JPH04260005A - Planar optical waveguide - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a planar optical waveguide. More specifically, the present invention relates to a planar optical waveguide with a novel configuration in which radiation loss in a curved portion with a small radius of curvature is reduced.

0002

2. Description of the Related Art Planar optical waveguides are widely used in optical devices such as directional optical couplers and optical switches. A planar optical waveguide is usually formed by diffusing metal along a pattern from the surface of a crystal substrate such as LiNbO3 or LiTaO3, which has an electro-optic effect, or by performing ion exchange to make the refractive index higher than that of the surrounding area. Ru. Planar optical waveguides used in such optical devices are generally rarely straight, but have curved portions. Particularly, in order to integrate various optical elements, it is advantageous to reduce the radius of curvature of the curved portion.

[0003] Since the planar optical waveguide is formed by a method such as thermal diffusion of a metal such as Ti or ion exchange as described above, there are relatively loose restrictions on the shape from the viewpoint of the manufacturing method. However, as described in Ohmsha Publishing: Photonic Integrated Circuits, Nishihara et al., pages 47-49, LiNbO3
Planar optical waveguides fabricated by diffusing Ti into a crystal substrate have a radius of curvature of several tens of meters because the difference in refractive index with the surrounding area is small.
If it is not greater than m, radiation loss will become excessive. As a countermeasure to this problem, along the outer periphery of the curved part, LiN such as Mg
A planar optical waveguide in which a substance that lowers the refractive index of a bO3 crystal substrate is diffused is disclosed in JP-A-63-157109.

0004

[Problem to be solved by the invention] However, LiN
When a metal is locally diffused into a bO3 crystal substrate to change the refractive index of that portion, it is difficult to obtain a steep change in the refractive index because the refractive index distribution correlates with the concentration distribution of the diffused metal. Therefore, even if a substance that lowers the refractive index of the substrate is diffused along the outer periphery of the curved portion of the planar optical waveguide, a desired refractive index distribution cannot be obtained and it is not effective. Further, the diffused Mg may have an adverse effect on the planar optical waveguide.

SUMMARY OF THE INVENTION An object of the present invention is to provide a planar optical waveguide having a novel configuration that solves the problems of the prior art and does not cause excessive radiation loss even when the radius of curvature of the curved portion is reduced.

[0006]

[Means for Solving the Problems] According to the present invention, an optical waveguide portion is formed on the surface of an electro-optic crystal substrate and has a curved portion, and a refraction of an outer portion of the curved portion of the optical waveguide portion of the electro-optic crystal substrate is provided. and an electrode arranged to be able to apply an electric field to the electro-optic crystal substrate to reduce the rate.

The planar optical waveguide of the present invention is formed on a surface perpendicular to the optical axis of an electro-optic crystal substrate, and includes an optical waveguide portion having a curved portion, and a planar optical waveguide arranged along the outside of the curved portion of the optical waveguide portion. It is preferable that the electro-optical crystal substrate has a structure including a first electrode and a second electrode disposed on the back surface of the electro-optic crystal substrate including a portion immediately below the first electrode.

[0008]

[Operation] In the planar optical waveguide of the present invention, an electric field can be applied to the electro-optic crystal substrate to reduce the refractive index of the outer part of the curved part of the optical waveguide formed on the surface of the electro-optic crystal substrate. Its main feature is that it has electrodes arranged in this way. In the planar optical waveguide of the present invention, the refractive index can be reduced by applying an electric field to the outside portion of the curved portion of the optical waveguide portion of the electro-optic crystal substrate. Therefore, in the curved part, the difference in refractive index between the optical waveguide part and the outer part thereof becomes large, and even if the radius of curvature is made small, the radiation loss does not become excessive.

In order to reduce the voltage applied between the electrodes in the planar optical waveguide of the present invention, it is preferable to apply the voltage in the negative direction of the optical axis of the electro-optic crystal substrate. For this purpose, an optical waveguide section is formed on the surface perpendicular to the optical axis of the electro-optic crystal substrate, a first electrode is arranged outside the curved section of the optical waveguide section, and a first electrode is placed on the outside of the curved section of the optical waveguide section. It is preferable to arrange the second electrode in a portion including directly below the first electrode.

The planar optical waveguide of the present invention is made of LiNbO3
This can be realized using an optical waveguide section formed by thermally diffusing Ti on the Z plane of a crystal substrate and a Cr/Au electrode formed by vapor deposition. The electro-optic crystal substrate also has L
iTaO3, GaAs, etc. can be used.

[0011] Hereinafter, the present invention will be explained in more detail with reference to examples, but the following disclosure is merely an example of the present invention and is not intended to limit the technical scope of the present invention in any way.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of a planar optical waveguide according to the present invention. FIG. 1(a) is a plan view of the curved portion of the planar optical waveguide of the present invention, and FIG. 1(b) is a sectional view taken along line BB in FIG. 1(a). As shown in FIGS. 1(a) and 1(b), the planar optical waveguide of the present invention is made of Li cut out perpendicularly to the optical axis.
It includes an optical waveguide section 2 formed on a NbO3 substrate 1, and an electrode 3 disposed along the optical waveguide section 2 on the outside of the curved section of the optical waveguide section 2. The optical waveguide section 2 is made of LiNbO
3 Deposit Ti on the substrate 1 to a thickness of 80 nm and a width of 7 μm.
A waveguide pattern of m was formed and thermal diffusion was performed at 1000° C. for 5 hours. Thereafter, a Cr/Au electrode 3 with a width of 7 μm was formed on the outside of the curved part of the optical waveguide section 2, and a LiNb
An electrode 4 having the same shape as the electrode 3 was also formed on the lower side of the electrode 3 on the back surface of the O3 substrate 1.

When using the planar optical waveguide of the present invention, an electric field 5 is generated between the electrodes 3 and 4 in the negative direction of the optical axis of the LiNbO3 substrate 1. The refractive index of the portion of the LiNbO3 substrate 1 to which the electric field is applied outside the curved portion of the optical waveguide portion 2 becomes smaller due to the electro-optic effect. The refractive index distribution at this time is shown in FIG. 1(c). As can be seen from FIG. 1(c), in the planar optical waveguide of the present invention, the refractive index on the outside of the curved part of the optical waveguide section 2 becomes smaller than the refractive index NS of the LiNbO3 substrate 1 during use, so the difference in refractive index becomes large, and light emitted to the outside of the curved portion of the optical waveguide section 2 is suppressed. Therefore, even if the radius of curvature of the curved portion of the optical waveguide section 2 is made small, the loss does not become excessive.

The radiation loss at the curved portion of the planar optical waveguide of the present invention was measured. LiNbO with a thickness of 0.2 mm
3. The substrate 1 was used, and the radius of curvature of the curved portion of the optical waveguide section 2 was set to 20 mm. When no voltage was applied between electrodes 3 and 4, there was a loss of 7 dB, but there was a loss of 10 dB between electrodes 3 and 4.
When a voltage of V was applied, the loss could be reduced to 2 dB.

In this embodiment, a planar optical waveguide formed on a LiNbO3 substrate was explained, but the present invention can be applied to any planar optical waveguide formed on an electro-optic crystal substrate. For example, the present invention is effective for planar optical waveguides formed on ferroelectric substrates such as LiTaO3 and semiconductor substrates such as GaAs. Furthermore, the arrangement of the electrodes is not limited to this example. In particular, the shape of the electrodes arranged on the back side of the substrate can be varied considerably, and the electrodes can also be arranged on the entire back side of the substrate. Furthermore, for the optical waveguide section near the edge of the substrate, it is also possible to arrange electrodes on the side surface of the substrate instead of on the back surface.

[0016]

[Effects of the Invention] As explained above, according to the present invention,
A planar optical waveguide with reduced radiation loss at the curved portion can be realized. The planar optical waveguide of the present invention has a structure in which electrodes are arranged along the outer periphery of the curved part, and a substance that lowers the refractive index is not diffused into the substrate, so it is easy to manufacture and has characteristics of the planar optical waveguide. There is no risk of deterioration. According to the present invention, it is possible to reduce the radius of curvature of the curved portion of the planar optical waveguide, thereby facilitating higher density optical integrated circuits.

[Brief explanation of the drawing]

FIG. 1 (a) is a plan view of an example of a planar optical waveguide of the present invention, (b) is a cross-sectional view taken along BB in (a), and (c) corresponds to (b). 3 is a graph showing the refractive index distribution.

[Explanation of symbols]

1 Electro-optic crystal substrate 2 Optical waveguide section 3, 4 Electrode

Claims (2)

[Claims] 1. An optical waveguide portion formed on a surface of an electro-optic crystal substrate and having a curved portion, and an electric field that reduces the refractive index of an outer portion of the curved portion of the optical waveguide portion of the electro-optic crystal substrate. 1. A planar optical waveguide, comprising: an electrode arranged so as to be able to apply an electric voltage to a crystal substrate. 2. An optical waveguide portion formed on a surface perpendicular to the optical axis of an electro-optic crystal substrate and having a curved portion; a first electrode disposed along the outside of the curved portion of the optical waveguide portion; 2. The planar optical waveguide according to claim 1, further comprising a second electrode disposed on the back surface of the electro-optic crystal substrate including a portion directly below the first electrode.