JPH04288531A - Optical switch - Google Patents

Abstract

PURPOSE:To realize a low driving voltage and a wide band without spoiling the coupling between two optical waveguides of a directional coupler, etc., by etching only one side between the two optical waveguides. CONSTITUTION:Only one side between the two optical waveguides 12 is etched to form a single projection part 12A on a nearby substrate 1 between a center electrode 14 and an earth conductor 15. The two directional coupler type optical waveguides 12 are arranged on the one projection part 12A, one of the optical waveguides 12 is arranged right below the center electrode 14 across a buffer layer 13, and the other optical waveguide 12 is arranged right below the earth conductor 15. Thus, no gap is formed between the two optical waveguides 12 by etching only one side between the optical waveguides 12 of the directional coupler, etc., so the coupling between the two optical waveguides 12 is not spoiled and the low driving voltage and wide band are, therefore, obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wideband optical switch that operates at a low driving voltage.

0002

[Prior Art] Ti thermal diffusion LiNbO3 has a significantly wider optical modulation band than conventional general optical modulators.
(Lithium niobate) As an optical modulator, there is a shield type velocity matching optical modulator as shown in the plan view of FIG. 1 and the cross-sectional view of FIG. 2 (Kono et al.: Electron. Let
t. , vol. 25, pp. 1382-1383, 19
89). In this example, z-cut L with electro-optic effect
A Mach-Zehnder optical waveguide 2 is formed on an iNbO3 substrate 1 by Ti thermal diffusion. A SiO2 buffer layer 3 having a thickness of D is formed on the substrate 1, and a coplanar waveguide (CPW) consisting of a center conductor (center electrode) and a ground conductor (earth electrode) 5 is formed on the buffer layer 3. ) shaped traveling wave electrode is formed. 6 is a terminating resistor connected between the CPW electrodes 4 and 5, and 7 is a power feeding coaxial line connected to the electrodes 4 and 5 to supply a modulating microwave signal to the electrodes 4 and 5. Further, in the vicinity of the region where the optical waveguide 2 and the CPW traveling wave electrode interact, the shield conductor 9 is connected to the center conductor 4 via an overlay 8 with a low dielectric constant, such as air.
It is fixed to the ground conductor 5 in such a way as to enclose it therein. In this example, the width 2W of the center conductor 4 is 8 μm.
, the gap 2G between the conductors 4 and 5 is 15 μm, the thickness D of the SiO2 buffer layer 3 is 1.2 μm, and the thickness T of the conductors 4 and 5 constituting the traveling wave electrode is 4 μm.

When driving power is supplied from the modulating microwave signal feed line 7, an electric field is applied between the center conductor 4 and the ground conductor 5 in this optical modulator. LiNbO3 substrate 1
has an electro-optic effect, so this electric field causes a change in the refractive index. As a result, a shift occurs in the phase of the light propagating through the two optical waveguides 2. When this shift becomes π, a higher-order mode is excited in the multiplexing section of the Mach-Zehnder optical waveguide 2, and the light is turned off.

In the case of this optical modulator, since the CPW electrodes 4 and 5 are configured as traveling wave electrodes, there is a speed difference between the modulating microwave signal propagating through the CPW electrodes and the light propagating through the optical waveguide 2. If there is no difference, ideally there is no limit to the optical modulation band.

However, in reality, the modulation band is limited by the difference in speed between the microwave signal wave and the light, in addition to the microwave propagation loss. The microwave effective refractive index of the CPW electrode for signal waves is nm, and the optical waveguide 2 for light is
If the effective refractive index of is n0, then the 3dB optical modulation band Δf
is inversely proportional to 1/(nm −n0 ). However, this equation of proportionality ignores microwave propagation loss. Therefore, in order to expand the optical modulation band, it is essential to bring the effective refractive indexes of microwaves and light closer to each other, that is, to match the speeds of microwaves and light.

To this end, in the conventional examples shown in FIGS. 1 and 2, the effective refractive indexes of microwave and light are made close to each other by increasing the thickness D of the SiO2 buffer layer 3. Further, by using a shield conductor 9 provided with an overlay 8, the thickness of the overlay 8 is increased to achieve perfect speed matching between microwave and light. In this way, this optical modulator uses the thick buffer layer 3 and the shield conductor 9 to match the speeds of the microwave and light, and the propagation loss of the microwave is kept low, resulting in a significantly wider band. The driving voltage of the device was almost the same as that of other conventional devices.

[0007] Therefore, in order to realize both a wide band and a low driving voltage, a shielded ridge optical modulator as shown in FIGS. 3 and 4, which employs a ridge structure in the shielded optical modulator, has been proposed. (Kono et al., Light Modulation Element: Patent Application No. 2-22208). In this example, in addition to the configuration shown in FIGS. 1 and 2, at least one optical waveguide 2 is formed on the substrate 1 by reducing the thickness of the portion of the substrate 1 near the traveling wave electrode. This increases the thickness of the buffer layer 3 in the vicinity between the traveling wave electrodes, and prevents the traveling wave electrode from contacting the substrate 1 in the region where light and microwave interact. wave electrode 4
, 5 and the substrate 1 are arranged. In addition, at least one of the optical waveguides 2 is arranged directly below the center conductor 4 via the buffer layer 3, and the width of the center conductor 4 is set at least one of the optical waveguides 2 directly below the center conductor 4. The width is set to be approximately equal to or slightly wider than the width of the protruding portion 2A corresponding to one optical waveguide 2. In this structure, 10 is an etching groove dug into the substrate 1 by etching.

[0008]

[Problems to be Solved by the Invention] However, although the optical modulators shown in FIGS. 3 and 4 have a low driving voltage and a sufficiently wide optical modulation band, the gap between the two optical waveguides 2 is also etched. Therefore, the coupling between the two optical waveguides 2 becomes extremely loose. Therefore, although this optical modulator can be applied to an intensity optical modulator or a phase modulator using a Mach-Zehnder type optical waveguide, it cannot actually be applied to an optical switch using a directional coupler.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art and prior art examples and to provide an optical switch with significantly improved drive voltage and optical modulation band characteristics.

0010

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a substrate having an electro-optic effect and having a directional coupler consisting of at least two optical waveguides, and a substrate having an electro-optic effect on the substrate. In an optical switch having a buffer layer disposed on the buffer layer and a traveling wave electrode disposed on the buffer layer, only the portion of the substrate outside the directional coupler is etched and thinned to form two light guides. The area between the wave paths is not etched, or even if it is etched, the etching is shallow compared to the depth of the etching groove outside the directional coupler.

[0011] Here, a shield conductor can be disposed via an overlay near a region where the light propagating through the optical waveguide and the microwave applied to the traveling wave electrode interact.

0012

[Operation] In the present invention, since etching is performed only on one side of each of two optical waveguides such as a directional coupler, two
Both lower driving voltage and wider bandwidth can be achieved without impairing the coupling of the optical waveguide, thereby making it possible to construct an optical switch.

[0013]

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

A plan view and a cross-sectional view of a first embodiment of the invention are shown in FIGS. 5 and 6, respectively. Here, the same parts as in FIGS. 1 to 4 are given the same reference numerals. 12 are two directional coupler type optical waveguides that do not intersect with each other;
4 is a center electrode of the asymmetric coplanar strip, and 15 is a ground conductor of the asymmetric coplanar strip. As shown in FIG. 6, in this embodiment, only one side of each of the two optical waveguides 12 is etched, and a single protrusion 12A is formed on the substrate 1 near the center electrode 14 and the ground conductor 15. formed on top. Two directional coupler type optical waveguides 12 are arranged on this one protruding portion 12A, and one of the optical waveguides 12 is connected to the center electrode 14 through the buffer layer 3. The remaining optical waveguide 12 is placed directly below the ground conductor 15. Note that the width of the center electrode 14 may be approximately the same as the width of the optical waveguide 12. In this way, in this example, only one side of the directional coupler is etched, and the space between the two optical waveguides 12, 12 constituting the directional coupler is not etched. are combined. Therefore, a directional coupler type optical switch can be realized.

In this embodiment, the etching groove 10
FIG. 7 shows the result of calculating the product of the driving voltage (Vπ) and the coupling length L of the optical waveguide (Vπ·L) when the depth T of the optical waveguide is taken as a variable. It can be seen from FIG. 7 that the drive voltage (Vπ) can be reduced by performing the etching process in the manner shown in FIG.

Furthermore, FIG. 8 shows the calculation results of the effective refractive index nm of microwaves when the depth T of the etching groove 10 is taken as a variable in this embodiment. As shown in FIG. 8, by performing the etching process shown in FIG. 6, the effective refractive index nm of microwaves can be reduced. Therefore, in this embodiment, the speed mismatch between microwave and light is alleviated, and the optical switch can be made to have a wider band.

FIGS. 9 and 10 show a plan view and a cross-sectional view of a second embodiment of the invention. In this example, FIGS. 5 and 6
In addition to the configuration of the first embodiment shown in , a shield conductor 9 is arranged. That is, the optical waveguide 12 and the traveling wave electrode 1
4 and 15, one end of the seed conductor 9 is connected to the ground conductor 15 through an overlay 8 of low dielectric constant, such as air, so as to enclose the center electrode 14. It is fixed to the buffer layer 3 on the substrate 1. The remaining parts are the same as in the first embodiment. Note that the width of the center electrode 14 may be approximately the same as the width of the optical waveguide 12.

In the second embodiment, the effective refractive index of microwaves is reduced by providing etching grooves 10 as in the first embodiment, so that the shield conductor 9 can easily reduce the speed of microwaves and light. It is possible to achieve consistency. Therefore, according to this embodiment, it is possible to more easily realize a broadband optical switch with a low driving voltage. Particularly in this case, the characteristic impedance can be increased by forming the etched grooves 10, which is advantageous in suppressing the decrease in characteristic impedance caused by providing the shield conductor 9.

In the first and second embodiments described above, the areas 12, 12 between the two optical waveguides are not etched;
Even if the space between the optical waveguides is etched, the effect of the present invention can be obtained as long as the etching is quite shallow compared to the depth of the etching groove 10 outside the directional coupler. It is clear that the present invention also includes the following.

It is clear that the traveling wave electrode is not limited to the asymmetric coplanar strip described above, but other microwave electrodes such as a symmetrical coplanar strip or a coplanar wave guide may be used. In addition, in the above embodiment, the substrate is z-cut LiNbO.
3 substrate was used, but in the present invention, x-cut LiNbO
3. Substrates with other orientations, such as a substrate 3, or other substrates having an electro-optic effect may also be used.

[0021]

As explained above, in the present invention, by etching only one side of an optical waveguide such as a directional coupler, it is possible to prevent a gap between two optical waveguides. This has the effect of realizing an optical switch with low driving voltage and wide bandwidth without damaging the coupling of the optical waveguide.

[Brief explanation of the drawing]

FIG. 1 is a plan view of a conventional shielded velocity matching optical modulator.

FIG. 2 is a cross-sectional view of a conventional shielded velocity matching optical modulator.

FIG. 3 is a plan view of a shielded optical modulator having a ridge structure, which is an embodiment of the prior application.

FIG. 4 is a plan view of a shield type optical modulator having a ridge structure according to the prior application.

FIG. 5 is a plan view showing a first embodiment of the present invention.

FIG. 6 is a cross-sectional view showing a first embodiment of the present invention.

FIG. 7 is a diagram showing the relationship between the depth of an etching groove and driving voltage in the first embodiment of the present invention.

FIG. 8 is a diagram showing the relationship between etching depth and microwave effective refractive index in the first example of the present invention.

FIG. 9 is a plan view showing a second embodiment of the invention.

FIG. 10 is a cross-sectional view showing a second embodiment of the invention.

[Explanation of symbols]

1 Z-cut LiNbO3 substrate 2 Ti thermal diffusion optical waveguide 2A Protruding portion of substrate 3 SiO2 buffer layer 4 Center conductor of coplanar wave guide 5 Ground conductor of coplanar wave guide 6 Terminating resistor 7 Microwave signal feed line for modulation 8 Overlay 9 Shield conductor 10 Etching groove 12 Directional coupler type optical waveguide 12A Projection part of substrate

Claims (2)

[Claims] 1. A substrate having an electro-optic effect and having a directional coupler consisting of at least two or more optical waveguides;
In an optical switch having a buffer layer disposed on the substrate and a traveling wave electrode disposed on the buffer layer, only the substrate portion outside the directional coupler is etched and thinned; An optical switch characterized in that the area between the two optical waveguides is not etched, or even if it is etched, the etching is shallower than the depth of the etching groove outside the directional coupler. 2. A shield conductor is disposed via an overlay in the vicinity of a region where the light propagating through the optical waveguide and the microwave applied to the traveling wave electrode interact. Optical switch described in.