JP7214527B2 - multicore optical amplifying fiber, multicore optical fiber amplifier and optical communication system - Google Patents

Description

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

For example, in applications such as submarine optical communication, it is expected that power consumption of optical amplifiers will be reduced by using multi-core EDFAs (Erbium-Doped optical fiber amplifiers) as optical amplifiers.

As for the multi-core EDFA, a configuration is known in which a double-clad multi-core EDF is used as a multi-core optical amplification fiber, and erbium (Er), which is a rare earth element contained in the core portion, is optically pumped by a clad pumping method (Non-Patent Document 1, 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, even more suitable characteristics of the multi-core optical amplifying fiber are required in order to increase the communication capacity.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a multi-core optical amplifying fiber having suitable characteristics, and a multi-core optical fiber amplifier and optical communication system using the same.

In order to solve the above-described problems and achieve the object, a multi-core optical amplifying fiber according to one aspect of the present invention provides a plurality of cores doped with a rare earth element, and , an inner cladding portion having a lower refractive index than the maximum refractive index of each core portion; and an outer cladding portion located on the outer periphery of the inner cladding portion and having a lower refractive index than the inner cladding 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, and the outer edge of the inner cladding portion among the plurality of core portions in the optical amplification wavelength band The clad thickness is such that the leakage loss of the core portion closest to the is 0.1 dB/100 m or less.

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

In the multi-core optical amplifying fiber according to one aspect of the present invention, the separation 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 in the optical amplification wavelength band. It is characterized in that it is set to be

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

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

In the multi-core optical amplifying 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 cladding portion in a cross section perpendicular to the longitudinal direction is greater than 1.460%. Characterized by

A multi-core optical amplifying fiber according to an 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.

A multi-core optical amplifying fiber according to an aspect of the present invention is characterized in that the rare earth element contains erbium.

A multi-core optical amplifying fiber according to an aspect of the present invention is characterized in that the rare earth element contains only erbium.

A multi-core optical amplifying fiber according to an aspect of the present invention is characterized in that the cross section of the inner cladding portion is non-circular.

In the multi-core optical amplifying 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 trench portion has a refractive index different from that of the inner clad portion. is characterized by having a lower refractive index than the portion of .

A multi-core optical fiber amplifier according to an aspect of the present invention includes: the multi-core optical amplifying fiber; a pumping light source that outputs pumping light for optically pumping the rare earth element of the multi-core optical amplifying fiber; and an optical coupler for optical coupling.

An optical communication system according to an aspect of the present invention is characterized by comprising the multi-core optical fiber amplifier.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to realize a multi-core optical amplifying fiber having favorable characteristics.

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

Embodiments will be described below with reference to the drawings. In addition, this invention is not limited by this embodiment. In addition, in the description of the drawings, the same or corresponding elements are given the same reference numerals as appropriate. Also, it should be noted that the drawings are schematic, and the relationship of dimensions of each element, the ratio of each element, and the like may differ from reality. Even between the drawings, there are cases where portions with different dimensional relationships and ratios are included. Further, in this specification, the cut-off wavelength refers to ITU-T (International Telecommunications Union) G.35. 650.1 means the cable cutoff wavelength. For other terms not specifically defined in this specification, see G.I. 650.1 and G.I. 650.2 shall comply with the definition and measurement method.

(Embodiment 1)
FIG. 1 is a schematic cross-sectional view of a multicore optical amplifying fiber according to Embodiment 1, showing a cross section perpendicular to the longitudinal direction of the multicore optical amplifying fiber. The multi-core optical amplifying fiber 1 includes seven core portions 1aa and 1ab, an inner clad portion 1b located on the outer periphery of each of the core portions 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, seven-core multi-core optical fiber.

The core portions 1aa and 1ab are arranged in a triangular lattice that achieves the closest 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 form corners of a regular hexagon. Core portions 1aa and 1ab contain, for example, germanium (Ge) or aluminum (Al) as a refractive index adjusting dopant that increases the refractive index. Moreover, 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 the 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 concentration quenching of Er.

The inner cladding portion 1b has a refractive index lower than the maximum refractive index of each core portion 1aa, 1ab. The inner cladding portion 1b is made of, for example, pure silica glass containing no dopant for adjusting the refractive index. As a result, the refractive index profiles of the core portions 1aa and 1ab and the inner cladding portion 1b are of a step index type. In addition, the inner clad portion 1b may have a trench portion positioned on the outer periphery of each of the core portions 1aa and 1ab. In this case, the trench portion is made of silica glass to which a refractive index adjusting dopant such as fluorine (F) is added to lower the refractive index, and the refractive index of the trench portion is the same as that of the other portion of the inner clad portion 1b made of pure silica glass. has a lower refractive index than that of In this case, the refractive index profiles of the core portions 1aa and 1ab and the inner cladding portion 1b are trench type.

A relative refractive index difference between the core portions 1aa and 1ab with respect to the inner clad portion 1b is defined as a core Δ. In this 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 resin, for example. When the inner cladding portion 1b has a trench portion, the refractive index of the outer cladding portion 1c may be higher than the refractive index of the trench portion. It is lower than the average refractive index of the cladding portion 1b.

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

When pumping light having a wavelength capable of optically exciting Er, for example, pumping light in a 900 nm wavelength band, is input to the inner clad portion 1b, the pumping light propagates inside the inner clad portion 1b and is doped into the core portions 1aa and 1ab. photo-excited Er. As a result, the core portions 1aa and 1ab can optically amplify the signal light input to the core portions 1aa and 1ab. In the case of Er, the optical amplification wavelength band that can be optically amplified is, for example, 1530 nm to 1565 nm called C band and 1565 nm to 1625 nm called L band.

In the multi-core optical amplifying fiber 1, the core portions 1aa and 1ab are designed so that the cutoff wavelength is equal to or shorter than the Er optical amplification wavelength band. That is, the cutoff wavelength is set to a wavelength of 1530 nm or less, for example, when the optical amplification band includes the C band. The cutoff wavelength may be 1530 nm. Also, considering the manufacturing error of the multi-core optical amplifying fiber 1, it may be set to around 1500 nm, for example. When only L-band signal light is amplified by the multi-core optical amplifying fiber 1, the cutoff wavelength is set to a wavelength of 1565 nm or less, for example. Also, considering the manufacturing error of the multi-core optical amplifying fiber 1, it may be set to around 1535 nm, for example. As a result, the multi-core optical amplifying fiber 1 can propagate the signal light in the Er optical amplification wavelength band in a single mode through the core portions 1aa and 1ab. In order to design each core portion 1aa, 1ab to have such a cutoff wavelength, the core Δ and the core diameter 2a of each core portion 1aa, 1ab 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 of the step index type. In this way, in order to achieve a predetermined cutoff wavelength, it is sufficient to appropriately combine the core Δ and the core diameter. 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. good.

Also, in the multi-core optical amplifying fiber 1, it is important to optimize the clad thickness Tc1 in order to achieve favorable characteristics. That is, if the clad thickness Tc1 is thin, the light confinement effect on the core portion 1ab closest to the outer edge of the inner clad portion 1b is reduced, resulting in leakage loss and reduced amplification efficiency. On the other hand, if the cladding thickness Tc1 is too thick, the cross-sectional area of the multi-core optical amplifying fiber 1 becomes excessively large.

The multi-core optical amplifying fiber 1 has a clad thickness Tc1 at 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 optical energy due to leakage loss is suppressed to an allowable level, so that the reduction in amplification efficiency can also be suppressed.

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

In addition, in a communication system, the optical fiber transmission line is long and the number of repeaters increases according to the transmission distance. Affects communication quality. Therefore, the multi-core optical amplifying fiber 1 for a multi-core optical fiber amplifier used in an optical communication system also preferably has low inter-core crosstalk in the optical amplification wavelength band.

Here, the crosstalk between cores means that when a signal light having a predetermined power is input to one of the cores 1aa and 1ab and propagated, part of the power of the signal light is In the case of leakage 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)

The pitch P1 of the multi-core optical amplifying fiber 1 is preferably set so that the inter-core crosstalk is −30 dB/100 m or less. If the crosstalk between cores is −30 dB/100 m or less, the multi-core optical amplifying fiber 1 can be suitably used for multi-core optical fiber amplifiers used in optical communication systems with transmission distances of several hundred kilometers, for example.

Also, if the pitch P1 of the multi-core optical amplifying fiber 1 is set so that the inter-core crosstalk is −40 dB/100 m or less, the multi-core optical fiber amplifier used in an optical communication system with a transmission distance of several thousand kilometers, for example. It can be suitably used for

Also, if the pitch P1 is set so that the inter-core crosstalk is −50 dB/100 m or less, the multi-core optical amplifying fiber 1 can be used in an optical communication system with a transmission distance of 10,000 km across the Pacific Ocean, for example. It can be suitably used for multi-core optical fiber amplifiers.

It should be noted that the inter-core crosstalk value described above is an example and is not limited.

Furthermore, according to the intensive 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 cladding portion 1b in the cross section perpendicular to the longitudinal direction of the multi-core optical amplifying fiber 1 (hereinafter referred to as When the ratio of core/cladding area) is relatively large, the proportion of the power of the pumping light that contributes to optical amplification in the core portions 1aa and 1ab increases, and the pumping efficiency increases.

In a known 7-core type multi-core optical amplification fiber, the core/clad area ratio is about 1%, but according to the inventors' intensive study, the core/clad area ratio is greater 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 will decrease and the leakage loss will increase, or the pitch P1 will decrease and inter-core crosstalk will occur. Therefore, it is preferable to set the core/cladding area ratio in consideration of various characteristics required for the multi-core optical amplifying fiber 1 .

An example of a design method for the multi-core optical amplifying fiber 1 will now be described. First, as step 1, the number and arrangement of core portions in the multi-core optical amplifying fiber are set according to the application. The multi-core optical amplifying fiber 1 has seven cores arranged in a triangular lattice. At this time, the refractive index profile of the core portion may also be set.

Subsequently, in step 2, a cutoff wavelength is set according to the optical amplification wavelength band. As described above, the cutoff wavelength is set so that the signal light in the optical amplification wavelength band can be propagated in single mode. Further, as described above, the higher the core/cladding 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 within a 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 longer within the range from 1530 nm to 100 nm.

By setting the cutoff wavelength in this way, the relationship between the core Δ and the core diameter as shown in FIG. 2 is determined.

Next, in step 3, the core Δ and the core diameter are set so as to achieve a desired bending loss. This determines the lower limit of the core Δ and the upper limit of the core diameter. For example, the bending loss is set to a value smaller than 0.1 dB/100 m when bent with a diameter of 50 mm in the optical amplification wavelength band. Since the longer the wavelength, the greater the bending loss, it is sufficient to set the bending loss at the maximum wavelength in the optical amplification wavelength band to 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 such that the bending loss at the wavelength of 1625 nm is less than 0.1 dB/100 m.

Next, in step 4, a desired inter-core crosstalk is set. Inter-core crosstalk is set according to the use of the multi-core optical amplifying fiber 1 and the like. 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 0.1 dB/100 m or less, for example. Since the longer the wavelength, the greater the leakage loss, it is sufficient to set the leakage loss at the maximum wavelength in the optical amplification wavelength band to a desired value. This determines 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 the set value. Furthermore, the clad diameter Dc1 is determined by determining the pitch P1 and the clad thickness Tc1. Furthermore, the core/cladding area ratio is determined.
Subsequently, in step 6, it is determined whether the core/cladding area ratio is greater than a desired value, such as 1.460%. If the value is greater than or equal to the desired value, the step is terminated. If the value is less than the desired value, for example, the cutoff wavelength is set to a longer wavelength, or the conditions for bending loss, crosstalk between cores, and leakage loss are relaxed. Then, return to any one of steps 2 to 5 above, and repeat the settings after the return step.

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

As described above, the multi-core optical amplifying fiber 1 according to Embodiment 1 has favorable characteristics.

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

The core portions 2aa, 2ab, and 2ac are arranged in a triangular lattice that achieves the closest 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 form corners of a regular hexagon. The 12 core portions 2ac are arranged around the core portion 2ab so as to be positioned at the centers of corners or sides of the regular hexagon. 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 amplifying fiber 1, and thus the description thereof is omitted.

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

The pitch P2 is the distance between the core portions 2aa, 2ab, and 2ac in the cross section of FIG. Also, the clad diameter of the inner clad portion 2b is defined as a clad diameter Dc2. Among the core portions 2aa, 2ab, and 2ac, the core portion closest to the outer edge of the inner clad portion 2b is one of the six core portions 2ac located at the corners of the regular hexagon among the twelve core portions 2ac. However, in this embodiment, all six core portions 2ac are assumed to be equidistant from the outer edge of the inner clad portion 2b. The shortest distance from the center of any one of the six core portions 2ac to the outer edge of the inner clad portion 2b is defined as clad thickness Tc2.

When excitation light having a wavelength capable of optically exciting Er is input to the inner clad portion 2b, the excitation light optically excites Er doped in each of the core portions 2aa, 2ab, and 2ac while propagating inside the inner clad portion 1b. . As a result, the core portions 2aa, 2ab, and 2ac can optically amplify the signal lights input to the core portions 2aa, 2ab, and 2ac.

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

Moreover, the multi-core optical amplifying fiber 2 has a clad thickness Tc2 at 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 optical energy due to leakage loss is suppressed to an allowable level, so that the reduction in amplification efficiency can also be suppressed.

Also, for example, the multi-core optical amplifying 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.

The pitch P2 of the multi-core optical amplifying fiber 2 is preferably set so that the inter-core crosstalk is -30 dB/100 m or less, -40 dB/100 m or less, or -50 dB/100 m or less. Thereby, the multi-core optical amplifying fiber 2 can be suitably used according to the transmission distance of the optical communication system. However, the inter-core crosstalk value is an example and not a limitation.

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

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

As described above, the multi-core optical amplifying fiber 2 according to Embodiment 2 has favorable characteristics.

(Other embodiments)
4A to 4G are schematic cross-sectional views of multi-core optical amplification fibers according to other embodiments. The multi-core optical amplifying fibers 3-8 shown in FIGS. 4(a)-(f) have a plurality of, for example, 4, 7 or 19 cores, inner claddings 3b-8b, and outer claddings 3c-8c. It is a double-clad and multi-core optical fiber. As for the core portions, only one core portion 3a to 8a is shown as a representative.

In the multi-core optical amplifying fiber 3, the centers of gravity of the core portions are eccentric with respect to the center of the inner clad portion 3b. The multi-core optical amplifying fiber 4 has an inner cladding portion 4b with a D-shaped cross section. The multi-core optical amplifying fiber 5 has an inner clad portion 5b with a hexagonal cross section. The multi-core optical amplifying fiber 6 has an elliptical cross section in the inner clad portion 6b. The multi-core optical amplifying fiber 7 has an inner cladding portion 7b with a rectangular cross section. The multi-core optical amplifying fiber 8 has an inner cladding portion 8b having a petal-shaped cross section. The multi-core optical amplifying fibers 3 to 8 can improve the pumping efficiency by eccentrically centering the cores or by making the cross section of the inner cladding non-circular.

The multi-core optical amplifying fiber 9 shown in FIG. 4(g) comprises 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, multi-core optical fiber. As for the core portions, only one core portion 9a is shown as a representative. 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 cladding portion 9b and the second inner cladding portion 9c propagate the pumping light. Thus, the multi-core optical amplifying fiber according to the embodiment may be a triple-clad multi-core optical fiber.

(calculation example)
A simulation calculation was performed on the characteristics of the multi-core optical amplifying fiber according to the second embodiment. In this calculation, 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 bending is performed with a diameter of 50 mm The calculation was performed under the condition that the bending loss of is smaller than 0.1 dB/100m.

Tables 1 and 2 show the calculation results. In Tables 1 and 2, "LP01 cutoff wavelength" means the cutoff wavelength of the LP01 mode, which is the fundamental propagation mode. 650.1 is the cable cutoff wavelength. "Pitch" is pitch P1. "XT" is inter-core crosstalk. "Leakage Loss" and "XT" are values in Table 1 at a wavelength of 1550 nm in the C-band and in Table 2 at a wavelength of 1625 nm in the L-band.

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

When the core Δ is 1.2% and 2a is 5.1 μm, the core/cladding area ratio is 1.505%. If the core Δ is 0.80% or less and 2a is 6.3 μm or more, the core/cladding area ratio is 2.00% or more. For example, when the core Δ is 0.85% and 2a is 6.1 μm, the core/cladding area ratio is 2.098%. When the core Δ is 0.5% and 2a is 7.9 μm, the core/cladding 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, XT is −30 dB/100 m or less, and if Δ is 0.45% or more, XT is −40 dB/100 m or less. It was confirmed that if the distance becomes 100 m or less and Δ is 0.55% or more, XT becomes -50 dB/100 m or less. It was also confirmed that if 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 with a clad thickness of 15 μm.

When the core Δ is 1.2% and 2a is 5.1 μm, the core/cladding area ratio is 1.460%. If the core Δ is 0.80% or less and 2a is 6.3 μm or more, the core/cladding 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/cladding area ratio is 2.037%. When the core Δ is 0.5% and 2a is 7.9 μm, the core/cladding 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, XT is -30 dB/100 m or less, Moreover, the core/cladding area ratio can be 2% or more.

(Embodiment 3)
FIG. 5 is a schematic diagram showing the configuration of a multi-core optical fiber amplifier according to Embodiment 3. FIG. 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 amplifying fiber 1 according to the first embodiment, a pump stripper 50, an optical fiber fan-out. (FAN OUT) 60 and seven optical isolators 70 are provided. In the figure, the symbol "x" indicates the fusion splicing point of the optical fiber.

The optical fiber fan-in 20 comprises seven bundled single-mode optical fibers and one multi-core fiber having seven cores, each core of the seven single-mode optical fibers at the joint. A section is configured to optically couple to each core section of the multicore fiber. It should be noted that the seven single-mode optical fibers are, for example, ITU-TG. 652 standard single mode optical fiber, each provided with an optical isolator 10 . Optical isolators 10 and 70 allow light to pass in the direction indicated by the arrows and block light from passing in the opposite direction. A multi-core fiber of the optical fiber fan-in 20 is connected to an optical coupler 40 . The end surfaces of the bundled seven single-mode optical fibers and the multi-core fibers to be optically coupled are processed obliquely with respect to the optical axis for reflection suppression, but may be perpendicular to the optical axis. . Instead of the seven optical isolators 10 and 70, an optical isolator in which a plurality of (seven in this embodiment) single-mode optical fibers are integrated may be used.

The multi-core fiber of the optical fiber fan-in 20, like the multi-core optical amplifying fiber 1, has seven cores arranged in a triangular lattice pattern, and located on the outer periphery of each core. and a cladding 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.

A semiconductor laser 30, which is an excitation light source, is a transverse 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 of Er in the 900 nm wavelength band. This allows the excitation light to photoexcite the erbium ions. A semiconductor laser 30 outputs excitation light from a multimode optical fiber. This multimode optical fiber is a step index type with a core diameter/cladding diameter of 105 μm/125 μm, for example, and an NA of 0.16 or 0.22, for example.

The optical coupler 40 includes a main optical fiber and an excitation light supply optical fiber. The main optical fiber has seven cores arranged in a triangular lattice like the cores of the multi-core fiber of the optical fiber fan-in 20, and the cores located on the outer periphery of each core. The optical fiber is a double clad type optical fiber comprising an inner clad portion having a lower refractive index than the inner clad portion and an outer clad portion positioned 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-based glass, and the outer clad portion is made of resin.

The optical fiber for pumping light supply is a multimode optical fiber of the same type, one end of which is connected to the multimode optical fiber of the semiconductor laser 30, and is a step index type with a core diameter/cladding diameter of, for example, 105 μm/125 μm, NA is, for example, 0.16 or 0.22. The pumping light supplying optical fiber receives the pumping light from the semiconductor laser 30 and supplies the pumping light to the main optical fiber. The inner cladding propagates pumping 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 propagated through each core portion of the multi-core fiber is optically coupled to each core portion when input to the main optical fiber. Each core propagates each signal light. Pumping light and signal light are output from the main optical fiber to the multi-core optical amplifying fiber 1 .

One end of the multi-core optical amplifying fiber 1 is connected to the main optical fiber of the optical coupler 40 . Each core portion 1aa, 1ab of the multi-core optical amplifying fiber 1 is connected to each core portion of the main optical fiber. Also, the inner clad portion 1b of the multi-core optical amplifying fiber 1 is connected to the inner clad portion of the main optical fiber. Therefore, when each signal light and pumping light propagated through the main optical fiber are input to the multi-core optical amplifying fiber 1, they propagate through the core portions 1aa and 1ab and the inner clad portion 1b in the same direction. The excitation light optically excites Er in each of the core portions 1aa and 1ab while propagating through the inner clad portion 1b. Each signal light propagating through each of the core portions 1aa and 1ab is optically amplified by the stimulated emission of Er. The multi-core optical amplifying fiber 1 outputs each optically amplified signal light and the pumping light that did not contribute to the optical amplification.

Pump stripper 50 is a known device that removes pump light that has not contributed to light amplification. The pump stripper 50 has, for example, a portion of the outer clad of a double-clad multi-core fiber having seven cores removed, extracts pumping light from the surface of the inner clad of the removed portion, and supplies it to a radiator plate or the like. It has a configuration in which the energy of the excitation light is converted into heat energy by irradiation and absorption, and the heat is released. The pump stripper 50 propagates each signal light through the multi-core fiber, and reduces the power of the pump light to a level that causes no problem even if it is output from the optical amplifier 100 .

The fiber optic fan-out 60, similar to the fiber optic fan-in 20, comprises seven bundled single-mode optical fibers and one multi-core fiber having 7 cores, with 7 cores at the joint. Each core portion of the single-mode optical fiber is configured to be optically coupled to each core portion of the multi-core fiber. Each single-mode optical fiber is provided with an optical isolator 70 . The multicore fiber is connected to pump stripper 50 . The end surfaces of the bundled seven single-mode optical fibers and the multi-core fibers to be optically coupled are processed obliquely with respect to the optical axis for reflection suppression, 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 fan-out 60, each signal light propagates through each core portion of each single-mode optical fiber and passes through the optical isolator 70. Output.

Since this optical amplifier 100 performs optical amplification using the multi-core optical amplifying fiber 1 having suitable characteristics, it is possible to realize suitable optical amplification.

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

40 A of optical couplers have the structure which changed the structure of the optical coupler 40, and changed so that the optical fiber 90 for pumping light collection|recovery was connected.

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

The pumping light recovery optical fiber 90 is composed of, for example, a commercial stuck-index optical fiber or graded-index optical fiber from which the cladding is removed and only a core. The pumping light recovery optical fiber 90 optically connects the optical coupler 80 and the optical coupler 40 .

In this optical amplifier 100A, of the pumping light output from the semiconductor laser 30 and supplied to the multicore optical amplifying fiber 1 via the optical coupler 40, at least the pumping light that did not contribute to the optical pumping in the multicore optical amplifying fiber 1 A portion is recovered by optical coupler 80 . The collected pumping light is input to the optical coupler 40 through the pumping light collecting optical fiber 90 to be regenerated as pumping light and supplied to the multi-core optical amplifying fiber 1 again. Thereby, the pumping efficiency in the optical amplifier 100A can be improved.

(Embodiment 5)
FIG. 7 is a schematic diagram showing the configuration of an optical communication system according to the fifth embodiment. An optical communication system 1000 includes an optical transmitter 1010, an optical receiver 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 comprises seven transmitters 1011-1017. Transmitters 1011 to 1017 each transmit signal light. The seven optical transmission fibers 1031 to 1037 transmit the signal lights output from the transmitters 1011 to 1017 respectively and input them to the optical amplifier 100 . The optical amplifier 100 collectively optically amplifies the seven signal lights input from the optical transmission fibers 1031 to 1037 and outputs them to the seven optical transmission fibers 1041 to 1047, respectively. The optical transmission fibers 1041 to 1047 transmit the amplified signal light and input it to the optical receiver 1020 . The optical receiver 1020 has seven receivers 1021-1027. The receivers 1021-1027 receive the amplified signal light transmitted by the optical transmission fibers 1041-1047 and convert it into an electrical signal.

Since the optical communication system 1000 uses the optical amplifier 100 capable of achieving suitable optical amplification, it is possible to achieve suitable optical communication. In this embodiment, the optical transmission fiber is seven single-core optical fibers, but an optical transmission fiber made up of one seven-core multi-core fiber may be used.

If the optical communication system 1000 is a long distance communication system, etc., the optical amplifier 100 can be used as a repeater amplifier, preamplifier, or 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 using the multi-core optical amplification fiber 1 according to the first embodiment, but the multi-core optical amplification fiber 2 according to the second embodiment and the multi-core optical amplification according to other embodiments are used. Optical amplifiers may be constructed using fibers. Further, in the above embodiments 1 and 2, the multi-core optical amplification fiber contains only Er as a rare earth element, but it may contain only a rare earth element other than Er, such as ytterbium (Yb), or both Er and Yb. may contain

Moreover, in the first and second embodiments, the core portions in the multi-core optical amplifying fiber are arranged in a triangular lattice pattern, but they may be arranged in a square lattice pattern. The number of core portions in the multi-core optical amplifying fiber is also not particularly limited as long as it is plural.

Moreover, the present invention is not limited by the above embodiments. The present invention also includes those configured by appropriately combining the respective constituent elements described above. Further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible.

1, 2, 3, 4, 5, 6, 7, 8, 9 multicore optical amplification fiber 1aa, 1ab, 2aa, 2ab, 2ac, 3a, 4a, 5a, 6a, 7a, 8a, 9a core portions 1b, 2b, 3b, 4b, 5b, 6b, 7b, 8b Inner clad portions 1c, 2c, 3c, 4c, 5c, 6c, 7c, 8c, 9d Outer clad portion 9b First inner clad portion 9c Second inner clad portions 10, 70 Light Isolator 20 Optical fiber fan-in 30 Semiconductor lasers 40, 40A, 80 Optical coupler 50 Pump stripper 60 Optical fiber fan-out 90 Pumping light recovery optical fiber 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 (14)

a plurality of core portions to which a rare earth element is added;
an inner cladding portion located on the outer periphery of each of the plurality of core portions and having a lower refractive index than the maximum refractive index of each core portion;
an outer cladding portion located on the outer periphery of the inner cladding portion and having a lower refractive index than the inner cladding portion;
and the core portion is designed so that the cutoff wavelength is equal to or less than the optical amplification wavelength band of the rare earth element, and in the optical amplification wavelength band, the inner cladding portion of the plurality of core portions The cladding thickness is such that the core closest to the outer edge has a leakage loss of 0.1 dB/100 m or less, and the total cross-sectional area of the plurality of cores relative to the cross-sectional area of the inner cladding in a cross section perpendicular to the longitudinal direction ratio is greater than 2.0% ,
The cladding thickness is 28.3 μm or less,
The cutoff wavelength is 1.493 μm or less,
The clad diameter, which is the outer diameter of the inner clad portion, is 210.6 μm or less
A multi-core optical amplification fiber characterized by: 2. The multi-core optical amplifying fiber according to claim 1, wherein the bending loss when bent at a diameter of 50 mm is less than 0.1 dB/100 m in the optical amplifying wavelength band. A separation distance between each of the plurality of core portions in a cross section perpendicular to the longitudinal direction is set so that inter-core crosstalk is −30 dB/100 m or less in the optical amplification wavelength band. 3. The multi-core optical amplifying fiber according to claim 1 or 2. 4. The multi-core according to claim 3, wherein a distance between each of said plurality of core portions is set so that inter-core crosstalk is -40 dB/100 m or less in said optical amplification wavelength band. Optical amplification fiber. 5. The multi-core according to claim 4, wherein the distance between each of the plurality of core portions is set so that inter-core crosstalk is −50 dB/100 m or less in the optical amplification wavelength band. Optical amplification fiber. 6. The multi-core optical amplifying fiber according to claim 1, wherein said clad thickness is 23.4 μm or less. 7. The multi-core optical amplifying fiber according to claim 1, wherein the centers of gravity of said plurality of cores are eccentric with respect to the center of said inner clad. 8. The multi-core optical amplifying fiber according to claim 1, wherein said cutoff wavelength is within a range of 100 nm from the shortest wavelength of said optical amplification wavelength band. 9. The multi-core optical amplifying fiber according to claim 1, wherein said rare earth element contains erbium. 10. The multi-core optical amplifying fiber of claim 9, wherein said rare earth element includes only erbium. 11. The multi-core optical amplifying fiber according to any one of claims 1 to 10, wherein the inner cladding has a non-circular cross section. The inner cladding portion has a trench portion positioned on the outer periphery of each of the plurality of core portions, and the trench portion has a refractive index lower than that of other portions of the inner cladding portion. The multi-core optical amplifying fiber according to any one of claims 1 to 11, characterized by: A multi-core optical amplifying fiber according to any one of claims 1 to 12;
a pumping light source that outputs pumping light for optically pumping the rare earth element of the multi-core optical amplifying fiber;
an optical coupler that optically couples the excitation light to the inner cladding;
A multi-core optical fiber amplifier comprising: An optical communication system comprising the multi-core optical fiber amplifier according to claim 13.

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