KR20030051635A - Negative-dispersion optical fiber and optical transmission line incorporating the same - Google Patents

Abstract

The present invention relates to a negative-dispersion optical fiber and a light transmission line employing the same to compensate for color dispersion of the positive-dispersion optical fiber in a shorter length in the signal wavelength band. A negatively-dispersed optical fiber has the following characteristics at a wavelength of 1550 nm: color dispersion (D) of -150 ps / nm / km or less; A dispersion gradient that satisfies the condition that its ratio (S / D) to color dispersion (D) is at least 2.0 × 10 ⁻³ / nm and at most 4.7 ps / nm / km; 12㎛ 25㎛ ² or more and ² or less of the effective area. To satisfy these properties, negative dispersion optical fiber is in the order technology towards the outer periphery from the center, the first cladding of the maximum refractive index (n ₁₎ the core region, the refractive index _{(n 2) (<n 1} ) of refraction A second cladding with an index n ₃ (> n ₂ ) and a third cladding with an index of refraction n ₄ (<n ₃ ).

Description

Negative-dispersion optical fiber and optical transmission line incorporating the same

An optical transmission system transmits signals of multiple channels through an optical transmission line composed of optical fibers to enable long distance and large capacity communication. Silica-based optical fibers commonly applied to optical transmission lines exhibit their minimum transmission loss near a wavelength of 1.55 μm. On the other hand, an Erbium-Doped Fiber Amplifier (EDFA) capable of amplifying signals in the vicinity of the wavelength of 1.55 mu m can be practically used as the optical amplification means. For this reason, the C-band (1530 nm to 1560 nm) is mainly used as the signal wavelength band.

Since EDFA has also been recently developed which can amplify signals around a wavelength of 1.58 mu m, the L-band (1570 nm to 1610 nm) is now also used as the signal wavelength band. In order to realize higher capacity transmission, the use of the S-band (1450 nm to 1530 nm) as the signal wavelength band is also under development.

Additionally, wavelength division multiplexing (WDM) optical transmission systems are systems for transmitting multiplexed signals of a plurality of channels included in the S-band, C-band or L-band. It is a possible system. With respect to these WDM light transmission systems, there is a need for an additional increase in information content, which requires keeping the absolute values of chromatic dispersion small over a wider wavelength band across the entire light transmission line.

However, optical fibers generally applied to optical transmission lines have a positive color dispersion, and have a positive sign of dispersion in either the S-band, the C-band, or the L-band. For example, near a wavelength of 1.3 μm, a standard single mode optical fiber with zero dispersion wavelength has a color dispersion of about +16 ps / nm / km to +21 ps / nm / km at a wavelength of 1.55 μm. A non-zero dispersion-shifted fiber (NZ-DSF) having a zero dispersion wavelength in the vicinity of a wavelength of 1.55 µm is about +2 ps / nm / km at a wavelength of 1.55 µm. It has a color dispersion of +12 ps / nm / km. Both of these single mode optical fibers and nonzero distributed displacement optical fibers have positive dispersion slopes in the S-band, C-band and L-band.

As described above, when the optical transmission line is constructed by applying only optical fibers having a positive color dispersion (hereinafter referred to as bi-dispersion optical fibers), the optical transmission line has a large cumulative color dispersion. This results in degradation of the waveform of the signals, and therefore it is difficult to realize long distance and large capacity optical transmission. Therefore, in order to compensate for color dispersion of positively-dispersed optical fibers, the application of optical fibers having a negative color dispersion (hereinafter referred to as negative-disperse optical fibers) is being studied (for example, Japanese Patent Laid-Open Nos. H6-11620, H8). -136758, H8-313750, etc.).

The present invention relates to a negatively distributed optical fiber for compensating for the color dispersion of a positively distributed optical fiber having a positive color dispersion in a signal wavelength band and an optical transmission line including the same.

1A to 1D are diagrams for explaining specific configurations of an optical transmission line according to the present invention.

2A and 2B show the cross-sectional structure and exponential profile of a negatively distributed optical fiber according to the present invention;

3 to 5 are graphs showing the relationship between chromatic dispersion and dispersion gradient of respective embodiments of negatively-dispersed optical fibers according to the present invention.

FIG. 6 is a table showing the specifications of samples (fibers A to G) corresponding to embodiments of negatively-dispersed optical fibers in accordance with the present invention. FIG.

FIG. 7 is a graph showing the color dispersion characteristics of each of the fibers A and B among the fibers A to G shown in the table of FIG. 6.

FIG. 8 is a graph showing chromatic dispersion characteristics of the fibers C to F among the fibers A to G shown in the table of FIG. 6;

FIG. 9 is a graph showing the color dispersion characteristics of the fibers G among the fibers A to G shown in the table of FIG. 6.

FIG. 10 is a graph showing color dispersion characteristics of respective transmission lines to which fibers A and B of the fibers A to G indicated in the table of FIG. 6 are respectively applied.

FIG. 11 is a graph showing the color dispersion characteristics of respective transmission lines to which the fibers C to F of the fibers A to G indicated in the table of FIG. 6 are respectively applied.

FIG. 12 is a graph showing color dispersion characteristics of respective transmission lines to which the fiber G is applied among the fibers A to G shown in the table of FIG. 6.

FIG. 13 is a table showing various characteristics of each transmission line to which the fibers A to G indicated in the table of FIG. 6 are applied.

14 shows an exponential profile of negatively-dispersed optical fibers prepared as comparative examples.

FIG. 15 is a graph showing the color dispersion characteristics of the entire transmission line composed of the negatively-dispersed and positively-dispersed optical fibers shown in FIG.

(Disclosure of this invention)

The present inventors have studied the above-described prior art and found the following problems. That is, it is generally known that negatively distributed optical fibers have greater transmission losses than positively distributed optical fibers. Therefore, there is a problem that transmission loss becomes large when using long negative-dispersion optical fibers. According to the knowledge of the inventors, an optical transmission line composed of positively-dispersed and negatively-dispersed optical fibers has an average color dispersion of the entire optical transmission line as zero near zero dispersion wavelength, but the absolute values of the color dispersion are deviations from the zero dispersion wavelength. There is a tendency to increase accordingly. Since conventional transmission lines have a large deviation of color dispersion in the signal wavelength band as described, there are limitations in implementing long distance and bulk WDM optical transmissions.

The present invention has been carried out to solve the above problems, and an object of the present invention is a short-length, negative-dispersion optical fiber that can compensate for the color dispersion of the positive-dispersion optical fiber of the signal wavelength band, and employing the long-distance and large capacity WDM It is to provide an optical transmission line that enables optical transmission.

In order to achieve the above object, the negative-dispersion optical fiber according to the present invention has the following characteristics at a wavelength of 1550 nm: -150 ps / nm / km or less, more preferably -180 ps / nm / km or less (D); A dispersion gradient S satisfying a condition in which the ratio S / D to the color dispersion D is 2.0 × 10 ⁻³ / nm or more and 4.7 × 10 ⁻³ / nm or less; And an effective area of at least 12 μm ² and at most 25 μm ² , more preferably at most 20 μm ² . Another negatively-dispersed optical fiber according to the present invention may have the following characteristics at a wavelength of 1550 nm: color dispersion (D) of -200 ps / nm / km or less; And dispersion gradient (S) which satisfies the condition that the ratio (S / D) to the color dispersion (D) is not less than 2.0 × 10 ⁻³ / nm and not more than 4.7 × 10 ⁻³ / nm.

Since the negative-dispersion optical fiber has a small chromatic dispersion D (the sign is negative and its absolute value is large) as described above, the optical transmission line composed of the positive-dispersion optical fiber and the negative-dispersion optical fiber has a small ratio. It can consist of a length of negative-dispersion optical fiber. This suppresses the increase in transmission loss due to the insertion of negatively-dispersed optical fibers into the optical transmission line, and makes it possible to construct the optical transmission line at low cost. Since the ratio S / D is at least 2.0 × 10 ⁻³ / nm and at most 4.7 × 10 ⁻³ / nm, the dispersion gradient compensation rate is about 60% to 140%, thereby an average over the entire optical transmission line It is suitable for making both the absolute values of the dispersion gradient and the average color variance small, and the deviation (maximum-minimum) of the average color variance between wavelengths for the entire optical transmission line within the signal wavelength band. 12 µm ² or larger, the effective area is greater than or equal to those of conventional negative-dispersion optical fibers, and can effectively suppress nonlinear light phenomena. An effective area of 25 μm ² , more preferably 20 μm ² or less, can effectively suppress an increase in loss of negative-dispersion optical fiber even in a cabled form as a bundle of optical fibers or in a modular form wound in coil form. .

The effective area A _eff is given by the following equation, as described in Japanese Patent Laid-Open No. H8-248251 (EP 0 724171A2).

In this equation, E represents the electric field caused by propagating light, and r represents the radial distance from the center of the core.

In the negative-dispersion optical fiber according to the present invention, the ratio S / D of the dispersion gradient S to the color dispersion D is not less than 2.7 x 10 ^-3 / nm and not more than 4.0 x 10 ^-3 / nm. In this case, the dispersion slope compensation ratio is about 80% to 120%, which makes both the average dispersion slope and the average absolute values of the average color dispersion of the entire optical transmission line employing the negative-dispersion optical fiber smaller, In the signal wavelength band, the deviation of the average color dispersion between the wavelengths of the entire optical transmission line is made smaller.

In the negative-dispersion optical fiber according to the present invention, the cutoff wavelength at a length of 2 m (CCITT standard) is suitably 1.0 mu m or more and 2.0 mu m or less. In this case, the band loss of the negatively compensated optical fiber can be controlled at a small level.

In the negative-dispersion optical fiber according to the present invention, the transmission loss at 1550 nm is suitably 1.0 dB / km or less, more preferably 0.7 dB / km or less. The reason is that an increase in transmission loss over the whole of the optical transmission line can be more effectively suppressed.

In order to realize the various characteristics as described above, the negative-dispersion optical fiber according to the present invention includes a core region extending along a predetermined axis and having a predetermined maximum index of refraction; A first cladding region surrounding the core region and having a refractive index lower than the maximum index of refraction of the core region; A second cladding region having a higher index of refraction than the first cladding region and surrounding the first cladding region; And a third cladding region having a refractive index lower than that of the second cladding region and surrounding the second cladding region.

In this exponential profile, negatively-dispersed optical fibers are realized with the various characteristics described above, and are particularly suitable because the band loss can be effectively reduced while lengthening the cutoff wavelength. In negative-dispersion optical fibers, the maximum relative refractive index difference of the core region with respect to the third cladding region is suitably 1.8% or more and 3.0% or less. In this case, the band loss can be easily reduced by lengthening the cutoff wavelength.

The optical transmission line according to the present invention includes a negative-dispersion optical fiber having the above-described structure, and a positive-dispersion optical fiber having the following characteristics at a wavelength of 1550 nm: +15 ps / nm / km or more and + 21ps / nm / km Color dispersion less than or equal to; And a dispersion slope of at least +0.05 ps / nm ² / km and at most + 0.07ps / nm ² / km. This optical transmission line is configured to compensate for the color dispersion of the positively-dispersed optical fiber by the negatively-dispersed optical fiber having color dispersion and dispersion gradients both of which become small (negative sign and large absolute values) in the signal wavelength band. This structure can reduce the ratio of the length of the negative-dispersion optical fiber to the entire transmission line, and can effectively suppress the increase in transmission loss of the entire transmission line. Since both chromatic dispersion and dispersion gradient in the optical transmission line are compensated by the application of negatively-dispersed optical fibers having various characteristics as described above, the absolute values of chromatic dispersion can be kept small throughout the entire signal wavelength band, which is It is suitable for implementing long distance and large capacity WDM optical transmission.

The optical transmission line according to the present invention includes a negatively distributed optical fiber (negatively distributed optical fiber according to the present invention) and a positively distributed optical fiber as described above, and a relay station comprising a transmitting station and a receiving station, a transmitting station and an optical amplifier, and the like. And at least one of the relay stations or between the relay station and the receiving station. The negatively distributed optical fiber integrated in the optical transmission line can be located at the relay station. Each of the negative-dispersion optical fibers and the positive-dispersion optical fibers constituting the optical transmission line may be composed of a plurality of fusion-spliced optical fibers.

Further, in the optical transmission line according to the present invention, the deviation of the average color dispersion between wavelengths on the entire optical transmission line in the wavelength band of 1530 nm to 1560 nm is 0.5 ps / nm / km or less, and the total light in the wavelength band of 1450 nm to 1560 nm. The deviation of the average color dispersion between the wavelengths on the transmission line is 2.0 ps / nm / km, and the deviation of the average color dispersion between the wavelengths on the entire optical transmission line is less than 4.0 ps / nm / km in the wavelength band of 1450 nm to 1610 nm. More preferably, it is 2.0 ps / nm / km or less.

The invention will be more clearly understood from the following detailed description and the accompanying drawings, which are provided merely to illustrate the invention and should not be considered as limiting the invention.

Other scopes of applicability of the present invention will become apparent from the following detailed description. However, it will be apparent to those skilled in the art that various changes and modifications within the concept and scope of the present invention can be apparent from the detailed description, and thus the detailed description and the specific embodiments represent the preferred embodiments of the present invention. It should be understood that the examples are given.

Each embodiment of the negatively-dispersed optical fibers and the optical transmission line including the same according to the present invention are described below with reference to FIGS. 1A to 2B and 3 to 15. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

1A is a diagram showing the structure of an optical transmission line 1 according to the present invention. This optical transmission line 1 is a positive-dispersion optical fiber 20 having a positive color dispersion at a wavelength of 1550 nm, and a negative-dispersion optical fiber 10 having a negative color dispersion at a wavelength of 1550 nm (negative according to the present invention). -Distributed optical fiber). Generally, negatively distributed optical fiber 10 has a smaller effective area than positively distributed optical fiber 20. Thus, in order to suppress the occurrence of nonlinear light phenomena, it is suitable to allow the signal to propagate through the negative-dispersion optical fiber 10 after it has propagated through the positive-dispersion optical fiber 20.

The negative-dispersion optical fiber 10 constituting the optical transmission line 1 is composed of a plurality of optical fibers 10a to 10e (having substantially the same respective optical characteristics of each other) fusion-spliced with each other, as shown in FIG. 1B. It may be configured as shown in Figure 1b. In addition, the bi-dispersion optical fiber 20 may be composed of a plurality of optical fibers 20a to 20e (having substantially the same respective optical characteristics of each other) fusion-spliced with each other, as shown in FIG. 1C. Further, the optical transmission line 1 is an optical fiber transmission line disposed at at least one of a transmitting station for transmitting signals of a plurality of channels, between a receiving station, between a transmitting station and a relay station, between relay stations or between a relay station and a receiving station. . The negative-dispersion optical fiber 10 included in the optical transmission line 1 can be housed in the relay station 2, as shown in FIG. 1D. In this case, the negative dispersion optical fiber 10 may be located between the multistage amplifiers 3a, 3b.

The bi-disperse optical fiber 20 is a single-mode optical fiber generally disposed between relay stations with a zero dispersion wavelength around a wavelength of 1.3 mu m. That is, the bi-dispersion optical fiber 20 has a color dispersion D ₁ of +15 ps / nm / km or more and +21 ps / nm / km or less and a wavelength of 1.55 μm and +0.05 ps / nm ² / km or more. And a dispersion slope S ₁ of +0.07 ps / nm ² / km or less.

The negative-dispersion optical fiber 10 may be fused and connected to the positive-dispersion optical fiber 20 to be disposed at the relay station, or may be wound in a coil shape and modularized to be disposed at the relay station or the receiving station (FIG. 1D). When the negatively distributed optical fiber 10 is disposed with the positively distributed optical fiber 20 between the relay stations, the cumulative transmission loss is small and suitable. The positively-dispersed optical fiber 20 and the negatively-dispersed optical fiber 10 are suitably spliced together by fusion splicing. In this case, because of the heat during the fusion splicing operation, the mode field diameters of these fibers increase, so that the splice loss is kept small.

Negative-dispersed optical fiber 10 has the following characteristics at a wavelength of 1550 nm: color dispersion D ₂ of -150 ps / nm / km or less, more preferably -180 ps / nm / km or less; Dispersion gradient S ₂ , which satisfies the condition that the ratio S ₂ / D ₂ to color dispersion D ₂ is 2.0 × 10 ⁻³ / nm or more and 4.7 × 10 ⁻³ / nm or less; And an effective area of at least 12 μm ² and at most 25 μm ² . As another example, the negative-dispersion optical fiber 10 has the following characteristics at a wavelength of 1550 nm: color dispersion D ₂ of -200 ps / nm / km or less; And dispersion slope (S ₂ ) which satisfies the condition that the ratio (S ₂ / D ₂ ) to color dispersion (D ₂ ) is not less than 2.0 × 10 ⁻³ / nm and not more than 4.7 × 10 ⁻³ / nm.

The smaller the color dispersion D ₂ (the absolute value larger than that in the negative sign), the smaller the ratio of the length of the negative-dispersion optical fiber 10 in the optical transmission line 1. Therefore, smaller color dispersion is suitable, since the reduction in the manufacturing cost of the optical transmission line 1 can be realized and the transmission loss on the entire optical transmission line 1 can be reduced. Since the ratio S ₂ / D ₂ is 2.0 × 10 ⁻³ / nm or more and 4.7 × 10 ⁻³ / nm or less, the dispersion gradient compensation ratio η is 60% to 140%. More preferably, the ratio S ₂ / D ₂ is at least 2.7 × 10 ⁻³ / nm and at most 4.0 × 10 ⁻³ / nm, where the dispersion gradient compensation ratio η is about 80% to 120%. The dispersion slope compensation rate η (%) is defined as in Equation 1 below.

That is, when the dispersion slope compensation ratio η is closer to 100%, both the absolute values of each average color dispersion and the average dispersion slope on the entire optical transmission line 1 become smaller, and thus the total within the signal wavelength band. The deviation (maximum value-minimum value) of the average color dispersion between wavelengths on the optical transmission line 1 also becomes smaller.

The negatively-dispersed optical fiber 10 has a greater resistance to bending, with a decrease in the effective area. When the effective area is less than 25 μm ² , the transmission loss is small even in the form of a cable as a bundle of a plurality of optical fibers having properties equivalent to those of the coiled modular or negatively distributed optical fiber 10. When the effective area is 12 µm ² or more, it is equivalent to or larger than the effective area of conventional optical fibers, and is sufficient to effectively suppress the occurrence of nonlinear optical phenomena in the negative-dispersion optical fiber 10.

In the negative-dispersion optical fiber 10, the cutoff wavelength at a length of 2 m is suitably 1.0 m or more and 2.0 m or less. When the cutoff wavelength is set in this range, the bending loss becomes small. Even when the cutoff wavelength is longer than the signal optical wavelength but less than 2.0 μm, the effective cutoff wavelength is shorter due to the distance depending on the cutoff wavelength or due to the loss of higher modes in the coiled module form, the single mode being It is guaranteed at single wavelengths in the negatively distributed optical fiber 10. In addition, in the negative-dispersion optical fiber 10, the transmission loss at 1550 nm is 1.0 dB / km or less, preferably 0.7 dB / km or less, whereby the transmission loss on the entire optical transmission line 1 becomes small. .

This optical transmission line 1 uses a negative-dispersion optical fiber 10 with small (negative sign and large absolute values) both chromatic dispersion and dispersion gradient in the signal wavelength band, whereby the positive-dispersion optical fiber 20 ) To compensate for color dispersion. The ratio of the length of the negative-dispersion optical fiber 10 is reduced in the entire transmission line 1, thereby keeping the transmission loss on the entire optical transmission line 1 small. Since both chromatic dispersion and dispersion gradient are compensated in the optical transmission line 1, the absolute values of chromatic dispersion remain small throughout the entire signal wavelength band, which is desirable with regard to the implementation of long distance and large capacity WDM optical transmissions.

In particular, the optical transmission line 1 yields good transmission characteristics because of variations in color dispersion in the S-band (1450 nm to 1530 nm), the C-band (1530 nm to 1560 nm) or the L-band (1570 nm to 1610 nm). Because it is small. More specifically, in the wavelength band of 1530 nm to 1560 nm, it is suitable that the deviation of the average color dispersion between the wavelengths on the entire light transmission line 1 is 0.5 ps / nm / km or less. In the wavelength band of 1450 nm to 1560 nm, it is suitable that the deviation of the average color dispersion between the wavelengths on the entire optical transmission line 1 is 2.0 ps / nm / km or less. In the wavelength band of 1450 nm to 1610 nm, the deviation of the average color dispersion between the wavelengths on the entire light transmission line 1 is suitably 4.0 ps / nm / km or less, more preferably 2.0 ps / nm / km or less.

2A is a diagram showing a cross-sectional structure of an embodiment of a negatively-dispersed optical fiber according to the present invention. This negative-dispersion optical fiber 200 has a core region 210 extending along a predetermined axis and a cladding region 220 provided on the outer periphery of the core region. The cladding region 220 includes a first cladding region 221 provided on the outer circumference of the core region 210, a second cladding region 222 and a second cladding region 222 provided on the outer circumference of the first cladding region 221. It consists of a 3rd cladding area provided on the outer periphery of the 2 cladding area 222. The core region 210 has a maximum index of refraction n ₁ and an outer diameter 2a. The first cladding region 221 has a refractive index n ₂ (<n ₁ ) and an outer diameter 2b. The second cladding region 222 has a refractive index n ₃ (> n ₂ ) and an outer diameter 2c. The third cladding region 223 has an index of refraction n ₄ (<n ₃ ) and an outer diameter of 125 μm.

This negative-dispersion optical fiber 200 uses silica-based glass as the base material, for example, doping the core region 210 and the second cladding region 222 with an appropriate amount of GeO ₂ , respectively, and the first cladding region. 221 is prepared by doping with an appropriate amount of element (F). As a reference, relative to the refractive index n ₄ of the second cladding region 223, the relative refractive index index deviation of the core region 210 is given by Δ ₁ (= (n ₁ −n ₄ ) / n ₄ ) The relative refractive index of the first cladding region 22 is given by Δ ₂ (= (n ₂ −n ₄ ) / n ₄ ), and the deviation of the relative refractive index of the second cladding region is Δ ₃ (= (n ₃ -n ₄ ) / n ₄ ) (relative refractive index index deviations herein represent a percentage).

2B shows an exponential profile 250 of a negatively distributed optical fiber 200 having the structure described above. This exponential profile 250 shows the refractive indices along the line L (see FIG. 2A) passing through the optical axis of the respective portions within the negatively-dispersed optical fiber 200. Thus, in exponential profile 250, region 251 represents the refractive indices of the respective portions on line L in core region 210, and region 252 represents line L in first cladding region 221. ), And region 253 represents those on line L in second cladding region 222, and region 254 represents those on line L in third cladding region 223.

The negatively distributed optical fiber 10 according to the present invention has a structure similar to that of the negatively distributed optical fiber 200 shown in FIG. 2A, and has an exponential profile similar to the exponential profile 250 shown in FIG. 2B, whereby , To realize the above-mentioned characteristics. In particular, the negative-dispersion optical fiber 10 having an exponential profile as shown in FIG. 2B has a cutoff wavelength to reduce bending loss, compared to the negative-dispersion optical fiber having an exponential profile of the comparative example described below (see FIG. 14). Can be lengthened. The relative refractive index index difference Δ ₁ of the core region 210 with respect to the third cladding region 223 is suitably 1.8% or more and 3.0% or less with respect to elongation of the cutoff wavelength to reduce bending loss.

Specific embodiments of the negatively-dispersed optical fiber 10 according to the present invention are described below. Each embodiment described below has a cross-sectional structure shown in FIG. 2A and an index profile shown in FIG. 2B. In the following description, Ra represents the ratio of the outer diameters of the second cladding region 222 and the core region 210, and Rb represents the ratio of the outer diameters of the first cladding region 221 and the second cladding region 222. . That is, the outer diameter ratios Ra and Rb are represented by Equations 2a and 2b, respectively.

3 is a graph showing the relationship between color dispersion and dispersion gradient for each of the embodiments of negative-dispersion optical fibers in accordance with the present invention. In each of the embodiments shown in this graph, with respect to the third cladding region 223, the relative refractive index difference Δ ₁ of the core region 210 is 2.4%, so that the relative refractive index of the first cladding region is 2.4%. The difference Δ ₂ is set to −0.5%, and the relative refractive index index difference Δ ₃ of the second cladding region is set to 0.2%, respectively.

Ra = 0.20 and Rb = 0.48, Ra = 0.20 and Rb = 0.50, Ra = 0.20 and Rb = 0.52, Ra = 0.20 and Rb = 0.55 and Ra = 0.20 and Rb = For each of the examples of 0.60, the inventors obtained the respective values of chromatic dispersion D ₂ and dispersion gradient S ₂ of each example at a wavelength of 1550 nm while varying the outer diameter 2c of the second cladding region. In Fig. 3, curve C510 shows the relationship between the color dispersion and the dispersion gradient of the embodiment Ra = 0.20 and Rb = 0.48, curve C520 is that of the embodiment Ra = 0.20 and Rb = 0.50, and C530 Ra = 0.20 and Rb. In the example of 0.52, C540 represents that of Ra = 0.20 and Rb = 0.55, and C550 represents that of Ra = 0.20 and Rb = 0.60. In Figure 3, line R1 = ratio (S / D) = 2.0 x 10 ^-3 , line R2 = ratio (S / D) = 2.7 x 10 ^-3 , and line R3 = ratio (S / D) = 4.0 x 10 ⁻³ and the line R4 represent the ratio (S / D) = 4.7 × 10 ⁻³ .

4 is a graph showing the relationship between color dispersion and dispersion gradient for respective embodiments of negative-dispersion optical fibers according to the present invention. In each of the embodiments shown in this graph, with respect to the third cladding region 223, the relative refractive index index difference Δ ₁ of the core region 210 is set to 2.7%, and the relative refractive index of the first cladding region is set. The refractive index difference Δ ₂ is set to −0.5%, and the relative refractive index difference Δ ₃ of the second cladding region is set to 0.3%, respectively.

For each of the embodiments with Ra = 0.20 and Rb = 0.46, the embodiments with Ra = 0.20 and Rb = 0.50, the embodiments with Ra = 0.20 and Rb = 0.54 and the embodiments with Rb = 0.60, we found a second cladding region The values of chromatic dispersion (D ₂ ) and dispersion gradient (S ₂ ) of each example were obtained at a wavelength of 1550 nm while varying the outer diameter (2c) of. In FIG. 4, curve C610 shows the relationship between the color dispersion and the dispersion slope of the embodiment Ra = 0.20 and Rb = 0.46, curve C620 is that of the embodiment Ra = 0.20 and Rb = 0.50, and C630 is Ra = 0.20 and Rb C640 represents an embodiment of Ra = 0.20 and Rb = 0.60. In Figure 4, line R1 = ratio (S / D) = 2.0 x 10 ^-3 , line R2 = ratio (S / D) = 2.7 x 10 ^-3 , and line R3 = ratio (S / D) = 4.0 x 10 ⁻³ and the line R4 represent the ratio (S / D) = 4.7 × 10 ⁻³ .

5 is a graph showing the relationship between color dispersion and dispersion gradient for each embodiment of negatively-dispersed optical fibers in accordance with the present invention. In each of the embodiments shown in this graph, relative to the third cladding region 223, the relative refractive index index difference Δ ₁ of the core region 210 is set to 2.1%, and the relative refractive index of the first cladding region is set. The refractive index difference Δ ₂ is set to −0.5%, and the relative refractive index index Δ ₃ of the second cladding region is set to 0.2%, respectively.

For each embodiment with Ra = 0.20 and Rb = 0.46, an embodiment with Ra = 0.20 and Rb = 0.50, and an embodiment with Ra = 0.20 and Rb = 0.54, the inventors have determined the outer diameter 2c of the second cladding region. Each value of chromatic dispersion (D ₂ ) and dispersion gradient (S ₂ ) of each example was obtained at a wavelength of 1550 nm while changing. In Fig. 5, curve C710 shows the relationship between the color dispersion and the dispersion gradient of the embodiment Ra = 0.20 and Rb = 0.46, curve C720 is that of the embodiment Ra = 0.20 and Rb = 0.50, and C730 is Ra = 0.20 And Rb = 0.54. In Figure 5, line R1 = ratio (S / D) = 2.0 x 10 ^-3 , line R2 = ratio (S / D) = 2.7 x 10 ^-3 , and line R3 = ratio (S / D) = 4.0 x 10 ⁻³ and the line R4 represent the ratio (S / D) = 4.7 × 10 ⁻³ .

In each of these FIGS. 3 to 5, the shaded region has a color dispersion (D ₂ ) of -150 nm ps / nm / km or less at a wavelength of 1550 nm, and a dispersion gradient (S ₂ ) with respect to color dispersion (D ₂ ) at a wavelength of 1550 nm. ) Ratio (S ₂ / D ₂ ) is 2.0 x 10 ^-3 / nm or more and 4.7 x 10 ^-3 / nm or less. As can be seen from the figures, by appropriately setting the values of the respective parameters Δ ₁ , Δ ₂ , Δ ₃ , Ra, Rb, 2c in the exponential profile 250 shown in FIG. 2B, at a wavelength of 1550 nm The color dispersion of D ₂ may be -150ps / nm / km or less, -180ps / nm / km or less, and further, -200ps / nm / km or less. Dispersion slope of the chromatic dispersion (D ₂₎ from the 1550nm wavelength ratio of the _{(S 2) (S 2 /} D 2) is 2.0 x 10 ^-3 / nm or more and 4.7 x 10 ^-3 / nm, and the less, even 2.7 x 10 ^-3 / nm or more and 4.0 x 10 ^-3 / nm or less may be sufficient.

6 is a table showing the specifications of samples (fibers A to G) corresponding to embodiments of negative-dispersion optical fibers in accordance with the present invention. Fibers A and B respectively correspond to each embodiment shown in FIG. 3. The fibers C to F each correspond to the respective embodiments shown in FIG. 4. Fiber G corresponds to the embodiment shown in FIG. 5.

In the fiber A, relative to the third cladding region 223, the relative refractive index index difference Δ ₁ of the core region 210 is set to 2.4%, and the relative refractive index index difference of the first cladding region 221 ( Δ ₂ ) is set to −0.5%, and the relative refractive index index difference Δ ₃ of the second cladding region 222 is set to 0.2%. Ra is 0.20, Rb is 0.52, and the outer diameter 2c of the second cladding region 222 is 15.4 mu m. Fiber A of this structure has the following properties at a wavelength of 1550 nm: color dispersion D ₂ of −200 ps / nm / km; Dispersion gradient S ₂ of −0.69 ps / nm ² / km; A ratio of 3.5 × 10 ⁻³ / nm (S ₂ / D ₂ ); Effective area of 17.5 μm ² ; Bending loss of 4 dB / m at a bending diameter of 20 mm; And transmission loss of 0.52 dB / km. The cutoff wavelength is 1.51 μm in 2 m long fiber A.

In the fiber B, with respect to the third cladding region 223, the relative refractive index index difference Δ ₁ of the core region 210 is set to 2.4%, and the relative refractive index index difference of the first cladding region 221 ( Δ ₂ ) is set to −0.5%, and the relative refractive index index difference Δ ₃ of the second cladding region 222 is set to 0.2%. Ra is 0.20, Rb is 0.48, and the outer diameter 2c of the second cladding region 222 is 15.6 mu m. Fiber B of this structure has the following properties at a wavelength of 1550 nm: color dispersion D ₂ of −185 ps / nm / km; Dispersion slope (S ₂ ) of -0.43 ps / nm ² / km; A ratio of 2.3 × 10 ⁻³ / nm (S ₂ / D ₂ ); Effective area of 17.7 μm ² ; Bending loss of 1 dB / m at a bending diameter of 20 mm; And transmission loss of 0.51 dB / km. The cutoff wavelength is 1.30 mu m in the 2 m long fiber B.

In the fiber C, with respect to the third cladding region 223, the relative refractive index index difference Δ ₁ of the core region 210 is set to 2.7%, and the relative refractive index index difference of the first cladding region 221 ( Δ ₂ ) is set to −0.5%, and the relative refractive index index difference Δ ₃ of the second cladding region 222 is set to 0.3%. Ra is 0.20, Rb is 0.46, and the outer diameter 2c of the second cladding region 222 is 15.2 mu m. Fiber (C) of this structure has the following properties at a wavelength of 1550 nm: color dispersion (D ₂ ) of -182 ps / nm / km; Dispersion gradient S ₂ of −0.39 ps / nm ² / km; A ratio of 2.1 × 10 ⁻³ / nm (S ₂ / D ₂ ); Effective area of 14.8 μm ² ; Bending loss of 0.001 dB / m at a bending diameter of 20 mm; And transmission loss of 0.65 dB / km. The cutoff wavelength is 1.70 m in the 2 m long fiber (C).

In the fiber D, relative to the third cladding region 223, the relative refractive index index difference Δ ₁ of the core region 210 is set to 2.7%, and the relative refractive index index difference of the first cladding region 221 ( Δ ₂ ) is set to −0.5%, and the relative refractive index index Δ ₃ of the second cladding region 222 is set to 0.3%. Ra is 0.20, Rb is 0.50, and the outer diameter 2c of the second cladding region 222 is 15.0 mu m. Fiber (D) of this structure has the following properties at a wavelength of 1550 nm: color dispersion (D ₂ ) of -189 ps / nm / km; Dispersion slope (S ₂ ) of -0.58 ps / nm ² / km; A ratio of 3.1 × 10 ⁻³ / nm (S ₂ / D ₂ ); Effective area of 14.4 μm ² ; Bending loss of 0.01 dB / m at a bending diameter of 20 mm; And transmission loss of 0.66 dB / km. The cutoff wavelength is 1.61 mu m in the 2 m long fiber D.

In the fiber E, with respect to the third cladding region 223, the relative refractive index index difference Δ ₁ of the core region 210 is set to 2.7%, and the relative refractive index index difference of the first cladding region 221 ( Δ ₂ ) is set to −0.5%, and the relative refractive index index difference Δ ₃ of the second cladding region 222 is set to 0.3%. Ra is 0.20, Rb is 0.54, and the outer diameter 2c of the second cladding region 222 is 14.8 mu m. Fiber E of this structure has the following properties at a wavelength of 1550 nm: color dispersion D ₂ of −194 ps / nm / km; Dispersion gradient S ₂ of −0.78 ps / nm ² / km; The ratio (S ₂ / D ₂ ) of 4.0 × 10 ⁻³ / nm; Effective area of 14.1 μm ² ; Bending loss of 0.06 dB / m at a bending diameter of 20 mm; And transmission loss of 0.67 dB / km. The cutoff wavelength is 1.51 mu m in the 2 m long fibers E.

In the fiber F, relative to the third cladding region 223, the relative refractive index index difference Δ ₁ of the core region 210 is set to 2.7%, and the relative refractive index index difference of the first cladding region 221 ( Δ ₂ ) is set to −0.5%, and the relative refractive index index difference Δ ₃ of the second cladding region 222 is set to 0.3%. Ra is 0.20, Rb is 0.54, and the outer diameter 2c of the second cladding region 222 is 14.6 mu m. The fiber F of this structure has the following characteristics at a wavelength of 1550 nm: color dispersion D ₂ of −216 ps / nm / km; Dispersion slope S ₂ of −0.65 ps / nm ² / km; A ratio of 3.0 × 10 ⁻³ / nm (S ₂ / D ₂ ); Effective area of 15.5 μm ² ; Bending loss of 0.2 dB / m at a bending diameter of 20 mm; And transmission loss of 0.67 dB / km. The cutoff wavelength is 1.49 mu m in 2 m long fiber F.

In the fiber G, with respect to the third cladding region 223, the relative refractive index index difference Δ ₁ of the core region 210 is set to 2.1%, and the relative refractive index index difference of the first cladding region 221 ( Δ ₂ ) is set to −0.5%, and the relative refractive index index difference Δ ₃ of the second cladding region 222 is set to 0.2%. Ra is 0.20, Rb is 0.50, and the outer diameter 2c of the second cladding region 222 is 17.0 mu m. Fiber G of this structure has the following properties at a wavelength of 1550 nm: color dispersion D ₂ of −206 ps / nm / km; Dispersion slope (S ₂ ) of -0.68 ps / nm ² / km; A ratio of 3.3 × 10 ⁻³ / nm (S ₂ / D ₂ ); Effective area of 21.3 μm ² ; Bending loss of 9.7 dB / m at a bending diameter of 20 mm; And a transmission loss of 0.49 dB / km. The cutoff wavelength is 1.37 mu m in the fiber G of 2 m length.

Each of the fibers A to G having the above specifications has the following characteristics at a wavelength of 1550 nm: color dispersion D ₂ of -180 ps / nm / km or less; The ratio (S ₂ / D ₂ ) of the dispersion gradient S ₂ to the color dispersion D _{2 of not} less than 2.0 × 10 ⁻³ / nm and not more than 4.7 × 10 ⁻³ / nm; 12㎛ 25㎛ ² or higher, and the effective area of ² or less; Transmission loss of 0.7 dB / km or less; And a cutoff wavelength of at least 1.0 μm and at most 2.0 μm at a length of 2 m. In particular, each of the fibers A, F has a color dispersion of -200 ps / nm / km or less. Each of the fibers A, D, E and F has a ratio (S ₂ / D ₂ ) that is at least 2.7 × 10 ⁻³ / nm and at most 4.0 × 10 ⁻³ / nm. Each of the fibers D to F having a relative refractive index difference Δ ₃ of the second cladding region 222 with respect to the third cladding region 223 has a relative refractive index difference Δ ₃ of 0.2%. It has a smaller effective area than the fibers A and B, a smaller bending loss at a bending diameter of 20 mm and a longer cutoff wavelength at a length of 2 m.

FIG. 7 is a graph showing the color dispersion characteristics of the fibers A and B among the fibers A to G shown in the table of FIG. 6. FIG. 8 is a graph showing the color dispersion characteristics of the fibers C to F among the fibers A to G shown in the table of FIG. 6. FIG. 9 is a graph showing color dispersion characteristics of the fibers G among the fibers A to G shown in the table of FIG. 6. In FIG. 7, curve C910 represents the color dispersion characteristic of fiber B, and curve C920 represents the color dispersion characteristic of fiber A. In FIG. In FIG. 8, curve C1010 represents the color dispersion characteristic of fiber C, curve C1020 represents the color dispersion characteristic of fiber D, curve C1030 represents the color dispersion characteristic of fiber E, and curve C1040 represents the color dispersion characteristic of fiber F. Indicates.

10 is a graph showing wavelength dispersion characteristics of each optical transmission line to which fibers A and B are applied, among the fibers A to G shown in the table of FIG. 6. FIG. 11 is a graph illustrating wavelength dispersion characteristics of respective optical transmission lines to which the fibers C to F of the fibers A to G shown in the table of FIG. 6 are applied. FIG. 12 is a graph illustrating wavelength dispersion characteristics of an optical transmission line to which the fiber G is applied, among the fibers A to G shown in the table of FIG. 6. FIG. 13 is a table illustrating various characteristics of respective optical transmission lines to which the fibers A to G shown in the table of FIG. 6 are applied. In FIG. 10, curve 1210 represents the color dispersion characteristics of the optical transmission line including fiber A, and curve 1220 represents the color dispersion characteristics of the optical transmission line including fiber B. In FIG. In FIG. 11, curve 1310 shows the color dispersion characteristics of the optical transmission line including fiber C, curve 1320 shows the color dispersion characteristics of the optical transmission line including fiber D, and curve 1330 shows the color of the optical transmission line including fiber E. In FIG. Dispersion characteristics, curve 1340 represents the color dispersion characteristics of the optical transmission line including the fiber F.

In each of FIGS. 10 to 13, the bi-dispersion optical fiber as another fiber constituting the optical transmission line has the following characteristics at a wavelength of 1550 nm, that is, color dispersion of +17 ps / nm / km and +0.057 ps / nm ² / km Has a dispersion slope of and has a length of 80 km. The insertion loss of FIG. 13 also includes splice losses due to connecting conventional short bi-disperse optical fibers to both ends of the module consisting of each of the fibers A to G described above.

In the optical transmission line including the fiber A, the length of the fiber A is 7.4 km, the insertion loss is 4.8 dB at the wavelength of 1550 nm, and the deviation of the color dispersion in the wavelength band (C-band) of 1530 nm to 1560 nm is 0.35 ps. / nm / km, deviation of color dispersion in wavelength band (S-band) from 1450nm to 1560nm is 0.94ps / nm / km, deviation of color dispersion in wavelength band (S + C + L band) from 1450nm to 1610nm Is 1.62 ps / nm / km.

In the optical transmission line including the fiber B, the length of the fiber B is 7.6 km, the insertion loss is 4.9 dB at the wavelength of 1550 nm, and the deviation of the color dispersion in the wavelength band (C-band) of 1530 nm to 1560 nm is 0.32 ps. / nm / km, the deviation of the color dispersion in the wavelength band (S-band) of 1450nm to 1560nm is 0.80ps / nm / km, and the deviation of the color dispersion in the wavelength band (S + C + L band) of 1450nm to 1610nm Is 3.18 ps / nm / km.

In the optical transmission line including fiber C, the length of the fiber C is 7.6 km, the insertion loss is 5.9 dB at a wavelength of 1550 nm, and the deviation of chromatic dispersion in the wavelength band (C-band) of 1530 nm to 1560 nm is 0.49 ps. / nm / km, the deviation of the color dispersion in the wavelength band (S-band) of 1450nm to 1560nm is 1.51ps / nm / km, and the deviation of the color dispersion in the wavelength band (S + C + L band) of 1450nm to 1610nm Is 3.64 ps / nm / km.

In the optical transmission line including fiber D, the length of fiber D is 7.5 km, the insertion loss is 6.0 dB at a wavelength of 1550 nm, and the deviation of chromatic dispersion in the wavelength band (C-band) of 1530 nm to 1560 nm is 0.04 ps. / nm / km, the deviation of the color dispersion in the wavelength band (S-band) of 1450nm to 1560nm is 0.44ps / nm / km, the deviation of the color dispersion in the wavelength band (S + C + L band) of 1450nm to 1610nm Is 1.72ps / nm / km.

In the optical transmission line including the fiber E, the length of the fiber E is 7.4 km, the insertion loss is 6.0 dB at the wavelength of 1550 nm, and the deviation of the color dispersion in the wavelength band (C-band) of 1530 nm to 1560 nm is 0.48 ps. / nm / km, deviation of color dispersion in wavelength band (S-band) from 1450nm to 1560nm is 0.88ps / nm / km, deviation of color dispersion in wavelength band (S + C + L band) from 1450nm to 1610nm Is 1.02 ps / nm / km.

In the optical transmission line including the fiber F, the length of the fiber F is 6.6 km, the insertion loss is 5.4 dB at the wavelength of 1550 nm, and the deviation of the color dispersion in the wavelength band (C-band) of 1530 nm to 1560 nm is 0.10 ps. / nm / km, the deviation of the color dispersion in the wavelength band (S-band) of 1450nm to 1560nm is 0.41ps / nm / km, and the deviation of the color dispersion in the wavelength band (S + C + L band) of 1450nm to 1610nm Is 2.10 ps / nm / km.

In the optical transmission line including the fiber G, the fiber G is 7.0 km in length, the insertion loss is 4.4 dB at a wavelength of 1550 nm, and the deviation of color dispersion in the wavelength band (C-band) of 1530 nm to 1560 nm is 0.43 ps. / nm / km, the deviation of the color dispersion in the wavelength band (S-band) of 1450nm to 1560nm is 1.88ps / nm / km, the deviation of the color dispersion in the wavelength band (S + C + L band) of 1450nm to 1610nm Is 3.13 ps / nm / km.

The optical transmission lines (including each one of the fibers A to G, respectively) having various characteristics as described above exhibit the following deviations: the deviation of the overall average color dispersion in the wavelength band of 1530 nm to 1560 nm (wavelength Change between them) is 0.5 ps / nm / km or less; The deviation (change between wavelengths) of the overall average color dispersion in the wavelength band of 1450 nm to 1560 nm is 2.0 ps / nm / km or less; The deviation (change between wavelengths) of the overall average color dispersion in the wavelength band of 1450 nm to 1610 nm is 4.0 ps / nm / km or less. In the case of optical transmission lines comprising the respective fibers A, D and E, the deviation (change between wavelengths) of the overall average color dispersion in the wavelength band of 1450 nm to 1610 nm is 2.0 ps / nm / km or less.

As a comparative example, FIG. 14 shows the exponential profile of a typical negative-dispersion optical fiber. Further, as can be seen from this exponential profile, the negatively-dispersed optical fiber prepared as a comparative example includes a core region (a portion corresponding to the region 101 of the exponent profile 100) and a first cladding region (exponent profile 100 ) And a second cladding region (a portion corresponding to region 103 of exponential profile 100). The core region has a maximum index of refraction n ₁ and an outer diameter 2a. The first cladding region has a refractive index n ₂ (<n ₁ ) and an outer diameter 2b. The second cladding region has an index of refraction n ₃ (> n ₂ and <n ₁ ).

More specifically, the negative-dispersion optical fiber of this comparative example has, for example, an outer diameter 2a of the core region of 3.2 mu m and an outer diameter 2b of the first cladding region of 8.1 mu m. With respect to the refractive index n ₃ of the second cladding region, the relative refractive index difference Δ ₁ of the core region is 2.1%, and the relative refractive index difference Δ ₂ of the first cladding region is −0.35%. This negative-dispersion optical fiber has the following characteristics at a wavelength of 1550 nm: color dispersion of -88 ps / nm / km; Dispersion gradient of −019 ps / nm ² / km; Effective area of 16.2 μm ² ; Bending loss of 6 dB / m at a bending diameter of 20 mm; And transmission loss of 0.39 dB / km. The cutoff wavelength at the length of 2m (the cutoff wavelength in LP ₁₁ mode with the 2m-long optical fiber loosely wound with a radius of 140mm) is 0.74 mu m.

On the other hand, a bi-dispersion optical fiber has, for example, the following characteristics at a wavelength of 1550 nm. Color dispersion of +17 ps / nm / km and dispersion gradient of +0.057 ps / nm ² / km. Assuming that the color dispersion of the positively-dispersed optical fiber is compensated by the negatively-dispersed optical fiber of the comparative example, when the length of this positively-dispersed optical fiber is 80 km, the negatively distributed optical fiber needs to have a length of 15.9 km.

FIG. 15 shows the overall average color dispersion characteristics of a light transmission line composed of a positively-dispersed optical fiber and a negatively-dispersed optical fiber (comparative example) as described above. As shown in Fig. 15, the average color variance is 0 at the wavelength of 1540 nm, but the absolute values of the average color variance increase with the deviation of this length. Thus, the deviation (maximum value-minimum value) of the average color dispersion between wavelengths in the wavelength band of 1530 nm to 1560 nm is 0.68 ps / nm / km, and the variation in average color dispersion between wavelengths in the wavelength band of 1450 nm to 1560 nm is 3.70 ps / nm / km, and the deviation of the average color dispersion between wavelengths in the wavelength band of 1450 nm to 1610 nm is 4.18 ps / nm / km. Since the deviations between the wavelengths of the signal wavelength bands are large as described above, the optical transmission line including the negative-dispersion optical fiber as the comparative example has limitations for the implementation of long distance and large capacity WDM optical transmission.

As described above, the present invention provides a negatively distributed optical fiber in an optical transmission line including a negatively distributed optical fiber and a negatively distributed optical fiber having a small color dispersion D (that is, a color dispersion having a negative sign and a large absolute value). The ratio of the length of the can be reduced. This can reduce the average transmission loss of the optical transmission line and enables low cost manufacturing. Since the ratio of the dispersion slope (S / D) to the color dispersion (D) of the negatively-dispersed optical fiber is at least 2.0 × 10 ⁻³ / nm and 4.7 × 10 ⁻³ / nm, the dispersion gradient compensation ratio is about 60% to 140 %, Which can reduce both the average dispersion slope of the entire optical transmission line and the absolute values of each of the average color dispersion, and the deviation (maximum value −) of the average color dispersion between wavelengths across the optical transmission line in the signal wavelength band. Decrease the minimum value). An effective area of 12 μm ² or more is equal to or larger than that of conventional negative-dispersion optical fibers, and can effectively suppress the occurrence of nonlinear optical phenomena in the negative-dispersion optical fibers. In addition, the effective area of the 25㎛ ^2, preferably 20㎛ is less than ^2, the rolled shape or sound module into a coil-shape as a negative in the cable bundle of optical fibers having a structure similar to that of the optical fiber dispersion-dispersion in the loss of the optical fiber This is small.

In addition, since both chromatic dispersion and dispersion gradient are compensated in the optical transmission line according to the present invention, the absolute values of chromatic dispersion can be kept small throughout the entire signal wavelength band, realizing long distance and large capacity WDM optical transmission. It becomes possible.

Claims (26)

Color dispersion (D) of -150 ps / nm / km or less; A dispersion gradient (S) that satisfies the condition that the ratio S / D to the color dispersion D is 2.0 × 10 ⁻³ / nm or more and 4.7 × 10 ⁻³ / nm or less; And 12㎛ ² or more and the sound having the characteristics of 25㎛ ² is less than the effective area at the wavelength of 1550nm - fiber dispersion. The method of claim 1, The color dispersion (D) is less than -180 ps / nm / km, negative-dispersion optical fiber. The method of claim 1, And the effective area is less than 20 μm ² . The negative-dispersion optical fiber of claim 1, wherein the ratio S / D is at least 2.7 × 10 ⁻³ / nm and at most 4.0 × 10 ⁻³ / nm. The method of claim 1, A negatively-dispersed optical fiber further having a cutoff wavelength of 1.0 m or more and 2.0 m or less at a length of 2 m. The method of claim 1, And further having a transmission loss of 1.0 dB / km or less for light having a wavelength of 1550 nm. The method of claim 6, The transmission loss is less than 0.7 dB / km. The method of claim 1, A core region having a predefined maximum index of refraction and extending along a predefined axis; A first cladding region having a refractive index lower than the maximum refractive index of the core region, the first cladding region provided in an outer periphery of the central core region; A second cladding region having a refractive index higher than the refractive index of the first cladding region and provided on an outer periphery of the first cladding region; And And a third cladding region provided on an outer periphery of the second cladding region, the refractive index being lower than the index of refraction of the second cladding region. The method of claim 8, And the maximum relative refractive index difference of the core region relative to the third cladding region is at least 1.8% and at most 3.0%. In the optical transmission line, Color dispersion of more than + 15ps / nm / km and less than + 21ps / nm / km, and dispersion gradients of more than +0.05 ps / nm ² / km and less than + 0.07ps / nm ² / km at 1550nm Positive-dispersion optical fiber; And An optical transmission line comprising the negative-dispersion optical fiber according to claim 1. The method of claim 10, In the wavelength band of 1530 nm to 1560 nm, the deviation of the average color dispersion between the wavelengths over the entire optical transmission line is 0.5 ps / nm / km or less. The method of claim 10, In a wavelength band of 1450 nm to 1560 nm, the variation in average color dispersion between wavelengths over the entire optical transmission line is 2.0 ps / nm / km or less. The method of claim 10, In the wavelength band of 1450 nm to 1610 nm, the deviation of the average color dispersion between the wavelengths over the entire optical transmission line is less than 4.0 ps / nm / km. The method of claim 13, In the wavelength band of 1450 nm to 1610 nm, the deviation of the average color dispersion between the wavelengths over the entire optical transmission line is 2.0 ps / nm / km or less. Color dispersion (D) of -200 ps / nm / km or less; And Having a characteristic of dispersion gradient (S) at a wavelength of 1550 nm that satisfies the condition that the ratio (S / D) to the color dispersion (D) is not less than 2.0 x 10 ^-3 / nm and not more than 4.7 x 10 ^-3 / nm. Well-dispersed fiber optics. The method of claim 15, Wherein said ratio (S / D) is at least 2.7 x 10 ^-3 / nm and at most 4.0 x 10 ^-3 / nm. The method of claim 15, A negatively-dispersed optical fiber further having a cutoff wavelength of 1.0 m or more and 2.0 m or less at a length of 2 m. The method of claim 15, And further having a transmission loss of 1.0 dB / km or less for light having a wavelength of 1550 nm. The method of claim 18, The transmission loss is less than 0.7 dB / km. The method of claim 15, A core region having a predefined maximum index of refraction and extending along a predefined axis; A first cladding region having a refractive index lower than the maximum refractive index of the core region, the first cladding region provided in an outer periphery of the core region; A second cladding region having the refractive index higher than the refractive index of the first cladding region and provided on an outer periphery of the first cladding region; And And a third cladding region having a refractive index lower than the refractive index of the second cladding region, the third cladding region provided on an outer periphery of the second cladding region. The method of claim 20, And the maximum index of refraction of the core region relative to the third cladding region is greater than or equal to 1.8% and less than or equal to 3.0%. In the optical transmission line, Color dispersion above +15 ps / nm / km and below +21 ps / nm / km; And + 0.05ps / nm ² / km or more and + 0.07ps / nm ² / km dispersion slope characteristics of 1550nm, the amount that has the wavelength of less than-dispersion optical fiber; And 16. An optical transmission line comprising the negatively-dispersed optical fiber according to claim 15. The method of claim 22, In a wavelength band of 1530 nm to 1560 nm, the deviation of the average color dispersion between the wavelengths of the entire optical transmission line is 0.5 ps / nm / km or less. The method of claim 22, In a wavelength band of 1450 nm to 1560 nm, the deviation of the average color dispersion between the wavelengths of the entire optical transmission line is 2.0 ps / nm / km or less. The method of claim 22, In the wavelength band of 1450 nm to 1610 nm, the deviation of the average color dispersion between the wavelengths of the entire optical transmission line is 4.0 ps / nm / km or less. The method of claim 25, In the wavelength band of 1450 nm to 1610 nm, the deviation of the average color dispersion between the wavelengths of the entire optical transmission line is 2.0 ps / nm / km or less.