JP2018017808A - Optical waveguide device and optical circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide device capable of minimizing multiple reflection of return light in a multi-mode interference section, and to provide an optical circuit.SOLUTION: An optical waveguide comprises a multi-mode interference section 13, an input waveguide 11 for directing input light 12 to the multi-mode interference section, output waveguides 14, 15 for directing output light from the multi-mode interference section, and return light waveguides 17, 18 for guiding return light 19 travelling from the output waveguide side to the input waveguide side in the multi-mode interference section to outside the multi-mode interference section.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device having a multimode interference part.

  With the realization of high-speed digital signal processing, digital coherent technology that can recover signal degradation due to optical fiber dispersion and nonlinearity by digital processing has been developed. In the digital coherent technology, in order to improve the frequency utilization efficiency of signal light, polarization multiplexing and multilevel phase modulation technology are used in addition to wavelength multiplexing.

  In an optical fiber transmission system employing a multi-level phase modulation technique, signal light transmitted through an optical fiber interferes with local oscillation light (hereinafter referred to as local light) having a constant amplitude and phase that is not modulated. Thus, a coherent mixer that converts a phase modulation signal into an intensity modulation signal is used.

  In such a coherent mixer, a multi-mode interference (MMI) type optical branching device is used to branch signal light and local light.

JP2013-137360A International Publication No. 2007/080891

  On the other hand, in multimode interference type optical branching devices, there is a problem of return light that affects device characteristics.

  FIG. 10 is an explanatory diagram showing the return light of the multimode interference type optical branching device. The multimode interference unit 13 of the optical branching device shown in FIG. 10 is connected to the input waveguide 11, the first output waveguide 14, and the second output waveguide 15. The input light 12 guided through the input waveguide 11 is branched into two by the multimode interference unit 13, and is coupled to the first output waveguide 14 and the second output waveguide 15, respectively, and output as output light.

  Return light that affects the device characteristics of the optical branching device is generated inside and outside the multimode interference unit 13. An example of return light generated inside the multimode interference unit 13 is return light 41 generated on the output side end face between the first output waveguide 14 and the second output waveguide 15. The return light 41 is light that is not coupled to the first output waveguide 14 and the second output waveguide 15 in the multimode interference unit 13 and is reflected from the output side end face of the multimode interference unit 13.

  An example of the return light generated outside the multimode interference unit 13 is an optical element in which the output light that is guided by being coupled to the first output waveguide 14 or the second output waveguide 15, respectively, is connected to the optical element. The return light 42 and the return light 43 are generated by reflection at the connection point.

  The return lights 41, 42, and 43 are subjected to multiple reflection at the reflection area 44, the reflection area 45, and the side surface (upper side or lower side of the multimode interference section 13 shown in FIG. 10) of the input side end face of the multimode interference section 13. Repeatedly, the first output waveguide 14 and the second output waveguide 15 are coupled again.

  When the input light 12 of the input waveguide 11 is signal light, the return lights 41, 42, and 43 are coupled to the main signal light, resulting in a ripple in the wavelength dependence of the optical branching device, or the optical phase of the main signal light The device characteristics will be affected.

  Patent Document 1 describes that a tapered multimode waveguide is provided between the multimode interference unit and each port waveguide in order to suppress the generation of return light generated inside the multimode interference unit. Has been.

  However, the disclosure in Patent Document 1 is only the light intensity of the return light at the output-side end face of the multimode interference unit, and the influence of multiple reflection accompanying reflection of the return light at the reflection areas 44 and 45 on the input-side end face. Is not considered.

  Further, even if the reflection intensity at the end face on the input waveguide side is reduced in the configuration of Patent Document 1, the power of the return light is not lost inside the multimode interference unit due to the energy conservation law. For this reason, the return light is repeatedly reflected and coupled to the output waveguide.

  An object of the present invention is to provide an optical waveguide device capable of suppressing multiple reflections of return light in a multimode interference section.

  The optical waveguide device of the present invention includes a multimode interference unit, an input waveguide that propagates input light to the multimode interference unit, an output waveguide that propagates output light from the multimode interference unit, and the output guide. A return optical waveguide is provided that guides return light propagating through the multimode interference unit from the waveguide side to the input waveguide side from the inside of the multimode interference unit.

  The optical circuit of the present invention includes a first branching element that splits signal light and outputs two lights that are in phase, a second branching element that splits local light and outputs two lights that are phase-shifted, From one light from the first branch element, a first multiplexing element that combines one light from the second branch element, another light from the first branch element, and from the second branch element An optical circuit including a second multiplexing element for multiplexing other light, wherein the first branching element or the second branching element is the optical waveguide device.

  The present invention can reduce the influence of return light on device characteristics.

It is a figure which shows the structural example of the optical branching device of 1st Embodiment. It is a figure which shows the structural example which removes the return light which guides a 1st return optical waveguide. It is a top view which shows an example of the removal mechanism of a 1st return optical waveguide. It is a top view which shows an example of the removal mechanism of a 1st return optical waveguide. It is a figure which shows the structural example of the optical branching device of 2nd Embodiment. It is a figure which shows the structural example of the optical branching device of 3rd Embodiment. It is a figure which shows the structural example of the optical branching device of 4th Embodiment. It is a figure which shows the structural example of the coherent mixer of 5th Embodiment. It is sectional drawing which shows the structure of the optical waveguide applicable to the optical branching device of 1st-5th embodiment. It is explanatory drawing which shows the return light of a multimode interference type optical branching device.

(First embodiment)
The optical waveguide device according to the first embodiment will be described with reference to the drawings. An example of the optical waveguide device of the first embodiment is an optical branching device having a multimode interference unit.

  FIG. 1 is a diagram illustrating a configuration example of the optical branching device according to the first embodiment. As shown in FIG. 1, the optical branching device has a multimode interference unit 13. The multimode interference unit 13 is connected to the input waveguide 11, the first output waveguide 14, and the second output waveguide 15.

  The input light 12 guided through the input waveguide 11 is branched into two by the multimode interference unit 13, and is coupled to the first output waveguide 14 and the second output waveguide 15, respectively, and is output as the output light 16. . Specifically, the input light 12 is signal light or local light.

  Furthermore, the first return optical waveguide 17 and the second return optical waveguide 18 are connected to the input side end face of the multimode interference unit 13. Specifically, the first return optical waveguide 17 and the second return optical waveguide 18 are connected to the reflection region on the input side end face of the multimode interference unit 13. The reflection region on the input side end surface is, for example, an end surface other than the end surface to which the input waveguide 11 is connected, among the input side end surfaces.

  More preferably, the first return optical waveguide 17 and the second return optical waveguide 18 return light from the output side end face of the multimode interference unit 13 or from the first output waveguide 14 and the second output waveguide 15. It is arranged at a position where it couples with the return light. The first return optical waveguide 17 and the second return optical waveguide 18 guide the return light from the multimode interference unit 13 to the outside.

  An example of the return light generated inside the multimode interference unit 13 is the return light generated on the output side end face between the first output waveguide 14 and the second output waveguide 15.

  Another example of the return light generated outside the multimode interference unit 13 is light in which output light guided by being coupled to the first output waveguide 14 or the second output waveguide 15 is connected at the other end. This is return light that is generated by reflection at the connection point with the element.

  The return light 19 coupled to the first return optical waveguide 17 and the second return optical waveguide 18 propagates in the opposite direction to the input light 12 propagating through the input waveguide 11. Further, each of the first return optical waveguide 17 and the second return optical waveguide 18 has a removal mechanism for removing the return light 19.

  Hereinafter, the removal of the return light 19 from the first return optical waveguide 17 will be described with reference to FIGS. 2, 3, and 4. The same applies to the removal of the return light 19 from the second return optical waveguide 18.

  FIG. 2 is a diagram illustrating a configuration example in which return light guided through the first return optical waveguide 17 is removed. FIG. 2A is a top view showing an example of a mechanism for removing the first return optical waveguide 17. 2B and 2C are cross-sectional views taken along lines A-A ′ and B-B ′ in FIG.

  The removal mechanism of the return light 19 shown in FIG. 2A is a light emission region provided in the first return optical waveguide 17. The light emission region has a configuration in which the width of the optical waveguide of the first return optical waveguide 17 is narrowed in the propagation direction of the return light 19 so that the effective refractive index of the core of the optical waveguide approaches the refractive index of the cladding. When the first return optical waveguide 17 is a rib-type waveguide, as shown in FIGS. 2B and 2C, the rib width of the upper clad 20 becomes narrow so that the slab mode is obtained. The return light 19 is emitted and removed.

  FIG. 3 is a top view showing an example of the removal mechanism of the first return optical waveguide 17. The removal mechanism of the return light 19 shown in FIG. 3 has a configuration in which an antireflective coating 22 is disposed on the chip end surface 21 of the first return optical waveguide 17. Thereby, the return light 19 propagating through the first return optical waveguide 17 is prevented from returning to the multimode interference unit 13.

  FIG. 4 is a top view showing an example of the removal mechanism of the first return optical waveguide 17. The removal mechanism of the return light 19 shown in FIG. 4 has a configuration in which a light absorption region 23 that absorbs the return light 19 is arranged in the first return optical waveguide 17. For example, the light absorption region 23 is configured to use infrared light absorption by free carrier absorption by adding impurities to the core of the optical waveguide. Specifically, when the core of the optical waveguide is made of silicon (Si), the light absorption region 23 is formed by ion implantation of boron (B), phosphorus (P), antimony (Sb), or the like. Can do.

  The waveguide width of the first return optical waveguide 17 or the second return optical waveguide 18 is preferably equal to or greater than the waveguide width of the input waveguide 11 at the input side end face.

(Effects of the first embodiment)
According to the first embodiment, it is possible to suppress the multiple reflection of the return light in the multimode interference unit. Thereby, in the optical branching device having the multimode interference unit, it is possible to reduce the influence of the return light on the device characteristics.

  The reason is that the return light is guided out of the multimode interference unit 13 by the first return optical waveguide 17 and the second return optical waveguide 18 connected to the multimode interference unit 13. Further, the return light coupled to the first return optical waveguide 17 and the second return optical waveguide 18 is removed.

(Second Embodiment)
Next, an optical waveguide device according to a second embodiment will be described with reference to the drawings. Similar to the first embodiment, the optical waveguide device of the second embodiment is an optical branching device having a multimode interference unit. The second embodiment is different from the first embodiment in that the return optical waveguide connected to the input side end face of the multimode interference unit has a tapered shape. Hereinafter, in the description of the optical branching device of the second embodiment, the same components as those of the optical branching device of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  FIG. 5 is a diagram illustrating a configuration example of the optical branching device according to the second embodiment. As shown in FIG. 5, the optical branching device has a multimode interference unit 13. The multimode interference unit 13 is connected to the input waveguide 11, the first output waveguide 14, and the second output waveguide 15.

  The first return optical waveguide 24 and the second return optical waveguide 25 are connected to the input side end face of the multimode interference unit 13. Specifically, the first return optical waveguide 24 and the second return optical waveguide 25 are connected to the reflection region on the input side end face of the multimode interference unit 13.

  More preferably, the first return optical waveguide 24 and the second return optical waveguide 25 are the return light from the output side end face of the multimode interference unit 13, or from the first output waveguide 14 and the second output waveguide 15. It is arranged at a position where it couples with the return light. The first return optical waveguide 24 and the second return optical waveguide 25 guide return light from the multimode interference unit 13 to the outside.

  Furthermore, in the second embodiment, the first return optical waveguide 24 or the second return optical waveguide 25 has a tapered shape. The tapered shape is a shape that is wide on the multimode interference unit 13 side and narrows in the propagation direction of the return light 19. Further, each of the first return optical waveguide 24 and the second return optical waveguide 25 has a removal mechanism for removing the return light 19.

  By using a tapered shape to connect the first return optical waveguide 24, the second return optical waveguide 25, and the multimode interference unit 13, the return light in the multimode interference unit 13 is converted into the first return optical waveguide 24, the second return optical waveguide 24, and the second return optical waveguide 24. It becomes easy to couple to the return optical waveguide 25.

(Effect of 2nd Embodiment)
According to the second embodiment, multiple reflections of return light in the multimode interference unit can be suppressed. Thereby, in the optical branching device having the multimode interference unit, it is possible to reduce the influence of the return light on the device characteristics.

  The reason is that the return light is guided out of the multimode interference unit 13 by the first return optical waveguide 24 and the second return optical waveguide 25 connected to the multimode interference unit 13. Further, the return light coupled to the first return optical waveguide 24 and the second return optical waveguide 25 is removed.

  Furthermore, in the second embodiment, since the first return optical waveguide 24 or the second return optical waveguide 25 has a tapered shape, it becomes easy to couple with the return light at the input side end face.

(Third embodiment)
Next, an optical waveguide device according to a third embodiment will be described with reference to the drawings. Similar to the first embodiment, the optical waveguide device of the third embodiment is an optical branching device having a multimode interference unit. In the third embodiment, a multimode waveguide 27 is provided between the input waveguide 11 and the multimode interference unit 26, and the first return optical waveguide 17 and the second return optical waveguide 18 are the same as those of the multimode waveguide 27. It is different from the first embodiment in that it is connected to the input side end face. Hereinafter, in the description of the optical branching device of the third embodiment, the same components as those of the optical branching device of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  FIG. 6 is a diagram illustrating a configuration example of the optical branching device according to the third embodiment. As shown in FIG. 6, the multimode waveguide 27 of the optical branching device is connected between the input waveguide 11 and the multimode interference unit 26, and has a shape in which the width becomes wider near the multimode interference unit 26. .

  The input light 12 guided through the input waveguide 11 is branched into two by the multimode interference unit 13 via the multimode waveguide 27 and is coupled to the first output waveguide 14 and the second output waveguide 15, respectively. , And output as output light 16.

  Further, the first return optical waveguide 17 and the second return optical waveguide 18 are connected to the input side end face of the multimode waveguide 27. Specifically, the first return optical waveguide 17 and the second return optical waveguide 18 are connected to the reflection region on the input side end face of the multimode waveguide 27. The reflection region on the input side end surface is, for example, an end surface other than the end surface to which the input waveguide 11 is connected, among the input side end surfaces.

  More preferably, the first return optical waveguide 17 and the second return optical waveguide 18 return light from the output side end face of the multimode interference unit 13 or from the first output waveguide 14 and the second output waveguide 15. It is arranged at a position where it combines with the return light. The first return optical waveguide 17 and the second return optical waveguide 18 guide the return light from the multimode interference unit 13 to the outside.

(Effect of the third embodiment)
According to the third embodiment, multiple reflections of return light in the multimode interference unit can be suppressed. Thereby, in the optical branching device having the multimode interference unit, it is possible to reduce the influence of the return light on the device characteristics.

  The reason is that the return light is transmitted through the multimode waveguide 27 by the first return optical waveguide 17 and the second return optical waveguide 18 connected to the input type end face of the multimode waveguide 27. This is because it is guided from the inside of 26. Further, the return light coupled to the first return optical waveguide 17 and the second return optical waveguide 18 is removed.

(Fourth embodiment)
An optical waveguide device according to a fourth embodiment will be described with reference to the drawings. An example of the optical waveguide device of the fourth embodiment is an optical branching device having a multimode interference unit. The fourth embodiment is an example of an optical branching device having a 2 × 2 multimode interference unit.

  FIG. 7 is a diagram illustrating a configuration example of the optical branching device according to the fourth embodiment. As shown in FIG. 7, the optical branching device has a multimode interference unit 28. The multimode interference unit 28 is connected to the input waveguide 11, the first output waveguide 14, and the second output waveguide 15.

  The input light 12 guided through the input waveguide 11 is branched into two by the multimode interference unit 28, and is coupled to the first output waveguide 14 and the second output waveguide 15, respectively, and is output as the output light 16. .

  Further, the first return optical waveguide 17 is connected to the input side end face of the multimode interference unit 28. Specifically, the first return optical waveguide 17 is connected to the reflection region on the input side end face of the multimode interference unit 28. The reflection region on the input side end surface is, for example, an end surface other than the end surface to which the input waveguide 11 is connected, among the input side end surfaces.

  More preferably, the first return optical waveguide 17 is coupled to the return light from the output side end face of the multimode interference unit 28 or the return light from the first output waveguide 14 and the second output waveguide 15. Be placed. Return light is guided from the inside of the multimode interference unit 28 to the outside by the first return optical waveguide 17.

  The return light 19 coupled to the first return optical waveguide 17 propagates in the opposite direction to the input light 12 propagating through the input waveguide 11. Further, the first return optical waveguide 17 has a removal mechanism for removing the return light 19.

  The fourth embodiment has been described as an example of an optical branching device having a 2 × 2 multimode interference unit. However, the first return optical waveguide 17 has a first output waveguide as long as it is in a position where it is coupled with the return light. It may not be arranged coaxially with the waveguide 14.

  The tapered first return optical waveguide 24 described in the second embodiment can also be applied to the first return optical waveguide 17 of the fourth embodiment.

(Effect of the fourth embodiment)
According to the fourth embodiment, multiple reflection of return light in the multimode interference unit can be suppressed. Thereby, in the optical branching device having the multimode interference unit, it is possible to reduce the influence of the return light on the device characteristics.

  This is because the return light is guided from the inside of the multimode interference unit 28 to the outside by the first return optical waveguide 17 connected to the multimode interference unit 28. Further, the return light coupled to the first return optical waveguide 17 is removed.

(Fifth embodiment)
Next, an optical circuit according to a fifth embodiment will be described with reference to the drawings. The optical circuit of the fifth embodiment is an example in which the optical branching device of the first embodiment and the fifth embodiment is applied to a coherent mixer. A coherent mixer is an optical circuit for converting the phase state of signal light into a light intensity signal.

  FIG. 8 is a diagram illustrating a configuration example of the coherent mixer according to the fifth embodiment. In the hybrid mixer of the fifth embodiment shown in FIG. 8, the signal light propagates through the input waveguide 51 and is input to the 3 × 2 MMI 53, and the 3 × 2 MMI 53 converts the input signal light into a branching ratio of 1: 1. Branch at. The phase difference of the signal light branched by the 3 × 2 MMI 53 is equiphase. The branched signal light is propagated through the distribution optical waveguides 55 and 56. The 3 × 2 MMI 53 is also called a first branch element.

  The local light propagates through the input waveguide 52 and is input to the 2 × 2 MMI 54. The 2 × 2 MMI 54 branches the input local light at a branching ratio of 1: 1. The phase difference of the local light branched by the 2 × 2 MMI 54 is 90 degrees. The branched local light is propagated in the distribution optical waveguides 57 and 58. The 2 × 2 MMI 54 is also called a second branch element.

  Whether the output light output from the MMI has the same phase or a 90 ° phase shift can be set using the cross-sectional position (presence of offset) of the input waveguide and the output waveguide and the MMI propagation length.

  The 2 × 2 MMI 59 causes the signal light that has propagated through the distribution optical waveguide 55 to interfere with the local light that has propagated through the distribution optical waveguide 57. The 2 × 2 MMI 60 causes the signal light that has propagated through the distribution optical waveguide 56 to interfere with the local light that has propagated through the distribution optical waveguide 58. The 2 × 2 MMI 59 is also called a first multiplexing element. The 2 × 2 MMI 60 is also called a second multiplexing element.

  The hybrid mixer 10 can form a silicon optical waveguide using, for example, silicon or silicon dioxide.

  In the coherent mixer of the fifth embodiment, a return optical waveguide 61 and a return optical waveguide 62 for removing the return light are connected to the 3 × 2 MMI 53 that branches the signal light, and the return light is returned to the 2 × 2 MMI that branches the local light. A return optical waveguide 63 for removing is connected.

  By removing the return light coupled to the return optical waveguides 61 and 62 of the 3 × 2 MMI 53, the influence on the device characteristics of the 3 × 2 MMI 53 can be reduced. Specifically, by suppressing multiple reflections due to return light in the MMI 53, it is possible to prevent loss ripple from occurring in the output signal light.

  Further, by removing the return light coupled to the return optical waveguide 63 of the 2 × 2 MMI 54, the influence on the device characteristics of the 2 × 2 MMI can be reduced. Specifically, by suppressing the multiple reflection due to the return light in the MMI 54, it is possible to prevent the phase change of the local light emission due to the return light.

(Effect of 5th Embodiment)
According to the coherent mixer of the fifth embodiment, the influence of the transmission characteristics of the coherent mixer can be reduced by suppressing the multiple reflection caused by the return light in the 3 × 2 MMI 53 and the 2 × 2 MMI 54.

(Optical waveguide structure)
FIG. 9 is a cross-sectional view showing the structure of an optical waveguide applicable to the optical branching devices of the first to fifth embodiments. FIGS. 9A to 9D are cross-sectional views in a plane perpendicular to the light guiding direction of the light in the optical waveguide.

  FIGS. 9A and 9B are cross-sectional views showing the structure of a rib-type optical waveguide. Specifically, it is an SOI (Silicon On Insulator) rib type optical waveguide in which a silicon oxide layer 32 and a silicon layer 33 are formed on a silicon substrate 31.

  9C and 9D are cross-sectional views showing the structure of the ridge type optical waveguide. Specifically, a ridge type optical waveguide in which an InP lower clad layer 35, an InGaAsP core layer 36, and an InP upper clad layer 37 are formed on an InP substrate 34.

  FIGS. 9A and 9C show an optical waveguide structure in the input waveguide 11 of the first embodiment shown in FIG. 1, for example. FIGS. 9B and 9D show, for example, an optical waveguide structure in the multimode interference unit 13 of the first embodiment shown in FIG.

  While the present invention has been described with reference to the embodiments, the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

DESCRIPTION OF SYMBOLS 11 Input waveguide 12 Input light 13 Multimode interference part 14 1st output waveguide 15 2nd output waveguide 16 Output light 17 1st return optical waveguide 18 2nd return optical waveguide 19 Return light 20 Upper clad 21 Chip end surface 22 None Reflective coating 23 Light absorption region 24 First return optical waveguide 25 Second return optical waveguide 26 Multimode interference portion 27 Multimode waveguide 28 Multimode interference portion 31 Silicon substrate 32 Silicon oxide layer 33 Silicon layer 34 InP substrate 35 InP lower clad Layer 36 InGaAsP core layer 37 InP upper clad layer 41 return light 42 return light 43 return light 44 reflection region 45 reflection region 51 input waveguide 52 input waveguide 53 3 × 2 MMI
54 2 × 2 MMI
55 Distribution optical waveguide 56 Distribution optical waveguide 57 Distribution optical waveguide 58 Distribution optical waveguide 59 2 × 2 MMI
60 2 × 2 MMI
61 Return optical waveguide 62 Return optical waveguide 63 Return optical waveguide

Claims (9)

A multimode interference unit;
An input waveguide for propagating input light to the multimode interference unit;
An output waveguide for propagating output light from the multimode interference unit;
A return optical waveguide that guides return light propagating through the multimode interference unit from the output waveguide side to the input waveguide side from the inside of the multimode interference unit;
Optical waveguide device. The return optical waveguide is connected to the multimode interference unit at an end face of the multimode interference unit on the side to which the input waveguide is connected, except at a connection point between the input waveguide and the multimode interference unit. ,
The optical waveguide device according to claim 1. The return light includes the output waveguide, the multimode interference unit, and the input light input to the multimode interference unit, of the end surface of the multimode interference unit on the side to which the output waveguide is connected. It is the light reflected from other than the connection point of
The optical waveguide device according to claim 1. The return light is light that is reflected from the connection portion between the output waveguide and the optical element, and the output light propagating through the output waveguide.
The optical waveguide device according to claim 1. The return optical waveguide includes a removal mechanism for removing the return light propagating through the return optical waveguide.
The optical waveguide device according to any one of claims 1 to 4. The removal mechanism is a light emission region provided in the return optical waveguide, a non-reflective coating provided on an end surface of the return optical waveguide, or a light absorption region provided in the return optical waveguide.
The optical waveguide device according to claim 5. A multimode waveguide that is connected between the input waveguide and the multimode interference section, and has a shape that is wider toward the portion closer to the multimode interference section;
The return optical waveguide is connected to the multimode waveguide at an end face of the multimode waveguide on the side to which the input waveguide is connected, except at a connection point between the input waveguide and the multimode waveguide. ,
The optical waveguide device according to any one of claims 1 to 6. The return optical waveguide has a tapered shape that is wide on the multimode interference part side and narrows toward the propagation direction of the return light,
The optical waveguide device according to claim 1. A first branch element for branching the signal light and outputting two lights having the same phase;
A second branch element for branching the local light and outputting two lights that are phase-shifted;
A first combining element that combines one light from the first branch element and one light from the second branch element;
An optical circuit including a second multiplexing element that multiplexes other light from the first branch element and other light from the second branch element,
The optical circuit in which the first branch element or the second branch element is the optical waveguide device according to any one of claims 1 to 8.

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