JP2013068909A - Optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To split output light in any proportion while using a 3dB directional coupler in a Mach-Zehnder interferometer.SOLUTION: An optical waveguide 11 including a clad 12 and a core 18, comprises a first and a second optical waveguide 16a and 16b, and has a first and a second directional coupler 32L and 32R composed of the first and second optical waveguides arranged in parallel with each other, and an arm part 32C composed of the first and second optical waveguides intervening between the first and second directional couplers. The first and second directional couplers function as 3dB couplers in regard to input light, and include a first width taper part 16T1 at the first or the second optical waveguide in the first directional coupler, which narrows from a first width W1 to a second width W2, and a second width taper part 16T2 at the first or the second optical waveguide in the second directional coupler, which widens from the second width to the first width. The arm part gives a phase difference of (2m+z)π to the input light (where z satisfies 0<z<1).

Description

  In the present invention, when performing two-way communication using two types of light having different wavelengths propagating through one optical fiber, the combined demultiplexing of the light output from the light emitting element and the light input to the light receiving element is performed. It is related with the optical element to perform.

  In an optical subscriber communication system in which optical transmission from the subscriber side to the station side (uplink communication) and optical transmission from the station side to the subscriber side (downlink communication) are performed with one optical fiber, Communication and downlink communication may be performed with light of different wavelengths. In this case, an optical element (hereinafter also referred to as an optical multiplexing / demultiplexing element) that multiplexes / demultiplexes light of different wavelengths is required on both the station side and the subscriber side.

  2. Description of the Related Art A subscriber-side terminal unit (ONU: Optical Network Unit) used in an optical subscriber communication system includes an optical multiplexing / demultiplexing element, a light emitting element, and a light receiving element that are spatially optically optically aligned. However, in recent years, an optical multiplexing / demultiplexing element constituted by an optical waveguide has been developed in order to reduce the labor for aligning the optical axis (see, for example, Patent Documents 1 to 5). In the optical multiplexing / demultiplexing element using the optical waveguide (hereinafter also referred to as a waveguide type optical element), the light propagation path is limited to a pre-made optical waveguide. It is not necessary to align the optical axis of a lens or mirror. Further, in the waveguide type optical element, the light emitting element and the light receiving element may be aligned with the input / output end of the optical waveguide with reference to a mark previously created in the optical multiplexing / demultiplexing element. Therefore, the labor of strict optical axis alignment of the light beams entering and exiting the light emitting element and the light receiving element is greatly reduced.

Recently, a cladding SiO ₂ as a material, an optical waveguide with a core refractive index difference between the SiO ₂ to a large Si and material (hereinafter, also referred to as Si optical waveguides.) Configuration and waveguide-type optical element is reported (For example, see Non-Patent Documents 1 to 3).

  Since the refractive index of the core of the Si optical waveguide is much larger than the refractive index of the cladding, light can be strongly confined in the optical waveguide. Further, by utilizing this large refractive index difference, a curved optical waveguide that bends light with a small radius of curvature of about 1 μm can be realized. Furthermore, since a processing technique using a Si electronic device can be used at the time of manufacturing, a very fine submicron cross-sectional structure can be realized. For these reasons, the waveguide type optical device can be miniaturized by using the Si optical waveguide.

  As a waveguide type optical element using a Si optical waveguide, a one-stage Mach-Zehnder interferometer (hereinafter also referred to as an MZ interferometer) that also functions as a wavelength separation element is disclosed (for example, see Non-Patent Document 4). . The MZ interferometer of this document constitutes a directional coupler by an optical waveguide whose width changes gradually (hereinafter also referred to as a width taper waveguide). As a result, the MZ interferometer in the literature is excellent in resistance to a dimensional error in the width direction of the optical waveguide (hereinafter also referred to as width error), and has excellent characteristics such as operation independent of polarization. ing.

Photonics Technology Letters vol. 18, no. 22, p. 2392, November 2006 Photonics Technology Letters vol. 20, no. 23, p. 1968, December 2008 Optics Express vol. 18, no. 23, p. 23891, November 2010 International Conference OFC 2011 Proceedings, Lecture Number OThM3

U.S. Pat. No. 4,860,294 US Pat. No. 5,764,826 US Pat. No. 5,960,135 US Pat. No. 7,072,541 JP-A-8-163028

  However, a multi-stage MZ interferometer type wavelength separation element with better characteristics could not be constructed with this directional coupler. This is because the function of this directional coupler is limited to a 3 dB coupler. That is, in a multistage MZ interferometer type wavelength separation element, an arbitrary distribution ratio is required for each MZ interferometer. In other words, when the MZ interferometer is configured with a directional coupler that functions as a 3 dB coupler, the distribution ratio is limited to 1: 0, so light can be output to the two output ports at an arbitrary intensity ratio. could not.

  The present invention has been made in view of such problems. Therefore, in the present invention, while using a directional coupler type 3 dB coupler, the output light can be distributed at an arbitrary ratio, and thus can be used for a multistage MZ interferometer type wavelength separation element. It aims at obtaining the optical element as a meter.

  In order to achieve the above-described object, the inventors have intensively studied and have come up with the idea that a predetermined phase difference is generated in the light propagating through the two waveguides constituting the arm portion of the directional coupler. Therefore, the optical element of the present invention includes an optical waveguide composed of a clad provided on the main surface side of the substrate and a core provided in the clad. The optical waveguide includes first and second optical waveguides, and the first and second optical waveguides constitute the first and second directional couplers and the arm portion.

  Each of the first and second directional couplers includes first and second optical waveguide portions arranged in parallel to each other at a distance allowing optical coupling. The arm portion is composed of portions of first and second optical waveguides interposed between the first and second directional couplers.

The first and second directional couplers function as a 3 dB coupler for the first light with the first wavelength λ ₁ . Further, the first or second optical waveguide constituting the first directional coupler has a width measured in a direction perpendicular to the light propagation direction and parallel to the main surface from the first width to the first width. A first width taper portion that is reduced along the light propagation direction is formed to a smaller second width. Similarly, a second width taper portion whose width increases from the second width to the first width along the light propagation direction is formed in the first or second optical waveguide constituting the second directional coupler. Yes.

  The arm unit has a phase difference of (2m + z) π with respect to the first light propagating through the first and second optical waveguides of the arm unit (m is an integer greater than or equal to 0, and z is a real number where 0 <z <1. ).

  The optical element of the present invention is configured to generate a predetermined phase difference in the arm portion of the directional coupler. As a result, according to the present invention, output light can be distributed at an arbitrary ratio while using a directional coupler as a 3 dB coupler, and therefore can be used for a multi-stage MZ interferometer type wavelength separation element. An optical element as an MZ interferometer is obtained.

(A) is a top view which shows roughly the structure of the optical element of Embodiment 1, (B) is an enlarged plan view of a 1st directional coupler, (C) is a 2nd directional coupler. It is an enlarged top view, (D) is the end elevation which cut | disconnected (A) along the AA line. (A) and (B) are schematic diagrams schematically depicting the behavior of light propagating through a directional coupler, and (C) schematically depicts the behavior of light propagating through the optical element of Embodiment 1. It is a schematic diagram. 6 is a plan view schematically showing a structure of an optical element according to Embodiment 2. FIG. It is the schematic diagram which drew the behavior of the light which propagates the optical element of Embodiment 2 typically. (A) And (B) is a top view which shows typically the structure of the modification of the optical element of Embodiment 2. FIG. FIG. 11 is a characteristic diagram illustrating operation characteristics of a modified example of the second embodiment. FIG. 11 is a characteristic diagram illustrating operation characteristics of a modified example of the second embodiment. FIG. 10 is a plan view schematically showing the structure of another modification example of Embodiment 2. 6 is a plan view schematically showing the structure of an optical element according to Embodiment 3. FIG. FIG. 10 is a plan view schematically showing the structure of a modification example of the optical element of Embodiment 3. 10 is a plan view schematically showing the structure of another modification of the optical element of Embodiment 3. FIG.

  Embodiments of the present invention will be described below with reference to the drawings. In each figure, the shape, size, and arrangement relationship of each component are schematically shown to the extent that the present invention can be understood. Moreover, although the preferable structural example of this invention is demonstrated hereafter, the material of each component, a numerical condition, etc. are only a suitable example. Therefore, the present invention is not limited to the following embodiments. Moreover, in each figure, the same code | symbol is attached | subjected to a common component and the description may be abbreviate | omitted.

[Embodiment 1]
Hereinafter, the optical element of Embodiment 1 will be described with reference to FIGS. 1 and 2. FIG. 1A is a plan view schematically showing the structure of the optical element. FIG. 1B is an enlarged plan view of the first directional coupler. FIG. 1C is an enlarged plan view of the second directional coupler. FIG. 1D is an end view of FIG. 1A cut along the line AA. 2A and 2B are schematic diagrams schematically illustrating the behavior of light propagating through the directional coupler. FIG. 2C is a schematic diagram schematically depicting the behavior of light propagating through the optical element. In FIG. 1A, the core 18 constituting the optical element is covered with the clad 12 and thus cannot be directly observed, but is drawn with a solid line for emphasis. 2A to 2C, drawing of the substrate 8 and the clad 12 is omitted.

(Construction)
The structure of the optical element 10 will be described with reference to FIGS. The optical element 10 includes an optical waveguide 11 composed of a clad 12 and a core 18. The optical waveguide 11 includes an MZ interferometer 32. The optical waveguide 11 further includes an input unit 24 and an output unit 26 as optional elements.

  The clad 12 is a film body that extends to the main surface 8 a side of the substrate 8 with a uniform thickness. More specifically, the clad 12 is provided on the main surface 8a and constitutes the optical waveguide 11 together with the core 18 included therein.

  Hereinafter, regarding the direction and dimensions of the optical element, a direction perpendicular to the light propagation direction and parallel to the main surface 8a is referred to as a “width direction”, and a length measured along the width direction is referred to as a “width”. A direction perpendicular to the main surface 8a is referred to as a “thickness direction”, and a length measured along the thickness direction is referred to as a “thickness”. Similarly, the geometric length measured along the light propagation direction is referred to as “length”. A section perpendicular to the light propagation direction of a predetermined structure is referred to as a “cross section”.

The material constituting the clad 12 is, for example, SiO ₂ having a refractive index of about 1.44. The thickness of the clad 12 is about 3 μm. The core 18 is arranged at a depth of about 1.5 μm from the main surface 8a. In order to prevent undesired coupling of light to the substrate 8, it is preferable to interpose a cladding 12 having a thickness of 1 μm or more between the core 18 and the substrate 8. The substrate 8 is made of Si, for example.

  The core 18 is formed of a material having a refractive index that is 40% or more larger than the refractive index of the cladding 12. In the example shown in this embodiment, the core 18 is made of Si having a refractive index of about 3.5.

  The input unit 24, the MZ interferometer 32, and the output unit 26 constituting the optical waveguide 11 are connected in this order. More specifically, the input optical waveguide 24a of the input unit 24 is connected to the input end IN16a of the first optical waveguide 16a of the MZ interferometer 32. The output end OUT16a of the first optical waveguide 16a of the MZ interferometer 32 is connected to the first output optical waveguide 26a of the output unit 26. The output end OUT16b of the second optical waveguide 16b of the MZ interferometer 32 is connected to the second output optical waveguide 26b of the output unit 26.

  In the optical waveguide 11, all the components are channel-type waveguides having a rectangular cross section. Moreover, the optical waveguide 11 sets thickness D1 of all the components to 300 nm. Further, except for first and second directional couplers 32L and 32R described later, the optical waveguide 11 has a component width W of 300 nm. As described above, the first and second directional couplings are obtained by making the cross-sectional shape of the components of the optical waveguide 11 excluding the first and second directional couplers 32L and 32R into a square shape having a width of 300 nm and a thickness of 300 nm. The components of the optical waveguide 11 excluding the devices 32L and 32R can be made polarization independent.

The input unit 24 constituting the optical waveguide 11 includes an input optical waveguide 24a and a dummy waveguide 24b. One end of the input optical waveguide 24 a is exposed from the side surface of the clad 12. First light Lt1 of the first wavelength lambda ₁ is inputted from one end. The other end of the input optical waveguide 24 a is connected to the first optical waveguide 16 a constituting the MZ interferometer 32. One end of the dummy waveguide 24b is exposed from the side surface of the clad 12, and the other end is connected to the second optical waveguide 16b. The dummy waveguide 24b is not substantially related to the operation of the optical element 10.

  The MZ interferometer 32 that constitutes the optical waveguide 11 can arbitrarily input the first light Lt1 input to the input end IN16a of the first optical waveguide 16a from the output ends OUT16a and OUT16b of the first and second optical waveguides 16a and 16b. Is output at a distribution ratio of. More specifically, the first light Lt1 is distributed and output to the output terminals OUT16a and OUT16b at a distribution ratio corresponding to the phase difference φ generated in the first light Lt1 in the arm portion 32C. The distribution ratio is x when the input light is output from each of the two output ports at an intensity ratio of x: (1-x) (where x is 0 <x <1). Indicates. The distribution ratio x when the MZ interferometer 32 functions as a 3 dB coupler is 0.5.

  The MZ interferometer 32 is structurally composed of first and second optical waveguides 16a and 16b which are two optical waveguides arranged in parallel. Functionally, it includes first and second directional couplers 32L and 32R formed by first and second optical waveguides 16a and 16b, and an arm portion 32C.

  The first optical waveguide 16a, which is a structural element of the MZ interferometer 32, includes two first linear portions 16aL and 16aR and a first bending portion 16aC. The first curved portion 16aC is interposed between the first straight portions 16aL and 16aR. The second optical waveguide 16b includes two second linear portions 16bL and 16bR and a second bending portion 16bC. The second bending portion 16bC is interposed between the second straight portions 16bL and 16bR. In the following description, unless otherwise specified, the “g-th straight part” or “g-th curved part” (g = 1 or 2) means the h-th light beam constituting the g-th straight part or the g-th curved part. A partial region of the waveguide (h = 1 or 2) is shown.

  The first directional coupler 32L, the arm part 32C, and the second directional coupler 32R, which are functional elements of the MZ interferometer 32, are arranged in series in this order. The first directional coupler 32L includes first and second straight portions 16aL and 16bL. The second directional coupler 32R includes first and second straight portions 16aR and 16bR. Similarly, the arm portion 32C includes first and second bending portions 16aC and 16bC.

  The first directional coupler 32L includes first and second linear portions 16aL and 16bL that are separated from each other by a distance that allows optical coupling and are parallel to each other. The first straight portion 16aL is formed with a first width taper portion 16T1 whose width decreases from the first width W1 to the second width W2 (<W1) along the light propagation direction. More specifically, the width of the first width taper portion 16T1 is reduced to an isosceles trapezoidal shape from the input portion 24 toward the arm portion 32C. The second straight portion 16bL has a third width W3, and is disposed with a constant interval Sp between the second straight portion 16bL and the first width tapered portion 16T1. That is, the side surface of the second linear portion 16bL and the side surface of the first width taper portion 16T1 are parallel.

  The second directional coupler 32R includes first and second linear portions 16aR and 16bR that are separated from each other by a distance that allows optical coupling and are parallel to each other. The second linear portion 16bR is formed with a second width taper portion 16T2 whose width increases from the second width W2 to the first width W1 along the light propagation direction. More specifically, the width of the second width taper portion 16T2 expands into an isosceles trapezoidal shape equal to the first width taper portion 16T1 as it goes from the arm portion 32C to the output portion 26. The first straight portion 16aR has a third width W3, and is disposed with a constant interval Sp between the first straight portion 16aR and the second width tapered portion 16T2.

  Although details will be described later in the section of (Operation), the first and second directional couplers 32L and 32R function as a polarization-independent 3 dB coupler with respect to the first light Lt1. That is, the first light Lt1 input to the first straight line portion 16aL is equally distributed from the first directional coupler 32L to the first and second straight line portions 16aL and 16bL, so that both the straight line portions 16aL and 16bL. Is output at a distribution ratio of 1: 1. Similarly, the second directional coupler 32R outputs the first light Lt1 input from the first linear portion 16aR from the first and second linear portions 16aR and 16bR at a distribution ratio of 1: 1.

  In the first and second width taper portions 16T1 and 16T2, the first width W1 is about 360 nm. The second width W2 is about 320 nm. The third width W3 is about 320 nm. Further, the interval Sp between the side surfaces between the first width taper portion 16T1 and the second linear portion 16bL and the interval Sp between the side surfaces between the second width taper portion 16T2 and the first linear portion 16aR are equal to each other. About 410 nm. The total length L1 of the first and second width taper portions 16T1 and 16T2, that is, the lengths of the first and second directional couplers 32L and 32R are, for example, about 100 μm.

  The arm portion 32C includes first and second bending portions 16aC and 16bC arranged in parallel. The arm portion 32C has a phase difference φ given to the first light Lt1 after propagating through the first and second optical waveguides 16aC and 16bC as (2m + z) π (m is an integer greater than or equal to 0, and z is a real number where 0 <z <1. ) (Hereinafter also referred to as a phase difference condition). That is, the arm portion 32C is configured to give a phase difference φ greater than 0 and less than π to the first light Lt1.

  The interference conditions regarding the first light Lt1 propagating through the MZ interferometer 32 are given by φ = 2mπ and φ = (2m + 1) π. These two expressions indicate that the above-described distribution ratio x is 0 or 1 when the phase difference φ imparted to the first light Lt1 is equal to 2mπ or (2m + 1) π. That is, the case where the phase difference φ is 2 mπ corresponds to the distribution ratio x = 0, and the first light Lt1 is output only from the output end OUT16b. On the other hand, the case where the phase difference φ is (2m + 1) π corresponds to the distribution ratio x = 1, and the first light Lt1 is output only from the output end OUT16a.

  By the way, the phase difference φ is equal to 2mπ corresponds to the case where z = 0 in the above phase difference condition, and the phase difference φ is equal to (2m + 1) π when the phase difference condition is z = 1. Correspond. Accordingly, the inventor has conceived that the phase difference φ given to the first light Lt1 can be set to 2mπ <φ <(2m + 1) π by setting z to 0 <z <1. As a result, the optical element 10 can output the first light Lt1 at an arbitrary distribution ratio x even though the first and second directional couplers 32L and 32R, which are 3 dB couplers, are used.

  As will be described in detail later, in this embodiment, the first light Lt1 propagates through the arm portion 32C as symmetric mode light and antisymmetric mode light. Therefore, by providing the phase difference φ, a part of the symmetric mode light is converted into antisymmetric mode light at a ratio corresponding to the phase difference φ. Similarly, a part of the antisymmetric mode light is also converted into symmetric mode light at a ratio corresponding to the phase difference φ. Therefore, the first light Lt1 is output as a mixed light of the antisymmetric mode light having the intensity ratio x and the symmetric mode light having the intensity ratio (1-x) after propagating through the arm portion 32C. Hereinafter, both symmetric mode light and antisymmetric mode light are collectively referred to as both mode light.

In order to generate the phase difference φ of 2mπ <φ <(2m + 1) π with respect to the first light Lt1, in the optical element 10, the optical path lengths of the first and second bending portions 16aC and 16bC constituting the arm portion 32C Are different. The “optical path length” generally indicates an optical length obtained by correcting the geometric length P of the optical waveguide with the equivalent refractive index q of the optical waveguide that the light of a certain wavelength senses. When the optical path length is S, the following formula (1) is established between S, P, and q.
S = P × q (1)

Here, the optical path length of the first bending portion 16aC is Sa, and the optical path length of the second bending portion 16bC is Sb. Further, the optical path length difference between the first and second curved portions 16aC and 16bC a (Sa-Sb), and [Delta] S _1. By the way, since 2πΔS ₁ / λ ₁ = φ holds between the phase difference φ and the optical path length difference ΔS ₁ , ΔS ₁ may be set so that the following formula (2) is satisfied from the phase difference condition. .
2ΔS ₁ / λ ₁ = 2m + z (2)

Incidentally, the lengths of the first and second bending portions 16aC and 16bC has a [Delta] S ₁ obtained from Equation (2), the first and second curved portions 16aC and 16bC equivalent refractive index of about the first light Lt1, wherein What is necessary is just to obtain | require by (1).

  Returning to the description of the configuration of the optical waveguide 11 again, the output unit 26 includes a first output optical waveguide 26a and a second output optical waveguide 26b. One end of the first output optical waveguide 26a is connected to the first linear portion 16aR of the second directional coupler 32R. One end of the second output optical waveguide 26b is connected to the second linear portion 16bR of the second directional coupler 32R. The first light Lt1 is output from the first and second output optical waveguides 26a and 26b at a distribution ratio of x: (1-x), respectively.

  In this embodiment, the difference in length between the first and second bending portions 16aC and 16bC constituting the arm portion 32C is set to 128 nm. This corresponds to a phase difference condition where m is 0 and z is 1/2, thereby generating a phase difference φ of π / 2. The distribution ratio x at this time is 0.5.

(Operation)
Hereinafter, the operation of the optical element 10 will be described. Prior to the description of the overall operation of the optical element 10, the operation of the first and second directional couplers 32L and 32R alone will be described with reference to FIGS. 2 (A) and 2 (B). By the way, the first and second directional couplers 32L and 32R operate substantially equally although there is a formal difference in structure. Therefore, hereinafter, as a representative of the bidirectional couplers 32L and 32R, a description will be given using a directional coupler 32A configured similarly to the first directional coupler 32L.

  In general, in a directional coupler having a width taper waveguide as a constituent element (hereinafter also referred to as a width taper directional coupler), such as the directional coupler 32A, the light depends on the optical waveguide to which light is input. Are known to be separated into a symmetric mode or an anti-symmetric mode in a polarization-independent manner.

  That is, as shown in FIG. 2A, when the light LtIN1 is input from the input end IN16a of the first optical waveguide 16a having the first width tapered portion 16T, the output side OUT16 is caused by the action of the width tapered waveguide. Furthermore, the symmetric mode light LtOUT1 distributed across both the optical waveguides 16a and 16b is excited. When this symmetric mode light LtOUT1 is output to two optical waveguides arranged at intervals not optically coupled, such as an arm portion, it is distributed with equal intensity to these optical waveguides. That is, for the light LtIN1, the directional coupler 32A functions as a 3 dB coupler.

  Similarly, as shown in FIG. 2B, when the light LtIN2 is input to the input end IN16b of the second optical waveguide 16b whose width does not change, both optical signals are input to the output side OUT16 by the action of the width taper waveguide. The antisymmetric mode light LtOUT2 distributed across the waveguides 16a and 16b is excited. When the antisymmetric mode light LtOUT2 is also output to the two optical waveguides arranged at intervals not optically coupled, they are distributed with equal intensity to these optical waveguides. That is, the directional coupler 32A functions as a 3 dB coupler also for the light LtIN2. Accordingly, the directional coupler 32A functions as a 3 dB coupler with respect to light input from the input ends IN16a and IN16b.

  When the lights LtIN1 and LtIN2 are simultaneously input to both input terminals IN16a and IN16b of the directional coupler 32A, the symmetric mode light LtOUT1 and the antisymmetric mode light LtOUT2 are mixed at the output side OUT16 of the directional coupler 32A. Light is excited. Here, the intensity ratio between the symmetric mode light LtOUT1 and the antisymmetric mode light LtOUT2 excited on the output side OUT16 is equal to the intensity ratio between the light LtIN1 and the light LtIN2.

  In general, the reverse process holds for light propagation. Therefore, when the mixed light of the symmetric mode light LtOUT1 and the antisymmetric mode light LtOUT2 is input from the output side OUT16 side so as to straddle both the optical waveguides 16a and 16b, the symmetric mode light LtOUT1 has the width taper portion 16T. The light is concentrated on the first optical waveguide 16a and output as light LtIN1 from the input end IN16a. Further, the antisymmetric mode light LtOUT2 concentrates the light on the second optical waveguide 16b, which is an equal-width optical waveguide, and is output from the input terminal IN16b as the light LtIN2. When the intensity ratio between the input symmetric mode light LtOUT1 and the antisymmetric mode light LtOUT2 is x: (1-x), the intensity ratio between the output light LtIN1 and the light LtIN2 is also x: (1-x). .

  Based on the above, the overall operation of the optical element 10 will be described with reference to FIG.

First light Lt1 input to the input unit 24, the first linear portion 16aL of the first directional coupler 32L propagates through the input optical waveguides 24a, is inputted as a light Lt1 _1.

Since the first directional coupler 32L is width tapered directional coupler, light Lt1 ₁ is input to the first linear portion 16AL, as described above, to excite the light Lt1 ₂ is a symmetric mode light output side.

The first directional coupler 32L, so functions as 3dB couplers, light Lt1 ₂ excited in this way, it is equally distributed to each of the first and second curved portions 16aC and 16bC which constitute the arm portion 32C The The optical path length difference ΔS ₁ between the first and second bending portions 16aC and 16bC constituting the arm portion 32C is set as described above. Therefore, the phase difference φ given by the arm portion 32C, light Lt1 ₂ after propagating arm portion 32C is a light Lt1 ₃ is a mixed light of the symmetric mode light and the antisymmetric mode light. Since the phase difference φ is set as described above, the intensity ratio between the symmetric mode light and the antisymmetric mode light included in the light Lt1 ₃ is equal to the distribution ratio, and (antisymmetric mode) :( symmetric mode) = x : (1-x).

Subsequently, the light Lt1 ₃ is input to the second directional coupler 32R. The second directional coupler 32R is a width-tapered directional coupler configured similarly to the first directional coupler 32L. Therefore, symmetric mode light contained in the light Lt1 the intensity to the second linear portion 16bR having a second width tapered portions 16T2 concentrated, the distribution ratio (1-x) of the second output optical waveguide as the light Lt1 ₅ 26b is output in a cross state. Similarly, antisymmetric mode light contained in the first light Lt1 ₃ the intensity to the first straight portion 16aR concentrated, is output in the bar state from the first output optical waveguide 26a as a light Lt1 ₄ distribution ratio x The

  Here, “output in the cross state” means that the light input from the input end IN16a has its power transferred to the second optical waveguide 16b and is output from the output end OUT16b. Further, “output in the bar state” means that light input from the input end IN16a is output from the output end OUT16a without causing power transfer to the second optical waveguide 16b.

  The operation of the optical element 10 described above can be mathematically explained by a transfer matrix. That is, when the first light Lt1 input only to the input terminal IN16a is displayed in a matrix, it is expressed by the following formula (3). In addition, when the 1st light Lt1 is input only into the 2nd linear part 16bL, it represents with the matrix which replaced 0 and 1 of following formula (3).

  At this time, the transmission matrix of the first directional coupler 32L is expressed by the following formula (4).

  Further, the transmission matrix of the arm portion 32C is represented by the following formula (5).

  Therefore, the operation of the MZ interferometer 32 as a whole is expressed as the following equation (6) using equations (4) and (5).

  Expression (6) indicates that the MZ interferometer 32 can change the distribution ratio x in accordance with the value of the phase difference φ with respect to the first light Lt1. For example, when the phase difference φ is π / 2, the following equation (7) is obtained from the equation (6).

  Expression (7) indicates that when the phase difference φ is π / 2, the MZ interferometer 32 functions as a 3 dB coupler for the first light Lt1.

  As described above, the optical waveguide 11 excluding the first and second directional couplers 32L and 32R operates in a polarization-independent manner with respect to the first light Lt1. Further, as described above, the first and second directional couplers 32L and 32R also operate in a polarization independent manner with respect to the first light Lt1. Therefore, the optical element 10 operates independent of polarization with respect to the first light Lt1. That is, the TE polarization component and the TM polarization component of the first light Lt1 have the same distribution ratio x.

  As described above, the optical element 10 of this embodiment uses the first and second directional couplers 32L and 32R, which are 3 dB couplers, and outputs the first light Lt1 at an output end with an arbitrary distribution ratio. It can output from OUT16a and OUT16b. Therefore, the MZ interferometer 32 of the optical element 10 can be used as an element constituting a multistage MZ interferometer type wavelength separation element.

(Modification)
Hereinafter, modified examples of the optical element 10 will be described.

(Modification 1)
In this embodiment, the case where a material having a refractive index 40% or more larger than the refractive index of the cladding 12 is used as the core 18 has been described. However, if light can be confined in the core with sufficient intensity, the refractive index of the core does not need to be 40% or more higher than the refractive index of the cladding. Such an optical element including an optical waveguide composed of a core and a clad can also output the first light Lt1 from the output terminals OUT16a and OUT16b at an arbitrary distribution ratio.

(Modification 2)
In this embodiment, the case where the optical element 10 is made polarization independent has been described. However, polarization independence is not a necessary condition that the optical element 10 should satisfy. Therefore, an optical element constituted by an optical waveguide in which the polarization dependency is generated by changing the width and the thickness is also included in the scope of the present invention.

[Embodiment 2]
Hereinafter, the optical element of Embodiment 2 will be described with reference to FIGS. FIG. 3 is a plan view schematically showing the structure of the optical element. FIG. 4 is a schematic diagram schematically illustrating the behavior of light propagating through the optical element. 5A and 5B are plan views schematically showing the structure of the modification. 6 and 7 are characteristic diagrams showing operation characteristics of the modification. FIG. 8 is a plan view schematically showing the structure of another modified example.

  In FIG. 3, the core 18 constituting the optical element is covered with the clad 12 and cannot be directly observed, but is drawn with a solid line for emphasis. In FIG. 8, drawing of the substrate 8 and the clad 12 is omitted. Moreover, in the above drawings, in order to prevent complication, the code | symbol of the one part component which is not required for description is abbreviate | omitted.

(Construction)
The structure of the optical element 50 will be described with reference to FIG. The optical element 50 corresponds to what includes the optical element 10 of Embodiment 1 as one component. More precisely, the optical element 50 includes the MZ interferometer 32 of the first embodiment as the first and second meta directional couplers 32 ₁ and 32 ₂ . Therefore, the first and second components of the meta directional coupler 32 ₁ and 32 _2, with subjecting the MZ interferometer 32 and the equivalent code, subscript subscript that denoted by reference numeral "1" or "2" To distinguish between the first and second meta-directional couplers 32 ₁ and 32 ₂ .

The optical waveguide 11 constituting the optical element 50 includes an optical unit 40. The optical unit 40 includes first and second meta directional couplers 32 ₁ and 32 ₂ and a meta arm unit 32 ₃ . As will be described in detail later, the optical unit 40 as a whole functions as one MZ interferometer including two directional couplers and an arm unit. That is, the first and second meta directional coupler 32 ₁ and 32 ₂ operate equivalent to two directional couplers, Metaamu 32 ₃ operates equivalent to the arm portion. Therefore, hereinafter, the optical unit 40 may be referred to as a Meta Mach-Zehnder interferometer (MMZ interferometer) 40.

In this embodiment, the case where the MMZ interferometer 40 operates as a wavelength separation element will be described. That MMZ interferometer 40 includes a first first light Lt1 wavelength lambda ₁ that has been input, the mixed light of the second light Lt2 different second wavelength lambda ₂ and the first wavelength lambda _1, and wavelength separation Output from different output ports. In this embodiment, the first wavelength λ ₁ of the first light Lt 1 is 1.49 μm that is generally used as downlink communication light in the optical subscriber communication system. Also, the second wavelength λ ₂ of the second light Lt2 is set to 1.31 μm that is generally used as upstream communication light in the optical subscriber communication system.

  Returning to the description of the configuration again, the optical waveguide 11 further includes an input unit 24 and an output unit 26 configured as optional elements in the same manner as in the first embodiment.

The input unit 24, the first meta directional coupler 32 ₁ , the meta arm unit 32 ₃ , the second meta directional coupler 32 ₂ and the output unit 26 constituting the optical waveguide 11 are connected in this order. More specifically, the input optical waveguide 24a of the input unit 24 is connected to the first meta-directional coupler 32 ₁ of the first optical waveguide 16a ₁ of the input IN16a. The second meta directional coupler 32 ₂ of the first output terminal OUT16a of the optical waveguide 16a ₂ is connected to the first output optical waveguide 26a of the output section 26. The second meta directional coupler 32 ₂ of the second output terminal OUT16b of the optical waveguide 16b ₂ is connected to the second output optical waveguide 26b of the output section 26.

Then, and a first optical waveguide 16a ₃ first optical waveguide 16a ₁ and 16a ₂ and Metaamu portion 32 ₃ which the first and second meta directional coupler 32 ₁ and 32 ₂ are provided is provided are connected, the first and a second optical waveguide 16b ₃ of the second optical waveguide 16b ₁ and 16b ₂ and Metaamu portion 32 ₃ provided is connected to the second meta directional coupler 32 ₁ and 32 ₂ are provided.

The first meta-directional coupler 32 ₁ is configured such that the arm portion 32 ₁ C gives a phase difference φ of (2m + 1/2) π to the first light Lt1. That is, the arm portion 32 ₁ C sets z to ½ in the above-described phase difference condition φ = (2m + z) π. Thereby, the arm portion 32 ₁ C substantially gives a phase difference of π / 2 to the first light Lt1. As a result, the first meta directional coupler 32 _first distribution ratio x is 0.5, the first meta directional coupler 32 ₁ functions as a 3dB coupler with respect to the first light Lt1. That is, ₁ first meta directional coupler 32 operates equivalent to the first directional coupler 32L of the first embodiment.

In order to give this phase difference φ, the optical path length difference ΔS ₁ between the first and second curved portions 16a ₁ C and 16b ₁ C of the arm portion 32 ₁ C is expressed by the equation (2) under the condition of z = 1/2. Use to set.

The arm portion 32 ₁ C is designed to give a substantially equivalent phase difference φ to light in a wide wavelength range. Therefore, although designed based on the first light Lt1, the first meta-directional coupler 32 ₁ is practically sufficient for the second light Lt2 having the second wavelength λ ₂ (≠ λ ₁ ). Function as a 3 dB coupler.

The second meta directional coupler 32 ₂ is substantially the same structure as the first meta directional coupler 32 _1, operates equivalently to a first directional coupler 32R of the first embodiment.

Here, explaining the arrangement relationship between the first and second components of the meta directional coupler 32 ₁ and 32 _2. Schematically, the constituent elements of the first and second meta-directional couplers 32 ₁ and 32 ₂ are arranged point-symmetrically with the center point of the MMZ interferometer 40 as the center of symmetry. Here, the center point of the MMZ interferometer 40 refers to the first and second optical waveguides 16a and 16b constituting the first and second directional couplers 32 ₁ L, 32 ₂ L, 32 ₁ R, and 32 ₂ R. And the center line in the full length direction of the MMZ interferometer 40 intersects.

  More specifically, the “point symmetry arrangement relationship” indicates an arrangement relationship that satisfies the following two conditions.

(Condition 1)
In the first meta directional coupler 32 ₁ , the first width tapered portion 16T1 _{1 of} the first directional coupler 32 ₁ L is provided in the f ₁ optical waveguide (f ₁ is 1 or 2), and the second direction It is assumed that the second width taper portion 16T2 ₁ of the sex coupler 32 ₁ R is provided in the f ₂ optical waveguide (f ₂ is 1 or 2). In this case, provided the first width tapered portion 16T1 ₂ of the first directional coupler 32 ₂ L in the second meta directional coupler 32 ₂ to the _{u 2} optical waveguide _{(u 2 = 3-f 2} ), and the The second width tapered portion 16T2 ₂ of the bidirectional coupler 32 ₂ R is provided in the u ₁ optical waveguide (u ₁ = 3-f ₁ ). FIG. 3 corresponds to the case of f ₁ = 1, f ₂ = 2, u ₁ = 2 and u ₂ = 1.

(Condition 2)
The optical path length difference between the first and second optical waveguides 16a ₁ C and 16b ₁ C in the arm portion 32 ₁ C of the first meta directional coupler 32 ₁ is ΔS ₁ 1, and the second meta directional coupler 32 _{2 is used.} of the first and the optical path length difference between the second optical waveguide between 16a ₂ C and 16b ₂ C of the arm portion 32 ₂ C and [Delta] S ₁ 2. At this time, ΔS ₁ 1 and ΔS ₁ 2 have the same absolute value and the signs are reversed. The arm portions 32 ₁ C and 32 ₂ C are arranged.

As described above, the peak wavelength band of the first light Lt1 cross-outputted from the MMZ interferometer 40 can be obtained by arranging the components of the first and second meta-directional couplers 32 ₁ and 32 _{2 in} a point-symmetric manner. Can be spread.

Returning to the description of the configuration again, Metaamu 32 ₃ broadly comprises imparting a phase difference Φ for wavelength separation into first and second light Lt1 and Lt2. In other words, the meta arm portion 32 ₃ is configured to give a phase difference Φ of 2Mπ (M is an integer of 0 or more) to the first light Lt1 propagating through the first and second optical waveguides 16a ₃ and 16b _3. Yes. In other words, the first light Lt1 in order to cross output, Metaamu portion 32 ₃ is a phase difference [Phi that applied to the first light Lt1, set so as to satisfy the Φ = 2Mπ an interference condition.

Here, the optical path length of the first bending portion 16a ₃ C constituting the Metaamu portion 32 ₃ and _{S 2} a, the optical path length of the second curved portion 16b ₃ C and _{S 2} b. Further, the optical path length difference (S ₂ a−S ₂ b) between the first and second curved portions 16a ₃ C and 16b ₃ C is set to ΔS ₂ . By the way, since 2πΔS ₂ / λ ₁ = Φ holds between the phase difference Φ and the optical path length difference ΔS ₂ for the first light Lt ₁ having the wavelength λ ₁ , ΔS ₂ is expressed by the following formula (8 ) Should be established.
2ΔS ₂ / λ ₁ = 2M (8)

Like the first light Lt1, in order to the second light Lt2 is bar output Metaamu portion 32 ₃ is a phase difference [Phi imparted to the second light Lt2, an interference condition Φ = (2M + 1) π further satisfaction It is preferable to set so as to. By the way, since 2πΔS ₂ / λ ₂ = Φ is established between the phase difference Φ and the optical path length difference ΔS ₂ with respect to the second light Lt ₂ having the wavelength λ ₂ , ΔS ₂ is expressed by the following formula (9 ) Is further satisfied.
2ΔS ₂ / λ ₂ = 2M + 1 (9)

Note that the lengths of the first and second bending portions 16a ₃ C and 16b ₃ C are ΔS ₂ determined to satisfy the expressions (8) and (9), and the first and second lights Lt1 and Lt2. and first and second curved portions 16a ₃ C and 16b ₃ C equivalent refractive index of about, may be determined by equation (1).

The meta arm part 32 ₃ configured in this way has a phase difference of 2Mπ with respect to the first light Lt1 propagating through the first and second bending parts 16a ₃ C and 16b ₃ C after propagation of the meta arm part 32 _3. Give. Further, a phase difference of (2M + 1) π is given to the second light Lt2.

(Operation)
Hereinafter, the operation of the MMZ interferometer 40 will be described with reference to FIG. As described above, the arm portions 32 ₁ C and 32 ₂ C of the first and second meta-directional couplers 32 ₁ and 32 ₂ are substantially equal to the first and second lights Lt 1 and Lt 2. A phase difference φ is given. As a result, the behavior when the first and second lights Lt1 and Lt2 propagate through the first and second meta-directional couplers 32 ₁ and 32 ₂ is substantially the same. Therefore, in the following, first, the behavior when the first light Lt1 propagates through the MMZ interferometer 40 will be described, and then the propagation behavior of the second light Lt2 will be centered on differences from the first light Lt1. explain.

Behavior of the first light Lt1 propagating the input section 24 and the first meta-directional coupler 32 _1, the distribution ratio of the optical Lt1 ₄ and the light Lt1 ₅ except the point equal 0.5, embodiment 1 (Operation ). Therefore, in this section we describe the propagation behavior of the first light Lt1 in Metaamu portion 32 ₃ and later.

The optical path length difference ΔS ₂ between the first and second bending portions 16a ₃ C and 16b ₃ C constituting the meta arm portion 32 ₃ C is set so as to satisfy the equation (8). Therefore, the light Lt1 ₄ and Lt1 _5, after the propagation of Metaamu portion 32 ₃ C, as a light Lt1 ₆ and Lt1 ₇ that the phase difference is imparted in 2Emupai, are output to the second meta directional coupler 32 ₂ .

Subsequently, the light Lt1 ₆ and Lt1 ₇ are respectively the first and second straight portions 16a ₂ L and 16b ₂ L of the first directional coupler 32 ₂ L constituting the second meta directional coupler 32 ₂ Input Is done. Since the first directional coupler 32 ₂ L is a width-tapered directional coupler configured in the same manner as the first directional coupler 32 ₁ L, the light Lt1 ₆ input to the first linear portion 16a ₂ L. Excites a symmetric mode light component on the output side. Similarly, the light Lt1 ₇ input to the second linear portion 16b ₂ L excites an antisymmetric mode light component on the output side. That is, on the output side of the first directional coupler 32 ₂ L, light in which a symmetric mode light component derived from the light Lt1 ₆ and an antisymmetric mode light component derived from the light Lt1 ₇ are mixed at an intensity ratio of 1: 1. Lt1 ₈ is excited.

Subsequently, the light Lt1 ₈ is input to the second meta directional coupler 32 _{and second} arm portions 32 ₂ C. Since the arm portion 32 ₂ C is configured in the same manner as the arm portion 32 ₁ C of the first meta-directional coupler 32 ₁ , the light Lt1 that propagates through the first and second optical waveguides 16a ₂ C and 16b ₂ C ₈ is given a phase difference φ. In other words, it provides a phase difference φ of [pi / 2 between the two component light of the first and second curved portions 16a ₂ C and 16b ₂ light Lt1 ₈ to each being equally distributed propagated to C. As a result, the antisymmetric mode light component contained in these component lights is converted into a symmetric mode light component, and the light Lt1 ₉ including only the symmetric mode light is output from the arm portion 32 ₂ C.

Subsequently, the light Lt1 ₉ is input to the second directional coupler 32 ₂ R. The second directional coupler 32 ₂ R is a width-tapered directional coupler configured in the same manner as the second directional coupler 32 ₁ R. Therefore, the light Lt1 ₉ is a symmetric mode light input to the input side, the intensity in the second linear portion 16b 2 _R having a second width tapered portions 16T2 ₂ concentrate, optical waveguide for the second output of the output section 26 The light Lt1 ₁₁ is output from 26b. As described above, the MMZ interferometer 40 cross-outputs the first light Lt1.

Next, the behavior when the second light Lt2 propagates through the MMZ interferometer 40 will be described mainly focusing on the differences from the first light Lt1. Behavior when the second light Lt2 is propagating through the first meta directional coupler 32 ₁ is the same as that of the first light Lt1.

Subsequently, the second light Lt2 is input to the meta arm unit 32 ₃ C. The optical path length difference ΔS ₂ of the meta arm portion 32 ₃ C is set so as to satisfy the expression (9). Therefore, the input light Lt1 ₄ and Lt1 ₅ derived from the second light Lt2 are the second light Lt1 ₆ and Lt1 ₇ to which the phase difference Φ of (2M + 1) π is given after propagation of the meta arm portion 32 ₃ C. are output to the meta directional coupler 32 _2.

Light Lt1 ₆ and Lt1 ₇ from the second light Lt2 is the behavior at the time of propagating the second meta directional coupler 32 ₂ of the first directional coupler 32 ₂ L are the same as the first light Lt1. Therefore, on the output side of the first directional coupler 32 ₂ L, a symmetric mode light component and an antisymmetric mode light component, each having a phase difference Φ of (2M + 1) π, are provided at an intensity ratio of 1: 1. mixed, light Lt1 ₈ from the second light Lt2 is excited.

Subsequently, the light Lt1 ₈ from the second light Lt2 is input to the second meta directional coupler 32 _{and second} arm portions 32 ₂ C. As described above, the arm portion 32 ₂ C gives a phase difference φ of π / 2 to the light Lt1 ₈ propagating through the first and second optical waveguides 16a ₂ C and 16b ₂ C. In other words, it provides a phase difference φ of [pi / 2 between the two component light of the first and second curved portions 16a ₂ C and 16b ₂ light Lt1 ₈ to each being equally distributed propagated to C. As a result, the provision of the phase difference φ at the arm portion 32 ₂ C and the provision of the phase difference Φ at the meta arm portion 32 ₃ C are combined, and the symmetric mode component light included in these component lights is antisymmetric. is converted to mode component light, from the arm portion 32 2 _C, light Lt1 ₉ containing only the antisymmetric mode light from the second light Lt2 is output.

Subsequently, the light Lt1 ₉ is input to the second directional coupler 32 ₂ R. The light Lt1 ₉ which is the antisymmetric mode light input to the input side of the second directional coupler 32 ₂ R concentrates the intensity on the first linear portion 16a ₂ R which is a uniform-width channel type optical waveguide, and outputs it. The light Lt1 ₁₀ is output from the first output optical waveguide 26a of the section 26. Thus, the MMZ interferometer 40 outputs the second light Lt2 as a bar.

As described above, the MMZ interferometer 40 functions in the same manner as the directional coupler and uses the first and second meta directional couplers 32 ₁ and 32 ₂ that can control the distribution ratio more precisely. The element is configured. Therefore, the MMZ interferometer 40 is superior in crosstalk characteristics as compared with an MZ interferometer type wavelength separation element using a normal directional coupler.

The MMZ interferometer 40 is configured using the directional couplers 32 ₁ L, 32 ₁ R, 32 ₂ L, and 32 ₂ R of the first embodiment. As a result, the operation is independent of the polarization, and the tolerance to the width error is excellent as compared with the MZ interferometer type wavelength separation element using a normal directional coupler.

(Modification)
Hereinafter, modified examples of the MMZ interferometer 40 will be described. The MMZ interferometer 40 can be modified in the same manner as the optical element 10 and further can be modified as listed below.

(Modification 1)
Hereinafter, a modification of the optical element 50 will be described with reference to FIGS. 5A and 5B, components similar to those in FIG. 3 are denoted by the same reference numerals, and description thereof is omitted as appropriate.

Optical device 50A shown in FIG. 5 (A), that the center axis C32 ₁ and C32 ₂ of the first and second meta directional coupler 32 ₁ A and 32 ₂ A are parallel to each other, and Metaamu portion The configuration is the same as that of the optical element 50 described above except that the shape of 32 ₃ A is different.

That is, the optical element 50A has a structure in which the first and second meta directional couplers 32 ₁ A and 32 ₂ A are bent into a hairpin shape with the meta arm portion 32 ₃ A as a bent portion. Thus, the total length of the optical element 50A can be reduced by bending the first and second meta-directional couplers 32 ₁ A and 32 ₂ A.

  Thus, since the difference between the optical element 50A and the optical element 50 is formal, the optical element 50A operates in the same manner as the optical element 50.

  Here, with reference to FIG.6 and FIG.7, the simulation implemented about 50 A of optical elements is demonstrated. For the reasons described above, the description of FIGS. 6 and 7 applies to the optical element 50 as well.

  FIG. 6 is a characteristic diagram showing the operating characteristics of the optical element 50A obtained by a two-dimensional FDTD (Finite Difference Time Domain) method. Schematically speaking, in FIG. 6, the input light Lt having a changed wavelength is input from the input terminal IN16a, and the intensity ratio between the bar state light and the cross state light output from the output terminals OUT16a and OUT16b is obtained. ing. In FIG. 6, the vertical axis represents the light intensity ratio (dB) of the bar state light (curve I) and the cross state light (curve II) to the input light Lt, and the horizontal axis represents the wavelength (μm).

In obtaining FIG. 6, the optical element 50A was designed to operate as a wavelength separation element that cross-outputs the first light Lt1 having the first wavelength λ ₁ = 1.49 μm (distribution ratio x = 0). That is, the difference in length between the first and second optical waveguides of the meta arm portion 32 ₃ A was set to 1.35 μm. Further, the curvature of all the bent portions included in the meta arm portion 32 ₃ A was set to 5 μm. Here, the bent portion indicates a region where the first and second optical waveguides of the meta arm portion 32 ₃ A are bent at a right angle. Other numerical conditions, that is, the refractive indexes of the core 18 and the cladding 12, the first to third widths W1 to W3 of the width taper directional coupler constituting the optical element 50A, and the width taper directional coupler are configured. The gap Sp between the side surfaces between the width taper waveguide and the equal width waveguide, the total length L1 of the width taper directional coupler, and the difference between the lengths of the first and second curved portions constituting the arm portion have already been described. The values obtained were used.

  Referring to FIG. 6, it can be seen from curve I that light having a wavelength of about 1.11 μm and a wavelength of about 1.42 μm (referred to as a bar peak wavelength) is output from the output terminal OUT16a with a high distribution ratio. Further, it can be seen from the curve II that light having a wavelength of about 1.25 μm and a wavelength of about 1.58 μm (referred to as a cross peak wavelength) is output from the output terminal OUT16b with a high distribution ratio. From this, it can be seen that the optical element 50A operates as a wavelength separation element that wavelength-separates mixed light of cross peak wavelength light and bar peak wavelength light.

  FIG. 7 is a characteristic diagram showing operating characteristics of the optical element 50A when a width taper directional coupler different from that in FIG. 6 is used. The calculation method for obtaining FIG. 7 and the dimensional conditions other than the width taper directional coupler are the same as those in FIG. Also, the vertical and horizontal axes in FIG. 7 and the two drawn curves I and II have the same meaning as in FIG.

  In FIG. 7, the total length of the width taper directional coupler is 200 μm. Further, the interval Sp between the side surfaces between the width taper waveguide and the equal width waveguide is changed to a taper shape. That is, as the width of the width taper waveguide constituting the width taper directional coupler becomes narrower, the waveguide interval is gradually narrowed. Specifically, the distance Sp between the side surfaces between the width taper waveguide and the equal width waveguide is changed from 500 nm to 300 nm. In addition, the first width W1 of the width taper directional coupler was 340 nm, and the second width W2 was 300 nm. Further, the difference in length between the first and second bending portions constituting the arm portion was set to 170 nm.

  Referring to FIG. 7, it can be seen from curve I that light having a wavelength of about 1.25 μm and a wavelength of about 1.66 μm is output from the output terminal OUT16a with a high distribution ratio. Further, it can be seen from the curve II that light having a wavelength of about 1.41 μm is output from the output terminal OUT16b with a high distribution ratio.

  In FIG. 7, the intensity ratio of the cross state light (curve II) to the bar state light (curve I) at the bar peak wavelength, and the intensity of the bar state light (curve I) relative to the cross state light (curve II) at the cross peak wavelength. It can be seen that the ratio is smaller than that of FIG. 6 on average.

  From this, it can be seen that the optical element 50A according to the configuration of FIG. 7 operates as a wavelength separation element having better crosstalk characteristics than the optical element 50A according to the configuration of FIG.

Next, another modification of the optical element 50 will be described with reference to FIG. The optical element 50B shown in FIG. 5B is configured in the same manner as the optical element 50A except that the shape of the meta arm portion 32 ₃ B is different. That is, the optical element 50B has succeeded in making the size smaller than that of the optical element 50A by devising the arrangement of the bent portions in the meta arm portion 32 ₃ B.

In the optical device 50A and 50B, although the respective center axis C32 ₁ and C32 ₂ parallel to one another, to the center axis C32 ₁ and C32 ₂ parallel is not an essential condition. Also by the center axis C32 ₁ and C32 ₂ places the first and second meta directional coupler 32 ₁ and 32 ₂ so as to intersect at a predetermined angle, it can reduce the size of the optical elements 50A and 50B.

(Modification 2)
In this embodiment, the distribution ratio x of the first and second meta-directional couplers 32 ₁ and 32 ₂ is set to 0.5 to operate as a 3 dB coupler, and the single MMZ interferometer 40 has a wavelength separation element. The case of configuring has been described. However, the first and second distribution ratio x of meta directional coupler 32 ₁ and 32 ₂ can be set to any value. As will be described later in Embodiment 3, with reference to MMZ interferometer 40 adjusting the first and second meta directional coupler 32 ₁ and 32 ₂ of the distribution ratio x to a value other than 0.5, the so-called multi-stage MZ An interferometer type wavelength separation element can be configured.

(Modification 3)
Hereinafter, a modification of the optical element 50 will be described with reference to FIG. As is apparent from comparison between FIG. 8 and FIG. 3, the optical element 50C is ₁ and R first meta directional coupler 32 ₁ of the second directional coupler 32, a Metaamu portion 32 _3, the second meta direction (indicated by reference numeral 64 in FIG. 4.) the first element portion consisting of a sex coupler 32 ₂ of the first directional coupler 32 2 _L is, except that it is substituted by a multimode optical waveguide 52, an optical element 50 is configured in the same manner.

Referring to FIG. 4, the first element portion 64, the light Lt1 ₃ containing both mode light is converted into light Lt1 ₄ and Lt1 ₅ propagating the two optical waveguides 16a and 16b, the wavelength by Metaamu 32 ₃ on which a retardation Φ corresponding, is converted into the light Lt1 ₈ containing both mode light again. That is, in the first element portion 64, in order to provide the phase difference Φ according to the wavelength, the light Lt1 ₃ is once converted into the light Lt1 ₄ and Lt1 ₅ which are single mode lights, and after the phase difference Φ is applied. Lights Lt1 ₆ and Lt1 ₇ were reconverted to obtain light Lt1 ₈ including both mode lights.

In general, in the multimode optical waveguide, the propagation constants are different between the two mode lights. Therefore, if the first element portion 64 is replaced with the multimode optical waveguide 52, the light Lt1 ₃ is not converted into the light Lt1 ₄ and Lt1 _5. However, the light Lt1 ₈ can be obtained by directly providing the light Lt1 ₃ with the phase difference Φ.

  Thus, by replacing the first element portion 64 with the multimode optical waveguide 52, the structure of the optical element 50 can be greatly simplified.

The multi-mode waveguide 52 can be designed using the following formula (10) that represents the interference condition related to the waveguide 52.
2πL _M Δn _M / λ ₁ = 2m _M π (10)

Here, L _M is the total length of the multi-mode waveguide 52, Δn _M is an equivalent refractive index difference between both mode lights with respect to the multi-mode waveguide 52, and m _M is an interference that takes an integer value of 0 or more. Is the order. Δn _M can be calculated from the width and thickness of the multimode waveguide 52.

Here, when the wavelength difference between the first and second light Lt1 and Lt2 should wavelength separating (lambda ₁ 1-? ₂₎ and [Delta] [lambda] _1, the total length _{L M} from equation (10) deformed following formula (11) Can be sought.
L _M = (λ ₁ ² / Δλ ₁ ) × (Δn _M −λ ₁ × (dΔn _M / dλ ₁ )) (11)

Multi-mode waveguide 52 thus set the overall length L _M operates in the same manner as the first element portion 64.

[Embodiment 3]
Hereinafter, the optical element of Embodiment 3 will be described with reference to FIGS. 9 to 11. FIG. 9 is a plan view schematically showing the structure of the optical element. FIG. 10 is a plan view schematically showing the structure of the modification. FIG. 11 is a plan view schematically showing the structure of another modified example.

  In FIG. 9, the core 18 constituting the optical element is covered with the clad 12 and cannot be directly observed, but is drawn with a solid line for emphasis. 10 and 11, the drawing of the substrate 8 and the clad 12 is omitted. Moreover, in the above drawings, in order to prevent complication, the code | symbol of the one part component which is not required for description is abbreviate | omitted.

(Construction)
The structure of the optical element 60 will be described with reference to FIG. The optical element 60 is a multi-stage MZ interferometer-type wavelength separation element, and corresponds to a plurality of MMZ interferometers 40 of the second embodiment connected in series. More precisely, the optical element 60 includes first to i-th MMZ interferometers 40-1 to 40-i (i is an integer equal to or greater than 2) configured in substantially the same manner as the MMZ interferometer 40. Therefore, the components of the first to i-th MMZ interferometers 40-1 to 40-i are denoted by the same reference numerals as those of the MMZ interferometer 40, and the subscripts “-i” attached to the end of the reference numerals indicate The i-MMZ interferometers 40-1 to 40-i are distinguished.

  The optical waveguide 11 constituting the optical element 60 includes first to i-th MMZ interferometers 40-1 to 40-i arranged in series in this order. The optical waveguide 11 further includes an input unit 24 and an output unit 26 configured as optional elements in the same manner as in the first embodiment. The optical element 60 is configured to output the first light Lt1 in the cross state and output the second light Lt2 in the bar state.

  The first to i-th MMZ interferometers 40-1 to 40-i are configured in the same manner as the MMZ interferometer 40 of the second embodiment except that the distribution ratio of the MMZ interferometer with respect to the first light Lt1 is different. That is, in order to cross-output the first light Lt1, the sum of the distribution ratios related to the first light of each of the first to iMMZ interferometers 40-1 to 40-i may be set to 1. Therefore, the distribution ratio x regarding the first light Lt1 of the first to i-th MMZ interferometers 40-1 to 40-i is 1 / i, respectively. Since the second light Lt2 is output as a bar, it is not necessary to consider the distribution ratio of each of the first to i-th MMZ interferometers 40-1 to 40-i.

By the way, the arbitrary rMMZ interferometer 40-r (r is an integer of 1 to i) includes two meta directional couplers 32 ₁ -r and 32 ₂ -r, respectively. Therefore, in order to set the distribution ratio of the r-th MMZ interferometer 40-r to 1 / i, the distribution ratios of the meta-directional couplers 32 ₁ -r and 32 ₂ -r may be set to 1 / (2i). .

After that, in the same manner as in the first embodiment, the phase difference introduced into the arm part 32 ₁ Cr of the meta directional coupler 32 ₁ -r is obtained so that the distribution ratio becomes 1 / (2i). Then, the optical path length difference of the arm portion 32 ₁ C-r is obtained from this phase difference, and the first and second bending portions 16a ₁ C- constituting the arm portion 32 ₁ C-r so as to achieve this optical path length difference. Determine the lengths of r and 16b ₁ Cr.

  Here, a combination of j-th and (j + 1) -th MMZ interferometers 40-j and 40- (j + 1) (j is an integer of 1 to i-1) adjacent to each other is considered. A structure including the jth and (j + 1) th MMZ interferometers 40-j and 40- (j + 1) is defined as a jth pair structure 42-j. At this time, the first and second optical waveguides 16a-j and 16b-j constituting the jth MMZ interferometer 40-j, and the first and second optical waveguides constituting the (j + 1) th MMZ interferometer 40- (j + 1). The waveguides 16a- (j + 1) and 16b- (j + 1) are arranged point-symmetrically with the center point of the j-th pair structure 42-j in FIG. 9 as the center of symmetry. Here, the center point of the j-th pair structure 42-j corresponds to the center of gravity of all the components of the j-th pair structure 42-j.

  As described above, the jth and (j + 1) th MMZ interferometers 40-j and 40- (j + 1) are arranged in a point-symmetric manner, so that the peak wavelength band of the first light Lt1 that is cross-outputted can be widened.

(Modification)
Hereinafter, modified examples of the optical element 60 will be described. The optical element 60 can be modified in the same manner as the optical element 10 and the MMZ interferometer 40, and further can be modified as listed below.

(Modification 1)
Hereinafter, a modification of the optical element 60 will be described with reference to FIG. FIG. 10 is an enlarged plan view of the j-th pair structure 42-j in FIG. Referring to FIG. 10, the optical element 60 </ b> A replaces the second element part 42 </ b> A-j (FIG. 9) in the j-th pair structure 42-j and functions in the same way as the second element part 42 </ b> A-j. The optical device 60 is different from the optical device 60 in that the arm portion 62-j is provided. That is, in the optical element 60A, adjacent jth and j + 1th MMZ interferometers 40-j and 40- (j + 1) share one jth equivalent arm portion 62-j.

Referring to FIG. 9, the second element portion 42A-j refers to the arm portion 32 ₂ Cj and the second directional coupler of the second meta directional coupler 32 ₂ -j in the j th MMZ interferometer 40-j. 32 ₂ R-j, and the arm part 32 ₁ C- (j + 1) and the first directional coupler 32 ₁ L of the first meta directional coupler 32 _1- (j + 1) in the j + 1 MMZ interferometer 40- (j + 1). -(J + 1) is the portion of the optical element 60.

Returning to FIG. 10 again, the j-th equivalent arm portion 62-j includes the third and fourth curved portions 16a ₄ C-j and 16b ₄ C as the portions of the first and second optical waveguides 16a and 16b arranged in parallel. -J. The j-th equivalent arm portion 62-j has a phase difference φ _A different from the above-described phase difference φ in the first light Lt1 after propagating through the third and fourth bending portions 16a ₄ Cj and 16b ₄ Cj. Configured to give Here, the phase difference φ _A given by the j-th equivalent arm unit 62-j is twice as large as the phase difference φ given by the arm unit 32 ₁ Cr provided in the rMMZ interferometer 40-r (FIG. 9). To do.

The point that the j-th equivalent arm portion 62-j is equivalent to the arm portion 32 ₁ C-r that doubles the phase difference φ will be described using the above-described transmission matrix. Here, the second meta-directional coupler 32 ₂ -j in the j-th MMZ interferometer 40 -j and the first meta-directional coupler 32 _1- in the j + 1 MMZ interferometer 40-(j + 1) shown in FIG. (j + 1) and the transfer characteristics of the series structure are connected in series represented by the transfer matrix M _31. Further, representing the transfer characteristic of the j equivalent arm portion 62-j in transmission matrix _{M 33.} In this _{case, M 31,} using the transfer matrix _{M s} and _{M f} of the above equation (4) and (5), represented by the following formula (12).
M ₃₁ = M _s M _f M _s M _s M _f M _s (12)

In Expression (12), M _f M _s M _s M _f , which is a product of a matrix of 2 to 5 items, corresponds to the transfer characteristic M ₃₃ of the j-th equivalent arm unit 62-j. Here, since M _s M _s in the equation (12) is a unit matrix, the equation (12) can be simplified as the following equation (13).
M ₃₁ = M _s M _f M _f M _s (13)

Furthermore, in Formula (13), M _f M _f is expressed as in the following Formula (14).

Here, the _{M 33} of formula (14), comparing the _{M f} of formula (6), but the coefficient 1/2 is hooked to a phase difference φ in phase term of each the _{M f elements, M 33} In the phase term, the phase difference φ is not multiplied by a coefficient. In other words, the difference in coefficients of the phase term, M ₃₃ is equal to the transfer characteristic of the M _f given a phase difference 2 [phi. This shows that, if the phase difference phi _A of the j equivalent arm portion 62-j to twice the phase difference phi of the arm 32 1 _C-r, it can be seen that both operate similarly.

Using this result, Expression (12) is expressed as Expression (15) below.
M ₃₁ = M _s M ₃₃ M _s (15)

  From the equation (15), it can be seen that the series structure and the series structure partially substituted with the j-th equivalent arm portion 62-j have the same transfer matrix and perform the same operation.

Thus, by replacing the second element portion 42A-j with the j-th equivalent arm portion 62-j, the first and second directional couplers 32 ₁ L- (j + 1) having a length on the order of 100 μm. And 32 ₂ Rj can be omitted, and the overall length of the optical element 60A can be greatly shortened.

(Modification 2)
Hereinafter, a modification of the optical element 60 will be mainly described with reference to FIG. The optical element 60B shown in FIG. 11 corresponds to a structure in which the multimode optical waveguide 52 shown in FIG. 8 and the j-th equivalent arm portion 62-j shown in FIG. 10 are used in combination.

  More specifically, the optical element 60B is obtained by replacing the first element portions 64-j and 64- (j + 1) in FIG. 10 with multimode optical waveguides 52-j and 52- (j + 1), respectively.

Furthermore, in the optical element 60B, the first and second optical waveguides 16a and 16b are bent at a right angle while maintaining parallelism, and the difference in length between the optical waveguides 16a and 16b generated between the outside and the inside of the bent portion Are used to form the arm portions 32 ₁ C-j and 32 ₂ C- (j + 1).

  As a result of configuring the optical element 60B in this way, the element size can be further reduced as compared with the optical element 60A.

(Modification 3)
In this embodiment, the case where the first and second optical waveguides constituting the j-th pair structure 42-j are arranged point-symmetrically with the center point as the center of symmetry has been described. However, the first and second optical waveguides constituting the j-th pair structure 42-j do not need to be arranged point-symmetrically, and may be arranged so as to overlap each other when translated. Even if comprised in this way, the optical element 60 has a wavelength separation capability sufficient practically.

8 Substrate 8a Main surface 10, 30, 50, 50A, 50B, 50C, 60, 60A, 60B Optical element 11 Optical waveguide 12 Clad 18 Core 32 Mach-Zehnder interferometer (MZ interferometer)
32 ₁ , 32 ₁ -r, 32 ₁ A, 32 ₁ B, 32 _1- (j + 1) first meta directional couplers 32 ₂ , 32 ₂ -r, 32 ₂ A, 32 ₂ B, 32 ₂ -j 2 meta directional couplers 32 ₃ , 32 ₃ A, 32 ₃ B, 32 ₃ C, 32 ₃ C-j, 32 ₃ C- (j + 1) meta arm portions 22 L, 32 L, 32 ₁ L, 32 ₂ L, 32 ₂ L-j, 32 ₁ L-j, 32 ₁ L- (j + 1), 32 ₂ L- (j + 1) first directional couplers 22R, 32R, 32 ₁ R, 32 ₂ R, 32 ₂ R-j, 32 ₁ R-j, 32 ₁ R- (j + 1), 32 ₂ R- (j + 1) Second directional couplers 22C, 32C, 32 ₁ C, 32 ₂ C, 32 ₁ C-r, 32 ₂ C-j, 32 ₁ C-j, 32 ₁ C- (j + 1), 32 ₂ C- (j + 1) Arm portions 16aL, 1 6aR, 16a ₁ L, 16a ₁ R, 16a ₂ L, 16a ₂ R 1st straight line part 16bL, 16bR, 16b ₁ L, 16b ₁ R, 16b ₂ L, 16b ₂ R 2nd straight line part 16a ₄ Cj 1st 3 bending portion 16b ₄ Cj 4th bending portion 16aC, 16a ₁ C, 16a ₂ C, 16a ₃ C, 16a ₁ C-r 1st bending portion 16bC, 16b ₁ C, 16b ₂ C, 16b ₃ C, 16b ₁ C-r second curved portion _{16a, 16a 1, 16a 2,} 16a 3, 16a-j, 16a- (j + 1) first optical waveguide _{16b, 16b 1, 16b 2,} 16b 3, 16b-j, 16b- ( j + 1) second optical waveguide 16T, 16T1,16T1 _1, 16T1 ₂ first width tapered portions 16T2,16T2 _1, 16T2 ₂ second width tapered portions _C32 1, C32 ₂ central axis IN16a IN16b input end OUT16 output side OUT16a, OUT16b output end 24 input unit 24a input optical waveguide 24b dummy waveguide 26 output unit 26a first output optical waveguide 26b second output optical waveguide 40 optical unit (meta Mach-Zehnder (MMZ) interference) Total)
40-r rMMZ interferometer 40-j jMMZ interferometer 40- (j + 1) (j + 1) MMZ interferometer 42-j j-pair structure 42A-j second element parts 52, 52-j, 52- ( j + 1) Multimode optical waveguide 62-j jth equivalent arm portion 64, 64-j, 64- (j + 1) first element portion

Claims (13)

An optical waveguide comprising a clad provided on the main surface side of the substrate and a core provided in the clad;
The optical waveguide comprises first and second optical waveguides;
First and second directional couplers each composed of a portion of the first and second optical waveguides separated from each other by a distance capable of optical coupling;
An arm portion composed of portions of the first and second optical waveguides interposed between the first and second directional couplers,
The first and second directional couplers function as a 3 dB coupler for the first light with the first wavelength λ ₁ , and
The first or second optical waveguide constituting the first directional coupler has a width measured in a direction perpendicular to the light propagation direction and parallel to the main surface from the first width. A first width taper portion that is reduced along the light propagation direction to a second width smaller than the width is formed,
The first or second optical waveguide constituting the second directional coupler is formed with a second width taper portion whose width increases from the second width to the first width along the light propagation direction. And
The arm portion has a phase difference of (2m + z) π with respect to the first light propagating through the first and second optical waveguides of the arm portion (m is an integer equal to or greater than 0, and z is 0 <z <1. A real number of the optical element.   In the first directional coupler, an interval between the first and second optical waveguides is narrowed along a light propagation direction, and in the second directional coupler, an interval between the first and second optical waveguides. The optical element according to claim 1, wherein the optical element is spread along a light propagation direction. The optical element according to claim 1, wherein an optical path length difference ΔS ₁ between the first and second optical waveguides constituting the arm portion is set so as to satisfy the following formula (1).
2ΔS ₁ / λ ₁ = 2m + z (1) The material constituting the core and Si, the optical element according to any one of claims 1 to 3, characterized in that the material constituting the clad and SiO _2.   The optical element according to any one of claims 1 to 4, wherein the material constituting the core has a refractive index that is 40% or more larger than the refractive index of the material constituting the clad. . While comprising the optical element according to any one of claims 1 to 5 as a first and a second meta-directional coupler,
A meta arm portion composed of first and second optical waveguides interposed between the first and second meta directional couplers;
The arm portions of the first and second meta-directional couplers are configured to give a phase difference of (2m + 1/2) π to the first light propagating through the respective arm portions;
The meta arm unit gives a phase difference of 2Mπ (M is an integer of 0 or more) to the first light propagating through the meta arm unit,
A first optical waveguide included in the first and second meta directional couplers is connected to a first optical waveguide included in the meta arm unit, and a second optical light included in the first and second meta directional couplers. An optical element, wherein a waveguide and a second optical waveguide provided in the meta arm portion are connected. The optical element according to claim 6, wherein an optical path length difference ΔS ₂ between the first and second optical waveguides constituting the meta arm portion is set so as to satisfy the following formula (2).
2ΔS ₂ / λ ₁ = 2M (2) Second light having a second wavelength λ ₂ different from the first wavelength λ ₁ is further input to the optical element,
The optical element according to claim 7, wherein the optical path length difference ΔS ₂ is set so as to further satisfy the following expression (3) for the second light.
2ΔS ₂ / λ ₂ = 2M + 1 (3)   The optical element according to any one of claims 6 to 8, wherein central axes of the first and second meta-directional couplers intersect or are parallel to each other. In the first meta directional coupler, the first width taper portion of the first directional coupler is provided in an f ₁ optical waveguide (f ₁ is 1 or 2), and the second directional coupler. When the second width taper portion is provided in the f ₂ optical waveguide (f ₂ is 1 or 2),
The first width tapered portion of the first directional coupler in the second meta directional coupler is provided in a u ₂ optical waveguide (u ₂ = 3-f ₂ ), and the second directional coupler Is provided in the u ₁ optical waveguide (u ₁ = 3-f ₁ ),
The optical path length difference between the first and second optical waveguides in the arm portion of the first meta directional coupler and the optical path length difference between the first and second optical waveguides in the arm portion of the second meta directional coupler. The optical element according to claim 6, wherein the absolute values are equal and the signs are inverted.   In place of the first element portion consisting of the second directional coupler of the first meta directional coupler, the meta arm portion, and the first directional coupler of the second meta directional coupler, The optical element according to claim 6, further comprising a multimode optical waveguide that functions in the same manner as the first element part. The optical element according to any one of claims 6 to 11 is provided as a first to i-th Meta Mach-Zehnder interferometer (i is an integer of 2 or more),
The first to i-th Meta Mach-Zehnder interferometers are connected to each other in this order,
The first optical waveguides of the jth and j + 1th Meta Mach-Zehnder interferometers (j is an integer from 1 to i-1) are connected to each other, and the second light of the jth and j + 1th Meta Mach-Zehnder interferometers is connected. An optical element characterized in that waveguides are connected to each other.   The arm portion of the second meta directional coupler and the second directional coupler in the jth meta Mach-Zehnder interferometer, and the first meta directional coupler in the j + 1 meta Mach-Zehnder interferometer. 13. A j-th equivalent arm portion that functions in the same manner as the second element portion is provided instead of the second element portion including the arm portion and the first directional coupler. Optical element.