(19)
(11)EP 3 627 197 A1

(12)EUROPEAN PATENT APPLICATION
published in accordance with Art. 153(4) EPC

(43)Date of publication:
25.03.2020 Bulletin 2020/13

(21)Application number: 18827612.5

(22)Date of filing:  03.07.2018
(51)International Patent Classification (IPC): 
G02B 6/036(2006.01)
H04B 10/80(2013.01)
G02B 6/02(2006.01)
(86)International application number:
PCT/JP2018/025192
(87)International publication number:
WO 2019/009284 (10.01.2019 Gazette  2019/02)
(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
Designated Extension States:
BA ME
Designated Validation States:
KH MA MD TN

(30)Priority: 03.07.2017 JP 2017130725

(71)Applicant: Nippon Telegraph and Telephone Corporation
Tokyo 100-8116 (JP)

(72)Inventors:
  • SAGAE, Yuto
    Musashino-shi, Tokyo 180-8585 (JP)
  • NAKAJIMA, Kazuhide
    Musashino-shi, Tokyo 180-8585 (JP)
  • MATSUI, Takashi
    Musashino-shi, Tokyo 180-8585 (JP)

(74)Representative: Brevalex 
95, rue d'Amsterdam
75378 Paris Cedex 8
75378 Paris Cedex 8 (FR)

  


(54)OPTICAL FIBER AND OPTICAL TRANSMISSION SYSTEM


(57) An optical fiber includes a core, a first clad that is provided on an outer circumferential portion of the core and has a refractive index lower than that of the core, and a second clad that is provided on an outer circumferential portion of the first clad and has a refractive index lower than that of the first clad. In the optical fiber, a mode field diameter at a wavelength of 1.55 µm is equal to or greater than 11.5 µm, a cutoff wavelength is equal to or less than 1.53 µm, a bending loss at a bending radius of 30 mm and a wavelength of 1.625 µm is equal to or less than 2.0 dB/100 turns, and a delay time of transmission light per unit length at a wavelength of 1.55 µm is equal to or less than 4.876 µs/km.




Description

[Technical Field]



[0001] The invention relates to an optical fiber and an optical transmission system. Priority is claimed on Japanese Patent Application No. 2017-130725, filed July 3, 2017, the content of which is incorporated herein by reference.

[Background Art]



[0002] With recent diversified use of communication networks, there is a demand for a reduction of transmission delay. For example, in communication between computers which is frequently used for financial transactions which are carried out on an international scale, a reduction in a transmission delay of 1 ms has a great influence on the transmission performance of communication, financial transaction services, and customer profit or loss. In the future, the demand for further reduction of a transmission delay is expected to accelerate.

[0003] In long-distance communication networks such as a submarine optical cable network crossing the Pacific Ocean, a construction length of a communication line amounts to several thousands of kilometers. For improvement in transmission performance of a long-distance communication network, it is important to reduce a delay which occurs in a transmission line. In a submarine optical cable network, effort to reduce a delay in a communication line by optimizing a constructed route of an intercontinental submarine optical cable network are made. By this effort, it has been reported that a transmission delay of a long-distance communication network is reduced by about several ms.

[0004] A delay of a communication network includes a delay which is caused in a device such as a transmission device and a delay which is caused in a transmission line. In a long-distance communication network, a delay time which is caused in a transmission line occupies most of a delay time which occurs in the whole network and becomes too long to ignore.

[0005] A delay time of an optical fiber per unit time in a transmission line of a communication network is mainly determined according to a refractive index of a medium of the optical fiber. In order to reduce a transmission delay of a communication network, it is effective to use a medium with a low refractive index as a medium of an optical fiber. A cutoff shift fiber which is used for a submarine optical cable network in the related art includes a core which is formed of silica glass with high purity. Accordingly, in a submarine optical cable network in the related art, light can be transmitted with a delay time of about 4.876 µs/km in a wavelength band including 1.55 µm. In Non-Patent Literature 1, it is reported that a delay time of a photonic band gap fiber is reduced by about 3.448 µs/km. The photonic band gap fiber includes a hollow core in which a refractive index of a medium is lowered to the utmost limit.

[Citation List]


[Non-Patent Literature]



[0006] [Non-Patent Literature 1]
N. V. Wheeler et al., "Wide-bandwidth, low-loss, 19-cell hollow core photonic band gap fiber and its potential for low latency data transmission," OFC/NFOEC Post deadline Papers PDP5A.2 2012.

[Summary of Invention]


[Technical Problem]



[0007] As described above, in a long-distance communication network, a delay time is improved by optimization of a constructed route. However, geometrical conditions or construction costs are limited in actually optimizing a constructed route, and there is a problem in that a reduction amount in delay time due to optimization of a constructed route is decreased by this limitation.

[0008] In a photonic band gap fiber, a hollow core with a low refractive index is formed. However, since a transmission loss of a photonic band gap fiber amounts to about several dB/km, there is a problem in that the photonic band gap fiber is not suitable for a transmission line of a long-distance communication network.

[0009] The invention is made in consideration of the above-mentioned problems and an objective thereof is to provide an optical fiber that can be applied to a long-distance communication network, has a mode field diameter (MFD) and a bending loss which are equivalent to an MFD and a bending loss of a cutoff shifted fiber according to the related art, and has a delay time which is less than a delay time of the cutoff shifted fiber.

[0010] The invention provides an optical transmission system of which the optical fiber has excellent characteristics.

[Solution to Problem]



[0011] The inventors newly found design conditions and structures of an optical fiber having five features: (1) the optical fiber includes a core, a first clad which is adjacent to an outer circumferential portion of the core, and a second clad which is adjacent to an outer circumferential portion of the first clad, (2) a radius of the core is equal to or less than 4 µm, (3) a relative refractive index difference of the first clad from the core is equal to or less than 0.0%, (4) a mode field diameter (MFD) at a wavelength of 1.55 µm is equal to or greater than 11.5 µm, and (5) a bending loss at a bending radius of 30 mm and a wavelength of 1.625 µm is equal to or less than 2.0 dB/100 turns as an optical fiber for solving the above-mentioned problems.

[0012] An optical fiber according to the invention includes a core, a first clad that is provided on an outer circumferential portion of the core and has a refractive index lower than that of the core, and a second clad that is provided on an outer circumferential portion of the first clad and has a refractive index lower than that of the first clad. A mode field diameter of the optical fiber according to the invention at a wavelength of 1.55 µm is equal to or greater than 11.5 µm. A cutoff wavelength of the optical fiber according to the invention is equal to or less than 1.53 µm. A bending loss of the optical fiber according to the invention at a bending radius of 30 mm and a wavelength of 1.625 µm is equal to or less than 2.0 dB/100 turns. A delay time of transmission light of the optical fiber according to the invention per unit length at a wavelength of 1.55 µm is equal to or less than 4.876 µs/km.

[0013] In the optical fiber according to the invention, a radius of the core may be equal to or less than 1.0 µm and equal to or greater than 4.3 µm, and a radius of the first clad may satisfy Equations (1) and (2).
[Equation 1]


[Equation 2]



[0014] In Equations (1) and (2), a1 represents the radius [µm] of the core. a2 represents the radius [µm] of the first clad. Δ1 represents a relative refractive index difference [%] of the first clad from the core.

[0015] In the optical fiber according to the invention, the relative refractive index difference of the second clad from the first clad may satisfy Equation (3).



[0016]  In the optical fiber according to the invention, in a sectional view, a plurality of low-delay cores including the core as a first core and the first clad provided on an outer circumferential portion of the first core may be disposed on concentric circles at the center of the second clad.

[0017] In the optical fiber according to the invention, the core may be disposed as a second core at the center of the second clad.

[0018] In the optical fiber according to the invention, in a sectional view, a low-delay core including the core as a first core and the first clad provided on an outer circumferential portion of the first core may be disposed at the center of the second clad, and the core may be disposed as a third core on a concentric circle at the center of the low-delay core.

[0019] In the optical fiber according to the invention, in a sectional view, a low-delay core including the core as a first core and the first clad provided on an outer circumferential portion of the first core may be disposed at the center of the second clad, and the cores may be packed most closely as fourth cores around the low-delay core.

[0020] An optical transmission system according to the invention includes the above-mentioned optical fiber, a transmitter that is connected to one end of the optical fiber and a receiver that is connected to the other end of the optical fiber.

[Advantageous Effects of Invention]



[0021] According to the invention, it is possible to provide an optical fiber that has an MFD and a bending loss which are equivalent to an MFD and a bending loss of a cutoff shifted fiber according to the related art and has a delay time which is less than a delay time of the cutoff shifted fiber. According to the invention, since compatibility between the optical fiber and an existing submarine optical cable network is acquired, the delay time of the optical fiber decreases. According to the invention, since a transmission line is constructed with the optical fiber according to the invention, a delay time in the transmission line of the optical transmission system decreases and a delay time in the whole optical transmission system decreases.

[Brief Description of Drawings]



[0022] 

FIG. 1 is a graph showing a relationship between optical characteristics and a delay time in a silica-core fiber according to the related art.

FIG. 2 is a diagram showing a refractive index distribution of a single-mode optical fiber (SMF) according to the invention.

FIG. 3 is a graph showing conditions of a radius a2 and a relative refractive index difference Δ1 of an optical fiber according to the invention in which all of a plurality of conditions in which a cutoff wavelength is 1.53 µm, an MFD is equal to or greater than 11.5 µm, a bending loss is equal to or less than 2 dB/100 turns, and a delay time is equal to or less than a delay time of a cutoff shift fiber are satisfied when a radius of a core is 1.0 µm.

FIG. 4 is a graph showing conditions of the radius a2 and the relative refractive index difference Δ1 of the optical fiber according to the invention in which all of the plurality of conditions in which the cutoff wavelength is 1.53 µm, the MFD is equal to or greater than 11.5 µm, the bending loss is equal to or less than 2 dB/100 turns, and the delay time is equal to or less than the delay time of the cutoff shifted fiber are satisfied when the radius of the core is 1.5 µm.

FIG. 5 is a graph showing conditions of the radius a2 and the relative refractive index difference Δ1 of the optical fiber according to the invention in which all of the plurality of conditions in which the cutoff wavelength is 1.53 µm, an MFD is equal to or greater than 11.5 µm, the bending loss is equal to or less than 2 dB/100 turns, and the delay time is equal to or less than the delay time of the cutoff shift fiber are satisfied when the radius of the core is 2.0 µm.

FIG. 6 is a graph showing a relationship between a fitting coefficient of the delay time and a radius of a core of an SMF according to the invention in which predetermined required conditions are satisfied.

FIG. 7 is a graph showing a relationship between a fitting coefficient of a MFD and the radius of the core of the SMF according to the invention in which predetermined required conditions are satisfied.

FIG. 8 is a graph showing a relationship between a fitting coefficient of a bending loss and the radius of the core of the SMF in the invention in which predetermined required conditions are satisfied.

FIG. 9 is a graph showing conditions of a radius a2 and a relative refractive index difference Δ1 of an optical fiber according to the invention in which all of a plurality of conditions in which a cutoff wavelength is 1.53 µm, an MFD is equal to or greater than 11.5 µm and equal to or less than 12.5 µm, a bending loss is equal to or less than 2 dB/100 turns and a delay time is equal to or less than a delay time of a cutoff shift fiber are satisfied when a radius of a core is 1.0 µm.

FIG. 10 is a graph showing conditions of the radius a2 and the relative refractive index difference Δ1 of the optical fiber according to the invention in which all of a plurality of conditions in which the cutoff wavelength is 1.53 µm, the MFD is equal to or greater than 11.5 µm and equal to or less than 12.5 µm, the bending loss is equal to or less than 2 dB/100 turns and the delay time is equal to or less than the delay time of the cutoff shift fiber are satisfied when the radius of the core is 1.5 µm.

FIG. 11 is a graph showing conditions of the radius a2 and the relative refractive index difference Δ1 of the optical fiber according to the invention in which all of a plurality of conditions in which the cutoff wavelength is 1.53 µm, the MFD is equal to or greater than 11.5 µm and equal to or less than 12.5 µm, the bending loss is equal to or less than 2 dB/100 turns and the delay time is equal to or less than the delay time of the cutoff shift fiber are satisfied when the radius of the core is 2.0 µm.

FIG. 12 is a graph showing a relationship between a fitting coefficient of a bending loss and a radius of a core of the SMF according to the invention in which predetermined required conditions are satisfied.

FIG. 13 is a graph showing a relationship between the radius a2 and the relative refractive index difference Δ1 of the SMF according to the invention in which the cutoff wavelength is equal to or less than 1.53 µm.

FIG. 14 is a graph showing a relationship between fitting coefficients κ9 and κ10 of the cutoff wavelength, a radius a1 of a core and a relative refractive index difference Δ1 of a first clad from a core area of the SMF according to the invention.

FIG. 15 is a graph showing a relationship between fitting coefficients κ11 and κ12 of the cutoff wavelength and the radius a1 of the core of the SMF according to the invention.

FIG. 16 is a graph showing a relationship between a fitting coefficient κ13 of the cutoff wavelength and the radius a1 of the core of the SMF according to the invention.

FIG. 17 is a diagram showing a refractive index distribution of the SMF according to the invention having a low-index layer in a second clad.

FIG. 18 is a diagram showing an example of an optical transmission system including the SMF according to the invention.

FIG. 19A is a sectional view of a first example of an optical fiber according to the invention including a plurality of cores.

FIG. 19B is a sectional view of a second example of an optical fiber according to the invention including a plurality of cores.

FIG. 19C is a sectional view of a third example of an optical fiber according to the invention including a single core.

FIG. 19D is a sectional view of a fourth example of an optical fiber according to the invention including a single core.

FIG. 20 is a diagram showing an example of an optical transmission system using the optical fiber according to the invention.

FIG. 21 is a graph showing a relationship between a radius of a core and a Rayleigh scattering loss which is applied to an electric field distribution by the core and a first clad when a mode field diameter is 11.5 µm in the optical fiber according to the invention.

FIG. 22 is a graph showing a relationship between a radius a1 and a Rayleigh scattering loss which is applied to an electric field distribution by a core and a first clad when a mode field diameter is 15.0 µm in the optical fiber according to the invention.

FIG. 23 is a graph showing a relationship between a Rayleigh scattering loss αR, an MFD, a group delay time of light which is transmitted in a core per unit length and the radius a1 in the optical fiber according to the invention based on Recommendation G.654.D.

FIG. 24 is a graph showing a relationship between a Rayleigh scattering loss αR, an MFD, a group delay time of light which is transmitted in a core per unit length and the radius a1 in the optical fiber according to the invention based on Recommendation G.654.E.

FIG. 25 is a diagram showing a refractive index distribution of an optical fiber according to the invention which has been manufactured by trial.

FIG. 26 is a diagram showing results of measurement and evaluation of optical characteristics in the optical fiber according to the invention which has been manufactured by trial, a conventional SMF and a CSF (various fibers).

FIG. 27 is a graph showing results of measurement of wavelength dependency of an MFD in the various fibers.

FIG. 28 is a graph showing results of measurement of wavelength dependency (a loss wavelength spectrum) of a transmission loss in the various fibers.

FIG. 29 is a graph showing plots with negative fourth power of wavelengths in the loss wavelength spectrum in the various fibers and fitting lines with the plots.

FIG. 30 is a graph showing measurement results of incident light power dependency of a phase shift amount based on a CW-SPM method in nonlinear coefficient evaluation of the various fibers and fitting lines of the measurement results.

FIG. 31 is a graph showing measurement results of the group delay time based on an impulse response approach in the various fibers.

FIG. 32 is a graph showing measurement results of wavelength dependency of the group delay time based on the impulse response approach in the various fibers.


[Description of Embodiments]



[0023] Hereinafter, embodiments of the invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the invention, but the invention is not limited to these embodiments. In the specification and the drawings, elements having the same functions will be referred to by the same reference signs and description thereof will not be repeated.

[0024] A silica-core fiber according to the related art includes a core (which may be referred to as a core area) formed of silica glass (SiO2) with a high purity of 99.8 wt% or more and has a conventional step index type refractive index distribution. As is well known, in a step index type refractive index distribution, a refractive index of a core that transmits light and a refractive index of a clad (which may be referred to as a clad area) are uniform. A relationship between optical characteristics (a radius a [µm] of a core formed of silica and a relative refractive index difference Δ[%] of a clad from the core) and a delay time in a silica -core fiber according to the related art is shown in FIG. 1. The refractive index of silica is 1.444377 at a wavelength of 1.55 µm.

[0025] Optical characteristics of a cutoff shifted fiber which is used mainly for a submarine optical cable are prescribed in Recommendation G.654.D of International Telecommunication Union-Telecommunication Standardization Sector (ITU-T). In Recommendation G.654.D, it is prescribed that a mode field diameter (MFD) of a cutoff shift fiber at a wavelength of 1.55 µm is equal to or greater than 11.5 µm and equal to or less than 15.0 µm. In Recommendation G.654.D, it is prescribed that a bending loss of the cutoff shift fiber at a wavelength of 1.625 µm and a bending radius of 30 mm is equal to or less than 2.0 dB/100 turns and that a cutoff wavelength of the cutoff shift fiber is equal to or less than 1.53 µm.

[0026] Optical characteristics of an optical fiber which is used for a land core network of long-distance communication are prescribed in Recommendation G.654.E of the ITU-T. In Recommendation G.654.E, it is prescribed that the MFD of the optical fiber for the land core network at a wavelength of 1.55 µm is equal to or greater than 11.5 µm and equal to or less than 12.5 µm. In Recommendation G.654.E, it is prescribed that a bending loss of the optical fiber for the land core network at a wavelength of 1.625 µm and a bending radius of 30 mm is equal to or less than 0.1 dB/100 turns and that the cutoff wavelength of the optical fiber for the land core network is equal to or less than 1.53 µm.

[0027] A solid line (A) in FIG. 1 indicates a relationship between a radius a [µm] of a core and a relative refractive index difference Δ[%] of a clad from the core of a silica-core fiber of which an MFD is 11.5 µm as prescribed in Recommendation G.654.D. A solid line (B) in FIG. 1 indicates a relationship between a radius a and a relative refractive index difference Δ of a silica-core fiber of which a banding loss αb is 2.0 dB/100 turns. A solid line (C) in FIG. 1 indicates a relationship between a radius a of a core and a relative refractive index difference Δ of a silica-core fiber of which a cutoff wavelength λc is 1.53 µm.

[0028] As indicated by an arrow in FIG. 1, an area in which conditions satisfying the prescriptions of Recommendation G.654.D, that is, conditions in which the MFD is equal to or greater than 11.5 µm, the bending loss αb is equal to or less than 2.0 dB/100 turns, and the cutoff wavelength λc is equal to or greater than 1.53 µm, is an area in which a part in which the relative refractive index difference Δ is lower than that of the solid line (A) (that is, a part in which the relative refractive index difference Δ is close to 0 and an upper part in the graph of FIG. 1), a part in which the radius a is greater than that of the solid line (B), and a part in which the radius a is less than that of the solid line (C) overlap. It can be seen that a silica-core fiber having a structure satisfying the hatched part in FIG. 1 satisfies all the conditions required for an optical fiber for a long-distance communication network and has a core with a radius of 4.4 µm or more.

[0029] Dotted lines in FIG. 1 indicate relationships between the radius a and the relative refractive index difference Δ in silica-core fibers in which a group delay time (GD) (which may be simply referred to as a delay time) is 4.861 µs/km, 4.87 µs/km, 4.873 µs/km, 4.876 µs/km and 4.879 µs/km. In a silica-core fiber satisfying prescriptions of Recommendation G.654.E, a bending loss which is lower than that of a silica-core fiber satisfying the prescriptions of Recommendation G.654.D is required. An area satisfying the prescriptions of Recommendation G.654.E changes toward a part in which the radius a is greater than that of the hatched part in FIG. 1.

[0030] As can be seen from a position relative to the hatched part in FIG. 1, a minimum group delay time which can be achieved in a cutoff shifted fiber is 4.876 µs/km. It can also be seen that the group delay time which is achieved decreases as the radius a decreases. When the radius a is equal to or less than 4.4 µm, the bending loss increases and it is difficult to reduce the group delay time. A reason why it is difficult to reduce the group delay time is that the most electric field distribution of transmission light is confined to the core and the speed of the transmission light is dominantly determined according to a refractive index of silica-glass with high purity, which is a material of the core. In a silica-core fiber with a refractive index distribution other than a step index type which has been developed to enable simultaneous enlargement of the core and reduction in loss, an electric field distribution is confined well to the core and thus a delay time thereof is equal to that of a step index type silica-core fiber or less than that of the step index type silica-core fiber. An example of the refractive index distribution other than a step index type is a W-shaped refractive index distribution.

[0031] A distribution of a refractive index n of a single-mode optical fiber (SMF) (an optical fiber) according to the invention is shown in FIG. 2. In the invention, an absolute value of a relative refractive index difference Δ is not used and the value is basically a negative value. An optical fiber according to the invention includes a core (r≤a1), a first clad (a1<r≤a2) and a second clad (r>a2) in a direction overlapping the radius r from the center of a section crossing a longitudinal direction. That is, the first clad is provided on an outer circumferential portion of the core, and the second clad is provided on an outer circumferential portion of the first clad. A refractive index n1 of the core is equivalent to a refractive index (a refractive index nSiO2=1.444377 at a wavelength of 1.55 µm) of silica glass with high purity or equal to or less than the refractive index of silica glass with high purity. A refractive index n2 of the first clad is less than the refractive index n1. A refractive index n3 of the second clad is less than the refractive index n2.

[0032] In an SMF in which a radius a1 is small, a delay time is expected to decrease and confinement of transmission light to the core is achieved by the second clad. Particularly, confinement of light of a first high-order mode having a characteristic in which an electric field distribution of light expands relatively has a small influence on transmission of light in the core. Accordingly, by designing the radius a1 and a relative refractive index difference Δ2 of the second clad from the first clad as will be described later, a cutoff wavelength of the SMF according to the invention is optimized. On the other hand, a fundamental mode is affected by a refractive index distribution in the vicinity of the center of a section of an optical fiber. In consideration thereof, the MFD or the bending loss of the SMF according to the invention are appropriately set by adjusting the radii a1 and a2.

(First embodiment)



[0033] A relationship between optical characteristics and a delay time in an SMF according to a first embodiment of the invention is shown in FIG. 3. A radius a1 of an SMF according to the first embodiment is set to 1.0 µm. In consideration of a relative refractive index difference Δ2 [%] and a radius a2, a cutoff wavelength of the SMF according to the first embodiment is set to be equal to or less than 1.53 µm. A solid line (D) in FIG. 3 represents a relationship between the radius a2 and a relative refractive index difference Δ1 of the SMF according to the first embodiment in which an MFD is equal to or greater than 11.5 µm. A solid line (E) in FIG. 3 represents a relationship between the radius a2 and the relative refractive index difference Δ1 of the SMF according to the first embodiment in which a bending loss is equal to or less than 2.0 dB/100 turns.

[0034] As can be seen from the result of comparison between FIGS. 1 and 3, the relative refractive index difference Δ1 of the SMF according to the first embodiment having a delay time (4.876 µs/km) equal to that of a cutoff shifted fiber according to the related art varies at a part in which the relative refractive index difference Δ1 increases with an increase of the radius a2 (that is, a part in which the relative refractive index difference Δ1 moves away from 0 and a lower part in the graph of FIG. 3). The radius a2 of the SMF according to the first embodiment having a delay time equal to that of the cutoff shift fiber is expressed by Equation (4) using functions κ0(a1), κ1(a1) and κ2(a1) of the relative refractive index difference Δ1 and the radius a1. In this specification, a function refers to a fitting function and arguments thereof may be omitted.
[Equation 4]



[0035] As described above, when the radius a1 of the SMF according to the first embodiment is 1.0 µm, the value of the function κ0(a1) is 2.00, the value of the function κ1(a1) is -1.42 and the value of the function κ2(a1) is 0.50. As shown in FIG. 3, in a boundary (a solid line (D) in FIG. 3) of the SMF in which the MFD is equal to or greater than 11.5 µm, the radius a2 when the relative refractive index difference Δ1 is 0.0 % is 5.56 µm.

[0036] With an increase of the radius a2, the influence of the first clad on the MFD decreases and the refractive index distribution of the SMF approaches that of the step index type. In this case, the MFD is determined according to only a structure (that is, parameters such as the radius a1 or the refractive index n1) of a core. Accordingly, the relative refractive index difference Δ1 converges on a value depending on the radius a1 with an increase of the radius a2. Accordingly, the radius a2 at which the MFD of the SMF according to the first embodiment is 11.5 µm is expressed by Equation (5) using a function of κ3(a1) of the relative refractive index difference Δ1 and the radius a1 and a function κ4(a1) in which the relative refractive index difference Δ1 converges with an increase of the radius a2.
[Equation 5]



[0037] When the radius a1 of the SMF according to the first embodiment is 1.0 µm, the value of the function κ3(a1) is -3.94 and the value of the function κ4(a1) is -0.88. As shown in FIG. 3, in a boundary (the solid line (E) in FIG. 3) of the SMF in which the bending loss is equal to or less than 2.0 dB/100 turns, when the relative refractive index difference Δ1 is 0.0%, the radius a2 is 7.68 µm.

[0038] When the refractive index distribution of the SMF approaches a simple step index type with an increase of the radius a2 as described above, the bending loss is also determined according to only the structure of the core. Since the relative refractive index difference Δ1 converges on the value determined according to the radius a1 with an increase of the radius a2, the radius a2 in which the bending loss SMF is 2.0 dB/100 turns is expressed by Equation (6) using functions of κ5(a1) and κ6(a1) of the relative refractive index difference Δ1 and the radius a1.
[Equation 6]



[0039] When the radius a1 of the SMF according to the first embodiment is 1.0 µm, the value of the function κ5(a1) is -1.08 and the value of the κ6(a1) is -0.82.

[0040] A hatched part in FIG. 3 represents a design area of the SMF according to the first embodiment which satisfies the prescriptions of Recommendation G.654.D and which can realize a delay time (4.876 µs/km) equal to or less than that of the cutoff shifted fiber.

[0041] As described above, a selection range of the radius a2 with respect to the relative refractive index difference Δ1 is limited by the conditions of the delay time, the MFD, and the bending loss. By employing the structure corresponding to the hatched part of FIG. 3, the SMF according to the first embodiment realizes optical characteristics equal to those of a cutoff shift fiber according to the related art and realizes a delay time equal to or less than the delay time of the cutoff shift fiber. As the relative refractive index difference Δ1 decreases, the delay time of the SMF according to the first embodiment decreases.

[0042] A relationship between the radius a2 and the relative refractive index difference Δ1 in the SMF according to the first embodiment of the invention in which the radius a1 is 1.5 µm is shown in FIG. 4. Solid lines (D) and (E) in FIGS. 4, 5 and FIGS. 9 to 11 which will be described later represent the same details as the solid lines (D) and (E) in FIG. 3. A boundary of a selection range of the radius a2 with respect to the relative refractive index difference Δ1 in the SMF in which the delay time is equal to or less than the delay time of a cutoff shifted fiber is expressed by Equation (4). A boundary of a selection range of the radius a2 with respect to the relative refractive index difference Δ1 in the SMF in which the MFD is equal to or greater than 11.5 µm is expressed by Equation (5). A boundary of a selection range of the radius a2 with respect to the relative refractive index difference Δ1 in the SMF in which the bending loss is equal to or less than 2.0 dB/100 turns is expressed by Equation (6). When the radius a1 is 1.5 µm, the value of the function κ0(a1) is 2.00, the value of function κ1(a1) is -0.86, the value of the function κ2(a1) is 0.43, the value of the function κ3(a1) is -3.94 and the value of the function κ4(a1) is -0.50. The hatched part in FIG. 4 represents a design area of the SMF according to the first embodiment satisfying the prescriptions of Recommendation G.654.D.

[0043] A relationship between the radius a2 and the relative refractive index difference Δ1 of the first clad in the SMF according to the first embodiment of the invention in which the radius a1 of the core area is 2.0 µm is shown in FIG. 5. A boundary of a structure in which the delay time is equal to or less than the delay time of the cutoff shift fiber is expressed by Equation (4) described above. A boundary of a structure in which the MFD is equal to or greater than 11.5 µm is expressed by Equation (5) described above. A boundary of a structure in which the bending loss is equal to or less than 2.0 dB/100 turns is expressed by Equation (6). When the radius a1 of the core area is 2.0 µm, the value of the function κ0(a1) is 2.00, the value of function κ1(a1) is -0.50, the value of the function κ2(a1) is 0.38, the value of the function κ3(a1) is -3.94 and the value of the function κ4(a1) is -0.36. The hatched part in FIG. 5 represents a design area of the SMF satisfying the prescriptions of Recommendation G.654.D of the ITU-T by the single-mode optical fiber according to the invention.

[0044] As shown in FIGS. 3 to 5, the relative relationship between the relative refractive index difference Δ1 and the radius a2 varies depending on the radius a1. The design area of the SMF according to the first embodiment is specified by expressing the coefficients of the functions representing the boundaries surrounding the hatched parts in FIGS. 3 to 5 as the functions of the radius a1. The invention is based on the premise that the radius a1 is equal to or greater than 1.0 µm in consideration of conditions for easily manufacturing the optical fiber.

[0045] In the boundary (that is, the boundary expressed by Equation (1)) of the structure in the SMF in which the delay time equal to or less than the delay time of the cutoff shift fiber is achieved, the value of the function κ0(a1) is 2.00 regardless of the radius a1 of the core area. A relationship between the values of the functions κ1(a1) and κ2(a1) and the radius a1 is shown in FIG. 6. As shown in FIG. 6, the function κ1(a1) is expressed by Equation (7) and the function κ2(a1) is expressed by Equation (8).
[Equation 7]


[Equation 8]



[0046]  By expressing the boundary of the structure in which the delay time is 4.876 µs/km using the radius a1, the relative refractive index difference Δ1 and the radius a2, the design area (that is, an area representing optical characteristics) of the SMF according to the first embodiment is expressed by Equation (1).
[Equation 9]



[0047] In the boundary (the boundary expressed by Equation (5)) of the structure in which the MFD is equal to or greater than 11.5 µm, the value of the function κ3(a1) is -3.94 regardless of the radius a1. A relationship between the value of the function κ4(a1) and the radius a1 is shown in FIG. 7. As shown in FIG. 7, the function κ4(a1) is expressed by Equation (10).
[Equation 10]



[0048] By expressing the boundary of the structure in which the MFD is 11.5 µm using the radius a1, the relative refractive index difference Δ1 and the radius a2, the design area of the SMF according to the first embodiment is expressed by Equation (11).
[Equation 11]



[0049] Regarding the boundary of the structure (the boundary expressed by Equation (6)) in which the bending loss is equal to or less than 2.0 dB/100 turns, a relationship between the values of the functions κ5(a1) and κ6(a1) and the radius a1 is shown in FIG. 8. As shown in FIG. 8, the function κ5(a1) is expressed by Equation (12) and the function κ6(a1) is expressed by Equation (13).
[Equation 12]


[Equation 13]



[0050] By expressing the boundary of the structure in which the bending loss is 2.0 dB/100 turns using the radius a1, the relative refractive index difference Δ1 and the radius a2, the design area of the SMF according to the first embodiment is expressed by Equation (9).
[Equation 14]



[0051] Based on the above description, the SMF according to the first embodiment includes a core of which the radius a1 is equal to or greater than 1.0 µm and equal to or less than 4.3 µm, and satisfies Equation (1) described above and Equation (2) described below as the relationship between the radius a2 and the relative refractive index difference Δ1.
[Equation 15]



[0052]  When the radius a1 and the radius a2 with respect to the relative refractive index difference Δ1 satisfy the above-mentioned conditions, the SMF according to the first embodiment realizes the same optical characteristics as those of the cutoff shifted fiber and realizes the delay time which is equal to or less than the delay time of the cutoff shifted fiber.

[0053] Table 1 shows an example of design parameters of the SMF according to the first embodiment. In the design shown in Table 1, the same optical characteristics as those of the cutoff shift fiber are achieved and a reduction in the delay time of 0.05 µs/km is achieved. By using the SMF designed as described above, a reduction in delay time of about 1 ms is achieved in a long-distance network of which the network length amounts to about 10000 km such as a submarine optical cable crossing the Pacific Ocean. The design parameters shown in Table 1 are an example satisfying the above-mentioned conditions and the same advantageous effects as in the SMF having the design parameters shown in Table 1 are achieved by the SMF having the structure satisfying the above-mentioned conditions.
[Table 1]
Structure parameterOptical characteristics
a1 1.0 µm MFD@λ=1.55 µm 11.8 µm
a2 8.5 µm λc 1.53 µm
Δ1 -0.70% αb@1.625 µm 1.2 dB/100 turns
    R=30 mm  
Δ2 -0.17% GD@1.55 µm 4.847 µs/km

(Second embodiment)



[0054] A relationship between the radius a2 and the relative refractive index difference Δ1 when the radius a1 is 1.0 µm and a design area of an SMF according to a second embodiment of the invention satisfies Recommendation G.654.E is shown in FIG. 9. In consideration of the relative refractive index difference Δ2 and the radius a2, the cutoff wavelength of the SMF according to the second embodiment is set to be equal to or less than 1.53 µm. A solid line (D) in FIG. 9 represents a boundary of a structure in which the MFD is 11.5 µm similarly to the first embodiment. A solid line (E) in FIG. 9 represents a boundary of a structure in which the bending loss is equal to or less than 0.1 dB/100 turns similarly to the first embodiment. A hatched part in FIG. 9 represents a design area in which all the above-mentioned conditions with respect to the cutoff wavelength, the MFD, and the bending loss are satisfied and the delay time (equal to or less than 4.876 µs/km) which is equal to or less than the delay time of the cutoff shifted fiber can be realized.

[0055] When the radius a1 is 1.0 µm, the boundary in which the delay time is equal to or less than the delay time of the cutoff shifted fiber is expressed by Equation (4) similarly to the first embodiment. The values of the functions κ0(a1), κ1(a1) and κ2(a1) are equal to the values of the functions κ0(a1), κ1(a1), and κ2(a1) described above in the first embodiment. The boundary in which the MFD is equal to or greater than 11.5 µm is expressed by Equation (5). The values of the functions κ3(a1) and κ4(a1) are equal to the values of the functions κ3(a1) and κ4(a1) described above in the first embodiment.

[0056] In the boundary of the structure in which the bending loss is equal to or less than 0.1 dB/100 turns, when the relative refractive index difference Δ1 is 0.0%, the radius a2 is 6.88 µm. As the radius a2 increases, the influence of the first clad on the bending loss decreases and the bending loss is determined according to only the structure of the core area. With the increase of the radius a2, the relative refractive index difference Δ1 converges on a value which is determined according to the radius a1 of the core area. Accordingly, the radius a2 at which the bending loss is 0.1 dB/100 turns in the SMF according to the invention is expressed by Equation (14) using the functions κ7(a1) and κ8(a1) of the relative refractive index difference Δ1 and the radius a1.
[Equation 16]



[0057] When the radius a1 is 1.0 µm, the value of the function κ7(a1) is -3.74 and the value of the function κ8(a1) is -1.36. The design area in which the relative refractive index difference Δ1 is high is limited by an increase of the delay time. Design ranges of the MFD and the radius a2 are limited by required conditions of the bending loss. As shown in FIG. 9, the delay time decreases as the relative refractive index difference Δ1 decreases.

[0058] A relationship between the radius a2 and the relative refractive index difference Δ1 when the radius a1 is 1.5 µm and a design area of the SMF according to the second embodiment satisfying Recommendation G.654.E is shown in FIG. 10. Even when the radius a1 is 1.5 µm, the boundary in which the delay time is equal to or less than the delay time of the cutoff shift fiber is expressed by Equation (4). The values of the functions κ0(a1), κ1(a1) and κ2(a1) are equal to the values of the functions κ0(a1), κ1(a1), and κ2(a1) described above in the first embodiment. The boundary in which the MFD is equal to or greater than 11.5 µm is expressed by Equation (5). The values of the functions κ3(a1) and κ4(a1) are equal to the values of the functions κ3(a1) and κ4(a1) described above in the first embodiment.

[0059] The boundary of the structure in which the bending loss is equal to or less than 0.1 dB/100 turns is expressed by Equation (14). Since the radius a1 is 1.5 µm, the value of the function κ7(a1) is -5.60 and the value of the function κ8(a1) is -0.87. The hatched part in FIG. 10 represents a design area in which all the above-mentioned conditions with respect to the cutoff wavelength, the MFD and the bending loss are satisfied and the delay time equal to or less than the delay time of the cutoff shifted fiber is achieved.

[0060]  A relationship between the radius a2 and the relative refractive index difference Δ1 when the radius a1 is 2.0 µm and a design area of the SMF according to the second embodiment satisfying Recommendation G.654.E is shown in FIG. 11. Even when the radius a1 is 2.0 µm, the boundary in which the delay time is equal to or less than the delay time of the cutoff shifted fiber is expressed by Equation (4). The values of the functions κ0(a1), κ1(a1) and κ2(a1) are equal to the values of the functions κ0(a1), κ1(a1), and κ2(a1) described above in the first embodiment. The boundary in which the MFD is equal to or greater than 11.5 µm is expressed by Equation (5). The values of the functions κ3(a1) and κ4(a1) are equal to the values of the functions κ3(a1) and κ4(a1) described above in the first embodiment.

[0061] The boundary of the structure in which the bending loss is equal to or less than 0.1 dB/100 turns is expressed by Equation (14). Since the radius a1 is 2.0 µm, the value of the function κ7(a1) is -6.25 and the value of the function κ8(a1) is -0.61. The hatched part in FIG. 11 represents a design area in which all the above-mentioned conditions associated with the cutoff wavelength, the MFD and the bending loss are satisfied and the delay time equal to or less than the delay time of the cutoff shift fiber is achieved.

[0062] As shown in FIGS. 9 to 11, the relative relationship between the relative refractive index difference Δ1 and the radius a2 depends on the radius a1. The boundaries in which the MFD is equal to or greater than 11.5 µm and which are shown in FIGS. 9 to 11 are the same as the boundaries which are shown in FIGS. 3 to 5. The structure of the SMF in which the MFD is equal to or greater than 11.5 µm is expressed by Equation (9). The boundaries in which the delay time is equal to or less than the delay time of the cutoff shift fiber and which are shown in FIGS. 9 to 11 are the same as the boundaries which are shown in FIGS. 3 to 5. The structure of the SMF in which the delay time is equal to or less than the delay time of the cutoff shift fiber is expressed by Equations (9) and (11).

[0063] The boundary of the structure in which the bending loss is equal to or less than 0.1 dB/100 turns is expressed by Equation (14). Variations of the function κ7 (the left axis in FIG. 12) and the function κ8 (the right axis in FIG. 12) with respect to the radius a1 are shown in FIG. 12. As shown in FIG. 12, the function κ7(a1) is expressed by Equation (15) and the function κ8(a1) is expressed by Equation (16).
[Equation 17]


[Equation 18]



[0064] As described above, the design area of the radius a2 of the SMF according to the second embodiment in which the bending loss is equal to or less than 0.1 dB/100 turns is expressed by Equation (17).
[Equation 19]



[0065] As described above, the SMF according to the second embodiment includes a core in which the radius a1 is equal to or greater than 1.0 µm and equal to or less than 4.3 µm and includes a design area in which Equations (1) and (2) are satisfied as the relationship between the radius a2 and the relative refractive index difference Δ1.
[Equation 20]


[Equation 21]



[0066] When the radius a1 and the radius a2 with respect to the relative refractive index difference Δ1 satisfy the above-mentioned conditions, the SMF according to the second embodiment achieves the same optical characteristics as the cutoff shift fiber and achieves a delay time equal to or less than the delay time of the cutoff shift fiber.

[0067] In FIG. 13, when the radius a1 of the core area is 1.0 µm and when the radius a1 of the core area is 2.0 µm, boundaries in which the cutoff wavelength is equal to or less than 1.53 µm with respect to the radius a2 and the relative refractive index difference Δ2 when the relative refractive index difference Δ1 is set to -0.1% and the relative refractive index difference Δ1 is set to -0.8% are shown. In a part in which the relative refractive index difference Δ2 is lower than the boundaries (that is a part in which the relative refractive index difference Δ2 approaches 0 and an upper part in the graph which is shown in FIG. 13), the above-mentioned required conditions with respect to the cutoff wavelength, the MFD and the bending loss are satisfied. As shown in FIG. 9, the boundary in which the cutoff wavelength is equal to or less than 1.53 µm varies depending on the radius a1 of the core and the relative refractive index difference Δ1. Accordingly, the relative refractive index difference Δ2 in which the cutoff wavelength is 1.53 µm is expressed by the radius a1, the relative refractive index difference Δ1 and the radius a2.

[0068] When the radius a2 increases, the boundary in which the cutoff wavelength is 1.53 µm varies to a part in which the relative refractive index difference Δ2 is low. When the radius a2 increases, an influence of the structure of the core on the cutoff wavelength decreases and the relative refractive index difference Δ2 converges regardless of the radius a1 and the relative refractive index difference Δ1. The converging value of the relative refractive index difference Δ2 is -0.033%. When the converging value of the relative refractive index difference Δ2 and the functions κ9(a1, Δ1) and κ10(a1, Δ1) with the radius a1 and the relative refractive index difference Δ1 as variables, the relative refractive index difference Δ2 in which the cutoff wavelength is 1.53 µm is expressed by Equation (20).
[Equation 20]



[0069] Variations of the function κ9 (the left axis in FIG. 14) and the function κ10 (the right axis in FIG. 14) with respect to the relative refractive index difference Δ1 when the radius a1 is set to 1.0 µm, 2.0 µm and 4.3 µm are shown in FIG. 14. When the relative refractive index difference Δ1 is 0.0% as described above, the relationships between the relative refractive index difference Δ1 and the functions κ9 and κ10 are expressed by Equations (21) and (22) using the functions κ11(a1), κ12(a1) and κ13(a1) having the radius a1 as a variable.
[Equation 23]


[Equation 24]



[0070] When the radius a1 is 4.3 µm and the relative refractive index difference Δ1 is equal to or less than -0.531%, the cutoff wavelength is equal to or greater than 1.53 µm regardless of the radius a2 and the relative refractive index difference Δ2. Accordingly, when the radius a1 is 4.3 µm, Equations (21) and (22) are applied to a structure of the SMF in which the relative refractive index difference Δ1 is equal to or higher than -0.531%.

[0071]  Variations of the function κ11 (the left axis in FIG. 15) and the function κ12 (the right axis in FIG. 15) with respect to the radius a1 are shown in FIG. 15. When the radius a1 is 0.0 µm, both the functions κ11 and κ12 lose dependency on the relative refractive index difference Δ1 and thus the functions κ11 and κ12 are 0 (zero). The curve which is shown as a solid line in FIG. 15 is expressed by Equation (23). The curve which is shown as a dotted line in FIG. 15 is expressed by Equation (24).
[Equation 25]


[Equation 26]



[0072] A variation of the function κ13 with respect to the radius a1 is shown in FIG. 16. When the radius a1 is 0.0 µm, the function κ13 loses dependency on the relative refractive index difference Δ1 and thus the function κ13 is 0 (zero). The curve which is shown as a solid line in FIG. 16 is expressed by Equation (25).
[Equation 27]



[0073] Based on the above description, by expressing the boundary of the structure of the SMF according to the second embodiment in which the cutoff wavelength is equal to or less than 1.53 µm using the radius a1, the relative refractive index difference Δ1 and the radius a2, the relative refractive index difference Δ2 is expressed by Equation (3).
[Equation 28]



[0074] The above-mentioned SMF according to the second embodiment is designed such that the radii a1 and a2 and the relative refractive index differences Δ1 and Δ2 satisfy the above-mentioned suitable conditions.

[0075] In the invention, by performing design such that the radius a1 is in a range of equal to or greater than 1.0 µm and equal to or less than 4.3 µm, suitable design areas of the radius a2 and the relative refractive index difference Δ1 which are expressed by Equations (1) and (2) are specified in the SMF based on Recommendation G.654.D. Similarly, in the SMF based on Recommendation G.654.E, suitable design areas of the radius a2 and the relative refractive index difference Δ1 which are expressed by Equations (18) and (19) are specified. In the suitable areas, the achievable delay time of the SMF is roughly determined according to the relative refractive index difference Δ1 and increases as the relative refractive index difference Δ1 decreases. By appropriately selecting the radii a1 and a2 and the relative refractive index difference Δ1 in the above-mentioned suitable areas, the cutoff wavelength satisfies the conditions which are required to apply the SMF to a long-distance communication network and the relative refractive index difference Δ2 is determined according to Equation (3).

[0076] The second clad of the SMF according to the second embodiment is not limited to the second clad with the refractive index distribution which is shown in FIG. 2. An example of a refractive index distribution in which a low-index area in which the refractive index is defined as n4 and a relative refractive index difference from the second clad is defined as Δ3 is provided in an area with a width w from a radius a3 to a radius a4 of the second clad which is shown in FIG. 2 is shown in FIG. 17. That is, the radius a3 is greater than the radius a2 and the refractive index n4 is lower than the refractive index n3. It is known that the low-index area of the second clad relaxes trade-off which is caused in the optical characteristics such as the MFD or the bending loss. With the SMF according to the second embodiment, the design area is expected to be suitably enlarged. By forming a hole instead of the low-index area in the second clad, the same advantageous effects as in the SMF including the low-index area in the second clad are obtained.

(Third embodiment)



[0077] A configuration of an optical transmission system 100 according to the invention is shown in FIG. 18. The optical transmission system 100 includes a transmitter 102, the optical fiber (SMF) 104 according to the invention and a receiver 106. The transmitter 102 and the optical fiber 104 are connected to each other via a connector. The optical fiber 104 and the receiver 106 are connected to each other via a connector. Since the optical transmission system 100 includes the optical fiber 104, a transmission delay of the optical transmission system 100 decreases. Accordingly, the optical transmission system 100 can respond to a demand for a reduction in delay time between the transmitter 102 and the receiver 106.

(Fourth embodiment)



[0078] Structures of optical fibers 51, 52, 53 and 54 which are single-conductor optical fibers and in which a plurality of cores are disposed in a sectional view crossing the longitudinal direction are shown in FIGS. 19A, 19B, 19C and 19D. One or more cores of the optical fibers 51, 52, 53 and 54 are low-delay cores which satisfy the conditions described above in the first embodiment or the second embodiment and which can reduce a delay time of the optical fiber.

[0079] A sectional surface of an optical fiber in which only four low-delay cores 60A are disposed in a second clad 66 (that is, the second clad in the first embodiment and the second embodiment) with a diameter of 125 µm is shown in FIG. 19A. Each low-delay core 60A includes a core (a first core) 62 at the center in a sectional view and a first clad 64 that is disposed on an outer circumferential portion of the core 62. A sectional surface of an optical fiber in which a core (a second core) 60B is disposed at the center of a second clad 66 with an outer diameter of 125 µm and four low-delay cores 60A are disposed on a concentric circle centered on the core 60B is shown in FIG. 19B. A sectional surface of an optical fiber including a second clad 66 with a diameter of 175 µm and a coating 70 with a diameter of 250 µm that is disposed on an outer circumferential portion of the second clad 66 is shown in FIG. 19C. In the SMF which is shown in FIG. 19C, a low-delay core 60A is disposed at the center of the second clad 66 and cores (third cores) 60C are disposed on a concentric circle centered on the low-delay core 60A. A sectional surface of an optical fiber in which a low-delay core 60A is disposed at the center of a second clad 66 with a diameter of 250 µm which is thought to be a maximum diameter of the clad with which reliability can be secured and a plurality of cores (fourth cores) 60D are disposed in a hexagonal shape with intervals interposed therebetween (that is, packed most closely) in the second clad outside the low-delay core 60A is shown in FIG. 19D.

[0080] With the optical transmission system including the optical fibers 51, 52, 53 and 54 which are shown in FIGS. 19A to 19D, the optical communication according to the related art and optical communication with a reduced delay time can be simultaneously and suitably realized.

(Fifth embodiment)



[0081] An optical transmission system 200 including the optical fiber according to the invention is shown in FIG. 20. The optical transmission system 200 includes an SMF (an optical fiber) 150, a plurality of transmitters 172 and a plurality of receivers 174. The SMF 150 may be one of the SMFs according to the first to third embodiments and the optical fibers 51, 52, 53 and 54 according to the fourth embodiment. At least one of the plurality of transmitters 172 is coupled to one end of the SMF 150 via a fan-in device. At least one of the plurality of receivers 174 is coupled to the other end of the SMF 150 via a fan-in device. With the optical transmission system 200, the optical communication according to the related art and the optical communication with a decreased transmission delay can be simultaneously realized. Accordingly, the optical transmission system 200 is capable of flexibly responding to a demand for a reduction of delay time in a transmission line.

(Preferable design conditions of the optical fiber)



[0082] A result of numerical calculation of the relationship between the radius a1 and the relative refractive index difference Δ1 with respect to a Rayleigh scattering loss αR due to an influence of the first clad on an electric field distribution of light which is transmitted by the core of the SMF when the MFD is 11.5 µm is shown in FIG. 21. A result of numerical calculation of the relationship between the radius a1 and the relative refractive index difference Δ1 with respect to the Rayleigh scattering loss αR due to an influence of the first clad on the electric field distribution of light which is transmitted by the core of the SMF when the MFD is 15.0 µm is shown in FIG. 22. As shown in FIGS. 21 and 22, it can be seen that the Rayleigh scattering loss αR of the SMF increases by decreasing the radius a1 and decreasing the relative refractive index difference Δ1.

[0083] A relationship between the Rayleigh scattering loss αR, the MFD, the group delay time of light which is transmitted by the core per unit length, and the radius a1 in the SMF based on Recommendation G.654.D based on the results of numerical calculation which are shown in FIGS. 21 and 22 is shown in FIG. 23. A relationship between the Rayleigh scattering loss αR, the MFD, the group delay time of light which is transmitted by the core per unit length, and the radius a1 in the SMF based on Recommendation G.654.E based on the results of numerical calculation which are shown in FIGS. 21 and 22 is shown in FIG. 24.

[0084] The Rayleigh scattering loss αR of a conventional SMF is considered to be about 0.17 dB/km. In consideration that the group delay time of the cutoff shift fiber (CSF) is about 4.877 µs/km, the optical characteristics based on Recommendation G.654.D and Recommendation G.654.E and low loss and low delay time can be simultaneously realized and a more suitable design area of the optical fiber according to the invention can be provided, by setting the radius a1 and the relative refractive index difference Δ1 corresponding to the hatched parts in FIGS. 23 and 24. The hatched parts in FIGS. 23 and 24 represent a range in which the boundary in which the MFD is equal to or greater than 11.5 µm and the boundary in which the Rayleigh scattering loss αR is 0.17 dB/km overlap each other toward a part in which the relative refractive index difference Δ1 is lower than the boundaries (that is, a part in which the relative refractive index difference Δ1 approaches 0 and an upper part in the graph which is shown in FIG. 13) such that the recommendations are satisfied. That is, in the above-mentioned embodiments, by additionally including conditions in which the Rayleigh scattering loss αR is equal to or less than 0.17 dB/km, the optical characteristics based on Recommendation G.654.D and Recommendation G.654.E and the low loss and low delay time can be simultaneously realized and a more suitable design area can be provided.

(Examples)



[0085] An SMF (an optical fiber) according to the invention was manufactured by trial in consideration of the design area, the relative relationship between parameters and the suitable conditions which have been described above in the embodiments. A refractive index distribution of the trial-manufactured SMF is shown in FIG. 25. Evaluation of characteristics of a conventional SMF and a CSF along with the trial-manufactured SMF according to the invention was carried out.

[0086] A wavelength serving in measuring an MFD, an effective sectional area Aeff, a bending loss per winding with a bending radius of 15 mm, a transmission loss, a Rayleigh scattering loss, a chromatic dispersion and a nonlinear coefficient, measurement results of each optical property and measurement methods used for various measurements in the optical fiber according to the invention which was manufactured by trial (hereinafter also referred to as a trial-manufactured optical fiber) are shown in FIG. 26. For the purpose of comparison, the same measurement as in the trial-manufactured optical fiber according to the invention (a "trial-manufactured optical fiber" in FIG. 26) was performed on the conventional SMF and the cutoff shift fiber (CSF) based on Recommendation G.654.D.

[0087] A structure of an optical fiber which was manufactured by trial in a design area of structure parameters based on Recommendation G.654.D in which characteristics of a cutoff shift fiber specified based on results of numerical calculation are prescribed is shown in FIG. 25. The trial-manufactured optical fiber includes a core which is formed of pure silica glass. The radius of the core is 1.0 µm and the radius of the first clad is 6.4 µm. The relative refractive index difference between the core and the first clad is -0.38%, and the relative refractive index difference between the first clad and the second clad is -0.24%.

[0088] As shown in FIG. 26, as designed, the trial-manufactured optical fiber has the equivalent MFD, the equivalent Aeff, the equivalent cutoff wavelength, and the equivalent bending loss as in the CSF. The transmission loss of the trial-manufactured optical fiber is as low as in the conventional SMF. The wavelength dispersion and the nonlinear coefficient of the trial-manufactured optical fiber have the equivalent values as in the CSF.

[0089] Results of measurement of wavelength characteristics of the MFD of various fibers are shown in FIG. 27. In the trial-manufactured optical fiber, the MFD is enlarged to the similar extent to that of the CSF in the whole band from C-band to L-band. The tendency of enlargement of the MFD matches the results of numerical calculation represented by solid lines.

[0090] Results of measurement of a loss wavelength spectrum of various fibers are shown in FIG. 28. The shape of the loss wavelength spectrum of the trial-manufactured optical fiber is the similar to the shapes of the conventional SMF and the CSF. The transmission loss of the trial-manufactured optical fiber is equivalent to that of the conventional SMF in the whole range of measurement wavelengths.

[0091] Results of a plot with [wavelength λ]-4 in the loss wavelength spectrum which is shown in FIG. 28 are shown in FIG. 29. When the Rayleigh scattering loss at the wavelength of 1.55 µm is analyzed based on the slopes of fitting lines in the range of from 0.52 µm-4 to 0.80 µm-4 of λ-4, the Rayleigh scattering loss is 0.161 dB/km in the trial-manufactured optical fiber, is 0.166 dB/km in the conventional SMF, and is 0.146 dB/km in the CSF. From these results, it is confirmed that the Rayleigh scattering loss of the trial-manufactured optical fiber is equivalent to that of the conventional SMF.

[0092] Measurement results of incident light power dependency of a phase shift based on a CW-SPM method, which correspond a nonlinear coefficient evaluation of various fibers are shown in FIG. 30. In the CW-SPM, the nonlinear coefficient (n2/Aeff) can be analyzed from the slope of the incident light power dependency of a phase shift amount using Equation (26).



[0093] In Equation (26), ΦSPM represents a phase shift amount, λ represents a wavelength, Leff represents an effective length of various fibers, and Pin represents power of incident light on various fibers. The nonlinear coefficient which was analyzed based on the slope of the fitting line represented by a dotted line is 1.79×10-10 /W in the trial-manufactured optical fiber, 2.95×10-10 /W in the conventional SMF, and 1.90×10-10 /W in the CSF. From these results, it is confirmed that the trial-manufactured optical fiber has substantially the equivalent low nonlinearity to that in the CSF.

[0094] Measurement results of the group delay time based on an impulse response approach in various fibers are shown in FIG. 31. The horizontal axis of the graph which is shown in FIG. 31 represents the measured group delay time in terms of a group delay time per unit length. In measurement, a pulse width of a pulse which is emitted from a pulse light source was modulated in 100 ps and the length of the optical fiber to be measured was set to 350 m. In the CSF, the group delay time was reduced by 0.018 µs/km in comparison with the conventional SMF. On the other hand, in the trial-manufactured optical fiber, it is confirmed that the group delay time was reduced additionally by 0.016 µs/km in comparison with that of the CSF.

[0095] Measurement results of wavelength dependency in the range from C-band to L-band of the measurement result of the group delay time which is shown in FIG. 31 are shown in FIG. 32. It is confirmed that the group delay time of the trial-manufactured optical fiber was reduced by about 0.016 µs/km from that of the CSF in the whole range from C-band to L-band.

[0096] The conventional SMF has a three-layered structure including a core, a first clad, and a second clad, and the following parameters were used for the above-mentioned numerical calculation for the conventional SMF.
  • Radius of core: 3.5 µm
  • Radius of first clad: 6.5 µm
  • Relative refractive index difference between core and second clad: 0.38%
  • Radius of second clad: 62.5 µm
  • Relative refractive index difference between first clad and second clad: 0.05%
  • Refractive index of second clad (wavelength of 1.55 µm): 1.444377


[0097] The CSF has a three-layered structure including a core, a first clad, and a second clad and the following parameters were used for the above-mentioned numerical calculation for the CSF.
  • Radius of core: 6 µm
  • Refractive index of core (wavelength of 1.55 µm): 1.444377
  • Radius of first clad: 25 µm
  • Relative refractive index difference between first clad and core: -0.35%
  • Radius of second clad: 62.5 µm
  • Relative refractive index difference between second clad and the core: -0.25%

[Industrial Applicability]



[0098]  The invention is capable of being widely applied to optical fibers in areas of application in which a reduction of transmission delay is mainly required such as optical fibers for a long-distance communication network. The invention is capable of being applied to communication between terminals in an optical communication system.

[Reference Signs List]



[0099] 
60A:
Low-delay core
60B:
Core (second core)
60C:
Core (third core)
60D:
Core (fourth core)
62:
Core (first core)
150:
SMF (optical fiber)
172:
Transmitter
174:
Receiver
a1, a2, a3:
Radius
Δ1, Δ2, Δ3:
Relative refractive index difference



Claims

1. An optical fiber, comprising:

a core;

a first clad that is provided on an outer circumferential portion of the core and has a refractive index lower than that of the core; and

a second clad that is provided on an outer circumferential portion of the first clad and has a refractive index lower than that of the first clad,

wherein a mode field diameter at a wavelength of 1.55 µm is equal to or greater than 11.5 µm,

wherein a cutoff wavelength is equal to or less than 1.53 µm,

wherein a bending loss at a bending radius of 30 mm and a wavelength of 1.625 µm is equal to or less than 2.0 dB/100 turns, and

wherein a delay time of transmission light per unit length at a wavelength of 1.55 µm is equal to or less than 4.876 µs/km.


 
2. The optical fiber according to Claim 1, wherein a radius of the core is equal to or less than 1.0 µm and equal to or greater than 4.3 µm, and
wherein a radius of the first clad satisfies Equations (1) and (2):
[Equation 1]

and
[Equation 2]

where a1 in Equations (1) and (2) represents the radius [µm] of the core, a2 represents the radius [µm] of the first clad, and Δ1 represents a relative refractive index difference [%] of the first clad from the core.
 
3. The optical fiber according to Claim 2, wherein the relative refractive index difference of the second clad from the first clad satisfies Equation (3):
[Equation 3]


 
4. The optical fiber according to any one of Claims 1 to 3, wherein, in a sectional view, a plurality of low-delay cores including the core as a first core and the first clad provided on an outer circumferential portion of the first core are disposed on concentric circles at the center of the second clad.
 
5. The optical fiber according to Claim 4, wherein the core is disposed as a second core at the center of the second clad.
 
6. The optical fiber according to any one of Claims 1 to 3, wherein, in a sectional view, a low-delay core including the core as a first core and the first clad provided on an outer circumferential portion of the first core is disposed at the center of the second clad, and
wherein the core is disposed as a third core on a concentric circle at the center of the low-delay core.
 
7. The optical fiber according to any one of Claims 1 to 3, wherein, in a sectional view, a low-delay core including the core as a first core and the first clad provided on an outer circumferential portion of the first core is disposed at the center of the second clad, and
wherein the cores are packed most closely as fourth cores around the low-delay core.
 
8. An optical transmission system, comprising:

the optical fiber according to any one of Claims 1 to 7;

a transmitter that is connected to one end of the optical fiber; and

a receiver that is connected to the other end of the optical fiber.


 




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Cited references

REFERENCES CITED IN THE DESCRIPTION



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Patent documents cited in the description




Non-patent literature cited in the description