JP2001021745A - Manufacture of embedded optical waveguide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a refractive index increasing portion of an axially symmetric shape excellent in consistency with an optical fiber, irrespective of a deep embedded depth, using two-step ion exchanging method. SOLUTION: Such three processes are conducted in order as the first ion exchange process for forming an ion exchange controlling membrane on a glass substrate to be immersed into the first molten salt to conduct thermal ion exchange so as to form a refractive index increasing portion, an etching process for removing the ion exchange control membrane, and the second ion exchange process for impressing an electric field to be perpendicular to the substrate while immersed into the second molten salt so as to embed the refractive index increasing portion along a depth direction of the substrate. The first method makes impressed electric field intensity in the second ion exchange process lowered with the lapse of time. In the second method, the third ion exchange process is conducted to impress an electric field reverse-directional to that in the second ion exchange process while immersed into the second molten salt after the second ion exchange process. In the third method, a heat- treatment process is conducted to hold heatingly the substrate under a no- electric-field condition after the second ion exchange process.

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 a low-loss embedded optical waveguide by a two-stage ion exchange method. More specifically, the present invention relates to a method of manufacturing a refractive index increasing portion embedded in a glass substrate to a sufficient depth. The present invention relates to a method for making a cross-sectional ion concentration distribution shape substantially symmetrical in a substrate depth direction.

[0002]

2. Description of the Related Art Various types of transmission elements for optical communication, optical integrated circuits,
Alternatively, for an optical sensor element or the like, a technique for forming an optical waveguide in a substrate made of a glassy substance is required. Conventionally, an ion exchange method has been employed for forming this type of optical waveguide. In forming an optical waveguide by this ion exchange method, a two-stage ion exchange method in which an optical waveguide is embedded in the substrate to reduce propagation loss due to scattering on the substrate surface and for mode field matching with an optical fiber is used. General.

[0003] The two-stage ion exchange method includes a first ion exchange step of forming a refractive index increasing portion by thermal ion exchange;
This is a method in which a second ion exchange step of embedding the increased refractive index portion in the depth direction of the substrate by applying an electric field is combined.
In the first ion exchange step, an ion exchange control film having a predetermined optical waveguide pattern is formed on a glass substrate containing alkali ions, and nitrate or sulfuric acid containing monovalent ions such as Ag or Tl that increases the refractive index is used. The salt is immersed in a molten salt for an appropriate period of time, and thermal ion exchange is performed through the ion exchange control membrane to form a refractive index increasing portion. After the first ion exchange step, the ion exchange control film is removed by etching. Next, in the second ion exchange step,
A glass substrate is immersed in a molten salt containing Na or K ions, ion-exchanged while applying an electric field, and Ag or T
The portion where the refractive index is increased by the 1 ion is embedded in the depth direction of the glass substrate.

[0004] The refractive index increasing portion thus formed functions as an optical waveguide. When the optical waveguide is formed in the vicinity of the substrate surface, scattering or absorption of the guided light occurs due to the surface of the substrate surface or the material on the substrate surface. this is,
This is because the exudation of the energy of the guided light occurs even on the substrate surface. In order to avoid this effect, the optical waveguide is embedded in the substrate as described above.

[0005]

In general, the larger the buried depth, the less affected by the substrate surface, and a low-loss optical waveguide can be realized. The necessary embedding depth varies depending on the wavelength used, the design of the optical waveguide, and the like. However, in consideration of the application in the communication wavelength range, the approximate embedding depth is about 10 μm. However, the larger the embedding depth,
The sectional ion concentration distribution shape of the portion where the refractive index increases is distorted, and is far from a perfect circle. Here, “embedded depth” refers to the depth from the substrate surface at the point where the concentration of ions that increase the refractive index in the refractive index increasing portion is the highest.

Meanwhile, the diffusion coefficient and the mobility of monovalent ions, such as Ag and Tl, which increase the refractive index in glass have more or less concentration dependence. Specifically, a portion having a high ion concentration in the glass has a large diffusion coefficient and mobility, and a portion having a low ion concentration has a small diffusion coefficient and mobility. Therefore, in the second ion exchange by applying an electric field, it is inevitable that as the buried depth increases, the refractive index increasing portion becomes vertically asymmetric in the substrate depth direction due to the dependency, and the mode field itself also increases. Naturally it becomes asymmetric. For this reason, it has been very difficult to realize an optical waveguide in which the cross-sectional ion concentration distribution of the portion where the refractive index is increased is a perfect circle and the mode is a single mode. Therefore, there is a defect that the matching with the optical fiber is inferior and the loss at the time of coupling with the optical fiber is large.

[0007] An object of the present invention is to form an axially symmetric refractive index increasing portion having good matching with an optical fiber by using a two-stage ion exchange method, despite the fact that the embedding depth is increased. To provide a method for manufacturing a buried optical waveguide.

[0008]

According to the present invention, an ion exchange control film having a predetermined optical waveguide pattern is formed on a substrate made of an ion-exchangeable glassy material containing monovalent ions, The first one that can increase the refractive index of the substrate
A first ion exchange step of immersing in a first molten salt containing a valent ion and performing thermal ion exchange through the ion exchange control film to form a refractive index increasing portion; and removing the ion exchange control film from the substrate. An etching step for removing, and a second substrate which can lower the refractive index of the portion where the refractive index has been increased by the first ion exchange step.
While immersed in the second molten salt containing a valence ion, the side on which the ion exchange control film was formed was set to a positive potential, and the opposite side was set to a negative potential, and an electric field was applied almost perpendicularly to the substrate to thereby perform the first step. And a second ion exchange step of burying the portion of increased refractive index due to monovalent ions in the depth direction of the substrate in this order on the premise of a method of manufacturing a buried optical waveguide.

In the first method of the present invention, the electric field intensity applied in the second ion exchange step is reduced with time, and the sectional ion concentration distribution shape of the portion where the refractive index is increased due to the embedded first monovalent ions is reduced. Is embedded while shaping it into an almost axially symmetric shape. During the second ion exchange, when an electric field of a constant intensity is applied regardless of time, the high ion concentration region is deep and the low concentration region is buried shallowly due to the concentration dependence of ion mobility, and the ion concentration distribution is asymmetric. Had become. On the other hand, as described above, by reducing the applied electric field strength with time in the second ion exchange step, the effect of ion diffusion due to heat on the ion mobility due to the electric field can be increased, and the refractive index increase portion can be increased. Can reduce the asymmetry of the cross-sectional shape of the ion concentration distribution.

[0010] In a second method of the present invention, after the second ion exchange step, while applying the electric field in a direction opposite to that of the second ion exchange step, the substrate is immersed in a second molten salt. 3) the ion exchange step, wherein the cross-sectional shape of the ion concentration distribution of the portion where the refractive index is increased by the first monovalent ions embedded in the second ion exchange step is substantially axially symmetric by the third ion exchange step. It is a way to shape it. By the third ion exchange step of applying a reverse electric field,
It is possible to selectively move only the high ion concentration region of the refractive index increasing portion, particularly, to reduce the asymmetry of the sectional distribution shape of the refractive index increasing ion concentration. Here, in the third ion exchange step of applying a reverse electric field, the mobility of the exchange ions due to heat is suppressed as much as possible, and the mobility of the high ion concentration region due to the electric field is further increased.

[0011] A third method of the present invention comprises a heat treatment step of heating and holding the substrate in an electric field-free state after the second ion exchange step, and the first ion implantation step embedded in the second ion exchange step. This is a method of shaping the cross-sectional shape of the ion concentration distribution of the portion where the refractive index is increased by the valence ions to be substantially axially symmetric by the heat treatment step. By the heat treatment after the second ion exchange step, a portion having a large spatial ion concentration gradient is selectively diffused. Also, from the concentration dependence of the diffusion constant, selective diffusion of a portion having a high ion concentration is realized. Furthermore, the portion which has once been moved by diffusion and reduced in concentration has the effect of stabilizing by realizing a steady state with little temporal change in shape, considering the concentration dependence of the diffusion constant. This heat treatment can be easily performed by immersing the substrate after the second ion exchange in the second molten salt used in the second ion exchange step for a certain period of time.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention has a
This is effective for an optical waveguide of 0 μm or more. This is because, in the conventional two-stage ion exchange method, as the burying depth increases, the asymmetry of the cross-sectional shape of the ion concentration distribution in the portion where the refractive index increases increases, and the necessity to improve the asymmetry increases. The change pattern and time of the applied electric field strength in the first method, the applied reverse electric field strength and the time in the second method, the temperature and the time in the third method, etc. are all the same as the material of the substrate, the type of ions used, What is necessary is just to determine suitably according to the design value of a wave path, etc.

The first monovalent ions capable of increasing the refractive index of the substrate include Ag ions, Tl ions, Cs ions,
At least one of Rb ions, K ions, and Li ions is used. In addition, the second one that can lower the refractive index of the refractive index increasing portion formed by the first monovalent ion.
As the valence ion, at least one ion of Na ion or K ion is used.

As a substrate made of a glassy substance, for example, any one of aluminoborosilicate glass, aluminosilicate glass, and borosilicate glass is used.

[0015]

EXAMPLES (Example 1-1) Na as an alkali ion
Ti having an opening of a predetermined optical waveguide pattern on an aluminoborosilicate glass substrate containing only ions
AgNO ₃ and NaNO ₃
Of molten salt (AgNO ₃ concentration 10%)
The first ion exchange in which the substrate is immersed at 90 ° C. for 90 minutes is performed to form a refractive index increasing portion on the substrate surface. Subsequently, the ion exchange control film on the substrate surface is removed by etching. afterwards,
A second ion exchange with an electric field applied is performed in the NaNO ₃ molten salt to bury the refractive index increasing portion formed on the substrate surface in the substrate.

In the second ion exchange step, the intensity of the applied electric field was temporally changed in the following pattern. (A) Pattern: 150 V / mm at a molten salt temperature of 255 ° C.
From the electric field intensity of 0 V / mm linearly over 180 minutes. (B) Pattern: 150 V / mm at a molten salt temperature of 255 ° C.
Is a half value linearly from the electric field strength of
Perform while lowering to V / mm. (C) Pattern: For comparison, molten salt temperature 255 ° C
For 180 minutes while maintaining the electric field strength of 150 V / mm constant (prior art).

FIG. 1 shows the result of simulating the concentration profile of the ion of increasing the refractive index of the optical waveguide of 3.5 μm manufactured under these conditions. In FIG. 1, symbols (a),.
(C) shows the ion concentration distribution corresponding to each of the above patterns. From these results, it is understood that the present invention in which the applied electric field intensity is reduced in the second ion exchange step has an ion concentration distribution that is sufficiently large so that the embedding depth is 10 μm or more and has better symmetry in the depth direction. It can be seen that the optical waveguide can be shaped.

FIGS. 2 and 3 show cross-sectional concentration profiles of ions having increased refractive index. FIG. 2 shows the method of the present invention (Example 1 above).
FIG. 3 shows a case of the prior art (pattern (c) of Example 1-1: Comparative Example). In FIG. 3 (prior art), the degree of asymmetry in the vertical direction (depth direction) is extremely high, and the central portion is also crushed to have an elliptical shape. However, in FIG. The properties are good, and especially in the central part, it is almost a perfect circle.

(Example 1-2) The refractive index increasing portion is buried in the substrate in the same procedure as in Example 1-1. The second ion exchange step was performed by changing only the processing time and changing the electric field strength over time as follows. (D) Pattern: 150 V / mm at a molten salt temperature of 255 ° C.
From the electric field strength of 0 to 120 V / mm linearly over 120 minutes. (E) Pattern: 150 V / mm at a molten salt temperature of 255 ° C.
Is a half value linearly from the electric field strength of
Perform while lowering to V / mm. (F) Pattern: for comparison, molten salt temperature 255 ° C
For 120 minutes while keeping the electric field strength of 150 V / mm constant (prior art).

FIG. 4 shows the result of simulating the concentration profile of the refractive index increasing ion of the 3.5 μm optical waveguide manufactured under the above conditions. In FIG. 4, symbols (d),.
(F) shows the ion concentration distribution corresponding to each of the above patterns. From these results, it is understood that the present invention in which the applied electric field intensity is reduced in the second ion exchange step has an ion concentration distribution that is sufficiently large so that the embedding depth is 10 μm or more and has better symmetry in the depth direction. It can be seen that the optical waveguide can be shaped.

(Example 2-1) N was used as an alkali ion.
T having an opening of a predetermined optical waveguide pattern on an aluminoborosilicate glass substrate containing only a ion
forming an ion exchange control membrane of i, AgNO ₃ and NaNO
During consisting ₃ molten salt (AgNO ₃ concentration of 10%), 25
The first ion exchange in which the substrate is immersed at 5 ° C. for 90 minutes is performed to form a refractive index increasing portion on the substrate surface. Subsequently, the ion exchange control film on the substrate surface is removed by etching. Thereafter, a second ion exchange with an electric field applied in a NaNO ₃ molten salt was carried out at 255 ° C. under an electric field strength of 120 V / mm.
This is performed for 0 minutes, and the portion where the refractive index is increased formed on the substrate surface is embedded in the substrate.

Further, the same temperature as in the second ion exchange step,
In the same molten salt, a third ion exchange was performed in a direction opposite to the second ion exchange under a higher electric field of 150 V / mm for up to 20 minutes.

FIG. 5 shows the result of simulating the concentration profile of the refractive index increasing ion of the 3.5 μm optical waveguide manufactured under the above conditions. In FIG. 5, each curve represents the processing time. The unprocessed data is the data at the end of the second ion exchange, and corresponds to the prior art. From this result,
According to the present invention in which the electric field strength in the reverse direction is applied in the third ion exchange step, the refraction is performed so that the burying depth is sufficiently large so as to be 10 μm or more, and the ion concentration distribution is more symmetric with respect to the depth direction. It can be seen that the rate increasing portion can be shaped.

FIG. 6 shows the method of the present invention (above-mentioned Embodiment 2).
5 shows a cross-sectional concentration profile of the refractive index increasing ion in the case of 1). As is clear from comparison with the prior art of FIG.
In the (method of the present invention), the degree of asymmetry in the vertical direction (depth direction) is greatly improved, and it can be seen that the shape becomes substantially a perfect circle especially at the center.

(Example 2-2) A refractive index increasing portion was embedded in the substrate by the same procedure as in Example 2-1. The third ion exchange step was performed while changing the electric field intensity and the processing time.
That is, at the same temperature and in the same molten salt as the second ion exchange,
A third ion exchange was performed for 40 minutes under an electric field of 100 V / mm in the opposite direction to the second ion exchange. FIG. 7 shows the result of simulating the concentration profile of the refractive index increasing ion of the 3.5 μm optical waveguide manufactured under these conditions.

Example 2-3 A refractive index increasing portion was embedded in the substrate by the same procedure as in Example 2-1. The third ion exchange step was performed while changing the electric field intensity and the processing time.
That is, at the same temperature and in the same molten salt as the second ion exchange,
8 under an electric field of 50 V / mm in the opposite direction to the second ion exchange.
A third ion exchange for 0 minutes was performed. FIG. 8 shows the result of simulating the concentration profile of the refractive index increasing ions of the 3.5 μm optical waveguide manufactured under these conditions.

From these results, it can be seen that the present invention in which an electric field in the opposite direction is applied in the third ion exchange step has an ion concentration distribution which is sufficiently large so that the embedding depth is 10 μm or more and is more symmetrical in the depth direction. It can be seen that the optical waveguide can be shaped to have

(Example 3) An ion exchange control film of Ti having an opening of a predetermined optical waveguide pattern is formed on an aluminoborosilicate glass substrate containing only Na ions as alkali ions, and AgNO ₃ and NaNO ₃ First ion exchange is performed by immersion in a molten salt (AgNO ₃ concentration: 5%) at 280 ° C. for 120 minutes to form a portion having an increased refractive index on the substrate surface. Subsequently, the ion exchange control film on the substrate surface is removed by etching. Then N
The second ion exchange in which an electric field is applied in a molten salt of aNO ₃ is performed at 280 ° C. under an electric field intensity of 150 V / mm for 120 minutes to bury the refractive index increasing portion formed on the substrate surface in the substrate. Further, heat treatment was performed for 120 minutes in the same molten salt at the same temperature as the second ion exchange.

The 3.5 μm optical waveguide fabricated under these conditions has a BSE (back
FIG. 9 shows the result of analysis by scattered electron). FIG. 10 shows the cross-sectional concentration profile of the refractive index increasing ions in the case of the method of the present invention (Example 3 above). From these results, according to the present invention in which the heat treatment is performed after the second ion exchange step, an optical waveguide having a refractive index distribution which is sufficiently large so that the embedding depth is 10 μm or more and has a better symmetry in the depth direction can be obtained. You can see that it can be shaped. As is clear from comparison with the prior art of FIG. 3, FIG.
In the (method of the present invention), the degree of asymmetry in the vertical direction (depth direction) is greatly improved, and it can be seen that the shape is almost a perfect circle especially at the center.

In the above example, the heat treatment step is performed while immersing in the same molten salt used for the second ion exchange. May be performed.

[0031]

As described above, the present invention utilizes the method of manufacturing a buried optical waveguide using a two-step ion exchange method in principle, and the second ion exchange step or the second ion exchange step By devising the later processing, the increased refractive index portion can be embedded at a sufficient depth, so that the loss can be reduced, and the increased refractive index portion can be shaped so as to have good axial symmetry. The consistency with the optical fiber can be improved.

[Brief description of the drawings]

FIG. 1 is a concentration profile of a refractive index increasing ion of an optical waveguide in Example 1-1.

FIG. 2 is a sectional concentration profile of a refractive index increasing ion of an optical waveguide according to the method of the present invention.

FIG. 3 is a sectional concentration profile of a refractive index increasing ion of an optical waveguide according to a conventional technique.

FIG. 4 is a concentration profile of a refractive index increasing ion of an optical waveguide in Example 1-2.

FIG. 5 is a concentration profile of a refractive index increasing ion of an optical waveguide in Example 2-1.

FIG. 6 is a cross-sectional concentration profile of a refractive index increasing ion of an optical waveguide according to the method of the present invention.

FIG. 7 is a concentration profile of ions having an increased refractive index in an optical waveguide in Example 2-2.

FIG. 8 is a concentration profile of a refractive index increasing ion of an optical waveguide in Example 2-3.

FIG. 9 is a concentration profile of a refractive index increasing ion of an optical waveguide in Example 3.

FIG. 10 is a cross-sectional concentration profile of a refractive index increasing ion of an optical waveguide according to the method of the present invention.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tatsushi Kuno 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd. (72) Inventor Yasuhiro Anma 5-36-11 Shimbashi, Minato-ku, Tokyo Fuji Electrochemical Co., Ltd. F term (reference) 2H047 KA04 PA13 PA21 QA04 TA36

Claims (7)

[Claims] 1. An ion-exchange control film having a predetermined optical waveguide pattern formed on a substrate made of a glassy substance capable of ion exchange containing monovalent ions to increase the refractive index of the substrate. A first ion exchange step of immersing in a first molten salt containing a first monovalent ion and performing thermal ion exchange through the ion exchange control membrane to form a refractive index increasing portion; An etching step of removing the exchange control film; and a second step of reducing the refractive index of the etched substrate by increasing the refractive index in the portion where the refractive index has been increased by the first ion exchange step.
While immersing in a second molten salt containing monovalent ions of
The side on which the ion exchange control film was formed was set to a positive potential, and the opposite side was set to a negative potential, and an electric field was applied almost perpendicularly to the substrate to increase the refractive index increased by the first monovalent ions to the depth of the substrate. A second ion exchange step of embedding in the direction,
In the method of manufacturing a buried optical waveguide performed in this order, the electric field intensity applied in the second ion exchange step is reduced with time, and the cross-sectional ion concentration distribution of a portion where the refractive index is increased due to the buried first monovalent ions. A method of manufacturing a buried optical waveguide, wherein a shape is buried while being shaped so as to approach an axially symmetric shape. 2. An ion exchange control film having a predetermined optical waveguide pattern is formed on a substrate made of a glassy substance which can exchange ions and contains monovalent ions, thereby increasing the refractive index of the substrate. A first ion exchange step of immersing in a first molten salt containing a first monovalent ion and performing thermal ion exchange through the ion exchange control membrane to form a refractive index increasing portion; An etching step of removing the exchange control film; and a second step of reducing the refractive index of the etched substrate by increasing the refractive index in the portion where the refractive index has been increased by the first ion exchange step.
While immersing in a second molten salt containing monovalent ions of
The side on which the ion exchange control film was formed was set to a positive potential, and the opposite side was set to a negative potential, and an electric field was applied almost perpendicularly to the substrate to increase the refractive index increased by the first monovalent ions to the depth of the substrate. A second ion exchange step of embedding in the direction,
In the method for manufacturing a buried optical waveguide performed in this order, after the second ion exchange step, an electric field in a direction opposite to that of the second ion exchange step is applied while the substrate is immersed in a second molten salt. A third ion exchange step, wherein a sectional ion concentration distribution shape of a refractive index increasing portion formed by the first monovalent ions embedded in the second ion exchange step is made axially symmetric by the third ion exchange step. A method for manufacturing a buried optical waveguide, wherein the optical waveguide is shaped so as to approach. 3. An ion exchange control film having a predetermined optical waveguide pattern is formed on a substrate made of a glass material containing ion exchangeable ions containing monovalent ions to increase the refractive index of the substrate. A first ion exchange step of immersing in a first molten salt containing a first monovalent ion and performing thermal ion exchange through the ion exchange control membrane to form a refractive index increasing portion; An etching step of removing the exchange control film; and a second step of reducing the refractive index of the etched substrate by increasing the refractive index in the portion where the refractive index has been increased by the first ion exchange step.
While immersing in a second molten salt containing monovalent ions of
The side on which the ion exchange control film was formed was set to a positive potential, and the opposite side was set to a negative potential, and an electric field was applied almost perpendicularly to the substrate to increase the refractive index increased by the first monovalent ions to the depth of the substrate. A second ion exchange step of embedding in the direction,
In the method for manufacturing a buried optical waveguide performed in this order, a heat treatment step of heating and holding the substrate in an electric field-free state after the second ion exchange step is provided, and the first buried optical waveguide embedded in the second ion exchange step is provided. Wherein the shape of the cross-sectional ion concentration distribution at the portion where the refractive index is increased due to the monovalent ions is shaped so as to approximate an axially symmetric shape by the heat treatment step. 4. The method according to claim 1, further comprising the step of:
4. The method of manufacturing an embedded optical waveguide according to claim 3, wherein the method is performed by immersing the embedded optical waveguide in a molten salt. 5. The depth of the point where the concentration of the ion which increases the refractive index in the refractive index increasing portion is highest from the substrate surface is 1 point.
The method for manufacturing a buried optical waveguide according to any one of claims 1 to 4, wherein the portion having an increased refractive index is buried so as to have a thickness of 0 µm or more. 6. The first monovalent ions capable of increasing the refractive index of the substrate include Ag ions, Tl ions, Cs ions,
As a second monovalent ion that can lower the refractive index of the refractive index increasing portion formed by the first monovalent ion using at least one ion of Rb ion, K ion, and Li ion, Na ion or 6. The method for manufacturing a buried optical waveguide according to claim 1, wherein at least one kind of K ions is used. 7. An aluminoborosilicate-based glass, an aluminosilicate-based glass,
The method for manufacturing a buried optical waveguide according to any one of claims 1 to 6, wherein one of borosilicate glass is used.