JP3570735B2 - Optical waveguide device - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to an optical waveguide device used for an ultra-high-speed external modulator, switch, and electric field sensor for optical communication. In particular, the present invention relates to a structure of an optical waveguide device capable of suppressing chirping and facilitating a low driving voltage.
[0002]
[Prior art]
Generally, in a high-speed optical communication system, for example, in a system up to a frequency of 1.6 GHz, a method of directly modulating a laser diode is used. Then, as the modulation frequency becomes higher, a chirping phenomenon in which the wavelength of the output light of the laser diode slightly fluctuates with time becomes remarkable. This has been the limit of long-distance communication due to the dispersion characteristics of optical fibers. On the other hand, an external modulation method in which a laser diode is set to a constant output light and an external modulator is installed can substantially reduce chirping, and is considered to be suitable for ultra-high-speed long-distance optical communication. Experiments have begun. An external modulator in which a Mach-Zehnder waveguide is formed on a LiNbO ₃ substrate is a typical example.
[0003]
However, even if the external modulator is essentially low in chirping, there is slight chirping, and an external modulator with low chirping is desired for use in an ultra-high-speed long-distance communication system. At the same time, an external modulator having a lower driving voltage is desired to convert an ultra-high-speed electric signal into an optical signal.
[0004]
As a high-speed light modulation method, as described above, an external modulation method for modulating laser light externally is known in addition to the direct modulation of the laser diode. In particular, a Mach-Zehnder optical modulator in which Ti is thermally diffused into a substrate having an electro-optical effect, for example, LiNbO ₃ to form a branch optical waveguide, is well known as an external modulator. FIG. 1 shows an example of a Mach-Zehnder type optical modulator. The optical waveguide 4 branches once into the branch waveguides 2 and 3, and a signal electrode 5 and a ground electrode 6 are provided on each waveguide. When modulation is performed at an ultra-high speed, the electrode is formed thick by plating or the like and is treated as a traveling wave electrode. A buffer layer such as SiO ₂ is provided between the electrode and the waveguide in order to suppress absorption of the waveguide by the electrode. Light waves that have propagated through the branch waveguides 2 and 3 are combined and coupled to the optical waveguide 4. The phase of the light wave propagating in each of the branch waveguides 2 and 3 is changed by the microwave electric field (see FIG. 2) applied to the signal electrode 5, and the light wave propagated in the optical waveguide is multiplexed in the waveguide 4. By doing so, intensity modulation can be performed.
[0005]
Usually, an electric signal applied to this type of modulator uses a microwave band high frequency signal, and in order to obtain electrical stability at such a high frequency, a ground electrode 6 is provided as shown in the figure. It is sufficiently larger than the signal electrode 5. The width of the signal electrode 5 is made substantially equal to the width of the waveguide in order to efficiently interact with the electric field distribution of the light wave propagating in the waveguide and the lines of electric force of the electric signal.
In this type of modulator, as described above and as shown in FIGS. Is asymmetrical with respect to the branch waveguides 2 and 3 as indicated by 7 in FIG. As described above, when the electric lines of force applied to the respective branch waveguides are asymmetric, the modulation efficiencies in the respective branch waveguides 2 and 3 are not equal, and chirping due to the asymmetry of the modulation efficiency occurs. Further, since the electric lines of force below the ground electrode are wider than the electric field distribution of the guided light, the modulation efficiency is reduced and the driving voltage of the modulator is increased.
[0006]
Therefore, as shown in FIG. 5, there has been proposed a structure in which a signal electrode 5 is provided directly above each of the branching waveguides 2 and 3, and a ground electrode 6 is provided outside each of the branch waveguides 2 and 3 (Japanese Patent Laid-Open No. Hei. 196212). With such a structure, chirping caused by the asymmetry of the modulation efficiency is reduced, and the interaction between the guided light and the electric flux lines applied to the optical waveguide is improved, so that the driving voltage of the modulator can be reduced. Can be.
However, when two or more signal electrodes are used in this way, it is necessary to control the microwave signal applied to the signal electrode for each optical waveguide to the opposite potential and to control the signal generation timing with extremely high precision. However, there is a problem that the driving system of the modulator becomes very complicated.
[0007]
Further, in the conventional electrode configuration as shown in FIG. 2, the electric flux lines 7 are asymmetric with respect to the branch waveguides 2 and 3, the modulation efficiency in each waveguide is different, and chirping becomes large. There was a problem. Further, there is a problem that the modulation efficiency in the waveguide below the ground electrode is low and the driving voltage is high.
[0008]
[Problems to be solved by the invention]
Therefore, the present invention suppresses the occurrence of chirping, particularly in a very high frequency band, in the conventional optical waveguide device as described above, and at the same time, can prevent a decrease in modulation efficiency and suppress a drive voltage. It is an object to provide an optical waveguide device. Another object of the present invention is to provide an optical waveguide device in which such suppression of chirping can be easily achieved without complicating the electrode structure and avoiding complication of the driving system.
[0009]
[Means for Solving the Problems]
Accordingly, the present invention has been made to solve the above-mentioned technical problems, and has been made on the substrate having the electro-optic effect, and has a branch waveguide formed thereon and a signal for controlling guided light. An optical waveguide device comprising an electrode and a ground electrode , wherein a center line between the signal electrode and the ground electrode and a center line of the branch waveguide are arranged so as to coincide with a center line of the substrate, and The branch waveguides are arranged symmetrically with respect to the center line, and the shape of the substrate is also symmetrical with respect to the center line, and an electric signal generated in the substrate by applying an electric signal to the signal electrode. The optical waveguide device according to claim 1, wherein a gap is provided between a part of the bottom surface of the ground electrode and a substantial part of the substrate so that a force line is generated symmetrically with respect to the center line .
The gap is preferably made of a derivative. Preferably, the gap is made of air.
[0010]
[Action]
The LiNbO ₃ optical waveguide device according to the present invention has a structure in which the ground electrode of the optical waveguide has a gap between a part of the bottom surface of the ground electrode and a substantial part of the substrate as shown in FIGS. Then, the electric field generated at the ground electrode concentrates on the electrode portion immediately above the waveguide because the dielectric constant of the gap portion below the ground electrode is much smaller than that of the substrate LiNbO ₃ . Accordingly, the lines of electric force in the substrate (1) are symmetric with respect to the branch waveguides (2, 3), the modulation efficiency in each waveguide is equal, and chirping based on the asymmetry of the modulation efficiency is eliminated. In addition, the size of the ground electrode is the same as that of the related art, and it can be used electrically stably even at a high frequency such as a microwave as in the related art.
[0011]
Further, in the past, the modulation efficiency under the ground electrode was low, but the electrode of the structure of the present invention allows the interaction between the electric flux lines and the light wave propagating through the optical waveguide to be efficient even under the ground electrode. And a modulator having a lower driving voltage can be realized. Further, in the optical waveguide device having the configuration of the present invention, it is not necessary to use two or more signal electrodes, and therefore, there is no problem such as a complicated driving system. As described above, with the optical waveguide device of the present invention, it is possible to provide an optical modulator that can lower the driving voltage and suppress chirping without complicating the driving system.
[0012]
Next, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.
[0013]
Embodiment 1
FIG. 3 is a cross-sectional view for explaining an optical waveguide device according to the present invention using a Mach-Zehnder type optical modulator as an example.
That is, as shown in FIG. 3A, a formed branch waveguide 2 is formed on a substrate having an electro-optical effect: LiNbO ₃ substrate 1, and a signal electrode 5 for controlling guided light is formed thereon. And a structure in which a gap 8 is provided between a part of the bottom surface of the ground electrode 6: a part opposite to the signal electrode 5 and a substantial part of the substrate. It is. That is, if the area or width of the portion of the ground electrode 6 which is in contact with the substrate is made substantially the same as the area or width of the signal electrode 5, the electric lines of force 7 are formed symmetrically as shown in FIG. 2, the modulation efficiency becomes equal, and chirping can be suppressed.
[0014]
FIG. 3b is not within the scope of the present invention because it is asymmetric with respect to the center line of the substrate.
[0015]
In this optical waveguide, metal Ti is vapor-deposited on a Z-cut LiNbO ₃ substrate 1 by a photo process in a pattern (2 ′, 3 ′) with a thickness of about 800 ° and a width of 7 μm (1 in FIG. 6), and after lift-off, The linear waveguide and the optical waveguides 2 and 3 are formed by thermal diffusion in air at about 1000 ° C. for 20 hours (2 in FIG. 6).
Thereafter, in order to prevent absorption loss of light by the electrodes, a SiO ₂ buffer layer 10 was formed in a thickness of 0.1 to 1.0 μm (3 in FIG. 6). Here, the part of the bottom portion of the ground electrode, to form a gap 8, the pattern 11 is formed as shown in 4 of FIG. 6 with a photoresist, 1 to 4 [mu] m deposited SiO ₂ from above the (12) (5 in FIG. 6). Thereafter, lift-off is performed to leave only the gap portion (12) of the ground electrode portion (6 in FIG. 6). Although not shown in the drawing, Ti and Au are vapor-deposited on the entire surface in order to form an electrode. The pattern of the electrode (13) was formed by a photo process (7 in FIG. 6). At this time, the width of the signal electrode 5 is about 7 μm, and the interval between the ground electrode 6 and the signal electrode 5 is 15 μm.
[0016]
Here, the thickness of the signal electrode 5 and the ground electrode 6 was formed to be at least 10 μm by a conventional electroplating method (8 in FIG. 6). By creating such an optical waveguide structure, a gap 8 (12) made of SiO ₂ can be formed under the ground electrode 6 to have a width of 1 to 4 μm. As described above, the optical waveguide device having the structure shown in FIG. 3A is manufactured.
[0017]
Embodiment 2
FIG. 4 is a sectional view showing another embodiment of the present invention. That is, an electrode having a structure in which MgO was vapor-deposited at 0.1 to 1 μm instead of SiO _{2 in} the production method as shown in FIG. 6 was produced. Then, as described above, after being manufactured in the steps described with reference to FIG. 6, MgO is removed by etching with acetic acid, and an optical waveguide device having an air gap (gap interval of 0.1 to 1 μm) as shown in FIG. Was prepared. That is, as shown in FIG. 4, a substrate having an electro-optic effect: a formed branch waveguide 2 is formed on a LiNbO ₃ substrate 1, and a signal electrode 5 for controlling guided light is formed thereon. The optical waveguide device having the ground electrode 6 formed thereon has a structure in which an air gap 8 is provided between a part of the bottom surface of the ground electrode 6: a part opposite to the signal electrode 5 and a substantial part of the substrate. is there.
[0018]
The optical waveguide device having the structure of the first and second embodiments is fixed to the case, the signal electrode and the ground electrode are respectively wired, and fibers for inputting and outputting light are attached to complete the optical waveguide module. it can. A DFB laser beam of, for example, 1.55 μm is connected to the incident fiber side of this module, and the output intensity of the laser beam is kept constant. Here, when a digital signal of, for example, a high frequency of 10 Gbit / sec is applied to the signal electrode, an electric field is generated in the LiNbO ₃ substrate 1 by the electric signal. The state of this electric field is shown in FIGS. Compared to the conventional FIGS. 2 and 5, the lines of electric force concentrate on the electrode directly above the waveguide under the ground electrode, and the distribution becomes symmetric with respect to the branch waveguides 2 and 3.
[0019]
For this reason, the modulation efficiency under the conventional ground electrode is reduced, but a modulator with a lower driving voltage can be realized. Further, it can be seen that chirp based on the asymmetry of the modulation efficiency in each branch waveguide is also improved by adopting this configuration. On the other hand, the ground electrode can be made sufficiently large as compared with the signal electrode as before, so that it can be used electrically stably even at high frequencies.
[0020]
In the above embodiments, the description has been made using the Mach-Zehnder type optical modulator, but it is apparent that the present invention can be similarly applied to an optical switch and the like. Further, it goes without saying that any material having an electro-optical effect such as LiTaO ₃ or PLZT can be used as the substrate material in addition to the LiNbO ₃ crystal. Further, the present invention can be used for all materials that can form an optical waveguide, such as PLZT. Further, the manufacturing process described in the embodiment is an example, and various combinations, such as materials to be used, configurations, and manufacturing processes, or different processes may be used as long as the objects of the present invention are satisfied.
[0021]
【The invention's effect】
As described above, the following remarkable technical effects were obtained by the structure of the optical waveguide device of the present invention.
First, it is possible to provide an optical waveguide device with a lower driving voltage and reduced chirping without complicating the driving system of the optical modulator.
Second, therefore, it is possible to realize ultra-high-speed long-distance communication using an optical fiber.
[Brief description of the drawings]
FIG. 1 is a plan view showing a structure of an example of a conventional Mach-Zehnder modulator.
FIG. 2 is a sectional view showing the structure of a conventional Mach-Zehnder modulator.
FIG. 3 is a cross-sectional view showing a specific example of the structure of the optical waveguide device of the present invention.
FIG. 4 is a sectional view showing another specific example of the structure of the optical waveguide device of the present invention.
FIG. 5 is a sectional view showing an example of a conventional Mach-Zehnder modulator.
FIG. 6 is a cross-sectional view showing a manufacturing process of the optical waveguide device of the present invention in the order of each process.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 LiNbO ₃ board | substrate 2, 3 Branch optical waveguide 5 Control electrode 6 Ground electrode 7 Electric field line 8 Gap

Claims (3)

An optical waveguide device comprising a signal electrode and a ground electrode for controlling branched light and guided light formed on a substrate having an electro-optic effect,
The center line between the signal electrode and the ground electrode and the center line of the branch waveguide are arranged so as to coincide with the center line of the substrate, and the branch waveguide is symmetric with respect to the center line. The shape of the substrate is also symmetrical with respect to the center line, and lines of electric force in the substrate generated by applying an electric signal to the signal electrode are symmetrical with respect to the center line. The optical waveguide device according to claim 1, wherein a gap is provided between a part of the bottom surface of the ground electrode and a substantial part of the substrate so as to generate a gap . The optical waveguide device according to claim 1, wherein the gap is made of a dielectric. The optical waveguide device according to claim 1, wherein the gap is made of air.

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