JPH07191352A - Optical waveguide device - Google Patents

Abstract

PURPOSE:To provide the optical waveguide device, such as an optical switch and an optical modulator capable of being effectively controlled with low voltage without necessitating precision for the positioning of an optical waveguide and an electrode with simple electrode structure as heretofore, and to suppress DC drift. CONSTITUTION:The optical waveguide device has plural optical waveguides formed on a substrate (a) formed of electrooptic crystal; and, for example, two waveguides 1 and 2 form a couple of optical waveguide groups which mutually exchanges waveguide energy in a coupling area of the substrate, the electrooptic axes (s) of the optical waveguides are formed in opposite directions between the couple of optical waveguide groups, and positive and negative electrodes 4 and 5 are arranged across the coupling area so that a parallel and uniform electric field E operates equally on the optical waveguides 1 and 2 in the coupling area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide type device having functions such as light switching, distribution, interference, modulation, branching and diversion.

[0002]

2. Description of the Related Art An optical waveguide type device is an element that handles optical signals and guided energy. A control electric field is applied to a light beam input to an optical waveguide in the element to switch or switch between a plurality of waveguides. It is used to perform operations such as modulation. The principle and operation of transmission and reception of guided energy and interference between the waveguides will be briefly described below.

Between two adjacent optical waveguides, when the propagation constants of substances forming both optical waveguides change so as to have the same value or different values, a specific length called a coupling length along the optical waveguides. At a distance, movement or interference of guided energy occurs between one optical waveguide and the other optical waveguide. The value of this propagation constant is mainly determined by the refractive index of the substance forming the optical waveguide. On the other hand, crystals exhibiting electro-optical effects such as Pockels effect and Kerr effect, or nonlinear optical effects are known. Such a crystal has a property that when an electric field is applied to a specific crystal axis among the crystal axes of the crystal, the refractive index changes according to the direction and strength of the electric field. That is, the value of the propagation constant can be controlled by applying an electric field from the outside. Therefore, if a substrate is formed using the crystal as described above and two optical waveguides are formed close to each other on the substrate,
By operating an electric field from the outside so that the propagation constants of the optical waveguides have the same value or different values, the amount of energy transfer and the degree of interference between the optical waveguides can be controlled.

In the present specification, a crystal having the above-mentioned electro-optical effect is referred to as an "electro-optic crystal", and a specific crystal axis of the electro-optic crystal to which an electric field is applied is defined as It is called an "electro-optic axis", and a substrate made of an electro-optic crystal formed so that the electro-optic axis is in a predetermined direction is called a "crystal substrate". In addition, a space required for movement or interference of guided energy between the optical waveguides, and a region to which a control electric field is applied is referred to as a “guided energy coupling region (hereinafter, simply“ coupling region ”). Say.
An optical waveguide to be controlled is included in the coupling region in both the width direction and the depth direction. Further, the longitudinal direction includes a specific distance required for movement or interference of guided energy between the optical waveguides, for example, a coupling length in directional coupling. Furthermore, the movement, interference, branching, direction change, etc. of part or all of the guided energy between the optical waveguides is referred to as “exchange of guided energy with each other”.

The structure of the conventional optical waveguide type device utilizing the electro-optical effect as described above and its problems will be described below. Here, the crystal substrate is made of Li Nb O ₃ crystal, and a directional coupling type optical switch in which two optical waveguides are formed close to and parallel to each other on the surface layer will be described as an example.
The Li Nb O ₃ crystal is a ferroelectric material known as one of the electro-optic crystals useful for the optical waveguide type device, and its optical axis “Z axis” is the “electro-optical crystal” in the electro-optic crystal described above. It corresponds to the "optical axis". The crystal substrate of this example is Li
Nb O ₃ crystal substrate in a plane perpendicular to the Z axis (Z plane) is formed such that the front and back surfaces of the substrate (hereinafter, "Z plate Li Nb O
₃ ”. ).

FIG. 6 is a diagram schematically showing a cross section perpendicular to the optical waveguide in the coupling region of the conventional optical waveguide type device. In the figure, a is Z plate Li Nb O _3, the optical waveguide 1 in parallel, such as by an impurity diffusion method to the surface layer,
2 is formed, and the electric fields E1 for control by the electrodes 4 and 5 are applied to these in opposite directions, whereby the propagation constants of the optical waveguides 1 and 2 are controlled to different values. . However, in the figure, s represents the Z axis,
The direction indicated by the arrow is the positive direction of the Z axis. A buffer layer 3 such as a SiO ₂ film is provided as a protective film in order to reduce absorption of guided light by the electrodes. In the above structure, in order to apply the control electric field E1 to the optical waveguides 1 and 2 in opposite directions, positive and negative plate electrodes 4 and 5 are formed on the same surface of the substrate in the same direction, Positive electrode 4
To generate a curved electric field E1 from the negative electrode 5 to the negative electrode 5. That is, with this configuration, the electric field E1 is applied to the optical waveguide 1
The Z axis of the optical waveguide 2 acts in the + Z direction, the Z axis of the optical waveguide 2 operates in the −Z direction, and the optical waveguide 2 acts approximately in the opposite direction to the Z axis.

[0007]

However, in such a conventional configuration, first, the action direction of the electric field E1 with respect to the Z-axis direction of each optical waveguide is merely approximately the opposite direction, and the complete opposite direction. The loss of electric field action is larger than that of.
Further, in order to make the electric field act most effectively on both optical waveguides, the electrodes 4, 5 are placed on the optimum positions near the respective centers of the optical waveguides so that the dense portions of the electric field E1 are concentrated on the respective optical waveguides.
Precise alignment is required, such as matching the edges of each. Such highly accurate alignment with a fine optical waveguide is extremely difficult, which is an obstacle to improving productivity. Furthermore, since both the positive and negative electrodes 4 and 5 are formed side by side on the same surface of the crystal substrate a, the buffer layer 3 existing between the electrodes 4 and 5 and the crystal substrate a are not formed. A phenomenon (DC drift) in which the operating voltage fluctuates due to charge transfer is a serious problem in practical use.

The object of the present invention is to solve the above problems, to provide a simple electrode structure, which does not require precision as in the prior art for the alignment of the optical waveguide and the electrode, and which can provide effective waveguide energy at a low voltage. (EN) Provided are controllable optical waveguide devices such as optical switches and optical modulators.

[0009]

SUMMARY OF THE INVENTION The inventor has found that the electro-optic axes of optical waveguides that are in a relationship of exchanging guided energies are formed so as to be in an opposite relationship to each other so that they are parallel to each other. The present invention has been completed by finding that the exchange of waveguide energy can be effectively controlled only by applying a linear and uniform parallel electric field formed between plate electrodes to all optical waveguides in the same manner. That is, the optical waveguide type device of the present invention is an optical waveguide type device in which a plurality of optical waveguides are formed on a crystal substrate, and the plurality of optical waveguides exchange waveguide energy with each other in a coupling region of the crystal substrate. Forming a pair of optical waveguide groups, and the specific crystal axes of the optical waveguides belonging to one optical waveguide group and the specific crystal axes of the optical waveguides belonging to the other optical waveguide group are opposite to each other. The positive and negative electrodes are arranged so that the parallel and uniform electric field acts on all the optical waveguides in the coupling region in the same manner. .

[0010]

By forming the two optical waveguides so that the electro-optical axes thereof are opposite to each other, a linear and uniform electric field generated between opposing electrodes having a simple structure is applied to these optical waveguides. By just acting in the same manner, electric fields in opposite directions act on the two optical waveguides having the same electro-optic axis direction.

The number of optical waveguides on the crystal substrate targeted by the present invention is plural. Even if the optical waveguides are in an integrated state in which they are not mechanically separated, if the light beams are unevenly distributed in the optical waveguides due to the electro-optical effect, etc. It is called a waveguide. For example,
A portion before Y-branching and an intersection at X-shaped intersection are two optical waveguides even if the optical waveguides are integrated. Further, even if each of the optical waveguide has one inlet and one outlet, this is also a plurality of optical waveguides as long as it is branched into a plurality of regions to be controlled.

Although the waveguide energy is exchanged between the optical waveguides, the waveguide energy exchange controlled by the present invention is performed regardless of how many optical waveguides are formed.
Be one of a pair of optical waveguide groups,
It is assumed that the waveguide energy is exchanged between the pair of optical waveguide groups. For example, when there are five optical waveguides, they are divided into a pair of optical waveguide groups with a ratio of 1: 4, 2: 3, etc., and guided energy is controlled to be exchanged between the groups. Further, when the crystal substrate is sufficiently wide or for the purpose of obtaining a special effect, a plurality of pairs of optical waveguide groups may be formed in the coupling region.
Hereinafter, for convenience, the two optical waveguides will be described.

The directions of the electro-optical axes of the optical waveguides to be inverted are opposite to each other when there are two optical waveguides, but when there are three or more optical waveguides, the same optical waveguides belong to the optical waveguides as described above. All are in the same direction within the waveguide group,
The two groups are formed in opposite directions. In other words, which of the pair of optical waveguide groups the plurality of optical waveguides belong to can be known from the direction of the electro-optical axis.

The positive and negative electrodes may be of any type as long as they can apply a parallel and uniform electric field. However, flat plate electrodes are particularly preferable, and these electrodes are arranged in parallel to each other. By doing so, a parallel and uniform electric field is easily formed between the electrodes. The area of the positive and negative electrodes is preferably set such that the overlap between the electromagnetic field of light propagating in the waveguide and the control electric field is maximized. The positive and negative electrodes have a coupling region between the electrodes and are disposed at positions where a parallel and uniform electric field can act similarly on all the optical waveguides in the coupling region. The positive and negative electrodes are arranged in the following directions: (1) The crystal plane expansion direction of the crystal substrate. (2) The direction perpendicular to the substrate surface of the crystal substrate. (3) A direction that forms an arbitrary angle with the substrate surface of the crystal substrate (for the purpose of special crystal axis or special effect). Etc. are exemplified, and positive and negative electrodes are arranged facing each other in this direction.

[0015]

EXAMPLES The present invention will be described more specifically below based on examples. In this example, as an example of the optical waveguide type device according to the present invention, a configuration is shown in which two optical waveguides are formed in the surface layer of a Z plate Li NbO ₃ crystal substrate to control various exchanges of waveguide energy. FIG. 1 is a diagram schematically showing a cross section perpendicular to an optical waveguide in a coupling region of an optical waveguide type device according to the present invention. As shown in the figure, Z
Optical waveguides 1 and 2 are formed on the surface layer of a crystal substrate a made of a plate Li Nb O ₃ , and a buffer layer 3 is formed thereon as a kind of protective layer. The Z-axis directions of the crystals forming both optical waveguides are mutually The parallel and flat positive and negative electrodes 4 and 5 facing each other are formed so that the parallel and uniform electric field E acts in the direction opposite to the entire coupling region in the direction perpendicular to the substrate surface. It is configured to be arranged on the front and back. Further, in the example of the figure, the arrow s means the Z axis which is the electro-optic axis, and the direction indicated by the arrow is the positive direction of the Z axis. With such a configuration, a linear and uniform parallel electric field E formed between the flat plate electrodes 4 and 5 facing each other is merely applied, and the opposite directions with respect to the Z axes of the optical waveguides 1 and 2 are obtained. Will act on.

The material used for the crystal substrate is
Any material can be used as long as it has a refractive index that is changed by the action of an external voltage or electric field.
In addition to O _3, and Li Ta O _3, inorganic optical materials such KTi OPO _4, and organic optical materials such as Porudoporima are exemplified. In particular, ferroelectric materials such as Li Nb O ₃ and Li Ta O ₃ exhibit a remarkable electro-optical effect and are useful substrate materials for the optical waveguide device of the present invention. As a method of forming the crystal substrate, a known crystal growth method most suitable for the material such as a vapor phase / liquid phase epitaxial growth method may be used.

A known method may be used to form the optical waveguide on the crystal substrate. For example, a method of diffusing Ti or the like into a crystal substrate (impurity diffusion method), a method of arbitrarily changing the refractive index of a predetermined portion in the crystal substrate by an ion exchange method or the like to form an optical waveguide, or an optical waveguide Examples include a method of growing a crystal having a different component composition only in a part.

As the mode of the optical waveguide formed on the crystal substrate, in addition to the mode in which a plurality of optical waveguides are arranged on the same surface of the substrate as in the above-mentioned example, the optical waveguides are formed in a layered manner to form different layers. Mode of exchanging guided wave energy with
Furthermore, the combined aspect etc. are mentioned. Of these modes, in particular, various waveguide patterns in the case where two optical waveguides are arranged on the same plane are shown in FIGS.
It shows in (d). In the same figure, the region f surrounded by the alternate long and short dash line is the coupling region, and it is indicated by hatching that the electro-optic axes are in opposite directions.
FIG. 3A shows an example of the directional coupler, which includes the optical waveguides 1 and 2.
It has functions such as switching and distribution of guided wave energy. FIG. 3B shows an example of the interferometer, which includes the optical waveguides 1,
It causes wave interference between the two and has a function of a filter or the like. Although FIGS. 3A and 3B show the case where both optical waveguides are parallel to each other, a certain angle may be intentionally set. FIG. 3C shows an example of branching, and FIG. 3D shows an example of crossing, both of which have functions such as direction conversion and distribution of guided energy.

The electro-optic axis has a unique property for each electro-optic crystal. As described above, the refractive index of the crystal varies depending on the direction and strength of the electric field acting on the electro-optic axis. Show changes. Among various electro-optic axes possessed by the electro-optic material, the Z-axis in ferroelectrics such as Li Nb O ₃ and Li Ta O ₃ is an example of the most useful electro-optic axis. The Z axis in this case corresponds to the main axis of the index ellipsoid, exerts the maximum electro-optical effect on the electric field applied from the outside, and coincides with the polarization direction of the crystal. Another crystal having such an electro-optic axis is KTiO.
An example is PO ₄ .

The range in which the electro-optic axis is reversed may be a range including at least one optical waveguide in the coupling region, but when a control electric field is applied to the entire region, the waveguide energy is exchanged from surrounding crystals. In order to suppress an adverse effect that hinders the optical waveguide, it is preferable that the crystal around the optical waveguide is also inverted so as to be in the same direction as the optical waveguide. For example,
As shown in FIG. 1, when the electro-optical axis s is perpendicular to the crystal substrate, if the electro-optical axis on the optical waveguide 2 side is reversed,
Most preferably, the optical waveguide 2 is appropriately included in the width direction, and all the front and back sides of the substrate are inverted in the depth direction.

Various combinations of the direction of the electro-optic axis with respect to the surface of the crystal substrate and the direction of the optical waveguide are possible depending on the type of crystal. FIG. 4 is a diagram showing a combination of the electro-optical axis direction of the crystal substrate and the mode of the optical waveguide. In the figure, the direction indicated by the arrow is the positive direction of the electro-optic axis. FIG. 4A shows the case where the electro-optic axis is perpendicular to the crystal substrate surface, which has the same configuration as that in FIG. FIG. 4B shows a case where the electro-optic axis is the same as the plane expansion direction of the crystal substrate and is also perpendicular to the transmission direction of the optical waveguide. FIG. 4C shows the case where the electro-optic axis is the same as the plane expansion direction of the crystal substrate and the same as the transmission direction of the optical waveguide. These are examples of the case where the optical waveguide is formed on the same surface of the crystal substrate, but the relationship between the electro-optical axis direction and the optical waveguide is the same even when the optical waveguide is formed in layers. .

As a method of making the directions of the electro-optic axes opposite to each other, various methods can be exemplified as follows. Ferroelectric materials such as Li Nb O ₃ and Li Ta O ₃ are used. In such a crystal substrate, a method in which the crystal structure of one of the two optical waveguides is a polarization inversion structure is useful. The polarization reversal structure in a ferroelectric crystal means that the direction of spontaneous polarization of the ferroelectric crystal is controlled by an external electric field, heat,
It is a structure that is reversed by applying force such as pressure. For example, the polarization inversion structure in the Z-axis of the Z-plate Li Nb O ₃ crystal is obtained by performing electron beam irradiation under appropriate conditions from the -Z surface of the substrate provided with an electrode on the + Z side. Further, in this case, it is possible to form a deep inversion structure that penetrates not only the surface layer of the substrate but also the back surface of the substrate. Other polarization inversion method, Li Nb O ₃ how crystal substrate by diffusing Ti ions heat treatment, the electrodes are arranged in front and back surface of the substrate, a method of applying an electric field due to the high voltage power source, Si O ₂ film A method of loading and heat-treating is exemplified. Further, as a method of reversing the electro-optical axes of not only the above-mentioned ferroelectrics but all the crystal substrates useful in the present invention, first the crystal substrates are mechanically divided so that the electro-optical axes are perpendicular to the substrates. Examples include a method of reversing the other side if any, and a method of rotating the other axis direction by 180 degrees and joining again if the direction of the electro-optic axis is the extension direction of the substrate surface. In the above example, the optical waveguide may be formed either before or after the reversal of the electro-optical axis.

The positions of the electrodes are arranged so that a parallel and uniform electric field acts on all the optical waveguides in the coupling region in the same manner, in the substrate surface extension direction of the crystal substrate, in the direction perpendicular to the substrate surface, or in these directions. Are arranged so as to face each other in an intermediate direction. Figure 5
It is a schematic diagram which shows the example of a combination of the formation direction of an optical waveguide, and the arrangement direction of an electrode. FIG. 5A is an example in which electrodes are arranged in a direction perpendicular to the substrate surface, that is, on the front and back surfaces of the substrate, with respect to two optical waveguides formed on the same substrate surface. Figure 5
(B) is an example in which the electrodes are arranged in the substrate surface expansion direction of the crystal substrate, that is, at both end edges of the substrate.

As described above, the area of the positive and negative electrodes is preferably set so that the overlap between the electromagnetic field of the light propagating in the waveguide and the control electric field is maximized. However, in actual use, it is considered that the effective part of the electromagnetic field of light is concentrated in a specific area near the central axis of the optical waveguide in the entire electromagnetic field of light that spreads infinitely. It may be effective to concentrate the electric field only on the crystal substrate. FIG. 2 is a diagram schematically showing a structural example of an electrode that concentrates an electric field on a specific portion near the central axis of such an optical waveguide. Similar to FIG. 1, this figure shows a cross section perpendicular to the optical waveguide in the coupling region of the optical waveguide type device, and is the same except for the electrode structure. As shown in the figure, an electrode 4a is provided on the optical waveguide 1, and an electrode 4b (these are positive electrodes in this example) are provided on the optical waveguide 2, and different electrodes are provided so as to correspond to the electrodes 4a and 4b, respectively. The electrodes 5a and 5b are arranged on the back surface of the substrate. The above electrode configuration can be one of the device structures capable of controlling the guided energy most effectively with a small electric power.

In addition to the above-described combination of the optical waveguide forming direction and the electrode arranging direction shown in FIG. 5, the case where the optical waveguide is formed in a layered form is added. There are combinations of electro-optical axis directions shown in a) to (c). Furthermore, for the purpose of special effects,
There are compound, intermediate angles and structures of these combinations. To determine these combinations, the properties of the electro-optic crystal and the electro-optic axis, the shape and function required of the device, etc. are referred to.

[Performance Comparison Experiment] In order to compare the optical waveguide type device according to the present invention with a conventional optical waveguide type device, a directional coupler was formed on a Z plate Li Nb O ₃ crystal substrate to confirm the operation. . As the sample of the optical waveguide type device according to the present invention, the one having the structure shown in FIG. 1 is used,
A conventional sample having the structure shown in FIG. 6 was used. Comparing the applied voltages required to operate the above two directional couplers and obtain the same switching effect, it is found that the device of the present invention shows similar operation at the applied voltage of about 75% of the conventional device. Was confirmed. Also,
In producing these samples, the conventional sample required a long time for the electrode position adjustment, whereas the sample of the present invention has a structure in which the electrode position adjustment does not require the precision as high as the conventional one. The production was completed in a short time.

[0027]

The optical waveguide device of the present invention has the following effects. Since it is possible to cause a difference in propagation constant between the optical waveguides by simply applying a linear and uniform parallel electric field generated between the opposing electrodes having a simple structure to the two optical waveguides in the same manner, The precision of the conventional alignment process is not required, and the productivity is improved.
Further, in contrast to the conventional curved control electric field acting at an angle with respect to the electro-optic axis, in the structure of the present invention, since the direction of the control electric field and the direction of the electro-optic axis coincide, the control electric field Acts most efficiently on the electro-optic axis, allowing operation at lower voltages. Furthermore, since the electrode structure has a facing structure with a dielectric material sandwiched between them, it is possible to suppress the DC drift phenomenon that occurs between the electrodes, which has been a problem in the related art.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a cross section perpendicular to an optical waveguide in a coupling region of an optical waveguide type device according to the present invention.

FIG. 2 is a diagram schematically showing a structural example of an electrode that concentrates an electric field on a specific portion near the central axis of an optical waveguide.

FIG. 3 shows an optical waveguide device according to the present invention, in which 2
It is a figure which shows typically various structural examples in case two optical waveguides are formed on the same surface.

FIG. 4 is a schematic diagram showing an example of a combination of an electro-optical axis direction of a crystal substrate and an arrangement direction of electrodes in an optical waveguide device according to the present invention.

FIG. 5 is a schematic view showing an example of a combination of an optical waveguide forming direction with respect to a crystal substrate and an electrode arranging direction in the optical waveguide type device according to the present invention.

FIG. 6 is a diagram schematically showing a cross section of a conventional optical waveguide type device, which is perpendicular to the optical waveguide in a coupling portion of guided wave energy.

[Explanation of symbols]

 a crystal substrate s electro-optic axis E electric field 1 optical waveguide 2 optical waveguide 4 positive electrode 5 negative electrode

Claims (3)

[Claims] 1. An optical waveguide type device comprising a plurality of optical waveguides formed on a substrate made of an electro-optic crystal whose refractive index changes according to the strength of an electric field with respect to a specific crystal axis. A pair of optical waveguide groups for exchanging guided energies with each other are formed in the guided wave energy coupling region of the substrate, and the optical axes of the optical waveguides belonging to one of the optical waveguide groups are connected to the specific crystal axis and the other optical waveguide group. The positive and negative electrodes are formed such that parallel and uniform electric fields are formed so as to be opposite to the above-mentioned specific crystal axes of the optical waveguides to which the optical waveguides belong in the same manner to all optical waveguides in the waveguide energy coupling region. An optical waveguide type device having a wave energy coupling region between electrodes. 2. The substrate is formed so as to have a specific crystal axis in a direction perpendicular to the substrate surface, and positive and negative electrodes are provided on the front surface side and the back surface side of the crystal substrate, respectively. The optical waveguide device according to claim 1. 3. The optical waveguide device according to claim 1, wherein exchanging guided energy with each other is movement, interference, branching, or diversion of a part or all of guided energy.