JP2018146754A - Mode exchanger - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mode exchanger that compensates for DMD (group delay difference among low modes) occurring on a transmission path and difference between modes of various optical characteristics without making the transmission path complicated in a mode multiplexing transmission system using a multimode optical fiber and thereby has little impact on the transmission capacity or transmission distance of a communication system.SOLUTION: A mode exchanger 301 has a coupling part coupling between different modes by arrangement of waveguides and a mode rotor 50 made on a planar light-wave circuit (PLC), and makes mode exchanges between input signal lights with different modes in a cyclic manner. Because the mode exchanger can make cyclic exchange of modes, it can compensate for DMD and difference among modes of various optical characteristics occurring on the transmission path by only inserting the mode exchanger into the transmission path.SELECTED DRAWING: Figure 4

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 optical fiber communication systems, non-linear effects and fiber fuses that occur in optical fibers are problematic, and 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, bending loss reduction, single mode operation area expansion, and effective area expansion are in a trade-off relationship with each other, and there is a problem that the amount of effective area expansion under certain conditions is limited. Therefore, a mode multiplex transmission system that uses a multimode fiber as a transmission fiber and performs parallel transmission using a plurality of modes to propagate has been studied as a technology for realizing a dramatic increase in capacity (for example, non-patent literature). 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 (see, for example, Non-Patent Documents 3 and 4). ).

  In addition, optical MIMO transmission has been proposed in which coupling between modes occurring in a transmission path is compensated by MIMO signal processing at the receiving end. Moreover, since the load of MIMO signal processing increases when the group delay difference between modes is large, studies have been made on low mode group delay difference (DMD) fibers (see, for example, Non-Patent Documents 5 and 6). .)

T.A. Matsui, et al. , “Applicability of Photonic Crystal Fiber With Uniform Air-Hole Structure to High-Speed and Wide-Band Transmission OverTownConventionmm. Lightwave Technol. 27, 5410-5416, 2009. N. Hanzawa et al. , “Demonstration of mode-division multiplexing transmission 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 transmission", OFC2012, OTU1l. 4). N. Hanzawa et al, "Mode multi / demultiplexing with parallel waveguide for mode division multiplexed transmission", Opt. Express vol. 22, pp. 29321-29330 (2014) R. Ryf et al. “Mode-division multiplexing over 96 km of two-mode fiber using coherent 6 × 6 MIMO processing”, J. et al. Lighttw. Technol. , Vol. 30, pp. 521-531 (2012). T.A. Mori et al. “Few-mode fibers supporting more than two LPs for mode-division-multiplexed transmission with MIMO DSP”, J. Lighttw. Technol. , Vol. 32, pp. 2468-2479 (2014). : K. Saith et al. “PLC-based LP11 mode rotator for mode-division 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 to the reduction in fiber DMD due to fiber manufacturing errors. Furthermore, in the DMD compensation transmission line described in Non-Patent Document 6, two types of optical fibers are required, and there is 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 also different between modes, and this mode-dependent characteristic difference can be obtained even if MIMO signal processing is performed. Incurs quality degradation. That is, the deterioration of the transmission quality in the transmission path using two types of optical fibers has a problem of limiting the transmission capacity and 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, which does not complicate the transmission path, and does not complicate the DMD generated in the transmission path or between modes of various optical characteristics. An object of the present invention is to provide a mode exchanger that compensates for the difference and has little influence on the transmission capacity and transmission distance of a communication system.

  In order to achieve the above object, a mode converter according to the present invention has a coupling part or mode rotator that couples between different modes depending on the arrangement of waveguides in a planar optical waveguide (PLC) for input. The mode is cyclically exchanged between signal lights having different modes.

Specifically, the first mode exchanger according to the present invention is:
A first waveguide formed on a substrate and capable of propagating three modes LP01, LP11a and LP11b;
A second waveguide in parallel with the first waveguide on the substrate;
Three coupling portions configured by the first waveguide and the second waveguide on the substrate and transferring light power between the first waveguide and the second waveguide;
A mode rotator connected to one end of the first waveguide and mode-converting LP11a and LP11b;
With
Two of the coupling portions have an effective refractive index of the LP01 mode propagating through the second waveguide and an effective refractive index of the LP11a mode propagating through the first waveguide, and are in the light propagation direction. The length is a length that completely transfers the light power from one waveguide to the other,
The coupling part sandwiched between the two coupling parts among the coupling parts has an effective refractive index of the LP01 mode propagating through the first waveguide and an effective refractive index of the LP01 mode propagating through the second waveguide, The length of the light propagation direction is a length that completely transfers the light power from one waveguide to the other waveguide.

The second mode exchanger according to the present invention is:
A first waveguide formed on a substrate and capable of propagating three modes LP01, LP11a and LP11b;
A second waveguide in parallel with the first waveguide on the substrate;
A third waveguide parallel to the first waveguide on the substrate;
Three first coupling portions configured by the first waveguide and the second waveguide on the substrate and transferring light power between the first waveguide and the second waveguide;
The first waveguide and the third waveguide on the substrate are arranged so as to sandwich the three first coupling portions, and light is transmitted between the first waveguide and the third waveguide. Two second coupling parts that transfer the power of
A first mode rotor connected to one end of the first waveguide and mode-converting LP11a and LP11b;
Two second mode rotators located between the two second coupling portions and disposed on the first waveguide so as to sandwich the three first coupling portions, and mode-converting LP11a and LP11b;
With
Two of the first coupling portions have the same effective refractive index of the LP01 mode propagating through the second waveguide and the effective refractive index of the LP11a mode propagating through the first waveguide, and light propagation. The length of the direction is a length that completely transfers the light power from one waveguide to the other,
Of the first coupling portion, the coupling portion sandwiched between the two coupling portions has an effective refractive index of the LP01 mode propagating through the first waveguide and an effective refractive index of the LP01 mode propagating through the second waveguide. In addition, the length of the light propagation direction is a length that completely transfers the light power from one waveguide to the other,
In the two second coupling portions, the effective refractive index of the LP01 mode propagating through the third waveguide matches the effective refractive index of the LP11a mode propagating through the first waveguide, and the length in the light propagation direction is the same. The length of the light is such that the power of the light is completely transferred from one waveguide to the other.

The third mode exchanger according to the present invention is:
A first waveguide formed on a substrate and capable of propagating three modes LP01, LP11a and LP11b;
A second waveguide in parallel with the first waveguide on the substrate;
A third waveguide parallel to the first waveguide on the substrate;
A coupling portion B, a coupling portion C, and a coupling, which are configured by the first waveguide and the second waveguide on the substrate and transfer light power between the first waveguide and the second waveguide. Part E,
A coupling portion A and a coupling portion D configured by the first waveguide and the third waveguide on the substrate and transferring light power between the first waveguide and the third waveguide;
Two mode rotators disposed on the first waveguide and mode-converting LP11a and LP11b;
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 one of the two mode rotors is the It is arranged between the coupling part A and the coupling part B, and the other of the two mode rotors is arranged between the coupling part D and the coupling part E,
The coupling unit A, the coupling unit B, the coupling unit D, and the coupling unit E propagate the effective refractive index of the LP01 mode that propagates in the second waveguide or the third waveguide and the first waveguide. The effective refractive index of the LP11a mode is the same, and the length of the light propagation direction is a length that completely transfers the light power from one waveguide to the other waveguide,
In the coupling part C, the effective refractive index of the LP01 mode propagating through the first waveguide matches the effective refractive index of the LP01 mode propagating through the second waveguide, and the length of the light propagation direction is equal to that of the light. The length is such that the power is completely transferred from one waveguide to the other.

The fourth mode exchanger according to the present invention is:
One waveguide formed on the substrate and capable of propagating the three modes LP01, LP11a and LP11b;
A grating portion formed in the waveguide and mode-converting LP01 and LP11a by periodically changing the width, or mode-converting LP01 and LP11b by periodically changing the height;
A mode rotator formed in the waveguide and mode-converting LP11a and LP11b;
Is provided.

The fifth mode exchanger according to the present invention is:
Two mode multiplexer / demultiplexers having three demultiplexing ports and one multiplexing port;
A cross waveguide section for connecting the different demultiplexing ports in a 1: 1 ratio between the two mode multiplexers and demultiplexers;
With
The mode multiplexer / demultiplexer is
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 LP01, LP11a, and LP11b;
Two second waveguides parallel to the first waveguide on the substrate and having one end connected to the other two demultiplexing ports,
Two coupling portions configured by the first waveguide on the substrate and the respective second waveguides, and transferring light power between the first waveguide and the second waveguide;
A mode rotator disposed on the first waveguide between the two coupling portions and mode-converting LP11a and LP11b;
With
In the coupling portion, the effective refractive index of the LP01 mode propagating through the second waveguide matches the effective refractive index of the LP11a mode propagating through the first waveguide, and the length of the light propagation direction is the power of the light. The length is such that it completely shifts from one waveguide to the other.

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

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

  The present invention relates to a transmission capacity of a communication system in a mode multiplexing transmission system using a multimode optical fiber, compensating for differences between modes of DMD and various optical characteristics generated in the transmission path without complicating the transmission path. It is possible to provide a mode exchanger with little influence on the transmission distance.

It is a figure explaining the coupling | bond 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 which concerns on this 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 composition of the mode exchanger concerning the present 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 composition of the mode exchanger concerning the present invention. It is a figure explaining the 4th composition of the mode exchanger concerning the present 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 splitter 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 the present specification and drawings, the same reference numerals denote the same components.

(Mode multiplexer / demultiplexer)
A coupling unit included in the mode converter of this embodiment will be described. FIG. 1A illustrates a 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-based material is assumed. The waveguide WG1 is a waveguide through which two modes propagate, and the waveguide WG2 is a waveguide through which one mode propagates. Here, the two modes are 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, width, and effective refractive index of two waveguides. The two waveguides have a region L2 (coupling portion) that is partially adjacent. 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 through the waveguide WG1 and the effective refractive index propagating through the LP01 mode propagating through the waveguide WG2 coincide. Yes. In general, in order to match the effective refractive index between different LP modes, an asymmetric parallel waveguide structure in which the waveguide structures are not the same is required.

  By matching the effective refractive indexes, mode conversion occurs in a region where the waveguides are close to each other, and light propagating through the waveguide WG2 is periodically transferred to the waveguide WG1. Here, when the length of the region L2 is adjusted so that the light 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. Is done.

  On the other hand, the LP01 mode light incident from the Port 2 does not cause mode conversion because the effective refractive index of the LP01 mode of the waveguide WG1 and the LP01 mode of the waveguide WG2 does not match in the region L2, and the LP01 mode Is output to Port4. As a result, an optical signal in which the two modes (LP01 and LP11) are multiplexed in Port 4 can be obtained by making the LP01 mode optical signal incident on Port1 and Port2. In addition, by inputting mode-multiplexed optical signals (LP01 and LP11) from Port 4, the optical signals can be separated into Port 1 and Port 2, respectively. 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 having the structure of FIG. 1A is an 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 is described in Non-Patent Document 4.

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

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 relates to a mode exchanger that combines these mode multiplexers / demultiplexers and mode rotators in order to compensate for differences between modes of DMD and various optical characteristics generated in a transmission path, and cyclically exchanges the modes. I will provide a. FIG. 3 is a diagram illustrating the function of the mode exchanger according to the present invention. A mode exchanger according to the present invention is inserted in the middle of several mode fiber or multimode fiber, and the mode is cyclically exchanged to compensate for DMD and optical characteristic difference generated in the transmission path.

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

(Configuration example 1)
FIG. 4 is a diagram for explaining the structure of the mode exchanger 301 of the present 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 parallel to the first waveguide WG1 on the substrate;
Three coupling portions (region II, region) configured by the first waveguide WG1 and the second waveguide WG2 on the substrate and transferring light power between the first waveguide WG1 and the second waveguide WG2. IV, region VI)
A mode rotor 50 connected to one end of the first waveguide WG1 and mode-converting LP11a and LP11b;
Is provided.
Two of the coupling portions (region II and region VI) have the same effective refractive index of the LP01 mode propagating through the second waveguide WG2 and the effective refractive index of the LP11a mode propagating through the first waveguide WG1. In addition, the length in the light propagation direction is the length l ₁ at which the power of the light is completely transferred from one waveguide to the other.
Of the coupling parts, the coupling part (region IV) sandwiched between the two coupling parts has an effective refractive index of the LP01 mode propagating through the first waveguide WG1 and an effective refractive index of the LP01 mode propagating through the second waveguide WG2. And the length in the light propagation direction is the length l ₂ that completely transfers the light power from one waveguide to the other.

  In the mode exchanger 301, the height of each waveguide is constant, the waveguide WG1 capable of propagating the three modes, the waveguide WG2 parallel to the waveguide WG1, and the waveguide WG1 arranged in parallel are guided. There are three coupling portions (region II, region IV, region VI) close to the waveguide WG2.

  In the coupling part 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, similarly to the mode multiplexer / demultiplexer described above. Has been. 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 to the counterpart waveguide as the LP01 mode between the waveguide WG1 and the waveguide WG2, and the LP11b mode of the waveguide WG1 is once converted to the LP11b of the waveguide WG2, and then guided again. The coupling length is designed so as to be converted back into the LP11b mode of the waveguide WG1.

  In addition, in the curved waveguides and the waveguide width converters in the regions I, III, V, and VII, the propagation modes are bent at a sufficiently gentle ratio so that the modes are not converted to other modes, or the waveguide width is changed. Designed to do.

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

The mode converter 301 operates as follows. Although the light propagation direction will be described from left to right, the same applies to the reverse case.
(1) Light in LP01 Mode Light propagates through 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 in the coupling portion of the region IV, propagates in the waveguide WG2 in the LP01 mode in the region V, and is converted from the waveguide WG2 to the LP11a mode of the waveguide WG1 in the region VI. In the region VII, the light propagates through the waveguide WG1 in the LP11a mode, is converted into the LP11b mode by the mode rotor 50, and is output.
(2) Light in LP11a mode Light propagates through the waveguide WG1 in the region 11 in the LP11a mode. The light is converted into the LP01 mode of the waveguide WG2 in the coupling part of the region II, propagates in the waveguide WG2 in the LP01 mode in the region III, and is converted from the waveguide WG2 to the LP01 mode of the waveguide WG1 in the coupling part of the region IV. Is done. Light propagates in the waveguide WG1 in the LP01 mode in the regions V, VI, and VII, and is not mode-converted by the mode rotator 50 and is output as the LP01 mode.
(3) Light in LP11b mode Light propagates in 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 portion in 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, and is converted into the LP11a mode by the mode rotator 50 and output.

  As described above, the mode switch 301 can convert the incoming LP01 into LP11b, LP11a into LP01, and LP11b into LP11a, and can implement mode cyclic switching.

(Configuration example 2)
FIG. 5 is a diagram for 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 parallel to the first waveguide WG1 on the substrate;
A third waveguide WG3 parallel to the first waveguide WG1 on the substrate;
Three first coupling portions (B, B, which are constituted by the first waveguide WG1 and the second waveguide WG2 on the substrate and transfer the light 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. Two second coupling parts (A, E) for transferring the optical power to and from WG3;
A first mode rotator 51 connected to one end of the first waveguide WG1 and mode-converting LP11a and LP11b;
Located between the two second coupling portions (A, E) and arranged on the first waveguide WG1 so as to sandwich the three first coupling portions (B, C, D), and mode conversion between LP11a and LP11b Two second mode rotors (52, 53) to
Is provided.
Two coupling portions (B, D) of the first coupling portions (B, C, D) are an LP11a mode that propagates through the first waveguide WG1 and an effective refractive index of the LP01 mode that propagates through the second waveguide WG2. The effective refractive indices of the light beams coincide with each other, and the length in the light propagation direction is a length that completely transfers the light power from one waveguide to the other waveguide.
Among the first coupling portions (B, C, D), the coupling portion C sandwiched between two coupling portions (B, D) has an LP01 mode effective refractive index propagating through the first waveguide WG1 and the second waveguide WG2. The effective refractive index of the LP01 mode propagating through the light beam coincides with each other, and the length in the light propagation direction is a length that completely transfers the light power from one waveguide to the other waveguide.
In the two second coupling portions (A, E), the effective refractive index of the LP01 mode propagating through the third waveguide WG3 matches the effective refractive index of the LP11a mode propagating through the first waveguide WG1, and the light propagation direction Is a length that completely transfers the light power from one waveguide to the other.

  In the mode exchanger 302, the height of each waveguide is constant, the waveguide WG1 capable of propagating three modes, the waveguide WG2 and the waveguide WG3 parallel to the waveguide WG1, and the waveguide WG2 or the waveguide WG3. Has a coupling portion (A, B, C, D, E) configured close to the waveguide WG1.

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

  In addition, in the bent waveguide and the waveguide width conversion section in the waveguide WG2 and the waveguide WG3, the propagation mode is bent at a sufficiently gentle ratio or the waveguide width changes so that the propagation mode is not converted to another mode. It is designed as follows.

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

The mode converter 302 operates as follows. Although the light propagation direction will be described from left to right, the same applies to the reverse case.
(1) Light in LP01 Mode Light propagates in the waveguide WG1 in the LP01 mode until reaching 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 until reaching the coupling portion D, and is converted into the LP11a mode of the waveguide WG1 in the coupling portion D. The light is converted into LP11b by the mode rotator 53 and output.
(2) Light in 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 LP11b mode at the coupling portion A, 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 in the coupling portion B, and is converted from the waveguide WG2 to the LP01 mode of the waveguide WG1 in the coupling portion C. Light propagates through the waveguide WG1 without being subjected to mode conversion by the coupling portions D and E and the rotor 53, and is output.
(3) Light in LP11b Mode Light is converted from the LP11b mode to the LP11a mode by the mode rotator 51, and converted from the waveguide WG1 to the LP01 mode of the waveguide WG3 in the coupling portion A. The light propagates through the waveguide WG3 in the LP01 mode, is converted from the waveguide WG2 to the LP11a mode of the waveguide WG1 in the coupling portion E, and is output.

  As described above, the mode switch 302 can convert the incoming LP01 to LP11b, LP11a to LP01, and LP11b to LP11a, and can implement mode cyclic switching.

Subsequently, propagation simulation in each mode is performed for the mode exchanger 302. FIG. 6 shows design parameters for each waveguide of the mode exchanger 302. FIG. 7 shows the results of propagation simulation in each mode with this design parameter.
FIG. 7A shows a state where incident LP01 mode light is converted to LP11b mode.
FIG. 7B shows how incident LP11a mode light is converted to LP01 mode.
FIG. 3C shows a state in which incident LP11b mode light is converted to the LP11a mode.

(Configuration example 3)
FIG. 8 is a diagram for explaining the structure of the mode exchanger 303 of the present 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 parallel to the first waveguide WG1 on the substrate;
A third waveguide WG3 parallel to the first waveguide WG1 on the substrate;
A coupling part B, a coupling part C, and a coupling composed of the first waveguide WG1 and the second waveguide WG2 on the substrate and transferring the light power between the first waveguide WG1 and the second waveguide WG2. Part E,
A coupling unit A and a coupling unit D, which are configured by the first waveguide WG1 and the third waveguide WG3 on the substrate and transfer light power between the first waveguide WG1 and the third waveguide WG3,
Two mode rotators (54, 55) disposed on the first waveguide WG1 and mode-converting LP11a and LP11b;
Is provided.
The coupling part A, the coupling part B, the coupling part C, the coupling part D, and the coupling part E are arranged in order from one end to the other end, and the mode rotor 54 is arranged between the coupling part A and the coupling part B. The mode rotor 55 is disposed between the coupling portion D and the coupling portion E.
The coupling unit A, the coupling unit B, the coupling unit D, and the coupling unit E include an effective refractive index of the LP01 mode that propagates through the second waveguide WG2 or the third waveguide WG3, and an effective LP11a mode that propagates through the first waveguide WG1. The refractive indexes are the same, and the length in the light propagation direction is a length that completely transfers the light power from one waveguide to the other waveguide.
In the coupling part C, the effective refractive index of the LP01 mode propagating through the first waveguide WG1 matches the effective refractive index of the LP01 mode propagating through the second waveguide WG2, and the length of the light propagation direction is the power of the light. Is the length of complete transition from one waveguide to the other.

  The mode exchanger 303 has a waveguide with a constant height, a waveguide WG1 capable of propagating three modes, a waveguide WG2 and a waveguide WG3 parallel to the waveguide WG1, and a waveguide WG2 or a waveguide WG3. It has a coupling part (A, B, C, D, E) configured close to the waveguide WG1.

  In the coupling portions (A, B, D, E), the conversion is performed between the LP11a of the waveguide WG1 and the LP01 mode of the waveguide WG2 or WG3, as in the mode multiplexer / demultiplexer described above. The bond length is designed. Further, in the coupling portion C, the waveguide widths of the waveguide WG1 and the waveguide 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 addition, in the bent waveguide and the waveguide width conversion section in the waveguide WG2 and the waveguide WG3, the propagation mode is bent at a sufficiently gentle ratio or the waveguide width changes so that the propagation mode is not converted to another mode. It is designed as follows.

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

The mode converter 303 operates as follows. Although the light propagation direction will be described from left to right, the same applies to the reverse case.
(1) Light in LP01 Mode Light propagates in the waveguide WG1 in the LP01 mode until reaching 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 until reaching the coupling portion E, and is converted into the LP11a mode of the waveguide WG1 in the coupling portion E and output.
(2) LP11a Mode Light Light is converted from the waveguide WG1 to the LP01 mode of the waveguide WG2 in the coupling portion A. Light propagates through the waveguide WG3 in the LP01 mode, and is converted from the waveguide WG3 to the LP11a mode of the waveguide WG1 in 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 in LP11b mode The light propagates through the waveguide WG1 without undergoing mode conversion at the coupling part A, is converted from the LP11b mode to the LP11a mode by the mode rotor 54, and is guided from the waveguide WG1 into the waveguide at the coupling part B. It is converted to LP01 mode of WG2. Light propagates through the waveguide WG2 in the LP01 mode until it reaches the coupling part C, and is converted from the waveguide WG2 into the LP01 mode of the waveguide WG1 by the coupling part C. The light propagates through the waveguide WG1 without being subjected to mode conversion by the coupling portions D and E and the rotor 55, and is output.

  As described above, the mode exchanger 303 can convert the incoming LP01 into LP11a, LP11a into LP11b, and LP11b into LP01, and can implement mode cyclic switching.

Subsequently, a 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 results of propagation simulation in each mode with this design parameter.
FIG. 10A shows a state where incident LP01 mode light is converted to LP11a mode.
FIG. 10B shows a state in which incident LP11a mode light is converted to LP11b mode.
FIG. 10C shows a state where incident LP11b mode light is converted to LP01 mode.

(Configuration example 4)
FIG. 11 is a diagram illustrating the structure of the mode exchanger 304 of the present embodiment. The mode exchanger 304 is
One waveguide WG formed on the substrate and capable of propagating three modes LP01, LP11a and LP11b;
A grating section 41 formed in the waveguide WG and mode-converting LP01 and LP11a by periodically changing the width, or mode-converting LP01 and LP11b by periodically changing the height;
A mode rotator 56 formed in the waveguide GW and mode-converting LP11a and LP11b;
Is provided.

  The grating section 41 is designed so that the width of the waveguide is periodically changed to convert LP01 and LP11a, and the mode rotator 56 is similar to the above-described mode rotator in that LP11a and LP11b. Designed to convert between modes.

The mode converter 304 operates as follows. Although the light propagation direction will be described from left to right, the same applies to the reverse case.
(1) Light in LP01 Mode Light is converted into the LP11a mode in the grating section 41, and converted from the LP11a mode into the LP11b mode in the mode rotator 56.
(2) Light in LP11a Mode Light is converted into the LP01 mode in the grating section 41, and is not converted into any mode in the mode rotator 56 and is output as the LP01 mode.
(3) Light in LP11b Mode Light is not converted into any mode in the grating section 41, but is converted from the LP11b mode to the LP11a mode in the mode rotor 56.

  As described above, the mode switch 304 can convert the incoming LP01 to LP11b, LP11a to LP01, and LP11b to LP11a, and can implement mode cyclic switching.

  The grating section 41 is the above-described cyclic exchange in the case where the horizontal width of the waveguide is changed, but in the case where the vertical height is periodically changed, the LP01 mode is used. And LP11b mode are converted. Therefore, the mode switch 304 converts the incident LP01 to LP11a, LP11a to LP11b, and LP11b to LP01, thereby realizing cyclic switching of modes.

  Note that the fluctuation period of the waveguide structure of the grating portion 41 is such that, as shown in Non-Patent Document 8, a refractive index fluctuation having a specific period corresponding to the effective refractive index of mode 1 and mode 2 is applied at wavelength λ. Thus, signals propagated in the respective modes can be converted.

The fiber grating interval Λ of the grating section 41 is determined by the waveguide structure parameters to be used, the wavelength, and the mode order to be converted. The grating interval Λ for mode conversion is given by equation (2). In Equation (2), n ₁ and n ₂ indicate 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 formula (2) to obtain the necessary grating. The period Λ can be determined.

  Further, with respect to the length of the grating section 41, the mode exchange function can be realized by calculating the coupling length and designing it appropriately.

A mode exchanger that can exchange 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, and 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 periodically changes;
A mode rotator 56 formed in the waveguide and converting one of the degenerate modes to the other degenerate mode;
Each of the n input modes is converted into a mode different from the input mode.

  For example, in the case of a mode switch that can exchange six modes, an LPG that exchanges two modes of LP01, LP11a / LP11b, LP21a / LP21b, and LP02 modes is designed based on Equation (1), and all modes are cyclic The grating portions 41 are formed by connecting a plurality of LPGs in multiple stages so that they are exchanged. By connecting such LPGs, the number of modes can be easily expanded. Furthermore, this mode exchanger has a mode rotator 56 for LP11a / LP11b and a mode rotator for LP21a / LP21b.

(Configuration example 5)
FIG. 12 is a diagram illustrating the structure of the mode exchanger 305 of the present embodiment. The mode exchanger 305 has a crossed waveguide section 103 between the three-mode duplexer 101 and the three-mode multiplexer 102. Specifically, the mode exchanger 305 is
Two mode multiplexer / demultiplexers (101, 102) having three demultiplexing ports (DP1, DP2, DP3) and one multiplexing port MP1;
A cross waveguide unit 103 for connecting the different demultiplexing ports in a 1: 1 ratio between two mode multiplexers / demultiplexers (101, 102);
Is provided.

  “To connect the two different demultiplexing ports 1: 1 between the two mode multiplexers / demultiplexers (101, 102)” is, for example, coupled to the demultiplexing port DP2 of the demultiplexer 101. The demultiplexing port DP3 of the demultiplexer 101 is connected to the demultiplexing port DP1 of the demultiplexer 101, and the demultiplexing port DP2 of the demultiplexer 101 is connected to the demultiplexing port DP2 of the demultiplexer 101. This means that the demultiplexing port DP1 is connected. Mode switching can be realized by connecting each of the three output waveguides from the mode duplexer 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, the other end connected to a multiplexing port MP1, and capable of propagating three modes LP01, LP11a, and LP11b;
Two second waveguides (WG2, WG3) parallel to the first waveguide WG1 on the substrate and connected at one end to the other two demultiplexing ports (DP2, DP3),
The first waveguide WG1 on the substrate and each of the second waveguides (WG2, WG3) are configured, and the optical power is transmitted between the first waveguide WG1 and the second waveguide (WG2, WG3). Two transition parts (A, B) to be transferred;
A mode rotator 57 disposed on the first waveguide WG1 between the two coupling portions (A, B) and mode-converting LP11a and LP11b;
Is provided.
In the coupling portions (A, B), the effective refractive index of the LP01 mode propagating through the second waveguide (WG2, WG3) matches the effective refractive index of the LP11a mode propagating through the first waveguide WG1, and the light propagation direction Is a length that completely transfers the light power from one waveguide to the other.

For the mode multiplexer 102, for example, the configuration of FIG. 13 can be used.
It is configured by a waveguide WG1 capable of propagating three modes and waveguides (WG2, WG3) arranged in parallel therewith. In the coupling portions (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 rotator 57 performs mode conversion 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 while being in the LP01 mode, and is output from the multiplexing port MP1.
The LP01 mode light input to the demultiplexing port DP2 is converted by the coupling portion A from the waveguide WG2 to the LP11a mode of the waveguide WG1. 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 by the coupling unit B and output from the multiplexing port MP1.

  Further, as the mode demultiplexer 101, the configuration of FIG. In this configuration, the configuration of the mode multiplexer 102 is inverted, and light in the LP01, LP11a, and LP11b modes input from the multiplexing port MP1 propagates through the waveguide WG1 in the LP01 mode and is sent to the demultiplexing port DP1. The waveguide WG3 propagates in the LP01 mode and propagates in the demultiplexing port DP3, and the waveguide WG2 propagates in the LP01 mode and is output to the demultiplexing port DP2.

  The mode converter 305 receives light from the demultiplexing ports (DP 1, DP 2, DP 3) of the mode demultiplexer 101 in the intersecting waveguide unit 103, respectively, and the demultiplexing ports (DP 2, DP 3, DP 1) of the mode multiplexer 102. Or it connects to (DP3, DP1, DP2), and the mode cyclic exchange is realized.

  In this embodiment, an example in which the number of modes is 3 is shown. However, the number of propagation modes of the input / output waveguides is n (n is an integer of 3 or more), and the number of modes in which the number of parallel waveguides is to be exchanged. By installing only the number, the number of corresponding modes can be increased with a configuration similar to the first to fifth embodiments. For example, an n-mode exchanger can be easily realized by setting the mode multiplexer / demultiplexer described in the fifth embodiment to an n-mode multiplexer / demultiplexer and setting the number of crossing waveguides to n.

(Other examples)
In the above-described embodiment, the configuration example related to the planar lightwave circuit using the SiO ₂ -based material (quartz-based material) is described, but the material may naturally be other. For example, even in the case of a planar lightwave circuit using a semiconductor such as Si-based or InGaAsP, or an organic material such as a polymer, the same effects as those of the embodiments described in this specification can be obtained. The wavelength band to be used is about 1.5 to 1.6 μm in the above embodiment, but it may be a mid-infrared region (2 μm or more) having a longer wavelength or a visible light band.

[Appendix]
The following describes the mode converter of this embodiment.
The present invention is a mode exchange that guarantees DMD between nodes and various optical characteristics by combining parallel waveguides, coupling portions where the waveguides are close to each other, and a mode rotator to cyclically convert the propagation mode. 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 three modes;
A waveguide WG2 parallel to the waveguide WG1,
The waveguide WG1 and the waveguide WG2 arranged in parallel have three coupling portions adjacent to each other,
The waveguide WG2 has a region where the waveguide width is the same as the waveguide WG1;
The waveguide WG1 includes a mode rotator for converting the LP11a mode into the LP11b mode in the region after the coupling portion,
The LP11a mode propagating in the waveguide WG1 in the first coupling portion is converted into the LP01 mode of the waveguide WG2,
The LP01 mode propagating through the waveguide WG1 in 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 in the waveguide WG2 to the LP11a mode of the waveguide WG1 in the third coupling portion,
It is a mode exchanger characterized in that the incident LP01 is converted into LP11b, LP11a is converted into LP01, and LP11b is converted 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 three modes, and waveguides WG2, 3 parallel to the waveguide WG1,
Each of the waveguide WG1 and the waveguides WG2, 3 arranged in parallel has five coupling portions adjacent to each other,
The waveguide WG1 includes a mode rotator that converts LP11a mode to LP11b mode,
There are a total of three mode rotors before and after the coupling part A and before the coupling part E,
In the coupling part A, the LP11a mode propagating through the waveguide WG1 is converted into the LP01 mode of the waveguide WG2,
In the coupling part B, the LP11a mode propagating through the waveguide WG1 is converted into the LP01 mode of the waveguide WG3,
The LP01 mode propagating through the waveguide WG1 or 3 in the coupling unit C is converted into the LP01 mode of the waveguide WG3 or 1,
The LP01 mode propagating through the waveguide WG3 in the coupling part D is converted into the LP11a mode of the waveguide WG1,
By converting the LP01 mode propagating through the waveguide WG2 at the coupling portion E into the LP11a mode of the waveguide WG1,
It is a mode exchanger characterized in that the incident LP01 is converted into LP11b, LP11a is converted into LP01, and LP11b is converted 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 three modes, waveguides WG2 and 3 parallel to the waveguide WG1, and
Each of the waveguide WG1 and the waveguides WG2, 3 arranged in parallel has five coupling portions adjacent to each other,
The waveguide WG1 includes a mode rotator that converts LP11a mode to LP11b mode,
A total of two mode rotors after coupling A and after coupling D;
In the coupling part A, the LP11a mode propagating through the waveguide WG1 is converted into the LP01 mode of the waveguide WG2,
In the coupling part B, the LP11a mode propagating through the waveguide WG1 is converted into the LP01 mode of the waveguide WG3,
The LP01 mode propagating through the waveguide WG1 or 3 in the coupling unit C is converted into the LP01 mode of the waveguide WG3 or 1,
In the coupling part D, the LP01 mode propagating through the waveguide WG2 is converted into the LP11a mode of the waveguide WG1,
By converting the LP01 mode propagating through the waveguide WG3 at the coupling portion E to the LP11a mode of the waveguide WG1,
It is a mode exchanger characterized in that the incident LP01 is converted into LP11b, LP11a is converted into LP01, and LP11b is converted 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 capable of propagating three modes,
A grating waveguide section in which the waveguide width changes periodically;
Including a mode rotator for converting LP11a mode to LP11b mode;
Designed to convert LP01 and LP01a in the grating waveguide section,
It is a mode exchanger characterized in that the incident LP01 is converted into LP11b, LP11a is converted into LP01, and LP11b is converted 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 three modes, waveguides WG2 and 3 parallel to the waveguide WG1, and
Each of the waveguide WG1 and the waveguides WG2, 3 arranged in parallel has two coupling portions adjacent to each other,
A mode rotator for converting the LP11a mode to the LP11b mode after the first coupling portion 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 through the waveguides WG2 and 3 is designed to be converted into the LP11a mode of the waveguide WG1.
A mode multiplexer that splits the LP01, LP11a, and LP11b modes propagating through the waveguide into the waveguides WG1, 2, and 3 as the LP01 mode;
A mode multiplexer / demultiplexer that is designed in the same way and is combined with the waveguide WG1 as the LP01, LP11a, and LP11b modes are input from the waveguides WG1, 2, 3
The output waveguides WG1, 2, 3 of the mode duplexer have a crossed waveguide structure connected to the input waveguides WG2, 3, 1 or 3, 1, 2 of the mode multiplexer. Mode exchanger.

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

41: grating sections 50-57: mode rotor 101: 3-mode duplexer 102: 3-mode multiplexer 103: crossed waveguide sections 301-305: mode converter

Claims (6)

A first waveguide formed on a substrate and capable of propagating three modes LP01, LP11a and LP11b;
A second waveguide in parallel with the first waveguide on the substrate;
Three coupling portions configured by the first waveguide and the second waveguide on the substrate and transferring light power between the first waveguide and the second waveguide;
A mode rotator connected to one end of the first waveguide and mode-converting LP11a and LP11b;
With
Two of the coupling portions have an effective refractive index of the LP01 mode propagating through the second waveguide and an effective refractive index of the LP11a mode propagating through the first waveguide, and are in the light propagation direction. The length is a length that completely transfers the light power from one waveguide to the other,
The coupling part sandwiched between the two coupling parts among the coupling parts has an effective refractive index of the LP01 mode propagating through the first waveguide and an effective refractive index of the LP01 mode propagating through the second waveguide, A mode converter characterized in that the length of the light propagation direction is a length that completely transfers the light power from one waveguide to the other waveguide. A first waveguide formed on a substrate and capable of propagating three modes LP01, LP11a and LP11b;
A second waveguide in parallel with the first waveguide on the substrate;
A third waveguide parallel to the first waveguide on the substrate;
Three first coupling portions configured by the first waveguide and the second waveguide on the substrate and transferring light power between the first waveguide and the second waveguide;
The first waveguide and the third waveguide on the substrate are arranged so as to sandwich the three first coupling portions, and light is transmitted between the first waveguide and the third waveguide. Two second coupling parts that transfer the power of
A first mode rotor connected to one end of the first waveguide and mode-converting LP11a and LP11b;
Two second mode rotators located between the two second coupling portions and disposed on the first waveguide so as to sandwich the three first coupling portions, and mode-converting LP11a and LP11b;
With
Two of the first coupling portions have the same effective refractive index of the LP01 mode propagating through the second waveguide and the effective refractive index of the LP11a mode propagating through the first waveguide, and light propagation. The length of the direction is a length that completely transfers the light power from one waveguide to the other,
Of the first coupling portion, the coupling portion sandwiched between the two coupling portions has an effective refractive index of the LP01 mode propagating through the first waveguide and an effective refractive index of the LP01 mode propagating through the second waveguide. In addition, the length of the light propagation direction is a length that completely transfers the light power from one waveguide to the other,
In the two second coupling portions, the effective refractive index of the LP01 mode propagating through the third waveguide matches the effective refractive index of the LP11a mode propagating through the first waveguide, and the length in the light propagation direction is the same. A mode converter characterized by having a length that completely transfers the light power from one waveguide to the other. A first waveguide formed on a substrate and capable of propagating three modes LP01, LP11a and LP11b;
A second waveguide in parallel with the first waveguide on the substrate;
A third waveguide parallel to the first waveguide on the substrate;
A coupling portion B, a coupling portion C, and a coupling, which are configured by the first waveguide and the second waveguide on the substrate and transfer light power between the first waveguide and the second waveguide. Part E,
A coupling portion A and a coupling portion D configured by the first waveguide and the third waveguide on the substrate and transferring light power between the first waveguide and the third waveguide;
Two mode rotators disposed on the first waveguide and mode-converting LP11a and LP11b;
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 one of the two mode rotors is the It is arranged between the coupling part A and the coupling part B, and the other of the two mode rotors is arranged between the coupling part D and the coupling part E,
The coupling unit A, the coupling unit B, the coupling unit D, and the coupling unit E propagate the effective refractive index of the LP01 mode that propagates in the second waveguide or the third waveguide and the first waveguide. The effective refractive index of the LP11a mode is the same, and the length of the light propagation direction is a length that completely transfers the light power from one waveguide to the other waveguide,
In the coupling part C, the effective refractive index of the LP01 mode propagating through the first waveguide matches the effective refractive index of the LP01 mode propagating through the second waveguide, and the length of the light propagation direction is equal to that of the light. A mode converter characterized by having a length for completely transferring power from one waveguide to the other. One waveguide formed on the substrate and capable of propagating the three modes LP01, LP11a and LP11b;
A grating portion formed in the waveguide and mode-converting LP01 and LP11a by periodically changing the width, or mode-converting LP01 and LP11b by periodically changing the height;
A mode rotator formed in the waveguide and mode-converting LP11a and LP11b;
A mode converter comprising: Two mode multiplexer / demultiplexers having three demultiplexing ports and one multiplexing port;
A cross waveguide section for connecting the different demultiplexing ports in a 1: 1 ratio between the two mode multiplexers and demultiplexers;
With
The mode multiplexer / demultiplexer is
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 LP01, LP11a, and LP11b;
Two second waveguides parallel to the first waveguide on the substrate and having one end connected to the other two demultiplexing ports,
Two coupling portions configured by the first waveguide on the substrate and the respective second waveguides, and transferring light power between the first waveguide and the second waveguide;
A mode rotator disposed on the first waveguide between the two coupling portions and mode-converting LP11a and LP11b;
With
In the coupling portion, the effective refractive index of the LP01 mode propagating through the second waveguide matches the effective refractive index of the LP11a mode propagating through the first waveguide, and the length of the light propagation direction is the power of the light. A mode converter characterized in that it has a length that completely shifts from one waveguide to the other. A waveguide formed on a substrate, having a constant height from the substrate, and capable of propagating n (n is an integer of 4 or more) modes;
A grating portion formed in the waveguide, the width of which periodically changes;
A mode rotator formed in the waveguide and converting one of the degenerate modes to the other degenerate mode;
A mode converter comprising: converting each of n input modes to a mode different from the input mode.