JPH05249330A - Optical waveguide - Google Patents

Abstract

PURPOSE:To provide the optical waveguide which can be easily produced and has a small propagation loss. CONSTITUTION:A core part 2 consisting of high-refractive index borosilicate glass having a refractive index Nf=1.59 is formed on an Si/SiO2 substrate 4 formed with about 15mum buffer layer 3 and a clad layer 1 consisting of borosilicate glass ('C-7059(R)') having a refractive index Nc=1.54 is laminated thereon. The example of the sizes to satisfy a single mode condition is the thickness of the core part T=0.4mum and the width of the core part W=2.0mum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide which is a main constituent element of an optical integrated circuit being developed in each industrial field, and more particularly to a structure of a three-dimensional waveguide called a ridge type.

[0002]

2. Description of the Related Art In recent years, optical integrated circuits that have been researched and developed in various fields include an optical waveguide, a laser diode as a light source, a switch as a functional element, a modulator, and a photodetector on one substrate. The diode etc. which are the above are integrated to have a desired function as a whole,
The integration makes it possible to reduce the size and weight of the optical system, stabilize it, and improve its performance.

An optical waveguide, which is a main component of such an optical integrated circuit, has a core portion formed on an optical substrate and a clad portion having a lower refractive index than the core portion laminated on the core portion, The configuration is such that light is confined and transmitted in the core portion having a relatively high refractive index.

Therefore, in order to manufacture an optical waveguide, it is basically necessary to use two or more kinds of transparent materials having different refractive indexes in the light wavelength region to be used, and further, the material to be used can be easily thinned. Is required. Conventionally, ferroelectric materials, organic materials, semiconductors and the like have been used as optical waveguide materials, and a combination of materials having an appropriate refractive index difference has been selected according to the purpose.

Optical waveguide materials that have been reported to date include, for example, alkali zinc borosilicate glass (refractive index N _f
= 1.51), barium borosilicate glass (N _f = 1.5
4), multi-component glass such as phosphosilicate glass (N _f = 1.47). Among these multi-component glasses, the refractive index is 1.
54 barium borosilicate glass (trade name C-7059, manufactured by Corning Incorporated: hereinafter referred to as C-7059 glass) is most frequently used.

Under such circumstances, in an optical integrated circuit having a structure in which Si is used as a substrate and a buffer layer made of SiO ₂ is provided on the substrate surface, a photodetector, an electronic circuit and the like can be monolithically integrated with an optical waveguide device. In addition to being able to obtain a high-precision, strain-free polished Si wafer, it is possible to form a highly pure SiO ₂ layer with few defects by thermal oxidation. It is attracting attention as being advantageous.

[0007]

However, in the prior art as described above, there were the following problems in forming the optical waveguide on the Si / SiO ₂ substrate.

First, since the refractive index of Si itself is as large as about 3.85, in order to prevent the guided light from being emitted to the substrate,
SiO ₂ layer as a buffer layer (refractive index of about 1.46)
Has to be very thick. As an example, a report by Y. Shani et al. (Appl. Phys. Lett. 52 (1990)
According to 120), in the case of a phosphosilicate glass waveguide, a buffer layer thickness of 15 μm is used. Therefore, Si /
In producing a waveguide using a SiO ₂ substrate,
Since it takes a long time to grow the SiO ₂ layer, or a high-pressure oxidation furnace is separately used, the productivity cannot be improved in any case.

On the other hand, if the refractive index of the core is increased, the buffer layer may be thin, but the stronger the confinement of the guided light, the more the single mode propagation condition becomes extremely critical, and the dimensional accuracy of the core is extremely high. The problem arises that it becomes severe.

Second, in the conventional waveguide using the Si / SO ₂ substrate, required low propagation loss (radiation loss to the substrate, total loss of the waveguide such as absorption loss, scattering loss, etc.) There is a problem that (loss) cannot be cleared.
As a film forming method for manufacturing an optical waveguide, an RF sputtering method (PVD) or a CVD (chmical vapor depo
sition) method, etc., but in the experiments conducted by the inventors of the present application (details will be described in comparative examples), C, which has been most frequently used in the past, is used.
Propagation loss in the case of forming a waveguide by the RF sputtering method using -7059 glass was only 9.7 dB / cm, which was a bad result.

Thirdly, the multi-component glass conventionally used as an optical waveguide material has a problem that film formation is not easy. The above-mentioned three multi-component glass is difficult to obtain a material having a thickness of about 5 mm required as a sputtering target, and lacks applicability to a general sputtering apparatus. For this reason, thin materials have been laminated and used as a target, and the efficiency of the film forming process cannot be improved.

The present invention has been made in view of the above problems, and an object thereof is to provide an optical waveguide which is easy to manufacture and has a small propagation loss.

[0013]

According to the present invention, in an optical waveguide having a core portion formed on a substrate and a clad portion laminated on the core portion and having a refractive index lower than that of the core portion, The core portion has a refractive index N _f of 1.54 <N _f <
The above object is achieved by using a high-refractive-index barium borosilicate glass having a refractive index of 1.60 and forming the clad portion with a barium-borosilicate glass having a refractive index N _{c of} approximately 1.54. ..

In the optical waveguide of the present invention, specifically, a Si / SiO ₂ substrate provided with a SiO ₂ layer as a buffer layer is used, and the core portion has a high refractive index barium boron having a refractive index N _f = 1.59. More preferably, it is made of silicate glass.

In this specification, the refractive index of the core portion is N
_f and the refractive index of the clad part are denoted by N _c .

[0016]

The operation of the present invention will be described with reference to FIG. FIG. 2 shows the dependency of the refractive index of the optical waveguide (core part) on the minimum film thickness of the SiO ₂ layer required to reduce the propagation loss to 0.15 dB / cm or less. Each curve in FIG. 2 has a core portion refractive index N _f = 1.50, N _f = 1.
The calculation result in the TE basic next (TE ₀ ) mode in the case of 60, N _f = 2.00 is shown.

From the figure, it can be seen that the higher the refractive index of the core, the more SiO ₂
It can be seen that the required film thickness of the layer can be made thin and there is a large difference in the required thickness of the SiO ₂ layer when the film thickness of the core portion is about 0.4 μm. However, if the refractive index of the core portion becomes high, the single mode propagation condition becomes critical as described above, and the dimensional accuracy of the core portion becomes strict. Further, in general, when a glass having a high refractive index is used, the chemical resistance tends to decrease and the propagation loss tends to increase. Therefore, conventionally, C-7059 glass (refractive index N _f = 1.5 is used.
No material with a higher refractive index than 4) has been used.

In such a situation, the inventors of the present invention constituted the core portion with the high-refractive-index barium borosilicate glass (refractive index N _f = 1.59) having a higher refractive index than the conventional one, and the clad portion with When the barium borosilicate glass having a refractive index N _c = 1.54 is used, the propagation loss can be significantly reduced as compared with the conventional one, and the single mode condition can be satisfied without the need for advanced fine processing, and the durability can be improved. The inventors have found that there is no practical problem with respect to chemical properties, and have completed the present invention.

In the optical waveguide of the present invention, since the core portion has a higher refractive index than the conventional one, the guided light is less likely to leak to the Si substrate, the SiO ₂ layer can be thinned, and the buffer layer formation time can be shortened. can do. Specifically, according to the present invention, the thickness of the buffer layer can be set to about 1.5 μm, and it can be formed in a general electric furnace in about 3 to 4 hours. The refractive index of the core portion (high-refractive-index barium borosilicate glass) in the present invention is preferably N _f = 1.59, but even if the refractive index is 1.54 <N _f <1.59, The effect of thinning the buffer layer by the higher refractive index than that of the conventional core can be obtained.

The high-refractive-index barium borosilicate glass used in the present invention has a low melting point, the glass itself is easily manufactured, and a thick material is easily available. It is convenient.

The optical waveguide according to the present invention does not necessarily need to use the Si substrate, and the substrate can be appropriately selected according to the application. At this time, needless to say, when the refractive index of the substrate itself is lower than that of the core portion, the core portion may be directly formed on the substrate without providing the buffer layer.

The buffer layer is not limited to SiO ₂ and may be made of other materials. The refractive index of the buffer layer is 1.46 to 1.5, which is lower than that of the core part.
It may be in the range of about 4, for example, the same C as the clad part.
The buffer layer may be formed of -7059 glass. However, since the C-7059 glass has a higher refractive index than SiO _2, required thickness than SiO ₂ is increased.

[0023]

1 is a sectional view showing the structure of an optical waveguide according to an embodiment of the present invention. In this example, the washed S
The i substrate 4 was steam-oxidized in an electric furnace at 1150 ° C. for about 4 hours to form a buffer layer (SiO ₂ layer) 3 of about 1.5 μm. After that, high refractive index barium borosilicate glass (N _f = 1.59) was deposited on the buffer layer 3 by the RF sputtering method. Recommended sputter condition is RF power: 5
00W, sputtering gas thickness: 0.43 Pa, Ar /
It is preferable to carry out reactive sputtering with O ₂ gas.

Next, in order to form the core portion 2 into a desired shape, a photolithography process using a highly heat-resistant resist is performed, and thereafter, reactive ion etching with a CF ₄ type gas is performed to obtain the figure. A convex core portion 2 as shown was formed.

Then, after removing the resist, C-70
Sputtering was performed under the above conditions using 59 glass to form the clad layer 1 to obtain a ridge-type three-dimensional optical waveguide as shown in FIG.

Examples of dimensions of the core portion 2 for satisfying the single mode condition in such an optical waveguide include a core portion thickness T: 0.4 μm and a core portion width W: 2.0 μm.
No particularly difficult fine processing is required.

Next, by the same RF sputtering method as in the above-mentioned embodiment, a high refractive index barium borosilicate glass (N _f = 1.59) as a core and a C as a clad are formed on a Si / SiO ₂ substrate. A two-dimensional waveguide in which -7059 glass was laminated was prepared, and the two-dimensional waveguide propagation loss was measured by the prism coupling method. As a result of measurement, TE ₀ mode propagation loss was 1.9 dB / cm, and TM ₀ mode propagation loss was 1.0 dB / cm.

As a comparative example, the core portion is made of C-705.
Two-dimensional waveguide propagation loss was similarly measured for the conventional optical waveguide made of 9 glass. This measurement result shows that TE ₀ mode propagation loss is 9.7 dB / cm and TM ₀
The mode propagation loss was 7.6 dB / cm.

As described above, the propagation loss of the optical waveguide according to the present invention is much lower in both TE ₀ mode and TM ₀ mode. In the optical waveguide of the present invention, the propagation loss is 1.0 dB / c by further finely adjusting the film forming conditions.
It can be set to m or less.

[0030]

As described above, in the present invention, the core portion is made of high refractive index barium borosilicate glass (1.54 <N _f <
1.60), the clad portion is made of C-7059 glass (N _c
= 1.54), the buffer layer (S
It is possible to reduce the thickness of the iO ₂ layer), and it is possible to shorten the formation time of the buffer layer.

Further, since the optical waveguide of the present invention has a very low propagation loss and does not require any special fine processing to satisfy the single mode condition, it is currently under active development. It is very useful as an optical waveguide element that constitutes an optical integrated circuit.

Further, the barium borosilicate glass having a high refractive index used in the present invention has advantages that it is easy to obtain a material having a large plate thickness, is versatile for an apparatus (sputtering apparatus, etc.), and is easy to form a film. There is also.

[Brief description of drawings]

FIG. 1 is a sectional view showing a configuration of an optical waveguide according to an embodiment of the present invention.

FIG. 2 is a graph showing the dependency of the refractive index of the optical waveguide (core portion) on the minimum film thickness of the SiO ₂ layer required to reduce the propagation loss to 0.15 dB / cm or less.

[Explanation of symbols]

1 clad part 2 core part 3 buffer layer (SiO ₂ layer) 4 Si substrate

Claims (3)

[Claims] 1. In an optical waveguide having a core portion formed on a substrate and a clad portion laminated on the core portion and having a refractive index lower than that of the core portion, the core portion has a refractive index N _f. Is 1.54 <N _f <1.60 and is made of high-refractive-index barium borosilicate glass, and the cladding portion has a refractive index N
An optical waveguide comprising barium borosilicate glass having _{c of} approximately 1.54. 2. The optical waveguide according to claim 1, wherein a buffer layer having a refractive index smaller than that of the core portion is provided on the surface of the substrate. 3. The optical waveguide according to claim 2, wherein the substrate is a Si substrate and the buffer layer is a SiO ₂ layer.