Mode exchanger and optical transmission system

Abstract

A mode exchanger (1) comprises: a cladding (11); and two or more waveguides (21, 22, 23) that each have a refractive index greater than the refractive index of the cladding and are disposed in the cladding. The waveguides each guide light introduced from one end of the mode exchanger to the other end of the mode exchanger, and when a portion located between the one end and the other end of the mode exchanger is defined as an intermediate portion (RC), the distance between the waveguides in a cross section perpendicular to a longitudinal direction from the one end to the other end increases from the one end toward the intermediate portion, and decreases from the intermediate portion toward the other end. In a section (R1) from the one end to the intermediate portion, at least some waveguides have a different waveguide width and / or refractive index compared to the other waveguides, and in a section (R2) from the intermediate portion to the other end, at least some waveguides have a different waveguide width and / or refractive index compared to the other waveguides.

Description

Mode changer and optical transmission system

[0001] This disclosure relates to a mode switcher and an optical transmission system.

[0002] In recent years, with the advancement of optical communication technology, there has been a growing demand for faster and higher-capacity information transmission. Mode-division multiplexing (MODD), which transmits signals by multiplexing them across multiple modes within an optical fiber, is being considered as one of the promising methods for increasing communication capacity.

[0003] In mode division multiplexing transmission, transmission characteristics can deteriorate due to the difference in mode loss (MDL) caused by differences in propagation characteristics for each mode. Patent documents 1 and 2 disclose a technique for equalizing characteristic differences by dividing the transmission path into sections and converting each section to a different mode.

[0004] Japanese Patent Publication No. 2018-189889 Japanese Patent Publication No. 2019-154004

[0005] According to the technology described in Patent Documents 1 and 2, since two multiplexers / demultiplexers are combined to exchange modes, the same number of connection points as the number of modes are required between the multiplexers / demultiplexers. This results in a large device size and also presents problems with potential losses at the connection points.

[0006] This disclosure has been made in view of the above-mentioned problems. Its purpose is to provide a mode switcher and an optical transmission system that can suppress an increase in the overall space of the device and suppress an increase in the loss of optical signals passing through it.

[0007] To solve the problems described above, a mode switcher according to one aspect of the present disclosure comprises a cladding and two or more waveguides disposed within the cladding, each having a refractive index greater than that of the cladding. The waveguides guide light introduced from one end of the mode switcher to the other end of the mode switcher, with the portion of the mode switcher located between the two ends being the intermediate section. The distance between the waveguides in a cross section perpendicular to the longitudinal direction from one end to the other end increases as the light progresses from the one end to the intermediate section and decreases as the light progresses from the intermediate section to the other end. At least one of the waveguide width and refractive index differs for at least some waveguides compared to other waveguides in the section from one end to the intermediate section, and at least some waveguides differ compared to other waveguides in the section from the intermediate section to the other end.

[0008] An optical transmission system according to one aspect of the present disclosure comprises a mode switch and a transmission path connected to the mode switch and guiding a plurality of optical propagation modes guided by a waveguide.

[0009] According to this disclosure, it is possible to suppress an increase in the overall space of the device and to suppress an increase in the loss of passing optical signals.

[0010] This is a plan view of a mode converter according to an embodiment of the present disclosure. This is a cross-sectional view of the end of the mode converter. This is a cross-sectional view of the middle portion of the mode converter. This is a plan view of a mode converter according to a first modified example of an embodiment of the present disclosure. This is a table showing a list of parameters used in the calculations. This is a diagram showing the relationship between the distance between waveguides and the conversion efficiency. This is a cross-sectional view of the end showing a second modified example regarding the arrangement of waveguides. This is a cross-sectional view of the end showing a third modified example regarding the arrangement of waveguides.

[0011] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, identical components are denoted by the same reference numerals, and redundant explanations will be omitted.

[0012] [Mode Converter Configuration] Figure 1 is a plan view of a mode converter according to an embodiment of the present disclosure.

[0013] The mode exchanger 1 comprises a cladding 11 and two or more waveguides 21, 22, 23. The refractive index of the waveguides 21, 22, 23 is higher than the refractive index of the cladding 11. The waveguides 21, 22, 23 guide light introduced from one end of the mode exchanger 1 to the other end of the mode exchanger 1. The number of waveguides included in the mode exchanger 1 is not limited to three. The number of waveguides included in the mode exchanger 1 may be two or more.

[0014] Note that a few-mode optical fiber FB1 is connected to one end of the mode exchanger 1. A few-mode optical fiber FB2 is connected to the other end of the mode exchanger 1. The few-mode optical fiber FB1 and the few-mode optical fiber FB2 constitute a transmission path that guides a plurality of light propagation modes guided by the waveguides provided in the mode exchanger 1. An optical transmission system may be configured by the mode exchanger 1 and the few-mode optical fibers FB1, FB2.

[0015] The few-mode optical fiber FB1 and the few-mode optical fiber FB2 each include a core MC. Light including a plurality of propagation modes propagates through the core MC.

[0016] Light including a plurality of propagation modes guided by the few-mode optical fiber FB1 is introduced into one end of the mode exchanger 1 and guided by the plurality of waveguides 21, 22, 23 of the mode exchanger 1. Light including a plurality of propagation modes guided by the plurality of waveguides 21, 22, 23 of the mode exchanger 1 is output from the other end of the mode exchanger 1 and guided by the few-mode optical fiber FB2.

[0017] In other words, the plurality of waveguides 21, 22, 23 provided in the mode exchanger 1 are optically coupled to one input transmission path (the core MC of the few-mode optical fiber FB1) at one end of the mode exchanger 1. The plurality of waveguides 21, 22, 23 provided in the mode exchanger 1 are optically coupled to one output transmission path (the core MC of the few-mode optical fiber FB2) at the other end of the mode exchanger 1.

[0018] Waveguide 21 consists of an input waveguide 21A and an output waveguide 21B, which are optically coupled to each other in the intermediate section RC. Waveguide 22 consists of an input waveguide 22A and an output waveguide 22B, which are optically coupled to each other in the intermediate section RC. Waveguide 23 consists of an input waveguide 23A and an output waveguide 23B, which are optically coupled to each other in the intermediate section RC. The intermediate section RC is located between one end and the other end of the mode changer 1. The input waveguides 21A, 22A, and 23A are arranged in section R1 from one end of the mode changer 1 to the intermediate section RC. The output waveguides 21B, 22B, and 23B are arranged in section R2 from the intermediate section RC to the other end of the mode changer 1.

[0019] Each input waveguide 21A, 22A, and 23A couples with one of the multiple propagation modes guided by the multimode optical fiber FB1. The coupled propagation mode is then guided as the fundamental mode of each input waveguide 21A, 22A, and 23A. Each input waveguide 21A, 22A, and 23A may have a structure (for example, width and refractive index) corresponding to different propagation modes. Immediately after being incident from one end of the mode switcher 1, the propagation mode couples with the multiple input waveguides, but as the distance between the input waveguides increases from one end of the mode switcher 1 toward the intermediate section RC, each propagation mode couples with the fundamental mode (LP) of one of the input waveguides. ０１ It will now be able to connect to (mode).

[0020] Each output waveguide 21B, 22B, and 23B couples with one of the multiple propagation modes guided by the multimode optical fiber FB2. The coupled propagation mode is then guided as the fundamental mode of each output waveguide 21B, 22B, and 23B. Each output waveguide 21B, 22B, and 23B may have a structure (for example, width and refractive index) corresponding to different propagation modes. Immediately after passing through the intermediate section RC, each propagation mode becomes the fundamental mode (LP) of one of the output waveguides. ０１Although the modes are coupled, as the distance between input waveguides decreases from the intermediate section RC toward the other end of the mode changer 1, the propagating modes become coupled to multiple output waveguides.

[0021] In a cross-section perpendicular to the longitudinal direction from one end to the other of the mode switcher 1, the distance between input waveguides 21A, 22A, and 23A increases as one moves from one end of the mode switcher 1 to the intermediate section RC. In other words, in section R1, the distance between input waveguides 21A, 22A, and 23A increases as one moves from one end of the mode switcher 1 to the intermediate section RC. As shown in Figure 1, the distance between input waveguides 21A, 22A, and 23A in the intermediate section RC is greater than the distance at one end of the mode switcher 1.

[0022] In a cross-section perpendicular to the longitudinal direction from one end to the other of the mode switcher 1, the distance between the output waveguides 21B, 22B, and 23B decreases as you proceed from the intermediate section RC to the other end of the mode switcher 1. In other words, in section R2, the distance between the output waveguides 21B, 22B, and 23B decreases as you proceed from the intermediate section RC to the other end of the mode switcher 1. As shown in Figure 1, the distance between the output waveguides 21B, 22B, and 23B at the other end of the mode switcher 1 is smaller than the distance at the intermediate section RC.

[0023] With respect to the waveguides 21, 22, and 23 provided by the mode switcher 1, at least one of the waveguide width and refractive index differs from the other waveguides in the section from one end to the middle section for at least some waveguides, and differs from the other waveguides in the section from the middle section to the other end for at least some waveguides.

[0024] For example, among the input waveguides 21A, 22A, and 23A, at least one of the width and refractive index of an input waveguide differs from that of the other input waveguides for at least some of the input waveguides. Therefore, the propagation constants of at least some of the input waveguides 21A, 22A, and 23A differ from the propagation constants of the other input waveguides.

[0025] Similarly, among the output waveguides 21B, 22B, and 23B, at least one of the width and refractive index of the output waveguides differs from that of the other output waveguides for at least some of the output waveguides. Therefore, the propagation constants of at least some of the output waveguides 21B, 22B, and 23B differ from the propagation constants of the other output waveguides.

[0026] Figure 2A is a cross-sectional view of the end of the mode converter. At one or the other end of the mode converter 1, waveguides 21, 22, and 23 are arranged in close proximity to the center of the core MC of the multimode optical fibers FB1 and FB2. By aligning the waveguides 21, 22, and 23 with the position of the core MC of the multimode optical fibers FB1 and FB2 connected to the mode converter 1, the waveguides 21, 22, and 23 efficiently optically couple to the core MC.

[0027] The arrangement of waveguides at one end and the other end of the mode switcher 1 is important for achieving efficient mode coupling with the multimode optical fibers FB1 and FB2. For example, the density and spatial distribution of the waveguide arrangement are designed to match the mode field distribution of the optical fibers FB1 and FB2.

[0028] When multimode optical fibers FB1 and FB2 perform 3-mode transmission, the propagation mode is LP ０１ Mode, LP １１ａ Mode, LP １１ｂ Waveguides are positioned considering the field distribution of the modes. This ensures that each propagation mode propagating through the core MC is efficiently coupled to its corresponding waveguide.

[0029] Figure 2B is a cross-sectional view of the intermediate section of the mode converter. In the intermediate section RC, waveguides 21, 22, and 23 are arranged further apart from each other than they are at one or the other end of the mode converter 1. By arranging waveguides 21, 22, and 23 further apart from each other, the propagating modes propagating through each waveguide are made independent of each other, and unintended mode coupling is suppressed. As shown in Figure 2B, the waveguides are arranged far enough apart that the coupling coefficient between them becomes sufficiently small (typically far enough that the coupling coefficient becomes 0.15 or less).

[0030] As an example, a case where the fundamental mode of the input-side waveguide 21A and the fundamental mode of the output-side waveguide 21B are different will be described. At one end of the mode converter 1, a low-order propagation mode (e.g., LP ０１ mode) may be designed to have a propagation constant close to that of the fundamental mode of the input-side waveguide 21A. Accordingly, the LP that has entered from the few-mode optical fiber FB1 ０１ mode is coupled to the input-side waveguide 21A. Then, in the intermediate portion RC, the input-side waveguide 21A and the output-side waveguide 21B are optically coupled to each other.

[0031] The output-side waveguide 21B has different structural characteristics from the input-side waveguide 21A at the other end of the mode converter 1. For example, the output-side waveguide 21B has a refractive index different from that of the input-side waveguide 21A. Accordingly, a higher-order propagation mode different from the propagation mode propagating through the input-side waveguide 21A (e.g., LP １１ mode) may be designed to have a propagation constant close to that of the fundamental mode of the output-side waveguide 21B. As a result, the propagation mode is converted while light propagates through the waveguide 21. For example, the LP that has propagated through the few-mode optical fiber FB1 ０１ mode is converted to an LP １１ mode when being coupled to the few-mode optical fiber FB2.

[0032] Here, while light propagates through the waveguide 21, an LP ０１ mode is converted to an LP １１ mode is taken as an example, but conversion of other propagation modes may also be applied. For example, a high-order propagation mode may be converted into a low-order propagation mode. Alternatively, instead of performing propagation mode conversion through a change in refractive index, propagation mode conversion may be performed through a change in the shape of the waveguide.

[0033] As described above, the conversion of propagation modes propagating through the waveguides 21, 22, and 23 is realized by a combination of a change in the structure of the waveguides (a shape such as refractive index or width) and optical coupling between the waveguides.

[0034] Figure 3 is a plan view of a mode converter according to a modified embodiment of the present disclosure. According to the modified embodiment shown in Figure 3, a transition section TR is provided in the intermediate section RC. In the transition section TR, at least one of the waveguide width and refractive index is continuously or stepped.

[0035] Similar to the mode switcher 1 shown in Figure 1, in the mode switcher 1 shown in Figure 3, in section R1, the distance between waveguides increases as you proceed from one end of the mode switcher 1 to the intermediate section RC, and in section R2, the distance between waveguides decreases as you proceed from the intermediate section RC to the other end of the mode switcher 1.

[0036] The transition section TR gradually changes the characteristics of the waveguide from one state to another. This characteristic change is achieved by changing the cross-sectional size (width and height), refractive index, or both of the waveguide. For example, gradually changing the width of the waveguide causes the effective refractive index and electric field distribution of the waveguide modes to change continuously. This allows for a smooth conversion from the propagation mode input from one end of the mode converter 1 to the propagation mode output from the other end of the mode converter 1, significantly reducing losses associated with mode conversion.

[0037] The change in refractive index in the transition section TR can be achieved with various profiles, such as linear change, quadratic change, and stepwise change. For example, in the case of a linear change, the refractive index n(z) changes with distance z as follows: "n(z) = n1 + (n2 - n1) * z / L". Here, n1 is the refractive index at the incident end of the transition section TR, n2 is the refractive index at the exit end, and L is the length of the transition section TR. Similarly, the cross-sectional size (width and height) of the waveguide in the transition section TR may also be changed.

[0038] The transition region TR may be manufactured by femtosecond laser processing. Femtosecond laser processing allows for localized refractive index changes by precisely controlling parameters such as the laser pulse energy, scanning speed, and focal position. For example, by gradually adjusting the laser scanning speed, the refractive index can be changed stepwise along the longitudinal direction.

[0039] By providing a transition section TR, it is possible to improve mode conversion efficiency, reduce insertion loss, and improve mode purity. In particular, providing a transition section TR is effective when there are large structural differences between waveguide structures and losses occur in the intermediate section RC due to mode mismatch.

[0040] [Example of calculation results] The results of calculating the effects realized by the mode converter shown in Figure 1 will be explained using Figures 4A and 4B.

[0041] Figure 4A is a table showing a list of parameters used in the calculation. Figure 4A shows the design parameters when used in a three-mode transmission line, where the radii of the input waveguides 21A, 22A, and 23A are set to r ｉｎ１ ,r ｉｎ２ ,r ｉｎ３ This is shown by [the following]. In addition, the refractive indices of the input waveguides 21A, 22A, and 23A are given by n, respectively. ｉｎ１ ,n ｉｎ２ ,n ｉｎ３ This is shown by the radii of the output waveguides 21B, 22B, and 23B, respectively, r ｏｕｔ１ ,r ｏｕｔ２ ,r ｏｕｔ３ This is shown by the refractive indices of the output waveguides 21B, 22B, and 23B, respectively, n ｏｕｔ１ ,n ｏｕｔ２ ,n ｏｕｔ３ This is shown by [the source].

[0042] Furthermore, distance s1 is the distance (in μm) between the center of the mode switcher 1 (fiber center) and the waveguide center at one end and the other end of the mode switcher 1. Distance s2 is the distance between the center of the mode switcher 1 (fiber center) and the waveguide center at the intermediate section RC. In particular, distance s2 represents the maximum distance between waveguide centers. The refractive index of the cladding is n ｃｌａｄ This is shown by L ＰＬ This indicates the total length from one end to the other of the mode switcher 1.

[0043] These parameter settings are based on the following conditions: cladding refractive index of 1.444, multimode optical fibers FB1 and FB2 refractive index of 1.44981, and core radius of 7 μm. ０１ Mode, LP １１ａMode, LP １１ｂ This is optimized for the efficient propagation of the three propagation modes of the mode. LP ０１ The mode is LP １１ａ Mode and LP １１ｂ It has a higher effective refractive index than the standard mode, and is intended for use at a wavelength of 1550 nm (C-band).

[0044] Figure 4B shows the relationship between the distance between waveguides and the conversion efficiency. The horizontal axis represents the distance between waveguides. The vertical axis on the left represents the conversion efficiency (solid line), and the vertical axis on the right represents the coupling coefficient (dashed line).

[0045] As shown in Figure 4B, the coupling coefficient decreases exponentially as the distance between waveguides increases. Specifically, the conversion efficiency increases with increasing waveguide distance, reaching a maximum value in a specific distance range. For example, it can be seen that a conversion efficiency of 90% or more is obtained in the region where the waveguide distance is 7 μm or more. In this case, the coupling coefficient is 0.15, so it can be seen that selective mode exchange is possible by widening the core spacing until the coupling coefficient becomes 0.15 or less.

[0046] Furthermore, it was found that when the waveguide distance is approximately 9 μm or more, the coupling coefficient becomes sufficiently small (0.1 or less), and at the same time, the conversion efficiency stabilizes at a high value of 90% or more. This indicates that by appropriately setting the waveguide distance, it is possible to achieve highly efficient mode conversion while suppressing crosstalk between modes.

[0047] The results in Figure 4B provide important guidelines for determining the rate of change of the waveguide distance in sections R1 and R2, and for determining the optimal waveguide distance in the intermediate RC section. This design demonstrates that by setting the waveguide distance in the intermediate RC section to approximately 9 to 11 μm, it is possible to simultaneously achieve a high conversion efficiency of 0.95 or higher and a sufficiently low coupling coefficient (0.1 or less).

[0048] These calculation results provide concrete design guidelines for the practical application of mode converters and theoretically support the idea that highly efficient mode conversion can be achieved by optimizing the waveguide structure. Furthermore, since a high-efficiency mode conversion of over 90% can be achieved, it can be confirmed that the mode converter of the present invention has superior performance compared to conventional methods using two multiplexers / demultiplexers.

[0049] [First Modification] Figure 5 is an end cross-sectional view showing a second modification regarding the arrangement of waveguides. In this modification, the mode switcher 1 shows a configuration in which six waveguides 41 to 46 are arranged within the cladding 11. In Figure 5, the six waveguides 41 to 46 are arranged in a ring shape, but the arrangement of the multiple waveguides is not limited to the example of a ring shape.

[0050] By arranging multiple waveguides, it is possible to handle a greater number of propagation modes. In particular, it is highly compatible with the circularly symmetric structure of optical fibers, and LP ０１ LP １１ａ LP １１ｂ LP ２１ａ LP ２１ｂ LP ０２ High-efficiency coupling with groups of propagation modes like these can be expected.

[0051] For example, the propagation modes guided by waveguides 41-46 at one end of mode switcher 1 are, respectively, LP ０１ LP １１ LP ２１ LP ２１ LP ０２ LP １１ As such, the propagation modes guided by waveguides 41 to 46 at the other end of mode switcher 1 are, respectively, LP ０２ LP ２１ LP １１ LP ０１ LP １１ LP ２１ The propagation mode may be converted in such a way that the propagation mode with large characteristic differences (LP ０１ Mode and LP ０２ This enables exchange between modes, and the maximum difference in characteristics between propagation modes can be reduced by a single mode changer.

[0052] The methods of converting the propagation mode are not limited to the examples given herein. For example, the propagation mode guided by waveguides 41-46 may be LP ０１ LP １１ LP ２１ LP ０２ LP ０１ The modes may be converted to cycle through them in order.

[0053] As the number of waveguides increases, the number of modes that can be processed also increases. In the six-waveguide configuration shown in Figure 5, the aforementioned six types of modes can be processed simultaneously, making it possible to significantly improve the overall transmission capacity of the system. The main advantage of this arrangement is that the distance between waveguides can be kept more uniform, enabling the processing of multiple modes while minimizing inter-mode crosstalk.

[0054] Compared to the embodiments shown in Figures 1-4, this modified configuration increases the number of propagation modes that can be processed and allows for more efficient processing of higher-order modes. In particular, it is a configuration suitable for multimode optical communication systems and for increasing the capacity of mode division multiplexing transmission.

[0055] [Second Modification] Figure 6 is an end cross-sectional view showing a third modification regarding the arrangement of waveguides. In this modification, the mode switcher 1 is connected to a 3-mode 4-core fiber used as a transmission line. The arrangement shown in Figure 6 represents a group of waveguides corresponding to one core of a multi-core fiber. When corresponding to an actual 3-mode 4-core fiber, the arrangement of three waveguides shown in Figure 6 will be arranged in four locations.

[0056] In optical transmission systems using multicore fibers, the use of mode switchers is as follows. For example, if the transmission path is an N-mode × M-core optical fiber (each core supporting N-mode), a device with M pairs of mode switchers is required. The configuration shown in Figure 6 is an example of replacing channels in a 3-mode 4-core fiber, and the structure at one end and the other end of the mode switcher 1 requires three waveguides to be placed in close proximity to each core of the multicore fiber. In Figure 6, waveguides 51, 52, and 53 are placed in close proximity to the positions of each core MC of the multicore fiber.

[0057] For example, the three modes within each core MC (e.g., LP ０１ LP １１ａ LP １１ｂ The mode is coupled to the three corresponding waveguides (corresponding to waveguides 51, 52, and 53 in Figure 6).

[0058] Furthermore, in the intermediate section RC of the mode switcher 1, by connecting a waveguide located close to one core MC with a waveguide located close to another core MC, mode switching becomes possible not only between the modes of each core MC but also between different core MCs. For example, the LP of the first core MC ０１ Mode 2 Core MC LP １１ It is also possible to change both the core MC and the mode simultaneously, such as by switching to a different mode.

[0059] In such a multi-core, multi-mode configuration, the spatial division multiplexing is greatly improved, increasing the overall transmission capacity of the system. For example, in a 3-mode, 4-core configuration, a total of 12 spatial channels can be used, theoretically enabling up to 12 times more information transmission compared to a single-mode, single-core fiber.

[0060] [Other] Notes are provided regarding methods for realizing the structure of a mode converter. Three examples of methods for realizing the structure of a mode converter are given below.

[0061] (a) Planar light wave circuits (PLCs and silicon photonics): Planar light wave circuits use a process in which patterns of optical waveguides and optical elements are formed using photolithography or etching techniques, and then completed by metal deposition or connection. In this case, the effective refractive index between waveguides is controlled by changing the waveguide width.

[0062] (b) Bundled fibers: Photonic lanthanum can be produced by melt-drawing multiple fibers in a capillary (glass tube). The effective refractive index can be controlled by bundling fibers with different structures.

[0063] (c) Femtosecond laser processing: By irradiating quartz glass with femtosecond pulses, it is possible to mount it inside a coreless fiber.

[0064] [Effects of the Embodiment] As described in detail above, the mode changer according to the present disclosure comprises a cladding and two or more waveguides disposed within the cladding, each having a refractive index greater than that of the cladding. The waveguides guide light introduced from one end of the mode changer to the other end of the mode changer, with the portion of the mode changer located between the one end and the other end being the intermediate section. The distance between the waveguides in a cross section perpendicular to the longitudinal direction from one end to the other end increases as one moves from the one end to the intermediate section and decreases as one moves from the intermediate section to the other end. At least one of the waveguide width and refractive index differs for at least some waveguides compared to other waveguides in the section from one end to the intermediate section, and at least some waveguides differ compared to other waveguides in the section from the intermediate section to the other end.

[0065] This helps to suppress an increase in the overall space required for the device and to prevent an increase in the loss of optical signals passing through it. Specifically, by consolidating the mode-switching function into a single device, miniaturization becomes possible, and the choice of installation location becomes more flexible. When mode switching is achieved by combining two multiplexers / demultiplexers, the same number of connection points as the number of modes are required between the multiplexers / demultiplexers, and connection loss occurs each time. On the other hand, according to this disclosure, the insertion loss of the entire system can be reduced by eliminating these connection points.

[0066] Furthermore, by controlling the distance between waveguides within a single device, mode coupling can be precisely managed, enabling highly efficient mode conversion. The waveguide structure and distance settings of this disclosure allow for highly efficient mode conversion of over 90%. This equalizes the loss difference between each mode and improves the overall transmission characteristics of the system. In addition, the mode switcher of this disclosure contributes to simplifying the manufacturing process. While methods combining multiplexers and demultiplexers require high-precision alignment of multiple components, resulting in a complex manufacturing process, this disclosure integrates the mode conversion function within a single device, reducing the alignment process and leading to lower manufacturing costs and improved yield.

[0067] In the mode converter according to this disclosure, the intermediate section may include a transition section in which at least one of the waveguide width and refractive index changes continuously or stepwise. This improves performance in terms of both mode conversion efficiency and insertion loss. When there is a large structural difference between waveguide structures, abrupt changes result in losses due to mode mismatch, but these losses can be significantly reduced by gradually changing the structure in the transition section. Furthermore, mode conversion efficiency can be improved by selecting an optimal refractive index change profile (linear change, quadratic change, stepwise change, etc.).

[0068] In the mode switcher according to this disclosure, the coupling coefficient between waveguides at the position where the distance between waveguides is maximum along the longitudinal direction may be 0.15 or less. This enables a significant improvement in mode purity and a reduction in unwanted crosstalk. When the coupling coefficient is reduced to 0.15 or less, unintended energy transfer between waveguides is suppressed, and each waveguide functions as an independent mode. As is clear from the results shown in Figure 4B, when the coupling coefficient is 0.15 or less (around 0.1), the conversion efficiency reaches a high value of 90% or more, and when it is 0.05 or less, an even better conversion efficiency of 95% or more can be obtained. In terms of the impact on actual system performance, it becomes possible to suppress intermode crosstalk to -20 dB or less (less than 1%), and the bit error rate (BER) during long-distance transmission can be reduced to 10 －１２ The signal-to-noise ratio (SNR) margin required to maintain the following can be reduced by approximately 1 dB. Furthermore, by appropriately controlling the coupling coefficient between waveguides, intentional mode conversion and accidental mode coupling can be clearly distinguished, ensuring stable transmission characteristics even in complex mode-multiplexing transmission systems.

[0069] In the mode switcher according to this disclosure, multiple waveguides may be optically coupled to one input transmission path at one end and optically coupled to one output transmission path at the other end. This significantly improves the overall system integration and efficiency. In this configuration, since the core of a multimode optical fiber capable of transmitting multiple propagation modes functions as the input / output transmission path, the number of connection points in the input / output section can be minimized, simultaneously achieving reduced connection loss and simplified installation. Furthermore, since mode conversion functionality can be added while maintaining high compatibility with existing multimode optical fiber systems, the barrier to introduction into existing optical communication infrastructure is low, and gradual network expansion is possible. In addition, by optimizing the arrangement density and spatial distribution of waveguides in the input / output section, high consistency with the field distribution of each mode can be achieved, and input / output coupling efficiency can be improved. In particular, in a mode division multiplexing transmission system using multimode optical fiber, the complexity of the overall system can be suppressed while maximizing the information density per transmission path, making it possible to achieve both high-speed, high-capacity communication and economic efficiency.

[0070] The optical transmission system according to this disclosure comprises a mode switch and a transmission path connected to the mode switch that guides multiple optical propagation modes guided by waveguides. This makes it possible to achieve high-speed, high-capacity optical communication while effectively suppressing the degradation of transmission characteristics caused by differences in characteristics between modes. By appropriately arranging the mode switch in the transmission path, the transmission path is divided into sections, and each section is converted to a different mode, so that a specific signal circulates through various modes throughout the entire transmission path. For example, in three-mode transmission, if the transmission path is divided into three sections, each signal propagates through three different modes for equal distances, and characteristic differences such as mode-dependent loss (MDL), mode dispersion (DMD), and mode-dependent gain (MDG) are averaged. This makes it possible to extend the transmission distance that was determined by the worst-case mode and to reduce the number of optical amplifiers. In a measured example, the maximum inter-mode loss difference was reduced from 10 dB to 2 dB or less in a 100 km transmission path, and the receiving sensitivity was improved by approximately 3 dB. Furthermore, by combining it with spatial division multiplexing technologies such as multicore fibers, it becomes possible to increase the overall transmission capacity of the system by tens of times compared to conventional methods, making a significant contribution to increasing the capacity of communication infrastructure.

[0071] While the contents of this disclosure have been described above in accordance with the embodiments, it will be obvious to those skilled in the art that this disclosure is not limited to these descriptions and that various modifications and improvements are possible. The discussions and drawings that constitute part of this disclosure should not be understood as limiting this disclosure. Various alternative embodiments, examples and operational techniques will become apparent to those skilled in the art from this disclosure.

[0072] This disclosure naturally includes various embodiments and other features not described herein. Therefore, the technical scope of this disclosure is determined solely by the inventive features relating to the claims that are reasonable given the above description.

[0073] 1 Mode Switcher 11 Cladding 21-23, 41-43, 51-53 Waveguides 21A-23A Input Waveguides 21B-23B Output Waveguides FB1, FB2 Multimode Optical Fiber MC Core R1, R2 Section RC Intermediate Section TR Transition Section

Claims

1. A mode changer comprising: a cladding; and two or more waveguides disposed within the cladding, each having a refractive index greater than that of the cladding, wherein the waveguides guide light introduced from one end of the mode changer to the other end of the mode changer, with a portion of the mode changer located between the one end and the other end designated as an intermediate section, the distance between the waveguides in a cross section perpendicular to the longitudinal direction from the one end to the other end increases as one proceeds from the one end to the intermediate section, and decreases as one proceeds from the intermediate section to the other end, and at least one of the width and refractive index of the waveguides differs from the other waveguides with respect to at least a portion of the waveguides in the section from the one end to the intermediate section, and differs from the other waveguides with respect to at least a portion of the waveguides in the section from the intermediate section to the other end.

2. The mode switcher according to claim 1, wherein the intermediate portion includes a transition portion in which at least one of the width and refractive index of the waveguide changes continuously or in a stepwise manner.

3. The mode switch according to claim 1, wherein the coupling coefficient between the waveguides at the position where the distance between the waveguides is maximum along the longitudinal direction is 0.15 or less.

4. The mode switch according to claim 1, wherein the plurality of waveguides are optically coupled to one input transmission line at one end and optically coupled to one output transmission line at the other end.

5. An optical transmission system comprising: a mode changer according to any one of claims 1 to 4; and a transmission path connected to the mode changer and guiding a plurality of light propagation modes guided by the waveguide.

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