JP2010085789A - Optical waveguide element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical waveguide element in which impedance matching is performed between a light wave and a microwave while performing speed matching, even when the thickness of a substrate having an electro-optical effect is 10 μm or smaller, further a driving voltage Vπ is reduced. <P>SOLUTION: The optical waveguide element has: a substrate 1 composed of a material having the electro-optical effect and the thickness of 10 μm or smaller; an optical waveguide 2 formed on the substrate; and a control electrode for modulating and controlling the light wave propagating in the optical waveguide, wherein the control electrode includes a signal electrode 3 and a grounding electrode 4, the gap between the signal electrode and the grounding electrode is ≥15 μm and ≤25 μm, and the thickness of the control electrode is ≥10 μm and ≤40 μm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device having a substrate thickness of 10 μm or less.

Conventionally, in the optical communication field and the optical measurement field, an optical waveguide element in which an optical waveguide is formed on a substrate having an electro-optic effect, such as an optical switch or an optical modulator, is often used as a means for controlling an optical wave.
In order to increase the processing speed of the optical waveguide device, the optical waveguide device is used to perform speed matching between the light wave propagating through the optical waveguide and the electric signal for controlling the light wave, and to secure a power source capable of high-speed driving. Therefore, it is required to reduce the driving voltage of the above.

Patent Document 1 discloses that the back surface of a substrate on which an electrode is formed is locally thin to improve the driving speed. Specifically, by setting the substrate thickness of the electrode portion to 5 to 50 μm and the other substrate portions to 150 to 1000 μm, the 3 dB bandwidth can be set to 15 GHz · cm or more.
Japanese Patent Laid-Open No. 10-133159

  However, Patent Document 1 discloses a substrate processing method, but hardly describes the configuration of electrodes. Further, in the method of processing an LN substrate using an excimer laser as in Patent Document 1, it is possible to process an arbitrary place. However, since processing is performed for each chip, the processing shape (width and depth) varies. In addition, the substrate is easily cracked due to processing distortion.

On the other hand, in Patent Document 2, with respect to an optical waveguide element having a cross-sectional shape as shown in FIG. 1, when the thickness of the substrate is 10 μm or more, the speed matching condition can be achieved while simultaneously reducing the drive voltage Vπ. It is disclosed.
JP 2001-66561 A

According to Patent Document 2, when an optical waveguide 2 is formed on a substrate 1 having an electro-optic effect, and a signal electrode 3 and a ground electrode 4 for controlling a light wave propagating through the optical waveguide 2 are arranged as shown in FIG. Discloses that the thickness d of the substrate is 10 μm or more, the thickness T of the electrode film is 20 μm or more, and the gap G between the signal electrode 3 and the ground electrode 4 is 25 μm or more. The width W of the signal electrode 3 is not particularly limited, but is assumed to be 10 μm.
Patent Document 2 also intends to avoid the occurrence of cracks in the substrate and the problem that the optical waveguide mode becomes flat when the thickness d of the substrate is less than 10 μm.

  When a substrate having a form as shown in FIG. 1 is used, the effective refractive index nmw of the microwave is lowered by leaching a part of the electric field in the recess 5 (air layer). And the part which acts on the production | generation of the electric field of each electrode film 3 and 4 exists on the thin part with a small thickness of a board | substrate, ie, the recessed part 5. FIG. However, if the thickness T of the electrode films 3 and 4 is increased as in the configuration described in Patent Document 2, the electric field moves upward, and the electric field does not easily leak into the air layer (recessed portion 5). It was found that the rate hardly changed and the drive voltage Vπ did not depend on the thickness T of the electrode films 3 and 6.

  Further, in Patent Document 2, by setting the gap G between the signal electrode 3 and the ground electrode 4 to 25 μm or more, the effective refractive index nmw of the microwave is remarkably lowered. Further, as shown in FIG. The drive voltage is also reduced by not providing a buffer layer between the electrode layer 3 and the electrode layer 3.

However, as in Patent Document 2, setting the gap G between the signal electrode 3 and the ground electrode 4 to 25 μm or more causes the drive voltage Vπ to increase remarkably and realizes ultrahigh speed drive exceeding several tens of GHz. It becomes a big obstacle in doing.

On the other hand, as shown in Patent Document 3, it has been proposed to increase the mechanical strength of the optical waveguide element by providing the adhesive layer 6 and the reinforcing plate 7 even when the thickness d of the substrate is 30 μm or less. .
By using a low dielectric constant layer as the adhesive layer, the speed matching between the light wave and the microwave is realized by taking advantage of the thin thickness of the substrate.
JP 2003-215519 A

  In order to perform speed matching, the thickness e of the adhesive layer is 10 μm or more, more preferably 30 μm or more. However, if the adhesive layer is thickened, a stress due to a difference in thermal expansion coefficient between the adhesive layer 6 and the substrate 1 is generated, and there is a risk of damaging the substrate 1. For this reason, the thickness e of the adhesive layer 6 needs to be 100 μm or less.

  As described above, it is not easy to achieve both speed matching and impedance matching between the light wave and the microwave, which are indispensable for an optical waveguide device capable of high-speed driving, and further reduction in driving voltage.

  The problem to be solved by the present invention is to solve the above-mentioned problems, and achieve impedance matching while realizing speed matching between light waves and microwaves even when the thickness of the substrate having the electro-optic effect is 10 μm or less. And providing an optical waveguide device capable of further reducing the drive voltage Vπ.

  In order to solve the above problems, the invention according to claim 1 is made of a material having an electro-optic effect, has a thickness of 10 μm or less, an optical waveguide formed on the substrate, and propagates through the optical waveguide. In an optical waveguide device having a control electrode for modulating and controlling a light wave, the control electrode is composed of a signal electrode and a ground electrode, and a gap between the signal electrode and the ground electrode is 15 μm or more and less than 25 μm. The thickness of the control electrode is 10 μm or more and 40 μm or less. In particular, even when the gap between the signal electrode and the ground electrode is less than 25 μm, by setting the thickness of the control electrode to 10 μm or more and 40 μm or less, the impedance in the optical waveguide element is set to around 50Ω, at least within the range of 30Ω to 70Ω. The present inventors have found that this is possible, and have completed the present invention.

  The invention according to claim 2 is the optical waveguide device according to claim 1, wherein a reinforcing plate is bonded to the back surface of the substrate via an adhesive layer.

  The invention according to claim 3 is the optical waveguide element according to claim 2, wherein the dielectric constant of the adhesive layer is 10 or less, and the thickness of the adhesive layer is 20 μm or more.

  According to the first aspect of the present invention, a substrate made of a material having an electro-optic effect and having a thickness of 10 μm or less, an optical waveguide formed on the substrate, and a light wave propagating through the optical waveguide are modulated and controlled. In an optical waveguide device having a control electrode, the control electrode is composed of a signal electrode and a ground electrode, and a gap between the signal electrode and the ground electrode is 15 μm or more and less than 25 μm, and the thickness of the control electrode Since it is 10 μm or more and 40 μm or less, speed matching between the light wave and the microwave can be realized, and the drive voltage Vπ can be reduced. In particular, by setting the gap between the signal electrode and the ground electrode to be less than 25 μm, not only a reduction in driving voltage due to a decrease in the distance between the signal electrode and the ground electrode, but also a microwave electrode due to microwave impedance matching. The drive voltage can be reduced by increasing the application efficiency to the same time, and more effective drive voltage reduction can be realized.

  Further, although the impedance increases when the thickness of the substrate is reduced, the gap between the signal electrode and the ground electrode is not less than 15 μm and less than 25 μm. Therefore, even when the thickness of the substrate is 10 μm or less, The impedance can be appropriately maintained in the range of 30Ω to 70Ω. Furthermore, the reduction in impedance due to the increase in the thickness of the control electrode is suppressed by setting the thickness of the control electrode to 40 μm or less. Note that increasing the surface area of the electrode film also contributes to reducing the electrode loss of the microwave, so it is necessary to consider that the thickness of the control electrode is not too thin, such as 10 μm or more.

  According to the invention of claim 2, since the reinforcing plate is bonded to the back surface of the substrate via the adhesive layer, it is possible to provide an optical waveguide element having high mechanical strength even if the thickness of the substrate is 10 μm or less. It becomes possible.

  According to the invention of claim 3, since the dielectric constant of the adhesive layer is 10 or less and the thickness of the adhesive layer is 20 μm or more, the dielectric constant of the reinforcing plate can be felt and the impedance is lowered. When the thickness of the adhesive layer is 20 μm or more, the impedance change with respect to the thickness is reduced, and the manufacturing variation is reduced.

Hereinafter, the present invention will be described in detail using preferred examples.
The configuration of the optical waveguide device of the present invention is basically the same as the configuration shown in FIG.
The optical waveguide element of the present invention is made of a material having an electro-optic effect, and modulates and controls a substrate 1 having a thickness of 10 μm or less, an optical waveguide 2 formed on the substrate, and a light wave propagating through the optical waveguide. In the optical waveguide device having the control electrode for the control electrode, the control electrode is composed of the signal electrode 3 and the ground electrode 4, and the gap between the signal electrode and the ground electrode is 15 μm or more and less than 25 μm, The thickness of the control electrode is 10 μm or more and 40 μm or less.

  As the substrate 1 having an electro-optic effect, for example, lithium niobate, lithium tantalate, and combinations thereof can be used. In particular, lithium niobate (LN) or lithium tantalate (LT) crystals having a high electro-optic effect are preferably used.

The optical waveguide can be formed by doping a high refractive index material such as Ti into the substrate from the substrate surface by a thermal diffusion method or a proton exchange method.
Note that the high refractive index material in the present invention means not only a material having a higher refractive index than the material constituting the substrate, but also by doping a specific material into the substrate so that the refractive index of the doped region is other than that. In the case where the refractive index is higher than that of the region, the specific material is also included in the high refractive index material.

As shown in FIG. 2, the control electrode is disposed so as to sandwich the optical waveguide 2 in order to control the modulation of the light wave propagating through the optical waveguide 2.
The control electrode can be formed on the front surface or the back surface of the substrate 1 by forming a Ti / Au electrode pattern, a gold plating method, or the like. The control electrode includes a signal electrode that propagates a modulation signal (microwave) and a ground electrode that is disposed around the signal electrode.

In the optical waveguide device of the present invention, a buffer layer such as SiO ₂ is not formed between the substrate 1 and the control electrode. The presence of the buffer layer effectively prevents the light wave propagating through the optical waveguide from being absorbed or scattered by the control electrode, and also guides the microwave applied from the control electrode and the inside of the optical waveguide. It also contributes to velocity matching with light waves. However, in the present invention, there is concern about an increase in driving voltage due to the presence of the buffer layer, absorption or scattering by the electrode is avoided by the arrangement relationship between the optical waveguide and the electrode, and the substrate is thinned for speed matching. It supplements with.

  In the present invention, since the substrate 1 formed of a material having an electro-optic effect is a thin plate having a thickness of 10 μm or less, the mechanical strength is insufficient. In order to compensate for this, a reinforcing plate 7 is bonded to the substrate 1 via an adhesive layer 6.

  As the material used for the reinforcing plate 7, various materials can be used. For example, in addition to using the same material as the thin plate, a material having a lower dielectric constant than the thin plate such as quartz, glass, and alumina is used. It is also possible to use a material having a crystal orientation different from that of the thin plate. However, it is preferable to select a material having a linear expansion coefficient equivalent to that of the thin plate in order to stabilize the modulation characteristics of the optical modulator with respect to temperature changes. If it is difficult to select an equivalent material, it is preferable to select a material having a linear expansion coefficient equivalent to that of the thin plate as an adhesive for joining the thin plate and the reinforcing plate as in Patent Document 2.

  For bonding the substrate (thin plate) 1 and the reinforcing plate 7, as an adhesive layer 6, an epoxy adhesive, a thermosetting adhesive, an ultraviolet curable adhesive, solder glass, thermosetting, photocuring or photosensitizing It is possible to use various adhesive materials such as a viscous resin adhesive sheet. In particular, it is necessary to select a material having a lower dielectric constant than that of the substrate 1 for the adhesive layer 6.

  In order to realize speed matching between the light wave and the microwave, the dielectric constant of the adhesive layer is preferably 10 or less, and the thickness of the adhesive layer is preferably 20 μm or more.

  As a method for manufacturing a thin plate constituting an optical modulator, there is a method in which the above-described optical waveguide or modulation electrode is formed on a substrate having a thickness of several hundreds μm, the back surface of the substrate is polished, and finished to a thickness of 10 μm or less, for example. . The optical waveguide and the modulation electrode can be made after the thin plate is made, but the thermal impact during the formation of the optical waveguide and the mechanical impact due to the handling of the thin film during various treatments are added. Therefore, it is preferable to polish the back surface of the substrate after forming the optical waveguide and the modulation electrode.

In order to reduce the driving voltage even when the substrate thickness is 10 μm or less as in the optical waveguide device of the present invention, the conditions necessary for impedance matching were simulated using the optical waveguide device shown in FIG. Impedance, considering the connection with 50Ω of external circuits, the reflection attenuation coefficient _{S 11} is to usable range 30~70Ω reduced below -10 dB.

The substrate was an X-cut lithium niobate substrate, and the thickness d of the substrate was changed in the range of 4 to 20 μm.
In addition, the following conditions were set as numerical values related to the control electrode constituted by the signal electrode 3 and the ground electrode 4.
(1) Width W of signal electrode: 5 to 30 μm
(2) Gap G between signal electrode and ground electrode; 5 to 50 μm
(3) Thickness T of control electrode (signal electrode and ground electrode): 10 to 50 μm

In order to evaluate the influence of each parameter on the impedance, the relationship between the main parameter and the impedance change is shown in FIGS.
FIG. 4 shows the effect of changes in the substrate thickness d on impedance changes. FIG. 5 shows the influence of a change in the gap (electrode spacing) G on the impedance change. Further, FIGS. 6 to 8 show the influence of the change in the thickness T of the control electrode on the impedance change.

  Referring to FIG. 4, when the thickness d of the substrate is 10 μm or less, the impedance can be adjusted to a numerical value range (30 to 70Ω) including 50Ω even when the gap G between the signal electrode and the ground electrode is 25 μm or less. Is easily understood.

  Further, as shown in FIG. 5, as the numerical value of the gap G becomes smaller, the impedance tends to decrease as a whole. Therefore, the gap between the signal electrode and the ground electrode may be in the range of 15 μm or more and less than 25 μm. preferable. However, in order to increase the impedance, it is possible to make the width W of the signal electrode as small as 5 μm and the thickness T of the electrode as low as 10 μm (see FIGS. 6 to 8). It is not preferable to reduce the surface area of the signal electrode because it causes an increase in microwave electrode loss.

  6 to 8, the electrode thickness T is preferably 40 μm or less, and the impedance T tends to increase as the thickness T decreases.

  As described above, in the optical waveguide device according to the present invention, when a substrate having a thickness of 10 μm or less is used, the gap between the signal electrode constituting the control electrode and the ground electrode is 15 μm or more and less than 25 μm, It is easily understood that the impedance can be adjusted in the range of 30 to 70Ω by setting the thickness of the control electrode to 10 μm or more and 40 μm or less. More preferably, the substrate thickness d is 3 to 8 μm, the width W of the signal electrode is 5 to 30 μm, the gap G of the electrode is 15 to 25 μm, and the thickness T of the control electrode is 10 to 40 μm. It becomes possible to approach 50Ω.

  According to the present invention, even when the thickness of a substrate having an electro-optic effect is 10 μm or less, impedance matching is performed while speed matching between the light wave and the microwave is realized, and further, the drive voltage Vπ can be reduced. It is possible to provide a possible optical waveguide element.

It is explanatory drawing which shows each parameter of the optical waveguide element disclosed by patent document 1. FIG. It is explanatory drawing which shows each parameter of the optical waveguide element using a reinforcement board. It is explanatory drawing which shows the simulation model for evaluating each parameter of an optical waveguide element. It is a graph which shows the influence which the change of the thickness d of a board | substrate has on an impedance change. It is a graph which shows the influence which the change of gap (electrode space | interval) G has on an impedance change. It is a graph which shows the influence which the change of the thickness T of a control electrode has on the impedance change when the thickness d of a board | substrate shall be 10 micrometers. It is a graph which shows the influence which the change of the thickness T of a control electrode has on the impedance change when the thickness d of a board | substrate shall be 7 micrometers. It is a graph which shows the influence which the change of the thickness T of a control electrode has on the impedance change when the thickness d of a board | substrate shall be 5 micrometers.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Board | substrate 2 Optical waveguide 3 Signal electrode 4 Ground electrode 5 Recessed part (air layer)
6 Adhesive layer 7 Reinforcing plate

Claims (3)

A substrate made of a material having an electro-optic effect and having a thickness of 10 μm or less;
An optical waveguide formed on the substrate;
In an optical waveguide device having a control electrode for modulating and controlling a light wave propagating through the optical waveguide,
The control electrode is composed of a signal electrode and a ground electrode,
The gap between the signal electrode and the ground electrode is not less than 15 μm and less than 25 μm,
An optical waveguide element having a thickness of the control electrode of 10 μm or more and 40 μm or less.   The optical waveguide device according to claim 1, wherein a reinforcing plate is bonded to the back surface of the substrate via an adhesive layer.   3. The optical waveguide device according to claim 2, wherein the dielectric constant of the adhesive layer is 10 or less, and the thickness of the adhesive layer is 20 μm or more.

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