JP2001242337A - Connecting structure for optical waveguide different in mode field diameter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To connect optical waveguides different in mode field diameter by reducing a connection loss owing to the uncoincidence of the mode field diameter without causing damage by heating to a substrate and the optical waveguide. SOLUTION: In connecting structure for the optical waveguide wherein the end surface of a second waveguide having the mode field diameter larger than the mode field diameter of a first optical waveguide is connected to the end surface of the first optical waveguide formed on the substrate by aligning optical axes of both optical waveguides, the mode field diameter of the first optical waveguide is brought close to the mode field diameter of the second optical waveguide by heating the vicinity of the end surface of the first optical waveguide and reducing the difference of refractive indexes between a core part 2 and a clad part 1 in the vicinity of the end surface. Thus, the connection loss owing to the uncoincidence of the mode field diameter in the connecting part is reduced, and since heating at a high temperature is not necessitated, damage to the substrate and the optical waveguide is reduced to an extent free from problems in practice.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a connection structure between optical waveguides having different mode field diameters, for example, an optical waveguide and an optical fiber, or an optical waveguide and an optical device, and more particularly to an optical waveguide having a small mode field diameter. The present invention relates to a connection structure for optical waveguides having different mode field diameters, in which the coupling loss at the connection portion is reduced by bringing the mode field diameter of the optical waveguide closer to the mode field diameter of the optical waveguide having a large mode field diameter.

[0002]

2. Description of the Related Art Conventionally, an optical connection between optical waveguides having different mode field diameters, for example, an optical waveguide having a three-dimensional waveguide shape formed on a substrate and an optical fiber or an optical device, has the following connection structure. To reduce the coupling loss in the connection.

As shown in the cross-sectional view of the connecting portion in FIG. 5, when the optical waveguide is connected to an optical fiber or an optical device, the mode field diameter of the silica-based optical waveguide core portion 12 is changed to that of the silica-based optical waveguide. The end faces of the core portion 14 of the optical fiber having a mode field diameter smaller than the mode field diameter of the optical waveguide are butted, and after adjusting the optical axes of both, the CO ₂ laser beam 15 is applied to the butted portion to about 2000 ° C. Is heated at a temperature of 3 to 6 seconds, and the two are fusion-spliced. In FIG. 5, reference numeral 11 denotes a clad portion of the silica-based optical waveguide, and reference numeral 13 denotes a clad portion of the optical fiber.

In the connection structure according to this method, an additive for controlling the refractive index added to a core portion 14 of the optical fiber is provided in a cladding portion 13 at a connection portion of the optical fiber fused to the silica-based optical waveguide. Is diffused such that the region gradually expands from the vicinity of the connection end to the end face, and FIG.
As shown in the figure, an enlarged portion is formed near the end face of the core portion 14 at the connection portion, and as a result, the mode field diameter of the optical fiber expands as the diameter of the core portion in the enlarged portion 16 increases, and the quartz optical waveguide It becomes almost the same as the mode field diameter. Thus, the core 12 of the optical waveguide is
The coupling loss due to the mismatch of the mode field diameter between the optical fiber and the core part 14 of the optical fiber is reduced.

[0005]

However, in the optical waveguides having different mode field diameters as described above, that is, in the connection structure between the silica-based optical waveguide and the optical fiber or the optical device, the connection between the silica-based optical waveguide and the optical fiber is made. The butted portion is heated to about 2000 ° C. by irradiating a CO ₂ laser beam to perform fusion splicing of the two. For this reason, the optical waveguide and the optical fiber are limited to materials that can withstand heating of about 2000 ° C., and in reality are made of quartz-based materials. Therefore, a photoelectric mixed substrate, for example, O
For a substrate that is damaged at such a high temperature, such as an integrated circuit substrate in which an opto-electronic circuit such as an E-MCM (photo-electron mixed multi-chip module) is built,
Due to the connection structure as described above, there is a problem that optical waveguides having different mode field diameters cannot be connected to each other, and a desired optoelectronic circuit cannot be formed.

The present invention has been made in view of the above-mentioned problems in the prior art, and has as its object to provide an optical waveguide having a different mode field diameter such as a connection between an optical waveguide formed on a photoelectric mixed substrate and an optical fiber. It is an object of the present invention to provide a connection structure capable of connecting the devices to each other while suppressing damage to an underlying substrate and an optical waveguide while reducing coupling loss due to mismatch of mode field diameters.

[0007]

According to the present invention, a connection structure for optical waveguides having different mode field diameters has a mode field diameter larger than the mode field diameter at an end face of a first optical waveguide formed on a substrate. In a connection structure of optical waveguides having different mode field diameters for connecting the end faces of the second optical waveguide while aligning the optical axes of the two optical waveguides, the vicinity of the end face of the first optical waveguide is heated and the vicinity of the end face is heated. The mode field diameter of the core part and the clad part are reduced by reducing the difference in refractive index between the core part and the clad part, so as to approach the mode field diameter of the second optical waveguide.

Further, according to the present invention, there is provided a connection structure for optical waveguides having different mode field diameters, wherein
Is characterized in that at least the core portion is made of a siloxane-based polymer containing a metal.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the connection structure of optical waveguides having different mode field diameters according to the present invention, an optical waveguide having a small mode field diameter formed on a substrate and an optical waveguide having a large mode field diameter formed on the same substrate are provided. As a structure for connecting the optical waveguide or the optical fiber or the optical device, the vicinity of the end face of the connection portion of the first optical waveguide is heated to change the refractive index of the core portion and / or the clad portion near the end face, By reducing the difference in refractive index between the two, the mode field diameter in the vicinity of the end face is enlarged, and the optical fiber or optical device as a second optical waveguide having a mode field diameter larger than the original mode field diameter is used. Since the mode field diameters can be almost matched, the connection due to the mode field diameter mismatch It is possible to reduce the loss. In addition, in order to increase the mode field diameter near the end face of the first optical waveguide, the heating for changing the refractive index difference between the core portion and the clad portion is performed by the conventional fusion splicing of the silica-based optical waveguide. Since high-temperature heating such as about 2000 ° C. is not necessary as described above, damage to an underlying substrate such as an opto-electron mixed substrate on which the first optical waveguide is formed is practically less compared to a connection structure using such high-temperature heating. It can be reduced to a level where there is no problem. In the connection structure of the optical waveguide of the present invention, it is not necessary to fuse and connect the end face of the first optical waveguide and the end face of the second optical waveguide at a high temperature like a conventional silica-based optical waveguide. It is not always necessary that the end faces be bonded to each other with the optical axes aligned using, for example, an ultraviolet-curable optical adhesive or the like.

Further, according to the connection structure of the optical waveguides having different mode field diameters of the present invention, depending on the difference in the size of the mode field diameter of the two optical waveguides connected to the optical material of the core portion and the cladding portion and the combination thereof. In the case of an optical waveguide having a small mode field diameter as in the conventional connection structure, the diameter of the core is directed toward the connection end in order to match the diameter of the core with the diameter of the core of the optical waveguide having a large mode field diameter. In contrast to a tapered optical waveguide which is gradually enlarged to be larger, or vice versa, a first tapered optical waveguide or the like is formed without forming such a tapered optical waveguide. Since the mode field diameter of the second optical waveguide can be adjusted by expanding the mode field diameter of the optical waveguide, the light using the optical waveguide connection structure of the present invention can be adjusted. It is possible to reduce the waveguide circuit.

Hereinafter, a connection structure of an optical waveguide according to the present invention will be described in detail with reference to the drawings.

FIGS. 1 and 2 are a sectional view and a perspective view, respectively, showing an embodiment of a connection structure for optical waveguides having different mode field diameters according to the present invention. In these figures, reference numeral 2 denotes a core portion of a first optical waveguide which is formed on a substrate (not shown) and forms an optical waveguide circuit, and 1 denotes a first optical waveguide formed around the core portion 2. This is the cladding of the wave path. The clad part 1 and the core part 2 form a first optical waveguide. Reference numeral 4 denotes a core portion of the optical fiber as a second optical waveguide, and reference numeral 3 denotes a cladding portion of the optical fiber surrounding the core portion 4. Reference numeral 5 denotes irradiation light of an infrared lamp for heating the vicinity of the end face of the first optical waveguide.

As described above, the end face of the core section 2 of the first optical waveguide and the end face of the core section 4 of the second optical waveguide are butted with their optical axes aligned, and irradiated near the end face of the first optical waveguide. By irradiating and heating the light 5, the vicinity of the connection end of the first optical waveguide is heated, the refractive index of the core 2 and / or the cladding 1 changes, and the difference between the refractive indices of the two decreases. As a result, the mode field diameter increases, and the diameter of the core section 4 and the mode field diameter generally coincide with each other. However, as shown in FIG. The second optical waveguide, which is larger than the first optical waveguide, can be adjusted so that the mode field diameters of the two optical waveguides are almost equal to each other in connection with the optical fiber, thereby reducing the coupling loss due to the mismatch of the mode field diameters. Can be done.

Heating for reducing the refractive index difference between the core portion 2 and the clad portion 1 of the first optical waveguide does not require a high temperature unlike the case where the silica optical waveguides are fusion-spliced. Since the temperature may be about 100 ° C. to 400 ° C. as described later, the substrate is hardly damaged by heating.

[0015] The reference numeral 6 indicates that the above-mentioned heating causes the refractive index difference between the core portion 2 and the cladding portion 1 of the first optical waveguide to change continuously, and the mode field diameter of the first optical waveguide changes to the mode of the second optical waveguide. FIG. 4 shows a change region that gradually increases as approaching the field diameter. The range of the change region 6 depends on the difference between the mode field diameters of the first and second optical waveguides to be connected and the core portion 2 and the clad portion after heating near the end face of the first optical waveguide. It is determined by the amount of change in the refractive index difference from 1.

Here, it is preferable that the cladding part 1 and the core part 2 of the first optical waveguide be made of an organic optical material whose refractive index changes by heating. In particular, according to an optical waveguide using a siloxane-based polymer for the cladding portion 1 and a metal, for example, a siloxane-based polymer containing titanium (Ti) for the core portion 2, both refractive indexes are linear according to the heating temperature. Therefore, by controlling the heating condition, the diameter can be changed to a desired mode field diameter so as to be close to the mode field diameter of the second optical waveguide.

The siloxane polymer suitably used for the first optical waveguide may be any resin having a siloxane bond in the polymer skeleton, such as polyphenylsilsesquioxane / polymethyl. Examples include phenylsilsesquioxane and polydiphenylsilsesquioxane. Further, the metal contained in the core portion 2 is not limited to titanium, and germanium, gallium and the like can be used. In order to form the core portion 2 containing these metals, the solution for forming a siloxane-based polymer film to which the metal alkoxide is added is thermally polymerized to form a metal-containing siloxane-based polymer layer. It may be processed to dimensions.

The siloxane-based polymer used for the cladding portion 1 may contain the same metal as described above. In this case, a difference in the refractive index between the core portion 2 and the metal may be provided. . Needless to say, these siloxane-based polymers may also be used for the second optical waveguide.

Further, as the first optical waveguide to which the optical waveguide connection structure of the present invention can be applied, an optical waveguide formed on a substrate and using an organic optical material is preferable. The waveguide may be an optical waveguide using a conventionally used quartz optical material or an organic optical material, and the core portion and the cladding portion are configured by combining the quartz optical material and the organic optical material. An optical waveguide may be used. The shape of these optical waveguides may be a three-dimensional optical waveguide, a film-shaped optical waveguide, or the like, in addition to the above-described optical fiber, and can be applied to various materials, shapes, and forms.

The means and method for heating the vicinity of the end face of the first optical waveguide are not limited to the irradiation light 5 of the infrared lamp as shown in FIG. Various light sources or heat sources can be used as long as they can heat the change region 6 near the connection end to a required temperature in a spot manner. For example, a condensed halogen lamp, infrared laser / visible light, or a locally heated small hot plate or small heater can be used. When the heating temperature is controlled, for example, when infrared laser light is used, the heating temperature can be controlled by the power of the infrared laser light and the irradiation time.

The irradiation amount and the heating amount for heating the change region 6 to the required temperature in a spot manner with respect to the first optical waveguide depend on the material forming the first optical waveguide and the first and second materials. An experiment was previously performed based on the difference in the size of the mode field diameter of the optical waveguide and the second optical waveguide connected to the optical waveguide, and the amount of change in the refractive index difference between the core and the clad due to the heating conditions for the first optical waveguide. I will ask for it.

Here, taking a siloxane-based polymer as an example, an example of how the refractive index changes with heating of the core 2 and the clad 1 is shown in FIG. In FIG.
The horizontal axis represents the heating temperature, and the vertical axis represents the refractive index. White circles indicate the measured values of the TE mode component of the refractive index of the titanium-containing siloxane-based polymer, and black circles indicate the measured values of the TM mode component of the refractive index of the titanium-containing siloxane-based polymer. , The open square indicates the measured value of the TE mode component of the refractive index of the siloxane-based polymer, the black square indicates the measured value of the TM mode component of the refractive index of the siloxane-based polymer, and the solid line indicates the asymptotic value of the measured value of the TE mode. The broken line shows the asymptote of the measured value in the TM mode.

As shown in FIG. 3, the refractive indices of the core portion and the clad portion change almost linearly with respect to the heating temperature, and by heating to the temperature indicated by the horizontal axis,
The refractive index difference between the core portion and the clad portion also changes in accordance with the temperature. In the drawing, the refractive index difference is represented by the size between two asymptotes. It also shows that heating at a higher temperature gradually reduces the refractive index difference between the core portion and the clad portion, and accordingly, the mode field diameter of the optical waveguide can be gradually changed greatly. Therefore, an optical waveguide having an arbitrary mode field diameter at the connection end can be manufactured.

[0024]

Next, a specific example of a connection structure of an optical waveguide according to the present invention will be described.

Example 1 A clad part 1 was formed on a silicon substrate.
Was formed of a siloxane-based polymer, and the core portion 2 was formed of a titanium-containing siloxane-based polymer to form a step index type optical waveguide. On the other hand, as shown in FIGS. 1 and 2, a spot diameter of 2.4 near the connection end of the first optical waveguide on the side to which the optical fiber as the second optical waveguide is connected.
The vicinity of the connection end is heated by irradiating the irradiation light 5 of an infrared lamp having a diameter of 2 mm to form the core portion 2 of the first optical waveguide.
And the refractive index of the cladding part 1 was changed. At this time, the power of the infrared lamp was 30 W, and the irradiation time was 3 seconds.
This condition corresponds to heating the portion irradiated by the irradiation light 5 to about 300 ° C.

In the first optical waveguide, the refractive index of the siloxane-based polymer in the cladding part 1 is n1 = 1.442, and the refractive index of the titanium-containing siloxane-based polymer in the core part 2 is n2 = 1.453.
The difference in refractive index between the two was Δn = 0.7%, but the refractive index of the cladding 1 near the connection end after heating by the irradiation light 5 was n1 ′ = 1.441, and the refractive index of the core 2 was n2 ′ = 1.4.
The refractive index difference was changed to 456, and the difference in refractive index between the two was reduced to Δn = 0.3%.

As a result, the mode field diameter at the connection end face of the first optical waveguide before heating by the irradiation light 5 is 5.5.
However, the mode field diameter at the connection end face of the first optical waveguide after being heated by the irradiation light 5 is increased to about 9 μm, and is substantially equal to 9 μm, which is the mode field diameter of the optical fiber as the second optical waveguide. Could be matched. As a result, the coupling loss due to the mode field diameter mismatch could be reduced by about 0.5 dB.

[Example 2] In the same manner as in [Example 1], a first optical waveguide of a step index type in which a cladding part 1 is made of a siloxane-based polymer and a core part 2 is made of a titanium-containing siloxane-based polymer is formed on a silicon substrate. Formed. And figure
As in another example of the embodiment of the optical waveguide connection structure of the present invention shown in a cross-sectional view in FIG. 4, the second optical waveguide connection structure is provided in the middle of the first optical waveguide.
The location where the optical device 7 as the optical waveguide is provided is processed, and irradiation light 5 of an infrared laser having a spot diameter of 5 mm is applied to the vicinity of the connection end of the first optical waveguide on the side connected to the optical waveguide of the optical device 7. By doing so, the cladding 1 and the core 2 near the connection end were heated to change the refractive index, and the difference in the refractive index between the core 2 and the cladding 1 was reduced. At this time, the power of the infrared laser was 30 W and the irradiation time was 3 seconds. This condition corresponds to heating the portion heated by the irradiation light 5 to about 300 ° C. In FIG. 4, reference numeral 8 denotes a portion corresponding to the core portion (4) of the second optical waveguide in the optical device 7.

In this example, in the first optical waveguide, the refractive index of the siloxane-based polymer in the cladding 1 is n1 = 1.44.
2. The refractive index of the titanium-containing siloxane-based polymer in the core portion 2 was n2 = 1.453, and the difference in refractive index was Δn = 0.7%.
After being irradiated with infrared laser light and heated, the refractive index of the cladding 1 near the connection end changes to n1 ′ = 1.441, the refractive index of the core 2 changes to n2 ′ = 1.449, and the difference in refractive index Δn
= 0.41%.

As a result, the mode field diameter at the connection end of the first optical waveguide before the irradiation of the infrared laser light is
Although the diameter was 5.5 μm, the mode field diameter at the connection end of the first optical waveguide after being irradiated with the infrared laser light and heated increased to about 9 μm, and almost matched the mode field diameter of the optical device 7 of 9 μm. did. As a result, the coupling loss due to the mode field diameter mismatch could be reduced by about 0.5 dB.

Further, by increasing the beam spot diameter of the irradiation light 5 of the infrared laser, two portions near the connection end of the first optical waveguide connected to the light receiving / emitting portions located on both sides of the optical device 7 are simultaneously heated. As a result, good optical connection could be made on both sides.

In the above embodiment, a silicon substrate is used as the substrate. However, the present invention is not limited to the silicon substrate. For example, an AlN substrate, an Al ₂ O ₃ substrate, a glass ceramic substrate, or the like may be used.

It should be noted that the above is only an example of the embodiment of the present invention, and the present invention is not limited to the embodiment. Various changes and improvements may be made without departing from the gist of the present invention. No problem.

[0034]

As described above, according to the connection structure for optical waveguides having different mode field diameters according to the present invention, the vicinity of the end face of the connection portion of the first optical waveguide having a small mode field diameter formed on the substrate. Is heated to change the refractive index of the core and / or cladding in the vicinity of the end face, thereby reducing the difference in refractive index between the two, thereby increasing the mode field diameter near the end face and increasing the second optical waveguide. Since the mode field diameters of the two optical waveguides are close to each other, it is possible to make the mode field diameters of both optical waveguides substantially equal to each other, so that the coupling loss due to the mismatch of the mode field diameters at the connection portion can be reduced. In addition, in order to increase the mode field diameter near the end face of the first optical waveguide, the heating for changing the refractive index difference between the core portion and the clad portion is performed by the conventional fusion splicing of the silica-based optical waveguide. Since high-temperature heating such as about 2000 ° C. is not necessary as described above, damage to an underlying substrate such as an opto-electron mixed substrate on which the first optical waveguide is formed is practically less compared to a connection structure using such high-temperature heating. It can be reduced to a level where there is no problem.

Further, according to the connection structure of the optical waveguides having different mode field diameters of the present invention, depending on the difference in the size of the mode field diameter of the two optical waveguides connected to the optical material of the core portion and the cladding portion and the combination thereof. Compared with a conventional connection structure in which an inverted tapered optical waveguide or a tapered optical waveguide is formed and connected, the first optical waveguide can be formed without forming such a tapered optical waveguide or the like. Since the mode field diameter of the second optical waveguide can be matched by increasing the mode field diameter, the optical waveguide circuit using the optical waveguide connection structure of the present invention can be shortened.

Further, when a siloxane-based polymer containing a metal is used in at least the core portion as the first optical waveguide, the refractive index of the core portion and / or the cladding portion linearly changes according to the heating temperature. Therefore, by controlling the heating conditions, the refractive index can be reduced as desired, changed to a desired mode field diameter, and can be accurately brought close to the mode field diameter of the second optical waveguide. Become.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of an embodiment of a connection structure for optical waveguides having different mode field diameters according to the present invention.

FIG. 2 is a perspective view of the connection structure shown in FIG.

FIG. 3 is a diagram showing a change in refractive index with respect to a heating temperature of a clad portion and a metal-containing core portion made of a siloxane-based polymer.

FIG. 4 is a sectional view showing another example of the embodiment of the connection structure of the optical waveguides having different mode field diameters according to the present invention.

FIG. 5 is a cross-sectional view showing an example of a conventional connection structure between an optical waveguide and an optical fiber.

[Explanation of symbols]

 1... Clad portion of first optical waveguide 2... Core portion of second optical waveguide 3... Clad portion of optical fiber (second optical waveguide) 4. ..Core portions of optical fibers (second optical waveguides)

Claims (2)

[Claims] 1. An end face of a second optical waveguide having a mode field diameter larger than its mode field diameter is connected to an end face of a first optical waveguide formed on a substrate so that both optical waveguides have their optical axes aligned. In the connection structure of optical waveguides having different mode field diameters, by heating the vicinity of the end face of the first optical waveguide to reduce the refractive index difference between the core portion and the clad portion near the end face, the mode field is reduced. A connection structure for optical waveguides having different mode field diameters, wherein the diameter is close to the mode field diameter of the second optical waveguide. 2. The connection structure for optical waveguides having different mode field diameters according to claim 1, wherein the first optical waveguide is made of a siloxane-based polymer containing a metal at least in a core portion.