The cladding region (corresponding to the cladding region 100B in FIG. 1), which is provided so as to surround the outer periphery of the core region, has a refractive index n5 (>n3, <n4). As a consequence, the respective refractive indices of the individual regions have the relationship in terms of magnitude of n1n3<n5<n4<n2. It is not always necessary for the refractive index n1 of the center core and the refractive index n3 of the second intermediate core to coincide with each other. 0097 The outside diameter a1 of the center core is 5.7 m, the outside diameter a2 (equal to the outside diameter a of the core region) of the outer core surrounding the center core is 19.1 m, the outside diameter a3 of the first intermediate core provided between the center core and the outer core is 9.6 m, and the outside diameter a4 of the second intermediate core provided between the first intermediate core and the outer core is 16.2 m. On the other hand, the outside diameter b of the cladding surrounding the outer core is 125 m. The relative refractive index difference of each region is given by expression (3) while the cladding region having the refractive index n5 is employed as the reference area, whereby the relative refractive index difference n5 of the center core and second intermediate core with respect to the cladding region is 1.0%, the maximum value n6 of relative refractive index difference of the first intermediate core with respect to the cladding region is 0.95%, and the relative refractive index difference n7 of the outer cladding with respect to the cladding region is 0.4%. 0098 Upon measurement of various characteristics of the dispersion-shifted optical fiber according to the ninth embodiment, the zero-dispersion wavelength and the cutoff wavelength at a length of 2 m were 1625 nm and 1.68 m, respectively. Also, with respect to light having a wavelength of 1550 nm, the effective area, the dispersion slope, the dispersion and the bending loss at a diameter of 32 mm were 94 m2, 0.07 ps/nm2/km, 4.9 ps/nm/km and 4104 dB/turn, respectively. 0099 Tenth Embodiment 0100 FIG. 14 is a view showing the refractive index profile of the dispersion-shifted optical fiber according to the tenth embodiment. This refractive index profile 200 corresponds to the refractive index of each part on the line L passing the center axis O of the dispersion-shifted optical fiber 100 shown in FIG. 1. 0101 As with the dispersion-shifted optical fiber 100 shown in FIG. 1, the dispersion-shifted optical fiber according to the tenth embodiment comprises a core region having an outside diameter a and a cladding region having an outside diameter b. The core region (corresponding to the core region 100A in FIG. 1) has a multilayer core structure constituted by a center core having a maximum refractive index n1; an intermediate core provided so as to surround the outer periphery of the center core and having a refractive index n2 (<n1); and an outer core provided so as to surround the outer periphery of the intermediate core and having a refractive index n3 (>n2, <n1). The cladding region (corresponding to the cladding region 100B in FIG. 1), which is provided so as to surround the outer periphery of the core region, has a refractive index n4 (n2), whereby the respective refractive indices of the individual regions have the relationship in terms of magnitude of n2n4<n3<n1. It is not always necessary for the refractive index n2 of the intermediate core and the refractive index n4 of the cladding region to coincide with each other. 0102 The outside diameter a1 of the center core is 5.2 m, the outside diameter a2 (equal to the outside diameter a of the core region) of the outer core surrounding the center core is 18.6 m, the outside diameter a3 of the intermediate core positioned between the center core and the outer core is 12.6 m, and the outside diameter b of the cladding region surrounding the outer core is 125 m. The relative refractive index difference of each region is given by expression (3) while the cladding region having the refractive index n4 is employed as the reference area, whereby the maximum value n1 of relative refractive index difference of the center core with respect to the cladding region is 0.95%, and the relative refractive index difference n3 of the outer core with respect to the cladding region is 0.25%. 0103 Upon measurement of various characteristics of the dispersion-shifted optical fiber according to the tenth embodiment, the zero-dispersion wavelength and the cutoff wavelength at a length of 2 m were 1708 nm and 1.62 m, respectively. Also, with respect to light having a wavelength of 1550 nm, the effective area, the dispersion slope, the dispersion and the bending loss at a diameter of 32 mm were 47 m2, 0.08 ps/nm2/km, 16.0 ps/nm/km and 0.01 dB/turn, respectively. 0104 FIG. 15 is a table listing various characteristics of the dispersion-shifted optical fibers according to the first to tenth embodiments explained in the foregoing. Each of the dispersion-shifted optical fibers according to these embodiments has a zero-dispersion wavelength of 1610 nm or more but 1750 nm or less, and a cutoff wavelength of 1.1 m or more at a length of 2 m, and has, with respect to light having a wavelength of 1550 nm, an effective area of 45 m2 or more and a dispersion slope of 0.15 ps/nm2/km or less. 0105 Therefore, each of the dispersion-shifted optical fibers is employable in WDM transmission systems including an optical fiber amplifier adapted to amplify light signals in a wavelength band of 1530 nm to 1610 nm, and effectively suppresses the occurrence of nonlinear phenomena such as four-wave mixing, self phase modulation, and the like, thereby realizing high-quality optical transmission. In particular, each of the dispersion-shifted optical fibers according to the second, fourth, and seventh to ninth embodiments has an effective area of 70 m2 or more, thus allowing the light signals propagating through the dispersion-shifted optical fiber to keep the power density at a lower level, thereby being able to further suppress the occurrence of nonlinear phenomena. 0106 Also, each of the dispersion-shifted optical fibers has an excellent bending characteristic. Here, each of the dispersion-shifted optical fibers according to the fifth and ninth embodiments has a cutoff wavelength longer than the signal wavelength. In view of the fact that the actual transmission distance of light signals is several hundreds to several thousands of kilometers, there are no problems since higher-order modes attenuate. 0107 Without being restricted to the above-mentioned embodiments, the present invention can be modified in various manners. For instance, each of FIGS. 2 and 6 to 14 just shows an example of refractive index profiles, and various other shapes can be employed as the refractive index profile for realizing the dispersion-shifted optical fiber according to the present invention. 0108 As in the foregoing, the dispersion-shifted optical fiber according to the present invention has a zero-dispersion wavelength of 1610 nm or more but 1750 nm or less, and a cutoff wavelength of 1.1 m or more at a length of 2 m, and has, with respect to light having a wavelength of 1550 nm, an effective area of 45 m2 or more and a dispersion slope of 0.15 ps/nm2/km or less. Therefore, even in the case where each light signal belonging in the wavelength band of 1530 nm to 1610 nm that can optically be amplified by an optical fiber amplifier having an expanded amplification wavelength band propagates through this dispersion-shifted optical fiber, the occurrence of nonlinear phenomena such as four-wave mixing, self phase modulation, and the like is effectively suppressed. Also, since the dispersion-shifted optical fiber according to the present invention has an excellent bending characteristic, high-performance optical transmission can sufficiently be realized over a wide band. 0109 In the case where the effective area is 70 m2 or more, the power density of each light signal propagating through the dispersion-shifted optical fiber is kept lower, whereby the occurrence of nonlinear phenomena is further effectively suppressed, and higher-quality optical transmission can be realized. 0110 From the invention thus described, it will be obvious that the embodiments 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.