JPH09185025A - Optical control element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical control element which is small in driving voltage and with which high-speed operation is possible by forming progressive wave type electrodes formed via a buffer layer on an optical waveguide formed on the surface of an optical substrate of plural layers varying in shapes. SOLUTION: The progressive wave type electrodes of the optical control element which have an optical substrate 1 having an electrooptic effect and the progressive wave type electrodes 21, 22 formed via the buffer layer 3 on the optical waveguide 2 formed on the surface of this optical substrate 1 are composed of the plural layers varying in shapes. The first progressive wave type electrode 21 consists of three parts 21a, 21b, 21c and the second progressive wave type electrode 22 disposed in contact with the buffer layer 3 consists of three parts 22a, 22b, 22c and is laminated and disposed on the first electrode 21. According thereto, the progressive wave type electrodes required to be thick films as a whole may be settable at the small thickness of the electrode layer in contact with the buffer layer and the formation of the thin resist layer for forming such electrode layers is possible as well.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical control element such as an optical modulator or an optical switch.

[0002]

2. Description of the Related Art In a high-speed and large-capacity optical transmission system, optical switching system, etc., an optical control element having a low driving voltage and capable of high-speed operation is useful. Examples of this type of light control element include an optical switch, a phase modulator, a light intensity modulator, and the like, and there is a light control element using a Ti-diffused LiNbO ₃ waveguide.

FIG. 1 shows an example of a conventional light control element, here, a Mach-Zehnder type light intensity modulator.
3A is a plan view, and FIG. 1B is a sectional view taken along the line AA ′ in FIG.

In FIG. 1, reference numeral 1 denotes a z-plate LiNbO ₃ (LN) substrate having an electro-optical effect, which is branched into two (2a, 2b) so as to form a Mach-Zehnder interferometer in the middle thereof by Ti thermal diffusion, for example. The formed optical waveguide 2 is formed. A SiO ₂ buffer layer 3 having a thickness of about 0.3 to 1 μm is formed on the substrate 1, and a traveling wave type electrode 4 for modulation composed of three portions 4 a, 4 b and 4 c is further formed on the buffer layer 3. ing. In addition, the traveling wave electrode 4a
A microwave voltage is supplied from the power supply line 5 and a terminating resistor 6 is connected between the terminals 4 and 4b.

Here, when the incident light 7 having a constant intensity is incident on the optical waveguide 2, the light propagates with its power distributed to the two optical waveguides 2a and 2b.
In the region 8 where the voltage supplied to the device interacts with each other, the refractive index of each of the optical waveguides 2a and 2b, that is, the phase of the light changes according to the voltage, so that the combined lights interfere with each other and are intensity-modulated. Emitted light 9.

In order to increase the speed of this optical modulator, it is essential that the propagation speed of light and the propagation speed of an electric signal on a traveling-wave electrode be substantially equal (speed matching). It is necessary to set the thickness to 10 to 30 μm (reference: 1995 Electronics Society Conference of the Institute of Electronics, Information and Communication Engineers C-153). Therefore, a technique for forming a thick film electrode exceeding 10 μm is required.
If an electrode with a thickness of 20 μm and a thickness of 20 μm is actually manufactured based on a design that optimizes the electrode characteristics and drive voltage, it will have an inverted trapezoidal cross section as shown in FIG. 1 (b). . The reason for this will be briefly described below.

As a conventional method for forming a thick film electrode, a gold plating method along a thick resist guide is well known.
This method is characterized in that the transfer pattern is formed by directly reflecting the resist shape on the electrode shape. FIG. 2 shows an electrode forming process of about 20 μm by the conventional gold plating method.

First, a base electrode 11 for plating and a resist 12 are formed on a substrate 1 on which an optical waveguide 2 and a buffer layer 3 are formed in advance. In this case, the thickness of the resist 12 is 20
At least μm is required. The thick resist 12 is exposed to ultraviolet rays 14 through a mask 13 corresponding to the shape of the portion where no electrode is provided (FIG. 2 (a)). In general, light such as ultraviolet rays has an intensity distribution in the lateral direction and the depth direction due to diffraction and absorption, so that even if the resist 12 is developed, the side surfaces of the resist pattern 12 'are not vertical and become trapezoidal (see FIG. 2). (b)
).

When gold plating is performed using this resist pattern 12 ', the formed gold electrode 15 has an inverted trapezoidal cross section (FIG. 2 (c)). The resist pattern 12 'and the unnecessary underlying electrode 11 thereunder are removed to complete the process (FIG. 2 (d)).
However, at this time, the gold electrode 15 and the remaining base electrode 11 constitute the electrode 4 described above.

The trapezoidal shape of the resist pattern described above is
It is known that the same occurs when an electron beam is used instead of ultraviolet rays.

[0011]

Since the driving voltage of the optical modulator depends on the magnitude of the electric field near the optical waveguide, it largely depends on the width of the electrode lower portion adjacent to the optical waveguide. FIG. 3 shows an example of calculation of the product VπL of the driving voltage and the working length of the electrode with respect to the width W of the lower part of the electrode.

Usually, when the width W of the lower portion of the electrode is about the same as the width of the optical waveguide, the driving voltage becomes the lowest. However, as described above, the electrode shape is an inverted trapezoidal shape, and the width of the lower portion of the electrode becomes smaller than the width of the upper portion of the electrode, which causes a problem that the driving voltage increases. Further, if the width of the mask 13 is set to be slightly wider in order to widen the width of the lower part of the electrode, the microwave characteristic is deteriorated (for example, speed mismatch becomes large, characteristic impedance is lowered, etc.). was there.

An object of the present invention is to provide a light control element which has a low driving voltage and can be operated at high speed.

[0014]

In order to solve the above problems, the present invention provides an optical substrate having an electro-optical effect, an optical waveguide formed on the surface of the optical substrate, and a buffer layer on the optical waveguide. In the light control element including the traveling-wave electrode formed as above, the traveling-wave electrode is composed of a plurality of layers having different shapes.

According to the present invention, it is possible to set the thickness of the electrode layer in contact with the buffer layer to be small by forming the traveling-wave electrode, which is required to be a thick film as a whole, by a plurality of layers. As a result, the resist for forming this thin electrode layer can also be made thinner, and it becomes possible to form a resist pattern that is not affected by the diffraction or absorption of light for exposure, and the width as designed can be achieved. It becomes possible to form an electrode having the same. Therefore, problems such as an increase in driving voltage and a decrease in characteristic impedance can be avoided.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings.

FIG. 4 shows an example of a Mach-Zehnder type optical intensity modulator similar to the first embodiment of the light control element of the present invention, in which the same components as those of the conventional example are shown. The same symbols are used. That is, 1 is a z-plate LiNbO ₃
The substrate 2 is an optical waveguide, 3 is a SiO ₂ buffer layer, 21 is a first traveling wave type electrode for modulation, and 22 is a second traveling wave type electrode for modulation.

The first traveling wave electrode 21 has three parts 21.
a, 21b, 21c, and is provided in contact with the buffer layer 3. The second traveling wave electrode 22 has three parts 22.
a, 22b, and 22c, which are stacked on the first electrode 21.

FIGS. 5A and 5B show an example of calculation of the influence of the thickness of the first electrode 21 on the drive voltage and microwave (high frequency) characteristics. In each electrode, the first electrode 21a has a width of 8 μm, the second electrode 22a has an upper width of 8 μm, and a lower electrode 4a.
μm, and the total thickness of the first electrode 21 and the second electrode 22 was 20 μm. The horizontal axis represents the thickness of the first electrode 21, and the vertical axis represents the product of the driving voltage and the working length of the electrode and the high frequency characteristics such as a high frequency attenuation constant, a characteristic impedance and an effective refractive index. The case where the thickness of the first electrode 21 is 0 is the same as the case of the conventional example.
When the thickness of the second electrode is 20 μm, the second electrode 2 has a rectangular shape.
Indicates that 1 is not present.

As can be seen from FIG. 5, the first electrode 21
If, for example, about 1 μm is formed, the product of the driving voltage and the working length of the electrode and the high-frequency characteristics are almost the same as in the case of a rectangular shape with a width of 8 μm, and lower driving voltage and higher speed can be achieved. At this time, the thickness of the second electrode 22 is 1
It becomes 9 μm.

FIGS. 6 and 7 show an example of a method of manufacturing the light control element shown in FIG. Similar to the conventional example, the optical waveguide 2 and the buffer layer 3 are formed on the substrate 1 in advance.

A base electrode 31 for plating is formed on the optical waveguide 2 or the buffer layer 3 to a thickness of about 10 to 500 nm, and a first resist 32 is further formed. This first
The resist 32 is exposed to ultraviolet rays 34 through a mask 33 corresponding to the shape of the portion where no electrode is provided (FIG. 6 (a)).
). The first resist 32 is developed to form a resist pattern 32 '(FIG. 6 (b)), and gold plating is applied using this to form a gold electrode 35 (FIG. 6 (c)).

Next, the resist pattern 32 'is removed,
A second resist 36 is formed, and the second resist 36 is formed.
The ultraviolet rays 38 are exposed through the mask 37 corresponding to the shape of the portion where the electrodes are not provided (FIG. 6 (d)). The second resist 36 is developed to form a resist pattern 36 '(FIG. 7 (e)), and gold plating is applied using this to form a gold electrode 39 (FIG. 7 (f)).

Finally, the resist pattern 36 'and the unnecessary underlying electrode 31 thereunder are removed to complete the process (FIG. 7).
(g)) is the gold electrode 35 and the remaining base electrode 31 at this time.
Constitutes the first electrode 21 described above, and the gold electrode 39 is the second electrode.
The electrode 22 of.

FIG. 8 shows another example of a method of manufacturing the light control element shown in FIG. Similar to the conventional example, the optical waveguide 2 and the buffer layer 3 are formed on the substrate 1 in advance.

A base electrode 41 for plating is formed on the optical waveguide 2 or the buffer layer 3 to a thickness of about 10 to 500 nm, and the first resist 42 and the second resist 4 are further formed.
Form 3 An ultraviolet ray 45 is passed through a mask 44 corresponding to the shape of the portion where the electrode is not provided on the second resist 43.
Is exposed (FIG. 8 (a)). The second resist 43 is developed to form a resist pattern 43 '(FIG. 8 (b)).
At this time, for the first resist 42 and the second resist 43, materials having different exposure characteristics and dissolution characteristics with respect to a developing solution are selected.

Next, the first resist 42 is exposed to ultraviolet rays 46 using the resist pattern 43 'as a mask (FIG. 8).
(b)). The first resist 42 is developed to form a resist pattern 42 '(FIG. 8 (c)). After that, a gold electrode 47 is formed by applying gold plating (FIG. 8 (d)), and the resist patterns 42 ', 43' and the unnecessary underlying electrode 41 thereunder are removed (FIG. 8 (e)). But gold electrode 4
7 and the remaining base electrode 41 constitute the above-mentioned first and second electrodes 21 and 22, and in this case, the first resist 4
The width of the portion corresponding to 2 is wider than the width of the portion corresponding to the second resist 43.

FIG. 9 shows a second example of the light control element of the present invention.
In this embodiment, an example of a Mach-Zehnder type optical intensity modulator in which the traveling wave type electrode has a three-layer structure is shown. In the figure, 5
1 is a first traveling wave type electrode for modulation, 52 is a second traveling wave type electrode
Is a traveling wave type electrode, and 53 is a third traveling wave type electrode for modulation.

The first traveling wave electrode 51 has three parts 51.
a, 51b, 51c, and is provided in contact with the buffer layer 3. The second traveling wave electrode 52 has three parts 52.
a, 52b, 52c, which are stacked on the first electrode 51. The third traveling-wave electrode 53 is composed of three parts 53a, 53b, and 53c, and is laminated and provided on the second electrode 52.

The width of the first electrode 51a is equal to the width of the second electrode 52a.
It is wider than the width of. As in the case of the first embodiment, the product of the driving voltage and the working length is almost determined by the width of the first electrode 51a, and the microwave characteristics are the second and third electrodes 52.
Since it is determined by the shapes of a and 53a, the same effect can be obtained.

In the present example, the second electrode has an inverted trapezoidal cross section, but the effect does not change even if the second electrode has a bulged shape or a narrowed shape. Further, although the electrode has a three-layer structure in this example, it may have four or more layers.

Although the optical waveguide by the Ti thermal diffusion method has been described so far, the same applies to the optical waveguide by the ion exchange method or the ridge type optical waveguide. Further, the phase modulator can be configured by using a linear waveguide as the optical waveguide. Also, as the material of the buffer layer, a dielectric material such as alumina or Teflon or a semi-insulating material can be used. Although the CPW electrode has been described as the traveling-wave electrode, an electrode structure such as an ACPS electrode may be used. Furthermore, although the gold plating method has been described as the electrode forming method, a vapor deposition method or the like may be used, and copper,
You may form using metals, such as silver and aluminum.

Up to now, the light control element using the electro-optic effect in the z-plate LiNbO ₃ substrate has been described. However, semiconductors such as x-plate and y-plate LiNbO ₃ substrates and other ferroelectrics can be used. Needless to say, the present invention is also very effective for a light control element using the electro-optical effect of a substrate having anisotropy such as organic matter.

[0034]

As described above, according to the present invention,
By constructing the traveling-wave electrode, which is required to be a thick film as a whole, with a plurality of layers, it is possible to set the thickness of the electrode layer in contact with the buffer layer to be small, which allows the thin electrode layer to be formed. The resist for forming the film can also be thinned, and it becomes possible to form a resist pattern that is not affected by the diffraction and absorption of light for exposure, and it is possible to form an electrode having a width as designed. For this reason,
Problems such as an increase in driving voltage and a decrease in characteristic impedance can be avoided, and the driving voltage can be reduced without impairing the high frequency characteristics, so that a light control element with a small driving voltage and capable of high-speed operation can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a conventional Mach-Zehnder type optical intensity modulator.

FIG. 2 is a diagram showing a method of manufacturing a conventional Mach-Zehnder type optical intensity modulator.

3 is a diagram showing a calculation example of a product of a driving voltage and an action length of an electrode with respect to a width of an electrode lower portion in the optical modulator of FIG.

FIG. 4 is a diagram showing a first embodiment of a light control element of the present invention.

5 is a diagram showing an example of calculation of a product of a driving voltage and an action length of an electrode with respect to a thickness of a first electrode and a high frequency characteristic in the light control element of FIG.

FIG. 6 is a diagram showing an example of a method of manufacturing the light control element of FIG.

7 is a diagram showing an example of a method for manufacturing the light control element in FIG.

FIG. 8 is a diagram showing another example of a method for manufacturing the light control element in FIG.

FIG. 9 is a diagram showing a second embodiment of the light control element of the present invention.

[Explanation of symbols]

1 ... LiNbO ₃ substrate, 2 ... Optical waveguide, 3 ... SiO ₂ buffer layer, 21, 51 ... First traveling wave type electrode, 22, 5
2 ... 2nd traveling wave type electrode, 53 ... 3rd traveling wave type electrode.

Claims (3)

[Claims] 1. An optical control comprising an optical substrate having an electro-optical effect, an optical waveguide formed on the surface of the optical substrate, and a traveling wave type electrode formed on the optical waveguide via a buffer layer. In the device, the traveling wave electrode is composed of a plurality of layers having different shapes, which is an optical control device. 2. A first of the plurality of layers that is in contact with the buffer layer
2. The light control element according to claim 1, wherein said layer has a thickness of 1 μm or less. 3. The light control element according to claim 2, wherein the second layer laminated on the first layer has an inverted trapezoidal cross section.