JP6445938B2 - Multi-core optical fiber and optical amplifier - Google Patents

Description

  The present invention relates to a multicore optical fiber and an optical amplifier in spatial multiplexing transmission.

  In recent years, Internet traffic has continued to increase due to the diversification of services, and the transmission capacity has been dramatically increased due to the increase in the number of wavelength multiplexing due to the increase in transmission speed and wavelength division multiplexing (WDM) technology. . Further, 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 multilevel phase modulation signals, but higher signal-to-noise ratios are required. However, in a transmission system using a single mode optical fiber (SMF), the transmission capacity is expected to saturate at 100 Tbit / sec due to input power limitation due to nonlinear effects in addition to the theoretical limit. Therefore, further increase in capacity has become difficult.

  In order to further increase the transmission capacity in the future, a medium that realizes innovative transmission capacity expansion is required. Therefore, mode multiplex transmission using a multi-core optical fiber (Multi Core Fiber, MCF) that can be expected to improve the signal-to-noise ratio and the space utilization efficiency by using a plurality of propagation modes in an optical fiber as a channel is attracting attention. (For example, refer nonpatent literature 1 and 2.).

  A multi-core-erbium-doped fiber amplifier (MC-EDFA) for amplifying light propagating through each core is an indispensable device for extending the transmission distance using a multi-core optical fiber. Become. As reported in Non-Patent Document 1, the MC-EDFA uses a core excitation means for inputting excitation light for each core and performing optical amplification. In addition, since individual pumping requires a pumping light source for each core, a cladding-pumped MC-EDFA using a multi-core optical fiber with a double-clad structure is proposed as a method that can be expected to reduce size and power consumption. (For example, see Non-Patent Document 2).

T.A. Kobayashi et al. , “2 × 344 Tb / s propagation-direction interleaved transmission over 1500-km MCF enhanced by multi-fullerial-field digital prop. ECOC2013, paper PD3. E. 4). H. Ono et al. "12-Core Double-Clad Er / Yb-Doped Fiber Amplifier Amplifying Free-space Coupling Pump / Signal Combiner Module" EcoC 2013 We. 4). A. 4 (2013). Junichi Sakai, “Guided-wave optics”, Kyoritsu Shuppan, February 15, 2004, p. 162-163 K. Okamoto, “Basics of Optical Waveguides”, Corona, October 20, 1992, p. 150-151

  However, in the clad excitation type MC-EDFA, the excitation intensity cannot be adjusted for each core because a plurality of cores are excited using a light source having the number of cores or less, and a gain difference between cores (Core dependent gain, CDG) is generated. To do.

  Accordingly, an object of the present invention is to suppress the occurrence of a gain difference between cores without inputting pumping light for each core when amplifying the propagation light of a multi-core optical fiber.

Specifically, the multi-core optical fiber of the present invention is
A multi-core optical fiber in which a plurality of cores doped with rare earth ions are disposed in a first cladding,
A second cladding disposed around the first cladding and reflecting excitation light having a wavelength for exciting the rare earth ions;
The plurality of cores have inter-core distances where propagation light is coupled.

In the multi-core optical fiber of the present invention,
The plurality of cores may propagate two or more modes.

In the multi-core optical fiber of the present invention,
The plurality of cores are:
When the coupling length of the excitation light is L _c ,
The coupling coefficient κ between the cores may be π / (2L _c ) or more.

In the multi-core optical fiber of the present invention,
The inter-core distance may be less than 35 μm.

Specifically, the optical amplifier of the present invention is
A multi-core optical fiber according to the present invention;
An excitation light source that emits excitation light having a wavelength for exciting the rare earth ions;
A multiplexer that couples the excitation light to at least one of the plurality of cores;
Is provided.

  The present invention contributes to the realization of mode multiplexing transmission by reducing the gain difference between the cores as described above. Further, in the multicore optical fiber for amplification that has been studied so far, it is necessary to provide a sufficient inter-core distance and a cladding outer diameter in order to reduce cross-core crosstalk of signal light. However, in the present invention that actively uses the coupling between the cores, the configuration allows the crosstalk between the cores, so that the outer diameter of the cladding and the core interval can be reduced, and the efficiency of coupling the excitation light to the core increases. Reduction of power can be expected. Therefore, according to the present invention, when the propagation light of the multi-core optical fiber is amplified, it is possible to suppress the occurrence of the gain difference between the cores without inputting the excitation light for each core.

An example of the relationship between the coupling amount per meter to the adjacent core and the gain difference between the cores is shown. An example of the schematic diagram of the multi-core optical fiber for amplification is shown. An example of the schematic diagram of a 2-core multi-core optical fiber is shown. An example of the relationship between the core radius a, the relative refractive index difference Δ, and the coupling length L _c (m) when the inter-core distance D = 25 μm is shown. An example of the relationship between the core radius a, the relative refractive index difference Δ, and the coupling length L _c (m) when the inter-core distance D = 35 μm is shown. An example of the relationship between the ratio of the amount of light coupled to each core when a Gaussian beam is incident on the first cladding, the core radius a, the relative refractive index difference Δ, and the inter-core distance D is shown. An example of a structure of an optical amplifier is shown. An example of the structure of the optical amplifier which amplifies by front and back pumping is shown.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited to embodiment shown below. These embodiments are merely examples, and the present invention can be implemented in various modifications and improvements based on the knowledge of those skilled in the art. In the present specification and drawings, the same reference numerals denote the same components.

(Embodiment 1)
Hereinafter, a multicore optical fiber for amplification for realizing CDG reduction in the optical amplifier according to the present invention will be described in detail. In addition, since the multi-core optical fiber according to the present embodiment needs to couple the modes propagating between the cores to the adjacent cores, the structure for coupling the adjacent cores will be described in detail.

  First, the relationship between inter-core coupling and CDG reduction in a multi-core optical fiber will be described. Here, for simplicity, a 2-core multi-core EDF is considered. Here, signal light propagates as propagation light to the cores 11 # 1 and 11 # 2 which are the two cores 11, and the rare earth element added to the cores 11 # 1 and 11 # 2 is erbium (Er). Proceed.

ここでＰｓは信号光強度、ｚはＥＤＦにおける光の伝搬方向での位置、νは信号光の波長である。 When the rate equation when Er is considered as a three-level system is solved and the propagation equation of EDF is obtained, it can be expressed as the following equation (1).

Here, Ps is the signal light intensity, z is the position of the EDF in the light propagation direction, and ν is the wavelength of the signal light.

ここで、Ｎ_２（ｒ，θ，ｚ）、Ｎ_１（ｒ，θ，ｚ）は、ぞれぞれ位置ｚにおけるマルチコア光ファイバ断面の１点を動径ｒと偏角をθで表した時のＥｒ^３＋励起準位と基底準位のイオン数を表す。 The signal light gain coefficient γ _e (z, ν) and the absorption coefficient γ _a (z, ν) are the normalized electric field distribution φ _s (ν, r, θ) of the signal light, and the emission cross-sectional area σ for the signal light and the excitation light. ^{Using S} _es and absorption cross section σ ^S _as , they can be expressed as in the following formulas (2) and (3), respectively.

Here, N ₂ (r, θ, z) and N ₁ (r, θ, z) represent one point on the cross section of the multi-core optical fiber at each position z, and the radius r and the declination angle are represented by θ. This represents the number of ions of the Er ³⁺ excited level and the ground level at the time.

  By solving the propagation equation of Formula (1) in each propagation mode, the gain of the signal light due to the difference in the mode of the excitation light can be found. The excitation light of the amplification multi-core optical fiber (EDF) doped with Er uses a wavelength of 980 nm band, and the excitation intensity intentionally gives a gain difference between the cores. Therefore, the core 11 # 1 has 70 mW, the core 11 # 2 120 mW is input, the Er concentration to be added is 400 ppm, and the length of the EDF used is 20 m. The signal light is 1550 nm and the input power is -15 dBm.

  As a result of calculation, when there is no coupling, the difference in gain between the core 11 # 1 and the core 11 # 2 is 2 dB. A study will be made as to what the gain difference between the cores 11 becomes when the core 11 # 1 and the core 11 # 2 are coupled. FIG. 1 shows fluctuations in the gain difference between the cores 11 with respect to the coupling amount to the adjacent cores 11 when propagating through the EDF for 1 m. From FIG. 1, it can be seen that the gain difference between the cores 11 decreases as the coupling amount to the adjacent cores increases in the EDF. In the present embodiment, the same characteristics can be obtained regardless of the number of cores 11.

  Next, the relationship between the coupling of light propagating through the core 11 and the parameters of the core 11 will be clarified. FIG. 2 shows a cross-sectional view of the amplification multi-core optical fiber 91 according to this embodiment. FIG. 2 is a cross-sectional view perpendicular to the longitudinal direction of the multi-core optical fiber 91 according to the present embodiment. As shown in FIG. 2, the multi-core optical fiber 91 according to this embodiment includes a core 11 to which a plurality of rare earth ions # 1 to # 6 are added, and a first cladding 12 surrounding the cores 11 to # 1 to # 6. And a second cladding 13 surrounding the first cladding 12.

  In FIG. 2, the number of cores 11 provided in the multi-core optical fiber 91 is six, but the number of cores 11 is arbitrary. In addition, each of the cores 11 included in the multicore optical fiber 91 may propagate the propagation light in a plurality of propagation modes. The multi-core optical fiber 91 may further include a coating (not shown) that contacts the second cladding 13 and surrounds the second cladding 13. In the present embodiment, a case where the multi-core optical fiber 91 is a step index (SI) type in which the core radius of the core 11 is a and the relative refractive index difference is Δ will be described.

  In the multi-core optical fiber 91, when the adjacent cores 11 are sufficiently separated and have no interaction, the signal light that is the propagation light in each core 11 propagates in the eigenmode. On the other hand, as the distance between the adjacent cores 11 approaches, the electric field distribution that exudes from each core 11 has an effect, and a power transition occurs between the cores 11. Hereinafter, for simplicity, the case where the multi-core optical fiber 91 has a two-core structure of the core 11 # 1 and the core 11 # 2 as shown in FIG. 3 will be described.

Here, as shown in FIG. 3, the refractive index of the cores 11 of # 1 and # 2 is n ₁ , the refractive index of the first cladding 12 is n ₀ , and the distance between the centers of the cores 11 # 1 and 11 # 2 is The inter-core distance D is assumed. In this case, when the structures of the two cores 11 # 1 and 11 # 2 are equal and the propagation constants of the fundamental modes propagating through the cores 11 # 1 and 11 # 2 are equal, the core 11 # 1 is coupled to the core 11 # 2. The electric field components are in phase. By maintaining this phase relationship over a long distance, a power transition occurs between the core 11 # 1 and the core 11 # 2.

  In the above description, the same core structure and the same propagation constant are described. However, in the core structure having different propagation constants, electric field components are mismatched between adjacent cores 11 along with propagation, so that a part of the power is shifted. become. In the two core structures, the core 11 # 1 and the light propagating through the core 11 # 2 in the z direction will be discussed.

ここで、Ａ（ｚ）、Ｂ（ｚ）は、それぞれ、位置ｚにおけるコア１１＃１、コア１１＃２における電界振幅係数である。κは、２コア１１＃１及びコア１１＃２間のモード結合係数である。 The optical power of the forward wave after propagating through the core 11 # 1 and the core 11 # 2 by the position z is expressed by the following equations (4) and (5), respectively. (See Non-Patent Document 3.)

Here, A (z) and B (z) are electric field amplitude coefficients in the core 11 # 1 and the core 11 # 2 at the position z, respectively. κ is a mode coupling coefficient between the two cores 11 # 1 and the core 11 # 2.

When light is incident on only one of the two cores, the optical power in the core 11 # 1 and the core 11 # 2 changes periodically as the position z changes. The maximum value of the relative intensity | B (z) / A (0) | ² expressed by the equation (5) is (2κ / δβ) ² , and the coupling length L _c is between the two cores 11 # 1 and 11 # 2. The optical power shifts every time.

The coupling length L _c of the core 11 adjacent to the core 11 is expressed by the following equation.
(Equation 6)
L _c = π / (δβ) (6)

In particular adjacent cores 11 # 1 and 11 # are equal 2 core structure, when the propagation constant of the propagation modes are the same, the coupling length L _c can be expressed by the following equations.
(Equation 7)
L _c = π / (2κ) (7)
In-bound amplified multi-core optical fiber 91 for, in order to generate sufficient mode coupling in between adjacent cores 11, it fiber length is the length of the multi-core optical fiber 91 to be used is the coupling length L _c or Is required.

Hereinafter, an example of the calculation of the relationship between the core radius a and the relative refractive index difference Δ, which are the core parameters, the coupling coefficient κ, and the coupling length L _c will be described. As shown in Non-Patent Document 4, the coupling coefficient κ of the fundamental mode propagating through the two parallel cores 11 is expressed by the following equation.

ｖはＶ値、Ｋはベッセル関数、ｋは波数、βは伝搬モードの伝搬定数を表す。ここで、コア半径ａ、比屈折率差Δ、コア間距離Ｄをそれぞれ、４．０〜８．０μｍ、０．４〜０．８％、２５および３５μｍとした際の結合長Ｌ_ｃを求めた。 Here, u and w can be expressed by the following equations (9) and (10).

v is a V value, K is a Bessel function, k is a wave number, and β is a propagation constant of a propagation mode. Here, the coupling length L _c is obtained when the core radius a, the relative refractive index difference Δ, and the inter-core distance D are 4.0 to 8.0 μm, 0.4 to 0.8%, 25, and 35 μm, respectively. It was.

FIG. 4 shows the value of the coupling length L _c (m) when the core radius a and the relative refractive index difference Δ are used as a function when D = 25 μm. In Figure 4, L405 is _L c = 5m, L410 is _L c = 10m, L415 is _L c = 15m, L420 is _L c = 20m, L425 is _L c = 25m, L430 is _L c = 30m, L435 is _{L c} = 35 m, L440 is L _c = 40 m, L445 is L _c = 45 m, L450 is L _c = 50 m, L455 is L _c = 55 m, L460 is L _c = 60 m, L465 is L _c = 65 m, L470 is L _c = 70 m, L475 is L _c = 75 m, L480 is L _c = 80 m, L485 is L _c = 85 m, L490 is L _c = 90 m, and L495 is L _c = 95 m.

From the results shown in FIG. 4, the coupling length L _c as will high region and a small area of the core radius a of the relative index difference Δ is larger tendency. In general, an Er ³⁺ doped multi-core optical fiber used for C-band optical amplification has a strip length of about several tens of meters. For this reason, it is understood that mode coupling occurs sufficiently in the multi-core optical fiber 91 by using the optimum fiber parameters.

FIG. 5 shows the value of the coupling length L _c when the core radius a and the relative refractive index difference Δ are used as a function when the inter-core distance D = 35 μm. In FIG. 5, L510 indicates a value at which L _c = 100 m, L520 indicates a value at which L _c = 200 m, L530 indicates a value at which L _c = 300 m, and L540 indicates a value at which L _c = 400 m. L550 indicates a value where L _c = 500 m, L560 indicates a value where L _c = 600 m, L570 indicates a value where L _c = 700 m, L580 indicates a value where L _c = 800 m, L590 indicates a value at which L _c = 900 m.

From the comparison between FIG. 5 and FIG. 4, when the inter-core distance D is increased, the required coupling length L _c is increased in all regions, and when the inter-core distance D is about 35 μm, the coupling amplification is performed. It turns out that it is not suitable as a multi-core optical fiber. Therefore, the inter-core distance D is preferably less than 35 μm. As described above, the multi-core optical fiber 91 for coupling amplification can be designed by optimally adjusting the parameters of the multi-core optical fiber 91 such as the inter-core distance D, the core radius a, and the relative refractive index difference Δ. .

  Next, the excitation efficiency at which propagating light is coupled to each core 11 in the 6-core multi-core structure shown in FIG. FIG. 6 shows the relationship between the ratio of the amount of light coupled to each core 11 and the core radius a, the relative refractive index difference Δ, and the inter-core distance D when a Gaussian beam is incident on the first cladding 12 as excitation light. The excitation light incident on the first cladding 12 is reflected at the interface between the first cladding 12 and the second cladding 13 and propagates inside the first cladding 12.

  In FIG. 6, L61 represents the case where the core diameter 2a = 10 μm and the relative refractive index difference Δ = 0.7%, and L62 represents the case where the core diameter 2a = 14 μm and the relative refractive index difference Δ = 0.4%. L63 indicates a case where the core diameter is 2a = 14 μm and the relative refractive index difference Δ is 0.7%. The outer diameter of the first cladding 12 was 125 μm.

  From FIG. 6, it can be seen that the smaller the inter-core distance D is, the larger the ratio of the amount of light coupled to the core 11 is. Further, comparing L63 and L61, it can be seen that the ratio of the amount of light coupled to the core 11 increases as the core diameter 2a increases. Further, comparing L63 and L62, it can be seen that the larger the relative refractive index difference Δ is, the larger the ratio of the amount of light coupled to the core 11 is.

  Therefore, since the multi-core optical fiber 91 according to this embodiment uses the coupling of the cores 11 # 1 to 11 # 6, when amplifying the propagation light of the multi-core optical fiber 91, the pump light is not input to each core 11 The occurrence of a gain difference between the cores 11 can be suppressed.

  In addition, although this embodiment demonstrated the case where rare earth ions were erbium (Er), the invention which concerns on this embodiment is not limited to this. For example, as the rare earth ion, any rare earth element capable of exciting signal light such as thulium (Tm) or praseodymium (Pr) can be used.

(Embodiment 2)
FIG. 7 shows a configuration example of the optical amplifier according to the present embodiment. The optical amplifier according to the present embodiment includes a multi-core optical fiber 91 according to the first embodiment, a pumping light source 92, an optical isolator 94 for preventing oscillation in the multi-core optical fiber 91, an optical multiplexer 93, and a multi-core optical fiber. 97 # 1 and 97 # 2.

  In the optical amplifier according to the present embodiment, a mode multiplexed transmission signal using a plurality of propagation modes in the optical fiber as channels is incident on the optical multiplexer 93 as the signal light S1. The multi-core optical fibers 97 # 1 and 97 # 2 include the same number of cores as the multi-core optical fiber 91 as shown in FIG. The connection unit 99 # 1 makes signal light propagating through each core included in the multicore optical fiber 97 # 1 enter each core 11 of the multicore optical fiber 91. Connection unit 99 # 2 makes signal light propagating through each core 11 of multi-core optical fiber 91 enter each core of multi-core optical fiber 97 # 2.

  The excitation light source 92 emits excitation light that is forward excitation light. The excitation light is light that excites rare earth ions added to the core 11 provided in the multi-core optical fiber 91. For example, when the rare earth ion is erbium, the excitation light source 92 emits light having a wavelength of 980 nm.

  The optical multiplexer 93 multiplexes the signal light S1 propagating through the multi-core optical fiber 97 # 1 and the excitation light E1 emitted from the excitation light source 92. As a result, the combined light obtained by combining the signal light S1 and the pumping light E1 enters the multi-core optical fiber 91 via the connection unit 99 # 1.

  The multi-core optical fiber 91 amplifies the signal light S1 using the excitation light E1. The amplified signal light is emitted from the connection section 99 # 2 to the multi-core optical fiber 97 # 2. The optical isolator 94 receives light emitted from the multi-core optical fiber 97 # 2 and allows the light to pass only in one direction.

  The optical multiplexer 93 makes the excitation light incident on the clad of the multi-core optical fiber 97 # 1. The excitation light incident on the multi-core optical fiber 91 via the multi-core optical fiber 97 # 1 excites the rare earth ions added to the cores 11 # 1 to 11 # 6 when propagating through the multi-core optical fiber 91.

  The optical multiplexer 93 may make the excitation light enter one of the cores 11 # 1 to 11 # 6 of the multicore optical fiber 97 # 1. The pumping light incident on the multicore optical fiber 91 via the multicore optical fiber 97 # 1 exits from the cores 11 # 1 to 11 # 6 to the first cladding 12 and is reflected by the second cladding 13 while being reflected by the multicore optical fiber. 91 is propagated.

  Therefore, since the optical amplifier according to the present embodiment uses the coupling of the cores 11 # 1 to 11 # 6 provided in the multicore optical fiber 91, when the propagation light of the multicore optical fiber 91 is amplified, the pumping light is supplied to each core 11. Generation of a gain difference between the cores 11 can be suppressed without input.

(Embodiment 3)
FIG. 8 shows a configuration example of the optical amplifier according to the present embodiment. The optical amplifier according to the present embodiment amplifies propagating light propagating through the multi-core optical fiber 91 by forward pumping and backward pumping. Specifically, the optical amplifier according to the present embodiment includes a multi-core optical fiber 91 according to the first embodiment, pumping light sources 92 # 1 and 92 # 2, an optical isolator 94, and optical multiplexers 93 # 1 and 93. # 2 and multi-core optical fibers 97 # 1 and 97 # 2.

  The excitation light source 92 # 2 emits backward excitation light, which is excitation light for exciting rare earth ions added to the core 11 included in the multi-core optical fiber 91, similarly to the excitation light source 92 # 1. The optical multiplexer 93 # 2 causes the light emitted from the excitation light source 92 # 2 to enter the multi-core optical fiber 91, and emits the light incident from the multi-core optical fiber 91 to the optical isolator 94.

  The wavelength of the backward pumping light is a wavelength that can excite the rare earth element added to the core 11, and when erbium is added to the core 11, for example, the wavelength of the 980 nm band. The wavelength of the backward pumping light emitted from the excitation light source 92 # 2 may be different from the wavelength of the forward pumping light emitted from the excitation light source 92 # 1. For example, the wavelength of the backward excitation light is 1480 nm.

  The optical multiplexers 93 # 1 and 93 # 2 cause excitation light to enter the first cladding 12 of the multi-core optical fibers 97 # 1 and 97 # 2. Further, the optical multiplexers 93 # 1 and 93 # 2 may make the excitation light incident on any of the plurality of cores provided in the multi-core optical fibers 97 # 1 and 97 # 2. In this embodiment, as shown in FIG. 8, it is possible to obtain a good noise figure and high output gain characteristics by using bidirectional excitation.

  Further, in the coupled amplification multicore optical fiber 91 according to the present embodiment, by making each core 11 a core 11 capable of propagating several modes, it is possible to suppress CDG generated in a higher-order mode. . Since the coupling length in the higher mode is generally shorter than that in the fundamental mode, the same effect as in the fundamental mode is obtained if the coupling length in the fundamental mode is equal to or shorter than the length of the multi-core optical fiber 91 for coupled amplification. be able to.

  The multi-core optical fiber and the optical amplifier of the present invention can be applied to the communication industry.

11: Core 12: First clad 13: Second clad 91: Multi-core optical fiber 92: Excitation light source 93: Optical multiplexer 94: Optical isolator 97: Multi-core optical fiber 99: Connection portion

Claims (5)

A multi-core optical fiber in which a plurality of cores doped with rare earth ions are disposed in a first cladding,
A second cladding disposed around the first cladding and reflecting excitation light having a wavelength for exciting the rare earth ions;
In the plurality of cores, the distance between adjacent cores is a distance at which signal light that is propagating light is coupled.
Multi-core optical fiber. The plurality of cores propagate more than one mode;
The multi-core optical fiber according to claim 1. The plurality of cores are:
When the coupling length of the excitation light is L _c ,
The coupling coefficient κ between the cores is π / (2L _c ) or more.
The multi-core optical fiber according to claim 1 or 2. The inter-core distance is less than 35 μm,
The multi-core optical fiber according to claim 1. The multi-core optical fiber according to any one of claims 1 to 4,
An excitation light source that emits excitation light having a wavelength for exciting the rare earth ions;
A multiplexer that couples the excitation light to at least one of the plurality of cores;
An optical amplifier.

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