JPH06100694B2 - Method of forming optical waveguide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] As a method of forming an optical waveguide having a small radius of curvature on an electro-optic crystal substrate, a curved region having a large refractive index is formed inside the optical waveguide formation planned position by thermal diffusion, A method of forming an optical waveguide on this refractive index gradient by thermal diffusion as in the conventional method.

[Industrial application field]

The present invention relates to a method of forming an optical waveguide having a small radius of curvature on an electro-optic crystal substrate.

As the optical waveguide, an optical fiber cable (optical transmission fiber) that uses transparent quartz as the core and glass with a lower refractive index than this as the cladding.

Transparent polymer with different refractive index (high molecular organic compound)
A plastic optical fiber cable with a core and a clad.

An optical waveguide in which a line having a higher refractive index than the substrate is formed by diffusing a metal on a transparent electro-optic crystal substrate including the optical axis on the surface.

An optical waveguide with a higher refractive index than the substrate obtained by forming a line made of a metal film on a glass substrate and then immersing it in molten salt for ion exchange.

and so on.

Here, an optical fiber cable is mainly used as a waveguide used for optical communication, and is used in combination with optical elements such as an optical switch, a polarization separation element, and a lens.

However, since these waveguides and optical elements are formed separately, they require fine alignment with each other, which increases the loss due to mismatching and cannot be avoided. Stick to

In order to avoid this, an optical circuit element in which a waveguide and an optical element are integrated and integrated is put into practical use.

Here, an optical circuit element is made using a transparent uniaxial crystal substrate such as lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ), but since LiNbO ₃ has a particularly large electro-optical effect, It is used as a substrate for optical circuit elements.

[Conventional technology]

LiNbO ₃ is used as a substrate, and a plurality of optical elements are formed and integrated on the substrate.

In this case, an optical waveguide for connecting optical elements such as integrated switches to each other needs to be bent to the optical element to form a pattern in order to shorten the connection length.

However, in order to carry out low-loss transmission, a radius of curvature of a certain value or more is required, and when the radius of curvature is small, there is a problem that the light leaks out of the waveguide and the loss increases abnormally.

In addition, there is a method of using total reflection as a method of forming a curved waveguide, but in this case as well, loss increases due to scattering on the reflecting surface.

Therefore, in order to transmit light with low loss, it is necessary to keep the radius of curvature of the optical waveguide at 30 to 40 mm, which limits the improvement of the degree of integration.

[Problems to be solved by the invention]

As described above, it is necessary to form the waveguide that is connected to the optical element that is integrated on the LiNbO ₃ substrate to be curved, but in order to transmit light with low loss, the radius of curvature is limited. However, there is a problem that sufficient integration cannot be performed for that reason.

[Means for solving problems]

The above problem uses the same metal material as the metal forming the waveguide by diffusion on the LiNbO ₃ substrate and forms a metal pattern having the same radius of curvature as the waveguide inside the curved position where the waveguide is to be formed. After thermally diffusing the metal into the electro-optic crystal substrate to form the curved first diffusion region having a large refractive index,
The problem can be solved by providing a waveguide pattern made of the metal in the refractive index gradient generated in this region and thermally diffusing the metal to form an optical waveguide made of the second diffusion region.

[Action]

According to the present invention, a refractive index gradient due to a diffusion metal is provided in advance in a region formed by bending an optical waveguide on a substrate, and metal is diffused in this gradient region as in the conventional case. In this case, the refractive index difference with the optical waveguide becomes large, and therefore, the escape of light from the waveguide is prevented.

FIG. 1 is a principle view of the present invention, in which the same metal as the optical waveguide forming metal is previously thermally diffused inside the position where the curved optical waveguide is formed.

In this case, the metal atoms are radially diffused around the metal pattern, and the atomic concentration decreases as it expands, so that the concentration gradient 1 of the refractive index is generated below the optical waveguide formation region.

When a waveguide pattern made of a metal film is formed on such a region as in the prior art and then thermally diffused to form an optical waveguide 2 having a locally high refractive index as shown in the figure, a curved optical path is formed. Since the refractive index difference is large in the outer peripheral portion of the waveguide 2, it is possible to suppress the escape of light from the optical waveguide 2.

The present invention is to confine light by forming an optical waveguide having a higher refractive index in a region having such a refractive index gradient,
In order to do this, it is necessary to make the concentration gradient 1 of the refractive index as large as possible.

FIG. 2 is a plan view (A) and X- for explaining a method for carrying out the present invention.
It is a cross-sectional view (B) taken along the line X ', in which the first titanium (Ti) metal pattern 4 is first formed on the LiNbO ₃ substrate 3 and thermal diffusion is performed to form a first diffusion region.

Next, the second Ti is placed at the refractive index gradient position generated in this way.
The metal pattern 5 is formed and thermal diffusion is performed to form a second diffusion region to be an optical waveguide.

FIG. 3 is a cross-sectional view (A) specifically explaining formation of the first diffusion region and a refractive index profile (B) generated by thermal diffusion.
When the first Ti metal pattern 4 is heat-treated, Ti ions are diffused as shown by the broken line to form the first diffusion region 6, and the refractive index profile as shown in FIG.

Then, a refractive index gradient 7 appears on both sides. The present invention is the fourth
As shown in the figure, the second Ti metal pattern 5 is provided at the position of this gradient 7 and the second diffusion region 8 to be an optical waveguide is formed by diffusing Ti.

〔Example〕

In the embodiment shown in FIG. 2, the first Ti metal pattern 4
Was formed by electron beam evaporation method and lift-off method with a width of 30 μm and a thickness of 800 Å and a radius of curvature of 5 mm.

Then, heat treatment was performed at a temperature of 1050 ° C. for 6 hours in a moist oxygen (O ₂ ) atmosphere to diffuse Ti and form a first diffusion region 6.

Here, the refractive index of LiNbO ₃ is 2.14, but the refractive index of the diffusion region is increased by about 1% just below the metal pattern,
This diffusion region is widely spread around this.

Next, similarly, a second Ti metal pattern 5 having a width of 7.5 μm and a thickness of 700 Å was formed by using the electron beam evaporation method and the lift-off method.

It should be noted that the second Ti metal pattern 5 and the first Ti metal pattern 4 were formed with three types of intervals of 2 μm, 0 and −2 μm, and they were formed using different substrates.

Here, the interval 0 is when both are in contact, and -2 μm is 2 μ.
This is the case when the patterns overlap by m.

Then, similarly to the above, heating was performed at 1050 ° C. for 6 hours in a moist O ₂ atmosphere to diffuse Ti atoms, thereby forming an optical waveguide.

Next, as an evaluation method of optical loss, an optical waveguide is formed.
Output is 1 mW and wavelength is 1.3 μm on the input side end face of LiNbO ₃ substrate.
The laser diode (LD) was set and the optical loss was measured by placing a parameter meter on the output side end face.

As a result, while the loss when the optical waveguide is straight is about 1 dB, the result of this experiment shows that the optical loss varies in the range of 1.5 to 2 dB. We got the least loss.

A radius of curvature of 5 mm formed without carrying out the present invention
It was confirmed that the optical waveguide had a loss of 30 dB, and that most of the light escaped at the curved portion.

〔The invention's effect〕

As described above, it becomes possible to reduce the previously required radius of curvature of 30 to 40 mm to 5 mm by implementing the present invention,
It is possible to improve the degree of integration of optical circuit elements.

[Brief description of drawings]

FIG. 1 is a principle view of the present invention, FIG. 2 is a plan view (A) and a sectional view (B) illustrating an embodiment of the present invention, and FIG. 3 is a sectional view illustrating formation of a first diffusion region. (A)
And refractive index profile (B), and FIG. 4 is a cross-sectional view (A) for explaining the formation of the second diffusion region.
And the refractive index profile (B). In the figure, 1 is a concentration gradient of refractive index, 2 is an optical waveguide, 4 is a first Ti metal pattern, 5 is a second Ti metal pattern, 6 is a first diffusion region, 8 is a second diffusion region, Is.

Claims (1)

[Claims] 1. A metal material that is the same as a metal that forms a waveguide by diffusion on an electro-optic crystal substrate, and a metal pattern having the same curvature as that of the waveguide is formed inside a curved waveguide formation planned position. After the metal is diffused into the electro-optic crystal substrate by heat treatment to form a first diffusion region having a large refractive index, a waveguide pattern made of the metal is provided on the refractive index gradient generated in the region, A method for forming an optical waveguide, characterized in that a metal is thermally diffused to form a second diffusion region, which is used as a waveguide.