JPH0477293B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a light modulation element using a material whose refractive index changes upon application of an electric field. [Summary] The present invention provides an optical modulation element having a structure in which a high-frequency electric field is applied to an optical waveguide whose refractive index changes due to the application of an electric field via an insulating buffer layer. This makes it possible to apply a strong modulated electric field to locations where the intensity is strong. [Prior art] Lithium niobate and lithium tantalate exhibit an electro-optic effect in which the refractive index changes when an electric field is applied, and light modulation elements that utilize this property have been known for a long time. . FIG. 3 is a cross-sectional view of a conventional optical modulation element using lithium niobate. An optical waveguide 2 is formed on a lithium niobate substrate 1, and an SiO ₂ insulating buffer layer 3 is formed on the surface of the substrate 1.
is formed, and electrodes 4 and 4' are provided on the surface of the insulating buffer layer 3 in order to apply a high frequency electric field to the optical waveguide 2 via the insulating buffer layer 3. When high frequency waves, particularly microwaves, are supplied between the electrodes 4 and 4', a high frequency electric field is applied to the optical waveguide 2 portion. This electric field changes the refractive index of the optical waveguide 2, making it possible to phase modulate the light propagating inside the waveguide. [Problems to be Solved by the Invention] However, in the light modulation element having the conventional structure, the electric field strength applied under the electrode is weak, and moreover, the electric field distribution does not match the light intensity distribution. An object of the present invention is to solve the above problems and provide a light modulation element that can apply a strong modulation electric field to a place where the intensity of light is strong. [Means for Solving the Problems] In the optical modulation element of the present invention, one or more grooves are provided along the optical waveguide on the surface of the insulating buffer layer near the optical waveguide, and the electrode has grooves complementary to the grooves. It is characterized by having a convex shape. [Operation] Since the electrode has a convex shape on the optical waveguide side,
The electric field can be concentrated in the optical waveguide. Thereby, modulation efficiency can be increased and drive voltage can be lowered. [Embodiment] FIG. 1 shows a perspective view of a light modulation element according to a first embodiment of the present invention, and FIG. 2 shows a sectional view. This example uses lithium niobate LiNbO ₃ as a material whose refractive index changes with the application of an electric field.
An optical waveguide 2 is formed on a part of a LiNbO ₃ substrate 1. This optical modulation element further includes an optical waveguide 2.
two substantially parallel electrodes 4, 4' arranged along the optical waveguide 2 and applying a high frequency electric field to the optical waveguide 2;
Each of these two electrodes 4, 4' and the optical waveguide 2
and a SiO ₂ insulating buffer layer 3 provided between. A power source 5 is connected to one end of the electrodes 4, 4', and a terminating resistor 6 is connected to the other end. When high frequency, especially microwave, is input from the power source 5 to one end of the electrodes 4, 4', this high frequency is transmitted to the electrodes 4, 4'.
4' and is transmitted to the terminating resistor 6 connected to the other ends of the electrodes 4, 4'. At this time, a high frequency electric field is transmitted to the optical waveguide 2 through the insulating buffer layer 3, changing the refractive index of that region. As a result, the phase of the light propagating through the optical waveguide 2 changes. The modulation sensitivity at this time is determined by the state of overlap between the optical power distribution and the high-frequency electric field, and by concentrating the strong high-frequency electric field on the stronger part of the light, low-voltage driving becomes possible. Therefore, in this embodiment, a groove 7 is provided along the optical waveguide 2 on the surface of the insulating buffer layer 3 near the optical waveguide 2, and a convex shape complementary to the groove 7 is provided in the electrode 4. With this structure, a high-frequency electric field is efficiently applied directly below the insulating buffer layer 3, that is, to the portion of the optical waveguide 2, and the modulation efficiency is increased. 1 and 2 show examples in which grooves and convex shapes are provided in the portion adjacent to the optical waveguide 2, but in addition to this, grooves and convex shapes are also provided on the side of the electrode 4' that is distant from the optical waveguide 2. A convex shape may also be provided. 4 to 6 show modifications of the shapes of the electrodes 4 and grooves 7. FIG. FIG. 1 and FIG. 2 show an example in which the cross-sectional shape of the groove is rectangular. The present invention is not limited to this shape; for example, in the example shown in FIG. 4, the cross-sectional shape of the groove and the convex shape of the electrode are triangular. In the example shown in FIG. 5, the shape is a rectangle with the corners cut off. In the example shown in FIG. 6, the cross-sectional shape of the groove and the convex shape of the electrode are rectangular, but the other part of the electrode is separated from the insulating buffer layer. FIG. 7 is a sectional view showing a light modulation element according to a second embodiment of the present invention. In this embodiment, the present invention is implemented in an MZ type intensity modulator, and an optical waveguide 2' is branched from the optical waveguide 2 on the input side, passes through the electrode 4' side, and then merges with the optical waveguide 2 again. Insulating buffer layer 3
According to the first embodiment, the surface near the optical waveguide 2' is provided with a groove 7' along the optical waveguide 2', and the electrode 4' is provided with a convex shape complementary to the groove 7'. different from. The MZ intensity modulator is also called a branching interference type modulator, and it splits light into two on the input side, changes the phase of at least one of them, and then combines the two lights and causes them to interfere. FIG. 7 shows a cross section of a portion where the optical waveguide branches into two. Details of such a modulator are shown, for example, in Nishihara et al., published by Ohm Publishing, Optical Integrated Circuits, 1st edition, pages 300 to 301. FIG. 8 is a sectional view showing a light modulation element according to a third embodiment of the present invention. In this embodiment, the insulating buffer layer 3 is connected to the electrode 4,
Grooves 7 and 7' are formed in the insulating buffer layer 3 on each side of the electrodes 4 and 4'.
This embodiment differs from the first embodiment in that a convex shape is provided on each of the electrodes 4 and 4' correspondingly. The applicant has already filed a patent application for an optical modulation element in which the width of the insulating buffer layer 3 is narrower than that of the electrodes 4, 4' (Japanese Patent Application No.
211013, patent application No. 1-311467). This embodiment combines the present invention with the structure shown in the earlier patent application. The modulation sensitivity of an optical modulation element using the electro-optic effect, that is, the drive voltage (hereinafter referred to as "half-wave voltage") required to generate a phase change of π, Vπ, is 1/Vπ∝∬ E(x,y)P It is expressed as (x,y)dxdy. However, E(x, y): Modulated electric field distribution P(x, y): Light intensity distribution. That is, if the modulation electric field distribution can be controlled so that a strong modulation electric field is applied to a location where the light intensity is strong, the half-wavelength voltage Vπ can be lowered. In fact, the intensity distribution of light within an optical waveguide is concentrated near the center of the optical waveguide, so if a strong electric field is applied to this part, the half-wave voltage Vπ can be lowered. 9 and 10 show calculated values of electric field distribution. FIG. 9 shows the electric field distribution of the conventional example, and FIG. 10 shows the electric field distribution of the third embodiment. This calculation was performed using the finite element method. In this calculation, substrate 1
The material of the optical waveguide 2 is LiNbO ₃ , the material of the insulating buffer layer 3 is SiO ₂ , and the material of the electrodes 4 and 4' is gold (Au).
And so. In addition, the dimensions of each part are as follows: Width W of electrode 4 = 10 μm Distance G between electrodes 4 and 4' = 10 μm Thickness of electrodes 4 and 4' t ₁ = 10 μm Thickness of insulating buffer layer 3 t ₂ = 1.2 μm (Fig. 9) Thickness of the thick part of the insulating buffer layer 3 t ₂ = 1.6 μm (Fig. 10) Thickness of the thin part of the insulating buffer layer 3 t ₃ = 0.4 μm Width a of the insulating buffer layer 3 under the electrode 4 ₁ = 8 μm, the width a ₂ of the groove 7 = 2 μm, and in the case of FIG. did. As shown in FIG. 9, in the case of the conventional example, the electric field distribution is uniform in the portion directly below the electrode, resulting in the electric field being applied even to unnecessary portions.
In contrast, in the embodiment shown in FIG. 10, by incorporating a convex shape, a strong electric field is applied directly below the convexity. By utilizing this effect and arranging the optical waveguide directly under the convexity, it becomes possible to lower the half-wavelength voltage Vπ. Figures 11 to 13 show the half-wavelength voltage Vπ when the interaction length between the light and the modulating electric field is 1 cm,
The relationship between characteristic impedance Z ₀ and modulation band Δf is shown. FIG. 11 shows values calculated for the conventional structure shown in FIG. 3 using the thickness t ₂ of the insulating buffer layer 3 as a parameter. Also, the 12th
The figure and FIG. 13 show values determined for the structure of the third embodiment shown in FIG. 8 using the thickness t ₃ of the thin portion of the insulating buffer layer 3 and the width a ₂ of the groove 7 as parameters, respectively. The values in FIGS. 12 and 13 are as follows: Width of electrode 4 W = 10 μm Distance between electrodes 4 and 4' G = 10 μm Thickness of electrodes 4 and 4' t ₁ = 10 μm Thickness of thick portion of insulating buffer layer 3 t ₂ = 2 μm (Figs. 12 and 13) Thickness of the thin part of the insulating buffer layer 3 t ₃ = 0.4 μm (Fig. 13) Width of the insulating buffer layer 3 under the electrode 4 a ₁ = 8 μm (Fig. 13) (Figs. 12 and 13) The width a ₂ of the groove 7 is 4 μm (Fig. 12), and in the case of Figs. 12 and 13, the insulating buffer layer 3 is placed in the center of the electrode 4, It was determined that the insulating buffer layer 3 was placed at the center. In addition, for calculation of half-wave voltage Vπ, depth
Optical waveguide 2 with spot size of 5.5μm and width of 11μm
The target is light that propagates. Here, the characteristic impedance is 50Ω, and the 11th
Referring to Figures 13 to 13, the half-wave voltage Vπ
and the modulation band Δf are as shown in the table below.

〔Effect of the invention〕

As explained above, the light modulation element of the present invention is
It has the effect of having high modulation efficiency and being able to operate with a small modulation voltage (power). Furthermore, by selecting the shapes of the grooves and electrodes of the insulating buffer, it is possible to set the voltage intensity distribution profile to a desired shape.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a light modulation element according to a first embodiment of the present invention. Figure 2 is a sectional view. FIG. 3 is a cross-sectional view of a conventional optical modulation element. FIGS. 4 to 6 are diagrams showing modified examples of the shapes of electrodes and grooves. FIG. 7 is a sectional view showing a light modulation element according to a second embodiment of the present invention. FIG. 8 is a sectional view showing a light modulation element according to a third embodiment of the present invention. 9th
The figure shows calculated values of electric field distribution in a conventional example.
FIG. 10 is a diagram showing calculated values of electric field distribution in the third embodiment. Figure 11 shows the half-wave voltage in the conventional example.
A diagram showing the relationship between Vπ, characteristic impedance Z ₀ and modulation band Δf. FIG. 12 is a diagram showing the relationship between half-wavelength voltage Vπ, characteristic impedance Z ₀ and modulation band Δf in the third embodiment. Figure 13 shows the half-wave voltage Vπ and characteristic impedance Z ₀ in the third embodiment.
and a diagram showing the relationship between modulation band Δf. FIG. 14 is a diagram showing calculated values of electric field distribution when there are a plurality of grooves. 1... Substrate, 2, 2'... Optical waveguide, 3... Insulating buffer layer, 4, 4'... Electrode, 5... Power supply,
6...Terminal resistor, 7,7'...groove.

Claims (1)

[Claims] 1. An optical waveguide formed of a material whose refractive index changes when an electric field is applied, and two substantially parallel electrodes arranged along the optical waveguide and applying a high-frequency electric field to the optical waveguide. , in an optical modulation element comprising an insulating buffer layer provided between each of the two electrodes and the optical waveguide, the surface of the insulating buffer layer near the optical waveguide has a layer along the optical waveguide. 1. A light modulation element, characterized in that one or more grooves are provided, and one of the two electrodes in contact with the groove is provided with a convex shape complementary to the groove. 2. The light modulation according to claim 1, wherein the insulating buffer layer has a shape divided into two regions corresponding to each of the two electrodes, and the width of each region is equal to or narrower than the width of the corresponding electrode. element.