JP2013068908A - 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 device comprises an optical waveguide 11 composed of a clad 12 provided on a main face 8a side of a substrate 8 and a core 18 provided in the clad. The optical waveguide includes a first and a second optical waveguide 16a and 16b, and has a first and a second directional coupler 22L and 22R composed of the first and second optical waveguides that are arranged in parallel and separated away from each other by a distance allowing optical coupling, and an arm part 22C 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 first light of a first wavelength λ. The arm part gives a phase difference of (2m+z)π to the first light propagating through the arm part (where m is an integer more than 0, and z is a real number satisfying 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 during 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 made of 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 waveguide optical element having more excellent characteristics could not be constituted by this directional coupler. This is because the function of this directional coupler is limited to a 3 dB coupler. That is, when a multi-stage MZ interferometer-type waveguide optical element functions as a 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, according to the present invention, while using a directional coupler type 3 dB coupler, the output light can be distributed at an arbitrary ratio, and therefore can be applied to 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 optical 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.

  The first and second optical waveguides constitute first and second directional couplers and arm portions. 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 constituted by the first and second optical waveguide portions 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 . 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, while using a directional coupler as a 3 dB coupler, it is possible to distribute output light at an arbitrary ratio, and therefore it is applicable to a multistage MZ interferometer type wavelength separation element. An optical element as an interferometer is obtained.

FIG. 2A is a plan view schematically showing the structure of the optical element of Embodiment 1. (B) is the end elevation which cut (A) along the AA line.

(A) is a top view which shows roughly the structure of the optical element of Embodiment 2. FIG. (B) is an enlarged plan view of a first directional coupler portion in (A). (C) is an enlarged plan view of a second directional coupler portion in (A). It is a characteristic view which shows the characteristic of the directional coupler used for the optical element of Embodiment 2. It is a characteristic view with which it uses for description of the effect of the directional coupler used for the optical element of Embodiment 2. FIG. (A) And (B) is a characteristic view which shows the operating characteristic of the optical element of Embodiment 2. FIG. (A) And (B) is a characteristic view which shows the operating characteristic of the optical element of Embodiment 2. FIG. 6 is a plan view schematically showing the structure of an optical element according to Embodiment 3. FIG. 6 is a plan view schematically showing the structure of an optical element according to Embodiment 4. 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 FIG. FIG. 1A is a plan view schematically showing the structure of the optical element. FIG. 1B is an end view of FIG. 1A cut along the line AA. 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.

(Construction)
With reference to FIGS. 1A and 1B, the structure of the optical element 10 will be described. 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 22. 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 geometric length measured along the width direction is referred to as a “width”. In addition, a direction perpendicular to the main surface 8a is referred to as a thickness direction, and a geometric length measured along the thickness direction is referred to as “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 3.47.

  The input unit 24, the MZ interferometer 22, and the output unit 26 constituting the optical waveguide 11 are connected in this order. More specifically, the input optical waveguide 24 a of the input unit 24 is connected to the input end IN 16 a of the first optical waveguide 16 a of the MZ interferometer 22. The output end OUT16a of the first optical waveguide 16a of the MZ interferometer 22 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 22 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 22L and 22R described later, the optical waveguide 11 has a component width W1 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 22L and 22R 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 22L and 22R 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 22. One end of the dummy waveguide 26 b is exposed from the side surface of the clad 12, and the other end is connected to the second optical waveguide 16 b constituting the MZ interferometer 22. The dummy waveguide 24b is not substantially related to the operation of the optical element 10.

  The MZ interferometer 22 constituting the optical waveguide 11 is configured to arbitrarily select 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 to the output ends OUT16a and OUT16b and output at a distribution ratio corresponding to the phase difference Δφ generated in the first light Lt1 in the arm portion 22C. The distribution ratio is the ratio x when the input light is output from each of the two output ports at a ratio of x: (1-x) (where x is 0 <x <1). Indicates. The distribution ratio x when the MZ interferometer 22 functions as a 3 dB coupler is 0.5.

  The MZ interferometer 22 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 22L and 22R formed by the first and second optical waveguides 16a and 16b, and an arm portion 22C.

  The first optical waveguide 16a that is a structural element of the MZ interferometer 22 includes two first linear portions 16aL and 16aR and a first curved 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 curved 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 22L, the arm portion 22C, and the second directional coupler 22R, which are functional elements of the MZ interferometer 22, are arranged in series in this order. The first directional coupler 22L includes first and second straight portions 16aL and 16bL. The second directional coupler 22R includes first and second straight portions 16aR and 16bR. Similarly, the arm portion 22C includes first and second bending portions 16aC and 16bC.

  The first directional coupler 22L includes first and second straight portions 16aL and 16bL that are arranged in parallel with each other at a distance allowing optical coupling. The lengths L1 of the first and second linear portions 16aL and 16bL are equal to each other and are ½ of the coupling length related to the first light Lt1. The widths W2 of the first and second straight portions 16aL and 16bL are equal to each other and are about 285 nm. The thickness of the first and second straight portions 16aL and 16bL is D1 described above. The waveguide interval between the first and second straight portions 16aL and 16bL, that is, the center-to-center distance between the first and second straight portions 16aL and 16bL is about 600 nm.

  Here, the “coupling length” means that light input from one optical waveguide is completely transferred to the other optical waveguide in a directional coupler composed of two linear optical waveguides parallel to each other. Is a geometric length along the light propagation direction of the directional coupler.

  The second directional coupler 22R is configured in the same manner as the first directional coupler 22L including dimensions.

  By setting the length L1 of the first and second directional couplers 22L and 22R to ½ of the coupling length of the first light Lt1, the first and second directional couplers 22L and 22R The optical Lt1 functions as a 3 dB coupler. That is, the first light Lt1 input from the first straight line portion 16aL is equally distributed in power to the first and second straight line portions 16aL and 16bL, and is output from the straight line portions 16aL and 16bL at a distribution ratio of 1: 1. Is done. Similarly, the second directional coupler 22R 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.

  Further, as described above, the first and second directional couplers 22L and 22R have a thickness D1 of 300 nm and a width W2 of 285 nm, thereby making the first and second directional couplers 22L and 22R non-polarized. Can be operated with dependence.

  Returning to the description of the functional elements of the MZ interferometer 22, the arm portion 22C is composed of the first and second bending portions 16aC and 16bC arranged in parallel. The arm unit 22C has a condition (hereinafter referred to as a phase difference) where the phase difference Δφ given to the first light Lt1 after propagating itself is (2m + z) π (m is an integer greater than or equal to 0, and z is a real number where 0 <z <1). (Also referred to as a condition). That is, the arm portion 22C is configured to give a phase difference Δφ greater than 0 and less than π to the first light Lt1 propagating through the first and second optical waveguides 16a and 16b.

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

  Incidentally, the phase difference Δφ is equal to 2mπ corresponds to the case where z = 0 in the above-described phase difference condition, and the phase difference Δφ is equal to (2m + 1) π when the phase difference condition is z = 1. Correspond. Accordingly, the inventor has conceived that the phase difference Δφ given to the first light Lt1 can be set to 2mπ <Δφ <(2m + 1) π by setting z to 0 <z <1. As a result, although the first and second directional couplers 22L and 22R that are 3 dB couplers are used, the optical element 10 can output the first light Lt1 at an arbitrary distribution ratio x.

In order to generate the phase difference Δφ in this range 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 22C are made 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 = Δφ holds between the phase difference Δφ 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 22R. One end of the second output optical waveguide 26b is connected to the second linear portion 16bR of the second directional coupler 22R. The first light Lt1 is output from the first and second output optical waveguides 26a and 26b at an intensity ratio of x: (1-x), respectively.

(Operation)
Hereinafter, the operation of the optical element 10 will be described with reference to FIG.

  The first light Lt1 input to the input unit 24 propagates through the input optical waveguide 24a and reaches the first linear portion 16aL of the first directional coupler 22L.

  The length of the first directional coupler 22L is set to L1 as described above. As a result, the first light Lt1 has half the power at the second linear portion 16bL when the propagation of the first directional coupler 22L is completed due to the interaction between the first and second linear portions 16aL and 16bL. Transition. That is, the first directional coupler 22L functions as a 3 dB coupler.

Subsequently, the first light Lt1 propagates through the arm portion 22C. The optical path length difference ΔS ₁ between the first and second light bending portions 16aC and 16bC constituting the arm portion 22C is set as described above. Therefore, the phase difference of Δφ described above is given to the first light Lt1 output from the first and second light bending portions 16aC and 16bC after propagating through the arm portion 22C.

  Subsequently, the first light Lt1 is input to the second directional coupler 22R. The second directional coupler 22R is configured similarly to the first directional coupler 22L. For this reason, the optical component having a phase difference of 2mπ included in the first light Lt1 is shifted in power from the second output waveguide 26b at the distribution ratio (1-x) with the power transferred to the second linear portion 16bR. Is done. Similarly, the optical component of the phase difference (2m + 1) π included in the first light Lt1 is shifted in power to the first linear portion 16aR and output in a bar state from the first output waveguide 26a with the distribution ratio x. Is done.

  Here, “output in the cross state” means that all the power of light input from the input end IN16a is transferred to the second optical waveguide 16b and 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.

  As a result of setting the width and thickness of the optical waveguide 11 as described above, the optical element 10 operates in a polarization-independent manner 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 22L and 22R, 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 22 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. 2A is a plan view schematically showing the structure of the optical element. FIG. 2B is an enlarged plan view of the first directional coupler portion in FIG. FIG. 2C is an enlarged plan view of the second directional coupler portion in FIG. FIG. 3 is a characteristic diagram showing characteristics of the directional coupler used in the optical element of this embodiment. FIG. 4 is a characteristic diagram for explaining the effect of the directional coupler used in the optical element of this embodiment. FIGS. 5A and 5B and FIGS. 6A and 6B are characteristic diagrams showing the operating characteristics of the optical element of this embodiment. 2A to 2C, 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.

(Construction)
The structure of the optical element 30 will be described with reference to FIGS. The optical element 30 is different from the first and second directional couplers 22L and 22R of the first embodiment in the structure of the first and second directional couplers 32L and 32R constituting the MZ interferometer 32. Except for this, the configuration is the same as that of the optical element 10. Therefore, in FIG. 2, the same components as those in FIG. In order to clarify the distinction from the directional coupler of the first embodiment, in the following description, the first and second directional couplers 32L and 32R of the present embodiment will be referred to as the first and second width difference directions. Also referred to as sex couplers 32L and 32R.

Referring to FIG. 2 (B), the first linear portion 16aL of the first directional coupler 32L includes a first wide area 16aL1 the width of the first width _{W W,} smaller second width than the first width _{W W} the first width of W _N and a narrow region 16AL2. The first wide region 16aL1 and the first narrow region 16aL2 are connected in series in this order. The length of the first wide area 16aL1 a first length _{L W,} the length of the first narrow region 16aL2 a second length _{L N.} A center line along the light propagation direction of the first straight line portion 16aL is defined as A16aL.

Similarly, the second linear portion 16bL of the first directional coupler 32L includes width and the second narrow region 16bL1 the second width _{W N,} and a second wide area 16bL2 the first width _{W W} . The second narrow region 16bL1 and the second wide region 16bL2 are connected in series in this order. The length of the second wide area 16bL2 a first length _{L W,} the length of the second narrow region 16bL1 a second length _{L N.} A center line along the light propagation direction of the second straight line portion 16bL is defined as A16bL.

Thus, the first and second wide area 16aL1 and 16bL2 are equal first width _{W W} and the first length _{L W} each other, first and second narrow region 16aL2 and 16bL1 also and second widths _{W N} The second lengths L _N are equal to each other. The first and second straight portions 16aL and 16bL are arranged such that the center lines A16aL and A16bL are parallel to each other.

  The first and second wide regions 16aL1 and 16bL2 and the first and second narrow regions 16aL2 and 16bL1 are arranged symmetrically with respect to the center point 32LO of the first directional coupler 32L.

Here, the center point 32LO of the first directional coupler 32L refers to the first straight line A1 that passes through the middle point in the length direction of the first directional coupler 32L and extends in the width direction, and the first directional coupler. This indicates an intersection with the second straight line A2 that passes through the midpoint in the width direction of 32L and extends in the length direction. Here, the midpoint in the length direction of the first directional coupler 32L is a point corresponding to the midpoint L _A / 2 of the total length L _A (= L _W + L _N ) of the first directional coupler 32L. is there. The midpoint in the width direction of the first directional coupler 32L is a point corresponding to ½ of the distance between the center lines A16aL and A16bL.

  Referring to FIG. 2C, the second directional coupler 32R is configured in the same manner as the first directional coupler 32L. That is, the first straight portion 16aR of the second directional coupler 32R and the first straight portion 16aL of the first directional coupler 32L are configured to be equal. Further, the second straight portion 16bR of the second directional coupler 32R and the second straight portion 16bL of the first directional coupler 32L are configured to be equal. Therefore, further description of the second directional coupler 32R is omitted.

Here, an example of the dimensions of the first and second directional couplers 32L and 32R will be given. For the first and second wide regions 16aL1, 16bL2, 16aR1 and 16bR2 constituting the first and second directional couplers 32L and 32R, the first width _WW is about 320 nm, for example, and the first length L _W is For example, about 10 μm. Regarding the first and second narrow regions 16aL2, 16bL1, 16aR2, and 16bR1, the second width W _N is, for example, about 280 nm, and the second length L _N is, for example, about 10 μm. Further, the distance between the center lines A16aL and A16bL and the distance between the center lines A16aR and A16bR are about 600 nm, for example. The thickness D1 of the first and second optical waveguides 16a and 16b constituting the first and second directional couplers 32L and 32R is set to 300 nm.

Next, a method for determining the first and second widths W _W and W _N and the first and second lengths L _W and L _N will be described. The first width W _W and the first length L _{W of} the first and second wide regions 16aL1 and 16bL2 constituting the first directional coupler 32L, and the second width of the first and second narrow regions 16aL2 and 16bL1 W _N and the second length L _N are preferably determined by simulation. That is, it is preferable to determine the dimensions of the first and second wide regions 16aL1 and 16bL2 and the first and second narrow regions 16aL2 and 16bL1 so that the first directional coupler 32L functions as a 3 dB coupler. .

  More specifically, first, the first directional coupler 32L is divided into a first half portion and a second half portion, and a simulation is executed. Here, the first half portion refers to a structure including the first wide region 16aL1 and the second narrow region 16bL1. Further, the latter half portion indicates a structure composed of the first narrow region 16aL2 and the second wide region 16bL2.

First, in the first half, simulation is performed, and the first and second widths and the first and second lengths are provisionally determined so that the distribution ratio x is maximized with respect to the first light Lt1. Then, the first half portion is combined with the second half portion to perform further simulation, and the first directional coupler 32L as a whole has a distribution ratio x with respect to the first light Lt1 of 0.5 and functions as a 3 dB coupler. And the second widths W _W and W _N and the first and second lengths L _W and L _N are adjusted.

(Effect of directional coupler)
According to the inventor's evaluation, the first and second directional couplers 32L and 32R in which the wide region and the narrow region are arranged point-symmetrically in this way are provided in the first and second directional characteristics of the optical element 10. It has become clear that the tolerance to the width error is greater than that of the couplers 22L and 22R. Hereinafter, this point will be described with reference to FIGS. 3 and 4. As described above, since the first and second directional couplers 32L and 32R have the same configuration, in the following description, the first directional coupler 32L will be described as an example. Therefore, unless otherwise specified, the following description holds true for the second directional coupler 32R as well.

FIG. 3 is a characteristic diagram showing the operating characteristics of the first directional coupler 32L obtained by the FDTD (Finite Difference Time Domain) method. Schematically speaking, in FIG. 3, the first and second widths W _W and W _N are changed by introducing a width error, and output in the bar state and the cross state from the first directional coupler 32L. Each of the light intensity ratios is obtained. Here, the light output from the first straight portion 16aL of the first directional coupler 32L in the bar state is referred to as bar state light (curve I). Further, the light output from the second linear portion 16bL of the first directional coupler 32L in the cross state is referred to as cross state light (curve II). In FIG. 3, the vertical axis represents the intensity ratio (dB) of the bar state light and the cross state light to the input light, and the horizontal axis represents the width error (nm) introduced into the first and second widths W _W and W _N. It is.

In obtaining FIG. 3, it is assumed that the wavelength of the input light input to the first linear portion 16aL of the first directional coupler 32L is 1.49 μm. Further, except for the first and second widths W _W and W _N, the size and the refractive index of the components of the first directional coupler 32L was used the above-mentioned values. The width errors introduced into the first and second widths _{W W} and _{W N} were varied between -20～20Nm. Note that the "width error is Enm" (E is a real number), respectively that the first width is changed to _W W + E from _{W W,} the second width is changed to _W N + E from _{W N} Show.

  Referring to FIG. 3, curves I and II are both constant at about −3 dB (distribution ratio x = 0.5) within a width error range of −20 to 20 nm. That is, even if there is a width error in this range, it can be seen that the distribution ratio x of the first directional coupler 32L is kept constant at 0.5 and functions as a 3 dB coupler with a practically sufficient level.

  FIG. 4 is a characteristic diagram in which the same calculation as in FIG. 3 is performed on the first directional coupler 22L of the first embodiment, and the first directional coupler 32L of this embodiment is superior to the width error. It is for showing that it has tolerance. That is, in FIG. 4, the intensity ratio of the bar state light and the cross state light output from the first directional coupler 22L is obtained while changing the width W2 by introducing the width error. Here, the bar state light (curve III) indicates light output from the first linear portion 16aL of the first directional coupler 22L. The cross state light (curve IV) indicates light output from the second linear portion 16bL of the first directional coupler 22L. The vertical axis and horizontal axis in FIG. 4 represent the same meaning as in FIG.

  In obtaining FIG. 4, in order to make the first directional coupler 22L function as a 3 dB coupler, the total length L1 is set to 9.7 μm. The numerical conditions and calculation methods other than the total length L1 were the same as those in FIG.

  Referring to FIG. 4, when the width error is 0 nm, the curves III and IV coincide with each other at −3 dB. From this, it can be seen that when the width error is 0 nm, the first directional coupler 22L functions as a 3 dB coupler. However, in width errors other than 0 nm, the curves III and IV do not match, and it can be seen that the distribution ratio x of the first directional coupler 22L is greatly deviated from 0.5. That is, it can be seen that the first directional coupler 22L does not function sufficiently as a 3 dB coupler with a width error other than 0 nm.

  From the comparison between FIG. 3 and FIG. 4, it is clear that the first directional coupler 32L of this embodiment is more resistant to the width error than the first directional coupler 22L of the first embodiment. That is, the first directional coupler 32L in which the wide portions 16aL1 and 16bL2 and the narrow portions 16aL2 and 16bL1 are arranged point-symmetrically can keep the distribution ratio x constant at 0.5 even if there is a width error. it can.

  Since the optical element 30 operates in the same manner as the optical element 10, the description thereof is omitted.

(Effect of optical element)
Next, the effect of the optical element 30 will be described with reference to FIGS. 5 (A) and 5 (B) and FIGS. 6 (A) and 6 (B). 5 and 6, while changing the width error, enter the first light Lt1 of changing the wavelength lambda ₁ to the input terminal IN16a, the intensity of the light output from the respective output terminals OUT16a and OUT16b, FDTD It was obtained by law.

  5 and 6, the vertical axis represents the intensity ratio (dB) of the light output from the output terminals OUT16a and OUT16b to the first light Lt1, and the horizontal axis represents the wavelength (μm). Curves I and 5 in FIG. 6 indicate bar state light output from the output end OUT16a, and a curve II indicates cross state light output from the output end OUT16b.

5 and 6, the optical element 30 is operated 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). Designed. That is, the optical path length difference ΔS ₁ between the first and second bending portions 16aC and 16bC in the arm portion 32C is set to follow 2ΔS ₁ / λ ₁ = 2m. Specifically, ΔS ₁ was set to 1.35 μm. Further, the first and second lengths L _W and L _N of the first and second directional couplers 32L and 32R were set to 2.8 μm, respectively. The other dimensions and refractive index were the same as in FIG.

  5A shows a case where the width error introduced into the optical element 30 is −40 nm, and FIG. 5B shows a case where the width error introduced into the optical element 30 is −20 nm. FIG. 6A shows the case where the width error introduced into the optical element 30 is 20 nm, and FIG. 6B shows the case where the width error introduced into the optical element 30 is 40 nm. The width error was introduced into the first and second directional couplers 32L and 32R.

  5 and 6, it can be seen that the optical element 30 functions as a wavelength separation element regardless of the magnitude of the width error. The peak wavelength is shifted in each figure because the equivalent refractive index of the optical waveguide 11 changes depending on the size of the width error. However, in each figure, there is no significant change in the ratio between the peak intensity at the predetermined wavelength and the bottom intensity at the wavelength. From this, it can be seen that in the optical element 30, the crosstalk characteristics are maintained well within the range of the width error. Here, the crosstalk characteristic is a separation capability when the first light Lt1 is divided and output to the output terminals OUT16a and OUT16b.

  As described above, the optical element 30 of this embodiment has high resistance to the width error in addition to the effect exhibited by the optical element 10. Specifically, as apparent from FIG. 2, the optical element 30 is configured by using the first and second directional couplers 32L and 32R in which the distribution ratio x is kept constant even if there is a width error. Yes. As a result, when the optical element 30 including the first and second directional couplers 32L and 32R is used as a wavelength separation element, crosstalk characteristics can be improved.

(Modification)
Hereinafter, modified examples of the optical element 30 will be described. The optical element 30 can be modified in the same manner as the optical element 10 and further can be modified as listed below.

(Modification 1)
In this embodiment, the case where the optical element 30 includes the first and second directional couplers 32L and 32R in which the wide region and the narrow region are arranged point-symmetrically on both sides of the arm portion 32C has been described. . However, the total number of the first and second directional couplers 32L and 32R is not limited to two. An optical element provided with only one of the first and second directional couplers 32L and 32R can also maintain a tolerance for width error to an acceptable level for practical use.

(Modification 2)
In this embodiment, when the first directional coupler 32L and the second directional coupler 32R are translated, the wide area and the narrow area are arranged in such a manner that they completely overlap each other. (Hereinafter, this arrangement mode is referred to as an overlap mode). That is, when the first and second directional couplers 32L and 32R are translated, the first wide regions 16aL1 and 16aR1, the second wide regions 16bL2 and 16bR2, the first narrow regions 16aL2 and 16aR2, In addition, the second narrow regions 16bL1 and 16bR1 overlap each other.

  However, the arrangement mode of the wide region and the narrow region constituting the first and second directional couplers 32L and 32R is not limited to the overlapping mode. That is, the arrangement mode of the wide region and the narrow region constituting the first and second directional couplers 32L and 32R is a reflection-related arrangement mode (hereinafter, this arrangement mode is referred to as a mirror mode). There may be. The optical element configured as described above can also maintain a practically sufficient tolerance against the width error.

(Modification 3)
In this embodiment, a first length _{L W} of the first and second wide area 16aL1,16bL2,16aR1 and 16BR2, and a second length _{L N} of the first and second narrow region 16aL2,16bL1,16aR2 and 16bR1 The case where they are equal has been described. However, it is not necessary to make the first and second lengths L _W and L _N equal, and an appropriate value can be selected according to the design. The optical element configured as described above can also maintain a practically sufficient tolerance against the width error.

[Embodiment 3]
Hereinafter, the optical element of Embodiment 3 will be described with reference to FIG. FIG. 7 is a plan view schematically showing the structure of the optical element. In FIG. 7, 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.

(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 30 of the second embodiment as one component. More precisely, the optical element 50 includes the MZ interferometer 32 of the second embodiment as 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" The two are distinguished. Further, in order to prevent the drawing from being complicated, the reference numerals of some components are omitted in FIG.

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 is also 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 wavelength lambda ₁ of the first light Lt1, enter the mixed light of the second light Lt2 the second wavelength lambda ₂ of the wavelength different from the first wavelength lambda _1, 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, a first optical waveguide 16a ₁ and 16a ₂ to the first and second meta directional couplers 32 ₁ and 32 ₂ are provided, a first optical waveguide 16a ₃ provided in Metaamu portion 32 ₃ are connected, a second optical waveguide 16b ₁ and 16b ₂ of the first and second meta directional coupler 32 ₁ and 32 ₂ are provided, are connected to the second optical waveguide 16b ₃ provided in Metaamu portion 32 _3.

The first meta-directional coupler 32 ₁ is configured such that the arm portion 32 ₁ C gives a phase difference Δφ 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 Δφ = (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, the first meta directional coupler 32 ₁ operates similarly to the first directional coupler 22L described in the first embodiment. In order to give this phase difference Δφ, 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 above formula (2 ) To set.

In this embodiment, the second light Lt2 is not considered in designing the arm portion 32 ₁ C. In general, in a directional coupler, the cross output light (first light Lt1) needs to be designed in strict accordance with the interference condition, but the bar output light (second light Lt2) has to be strict in the interference condition. This is because even if not satisfied, the bars are output with a practically sufficient distribution ratio. Therefore, the arm portion 32 ₁ C is designed based on the first light Lt1, and the second light Lt2 is not considered.

The second meta directional coupler 32 ₂ is configured similarly to the first meta-directional coupler 32 _1, it operates similarly to the second directional coupler 22R described in the first embodiment. In the second meta directional coupler 32 _2, wide area and a narrow area is located in the first meta-directional coupler 32 ₁ duplicates embodiment.

Metaamu portion 32 _3, (the M 0 or an integer) 2Emupai for the first light Lt1 propagating the first and second optical waveguides 16a ₃ and 16b ₃ are configured to provide a phase difference ΔΦ of. In other words, the first light Lt1 in order to cross output, Metaamu portion 32 ₃ is a phase difference .DELTA..PHI to impart to the first light Lt1, set so as to satisfy the the interference condition ΔΦ = 2Mπ.

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 . Incidentally, with respect to the first light Lt1 wavelength lambda _1, between the phase difference .DELTA..PHI the optical path length difference [Delta] S _2, because holds true 2πΔS ₂ / λ ₁ = ΔΦ, the phase difference condition, [Delta] S ₂ is represented by the following formula (3 ) Should be established.
2ΔS ₂ / λ ₁ = 2M (3)

To the second light Lt2 is bar output is preferably set to Metaamu portion 32 ₃ is a phase difference .DELTA..PHI to impart to the second optical Lt2, an interference condition ΔΦ = (2M + 1) π is further satisfied . Incidentally, with respect to the second light Lt2 wavelength lambda _2, between the phase difference .DELTA..PHI the optical path length difference [Delta] S _2, because holds true 2πΔS ₂ / _{λ 2} = ΔΦ, the phase difference condition, [Delta] S ₂ is represented by the following formula (4 It is preferable to set so as to satisfy the above.
2ΔS ₂ / λ ₂ = 2M + 1 (4)

The meta arm part 32 ₃ configured as described above has a phase difference of 2Mπ with respect to the first light Lt1 distributed in 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. First and second light Lt1 and Lt2 input to the input unit 24, reaches the first meta directional coupler 32 ₁ propagated through the input optical waveguide 24a.

Since the first meta directional coupler 32 ₁ is configured as described above, the power of the first light Lt1 is transferred to the second optical waveguide 16b ₁ after propagation through the first meta directional coupler 32 _1. As a result, the first and second linear portions 16a ₁ R and 16b ₁ R are equally distributed and output. On the other hand, the second light Lt2 is output from the first linear portion 16a ₁ R with almost no power transfer to the second optical waveguide 16b ₁ . That is, the first meta directional coupler 32 ₁ functions as a 3dB coupler with respect to the first light Lt1.

Subsequently, the first and second lights Lt1 and Lt2 propagate through the meta arm portion 32 ₃ C. The optical path length difference ΔS ₂ between the first and second light bending portions 16a ₃ C and 16b ₃ C constituting the meta arm portion 32 ₃ C is set as described above. Therefore, a phase difference of 2Mπ is given to the first light Lt1 output from the first and second light bending portions 16a ₃ C and 16b ₃ C after propagating through the meta arm portion 32 ₃ C. On the other hand, the second light Lt2 is mostly the component output from the first light bending portion 16a ₃ C, and the component output from the second light bending portion 16b ₃ C is small, but both components are (2M + 1). ) A phase difference of π is given.

Subsequently, the first and second lights Lt 1 and Lt ₂ are input to the second meta-directional coupler 322. The second meta directional coupler 32 _2, like the first meta directional coupler 32 ₁ is configured as a 3dB coupler. As a result, all the power of the first light Lt1 is transferred to the second linear portion 16b ₂ R, and is output in a cross state from the second output optical waveguide 26b. On the other hand, in the second light Lt2, a small amount of component light propagating through the second linear portion 16b ₂ R is transferred to the first linear portion 16a ₂ R, and is output from the first output optical waveguide 26a in a bar state. The

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 directional couplers 32 ₁ L, 32 ₁ R, 32 ₂ L, and 32 ₂ R similar to the directional coupler of the second embodiment. As a result, compared to an MZ interferometer type wavelength separation element using a normal directional coupler, resistance to width errors is excellent.

(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 elements 10 and 30, and further can be modified as listed below.

(Modification 1)
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 described later in the fourth embodiment, by using the 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 2)
In this embodiment, the case where the MZ interferometer 32 of the _second embodiment is used as the first and second meta-directional couplers 32 ₁ and 32 ₂ has been described. However, other 3 dB couplers may be used as the first and second meta-directional couplers 32 ₁ and 32 ₂ . For example, the MZ interferometer 22 of Embodiment 1 may be used as the first and second meta-directional couplers 32 ₁ and 32 ₂ .

[Embodiment 4]
Hereinafter, the optical element of Embodiment 4 will be described with reference to FIG. FIG. 8 is a plan view schematically showing the structure of the optical element. In FIG. 8, 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.

(Construction)
The structure of the optical element 60 will be described with reference to FIG. In other words, the optical element 60 is a multi-stage MZ interferometer type wavelength separation element, and corresponds to a configuration in which a plurality of MMZ interferometers 40 of the third embodiment are 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. Further, in order to prevent the drawing from being complicated, the reference numerals in FIG. 8 are omitted except for the components necessary for the description.

  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 third 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 of the first to iMMZ interferometers 40-1 to 40-i with respect to the first light Lt1 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.

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 described in the first embodiment, the phase difference introduced into the arm portion 32 ₁ Cr of the meta directional coupler 32 ₁ -r is set so that the distribution ratio becomes 1 / (2i). Ask. 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 O42-j of the j-th pair structure 42-j as the center of symmetry. Here, the center point O42-j of the j-th pair structure 42-j corresponds to the center of gravity of all the constituent elements 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.

  The operation of the optical element 60 is the same as that of the MMZ interferometer 40 of the third embodiment except that the distribution ratio of the first light Lt1 after propagating through the rMMZ interferometer 40-r is 1 / i. . Therefore, description of the operation of the optical element 60 is omitted.

  As described above, the optical element 60 constitutes a wavelength separation element using the first to i-th MMZ interferometers 40-1 to 40-i that can set the distribution ratio precisely to an arbitrary value. As a result, the optical element 60 can broaden the peak wavelength band of the cross output light and the bar output light as compared with the one-stage MZ interferometer type wavelength separation element.

The optical element 60 is configured using the directional couplers 32 ₁ L, 32 ₁ R, 32 ₂ L, and 32 ₂ R of the third embodiment. As a result, compared to an MZ interferometer type wavelength separation element using a normal directional coupler, resistance to width errors is excellent.

(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 elements 10 and 30 and the MMZ interferometer 40, and can be further modified as follows.

(Modification 1)
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 O42-j 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 have to be arranged point-symmetrically, and may be arranged in an overlapping manner. 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, 60 Optical element 11 Optical waveguide 12 Clad 18 Core 22, 32, Mach-Zehnder interferometer (MZ interferometer)
32LO, 32RO, O42-j Center points 32 ₁ , 32 ₁ -r 1st meta directional coupler 32 ₂ , 32 ₂ -r 2nd meta directional coupler 32 ₃ , 32 ₃ C meta arm portions 22L, 32L, 32 ₁ L, 32 ₂ L 1st directional coupler 22R, 32R, 32 ₁ R, 32 ₂ R 2nd directional coupler 22C, 32C, 32 ₁ C, 32 ₂ C, 32 ₁ C-r arm part 16aL, 16aR, 16a ₁ L, 16a ₁ R, 16a ₂ L, 16a ₂ R 1st straight line portion 16bL, 16bR, 16b ₁ L, 16b ₁ R, 16b ₂ L, 16b ₂ R 2nd straight line 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 2nd bending portion 16a, 16a ₁ , 16a ₂ , 1 _{6a 3, 16a-j, 16a-} (j + 1) first optical waveguide _{16b, 16b 1, 16b 2,} 16b 3, 16b-j, 16b- (j + 1) second optical waveguide 16aL1,16aR1 first wide area 16aL2,16aR2 1st narrow region 16bL1, 16bR1 2nd narrow region 16bL2, 16bR2 2nd wide region A16aL, A16bL, A16aR, A16bR Center line A1 1st straight line A2 2nd straight line IN16a Input end OUT16a, OUT16b Output end 24 Input section 24a Input Optical waveguide 24b dummy waveguide 26 output portion 26a first output optical waveguide 26b second output optical waveguide 40 optical unit (meta Mach-Zehnder (MMZ) interferometer)
40-r rMMZ interferometer 40-j jMMZ interferometer 40- (j + 1) (j + 1) MMZ interferometer 42-j j-th pair structure

Claims (12)

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 arranged in parallel with each other at 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 of the first wavelength λ ₁ ,
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. 2. 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)   3. The geometric length measured along the light propagation direction of the first and second directional couplers is set to ½ of the coupling length of the first light, respectively. An optical element according to 1. A first optical waveguide constituting the first or second directional coupler is a first wide region having a first width that is a length measured in a direction perpendicular to the light propagation direction and parallel to the main surface; A first narrow region having a second width smaller than the first width,
A second optical waveguide constituting the first or second directional coupler includes a second wide region having the first width and a second narrow region having the second width;
The first and second wide regions and the first and second narrow regions are arranged point-symmetrically with a center point of the first or second directional coupler as a center of symmetry. The optical element according to any one of claims 1 to 3. A first optical waveguide constituting the first and second directional couplers is a first wide region having a width that is a length measured in a direction perpendicular to a light propagation direction and parallel to the main surface; A first narrow region having a second width smaller than the first width,
The second optical waveguide constituting the first and second directional couplers includes a second wide region having the first width and a second narrow region having the second width,
The first and second wide regions and the first and second narrow regions are arranged point-symmetrically with respect to a center point of the first and second directional couplers, The optical element according to any one of claims 1 to 3. 6. The optical element according to claim 1, wherein the material constituting the core is Si, and the material constituting the clad is SiO ₂ .   The optical element according to any one of claims 1 to 6, wherein the material constituting the core has a refractive index greater by 40% or more than the refractive index of the material constituting the clad. . While comprising the optical element according to any one of claims 1 to 7 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 8, 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 having} a wavelength different from the first wavelength λ ₁ is further input to the optical element,
The optical element according to claim 9, 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 8 to 10 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 arranged in series 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. For all pairs consisting of the jth and j + 1th Meta Mach-Zehnder interferometers,
The first and second optical waveguides of the jth meta Mach-Zehnder interferometer and the first and second optical waveguides of the j + 1 meta Mach-Zehnder interferometer are the j-th and j + 1-th metamachers. The optical element according to claim 11, wherein the optical element is arranged point-symmetrically with a center point of a structure formed of an ender interferometer as a center of symmetry.