阅读说明：本技术 光波导元件 (Optical waveguide element ) 是由 片冈优 宫崎德一 于 2019-09-27 设计创作，主要内容包括：为了提供一种能够减少与光纤耦合的耦合部的耦合损失并能够减少光波导的传播损失的光波导元件,光波导元件的特征在于,具备支承基板和层叠在该支承基板上且由具有电光效应的材料构成的波导层,用于形成光波导的脊部突出设置于波导层上表面,在脊部的一部分的正下方,在支承基板的上表面形成有槽部,在槽部填充有具有与波导层的材料相同程度的有效折射率的材料。(In order to provide an optical waveguide element capable of reducing coupling loss of a coupling section coupled to an optical fiber and reducing propagation loss of an optical waveguide, the optical waveguide element is characterized by comprising a support substrate and a waveguide layer laminated on the support substrate and made of a material having an electro-optic effect, wherein a ridge portion for forming the optical waveguide is provided protruding from an upper surface of the waveguide layer, a groove portion is formed in the upper surface of the support substrate directly below a part of the ridge portion, and the groove portion is filled with a material having an effective refractive index equivalent to that of the material of the waveguide layer.)

1. An optical waveguide device comprising a support substrate and a waveguide layer laminated on the support substrate and made of a material having an electro-optical effect,

a ridge protrusion for forming an optical waveguide is provided on an upper surface of the waveguide layer,

a groove portion is formed on the upper surface of the support substrate directly below a part of the ridge portion,

the trench is filled with a material having an effective refractive index to the same extent as the material of the waveguide layer.

2. The optical waveguide element according to claim 1,

the cross-sectional area of the groove portion continuously changes along the extending direction of the ridge portion.

3. The optical waveguide element according to claim 2,

the cross-sectional area of the groove portion changes so as to decrease continuously with respect to the propagation direction of the optical wave propagating through the optical waveguide.

4. The optical waveguide element according to claim 2,

the cross-sectional area of the groove portion changes so as to increase continuously with respect to the propagation direction of the optical wave propagating through the optical waveguide.

5. The optical waveguide element according to claim 3 or 4,

the dimension of the groove in the width direction continuously changes with respect to the propagation direction of the optical wave propagating through the optical waveguide.

6. The optical waveguide element according to claim 3 or 4,

the dimension of the groove in the height direction continuously changes with respect to the propagation direction of the optical wave propagating through the optical waveguide.

7. The optical waveguide element according to any one of claims 1 to 6,

in the coupling portion in which the optical waveguide element is coupled to an input-side optical fiber that introduces the optical wave into the optical waveguide, and the coupling portion in which the optical waveguide element is coupled to an output-side optical fiber that leads the optical wave out from the optical waveguide, a dimension in the width direction of the groove portion is substantially the same as a dimension in the width direction of the ridge portion, and a dimension in the height direction from the upper surface of the ridge portion to the bottom surface of the groove portion is substantially the same as a dimension in the width direction of the ridge portion.

8. The optical waveguide element according to claim 7,

the optical waveguide element has a modulation section for modulating the optical wave propagating through the optical waveguide,

a cross-sectional area of the groove portion changes so as to decrease continuously in a part of the optical waveguide from a coupling portion where the optical waveguide element is coupled to the input-side optical fiber to the modulation portion,

in the optical waveguide from the modulation section to the coupling section where the optical waveguide element is coupled to the output side optical fiber, the cross-sectional area of the groove section changes so as to increase continuously.

9. The optical waveguide element according to any one of claims 1 to 8,

the dimension in the height direction of the ridge portion is larger than half of the dimension in the height direction of the groove portion and smaller than 2 times the dimension in the height direction of the groove portion.

10. The optical waveguide element according to any one of claims 1 to 9,

the dimension in the width direction of the ridge portion is larger than a half of the dimension in the height direction from the upper surface of the ridge portion to the bottom surface of the groove portion and smaller than 2 times the dimension in the height direction from the upper surface of the ridge portion to the bottom surface of the groove portion.

11. The optical waveguide element according to any one of claims 1 to 10,

the waveguide layer is made of lithium niobate,

the material occupying the grooves is any of lithium niobate, silicon nitride having an effective refractive index that is approximately equal to that of lithium niobate, and a resin adjusted to have an effective refractive index that is approximately equal to that of lithium niobate.

12. The optical waveguide element according to any one of claims 1 to 11,

the waveguide layer is attached to the support substrate.

Technical Field

The present invention relates to an optical waveguide device including a waveguide layer which is laminated on a support substrate and is made of a material having an electro-optical effect.

Background

Conventionally, in the field of optical communication and optical measurement, an optical waveguide element in which an optical waveguide is formed on a substrate having an electro-optical effect is used. For example, patent document 1 listed below discloses an optical element having a ridge waveguide in which an optical waveguide portion is protruded and the other substrate region is thinned.

Prior art documents

Patent document

Patent document 1: japanese patent laid-open publication No. 2006 and 284964

Disclosure of Invention

Problems to be solved by the invention

The optical waveguide element is coupled to an optical fiber for introducing a light wave into the optical waveguide element on a light wave input side, and is coupled to an optical fiber for outputting a light wave from the optical waveguide element on a light wave output side. In the case of a single mode, for example, the light intensity distribution in a cross section perpendicular to the propagation direction is substantially a perfect circle, and has a beam shape with little distortion. On the other hand, the light wave propagating through the ridge waveguide formed in the optical waveguide element has a beam shape in which the light intensity distribution is distorted in a cross section perpendicular to the propagation direction in accordance with the cross section shape of the ridge protruding toward the upper surface of the waveguide layer.

Thus, the optical fiber and the optical waveguide element have different beam shapes of the propagating light waves in light intensity distribution. Such a mismatch in the beam shape has a problem that a large coupling loss may occur in the coupling portion between the optical fiber and the optical waveguide element.

As described above, the light wave propagating through the ridge waveguide has a beam shape corresponding to the cross-sectional shape of the ridge. In this case, when the waveguide layer and the boundary between the ridge and the outside air are not sufficiently processed (when so-called processing roughness occurs), the light waves propagating through the ridge waveguide may overlap at the boundary and scatter. As a result, there is a problem that a large propagation loss may occur in the ridge waveguide of the optical waveguide element.

The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an optical waveguide element capable of reducing a coupling loss at a coupling portion coupled to an optical fiber and reducing a propagation loss of an optical waveguide.

Means for solving the problems

In order to solve the above-described problems, an optical waveguide element according to the present invention has the following features.

(1) In order to achieve the above object, an optical waveguide device of the present invention includes a support substrate and a waveguide layer laminated on the support substrate and made of a material having an electro-optical effect,

a ridge protrusion for forming an optical waveguide is provided on an upper surface of the waveguide layer,

a groove portion is formed on the upper surface of the support substrate directly below a part of the ridge portion,

the trench is filled with a material having an effective refractive index to the same extent as the material of the waveguide layer.

With this configuration, the light wave propagating through the optical waveguide can be expanded in the direction of the lower groove portion, and the beam shape of the light wave propagating through the optical waveguide of the optical waveguide element can be formed into a shape with less distortion. As a result, the beam shape of the optical wave propagating through the optical waveguide of the optical waveguide element can be made close to the beam shape of a single-mode optical wave propagating through the optical fiber, for example, and thus it is possible to suppress the mismatch of the beam shapes and reduce the coupling loss at the coupling portion where the optical fiber and the optical waveguide element are coupled.

In addition, with this configuration, the height position of the light wave propagating through the optical waveguide of the optical waveguide element can be arranged at a position close to the lower groove portion. As a result, the region where the light wave propagating through the optical waveguide of the optical waveguide element overlaps at the boundary portion between the waveguide layer and the ridge portion and the outside air can be reduced, and scattering caused by the boundary portion can be suppressed, thereby reducing the propagation loss of the optical waveguide element.

(2) In the optical waveguide element according to the above (1), a cross-sectional area of the groove portion continuously changes along an extending direction of the ridge portion.

With this configuration, scattering that occurs when the beam shape of the light wave propagating through the optical waveguide of the optical waveguide element discontinuously changes can be prevented. As a result, scattering due to discontinuous changes in the beam shape of the optical wave can be suppressed, and the propagation loss of the optical waveguide element can be reduced.

(3) In the optical waveguide element according to the above (2), a cross-sectional area of the groove portion changes so as to decrease continuously with respect to a propagation direction of an optical wave propagating through the optical waveguide.

With this configuration, the beam shape of the optical wave propagating through the optical waveguide in the portion where the groove portion is formed can be changed to gradually approach the beam shape of the optical wave propagating through the normal ridge waveguide where no groove portion is formed. As a result, scattering due to discontinuous changes in the beam shape of the optical wave can be suppressed, propagation loss of the optical waveguide element can be reduced, and the optical wave at the portion where the groove portion is formed can be smoothly guided from the normal ridge waveguide where the groove portion is not formed with low loss.

(4) In the optical waveguide element according to the above (2), a cross-sectional area of the groove portion changes so as to increase continuously with respect to a propagation direction of an optical wave propagating through the optical waveguide.

With this configuration, the beam shape of the light wave propagating through the normal ridge optical waveguide in which the groove portion is not formed can be changed so as to gradually approach the beam shape of the light wave propagating through the optical waveguide in the portion in which the groove portion is formed. As a result, scattering due to discontinuous changes in the beam shape of the optical wave can be suppressed, propagation loss of the optical waveguide element can be reduced, and conversion from a normal ridge waveguide in which the groove portion is not formed to an optical waveguide in the portion in which the groove portion is formed can be performed smoothly with low loss.

(5) The optical waveguide element according to the above (3) or (4), wherein a dimension of the groove portion in the width direction continuously changes with respect to a propagation direction of the optical wave propagating through the optical waveguide.

With this structure, the cross-sectional area of the groove can be continuously changed by appropriately designing the dimension of the groove in the width direction. As a result, scattering due to discontinuous changes in the beam shape of the optical wave can be suppressed, and the propagation loss of the optical waveguide element can be reduced.

(6) The optical waveguide element according to the above (3) or (4), wherein a dimension of the groove portion in a height direction continuously changes with respect to a propagation direction of the optical wave propagating through the optical waveguide.

With this structure, the sectional area of the groove can be continuously changed by appropriately designing the dimension of the groove in the height direction. As a result, scattering due to discontinuous changes in the beam shape of the optical wave can be suppressed, and the propagation loss of the optical waveguide element can be reduced.

(7) The optical waveguide element according to any one of the above (1) to (6), wherein a dimension in a width direction of the groove portion is substantially the same as a dimension in a width direction of the ridge portion, and a dimension in a height direction from an upper surface of the ridge portion to a bottom surface of the groove portion is substantially the same as a dimension in the width direction of the ridge portion, in a coupling portion where the optical waveguide element is coupled to an input-side optical fiber that introduces the optical wave into the optical waveguide, and a coupling portion where the optical waveguide element is coupled to an output-side optical fiber that leads the optical wave out from the optical waveguide.

With this configuration, the beam shape of the optical wave propagating through the optical waveguide of the optical waveguide element can be made to be close to a substantially perfect circle in the coupling portion coupled to the input-side optical fiber and the coupling portion coupled to the output-side optical fiber. As a result, it is possible to suppress the beam shape mismatch of the optical wave at the coupling portion where the optical fiber and the optical waveguide element are coupled, and to reduce the coupling loss at the coupling portion where the optical fiber and the optical waveguide element are coupled.

(8) The optical waveguide element according to the above (7), wherein,

the optical waveguide element has a modulation section for modulating the optical wave propagating through the optical waveguide,

a cross-sectional area of the groove portion changes so as to decrease continuously in a part of the optical waveguide from a coupling portion where the optical waveguide element is coupled to the input-side optical fiber to the modulation portion,

in the optical waveguide from the modulation section to the coupling section where the optical waveguide element is coupled to the output side optical fiber, the cross-sectional area of the groove section changes so as to increase continuously.

With this configuration, the coupling loss of the coupling portion where the input-side optical fiber and the output-side optical fiber are coupled to the optical waveguide element can be reduced, and the propagation loss of the optical waveguide from the coupling portion coupled to the input-side optical fiber to the modulation portion and from the modulation portion to the coupling portion coupled to the output-side optical fiber can be reduced. Further, the modulation section can select a configuration with higher modulation efficiency of the optical wave, and thus the optical wave can be efficiently modulated in the modulation section.

(9) The optical waveguide element according to any one of the above (1) to (8), wherein a height direction dimension of the ridge portion is larger than a half of a height direction dimension of the groove portion and smaller than 2 times the height direction dimension of the groove portion.

With this configuration, the dimensions of the ridge portion and the groove portion can be appropriately determined, and a single-mode light wave can be appropriately sealed in the optical waveguide of the optical waveguide element.

(10) The optical waveguide element according to any one of the above (1) to (9), wherein a dimension in a width direction of the ridge portion is larger than a half of a dimension in a height direction from an upper surface of the ridge portion to a bottom surface of the groove portion and is smaller than 2 times the dimension in the height direction from the upper surface of the ridge portion to the bottom surface of the groove portion.

With this configuration, the dimensions of the ridge portion and the groove portion can be appropriately determined, and a single-mode light wave can be appropriately sealed in the optical waveguide of the optical waveguide element.

(11) The optical waveguide device according to any one of the above (1) to (10), wherein a material of the waveguide layer is lithium niobate,

the material occupying the grooves is any of lithium niobate, silicon nitride having an effective refractive index that is approximately equal to that of lithium niobate, and a resin adjusted to have an effective refractive index that is approximately equal to that of lithium niobate.

With this configuration, the material of the waveguide layer and the material occupying the groove can be determined appropriately, so that the coupling loss of the coupling section for coupling the optical fiber and the optical waveguide element can be reduced, and the propagation loss of the optical waveguide element can be reduced.

(12) The optical waveguide device according to any one of the above (1) to (11), wherein the waveguide layer is bonded to the support substrate.

With this structure, the material for filling the groove portion 5 can be selected appropriately, and the optical waveguide element can be manufactured by a simple process.

Effects of the invention

According to the present invention, in the optical waveguide element, the coupling loss of the coupling portion coupled to the optical fiber can be reduced, and the propagation loss of the optical waveguide can be reduced.

Drawings

Fig. 1 is a perspective view schematically showing a structure in the vicinity of an optical waveguide element according to a first embodiment of the present invention.

Fig. 2 is a schematic cross-sectional view for explaining the dimensions of each part of the optical waveguide element according to the first embodiment of the present invention.

Fig. 3A is a plan view of a structure in the vicinity of the optical waveguide element shown in fig. 1.

Fig. 3B is a front view of a structure in the vicinity of an optical waveguide of the optical waveguide element shown in fig. 1.

Fig. 3C is a side view of a structure in the vicinity of the optical waveguide element shown in fig. 1.

Fig. 4A is a diagram showing a cross-sectional structure of the vicinity of the optical waveguide element according to the first embodiment of the present invention, and is a cross-sectional view a-a in fig. 3C.

Fig. 4B is a diagram showing a cross-sectional structure of the vicinity of the optical waveguide element according to the first embodiment of the present invention, and is a cross-sectional view B-B in fig. 3C.

Fig. 4C is a diagram showing a cross-sectional structure of the vicinity of the optical waveguide element according to the first embodiment of the present invention, and is a cross-sectional view taken along line C-C of fig. 3C.

Fig. 5A is a diagram showing the result of electric field intensity simulation of the optical waveguide element according to the first embodiment of the present invention, and is a diagram showing a light intensity distribution in the cross-sectional structure of fig. 4A.

Fig. 5B is a diagram showing the result of electric field intensity simulation of the optical waveguide element according to the first embodiment of the present invention, and is a diagram showing a light intensity distribution in the cross-sectional structure of fig. 4B.

Fig. 5C is a diagram showing the result of electric field intensity simulation of the optical waveguide element according to the first embodiment of the present invention, and is a diagram showing a light intensity distribution in the cross-sectional structure of fig. 4C.

Fig. 6 is a plan view showing a first configuration example of an optical modulator including an optical waveguide element according to the first embodiment of the present invention.

Fig. 7 is a plan view showing a second configuration example of an optical modulator including the optical waveguide element according to the first embodiment of the present invention.

Fig. 8A is a diagram showing a first example of a process for manufacturing an optical waveguide element according to the first embodiment of the present invention, and is a diagram showing a state after the first step.

Fig. 8B is a diagram showing a first example of the process for manufacturing an optical waveguide element according to the first embodiment of the present invention, and is a diagram showing a state after the second step.

Fig. 8C is a diagram showing a first example of the process for manufacturing an optical waveguide element according to the first embodiment of the present invention, and is a diagram showing a state after the third step.

Fig. 9A is a diagram showing a second example of the manufacturing process of the optical waveguide element according to the first embodiment of the present invention, and is a diagram showing a state after the first step.

Fig. 9B is a diagram showing a second example of the manufacturing process of the optical waveguide element according to the first embodiment of the present invention, and is a diagram showing a state after the second step.

Fig. 9C is a diagram showing a second example of the process for manufacturing an optical waveguide element according to the first embodiment of the present invention, and is a diagram showing a state after the third step.

Fig. 9D is a diagram showing a second example of the process for manufacturing an optical waveguide element according to the first embodiment of the present invention, and is a diagram showing a state after the fourth step.

Fig. 10 is a perspective view schematically showing the vicinity of an optical waveguide element according to a second embodiment of the present invention.

Fig. 11A is a plan view of the structure in the vicinity of the optical waveguide element shown in fig. 10.

Fig. 11B is a front view of the structure in the vicinity of the optical waveguide element shown in fig. 10.

Fig. 11C is a side view of a structure in the vicinity of the optical waveguide element shown in fig. 10.

Fig. 12A is a view showing a cross-sectional structure of the vicinity of the optical waveguide element according to the second embodiment of the present invention, and is a cross-sectional view taken along line D-D in fig. 11C.

Fig. 12B is a view showing a cross-sectional structure of the vicinity of the optical waveguide element according to the second embodiment of the present invention, and is a cross-sectional view from E to E in fig. 11C.

Fig. 12C is a view showing a cross-sectional structure of the vicinity of the optical waveguide element according to the second embodiment of the present invention, and is a cross-sectional view F-F in fig. 11C.

Fig. 13A is a diagram showing the result of electric field intensity simulation of the optical waveguide element according to the second embodiment of the present invention, and is a diagram showing a light intensity distribution in the cross-sectional structure of fig. 12A.

Fig. 13B is a diagram showing the result of electric field intensity simulation of the optical waveguide element according to the second embodiment of the present invention, and is a diagram showing a light intensity distribution in the cross-sectional structure of fig. 12B.

Fig. 13C is a diagram showing the result of electric field intensity simulation of the optical waveguide element according to the second embodiment of the present invention, and is a diagram showing a light intensity distribution in the cross-sectional structure of fig. 12C.

Detailed Description

Embodiments of the present invention will be described below with reference to the drawings.

[ first embodiment ]

An optical waveguide element according to a first embodiment of the present invention will be described.

Fig. 1 is a perspective view schematically showing the vicinity of an optical waveguide element 1 according to a first embodiment of the present invention. In the drawings, the width direction of the optical waveguide element 1 is defined as an X axis, the length direction of the optical waveguide element 1 is defined as a Y axis, and the height direction of the optical waveguide element 1 is defined as a Z axis.

The optical waveguide device 1 shown in fig. 1 includes a support substrate 2 and a waveguide layer 3 laminated on the support substrate 2.

The support substrate 2 is a substrate that can stably support the waveguide layer 3 by supplementing the strength of the waveguide layer 3. The dimension of the support substrate 2 in the height direction is not particularly limited, and is, for example, several hundred μm. The material of the support substrate 2 is not particularly limited, and is, for example, a ceramic such as quartz glass.

The waveguide layer 3 is a thin plate formed of a material having an electro-optical effect and laminated on the support substrate 2. In the waveguide layer 3, for example, lithium niobate (LiNbO) can be used₃: hereinafter, referred to as LN) as a material having an electro-optical effect, lithium tantalate, lead lanthanum zirconate titanate (PLZT), a semiconductor, or the like may be used. The material to be filled in the waveguide layer 3 preferably has a transmittance to such an extent that propagation loss is not generated, and particularly, a material having a high transmittance at the wavelength of the light wave propagating through the optical waveguide is preferably selected.

A ridge 4 made of the same material as the waveguide layer 3 is provided on the upper surface of the waveguide layer 3. The ridge 4 is provided so as to protrude from the upper surface of the waveguide layer 3 and is formed so as to have a cross-sectional convex shape. The ridge portion 4 has a function of enclosing a light wave, and is used as an optical waveguide. The extending direction of the optical waveguide coincides with the extending direction of the ridge portion 4 (Y-axis direction in fig. 1).

The propagation direction of the optical wave propagating through the optical waveguide may be a positive direction of the Y axis or a negative direction. For example, in the perspective view of fig. 1, a light wave may propagate in the optical waveguide formed by the ridge portion 4 from the front side toward the rear side in the drawing, or conversely, may propagate in the optical waveguide formed by the ridge portion 4 from the rear side toward the front side in the drawing. As will be described later, in the optical waveguide element 1 incorporated into the optical modulator 10A shown in fig. 6, for example, the light wave propagates in the positive direction of the Y axis shown in fig. 1 in the section S1, and propagates in the negative direction of the Y axis shown in fig. 1 in the section S2.

In this specification, the ridge portion 4 that protrudes perpendicularly to the upper surface of the waveguide layer 3 and has a rectangular cross-sectional shape is shown as an example. However, the ridge portion 4 is not limited to this configuration, and a ridge portion 4 having a trapezoidal cross-sectional shape may be provided, for example.

A groove portion 5 is provided on the upper surface of the support substrate 2 located directly below the ridge portion 4. The groove portion 5 is formed so as to have a concave cross-section by defining a groove by digging a dug surface formed by digging the upper surface of the support substrate 2. In fig. 1, in order to show the shape of the groove 5 in the drawing, the support substrate 2, the waveguide layer 3, and the ridge portion 4 are shown only as outlines and made transparent, and the material filling the groove 5 is shown by a grid.

The cross-sectional area of the groove defined by the groove portion 5 is set so as to continuously vary along the extending direction of the optical waveguide, that is, the extending direction of the ridge portion 4. The continuous change means a change in which the position is not changed discontinuously but smoothly connected, and means a gradual change at a constant rate, for example.

In the first embodiment of the present invention, the dimension of the groove portion 5 in the height direction is set to be constant along the extending direction of the ridge portion 4, and the sectional area of the groove is set to be continuously changed by continuously changing the dimension of the groove portion 5 in the width direction. The dimension of the groove portion 5 in the width direction is set to be large on the front side of the perspective view of fig. 1, and the dimension of the groove portion 5 in the width direction gradually decreases as it goes to the back side of the drawing, and the dimension of the groove portion 5 in the width direction becomes zero on the back side of the drawing. The dimension of the groove portion 5 in the width direction being zero means that the groove portion 5 is not formed or the groove portion 5 disappears.

In the present description, the groove portion 5 is illustrated as an example, which is vertically cut with respect to the upper surface of the support substrate 2 and has a rectangular cross-sectional shape. However, the groove portion 5 is not limited to this configuration, and for example, a groove portion 5 having a trapezoidal cross-sectional shape may be provided. The groove portion 5 may be formed in a shape corresponding to the shape of the ridge portion 4, such as a shape in which the ridge portion 4 is inverted upside down.

The trench portion 5 is preferably filled with a material having the same effective refractive index as that of the material of the waveguide layer 3. That is, the refractive index of the material of the waveguide layer 3 is n₁N represents a refractive index of a material filling the groove portion 5₂In the case of (2), n is preferably n₁≈n₂. The material having the effective refractive index of the same level as that of the material of the waveguide layer 3 may be the same material as that of the material constituting the waveguide layer 3, such as LN, or may be silicon nitride (SiN), resin, or the like. N represents a refractive index of the outside air existing above the ridge portion 4₀Let the refractive index of the material of the support substrate 2 be n₃In the case of (2), n is preferably n₀<n₁And n₃<n₁。

Fig. 2 is a schematic cross-sectional view for explaining the dimensions of each part of the optical waveguide element 1 according to the first embodiment of the present invention. Dimension (height h) of ridge 4 in height direction₁) For example, 2 μm or less. The height dimension (depth h) of the groove 5₂) Is set to a dimension (height h) in the height direction of the ridge portion 4, for example₁) Are substantially identical. The dimension (width w) of the ridge 4 in the width direction₁) For example, the thickness is set to 4 μm or less. The dimension (height h) in the height direction from the upper surface of the ridge portion 4 to the bottom surface of the groove portion 5₃) Is set to a dimension (width w) in the width direction of the ridge portion 4, for example₁) Are substantially identical.

The width dimension (width w) of the groove 5₂) The values are set to be different depending on the extending direction of the ridge portion 4. The dimension (width w) of the groove portion 5 in the width direction₂) When zero, the groove portion 5 is not formed. Further, as described later, the width dimension (width w) of the groove portion 5 in the width direction at the coupling portion to be coupled with the optical fiber₂) Is set to have a dimension (width) in the width direction of, for example, the ridge portion 4Degree w₁) Are substantially identical.

Dimension (height h) of ridge 4 in height direction₁) Preferably, the dimension (depth h) is set to be larger than the height direction of the groove part 5₂) Is greater than half and is greater than the dimension (depth h)₂) 2 times smaller. That is, h is preferable₂/2<h₁<2×h₂. The width dimension (width w) of the ridge 4₁) Preferably, the dimension (height h) in the height direction from the upper surface of the ridge portion 4 to the bottom surface of the groove portion 5 is set to be larger than₃) Is greater than half and is greater than the dimension (height h)₃) 2 times smaller. That is, h is preferable₃/2<w₁<2×h₃. By appropriately sizing the ridge portion 4 and the groove portion 5 in this manner, a single-mode light wave can be appropriately sealed into the optical waveguide of the optical waveguide element 1.

Fig. 3A to 3C are three views of the structure in the vicinity of the optical waveguide element 1 shown in fig. 1. Fig. 3A is a plan view of the structure in the vicinity of the optical waveguide element 1 shown in fig. 1, fig. 3B is a front view of the structure in the vicinity of the optical waveguide element 1 shown in fig. 1, and fig. 3C is a side view of the structure in the vicinity of the optical waveguide element 1 shown in fig. 1. In fig. 3A to 3C, similarly to fig. 1, in order to express the shape of the groove portion 5 in the drawing, the support substrate 2, the waveguide layer 3, and the ridge portion 4 are made transparent by showing only outlines, and the material filling the groove portion 5 is shown by a grid.

The state in which the dimension in the width direction of the groove portion 5 continuously changes along the extending direction of the ridge portion 4 is particularly clearly shown in the plan view of fig. 3A. The width-directional dimension of the groove portion 5 is set to be gradually reduced in the Y-axis direction of fig. 3A, and the groove portion 5 has a wedge shape whose tip is tapered in the Y-axis direction in the plan view of fig. 3A. The dimension of the groove portion 5 in the Y axis direction (the dimension in the longitudinal direction of the wedge shape) is preferably a length to such an extent that propagation loss due to a rapid change in the beam shape does not occur, and is set to 500 μm or more, for example.

Further, with reference to the cross-sectional views shown in fig. 4A to 4C, a case where the groove portion 5 has a wedge shape in the extending direction of the ridge portion 4 will be described.

Fig. 4A is a sectional view a-a in the side view of fig. 3C, showing the sectional structure of the front side in the perspective view of fig. 1. In the cross-sectional view A-A in FIG. 4A, the dimension in the height direction from the upper surface of the ridge portion 4 to the bottom surface of the groove portion 5 (height h in FIG. 2)₃) The dimension in the width direction of the ridge 4 (width w in fig. 2)₁) Are substantially identical. The height dimension of the groove 5 (depth h in FIG. 2)₂) The dimension in the height direction of the ridge 4 (height h in fig. 2)₁) Substantially the same as the width dimension (width w in FIG. 2) of the groove portion 5 in the width direction₂) The dimension in the width direction of the ridge 4 (width w in fig. 2)₁) Are substantially identical. That is, the cross-sectional shape of the ridge portion 4 protruding from the waveguide layer 3 and the cross-sectional shape of the groove portion 5 dug in the support substrate 2 are symmetrical with respect to the waveguide layer 3.

Fig. 4B is a sectional view B-B of the side view of fig. 3C, and the perspective view of fig. 1 shows a sectional structure of a substantially central portion. In the perspective view of fig. 1, the dimension of the groove portion 5 in the width direction continuously decreases as the groove portion advances from the front side to the back side in the Y-axis direction. In the cross-sectional structure of the substantially central portion shown in the cross-sectional view B-B of fig. 4B, the dimension of the groove portion 5 in the width direction (width w of fig. 2)₂) The dimension in the width direction of the ridge portion 4 (width w in fig. 2)₁) About half of that.

Fig. 4C is a C-C sectional view in the side view of fig. 3C, showing the innermost sectional structure in the perspective view of fig. 1. As the width-directional dimension of the groove portion 5 further decreases as the Y-axis direction advances, the width-directional dimension of the groove portion 5 (width w2 in fig. 2) becomes zero in the innermost cross-sectional structure shown in the C-C cross-sectional view in fig. 4C, and the groove portion 5 disappears. The sectional structure shown in the C-C sectional view of fig. 4C is the same as that of a general ridge waveguide.

Fig. 5A to 5C are diagrams showing the results of electric field intensity simulations of the optical waveguide element 1 according to the first embodiment of the present invention. Fig. 5A is a diagram showing a light intensity distribution of the cross-sectional structure of fig. 4A. Fig. 5B is a diagram showing a light intensity distribution of the cross-sectional structure of fig. 4B. Fig. 5C is a diagram showing a light intensity distribution of the cross-sectional structure of fig. 4C. In fig. 5A to 5C, the light intensity distribution is represented by a shade, and the thicker the concentration is, the higher the light intensity is, and the thinner the concentration is, the weaker the light intensity is.

In the present specification, the direction in which the entire light wave travels in the transmission medium is referred to as the propagation direction, and the shape represented by the light intensity distribution in a cross section perpendicular to the propagation direction of the light wave is referred to as the beam shape. The beam shape includes, in addition to the geometric shape, the size of the shape. For example, matching the beam shapes of two light waves means that the geometric shape and size of the light intensity distribution of one light wave match the geometric shape and size of the light intensity distribution of the other light wave.

In the cross-sectional structure shown in fig. 4A, the dimension in the height direction from the upper surface of the ridge portion 4 to the bottom surface of the groove portion 5 is substantially the same as the dimension in the width direction of the ridge portion 4, and the dimension in the width direction of the ridge portion 4 is substantially the same as the dimension in the width direction of the groove portion 5. That is, the region surrounded by the side surfaces of the ridge portion 4 and the side surfaces of the groove portion 5 has an aspect ratio of 1: 1 is substantially square. As a result, the beam shape of the light wave propagating through the optical waveguide becomes approximately a perfect circle as shown in fig. 5A.

In the cross-sectional structure shown in fig. 4B, the dimension of the groove portion 5 in the width direction is reduced as compared with fig. 4A. Thereby, the region that expands toward the groove portion 5 is reduced and the region that expands toward the ridge portion 4 is increased with respect to the light wave propagating through the optical waveguide. The beam shape of the light wave is slightly deformed as shown in fig. 5B.

The cross-sectional structure shown in fig. 4C has the same shape as a normal ridge waveguide, with no groove 5. Therefore, as shown in fig. 5C, the beam shape of the light wave is deformed so as to enter the ridge portion 4.

When the light wave propagates in the positive direction of the Y axis of fig. 1, the beam shape of the light wave gradually deforms from a shape close to a substantially perfect circle (the state shown in fig. 5A) (the state shown in fig. 5B), and continuously changes to the same beam shape as the ridge waveguide (the state shown in fig. 5C). Since the dimension of the groove portion 5 in the width direction continuously changes, the beam shape shown in fig. 5A to 5C also continuously changes.

In the first embodiment of the present invention, by providing the groove portion 5 such that the dimension of the groove portion 5 in the width direction is continuously reduced with respect to the propagation direction of the optical wave, the beam shape of the optical wave propagating through the optical fiber (shape close to a substantially perfect circle) can be continuously changed to the beam shape of the optical wave propagating through a normal ridge waveguide. Since the beam shape of the light wave changes continuously, it is possible to prevent the occurrence of loss due to discontinuous and abrupt changes.

Further, by providing the groove portion 5 formed so that the beam shape of the optical wave becomes close to a substantially perfect circle (the state shown in fig. 5A) in the coupling portion coupled to the optical fiber that introduces the optical wave into the optical waveguide element 1, it is possible to suppress the mismatch of the beam shape between the optical wave introduced from the optical fiber and propagating through the optical waveguide of the optical waveguide element 1. As a result, the coupling loss at the coupling portion where the optical fiber and the optical waveguide element 1 are coupled can be reduced.

On the other hand, when the light wave propagates in the negative direction of the Y axis of fig. 1, the beam shape of the light wave gradually changes from the same beam shape as the ridge waveguide (the state shown in fig. 5C) to a shape with little distortion (the state shown in fig. 5B), and continuously changes to a shape close to a substantially perfect circle (the state shown in fig. 5A). Since the dimension of the groove portion 5 in the width direction continuously changes, the beam shape shown in fig. 5A to 5C also continuously changes.

In the first embodiment of the present invention, by providing the groove portion 5 such that the dimension of the groove portion 5 in the width direction continuously increases with respect to the propagation direction of the optical wave, the beam shape of the optical wave propagating through the normal ridge waveguide can be continuously changed to the beam shape of the optical wave propagating through the optical fiber (shape close to a substantially perfect circle). Since the beam shape of the light wave changes continuously, it is possible to prevent the occurrence of loss due to discontinuous and abrupt changes.

Further, by providing the groove portion 5 formed so that the beam shape of the optical wave becomes close to a substantially perfect circle (the state shown in fig. 5A) in the coupling portion coupled to the optical fiber that guides the optical wave from the optical waveguide element 1, it is possible to suppress the beam shape mismatch between the optical wave that is guided from the optical fiber and propagates through the optical waveguide of the optical waveguide element 1 and the optical wave that is guided from the optical fiber. As a result, the coupling loss at the coupling portion between the optical fiber and the optical waveguide element 1 can be reduced.

In addition, when the groove portion 5 is provided, since the light wave spreads so as to enter the groove portion 5, the position of the light wave (for example, the position where the light intensity is extremely large) is separated from the ridge portion 4 and approaches the groove portion 5. According to the first embodiment of the present invention, the position of the light wave can be moved to the groove portion 5 side by providing the groove portion 5. As a result, the region where the light waves overlap at the boundary between the waveguide layer 3 and the ridge portion 4 and the outside air can be reduced, and scattering caused by the boundary can be suppressed, thereby reducing the propagation loss of the light waveguide of the optical waveguide element 1.

Next, a configuration example of an optical modulator including the optical waveguide element 1 according to the first embodiment of the present invention will be described.

Fig. 6 is a plan view showing a first configuration example of an optical modulator including the optical waveguide element 1 according to the first embodiment of the present invention. In fig. 6, in order to express the shape of the groove portion 5 in the drawing, the waveguide layer 3 and the ridge portion 4 are made transparent, the optical waveguide formed directly below the ridge portion 4 is indicated by a line, and the material to be filled in the groove portion 5 is indicated by a grid.

The optical modulator 10A shown in fig. 6 is an optical modulator having a mach-zehnder type optical waveguide, and includes an optical waveguide element 1, and an input-side optical fiber 20 and an output-side optical fiber 30 connected to the optical waveguide element 1.

The input-side optical fiber 20 is an optical fiber for introducing light waves into the optical waveguide of the optical waveguide element 1. The output side optical fiber 30 is an optical fiber for guiding light waves from the optical waveguide of the optical waveguide element 1. In fig. 6, the propagation direction of the optical wave in the input-side optical fiber 20 and the output-side optical fiber 30 is shown by arrows.

The optical waveguide element 1 has a ridge portion 4 (not shown in fig. 6) to form a mach-zehnder type optical waveguide. The optical waveguide formed in the optical waveguide device 1 is composed of an input waveguide 11, two first branch waveguides 13, two parallel waveguides 15, two second branch waveguides 17, and an output waveguide 19.

The input waveguide 11 is connected to an input-side optical fiber 20 at an input-side end, and light waves are guided from the input-side optical fiber 20. The input waveguide 11 is branched into two first branch waveguides 13 at a branch portion 12 formed of an optical coupler or the like. The two first branch waveguides 13 are bent at the first bend portion 14, respectively, to become two parallel waveguides 15 parallel to each other. The two parallel waveguides 15 are bent at second bent portions 16 to become two second branch waveguides 17. The two second branch waveguides 17 are recombined in a combining section 18 constituted by an optical coupler or the like to become one output waveguide 19. The output waveguide 19 is connected to the output-side optical fiber 30 at the output-side end, and guides the light wave synthesized by the synthesizing section 18 to the output-side optical fiber 30.

In the mach-zehnder type optical waveguide, generally, the modulation sections 15a are provided in the two parallel waveguides 15, respectively. The modulation section 15a adjusts the phase of the optical wave by appropriately modulating the optical wave propagating through each of the two parallel waveguides 15. Note that, in order to modulate an optical wave, a modulation electrode including a signal electrode and a ground electrode, a transmission line for transmitting an electric signal to the modulation electrode, and the like are arranged on the waveguide layer 3, but in fig. 6, these components are not illustrated.

In the optical waveguide element 1 shown in fig. 6, a groove 5 is formed in a part of the input waveguide 11 (section S1 in fig. 6). The section S1 in which the groove portion 5 is formed is, for example, a section from the input-side end of the input waveguide 11 to which the input-side optical fiber 20 is connected to a halfway point of the input waveguide 11. The size of the section S1 is preferably a length to which propagation loss due to a rapid change in the beam shape does not occur, and is set to be, for example, about 500 μm or more, although it depends on the size of the waveguide layer 3, the ridge portion 4, and the like.

In the optical waveguide element 1 shown in fig. 6, the groove portion 5 is formed in a part of the output waveguide 19 (the section S2 in fig. 6). The section S2 in which the groove portion 5 is formed is, for example, a section from the middle of the output waveguide 19 to the output-side end of the output waveguide 19 connected to the output-side optical fiber 30. The size of the section S2 is preferably a length to which propagation loss due to a rapid change in the beam shape does not occur, and is set to be, for example, about 500 μm or more, although it depends on the size of the waveguide layer 3, the ridge portion 4, and the like.

On the other hand, the optical waveguide of the optical waveguide element 1 other than the section S1 and the section S2 may have any structure, for example, a structure of a normal ridge waveguide in which the groove portion 5 is not formed. In particular, the modulation section 15a can select a configuration with higher modulation efficiency of the optical wave, and thus the optical wave of the modulation section 15a can be modulated efficiently.

In the section S1, the groove 5 formed at the input-side end of the input waveguide 11 is formed such that the height dimension from the upper surface of the ridge 4 to the bottom surface of the groove 5 is substantially the same as the width dimension of the ridge 4, and the width dimension of the ridge 4 is substantially the same as the width dimension of the groove 5, as in the cross-sectional structure shown in fig. 4A. By forming the groove portion 5 in this way, the light wave introduced from the input-side optical fiber 20 is introduced into the input waveguide 11 of the optical waveguide element 1 while maintaining a substantially perfect circular beam shape. This can reduce the coupling loss at the coupling portion where the input-side optical fiber 20 and the optical waveguide element 1 are coupled.

In the section S1, the groove portion 5 is formed such that the width dimension thereof continuously decreases along the propagation direction of the optical wave, and the width dimension thereof is zero at a point in the input waveguide 11, so that the groove portion 5 disappears. By forming the groove portion 5 in this way, it is possible to prevent propagation loss from occurring in the input waveguide 11, and to change the light beam shape close to a substantially perfect circle to a light beam shape suitable for a ridge waveguide, and to smoothly guide a light wave to a normal ridge waveguide.

In the section S2, the groove portion 5 is formed such that the groove portion formation start position is located in the middle of the output waveguide 19, and the width dimension continuously increases from the start position in the propagation direction of the optical wave. By forming the groove portion 5 in this way, it is possible to prevent propagation loss from occurring in the output waveguide 19, and to change the beam shape suitable for the ridge waveguide to a beam shape close to a substantially perfect circle, and to smoothly guide a light wave from a normal ridge waveguide fiber.

In the section S2, the groove 5 formed at the output-side end of the output waveguide 19 is formed such that the height dimension from the upper surface of the ridge 4 to the bottom surface of the groove 5 is substantially the same as the width dimension of the ridge 4, and the width dimension of the ridge 4 is substantially the same as the width dimension of the groove 5, as shown in the cross-sectional structure of fig. 4A. By forming the groove portion 5 in this way, the light wave is guided from the output waveguide 19 to the output side optical fiber 30 in a state of being in a beam shape close to a substantially perfect circle. This can reduce the coupling loss at the coupling portion where the output side optical fiber 30 and the optical waveguide element 1 are coupled.

Fig. 7 is a plan view showing a second configuration example of an optical modulator including the optical waveguide element 1 according to the first embodiment of the present invention. In fig. 7, similarly to fig. 6, in order to express the shape of the groove portion 5 in the drawing, the waveguide layer 3 and the ridge portion 4 are made transparent, the optical waveguide formed directly below the ridge portion 4 is indicated by a line, and the material filled in the groove portion 5 is indicated by a grid.

In comparison with the optical modulator 10A shown in fig. 6, the optical modulator 10B shown in fig. 7 differs in the section in which the groove portion 5 is formed in the optical waveguide element 1. The groove portion 5 formed in the optical waveguide element 1 shown in fig. 7 will be described below.

In the optical waveguide element 1 shown in fig. 7, the groove 5 is formed in the input waveguide 11 (section S3 in fig. 7), the first branch waveguide 13 (section S4 in fig. 7), and a part of the parallel waveguide 15 (section S5 in fig. 7). The section S3 is a section including the entire input waveguide 11. The section S4 is a section including the entire first branch waveguide 13. The section S5 is a section from the first bend 14 to halfway the parallel waveguide 15. However, the section S5 does not overlap the modulator 15a, and the end of the section S5 is located between the first bending portion 14 and the modulator 15 a. The size of the section S5 is preferably a length to which propagation loss due to a rapid change in the beam shape does not occur, and is set to be, for example, about 500 μm or more, depending on the size of the waveguide layer 3, the ridge portion 4, and the like. In the section where the groove portion 5 is provided, the position of the light wave can be moved toward the groove portion 5. As a result, the region where the light waves overlap at the boundary between the waveguide layer 3 and the ridge portion 4 and the outside air can be reduced, scattering caused by the boundary can be suppressed, and the propagation loss of the light waveguide element 1 can be reduced.

In the optical waveguide element 1 shown in fig. 7, the groove portion 5 is formed in a part of the parallel waveguide 15 (the section S6 in fig. 7), the second branch waveguide 17 (the section S7 in fig. 7), and the output waveguide 19 (the section S8 in fig. 7). The section S6 is a section from the middle of the modulation section 15a to the second bending section 16. However, the section S6 does not overlap the modulator 15a, and the start end of the section S6 is located between the modulator 15a and the second bend 16. The size of the section S6 is preferably a length to which propagation loss due to a rapid change in the beam shape does not occur, and is set to be, for example, about 500 μm or more, depending on the size of the waveguide layer 3, the ridge portion 4, and the like. The section S7 is a section including the entirety of the second branch waveguide 17. The section S8 is a section including the entire output waveguide 19. In the section where the groove portion 5 is provided, the position of the light wave can be moved toward the groove portion 5. As a result, the region where the light waves overlap at the boundary between the waveguide layer 3 and the ridge portion 4 and the outside air can be reduced, scattering caused by the boundary can be suppressed, and the propagation loss of the light waveguide element 1 can be reduced.

On the other hand, the optical waveguide of the optical waveguide element 1 other than the sections S3 to S8 may have any structure such as a structure of a normal ridge waveguide in which the groove portion 5 is not formed. In particular, the modulation section 15a can select a configuration with higher modulation efficiency of the optical wave, and thus the optical wave of the modulation section 15a can be modulated efficiently.

In the section S3, the groove 5 formed at the input-side end of the input waveguide 11 is formed such that the height dimension from the upper surface of the ridge 4 to the bottom surface of the groove 5 is substantially the same as the width dimension of the ridge 4, and the width dimension of the ridge 4 is substantially the same as the width dimension of the groove 5, as shown in the cross-sectional structure of fig. 4A. By forming the groove portion 5 in this way, the light wave introduced from the input-side optical fiber 20 is introduced into the input waveguide 11 of the optical waveguide element 1 while maintaining a substantially perfect circular beam shape. This can reduce the coupling loss at the coupling portion between the input-side optical fiber 20 and the optical waveguide element 1.

In the section S3, the groove portion 5 is formed to have the same dimension in the width direction as that formed at the input-side end portion along the propagation direction of the optical wave. That is, in the section S3, the width-directional dimension of the groove portion 5 is not changed and is kept constant. By forming the groove portion 5 in this way, it is possible to propagate the optical wave while maintaining the shape of the optical beam close to a substantially perfect circle in the input waveguide 11, and it is possible to prevent the occurrence of propagation loss.

In the section S4, the groove portion 5 is formed to have the same width-directional dimension as that of the section S3, without changing the width-directional dimension along the propagation direction of the optical wave. By forming the groove portion 5 in this way, the optical wave can be propagated in the first branch waveguide 13 while maintaining the beam shape close to a substantially perfect circle, and the occurrence of propagation loss can be prevented.

In the section S5, the groove portion 5 formed in the first bend 14 that becomes the starting end of the section S5 is formed such that the dimension in the height direction from the upper surface of the ridge portion 4 to the bottom surface of the groove portion 5 is substantially the same as the dimension in the width direction of the ridge portion 4, and the dimension in the width direction of the ridge portion 4 is substantially the same as the dimension in the width direction of the groove portion 5, as in the cross-sectional structure shown in fig. 4A. By forming the groove portion 5 in this way, the optical wave propagating through the first branch waveguide 13 is introduced into the parallel waveguide 15 while maintaining a substantially perfect circular beam shape. This can reduce the loss when the optical wave is introduced from the first branch waveguide 13 (section S4) to the parallel waveguide 15 (section S5).

In the section S5, the groove portion 5 is formed so that the dimension in the width direction decreases continuously along the propagation direction of the optical wave, and the groove portion 5 disappears when the dimension in the width direction is zero in the middle of the parallel waveguide 15 (before the modulation portion 15 a). By forming the groove portion 5 in this way, it is possible to prevent propagation loss from occurring in the parallel waveguide 15, and to change the beam shape close to a substantially perfect circle to a beam shape suitable for the ridge waveguide, and to smoothly guide the optical wave to the normal ridge waveguide.

In the section S6, the groove portion 5 is formed such that the middle of the parallel waveguide 15 (the rear stage of the modulation portion 15 a) is the start position of the groove portion formation, and the dimension in the width direction continuously increases from the start position along the propagation direction of the optical wave. By forming the groove portion 5 in this way, it is possible to prevent propagation loss from occurring in the parallel waveguide 15, change the beam shape suitable for the ridge waveguide to a beam shape close to a substantially perfect circle, and smoothly guide the light wave from the normal ridge waveguide fiber.

In the section S6, the groove portion 5 formed in the second bend portion 16 that is the terminal end of the section S6 is formed such that the dimension in the height direction from the upper surface of the ridge portion 4 to the bottom surface of the groove portion 5 is substantially the same as the dimension in the width direction of the ridge portion 4, and the dimension in the width direction of the ridge portion 4 is substantially the same as the dimension in the width direction of the groove portion 5, as in the cross-sectional structure shown in fig. 4A. By forming the groove portion 5 in this way, the optical wave propagating through the parallel waveguide 15 is introduced into the second branch waveguide 17 while maintaining a substantially perfect circular beam shape. This can reduce the loss when the optical wave is guided from the parallel waveguide 15 (section S6) to the second branch waveguide 17 (section S7).

In the section S7, the groove portion 5 is formed to have the same dimension in the width direction along the propagation direction of the optical wave as the dimension formed at the terminal end of the section S6. That is, in the section S7, the width-directional dimension of the groove portion 5 is not changed and is kept constant. By forming the groove portion 5 in this way, the optical wave can be propagated in the second branch waveguide 17 while maintaining the beam shape close to a substantially perfect circle, and the occurrence of propagation loss can be prevented.

In the section S8, the groove portion 5 is similarly formed to have the same width-directional dimension as that of the section S7 without changing the width-directional dimension along the propagation direction of the optical wave. By forming the groove portion 5 in this way, it is possible to propagate the optical wave while maintaining the beam shape close to a substantially perfect circle in the output waveguide 19, and it is possible to prevent the occurrence of propagation loss. The light wave is then guided from the output waveguide 19 to the output side optical fiber 30 in a state of being in a substantially perfect circular beam shape. This can reduce the coupling loss at the coupling portion where the output side optical fiber 30 and the optical waveguide element 1 are coupled.

In the optical modulator 10A shown in fig. 6, a section S1 in which the groove 5 has a wedge shape is provided in the input branch path 15, and a section S2 in which the groove 5 has an inverted wedge shape (trumpet shape) is provided in the output branch path 19. In the optical modulator 10B shown in fig. 7, a section S5 in which the groove 5 has a wedge shape and a section S6 in which the groove 5 has an inverted wedge shape (trumpet shape) are provided in the parallel branch path 15. However, the section in which the slot portion 5 has the wedge shape or the inverted wedge shape may be provided at any position not overlapping with the modulation portion 15, and may be provided in the first branch waveguide 13 or the second branch waveguide 17, for example.

Hereinafter, a manufacturing process of the optical waveguide element 1 according to the first embodiment of the present invention will be described.

A first example of the manufacturing process of the optical waveguide element 1 will be described with reference to fig. 8A to 8C. Fig. 8A to 8C illustrate an example in which the dimension in the width direction of the groove portion 5 is substantially the same as the dimension in the width direction of the ridge portion 4, but the dimension in the width direction of the groove portion 5 is appropriately set in the optical waveguide in accordance with a desired beam shape.

In the first step, the groove portion 5 is formed by removing a part of the support substrate 2 by, for example, dry etching or the like. Fig. 8A shows a state after the first step.

In the second step, a waveguide layer 3 made of a material having an electro-optical effect, such as LN, is crystal-grown on the support substrate 2 in which the trench portions 5 are formed, for example, by epitaxial growth or the like. Fig. 8B shows a state after the second step.

In the third step, the ridge portion 4 is formed in the waveguide layer 3 by removing a portion other than the ridge portion 4 by, for example, dry etching or the like. Fig. 8C shows a state after the third step.

The manufacturing process of the optical waveguide element 1 described with reference to fig. 8A to 8C is suitable when the material for filling the groove portion 5 is the same as the material (for example, LN) of the waveguide layer 3. In the second step, the filling of the material into the trench portion 5 and the growth of the waveguide layer 3 can be performed simultaneously, and the optical waveguide element 1 according to the first embodiment of the present invention can be manufactured with a small number of steps.

A second example of the manufacturing process of the optical waveguide element 1 will be described with reference to fig. 9A to 9D. Fig. 9A to 9D illustrate an example in which the dimension in the width direction of the groove portion 5 is substantially the same as the dimension in the width direction of the ridge portion 4, and the dimension in the width direction of the groove portion 5 is appropriately set in the optical waveguide according to a desired beam shape.

In the first step, the groove portion 5 is formed by removing a part of the support substrate 2 by, for example, dry etching or the like. Fig. 9A shows a state after the first step.

In the second step, the groove portion 5 is filled with a desired material. In addition, by filling the groove portions 5 with a desired material and then polishing and smoothing the surface of the support substrate 2, it is easy to attach the substrate serving as the base of the waveguide layer 3 to the support substrate 2 in the third step to be described later. Fig. 9B shows a state after the second step.

In the third step, a substrate serving as a base of the waveguide layer 3 is attached to the support substrate 2 in which a desired material is embedded in the groove portion 5. Fig. 9C shows a state after the third step.

In the fourth step, the ridge portion 4 is formed in the waveguide layer 3 by removing a portion other than the ridge portion 4 by, for example, dry etching or the like. Fig. 9D shows a state after the fourth step.

The manufacturing process of the optical waveguide element 1 described with reference to fig. 9A to 9D is suitable when the material for filling the groove portion 5 is appropriately selected. As a material to be filled into the trench portion 5 in the second step, for example, the same material as the waveguide layer 3 (for example, LN, etc.) may be selected, or SiN, a resin, or the like may be selected. In addition, the waveguide layer 3 can be laminated on the support substrate 2 by the bonding of the substrates in the third step, and the optical waveguide element 1 according to the first embodiment of the present invention can be manufactured by a simple process.

[ second embodiment ]

An optical waveguide element according to a second embodiment of the present invention will be described.

In the optical waveguide element 1 according to the first embodiment of the present invention, the dimension of the groove portion 5 in the height direction is kept constant along the extending direction of the ridge portion 4, and the cross-sectional area of the groove is set to be continuously varied by continuously varying the dimension of the groove portion 5 in the width direction. On the other hand, as shown in fig. 10, in the optical waveguide element 41 according to the second embodiment of the present invention, the dimension of the groove portion 5 in the width direction is kept constant along the extending direction of the ridge portion 4, and the dimension of the groove portion 5 in the height direction is continuously changed to set the sectional area of the groove to be continuously changed.

The optical waveguide element 41 according to the second embodiment of the present invention has the same configuration as the optical waveguide element 1 according to the first embodiment of the present invention, except that the dimension of the groove portion 5 in the width direction is kept constant and the dimension of the groove portion 5 in the height direction is continuously changed. Here, the same structure will not be described.

Fig. 10 is a perspective view schematically showing the vicinity of the optical waveguide element 41 according to the second embodiment of the present invention. In the drawings, the width direction of the optical waveguide element 41 is defined as an X axis, the length direction of the optical waveguide element 41 is defined as a Y axis, and the height direction of the optical waveguide element 41 is defined as a Z axis. In fig. 10, similarly to fig. 1, in order to express the shape of the groove portion 5 in the drawing, the support substrate 2, the waveguide layer 3, and the ridge portion 4 are made transparent by showing only outlines, and the material filling the groove portion 5 is shown by a grid.

In the front side of the perspective view of fig. 10, the height dimension of the groove portion 5 is set to be large, the height dimension of the groove portion 5 gradually decreases as it goes to the back side of the drawing, and the height dimension of the groove portion 5 is set to be zero at the innermost side of the drawing. The dimension of the groove portion 5 in the height direction being zero means that the groove portion 5 is not formed or the groove portion 5 disappears.

Fig. 11A to 11C are three views of the structure in the vicinity of the optical waveguide element 41 shown in fig. 10. Fig. 11A is a plan view of the structure in the vicinity of the optical waveguide element 41 shown in fig. 10, fig. 11B is a front view of the structure in the vicinity of the optical waveguide element 41 shown in fig. 10, and fig. 11C is a side view of the structure in the vicinity of the optical waveguide element 41 shown in fig. 10. In fig. 11A to 11C, similarly to fig. 10, in order to express the shape of the groove portion 5 in the drawing, the support substrate 2, the waveguide layer 3, and the ridge portion 4 are made transparent by showing only outlines, and the material filling the groove portion 5 is shown by a grid.

The state in which the height direction dimension of the groove portion 5 continuously changes along the extending direction of the ridge portion 4 is particularly clearly shown in the side view of fig. 11C. The dimension of the groove portion 5 in the height direction is set to be gradually reduced along the Y-axis direction in fig. 11A, and the groove portion 5 has a tapered shape whose tip is tapered along the Y-axis direction in the plan view of fig. 11C. The dimension of the groove portion 5 in the Y axis direction (the dimension in the longitudinal direction of the wedge shape) is preferably a length to such an extent that propagation loss due to a rapid change in the beam shape does not occur, and is set to 500 μm or more, for example.

Further, with reference to the cross-sectional views shown in fig. 12A to 12C, a case where the groove portion 5 has a wedge shape in the extending direction of the ridge portion 4 will be described.

Fig. 12A is a D-D sectional view in the side view of fig. 11C, showing a sectional structure of the front side in the perspective view of fig. 10. In the cross-sectional view of fig. 12A taken along the line D-D, the dimension in the height direction (height h3 in fig. 2) from the upper surface of the ridge portion 4 to the bottom surface of the groove portion 5 is substantially the same as the dimension in the width direction (width w1 in fig. 2) of the ridge portion 4. The height dimension (depth h2 in fig. 2) of the groove portion 5 is substantially the same as the height dimension (height h1 in fig. 2) of the ridge portion 4, and the width dimension (width w2 in fig. 2) of the groove portion 5 is substantially the same as the width dimension (width w1 in fig. 2) of the ridge portion 4. That is, the cross-sectional shape of the ridge portion 4 protruding from the waveguide layer 3 and the cross-sectional shape of the groove portion 5 dug in the support substrate 2 are symmetrical with respect to the waveguide layer 3.

Fig. 12B is a cross-sectional view E-E in the side view of fig. 11C, and the perspective view of fig. 10 shows a cross-sectional structure of a substantially central portion. In the perspective view of fig. 10, the dimension of the groove portion 5 in the height direction continuously decreases as it goes from the front side to the back side in the Y-axis direction. In the cross-sectional structure of the substantially central portion shown in the cross-sectional view E-E of fig. 12B, the height dimension (depth h2 in fig. 2) of the groove portion 5 is about half of the height dimension (height h1 in fig. 2) of the ridge portion 4.

Fig. 12C is a sectional view F-F in the side view of fig. 11C, and the perspective view of fig. 10 shows the innermost sectional structure. As the Y-axis direction advances, the height dimension of the groove portion 5 further decreases, and the groove portion 5 disappears in the innermost cross-sectional structure shown in the F-F cross-sectional view of fig. 12C, because the height dimension of the groove portion 5 (the depth h2 in fig. 2) is zero. The sectional structure shown in the sectional F-F view of fig. 12C is the same as that of a typical ridge waveguide.

Fig. 13A to 13C are diagrams showing the results of electric field intensity simulations of the optical waveguide element 41 according to the second embodiment of the present invention. Fig. 13A is a diagram showing a light intensity distribution of the cross-sectional structure of fig. 12A. Fig. 13B is a diagram showing a light intensity distribution of the cross-sectional structure of fig. 12B. Fig. 13C is a diagram showing a light intensity distribution of the cross-sectional structure of fig. 12C. In fig. 13A to 13C, the light intensity distribution is represented by a shade, and the thicker the concentration is, the higher the light intensity is, and the thinner the concentration is, the weaker the light intensity is.

In the cross-sectional structure shown in fig. 13A, the dimension in the height direction from the upper surface of the ridge portion 4 to the bottom surface of the groove portion 5 is substantially the same as the dimension in the width direction of the ridge portion 4, and the dimension in the width direction of the ridge portion 4 is substantially the same as the dimension in the width direction of the groove portion 5. That is, the region surrounded by the side surfaces of the ridge portion 4 and the side surfaces of the groove portion 5 has an aspect ratio of 1: 1 is substantially square. As a result, the beam shape of the light wave propagating through the optical waveguide becomes a nearly perfect circle shape similar to the light wave propagating through the optical fiber, as shown in fig. 13A.

In the cross-sectional structure shown in fig. 13B, the dimension of the groove portion 5 in the height direction is reduced as compared with fig. 13A. This reduces the area of the light wave propagating through the optical waveguide that spreads to the groove portion 5 and increases the area that spreads to the ridge portion 4. The beam shape of the light wave is slightly deformed as shown in fig. 13B.

In the cross-sectional structure shown in fig. 13C, the groove portion 5 disappears and has the same shape as a normal ridge waveguide. Therefore, as shown in fig. 13C, the beam shape of the light wave is deformed so as to enter the ridge portion 4.

When the light wave propagates in the positive direction of the Y axis of fig. 10, the beam shape of the light wave is gradually deformed (the state shown in fig. 13B) from a shape close to a substantially perfect circle (the state shown in fig. 13A), and continuously changes to the same beam shape as the ridge waveguide (the state shown in fig. 13C). Since the dimension of the groove portion 5 in the height direction continuously changes, the beam shape shown in fig. 13A to 13C also continuously changes.

In the second embodiment of the present invention, by providing the groove portions 5 such that the dimension of the groove portions 5 in the height direction thereof continuously decreases with respect to the propagation direction of the optical wave, the beam shape of the optical wave propagating through the optical fiber (shape close to a substantially perfect circle) can be continuously changed to the beam shape of the optical wave propagating through a normal ridge waveguide. Since the beam shape of the light wave changes continuously, it is possible to prevent the occurrence of loss due to discontinuous and abrupt changes.

Further, by providing the groove portion 5 formed so that the beam shape of the optical wave becomes close to a substantially perfect circle (the state shown in fig. 13A) in the coupling portion coupled to the optical fiber that introduces the optical wave into the optical waveguide element 41, it is possible to suppress the mismatch of the beam shape between the optical wave introduced from the optical fiber and propagating through the optical waveguide of the optical waveguide element 41. As a result, the coupling loss at the coupling portion where the optical fiber and the optical waveguide element 41 are coupled can be reduced.

On the other hand, when the light wave propagates in the negative direction of the Y axis of fig. 10, the beam shape of the light wave gradually changes from the same beam shape as the ridge waveguide (the state shown in fig. 13C) to a shape with little distortion (the state shown in fig. 13B), and continuously changes to a shape close to a substantially perfect circle (the state shown in fig. 13A). Since the dimension of the groove portion 5 in the height direction continuously changes, the beam shape shown in fig. 13A to 13C also continuously changes.

In the second embodiment of the present invention, by providing the groove portion 5 such that the dimension of the groove portion 5 in the height direction continuously increases with respect to the propagation direction of the optical wave, the beam shape of the optical wave propagating through the normal ridge waveguide can be continuously changed to the beam shape of the optical wave propagating through the optical fiber (shape close to a substantially perfect circle). Since the beam shape of the light wave changes continuously, it is possible to prevent the occurrence of loss due to discontinuous and abrupt changes.

Further, by providing the groove portion 5 formed so that the beam shape of the optical wave becomes close to a substantially perfect circle (the state shown in fig. 13A) in the coupling portion coupled to the optical fiber from which the optical wave is guided out of the optical waveguide element 41, it is possible to suppress the beam shape mismatch between the optical wave guided into the optical waveguide of the optical waveguide element 41 from the optical fiber and the optical wave guided out of the optical fiber. As a result, the coupling loss at the coupling portion where the optical fiber and the optical waveguide element 41 are coupled can be reduced.

In addition, in the case where the groove portion 5 is provided, since the light wave spreads so as to enter the groove portion 5, the position of the light wave (for example, the position where the light intensity becomes extremely large) is separated from the ridge portion 4 and approaches the groove portion 5. According to the second embodiment of the present invention, as in the first embodiment of the present invention, the position of the light wave can be moved toward the groove portion 5 by providing the groove portion 5. As a result, the region where the light wave overlaps at the boundary between the waveguide layer 3 and the ridge portion 4 and the outside air can be reduced, scattering caused by the boundary can be suppressed, and the propagation loss of the light waveguide element 41 can be reduced.

The optical waveguide element 41 according to the second embodiment of the present invention can realize an optical modulator similar to the optical modulator 10A (see fig. 6) and the optical modulator 10B (see fig. 7) according to the first embodiment of the present invention. However, in the second embodiment of the present invention, the groove portion 5 is formed so that the dimension in the height direction continuously changes.

In the optical modulator according to the second embodiment of the present invention, the groove portions 5 are formed so that the dimension in the height direction is continuously decreased with respect to the propagation direction of the optical wave in the section S1 of the optical waveguide included in the optical modulator 10A shown in fig. 6, and the groove portions 5 are formed so that the dimension in the height direction is continuously increased with respect to the propagation direction of the optical wave in the section S2.

In the optical modulator according to the second embodiment of the present invention, the groove portions 5 are formed so that the dimension in the height direction is continuously decreased with respect to the propagation direction of the optical wave in the section S5 of the optical waveguide included in the optical modulator 10B shown in fig. 7, and the groove portions 5 are formed so that the dimension in the height direction is continuously increased with respect to the propagation direction of the optical wave in the section S6.

The optical waveguide element 41 according to the second embodiment of the present invention can be manufactured by the same steps as those described in the first embodiment of the present invention (see fig. 8A to 8C and fig. 9A to 9D). However, in the second embodiment of the present invention, the dimension of the groove portion 5 in the height direction is appropriately set in the optical waveguide according to a desired beam shape.

In the present specification, the first and second embodiments of the present invention have been described separately, but the first and second embodiments may be combined. For example, the groove portion 5 may be formed so that both the height direction dimension and the width direction dimension continuously change along the extending direction of the ridge portion 4.

The present invention is not limited to the above-described embodiments and modifications, and various modifications, design changes, and the like within the scope not departing from the technical spirit of the present invention are included in the technical scope of the present invention.

Industrial applicability

The invention provides an optical waveguide element which can reduce coupling loss of a coupling part coupled with an optical fiber and propagation loss of an optical waveguide, and is applicable to the fields of optical communication, optical measurement and the like.

Description of the reference symbols

1. 41 optical waveguide element

2 supporting substrate

3 waveguide layer

4 ridges

5 groove part

10A, 10B optical modulator

11 input waveguide

12 branch part

13 first branch waveguide

14 first bend

15 parallel waveguide

16 second bend

17 second branch waveguide

18 synthesis part

19 output waveguide

20 input side optical fiber

30 output side optical fiber