JP2011075645A - Optical wave guide and method of manufacturing of optical waveguide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that a spot size transformer is necessary because diameter of transmission of light is different when a Si thin wire waveguide is connected to the external part of an optical waveguide such as an optical fiber using an optical signal, but optical coupling efficiency is insufficient and a variation in manufacturing exists thus the manufacturing of the spot size transformer is not easy. <P>SOLUTION: An optical coupling block region having a structure made of a material which is the same as that of a clad layer and projected from the clad layer is disposed between the end face of the optical waveguide and the end face of the tapered structure region of the Si thin wire optical waveguide. Further, the tapered structure region is manufactured, the formed tapered structure section is referred to and a mask for forming the optical coupling block region is positioned. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical waveguide and a method for manufacturing the optical waveguide, and in particular, an optical waveguide having a spot size conversion structure used when connecting an optical waveguide using a thin silicon wire to a larger optical component such as an optical fiber, and the like It relates to a manufacturing method.

In recent years, a technique using silicon (Si: silicon, hereinafter abbreviated as Si) as an optical waveguide material has begun to attract attention for miniaturization and mass productivity. An optical waveguide having a structure in which the entire outer periphery of a core made of Si is covered with a clad having a lower refractive index, such as silica (SiO ₂ ), is referred to as a Si thin-wire optical waveguide.
In the above-mentioned structure made of this material, the Si wire optical waveguide has a very large difference in refractive index between the core and the clad, so that light can be strongly confined in the core. There is a feature that can realize the structure. Further, it is possible to realize an optical waveguide having a curved structure in which light is bent sharply with the large refractive index difference, and the bending radius can be reduced to about 1 micron.
From these facts, this Si wire optical waveguide can realize optical circuit devices of the same size as Si electronic devices, which is impossible with other structures, and fuses light and electrons on the device (chip). It is attracting attention as a leading technology. However, in order to connect to the outside of the device such as an optical fiber (as a rough value, the core is 8 μmφ and the outer diameter is 125 μmφ), the Si thin wire optical waveguide has a very small cross section (the approximate value of the optical waveguide core is 0.3 μm □). ) The spot size of the light passing through must be enlarged.

For this reason, various spot size converters for enlarging this light have been devised, and these are disclosed in Patent Documents 1-7. As a method for increasing the spot size of light, a method of reducing the optical waveguide core into a tapered shape (a shape that narrows toward the tip) is conventionally known. Here, the taper width of the optical waveguide core is disclosed in Patent Documents 1, 2, 3, and 7, and the taper thickness of the optical waveguide core is disclosed in Patent Documents 4, 5, 6. In addition to this, a taper-shaped three-dimensional structure in which the waveguide cross-sectional area is enlarged, or one using a grating (formation of a lattice pattern) has been proposed.
Among these, the tapered taper is easy to manufacture and has a wide wavelength band. In addition, since the doubled optical waveguide core disclosed in Patent Document 6 also guides an enlarged spot, the wavefront is fixed and light input / output with little loss can be performed.

FIG. 4 is a structural diagram of a spot size converter (taper structure region 4) when an optical signal is propagated from the optical waveguide 40 using a conventional Si thin wire to the optical fiber 5. In FIG. 4, first, an SOI (Silicon on Insulator) substrate as a substrate (including Si layer 1 and SiO ₂ layer 23 (SiO ₂ layer 2 (FIG. 2)) and Si layer 3 (FIG. 2) corresponding to optical waveguide core 3G). Is used. An optical waveguide core 3G having Si as a core is formed on this SOI substrate.
The optical waveguide 40 is provided with a tapered spot size converter (taper structure region 4) in the vicinity of the light incident / exit end face 2s. Here, the width of the core 3G of the Si optical waveguide is narrowed toward the tip, and the thickness is constant. The optical fiber 5 and the end face of the optical fiber core 5c are in contact with the light incident / exit end face 2s of the optical waveguide 40 in order to input light passing through the optical waveguide core 3G.
Here, for the sake of simplicity, the optical fiber 5 is simply a butt joint type (Butt Joint type) joint in which the optical fiber 5 is abutted against the optical waveguide 40, but in general, the optical coupling efficiency is good. In many cases, the tip of the optical fiber 5 is processed so as to obtain a tip-end optical fiber (having a lens structure). The spot size of the light propagating through the optical waveguide core 3G is submicron, and the spot size of the propagation light of the optical fiber 5 is several microns. Therefore, the optical waveguide 40 or the optical waveguide core is left as it is due to size mismatching. Light is not efficiently sent from 3G to the optical fiber 5. However, if the tapered structure region 4 is provided, the light spreads in this portion, so that it can be efficiently fed into the optical fiber 5. However, since light spreads rapidly between the front end (thin side) of the tapered structure region 4 and the light incident / exit end face 2s of the optical waveguide 40, the optical coupling efficiency varies unless this distance is constant. The resulting structure.

JP 2002-162528 A JP 2000-235128 A US Pat. No. 6,684,011 Japanese Patent Laid-Open No. 9-15435 JP 2005-326876 A JP-A-7-63935 JP 2003-207684 A

However, the technology disclosed in FIG. 4 and Patent Documents 1 to 5 that are simply tapered changes the distance between the tapered structure region and the light incident / exit end face of the optical waveguide due to variations in the manufacturing process. As a result, there is a problem that the optical coupling efficiency fluctuates.
Further, the techniques disclosed in Patent Documents 6 and 7 formed in a double core or stepped shape have a problem that the production is not easy and the manufacturing cost increases.

  Accordingly, the present invention has been made to solve the above-described problem, and an optical waveguide having a small change in optical coupling efficiency due to the distance between the tapered structure region and the input / output end face of the optical waveguide, and its manufacture It aims to provide a method.

In order to achieve the above object, each invention is configured as follows.
That is, in an optical waveguide having a core layer and a cladding region covering the periphery of the core layer, a tapered structure region in which a core width is narrowly formed along the propagation direction at one end of the core layer, a light incident / exit end face of the optical waveguide, and the tapered structure And an optical coupling block region having a structure projecting in a normal direction from the cladding region between the tip of the region.

  With such a configuration, the conversion of the spot size of light from the optical waveguide to the optical fiber is efficiently performed, and the optical coupling loss is reduced.

  A core layer and a cladding region covering the periphery of the core layer; a tapered structure region in which a core width is narrowly formed in one end of the core layer along a propagation direction; a light incident / exit end surface of an optical waveguide; An optical coupling block region having a structure projecting in a direction normal to the cladding region, and producing the tapered structure region, and then refer to the formed tapered structure region Then, the mask for forming the optical coupling block region is aligned.

  With this method, even if the tip position of the tapered structure region changes due to the narrowing of the manufacturing process, the position where the optical coupling block region is formed can be adjusted at the stage of mask alignment, and the positional relationship between the tapered structure region and the optical coupling block region Keep it optimal.

According to the present invention, it is possible to reduce the change in optical coupling efficiency due to the distance between the tapered structure region and the incident / exit end face of the optical waveguide.
For this reason, a spot size converter can be easily manufactured at low cost, and an optical waveguide having stable characteristics can be obtained.

1 is a structural diagram showing a structure of an optical waveguide according to an embodiment of the present invention. It is a figure which shows how to produce the optical waveguide which concerns on embodiment of this invention. It is a characteristic view of the optical coupling efficiency between the spot size converter including the optical coupling block area | region of the optical waveguide which concerns on embodiment of this invention, and an optical fiber. It is a structural diagram which shows the structure of the conventional optical waveguide.

Hereinafter, embodiments of the present invention will be described in detail.
FIG. 1 is a structural diagram of an optical waveguide according to an embodiment of the present invention. In FIG. 1, an SOI substrate having a structure in which both sides of an insulating layer made of SiO ₂ are sandwiched between Si is used as a substrate (Si layer 1 + SiO ₂ layer 23 + Si layer 3G). The optical waveguide 10 is formed by processing the SOI substrate, and includes an optical waveguide core 3G made of an Si layer and a cladding region 23 made of an SiO ₂ layer. Further, a tapered structure region 4 made of a Si layer is formed at the tip of the optical waveguide core 3G at the same time. In FIG. 1, the portion indicated by the optical waveguide core 3G and the tapered structure region 4 is a region made of an Si layer, and there is a cladding region made of an SiO ₂ layer around it. The lower clad region 23, which is a part of this clad region, is composed of an SiO ₂ layer and is also shown in FIG. 1, but the clad region composed of the upper and lateral SiO ₂ layers is shown in FIG. Not shown for ease of viewing. However, it is shown in FIG.

The refractive index of the refractive index difference is large (Si light between the core and the cladding in the optical waveguide 10 by the optical waveguide core 3G and cladding region 23 made of SiO ₂ formed of the Si is about 3.5, also SiO _{2 has} a refractive index of approximately 1.46), the light propagates inside the optical waveguide core 3G without leaking to the outside.
A tapered structure region 4 is formed at the end of the optical waveguide core 3G. In the taper structure region 4, the width of the core made of the Si layer is narrowed toward the tip. It is known that there is a region where the spot size of light becomes wider when the core width is narrowed to a certain extent, and the optical coupling loss of both can be reduced by matching the spread spot size with the spot size of the optical fiber. . Therefore, the optical waveguide 10 is provided with the tapered structure region 4, and the light spot size is easily converted at the tip by advancing light therein.

The structure of the optical waveguide core 3G and the tapered structure region 4 so far is the same as that shown in FIG. In FIG. 4 of the conventional example, the tip of the taper structure region 4 is coupled to the optical fiber 5.
In this embodiment, further in FIG. 1, the optical waveguide 10 has an optical coupling block region 21 made of the same material (SiO ₂ ) as the cladding region 23, between the tapered structure region 4 and the light incident / exit end face 2s of the optical waveguide. The structure is provided so as to protrude to the surface side of the cladding region 23. The optical coupling block region 21 will be described in detail later.

Note that the refractive index of SiO ₂ varies depending on the crystal structure and the temperature and pressure during crystallization, and the refractive index of 1.46 is an example of a typical value of quartz (SiO ₂ ).
Further, in the relationship between the refractive index of the core and the clad, the refractive index of Si is about 3.5 and the refractive index of SiO ₂ is about 1.46. At this time, the refractive index of the core (Si) is about 2.4 times the refractive index of the cladding (SiO ₂ ). Thus, when there is a large difference in refractive index, the light confinement effect is large, and light propagates in the optical waveguide core 3G of the optical waveguide 10 without leaking outside. However, even without such a large difference in refractive index, the light confinement effect of the optical waveguide may be sufficient. In general, if the refractive index of the core is 1.4 times or more (40% or more larger) than the refractive index of the cladding, it is practical as an optical waveguide having a dimension of about 1 micron or less.

(Optical coupling block area)
The optical coupling block region 21 that is a feature of the present embodiment will be described.
As shown in FIG. 1, the optical coupling block region 21 has a rectangular parallelepiped structure protruding from the upper surface of the cladding region 23. The light incident / exit end face 2s of the optical waveguide is a flat surface. Further, the end face 21sa of the optical coupling block region 21 on the tapered structure region 4 side made of the SiO ₂ layer is a plane perpendicular to the upper surface of the cladding region 23 and perpendicular to the direction of the optical waveguide core 3G. An end surface 21sb on the side surface of the coupling block region 21 parallel to the extending direction of the optical waveguide core 3G is a plane perpendicular to the upper surface of the cladding 23 region. An upper end surface 21 sc of the optical coupling block region 21 is a plane parallel to the surface of the cladding region 23. Further, each side surface of the optical coupling block region 21 may be a curved surface or may not be vertical. However, if the optical coupling block region 21 has a rectangular parallelepiped structure as shown in FIG. 1, the manufacturing process is relatively easy.

The upper part of the optical coupling block region 21 and the outside of the side face are the outside of the optical waveguide 10 and are usually filled with air. Therefore, the optical coupling block region 21 is a SiO ₂ layer having a refractive index of about 1.46, and the periphery is an air layer having a refractive index of about 1.0. If the length of the taper structure region 4 is long to some extent, the spread angle of light coming out of the taper structure region 4 is small, and light is totally reflected by the total reflection of light due to the refractive index difference between the light block region 21 and the surrounding air layer. There is no leakage from the optical coupling block region 21 to the surrounding air layer.

  Next, the role of the optical coupling block region 21 will be qualitatively described. The light incident on the tapered structure region 4 from the optical waveguide core 3G changes in light spot size as the taper width decreases, and the end face of the optical coupling block region 21 is located at a position that matches the spot size of the optical coupling block region 21. It is joined to 21sa. Therefore, light propagates efficiently from the tapered structure region 4 to the optical coupling block region 21. The light transferred to the optical coupling block region 21 spreads in the optical coupling block region 21 and the light spot size also increases. And the light of the optical coupling block area | region 21 propagates efficiently to the optical fiber 5 with which the spot size did not have much difference. In other words, the optical coupling block region 21 functions to smoothly propagate light between the tapered structure region 4 and the optical fiber 5.

Hereinafter, the operation of providing the optical coupling block region 21 will be described. In the case where the optical coupling block region 21 is not provided, the light spot size changes when light entering the tapered structure region 4 from the optical waveguide core 3G moves toward the tip (narrow side) of the tapered structure region 4. . In an appropriate taper width, there is a region where the spot size widens, and when the spot size of light in the taper structure region 4 approaches the spot size of the optical fiber 5, it is joined to the optical fiber 5 and optically coupled. Propagate light.
However, the optical fiber 5 is another element as viewed from the optical waveguide 10 and is an external component. Therefore, the distance between the optical fiber 5 and the optical waveguide 10 varies when they are joined. If this distance varies, the spot size of the light emitted from the tip of the tapered structure region 4 also changes depending on this distance, so that it does not match the spot size of the optical fiber, and the optical coupling efficiency also varies.

  On the other hand, the optical coupling block region 21 of this embodiment is included in the optical waveguide 10 as the same component elements as the optical waveguide core 3G and the tapered structure region 4, and is manufactured in the same manufacturing process. Therefore, the distance between the tip of the taper structure region 4 and the end surface 21sa of the optical coupling block region 21 can be manufactured stably. Although the position of the tip of the taper structure region 4 varies somewhat due to variations in the etching process of Si for producing the taper structure region 4 to be described later, when compared with the variation in aligning the optical fiber 5 that is an external component, It is very stable because it can be manufactured at once with a wafer (SOI substrate). That is, since the distance between the tip of the taper structure region 4 and the end surface 21sa of the optical coupling block region 21 is stable, the optical coupling efficiency can be stably maintained at a high value. The light that has moved to the optical coupling block region 21 does not change much between the light spot size of the optical coupling block region 21 and the light spot size of the optical fiber 5. The light is smoothly propagated. Therefore, the optical coupling efficiency can be stably maintained at a high value even when there are manufacturing variations and when the external component (optical fiber 5) is attached. That is, by providing the optical coupling block region 21 and propagating light through the optical coupling block region 21, high optical coupling efficiency can be stably obtained.

  As described above, the qualitative reason for the smooth propagation of light from the optical waveguide 10 to the optical fiber 5 by providing the optical coupling block region 21 has been described. Further, what shape the optical coupling block region 21 has and how much effect it has will be described later from a quantitative viewpoint of optical coupling efficiency characteristics.

(Outline of manufacturing method of optical waveguide including optical coupling block region)
Next, a method of manufacturing the optical waveguide 10 including the optical coupling block region 21 will be described with reference to FIGS.
In FIG. 2, (a), (b), (c), and (d) represent manufacturing steps in order. In each of the drawings (a) to (d), the upper figure is a plan view of the optical waveguide 10 as viewed from above, and the lower figure shows a cross section of the optical waveguide 10 near the light incident / exit end face 2s. FIG. In order to make the structure easy to understand, the structure of the part slightly away from the cross section is also illustrated virtually assuming that the materials constituting the optical waveguide 10 are Si and SiO ₂ are translucent.

The optical waveguide 10 is manufactured by the following process.
(1) The manufacturer first prepares an SOI substrate. An SOI substrate already prepared as a product can be purchased from a dedicated manufacturer. As shown in FIG. 2A, the SOI substrate has a structure of an Si layer 1, an SiO ₂ layer 2, and an Si layer 3 in order from the lower layer.
(2) In order to form the structure of the optical waveguide core 3G and the tapered structure region 4 in FIG. 2B, the resist is patterned (formed) using a mask having a shape as viewed from above. Then, unnecessary portions of the Si layer 3 are removed according to the shape of the mask by dry etching from above the resist, and the core portion 3G of the waveguide structure made of Si and the tapered structure region 4 are formed as shown in FIG. It is formed as follows.
(3) Next, in FIG. 2 (c), an SiO ₂ layer 22 (FIG. ₂ ) that becomes the upper cladding region with the same material (SiO ₂ ) as the lower cladding region 2 from above the structure of FIG. 2 (b). 2 (c)) is formed by depositing SiO ₂ using a CVD (Chemical Vapor Deposition) method.
(4) In order to form the structure of the optical coupling block region 21, which is a protruding portion in FIG. 2D, the resist is patterned (formed) using a mask having a shape as viewed from above. Then, by dry etching from above the resist, unnecessary portions of the SiO ₂ layer 22 are removed according to the shape of the mask, and the protruding portions 21 are formed. This protrusion 21 becomes the optical coupling block region 21. Note that the remainder obtained by removing unnecessary portions from the SiO ₂ layer 22 becomes the SiO ₂ layer 23.

As described above, the optical coupling block region 21 is formed between the tapered structure region 4 of the Si wire waveguide structure and the light incident / exit end face 2 s of the optical waveguide 10. Further, in portions other than the optical coupling block region 21, the lower cladding and the upper cladding are integrated to form a cladding region 23. Thereby, the optical waveguide 10 by the Si thin wire which covered the optical waveguide core 3G with the lower clad and the upper clad can be produced.
The position of the optical coupling block region 21 may be set in advance, but the end face 21sa (FIG. 1) of the optical coupling block region 21 is aligned with the tip (thin side) of the already formed tapered structure region 4. It can be considered that the mask is aligned. This is because the tip position of the tapered structure region 4 may change due to the narrowing due to the process.

In addition, when the process for producing the optical waveguide 10 is further described, the optical waveguide core 3G made of the Si layer described with reference to FIG. 2B has a vertical or horizontal dimension of 0.5 microns (μm). ) Set to single mode as below. It is known that when the vertical and horizontal directions are 0.3 microns, a single mode is obtained in the vertical and horizontal directions. When optical loss is considered, it is better to increase the optical confinement of the fundamental mode to the core to about 0.5 microns. The higher-order mode is not a problem because of a large loss, and operates as a single mode. The distance between the Si core layer 3G and the Si layer 1 for the light of the Si waveguide 10 is prevented from escaping into the Si layer 1 of the substrate, that is an insulating layer made of SiO ₂ (SiO ₂ layer 2) is 1 micron It is good to do it above.

(Optical coupling efficiency characteristics calculation example)
Next, the effect of using the optical coupling block region 21 will be described from a quantitative viewpoint.
FIG. 3 shows an example in which the characteristics of the optical coupling efficiency in the configuration of the optical waveguide 10 having the optical coupling block region 21 are calculated. In FIG. 3, the vertical axis represents the light coupling efficiency under the conditions described later, and the horizontal axis represents the distance between the light incident / exit end face 2 s of the optical waveguide 10 and the tip of the tapered structure region 4, and the characteristics are graphed. .

The wavelength of the propagating light is 1.55 microns. The thickness of the residual clad on the upper portion of the optical waveguide core 3G is 1 micron, and the width of the optical coupling block region 21 is 6 microns. Also, the height was set to T, and the case was divided into 1 micron and 2 microns. The length of the tapered structure region 4 is 200 microns. The cladding regions 2 and 23 and the optical coupling block region 21 are assumed to be quartz (SiO ₂ ) having a refractive index of 1.46. The optical waveguide core 3G is 0.3 micron-square Si and has a refractive index of 3.5.

  Under the above conditions, the light of the optical fiber 5 having a spot size of 8 microns in diameter is irradiated onto the block 21 at the light incident / exit end face 2s of the optical waveguide 10, and the proportion of the light in the optical waveguide core 3G. Is excited using a three-dimensional BPM method (Beam Propagation Method).

  In the characteristic diagram of FIG. 3, it can be seen that the influence of the distance (the distance between the light incident / exit end surface 2 s of the optical waveguide 10 and the tip of the tapered structure region 4) varies depending on the height of the optical coupling block region 21. When there is no optical coupling block region 21 (T = 0 micron), the efficiency decreases almost linearly as the distance increases. Further, when the optical coupling block region 21 is present (T = 1 or 2 microns), even if the distance is 200 microns, it is still 70% relative to the maximum (the distance between the tip of the tapered portion and the end face of the optical waveguide is 0). Maintain efficiency.

  As described above, by providing the optical coupling block region 21, high optical coupling efficiency can be maintained even when the optical fiber 5 is coupled at a distance of 200 microns from the tip of the tapered structure region 4 (the best value (tapered structure region 4 70% of the value at the front end of the optical fiber 5), and according to this embodiment, a spot size converter for connecting a large optical component such as the optical waveguide 5 and the optical fiber 5 of the Si thin wire having a minute cross-sectional area. Can be easily manufactured.

  It is predicted that there will be more and more promising markets in the future for the integration of silicon optical waveguides with excellent miniaturization and mass productivity, and silicon electronic devices. Among them, the present invention can provide a spot size conversion means for efficiently transmitting an optical signal from a silicon optical waveguide to an optical fiber with a stable performance and can be widely used.

1, 3 Si layer _2, 22, 23 SiO ₂ layer (cladding, cladding region)
2s Light input / output end face of optical waveguide 3G Optical waveguide core, core 4 Tapered structure region 5 Optical fiber 5c Optical fiber core 10, 40 Optical waveguide 21 Optical coupling block region 21sa, 21sb, 21sc SiO ₂ constituting optical coupling block region End face of layer

Claims (5)

In an optical waveguide having a core layer and a cladding region covering its periphery,
A tapered structure region in which the core width is narrowly formed along the propagation direction at one end of the core layer;
An optical coupling block region having a structure protruding in a normal direction from the cladding region between a light incident / exit end surface of the optical waveguide and a tip of the tapered structure region;
An optical waveguide comprising:   The optical waveguide according to claim 1, wherein the refractive index of the core layer is 40% or more larger than the refractive index of the cladding region.   The optical waveguide according to claim 1, wherein the optical coupling block region and the cladding region are made of the same material.   The optical waveguide according to any one of claims 1 to 3, wherein the optical coupling block region is formed in a rectangular parallelepiped shape. Between the core layer and the cladding region that covers the periphery of the core layer, a tapered structure region in which the core width is narrowly formed in one end of the core layer along the propagation direction, and between the light incident / exit end face of the optical waveguide and the tip of the tapered structure region In an optical waveguide manufacturing method comprising: an optical coupling block region having a structure protruding in a normal direction from the cladding region;
A method of manufacturing an optical waveguide, comprising: forming the tapered structure region, and then aligning a mask for forming the optical coupling block region with reference to the formed tapered structure region.

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