JPH0721595B2 - Waveguide type optical switch - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical control element for optical communication, and more particularly to a waveguide type optical control device whose characteristics do not depend on the polarization state of incident light.

(Prior Art) Practical application of optical communication systems has progressed, and development has been advanced into more advanced systems having large capacity and multiple functions. New functions such as a switching function of an optical transmission network, a high-speed connection between terminals in an optical data bus, and switching are required, and the need for an optical switching network that enables them is increasing.

Currently used optical switches are prisms, mirrors,
It is a means for mechanically moving a fiber or the like, and has drawbacks such as low speed and insufficient reliability, large shape, and unsuitable for matrix formation. What is being developed as a means to solve this is a waveguide type switch that uses an optical waveguide installed on a substrate, and has features such as high speed, multi-element integration, and high reliability. . In particular, a material using a ferroelectric material such as LiNbO ₃ crystal has features such as low light absorption and low loss, and high efficiency because it 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 transmitted through an optical fiber. In a high speed, large capacity optical communication system, a single mode optical fiber is used as the optical fiber. A light wave that has passed through a single-mode fiber is generally elliptically polarized, and its polarization state also changes with time. In conventional waveguide type optical switches, switch characteristics such as voltage and crosstalk largely depend on the polarization state of incident light. Therefore, it is difficult to insert this waveguide type optical switch in the middle of the optical fiber transmission line.

The cause of the polarization dependence of the characteristics of the conventional waveguide type optical switch is that the refractive index of the crystal substrate and the electro-optic constant used have anisotropy. For example, as a typical example, z
A light wave mode T propagating in a waveguide is introduced through a Ti diffusion optical waveguide in a plate lithium niobate crystal and changes in the refractive index Δn _Z and Δn _Y that occur through the electrolytic E _Z between the electrodes provided on the waveguide.
Assuming a device that changes the phase constant of M and TE waves, the electrolytic E _Z and the refractive index changes Δn _Z and Δn _Y caused by it
The proportional constants that connect between and are (ne ³ r ₃₃ ) / 2, (n ₀ ³ r
₁₃ ) / 2. The ratio of these magnitudes is approximately 3: 1 and the phase difference caused by the electric field is three times different, so TM
Even if the electric field strength is sufficient for waves, it is necessary to double the electric field strength for TE waves.

In order to improve this, as a device method using a crystal orientation with little anisotropy, a y-plate lithium niobate crystal is used as a substrate, and a waveguide direction, that is, a light transmission direction is selected as the z axis to form a directional coupling type optical switch. There is an invention to do. In this device system, the refractive index of crystals related to TM and TE waves is the ordinary ray refractive index n ₀ , and the electro-optic constants involved in the applied electric field component E _Y in the depth direction of the substrate are both Combined with the fact that both are r ₂₂ , the absolute values of the changes in the refractive index caused by the electric field E _Y are almost equal, and it is expected that a waveguide device that does not depend on the polarization of incident light can be constructed.

However, as has already been clarified, for the purpose of forming a channel-shaped waveguide in a y-plate lithium niobate crystal, a Ti filament pattern having a longitudinal direction in the z-axis direction was formed, and this was subjected to thermal diffusion. In this case, since the diffusion coefficient of Ti in the direction orthogonal to the longitudinal direction of the pattern, that is, in the x-axis direction is extremely large, it spreads several times wider than the initial linear pattern width and has an effective light confinement effect. It is difficult to build an optical waveguide. Therefore, it is not possible to form a directional coupling waveguide having a gap of several μm, and only a coupling waveguide having an extremely wide waveguide spacing can be formed. For this reason,
The switching voltage becomes high, and it is difficult to obtain a small device.

As described above, in the conventional waveguide type optical switch, the device characteristics greatly differ depending on the polarization state of the incident light.
As mentioned above, it is difficult to insert this device in the middle of the optical fiber transmission line.

(Object of the Invention) An object of the present invention is to eliminate the above-mentioned drawbacks of the conventional waveguide type optical switch and to provide an optical switch whose characteristics do not depend on the polarization state of incident light.

(Structure of the Invention) According to the present invention, a directional coupler composed of two channel optical waveguides formed in a lithium niobate crystal plate and adjacent to each other, and an electrode installed in the vicinity of the directional coupler. The light transmission direction of the directional coupler is set to be the same as the C axis of the crystal plate, impurities are diffused over the entire surface of the substrate, and ion exchange is performed from the surface of the substrate to a portion other than a portion to be a channel. By providing the optical waveguide forming the directional coupler, a waveguide type optical switch whose characteristics do not depend on the polarization state of incident light can be obtained.

According to a second aspect of the present invention, the directional coupler includes two channel optical waveguides formed on a lithium niobate crystal plate and adjacent to each other, and an electrode installed near the directional coupler. The direction of light transmission of the directional coupler is made equal to the C-axis of the crystal plate, impurities are diffused from the substrate surface only in the portions to be channels, and ion exchange is performed from the substrate surface to the portions other than the portions. By providing the optical waveguide forming the directional coupler, a waveguide optical switch whose characteristics do not depend on the polarization state of incident light can be obtained.

Example 1 The present invention will be described in detail below based on examples with reference to the drawings.

FIG. 1 is a top view showing the structure of a directional coupler type optical switch which is an embodiment of the present invention. 11 is a LiNbO ₃ crystal plate, the substrate orientation is the y plate (that is, the normal line standing on the substrate is the y axis),
The light transmission direction is chosen for the z-axis of the crystal. Two optical waveguides close to each other on the crystal substrate and provided on these waveguides,
In the meantime, it is composed of electrodes 4 and 5 for applying a switch voltage. The light 6 whose path is switched by this directional coupler enters from one waveguide 3. The length of this directional coupler in the light transmission direction is almost perfect coupling length for both TE and TM modes of incident light 6 whose polarization state is indefinite (for a certain mode, the light in one waveguide is The length of the coupler required to transfer all the power to the other waveguide is set), and in the state where no voltage is applied to the above electrode, the light is incident from one waveguide 3 now. All the light has its power transferred to the other waveguide 2. When a voltage is applied to the above electrode, the light incident from one of the waveguides 3 does not move to the adjacent optical waveguide 2 and all the power travels through the waveguide and exits from this directional coupler. The reason why the guided light with undefined polarization state having both TE and TM components is switched to the waveguides adjacent to each other without leaving energy, that is, the switching characteristic independent of the polarization state is obtained is as follows.

FIG. 2 is a sectional view taken along line AA ′ of the directional coupler type optical switch of FIG. 1, which is a configuration diagram of an embodiment of the present invention. A structure of a waveguide which enables the above-mentioned directional coupling having no polarization dependence, and the directional coupling waveguide is formed as follows. A Ti film is provided on the entire y surface of the lithium niobate substrate 1 by vapor deposition or the like, and thermal diffusion is performed at about 1000 ° C. On the TiN-diffused LiNbO ₃ substrate, a thin film pattern such as Ti, Ta, etc. having the waveguide pattern shape shown in FIG. Proton (H ⁺ ) exchange is performed. After removing the thin film pattern used as a mask at the time of H ⁺ exchange with acid, etc.,
A SiO ₂ film is provided on the entire surface of the substrate, and electrodes 4 and 5 are provided on the waveguide. The refractive index of the LiNbO ₃ surface layer that diffused Ti slightly increased,
Form a planar waveguide. In order to suppress the lateral spread of light in this planar waveguide and form a channel-shaped waveguide, H ⁺ exchange is performed from the surface except the region to be a channel-shaped optical waveguide. In the region 8 subjected to H ⁺ exchange, the refractive index _nφ for ordinary rays is reduced by 0.04. The crystal plate used here is the y plate, and the light transmitted in the z-axis direction should be TE waves.
Since it is an ordinary ray even for the TM wave, the refractive index in the region subjected to H ⁺ exchange is low for any knitting wave component. That is, the channel-shaped waveguides 2 and 3 are formed by the two effects of the high refractive index layer 7 by Ti diffusion in the depth direction of the substrate and the shallow H ⁺ exchange layer 8 having a low refractive index in the lateral direction.

Since the channel-shaped optical waveguide formed in this way is a waveguide having almost the same equivalent refractive index for both TE and TM waves, the complete coupling length when a directional coupler is formed is equal for both waves. Therefore, by setting the length of the directional coupler equal to this perfect coupling length, all the incident light 6 is 2 when no voltage is applied between the electrodes 4 and 5 shown in FIG. The light is coupled to the waveguide and emitted. further,
The electro-optic constants involved in applying an electric field to cause a change in the refractive index are equal to r _{22 for} both waves, so the way in which the combined light intensity changes with applied voltage is the same, and the optical switching state at equal voltages is the same. That is, TE and TM are all emitted from the waveguide 3.

Second Embodiment The present invention will be described in detail below with reference to the drawings based on an embodiment.

FIG. 3 is a top view showing the structure of a directional coupler type coupler type optical switch according to an embodiment of the present invention. Reference numeral 1 is a LiNbO ₃ crystal plate, the substrate orientation is selected as the y plate (that is, the normal line standing on the substrate is the y axis), and the light transmission direction is selected as the z axis of the crystal. It is composed of two optical waveguides close to each other on the crystal substrate and electrodes provided on these waveguides and for applying a switch voltage therebetween. The light 6 whose path is switched by this directional coupler enters from one waveguide 3. The length of this directional coupler in the light transmission direction is almost perfect coupling length for both TE and TM modes of incident light 6 whose polarization state is indefinite (for a certain mode, the light in one waveguide is The length of the coupler required to transfer all the power to the other waveguide is set), and in the state where no voltage is applied to the above electrode, the light is incident from one waveguide 3 now. Light 6
All the power is transferred to the other waveguide 2. When a voltage is applied to the above electrode, the light 6 incident from one of the waveguides 3 does not move to the adjacent optical waveguide 2 and all the power goes through its own waveguide 3 and exits from this directional coupler. To do. The reason why the guided light with undefined polarization state having both TE and TM components is switched to the waveguides adjacent to each other without leaving energy, that is, the switching characteristic independent of the polarization state is obtained is as follows.

FIG. 4 is a sectional view taken along the line AA ′ of the directional coupler type optical switch of FIG. 1, which is a block diagram of an embodiment of the present invention. It is a structure of a waveguide which allows the above-mentioned directional coupling having no polarization dependence, and is formed like this directional coupling waveguide. Using a vapor deposition and lithographic technique, a Ti film was formed in the shape of the waveguide pattern shown in Fig. 3 at the position where the waveguide should be provided on the y-plane of the lithium niobate substrate of No. 1, and the Ti film was heated at a temperature of over 1000 ° C. Spread. A thin film of Ti, Ta or the like is further formed on the Ti-diffused waveguide 7 by aligning it with the pattern, and is immersed in benzoic acid, phosphoric acid or the like which is heated using this as a mask, and the proton (H ⁺ ) Replace it. After that, the thin film pattern used as a mask at the time of H ⁺ exchange is removed by dissolution with an acid, etc., and a SiO ₂ film 9 is provided on the entire surface of the substrate, and electrodes 4, 5 are further provided on the waveguide.

The region where Ti is diffused forms the channel-shaped waveguide 7 whose refractive index is slightly increased. However, as described above, since the lateral spread of Ti diffusion is large, it spreads several times wider than the initial line pattern width. However, the optical waveguide does not have an effective optical confinement effect. In order to suppress this and form a channel-shaped waveguide corresponding to the initial linear pattern width, H ⁺ AC is applied from the surface except the region to be the optical waveguide. After that, in the region 8 subjected to H ⁺ exchange, the refractive index _nφ with respect to the ordinary ray decreases by 0.04. Crystal plate used here is the y plate, since light passing through the z-axis direction is ordinary ray white to TM wave white in TE wave, H ⁺ region subjected to replacement in any of the polarization components On the other hand, the refractive index is lowered. That is, Ti in the depth direction of the substrate
High refractive index layer due to diffusion, shallow H ⁺ with low refractive index in the lateral direction
Channel-shaped waveguides 2 and 3 are formed by two effects with the exchange layer.

Since the channel-shaped optical waveguide formed in this way is a waveguide having almost the same equivalent refractive index for both TE and TM waves, the complete coupling length when a directional coupler is formed is equal for both waves. Therefore, by setting the length of the directional coupler equal to this complete coupling length, all the incident light 6 is 2 when no voltage is applied between the electrodes 4 and 5 shown in FIG. The light is coupled to the waveguide and emitted. further,
The electro-optic constants involved in applying an electrolysis to cause a change in the refractive index are equal to r _{22 for} both waves, so the way in which the combined light intensity changes with applied voltage is the same, and the switch state of light at equal voltages is the same. That is, TE and TM are all emitted from the waveguide 3.

In this embodiment, in order to form a channel-shaped waveguide,
Both Ti diffusion and H ⁺ exchange are patterned. Ti diffusion does not necessarily have to be patterned, and it may be performed on the entire surface of the substrate, but since the phenomenon that the substrate shows swelling can be used in the region where Ti diffusion is performed, when forming an electrode pattern in a later process. It has the advantage that it is convenient for aligning the mask.

(Effect of the Invention) As described above, according to the present invention, it is possible to obtain a waveguide type directional switch in which the switch characteristic does not depend on the polarization state of incident light, that is, so-called polarization independent.

In the description of the above embodiment, the case where the y-plate is used has been described, but the same polarization-independent optical switch can be obtained even when the x-plate is used and the light transmission direction is selected for the z-axis.

[Brief description of drawings]

1 and 3 are top views for explaining the structure of an embodiment of the present invention, and FIGS. 2 and 4 are cross-sectional views thereof. In the figure, 1 is a LiNbO ₃ y plate, 2 and 3 are waveguides, 4 and 5 are electrodes, 7 is a Ti diffusion part, and 8 is an ion exchange region.

Claims (2)

[Claims] 1. A directional coupler comprising a directional coupler formed of two channel optical waveguides adjacent to each other formed on a lithium niobate crystal plate and an electrode provided in the vicinity of the directional coupler. The light transmission method is the same as the C-axis of the crystal plate, and the optical waveguide forming the directional coupler diffuses impurities over the entire surface of the substrate and ion-exchanges from the substrate surface to a portion other than a portion to be a channel. A waveguide type optical switch characterized by being provided with. 2. A directional coupler comprising a directional coupler composed of two channel optical waveguides formed adjacent to each other on a lithium niobate crystal plate and an electrode provided in the vicinity of the directional coupler. Has the same light transmission direction as the C-axis of the crystal plate, and in the optical waveguide forming the directional coupler, the impurities are diffused only in the portion to be the channel, and the portion other than the portion is ion-deposited from the substrate surface. A waveguide type optical switch characterized by being exchanged.