JPH07270736A - Waveguide type acousto-optical element - Google Patents

Abstract

PURPOSE:To provide a waveguide type acousto-optical element which is formable to have a small chip size without increasing the loss of necessary signals. CONSTITUTION:A slab waveguide 2 for two-dimensionally confining light is formed in the central part of a substrate 1. This slab waveguide 2 is internally provided with a crossed finger type transducer (IDT) 3 for generating surface acoustic waves (SAW). and fourth channel waveguides are coupled through first and second tapered waveguides to one end of an acousto-optical effect imparting region near this IDT 3. Second and third channel waveguides are coupled through the second and third tapered waveguides to the other end of this acousto-opticsal effect imparting region. Further, the first channel waveguide 6a and the second channel waveguide 10a have the same curvilinear shape and the third channel waveguide 11a and the fourth channel waveguide 7a have the same curvilinear shape. The min. radius of curvature of the curvilinear shape of the former is set larger than the min. radius of curvature of the latter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type acousto-optic device used as an optical switch, an optical modulator, an optical frequency shifter or the like in the fields of optical measurement and optical information processing.

[0002]

2. Description of the Related Art FIG. 3 is a diagram showing a structure of a conventional waveguide type acousto-optic device.

In FIG. 3, reference numeral 1 is a substrate, and this substrate 1 is composed of a Z-cut lithium tantalate (LT) plate or the like. A slab waveguide 2 for confining light in a two-dimensional manner is formed in the center of the substrate 1. Inside the slab waveguide 2, a surface acoustic wave (SA) is generated in the slab waveguide 2.
W) producing interdigital transducer (IDT)
3 is provided, and an acousto-optic effect imparting region capable of imparting a good acousto-optic effect is formed in the vicinity of the IDT 3.

At one end of the acousto-optic effect imparting region of the slab waveguide 1, that is, at the left end in the drawing, the first tapered waveguide 4 is formed in which the width of the light propagation region is tapered.
And the open end portions of the fourth tapered waveguide 5 are coupled to each other, and the other end portions of the first tapered waveguide 4 and the fourth tapered waveguide 5 respectively confine and propagate light three-dimensionally. The first channel waveguide 6 and the fourth channel waveguide 7 are coupled to each other, and the other end portions of these channel waveguides are opened to the end surface of the substrate 1.

Similarly, the open ends of the second tapered waveguide 8 and the third tapered waveguide 9 are coupled to the other end of the acousto-optic effect imparting region, that is, the right end in the figure. A second channel waveguide 10 and a third channel waveguide 11 are respectively coupled to the other end portions of the second tapered waveguide 8 and the third tapered waveguide 9, and the other end portions of these channel waveguides are coupled. Are opened at the end face of the substrate 1.

Here, the acousto-optic effect imparting region is relatively narrow, but in order to connect it to an external optical fiber or the like, a space larger than a predetermined space is required for attaching the connecting jig or the like. Therefore, each channel waveguide has a curved shape such that the distance between adjacent channel waveguides is small on the end side where it is coupled to the acousto-optic effect imparting region, and gradually increases toward the external coupling section side. Has been formed. That is, each channel waveguide is shown in FIG.
As shown in FIG. 3, it is composed of a first straight waveguide portion for external connection, a second straight linear waveguide portion for connection with a tapered waveguide, and a curved waveguide portion for connecting them. . The first linear waveguide portion for connection to the outside is the substrate 1
The second linear waveguide portion for connecting to the tapered waveguide is formed so as to form a diffraction angle (Bragg angle, θB) with the wavefront of the SAW. Has been formed. The waveguides can be formed by the well-known proton exchange method (PE).

In this case, the curved waveguide portions of the four channel waveguides are all formed to have the same shape, for example, the shape expressed by the following equation in the XY coordinates as shown in FIG. ing.

Y = h / 2 {1-cos (πX / L)} However, in the above equation, for example, h = 357 μm,
L = 13300 μm, the length of the curved waveguide portion is about 12 mm, the minimum polar radius is 100 mm, the length of the first straight waveguide portion for external connection is about 2 mm, and the tapered waveguide is The length of the second linear waveguide portion for connection of is about 0.5 mm, the width of the slab waveguide 2 is about 8 mm, and the length of the entire substrate (chip) is 51 mm.

In the above structure, the light incident on the first channel waveguide 6 from an external optical fiber (not shown) or the like has its beam width expanded by the first taper waveguide 4 to become substantially parallel light, and is slab-guided. The light is emitted to the waveguide 2. SAW is not excited in this slab waveguide 2 (off)
In the state, the parallel light is condensed by the third taper waveguide 9, guided to the third channel waveguide 11, and taken out to the outside. When the SAW is excited (on), the collimated light receives the acousto-optic effect, is diffracted, is deflected by a predetermined angle, is guided to the second channel waveguide 10 by the second tapered waveguide 8, and is externally guided. Taken out.

That is, the switching between the second and third channel waveguides and the intensity modulation can be performed by the on-off of the SAW. Further, since the optical frequency of the diffracted light changes due to the Doppler effect, it can be used as an optical frequency shifter.

[0011]

By the way, one of the demands for the waveguide type acousto-optic device is a demand for miniaturization of the device (chip).
It was said that miniaturization had reached the limit.

However, the following facts have been found out by the research conducted by the present inventors. That is, in the above-described conventional waveguide type acousto-optic device, the curved waveguide portions of the four channel waveguides are all formed in the same shape. Here, it is known that the loss of light propagating in the curved waveguide portion of the channel waveguide depends on the minimum radius of curvature of the curve, and increases as the minimum radius of curvature decreases. For this reason, it has been necessary to set the radius of curvature of all channel waveguides to at least a radius of curvature greater than or equal to the minimum radius of curvature of the channel waveguides in which it is necessary to minimize loss. That is, among these channel waveguides, even those that do not necessarily need to have a large radius of curvature, those that have to have the same large radius of curvature as the ones that need to minimize loss are required. Is. In other words, some of the channel waveguides of the conventional waveguide type acousto-optic device include ones that can be used even if the radius of curvature is made smaller. It was found that there is a possibility that the size of the entire chip can be further reduced by forming the chip as small as possible within the allowable loss range.

The present invention has been made based on the above-mentioned clarified facts, and provides a waveguide type acousto-optic device which can be formed in a small chip size without increasing a loss of a required signal. The purpose is to do.

[0014]

In order to solve the above-mentioned problems, a waveguide type acousto-optic device according to the present invention comprises: (Structure 1) A surface acoustic wave is generated in the waveguide, A slab waveguide that imparts an acousto-optical effect to propagating light, and one end of the slab waveguide is coupled to one end of an acousto-optic effect imparting region of the slab waveguide, and the other end is coupled to the outside 1 or 2 Two or more input / output channel waveguides, one end of which is coupled to the other end of the acousto-optic effect imparting region of the slab waveguide and the other end of which is coupled to the outside. A channel waveguide, and the channel waveguide has a distance between adjacent channel waveguides that is small on the end side coupled to the acousto-optic effect imparting region and gradually increases toward the external coupling portion side. Is formed into a curved shape so that In the waveguide type acousto-optical element, a structure which is characterized in that than the minimum radius of curvature of one of the channel waveguides of the adjacent channel waveguides were formed larger minimum radius of curvature of the other channel waveguide,
As a mode of this configuration 1, (configuration 2) In the waveguide type acousto-optic device of configuration 1,
The channel waveguide formed with a large minimum radius of curvature,
The configuration is characterized in that the curve representing the shape does not have an inflection point except at the ends.

[0015]

According to the above configuration 1, the minimum radius of curvature of one channel waveguide of the adjacent channel waveguides is made larger than the minimum radius of curvature of the other channel waveguide, and the radius of curvature can be reduced. With regard to the above, by making the size as small as possible within the range of the allowable loss, it becomes possible to make the size of the entire chip smaller.

According to the structure 2, the channel waveguide having the large minimum radius of curvature is formed into a curved shape having no inflection point, so that loss can be suppressed and the size can be further reduced. .

[0017]

FIG. 1 is a diagram showing the configuration of a waveguide type acousto-optic device according to an embodiment of the present invention.

An embodiment will be described below with reference to FIG. Since this embodiment has many parts in common with the configuration of the conventional example shown in FIG. 3 described above, the same parts are designated by the same reference numerals. The difference between this embodiment and the above-mentioned conventional example is that the first channel waveguide 6a and the second channel waveguide 10a have the same curved shape,
In addition, the third channel waveguide 11a and the fourth channel waveguide 7a have the same curved shape, and the minimum curvature radius of the former curved shape is larger than the minimum curvature radius of the latter. Since the other points are almost the same as the above-mentioned conventional example, a part of the description of the same parts will be omitted.

In FIG. 1, a substrate 1 is a Z-cut lithium tantalate (LT) single crystal plate, and each waveguide is formed by patterning an Al film on the substrate using a mask having a predetermined shape. Later, PE was formed using benzoic acid.

The IDT 3 was formed of an Al film having a thickness of 0.3 μm, and had an electrode width of 2.75 μm and a pitch of 5.5 μm so that the center frequency was 300 MHz. The wavelength used is 633 nm, and the Bragg angle (θB) is about 1.5 degrees.

Each channel waveguide has a straight line portion for connection with the outside, a straight line portion for connection with the tapered waveguide,
The first channel waveguide 6a and the second channel waveguide 10a having a large minimum radius of curvature are the same as those of the conventional example in that they are composed of curved portions for connecting them, and
The third channel waveguide 11a and the fourth channel waveguide 7 having a small minimum radius of curvature are designated for propagating the input light of light and the diffracted light of the SAW, respectively.
The point a is different from the conventional example in that it is designated to propagate undiffracted light.

First and second channel waveguides 6a and 10a
The shapes of the curved lines in are the same, and
Both of the coordinates are curves represented by the following equation.

Y = h {(X / L)-(1 / 2π) sin
(2πX / L)} However, in the above equation, h = 54.4 μm, L = 8
270 μm, and the length of the curved waveguide is about 4.
13 mm, minimum polar radius is 100 mm, the length of the first straight waveguide portion for connection with the outside is about 1.9 mm, and the length of the second straight waveguide portion for connection with the tapered waveguide is About 3
mm, the length of the tapered waveguide is about 7 mm, the slab waveguide 2
Has a width of about 8 mm.

Also, the third and fourth channel waveguides 11
The shapes of the curved portions in a and 7a are the same and are both represented by the following equation.

Y = h / 2 {1-cos (πX / L)} However, in the above equation, h = 638 μm, L = 68
The length of the curved waveguide is about 6.6.
7 mm, minimum polar radius is 15 mm, the length of the first straight waveguide portion for connection with the outside is about 1.9 mm, and the length of the second straight waveguide portion for connection with the tapered waveguide is About 0.5
mm, the length of the tapered waveguide is about 7 mm, the slab waveguide 2
Has a width of about 8 mm.

The entire length of the waveguide type acousto-optic device having the above structure is 40 mm.

According to the above-mentioned embodiment, the minimum polar radius of the channel waveguide in which the incident light and the diffracted light propagate, which is strictly required to suppress the propagation loss, is not reduced, that is, in other words, The chip size can be reduced without increasing the propagation loss due to scattering or the like. Moreover, since the chip size is small, more chips can be manufactured from the same wafer.

Further, in the above embodiment, since the curved portions of the first and second channel waveguides do not have an inflection point,
It is possible to suppress the meandering of light in the channel waveguide and reduce the radiation loss. In addition, since the meandering is small, the incident light is
When the light is emitted from the channel waveguide and the tapered waveguide to the slab waveguide, the light can be prevented from being bent and emitted, and the diffraction efficiency due to the SAW is not reduced. Further, the light emitted in a curved manner reaches a position deviated from the second and third tapered waveguides to cause an increase in loss and a deterioration in extinction ratio, but the light is not emitted in a curved manner. Moreover, such a problem does not occur.

In the above embodiment, either the third channel waveguide 11a or the fourth channel waveguide 7a does not necessarily have to be provided, but functions as an optical switch, an optical modulator, an optical frequency shifter, or the like. A waveguide-type acousto-optic device capable of satisfying the requirements is obtained.

Further, as shown in FIG. 2, a first waveguide lens 4a is used instead of the first tapered waveguide 4 and the fourth tapered waveguide 5, and the second tapered waveguide 8 and the third tapered waveguide 8 are used. The second waveguide lens 8a may be used instead of the waveguide 9.

Further, as the curved shape of the channel waveguide, in addition to the shape of the above-mentioned embodiment, a part of the circumference is used, a tangent function, a suspension curve, etc. In short, the direction is changed by a predetermined angle. Anything can be used.

In addition, the linear portion of each channel waveguide (first
2 linear waveguide sections) may be omitted.

Further, in the above-mentioned embodiment, the LT is formed on the substrate.
However, a substrate made of another piezoelectric material such as lithium niobate (LN) may be used. Further, it may have a structure in which a thin film having piezoelectricity such as ZnO and capable of propagating light is formed on a quartz substrate.

[0034]

As described in detail above, according to the waveguide type acousto-optic device of the present invention, the minimum curvature radius of one channel waveguide of the adjacent channel waveguides is smaller than the minimum curvature radius of the other channel waveguide. If the radius is made large and the radius of curvature can be made small, by making it as small as possible within the allowable loss range,
The size of the entire chip can be made smaller without increasing the loss.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a waveguide type acoustooptic device according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a modified example of the waveguide type acoustooptic device according to the embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a waveguide type acoustooptic device according to a conventional example of the present invention.

FIG. 4 is an explanatory view of the shape of a channel waveguide.

FIG. 5 is a graph showing the relationship between the radius of curvature of the channel waveguide and the loss.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate, 2 ... Slab waveguide, 3 ... Interdigital transducer (IDT), 4 ... 1st taper waveguide, 5 ... 2nd taper waveguide, 6a ... 1st channel waveguide, 7a ... 4th
Channel waveguide, 8 ... second taper waveguide, 9 ... third taper waveguide, 10a ... second channel waveguide, 11a ...
Third channel waveguide.

Claims (2)

[Claims] 1. A slab waveguide for generating a surface acoustic wave in a waveguide to impart an acoustooptic effect to light propagating in the waveguide, and one end of an acoustooptic effect imparting region of the slab waveguide. One end is connected to the part, and the other end is connected to the outside 1
Alternatively, two or more input / output channel waveguides and two or more input / output channel waveguides, one end of which is coupled to the other end of the acousto-optic effect imparting region of the slab waveguide, and the other end of which is coupled to the outside.
And a channel waveguide for emission, wherein the channel waveguide has a distance between adjacent channel waveguides that is small on the end side coupled to the acousto-optic effect imparting region and gradually increases toward the external coupling section side. In a waveguide type acousto-optic device formed in a curved shape so as to be gradually larger, the minimum radius of curvature of one channel waveguide of the adjacent channel waveguides is made larger than the minimum radius of curvature of the other channel waveguide. A waveguide type acousto-optic device characterized by the above. 2. The waveguide type acousto-optic device according to claim 1, wherein the channel waveguide having a large minimum curvature radius,
A waveguide type acousto-optic device characterized in that a curve representing its shape does not have an inflection point except at an end.