JPH01178917A - Light control circuit and production thereof - Google Patents

Abstract

PURPOSE:To lower voltage and losses and to eliminate a DC drift without requiring a buffer layer by introducing a 2nd metal ion to decrease the refractive index of a substrate surface into the substrate surface thereby embedding light guides into the substrate. CONSTITUTION:The buffer layer is not formed between transparent electrodes (ITO) 103 and the LiNbO3 substrate 101 and an Mg-diffused layer 104 diffused with the Mg ion is formed on the surface of the LiNbO3 substrate 101. The light guides 102 are formed by diffusing Ti into the LiNbO3 substrate 101 and hereafter, the Mg which is the ion to lower the refractive index of the LiNbO3 substrate 101 conversely from the Ti ion is diffused into the substrate surface. The refractive index direction in the depth direction is then symmetrized and the position where the refractive index is max. can be made as the position of the depth of several mum in the substrate. The embedded guides 102 having the position of the max. light intensity in the depth of several mum in the substrate can be formed by selecting the diffusion depth of the Mg at this time. The voltage required for switching is thus lowered by the effect of removing the buffer layer and the phenomenon of the DC drift is prevented as well.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an optical circuit that modulates a light wave, switches an optical path, and converts a mode of a light wave, and particularly relates to an optical circuit that performs control using an optical waveguide provided in a substrate. Regarding wave-type optical circuits.

(Prior art and its problems) In recent years, as optical communication systems and optical signal systems have become more practical, systems that can process even larger capacity optical signals and have higher functionality are required. There is a growing need for low-loss, low-voltage, high-speed optical control elements. As a high-speed optical control element, there is a method in which a Ti-diffused optical waveguide is formed in a LiNbO3 crystal substrate that has a large electro-optic effect, and the refractive index distribution of the waveguide is controlled by changing it with an electric field using the electro-optic effect. These optical control elements are well known, and reports have been made regarding directional coupler type optical modulators or switches, total internal reflection type switches, branching interference optical modulators or switches, etc. For example, LiN
In the optical waveguide formed by diffusing Ti in the bO3 crystal, 0.1 to 0.2 for propagating light with a wavelength of 1.3 pm
A small propagation loss of dB/am has been obtained, and since the spot size of the Ti-diffused LiNbO5 optical waveguide can be relatively easily matched to the spot size of the single-mode fiber output light, the coupling loss with the single-mode fiber can be reduced. It has the advantage of being relatively small. However, because
This value of propagation loss of 0.1 to 0.2 dB/cm is obtained when a metal electrode for applying an electric field is not formed on the substrate.
Alternatively, a buffer layer 601 as shown in FIG. 6(b) (-A-A' cross-sectional view of a directional coupler type optical switch is shown as an example) is formed between the electrode and the substrate. It is a value. LiNb0 usually used for light control elements
For the orientation of the a-substrate and the polarization of the guided light, two plates and TM mode are selected so that r33, which is the largest electro-optic constant, can be used.

At this time, as shown in FIG. 6(e), if a metal electrode is directly formed on the substrate without a buffer layer, the guided light will be greatly absorbed by the metal electrode, and the propagation loss will increase significantly. This is because the refractive index distribution in the depth direction of the Ti diffused optical waveguide is such that the maximum refractive index is at the substrate surface, and the depth direction energy distribution of the guided light is also concentrated on the substrate surface. It is. Although it is possible to use a transparent electrode as the electrode, the absorption coefficient of currently known transparent electrodes is still large at several hundred cm-'', so it is assumed that the depthwise energy distribution of the guided light is concentrated on the substrate surface. A large absorption loss still occurs.As a method to eliminate such absorption loss in the electrode, a buffer layer using a dielectric film such as 5i02 is placed between the electrode and the LiNb0a substrate as shown in FIG. 6(b). However, since the dielectric constant of a dielectric buffer layer such as 5i02 is usually larger than that of LiNb0a, the electric field strength actually applied to the LiNbO3 substrate is lower than that in the case without a buffer layer. There is a problem in that the operating voltage of the device increases as the dielectric buffer layer decreases considerably.In addition, by introducing the dielectric buffer layer, space charges accumulate at the interface between the metal electrode and the buffer layer and the interface between the buffer layer and the LiNb0a substrate. However, it is known that a so-called DC drift phenomenon occurs in which the operating state of an element changes over time, and this is also a problem.

One way to eliminate the above problems is to
There is a method of eliminating the buffer layer by embedding a Ti-diffused optical waveguide by diffusing ions that reduce the refractive index of LiNbO3 onto the substrate surface after forming the i-diffused optical waveguide.

Mg ions are ions that reduce the refractive index of LiNb0a, and Mg ions are added after forming the Ti diffused optical waveguide.
By thermally diffusing the gO film into the LiNbO3 substrate,
According to the present application, the refractive index of the substrate surface of the Ti-diffused optical waveguide decreases, and the position where the energy of guided light is maximum changes from the substrate surface to a position at a certain depth in the substrate, making it possible to embed the Ti-diffused optical waveguide. The inventors attended the 8th Topical Meet on Integrated Optics and Waveguide Optics.
ing on Integrated 0ptics
nd Guided-Wave Optics) Technical Digest FDP-2. However, it is possible to completely embed the waveguide by diffusing Mg ions after forming the Ti-diffused optical waveguide, that is, to embed the waveguide so that no energy leaks into the electrodes formed directly on the substrate surface without a buffer layer. is extremely difficult. Furthermore, even if it were possible to completely bury the light, the position of maximum energy in the depth distribution of light energy would be located deep into the substrate, resulting in an increase in operating voltage, so this method is also not suitable. I can't say it's a good idea.

(Means for Solving the Problems) The first invention of the present application provides an optical waveguide formed by introducing metal ions that increase the refractive index of the substrate into a dielectric substrate;
a light control circuit comprising a transparent electrode, at least a diffusion layer formed on the surface of the dielectric substrate in the optical waveguide portion under the transparent electrode by introducing a second metal ion that acts to reduce the refractive index of the substrate; This is an optical control circuit characterized in that it is provided with.

The second invention of the present application is to form a first thin film containing a metal element that increases the refractive index of the substrate on a dielectric substrate in a desired pattern, and then heat the substrate to form the thin film pattern in the substrate. a second thin film pattern containing a metal element that reduces the refractive index of the substrate on at least a partial region of the substrate, and heating the substrate again to form the second thin film pattern. The method includes at least the steps of: forming an optical waveguide by diffusing the first and second thin film patterns into the substrate; and forming a transparent electrode on the optical waveguide on which the first and second thin film patterns are diffused. This is a characteristic method of manufacturing an optical control circuit.

(Function) In the present invention, as described above, the first metal ions that increase the refractive index of the substrate of the optical waveguide constituting the optical control circuit are introduced into the substrate to reduce the refractive index of the substrate surface. The optical waveguide is embedded to some extent in the substrate by introducing second metal ions into the substrate surface, and a transparent electrode is used as an electrode for applying an electric field to control the light propagating through the waveguide. By adopting this transparent electrode, the electrode absorption loss hardly increases even if the waveguide is not completely buried by introducing the second metal ion, so there is no need for a buffer layer, low voltage, low loss, and no DC drift. A light control circuit can be obtained.

(Example) The present invention will be described in detail below with reference to the drawings.

FIG. 1(a) is a perspective view of a Ti-diffused LiNbO3 directional coupler type optical control circuit according to the present invention, and FIG. 1(b) is a sectional view taken along line AA'.

In FIG. 1, Ti is contained in the LiNbO3z substrate 101.
A band-shaped Ti diffused optical waveguide 102 formed by diffusing
, -pair of transparent electrodes (ITO) 103 constitute an optical directional coupler. Here, as shown in the AA' cross section in FIG. 1(b), the transparent electrode (ITO) 103
No buffer layer is formed between the LiNbO3 substrate 101 and the LiNbO3 substrate 101, and an Mg diffusion layer in which Mg ions are diffused is formed on the surface of the LiNbO3 substrate 101.

In the optical directional coupler whose configuration is shown in FIG.
03, the length of the directional coupler is made to match the perfect coupling length, so the output side optical waveguide 106a (the input side optical waveguide 105b
However, when a voltage is applied between the transparent electrodes 103, the refractive index of the Ti diffused optical waveguide changes due to the electro-optic effect, and the two Ti diffused Since a phase mismatch occurs between the optical waveguides 102, the light wave incident from the waveguide 105a is emitted to the output side optical waveguide 106b, and the voltage applied to the transparent electrode 103 can cause light switching.

A method of manufacturing the optical directional coupler shown in FIG. 1 will now be described. First, an optical waveguide pattern of a directional coupler made of a Ti film is formed on a LiNbO3 substrate 101 using a normal photolithography technique. That is, by lift-off method or etching, the thickness is 500 to 1500, and the width is several.
A waveguide pattern of about 1011 m of Ti film 201 is formed (FIG. 2(a)). The substrate on which the waveguide pattern is formed with Ti is heated at 1000-1100°C15-10
Ti is converted into LiN by heating in a diffusion furnace for about an hour.
It is diffused into the b0a substrate, and the refractive index of that portion increases slightly, forming an optical waveguide 103 (FIG. 2(b)). Next, about 100 to 500 MgO thin films 202 are formed on the entire surface of the substrate by sputtering or the like. As shown in FIG. 2(C), the substrate on which the MgO thin film 202 is formed has a thickness of 700 to 1000
By heating in a diffusion furnace for 12 to 10 hours at °C, Mg ions are diffused into the LiNbO3 substrate, and an Mg diffusion layer 104 is formed on the substrate surface (Fig. 2(d)).
. Here, it goes without saying that the Mg ions are also diffused on the surface of the Ti diffused optical waveguide 102 formed by diffusing Ti. Thereafter, a pattern of a transparent electrode 103 made of ITO is formed using a conventional photolithography technique (FIG. 2(e)), and finally, an optical input/output end face is formed by polishing or cleaving perpendicular to the input/output optical waveguide. The above is the method for manufacturing a directional coupler type optical control circuit, and the directional coupler type optical control circuit shown in FIG. 1 is formed by the above manufacturing method.

Next, the principle of obtaining a directional coupler type optical control circuit with low loss, low voltage, and no DC drift according to the present invention will be explained. In a conventional optical waveguide formed by diffusing only Ti into a LiNb0a substrate, the refractive index distribution in the depth direction has the maximum refractive index n2 at the substrate surface, as shown in Figure 3(e). As shown in FIG. 3(d), the light intensity distribution in the horizontal direction oozes out to the electrode side, and if the electrode is a metal electrode, the ooze out portion is absorbed. In this case, even if the electrode is a transparent electrode, the seepage toward the electrode side is quite large, so the electrode absorption is still large considering the current absorption coefficient of the transparent electrode.

After Ti is diffused into the LiNbO3 substrate 101 to form an optical waveguide, when Mg, which is an ion that lowers the refractive index of the LiNbO3 substrate in contrast to Ti ions, is diffused to the substrate surface, as shown in FIG. The refractive index distribution becomes symmetrical,
The position of the maximum refractive index can be located at a depth of several 11 meters within the substrate. At this time, by selecting the diffusion depth of Mg, it is possible to form a buried waveguide having a position of maximum light intensity at a depth of several pm in the substrate, as shown in FIG. 2(b). At this time, although there is some seepage into the electrodes in the depth direction light intensity distribution, the amount of seepage is considerably smaller than in the case of an optical waveguide formed by diffusing only Ti. Therefore, if the electrode is a metal, a non-negligible absorption loss occurs, but if the electrode is a transparent electrode, the absorption loss is very small. The light intensity distribution at this time is as shown in Figure 3(b), where the position of maximum light intensity is T.
Compared to the case of only i-diffusion, the depth is only a few pm, and the light intensity distribution hardly spreads. Therefore, the overlap between the guided light and the electric field is only slightly degraded compared to the case of only Ti diffusion. The increase in operating voltage due to the deterioration of this overlap can be sufficiently compensated by the effect of eliminating the need for a buffer layer according to the present invention, and also because of the effect of removing the buffer layer, the voltage required for switching is reduced only by Ti diffusion, and the dielectric buffer layer can be reduced from what would otherwise be required. Further, according to the present invention, since the buffer layer can be removed, it goes without saying that the phenomenon of DC drift can be prevented. The diffusion depth of Mg is shown in Figure 3 (a
) and (b), the position of the maximum refractive index is made deeper as shown in Fig. 3(e), and the refractive index is buried deep into the substrate as shown in Fig. 3(D). It is also conceivable to reduce the absorption loss in the electrode by forming an optical waveguide.However, in this case, the
As shown in , not only is the position of the maximum light intensity deep within the substrate, but also the half-width of the depth direction light intensity distribution is
Since the area is considerably wider than in the case of only diffusion, the overlap between the electric field and the guided light is greatly reduced, which leads to an undesirable increase in the operating voltage.

The present application shows that the coupling loss between the waveguide and the single mode fiber can be significantly reduced by diffusing Mg ions onto the substrate surface after forming the Ti-diffused optical waveguide to make the optical intensity distribution in the depth direction symmetrical. 8th Conference on Integrated Optics and Waveguide Optics (sthTopical Meet
ting on Integrated and Gu
It is reported in the Technical Digest FDP-2 of IDE-WaveOptics), and it is also clear that an optical control element (optical phase modulator) manufactured using this method can achieve a low insertion loss of only 1.1 dB. At the Optical Fiber Communications Conference in 1987, the inventors of the present application
It was reported in the Technical Digest WK5 of the 2011 Technical Conference. Therefore, it goes without saying that according to the present invention, it is possible to couple not only low voltage and low absorption loss but also single mode fiber with low loss. Note that the element manufactured in the above report focuses on reducing coupling loss with a single mode fiber, and the electrode is made of metal and has a dielectric buffer layer. By using the configuration of the present invention and using transparent electrodes and removing the buffer layer, it is possible to lower the voltage while maintaining the insertion loss.

FIG. 5(a) is a perspective view of a Ti-diffused LiNbO3 branching interference type optical modulator which is another embodiment of the optical control circuit according to the present invention, and FIG. 5(b) is a cross-sectional view taken along the line AA'. be.

In FIG. 5, the light incident from the incident side optical waveguide 105a is branched into two optical waveguides at a 1:1 ratio at a 3 dB branching section 501. When no voltage is applied to the transparent electrode, the light branched into the two optical waveguides passes through the convergence section 502.
Since they are merged in the same phase at , they are emitted as they are from the output side optical waveguide 106a. Transparent electrode (ITO) 103
When a voltage is applied to the electrode, the refractive index of the waveguide directly under the electrode changes due to the electro-optic effect, but the phase of the light changes by n/2 at the electrode.
(Vn/half of the half-wavelength voltage Vn)
When 2) is applied, the light having opposite phases is combined at the merging section 502, so that the guided light is radiated into the substrate at this time, and no light is emitted from the output side optical waveguide 106a. Therefore, the light control element shown in FIG. 1 operates as an optical modulator capable of 0N10FF of incident light depending on the voltage.

In the branching interference type optical modulator shown in FIG. 1(a),
a Ti diffused optical waveguide 102 formed by diffusing Ti;
A branching interference type optical modulator is formed by the transparent electrode (ITO) 103. Note that the Ti diffused optical waveguide pattern includes a 3 dB branching section 501 and a merging section 502. Here, as shown in the AA' cross section in FIG. 1(b), a transparent electrode (ITO) 103 and a LiNb0a substrate 101
No buffer layer is formed between LiNbO
A Mg diffusion layer in which Mg ions are diffused is formed on the surface of the third substrate 101. Ti-diffused LiN shown in Figure 5
The manufacturing method of the b0a branching interference type optical modulator is the same as that of the first method except that the Ti diffused optical waveguide and electrode patterns are different.
Since this is the same as the directional coupler type optical control circuit in the embodiment, the explanation will be omitted here.

In this example as well, after Ti is diffused into the LiNbO3 substrate 101 to form an optical waveguide, Mg, which is an ion that lowers the refractive index of the LiNbO3 substrate in contrast to Ti ions, is diffused to the substrate surface to increase the depth. A buried optical waveguide is formed in which the directional refractive index distribution is made symmetrical and the position of the maximum refractive index is located at a depth of several pm within the substrate. At this time, in the depth direction light intensity distribution, although some energy seeps into the electrode, the amount of seepage is considerably smaller than in the case of an optical waveguide formed by diffusing only Ti. Therefore, if the electrode is a metal, a non-negligible absorption loss occurs, but if the electrode is a transparent electrode, the absorption loss is very small. As shown in FIG. 3(b), the light intensity distribution at this time is only several tens of meters deeper than in the case where only Ti diffusion is used, and the light intensity distribution hardly spreads. Therefore, the overlap between the guided light and the electric field is only slightly degraded compared to the case of only Ti diffusion. The increase in operating voltage due to the deterioration of this overlap can be sufficiently compensated by the effect of eliminating the need for a buffer layer according to the present invention, and also because of the effect of removing the buffer layer, the voltage required for switching is reduced only by Ti diffusion, and the dielectric buffer layer can be reduced from what would otherwise be required. Further, according to the present invention, since the buffer layer can be removed, it goes without saying that the phenomenon of DC drift can be prevented. Moreover, the optical intensity distribution in the depth direction becomes symmetrical due to the additional diffusion of Mg ions, thereby significantly reducing the coupling loss between the waveguide and the single mode fiber.

(Effects of the Invention) As described above, according to the present invention, absorption loss at the electrode is small, operation is performed at low voltage, coupling loss with a single mode fiber is significantly reduced, and there is no DC drift. Optical control circuits such as optical switches and optical modulators can be obtained.

The invention is not limited to the above embodiments. The present invention is applicable to optical control circuits of any type, such as optical phase modulators, cross-type optical switches, optical phase shifters, optical mode converters, and other optical control elements, and between these optical control elements or between these optical control elements and other optical control elements. It is also applicable to an optical control circuit in which optical elements are integrated on the same substrate.

The substrate material, the shape of the optical waveguide, and the shape of the electrodes in the present invention are not limited to those in the above embodiments, and a ferroelectric crystal such as LiTaO3 crystal is used as the substrate material, and a slab waveguide is used as the optical waveguide. As for the shape, a comb-shaped electrode or the like can also be used.

In addition, Yomaki, Komatsu, and Ota convened the 7th Topical Meeti on Integrated Optics and Waveguide Optics.
ng onIntegrated and Guide
As stated in Technical Digest TuA-5 of d-Wave Optics), a thin film containing a first metal ion that diffuses between the optical waveguide constituting the optical control section and the optical waveguide of the main part. By setting the film thickness separately and stacking the layers on the substrate with the reconsidered boundary in the Taber shape, the metal atoms are diffused into the substrate, and a thin film containing a second metal ion that reduces the refractive index is further spread over the entire surface of the substrate. By forming an optical waveguide by thermally diffusing this thin film into the substrate after forming it to a uniform thickness, the refractive index distribution of the optical waveguide in the input/output section and the optical waveguide in the optical control element section can be controlled more precisely. Coupling loss with single mode fiber can be further reduced without increasing operating voltage. At this time, it goes without saying that if the thickness of the thin film containing the second metal ion is set separately for the input/output section and the light control element section, the refractive index distribution of the optical waveguide in each section can be controlled more precisely.

[Brief explanation of the drawing]

FIG. 1(a) is a diagram showing the configuration of a directional coupler type optical control circuit which is a first embodiment of the optical control element according to the present invention.
Figure (b) is a concave cross-sectional view taken along line A-A' of the directional coupler type optical control circuit shown in Figure 1 (a), and Figure 2 is a diagram for explaining the manufacturing method of the optical control circuit according to the present invention. Figures 3 and 4 are diagrams for explaining the principle of obtaining an optical control circuit that operates at low voltage, has low loss, and has no DC drift according to the present invention, and Figure 5 (a) is a diagram according to the present invention. A diagram showing the configuration of a branching interference type optical modulator which is another embodiment of the optical control element, FIG. 5(b) is A of the branching interference type optical modulator shown in FIG. 5(a).
-A' is a diagram showing a concave cross section, and FIG. 6 is a diagram for solving the problems of the prior art. In the figure, 101.LiNb0a substrate 102...-Ti diffused optical waveguide 103...Transparent electrode (ITO) 104...Mg diffused layer 105a, 105b...Incidence side optical waveguide 106a, 1
06b... Output side optical waveguide 201... Ti thin film 202... MgO thin film 501... 3 dB branch part 502... Merging part 601... 5i02 Buffer layer 603... Metal electrode

Claims (2)

[Claims] (1) In an optical control circuit comprising an optical waveguide formed by introducing metal ions that increase the refractive index of the substrate into a dielectric substrate, and a transparent electrode, at least the optical waveguide portion under the transparent electrode is 1. A light control circuit comprising a dielectric substrate surface provided with a diffusion layer formed by introducing a second metal ion that acts to reduce the refractive index of the substrate. (2) After forming a first thin film containing a metal element that increases the refractive index of the substrate on the dielectric substrate in a desired pattern,
heating the substrate to diffuse the thin film pattern into the substrate, and then forming a second thin film pattern on at least a partial region of the substrate that includes a metal element that reduces the refractive index of the substrate; heating again to diffuse the second thin film pattern into the substrate to form an optical waveguide, and forming a transparent electrode on the optical waveguide where both the first and second thin film patterns are diffused. 1. A method of manufacturing an optical control circuit, comprising at least the step of forming.