WO2024203416A1

WO2024203416A1 - Optical waveguide and optical waveguide module

Info

Publication number: WO2024203416A1
Application number: PCT/JP2024/010135
Authority: WO
Inventors: 彩音東條; 知幸赤星
Original assignee: 京セラ株式会社
Priority date: 2023-03-24
Filing date: 2024-03-14
Publication date: 2024-10-03

Abstract

This optical waveguide is optically coupled to another optical waveguide including a first core having a first refractive index, and comprises a core and a cladding. The core has a refractive index different from the first refractive index. The cladding is disposed so as to cover at least part of the periphery of the core. The core is provided with an end surface and a side surface, has an elongated shape, and has, in order, a coupling part and an extension part adjacent to the coupling part along the longitudinal direction. The coupling part has an optical coupling surface located on the side surface, exposed from the cladding, and optically coupled to the first core. In the extension part, when two cross sections located with a predetermined space in the longitudinal direction therebetween are defined as a first cross section and a second cross section, in order from the one closer to the coupling part, a second area that is the area of the second cross section is larger than a first area that is the area of the first cross section.

Description

Optical waveguide and optical waveguide module

The disclosed embodiments relate to optical waveguides and optical waveguide modules.

　Conventionally, optical waveguide modules have been known in which two optical waveguides with different refractive indices are optically coupled to each other. The two optical waveguides that are optically coupled are, for example, a silicon optical waveguide and a polymer optical waveguide that has a smaller refractive index than the silicon optical waveguide.

The polymer optical waveguide is composed of a core made of, for example, a polymer material and a cladding that surrounds the core. A portion of the upper surface of the core of the polymer optical waveguide is exposed from the cladding and forms an optical coupling surface that is optically coupled with the tapered core of the silicon optical waveguide. The core of the polymer optical waveguide extends within the cladding in a direction away from the silicon optical waveguide to the end face of the cladding.

JP 2002-122750 A

The optical waveguide according to one aspect of the embodiment is an optical waveguide that is optically coupled to another optical waveguide that includes a first core having a first refractive index, and has a core and a clad. The core has a refractive index different from the first refractive index. The clad is arranged to cover at least a portion of the periphery of the core. The core has end faces and side faces, is elongated, and has, in order along the longitudinal direction, a coupling portion and an extension portion adjacent to the coupling portion. The coupling portion has an optical coupling surface that is located on the side face and is exposed from the clad and is optically coupled to the first core. When two cross sections located at a predetermined distance in the longitudinal direction in the extension portion are referred to as a first cross section and a second cross section in order from the one closest to the coupling portion, the second area that is the area of the second cross section is larger than the first area that is the area of the first cross section.

FIG. 1 is a schematic perspective view of an optical waveguide module according to a first embodiment. FIG. 2 is a plan view of the optical waveguide module according to the first embodiment as viewed from the positive direction of the Z axis. FIG. 3 is a cross-sectional view taken along line III-III in FIG. FIG. 4 is a side view of the optical waveguide module according to the first embodiment, as viewed from the negative direction of the X-axis. FIG. 5 is a diagram showing an example of the relationship between the optical coupling loss in the second optical waveguide and the size of the core cross section. FIG. 6 is a diagram showing an example of the relationship between the propagation loss of light in the second optical waveguide and the size of the core cross section. FIG. 7 is a diagram showing the effect of the optical waveguide module according to the first embodiment (simulation results of optical loss according to changes in cross-sectional size). FIG. 8 is a plan view of the optical waveguide module according to the second embodiment as viewed from the positive direction of the Z axis. FIG. 9 is a cross-sectional view taken along line IX-IX in FIG. FIG. 10 is a plan view of the optical waveguide module according to the third embodiment as viewed from the positive direction of the Z axis. FIG. 11 is a cross-sectional view taken along line XI-XI of FIG.

Below, with reference to the attached drawings, an embodiment of the optical waveguide and optical waveguide module disclosed in the present application will be described. Note that the present disclosure is not limited to the embodiments described below. Furthermore, each embodiment can be appropriately combined to the extent that there is no contradiction in the processing content. Furthermore, the same parts in each of the following embodiments are given the same reference numerals, and duplicate explanations will be omitted.

In addition, in the embodiments described below, expressions such as "constant," "orthogonal," "vertical," and "parallel" may be used, but these expressions do not necessarily mean "constant," "orthogonal," "vertical," or "parallel" in the strict sense. In other words, each of the above expressions allows for deviations due to, for example, manufacturing precision, installation precision, and the like.

The figures referenced below are schematic for the sake of convenience. Therefore, details may be omitted and the dimensional proportions may not necessarily correspond to the actual ones.

In addition, in order to make the explanation easier to understand, the drawings referenced below may show an orthogonal coordinate system that defines the X-axis, Y-axis, and Z-axis directions that are mutually perpendicular, with the positive Z-axis direction being the vertically upward direction.

First Embodiment
First, the configuration of an optical waveguide module 1 according to the first embodiment will be described with reference to Fig. 1. Fig. 1 is a schematic perspective view of the optical waveguide module 1 according to the first embodiment.

As shown in FIG. 1, the optical waveguide module 1 has a first optical waveguide 2 and a second optical waveguide 3. The first optical waveguide 2 and the second optical waveguide 3 are joined by a joining material 4 made of, for example, a photocurable resin.

The first optical waveguide 2 has a substrate 21 and a first core 22.

The substrate 21 is a rectangular parallelepiped substrate having a thickness in the vertical direction (Z-axis direction). The substrate 21 is made of, for example, ceramics. The ceramics constituting the substrate 21 are, for example, _sintered bodies containing aluminum nitride (AlN), aluminum _oxide ( _Al2O3 , alumina), silicon carbide (SiC), silicon nitride ( _Si3N4 ), or the like as main components.

The first core 22 is located on the underside of the substrate 21 and extends in one direction (the X-axis direction) along the underside of the substrate 21. The first core 22 has a tapered shape (see FIG. 2) that narrows toward the tip. The first core 22 is a passage for transmitting light, and is made of a material that contains, for example, silicon.

The second optical waveguide 3 is an optical waveguide that is optically coupled to the first optical waveguide 2, and has a second core 31 and a cladding 32.

The second core 31 is located at a position where a part of it overlaps with the first core 22 of the first optical waveguide 2 in a plan view seen from the top-bottom direction (Z-axis direction). One end face of the second core 31 is exposed at the end face (the end face on the positive X-axis side) of the cladding 32 located opposite the first core 22, forming a coupling surface that can be coupled to an optical fiber (not shown). The second core 31 is a passage for transmitting light, and is made of a material with a refractive index different from that of the first core 22. The material that makes up the second core 31 has a lower refractive index than the material that makes up the first core 22, for example. The material that makes up the second core 31 can be, for example, a polymer with a lower refractive index than silicon.

The cladding 32 is positioned so as to surround the second core 31. The cladding 32 is made of a material with a different refractive index than the second core 31, and totally reflects the light inside the second core 31 at the boundary surface with the second core 31.

Next, the details of the configuration of the second core 31 will be described with reference to Figs. 2 to 4. Fig. 2 is a plan view of the optical waveguide module 1 according to the first embodiment, as viewed from the positive direction of the Z axis. Fig. 3 is a cross-sectional view taken along line III-III in Fig. 2. Fig. 4 is a side view of the optical waveguide module 1 according to the first embodiment, as viewed from the negative direction of the X axis.

As shown in Figures 2 to 4, the second core 31 has a connecting portion 311 and an extension portion 312.

The coupling portion 311 has an exposed surface 311a exposed from the clad 32 at a cutout portion 32a formed by cutting out a portion of the upper surface of the clad 32. The exposed surface 311a is overlapped and joined to the first core 22 of the first optical waveguide 2, thereby forming an optical coupling surface that is optically coupled to the first core 22.

The extension section 312 extends from the side surface adjacent to the exposed surface 311a of the coupling section 311 inside the cladding 32 to the end face (the end face on the positive X-axis direction) of the cladding 32 located on the opposite side to the coupling section 311, and propagates light between the coupling section 311 and the end face of the cladding 32. A portion of the extension section 312 has a cross-sectional size perpendicular to the longitudinal direction (X-axis direction) larger than that of the coupling section 311. Here, the cross-sectional size perpendicular to the longitudinal direction (X-axis direction) is a parameter that defines the cross section, and is at least one parameter selected from a group of parameters including thickness and width (diameter). In the examples of Figures 2 to 4, a portion of the extension section 312 has a thickness and width larger than that of the coupling section 311.

Regarding the extension portion 312, in other words, first, the second core 31 has an elongated shape. Here, elongated does not only refer to a thin, so-called band-like shape, but also includes any shape that is elongated and has a predetermined thickness. The shape includes a rectangular shape, including one with a square cross-sectional shape. Furthermore, the shape may include portions with different cross-sectional areas at multiple locations in the longitudinal direction of the second core 31. In this case, the joint portion 311 and the extension portion 312 that constitute the second core 31 are adjacent to each other in the longitudinal direction of the second core 31. The surface along the longitudinal direction of the extension portion 312 is called the side surface 312cc.

Here, the extension portion 312 will be further explained using the symbols shown in Figures 2 and 3. Consider the two planes that intersect with the longitudinal direction of the extension portion 312. In other words, the two intersecting planes are two cross sections perpendicular to the longitudinal direction of the second core 31. The two cross sections are planes that intersect with the side surface 312cc. In particular, the two cross sections are planes parallel to the end surface of the extension portion 312. Note that, as shown in Figures 2 and 3, even if the shape of the extension portion 312 is such that the diameter (width) changes in the longitudinal direction with respect to the side surface 312cc, the two cross sections are planes parallel to the end surface of the extension portion 312.

The two cross sections are located at a predetermined distance in the longitudinal direction of the extension portion 312. In Figures 2 and 3, the two cross sections are first cross section 312aa and second cross section 312bb. In other words, when the cross section of the extension portion 312 located at a predetermined distance in the longitudinal direction on the joining portion 311 side is represented as first cross section 312aa, and the cross section located farther from the joining portion 311 than the first cross section 312aa is represented as second cross section 312bb, the area of second cross section 312bb is larger than the area of first cross section 312aa.

The areas of the first cross section 312aa and the second cross section 312bb in the extension portion 312 can be determined, for example, by analyzing an image of the cross section taken by an electron microscope. The photographs used for image analysis may be general scanning electron microscope photographs (SEM photographs), or may be photographs of reflected electron images obtained by the scanning electron microscope. Furthermore, TOF-SIMS (time-of-flight secondary ion mass spectrometry) may be used to distinguish the second core 31 and the cladding 32, including the first core 22.

Furthermore, the direction along the plane of the first cross section 312aa and the second cross section 312bb (in other words, the direction of expansion within the plane) is perpendicular to the side surface 312cc (of the second portion 312b).

Incidentally, in the second optical waveguide 3 that is optically coupled to the first optical waveguide 2, the optimal core cross-sectional size for reducing optical coupling loss is different from the optimal core cross-sectional size for reducing optical propagation loss.

Figure 5 is a diagram showing an example of the relationship between the optical coupling loss in the second optical waveguide 3 and the size of the core cross section. Note that in Figure 5, the "core cross section size" on the horizontal axis refers to the size of the cross section perpendicular to the longitudinal direction (X-axis direction) of the coupling part 311 of the second core 31, and refers to the size of the cross section when the thickness and width of the coupling part 311 are equal.

As shown in FIG. 5, the smaller the core cross-sectional size of the second optical waveguide 3, the lower the optical coupling loss in the second optical waveguide 3. This is thought to be because the smaller the core cross-sectional size of the second optical waveguide 3, the wider the mode field diameter of the light, facilitating the movement of light between the coupling portion 311 of the first core 22 and the second core 31. Therefore, from the viewpoint of coupling loss, it is desirable for the core cross-sectional size of the second optical waveguide 3 to be relatively small.

FIG. 6 is a diagram showing an example of the relationship between the propagation loss of light in the second optical waveguide 3 and the size of the core cross section. In FIG. 6, the "core cross section size" on the horizontal axis refers to the size of a cross section perpendicular to the longitudinal direction (X-axis direction) of the extension portion 312 of the second core 31, and indicates the size of the cross section when the thickness and width of the extension portion 312 are equal. In the example of FIG. 6, the surface roughness Rq of the second core 31 is 0.6 μm.

As shown in Figure 6, the propagation loss of light in the second optical waveguide 3 increases as the core cross-sectional size of the second optical waveguide 3 becomes smaller. This is thought to be because the smaller the core cross-sectional size of the second optical waveguide 3, the greater the effect of the surface roughness Rq of the outer core surface, causing scattering of light on the outer core surface. Therefore, from the perspective of propagation loss, it is clear that it is desirable for the core cross-sectional size of the second optical waveguide 3 to be relatively large.

Here, the polymer optical waveguide that is optically coupled to the silicon optical waveguide generally has a uniform core cross-sectional size along the longitudinal direction. For this reason, in a typical polymer optical waveguide, if the core cross-sectional size is set to a relatively small size from the perspective of coupling loss, there is a risk of increased propagation loss.

In contrast, in the second optical waveguide 3 according to this embodiment, the cross-sectional size of the coupling portion 311 in the second core 31 is relatively small, and the cross-sectional size of the extension portion 312 is relatively large. By making the cross-sectional size of the coupling portion 311 in the second core 31 relatively small, it is possible to reduce coupling loss compared to when the core cross-sectional size of the second optical waveguide 3 is made uniform in the longitudinal direction. In addition, by making the cross-sectional size of the extension portion 312 in the second core 31 relatively large, it is possible to reduce propagation loss compared to when the core cross-sectional size of the second optical waveguide 3 is made uniform in the longitudinal direction.

Returning to the explanation of Figures 2 to 4, the extension portion 312 has a first portion 312a and a second portion 312b. The first portion 312a is connected to the connecting portion 311. The cross-sectional size (area) of the first portion 312a increases with increasing distance from the connecting portion 311. Specifically, the cross-sectional size (area) of the first portion 312a gradually increases with increasing distance from the connecting portion 311. The second portion 312b is connected to the first portion 312a, and has a constant cross-sectional size.

In this way, by providing the first portion 312a in the extension portion 312, light can be transferred stably from the joint portion 311 to the extension portion 312, and scattering of light in the extension portion 312 can be reduced. Furthermore, by connecting the second portion 312b to the first portion 312a, light propagation in the extension portion 312 can be made smoother, and scattering of light in the extension portion 312 can be reduced.

Note that, although an example has been shown here in which the cross-sectional size of the first portion 312a gradually increases with increasing distance from the connecting portion 311, this is not limiting, and the cross-sectional size of the first portion 312a may increase in stages with increasing distance from the connecting portion 311.

In addition, while an example in which the second portion 312b is connected to the first portion 312a has been shown here, the present invention is not limited to this, and the second portion 312b may be omitted. In other words, the cross-sectional size (area) of the extension portion 312 may increase the farther away from the connecting portion 311 over the entire length of the extension portion 312. This allows light to move more stably from the connecting portion 311 to the extension portion 312, and further reduces scattering of light in the extension portion 312.

Next, the effect of the optical waveguide module 1 according to the first embodiment (simulation results of optical loss according to changes in cross-sectional size) will be described with reference to Figure 7. Figure 7 is a diagram showing the effect of the optical waveguide module 1 according to the first embodiment (simulation results of optical loss according to changes in cross-sectional size).

7, "Example" is a simulation result of measuring optical loss when light is propagated from the first optical waveguide 2 to the second optical waveguide 3 in the optical waveguide module 1 according to this embodiment. The optical loss is the sum of the coupling loss and the propagation loss. The simulation conditions used in the example are as follows:
Width of the start end of the extension part 312: 5 μm
Width of the end of the extension 312: 9 μm
Thickness of the start end of the extension part 312: 5 μm
Thickness of the end of the extension 312: 9 μm
Length of second core 31 (=length of joint portion 311 + length of extension portion 312): 6000 μm
Length of extension portion 312: 3000 μm

7, "Comparative Example" is a simulation result of measuring optical loss when light is propagated from the first optical waveguide 2 to the second optical waveguide 3 in the optical waveguide module 1 according to the comparative example. The optical waveguide module 1 according to the comparative example employs a structure in which the core cross-sectional size of the second optical waveguide 3 is uniform in the longitudinal direction, that is, a structure in which the core cross-sectional sizes of the coupling portion 311 and the extension portion 312 of the second core 31 are the same. The simulation conditions used in the comparative example are as follows.
Width of the start end of the extension part 312: 5 μm
Width of the end of the extension 312: 5 μm
Thickness of the start end of the extension part 312: 5 μm
Thickness of the end of the extension 312: 5 μm
Length of second core 31 (=length of joint portion 311 + length of extension portion 312): 6000 μm
Length of extension portion 312: 3000 μm

As is clear from the simulation results in Figure 7, the embodiment was able to reduce optical loss compared to the comparative example. From these simulation results, it can be concluded that by making the cross-sectional size of the coupling portion 311 relatively small and the cross-sectional size of the extension portion 312 relatively large in the second core 31, both the coupling loss and the propagation loss can be reduced.

Second Embodiment
Fig. 8 is a plan view of the optical waveguide module 1 according to the second embodiment as viewed from the positive direction of the Z axis. Fig. 9 is a cross-sectional view taken along line IX-IX in Fig. 8.

As shown in Figures 8 and 9, in the optical waveguide module 1 according to the second embodiment, the size of the cross section of the coupling portion 311 of the second core 31 perpendicular to the longitudinal direction (X-axis direction) decreases with increasing distance from the extension portion 312. Specifically, the cross section of the coupling portion 311 gradually decreases with increasing distance from the extension portion 312.

In this way, the cross-sectional size of the coupling portion 311 may become smaller as it moves away from the extension portion 312. By adopting such a configuration, the cross-sectional size of the coupling portion 311 in the second core 31 can be made smaller, and the coupling loss can be further reduced.

Third Embodiment
Fig. 10 is a plan view of the optical waveguide module 1 according to the third embodiment as viewed from the positive direction of the Z axis, and Fig. 11 is a cross-sectional view taken along line XI-XI in Fig. 10.

As shown in Figures 10 and 11, in the optical waveguide module 1 according to the third embodiment, the extension portion 312 of the second core 31 has a cross-sectional size perpendicular to the longitudinal direction (X-axis direction) that is generally larger than that of the coupling portion 311. In the example of Figures 10 and 11, the width of the extension portion 312 is generally larger than that of the coupling portion 311, and the thickness of the extension portion 312 is generally larger than that of the coupling portion 311. In other words, the extension portion 312 is discontinuously connected to the coupling portion 311 via a step in the width direction (Y-axis direction) and a step in the thickness direction (Z-axis direction).

In this way, the extension portion 312 may be discontinuously connected to the coupling portion 311 via a step. With this configuration, the propagation loss in the second core 31 can be further reduced.

As described above, the optical waveguide (for example, the second optical waveguide 3) according to the embodiment is an optical waveguide that is optically coupled to another optical waveguide (for example, the first optical waveguide 2) that includes a first core (for example, the first core 22) having a first refractive index, and has a core (for example, the second core 31) and a clad (for example, the clad 32). The core has a refractive index different from the first refractive index. The clad is arranged so as to cover at least a portion of the periphery of the core. The core has end faces and side faces, is elongated, and has, in order along the longitudinal direction, a coupling portion (for example, the coupling portion 311) and an extension portion (for example, the extension portion 312) adjacent to the coupling portion. The coupling portion has an optical coupling surface (for example, the exposed surface 311a) that is located on the side face and is exposed from the clad to be optically coupled to the first core. In the extension section, when two cross sections positioned at a predetermined distance in the longitudinal direction are designated, in order from the one closest to the coupling section, as a first cross section (for example, first cross section 312aa) and a second cross section (for example, second cross section 312bb), the second area, which is the area of the second cross section, is larger than the first area, which is the area of the first cross section. As a result, according to the optical waveguide of the embodiment, it is possible to reduce both coupling loss and propagation loss.

The extension portion may have a first portion connected to the coupling portion and a second portion connected to the first portion. The first portion may include a first cross section and a second cross section, and the cross-sectional area of the first portion may increase with increasing distance from the coupling portion. As a result, the optical waveguide according to the embodiment can reduce scattering of light in the extension portion.

The cross-sectional area of the first portion may also gradually increase with increasing distance from the coupling portion. This allows the optical waveguide of the embodiment to reduce light scattering in the extension portion.

The cross-sectional area of the first portion may also increase stepwise as it moves away from the coupling portion. This allows the optical waveguide according to the embodiment to reduce light scattering in the extension portion.

The cross-sectional area of the extension portion may also increase over the entire length of the extension portion as it moves away from the coupling portion. This allows the optical waveguide according to the embodiment to further reduce light scattering in the extension portion.

The cross-sectional area of the coupling portion may also decrease with increasing distance from the extension portion. This allows the optical waveguide according to the embodiment to further reduce coupling loss.

The extension portion may be discontinuously connected to the coupling portion via at least one of a step in the width direction and a step in the thickness direction. This allows the optical waveguide according to the embodiment to further reduce propagation loss in the core.

Further advantages and alternative embodiments may be readily derived by those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described above. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and equivalents thereof.

1 Optical waveguide module 2 First optical waveguide 3 Second optical waveguide 4 Bonding material 21 Substrate 22 First core 31 Second core 32 Clad 311 Coupling portion 311a Exposed surface 312 Extended portion 312a First portion 312aa First cross section 312bb Second cross section 312b Second part 312cc Side

Claims

An optical waveguide optically coupled to another optical waveguide including a first core having a first refractive index, the core having a refractive index different from the first refractive index;
and a cladding disposed so as to cover at least a portion of the periphery of the core,
The core has an end surface and a side surface, is elongated, and has, in order along a longitudinal direction, a joint portion and an extension portion adjacent to the joint portion,
the coupling portion has an optical coupling surface that is located on the side surface, is exposed from the clad, and is optically coupled to the first core,
An optical waveguide, in which when two cross sections located at a predetermined distance in the longitudinal direction in the extension portion are referred to as a first cross section and a second cross section, in order from the one closest to the coupling portion, a second area that is the area of the second cross section is larger than a first area that is the area of the first cross section.
The extension portion is
A first portion connected to the coupling portion;
a second portion connected to the first portion,
the first portion includes the first cross section and the second cross section;
The optical waveguide of claim 1 , wherein a cross-sectional area of the first portion increases with distance from the coupling portion.
The optical waveguide of claim 2, wherein the cross-sectional area of the first portion gradually increases with increasing distance from the coupling portion.
The optical waveguide of claim 2, wherein the cross-sectional area of the first portion increases stepwise with increasing distance from the coupling portion.
The optical waveguide of claim 1, wherein the cross-sectional area of the extension increases with increasing distance from the coupling portion over the entire length of the extension.
The optical waveguide of claim 1, wherein the cross-sectional area of the coupling portion decreases with increasing distance from the extension portion.
The optical waveguide of claim 1, wherein the extension portion is discontinuously connected to the coupling portion via at least one of a step in the width direction and a step in the thickness direction.
The optical waveguide of claim 1, wherein the extension portion is discontinuously connected to the coupling portion via a step in the width direction and a step in the thickness direction.
a first optical waveguide including a first core having a first refractive index;
a second optical waveguide optically coupled to the first optical waveguide;
The second optical waveguide is
A second core having a refractive index different from the first refractive index;
and a cladding disposed so as to cover at least a portion of the periphery of the second core,
the second core has an end surface and a side surface, is elongated, and has, in order along a longitudinal direction, a joint portion and an extension portion adjacent to the joint portion;
the coupling portion has an optical coupling surface that is located on the side surface, is exposed from the clad, and is optically coupled to the first core,
an optical waveguide module, wherein when two cross sections located at a predetermined distance in the longitudinal direction in the extension section are referred to as a first cross section and a second cross section, in order from the one closest to the coupling section, a second area which is the area of the second cross section is larger than a first area which is the area of the first cross section.