Title of Invention	"DISPERSION-SHIFTED FIBER"
Abstract	A dispersion-shifted libber (100, 200, 300) having a zed-dispersion wavelength outside of a wavelength band of 1.53 10 1.56 (.mi, the libber having a core region (1 10, 120; 1 1 I, 121; 301, !02 *(H) and a cladding region (210, 220; 21 1, 221; 304. M)5); said core region comprises an inner core (I10; I II; 30l) and an outer core (120; 121; 303) disposed around the outer periphery of sail inner core, and said cladding region comprises an inner cladding (210; 21 1; MM: disposed around the outer periphery of said outer core, said inner cladding having a enactive index lower than thai of said outer core; and an outside diameter 2e; an outer cladding (220; 221; 305) disposed around the outer periphery oil said inner cladding, said outer cladding having a refractive index higher than Thai of said inner cladding.

Title of Invention

"DISPERSION-SHIFTED FIBER"

Abstract

A dispersion-shifted libber (100, 200, 300) having a zed-dispersion wavelength outside of a wavelength band of 1.53 10 1.56 (.mi, the libber having a core region (1 10, 120; 1 1 I, 121; 301, !02 *(H) and a cladding region (210, 220; 21 1, 221; 304. M)5); said core region comprises an inner core (I10; I II; 30l) and an outer core (120; 121; 303) disposed around the outer periphery of sail inner core, and said cladding region comprises an inner cladding (210; 21 1; MM: disposed around the outer periphery of said outer core, said inner cladding having a enactive index lower than thai of said outer core; and an outside diameter 2e; an outer cladding (220; 221; 305) disposed around the outer periphery oil said inner cladding, said outer cladding having a refractive index higher than Thai of said inner cladding.

Full Text	BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a single-mode dispersion-shifted 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. 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 pm or L.55 fjiti 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 to. 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. 2-141.704 propose a dispersion-shifted fioer 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 opticat 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 mentiored 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. Patent No. S ,483,612K 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 ir. a relaying transmission system. SUMMARY OF _ THE_ INVENTION As a result of studies concerning the above-menticned 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 srea Aaff is set on the order of 55 µm2. 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 qi.ality in the conventional transmission distance in view of further advance in wavelength multiplexing which will occur as communications become more sophisticated. Hero, as disclosed or. Japanese Patent Application Laid-Open No. 8-.49251, effective core cross-sectional area Aeff is given by the following expression: (Formula Removed) wherein S 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 1.53µm (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/kn in terrr.s of absolute value, a dispersion slope not greater than 0.13 ps/nm2'/km in terms of absolute value, an effective core crass-sertional area Aeff of 7G µm' or more, and a transmission loss not greater, than 0.25 dS/km with respect to light in a wave .ength band of 1.55 µm. Here,- when the dispersion level in terms of absolute value is smaller than 1.O ps/nm/km, waveform distortion caused by four-wave mixing, unstable modulation, and the like cannot practically ise t neglected in long-haul light transmission ovor 20 km or more- wh In the dispersion-shifted fiber according to the present invention, the absolute value of dispersion slope is lot greater than 0,13 ps/nm2/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./A,)- Accordingly, at the same propagating light condition, nonlinear optical effects are effsctively restrained from occurring when the nonlinear optical effect constant (N./Aeff) is made smaller. On the other hand, since nonlinear refractive index N: 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, 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 Aetr is increased to 70 µm.' 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-soct.ional area Aftf, and nonlinear optical constant (N./Aeff) in a dispersion-shifted fiber having a typical composition. From Fig. 1, it can be seen that nonlinear optical constant (N./Aeff), which is 5.8 x i(Ti0 (i/w) when effective core cress-sectional area Aeff is 55 µm2, becomes 4.6 * 10-10 (1/W) when effective core cross-sectional area Aeff is 70 µm2, thus being reduced by about 20%. Accordingly, as compared with the conventional dispersion-shifted fiber, the dispersion-shifted fiber according to tr.e praser.t invention can increase the degree of wavelength multiplexing in signal light. In general, refractive index N of a medium under ST.rang light changes depending on light intensity. Accordingiy, the minimum order of effect on ::e£ractive index N is expressed by: N - N0, + N2, • E2 wherein N,, is a linear refractive index, N2 is a nonlinear refractive index, and E is a field amplitude. Namely, under strong light, the refractive iidex N of the mediuin is given by the sum of N, and an increase which is in proportion to the square of field amplitude E. Tn particular, the constant of proportici N. (unit: m2/v2) 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 201 (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 th? case where the. conventional repeater spacing is 50 km, for example, "he number of repeaters can be reduced by dDOUl 101. Furtier, 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 A9ff, 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, Ihe dispersicn-shifted fiber according to the present invent.ior 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 tending generated at the cable-forming step or the like. Preferably, in the dispersion-shifted liber Arrordinq to the present invention, the absolute value of dispersion slope is 0.09 ps/nm2/km or mere. The smaller the dispersion slope is, the less becomes the variation in the amount of waveform distortion caused by wavelength dispersion in the signal lightis. 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 ;5lope is preferably at least 0.09 ps/nm2/km but not greater than 0.13 ps/nm:/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 dispers Ion-shifted fiber according to the present invention has a depressed ciadding/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 outride diameter of 2b; an inr.er 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 Aeff is increased in a d:.spersion-shitted fiber having a simple dual-shape core type refractive index profile without a depression cladding structure, namely, non-depressed cladding/du.il-shape core type refractive index profile, in a sta-:e where its absolute value of dispersion is set to 1.0 to 4.5 ps/nm/km at the wavelength of 1,5W nm, its cutoff wavelengtin 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 wavel.enqtn, 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 cere type refractive index is lower than that of a disversion-shifted fiber havinq 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 conliyuiation 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 rispersion-shifted fiber satisfies the following relationships: a/b ≤ 0.15 (I) 0.8% ≤ ▲n. ≤ 1.2% (2) 0.12% ≤ ▲n, ≤, 0.30% (3) ▲n,▲n: ≤ 0.95 (4) wherein ;in, is a relative refractive index difference of the inner core with respect to the inner cladding/ uns is a relative refractive index difference of the outer core with respect to the inner cladding, anc in, 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 absclute value of dispei sion slope to become at least 0.09 ps/nm2/kn but not larger than 0.13 ps/nm2/km. Here,- the relative refractive index difference ▲n: of the inner cere with respect to the inner cladding, relative refractive index difference ▲n, of the outer core with respect to the inner cladding, and relative refractive index difference ▲n. of the outer cladding with respect to the inner cladding are respectively deftn«d as follows: ▲n1 = (n.2 ≤n3J)/(2n.2) (5) ▲n2. = (n.: -n,:)/(2n:3') (6) ▲n, = (n4 -n/)/(2n.:) (7) wh«rein n is the refractive index of the inrer core, n{ is the refractive index of the outer core, n. is the refractive index of the inner cladding, and n4 is the refractive index of the outer cladding. In -:his specification, each relative refractive index differencs is expressed in terms of percentage. Prafsrably, the first embodiment of the dispersion-shifted fiber further satisfies t:ie following relationships 1.2 ≤ c/b ≤ 3.5 (8) wherein 2- is an outside diameter of the i 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 ..nner cladding is too thick, it functions in a I way similar to a normal cladding and fails to yield the i cutoff-wavelength-shortening effect of the depressed cladding type refractive index. When the dispersion-shifted fiber satisfies the relationship of /b ≥ 1.2, the bending loss in the case where it is bent at a diameter of 32 nun can become 0.5 dB/turn or 5ess. 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 mad* shorter, thereby making it easy to secure a wavelength range of signal-light which allows single-moc.e 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 ot the dispersion-shitted 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 nuts ide 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 ojter 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% ≤ An. ≤ 1.1% (11) 0.2% ≤ An3 - An. s 0.7% (12) ▲n4/▲n, s 0.95 (13) wherein An. is a relative refractive index difference of said inner core with respect to said inner cladding, An, is a relative refractive index difference of said intermediite core with respect to said inner cladding.. An, is a relative refractive index difference' of said outer cor* with respect to said inner claddi iq, and '-.n, is a relative refractive index difference of said o-.oater cladding with respect to said inner cladding. The .above-relationships (9) and (10) an? condition:; to satisfy the effective core croiss-sect tan area Aeff of 70 µm:. The relative refractive index difference in. of said inner core with respert to said inner cladding is preferably 0.5 % or more .an 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 An, is 1.1 % or less, the dispersion slope at wavelength of 1,550 nat falls within a range of 0.09 to n.13 ps/nmVkm. The value (▲n3 - ▲n, ) should be C.2 % or more in order to satisfy the condition that the bending loss when bent at a diameter of 32 mm becomes; 0.5 dB/turn or less, and it should be 0.7 % or loss in order to nake cutoff wavelength at a length of 2 m set 2.2 pm 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 dU/kra. Further, the second embodiment of the dispersion-shifted f:.ber satisfies the following relatironship: 1 .2 ≤, d/c ≤3.5 (14) wherein 2d is an outside diameter of the inneir cladding. The bending loss when bent at a diameter- of 12 mm becomes 0.5 dB/kre 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 cnly, 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. Accordingly, the present invention relates to a dispersion-shifted fiber (100, 200, 300) having a zero-dispersion wavelength outside of a wavelength band of 1.53 to 1.56 nm, the fiber having a core region (110, 120, 111, 121, 301, 302, 303) and a cladding region (210, 220, 211, 221, 304, 305), said core region comprises an inner core (110, 111, 301) having a predetermined refractive index and an outside diameter of 2a, and an outer core (120, 121, 303) disposed around the outer periphery of said inner core, said outer core having a refractive index lower than that of said inner core and an outside diameter of 2b, and said cladding region comprises an inner cladding (210, 211, 304) disposed around the outer periphery of said outer core, said inner cladding having a refractive index lower than that of said outer core and an outside diameter 2c; an outer cladding (220, 221, 305) disposed around the outer periphery of said inner cladding, said outer cladding having a refractive index higher than that of said inner cladding, said dispersion-shifted fiber satisfies the following relationships: a/b ≤0.15 0.8% ≤ Am ≤1.2% 0.12 ≤An2 ≤0.30% Ana/Ana whereby said dispersion-shifted fiber has a dispersion level of 1.0 to 4.5ps/nm/km in terms of absolute value at a wavelength of 1,550 nm, a dispersion slope not greater than 0.13 ps/nm2/km in terms of absolute value at the wavelength of 1,550 nm, a dispersion slope not greater than 0.13 ps/nm2/km in terms of absolute value at the wavelength of 1,550 nm, an effective area not less than 70 \|^m2 at the wavelength of 1,550 nm, a transmission loss not greater than 0.25 dB/km with respect to light in a 1.55 - µm wavelength band and a cutoff wavelength c (Lo) at a length of 2m satisfying the following condition: c (Lo) ≤ 2.2µm. BHIEF. .DBS-CRI2IIQH..OF THEAccompanying DRAWINGS Fig. 1 is a graph showing an example of relationship between effective core cross-sectional area Aeff and nonlinear optical constant (N,/Aiff); Fig. 2 is a view showing a cross-sectioral configuration of a typical embodiment of the dispersion-shifted fiber according to the present invention and its refractive index profile (depressed cladding/cual-shape core type); Fig. 3 is a graph showing a relationship between value fa/b) and effective core cross-sectional area Aeff V Fig. 4 is a graph showing a relationship between relative refractive index difference in; 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 In. and dispersion slope; Fig. 6 is a graph showing a relationship between relative refractive index difference .In, and bending loss generated when bent at a diameter of 32 nun; Pig. 7 is a graph showing a relationship between relative refractive index difference ▲n2 and cutoff wavelength at a reference length of 2 m; Pig. 8 is a graph showing a relationship between value (An.Mn.) and transmission loss; Fig. 9 is a graph showing a relationship between value (cYb) 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 f:.ber according to the present invention and its refractive index profile (depressed cladding/segineted-core type); Pig. 13 is a view showing a first application of the depressed cladding/segmented-core type refractive •index profile in the second embodiment of tho 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 tho 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. Pig. 2 is a view showing a cross-sectional configuration of a typica". 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. as its maximum refractive index and an outside diameter 2a; an outer cora 120, disposed around the outer periphery of the inner core 110, having a refractive index n; ( n3). As a result of tiis configuration, the dispersion-shifted fiber '.00 realizes a depressed ciadding/dual-shape core type refractive index profile 101. This dispersion-shifted fiber satisfies the following relationships: a/b ≤ 0.15 (1) 0.8% 4 L'.n, ≤ 1.2% (2; 0.12% 4 An, ≤ 0.30% (3) ▲nj/in: ≤ 0.95 (4) 1.2 ≤. c/b ≤ 3.5 (8) wherein An, is a relative refractive index difference of the inner core 110 with respect to the inner cladding 210, in. is a relative refractive index difference of the outer core 120 with respect to the inner cladding 210, and ▲n3 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 A9,f, In Fig. 3, while &H! is 1.0% and An2 is 0.2%, the outside: diameter 2a and the outside diameter 2b of the inner sore 110 are changed so as to attain a zero-dispersion wavelength of 1,580 nm. It can be seen from ?ig. l that A,fi becomes 70 µm.: or more when (a/b) dees not exceed 0.15. Fig. 4 is a graph showing a relationship between the relative refractive index difference ▲n1 of the inner cors 110 with respect to the inner cladding 210 and absolute value JDJ of dispersion level D with respect to light having a wavelength of 1,55D nm. in Fig. 4, tie outside diameter 2a of rhe inner core 110, outside diameter 2b of the outer core 120, relative refractive index difference an. of the inner cor* 110 with respect to the inner cladding 210, and -elative refractive index difference An: of the outer cor« 120 with respect to the inner cladding 210 are changed so as to satisfy that (a/b) is C.13 and A,,, is £0 ^m. Tr can be seen from Fig. 4 that \|D is equal to or larger than 1.C ps/nm/km when In, exceeds O.S%, and that ;Dj is equal to or smaller than 4.5 ps/run/km when ai^ is approximately 1.2%. Fig. 5 is a .graph showing a relationship between the relative refractive index difference -n. of the inner core 110 with rsspect to the inner cladding 210 and dispersion slope. In Fig, 5, the outside diameter 2a of the inner core 110,.oulside diameter 2h of the i outer core 120, relative refractive index difference ▲nt of the inner core 110 with respect to the inner cladding 210, and relative refractive index difference _n. ot the outer core 120 with respect to the inner cladding 210 are changed so as to satisfy 0.13 tf (a/b), 80 ym" of Aett, 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 F'ig. 5 that the dispersion slope becomes.. 0.13 ps/nm2/km or ;more when An, is equal to or greater than 1.2%. Fig. 6 is a graph showing a relationship between the relative refractive index difference ▲n2 af the outer core 120 with respect to the inner cladding 210 and bending loss generated when bent at a diameter of 32 mm. Ir. Fig. 6, the outside diameter 2a of the inner core 110, outside diameter 2b of the outer cere 120, and relative refractive index diffrence .'.n. of the Outer core 12C with respect to the inner cladding 210 are changed so as to satisfy that relative refractive index difference ±n, is l.C% in the inner core 110 with respect to the inner cladding 210, (a/b) is 0.13, Aeff is 80 ^m2, and zero-dispersion wavelength is 1,580 nm. It can be seen from Fig. 6 that the bending oss upon bending at a diameter of 32 nun is 0.5 dB/tum or less when an2 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 11 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 An. of the outer core 120 with respect to the inner clacding 210 and cutoff wavelength at a reference length cf 2 m. In Fig. 7, the outside diameter 2a of the inner core 110, outside diameter 2b of the outer core 120, relative refractive zndex difference in, of the outer rore 1?0 • with respect to the inner cladding 210, and relative refractive index difference ins of the outer cladding 220 with respect to the inner cladding 210 are changed so as to satisfy that in, is 1.0% in the inner core 110 with respect to the inner cladding 210, (a/b) is 0.13, Aeff, is 80 µm, zero-dispersion wavelength is ' ,580 nm, and ▲n,/▲n, is 0.8. Normally, cutoff wavelength 2 of an optical fiber is measured according to a bending process performed at a length of 2 m, which is recommended by CCI':T-G. 650. When a S-node optical fiber has a length L or. 2 m, the cutoff wavelength , as the lowest wavelength allowing single-mode transmission, coincides with the result of the above-mentioned measurement. Tt has been known that, as the length L increases, the cutoff wavelength  changes according to the following expression (15): ▲C(L) • C(L, = 2 m) - 0.184 x log..{L/L0) (IS) (see T. Koto et al., OECC'96 Technical Digest. July! 1966, Makuhari Messe, pp. 160-161). On the other hand, the dispersion-shifted fiber according to the present invention is assumec to transmit signal light having a wavelength in the band of 1.55 ]JEi over a long distance of 20 km or nore. Consequently, it is necessary for the cutoff wavelength AC(L0) at a length of 2 m to satisfy the following expression (16): XC(L) From Fig. 7, it can be seen that the cutoff wavelength ,\C(L.) is equal to or less than 1.', ;jm at a length of 2 T when An. i'ig. 8 is a graph showing a relationship between value (£n,/£n:) and transmission loss. Tn Fie. 8, the relative refractive index difference in} of the outer cladding 220 with respect to the inner cladding 210 is changed under the following conditions: relative refractive index difference ▲n, - 1.0%; relative refractive index difference ▲n, = 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 ;▲n2,/▲n2.) is greater than 0.9S. Fig. 9 is a graph showing a relationship between value (c/b) and bending less at a diameter o-° 32 ram,. In Fig. 9, the outside diameter 2c of the inner cladding 210 is changed under the following conditions: relative refractive index difference ir^ 1.0%; relative refractive index difference An, = 0.20%; relative -refractive index difference ▲n3. = 0.12%; radius a -- 2.1 µm; and radius b >s 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 d3/turr. when (c/b) dope not exceed 1.2- Fig. 1C is a graph showing a relationship between value (c/b) and cutoff wavelength at a length of 2 m. In Fig. 1C, the outside diameter 2c of the inner cladding 210 is changed under the following conditions: relative refractive index difference in, = l.10%; relative retractive index difference ▲n2, = 0,20%; relative refractive index difference ▲n3, - 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 indt>x profile, it satisfies the following relationships: a/b ≤ 0.15 (1) 0.8% ≤ An4 s 1.21 (2) 0.12% ≤ An- ≤ 0.30% (3) ▲n3/▲n; ≤ 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/mr;Vkm in terms of absolute value, an effective core cross-sectional area Aefr of 70 µm1 or move, a transmission loss not greater than 0.25 dB/kre with respect to light in the wavelength band of 1.55 ;µm, and a bending loss of 0.5 d3/turn or less when bent at a diameter of 32 mm. Thus, favorably realized is a dispers ioi-shi ft.d 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 hav.ng 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 lover than that of the outer core and an outside' diameter of 2d; and an outer cladding, disposed around the otrer periphery o≤ the inner cladding, having a refractive index higher than that of the inner cladding. Accordingly, the dispersion-shifted fibsr having the above-mentioned various characteristics should satisty the following relationshipsi a/c ≤ 0 .42 (9) b/c ≥ 0.60 (10) 0.5% ≤ ▲n, ≤ 1.1 (11 ) 0.2% ≤ ▲n, - ▲n2- ▲nH/▲nj 1.2 ≤ d/C ≤ 3.5 (14) wherein in. is a relative refractive index difference of said inner core with respect to said inner cladding, ▲n2 is a relative refractive index difference of said intermediate cere with respect to said inner cladding, ▲n3, is a relative refractive index difference of said outer core with respect to said inner claddirg, and ▲n4, is a relative refractive index difference of said outer cladding with respect to said inner cladding, wherein 2c; 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: x-1,2,3,4 wherein n. is the refractive index of the inner core, n. is the refractive index of the intermediate core, n. is the refractive index of the outer core, and r.. is the refractive index of the outer cladding. Fig. 11 is a view showing a cress-sectional configuration of a first typical example (dual-shape core type) of a dispersion-shifted fiber according to the present invention and its refractive indax profile. As shown in Fig. 11, the dispersion-shifted fiber 200 has a depressed cladding/dual-shape core typs refractive index profile 201 and comprises ai 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 3? 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 2 11. Also, the first embodiment of the dispersion-shifted fxber 200 satisfies the above-mentioned relational, expressions (1) to (4) and (8) as follows: a/b = 0.13 0.8% 0.12* ▲n, s 0.12% ▲n,/▲n. = 0.6 1.2 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/im/tan; dispersion slope • 0.111 ps/nm2/km; effective core cross-sectional area Ae,t * 78.2 pm; cutoff wavelength * 1.59 ^m- 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 P.g. 12, the second embodiment of the dispersion-shifted fiber 300 has a segnented-core type refractive index profile and comprises an inner core 301 having an outside', diameter 2a. ot 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 core1 303, disposed around the outer periphery of the intermediate protiie, it can reduce bending loss ar.d favorably realize the aimed dispersion-shifted fiber. From the invention thus described, it will be obvious t.iat the implementation of the invention may b* varied in many ways. Such variations are no: to be regarded is a departure from the spirit and scope o* 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. Further, characteristics of the second embodiment of the dispersion-shifted fiber measured at -;he 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 pa/nm2/km; effective core cross-sectional area Aart= 81.8 µm; cutoff wavelength • 1.74 µm; bending loss = 0.1 dB/turn when bent at a diameter ot 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 µm2 or greater, nonlinear optical effects are effectively restrained from occurring. Accordingly, it is possible to favorably realize a -dispersion-shifted fiber, suitable for long-taul 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 prorile, it can reduce bending less and favorably realize the aimed dispersion-shifted fiber. From the invention thus described, it will be obvious that the implementation of the inver. cion may be varied in many ways. Such variations are no-: to be reqarded .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. WE CLAIM: A dispersion-shifted fiber (100, 200, 300) having a zero-dispersion wavelength outside of a wavelength band of 1.53 to 1.56 jam, the fiber having a core region'(110, 120; 111, 121; 301, 302, 303) and a cladding region (210, 220; 211, 221; 304, 305); said core region comprises: an inner core (110; 111; 301) having a predetermined refractive index and an outside diameter of 2a; and an outer core (120; 121; 303) disposed around the outer periphery of said inner core, said outer core having a refractive index lower than that of said inner core and an outside diameter of 2b; and said cladding region comprises: an inner cladding (210; 211; 304) disposed around the outer periphery of said outer core, said inner cladding having a refractive index lower than that of said outer core; and an outside diameter 2c; an outer cladding (220; 221; 305) disposed around the outer periphery of said inner cladding, said outer cladding having a refractive index higher than that of said inner cladding; said dispersion-shifted fiber satisfies the following relationships: a/b £0.15 0.8% ≤ ▲Ani ≤ 1.2% 0.12 ≤▲n2 ≤0.30% ▲n3/An2 ≤ 0.95 whereby said dispersion-shifted fiber has: a dispersion level of 1,0 to 4.5ps/nm/km in terms of absolute value at a wavelength of 1,550 nm; a dispersion slope not greater than 0.13 ps/nm2/km in terms of absolute value at the wavelength of 1,550 nm; an effective area not less than 70 µm2 at the wavelength of 1,550 nm; a transmission loss not greater than 0.25 dB/km with respect to light in a 1.55 - µm wavelength band; and a cutoff wavelength .c (L0) at a length of 2m satisfying the following condition: c (L0) £ 2.2 2. A dispersion-shifted fiber as claimed in claim 1, wherein said dispersion- shifted fiber has a bending loss not greater than 0.5 dB/turn with respect to light in the 1.55 - µm wavelength band when bent at a diameter of 32 mm. 3. A dispersion-shifted fiber as claimed in claim 1, wherein said dispersion- shifted fiber has a dispersion slope in terms of absolute value not less than 0.09 ps/nm-/km at the wavelength of 1,550 nm. 4. A dispersion-shifted fiber as claimed in claim 1, wherein said dispersion- shifted fiber satisfies the following relationship: 1.2 ic/b-J. 3.5 wherein 2c is an outside diameter of said inner cladding (210; 211; 304). 5. A dispersion-shifted fiber as claimed in claim 1, wherein said cutoff wavelength at a length of 2 m is not less than 1.59 µm. and not greater than 2.2 ft. A dispersion-shifted fiber substantially as hereinbefore described with reference to the accompanying drawings.

Full Text

BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a single-mode dispersion-shifted 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. 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 pm or L.55 fjiti 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 to. 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. 2-141.704 propose a dispersion-shifted fioer 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 opticat 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 mentiored 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. Patent No. S ,483,612K 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 ir. a relaying transmission system. SUMMARY OF _ THE_ INVENTION
As a result of studies concerning the above-menticned 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 srea Aaff is set on the order of 55 µm2. 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 qi.ality in the conventional transmission distance in view of further advance in wavelength multiplexing which will occur as communications become more sophisticated.
Hero, as disclosed or. Japanese Patent Application
Laid-Open No. 8-.49251, effective core cross-sectional area Aeff is given by the following expression:
(Formula Removed) wherein S 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 1.53µm (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/kn in terrr.s of absolute value, a dispersion slope not greater than 0.13 ps/nm2'/km in
terms of absolute value, an effective core crass-sertional area Aeff of 7G µm' or more, and a transmission loss not greater, than 0.25 dS/km with respect to light in a wave .ength band of 1.55 µm.
Here,- when the dispersion level in terms of absolute value is smaller than 1.O ps/nm/km, waveform distortion caused by four-wave mixing, unstable modulation, and the like cannot practically ise
t
neglected in long-haul light transmission ovor 20 km or more- wh In the dispersion-shifted fiber according to the present invention, the absolute value of dispersion slope is lot greater than 0,13 ps/nm2/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./A,)- Accordingly, at the same propagating light condition, nonlinear optical effects are effsctively
restrained from occurring when the nonlinear optical effect constant (N./Aeff) is made smaller. On the other hand, since nonlinear refractive index N: 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, 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 Aetr is increased to 70 µm.' 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-soct.ional area Aftf, and nonlinear optical constant (N./Aeff) in a dispersion-shifted fiber having a typical composition. From Fig. 1, it can be seen that nonlinear optical constant (N./Aeff), which is 5.8 x i(Ti0 (i/w) when effective core cress-sectional area Aeff is 55 µm2, becomes 4.6 * 10-10 (1/W) when effective core cross-sectional area Aeff is 70 µm2, thus being reduced by about 20%. Accordingly, as compared with the conventional dispersion-shifted fiber, the dispersion-shifted fiber according to tr.e praser.t invention can increase the degree of wavelength
multiplexing in signal light.
In general, refractive index N of a medium under ST.rang light changes depending on light intensity. Accordingiy, the minimum order of effect on ::e£ractive index N is expressed by:
N - N0, + N2, • E2
wherein N,, is a linear refractive index, N2 is a nonlinear refractive index, and E is a field amplitude. Namely, under strong light, the refractive iidex N of the mediuin is given by the sum of N, and an increase which is in proportion to the square of field amplitude E. Tn particular, the constant of proportici N. (unit: m2/v2) 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 201 (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 th? case where the. conventional repeater spacing is 50 km, for example, "he number of repeaters can be reduced by
dDOUl 101.
Furtier, 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 A9ff, 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, Ihe dispersicn-shifted fiber according to the present invent.ior 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 tending generated at the cable-forming step or the like.
Preferably, in the dispersion-shifted liber Arrordinq to the present invention, the absolute value of dispersion slope is 0.09 ps/nm2/km or mere. The
smaller the dispersion slope is, the less becomes the variation in the amount of waveform distortion caused by wavelength dispersion in the signal lightis. 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 ;5lope is preferably at least 0.09 ps/nm2/km but not greater than 0.13 ps/nm:/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 dispers Ion-shifted fiber according to the present invention has a depressed ciadding/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 outride diameter of 2b; an inr.er 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 Aeff is increased in a d:.spersion-shitted fiber having a simple dual-shape core type refractive index profile without a depression cladding structure, namely, non-depressed cladding/du.il-shape core type refractive index profile, in a sta-:e where its absolute value of dispersion is set to 1.0 to 4.5 ps/nm/km at the wavelength of 1,5W nm, its cutoff wavelengtin 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 wavel.enqtn, 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 cere type refractive index is lower than that of a disversion-shifted fiber havinq 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 conliyuiation 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 rispersion-shifted fiber satisfies the following relationships:
a/b ≤ 0.15 (I)
0.8% ≤ ▲n. ≤ 1.2% (2)
0.12% ≤ ▲n, ≤, 0.30% (3)
▲n,▲n: ≤ 0.95 (4)
wherein ;in, is a relative refractive index difference of the inner core with respect to the inner cladding/ uns is a relative refractive index difference of the outer core with respect to the inner cladding, anc in, 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 absclute value of dispei sion slope to become at least 0.09 ps/nm2/kn but not larger than 0.13 ps/nm2/km.
Here,- the relative refractive index difference ▲n: of the inner cere with respect to the inner cladding, relative refractive index difference ▲n, of the outer core with respect to the inner cladding, and relative refractive index difference ▲n. of the outer cladding with respect to the inner cladding are respectively deftn«d as follows:
▲n1 = (n.2 ≤n3J)/(2n.2) (5) ▲n2. = (n.: -n,:)/(2n:3') (6) ▲n, = (n4 -n/)/(2n.:) (7)
wh«rein n is the refractive index of the inrer core, n{ is the refractive index of the outer core, n. is the refractive index of the inner cladding, and n4 is the refractive index of the outer cladding. In -:his specification, each relative refractive index differencs is expressed in terms of percentage.
Prafsrably, the first embodiment of the dispersion-shifted fiber further satisfies t:ie following relationships
1.2 ≤ c/b ≤ 3.5 (8)
wherein 2- is an outside diameter of the i
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 ..nner cladding is too thick, it functions in a
I way similar to a normal cladding and fails to yield the
i
cutoff-wavelength-shortening effect of the depressed cladding type refractive index. When the dispersion-shifted fiber satisfies the relationship of /b ≥ 1.2, the bending loss in the case where it is bent at a diameter of 32 nun can become 0.5 dB/turn or 5ess.
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 mad* shorter, thereby making it easy to secure a wavelength range of signal-light which allows single-moc.e 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 ot the dispersion-shitted 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 nuts ide 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 ojter 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% ≤ An. ≤ 1.1% (11)
0.2% ≤ An3 - An. s 0.7% (12)
▲n4/▲n, s 0.95 (13)
wherein An. is a relative refractive index difference of said inner core with respect to said inner cladding, An, is a relative refractive index difference of said intermediite core with respect to said inner cladding.. An, is a relative refractive index difference' of said outer cor* with respect to said inner claddi iq, and '-.n, is a relative refractive index difference of said o-.oater cladding with respect to said inner cladding.
The .above-relationships (9) and (10) an? condition:; to satisfy the effective core croiss-sect tan area Aeff of 70 µm:. The relative refractive index difference in. of said inner core with respert to said inner cladding is preferably 0.5 % or more .an 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 An, is 1.1 % or less, the dispersion slope at wavelength of 1,550 nat falls within a range of 0.09 to n.13 ps/nmVkm. The value (▲n3 - ▲n, ) should be C.2 % or more in order to satisfy the condition that the bending loss when bent at a diameter of 32 mm becomes; 0.5 dB/turn or less, and it should be 0.7 % or loss in order to nake cutoff wavelength at a length of 2 m set 2.2 pm 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 dU/kra.
Further, the second embodiment of the dispersion-shifted f:.ber satisfies the following relatironship:
1 .2 ≤, d/c ≤3.5 (14)
wherein 2d is an outside diameter of the inneir cladding.
The bending loss when bent at a diameter- of 12 mm becomes 0.5 dB/kre 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 cnly, 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.
Accordingly, the present invention relates to a dispersion-shifted fiber (100, 200, 300) having a zero-dispersion wavelength outside of a wavelength band of 1.53 to 1.56 nm, the fiber having a core region (110, 120, 111, 121, 301, 302, 303) and a cladding region (210, 220, 211, 221, 304, 305), said core region comprises an inner core (110, 111, 301) having a predetermined refractive index and an outside diameter of 2a, and an outer core (120, 121, 303) disposed around the outer periphery of said inner core, said outer core having a refractive index lower than that of said inner core and an outside diameter of 2b, and said cladding region comprises an inner cladding (210, 211, 304) disposed around the outer periphery of said outer core, said inner cladding having a refractive index lower than that of said outer core and an outside diameter 2c; an outer cladding (220, 221, 305) disposed around the outer periphery of said inner cladding, said outer cladding having a refractive index higher than that of said inner cladding, said dispersion-shifted fiber satisfies the following relationships:
a/b ≤0.15
0.8% ≤ Am ≤1.2%
0.12 ≤An2 ≤0.30%
Ana/Ana whereby said dispersion-shifted fiber has a dispersion level of 1.0 to 4.5ps/nm/km in terms of absolute value at a wavelength of 1,550 nm, a dispersion slope not greater than 0.13 ps/nm2/km in terms of absolute value at the wavelength of 1,550 nm, a dispersion slope not greater than 0.13 ps/nm2/km in terms of absolute value at the wavelength of 1,550 nm, an effective area not less than 70 |^m2 at the wavelength of 1,550 nm, a transmission loss not greater than 0.25 dB/km with respect to light in a 1.55 - µm wavelength band and a cutoff wavelength c (Lo) at a length of 2m satisfying the following condition: c (Lo) ≤ 2.2µm.
BHIEF. .DBS-CRI2IIQH..OF THEAccompanying DRAWINGS
Fig. 1 is a graph showing an example of relationship between effective core cross-sectional area Aeff and nonlinear optical constant (N,/Aiff);
Fig. 2 is a view showing a cross-sectioral configuration of a typical embodiment of the dispersion-shifted fiber according to the present invention and its refractive index profile (depressed cladding/cual-shape core type);
Fig. 3 is a graph showing a relationship between
value fa/b) and effective core cross-sectional area
Aeff
V
Fig. 4 is a graph showing a relationship between relative refractive index difference in; 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 In. and dispersion slope;
Fig. 6 is a graph showing a relationship between relative refractive index difference .In, and bending loss generated when bent at a diameter of 32 nun;
Pig. 7 is a graph showing a relationship between relative refractive index difference ▲n2 and cutoff wavelength at a reference length of 2 m;
Pig. 8 is a graph showing a relationship between value (An.Mn.) and transmission loss;
Fig. 9 is a graph showing a relationship between value (cYb) 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 f:.ber according to the present invention and its refractive index profile (depressed cladding/segineted-core type);
Pig. 13 is a view showing a first application of the depressed cladding/segmented-core type refractive •index profile in the second embodiment of tho 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 tho 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.
Pig. 2 is a view showing a cross-sectional configuration of a typica". 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. as its maximum refractive index and an outside diameter 2a; an outer cora 120, disposed around the outer periphery of the inner core 110, having a refractive index n; ( n3). As a result of tiis configuration, the dispersion-shifted fiber '.00 realizes a depressed ciadding/dual-shape core type refractive index profile 101.
This dispersion-shifted fiber satisfies the following relationships:
a/b ≤ 0.15 (1)
0.8% 4 L'.n, ≤ 1.2% (2;
0.12% 4 An, ≤ 0.30% (3)
▲nj/in: ≤ 0.95 (4)
1.2 ≤. c/b ≤ 3.5 (8)
wherein An, is a relative refractive index difference of the inner core 110 with respect to the inner cladding 210, in. is a relative refractive index difference of the outer core 120 with respect to the inner cladding
210, and ▲n3 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 A9,f, In Fig. 3, while &H! is 1.0% and An2 is 0.2%, the outside: diameter 2a and the outside diameter 2b of the inner sore 110 are changed so as to attain a zero-dispersion wavelength of 1,580 nm. It can be seen from ?ig. l that A,fi becomes 70 µm.: or more when (a/b) dees not exceed 0.15.
Fig. 4 is a graph showing a relationship between the relative refractive index difference ▲n1 of the inner cors 110 with respect to the inner cladding 210 and absolute value JDJ of dispersion level D with respect to light having a wavelength of 1,55D nm. in Fig. 4, tie outside diameter 2a of rhe inner core 110, outside diameter 2b of the outer core 120, relative refractive index difference an. of the inner cor* 110 with respect to the inner cladding 210, and -elative refractive index difference An: of the outer cor« 120 with respect to the inner cladding 210 are changed so as to satisfy that (a/b) is C.13 and A,,, is £0 ^m*. Tr can be seen from Fig. 4 that |D is equal to or larger

than 1.C ps/nm/km when In, exceeds O.S%, and that ;Dj is equal to or smaller than 4.5 ps/run/km when ai^ is approximately 1.2%.
Fig. 5 is a .graph showing a relationship between the relative refractive index difference -n. of the inner core 110 with rsspect to the inner cladding 210 and dispersion slope. In Fig, 5, the outside diameter
2a of the inner core 110,.oulside diameter 2h of the
i
outer core 120, relative refractive index difference ▲nt of the inner core 110 with respect to the inner cladding 210, and relative refractive index difference _n. ot the outer core 120 with respect to the inner cladding 210 are changed so as to satisfy 0.13 tf (a/b), 80 ym" of Aett, 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 F'ig. 5 that the dispersion slope becomes.. 0.13 ps/nm2/km or ;more when An, is equal to or greater than 1.2%.
Fig. 6 is a graph showing a relationship between the relative refractive index difference ▲n2 af the outer core 120 with respect to the inner cladding 210 and bending loss generated when bent at a diameter of 32 mm. Ir. Fig. 6, the outside diameter 2a of the inner core 110, outside diameter 2b of the outer cere 120, and relative refractive index diffrence .'.n. of the Outer core 12C with respect to the inner cladding 210
are changed so as to satisfy that relative refractive index difference ±n, is l.C% in the inner core 110 with respect to the inner cladding 210, (a/b) is 0.13, Aeff is 80 ^m2, and zero-dispersion wavelength is 1,580 nm. It can be seen from Fig. 6 that the bending oss upon bending at a diameter of 32 nun is 0.5 dB/tum or less when an2 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 11 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 An. of the outer core 120 with respect to the inner clacding 210 and cutoff wavelength at a reference length cf 2 m. In Fig. 7, the outside diameter 2a of the inner core 110, outside diameter 2b of the outer core 120, relative refractive zndex difference in, of the outer rore 1?0
•
with respect to the inner cladding 210, and relative refractive index difference ins of the outer cladding 220 with respect to the inner cladding 210 are changed so as to satisfy that in, is 1.0% in the inner core 110 with respect to the inner cladding 210, (a/b) is 0.13, Aeff, is 80 µm, zero-dispersion wavelength is ' ,580 nm, and ▲n,/▲n, is 0.8.
Normally, cutoff wavelength 2 of an optical fiber is measured according to a bending process performed at a length of 2 m, which is recommended by CCI':T-G. 650. When a S-node optical fiber has a length L or. 2 m, the cutoff wavelength , as the lowest wavelength allowing single-mode transmission, coincides with the result of the above-mentioned measurement. Tt has been known that, as the length L increases, the cutoff wavelength  changes according to the following expression (15):
▲C(L) • C(L, = 2 m) - 0.184 x log..{L/L0) (IS)
(see T. Koto et al., OECC'96 Technical Digest. July! 1966, Makuhari Messe, pp. 160-161).
On the other hand, the dispersion-shifted fiber according to the present invention is assumec to transmit signal light having a wavelength in the band of 1.55 ]JEi over a long distance of 20 km or nore. Consequently, it is necessary for the cutoff wavelength AC(L0) at a length of 2 m to satisfy the following expression (16):
XC(L) From Fig. 7, it can be seen that the cutoff wavelength ,\C(L.) is equal to or less than 1.', ;jm at a length of 2 T when An. i'ig. 8 is a graph showing a relationship between value (£n,/£n:) and transmission loss. Tn Fie. 8, the
relative refractive index difference in} of the outer cladding 220 with respect to the inner cladding 210 is changed under the following conditions: relative refractive index difference ▲n, - 1.0%; relative refractive index difference ▲n, = 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 ;▲n2,/▲n2.) is greater than 0.9S.
Fig. 9 is a graph showing a relationship between value (c/b) and bending less at a diameter o-° 32 ram,. In Fig. 9, the outside diameter 2c of the inner cladding 210 is changed under the following conditions: relative refractive index difference ir^ * 1.0%; relative refractive index difference An, = 0.20%; relative -refractive index difference ▲n3. = 0.12%; radius a -- 2.1 µm; and radius b >s 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 d3/turr. when (c/b) dope not exceed 1.2-
Fig. 1C is a graph showing a relationship between value (c/b) and cutoff wavelength at a length of 2 m. In Fig. 1C, the outside diameter 2c of the inner cladding 210 is changed under the following conditions:
relative refractive index difference in, = l.10%; relative retractive index difference ▲n2, = 0,20%; relative refractive index difference ▲n3, - 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 indt>x profile, it satisfies the following relationships:
a/b ≤ 0.15 (1)
0.8% ≤ An4 s 1.21 (2)
0.12% ≤ An- ≤ 0.30% (3)
▲n3/▲n; ≤ 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/mr;Vkm in terms of absolute value, an effective core cross-sectional area Aefr of 70 µm1 or move, a transmission loss not greater than 0.25 dB/kre with respect to light in the wavelength band of 1.55 ;µm, and a bending loss of 0.5 d3/turn or less when bent at a
diameter of 32 mm.
Thus, favorably realized is a dispers ioi-shi ft.*d 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 hav.ng 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 lover than that of the outer core and an outside' diameter of 2d;
and an outer cladding, disposed around the otrer periphery o≤ the inner cladding, having a refractive index higher than that of the inner cladding.
Accordingly, the dispersion-shifted fibsr having the above-mentioned various characteristics should satisty the following relationshipsi
a/c ≤ 0 .42 (9)
b/c ≥ 0.60 (10)
0.5% ≤ ▲n, ≤ 1.1* (11 )
0.2% ≤ ▲n, - ▲n2- ▲nH/▲nj 1.2 ≤ d/C ≤ 3.5 (14)
wherein in. is a relative refractive index difference of said inner core with respect to said inner cladding, ▲n2 is a relative refractive index difference of said intermediate cere with respect to said inner cladding, ▲n3, is a relative refractive index difference of said outer core with respect to said inner claddirg, and ▲n4, is a relative refractive index difference of said outer cladding with respect to said inner cladding, wherein 2c; 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:
x-1,2,3,4
wherein n. is the refractive index of the inner core, n. is the refractive index of the intermediate core, n. is the refractive index of the outer core, and r.. is the refractive index of the outer cladding.
Fig. 11 is a view showing a cress-sectional configuration of a first typical example (dual-shape core type) of a dispersion-shifted fiber according to the present invention and its refractive indax profile. As shown in Fig. 11, the dispersion-shifted fiber 200 has a depressed cladding/dual-shape core typs refractive index profile 201 and comprises ai 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 3? 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 2 11.
Also, the first embodiment of the dispersion-shifted fxber 200 satisfies the above-mentioned relational, expressions (1) to (4) and (8) as follows:
a/b = 0.13 0.8% 0.12* ▲n, s 0.12%
▲n,/▲n. = 0.6 1.2 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/im/tan;
dispersion slope • 0.111 ps/nm2/km;
effective core cross-sectional area Ae,t * 78.2 pm;
cutoff wavelength * 1.59 ^m-
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 P.g. 12, the second embodiment of the dispersion-shifted fiber 300 has a segnented-core type refractive index profile and comprises an inner core 301 having an outside', diameter 2a. ot 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 core1 303, disposed around the outer periphery of the intermediate
protiie, it can reduce bending loss ar.d favorably realize the aimed dispersion-shifted fiber.
From the invention thus described, it will be obvious t.iat the implementation of the invention may b* varied in many ways. Such variations are no: to be regarded is a departure from the spirit and scope o* 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.

Further, characteristics of the second embodiment of the dispersion-shifted fiber measured at -;he 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 pa/nm2/km;
effective core cross-sectional area Aart= 81.8 µm;
cutoff wavelength • 1.74 µm;
bending loss = 0.1 dB/turn when bent at a diameter ot 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 µm2 or greater, nonlinear optical effects are effectively restrained from occurring. Accordingly, it is possible to favorably realize a -dispersion-shifted fiber, suitable for long-taul 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

prorile, it can reduce bending less and favorably realize the aimed dispersion-shifted fiber.
From the invention thus described, it will be obvious that the implementation of the inver. cion may be varied in many ways. Such variations are no-: to be reqarded .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.

WE CLAIM:
A dispersion-shifted fiber (100, 200, 300) having a zero-dispersion
wavelength outside of a wavelength band of 1.53 to 1.56 jam, the fiber having
a core region'(110, 120; 111, 121; 301, 302, 303) and a cladding region (210,
220; 211, 221; 304, 305); said core region comprises:
an inner core (110; 111; 301) having a predetermined refractive index and an
outside diameter of 2a; and
an outer core (120; 121; 303) disposed around the outer periphery of said
inner core, said outer core having a refractive index lower than that of said
inner core and an outside diameter of 2b;
and said cladding region comprises:
an inner cladding (210; 211; 304) disposed around the outer periphery of
said outer core, said inner cladding having a refractive index lower than that
of said outer core; and an outside diameter 2c;
an outer cladding (220; 221; 305) disposed around the outer periphery of
said inner cladding, said outer cladding having a refractive index higher than
that of said inner cladding;
said dispersion-shifted fiber satisfies the following relationships:
a/b £0.15
0.8% ≤ ▲Ani ≤ 1.2%
0.12 ≤▲n2 ≤0.30%
▲n3/An2 ≤ 0.95
whereby said dispersion-shifted fiber has:
a dispersion level of 1,0 to 4.5ps/nm/km in terms of absolute value at a
wavelength of 1,550 nm;
a dispersion slope not greater than 0.13 ps/nm2/km in terms of absolute
value at the wavelength of 1,550 nm;
an effective area not less than 70 µm2 at the wavelength of 1,550 nm;
a transmission loss not greater than 0.25 dB/km with respect to light in a
1.55 - µm wavelength band; and
a cutoff wavelength .c (L0) at a length of 2m satisfying the following
condition:
c (L0) £ 2.2

2. A dispersion-shifted fiber as claimed in claim 1, wherein said dispersion-
shifted fiber has a bending loss not greater than 0.5 dB/turn with respect to
light in the 1.55 - µm wavelength band when bent at a diameter of 32 mm.
3. A dispersion-shifted fiber as claimed in claim 1, wherein said dispersion-
shifted fiber has a dispersion slope in terms of absolute value not less than
0.09 ps/nm-/km at the wavelength of 1,550 nm.
4. A dispersion-shifted fiber as claimed in claim 1, wherein said dispersion-
shifted fiber satisfies the following relationship:
1.2 ic/b-J. 3.5
wherein 2c is an outside diameter of said inner cladding (210; 211; 304).
5. A dispersion-shifted fiber as claimed in claim 1, wherein said cutoff
wavelength at a length of 2 m is not less than 1.59 µm. and not greater than
2.2
ft. A dispersion-shifted fiber substantially as hereinbefore described with reference to the accompanying drawings.

Documents:

3774-del-1997-abstract.pdf

3774-del-1997-claims.pdf

3774-del-1997-correspondence-others.pdf

3774-del-1997-correspondence-po.pdf

3774-del-1997-description (complete).pdf

3774-del-1997-drawings.pdf

3774-del-1997-form-1.pdf

3774-del-1997-form-13.pdf

3774-del-1997-form-19.pdf

3774-del-1997-form-2.pdf

3774-del-1997-form-29.pdf

3774-del-1997-form-3.pdf

3774-del-1997-form-4.pdf

3774-del-1997-form-6.pdf

3774-del-1997-gpa.pdf

3774-del-1997-petition-137.pdf

3774-del-1997-petition-138.pdf

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Patent Number

216120

Indian Patent Application Number

3774/DEL/1997

PG Journal Number

12/2008

Publication Date

21-Mar-2008

Grant Date

10-Mar-2008

Date of Filing

24-Dec-1997

Name of Patentee

SUMITOMO ELECTRIC INDUSTRIES LTD

Applicant Address

5-33 KITAHAMA 4-CHOME, CHUO-KU, OSAKA-SHI, OSAKA 541, JAPAN

Inventors:

#	Inventor's Name	Inventor's Address
1	TAKATOSHI KATO	C/O YOKOHAMA WORKS OF SUMITOMO ELECTRIC INDUSTRIES LTD., OF 1 TAYA-CHO, SAKAE-KU, YOKOHAMA-SHI, KANAGAWA 244, JAPAN
2	EISUKE SASAOKA	C/O YOKOHAMA WORKS OF SUMITOMO ELECTRIC INDUSTRIES LTD., OF 1 TAYA-CHO, SAKAE-KU, YOKOHAMA-SHI, KANAGAWA 244, JAPAN
3	SHINJI ISHIKAWA	C/O YOKOHAMA WORKS OF SUMITOMO ELECTRIC INDUSTRIES LTD., OF 1 TAYA-CHO, SAKAE-KU, YOKOHAMA-SHI, KANAGAWA 244, JAPAN

PCT International Classification Number

G02B 006/16

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	350691/1996	1996-12-27	Japan