JP2982448B2 - Light control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical control device for modulating light waves, switching light, and the like, and more particularly to a waveguide type optical control device for controlling using an optical waveguide provided in a substrate.

[0002]

2. Description of the Related Art As optical communication systems have been put into practical use,
Further, there is a demand for an advanced system having a large capacity and multiple functions, and it is necessary to add new functions such as generation of an advanced optical signal and switching and switching of an optical transmission line.
In the current practical system, the optical signal is obtained by directly modulating the injection current of a semiconductor laser or a light emitting diode.
There are drawbacks in that high-speed modulation of about 0 GHz or more is difficult, and it is difficult to apply to a coherent optical transmission system because of wavelength fluctuation.

As a means for solving this problem, there is a method using an external modulator. In particular, a waveguide type optical modulator constituted by an optical waveguide formed in a substrate has features of small size, high efficiency, and high speed. is there. On the other hand, an optical switch is used as a means for obtaining an optical transmission line switching or network switching function.

An optical switch currently in practical use is one that mechanically moves a prism, a mirror, a fiber, and the like, and has disadvantages such as low speed, insufficient reliability, a large shape, and unsuitable for matrix formation. There is.

What is being developed as a means for solving this problem is a waveguide type optical switch using an optical waveguide, which has features such as high speed, integration of many elements, and high reliability. In particular, a material using a ferroelectric material such as lithium niobate (hereinafter referred to as LiNbO ₃ ) crystal has a small light absorption and a low loss, and has a large electro-optic effect and thus has high efficiency. Various types of optical control elements such as a directional coupler type optical modulator / switch, a total reflection type optical switch, and a Mach-Zehnder type optical modulator have been reported.

FIG. 6 is a plan view of a directional coupler type optical switch as an example of a conventional light control device, and FIG. 7 is a sectional view taken along the line AA 'of FIG. In FIG. 6, striped optical waveguides 52 and 53 are formed on a LiNbO ₃ crystal substrate 51 by diffusing Ti to have a larger refractive index than that of the substrate.
Numerals 3 are close to each other by about several μm at a central portion to form a directional coupler 54.

Light control electrodes 55 and 56 are formed on the optical waveguide constituting the directional coupler 54 via a buffer layer 57 for preventing light absorption by the electrodes as shown in FIG.

When the light control electrodes 55 and 56 have the same potential, the incident light 1 incident on the optical waveguide 52 gradually transfers the light energy to the adjacent optical waveguide 53 as it propagates through the directional coupler 54. After passing through the directional coupler 54, almost 100% of the energy is transferred to the optical waveguide 53, and the emitted light 2 is obtained.

On the other hand, when a voltage is applied between the electrodes 55 and 56, the refractive index of the optical waveguide below the control electrode changes due to the electro-optic effect of LiNbO ₃ due to the electric field generated between the electrodes, and the light propagates through the optical waveguides 52 and 53. A phase velocity mismatch occurs between the two guided modes, and the coupling state between the two changes. The intensity of the outgoing light 2 from the optical waveguide 53 decreases with an increase in the applied voltage, and takes a minimum value at a specific voltage (hereinafter referred to as V _S ).

At this time, the intensity of light emitted from the other optical waveguide 52 becomes maximum. The same applies to the case where the polarity of the applied voltage is reversed.
Minimum at _S FIG. 4 shows an example of a change of the emitted light 2 with respect to the applied voltage by a solid line 3. Therefore, when performing switching of optical transmission line, to select a destination of the optical signal by the voltage between the electrodes to 0 or V _S.

When such a waveguide type optical control element is applied to an actual optical communication system, stability of operation characteristics is particularly important in addition to basic performance such as low loss and high speed.
However, the conventional waveguide type optical control device does not provide sufficient characteristics with respect to stability of operation characteristics.

[0012]

In a conventional optical switch, if a voltage is continuously applied for switching the transmission line, a phenomenon occurs in which the optical output-voltage characteristic of the switch drifts in the applied voltage direction ( Hereinafter, this phenomenon is called DC drift). An example of the voltage-optical output characteristics of the optical switch changed by the DC drift is shown by a broken line 4 in FIG. D
When the C drift occurs, the voltages at which the maximum value and the minimum value of the outgoing light 2 are obtained shift from 0 and V _S to ΔV and V _S + ΔV, respectively.

The drift voltage ΔV changes depending on the voltage application time and the applied voltage, and the drift amount saturates in a certain time as shown in FIG. Further, DC drift is a reversible phenomenon and decreases when voltage is removed, and ΔV returns to zero.

The cause of the DC drift is that the impurity ions contained in the substrate or the SiO ₂ buffer layer are attracted by the electric field between the electrodes and move due to the saturation characteristics and reversibility of the drift amount to form an anti-electric field, thereby causing electro-optics. It is estimated that the electric field involved in the change in the refractive index due to the effect is effectively reduced.

When the DC drift described above occurs,
The characteristics of the optical switch fluctuate according to the history of the switching state. For example, when the operating voltage is set to be constant, crosstalk degradation occurs and stable operation cannot be expected, which is a major problem in practical use.

It is an object of the present invention to provide a light control circuit capable of suppressing fluctuations in characteristics due to a DC drift occurring in the above-described conventional light control circuit and achieving a stable operation.

[0017]

In order to achieve the above object, an optical control circuit according to the present invention comprises an optical waveguide formed on a dielectric substrate having an electro-optic effect, and an electric field provided near the optical waveguide. Controlling at least a pair of light control electrodes for changing the refractive index of the optical waveguide, and a bias electrode biased with respect to the light control electrode, the light control electrode of the dielectric substrate On the forming surface
On the back side, and at the position facing the light control electrode
Having a groove and having the bias electrode on the bottom surface of the groove
It is .

[0018]

[0019]

According to the present invention, by constantly applying a bias voltage between the light control electrode and the bias electrode, it is possible to suppress the movement of impurity ions that cause DC drift.
Therefore, an optical control circuit that suppresses drift during switching or modulation and operates stably can be obtained.

Further, a groove is partially formed on the back surface of the substrate to reduce the distance between the light control electrode and the bias electrode, thereby increasing the electric field strength in the substrate and lowering the bias voltage necessary for suppressing the movement of impurity ions. can do.

[0021]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing an embodiment of the present invention. FIG. 2 is a plan view. FIG. 3 is a cross-sectional view taken along a cutting line AA ′ in FIG.

In FIG. 1, Z-Cut LiNbO ₃
900 to 11 stripes of Ti on the Z-plane of the substrate 11
By diffusing at 00 ° C. for several hours, the optical waveguides 12 and 13 having a width of about 3 to 10 μm are formed. The optical waveguides 12 and 13 are close to each other by about several μm at the center to form a directional coupler 14.

The length of the directional coupler 14 is set so that the movement of light between the optical waveguides becomes 100%. The light control electrode 1 is provided on the optical waveguide constituting the directional coupler 14 via a buffer layer 17 for preventing light absorption by the electrode.
5, 16 are formed.

Also, the back side of the LiNbO ₃ substrate 11
A groove 22 is formed on the Z plane by ion milling. The position of the groove 22 is opposed to the light control electrodes 15 and 16, and the depth is set such that the thickness of the remaining substrate is about 5 to 10 times the thickness of the optical waveguide. Further, a bias electrode 18 is formed on the bottom surface of the groove 22.

The light control electrode 16 is grounded and the light control electrode 1
A switch power supply 20 is connected between 5 and 16 so that a switch voltage V _S can be applied by switching a switch 19. Bias electrode 18 and light control electrodes 15 and 16
Negative bias voltage V _B sufficiently larger than the switch voltage V _S is always applied between.

When the potential between the light control electrodes 15 and 16 is the same,
The incident light 1 incident on the optical waveguide 12 is directed to a directional coupler 14.
As the light propagates through the portion, the light energy gradually shifts to the optical waveguide 13 adjacent thereto, and after passing through the directional coupler 14, almost 100% of the energy shifts to the optical waveguide 13 to become the outgoing light 2.

On the other hand, when a switch voltage V _S is applied between the light control electrodes 15 and 16, the refractive index of the optical waveguide under the control electrode changes due to the electro-optic effect of LiNbO ₃ due to the electric field generated between the electrodes, and A phase velocity mismatch occurs between the waveguide modes propagating in the waveguides 12 and 13, and the coupling state between the two changes, and the intensity of the emitted light 2 becomes almost zero.

[0028] In the configuration of the present invention, since the application of the sufficiently large negative bias voltage V _B than constantly switch voltage V _S between the light control electrode and bias electrode, a positive impurity ions bias voltage included in the substrate As the impurity ions disappear from the vicinity of the light control electrode due to the movement of the impurity ions, the effective electric field decreases due to the movement of the impurity ions due to the presence or absence of a voltage between the light control electrodes, that is, the DC drift also disappears.

The force for attracting impurity ions to the bias electrode is proportional to the electric field intensity distribution. Therefore usually 0.5-1m
By reducing the distance between the bias control electrode and the light control electrode having a groove formed in the m LiNbO ₃ substrate to about 50 to 100 μm, the electric field strength inside the substrate is increased and the bias voltage is reduced to 1/10 or less. Can be.

Although the substrate is easily broken by partially reducing the thickness of the substrate, in the present embodiment, a groove is formed on the back surface of the substrate after Ti is thermally exposed and thermally diffused most likely to be damaged. Therefore, the possibility of cracking is low and safe.

In the present invention, the refractive index of the optical waveguide changes due to the electro-optic effect caused by the bias voltage. However, the intensity and direction of the electric field applied to the optical waveguides 12 and 13 become equal, so that the light propagating through both optical waveguides is guided. Since there is no phase velocity mismatch during the wave mode, the switch operation is not affected at all.

Although the case of the Ti-diffused LiNbO ₃ optical waveguide has been described as an example, similar effects can be obtained when the present invention is applied to a dielectric substrate having another electro-optical effect or an optical waveguide. The same effect can be obtained not only with the lumped constant type electrode but also with the traveling wave type electrode. In this embodiment, the case where a negative bias voltage is applied has been described.
When the polarity of the impurity ions contained in the NbO ₃ substrate is negative, a similar effect can be obtained by applying a positive bias voltage.

[0033]

As described above, according to the present invention, the movement of impurity ions causing a DC drift can be suppressed by constantly applying a bias voltage between the light control electrode and the bias electrode.

Therefore, it is possible to provide a light control circuit that suppresses fluctuations in characteristics due to DC drift and obtains stable operation.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of the present invention.

FIG. 2 is a plan view showing one embodiment of the present invention.

FIG. 3 is a sectional view showing one embodiment of the present invention.

FIG. 4 is a diagram illustrating an example of a voltage-optical output characteristic of a LiNbO ₃ optical switch before DC drift occurs and after a DC drift occurs. Time-DC of LiNbO ₃ optical switch
FIG. 4 is a diagram illustrating an example of a drift amount characteristic.

FIG. 5 is a diagram illustrating an example of a time-DC drift amount characteristic of a LiNbO ₃ optical switch.

FIG. 6 is a plan view showing a conventional example.

FIG. 7 is a sectional view showing a conventional example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Incident light 2 Outgoing light 3 Voltage-optical output characteristic before DC drift generation 4 Voltage-optical output characteristic after DC drift generation 11, 51 LiNbO ₃ substrate 12, 13, 52, 53 Optical waveguide 14, 54 Directional coupler 15, 16, 55, 56 Light control electrode 17, 57 Buffer layer 18 Bias electrode 19, 59 Switch 20, 60 Switch power supply 21 Bias power supply 22 Groove 23, 24 Electrode lead wire

Claims (1)

(57) [Claims] 1. An optical waveguide formed on a dielectric substrate having an electro-optic effect, and at least one pair of lights provided near the optical waveguide for controlling an electric field to change a refractive index of the optical waveguide. A control electrode, having a bias electrode biased with respect to the light control electrode, a back surface with respect to a surface of the dielectric substrate on which the light control electrode is formed,
And has a groove provided at a position facing the light control electrode,
A light control circuit comprising the bias electrode on a bottom surface of the groove .