JP2020161600A - Multi-core optical amplification fiber, multi-core optical fiber amplifier and optical communication system - Google Patents

Abstract

To provide a multi-core optical amplification fiber having a preferred characteristic, a multi-core optical fiber amplifier and an optical communication system each using the multi-core optical amplification fibers.SOLUTION: The multi-core optical amplification fiber comprises: a plurality of core portions to which a rare earth element has been added; an inner clad portion which is positioned at an outer periphery of each of the plurality of core portions, and has a refractive index lower than the maximum refractive index of each clad portion; and an outer clad portion which is positioned at the outer periphery of the inner clad portion, and has the refractive index lower than that of the inner clad portion. Each clad portion is designed so that a cut-off wavelength becomes a wavelength of an optical amplification wavelength or less of the rare earth element. The multi-core optical amplification fiber has such a clad thickness that leakage loss in a core portion nearest to an outer edge of the inner clad portion out of the plurality of core portions becomes 0.1 dB/100 m or less in the optical amplification wavelength.SELECTED DRAWING: Figure 1

Description

The present invention relates to a multi-core optical fiber amplifier, a multi-core optical fiber amplifier and an optical communication system.

For example, in applications such as submarine optical communication, it is expected that the power consumption of the optical amplifier can be reduced by using a multi-core EDFA (Erbium-Doped optical Fiber Amplifier) as the optical amplifier.

Regarding the multi-core EDFA, a configuration is known in which a double-clad type multi-core EDF is used as a multi-core optical amplification fiber to photoexcit the rare earth element erbium (Er) contained in the core portion by a clad excitation method (non-patent documents). See 1 and 2).

Kazi S Abedin et al, “Multimode Erbium Doped Fiber Amplifiers for Space Division Multiplexing Systems”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.32, NO.16, AUGUST 15,2014 pp.2800-2808. Kazi S Abedin et al, “Cladding-pumped erbium-doped multicore fiber amplifier”, OPTICS EXPRESS Vol.20, No.18 27 August 2012 pp.20191-20200.

However, since communication traffic is constantly increasing, more suitable characteristics of the multi-core optical amplifier fiber are required in order to increase the communication capacity.

The present invention has been made in view of the above, and an object of the present invention is to provide a multi-core optical amplification fiber having suitable characteristics, and a multi-core optical fiber amplifier and an optical communication system using the same.

In order to solve the above-mentioned problems and achieve the object, the multi-core optical amplification fiber according to one aspect of the present invention is located on a plurality of core portions to which a rare earth element is added and on the outer periphery of each of the plurality of core portions. An inner clad portion having a refractive index lower than the maximum refractive index of each core portion, and an outer clad portion located on the outer periphery of the inner clad portion and having a refractive index lower than the refractive index of the inner clad portion. The core portion is designed so that the cutoff wavelength is equal to or lower than the optical amplification wavelength band of the rare earth element. In the optical amplification wavelength band, the outer edge of the inner clad portion among the plurality of core portions is provided. It is characterized by having a clad thickness in which the leakage loss of the core portion closest to is 0.1 dB / 100 m or less.

The multi-core optical amplifier fiber according to one aspect of the present invention is characterized in that the bending loss when bent at a diameter of 50 mm is smaller than 0.1 dB / 100 m in the optical amplifier wavelength band.

In the multi-core optical amplification fiber according to one aspect of the present invention, the distance between each of the plurality of core portions in the cross section perpendicular to the longitudinal direction is -30 dB / 100 m or less between the cores in the optical amplification wavelength band. The feature is that it is set to be.

In the multi-core optical amplification fiber according to one aspect of the present invention, the separation distance between each of the plurality of core portions is set so that the crosstalk between cores is −40 dB / 100 m or less in the optical amplification wavelength band. It is characterized by being.

In the multi-core optical amplification fiber according to one aspect of the present invention, the separation distance between each of the plurality of core portions is set so that the crosstalk between cores is -50 dB / 100 m or less in the optical amplification wavelength band. It is characterized by being.

In the multi-core optical amplifier fiber according to one aspect of the present invention, the ratio of the total cross-sectional area of the plurality of core portions to the cross-sectional area of the inner clad portion in the cross section perpendicular to the longitudinal direction is larger than 1.460%. It is a feature.

The multi-core optical amplifier fiber according to one aspect of the present invention is characterized in that the cutoff wavelength is within a range of 100 nm from the shortest wavelength of the optical amplification wavelength band.

The multi-core optical amplifier fiber according to one aspect of the present invention is characterized in that the rare earth element contains erbium.

The multi-core optical amplifier fiber according to one aspect of the present invention is characterized in that the rare earth element contains only erbium.

The multi-core optical amplifier fiber according to one aspect of the present invention is characterized in that the cross section of the inner clad portion is non-circular.

In the multi-core optical amplifier fiber according to one aspect of the present invention, the inner clad portion has a trench portion located on the outer periphery of each of the plurality of core portions, and the refractive index of the trench portion is other than that of the inner clad portion. It is characterized by having a refractive index lower than that of the portion of.

The multi-core optical fiber amplifier according to one aspect of the present invention includes the multi-core optical amplification fiber, an excitation light source that outputs excitation light for photoexciting the rare earth element of the multi-core optical amplification fiber, and the excitation light on the inner clad portion. It is characterized by including an optical coupler for optical coupling.

The optical communication system according to one aspect of the present invention is characterized by including the multi-core optical fiber amplifier.

According to the present invention, a multi-core optical amplifier fiber having suitable characteristics can be realized.

FIG. 1 is a schematic cross-sectional view of the multi-core optical amplifier fiber according to the first embodiment. FIG. 2 is a diagram showing an example of the relationship between the core Δ and the core diameter. FIG. 3 is a schematic cross-sectional view of the multi-core optical amplifier fiber according to the second embodiment. FIG. 4 is a schematic cross-sectional view of a multi-core optical amplifier fiber according to another embodiment. FIG. 5 is a schematic view showing the configuration of the multi-core optical fiber amplifier according to the third embodiment. FIG. 6 is a schematic view showing the configuration of the multi-core optical fiber amplifier according to the fourth embodiment. FIG. 7 is a schematic diagram showing the configuration of the optical communication system according to the fifth embodiment.

Hereinafter, embodiments will be described with reference to the drawings. The present invention is not limited to this embodiment. Further, in the description of the drawings, the same or corresponding elements are appropriately designated by the same reference numerals. In addition, it should be noted that the drawings are schematic, and the relationship between the dimensions of each element, the ratio of each element, and the like may differ from the reality. Even between drawings, there may be parts where the relationship and ratio of dimensions are different from each other. Further, in the present specification, the cutoff wavelength is referred to as ITU-T (International Telecommunication Union) G.I. It means the cable cutoff wavelength defined in 650.1. In addition, for other terms not specifically defined in this specification, G.I. 650.1 and G.M. The definition and measurement method in 650.2 shall be followed.

(Embodiment 1)
FIG. 1 is a schematic cross-sectional view of the multi-core optical amplifier fiber according to the first embodiment, and shows a cross section perpendicular to the longitudinal direction of the multi-core optical amplifier fiber. The multi-core optical amplification fiber 1 includes seven core portions 1aa and 1ab, an inner clad portion 1b located on the outer periphery of each core portion 1aa and 1ab, and an outer clad portion 1c located on the outer periphery of the inner clad portion 1b. It is a double-clad type and 7-core type multi-core optical fiber provided.

The core portions 1aa and 1ab are arranged in a triangular lattice pattern to realize a close-packed state. The core portion 1aa is arranged near the center of the inner clad portion 1b. The six core portions 1ab are arranged around the core portion 1aa so as to be at the corners of a regular hexagon. The core portions 1aa and 1ab contain, for example, germanium (Ge) or aluminum (Al) as a refractive index adjusting dopant for increasing the refractive index. Further, the core portions 1aa and 1ab contain Er as a rare earth element which is an amplification medium. Er is added at a concentration such that the peak of the absorption coefficient near a wavelength of 1530 nm is 2.5 dB / m to 11 dB / m, but the absorption coefficient is not particularly limited. Al also has a function of suppressing the concentration quenching of Er.

The inner clad portion 1b has a refractive index lower than the maximum refractive index of each core portion 1aa and 1ab. The inner clad portion 1b is made of, for example, pure quartz glass containing no dopant for adjusting the refractive index. As a result, the refractive index profile of each core portion 1aa and 1ab and the inner clad portion 1b becomes a step index type. The inner clad portion 1b may have trench portions located on the outer circumferences of the core portions 1aa and 1ab. In this case, the trench portion is made of quartz glass to which a refractive index adjusting dopant that lowers the refractive index such as fluorine (F) is added, and the refractive index of the trench portion is the other portion of the inner clad portion 1b made of pure quartz glass. It has a refractive index lower than that of. In this case, the refractive index profile of each core portion 1aa and 1ab and the inner clad portion 1b is a trench type.

The difference in the specific refractive index of each core portion 1aa and 1ab with respect to the inner clad portion 1b is defined as the core Δ. In the present embodiment, the cores Δ of the core portions 1aa and 1ab are substantially equal.

The outer clad portion 1c has a refractive index lower than that of the inner clad portion 1b, and is made of, for example, a resin. When the inner clad portion 1b has a trench portion, the refractive index of the outer clad portion 1c may be higher than the refractive index of the trench portion, but the refractive index of other portions of the inner clad portion 1b and the inner side. It is lower than the average refractive index of the clad portion 1b.

The separation distance between the core portions 1aa and 1ab in the cross section of FIG. 1 is defined as the pitch P1. The pitch P1 corresponds to the length of one side of the triangular lattice. Further, the outer diameter (clad diameter) of the inner clad portion 1b is defined as the clad diameter Dc1. Further, among the core portions 1aa and 1ab, the core portion closest to the outer edge of the inner clad portion 1b is any of the six core portions 1ab, but in the present embodiment, any of the six core portions 1ab Is equidistant from the outer edge of the inner clad portion 1b. The shortest distance from the center of any of the six core portions 1ab to the outer edge of the inner clad portion 1b is defined as the clad thickness Tc1.

When excitation light having a wavelength capable of photoexciting Er, for example, excitation light in the 900 nm wavelength band is input to the inner clad portion 1b, the excitation light is added to the core portions 1aa and 1ab while propagating inside the inner clad portion 1b. The generated Er is photoexcited. As a result, the core units 1aa and 1ab can photo-amplify the signal light input to the core units 1aa and 1ab. In the case of Er, the optical amplification wavelength band that can be photoamplified is, for example, 1530 nm to 1565 nm, which is called the C band, or 1565 nm to 1625 nm, which is called the L band.

In the multi-core optical amplification fiber 1, the core portions 1aa and 1ab are designed so that the cutoff wavelength is equal to or lower than the optical amplification wavelength band of Er. That is, the cutoff wavelength is set to, for example, a wavelength of 1530 nm or less when the optical amplification band includes the C band. The cutoff wavelength may be 1530 nm. Further, in consideration of the manufacturing error of the multi-core optical amplifier fiber 1, for example, it may be set to around 1500 nm. When only the L band signal light is amplified by the multi-core optical amplifier fiber 1, the cutoff wavelength is set to, for example, a wavelength of 1565 nm or less. Further, in consideration of the manufacturing error of the multi-core optical amplifier fiber 1, for example, it may be set to around 1535 nm. As a result, the multi-core optical amplification fiber 1 can propagate the signal light of the optical amplification wavelength band of Er in each core portion 1aa and 1ab in a single mode. In order to design each core portion 1aa and 1ab for such a cutoff wavelength, the core Δ of each core portion 1aa and 1ab and the core diameter 2a may be appropriately combined and set.

For example, FIG. 2 is a diagram showing an example of the relationship between the core Δ and the core diameter when the cutoff wavelength is 1500 nm. FIG. 2 shows the core diameter when the refractive index profile is a step index type. As described above, in order to realize a predetermined cutoff wavelength, the combination of the core Δ and the core diameter may be appropriate. Even when the refractive index profile is a trench type, the relationship between the core Δ and the core diameter is required according to the cutoff wavelength. Therefore, if the combination of the core Δ and the core diameter is appropriately adjusted according to the relationship. Good.

Further, in the multi-core optical amplifier fiber 1, it is important to make the clad thickness Tc1 appropriate in order to realize suitable characteristics. That is, when the clad thickness Tc1 is thin, the action of light confinement on the core portion 1ab closest to the outer edge of the inner clad portion 1b becomes small, leakage loss occurs, and the amplification efficiency decreases. On the other hand, if the clad thickness Tc1 is made too thick, the cross-sectional area of the multi-core optical amplifier fiber 1 becomes excessively large.

The multi-core optical amplification fiber 1 has a clad thickness Tc1 in which the leakage loss of the core portion 1ab is 0.1 dB / 100 m or less in the optical amplification wavelength band. As a result, the loss of light energy due to the leakage loss is suppressed to an acceptable level, so that the decrease in amplification efficiency can also be suppressed.

When a multi-core optical fiber amplifier is configured by using the multi-core optical amplification fiber 1, the multi-core optical amplification fiber 1 has a length suitable for the required gain and optical output, for example, 1 to 100 m, depending on its absorption coefficient. In many cases, it is adjusted to the above and is housed in the housing in a state of being wound around a bobbin or the like. Therefore, it is preferable that the bending loss due to winding is small. For example, the multi-core optical amplifier fiber 1 preferably has a bending loss smaller than 0.1 dB / 100 m when bent at a diameter of 50 mm in the optical amplifier wavelength band. Since the bending loss tends to increase when the core Δ is low, it is preferable to set the core Δ so as to realize a bending loss smaller than 0.1 dB / 100 m for the multi-core optical amplifier fiber 1.

Further, in a communication system, the optical fiber transmission line is long and the number of repeaters also increases according to the transmission distance, so that crosstalk in each of the optical elements inserted in the middle of the optical fiber transmission line is accumulated. Affects communication quality. Therefore, even as the multi-core optical amplification fiber 1 for a multi-core optical fiber amplifier used in an optical communication system, it is preferable that the crosstalk between cores is small in the optical amplification wavelength band.

Here, crosstalk between cores means that when a signal light having a predetermined power is input to one of the core parts 1aa and 1ab and propagated, a part of the power of the signal light is part of the other core. When leaking to a part, for example, it is specified as follows.
(Crosstalk between cores)
= (Power of signal light leaked to other cores) / (Power of input signal light)

In the multi-core optical amplifier fiber 1, the pitch P1 is preferably set so that the crosstalk between cores is −30 dB / 100 m or less. When the inter-core crosstalk is −30 dB / 100 m or less, the multi-core optical amplification fiber 1 can be suitably used for, for example, a multi-core optical fiber amplifier used in an optical communication system having a transmission distance of several hundred km.

Further, the multi-core optical fiber amplifier 1 is a multi-core optical fiber amplifier used in an optical communication system having a transmission distance of several thousand km, for example, if the pitch P1 is set so that the crosstalk between cores is −40 dB / 100 m or less. Can be suitably used for.

Further, the multi-core optical amplification fiber 1 is used in an optical communication system having a transmission distance of 10000 km, for example, across the Pacific Ocean, if the pitch P1 is set so that the crosstalk between cores is -50 dB / 100 m or less. It can be suitably used for a multi-core optical fiber amplifier.

The above-mentioned values of crosstalk between cores are examples and are not limited.

Furthermore, according to the diligent studies of the present inventors, the ratio of the total cross-sectional area of the core portions 1aa and 1ab to the cross-sectional area of the inner clad portion 1b in the cross section perpendicular to the longitudinal direction of the multi-core optical amplifier fiber 1 (hereinafter referred to as When the core / clad area ratio) is relatively large, the ratio of the power of the excitation light that contributes to the optical amplification in the core portions 1aa and 1ab is large, and the excitation efficiency is high.

In a known 7-core type multi-core optical amplifier fiber, the core / clad area ratio is about 1%, but according to the diligent studies of the present inventors, the core / clad area ratio is larger than 1.460%. Preferably, 2% or more is more preferable.

However, if the core / clad area ratio is increased while the value of the clad diameter Dc1 of the inner clad portion 1b is fixed, the clad thickness Tc1 becomes smaller and the leakage loss becomes larger, or the pitch P1 becomes smaller and the inter-core crosstalk Therefore, it is preferable to set the core / clad area ratio in consideration of various characteristics required for the multi-core optical amplification fiber 1.

Here, an example of a design method of the multi-core optical amplifier fiber 1 will be described. First, as step 1, the number and arrangement of core portions in the multi-core optical amplifier fiber are set according to the application and the like. In the multi-core optical amplifier fiber 1, the number of core portions is 7, and they are arranged in a triangular lattice pattern. At this time, the refractive index profile of the core portion may also be set.

Subsequently, as step 2, the cutoff wavelength corresponding to the optical amplification wavelength band is set. As described above, the cutoff wavelength is set so that the signal light in the optical amplification wavelength band can be propagated in a single mode. Further, as described above, the larger the core / clad area ratio, the higher the excitation efficiency. Therefore, the cutoff wavelength may be relatively long so that the core diameter can be increased. For example, it may be in the range of 100 nm from the shortest wavelength of the optical amplification wavelength band. For example, when the optical amplification wavelength band includes the C band, the cutoff wavelength may be 1430 nm or more in the range of 1530 nm to 100 nm.

When the cutoff wavelength is set in this way, the relationship between the core Δ and the core diameter as shown in FIG. 2, for example, is determined.

Subsequently, in step 3, the core Δ and the core diameter are set so as to realize a desired bending loss. As a result, the lower limit of the core Δ and the upper limit of the core diameter are determined. For example, the bending loss is set to a value smaller than 0.1 dB / 100 m when bent at a diameter of 50 mm in the optical amplification wavelength band. Since the longer the wavelength, the larger the bending loss, it is sufficient to set the bending loss at the maximum wavelength of the optical amplification wavelength band to be a desired value. For example, when the optical amplification wavelength band includes the L band, the core Δ and the core diameter may be set so that the bending loss at the wavelength of 1625 nm is smaller than 0.1 dB / 100 m.

Subsequently, as step 4, a desired crosstalk between cores is set. The inter-core crosstalk is set according to the application of the multi-core optical amplifier fiber 1. By setting the crosstalk between cores, the pitch P1 for realizing this is determined.

Subsequently, as step 5, the leakage loss for the core portions 1aa and 1ab is set to, for example, 0.1 dB / 100 m or less. Since the longer the wavelength, the larger the leakage loss, it is sufficient to set the leakage loss at the maximum wavelength of the optical amplification wavelength band to a desired value. From this, the clad thickness Tc1 at which the leakage loss of the core portion 1ab closest to the outer edge of the inner clad portion 1b is a set value is determined. Further, the clad diameter Dc1 is determined by determining the pitch P1 and the clad thickness Tc1. Furthermore, the core / clad area ratio is determined.
Subsequently, as step 6, it is determined whether the core / clad area ratio is larger than a desired value, for example 1.460%. If it is above the desired value, the step is completed, and if it is less than the desired value, for example, the cutoff wavelength is set to a longer wavelength, and the conditions of bending loss, crosstalk between cores, and leakage loss are alleviated. Above, the process returns to any one of steps 2 to 5 above, and the settings after the returned step are repeated.

According to the steps of the above design method, the multi-core optical amplifier fiber 1 having suitable characteristics can be designed. However, the design method is not limited to the above. For example, bending loss, inter-core crosstalk, and core / clad area ratio may be omitted as appropriate.

As described above, the multi-core optical amplifier fiber 1 according to the first embodiment has suitable characteristics.

(Embodiment 2)
FIG. 3 is a schematic cross-sectional view of the multi-core optical amplifier fiber according to the second embodiment, showing a cross section perpendicular to the longitudinal direction of the multi-core optical amplifier fiber. The multi-core optical amplification fiber 2 includes 19 core portions 2aa, 2ab, 2ac, an inner clad portion 2b located on the outer periphery of each core portion 2aa, 2ab, 2ac, and an outer clad portion located on the outer periphery of the inner clad portion 2b. It is a double clad type and 19 core type multi-core optical fiber provided with 2c.

The core portions 2aa, 2ab, and 2ac are arranged in a triangular lattice pattern to realize a close-packed state. The core portion 2aa is arranged near the center of the inner clad portion 2b. The six core portions 2ab are arranged around the core portion 2aa so as to be at the corners of a regular hexagon. The twelve core portions 2ac are arranged around the core portion 2ab so as to be located at the corners of a regular hexagon or the center of the sides. Since the composition and refractive index of the core portions 2aa, 2ab and 2ac are the same as those of the core portions 1aa and 1ab of the multi-core optical amplifier fiber 1, the description thereof will be omitted.

Since the composition and the refractive index of the inner clad portion 2b are the same as those of the inner clad portion 1b, the description thereof will be omitted. The refractive index profile of each core portion 2aa, 2ab, 2ac and the inner clad portion 2b is a step index type, but may be a trench type. Further, since the composition and the refractive index of the outer clad portion 2c are the same as those of the outer clad portion 1c, the description thereof will be omitted.

The separation distance between the core portions 2aa, 2ab, and 2ac in the cross section of FIG. 3 is defined as the pitch P2. Further, the clad diameter of the inner clad portion 2b is defined as the clad diameter Dc2. Further, among the core portions 2aa, 2ab, and 2ac, the core portion closest to the outer edge of the inner clad portion 2b is any of the six core portions 2ac located at the corners of the regular hexagon among the 12 core portions 2ac. However, in the present embodiment, it is assumed that all of the six core portions 2ac are equidistant from the outer edge of the inner clad portion 2b. The shortest distance from the center of any of the six core portions 2ac to the outer edge of the inner clad portion 2b is defined as the clad thickness Tc2.

When excitation light having a wavelength capable of photoexciting Er is input to the inner clad portion 2b, the excitation light photoexcites Er added to each core portion 2aa, 2ab, and 2ac while propagating inside the inner clad portion 1b. .. As a result, each core portion 2aa, 2ab, 2ac can photoamplify the signal light input to each core portion 2aa, 2ab, 2ac.

In the multi-core optical amplification fiber 2, the core portions 2aa, 2ab, and 2ac are designed so that the cutoff wavelength is equal to or lower than the optical amplification wavelength band of Er. As a result, the multi-core optical amplification fiber 2 can propagate the signal light of the optical amplification wavelength band of Er in each core portion 2aa, 2ab, 2ac in a single mode. In order to design the core portions 2aa, 2ab, and 2ac for such a cutoff wavelength, the core Δ of each core portion 2aa, 2ab, and 2ac and the core diameter 2a are appropriately combined and set as in the first embodiment. Just do it.

Further, the multi-core optical amplification fiber 2 has a clad thickness Tc2 in which the leakage loss of the six core portions 2ac is 0.1 dB / 100 m or less in the optical amplification wavelength band. As a result, the loss of light energy due to the leakage loss is suppressed to an acceptable level, so that the decrease in amplification efficiency can also be suppressed.

Further, for example, the multi-core optical amplification fiber 2 preferably has a bending loss smaller than 0.1 dB / 100 m when bent at a diameter of 50 mm in the optical amplification wavelength band.

Further, in the multi-core optical amplifier fiber 2, the pitch P2 is preferably set so that the crosstalk between cores is -30 dB / 100 m or less, -40 dB / 100 m or less, or -50 dB / 100 m or less. As a result, the multi-core optical amplification fiber 2 can be suitably used according to the transmission distance of the optical communication system. However, the value of crosstalk between cores is an example and is not limited.

Further, the multi-core optical amplifier fiber 2 preferably has a core / clad area ratio of more than 1.460%, more preferably 2% or more. However, it is preferable to set the core / clad area ratio in consideration of various characteristics required for the multi-core optical amplifier fiber 2.

The multi-core optical amplifier fiber 2 can be designed by the same design method as the multi-core optical amplifier fiber 1, and the clad diameter Dc2 and the like can be determined.

As described above, the multi-core optical amplifier fiber 2 according to the second embodiment has suitable characteristics.

(Other embodiments)
4 (a) to 4 (g) are schematic cross-sectional views of a multi-core optical amplifier fiber according to another embodiment. The multi-core optical amplifier fibers 3 to 8 shown in FIGS. 4 (a) to 4 (f) have a plurality of, for example, 4, 7 or 19 core portions, inner clad portions 3b to 8b, and outer clad portions 3c to 8c. It is a double-clad type and multi-core optical fiber. The core portion is shown on behalf of only one core portion 3a to 8a.

In the multi-core optical amplifier fiber 3, the centers of gravity of the plurality of core portions are eccentric with respect to the center of the inner clad portion 3b. In the multi-core optical amplifier fiber 4, the inner clad portion 4b has a D-shaped cross section. In the multi-core optical amplifier fiber 5, the inner clad portion 5b has a hexagonal cross section. The multi-core optical amplifier fiber 6 has an inner clad portion 6b having an elliptical cross section. The multi-core optical amplifier fiber 7 has an inner clad portion 7b having a rectangular cross section. The multi-core optical amplifier fiber 8 has a petal-shaped cross section of the inner clad portion 8b. In the multi-core optical amplifier fibers 3 to 8, the excitation efficiency can be improved because the centers of gravity of the plurality of core portions are eccentric or the cross section of the inner clad portion is non-circular.

The multi-core optical amplification fiber 9 shown in FIG. 4 (g) includes a plurality of, for example, 4, 7 or 19 core portions, a first inner clad portion 9b, a second inner clad portion 9c and an outer clad portion 9d. It is a triple clad type and multi-core optical fiber. The core portion is shown on behalf of only one core portion 9a. The core portion 9a, the first inner clad portion 9b, and the second inner clad portion 9c are made of quartz glass, and the outer clad portion 9d is made of resin. The first inner clad portion 9b and the second inner clad portion 9c propagate the excitation light. As described above, the multi-core optical amplification fiber according to the embodiment may be a triple-clad type multi-core optical fiber.

(Calculation example)
A simulation calculation was performed on the characteristics of the multi-core optical amplifier fiber according to the second embodiment. In this calculation, when the core Δ and 2a are set so that the cutoff wavelength is about 1550 nm, the pitch P1 is fixed at 38.5 μm, the leakage loss is 0.001 dB / km, and the diameter is 50 mm. The calculation was performed on the condition that the bending loss of was smaller than 0.1 dB / 100 m.

The calculation results are shown in Tables 1 and 2. In Tables 1 and 2, the “LP01 cutoff wavelength” means the cutoff wavelength of the LP01 mode, which is the base propagation mode, and G.I. This is the cable cutoff wavelength defined in 650.1. "Pitch" is pitch P1. "XT" is a crosstalk between cores. “Leakage loss” and “XT” are values at a wavelength of 1550 nm in the C band in Table 1 and values at a wavelength of 1625 nm in the L band in Table 2.

As shown in Table 1, at a wavelength of 1550 nm, if the core Δ is 0.35% or more, the XT is -30 dB / 100 m or less, and if the Δ is 0.4% or more, the XT is -40 dB / 100 m or less. It was confirmed that when Δ was 0.45% or more, XT was -50 dB / 100 m or less. Further, it was confirmed that when the core Δ is 1.2% and 2a is 5.1 μm, a low XT of −126 dB / 100 m can be obtained even if the clad thickness is 13.6 μm.

When the core Δ is 1.2% and 2a is 5.1 μm, the core / clad area ratio is 1.505%. Further, if the core Δ is 0.80% or less and 2a is 6.3 μm or more, the core / clad area ratio is 2.00% or more. For example, when the core Δ is 0.85% and 2a is 6.1 μm, the core / clad area ratio is 2.098%. When the core Δ is 0.5% and 2a is 7.9 μm, the core / clad area ratio is 2.941%.

Therefore, at a wavelength of 1550 nm, if the core Δ is 0.35% or more and 0.80% or less and the core diameter is 6.3 μm or more and 9.5 μm or less, the XT is -30 dB / 100 m or less and the core / The clad area ratio can be 2% or more.

Further, as shown in Table 2, at a wavelength of 1625 nm, if the core Δ is 0.35% or more, the XT is -30 dB / 100 m or less, and if the Δ is 0.45% or more, the XT is -40 dB /. It was confirmed that the XT was -50 dB / 100 m or less when the value was 100 m or less and Δ was 0.55% or more. Further, it was confirmed that when the core Δ is 1.2% and 2a is 5.1 μm, a low XT of -112.8 dB / 100 m can be obtained even if the clad thickness is 15 μm.

When the core Δ is 1.2% and 2a is 5.1 μm, the core / clad area ratio is 1.460%. Further, when the core Δ is 0.80% or less and 2a is 6.3 μm or more, the core / clad area ratio is preferably 2.00% or more. For example, when the core Δ is 0.85% and 2a is 6.1 μm, the core / clad area ratio is 2.037%. When the core Δ is 0.5% and 2a is 7.9 μm, the core / clad area ratio is 2.844%.

Therefore, at a wavelength of 1650 nm, if the core Δ is 0.35% or more and 0.80% or less and the core diameter is 6.3 μm or more and 9.5 μm or less, the XT is -30 dB / 100 m or less. Moreover, the core / clad area ratio can be 2% or more.

(Embodiment 3)
FIG. 5 is a schematic view showing the configuration of the multi-core optical fiber amplifier according to the third embodiment. Hereinafter, the multi-core optical fiber amplifier may be simply referred to as an optical amplifier. The optical amplifier 100 includes seven optical isolators 10, an optical fiber fan-in (FAN IN) 20, a semiconductor laser 30, an optical coupler 40, a multi-core optical amplifier fiber 1 according to the first embodiment, a pump stripper 50, and an optical fiber fan-out. (FAN OUT) 60, 7 optical isolators 70, are provided. The symbol "x" in the figure indicates the fusion splicing point of the optical fiber.

The optical fiber fan-in 20 includes seven bundled single-mode optical fibers and one multi-core fiber having seven core portions, and each core of the seven single-mode optical fibers at the coupling portion. The portions are configured to be optically coupled to each core portion of the multi-core fiber. The seven single-mode optical fibers are, for example, ITU-TG. It is a standard single-mode optical fiber defined in 652, each of which is provided with an optical isolator 10. The optical isolators 10 and 70 allow light to pass in the direction indicated by the arrow and block the passage of light in the opposite direction. The multi-core fiber of the optical fiber fan-in 20 is connected to the optical coupler 40. The end faces of the seven bundled single-mode optical fibers and multi-core fibers that are optically coupled are processed diagonally with respect to the optical axis to suppress reflection, but may be perpendicular to the optical axis. .. In addition, instead of the seven optical isolators 10 and 70, an optical isolator having a configuration in which a plurality of (7 in this embodiment) single-mode optical fibers are integrated may be used.

Similar to the multi-core optical amplification fiber 1, the multi-core fiber of the optical fiber fan-in 20 is located on the outer periphery of each core portion and seven core portions arranged in a triangular lattice pattern, and is based on the maximum refractive index of each core portion. Also has a clad portion with a low refractive index. When signal light is input to each single-mode optical fiber of the optical fiber fan-in 20, each optical isolator 10 passes each signal light, and each core portion of the multi-core fiber propagates each signal light.

The semiconductor laser 30 which is an excitation light source is a horizontal multimode semiconductor laser and outputs excitation light. The wavelength of the excitation light is 975 nm, which is substantially the same as the wavelength of the absorption peak in the 900 nm wavelength band of Er. As a result, the excitation light can photoexcitate the erbium ion. The semiconductor laser 30 outputs excitation light from a multimode optical fiber. This multimode optical fiber is a step index type having a core diameter / clad diameter of, for example, 105 μm / 125 μm, and has an NA of, for example, 0.16 or 0.22.

The optical coupler 40 includes a main optical fiber and an optical fiber for supplying excitation light. The main optical fiber is located on the outer periphery of each core portion and seven core portions arranged in a triangular lattice like the core portion of the multi-core fiber of the optical fiber fan-in 20, and is based on the maximum refractive index of each core portion. Is a double clad type optical fiber having an inner clad portion having a low refractive index and an outer clad portion located on the outer periphery of the inner clad portion and having a lower refractive index than the inner clad portion. The core portion and the inner clad portion are made of quartz glass, and the outer clad portion is made of resin.

The excitation light supply optical fiber is a multimode optical fiber of the same type in which another end is connected to the multimode optical fiber of the semiconductor laser 30, and is a step index type having a core diameter / clad diameter of, for example, 105 μm / 125 μm. The NA is, for example, 0.16 or 0.22. In the optical fiber for supplying excitation light, the excitation light is input from the semiconductor laser 30, and the excitation light is supplied to the main optical fiber. The inner clad portion propagates the excitation light.

One end of the main optical fiber of the optical coupler 40 is connected to the multi-core fiber of the optical fiber fan-in 20. Each core portion of the multi-core fiber is connected to each core portion of the main optical fiber. Therefore, each signal light propagating in each core portion of the multi-core fiber is optically coupled to each core portion when input to the main optical fiber. Each core portion propagates each signal light. The excitation light and the signal light are output from the main optical fiber to the multi-core optical amplification fiber 1.

One end of the multi-core optical amplification fiber 1 is connected to the main optical fiber of the optical coupler 40. Each core portion 1aa and 1ab of the multi-core optical amplification fiber 1 is connected to each core portion of the main optical fiber. Further, the inner clad portion 1b of the multi-core optical amplification fiber 1 is connected to the inner clad portion of the main optical fiber. Therefore, when the signal light and the excitation light propagating in the main optical fiber are input to the multi-core optical amplifier fiber 1, they propagate in the same direction in the core portions 1aa and 1ab and the inner clad portion 1b, respectively. The excitation light photoexcites Er in each core portion 1aa and 1ab while propagating in the inner clad portion 1b. Each signal light propagating in each core portion 1aa and 1ab is photoamplified by the action of stimulated emission of Er. The multi-core optical amplification fiber 1 outputs each light-amplified signal light and excitation light that does not contribute to optical amplification.

The pump stripper 50 is a known device that eliminates excitation light that did not contribute to optical amplification. In the pump stripper 50, for example, a part of the outer clad of the double clad type multi-core fiber having seven cores is removed, and excitation light is taken out from the surface of the inner clad part of the removed part and used as a heat radiating plate or the like. It has a configuration in which it is irradiated and absorbed to convert the energy of excitation light into heat energy and dissipate heat. The pump stripper 50 propagates each signal light by the multi-core fiber, and reduces the excitation light to a power that does not cause any problem even if it is output from the optical amplifier 100.

Similar to the optical fiber fan-in 20, the optical fiber fan-out 60 includes seven bundled single-mode optical fibers and one multi-core fiber having seven core portions, and has seven at the coupling portion. Each core portion of the single-mode optical fiber of the book is configured to be optically coupled to each core portion of the multi-core fiber. An optical isolator 70 is provided in each single-mode optical fiber. The multi-core fiber is connected to the pump stripper 50. The end faces of the seven bundled single-mode optical fibers and multi-core fibers that are optically coupled are processed diagonally with respect to the optical axis to suppress reflection, but may be perpendicular to the optical axis. ..

When signal light is input from each core portion of the multi-core fiber of the pump stripper 50 to each core portion of the optical fiber fanout 60, each signal light propagates through each core portion of each single-mode optical fiber and passes through the optical isolator 70. Output.

Since the optical amplifier 100 performs optical amplification using the multi-core optical amplification fiber 1 having suitable characteristics, suitable optical amplification can be realized.

(Embodiment 4)
FIG. 6 is a schematic view showing the configuration of the multi-core optical fiber amplifier according to the fourth embodiment. The optical amplifier 100A has a configuration in which the optical coupler 40 is replaced with the optical coupler 40A and the optical coupler 80 and the excitation light recovery optical fiber 90 are added in the configuration of the optical amplifier 100.

The optical coupler 40A has a modified configuration in which the configuration of the optical coupler 40 is modified so that the excitation light recovery optical fiber 90 is connected.

The optical coupler 80 has the same configuration as the optical coupler 40, and has a modified configuration so that the excitation light recovery optical fiber 90 is connected.

The excitation light recovery optical fiber 90 is composed of, for example, a commercial step-index type optical fiber or a graded-index type optical fiber from which the clad is removed and only the core is formed. The excitation light recovery optical fiber 90 optically connects the optical coupler 80 and the optical coupler 40.

In this optical amplifier 100A, among the excitation lights output from the semiconductor laser 30 and supplied to the multi-core optical amplification fiber 1 via the optical coupler 40, at least the excitation light that did not contribute to the optical excitation in the multi-core optical amplification fiber 1 A part is recovered by the optical coupler 80. The recovered excitation light is input to the optical coupler 40 through the excitation light recovery optical fiber 90, regenerated as excitation light, and is supplied to the multi-core optical amplification fiber 1 again. Thereby, the excitation efficiency in the optical amplifier 100A can be improved.

(Embodiment 5)
FIG. 7 is a schematic diagram showing the configuration of the optical communication system according to the fifth embodiment. The optical communication system 1000 includes an optical transmission device 1010, an optical reception device 1020, an optical amplifier 100 according to the third embodiment, and optical transmission fibers 1031 to 1037 and 1041 to 1047, which are 14 single-core optical fibers. I have.

The optical transmitter 1010 includes seven transmitters 101 to 1017. The transmitters 101 to 1017 each transmit signal light. The seven optical transmission fibers 1031 to 1037 transmit the signal light output from each of the transmitters 101 to 1017 and input the signal light to the optical amplifier 100. The optical amplifier 100 collectively photoamplifies the seven signal lights input from the optical transmission fibers 1031 to 1037 and outputs them to each of the seven optical transmission fibers 1041 to 1047. The optical transmission fibers 1041 to 1047 transmit the amplified signal light and cause the optical receiver 1020 to input the amplified signal light. The optical receiver 1020 includes seven receivers 1021-1027. The receivers 1021 to 1027 receive the amplified signal light transmitted by the optical transmission fibers 1041 to 1047 and convert it into an electric signal.

Since the optical communication system 1000 uses an optical amplifier 100 capable of realizing suitable optical amplification, suitable optical communication can be realized. In the present embodiment, the optical transmission fiber is seven single-core optical fibers, but an optical transmission fiber composed of one 7-core multi-core fiber may be used.

If the optical communication system 1000 is a long-range communication system or the like, the optical amplifier 100 can be used as a repeater amplifier, a preamplifier, or a booster amplifier. If the optical communication system 1000 is a network system using a ROADM (Reconfigurable Optical Add / Drop Multiplexer), the optical amplifier 100 can be used for loss compensation.

In the third embodiment, the optical amplifier is configured by using the multi-core optical amplifier fiber 1 according to the first embodiment, but the multi-core optical amplifier fiber 2 according to the second embodiment and the multi-core optical amplifier according to other embodiments are used. An optical amplifier may be configured using fibers. Further, in the above embodiments, in 1 and 2, the multi-core optical amplifier fiber contains only Er as a rare earth element, but may contain only a rare earth element other than Er, for example, ytterbium (Yb), or both Er and Yb. May include.

Further, in the first and second embodiments, the core portions of the multi-core optical amplifier fiber are arranged in a triangular lattice shape, but may be arranged in a square lattice shape. The number of cores in the multi-core optical amplifier fiber is not particularly limited as long as it is plural.

Moreover, the present invention is not limited by the above-described embodiment. The present invention also includes a configuration in which the above-mentioned components are appropriately combined. Further, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspect of the present invention is not limited to the above-described embodiment, and various modifications can be made.

1,2,3,4,5,6,7,8,9 Multi-core optical amplification fiber 1aa, 1ab, 2aa, 2ab, 2ac, 3a, 4a, 5a, 6a, 7a, 8a, 9a Core part 1b, 2b, 3b, 4b, 5b, 6b, 7b, 8b Inner clad part 1c, 2c, 3c, 4c, 5c, 6c, 7c, 8c, 9d Outer clad part 9b 1st inner clad part 9c 2nd inner clad part 10,70 Light Isolator 20 Optical fiber fan-in 30 Semiconductor laser 40, 40A, 80 Optical coupler 50 Pump stripper 60 Optical fiber fan-out 90 Optical fiber for excitation light recovery 100, 100A Optical amplifier 1000 Optical communication system 1010 Optical transmitter 1020 Optical receiver 1011 -1017 Transmitter 1021-1027 Receiver 1031-1037, 1041-1047 Optical transmission fiber

Claims (13)

Multiple cores with rare earth elements added,
An inner clad portion located on the outer periphery of each of the plurality of core portions and having a refractive index lower than the maximum refractive index of each core portion,
An outer clad portion located on the outer periphery of the inner clad portion and having a refractive index lower than that of the inner clad portion,
The core portion is designed so that the cutoff wavelength is equal to or lower than the optical amplification wavelength band of the rare earth element. In the optical amplification wavelength band, the inner clad portion of the plurality of core portions A multi-core optical amplifier fiber having a clad thickness such that the leakage loss of the core portion closest to the outer edge is 0.1 dB / 100 m or less. The multi-core optical amplifier fiber according to claim 1, wherein in the optical amplification wavelength band, the bending loss when bent at a diameter of 50 mm is smaller than 0.1 dB / 100 m. The distance between each of the plurality of core portions in the cross section perpendicular to the longitudinal direction is set so that the inter-core crosstalk is -30 dB / 100 m or less in the optical amplification wavelength band. The multi-core optical amplifier fiber according to claim 1 or 2. The multi-core according to claim 3, wherein the separation distance between each of the plurality of core portions is set so that the inter-core crosstalk is -40 dB / 100 m or less in the optical amplification wavelength band. Optical amplification fiber. The multi-core according to claim 4, wherein the separation distance between each of the plurality of core portions is set so that the inter-core crosstalk is -50 dB / 100 m or less in the optical amplification wavelength band. Optical amplification fiber. Any one of claims 1 to 5, wherein the ratio of the total cross section of the plurality of core portions to the cross section of the inner clad portion in the cross section perpendicular to the longitudinal direction is larger than 1.460%. The multi-core optical amplification fiber according to. The multi-core optical amplification fiber according to any one of claims 1 to 6, wherein the cutoff wavelength is within a range of 100 nm from the shortest wavelength of the optical amplification wavelength band. The multi-core optical amplifier fiber according to any one of claims 1 to 7, wherein the rare earth element contains erbium. The multi-core optical amplifier fiber according to claim 8, wherein the rare earth element contains only erbium. The multi-core optical amplifier fiber according to any one of claims 1 to 9, wherein the cross section of the inner clad portion is non-circular. The inner clad portion has a trench portion located on the outer periphery of each of the plurality of core portions, and the refractive index of the trench portion has a refractive index lower than that of other portions of the inner clad portion. The multi-core optical amplification fiber according to any one of claims 1 to 10. The multi-core optical amplifier fiber according to any one of claims 1 to 11.
An excitation light source that outputs excitation light that photoexcites the rare earth element of the multi-core optical amplification fiber, and
An optical coupler that optically couples the excitation light to the inner clad portion,
A multi-core optical fiber amplifier characterized by comprising. An optical communication system comprising the multi-core optical fiber amplifier according to claim 12.

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