JPH0473605A - Optical waveguide - Google Patents

Abstract

PURPOSE:To reduce the loss in a cross waveguide by applying a material which decreases the refractive index of an optical waveguide to the crossing part of the optical waveguide. CONSTITUTION:Two Ti diffusion optical waveguides 2 are formed while crossing on a lithium niobate crystal substrate 1 whose Z-axis is set perpendicularly to a base plate surface by diffusing Ti. Furthermore, a magnesium oxide(MgO) diffusion layer 4 is formed on the surface of the waveguide by diffusing additionally MgO provided with an effect to decrease the refractive index of the base plate on the crossing part 3 of the two optical waveguides to weaken the light confinement of waveguide light in a propagating direction at the rhombic crossing part 3. Thereby, the generation of a multimode component at the crossing part can be suppressed, and the loss due to mode conversion and mode coupling that is the factor of crossover loss can be reduced by crosstalk on crossing optical waveguide.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical waveguide formed in the shape of a channel in a substrate.

[Conventional technology]

An optical waveguide is formed as a high refractive index layer near the surface of a substrate by thermal diffusion, ion exchange, ion implantation, etc., and confines light within the optical waveguide.It is an optical integrated circuit that integrates functional elements such as optical switches and optical modulators. can be configured. In particular, in order to reduce the size and increase the scale of integration of optical integrated circuits, intersecting waveguides in which optical waveguides intersect are essential. At this time, crossover loss occurs due to mode conversion at the intersection and crosstalk to the optical waveguides that intersect. Reducing this cross-over loss is an issue in waveguide-type low-loss optical circuits.

[Problem to be solved by the invention]

As described above, in conventional crossing waveguides, crossing loss occurs due to mode conversion when guided light passes through the crossing portion and crosstalk to the crossing optical waveguides. In other words, the guided light that has propagated in a single mode to the vicinity of the intersection becomes multimode at the intersection, and after passing through the intersection,
Since it becomes a single mode again, mode conversion, mode coupling, and crosstalk to intersecting waveguides occur.

Therefore, in order to reduce the loss of the crossed waveguides, it is necessary to make the crossing portion into a single mode.

[Means to solve the problem]

The present invention provides that, in optical waveguides formed on a substrate that intersect with each other, a substance that reduces the refractive index of the substrate is applied to at least the intersecting portions of the optical waveguides, or the optical waveguides around the optical waveguides excluding the intersecting portions. It is an optical waveguide characterized by:

[Effect]

For example, in an optical waveguide formed by thermal diffusion in a crystal having optical anisotropy such as lithium niobate or lithium tantalate, the optical waveguide at the intersection where two optical waveguides intersect, or around the intersection By applying additional diffusion of magnesium oxide to the area, the confinement in the propagation direction of the guided light in the area subjected to additional diffusion of magnesium oxide becomes weaker, and the generation of multimode components can be suppressed. Therefore, the intersection region can be made into a single mode, and the intersection loss can be reduced.

[Embodiment] FIG. 1 is a plan view of an embodiment of the present invention. Ti is diffused into a lithium niobate crystal substrate 1 whose Z axis is perpendicular to the substrate surface to form two Ti-diffused optical waveguides 2 intersecting each other. T
The diffusion of i is approximately 1000 in a gas atmosphere containing water vapor.
Perform at a temperature of °C. Furthermore, in order to weaken the optical confinement in the propagation direction of the guided light at the diamond-shaped intersection 3, magnesium oxide (MgO), which has the effect of lowering the refractive index of the substrate, is additionally diffused at the intersection 3 of the two optical waveguides. Then, as shown in FIG. 2, an Mg◯ diffusion layer 4 is formed on the waveguide surface. Since the additional diffusion of MgO is performed at about 900° C., the distribution of diffused Ti is not changed, but the refractive index near the surface where MgO is diffused is decreased. By doing this,
The generation of multimode components at intersections is suppressed, and loss is reduced due to mode conversion, mode coupling, and crosstalk to intersecting optical waveguides, which are the causes of intersection loss.6 Figure 2 is a cross-sectional view taken along line AA in Figure 1. It is. By additionally diffusing magnesium oxide, the light confinement at the intersection 3 is weakened,
Multimode components can be suppressed.

In Figure 3, additional diffusion of magnesium oxide is performed at the intersection 3.
In the case of FIG. 3, which shows a perspective view of the two optical waveguides immediately before and after the intersection, it is possible to further reduce losses due to mode coupling, etc., immediately before and after the intersection between the two optical waveguides. .

FIG. 4 is a plan view of another embodiment of the invention.

2 on a lithium niobate crystal substrate with the Z axis perpendicular to the substrate surface.
Two Ti-diffused optical waveguides are formed to cross each other. Diffusion is performed in a gas atmosphere containing water vapor at a temperature of about 1000°C.Furthermore, in order to weaken the optical confinement in the propagation direction of the guided light around the crossing diamond part 3, the crossing diamond shape of the two optical waveguides is used. Magnesium oxide (MgO), which has the effect of lowering the refractive index of the substrate, is additionally diffused into the optical waveguide around the portion 3 to form a magnesium oxide diffusion layer 4 . Since the additional diffusion is performed at about 900° C., the Ti diffusion distribution is not changed or the refractive index near the surface where M g O is diffused is decreased. By doing this, the generation of multimode components at intersections is suppressed, and mode conversion, which is a cause of intersection loss, is suppressed.
Losses due to mode coupling and intersecting optical waveguides are reduced. FIG. 5 is a sectional view along line AA in FIG. 4.

By additionally diffusing magnesium oxide, it is possible to weaken the optical confinement of the optical waveguide around the intersection rhombus and suppress the generation of multimode components at the intersection.

〔Effect of the invention〕

As explained above, the present invention is characterized in that, in optical waveguides formed on a substrate that intersect with each other, a substance that lowers the refractive index of the substrate is introduced from the substrate surface into at least the intersection of the optical waveguides or the optical waveguide around the intersection. Therefore, the optical confinement in the optical waveguides at or around the intersection becomes weaker, suppressing the generation of multimode components, and reducing mode conversion and mode coupling, which are the causes of intersection loss, and crosstalk between the two intersecting optical waveguides. be.

[Brief explanation of the drawing]

FIG. 1 is a plan view showing an embodiment of the present invention, FIG. 2 is a sectional view taken along the line AA in FIG. 6. FIG. 4 is a plan view showing one embodiment of the present invention, and FIG. 5 is a sectional view taken along line AA in FIG. 4. DESCRIPTION OF SYMBOLS 1... Lithium niobate, 2... Ti diffused optical waveguide, 3... Crossing part, 4... Magnesium oxide diffusion layer.

Claims (1)

[Claims] 1. In a plurality of intersecting optical waveguides each consisting of a region having a higher refractive index than the surrounding area, a substance that lowers the refractive index of the optical waveguide is applied at least to the intersection of the optical waveguides. An optical waveguide characterized by: 2. In a plurality of intersecting optical waveguides each consisting of a region having a higher refractive index than the surrounding area, at least the optical waveguide region around the intersecting rhombus of the optical waveguide is coated with a substance that reduces the refractive index of the optical waveguide. An optical waveguide featuring: