JPS6266222A - Light modulator - Google Patents

Abstract

PURPOSE:To attain the super high speed modulating action by eliminating the receiving of the limit of the modulating band width by the electrode capacity and executing the progressive wave action to the modulating electric signal. CONSTITUTION:At the upper part of a linear wave path 19, a slot line 24 composed excluding a part of a metallic conductor 23 is constituted. At both edges of the slot line 24, central conductors 26 and 28 of two semi-ridged cables are connected so that they can be electrically linked. The connection is executed by connecting the central conductors 26 and 28 of the semi-ridged cable with the metallic conductor 23 by crossing the slot line 24. Since the characteristic impedance of the slot line is determined by the dielectric constant of a substrate 14 and the narrower width of the slot line 24, to the given substrate 14, the width is suitably set and can be equal to the characteristic impedance of the coaxial cable. Consequently, the slot line 24 constitutes the progressive wave electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates to an optical modulator that electrically controls the intensity of light waves guided in an optical integrated circuit.

BACKGROUND OF THE INVENTION A conventional example of an optical modulator using crossed optical waveguides is shown in FIG. Optical waveguides 2 and 3 are formed so as to intersect on the surface of a substrate 1 having an electro-optic effect.

Electrodes 8.9 are provided at the intersections of these optical waveguides, and these electrodes are further connected to electrode pads 10.11 at the ends of the substrate.
A voltage from a signal source 13 is applied through a resistor 12 between these electrode pads 10 and 11 (for example, R.A. Beck.
Double Ni Nis Xi Chang, Applied Optics.

Volume 18, No. 19, 3296-33oO page, October 1
979 CR, person, Beaker and W, S,
ClChang.

Applied Optics vol, 1B, A19
．． pp3296-3300, Oct. 1979)
.

It is well known that an element with such a structure operates as an optical switch or optical modulator. In other words, when light enters from terminal 4, for example, and no voltage is applied from the outside, the light travels straight toward terminal 5. do. - When a voltage is applied from the outside, the electric field generated between the electrodes 8 and 9 causes a change in the refractive index based on the electro-optic effect at the intersection of the optical waveguides, and the light incident from the terminal 4 is transferred to the terminal 7. You will be able to move towards others. Therefore, optical switching becomes possible.

Further, when focusing on the output light from the terminal 6, intensity-modulated light is extracted, and it can also be used as an optical modulator.

Problems to be Solved by the Invention In an optical switch (or optical modulator) using crossed waveguides like the conventional example, for example, when light is input from terminal 4, the light extracted from terminal 6 is turned on. It is known that the switching (or modulation) voltage required to turn it off is higher than that of a directional coupler type optical switch (or optical modulator). This is because a directional coupler type optical switch (or optical modulator) can lower the switching (or modulation) voltage by increasing the coupling length, whereas a crossed waveguide type optical switch (or optical modulator) can lower the switching (or modulation) voltage by increasing the coupling length.
This is because only the intersection portion acts as the bond length, so it is essentially impossible to make the bond length long.

In addition, in the electrode configuration of a conventional optical switch (or optical modulator), when the frequency of the modulation signal becomes high, the electrode 8
．． Because of the time constant determined by the electrode capacitance and load resistance, the electric field applied to 9 cannot follow changes in the external voltage.

Furthermore, since high-frequency electrical signals are usually supplied using coaxial cables, this kind of electrode configuration makes it difficult to connect to coaxial cables, and even if a temporary connection is made, impedance increases between the electrode pads and the coaxial cables. If a mismatch occurs, - the power of the external switching signal will not be efficiently supplied to the electrode section.Therefore, ultra-high speed modulation is not possible with a device configured in this way.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an optical modulator having a configuration that enables ultrahigh-speed optical modulation with smaller electric signal power.Means for Solving the Problems In order to solve the above-mentioned problems, the invention lengthens the crossing portion of the crossed waveguides to increase the coupling length and lower the switching (or modulation) voltage, and also provides a gap portion approximately in the center of the crossing portion. A slot line is provided so that the slot lines overlap, and optical modulation is performed by the electric field generated in the gap between the slots.The slot line and two semi-rigid cables are electrically coupled, and the semi-rigid cable and the slot line are connected to one continuous cable. By using two transmission lines, traveling wave operation is performed on the modulated electrical signal.

Operation In the configuration of the present invention, the length of the crossing portion of the crossed waveguides can be appropriately selected to optimize the coupling length and reduce the switching (or modulation) voltage. Since the semi-rigid cable and the slot line form one continuous transmission line, the characteristic impedance of the slot line is selected to be equal to the characteristic impedance of the semi-rigid cable, thereby reducing the connection between the external power source and the optical modulator electrode. Impedance can be matched over a wide band. That is, unlike the conventional example, the modulation bandwidth is not limited by the electrode capacitance, and traveling wave operation can be performed on the modulated electrical signal, making it possible to perform ultra-high-speed modulation operation.

Embodiment An embodiment of the present invention is shown in FIG. FIG. 1(A) is a top view, and FIG. 1(B) is a sectional view taken along the line I--I shown in FIG. 1(A). In FIG. 1, 14 is a substrate made of a substance having an electro-optic effect, 15.16 is one Y-branch optical waveguide, 17.18 is the other Y-branch optical waveguide, and 19 is a straight optical waveguide. In the conventional example, the length of the intersection is uniquely determined by the width of one waveguide and the intersection angle, but in the present invention, the length of the intersection can be set completely freely. That is, it is possible to choose the length of the intersection so that the switching or modulation voltage is low. A slot line 24 is formed above the straight waveguide 19 by removing a portion of the metal conductor 23. Center conductors 26 and 28 of two semi-ridged cables are connected to both ends of this slot line 24 so as to be electrically coupled. This connection is made by connecting the central conductor 26,28 of the semi-ridged cable across the slotted line 24 to the metal conductor 23. The characteristic impedance of the slot line is the substrate 14
The dielectric constant of the slot line 24 is determined by the width of the narrower side of the slot line 24, so for a given board 14, this width is appropriately determined to determine the characteristic impedance of the coaxial cable (for example, 5oΩ).
can be made equal to Therefore, in FIG. 1, the slot line 24 constitutes a traveling wave electrode, and if one of the two semi-ridged cables is used as an input cable for the modulated electrical signal and the other is used as an output cable, the semi-ridged
The cable and slot line form one continuous transmission line. Therefore, by terminating the output cable with a resistor equal to its characteristic impedance, the input impedance seen from the input cable side can be kept at a constant value over a wide frequency range, allowing broadband operation of the optical modulator.

Effects of the Invention According to the present invention, switching or modulation voltage can be lowered by appropriately selecting the length of the intersection of optical waveguides, and modulation can be performed using a low-power electrical signal even at extremely high frequencies. In addition, a slot line is used as the electrode, a semi-ridged cable is connected to this slot line, and the modulated voltage is supplied from this semi-ridged cable, so ultra-high frequencies can be transmitted without reflection due to discontinuity of the connection. modulation,! Air signals can be efficiently supplied to the electrodes, and ultra-high-speed modulation operations can be performed. In addition, since the semi-ridged cable and the slot line form one continuous transmission line, traveling wave operation is performed on the modulated electrical signal, and the modulation bandwidth is not limited by electrode capacitance, allowing ultra-high frequency transmission. A light modulator can be operated.

[Brief explanation of drawings]

Fig. 1 (M), ■) is a top view and a sectional view taken along line I-I of an optical modulator according to an embodiment of the present invention, and Fig. 2 is a basic configuration diagram of a conventional optical modulator. ... Substrate made of a substance having an electro-optic effect, 23 ... Slot line grounding conductor, 24 ...
... Slot line, 26.27 ... Seven-millimeter rigid cable outer conductor, 26.28 ... Semi-rigid cable outer conductor. Name of agent: Patent attorney Toshio Nakao and 1 other person
2 figure

Claims (1)

[Claims] Near the surface of a substrate made of a substance that has an electro-optic effect,
A straight optical waveguide having two Y-branched optical waveguides separated by a certain distance from each other is formed, and a slot line is provided approximately at the center of the straight optical waveguide between the two branch points of the Y-branched optical waveguide, and the slot line Two semi-ridged rails at both ends of the track.
Connect the cables electrically and connect the two semi-ridged
One of the cables is used as the input terminal for the modulated electrical signal, and the other
1. An optical modulator, characterized in that two semi-ridged cables are used as output terminals for modulated electrical signals, and the semi-ridged cables and the slot line constitute a continuous transmission line.