JPH0750285B2 - Optical switching method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical switch that modulates a light wave, switches an optical path, and the like in optical communication and the like, and particularly relates to a waveguide type using an optical waveguide formed on a substrate. Regarding optical switches.

[Prior art and its problems]

Optical communication systems are being put to practical use, and development is being advanced to more advanced systems with large capacity and multiple functions. New functions such as switching function of optical transmission line network, high-speed connection between terminals in optical data bus, and switching are required, and the need for an optical switching network that enables them is increasing. Currently used optical switches mechanically move prisms, mirrors, fibers, etc., and have drawbacks such as low speed, insufficient reliability, large shape, and unsuitable for matrix formation. There is. What is being developed as a means to solve this is a waveguide type optical switch using an optical waveguide installed on a substrate, and has features such as high speed, multi-element integration, and high reliability. In particular, the one using a ferroelectric material such as LiNbO ₃ crystal is
It has features such as high efficiency because it has low light absorption and low loss and has a large electro-optical effect.

In general, an optical switch is often inserted in an optical transmission path and used to switch the optical path of an optical signal propagating in an optical fiber. In a high-speed, large-capacity optical communication system, a single-mode optical fiber is used as an optical fiber, and a semiconductor laser is used as a light source. The semiconductor laser light emits linearly polarized light, but the light wave propagated in the single-mode optical fiber generally becomes elliptical polarized light, and its polarization state also fluctuates with time. On the other hand, in the above-mentioned waveguide type optical switch,
In the case of a normal configuration, there is a drawback that characteristics such as switch voltage and crosstalk largely depend on the polarization state of incident light. FIG. 2 is a perspective view showing a directional coupling type optical switch which is an example of a conventional waveguide type optical switch. LiNbO ₃ crystal substrate cut and shaped perpendicular to the optical axis, that is, the z-axis direction
Optical waveguides 32 and 33 are formed on 31 by diffusing a metal such as Ti. The optical waveguides 32 and 33 are arranged close to each other at an interval of about several μm to form an optical directional coupler 34, and a SiO ₂ film ( _second layer) that is a buffer layer is formed on the optical waveguides 32 and 33.
Control electrodes 35 and 39 are installed via (not shown). The basic operating principle of this optical switch is that one side optical waveguide, for example, the light wave 16 incident from the end face of 32 is the optical waveguide.
When propagating through 32, energy is transferred to the optical waveguide 33 which is close to the optical directional coupler 34, and the length of the optical directional coupler 34 is matched with the complete coupling length L _c , almost 100. % Of the energy is transferred to the optical waveguide 33 and becomes emitted light 37. On the other hand, when a voltage is applied between the control electrodes 35 and 39, the refractive indexes of the optical waveguides 32 and 33 change due to the electro-optic effect, and the refractive indexes of the two become asymmetric, resulting in phase difference between the light waves propagating through both. Matching occurs and the coupling state changes, and under an appropriate applied voltage, energy is transferred to the original optical waveguide 32 and becomes emitted light 38.
Here, the propagation light of the optical waveguide formed on the substrate is generally separated into two independent modes, that is, a TM mode whose polarization direction is perpendicular to the substrate surface and a TE mode having a polarization component orthogonal to the TM mode. Usually, the amount of change in the refractive index that changes due to the electro-optical effect differs depending on the polarization direction, and as a result, the switch voltage also greatly changes depending on the polarization direction. For example, in the case of FIG. 2, the refractive index change amounts obtained for TM mode and TE mode are Becomes Where r ₃₃ and r ₁₃ are electro-optic constants,
n _e and n _o are refractive indices for extraordinary light and ordinary light, and E _z is Z
The electric field strength applied in the direction. In the case of LiNbO ₃ crystal,
Since r ₃₃ > 3r ₁₃ , δn _TM > 3δn _TE , and the TE mode switch voltage is three times or more the TE mode switch voltage. Therefore, it is usually necessary to match the incident light with the polarization state of either the TM mode or the TE mode, and the optical switch shown in FIG. 2 cannot be used by inserting it into a single-mode optical fiber transmission line. .

In addition to the directional coupling type shown here, there are total reflection type, balanced brush type, Y-branch type, etc. in the waveguide type optical switch. Compare the switch voltage and crosstalk that are important for the optical switch. The directional coupling type is the one that can be easily lowered and is the simplest to construct.

An object of the present invention is to provide an optical switch which does not depend on the polarization state of incident light, has a low switch voltage and is easy to manufacture, except for the above-mentioned drawbacks of the conventional waveguide type optical switch.

[Means for solving problems]

In order to achieve the above object, the present invention provides an optical directional coupler formed on a crystal substrate having an electro-optical effect, the optical directional coupler including two optical waveguides close to each other, and the optical waveguide on each of the two optical waveguides. An optical switch composed of installed first and second control electrodes and a third control electrode which is installed facing the second control electrode and in proximity to the optical waveguide is used. The optical directional coupler is set so that the complete coupling lengths for both TM and TE modes are substantially equal to each other, and
By applying a voltage that causes a potential difference between the third control electrode and the first control electrode, an electric field component perpendicular to the surface of the substrate and a horizontal direction in the optical waveguide below the second control electrode are applied. The electric field components are simultaneously generated to simultaneously control the outputs of both the TM and TE modes.

[Action]

In a conventional directional coupling type optical switch, electrodes are installed only on two optical waveguides forming a directional coupler, and one direction (for example, the second direction) is used as an electric field component that causes a change in refractive index.
In the optical switch of the present invention, the third control electrode is installed to effectively use the electric field components in two directions orthogonal to each other while the electric field component in the z direction (in the figure) is used. Thus, the amount of change in the refractive index for both TE modes can be made closer, and the switch voltage can be reduced.

〔Example〕

Next, the present invention will be described with reference to FIG. First
FIG. 1 is a perspective view showing an embodiment of the optical switching method according to the present invention. A Ti film pattern having a thickness of several hundred to 1,000 Å and a width of several to several tens of μm was formed by thermal diffusion on a LiNbO ₃ substrate 11 having the same shape as the conventional directional coupling type switch shown in FIG. Optical directional coupler 14 with optical waveguides 12 and 13 installed close to each other
Are configured. First and second control electrodes 15 and 19 are installed on the optical waveguides 12 and 13, respectively, and a third control electrode 20 is installed to face the control electrode 19 and close to the optical waveguide 13. . The length of the optical directional coupler 14 is the same as in the conventional example,
It is set to be almost equal to the full coupling length for both TM and TE modes. Its length is usually several mm to several tens of mm. The basic operation principle is the same as that described with reference to FIG. 2, but in this embodiment, the refractive index change amount δn _TE due to the applied voltage for the TE mode is a value due to the electric field component E _{z in the} z direction. And the amount of change due to the electric field component E _{y in the} y direction Is the sum of For example, when a positive voltage V ₁ is applied to the first control electrode 15, a second control electrode 19 is grounded, and a negative voltage V ₂ is applied to the control electrode 20, a positive voltage V ₁ is applied to the optical waveguide 13 at the same time as a z-direction electric field component. Electric field E _{y in the} y direction caused by the potential difference between ₁ and the negative voltage V ₂
Occurs, and the above-described refractive index change amount δn _TE ″ occurs via the electro-optic constant r _22. The value of δn _TE ″ is a negative voltage V ₂
It can be adjusted by the value of. That is, since the conventional optical switch shown in FIG. 2 uses only the change in the refractive index due to the electric field component E _{z in} the z direction, the refractive index change amount δn _TE for the TE mode is the refractive index change amount δn for the TM mode. _TM
It is about 1/3 of the above, and it is impossible to bring the refractive index change amounts δn _TE and δn _TM close to each other. However, in this embodiment, the refraction amount is increased by applying an applied voltage to the third control electrode 20. rate variation .DELTA.n _TM becomes only can be increased, closer to the refractive index change .DELTA.n _TE and .DELTA.n _TM, it is possible to obtain a light switching method polarization dependence at a low voltage.

Although a Z-plate crystal (a crystal whose z-axis is perpendicular to the substrate surface) was used in this example, a Y-plate or an X-plate crystal is used to generate a lateral electric field E _z and a depth electric field E _y or E _x . Even if both electric field components are used, the same effect as this embodiment can be obtained.

〔The invention's effect〕

As described above, according to the present invention, an optical switching method that does not depend on the polarization state of incident light, has a low switch voltage, and is easy to manufacture can be obtained.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of an optical switch according to the present invention, and FIG. 2 is a perspective view showing an example of a conventional optical switch. 11,31 …… LiNbO ₃ crystal substrate, 12,13,32,33 …… Optical waveguide,
14,34 …… Optical directional coupler, 15,19,20,35,39 …… Control electrode, 16 …… Light wave, 17,18,37,38 …… Outgoing light.

Claims (1)

[Claims] 1. An optical directional coupler formed on a crystal substrate having an electro-optical effect, the optical directional coupler including two optical waveguides adjacent to each other, and first and first optical waveguides respectively installed on the two optical waveguides. An optical switch composed of two control electrodes and a third control electrode facing the second control electrode and arranged close to the optical waveguide is used, and the optical directional coupler is The complete coupling lengths for both TM and TE modes are set to be substantially equal to each other, and a voltage is applied between the third control electrode and the first control electrode to generate a potential difference. An optical switching method characterized in that an electric field component perpendicular to a substrate surface and a horizontal electric field component are simultaneously generated in an optical waveguide under a second control electrode, and outputs of both the TM and TE modes are controlled simultaneously.