JPH07270634A - Manufacture of optical waveguide and optical waveguide substrate - Google Patents

Abstract

PURPOSE:To provide a manufacturing method for an optical waveguide by which the optical waveguide whose substrate is not deformed is formed without increasing the number of manufacturing processes and an optical waveguide whose substrate is not deformed. CONSTITUTION:At a step 2, the film of a mask material 3 is formed on the substrate 1. At a step 3, the mask material 3 is patterned so that the width of the optical waveguide becomes 4mum and the width of a mask becomes 15mum. At a step 4, the substrate 1 covered with the mask material 3 is immersed in benzoic acid 5 and ion is exchanged. At a step 5, the substrate 1 is annealed in an electric furnace and the exchanged ion is diffused. Thus, the optical waveguides 2a and 2b are formed at the aperture parts 4a and 4b of the mask material 3 on the surface of the substrate 1 and all the ion is exchanged at the other aperture parts 4c, 4d and 4e. Besides, the ion is exchanged on the whole of the back surface. As the result, the difference of the area of the parts where the ion is exchanged, becomes small on the front surface and the back surface and the deformation of the substrate 1 such as warp is not caused. Besides, the substrate 1 is manufactured by a small number of processes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical waveguide, and more particularly to a method for manufacturing an optical waveguide by covering the surface of a substrate with a mask material and forming an optical waveguide by ion exchange from an opening of the mask material. Further, the present invention relates to an optical waveguide substrate having an optical waveguide, and particularly to an optical waveguide substrate having an optical waveguide formed by ion exchange.

[0002]

2. Description of the Related Art As a general optical waveguide manufacturing method, there is a method of forming an optical waveguide by ion exchange. This method is based on the property that substances such as crystals such as lithium tantalate (LiTaO ₃ ) and glass are ion-exchanged with ions having a relatively high refractive index, and the refractive indices of those substances also increase. There is. Specifically, a mask material having an optical waveguide pattern is provided on the substrate on which the optical waveguide is formed,
By exchanging ions such as protons (H ⁺ ) with ions in the substrate through the openings of the pattern of the mask material and diffusing the ions, an optical waveguide having a desired refractive index can be obtained.

FIG. 6 is a sectional view showing a first example of an optical waveguide formed by a conventional ion exchange method. Board 1
The surface of No. 1 (the surface on which the optical waveguide is formed is the front surface and the opposite surface is the back surface) is covered with the pattern of the mask material 31 for preventing ion exchange, and the opening 4 of the pattern is formed.
The substrates 11 are ion-exchanged at 1a and 41b to form optical waveguides 21a and 21b. The entire back surface is exposed, and the ion exchange portion 21c extends over the entire back surface. The side surface is also the ion exchange part 21.
d, 21e.

FIG. 7 is a sectional view showing a second example of an optical waveguide formed by a conventional ion exchange method. Board 1
The surface of No. 2 is covered with a pattern of a mask material 32a for preventing ion exchange, and the openings 42a, 42 of the pattern are formed.
In step b, the substrate 12 is ion-exchanged and the optical waveguide 22a,
22b is formed. The entire back surface is covered with the mask material 32b. The side surfaces are the ion exchange portions 21c and 21d.

Although the optical waveguide can be formed in this way, since the ionic radii of the invading ions during ion exchange and the ions constituting the substrate are different, expansion or contraction occurs at the ion-exchanged portion, resulting in strain. Occurs. As in the above two examples, when the ion exchange state is significantly different between the front surface and the back surface of the substrate, the substrate is deformed such as warped due to the stress based on the strain difference between the both surfaces of the substrate. This deformation of the substrate causes deterioration of the characteristics of the optical waveguide.

FIG. 8 is a sectional view showing a third example of an optical waveguide formed by a conventional ion exchange method. This example is a method for preventing deformation such as warpage of the substrate as in the above two examples.

In the figure, the surface of the substrate 13 is covered with a pattern of a mask material 33a for preventing ion exchange, and the substrate 13 is formed in the openings 43a and 43b of the pattern.
Are ion-exchanged to form optical waveguides 23a and 23b on the surface. The back surface is covered with the same pattern of the mask material 33b as the front surface, and the openings 43c, 43 of the pattern are formed.
Since the substrate 13 is ion-exchanged at d, an ion-exchange portion 23c similar to the surface optical waveguides 23a and 23b,
23d is formed. The ion exchange part 23
c and 23d are provided only to prevent deformation of the substrate 13 such as warpage, and are not used as optical waveguides. The side surfaces also become the ion exchange portions 23e and 23f.

As a result, the same ion exchange state can be obtained on the front surface and the back surface of the substrate, so that the substrate is not deformed. Japanese Patent Publication No. 5-55041 discloses such an example.

[0009]

However, in the method shown in FIG. 8, a mask material pattern must be formed on the back surface of the substrate. To form the mask material pattern,
It is necessary to use a film forming technique or a photolithography technique, and using such a technique even on the back surface of the substrate leads to an increase in manufacturing steps.

Further, the surface forming the optical waveguide must be mirror-polished to prevent scattering and absorption of the guided light so as to maintain a clean surface state free from dirt such as dust adhesion. Since the back surface is also processed in the above method, it is necessary to be very careful when using a jig or the like during processing.

As described above, there are problems such as an increase in manufacturing time and cost due to an increase in manufacturing steps, and an adverse effect on yield due to difficulty in maintaining a clean surface state.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method of manufacturing an optical waveguide which can form an optical waveguide without deforming the substrate without increasing the number of manufacturing steps. . Another object of the present invention is to provide an optical waveguide substrate in which the substrate is not deformed and propagation loss of the optical waveguide is suppressed.

[0013]

In the present invention, in order to solve the above-mentioned problems, the front surface of the substrate is covered with a mask material, and the back surface of the substrate and the surface portion of the substrate corresponding to the opening of the mask material are provided. In the method of manufacturing an optical waveguide in which the optical waveguide is formed on the surface by ion exchange, the width within which the covering portion is within 10% of the surface area and which can substantially suppress the leakage of guided light from the optical waveguide. Using the mask material of
Provided is a method of manufacturing an optical waveguide that forms an optical waveguide.

Further, in an optical waveguide substrate having an optical waveguide, an optical waveguide is formed on the surface of the optical waveguide substrate.
% Or more, and the entire back surface is ion-exchanged, to provide an optical waveguide substrate.

[0015]

By using a mask material whose covering portion is within 10% of the surface area and has a width capable of substantially suppressing the leakage of guided light from the optical waveguide, the area of the ion-exchanged portion can be reduced. Since it is 90% or more of the area, the difference in the area of the ion exchange portion between the front surface and the back surface is small, and the deformation of the substrate can be prevented.

Further, by making the optical waveguide substrate in which the area of the ion-exchanged portion is 90% or more of the surface area, the optical waveguide substrate is not deformed.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 is a plan view of a Mach-Zehnder type (MZ type) optical waveguide manufactured according to the present invention. On the substrate 1,
The optical waveguide 2 is formed so as to have one at both ends, branch into two in the middle, and be coupled again. M like this
The Z-type optical waveguide can be used for an optical modulator utilizing the electro-optical effect. The electro-optic effect is a phenomenon in which the refractive index changes with an applied electric field. When an electric field is applied to one of the branched optical waveguides 2, the optical path length of light passing through the optical waveguide changes due to the electro-optic effect. As a result, the phase of light at the output end is controlled, and a phase modulator or optical switch is constructed.

FIG. 1 is a diagram showing steps of the method for manufacturing an optical waveguide of the present invention. This example is for manufacturing an MZ type optical waveguide, and shows an AA sectional view of the MZ type optical waveguide shown in FIG. The procedure is divided into 5 steps from step 1 to step 5, and the step number is S1 in the figure.
~ Display in S5.

In step 1, lithium tantalate (LiT
A substrate 1 made of aO ₃ ) is prepared. In step 2, a mask material 3 for preventing ion exchange is formed on the substrate 1 by a vacuum vapor deposition method. Aluminum (Al) is used for the mask material 3.

In step 3, the formed mask material 3 is processed into a pattern having an optical waveguide width of 4 μm and a mask width of 15 μm by a photolithography technique. In step 4, the substrate 1 covered with the mask material 3 is immersed in benzoic acid 5 heated to 240 ° C. for 25 minutes to exchange protons (H ⁺ ) in benzoic acid with lithium ions (Li ⁺ ) in the substrate. .

In step 5, 30 at 400 ° C. in an electric furnace.
Annealing is performed for a minute to diffuse protons (H ⁺ ). As a result, on the surface of the substrate 1, the openings 4 of the mask material 3 are formed.
a and 4b form the optical waveguides 2a and 2b,
In the other openings 4c, 4d, and 4e, the ion exchange portion 2 is used.
c, 2d, and 2e. On the back surface, the entire surface is the ion exchange portion 2f. The optical waveguide 2 formed in this way
Is an optical waveguide of single mode propagation having a wavelength of 633 nm. Then, the optical waveguide can be manufactured by a small number of steps as described above. In addition, the side surfaces are also the ion exchange parts 2g, 2h
Becomes

FIG. 3 is a plan view showing the pattern of the mask material formed on the substrate. This is the state after performing step 3 in FIG. Mask materials 3a, 3b, and 3c having a width of 15 μm are provided on the substrate 1 so as to surround the periphery of the portion to be the MZ type optical waveguide.

Next, a method of determining the width of the mask material 3 will be described. The light propagating through the optical waveguide causes power transfer to this layer when a layer having a higher refractive index than the substrate such as another optical waveguide exists near the optical waveguide. This is called evanescent coupling, and the coupling coefficient is a value that represents the state of this coupling. The coupling coefficient is determined by various characteristics such as the wavelength, mode, waveguide width, refractive index, and waveguide spacing of the optical waveguide. The reciprocal of this coupling coefficient is the complete coupling length. The complete coupling length is the length of the optical waveguide required to transfer 100% of the guided light power. The power transfer due to the coupling decreases as the distance between the ionic coupling portion and the optical waveguide increases, and increases as the optical waveguide length increases. Therefore, the minimum value of the width of the mask material is determined by the complete coupling length, the optical waveguide length, and the allowable transfer power between the optical waveguides.

As for the width of the mask material, the complete coupling length is Lp, the optical waveguide length is Lw, and the allowable transfer power between the optical waveguides is Pa.
(%), The value can be determined so as to satisfy Lp ≧ (Lw / Pa) × 100.

FIG. 4 is a graph showing the relationship between the distance between the optical waveguides and the coupling coefficient. This is a proton exchange LiT
Data in the case of aO ₃ optical waveguide (wavelength 633 nm, single mode propagation), where the horizontal axis is the interval of optical waveguide (μ
m), and the vertical axis is the coupling coefficient (1 / μm). As shown in the figure, the coupling coefficient sharply decreases when the distance between the optical waveguides is 3.0 μm to 8.0 μm. The optical waveguide spacing is 8.
Even with a value of 0 μm or more, the coupling coefficient decreases as the spacing increases.

The coupling coefficient is 1.17 × 10 ^-7 when the distance between the optical waveguides is 15.0 μm. Since the reciprocal of the coupling coefficient is the perfect bond length, the perfect bond length at this time is about 10 m. The length of the MZ type optical waveguide shown in FIG. 2 is about 5 cm. Therefore, the power transfer caused by setting the width of the mask material to 15 μm is about 0.5%. Since the power transfer of about 0.5% is within the allowable range,
It has no practical effect. In this way, the width of the mask material in the above embodiment is determined.

If the area not ion-exchanged is 10% or less of the surface area of the substrate, the deformation of the substrate can be prevented. If the area on the surface of the substrate that is ion-exchanged is too small, the difference in the area of the ion-exchanged portion between the front surface and the back surface becomes large, and the substrate is deformed such as warped. Therefore, the width of the mask material cannot be made wider than necessary. Therefore, the width of the mask material is determined so that the area covered with the mask material is 10% or less of the surface area of the substrate.

Even if the area not ion-exchanged is larger than 10% of the surface area of the substrate, the effect of suppressing the warpage of the substrate is not small by ion-exchanging the portions other than the optical waveguide. Of course.

By the method described above, a total length of 60 m
When five MZ type optical waveguides each having a width of m and a mask material of 15 μm are formed in a substrate of 3 inches square, the total area of the non-ion-exchanged portion is about 0.1% of the surface area of the substrate.

When the planar state of this substrate was observed by an optical interferometer, no interference fringes due to warpage were observed. Further, light was propagated through the optical waveguide, and its propagation loss was examined, and it was about 0.6 dB / cm.

Next, under the same manufacturing conditions as the above-mentioned optical waveguide,
A 50 mm long waveguide is 1 m on a 50 mm x 5 mm substrate.
Three types were produced: 5 at m intervals, 10 at 500 μm intervals, and 17 at 250 μm intervals. The ratio of the region of the waveguide substrate not ion-exchanged by the mask to the substrate front surface (back surface) is 3%, 6%, and 9.5%, respectively.
Met. The surface flatness of these waveguide substrates was examined by an optical interferometer, but almost no interference fringes were observed. Moreover, when the propagation loss of light was examined, it was about 0.6 to 0.8 dB / cm.

Next, as a comparative example, 20 waveguides were formed on a substrate of 50 mm × 5 mm at intervals of 250 μm. The ratio of the non-ion-exchanged region on the surface of this substrate to the front surface (back surface) was 13%. When the flatness of this substrate was examined by an optical interferometer, interference fringes were observed, and the optical loss was 1 dB / cm, which was higher than that in the above-mentioned examples.

Therefore, the optical waveguide substrate in which the ratio of the non-ion-exchanged region to the substrate surface is 10% or less is free from deformation such as warpage and the propagation loss of the optical waveguide can be suppressed.

Although the MZ type optical waveguide of proton-exchanged lithium tantalate has been described in the above embodiments, the manufacturing method of the present invention can be used regardless of the material as long as the optical waveguide is manufactured by ion exchange. . Instead of lithium tantalate, the substrate may be glass containing lithium niobate, Na or K.

FIG. 5 is a plan view showing a directional coupler type optical waveguide manufactured according to the present invention. This is a directional coupling type optical waveguide using ion exchange glass as the substrate 10,
The two optical waveguides 20a and 20b are juxtaposed very close to each other, and when light is incident on one optical waveguide, the power of the light is transferred to the other optical waveguide by evanescent coupling.
If the coupling state between the distributed coupling optical waveguides is modulated, the amount of light emitted from each optical waveguide changes and it can be used as an optical modulator or an optical switch.

Aluminum (Al) is used as the mask material.
Used pyrophosphoric acid, or adipic
Acids may be used. In addition to the proton ions,
For on-exchange, for example, RbN containing Rb ions
O₃, TlNO containing Tl ions ₃, Including Ag ions
AgNO₃Etc. may be used.

[0037]

As described above, according to the present invention, since the mask having the covering portion within 10% of the whole area and having the width capable of substantially suppressing the leakage of the guided light from the optical waveguide is used, the ion exchange is performed. 90% of surface area
As described above, the difference in the area of the ion-exchanged portion between the front surface and the back surface of the substrate is reduced. As a result, it is possible to manufacture an optical waveguide in which the substrate does not deform in a few steps,
The production time and cost can be reduced, and the yield can be improved by easily maintaining a clean surface state.

Further, the optical waveguide substrate made by using this optical waveguide is not deformed, and the propagation loss of the optical waveguide can be suppressed.

[Brief description of drawings]

FIG. 1 is a diagram showing steps of a method for manufacturing an optical waveguide of the present invention.

FIG. 2 is a Mach-Zehnder type (M
It is a top view of a (Z type) optical waveguide.

FIG. 3 is a plan view showing a pattern of a mask material formed on a substrate.

FIG. 4 is a diagram showing a graph of the relationship between the distance between optical waveguides and the coupling coefficient.

FIG. 5 is a plan view of a directional coupler type optical waveguide manufactured according to the present invention.

FIG. 6 is a cross-sectional view showing a first example of an optical waveguide formed by a conventional method.

FIG. 7 is a sectional view showing a second example of an optical waveguide formed by a conventional method.

FIG. 8 is a sectional view showing a third example of an optical waveguide formed by a conventional method.

[Explanation of symbols]

 1 Substrate 2, 2a, 2b Optical Waveguide 2c, 2d, 2e, 2f Ion Exchange Part 3 Mask Material 4a, 4b, 4c, 4d, 4e Opening 5 Benzoic Acid

Claims (3)

[Claims] 1. An optical waveguide is formed on a surface of a substrate by covering the surface of the substrate with a mask material and performing ion exchange between the back surface of the substrate and the surface portion of the substrate corresponding to the opening of the mask material. In the method for manufacturing an optical waveguide, an optical waveguide is formed by using the mask material having a covering portion within 10% of the surface area and having a width capable of substantially suppressing leakage of guided light from the optical waveguide. Waveguide manufacturing method. 2. The width of the mask material has a perfect bond length of L
p, the optical waveguide length is Lw, the waveguide on the surface of the substrate,
Allowable transfer power between other ion exchange regions is Pa
The method for manufacturing an optical waveguide according to claim 1, wherein (%) is determined so as to satisfy Lp ≧ (Lw / Pa) × 100. 3. An optical waveguide substrate having an optical waveguide, wherein an optical waveguide is formed on the surface, 90% or more of the entire surface is ion-exchanged, and the entire back surface is ion-exchanged.