JP2001356229A - Optical integrated circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical integrated circuit which can be made small in size and in which input and output of light is rather easily performed and an optical circuit including passive devices can be formed. SOLUTION: A PhC(photonic crystal) circuit 1 and a Si optical waveguide 10 are integrated by connecting the Si optical waveguide 10 to the optical waveguide formed by the PhC structure of the PhC circuit 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical integrated circuit having an optical waveguide used for optical communication and optical signal processing.

[0002]

2. Description of the Related Art The bending radius of an ordinary optical waveguide is limited by the difference in refractive index between the optical waveguide portion and the periphery thereof. Si
Since a semiconductor material such as (silicon) has a high refractive index, it is easy to obtain a large refractive index difference from the periphery.
It can be said that this material is suitable for an optical waveguide that requires miniaturization and high integration. In particular, the combination of Si and SiO ₂ (silicon oxide) has a great advantage in terms of manufacturing because mass production technology cultivated in LSI (large-scale integrated circuit) and high-precision fine processing technology can be applied.

A conventional semiconductor optical waveguide is, for example, shown in FIG.
As shown in FIG. 2A, an SiO ₂ layer 14 is formed on a Si substrate 13, and a Si layer 15 is further formed on the SiO ₂ layer 14, using a so-called SOI (Silicon on Insulator) substrate.
Etching was performed leaving a part of the layer 15 to form the core 11 and the rib 16 as shown in FIG. 6B. FIG. 7A shows a cross-sectional structure of a rib-type optical waveguide, and FIG. 7B shows a cross-sectional structure of a buried rib-type optical waveguide in which a core 11 and a rib 16 are embedded with a cladding layer 17 such as SiO ₂ .

The reason why the semiconductor optical waveguide is formed into a rib shape as shown in FIGS. 6 (b), 7 (a) and 7 (b) is as follows. In an optical waveguide having Si as a core and SiO ₂ as a clad, Si in a communication wavelength band of 1.55 μm is used.
(Refractive index n ₁ = 3.48) and SiO ₂ (refractive index n ₂ =
The relative refractive index difference Δ = (n ₁ −n ₂ ) / n ₁ with 1.44) is large. Therefore, an optical waveguide of a type such that the cladding completely surrounds the core such as a buried rectangular optical waveguide,
To make a single mode optical waveguide suitable for large capacity transmission, the core diameter, that is, the thickness and width of the optical waveguide are about 0.2.
It becomes about 5 μm, and it becomes difficult to input and output light (connection to an optical fiber).

On the other hand, in the rib type optical waveguide, the in-plane lateral direction (x
Direction), the equivalent refractive index of the clad-equivalent portion increases, and the relative refractive index difference with respect to the core decreases, so that the width of the optical waveguide for making a single mode increases to some extent, making processing and input / output of light relatively easy. Become.

Conventionally used quartz-based planar lightwave circuits (P
Passive devices such as an optical multiplexer / demultiplexer and an arrayed waveguide diffraction grating realized in a laner lightwave circuit (hereinafter, PLC) can also be manufactured.

For these reasons, research and development of a rib-type optical waveguide in consideration of light input / output with the outside in a combination in which the relative refractive index difference between the original materials is large has been promoted.

On the other hand, a photonic crystal structure (hereinafter referred to as a PhC structure), which has been actively studied recently, is formed by forming a fine periodic structure having a period of about half the wavelength of light.
This generates a so-called light band gap, in which light of that wavelength cannot exist.

Accordingly, by forming a defect in the PhC structure along the path through which light propagates, the defective portion functions as an optical waveguide. In light propagation by this method, 1
Since a confinement effect of 00% can be expected, sharp bending is also possible, and miniaturization of the optical circuit is expected.

Originally, the PhC structure is formed three-dimensionally and a three-dimensional band gap is generated. However, as shown in the cross-sectional structure in FIG. 8, the materials 7a, 7b,
The two-dimensional PhC structure in which light is confined by a refractive index difference of 7c and only two-dimensional confinement is performed by the in-plane PhC structure 4 is relatively easy to fabricate, and is being studied.

A diagram of an example of a two-dimensional PhC structure as viewed from above,
FIGS. 9A and 9B and FIG. Thus, it is sufficient that the structure has the structure in which the holes 8a are periodically arranged or the structure 4 in which the columns 8b are arranged, and the arrangement is not limited to one. 9 and 10, the cross-sectional shapes of the holes 8a and the columns 8b are represented by circles. However, actually, the shapes are not limited to circles, and hexagons, quadrangles, and the like have been tried.

FIGS. 11 and 12 show examples in which the optical waveguides 2 and 3 are formed by giving defects to the two-dimensional PhC structure 4. FIG. The optical waveguide 2 is linear, and the optical waveguide 3 has a bent portion. These show how to give a defect to the structure and are not necessarily examples of actual lightwave circuits.

A two-dimensional PhC structure (hereinafter simply referred to as a PhC structure) 4 may be formed by using a single material having a large refractive index, such as Si, for example, to form a fine periodic structure as shown in FIGS. However, in consideration of the expandability in the three-dimensional direction and the combination with other devices, the holes 8a or the gaps between the columns 8b are filled with another material, and a low-refractive-index substance is placed on the two-dimensional structure. There is also a method of stacking. In such a case, the combination of the materials is problematic, but the combination of Si and its oxide, SiO ₂ , is the most promising combination because of its good material consistency and easy processing.

[0014]

SUMMARY OF THE INVENTION As described above, SO
The rib type optical waveguide using the I-substrate has advantages in manufacturing and Si
Due to the expectation of integration with devices, studies are proceeding as inexpensive optical waveguides. However, this method has a problem that the effective non-refractive index difference between the core and the clad is set to be small, so that the confinement effect is small, and the minimum bending radius increases accordingly, which hinders miniaturization of the optical integrated circuit. There is a point.

On the other hand, an optical waveguide using a PhC structure capable of sharply bending has just begun to be studied, and an experiment for propagating light to the optical waveguide is currently being conducted. Further research is needed on how to implement the passive circuit that has been implemented, and at present it is still a difficult situation from both sides of design and processing. In addition to the problem of the circuit configuration, there is a problem of the spot size.

For example, in the case of a normal single mode optical fiber, the spread of light propagating through the fiber has a mode field diameter of about 8 to 10 μm. On the other hand, in an optical waveguide having a PhC structure, it depends on the design. Since the spot diameter is as small as about 0.3 μm, it is not easy to efficiently input light into the optical waveguide. In other words, the coupling loss due to mode mismatch with the optical fiber is too large. That is, since the spot size becomes very small in the optical waveguide having the PhC structure, there is a problem that it becomes very difficult to input and output light to and from the outside.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has as its object to reduce the size, relatively easily input and output light, and configure an optical circuit including a passive device. It is to provide technology.

[0018]

According to the present invention, the above problems are solved by combining a general optical waveguide such as a normal semiconductor optical waveguide with an optical waveguide having a photonic crystal structure. An optical integrated circuit, such as an optical waveguide or a lightwave circuit, having both ease of manufacture and stability and a high optical confinement effect of a photonic crystal structure is realized.

That is, according to the first aspect of the present invention, there is provided a first optical integrated circuit portion having a first optical waveguide structure formed of a fine periodic structure formed in a light propagation layer, and connected to the first optical waveguide structure. And a second optical integrated circuit portion having a second optical waveguide structure in which a core as a light propagation layer is disposed adjacent to a region having a lower refractive index than the core.

According to a second aspect of the present invention, in the optical integrated circuit, a bent waveguide structure is introduced into the first optical waveguide structure, and the bent waveguide is formed by a strong light confinement effect of the first optical waveguide structure. 4. The optical integrated circuit according to claim 3, wherein a bending radius of the structure is reduced.
The optical integrated circuit of the invention according to claim 4, wherein
6. The invention according to claim 5, wherein the spot size of the guided light is converted by continuously changing the size of at least one of the first optical waveguide structure and the second optical waveguide structure. In the optical integrated circuit of the above, the material of the core is Si
The optical integrated circuit according to the invention according to claim 6 is characterized in that the second optical waveguide structure is a rib-type optical waveguide.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following optical integrated circuit will be mainly described as an embodiment of the present invention. (1) Fine periodic structure formed in the light propagation layer, so-called two-dimensional photonic crystal structure (hereinafter simply referred to as PhC structure)
A method of confining light in a defect part by providing a defect in the light, and increasing the refractive index of only the light propagation layer with respect to the upper and lower layers in the direction perpendicular to the plane including the light traveling direction. And an optical integrated circuit in which light is confined by providing a core of a substance having a high refractive index along a path through which light travels in the plane. (2) The main optical circuit portion or the light propagation portion is formed of an optical waveguide with a high refractive index core, and the bending radius of a portion requiring a strong confinement effect such as a bent portion of the optical waveguide is applied by applying a PhC structure. An optical integrated circuit that has been made smaller and smaller overall. (3) The main optical circuit portion or the light propagation portion is formed by confining light by the PhC structure, and optical connection with the optical input / output portion with the outside or other circuit portion having a relatively large light spot size on the same substrate. The part is an optical integrated circuit using an optical waveguide forming a core made of a high refractive index material. (4) The connection efficiency between the PhC structure portion and the ordinary optical waveguide portion is provided by attaching a spot size conversion function to the connection portion between the light propagation path using light confinement by the PhC structure and the high refractive index core type optical waveguide. Optical integrated circuit with enhanced performance.

Such an optical integrated circuit realizes an optical input / output with a large spot size using an optical waveguide having a high refractive index core, such as a normal semiconductor optical waveguide, while maintaining the PhC.
Due to the structure, the minimum bending radius can be kept small, the size can be reduced, and the external input / output can be made efficient.

[Embodiment 1] FIGS. 1 and 2 show a first embodiment of the present invention in which a Si optical waveguide is applied to an optical input / output unit when an optical integrated circuit is formed by a PhC structure. Is shown.

In FIGS. 1 and 2, a PhC circuit portion 1 formed by a PhC structure has an optical waveguide structure formed by providing a defect in the PhC structure (see optical waveguides 2 and 3 in FIGS. 11 and 12). And the optical waveguide structure has Si
The cores of the optical waveguide 10 (see the core 11 in FIGS. 6 and 7) are integrated so as to be connected. The content of the PhC circuit unit 1 is omitted in the sense that the circuit configuration is diversified. However, optical resonators, optical filters, optical switches, and the like using a PhC structure have been proposed. A possible lightwave circuit shall be included. As the Si optical waveguide 10, the rib-type Si optical waveguide shown in FIGS. 6B and 7A or the buried rib-type Si optical waveguide shown in FIG. 7B is used.

In the optical integrated circuit shown in FIG. 1, the periphery of the PhC circuit section 1 is simply connected to an external input / output such as an optical fiber by a Si optical waveguide 10 connected to an optical waveguide having a PhC structure. .

The optical integrated circuit shown in FIG.
The other planar lightwave circuit unit 20 is also connected to the PhC circuit unit 1 via the “0”.

The combination of the PhC circuit section 1 and the Si optical waveguide 10 or the combination of the PhC circuit section 1 and the Si optical waveguide 10 and another light wave circuit section 20 forms an optical integrated circuit by a PhC structure. This is effective when the optical input / output unit needs to be connected to an optical fiber, or when the connection destination is not an optical fiber, to efficiently input / output light to / from the PhC circuit unit 1.

As an example, the condition of the spot size (spot diameter) is set as follows.
3 μm, optical fiber spot size is 10 μm, Si
The effectiveness will be described assuming that the spot size of the optical waveguide is 3 μm.

In this case, as in the conventional case, the spot size of 10 μm is added to the PhC circuit section 1 having the spot size of 0.3 μm.
When the m optical fibers are directly connected, the mode mismatch loss caused by the large difference in the spot diameter is 24 db on one side when referring to FIG. 13 and 49 db when considering both the input and output sides.

On the other hand, as shown in FIGS. 1 and 2, the PhC circuit section 1 having the spot size of 0.3 μm and the optical fiber having the spot size of 10 μm are connected via the Si optical waveguide 10 having the spot size of 3 μm. In this case, even if the mode mismatch loss at two locations added through the mediation of the Si optical waveguide 10 is added, the sum is 19 db on one side and 5 d on one side.
b, 10 db loss was reduced on both sides.

Further, by optimizing the spot size of the mediating Si optical waveguide 10, the mode mismatch loss can be reduced as shown in FIG. From FIG. 13, the optimum value of the mediating spot diameter under the above conditions is 1.75 μm.
At this time, the mode mismatch loss is 38 db.

As described above, by using the Si optical waveguide 10 for the optical input / output unit of the PhC circuit unit 1, there is an effect that the loss due to mode mismatch can be reduced.

[Embodiment 2] FIG. 3 shows a second embodiment of the present invention, in which, for example, a quartz-based planar lightwave circuit (PLC), which is usually made, is directly manufactured using a Si optical waveguide. An optical integrated circuit in which a PhC structure is applied to a bending circuit portion is shown. In FIG. 3, the ordinary optical circuit portion is omitted, and only the bent portion is shown in an enlarged manner.

In the optical integrated circuit shown in FIG. 3, most of the PLC 30 is formed by the Si optical waveguide 10 whose cross section is shown in FIG. The PhC structure 4 is applied to the portion 32, and a defect is provided along the place 31 to be bent, and the bent optical waveguide 5 is formed.
The Si optical waveguide 10 is used for optical input / output with an optical fiber, for example.

In general, the current silica-based PLC has a waveguide width of 1
0 μm and the allowable minimum bending radius is about 5 μm.

On the other hand, the Si optical waveguide 10 described above can be designed to have a single waveguide with a narrower waveguide width by adjusting structural parameters such as the thickness of the core and the height of the rib.

For example, as shown in FIG.
_{2 In the} embedded Si optical waveguide, the front thickness t of the core is 4 μm,
Under the condition that the rib height h is 2 μm, the width of the waveguide to be a single mode is 4 μm. This waveguide width 4μ
The spot size based on m is about 4 μm, which is a spot size that matches the size of the light receiving surface of the photodetector (PD) when light is introduced into the photodetector (PD) by, for example, an optical waveguide.

In FIG. 3, S having a spot size of 4 μm
In the connection between the i optical waveguide 10 and the optical fiber having a mode field diameter of 10 μm, the coupling loss due to mode mismatch is 3.2 db, and the PhC having a spot size of 0.3 μm is used.
It can be said that the optical waveguide structure has a very small loss as compared with a coupling loss of 24 db due to mode mismatch when the optical waveguide 5 having the structure and the optical fiber are directly connected.

Further, the Si optical waveguide 1 under the above conditions is used.
The minimum bending radius of 0 is about 4 mm.

That is, the core material of the optical waveguide is SiO_TwoFrom
Cladding SiO by changing to Si _TwoRelative refractive index difference
The waveguide width becomes smaller because it becomes larger (from 10 μm
4 μm), but the bend radius is not very small (5 m
from 4m to 4mm)
There is no merit.

On the other hand, in the present embodiment, the circuit of the PLC 30 is manufactured as it is with the Si optical waveguide 10, the PhC structure 4 is applied to the bent circuit portion, and the optical waveguide 5 due to the defect of the PhC structure 4 is introduced. The waveguide width and the spot size can be kept as they are, that is, the coupling loss in the optical input / output portion with the outside can be suppressed, and only the allowable bending radius can be reduced.

The PhC structure completely confines light, and its effect on bending has been proved. The optical waveguide 10 itself having the above-described structural parameters has, for example, a bending radius of 0.1 mm.
If it is attempted to bend at 5 mm, the radiation loss due to the bending is large and it is difficult to use it as an optical waveguide. However, by applying the PhC structure 4 to the bent portion as in this example, the loss is suppressed and the radius is small. Bending can be realized. That is, according to the technique of this example, miniaturization can be achieved without sacrificing coupling loss.

[Embodiment 3]

FIGS. 4 and 5 show an optical integrated circuit having a spot size conversion (SS conversion) structure between an optical waveguide having a PhC structure and a Si optical waveguide as a fourth embodiment of the present invention.

Each of the optical integrated circuits illustrated in FIGS. 4 and 5 has the same structure as that of the first embodiment shown in FIGS. 1 and 2 in the case where the optical integrated circuit is formed by the PhC structure. This is an example in which the optical waveguide 10 is applied. The difference from the first embodiment is that a spot size conversion (SS conversion) structure is provided between the optical waveguide 2 having a PhC structure and the Si optical waveguide 10. is there.

In the spot size conversion structure 12 of the optical integrated circuit illustrated in FIG. 4, the waveguide width of the Si optical waveguide 10 is set to Ph.
At the connection portion with the optical waveguide 2 by the C structure 4, PhC
The size is reduced according to the structure 4, and the width of the waveguide is gradually increased continuously to change the spot size. In FIG. 4, the spot size conversion structure 12 on the left has a constant width from the middle.

Further, the spot size conversion structure 6 of the optical integrated circuit illustrated in FIG. 5 continuously and gradually increases the optical confinement width of the optical waveguide 2 by the PhC structure 4 for connection with the Si optical waveguide 10. Enlarge and convert spot size, S
The structure connects to the i optical waveguide 10.

In FIG. 5, the right Si optical waveguide 10 is provided with a spot size conversion structure 12 as shown in FIG.

It is apparent that the coupling efficiency is increased in both the optical integrated circuits of FIGS. 4 and 5 as compared with the direct connection between the spot size of 0.3 μm and the spot size of 4 μm.
Table 1 shows the relationship between the connection spot size and the coupling loss.

[Table 1]

For example, a Si optical waveguide 10 having a spot size of 1 μm is connected to the optical waveguide 6 having a spot size of 0.3 μm in the PhC structure 4, and the spot size conversion structures 6 and 12 are used from this connection portion. To 4 μm and connect to an optical fiber. By this, originally 2
The coupling loss from 0 db (16.5 db + 3, 2 db) can be suppressed to 8.4 db (5.2 db + 3, 2 db).

That is, the optical waveguide 2 of the PhC structure 4 and S
Changing the spot size of the guided light by changing the size of at least one of the i-optical waveguide 10 has the effect of reducing the coupling loss.

In each of the above embodiments, the semiconductor optical waveguide is considered to have a particularly great advantage in fabrication.
Although the optical waveguide has been mainly described as an example, actually, the optical waveguide and the PhC structure do not necessarily need to be made of Si.
Any material that can maintain a difference in refractive index, such as a compound semiconductor such as aAs or InP, or a ternary or quaternary material obtained by adding Al or the like thereto, can be used.

[0053]

As described above, according to the optical integrated circuit of the present invention, the first optical waveguide structure having the PhC structure and the core as the light propagation layer are adjacent to the region having a lower refractive index than the core. By combining and combining the second optical waveguide structure of the arranged structure, it is possible to reduce the size by using the PhC structure, and the second optical waveguide structure allows easy and efficient light input and output with the outside and the like. Therefore, there is a great effect that a small-sized and low-loss optical integrated circuit can be formed.

Further, by introducing a bent waveguide structure into the first optical waveguide structure, the strong optical confinement of the first optical waveguide structure is improved as compared with the case where a bent waveguide structure is introduced into the second optical waveguide structure. The bending radius can be reduced by the action.

In addition, by having the second optical waveguide structure as an optical input / output structure, compared with the case where the first optical waveguide structure is provided as an optical input / output structure, it is possible to connect to the outside or other circuit parts within the same substrate. Coupling loss is reduced.

Further, by changing the spot size by continuously changing at least one of the sizes of the first and second optical waveguide structures, the coupling loss between the two optical waveguide structures is reduced.

Further, the core material of the second optical waveguide structure is Si
By doing so, there is an advantage in manufacturing that mass production technology of LSI and high-precision fine processing technology can be applied.

[Brief description of the drawings]

FIG. 1 is a plan view showing an optical integrated circuit in which a PhC circuit portion having an optical waveguide structure based on a PhC structure and a Si optical waveguide are combined as a first embodiment of the present invention.

FIG. 2 is a plan view showing, as a first embodiment of the present invention, an optical integrated circuit in which, in addition to the circuit of FIG. 1, another lightwave circuit unit connected to a PhC circuit unit via a Si optical waveguide is combined; .

FIG. 3 is a diagram showing an optical integrated circuit in which an optical waveguide structure having a PhC structure is applied to a bending circuit portion of a Si optical waveguide as a second embodiment of the present invention.

FIG. 4 is a diagram showing an optical integrated circuit in which the size of a Si optical waveguide changes continuously as a third embodiment of the present invention.

FIG. 5 is a diagram showing an optical integrated circuit in which the size of an optical waveguide having a PhC structure and the size of a Si optical waveguide change continuously as a third embodiment of the present invention.

FIG. 6 is a diagram showing an example of an SOI substrate and a Si optical waveguide formed using the SOI substrate.

FIG. 7 is a diagram showing an example of a cross-sectional structure of a rib-type optical waveguide and a buried rib-type optical waveguide.

FIG. 8 is a diagram showing an example of a cross-sectional structure of a two-dimensional PhC structure.

FIG. 9 is a diagram showing an example of a periodic arrangement in a two-dimensional PhC structure.

FIG. 10 is a diagram showing another example of a periodic array in the two-dimensional PhC structure.

FIG. 11 is a diagram showing an example of forming an optical waveguide by a defect of a two-dimensional PhC structure.

FIG. 12 is a diagram showing another example of the formation of the optical waveguide due to the defect of the two-dimensional PhC structure.

FIG. 13 is a diagram showing a mode mismatch loss when connecting an optical waveguide and an optical fiber having a PhC structure.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 PhC circuit 2 Linear optical waveguide by PhC structure 3 Optical waveguide having bent portion by PhC structure 4 PhC structure 5 Optical waveguide by PhC structure applied to SiO ₂ optical waveguide 6 Size of optical waveguide by PhC structure is continuous varying structure 8a hole 8b pillar 10 SiO ₂ optical waveguide 11 core 12 SiO ₂ waveguide size continuously altered structure 16 ribs 20 other lightwave circuit 30 PLC circuit 31 bent circuit portion 32 bent in the PLC circuit in Peripheral part of circuit part

Continued on the front page (72) Inventor Tsutomu Kito 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Akio Sugita 2-3-1 Otemachi, Chiyoda-ku, Tokyo Sun Within the Telegraph and Telephone Corporation (72) Inventor Chiharu Takahashi 2-3-1 Otemachi, Chiyoda-ku, Tokyo Japan Within the Telegraph and Telephone Corporation (72) Inventor Yoji Omori 2-3-1, Otemachi, Chiyoda-ku, Tokyo No. Nippon Telegraph and Telephone Corporation F-term (reference) 2H047 KA01 KA12 QA02 TA01 TA31

Claims (6)

[Claims] A first optical integrated circuit portion having a first optical waveguide structure based on a fine periodic structure formed in a light propagation layer;
A second optical integrated circuit portion having a second optical waveguide structure in which a core as a light propagation layer connected to the first optical waveguide structure is disposed adjacent to a region having a lower refractive index than the core. Optical integrated circuit. 2. A bent waveguide structure is introduced into the first optical waveguide structure, and a bending radius of the bent waveguide structure is reduced by a strong light confinement effect of the first optical waveguide structure. The optical integrated circuit according to claim 1. 3. The optical integrated circuit according to claim 1, wherein the second optical waveguide structure has a light input structure or a light output structure. 4. The spot size of guided light is converted by continuously changing the size of at least one of the first optical waveguide structure and the second optical waveguide structure. Item 4. The optical integrated circuit according to item 1, 2, or 3. 5. The optical integrated circuit according to claim 1, wherein a material of the core is Si. 6. The optical integrated circuit according to claim 1, wherein the second optical waveguide structure is a rib-type optical waveguide.