JPH0862555A - Optical functional device - Google Patents

Abstract

PURPOSE: To enhance the efficiency of mode transformation by means of an electric field and to reduce an impressed voltage in an optical functional device in which electric optical efficiency is utilized. CONSTITUTION: This optical functional device having a comb-line electrode 30 for periodically adding the electric field along extending direction with respect to an optical waveguide 20 on the surface of a substrate constituted of an electric optical material on the surface layer of which the optical waveguide 20 is formed is provided with a conductor 50 which is partially superposed on the optical waveguide 20 and which is separated from the electrode 30 in arrangement by plane view between respective electrode fingers 31 constituting the electrode 30.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical functional device utilizing the electro-optic effect. As the construction of multimedia information networks by optical transmission progresses, it is desired to improve the practicality of optical functional devices such as mode converters, wavelength filters, and optical modulators.

[0002]

2. Description of the Related Art A polarization mode of light propagating in an optical waveguide is changed from a TE mode to a TM mode or a TM mode by periodically applying an electric field to the optical waveguide formed in a substrate of an electro-optical material by a comb-shaped electrode. Mode can be converted to TE mode. Then, by appropriately selecting the arrangement period Λ of the electrode fingers of the comb-shaped electrodes, it is possible to selectively perform mode conversion of the light having the wavelength λ expressed by the equation (1). A device having a structure in which an analyzer is provided at the rear end of the optical waveguide and the light after mode conversion is sent to the subsequent stage through the analyzer functions as a wavelength filter.

Λ = Λ | N _TE −N _TM | (1) N _TE : Effective refractive index of TE mode N _TM : Effective refractive index of TM mode For example, in lithium niobate (LiNbO ₃ ), the birefringent index | N _TE −N _TM | is about 0.072 and the wavelength is 1.5.
When selecting the light of μm, the arrangement period Λ of the electrode fingers may be set to about 20.8 μm.

[0004]

The applied voltage (electric field strength) required for mode conversion is inversely proportional to the length of the comb-shaped electrode in the light propagation direction (the number of electrode fingers arranged). In other words, in rotating the polarization direction by π / 2, if the substrate size is shortened in order to downsize the device and reduce the material cost, a higher voltage must be applied, and the withstand voltage cannot be secured. It gets harder.

For example, when Z-cut lithium niobate is used as the substrate material and the length of the comb electrodes in the light propagation direction (Y direction) is 5 mm, a voltage of 100 V or more is used for mode conversion of light having a wavelength of 1.5 μm. Had to be applied.

The X-cut lithium niobate can reduce the applied voltage as compared with the case of using the Z-cut. However, in the X-cut crystal, the cross-sectional shape of the optical waveguide becomes flat due to the anisotropy of the impurity diffusion coefficient, and the loss in connection with an external transmission line such as an optical fiber is large.

The present invention has been made in view of the above problems, and an object thereof is to improve the efficiency of mode conversion by an electric field and reduce the applied voltage.

[0008]

In order to solve the above-mentioned problems, the device of the present invention comprises an electro-optical material having an optical waveguide formed in the surface layer portion as shown in FIGS. 1 and 2. An optical functional device having a comb-shaped electrode for periodically applying an electric field on the surface of a substrate along the extension direction of the optical waveguide, the device being provided between the electrode fingers constituting the comb-shaped electrode. A conductor that partially overlaps the optical waveguide in a plan view and is separated from the comb-shaped electrode is provided.

According to a second aspect of the present invention, the electrode paired with the comb-shaped electrode is a linear electrode extending in the arrangement direction of the electrode fingers, and the conductor is electrically integrated with the linear electrode. Become.

In the device of the third aspect of the present invention, the conductor is made of a transparent conductive material having a light-transmitting property with respect to the guided light of the optical waveguide.

[0011]

As shown in FIG. 1 showing the principle of the present invention, a predetermined voltage V _MC is applied between a pair of electrodes sandwiching the optical waveguide 20 during mode conversion. At this time, at least one of the electrodes is a comb-shaped electrode 30 having a plurality of electrode fingers (comb teeth) 31.
And the extension direction (light propagation direction) of the optical waveguide 20.
An electric field is periodically applied along.

When the conductor 50 exists between the electrode fingers 31, the potentials of the conductor 50 and its vicinity are substantially uniform regardless of the conductive connection state including the floating state, so that the conductor 50 below the conductor 50 in the optical waveguide 20. An electric field is not generated in the area. That is, the electric field is concentrated between each electrode finger 31 and the other electrode facing them.

Therefore, the periodical change of the electric field strength in the light propagation direction in the optical waveguide 20 becomes extreme, and the electro-optical effect becomes remarkable for each array period Λ of the electrode fingers 31, so that the applied electric field acts more effectively. The mode conversion efficiency is improved.

On the other hand, when there is no conductor between the electrode fingers 31, the electric field spreads in the arrangement direction of the electrode fingers 31. As a result, as indicated by the chain line in the figure, the periodic change in the electric field strength in the optical waveguide 20 becomes slow.

[0015]

FIG. 2 is a perspective view of the wavelength filter 1 of the first embodiment, schematically showing the structure of the mode conversion portion. In the figure, the same reference numerals are given to the components corresponding to those in FIG. The following figures are also the same.

The base (substrate) 10 of the wavelength filter 1 is Z
It is a cut lithium niobate crystal with an outer dimension of 5 m.
It is a plate-like body of m (X) × 30 mm (Y) × 1 mm (Z). An optical waveguide 20 extending over the entire length in the Y direction is formed in the surface layer portion of the substrate 10 in the Z direction by thermal diffusion of titanium (Ti). The width of the optical waveguide 20 is about 7 μm. The upper surface of the substrate 10 including the optical waveguide 20 is covered with a transparent dielectric layer (buffer layer) 25 made of silicon dioxide (SiO ₂ ), which absorbs light generated when the electrode metal and the optical waveguide 20 are in contact with each other. Is prevented. The thickness of the buffer layer 25 is 0.5 μm.

A pair of comb-shaped electrodes 30 are provided on the buffer layer 25 in order to periodically apply an electric field to the optical waveguide 20 along the propagation direction thereof. Each comb-shaped electrode 30 is a metal film having a thickness of 0.1 μm formed by vacuum deposition of gold (Au), and each electrode finger (comb-tooth portion) 31 is patterned so as to face each other with the optical waveguide 20 in between. ing. The width w of the electrode finger 31 is about 5.3 μm, and the distance between the electrodes (the facing gap of the comb-shaped electrode pair) is 7 μm.

Further, in this embodiment, the period Λ of the array of the electrode fingers 31 is 21.1 μm. That is, the selection wavelength λ is set to 1.5 μm. Further, the length L of the comb-shaped electrode 30 that defines the applied voltage V _MC is 5 mm.

In addition to the basic components described above, in the wavelength filter 1, each electrode finger 31 that constitutes the comb-shaped electrode 30.
A strip-shaped conductor 50 that crosses the optical waveguide 20 and is separated from the comb-shaped electrode 30 in a plan view is provided in the central portion between the two. The conductor 50 is patterned at the same time as the comb-shaped electrode 30 and has a width of about 5.3 μm and a length of about 30 μm.

When using the wavelength filter 1, each comb electrode 30 is connected to a DC power source. The conductor 50 is in a floating state. When a voltage V _MC of 30 V was actually applied, the light having a wavelength λ of 1.536 μm was selectively mode-converted, and its conversion efficiency (input / output intensity ratio) was 50%.

As a comparative example, without providing the conductor 50,
When a sample having the same structure as the wavelength filter 1 was produced for the others and a voltage V _MC of 30 V was applied in the same manner, the mode conversion efficiency for light having a wavelength of 1.536 μm was almost half that of the wavelength filter 1. . From this, it was confirmed that the conductor 50 suppresses the spread of the electric field and enhances the conversion efficiency.

FIG. 3 is a schematic diagram showing the structure of the wavelength filter 2 of the second embodiment. The wavelength filter 2 partially overlaps the substrate 10 made of Z-cut lithium niobate crystal in which the Y-propagation type optical waveguide 20 is formed, the comb-shaped electrode 30 having the electrode fingers 31 arranged at equal intervals, and the optical waveguide 20. Linear electrode 35, tuning electrode 3 parallel to linear electrode 35
6 and the strip-shaped conductor 5 arranged between the electrode fingers 31
It is composed of 0 and the like.

By applying a voltage V _MC between the comb-shaped electrode 30 and the linear electrode 35, an electric field in the X direction is periodically applied to the optical waveguide 20 along the extension direction thereof, and the electrode finger 31 moves. It is possible to perform mode conversion of light having a wavelength that depends on the array period.

By applying an appropriate voltage V _T between the linear electrode 35 and the tuning electrode 36, an electric field in the Z direction is applied to the optical waveguide 20 so that the birefringence (|
N _TE −N _TM |) can be changed to adjust the selected wavelength λ for mode conversion.

Although each conductor 50 is separated from the comb-shaped electrode 30, one end side thereof is connected to the linear electrode 35. Then, the conductor 50 and the linear electrode 35 which are integrated in this way.
Is made of a transparent conductive material such as ITO or NESA. Therefore, since there is no light absorption by the conductor 50 and the linear electrode 35, the buffer layer is not provided in the wavelength filter 2, and the electrodes 30, 3 are directly provided on the surface of the substrate 10.
5, 36 and the conductor 50 are deposited. By omitting the buffer layer, it is possible to stabilize the electric field and suppress variations in mode conversion characteristics.

In the wavelength filter 2, the conductor 50 is kept at the same potential as the linear electrode 35, which suppresses the spread of the electric field in the light propagation direction and enhances the mode conversion efficiency as in the above-described embodiment.

FIG. 4 is a schematic diagram showing the structure of the wavelength filter 3 of the third embodiment. The wavelength filter 3 includes a substrate 10 made of Z-cut lithium niobate crystal in which a Y-propagation type optical waveguide 20 is formed, a comb-shaped electrode 30 having electrode fingers 31 arranged at equal intervals, and a comb-shaped electrode sandwiching the optical waveguide 20. It is composed of a linear heating electrode 38 opposed to 30, a strip-shaped conductor 50 arranged between each electrode finger 31, and the like.

By applying a voltage V _MC between the comb-shaped electrode 30 and the heating electrode 38, an electric field is periodically applied to the optical waveguide 20, and the light having a wavelength depending on the arrangement period of the electrode fingers 31 is changed into a mode. Can be converted.

Further, by applying an appropriate heating voltage V _H to both ends of the heating electrode 38, the birefringence (| N _TE −N _TM |) is changed by utilizing the thermo-optic effect to select the mode conversion. The wavelength λ can be adjusted.

Although each conductor 50 is separated from the comb-shaped electrode 30, one end side is connected to the heating electrode 38. Then, the conductor 50 and the linear electrode 35 which are integrated in this way.
Together with the comb-shaped electrode 30 are formed by patterning a gold thin film. The heating electrode 38 is formed by plating gold with a patterned metal film as a base. When the length of the heating electrode 38 is 5 mm and the thickness of the gold plating is 3 μm, the electric resistance between both ends is 10Ω.

In the wavelength filter 3, the conductor 50 is kept at the same potential as that of each part of the heating electrode 38, so that the spread of the electric field in the light propagation direction is suppressed and the mode conversion efficiency is enhanced as in the above-described embodiment.

According to the above-mentioned embodiment, when the Z-cut lithium niobate crystal is used as the electro-optical material, the insertion loss (the loss when it is inserted into the optical transmission line constituted by the optical fiber) is small. By utilizing the advantage of the Z-cut-Y propagation method, it is possible to realize highly efficient mode conversion with a low applied voltage.

In the above-described embodiment, the shape of the conductor 50 in plan view is not limited to the illustrated strip shape, but various modifications can be made according to the shape of the comb-shaped electrode 30. For example, a square,
The shape may be a circle, an ellipse, a triangle, a rhombus, or a frame shape with these shapes being framed. However, it is necessary to be separated from the comb-shaped electrode 30 by a predetermined distance for insulation. The conductor 5
When 0 is a quadrangle or the like, the withstand voltage can be increased by rounding the corners.

In the above-described embodiment, the period (arrangement pitch) Λ of the electrode fingers 31 and the length L of the comb-shaped electrode 30 in the light propagation direction may be appropriately selected according to the application. In addition, the material, shape, size, arrangement structure, forming method, etc. of each element can be variously modified within the scope of the gist of the present invention.
In particular, the structure in which the silicon layer is provided between the buffer layer 25 and various electrodes contributes to the stabilization of the electric field against temperature changes,
It is effective for improving temperature stability.

[0035]

According to the inventions of claims 1 to 3,
The applied voltage for mode conversion can be reduced, dielectric breakdown can be prevented, and driving can be facilitated.

In particular, according to the invention of claim 3, the buffer layer, which is a transparent dielectric layer for preventing light absorption, can be omitted, and the change of the electric field due to the interposition of the dielectric can be eliminated and the characteristics can be eliminated. The stability of can be increased.

[Brief description of drawings]

FIG. 1 is a principle diagram of the present invention.

FIG. 2 is a perspective view of the wavelength filter according to the first embodiment.

FIG. 3 is a schematic diagram showing a configuration of a wavelength filter according to a second embodiment.

FIG. 4 is a schematic diagram showing a configuration of a wavelength filter according to a third embodiment.

[Explanation of symbols]

 1, 2 and 3 wavelength filter (optical functional device) 10 substrate 20 optical waveguide 30 comb-shaped electrode 31 electrode finger 35 linear electrode 38 heating electrode (linear electrode) 50 conductor

Claims (3)

[Claims] 1. A comb-shaped electrode for periodically applying an electric field to the optical waveguide along its extension direction is provided on the surface of a substrate made of an electro-optical material having an optical waveguide formed in the surface layer portion. An optical functional device, characterized in that a conductor, which partially overlaps with the optical waveguide in a plan view arrangement and is separated from the comb-shaped electrode, is provided between each electrode finger forming the comb-shaped electrode. Optical function device. 2. The electrode paired with the comb-shaped electrode is a linear electrode extending in the arrangement direction of the electrode fingers, and the conductor is electrically integrated with the linear electrode. Item 2. The optical functional device according to Item 1. 3. The optical functional device according to claim 1, wherein the conductor is made of a transparent conductive material having a light-transmitting property with respect to the guided light of the optical waveguide.