Mode rotator and mode multiplexer / demultiplexer

Abstract

To reduce processing accuracy of a depth of a trench formed by etching.SOLUTION: A mode rotator 10 comprises: an L-shaped part 12 which is formed on a quartz substrate as a planar Lightwave circuit (PLC) and provided in a waveguide 101 that propagates four modes E11, E12, E21, and E22, and the cross section of which is formed in a shape of the letter L, with a trench 12b formed on the top face 12a thereof; and a convex part 11 which adjoins the L-shaped part 12 and the cross section of which is formed in a shape of a convex, with a trench 11b formed on the top face 11a thereof. Each of the L-shaped part 12 and the convex part 11 is capable of switching thermal mode conversion in the direction of propagation.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a mode rotator and a mode multi / demultiplexer. [Background technology]

[0002] Mode multiplexing transmission technology is known that increases communication capacity by utilizing multiple modes propagating through an optical fiber. A mode multiplexer / demultiplexer (MUX / DEMUX) is indispensable for exciting the modes propagating through the fiber. So far, MUX / DEMUX using a spatial light modulator (SLM) (see Non-Patent Document 1) and the like have been reported. In addition, a planar lightwave circuit (PLC) type mode multiplexer / demultiplexer using a silica-based material (see Non-Patent Document 2) has the characteristics of low loss, small size, and easy manufacturing. The first higher-order mode of the waveguide used in PLC is the LP11 mode, and there are two modes, LP11a mode and LP11b mode, with degenerate propagation constants. Due to the symmetry of the electric field distribution, it is difficult to directly excite the LP11b mode from the LP01 mode using an asymmetric directional coupler, so a PLC type mode rotator using an L-shaped waveguide for exciting the LP11b mode has been proposed (see Cited Document 3). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] R. Ryf et al., “Mode-division multiplexing over 96 km of few-mode fiber using coherent 6×6 MIMO processing,” J. Lightw. Technol. 30(4), 521-531 (2012) [Non-Patent Document 2] N. Hanzawa et al., “PLC-based four-mode multi / demultiplexer with LP 11 mode rotator on one chip,” J. Lightw. Technol. 33(6), 1161-1165 (2015) [Non-Patent Document 3] T. Fujisawa et al., “Scrambling-type three-mode PLC multiplexer based on cascaded Y-branch waveguide with integrated mode rotator,” J. Lightw. Technol. 36(10), 1985-1992 (2018) Summary of the Invention [Problem to be solved by the invention]

[0004] The L-shaped mode rotator in Non-Patent Document 3 required a two-step etching process to form a trench and make it L-shaped in order to maintain its performance as an optical device. However, it was difficult to etch the trench to the designed depth by the two-step etching process because strict processing accuracy was required.

[0005] The present disclosure has been proposed in consideration of the above-mentioned circumstances, and aims to provide a mode rotator in which the processing accuracy requirement for the depth of the trench formed by etching is relaxed, and a mode multiplexer / demultiplexer using such a mode rotator. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, the mode rotator of the present disclosure is a mode rotator formed on a substrate and provided in a waveguide capable of propagating n modes, where n is an integer greater than or equal to 3, and includes an L-shaped portion having a trench formed on its top surface so that its cross section has an L-shape, and a convex portion adjacent to the L-shaped portion and having a trench formed on its top surface so that its cross section has a convex shape, and the L-shaped portion and the convex portion are each capable of adiabatic mode conversion in the propagation direction. Effect of the Invention

[0007] According to the present disclosure, the processing precision required for the depth of the trench formed by etching is relaxed. [Brief description of the drawings]

[0008] [Figure 1] 1 is a diagram showing a configuration of a mode rotator according to an embodiment of the present invention; [Diagram 2] 11 is a diagram showing a transition of the effective refractive index of a mode propagating through the mode rotator of the present embodiment. FIG. [Diagram 3] 5 is a graph showing the transmittance at a convex portion of the mode rotator of the present embodiment. [Figure 4] 4 is a graph showing the transmittance in an L-shaped portion of the mode rotator of the present embodiment. [Diagram 5] 11 is a diagram showing the relationship between the change in trench depth and the transmittance in the mode rotator of the present embodiment. FIG. [Figure 6] FIG. 13 is a diagram showing a mode rotator of a modified example. [Figure 7] 13 is a transmission spectrum of a modified mode rotator. [Figure 8] 1 is a diagram illustrating a mode multiplexer / demultiplexer according to an embodiment of the present invention; [Figure 9] FIG. 13 is a diagram showing the configuration of a mode rotator of a comparative example. [Figure 10] 13 is a transmission spectrum of a mode rotator of a comparative example. [Figure 11] FIG. 13 is a diagram showing the relationship between the change in trench depth and the transmittance in a mode rotator of the comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] (mode rotator) The mode rotator and mode multiplexer / demultiplexer of the present embodiment will be described in detail below with reference to the drawings. Fig. 1 is a diagram showing a schematic configuration of a mode rotator of the present embodiment. The center of Fig. 1 shows a plan view of the mode rotator, and the right column shows cross-sectional shapes along several cutting lines shown on the mode rotator at the center. The left column of Fig. 1 shows the transition of the electric field distribution of the excitation mode propagating through the mode rotator 10.

[0010] The mode rotator 10 of the present application is formed along and integral with a waveguide 101 of a silica-based planar lightwave circuit (PLC). The waveguide 101 is embedded in a cladding that spreads horizontally on a quartz substrate, and is formed of a core with a rectangular cross section having a predetermined width and height. For convenience, an orthogonal coordinate system is set in which the direction in which light propagates through the waveguide 101 is the z direction, the horizontal direction is the x direction, and the downward vertical direction is the y direction.

[0011] The waveguide 101 can propagate four excitation modes of electric field distribution, E11, E12, E21, and E22 (hereinafter, simply referred to as modes). Here, the subscripts of the Emn modes indicate the orders of the electric field distribution in the horizontal direction (m) and vertical direction (n). The waveguide 101 is connected to optical fibers of 2LP (linear polarization) modes (not shown) on the upstream and downstream sides. These optical fibers of 2LP modes can propagate three modes, LP01, LP11a, and LP11b. The subscripts of LPmn indicate the electric field distribution in the angular direction (m) and radial direction (n). The subscripts a or b of LP11 indicate that the polarization plane is horizontal or vertical. These LP01, LP11a, and LP11b modes correspond to the E11, E21, and E12 modes in the waveguide 101, respectively.

[0012] The mode rotator 10 is composed of a convex portion 11 having an inverted T-shaped or convex cross-sectional shape, and an L-shaped portion 12 having an L-shaped cross-sectional shape in contact with the convex portion 11, in the direction in which light propagates through the waveguide 101 shown as the z direction in the figure. In the convex portion 11, the distal end from the L-shaped portion 12 corresponds to the incident end, and the boundary with the L-shaped portion 12 corresponds to the output end. In the L-shaped portion 12, the boundary with the convex portion 11 corresponds to the incident end, and the distal end from the convex portion 11 corresponds to the output end. The convex portion 11 and the L-shaped portion 12 each have a predetermined length in the z direction to enable adiabatic conversion.

[0013] The convex portion 11 has a trench 11b of a predetermined depth formed in the top surface 11a so that the cross section is convex. The top surface 11a is separated from the trench 11b by linear edges 11c on both sides, and has a tapered shape that gradually narrows in width as it proceeds in the z direction. At the input end, the top surface 11a occupies the entire width, but the width of the top surface 11a gradually narrows as it proceeds in the z direction, and when it reaches the output end, the edges 11c on both sides of the top surface 11a join at approximately the center in the width direction, and the trench 11b occupies the entire width.

[0014] In other words, in the convex portion 11, the top surface 11a occupies the entire width at the end distal from the L-shaped portion 12, and the top surface 11a separated from the trench 11b by the linear edges 11c on both sides has a tapered shape that gradually narrows as it approaches the boundary with the L-shaped portion 12 from the end distal from the L-shaped portion 12. At the boundary with the L-shaped portion 12, the edges 11c on both sides sandwiching the top surface 11a join approximately in the center in the width direction, and the trench 11b occupies the entire width.

[0015] The L-shaped portion 12 has a trench 12b of a predetermined depth formed in the top surface 12a so that the cross section has an L shape. The top surface 12a is separated from the trench 12b by a linear edge 12c and has an inverse tapered shape that gradually widens in the z direction. At the input end, the trench 12b occupies the entire width, but the top surface 12a gradually widens in the z direction, and at the output end, the top surface 12a occupies the entire width.

[0016] In other words, in the L-shaped portion 12, the top surface 12a occupies the entire width at the end distal from the convex portion 11, and the top surface 12a separated from the trench 12b by the linear edge 12c has a tapered shape that gradually narrows as it approaches the boundary with the convex portion 11 from the end distal from the convex portion 11. At the boundary with the convex portion 11, the trench 12b occupies the entire width.

[0017] The left column in the figure shows the transition of the electric field distribution of the E21 mode incident on the mode rotator 10. This electric field distribution was obtained by the finite element method (FEM), and shows the electric field distribution in the cross section of the core of the waveguide 101 and the surrounding cladding. In the mode rotator 10, if the propagation constants of the E21 mode and the E12 mode are βa and βb, respectively, and the length of the mode rotator 10 in the z direction is L, the E21 mode and the E12 mode can be rotated to the E12 mode and the E21 mode, respectively, by adjusting as shown in the following formula 1. In the mode rotator 10, the propagation constant is transitioned by gradually changing the structure in the z direction so as to satisfy formula 1, making it possible to convert the mode to rotate adiabatically.

[0018]

number

[0019] In the mode rotator 10, the upstream waveguide 101 propagates the E21 mode corresponding to the LP11a mode incident from an optical fiber (not shown) toward the mode rotator 10. The E21 mode incident on the mode rotator 10 does not rotate in the convex portion 11 symmetrical in the x direction, but rotates 90° around the z axis in the L-shaped portion 12 asymmetrical in the x direction, and is converted into the E12 mode. The downstream waveguide 101 propagates the E12 mode output from the mode rotator 10 toward an optical fiber (not shown) connected downstream. This E21 mode excites the LP11b mode of the optical fiber and is guided. Although not shown, the E12 mode corresponding to the LP11b mode of the optical fiber does not rotate in the convex portion 11, but rotates 90° around the z axis in the L-shaped portion 12, and is converted into the E21 mode, and excites the LP11a of the optical fiber.

[0020] 1 shows the mode rotor 10 in which the convex portion 11 and the L-shaped portion 12 are arranged in this order in the z direction, but the configuration of the mode rotor 10 is not limited to this, and the L-shaped portion 12 and the convex portion 11 may be arranged in this order in the z direction. Such a mode rotor 10 can be constructed, for example, by rotating the mode rotor 10 in FIG. 1 by 180° around the y axis.

[0021] FIG. 2 shows the effective refractive index n off 2(a) shows a plan view of the mode rotator 10, and FIG. off The size of each part of the mode rotator 10 is shown in FIG. 2(c). The above-mentioned change in the propagation constant is caused by the effective refractive index n off It can be set by:

[0022] As shown in Fig. 2(c), the core embedded in the cladding in the PLC to form the waveguide 101 has the following dimensions for the convex portion 11: width w, height h, width wt1 of the shoulder trench 11b in the x-direction, width wt2 of the shoulder trench 11b in the direction opposite to the x-direction, and height ht of the top surface 11a from the bottom of the trench 11b. Since the width wt1 of the shoulder trench 11b in the x-direction and the width wt2 of the shoulder trench 11b in the direction opposite to the x-direction are equal, the widths of both trenches can be expressed by the trench width wt, such as wt1 = wt2 = wt. For the L-shaped portion 12, the width wt1 of the shoulder trench 11b in the x-direction = 0, and the width wt2 of the shoulder trench 11b in the direction opposite to the x-direction can be expressed by wt2 = wt.

[0023] FIG. 2(b) shows the effective refractive index n off 2(b) also shows the electric field distribution of the mode in cross sections at several positions on the mode rotator 10, along with the transition of the trench width wt in the z direction of the mode rotator 10.

[0024] As shown by the dashed line in the figure, the effective refractive index n off decreases as it progresses through convex portion 11, reaches a minimum value at the boundary between convex portion 11 and L-shaped portion 12, and then increases as it progresses through L-shaped portion 12. Referring to the transition of the electric field distribution of the first mode in the figure, the first mode, which was E21 mode when it entered mode rotator 10, gradually shrinks in the y direction as the trench width Wt increases in convex portion 11, and is rotated around the z axis in L-shaped portion 12 as the trench width Tw decreases, converting to E12 mode.

[0025] As shown by the solid line in the figure, the effective refractive index n off decreases as one progresses through the convex portion 11, reaches a minimum value at the boundary between the convex portion 11 and the L-shaped portion 12, and then increases as one progresses through the L-shaped portion 12. The effective refractive index n offis the effective refractive index n of the first mode at the input end of the mode rotator 10. off However, after the light beam crosses the convex portion 11, the effective refractive index n off Referring to the transition of the electric field distribution of the second mode in the figure, the second mode, which was the E12 mode when incident on the mode rotator 10, is reduced in the y-axis direction in the convex portion 11 as the trench width Wt increases, and is rotated around the z-axis in the L-shaped portion 12 as the trench width Wt decreases, and is converted to the E21 mode.

[0026] FIG. 3 is a graph showing the transmittance at the convex portion 11 of the mode rotator 10. FIG. 3(a) shows the case where the input is E21 mode, and FIG. 3(b) shows the case where the input is E12 mode. The transmittance on the vertical axis of the graph was calculated using the local coupled mode theory (LCMT) and the beam propagation method (BPM). Here, the tapered shape with the edges 11c on both sides on the top surface 11a of the convex portion 11 is formed from a reference position that is a distance d from the entrance end in the z direction. The horizontal axis Ltp of the graph is the distance from the reference position in the z direction.

[0027] As shown in Fig. 3(a), the transmittance of the E21 mode input to the convex portion 11 increases from approximately 90% at the reference position to approximately 100% at Ltp = 0.5 mm, and remains approximately 100% until Ltp = 1.5 mm is reached. On the other hand, the E22 mode decreases from a few percent at the reference position to approximately 0% at Ltp = 0.5 mm, and remains approximately 0% until Ltp = 1.5 mm is reached. It is observed that the results of LCMT and BPM are in good agreement regardless of whether the input is E21 mode or E12 mode.

[0028] As shown in FIG. 3(b), the transmittance of the E12 mode input to the convex portion 11 increases from approximately 90% for LCMT or approximately 7% for BPM at the reference position to approximately 100% at Ltp=0.5 mm, and remains approximately 100% until Ltp=1.5 mm is reached. On the other hand, the E11 mode decreases from approximately 10% at the reference position to approximately 0% at Ltp=0.5 mm, and remains approximately 0% until Ltp=1.5 mm is reached. It is observed that the results of LCMT and BPM are in good agreement regardless of whether the input is E12 mode or E21 mode, except that some deviation is observed for the E12 mode in the range from the reference position to Ltp=approximately 0.5 mm.

[0029] It is preferable that the transmittance of the convex portion 11 is as high as possible, and in general-purpose optical devices, it is required to be 95% or more. With reference to Figures 3(a) and 3(b), it is observed that the transmittance of 95% or more is satisfied if Lpt ≧ 0.5 mm. In other words, by making the length of the convex portion 11 in the z direction 0.5 mm or more, a transmittance of 95% or more can be ensured, and adiabatic conversion can be realized.

[0030] Fig. 4 is a graph showing the transmittance in the L-shaped portion 12 of the mode rotator 10. Fig. 4(a) shows the case where the input is in E21 mode, and Fig. 4(b) shows the case where the input is in E12 mode. The transmittance on the vertical axis of the graph was obtained by LCMT and BPM, as in Fig. 3. The horizontal axis Ltp of the graph is the distance traveling in the z direction from the incident end.

[0031] As shown in Fig. 4(a), the transmittance of the E21 mode input to the L-shaped section 12 decreases from approximately 90% at the input end to approximately 0% at a distance Ltp of approximately 3 mm through a minimum and maximum, and maintains approximately 0% until it reaches Ltp = 1.5 mm. On the other hand, the E12 mode increases from 0% at the input end to approximately 100% at a distance Ltp of approximately 3 mm through a maximum and minimum, and maintains approximately 100% until it reaches Ltp = 1.5 mm. It is observed that the results of LCMT and BPM are in good agreement, except for a slight deviation observed near the input end of the E21 mode.

[0032] As shown in FIG. 4(b), the E12 mode input to the L-shaped section 12 goes from about 85% of LCMT or about 65% of BPM at the input end, passes through a minimum and a maximum, and then drops to about 0% at a distance Ltp=about 3 mm, and maintains about 0% until it reaches Ltp=1.5 mm. On the other hand, the E21 mode goes from 0% at the input end, passes through a maximum and a minimum, and then increases to about 100% at a distance Ltp=about 3 mm, and maintains about 100% until it reaches Ltp=1.5 mm. It is observed that the results of LCMT and BPM are in good agreement regardless of whether the input is E12 mode or E21 mode, except for a slight deviation observed near the input end of the E12 mode.

[0033] It is preferable that the transmittance of the L-shaped portion 12 is as high as possible, and in general-purpose optical devices, it is required to be 95% or more. With reference to Figures 4(a) and 4(b), it can be observed that the transmittance of 95% or more is satisfied if Lpt ≧ 3 mm. In other words, by making the length of the L-shaped portion 12 in the z direction 3 mm or more, a transmittance of 95% or more can be ensured, and adiabatic conversion can be realized.

[0034] FIG. 5 is a diagram showing the relationship between the change in trench depth and the transmittance in the mode rotator 10 of this embodiment. FIG. 5(a) is a cross-sectional view showing the relationship between the depth of the trench 10b in the mode rotator 10 of this embodiment and the amount of etching. FIG. 5(b) is a graph showing the relationship between the wavelength and the transmittance. The mode rotator 10 of this embodiment is formed on the top surface of the waveguide 101 made of a core embedded in the cladding of the PLC, so two etching steps are required: a step of forming the top surface of the waveguide 101, and a step of forming a trench on the top surface.

[0035] 5(a), when the trench 10b of the mode rotator 10 is formed to a designed predetermined depth, the ideal shape is assumed to be a displacement of the trench 10b from the predetermined depth in the depth direction, Δht. When Δht>0, etching is excessive, and the depth of the trench 10b becomes deeper than the predetermined depth, and when Δht<0, etching is insufficient, and the depth of the trench 10b becomes smaller than the predetermined depth.

[0036] The graph in Figure 5(b) shows the transmittance versus wavelength for the ideal case of Δht=0, over-etching of Δht=+0.2μm, and under-etching of Δht=-0.2μm. The solid line in the figure shows the case of conversion from E12 mode to E21 mode, and the dashed line shows the case of conversion from E21 mode to E12 mode. Referring to the graph, it can be observed that if the absolute value of the displacement Δht is 0.2μm, i.e., 200nm or less, the transmission loss can be suppressed to 0.2dB or less.

[0037] As described above, the mode rotator 10 of this embodiment is formed by a two-stage etching process. If the displacement of the trench 10b of the mode rotator 10 from the designed predetermined depth is 200 nm or less, the transmission loss can be suppressed to 0.2 dB or less. This allows a relatively large margin in processing accuracy, and the processing accuracy required to ensure performance such as transmission loss is relaxed. This reduces the burden of manufacturing the mode rotator 10, and the manufacturing cost of the mode rotator 10 can also be reduced. Since the mode rotator 10 can be mass-produced on a substrate, the manufacturing process becomes easier, and the yield rate is improved, which is expected to lead to economical production. In addition, the mode rotator 10 of this embodiment has high mode conversion efficiency and wideband characteristics.

[0038] In the mode rotator 10 of the present embodiment, the three modes E11, E21, and E12 have been described, but the present invention is not limited to this, and n may be an integer of 3 or more, and may correspond to n modes. In this case, the mode rotator 10 maintains the fundamental mode E11 mode among the n modes, converts the higher modes E21 and E12 modes so as to be adiabatically exchanged, and can maintain any other higher modes. When n is greater than 4, the mode rotator 10 can also adiabatically exchange the E21 mode and a symmetrical higher mode in place of the E21 mode.

[0039] (Modification) Fig. 6 is a diagram showing a modified mode rotator 20. Fig. 6(a) is a plan view showing the modified mode rotator 20. Fig. 6(b) is a plan view showing the mode rotator 10 of the present embodiment for comparison, and Fig. 6(c) is a graph showing the conversion efficiency etc. of the modified mode rotator 20.

[0040] The mode rotator 20 of the modified example is different in that the edge 22c of the L-shaped portion 22, which was linear in the mode rotator 10 of the present embodiment, is changed to a curved shape designed by a method called FAQUAD (fast quasi-adiabatic). The other configurations of the mode rotator 20 of the modified example are the same as those of the mode rotator 10 of the present embodiment. In the mode rotator 20 of the modified example, the L-shaped portion 22 and the convex portion 21 are arranged in the opposite order in the z direction to the mode rotator 10 of the present embodiment shown in FIG. 1, but the order in which the L-shaped portion 22 and the convex portion 21 are arranged does not affect the function of mode conversion. The mode rotator 10 of the present embodiment shown in FIG. 6(b) is also arranged in the order of the L-shaped portion 22 and the convex portion 21 in the z direction to match the configuration of the modified example.

[0041] As shown in FIG. 6(a), in the L-shaped portion 22 of the modified example, the top surface 22a is separated from the trench 22b by the FAQUAD edge 22c so that the cross section is L-shaped, and the top surface 22a occupies the entire width at the entrance end, but the width of the top surface 22a gradually narrows as it advances in the z direction, and when it reaches the exit end, the trench 22b occupies the entire width. The length of the L-shaped portion 22 in the z direction is 1.2 mm. Note that the shape of the edge 22c of the modified example is designed by the FAQUAD method, so the tapered shape of the modified example is sometimes called a FAQUAD taper. Similarly, the linear tapered shape of the L-shaped portion 22 of this embodiment is sometimes called a linear taper.

[0042] 6(b) of the present embodiment for comparison, in order to ensure a transmittance of 95% as described above, it was necessary to ensure a length in the z direction of 0.5 mm for the convex portion 21 and 3 mm for the L-shaped portion 22. Compared to such mode rotator 10 of the present embodiment, the length of 1.2 mm of the L-shaped portion 22 of the modified example can be said to be short.

[0043] Fig. 6(c) is a graph showing the mode conversion efficiency and mode residual ratio of the L-shaped portion 22 of the modified example. For comparison, the graph also shows the mode rotator 10 of the present embodiment. In Fig. 6(c), the L-shaped portion 22 of the modified example is referred to as a FAQUAD taper, and the L-shaped portion 22 of the mode rotator 10 of the present embodiment is referred to as a linear taper.

[0044] 6(c), the conversion efficiency of the L-shaped portion 22 of the modified example, which converts the E12 mode into the E21 mode when the L-shaped portion 22 is incident thereon, is similar to that of the L-shaped portion 22 of the present embodiment, but it is observed that the conversion efficiency is higher than that of the present embodiment at most positions in the z direction. The mode residual ratio of the L-shaped portion 22 of the modified example shows a tendency to decrease generally in the z direction, similar to that of the L-shaped portion 22 of the present embodiment.

[0045] FIG. 7 shows the transmission spectrum of the modified mode rotator 20. FIG. 7(a) shows the transmission spectrum of the modified mode rotator 20, and FIG. 7(b) shows the crosstalk. The horizontal axis of the graphs in FIG. 7(a) and FIG. 7(b) is the wavelength of 1500 to 1600 nm. Referring to the graphs, it is observed that the transmission loss is suppressed to about 0.05 dB in the relevant band. As shown in FIG. 7(b), it is observed that the crosstalk is suppressed to less than −25 dB in the relevant band. This crosstalk corresponds to the extinction ratio, and is an index of whether the mode has been rotated.

[0046] The modified mode rotator 20 can be designed using the FAQUAD method to reduce the length in the z direction. This allows the modified mode rotator 20 to be made smaller and lighter. In addition, the modified mode rotator 20 can achieve a higher conversion efficiency than the mode rotator 10 of the present embodiment.

[0047] (Mode multiplexer / demultiplexer) 8 is a plan view showing a schematic configuration of a mode multiplexer / demultiplexer 50 of the present embodiment. Here, an example of configuring a mode multiplexer is shown as the mode multiplexer / demultiplexer 50. The mode multiplexer / demultiplexer 50 is an application of the mode rotator 10 of the present embodiment, and is configured by a PLC. For convenience, an orthogonal coordinate system is set in which the direction in which light propagates through the first waveguide 101 of the main waveguide is the z direction, the horizontal direction is the x direction, and the downward vertical direction is the y direction.

[0048] A first waveguide 101 of a main waveguide, and a second waveguide 102 and a third waveguide 103 of a sub-waveguide are connected to the mode multiplexer / demultiplexer 50 from the upstream side. A mode rotator 10 is provided on the first waveguide 101 extending linearly. A first coupling portion 51 is provided in a predetermined range on the upstream side of the mode rotator 10, the first coupling portion 51 facing a second waveguide 102, which is parallel to the first waveguide 101 on the opposite side to the x direction, with a gap therebetween. A second coupling portion 52 is provided in a predetermined range on the downstream side of the mode rotator 10, the second coupling portion 52 facing a third waveguide 103, which is parallel to the first waveguide 101 on the y direction side, with a gap therebetween. The second waveguide 102 and the third waveguide 103 are terminated on the downstream side of the first coupling portion 51 and the second coupling portion 52, respectively.

[0049] An E11 mode corresponding to the LP01 mode of an optical fiber (not shown) connected upstream propagates through the first waveguide 101 toward the mode rotator 10. Similarly, an E11 mode corresponding to the LP01 mode propagates through the second waveguide 102. An E12 mode is multiplexed into the first waveguide 101 from the second waveguide 102 via the first coupling portion 51. With regard to the incident E11 and E12 modes, the mode rotator 10 converts the E12 mode into an E21 mode corresponding to the LP11a mode, but maintains the E11 mode.

[0050] The E11 mode and the E12 mode are output from the mode rotator 10 and propagate through the first waveguide 101 on the downstream side. The E11 mode, which corresponds to the LP01 mode of the optical fiber (not shown) connected on the upstream side, also propagates through the third waveguide 103. The E21 mode is multiplexed from the third waveguide 103 through the second coupling portion 52 into the first waveguide 101. Therefore, the E11 mode propagating through the first waveguide 101, the E21 mode multiplexed from the second waveguide 102, and the E12 mode multiplexed from the third waveguide 103 are superimposed and propagated through the first waveguide 101. These E11, E21, and E12 modes are incident on the optical fiber (not shown) connected on the downstream side to the first waveguide 101, and are guided by exciting the LP01, LP11a, and LP11b modes, respectively.

[0051] Although an example of a mode multiplexer is shown as the mode multiplexer / demultiplexer 50 here, a mode multiplexer having not only the function of a mode multiplexer but also the function of a mode demultiplexer can be configured. In this case, the second waveguide 102 and the third waveguide 103 are not terminated downstream of the first coupling portion 51 and the second coupling portion 52. Not only are the first waveguide 101 of the main waveguide and the second waveguide 102 and the third waveguide 103 of the sub-waveguide connected to the mode multiplexer / demultiplexer 50 from the upstream side, but the first waveguide 101 of the main waveguide and the second waveguide 102 and the third waveguide 103 of the sub-waveguide are also connected from the downstream side.

[0052] In such a mode multiplexer / demultiplexer, the function of the mode multiplexer is as described above, but the function of the mode demultiplexer is as follows: In the first waveguide 101 toward the mode rotator 10, the modes E11, E21, and E12, which correspond to the modes LP01, LP11a, and LP11b, respectively, of the optical fiber (not shown) connected upstream, are superimposed and propagated. Of these modes, the E12 mode is demultiplexed into the second waveguide 102 via the first coupling part 51, and propagates through the second waveguide 102 as the E12 mode.

[0053] The mode rotator 10 converts the E11, E12, and E21 modes incident from the first waveguide 101 so as to exchange the higher-order E12 and E21 modes, but maintains the fundamental E11 mode. Therefore, the E21 propagating through the first waveguide 101 is converted to the E12 mode by the mode rotator 10. This E12 mode is branched to the third waveguide 103 via the second coupling portion 52, and propagates through the third waveguide as the E12 mode.

[0054] Since mode multiplexer / demultiplexer 50 of the present embodiment uses mode rotator 10 of the present embodiment, a comparatively large margin of error is provided in terms of processing accuracy, and the processing accuracy required to ensure performance such as transmission loss is relaxed. This reduces the burden of manufacturing mode multiplexer / demultiplexer 50, and the manufacturing costs of mode multiplexer / demultiplexer 50 can also be reduced.

[0055] In the present embodiment, the mode multiplexer / demultiplexer 50 has been described with respect to three modes, E11, E21, and E12, which correspond to LP01, LP11a, and LP11b, but the present invention is not limited thereto and may be adapted to n modes, where n is an integer of 3 or more. In this case, the mode multiplexer / demultiplexer 50 can convert the n modes so that it multiplexes / demultiplexes the higher-order modes E21 and E21 modes with respect to the fundamental mode E11 mode, and maintains other higher-order modes, if any. When n is greater than 4, the mode rotator 10 can also multiplex / demultiplex symmetrical higher-order modes instead of the E21 and E21 modes.

[0056] In addition, while an example has been shown in which the mode rotator 10 of the present embodiment is applied to the mode multiplexer / demultiplexer 50, the mode rotator 20 of the modified example can also be applied instead of the mode rotator 10 of the present embodiment. Because the mode rotator 20 of the modified example has a shorter length in the z direction than the mode rotator 10 of the present embodiment, applying the mode rotator 20 of the modified example can also reduce the size and weight of the mode multiplexer / demultiplexer 50.

[0057] (Comparative Example) 9 is a diagram showing the configuration of a mode rotator 30 of the comparative example. The mode rotator 30 of the comparative example has a predetermined length in the z direction, and a trench 30b of a predetermined depth is formed in the top surface 30a so that the cross section is L-shaped. The top surface 30a of the mode rotator 30 of the comparative example is separated from the trench 30b by a linear edge 30c, and has a rectangular shape extending in the z direction while maintaining a predetermined width between the edge 30c and the edge that the top surface 30a forms with the side surface in the x direction. The mode rotator 30 of the comparative example is also formed as a PLC on a quartz substrate, similar to the mode rotator 10 of the present embodiment.

[0058] Referring to the transition of the electric field distribution of the modes shown in the left column in the figure, the E21 mode incident on the mode rotator 30 from the upstream waveguide 101 excites two tilt modes at the incident end, and one of the two tilt modes propagating through the mode rotator 30 is rotated 180° around the z-axis while the other is maintained and output from the mode rotator 30. The E12 mode excited by these modes propagates in the waveguide 101 connected on the downstream side.

[0059] FIG. 10 shows the transmission spectrum of the mode rotator 30 of the comparative example. FIG. 10(a) shows the transmission loss of the mode rotator 30 of the comparative example, and FIG. 10(b) shows the crosstalk. The horizontal axis of the graphs of FIG. 10(a) and FIG. 10(b) is the wavelength of 1500 to 1600 nm. Referring to the graphs, it is observed that the transmission loss is a maximum of about 0.1 dB. As shown in FIG. 10(b), it is observed that the crosstalk is about -25 dB.

[0060] Fig. 11 is a diagram showing the relationship between the change in depth of the trench 30b and the transmittance in the mode rotator 30 of the comparative example. Fig. 11(a) is a cross-sectional view showing the relationship between the depth of the trench 30b of the mode rotator 30 of the comparative example and the amount of etching. Fig. 11(b) is a graph showing the relationship between the wavelength and the transmittance.

[0061] 11(a), when the trench 30b of the mode rotator 30 is formed to a designed predetermined depth, the displacement of the trench 30b in the depth direction from the ideal shape is Δht. When Δht>0, etching is excessive and the depth of the trench 30b is greater than the predetermined depth, and when Δht<0, etching is insufficient and the depth of the trench 30b is smaller than the predetermined depth.

[0062] The graph in Figure 11(b) shows the transmittance versus wavelength for the ideal case of Δht = 0, over-etching of Δht = +0.2 μm, and under-etching of Δht = -0.2 μm. It can be seen that the transmission loss exceeds 0.2 dB when the absolute value of the displacement Δht is 0.2 μm, i.e., 200 nm. [Explanation of symbols]

[0063] 10 mode rotor 11 Convex part 11a Top surface 11b Trench 11c Edge 12 L-shaped part 12a Top surface 12b Trench 12c Edge 101 Waveguide

Claims

1. A mode rotator formed on a substrate and provided in a waveguide through which n modes can propagate, n being an integer equal to or greater than 3, comprising: an L-shaped portion having a trench formed on a top surface thereof so that the cross section thereof has an L-shape; a convex portion having a trench formed on a top surface thereof so that the convex portion is in contact with the L-shaped portion and has a cross section of a convex shape; Including, The L-shaped portion and the convex portion are mode rotators capable of adiabatic mode conversion in the propagation direction.

2. 2. A mode rotator as described in claim 1, wherein the trench of the L-shaped portion has a tapered shape such that the width of the top surface gradually narrows from the distal end from the convex portion to the boundary with the convex portion, and the trench of the convex portion has a tapered shape such that the width of the top surface gradually narrows from the distal end from the L-shaped portion to the boundary.

3. 3. The mode rotator of claim 2, wherein in the L-shaped portion, the distal end is entirely formed by the top surface and the boundary is entirely formed by the trench, and in the convex portion, the distal end is entirely formed by the top surface and the boundary is entirely formed by the trench.

4. 2. The mode rotator of claim 1, wherein the edge of said L-shaped portion that separates said top surface and said trench is straight.

5. 2. The mode rotator of claim 1, wherein in said L-shaped portion, an edge separating said top surface and said trench is designed by the FAQUAD methodology.

6. 2. The mode rotator according to claim 1, wherein the L-shaped portion and the convex portion each have a length in the propagation direction such that the transmittance of all propagating modes is 95% or more.

7. 7. The mode rotator according to claim 6, wherein the L-shaped portion has a length of 1.2 mm or more in the propagation direction, and the convex portion has a length of 0.5 mm or more in the propagation direction.

8. A mode multiplexer / demultiplexer using the mode rotator according to claim 1, the mode multiplexer / demultiplexer being formed on a substrate, a main waveguide having the mode rotator; At least one secondary waveguide that is parallel to the primary waveguide and that propagates a mode that is at least one of multiplexed and demultiplexed with respect to the primary waveguide; a coupling portion for coupling the main waveguide and the at least one sub-waveguide, A mode multiplexer / demultiplexer including:

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