JPH10300964A - Formation of light guide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress abrupt coagulation of a proton exchange solution at certain temperature (coagulation point) and to prevent the crystal state of a substrate exposed to the proton exchange liquid from being spoiled owing to the application of stress by using the proton exchange solution prepared by mixing at least two kinds of carboxylic acid for the formation of the light guide by a proton exchanging method. SOLUTION: The proton exchange solution 13 prepared by mixing at least two kinds of straight chain saturated carboxylic acid while a substrate 11 with electrooptic effect is dipped is heated to form the light guide 14 of a proton exchange layer in the surface layer of the substrate 11. Then the proton exchange solution 13 in which the substrate 11 is still dipped is left as it is and cooled down to room temperature. Consequently, the temperature range of coagulation of the proton exchange solution 13 in the cooling process is widened to suppress the abrupt coagulation of the proton exchange solution 13 and abrupt heat generation and volume contraction accompanying the abrupt coagulation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an optical waveguide used in an optical communication device.

[0002]

2. Description of the Related Art A method of forming an optical waveguide by a proton exchange method is described in Rep. Prog. Phys., 1993 (UK) M. Lawrence “Lithium
As described in niobate integrated optics "p. 400-401, first, a mask pattern is formed on a substrate having an electro-optic effect, and then the resultant is dissolved in benzoic acid dissolved by heating to 140 to 300C. According to this method, the protons released from benzoic acid act on the surface layer portion of the substrate exposed from the mask pattern, thereby exposing the surface side of the substrate for one hour or more. An optical waveguide having an increased refractive index is formed.

[0003]

However, the above-described method of forming an optical waveguide by the proton exchange method has the following problems. That is, in the above-described forming method, after exposing the substrate to heated benzoic acid to form an optical waveguide, the substrate and the benzoic acid are kept in a state where the substrate is exposed to benzoic acid in order to prevent rapid cooling of the substrate. Is cooled to room temperature. However, since there is a freezing point of benzoic acid in the cooling process to room temperature, in the cooling process, benzoic acid solidifies with a rapid exothermic reaction and volume shrinkage. Therefore, stress due to rapid solidification of benzoic acid is applied to the surface of the substrate, and the crystal structure of the substrate is damaged.

[0004] The damage impairs the non-linearity of the electro-optic effect on the substrate, and in the optical waveguide formed by the above method, the outgoing light becomes weaker with respect to the incident light (so-called extinction), and the cross-sectional shape of the incident light is reduced. This causes a problem that the sectional shape of the emitted light is distorted.

[0005]

According to a method for forming an optical waveguide of the present invention for solving the above-mentioned problems, in forming an optical waveguide by a proton exchange method, a proton exchange solution in which at least two kinds of carboxylic acids are mixed is used. It is characterized by: At this time, the proton exchange solution is heated while the surface of the substrate having the electro-optic effect is exposed to the proton exchange solution.

In the above forming method, a proton exchange solution in which at least two kinds of carboxylic acids are mixed is used,
The temperature region where coagulation occurs in the proton exchange solution is widened. Therefore, when the heated proton exchange solution is cooled, rapid solidification of the proton exchange solution at a certain temperature (freezing point) and rapid heat generation and volume shrinkage of the proton exchange solution due to the solidification are suppressed. For this reason, when the proton exchange solution is cooled while the surface is exposed to the heated proton exchange solution, the stress applied to the substrate by the solidification of the proton exchange solution is suppressed to a small value.

[0007]

1 (1) to 1 (4) are process diagrams for explaining a method for forming an optical waveguide according to the present invention. An embodiment of the present invention will be described below with reference to these drawings. First, as shown in FIG. 1A, a metal mask 12 having a slit pattern is formed on a substrate 11 having an electro-optical effect. However, the drawing shows the metal mask 12.
Is a cross-sectional view substantially perpendicular to the longitudinal direction of the slit pattern. As the substrate 11, for example, Lithium niob
ate, Lithium tantalate, Lithium borate, Potassium
niobate, Potassium Titanyl Phosphate, Bismuth Sili
con Oxide, Barium Titanate, etc. will be used. The metal mask 12 is made of chromium (Cr), tantalum (Ta), or the like.

Next, as shown in FIG. 1B, the substrate 11 is immersed in a solution used for proton exchange, ie, a proton exchange solution 13, and the surface of the substrate 11 exposed from the metal mask 12 is subjected to proton exchange. Expose to solution 13. As the proton exchange solution 13, a mixture of at least two kinds of carboxylic acids is used. As the carboxylic acid, one that is in a liquid state in a temperature range (140 ° C. to 300 ° C.) in which proton exchange occurs in the surface layer of the substrate 11 is used. More preferably, a straight-chain saturated carboxylic acid having 16 or more carbon atoms is used.

As the straight-chain saturated carboxylic acids having 16 or more carbon atoms, those available as reagents include palmitic acid, heptadecanoic acid, stearic acid, nonadecanoic acid,
It is preferable to use arachiic acid, heneicosanoic acid, behenic acid, tricosanoic acid, lignoceric acid, pentacosanoic acid, cerotic acid, heptacosanoic acid, octacosanoic acid, nonacosanoic acid or melicic acid.

When two kinds of carboxylic acids are used, the mixing ratio is about 2: 8 to 8: 2 (weight ratio).

Next, the proton exchange solution 13 in which the substrate 11 is immersed is placed in a temperature range in which proton exchange occurs (140 ° C.).
To 300 ° C.) and to a temperature at which the proton exchange solution 13 is maintained in a liquid state. Then, this heating state is maintained for about 10 minutes to 24 hours, and the proton exchange solution 1
The protons released from each of the carboxylic acids constituting 3 act on the exposed surface layer of the substrate 11. by this,
An optical waveguide made of a proton exchange layer is formed on the exposed surface layer of the substrate.

Thereafter, as shown in FIG. 1C, the proton exchange solution 13 is left to cool to room temperature. Here, when a carboxylic acid having a freezing point higher than the room temperature is used as the proton exchange solution 13, for example, when a linear saturated carboxylic acid having 16 or more carbon atoms is used as described above, the proton The exchange solution 13 solidifies.
Then, the substrate 11 is a coagulated product 1 of the proton exchange solution 13.
3a.

Next, as shown in FIG. 1D, the proton exchange solution 13 or the proton exchange solution 13 is washed by using a solvent that dissolves the coagulated product 13a of the proton exchange solution 13. Coagulate 13
The substrate 11 is taken out from a. The substrate 11 has an optical waveguide 14 provided on a surface layer.

In the method for forming an optical waveguide, a proton exchange solution 13 is obtained by mixing at least two kinds of carboxylic acids. Here, FIG. 2 shows lignoceric acid (having 2 carbon atoms).
4) and DSC (differen) of cerotic acid (C26)
3 shows a tial scanning calorimetry) curve. Each DS in FIG.
In the C curves a to e, the mixing ratio (weight ratio) of lignoceric acid and serotic acid is as follows: a: lignoceric acid / serotinic acid = 10/0, b: lignoceric acid / serotinic acid = 8 /
2, c: lignoceric acid / serotinic acid = 5/5, d: lignoceric acid / serotinic acid = 2/8, e: lignoceric acid / serotinic acid = 0/10. FIG. 3 shows a DSC curve of benzoic acid used for the conventional proton exchange method as a comparison.

As shown in these figures, a single carboxylic acid (lignoceric acid of a in FIG. 2; e in FIG. 2)
(Cerotic acid and benzoic acid in FIG. 3) generate rapid heat near each freezing point, and it is confirmed that the carboxylic acid rapidly solidifies near these freezing points.
In contrast, in the case of a mixture of lignoceric acid and serotonic acid, no rapid heat generation at a constant temperature was observed, and the temperature region in which coagulation occurred was widened, and rapid volume shrinkage due to rapid coagulation was suppressed. It is confirmed that.

As described above, in the above-described forming method using the proton exchange solution obtained by mixing at least two kinds of carboxylic acids, the proton exchange solution was cooled while the substrate was exposed to the heated proton exchange solution. In this case, the stress applied to the substrate due to the solidification of the proton exchange solution can be reduced. Further, by making the mixing ratio (weight ratio) of the two types of carboxylic acids approximately from 2: 8 to 8: 2, the temperature range can be sufficiently widened as is apparent from FIG. The effect of suppressing the stress can be obtained.

By using a linear saturated carboxylic acid as the carboxylic acid, the temperature range can be sufficiently widened. Thus, by using a proton exchange solution obtained by mixing two types of linear carboxylic acids, the stress applied to the substrate 11 can be sufficiently suppressed.

Further, the carboxylic acid has 16 to 3 carbon atoms.
By using one carboxylic acid, proton exchange can be performed in a higher temperature range in a temperature range in which proton exchange occurs, and proton exchange can be performed efficiently.

FIG. 4A is a perspective view of the optical waveguide 14 formed on the surface layer of the substrate 11, and FIG. 4B is a sectional view taken along line AA 'of FIG. This optical waveguide 14
Then, when the light h emitted from the optical fiber 41 is made incident from one end side, the light h passes through the optical waveguide 14, is emitted from the other end side, and is guided into the other optical fiber 41. Then, by providing an optical element using the Mach-Zehnder interference effect, for example, on the optical waveguide 14, an optical switch can be manufactured.

Next, another example of a method for forming an optical waveguide will be described with reference to FIGS. Note that the same components as those in the procedure for forming the optical waveguide described with reference to FIG. 1 are denoted by the same reference numerals, and redundant description is omitted. First, FIG.
As shown in (1), the substrate 1 is formed by lithography.
1 is formed with a resist pattern 51. Next, FIG.
As shown in (2), a titanium (Ti) film 52 is formed on the substrate 11 on which the resist pattern 51 is formed by an evaporation method or a sputtering method.

Thereafter, as shown in FIG.
1 is removed, and a titanium pattern 52 a is formed on the substrate 11. Then, a heat treatment is performed at 1050 ° C. for about 8 hours in an electric furnace kept in an oxygen atmosphere, and a first optical waveguide 53 formed by diffusing titanium from the titanium pattern 52 a into the surface layer of the substrate 11 is formed.
To form

Thereafter, the titanium pattern 5 on the substrate 11 is
2a is removed. After the first optical waveguide 53 formed by diffusing titanium in the surface layer of the substrate 11 as described above, as shown in FIG. The second optical waveguide 14 made of a proton diffusion layer is formed in a portion sandwiched between the waveguides 53. The formation of the second optical waveguide 14 is shown in FIGS. 1 (1) to 1 (4).
Will be performed in the same manner as the method of forming the optical waveguide described with reference to FIG.

As described above, the first optical waveguide 53 made by diffusing titanium and the second optical waveguide 53 made of a proton exchange layer
The optical waveguide 14 was obtained. FIG. 6 shows a plan view of FIG. Here, a TM wave propagates in the first optical waveguide 53, and a TE wave propagates in the second optical waveguide 14. Therefore, by applying an electric field to each of the optical waveguides 53 and 14 to change the phase of the light h incident on the waveguides,
The output position of the light can be switched.
Then, a waveguide-type light control element can be manufactured using such first optical waveguide 53 and second optical waveguide 14.

The materials and numerical values shown in each of the above embodiments are merely examples, and the present invention is not limited by these materials and numerical values.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for forming an optical waveguide according to the present invention will be described below with reference to FIG. 1 used in the above embodiment. First, in the process described with reference to FIG.
A metal mask 12 made of chromium was formed on a substrate 11 made of m niobate (lithium niobate: LiNbO ₃ ). Next, in the step described with reference to FIG. 1 (2), a proton exchange solution 13 in which lignoceric acid and serotinic acid were mixed at a ratio of 5: 5 (weight ratio) and dissolved by heating was prepared. The substrate 11 was immersed in the exchange solution 13. In this state, the proton exchange solution 13 is heated at 200 ° C. for 1 hour.
Heated for hours.

Thereafter, the steps described with reference to FIGS. 1 (3) and 1 (4) are performed in the same manner as in the above embodiment, and the substrate 11
An optical waveguide 14 was formed on the surface layer.

The results of comparing the extinction ratio and the cross-sectional shape of the emitted light between the optical waveguide obtained in the above example and the optical waveguide obtained by using benzoic acid as a proton exchange solution (comparative example) are shown. It is shown in Table 1 below. The cross-sectional shape of the emitted light is such that when light having a concentric cross-section is incident on each optical waveguide, the light emitted from these optical waveguides (emitted light)
Is the cross-sectional shape.

[0028]

[Table 1]

As shown in Table 1, according to the above embodiment, an optical waveguide having a smaller extinction ratio than the conventional proton exchange method using benzoic acid and having no distortion in the shape of the emitted light could be obtained. .

[0030]

As described above, according to the method for forming an optical waveguide of the present invention, a constant temperature can be obtained by using a proton exchange solution in which at least two kinds of carboxylic acids are mixed in forming an optical waveguide by a proton exchange method. It is possible to suppress rapid solidification of the proton exchange solution at the (solidification point) and prevent the substrate exposed to the proton exchange solution from being stressed and the crystal state thereof being damaged. For this reason, the nonlinearity of the electro-optic effect of the substrate can be ensured, and an optical waveguide free from extinction and distortion in the cross-sectional shape of guided light can be obtained.

[Brief description of the drawings]

FIG. 1 is a sectional process view illustrating a method for forming an optical waveguide of the present invention.

FIG. 2 is a DSC curve of lignoceric acid and serotic acid.

FIG. 3 is a DSC curve of benzoic acid.

FIG. 4 is a diagram (part 1) for explaining the function of the optical waveguide;

FIG. 5 is a sectional process view illustrating another example of the method for forming an optical waveguide of the present invention.

FIG. 6 is a diagram (part 2) for explaining the function of the optical waveguide;

[Explanation of symbols]

 11 Substrate 13 Proton exchange solution 14 Optical waveguide

Claims (3)

[Claims] 1. A method for forming an optical waveguide in a surface layer of a substrate by a proton exchange method, comprising using a proton exchange solution obtained by mixing at least two types of carboxylic acids. 2. The method for forming an optical waveguide according to claim 1, wherein the carboxylic acid is a straight-chain saturated carboxylic acid. 3. The method for forming an optical waveguide according to claim 2, wherein the carboxylic acid is a carboxylic acid having 16 to 31 carbon atoms.