JPWO2015133344A1 - Mode multiplexer / demultiplexer - Google Patents

Abstract

In the mode multiplexer / demultiplexer, the degeneration between the TE01 mode and the TE10 mode and the degeneration between the TM01 mode and the TM10 mode are solved and separated in the rectangular waveguide. Asymmetrically tapered coupled mode transition waveguide formed by multimode waveguide and single-mode waveguide, adiabatic between higher-order mode of multimode waveguide and fundamental mode of singlemode waveguide Mode conversion by mode interaction. The magnitude relationship between the propagation constant of the higher-order mode of the multi-mode waveguide and the fundamental mode propagation constant of the single-mode waveguide is changed along the propagation direction. Release degeneration or generate a reduced higher-order mode. In canceling the degeneration of the higher-order mode, the primary mode of the TE mode and the TM mode degenerated in the square waveguide is separated into a higher-order mode in the substrate parallel direction and a higher-order mode in the substrate vertical direction. [Selection] Figure 8

Description

  The present invention relates to an optical device used for multimode transmission, and more particularly to a mode multiplexer / demultiplexer for multiplexing and demultiplexing (separating) optical signals between a single mode fiber and a few mode fiber.

  In large-capacity optical communication technology, a mode multiplexing number mode fiber having a larger communication capacity is being used in addition to a single mode fiber as a transmission medium. As a fiber that enables transmission in a plurality of modes, there are a multimode fiber having a large core diameter and a number mode fiber having a core diameter intermediate between a single mode fiber and a multimode fiber. Although the number of modes that can be transmitted by the several mode fiber is said to be about 2 to 3 to 10, for example, the specific number of modes is not specified.

  FIG. 1 is a diagram for explaining the outline of a single mode fiber and a number mode fiber. The core diameter of the several-mode fiber is larger than the core diameter of the single-mode fiber, and mode multiplex transmission is performed by placing different information on a plurality of propagation modes. In addition, the numerical value of the core diameter shown in FIG. 1 is an example, and is not limited to this numerical value.

  In mode multiplex transmission, a single mode signal is formed, and during transmission, a plurality of single mode signals are combined with the mode multiplex signal and transmitted as a mode multiplex signal over a multimode transmission line, and received at the receiving side. Are demultiplexed into a plurality of single mode signals and then decoded.

  In mode multiplexing communication, a plurality of single mode signals are transmitted in the form of mode multiplexed signals in one multimode fiber or several mode fibers. It is necessary to multiplex and demultiplex (separate) between them.

In a waveguide mode propagating through an optical fiber, a mode group in which propagation constants are substantially equal and linear polarization can be formed by superposition is called an LP mode. The LP mode includes a basic mode (0th order mode) and a higher order mode. FIG. 2A shows the LP ₀₁ mode of the 0th order mode. A single mode fiber is a fiber that propagates only the LP ₀₁ mode, and a few mode fiber is a fiber that propagates higher order modes such as the LP ₁₁ mode in addition to the LP ₀₁ mode.

FIGS. 2B and 2C show electromagnetic field intensity distributions of the LP ₁₁ ^even mode and the LP ₁₁ ^odd mode, which are the first-order LP modes, and FIGS. 2D and ^2E show LP ₁₁ ^even and LP _11. ^{The odd-} field electromagnetic field amplitude distribution is shown. The LP ₁₁ ^even mode and the LP ₁₁ ^odd mode have the same propagation constant and are degenerated. Since the LP ₁₁ ^even mode and the LP ₁₁ ^odd mode each have two electromagnetic field distributions whose polarization directions are different by 90 degrees, the four degenerate states are caused by four different electromagnetic field distributions having the same propagation constant. It has become.

Note that the definitions of the LP ₁₁ ^even mode and the LP ₁₁ ^odd mode are represented here by the cos function when the angle coordinate is used from the longitudinal axis in the core cross section (y axis in FIG. 3) using the cylindrical coordinate system. The amplitude distribution is defined as an even mode, a mode represented by a sin function, and an odd mode. Therefore, when the x-axis and the y-axis are defined as shown in FIG. 3, the LP ₁₁ ^even mode has an even function electromagnetic field amplitude distribution with respect to the x axis direction, and the LP ₁₁ ^odd mode has an odd function electromagnetic field amplitude distribution with respect to the x axis. have.

  Patent Documents 1 and 2 are known as techniques of multiplexing and demultiplexing (separation) performed between a single mode signal and a mode multiplexed signal.

  Patent Document 1 discloses a configuration in which a single mode waveguide and a multimode waveguide are adiabatically close to each other and a signal is shifted between the waveguides by an interaction between fundamental modes.

Patent Document 2 discloses an optical mode conversion / multiplexing separator composed of a planar lightwave circuit including a directional coupler composed of two translated waveguides having different waveguide widths. In this optical mode conversion / multiplexing / separating device, the high-order mode light is controlled in the horizontal direction and the vertical direction by defining the core thickness and bending radius of the planar lightwave circuit. by optically connected at different 90 degrees to the substrate XY plane, it has been shown to be associated from the E ₁₁ mode into six modes of propagation of E ₃₂ modes.

  Further, Patent Document 3 is known as a technique related to mode combining and separation in a rectangular waveguide. In Patent Document 3, there is a mirror-related mode distribution (an evenly symmetric electric field distribution in the vertical direction) and a point symmetric mode distribution in the core vertical direction (an oddly symmetric electric field distribution in the vertical direction). In a mode multiplexer / demultiplexer using a quartz planar lightwave circuit (PLC), the mode distribution having a different electric field distribution from the above is distributed between the single mode waveguide and the multimode waveguide. In addition, a configuration is shown in which a dielectric constant adjustment unit that makes the dielectric constant distribution asymmetric in the thickness direction of the planar lightwave circuit is provided.

JP 2004-157506 A JP 2013-152272 A JP 2014-2605A

In a waveguide having a rectangular core cross-sectional shape, the electromagnetic field distributions TE ₀₁ and TM ₀₁ corresponding to the LP ₁₁ ^even mode of the several-mode fiber, and the electromagnetic field distributions TE ₁₀ and TM ₁₀ corresponding to the LP ₁₁ ^odd mode are obtained. Prepare. 3A and 3C show the TE ₀₁ mode and the TE ₁₀ mode in which the electric field has only the x direction, and FIGS. 3B and 3D show the TM ₀₁ and TM _{10 in} which the electric field has only the y direction. Show.

In a rectangular waveguide with a square cross-section of the core in a rectangular waveguide, the TE ₀₁ mode and the TE ₁₀ mode, and the TM ₀₁ mode and the TM ₁₀ mode are degenerate because the propagation constants are the same. .

In the rectangular waveguide, strictly speaking, the polarization direction is not 100% polarized in the x direction or the y direction, and a component in the orthogonal coordinate direction theoretically exists, but the relative refractive index difference Δ is 1. When it is as small as% or less, the electric field is polarized substantially in the x-direction or y-direction, so it is called the TE mode or TM mode for convenience. Alternatively, it may be called TE-like mode or TM-like mode. On the other hand, for the mode number (or mode label), the x and y axes are defined as shown in FIG. 3, and the first subscript i of TE _ij is the mode order in the x direction, and the second subscript. The letter j represents the mode order in the y direction.

  Even when the cross-sectional shape of the core is not square, when the relative refractive index difference Δ is as small as about 1%, the propagation constant of the TE mode and the TM mode is almost zero because the polarization dependence of the propagation constant is small. Match and degenerate.

  Therefore, the TE mode and the TM mode are degenerated in a square waveguide having a square core cross section or a rectangular waveguide having a relative refractive index difference Δ of about 1% even when the core cross section is a rectangle. To do.

Therefore, in the configuration in which the number mode fiber and the rectangular waveguide are abutted, the LP ₁₁ ^even mode and the LP ₁₁ ^odd mode, which are in a degenerated state in the number mode fiber, are TE ₀₁ mode, TE ₁₀ mode, and TM in the square waveguide, respectively. _The degenerate modes of the ₀₁ mode and the TM ₁₀ mode are mixed. Also, the TE ₀₁ mode, TM ₀₁ mode, TE ₁₀ mode, and TM ₁₀ mode, which are in a degenerated state in a rectangular waveguide, correspond to the LP ₁₁ ^even mode and the LP ₁₁ ^odd mode, respectively, in a few mode fiber.

FIG. 4 is a diagram for explaining degeneration of a square waveguide. FIG. 4 shows the normalized propagation constant b (vertical axis) with respect to the V parameter value (horizontal axis) in each Nth-order mode. In the 0th order mode (N = 0), TE ₀₀ and TM ₀₀ are degenerated and are on the same characteristic curve. In the 1st order mode (N = 1), the TE ₀₁ mode, the TE ₁₀ mode, the TM ₀₁ mode, and the TM The four modes of the ₁₀ modes are on the same characteristic curve for degeneration.

The techniques disclosed in Patent Documents 1 and 2 described above relate to a mode multiplexer / demultiplexer that performs mode multiplexing or demultiplexing (separation) for each order mode between a single mode and multiple modes. . Patent Documents 1 and 2 show a method of separating the TE ₀₁ mode and the TE ₁₀ mode, or the TM ₀₁ mode and the TM ₁₀ mode. Patent Document 1 does not indicate that separation of a higher-order mode in the vertical direction perpendicular to the substrate is performed. Further, since a directional coupler is used in Patent Document 2, it is necessary to accurately design a coupling coefficient between degenerate modes (TE ₀₁ mode and TE ₁₀ mode).

Further, in Patent Document 3 described above, a mirror-related mode distribution (an evenly symmetric electric field distribution in the vertical direction) and a point symmetric mode distribution in the vertical direction of the core (odd symmetry in the vertical direction). A mode-synthesizing and separating mode distribution having a different electric field distribution from the TE ₀₁ mode and the TE ₁₀ mode is disclosed, but a quartz planar lightwave circuit (PLC) is disclosed. In the mode multiplexer / demultiplexer using (1), it is necessary to provide a dielectric constant adjusting section that makes the dielectric constant distribution asymmetric in the thickness direction of the planar lightwave circuit between the single mode waveguide and the multimode waveguide. There's a problem.

Therefore, the present invention solves the above-described conventional problems, and eliminates the need for a directional coupler in the degeneration of the TE ₀₁ mode and the TE ₁₀ mode and the degeneration of the TM ₀₁ mode and the TM ₁₀ mode in the rectangular waveguide. An object of the present invention is to eliminate the need for strict design and manufacture of the coupling coefficient between degenerate modes (TE ₀₁ mode and TE ₁₀ mode).

In addition, in a rectangular waveguide, without using a dielectric constant adjustment unit, the object is to separate and resolve the degeneration of the TE ₀₁ mode and the TE ₁₀ mode and the degeneration of the TM ₀₁ mode and the TM ₁₀ mode by the configuration of the waveguide itself. And

Further, due to the reciprocity of light, the TE ₀₁ mode and the TE ₁₀ mode, and the TM ₀₁ mode and the TM ₁₀ mode are combined into the rectangular waveguide, and the TE itself is configured without using the dielectric constant adjustment unit. _The purpose is to combine the ₀₁ mode and the TE ₁₀ mode, and the TM ₀₁ mode and the TM ₁₀ mode.

  The mode multiplexer / demultiplexer of the present invention uses an asymmetric tapered coupled mode transition waveguide formed by a multimode waveguide and a single mode waveguide, and is adiabatic between the multimode and the single mode. The mode conversion is performed by mode interaction, and the magnitude relationship between the propagation constant of the multimode waveguide and the propagation constant of the single mode waveguide is changed along the propagation direction, and the optical power transition caused by this propagation constant change Thus, the degeneration of the higher-order mode is released and separated, or the reduced higher-order mode is generated.

The mode multiplexer / demultiplexer of the present invention insulates the propagation constant in the propagation direction by making the core width or thickness of the core of the multimode waveguide, or both the core width and core thickness into a tapered shape along the propagation direction. Change. In canceling the degeneration of the higher-order mode, the TE mode and the TM mode degenerate in the square waveguide, that is, the TE ₀₁ mode, the TE ₁₀ mode, the TM ₀₁ mode, and the TM ₁₀ mode are changed in the substrate parallel direction. The primary mode, that is, the TE ₁₀ mode and the TM ₁₀ mode, and the primary mode in the substrate vertical direction, that is, the TE ₀₁ mode and the TM ₀₁ mode are separated. On the other hand, in the generation of the degenerated higher-order mode, the first-order mode of the TE mode and the TM mode is combined in the rectangular waveguide.

  The mode multiplexer / demultiplexer of the present invention includes an asymmetric tapered coupled mode transition waveguide formed of a single mode waveguide and a multimode waveguide whose core cross-sectional shape is rectangular on a substrate.

  The multimode waveguide is configured in a tapered shape in which the ratio of the core width in the direction parallel to the substrate and the core thickness in the direction perpendicular to the substrate changes gradually along the propagation direction, and this taper shape changes the propagation constant along the propagation direction. Can be changed gradually. In the case of a square cross section having the same core width and core thickness, the multimode waveguide is degenerated because the propagation constants of the higher-order modes are equal. On the other hand, when the ratio between the core width and the core thickness changes, the degeneration is solved because a difference occurs in the propagation constant of the higher mode of the multimode waveguide.

  The mode multiplexer / demultiplexer of the present invention uses the change of the propagation constant due to the change in the ratio of the core width and the core thickness to separate the higher mode degeneration of the multimode waveguide, Combine higher-order modes.

  A single-mode waveguide is a curved shape that is curved on the substrate, and has a tapered shape in which the core width gradually increases along the propagation direction with respect to the substrate, whereby the propagation constant gradually changes along the propagation direction. The distance from the juxtaposed multimode waveguides can be gradually changed.

  In the arrangement of the multimode waveguide and the single mode waveguide, of the two single mode waveguides, the first single mode waveguide is in the propagation direction with respect to the plane in the core width direction of the multimode waveguide. Are juxtaposed along the propagation direction with respect to the surface of the multimode waveguide in the core thickness direction, and the second single mode waveguide is stacked in the core thickness direction. The

  In the juxtaposition of a multimode waveguide and a single mode waveguide, two juxtaposed single mode waveguides can be obtained by using the curved shape of the single mode waveguide so that the single mode waveguide is compared with the multimode waveguide. Thus, the distance can be close to a distance exhibiting adiabatic mode interaction, and then separated.

  Adiabatic transition of modes between a multimode waveguide and a single mode waveguide with a change in the magnitude relationship between the propagation constants of the higher-order modes of the multimode waveguide and the propagation constants of the fundamental mode of the single mode waveguide Is done. The mode multiplexer / demultiplexer of the present invention uses a mode transition accompanying a change in the magnitude relationship of propagation constants to separate a higher-order mode separated from degeneracy in a multi-mode waveguide into a single-mode waveguide, or The fundamental mode of the single mode waveguide is demultiplexed into the multimode waveguide.

The mode multiplexer / demultiplexer of the present invention includes a change in the degenerate state of a higher-order mode due to a change in the ratio between the core width and the core thickness of the multimode waveguide, and the adiabatic transition of the mode between the multimode waveguide and the single mode waveguide. Can be used to multiplex or demultiplex (separate) the TE ₁₀ mode and the TE ₀₁ mode, and the TM ₁₀ mode and the TM ₀₁ mode, which are the primary and horizontal primary modes of TE and TM.

  In the two single mode waveguides included in the mode multiplexer / demultiplexer of the present invention, the first single mode waveguide juxtaposed on the surface of the multimode waveguide in the core width direction is between the multimode waveguide and The mode transition is made in the mode distribution in the substrate parallel direction. On the other hand, the second single mode waveguide juxtaposed on the surface of the multimode waveguide in the core thickness direction mode-shifts the mode distribution in the substrate vertical direction with the multimode waveguide.

  Therefore, the mode multiplexer / demultiplexer of the present invention has a configuration in which the single mode waveguide is juxtaposed in the core width direction and the core thickness direction with respect to the multimode waveguide, so that the mode distribution in the substrate parallel direction and the substrate vertical The mode distribution of the direction can be changed between different single-mode waveguides.

  In the juxtaposition of the multimode waveguide and the single mode waveguide, the single mode waveguide is separated after being close to the multimode waveguide by a curved shape.

  At this time, a portion where the single mode waveguide and the multimode waveguide portion are close to each other forms a coupling branch portion. In this coupling branch, the single mode waveguide and the multimode waveguide are close to a distance exhibiting adiabatic mode interaction. The mode transition between the multimode waveguide and the single mode waveguide takes place within the coupling branch.

  It should be noted that the range of the coupling branch portion is expressed as indicating a range in which the two waveguides are close to a distance exhibiting adiabatic mode interaction in each propagation direction, and this range is not critical. For example, when the degree of mode interaction is expressed by electric field strength, it can be determined based on comparison with a set value that is arbitrarily determined.

In the coupling / branching portion of the mode multiplexer / demultiplexer of the present invention, the mode transition between the multimode waveguide and the single mode waveguide can be set according to the magnitude relationship of the propagation constant of each waveguide. Since the equivalent refractive index n _eq is a value obtained by dividing the normalized propagation constant β by the propagation constant k ₀ in vacuum, it can be set by the magnitude relation of the equivalent refractive index instead of the magnitude relation of the propagation constant.

  Propagation constants for mode transition are set so that the magnitude relationship between the propagation constants of the higher-order modes of the multimode waveguide and the propagation constants of the fundamental modes of the single-mode waveguide are switched within the range of the coupling branch. .

  If the propagation constants of the higher-order mode of the multimode waveguide and the fundamental mode of the single-mode waveguide match, the mode constant does not occur between the waveguides because the propagation constants match, and mode coupling is not Get up. On the other hand, when the propagation constants of the multimode waveguide and the single mode waveguide are different, the electromagnetic field distribution is localized in the waveguide having a large propagation constant.

  Therefore, the multi-mode waveguide and the single-mode waveguide are arranged side by side so that the propagation constant of the higher-order mode of the multi-mode waveguide is larger than the propagation constant of the fundamental mode of the single-mode waveguide at the incident end. And is set so that the magnitude relationship between the propagation constants of the two waveguides is switched in the coupling branch. The mode can be separated from the higher order mode of the waveguide.

  The taper shape of the multimode waveguide can set the change in the thickness of the cross section in two forms.

  In the first form of the tapered shape, the tapered shape of the multimode waveguide is formed narrower from the end side where the degenerated mode is incident or exited toward the end side where the degenerated mode is released or incident. . On the other hand, the taper shape of the single mode waveguide is formed so as to be thicker toward the end side where the degenerated mode is emitted or incident.

  In the second form of the taper shape, the taper shape of the multimode waveguide is formed thicker toward the end side where the degenerated mode is released from the end side where the degenerated mode is incident or emitted, and toward the end side where the degenerate mode is released. . On the other hand, the tapered shape of the single-mode waveguide is formed thicker toward the end side where the degenerated mode is emitted or incident, as in the first embodiment.

  In the first and second embodiments, for example, when the core thickness of the waveguide is constant, the taper shape is reduced by reducing the core width of the waveguide relative to the core thickness. The width can be increased by increasing the width relative to the core thickness. In addition, the taper shape is obtained by reducing the waveguide core thickness relative to the core width when the waveguide core width is constant, and increasing the waveguide core thickness relative to the core width. Can be thick.

  In the first form with the taper shape in which the core thickness is constant, the core thickness can be made constant in the propagation direction of the waveguide, so the core thickness is the same as that in the second form with the taper shape. Since there is no need to change the direction of propagation, the waveguide can be easily formed on the substrate.

  The mode multiplexer / demultiplexer according to the present invention receives a degenerated mode that propagates through an optical fiber, demultiplexes the degenerated mode into modes of each order, and separates the demultiplexed modes of each order into respective ports and emits them. In addition to being used as a wave combiner, it can be used as a mode combiner that multiplexes the modes incident from each port and emits the combined mode to a several-mode optical fiber.

  When the mode is separated from the port, the demultiplexed mode includes a TE mode and a TM mode whose polarization directions are different by 90 °. In order to separate the TE mode and the TM mode, a polarization separation member can be used.

  When installing an optical fiber and a polarization separating member for a mode multiplexer / demultiplexer, an optical fiber having a circular cross section is connected to one end of the multimode waveguide, and a polarization separating member is disposed at one end of the single mode waveguide. . Alternatively, a polarization separation member is disposed at the exit end of the optical fiber having a circular cross-sectional shape, and then one end of the multimode waveguide of the mode multiplexer / demultiplexer is butted against the end of the polarization separation member that emits one polarized light. One end of a multimode waveguide of a similar mode multiplexer / demultiplexer can also be butt-connected to the other end that emits polarized light.

  In the coupling between the optical fiber and the multimode waveguide, the cross-sectional shape of the end of the multimode waveguide coupled to the circular optical fiber is a square shape, and the core width in the direction parallel to the substrate and the core in the direction perpendicular to the substrate In addition to the configuration in which the ratio with the thickness is gradually changed from the end surface along the propagation direction to form a rectangular shape, the end cross section is formed into a vertically or horizontally long rectangular shape without going through a square shape, The ratio between the rectangular core width and the core thickness may be gradually changed from the end face along the propagation direction.

As described above, according to the mode multiplexer / demultiplexer of the present invention, the directional coupler can be used in the degeneration of the TE ₀₁ mode and the TE ₁₀ mode and the degeneration of the TM ₀₁ mode and the TM ₁₀ mode in the rectangular waveguide. Therefore, it is possible to eliminate the need for strict design and manufacture of the coupling coefficient between the degenerated modes (TE ₀₁ mode and TE ₁₀ mode).

In a rectangular waveguide, demultiplexing (separating) by solving the degeneration of the TE ₀₁ mode and the TE ₁₀ mode and the degeneration of the TM ₀₁ mode and the TM ₁₀ mode according to the configuration of the waveguide itself without using a dielectric constant adjustment unit. Can do.

In addition, from the reciprocity of light, the TE ₀₁ mode and the TE ₁₀ mode, and the TM ₀₁ mode and the TM ₁₀ mode can be combined into a rectangular waveguide by the configuration of the waveguide itself without using a dielectric constant adjustment unit. it can.

It is a figure for demonstrating the outline of a single mode fiber and several mode fiber. It is a figure for demonstrating the electromagnetic field distribution of several mode fiber. It is a figure for demonstrating the electromagnetic field distribution of a rectangular waveguide. It is a figure for demonstrating degeneration of the propagation constant of a square waveguide. It is a figure for demonstrating a symmetrical 3 layer flat plate waveguide. It is a figure for demonstrating the polarization mode (polarization mode) of a flat plate waveguide. It is a figure for demonstrating the dispersion curve and polarization | polarized-light dependence of a flat plate waveguide. It is a perspective view which shows schematic structure of the 1st structural example of the mode multiplexer / demultiplexer of this invention. It is the figure seen from the core thickness direction which shows schematic structure of the 1st structural example of the mode multiplexer / demultiplexer of this invention. It is a figure for demonstrating the dispersion curve of a rectangular (rectangular) waveguide. It is a figure for demonstrating cancellation | release of mode degeneration of a mode multiplexer / demultiplexer of this invention, and operation | movement of a demultiplexing. It is a figure which shows the propagation analysis of the mode in each coupling branch part of the mode multiplexer / demultiplexer of this invention. It is a figure which shows schematic structure of the 2nd, 3rd structural example of the mode multiplexer / demultiplexer of this invention. It is a figure for demonstrating cancellation | release of degeneration and the operation | movement of a demultiplexing of the 2nd structural example of the mode multiplexer / demultiplexer of this invention. It is a figure for demonstrating cancellation | release of degeneracy and the operation | movement of a demultiplexing of the 3rd structural example of the mode multiplexer / demultiplexer of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Hereinafter, the flat plate waveguide used in the mode multiplexer / demultiplexer of the present invention will be described with reference to FIGS. 5 to 6, and the first configuration example of the mode multiplexer / demultiplexer of the present invention will be described with reference to FIGS. The second and third configuration examples of the mode multiplexer / demultiplexer of the present invention will be described with reference to FIGS.

[Schematic structure of flat waveguide]
First, a rectangular waveguide used in the mode multiplexer / demultiplexer of the present invention and a flat waveguide that is the most basic optical waveguide will be described. FIG. 5 is a schematic diagram for explaining the structural parameters of the plate waveguide, and FIG. 6 is a schematic diagram for explaining the polarization mode (polarization mode) of the plate waveguide.

  FIG. 5 shows a symmetric three-layer plate waveguide as a plate waveguide. The symmetric three-layer flat plate waveguide has a three-layer structure in which a core is sandwiched in the vertical direction (y-axis direction in the figure) by a clad on the substrate, and the horizontal direction (x direction in the figure) is the direction of the core width. The longitudinal direction (y-axis direction in the figure) is the core thickness direction, and the depth direction (z-axis direction in the figure) is the propagation direction.

Here, the core refractive index of a region where light is mainly guided to the n _1, and the refractive index of the cladding which is a region surrounding the core and n _2, the relative refractive index difference Δ is the following formula (1 ).
Δ = (n ₁ ² −n ₂ ² ) / 2n ₁ ² (1)
The core half width (half thickness) a of the flat plate waveguide corresponds to the core diameter (radius) a of the optical fiber.

The V parameter of the waveguide parameter and the normalized propagation constant b are expressed by the following equations (2) and (3), respectively.
V = κ ₀ n ₁ a (2Δ) ^1/2 = (2π / λ) n ₁ a (2Δ) ^1/2 (2)
b = ((β / k ₀ ) ² −n ₂ ² ) / (n ₁ ² −n ₂ ² ) (3)
Here, k ₀ is a propagation constant in vacuum, β is a normalized propagation constant, and λ is a wavelength.

  In the planar waveguide, a TE mode (Transverse Electric mode) in which the electric field has a component only in the y direction and a TM mode (Transverse Magnetic mode) in which the magnetic field has a component only in the y direction are propagated. 6A and 6B are diagrams for explaining the polarization mode (polarization mode) of the planar waveguide. FIG. 6A shows the TE mode (S wave), and FIG. 6B shows the TM mode (P wave). Is shown. In the figure, the propagation direction is from left to right. The TE mode and the TM mode are different in polarization (polarization) direction by 90 °.

In the high-order mode of the rectangular waveguide, the TE mode has a TE ₀₁ mode and a TE ₁₀ mode in which the direction in which the electromagnetic field distribution can be knotted is 90 °, and the TM mode similarly has a direction in which the node can be noded in the electromagnetic field distribution. ° There are different TM ₀₁ and TM ₁₀ modes. In a square waveguide having a square cross-sectional shape of the core, the TE ₀₁ mode and the TE ₁₀ mode, and the TM ₀₁ mode and the TM ₁₀ mode have the same propagation constant, and are degenerated.

Note that, as described above, the strict mode in the rectangular waveguide is not strictly polarized with 100% polarization in the x direction or the y direction, and the components in the orthogonal coordinate directions exist theoretically. However, when the relative refractive index difference Δ is as small as 1% or less, the electric field is substantially polarized in the x direction or the y direction, and is therefore referred to as a TE mode or a TM mode for convenience. Alternatively, it may be called a TE-like mode or a TM-like mode. On the other hand, for the mode number (or mode label), the x and y axes are defined as shown in FIG. 3, and the first subscript i of TE _ij is the mode order in the x direction, and the second subscript. The letter j represents the mode order in the y direction.

  In the flat waveguide, the TE mode propagation constant does not depend on the relative refractive index difference Δ, but the TM mode has a smaller propagation constant than the TE mode propagation constant when the relative refractive index difference Δ is large. Therefore, when the relative refractive index difference Δ is large, a difference occurs in the propagation constant between the TE mode and the TM mode.

  On the other hand, when the relative refractive index difference Δ is small, the propagation constants of the TE mode and the TM mode are almost the same, and the relative refractive index difference Δ is 1% even when the cross-sectional shape of the core is not square. When the degree is small, since the polarization dependence of the propagation constant is small, the propagation constants of the TE mode and the TM mode almost coincide and degenerate. Therefore, in the case of a square waveguide having a square cross section of the core, or a rectangular waveguide having a small relative refractive index difference Δ of about 1% even when the core has a rectangular cross section, the TE mode is used. TM mode is degenerated.

The TE ₀₁ mode and TM ₀₁ mode of the rectangular waveguide correspond to the LP ₁₁ ^even mode of the few mode fiber, and the TE ₁₀ mode and TM ₁₀ mode of the rectangular waveguide correspond to the LP ₁₁ ^odd mode of the few mode fiber. Yes.

  FIG. 7 is a diagram for explaining a dispersion curve of a plate waveguide. FIG. 7A shows a dispersion curve of the plate waveguide when the relative refractive index difference Δ is small, and FIG. The dispersion curve of the flat plate waveguide when the rate difference Δ is large is shown. The dispersion curve in FIG. 7 shows the normalized propagation constant b with respect to the V parameter.

[First configuration example of mode multiplexer / demultiplexer]
Next, a first configuration example of the mode multiplexer / demultiplexer of the present invention will be described. 8 and 9 are a perspective view showing a schematic configuration of the first configuration example of the mode multiplexer / demultiplexer and a diagram seen from the core thickness direction.

  The mode multiplexer / demultiplexer 1A includes a multi-mode waveguide 2 having a rectangular core cross-sectional shape and first and second single-mode waveguides 3 and 4 formed on a substrate (not shown). The multimode waveguide 2 and the first and second single mode waveguides 3 and 4 constitute an asymmetric taper coupled mode transition type waveguide.

  In the multimode waveguide 2, the ratio of the core width in the parallel direction (x-axis direction in the figure) to the core thickness in the vertical direction (y-axis direction in the figure) is the propagation direction with respect to the substrate (not shown). The taper shape gradually changes along the z-axis direction (in the drawing) and forms a bus line through which the degenerated mode propagates.

  The first and second single-mode waveguides 3 and 4 are formed on a side surface (A in the drawing) of the multimode waveguide 2 in the core width direction (x-axis direction in the drawing) on the substrate (not shown). Is a curved shape that is curved in a direction away from the side surface of the multimode waveguide, and the core width (length in the x-axis direction in the figure) is the propagation direction (in the figure). The taper shape gradually increases along the z-axis direction), and is gradually separated after the distance from the multimode waveguide 2 approaches in the propagation direction.

  The first and second single-mode waveguides 3 and 4 form a drop line that emits a mode signal that has undergone mode transition when the higher-order mode separated from the multimode waveguide 2 undergoes mode transition. . On the other hand, the first and second single-mode waveguides 3 and 4 form an inline in which the fundamental mode is incident when the fundamental mode makes a mode transition to the multi-mode waveguide 2.

  In the arrangement of the multimode waveguide 2 and the first and second single mode waveguides 3 and 4, the first single mode waveguide 3 is arranged in the core width direction of the multimode waveguide 2 (the x axis in the figure). Direction) is juxtaposed along the propagation direction (z-axis direction in the drawing) with respect to the side surface (A surface in the drawing), and the second single-mode waveguide 4 is in the core thickness direction of the multimode waveguide 2 ( They are stacked and juxtaposed along the propagation direction with respect to the upper surface (the B surface in the figure) in the y-axis direction in the figure. The first single mode waveguide 3 and the second single mode waveguide 4 are stacked in the core thickness direction.

  The first single mode waveguide 3 may be juxtaposed on the side surface of the multimode waveguide 2 on the side opposite to the A plane in the drawing, and the second single mode waveguide 4 is shown in the drawing. You may juxtapose with the lower surface of the multimode waveguide 2 on the opposite side to the B surface.

  In the juxtaposition of the multimode waveguide 2 and the first and second single mode waveguides 3 and 4, the first and second single mode waveguides 3 and 4 are curved with respect to the multimode waveguide 2 due to the curved shape. Separate after approaching. In this proximity, the first and second single mode waveguides 3 and 4 and the multimode waveguide 2 are close to a distance exhibiting adiabatic mode interaction, so that the coupling branches C and D are formed. .

Due to the tapered shape of the multimode waveguide 2 at the coupling branch portions C and D, the cross section is not square, so that there is a difference in the propagation constant between the TE ₀₁ mode and the TE ₁₀ mode or between the TM ₀₁ mode and the TM ₁₀ mode. Occurs, and the mode degeneration is solved. On the other hand, since the cross section is changed from a rectangle to a square, the mode propagation constants coincide with each other, and the mode degenerates.

  In addition, in the coupling branches C and D, the magnitude relationship between the propagation constant of the higher mode of the multimode waveguide 2 and the propagation constant of the fundamental mode of the first and second single mode waveguides 3 and 4 changes. Accordingly, mode adiabatic transition is performed between the waveguides, and the localized portion of the optical power transits by adiabatic interaction from the waveguide having a small propagation constant toward the waveguide having a large propagation constant. Therefore, in the coupling branch portions C and D, the mode separation from the multimode waveguide to the single mode waveguide and the mode multiplexing from the single mode waveguide to the multimode waveguide are performed.

  In a state in which a higher-order mode in which the degeneracy is solved in the multimode waveguide is propagating, the magnitude relationship of the propagation constants of the multimode waveguide and the juxtaposed single mode waveguide is switched, and the single mode waveguide When the propagation constant of the fundamental mode becomes larger than the propagation constant of the higher order mode of the multimode waveguide, the higher order mode propagating through the multimode waveguide makes a mode transition to the fundamental mode of the single mode waveguide.

  In addition, in the state where the mode signal is propagated to the single mode waveguide, the magnitude relationship between the propagation constant of the fundamental mode of the single mode waveguide and the propagation constant of the higher mode of the multimode waveguide is switched. When the propagation constant of the higher-order mode of the waveguide is larger than the propagation constant of the fundamental mode of the single-mode waveguide, the fundamental mode of the single-mode waveguide transitions to the higher-order mode of the multimode waveguide. To do.

There are TE ₁₀ mode, TE ₀₁ mode, TM ₁₀ mode and TM ₀₁ mode as the primary modes of TE and TM of the multimode waveguide. The mode multiplexer / demultiplexer of the present invention solves the degeneration of these primary modes. In addition to splitting (separating) and mode transition to the fundamental mode of a single mode waveguide, the fundamental mode of multiple single mode waveguides can be mode transitioned to a multimode waveguide and combined, A plurality of modes can be propagated in the multimode waveguide.

  In the first configuration example, a degenerated mode propagating through the optical fiber 11 is incident and demultiplexed into modes of each order, and the demultiplexed modes are first and second single-mode waveguides 3 and 3. In addition to being used as a mode demultiplexer 4 that is separated from each port G and emitted, a mode multiplexer that combines the modes incident from each port G, combines the combined modes, and outputs the combined mode to the optical fiber 11. Can be used as

  When the mode is separated from the port G, the demultiplexed modes include a TE mode and a TM mode whose polarization directions are different by 90 °. In order to separate the TE mode and the TM mode, a polarization separation member 12 can be provided at each port F.

  When an optical fiber 11 is connected to one end E of the multimode waveguide 2, a mode signal is exchanged between the optical fiber 11 and the multimode waveguide 1, and the mode duplexer is used. Used as an incident end, when used as a mode multiplexer, it is used as an exit end for emitting the combined higher-order mode to the optical fiber 11 side.

  When the other end F of the multimode waveguide 2 is used as a mode demultiplexer, the fundamental mode remaining after being demultiplexed (separated) into the first and second single mode waveguides 3 and 4 is provided. It is used as the exit end to be emitted, and is used as the entrance end when used as a mode multiplexer. The polarization separation member 12 is also installed at the port of the end F.

  In the installation of the optical fiber 11 and the polarization separating member 12 with respect to the mode multiplexer / demultiplexer 1A, the optical fiber 11 having a circular cross section is abutted and coupled to one end E of the multimode waveguide 2 so that the first and second singles The polarization separation member 12 is coupled to the port G at one end of the mode waveguides 3 and 4.

  Alternatively, a polarization separation member is disposed at the exit end of the optical fiber having a circular cross-sectional shape, and then one end of the multimode waveguide of the mode multiplexer / demultiplexer is butted against the end of the polarization separation member that emits one polarized light. One end of a multimode waveguide of a similar mode multiplexer / demultiplexer can also be butt-connected to the other end that emits polarized light.

  The installation position of the polarization separation member may be configured to be installed at the incident end E in addition to the separation port. According to this configuration, orthogonal polarized light can be separated by the polarization separation member provided at the incident end E, and then mode separation can be performed. Also in multiplexing, a polarization multiplexer may be installed at the end E.

  In the connection between the optical fiber 11 and the multimode waveguide 2, the cross-sectional shape of the end E of the multimode waveguide 2 connected to the circular optical fiber 11 is a square shape, and the core width in the direction parallel to the substrate is In addition to a configuration in which the ratio of the core thickness in the vertical direction is gradually changed from the surface of the end portion E along the propagation direction to form a rectangular shape, the cross section of the end portion E can be elongated vertically without going through a square shape. A horizontally long rectangular shape may be used, and the ratio of the core width to the core thickness of the rectangle may be gradually changed from the surface of the end E along the propagation direction.

  Next, an operation example of the first configuration example will be described with reference to FIGS.

FIG. 10 shows a dispersion curve of a rectangular (rectangular) waveguide. In the illustrated example, in a rectangular waveguide having a core refractive index n ₁ = 1.4678 and a clad refractive index n ₂ = 1.4530, the equivalent is obtained when the core thickness h is 8 μm and the core width W is changed. Refractive index n _eq is shown. Since the equivalent refractive index n _eq is a value obtained by dividing the normalized propagation constant β by the propagation constant k ₀ in vacuum, FIG. 10 shows the change of the propagation constant with respect to the ratio between the core width and the core thickness of the rectangular waveguide. ing.

The dispersion curve in FIG. 10 shows the propagation constants of the TE _{00 in the} 0th order mode, the TE ₀₁ mode in the 1st order mode, the TE ₁₀ mode, and the TE ₁₁ mode. Here, TM ₀₀ mode, TM ₀₁ mode, TM ₁₀ mode, and TM ₁₁ mode of different polarizations are not shown.

Since the core thickness h is 8 μm, a rectangular waveguide having a core width W of 8 μm corresponds to a square waveguide having a square cross-sectional shape of the core. In this square waveguide, the TE ₀₁ mode and the TE ₁₀ mode of the primary mode, and the TM ₀₁ mode and the TM ₁₀ mode are degenerated because the propagation constants are the same. In FIG. 10, the point where the TE ₀₁ mode and the TE ₁₀ mode intersect is in a degenerated state.

When the ratio of the core width W to the core thickness h is deviated from 1: 1, the cross-sectional shape of the core of the rectangular waveguide is changed from a square to a rectangle. Thus, when the ratio between the core width W and the core thickness h of the rectangular waveguide deviates from 1: 1, the degeneration is solved because the propagation constants of the TE ₀₁ mode and the TE ₁₀ mode are different. In FIG. 10, when the core width W is larger than 8 μm (the right side in the drawing from the intersection P between the TE ₀₁ mode and the TE ₁₀ mode), the equivalent refractive index n _eq of the TE ₁₀ mode is equivalent to that of the TE ₀₁ mode. It becomes larger than the refractive index n _eq . On the other hand, when the core width W is smaller than 8 μm (on the left side in the drawing from the intersection P between the TE ₀₁ mode and the TE ₁₀ mode), the equivalent refractive index n _eq of the TE ₀₁ mode is the equivalent refractive index n of the TE ₁₀ mode. _It becomes larger than _eq .

In the mode multiplexer / demultiplexer of the present invention, when the ratio of the core width W to the core thickness h of the rectangular waveguide is deviated from 1: 1 and the cross-sectional shape is changed from a square to a rectangle, the TE ₀₁ mode and the TE ₁₀ mode By utilizing the fact that the degeneration is solved with different propagation constants, each of the TE ₀₁ mode and the TE ₁₀ mode in which the degeneration is solved is adiabatically transitioned to a single mode waveguide juxtaposed to the multimode waveguide. Demultiplex (separate) modes.

  Also, the reverse action can be performed in the same way due to the reciprocity of light, and the modes can be combined in the multimode waveguide by adiabatic transition from the single mode waveguide to the multimode waveguide.

  The operation of demultiplexing a single mode waveguide by solving mode degeneration will be described with reference to FIG. FIG. 11 illustrates an example of a waveguide in which the core width W of the rectangular waveguide of the multimode waveguide gradually becomes smaller than the core thickness h and becomes tapered.

  FIG. 11A shows the dispersion curve of the multimode waveguide similarly to FIG. 10, and FIG. 11B shows the dispersion curves of the first and second single mode waveguides. The rectangles shown below the horizontal axes in FIGS. 11A and 11B schematically show changes in the cross-sectional shape of the core.

In the multimode waveguide of FIG. 11A, the degeneration of the TE ₀₁ mode and the TE ₁₀ mode is solved as the core width W of the core cross section gradually becomes smaller and becomes a tapered rectangle, and the equivalent refraction of the TE ₀₁ mode is solved. The rate n _eq (indicated by a broken line in the figure) is larger than the equivalent refractive index n _eq of TE ₁₀ mode (indicated by a thick solid line in the figure).

In the first and second single mode waveguides of FIG. 11B, the equivalent refractive index n _eq increases as the core width W of the core cross section gradually increases.

FIGS. 11C to 11E show the multimode waveguide (TE) when the multimode waveguide and the first and second single mode waveguides are brought close to each other so that adiabatic mode transition occurs. ₀₁ mode and TE ₁₀ mode) and a mode transition between single mode waveguides. The TE ₀₁ mode is indicated by a broken line, the TE ₁₀ mode is indicated by a solid line, and the fundamental mode of the single mode waveguide is indicated by a one-dot chain line.

FIG. 11C shows the dispersion curves of the TE ₀₁ mode, TE ₁₀ mode, and TE ₁₁ mode of the multimode waveguide of FIG. 11A and the first and second single-mode waveguides of FIG. A state in which the dispersion curve of the waveguide is superimposed is shown. Note that in the TE _10- mode separation part (C in FIGS. 8 and 9), the core thickness h of the first single-mode waveguide is the same as the core thickness of the multi-mode waveguide. The dispersion curve of the single mode waveguide is the same as the dispersion curve of the TE ₀₀ mode of the multimode waveguide of FIG. On the other hand, in the TE ₀₁ mode separation part (D in FIGS. 8 and 9), the core thickness h of the second single mode waveguide does not necessarily have to be the same as the core thickness of the multimode waveguide. For simplicity of explanation, it is assumed that the dispersion curve of the second single mode waveguide of FIG. 11 (b) is the same as the dispersion curve of the TE ₀₀ and mode of the multimode waveguide of FIG. 11 (a). explain.

When the core thickness h of the second single mode waveguide of the TE ₀₁ mode separation section (D in FIGS. 8 and 9) is different from the core thickness of the multimode waveguide, FIGS. The dispersion curve of the second single-mode waveguide of FIG. 11 is different from the TE ₀₀ and mode dispersion curves of the multimode waveguide of FIG. 11A, but the normalized propagation constant is zero when the core width is zero. The point where it becomes zero is common, and the basic principle of the operation of the mode multiplexer / demultiplexer of the present invention is the same.

In FIG. 11 (c), the dispersion curve (the one-dot chain line in the figure) of the first single-mode waveguide is a process in which the cross-sectional shape is changed from a thin state to a thickening direction (rightward on the horizontal axis). First, the TE ₁₀ mode dispersion curve (thick solid line in the figure) where the degeneration of the multimode waveguide has been solved and the point Q are interchanged, and then the second single mode dispersion curve (dotted line in the figure) ) Is switched between the TE ₀₁ mode dispersion curve (broken line in the figure) and the point R.

FIG. 11 (d) shows a change in the magnitude relationship of the propagation constant between the dispersion curve of the fundamental mode of the first single-mode waveguide and the dispersion curve of the TE ₁₀ mode of the multimode waveguide.

In the direction in which the cross-sectional shape with the multimode waveguide becomes narrow (the left direction of the horizontal axis), the first single-mode waveguide has a cross-sectional shape that is initially thin and thick in the propagation direction. Before the point Q where the dispersion curve of the fundamental mode of the mode waveguide and the dispersion curve of the TE ₁₀ mode of the multi-mode waveguide change in the relationship of the propagation constants (on the right side of the point Q in the figure), the first Since the propagation constant of the fundamental mode of the single mode waveguide is smaller than the propagation constant of the TE ₁₀ mode, no branching due to adiabatic transition from the multimode waveguide to the first single mode waveguide occurs.

Next, after the point Q (on the left side of the point Q in the figure), the magnitude of the propagation constant is switched, and the propagation constant of the fundamental mode of the first single mode waveguide is the TE of the multimode waveguide. _Since it becomes larger than the propagation constant of ₁₀ modes, the TE ₁₀ mode is demultiplexed into the first single mode waveguide by the adiabatic transition from the multimode waveguide to the first single mode waveguide.

In the direction in which the cross-sectional shape of the multimode waveguide becomes narrower (leftward in the horizontal axis), the dispersion curve of the fundamental mode of the second single-mode waveguide is the dispersion curve of the TE ₀₁ mode at point R next to point Q. And the relationship of magnitude changes.

FIG. 11E shows that the magnitude relationship between the dispersion curve of the fundamental mode of the second single mode waveguide and the dispersion curve of the TE ₀₁ mode of the multimode waveguide is interchanged.

In the direction in which the cross-sectional shape of the multimode waveguide is thinner (the left direction of the horizontal axis), the dispersion curve of TE ₀₁ mode of dispersion and multimode waveguide fundamental mode of the second single-mode waveguide size relationship Before the point R at which the is switched (on the right side of the point R in the figure), the propagation constant of the fundamental mode of the second single mode waveguide is smaller than the propagation constant of the TE ₀₁ mode. No branching due to adiabatic transition to the second single-mode waveguide occurs.

Next, after the point R (on the left side of the point R in the figure), the magnitude of the propagation constant is switched, and the propagation constant of the fundamental mode of the second single mode waveguide is the propagation constant of the TE ₀₁ mode. The TE ₀₁ mode is demultiplexed into the second single mode waveguide by adiabatic transition from the multimode waveguide to the second single mode waveguide. However, since the mode transition in this case occurs in the vertical direction (y direction in FIG. 8), the second single mode waveguide is arranged on the upper side or the lower side of the multimode waveguide as shown in FIG.

From the above operation, after the multimode waveguide is degenerated, the TE ₁₀ mode first transits to the fundamental mode of the first single-mode waveguide, and then the TE ₀₁ mode becomes the second single-mode waveguide. Transition to the basic mode.

In the configuration example of the mode multiplexer / demultiplexer shown in FIGS. 8 and 9, the TE ₁₀ mode is demultiplexed (separated) from the multi-mode waveguide 2 to the first single mode waveguide 3, and the TE ₀₁ mode is changed to the second mode. The single mode waveguide 4 is demultiplexed (separated). The remaining TE ₀₀ mode obtained by demultiplexing (separating) the TE ₁₀ mode and the TE ₀₁ mode is emitted from the tapered end F of the multimode waveguide 2.

FIG. 12 shows the propagation analysis of the modes in the coupling branches C and D of the mode multiplexer / demultiplexer, and the electromagnetic field strengths in the x and y directions of the multimode waveguide and single mode waveguide between 2 mm in the propagation direction. The distribution shows a state where the LP ₁₁ ^even mode and the LP ₁₁ ^odd mode of the several mode fiber are incident at the position of z = 0 μm.

FIG. 12A shows a TE ₁₀ mode propagation analysis in the coupling branch C, and shows a state in which the TE ₁₀ mode transitions from the multimode waveguide 2 to the first single mode waveguide 3 in the coupling branch C. ing. The mode branching ratio of this coupling branch C is 25 dB.

FIG. 12B shows a TE ₀₁ mode propagation analysis in the coupling branch D, and shows a state in which the TE ₀₁ mode transitions from the multimode waveguide 2 to the first single mode waveguide 3 in the coupling branch D. ing. The mode branching ratio of the coupling branch part D is 19 dB.

  The coupling branch C is composed of the multimode waveguide 2 and the first single mode waveguide 3 juxtaposed in the x direction with respect to the multimode waveguide 2, and the coupling branch D is composed of the multimode waveguide D. The waveguide 2 and the multimode waveguide 2 are configured by a second single mode waveguide 4 stacked in parallel in the y direction.

[Second configuration example of mode multiplexer / demultiplexer]
Next, a second configuration example of the mode multiplexer / demultiplexer of the present invention will be described. FIG. 13A is a diagram showing a schematic configuration of the second configuration example of the mode multiplexer / demultiplexer, and shows a state viewed from the core thickness direction.

  Similarly to the mode multiplexer / demultiplexer 1A, the mode multiplexer / demultiplexer 1B includes a multimode waveguide 2 having a rectangular core cross section on a substrate (not shown) and first and second single-mode waveguides. The waveguides 3 and 4 are formed, and the multimode waveguide 2 and the first and second single mode waveguides 3 and 4 constitute an asymmetric taper coupled mode transition type waveguide.

  The configuration of the first and second single mode waveguides 3 and 4 can be the same as that of the first configuration example, and the first configuration example is the first and second single mode waveguides 3 and 3. 4 is different in the arrangement position of the multimode waveguide 2.

  In the first configuration example, the first single mode waveguide 3 and the second single mode waveguide 4 are arranged at positions shifted in the propagation direction of the multimode waveguide 2, whereas the second In the configuration example, the first single mode waveguide 3 and the second single mode waveguide 4 are arranged at substantially the same position in the propagation direction of the multimode waveguide 2.

  With reference to FIG. 14, an operation of demultiplexing a single mode waveguide by solving mode degeneration in the second configuration example will be described.

  14A shows the dispersion curve of the multimode waveguide as in FIG. 11A, and FIG. 14B shows the first and second single mode waveguides as in FIG. 11A. The dispersion curve is shown. 14A and 14B schematically show changes in the cross-sectional shape of the core. FIG. 14B shows the case where the core thicknesses of the first and second single mode waveguides are different.

FIGS. 14C to 14E show the multimode waveguide (TE) when the multimode waveguide and the first and second single mode waveguides are brought close enough to cause adiabatic mode transition. ₍₀₁ mode and TE ₁₀ mode) and mode transitions between the first and second single mode waveguides. The TE ₀₁ mode is indicated by a broken line, the TE ₁₀ mode is indicated by a thick solid line, and the fundamental modes of the first and second single mode waveguides are indicated by an alternate long and short dash line, respectively.

FIG. 14C shows the dispersion curves of the TE ₀₁ mode, TE ₁₀ mode, and TE ₁₁ mode of the multimode waveguide of FIG. 14A and the dispersion curves of the first and second single mode waveguides. The superimposed state is shown. Here, the dispersion curves of the two single mode waveguides are indicated by a one-dot chain line and a two-dot chain line.

In FIG. 14C, the dispersion curves of the two single mode waveguides are at the incident end E in the direction from the point P toward the direction in which the cross-sectional shape of the multimode waveguide becomes narrower (the left side of the horizontal axis). At a close position, the TE ₁₀ mode dispersion curve (thick solid line in the figure) and the TE ₀₁ mode dispersion curve (dashed line in the figure) where the degeneration of the multimode waveguide is solved are set smaller.

FIG. 14D shows the case of the first single mode waveguide 3 arranged in the x direction with respect to the multimode waveguide. The propagation constant of the fundamental mode of the single mode waveguide 3 is smaller than the TE ₁₀ mode propagation constant of the multimode waveguide at a position close to the incident end E, and gradually increases in the propagation direction. for greater than TE ₁₀ mode of propagation constant, by adiabatic transition from the multi-mode waveguide to the single-mode waveguide, TE ₁₀ mode of the multimode waveguide is a single mode waveguide 3 demultiplexed.

FIG. 14E shows the case of the second single-mode waveguide 4 that is arranged and laminated in the y-direction with respect to the multimode waveguide. The propagation constant of the fundamental mode of the single mode waveguide 4 is smaller than the propagation constant of the TE ₀₁ mode of the multimode waveguide at a position close to the incident end E, and gradually increases in the propagation direction. for greater than TE ₀₁ mode of propagation constant, by adiabatic transition from the multi-mode waveguide to the single-mode waveguide, TE ₀₁ mode of the multimode waveguide is a single mode waveguide 4 demultiplexed.

From the above operation, after the multimode waveguide is degenerated, the TE ₁₀ mode transitions to the first single mode waveguide 3 arranged in the x direction with respect to the multimode waveguide, and the TE ₀₁ mode is The mode transition is made to the second single mode waveguide 4 arranged in the y direction with respect to the mode waveguide. Here, _TE by performing a single-mode waveguides having different ₁₀ modes Mode mode transition of the _{TE 01} mode, can be _{TE 10} mode and the _{TE 01} mode demultiplexing (separation).

[Third configuration example of mode multiplexer / demultiplexer]
Next, a third configuration example of the mode multiplexer / demultiplexer according to the present invention will be described. FIG. 13B is a diagram showing a schematic configuration of the third configuration example of the mode multiplexer / demultiplexer, and shows a state viewed from the core thickness direction.

  Similarly to the mode multiplexer / demultiplexer 1A, the mode multiplexer / demultiplexer 1C includes a multimode waveguide 2 having a rectangular core cross-section on a substrate (not shown) and first and second single-mode waveguides. The waveguides 3 and 4 are formed, and the multimode waveguide 2 and the first and second single mode waveguides 3 and 4 constitute an asymmetric taper coupled mode transition type waveguide.

  The multimode waveguide 2 of the first configuration example is tapered in the propagation direction from the end where the optical fiber 11 is coupled to the other end, whereas the multimode waveguide of the third configuration example is tapered. Reference numeral 5 denotes a configuration with a tip.

  With reference to FIG. 15, an operation of demultiplexing a single mode waveguide by solving mode degeneration in the third configuration example will be described.

  15A shows the dispersion curve of the multimode waveguide as in FIG. 11A, and FIG. 15B shows the first and second single mode waveguides as in FIG. 14B. A dispersion curve is shown. The rectangles shown below the horizontal axes in FIGS. 15A and 15B schematically show changes in the cross-sectional shape of the core.

In the multimode waveguide of FIG. 15A, the TE ₀₁ mode and the TE ₁₀ mode increase from a square shape toward a thickened rectangle toward the direction in which the core width W increases from the point P (right direction in the figure). The degeneration is solved and the equivalent refractive index n _eq of TE ₁₀ mode (indicated by a broken line in the figure) is larger than the equivalent refractive index n _eq of TE ₀₁ mode (indicated by a thick solid line in the figure).

In the first and second single mode waveguides of FIG. 15B, the equivalent refractive index n _eq increases as the core width W of the core cross section gradually increases and becomes a tapered rectangle.

FIGS. 15C to 15E show the multimode waveguide (TE) when the multimode waveguide and the first and second single mode waveguides are brought close enough to cause adiabatic mode transition. ₍₀₁ mode and TE ₁₀ mode) and mode transitions between the first and second single mode waveguides. The TE ₀₁ mode is indicated by a broken line, the TE ₁₀ mode is indicated by a thick solid line, and the fundamental mode of the single mode waveguide is indicated by a one-dot chain line.

FIG. 15C shows the dispersion curves of the TE ₀₁ mode, TE ₁₀ mode, and TE ₁₁ mode of the multimode waveguide of FIG. 15A, and the first and second single-mode waveguides of FIG. A state in which the dispersion curve of the waveguide is superimposed is shown. In FIG. 15C, the right side in the drawing from the point P is a range in which the cross-sectional shape of the multimode waveguide becomes thicker.

In FIG. 15C, the dispersion curve of the first single-mode waveguide (the one-dot chain line in the figure) shows that the multimode waveguide is first degenerated in the direction in which the cross-sectional shape becomes thicker (the right direction of the horizontal axis). The dissolved TE ₁₀ mode dispersion curve (thick solid line in the figure) and the propagation constant at the point S are interchanged, and then the second single mode waveguide dispersion curve (two-dot chain line in the figure) is TE The magnitude relationship of the propagation constants is switched between the dispersion curve of the ₀₁ mode (broken line in the figure) and the point T.

FIG. 15 (d) shows the change in the magnitude relationship of the propagation constant between the dispersion curve of the fundamental mode of the first single-mode waveguide and the dispersion curve of the TE ₁₀ mode of the multimode waveguide.

Propagation constant between the dispersion curve of the fundamental mode of the first single-mode waveguide and the dispersion curve of the TE ₁₀ mode of the multi-mode waveguide in the direction in which the cross-sectional shape of the multi-mode waveguide becomes thicker (right direction of the horizontal axis). Since the propagation constant of the fundamental mode of the first single-mode waveguide is smaller than the propagation constant of the TE ₁₀ mode before the point S at which the magnitude relationship of the two is switched (on the left side of the point S in the figure), the multimode No demultiplexing due to adiabatic transition from the waveguide to the fundamental mode of the first single mode waveguide occurs.

Next, after the point S (on the right side of the point S in the figure), the propagation constants are switched, and the propagation constant of the fundamental mode of the first single mode waveguide is the propagation constant of the TE ₁₀ mode. Therefore, the TE ₁₀ mode of the multimode waveguide is demultiplexed into the single mode waveguide by the adiabatic transition from the multimode waveguide to the first single mode waveguide. At this time, the width of the multimode waveguide is assumed not to be larger than the cutoff point M at which the TE ₁₁ mode starts to propagate, and the width of the first single mode waveguide is cut at which the TE ₁₀ mode starts to propagate. It is assumed that it is not thicker than the off point L.

In the direction in which the cross-sectional shapes of the multimode waveguide and the single mode waveguide become thicker (rightward on the horizontal axis), the dispersion curve of the fundamental mode of the first single mode waveguide is TE ₁₀ mode dispersion at point S. The magnitude relationship between the curve and the propagation constant is switched, and the second single mode waveguide is switched at the point T between the TE ₀₁ mode dispersion curve and the propagation constant.

FIG. 15 (e) shows the change in the magnitude relationship of the propagation constant between the dispersion curve of the fundamental mode of the second single-mode waveguide and the dispersion curve of the TE ₀₁ mode of the multimode waveguide.

In the direction in which the cross-sectional shapes of the multimode waveguide and the single mode waveguide become thicker (rightward on the horizontal axis), the dispersion curve of the fundamental mode of the single mode waveguide and the dispersion curve of the TE ₀₁ mode of the multimode waveguide Since the propagation constant of the fundamental mode of the second single-mode waveguide is smaller than the propagation constant of the TE ₀₁ mode before the point T where the magnitude relationship of the propagation constants is switched (on the left side of the point T in the figure), Demultiplexing due to adiabatic transition from the multimode waveguide to the single mode waveguide does not occur.

Next, after the point T (right side of the point T in the figure), the magnitude of the propagation constant is switched, and the propagation constant of the fundamental mode of the second single mode waveguide is the TE of the multimode waveguide. _Since it becomes larger than the propagation constant of the ₀₁ mode, the TE ₀₁ mode is demultiplexed into the single mode waveguide by the adiabatic transition from the multimode waveguide to the single mode waveguide. At this time, the width of the multi-mode waveguide should not be thicker than the cut-off point M at which the TE ₁₁ mode starts to propagate, and the width of the second single-mode waveguide is propagated by the TE ₁₀ mode. It is assumed that it is not thicker than the cutoff point L to be started.

From the above operation, after the multimode waveguide is degenerated, the TE ₁₀ mode first transits to the fundamental mode of the first single-mode waveguide, and then the TE ₀₁ mode becomes the second single-mode waveguide. Transition to the basic mode.

  The present invention is not limited to the above embodiments. Various modifications can be made based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

  The mode multiplexer / demultiplexer of the present invention can be applied to mode multiplexing transmission of optical fiber communication.

1A mode multiplexer / demultiplexer 1B mode multiplexer / demultiplexer 1C mode multiplexer / demultiplexer 2 multimode waveguide 3 first single mode waveguide 4 second single mode waveguide 5 multimode waveguide 11 optical fiber 12 Polarization separating member C, D Coupling branch E End F End G Port h Core thickness L TE ₁₀ Mode cut-off point M TE ₁₁ Mode cut-off point P Two higher orders of multimode waveguide Intersection where mode propagation constants coincide Q The point where the propagation constant of one higher-order mode of a multimode waveguide matches the propagation constant of the fundamental mode of a single-mode waveguide R One higher-order mode of a multimode waveguide The point where the propagation constant matches the propagation constant of the fundamental mode of the single mode waveguide S The point where the propagation constant of one higher order mode of the multimode waveguide matches the propagation constant of the fundamental mode of the single mode waveguide T Mode guidance The point where the propagation constant of one higher-order mode of the waveguide matches the propagation constant of the fundamental mode of the single-mode waveguide. TE ₀₁ , TE ₁₀ Electromagnetic field distribution W Core width

Claims (7)

A mode multiplexer / demultiplexer including an asymmetric tapered coupled mode transition waveguide formed of two single mode waveguides and a multimode waveguide whose core cross-sectional shape is rectangular on a substrate,
The multi-mode waveguide has a tapered shape in which the ratio of the core width in the parallel direction to the core thickness in the direction perpendicular to the substrate gradually changes along the propagation direction,
The single-mode waveguide has a curved shape curved on the substrate, and has a tapered shape in which the core width gradually increases along the propagation direction with respect to the substrate.
In the arrangement of the multimode waveguide and the two single mode waveguides,
Of the two single-mode waveguides, the first single-mode waveguide is juxtaposed along the propagation direction with respect to the surface of the multi-mode waveguide in the core width direction, and the second single-mode waveguide is Laminated and juxtaposed along the propagation direction with respect to the plane of the core thickness direction of the multimode waveguide,
In the juxtaposition, the first and second single mode waveguides are separated after being close to the multimode waveguide by the curved shape, and the single mode waveguide and the multimode waveguide are adiabatic. A coupling branch is formed close to the distance exhibiting the mode interaction,
In the coupling branch, the higher-order mode is degenerated and / or degenerated due to the tapered shape of the multi-mode waveguide, the higher-order mode propagation constant of the multi-mode waveguide, and the fundamental mode of the single-mode waveguide. Due to the adiabatic transition of the modes between the waveguides due to the change in the magnitude relationship of the propagation constant, the TE ₁₀ mode and the TE ₀₁ mode, the TM ₁₀ mode and the TM ₀₁ mode, which are the primary and horizontal primary modes of TE and TM, are combined. A mode multiplexer / demultiplexer characterized by performing wave and / or demultiplexing. In the two single mode waveguides,
A first single mode waveguide juxtaposed on a surface of the multimode waveguide in the core width direction mode-shifts a mode distribution in a substrate parallel direction with the multimode waveguide;
The second single mode waveguide juxtaposed on the surface of the multimode waveguide in the core thickness direction mode-shifts the mode distribution in the substrate vertical direction between the multimode waveguide and the second mode waveguide. The mode multiplexer / demultiplexer according to 1.   3. The mode coupling according to claim 1, wherein in the coupling branch portion, a magnitude of a propagation constant of a higher-order mode of the multi-mode waveguide and a propagation constant of a fundamental mode of the single-mode waveguide are interchanged. Duplexer. The multi-mode waveguide has a tapered shape that becomes narrower toward the end side where the degenerated mode is emitted or incident from the end side where the degenerated mode is incident or emitted.
The said single mode waveguide is a taper shape which becomes thick toward the edge part side which radiate | emits or enters the mode by which the degeneracy was released | excluded, It is any one of Claim 1 to 3 characterized by the above-mentioned. Mode multiplexer / demultiplexer. The multimode waveguide has a tapered shape that becomes thicker toward the end side where the degenerated mode is emitted or incident from the end side where the degenerated mode is incident or emitted.
The said single mode waveguide is a taper shape which becomes thick toward the edge part side which radiate | emits or enters the mode by which the degeneracy was released | excluded, It is any one of Claim 1 to 3 characterized by the above-mentioned. Mode multiplexer / demultiplexer. An optical fiber having a circular cross-sectional shape is abutted and coupled to one end of the multimode waveguide,
The mode coupling according to any one of claims 1 to 5, wherein a polarization separation member is coupled between the multimode waveguide and the optical fiber, or at one end of the single mode waveguide. Duplexer.   The cross-sectional shape of the end portion of the multimode waveguide coupled to the circular optical fiber is a square shape, a vertically long shape, or a horizontally long rectangular shape, and has a core width parallel to the substrate and a core thickness perpendicular to the substrate. The mode multiplexer / demultiplexer according to claim 6, wherein the ratio of λ changes gradually along the propagation direction from the end face.