JPH04237016A - Light control device - Google Patents

Abstract

PURPOSE:To obtain a sufficient crosstalk characteristic for each polarized light beam with the same applied voltage without any polarization dependency of loss. CONSTITUTION:A directional coupler 5 is formed by injecting impurities into a ferroelectric crystal substrate 2 with electrooptic effect as a couple of optical waveguides 1 which have parallel optical axes. A couple of control electrodes 4 are provided on the directional coupler 5 across a buffer layer 3 and the voltage applied to the control electrodes 4 matches a polarization mode (TM mode) wherein an electric field component is perpendicular to the crystal substrate 2 and a polarization mode (TE mode) wherein the electric field component is parallel to the crystal substrate 2; and the length of the directional coupler 5 and the injection amount of the impurities are so set that when the applied voltage is zero, incident light and projection light are nearly equal in quantity in each polarization mode.

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 used in optical communication and optical measurement, and more particularly to an optical control device having an optical waveguide switch used as an optical shutter function.

0002

[Background Art] As the practical use of optical communication systems progresses,
In addition, advanced systems with larger capacity and multiple functions are required, and there is a need to add new functions such as generation of more sophisticated optical signals and switching and exchanging optical transmission lines. In current practical systems, optical signals are obtained by directly modulating the injection current of semiconductor lasers or light-emitting diodes, but direct modulation is difficult to achieve due to effects such as relaxation oscillation.
It has drawbacks such as the difficulty of high-speed modulation at around GHz or higher, and the difficulty of applying it to a coherent optical transmission system due to wavelength fluctuations. One way to solve this problem is to use an external modulator. In particular, a waveguide-type optical modulator constructed from an optical waveguide formed in a substrate has the advantage of being small, highly efficient, and fast. On the other hand, optical switches are used as means for switching optical transmission lines and providing network switching functions. Optical switches currently in use mechanically move prisms, mirrors, fibers, etc., and have drawbacks such as slow speed, insufficient reliability, large size, and unsuitability for matrix formation. A waveguide type optical switch using an optical waveguide is currently being developed as a means to solve this problem, and has features such as high speed, ability to integrate multiple elements, and high reliability. In particular, materials using ferroelectric materials such as lithium niobate (LiNbO3) crystals have features such as low light absorption and low loss, and high efficiency due to large electro-optic effects. Various types of optical control elements have been reported, including directional coupler type optical modulators/switches, total internal reflection type optical switches, and Mach-Zehnder type optical modulators. When using such a waveguide type optical control element as an optical shutter, it is required that the polarization dependence of the insertion loss is small and that there is sufficient crosstalk regardless of the polarization state of the incident light at an appropriate voltage. be done.

FIG. 5 shows a plan view of a conventional optical control device. In order to reduce the polarization dependence of insertion loss when an optical shutter is constructed using a directional coupler type optical waveguide 1a, as shown in FIG. The relationship between the impurity amount C and the complete coupling length LC (the optical waveguide length necessary for the light incident from port 1 of two adjacent optical waveguides 1a to completely exit from port 4) for each incident light polarization is expressed as follows. The optical waveguide 1a was formed with an impurity amount C whose perfect bond lengths matched each other in the polarization state of each incident light.

0004

[Problems to be Solved by the Invention] The conventional optical control device described above has a control electrode 4 formed on the optical waveguide 1a.
The amount of light incident on port 1 and exiting from port 4 shown in Figure 5 for the voltage applied to a is determined in order to achieve the best crosstalk for each polarization of TE and TM as shown in Figure 7. switching voltage (VTM, VTE
, VTM2) are different.

[0005]

[Means for Solving the Problems] The optical control device of the present invention includes a ferroelectric crystal substrate having an electro-optic effect and a pair of optical waveguides whose optical axes are parallel to each other by implanting impurities into the crystal substrate. and a pair of control electrodes provided on the directional coupler via a buffer layer, the light entering from one of the optical waveguides exiting from the other of the optical waveguides. When the electric field component matches the polarization mode perpendicular to the crystal substrate and the polarization mode parallel to the crystal substrate, and the applied voltage is zero, The length of the directional coupler and the amount of impurity implanted are set so that the amounts of the incident light and the output light are approximately equal for each of the polarization modes.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be explained with reference to the drawings. FIG. 1 is a plan view of one embodiment of the present invention.

This embodiment is a directional coupler 5 in which a ferroelectric crystal substrate 2 having an electro-optic effect and a pair of optical waveguides 1 whose optical axes are parallel are formed by implanting impurities into the crystal substrate 2. and 1 provided on the directional coupler 5 via the buffer layer 3.
A pair of control electrodes 4 are provided, and the electric field component is applied to the control electrode 4 necessary for the light incident from port 1 of the optical waveguide 1 to be emitted from the port 2 of the optical waveguide 1. The polarization mode perpendicular to the crystal substrate 2 (TM mode) and the polarization mode parallel to the crystal substrate 2 (TE mode) coincide, and when the applied voltage is zero, the incident light and the output light for each polarization mode are the same. The length of the directional coupler 5 and the amount of impurity implanted are set so that the amounts of light are almost the same.

FIG. 2 is a correlation diagram showing the relationship between the amount of impurity and the bond length of the directional coupler of this example, and FIG. 3 shows the relationship between the bond length and applied voltage of the directional coupler of this example. Correlation Diagram FIG. 4 is a correlation diagram showing the applied voltage and the amount of emitted light for each polarized light in this example.

In this example, a lithium niobate crystal substrate 2 cut parallel to the optical axis (Z-axis) is used, and after forming a film of Ti (titanium) as an impurity to a thickness of 45 nm by sputtering, a film is formed by photolithography. The optical waveguide width of the directional coupler 5 and the width between the optical waveguides were set to 9 μm, and thermal diffusion was performed at a temperature of 1050° C. for 8 hours to form an optical waveguide. Under these conditions, the complete coupling lengths for TE polarized light and TM polarized light are 18 mm and 30 mm, respectively.If a directional coupler 5 with a coupling length of 23 mm is formed under these conditions, the incident light will enter from port 1 regardless of its polarization state. The light emitted from ports 3 and 4 is emitted at a ratio of approximately 1:4, and the amount of light emitted from port 4 has an increase in loss by approximately 0.7 dB compared to the amount of light emitted from port 3, but the loss is polarization dependent. It is possible to achieve a state without

In order to avoid light absorption of TM polarized light by the control electrode on the directional coupler type optical waveguide having such characteristics, after forming an SiO2 film as an optically transparent buffer layer, the control electrode is A metal film is patterned and etched on the sexual coupler using photolithography. When voltage is applied to this control electrode, port 1
As shown in FIG. 4, the incident light is output from the port 4, and the switching voltages for the TE polarized light and the TM polarized light are almost the same, and sufficient crosstalk characteristics can be obtained.

[0011]

Effects of the Invention As explained above, the present invention enables the applied voltage to be applied to the control electrode necessary for light incident from one side of the optical waveguide to be emitted from the other side of the optical waveguide to have an electric field component perpendicular to the crystal substrate. The directionality is such that the polarization mode parallel to the crystal substrate matches the polarization mode parallel to the crystal substrate, and the amount of light entering the optical waveguide and the light outputting it almost match for each polarization mode when the applied voltage is zero. By setting the length of the coupler and the amount of impurity implanted, there is an effect that there is no polarization dependence of loss and sufficient crosstalk characteristics can be obtained with the same applied voltage for each polarization.

[Brief explanation of the drawing]

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

FIG. 2 is a correlation diagram showing the relationship between the amount of impurities and the bond length of the directional coupler of this example.

FIG. 3 is a correlation diagram showing the relationship between the coupling length and applied voltage of the directional coupler of this example.

FIG. 4 is a correlation diagram showing the applied voltage and the amount of emitted light for each polarized light in this example.

FIG. 5 is a plan view of an example of a conventional light control device.

FIG. 6 is a correlation diagram showing the relationship between the amount of impurities and the bond length of a conventional directional coupler.

FIG. 7 is a correlation diagram showing the applied voltage and the amount of emitted light for each polarized light in a conventional example.

[Explanation of symbols] 1 Optical waveguide 2 Ferroelectric crystal substrate 3 Buffer layer 4 Control electrode 5 Directional coupler

Claims (1)

[Claims] 1. A directional coupler comprising a ferroelectric crystal substrate having an electro-optic effect, a pair of optical waveguides with parallel optical axes formed by implanting impurities into the crystal substrate, and the directional coupling. and a pair of control electrodes provided on the device via a buffer layer, and applies an application to the control electrodes necessary for light incident from one of the optical waveguides to be emitted from the other optical waveguide. When the applied voltage is such that the electric field components match for the polarization mode perpendicular to the crystal substrate and the polarization mode parallel to the crystal substrate, and the applied voltage is zero, the incident light and the A light control device characterized in that the length of the directional coupler and the amount of the impurity implanted are set so that the amount of light substantially matches that of the emitted light.