JPH06160785A - Optical modulator - Google Patents

Abstract

PURPOSE:To provide an optical modulator capable of reducing a noise and improving withstand voltage performance. CONSTITUTION:An optical waveguide 2 is formed on a substrate 1 and thin-line state electrodes 3 and 4 are respectively formed to be opposed to each other on both sides of the optical waveguide. The edge of an end part on the indient side and the outgoing side of at least either of two electrodes is formed to be curved. Thus, the concentration of electric field at the end of the electrode is avoided. Therefore, the noise is reduced and the withstand voltage performance is improved further.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator utilizing the electro-optical effect, and more particularly to an optical modulator designed to reduce noise and improve withstand voltage characteristics.

[0002]

2. Description of the Related Art An optical modulator is known in which an optical waveguide is formed on an electro-optic crystal substrate such as LiNbO ₃ by, for example, a proton exchange method, and strip-shaped electrodes facing each other are formed along the optical waveguide. For example, it has been put to practical use as a phase modulator, an intensity modulator, an optical switch and the like. In the optical modulator, a signal voltage is applied between the electrodes, and the refractive index of the region sandwiched by the electrodes of the optical waveguide is selectively changed according to the input signal to perform the optical modulation.

[0003]

In the above-mentioned type of optical modulator, there is a strong demand for noise reduction and further improvement of withstand voltage performance. To achieve this purpose, it is necessary to form a uniform refractive index distribution according to the signal voltage over the modulation region of the optical waveguide. However, in the conventional optical modulator, since the edges of the electrodes to which the signal voltage is applied, especially the entrance side edge and the exit side edge are formed at right angles, the electric field is concentrated in this part,
A strong electric field was locally generated when the signal voltage was applied. When a strong electric field is locally generated, the refractive index of this portion becomes abnormally high, and a part of the incident light wave is reflected at this portion, which causes, for example, light source noise. Similarly, a strong electric field is locally formed at the edge portion on the emitting side, which causes noise. In addition, if the electric field is locally concentrated, dielectric breakdown occurs, and the withstand voltage performance also deteriorates.

Therefore, the object of the present invention is to eliminate the above-mentioned drawbacks, to form a uniform refractive index distribution over the optical modulation area of the optical waveguide, reduce the signal noise and further improve the withstand voltage performance. It is to provide an optical modulator.

[0005]

In the optical modulator according to the present invention, an optical waveguide is formed on an electro-optic crystal substrate, and a plurality of strip-shaped electrodes facing each other are formed along the optical waveguide. An optical modulator for selectively changing the refractive index of a waveguide with respect to incident light propagating in an optical waveguide by applying a signal voltage between the electrodes, and at least one electrode of the plurality of electrodes It is characterized in that any one of the corners on the side opposite to is curvedly formed.

By curving the edges of the electrodes on the incident side and the emitting side of the electrodes facing the optical waveguide, the distance between the electrodes, that is, the gap is gradually shortened along the light propagation direction. The electric field formed in the portion gradually increases. As a result, the refractive index of the optical waveguide when a signal voltage is applied gradually changes along the propagation direction of light, and therefore, it is possible to avoid the disadvantage that the refractive index locally rapidly increases. Moreover, the local electric field concentration is prevented, and the withstand voltage performance is further improved.

[0007]

1 is a diagram showing the construction of an example of an optical modulator according to the present invention. In this example, an optical modulator including a lumped constant electrode used for a phase modulator will be described. For example, the optical waveguide 2 is formed on the electro-optic crystal substrate 1 such as LiNbO ₃ by the Ti diffusion method or the proton exchange method. First and second strip electrodes 3 and 4 are formed on both sides of the optical waveguide 2. These electrodes can be formed by forming a resist pattern of the electrodes by photolithography, then sputtering aluminum and using the lift-off method. The gap between the first electrode and the second electrode is, for example, 10
The width of the electrode itself is 50 μm. First electrode 3
Is connected to the signal source 5. Further, both ends of the second electrode 4 are grounded. The light beam to be phase-modulated shall be incident and propagate along the direction of the arrow. The incident light beam propagates along the optical waveguide 2, and the first electrode 3 and the second electrode 4
In the areas where and are opposite to each other, the light is phase-modulated according to the signal voltage applied from the signal source 5 and emitted. That is, when a signal voltage from the signal source 5 is applied, an electric field is formed between the first electrode 3 and the second electrode 4, and this electric field changes the refractive index of this region of the optical waveguide 2, The incident light due to the change in the refractive index is phase-modulated according to the input signal. At this time, if the edge portion 3a at the incident side end portion and the edge portion 3b at the emitting side end portion of the first electrode 3 are formed at a right angle, the electric field concentrates on this portion. Therefore, in this example,
The edge portion of the incident side end portion 3a facing the optical waveguide 2 is curved so as to gradually approach the optical waveguide along the propagation direction of the incident light, and the exit side end portion 3b is curved so as to gradually move away. Form. In this way, by forming the edges of the entrance and exit ends in a curved shape, the strength of the electric field formed at the end of each electrode when a signal is input is smoothly formed along the light propagation direction. It is possible to prevent local electric field concentration.

Next, with respect to the withstand voltage performance of the optical modulator according to the present invention, a comparison test with a conventional electrode structure will be conducted, and the result will be described. The light modulator of the present invention having the above-mentioned structure is used, and the incident side and emission side end portions 3a are used as a conventional electrode structure.
And 3b with the edges formed at right angles were used. The experimental results are shown in Table 1.

[0009]

[Table 1]

As shown in Table 1, in the conventional electrode structure,
A breakdown occurred at 22V, but the withstand voltage of the present invention is 10
It is 4V, and the withstand voltage performance has increased about 5 times. Also,
The modulation bands were both 0 to 3.0 GHz, and the modulation bands were the same.

FIG. 2 shows a modification of the optical modulator according to the present invention. In this example, an example in which the present invention is applied to a traveling wave electrode is shown. In the figure, the same members as those used in FIG. One end side of the first electrode 3 is connected to the signal source 5, and the emission side end portion is connected to the second electrode 4 via a termination impedance of 50Ω. In this example, the edges of the incident side and emission side end portions 3a and 3b of the first electrode 3 are curved.

3 (a) to 3 (d) show the electrode structure of the type of the optical modulator according to the present invention. In the figure, only the first electrode 3 and the second electrode 4 are shown. FIG. 3A shows an example in which both ends of the first and second electrodes are both formed in an arc shape having a radius equal to the width of the electrodes. Further, FIG. 3B shows an example in which both ends are formed into an arc having a diameter equal to the width of the electrode. Furthermore, FIG. 3 (c) shows the first
An example in which both ends of the electrode 3 are formed in an arc shape is shown. Further, FIG. 3D shows an example in which the entire incident side and outgoing side end portions of the second electrode 4 are formed in an arc shape.

FIG. 4 shows another modification of the optical modulator according to the present invention. FIG. 4 (a) is a plan view and FIG. 4 (b) is a sectional view taken along a plane orthogonal to the optical waveguide. Is. In this example, a signal electrode 10 is formed on the optical waveguide 2 along the optical waveguide, and two ground electrodes 11a and 11 are provided on both sides of the signal electrode 10.
b, the signal electrode 10 is connected to the signal source 5, and the ground electrode
Both 11a and 11b are grounded. When the modulation signal is input to the signal electrode 10, an electric field is formed between the signal electrode 10 and the ground electrodes 11a and 11b, and the bending ratio of the region sandwiched by these electrodes of the optical waveguide is selectively changed according to the input signal. Change. Therefore, also in the present example, the corners of the signal electrode 10 and the counter electrodes 11a and 11b on the sides facing each other are formed angularly, and electric field concentration occurs at this portion. For this reason, also in this example, the corners of the light incident side and the emission side on the side of each electrode facing the other electrode are curved.

The present invention is not limited to the above-mentioned embodiments, and various changes and modifications can be made. In the above-described embodiments, the optical modulator that performs the phase modulation has been described, but the present invention is not limited to the phase modulator, and can be applied to various optical modulators such as an intensity modulator and an optical switch.
Further, although LiNbO ₃ is used as the substrate material in the description, various electro-optic crystal materials can also be used.

[0015]

As described above, according to the present invention, since the incident side and the emitting side end portions of at least one of the strip electrodes formed on both sides of the optical waveguide are curved, the signal voltage is reduced. It is possible to form a smoothly changing electric field at the incident side and the emitting side end portions when a voltage is applied. As a result, the inconvenience such as light reflection caused by the local increase in the refractive index is eliminated, and the generation of noise can be effectively reduced. Moreover, since electric field concentration can be prevented, the withstand voltage performance is further improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an example of an optical modulator according to the present invention.

FIG. 2 is a diagram showing a modified example of the optical modulator according to the present invention.

FIG. 3 is a diagram showing various electrode structures of the optical modulator according to the present invention.

FIG. 4 is a diagram showing another modification of the optical modulator according to the present invention.

[Explanation of symbols]

 1 substrate 2 optical waveguide 3 first electrode 4 second electrode 5 signal source

Claims (2)

[Claims] 1. An optical waveguide is formed on an electro-optic crystal substrate,
A plurality of strip electrodes facing each other are formed along the optical waveguide, and a signal voltage is applied between these electrodes to selectively change the refractive index of the waveguide with respect to incident light propagating in the optical waveguide. In the optical modulator, at least one of the plurality of electrodes is curved to form one corner on the side facing the other electrode. 2. An optical waveguide is formed on an electro-optic crystal substrate,
An optical modulator that forms strip-shaped electrodes that face each other across this optical waveguide, and applies a signal voltage between these electrodes to selectively change the refractive index of the waveguide with respect to incident light to be modulated. The light-incident side facing edge of at least one of the two electrodes is curved so as to gradually approach the waveguide along the propagation direction of the light to be modulated, and An optical modulator characterized in that the opposite edge portion of is curved so as to gradually move away from the optical waveguide along the propagation direction of the modulated light.