JP6654064B2 - Mode converter, optical amplifier and optical transmission system - Google Patents

Description

  The present invention relates to an optical fiber, a mode converter, an optical amplifier, and an optical transmission system used for mode multiplex transmission.

  In recent years, Internet traffic has been still increasing due to diversification of services, and transmission capacity has been dramatically increased due to an increase in transmission speed and an increase in the number of wavelength multiplexes by wavelength division multiplexing (WDM) technology. . In recent years, further expansion of transmission capacity is expected by digital coherent technology, which has been actively studied. In digital coherent transmission systems, frequency utilization efficiency has been improved by using multi-level phase modulation signals, but a higher signal-to-noise ratio is required. However, in a conventional transmission system using a single mode fiber (SMF), the transmission capacity is expected to saturate at 100 Tbit / sec due to the input power limitation caused by the nonlinear effect in addition to the theoretical limit. Therefore, it is becoming difficult to further increase the capacity.

  In order to further increase the transmission capacity in the future, a medium for realizing an innovative transmission capacity expansion is required. Therefore, mode multiplex transmission using a multi-mode fiber (Multi Mode Fiber: MMF), which can be expected to improve a signal-to-noise ratio and space utilization efficiency by using a plurality of propagation modes in an optical fiber as a channel, has attracted attention. . Until now, higher-order modes propagating in a fiber have been a cause of signal degradation, but active use is being studied in the development of digital signal processing and multiplexing / demultiplexing techniques (for example, Non-Patent Document 1, 2).

  In the mode multiplexing transmission technology, a mode converter becomes important in the construction of a transmission system. Therefore, various devices such as a method using a phase mask and a waveguide type mode converter have been proposed (for example, Non-patented). See references 3 and 4.)

N. Hanzawa et al. , “Demonstration of Mode-Division multiplexing Transmission Over 10 km Two-mode Fiber with Mode Coupler”, OFC2011, paper OWA4 T. Sakamoto et al. , "Modal Dispersion Technology for Long-haul Transmission over Few-mode Fiber with SIMO Configuration", ECOC 2011, We. 10. P1.82 R. G. FIG. H van Uden et al, "Phase Plate Tolerances in a Tri-Mode Demultiplexer", Summer topicals 2012 WC1.3. N. Hanzawa, et al, “Two-mode PLC-based mode multi / demultiplexer for mode and wavelength division multiple multiplexed, Opt. C. D. Poole et al. “Helical-Grating Two-Mode Fiber Spatial-Mode Coupler”, Journal of lightwave technology, vol. 9, no. 5, 598-604 (1991) P. Sillard et al, “50 μm Multimode Fibers for Mode Division Multiplexing”, ECOC2015, paper Mo. 4.1.2 Wada et al., “Study of Multimode EDFA with Mode Scramble Function”, IEICE General Conference 2015, B-13-31 Labato, A .; Ferreira, F .; Rabe, J .; Kuschnerov, M .; Spinnler, B .; Rankl, B .; , "Mode scramblers and reduced-search maximum-likelihood detection for mode-dependent-loss-impaired transmission communication, E-communication Communications, E20C. 1-3, 22-26 Sept. 2013

  A method using a long-period grating (LPG) as a device for converting a mode is known, but as described in Non-Patent Document 5, a conversion band is limited. Further, the LPG formed in the MMF has such characteristics that the wavelength at which mode conversion is possible is uniquely determined by the propagation constant difference between the modes, and that the propagation constant difference between the modes is generally different for each selected mode. For this reason, there is a problem that it is difficult to simultaneously convert a plurality of modes in a wide wavelength band in the MMF in which the LPG having one kind of grating pitch is formed.

  Therefore, an object of the present invention is to provide an optical fiber, a mode converter, an optical amplifier, and an optical transmission system capable of simultaneously converting a plurality of modes in a wide wavelength band in order to solve the above-mentioned problems.

  In order to achieve the above object, a mode converter, an optical amplifier, and an optical transmission system according to the present invention employ a graded index fiber in which a parameter and a radius of a core are within predetermined ranges.

Specifically, the optical fiber according to the present invention, the refractive index distribution of the core is a graded index type optical fiber,
Light can be propagated in a plurality of propagation modes in a wavelength range of 1530 to 1625 nm,
The relationship between the α parameter and the core radius r is
2.0-1.4 × 10 ⁻² r + 1.4 × 10 ⁻⁴ r ² ≦ α
≦ 2.0 + 4.3 × 10 ⁻³ r + 1.9 × 10 ⁻⁴ r ² ,
Or 2.0−7.0 × 10 ⁻⁴ r + 8.0 × 10 ⁻⁵ r ² ≦ α
≦ 2.0 + 5.0 × 10 ⁻⁴ r + 1.0 × 10 ⁻⁴ r ²
Is satisfied.

  An optical fiber according to the present invention is characterized in that erbium is added to the core.

  The mode converter according to the present invention is characterized in that a long-period grating is formed in the optical fiber.

  An optical amplifier according to the present invention is characterized in that two amplification optical fibers are connected by the mode converter.

  An optical transmission system according to the present invention includes an optical amplifier having the optical fiber, a transmission line in which the mode converter is arranged, or the optical amplifier.

  By setting the α parameter and the core radius of an optical fiber having a graded index type refractive index distribution within the range of the above formula, the propagation constant difference between a plurality of modes can be reduced to 20 rad / m or less. For this reason, in this optical fiber, since the propagation constant difference between various modes is close, the wavelengths that can be mode-converted are close.

  Further, by employing a graded index optical fiber having a small wavelength dependence of the propagation constant difference, it is possible to reduce the fluctuation of the propagation constant difference over a wide wavelength range. This can cause mode conversion in a wide wavelength range. Therefore, the present invention can provide an optical fiber capable of simultaneously converting a plurality of modes in a wide wavelength band. By forming an LPG on the optical fiber, it is possible to provide a mode converter, an optical amplifier, and an optical transmission system capable of simultaneously converting a plurality of modes in a wide wavelength band.

  The present invention can provide an optical fiber, a mode converter, an optical amplifier, and an optical transmission system capable of simultaneously converting a plurality of modes in a wide wavelength band.

FIG. 3 is a diagram illustrating the wavelength dependence of Δβ for SI fiber and GI fiber. FIG. 3 is a diagram illustrating a relative refractive index Δ of a core with respect to a clad in a radial direction of a GI fiber. FIG. 4 is a diagram illustrating a relationship between a relative refractive index difference Δ and Δβ between modes in a GI fiber. FIG. 6 is a diagram illustrating the relationship between the α parameter and the deviation of Δβ between modes in the C band. FIG. 9 is a diagram illustrating the relationship between the α parameter in the C + L band and the deviation of Δβ between the modes. FIG. 5 is a diagram illustrating a region where an α parameter and a core radius satisfy Δβ ≦ 10 in a C band and a C + L band. FIG. 2 is a diagram illustrating a multimode optical amplifier according to the present invention. FIG. 1 is a diagram illustrating an optical transmission system according to the present invention.

  An embodiment 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.

λ_０：モード変換を起こす共鳴波長
ｒ：グレーデッドインデックス型光ファイバのコア半径（μｍ）
ｘ：グレーデッドインデックス型光ファイバのコア中心からの距離（μｍ）
ｎ１：グレーデッドインデックス型光ファイバのコア中心の屈折率
ｃ：光速（ｍ／ｓ）
Ｌ：グレーティング長（ｍ）
α：αパラメータ、次式の屈折率分布ｎ（ｘ）内で使用される半径方向に対する位置の乗数である。

[Definition]
The parameters used in this specification are as follows.
Δ: maximum relative refractive index between the core and the clad of the graded index optical fiber Δβ: difference in propagation constant between two propagation modes Δβ ′: mode dispersion (dΔβ / dλ) (ps / km)
Δλ: full width at half maximum (nm) of the mode conversion band of LPG, expressed by the following equation.

λ ₀ : resonance wavelength causing mode conversion r: core radius (μm) of graded index optical fiber
x: distance (μm) from the center of the core of the graded index optical fiber
n1: Refractive index at the center of the core of the graded index optical fiber c: Speed of light (m / s)
L: Grating length (m)
α: α parameter, a position multiplier in the radial direction used in the refractive index distribution n (x) in the following equation.

(Embodiment 1)
In the present embodiment, the refractive index distribution of the core is a graded index type optical fiber,
Light can be propagated in a plurality of propagation modes in a wavelength range of 1530 to 1625 nm,
The relationship between the α parameter and the core radius r is
2.0-1.4 × 10 ⁻² r + 1.4 × 10 ⁻⁴ r ² ≦ α
≦ 2.0 + 4.3 × 10 ⁻³ r + 1.9 × 10 ⁻⁴ r ² ,
Or 2.0−7.0 × 10 ⁻⁴ r + 8.0 × 10 ⁻⁵ r ² ≦ α
≦ 2.0 + 5.0 × 10 ⁻⁴ r + 1.0 × 10 ⁻⁴ r ²
An optical fiber characterized by satisfying the following will be described.

  An LPG can be formed in an optical fiber and used as a mode converter of a propagation mode. As a method of forming an LPG on an optical fiber, a method of irradiating the optical fiber with ultraviolet light or electric discharge to create a periodic refractive index change in a longitudinal direction of the optical fiber, or a method of realizing a periodic grating plate using an optical fiber. For example. In the present embodiment, parameters that determine the characteristics of a mode converter in which an LPG is formed on an optical fiber will be described.

[Allowable propagation constant difference]
In converting from mode 1 to mode 2 using LPG, it is necessary to produce a periodic structure according to the propagation constant difference Δβ between mode 1 and mode 2. Assuming that the manufacturing error of the LPG is ± 1 μm, it is assumed that the LPG is manufactured on a graded index (GI) fiber having Δ = 1.5%, a = 15 μm, and α = 2.0. At this time, the propagation constant difference Δβ between the two corresponding propagation modes has a deviation of about 10. Therefore, Δβ allowed during mode conversion is set to ± 10.

[Wavelength dependence of propagation constant difference]
Here, the reason that the optical fiber used in the present embodiment is a GI fiber will be described.
FIG. 1 shows the LP01 mode and LP11 of a step index (SI) fiber having a core radius of 7.8 μm and a relative refractive index difference of 1.0% and a GI fiber having a core radius of 10 μm, a relative refractive index difference of 1% and α = 2.0. The relation between Δβ and the wavelength when the wavelength of the mode is 1565 nm is shown. The normalized frequencies of the two fibers are the same. That is, it is a comparison between the SI fiber and the GI fiber adjusted so that Δβ = 0 at the wavelength of 1565 nm.

  As shown in FIG. 1, the slope of the wavelength dependency of Δβ is gentler in the GI fiber than in the SI fiber. When the deviation of Δβ is allowed to be about 10, a wideband mode conversion can be expected with a GI fiber.

The full width at half maximum Δλ of the mode conversion band of the long-period grating can be expressed by Expression (1) as described in Non-Patent Document 5. Assuming that L = 2 cm, if Δβ ′ = 3500 ps / km or less, Δλ which is a wavelength band capable of mode conversion covers the C band and the L band. Here, the GI fiber of α = 2.0, Δ = 1% and a = 10 μm and the calculation results of Δβ ′ of α = 2.0, Δ = 1% and a = 25 μm are shown in FIGS. 9 and 10, respectively. . Both figures show Δβ ′ in the LPm1 mode and the LPn1 mode (m and n are integers of 0 or more, and m ≠ n).
From the results of FIGS. 1, 9 and 10, it can be seen that if LPG is formed on the GI fiber, the wavelength dependence of various propagation constant differences Δβ is small. That is, it is possible to design a mode converter in which various Δβs are reduced (Δβ ′ ≦ 3500 ps / km) over the C band and the L band by forming the LPG on the GI fiber.

[Difference in propagation constant between propagation modes]
Here, structural parameters of the GI fiber that can reduce the propagation constant difference Δβ between various propagation modes will be described.
Although the number of modes that can be propagated varies depending on the structure of the GI fiber, it is known that a mode group is formed by higher-order modes (for example, Non-Patent Document 6). In a GI fiber that can propagate over 20 LP modes,
(LP01),
(LP11),
(LP21, LP02),
(LP12, LP31),
(LP03, LP22, LP41),
(LP13, LP32, LP51),
(LP04, LP23, LP42, LP61),
(LP14, LP33, LP52, LP71)
Are formed. Modes enclosed in parentheses () have very small differences in propagation constants and frequently generate mode coupling. Looking at these groups, each group includes the LPm1 mode (m is an integer of 0 or more). Therefore, in this study, the LPm1 mode is used as a mode to be studied when two or more modes are simultaneously excited using one LPG. In other words, the study is advanced among propagation modes in which mode coupling is difficult. Specifically, a calculation study is performed assuming mode conversion among LP01-LP11, LP11-LP21, LP21-LP31, LP31-LP41, LP41-LP51, and LP61-LP71.

1. The maximum relative refractive index Δ between the core and the clad of the GI fiber has a small effect on the propagation constant difference Δβ.
FIG. 2 is a diagram illustrating the relative refractive index Δ of the core with respect to the cladding in the radial direction of the GI fiber. The variables for designing a GI fiber are α, a and Δ. First, FIG. 3 shows Δβ between modes when a = 25 μm, α = 2.0, and Δ is a variable. Δ was calculated in the range of 1.0% to 2.5%. It can be confirmed that the variation of Δβ between modes with the increase of Δ is as small as 1 or less. Therefore, in this study, Δ is ignored as a variable for designing Δβ, and the variables are used as the core radius and the α parameter. Hereinafter, Δβ between the propagation modes is calculated in the range of the α parameter of 1.5 to 2.5 and the core radius of 15 to 30 μm.

2. There is a range of α parameters for suppressing the propagation constant difference Δβ between various propagation modes to within ± 10. However, the range of the α parameter has wavelength dependence.
FIG. 4 shows the result of evaluating the α parameter dependence of the propagation constant difference Δβ in a GI fiber having a core radius r = 22 μm. The wavelength is 1550 nm. As described above, if the deviation of Δβ between the modes is allowed up to 10, the α parameter is in the range of 1.78 to 2.18. Similarly, the result of expanding the band to the C + L band is shown in FIG. The α parameter ranges from 1.88 to 2.03.
[Supplement]
FIG. 4 shows the α parameter dependence of Δβ in the GI fiber for the C band (1530-1565 nm), and FIG. 5 shows the α parameter dependence of Δβ for the C + L band (1530-1625 nm). If the wavelength gradually changes from 1530 nm to 1625 nm, the interval between the contour lines of Δβ in FIG. 4 gradually narrows, as shown in FIG. Since there are two types of GI fibers, one for the C band and the other for the C + L band, the α parameter is set so as to cover each wavelength band.

4 and 5 were performed in a core radius of 15 to 30 μm. FIG. 6 is a diagram summarizing how the range of the α parameter that suppresses the propagation constant difference Δβ between various propagation modes to within ± 10 in the range of a radius of 15 to 30 μm changes.
In the C band, the range of the core radius and the α parameter for suppressing the propagation constant difference Δβ between various propagation modes to within ± 10 is as follows.
2.0-1.4 × 10 ⁻² r + 1.4 × 10 ⁻⁴ r ² ≦ α
≦ 2.0 + 4.3 × 10 ⁻³ r + 1.9 × 10 ⁻⁴ r ²
And
In the C + L band,
2.0-7.0 × 10 ⁻⁴ r + 8.0 × 10 ⁻⁵ r ² ≦ α
≦ 2.0 + 5.0 × 10 ⁻⁴ r + 1.0 × 10 ⁻⁴ r ²
It is.

  By forming the LPG on the GI fiber having the above range of the core radius and the α parameter, a wide-band propagation mode converter can be realized, and mode conversion can be realized between two or more propagation modes even at the same grating pitch.

(Embodiment 2)
The optical fiber described in the first embodiment can be used for a multimode optical amplifier (for example, Non-Patent Document 7). FIG. 7 is a diagram illustrating a multi-mode optical amplifier 301 including the optical fiber described in the first embodiment. The multi-mode optical amplifier 301 includes a pumping light source 10, a mode multiplexing optical coupler 20 for multiplexing an input optical signal with the pumping light from the pumping light source 10, and an optical signal pumped by the pumping light. A multimode amplification optical fiber 30 for amplification and an isolator 50 for outputting only the amplified optical signal are provided. The multimode optical amplifier 301 connects the two multimode amplification optical fibers 30 by the mode converter 40, and the mode converter 40 is the optical fiber described in the first embodiment in which the LPG is formed.

  Since the multimode optical amplifier 301 has a mode converter between the multimode amplification optical fibers 30, it has an effect of reducing a gain deviation between modes in amplification, and can reduce a difference in transmission quality between signals.

  Note that erbium may be added to the core of the optical fiber described in the first embodiment and used as the multimode amplification optical fiber 30. Further, if an LPG is formed on the optical fiber, the configuration is not such that the mode converter 40 connects the two multimode amplification optical fibers 30 as shown in FIG. Can be realized.

(Embodiment 3)
When the differential mode delay (DMD) between modes at the receiving end is large, the load of digital processing (DSP) related to MIMO increases, and the DSP load must be reduced in order to realize long-distance transmission. Will be an issue. In order to mitigate the influence of DMD, use of a mode scrambler for generating coupling between modes has been proposed (for example, Non-Patent Document 8).

  Broadband mode scrambling can be realized by introducing the mode converter 40 and the multimode optical amplifier 301 described in the second embodiment in the longitudinal direction into such a transmission line requiring active mode coupling. . FIG. 8 is a diagram illustrating an optical transmission system 401 including the mode converter 40 and the multi-mode optical amplifier 301 described in the second embodiment. The optical transmission system 401 connects the optical transmitter 60 and the optical receiver 70 with a transmission fiber 80. The transmission fiber 80 includes a multi-mode optical amplifier 301 and a mode converter 40 on the way (either one may be used).

  The optical transmission system 401 can reduce the DSP load by introducing the mode converter 40 and the multi-mode optical amplifier 301 as a mode scrambler.

  The present invention relates to a mode conversion optical fiber and a mode converter, and realizes gain adjustment for each propagation mode and extension of a transmission distance in transmission using a plurality of modes.

10: Light source for pumping 20: Optical coupler for mode multiplexing 30: Optical fiber for multi-mode amplification 40: Mode converter 50: Isolator 60: Optical transmitter 70: Optical receiver 80: Transmission fiber 301: Multi-mode optical amplifier 401: Optical transmission system

Claims (6)

A mode converter in which a refractive index distribution of a core has a long-period grating formed in a graded-index type optical fiber,
The graded index optical fiber,
The core radius r is not less than 15 μm and not more than 30 μm;
relationship of the core radius r and α parameter,
When light can be propagated in a plurality of propagation modes in a wavelength range of 1530 to 1565 nm ,
2.0-1.4 × 10 ⁻² r + 1.4 × 10 ⁻⁴ r ² ≦ α
≦ 2.0 + 4.3 × 10 ⁻³ r + 1.9 × 10 ⁻⁴ r ² ,
When light can be propagated in a plurality of propagation modes in a wavelength range of 1530 to 1625 nm,
2.0-8.0 × 10 ⁻³ r + 1.0 × 10 ⁻⁴ r ² ≦ α
≦ 2.0−5.0 × 10 ⁻⁴ r + 1.0 × 10 ⁻⁴ r ²
The filling,
The long period grating,
A mode converter characterized in that mode conversion can be performed between a plurality of propagation modes at the same grating pitch. The mode converter according to claim 1, wherein the graded index optical fiber has a uniform refractive index of a cladding. The mode converter according to claim 1, wherein the graded index optical fiber has 20 or more propagation modes. The mode converter according to any one of claims 1 to 3, wherein erbium is added to the core. 4. An optical amplifier, wherein two amplifying optical fibers are connected by the mode converter according to claim 1. An optical transmission system comprising the transmission line in which the mode converter according to claim 1 is arranged, or an optical amplifier according to claim 5 .

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