Communication system

Abstract

Provided is a method for suppressing an output light intensity difference due to inter-core crosstalk in order to reduce signal processing of a coupled MCF as much as possible.　The present invention has a coupled multicore fiber (101), input-side single-mode fibers (104) and output-side single-mode fibers (105) that correspond to the number of cores of the coupling-type multicore fiber, a fan-in element (102) that connects the input-side single-mode fibers and the coupled multicore fiber, a fan-out element (103) that connects the coupled multicore fiber and the output-side single-mode fibers, and an optical element (106), the optical element being disposed between the input-side single-mode fibers and the fan-in element, or between the fan-in element and the coupled multicore fiber, and changing either or both of the polarization and the phase of incident light.

Description

communication systems

[0001] The present invention relates to a communication system, and more particularly to reducing the load of signal processing in a coupled multi-core fiber capable of large-capacity communication.

[0002] In the information society of recent years, the development of communication devices has led to a rapid increase in data traffic. Furthermore, as seen in the so-called 5G (Generation 2) and 6G, not only is communication volume increasing, but low latency and the ability to connect multiple devices such as smartphones are becoming possible, and the use of AI (Artificial Intelligence) is becoming more commonplace, it is expected that data traffic will continue to increase.

[0003] In the field of high-capacity communications, progress has been made in developing technologies to increase capacity even for the currently mainstream wavelength division multiplexing method (transmitting light of different wavelengths over a single optical fiber). However, there is a limit to the number of wavelengths that can be used with wavelength division multiplexing, and it is highly likely that this method will not be able to accommodate the rapid increase in communication capacity that is expected in the future.

[0004] To date, research and development of multi-core fibers (MCFs) having multiple cores within one fiber has been underway with the aim of increasing the transmission capacity per optical fiber.

[0005] MCFs include uncoupled MCFs, which are optical fibers in which the distance between cores is increased to reduce inter-core crosstalk, and coupled MCFs, which are optical fibers in which the distance between cores is shortened to achieve high core density.

[0006] Since each core of a non-coupled MCF is an independent optical transmission path, the optical communication transmission technology and signal processing technology that have been developed up to now can be utilized as is.

[0007] On the other hand, a coupled MCF is an optical fiber with a higher core density achieved by shortening the core spacing. However, crosstalk occurs between the cores, and therefore optical signals must be received taking this into consideration, necessitating signal processing using a digital signal processor (DSP) or the like.

[0008] Patent Document 1 proposes a method for stably controlling an optical amplifier in a coupled MCF.

[0009] International Publication No. 2020 / 171103

[0010] The signal processing of coupled MCF requires more complex processing than conventional signal processing or uncoupled MCF, which leads to increased complexity of the signal circuit and power consumption, which is likely to hinder practical application.

[0011] In view of these, an object of the present invention is to provide a method for suppressing the difference in output light intensity due to inter-core crosstalk in order to reduce the signal processing of a coupled MCF as much as possible.

[0012] To achieve this object, the present invention has the following configuration.

[0013] [1] A communication system comprising a coupled multicore fiber, an input single-mode fiber and an output single-mode fiber corresponding to the number of cores of the coupled multicore fiber, a fan-in element connecting the input single-mode fiber to the coupled multicore fiber, a fan-out element connecting the coupled multicore fiber to the output single-mode fiber, and an optical element, wherein the optical element is disposed between the input single-mode fiber and the fan-in element or between the fan-in element and the coupled multicore fiber, and changes either the polarization or the phase of incident light, or both. [2] The communication system according to [1], wherein the optical element is a patterned retardation element having two or more slow axis directions in its plane, and light from each core after passing through the optical element is P-polarized or S-polarized. [3] The communication system according to [1], wherein the optical element is an optical element having a patterned retardation element having two or more slow axis directions in its plane and a polarizing plate, and light from each core after passing through the optical element is P-polarized or S-polarized. [4] The communication system according to [1], wherein the optical element is a patterned polarizing plate having two or more transmission axes in a plane, and the light of each core after passing through the optical element is P-polarized or S-polarized. [5] The communication system according to [1], wherein the phase of the light of each core after passing through the optical element is (phase of the light before passing) -20 degrees ≦ (phase of the light after passing) ≦ (phase of the light before passing) + 20 degrees, or (phase of the light before passing) + 160 degrees ≦ (phase of the light after passing) ≦ (phase of the light before passing) + 200 degrees.

[0014] [6] The communication system according to [1], further comprising: another optical element disposed between the coupled multicore fiber and the fan-out element, or between the fan-out element and the output-side single-mode fiber, the other optical element changing the intensity of the output light. [7] The communication system according to [1], wherein the other optical element is an optical element having a patterned retardation element having two or more slow axis directions in its plane and a polarizer, and wherein the difference in intensity of the light of each core after passing through the other optical element is within ±10%. [8] The communication system according to [1], wherein the other optical element is a patterned polarizer having two or more transmission axes in its plane, and wherein the difference in intensity of the light of each core after passing through the other optical element is within ±10%.

[0015] [9] A communication system comprising a coupled multicore fiber, an input-side single-mode fiber and an output-side single-mode fiber corresponding to the number of cores of the coupled multicore fiber, a fan-in element connecting the input-side single-mode fiber to the coupled multicore fiber, a fan-out element connecting the coupled multicore fiber to the output-side single-mode fiber, and an optical element, wherein the optical element is disposed between the coupled multicore fiber and the fan-out element or between the fan-out element and the output-side single-mode fiber and is an element for changing the intensity of output light.

[10] The communication system according to [9], wherein the optical element is an optical element having a patterned retardation element having two or more in-plane slow axis directions and a polarizing plate, and wherein the difference in intensity of light among the cores after passing through the optical element is within ±10%.

[11] The communication system according to [9], wherein the optical element is a patterned polarizing plate having two or more in-plane transmission axes, and wherein the difference in intensity of light among the cores after passing through the optical element is within ±10%.

[12] A communication system according to any one of [1] to

[11] , wherein the fan-in element, or the fan-out element, or both, are elements including a liquid crystal diffraction element, or a liquid crystal lens element, or both.

[0016] According to the present invention, it is possible to suppress the difference in output light intensity due to inter-core crosstalk in a coupled MCF.

[0017] FIG. 1 is a schematic diagram showing a first example of a communication system using a four-core coupled MCF in which an optical element of the present invention is arranged. FIG. 2 is a schematic diagram showing an example of a pattern phase difference in the optical element, with the arrow direction representing the slow axis direction. FIG. 3 is a schematic diagram showing an example of a pattern phase difference in the optical element, with the arrow direction representing the slow axis direction. FIG. 4 is a schematic diagram showing an example of a patterned polarizing plate in which the optical element is arranged, with the arrow direction representing the transmission axis direction. FIG. 5 is a schematic diagram showing a second example of a communication system using a four-core coupled MCF in which an optical element of the present invention is arranged. FIG. 6 is a schematic diagram showing a third example of a communication system using a four-core coupled MCF in which an optical element of the present invention is arranged.

[0018] A preferred embodiment of the communication system of the present invention will be described in detail below.

[0019] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. Furthermore, unless otherwise specified, a specific angle includes a generally accepted error range in the relevant technical field.

[0020] 1 is a schematic diagram showing a first example of a communication system using a four-core coupled MCF in which an optical element of the present invention is arranged. In the communication system 100, a coupled multicore fiber 101 (hereinafter referred to as coupled MCF 101) is connected to an input single-mode fiber 104 and an output single-mode fiber 105 via a fan-in element 102 and a fan-out element 103. An optical element 106 is arranged between the input single-mode fiber 104 and the fan-in element 102.

[0021] FIG. 1 shows an example of the arrangement of the optical element 106. As described above, the optical element 106 is arranged between the input single-mode fiber 104 and the fan-in element 102. However, the optical element 106 may also be arranged between the fan-in element 102 and the coupled MCF 101.

[0022] With respect to the light that has passed through the optical element 106, it is preferable from the viewpoint of suppressing crosstalk to make the light of one core P-polarized and the light of the other core S-polarized.

[0023] In the case of an MCF having three or more cores, it is preferable that the polarization states of a certain core 1 and its nearest neighboring core 2 are P-polarized and S-polarized, and it is preferable that the polarization state of the light of the next nearest neighboring core 3 is different from that of the light of that core 3 and its nearest neighboring core.

[0024] When there are a large number of cores, depending on their arrangement and distance, P-polarized light or S-polarized light may be adjacent to each other, but it is desirable to control the polarization so that P-polarized light or S-polarized light is not adjacent to each other as much as possible.

[0025] Furthermore, with regard to the light after passing through the optical element 106, it is preferable from the viewpoint of suppressing crosstalk that the phase of the light from one core is shifted by 180 degrees with respect to the light from the other core.

[0026] In the case of an MCF having three or more cores, it is preferable that the phases of the cores adjacent to each other are shifted by 180 degrees, as described above. Even when the number of cores is large, it is desirable to control the polarization so that P-polarized light or S-polarized light does not adjoin each other as much as possible, as described above.

[0027] For example, in the case of four cores shown in Figure 1, adjacent cores may be shifted in phase by 180 degrees, and diagonally opposite cores may be in phase with each other. When the cores are in phase with each other, the phase shift may be within 40 degrees, preferably within 20 degrees, and more preferably within 10 degrees. Furthermore, the phase shift between adjacent cores may be within 160 to 200 degrees, preferably within 170 to 190 degrees, and more preferably within 175 to 185 degrees.

[0028] Furthermore, it is preferable that the phase of light of each core after passing through the optical element satisfies (phase of light before passing) -20 degrees ≦ (phase of light after passing) ≦ (phase of light before passing) +20 degrees, or (phase of light before passing) +160 degrees ≦ (phase of light after passing) ≦ (phase of light before passing) +200 degrees.

[0029] The optical element 106 may be selected appropriately from among a patterned retardation element, a combination of a patterned retardation element and a polarizing plate, and a patterned polarizing plate depending on the polarization state of the incident light, etc.

[0030] The patterned retardation element is a retardation element having two or more slow axis directions in a plane, and may be formed by laminating retardation elements so that the slow axis directions are different, or may be formed by, for example, performing rubbing in different directions multiple times, or irradiating a photo-alignment film with polarized UV (Ultra Violet) in different directions multiple times, and then arranging a refractive index anisotropic material, but the latter is preferable from the viewpoint of avoiding adverse effects due to the boundary and from the viewpoint of productivity. In addition, in the latter case, an example of the refractive index anisotropic material may be a liquid crystal compound.

[0031] 2 and 3 are schematic diagrams showing examples of patterned retardation elements, in which the arrow direction indicates the slow axis direction.

[0032] A patterned polarizing plate is a polarizing plate having two or more transmission axis directions in its plane, and may be formed by laminating polarizing plates so that the transmission axis directions are different, or may be formed by, for example, changing the direction of a wire-grid polarizing plate, performing rubbing multiple times in different directions, or irradiating a photo-alignment film with polarized UV light multiple times in different directions, followed by arranging a dichroic dye or the like. The latter is preferred from the viewpoints of avoiding adverse effects due to boundaries and productivity.

[0033] A schematic diagram showing an example of a patterned polarizer is shown in Figure 4. The arrow direction indicates the transmission axis direction.

[0034] FIG. 5 is a schematic diagram showing a second example of a communication system using a four-core coupled MCF in which the optical element of the present invention is disposed. In FIG. 5, components identical to those in the communication system 100 shown in FIG. 1 are designated by the same reference numerals, and detailed descriptions thereof will be omitted. The communication system 100a shown in FIG. 5 has the same configuration as the communication system 100 shown in FIG. 1, except that the position of the optical element 106 is different from that of the communication system 100 shown in FIG. 1. The optical element 106 is disposed between the fan-out element 103 and the output-side single-mode fiber 105. FIG. 5 shows an example of the arrangement of the optical element 106. As described above, the optical element 106 is disposed between the fan-out element 103 and the output-side single-mode fiber 105. However, the optical element 106 may also be disposed between the coupled MCF 104 and the fan-out element 103.

[0035] From the viewpoint of reducing the load of signal processing, it is preferable to keep the difference in light intensity between the cores within ±10% for the light that has passed through the optical element 106. For example, by keeping the difference in light intensity within ±10%, the rate of crosstalk between the cores can be made constant, which makes it easier to detect the main component by signal processing and to set the light intensity of the amplification laser.

[0036] Furthermore, from the viewpoint of reducing the load of signal processing, it is more preferable to block the polarization of crosstalk light between the cores and keep the difference in light intensity within ±10% for the light that has passed through the optical element 106. This reduces the crosstalk itself between the cores, making it easier to detect the main component by signal processing and to set the light intensity of the amplification laser.

[0037] In this case, the optical element 106 may be a combination of a patterned retardation element and a polarizing plate, and if it is a patterned polarizing plate, it may be selected appropriately depending on the polarization state of the emitted light.

[0038] The patterned retardation element and the patterned polarizing plate may be the same as those described above.

[0039] It is also preferable that the fan-in element, or the fan-out element, or both, are elements including a liquid crystal diffraction element, or a liquid crystal lens element, or both.

[0040] As mentioned above, the fan-in element and the fan-out element are elements for connecting a multi-core fiber and a single-mode fiber.

[0041] There are several connection methods, including spatial coupling, which uses optical refractive elements such as prisms and regular lenses, and fiber bundles, which glue fiber cores together. With these methods, connection loss occurs due to misalignment of each fiber core. Furthermore, the amount of loss varies depending on the amount of misalignment, which causes differences in the output light intensity. Suppressing this loss requires controlling the position of each core, but this is extremely difficult to control because each core's path is handled by a single refractive element and misalignment of one core affects the misalignment of the other cores.

[0042] Another connection method is the optical waveguide type, but this can cause differences in output light intensity because of mode loss due to differences in the shapes of the optical fiber core and the waveguide, and connection loss due to bending loss that occurs at the bent parts of the waveguide.

[0043] In response to this, for example, by using liquid crystal diffraction elements with different diffraction directions and patterning them in different locations within the plane, it is possible to design them to suit each core, thereby independently controlling the light passing through each core. Furthermore, since the diffraction efficiency can be increased, the loss corresponding to the bending loss of the optical waveguide can be reduced. Furthermore, since there are no surface irregularities, the cores can be bonded together, suppressing interfacial loss. These features reduce the connection loss between each core and the difference in output light intensity.

[0044] This element can also be applied to fan-in elements, since the loss difference on the input side can directly lead to the loss difference on the output side. Naturally, it can also be applied to both fan-in and fan-out elements.

[0045] FIG. 6 is a schematic diagram showing a third example of a communication system using a four-core coupled MCF in which the optical element of the present invention is disposed. In FIG. 6, components identical to those in the communication system 100 shown in FIG. 1 are designated by the same reference numerals, and detailed descriptions thereof will be omitted. The communication system 100b shown in FIG. 6 has the same configuration as the communication system 100 shown in FIG. 1, except that it includes another optical element 106b in addition to the optical element 106a. The communication system 100b is a combination of the communication system 100 shown in FIG. 1 and the communication system 100a shown in FIG. 5. In the communication system 100b, the optical element 106a is disposed between the input single-mode fiber 104 and the fan-in element 102. The other optical element 106b is disposed between the fan-out element 103 and the output single-mode fiber 105. In the communication system 100b shown in FIG. 6, the optical element 106a may be disposed between the coupled MCF 104 and the fan-out element 103 as described above, and another optical element 106b may be disposed between the coupled MCF 104 and the fan-out element 103.

[0046] Although typical embodiments of the present invention have been described above, the present invention is not limited to these and various embodiments that can be understood by those skilled in the art can be applied as long as they fall within the scope of the claims of the present invention.

[0047] 100, 100a, 100b Communication system 101 Coupled multi-core fiber 102 Fan-in element 103 Fan-out element 104 Input side single mode fiber 105 Output side single mode fiber 106, 106a Optical element 106b Other optical element

Claims

1. A communication system comprising: a coupled multicore fiber; an input single mode fiber and an output single mode fiber corresponding to the number of cores of the coupled multicore fiber; a fan-in element connecting the input single mode fiber and the coupled multicore fiber; a fan-out element connecting the coupled multicore fiber and the output single mode fiber; and an optical element, wherein the optical element is disposed between the input single mode fiber and the fan-in element or between the fan-in element and the coupled multicore fiber, and is an element that changes either the polarization or the phase, or both, of incident light.

2. The communication system according to claim 1, wherein the optical element is a pattern phase difference element having two or more slow axis directions in a plane, and the light of each core after passing through the optical element is P-polarized or S-polarized.

3. The communication system according to claim 1, wherein the optical element is an optical element having a pattern phase difference element having two or more slow axis directions in a plane and a polarizing plate, and the light of each core after passing through the optical element is P-polarized or S-polarized.

4. The communication system according to claim 1, wherein the optical element is a patterned polarizer having two or more transmission axes in a plane, and the light of each core after passing through the optical element is P-polarized or S-polarized.

5. The communications system of claim 1, wherein the phase of light of each core after passing through the optical element is: (phase of the light before passing) -20 degrees ≦ (phase of the light after passing) ≦ (phase of the light before passing) + 20 degrees, or (phase of the light before passing) + 160 degrees ≦ (phase of the light after passing) ≦ (phase of the light before passing) + 200 degrees.

6. The communication system according to claim 1, further comprising another optical element disposed between the coupled multicore fiber and the fan-out element, or between the fan-out element and the output-side single mode fiber, the other optical element changing the intensity of the output light.

7. The communication system according to claim 6, wherein the other optical element is an optical element having a pattern retardation element having two or more slow axis directions in its plane and a polarizing plate, and the difference in light intensity of each core after passing through the other optical element is within ±10%.

8. The communication system according to claim 6, wherein the other optical element is a patterned polarizing plate having two or more transmission axes in a plane, and the difference in intensity of light of each core after passing through the other optical element is within ±10%.

9. A communication system comprising: a coupled multicore fiber; an input side single mode fiber and an output side single mode fiber corresponding to the number of cores of the coupled multicore fiber; a fan-in element connecting the input side single mode fiber and the coupled multicore fiber; a fan-out element connecting the coupled multicore fiber and the output side single mode fiber; and an optical element, wherein the optical element is disposed between the coupled multicore fiber and the fan-out element or between the fan-out element and the output side single mode fiber, and is an element that changes the intensity of output light.

10. The communication system according to claim 9, wherein the optical element is an optical element having a pattern retardation element having two or more slow axis directions in a plane and a polarizing plate, and the difference in light intensity of each core after passing through the optical element is within ±10%.

11. The communication system according to claim 9, wherein the optical element is a patterned polarizing plate having two or more transmission axes in a plane, and the difference in intensity of light of each core after passing through the optical element is within ±10%.

12. A communication system according to any one of claims 1 to 11, wherein the fan-in element, or the fan-out element, or both, are elements including a liquid crystal diffraction element, or a liquid crystal lens element, or both.

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