JP2000227581A - Waveguide type optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the new waveguide type optical modulator which can reduce the driving voltage of an attenuator part as a waveguide type optical modulator equipped with an optical modulation part and an attenuator part. SOLUTION: The optical modulation part 2 is constituted by forming a Mach- Zehnder type 1st optical waveguide 5 in a substrate 4 having electrooptic effect and forming a 1st signal electrode 6 and a 1st ground electrode 7 after forming a buffer layer 22 on the substrate 4. A 2nd optical waveguide 8 is formed in the substrate 4 so that it is connected in series with the 1st optical waveguide 5, and the 2nd signal electrode 9 and 2nd ground electrode 10 are formed without forming a buffer layer to constitute the attenuator part 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a waveguide type optical modulator, and more particularly,
The present invention relates to a waveguide type optical modulator having an attenuator, which can be suitably used for a wavelength division multiplexing transmission optical network system or the like.

TECHNICAL FIELD OF THE INVENTION

[0001]

2. Description of the Related Art With the development of optical communication systems in recent years, there has been a demand for the development of communication systems having not only high speed and large capacity but also high multifunctionality. In particular, functions such as wavelength multiplexing within the same optical transmission line, switching and switching of optical transmission lines, and the like are required. This communication system employs an erbium doped fiber amplifier (hereinafter abbreviated as E).
Wavelength multiplexing transmission (wave
length division multiplexing: hereinafter, may be abbreviated as WDM system).

In the WDM system, different signals are respectively put on light waves from a plurality of light sources having different wavelengths and transmitted by a single optical fiber. That is, an arbitrary signal modulated by an optical modulator connected to each of the plurality of light sources is transmitted through a single optical fiber. The EDFA is provided for amplifying a signal in the transmission path. Thus, the transmission capacity of the entire system can be increased without increasing the number of optical fibers and the bit rate of each signal.

In the WDM system, it is required that the transmission state at each wavelength be constant. However, E
Since the gain of the DFA has wavelength dependence and the output of each light source is not constant but changes with time, there has been a problem that the light receiving level at the receiving end differs for each wavelength. In view of such a problem, an attempt has been made to connect an attenuator to the optical modulator. FIG.
FIG. 2 is a top view showing an example of a conventional waveguide type optical modulator to which an attenuator is connected, and FIG. 2 is a sectional view of the optical modulator shown in FIG. FIG. 2A is a cross-sectional view of the light modulation section taken along the line AA ′ in FIG.
(B) is a cross-sectional view of the attenuator section taken along the line BB 'in FIG.

In the conventional waveguide type optical modulator 1 shown in FIGS. 1 and 2, Mach-Zehnder type first optical waveguides 5 and 5 are formed by thermally diffusing titanium onto the surface of a substrate 4 having an electro-optic effect. The second optical waveguide 8 is formed so as to be connected in series. Then, a buffer layer 14 made of silicon dioxide or the like is formed on the substrate 4. On the buffer layer 14, a modulation electrode composed of the first signal electrode 6 and the first ground electrode 7 is formed.
The attenuator electrode including the signal electrode 9 and the second ground electrode 10 is formed. One ends of the first signal electrode 6 and the second signal electrode 9 are connected to external power supplies 12 and 13, and the other end of the first signal electrode 6 is connected to a resistor R and a capacitor C.
Grounded. Further, a polarizer 11 is provided on the incident side and the exit side of the optical modulator 1. The arrows in the figure indicate the traveling direction of the light wave.

[0005] The first optical waveguide 5, the first signal electrode 6 and the first ground electrode 7 constitute an optical modulator 2, and the second optical waveguide 8, the second signal electrode 9, and the second Constitutes the attenuator section 3. The buffer layer 14 is formed to prevent light waves guided through the optical waveguide from being absorbed by the modulation electrode and the attenuator electrode.

[0006] When a lightwave of wavelength λ1 on which a predetermined signal is superimposed is incident on the waveguide type optical modulator 1, it is turned on / off in an optical modulator 2 and then reaches an attenuator 3 where it undergoes intensity modulation. That is, the signal intensity of each wavelength in the entire communication system is made constant by forcibly attenuating the signal of the specific wavelength having a high output value by the attenuator.

[0007]

However, in the waveguide type optical modulator 1 as shown in FIG. 1, it is necessary to increase the size of the optical modulator 2 in order to enable high-speed optical modulation. However, if the light modulator 2 occupies most of the substrate 4 having a predetermined size, the attenuator 3 cannot be made sufficiently large. Therefore, the attenuator unit 3
It has become necessary to make the drive voltage applied to the horn very high. When the driving voltage is increased, discharge occurs in the attenuator unit 3, and the waveguide type optical modulator 1 itself may be damaged. Therefore, there is a problem that the reliability of such an optical modulator cannot be sufficiently obtained. A higher driving voltage may cause a relatively large DC drift due to the buffer layer 14, and has a practical problem.

An object of the present invention is to provide a waveguide type optical modulator having an optical modulator and an attenuator, which can reduce the driving voltage of the attenuator. Aim.

[0009]

According to the present invention, there is provided an optical modulator having a Mach-Zehnder type first optical waveguide formed on a substrate having an electro-optical effect and a modulation electrode, and an optical modulator formed on the substrate. A waveguide-type optical modulator comprising: a branch-type second optical waveguide connected in series with the first optical waveguide; and an attenuator unit having an attenuator electrode.
A buffer layer was formed on the substrate of the attenuator unit and the substrate of the light modulation unit, and the thickness of the buffer layer formed on the substrate of the attenuator unit was formed on the substrate of the light modulation unit. A waveguide type optical modulator characterized in that the thickness is smaller than the thickness of the buffer layer.

As a result of intensive studies to reduce the drive voltage of the attenuator, the inventors have found the following facts.
FIG. 3 is a graph showing the relationship between the thickness T of the buffer layer in the attenuator section discovered by the present inventors and the half-wavelength voltage Vπ as the driving voltage. As is clear from this graph, when the thickness T of the buffer layer is reduced,
Surprisingly, it can be seen that the value of the half-wave voltage Vπ decreases almost uniformly and linearly without saturation. Further, it was found that even when the buffer layer was not formed in the attenuator portion, the absorption of the light wave by the attenuator electrode was extremely small. The present invention has been made based on the above findings.

According to the present invention, since the thickness of the buffer layer in the attenuator is made smaller than the thickness of the buffer in the optical modulator, the driving voltage of the attenuator in the waveguide type optical modulator can be reduced. As a result, discharge of the attenuator section as described above can be prevented. Further, since the buffer layer of the attenuator section is not formed, DC drift caused by the buffer layer can be suppressed. As a result, it is possible to provide a waveguide type optical modulator satisfying practicality and reliability.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments of the present invention. FIG. 4 is a sectional view showing an example of the waveguide type optical modulator of the present invention. FIGS. 4A and 4B show an optical modulator and an attenuator in a waveguide type optical modulator corresponding to FIGS. 2A and 2B, respectively. The configuration of the conventional waveguide optical modulator 1 shown in FIGS. 1 and 2 and the waveguide optical modulator 21 of the present invention shown in FIG.
The difference from the above configuration is whether or not a buffer layer is formed in the attenuator section. Therefore, the following description will be made with reference to FIGS.

As described above, FIG. 4 shows a waveguide type optical modulator in which a buffer layer is not formed in an attenuator as a preferred embodiment of the waveguide type optical modulator of the present invention. As a result, the drive voltage of the attenuator section can be further reduced, and DC drift due to the buffer layer can be almost prevented. However, the formation of the buffer layer in the attenuator section is not necessarily prohibited. However, in this case, it is necessary that the thickness of the buffer layer of the attenuator section is smaller than the thickness of the buffer layer of the light modulation section according to the present invention. Specifically, when the thickness of the buffer layer in the light modulation section is 1, the thickness of the buffer layer in the attenuator section is set to preferably 0.5 or less, more preferably 0.3 or less.

The waveguide type optical modulator of the present invention is manufactured as follows. A waveguide pattern is formed by a photoresist or the like on a substrate 4 having an electro-optical effect such as lithium niobate (LiNbO ₃ ). Next, titanium (Ti) is deposited on this waveguide pattern by a vapor deposition method or the like.
Etc. are deposited. Next, the first optical waveguide 5 and the second optical waveguide 8 are formed so as to be connected in series by thermally diffusing the titanium into the substrate 4. Subsequently, a uniform buffer layer 22 made of silicon dioxide (SiO ₂ ) is formed on the substrate 4. Thereafter, a portion corresponding to the attenuator unit 3 is removed by dry etching or the like. Next, a first signal electrode 6 and a first ground electrode 7 made of gold (Au) or the like, and a second signal electrode 9 and a second ground electrode 10 are formed. In addition to the above silicon dioxide, alumina (Al ₂ O ₃ )
Such a material can be used.

When the waveguide type optical modulator of the present invention is used, optical modulation is performed as follows. The lightwave of wavelength λ1 on which the predetermined signal is superimposed enters the waveguide type optical modulator 1, passes through the polarizer 11, and is turned on / off in the optical modulation unit 2. That is, when the signal superimposed on the lightwave of wavelength λ1 is turned off and is not taken out as a signal of the communication system, the lightwave is branched in the first optical waveguide 5 of the Mach-Zehnder type, By the electrode 6 and the first ground electrode 7, only the phase of the light wave passing through one of the optical waveguides is shifted by a half wavelength.
Then, when the light waves that have passed through each of the branched first optical waveguides 5 are combined, they are canceled out because their phases are shifted by half a wavelength.

On the other hand, when the signal is turned on and is taken out as a signal of the communication system, the light wave is passed through the first optical waveguide 5 without any modulation.
In this case, the lightwave having the wavelength λ1 directly reaches the attenuator section 3 and is branched in the second optical waveguide 8. Then, a modulation signal is applied between the second signal electrode 9 and the second ground electrode 10 for the light wave passing through one optical waveguide, and the phase of the light wave passing through the one optical waveguide is changed. Shift by an arbitrary amount. As a result, when the lightwaves that have passed through each of the branched second optical waveguides 8 are combined, the phases of the lightwaves are shifted by an arbitrary amount, so that the intensity of the lightwave of the wavelength λ1 is attenuated by the corresponding interference effect. Will be done. After passing through the polarizer 11, the intensity-modulated light wave is received as a predetermined signal of the communication system.
In the communication system, such a waveguide type optical modulator is provided for each light wave having a different wavelength so that the signal intensity of the entire communication system is made constant.

As described above, in the waveguide type optical modulator according to the present invention, both the optical modulator 2 and the attenuator 3 modulate the intensity of the light wave. Therefore, it is necessary that the optical waveguides constituting these are of a branch type. FIGS. 1 and 4 exemplify a Mach-Zehnder first optical waveguide 5 and a second optical waveguide 8 as preferred embodiments of such a branched optical waveguide. However, the optical waveguide forming the attenuator section 3 can use a directional coupler type instead of the Mach-Zehnder type.

In FIGS. 1 and 4, the polarizer 11
Are provided on both the entrance side and the exit side of the waveguide type optical modulator 21. However, in the waveguide type optical modulator of the present invention, a polarizer is not always required.
Therefore, the polarizer can be provided only on either the entrance side or the exit side of the waveguide type optical modulator. Further, even when no polarizer is provided, the object of the present invention can be sufficiently achieved.

[0019]

The present invention will be specifically described below with reference to examples. Example In this example, the waveguide type optical modulator 21 having the configuration shown in FIGS. 1 and 4 was manufactured by the method described in the above-mentioned "Embodiment of the invention". As the substrate 4, an X-cut plate of lithium niobate was used. The first optical waveguide 5 and the second optical waveguide 8 were formed by thermally diffusing titanium. The first signal electrode 6 and the first ground electrode 7, and the second signal electrode 9 and the second ground electrode 10 were formed by performing plating after depositing gold.
The electrode length L of the first signal electrode 6 and the electrode length 1 of the second signal electrode 9 were set to 4 cm and 2 cm, respectively. Further, the buffer layer 22 of the light modulation section 2 was formed to have a thickness of 1.1 μm using silicon dioxide.

In the waveguide type optical modulator 21 thus formed, the second signal electrode 9 of the attenuator 3 is
A drive voltage was examined by applying a voltage from the external power supply 13. As a result, the half-wave voltage Vπ was 5.1 V. Further, a high-temperature screening test at 80 ° C. is performed on the waveguide type optical modulator 21 to change the DC drift voltage with time.
I checked the drift. FIG. 5 shows the obtained results. As is clear from FIG. 5, the waveguide type optical modulator of the present invention shows almost no change over time in the half-wavelength voltage Vπ and hardly any DC drift.

COMPARATIVE EXAMPLE In this comparative example, a waveguide type optical modulator 1 as shown in FIGS. 1 and 2 was manufactured by the method described in "Embodiments of the Invention". However, the buffer layer 14 made of silicon dioxide was formed on the substrate 4 to a uniform thickness of 1.1 μm without performing dry etching. The optical waveguide, the signal electrode, and the like were formed in the same manner as in the example.

The drive voltage was examined by applying a voltage from the external power supply 13 to the second signal electrode 9 of the attenuator section 3 in the waveguide type optical modulator 1 thus formed. As a result, the half-wave voltage Vπ was 11.4 V. Further, a screening test at a high temperature of 80 ° C. is performed on the waveguide type optical modulator 1 to change the DC drift voltage with time, that is, D D
The C drift was examined. FIG. 6 shows the obtained results. As can be seen from FIG. 6, in the conventional waveguide type optical modulator, the DC drift voltage increases with time and DC drift occurs.

As is clear from the examples and comparative examples, according to the waveguide type optical modulator of the present invention, it is possible to reduce the driving voltage of the attenuator and to prevent the occurrence of DC drift. Can also.

As described above, the present invention has been described in the embodiments of the present invention with reference to specific examples. However, the present invention is not limited to the above-described contents, and any modifications may be made without departing from the scope of the present invention. And deformations are possible.

[0025]

As described above, in the waveguide type optical modulator according to the present invention, the buffer layer is formed only in the optical modulation section, and the buffer layer is not formed in the attenuator section. Therefore, the driving voltage of the attenuator can be reduced without impairing the function of the buffer layer of preventing absorption of light waves by the electrodes. Further, since no buffer layer is formed in the attenuator portion, it is possible to prevent DC drift from occurring in this portion.

[Brief description of the drawings]

FIG. 1 is a top view showing an example of the present invention and a conventional waveguide optical modulator.

FIG. 2 is a cross-sectional view illustrating an example of a conventional waveguide type optical modulator.

FIG. 3 is a graph showing a relationship between a thickness of a buffer layer and a driving voltage in an attenuator section.

FIG. 4 is a cross-sectional view illustrating an example of a waveguide type optical modulator according to the present invention.

FIG. 5 is a graph showing a change over time of a DC drift voltage in the waveguide type optical modulator of the present invention.

FIG. 6 is a graph showing a change over time of a DC drift voltage in a conventional waveguide optical modulator.

[Explanation of symbols]

 1, 21 waveguide optical modulator 2 optical modulator 3 attenuator 4 substrate 5 first optical waveguide 6 first signal electrode 7 first ground electrode 8 second optical waveguide 9 second signal electrode 10 2 ground electrode 11 polarizer 12, 13 external electrode 14, 22 buffer layer

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Yoshihiro Hashimoto 585 Tomimachi, Funabashi-shi, Chiba F-term in the New Technology Research Laboratories, Sumitomo Osaka Cement Co., Ltd. 2H079 AA02 AA12 BA01 CA04 DA03 EA05 EB05 GA01 HA12 HA23 2K002 AA02 AB04 AB09 BA06 CA03 DA08 EB01 EB09 GA03 HA03

Claims (4)

[Claims] An optical modulator having a Mach-Zehnder type first optical waveguide formed on a substrate having an electro-optic effect and a modulation electrode; and a first optical waveguide formed on the substrate and having a first optical waveguide. What is claimed is: 1. A waveguide type optical modulator comprising: a branch-type second optical waveguide connected in series; and an attenuator unit having an attenuator electrode. A buffer layer formed on the substrate of the attenuator section, wherein a thickness of the buffer layer formed on the substrate of the light modulation section is smaller than a thickness of the buffer layer formed on the substrate of the light modulation section. Waveguide optical modulator. 2. The waveguide type optical modulator according to claim 1, wherein a buffer layer is not formed on the substrate of the attenuator section. 3. The optical waveguide according to claim 1, wherein said second optical waveguide is a Mach-Zehnder optical waveguide.
3. The waveguide type optical modulator according to 1. 4. The waveguide type optical modulator according to claim 1, wherein the second optical waveguide is a directional coupler type optical waveguide.