JPH04265950A - Optical directional coupler - Google Patents

Abstract

PURPOSE:To improve the accuracy of an optical coupler and an optical branching device by controlling the branching ratio and coupling ratio of the directional coupler. CONSTITUTION:Three-dimensional waveguides 2 and 3 which have coupling parts 4 close to each other in parallel nearby their center parts are arranged on the surface of an LiNbO3 crystal substrate 1 which is perpendicular to the optical axis. An electrode 5 is arranged between the three-dimensional waveguides 2 and 3 at the coupling parts 4 and an electrode 6 is arranged right below the electrode 5 on the rear of the LiNbO3 crystal substrate 1. When an electric field 7 is applied between the electrodes 5 and 6, the refractive index of the part varies to control the branching ratio and coupling ratio of the three-dimensional optical waveguides 2 and 3.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical directional coupler. More specifically, the present invention relates to an optical directional coupler that can accurately control branching ratios and coupling ratios.

0002

2. Description of the Related Art Optical directional couplers skillfully utilize the properties of optical waveguides, and are widely applied to optical modulators such as optical splitters and optical couplers. A schematic diagram of a conventional optical directional coupler is shown in FIG. The optical directional coupler in FIG. 4 is LiNbO3
It is composed of two three-dimensional optical waveguides 2 and 3 formed by thermally diffusing Ti onto a substrate 1. As shown in FIG. 4, portions of the three-dimensional optical waveguides 2 and 3 having a predetermined length along the light propagation direction are parallel and close to each other to form a coupling portion 4. The light propagating through the three-dimensional optical waveguides 2 and 3 is configured to mutually transfer to the other three-dimensional optical waveguide at this coupling portion 4.

In the above optical directional coupler, the width of the three-dimensional optical waveguides 2 and 3, the length of the coupling part 4, the distance between the three-dimensional optical waveguides 2 and 3 in the coupling part 4, the three-dimensional optical waveguide The branching ratio and the coupling ratio are determined by the difference between the refractive index of 2 and 3 and the refractive index of the substrate. However, these values are minute, and even a slight difference will affect the branching ratio and coupling ratio of the optical directional coupler. On the other hand, there are inevitable errors in the manufacturing process, and it has been difficult to produce an optical directional coupler with a predetermined branching ratio and coupling ratio.

Therefore, Japanese Patent Application Laid-Open No. 61-241706 discloses that a part of the substrate between the three-dimensional optical waveguides in the coupling part of an optical directional coupler is subjected to ion exchange to change the refractive index to a predetermined value. A method for achieving branching ratios and coupling ratios is disclosed. FIG. 3 shows a schematic diagram of an optical directional coupler disclosed in Japanese Patent Application Laid-Open No. 61-241706. The optical directional coupler of FIG. 3, like the one of FIG. 4, includes three-dimensional optical waveguides 2 and 3 formed in a shape having a coupling portion 4 by thermally diffusing Ti into a LiNbO3 substrate 1. Between the three-dimensional optical waveguides 2 and 3 of the coupling portion 4, a region 8 is formed in which some of the Li ions of the LiNbO3 substrate 1 are replaced with H ions. The refractive index of the ion exchange region 8 is LiN
bO3 is small compared to other parts of the substrate 1, and the size and refractive index of the ion exchange region 8 can be controlled by adjusting the heating time for H ion thermal diffusion performed after ion exchange. can.

[0005]

Problems to be Solved by the Invention The method of changing the refractive index of the substrate of the bonding portion by ion exchange as described above is time-consuming, as it requires further ion diffusion after ion exchange. In addition, this method can only be applied when reducing the refractive index of the substrate of the bonding portion. Further, there are problems such as the inability to apply the method to optical directional couplers using semiconductor substrates such as GaAs and InP.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical directional coupler which solves the problems of the prior art and allows precise control of the branching ratio and coupling ratio.

[0007]

[Means for Solving the Problems] According to the present invention, portions of a plurality of three-dimensional optical waveguides formed on a substrate, each having a predetermined length along the direction in which light propagates, are arranged in parallel and close to each other. In an optical directional coupler having a coupling part, the refractive index of the part can be changed by applying an electric field or flowing a current to the substrate in the part between the three-dimensional optical waveguides of the coupling part. An optical directional coupler is provided, the optical directional coupler comprising an electrode disposed in the direction of the directional coupler.

[0008] The optical directional coupler of the present invention includes the plurality of three
A dimensional optical waveguide is formed on a surface perpendicular to the optical axis of the electro-optic crystal substrate, a first electrode disposed between the three-dimensional optical waveguides of the coupling portion, and a portion directly below the first electrode. It is preferable to include a second electrode disposed on the back surface of the optical crystal substrate.

Further, in the optical directional coupler of the present invention, the plurality of three-dimensional optical waveguides are formed on the surface of a semiconductor substrate whose refractive index changes due to carrier injection, and the plurality of three-dimensional optical waveguides are arranged between the three-dimensional optical waveguides of the coupling part. a first electrode disposed on the back surface of the semiconductor substrate including a portion immediately below the first electrode;
The configuration may include the following electrodes.

0010

[Operation] The optical directional coupler of the present invention is equipped with an electrode that can apply an electric field to the substrate between the three-dimensional optical waveguides of the coupling part or apply a current to the substrate to change the refractive index of that part. has its main characteristics. In the optical directional coupler of the present invention, when an electro-optic crystal is used for the substrate, an electric field is applied to the substrate between the three-dimensional optical waveguides of the coupling part, and the refractive index is changed by injecting carriers such as InP. When a semiconductor crystal that is easily refracted is used for the substrate, a current is passed through the substrate between the three-dimensional optical waveguides of the coupling portion to change the refractive index.

In the optical directional coupler of the present invention, LiN
When using an electro-optic crystal substrate such as bO3 or GaAs, it is preferable to arrange the electrodes so that the electric field can be applied in the crystal direction where the effect of the electric field is greatest. Specifically, the electrodes are arranged so that an electric field is applied parallel to the optical axis of the electro-optic crystal. For this purpose, for example, a plurality of three-dimensional optical waveguides are formed so as to form a coupling part on a plane perpendicular to the optical axis of the electro-optic crystal substrate, and an electrical An electrode is placed on the back side of the optical crystal substrate.

In the optical directional coupler of the present invention, it is possible to control the refractive index of the substrate between the three-dimensional optical waveguides of the coupling portion by changing the direction and magnitude of the applied electric field or current. Therefore, unlike conventional devices, it is possible to adjust the refractive index to the optimum value while actually installing the device.

[0013] The present invention will be explained in more detail with reference to examples below. However, 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.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 each show a schematic diagram of an example of an optical directional coupler of the present invention. The optical directional coupler of FIG. 1 uses a LiNbO3 substrate that has an electro-optic effect. FIG. 1(a) is a perspective view, and FIG. 1(b) is a perspective view of FIG.
(a) It is a sectional view taken along BB. The optical directional coupler shown in FIG. 1 is formed by thermally diffusing Ti on a Z-axis cut LiNbO3 substrate 1, as in the conventional optical directional coupler.
It includes dimensional optical waveguides 2 and 3. 3D optical waveguide 2
and 3, similar to the conventional optical directional coupler shown in FIGS. 3 and 4, portions of a predetermined length near the center are parallel and close to each other to form a coupling portion 4. An electrode 5 is arranged between the three-dimensional optical waveguides 2 and 3 of the coupling part 4, and the Li
An electrode 6 is placed directly below the electrode 5 on the back side of the NbO3 substrate 1.

The above optical directional coupler of the present invention was manufactured as follows. First, a Ti waveguide pattern with a thickness of 70 nm is formed on a LiNbO3 substrate 1 by a lift-off method. The width of the pattern is 7μm, and the spacing between the joints is 8μm.
m, and the bond length was 10 mm. The LiNbO3 substrate 1 on which the waveguide pattern was formed was heated at 1000°C for 5 hours,
Three-dimensional optical waveguides 2 and 3 were formed by diffusing Ti. Next, a portion of the coupling portion 4 between the three-dimensional optical waveguides 2 and 3 is patterned to a width of 5 μm by photolithography, and Cr and Au are deposited to form the electrode 5. Furthermore, patterning was performed on the back surface of the LiNbO3 substrate 1 directly below the electrode 5, and Cr and Au were vapor-deposited to form the electrode 6.

[0016] The optical directional coupler produced as described above is
The theoretical branching ratio is 50:50 when no voltage is applied between the electrodes 5 and 6, but due to manufacturing errors, the branching ratio actually ranges from 45:55 to 55:45. However, when the LiNbO3 substrate 1 is 250 μm thick, by applying a voltage within 5 V between the electrodes 5 and 6 to generate an electric field 7, the electrodes 5 and 6 of the substrate 1 are
The refractive index between each optical directional coupler changes, and each optical directional coupler has a
It was possible to obtain a branching ratio of 0:50.

On the other hand, the optical directional coupler shown in FIG. 2 uses an InP substrate whose refractive index changes by carrier injection. FIG. 2(a) is a perspective view, and FIG. 2(b) is a perspective view of FIG. 2(a).
It is a sectional view taken along BB. The optical directional coupler shown in FIG. 2 has an InGaAsP waveguide layer 11 on an InP substrate 1, an InP
It includes ridge-type three-dimensional optical waveguides 2 and 3 formed by stacking a cladding layer 12 and an InGaAsP cap layer 13. Similar to the conventional optical directional coupler shown in FIGS. 3 and 4, the three-dimensional optical waveguides 2 and 3 have portions of a predetermined length near the center that are close to each other in parallel to form a coupling portion 4. ing. Between the three-dimensional optical waveguides 2 and 3 of the coupling part 4, a carrier injection region 14 is formed by Zn diffusion,
An electrode 5 is arranged thereon. An electrode 6 is arranged on the front surface of the back side of the InP substrate 1 .

The above optical directional coupler of the present invention was manufactured as follows. First, an InGa film with a thickness of 1.0 μm was formed on an InP substrate 1 by liquid phase epitaxial (LPE) method.
sP film, 0.5 μm thick InP film, 0.5 μm thick
InGaAsP films are successively formed and laminated. Next, C
Ridge-type three-dimensional optical waveguides 2 and 3 were formed by reactive ion beam etching using l2 gas. The width of the ridge-type three-dimensional optical waveguides 2 and 3 is 6 μm, and the spacing between the coupling parts is 5 μm.
The height of the ridge portion was 1.5 μm. Next, Zn is diffused into a portion of the coupling portion 4 between the three-dimensional optical waveguides 2 and 3 to form a carrier injection region 14, and Cr and Au are deposited thereon to form the electrode 5. Further, Cr and Au were also deposited on the back surface of the InP substrate 1 to form an electrode 6.

The optical directional coupler produced as described above is
The theoretical branching ratio is 50:50 when no current is passed between the electrodes 5 and 6, but due to manufacturing errors, the branching ratio is actually distributed in the range of 45:55 to 55:45. However, by passing a maximum current of 120 mA through the InP substrate 1, the refractive index of the carrier injection region 14 changes,
It was possible to obtain a branching ratio of approximately 50:50 with each optical directional coupler.

[0020]

As explained above, in the optical directional coupler of the present invention, an electric field is applied to the substrate between the three-dimensional optical waveguides in the coupling part, or a current is applied to the substrate to change the refractive index of that part. It is possible to control the branching ratio and coupling ratio. According to the present invention, a highly accurate optical branching/coupling device is provided.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an example of an optical directional coupler of the present invention.

FIG. 2 is a schematic diagram of another embodiment of the optical directional coupler of the present invention.

FIG. 3 is a schematic diagram of a conventional improved optical directional coupler.

FIG. 4 is a schematic diagram of a conventional optical directional coupler.

[Explanation of symbols]

1　　　　Substrate 2, 3 Three-dimensional optical waveguide ４　　　　Connecting part 5, 6 Electrode

Claims (3)

[Claims] Claim 1: A portion of a plurality of three-dimensional optical waveguides formed on a substrate having a predetermined length along the direction in which light propagates,
In an optical directional coupler having coupling parts arranged close to each other in parallel, an electric field or current is applied to the substrate in the part between the three-dimensional optical waveguides of the coupling part to change the refractive index of the part. 1. An optical directional coupler comprising electrodes arranged so as to be able to change. 2. The optical directional coupler according to claim 1, wherein the plurality of three-dimensional optical waveguides are formed on a surface perpendicular to the optical axis of an electro-optic crystal substrate, and the three-dimensional optical waveguides of the coupling portion An optical directional coupler comprising: a first electrode disposed between the two electrodes; and a second electrode disposed on the back surface of the electro-optic crystal substrate including a portion immediately below the first electrode. 3. The optical directional coupler according to claim 1, wherein the plurality of three-dimensional optical waveguides are formed on a surface of a semiconductor substrate in which a refractive index change occurs due to carrier injection, and the three-dimensional optical waveguides of the coupling portion a first electrode disposed between;
and a second electrode disposed on the back surface of the semiconductor substrate including a portion directly below the first electrode.