JPH04110830A - Optical control device - Google Patents

Abstract

PURPOSE:To realize high crosstalk and a high extinction ratio by nearly equalizing the width of an electrode of one of two optical waveguides on a substrate to the width of an optical waveguide and making the width of the electrode of the other optical waveguide much larger than the width of the optical waveguide. CONSTITUTION:This optical control device has the directional coupler 11 composed of the two optical waveguides 11a and 11b and electrodes 13 and 14 are formed on the optical waveguides 11a and 11b across a buffer layer 12. The Y-axial widths WE1 and WE2 of the electrodes 13 and 14 are not equal and one electrode 14 is formed wider than the other electrode 13. When light waves propagated in the optical waveguides 11a and 11b are TE polarized light, this optical control device can use both a longitudinal electrooptic constant and a lateral electrooptic constant, so the refractive index variation of ordinary light in the optical waveguides 11a and 11b formed below the electrodes 13 and 14 can be made large although an applied voltage is equal to that of conventional structure. Low-voltage operation is therefore realized.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical control device that modulates light waves and switches optical paths, and particularly relates to a waveguide that performs control using an optical waveguide formed in an electro-optic crystal substrate. The present invention relates to a type light control device.

[Conventional technology]

As optical communication systems become more practical, advanced systems with higher capacity and more functionality are required, and new functions such as generation of faster optical signals and switching and switching of optical transmission lines are needed. has been done. In current practical systems, optical signals are obtained by directly modulating the injection current of semiconductor lasers or light emitting diodes, but with direct modulation, high-speed modulation of several GHz or more is difficult due to effects such as relaxation oscillation, and wavelength fluctuations The coherent optical transmission method has drawbacks such as difficulty in application due to the occurrence of

One way to solve this problem is to use an external modulator. In particular, a waveguide type optical modulator consisting of an optical waveguide formed in an electro-optic crystal substrate is small, highly efficient,
It has the advantage of high speed.

On the other hand, optical switches are used as means for switching optical transmission lines and providing network switching functions. Optical switches currently in practical use switch optical paths by mechanically moving prisms, mirror fibers, etc., and have drawbacks such as slow speed and large size, making them unsuitable for matrix formation.

As a means to solve this problem, the development of waveguide type optical switches using optical waveguides is progressing, and these switches have characteristics such as high speed, ability to integrate multiple elements, and high reliability. In particular, materials using ferroelectric materials such as lithium niobate (LiNbO2) crystals have features such as low light absorption and low loss, and high efficiency due to their large electro-optical effect. Various types of optical control devices have been reported, such as directional coupler type optical modulators or optical switches, total internal reflection type optical switches, and Matsuhatsu Enda type optical modulators.

Among them, optical modulators and optical switches using directional couplers are being actively researched and developed because they are easier to obtain low crosstalk and high extinction ratio than other methods.

In recent years, research and development on high-density integration of waveguide optical switches using this directional coupler has been actively conducted, and four
According to the literature by Yu et al., Institute of Electronics, Information and Communication Engineers ○QE88-147, an 8×8 matrix optical switch was obtained in which 64 directional coupler type optical switches were integrated using a tNf, 2-board (DL]Nb03 substrate. In addition, the 8×8
The matrix optical switch is a 64-element directional coupling optical switch that is integrated so that it can perform switching functions for any polarization component.Each of the 64 element directional coupling optical switches can perform switching for any polarization plane component. (Hereafter,
(called polarization-independent operation). This polarization-independent operation is realized by matching the switching voltages of the TE polarization component and the TM polarization component, whose polarization planes are orthogonal to each other in a light wave having a random polarization plane propagating through an optical waveguide.

That is, it is sufficient that the applied voltages for obtaining the par state of the directional coupler match in any plane of polarization.

On the other hand, research and development of devices consisting of a single optical switching element, such as external optical modulators, is also actively progressing.

The characteristics of such optical switch devices include switching voltage (power), crosstalk, extinction ratio, loss, switching speed, intensity and phase modulation frequency bands, and operational stability against environments such as temperature, humidity, and shock. . Furthermore, in the aforementioned optical switch that operates without polarization dependence, it is important that the switching voltages of arbitrary polarizations match.

[Problem to be solved by the invention]

Among the above-mentioned characteristic items, reduction of the switching voltage and matching of the switching voltage for arbitrary polarization are the most important issues in realizing polarization-independent operation.

Here, the conventional technology will be explained using drawings. FIG. 2 is a sectional view showing the structure of a conventional optical switch using a directional coupler. In FIG. 2, the Z plate L ]N b Os
The substrate is used as an electro-optic crystal substrate 1. A directional coupler 2 consisting of two optical waveguides 2a and 2b is formed on this electro-optic crystal substrate 1. At this time, the direction in which the light waves propagate through the optical waveguides 2a and 2b is the X axis. (Hereafter, X
(called axial propagation).

Generally, the two adjacent optical waveguides 2a and 2b forming the directional coupler 2 are designed and manufactured so that the propagation constants are equal to each other. In this case, if the length of the directional coupler 2 is set to a certain length, the light wave propagating through one optical waveguide 2a or 2b will be transmitted through the adjacent optical waveguide 2b or 2a.
The energy of the light wave gradually transfers to the adjacent optical waveguides 2a and 2b. 100% of light wave energy
The length of the directional coupler 2 when the waveguide transitions to the other adjacent optical waveguide 2a12b is called the perfect coupling length, and in FIG. 2, the directional coupler 2 whose length is set to this perfect coupling length is used. ing.

Further, a buffer layer 3 is provided on the electro-optic crystal substrate 1 described above.
is loaded, and electrodes 4a and 4b (hereinafter referred to as metal electrodes) made of a metal material are formed on the optical waveguide 2a12b via the buffer layer 3. The buffer layer 3 described above is a metal electrode 4a.

The metal electrodes 4a and 4b are used as an optical buffer layer to prevent absorption of light by the metal electrodes 4a and 4b, and those having a small volume resistivity are used to enable high-speed operation. Also,
two electrodes 4a formed on the optical waveguides 2a and 2b,
The widths of the optical waveguides 4b are equal in WE, and are approximately the same as the width WG of the optical waveguides 2a and 2b in the direction perpendicular to the light wave propagation direction.

In the optical control device with this structure, the electric field component that controls light waves is the electric field component perpendicular to the surface of the electro-optic crystal substrate 1 using the Z-plate LiNbO3 substrate, that is, the electric field component in the Z-axis direction (hereinafter, vertical (called electric field component). In this case, the electro-optic constant that causes a change in the refractive index of the optical waveguide 2a12b used to control light waves is one in a direction parallel to the longitudinal electric field (hereinafter referred to as a longitudinal electro-optic constant). Therefore, the switching voltage is this w!
It is almost uniquely determined by the size of the electro-optic constant, and only a specific switching voltage can be obtained. This longitudinal electro-optical constant is r33 for TM polarized light and r13 for TE polarized light in the Z-plate LiNbO3 substrate and χ-axis propagation.

In addition, as one of the means for operating the optical switch having the conventional structure shown in FIG. When the potential difference between the electrodes 4a and 4b is zero, that is, when no longitudinal electric field is generated, the optical waveguide 2a12
100% of the light waves propagating through b are transferred from one optical waveguide to the other. This is called a cross state. When a potential difference is applied between the two electrodes 4a and 4b, the two optical waveguides 2a with the same propagation constant
, 2b, a phase mismatch occurs, and the energy transfer of light waves to the adjacent optical waveguides 2a, 2b becomes difficult.

Then, at a specific potential difference, no energy transfer of the light wave occurs, and the light wave propagates as it is through the optical waveguides 2a and 2b in which it was initially propagating at t. This is called the bar state. The potential difference that produces this bar state is the switching voltage. There is no single potential difference that can produce a bar state; it is sufficient that the potential difference causes a complete phase mismatch.

Therefore, the potential difference to obtain the first bar state is VS
If so, a bar state can be obtained even with a potential difference of 2.24VS, 3.42VS, etc. To obtain polarization-independent operation,
Obtain the above-mentioned cross state and reduce the switching voltage to T.
It is necessary to match the M and TE polarizations. On the other hand, the switching voltage of each polarized light is determined by the longitudinal electro-optic constant described above, that is, r33 for TM polarized light and r33 for TE polarized light.
It is uniquely determined by the size of 13. In this case, the longitudinal electro-optic constant r33 causes a change in the extraordinary refractive index ne,
Another longitudinal electro-optic constant r13 causes a change in the ordinary refractive index no. Therefore, in order to realize a potential difference that allows a bar state to be obtained simultaneously between each polarization of TM and TE, r
The ratio of 33 and r13 (r33/r, 3) is 1.0.
It needs to be a ratio such as 2.24.3.42. However, the ratio of the magnitudes of the longitudinal electro-optic constants r33 and r13 (r'+
3/r13) is not the value mentioned above but is approximately 2.9 to 3.3. This longitudinal electro-optic constant is a constant specific to the Z plate LiNbO3 substrate and cannot be adjusted. In other words, a perfect bar state cannot be obtained for light waves with random polarization planes. That is, one optical waveguide 2a or 2b
A drawback is that it is not possible to completely eliminate the energy transfer of light waves from one optical waveguide to the adjacent optical waveguide 2b or 2a. Therefore, when applied to an optical switch that switches optical paths, this appears as a deterioration of crosstalk, and when applied to an optical intensity modulator, it causes a problem of deterioration of the extinction ratio.

An object of the present invention is to provide an optical control device that can operate at a low voltage, have matching switching voltages for arbitrary planes of polarization, and achieve polarization-independent operation capable of high crosstalk and high extinction ratio.

[Means to solve the problem]

The optical control device according to the present invention includes a directional coupler consisting of two adjacent optical waveguides formed on a crystal substrate having an electro-optic effect, and electrodes formed on each of the two optical waveguides. In the optical control device, the width of the electrodes formed on each of the two optical waveguides in the direction perpendicular to the light wave propagation direction is equal to the width of each electrode formed on each of the two optical waveguides. Differently, the width of the electrode formed on one of the two optical waveguides is approximately the same as the width of the optical waveguide in the direction perpendicular to the light wave propagation direction, and The present invention is characterized in that the width of the electrode formed on the optical waveguide is sufficiently wider than the width of the optical waveguide.

1 Effect] By using an optical control device using a directional coupler according to the present invention, a low voltage can be obtained, and an optical switch or modulator with high crosstalk and high extinction ratio can be obtained. That is, in the present invention, unlike the conventional structure, one of the two electrodes is made sufficiently wider than the width of the optical waveguide, and a horizontal electric field is used in addition to a vertical electric field.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIGS. 1A and 1B are cross-sectional views showing the structure of a light control device according to an embodiment of the present invention. Figure 1 (A), (
In B), a Z-plate LiNbO3 substrate is used as the electro-optic crystal substrate 10, and the propagation direction of the light wave is parallel to the X-axis.

The optical control device in FIG. 1(A) has two optical waveguides 11a.
, llb is provided, and electrodes 13 and 14 are formed on the optical waveguides 11a and llb via a nox7 fiber layer 12). At this time, the widths 'v''v'E1 and WB2 in the Y-axis direction of the respective electrodes 13.14 formed on the two optical waveguides 11a and llb are not equal, and one electrode 14 is different from the other electrode 13.14. At this time, since the electrodes 13.14 completely cover the optical waveguides 11a and llb, the width of the electrodes 13.14 is wider than the width WC of the optical waveguides 11a and llb.
3.14 overlap with the optical waveguides 11a and 11b is as follows:
Two optical waveguides 11a, l forming the directional coupler 11
In the region between 1b and 1b, both electrodes 13 and 14 are equal.

On the other hand, outside the directional coupler 11, the two electrodes 13 and 14 have different degrees of overlap. That is, one electrode 14
The width of the electrode 13 is wider than that of the other electrode 13.

Regarding one electrode 14, the outer part of the directional coupler 11 is connected to the optical waveguide 1 formed under the electrode 14 described above.
The width WE2 of the electrode 14 described above is sufficiently wider than the width WG of the optical waveguide 11b formed under the electrode 14. Regarding the other electrode 13, the outer part of the directional coupler 11 completely covers the optical waveguide 11a formed under the electrode 13,
The width WEI of the electrode 13 described above is slightly wider than the width WG of the optical waveguide 11a formed therebelow.

In such an electrode structure, in addition to the m electric field components, there are electric field components perpendicular to the propagation direction of the light wave and parallel to the surface of the electro-optic crystal substrate 10 made of the Z plate L I N b 03 substrate.
That is, an electric field component in the Y-axis direction (hereinafter referred to as a transverse electric field component) is used. The electro-optic constant corresponding to this transverse electric field component is r22 (hereinafter referred to as the transverse electro-optic constant),
This causes a change in the ordinary refractive index no. That is, it contributes only to TE polarization. Therefore, the optical waveguides 11a, ll
When the light wave propagating in b is TE polarized, the structure of the optical control device according to the present invention allows use of both the longitudinal electro-optic constant and the transverse electro-optic constant. Even if they are the same, the change in the refractive index of the ordinary light of the optical waveguides 11a and 11b formed under the electrodes 13 and 14 can be increased. Therefore, lower voltage operation can be obtained compared to conventional structures.

Conversely, if the direction of the transverse electric field component is reversed, that is, if a structure is adopted in which one electrode 13 and the other electrode 14 are interchanged, the ordinary refractive index no.
changes can be made smaller than with conventional structures. therefore,
Since the switching voltage of TE polarized light can be made smaller or larger, it is possible to make the switching voltage of TE polarized light smaller or larger.
TM.

The switching voltages of each of the TE polarized lights can be matched. That is, the switching voltage for TE polarization can be adjusted to the switching voltage for TM polarization, which is a certain constant value. Therefore, in polarization-independent operation, a high crosstalk can be obtained in the application of a switch for switching optical paths, and a high extinction ratio can be obtained in the application of an optical intensity modulator.

Note that the buffer layer 12 is 5102 series, A1203
, ~4g F2 Si○N, Si3 N4, etc. are used, and the electrodes 13 and 14 are Au and An.

MO1IT○, ZnC-based materials, conductive polymers, etc. are used. Note that if the electrodes 13 and 14 do not absorb light, the buffer layer 12 may not be used.

[Effect of Hathu]

As described above, by using the present invention, in an optical control device using a directional coupler consisting of two adjacent optical waveguides, unlike the conventional structure, two optical waveguides formed on two adjacent optical waveguides can be used. One of the electrodes is formed sufficiently wider than the width of the optical waveguide, and a horizontal electric field is used in addition to the vertical electric field, resulting in low voltage and high crosstalk and high extinction in polarization-independent operation. This has the excellent effect of providing a high-speed optical switch and modulator.

[Brief explanation of drawings]

FIGS. 1A and 1B are cross-sectional views of a light control device according to the present invention, and FIG. 2 is a cross-sectional view showing an example of a conventional light control device. 10... Electro-optic crystal substrate, 11 aS'i i b'--optical waveguide, 11... Directional coupler, 13.14... Electrode.

Claims (1)

[Claims] The above-described optical control device includes a directional coupler consisting of two adjacent optical waveguides formed on a crystal substrate having an electro-optic effect, and an electrode formed on each of the two optical waveguides. The width of the electrodes formed on each of the two optical waveguides in the direction perpendicular to the light wave propagation direction is different for each of the electrodes formed on each of the two optical waveguides. An electrode formed on one of the waveguides whose width is approximately the same as the width in the direction perpendicular to the light wave propagation direction of the optical waveguide, and an electrode formed on the other optical waveguide. An optical control device characterized in that the width of the optical waveguide is formed to be sufficiently wider than the width of the optical waveguide.