JP2003075791A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator in which the occurrence of a bias microjump phenomenon is prevented and the fluctuation in an instantaneous control voltage is made less. SOLUTION: The modulator is provided with a substrate 1 which is made of a material having an electrooptical effect, optical waveguides 2 formed on the substrate and electrodes 5 and 6 which are formed on the substrate to modulate the strength of the light beams that pass through a portion of the substrate and are applied with a DC voltage. The electrodes are constituted of a signal electrode 5 and ground electrodes 6. Electrifying preventing treatment 7 is provided in the vicinity of the electrodes 6 and in the edge portions of the substrate 1.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical modulator,
In particular, the present invention relates to an optical modulator to which a DC voltage is applied, which is used in a DC section of an AC / DC separation type intensity modulator, an attenuator section of a waveguide type intensity modulator, and the like.

[0002]

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 high speed, large capacity, and high multifunctionality. In particular, functions such as wavelength multiplexing in the same optical transmission line, switching and exchange of optical transmission lines are required. This communication system includes an optical fiber amplifier (erbium doped fiber amplifier).
Wavelength division transmission (wave length division) using a
This is being realized by the multiplexing (hereinafter referred to as WDM) system.

In this WDM system, different signals are put on light waves from a plurality of light sources having different wavelengths and the signals are 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 to amplify the signal in this transmission path. As a result, the transmission capacity of the entire system can be increased without increasing the number of optical fibers or the bit rate of each signal.

In the conventional modulator used in the WDM system, the bias T is used to apply the RF signal and the DC control voltage to one electrode, which has a problem of reducing the mounting space. . Therefore, in order to cope with this problem, an AC / DC separation type intensity modulator or the like which can eliminate the bias T and reduce the mounting space by providing a dedicated electrode for each of the RF signal and the DC control voltage is proposed. It is used. In the WDM system,
It is required that the transmission state at each wavelength be constant. However, since the gain of the EDFA has wavelength dependency and the output of each light source is not constant and changes with time, there is a problem that the light receiving level at the receiving end is different for each wavelength. In order to deal with such a problem, there has been attempted a method of flattening each signal output by integrating an attenuator after the optical modulator, as in a waveguide type intensity modulator.

[0005]

A DC voltage is usually applied to the electrodes constituting the DC portion of the AC / DC separation type intensity modulator and the attenuator portion of the waveguide type intensity modulator. For this reason, there is a case where the electric charge gradually accumulated in the edge portion of the substrate or the like jumps into the ground electrode at a certain moment (called a bias micro-jump phenomenon).

Due to the bias micro-jump phenomenon, A
In the C / DC separation type intensity modulator, as shown in FIG. 1, a solid line graph showing the relationship between the operating point voltage and the optical output becomes a broken line graph slightly shifted to the left, and as a result, on- The off optical output level changes as shown on the right side of FIG. 1, the on-off level difference is reduced in some cases, and it becomes difficult to distinguish on-off. Further, in the waveguide type intensity modulator, as shown in FIG. 10, when the bias micro-jump phenomenon occurs, the relationship between the operating point voltage and the output in the attenuator section shifts from the solid line graph to the broken line graph, As a result, even with the same control voltage, the output fluctuation (ΔP) occurs at high attenuation (control voltage V2).
2) becomes large.

The problem to be solved by the present invention is to provide an optical modulator which prevents the occurrence of the bias micro-jump phenomenon as described above and has a small instantaneous fluctuation of the control voltage.

[0008]

In order to solve the above problems, an optical modulator according to a first aspect of the present invention is a substrate made of a material having an electro-optical effect, and intensity modulation of light passing through a part of the substrate. An electrode formed on the substrate for the purpose of applying a DC voltage to the electrode, the electrode is composed of a signal electrode and a ground electrode, and is in the vicinity of the ground electrode. In addition, the edge portion of the substrate is characterized by being subjected to antistatic treatment. The DC voltage applied to the electrode includes a case where a small amplitude signal (for example, several MHz) is superimposed in order to confirm whether the control signal of the DC voltage is actually functioning.

An optical modulator according to a second aspect is the optical modulator according to the first aspect, wherein the antistatic treatment is a treatment for providing a conductive film electrically connected to the ground electrode. And

An optical modulator according to a third aspect is the optical modulator according to the first aspect, wherein the antistatic treatment is performed so that the minimum angle forming the edge portion of the substrate is always an obtuse angle. It is characterized in that it is a process of processing a portion.

An optical modulator according to a fourth aspect is the optical modulator according to any one of the first to third aspects, wherein the optical modulator constitutes a DC section of an AC / DC separation type intensity modulator. Is characterized by.

An optical modulator according to a fifth aspect is the optical modulator according to any one of the first to third aspects, wherein the optical modulator includes a waveguide-type intensity modulation including an optical modulator and an attenuator. It is characterized in that it constitutes an attenuator portion of the container.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to preferred examples. 2 is a plan view of a conventional AC / DC separation type intensity modulator, and FIG. 4 is an AC / DC to which the present invention is applied.
The top view of a DC isolation | separation type | mold intensity modulator is shown. In addition, FIG.
4 is a cross-sectional view taken along the line AA ′ of FIG. 2, and FIG.
3 is a sectional view taken along line AA ′ of FIG.

The substrate 1 used in the AC / DC separation type intensity modulator is made of a material having an electro-optic effect, such as lithium niobate (LiNbO ₃ ; hereinafter referred to as LN), lithium tantalate (LiTaO ₃ ), PLZT ( Lead lanthanum zirconate titanate), and a quartz-based material. Specifically, an X-cut plate of these single crystal materials,
It is composed of a Y-cut plate and a Z-cut plate. In particular, it is preferable to use a LiNbO ₃ crystal, a LiTaO ₃ crystal, or a solid solution crystal composed of LiNbO ₃ and LiTaO ₃ because it is easy to configure as an optical waveguide device and has large anisotropy. In this example, an example using an X-cut plate of lithium niobate (LN) will be mainly described.

The optical waveguide 2 can be formed by a thermal diffusion method, a proton exchange method or the like, but here, the optical waveguide 2 is formed by thermally diffusing Ti into the LN substrate 1. A
Electrodes such as the C-applying electrode 3, the DC-applying electrode 5, and the ground electrodes 4 and 6 may be directly formed on the substrate 1 on which the optical waveguide 2 is formed, or may be formed via a SiO ₂ buffer layer ( 2 and 4, the structure in which the buffer layer is omitted is shown in order to make the structure easy to understand). In this example,
A buffer layer is formed below the DC application electrode 5 by vapor deposition in consideration of DC drift, and a buffer layer is formed below the other electrodes by a sputtering method.

The AC / DC separation type intensity modulator is divided into an AC section and a DC section, and the light wave λ incident on the optical waveguide 2 is
It is turned on-off according to an electric signal applied to the AC applying electrode 3 of the AC section. Next, a control signal applied to the DC application electrode 5 of the DC section controls a phenomenon (DC drift phenomenon) in which the light output fluctuates due to the change over time of the driving point voltage. In the conventional AC / DC separation type intensity modulator, as shown in FIG.
As shown in FIG. 1, charges are likely to be accumulated in the edge portion of the substrate 1 in the vicinity of the ground electrode 6 in the DC portion, and a part of the accumulated charges jumps into the ground electrode 6, so that the bias micro-jump shown in FIG. The phenomenon was occurring.

In the AC / DC separation type intensity modulator of the present invention, a conductive film 7 of Au or the like as shown in FIGS. 4 and 5 is provided in order to suppress the accumulation of charges at the edge portion of the substrate 1. Furthermore, by energizing the conductive film 7 and the ground electrode 6, the accumulation of charges can be completely suppressed. The conductive film 7 is not limited to a metal film, and a known material can be selected as long as it has conductivity and can withstand the usage environment of the optical modulator.

Further, in order to suppress the accumulation of charges at the edge portion of the substrate 1, the edge portion of the substrate 1 is cut off by cutting or the like as shown in FIG. 13 to make the angle forming the edge portion obtuse.
As a result, it is possible to suppress the concentration of electric charges and reduce the accumulation of electric charges.

FIG. 6 (conventional example) and FIG. 7 (inventive example) show the results of measuring the characteristics of the conventional AC / DC separated intensity modulator (FIGS. 2 and 3) and the present invention (FIGS. 4 and 5). Shown in. In the conventional example of FIG. 6, the potential difference (D
Looking at the fluctuation of the C port voltage), it can be seen that the control voltage greatly fluctuates each time the bias micro-jump phenomenon occurs. On the other hand, in the embodiment of FIG.
The bias micro-jump phenomenon is suppressed, and the optical output fluctuation shows a relatively stable waveform.

Next, an example of the waveguide type intensity modulator will be described. FIG. 8 is a plan view of a conventional waveguide type intensity modulator, and FIG. 11 is a plan view of a waveguide type intensity modulator to which the present invention is applied. 9 is a sectional view taken along the line BB 'in FIG. 8, and FIG. 12 is a sectional view taken along the line BB' in FIG. Regarding the structure of the waveguide type intensity modulator, as in the case of the AC / DC separation type intensity modulator, the LN substrate 11 of the X-cut plate is used, and the optical waveguide 12 is also formed by thermal diffusion of Ti. ing. The buffer layer may be provided on the entire surface of the LN substrate,
In this embodiment, the SiO ₂ layer is formed only in the light modulation portion (in FIGS. 8 and 11, the structure in which the buffer layer is omitted is shown in order to make the structure easy to understand).

The waveguide type intensity modulator is divided into an optical modulator and an attenuator, and a light wave λ which is incident on the optical waveguide 12 is formed.
Is turned on-off in response to an electric signal applied to the first signal electrode 13 of the light modulation section, and then subjected to intensity modulation by a control signal applied to the second signal electrode 15 of the attenuator section. The electric signal applied to the first signal electrode 13 is a high frequency microwave, while the DC control voltage is applied to the second signal electrode. As described above, even in the waveguide type intensity modulator, the bias micro-jump phenomenon occurs in the attenuator portion, and the optical output greatly changes as shown in FIG. 10 (see ΔP2).

In order to prevent this bias micro-jump phenomenon, as in the case of the AC / DC separation type intensity modulator, as shown in FIG.
Conductive films 17 such as 1 and 12 are provided near the ground electrode 16 and at the edge portion of the substrate 11, and the conductive film 17 is configured to energize the ground electrode 16 as necessary. Further, the edge portion 8 of the substrate 11 may be cut off as shown in FIG.

The present invention is based on the AC / DC as described above.
The configuration of the present invention can be applied to any optical modulator that applies a DC voltage to perform optical modulation, without being limited to the separation type intensity modulator and the waveguide type intensity modulator. Moreover, although the X-cut plate of LN has been mainly described as the substrate of the optical modulator, the present invention is not limited to this, and it is obvious that the present invention can be applied to other materials such as a Z-cut plate.

[0024]

As described above, according to the optical modulator of the first aspect, since the antistatic treatment is applied to the edge portion of the substrate near the ground electrode, the light is accumulated in these portions. It is possible to prevent a bias micro-jump phenomenon in which electric charges jump into the ground electrode, and it is possible to suppress a momentary change in the control voltage due to this phenomenon.

According to the optical modulator of the second aspect, since the conductive film electrically connected to the ground electrode is provided as the antistatic treatment, the bias micro-jump phenomenon can be effectively prevented by a simple process.

According to the optical modulator of claim 3, as the antistatic treatment, the edge portion is processed so that the minimum angle forming the edge portion of the substrate is always an obtuse angle. Therefore, a conductive film such as Au is used. The treatment can be carried out inexpensively and easily as compared with the case where it is necessary.

According to the optical modulator of claim 4, AC / DC
AC / D that prevents bias micro-jump phenomenon in separated intensity modulator and has little instantaneous fluctuation of control voltage
A C separation type intensity modulator can be provided.

According to the optical modulator of the fifth aspect, it is possible to provide the waveguide type intensity modulator which prevents the bias micro-jump phenomenon in the waveguide type optical modulator and has a small instantaneous fluctuation of the control voltage.

[0029]

[Brief description of drawings]

FIG. 1 is a graph illustrating the influence of a bias micro-jump phenomenon in an AC / DC separation type intensity modulator.

FIG. 2 is a plan view of a conventional AC / DC separation type intensity modulator.

FIG. 3 is a sectional view of a conventional AC / DC separation type intensity modulator.

FIG. 4 is a plan view of an AC / DC separation type intensity modulator of the present invention.

FIG. 5 is a sectional view of an AC / DC separated intensity modulator of the present invention.

FIG. 6 is a graph showing fluctuations in DC port voltage in a conventional AC / DC separation type intensity modulator.

FIG. 7 is a graph showing variations in DC port voltage in the AC / DC separation type intensity modulator of the present invention.

FIG. 8 is a plan view of a conventional waveguide type intensity modulator.

FIG. 9 is a sectional view of a conventional waveguide type intensity modulator.

FIG. 10 is a graph for explaining the influence of the bias micro-jump phenomenon in the waveguide type intensity modulator.

FIG. 11 is a plan view of a waveguide type intensity modulator of the present invention.

FIG. 12 is a sectional view of a waveguide type intensity modulator of the present invention.

FIG. 13 is a sectional view of an AC / DC separation type intensity modulator showing an application example of the present invention.

[Explanation of symbols]

1 LN board 2 Optical waveguide 3 AC application electrode 4 ground electrode 5 DC application electrode 6 ground electrode 7 Conductive film 8 Edge part 11 LN board 12 Optical waveguide 13 First signal electrode 14 Ground electrode 15 Second signal electrode 16 Ground electrode 17 Conductive film

Continued front page    (72) Inventor Takashi Kamiki             28 Sumitomo Osaka, 6-6 Rokubancho, Chiyoda-ku, Tokyo             Inside Cement Co., Ltd. (72) Inventor Toru Sugamata             28 Sumitomo Osaka, 6-6 Rokubancho, Chiyoda-ku, Tokyo             Inside Cement Co., Ltd. F-term (reference) 2H079 AA02 AA12 BA01 CA04 DA03                       EA05 EA24 EB05 HA21

Claims (5)

[Claims] 1. A substrate made of a material having an electro-optical effect, an optical waveguide formed on the substrate, and an electrode formed on the substrate to modulate the intensity of light passing through a part of the substrate. In the optical modulator having a DC voltage applied to the electrode, the electrode is composed of a signal electrode and a ground electrode, and an antistatic treatment is applied to an edge portion of the substrate in the vicinity of the ground electrode. An optical modulator characterized in that 2. The optical modulator according to claim 1, wherein the antistatic treatment is a treatment of providing a conductive film electrically connected to the ground electrode. 3. The optical modulator according to claim 1, wherein the antistatic treatment is a treatment of the edge portion of the substrate such that the minimum angle forming the edge portion is always an obtuse angle. An optical modulator characterized by. 4. The optical modulator according to claim 1, wherein the optical modulator constitutes a DC section of an AC / DC separation type intensity modulator. 5. The optical modulator according to any one of claims 1 to 3, wherein the optical modulator constitutes an attenuator section of a waveguide type intensity modulator including an optical modulator section and an attenuator section. An optical modulator characterized by.