JP2004233976A - Optical modulator, matching method of impedance and speed of optical modulator, and producing method of optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simplify the matching of impedance and speed with a sufficient intensity and stability of an optical modulator so that a producing process of the optical modulator can be simplified. <P>SOLUTION: A Mach-Zehnder type optical waveguide 12 is formed on a main face 11A of a substrate 11 made of materials having electrooptic effects, and a coplanar (CPW) type modulators 13 and 14 are formed on the main face 11A. On a backside 11B of the substrate 11, a low dielectric constant layer 16 is formed as to contact with the main face, and a support substrate 17 is formed as to contact with the low dielectric constant layer 16. A concave part 18 is set on the area on which a modulation region P is laid. When a dielectric constant of a body of the support substrate is ε<SB>r</SB>, and the dielectric constant of an internal space of the concave part 18 is ε<SB>S</SB>, a relational expression ε<SB>r</SB>>ε<SB>S</SB>is satisfied. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to an optical modulator, a speed and impedance matching method of the optical modulator, and a method of manufacturing the optical modulator, which can be suitably used in a high-speed, large-capacity optical fiber communication system.

  In recent years, with the progress of high-speed, large-capacity optical fiber communication systems, high-speed optical modulators using optical waveguide elements, such as external modulators, have been put to practical use and are being widely used. In such an optical modulator, a light wave guided in an optical waveguide is modulated using, for example, a modulation signal in a microwave region. Therefore, speed matching and impedance matching between the light wave and the modulation signal can be achieved. It becomes important.

  In view of such a viewpoint, Japanese Patent Application Laid-Open No. 9-211402 and the like disclose a support substrate provided on the back surface side of a substrate constituting a light modulation element, and at a position on the support substrate corresponding to a modulation region of the light modulation element. Providing a cavity, reducing the effective refractive index for the modulation signal, suppressing a reduction in the phase velocity of the modulation signal, a light wave guided in the optical waveguide of the light modulation element, and a modulation electrode of the light modulation element Attempts have been made to achieve speed matching with a modulation signal applied from a device and impedance matching.

  However, in the above-mentioned method, in order to realize sufficient speed matching and impedance matching, it is necessary to reduce the thickness of the substrate constituting the light modulation element to, for example, several μm. Since a sufficient processing accuracy is required for such a thinning process, it has been extremely difficult to manufacture an optical modulator including the above-described optical modulator. In the case where a cavity or the like is provided, stress concentrates on such a portion, causing a DC drift or a temperature drift, which may make stable operation of the optical modulator difficult. Further, the light modulation element, that is, the light modulator may be broken by the stress concentration described above.

  SUMMARY OF THE INVENTION It is an object of the present invention to easily achieve speed matching and impedance matching while ensuring sufficient strength and stability of an optical modulator, and to further simplify a manufacturing process of the optical modulator.

In order to achieve the above object, the present invention provides:
A substrate made of a material having an electro-optic effect, an optical waveguide provided on the main surface side of the substrate, and a light wave guided on the optical waveguide provided on the main surface of the substrate and modulated in the optical waveguide; A light modulation element having a modulation electrode for applying a signal,
The light modulation element, a dielectric layer provided such that the main surface is in contact with the back surface side of the substrate,
Provided on the back side of the dielectric layer, the light modulation element, comprising a support substrate having a concave portion at least at a position corresponding to the modulation region,
When the dielectric constant of the support substrate is ε _r , and the dielectric constant in the concave portion is ε _S ,
ε _r > ε _S
The present invention relates to an optical modulator characterized by satisfying the following relationship:

Also, the present invention
A substrate made of a material having an electro-optic effect and having a dielectric constant of ε _LN; an optical waveguide provided on the main surface side of the substrate; and an optical waveguide provided on the main surface of the substrate. A step of preparing a light modulation element having a modulation electrode for applying a modulation signal to a light wave guided through,
A step of providing a dielectric layer such that the main surface of the light modulation element is in contact with the back surface of the substrate,
Wherein the back surface side of the dielectric layer, said light modulation element, in a position corresponding to at least the modulation region, the dielectric constant of the internal space has a recess which is epsilon _S, the dielectric constant is a epsilon _r, epsilon _r > comprising: providing a supporting substrate that satisfies epsilon _S the relationship,
And a method for matching the speed and impedance of the optical modulator.

Further, the present invention provides
A substrate made of a material having an electro-optic effect and having a dielectric constant of ε _LN; an optical waveguide provided on the main surface side of the substrate; and an optical waveguide provided on the main surface of the substrate. A step of preparing a light modulation element having a modulation electrode for applying a modulation signal to a light wave guided through,
A step of providing a dielectric layer such that the main surface of the light modulation element is in contact with the back surface of the substrate,
Wherein the back surface side of the dielectric layer, said light modulation element, in a position corresponding to at least the modulation region, the dielectric constant of the internal space has a recess which is epsilon _S, the dielectric constant is a epsilon _r, epsilon _r > comprising: providing a supporting substrate that satisfies epsilon _S the relationship,
And a method of manufacturing an optical modulator.

  According to the present invention, a dielectric layer and a support substrate are provided on the back surface side of the light modulation element, and a concave portion having a dielectric constant smaller than that of the dielectric layer is provided on the support substrate so that a desired light modulation We are making vessels. In order to achieve speed matching of the optical modulator, the thickness of the substrate of the optical modulator must be reduced to some extent to reduce the effective refractive index for a modulation signal. Due to the low dielectric constant, the effective refractive index can be sufficiently reduced even if the thickness is set relatively large.

  Further, by configuring the dielectric layer from a low dielectric constant material, the effective refractive index can be sufficiently reduced even if the substrate is set to be thicker, and the dielectric constant is reduced by the low dielectric constant layer. In addition, the strength of the substrate can be reinforced.

  Therefore, the speed matching and the impedance matching can be easily achieved after sufficiently securing the intensity of the optical modulator, and further sufficiently suppressing the DC drift and the temperature drift and securing the stability. Further, the manufacturing process of the optical modulator can be simplified.

  As described above, according to the present invention, while sufficiently securing the strength and stability of the optical modulator, speed matching and impedance matching are easily achieved, and further, the manufacturing process of the optical modulator is simplified. be able to.

Hereinafter, the present invention will be described in detail with reference to embodiments of the present invention.
FIG. 1 is a plan view showing an example of the optical modulator of the present invention, and FIG. 2 is a cross-sectional view of the optical modulator shown in FIG. 1 taken along line AA. In these figures, in order to clarify the features of the present invention, the size of each component and the like are drawn so as to be different from actual ones.

  The optical modulator 10 shown in FIGS. 1 and 2 has a Mach-Zehnder optical waveguide 12 on the main surface 11A side of a substrate 11 made of a material having an electro-optical effect, and a signal electrode 13 on the main surface 11A. And a ground electrode 14 on both sides of the signal electrode 13. The signal electrode 13 and the ground electrode 14 constitute a so-called coplanar (CPW) type modulation electrode. The modulation electrode including the substrate 11, the optical waveguide 12, and the signal electrode 13 and the ground electrode 14 constitute a light modulation element 15. Note that a region P surrounded by a broken line is a modulation region in which a light wave guided through the optical waveguide 12 and a modulation signal from the modulation electrode act on each other to modulate the light wave.

  On the back surface 11B side of the substrate 11, a dielectric layer 16 provided so that the main surface is in contact, and a support substrate 17 provided so as to be in contact with the dielectric layer 16 are provided. A concave portion 18 is provided in a region of the support substrate 17 where the modulation region P is located.

In the optical modulator 10 shown in FIGS. 1 and 2, when the dielectric constant of the support substrate 17 is ε _r and the dielectric constant of the internal space of the concave portion 18 is ε _S , the relationship of ε _r > ε _S is satisfied. I do.

  In the optical modulator 10 shown in FIGS. 1 and 2, the dielectric constant of the substrate 11, the dielectric layer 16 and the space in the recess, the height h and the width d of the signal electrode 13, and the signal electrode 13 and the ground electrode By changing the thickness t of the substrate 11, the thickness T of the dielectric layer 16, and the width W and the depth D of the concave portion 18 in consideration of the gap g with respect to the speed 14, the speed matching and the impedance matching. So that

  In order to achieve speed matching and impedance matching, it is necessary to reduce the thickness of the substrate 11 to reduce the effective refractive index for the modulation signal applied from the modulation electrode. In the optical modulator 10 shown in FIGS. 1 and 2, a concave portion 18 having a space that satisfies the above-described relational expression of the dielectric constant is provided in a support substrate 17 provided below the optical modulation element 15. Since the modulation signal leaks from the substrate 11 to the outside and reaches the concave portion 18, the dielectric constant of the internal space of the concave portion 18 also affects the propagation state, and the low dielectric constant of the internal space is reduced. Accordingly, the effective refractive index for the modulation signal can be reduced.

  Therefore, even when the thickness of the substrate 11 is relatively large, the effective refractive index for the modulation signal can be reduced due to the low dielectric constant inside the concave portion 18.

  Further, the dielectric layer 16 is directly provided on the back surface 11B side of the substrate 11, so that the thinned substrate 11 can be reinforced. Furthermore, since the modulation signal leaks from inside the substrate 11 to the outside, the effective refractive index for the modulation signal can be reduced due to the low dielectric constant of the dielectric layer 16. Therefore, even when the thickness of the substrate 11 is relatively large, the effective refractive index for the modulation signal can be kept sufficiently small.

  Specifically, by adopting the configuration shown in FIGS. 1 and 2, even when the thickness of the substrate 11 is increased to about 30 μm, it is possible to sufficiently achieve the speed matching and the impedance matching. become.

  As described above, in the optical modulator 10 shown in FIGS. 1 and 2, the intensity of the optical modulator is sufficiently secured, and as a result, DC drift, temperature drift, and the like are sufficiently suppressed, and the stability is sufficiently secured. Above, speed matching and impedance matching can be easily achieved. Further, the manufacturing process of the optical modulator can be simplified.

  As described above, the substrate 11 is required to be made of a material having an electro-optical effect, and is made of, for example, lithium niobate, potassium lithium niobate, lithium tantalate, KTP, glass, silicon, GaAs, and quartz. In particular, it is preferable to use lithium niobate.

  The dielectric layer 16 can be made of a material having a low dielectric constant, such as glass, epoxy resin, and polyimide. Since these materials have a sufficiently low dielectric constant as compared with the above-described material of the substrate 11, the above-described relational expression regarding the dielectric constant is satisfied, and the above-described effects can be realized.

  Further, the thickness T of the dielectric layer 16 is not particularly limited, but in the present invention, by setting the thickness T in the range of 0.1 μm to 200 μm, the strength and stability of the optical modulator are secured. Thus, speed matching and impedance matching can be improved.

  The dielectric layer 16 needs to be formed on the back surface 11B side of the substrate 11, but the formation method is not particularly limited, and is appropriately selected according to the type of material constituting the dielectric layer. For example, when the thickness of the dielectric layer 16 is relatively large, a sheet-like member made of the above-described material is separately prepared, and the sheet-like member is formed by attaching the sheet-like member to the back surface 11A of the substrate 11. can do. In this case, the dielectric layer 16 can be easily formed without using a complicated film forming apparatus.

The internal space of the concave portion 18 formed in the support substrate 17 can be set in a hollow state, or can be buried by filling with some low dielectric constant material. In this case, the dielectric constant of the low dielectric constant material .epsilon.2, the dielectric constant epsilon _LN substrate _11, and in the case where the dielectric constant of the dielectric layer 16 was set to epsilon _1, the ε _LN> ε _1> ε ₂ the relationship It is necessary to be satisfied.

  When the inside of the concave portion 18 is maintained in a hollow state, the inside of the concave portion 18 is filled with air. Therefore, the dielectric constant in the internal space of the concave portion 18 is about 1, and a sufficiently low dielectric constant can be achieved.

  On the other hand, when the low dielectric material is buried in the concave portion 18, the dielectric constant of the low dielectric material is generally higher than the dielectric constant (1) of air. The effect of lowering the dielectric constant is reduced as compared with the case of (1). However, the intensity of the optical modulator 10 itself can be increased, and the intensity and stability of the optical modulator 10 can be more sufficiently ensured.

  Examples of the low dielectric substance include glass, epoxy resin, and polyimide.

  The optical waveguide 12 can be formed by using a known method such as a titanium diffusion method or a proton exchange method. The signal electrode 13 and the ground electrode 14 can be formed from a material such as gold, silver, or copper by using a plating method, a vapor deposition method, or the like. The support substrate 17 is preferably formed of a material having a linear expansion coefficient close to that of the substrate 11 and the low dielectric constant layer 16.

(Example)
In this example, an optical modulator as shown in FIGS. 1 and 2 was manufactured. An X-cut lithium niobate single crystal (FIG. 2, LN dielectric constant ε _LNx = 28 in the X direction, LN dielectric constant ε _LNy = 43 in the Y direction) is used as the substrate 11, and the optical waveguide 12 is formed by a titanium diffusion method. By using the plating method and the vapor deposition method together, the signal electrode 13 and the ground electrode 14 were formed, and the light modulation element 15 was manufactured. The height h of these electrodes was 20 μm, the width d of the signal electrode 13 was 30 μm, and the gap g between the signal electrode 13 and the ground electrode 14 was 30 μm.

Next, a sheet-like member (dielectric constant ε ₁ = 3.8) made of epoxy resin is attached to the back surface 11B side of the substrate 11 to form a dielectric layer 16, and a region located in the modulation region of the light modulation element 15 is formed. Then, a supporting substrate 17 made of lithium niobate having a concave portion 18 was formed, and the optical modulator 10 was manufactured. The interior of the recess 18 was kept in a hollow state (dielectric constant ε _S = 1).

  In the optical modulator 10 obtained as described above, the thickness of the substrate 11 is 10 μm, the thickness of the dielectric layer 16 is 5 μm, the width W of the concave portion 18 is 500 μm, and the depth D of the concave portion 18 is 500 μm. As a result, speed matching and impedance matching were realized.

(Comparative example)
An optical modulator was manufactured in the same manner as in the example except that no concave portion was provided in the supporting substrate. In this case, when the thickness of the substrate 11 was 0.3 μm and the thickness of the low dielectric constant layer 16 was 5 μm, speed matching and impedance matching could be realized.

  As is clear from the examples and the comparative examples, it can be seen that in the optical modulator of the present invention, even when the thickness of the substrate 11 is sufficiently large, speed matching and impedance matching can be realized. That is, according to the present invention, it is understood that the speed matching and the impedance matching can be realized while securing the strength and the stability of the optical modulator more sufficiently. Further, since the thickness of the substrate 11 is relatively large, processing becomes easy, and a target optical modulator can be easily manufactured.

  As described above, the present invention has been described in detail based on the embodiments of the present invention by giving specific examples. However, the present invention is not limited to the above contents, and may be any form within the scope of the present invention. Changes and modifications are possible. For example, in the above embodiment, the CPW type modulation electrode is arranged, but an ACPS type modulation electrode may be arranged. Elements such as Mg, Zn, Sc, and In can be added to the substrate 11 in order to improve the light damage resistance. Furthermore, a buffer layer may be provided between the substrate 11 and the signal electrode 13 and the ground electrode 14.

It is a top view showing an example of the optical modulator of the present invention. FIG. 2 is a cross-sectional view of the optical modulator shown in FIG. 1 taken along line AA.

Explanation of reference numerals

  Reference Signs List 10 light modulator, 11 substrate, 12 optical waveguide, 13 signal electrode, 14 ground electrode, 15 light modulation element, 16 dielectric layer, 17 support substrate, 18 recess

Claims (13)

A substrate made of a material having an electro-optic effect, an optical waveguide provided on the main surface side of the substrate, and a light wave provided on the main surface of the substrate and modulated on a light wave guided through the optical waveguide; A light modulation element having a modulation electrode for applying a signal,
The light modulation element, a dielectric layer provided such that the main surface is in contact with the back surface side of the substrate,
Provided on the back side of the dielectric layer, the light modulation element, comprising a support substrate having a concave portion at least at a position corresponding to the modulation region,
When the dielectric constant of the support substrate is ε _r , and the dielectric constant in the concave portion is ε _S ,
ε _r > ε _S
An optical modulator characterized by satisfying the following relationship:   The optical modulator according to claim 1, wherein the dielectric layer has a sheet shape.   The optical modulator according to claim 1, wherein the inside of the recess is a cavity.   The optical modulator according to claim 1, wherein the dielectric layer is made of a material having a smaller dielectric constant than a material of the electro-optic substrate. The dielectric constant of the electro-optical substrate is ε _LN , and the dielectric constant of the dielectric layer is ε _1, and the dielectric constant is ε ₂ ,
ε _LN > ε ₁ > ε ₂
The optical modulator according to claim 1, wherein a low dielectric constant material satisfying the following relationship is buried in the concave portion.   The optical modulator according to claim 1, wherein the electro-optic substrate is made of lithium niobate.   The optical modulator according to any one of claims 1 to 6, wherein a thickness of a modulation part of the electro-optical substrate or at least a part of the modulation part is 30 m or less. A substrate made of a material having an electro-optic effect and having a dielectric constant of ε _LN; an optical waveguide provided on the main surface side of the substrate; and an optical waveguide provided on the main surface of the substrate. A step of preparing a light modulation element having a modulation electrode for applying a modulation signal to a light wave guided through,
A step of providing a dielectric layer such that the main surface of the light modulation element is in contact with the back surface of the substrate,
Wherein the back surface side of the dielectric layer, said light modulation element, in a position corresponding to at least the modulation region, the dielectric constant of the internal space has a recess which is epsilon _S, the dielectric constant is a epsilon _r, epsilon _r > comprising: providing a supporting substrate that satisfies epsilon _S the relationship,
A method for matching the speed and impedance of an optical modulator, comprising:   The method of claim 9, wherein the recess is a cavity. In the case where the dielectric constant of the dielectric layer was set to epsilon _1, having a dielectric constant epsilon _2,
ε _LN > ε ₁ > ε ₂
10. The speed and impedance matching method of an optical modulator according to claim 9, wherein a low dielectric constant material satisfying the following relationship is embedded in the concave portion. A substrate made of a material having an electro-optic effect and having a dielectric constant of ε _LN; an optical waveguide provided on the main surface side of the substrate; and an optical waveguide provided on the main surface of the substrate. A step of preparing a light modulation element having a modulation electrode for applying a modulation signal to a light wave guided through,
A step of providing a dielectric layer such that the main surface of the light modulation element is in contact with the back surface of the substrate,
Wherein the back surface side of the dielectric layer, said light modulation element, in a position corresponding to at least the modulation region, the dielectric constant of the internal space has a recess which is epsilon _S, the dielectric constant is a epsilon _r, epsilon _r > comprising: providing a supporting substrate that satisfies epsilon _S the relationship,
A method for manufacturing an optical modulator, comprising:   The method according to claim 12, wherein the inside of the recess is a cavity. In the case where the dielectric constant of the dielectric layer was set to epsilon _1, having a dielectric constant epsilon _2,
ε _LN > ε ₁ > ε ₂
13. The method of manufacturing an optical modulator according to claim 12, wherein a low dielectric constant material satisfying the following relationship is embedded in the recess.

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