JP2019032440A - Optical fiber - Google Patents

Abstract

To provide a long-period optical fiber grating capable of efficiently promoting coupling between propagation modes in a mode multiplex optical transmission system using as a transmission path a few-mode optical fiber through which a plurality of propagation modes propagate.SOLUTION: A long-period optical fiber grating according to the present invention is configured to be such a long-period grating that is achieved by imparting a variation of a refractive index to a specified region (0.9a≤Lc≤1.1a) in a cross section of a core at an interval of a pitch 2π/Δβ±10 μm corresponding to the propagation constant difference Δβ between an LPm, n mode and an LPm+1, n mode with m≥1 and n≥1 and between an LP0, n mode and an LP1, n-1 mode with n≥2.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to an optical fiber having a long-period grating that couples a plurality of propagation modes.

  A number mode optical fiber using a plurality of propagation modes has been proposed as a technique for expanding the transmission capacity. In particular, mode multiplex transmission using a plurality of propagation modes is attracting attention as a new large-capacity transmission system because the transmission capacity can be improved several times the number of modes.

  In transmission using this number mode optical fiber, crosstalk between modes occurs in the transmission path, and therefore, a MIMO (Multiple-Input Multiple-Output) equalizer is used at the receiving end as compensation means. However, when a loss difference between modes (Mode dependent Loss: hereinafter referred to as MDL) exists, even if a MIMO equalizer is used, the performance degradation of the transmission system becomes a problem (see, for example, Non-Patent Document 1). . However, if the group delay difference (Differential Mode Delay: DMD) between modes is large at the receiving end, the load of digital processing (DSP) related to MIMO increases, and in order to realize long-distance transmission, the DSP Reduction of a load becomes a problem (for example, refer nonpatent literature 2). In order to alleviate the influence of MDL and DMD, use of a mode scrambler that causes coupling between modes has been proposed (see, for example, Non-Patent Document 3). In addition, a ring core type fiber has been proposed in order to actively cause coupling between modes in an optical fiber transmission line (see, for example, Non-Patent Document 4). Furthermore, a mode coupling is caused in the middle of a 2 km 6-mode fiber using a mode multiplexer / demultiplexer, and the DMD reduction effect has been experimentally confirmed (for example, see Non-Patent Document 5).

P. J. et al. Winzer, et al. "Mode-dependent loss, gain, and noise in MIMO-SDM systems," in Proc. ECOC 2014, paper Mo. 3.3.2, 2014. S. O. Arik, D.D. Askarov, J. et al. M.M. Kahn, Effect of mode coupling on signal processing complexity in mode-division multiplexing, J.A. Lightwave Technol. 31 (3) (2013) 423-431. Lobato, A.M. Ferreira, F .; Rabe, J .; Kuschnerov, M .; Spinnler, B .; Rankl, B .; , “Mode scramblers and reduced-search maximum-likelihood detection for mode-dependent-loss-implied transmission, and in Optical Communication (ECOC 2013). 1-3, 22-26 Sept. 2013 N. Fontaine, R.A. Ryf, M.M. Hirano, and T.R. Sasaki, “Experimental investment of cross accumulation in core-core fiber,” in Proc. IEEE Photon. Soc. Summer Top. Meeting Series, 2013, pp. 111-112. Y. Wakayama, D.W. Soma, K.K. Igarashi, H .; Taga, and T.M. Tsuritani, "Intermediate Mode Interchange for Reduction of Differential Mode-Group Delay Weakly-Coupled 6-Mode Fiber Transmission Line," in Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2016), paper M3E. 6). T.A. Mori, T .; Sakamoto, M .; Wada, T .; Yamamoto and F.M. Yamamoto, "Few-Mode Fibers Supporting More Than Two LP Modes For Mode-Division-Multiplexed Transmission With MIMO DSP" J. Lightwave Technol. 32 (14) (2014) 2468-2479. P. Sillard, K .; D. Jong, F.M. Achten and C.M. M.M. Okonkwo, “Low-Differential-Mode-Group-Delay 9-LP-Mode Fiber” Lightwave Technol. 34 (2) (2016) 425-430.

  However, Non-Patent Document 4 has a problem that mode coupling for obtaining a sufficient DMD reduction amount is insufficient. In addition, Δβ tends to increase in higher order modes, and it is difficult to cause sufficient mode coupling when the number of modes increases. Similarly, in the method described in Non-Patent Document 5, sufficient mode coupling does not occur in the higher-order mode. Non-Patent Document 6 uses 11 long-period gratings (LPG) to suppress the spread of the group delay difference in the 9LP mode fiber, but there is no discussion of combinations of modes that are easy to combine. There is a problem that it is unclear whether all modes are combined. Also, LPG design that maximizes coupling efficiency has not been studied.

  Therefore, in order to solve the above-described problems, an object of the present invention is to provide an optical fiber having a long-period grating in which all modes are efficiently coupled to an optical fiber in which a plurality of modes propagate.

  In order to achieve the above object, the optical fiber according to the present invention has an LPG having a structure in which a plurality of grating portions in which the refractive index is changed only in a part of the core in the cross section are arranged in the longitudinal direction.

Specifically, the optical fiber according to the present invention is an optical fiber in which a plurality of modes propagates,
The core has a long-period grating whose grating portion interval is Formula C1,
The grating portion is
In the cross section of the optical fiber, when a point at a distance Lc that satisfies the mathematical formula C2 from the outer periphery of the core on a straight line H connecting the center of the optical fiber cross section and a point on the outer periphery of the core is a point P, The refractive index of the region surrounded by the straight line V passing through the point P and the outer periphery of the core is changed.
(Formula C1)
2π / Δβ ± 10μm
However, Δβ is a propagation constant difference between LPm, n mode and LPm + 1, n mode (m ≧ 0 and n ≧ 1) or between LP0, n mode and LP1, n−1 mode (n ≧ 2). It is.
(Formula C2)
0.9a ≦ Lc ≦ 1.1a
Where a is the core radius.

  By forming the core of the grating part as described above and arranging the grating part in the longitudinal direction of the optical fiber at an interval of Formula C1, all modes are efficiently coupled to the optical fiber in which a plurality of modes propagate. A long period grating. Strong coupling occurs in all propagation modes by applying a refractive index change to a specific region of the core cross section at a pitch of 2π / Δβ ± 10 μm, which corresponds to a propagation constant difference Δβ between modes in which strong mode coupling is expected to occur. be able to. Therefore, the present invention can provide an optical fiber having a long period grating in which all modes are efficiently coupled to an optical fiber in which a plurality of modes propagate.

  The long-period grating of the optical fiber according to the present invention is characterized in that the intervals of the grating portions are different. In the case of a step index type optical fiber, the propagation constant difference Δβ is different for each wavelength, and there is an interval between grating portions that can be efficiently mode-coupled for each propagation wavelength. For this reason, when performing wavelength division multiplexing transmission using a step index type optical fiber, the interval between the grating portions is set for each wavelength to be multiplexed (the interval between the grating portions is not uniform).

  The optical fiber according to the present invention is a graded index type, and the long-period grating has the same interval between the grating portions. In the case of a graded index optical fiber, the wavelength dependence of the spacing between the grating portions that can efficiently perform mode coupling is small. For this reason, when performing wavelength division multiplexing transmission using a graded index optical fiber, the intervals between the grating portions can be made uniform.

  The present invention can provide an optical fiber having a long period grating in which all modes are efficiently coupled to an optical fiber in which a plurality of modes propagate.

It is a core sectional view of the optical fiber concerning the present invention, and is a figure explaining the refractive index change field of a grating part. It is a core sectional view of the optical fiber concerning the present invention, and is a figure explaining the refractive index change field of a grating part. It is a figure explaining an example of the refractive index profile of GI fiber which can propagate 6LP mode. It is a figure explaining the electric field distribution and mode group of 6LP mode. It is a figure explaining angle (phi) dependence of the refractive index change area | region of the mode coupling efficiency between each mode in the optical fiber which concerns on this invention. When the optical fiber which concerns on this invention is GI fiber which can propagate 6LP mode, it is a figure explaining the relationship between the refractive index variation width Lc / 2a with respect to the core diameter at the time of entering each mode, and mode coupling efficiency. It is a figure explaining the structure of the optical fiber which concerns on this invention, Comprising: A mode that the refractive index change of Lc = a is given to the cross section of a core with the period 2π / Δβ corresponding to the propagation constant difference Δβ between the modes of the adjacent mode group. It is a figure explaining. It is a figure explaining an example of the refractive index profile of GI fiber which can propagate 4LP mode. When the optical fiber which concerns on this invention is GI fiber which can propagate 4LP mode, it is a figure explaining the relationship between the refractive index change width Lc / 2a with respect to the core diameter at the time of entering each mode, and mode coupling efficiency. It is a figure explaining an example of the refractive index profile of GI fiber which can propagate 9LP mode. It is a figure explaining the electric field distribution and mode group of 9LP mode. When the optical fiber which concerns on this invention is GI fiber which can propagate 9LP mode, it is a figure explaining the relationship between refractive index variation width Lc / 2a with respect to the core diameter at the time of entering each mode, and mode coupling efficiency. It is a figure explaining an example of the refractive index profile of SI fiber which can propagate 6LP mode. When the optical fiber which concerns on this invention is SI fiber which can propagate 6LP mode, it is a figure explaining the relationship between the refractive index variation width Lc / 2a with respect to the core diameter at the time of entering each mode, and mode coupling efficiency. In each mode combination of 6LP mode GI fiber, (A) the relationship of mode coupling efficiency to Lc / 2a, (B) the mode coupling efficiency for each mode combination is 90 which is the maximum value obtained at L _c /2a=0.5. It is a figure explaining the width | variety of _Lc / 2a used as% or more. In each mode combination of 4LP mode GI fiber, (A) the relationship of mode coupling efficiency to Lc / 2a, (B) the mode coupling efficiency for each mode combination is 90% of the maximum value obtained at Lc / 2a = 0.5 It is a figure explaining the width | variety of Lc / 2a used as the above. In each mode combination of the 9LP mode GI fiber, (A) the relationship of mode coupling efficiency to Lc / 2a, and (B) the mode coupling efficiency for each mode combination is 90 which is the maximum value obtained at L _c /2a=0.5. It is a figure explaining the width | variety of _Lc / 2a used as% or more. In each mode combination of 6LP mode SI fiber, (A) the relationship of mode coupling efficiency to Lc / 2a, (B) the mode coupling efficiency for each mode combination is 90% of the maximum value obtained at Lc / 2a = 0.5 It is a figure explaining the width | variety of Lc / 2a used as the above. It is a figure explaining the relationship between the mode shift efficiency and the pitch shift from the ideal spacing of 2π / Δβ corresponding to the propagation constant difference Δβ between modes. It is a figure explaining the wavelength dependence of the effective refractive index of LP mode in 6LP mode SI fiber. It is a figure which shows the wavelength dependence of the pitch space | interval which coupling | bonding between LP modes produces in 6LP mode SI fiber. It is a figure which shows the wavelength dependence of the effective refractive index of LP mode in 6LP mode GI fiber. It is a figure which shows the wavelength dependence of the pitch space | interval which the coupling between LP modes produces in 6LP mode GI fiber. It is a table | surface explaining the combination of the mode which is easy to couple | bond in 6LP mode GI fiber. It is a table | surface explaining the combination of the mode which is easy to couple | bond in 4LP mode GI fiber. It is a table | surface explaining the combination of the mode which is easy to couple | bond in 9LP mode GI fiber. It is a table | surface explaining the combination of the mode which is easy to couple | bond in 6LP mode SI fiber. It is a core sectional view of the optical fiber concerning the present invention, and is a figure explaining the refractive index profile of a grating part.

  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. In the present specification and drawings, the same reference numerals denote the same components.

FIG. 7 is a diagram illustrating an optical fiber core 100 according to an embodiment described below. This optical fiber is an optical fiber in which a plurality of modes propagates,
The core 100 has a long-period grating 12 in which the interval between the grating portions 11 is a formula C1,
The grating section 11
In the cross section of the optical fiber, when a point at a distance Lc satisfying the formula C2 from the outer periphery of the core on a straight line H connecting the center of the optical fiber cross section and a point on the outer periphery of the core is a point P, the point is perpendicular to the straight line H. The refractive index of the region (refractive index changing region 21) surrounded by the straight line V passing through the point P and the outer periphery of the core is changed by Δn.
(Formula C1)
2π / Δβ ± 10μm
However, Δβ is a propagation constant difference between LPm, n mode and LPm + 1, n mode (m ≧ 0 and n ≧ 1) or between LP0, n mode and LP1, n−1 mode (n ≧ 2). It is.
(Formula C2)
0.9a ≦ Lc ≦ 1.1a
Where a is the core radius.
That is, the grating part 11 is composed of a refractive index changing region 21 and another region 22. Note that the refractive index profile of the refractive index changing region 21 is changed while maintaining the shape of the refractive index profile of the core other than the grating portion 11 (see FIG. 28).

(Embodiment 1)
FIG. 1 is a cross-sectional view of the core of the optical fiber according to the present embodiment, and is a view for explaining a refractive index changing region 21 of the grating portion 11. A point P is a point on the straight line H connecting the center O of the core 100 and the outer periphery of the core at a position Lc from the outer periphery in the cross section of the optical fiber in which at least two or more modes propagate. It is assumed that the refractive index change is given to the refractive index change region 21 (area S) surrounded by the straight line V perpendicular to the straight line H and passing through the point P and the outer periphery of the core. Considering the rotation of the angle φ around the center O of the core 100 with this refractive index change amount Δn (FIG. 2), the refractive index change in the cross section of the core is expressed by the following equation.
The mode coupling efficiency can be calculated by the overlap integral of the electric field distributions of the two modes coupled with the above refractive index change equation.

  This time, a graded index (GI) fiber (Non-patent Document 7) having a core radius of 12.5 μm and a relative refractive index difference of 0.84% as shown in FIG. 3 is considered as a fiber that causes mode coupling. It was. The vertical axis in FIG. 3 represents the relative refractive index difference when the silica level is zero. The modes that can propagate in this optical fiber are ten LP modes as shown in FIG. In these LP modes, if modes having substantially equal propagation constants are defined as mode groups, there are four mode groups from M = 1 to M = 4. The mode coupling efficiency was actually calculated by the overlap integration of the electric field distribution of these modes and the refractive index change shown in FIG.

First, the result of calculating the mode coupling efficiency with respect to the angle φ is shown in FIG. From FIG. 5, for example, when φ = 0 (rad), coupling between the LP ₀₁ and LP _11a modes occurs, and coupling efficiency between the LP ₀₁ and LP _11b modes does not occur. On the other hand, when φ = π / 2 (rad), mode coupling occurs between modes in which coupling does not occur at φ = 0 (rad). In the case of φ = π / 4 (rad) in the meantime, mode coupling occurs between any modes.

  Actually, since the electric field distribution of the mode can be changed by the length of the transmission fiber, the twist or the like, the angle φ is considered to change in the range of 0 to π. Therefore, in the following examination, the mode coupling efficiency by a plurality of grating portions (φ = 0 to π) is averaged, and φ is not considered.

Next, the relationship with the refractive index change width L _c / 2a with respect to the core diameter was obtained (FIG. 6). From the results of FIG.
Between LP ₀₁ mode and LP ₁₁ mode,
Between LP ₁₁ mode and LP ₂₁ mode or LP ₀₂ mode,
Between LP ₂₁ mode and LP ₁₁ mode or LP ₃₁ mode,
Between LP ₀₂ mode and LP ₁₁ mode or LP ₁₂ mode,
Between LP ₃₁ mode and LP ₂₁ mode,
Between LP ₁₂ mode and LP ₀₂ mode,
The mode coupling efficiency of was high.
The mode coupling efficiency between the LP _{01 and} LP ₁₁ modes is maximized at Lc = a. A combination of modes in which a mode coupling efficiency of 1/2 or more of the maximum value is obtained at Lc = a is indicated by “◯” in the table of FIG.

  From the above results, LPm, n mode and LP1, n-1 for LPm, n mode and LPm, n mode and LPm + 1, n mode for m ≧ 0 and n ≧ 1, LP0, n mode and LP1, n−1 for n ≧ 2. In a graded index fiber in which mode groups exist and the mode groups exist as shown in FIG. 4, mode coupling is likely to occur between adjacent mode groups.

In addition, among the modes between which the large mode coupling efficiency can be obtained, in any case between LPm, n mode and LPm + 1, n mode for m ≧ 0 and n ≧ 1, L _c /2a=0.5. It was found that the maximum mode coupling efficiency was obtained. Although coupling efficiency between LP ₀₂ mode and the _{LP 11} mode is not a maximum at _L c _/2a=0.5, coupling efficiency has a value sufficiently close to the maximum value in the L c /2a=0.5.

Therefore, as shown in FIG. 7, the pitch interval 2π / Δβ corresponding to the propagation constant difference Δβ between adjacent mode groups, including errors and tolerances (formula C1)
2π / Δβ ± 10μm
By disposing the grating portion 11 (providing half the refractive index change with respect to the core cross section), all modes can be strongly coupled. In this fiber, the propagation constant difference Δβ between adjacent mode groups is about 10,000, and the ideal pitch interval is about 600 μm. Therefore, a strong coupling with modes in adjacent mode groups can be generated by giving a refractive index change to one half region in the core cross section at intervals of about 600 μm, and all modes in an optical fiber capable of propagating multiple modes can be generated. Can be strongly coupled.

(Embodiment 2)
In the present embodiment, a GI fiber having the refractive index profile of FIG. 8 with a core radius of 12.5 μm and a relative refractive index difference of 0.6% was studied. The vertical axis in FIG. 8 represents the relative refractive index difference when the silica level is zero. In this embodiment as well, the mode coupling efficiency is averaged for φ as in the first embodiment, and the refractive index change width Lc for obtaining the maximum coupling efficiency is obtained. The GI fiber of FIG. 8 is a 4LP mode fiber, and the mode of M = 4 in FIG. 4 does not propagate, but the mode of M = 1 to 3 propagates.

  FIG. 9 shows the calculation result. From the results of FIG. 9, the combination of modes easy to couple in the 4LP mode fiber is LPm, n mode and LPm + 1, n mode for m ≧ 0 and n ≧ 1 for LPm and n mode as in the case of 6LP mode fiber. As a result, when m = 0 and n ≧ 2, coupling between the LP0, n mode and the LP1, n−1 mode is easy.

The mode coupling efficiency between the LP _{01 and} LP ₁₁ modes is maximized at Lc = a, and combinations of modes in which a mode coupling efficiency of ½ or more of the maximum value is obtained at Lc = a are shown in the table of FIG. “○”. It is most likely to be coupled to a mode in an adjacent county, and is not easily coupled to a mode of a distant mode group such as M = 1 and M = 3. In addition, among the modes between which the large mode coupling efficiency can be obtained, in any case between LPm, n mode and LPm + 1, n mode for m ≧ 0 and n ≧ 1, L _c /2a=0.5. It was found that the maximum mode coupling efficiency was obtained.

Although coupling efficiency between LP ₀₂ mode and the _{LP 11} mode is not a maximum at _L c _/2a=0.5, coupling efficiency has a value sufficiently close to the maximum value in the L c /2a=0.5. Even when a 4LP mode fiber is used, it is desirable to change the refractive index in a half region of the core cross section at a pitch interval of Formula C1 corresponding to Δβ between adjacent mode groups.

(Embodiment 3)
In the present embodiment, a GI fiber having the refractive index profile of FIG. 10 having a core radius of 14.1 μm and a relative refractive index difference of 1.0% was examined. The vertical axis in FIG. 10 represents the relative refractive index difference when the silica level is zero. In this embodiment as well, the mode coupling efficiency is averaged for φ as in the first embodiment, and the refractive index change width Lc for obtaining the maximum coupling efficiency is obtained. The fiber of FIG. 10 is a 9LP mode fiber, and can propagate a 15LP mode composed of five mode groups as shown in FIG.

FIG. 12 shows the calculation result. From the results of FIG.
Between LP01 mode and LP11 mode,
Between LP11 mode and LP21 mode,
Between LP21 mode and LP31 mode,
Between LP31 mode and LP41 mode,
Between LP12 mode and LP22 mode,
The mode coupling efficiency of was high.
Thus, even in the 9LP mode fiber, it can be said that the mode coupling efficiency increases between the LPm, n mode and the LPm + 1, n mode for m ≧ 0 and n ≧ 1 for the LPm, n mode.

further,
Between LP02 mode and LP11 mode,
Between LP03 mode and LP12 mode,
Has a large mode coupling efficiency.
For this reason, even in the 9LP mode fiber, when m = 0 and n ≧ 2, coupling between the LP0, n mode and the LP1, n−1 mode is easy.

In addition, among the modes between which the large mode coupling efficiency can be obtained, in any case between LPm, n mode and LPm + 1, n mode for m ≧ 0 and n ≧ 1, L _c /2a=0.5. It was found that the maximum mode coupling efficiency was obtained. Between LP ₀₂ mode and the _{LP 11} mode, but coupling efficiency between LP03 mode and the LP12 mode does not become a maximum at _L c _/2a=0.5, the coupling efficiency is close enough to the maximum value in the L c /2a=0.5 It is a value.

The mode coupling efficiency between the LP _{01 and} LP ₁₁ modes is maximized at Lc = a. A combination of modes in which a mode coupling efficiency of 1/2 or more of the maximum value is obtained at Lc = a is indicated by “◯” in the table of FIG. Even when a 9LP mode fiber is used, a refractive index change is given to a half region of the core cross section at a pitch interval of Formula C1 corresponding to the propagation constant difference Δβ between the combinations of modes indicated by “◯” in the table of FIG. desirable.

(Embodiment 4)
In the present embodiment, a step index (SI) fiber having the refractive index profile of FIG. 13 having a core radius of 12.0 μm and a relative refractive index difference of 0.4% was studied. The vertical axis in FIG. 13 represents the relative refractive index difference when the silica level is zero. In this embodiment as well, the mode coupling efficiency is averaged for φ as in the first embodiment, and the refractive index change width Lc for obtaining the maximum coupling efficiency is obtained.

FIG. 14 shows the result of the analysis. Compared to GI fiber, SI fiber
Between LP ₁₁ mode and LP ₃₁ mode,
Between LP ₀₂ mode and LP ₁₂ mode,
As a result, it became easier to combine.

  However, as in GI fiber, LPm, n mode for LPm, n mode and LPm, n mode and LPm + 1, n mode for m ≧ 1, and LP0, n mode for m = 0 and n ≧ 2. The tendency to be easily coupled between the LP1 and n-1 modes was the same as in the first to third embodiments.

In addition, among the modes between which the large mode coupling efficiency can be obtained, in any case between LPm, n mode and LPm + 1, n mode for m ≧ 0 and n ≧ 1, L _c /2a=0.5. It was found that the maximum mode coupling efficiency was obtained. Although coupling efficiency between LP ₀₂ mode and the _{LP 11} mode is not a maximum at _L c _/2a=0.5, coupling efficiency has a value sufficiently close to the maximum value in the L c /2a=0.5.

The mode coupling efficiency between the LP _{01 and} LP ₁₁ modes is maximized at Lc = a. A combination of modes in which a mode coupling efficiency of 1/2 or more of the maximum value is obtained at Lc = a is indicated by “◯” in the table of FIG. In the SI fiber, modes are not grouped, but a refractive index change is given to half of the core cross section at a pitch interval of Formula C1 corresponding to the propagation constant difference Δβ between the combinations of modes indicated by “◯” in the table of FIG. Thus, efficient mode coupling can be obtained.

(Embodiment 5)
In the first to fourth embodiments, when Lc / 2a is 0.5, the maximum coupling efficiency is obtained between LPm, n mode and LPm + 1, n mode (m ≧ 1 and n ≧ 1) having large mode coupling efficiency. explained. In this embodiment, the allowable range of Lc for obtaining a value of 90% of the maximum value of the mode coupling efficiency κ is calculated.

FIG. 15 shows the relationship between (A) mode coupling efficiency with respect to Lc / 2a and (B) mode coupling efficiency for each mode combination in the case of 6LP mode GI fiber and the mode combinations indicated by “◯” in the table of FIG. The width of L _c / 2a is 90% or more of the maximum value obtained when L _c /2a=0.5. In the 6LP mode GI fiber, the mode coupling efficiency between the LP ₀₂ mode and the LP ₁₂ mode is easily affected by the deviation of the value of Lc, and the allowable range of Lc / 2a (κ is 90% or more of the maximum value) is 0. .45 to 0.56.

FIG. 16 shows the relationship between (A) mode coupling efficiency with respect to Lc / 2a and (B) mode coupling efficiency for each mode combination in the case of 4LP mode GI fiber in the mode combinations indicated by “◯” in the table of FIG. The width of L _c / 2a is 90% or more of the maximum value obtained when L _c /2a=0.5. In the 4LP mode GI fiber, the mode coupling efficiency between the LP01 mode and the LP11a mode is easily affected by the deviation of the value of Lc, and the allowable range of Lc / 2a (κ is 90% or more of the maximum value) is 0.45. -0.56.

FIG. 17 shows (A) the relationship of mode coupling efficiency with respect to Lc / 2a and (B) the mode coupling efficiency for each mode combination in the 9LP mode GI fiber in the mode combinations indicated by “◯” in the table of FIG. The width of L _c / 2a is 90% or more of the maximum value obtained when L _c /2a=0.5. In 9LP mode GI fiber, the mode coupling efficiency between the LP ₀₁ mode and the LP ₁₁ mode, and between the LP ₀₂ mode and the LP ₁₂ mode is easily affected by the deviation of the value of Lc, and the allowable range of Lc / 2a ( κ was 90% or more of the maximum value) was 0.45 to 0.55.

FIG. 18 shows (A) the relationship of mode coupling efficiency to Lc / 2a and (B) the mode coupling efficiency for each mode combination in the case of 6LP mode SI fiber in the mode combinations indicated by “◯” in the table of FIG. The width of L _c / 2a is 90% or more of the maximum value obtained when L _c /2a=0.5. In the 6LP mode SI fiber, the mode coupling efficiency between the LP ₀₂ mode and the LP ₁₂ mode is easily affected by the deviation of the value of Lc, and the allowable range of Lc / 2a (κ is 90% or more of the maximum value) is 0. .45 to 0.56.

From the above, in any fiber, the value of Lc / 2a is 0.5 ± 0.05, that is, Lc (formula C2)
0.9a ≦ Lc ≦ 1.1a
By keeping within this range, the coupling efficiency between modes that can provide a large mode coupling efficiency can be set to 90% or more of the maximum value that can be taken when Lc / 2a is 0.5.

(Embodiment 6)
In order to cause mode coupling, it is necessary to periodically apply a refractive index change (grating portion) to the longitudinal direction of the fiber. The maximum conversion efficiency can be obtained when the period is a propagation constant difference of 2π / Δβ between modes to be converted. However, sufficient mode coupling is possible even if the period slightly deviates from 2π / Δβ. In general, the mode coupling strength ξ is expressed as follows.
Here, the propagation constants of mode l and mode m are set as βl and βm, respectively (that is, Δβ = βl−βm), and the ideal structure pitch is 2π / (βl−βm). The period of the structural change actually applied to the optical fiber was set to 2π / Ω, and the total length of the structural change was L.

When the structure period given to the optical fiber is 2π / Ω, the deviation from the ideal pitch is expressed by the following equation.

  For example, when considering an LPG having a structural change length of 1 cm, the relationship between the pitch shift Δpitch and the mode coupling strength ξ is expressed as shown in FIG. If Δpitch is other than 0 as shown in FIG. 19, mode coupling tends to be weak depending on the value of Δβ. Assuming a fiber with Δβ of 8000 or less, by suppressing the pitch deviation Δpitch to 10 μm or less, a mode coupling efficiency of 90% or more of the mode coupling efficiency when there is no pitch deviation can be obtained. Therefore, as described with reference to FIG. 7, it is preferable that the grating portion 11 arranged in the core 100 of the optical fiber has an interval of the formula C1.

(Embodiment 7)
The optical fiber of this embodiment is a step index type.
When wavelength multiplexing transmission is performed using an optical fiber having a long-period grating, it is necessary to provide a pitch interval of the grating portion corresponding to each wavelength. For example, when transmission is performed using the C band and the L band, the C band is in the range of 1530 to 1565 nm, and the L band is in the range of 1565 to 1625 nm. Therefore, the grating section needs to have a pitch interval corresponding to the band of 1530 to 1625 nm. There is.

14 shows an example of an SI fiber capable of propagating 6LP mode having the refractive index profile of FIG. FIG. 20 is a diagram illustrating the wavelength dependence of the effective refractive index neff for each mode of the SI fiber. Neff of the LP ₀₁ mode is large, and neff of the LP ₁₂ mode is small. In all modes, neff is small on the long wavelength side. The effective refractive index and the propagation constant β are in a proportional relationship of Equation 4.

FIG. 21 shows the result of calculating the pitch interval of the grating portions necessary for mode coupling the mode combinations indicated by “◯” in the table of FIG. 27 from the propagation constant difference Δβ. The pitch intervals of the grating portions necessary for coupling between the modes are as follows.
LP ₀₁ -LP ₁₁ : pitch interval of about 1600 μm,
LP ₁₁ -LP ₂₁ : pitch interval of about 1250 μm,
LP ₁₁ -LP ₀₂ : pitch interval of about 950 μm,
LP ₂₁ -LP ₃₁ : pitch interval of about 1050 μm,
LP ₀₂ -LP ₁₂ : Pitch interval is about 850 μm.
Therefore, in order to efficiently couple all ten propagation modes, it is necessary to provide a grating portion in the core at the above pitch interval.

In addition, when performing wavelength multiplexing transmission, it is necessary to provide a pitch interval of the grating portion corresponding to each wavelength. For example, when signal transmission is performed using the CL band, it is necessary to provide a pitch interval corresponding to a range of 1530 to 1625 nm. For example, the pitch interval of the grating part that causes mode coupling between LP ₀₁ and LP ₁₁ is 1647 μm at 1530 nm,
1581 μm at 1625 nm
Therefore, it is necessary to provide the grating portions at a plurality of pitch intervals in the range of 1581 μm to 1647 μm. Here, if a pitch deviation of ± 10 μm is allowed, mode coupling can occur between LP ₀₁ and LP ₁₁ over the entire CL band by providing grating portions at four pitch intervals of 1590 μm, 1610 μm, 1630 μm, and 1640 μm. It is. Further, by providing the pitch interval of the grating portion corresponding to the wavelength band in the same manner between other modes, it is possible to cause mode coupling over the entire CL band.
In the present embodiment, the step index type optical fiber has been described. However, even in the case of a GI type optical fiber, a plurality of pitch intervals of the grating portions may be provided according to the wavelength band.

(Embodiment 8)
The optical fiber of this embodiment is a graded index type.
When a GI fiber is used as a fiber that imparts a change in refractive index, all propagation modes can be easily and efficiently coupled with a small number of pitches. FIG. 22 shows the calculation result of the effective refractive index neff of the GI fiber capable of propagating the 6LP mode having the refractive index profile of FIG. Although the neff differs from mode to mode, the difference in neff between the modes is the same, and therefore the pitch interval 2π / Δβ, which is the period giving the change in refractive index, is also equal between the modes.

  FIG. 23 shows the result of calculating the pitch interval of the grating portions necessary for mode coupling the mode combinations indicated by “◯” in the table of FIG. 24 from the propagation constant difference Δβ. The pitch interval of the grating portion necessary for mode coupling between all the modes is about 600 μm, and all modes can be efficiently coupled by applying a refractive index change at this pitch interval.

In addition, in the case of an optical fiber having a refractive index profile as shown in FIG. 3, there is almost no wavelength dependence of the pitch interval of the grating portion. Can be combined. For example, since the pitch interval between the LP ₀₁ and LP ₁₁ modes is 609.8 μm at 1530 nm and 609.1 μm at 1625 nm, it is sufficient to arrange the grating portions with one pitch interval of 609 μm to cover the CL band. It is.

Regarding other modes, the pitch interval of the grating portion covering the CL band is as follows.
Between LP _{11 and} LP ₂₁ modes: pitch interval 601.4 μm to 602.3 μm,
Between LP _{11 and} LP ₀₂ modes: pitch interval 601.1 μm to 601.8 μm,
Between LP _{21 and} LP ₃₁ modes: pitch interval 601.4 μm to 602.3 μm,
Between LP _{02 and} LP ₁₂ modes: pitch interval 600.7 μm to 601.7 μm.
Here, if a pitch deviation of ± 10 μm is allowed, the 10 modes can be coupled with high efficiency over the entire CL band by providing the grating portions at one pitch interval of about 605 μm.
Even in the case of a step index type optical fiber, the grating portions may be provided at one pitch interval. For example, when only the LP01 and LP11 modes are transmitted with one wavelength using a step index type optical fiber, one type of pitch interval may be used.

(Embodiment 9)
In the first to eighth embodiments, a general SI fiber or GI fiber has been described as an optical fiber. However, the above-described grating unit is a low DMD transmission using a low DMD fiber, a ring core fiber that easily generates coupling, a DMD compensation transmission line, or the like. It may be formed on the road. In this case, it is possible to expect a further effect of reducing the difference in propagation time between modes due to mode coupling by the long period grating, and it is possible to provide an optical transmission system capable of transmission with a very small signal processing load.

(Embodiment 10)
Any method may be used as the method for providing the refractive index change region of the grating portion. For example, the refractive index may be changed by irradiating the optical fiber with laser light, or the refractive index may be changed by applying bending or stress. Regardless of which method is used to provide the refractive index change region, high-efficiency mode coupling can be generated.

  The present invention can realize a large capacity and a long distance of optical fiber transmission by using a higher-order mode in the fiber.

11: Grating part 12: Long period grating 21: Refractive index change area 22: Other area 100: Core

Claims (3)

An optical fiber in which multiple modes propagate,
The core has a long-period grating whose grating portion interval is Formula C1,
The grating portion is
In the cross section of the optical fiber, when a point at a distance Lc that satisfies the mathematical formula C2 from the outer periphery of the core on a straight line H connecting the center of the optical fiber cross section and a point on the outer periphery of the core is a point P, An optical fiber, wherein a refractive index of a region surrounded by a straight line V passing through the point P and the outer periphery of the core is changed.
(Formula C1)
2π / Δβ ± 10μm
However, Δβ is a propagation constant difference between LPm, n mode and LPm + 1, n mode (m ≧ 0 and n ≧ 1) or between LP0, n mode and LP1, n−1 mode (n ≧ 2). It is.
(Formula C2)
0.9a ≦ Lc ≦ 1.1a
Where a is the core radius.   The optical fiber according to claim 1, wherein the long-period grating has different intervals between the grating portions. The optical fiber is a graded index type,
The optical fiber according to claim 1, wherein the long-period grating has the same interval between the grating portions.