JP2760222B2 - Light modulation element and light modulation device using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-speed light modulation element which is a basic element in a high-speed optical communication or an optical signal processing system, and a method for driving the light modulation element.

[0002]

2. Description of the Related Art An optical modulation element is a basic element in high-speed information communication and an optical signal processing system, and it is considered that a demand for an optical modulation element that can operate at an ultra-high speed will increase more and more in the future. For such requests,
Direct modulation of a semiconductor laser, which has been conventionally used, is difficult to apply. Therefore, development of an external modulation type element capable of high-speed operation is urgently required. Among them, a so-called electro-optic modulator using a dielectric crystal having a particularly large Pockels effect can operate at ultra-high speed, and has very little phase disturbance due to modulation. It is very effective for distance optical fiber communication. Furthermore, there is a possibility that miniaturization and high efficiency can be realized by using the optical waveguide structure.

[0003] The general structure of this high-speed electro-optic modulation element is a transmission line for propagating a modulation signal on an electro-optic crystal as a modulation electrode, and an optical waveguide formed near the transmission line. An electric field induced around the modulation electrode is generated by the modulation signal, and the refractive index of the optical waveguide changes due to the electro-optic effect. Thereby, the phase of the light wave propagating in the optical waveguide changes with the modulation signal, and light modulation is realized.

[0004]

With respect to the modulation frequency of the optical modulation element, further high-speed optical modulation is required. Therefore, it is desired to realize efficient optical modulation in the ultrahigh frequency range of millimeter waves and submillimeter waves. It is rare.

However, in the configuration of the conventional optical modulation element described above, in such an ultra-high frequency range, not only the propagation loss of the modulated wave at the modulation electrode becomes very large, but also the efficiency of such a high frequency is reduced. It is very difficult to couple well with the modulation electrode. In particular, a waveguide is often used for such high-frequency transmission, and in such a case, a method of converting the waveguide into a coaxial cable and coupling it to the modulation electrode causes loss on the way. There was a problem that the size became extremely large and the feasibility was poor.

[0006] In an electro-optic light modulation element, the electro-optic coefficient which is the basis of light modulation is relatively small in a normal crystal. Therefore, it is important to efficiently apply an electric field to the optical waveguide, but there is also a problem that the modulation efficiency is low in the conventional electro-optic light modulation device.

The present invention provides an optical modulator capable of improving the modulation efficiency in an ultra-high frequency range and inputting a modulated wave without amplifying the same, and a method of driving an optical modulator capable of directly coupling a modulated wave from a waveguide. The purpose is to provide.

[0008]

According to the present invention, an optical waveguide having an electro-optic effect is exposed on the surface of a substrate on which a split ring type electrode is formed, and is passed through a slit portion of the split ring type electrode to form a split ring type electrode. The conventional problems have been solved by a light modulating element that intersects with the electrode portion of the electrode and a driving method for installing the light modulating element inside the waveguide tube.

[0009]

In the light modulation device of the present invention, the modulated wave is efficiently coupled to the modulation electrode in an ultra-high frequency region such as a millimeter wave or a submillimeter wave, and the electric field is efficiently concentrated on the optical waveguide. The modulation efficiency of modulation can be greatly improved.

[0010]

An embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a perspective view of one embodiment of the light modulation device of the present invention. In FIG. 1, for example, an optical waveguide 12 is formed on a surface of a substrate 11 having an electro-optical effect represented by lithium niobate by, for example, a metal diffusion method.
Thereafter, using a metal thin film of aluminum or the like, a split ring type electrode 13 (hereinafter, referred to as an electrode 13) composed of a ring type line having a slit portion 14 in a part as shown in the figure.
) Is formed, for example, by a method such as sputtering, photolithography, and reactive ion etching.

The input light 15 is guided to one of the optical waveguides 12 of the optical modulator, passes through the slit 14 of the electrode 13, and is output from the other end of the optical waveguide 12 as output light 16. At this time, a modulated wave is coupled to the electrode 13 by an appropriate method, and a current flows along the line.
A large electric field is generated in the slit part 14, and the refractive index of the waveguide 12 changes according to the electric field strength due to the electro-optic effect. Then, the phase of the output light 16 changes, and this light modulation element operates as a phase modulator.

FIG. 1 shows an example in which the electrode 13 has a circular shape. However, in practice, there is no problem if the electrode 13 has a rectangular or elongated shape as long as it is an annular line. However, if the opposing lines are too close to each other to cause coupling between the lines, or if the radius of curvature of the bent portion is small, the loss increases. These cause instability of operation and deterioration of modulation efficiency.

The split ring type electrode 13 constituting the light modulation element of the present invention has a structure in which the open ends of the lines face each other at the slits 14, so that a high-frequency current is induced in the lines. And the voltage amplitude at both ends of the line facing the slit portion 14 is the highest in the line, and since the voltages at both ends are in opposite phases with respect to time, the two ends of the line facing the slit portion 14 Has a very large potential difference. Therefore, since an extremely large electric field is induced in the slit portion 14, an optical waveguide is provided in the vicinity thereof, and a high-frequency current is induced in the electrode 13 by a modulation signal wave by an appropriate method.
Extremely efficient light modulation becomes possible.

Next, an embodiment of a method for driving the light modulation element will be described with reference to the drawings. It is extremely useful to apply the driving method of the present invention in order to drive the light modulation element of the present invention without impairing the excellent features.

FIG. 2 is a conceptual perspective view of a main part of an embodiment of a driving device for explaining a driving method of the light modulation element of the present invention. The light modulating element 21 is disposed, for example, perpendicularly to the wall surface in a circular waveguide 22 as shown in the figure, and a current is generated in the split ring electrode 24 by an electric field of an electromagnetic wave propagating in the waveguide, thereby achieving efficient light modulation. Is realized. For inputting and outputting light, it is effective to connect an optical fiber directly to both end portions of the optical waveguide 23 and to input and output light from outside the waveguide through the optical fiber.

Waveguides are suitable as transmission lines for ultrahigh frequencies such as millimeter waves and submillimeter waves in terms of propagation loss. Conventionally, in order to couple such an ultra-high frequency to the electrode of the light modulation element, the waveguide is once converted into a coaxial cable, and the coaxial cable is then coupled to the light modulation element. However, in the driving method of the light modulation element of the present invention, the modulated wave in the waveguide can be directly coupled to the electrode of the light modulation element.
In optical modulation using a modulation signal of an ultra-high frequency, efficient modulation is possible and the optical modulation system can be simplified.

When a circular waveguide is used as the waveguide and the modulated wave is coupled to the optical modulation element in the _TE0n mode, the coupling efficiency becomes highest and extremely high efficiency optical modulation can be realized. This is because the electric field in the _TE0n mode is oriented in the circumferential direction, and therefore, a high-frequency current is generated in the electrode most efficiently. In addition, since the _TE0n mode has a property that the loss in the waveguide decreases as the frequency increases, it is most _suitable for driving an optical modulation element in an ultra-high frequency range such as a millimeter wave and a submillimeter wave. is there.

By using a superconductor having a very low surface resistance at a high frequency in addition to a metal material such as aluminum as a material for the electrode, the attenuation of the current induced in the electrode is reduced, and the efficiency is further improved. Light modulation becomes possible.

The light modulation element shown in the embodiment operates as a so-called phase modulation element for modulating the phase of a light wave. Phase modulation is expected to be used in next-generation optical communication systems such as coherent optical communication. In order to use the light modulation element of the present invention as a light intensity modulation element mainly used in the current optical communication system, by using an optical waveguide,
By configuring the Mach-Zehnder interferometer on a substrate, it can be realized very easily.

Further, in the present optical modulator, an optical waveguide portion,
That is, it is only necessary that the range over which the electric field of the light wave reaches or a part thereof has the electro-optic effect. Therefore, as described in the present embodiment, even if a material having an electro-optical effect is not used for the substrate 11, for example, a material obtained by thinning a material having an electro-optical effect on a low refractive index substrate may be used. By forming an optical waveguide on the part, or forming a core part with a higher refractive index than the surroundings on the substrate surface and forming a material having an electro-optic effect as a clad part on it, it seeps from the core part It is similarly effective to perform optical modulation using the generated electric field.

[0021]

As can be seen from the above description, according to the present invention, an optical waveguide having an electro-optic effect is exposed on the surface of a substrate on which a split ring type electrode is formed, and is passed through a slit portion of the split ring type electrode. , Because it is an optical modulation element that intersects the electrode part of the split ring type electrode, the modulation efficiency is dramatically improved, especially in the ultra-high frequency range such as millimeter wave and submillimeter wave, compared to the conventional waveguide type optical modulation element. Can be improved.

Further, since the present invention is a method for driving an optical modulation element in which this optical modulation element is installed inside a waveguide, it is possible to directly couple a modulated wave from the waveguide and to use an ultra-high frequency Modulation efficiency in the frequency range, enabling optical modulation with low power, ultra-high speed and simplified optical communication system,
Higher performance is possible.

[Brief description of the drawings]

FIG. 1 is a conceptual perspective view showing a configuration of one embodiment of a light modulation element of the present invention.

FIG. 2 is a conceptual perspective sectional view of an essential part for explaining an embodiment of a method for driving a light modulation element according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Substrate 12 Optical waveguide 13 Split ring type electrode 15 Slit part 16 Input light 17 Output light 21 Substrate 22 Circular waveguide 23 Optical waveguide 24 Split ring type electrode 25 Modulation wave 26 Input light 27 Output light

Claims (5)

(57) [Claims] 1. A split ring-type electrode for generating a current by an electric field of an electromagnetic wave, and an optical waveguide having an electro-optical effect, wherein the split ring-type electrode is formed on a substrate, and the optical waveguide is A light modulation element exposed on a substrate surface, passing through a slit portion of the split ring type electrode, and crossing an electrode portion of the split ring type electrode. 2. The light modulation device according to claim 1, wherein the split ring type electrode is a superconducting thin film. 3. An optical modulator according to claim 1, wherein
A light modulation device arranged inside. 4. The optical modulation device according to claim 3, wherein a substrate is placed in a direction perpendicular to a wall surface of the waveguide, and a _TE0n mode is used as a waveguide mode . 5. The method according to claim 1, wherein the waveguide is a circular waveguide.
The light modulation device according to claim 3.