JP2001004967A - Optical waveguide element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide element having high reliability and high stability without the occurrence of an operating point shift. SOLUTION: This element 30 has a substrate 21 consisting of material having an electro-optic effect, a Mach-Zehnder type waveguide 27 formed on the main surface of this substrate 21, a signal electrode 24 and grounding electrodes 25-1 and 25-2 for controlling the light waves guided in this optical waveguide. The grounding electrode 25-2 is parted in a longitudinal direction of the Mach- Zehnder type waveguide 27 in such a manner the parting width d1 is equal to a gap g1 of the signal electrode 24 and the grounding electrode 25-1, by which the signal electrode 24 as well as the grounding electrodes 25-1 and 25-2 in the modulation region W1 of the Mach-Zehnder type waveguide 27 are made symmetrical with respect to the center of the branch optical waveguides 23-1 and 23-2.

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, and more particularly, to an optical waveguide device which can be suitably used for a high-speed optical modulator in a high-speed and large-capacity optical fiber communication system.

[0002]

2. Description of the Related Art 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 widely used. It has become to. Such an optical waveguide device generally has the following configuration. FIG. 1 is a sectional view schematically showing an example of an optical waveguide element used in a conventional high-speed optical modulator. FIG. 2 is a cross-sectional view schematically showing another example of the optical waveguide element used in the conventional high-speed optical modulator.

The optical waveguide device 10 shown in FIG. 1 has a substrate 1 made of a Z-cut plate made of a material having an electro-optic effect, and a buffer layer 2 formed thereon. further,
The substrate 1 has optical waveguides 3-1 and 3-2 formed by a titanium diffusion method or the like. This optical waveguide 3-1
And 3-2 form a pair, and constitute a Mach-Zehnder type optical waveguide.

On the buffer layer 2, a signal electrode 4 for applying a modulation signal to a light wave guided in the optical waveguide 3-1 and ground electrodes 5-1 and 5-2 are provided. I have. When a Z-cut plate is used for the substrate 1, the signal electrode 4 is formed so as to be located on the optical waveguide 3-1 as shown in FIG. The ground electrodes 5-1 and 5-2 serve as counter electrodes to the signal electrode 4, and the electrode impedance formed by them can be matched to the external impedance of 50Ω and the driving voltage can be reduced. Form as close as possible.

The optical waveguide device 20 shown in FIG. 2 has a substrate 11 made of an X-cut plate made of a material having an electro-optical effect, and a buffer layer 12 formed thereon. Further, similarly to FIG. 1, the substrate 11 has optical waveguides 13-1 and 13-2 that constitute a Mach-Zehnder type optical waveguide. Further, on the buffer layer 12, the signal electrode 14 and the ground electrodes 15-1 and 15-2 are provided in the same manner as described above. When an X-cut plate is used for the substrate 11, the signal electrode 14 is formed so as to be located on the side surface of the optical waveguide 13-1, so as to be a chirp type.
In addition, the ground electrodes 15-1 and 15-2 function as counter electrodes as described above.

The optical waveguide devices 10 and 2 shown in FIGS.
At 0, a predetermined half-wave voltage is applied to the signal electrodes 4 and 14, and the phase of the light wave guided in the optical waveguides 3-1 and 13-1 is changed to the phase in the optical waveguides 3-2 and 13-2. The optical signal is turned on / off by shifting the phase of the wave by π. The half-wave voltage is calculated in advance by setting a predetermined operating point on the light intensity modulation curve, and from the voltage at which the phase shifts by π based on the operating point.

[0007]

However, in the conventional optical waveguide elements as shown in FIGS. 1 and 2, when these elements are used for a long time as an optical modulator, the above operating point shifts. The problem had arisen. For this reason, the half-wavelength voltage applied to turn on / off the optical signal is deviated from the initially set value, and the optical signal cannot be turned on / off satisfactorily in some cases. Therefore, when these optical waveguide devices are used as an optical modulator for a high-speed, large-capacity optical fiber communication system, there are cases where the conditions of high reliability and high stability required for these systems cannot be satisfied. .

An object of the present invention is to provide a new optical waveguide device having high reliability without causing an operating point shift.

[0009]

An optical waveguide device according to the present invention comprises: a substrate made of a material having an electro-optic effect; a Mach-Zehnder type optical waveguide formed on a main surface of the substrate;
A modulation electrode for controlling a light wave guided through the optical waveguide. The modulation electrode serves as a signal electrode for modulating a light wave guided through one of the two branch optical waveguides constituting the Mach-Zehnder type optical waveguide, and has a role of a counter electrode with respect to the signal electrode. And a ground electrode. Further, a lower ground surface of the modulation electrode on the substrate side is substantially symmetric with respect to a center between the two branch optical waveguides in a modulation region of the Mach-Zehnder optical waveguide. And

The present inventors have conducted intensive studies in order to find out the cause of the operating point shift. Then, attention was paid to the fact that there are two types of DC drift and temperature drift as main factors that cause the shift of the operating point, and a detailed study was performed from these viewpoints. As a result, it has been found that the above-mentioned operating point shift occurs even when no DC bias is applied, and that the DC drift is not related to the above operating point shift. Then, the present inventors conducted further detailed studies from the viewpoint of temperature drift. Then, the optical waveguides 3-1 and 3-2 and the optical waveguide 13-1 in the optical waveguide devices 10 and 20 shown in FIGS.
And 13-2.

As a result, the inventors of the present invention have made the substrates 1 and 11
The operating point shift is significantly changed by changing the shape and size of the signal electrodes 4 and 14 and the ground electrodes 5-1 and 5-2, 15-1 and 15-2 formed thereon. Was found. That is, as the shape and size of the signal electrode and the ground electrode change, the stress applied to the substrate of these electrodes changes due to the temperature rise during the operation of the optical waveguide element. As a result, the refractive index around the optical waveguide changes, and this change in the refractive index also affects the optical waveguide, thereby causing the operating point shift as described above.

For example, in the case of the optical waveguide device 10 shown in FIG. 1, the signal electrode 4 and the ground electrodes 5-1 and 5
Since -2 differs in both shape and size, the stress exerted on the substrate 1 by the temperature rise also differs. Therefore, the change in the refractive index near the signal electrode 4 and the change in the refractive index near the ground electrodes 5-1 and 5-2 are different from each other.
The effect of the change in the refractive index on the lens 2 differs. Therefore, an operating point shift occurs in the optical waveguide element 10 due to a temperature drift.

The present invention has been made based on the results of many years of study as described above. According to the present invention, in an optical waveguide device having a Mach-Zehnder type optical waveguide, a contact surface of a modulation electrode composed of a signal electrode and a ground electrode with a substrate is formed by two of the Mach-Zehnder type optical waveguide. , For example, the optical waveguide 3- shown in FIG.
It is designed to be symmetrical about the center II between 1 and 3-2. Therefore, the stress applied to the substrate from the modulation electrode when the temperature rises becomes symmetrical with respect to the center of the two branch optical waveguides.
The influence of the change in the refractive index on each of the branch optical waveguides is the same. As a result, it is possible to prevent an operating point shift due to a temperature drift in the entire optical waveguide element.

As described above, the optical waveguide device of the present invention has high reliability and high stability, and therefore can be suitably used as a high-speed optical modulator for a high-speed and large-capacity optical fiber communication system.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments of the present invention. FIG. 3 is a schematic plan view showing an example of a preferred embodiment of the optical waveguide device of the present invention. FIG. 4 is a cross-sectional view of the optical waveguide device shown in FIG. 3 taken along the plane AA. 3 and 4 and the figures shown below, in order to clearly explain the features of the present invention,
The size and scale of each part are drawn differently from the actual ones.

The optical waveguide element 30 shown in FIGS. 3 and 4 is a substrate 21 made of a Z-cut plate made of a material having an electro-optical effect.
And a buffer layer 22 formed on the substrate 21. The substrate 21 has optical waveguides 23-1 and 23-2 formed by a titanium diffusion method or the like. The optical waveguides 23-1 and 23-2 form a pair with each other and constitute a Mach-Zehnder type optical waveguide 27.

A signal electrode 24 formed on the buffer layer 22 so as to be located on the optical waveguide 23-1 and a ground electrode 25-1 acting as a counter electrode to the signal electricity 24. And 25-2. Further, the ground electrode 25-2 is divided in the modulation region of the Mach-Zehnder type optical waveguide 27 in the length direction of the Mach-Zehnder type optical waveguide 27.
The cut portion is formed such that its width d1 is equal to the gap g1 between the signal electrode 24 and the ground electrode 25-1. Then, a thin film 26 made of a conductive material such as Al is formed on the divided portion. In the figure, W1
The region indicated by is the modulation region of the Mach-Zehnder type optical waveguide 27.

As described above, in the optical waveguide device 30 shown in FIGS. 3 and 4, the ground electrode 25-2 is divided in the longitudinal direction of the Mach-Zehnder type optical waveguide 27. The width d1 of the divided portion is equal to the gap g1 between the signal electrode 24 and the ground electrode 25-1. For this reason, FIG.
As shown in FIG. 3, the signal electrode 24 and the ground electrodes 25-1 and 25-2
5-2 is the center II- of the optical waveguides 23-1 and 23-2 as two branch optical waveguides constituting the Mach-Ender optical waveguide 27 in the modulation region W1 of the Mach-Ender optical waveguide 27. It is symmetric with respect to II. Therefore, since the change in the refractive index caused by the stress on the substrate such as the signal electrode 24 due to the temperature rise becomes the same in the optical waveguides 23-1 and 23-2, the optical waveguide can be highly reliable without operating point shift. Element 3
0 can be used.

The optical waveguide device 30 shown in FIGS.
5, a thin film 26 made of a conductive material is formed at a portion where the ground electrode 25-2 is divided. This makes it possible to compensate for the deterioration of the conductivity of the ground electrode 25-2 caused by the division, and to suppress the deterioration of the high-frequency characteristics. However, even when the thin film 26 is not formed, the object of the present invention can be sufficiently achieved. Further, since the thin film 26 is thinner than the signal electrode 24 and the like and has sufficiently low rigidity, the purpose of the present invention is not hindered by forming the thin film 26.
Examples of the conductive material that forms the thin film 26 include Cu, Ni—Cr, Au, and Ti in addition to Al.

Although the buffer layer 22 is provided on the substrate 21 in FIGS. 3 and 4, it is not always necessary to provide the buffer layer in the present invention. However,
By forming the buffer layer, it is possible to reduce the propagation loss of the guided light as described above.

FIG. 5 is a schematic plan view showing another preferred embodiment of the optical waveguide device of the present invention. FIG. 6 is a cross-sectional view when the optical waveguide element shown in FIG. 5 is cut along the BB plane. The optical waveguide element 40 shown in FIGS. 5 and 6 has a substrate 31 made of an X-cut plate made of a material having an electro-optic effect.
And a buffer layer 32 formed on the substrate 31. Then, similarly to the above, the optical waveguides 33-constituting the Mach-Zehnder type optical waveguide 37 are provided in the substrate 31.
1 and 33-2.

A signal electrode 34 formed on the buffer layer 32 so as to be located on the side of the optical waveguide 33-1 and a ground electrode 35 acting as a counter electrode to the signal electrode 34. -1 and 35-2. Further, the ground electrode 35-2 is divided in the length direction of the Mach-Zehnder optical waveguide 37 in the modulation region of the Mach-Zehnder optical waveguide 37. At this time, the width d2 of the divided portion has the signal electrode 34.
And the ground electrode 35-1 is formed to be equal to the gap g2. Then, a thin film 36 made of a conductive material is formed on the divided portion in the same manner as described above.
Note that a region indicated by W2 in the drawing indicates a modulation region of the Mach-Zehnder type optical waveguide 37.

The optical waveguide device 40 shown in FIGS. 5 and 6 differs from the optical waveguide device 30 shown in FIGS. 3 and 4 in that the cut surface of the material constituting the substrate and the arrangement of the signal electrodes and the like are slightly different. are doing.

Also in the optical waveguide device 40 shown in FIGS. 5 and 6, the ground electrode 35-2 is divided in the longitudinal direction of the Mach-Zehnder type optical waveguide 37, and its width d2 is determined by the signal electrode 34 and the ground electrode 35-1. Is formed to be equal to the gap g2 between Therefore, as shown in FIG. 6, in the modulation region W2 of the Mach-Zehnder type optical waveguide 37, the signal electrode 34 and the ground electrodes 35-1 and 35-2.
Are the two components constituting the Mach-Zehnder type optical waveguide 37.
Optical waveguides 33-1 and 33-2, which are two branch optical waveguides
Is symmetrical with respect to the center III-III. For this reason, the change in the refractive index caused by the stress on the substrate such as the signal electrode 34 due to the temperature rise becomes the same in the optical waveguides 33-1 and 33-2. A waveguide element 40 can be used.

Although the same effect as described above can be obtained by providing the thin film 36, it is not an essential element in the present invention. Also, regarding the thin film 36, the thin film 26
And the same conductive material. Further, even when the thin film 36 is provided as described above, the object of the present invention is not hindered. Also, as described above, the buffer layer 32 is not an essential element in the present invention,
Even when the buffer layer 32 is not provided, the object of the present invention can be sufficiently achieved.

In FIGS. 3 to 6, in the modulation region of the Mach-Zehnder optical waveguide, the modulation electrode including the signal electrode and the ground electrode is provided between the two branch optical waveguides constituting the Mach-Zehnder optical waveguide. Has been described as being symmetrical about the center. However, the modulation electrode itself, that is, the shape and size of the modulation electrode do not need to be symmetrical with respect to the center between the branch optical waveguides, and are in contact with the surface of the modulation electrode that is in contact with the substrate or the buffer layer of the modulation electrode. The plane only has to be symmetric with respect to the center.

[0027]

The present invention will be specifically described below with reference to examples. Example 1 In this example, an optical waveguide device 30 as shown in FIGS. 3 and 4 was manufactured. Using a Z-cut plate of lithium niobate single crystal as the substrate 21, a Mach-Zehnder type optical waveguide pattern was formed on the substrate by using a photoresist. Next, titanium was deposited on this pattern by an evaporation method. Then, the entire substrate is 950 to 1050
By heating at 10 ° C. for 10 to 20 hours, the titanium was diffused into the inside of the substrate 21 to produce the Mach-Zehnder optical waveguide 27. Next, a buffer layer 22 made of silicon oxide was formed on the substrate 21 to a thickness of 0.5 μm.

Thereafter, a signal electrode 24 made of gold (Au) and a ground electrode 25-are formed by using both a vapor deposition method and a plating method using a mask having an opening formed at a portion where a divided portion of the ground electrode is to be formed. 1 and 25-2 to a thickness of 15
It was formed to a thickness of μm. Next, a thin film 26 made of Al was formed to a thickness of 2000 ° on the divided portion by an evaporation method. Note that the width d1 of the divided portion is equal to the width of the signal electrode 24.
25 μm equal to the gap g1 between the ground electrode 25-1
Formed. Thereafter, an optical fiber was connected to the input / output port of the Mach-Zehnder type optical waveguide 27 of the optical waveguide device 30 manufactured in this way, and the operating point shift when the temperature was changed to 0 to 70 ° C. was examined. As a result, the operating point shift was 0.3V.

Example 2 In this example, an optical waveguide device 40 as shown in FIGS. 5 and 6 was manufactured. An optical waveguide element 40 was produced in the same manner as in Example 1, except that an X-cut plate of lithium niobate single crystal was used as the substrate 31. This optical waveguide element 4
As a result of examining the operating point shift of 0 in the same manner as in Example 1,
0.2 V.

Comparative Example 1 In this comparative example, an optical waveguide device 1 as shown in FIG.
0 was produced. The fabrication of the optical waveguide device 10 was performed in the same manner as in Example 1 except that the ground electrode was cut off to form no thin film made of a conductive material. The operating point shift of the obtained optical waveguide device 10 was examined in the same manner as in Example 1. As a result, the operating point shift was 1.8 V.

Comparative Example 2 In this comparative example, an optical waveguide device 2 as shown in FIG.
0 was produced. The fabrication of the optical waveguide device 20 was performed in the same manner as in Example 2 except that the ground electrode was not divided to form a thin film made of a conductive material. Further, the operating point shift of the obtained optical waveguide device 20 was examined in the same manner as in Example 1. As a result, the operating point shift was 1.0 V.

As is clear from the above Examples and Comparative Examples, the optical waveguide device obtained according to the present invention has a small operating point shift, and the stress exerted on the substrate by the modulation electrode due to a rise in temperature is substantially constant. It can be seen that the change in the refractive index is almost constant. Therefore, it is understood that the optical waveguide device of the present invention has high reliability and stability.

As described above, the present invention has been described in detail based on the embodiments of the present invention with specific examples, but the present invention is not limited to the above-described contents, and may be any form within the scope of the present invention. Deformation and modification are possible.

[0034]

As described above, the optical waveguide element of the present invention hardly causes an operating point shift due to a temperature drift even when used for a long time. Therefore, it can be suitably used for a high-speed and large-capacity optical fiber communication system requiring long-term reliability and stability.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a conventional optical waveguide device.

FIG. 2 is a cross-sectional view showing another example of a conventional optical waveguide device.

FIG. 3 is a schematic plan view showing an example of the optical waveguide device of the present invention.

4 is a cross-sectional view of the optical waveguide device shown in FIG. 3 taken along line AA.

FIG. 5 is a schematic plan view showing another example of the optical waveguide device of the present invention.

6 is a cross-sectional view of the optical waveguide device shown in FIG. 5 taken along line BB.

[Explanation of symbols]

1, 11, 21, 31 Substrate 2, 12, 22, 32 Buffer layer 3-1, 3-2, 13-1, 13-2, 23-1, 23
-2, 33-1, 33-2 (branch) optical waveguides 4, 14, 24, 34 Signal electrodes 5-1, 5-2, 15-1, 15-2, 25-1, 25
-2, 35-1, 35-2 Ground electrode 10, 20, 30, 40 Optical waveguide element 26, 36 Thin film

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yoshihiro Hashimoto 585 Tomicho, Funabashi-shi, Chiba F-term in the New Technology Research Laboratories, Sumitomo Osaka Cement Co., Ltd. 2H047 KA04 LA12 NA02 QA03 RA08 2H079 AA02 AA12 BA01 BA03 CA05 DA03 EA05 EB05 EB12 HA23

Claims (4)

[Claims] 1. A substrate made of a material having an electro-optical effect, a Mach-Zehnder optical waveguide formed on a main surface of the substrate, and a modulation electrode for controlling a light wave guided in the optical waveguide. The modulation electrode is a signal electrode for modulating a light wave guided through one of the two branch optical waveguides constituting the Mach-Zehnder type optical waveguide, and substantially with respect to the signal electrode. An optical waveguide element comprising a ground electrode having a role of a counter electrode, wherein a lower surface of the modulation electrode on the substrate side is provided between the two branch optical waveguides in a modulation region of the Mach-Zehnder optical waveguide. An optical waveguide element, which is substantially symmetrical with respect to the center of the optical waveguide. 2. The Mach-Zehnder optical waveguide according to claim 1, wherein the ground electrode is divided in a length direction of the Mach-Zehnder optical waveguide in a modulation region of the Mach-Zehnder optical waveguide. Optical waveguide device. 3. The thin film made of a conductive material is formed on a portion of the ground electrode which is divided in a length direction of the Mach-Zehnder optical waveguide. Optical waveguide device. 4. The modulation electrode is substantially symmetrical in a modulation region of the Mach-Zehnder type optical waveguide with respect to a center between two branch optical waveguides constituting the Mach-Zehnder type optical waveguide. The optical waveguide device according to any one of claims 1 to 3, wherein the optical waveguide device is formed.