JP4454301B2 - Optical modulator and method for manufacturing optical modulator - Google Patents

Description

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

  In recent years, with the progress of high-speed and large-capacity optical fiber communication systems, high-speed optical modulators using optical waveguide elements have been put into practical use and widely used, as represented by external modulators. In such an optical modulator, the light wave guided through the optical waveguide is modulated using, for example, a modulation signal in the microwave region, so that 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 provide a support substrate on the back side of the substrate constituting the light modulation element, and at a position corresponding to the modulation region of the light modulation element of the support substrate. A light wave guided through the optical waveguide of the light modulation element, and a modulation electrode of the light modulation element by providing a cavity, reducing an effective refractive index with respect to the modulation signal, suppressing a reduction in phase velocity of the modulation signal; Attempts have been made to achieve speed matching with the modulation signal applied from the above and impedance matching.

  However, in the above 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 such a thinning process requires a sufficient processing accuracy, it is extremely difficult to manufacture an optical modulator including the optical modulation element as described above. In addition, when a hollow portion or the like is provided, stress concentrates on such a portion, and DC drift or temperature drift occurs, which may make it difficult to perform stable operation of the optical modulator. Furthermore, the light modulation element, that is, the light modulator may be destroyed due to the stress concentration described above.

  An object of the present invention is to facilitate speed matching and impedance matching while ensuring sufficient strength and stability of an optical modulator, and to simplify the 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 is modulated. A light modulation element having a modulation electrode for applying a signal;
A dielectric layer provided so that a main surface is in contact with the back surface side of the substrate of the light modulation element;
A support substrate provided on the back side of the dielectric layer and having a recess at a position corresponding to at least the modulation region of the light modulation element;
When the dielectric constant of the support substrate is εr and the dielectric constant in the recess is εs,
εr> εs
While satisfying the relationship
The dielectric layer relates to an optical modulator, wherein the dielectric layer is made of a material having a dielectric constant smaller than a material constituting the electro-optic substrate .

Furthermore, the present invention provides
A substrate made of a material having an electro-optic effect and a dielectric constant of εLN, an optical waveguide provided on the main surface side of the substrate, and the optical waveguide provided on the main surface of the substrate Preparing a light modulation element having a modulation electrode for applying a modulation signal to the guided light wave;
Providing a dielectric layer composed of a material having a smaller dielectric constant than the material constituting the substrate so that the main surface of the light modulation element is in contact with the back surface side of the substrate ;
On the back side of the dielectric layer, there is a recess having a dielectric constant of εs in the internal space at a position corresponding to at least the modulation region of the light modulation element, the dielectric constant is εr, and εr> εs. Providing a support substrate that satisfies the relationship;
It is related with the manufacturing method of the optical modulator characterized by comprising.

  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 the dielectric constant is provided on the support substrate, so that the desired light modulation is achieved. A vessel is being made. In order to achieve speed matching of the optical modulator, it is necessary to reduce the effective refractive index with respect to the modulation signal by reducing the thickness of the substrate of the optical modulation element to some extent. 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 made sufficiently small even when the substrate is set to be thicker, and the dielectric layer is made thinner by the low dielectric constant layer. Further, the strength of the substrate can be reinforced.

  Therefore, it is possible to easily achieve speed matching and impedance matching after sufficiently ensuring the strength of the optical modulator and further sufficiently suppressing stability such as DC drift and temperature drift. Furthermore, the manufacturing process of the optical modulator can be simplified.

  As described above, according to the present invention, speed matching and impedance matching can be easily achieved with sufficient strength and stability of the optical modulator, and further, the manufacturing process of the optical modulator can be simplified. be able to.

Hereinafter, the present invention will be described in detail according to embodiments of the 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 the line AA. In these drawings, in order to clarify the features of the present invention, the size of each component is drawn differently from the actual one.

  The optical modulator 10 shown in FIGS. 1 and 2 has a Mach-Zender optical waveguide 12 on the main surface 11A side of the substrate 11 made of a material having an electro-optic effect, and a signal electrode 13 on the main surface 11A. In addition, a ground electrode 14 is provided 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 composed of the substrate 11, the optical waveguide 12, and the signal electrode 13 and the ground electrode 14 constitutes an optical modulation element 15. A region P surrounded by a broken line is a modulation region where the light wave guided through the optical waveguide 12 and the modulation signal from the modulation electrode interact with each other to modulate the light wave.

  On the back surface 11 </ b> B side of the substrate 11, a dielectric layer 16 provided so as to be in contact with the main surface and a support substrate 17 provided so as to be in contact with the dielectric layer 16 are provided. A recess 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 FIG. 1 and FIG. 2, when the dielectric constant of the support substrate 17 is ε _r and the dielectric constant of the internal space of the recess 18 is ε _S , the relationship ε _r > ε _S is satisfied. To 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 are provided. In consideration of the gap g with respect to the thickness 14, the thickness t of the substrate 11, the thickness T of the dielectric layer 16, and the width W and depth D of the recess 18 are changed, thereby speed matching and impedance matching. Try to plan.

  In order to achieve speed matching and impedance matching, it is necessary to reduce the effective refractive index for the modulation signal applied from the modulation electrode by reducing the thickness of the substrate 11. In the optical modulator 10 shown in FIGS. 1 and 2, a recess 18 having a space that satisfies the 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 thereof, thereby reducing the low dielectric constant of the internal space. Based on this, 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 recess 18.

  In addition, 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 the inside of 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 with respect to the modulation signal can be kept sufficiently small.

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

  As described above, in the optical modulator 10 shown in FIGS. 1 and 2, the strength of the optical modulator is sufficiently ensured, and as a result, the DC drift and the temperature drift are sufficiently suppressed, and the stability is sufficiently secured. In the above, speed matching and impedance matching can be easily achieved. Furthermore, 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-optic effect. For example, the substrate 11 is made of lithium niobate, potassium lithium niobate, lithium tantalate, KTP, glass, silicon, GaAs, and quartz. In particular, it is preferably composed of 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 material of the substrate 11 described above, the above-described operational effects can be realized while satisfying the relational expression related to the dielectric constant as described above.

  Further, the thickness T of the dielectric layer 16 is not particularly limited, but in the present invention, the thickness and the stability of the optical modulator are ensured by setting the thickness T in the range of 0.1 μm to 200 μm. 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, in the case where the thickness of the dielectric layer 16 is relatively large, a sheet-like member made of the material as described above is separately prepared and attached to the back surface 11A side of the substrate 11 or the like. 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 recess 18 formed in the support substrate 17 can be set in a hollow state, or can be filled 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 recess 18 is held in a hollow state, the recess 18 is filled with air. Therefore, the dielectric constant in the internal space of the recess 18 is about 1, and a sufficiently low dielectric constant can be achieved.

  On the other hand, in the case where a low dielectric constant material is embedded in the recess 18, the dielectric constant of the low dielectric constant material is generally larger than the dielectric constant (1) of air, so that the interior of the recess 18 is held in a cavity. Compared with the above, the effect of lowering the dielectric constant is reduced. 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 constant material include glass, epoxy resin, and polyimide.

  The optical waveguide 12 can be formed 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 by using a plating method, a vapor deposition method, or the like from materials such as gold, silver, and copper. 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 produced. An X-cut lithium niobate single crystal (FIG. 2, X direction LN dielectric constant ε _LNx = 28, Y direction LN dielectric constant ε _LNy = 43) is used as the substrate 11, and the optical waveguide 12 is formed by a titanium diffusion method. By using both the plating method and the vapor deposition method, the signal electrode 13 and the ground electrode 14 were formed, and the light modulation element 15 was produced. 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 an epoxy resin is attached to the back surface 11B side of the substrate 11 to form the dielectric layer 16, and a region located in the modulation region of the light modulation element 15 A support substrate 17 made of lithium niobate having recesses 18 was formed on the substrate, and an optical modulator 10 was manufactured. The inside of the concave portion 18 was held 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 recess 18 is 500 μm, and the depth D of the recess 18 is 500 μm. By doing so, speed matching and impedance matching could be realized.

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

  As can be seen from the examples and comparative examples, in the optical modulator of the present invention, it is understood that speed matching and impedance matching can be realized even when the thickness of the substrate 11 is sufficiently increased. That is, according to the present invention, it is understood that speed matching and impedance matching can be realized while ensuring the strength and stability of the optical modulator more sufficiently. Further, since the thickness of the substrate 11 is relatively large, the processing becomes easy, and the intended 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 with specific examples. However, the present invention is not limited to the above-described contents, and various modifications can be made without departing from 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. Further, elements such as Mg, Zn, Sc, and In can be added to the substrate 11 in order to improve the light damage resistance. Further, a buffer layer can be provided between the substrate 11 and the signal electrode 13 and the ground electrode 14.

It is a top view which shows an example of the optical modulator of this invention. It is sectional drawing cut along the AA line of the optical modulator shown in FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Optical modulator, 11 Substrate, 12 Optical waveguide, 13 Signal electrode, 14 Ground electrode, 15 Optical modulation element, 16 Dielectric layer, 17 Support substrate, 18 Recess

Claims (9)

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 in the optical waveguide provided on the main surface of the substrate is modulated. A light modulation element having a modulation electrode for applying a signal;
A dielectric layer provided so that a main surface is in contact with the back surface side of the substrate of the light modulation element;
A support substrate provided on the back side of the dielectric layer and having a recess at a position corresponding to at least the modulation region of the light modulation element;
When the dielectric constant of the support substrate is εr and the dielectric constant in the recess is εs,
εr> εs
While satisfying the relationship
The optical modulator, wherein the dielectric layer is made of a material having a dielectric constant smaller than that of the material constituting the electro-optic substrate .   The optical modulator according to claim 1, wherein the dielectric layer has a sheet shape.   The optical modulator according to claim 1, wherein the concave portion is a cavity. When the dielectric constant of the electro-optic substrate is εLN and the dielectric constant of the dielectric layer is ε1, the dielectric constant ε2
εLN>ε1> ε2
The optical modulator according to claim 1, wherein a low dielectric constant material satisfying the following relationship is embedded in the recess . The optical modulator according to claim 1, wherein the electro-optic substrate is made of lithium niobate . The optical modulator according to claim 1, wherein a thickness of at least a part of the modulation section of the electro-optic substrate or the modulation section is 30 μm or less . A substrate made of a material having an electro-optic effect and a dielectric constant of εLN, an optical waveguide provided on the main surface side of the substrate, and the optical waveguide provided on the main surface of the substrate Preparing a light modulation element having a modulation electrode for applying a modulation signal to the guided light wave;
Providing a dielectric layer composed of a material having a smaller dielectric constant than the material constituting the substrate so that the main surface of the light modulation element is in contact with the back surface side of the substrate;
On the back side of the dielectric layer, there is a recess having a dielectric constant of εs in the internal space at a position corresponding to at least the modulation region of the light modulation element, the dielectric constant is εr, and εr> εs. Providing a support substrate that satisfies the relationship;
A method of manufacturing an optical modulator, comprising: The method of manufacturing an optical modulator according to claim 7, wherein the recess is a cavity . When the dielectric constant of the dielectric layer is ε1, it has a dielectric constant ε2.
εLN>ε1> ε2
The method of manufacturing an optical modulator according to claim 8, wherein a low dielectric constant material satisfying the following relationship is embedded in the recess .

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