The present disclosure relates to an optical fiber for optical communication.
In a current long-distance transmission network, a dramatic increase in capacity is achieved by an optical coherent communication technology. In the optical coherent communication technology, information is allocated to a phase state of light. A phase state of signal light changes due to wavelength dispersion of an optical fiber constituting a transmission path and phase fluctuation of a signal light source, and thus signal quality deteriorates. Accordingly, in the optical coherent communication, digital signal processing (DSP) for removing phase noise in a receiver is indispensable. Although sufficient signal quality is currently ensured by the DSP, there is a problem that the calculation cost in the device increases.
In order to solve this problem, Non Patent Literature 1 proposes a system in which carrier phase estimation is performed using one channel in a transmission channel as a Master channel, and phase correction is performed by diverting an estimation result to other transmission channels. This transmission method is called Master-Slave CPE (MS-CPE). The MS-CPE can reduce signal processing cost for phase estimation by a Slave channel.
On the other hand, in recent optical communication, there is an increasing demand for reduction of communication delay time. According to Non Patent Literature 2, it is reported that the DSP processing time in the optical coherent communication technology is about 1 has. Reduction of delay time of the DSP processing is also one of solutions to the request for reduction of the communication delay time. Therefore, Non Patent Literature 3 describes a possibility of reducing the DSP processing delay time in the Slave channel by applying a transmission channel with a reduced group delay time to the Master channel of the MS-CPE.
Zhou, S. L. Woodward, R. Isaac, B. Zhu, T. F. Taunay, M. Fishteyn, J. M. Fini and M. F. Yan, “Joint Digital Signal Processing Receivers for Spatial Superchannels,” Photon. Technol. Lett., vol. 24, No. 21, 1957
The MS-CPE of Non Patent Literature 3 uses, as an optical fiber, a multicore fiber of a core and a step-type clad in which a refractive index distribution is reduced to about 1 μm for the transmission channel with the reduced group delay time. However, in the above-described core in which the refractive index distribution is reduced, the refractive index distribution is easily deformed due to a spinning tension or the like at the time of manufacturing the optical fiber (it is difficult to obtain the desired effect of MS-CPE), and there is a problem in manufacturability.
Therefore, in order to solve the above problems, an object of the present invention is to provide an optical fiber with improved manufacturability for MS-CPE.
In order to achieve the above object, the optical fiber according to the present invention uses a low delay core using a typical refractive index distribution employed in a general-purpose optical fiber as a Master channel.
Specifically, a first optical fiber according to the present invention is an optical fiber included in an optical communication system of an MS-CPE transmission method, characterized in that
[Mathematical Expression C1]
−1.05+0.37a−0.05a2<Δ<−1.02+0.26a−0.02a2
and
Δ>4.23+7.37Δτ+3.81Δτ2−(3.15+4.77Δτ+2.47Δτ2)a+(0.52+0.77Δτ+0.40Δτ2)a2 (C1)
Further, a second optical fiber according to the present invention is an optical fiber included in an optical communication system of an MS-CPE transmission method, characterized in that the optical fiber includes a core having a radius a1 (μm) for a master channel, a low refractive index layer surrounding a periphery of the core and having a relative refractive index difference Δ1 (%) with respect to the core, and a clad having a relative refractive index difference Δ2 (%) with respect to the core, and that the core, the low refractive index layer, and the clad have a W-type refractive index distribution structure and satisfy Mathematical Expression C3,
[Mathematical Expression C3]
0.23+0.66MFD−3.33(μ2/Δ1)<a1<0.13(Δ2/Δ1)+(1.11−0.14MFD)Δτ+0.10−0.41MFD (C3)
The optical fiber according to the present invention has an effect of improving manufacturability by forming the low delay core with a general-purpose refractive index distribution structure. Therefore, the present invention can provide an optical fiber with improved manufacturability for MS-CPE.
Further, the optical fiber according to the present invention is characterized in that the refractive index of the core is lower than that of pure silica glass. For example, the first optical fiber according to the present invention is characterized in that the radius a (μm) satisfies Mathematical Expression C4, the group delay time difference Δτmin (nm/km) that is minimum satisfies Mathematical Expression C5, and a relative refractive index difference ΔF (%) of the core with respect to the pure silica glass satisfies Mathematical Expression C6.
By reducing the refractive index of the core, a larger Δτ can be achieved, and the application region of the MS-CPE using the low delay signal can be expanded.
[Mathematical Expression C4]
−4.4+0.8MFD<a<−1.2+0.6MFD (C4)
[Mathematical Expression C5]
Δτmin=−18.93+7.00×103MFD−3.30 (C5)
[Mathematical Expression C6]
0.98−0.06MFD−(0.42+0.03MFD)a−(0.04+0.002MFD)a2<ΔF<0.03+1.16×103MFD−3.47+0.02Δτ−(0.01+1.88×105MFD−5.09)a+(0.64×10−3+1.89×104MFD−7.16)a2 (C6)
Here, the MFD is a mode field diameter (μm) of the core.
Further, an optical fiber according to the present invention is a multicore fiber including a plurality of cores, in which one of the plurality of cores is the core for the master channel.
Note that the respective inventions described above can be combined as much as possible.
The present invention can provide an optical fiber with improved manufacturability for MS-CPE.
Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components having the same reference numerals in the present description and the drawings indicate the same components.
It is known that the DSP processing time reaches about 1 μs. In order to effectively reduce the DSP processing time of the Slave channels by using the MS-CPE that uses the low delay signal disclosed in Non Patent Literature 3, it is desirable to set a signal arrival time difference between the low delay Master channel and the Slave channels in the receiver to 1 μs or more. Accordingly, when a signal arrival time difference between the Master and the Slave in the receiver is s (μs), a low delay Master channel group delay time τm (μs/km), a group delay time τs (μs/km) of the Slave channel, and a transmission system length (transmission path length) L (km), a requirement of τm is determined by Mathematical Expression (10).
If a group delay time difference between the slave channel and the master channel is Δτ (ns/km), Mathematical Expression (10) is expressed as follows.
In the present embodiment, a method of designing a core that achieves a low delay master channel in an optical fiber having an SI-type refractive index distribution structure suitable for the submarine optical communication system will be described. Here, the optical fiber is the optical transmission line 50 in
Parameters are a bending loss (αb), a mode field diameter (MFD), and Δτ when a channel (group delay time τs=4.88 μs/km) having properties equivalent to those of a general-purpose cut-off shift fiber (ITU-TG.654) is assumed for the slave channel. These are calculation results of the bending loss αb and the MFD at a wavelength of 1.625 μm. A dashed line and a one dot chain line are boundary lines of a structure that achieves a typical bending loss recommendation value of 2.0 dB/100 tuns or less and the MFD of 9.5 μm or more in the international standard ITU-T G.654 for a long-distance transmission fiber. The structure of the grey region enclosed by these curves has optical properties suitable for a long-distance transmission line and enables Δτ<−1.0 ns/km for general purpose cut-off shift fibers. In the present embodiment, a case where the slave channel is an SI-type optical fiber will be described, but the slave channel may be other than the SI-type optical fiber.
Note that a core structure in which αb is 2.0 dB/100 turns is expressed by
[Mathematical Expression 12]
Δ=−1.02+0.26a−0.02a2 (12)
[Mathematical Expression 13]
Δ=−1.05+0.37a−0.05a2 (13),
[Mathematical Expression 14]
Δ=0.60−0.81a+0.15a2 (14).
With respect to a core structure that achieves a delay time difference of Δτ, a relational expression between Δ and a is expressed by
[Mathematical Expression 15]
Δ=K0+K1a+K2a2 (15)
[Mathematical Expression 16]
K
0=423+7.37Δτ+3.81Δτ2 (16)
Similarly, from
[Mathematical Expression 17]
K
1−3.15−4.77Δτ−2.47Δτ2 (17)
Similarly, from
[Mathematical Expression 18]
K
2=0.52+0.77Δτ+0.40Δτ2 (18)
From the above, an SI-type pure quartz core fiber that satisfies
[Mathematical Expression 19]
−1.05+0.37a−0.05a2<Δ<−1.02+0.26a−0.02a2 (19)
and satisfies
[Mathematical Expression 20]
Δ>4.23+7.37Δτ+3.81Δτ2−(3.15+4.77Δτ+2.47Δτ2)a+(0.52+0.77Δτ+0.40τ2)a2 (20)
Note that, in a case of the optical cable and the tape fiber, the “SI-type pure quartz core fiber” means an optical fiber used for the master channel, and the “cut-off shift fiber” means an optical fiber used for the slave channel. Further, in the case of the multicore optical fiber, as will be described later, the “SI-type pure quartz core fiber” means a core used for the master channel, and the “cut-off shift fiber” means a core used for the slave channel.
In the present embodiment, a method of designing a core that achieves the low delay master channel in an optical fiber having a W-type refractive index distribution structure suitable for the submarine optical communication system will be described.
Here, the optical fiber is the optical transmission line 50 in
In a transmission path of several 1000 km class, an optical fiber having a W-type refractive index distribution having a low refractive index region around a core is often used. Here, considering application of the MS-CPE to the long-distance transmission line using the low delay Master channel, if a low delay channel is achieved by the W-type optical fiber, an existing manufacturing technology can be applied, which is preferable.
A ratio of the relative refractive index difference Δ1 of a low refractive index layer to a core refractive index and the relative refractive index difference Δ2 of a clad with respect to the core refractive index is defined as Δ1/Δ2.
A structure in which αb is 2.0 dB/100 tuns or less is a structure in which a1 is larger than the dashed line in the drawing, and a structure in which Δτ is 1.0 ns/km or more is a structure in which a1 is smaller than the solid line in the drawing. Thus, the structure in which αb is 2.0 dB/100 tuns or less and Δτ is 1.0 ns/km or more is the gray region in the drawing. Further, a structure in which αb of the dashed line is 2.0 dB/100 tuns satisfies the relationship of
[Mathematical Expression 21]
Δ2/Δ1−1.95−0.30a1 (21)
[Mathematical Expression 22]
Δ2/Δ1=27.91−7.70a1 (22)
Here, for the structure in which the group delay time difference is Δτ, the relationship between Δ2/Δ1 and the radius a1 is expressed as follows, for example.
[Mathematical Expression 23]
Δ2/Δ1=K3+7.7a1 (23)
[Mathematical Expression 24]
K
3=28.8+1.7Δτ (24)
By solving Mathematical Expression 21 for a1, the smallest designable a1 for Δ2/Δ1 is presented, and by substituting Mathematical Expression 24 into Mathematical Expression 23 and solving Mathematical Expression 23 for a1, the largest designable a1 for Δ2/Δ1 is presented. From the above, the W-type optical fiber having the pure quartz core satisfying the relationship of
[Mathematical Expression 25]
6.50−3.33(Δ2/Δ1)<a1<0.13(Δ2/Δ1)−0.22Δτ−3.74 (25)
Note that, in a case of the optical cable and the tape fiber, the “W-type pure quartz core fiber” means the optical fiber used for the master channel, and the “cut-off shift fiber” means the optical fiber used for the slave channel. Further, in the case of the multicore optical fiber, the “W-type pure quartz core fiber” means the core used for the master channel, and the “cut-off shift fiber” means the core used for the slave channel.
The structure in which αb is 2.0 dB/100 tuns or less is a structure in which a1 is larger than the dashed line in the drawing, and a structure in which Δτ is 1.0 ns/km or more is a structure in which a1 is smaller than the solid line in the drawing. Thus, the structure in which αb is 2.0 dB/100 tuns or less and Δτ is 1.0 ns/km or more is the gray region in the drawing. Further, the structure in which αb of the dashed line is 2.0 dB/100 tuns satisfies the relationship of
[Mathematical Expression 26]
Δ2/Δ1=2.01−0.30a1 (26)
[Mathematical Expression 27]
Δ2/Δ1=27.91−7.70a1 (27).
Here, for the structure in which the group delay time difference is Δτ, the relationship between Δ2/Δ1 and the radius a1 is expressed as follows.
[Mathematical Expression 28]
Δ2/Δ1=K4+7.7a (28)
[Mathematical Expression 29]
K
4=29.9+2.0Δτ (29)
By solving Mathematical Expression 26 for a1, the smallest designable a1 for Δ2/Δ1 is presented, and by substituting Mathematical Expression 29 into Mathematical Expression 28 and solving Mathematical Expression 29 for a1, the largest designable a1 for Δ2/Δ1 is presented. From the above, the W-type optical fiber having the pure quartz core satisfying the relationship of
[Mathematical Expression 30]
670−3.33(Δ2/Δ1)<a10.13(Δ2/Δ1)−0.26Δτ−3.88 (30)
Note that, in the case of the optical cable and the tape fiber, the “W-type pure quartz core fiber” means the optical fiber used for the master channel, and the “cut-off shift fiber” means the optical fiber used for the slave channel. Further, in the case of the multicore optical fiber, the “W-type pure quartz core fiber” means the core used for the master channel, and the “cut-off shift fiber” means the core used for the slave channel.
The structure in which αb is 2.0 dB/100 tuns or less is a structure in which a1 is larger than the dashed line in the drawing, and a structure in which Δτ is 1.0 ns/km or more is a structure in which a1 is smaller than the solid line in the drawing. Thus, the structure in which αb is 2.0 dB/100 tuns or less and Δτ is 1.0 ns/km or more is the gray region in the drawing. Further, the structure in which αb of the dashed line is 2.0 dB/100 tuns satisfies the relationship of
[Mathematical Expression 31]
Δ2/Δ1=2.05−0.30a1 (31)
[Mathematical Expression 32]
Δ2/Δ1=28.42−7.70a1 (32).
Here, for the structure in which the group delay time difference is Δτ, the relationship between Δ2/Δ1 and the radius a1 is expressed as follows.
[Mathematical Expression 33]
Δ2/Δ1=K5+7.7a (33)
[Mathematical Expression 34]
K
530.6±2.2Δτ (34)
By solving Mathematical Expression 31 for a1, the smallest designable a1 for Δ2/Δ1 is presented, and by substituting Mathematical Expression 34 into Mathematical Expression 33 and solving Mathematical Expression 33 for a1, the largest designable a1 for Δ2/Δ1 is presented. From the above, the W-type optical fiber having the pure quartz core satisfying the relationship of
[Mathematical Expression 35]
6.83−3.33(Δ2/Δ1)<a1<0.13(Δ2/Δ1)−0.29Δτ−3.94 (35)
Note that, in the case of the optical cable and the tape fiber, the “W-type pure quartz core fiber” means the optical fiber used for the master channel, and the “cut-off shift fiber” means the optical fiber used for the slave channel. Further, in the case of the multicore optical fiber, the “W-type pure quartz core fiber” means the core used for the master channel, and the “cut-off shift fiber” means the core used for the slave channel.
Here, the design lower limit of the core radius a1 in Mathematical Expressions 17, 22, and 27 is expressed by
[Mathematical Expression 36]
a
1
=K
6−0.33(Δ2/Δ1) (36)
[Mathematical Expression 37]
K
6=0.23+0.66MFD (37).
Similarly, the design upper limit of the core radius a1 in Mathematical Expressions 25, 30, and 35 is expressed by
[Mathematical Expression 38]
a
1=0.13(Δ2/Δ1)+K7Δτ+K8 (38)
[Mathematical Expression 39]
K
7=1.11−0.14MFD (39).
Further,
[Mathematical Expression 40]
K
8=0.10−0.41MFD (40).
From the above, the W-type optical fiber having the pure quartz core satisfying the relationship of
[Mathematical Expression 41]
0.23+0.66MFD−3.33(Δ2/Δ1)<a1<0.13(Δ2/Δ1)+(1.11−0.14MFD)Δτ+0.10−0.41MFD (41)
Note that, in the case of the optical cable and the tape fiber, the “W-type pure quartz core fiber” means the optical fiber used for the master channel, and the “cut-off shift fiber” means the optical fiber used for the slave channel. Further, in the case of the multicore optical fiber, the “W-type pure quartz core fiber” means the core used for the master channel, and the “cut-off shift fiber” means the core used for the slave channel.
The optical communication system of the present embodiment is characterized in that the refractive index of the core of the optical fiber of the optical transmission line 50 is lower than that of the pure silica glass.
In the first and second embodiments, the structural conditions of the optical fiber having the pure silica glass as a core have been described. In the SI-type and W-type structures illustrated in the first and second embodiments, fluorine-doped glass may be used as a core. In this case, a larger Δτ can be achieved by decreasing the core refractive index, and the application region of the MS-CPE using the low delay signal can be expanded.
In the present embodiment, a method of designing a core that achieves the low delay master channel in the optical fiber having the SI-type refractive index distribution structure using fluorine-doped glass as a core will be described.
The parameters are the group delay time difference Δτ at a wavelength of 1.55 μm, the bending loss αb at a wavelength of 1.625 μm, a cutoff wavelength λc, and a Rayleigh scattering loss αR at a wavelength of 1.55 μm. Here, the relative refractive index difference of the clad with respect to the pure silica glass is adjusted so that the MFD is 9.5 μm in each structure.
By a one dot chain line, 0.17 dB/km equivalent to the Rayleigh scattering loss in the general-purpose SMF is indicated. When the core radius a satisfies
[Mathematical Expression 43]
3.2 μm<a<4.7 μm (43),
In the range of the core radius of Mathematical Expression 43, the solid line can be expressed by the following Mathematical Expression
[Mathematical Expression 44]
ΔF=0.49−0.21a+0.02a2 (44)
[Mathematical Expression 45]
ΔF=0.32−0.27a+0.03a2 (45)
[Mathematical Expression 46]
0.32−0.27a+0.03a2<ΔF<0.49−0.21a+0.02a2 (46)
The structure of the gray region surrounded by curves illustrated in
Here, for the structure in which the group delay time difference is Δτ, the relationship between ΔF and the core radius a is expressed as follows.
[Mathematical Expression 47]
ΔF=K9−0.21a+0.02a2 (47)
[Mathematical Expression 48]
K
9=0.50+0.02Δτ (48)
From the above, a fluorine-doped core fiber satisfying the relationship of
[Mathematical Expression 49]
0.32−0.27a+0.03a2<ΔF<0.50+0.02Δτ−0.21a+0.02a2 (49)
Note that, in a case of the optical cable and the tape fiber, the “SI-type fluorine-doped core fiber” means the optical fiber used for the master channel, and the “cut-off shift fiber” means the optical fiber used for the slave channel. Further, in the case of the multicore optical fiber, the “SI-type fluorine-doped core fiber” means the core used for the master channel, and the “cut-off shift fiber” means the core used for the slave channel.
When the core radius a satisfies
[Mathematical Expression 50]
7.5 μm<a<8.1 μm (50),
In the range of the core radius of Mathematical Expression 50, the solid line can be expressed by the following Mathematical Expression
[Mathematical Expression 51]
ΔF=0.10−0.03a+0.13×10−2a2 (51)
[Mathematical Expression 52]
ΔF=−0.73+0.09a+0.01a2 (52)
[Mathematical Expression 53]
−0.73+0.09a+0.01a2<ΔF<0.10−0.03a+0.13×10−2a2 (53)
The structure of the gray region surrounded by the curves illustrated in
Here, for the structure in which the group delay time difference is Δτ, the relationship between ΔF and the core radius a is expressed as follows.
[Mathematical Expression 54]
ΔF=K10−0.03a+0.13×10−2a2 (54)
[Mathematical Expression 55]
K
10=0.13+0.02Δτ (55)
From the above, the fluorine-doped core fiber satisfying the relationship of
[Mathematical Expression 56]
−0.73+0.09a+0.01a2<ΔF<0.13+0.02Δτ−0.03a+0.13×10−2a2 (56)
Note that, in the case of the optical cable and the tape fiber, the “SI-type fluorine-doped core fiber” means the optical fiber used for the master channel, and the “cut-off shift fiber” means the optical fiber used for the slave channel. Further, in the case of the multicore optical fiber, the “SI-type fluorine-doped core fiber” means the core used for the master channel, and the “cut-off shift fiber” means the core used for the slave channel.
[Mathematical Expression 57]
a
min=−4.4+0.8MFD (57)
[Mathematical Expression 58]
a
max=1.2+0.6MFD (58)
[Mathematical Expression 59]
Δτmin=−18.93+7.00×103MFD−3.30 (59)
Here, when the core radius is amin<a<amax and the group delay time difference to be designed is Δτmin<Δτ<−1 ns/km, ΔFmin and ΔFmax with which αR is 0.17 dB/km can be expressed by the following Mathematical Expressions using coefficients K11, K12, K13, K14, K15, and K16 depending on the MFD, the core radius a, and the group delay time difference ΔT.
[Mathematical Expression 60]
ΔFmin=K11+K12a+K13a2 (60)
[Mathematical Expression 61]
ΔFmax=K14+0.02Δτ+K15a+K16a2 (61)
[Mathematical Expression 62]
K
11=0.98−0.06MFD (62)
[Mathematical Expression 63]
K
12=−0.42−0.03MFD (63)
[Mathematical Expression 64]
K
13=−0.04−0.002MFD (64)
[Mathematical Expression 65]
K
14=0.03+1.16×103MFD−3.47 (65)
[Mathematical Expression 66]
K
15=−0.01−1.88×105MFD−5.09 (66)
[Mathematical Expression 67]
K
16=0.64×10−3+1.89×104MFD−7.16 (67)
As described above, for the MFD of 9.5 μm or more and 15.0 μm or less, when the core radius is in the region of amin<a<amax expressed by Mathematical Expressions 57 and 58, and the group delay time difference Δτ to be designed satisfies Δτmin<Δτ<−1 ns/km using Δτmin expressed by Mathematical Expression 59, the fluorine-doped core fiber satisfying the relationship of
[Mathematical Expression 68]
0.98−0.06MFD−(0.42+0.03MFD)a−(0.04+0.002MFD)a2<ΔF<0.03+1.16×103MFD−3.47+0.02Δτ−(0.01+1.88×105MFD−5.09)a+(0.64×10−3+1.89×104MFD−7.16)a2 (68)
Note that, in the case of the optical cable and the tape fiber, the “SI-type fluorine-doped core fiber” means the optical fiber used for the master channel, and the “cut-off shift fiber” means the optical fiber used for the slave channel. Further, in the case of the multicore optical fiber, the “SI-type fluorine-doped core fiber” means the core used for the master channel, and the “cut-off shift fiber” means the core used for the slave channel.
In the present embodiment, a fluorine-doped core SI optical fiber capable of expanding an application region of MS-CPE using the low delay signal to a land relay system will be described. A transmission path length of about several 100 km is assumed in the land relay system, and thus it is desirable that Δτ be −3.4 ns/km or less from
The parameters are the group delay time difference Δτ at a wavelength of 1.55 μm, the bending loss αb at a wavelength of 1.625 μm, the cutoff wavelength λc, and the Rayleigh scattering loss αR at a wavelength of 1.55 μm. Here, the relative refractive index difference of the clad with respect to the pure silica glass is adjusted so that the MFD is 9.5 μm in each structure.
When the radius a satisfies
[Mathematical Expression 69]
3.5 μm<a<4.7 μm (69),
Δc<1.53 μm and αb<0.1 dB/100 turns can be achieved. Further, Δτ<−3.4 ns/km can be achieved in a region where ΔF is smaller than the solid line, and the Rayleigh scattering loss of 0.17 dB/km or less can be achieved in a region where ΔF is larger than the one dot chain line.
The solid line is obtained by setting Δτ=−3.4 ns/km in Mathematical Expressions 47 and 48 using the core radius a in the range of the core radius a of Mathematical Expression 69. The one dot chain line is expressed by Mathematical Expression 45. That is, Δτ<−3.4 ns/km and αR<0.17 dB/km can be achieved in the following Mathematical Expression.
[Mathematical Expression 70]
0.32−0.27a+0.03a2<ΔF<0.43−0.21a+0.02a2 (70)
When the core radius a satisfies
[Mathematical Expression 71]
7.9 μm<a<8.1 μm (71),
Δc<1.53 μm and αb<0.1 dB/100 turns can be achieved. Further, Δτ<−3.4 ns/km can be achieved in a region where ΔF is smaller than the solid line, and the Rayleigh scattering loss of 0.17 dB/km or less can be achieved in a region where ΔF is larger than the one dot chain line.
The solid line is obtained by setting Δτ=−3.4 ns/km in Mathematical Expressions 54 and 55 using the core radius a in the range of the core radius a of Mathematical Expression 71. The one dot chain line can be expressed by Mathematical Expression 52 in the range of the core radius of Mathematical Expression 71. That is, Δτ<−3.4 ns/km and αR<0.17 dB/km can be achieved in the following Mathematical Expression.
[Mathematical Expression 72]
−0.73+0.09a+0.01a2<ΔF<0.06−0.03a+0.13×10−2a2 (72)
Here, the core radius amax at which λc is 1.53 μm or less can be expressed by Mathematical Expression 58.
[Mathematical Expression 73]
a
min=−4.3+0.8MFD (73)
Furthermore,
[Mathematical Expression 74]
Δτmin=−20.0+2.41×102MFD−1.71 (74)
Here, when the core radius is amin<a<amax and the group delay time difference to be designed is Δτmin<Δτ<−3.4 ns/km, ΔFmin and ΔFmax at which αR is 0.17 dB/km can be expressed by Mathematical Expression 68.
In the structure described in the present embodiment, the application region of the MS-CPE using the low delay signal can be extended to about 300 km. In addition, designing as the W type is possible by adding a jacket having a refractive index lower than that of the core region and higher than that of the clad region so as to surround the structure of the present example.
In the present embodiment, an optical communication system in which the optical transmission line 50 is the multicore optical fiber will be described. The multicore optical fiber according to the present embodiment includes at least one core structure described in the first to fourth embodiments.
In the multicore optical fiber according to the present example, the two or more transmission cores (52, 53) are annularly arranged in the clad 51. Then, it is characterized in that at least one (core 52) of the transmission cores is the low delay core having the structure described in the first to fourth embodiments. In the multicore fiber according to the present example, the core 52 is set as the low delay master channel, and the other cores 53 are set as the slave channels. Thus, since noise due to disturbance is made common, MS-CPE using the low delay channel can be stably operated in the optical communication system 300.
In the multicore optical fiber according to the present example, the two or more transmission cores (52, 53) are arranged in the clad 51 in a hexagonal close-packed structure. Then, it is characterized in that at least one (core 52) of the transmission cores is the low delay core having the structure described in the first to fourth embodiments. In the multicore fiber according to the present example, the core 52 is set as the low delay master channel, and the other cores 53 are set as the slave channels. Thus, since noise due to disturbance is made common, MS-CPE using the low delay channel can be stably operated in the optical communication system 300.
The multicore optical fiber according to the present example is characterized in that the transmission core (core 52) described in the first to fourth embodiments is arranged at the center of the clad 51, and two or more transmission cores 53 are arranged in a square lattice pattern around the transmission core in the clad 51. In the multicore fiber according to the present example, the core 52 is set as the low delay master channel, and the other cores 53 are set as the slave channels. Thus, since noise due to disturbance is made common, MS-CPE using the low delay channel can be stably operated in the optical communication system 300.
The multicore optical fiber according to the present example is characterized in that the transmission core (core 52) described in the first to fourth embodiments is arranged at the center of the clad 51, and two or more transmission cores 53 are annularly arranged around the transmission core and in the clad 51. In the multicore fiber according to the present example, the core 52 is set as the low delay master channel, and the other cores 53 are set as the slave channels. Thus, since noise due to disturbance is made common, MS-CPE using the low delay channel can be stably operated in the optical communication system 300.
The multicore optical fiber according to the present example is characterized in that the transmission core (core 52) described in the first to fourth embodiments is arranged at the center of the clad 51, and four transmission cores 53 are arranged in a square lattice pattern around the transmission core in the clad 51. The core 52 is the low delay master channel, and the cores 53 are the slave channels. Here, the low delay master channel is assumed to be the SI-type, and the slave channels are assumed to be the W type including the low refractive index layer 54 on the outer periphery of the cores 53. The core radius of the low delay master channel is am1, the core radius of the slave channel is as1, and a radius of the low refractive index layer is as2. Further, the distance between the low delay master channel and the slave channel (the distance between the centers of the core 52 and the core 53) is Λ. Each core shares the clad 51, and the relative refractive index difference of the clad 51 with respect to the pure silica glass is Δc. Further, the relative refractive index difference of the low refractive index layer 54 with respect to the pure silica glass is defined as Δd.
A solid line indicates a structure in which αc is 0.01 dB/km, a one dot chain line indicates a structure in which XTs-s is −59 dB/km, and a dashed line indicates a structure in which XTm-s is −59 dB/km. It is known that communication of 10,000 km Quadrature phase shift keying (QPSK) modulation can be performed with sufficient transmission quality when both the crosstalk XTm-s and XTs-s are −59 dB/km. Here, am1 and Δc are 3.4 μm and −0.4%, respectively, on the basis of an optical fiber structure capable of achieving MS-CPE using the low delay master channel in the optical transmission system of 1,000 km or more described in the first embodiment. Further, as1 is adjusted so that the MFD of the slave channel is 9.5 μm at a wavelength of 1.55 μm. The as2 is set to 3as1 on the basis of an existing cut-off shift fiber (W type). In the gray structure in the drawing, it is possible to achieve both sufficient transmission quality and sufficient low loss in the long-distance transmission line.
At this time, the group delay time of the low delay master channel is 4.879 μs/km, and the group delay time of the slave channel is 4.883 μs/km. This is because a group delay time difference between the low delay master channel and the slave channel is 4 ns/km, and a transmission delay time difference of 1 μs or more can be achieved in an optical transmission line of 250 km or more.
As described above, by using this MCF as the optical transmission line 50, it is possible to achieve the MS-CPE optical transmission system using the low delay master channel even in a distance of 250 km or more.
The present invention is the optical communication system 301 including the optical fiber described in the first to sixth embodiments as an optical transmission line 50. That is, the optical communication system according to the present invention is as follows.
(1) An optical communication system of an MS-CPE transmission method including a transmitter, a receiver, and an optical transmission line connecting the transmitter and the receiver, characterized in that an optical fiber of the optical transmission line includes
−1.05+0.37a−0.05a2<Δ<−1.02+0.26a−0.02a2
and
Δ>4.23+7.37Δτ+3.81Δτ2−(3.15+4.77Δτ+2.47Δτ2)a+(0.52+0.77Δτ+0.40Δτ2)a2 (C1)
(2) An optical communication system of an MS-CPE transmission method including a transmitter, a receiver, and an optical transmission line connecting the transmitter and the receiver, characterized in that an optical fiber of the optical transmission line includes a core having a radius a1 (μm) for a master channel,
[Mathematical Expression C3]
0.23+0.66MFD−3.33(μ2/Δ1)<a1<0.13(Δ2/Δ1)+(1.11−0.14MFD)Δτ+0.10−0.41MFD (C3)
(3) In the optical fiber according to (1) and (2) above, the core has a refractive index equivalent to that of pure silica glass, but the core may be fluorine-doped glass and may have a refractive index lower than that of the pure silica glass.
(4) The optical fiber according to (1) above is characterized in that the core has a refractive index equivalent to that of the pure silica glass, the core is fluorine-doped glass, the refractive index is lower than that of the pure silica glass,
[Mathematical Expression C4]
−4.4+0.8MFD<a<−1.2+0.6MFD (C4)
[Mathematical Expression C5]
Δτmin=−18.93+7.00×103MFD−3.30 (C5)
[Mathematical Expression C6]
0.98−0.06MFD−(0.42+0.03MFD)a−(0.04+0.002MFD)a2<ΔF<0.03+1.16×103MFD−3.47+0.02Δτ−(0.01+1.88×105MFD−5.09)a+(0.64×10−3+1.89×104MFD−7.16)a2 (C6)
(5) The optical fiber according to (1) to (4) above is characterized by including a plurality of cores, in which one of the plurality of cores is the core for the master channel.
Further, the present invention is also a method for designing the optical fiber described in the first to sixth embodiments. That is, a design method according to the present invention is as follows (see
(6) A method for designing an optical fiber included in an optical communication system of an MS-CPE transmission method is characterized by including:
(7) A method for designing an optical fiber included in an optical communication system of an MS-CPE transmission method is characterized by including:
(8) The optical fiber according to (6) and (7) above is characterized in that the core has a refractive index equivalent to that of the pure silica glass, but the core is fluorine-doped glass and has a refractive index lower than that of the pure silica glass.
(9) The optical fiber according to (6) above is characterized in that, when the core has a refractive index equivalent to that of the pure silica glass but the optical fiber is fluorine-doped glass and a refractive index of the core is lower than that of the pure silica glass,
(10) The optical fiber according to (6) to (9) above is characterized in that, when having a plurality of cores, one of the plurality of cores is the core for the master channel.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/JP2021/000073 | 1/5/2021 | WO |