JP2005202440A - Optical fiber and optical communication system containing optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber having a structure suitable for a long-distance optical communication. <P>SOLUTION: The optical fiber (200) comprises a core area (210) and a clad area (220) disposed so as to surround an outer circumference of the core area (210). The clad area (220) is composed of an inner clad (221) that has a refractive index lower than the core area (210) and an outer clad (222) that has a refractive index higher than the inner clad (221). The optical fiber (200) has an effective cross section of 110 μm<SP>2</SP>or more at a wavelength of 1.55 μm, and, because of this enlarged effective cross section, suppresses an occurrence of a nonlinear optical phenomenon effectively even when a power of incident light is increased, allowing a longer-distance optical communication. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical fiber applicable as an optical transmission line of an optical communication system in a 1.55 μm wavelength band.

  Conventionally, a single mode optical fiber has been used as an optical transmission line in optical communication. This single mode optical fiber has a zero dispersion wavelength near the wavelength of 1.3 μm, a positive dispersion slope in the 1.55 μm wavelength band, and a dispersion of about 18 ps / nm / km at the wavelength of 1.55 μm.

The single-mode optical fiber having the optical characteristics as described above is defined in the ITU-T G652 standard and G654 standard, and has a simple refractive index profile including a core and a clad. Further, since silica glass, which is the main component of the optical fiber, has low absorption in the 1.55 μm wavelength band (1500 nm to 1600 nm), the 1.55 μm wavelength band is applied as the signal wavelength band. On the other hand, as described above, the single mode optical fiber has positive dispersion in the 1.55 μm wavelength band. Thus, in order to compensate for this positive dispersion, an example in which an optical communication system is constructed by combining a dispersion compensating optical fiber that generates negative dispersion having a large absolute value in the 1.55 μm wavelength band and the single mode optical fiber. Is introduced in Non-Patent Document 1, for example.
M. Murakami, et al., ECOC'98, pp.313-314 (1998)

As a result of examining the conventional optical fiber, the inventors have found the following problems. That is, the effective area of the single mode optical fiber defined in the G652 standard or the G654 standard is larger than that of the dispersion compensating optical fiber or the like, and is about 80 μm ² . Therefore, the single mode optical fiber is relatively effective for reducing the nonlinear optical phenomenon.

  By the way, in order to increase the repeat interval in the optical communication system, it is necessary to increase the power of the incident optical signal. At this time, by further increasing the effective cross-sectional area of the optical fiber used for the optical transmission line between the relay stations, non-linear optical phenomena are sufficiently generated even when a high-power optical signal propagates through the optical fiber. Must be suppressed.

  However, the optical fibers defined in the G652 standard and the G654 standard cannot sufficiently suppress the occurrence of nonlinear optical phenomena. Therefore, it has been difficult to perform further long-distance optical communication using a conventional optical fiber.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide an optical fiber having a structure suitable for long-distance optical communication and an optical communication system including the optical fiber.

  An optical fiber according to the present invention is provided between an optical transmitter that outputs an optical signal and an optical receiver that receives the optical signal, between an optical transmitter and a relay station including an optical amplifier, between relay stations, and between It is an optical waveguide mainly composed of silica glass, which is disposed at least between the station and the optical receiver. Examples of the optical fiber include an optical fiber having a matched refractive index profile obtained by forming a clad region that surrounds the outer periphery of the core region as a single layer, an inner clad at least in contact with the core region, and the inner clad. Any optical fiber having a depressed clad type refractive index profile obtained by configuring with an outer clad having a higher refractive index can be applied.

This optical fiber has a refractive index profile of 110 μm ² or more as a characteristic at a wavelength of 1.55 μm (1550 nm), regardless of which of the above-mentioned matched refractive index profile and depressed clad refractive index profile. It has an effective cross-sectional area, a dispersion of 18 to 23 ps / nm / km, and a dispersion slope of 0.058 to 0.066 ps / nm ² / km.

In particular, in an optical fiber having a matched refractive index profile, the relative refractive index difference of the core region with respect to the cladding region is preferably + 0.15% to + 0.30%. In this case, an optical fiber having a cut-off wavelength of 1.3 μm to 1.75 μm and an effective area of 110 μm ² or more at a wavelength of 1.55 μm is obtained. The optical fiber preferably has a transmission loss of 0.30 dB / km or less at a wavelength of 1.38 μm (1380 nm).

On the other hand, an optical fiber having a depressed cladding type refractive index profile includes a core region, an inner cladding region provided on the outer periphery of the core region, and an outer cladding provided so as to surround the outer periphery of the inner cladding. And an effective area of 110 μm ² or more at a wavelength of 1.55 μm. However, the inner cladding and the outer cladding constitute a cladding region that surrounds the outer periphery of the core region, the inner cladding has a lower refractive index than the core region, and the outer cladding is higher than the inner cladding. Has a refractive index.

In any optical fiber having any of the refractive index profiles described above, the effective area is preferably 120 μm ² or more, more preferably 150 μm ² or more at a wavelength of 1.55 μm. Such an increase in effective cross-sectional area effectively suppresses the occurrence of nonlinear optical phenomena even when the power of the incident optical signal (1.55 μm wavelength band) is increased. to enable.

The cut-off wavelength (LP ₁₁ mode cut-off wavelength measured with a 2 m long optical fiber wrapped around a mandrel with a radius of 140 mm once) is preferably 1.3 μm to 1.75 μm. . In this case, a single mode is ensured in the 1.55 μm wavelength band, and an increase in bending loss is suppressed (advantageous for cable formation). The transmission loss at a wavelength of 1.55 μm is preferably at least 0.180 dB / km or less in order to realize long-distance optical communication.

  The outer diameter of the core region is preferably 11.5 μm to 23.0 μm so as to satisfy the above-described condition regarding the cutoff wavelength. Further, by setting the outer diameter (fiber diameter) of the cladding region to 130 μm to 200 μm, the microbend loss can be reduced and the fracture probability can be reduced.

Furthermore, in an optical fiber having a depressed cladding type refractive index profile, the ratio 2b / 2a of the outer diameter 2b of the inner cladding to the outer diameter 2a of the core region is preferably 1.1-7. The cut-off wavelength can be shortened without increasing the bending loss, and the effective area can be increased with a single mode guaranteed in the 1.55 μm wavelength band even if the outer diameter of the core region is increased. Because you can. The relative refractive index difference of the core region with respect to the outer cladding and the relative refractive index difference of the inner cladding are + 0.15% to + 0.30% and −0.15% to −0.01%, respectively. Is preferred. Under such conditions, an optical fiber having a cutoff wavelength of 1.3 μm to 1.75 μm and an effective area of 110 μm ² or more at a wavelength of 1.55 μm is obtained.

  In the optical fiber according to the present invention, the core region is made of silica glass to which an additive is not intentionally added (hereinafter referred to as pure silica glass), and a cladding region (an optical fiber having a depressed cladding type refractive index profile). In this case, the inner clad and the outer clad) are preferably made of silica glass to which fluorine is added. In such a configuration, since an additive such as Ge element is not intentionally added to the core region, transmission loss is suppressed by about 0.02 dB / km as compared with an optical fiber in which Ge element is added to the core region. can do. However, in the configuration in which only the refractive index of the cladding region is controlled based on the core region in this way, an additive to the cladding region is used to increase the difference in refractive index between the core region and the cladding region. The amount of must be increased. Therefore, an increase in transmission loss when added to the core region, unlike Ge, Al, and P, increases the refractive index of the core region with respect to pure silica glass by adding less chlorine. Even if the amount of fluorine added to the cladding region is reduced, a sufficient difference in refractive index between the core region and the cladding region can be produced. That is, it is possible to reduce the amount of fluorine added that causes an increase in transmission loss without affecting the optical characteristics.

The refractive index profile of the optical fiber according to the present invention may be a shape that gradually changes from the center of the core region toward the outer periphery, particularly in the core region. Specifically, in the cross section of the core region, the relative refractive index difference Δn _{a of} a portion that is separated from the reference region of the cladding region by a distance r (0 ≦ r ≦ a) in the radial direction from the center of the core region. (R) is
Δn _a (r) = Δn _a (0) · (1− (r / a) ^α ) (1)
Δn _a (0): relative refraction at the center of the core region with respect to the reference region of the cladding region
The refractive index profile shape in the radial direction in the core region is controlled so as to be approximated by an expression of a real number having a rate difference α of 1 to 10. As described above, the refractive index profile in which the portion corresponding to the core region is expressed by the approximate expression (1) has a dome shape in which the portion corresponding to the core region is raised at the center portion rather than the peripheral portion.

Further, in the cross section of the core region, the relative refractive index difference Δn _a (r) at _a portion that is separated from the reference region of the cladding region by a distance r (0 ≦ r ≦ a) in the radial direction from the center of the core region. But,
Δn _a (r) = Δn _a (a) · (1−γ · (1−r / a) ^β ) (2)
Δn _a (a): equivalent to the outer periphery of the core region with respect to the reference region of the cladding region
The refractive index profile shape in the radial direction in the core region may be controlled so as to be approximated by an expression of a relative refractive index difference β: 1 to 10 and a real number γ: a positive real number. Thus, the refractive index profile in which the portion corresponding to the core region is represented by the approximate expression (2) has a shape in which the portion corresponding to the core region is raised at the peripheral portion rather than the central portion. However, the approximate expression (1), the relative refractive index difference [Delta] n _a in the core region in the case of either (2) is set based on the lower portion of the most refractive index. Therefore, in the case of an optical fiber having a matched refractive index profile, the reference region of the cladding region corresponds to a single cladding region itself, and in the case of an optical fiber having a depressed cladding type refractive index profile, the reference region of the cladding region. The region corresponds to the inner cladding.

  The optical fiber having the above-described structure is an optical communication that propagates an optical signal having a wavelength band of 1.35 to 1.52 μm in addition to a 1.55 μm wavelength band of 1530 to 1565 nm and a 1.58 μm wavelength band of 1570 to 1620 nm. Applicable to the system. Further, such an optical communication system may include an optical amplifier that is disposed upstream of the optical fiber and amplifies optical signals having a plurality of wavelengths. Such an optical amplifier may include an erbium-doped fiber amplifier provided with an amplification optical fiber doped with erbium, or may include a Raman amplifier.

The effective area A _eff is given by the following expression (3) as shown in Japanese Patent Laid-Open No. 8-248251 (EP 0724 171 A2).

  Here, E is the electric field associated with the propagating light, and r is the radial distance from the core center. In this specification, the dispersion slope is given by the slope of a graph showing the wavelength dependence of dispersion.

  According to this invention, since it has a large effective area at a wavelength of 1.55 μm, the occurrence of a nonlinear optical phenomenon is effectively suppressed even when a high-power optical signal (1.55 μm wavelength band) is transmitted.

  Embodiments of an optical fiber and an optical communication system including the optical fiber according to the present invention will be described below with reference to FIGS. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a diagram showing a schematic configuration of an optical communication system to which an optical fiber according to the present invention can be applied. The optical communication system according to the present invention has a configuration in which an optical fiber 30 is disposed between relay stations 10 and 20, as shown in FIG. However, each of the relay stations 10 and 20 may include optical amplifiers 11 and 21 in order to enable long-distance transmission of optical signals in the wavelength bands 1350 to 1520 nm, 1520 to 1565 nm, or 1570 to 1620 nm. Such optical amplifiers 11 and 21 may include erbium-doped fiber amplifiers including amplification optical fibers 12 and 22 doped with erbium, or may include Raman amplifiers. Further, at both ends of the optical fiber 30, an optical transmitter that transmits an optical signal and an optical receiver that receives the optical signal may be arranged instead of the relay stations 10 and 20. Therefore, in the optical communication system, the optical fiber 30 is at least between the optical transmitter and the optical receiver, between the optical transmitter and the relay station, between each relay station, and between the relay station and the optical receiver. Placed in either.

  The optical fiber 30 may have a structure in which a plurality of components 31 to 33 are fusion-connected as shown in FIG. In such an aspect, a configuration in which a plurality of optical fibers according to the present invention are prepared as a plurality of components 31 to 33, an optical fiber according to the present invention, and other optical fibers such as a dispersion compensating optical fiber and a dispersion shifted optical fiber. A combination of and is included.

  Next, each embodiment of the optical fiber according to the present invention will be described.

(First embodiment)
FIG. 2 is a diagram illustrating a cross-sectional structure and a refractive index profile 150 of the optical fiber 100 according to the first embodiment. The optical fiber 100 according to the first embodiment is applicable to the optical communication system shown in FIG. As shown in FIG. 2A, the optical fiber 100 according to the first embodiment includes a core region 110 having a refractive index n1 and an outer diameter 2a extending along a predetermined axis, and an outer periphery of the core region 110. And a cladding region 120 having a refractive index n2 lower than that of the core region 110.

Also, the refractive index profile 150 shown in FIG. 2B shows the refractive index in each part of the optical fiber 100 on the line L1 (line perpendicular to the predetermined axis) in FIG. Specifically, the region 151 indicates the refractive index of each part on the line L1 in the core region 110, and the region 152 indicates the refractive index of each part on the line L1 in the cladding region 120. In the first embodiment, the relative refractive index difference Δn ⁺ of the core region 110 with respect to the cladding region 120 (reference region) is defined as follows.
Δn ⁺ = (n1-n2) / n2

In this specification, the relative refractive index difference Δn ⁺ is expressed as a percentage.

Optical fiber 100 according to the first embodiment having the above structure has, as characteristics at a wavelength of 1.55 .mu.m, 110 [mu] m ² or more, preferably 120 [mu] m ² or more, more preferably a 150 [mu] m ² or more effective cross-sectional area, 18 It has a dispersion of 23 ps / nm / km and a dispersion slope of 0.058 to 0.066 ps / nm ² / km. The effective area at a wavelength of 1600 nm is 130 μm ² or more. In addition, by setting the relative refractive index difference Δn ⁺ of the core region 110 with respect to the cladding region 120 to + 0.15% to + 0.30%, the optical fiber 100 according to the first embodiment has a thickness of 1.3 μm to 1.75 μm cut-off wavelength (LP ₁₁ mode cut-off wavelength measured with a 2 m long optical fiber loosely wrapped around a mandrel with a radius of 140 mm) and an effective value of 110 μm ² or more at a wavelength of 1.55 μm It is possible to have a cross-sectional area A _eff . The optical fiber 100 according to the first embodiment preferably has a transmission loss of 0.30 dB / km or less at least at a wavelength of 1.38 μm in order to reduce the number of relay stations installed.

  FIG. 3 is a table showing structural parameters of samples 1 to 5 and optical characteristics at a wavelength of 1.55 μm of the optical fiber 100 according to the first embodiment having the above-described structure.

As can be seen from the table of FIG. 3, in the optical fiber according to Sample 1, the outer diameter of the core region 110 is set to 12.9 μm, and the relative refractive index difference Δn ⁺ of the core region 110 with respect to the cladding region 120 is set to 0.30%. Has been. The cut-off wavelength λc of the optical fiber according to Sample 1 is 1.39 μm. Further, at a wavelength of 1.55 μm, the optical fiber according to Sample 1 has an effective area A _eff of 110 μm ² , a dispersion of 19.8 ps / nm ² / km, a dispersion slope of 0.0610 ps / nm ² / km, It has a bending loss at a diameter of 20 mm of 0 dB / m and a transmission loss of 0.169 dB / km.

In the optical fiber according to Sample 2, the outer diameter of the core region 110 is set to 13.6 μm, and the relative refractive index difference Δn ⁺ of the core region 110 with respect to the cladding region 120 is set to 0.30%. The cut-off wavelength λc of the optical fiber according to Sample 2 is 1.47 μm. At a wavelength of 1.55 μm, the optical fiber according to Sample 2 has an effective area A _eff of 115 μm ² , a dispersion of 20.3 ps / nm / km, a dispersion slope of 0.0612 ps / nm ² / km, and 1. It has a bending loss at a diameter of 20 mm which is 4 dB / m and a transmission loss of 0.171 dB / km.

In the optical fiber according to Sample 3, the outer diameter of the core region 110 is set to 14.2 μm, and the relative refractive index difference Δn ⁺ of the core region 110 with respect to the cladding region 120 is set to 0.29%. The cut-off wavelength λc of the optical fiber according to Sample 3 is 1.51 μm. At a wavelength of 1.55 μm, the optical fiber according to Sample 3 has an effective area A _eff of 123 μm ² , a dispersion of 20.5 ps / nm / km, a dispersion slope of 0.0616 ps / nm ² / km, and 2. It has a bending loss at a diameter of 20 mm, which is 8 dB / m, and a transmission loss of 0.172 dB / km.

In the optical fiber according to sample 4, the outer diameter of the core region 110 is set to 14.8 μm, and the relative refractive index difference Δn ⁺ of the core region 110 with respect to the cladding region 120 is set to 0.28%. The cut-off wavelength λc of the optical fiber according to Sample 4 is 1.50 μm. In addition, at the wavelength of 1.55 μm, the optical fiber according to Sample 4 has an effective area A _eff of 130 μm ² , a dispersion of 20.7 ps / nm / km, a dispersion slope of 0.0618 ps / nm ² / km, It has a bending loss at a diameter of 20 mm which is 6 dB / m and a transmission loss of 0.171 dB / km.

Further, in the optical fiber according to Sample 5, the outer diameter of the core region 110 is set to 16.0 μm, and the relative refractive index difference Δn ⁺ of the core region 110 with respect to the cladding region 120 is set to 0.23%. The cut-off wavelength λc of the optical fiber according to Sample 5 is 1.47 μm. Further, at a wavelength of 1.55 μm, the optical fiber according to Sample 5 has an effective area A _eff of 155 μm ² , a dispersion of 20.8 ps / nm / km, a dispersion slope of 0.0622 ps / nm ² / km, and 6. It has a bending loss at a diameter of 20 mm, which is 2 dB / m, and a transmission loss of 0.172 dB / km.

  The optical fiber 100 according to the first embodiment described above is an optical fiber having a matched refractive index profile composed of only a single cladding region 120. The cladding region 120 includes an inner cladding and the inner cladding. An optical fiber having a depressed clad type refractive index profile constituted by an outer clad having a higher refractive index than the clad may be used.

(Second embodiment)
A second embodiment of the optical fiber according to the present invention is an optical fiber having a depressed cladding type refractive index profile, and FIG. 4 shows a cross-sectional structure and a refractive index profile 250 of the optical fiber 200 according to the second embodiment. FIG. The optical fiber 200 according to the second embodiment is also applicable to the optical communication system shown in FIG.

  As shown in FIG. 4A, the optical fiber 200 according to the second embodiment includes a core region 210 that extends along a predetermined axis and has a refractive index n1 and an outer diameter 2a, and an outer periphery of the core region 210. And a clad region 220 provided so as to surround the substrate. The clad region 220 is provided on the outer periphery of the core region 210 and has an inner clad 221 having a lower refractive index n2 and an outer diameter 2b than the core region 210, and is provided on the outer periphery of the inner clad 221 and from the inner clad 221. And an outer cladding 222 having a higher refractive index n3, forming a depressed cladding type refractive index profile.

Note that the refractive index profile 250 shown in FIG. 4B indicates the refractive index in each part of the optical fiber 200 on the line L2 shown in FIG. Specifically, the region 251 is the refractive index of each part on the line L2 in the core region 210, the region 252 is the refractive index of each part on the line L2 in the inner cladding 221, and the region 253 is the line in the outer cladding 222. The refractive index of each part on L2 is shown. Further, in this second embodiment, the outer cladding 222 relative refractive index of the relative refractive index difference [Delta] n ⁺ and the inner cladding 221 of the core region 210 difference with respect to (reference region) [Delta] n ^-, respectively are defined as follows.
Δn ⁺ = (n1-n3) / n3
Δn ⁻ = (n2−n3) / n3

However, the relative refractive index differences Δn ⁺ and Δn ⁻ are both expressed as percentages, and the parameters in the formula are in no particular order. Therefore, it means that the refractive index of the portion where the relative refractive index difference is a negative value is a portion lower than the cladding region which is the reference region.

  The optical fiber 200 having such a refractive index profile 250 can be realized by adding Ge element to the core region 210 and adding F element to the inner cladding 221 based on silica glass. Further, it can be realized by using pure silica glass for the core region 210 and silica in which F element is added to each of the inner cladding 221 and the outer cladding 222. In the latter case, since an additive such as Ge element is not added to the core region 210, the transmission loss is reduced by about 0.02 dB / km as compared with the optical fiber in which the Ge element is added to the core region 210. Therefore, for example, when the transmission path length between the relay stations is 50 km, the power of the optical signal reaching one relay station is increased by about 1 dB, so that the transmission quality of the entire optical communication system is improved. Further, since the F element is added to the outer cladding 222, the hydrogen resistance and radiation resistance are also improved.

The optical fiber 200 according to the second embodiment also has an effective area of 110 μm ² or more, preferably 120 μm ² or more, more preferably 150 μm ² or more at a wavelength of 1.55 μm. The effective area at a wavelength of 1600 nm is 130 μm ² or more. Therefore, the optical fiber 200 according to the second embodiment has an effective area of about 2 to 3 times that of the optical fiber defined in the G652 standard or the G654 standard. It can be suppressed by about 2 dB to 3 dB. As a result, the transmission quality of the entire optical communication system is improved. The optical fiber 200 according to the second embodiment is more preferable if the transmission loss at a wavelength of 1.55 μm is 0.180 dB / km or less.

FIG. 5 is a graph showing the relationship between the fiber length and the effective cutoff wavelength. The optical fiber prepared for the measurement of this graph has an effective area of 120 μm ² , a dispersion of +21.8 ps / nm / km, and a dispersion slope of +0.063 ps / nm ² / km at a wavelength of 1.55 μm. And a transmission loss of 0.170 dB / km. The effective cut-off wavelength is the cut-off wavelength of the LP ₁₁ mode in a state where an optical fiber having a length indicated on the horizontal axis is loosely wound once with a radius of 140 mm. As can be seen from this graph, if the cutoff wavelength at a fiber length of 2 m is 1.75 μm or less, the optical fiber becomes single mode when the transmission distance exceeds 1 km in 1.55 μm wavelength optical communication. Become. Thus, in order to satisfy the single mode condition at the wavelength of 1.55 μm, the cutoff wavelength at the fiber length of 2 m can be allowed to 1.75 μm in the case of an optical fiber having a length of 1 km or more. On the other hand, when the cutoff wavelength is short, the bending loss of the optical fiber in the 1.55 μm wavelength band increases. Therefore, an optical fiber having a cutoff wavelength of 1.30 μm to 1.75 μm (preferably 1.30 μm to 1.60 μm) is suitable for an optical transmission line for long-distance optical communication such as a submarine cable.

FIG. 6 is a graph for explaining a preferable range of each parameter in the optical fiber 200 according to the second embodiment. In addition, the horizontal axis of the graph of FIG. 6 is a cut-off wavelength (μm), and the vertical axis is an effective area A _eff (μm ² ). In the optical fiber prepared for this measurement, the ratio (2b / 2a) of the outer diameter 2b of the inner cladding 221 to the outer diameter 2a of the core region 210 is set to 4.0. Further, the difference (Δn ⁺ −Δn ⁻ ) between the relative refractive index difference Δn ⁺ of the core region 210 with respect to the outer cladding 222 and the relative refractive index difference Δn ⁻ of the inner cladding 221 is set to 0.3%.

In FIG. 6, each sample in which Δn ⁺ is + 0.30%, Δn ⁻ is −0.00%, and the outer diameter 2a of the core region 210 is 10.0 μm, 11.25 μm, and 12.5 μm. effective area _{a eff} and cutoff graph showing the relationship between the wavelength λc G100, Δn ⁺ is + 0.25%, Δn ^- is the outer diameter 2a of the core region 210 when a -0.05% 12.5 .mu.m, 13 Graph G200 showing the relationship between the effective area A _eff and the cutoff wavelength λc of each sample of .75 μm and 15.0 μm, Δn ⁺ is + 0.20%, Δn ⁻ is −0.10%, and the core region 210 outer diameter 2a is 13.75μm, 15.0μm, 16.25μm, 17.5μm and a graph showing the relationship between effective area _{a eff} and the cut-off wavelength λc of each sample is 18.75μm G300, further, delta ⁺ Is + 0.15%, [Delta] n ^- is the outer diameter 2a is 18.5μm in and the core region 210 is -0.15%, 20.0 .mu.m, each sample of the effective area _{A eff} which is 21.5μm and 23.0μm A graph G400 showing the relationship between and the cutoff wavelength λc is shown.

Further, in FIG. 6, preferred ranges of parameters in the optical fiber 200 according to the second embodiment are shown by hatching. For the above reasons, the preferable range of the cutoff wavelength is set to 1.3 μm to 1.75 μm, and the lower limit value of the effective area is set to 110 μm ² . Note that the upper limit of the effective area, the relative refractive index difference of the relative refractive index difference [Delta] n ⁺ and the inner cladding 221 of the core region 210 [Delta] n with respect to the outer cladding 222 ^- fundamental mode light in the case each of the absolute values are equal to each other This is limited by not propagating through the optical fiber.

As can be seen from the graph of FIG. 6, in order to obtain a preferable range indicated by hatching, the relative refractive index difference Δn ⁺ of the core region 210 with respect to the outer cladding 222 is in the range of + 0.15% to + 0.30%. The relative refractive index difference Δn ⁻ of the inner cladding 221 with respect to the outer cladding 222 is preferably in the range of −0.15% to −0.01%. The outer diameter 2a of the core region 210 is preferably in the range of 11.5 μm (more preferably 12.5 μm) to 23.0 μm.

Next, the relationship between the ratio 2b / 2a of the outer diameter 2b of the inner cladding 221 to the outer diameter 2a of the core region 210 and the cutoff wavelength will be described. FIG. 7 shows cutoff wavelengths of an optical fiber (having a depressed clad type refractive index profile) provided with the outer clad 222 and an optical fiber (having a matched type refractive index profile) not provided with the outer clad 222. It is the graph which plotted the difference of 2 with respect to ratio 2b / 2a. The optical fiber having a depressed cladding type refractive index profile prepared for measurement in this graph has an outer diameter 2a of the core region of 13.0 μm, and a relative refractive index difference Δn ⁺ of the core region with respect to the outer cladding is +0.25. %, The relative refractive index difference Δn ⁻ of the inner cladding with respect to the outer cladding is set to −0.10%. On the other hand, an optical fiber having a matched type refractive index profile, inner cladding relative refractive index difference [Delta] n ⁺ is + 0.35% in the core region with respect to (cladding region) ^- is set to ([Delta] n is 0). As can be seen from the graph of FIG. 7, the cutoff wavelength of the optical fiber having the depressed clad refractive index profile is shortened compared to the cutoff wavelength (μm) of the optical fiber having the matched refractive index profile. Is sufficiently obtained if the ratio 2b / 2a is 7 or less, and generally tends to increase as the ratio 2b / 2a decreases, and the ratio 2a / 2b is a value in the vicinity of 1.5 to 2.0. It becomes the biggest when there is. On the other hand, when the ratio 2b / 2a is 1.1 or less, the bending loss increases, and the transmission quality of the entire optical communication system deteriorates. Therefore, if the ratio 2b / 2a of the outer diameter 2b of the inner cladding to the outer diameter 2a of the core region is 1.1 to 7, the cut-off wavelength can be shortened without deteriorating the bending characteristics. Even if the outer diameter is increased, it is single mode in the 1.55 μm wavelength band and the effective area can be increased.

Next, seven samples (samples 6 to 12) of the optical fiber 200 according to the second embodiment will be described. FIG. 8 shows the relative refraction of the core region 210 with respect to the outer diameter 2a of the core region 210, the outer diameter 2b of the inner cladding 221 and the outer cladding 222 for each of the samples 6 to 12 of the optical fiber 200 according to the second embodiment. Ratio difference Δn ⁺ , relative refractive index difference Δn ⁻ of inner cladding 221 with respect to outer cladding, cutoff wavelength, effective area A _{eff at} wavelength 1.55 μm, dispersion (ps / nm / km), dispersion slope (ps / nm ²⁾ / Km), a table summarizing bending loss at a wavelength of 1.55 μm when bent at a diameter of 20 mm, and transmission loss at a wavelength of 1.55 μm. In all of the samples 6 to 12, the outer diameter of the outer cladding 222 is set to 125 μm.

In the optical fiber according to Sample 6, the outer diameter 2a of the core region 210 is 14.8 μm, the outer diameter 2b of the inner cladding 221 is 59.0 μm, and the relative refractive index difference Δn ⁺ of the core region 210 with respect to the outer cladding 222 is +0.23. %, The relative refractive index difference Δn ⁻ of the inner cladding 221 with respect to the outer cladding 222 is set to −0.07%. Further, the optical fiber according to Sample 6 has a cutoff wavelength of 1.45 μm, and various characteristics of a wavelength of 1.55 μm, an effective area A _eff of 153 μm ² , a dispersion of 21.8 ps / nm / km, It has a dispersion slope of 0.063 ps / nm ² / km, a bending loss at a diameter of 20 mm of 0.2 dB / m, and a transmission loss of 0.170 dB / km.

In the optical fiber according to Sample 7, the outer diameter 2a of the core region 210 is 16.25 μm, the outer diameter 2b of the inner cladding 221 is 65.0 μm, and the relative refractive index difference Δn ⁺ of the core region 210 with respect to the outer cladding 222 is +0.20. %, The relative refractive index difference Δn ⁻ of the inner cladding 221 with respect to the outer cladding 222 is set to −0.10%. Further, the optical fiber according to Sample 7 has a cutoff wavelength of 1.42 μm, and various characteristics of a wavelength of 1.55 μm, an effective area A _{eff of} 177 μm ² , a dispersion of 21.1 ps / nm / km, It has a dispersion slope of 0.063 ps / nm ² / km, a bending loss at a diameter of 20 mm of 0.1 dB / m, and a transmission loss of 0.173 dB / km.

In the optical fiber according to Sample 8, the outer diameter 2a of the core region 210 is 15.3 μm, the outer diameter 2b of the inner cladding 221 is 61.0 μm, and the relative refractive index difference Δn ⁺ of the core region 210 with respect to the outer cladding 222 is +0.23. %, The relative refractive index difference Δn ⁻ of the inner cladding 221 with respect to the outer cladding 222 is set to −0.12%. Further, the optical fiber according to Sample 8 has a cutoff wavelength of 1.46 μm, and various characteristics of a wavelength of 1.55 μm, an effective area A _eff of 154 μm ² , a dispersion of 22.2 ps / nm / km, It has a dispersion slope of 0.063 ps / nm ² / km, a bending loss at a diameter of 20 mm of 0.03 dB / m, and a transmission loss of 0.174 dB / km.

In the optical fiber according to Sample 9, the outer diameter 2a of the core region 210 is 13.8 μm, the outer diameter 2b of the inner cladding 221 is 66.0 μm, and the relative refractive index difference Δn ⁺ of the core region 210 with respect to the outer cladding 222 is +0.28. %, The relative refractive index difference Δn ⁻ of the inner cladding 221 with respect to the outer cladding 222 is set to −0.14%. Further, the optical fiber according to this sample 9 has a cutoff wavelength of 1.49 μm, and has various characteristics of a wavelength of 1.55 μm, an effective area A _eff of 122 μm ² , a dispersion of 22.1 ps / nm / km, It has a dispersion slope of 0.062 ps / nm ² / km, a bending loss at a diameter of 20 mm of 0.2 dB / m, and a transmission loss of 0.171 dB / km.

In the optical fiber according to the sample 10, the outer diameter 2a of the core region 210 is 12.4 μm, the outer diameter 2b of the inner cladding 221 is 55.0 μm, and the relative refractive index difference Δn ⁺ of the core region 210 with respect to the outer cladding 222 is +0.26. %, The relative refractive index difference Δn ⁻ of the inner cladding 221 with respect to the outer cladding 222 is set to −0.11%. Further, the optical fiber according to the sample 10 has a cutoff wavelength of 1.58 μm, and various characteristics of a wavelength of 1.55 μm, an effective area A _{eff of} 110 μm ² , a dispersion of 21.3 ps / nm / km, It has a dispersion slope of 0.061 ps / nm ² / km, a bending loss at a diameter of 20 mm of 0.02 dB / m, and a transmission loss of 0.169 dB / km.

In the optical fiber according to the sample 11, the outer diameter 2a of the core region 210 is 12.8 μm, the outer diameter 2b of the inner cladding 221 is 45.0 μm, and the relative refractive index difference Δn ⁺ of the core region 210 with respect to the outer cladding 222 is +0.25. %, The relative refractive index difference Δn ⁻ of the inner cladding 221 with respect to the outer cladding 222 is set to −0.09%. Further, the optical fiber according to this sample 11 has a cutoff wavelength of 1.45 μm, and various characteristics of a wavelength of 1.55 μm, an effective area A _eff of 119 μm ² , a dispersion of 21.3 ps / nm / km, It has a dispersion slope of 0.061 ps / nm ² / km, a bending loss at a diameter of 20 mm of 0.05 dB / m, and a transmission loss of 0.171 dB / km.

In the optical fiber according to the sample 12, the outer diameter 2a of the core region 210 is 12.0 μm, the outer diameter 2b of the inner cladding 221 is 48.0 μm, and the relative refractive index difference Δn ⁺ of the core region 210 with respect to the outer cladding 222 is +0.23. %, The relative refractive index difference Δn ⁻ of the inner cladding 221 with respect to the outer cladding 222 is set to −0.15%. Further, the optical fiber according to the sample 12 has a cutoff wavelength of 1.35 μm, and various characteristics of a wavelength of 1.55 μm, an effective area A _{eff of} 112 μm ² , a dispersion of 20.9 ps / nm / km, It has a dispersion slope of 0.060 ps / nm ² / km, a bending loss at a diameter of 20 mm of 0.10 dB / m, and a transmission loss of 0.173 dB / km.

As described above, each of the optical fibers according to the seven types of samples is guaranteed to have a single mode in the 1.55 μm wavelength band and has a sufficiently large effective area A _eff , and thus has a large power of 1.55 μm. Even if an optical signal in the wavelength band propagates through the optical fiber, the occurrence of a nonlinear optical phenomenon is effectively suppressed, and it is preferable as an optical transmission line in long-distance optical communication. Moreover, since these optical fibers all have a transmission loss of 0.180 dB / km or less at a wavelength of 1.55 μm, they are also suitable for optical transmission lines in long-distance optical communications such as submarine cables.

(First application example)
Next, a first application example of the optical fiber 200 according to the second embodiment will be described. FIG. 9 is a diagram showing a refractive index profile 350 of the first application example of the optical fiber according to the second embodiment. The optical fiber according to the first application example has the same structure as the cross-sectional structure shown in FIG. 4A, and the amount of fluorine added, which is an increase factor in transmission loss, without affecting the optical characteristics. It is characterized by having a structure that enables reduction.

That is, the optical fiber according to the first application example has a core region having an outer diameter 2a and a refractive index n1, and an outer diameter 2b, similar to the optical fiber 200 shown in FIG. An inner cladding having a lower refractive index n2 than the core region and an outer cladding having a higher refractive index n3 than the inner cladding are provided. Chlorine that increases the refractive index is added to the core region, and fluorine that decreases the refractive index is added to the inner cladding and the outer cladding. Further, the relative refractive index difference Δn1 of the core region, the relative refractive index difference Δn2 of the inner cladding, and the relative refractive index difference Δn3 of the outer cladding with respect to pure silica glass are respectively given by the following equations.
Δn1 = (n1−n0) / n0
Δn2 = (n2−n0) / n0
Δn3 = (n3−n0) / n0

  Here, the relative refractive index differences Δn1 to Δn3 are expressed as percentages, and n0 is the refractive index of pure silica glass. Also, the parameters in the above formulas are in no particular order, which means that the refractive index of the portion where the relative refractive index difference takes a negative value is lower than the refractive index n0 of pure silica glass.

  Further, in the refractive index profile 350 of FIG. 9, a region 351 is a refractive index of a portion corresponding to the core region 210 of FIG. 4A, and a region 352 is a refractive index of a portion corresponding to the inner cladding 221 of FIG. The region 353 indicates the refractive index of the portion corresponding to the outer cladding 222 of FIG.

  FIG. 10 is a table showing the structural parameters of the optical fiber samples 13 to 15 and the optical characteristics at a wavelength of 1.55 μm according to the first application example.

As can be seen from the table in FIG. 10, the optical fiber according to Sample 13 has an outer diameter 2a of the core region of 12.6 μm, an outer diameter 2b of the inner cladding of 43.8 μm, and the relative refraction of the core region with respect to pure silica glass. The rate difference Δn1 is 0.04%, the relative refractive index difference Δn2 of the inner cladding with respect to pure silica glass is −0.30%, and the relative refractive index difference Δn3 of the outer cladding with respect to the pure silica glass is −0.21%. Is set. Further, the optical fiber according to Sample 13 has an effective area A _eff of 115 μm ² , a cutoff wavelength of 1.42 μm, and various characteristics of a wavelength of 1.55 μm, a dispersion of 21.3 ps / nm / km, It has a dispersion slope of 0.061 ps / nm ² / km, a bending loss at a diameter of 20 mm of 0.3 dB / m, and a transmission loss of 0.169 dB / km.

The optical fiber according to Sample 14 has an outer diameter 2a of the core region of 12.9 μm, an outer diameter 2b of the inner cladding of 45.0 μm, a relative refractive index difference Δn1 of the core region with respect to pure silica glass is 0.08%, and pure silica. The relative refractive index difference Δn2 of the inner cladding with respect to the glass is set to −0.27%, and the relative refractive index difference Δn3 of the outer cladding with respect to the pure silica glass in the core region is set to −0.16%. Further, the optical fiber according to the sample 14 has an effective area A _eff of 117 μm ² , a cutoff wavelength of 1.45 μm, and various characteristics of a wavelength of 1.55 μm, a dispersion of 21.3 ps / nm / km, It has a dispersion slope of 0.061 ps / nm ² / km, a bending loss at a diameter of 20 mm of 0.2 dB / m, and a transmission loss of 0.167 dB / km.

The optical fiber according to Sample 15 has a core region outer diameter 2a of 12.6 μm, an inner cladding outer diameter 2b of 45.5 μm, a relative refractive index difference Δn1 of the core region with respect to pure silica glass is 0.11%, and pure silica. The relative refractive index difference Δn2 of the inner cladding with respect to glass is set to −0.23%, and the relative refractive index difference Δn3 of the outer cladding with respect to the pure silica glass in the core region is set to −0.14%. Further, the optical fiber according to Sample 15 has an effective area A _eff of 113 μm ² , a cutoff wavelength of 1.40 μm, and various characteristics of a wavelength of 1.55 μm, a dispersion of 21.2 ps / nm / km, It has a dispersion slope of 0.061 ps / nm ² / km, a bending loss at a diameter of 20 mm of 0.4 dB / m, and a transmission loss of 0.165 dB / km.

FIG. 11 shows a sample having a matched refractive index profile according to the first embodiment (having the structure shown in FIG. 2) and a depressed clad refractive index profile according to the second embodiment (shown in FIG. 4). _Is a graph plotting the relationship between the effective area A _eff (μm) and the microbend loss (dB / km). In this graph, white circle A indicates data of a sample having a matched refractive index profile, and black circle B indicates data of a sample having a depressed clad refractive index profile.

  From this graph, it can be seen that the optical fiber having a depressed cladding structure has a greater effect of reducing microbend loss. In the measurement for each plotted sample, an optical fiber was wound around a bobbin having a trunk diameter of 280 mm with JIS # 1000 sandpaper on the surface with a tension of 100 g, and the loss increase at a wavelength of 1.55 μm due to this was measured. The loss increase was defined as microbendros.

(Second application example)
The refractive index profile of the optical fiber according to the present invention may be a shape that changes from the center of the core region toward the outer periphery in the core region. FIG. 12 is a diagram showing a refractive index profile 450 of the second application example of the optical fiber 200 according to the second embodiment, and this refractive index profile 450 is in the core region as in the first application example. It has a shape in which the refractive index decreases from the center toward the periphery. The second application example also has a structure similar to the cross-sectional structure shown in FIG. That is, the optical fiber according to the second application example, like the optical fiber 200, has an outer diameter 2a and a core region having a maximum refractive index n1 in the central portion and an outer diameter 2b and lower than the core region. An inner cladding having a refractive index n2 and an outer cladding made of pure silica glass and having a higher refractive index n0 than the inner cladding. Germanium for increasing the refractive index is added to the core region, and fluorine for decreasing the refractive index is added to the inner cladding.

  In the refractive index profile 450 of the optical fiber according to the second application example shown in FIG. 12, the region 451 is the refractive index of the portion corresponding to the core region 210 shown in FIG. 4A, and the region 452 is FIG. The refractive index of a portion corresponding to the inner cladding 221 shown in a), and the region 453 shows the refractive index of a portion corresponding to the outer cladding 222 shown in FIG. In addition, AX1 shown in FIG. 12 is the central axis of the optical fiber according to the second application example.

Also, the relative refractive index difference Δn a _(0), and the relative refractive index difference [Delta] n _b of the outer cladding of the central portion of the core region relative to the inner cladding is given by the following equation, respectively.
Δn _a (0) = (n1−n2) / n2
Δn _b = (n0−n2) / n2

In addition, in the cross section of the core region, the relative refractive index difference Δn _a (r) of the portion of the inner cladding that is separated from the center of the core region by a distance r (0 ≦ r ≦ a) in the radial direction is:
Δn _a (r) = Δn _a (0) · (1− (r / a) ^α ) (4)
Δn _a (0): relative refractive index difference in the central portion of the core region with respect to the inner cladding α: 1 to 10
Is given by the following approximate expression.

  FIG. 13 is a table showing structural parameters and optical characteristics of optical fiber samples 16 to 24 according to the second application example having the above-described structure.

As can be seen from the table of FIG. 13, the optical fiber according to Sample 16 has an outer diameter 2a of the core region of 21.0 μm, an outer diameter 2b of the inner cladding of 50.2 μm, and a ratio of the core region center to the inner cladding. refractive index difference Δn _a (0) is + 0.40%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.08% with respect to the inner cladding. The parameter α in the approximate expression (4) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 1.0. The optical fiber according to this sample 16 has a dispersion of 19.25 ps / nm / km, a dispersion slope of 0.064 ps / nm ² / km, and an effective area A _eff of 120 μm ² as characteristics of a wavelength of 1.55 μm. And a cutoff wavelength of 1.45 μm.

In the optical fiber according to Sample 17, the outer diameter 2a of the core region is 19.3 μm, the outer diameter 2b of the inner cladding is 49.5 μm, and the relative refractive index difference Δn _a (0) at the center of the core region with respect to the inner cladding is +0. 37%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.08% with respect to the inner cladding. The parameter α in the approximate expression (4) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 1.5. The optical fiber according to the sample 17 has a dispersion of 19.94 ps / nm / km, a dispersion slope of 0.063 ps / nm ² / km, and an effective area A _eff of 120 μm ² as characteristics of a wavelength of 1.55 μm. And has a cut-off wavelength of 1.44 μm.

In the optical fiber according to the sample 18, the outer diameter 2a of the core region is 17.4 μm, the outer diameter 2b of the inner cladding is 49.0 μm, and the relative refractive index difference Δn _a (0) at the center of the core region with respect to the inner cladding is +0. 35%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.07% with respect to the inner cladding. The parameter α in the approximate expression (4) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 2.0. The optical fiber according to the sample 18 has, as characteristics at a wavelength of 1.55 μm, a dispersion of 20.12 ps / nm / km, a dispersion slope of 0.063 ps / nm ² / km, and an effective area A _eff of 118 μm ^2. And has a cut-off wavelength of 1.44 μm.

In the optical fiber according to the sample 19, the outer diameter 2a of the core region is 16.5 μm, the outer diameter 2b of the inner cladding is 51.4 μm, and the relative refractive index difference Δn _a (0) at the center of the core region with respect to the inner cladding is +0. 34%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.08% with respect to the inner cladding. The parameter α in the approximate expression (4) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 3.0. The optical fiber according to this sample 19 has a dispersion of 20.55 ps / nm / km, a dispersion slope of 0.062 ps / nm ² / km, and an effective area A _eff of 119 μm ² as characteristics of a wavelength of 1.55 μm. And a cutoff wavelength of 1.45 μm.

In the optical fiber according to the sample 20, the outer diameter 2a of the core region is 15.3 μm, the outer diameter 2b of the inner cladding is 51.0 μm, and the relative refractive index difference Δn _a (0) at the center of the core region with respect to the inner cladding is +0. 33%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.07% with respect to the inner cladding. The parameter α in the approximate expression (4) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 4.0. The optical fiber according to the sample 20 has a dispersion of 20.71 ps / nm / km, a dispersion slope of 0.062 ps / nm ² / km, and an effective area A _eff of 118 μm ² as characteristics of a wavelength of 1.55 μm. And a cutoff wavelength of 1.45 μm.

In the optical fiber according to the sample 21, the outer diameter 2a of the core region is 14.5 μm, the outer diameter 2b of the inner cladding is 50.2 μm, and the relative refractive index difference Δn _a (0) at the center of the core region with respect to the inner cladding is +0. 32%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.07% with respect to the inner cladding. The parameter α in the approximate expression (4) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 6.0. The optical fiber according to the sample 21 has a dispersion of 20.85 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 119 μm ² as characteristics of a wavelength of 1.55 μm. And a cutoff wavelength of 1.45 μm.

In the optical fiber according to the sample 22, the outer diameter 2a of the core region is 14.1 μm, the outer diameter 2b of the inner cladding is 49.8 μm, and the relative refractive index difference Δn _a (0) at the center of the core region with respect to the inner cladding is +0. 32%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.07% with respect to the inner cladding. The parameter α in the approximate expression (4) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 8.0. The optical fiber according to the sample 22 has a dispersion of 20.91 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 117 μm ² as characteristics of a wavelength of 1.55 μm. And has a cut-off wavelength of 1.44 μm.

In the optical fiber according to the sample 23, the outer diameter 2a of the core region is 13.7 μm, the outer diameter 2b of the inner cladding is 48.9 μm, and the relative refractive index difference Δn _a (0) at the center of the core region with respect to the inner cladding is +0. 32%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.08% with respect to the inner cladding. The parameter α in the approximate expression (4) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 10.0. The optical fiber according to the sample 23 has a dispersion of 20.97 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 119 μm ² as characteristics of a wavelength of 1.55 μm. And has a cut-off wavelength of 1.44 μm.

In the optical fiber according to the sample 24, the outer diameter 2a of the core region is 12.4 μm, the outer diameter 2b of the inner cladding is 50.1 μm, and the relative refractive index difference Δn _a (0) at the center of the core region with respect to the inner cladding is +0. 32%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.08% with respect to the inner cladding. The parameter α in the approximate expression (4) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to ∞. The optical fiber according to the sample 24 has a dispersion of 21.01 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 117 μm ² as characteristics of a wavelength of 1.55 μm. And a cutoff wavelength of 1.46 μm.

  FIG. 14 is a graph plotting the relationship between the parameter α and the dispersion (ps / nm / km) at a wavelength of 1.55 μm for the samples 16 to 24 described above. As can be seen from this graph, the appropriate range of the parameter α for setting the dispersion at a wavelength of 1.55 μm to 21 ps / nm / km or less is 1 to 10.

(Third application example)
In contrast to the refractive index profile of the second application example described above, the third application example of the optical fiber 200 according to the second embodiment has a shape in which the refractive index in the core region decreases from the periphery toward the center. It has a depressed cladding type refractive index profile. FIG. 15 is a diagram showing a refractive index profile 550 of an optical fiber according to the third application example. The optical fiber according to the third application example is also similar to the cross-sectional structure shown in FIG. Provide structure.

  That is, the optical fiber according to the third application example, like the optical fiber 200, has an outer diameter 2a and a core region having a maximum refractive index n1 in the peripheral portion, and an outer diameter 2b and lower than the core region. An inner cladding having a refractive index n2 and an outer cladding made of pure silica glass and having a higher refractive index n0 than the inner cladding. Chlorine that increases the refractive index is added to the core region, and fluorine that decreases the refractive index is added to the inner cladding.

  In the refractive index profile 550 according to the third application example shown in FIG. 15, the region 451 is the refractive index of the portion corresponding to the core region 210 shown in FIG. 4A, and the region 452 is shown in FIG. The refractive index of a portion corresponding to the illustrated inner cladding 221 and the region 453 indicate the refractive index of a portion corresponding to the outer cladding 222 illustrated in FIG. In addition, AX2 shown in FIG. 15 is the central axis of the optical fiber according to the third application example.

Further, the portion corresponding to the outer periphery of the core region relative to the inner cladding relative refractive index of the (core region from the center distance a distant sites) difference [Delta] n a _(a), and the relative refractive index difference [Delta] n _b of the outer cladding Are given by the following equations, respectively.
Δn _a (a) = (n1−n2) / n2
Δn _b = (n0−n2) / n2

In addition, in the cross section of the core region, the relative refractive index difference Δn _a (r) of the portion of the inner cladding that is separated from the center of the core region by a distance r (0 ≦ r ≦ a) in the radial direction is:
Δn _a (r) = Δn _a (a) · (1−γ · (1−r / a) ^β ) (5)
Δn _a (a): specific bending of the portion corresponding to the outer periphery of the core region with respect to the inner cladding
Folding difference β: 1 to 10
γ: given by an approximate expression of a positive real number.

  FIG. 16 is a table showing structural parameters and optical characteristics of optical fiber samples 25 to 34 according to the third application example having the above-described structure.

As can be seen from the table of FIG. 16, the optical fiber according to Sample 25 has an outer diameter 2a of the core region of 10.2 μm, an outer diameter 2b of the inner cladding of 51.0 μm, and a ratio of the outer periphery of the core region to the inner cladding. refractive index difference Δn _{a (a)} is + 0.58%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.08% with respect to the inner cladding. The parameter β in the approximate expression (5) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 1.0. The optical fiber according to this sample 25 has a dispersion of 19.48 ps / nm / km, a dispersion slope of 0.063 ps / nm ² / km, and an effective area A _eff of 116 μm ² as characteristics of a wavelength of 1.55 μm. And a cutoff wavelength of 1.45 μm.

In the optical fiber according to the sample 26, the outer diameter 2a of the core region is 10.6 μm, the outer diameter 2b of the inner cladding is 50.4 μm, and the relative refractive index difference Δn _a (a) of the outer periphery of the core region with respect to the inner cladding is +0. 49%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.08% with respect to the inner cladding. The parameter β in the approximate expression (5) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 1.5. The optical fiber according to the sample 26 has a dispersion of 19.99 ps / nm / km, a dispersion slope of 0.062 ps / nm ² / km, and an effective area A _eff of 117 μm ² as characteristics of a wavelength of 1.55 μm. And a cutoff wavelength of 1.46 μm.

In the optical fiber according to the sample 27, the outer diameter 2a of the core region is 10.8 μm, the outer diameter 2b of the inner cladding is 49.0 μm, and the relative refractive index difference Δn _a (a) of the outer periphery of the core region with respect to the inner cladding is +0. 44%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.07% with respect to the inner cladding. The parameter β in the approximate expression (5) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 2.0. The optical fiber according to the sample 27 has a dispersion of 20.28 ps / nm / km, a dispersion slope of 0.062 ps / nm ² / km, and an effective area A _eff of 118 μm ² as characteristics of a wavelength of 1.55 μm. And has a cut-off wavelength of 1.44 μm.

In the optical fiber according to the sample 28, the outer diameter 2a of the core region is 11.1 μm, the outer diameter 2b of the inner cladding is 49.2 μm, and the relative refractive index difference Δn _a (a) of the outer periphery of the core region with respect to the inner cladding is +0. 40%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.07% with respect to the inner cladding. The parameter β in the approximate expression (5) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 3.0. The optical fiber according to the sample 28 has, as characteristics at a wavelength of 1.55 μm, a dispersion of 20.45 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 116 μm ^2. And a cutoff wavelength of 1.45 μm.

In the optical fiber according to the sample 29, the outer diameter 2a of the core region is 11.4 μm, the outer diameter 2b of the inner cladding is 49.6 μm, and the relative refractive index difference Δn _a (a) of the outer periphery of the core region with respect to the inner cladding is +0. 37%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.08% with respect to the inner cladding. The parameter β in the approximate expression (5) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 4.0. The optical fiber according to the sample 29 has, as characteristics at a wavelength of 1.55 μm, a dispersion of 20.76 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 118 μm ^2. And a cutoff wavelength of 1.46 μm.

In the optical fiber according to the sample 30, the outer diameter 2a of the core region is 11.7 μm, the outer diameter 2b of the inner cladding is 49.6 μm, and the relative refractive index difference Δn _a (a) of the outer periphery of the core region with respect to the inner cladding is +0. 35%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.07% with respect to the inner cladding. The parameter β in the approximate expression (5) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 6.0. The optical fiber according to the sample 30 has, as characteristics at a wavelength of 1.55 μm, a dispersion of 20.84 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 118 μm ^2. And a cutoff wavelength of 1.46 μm.

In the optical fiber according to the sample 31, the outer diameter 2a of the core region is 11.8 μm, the outer diameter 2b of the inner cladding is 50.2 μm, and the relative refractive index difference Δn _a (a) of the outer periphery of the core region with respect to the inner cladding is +0. 34%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.07% with respect to the inner cladding. The parameter β in the approximate expression (5) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 8.0. The optical fiber according to this sample 31 has, as characteristics at a wavelength of 1.55 μm, a dispersion of 20.89 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 115 μm ^2. And a cutoff wavelength of 1.45 μm.

In the optical fiber according to the sample 32, the outer diameter 2a of the core region is 11.9 μm, the outer diameter 2b of the inner cladding is 49.4 μm, and the relative refractive index difference Δn _a (a) of the outer periphery of the core region with respect to the inner cladding is +0. 33%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.07% with respect to the inner cladding. The parameter β in the approximate expression (5) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 10.0. The optical fiber according to this sample 32 has a dispersion of 20.92 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 117 μm ² as characteristics of a wavelength of 1.55 μm. And a cutoff wavelength of 1.45 μm.

In the optical fiber according to Sample 33, the outer diameter 2a of the core region is 21.1 μm, the outer diameter 2b of the inner cladding is 50.4 μm, and the relative refractive index difference Δn _a (a) of the outer periphery of the core region with respect to the inner cladding is +0. 32%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.08% with respect to the inner cladding. The parameter β in the approximate expression (5) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to 15.0. The optical fiber according to this sample 33 has a dispersion of 20.97 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 118 μm ² as characteristics of a wavelength of 1.55 μm. And has a cut-off wavelength of 1.44 μm.

In the optical fiber according to the sample 34, the outer diameter 2a of the core region is 12.4 μm, the outer diameter 2b of the inner cladding is 50.1 μm, and the relative refractive index difference Δn _a (a) of the outer periphery of the core region with respect to the inner cladding is +0. 32%, the relative refractive index difference [Delta] n _b of the outer cladding is set to 0.08% with respect to the inner cladding. The parameter β in the approximate expression (5) representing the relative refractive index difference refractive index Δn _a (r) in the core region with respect to the inner cladding is set to ∞. The optical fiber according to this sample 34 has a dispersion of 21.01 ps / nm / km, a dispersion slope of 0.061 ps / nm ² / km, and an effective area A _eff of 117 μm ² as characteristics of a wavelength of 1.55 μm. And a cutoff wavelength of 1.46 μm.

  FIG. 17 is a graph plotting the relationship between the parameter β and the dispersion (ps / nm / km) at a wavelength of 1.55 μm for the samples 25 to 34 described above. As can be seen from this graph, the appropriate range of the parameter β for setting the dispersion at a wavelength of 1.55 μm to 21 ps / nm / km or less is 1 to 10.

  The shape of the refractive index profile of the optical fiber according to the present invention is not limited to the above-described shape, and various modifications are possible. For example, as shown in FIG. 18A, the refractive index profile shape of the core region is a shape in which the refractive index is maximum at a part away from the center of the core region by a predetermined distance, as shown in FIG. As shown in FIG. 18 (c), the refractive index gradually decreases at the boundary between the core region and the cladding region. As shown in FIG. 18 (d), the shape in which the refractive index is reduced near the center of the core region, as shown in FIG. 18 (e), near the center of the core region. A shape with a high refractive index is applicable. On the other hand, as the refractive index profile shape of the cladding region, as shown in FIG. 19A, a shape in which the refractive index of the inner cladding decreases from the center of the optical fiber toward the periphery, FIG. ), The refractive index of the inner cladding increases from the center of the optical fiber toward the periphery, and as shown in FIG. 19C, the refractive index of the outer cladding is A shape that decreases in the radial direction of the optical fiber in the vicinity of the interface with the cladding, as shown in FIG. 19D, the refractive index of the outer cladding is from the center of the optical fiber toward the periphery. Increasing shapes are applicable.

  Next, the optical fiber according to the present invention, in particular, the core region is made of pure silica glass, and a desired refractive index difference is obtained between the core region and the cladding region by adjusting the amount of fluorine added to the cladding region. The optical fiber having the generated structure is suitable for short wavelength optical communication using an optical signal having a wavelength band of 1.35 to 1.52 μm. The reason will be described below.

Ａ：レーリー散乱係数、λ：波長
で表されるレーリー散乱とＯＨ基に起因した損失が支配的となる。 Transmission loss of an optical fiber is generally caused by Rayleigh scattering, ultraviolet absorption, infrared absorption, absorption / scattering caused by additives, and the like. In the wavelength band of 1.0 to 1.6 μm,

A: Rayleigh scattering coefficient, λ: Rayleigh scattering represented by wavelength and loss due to OH group are dominant.

The Rayleigh scattering coefficient varies depending on the substance added to the silica glass and its concentration. In particular, when GeO ₂ is added by Δ%, the Rayleigh scattering coefficient A is
A = A _SiO2 · (1 + a _GeO2 · | Δ |) (6)
When the fluorine is added by Δ%, the Rayleigh scattering coefficient A is
A = A _SiO2 · (1 + a _F · | Δ |) (7)
It is empirically known that _{Here, A SiO2} is Rayleigh scattering coefficient of pure silica glass (SiO _{2), a GeO2} and _{a F} are constants. From these equations (6) and (7), it can be seen that the Rayleigh scattering coefficient increases as the concentration of GeO ₂ or fluorine increases.

The Rayleigh scattering coefficient in the optical fiber is experimentally expressed as follows by superimposing the Rayleigh scattering coefficient A (r) and the optical power distribution P (r) in a portion separated by a distance r in the radial direction from the center. .

When much of the optical power is confined in the core region, the optical fiber made of pure silica glass (SiO ₂ ) is transmitted compared to the optical fiber in which GeO ₂ is added to the core region. The fact that the loss is reduced is explained from the above equation (8).

  In the case of an optical fiber whose core region is made of pure silica glass, a desired difference in refractive index is generally obtained by adding fluorine to the cladding region. However, as the amount of fluorine added increases, the Rayleigh scattering coefficient increases, so the smaller the amount of fluorine added, the better. Conversely, the smaller the rate at which light oozes into the cladding region (the greater the difference in refractive index between the core region and the cladding region), the smaller the Rayleigh scattering coefficient and the smaller the transmission loss. The optical fiber according to the present invention has a higher light confinement in the core region than an optical fiber such as the ITU-T G654 standard, and can therefore reduce Rayleigh scattering.

  As described above, since the transmission loss due to Rayleigh scattering is proportional to the fourth power of the wavelength λ, the difference in the Rayleigh scattering coefficient becomes more noticeable as the wavelength is shorter. This also shows that the optical fiber according to the present invention is suitable for short wavelength optical communication using an optical signal in the wavelength band of 1.35 to 1.52 μm.

  Further, since the transmission loss due to the OH group has a loss peak at a wavelength of 1.38 μm as shown in FIG. 20, it is a factor that limits the optical communication in the short wavelength band of 1.35 to 1.52 μm as described above. It becomes. However, by performing dehydration in the optical fiber manufacturing process, etc., the transmission loss at a wavelength of 1.38 μm is suppressed to 0.3 dB / km or less, thereby constructing an optical communication system more suitable for optical communication in a shorter wavelength band. Is possible.

On the other hand, the ease of occurrence of the nonlinear optical phenomenon in the optical fiber is expressed by <N2> / A _eff , and it is known that the smaller the value, the more difficult the nonlinear optical phenomenon occurs. In other words, the nonlinear optical phenomenon is more likely to occur as the optical power of light incident on the optical fiber is larger. However, the nonlinear optical phenomenon is more likely to occur as the nonlinear refractive index <N2> is smaller and the effective area A _eff is larger from the above relational expression. It is preferable for suppressing the above.

Note that the refractive index <N> of the medium under strong light varies depending on the light intensity as described above. Therefore, the lowest order effect on this refractive index <N> is
<N> = <N0> + <N2> · | E | ²
Here, <N0>: refractive index with respect to linear polarization <N2>: second-order nonlinear refractive index with respect to third-order nonlinear polarization | E | ² : expressed by light intensity. That is, under strong light, the refractive index <N> of the medium is given by the sum of the normal value <N0> and an increment proportional to the square of the photoelectric field amplitude E. In particular, the proportional constant <N2> (unit: m ² / W) of the second term is called a second-order nonlinear refractive index. Further, the distortion of the signal light pulse is mainly affected by the second-order nonlinear refractive index of the nonlinear refractive index. In this specification, the nonlinear refractive index mainly means the second-order nonlinear refractive index. To do.

  As described above, since the optical fiber according to the present invention has a structure in which a nonlinear optical phenomenon is unlikely to occur, it is suitable for an optical transmission line for long-distance optical communication such as a submarine cable.

  Next, as shown in FIG. 1B, an optical communication system in which an optical fiber 30 is constituted by an optical fiber according to the present invention and a dispersion compensating optical fiber (hereinafter referred to as DCF) will be described.

Since the optical fiber according to the present invention has an effective area of 110 μm ² or more at a wavelength of 1.55 μm, a nonlinear optical phenomenon hardly occurs. Further, the nonlinear refractive index <N2> increases as the GeO ₂ concentration increases. As in the present invention, the core region is composed of pure silica glass (SiO ₂ ) alone or silica glass to which chlorine is added. In this case, the nonlinear refractive index <N2> is small. Therefore, the optical fiber according to the present invention has a small <N2> / A _eff , and even if it is used in a region having a relatively strong optical power such as a light source for an optical signal or the vicinity of the emission end of an optical amplifier, the nonlinear optical The phenomenon is difficult to occur.

On the other hand, since the effective area A _{eff of} DSF is as small as 10 to 30 μm ² and a large amount of GeO ₂ is added to the core region to compensate dispersion, the nonlinear refractive index <N2> is large. For this reason, the DCF is characterized in that a nonlinear optical phenomenon is likely to occur when used in a region where the optical power is large.

  From the above, the optical fiber according to the present invention is disposed in a region where the optical power is strong, such as in the vicinity of the light source for the optical signal and in the vicinity of the emission end of the optical amplifier, while the downstream side of the optical fiber in which the optical power is reduced. By arranging the DCF on the side and constructing the optical communication system, the occurrence of the nonlinear optical phenomenon can be effectively suppressed and good transmission quality can be guaranteed.

  Further, as shown in FIG. 1B, the optical fiber according to the present invention and a dispersion-shifted optical fiber (an optical fiber having a dispersion of 0 to −6 ps / nm / km at a wavelength of 1.55 μm, , An optical communication system in which the optical fiber 30 is configured.

  An NZ-DSF having a small absolute value and negative dispersion may be used as an optical transmission line for long-distance optical communication in order to prevent quality degradation of an optical signal due to modulation instability. In an optical communication system to which such an optical fiber is applied, it is necessary to compensate for dispersion accumulated by propagation of an optical signal over a long distance with an optical fiber having positive dispersion in a 1.55 μm wavelength band. Even in such an optical communication system, a configuration in which the optical fiber according to the present invention is arranged in the vicinity of the output end of an optical amplifier having strong optical signal power is effective.

Note that NZ-DSF has an effective area A _eff as small as 50 to 80 μm ² . The generation efficiency η of four-wave mixing is approximated by the following equation.

  Here, α is transmission loss and Disp is chromatic dispersion. As described above, when the absolute value of dispersion is small, the generation efficiency of four-wave mixing is large in the NZ-DSF where the dispersion is as small as 0 to −6 ps / nm / km. The quality of the product may deteriorate.

Next, the microbend loss of the optical fiber according to the present invention will be described. Generally, it is known that when the effective area A _eff increases, the microbend loss (dB / km) increases accordingly. Thus, in the optical fiber according to the present embodiment, it is important to increase the effective cross-sectional area while suppressing the increase in microbend loss within an allowable range. For example, in the case of applying an optical fiber to an optical fiber unit having a cross-sectional structure as shown in FIG. 21 or an optical fiber cable including the optical fiber unit, in order to prevent deterioration of transmission characteristics due to cable formation It is preferable to suppress the microbend loss of the applied optical fiber to about 1 dB / km or less.

  In the optical fiber unit 600 shown in FIG. 21A, the optical fiber 100 (200) covered with the ultraviolet curable resin 620 is disposed around the tensile strength wire 610, and the optical fiber 100 (200) is arranged. The structure is covered with an ultraviolet curable resin layer 630 and an ultraviolet curable resin layer 640 in order. Further, in the optical fiber cable 700 to which the optical fiber unit 600 having the above structure is applied, a plurality of optical fiber units 600 are covered with a waterproof compound 710 as shown in FIG. Further, a tensile strength wire 730 is disposed around the waterproof compound 710 via an iron three-divided pipe 720. Thus, the optical fiber unit 600 covered with the tensile strength wire 730 is accommodated in the copper tube 740, and the gap between the tensile strength wires 730 is filled with the waterproof compound 710. Further, the copper tube 740 is covered with a low density polyethylene layer 750 and a high density polyethylene layer 760 in order.

  As described above, microbendros is an increase in loss at a wavelength of 1.55 μm caused by wrapping an optical fiber with a tension of 100 g around a bobbin having a cylinder diameter of 280 mm with JIS # 1000 sandpaper on the surface. This microbend loss differs depending on the resin layer covering the periphery of the optical fiber and the fiber diameter of the optical fiber. Below, the relationship between the resin layer which coat | covers the circumference | surroundings of an optical fiber and microbend loss, and the relationship between the fiber diameter of an optical fiber and microbend loss are demonstrated.

  FIG. 22 is a cross-sectional view of an optical fiber covered with a resin layer. As shown in this figure, an optical fiber 100 (200) having a fiber diameter of 125 μm has a first resin layer 300 having a Young's modulus E1 and an outer diameter d1, and a Young's modulus E2 and an outer diameter d2. The second resin layer 400 is sequentially covered. The Young's moduli E1 and E2 are ratios T / T of the stress T in the axial direction of each of the first and second resin layers 300 and 400, which are elastic bodies, and the strain amount ε when the stress T is applied. is given by ε. Then, microbend loss was measured while variously changing the outer diameter and Young's modulus of each of the first and second resin layers 300 and 400. The results are shown in FIGS.

FIG. 23 is a table showing microbend loss (dB / km) and the like obtained when the Young's modulus E1 of the first resin layer 300 is changed. The sample prepared for the measurement has an outer diameter d1 of the first resin layer 300 of about 200 μm, a Young's modulus E2 of the second resin layer 400 of about 70 kg / mm ² , and an outer diameter of the second resin layer 400. d2 is set to about 250 μm. In these measurement samples, when the Young's modulus E1 of the first resin layer 300 is 0.06 kg / mm ² , the microbend loss is 0.50 dB / km, and the Young's modulus E1 of the first resin layer 300 is 0.12 kg / mm. microbend loss 1.0 dB / miles when ^2, the Young's modulus E1 of the first resin layer 300 is microbend loss when 0.20 kg / mm ² was 1.5 dB / miles.

FIG. 24 is a table showing microbend loss (dB / km) and the like obtained when the outer diameter d1 of the first resin layer 300 is changed. Samples prepared for the measurement, the Young's modulus E1 of the first resin layer 300 is about 0.12 kg / mm ^2, the Young's modulus E2 of the second resin layer 400 is about 70 kg / mm ^2, the second resin The outer diameter d2 of the layer 400 is set to about 250 μm. In these measurement samples, when the outer diameter d1 of the first resin layer 300 is about 180 μm, the microbend loss is 1.8 dB / km, and when the outer diameter d1 of the first resin layer 300 is about 200 μm, the microbendros is The microbend loss was 0.38 dB / km when the outer diameter d1 of the first resin layer 300 was about 209 μm and 0.85 dB / km.

FIG. 25 is a table showing microbend loss (dB / km) and the like obtained when the Young's modulus E2 of the second resin layer 400 is changed. In the sample prepared for the measurement, the Young's modulus E1 of the first resin layer 300 is about 0.12 kg / mm ² , the outer diameter d1 of the first resin layer 300 is about 200 μm, and the second resin layer 400 The outer diameter d2 is set to about 250 μm. In these measurement samples, when the Young's modulus E2 of the second resin layer 400 is about 0.2 kg / mm ² , the microbend loss is 0.12 dB / km, and the Young's modulus E2 of the second resin layer 400 is about 1 kg / mm. When mm ^2, the microbend loss is 0.31 dB / km, and when the Young's modulus E2 of the second resin layer 400 is about 10 kg / mm ² , the microbend loss is 0.72 dB / km and the Young of the second resin layer 400 When the rate E2 was about 70 kg / mm ² , the microbendros was 1.2 dB / km, and when the Young's modulus E2 of the second resin layer 400 was about 100 kg / mm ² , the microbendros was 1.4 dB / km. .

FIG. 26 is a table showing microbend loss (dB / km) and the like obtained when the Young's modulus E2 of the second resin layer 400 is changed. In the sample prepared for measurement, the Young's modulus E1 of the first resin layer 300 is about 0.12 kg / mm ² , the outer diameter d1 of the first resin layer 300 is about 290 μm, and the second resin layer 400 The outer diameter d2 is set to about 400 μm. In these measurement samples, when the Young's modulus E2 of the second resin layer 400 is about 0.2 kg / mm ² , the microbend loss is 0.45 dB / km, and the Young's modulus E2 of the second resin layer 400 is about 1 kg / mm ^2. When it is mm ^2, the microbendros is 0.96 dB / km, and when the Young's modulus E2 of the second resin layer 400 is about 10 kg / mm ² , the microbendros is 2.3 dB / km, and the Young of the second resin layer 400 The microbendros was 4.1 dB / km when the rate E2 was about 70 kg / mm ^{2, and} the microbendros was 4.5 dB / km when the Young's modulus E2 of the second resin layer 400 was about 100 kg / mm ² . .

FIG. 27 is a table showing microbend loss (dB / km) and the like obtained when the outer diameter d2 of the second resin layer 400 is changed. In the sample prepared for the measurement, the Young's modulus E1 of the first resin layer 300 is about 0.12 kg / mm ² , the outer diameter d1 of the first resin layer 300 is about 200 μm, and the second resin layer 400 Young's modulus E2 is set to about 70 kg / mm ² . In these measurement samples, when the outer diameter d2 of the second resin layer 400 is about 250 μm, the microbend loss is 8.2 dB / km, and when the outer diameter d2 of the second resin layer 400 is about 350 μm, the microbendros is When the outer diameter d2 of the second resin layer 400 is about 400 μm, the microbend loss is 0.95 dB / km, and when the outer diameter d2 of the second resin layer 400 is about 450 μm, the microbendros is It was 0.65 dB / km.

  As can be seen from the results shown in FIGS. 23 to 27, the smaller the Young's modulus E1 of the first resin layer 300 and the larger the outer diameter d1 of the first resin layer 300, the greater the second resin layer. The smaller the Young's modulus E2 of 400 or the larger the outer diameter d2 of the second resin layer 400, the smaller the microbend loss and the transmission characteristics of the optical fiber are improved. Therefore, by covering the optical fibers 100 and 200 according to the first and second embodiments with a resin layer having a small Young's modulus and a large outer diameter, a cable having a small microbend loss even if the effective sectional area is large. Is obtained.

FIG. 28 is a table showing the microbend loss (dB / km) and the like obtained by changing the fiber diameter of the optical fiber, and FIG. 29 is a graph showing the relationship between the fiber diameter and the microbend loss. is there. The measurement sample prepared to obtain the table of FIG. 28 has an effective area of about 150 μm ² , a cutoff wavelength of about 1.34 μm at ² m, a dispersion at a wavelength of 1.55 μm of about 21 ps / nm / km, The dispersion slope is set to about 0.060 ps / nm ² / km. In these measurement samples, the microbend loss is 1.5 dB / km when the fiber diameter is 125 μm, the microbend loss is 0.70 dB / km when the fiber diameter is 135 μm, and the microbend loss is 0.1 when the fiber diameter is 150 μm. The microbend loss was 0.05 dB / km when the fiber diameter was 28 μm and the fiber diameter was 180 μm.

FIG. 28 and FIG. 29 show that the microbend loss decreases as the fiber diameter of the optical fiber increases. In the case of an optical fiber having an effective area of about 150 μm ² , the fiber diameter needs to be 130 μm or more so that the microbend loss becomes 1 dB / km or less. On the other hand, the greater the fiber diameter of the optical fiber, the greater the strain generated on the cladding surface due to bending, and the fracture probability increases. However, if the fiber diameter is 200 μm or less, the fracture probability is 10 ⁻⁵ or less, which is not a practical problem. Therefore, in general, the fiber diameter of the optical fiber is 125 μm, but, as in the optical fibers according to the first and second embodiments described above, by setting the fiber diameter (outer diameter of the outer layer cladding region) to 130 μm to 200 μm, Even if the effective area is large, the microbend loss can be reduced, and the fracture probability can be reduced.

  In addition, this invention is not limited to the structure of the above-mentioned Example, A various deformation | transformation is possible. The specific sample corresponding to the optical fiber according to the present invention is not limited to the above-described sample structure.

  As described above, since the optical fiber according to the present invention has a large effective area at a wavelength of 1.55 μm, non-linear optical phenomenon occurs even when a high-power optical signal (1.55 μm wavelength band) is transmitted. It is effectively suppressed and is suitable for an optical transmission line in long-distance optical communication such as a submarine cable.

It is a figure which shows schematic structure of the optical communication system which concerns on this invention. It is a figure which shows the cross-section and refractive index profile of 1st Example of the optical fiber which concerns on this invention. It is a table | surface which shows the structural parameter and optical characteristic of several samples prepared as an optical fiber which concerns on 1st Example shown by FIG. It is a figure which shows the cross-sectional structure and refractive index profile of 2nd Example of the optical fiber which concerns on this invention. It is a graph which shows the relationship between fiber length (m) and an effective cut-off wavelength (micrometer). It is a graph for demonstrating the suitable range of each parameter in the optical fiber which concerns on 2nd Example. It is the graph which showed the difference (micrometer) of the cutoff wavelength of the optical fiber which has a depressed clad type refractive index profile, and the cutoff wavelength of the optical fiber which has a matched type refractive index profile with respect to ratio 2b / 2a. It is a table | surface which shows the structural parameter and optical characteristic of several samples prepared as an optical fiber which concerns on 2nd Example shown by FIG. It is a figure which shows the refractive index profile of the 1st application example of the optical fiber which concerns on 2nd Example. 10 is a table showing structural parameters and optical characteristics of a plurality of samples prepared as an optical fiber according to a first application example having the refractive index profile shown in FIG. 9. It is a graph which shows the relationship between effective area _Aeff (micrometer _< ² >) and microbend loss (dB / km). It is a figure which shows the refractive index profile of the 2nd application example of the optical fiber which concerns on 2nd Example. 13 is a table showing structural parameters and optical characteristics of a plurality of samples prepared as optical fibers according to a second application example having the refractive index profile shown in FIG. Relationship between the parameter α in the formula that approximates the refractive index profile of the core region of the optical fiber according to the second application example and the dispersion (ps / nm / km) at the wavelength of 1.55 μm of the optical fiber according to the second application example It is a graph which shows. It is a figure which shows the refractive index profile of the 3rd application example of the optical fiber which concerns on 2nd Example. 16 is a table showing structural parameters and optical characteristics of a plurality of samples prepared as an optical fiber according to a third application example having the refractive index profile shown in FIG. Relationship between the parameter β in the equation for approximating the refractive index profile of the core region of the optical fiber according to the third application example and the dispersion (ps / nm / km) at the wavelength of 1.55 μm of the optical fiber according to the third application example It is a graph which shows. It is the figure which showed schematically the applicable shape pattern as a refractive index profile of a core area | region. It is the figure which showed schematically the applicable shape pattern as a refractive index profile of a clad area. It is a graph which shows the relationship between a wavelength (nm) and transmission loss (dB / km). It is a figure which shows the cross-section of the optical fiber unit which can apply the optical fiber which concerns on this invention, and a cable including the same. It is a figure which shows the cross-section of the optical fiber coat | covered with the resin layer. The microbend loss (dB / km) obtained when the Young's modulus (kg / mm ² ) of the first resin was changed for three types of resin-coated optical fibers as shown in FIG. It is a table | surface which shows. FIG. 22 shows the microbend loss (dB / km) obtained when the outer diameter (μm) of the first resin layer is changed for three types of resin-coated optical fibers as shown in FIG. It is a table. The microbend loss (dB / km) obtained when the Young's modulus (kg / mm ² ) of the second resin was changed for five types of samples of optical fibers coated with resin as shown in FIG. It is a table | surface which shows. The microbend loss (dB / km) obtained when the Young's modulus (kg / mm ² ) of the second resin was changed for five types of samples of optical fibers coated with resin as shown in FIG. It is a table | surface which shows. FIG. 22 shows the microbend loss (dB / km) obtained when the outer diameter (μm) of the second resin layer is changed for four types of resin-coated optical fibers as shown in FIG. It is a table. It is a table | surface which shows the microbend loss (dB / km) obtained when the fiber diameter (micrometer) was changed about four types of optical fiber samples. It is a graph which shows the relationship between a fiber diameter (micrometer) and microbend loss (dB / km).

Explanation of symbols

100, 200: optical fiber, 110, 210: core region, 120, 220: cladding region, 221: inner cladding, 222 outer cladding.

Claims (9)

A core region having an outer diameter 2a extending along a predetermined axis;
An inner cladding provided on the outer periphery of the core region and having a lower refractive index than the core region, and an outer cladding provided on the outer periphery of the inner cladding and having a higher refractive index than the inner cladding With areas,
An optical fiber having an effective area of 110 μm ² or more at a wavelength of 1.55 μm. ^2. The optical fiber according to claim 1, wherein the optical fiber has an effective area of 150 μm ² or more at a wavelength of 1.55 μm. 2. The optical fiber according to claim 1, wherein the optical fiber has a cutoff wavelength of 1.3 [mu] m to 1.75 [mu] m. The optical fiber according to claim 1, wherein the optical fiber has a transmission loss of 0.180 dB / km or less at a wavelength of 1.55 µm. The optical fiber according to claim 1, wherein the core region has an outer diameter of 11.5 μm to 23.0 μm. The optical fiber according to claim 1, wherein the cladding region has an outer diameter of 130 μm to 200 μm. The optical fiber according to claim 1, wherein a ratio 2b / 2a of an outer diameter 2b of the inner cladding to an outer diameter 2a of the core region is 1.1-7. The relative refractive index difference of the core region with respect to the outer cladding is + 0.15% to + 0.30%, and the relative refractive index difference of the inner cladding with respect to the outer cladding is −0.15% to −0.01%. The optical fiber according to claim 1, wherein the optical fiber is provided. The optical fiber according to claim 1, wherein the core region is made of silica glass to which no additive is intentionally added, and the cladding region is made of silica glass to which fluorine is added.

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