A multi-core optical fiber and data center optical interconnect system based on weak coupling

Abstract

This application relates to a weakly coupled multi-core optical fiber and a data center optical interconnect system. The multi-core optical fiber includes a cladding and multiple fiber cores located within the cladding. Each fiber core has an isolation trench around its periphery. The multi-core optical fiber satisfies the following conditions: Condition 1: Apart from any two adjacent fiber cores with a spacing of D, there is no fiber core whose spacing with the two adjacent fiber cores is simultaneously less than or equal to D; Condition 2: Among any three adjacent fiber cores, the spacing between the two outermost fiber cores is less than or equal to 2D and greater than or equal to λD, where 0 ≤ λ < 2. This application can reduce the overall cross-sectional area, improve the fiber's cabling properties, bendability, and other mechanical properties, and achieve almost crosstalk-free bidirectional point-to-point data transmission in the fiber core channel.

Description

Technical Field

[0001] This application relates to the field of optical communication technology, and in particular to a data center optical interconnect system based on weakly coupled multi-core optical fiber. Background Technology

[0002] With the widespread application of cloud computing and big data, data centers, as the core infrastructure of cloud computing, have experienced exponential growth in computing power and internal data exchange capabilities. For data centers, internal servers and switches are generally interconnected using optical modules or AOC cables. To cope with the increasing data traffic and to accommodate more flexible expansion, upgrades, and redundancy backup capabilities, modern next-generation data centers have generally begun to adopt the Spine-Leaf network architecture advocated by Facebook. This results in stronger data exchange and throughput capabilities within the data center, while also creating a flatter and denser network structure. Therefore, modern large-scale data centers have a very high demand for optical modules and fiber optic resources. The cost of optical interconnects accounts for a large proportion of network costs, making the selection of appropriate technologies while balancing technological development with construction and maintenance costs a significant challenge. From a technical perspective, the selection of optical modules for data centers primarily considers transmission distance, speed, density, and power consumption.

[0003] In terms of transmission distance, its application scenarios can be mainly divided into the following aspects:

[0004] <20m, mainly used for interconnecting servers and TOR switches within a server rack. Currently, data rates are mainly 10G and 25G, and are transitioning to 50G or 100G. The implementation method is mainly DAC (direct connection copper cable) or AOC (active optical cable).

[0005] For distances under 500m, this is primarily used for interconnecting Leaf and Spine switches within the same data center. Currently, data rates are mainly 40G and 100G, with a transition towards 400G. Short distances under 100m primarily use 850nm multimode (MM) fiber; distances between 100m and 500m mostly use 1310nm wavelength single-mode (SM) fiber, with parallel single-mode (PSM) technology being the primary choice for cost considerations.

[0006] For distances under 10km, this is primarily used for interconnecting switches or routers between data center buildings. Currently, data rates are mainly 100G, with a transition towards 400G. At this distance, fiber optic costs are already significant, so wavelength division multiplexing (WDM) technology is commonly used to transmit more signals over a single fiber, primarily using coarse wavelength division multiplexing (CWDM) at 1310nm. PSM technology can still be used in some short-distance (500m–2km) applications.

[0007] For distances over 10km, the primary use is for Data Center Interconnection (DCI). Currently, the implementation primarily employs 100G+DWDM (Dense Wavelength Division Multiplexing). Future implementations for distances over 80km will mainly utilize 400G coherent communication + DWDM, while PAM4+DWDM is also used for distances under 80km. The transmission wavelength is primarily 1550nm, achieving transmission rates of 4T to 32T by multiplexing 40-80 wavelengths on a single fiber using DWDM. For some short-distance (10km-40km) applications with lower rate requirements, CWDM technology with a 1550nm wavelength can still be used.

[0008] In recent years, multi-core optical fibers have become a hot topic in optical fiber communication research. Data center optical interconnects, as a typical application scenario for point-to-point high-capacity optical transmission with relatively simple networks, are very suitable for using multi-core optical fibers to improve single-fiber capacity and port density.

[0009] Multi-core optical fibers are divided into two types: uncoupled and strongly coupled.

[0010] In uncoupled optical fibers, the power coupling between cores is almost negligible (typically below -40dB / km). Therefore, each core can be considered an independent channel, and crosstalk between channels does not need to be considered when transmitting optical signals of the same wavelength in each core, thus avoiding the use of complex digital signal processing algorithms. However, the manufacturing process and cost of uncoupled multi-core optical fibers are more complex due to the need to eliminate crosstalk between adjacent cores. Moreover, the wide isolation trenches required between adjacent cores increase the inter-core spacing, resulting in a larger overall cross-sectional area when the number of cores is large. This significantly reduces the mechanical properties such as cabling and bendability, limiting the application of uncoupled multi-core optical fibers in practical engineering.

[0011] The difference between strongly coupled multi-core optical fibers and uncoupled multi-core optical fibers lies in the fact that crosstalk between the cores cannot be ignored (generally greater than -10dB / km). The advantages of this type of fiber are its simple manufacturing process and lower cost. Furthermore, under the condition of the same core geometry and number of cores, because it does not require isolation trenches, the core spacing is much smaller than that of uncoupled multi-core optical fibers. Therefore, the overall cross-sectional area of ​​strongly coupled multi-core optical fibers can be significantly smaller than that of weakly coupled multi-core optical fibers, resulting in better transmission efficiency and cabling performance than uncoupled multi-core optical fibers. However, its biggest drawback is that when the optical signals transmitted by each core are in the same direction and have the same wavelength, significant signal crosstalk will exist between these transmitted signals. To eliminate this crosstalk, complex digital signal processing algorithms must be introduced into the transmitted signals, increasing the complexity and power consumption of the transceiver. It also causes the optical signals between each core to lose their independence, requiring simultaneous modulation and demodulation of channels in several cores, greatly increasing system complexity. Summary of the Invention

[0012] This application provides a data center optical interconnect system based on weakly coupled multi-core optical fiber, which can reduce the overall cross-sectional area, improve the mechanical properties of optical fiber such as cabling and bendability, and realize bidirectional point-to-point data transmission with almost no crosstalk in the fiber core channel.

[0013] In a first aspect, a weakly coupled multi-core optical fiber is provided, comprising a cladding and multiple fiber cores located within the cladding, each fiber core having an isolation trench on its periphery; and the multi-core optical fiber satisfies the following conditions:

[0014] Condition 1: Apart from any two adjacent fiber cores with a spacing of D, there is no fiber core whose spacing with the two adjacent fiber cores is simultaneously less than or equal to D.

[0015] Condition 2: In any three adjacent fiber cores, the distance between the two outermost fiber cores is less than or equal to 2D and greater than or equal to λD, where 0≤λ<2.

[0016] In some embodiments, λ is the square root of 2.

[0017] In some embodiments, the spacing between two adjacent fiber cores is D = 2(r + d1) + d2;

[0018] Where r is the radius of the fiber core, d1 is the width of the isolation trench, and d2 is the distance between the isolation trenches of two adjacent fiber cores.

[0019] In some embodiments, the width d1 of the isolation trench ranges from r / 2 to r / 4, where r is the radius of the fiber core.

[0020] In some embodiments, the distance d2 between the isolation trenches of two adjacent fiber cores ranges from 0 to 2r+2d1, where r is the radius of the fiber core and d1 is the width of the isolation trench.

[0021] In some embodiments, the distance d2 between the isolation trenches of two adjacent fiber cores is 0.

[0022] In some embodiments, the number of fiber cores is an even number.

[0023] In some embodiments, there are six fiber cores arranged in a hexagonal pattern.

[0024] In some embodiments, there are 24 fiber cores, of which 6 fiber cores are located at the center of the cladding and are arranged in a hexagonal pattern to form a central hexagonal structure, and the remaining 18 fiber cores are arranged outside the central hexagonal structure to form 6 peripheral hexagonal structures. Any two adjacent hexagonal structures in the central hexagonal structure and the 6 peripheral hexagonal structures share two adjacent fiber cores.

[0025] Secondly, a data center optical interconnect system is provided, comprising:

[0026] As described above, a multi-core optical fiber based on weak coupling;

[0027] Two fan-in and fan-out devices are respectively connected to the two ends of the multi-core optical fiber;

[0028] Multiple transceiver ends, each including an optical signal receiver and an optical signal transmitter connected to two fan-in and fan-out devices respectively. The optical signal receiver and the optical signal transmitter of the transceiver end are connected to one core of the multi-core optical fiber through the two fan-in and fan-out devices to form an optical transmission channel.

[0029] Furthermore, the optical signal receiver and optical signal transmitter on the fan-in / fan-out device are cross-distributed so that the transmission directions of the optical signals in adjacent optical transmission channels are opposite.

[0030] The beneficial effects of the technical solution provided in this application include:

[0031] This application provides a data center optical interconnect system based on weakly coupled multi-core optical fiber. Condition one is used to ensure that, except for the non-negligible crosstalk between the closest inter-core cores, the crosstalk between the multi-core optical fiber and the other cores is negligible due to the large inter-core distances. In use, by making the optical signal transmission directions of adjacent cores opposite, the crosstalk between the closest inter-core cores can be ensured to be negligible, thus making all inter-core crosstalk negligible. This minimizes the significant signal crosstalk present in strongly coupled multi-core optical fibers. The problem is to achieve bidirectional point-to-point data transmission with almost no crosstalk in the fiber core channel. At the same time, condition two is used to limit the crosstalk between the fiber core and other fiber cores, so as to overcome the problems of large overall cross-sectional area, poor cable laying and bendability of uncoupled multi-core optical fibers as much as possible. This is because the width of the isolation trench between each fiber core can be greatly reduced, and the spacing between the isolation trenches of two adjacent fiber cores can even be 0. This greatly reduces the distance between fiber cores, thereby reducing the overall cross-sectional area and improving the cable laying and bendability of the optical fiber. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1A schematic diagram (6 cores) of a multi-core optical fiber based on weak coupling provided for an embodiment of this application;

[0034] Figure 2 A schematic diagram (24 cores) of a multi-core optical fiber based on weak coupling provided for an embodiment of this application;

[0035] Figure 3 This is a schematic diagram of a data center optical interconnect system provided in an embodiment of this application.

[0036] In the diagram: 1. Cladding; 2. Isolation trench; 3. Fiber core; 4. Fan-in / fan-out devices; 5. Optical signal receiver; 6. Optical signal transmitter. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0038] See Figure 1 As shown, this application embodiment provides a weakly coupled multi-core optical fiber, which includes a cladding 1 and multiple fiber cores 3 located within the cladding 1. Each fiber core 3 has an isolation trench 2 on its periphery; and the multi-core optical fiber satisfies the following conditions:

[0039] Condition 1: Apart from any two adjacent fiber cores 3 with a spacing of D, there is no fiber core 3 whose spacing with the two adjacent fiber cores 3 is simultaneously less than or equal to D.

[0040] For example, see Figure 1 As shown, there are 6 fiber cores 3, each numbered in the diagram, such as core 1, core 2, ..., core 6. If core 1 and core 2 are positioned as two adjacent fiber cores 3, then among cores 3, ..., core 6, there will not be a single fiber core 3 whose distance from core 1 and core 2 is simultaneously less than or equal to D. Either the distance from core 1 and core 2 is greater than D, or the distance from one of core 1 and core 2 is equal to D, and the distance from the other is greater than D. For example, the distance between core 3 and core 1 is equal to D, but the distance between core 3 and core 2 is greater than D, or the distance between core 4 and core 2 is equal to D, but the distance between core 1 and core 4 is greater than D. Similarly, cores 5 and 6 have distances greater than D from both core 1 and core 2.

[0041] Condition 2: In any three adjacent fiber cores 3, the distance between the two outermost fiber cores 3 is less than or equal to 2D and greater than or equal to λD, where 0≤λ<2.

[0042] For example, see Figure 1 As shown, cores 3, 1, and 2 are three adjacent fiber cores 3, with cores 3 and 2 being the two outermost fiber cores 3.

[0043] This application employs the aforementioned condition one to ensure that, apart from the non-negligible crosstalk between the closest inter-core fibers, the crosstalk between the multi-core fiber and the other fibers is negligible due to the large distance between the fibers. In use, by reversing the optical signal transmission directions in adjacent fibers, the crosstalk between the closest inter-core fibers can be ensured to be negligible, thus making the crosstalk between all fibers negligible. This minimizes the significant signal crosstalk problem present in strongly coupled multi-core fibers, achieving bidirectional point-to-point data transmission with almost no crosstalk in the fiber core channel. Simultaneously, the aforementioned condition two ensures that, while the crosstalk between the fiber and the other fibers is negligible, the application strives to overcome the problems of large overall cross-sectional area and poor mechanical properties such as cabling and bendability in uncoupled multi-core fibers. This is because the width of the isolation trench between each fiber core can be significantly reduced, even to the point where the spacing between the isolation trenches of two adjacent fibers is zero. This greatly reduces the distance between fibers, thereby reducing the overall cross-sectional area and improving the fiber's cabling and bendability.

[0044] The multi-core optical fiber provided in this application has an even number of fiber cores 3. For example, see [link to example]. Figure 1 As shown, there are 6 fiber cores 3, and each fiber core is numbered in the figure, such as core 1, core 2, ..., core 6, and arranged in a hexagonal pattern.

[0045] For example, see also: Figure 2 As shown, there are 24 fiber cores 3, each numbered in the figure, such as core 1, core 2, ..., core 24. Six fiber cores 3 are located at the center of cladding 1 and are arranged in a hexagonal pattern to form a central hexagonal structure. The remaining 18 fiber cores 3 are arranged outside the central hexagonal structure to form six peripheral hexagonal structures. Any two adjacent hexagonal structures in the central hexagonal structure and the six peripheral hexagonal structures share two adjacent fiber cores 3.

[0046] Furthermore, the value of λ is the square root of 2, that is, λ = 1.41.

[0047] See Figure 1 As shown, the distance between two adjacent fiber cores 3 is D = 2(r + d1) + d2; where r is the radius of the fiber core 3, r is greater than 0, d1 is the width of the isolation groove 2, and d2 is the distance between the isolation grooves 2 of the two adjacent fiber cores 3.

[0048] The width d1 of the isolation trench 2 ranges from r / 2 to r / 4, where r is the radius of the fiber core 3.

[0049] The distance d2 between the isolation grooves 2 of two adjacent fiber cores 3 ranges from 0 to 2r+2d1, where r is the radius of the fiber core 3 and d1 is the width of the isolation groove 2.

[0050] To minimize the overall cross-sectional area, the distance d2 between the isolation trenches 2 of two adjacent fiber cores 3 is set to 0, for example... Figure 2 As shown, the isolation trenches 2 of two adjacent fiber cores 3 are close together.

[0051] See 1 and Figure 3 As shown in the embodiments of this application, a data center optical interconnect system is also provided. The system includes two fan-in / fan-out devices 4, multiple transceivers, and a multi-core optical fiber based on weak coupling provided in any of the above embodiments. The two fan-in / fan-out devices 4 are respectively connected to the two ends of the multi-core optical fiber. The transceivers include an optical signal receiver 5 and an optical signal transmitter 6 respectively connected to the two fan-in / fan-out devices 4. The optical signal receiver 5 and the optical signal transmitter 6 of the transceivers are connected to one fiber core 3 of the multi-core optical fiber through the two fan-in / fan-out devices 4 to form an optical transmission channel. The optical signal receiver 5 and the optical signal transmitter 6 on the fan-in / fan-out devices 4 are cross-distributed so that the transmission directions of the optical signals in adjacent optical transmission channels are opposite.

[0052] The optical signal receiver 5 and the optical signal transmitter 6 of the transceiver are respectively connected to two fan-in and fan-out devices 4, and the optical signal receiver 5 and the optical signal transmitter 6 on the fan-in and fan-out devices 4 are cross-distributed, so that the transmission directions of the optical signals in the two adjacent optical transmission channels are opposite.

[0053] For example, see Figure 3 As shown, the optical fiber has 6 fiber cores 3, and correspondingly, there are 6 transceiver ends, arranged from top to bottom. The optical signal transmitting end 6 of the first transceiver end is on the fan-in fan-out device 4 on the left, and the optical signal receiving end 5 of the first transceiver end is on the fan-in fan-out device 4 on the right. Core 1, together with the optical signal receiving end 5 and the optical signal transmitting end 6 of the first transceiver end, form the first optical transmission channel. In this optical transmission channel, the optical signal is transmitted along direction 1.

[0054] The optical signal transmitting end 6 of the second transceiver is on the fan-in fan-out device 4 on the right, and the optical signal receiving end 5 of the second transceiver is on the fan-in fan-out device 4 on the left. Chip 2, together with the optical signal receiving end 5 and the optical signal transmitting end 6 of the second transceiver, form a second optical transmission channel. In this optical transmission channel, the optical signal is transmitted along direction 2.

[0055] The optical signal transmitter 6 of the third transceiver is on the fan-in fan-out device 4 on the left, and the optical signal receiver 5 of the third transceiver is on the fan-in fan-out device 4 on the right. Chip 3, together with the optical signal receiver 5 and the optical signal transmitter 6 of the third transceiver, form the third optical transmission channel. In this optical transmission channel, the optical signal is transmitted along direction 1.

[0056] The optical signal transmitter 6 of the fourth transceiver is on the fan-in fan-out device 4 on the right, and the optical signal receiver 5 of the fourth transceiver is on the fan-in fan-out device 4 on the left. Chip 4, together with the optical signal receiver 5 and the optical signal transmitter 6 of the fourth transceiver, form the fourth optical transmission channel. In this optical transmission channel, the optical signal is transmitted along direction 2.

[0057] Similarly, the optical signals in adjacent optical transmission channels travel in opposite directions, meaning the fiber cores transmitting optical signals in the same direction are designed to be non-adjacent. Since the direction of the optical signal transmitted in each fiber core is opposite to that of its nearest neighbor, crosstalk between them is minimal. Crosstalk with other fiber cores transmitting in the same direction is negligible due to the large distance between the fiber cores and the presence of isolation trenches. This makes crosstalk between all fiber cores in the system negligible, allowing each fiber core to be considered an independent channel, eliminating the need for complex digital signal processing algorithms and significantly reducing network complexity.

[0058] Example

[0059] See Figure 1 As shown, Example 1 is a 6-core multi-core optical fiber. In this fiber core geometry, the radius r of each core is 6 μm, the width d1 of the isolation trench around each core is 2 μm, and the maximum distance d2 between adjacent core isolation trenches is 5 μm. Therefore, the distance D between adjacent cores is 21 μm. The geometric distribution structure of the multi-core optical fiber cores satisfies the following two conditions:

[0060] (1) Apart from any two fiber cores with a spacing of 21um, there is no fiber core whose spacing with these two fiber cores is simultaneously less than or equal to 21um.

[0061] (2) Among any three adjacent fiber cores, the distance between the two outermost fiber cores that are further apart is less than or equal to 42um and greater than or equal to 29.6um, and such a distance can ensure that the crosstalk between the two fiber cores can be ignored.

[0062] See Figure 3As shown, six optical signals of the same wavelength (1550nm) are transmitted in a 10km long 6-core optical fiber. The system selects fiber cores according to the principle that the optical signals transmitted in the same direction are not adjacent to each other. Fiber cores 1, 3, and 5 transmit optical signals in one direction (direction 1), while fiber cores 2, 4, and 6 transmit optical signals in the other direction (direction 2). Since the direction of the optical signal transmitted in each fiber core is opposite to the direction of the optical signal transmitted in its nearest neighboring fiber core, the crosstalk between them is minimal. The crosstalk between the fiber cores and the other co-directional transmission fiber cores is negligible due to the large distance between the fiber cores and the presence of isolation trenches. This makes the crosstalk between all fiber cores in the system negligible, allowing each fiber core to be considered an independent channel without the need for complex digital signal processing algorithms, and greatly reducing the complexity of the network.

[0063] In the description of this application, it should be noted that the terms "upper," "lower," etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Unless otherwise expressly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication between two elements. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0064] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0065] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A multi-core optical fiber based on weak coupling, characterized in that, It includes a cladding (1) and a plurality of fiber cores (3) located within the cladding (1), each fiber core (3) having an isolation trench (2) on its periphery; and the multi-core optical fiber satisfies the following conditions: Condition 1: Apart from any two adjacent fiber cores (3) with a spacing of D, there is no fiber core (3) whose spacing with the two adjacent fiber cores (3) is simultaneously less than or equal to D; Condition 2: In any three adjacent fiber cores (3), the distance between the two outermost fiber cores (3) is less than or equal to 2D and greater than or equal to λD, where 0≤λ<2; There are 24 fiber cores (3), of which 6 fiber cores (3) are located at the center of the cladding (1) and are arranged in a hexagonal pattern to form a central hexagonal structure. The remaining 18 fiber cores (3) are arranged outside the central hexagonal structure to form 6 peripheral hexagonal structures. Any two adjacent hexagonal structures in the central hexagonal structure and the 6 peripheral hexagonal structures share two adjacent fiber cores (3).

2. The multi-core optical fiber based on weak coupling as described in claim 1, characterized in that: λ takes the value of the square root of 2.

3. The multi-core optical fiber based on weak coupling as described in claim 1, characterized in that: The distance between two adjacent fiber cores (3) is D = 2(r + d1) + d2; Where r is the radius of the fiber core (3), d1 is the width of the isolation groove (2), and d2 is the distance between the isolation grooves (2) of two adjacent fiber cores (3).

4. The multi-core optical fiber based on weak coupling as described in claim 1, characterized in that: The width d1 of the isolation trench (2) ranges from r / 2 to r / 4, where r is the radius of the fiber core (3).

5. The multi-core optical fiber based on weak coupling as described in claim 1, characterized in that: The distance d2 between the isolation grooves (2) of two adjacent fiber cores (3) ranges from 0 to 2r+2d1, where r is the radius of the fiber core (3) and d1 is the width of the isolation groove (2).

6. The multi-core optical fiber based on weak coupling as described in claim 5, characterized in that: The distance d2 between the isolation trenches (2) of two adjacent fiber cores (3) is 0.

7. A data center optical interconnect system, characterized in that, It includes: The weakly coupled multi-core optical fiber as described in any one of claims 1 to 6; Two fan-in and fan-out devices (4) are respectively connected to the two ends of the multi-core optical fiber; Multiple transceivers, each including an optical signal receiver (5) and an optical signal transmitter (6) connected to two fan-in and fan-out devices (4) respectively, the optical signal receiver (5) and the optical signal transmitter (6) of the transceiver are connected to one of the fiber cores (3) of the multi-core optical fiber through the two fan-in and fan-out devices (4) to form an optical transmission channel; Furthermore, the optical signal receiving end (5) and optical signal transmitting end (6) on the fan-in fan-out device (4) are cross-distributed so that the transmission directions of optical signals in adjacent optical transmission channels are opposite.

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