JP2015088530A - Multi-core optical fiber for optical amplification and optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-core optical fiber for optical amplification which is compact and economical, and can perform optical amplification with high efficiency.SOLUTION: A multi-core optical fiber for optical amplification is configured so that a signal core 12 is arranged in a concentric shape at a position eccentric from the center of a fiber 10 and a neighborhood of the fiber 10 center encircled by the signal core 12 becomes an excitation light supply region 14 that is composed of a clad layer 11 and supplies excitation light. On an outer periphery of the signal core 12, a trench 13 that is smaller in refractive index than the clad layer 11 is formed, an equivalent refractive index of one of or both of the signal core 12 side and the excitation light supply region 14 is adjusted so that an equivalent refractive index on the signal core 12 side with respect to an excitation wavelength and an equivalent refractive index of the excitation light supply region 14 with respect to an excitation wavelength match, and optical amplification of the excitation light power inputted to the excitation light supply region 14 is performed by using resonance by mode coupling in the way that it is absorbed by the signal core 12 side.

Description

  The present invention relates to an optical amplification optical fiber that performs optical amplification in a fiber, and more particularly, to an optical fiber for multicore optical amplification using a multicore fiber provided with a plurality of signal cores for transmitting optical signals. .

  In optical fiber communication, with an increase in the amount of communication information, the communication capacity has been expanded by combining time division multiplexing and wavelength division multiplexing. However, it is approaching the theoretical limit calculated from the available wavelength bandwidth of the optical fiber. In order to exceed this limit, development of a multi-core fiber having a plurality of signal cores for transmitting signals in one optical fiber and development of a multi-core optical transmission technology using the same are progressing.

In an optical transmission technique using a multi-core fiber, it is necessary to perform optical amplification of signal light by supplying pumping light (amplification optical power) to all of a plurality of signal cores.
A method of supplying excitation light individually for each signal core when performing optical amplification is disclosed (for example, see Non-Patent Document 1). According to this method, optical amplification with high amplification efficiency can be performed. However, the same number of pumping light sources and optical couplers as the number of signal cores are required, and as a result of increasing the number of parts, it becomes difficult to make a compact configuration, and the optical amplifier as a whole becomes expensive. .

Therefore, in addition to the signal core, one excitation core for supplying the excitation light is provided in one optical fiber, and the excitation light is input from this to the plurality of signal cores in a lump. An optical amplifying apparatus using a multi-core optical amplifying optical fiber that performs optical amplification has been devised (see Patent Document 1). Hereinafter, a conventional example described in this document will be briefly described.
As shown in FIG. 8, the optical amplification device 100 includes a multi-core fiber 101 and a pumping light source 102. The multi-core fiber 101 includes a cladding layer 110, a pumping core 111, and a plurality of signal cores 112-1 to 112-n to which Er ³⁺ that is an optical amplification medium is added (indicated as 112 when not specified individually). ). The excitation core 111 is disposed at the center of the multi-core fiber 101 (center of the cladding layer 110), and the signal core 112 is disposed at a position spaced apart from each other around the excitation core 111. The periphery of the excitation core 111 and the signal core 112 is covered with a clad layer 110.
In the signal core 112, the refractive index with respect to the cladding layer 110 is sufficiently high so that the optical power of the signal light passing through the core is confined in the core. On the other hand, in the excitation core 111, the refractive index is lower than that of the signal core 112 and higher than that of the cladding layer 110 so that the optical power of the passing excitation light is not completely confined in the core. .

When pumping light is input from the pumping light source 102 to the pumping core 111 of the optical amplifying apparatus 100 via an optical coupler or the like, most of the pumping light stays in the pumping core 111, but one of the pumping light is emitted. The part oozes out from the excitation core 111 and propagates. A part of the excitation light propagating outside the excitation core 111 propagates isotropically in the radial direction from the excitation core 111, and a part of the excitation light reaches the position of the signal core 112. By exciting the Er ³⁺ ions added to 112, the signal light flowing through the signal core 112 is optically amplified.

H. Takahashi, T. Tsuritani, ELT de Gabory, T. Ito, WRPeng, K. Igarashi, K. Takeshima, Y. Kawaguchi, I. Morita, Y. Tsuchida, Y. Mimura, K. Maeda, T. Saito , K. Watanabe, K. Imamura, R. Sugizaki, and M. Suzuki, “First demonstration of MC-EDFA-repeatered SDM transmission of 40 x 128-Gbit / s PDM-QPSK signals per core over 6,160-km 7- core MCF ”Optics Express, vol.21 no.1, pp.789-795 (2013).

JP 2013-58651 A

  In the optical amplification technique using the multi-core fiber described in Patent Document 1 described above, a plurality of signal cores 112 can be optically amplified in a lump by inputting excitation light to the excitation core 111. Therefore, the number of components of the pumping light source and the optical coupler is reduced, the configuration of the optical amplifying device 100 can be made compact, and the device price can be reduced.

  However, most of the excitation light input to the excitation core 111 remains in the excitation core 111, and only a part of the excitation light propagates to the signal core 112 via the cladding layer 110. The light propagated from the cladding layer 110 to the cladding layer 110 is also propagated isotropically, so that the ratio of the optical power that reaches the signal core 112 and is used for optical amplification is small, and the efficiency of optical amplification is never high. There wasn't.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a multicore optical amplification optical fiber capable of performing optical amplification with high efficiency never obtained in the prior art and an optical amplification apparatus using the same.

  An optical fiber for multi-core optical amplification according to the present invention, which has been made to solve the above problems, includes at least three signal cores to which an amplification medium for amplifying pumping light is added, and a cladding layer covering the signal cores. Pumping light in which the signal core is concentrically arranged at a position eccentric from the center of the multicore fiber, and the vicinity of the center of the multicore fiber surrounded by the signal core is formed of a cladding layer. A multi-core optical amplifying optical fiber having a structure in which a trench having a refractive index lower than that of the cladding layer is formed on the outer periphery of each signal core. A signal core side based on the trench and the signal core for an excitation wavelength determined by the amplification medium of the signal core. The equivalent refractive index of either the signal core side or the excitation light supply region or both is adjusted so that the valence refractive index matches the equivalent refractive index of the excitation light supply region with respect to the excitation wavelength, and the excitation light supply Optical amplification is performed such that the pumping light power input to the region is absorbed by the signal core side using resonance due to mode coupling.

  From another point of view, the present invention can be an optical amplifying apparatus comprising the multi-core optical amplification optical fiber and an excitation light source that irradiates an excitation light supply region of the optical fiber with excitation light having an excitation wavelength. .

Here, the “amplification medium for amplifying the excitation light” is an active substance that is excited by the excitation light. Specifically, Er is preferable, but Nd, Ho, Tm, Pr, Yb, Eu, Ce is preferable. Other rare earth elements such as may be used.
For example, Er will be used as an optical amplification fiber for 1.55 μm band, Pr will be used for 1.3 μm band, Tm will be used for 1.45 μm band, and Yb will be used as an optical amplification fiber for 1 μm band. It will be.

In the optical fiber for multicore optical amplification according to the present invention, pumping light is input from a pumping light supply region that is a part of a cladding layer. The signal core side or the pump light supply so that the equivalent refractive index of the pump light supply region and the signal core side equivalent refractive index based on the signal core and the trench on the outer periphery thereof coincide with each other at the pump light pump wavelength. Either one or both of the region-side equivalent refractive indices are adjusted so that mode coupling occurs at the excitation wavelength. As a result, the optical power of the pumping light input to the pumping light supply region is given to the signal core side by the resonance action in the mode coupling. When the resonance effect due to mode coupling occurs, the pumping light power from the pumping light supply region is not propagated isotropically as in the conventional method but is absorbed by being drawn toward the signal core. .
In addition, in the present invention, the excitation core for inputting the excitation light is not formed in the fiber, and a trench having a lower refractive index than that of the cladding layer is provided on the outer periphery of the signal core so that the vicinity of the center of the fiber (ie, A pumping light supply region (of the cladding layer) where pumping light is input (near the center of the cladding layer) is provided, and a signal core surrounded by a trench is disposed so as to surround the pumping light supply region. I have to. Thus, in the past, due to the existence of the excitation core, only a part of the excitation light that oozes out from the excitation core to the cladding layer contributed to the optical amplification in the signal core, but the cladding layer near the center of the fiber. Since most of the optical power of the pumping light input to the signal becomes available and is directed to the signal core by the resonance action, the pumping light power can be transferred to the signal core side with high efficiency and can be amplified. It becomes like this.

  According to the present invention, the multi-core fiber is used to input the pumping light to the pumping light supply region composed of the cladding layer near the center of the fiber, and the optical amplification is performed using the resonance action by the mode coupling. The pumping light can be absorbed by the signal core with much higher efficiency than in the past, and highly efficient optical amplification becomes possible. In addition, since a multi-core fiber is used, compact and low-cost optical amplification becomes possible.

The typical sectional view showing the optical fiber for multi-core light amplification of one embodiment of the present invention. The figure which shows the relative refractive index difference distribution of the radial direction in the A-A 'line | wire crossing the signal core in the optical fiber for multi-core optical amplification of FIG. Explanatory drawing about the wavelength dependence of the equivalent refractive index of a signal core and an excitation light supply area | region when changing the addition amount of an additive as a parameter. Explanatory drawing about the wavelength dependence of the equivalent refractive index in the pumping light supply area | region and signal core side of the optical fiber for multi-core optical amplification adjusted so that mode coupling | bonding may arise at a pumping wavelength. The schematic diagram which shows the electric field distribution of the resonance state by the mode coupling in the optical fiber for multi-core optical amplification of FIG. The figure which shows the relationship between the length of the optical fiber for multi-core optical amplification of FIG. 1, and the ratio of the optical power which moves to a signal core. The figure which shows the relationship between the length of the optical fiber for multi-core optical amplification which has a pseudo pumping core, and the ratio of the optical power which moves to a signal core. The figure which shows the optical amplifier using the conventional optical fiber for multi-core optical amplification.

Hereinafter, the structure and operation of the optical fiber for multicore optical amplification of the present invention will be described with reference to the drawings.
In the following description, the terms “refractive index”, “equivalent refractive index”, and “relative refractive index difference” are used for the refractive index. Although a strict description is omitted, the “refractive index” refers to a refractive index that depends on the material characteristics of each substance.
"Equivalent refractive index" is a refractive index that light senses when light propagates through a structure composed of a plurality of materials such as an optical fiber. For example, the refractive index of each material is roughly divided into the power distribution of light. Is a weighted average.
The “relative refractive index difference” is given by a relative refractive index difference Δ≈ (n1−n2) / n1, where n1 is the refractive index of the substance 1 and n2 is the refractive index of the substance 2.

<Structure of optical fiber for multi-core optical amplification>
FIG. 1 is a schematic cross-sectional view showing a configuration of a multi-core optical fiber FA for amplification according to an embodiment of the present invention.
The multi-core optical fiber FA for amplification includes a fiber body 10 having a circular cross section. The fiber body 10 includes six signal cores 12, a trench 13 formed on the outer periphery of the signal core 12, and a signal including the trench 13. The clad layer 11 covers the core 12.

The six signal cores 12 are arranged concentrically so as to avoid the vicinity of the center of the fiber body 10, and a region of the clad layer 11 surrounded by the signal core 12 is formed near the center of the fiber body 10. It is. This region (the region indicated by the alternate long and short dash line in FIG. 1) is particularly called “excitation light supply region 14”. As in the conventional example of FIG. 8, pumping light is input to the pumping light supply region 14 from an external pumping light source via an optical coupler or the like.
The reason why the signal core 12 is arranged so as to surround the pumping light supply region 14 in this manner is that the mode coupling is less likely to occur by surrounding the pumping light supply region 14 with the signal core 12 (that is, the signal core 12 exists in the vicinity). This is because the mode coupling is caused in the entire periphery (omnidirectional) of the excitation light supply region 14 by eliminating the non-direction). As a result, it is possible to drastically reduce the excitation light that disappears outside the fiber body 10 without causing mode coupling.

  In the embodiment of FIG. 1, the six signal cores 12 are concentrically arranged at equal intervals to show the pumping light supply region 14, but the three signal cores 12 are concentrically arranged at equal intervals. In this case, since the pumping light supply region 14 can be surrounded, it is necessary to provide at least three signal cores in the present invention.

On the other hand, although it is a problem different from the efficiency of optical amplification, since the mutual interference (crosstalk) of signal light occurs when the distance between the signal cores 12 is too close, it is necessary to separate them so that mutual interference does not occur. The maximum number of signal cores 12 that can be arranged is limited. Providing the trench 13 has an effect of suppressing mutual interference. However, if it approaches to about 45 μm, the influence of mutual interference appears.
Therefore, in order to secure a distance between signal cores where no mutual interference occurs practically, the number of signal cores is selected with about 3 to 12 for a general fiber diameter (for example, 125 μm). I have to.
Further, in order to uniformly amplify the signal cores 12 at the time of optical amplification by mode coupling, they are arranged concentrically and spaced apart at equal intervals in the circumferential direction.

Er ³⁺ that is an optical amplification medium is added to the signal core 12. When excitation laser light (excitation light) having an excitation wavelength of 980 nm or 1480 nm enters the signal core 12, Er ³⁺ is excited and a signal is generated. As the signal light travels in the core 12, stimulated emission occurs and the light is amplified. In the case of an optical amplification medium other than the Er element, the excitation wavelength is selected according to each type.

  The trench 13 is an annular region whose refractive index is lower than that of the cladding layer 11. FIG. 2 is a diagram showing a relative refractive index difference distribution in the radial direction along the line AA ′ in FIG. 1, that is, a line connecting the center of the fiber body 10 and the center of the signal core 12, and the horizontal axis indicates the fiber center. The position in the radial direction from, the vertical axis is the relative refractive index difference with respect to the refractive index of the cladding layer 11.

  In the conventional multi-core fiber, the trench is formed only for the purpose of suppressing mutual interference (crosstalk) between signal cores. However, in the present invention, not only the purpose of suppressing mutual interference but also the equivalent refractive index based on the trench 13 and the signal core 12 at the excitation wavelength (that is, 980 nm or 1480 nm) (referred to as “the equivalent refractive index on the signal core 12 side”). It can also be used as one of the adjusting means. That is, by adjusting the relative refractive index difference and the radial width (thickness) of the trench 13 as parameters, the equivalent refractive index at the excitation wavelength on the signal core 12 side can be substantially changed.

  In the present invention, in the fiber main body 10 having the above-described structure, mode coupling occurs at the pumping wavelength. Therefore, the equivalent refractive index at the pumping wavelength in the pumping light supply region 14 and the pumping wavelength on the signal core 12 side are equivalent. Either or both of the equivalent refractive index of the pumping light supply region 14 and the equivalent refractive index on the signal core 12 side are adjusted so that the refractive index matches.

FIG. 3 illustrates the wavelength dependence of the equivalent refractive index (proportional to the propagation constant β) of the signal core 12 and the pumping light supply region 14 when the amount of additive having an effect of increasing or decreasing the refractive index is changed as a parameter. It is a figure to do.
Here, the equivalent refractive index is calculated on the assumption that the diameter of the signal core 12 is sufficiently smaller than the diameter of the pumping light supply region 14 (the average diameter of the region surrounded by the signal core 12). Specifically, the calculation is performed by numerical analysis with the structure of FIG. 1 with the diameter of the signal core 12 being 8 μm and the average diameter of the excitation light supply region being 70 μm.

  Due to the difference in diameter (difference in shape) between them, the equivalent refractive index on the side of the signal core 12 having a small diameter decreases more sharply as the signal wavelength increases than the equivalent refractive index of the excitation light supply region 14 having a large diameter. Tends to be a refractive index curve. Therefore, the equivalent refractive index can be matched at the intersection position of the two refractive index curves.

  In general, when Ge, Al or the like is added to a silica-based fiber material, the refractive index curve can be shifted upward, and when fluorine or the like is added, the refractive index curve can be shifted downward. Therefore, in FIG. 3, the equivalent refractive index is shifted up and down by adding an appropriate amount of these additives to either or both of the excitation light supply region 14 and the signal core 12. When the crossing position is adjusted to be at the excitation wavelength (for example, 1480 nm), mode coupling can be generated at the excitation wavelength.

  The adjustment method by adding an additive having the effect of increasing or decreasing the refractive index to the excitation light supply region 14 and the signal core 12 has been described above. As described above, the relative refraction of the trench 13 with respect to the cladding layer 11 is described. Since the equivalent refractive index can be adjusted by using the difference in the index and the width (thickness) in the radial direction as parameters, the equivalent refractive index may be adjusted by either or both methods.

<Operation of optical amplification by mode coupling>
Next, the operation of optical amplification by the multi-core optical amplification optical fiber FA will be described.
FIG. 4 shows the wavelength dependence of the equivalent refractive index (proportional to the propagation constant β) on the pumping light supply region 14 and the signal core 12 side of the optical fiber FA for multicore optical amplification adjusted so that mode coupling occurs at the pumping wavelength. It is a figure explaining. In this multicore optical amplifying optical fiber FA, the excitation wavelength is set to 1480 nm, and the two are adjusted so that their equivalent refractive indexes coincide with each other at this excitation wavelength.

  In the optical fiber FA for multi-core optical amplification, the excitation light supply region 14 and the signal core 12 are arranged close to each other. In a wavelength range sufficiently away from the excitation wavelength (1480 nm), light coupling is sparse and works as an independent optical transmission line.

  Since the signal light having a wavelength away from the excitation wavelength has a sparse optical coupling, the amount of movement of the optical power between the excitation light supply region 14 and the signal core 12 is small. Therefore, the signal light stays in the signal core 12, and the crosstalk between the signals between the two is small.

  On the other hand, mode coupling occurs in the vicinity of the pumping wavelength, and the optical power in the pumping light supply region 14 can be moved to the signal core 12 side by the resonance action by mode coupling. Also, the optical power on the signal core 12 side can be moved to the pumping light supply region 14 by the resonance action by mode coupling.

  FIG. 5 is a diagram schematically showing an electric field distribution in a resonance state due to mode coupling of the multi-core optical fiber for amplification FA shown in FIG. Since the movement of the optical power when mode coupling occurs is affected by the electric field distribution, the optical power does not move from the pumping light supply region 14 radially evenly in the entire circumferential direction. The optical power moves intensively as it is drawn in the existing direction. Therefore, the optical power is moved more efficiently than in the case where mode coupling is not used.

When light absorption in mode coupling between the signal core 12 and the pumping light supply region 14 is both weak, the movement from the signal core 12 to the pumping light supply region 14 and vice versa from the pumping light supply region 14 is reversed. When moving to the signal core 12, the same amount of optical power moves.
However, since Er ³⁺ (or other rare earth element) that is an optical amplification medium is added to the signal core 12, the optical power is absorbed larger than that of the pumping light supply region 14. Therefore, the optical power input to the pumping light supply region 14 moves unilaterally toward the signal core 12 side, and optical amplification is performed with high efficiency.

FIG. 6 shows the relationship between the fiber length in the axial direction of the multi-core optical fiber FA for amplification in FIG. 1 (that is, the fiber length at which the optical power is moved) and the ratio of the optical power moving to the signal core 12. FIG. The light absorption coefficient α of the amplification medium (Er) was set to 0.3 cm ⁻¹ .
The calculation is performed using the propagation analysis software “BeamPROP (registered trademark)” of SYNOPSYS.
The optical power in the pumping light supply region 14 decreases with the distance traveled in the axial direction and reaches 20% at 50 mm and 2.5% at 100 mm, so there is almost no loss (actually Most of the optical power is transferred to the signal core 12 at 80 mm at 50 mm and 97.5% at 100 mm.

  For reference, when the same calculation is performed with a multi-core optical amplification optical fiber having a pumping core (multi-core optical amplification optical fiber in FIG. 8), the result is that the efficiency of moving optical power is poor. Even when converted to a length of 1000 mm, it was only moved to the signal core side by about 1% or less. This difference is due to the presence of the excitation core and the movement of optical power due to mode coupling.

  Although one embodiment of the present invention has been described above, the present invention is not necessarily limited to the above-described embodiment, and can be appropriately modified and changed within the scope of achieving the object and not departing from the scope of the claims. Is possible.

(Modified Example)
In the embodiment of the optical fiber FA for multi-core optical amplification shown in FIG. 1, the mode coupling is generated and the pumping light is input from the cladding layer 11 instead of the pumping core. It has become possible to greatly increase.
Of these, it was also confirmed whether the efficiency of optical amplification can be increased even when only the adjustment of the equivalent refractive index for causing mode coupling is performed alone.

That is, in the embodiment described with reference to FIG. 1, the entire cladding layer 11 in the fiber body 10 is homogeneous. Therefore, the refractive index of the cladding layer near the center of the fiber to be the excitation light supply region 14 and the cladding layer outside the cladding layer. Was the same value.
On the other hand, as a modified embodiment, the refractive index of the pumping light supply region 14 is adjusted to be slightly higher than the refractive index of the cladding layer on the outer side, thereby giving a pseudo structure similar to that of the pumping core. However, it is the same as the example of FIG. 1 that the trench 13 is provided and that the equivalent refractive index of the pseudo excitation core and the equivalent refractive index of the signal core are matched at the excitation wavelength.

And the result of having performed the calculation similar to FIG. 6 is shown in FIG. FIG. 7 shows the fiber length in the axial direction of the multi-core optical fiber FA with a pseudo excitation core (that is, the fiber length at which the optical power is moved) and the optical power moving to the signal core 12. It is a figure which shows the relationship with the ratio. The light absorption coefficient α of the amplification medium (Er) was set to 0.5 cm ⁻¹ .
As a result, the amount of optical power in the pumping light supply region 14 is reduced to 75% at a traveling distance of 50 mm, and it is assumed that there is almost no loss due to the movement of the optical power due to mode coupling. Will move. Assuming that the optical power in the pumping light supply region 14 shifts to the signal core 12 with the same coupling rate, 94% of the optical power moves to the signal core 12 side at a travel distance of 100 mm.
Therefore, it has been found that even in the case of a multi-core optical amplification optical fiber having a pseudo excitation core, the efficiency of optical amplification can be remarkably improved by using mode coupling.

  In the above modified embodiment, the electric field distribution of the excitation light is unevenly distributed in the center, so that the length of the fiber necessary to achieve the same supply rate becomes long, but the matching of the equivalent refractive index becomes easy. have.

  Therefore, according to the present invention, it has been confirmed that a large effect can be obtained by performing optical amplification using the movement of optical power by mode coupling as compared with the case where mode coupling does not occur.

  The present invention can be used as an optical fiber for multi-core optical amplification used for optical amplification in optical fiber communication.

10: Fiber body 11: Clad layer 12: Signal core (Er addition)
13: Trench 14: Pumping light supply region (clad layer near the center of the fiber)
FA: Optical fiber for multi-core optical amplification

Claims (6)

A multi-core fiber having at least three signal cores to which an amplification medium for amplifying the excitation light is added, and a clad layer covering the signal cores;
The signal core is arranged concentrically at a position eccentric from the center of the multicore fiber, and pumping is performed to supply pumping light in the vicinity of the center of the multicore fiber surrounded by the signal core. A multi-core optical amplification optical fiber having a structure in which a trench having a refractive index lower than that of a cladding layer is formed on an outer periphery of each signal core, and configured to be a light supply region,
The signal core side equivalent refractive index based on the trench and the signal core with respect to the excitation wavelength determined by the amplification medium of the signal core matches the equivalent refractive index of the excitation light supply region with respect to the excitation wavelength. The equivalent refractive index of either the core side or the excitation light supply region or both is adjusted,
An optical fiber for multi-core optical amplification in which optical amplification is performed such that pumping light power input to the pumping light supply region is absorbed by a signal core side using resonance due to mode coupling. The equivalent refractive index of the signal core side or the excitation light supply region is adjusted by addition of an additive that increases or decreases the equivalent refractive index to the signal core, the trench, or the excitation light supply region. Multi-core optical fiber for optical amplification. 2. The multi-core optical amplification optical fiber according to claim 1, wherein the equivalent refractive index on the signal core side is adjusted by a radial width of the trench. The multi-core optical amplification optical fiber according to any one of claims 1 to 3, wherein the signal core amplification medium is a rare earth element. The multi-core optical amplifying fiber according to any one of claims 1 to 4, wherein the excitation light supply region is adjusted to have a refractive index higher than that of a cladding layer outside the excitation light supply region. 6. A light comprising: the multi-core optical amplification optical fiber according to claim 1; and an excitation light source that irradiates the excitation light supply region of the multi-core optical amplification optical fiber with the excitation light having the excitation wavelength. Amplification equipment.

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