JPS63313120A - Optical polarization control element - Google Patents

Abstract

PURPOSE:To obtain an element which allows low-voltage driving by forming a thin film having the dielectric constant lower than the dielectric constant of an electrooptic crystal substrate on the surface of said substrate, on which surface a light guide is provided, and providing electrodes on this thin film and the rear face of the substrate. CONSTITUTION:The light guide 11 formed by diffusion of Ti, etc., is provided on one face of the substrate 10 consisting of the electrooptic crystal such as LiNbO3 and further a buffer layer 12 consisting of SiO2, etc., is formed on the surface thereof. The two upper electrodes 13-1, 13-2 consisting of Au, etc., are formed on the layer 12 equidistantly from the light guide 11 and the one lower electrode 13-3 is formed on the rear face of the substrate 10 around the region nearest the light guide 11. Voltages are impressed between the electrodes 13-1, 13-2 and the electrodes 13-3 by power supplies 14, 15 to generate the rotating electric field in the direction perpendicular to the light guide 11, by which the direction of the polarization of the incident light on the light guide 11 is rotated perpendicularly to the plane of the figure.

Description

[Detailed Description of the Invention] [Summary] A low dielectric constant buffer layer is formed on the surface of a high dielectric constant electro-optic crystal provided with a leading waveguide.

Electrodes are provided on this thin film and on the back surface of the electro-optic crystal.

By generating a rotating electric field perpendicular to the optical axis of the guide waveguide, that is, the direction in which light travels in the guide waveguide, a low-voltage driven waveguide-type optical polarization control element is provided.

[Industrial application field]

The present invention relates to a waveguide type optical polarization control element used in coherent optical communication.

[Conventional technology]

Along with the development of optical communication technology, communication technology that utilizes the wave nature of light (coherent optical communication) is being developed. In current coherent optical communications, a method is generally used in which the signal light and other light (local oscillation light) are mixed at the receiving station, and the interference between the two is used to read the information carried by the signal light. There is. In this case, if the polarization states of the signal light and the local oscillation light do not match, a loss will occur, and in extreme cases, the signal will disappear completely. As a countermeasure for this,
Methods such as ``separating the signal light into two orthogonal polarized lights'' and ``mixing the signal light with circularly polarized light'' have been adopted, but losses are still unavoidable.

For this reason, a technique is expected to control the polarization state of one locally oscillated light so that it completely matches the polarization state of the signal light. In order to realize this, a means for converting an arbitrary polarization state into a certain polarization state is required. On the other hand, since the polarization state of the received signal light changes from moment to moment, the conversion means is required to be able to infinitely follow the polarization state of the receiving destination.

As a means for converting the above polarization state, a bulk type optical polarization control element having a structure as shown in FIGS. 2(a) and 2(b) is known. this is.

Electrodes 21-1, 21-2.
21-3, 21-4, and electrode 22-1.2.
2-2.22-3.22-4 and another set consisting of 2-2.22-3.22-4.

Next, electrodes 21-1 and 21-3.21 facing each other
-2 and 21-4, respectively, power supplies 23 and 24
Apply voltage. By controlling each voltage, the direction and strength of the electric field can be controlled, and 1
/2 wavelength plate produces a change in refractive index equivalent to that of a 2-wave plate. As a result, the direction of polarization of the linearly polarized light 25 passing through the electro-optic crystal block 20 in the direction of the arrow is rotated by an amount equivalent to passing through a 172-wave plate. In this way, the linearly polarized light 25 is converted into linearly polarized light tilted at an arbitrary angle.

Similarly, electrodes 22-1 and 22-3.22 facing each other
A voltage is applied between -2 and 22-4 by a power source (9) not shown, and by controlling these voltages, the direction and intensity of the electric field can be controlled, and the electric field can be passed through the quarter-wave plate in any direction. produces an equivalent refractive index change.

As a result, the phase of the linearly polarized light tilted as described above is changed and converted into elliptically polarized light having an arbitrary ellipticity. In this way, it is possible to control the ellipticity of polarized light, which corresponds to rotating a quarter-wave plate.

The above optical polarization control element is used to convert the three local oscillation lights (generated from the modulation laser at the receiving station) into elliptically polarized light that matches the signal light 5 transmitted through the optical fiber. Mix with signal light.

Note that the above-mentioned infinite tracking is possible by infinitely shifting the phase of each power supply voltage in time.

[Problem that the invention seeks to solve]

However, in the device shown in FIG. 2, it is difficult to reduce the thickness of the electro-optic crystal block 20;
The diameter is several millimeters, and even if precision machining is performed with a high level of skill, the diameter is approximately several 100 micrometers at most. For this reason, there is a problem in that a high operating voltage of, for example, several hundred volts is required. Therefore, there has been a demand for an optical polarization control element that can be driven at low voltage.

[Means for solving problems]

What are the problems with the above-mentioned conventional bulk type optical polarization control element?

An electro-optic crystal substrate having two surfaces consisting of planes parallel to each other; and a guide waveguide formed in the vicinity of the first surface in the substrate so as to have an optical axis parallel to the surfaces.

a buffer layer formed on the first surface and having a lower dielectric constant than the electro-optic crystal substrate;

At least one set is provided on each of the buffer layer and the second surface at two different positions on the leading waveguide, and each set is provided on each of the buffer layer and the second surface. It is characterized in that it is composed of three or more electrodes including a book, and includes two sets of electrode groups for generating a rotating electric field perpendicular to the optical axis of the leading waveguide at each corresponding position. This problem is solved by the optical polarization control element according to the present invention.

[Effect]

A thin film with a dielectric constant lower than that of the substrate is formed on the surface of the electro-optic crystal substrate where the optical waveguide is provided.

By providing electrodes on this thin film and on the back of the substrate,
A waveguide-type optical polarization control element that can be driven at low voltage is constructed.

〔Example〕

FIG. 1 is a sectional view showing one embodiment of the optical polarization control element according to the present invention. On one surface (first surface) of a substrate 10 made of an electro-optic crystal such as LiNbO3, for example,
An optical waveguide 11 formed by diffusing titanium (Ti) or the like is provided. Furthermore, a buffer layer 12 made of silicon dioxide (Si02), for example, is formed on this surface. Furthermore, on the buffer layer 12 there is a layer of 3, for example, gold (
Two upper electrodes 13-1 and 13.2 made of
are formed equidistantly from the leading waveguide 11, and on the other hand,
On the back surface (second surface) of the substrate 10, one lower electrode 13-3 is formed, centered on the area closest to the optical waveguide 11. By applying a voltage between each and the lower electrode 13-3 from the power sources 14 and 15, a rotating electric field is generated in a direction perpendicular to the leading waveguide 11. As a result, a rotating electric field is generated perpendicularly to the plane of the paper 3 and entering the leading waveguide 11. The direction of polarization of the light is rotated.

In this embodiment, a groove 16 parallel to the leading waveguide 11 is provided in the vicinity of the leading waveguide 11 on the back surface of the substrate IO. In this case, the lower electrode 13-3 is.

It is sufficient that it is formed at least on the bottom surface 16-1 of the groove 16. Note that the groove 16 does not necessarily have a bottom surface 16-1.
does not need to be provided parallel to the back surface of the substrate IO.

Another set of upper and lower electrodes is formed around the guide waveguide 11 in the same manner as described above.

By controlling the voltage applied between each set of upper and lower electrodes in the optical polarization control element constructed in this way, the same function as the element shown in FIG. 2 can be obtained.

The buffer layer 12 made of 5i02 has the inherent effect of a buffer layer of sufficiently isolating the electrode against light (preventing the light from seeping out of the leading waveguide 11 and being absorbed by the electrode), and has an improved property compared to the substrate 10 made of LiNbO3. Since its dielectric constant is as small as about 1/6, it has the effect of making it electrically equivalent to increasing the distance.

In the structure shown in FIG. 1, the thickness of the buffer layer 12 made of 5i02 is 1, for example, 4000, and the thickness of the substrate lO made of LiNbO3 at the groove 16 portion and the thickness of the groove 16 are as follows.
The width of

The driving voltage in this case, for example 50 μ-, is 40 volts, assuming an electrode length of 3 CI, which is reduced to 115 to 10 times that of the conventional device of FIG. In addition.

The groove 16 having the above dimensions can be formed with high precision using a dicing saw or the like.

FIG. 3 is a cross-sectional view showing a second embodiment of the optical polarization control element according to the present invention, in which the same parts as in FIG. 1 are designated by the same symbols as shown in FIG.

In this embodiment, a guiding waveguide 1 is provided on the back surface of the substrate 10.
Two lower electrodes 31 and 32 are formed equidistantly from each other with 1 in between. The lower electrodes 31 and 32 are connected to the optical waveguide 1
1 and may be formed inside grooves 33 and 34 provided equidistantly from each other with the leading waveguide 11 in between.

In this case, the lower electrodes 31 and 32 are small.

It is necessary that the grooves 33 and 34 are formed on the bottom surfaces 33-1 and 34-1.

In the case of the optical polarization control element shown in FIG. Apply.

Note that the conditions in this embodiment, such as the material and thickness of the buffer layer 12, the material of the substrate 10, and the thickness at the bottom of the grooves 33 and 34, are the same as described in the explanation of the embodiment of FIG.

FIG. 4 is a sectional view showing a third embodiment of the optical polarization control element according to the present invention. In the optical polarization control element of this example,
A reinforcing substrate 17 made of a material having similar thermal expansion characteristics to the electro-optic crystal of the substrate 10 is bonded onto the buffer N12 on which the upper electrodes 13-1 and 1:3-2 are formed. The reinforcement substrate 17 increases the mechanical strength of the optical polarization control element.

FIG. 5 is a side view showing the shape of a groove provided in the optical polarization control element according to the present invention. The groove 16 (FIGS. 1 and 4) and the grooves 33 and 34 (FIG. 3) provided in the substrate 10 in the optical polarization control element of the above embodiment are formed so as to traverse the entire substrate 10. However, as shown in FIG. 5, by having a structure in which both ends in the longitudinal direction are blocked by walls 18-1 and 18-2 made of the substrate 10, the mechanical structure of the optical polarization control element can be improved. can increase the strength of the target.

〔Effect of the invention〕

According to the present invention, it is suitable for coherent optical communication.

This has the effect of making it possible to provide an optical polarization control element that can be driven at low voltage.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing one embodiment of the optical polarization control element according to the present invention. FIG. 2 is a schematic diagram showing the structure of a conventional optical polarization control element. FIG. 3 is a sectional view showing a second embodiment of the optical polarization control element according to the present invention. FIG. 4 is a sectional view showing a third embodiment of the optical polarization control element according to the present invention. FIG. 5 is a side view showing the shape of a groove provided in the optical polarization control element according to the present invention. In fig. 10 is a substrate, 11 is a leading waveguide, and 12 is a buffer layer. 13-1 and 13-2 are upper electrodes. 13-3.3L 32 is the lower electrode. 14 and 15 are power supplies, 16.33.34 are grooves. 16-1.33-1.34-1 is the bottom surface, 17 is a reinforcing board. 18-1 and 18-2 are wall portions. It is. ``The invention of the invention is ￟￥1, the owner of the garden H, the ear of the fuichi, the eel ita' 1, the 3, the figure, the foot 3, the east, the Q, the 1,000, 2, the senbihatsu, H1, and the 3rd grade of the three points. 4 Fig. 1 element from east 1;

Claims (1)

[Claims] 1) An electro-optic crystal substrate having two surfaces consisting of planes parallel to each other, and having an optical axis parallel to the surfaces in the substrate near the first surface. an optical waveguide formed; a buffer layer having a dielectric constant smaller than that of the electro-optic crystal substrate; and a buffer layer formed on the first surface; and a buffer layer formed on the buffer layer at two different positions on the optical waveguide. and a second surface, each set comprising three or more electrodes, each set comprising at least one electrode provided on each of the buffer layer and the second surface. , two sets of electrode groups for generating rotating electric fields perpendicular to the optical axis of the optical waveguide at corresponding positions. 2) The optical polarization control element according to claim 1, wherein the electrodes formed on the buffer layer are two electrodes provided equidistantly from the optical waveguide. 3) The electrode formed on the second surface of the electro-optic substrate is
The optical polarization control element according to claim 1, characterized in that it is two electrodes provided equidistantly from the optical waveguide. 4) A groove having a bottom parallel to the optical axis of the optical waveguide is provided on a second surface of the electro-optic substrate, and the electrode formed on the second surface is formed on the bottom of the groove. An optical polarization control element according to claim 1, characterized in that: 5) The optical polarization control element according to claim 4, wherein the groove is formed only in the center of the substrate. 6) Claim 1, characterized in that another substrate having approximately the same thermal expansion coefficient as the electro-optic crystal substrate is bonded onto the buffer layer on which the two sets of electrodes are formed.
The optical polarization control element described in .