JPH09269468A - Optical waveguide device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide device which is less affected by surface charges accompanying a temp. change, is decreased in temp. drift and is highly reliable. SOLUTION: This device consists of a pyroelectric substrate 1 formed with optical waveguides 2 on its front layer, a dielectric layer 3 formed on one main surface of this pyroelectric substrate 1 and control electrodes 4 formed in one region on this dielectric substrate 3. In such a case, the dielectric layer 3 is formed to the thickness in the regions where the control electrodes 4 are not formed larger than the thickness of the regions where the control electrodes 4 are formed. The regions of the dielectric layer 3 where the control electrodes 4 are not formed are formed of plural layers.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide device such as an optical modulator or an optical switch used in an optical fiber communication system, an optical fiber sensor, an optical measuring instrument or the like.
More specifically, the present invention relates to an optical waveguide device that suppresses characteristic fluctuations due to temperature fluctuations.

[0002]

2. Description of the Related Art Conventionally, in an optical waveguide device using a ferroelectric substrate, it is known that various characteristics fluctuate due to ambient temperature changes because the substrate has pyroelectricity. As an example thereof, an optical modulator J1 shown in FIG. 7 using a pyroelectric substrate made of lithium niobate (LiNbO ₃ ) single crystal or the like will be described. The optical modulator J1 has a Y-branch type optical waveguide 2 formed on the surface layer of the substrate 1.
A buffer layer 3 made of SiO ₂ or the like is formed on one main surface of the substrate 1, and the optical waveguide 2 is provided so as to be substantially along the optical waveguide 2.
A control electrode 4 for applying an electric field is formed on.

Next, a method of manufacturing the optical modulator J1 will be described with reference to FIG. 8 which is a sectional view taken along the line AA in FIG. A metal such as titanium is patterned on a substrate 1 having pyroelectricity, such as a Z-cut lithium niobate single crystal having a large electro-optical effect, and a metal such as titanium is patterned, and thermal diffusion is performed at about 1050 ° C. to have a higher refractive index than the substrate 1. The optical waveguide 2 is formed.

In order to prevent electric field absorption by the control electrode 4, SiO is formed on the surface of the substrate 1 including the optical waveguide 2.
A buffer layer 3 such as ₂ is formed, and finally a control electrode 4 is formed.

Here, the substrate 1 is pyroelectric and has spontaneous polarization, but when the ambient temperature is constant, the surface of the substrate 1 is neutralized by electric charges in the air or external electric charges supplied through electrodes or the like. It is harmless and apparently not charged.

However, when the ambient temperature of the substrate 1 changes, the magnitude of the spontaneous polarization of the substrate 1 changes with the temperature change. At this time, in the region where the control electrode 4 is formed, the charge can be easily moved through the control electrode 4 through the buffer layer 3 that is dielectrically polarized, so that the charge is supplied according to the change in the magnitude of the spontaneous polarization. .

On the other hand, in the surface region 3a of the buffer layer 3 in which the control electrode 4 is not formed, abrupt charge movement is not possible, so that the charge is apparently positive (when the temperature rises) or negative (when the temperature falls). . The unnecessary electric field generated by this charge causes a temperature drift that changes the operating point of the optical modulator using the lithium niobate substrate.

Conventionally, the following method has been used as a method for preventing the characteristic variation of the optical waveguide device such as the optical modulator due to the temperature variation. That is,
A semiconductor film 5 of Si or the like is formed on the buffer layer 3 formed on the substrate 1 as in an optical modulator J2 shown in a sectional view of FIG. 9 at a position similar to that of FIG. By making the generated surface charges uniform on the surface of the substrate 1 (buffer layer 3), the temperature drift is reduced (see, for example, Japanese Patent Laid-Open No. 62-173428). Note that in FIG. 9, the description of the same configuration as that in FIG. 8 is omitted.

As another method, a method has been proposed in which the crystal axes of the substrate in the region where the control electrode is not formed are alternately inverted at regular intervals so that the charges generated by the pyroelectric effect cancel each other ( For example, JP-A-6-751
96).

[0010]

However, as shown in FIG. 9, in the method of forming the semiconductor film 5 in at least the region on the buffer layer 3 excluding the control electrode 4, the semiconductor film 5 is formed.
As a result, the resistance between the electrodes of the control electrode 4 is reduced to some extent, so that DC driving cannot be performed. As a result, there are problems that the modulation cannot be performed at a low speed, and the static characteristics (driving voltage, extinction ratio) cannot be measured in the inspection after manufacturing. Furthermore, since the resistivity of the semiconductor film 5 is controlled by high temperature annealing after film formation, there are problems that the reproducibility is poor and the process is complicated.

Further, in the method in which the crystal axes of the substrate 1 are alternately inverted at a constant interval, direct current drive is possible, but inversion of the crystal axes in a minute region requires expensive equipment such as an electron beam. There was still a problem with productivity.

Therefore, the present invention solves the above-mentioned problems and provides an optical waveguide device which is capable of reducing the influence of surface charges due to temperature changes and which has reduced temperature drift and is highly reliable. With the goal.

[0013]

In order to achieve the above object, an optical waveguide device of the present invention comprises a pyroelectric substrate having an optical waveguide formed on its surface layer, and a pyroelectric substrate formed on one main surface thereof. And a control electrode formed in one region on the dielectric layer, wherein the dielectric layer has a thickness in a region where the control electrode is not formed. It is characterized in that it is thicker than the region where the control electrode is formed. Further, it is preferable that the dielectric layer is thin at least in a region where the control electrode and the optical waveguide face each other, and thick in a region near the optical waveguide where the control electrode is not formed. Further, it is characterized in that the region of the dielectric layer in which the control electrode is not formed is a plurality of layers.

According to the optical waveguide device having the above structure, the region of the dielectric layer where the control electrode is not formed is made thicker than the region of the dielectric layer where the control electrode is formed, so that the control electrode is formed. It is possible to reduce the influence of the surface charge that accompanies the temperature change in the unheated region, and it is possible to reduce the temperature drift.

If the region of the dielectric layer where the control electrode is not formed is a single layer, there is no electric field absorption (transparent to the wavelength used) and insulation (especially between the electrodes). However, for high speed operation (higher frequency), it is necessary to select a material for the dielectric layer that satisfies the following three requirements: the relative permittivity ε is small, but the dielectric without the control electrode is formed. If the layer region is a plurality of layers, the condition of, is relaxed and the degree of freedom in design is increased.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. The same components as those in FIG. 7 are designated by the same reference numerals, and the description will be omitted or simplified. Further, description will be given based on FIG. 1C which is a cross-sectional view of an optical modulator having a configuration similar to that of the optical modulator shown in FIG.

FIG. 1 (c) is a part of FIG.
An optical modulator H1, which is a cross-sectional view taken along the line A, shows a substrate 1 made of, for example, lithium niobate, lithium tantalate, lithium tetraborate, or the like having an optical waveguide 2 formed on its surface layer. And the optical waveguide 2 on the substrate 1
A dielectric layer (also referred to as a buffer layer) 3 made of SiO ₂ , Al ₂ O ₃ or the like formed in a region including a layer, and titanium, platinum, gold, nickel, copper, formed in one region on the dielectric layer 3. The control electrode 4 is made of chromium or the like.

Here, when the interelectrode distance of the control electrode 4 is D, the dielectric layer 3 has at least a region along the optical waveguide 2 (in the vicinity where the optical waveguide 2 is formed). The thickness of the region not formed is made thicker than the thickness of the region where the control electrode 4 is formed. More preferably, the thickness of the region where the control electrode 4 is not formed is 1 μm.
It was found that good characteristics can be obtained when the thickness is set to m or more and the thickness is in the range of D / 4 to D / 2 and is larger than the thickness of the region where the control electrode 4 is formed.

By configuring the optical modulator as described above, it is possible to minimize the influence of the surface charge accompanying the temperature change in the region where the control electrode is not formed, and to reduce the temperature drift. .

[0020]

EXAMPLE A more specific example will be described below with reference to a sectional view taken at the same position as in FIG.

[Embodiment 1] First, Embodiment 1 will be described with reference to FIGS. A metal such as titanium is patterned on a Z-cut lithium niobate substrate 1 having a large electro-optic effect, and thermal diffusion is performed at about 1050 ° C. to form an optical waveguide 2 having a higher refractive index than the substrate 1.

Next, as shown in FIG. 1A, SiO ₂
The thickness is increased to about 5 .mu.m in the step of forming the buffer layer 3 which is a dielectric layer made of, for example. Next, FIG.
As shown in (b), a region for forming a control electrode 4 described later is formed by wet or dry etching.
The surface of the buffer layer 3 is removed by etching and its thickness is reduced to 1
Make it about μm.

Thereafter, as shown in FIG. 1 (c), a control electrode 4 made of a metal film such as gold is patterned into a predetermined shape in the above-mentioned etching region to form a thickness of about 5 to 10 μm. Finally, the optical modulator H1 is completed.

At this time, by selecting a mask material capable of sharing the etching mask of the buffer layer 3 with the patterning mask of the control electrode 4, the patterning step does not increase, so that the manufacturing of this structure is realized simply and quickly. it can.

Next, with respect to the manufactured optical modulator H1, the characteristic change due to temperature fluctuation was measured. As a result, the temperature drift could be reduced, and the characteristic was better than that of a conventional buffer layer formed uniformly. Was greatly improved.

[Second Embodiment] Next, a second embodiment, which is another embodiment, will be described with reference to FIGS.
As shown in FIG. 2A, the buffer layer 3 which is a dielectric layer
The process up to the formation of is similar to that of FIG. As shown in FIG. 2B, a control electrode 4 described later
2 is patterned between the regions for forming the buffer layer 3 and the same material as the buffer layer 3 or a dielectric layer having a higher resistivity, for example, a resistance layer 6 such as silicon nitride or silicon carbide.
The control electrode 4 was formed as shown in FIG. Here, the buffer layer 3 is formed of SiO ₂ , and high temperature annealing is performed at about 700 ° C. for 20 hours to increase the resistivity.

When the buffer layer 3 is thickened as in the first embodiment, the annealing time becomes long and the productivity is lowered. However, in the second embodiment, there is no such problem and the degree of freedom in material combination is high (necessary). Since the degree of freedom in selecting materials for satisfying various conditions is widened), it is very advantageous for improving the DC drift characteristic.

The optical modulator H2 produced in this example
Regarding the above, when the change in the characteristics due to the temperature change was measured, the characteristics were significantly improved as compared with the case where the buffer layer was uniformly formed as in Example 1 as described later.

[Third Embodiment] Next, still another embodiment will be described with reference to FIGS. FIG.
As shown in FIG. 3A, in this embodiment, after forming the waveguide 2, the resistance layer 6, which is a high-resistivity dielectric layer, is patterned while the control electrode 4 is formed, and then, as shown in FIG. As shown in FIG. 3, the buffer layer 3 which is a dielectric layer is formed, and then the control electrode 4 is formed as shown in FIG. 3C, and finally the optical modulator H3 is completed.

Also in this embodiment, the degree of freedom in selecting the material of the dielectric layer forming the buffer layer is high, which is advantageous for improving the DC drift characteristic. Also in the third embodiment,
When the change in the characteristics due to the temperature change was measured, the characteristics were significantly improved as compared with the case where the buffer layer was uniformly formed as in Example 1 as described later.

[Embodiment 4] Next, still another embodiment will be described. In Examples 1 to 3 described above, the method is to change only the thickness of the dielectric layer, and the surface is not removed. Therefore, the influence of surface charge due to temperature change should be completely eliminated. I couldn't.

Therefore, as shown in FIG. 4 (a) or FIG. 4 (b), if the semiconductor film 5 is partially divided between the control electrodes 4, direct current driving can be performed and the area where surface charges exist becomes small. . Further, in FIG. 4A, the place where the charge is generated is farthest from the waveguide, so that the influence of the surface charge is small. FIG. 4
In (b), the electric field is absorbed in the vicinity of the electrodes, and the influence of surface charges is small. In these examples as well, the characteristics were significantly improved as compared with the case where the buffer layer was uniformly formed as in Example 1, and the temperature drift could be made almost zero as described later.

Next, the results of measuring the temperature drift of the optical modulators of the above embodiments and the conventional example (FIG. 8) will be described. Temperature drift ΔD [%] (ΔD is defined by ΔD = ΔV × 100 / Vπ, as shown in FIG. 6, where ΔV is a drift voltage and Vπ is a half-wave voltage.)
The changes with time in temperature change are as shown in FIGS. 5 (a) and 5 (b). Here, FIG. 5B shows the relationship between the time and the temperature change. Before 0 hours on the horizontal axis, each optical modulator is put in an environment of an ambient temperature of 50 ° C. for a sufficient time. After the lapse of time, the temperature drift was measured when the sample was then put into an environment with an ambient temperature of 60 ° C.

As is apparent from FIG. 5, it has been found that the temperature drift is significantly improved in any of the above-mentioned examples as compared with the conventional example. In particular, in each of the examples, the temperature drift became almost zero after about 10 minutes had passed since the substrate was placed in an environment where the ambient temperature changes.

Although the optical modulator has been described as an example in the present embodiment, the present invention is not limited to this, and can be suitably applied to an optical waveguide device such as an optical switch.

[0036]

As described above, according to the optical waveguide device of the present invention, it is possible to drive the direct current, so that the influence of the surface charge can be prevented as much as possible, and the temperature drift can be reduced. It is possible to provide a highly reliable optical waveguide device.

Further, since it is extremely excellent in productivity, it is possible to inexpensively and rapidly supply a large amount of optical waveguide devices having excellent characteristics.

In particular, by using a plurality of dielectric layers, the degree of freedom in design for increasing the frequency and reducing the long-term DC drift is increased, and the characteristics can be improved by various methods. It will be possible.

[Brief description of drawings]

1A to 1C are cross-sectional views showing a manufacturing process of an example (Example 1) of an optical waveguide device according to the present invention.

2A to 2C are cross-sectional views showing a manufacturing process of an example (Example 2) of the optical waveguide device according to the present invention.

3A to 3C are cross-sectional views showing a manufacturing process of an example (Example 3) of the optical waveguide device according to the present invention.

4A and 4B are cross-sectional views showing an example (Example 4) of an optical waveguide device according to the present invention.

5A is a diagram illustrating a temperature change of an ambient temperature of the optical waveguide device, and FIG. 5B is a diagram illustrating a relationship between a temperature change of the optical waveguide device and a temperature drift.

FIG. 6 is a diagram illustrating the definition of temperature drift.

FIG. 7 is a plan view illustrating a conventional example and an optical modulator according to the present invention.

FIG. 8 is a sectional view illustrating a conventional optical modulator.

FIG. 9 is a sectional view illustrating another conventional optical modulator.

[Explanation of symbols]

 1 ・ ・ ・ Pyroelectric substrate 2 ・ ・ ・ Optical waveguide 3 ・ ・ ・ Buffer layer 4 ・ ・ ・ Control electrode 5 ・ ・ ・ Semiconductor film 6 ・ ・ ・ Resistive layer H1, H2, H3, H4 ・ ・ ・ Optical modulation Device (present invention) J1, J2 ... Optical modulator (conventional example)

Claims (2)

[Claims] 1. A pyroelectric substrate having an optical waveguide formed on a surface layer thereof, a dielectric layer formed in a region including the optical waveguide of the pyroelectric substrate, and formed on one region of the dielectric layer. And a control electrode, wherein the dielectric layer is thicker in a region where the control electrode is not formed than in a region where the control electrode is formed. Waveguide device. 2. The optical waveguide device according to claim 1, wherein the region of the dielectric layer where the control electrode is not formed is a plurality of layers.