JPH0697287B2 - Light control circuit manufacturing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical control element for modulating a light wave, switching an optical path, etc., and more particularly to a waveguide type optical control for controlling using an optical waveguide provided in a substrate. The present invention relates to a method of manufacturing a control circuit.

(Prior art and its problems) As the practical use of the optical communication system progresses in recent years, a system with higher capacity and higher function is required, and optical control elements such as modulators and optical switches for higher speed light waves. Is needed. In such an optical control element, its insertion loss may limit the transmission distance of an optical signal, so that high speed and low loss are important. As a high-speed optical control element, a waveguide is formed in a substrate such as LiNbO ₃ crystal that has a large electro-optic effect coefficient, and the refractive index distribution of the waveguide is controlled by changing the electric field using the electro-optic effect. There is a report on a directional coupling type optical modulator or switch, a total reflection type optical switch, a branching interference type optical modulator or switch, etc. For example, in an optical waveguide formed by diffusing Ti in a LiNbO ₃ crystal, a small propagation loss of 0.1 to 0.2 dB / cm is obtained for a wavelength of 1.3 μm. However, when applying such a waveguide type optical control element to an actual optical fiber transmission system, it is necessary to consider the coupling loss with the optical fiber. For this purpose, the optical waveguide is made so that the optical energy distribution of the propagation mode of the optical waveguide is as close as possible to the optical energy distribution of the propagation mode of the optical fiber. The loss chamber value when the optical waveguide is inserted between the optical fibers by the above means is about 2 dB. This is because the Ti diffused waveguide has different refractive index distributions in the direction perpendicular to the substrate and in the horizontal direction, and the optical energy distribution does not match that of an optical fiber having a circular refractive index distribution. On the other hand, the operating speed of the waveguide type optical control element greatly depends on the operating voltage thereof, and it is very important in practice to make the operating voltage as small as possible in order to increase the operating speed. However, in order to reduce the voltage of the light control element, it is necessary to concentrate the light energy of the propagating light in the vicinity of the electrode where the strength of the applied electric field is large. The condition for lowering the voltage is generally the above-mentioned coupling loss with the optical fiber. Is different from the condition for reducing

The light energy distribution of a single-mode optical fiber that is usually used has a width of about 6 to 8 μm at which the intensity becomes 1 / e. Therefore, when low coupling loss is aimed, the light energy distribution of the optical waveguide also becomes about the above value. To be chosen. This condition is, for example, buoy. Written by V.Ramaswamy, RCAlferness, and M.Divino in Electronics Letters, Vol. 18, No. 1, pp. 30-31. There is. On the other hand, in order to reduce the voltage, it is necessary to make the energy distribution of the propagation light in the optical waveguide smaller than the range of the low coupling loss condition with the optical fiber. Regarding the trade-off between the low voltage condition and the reduction condition of the coupling loss with the optical fiber, L. Riviere et al.
ternational Conference on Integrated Optics and Op
Technical Digest 29C4-4 (pages 362 to 363) of tical fiber communication.

In order to satisfy low loss and low voltage at the same time in an optical control element formed by diffusing a metal with a ferroelectric material in this way, the optical energy distribution of the propagation mode of the waveguide is changed at the coupling part with the optical fiber. It is necessary to match the light energy distribution of the propagation mode and circularize the light energy distribution, and it is necessary to concentrate the light energy of the propagation mode in the vicinity of the electrode where the strength of the applied electric field is large in the light control unit. However, the conventional manufacturing method, that is, a method of thermally diffusing a thin film pattern of one kind of metal atom such as Ti into the ferroelectric substrate with the same film thickness and the same pattern width in the input / output optical waveguide part and the optical control element part Since, the refractive index distributions of the input / output waveguide section and the light control element section cannot be set separately, it was impossible to simultaneously realize low loss and low voltage. On the other hand, Kondo, Komatsu, and Ota proposed the 7th Topical Meeting on Integrated and Guide- Wave
Optics) Technical digest TuA5-1, a thin film containing metal atoms that diffuses between the optical waveguide that constitutes the optical control element and the input / output optical waveguide that connects it to the optical input / output end face. By setting the film thickness of the waveguide pattern separately, the optical energy distribution of the waveguide at the coupling part with the optical fiber is brought close to the optical energy distribution of the optical fiber,
In some light control units, the light energy distribution of the waveguide is concentrated near the electrodes. However, in the above manufacturing method, the light energy distribution of the input / output optical waveguide is asymmetric in the depth direction of the substrate and is not circular, so the loss in coupling with the optical fiber has not reached the theoretical limit. Therefore, in order to further reduce the loss, a manufacturing method in which the optical energy distribution of the input / output optical waveguide is made symmetrical and circular in the depth direction of the substrate is required.

(Means for Solving the Problems) A method of manufacturing an optical control circuit according to the present invention is an optical control circuit that uses an optical waveguide formed by diffusing metal atoms on the surface of a dielectric substrate. A method of manufacturing an optical control circuit in which an optical field in a direction perpendicular to a substrate is made symmetrical to reduce coupling loss between the optical control circuit and an optical fiber without deteriorating the performance of an active element portion of the optical control circuit. And a step of laminating a thin film containing metal atoms having a function of increasing the refractive index of the substrate in a desired pattern shape on the substrate, and the substrate only on a portion corresponding to the input / output optical waveguide of the thin film. Further laminating a thin film containing a metal atom different from the metal having a function of decreasing the refractive index of the substrate, and heating the substrate to form the laminated thin film pattern in the substrate by one diffusion process. A step of forming an optical waveguide by diffusing and at least one optical control by installing an electrode on the optical waveguide formed by diffusing only a thin film containing metal atoms having a function of increasing the refractive index of the substrate. And a step of forming an optical input / output end face at an end of the optical waveguide part connected to the light control element part and having the two-layer thin film pattern diffused therein. Is characterized by.

According to the present invention, as described above, the thin film pattern that diffuses between the optical waveguide forming the light control element and the input / output waveguide connecting the optical waveguide and the optical input / output end face is formed of one layer and two layers. By making the refractive index distribution different, a circular refractive index distribution is set in the input / output optical waveguide part so as to give a propagation light energy distribution close to the optical energy distribution of the optical fiber, and the light control element is configured independently of it. By setting the refractive index distribution of the optical waveguide of the portion to be confined so that the propagating light energy distribution is sufficiently close to the electrodes, it is a method of manufacturing an optical control circuit capable of low loss coupling and operating at low voltage. .

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

FIG. 1 shows a method for manufacturing a directional coupling type optical control circuit according to the present invention in order to explain an embodiment of a method for manufacturing an optical control circuit according to the present invention. Hereinafter, a method of manufacturing the directional coupling type optical control circuit according to the present invention will be described step by step.

First, an optical waveguide pattern is formed on the LiNbO ₃ substrate 301 by using a normal photolithography technique. Ie Li
A photoresist is uniformly coated on the NbO ₃ substrate, the photoresist is exposed through a photomask having the same shape as the optical waveguide portion, and a phenomenon is formed to form a waveguide-shaped groove in the photoresist film. Here, the directional coupler optical waveguide pattern 302 is two waveguide patterns having a width of several tens of μm and a length of several tens of mm that are close to each other at an interval of several μm. The output optical waveguide pattern 304 is composed of two waveguides separated from each other by, for example, several tens to several hundreds of μm, so that coupling between the two waveguides does not occur.
In addition, the photomask is formed so that the distance between the two optical waveguides gradually increases from the end of the directional coupler portion to the input / output optical waveguide. After forming a waveguide-shaped groove in the photoresist film by using the photolithography technique, a Ti film is first formed on the entire surface from about 700 to 1100Å. Next, Mg is formed with a thickness of 500 Å or less only in the portions corresponding to the input optical waveguide, output, and optical waveguide. At this time, portions other than the input optical waveguide and the output optical waveguide are covered with a shield plate. After that, by dissolving the photoresist film, Ti and Mg are laminated in the input / output optical waveguide portion and an optical waveguide pattern of only Ti is formed in the light control element portion as shown in FIG. The boundary portion between the input / output optical waveguide portion and the light control element portion can be formed in an arbitrary taper shape by adjusting the distance between the shield plate and the substrate or by gradually moving the shield plate during vapor deposition. The substrate with the optical waveguide pattern as shown in Fig. 1 is heated in a diffusion furnace at 1000 to 1100 ° C for 5 to 10 hours to diffuse Ti and Mg into the LiNbO ₃ substrate and refract only that portion. The rate increases slightly to become an optical waveguide. After that, a SiO ₂ film of 2000 Å or less was formed on the LiNbO ₃ substrate to prevent light absorption at the electrode, and it was placed directly above the director of the directional coupler.
A pair of electrodes 4 as shown in FIG. _{2 in} which Cr and Au or Cr and Al are laminated on SiO ₂ is formed. Then, the optical input / output end face 1 is polished or cleaved in the direction perpendicular to the input / output optical waveguide.
Form 5,16,17,18. The SiO ₂ film is omitted in FIG. The above is the manufacturing method of the directional coupling type optical control circuit according to the present invention, and the directional coupling type optical control circuit shown in FIG. 2 is formed by the above manufacturing method.

In the manufacturing method according to the present invention, the directional coupler 3 includes only Ti as Li.
Since it is diffused in the NbO ₃ substrate, the refractive index distribution in the depth direction of the optical waveguide 2 of the directional coupler section 3 is large as shown in FIG. 3 (a), and the energy distribution of propagating light is the third. As shown in FIG. 3C, the size becomes small, and it is strong and confined in the optical waveguide, and the optical path can be switched at a low voltage. On the other hand, in the coupling of the optical waveguide and the optical fiber, the spot size of the optical fiber is 10 μm (1 /
e ² full width), the energy distribution of the light 12 emitted from the optical waveguide is broadened to some extent, and the light intensity distribution of the optical fiber is symmetrical. Symmetry in the depth direction is necessary for low loss coupling. In the manufacturing method according to the present invention, the input optical waveguides 5 and 6 and the output optical waveguides 7 and 8 are formed by stacking Mg, which is a metal ion that reduces the refractive index of the waveguide, on Ti and depositing it in a LiNbO ₃ substrate. It is formed by heat diffusion. Therefore, the maximum value of the refractive index in the input optical waveguides 5 and 6 and the output optical waveguides 7 and 8 is smaller than the refractive index of the directional coupler section 3 as shown in FIG. The distribution is wide as shown in FIG. 3 (d), and the light intensity distribution is symmetrical also in the depth direction.
Therefore, it becomes possible to couple with the optical fiber with low loss. The connecting portions 9 and 10 of the optical waveguide 2, the input optical waveguides 5 and 6, and the output optical waveguides 7 and 8 have a refractive index distribution shown in FIG. 3 (a) in order to reduce the loss due to the mode conversion of the mixed light. To (b) distribution is formed so as to gradually change from several hundred μm to several mm.

As described above, by using the method for manufacturing an optical control circuit of the present invention, the refractive index distributions of the input / output optical waveguide section and the directional coupler section optical waveguide section can be set separately. The energy distribution of the guided light can be a circular distribution that matches the energy distribution of the optical fiber,
In the directional coupler section, the energy distribution can be strongly confined to the substrate surface. Therefore, it is possible to further reduce the loss and voltage of the light control circuit as compared with the conventional manufacturing method. Moreover, the manufacturing method of the present invention is different from the conventional manufacturing method only in the step of stacking metal atoms for lowering the refractive index of the waveguide, and the manufacturing method is almost the same as the conventional method, and also difficult. Not accompanied.

FIG. 4 shows a method of manufacturing a branching interference type optical modulator according to the present invention in order to explain another embodiment of the method of manufacturing an optical control circuit according to the present invention. The method of manufacturing the branching interference type optical modulator according to the present invention will be described below.

First, a pattern of an optical waveguide is formed on the LiNbO ₃ substrate 401 by using a normal photolithography technique. Ie Li
A photoresist is uniformly coated on the NbO ₃ substrate, the photoresist is exposed through a photomask having the same shape as the optical waveguide portion, and developed to form a waveguide-shaped groove in the photoresist film. Here, the optical waveguide pattern has a width of several to several tens of μm. 3dB branch optical waveguide pattern 405
Is an input light Y-branch optical waveguide, its opening angle is several m rad, and the interval between the two phase modulator optical waveguide patterns 402 is several tens μm. The merging portion optical waveguide pattern 406 is also a Y branch optical waveguide pattern having an opening angle of m rad, similar to the 3 dB branching optical waveguide pattern 405. After forming a waveguide-shaped groove in the photoresist film by using the photolithography technique, a Ti film is first formed on the entire surface from about 700 to 1100Å.
Next, only the parts corresponding to the input optical waveguide and output optical waveguide
Form Mg403,404 less than 500Å. At this time, the parts other than the input optical waveguide and the output optical waveguide are covered with a shield. After that, by dissolving with the photoresist film, Ti and Mg in the input / output optical waveguide portion as shown in FIG.
Are laminated, and an optical waveguide pattern in which only Ti is formed is formed in the branch interference type optical modulator portion. The boundary portion between the input / output optical waveguide portion and the phase modulator portion is adjusted by adjusting the distance between the shield plate and the substrate, or by using the shield plate.
An arbitrary taper shape can be obtained by gradually moving when the Mg film is formed. The substrate with the optical waveguide pattern as shown in Fig. 4 is heated in a diffusion furnace at 1000 to 1100 ° C for 5 to 10 hours to diffuse Ti and Mg into the LiNbO ₃ substrate and refract it. The optical waveguides 502, 503, 507, 508 are formed by changing the ratio. Then, to prevent light absorption at the electrode, a SiO ₂ film of 2000 Å or less is formed on the LiNbO ₃ substrate, and Cr and Au are deposited on the SiO ₂ just above the waveguide of the phase modulator section.
Or an electrode 504 as shown in FIG. 5 in which Cr and Al are laminated.
To form. After that, the light input / output end face is formed by polishing or cleaving in the direction perpendicular to the input / output optical waveguide. The fifth
In the figure, the SiO ₂ film is omitted. The above is the method for manufacturing the branch interference type optical modulator according to the present invention, and the branch interference type optical modulator shown in FIG. 5 is formed by the above manufacturing method.

In the manufacturing method according to the present invention, the phase modulator part includes only Ti and Li.
Since it is diffused in the NbO ₃ substrate, the refractive index distribution in the depth direction of the optical waveguide of the phase modulator part is the same as that in FIG. 3 (a) and large, and the energy distribution of propagating light is in FIG. 3 (c). As shown in (1), it becomes small, and it is strong and confined in the optical waveguide, so that the light can be modulated at a low voltage. Further, in the manufacturing method according to the present invention, the input / output optical waveguide portions 507, 508
Is an ion that reduces the index of refraction of the waveguide on Ti
It is formed by stacking Mg and thermally diffusing it into a LiNbO ₃ substrate. Therefore, the input optical waveguide 507 and the output optical waveguide 5
In 08, the maximum value of the refractive index is smaller than that of the phase modulator optical waveguide as shown in FIG. 3 (b), and the energy distribution of the propagating light spreads as shown in FIG. 3 (d). Cage,
Moreover, the light intensity distribution becomes a symmetrical distribution in the depth direction. Therefore, it becomes possible to couple with the optical fiber with low loss.

By using the method for manufacturing an optical control circuit of the present invention as described above, the refractive index distributions of the input / output optical waveguide section and the phase modulator section optical waveguide section can be set separately, and the input / output optical waveguide section can be set to have a different refractive index distribution. The energy distribution of the wave light can be a circular distribution that matches the energy distribution of the optical fiber, and the energy distribution can be strongly confined to the substrate surface in the phase modulator section. Therefore, it is possible to manufacture an optical control element having lower loss and lower voltage than the conventional manufacturing method by this method. Moreover, compared with the conventional manufacturing method, only the step of laminating metal atoms for lowering the refractive index of the waveguide is increased, which is almost the same as the conventional method.

(Effect of the present invention) As described above, according to the present invention, an optical control circuit capable of low-loss optical fiber coupling and capable of low voltage operation can be obtained.

The present invention provides a low operating voltage characteristic and a low loss optical fiber coupling characteristic which are conventionally obtained by separate elements for any type of optical control circuit, such as an optical phase modulator or a crossed waveguide type optical switch. Both can be obtained with one element. The substrate material, the optical waveguide shape, the electrode shape, etc. used in the present invention are not limited to those in the above embodiments, and a ferroelectric crystal such as LiTaO ₃ crystal is used as the substrate material, and thermal diffusion and ion exchange are used as the optical waveguide. The optical waveguide and the like using both of the above can be used, and the electrode shape can be an electrode having a traveling waveform more suitable for higher speed.

Also, Kondo, Komatsu, and Ota (7th Topical Meeting on Integrated an
dA Guide-Wave Optics) technical digest TuA
As described in 5-1, the thicknesses of the thin film patterns containing the metal atoms diffused between the optical waveguide part and the input / output optical waveguide path that constitute the light control element part are set separately and The boundary is tapered and laminated on the substrate, and then a thin film containing metal atoms that reduces the refractive index is laminated only on the part corresponding to the input / output optical waveguide, and then the thin film pattern is thermally diffused into the substrate to form the optical waveguide. If formed, it is possible to independently and finely control the refractive index distribution of the input / output optical waveguide and the optical waveguide of the optical control element part, and to manufacture an optical control circuit that can be coupled to an optical fiber with low loss and can operate at low voltage. Is obtained.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a first embodiment of a method for manufacturing a light control circuit according to the present invention, and FIG. 2 is a diagram showing a configuration of a directional coupling type light control circuit obtained by the present invention. FIGS. 4A and 4B are diagrams for explaining the principle of the method for manufacturing an optical control circuit according to the present invention, and FIGS. 4 and 5 are diagrams for explaining a second embodiment of the present invention. In the figure, 301,401 …… LiNbO ₃ substrate 2,5,6,7,8,9,10,502,503 …… optical waveguide 4,504 …… electrode 302 …… directional coupler part waveguide pattern (Ti) 303,403 …… input optical waveguide pattern (Mg on Ti) 304,404 …… Output optical waveguide pattern (Mg on Ti) 405 …… 3dB branch 406 …… Merging section

Claims (1)

[Claims] 1. An optical control circuit using an optical waveguide formed by diffusing metal atoms on the surface of a dielectric substrate, wherein an optical field in a direction perpendicular to the substrate is made symmetrical only in the input / output waveguide portion, and the optical control circuit is provided. A method of manufacturing an optical control circuit, which reduces the coupling loss between the optical control circuit and an optical fiber without deteriorating the performance of the active element part of the optical fiber, and increases the refractive index of the substrate on the substrate. Laminating a thin film containing a metal atom having a desired pattern shape, and a metal atom different from the metal having a function of reducing the refractive index of the substrate only on a portion of the thin film corresponding to the input / output optical waveguide. A step of further laminating a thin film containing: a step of forming an optical waveguide by heating the substrate to diffuse the laminated thin film pattern into the substrate by one diffusion process, A step of forming an electrode on an optical waveguide formed by diffusing only a thin film containing metal atoms having the function of increasing the refractive index of a plate to form at least one light control element part; and the light control element And a step of forming an optical input / output end face at an end portion of the optical waveguide portion connected to the optical waveguide portion and having the two-layered thin film pattern diffused, the manufacturing method of the optical control circuit.