JP6699028B2 - Mode exchanger - Google Patents

Description

  The present disclosure relates to a mode control technique having a function of exchanging power between propagating modes in a mode multiplex transmission system using a multimode optical fiber.

  In the optical fiber communication system, the nonlinear effect and the fiber fuse generated in the optical fiber pose a problem, and the transmission capacity is limited. In order to relax these restrictions, it is necessary to reduce the density of the light guided to the optical fiber, and as shown in Non-Patent Document 1, a large core fiber has been studied.

  However, there is a trade-off relationship between the reduction of bending loss, the expansion of the single mode operation region, and the expansion of the effective cross-sectional area, and there is a problem that the expansion amount of the effective cross-sectional area under a predetermined condition is limited. Therefore, a mode multiplex transmission system in which a multimode fiber is used as a transmission fiber and parallel transmission is performed using a plurality of propagating modes has been studied as a technique for achieving a dramatic increase in capacity (for example, Non-Patent Document 1). See 2.).

  In a mode multiplex transmission system, a mode multiplexer/demultiplexer has been proposed in order to propagate a plurality of signals emitted from a transmitter as separate modes in an optical fiber (for example, see Non-Patent Documents 3 and 4). ).

  In addition, optical MIMO transmission has been proposed in which coupling between modes occurring in a transmission line is compensated by MIMO signal processing at the receiving end. Further, a large difference in group delay between modes increases the load of MIMO signal processing, and thus low inter-group delay difference (DMD) fibers have been studied (for example, see Non-Patent Documents 5 and 6). ..).

T. Matsui, et al. , "Applicability of Photonic Crystal Fiber with Uniform Uniform Air-Hole Structure to High-Speed and Wide-Band Transistion overconvection Conversation. Lightwave Technology. 27, 5410-5416, 2009. N. Hanzawa et al. , "Demonstration of mode-division multiplexing multiplexing transmission over 10 km two-mode fiber with mode coupler", OFC2011, paper OWA4 (2011). N. Hanzawa et al. , "Asymmetric parallel waveguide with mode conversion for mode and wavelength division multiplexing", OFC 2012, OTU11. 4. N. Hanzawa et al, "Mode multi/demultiplexing with parallel waveguide for mode division multiplex transmission", Opt. Express vol. 22, pp. 29321-29330 (2014) R. Ryf et al. , "Mode-division multiplexing over 96 km of few-fiber fiber using coherent 6x6 MIMO processing", J. Am. Lightw. Technol. , Vol. 30, pp. 521-531 (2012). T. Mori et al. , "Few-mode fibers supporting more than two LP modes for mode-division-multiplexed transmission with MIMO DSP", J. Am. Lightw. Technol. , Vol. 32, pp. 2468-2479 (2014). : K. Saith et al. , "PLC-based LP11 mode rotor for mode-division multiplexing multiplexing transmission," Opt. Express, vol. 22, pp. 19117-19130 (2014). S. Savin et al., "Tunable mechanically induced long-period fiber gratings", OPTICS LETTERS/Vol. 25, No. 10 / May 15, 2000

  However, there is a limit in reducing the DMD of the fiber due to the manufacturing error of the fiber. Furthermore, the DMD compensation transmission line described in Non-Patent Document 6 requires two types of optical fibers, which causes a problem that the transmission line becomes complicated. In a transmission line using two types of optical fibers, the optical characteristics such as transmission loss, chromatic dispersion, and polarization dispersion are different between modes, and this mode-dependent characteristic difference causes the restored signal even if MIMO signal processing is performed. It causes quality deterioration. That is, the deterioration of the transmission quality in the transmission path using the two types of optical fibers has a problem of limiting the transmission capacity and the transmission distance of the communication system. ..

  Therefore, in order to solve the above-described problems, the present invention provides a mode multiplex transmission system using a multimode optical fiber, in which a DMD between modes or various optical characteristics generated in the transmission line is not complicated. An object of the present invention is to provide a mode switch that compensates for the difference and has less influence on the transmission capacity and transmission distance of the communication system.

  In order to achieve the above-mentioned object, the mode converter according to the present invention is configured such that a coupling portion or a mode rotator that couples between different modes depending on the arrangement of waveguides is built in a planar optical waveguide (PLC) and input. It is decided to cyclically perform mode exchange between signal lights having different modes.

Specifically, the first mode exchanger according to the present invention is
A first waveguide that is formed on the substrate and can propagate three modes of LP01, LP11a, and LP11b;
A second waveguide arranged on the substrate in parallel with the first waveguide;
Three coupling portions which are composed of the first waveguide and the second waveguide on the substrate, and which transfer the optical power between the first waveguide and the second waveguide;
A mode rotator that is connected to one end of the first waveguide and performs mode conversion between LP11a and LP11b;
Equipped with
In two of the coupling portions, the effective refractive index of the LP01 mode propagating in the second waveguide and the effective refractive index of the LP11a mode propagating in the first waveguide are the same, and The length is a length at which the power of the light is completely transferred from one waveguide to the other waveguide,
In the coupling section sandwiched between the two coupling sections of the coupling section, the effective refractive index of the LP01 mode propagating in the first waveguide and the effective refractive index of the LP01 mode propagating in the second waveguide match, It is characterized in that the length of the light in the propagation direction is a length at which the power of the light is completely transferred from one waveguide to the other waveguide.

A second mode exchanger according to the present invention is
A first waveguide that is formed on the substrate and can propagate three modes of LP01, LP11a, and LP11b;
A second waveguide arranged on the substrate in parallel with the first waveguide;
A third waveguide arranged in parallel with the first waveguide on the substrate;
Three first coupling portions that are composed of the first waveguide and the second waveguide on the substrate, and that transfer the optical power between the first waveguide and the second waveguide;
It is composed of the first waveguide and the third waveguide on the substrate, is arranged so as to sandwich the three first coupling portions, and the light is provided between the first waveguide and the third waveguide. Two second couplings that transfer the power of
A first mode rotor connected to one end of the first waveguide for mode conversion between LP11a and LP11b;
Two second mode rotators that are between the two second coupling portions and that are arranged on the first waveguide so as to sandwich the three first coupling portions and that perform mode conversion between LP11a and LP11b;
Equipped with
In two of the first coupling portions, the effective refractive index of the LP01 mode propagating in the second waveguide and the effective refractive index of the LP11a mode propagating in the first waveguide match each other, and thus the propagation of light is performed. The length in the direction is a length at which the power of the light is completely transferred from one waveguide to the other waveguide,
The coupling portion sandwiched between the two coupling portions of the first coupling portion has an effective refractive index of the LP01 mode propagating in the first waveguide and an effective refractive index of the LP01 mode propagating in the second waveguide are equal to each other. However, the length of the light in the propagation direction is the length at which the power of the light is completely transferred from one waveguide to the other waveguide,
The two second coupling portions have the same effective refractive index in the LP01 mode propagating in the third waveguide and the effective refractive index in the LP11a mode propagating in the first waveguide, and have a length in the light propagation direction. It is characterized in that the power of the light is completely transferred from one waveguide to the other waveguide.

A third mode exchanger according to the present invention is
A first waveguide that is formed on the substrate and can propagate three modes of LP01, LP11a, and LP11b;
A second waveguide arranged on the substrate in parallel with the first waveguide;
A third waveguide arranged in parallel with the first waveguide on the substrate;
A coupling part B, a coupling part C, and a coupling which are composed of the first waveguide and the second waveguide on the substrate and which transfer the power of light between the first waveguide and the second waveguide. Part E,
A coupling section A and a coupling section D which are composed of the first waveguide and the third waveguide on the substrate, and which transfer the optical power between the first waveguide and the third waveguide;
Two mode rotators arranged on the first waveguide and mode-converting LP11a and LP11b;
Equipped with
The coupling part A, the coupling part B, the coupling part C, the coupling part D and the coupling part E are sequentially arranged from one end to the other end, and one of the two mode rotors is Is disposed between the coupling portion A and the coupling portion B, and the other of the two mode rotors is disposed between the coupling portion D and the coupling portion E,
The coupling portion A, the coupling portion B, the coupling portion D, and the coupling portion E propagate the effective refractive index of the LP01 mode propagating in the second waveguide or the third waveguide and the first waveguide. The effective refractive indices of the LP11a modes are the same, and the length in the light propagation direction is the length at which the power of the light is completely transferred from one waveguide to the other waveguide,
In the coupling portion C, the effective refractive index of the LP01 mode propagating in the first waveguide and the effective refractive index of the LP01 mode propagating in the second waveguide match, and the length in the light propagation direction is equal to that of the light. It is characterized in that the length is such that power is completely transferred from one waveguide to the other waveguide.

A fourth mode exchanger according to the present invention is
One waveguide formed on the substrate and capable of propagating three modes of LP01, LP11a and LP11b,
A grating portion formed in the waveguide and having a width that changes periodically to perform mode conversion between LP01 and LP11a, or a height that changes periodically to perform mode conversion between LP01 and LP11b.
A mode rotator formed in the waveguide for mode-converting LP11a and LP11b;
Equipped with.

A fifth mode exchanger according to the present invention is
A two-mode multiplexer/demultiplexer having three demultiplexing ports and one multiplexing port;
A crossed waveguide section that connects the demultiplexing ports different from each other between the two mode multiplexers/demultiplexers at a ratio of 1:1;
Equipped with
The mode multiplexer/demultiplexer,
A first waveguide formed on a substrate, having one end connected to one of the demultiplexing ports and the other end connected to a multiplexing port and capable of propagating three modes of LP01, LP11a, and LP11b;
Two second waveguides arranged in parallel with the first waveguide on the substrate and having one ends respectively connected to the other two demultiplexing ports;
Two coupling parts each of which is composed of the first waveguide and the respective second waveguides on the substrate, and which transfers the optical power between the first waveguide and the second waveguide;
A mode rotator arranged on the first waveguide between the two coupling portions, for mode-converting LP11a and LP11b;
Equipped with
In the coupling portion, the effective refractive index of the LP01 mode propagating in the second waveguide and the effective refractive index of the LP11a mode propagating in the first waveguide match, and the length in the light propagation direction is the power of the light. Is a length that completely transitions from one waveguide to the other waveguide.

A fifth mode exchanger according to the present invention is
A waveguide formed on a substrate, having a constant height from the substrate, capable of propagating n (n is an integer of 4 or more) modes;
A grating portion formed in the waveguide, the width of which changes periodically,
A mode rotator formed in the waveguide and converting one of the degenerate modes into the other degenerate mode,
And converting each of the n input modes into a mode different from the input mode.

  Since the mode exchanger according to the present invention is capable of cyclic switching of modes, it is possible to compensate for the difference between the modes of DMD and various optical characteristics generated in the transmission line simply by inserting the mode exchanger in the transmission line. it can.

  The present invention, in a mode multiplex transmission system using a multi-mode optical fiber, compensates for a difference between modes of DMD and various optical characteristics generated in the transmission line without complicating the transmission line, and thus the transmission capacity of the communication system. It is possible to provide a mode exchanger that has little influence on the transmission distance.

It is a figure explaining the coupling part of the parallel waveguide in a mode exchanger. It is a figure explaining the structure of a mode rotor. It is a figure explaining the function of the mode exchanger concerning the present invention. It is a figure explaining the 1st structure of the mode exchanger which concerns on this invention. It is a figure explaining the 2nd structure of the mode exchanger which concerns on this invention. It is an example of the structural parameter in the 2nd structure of the mode exchanger which concerns on this invention. It is a figure explaining the propagation calculation result of the mode in the 2nd structure of the mode exchanger which concerns on this invention. It is a figure explaining the 3rd structure of the mode exchanger which concerns on this invention. It is an example of the structural parameter in the 3rd structure of the mode exchanger which concerns on this invention. It is a figure explaining the propagation calculation result of the mode in the 3rd structure of the mode exchanger which concerns on this invention. It is a figure explaining the 4th structure of the mode exchanger which concerns on this invention. It is a figure explaining the 5th structure of the mode exchanger which concerns on this invention. It is a figure explaining the structural example of the 3 mode multiplexer of the 5th structure of the mode exchanger which concerns on this invention. It is a figure explaining the structural example of the 3 mode branching device of the 5th structure of the mode exchanger which concerns on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In this specification and the drawings, constituent elements having the same reference numerals indicate the same elements.

(Mode multiplexer/demultiplexer)
The coupling unit included in the mode converter of this embodiment will be described. FIG. 1A is a diagram illustrating the configuration of a two-mode multiplexer/demultiplexer that is one of the coupling units. Here, a planar lightwave circuit (PLC) type mode multiplexer/demultiplexer using a rectangular embedded waveguide using a quartz material is assumed. The waveguide WG1 is a waveguide in which two modes propagate, and the waveguide WG2 is a waveguide in which one mode propagates. Note that here, the two modes indicate the LP01 mode and the first higher-order mode LP11 mode, which are basic modes.

  FIG. 1B is a conceptual diagram illustrating the relationship between the relative refractive index, the width, and the effective refractive index of the two waveguides. The two waveguides have a region L2 (coupling portion) that is partially close to each other. In the region L2, the waveguide width and the relative refractive index difference are adjusted so that the effective refractive index (neff) of the LP11 mode propagating in the waveguide WG1 and the effective refractive index propagating in the LP01 mode propagating in the waveguide WG2 match. There is. Generally, in order to match the effective refractive index between different LP modes, an asymmetric parallel waveguide structure having different waveguide structures is required.

  Since the effective refractive indices match, mode conversion occurs in a region where the waveguides are close to each other, and the light propagating through the waveguide WG2 has its power periodically transferred to the waveguide WG1. Here, if the length of the region L2 is adjusted so that the optical power of the waveguide WG2 is completely transferred to the waveguide WG1, the LP01 mode signal light incident from the Port1 is output to the Port4 as the LP11 mode of the waveguide WG1. To be done.

  On the other hand, the LP01 mode light incident from the Port2 does not cause mode conversion because the LP01 mode of the waveguide WG1 and the LP01 mode of the waveguide WG2 do not match in effective refractive index in the region L2. Is output to Port4. As a result, by injecting the LP01 mode optical signal into Port1 and Port2, an optical signal in which two modes (LP01 and LP11) are multiplexed in Port4 can be obtained. Further, by inputting the mode-multiplexed optical signals (LP01 and LP11) from Port4, the optical signals can be separated into Port1 and Port2. At this time, both the Port 1 optical signal and the Port 2 optical signal are in the LP01 mode. Thus, the PLC of FIG. 1 functions as a mode multiplexer. A detailed design method of the mode multiplexer/demultiplexer is described in Non-Patent Document 3.

  The higher-order mode coupled by the PLC of the structure of FIG. 1A is the LP11a mode having two peaks in the horizontal direction of the waveguide, and the LP11b mode having two peaks in the vertical direction is coupled to any mode. do not do. The reason for this is described in Non-Patent Document 4.

(Mode rotor)
When an optical signal in the LP11b mode is coupled by the PLC having the structure of FIG. 1A, it is necessary to once convert the LP11b mode into the LP11a mode. A mode rotator is used for the conversion. FIG. 2 is a diagram illustrating the structure of the mode rotor. The mode rotator is a waveguide having a width w and a height h, and includes a trench layer in which a groove is provided in at least a part of the waveguide in the longitudinal direction. The mode rotator of FIG. 2 shows an example in which a trench having a depth ht and a width wt is provided at the upper end of the waveguide. The position and shape of the groove vary depending on the conversion mode.

At this time, when the propagation constants of the LP11a and LP11b modes in the mode rotor are βa and βb, respectively, the length L of the mode rotor is (equation 1).
(Βa-βb) L=π
When the position and shape of the groove are adjusted so that the LP11a or LP11b mode input to the mode rotor is converted to LP11b or LP11a at the output end. A detailed design method of the mode rotor is described in Non-Patent Document 7.

(Embodiment)
The present invention combines a mode multiplexer/demultiplexer and a mode rotator in order to compensate for differences between modes of DMD and various optical characteristics generated in a transmission line, and a mode exchanger that cyclically switches between modes. I will provide a. FIG. 3 is a diagram for explaining the function of the mode exchanger according to the present invention. By inserting the mode exchanger according to the present invention in the middle of a few-mode fiber or a multimode fiber and cyclically exchanging between modes, the DMD and optical characteristic difference generated in the transmission path are compensated.

  For example, when the light propagation direction is from left to right in FIG. 3, LP01 incident on the mode exchanger may be mode-converted to LP11a, LP11a may be converted to LP11b, and LP11b may be converted to LP01. Further, the incident LP01 may be mode-converted to LP11b, LP11a to LP01, and LP11b to LP11a for output.

(Structure example 1)
FIG. 4 is a diagram for explaining the structure of the mode exchanger 301 of this embodiment. The mode exchanger 301 is
A first waveguide WG1 formed on a substrate and capable of propagating three modes LP01, LP11a, and LP11b;
A second waveguide WG2 arranged in parallel with the first waveguide WG1 on the substrate,
Three coupling parts (region II, region) which are composed of the first waveguide WG1 and the second waveguide WG2 on the substrate, and which transfer the optical power between the first waveguide WG1 and the second waveguide WG2. IV, region VI),
A mode rotator 50 which is connected to one end of the first waveguide WG1 and performs mode conversion between LP11a and LP11b;
Equipped with.
Two of the coupling portions (region II and region VI) have the same effective refractive index of the LP01 mode propagating in the second waveguide WG2 and the LP11a mode propagating in the first waveguide WG1. The length of the light in the propagation direction is the length l ₁ at which the power of the light is completely transferred from one waveguide to the other waveguide.
Of the coupling portions, the coupling portion (region IV) sandwiched between the two coupling portions has an effective refractive index of LP01 mode propagating in the first waveguide WG1 and an effective refractive index of LP01 mode propagating in the second waveguide WG2. And the length in the light propagation direction is the length l ₂ at which the power of the light is completely transferred from one waveguide to the other waveguide.

  In the mode exchanger 301, the heights of the respective waveguides are constant, the waveguide WG1 capable of propagating the three modes, the waveguide WG2 parallel to the waveguide WG1, and the waveguide WG1 arranged parallel to the waveguide WG1. The waveguide WG2 has three adjacent coupling portions (region II, region IV, and region VI).

  In the coupling portions existing in the regions II and VI, the coupling length is designed so as to convert between the LP11a of the waveguide WG1 and the LP01 mode of the waveguide WG2, like the mode multiplexer/demultiplexer described above. Has been done. In the region IV, the waveguide widths of the waveguide WG1 and the waveguide WG2 are designed to be equal. Further, in the region IV, the LP01 mode is converted between the waveguides WG1 and WG2 as the LP01 mode to the other waveguide, the LP11b mode of the waveguide WG1 is once converted to the LP11b of the waveguide WG2, and the waveguide is again guided. The coupling length is designed so that the waveguide WG1 is converted back to the LP11b mode.

  Further, in the curved waveguide and the waveguide width conversion section in the regions I, III, V, and VII, the propagating mode is bent at a sufficiently gentle ratio so as not to be converted into another mode, or the waveguide width is changed. Is designed to

  The mode rotor 50 is designed to perform mode conversion between the LP 11a and the LP 11b, similarly to the mode rotor described above.

The mode converter 301 operates as follows. Note that the light propagation direction will be described from left to right, but the same applies even if the directions are reversed.
(1) Light in LP01 Mode Light propagates in the waveguide WG1 in the LP01 mode in the regions I, II, and III. The light is converted into the LP01 mode of the waveguide WG2 at the coupling portion of the region IV, propagates through the waveguide WG2 in the LP01 mode in the region V, and is converted into the LP11a mode of the waveguide WG1 in the region VI. The light propagates through the waveguide WG1 in the LP11a mode in the region VII, is converted into the LP11b mode by the mode rotator 50, and is output.
(2) Light in LP11a Mode In region I, light propagates in the waveguide WG1 in LP11a mode. The light is converted to the LP01 mode of the waveguide WG2 at the coupling part of the region II, propagates in the LP01 mode of the waveguide WG2 at the region III, and is converted to the LP01 mode of the waveguide WG1 at the coupling part of the region IV. To be done. The light propagates in the waveguide WG1 in the LP01 mode in the regions V, VI, and VII, is not mode-converted by the mode rotator 50, and is output as the LP01 mode.
(3) Light of LP11b Mode Light propagates through the waveguide WG1 in the LP11b mode in the regions I, II, and III. The light is converted into the LP11b mode of the waveguide WG2 at the coupling part of the region IV, but is converted again into the LP11b mode of the waveguide WG1. Light propagates in the waveguide WG1 in the LP11b mode in the regions V, VI, and VII, is converted into the LP11a mode in the mode rotator 50, and is output.

  As described above, in the mode exchanger 301, the incident LP01 can be mode-converted into LP11b, LP11a into LP01, and LP11b into LP11a, and cyclic switching of modes can be realized.

(Structure example 2)
FIG. 5: is a figure explaining the structure of the mode exchanger 302 of this embodiment. The mode exchanger 302 is
A first waveguide WG1 formed on a substrate and capable of propagating three modes LP01, LP11a, and LP11b;
A second waveguide WG2 arranged in parallel with the first waveguide WG1 on the substrate,
A third waveguide WG3 arranged in parallel with the first waveguide WG1 on the substrate,
Three first coupling portions (B, B, which are composed of the first waveguide WG1 and the second waveguide WG2 on the substrate, and transfer the optical power between the first waveguide WG1 and the second waveguide WG2. C, D),
The first waveguide WG1 and the third waveguide WG3 on the substrate are arranged so as to sandwich the three first coupling portions (B, C, D), and the first waveguide WG1 and the third waveguide WG1 are provided. Two second coupling parts (A, E) for transferring the optical power to and from the WG3,
A first mode rotator 51 connected to one end of the first waveguide WG1 for mode conversion between LP11a and LP11b;
It is placed on the first waveguide WG1 so as to sandwich the three first coupling portions (B, C, D) between the two second coupling portions (A, E), and the LP11a and LP11b are mode-converted. Two second mode rotors (52, 53)
Equipped with.
Two coupling parts (B, D) of the first coupling parts (B, C, D) have an effective refractive index of the LP01 mode propagating in the second waveguide WG2 and an LP11a mode propagating in the first waveguide WG1. Have the same effective refractive index, and the length in the light propagation direction is the length at which the power of the light is completely transferred from one waveguide to the other waveguide.
The coupling portion C sandwiched between the two coupling portions (B, D) of the first coupling portions (B, C, D) has the effective refractive index of the LP01 mode propagating through the first waveguide WG1 and the second waveguide WG2. The effective refractive index of the LP01 mode propagating in the light is the same, and the length in the light propagation direction is the length at which the power of the light is completely transferred from one waveguide to the other waveguide.
In the two second coupling portions (A, E), the effective refractive index of the LP01 mode propagating in the third waveguide WG3 and the effective refractive index of the LP11a mode propagating in the first waveguide WG1 match, and the light propagation direction Is the length at which the power of the light is completely transferred from one waveguide to the other waveguide.

  In the mode exchanger 302, the heights of the respective waveguides are constant, the waveguide WG1 capable of propagating the three modes, the waveguides WG2 and WG3 parallel thereto, and the waveguide WG2 or the waveguide WG3. Has a coupling portion (A, B, C, D, E) formed in proximity to the waveguide WG1.

  In the coupling portion (A, B, D, E), conversion is performed between the LP11a of the waveguide WG1 and the LP01 mode of the waveguide WG2 or the waveguide WG3, as in the mode multiplexer/demultiplexer described above. The bond length is designed. Further, in the coupling portion C, in order to convert the LP01 mode propagating in the waveguide WG1 or the waveguide WG2 into the LP01 mode propagating in the other waveguide, the waveguide width of the waveguide WG1 or the waveguide WG2 is made equal. The coupling length may be designed so that the power transitions to the other waveguide.

  In the curved waveguide and the waveguide width converter in the waveguide WG2 and the waveguide WG3, the propagating mode is bent at a sufficiently gentle ratio so that the propagating mode is not converted into another mode, or the waveguide width changes. Is designed for.

  The mode rotors (51, 52, 53) are designed to perform mode conversion between the LP 11a and the LP 11b, like the mode rotors described above.

The mode converter 302 operates as follows. Note that the light propagation direction will be described from left to right, but the same applies even if the directions are reversed.
(1) Light in LP01 Mode Light propagates in the waveguide WG1 in LP01 mode up to the coupling portion C. The light is converted into the LP01 mode of the waveguide WG2 at the coupling portion C. The light propagates in the waveguide WG2 in the LP01 mode up to the coupling portion D, and is converted into the LP11a mode in the waveguide WG1 at the coupling portion D. The light is converted into LP11b by the mode rotator 53 and is output.
(2) Light of LP11a Mode Light is converted from the LP11a mode to the LP11b mode by the mode rotator 51, propagates in the waveguide WG1 in the coupling section A in the LP11b mode, and is converted to the LP11a mode by the mode rotator 52. .. The light is converted from the waveguide WG1 to the LP01 mode of the waveguide WG2 at the coupling portion B, and converted from the waveguide WG2 to the LP01 mode of the waveguide WG1 at the coupling portion C. The light propagates through the waveguide WG1 without being mode-converted by the coupling portions D and E and the rotor 53, and is output.
(3) Light of LP11b Mode Light is converted from LP11b mode to LP11a mode by the mode rotator 51, and is converted from the waveguide WG1 to the LP01 mode of the waveguide WG3 at the coupling portion A. The light propagates in the waveguide WG3 in the LP01 mode, is converted from the waveguide WG2 to the LP11a mode in the waveguide WG1 at the coupling portion E, and is output.

  As described above, the mode exchanger 302 can perform mode conversion of the incident LP01 into LP11b, LP11a into LP01, and LP11b into LP11a, and cyclic switching of modes can be realized.

Subsequently, propagation simulation of each mode is performed on the mode exchanger 302. FIG. 6 shows design parameters for each waveguide of the mode exchanger 302. FIG. 7 shows the result of the propagation simulation of each mode with this design parameter.
FIG. 7A shows how incident light in the LP01 mode is converted into the LP11b mode.
FIG. 7B shows how incident LP11a mode light is converted to LP01 mode.
FIG. 6C shows how incident LP11b mode light is converted into LP11a mode.

(Configuration example 3)
FIG. 8 is a diagram for explaining the structure of the mode exchanger 303 of this embodiment. The mode exchanger 303 is
A first waveguide WG1 formed on a substrate and capable of propagating three modes LP01, LP11a, and LP11b;
A second waveguide WG2 arranged in parallel with the first waveguide WG1 on the substrate,
A third waveguide WG3 arranged in parallel with the first waveguide WG1 on the substrate,
A coupling part B, a coupling part C, and a coupling which are composed of the first waveguide WG1 and the second waveguide WG2 on the substrate, and transfer the optical power between the first waveguide WG1 and the second waveguide WG2. Part E,
A coupling section A and a coupling section D which are composed of the first waveguide WG1 and the third waveguide WG3 on the substrate, and which transfer the optical power between the first waveguide WG1 and the third waveguide WG3;
Two mode rotators (54, 55) arranged on the first waveguide WG1 and mode-converting the LP11a and the LP11b.
Equipped with.
The coupling portion A, the coupling portion B, the coupling portion C, the coupling portion D and the coupling portion E are sequentially arranged from one end to the other end, and the mode rotor 54 is disposed between the coupling portion A and the coupling portion B. The mode rotor 55 is arranged between the coupling portion D and the coupling portion E.
The coupling portion A, the coupling portion B, the coupling portion D, and the coupling portion E are effective in the effective refractive index of the LP01 mode propagating in the second waveguide WG2 or the third waveguide WG3 and the effective refractive index of the LP11a mode propagating in the first waveguide WG1. The refractive indexes are the same, and the length in the light propagation direction is the length at which the power of the light is completely transferred from one waveguide to the other waveguide.
In the coupling portion C, the effective refractive index of the LP01 mode propagating in the first waveguide WG1 and the effective refractive index of the LP01 mode propagating in the second waveguide WG2 match, and the length in the light propagation direction is the power of the light. Is the length of complete transition from one waveguide to the other.

  In the mode exchanger 303, the height of the waveguide is constant, the waveguide WG1 capable of propagating the three modes, the waveguide WG2 and the waveguide WG3 parallel to the waveguide WG1, and the waveguide WG2 or the waveguide WG3 are guided. It has a coupling portion (A, B, C, D, E) that is formed close to the waveguide WG1.

  In the coupling portion (A, B, D, E), conversion is performed between the LP11a of the waveguide WG1 and the LP01 mode of the waveguide WG2 or the waveguide WG3, as in the mode multiplexer/demultiplexer described above. The bond length is designed. In the coupling portion C, the waveguide widths of the waveguides WG1 and WG3 are made equal in order to convert the LP01 mode propagating in the waveguide WG1 or the waveguide WG3 into the LP01 mode propagating in the other waveguide. The coupling length may be designed so that the power transitions to the other waveguide.

  In the curved waveguide and the waveguide width converter in the waveguide WG2 and the waveguide WG3, the propagating mode is bent at a sufficiently gentle ratio so that the propagating mode is not converted into another mode, or the waveguide width changes. Is designed for.

  The mode rotors (54, 55) are designed to perform mode conversion between the LP 11a and the LP 11b, similarly to the mode rotors described above.

The mode converter 303 operates as follows. Note that the light propagation direction will be described from left to right, but the same applies even if the directions are reversed.
(1) Light in LP01 Mode Light propagates in the waveguide WG1 in LP01 mode up to the coupling portion C. The light is converted into the LP01 mode of the waveguide WG2 at the coupling portion C. The light propagates in the waveguide WG2 in the LP01 mode up to the coupling portion E, is converted into the LP11a mode of the waveguide WG1 in the coupling portion E, and is output.
(2) Light of LP11a Mode Light is converted from the waveguide WG1 to the LP01 mode of the waveguide WG2 at the coupling portion A. Light propagates in the waveguide WG3 in the LP01 mode, and is converted from the waveguide WG3 to the LP11a mode in the waveguide WG1 at the coupling portion D. The light is converted from the LP11a mode to the LP11b mode by the mode rotator 55 and output.
(3) Light of LP11b Mode Light propagates in the waveguide WG1 without being mode-converted in the coupling portion A, is converted from the LP11b mode to the LP11a mode by the mode rotator 54, and is guided in the coupling portion B from the waveguide WG1. It is converted to the LP01 mode of WG2. Light propagates in the waveguide WG2 in the LP01 mode to the coupling portion C, and is converted in the coupling portion C from the waveguide WG2 to the LP01 mode of the waveguide WG1. The light propagates through the waveguide WG1 without being subjected to mode conversion in the coupling portions D and E and the rotor 55, and is output.

  As described above, the mode exchanger 303 can perform mode conversion of the incident LP01 into LP11a, LP11a into LP11b, and LP11b into LP01, so that cyclic switching of modes can be realized.

Then, propagation simulation of each mode is performed for the mode exchanger 303. FIG. 9 shows design parameters for each waveguide of the mode exchanger 303. FIG. 10 shows the result of the propagation simulation of each mode with this design parameter.
FIG. 10A shows how incident light in the LP01 mode is converted into the LP11a mode.
FIG. 10B shows how incident light of LP11a mode is converted into LP11b mode.
FIG. 10C shows how incident LP11b mode light is converted into LP01 mode.

(Structure example 4)
FIG. 11 is a diagram for explaining the structure of the mode exchanger 304 of this embodiment. The mode exchanger 304 is
One waveguide WG formed on the substrate and capable of propagating three modes of LP01, LP11a, and LP11b,
A grating unit 41 formed in the waveguide WG and having a width that periodically changes to perform mode conversion between LP01 and LP11a, or a height that changes periodically to perform mode conversion between LP01 and LP11b,
A mode rotator 56 formed in the waveguide GW for mode-converting LP11a and LP11b;
Equipped with.

  The grating part 41 is designed so that the width of the waveguide periodically changes the width and LP01 and LP11a are converted, and the mode rotor 56 is similar to the mode rotators described above, and the LP11a and LP11b are the same. Designed to convert between modes.

The mode converter 304 operates as follows. Note that the light propagation direction will be described from left to right, but the same applies even if the directions are reversed.
(1) Light in LP01 Mode Light is converted into LP11a mode in the grating section 41, and converted from LP11a mode into LP11b mode in the mode rotator 56.
(2) Light in LP11a Mode Light is converted into LP01 mode in the grating section 41 and is converted into LP01 mode in the mode rotator 56 without any mode conversion.
(3) Light in LP11b Mode Light is not converted into any mode in the grating section 41, and is converted from LP11b mode to LP11a mode in the mode rotator 56.

  As described above, the mode exchanger 304 can perform mode conversion of the incident LP01 into LP11b, LP11a into LP01, and LP11b into LP11a, and cyclic switching of modes can be realized.

  It should be noted that the grating portion 41 performs the above cyclic exchange in the case of a structure in which the width of the waveguide in the horizontal direction changes, but the LP01 mode in the case of a structure in which the height in the vertical direction periodically changes. And the LP11b mode is converted. Therefore, the mode exchanger 304 mode-converts the incident LP01 to LP11a, LP11a to LP11b, and LP11b to LP01 to realize the cyclic exchange of modes.

  Note that the fluctuation period of the waveguide structure of the grating portion 41 should be such that, as shown in Non-Patent Document 8, a refractive index fluctuation of a specific period according to the effective refractive index of the mode 1 and the mode 2 is given at the wavelength λ. Then, the signal propagated in each mode can be converted.

The spacing Λ of the fiber gratings of the grating unit 41 is determined by the waveguide structure parameter used, the wavelength, and the mode order to be converted. The grating spacing Λ for mode conversion is given by equation (2). In Equation (2), n ₁ and n ₂ represent the effective refractive index of mode 1 and the effective refractive index of mode 2, respectively, at the wavelength λ. After determining the wavelength λ to be used, numerical analysis is performed from the waveguide structure parameters, the effective refractive index of each mode is calculated, and n ₁ and n ₂ obtained by the calculation are substituted into the equation (2) to obtain the required grating. The period Λ can be determined.

  Further, regarding the length of the grating portion 41, the function of mode exchange can be realized by calculating the coupling length and designing it appropriately.

Further, a mode exchanger capable of exchanging modes of four or more modes can be realized with the same structure. That is, the mode exchanger is
A waveguide formed on a substrate, having a constant height from the substrate, capable of propagating n (n is an integer of 4 or more) modes;
A grating portion 41 formed in the waveguide, the width of which changes periodically;
A mode rotator 56 formed in the waveguide for converting one of the degenerate modes into the other degenerate mode;
And converting each of the n input modes into a mode different from the input mode.

  For example, in the case of a mode exchanger capable of exchanging 6 modes, an LPG for exchanging any two modes of LP01, LP11a/LP11b, LP21a/LP21b, and LP02 modes is designed based on equation (1), and all modes are cyclic. A plurality of LPGs are connected in multiple stages so as to be exchanged to form the grating portion 41. By connecting such LPGs, the number of modes can be easily expanded. Further, the mode exchanger has, as the mode rotor 56, a mode rotating unit for LP11a/LP11b and a mode rotating unit for LP21a/LP21b.

(Structure example 5)
FIG. 12 is a diagram for explaining the structure of the mode exchanger 305 of this embodiment. The mode exchanger 305 has a cross waveguide portion 103 between the three-mode demultiplexer 101 and the three-mode multiplexer 102. Specifically, the mode exchanger 305 is
Two mode multiplexers/demultiplexers (101, 102) having three demultiplexing ports (DP1, DP2, DP3) and one multiplexing port MP1;
Between the two mode multiplexers/demultiplexers (101, 102), the crossing waveguide portion 103 that connects the demultiplexing ports different from each other at a ratio of 1:1;
Equipped with.

  It should be noted that "to connect the demultiplexing ports different from each other between the two mode demultiplexers (101, 102) at a ratio of 1:1" means, for example, that the demultiplexing port DP2 of the demultiplexer 101 is combined. The demultiplexer port DP3 of the demultiplexer 102 is connected, the demultiplexing port DP1 of the demultiplexer 101 and the demultiplexing port DP2 of the demultiplexer 102 are connected, and the demultiplexing port DP3 of the demultiplexer 101 and the multiplexer 102 are connected. This means connecting with the demultiplexing port DP1. Mode switching can be realized by connecting each of the three output waveguides from the mode demultiplexer 101 to a demultiplexing port of the mode multiplexer 102 different from the demultiplexing port of the mode demultiplexer 101.

The mode multiplexer/demultiplexer (101, 102)
A first waveguide WG1 formed on a substrate, having one end connected to one demultiplexing port DP1 and the other end connected to a multiplexing port MP1 and capable of propagating three modes of LP01, LP11a, and LP11b;
Two second waveguides (WG2, WG3), which are arranged in parallel with the first waveguide WG1 on the substrate and have one ends respectively connected to the other two demultiplexing ports (DP2, DP3);
It is composed of the first waveguide WG1 on the substrate and the respective second waveguides (WG2, WG3). Light power is generated between the first waveguide WG1 and the second waveguide (WG2, WG3). Two transitions (A, B) that migrate,
A mode rotator 57 which is arranged on the first waveguide WG1 between the two coupling portions (A, B) and which performs mode conversion between LP11a and LP11b;
Equipped with.
In the coupling portions (A, B), the effective refractive index of the LP01 mode propagating in the second waveguides (WG2, WG3) and the effective refractive index of the LP11a mode propagating in the first waveguide WG1 match, and the propagation direction of light Is the length at which the power of the light is completely transferred from one waveguide to the other waveguide.

As the mode multiplexer 102, for example, the configuration of FIG. 13 can be used.
It is composed of a waveguide WG1 capable of propagating three modes and waveguides (WG2, WG3) arranged in parallel with the waveguide WG1. At the coupling portion (A, B) where the waveguide WG1 and the waveguides (WG2, WG3) are close to each other, the LP11a mode of the waveguide WG1 and the LP01 mode of the waveguide (WG2, WG3) are mode-converted. The mode rotor 57 is mode-converted between the LP11a mode and the LP11b mode.

The mode multiplexer 102 operates as follows.
The LP01 mode light input to the demultiplexing port DP1 propagates through the waveguide WG1 in the LP01 mode, and is output from the multiplexing port MP1.
The LP01 mode light input to the demultiplexing port DP2 is converted from the waveguide WG2 to the LP11a mode of the waveguide WG1 at the coupling portion A. The light is converted from the LP11a mode to the LP11b mode by the mode rotator 57 and output from the multiplexing port MP1.
The LP01 mode light input to the demultiplexing port DP3 is converted from the waveguide WG3 to the LP11a mode of the waveguide WG1 at the coupling portion B, and output from the multiplexing port MP1.

  The configuration of FIG. 14 can be used as the mode demultiplexer 101. This configuration is an inversion of the configuration of the mode multiplexer 102. The LP01, LP11a, and LP11b mode light input from the multiplexing port MP1 propagates in the LP01 mode in the waveguide WG1 and is split into the demultiplexing port DP1. , And propagates in the waveguide WG3 in the LP01 mode and propagates in the demultiplexing port DP3, and propagates in the waveguide WG2 in the LP01 mode and is output to the demultiplexing port DP2.

  In the cross waveguide section 103, the mode converter 305 applies the lights from the demultiplexing ports (DP1, DP2, DP3) of the mode demultiplexer 101 to the demultiplexing ports (DP2, DP3, DP1) of the mode multiplexer 102, respectively. Alternatively, it is connected to (DP3, DP1, DP2) to realize cyclic switching of modes.

  Although the example in which the number of modes is 3 has been shown in the present embodiment, the number of propagation modes in the input/output waveguide is n (n is an integer of 3 or more), and the number of parallel waveguides is desired to be exchanged. By installing the same number, the number of corresponding modes can be increased with a configuration similar to that of the first to fifth embodiments. For example, the mode multiplexer/demultiplexer described in the fifth embodiment is an n-mode multiplexer/demultiplexer and the number of crossed waveguides is n, whereby an n-mode exchanger can be easily realized.

(Other embodiments)
In addition, in the above-mentioned embodiment, the configuration example of the planar lightwave circuit using the SiO ₂ -based material (quartz-based material) has been described, but the material may of course be another material. For example, even in a planar lightwave circuit using a semiconductor such as Si-based or InGaAsP, or an organic substance such as a polymer, the same effect as that of the embodiment described in this specification can be obtained. Also, regarding the wavelength band to be used, although it is set to about 1.5 to 1.6 μm in the above embodiment, it may be in the mid-infrared region (2 μm or more) having a longer wavelength or in the visible light band.

[Appendix]
The following describes the mode converter of this embodiment.
The present invention combines a parallel waveguide, a coupling portion where the waveguides are close to each other, and a mode rotator to cyclically convert a propagation mode, thereby performing DMD between nodes and mode exchange that guarantees various optical characteristics. It is a vessel. The configuration of this mode exchanger is as follows.

(1):
A PLC type device arranged in a transmission system using a three-mode fiber as a transmission line,
The height of the waveguide is constant,
A waveguide WG1 capable of propagating the three modes,
A waveguide WG2 parallel to the waveguide WG1,
The waveguide WG1 and the waveguide WG2 arranged in parallel have three coupling portions in proximity to each other,
The waveguide WG2 has a region in which the waveguide width is the same as that of the waveguide WG1,
The waveguide WG1 includes a mode rotator for converting the LP11a mode to the LP11b mode in the region after the coupling portion,
The LP11a mode propagating through the waveguide WG1 in the first coupling portion is converted into the LP01 mode of the waveguide WG2,
The LP01 mode propagating in the waveguide WG1 at the second coupling portion is converted into the LP01 mode of the waveguide WG2,
The LP01 mode propagating through the waveguide WG2 is converted into the LP01 mode of the waveguide WG1,
By converting the LP01 mode propagating through the waveguide WG2 in the third coupling portion into the LP11a mode of the waveguide WG1,
It is a mode exchanger characterized by converting the input LP01 into LP11b, LP11a into LP01, and LP11b into LP11a for output.

(2):
A PLC type device arranged in a transmission system using a three-mode fiber as a transmission line,
The height of the waveguide is constant,
A waveguide WG1 capable of propagating the three modes, and waveguides WG2 and WG3 parallel to the waveguide WG1,
Each of the waveguides WG1 and WG2, WG3 arranged in parallel has five coupling portions in proximity to each other,
The waveguide WG1 includes a mode rotator for converting the LP11a mode to the LP11b mode,
Having a total of three mode rotors in front of and behind the joint A and in front of the joint E,
The LP11a mode propagating through the waveguide WG1 in the coupling portion A is converted into the LP01 mode of the waveguide WG2,
The LP11a mode propagating in the waveguide WG1 at the coupling portion B is converted into the LP01 mode of the waveguide WG3,
The LP01 mode propagating through the waveguide WG1 or 3 in the coupling portion C is converted into the LP01 mode of the waveguide WG3 or 1,
The LP01 mode propagating through the waveguide WG3 in the coupling portion D is converted into the LP11a mode of the waveguide WG1,
By converting the LP01 mode propagating through the waveguide WG2 in the coupling portion E into the LP11a mode of the waveguide WG1,
It is a mode exchanger characterized by converting the input LP01 into LP11b, LP11a into LP01, and LP11b into LP11a for output.

(3):
A PLC type device arranged in a transmission system using a three-mode fiber as a transmission line,
The height of the waveguide is constant,
A waveguide WG1 capable of propagating the three modes, and waveguides WG2, 3 parallel to the waveguide WG1,
Each of the waveguides WG1 and WG2, WG3 arranged in parallel has five coupling portions in proximity to each other,
The waveguide WG1 includes a mode rotator for converting the LP11a mode to the LP11b mode,
Having a total of two mode rotors after the connection A and after the connection D,
The LP11a mode propagating through the waveguide WG1 in the coupling portion A is converted into the LP01 mode of the waveguide WG2,
The LP11a mode propagating in the waveguide WG1 at the coupling portion B is converted into the LP01 mode of the waveguide WG3,
The LP01 mode propagating through the waveguide WG1 or 3 in the coupling portion C is converted into the LP01 mode of the waveguide WG3 or 1,
The LP01 mode propagating through the waveguide WG2 at the coupling portion D is converted into the LP11a mode of the waveguide WG1,
By converting the LP01 mode propagating through the waveguide WG3 in the coupling portion E into the LP11a mode of the waveguide WG1,
It is a mode exchanger characterized by converting the input LP01 into LP11b, LP11a into LP01, and LP11b into LP11a for output.

(4):
A PLC type device arranged in a transmission system using a three-mode fiber as a transmission line,
The height of the waveguide is constant,
In a waveguide that can propagate three modes,
A grating waveguide portion in which the waveguide width changes periodically,
Includes a mode rotator that converts LP11a mode to LP11b mode,
Designed to convert LP01 and LP01a in the grating waveguide section,
It is a mode exchanger characterized by converting the input LP01 into LP11b, LP11a into LP01, and LP11b into LP11a for output.

(5):
A PLC type device arranged in a transmission system using a three-mode fiber as a transmission line,
The height of the waveguide is constant,
A waveguide WG1 capable of propagating the three modes, and waveguides WG2, 3 parallel to the waveguide WG1,
Each of the waveguides WG1 and WG2, WG3 arranged in parallel has two coupling portions in proximity to each other,
A mode rotator for converting the LP11a mode to the LP11b mode after the first coupling part of the waveguide WG1;
The two coupling portions are converted into the LP11a mode of the waveguide WG1 and the LP01 mode of the other waveguide,
Alternatively, the LP01 mode guided in the waveguides WG2, 3 is designed to be converted into the LP11a mode in the waveguide WG1,
A mode multiplexer that demultiplexes LP01, LP11a, and LP11b modes propagating in the waveguide into the waveguides WG1, WG2, and 3, respectively, as LP01 mode,
A mode multiplexer/demultiplexer having the same design, in which the LP01 mode input from the waveguides WG1, 2, and 3 is multiplexed into the waveguide WG1 as LP01, LP11a, and LP11b modes,
The output waveguides WG1, 2, 3 of the mode demultiplexer have a crossed waveguide structure connected to the input waveguides WG2, 3, 1 or 3, 1, 2 of the mode multiplexer. It is a mode exchanger that does.

  INDUSTRIAL APPLICABILITY The present invention can realize large-capacity/long-distance communication in a mode multiplex transmission system using a multimode optical fiber.

41: Grating unit 50-57: Mode rotator 101: 3-mode demultiplexer 102: 3-mode multiplexer 103: Crossing waveguide unit 301-305: Mode converter

Claims (3)

A first waveguide that is formed on the substrate and can propagate three modes of LP01, LP11a, and LP11b;
A second waveguide arranged on the substrate in parallel with the first waveguide;
Three coupling portions which are composed of the first waveguide and the second waveguide on the substrate, and which transfer the optical power between the first waveguide and the second waveguide;
A mode rotator that is connected to one end of the first waveguide and performs mode conversion between LP11a and LP11b;
Equipped with
In two of the coupling portions, the effective refractive index of the LP01 mode propagating in the second waveguide and the effective refractive index of the LP11a mode propagating in the first waveguide are the same, and The length is a length at which the power of the light is completely transferred from one waveguide to the other waveguide,
In the coupling section sandwiched between the two coupling sections of the coupling section, the effective refractive index of the LP01 mode propagating in the first waveguide and the effective refractive index of the LP01 mode propagating in the second waveguide match, A mode converter characterized in that the length of the light in the propagation direction is such that the power of the light is completely transferred from one waveguide to the other waveguide. A first waveguide that is formed on the substrate and can propagate three modes of LP01, LP11a, and LP11b;
A second waveguide arranged on the substrate in parallel with the first waveguide;
A third waveguide arranged in parallel with the first waveguide on the substrate;
Three first coupling portions that are composed of the first waveguide and the second waveguide on the substrate, and that transfer the optical power between the first waveguide and the second waveguide,
It is composed of the first waveguide and the third waveguide on the substrate, is arranged so as to sandwich the three first coupling portions, and the light is provided between the first waveguide and the third waveguide. Two second couplings that transfer the power of
A first mode rotor connected to one end of the first waveguide for mode conversion between LP11a and LP11b;
Two second mode rotators that are between the two second coupling portions and that are arranged on the first waveguide so as to sandwich the three first coupling portions and that perform mode conversion between LP11a and LP11b;
Equipped with
In two of the first coupling portions, the effective refractive index of the LP01 mode propagating in the second waveguide and the effective refractive index of the LP11a mode propagating in the first waveguide match each other, so that light propagation The length in the direction is a length at which the power of the light is completely transferred from one waveguide to the other waveguide,
The coupling portion sandwiched between the two coupling portions of the first coupling portion has an effective refractive index of the LP01 mode propagating in the first waveguide and an effective refractive index of the LP01 mode propagating in the second waveguide are equal to each other. However, the length of the light in the propagation direction is the length at which the power of the light is completely transferred from one waveguide to the other waveguide,
The two second coupling portions have the same effective refractive index in the LP01 mode propagating in the third waveguide and the effective refractive index in the LP11a mode propagating in the first waveguide, and have a length in the light propagation direction. A mode converter characterized in that it has a length such that the power of the light is completely transferred from one waveguide to the other waveguide. A first waveguide that is formed on the substrate and can propagate three modes of LP01, LP11a, and LP11b;
A second waveguide arranged on the substrate in parallel with the first waveguide;
A third waveguide arranged in parallel with the first waveguide on the substrate;
A coupling part B, a coupling part C, and a coupling which are composed of the first waveguide and the second waveguide on the substrate and which transfer the power of light between the first waveguide and the second waveguide. Part E,
A coupling section A and a coupling section D which are composed of the first waveguide and the third waveguide on the substrate, and which transfer the optical power between the first waveguide and the third waveguide;
Two mode rotators arranged on the first waveguide and mode-converting LP11a and LP11b;
Equipped with
The coupling part A, the coupling part B, the coupling part C, the coupling part D and the coupling part E are sequentially arranged from one end to the other end, and one of the two mode rotors is Is disposed between the coupling portion A and the coupling portion B, and the other of the two mode rotors is disposed between the coupling portion D and the coupling portion E,
The coupling portion A, the coupling portion B, the coupling portion D, and the coupling portion E propagate the effective refractive index of the LP01 mode propagating in the second waveguide or the third waveguide and the first waveguide. The effective refractive indices of the LP11a modes are the same, and the length in the light propagation direction is the length at which the power of the light is completely transferred from one waveguide to the other waveguide,
In the coupling portion C, the effective refractive index of the LP01 mode propagating in the first waveguide and the effective refractive index of the LP01 mode propagating in the second waveguide match, and the length in the light propagation direction is equal to that of the light. A mode converter characterized in that the length is such that power is completely transferred from one waveguide to the other waveguide.