JPH09159865A - Optical waveguide connection structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the connection structure which can reduce connection loss and reflection attenuation without requiring high precision for a device and operation for connection or requiring any spot size converter. SOLUTION: Optical fibers 22 having cores 22a worked increasing in sectional area gradually with the distance from the tips are arranged on a substrate type optical waveguide 21 having the top surface of a core 21a exposed so that their flanks come into contact with each other, thereby constituting a coupling part 24. At this time, the substrate type optical waveguide 21 and optical fibers 22 are so set that the cord 22a is a little higher in refractive index than the core 21a on the opposite side from the tip of the core 22a of the coupling part 24 or the core 22a is a little larger in sectional area than the core 21a when both the cores 21a and 22a are nearly equal in refractive index.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical waveguides having a core for guiding and propagating light and a clad having a refractive index slightly smaller than that of the core, particularly an optical waveguide formed on a substrate (hereinafter referred to as substrate type). (Referred to as an optical waveguide) and an optical fiber.

[0002]

2. Description of the Related Art An optical integrated circuit in which an optical circuit element such as a light emitting element, a light receiving element and an optical modulator is formed on a substrate together with an optical waveguide is capable of advanced optical signal processing, mass productivity and stability. From the viewpoint of cost efficiency, it is becoming a major component in the optical industry including optical fiber communication. When this optical integrated circuit is actually used, an optical fiber is generally used to input an optical signal to be processed from the outside or output the processed optical signal to the outside. Therefore,
In mounting an optical integrated circuit, optical connection between a substrate type optical waveguide in the optical integrated circuit and an optical fiber for optical input / output has become a very important problem.

The requirements for this optical connection are:
From a practical point of view, there are mainly the following two. The first is
Essentially, a part of the optical energy of the optical signal to be transmitted is lost at the connection portion, that is, the connection loss is minimized. Secondly, originally, a part of the optical energy of the optical signal to be transmitted is reflected at the connection portion and returns in the direction opposite to the transmission direction, that is, the reflected return light is reduced as much as possible. The expression is to minimize the return loss.

Optically connecting the substrate type optical waveguide and the optical fiber is to optically couple the waveguide modes propagating through the core of the substrate type optical waveguide and the optical fiber,
Conventionally, as a connection structure that realizes such coupling, (1)
A connection structure using butt coupling, (2) prism coupling, and (3) directional coupling has been proposed.

FIG. 1 shows an example of an optical waveguide connection structure using butt coupling, which includes a core 1a and a clad 1b.
The end face of the substrate type optical waveguide 1 composed of the substrate 1c and the end face of the optical fiber 2 composed of the core 2a and the clad 2b are butted and connected so that the cores 1a and 2a are on the same straight line.

This structure is very common and is used in almost all of the connection structures that are currently in practical use.

[0007]

However, in the single mode type substrate type optical waveguide and optical fiber which are expected to be used from now to the future, the width or height of the core of the substrate type optical waveguide, Fiber core diameter is 1
It is around 0 μm or less. Therefore, in practical use, in order to obtain a sufficiently low connection loss, positioning should be performed so that the optical axis deviation between the core of the substrate type optical waveguide and the core of the optical fiber when the end faces are butted to each other is approximately 1 μm or less. There is a need to do. Further, at this time, it is necessary to dispose the substrate-type optical waveguide and the optical fiber so that the deviation of the angle of the optical axis is about 1 ° or less. Therefore, with this connection structure, there is a drawback that the device and work for making the connection require very high accuracy, and the cost becomes higher accordingly.

Further, in this connection structure, when a substrate type optical waveguide and an optical fiber are butted against each other, a gap is generated between the end faces, so that reflected return light is generated. In order to prevent this, the end faces of the substrate type optical waveguide and the optical fiber are processed so as to be inclined by about 8 ° with respect to the plane orthogonal to the optical axis, or the matching has a refractive index matching the refractive index of the cores of both. It is necessary to fill the space between the end faces with a liquid or an adhesive, which has a drawback that the cost becomes higher.

Furthermore, when the spot sizes relating to the field distribution of the guided mode of the substrate type optical waveguide and the optical fiber are different, even if the optical axes of the core of the substrate type optical waveguide and the core of the optical fiber are highly accurate. Even if they are combined, the connection loss due to the coupling loss between the guided modes of the substrate type optical waveguide and the optical fiber cannot be avoided. In order to avoid such connection loss due to spot size mismatch, it is necessary to add a spot size converter to the substrate type optical waveguide or optical fiber to match the spot size at the connection point. Has the drawback of becoming even higher.

FIG. 2 shows an example of an optical waveguide connection structure using prism coupling. A prism 4 is installed on a core 3a of a substrate type optical waveguide 3 including a core 3a, a clad 3b and a substrate 3c. The light emitted from the core 5a of the optical fiber 5 including the core 5a and the clad 5b is condensed by the lens 6 and is incident on the core 3a via the prism 4 to optically couple as a waveguide mode. In addition,
Due to the reversibility of light, it can also be applied to the case where light is transmitted from the substrate type optical waveguide 3 to the optical fiber 5 contrary to the above.

However, even with this structure, it is necessary to adjust the position and angle between the substrate type optical waveguide and the optical fiber with the same high accuracy as in the structure shown in FIG. 1, and optical components such as prisms and lenses are added. However, there is a drawback that the cost becomes higher accordingly. Further, when the clad is present on the upper part of the core of the substrate type optical waveguide, it cannot be applied or it is necessary to remove the upper clad. Furthermore, even if the above-mentioned optical components and optical fibers can be positioned with high accuracy, the coupling efficiency between the light passing through the prism and the waveguide mode of the substrate-type optical waveguide cannot be sufficiently increased, and as shown in FIG. The connection loss cannot be as low as the structure. Therefore, this structure is actually used only for the measurement of the optical characteristics of the substrate type optical waveguide in the research on the substrate type optical circuit.

The tip of the optical fiber is processed into a peculiar shape, and the tip is brought into contact with the core of the substrate type optical waveguide, and the substrate type optical waveguide and the optical fiber are optically coupled by the principle of prism coupling as described above. The structure to be combined is Japanese Patent Publication Sho 58-577
No. 24 discloses this. However, with this structure,
Since it is difficult to process the tip of the optical fiber to accurately realize the prism coupling, and the above-mentioned other drawbacks remain the same except that the optical component is not necessary, the proposal is limited to the proposal.

FIG. 3 shows an example of an optical waveguide connection structure using directional coupling.
An example described in Japanese Patent Publication No. 57774 is shown. In this example, the end face of the optical fiber 7 including the core 7a and the clad 7b and the end face of the substrate type optical waveguide 8 including the core 8a, the clad 8b and the flexible substrate 8c are arranged in the same straight line in advance. The cores 8a of the substrate type optical waveguide 8 are connected to the substrate type optical waveguide 9 composed of the core 9a, the clad 9b and the substrate 9c by utilizing the flexibility of the flexible substrate 8c. Then, the optical fibers 7 and the substrate type optical waveguide 9 are connected by bringing them into contact with each other to realize the directional coupling between the substrate type optical waveguides 8 and 9. At this time, the two substrate type optical waveguides 8 and 9 have a constant structure along the light propagation direction at the contacting portions, and have the same propagation constant.

In this structure, it is essential to realize the directional coupling between the substrate type optical waveguides 8 and 9. In order to make the directional coupling between the substrate type optical waveguides 8 and 9 close to 100%, it is necessary to make the propagation constant of the substrate type optical waveguide 8 and the propagation constant of the substrate type optical waveguide 9 coincide with each other quite accurately. is there.
Further, the length in the longitudinal direction of the contact portion between the two substrate type optical waveguides 8 and 9 needs to be matched with the complete coupling length, that is, the length at which the optical energy of one optical waveguide is maximally transferred to the other optical waveguide. is there.

However, it is generally difficult to accurately control the propagation constants of the substrate type optical waveguides due to manufacturing errors of the waveguide structure, so that the propagation constants of the substrate type optical waveguides 8 and 9 should be exactly matched. Is quite difficult. Also, due to manufacturing errors in the waveguide structure of the substrate-type optical waveguides 8 and 9 and errors in the spacing between cores when they are in contact with each other, the complete coupling length when actually connected should differ greatly from the design value. Is expected. Therefore, in reality, it was very difficult to increase the coupling efficiency between the substrate type optical waveguides 8 and 9 to nearly 100%.

FIG. 4 shows another example of an optical waveguide connection structure using directional coupling.
Another example described in JP-A-8-57724 is shown. In this example, a part of the clad 10b and the core 10a of the optical fiber 10 including the core 10a and the clad 10b is removed in advance, and a part of the core 10a is exposed, and the core 11a,
The core 10a of the optical fiber 10 is brought into contact with the substrate type optical waveguide 11 composed of the clad 11b and the substrate 11c, and the optical fiber 10 and the substrate type optical waveguide 11 are connected by realizing directional coupling.

In this structure, the optical fiber and the substrate type optical waveguide are directly connected, but there is a drawback similar to the case of the example of FIG. Further, controlling the propagation constant of the optical fiber after removing the clad and the core to a specific value is considerably more difficult than controlling the propagation constant of the substrate type optical waveguide to a specific value.

The connection structure using the three types of optical coupling described so far is not only for connecting the substrate type optical waveguide and the optical fiber, but also for connecting the substrate type optical waveguides or the optical fibers. Although applicable, basically the same drawbacks also occur in those cases.

By the way, the input and output of light from the substrate type optical waveguide to the optical fiber is carried out through the end face of the substrate type optical waveguide in the case of using the butt coupling, and in the substrate using the prism coupling and the directional coupling. Through the upper surface of the mold optical waveguide. That is, in the butt-coupling type, light is input / output to / from the optical fiber from or near the end face of the optical integrated circuit including the substrate type optical waveguide.
With the prism coupling and the directional coupling, it is possible to input and output light to and from the optical fiber from an arbitrary position on the upper surface of the optical integrated circuit. However, considering the fact that the prism coupling and the directional coupling are not practically used and the above-mentioned drawbacks, the one using the prism coupling and the directional coupling is not particularly suitable for practical use as compared with the one using the butt coupling.

An object of the present invention is to require a spot size converter without requiring high precision in a device for connection and work, and even if there is a mismatch in a spot size of a waveguide mode of an optical waveguide. To provide an optical waveguide connection structure capable of reducing the connection loss and the return loss, and capable of inputting and outputting light from an arbitrary position on the upper surface of the substrate type optical waveguide. is there.

[0021]

In order to solve the above-mentioned problems, the present invention optically connects two optical waveguides each having a core for propagating and guiding light and a clad having a refractive index slightly smaller than that of the core. In the connection structure of the optical waveguide to
The cores of the two optical waveguides are arranged so that their side surfaces are substantially parallel to each other so that they are in contact with each other or are sufficiently close to each other to form a coupling part, and the equivalent refractive index of the coupling part is the light input side. From the state where the value in the optical waveguide that inputs light is larger than the value in the optical waveguide that outputs light, the value in the optical waveguide that inputs light becomes We propose a connection structure of optical waveguides in which the cross-sectional area or refractive index of the cores of the two optical waveguides are set so as to gradually change to a value smaller than the value in the output optical waveguide.

According to the present invention, in the coupling portion for optically coupling two optical waveguides, the state where the equivalent refractive index of the input side optical waveguide is large is gradually changed to the state where the equivalent refractive index of the output side optical waveguide is large. Because of the change, the guided mode also propagates according to the light propagation direction, and the guided mode of the input side optical waveguide is coupled to the guided mode of the output side optical waveguide, that is, adiabatically converted. This enables a lossless connection even when the spot sizes of the input-side and output-side optical waveguides do not match, and there is no abrupt waveguide structure change that causes reflection in the light propagation direction. The reflected return light at the coupling portion can be extremely reduced, the return loss can be reduced, and the two optical waveguides at the coupling portion do not need to be accurately adjusted to have a specific value. Since it is not necessary to strictly control the structure of the waveguide, and furthermore, it is not necessary to accurately adjust the relative positional relationship between the two optical waveguides in the coupling part, so high precision is required for the device and work for connection. There is nothing to do.

In the present invention, if a substrate type optical waveguide is used for one of the optical waveguides and an optical fiber is used for the other,
Light can be input and output from any position on the upper surface of the substrate type optical integrated circuit. Furthermore, in the present invention, when the resin is embedded and fixed in a resin having a refractive index lower than the core of any of the two optical waveguides, the coupling action between the two optical waveguides can be further strengthened.

[0024]

DETAILED DESCRIPTION OF THE INVENTION The optical waveguide connection structure of the present invention will be described in detail below with reference to the drawings.

FIG. 5 shows a first embodiment of the present invention. In the figure, 21 is a substrate type optical waveguide comprising a core 21a, a clad 21b and a silicon substrate 21c, and 22 is a core 22a and a clad 22b. The optical fiber 23 is a refractive index matching liquid.

The substrate type optical waveguide 21 and the optical fiber 22 are arranged such that the cores 21a and 22a are in contact with each other or are sufficiently close to each other in a state where the side surfaces of the cores 21a and 22a are substantially parallel to each other to form a coupling portion 24. At this coupling portion 24, the guided mode of the substrate type optical waveguide 21 and the guided mode of the optical fiber 22 are coupled. Reference numeral 25 is a schematic representation of the photoelectric field distribution in the longitudinal direction of the cross section at each point in the longitudinal direction of the coupling portion 24. Here, the case where light is input from the substrate type optical waveguide 21 and output from the optical fiber 22 is shown. ing.

The substrate type optical waveguide 21 is a silica type optical waveguide manufactured by a flame deposition method on a silicon substrate 21c having a flat surface, and has a core 21a having a rectangular cross section and a slight refraction from the core 21a. Low clad 21b
It has. In the substrate type optical waveguide 21, the clad 21b above the core 21a is removed in the vicinity of the region where the coupling portion 24 is formed, and the upper surface of the core 21a is exposed. However, it is necessary to be careful not to remove the upper portion of the core 21a so that a step is formed on the upper surface of the core 21a and a loss is not generated there. Core 22a of optical fiber 22
Since the photoelectric field distribution of the guided mode of the substrate type optical waveguide 21 should sufficiently exude to the core 22a of the optical fiber 22 when they are brought into contact with each other, part of the core 21a is removed when the clad 21b on the core 21a is removed. You may leave some on the top of. Such removal of the clad 21b can be performed by grinding, polishing, dry etching by reactive ion etching or the like, or wet etching by hydrofluoric acid solution or the like.

The optical fiber 22 is a silica single mode optical fiber, and has a core 22a having a circular cross section and a clad 22b having a refractive index slightly lower than that of the core 22a. In the optical fiber 22, the clad 22b is removed in the vicinity of the tip, the core 22a is exposed, and the exposed portion of the core 22a is sharp and tapered, that is, the cross-sectional area of the core 22a gradually increases in the direction away from the tip. It is processed into the shape. As a processing method for obtaining such a shape, a method of etching using a hydrofluoric acid-based solution under appropriate conditions is generally used. In this method, it is known that the tip of the core is approximately at the center of the cross section of the core, and the diameter of the tip can be reduced to 0.1 μm or less. As other processing methods, polishing, a method of locally heating and melting and stretching, or a method of combining them can be considered.

The refractive index matching liquid 23 is transparent so that the absorption of light energy is sufficiently small, and the refractive index matching liquid 23 has the same refractive index as that of the claddings 21b and 22b of the substrate type optical waveguide 21 and the optical fiber 22 and the core 21a and the core 21a. A material having a refractive index lower than that of 22a is used. The refractive index matching liquid 23 plays a role of a clad for the substrate type optical waveguide 21 and the optical fiber 22 in the coupling portion 24. In principle, the index matching liquid 2
3 is not necessarily used, but by using such a refractive index matching liquid 23, the core
Extrusion of the mutual photoelectric field distribution between a and the core 22a becomes large, and the optical coupling action between the substrate type optical waveguide 21 and the optical fiber 22 can be strengthened.

In the substrate type optical waveguide 21, the right end surface of the coupling section 24 is obliquely polished because the coupling type 24 has the optical fiber 22 extending from the substrate type optical waveguide 21.
This is to prevent the light that could not be coupled to the light from being reflected by the end surface and returning.

An optical fiber 22 is formed on the substrate type optical waveguide 21.
When the coupling portion 24 is formed by bringing the ends of the optical fiber 22 into contact with each other, when the optical fiber 22 is away from the substrate type optical waveguide 21, the substrate type optical waveguide 21 and the optical fiber 22 are aligned parallel to each other. The optical fiber 22 is tilted by an appropriate angle such that the tip of the optical fiber 22 approaches the substrate type optical waveguide 21, and they are brought into contact with each other. This allows the optical fiber 22
The upper surface of the core 21a and the bottom surface of the core 22a are in contact with each other in a substantially parallel state due to elastic bending deformation near the tip of the core 21a. It can be pressed with powerful force.

The equivalent refractive index is an effective refractive index sensed when a guided mode propagates, and is sometimes called an effective refractive index. In general, the equivalent refractive index of an optical waveguide becomes higher as the refractive index of its core becomes higher. In addition, the smaller the cross-sectional area of the core, the more the photoelectric field of the guided mode oozes out into the clad having the low refractive index, and the lower the equivalent refractive index.

In the coupling portion 24, the substrate type optical waveguide 21
Since the cores 21a and 22a of the optical fiber 22 are sufficiently close to each other, the guided mode shared by the substrate type optical waveguide 21 and the optical fiber 22 propagates. Therefore, the substrate type optical waveguide 21 and the optical fiber 22 in the coupling portion 24
The equivalent refractive index of is considered to be when they are sufficiently separated and independent.

If light is input from the substrate type optical waveguide 21 and output from the optical fiber 22 in the coupling section 24, the equivalent refractive index of the substrate type optical waveguide 21 is constant, but the optical fiber 22 has a constant refractive index. The equivalent refractive index gradually increases as the cross-sectional area of the core 22a increases. Since the cross-sectional area of the core 22a of the optical fiber 22 is small on the light input side of the coupling portion 24, the substrate optical waveguide 21 has a higher equivalent refractive index than the optical fiber 22. On the other hand, on the light output side of the coupling portion 24, the optical fiber 22 has a higher equivalent refractive index than the substrate type optical waveguide 21. For this purpose, the core 22a may have a slightly higher refractive index than the core 21a, or if the cores 21a and 22a have the same refractive index, the core 22a may have a slightly larger cross-sectional area than the core 21a. The substrate-type optical waveguide 21
And the optical fiber 22 are set.

In general, a certain waveguide mode propagates through a path having a high equivalent refractive index when the sectional structure of the waveguide gradually changes along the light propagation direction. The guided mode of is propagated toward the optical fiber 22. Further, due to the reversibility of light propagation, when light propagates in the opposite direction to the above, the waveguide mode of the optical fiber 22 propagates to the substrate type optical waveguide 21. As a result, the optical connection between the substrate type optical waveguide 21 and the optical fiber 22 is performed.

Looking at the above-described light propagation in the coupling section 24 in more detail, the guided mode propagating in the substrate type optical waveguide 21 is detected in the coupling section 24.
And a guided mode shared by the optical fiber 22. This waveguide mode gradually changes its photoelectric field distribution, while changing from the waveguide mode photoelectric field distribution form of the substrate type optical waveguide 21 on the input side to the optical fiber 22 waveguide mode form on the output side. And transition. In other words, the guided mode of the substrate type optical waveguide 21 is adiabatically coupled to the guided mode of the optical fiber 22. Therefore, even if the spot sizes of the photoelectric field distribution in the substrate type optical waveguide 21 and the optical fiber 22 are different, a low loss optical connection can be made between the substrate type optical waveguide 21 and the optical fiber 22. .

Here, the coupling loss is further reduced by increasing the length of the coupling portion 24 in the propagation direction to reduce the degree of mode conversion per unit length, that is, by making the mode conversion more adiabatic. can do. In order to obtain a coupling efficiency of 99% or more in optical energy between the substrate type optical waveguide 21 and the optical fiber 22, the length of the coupling portion 24 in the propagation direction is set to several hundreds μm or more.

According to the present embodiment, it is not necessary to adjust the length of the coupling portion 24 to a specific length as in the case of the conventional connection structure using directional coupling. Further, since there is no abrupt change in the waveguide structure that causes reflection in the light propagation direction, the return loss can be made extremely small. Further, if the coupling portion 24 is sufficiently long and the substrate-type optical waveguide 21 and the optical fiber 22 satisfy the magnitude relation of the equivalent refractive index as described above, low loss optical coupling can be achieved.
It is not necessary to strictly control the structures of the substrate type optical waveguide 21 and the optical fiber 22 and strictly set their equivalent refractive index to a specific value. Furthermore, when the optical fiber 22 is mounted on the substrate type optical waveguide 21 to form the coupling portion 24, the allowable range of the installation position of the tip end portion of the optical fiber 22 in the light propagation direction is wide.

The connection structure described above can be applied to a connection device for semipermanently connecting a substrate type optical waveguide and an optical fiber, and an optical connector capable of attaching and detaching the optical fiber to the substrate type optical waveguide more than once. Be expected. Also, instead of the refractive index matching liquid, an adhesive having the same refractive index and sufficiently small absorption of light energy can be used to firmly and permanently connect the substrate type optical waveguide and the optical fiber. .

FIG. 6 shows a second embodiment of the present invention, and here shows an example in which the cross-sectional area of the core of the substrate type optical waveguide is changed. That is, in the figure, 31 is the core 3
1a, a clad 31b and a substrate type optical waveguide comprising a silicon substrate 31c, 32 is a core 32a and a clad 32b
The optical fiber 33 is a refractive index matching liquid.

The substrate type optical waveguide 31 and the optical fiber 32 are arranged such that the side surfaces of the cores 31a and 32a thereof are in contact with each other or are sufficiently close to each other in an optically close state to form a coupling portion 34. At this coupling portion 34, the guided mode of the substrate type optical waveguide 31 and the guided mode of the optical fiber 32 are coupled. Reference numeral 35 is a schematic representation of the photoelectric field distribution in the longitudinal direction of the cross section at each point in the longitudinal direction of the coupling portion 34. Here, the case where light is input from the substrate type optical waveguide 31 and output from the optical fiber 32 is shown. ing.

The substrate-type optical waveguide 31 has the cladding 3 above the core 31a near the region where the coupling portion 34 is formed.
1b is removed and the upper surface of the core 31a is exposed.
Further, the core 31a is processed so that the thickness thereof gradually decreases along the light propagation direction. The tapered processing of the thickness of the core 31a can be performed by ion beam etching using an ion beam whose beam intensity is positionally inclined, grinding, or the like. Here, instead of reducing the thickness of the core 31a, it may be possible to reduce the width of the core 31a by keeping the thickness of the core 31a constant. In this case, the mask pattern for forming the core 31a is changed. It can be easily realized only by.

On the other hand, in the optical fiber 32, the cladding 32b is removed by grinding or polishing in the vicinity of the distal end so that the core 32a is appropriately exposed, and the cross-sectional structure of the core 32a changes in the longitudinal direction. There is no.

The coupling portion 34 is constructed by bringing the upper surface of the core 31a of the substrate type optical waveguide 31 and the exposed bottom surface of the core 32a of the optical fiber 32 into contact with each other by the elastic force of the optical fiber 32. A gap is partially formed between the upper surface of the core 31a and the bottom surface of the core 32a after the contact due to the undulations and roughness of these surfaces. When this gap is air, it prevents the photoelectric field distribution of the guided mode between the core 31a and the core 32a from exuding and reduces the efficiency of optical coupling. Therefore, before the core 31a and the core 32a are brought into contact with each other, the substrate type The refractive index matching liquid 33 is applied on the upper surface of the optical waveguide 31 to fill the gap. The cross section of the optical fiber 32 is reduced at the coupling portion 34 as shown in FIG. 7B, but at the portion away from the coupling portion 34, the optical fiber 32 gradually recovers along the longitudinal direction to a normal cross section. It's getting back.

The tip of the optical fiber 32 is chamfered because the light that could not be coupled from the optical fiber 32 to the substrate type optical waveguide 31 at the coupling portion 34 is reflected at the tip and does not return. To do so.

If light is input from the substrate type optical waveguide 31 and output from the optical fiber 32 in the coupling section 34, the equivalent refractive index of the optical fiber 32 is constant, but the equivalent refractive index of the substrate type optical waveguide 31 is constant. The rate gradually decreases as the cross-sectional area of the core 31a decreases. Since the cross-sectional area of the core 31a of the substrate type optical waveguide 31 is small on the output side of the coupling section 34, the optical fiber 32 is smaller than the substrate type optical waveguide 31.
Has a higher equivalent refractive index. On the other hand, on the input side of the coupling section 34, the substrate type optical waveguide 31
Is to have a higher equivalent refractive index. For this purpose, the core 31a may have a slightly higher refractive index than the core 32a, or if the cores 31a and 32a have the same refractive index, the core 31a may have a slightly larger cross-sectional area than the core 32a. The substrate type optical waveguide 31 and the optical fiber 32 are set so as to be large.

FIG. 7 shows a third embodiment of the present invention, and here shows an example in which a guide groove for positioning an optical fiber is formed in a substrate type optical waveguide. That is, in the figure,
Reference numeral 41 is a substrate type optical waveguide including a core 41a, a clad 41b and a substrate (not shown), 42 is an optical fiber including a core 42a and a clad 42b, and 43 is a guide groove. The refractive index matching liquid was omitted.

The guide groove 43 is formed by removing the clad 41b above the core 41a of the substrate-type optical waveguide 41 and removing the core 41a.
It is formed by appropriately setting the region for removing the clad 41b when exposing the upper surface of the. This makes it possible to position the optical fiber 42 that constitutes the coupling portion with high accuracy and easily. The guide groove 43 can be easily formed by using a lithography technique or an etching technique used when manufacturing the substrate type optical waveguide 41.

In the above description, the case of connecting the substrate type optical waveguide and the optical fiber has been described, but the present invention can be applied to the case of connecting the substrate type optical waveguides or the optical fibers.

[0050]

As described above, according to the present invention, in the coupling portion for optically coupling two optical waveguides, the equivalent refractive index of the output side optical waveguide changes from the state where the input side optical waveguide has a large equivalent refractive index. Since the rate gradually changes to a large state, the guided mode also propagates according to the light propagation direction, and the guided mode of the input side optical waveguide is coupled to the guided mode of the output side optical waveguide, that is, adiabatic conversion. As a result, even if the spot sizes of the input-side and output-side optical waveguides do not match, a lossless connection is possible, and there is no sudden change in the waveguide structure that causes reflection in the light propagation direction. Therefore, the reflected return light at the coupling portion can be extremely reduced, the return loss can be reduced, and the two optical waveguides at the coupling portion need to be accurately adjusted to have a specific equivalent refractive index. Therefore, it is not necessary to strictly control the structure of the optical waveguide, and further, since it is not necessary to accurately adjust the relative positional relationship between the two optical waveguides in the coupling portion, it is high in equipment and work for connection. There is no need for precision.

In the present invention, if a substrate type optical waveguide is used for one of the optical waveguides and an optical fiber is used for the other,
Light can be input and output from any position on the upper surface of the substrate type optical integrated circuit. Furthermore, in the present invention, when the resin is embedded and fixed in a resin having a refractive index lower than the core of any of the two optical waveguides, the coupling action between the two optical waveguides can be further strengthened.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an example of a conventional optical waveguide connection structure using butt coupling.

FIG. 2 is a configuration diagram showing an example of a conventional optical waveguide connection structure using prism coupling.

FIG. 3 is a configuration diagram showing an example of a conventional optical waveguide connection structure using directional coupling.

FIG. 4 is a configuration diagram showing another example of a conventional optical waveguide connection structure using directional coupling.

FIG. 5 is a configuration diagram showing a first embodiment of an optical waveguide connection structure of the present invention.

FIG. 6 is a configuration diagram showing a second embodiment of an optical waveguide connection structure of the present invention.

FIG. 7 is a configuration diagram showing a third embodiment of the optical waveguide connection structure of the present invention.

[Explanation of symbols]

21, 31, 41 ... Substrate type optical waveguide, 21a, 31a,
41a ... Core of substrate type optical waveguide, 21b, 31b, 41
b ... Clad of substrate type optical waveguide, 21c, 31c ... Silicon substrate, 22, 32, 42 ... Optical fiber, 22a, 3
2a, 42a ... Optical fiber cores, 22b, 32b, 4
2b ... Cladding of optical fiber, 23, 33 ... Refractive index matching liquid, 24, 34 ... Coupling part, 25, 35 ... Photoelectric field distribution,
43 ... Guide groove.

Claims (3)

[Claims] 1. A connection structure of optical waveguides for optically connecting two optical waveguides, each of which has a core for guiding and propagating light and a clad having a refractive index slightly smaller than that of the core. The side surfaces are arranged substantially parallel to each other such that they are in contact with each other or arranged so as to be sufficiently close to each other optically to form a coupling section, and the equivalent refractive index of the coupling section increases from the light input side to the light output side. , From the state where the value in the optical waveguide for inputting light is larger than the value in the optical waveguide for outputting light, the value in the optical waveguide for inputting light in the optical waveguide for outputting light A cross-sectional area or refractive index of the cores of the two optical waveguides is set so as to gradually change to a state smaller than the value of 1. 2. The optical waveguide connection structure according to claim 1, wherein one of the two optical waveguides is an optical waveguide formed on a substrate and the other is an optical fiber. 3. The optical waveguide connection structure according to claim 1, wherein the two optical waveguides are fixed by being embedded in a resin whose refractive index is lower than the refractive index of either core of the two optical waveguides. .