JPH03249618A - Optical modulator - Google Patents

Abstract

PURPOSE:To return a switching curve, shifted by applying a modulating voltage, to its initial state in a short time by providing a switching means, and applying the electrode of a waveguide with a DC voltage having the opposite polarity from the modulating voltage outputted from a driving circuit by the switching of the switching means. CONSTITUTION:This device has a waveguide substrate 2, an input-side optical fiber 5 and an output-side optical fiber 6 which are connected optically to the waveguide 1, the driving circuit 7 which is connected to the electrode 3, and a DC voltage source 9 which outputs a positive DC voltage. Then a relay circuit 10 as the switching means is provided between the driving circuit 7 connected to the electrode 3 and the DC voltage source 9. Therefore, the DC voltage having the opposite polarity from the modulating voltage can be applied in a period wherein the application of the modulating voltage is stopped by the switching of the switching means 10. Consequently, the switching curve which is shifted by the application of the modulating voltage can be returned forcibly to its initial state.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical modulation device, and particularly to an optical modulation device using a waveguide type optical device.

[Conventional technology]

A waveguide type optical device has a high refractive index portion formed in a substrate made of ferroelectric material or semiconductor material as a waveguide for confining and guiding light. Electrodes are formed for applying a modulating voltage.

By applying a modulating voltage to this electrode from the outside, the refractive index of the waveguide is changed, modulating the phase and intensity of light, or switching the optical path. An example of this waveguide type optical device is an optical device using a lithium niobate substrate, which has a relatively high electro-optic effect among ferroelectric materials. This optical device forms a titanium film on the substrate,
After patterning this titanium film into a waveguide pattern, a waveguide is formed by diffusing titanium at a high temperature of around 1000°C for several hours, and a silicon dioxide buffer layer is formed on this waveguide by a vapor deposition method. An electrode is formed on the top surface using a metal film. Such a waveguide type optical device can integrate a switching function for modulating light and switching an optical path on a substrate. In addition, since light can be modulated at high speed, development is progressing as an external modulator for large-capacity optical communication, an optical path switching switch in a break point measuring device, and the like.

4 and 5 show a conventionally widely known Matsuhatsu Enda type high-speed optical modulator. below,
Its structure and operating principle will be explained.

The waveguide 1 is formed on a waveguide substrate 2 made of lithium niobate. This waveguide l has two branch parts 1a, l
b, and two parallel paths 10.b in the center. It has ld. On top of each of these two parallel paths lc and ld, electrodes 3 and 4 made of metal layers made of chromium or gold are provided via a silicon dioxide buffer layer 8 formed over the entire surface of the waveguide substrate 2. It is formed. An input optical fiber 5 and an output optical fiber 6 are optically coupled to both end faces of the waveguide 1, respectively. An output terminal of a drive circuit 7 for applying a modulation voltage is connected to one electrode 3, and a DC voltage source 9 for supplying a positive DC voltage is connected to an input terminal of this drive circuit 7. There is. The other electrode 4 is grounded.

In such a configuration, when a voltage is externally applied to the electrode 3, an electric field is generated in the waveguide 1 in the direction indicated by the arrow as shown in FIG. 7(a), and the electro-optic property of lithium niobate is The effect changes the refractive index of the waveguide 1.

When the refractive index of the waveguide 1 changes, the phase of light propagating there changes, and this changes depending on whether the refractive index becomes high or low, depending on the voltages applied in opposite directions. When a pulse voltage as shown in FIG. 6 is applied to the electrode 3,
The electric fields generated in the parallel paths 1c and ld of the waveguide 1 are in opposite directions, and the phases of the light propagated through the respective paths 1c and ld change in opposite directions.

In addition, on the right side of FIG. 6, the period indicated by a is the modulation voltage (Vπ
) is applied, and the period indicated by b is a period during which modulation is suspended.

A solid line 31 in FIG. 8 represents a switching curve showing the relationship between the applied voltage and the optical output. When no voltage is applied, the light that is once branched and recombined has no phase difference, so propagation loss and branching loss are removed, and the light is output again. However, when the applied voltage is increased, a phase difference occurs in the light. Therefore, if a voltage is applied so that the phases of the two lights are exactly reversed, no light will be output. The Matsuhatsu Enda type high-speed optical modulator modulates light by turning on and off an applied voltage based on the principle explained above.

[Problem to be solved by the invention]

As mentioned above, in the conventional optical modulation device, voltage modulation is performed between the v=00 state where no voltage is applied and the state where a modulation voltage (Vπ) is applied. As shown in b), among the impurity ions contained in the silicon dioxide buffer layer 8 or the waveguide substrate 2,
Mobile ions such as sodium ions (Na+) begin to move in the direction of the electrode 4. As a result, even if the same voltage is applied, the electric field strength generated in the waveguide 1 becomes weaker, making it impossible to obtain the extinction ratio of light necessary for desired modulation. This causes the switching curve, which shows the relationship between the applied voltage and the optical output, to shift in the direction of the same polarity as the applied voltage, as shown by the broken line 30 in FIG. 8 (drift phenomenon).
is equivalent to .

Therefore, there was a problem in that the modulation voltage had to be increased in order to maintain the original characteristics.

This drift phenomenon is reversible, and if the impurity ions gathered near the electrode 4 diffuse into the silicon dioxide buffer layer 8 or the waveguide substrate 2 again, the switching curve will return to its initial state. Abandoned state (
When the voltage V=0), returning the switching curve shifted by voltage application to its initial state requires a standing time that is several times to several tens of times longer than the voltage application time.

The present invention has been made in view of such problems, and includes:
The purpose is to provide an optical modulation device that can forcibly return a switching curve shifted to its initial state in a short period of time by applying a modulation voltage, and can always maintain stable characteristics.

[Means to solve the problem]

The optical modulation device of the present invention includes a waveguide substrate having a waveguide and a modulation electrode near the waveguide, a DC voltage source that outputs a DC voltage necessary for modulation, and a DC voltage source that modulates the output of the DC voltage source. A drive circuit converts the voltage into a voltage and applies it to the electrodes, and the voltage supplied to the electrodes is switched to a modulated voltage output from the drive circuit or a DC voltage output from a DC voltage source, and voltages of opposite polarity are alternately applied to the electrodes. and switching means for applying the voltage.

Further, the optical modulation device of the present invention includes a waveguide substrate having a waveguide and a modulation electrode near the waveguide, a first DC voltage source that outputs a DC voltage necessary for modulation, and a first DC voltage source that outputs a DC voltage necessary for modulation.
a drive circuit that converts the output of the DC voltage source into a modulated voltage and applies it to the electrode; a second DC voltage source that outputs a DC voltage of opposite polarity to the output of the first DC voltage source; and switching means for switching the voltage output from the drive circuit to the modulated voltage output from the drive circuit or the DC voltage output from the second DC voltage source.

In an optical modulation device having such a configuration, by switching the switching means, it is possible to apply a DC voltage of opposite polarity to the modulation voltage during a period when the application of the modulation voltage is suspended, and thereby the modulation voltage is applied. By doing so, it becomes possible to forcibly return the shifted switching curve to its initial state.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a planar configuration of a waveguide type optical modulation device according to an embodiment of the present invention. Here, an input optical fiber 5 optically connected to the waveguide substrate 2 and the waveguide 1
The drive circuit 7 connected to the output side optical fiber 6 and the electrode 3, and the DC voltage source 9 that outputs a positive DC voltage have the same configuration as the conventional optical modulator shown in FIG. The explanation will be omitted. In this embodiment, a relay circuit 10 as a switching means is provided between a drive circuit 7 connected to the electrode 3 and a DC voltage source 9. The relay circuit 10 is provided with movable contacts 10a, lQb and fixed contacts 10C to 10e that interlock with each other. One movable contact 10a is grounded. The output end of the DC voltage source 9 is connected to the other movable contact 10b. Fixed contact 10C
A modulated voltage input terminal 7a of the drive circuit 7 is connected to the modulated voltage input terminal 7a of the drive circuit 7, and a modulated voltage output terminal 7b of the drive circuit 7 is connected to the electrode 3 and to the fixed contact 10e.

That is, in the optical modulation device of this embodiment, as shown in FIG.
are connected to the fixed contacts 10d and 10C, respectively, the electrode 4 is grounded, a positive DC voltage is supplied from the DC voltage source 9 to the drive circuit 7, and a modulated voltage is applied to the electrode 3. . When the movable contacts 10a and 10b switch from the fixed contacts 10d and IOC to the fixed contacts 10eS10d, the electrode 3 is grounded and the DC voltage source 9 is connected to the electrode 4.

FIG. 2 shows an example of a waveform diagram of the modulation voltage applied to the electrode 3 in this optical modulation device. In Figure 2 a
During the period indicated by , the relay circuit lO is in the state shown in FIG. 1, and a modulated voltage is applied to the electrode 3. During the rest period indicated by b in the same figure, the relay circuit 10 switches, and the movable contact 1
0a and 10b are connected to fixed contacts 10e and 10d,
As a result, a negative voltage is applied to the electrode 3. That is, by switching this relay circuit 10, the modulated voltage output from the drive circuit 7 and the output voltage of the DC voltage source 10 can be connected to the electrode 3 so that they have opposite polarities, and therefore, during the period when modulation is stopped, A negative voltage having a polarity opposite to that of the modulation voltage can be applied to the electrode 3. This results in Figure 7 (b)
The impurity ions accumulated in the vicinity of the electrode 4 shown in ) can be forcibly diffused into the silicon dioxide bulk layer 8 again. In other words, the switching curve indicated by the broken line 30 in FIG. 8 can be forcibly returned to the initial state indicated by the solid line 31.

Modulation voltage (Vπ) of the optical modulation device of this embodiment and the conventional example
The magnitude of the voltage is, for example, 6.5V.

FIG. 9 shows an investigation of the amount of voltage shift due to each drift phenomenon when these optical devices were used simultaneously for 2 hours and then stopped for 1 hour, and this was repeated. During modulation, a pulse voltage with a period of 1 ms and a duty of 30% was applied to the electrode 3. During the rest period, in the conventional optical modulator, the voltage = O
V, and in the optical modulation device of this embodiment, the relay circuit 10 is switched from the drive circuit 7 to the DC voltage source 9, and the DC voltage (6,
5V) was applied for about 20 minutes, and then OV was applied. In the conventional optical modulator, a voltage shift of about 3 V occurs after 10 hours as shown by a broken line 32, whereas in the optical modulator of this embodiment, a voltage shift of about 0.5 VL occurs as shown by a solid line 33. No problems occurred and stable characteristics were maintained.

FIG. 3 shows another embodiment of the invention.

In this embodiment, in addition to a DC voltage source 9 as a first DC voltage source that outputs a positive DC voltage, a negative voltage source 20 as a second DC voltage source that outputs a negative DC voltage and a relay are used. A circuit 21 is provided, and connections between the electrode 3, the drive circuit 7, and the negative voltage source 20 are switched by this relay circuit 2I. That is, the relay circuit 21
The movable contact 21a is connected to the electrode 3, and one fixed contact 21b is connected to the modulated voltage output terminal 7b of the drive circuit 7.
The other movable contact 21C is connected to the output end of the negative voltage source 20. The rest of the configuration is the same as the conventional example shown in FIG. 4, so a description thereof will be omitted.

In the optical modulation device of this embodiment, the drive circuit 7 and the negative voltage source 20 can be alternately connected to the electrode 3 by switching the relay circuit 21 .

As a result, the modulation voltage shown in FIG. 2 is applied to the electrode 3 in the same manner as in the embodiment shown in FIG. 1, and a negative voltage having the opposite polarity to the modulation voltage can be applied during the period when modulation is suspended. Therefore, similarly to the embodiment of FIG. 1, the shifted switching curve can be returned to its initial state.

〔Effect of the invention〕

As explained above, according to the optical modulation device of the present invention, the switching means is provided, and by switching the switching means, a DC voltage having a polarity opposite to that of the modulation voltage output from the drive circuit can be applied to the electrodes of the waveguide. , it is possible to apply a DC voltage of opposite polarity to the modulating voltage to the electrodes during the time when the application of the modulating voltage is stopped, and it is possible to forcibly return the switching curve that has been shifted due to the drift phenomenon to the initial state. Therefore, the voltage shift due to the drift phenomenon is not accumulated, and as a result, stable characteristics can be maintained in the result.

[Brief explanation of drawings]

FIG. 1 is a plan view showing the configuration of a light modulation device according to an embodiment of the present invention, FIG. 2 is a waveform diagram showing an example of a modulation voltage applied to the electrodes of this light modulation device, and FIG. FIG. 4 is a plan view showing the configuration of a conventional optical modulation device, and FIG. 5 is a partial plane taken along line AA' in FIG. 4. Figure 6 is a waveform diagram showing an example of a modulation voltage applied to a conventional optical modulation device, Figure 7 (a), (
b) are diagrams for explaining the state of the conventional optical modulation device when in use, where (a) is a cross-sectional view showing the initial state, and (b) is a cross-sectional view showing the state after use. , FIG. 8 is a diagram showing the relationship between the voltage applied to the electrode and the optical output, and FIG. 9 is a diagram showing the relationship between the elapsed time and the shifted voltage when using the embodiment of the present invention and the conventional optical modulation device. It is a diagram. 1... Waveguide, 2... Waveguide substrate, 3
．． 4... Electrode, 5... Fiber for human power, 6... Output fiber, 7...
Drive circuit, 9... DC voltage source (first DC voltage source), 10.20... Relay circuit,

Claims (1)

[Claims] 1. A waveguide substrate having a waveguide and a modulation electrode near the waveguide, a DC voltage source that outputs a DC voltage necessary for modulation, and a device that modulates the output of the DC voltage source. a drive circuit that converts the voltage into a voltage and applies it to the electrode; and a drive circuit that switches the voltage supplied to the electrode to a modulated voltage output from the drive circuit or a DC voltage output from the DC voltage source and applies it to the electrode in opposite directions. 1. A light modulation device comprising: switching means for alternately applying polarity voltages. 2. A waveguide substrate having a waveguide and a modulation electrode near the waveguide, a first DC voltage source that outputs the DC voltage necessary for modulation, and modulating the output of the first DC voltage source. a drive circuit that converts the voltage into a voltage and applies it to the electrode; a second DC voltage source that outputs a DC voltage of opposite polarity to the output of the first DC voltage source; An optical modulation device comprising a switching means for switching between the modulation voltage output from the drive circuit and the DC voltage output from the second DC voltage source.