JPH0345909A - Tapered waveguide type spot size converter and formation of its thin film - Google Patents

Abstract

PURPOSE:To decrease insertion losses while maintaining the short length of a tapered part by aligning the position where the tapered part of the core having a high refractive index starts and the front end of the tapered part of the core having an intermediate refractive index and thereby uniformly changing the refractive index of the equiv. core. CONSTITUTION:A clad mode removing layer 2, a low refractive index glass clad layer 6, a high refractive index glass core layer 1 of a tapered opposite superposition type, an intermediate refractive index glass core layer 5 and a low refractive index glass clad layer 4 are successively laminated on a substrate 3. The two slab waveguides having the high refractive index glass clad layer 1 and the intermediate refractive index glass clad layer 5 are so formed as to face each other to largely vary the refractive index difference in the propagation direction of light between the cores 1.5 in the waveguide of the high refractive index glass core layer 1 having the small core thickness and the waveguide of the intermediate refractive index glass core layer 5 having the large core thickness and the clads 4, 6 having low refractive indexes, by which the arbitrary changing of the conversion rate of a spot size is permitted. The taper angle is diminished in this way and the lower loss is attained without increasing the length of the converter.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a tapered waveguide-type spot size converter used when connecting a plurality of waveguide-type optical elements having different spot sizes in an optical waveguide. and its manufacturing method.

[Conventional technology]

FIG. 7 is a diagram showing an example of the use of a spot size converter in an optical integrated circuit.

As shown in FIG. 7, when optical integrated circuits 24 and 25 in which various waveguide type devices are integrated are inserted between a semiconductor laser 21 that serves as a light source and an optical fiber 22 that ultimately serves as an output of an optical signal. , spot size mismatch becomes a major problem. This is because the spot size of the semiconductor laser 21 is 0.
This is because, while the spot size of the optical fiber 22 is around 2 μm, the spot size of the optical fiber 22 is several μm, which is several ten times larger. When optical waveguides with greatly different spot sizes are directly connected by butt joint,
A coupling loss of 10 dB or more will occur. Therefore, a waveguide type spot size converter 23 that has low loss and can convert the spot size over a wide range is required.

Conventionally reported spot size converters include waveguide type converters that use ion exchange to form a tapered refractive index distribution in the light propagation direction on a glass substrate.
By making optical fibers and plastic waveguides into a tapered shape,
There is a waveguide type in which the refractive index or refractive index distribution of the core is slightly changed in the light propagation direction.

[Problem to be solved by the invention]

However, none of the above-mentioned conventional spot size converters can achieve a large spot size conversion ratio, which is only about twice as large at most. One of the causes
The first is that the core of the optical waveguide on the input side and the core of the optical waveguide on the output side of the spot size converter are made of the same material.

In other words, since the core is made of the same material, the refractive index difference between the core and cladding is almost constant, and even if the core thickness is made thinner to reduce the spot size, the field distribution will seep out from the core and the spot size will be too small. The problem is that it doesn't.

Therefore, in order to increase the spot size conversion ratio in these converters, it is necessary to greatly change the refractive index difference between the core and the cladding in the light propagation direction. However, in a tapered optical fiber, the refractive index distribution of the core is changed by diffusing the dopant, so the refractive index difference itself does not change much.

In addition, in the case of tapered plastic waveguides, the leading waveguide is fabricated using a photopolymerization method, and the refractive index is changed depending on the amount of exposure, but it is not possible to achieve a maximum refractive index difference of 0.02, and at this stage, A large conversion ratio for spot size cannot be expected.

The present invention solves the above-mentioned problems. It is possible to reduce the insertion loss while keeping the length of the tapered part short, and it is also possible to control the shape, length, and position of the tapered part. The object of the present invention is to provide a tapered waveguide type spot size converter and a method for forming a thin film thereof.

[Means to solve the problem]

To this end, the present invention provides a tapered waveguide type spot size converter that connects optical waveguides with different spot sizes. Two slab waveguides each having a thicker core with an intermediate refractive index lower than the core are overlapped with their tapered ends facing each other, and the tapered part start position of the high refractive index core and the tapered part of the intermediate refractive index core are stacked. It is characterized by a structure in which the tips of the shadow mask are made to coincide with each other so that the refractive index of the equivalent core changes uniformly.The thin film formation method involves lifting the shadow mask from the substrate via a spacer, and then removing the spacer from the substrate. A thin film that is recessed from the edge of the shadow mask, has the lower corner of the edge of the shadow mask cut off diagonally, and has a tapered part toward the inside of the edge of the shadow mask using a sputtering method. It is characterized by the formation of

[Effect]

In the tapered waveguide spot size converter of the present invention, two slab waveguides each having a core made of a different material are stacked with their tapered ends facing each other, and the tapered part of the high refractive index core starts. Since the position and the tip of the tapered part of the core with an intermediate refractive index are made to coincide with each other so that the refractive index of the equivalent core changes uniformly, the equivalent refractive index of the core and cladding is The refractive index difference can be changed largely and smoothly, the spot size conversion ratio can be increased, and the loss of light can be reduced. In addition, in this thin film formation method, a shadow mask and a spacer are used, the lower corner of the edge of the shadow mask is cut off diagonally, and a tapered thin film is formed by the shadow mask sputtering method. By changing the thickness of the spacer and the cutout at the lower corner of the edge of the shadow mask, the shape of the taper part can be smoothly controlled so that the refractive index of the equivalent core changes uniformly. The inclination angle can be reduced without changing the length of the part.

〔Example〕

Examples will be described below with reference to the drawings.

FIG. 1 is a diagram showing one embodiment of a tapered waveguide type spot size converter according to the present invention. In the figure, 1 is a high refractive index glass core layer, 2 is a cladding mode removal layer, 3 is a semiconductor substrate, 4 and 6 are low refractive index glass cladding layers, 5 is an intermediate refractive index glass core layer, and 7 is a shadow A mask, 8 a spacer 9 a substrate, and 10 a thin film.

The tapered waveguide spot size converter according to the present invention changes the refractive index of the core by changing the equivalent refractive index of the core when changing the core width in a tapered manner. This was devised based on the expectation that the same effect would be obtained as in the conventional case, and as shown in FIG. A superimposed type high refractive index glass core layer 1, intermediate refractive index glass core layer 5, and low refractive index glass cladding layer 4 are laminated in this order. In this way, two slab waveguides having a high refractive index glass core layer 1 and an intermediate refractive index glass core layer 5 each formed of different materials are made to face each other, and by changing the core material, In other words, the core (L5) and the low refractive index cladding (L5) are formed by a waveguide made of a high refractive index glass core layer 1 with a thin core thickness and a waveguide made of an intermediate refractive index glass core layer 5 with a thick core thickness.
4 and 6> in the light propagation direction, the spot size conversion ratio can be set arbitrarily. In particular, in order to integrate circuits, it is required to reduce the taper angle without increasing the length of the spot size converter. The goal is to make it shorter. Therefore, in the overlapping portion, basically the start position of the taper portion of the high refractive index glass core FJl and the tip of the taper portion of the intermediate refractive index glass core 5 are made to coincide with each other, thereby eliminating waste in the taper portion. Furthermore, the tapered membrane is made to have a nearly straight surface shape. By doing this, the waveguide can be configured so that the equivalent core refractive index changes uniformly, and the refractive index difference between the core and cladding can be greatly changed in the light propagation direction.
The spot size conversion ratio can be increased and insertion loss can be reduced. Note that a thin film having such a tapered portion can be easily formed by a sputtering method using a shadow mask as described below.

FIG. 2 is a diagram for explaining one embodiment of a thin film forming method for a tapered waveguide spot size converter according to the present invention;
Figure 3 shows the relationship between the shape of the edge of the shadow mask and the surface shape of the tapered film, Figure 4 shows the relationship between the thickness of the spacer and the length of the tapered part, and Figure 5 shows the edge of the shadow mask. FIG. 3 is a diagram showing the relationship between the height of the vertical portion of the shadow mask and the length of the tapered portions formed on the inside and outside of the shadow mask.

In the shadow mask sputtering method, as shown in FIG.
A shadow mask 7 such as a 00 μm metal plate is placed slightly above the substrate 9, and a thin film 1 of a dielectric etc. is placed on the substrate 9.
0 is formed by sputtering.

Originally, when the thin film IO is formed by sputtering, the molecules of the raw material are incident on the substrate 9 almost perpendicularly, but not all the molecules are completely perpendicular to the substrate 9. Therefore, the molecules that have come obliquely to the substrate 9 also enter the shadow part of the shadow mask 7, and the thin film 10 also enters the shadow part.
is formed.

The shape of the thin film 10 in this part becomes tapered as shown in FIG. 2 because the number of flying molecules decreases as it goes deeper from the edge of the shadow mask 7.

The length of the tapered part is determined by the distance between the shadow mask 7 and the substrate 9, and therefore the thickness of the spacer 8, and also the thickness of the beginning part of the tapered part, that is, the part that connects to the part with a -like thickness. is determined by the conditions for forming the film in the --like portion, such as the sputtering power input, the type of gas, the degree of vacuum, the distance between the substrate and the target, and the sputtering time.

However, even if the surface shape of the tapered film formed in this manner is different from that shown in FIG.
As shown in a), a discontinuous portion with a large slope appears near the center of the tapered portion, and the slope of this portion becomes considerably large. Such a large angle of inclination is undesirable because it causes radiation loss when guiding light.

Therefore, the basic configuration of the shadow mask sputtering method for forming the thin film of the tapered waveguide spot size converter according to the present invention is as shown in FIG. At that time, the spacer 8 is retracted sufficiently from the edge of the shadow mask 7 so that the tapered part is formed in the shadow part of the shadow mask 7, and the spacer 8 is retracted sufficiently from the edge of the shadow mask 7 so that the tapered part is formed in the shadow part of the shadow mask 7. The side corners are cut diagonally so that not only the length and position of the tapered part but also the angle of inclination can be controlled. Next, the relationship between each parameter and the formed thin film will be explained in detail.

According to the thin film forming method of the present invention, there are three parameters that are adjusted to control the shape of the tapered portion, the position where it is formed, and the like. That is, 1) the thickness of the spacer 8, 2) the shape of the end portion of the shadow mask 7, and 2) the overall thickness of the shadow mask 7 or the distance between the upper end surface of the shadow mask 7 and the surface of the substrate 9. As described below, each of these parameters is almost independently related to a part of the shape of the tapered portion to be formed.

First, we will discuss one problem when changing the thickness of the spacer shown in FIG. 2 among the three parameters mentioned above. In this case, as the thickness hs of the spacer increases, the length of the tapered portion increases. Therefore, the maximum value of the inclination angle of the tapered portion becomes small. This relationship is shown in Figure 4, where the horizontal axis is the thickness of the spacer hss, the vertical axis on the left is the length of the tapered part, and the vertical axis on the right is the maximum inclination angle of the tapered part, the thickness of the thick part. This value is normalized by . Mathematically, this normalization is like normalizing one corner of a right triangle by the length of its opposite side, and although it is not mathematically rigorous, since the taper angle is very small, the angle is − It is approximately proportional to the thickness of the various parts.

Next, a case will be described in which the shape of the edge portion of the shadow mask is changed. As shown in FIG. 2, if the lower corner of the edge of the shadow mask is cut diagonally, the normalized maximum taper angle can be made much smaller than if this portion was not cut.

For example, if the angle of the diagonally cut part is 45 degrees and the height he of the cut part is 400 μm, the surface shape of the tapered film formed in the shadow part of the shadow mask is shown in Fig. ). The conditions at this time, such as the thickness of the entire shadow mask and the thickness of the spacer, are the same as in FIG. 3(a).

As can be seen by comparing No. 31K (a) and 6), in the case of Fig. 31K (b) where the lower corner of the edge of the shadow mask is cut diagonally, the surface shape of the tapered film becomes smooth, The normalized maximum taper angle is very small. When this relationship is plotted in Figure 4, the points corresponding to Figure 3(a) are represented by white triangles, and the points corresponding to Figure 3(b) are represented by black triangles, and the normalized maximum taper angle is It has decreased to about one-fourth. On the other hand, the length of the tapered portion is represented by a white circle at the point corresponding to FIG. 3(a), and a black circle at the point corresponding to FIG. 3(6), and it can be seen that the length does not change much. From the above, if you cut the lower corner of the edge of the shadow mask diagonally, the length of the tapered shape that will be formed will hardly change.
It can be seen that the angle (maximum inclination angle) can be made very small. This is an important technique because it reduces radiation loss when forming a tapered leading waveguide, and also allows the element length to hardly change.

Finally, the relationship between the height of the vertical portion of the end of the shadow mask shown in FIG. 2 and the tapered shape will be described. If this height is increased, the length of the part outside the edge of the shadow mask will become longer within the length of the tapered part formed as shown in Figure 5, but it will be longer than the edge of the shadow mask. The length formed in the inner part, that is, the shadow part, hardly changes. Utilizing this fact, it is possible to adjust the relative positional relationship between the position of the end of the shadow mask and the center of the tapered shape formed therebelow.

In addition, in FIG. 5, the height of the vertical portion of the edge of the shadow mask was used as a parameter, but this relationship basically means that the distance between the upper end surface of the shadow mask and the substrate surface <h, +h, +h
If we keep = > constant, that is, if we reduce the thickness of the shadow mask by the amount that we increase the thickness of the spacer, then the length of the tapered part that is formed will be the length of the part inside the edge of the shadow mask, that is, the part of the shadow. The result is that the length becomes longer, and the length of the portion outside the edge of the shadow mask hardly changes.

Next, an overview of the manufacturing process will be explained.

FIG. 6 is a diagram showing the manufacturing process of the tapered waveguide type spot size converter according to the present invention.

Once each parameter is determined as described above, the manufacturing process begins by sequentially laminating a cladding mode removal layer and a low refractive index cladding layer on the substrate as shown in Figure 6.
! (a) to (C)), then install a shadow mask via a spacer, and layer a high refractive index core layer thinly to match the smaller spot size ((d) in the same figure). A shadow mask is positioned and installed on the opposite side via a spacer ((e) in the same figure), and a core layer having an intermediate refractive index is laminated ((f) in the same figure).

Finally, a cladding layer with a low refractive index is laminated (see figure (dark)).As shown in figure (e), the position of the shadow mask is adjusted to align the centers of the two tapered parts. It is also possible to align the centers of the two taper parts simply by aligning the two shadow masks so that they are almost in close contact with each other.By using the shadow mask sputtering method in this way, the shape and length of the taper parts, This is advantageous because the tilt angle and the like can be controlled.

Note that the present invention is not limited to the above embodiments, and various modifications are possible.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the tapered spot size converts the spot size between various waveguide type optical circuits having different spot sizes manufactured on the substrate using a semiconductor. Since the taper angle can be reduced without increasing the length of the converter, loss can be easily reduced. In addition, the shape and length of the taper part can be freely controlled, and the refractive index difference between the core and cladding can be greatly changed in the light propagation direction, so the spot size conversion ratio can be increased and the taper angle can be changed. It is possible to form a tapered part that satisfies the contradictory conditions of making the tapered part as small as possible and also making the length of the tapered part as short as possible from a practical standpoint. Furthermore, 2
Since the overlapping of the two tapered portions can be controlled with high precision, insertion loss can be easily reduced.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of a tapered waveguide spot size converter according to the present invention, and FIG. 2 is an embodiment of a thin film forming method for a tapered waveguide spot size converter according to the present invention. Figure 3 is a diagram showing the relationship between the shape of the edge of the shadow mask and the surface shape of the tapered film.
Figure 4 is a diagram showing the relationship between the thickness of the spacer and the length of the tapered part, and Figure 5 is a diagram showing the relationship between the height of the vertical part of the edge of the shadow mask and the length of the tapered part formed on the inside and outside of the shadow mask. FIG. 6 is a diagram showing the manufacturing process of the tapered waveguide spot size converter according to the present invention, and FIG. It is a figure which shows the example of use of a pot size converter. DESCRIPTION OF SYMBOLS 1... High refractive index glass core layer, 2... Cladding mode removal layer, 3... Semiconductor substrate, 4 and 6... Low refractive index glass cladding layer, 5... Intermediate refractive index glass・Core layer, 7... Shadow mask, 8... Spacer,
9-...Substrate, IO...Thin film. Applicant New Technology Development Corporation

Claims (2)

[Claims] (1) A tapered waveguide spot size converter that connects optical waveguides with different spot sizes, each made of a different material, with a core having a high refractive index and a thin core thickness compared to the cladding, and a core having a thinner core thickness than the cladding. Two slab waveguides with intermediate refractive index and thick cores are overlapped with their tapered ends facing each other, and the start position of the taper part of the high refractive index core is aligned with the tip of the taper part of the intermediate refractive index core. 1. A tapered waveguide spot size converter characterized in that the refractive index of the equivalent core changes uniformly. (2) A thin film forming method for a tapered waveguide spot size converter that connects optical waveguides with different spot sizes, in which a shadow mask is floated from the substrate via a spacer, and the spacer is moved from the edge of the shadow mask to the back. A taper characterized in that the shadow mask is arranged in a recessed manner, the lower corner of the edge of the shadow mask is cut off diagonally, and a thin film having a tapered portion is formed from the edge of the shadow mask inward by a sputtering method. Thin film formation method for waveguide spot size converter.