Multi-mode interference type multiplexer/demultiplexer and optical element using the same

文档序号：1676801 发布日期：2019-12-31 浏览：32次中文

阅读说明：本技术 多模干涉型合波分波器以及使用其的光学元件 (Multi-mode interference type multiplexer/demultiplexer and optical element using the same ) 是由高林正和西川智志秋山浩一于 2017-12-04 设计创作，主要内容包括：将光引导到单模波导,并且抑制多模干涉型合波分波器中的反射返回光。具备：多模波导(1),与第1端部连接的第1单模波导(2a),与第1单模波导相向的第2单模波导(2c),与第2端部连接的第3单模波导(2b),与第3单模波导相向的反射面(4),以及与侧端部连接的第4单模波导(5)。从第2或者第3单模波导入射的光在反射面反射,在第4单模波导的侧端部处的第1连接部(31)处成像。(Light is guided to a single-mode waveguide, and reflected return light in a multi-mode interference type multiplexer/demultiplexer is suppressed. The disclosed device is provided with: the multimode waveguide (1), a 1 st single-mode waveguide (2a) connected to the 1 st end portion, a 2 nd single-mode waveguide (2c) facing the 1 st single-mode waveguide, a 3 rd single-mode waveguide (2b) connected to the 2 nd end portion, a reflecting surface (4) facing the 3 rd single-mode waveguide, and a 4 th single-mode waveguide (5) connected to the side end portion. Light incident from the 2 nd or 3 rd single mode waveguide is reflected on the reflecting surface and forms an image at the 1 st connecting part (31) at the side end part of the 4 th single mode waveguide.)

1. A multi-mode interference type multiplexer/demultiplexer includes:

a multimode waveguide (1, 1c) having a 1 st end (1a), a 2 nd end (1b) which is an end opposite to the 1 st end (1a), and a 1 st end (10a) and a 2 nd end (10b) which face each other;

a 1 st single mode waveguide (2a, 102a) connected to the 1 st end (1a) of the multimode waveguide (1, 1 c);

a 2 nd single mode waveguide (2c) connected to a position facing the 1 st single mode waveguide (2a, 102a) at the 2 nd end (1b) of the multimode waveguide (1, 1 c);

a 3 rd single mode waveguide (2b, 300 to 303) connected to a position closer to the 1 st side end portion (10a) than a position at which the 2 nd single mode waveguide (2c) is connected, at the 2 nd end portion (1b) of the multimode waveguide (1, 1 c);

a reflecting surface (4, 4a, 4b, 4c, 4d) disposed at a position facing the 3 rd single-mode waveguide (2b, 300 to 303) in the multi-mode waveguide (1, 1 c); and

a 4 th single-mode waveguide (5, 5a, 5b, 5c, 5d, 105, 400 to 403) connected to the 1 st side end (10a) or the 2 nd side end (10b),

the direction connecting the 1 st end (1a) and the 2 nd end (1b) is set as the 1 st direction,

a direction intersecting the 1 st direction is set as a 2 nd direction,

the 1 st end (10a) and the 2 nd end (10b) are opposed to each other in the 2 nd direction,

light incident from the 2 nd single-mode waveguide (2c) or the 3 rd single-mode waveguide (2b, 300 to 303) is reflected at the reflection surface (4, 4a, 4b, 4c, 4d) and imaged at the 1 st connection portion (31, 31a, 31b, 31c, 32) of the 4 th single-mode waveguide (5, 5a, 5b, 5c, 5d, 105, 400 to 403),

the 1 st connection portion (31, 31a, 31b, 31c, 32) is a connection portion at the 1 st side end portion (10a) or a connection portion at the 2 nd side end portion (10 b).

2. The multi-mode interferometric wave multiplexer/demultiplexer of claim 1 wherein,

the 4 th single-mode waveguide (5) is connected to the 2 nd side end (10b),

the reflecting surface (4) is an inclined surface which approaches the 2 nd end (1b) as approaching the 1 st end (10 a).

3. The multi-mode interferometric wave multiplexer/demultiplexer of claim 1 wherein,

the 4 th single-mode waveguide (5a) is connected to the 1 st side end (10a),

the reflecting surface (4a) is an inclined surface which is separated from the 2 nd end (1b) as approaching the 1 st end (10 a).

4. The multi-mode interference type multiplexer/demultiplexer of any one of claims 1 to 3 wherein,

when a point at which light incident from the 2 nd single-mode waveguide (2c) or the 3 rd single-mode waveguide (2b, 300, 3001, 302, 303) is reflected on the reflection surface (4, 4a, 4b, 4c) is set as a reflection point (Z), a distance between the reflection point (Z) and the 1 st end portion (1a) in the direction along the 1 st direction is set as a 1 st distance (X), and a distance between the reflection point (Z) and the 1 st connection portion (31, 31a, 31b, 31c, 32) is set as a 2 nd distance (Y),

the 1 st distance (X) and the 2 nd distance (Y) are equal.

5. The multi-mode interference type multiplexer/demultiplexer of any one of claims 1 to 3 wherein,

when a point at which light incident from the 2 nd single-mode waveguide (2c) or the 3 rd single-mode waveguide (2b, 300, 3001, 302, 303) is reflected on the reflection surface (4d) is set as a reflection point (Z), a distance between the reflection point (Z) and the 1 st end portion (1a) in the direction along the 1 st direction is set as a 1 st distance (X), and a distance between the reflection point (Z) and the 1 st connection portion (31c) is set as a 2 nd distance (Y),

the 2 nd distance (Y) is greater than the 1 st distance (X).

6. The multi-mode interference type multiplexer/demultiplexer of claim 4 or 5 wherein,

passing through the reflection point (Z) along an axis (H) of the 4 th single mode waveguide (5, 5a, 5b, 5c, 5d, 105, 400 to 403) in its length direction.

7. The multi-mode interference type multiplexer/demultiplexer of any one of claims 1 to 6 wherein,

at least 1 of the 1 st single-mode waveguide (102a) and the 4 th single-mode waveguide (105) has a tapered shape.

8. The multi-mode interference type multiplexer/demultiplexer of any one of claims 1 to 6 wherein,

the optical waveguide device further comprises a 5 th single-mode waveguide (6), wherein the 5 th single-mode waveguide (6) is connected to an end portion of the 4 th single-mode waveguide (5b) on the side opposite to the 1 st connecting portion (31), and has an absorption layer (54) that absorbs incident light.

9. The multi-mode interferometric wave multiplexer/demultiplexer of claim 8 wherein,

an angle of a connection surface (101) between the 4 th single-mode waveguide (5b) and the 5 th single-mode waveguide (6) is a Brewster angle.

10. A multi-mode interference type multiplexer/demultiplexer according to any one of claims 1 to 9, comprising:

a plurality of said 3 rd single mode waveguides (300, 301, 302, 303); and

a plurality of the 4 th single-mode waveguides (400, 401, 402, 403) corresponding to the respective 3 rd single-mode waveguides (300, 301, 302, 303),

when the path lengths of light which is vertically incident on the multimode waveguide (1C) from each of the 3 rd single-mode waveguides (300, 301, 302, 303) and is reflected by the reflecting surfaces (4, 4a, 4B, 4C, 4D) and reaches the corresponding 4 th single-mode waveguide (400, 401, 402, 403) are respectively set as a length a, a length B, a length C and a length D, the length L of the multimode waveguide (1C) satisfies the requirement

[ formula 1]

The reflecting surfaces (4, 4a, 4B, 4C, 4D) and the 4 th single-mode waveguide (400, 401, 402, 403) are disposed at positions where L is a length a, and C is a length D.

11. An optical element comprising at least 1 multi-mode interference type multiplexer/demultiplexer according to any one of claims 1 to 10.

Technical Field

The technology disclosed in the present specification relates to suppression of reflected return light in a multi-mode interference type multiplexer/demultiplexer.

Background

Conventionally, a multi-mode interference type multiplexer/demultiplexer has been applied as a multiplexer/demultiplexer in an optical integrated circuit, and it is effective to reduce reflected return light in the multi-mode interference type multiplexer/demultiplexer in order to improve characteristics of elements such as an extinction ratio of an MZ (mach-zehnder) modulator.

For example, patent document 1 discloses a configuration in which an inclined surface is provided in a multi-mode interference type multiplexer/demultiplexer, and light that may be reflected return light is guided to a multi-mode waveguide.

Disclosure of Invention

In patent document 1, a position where the light that is reflected to the return light can form an image is an inclined surface. For this reason, the reflected light expanded from the imaging position cannot be guided to the single-mode waveguide but is guided to the multi-mode waveguide.

However, in the multi-mode waveguide, when a curved waveguide is formed, light is easily emitted, and handling of incident light becomes difficult. On the other hand, if light that may become reflected return light can be guided to a single-mode waveguide, it can be handled by a curved waveguide, so layout in an integrated circuit becomes easy.

The technique disclosed in the present specification is intended to solve the above-described problems, and an object thereof is to provide a technique for guiding light that may become reflected return light to a single-mode waveguide and suppressing the reflected return light in a multi-mode interference type multiplexer/demultiplexer.

The 1 st aspect of the technology disclosed in the present specification includes: a multimode waveguide having a 1 st end, a 2 nd end which is an end opposite to the 1 st end, and a 1 st end and a 2 nd end which face each other; a 1 st single mode waveguide connected to the 1 st end of the multimode waveguide; a 2 nd single mode waveguide connected to a position facing the 1 st single mode waveguide at the 2 nd end of the multimode waveguide; a 3 rd single mode waveguide connected to a position closer to the 1 st side end than a position at which the 2 nd single mode waveguide is connected, at the 2 nd end of the multimode waveguide; a reflecting surface arranged at a position facing the 3 rd single mode waveguide in the multi-mode waveguide; and a 4 th single-mode waveguide connected to the 1 st end portion or the 2 nd end portion, a direction connecting the 1 st end portion and the 2 nd end portion being a 1 st direction, a direction intersecting the 1 st direction being a 2 nd direction, the 1 st end portion and the 2 nd end portion facing each other in the 2 nd direction, light incident from the 2 nd single-mode waveguide or the 3 rd single-mode waveguide being reflected by the reflection surface, and an image being formed at a 1 st connection portion of the 4 th single-mode waveguide, the 1 st connection portion being a connection portion of the 1 st end portion or a connection portion of the 2 nd end portion.

The 2 nd aspect of the technology disclosed in the present specification includes at least 1 of the above-described multimode interference type multiplexer/demultiplexer.

In particular, according to claim 2, at least 1 of the above-described multi-mode interference type multiplexer/demultiplexer is provided. With such a configuration, the reflected return light can be suppressed in the optical element such as a mach-zehnder modulator or a 2-wavelength integrated modulator.

Objects, features, aspects and advantages related to the technology disclosed in the present specification will become more apparent from the detailed description and the accompanying drawings shown below.

Drawings

Fig. 1 is a plan view schematically illustrating the structure of a multimode interference type multiplexer/demultiplexer according to an embodiment.

Fig. 2 is a cross-sectional view at section a-a' of the single mode waveguide of fig. 1.

Fig. 3 is a plan view showing the structure of the multi-mode interference type multiplexer/demultiplexer illustrated in fig. 1 in detail.

Fig. 4 is a diagram illustrating results obtained by simulating propagation of light incident from a single-mode waveguide through a multi-mode waveguide in the case of a normal 2 × 2 multi-mode interference type multiplexer/demultiplexer.

Fig. 5 is a diagram illustrating a result obtained by simulating propagation of light incident from a single-mode waveguide in a multi-mode waveguide in the case of the multi-mode interference type multiplexer/demultiplexer according to the embodiment.

Fig. 6 is a plan view schematically illustrating a configuration of a modification of the multimode interference type multiplexer/demultiplexer according to the embodiment.

Fig. 7 is a plan view schematically illustrating the structure of the multi-mode interference type multiplexer/demultiplexer according to the embodiment.

Fig. 8 is a cross-sectional view illustrating a B-B' section of the single mode waveguide for absorption in fig. 7.

Fig. 9 is a plan view schematically illustrating the structure of an MZ modulator according to an embodiment.

Fig. 10 is a cross-sectional view illustrating a C-C' cross section of the arm portion at a position where the modulation electrode is formed in fig. 9.

Fig. 11 is a plan view schematically illustrating the structure of the 2-wavelength integrated modulator according to the embodiment.

Fig. 12 is a sectional view illustrating a section D-D' in fig. 11.

Fig. 13 is a plan view showing a modification of the structure of the multi-mode interference type multiplexer/demultiplexer.

Fig. 14 is a plan view showing a modification of the structure of the multi-mode interference type multiplexer/demultiplexer.

Fig. 15 is a plan view schematically illustrating the structure of the multi-mode interference type multiplexer/demultiplexer according to the embodiment.

Fig. 16 is a plan view schematically illustrating the structure of the multi-mode interference type multiplexer/demultiplexer according to the embodiment.

Fig. 17 is an enlarged view of a part of the simulation result illustrated in fig. 5.

Fig. 18 is a plan view schematically illustrating the structure of the multi-mode interference type multiplexer/demultiplexer according to the embodiment.

Fig. 19 is a plan view schematically illustrating the structure of a typical 4 × 4 multimode interference type multiplexer/demultiplexer.

Fig. 20 is a diagram illustrating a 4-wavelength integrated modulator to which the multi-mode interference type multiplexer/demultiplexer of fig. 18 is applied.

(symbol description)

1. 1c, 1 d: a multimode waveguide; 1a, 1 b: an end portion; 2a, 2b, 2c, 3, 102 a: a single mode waveguide; 4. 4a, 4b, 4c, 4 d: a reflective surface; 5. 5a, 5b, 5c, 5d, 105: a single mode waveguide for unwanted light; 6: a single mode waveguide for absorption; 10a, 10 b: a lateral end; 20a, 20b, 20c, 20 d: a multi-mode interference type wave-combining and wave-splitting filter; 30. 31, 31a, 31b, 31c, 32, 33c, 34, 35: a connecting portion; 41. 131: a modulation electrode; 41a, 41 b: a modulator; 42a, 42 b: an arm portion; 50: a substrate; 51: a lower cladding layer; 52: a wave-guiding layer; 53: an upper cladding layer; 54: an absorbing layer; 55: a multiple quantum well layer; 56: a diffraction lattice; 60: an incident end; 61. 71: an exit end; 100: no light is required; 101: a connecting surface; 131a, 131b, 231a, 231b, 231c, 231 d: an EA modulator; 132: an electrode for LD; 132a, 132 b: LD; 200. 201: an arrow; 300. 301, 302, 303: an input port; 400. 401, 402, 403: an output port; 500. 501, 502, 503: the path is long; E. g, H: an axis; f: a centerline; l: the multimode waveguide is long; w: the multimode waveguide is wide; x, Y: a distance; z: and (4) an intersection point.

Detailed Description

Hereinafter, embodiments will be described with reference to the attached drawings.

The drawings are schematically illustrated, and the configuration is omitted or simplified as appropriate for convenience of explanation. The mutual relationship between the size and the position of the structures and the like shown in the different drawings is not necessarily described correctly, and may be appropriately changed.

In the following description, the same components are denoted by the same reference numerals, and the same names and functions are also assumed. Therefore, detailed descriptions thereof may be omitted to avoid redundancy.

In the following description, even if terms indicating specific positions and directions such as "up", "down", "left", "right", "side", "bottom", "front", and "back" are used in some cases, these terms are terms that are used inexpensively to facilitate understanding of the contents of the embodiments and are terms that are not related to the directions in actual implementation.

In the following description, even if ordinal numbers such as "1 st" and "2 nd" are used, these terms are used inexpensively to facilitate understanding of the contents of the embodiments, and are not limited to terms such as the order in which the ordinal numbers can be generated.

< embodiment 1 >

Hereinafter, a multi-mode interference type multiplexer/demultiplexer according to the present embodiment will be described.

< Structure of multi-mode interference type multiplexer/demultiplexer >

Fig. 1 is a plan view schematically illustrating the structure of a multimode interference type multiplexer/demultiplexer according to the present embodiment. As illustrated in fig. 1, the multi-mode interference type multiplexer/demultiplexer according to the present embodiment includes a multi-mode waveguide 1, a single-mode waveguide 2a, a single-mode waveguide 2b, a single-mode waveguide 2c, and a single-mode waveguide 5 for unnecessary light. Further, a reflecting surface 4 is disposed at a position facing the single mode waveguide 2b of the multi-mode waveguide 1.

The multi-mode interference type multiplexer/demultiplexer according to the present embodiment demultiplexes light incident from the single-mode waveguide 2a and outputs the demultiplexed light to the single-mode waveguide 2b and the single-mode waveguide 2 c. The multi-mode interference type multiplexer/demultiplexer according to the present embodiment multiplexes the light incident from the single-mode waveguide 2b and the single-mode waveguide 2c and outputs the multiplexed light to the single-mode waveguide 2 a.

The single-mode waveguide 2a, the single-mode waveguide 2b, and the single-mode waveguide 2c, into or from which light enters or exits, are connected to the multimode waveguide 1 at connection portions, respectively. The connection portion is a boundary portion connecting the single-mode waveguide and the multi-mode waveguide, and is a portion connecting the two waveguides so as to allow light to enter or exit. The single-mode waveguide 2a is connected to an end 1a in the longitudinal direction of the multimode waveguide 1, and the single-mode waveguide 2b and the single-mode waveguide 2c are connected to the other end in the longitudinal direction of the multimode waveguide 1, that is, an end 1b on the opposite side of the end 1a to which the single-mode waveguide 2a is connected. The single-mode waveguide 2c is connected to a position opposed to the single-mode waveguide 2 a. The single-mode waveguide 2b is connected to the end 1b of the multi-mode waveguide 1 at a position closer to the side end 10a than the position where the single-mode waveguide 2c is connected. The reflecting surface 4 is located closer to the positive X-axis direction than the end 1 a.

A single-mode waveguide 5 for unnecessary light is connected to the side end portion 10b of the multimode waveguide 1. The single-mode waveguide 5 for the unwanted light is configured to guide the unwanted light reflected by the reflecting surface 4, that is, the unwanted light which may be reflected return light in the multi-mode interference type multiplexer/demultiplexer, and to remove the unwanted light.

The basic design of the multi-mode interference type multiplexer/demultiplexer according to the present embodiment is a 2 × 2 multi-mode interference type multiplexer/demultiplexer, but the unused single-mode waveguide 3 connected to the connection portion 30 is removed, and instead, the reflection surface 4 and the single-mode waveguide 5 for unnecessary light connected to the connection portion 31 are arranged.

When 2 normal 2 × 2 multi-mode interference type multiplexer/demultiplexer devices are connected to form an MZ interferometer, 1 unused single-mode waveguide is disposed on each of the incident side and the emission side. Therefore, such a single-mode waveguide can be connected to the side end portion 10b of the multimode waveguide 1 and used as the single-mode waveguide 5 for unnecessary light for removing the unnecessary light 100.

When light enters the single-mode waveguide 2a, the light is finally branched into 2 by the multi-mode waveguide 1 and is emitted from the single-mode waveguide 2b and the single-mode waveguide 2c, respectively.

When light enters from the single-mode waveguide 2b or the single-mode waveguide 2c, the light is finally branched into 2 by the multi-mode waveguide 1, one of the light is emitted from the single-mode waveguide 2a, and the other light is reflected by the reflecting surface 4 and coupled to the single-mode waveguide 5 for unwanted light.

Fig. 2 is a cross-sectional view at a section a-a' of the single-mode waveguide 2a in fig. 1. As illustrated in fig. 2, the single-mode waveguide 2a includes a substrate 50, a lower cladding layer 51 formed on the upper surface of the substrate 50, a waveguide layer 52 formed on the upper surface of the lower cladding layer 51, and an upper cladding layer 53 formed on the upper surface of the waveguide layer 52. The structure of the multimode waveguide 1 is the same as that of the single-mode waveguide 2a illustrated in fig. 2 in that the width in the Y-axis direction is wide.

The substrate 50 is, for example, an InP substrate. In this case, the lower clad layer 51 and the upper clad layer 53 are InP layers. Further, a bulk (bulk) layer or a multiple quantum well layer (MQW layer) made of an InGaAsP material can be selected and stacked as the waveguide layer 52.

The waveguide layer 52 can be formed into an arbitrary shape by photolithography, and in the present embodiment, a semiconductor waveguide pattern as illustrated in fig. 1 is formed.

Fig. 3 is a plan view showing the structure of the multi-mode interference type multiplexer/demultiplexer illustrated in fig. 1 in detail. The basic design of the multi-mode interference type multiplexer/demultiplexer according to the present embodiment is a 2 × 2 multi-mode interference type multiplexer/demultiplexer, and the length of the multi-mode waveguide corresponding to the width of the multi-mode waveguide 1 and the interval between the adjacent 2 single-mode waveguides are based on the basic design of the 2 × 2 multi-mode interference type multiplexer/demultiplexer.

The multimode waveguide length L is expressed by the following equation with respect to the multimode waveguide width W.

[ formula 1]

L＝2×neff×W²/(3×λ)

Here, n is_effDenotes the equivalent refractive index, and λ denotes the wavelength. The single-mode waveguides connected to the multi-mode waveguide 1 are arranged at ± W/6 from a center line F along the longitudinal direction of the multi-mode waveguide 1. Here, an axis along the longitudinal direction of the single-mode waveguide 2b is an axis E, and an axis along the longitudinal direction of the single-mode waveguide 2c is an axis G.

That is, in the multimode waveguide 1, the multimode waveguide width W and the multimode waveguide length L are set so that the light incident from the single-mode waveguide 2a forms an image at the connection portion 34 of the single-mode waveguide 2b or the connection portion 35 of the single-mode waveguide 2c, and conversely, the light incident from the single-mode waveguide 2b or the single-mode waveguide 2c forms an image at the connection portion 33 of the single-mode waveguide 2 a.

As a specific example of the multimode waveguide width W and the multimode waveguide length L, for example, it is assumed that the multimode waveguide length L is 204 μm in the case where the multimode waveguide width W is 12 μm or 460 μm in the case where the multimode waveguide width W is 18 μm (n in both cases)_effBoth 3.3 and λ both 1.55 μm).

In fig. 3, a center line connecting a single-mode waveguide 3, which is originally provided in a 2 × 2 multimode interference type multiplexer/demultiplexer but is removed because it is not used, and a single-mode waveguide 2b connected to face the single-mode waveguide 3 is an axis E. The intersection point of the axis E and the reflecting surface 4 is defined as an intersection point Z. The intersection point Z is a representative reflection point at which light incident from the single-mode waveguide 2c or the single-mode waveguide 2b is reflected by the reflection surface 4.

The distance between the intersection point Z and the connection portion 31 connecting the single-mode waveguide 5 for unwanted light is defined as a distance Y. In fig. 3, the distance Y is a distance along the Y-axis direction, and is a distance along the axis H of the single-mode waveguide 5 for the unwanted light, which is arranged such that the axis H, which is an axis along the longitudinal direction, passes through the intersection point Z.

The distance between the intersection point Z and the connection portion 30 connecting the single-mode waveguide 3 before removal to the multi-mode waveguide 1 is defined as a distance X. In fig. 3, the distance X is a distance along the X-axis direction.

In the multi-mode interference type multiplexer/demultiplexer according to the present embodiment, by arranging the reflecting surface 4 so that the distance X and the distance Y are equal to each other, the light to be imaged at the connecting portion 30 of the single-mode waveguide 3 is reflected by the reflecting surface 4, and then imaged at the connecting portion 31 at the side end portion 10b of the multi-mode waveguide 1. In fig. 3, the angle of the reflecting surface 4 with respect to the X-axis direction is set to 45 °.

Therefore, the light reflected on the reflecting surface 4 is imaged at the connecting portion 31 of the single-mode waveguide 5 for unwanted light arranged so that the axis H along the longitudinal direction passes through the intersection point Z. Then, the light is guided to the single-mode waveguide 5 for unnecessary light, and removed.

Here, the light incident from the single mode waveguide 2b and the single mode waveguide 2c travels in each direction by being reflected or interfered in the multi-mode waveguide 1, but is condensed at a position where the reflection surface 4 is arranged so as to form an image in the single mode waveguide 3. Therefore, in the reflection surface 4, the positions where the light incident from the single-mode waveguide 2b and the single-mode waveguide 2c is reflected are the intersection point Z and the periphery thereof.

Further, as described above, since the light incident from the single mode waveguide 2b and the single mode waveguide 2c is condensed by the reflection surface 4, the traveling direction of the light at the reflection surface 4 is substantially aligned with the X-axis negative direction, and when the angle of the reflection surface 4 with respect to the X-axis direction is 45 °, the traveling direction of the light reflected by the reflection surface 4 is also substantially aligned with the Y-axis negative direction.

< operation of the multi-mode interference type multiplexer/demultiplexer >

Light entering the multimode waveguide 1 from the single-mode waveguide propagates while being reflected or interfered at the side end portion of the multimode waveguide 1, and finally forms an image at 2 locations.

Fig. 4 is a diagram illustrating a result obtained by simulating propagation of light incident from the single-mode waveguide 2b in the multi-mode waveguide 1 in the case of a normal 2 × 2 multi-mode interference type multiplexer/demultiplexer.

As illustrated in fig. 4, in a typical 2 × 2 multi-mode interference type multiplexer/demultiplexer, that is, in a case where a single-mode waveguide 2a and a single-mode waveguide 3 are connected to an end 1a of a multi-mode waveguide 1, light incident from a single-mode waveguide 2b is formed into an image on the single-mode waveguide 2a and the single-mode waveguide 3, respectively. In addition, in the case of simulating the propagation of the light incident from the single-mode waveguide 2c in the multi-mode waveguide 1, the propagation is line-symmetric to the scheme illustrated in fig. 4, and the light incident from the single-mode waveguide 2c is imaged on the single-mode waveguide 2a and the single-mode waveguide 3, respectively.

Since light is imaged at 2 locations, the light is condensed toward the 2 points, but even if the light is reflected by the reflecting surface 4 on the way along which the light is condensed to change the direction of light transmission, the condensing angle does not change. Therefore, the reflected light is imaged at the same transmission distance as in the case where the reflecting surface 4 is not present.

Therefore, by arranging the reflecting surface 4 and the single-mode waveguide 5 for the unwanted light in such a manner that the distance X and the distance Y are equal as described above, the light incident from the single-mode waveguide 2b is imaged at the single-mode waveguide 5 for the unwanted light connected to the side end portion 10b of the multi-mode waveguide 1.

Further, since light imaged at this portion can be guided by the single-mode waveguide, it can be easily removed as unnecessary light.

Fig. 5 is a diagram illustrating a result obtained by simulating propagation of light incident from the single-mode waveguide 2b through the multi-mode waveguide 1 when the multi-mode interference type multiplexer/demultiplexer according to the present embodiment is used.

As illustrated in fig. 5, when the multi-mode interference type multiplexer/demultiplexer according to the present embodiment is used, one light, that is, light to be imaged in the single-mode waveguide 3 in fig. 4 is reflected by the reflecting surface 4, and is imaged as unwanted light in the single-mode waveguide 5 for unwanted light.

< modification of the multimode interference type multiplexer/demultiplexer >

In the structure illustrated in fig. 1, the single-mode waveguide 5 for the unnecessary light is connected to the side end portion 10b at a position distant from the connection portion 30 to which the single-mode waveguide 3 is connected, among the 2 side end portions of the multi-mode waveguide 1.

The reflecting surface 4 is formed obliquely so as to be closer to the end 1b of the multimode waveguide 1 as the reflecting surface 4 is farther from the center line F of the multimode waveguide 1.

On the other hand, as illustrated in fig. 6, the single-mode waveguide 5a for the unnecessary light may be connected to the connection portion 32 of the side end portion 10a located closer to the connection portion 30 to which the single-mode waveguide 3 is connected, among the 2 side end portions of the multi-mode waveguide 1.

In this case, the reflecting surface 4a is formed obliquely so as to be farther from the center line F of the multimode waveguide 1 and farther from the end 1b of the multimode waveguide 1. Here, fig. 6 is a plan view schematically illustrating the configuration of a modification of the multimode interference type multiplexer/demultiplexer according to the present embodiment.

Further, the tip of the single-mode waveguide 5a for the unwanted light or the tip of the single-mode waveguide 5 (see fig. 1) for the unwanted light may be inclined. When the tip of the single-mode waveguide for unwanted light has a slanted shape, the unwanted light may be emitted to the outside of the waveguide. Further, the unnecessary light may be emitted to the outside of the substrate 50 by disposing the end of the single-mode waveguide 5a for the unnecessary light or the end of the single-mode waveguide 5 (see fig. 1) for the unnecessary light so as to reach the end of the substrate 50.

According to the multi-mode interference type multiplexer/demultiplexer of the present embodiment, it is possible to guide unnecessary light, which is light that can be reflected return light, to a single-mode waveguide for the unnecessary light. Therefore, it is possible to handle by the curved waveguide, so layout in an integrated circuit or the like becomes easy.

In the case where it is assumed that unnecessary light is guided to the multimode waveguide 1, the waveguide width is large, so the occupied area is widened. Further, when the multi-mode waveguide 1 is formed in a curved shape, light is emitted to the outside of the multi-mode waveguide 1, and may become stray light affecting other elements. Therefore, a problem arises particularly in an integrated circuit in the case where unnecessary light is guided to the multimode waveguide 1.

According to the multi-mode interference type multiplexer/demultiplexer of the present embodiment, unnecessary light that may be reflected return light generated in the multi-mode interference type multiplexer/demultiplexer can be guided to the single-mode waveguide and removed.

< embodiment 2 >

The following describes a multi-mode interference type multiplexer/demultiplexer according to the present embodiment. In the following description, the same components as those described in the above-described embodiments are denoted by the same reference numerals, and detailed description thereof is omitted as appropriate.

< Structure of multi-mode interference type multiplexer/demultiplexer >

Fig. 7 is a plan view schematically illustrating the structure of the multi-mode interference type multiplexer/demultiplexer according to the present embodiment. The multi-mode interference type multiplexer/demultiplexer according to the present embodiment includes a multi-mode waveguide 1, a single-mode waveguide 2a, a single-mode waveguide 2b, a single-mode waveguide 2c, a single-mode waveguide 5b for unwanted light, and a single-mode waveguide 6 for absorption connected to a tip of the single-mode waveguide 5b for unwanted light. The reflecting surface 4 is disposed at a position facing the single-mode waveguide 2b of the multi-mode waveguide 1.

In the above structure, the light reflected at the reflecting surface 4 is imaged at the connecting portion 31 of the single-mode waveguide 5b for unwanted light, and further, is absorbed as unwanted light in the single-mode waveguide 6 for absorption connected at the end portion of the single-mode waveguide 5b for unwanted light on the side opposite to the connecting portion 31.

With this configuration, the possibility of stray light being generated in the module or the like by absorbing light is reduced as compared with the case where unnecessary light is emitted to the outside of the waveguide or the outside of the substrate 50. This structure is effective in a case where it is difficult to dispose the curved waveguide to the end of the substrate 50 or the like due to the relationship of the arrangement of elements in the integrated circuit.

Further, the connection surface 101 can be made reflection-free by inclining the angle of the connection surface 101 for the single-mode waveguide 5b for unnecessary light and the single-mode waveguide 6 for absorption to the brewster angle (for example, 45 °). In this case, it is also possible to completely absorb light guided to the single-mode waveguide 5b for unnecessary light in the single-mode waveguide 6 for absorption. When the angle of the connection surface 101 of the single-mode waveguide 5b for the unnecessary light and the single-mode waveguide 6 for absorption is not at the brewster angle, a slight reflected return light may be generated at the connection surface 101. On the other hand, in the case where the waveguide guiding the unnecessary light is a multimode waveguide, the light propagating in the multimode waveguide has a divergence angle, and even if the angle of the connection surface 101 of the multimode waveguide and the single-mode waveguide 6 for absorption is adjusted, it is difficult to effectively suppress reflection of the light at the connection surface 101.

Fig. 8 is a cross-sectional view illustrating a B-B' section of the single-mode waveguide 6 for absorption in fig. 7. As illustrated in fig. 8, the single-mode waveguide 6 for absorption includes a substrate 50, a lower cladding layer 51 formed on the upper surface of the substrate 50, an absorption layer 54 formed on the upper surface of the lower cladding layer 51, and an upper cladding layer 53 formed on the upper surface of the absorption layer 54.

The unnecessary light incident on the absorption single-mode waveguide 6 is absorbed and disappears while propagating through the absorption single-mode waveguide 6 having the absorption layer 54.

The substrate 50 is, for example, an InP substrate. In this case, the lower clad layer 51 and the upper clad layer 53 are InP layers.

As a manufacturing step, first, a lower clad layer 51 as an InP layer is laminated on the upper surface of a substrate 50 as an InP substrate. Then, a bulk layer made of, for example, an InGaAsP material can be selected and stacked as a waveguide layer on the upper surface of the lower clad layer 51.

The waveguide layer can be formed into an arbitrary shape by photolithography, and in the present embodiment, a waveguide pattern as illustrated in fig. 7 is formed in a region other than the single-mode waveguide 6 for absorption.

Next, for example, a multiple quantum well layer or the like made of an InGaAsP material is selected and stacked, and a single mode waveguide 6 for absorption illustrated in fig. 7 is patterned by photolithography.

After that, an upper cladding layer 53 as an InP layer is stacked, and patterned by photolithography. Further, the regions of the single-mode waveguide 2a, the single-mode waveguide 2b, the single-mode waveguide 2c, and the multimode waveguide 1 can form a cross section as illustrated in fig. 2, and the region of the single-mode waveguide 6 for absorption can form a cross section as illustrated in fig. 8. Through such steps, the multimode interference type multiplexer/demultiplexer according to the present embodiment can be manufactured.

Further, a structure is also conceivable in which a single-mode waveguide 5b for unnecessary light, which is connected to the single-mode waveguide 6 for absorption according to the present embodiment, is connected to the side end portion 10 a. In this case, as illustrated in fig. 6, for example, the reflecting surface is formed to be inclined so as to be farther from the center line F of the multimode waveguide 1 than the end 1b of the multimode waveguide 1.

< embodiment 3 >

A multi-mode interference type multiplexer/demultiplexer and an MZ modulator as an optical element using the same according to the present embodiment will be described. In the following description, the same components as those described in the above-described embodiments are denoted by the same reference numerals, and detailed description thereof is omitted as appropriate.

< Structure of MZ Modulator >

Fig. 9 is a plan view schematically illustrating the structure of an MZ modulator according to the present embodiment. Fig. 9 illustrates an MZ modulator including the multi-mode interference type multiplexer/demultiplexer illustrated in fig. 1 and having electrodes formed on 2 arm portions of the MZ interferometer.

As illustrated in fig. 9, the MZ modulator includes a multi-mode interference type multiplexer/demultiplexer 20a and a multi-mode interference type multiplexer/demultiplexer 20b which are respectively disposed on the upper surface of the substrate 50, and an arm 42a and an arm 42b which are connected in parallel to each other between the multi-mode interference type multiplexer/demultiplexer 20a and the multi-mode interference type multiplexer/demultiplexer 20 b.

The arm portion 42a is a portion connecting between the multimode interference type multiplexer/demultiplexer 20a and the multimode interference type multiplexer/demultiplexer 20b via the single mode waveguide 2b extending from each of the multimode interference type multiplexer/demultiplexer 20a and the multimode interference type multiplexer/demultiplexer 20b, and includes a modulator 41a partially formed in a path of the single mode waveguide 2 b.

The arm portion 42b is a portion connecting between the multimode interference type multiplexer/demultiplexer 20a and the multimode interference type multiplexer/demultiplexer 20b via the single mode waveguide 2c extending from each of the multimode interference type multiplexer/demultiplexer 20a and the multimode interference type multiplexer/demultiplexer 20b, and includes a modulator 41b partially formed in the path of the single mode waveguide 2 c.

In each of the multimode interference type multiplexer/demultiplexer 20a and the multimode interference type multiplexer/demultiplexer 20b, the waveguide layer in the single-mode waveguide 2b and the single-mode waveguide 2c is formed of a multiple quantum well layer (MQW layer). Since the phase of light in the arm 42a and the arm 42b changes by applying a voltage to the modulator 41a and the modulator 41b, intensity modulation or phase modulation can be performed.

Light enters from an entrance end 60 of the MZ modulator, and is branched into 2 pieces in the multi-mode interference type multiplexer/demultiplexer 20 a. Then, the light is modulated by the modulators 41a and 41b, and then multiplexed by the multi-mode interference type multiplexer/demultiplexer 20 b. Then, the light exits from the exit end 61 of the MZ modulator.

In the incident-side multi-mode interference type multiplexer/demultiplexer 20a and the outgoing-side multi-mode interference type multiplexer/demultiplexer 20b, light that should be coupled to the single-mode waveguide 3 (see fig. 1) arranged as a 2 × 2 multi-mode interference type multiplexer/demultiplexer is reflected by the reflection surface 4 (see fig. 1) and guided to the single-mode waveguide 5 for unwanted light. Then, the light is emitted as unwanted light to the outside of the substrate 50.

In order to prevent the light reflected by the reflection surface 4 (see fig. 1) from becoming stray light in the MZ modulator, a paint that absorbs light may be applied to the end of the substrate 50 to absorb the light. In addition, as in the case illustrated in embodiment 2, the single-mode waveguide 6 (see fig. 7) for absorption may be connected to the tip of the single-mode waveguide 5b (see fig. 7) for unwanted light, whereby unwanted light can be absorbed.

Fig. 10 is a cross-sectional view illustrating a C-C' cross section of the arm portion at a position where the modulation electrode is formed in fig. 9. As illustrated in fig. 10, the arm portion 42a includes a substrate 50, a lower cladding layer 51 formed on the upper surface of the substrate 50, a multiple quantum well layer 55 formed on the upper surface of the lower cladding layer 51, an upper cladding layer 53 formed on the upper surface of the multiple quantum well layer 55, and a modulation electrode 41 formed on the upper surface of the upper cladding layer 53.

The substrate 50 is, for example, an InP substrate. In this case, the lower clad layer 51 and the upper clad layer 53 are InP layers. The multiple quantum well layer 55 is made of an InGaAsP material. The modulation electrode 41 is an electrode for applying a modulation voltage.

As a manufacturing step, first, a lower clad layer 51 as an InP layer is laminated on the upper surface of a substrate 50 (an InP substrate). Then, a bulk layer made of, for example, an InGaAsP material can be selected and stacked as a waveguide layer on the upper surface of the lower clad layer 51.

The waveguide layer can be formed into an arbitrary shape by photolithography, and in the present embodiment, a waveguide pattern as illustrated in fig. 9 is formed in a region other than the modulators 41a and 41b, that is, in a region of the multi-mode interference type multiplexer/demultiplexer 20a and 20 b.

Next, a multiple quantum well layer or the like made of, for example, InGaAsP material is selected and stacked, and a multiple quantum well layer 55 is formed in regions of the modulator 41a and the modulator 41b illustrated in fig. 9 by photolithography.

Thereafter, an upper cladding layer 53 as an InP layer is stacked, and a single-mode waveguide and a multi-mode waveguide are formed by photolithography. Further, electrodes having cross sections as illustrated in fig. 10 may be formed in the regions of the modulators 41a and 41 b. Through such steps, the MZ modulator according to the present embodiment can be manufactured.

In fig. 10, the side surface of the waveguide is air, but SiO may be used to improve the long-term reliability₂Or organic material or the like to fill the side face of the waveguide.

According to the MZ modulator of the present embodiment, the reflected return light can be suppressed.

< embodiment 4 >

A multi-mode interference type multiplexer/demultiplexer and a 2-wavelength integrated modulator as an optical element using the same according to the present embodiment will be described. In the following description, the same components as those described in the above-described embodiments are denoted by the same reference numerals, and detailed description thereof is omitted as appropriate.

< Structure of 2 wavelength Integrated Modulator >

Fig. 11 is a plan view schematically illustrating the structure of the 2-wavelength integrated modulator according to the present embodiment. In order to cope with the increase in capacity of the optical communication system, a 2-wavelength integrated modulator can be used as a light source with a modulator capable of transmitting a wavelength-multiplexed signal.

As illustrated in fig. 11, the 2-wavelength integrated modulator includes: 2 LDs 132a and 132b having different oscillation wavelengths; 2 EA (electro-absorption) modulators 131a and 131b connected to each LD; and a multi-mode interference type multiplexer/demultiplexer 20c connected in parallel to the EA modulator 131a and the EA modulator 131b, respectively.

The lights emitted from the EA modulators 131a and 131b are multiplexed by the multi-mode interference type multiplexer/demultiplexer 20 c. Therefore, signal light having a wavelength of any one of LD132a and LD132b can be output from the same output port.

According to the 2-wavelength integrated modulator illustrated in fig. 11, it is possible to achieve a reduction in size of a module and simplification of an assembly process, compared to a configuration of a system in which 2 elements having different oscillation wavelengths are used and optical coupling output from these elements is performed using an optical system such as a prism.

When one of the LDs 132a and 132b is turned on or both of the LDs 132a and 132b are turned on, the continuous light emitted from the LD is modulated by the corresponding EA modulator. Further, the multi-mode interference type multiplexer/demultiplexer 20c is branched into 2 pieces.

At this time, light that is not emitted from the emission end 71 of the 2-wavelength integrated modulator, is reflected by the reflection surface 4 (see fig. 1), and is coupled to the single-mode waveguide 5 for unwanted light is removed as unwanted light.

When the reflected return light toward the LD occurs, good signal light cannot be obtained, but according to the 2-wavelength integrated modulator according to the present embodiment, good signal light can be obtained by suppressing the reflected return light.

Fig. 12 is a sectional view illustrating a section D-D' in fig. 11. As illustrated in fig. 12, the structure spanning LD132a and EA modulator 131a includes: a substrate 50; a lower cladding layer 51 formed on the upper surface of the substrate 50; a multiple quantum well layer 55 formed on the upper surface of the lower clad layer 51 at a position corresponding to the LD132a and the EA modulator 131 a; a waveguide layer 52 formed on the upper surface of the lower cladding layer 51 at a position corresponding to the single-mode waveguide 2 c; an upper cladding layer 53 formed on the upper surface of the multiple quantum well layer 55 and the upper surface of the waveguide layer 52; a diffraction grating 56 intermittently formed inside the upper cladding layer 53 for oscillating the LD132a at a certain wavelength; an LD electrode 132 formed at a position corresponding to the LD132a on the upper surface of the upper cladding layer 53, for passing a current through the LD132 a; and a modulation electrode 131 formed at a position corresponding to the EA modulator 131a on the upper surface of the upper cladding layer 53, for applying a modulation voltage.

A constant current flows through the LD132a via the LD electrode 132. Thereby, continuous light is emitted from the LD132 a. In addition, since ON/OFF control of the light output can be realized by controlling the voltage applied to the modulation electrode 131, an intensity modulation signal can be generated.

The substrate 50 is, for example, an InP substrate. In this case, the lower clad layer 51 and the upper clad layer 53 are InP layers. The multiple quantum well layer 55 and the waveguide layer 52 are made of InGaAsP material.

The waveguide layer can be formed into an arbitrary shape by photolithography, and in the present embodiment, a waveguide pattern as illustrated in fig. 11 is formed in a region other than the LD132a, the LD132b, the EA modulator 131a, and the EA modulator 131b, that is, in a region of the multi-mode interference type multiplexer/demultiplexer 20 c.

Next, multiple quantum well layers or the like made of, for example, InGaAsP materials are selected and stacked, and the multiple quantum well layers 55 are formed in regions of the LD132a, the LD132b, the EA modulator 131a, and the EA modulator 131b illustrated in fig. 11 by photolithography.

After that, an upper clad layer 53 as an InP layer is laminated. In the LD132a and LD132b regions, InGaAsP-based diffraction lattices are formed by photolithography and then embedded in the upper cladding layer 53 serving as an InP layer. Then, the LD electrode 132 and the modulation electrode 131 are formed on the upper surface of the upper cladding layer 53. Through such steps, the 2-wavelength integrated modulator according to the present embodiment can be manufactured.

According to the 2-wavelength integrated modulator according to the present embodiment, the reflected return light can be suppressed.

< embodiment 5 >

Fig. 15 is a plan view schematically illustrating the structure of the multi-mode interference type multiplexer/demultiplexer according to the present embodiment. The multi-mode interference type multiplexer/demultiplexer according to the present embodiment includes a multi-mode waveguide 1, a single-mode waveguide 102a, a single-mode waveguide 2b, a single-mode waveguide 2c, and a single-mode waveguide 105 for unnecessary light. The reflecting surface 4 is disposed at a position facing the single-mode waveguide 2b of the multi-mode waveguide 1.

In the above configuration, the light reflected on the reflecting surface 4 is imaged on the connecting portion 31c of the single-mode waveguide 105 for unwanted light, and is absorbed as unwanted light in the single-mode waveguide 6 for absorption connected to the end portion of the single-mode waveguide 105 for unwanted light on the side opposite to the connecting portion 31 c.

The single-mode waveguide 102a is connected to the multimode waveguide 1 at a connection portion 33 c. The single-mode waveguide 102a has a tapered shape expanding closer to the end connected to the multi-mode waveguide 1.

The single-mode waveguide 105 for unnecessary light is connected to the multimode waveguide 1 at the connection portion 31 c. The single mode waveguide 105 for unwanted light is a tapered shape that expands closer to the end connected to the multimode waveguide 1.

In a typical multimode interference type multiplexer/demultiplexer in which the single-mode waveguide 3 and the single-mode waveguide 2a are connected to the multimode waveguide 1, if the single-mode waveguide 3 and the single-mode waveguide 2a have a tapered structure having the single-mode waveguide 102a and the single-mode waveguide 105 for unnecessary light as described above, the gap between the single-mode waveguide 3 and the single-mode waveguide 2a is narrowed, and these cannot be processed satisfactorily.

When the machining is not performed satisfactorily, a problem occurs in that light incident from the single-mode waveguide 2a does not propagate as in a simulation, and light is not coupled to the single-mode waveguide 2b and the single-mode waveguide 2 c. In addition, the yield is deteriorated because variations occur in the processing of each manufactured element.

In contrast, according to the multi-mode interference type multiplexer/demultiplexer of the present embodiment, the single-mode waveguide 102a as 2 single-mode waveguides and the single-mode waveguide 105 for the unwanted light are connected to different end portions of the multi-mode waveguide 1. Further, since the single-mode waveguide 102a and the single-mode waveguide 105 for the unnecessary light are disposed separately, even when the connection portion between the single-mode waveguide and the multi-mode waveguide 1 is tapered, the single-mode waveguide does not deteriorate in workability.

On the other hand, by making the shape of the single-mode waveguide 102a at the connection portion 33c with the multi-mode waveguide 1a tapered shape, and similarly making the shape of the single-mode waveguide 105 for unwanted light at the connection portion 31c with the multi-mode waveguide 1a tapered shape, it is possible to reduce the transmission loss of light at these connection portions and the reflected return light from the connection portions. That is, according to the configuration of the present embodiment, it is possible to suppress the transmission loss of light and the reflected return light without deteriorating the workability.

The single-mode waveguide having a tapered shape may be at least one of the single-mode waveguide 102a and the single-mode waveguide 105 for unnecessary light. For example, in the case where the single-mode waveguide 105 for unwanted light has a tapered shape, more reflected return light can be guided and removed.

< embodiment 6 >

Fig. 16 is a plan view schematically illustrating the structure of the multi-mode interference type multiplexer/demultiplexer according to the present embodiment. The multi-mode interference type multiplexer/demultiplexer according to the present embodiment includes a multi-mode waveguide 1, a single-mode waveguide 102a, a single-mode waveguide 2b, a single-mode waveguide 2c, and a single-mode waveguide 105 for unnecessary light. The reflecting surface 4a is disposed at a position facing the single-mode waveguide 2b of the multi-mode waveguide 1.

Fig. 17 is an enlarged view of a part of the simulation result illustrated in fig. 5. When light is input from the single-mode waveguide 2b, the light is guided to the single-mode waveguide 2a and the single-mode waveguide 5 for unnecessary light, as illustrated in fig. 5 and 17.

The light guided to the single-mode waveguide 2a passes near the corner of the position of the arrow 200 in fig. 17. Therefore, a small amount of light strikes the corner, whereby loss or reflection of the returning light may occur.

In addition, even in the case where light is input from the single-mode waveguide 2a, the light is similarly transmitted, so the light passes near the corner of the position of the arrow 200, and a small amount of light is irradiated to the corner. Thus, loss or reflection of the returning light may occur.

In order to prevent the light from reaching this angle, the reflecting surface 4d in fig. 16 may be moved in parallel in the direction of the arrow 201, i.e., in the negative X-axis direction. However, the reflection point (i.e., the intersection point Z) is also moved in parallel by this.

Therefore, the single-mode waveguide 105 for the unnecessary light also needs to be moved in parallel in the X-axis negative direction by the same length as the length of the moving reflection surface 4d (i.e., the length in the Y-axis direction).

At this time, the imaging point of the light is also shifted by the same length in the negative X-axis direction, but the imaging point of the light is also shifted by the same length in the positive Y-axis direction, so that the imaging point exists inside the multimode waveguide 1. That is, the distance Y as the distance between the intersection Z and the connection portion 31c is longer than the distance X as the distance between the intersection Z and the connection portion 30.

In the case where the connection portion for the single-mode waveguide for the unwanted light is not formed in a tapered shape, that is, in the case where the single-mode waveguide 5 for the unwanted light is connected, the coupling loss of the single-mode waveguide 5 for the unwanted light and the multi-mode waveguide 1 increases due to the movement of the imaging point.

On the other hand, in the case where the connection portion of the single-mode waveguide for the unwanted light is formed in a tapered shape, that is, in the case where the single-mode waveguide 105 for the unwanted light is connected, even if the imaging position is slightly shifted due to the movement of the imaging point described above, it is easy to guide the light to the single-mode waveguide 105 for the unwanted light. Therefore, the light guided to the single-mode waveguide 105 for the unnecessary light can be removed as reflected return light without increasing the coupling loss.

Even when the imaging point is shifted by about 4 μm in the positive Y-axis direction, or when the single-mode waveguide is not tapered at the connection portion, the generated excess loss is about 0.6 dB. In the same case, if the connection portion of the single-mode waveguide is tapered, the excess loss is further reduced.

According to the multi-mode interference type multiplexer/demultiplexer of the present embodiment, it is possible to suppress loss of light incident on or emitted from the single-mode waveguide 102a and reflected return light without reducing the function of removing the reflected return light of the single-mode waveguide 105 for unnecessary light.

< embodiment 7 >

Fig. 18 is a plan view schematically illustrating the structure of the multi-mode interference type multiplexer/demultiplexer according to the present embodiment. In each of the above embodiments, the basic design is a 2 × 2 multimode interference type multiplexer/demultiplexer, but the number of single-mode waveguides on the input side and the output side may be increased to N × N. For example, as exemplified in the present embodiment, a 4 × 4 multimode interference type multiplexer/demultiplexer may be used.

As illustrated in fig. 18, an input port 300, an input port 301, an input port 302, and an input port 303 are connected to the multimode waveguide 1 c. Further, an output port 400, an output port 401, an output port 402, and an output port 403 are connected to the multimode waveguide 1 c.

On the other hand, fig. 19 is a plan view schematically illustrating the structure of a general 4 × 4 multimode interference type multiplexer/demultiplexer. The multimode waveguide 1d in fig. 19 has a quadrangular structure without a reflecting surface.

As illustrated in fig. 19, an input port 300, an input port 301, an input port 302, and an input port 303 are connected to the multimode waveguide 1 d. Further, an output port 400, an output port 401, an output port 402, and an output port 403 are connected to the multimode waveguide 1 d.

As in the case of the 2 × 2 multi-mode interference type multiplexer/demultiplexer, in the basic design of the 4 × 4 multi-mode interference type multiplexer/demultiplexer, if the width of the multi-mode waveguide is determined, the length of the multi-mode waveguide and the position where the single-mode waveguide is connected are also uniquely determined. For example, when the width of the multimode waveguide is set to 24 μm, the length of the multimode waveguide is 1200 μm and the interval of the single-mode waveguide is 6 μm.

In fig. 19, when light enters from any of 4 input ports, the light is branched by 4 and output from 4 output ports.

On the other hand, in the multi-mode interference type multiplexer/demultiplexer according to the present embodiment illustrated in fig. 18, light incident from the input port 300, the input port 301, and the input port 302 is reflected perpendicularly by the reflection surface and coupled to the output port 400, the output port 401, and the output port 402, respectively. This light is removed as unwanted light.

Light entering from the input port 303 is emitted from the output port 403 without being reflected by the reflection surface. The light is used as, for example, signal light.

By disposing the reflection surface on the output side in this way, the unnecessary light is transmitted in the direction perpendicular to the transmission direction of the signal light. By transmitting the unwanted light in the vertical direction, it is possible to suppress the unwanted light from being mixed into the signal light and to ensure signal quality.

In comparison between the case illustrated in fig. 19 and the case illustrated in fig. 18, in the case illustrated in fig. 18, for example, no other output port exists in the vicinity of the output port 403 that outputs the signal light. Therefore, the workability of the output port 403 is improved, and the yield is improved.

When the path length 500 is a, the path length 501 is B, the path length 502 is C, and the path length 503 is D in fig. 18, the connection position between the reflection surface and the single-mode waveguide for the unwanted light is set so that the multi-mode waveguide length L of the multi-mode waveguide 1C satisfies the following expression.

[ formula 2]

Length a, length B, length C, length D

Here, the length a (path length 500) is, for example, the sum of the length in the horizontal direction from the connection portion of the input port 300 with the multimode waveguide 1c to the reflection surface and the length in the vertical direction from the intersection of the line along the X-axis direction passing through the connection portion of the input port 300 and the reflection surface to the connection portion of the output port 400 with the multimode waveguide 1 c.

With the above setting, the light reflected at the reflecting surface is guided to the single-mode waveguide for the unnecessary light.

Fig. 20 is a diagram illustrating a 4-wavelength integrated modulator to which the multi-mode interference type multiplexer/demultiplexer of fig. 18 is applied. Fig. 11 illustrates a 2-wavelength integrated modulator to which a 2 × 2 multi-mode interference type multiplexer/demultiplexer is applied, and fig. 20 illustrates a 4-wavelength integrated modulator to which a 4 × 4 multi-mode interference type multiplexer/demultiplexer is applied. By multiplexing 4 signal lights having different wavelengths and outputting the multiplexed signal light, the data capacity can be increased by 2 times as compared with a 2-wavelength integrated modulator that multiplexes 2 signal lights.

As illustrated in fig. 20, the 4-wavelength integrated modulator includes: 4 LDs 232a, 232 LD232b, 232 LD232c, and 132 LD132d with different oscillation wavelengths; 4 EA modulators 231a, 231b, 231c, and 231d connected to the respective LDs; and a multi-mode interference type multiplexer/demultiplexer 20d connected in parallel to the EA modulator 231a, the EA modulator 231b, the EA modulator 231c, and the EA modulator 231d, respectively.

In the 4-wavelength integrated modulator according to the present embodiment, as in the 2-wavelength integrated modulator, since the unnecessary light is guided to the single-mode waveguide for the unnecessary light and is removed, it is possible to obtain good signal light without being reflected return light to the LD.

< effects produced by the above-described embodiments >

Next, the effects produced by the above-described embodiments will be exemplified. In the following description, the effects are described based on the specific configurations illustrated in the above-described embodiments, but may be replaced with other specific configurations illustrated in the present specification in a range where similar effects are produced.

In addition, the permutation may be performed across a plurality of embodiments. That is, the same effects may be produced by combining the respective configurations illustrated in the different embodiments.

According to the above-described embodiments, the multi-mode interference type multiplexer/demultiplexer includes the multi-mode waveguide 1, the 1 st single-mode waveguide, the 2 nd single-mode waveguide, the 3 rd single-mode waveguide, the reflecting surface, and the 4 th single-mode waveguide connected to the 1 st connecting portion. Here, the 1 st single-mode waveguide corresponds to, for example, the single-mode waveguide 2 a. The 2 nd single-mode waveguide corresponds to, for example, the single-mode waveguide 2 c. The 3 rd single-mode waveguide corresponds to, for example, the single-mode waveguide 2 b. The reflection surface corresponds to, for example, 1 of the reflection surfaces 4, 4a, 4b, and 4 c. In addition, the 4 th single-mode waveguide corresponds to, for example, any 1 of the single-mode waveguide 5 for unnecessary light, the single-mode waveguide 5a for unnecessary light, the single-mode waveguide 5b for unnecessary light, the single-mode waveguide 5c for unnecessary light, and the single-mode waveguide 5d for unnecessary light according to the reflection surface. The 1 st connection portion corresponds to, for example, 1 of the connection portions 31, 31a, 31b, and 32 according to the 4 th single-mode waveguide. The multimode waveguide 1 has a 1 st end, a 2 nd end opposite to the 1 st end, and a 1 st end and a 2 nd end facing each other. When the direction connecting the 1 st end and the 2 nd end is the 1 st direction and the direction intersecting the 1 st direction is the 2 nd direction, the 1 st end and the 2 nd end face each other in the 2 nd direction. Here, the 1 st end corresponds to, for example, the end 1 a. The 2 nd end corresponds to, for example, the end 1 b. The 1 st direction corresponds to, for example, the X-axis direction. The 2 nd direction corresponds to, for example, the Y axis direction. The 1 st side end corresponds to the side end 10a, for example. The 2 nd side end corresponds to the side end 10b, for example. The single-mode waveguide 2a is connected to the end 1a of the multimode waveguide 1. The single-mode waveguide 2c is connected to a position facing the single-mode waveguide 2a at the end 1b of the multi-mode waveguide 1. The single-mode waveguide 2b is connected at the end 1b of the multi-mode waveguide 1 to a position closer to the side end 10a than the position where the single-mode waveguide 2c is connected. The reflecting surface 4 is disposed at a position facing the single mode waveguide 2b in the multi-mode waveguide 1. A single-mode waveguide 5 for unnecessary light is connected to the side end portion 10 b. Further, light incident from the single-mode waveguide 2c or the single-mode waveguide 2b is reflected at the reflection surface 4, and is imaged at the connection portion 31, which is the connection portion, at the side end portion 10b of the single-mode waveguide 5 for unwanted light.

According to such a configuration, light that may become reflected return light is reflected on the reflecting surface 4 and is imaged at the connecting portion 31, whereby the light can be guided to the single-mode waveguide 5 for unnecessary light. Therefore, it is possible to easily form a layout including a curved waveguide for processing unnecessary light and suppress reflected return light in the multi-mode interference type multiplexer/demultiplexer.

In addition, since the single-mode waveguide 5 for the unnecessary light is connected to the side end portion 10b of the multi-mode waveguide 1, the single-mode waveguide 5 for the unnecessary light can be connected to a position relatively distant from the single-mode waveguide 2 a.

When the gaps between the 2 single-mode waveguides are narrowed to, for example, about 2 μm when patterning is performed by photolithography, the possibility that these gaps cannot be processed smoothly, and the lower cladding layer 51, the waveguide layer 52, and the upper cladding layer 53 to be removed remain as they are is increases. In addition, when the processing is not performed satisfactorily, a problem occurs in that light incident from the single-mode waveguide 2a does not propagate as in a simulation, and light is not coupled to the single-mode waveguide 2b and the single-mode waveguide 2 c.

In addition, other configurations than these configurations, which are exemplified in the present specification, can be appropriately omitted. That is, if at least these structures are provided, the above-described effects can be produced.

However, the above-described effects can be similarly produced even when at least 1 of the other configurations exemplified in the present specification is appropriately added to the above-described configuration, that is, when the other configurations exemplified in the present specification that are not described as the above-described configuration are added to the above-described configuration.

In addition, according to the above-described embodiment, the single-mode waveguide 5 for the unwanted light is connected to the side end portion 10 b. The reflecting surface 4 is an inclined surface which approaches the end portion 1b as approaching the end portion 10 a. According to such a structure, the light reflected at the reflecting surface 4 is imaged at the connecting portion 31 of the single-mode waveguide 5 for unwanted light connected to the side end portion 10 b.

In addition, according to the above-described embodiment, the single-mode waveguide 5a for the unwanted light is connected to the side end portion 10 a. The reflecting surface 4a is an inclined surface that is distant from the end 1b as it approaches the end 10 a. According to such a structure, the light reflected at the reflection surface 4a is imaged at the connection portion 32 of the single-mode waveguide 5a for unwanted light connected to the side end portion 10 a.

Further, according to the above-described embodiment, when a point at which light incident from the single mode waveguide 2c or the single mode waveguide 2b is reflected on the reflection surface 4 is defined as a reflection point, a distance between the reflection point and the end portion 1a in the direction along the X axis direction is defined as a 1 st distance, and a distance between the reflection point and the connection portion 31 is defined as a 2 nd distance, the 1 st distance and the 2 nd distance are equal to each other. The reflection point corresponds to the intersection point Z, for example. The 1 st distance corresponds to, for example, the distance X. The 2 nd distance corresponds to, for example, the distance Y. According to such a structure, light that should be imaged at the connection portion 30 of the single-mode waveguide 3 is reflected at the reflection surface 4, and then imaged at the connection portion 31 of the single-mode waveguide 5 for unwanted light at the side end portion 10b of the multi-mode waveguide 1 at the same transmission distance as in the case where the reflection surface 4 is not present.

In addition, according to the above-described embodiment, the intersection point Z is passed along the axis H in the longitudinal direction of the single-mode waveguide 5 for the unwanted light. According to such a structure, the light reflected at the reflecting surface 4 is imaged at the connecting portion 31 of the single-mode waveguide 5 for unwanted light, and guided into the single-mode waveguide 5 for unwanted light to be transmitted. At this time, the direction of light incident on the single-mode waveguide 5 for unwanted light is a direction along the axis H of the single-mode waveguide 5 for unwanted light, so reflection of light at the connection portion 31 can be suppressed.

Further, according to the above-described embodiment, the multi-mode interference type multiplexer/demultiplexer further includes the 5 th single-mode waveguide having the absorption layer 54 for absorbing incident light, which is connected to the end portion of the single-mode waveguide 5b for unwanted light on the side opposite to the connection portion 31. Here, the 5 th single-mode waveguide corresponds to, for example, a single-mode waveguide 6 for absorption. According to such a structure, the light guided into the single-mode waveguide 5b for unwanted light is absorbed in the single-mode waveguide 6 for absorption. Therefore, the possibility of stray light generation in the module or the like is reduced by absorbing unnecessary light. In particular, it is effective in the case where it is difficult to dispose the curved waveguide to the end of the substrate 50 or the like due to the relationship of the arrangement of elements in the integrated circuit.

In addition, according to the above-described embodiment, the angle of the connection surface 101 between the single-mode waveguide 5b for unwanted light and the single-mode waveguide 6 for absorption is the brewster angle. According to such a structure, reflection of light at the connection surface 101 of the single-mode waveguide 5b for unnecessary light and the single-mode waveguide 6 for absorption can be suppressed. Ideally, reflection at the connection surface 101 can be eliminated, and all unnecessary light is absorbed at the absorption layer 54.

Further, according to the above-described embodiment, at least 1 of the above-described multimode interference type multiplexer/demultiplexer devices is provided. With such a configuration, the reflected return light can be suppressed in the optical element such as a mach-zehnder modulator or a 2-wavelength integrated modulator. For example, when an MZ interferometer is configured by connecting 2 ordinary 2 × 2 multimode interference type multiplexer/demultiplexers, 1 unused single-mode waveguide is disposed on each of the incident side and the exit side. Such a single-mode waveguide can be connected to the side end portion of the multimode waveguide 1 and used as a single-mode waveguide for unwanted light for removing unwanted light.

In addition, according to the above-described embodiment, the distance Y is larger than the distance X. According to such a configuration, although an image forming point exists inside the multi-mode waveguide 1, the excessive loss due to the shift of the image forming point in the positive Y-axis direction is not large, and if the tapered shape is formed at the connection portion of the single-mode waveguide, the excessive loss is further reduced.

In addition, according to the above-described embodiment, at least 1 of the single-mode waveguide 102a and the single-mode waveguide 105 for unwanted light has a tapered shape. With this configuration, the transmission loss of light at the connection portion and the reflected return light from the connection portion can be reduced. That is, the transmission loss of light and the reflected return light can be suppressed without deteriorating the workability.

Further, according to the above-described embodiment, the input port 300, the input port 301, the input port 302, and the input port 303, which are single-mode waveguides, and the output port 400, the output port 401, the output port 402, and the output port 403, which are single-mode waveguides for unnecessary light, which correspond to the respective single-mode waveguides, are provided. When the path lengths of light that is vertically incident from each single-mode waveguide to the multi-mode waveguide 1C by being reflected by the reflecting surface and reaches the corresponding single-mode waveguide for unwanted light are set to be length a, length B, length C, and length D, respectively, length L of the multi-mode waveguide 1C satisfies

[ formula 3]

Length a, length B, length C, length D

Is provided with a reflective surface and a single mode waveguide for unwanted light. According to such a configuration, since the unnecessary light is guided to and removed from the single-mode waveguide for the unnecessary light, it is possible to obtain favorable signal light without being reflected return light to the LD.

< modification of the above-described embodiment >

In the above-described embodiment, the angle of the reflecting surface 4 with respect to the X-axis direction is set to 45 °, but the angle of the reflecting surface is not limited to this case.

Fig. 13 is a plan view showing the configuration of the multi-mode interference type multiplexer/demultiplexer in the case where the angles of the reflecting surfaces are different. As illustrated in fig. 13, even when the angle of the reflecting surface 4b is larger than that illustrated in fig. 3, the distance X and the distance Y maintain the same relationship. Further, the connection portion 31a of the single-mode waveguide 5c for the unwanted light is shifted to a position shifted in the X-axis negative direction along with the reflection direction of the light at the intersection point Z.

In this case, it is preferable that the connection angle of the single-mode waveguide 5c for unwanted light at the connection portion 31a is also inclined so that the axis H along the longitudinal direction of the single-mode waveguide 5c for unwanted light passes through the intersection point Z. This is to suppress reflection at the connecting portion 31a when the light reflected at the reflecting surface 4b is incident on the single-mode waveguide 5c for unwanted light.

Fig. 14 is a plan view showing the details of the structure of the multi-mode interference type multiplexer/demultiplexer in the case where the angles of the reflecting surfaces are different. As illustrated in fig. 14, when the angle of the reflecting surface 4c is smaller than that illustrated in fig. 3, the distance X and the distance Y are maintained in an equal relationship. Further, the connection portion 31b of the single-mode waveguide 5d for the unnecessary light is shifted to a position shifted in the X-axis positive direction along with the reflection direction of the light at the intersection point Z.

In this case, it is preferable that the connection angle of the single-mode waveguide 5d for unwanted light at the connection portion 31b is also inclined so that the axis H along the longitudinal direction of the single-mode waveguide 5d for unwanted light passes through the intersection point Z. This is to suppress reflection at the connecting portion 31b when the light reflected at the reflecting surface 4c is incident on the single-mode waveguide 5d for unwanted light.

In the above-described embodiments, materials, dimensions, shapes, relative arrangement, conditions for implementation, and the like of the respective constituent elements are described in some cases, but these are merely illustrative in all aspects and are not limited to the examples described in the present specification.

Therefore, a myriad of modifications and equivalents not illustrated can be conceived within the scope of the technology disclosed in the present specification. For example, the present invention includes a case where at least 1 component is modified, added, or omitted, and a case where at least 1 component in at least 1 embodiment is extracted and combined with components of other embodiments.

In addition, the constituent elements described as being provided with "1" in the above-described embodiments may be provided with "1 or more" as long as no contradiction occurs.

Further, each component in the above-described embodiments is a conceptual unit, and includes a case where 1 component is configured by a plurality of structures, a case where 1 component corresponds to a part of a certain structure, and a case where a plurality of components are provided in 1 structure within the scope of the technology disclosed in the present specification.

In addition, each component in the above-described embodiments includes a structure having another structure or shape as long as the same function is exerted.

In addition, the descriptions in the specification of the present application are referred to for all purposes related to the present technology, and should not be construed as prior art.

In the above-described embodiments, when a material name or the like is described without being particularly specified, other additives, for example, an alloy or the like are contained in the material as long as no contradiction occurs.

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Multi-mode interference type multiplexer/demultiplexer and optical element using the same

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