JPS5934286B2 - optical directional coupler - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical directional coupler in which the capacitance of electrodes is reduced in order to perform high-speed switching of the optical directional coupler.

FIG. 1 shows an optical switch element using a conventional optical directional coupler.

In Figure 1, 1 is a material whose refractive index changes with an electric field.
For example, LlNbO3 and LiTa03,2,3 are substances 1
an optical waveguide with a larger refractive index, 4 and 5 are electrodes, 6
is a signal source, 75 is input light, 8 is output light when an electric field is applied, and 9 is output light when no electric field is applied. In order to operate this, if the phase constants of the light propagating in the two optical waveguides 2 and 3 are equal, the two optical waveguides 2 and 3 must be
3 are brought close to each other in parallel, for example, the input light 7 gradually moves from the optical waveguide 8 to the optical waveguide 9.

This is called optical coupling, and 100% of the light is transmitted to the other optical waveguide.
The length of transition, that is, the maximum coupling length, is uniquely determined by the coupling efficiency of light. Therefore, assuming that the coupling portion between the two optical waveguides 2 and 153 is set to the maximum coupling length as shown in FIG. 1, the electrodes 4 and 5 on the two optical waveguides 2 and 3
When a voltage is applied to the optical waveguides 2 and 3, the refractive index of the optical waveguides 2 and 3 changes. As a result, the phase constant of the light propagating through the optical waveguide changes, and the light no longer transfers from the optical waveguide 2 to the optical waveguide 320. That is, when the voltage is zero, light propagates through the optical waveguide 3, and when a voltage is applied, the light propagates through the optical waveguide 2.
The light that travels inside is switched. By the way, the speed at which light is switched is determined by the electrostatic capacitance determined by the gap and length of the electrodes. It is a well-known fact that when the capacitance is small, the time constant derived from the product of the load resistance connected in parallel with the electrode becomes small, and high-speed switching is possible. However, the gap and length of the electrodes are determined by the shape of the optical coupling section. Since the optical waveguide gap is several 30 μm, it was unavoidable that the capacitance would increase. Therefore, high-speed optical switching also has the drawback of being severely restricted. Further, since only the electric field in the depth direction can be used as the electric field direction, when using LlNbO3, for example, the Z plate 35 must be used, and the crystal orientation is also defined. In order to solve the above-mentioned drawbacks, the present invention forms two optical waveguides close to each other on the surface of a substance whose refractive index is changed by an electric field, and has a refractive index larger than that of the substance. 1 for forming another optical waveguide between two optical waveguides and applying voltage to the two optical waveguides at both ends.
A pair of electrodes is installed. The present invention will be explained in detail below with reference to the drawings. FIG. 2 is a configuration diagram of one embodiment of the present invention, and 1 to 9 are the same as those in FIG.
0 is an optical waveguide having a length required for 100% of light to transfer from optical waveguide 2 to optical waveguide 3 between straight optical waveguides 2 and 3.

100 lights between optical waveguides 2 and 3? For coupling, the phase constant of the light passing through the optical waveguide 10 must also be equal to that of the other two optical waveguides. The operating principle of the optical switch using this optical directional coupler is exactly the same as the conventional method.
However, as shown in Figure 3, the optical waveguide 2 directly below the electrode
, 3 receive an electric field in the depth direction, and the central optical waveguide 10 receives a horizontal electric field acting on the electrode gap, so that the electric field distribution can be used effectively. Also, for example, LiNbO
When using the Z plate No. 3, the electric field in the depth direction applied to the optical waveguides 2 and 3 directly under the electrodes is used, as in the conventional case.
Electro-optic coefficient R3 showing a large electro-optic effect in this direction
3 can be used. On the other hand, when using a Y plate,
By utilizing a horizontal electric field applied to the optical waveguide 10, R33, which also exhibits a large electro-optic effect in this direction, can be used. Even if the crystal orientation is different in this way,
Since the direction of the electric field with the larger electro-optic effect can be selected, it is not restricted by the crystal orientation of the substrate. By the way, in an optical directional coupler using three optical waveguides, what is the maximum coupling length compared to a case with only two? It will be twice as long.

Correspondingly, the length of the electrode will also increase by a factor of approximately 2,000,000, and the capacitance will also increase. However, for example, even if it is assumed that the optical waveguide gap and the optical waveguide width are the same size, the electrode gap will triple, and the capacitance of the electrodes will eventually decrease to 1 to 0.5. As explained above, the optical directional coupler of the present invention can effectively use the electric field distribution and is not limited by the crystal orientation of the substrate.

Furthermore, since the electrode gap becomes larger, there is an advantage that not only the electrode capacitance is reduced but also the positioning of the electrodes and the optical waveguide becomes easier.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional optical directional coupler, Fig. 2 is a block diagram of an embodiment of an optical directional coupler according to the present invention, and Fig. 3 is a cross-sectional view of the optical directional coupler according to the present invention. FIG. 3 is a diagram showing the electric field distribution as seen. 1... Substrate, 2, 3, 10... Optical waveguide, 4, 5... Electrode, 6... Signal source,
7...Incoming light, 8...Output light when voltage is applied, 9...Output light when voltage is zero, 1
1... Electric field distribution.

Claims (1)

[Claims] 1 Two optical waveguides having a refractive index greater than the refractive index of the substance are formed adjacent to each other on the surface of a substance whose refractive index changes depending on an electric field, and another optical waveguide is formed between these two optical waveguides.
An optical directional coupler in which two optical waveguides are formed, and a pair of electrodes for applying a voltage are installed on the two optical waveguides at both ends.