JP2963989B1 - Light modulator - Google Patents

Abstract

An object of the present invention is to provide an optical modulator in which a large electric field is generated in an electro-optic crystal waveguide by a small applied voltage, and a large change in optical refractive index is caused, and a manufacturing method thereof. The present invention has a structure in which an optical waveguide (1) is sandwiched between modulation electrodes (4, 6, 7) from above and below, so that all components of an applied electric field contribute to an electro-optic effect to reduce a modulation voltage. Usually, since the waveguide having the electro-optic effect is a crystal, it is impossible to use a metal material for the lower electrode. Therefore, the above structure is achieved by making the lower electrode 4 a conductive crystal material and heteroepitaxially growing a waveguide crystal thereon.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates also relates to an optical modulator using an electro-optical effect of the crystal, an optical modulator and a method for manufacturing the same with a particularly conductive crystal as the lower electrode.

[0002]

2. Description of the Related Art Conventionally, an optical modulator of this type is shown in FIG. In these devices, two or more metal surface electrodes 33 for applying an electric field are loaded on the device surface via the SiO ₂ buffer layer 2 on the electro-optic crystal substrate 31. When a voltage is applied between the electrodes 33 by the power supply 34, the lines of electric force pass through as shown in the cross-sectional view of FIG. Due to the electro-optic effect in which the refractive index of the crystal with respect to light by the electric field changes, the refractive index of the optical waveguide 1 changes, and the phase of light propagating in the optical waveguide 1 changes. A light intensity modulator using a Mach-Zehnder waveguide and a modulator (optical switch) using a directional coupler waveguide are realized by utilizing the phase change and the interference effect of light.

[0003]

However, in the above-mentioned conventional optical modulator, since two or more metal surface electrodes 33 for modulation are loaded on the element surface, the applied electric field passes through the inside of the substrate 31; and only the component E _Z contribute direction electrooptic effect contributes to the light modulation. Since only a part of the applied electric field is effectively used as described above, a voltage of 5 to several tens of volts is required to perform sufficient modulation.
This is a practical problem from the viewpoint of energy saving.
In order to make the electric field act more effectively, a method has been proposed in which side electrodes 36 for modulation are provided on both sides of a ridge-type optical waveguide 35 as shown in FIG. When it is difficult to deposit an electrode material, and when two or more waveguides such as a Mach-Zehnder type and a directional coupler type are close to each other, it is impossible to deposit electrodes on both sides of each waveguide.

[0004]

In order to solve the above-mentioned conventional problems, the present invention has a structure in which a waveguide is sandwiched between modulation electrodes from above and below, so that all components of an applied electric field contribute to the electro-optic effect. , The modulation voltage is reduced. Usually, since the waveguide having the electro-optic effect is a crystal, it is impossible to use a metal material for the lower electrode. Therefore, the above structure is achieved by forming the lower electrode from a conductive crystal material and heteroepitaxially growing a waveguide crystal thereon.

According to the present invention, an electro-optic crystal waveguide having a thickness of several microns is sandwiched between a conductive crystal serving as a lower electrode and an upper electrode such as a metal. A large electric field is generated in the wave path, causing a large change in the optical refractive index.

[0006]

FIG. 1 shows the structure of a Mach-Zehnder type optical modulator as one embodiment of the present invention. First, a zinc oxide (hereinafter ZnO) crystal is grown on a z-cut lithium niobate (hereinafter LN) crystal substrate 5, which is used as a lower crystal electrode 4. If a ZnO crystal substrate with good crystallinity is available, the substrate can also serve as the lower electrode if it is used.
An LN crystal thin film 3 is heteroepitaxially grown thereon. A Mach-Zehnder type waveguide pattern mask is made of a tantalum thin film on the LN crystal thin film 3, and this sample is immersed in pyrophosphoric acid at about 200 ° C. for about 20 minutes to perform proton exchange. Further, annealing is performed in air at 300 ° C. for about 1 hour to produce a proton exchange optical waveguide 1. For the production of a waveguide, a titanium diffusion method in which a waveguide pattern of a titanium thin film is provided and this is thermally diffused in a high-temperature furnace for several hours can be used, but the process temperature is as high as about 1000 ° C. and ZnO is used.
Since there is a possibility that the interdiffusion at the interface between LN and LN proceeds, the proton exchange method that can be manufactured at a relatively low temperature is more preferable. Upper gold electrodes 6 and 7 are formed on the two branch lines of the manufactured Mach-Zehnder waveguide through a silicon oxide (SiO ₂ ) film 2 as a transparent insulator by vapor deposition. Lower electrode 4
Is connected to 0 V, a positive voltage is applied to the upper gold electrode 7 using the power supply device 9, and a negative voltage is applied to the upper gold electrode 6 using the power supply device 8. Since the electric field generated in the waveguide below the upper electrode 7 and the electric field generated in the waveguide below the upper electrode 6 have opposite directions, the sign of the refractive index change caused by the electro-optic effect is reversed, and the light passing through one of the waveguides is inverted. The phase is advanced and the phase of light passing through the other is delayed. Since these are superimposed again and output, the output light intensity changes depending on the applied voltage due to interference.

As described above, the optical modulator of the present invention has a structure in which the waveguide is sandwiched between the modulation electrodes from above and below, and reduces the modulation voltage by making all components of the applied electric field contribute to the electro-optic effect. . According to the present invention, an electro-optic crystal waveguide having a thickness of several microns is sandwiched between a conductive crystal as a lower electrode and an upper electrode such as a metal. A large electric field is generated, causing a large change in the refractive index of light.

The operation of the present invention will be described with reference to FIG. As shown on the left side of FIG. 2, in the case of a conventional surface electrode, the distance between the surface electrodes is determined by the accuracy of photolithography, and in the case of an optical element, the electrode spacing is about 1 to 2 microns or more. Generally, the electric field strength E is expressed by E = V / d using the voltage V between the electrodes and the distance d. In the case of a surface electrode, the electric field strength in the horizontal direction connecting the electrodes with a straight line is given by this equation. The electric field strength in the waveguide below the electrode is smaller than this value. Further, as shown in the lower left of the figure, the effective electric field _EZ has a smaller value because it is a component of the electric field in the z direction.

On the other hand, in the method according to the present invention shown on the right side of FIG. 2, the thickness of the electro-optic crystal thin film is about 1 to 2 μm, and the electrode spacing is almost the same as that of the conventional surface electrode. As seen in the lines of electric force, the resulting electric fields all work effectively. Therefore, in the method according to the present invention, the applied voltage required to obtain the same effective electric field strength as the conventional method is small, and the power consumption is small.

FIG. 3 shows the structure of a directional coupler type optical modulator according to an embodiment of the present invention. First, a ZnO crystal is grown on a z-cut lithium tantalate (LT) crystal substrate 15 and used as a lower crystal electrode 4. An LT crystal thin film 13 is further heteroepitaxially grown thereon.
The directional coupler type proton exchange optical waveguide 1 is manufactured in the same manner as in the above case. After forming an electrode pattern mask by photolithography on the two parallel line portions of the directional coupler type waveguide, a ZnO crystal thin film is again formed,
These are referred to as upper electrodes 16 and 17. The lower electrode 4 is connected to 0V, and a positive voltage is applied to the upper ZnO crystal electrode 17 using the power supply device 9 and a negative voltage is applied to the upper ZnO crystal electrode 16 using the power supply device 8. When the parallel line portion of the directional coupler is equal to an odd multiple of its interaction length, when no voltage is applied, the light incident on the left waveguide at the input end of the element is converted into the right waveguide at the output end. Inject from If the interaction length is changed by adjusting the applied voltage so that the length of the parallel line portion becomes an even multiple of the interaction length, all light exits from the left waveguide at the emission end, and the light travels Change is achieved. Also, by adjusting the applied voltage, the intensity of the output light from the two waveguides can be changed.

FIG. 4 shows the structure of a single waveguide phase modulator as one embodiment of the present invention. First, an indium oxide crystal is grown on a z-cut LN crystal substrate 5 and used as a lower crystal electrode 24. An LN crystal thin film 3 is further heteroepitaxially grown thereon. After the straight waveguide pattern is made of titanium, the surface of the LN crystal thin film is ground by reactive ion etching using CF ₄ + Ar gas or the like, and then the titanium mask is removed by chemical etching to manufacture the ridge type optical waveguide 21. . After forming an electrode pattern mask by photolithography, an indium oxide crystal thin film is formed again, and this is used as the upper electrode 27. Lower electrode 2
When the power supply 9 applies a voltage to the upper crystal electrode 27 by grounding 4, the refractive index of the LN waveguide changes due to the electro-optic effect, and the phase of light propagating in the waveguide changes.

[0012]

According to the optical modulator of the present invention, a structure sandwiching the modulation electrodes in the optical waveguide from above and below, all components of the applied electric field so as to contribute to the electro-optical effect, is performed to reduce the modulation voltage . The lower electrode is made of a conductive crystal material, and a waveguide crystal is heteroepitaxially grown thereon to achieve such a structure.

As a result, the driving voltage of the electro-optical modulator, which has been a problem in the past, can be reduced, and a high-speed optical modulator with low power consumption can be realized. In addition, wasteful heat generation is eliminated by lowering the power, and a highly integrated element can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration example of a Mach-Zehnder type light intensity modulator using the present invention.

FIG. 2 is a diagram showing the effect of the present invention.

FIG. 3 is a configuration example of a modulator that changes the path of directional coupler light using the present invention.

FIG. 4 is a configuration example of an optical phase modulator using the present invention.

FIG. 5 shows a configuration of a conventionally used optical modulator.

FIG. 6 shows a configuration of an optical modulator for modulation at a low voltage which has already been proposed.

[Explanation of symbols]

1 proton exchange optical waveguide 2 SiO ₂ insulating film 3 LN crystal thin film 4 lower ZnO crystal electrode 5 LN substrate 6,7 upper gold electrode 8,9 modulated power 13 LT crystal thin film 15 LT crystal substrate 16, 17 the upper ZnO crystal electrode 21 Ridge Type optical waveguide 24 Indium oxide crystal electrode 27 Upper crystal electrode

Continuation of the front page (56) References JP-A-62-154407 (JP, A) JP-A-61-182015 (JP, A) JP-A-9-329722 (JP, A) JP-A-3-196025 (JP) (A) JP-A-3-119311 (JP, A) (58) Fields investigated (Int.Cl. ⁶ , DB name) G02F 1/15-1/055 505 INSPEC (DIALOG) JICST file (JOIS)

Claims (5)

(57) [Claims] An electric field is generated in an optical waveguide by applying a voltage between electrodes, and the refractive index of an optical waveguide portion is changed by utilizing an electro-optic effect in which a refractive index of a crystal with respect to light is changed by the electric field. In the optical modulator that modulates the light, the zinc oxide grown on the lithium tantalate crystal substrate
A lower conductive crystal electrode composed of a crystal, and a tandem heteroepitaxially grown on the crystal electrode.
Consisting of a crystal waveguide having an electro-optic effect formed using lithium tantalate, and a zinc oxide crystal thin film loaded on the crystal waveguide
An optical modulator comprising: an upper modulation electrode; 2. The light modulation device according to claim 1, wherein the crystal waveguide is of a Mach-Zehnder type, and the upper electrode is individually loaded on each branch of the Mach-Zehnder-type waveguide to perform light intensity modulation. vessel. 3. The crystal waveguide is of a directional coupler type,
2. The optical modulator according to claim 1, further comprising individually loading the upper electrode on each waveguide of the directional coupler type waveguide to change a light path. 4. The crystal waveguide is configured as a single line,
2. The optical modulator according to claim 1, comprising performing phase modulation of light. 5. A voltage is applied between the electrodes to generate an electric field in the optical waveguide, and the electric field changes the refractive index of the optical waveguide portion by utilizing an electro-optic effect in which the refractive index of the crystal changes with respect to light. In a light modulator that modulates light by indium oxide, indium oxide grown on a lithium niobate crystal substrate
A lower conductive crystal electrode made of beam crystals, hetero Pita key Interstitial grown in position of the light on the said crystals electrode
Performs phase modulation and forms a single line from lithium niobate
A ridge-shaped waveguide, and an indium oxide crystal thin film loaded on the crystal waveguide .
An optical modulator comprising: an upper modulation electrode comprising: an upper modulation electrode;