JPWO2000002074A1 - optical fiber - Google Patents

Abstract

(57) [Abstract] This invention relates to an optical fiber having a structure that effectively suppresses at least an increase in microbend loss. The optical fiber is suitable for use as a dispersion-flattened fiber, a dispersion-compensating fiber, etc., and is single-mode compensated in the wavelength band in use. In particular, the optical fiber has a fiber diameter of 140 μm or more, which provides high rigidity and effectively suppresses an increase in microbend loss. Meanwhile, the fiber diameter is kept to 200 μm or less, which means that the probability of breakage due to bending strain is practically negligible. Furthermore, the optical fiber has a larger mode field diameter, which reduces the optical energy density per unit cross-sectional area and effectively suppresses the occurrence of nonlinear optical phenomena.

Description

[Detailed Description of the Invention] Optical Fiber Technical Field This invention relates to an optical fiber suitable for use as an optical transmission path in a wavelength division multiplexing (WDM) transmission system.

BACKGROUND ART Optical fibers are primarily used as transmission paths for optical transmission, particularly WDM transmission, which enables large-capacity and high-speed optical transmission. However, in recent years, degradation of signal light due to nonlinear optical phenomena, such as four-wave mixing, occurring in optical fibers between signal light beams, has become a problem in such WDM transmission systems. Therefore, it is important to effectively suppress the occurrence of nonlinear optical phenomena in WDM transmission systems. To achieve this, it is necessary to reduce the optical energy density per unit cross-sectional area by increasing the mode field diameter or effective cross-sectional area of the optical fiber. For example, Japanese Patent Laid-Open Publication No. 8-248251 discloses an optical fiber having an effective cross-sectional area (70 ^μm2 or more) larger than that of conventional dispersion-shifted fibers.

Disclosure of the Invention It is generally known that as the mode field diameter or effective area increases, the microbending characteristics deteriorate and the microbending loss due to cable construction increases.

For example, Figure 1 shows the refractive index profile of a dispersion-shifted fiber with a dual-core structure. This dispersion-shifted fiber has a core region composed of an inner core with a refractive index of n1 and an outer core with a refractive index of n2 (<n1), with a single cladding layer with a refractive index of n3 (<n2>) surrounding the core region. Figure 2 is a graph showing the relationship between the mode field diameter at a wavelength of 1.55 μm (1550 nm) and the loss increase due to microbending for an optical fiber having this dual-core refractive index profile. Note that in this specification, the mode field diameter refers to the Petermann-I mode field diameter, and this Petermann-I mode field diameter MFDI is given by the following equations (1a) and (1b), as shown in E. G. Neumann, "Single-Mode Fibers," pp. 225, 1988:

In equation (1b), r is the radial position variable with the core center as the origin, and E is the electric field amplitude, which is a function of the position variable r. Microbend loss is defined as the increase in loss when an optical fiber 250 m long is wound with a tension of 100 g around a bobbin with a diameter of 280 mm and JIS #1000 sandpaper laid on its surface.

Furthermore, theoretical studies have also shown that the relationship between the microbending loss Δα and the mode field diameter MFD1 is expressed by the following equations (2a) to (2c):

however, In these equations, R is the radius of curvature of the minute bend applied to the optical fiber, k is the wave number, n1 is the refractive index of the core region, and Lc is the correlation length of the minute bend applied to the optical fiber.

As can be seen from Figure 2 and equations (2a) to (2c), the larger the mode field diameter (MFD1), the greater the microbend loss. However, while conventional optical fibers are designed with macrobend loss in mind, they do not take microbend loss into consideration at all. Furthermore, as an indicator for assembling optical fiber into a cable, it is known that when the loss increase measured by winding the optical fiber around a bobbin with sandpaper wrapped around its surface exceeds approximately 1 dB/km, the loss increases due to cabling. Therefore, it is clear that the microbend loss increases when the optical fiber described above is cabling.

The present invention has been made to solve the above-mentioned problems, and aims to provide an optical fiber having a structure that can effectively suppress an increase in microbend loss.

To achieve the above-mentioned objectives, the optical fiber according to the present invention has a core region extending along a predetermined axis and a cladding region disposed around the core region, each composed of at least three glass layers with different refractive indices. Furthermore, the optical fiber is guaranteed to be substantially single-mode for light in the operating wavelength, e.g., the 1.55 μm wavelength band (1500 nm to 1600 nm), and has a fiber diameter of 140 μm to 200 μm. As described above, the optical fiber according to the present invention has a fiber diameter of 140 μm or more, so that even if the mode field diameter is large, the optical fiber has high rigidity and an increase in microbend loss is suppressed. Meanwhile, because the fiber diameter is 200 μm or less, the probability of optical fiber breakage due to bending strain is practically negligible.

In particular, when the 1.55 μm wavelength band is the wavelength band used in WDM transmission, the optical fiber according to the present invention preferably has an absolute value of chromatic dispersion at a wavelength of 1550 nm of 5 ps/nm/km or less. Furthermore, the Petermann-I mode field diameter is preferably 11 μm or greater. This is because, even when a WDM signal is transmitted, a mode field diameter of 11 μm or greater reduces the optical energy density per unit cross-sectional area, thereby effectively suppressing the occurrence of nonlinear optical phenomena.

The optical fiber according to the present invention can be applied as a single-mode optical fiber such as a dispersion-shifted fiber, a dispersion-flattened fiber, or a dispersion-compensating fiber.

In particular, when the optical fiber according to the present invention is applied as a dispersion-flattened fiber, the optical fiber preferably has a dispersion slope of 0.02 ps/ ^nm2 /km or less and an effective area of 50 ^μm2 or more at at least one wavelength within the wavelength band used, and it is particularly preferable that the absolute value of the dispersion slope is 0.02 ps/ ^nm2 /km or less.

Furthermore, when the optical fiber according to the present invention is applied as a dispersion compensating fiber, the optical fiber preferably has a chromatic dispersion of -18 ps/nm/km or less at at least one wavelength within the wavelength band used, and an effective area of 17 μm or ^more .

Furthermore, when the optical fiber according to the present invention is used as an optical fiber with an enlarged effective area, it preferably has an effective area of 110 ^μm2 or more at at least one wavelength within the wavelength band used. In this optical fiber, too, the optical energy density per unit cross-sectional area can be kept low, and the occurrence of nonlinear optical phenomena can be effectively suppressed.

The fiber diameter of the various optical fibers described above is also 140 μm or more and 200 μm or less. However, in the case of dispersion compensating fibers having the above-described characteristics, the microbending characteristics are particularly susceptible to deterioration, so the fiber diameter is preferably 150 μm or more and 200 μm or less.

When the optical fiber according to the present invention is applied to an optical cable, the optical fiber preferably has an effective area of 17 μm or ^more , a chromatic dispersion value of −83 ps/nm/km or more, and a fiber diameter of 140 μm or more and 1200 μm or less at at least one wavelength within the wavelength band used. Note that single-mode optical fibers such as dispersion-shifted fiber, dispersion-flattened fiber, and dispersion-compensating fiber can also be applied to such optical fibers intended for cable construction.

As described above, taking into consideration various applicable situations, the optical fiber according to the present invention is preferably an optical fiber having a fiber diameter of 140 μm or more and 200 μm or less, an effective area of 17 μm or more, and ^a chromatic dispersion value of -83 ps/nm/km or more at at least one wavelength within the used wavelength band, and more preferably an optical fiber having a fiber diameter of 140 μm or more and 200 μm or less, an effective area of 17 μm ^or more, and a chromatic dispersion value of -48 ps/nm/km or more at at least one wavelength within the used wavelength band. Furthermore, depending on the type of optical fiber to be applied, the fiber diameter is preferably 150 μm or more and 200 μm or less.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the refractive index profile of a dispersion-shifted fiber (dual-core structure).

Figure 2 is a graph showing the relationship between mode field diameter (Petermann-I) and loss increase due to microbending at a wavelength of 1.55 μm in the dispersion-shifted fiber shown in Figure 1.

FIG. 3A is a diagram showing a cross-sectional structure common to all embodiments of the optical fiber according to the present invention, and FIG. 3B is a diagram showing the refractive index profile of an optical fiber according to a fourth embodiment.

FIG. 4 is a table summarizing the specifications of four samples fabricated to illustrate the optical fiber according to the first embodiment.

FIG. 5 is a graph showing the evaluation results of four samples fabricated to illustrate the optical fiber according to the first example.

FIG. 6A is a diagram showing the cross-sectional structure of an optical fiber unit that constitutes part of an optical cable, and FIG. 6B is a diagram showing the cross-sectional structure of an optical cable that includes the optical fiber unit shown in FIG. 6A.

FIG. 7 is a table summarizing the specifications of three samples fabricated to illustrate the second embodiment of the optical fiber according to the present invention.

Figure 8 is a table summarizing the specifications of four samples fabricated to illustrate the third embodiment of the optical fiber according to the present invention.

FIG. 9 is a graph showing the evaluation results of four samples fabricated to illustrate the optical fiber according to the third example.

FIG. 10 is a graph showing the relationship between fiber diameter and breakage probability.

FIG. 11 is a table summarizing the specifications of two samples fabricated to illustrate the fourth embodiment of the optical fiber according to the present invention.

FIG. 12 is a graph showing the refractive index profile of a fifth embodiment of an optical fiber according to the present invention.

FIG. 13 is a graph showing refractive index profiles of sixth and seventh embodiments of the optical fiber according to the present invention.

FIG. 14 is a graph showing the relationship between chromatic dispersion and dispersion slope for the optical fiber according to the fifth embodiment shown in FIG. 12 (Δ ⁺ =0.9%, Δ ⁻ =−0.44%).

BEST MODE FOR CARRYING OUT THE INVENTION Each embodiment of the optical fiber according to the present invention will be described in detail below using Figures 1, 3A, 3B, 4, 5, 6A, 6B, and 7-14.

As shown in Figure 3A, optical fiber 100 according to the present invention comprises core region 110 having an outer diameter a extending along a predetermined axis, and cladding region 120 having an outer diameter b (equal to the fiber diameter) surrounding core region 110. In the embodiments described below, these core and cladding regions are composed of at least three glass layers each having a different refractive index. Furthermore, the optical fiber according to each of the following embodiments has an absolute value of chromatic dispersion at a wavelength of 1.55 μm (1550 nm) of 5 ps/nm/km or less, a Petermann-I mode field diameter of 11 μm or more, and a fiber diameter b of 140 μm or more and 200 μm or less.

The optical fibers according to the first to third embodiments have a refractive index profile of a dual-core structure similar to the refractive index profile shown in FIG. 1, while the refractive index profile of the optical fiber according to the fourth embodiment has a refractive index profile of a segmented core-depressed cladding structure as shown in FIG. 3B.

The refractive index profile shown in FIG. 1 indicates the refractive index at each location on line L in FIG. 3A. In the optical fibers of the first to third embodiments, the core region 110 having an outer diameter a is composed of an inner core with a refractive index n1 and an outer core with a refractive index n2 (<n1) disposed around the inner core. The cladding region 120 having an outer diameter b (equal to the fiber diameter) is composed of a single cladding having a refractive index n3 (<n2) disposed around the outer core. Thus, the optical fibers of the first to third embodiments are composed of three glass layers (inner core, outer core, and single cladding) and are guaranteed to be single-mode optical fibers in the wavelength band used.

On the other hand, the optical fiber according to the fourth embodiment, as shown in FIG. 3B, is an optical fiber having a refractive index profile 500 with a segmented core-depressed cladding structure. This refractive index profile 500 also shows the refractive index at each location on line L in FIG. 3A. In particular, in the refractive index profile 500, portion 510 represents the core region with outer diameter a, and portion 520 represents the cladding region with outer diameter b. In this fourth embodiment, the core region is composed of an inner core with refractive index n1, an intermediate core with refractive index n2 (<n1) located around the inner core, and an outer core with refractive index n3 (>n2) located around the intermediate core. Meanwhile, the cladding region is composed of an inner cladding with refractive index n4 (<n3) located around the outer core, and an outer cladding with refractive index n5 (>n4) located around the inner cladding. As described above, the optical fiber according to the fourth embodiment is composed of five glass layers (inner core, middle core, outer core, inner cladding, and outer cladding), and is an optical fiber that is guaranteed to be single-mode in the wavelength band used.

The optical fibers according to the first to fourth embodiments having the refractive index profiles described above will be described in order below.

First Example First, to explain the optical fiber according to the first example, four types of optical fibers (samples 1a to 1d) having substantially the same Petermann-I mode field diameter MFD1 but different fiber diameters b were fabricated and evaluated.

FIG. 4 is a table summarizing the specifications of four samples 1a to 1d fabricated to explain the optical fiber of the first embodiment.

The fiber diameter b is approximately 125 μm for sample 1a, approximately 140 μm for sample 1b, approximately 150 μm for sample 1c, and approximately 160 μm for sample 1d.

The Petermann-I mode field diameter MFD1 (11.73 to 11.88 μm), effective area (69.7 to 72.1 μm2), chromatic dispersion (−2.2 to −1.9 ps/nm/km), and cutoff wavelength (1.50 to 1.53 μm) given by the above formulas (1a) and (1b) are substantially identical among the four samples 1a to 1d.

These four samples 1a-1d, each with a different fiber diameter b, were obtained by drawing four preforms using core members of the same diameter and with different core-to-clad outer diameter ratios. Furthermore, each of the four samples 1a-1d was surrounded by a coating layer made of the same material and with an outer diameter of 250 μm. The mode field diameter MFD1, effective area, and chromatic dispersion were all measured at a wavelength of 1.55 μm (1550 nm).

Microbend loss was measured for each of these four samples 1a to 1d. For this measurement, a 250-m-long optical fiber was wound with a tension of 100 g around a bobbin with a diameter of 280 mm and a surface covered with JIS #1000 sandpaper, and the resulting increase in loss was defined as microbend loss. Figure 5 is a graph showing the evaluation results for each of the four samples experimentally produced to illustrate the optical fiber according to Example 1. This graph reveals that the larger the fiber diameter b, the smaller the increase in loss, i.e., the smaller the microbend loss.

When manufacturing an optical cable having the cross-sectional structure shown in Figures 6A and 6B, microbending loss increases during cable fabrication. To prevent this, it is necessary to keep the microbending loss below approximately 1 dB/km.

Therefore, as can be seen from FIG. 5, when the effective area is about 70 μm ² , the fiber diameter b needs to be 140 μm or more.

FIG. 6A shows the cross-sectional structure of an optical fiber unit, and FIG. 6B shows the cross-sectional structure of an optical cable incorporating this optical fiber unit 200. In FIG. 6A, the optical fiber 100 is coated with UV-curable resin 150 as a coating layer and integrated with UV-curable resin 152 around a tension member 151. UV-curable resin 153 is then coated around the UV-curable resin 152 to obtain the optical fiber unit 200. The optical fiber cable 300 incorporating this optical fiber unit 200 is obtained by encasing the optical fiber unit 200, which is covered in turn with a water-repellent compound 251, a three-section steel pipe 250, and a tensile piano wire 252, in a copper tube 253, and then covering the outer periphery of the copper tube 253 with low-density polyethylene 254 and high-density polyethylene 255, in that order.

Second Example Next, in the second example, a comparison will be made between the characteristics of a conventional optical fiber that has already been confirmed to have no problems when cabled and the characteristics of the optical fiber according to this second example. Figure 7 is a table summarizing the specifications of three samples 2a to 2c that were experimentally produced to explain the optical fiber according to the second example. Of these, sample 2a is a conventional optical fiber that has already been confirmed to have no problems when cabled.

The fiber diameter b is approximately 125 μm for Samples 2a and 2b and approximately 140 μm for Sample 2c. The Peterman n-I mode field diameter MFD1 given by the above formulas (1a) and (1b) is approximately 10 μm for Sample 2a and approximately 11 μm for Samples 2b and 2c. The effective area is approximately 55 ^μm for Sample 2a and approximately 65 ^μm for Samples 2b and 2c. The chromatic dispersion values (-2.2 to -2.0 ps/nm/km) and cutoff wavelengths (1.51 to 1.53 μm) are approximately identical for each of the three Samples 2a to 2c. Each of the three Samples 2a to 2c is surrounded by a coating layer made of the same material and having an outer diameter of 250 μm. The mode field diameter MFD1, effective area, and chromatic dispersion values are measured at a wavelength of 1.55 μm.

Using these three types of samples 2a to 2c, an optical fiber unit 200 having the cross-sectional structure shown in Figures 6A and 6B was fabricated. A water pressure of 100 atmospheres was applied to the resulting optical fiber unit 200 to simulate the external pressure applied to the optical fiber unit 200 during cabling. As a result, sample 2b, which had a fiber diameter b of approximately 125 µm and an effective cross-sectional area of approximately 65 ^µm2 , had a loss increase, i.e., microbend loss, of 20 mdB/km. In contrast, sample 2c, which had a fiber diameter b of approximately 140 µm and an effective cross-sectional area of approximately 65 ^µm2 , had a loss increase, i.e., microbend loss, of less than the measurement limit of 0.5 mdB/km, demonstrating characteristics equivalent to those of conventional sample 2a, which has already been confirmed to pose no problems when cabling.

Third Example Next, a third example will be described. In this third example, the effect of reducing microbend loss due to an increased fiber diameter was confirmed for an optical fiber with a larger effective area. Figure 8 is a table summarizing the specifications of four samples 3a to 3d that were fabricated to explain the optical fiber according to the third example.

The fiber diameter b is approximately 150 μm for samples 3a and 3c, and approximately 170 μm for samples 3b and 3d. The Petermann-I mode field diameter MFD1 given by the above formulas (1a) and (1b) is approximately 13.2 μm for samples 3a and 3b, and approximately 14.2 μm for samples 3c and 3d. The effective area is approximately 80 μm for samples 3a and 3b.², and about 90 μm for samples 3c and 3d.² On the other hand, the chromatic dispersion values (-2.0 to -2.2 ps/nm/km) and cutoff wavelengths (1.50 to 1.53 μm) are approximately the same for each of the four samples 3a to 3d. Each of the four samples 3a to 3d is surrounded by a coating layer made of the same material and having an outer diameter of 250 μm. The mode field diameter (MFD1), effective area, and chromatic dispersion values are all measured at a wavelength of 1.55 μm.

The microbend loss was measured for each of the four samples 3a to 3d. The method for measuring the microbend loss was the same as in the first example.

9 is a graph showing the evaluation results of four samples fabricated to explain the optical fiber according to the third embodiment. As can be seen from this graph, when the effective area is about 80 ^μm2 , if the fiber diameter b is about 150 μm or more, the microbend loss can be kept below the target value of about 1 dB/km. Also, when the effective area is about 90 ^μm2 , if the fiber diameter b is about 170 μm or more, the microbend loss can be kept below the target value of about 1 dB/km.

From the above, it can be seen that even if the effective area is large, as long as the fiber diameter is large, microbend loss can be suppressed to the target value of approximately 1 dB/km or less. This can also be explained as follows: Microbend loss in optical fiber occurs when external forces applied to the optical fiber cause random microbends in the longitudinal direction of the optical fiber's core region. This microbend loss is proportional to the root mean square of the reciprocal of the radius of curvature of the microbend. On the other hand, if the external force applied to the optical fiber is constant, microbends occurring in the optical fiber can be suppressed by increasing the optical fiber's rigidity. If the fiber diameter b of the optical fiber is D, the rigidity (bending moment) I of the optical fiber is given by the following equation (3):

Therefore, by increasing the fiber diameter D of the optical fiber, the rigidity I of the optical fiber is significantly improved, and as a result, microbending is suppressed and microbend loss is significantly reduced.

For example, an optical fiber was fabricated with a fiber diameter of 200.3 μm, an effective area ^of 90.4 μm, a chromatic dispersion of −2.1 ps/nm/km, a cutoff wavelength of 1.73 μm, and an outer diameter of the coating layer of 250 μm. When the microbend loss was measured using the same method as in Example 1, the value was 0.3 dB/km. This is comparable to the characteristics of conventional optical fibers that have already been confirmed to pose no problems when cabled.

However, as the fiber diameter b increases, the strain on the fiber surface (the surface of the outermost cladding) when the optical fiber is bent becomes greater than that of conventional optical fibers, increasing the probability of breakage due to bending. Therefore, we will estimate the range of fiber diameter b within which the breakage probability does not pose a practical problem. The breakage probability F of an optical fiber after passing the screening test is given by the following equation (4):

Here, L is the length of the optical fiber to which strain is applied in actual use, Np is the number of breaks per unit length during the screening test, σs is the strain during actual use, σp is the strain during screening, ts is the actual use time, tp is the screening time, n is the fatigue coefficient, and m is a parameter representing the slope of the Weibull plot.

During actual use, small-diameter bends in optical fiber occur in the excess length left for fusion splicing in repeaters, and in the worst case, one bend with a diameter of 30 mm exists in each repeater. Assume also that the total length of optical fiber in the optical transmission system is 9,000 km, with repeaters installed at an average interval of 50 km. In this case, the fiber length L subjected to a 30 mm diameter bend is 16.9 m throughout the entire optical transmission system. Assume also that the number of breaks per unit length Np during the screening test is 2 × 10 ⁻⁵ , and the strain σp during screening is 2.2%. Assume the actual use time ts is 25 years, and the screening time tp is 1 second. Assume the fatigue coefficient n is 20, and the parameter m representing the slope of the Weibull plot is 10.

Figure 10 is a graph showing the relationship between fiber diameter b and breakage probability based on the above equation (4) under the above assumptions. As can be seen from this graph, the larger the fiber diameter, the higher the breakage probability. However, if the fiber diameter is 200 μm or less, the breakage probability is 10 ⁻⁵ or less, which is not a problem in practical use.

Fourth Example Next, a fourth example will be described. In the evaluation of this fourth example, two types of samples 4a and 4b were prepared, each having a refractive index profile of a dispersion-shifted optical fiber having the refractive index profile of the segment-core depressed cladding structure shown in Figure 3B, and having substantially the same Petermann-I mode field diameter MFD1 but different fiber diameters b.

As mentioned above, the magnitude relationships among the refractive index n1 of the inner core, the refractive index n2 of the intermediate core, the refractive index n3 of the outer core, the refractive index n4 of the inner cladding, and the refractive index n5 of the outer cladding are n1 > n2, n3 > n2, and n5 > n4.

Figure 11 is a table summarizing the specifications of two samples fabricated to illustrate the optical fiber according to this fourth embodiment. The fiber diameter b is approximately 125 μm for sample 4a and approximately 150 μm for sample 4b. Meanwhile, the Petermann-I mode field diameter MFD1 (11.98, 12.17 μm), effective area (69.7, 72.1 μm), chromatic dispersion (-2.1, -2.2 ps/nm/km), and cutoff wavelength (1.53, 1.51 μm) given by equations (1a) and (1b) above are approximately the same for both samples 4a and 4b.

Thus, the two types of samples 4a and 4b, which have different fiber diameters b, were fabricated by drawing two types of preforms using core members of the same diameter and different outer diameter ratios between the core and cladding members. Furthermore, each of the two types of samples 4a and 4b was surrounded by a coating layer made of the same material and having an outer diameter of 250 μm. The mode field diameter MFD1, effective area, and chromatic dispersion were all measured at a wavelength of 1.55 μm.

The microbend loss of each of these two samples 4a and 4b was measured using the same method as in Example 1. As a result, the microbend loss of sample 4a, with a fiber diameter b of approximately 125μ, was 4.12 dB/km, while the microbend loss of sample 4b, with a fiber diameter b of approximately 150μ, was 0.74 dB/km, achieving the target microbend loss of approximately 1 dB/km or less that would prevent loss increase due to cabling.

12 shows the refractive index profile of a fifth embodiment of the optical fiber according to the present invention. This optical fiber is a dispersion-flattened fiber, with a core region 110 having an outer diameter a and consisting of an inner core with a refractive index n1 and an outer diameter of 3.75 μm, an outer core with a refractive index n2 (>n1) and an outer diameter of 8.25 μm, and an outer core with a refractive index n2 (>n1) and an outer diameter of 8.25 μm, which is disposed around the inner core. The cladding region 120 having an outer diameter b has a depressed cladding structure and consists of an inner cladding with a refractive index n3 (=n1) and an outer diameter of 15.0 μm, which is disposed around the outer core, and an outer cladding with a refractive index n4 (>n3, <n2) and an outer diameter b, which is disposed around the inner cladding.

12 shows the refractive index of each portion on the line L in FIG. 3A, where a portion 610 shows the refractive index in the core region 110 and a portion 620 shows the refractive index in the cladding region 120. Furthermore, in this fifth embodiment, the relative refractive index difference Δ ⁺ of the outer core (refractive index n2) with respect to the outer cladding (refractive index n4) is +0.63%, and the relative refractive index difference Δ ⁻ of the inner core (refractive index n1) and the inner cladding (refractive index n3) with respect to the outer cladding (refractive index n4) is −0.60%, which are given by the following equation (5):

Δ=(n _core ² -n _cld ² )/n _cld ² (5) In the above formula (5), n _core is the refractive index of the target glass region, _{and n cld is} the refractive index of the reference outer cladding. The refractive indices of the glass regions in formula (5) are in no particular order, and the relative refractive index of a region having a higher refractive index than the outer cladding is a positive value and represented by Δ ⁺ , while the relative refractive index of a region having a lower refractive index than the outer cladding is a negative value and represented by Δ ^- . In this specification, the relative refractive index difference is expressed as a percentage.

The samples prepared for evaluation of the fifth embodiment were of two types, with fiber diameters of 125 μm and 160 μm. Both samples were coated with a 250 μm outer diameter coating made of the same material. Apart from the fiber diameter, each sample had a chromatic dispersion value of 0.12 ps/nm/km at 1.55 μm, an effective cross-sectional area of 72 μm at ^1.55 μm, and a cutoff wavelength of 1.187 μm. The dispersion slopes were ^0.0096 ps/nm/km at 1530 nm, ^0.0120 ps/nm/km at 1550 nm, and ^0.0265 ps/nm/km at 1560 nm. The dispersion slope refers to the gradient of a graph showing chromatic dispersion values in a given wavelength band.

The microbend loss at a wavelength of 1.55 μm (1550 nm) was evaluated for each of these samples. The results showed that the loss was 1.1 dB/km for the sample with a fiber diameter of 125 μm, while it was 0.1 dB/km for the sample with a fiber diameter of 160 μm, confirming that the increase in loss due to cabling was sufficiently suppressed.

13 shows the refractive index profile of a sixth embodiment of an optical fiber according to the present invention. This optical fiber is a dispersion-compensating fiber, and its core region 110, with an outer diameter a, is composed of a single core with a refractive index n1 and an outer diameter a1. The cladding region 120, with an outer diameter b, has a depressed cladding structure and is composed of an inner cladding with a refractive index n2 (<n1) and an outer diameter b1, which is provided around the core, and an outer cladding with a refractive index n3 (>n2, <n1) and an outer diameter b, which is provided around the inner cladding.

13 shows the refractive index of each portion on the line L in FIG. 3A, where portion 710 shows the refractive index in the core region 110 and portion 720 shows the refractive index in the cladding region 120. Furthermore, in this sixth embodiment, the relative refractive index difference Δ ⁺ of the core (refractive index n1) with respect to the outer cladding (refractive index n3) and the relative refractive index difference Δ ⁻ of the inner cladding (refractive index n2) with respect to the outer cladding (refractive index n3) are each given by the above formula (5).

The samples prepared for evaluation of this sixth embodiment were optical fibers having the characteristics shown by point P in Fig. 14, with a chromatic dispersion value of -33 ^ps / ^nm /km at 1.55 µm (1550 nm), a dispersion slope of -0.10 ps/ ^nm2 /km, and a ratio Ra (=a1/b1) of the outer diameter a1 of the core region 110 to the outer diameter b1 of the inner cladding of 0.6, among the samples with Δ + = +0.9% and Δ - = -0.44% as shown in Fig. 14, and were of two types with fiber diameters of 125 µm and 160 µm, respectively. These samples also had a coating layer of the same material with an outer diameter of 250 µm on their outer peripheries.

The microbend loss at a wavelength of 1.55 μm (1550 nm) was evaluated for each of these samples. The results showed that the loss was 2.3 dB/km for the sample with a fiber diameter of 125 μm, while it was 0.3 dB/km for the sample with a fiber diameter of 160 μm, confirming that the increase in loss due to cabling was sufficiently suppressed.

Seventh Example In the seventh example, an optical fiber with a larger effective area was evaluated. The refractive index profile of the prepared sample was the same as that shown in FIG. 13, but the effective area was 122 μm ² (greater than 110 μm ² ). Two types of samples were prepared: fiber diameters of 125 μm and 160 μm. These samples also had a coating layer of the same material with an outer diameter of 250 μm around their outer peripheries. Each sample had, excluding the fiber diameter, a relative refractive index difference Δ ⁺ of the core (index of curvature n1) to the outer cladding (index of curvature n3) of +0.28%, a relative refractive index difference Δ - of the inner cladding (index of curvature n2) to the outer cladding (index of curvature n3) of -0.14%, a cutoff wavelength of 1.49 μm, an effective area at a wavelength ^of 1.55 μm of 122 μm ² , a chromatic dispersion at a wavelength of 1.55 μm of 22.1 ps/nm/km, and a chromatic dispersion at a wavelength of 1.55 μm of 2.0 ps/nm/km. The dispersion slope at 55 μm is 0.062 ps/nm ² /km.

The microbend loss at a wavelength of 1.55 μm (1550 nm) was evaluated for each of these samples. The results showed that the loss was 1.3 dB/km for the sample with a fiber diameter of 125 μm, while it was 0.2 dB/km for the sample with a fiber diameter of 160 μm, confirming that the increase in loss due to cabling was sufficiently suppressed.

As described above, the optical fiber according to the present invention has a fiber diameter of 140 μm or more and 200 μm or less, so that an increase in microbend loss is effectively suppressed and the probability of breakage due to bending strain can be reduced to a level that does not pose a practical problem. Furthermore, if the absolute value of chromatic dispersion at a wavelength of 1.55 μm is 5 ps/nm/km or less and the Petrmann-I mode field diameter is 11 μm or more, the occurrence of nonlinear optical phenomena is suppressed, making the optical fiber suitable for use as an optical transmission line in a WDM transmission system using the 1.55 μm wavelength band.

It should be noted that the present invention is not limited to the above-described embodiments and various modifications are possible. For example, the refractive index profile is not limited to a dual-core structure or a segmented-core structure and may be any structure. It may also be realized by a ring-core refractive index profile having a ring-shaped high-refractive-index ring core region surrounding a central low-refractive-index region.

INDUSTRIAL APPLICABILITY As described above, according to this invention, since the fiber diameter is 140 μm or greater, the optical fiber has high rigidity and suppresses increases in microbend loss. Meanwhile, since the fiber diameter is 200 μm or less, the probability of optical fiber breakage due to bending strain is practically negligible. Furthermore, if the absolute value of chromatic dispersion at a wavelength of 1.55 μm is set to 5 ps/nm/km or less, this wavelength band is suitable for WDM transmission using this wavelength band. Furthermore, according to this invention, since the mode field diameter is 11 μm or greater, the optical energy density per unit cross-sectional area is low, effectively suppressing the occurrence of nonlinear optical phenomena. Therefore, the optical fiber according to this invention is suitable as an optical transmission line in a WDM optical transmission system.

───────────────────────────────────────────────────── (Note) This publication is based on the publication published internationally by the International Bureau of Patents (WIPO). The effect of the international publication of the Japanese patent application (Japanese utility model registration application) related to this publication arises pursuant to Article 184-10, Paragraph 1 of the Patent Act (Article 48-13, Paragraph 2 of the Utility Model Act) and is unrelated to this publication.

Claims (6)

[Claims] 1. An optical fiber having a core region extending along a predetermined axis and a cladding region surrounding the core region, each composed of at least three glass layers with different refractive indices, which is substantially guaranteed to be single-mode for the wavelength of light used, and which has a fiber diameter of 140 μm or more and 200 μm or less. 2. An optical fiber having a dispersion slope of 0.02 ps/nm ² /km or less at at least one wavelength within the wavelength band used, an effective area of 50 μm ² or more, and a fiber diameter of 140 μm or more and 200 μm or less. 3. An optical fiber having a chromatic dispersion of -18 ps/nm/km or less at at least one wavelength within the wavelength band used, an effective area of 17 ^μm2 or more, and a fiber diameter of 140 μm or more and 200 μm or less. 4. An optical fiber having an effective area of 110 ^μm2 or more at at least one wavelength within the wavelength band used, and a fiber diameter of 140 μm or more and 200 μm or less. 5. The optical fiber according to any one of claims 1 to 4, characterized in that it has a chromatic dispersion having an absolute value of 5 ps/nm/km or less at a wavelength of 1550 nm and a Petermann-I mode field diameter of 11 µm or more. 6. An optical fiber in which a core region extending along a predetermined axis and a cladding region provided on the outer periphery of said core region are composed of at least three or more glass regions each having a different refractive index, and which is substantially guaranteed to be single-mode for light in a wavelength band used, said optical fiber having an effective area of 17 ^μm2 or more, a chromatic dispersion value of -83 ps/nm/km or more, and a fiber diameter of 140 μm or more and 200 μm or less, at least one wavelength in said wavelength band used.