JP2833816B2 - Light modulator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator, and more particularly to an optical modulator using a waveguide type optical device.

[Conventional technology]

In a waveguide type optical device, a portion having a high refractive index is formed as a waveguide for confining and guiding light in a substrate made of a ferroelectric material or a semiconductor material. An electrode is formed for applying a modulation voltage. By applying a modulation voltage from the outside to this electrode, the refractive index of the waveguide is changed to modulate the phase and intensity of light or to switch the optical path. As an example of this waveguide type optical device, there is an optical device using a lithium niobate substrate having a relatively high electro-optic effect among ferroelectric materials. This optical device forms a waveguide by forming a titanium film on a substrate, patterning the titanium into a waveguide pattern, and then diffusing titanium at a high temperature of about 1000 ° C. for several hours to form a waveguide. A silicon dioxide buffer layer is formed by a vapor deposition method, and an electrode is formed on the upper surface by a metal film. Such a waveguide type optical device can integrate a switching function for modulating light or switching an optical path on a substrate. Further, since light can be modulated at a high speed, development as an optical path switching switch in an external modulator for large-capacity optical communication, a break point measuring device, and the like has been promoted.

FIG. 4 and FIG. 5 show a Mach-Zehnder type high-speed optical modulator widely known in the prior art. Hereinafter, its structure and operation principle will be described.

The waveguide 1 is a waveguide substrate 2 made of lithium niobate.
Is formed. This waveguide 1 has two branch portions 1a and 1b.
And two parallel paths 1c and 1d in the center. On top of each of the two parallel paths 1c and 1d, a metal layer electrode 3 made of chromium or gold is placed via a silicon dioxide buffer layer 8 formed on the entire surface of the waveguide substrate 2.
And the electrodes 4 are formed. An input optical fiber 5 and an output optical fiber 6 are optically coupled to both end surfaces of the waveguide 1, respectively. One electrode 3 is connected to an output terminal of a drive circuit 7 for applying a modulation voltage, and an input terminal of the drive circuit 7 is connected to a DC voltage source 9 for supplying a positive DC voltage. . The other electrode 4 is grounded.

In such a configuration, when a voltage is applied to the electrode 3 from the outside, an electric field is generated in the waveguide 1 in the direction indicated by an arrow as shown in FIG. The effect changes the refractive index of the waveguide 1. When the refractive index of the waveguide 1 changes, the phase of light propagating therethrough changes. This changes depending on the voltages applied in opposite directions depending on whether the refractive index increases or decreases. 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 1d of the waveguide 1 are opposite to each other. Change in opposite directions. In FIG. 6, the modulation voltage (Vπ) is applied during a period indicated by a, and the modulation period is stopped during a period indicated by b.

The solid line 31 in FIG. 8 represents a switching curve showing the relationship between the applied voltage and the light output. In the state where no voltage is applied, the light that has been once branched and re-joined has no phase difference, so that light is output again except for propagation loss and branch loss. However, increasing the applied voltage causes a phase difference in the light. Therefore, if a voltage is applied so that the phases of the two lights are just inverted, no light is output. The Mach-Zehnder type high-speed light modulator modulates light by turning on and off with an applied voltage based on the principle described above.

[Problems to be solved by the invention]

As described above, in the conventional optical modulation device, voltage modulation is performed between the state of V = 0 where no voltage is applied and the state where the modulation voltage (Vπ) is applied.
As shown in FIG. 2B, the silicon dioxide buffer layer 8
Alternatively, mobile ions such as sodium ions (Na +) among the impurity ions contained in the waveguide substrate 2
Start moving in the direction. As a result, even if the same voltage is applied, the intensity of the electric field generated in the waveguide 1 is weakened, and the extinction ratio of light required for desired modulation cannot be obtained. This is equivalent to the fact that the switching curve showing the relationship between the applied voltage and the optical output shifts in the direction of the same polarity as the applied voltage (drift phenomenon), as shown by the broken line 30 in FIG. Therefore, there is a problem that the modulation voltage must be increased in order to maintain the original characteristics. This drift phenomenon is reversible, and impurity ions gathered in the vicinity of the electrode 4 are returned to the silicon dioxide buffer layer 8 again.
Alternatively, the switching curve returns to the initial state if it diffuses into the waveguide substrate 2, but generally, if the waveguide element is in the idle state (voltage V = 0), the switching curve shifted by voltage application is returned to the initial state. Requires a standing time several to several tens times longer than the voltage application time.

The present invention has been made in view of such a problem, and an object of the present invention is to make it possible to forcibly return a switching curve shifted by application of a modulation voltage to an initial state in a short time, and to always provide stable characteristics. An object of the present invention is to provide a light modulation device that can be maintained.

[Means for solving the problem]

According to the first aspect of the present invention, (a) a waveguide substrate having a waveguide and an electrode for modulation near the waveguide;
(B) a DC voltage source that outputs a DC voltage required for modulation;
(C) converting the output of the DC voltage source into a modulation voltage that is output intermittently with a predetermined pause period and that is boosted to a voltage higher than the value of the output voltage of the DC voltage source and applied to the electrodes; And (d) switching the voltage supplied to the electrodes to a modulated voltage output from the drive circuit during periods other than the idle period, and during the idle period, to a constant unmodulated voltage value output from the DC voltage source. The light modulation device is provided with switching means for switching to a DC voltage and alternately applying voltages of opposite polarities to the electrodes.

According to the second aspect of the present invention, there are provided (a) a waveguide and a waveguide substrate having a modulation electrode near the waveguide, and (b) a first DC voltage for outputting a DC voltage required for modulation. And (c) converting the output of the first DC voltage source into a modulation voltage which is output intermittently with a predetermined pause and raised to a voltage higher than the output voltage of the DC voltage source. (D) a second DC voltage source for outputting a DC voltage of a constant voltage value having a polarity opposite to that of the output of the first DC voltage source, and (e) applying a voltage to the electrode. Switching means for switching the applied voltage to a modulation voltage output from the drive circuit during a period other than the idle period, and switching to a constant DC value output from the second DC voltage source during the idle period. To be prepared.

That is, according to the first and second aspects of the invention,
By applying a DC voltage having a polarity opposite to that of the modulation voltage using a pause period in which the modulation voltage is suspended by switching the switching means, the switching common line shifted by application of the modulation voltage is forcibly initialized. It returns to the state. The drive circuit converts the modulated voltage into a higher voltage than the output voltage of the DC voltage source and applies the voltage to the electrodes.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows a planar configuration of a waveguide type optical modulator according to an embodiment of the present invention. Here, the input side optical fiber 5 and the output side optical fiber 6 optically connected to the waveguide substrate 2 and the waveguide 1 and the drive circuit 7 connected to the electrode 3 and a DC voltage source for outputting a positive DC voltage 9 is
Since the configuration is the same as that of the conventional light modulator shown in FIG. 4, the description thereof is omitted. In this embodiment, a relay circuit 10 as a switching means is provided between the drive circuit 7 connected to the electrode 3 and the DC voltage source 9. The relay circuit 10 includes movable contacts 10a and 10b and a fixed contact 10
c to 10e are provided. One movable contact 10a is grounded. The output terminal of the DC voltage source 9 is connected to the other movable contact 10b. The fixed contact 10c is connected to a modulation voltage input terminal 7a of the drive circuit 7, and the modulation 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 modulator of the present embodiment, as shown in FIG. 1, when the movable contacts 10a and 10b of the relay circuit 10 are connected to the fixed contacts 10d and 10c, respectively, the electrode 4
Are grounded, a positive DC voltage is supplied from the DC voltage source 9 to the drive circuit 7, and a modulation voltage is applied to the electrode 3. When the movable contacts 10a, 10b are switched from the fixed contacts 10d, 10c to the fixed contacts 10e, 10d, 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 a modulation voltage applied to the electrode 3 in this light modulation device. In the period indicated by a in FIG. 2, the relay circuit 10 is in the state shown in FIG. 1, and a modulation voltage is applied to the electrode 3. The relay circuit 10 switches during the idle period shown by b in FIG.
a, 10b are connected to the fixed contacts 10e, 10d side, so that a negative voltage is applied to the electrode 3. That is, this relay circuit
The modulation voltage output from the drive circuit 7 and the output voltage of the DC voltage source 10 can be connected to the electrodes 3 by switching the drive circuit 10 so that they have opposite polarities. A negative polarity voltage can be applied to the electrode 3. Thereby, the electrode 4 shown in FIG.
Can be forcibly diffused into the silicon dioxide buffer layer 8 again. That is, the switching curve indicated by the broken line 30 in FIG. 8 can be forcibly returned to the position of the solid line 31 in the initial state.

The modulation voltage (V
The magnitude of π) is, for example, 6.5V. FIG. 9 shows the results of examining the amount of voltage shift caused by each drift phenomenon when these optical devices are used simultaneously for 2 hours, paused for 1 hour, and repeated. During the modulation, a pulse voltage having a period of 1 ms and a duty of 30% was applied to the electrode 3. During the rest period, the voltage is set to 0 V in the conventional light modulator, and in the light modulator of the present embodiment, the relay circuit 10 is switched from the drive circuit 7 to the DC voltage source 9 to apply a DC voltage (6.5 V) for about 20 minutes. Then, it was set to 0V. 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, as shown by a solid line 33,
Only 0.5 V causes a voltage shift, and stable characteristics can be maintained.

FIG. 3 shows another embodiment of the present invention. In this embodiment, apart from the DC voltage source 9 as a first DC voltage source for outputting a positive DC voltage, a negative voltage source 20 as a second DC voltage source for outputting a negative DC voltage and a relay A circuit 21 is provided, and the connection between the electrode 3, the drive circuit 7, and the negative voltage source 20 is switched by the relay circuit 21. That is, the movable contact 21a of the relay circuit 21
Is connected to the electrode 3 and one fixed contact 21b is connected to the drive circuit 7
Connected to the modulation voltage output terminal 7b of the
c is connected to the output terminal of the negative voltage source 20. The other configuration is the same as that of the conventional example shown in FIG. 4, and a description thereof will be omitted.

In the light modulation device of the present 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 first electrode 3
2, the modulation voltage shown in FIG. 2 can be applied, and a negative voltage having a polarity opposite to that of the modulation voltage can be applied while the modulation is paused. Therefore, the shifted switching curve can be returned to the initial state similarly to the embodiment of FIG.

[Effects of the Invention] As described above, according to the optical modulation device of the present invention, the output of the DC voltage source is output intermittently with a predetermined pause, and the output voltage of the DC voltage source is In addition to preparing a drive circuit for converting the voltage supplied to the electrode to a modulation voltage that has been boosted to a higher voltage and applying the voltage to the electrode, the voltage supplied to the electrode is switched to the modulation voltage output from the drive circuit during periods other than the idle period, and during the idle period Switching means for switching to a DC voltage of a constant unmodulated voltage value output from a DC voltage source and alternately applying voltages of opposite polarities to the electrodes is provided. A DC voltage having a constant voltage value in the opposite direction can be applied, and the switching curve shifted by the drift phenomenon can be forcibly returned to the initial state. No special time required. Further, since the voltage for this forced return is a DC voltage, the switching curve can be returned to the initial state in a short time and stably, and stable characteristics can be always maintained.

Further, since the drive circuit boosts the output of the DC voltage source, the voltage source can also be used as a voltage source for generating a voltage to be applied during the idle period, and the cost of the device can be reduced.

[Brief description of the drawings]

FIG. 1 is a plan view showing the configuration of an optical modulator 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 the optical modulator, and FIG. FIG. 4 is a plan view showing a configuration of a light modulation device according to another embodiment of the present invention, FIG. 4 is a plan view showing a configuration of a conventional light modulation device, FIG. FIG. 6 is a waveform diagram showing an example of a modulation voltage applied to a conventional light modulation device, and FIG.
FIGS. 2B and 2B are diagrams for explaining states of the conventional light modulation device at the time of use. FIG. 1A is a cross-sectional view showing an initial state, and FIG. 1B is a cross-sectional view showing a state after use. FIG. 8 is a diagram showing the relationship between the voltage applied to the electrodes and the light output. 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 light modulator. FIG. DESCRIPTION OF SYMBOLS 1 ... waveguide, 2 ... waveguide board, 3,4 ... electrode, 5 ... input fiber, 6 ... output fiber, 7 ... drive circuit, 9 ... DC voltage source (first DC voltage source), 10,20 ... relay circuit, 21 ... Negative voltage source (second DC voltage source)

Claims (2)

(57) [Claims] A waveguide substrate having a waveguide and an electrode for modulation in the vicinity of the waveguide; a DC voltage source for outputting a DC voltage required for modulation; A drive circuit that is intermittently output with a period of time and converted to a modulation voltage boosted to a voltage higher than the output voltage value of the DC voltage source and applied to the electrode, and the voltage supplied to the electrode is During a period other than the pause period, the voltage is switched to the modulation voltage output from the drive circuit. During the pause period, the voltage is switched to the unmodulated DC voltage of a constant unmodulated voltage value output from the DC voltage source, and voltages of opposite polarities are applied to the electrodes. And a switching means for alternately applying the signals. 2. A waveguide substrate having a waveguide and an electrode for modulation in the vicinity of the waveguide, a first DC voltage source for outputting a DC voltage required for modulation, and a first DC voltage source for the first DC voltage source. A drive circuit that converts the output into a modulation voltage that is output intermittently with a predetermined pause period and that is boosted to a voltage higher than the output voltage value of the DC voltage source and applies the modulation voltage to the electrode; A second DC voltage source that outputs a DC voltage having a constant voltage value of the opposite polarity to the output of the DC voltage source, and a voltage applied to the electrode is output from the drive circuit during a period other than the idle period. Switching means for switching to a constant modulation voltage, and switching to a constant DC voltage output from the second DC voltage source during the idle period.