GB2292468A - Manufacturing method for an optical waveguide - Google Patents

Abstract

A method of manufacturing an optical waveguide whose refractive index distribution exhibits a step index profile comprises: forming a first layer (2) of fine glass particles to serve as a lower cladding on a substrate (1) by flame hydrolysis deposition; heat-treating the first layer (2) at 800 DEG C or more; forming a second layer (3) of fine glass particles and a refractive index increasing material e.g. P2O5 or GeO2, on the first layer (2) by flame hydrolysis deposition; heat-treating the first and second layers (2, 3) together, thereby converting the layers into transparent glass layers; applying photolithography and then dry-etching layer (3) to form a core (5) on the layer (2A); burying the core (5) by forming a third layer (6) of fine glass particles to serve as an upper cladding by flame hydrolysis deposition; and a step of heat-treating the third layer (6), to convert it into a transparent glass layer. <IMAGE>

Description

MANUFACTURING METHOD FOR AN OPTICAL WAVEGUIDE The present invention relates to a manufacturing method for an optical waveguide, and more specifically, to a method for manufacturing an optical waveguide of a silica glass type, whose sectional configuration has a refractive index distribution exhibiting a step index profile, and which can reduce propagation light losses.

In general, an optical waveguide formed of silica glass is manufactured in the following manner.

First, a first layer 2 of fine particles of silica glass with a predetermined thickness is formed on a substrate 1 of, e.g., single-crystal silicon by a flame hydrolysis deposition process, as shown in FIG. 1.

The flame hydrolysis deposition process used in this case is a method in which a burner A, for example, is supplied with SiC14, oxygen, and hydrogen, for use as source materials, in predetermined ratios, and is ignited so that combustion flames from the burner A are blown against the surface of the substrate 1, whereupon fine particles of silica glass produced by combustion are deposited on the substrate surface.

Thus, the first layer 2 is converted into a transparent glass layer by heat treatment, which will be mentioned later, and constitutes a lower cladding of the optical waveguide to be manufactured.

Then, a second layer 3 of fine particles of another silica glass with a predetermined thickness is formed on the first layer 2 by a flame hydrolysis deposition process again, as shown in FIG. 2.

In this case, the flame hydrolysis deposition process is applied in a manner such that the burner A, for example, continues to be supplied with a predetermined quantity of source materials, including So13, POC13, GeC14, etc., as well as the aforesaid combination of SiCl4, oxygen, and hydrogen.

At the time of combustion, these materials are converted into oxide particles, such as B203, P205, GeO2, etc., which are deposited together with fine glass particles.

When the second layer 3 is heat-treated into transparent glass, the oxides are solidly dissolved in the glass, whereupon the refractive index of the glass increases. Thus, the transparent glass serves as a core. Hereinafter, therefore, the aforesaid oxides will be referred to as refractive index increasing materials.

Subsequently, the whole structure is heat-treated at a temperature of 1,300 to 1,400T, whereby the first layer 2, which is formed of fine particles of silica glass, and the second layer 3, which is doped with the refractive index increasing materials, are converted into transparent glass layers 2A and 3A, respectively (FIG. 3).

Thus, the lower cladding layer formed of the transparent glass layer 2A and the core layer formed of the transparent glass layer 3A, whose refractive index is higher than that of the glass layer 2A, are successively stacked in layers on the substrate 1.

Then, a mask 4 designed in a3xrd=re with the nodel of the plane pattern of the optical waveguide to be obtained is formed on the surface of the transparent glass layer 3A by photolithography, as shown in FIG. 4.

Thereafter, all the other portions of the transparent glass layer 3A than the portion thereof corresponding to the optical waveguide to be formed are removed by dry etching, e.g., reactive ion etching.

Thereupon, a core 5 of high-refraction transparent glass is formed as an optical waveguide with a predetermined plane pattern on the transparent glass layer 2A, as shown in FIG. 5.

Then, a third layer 6 of fine particles of silica glass with a predetermined thickness is formed on the transparent glass layer 2A by a flame hydrolysis deposition process so as to have the core 5 buried therein, as shown in FIG. 6.

In the flame hydrolysis deposition process for this case, the fine particles of silica glass are deposited without supplying the aforesaid refractive index increasing materials. During heat treatment, therefore, the third layer 6 is converted into transparent glass whose refractive index is lower than that of the core 5, and constitutes an upper cladding of the optical waveguide to be manufactured.

Finally, the third layer 6 is heat-treated into transparent glass at a temperature of, e.g., 1,000 to 1,200"C. In consequence, the desired optical waveguide is obtained in a manner such that the core 5 with the predetermined plane pattern is buried in the transparent glass layers 2A and 6A which are lower in refractive index than the core 5, as shown in FIG. 7.

According to the optical waveguide manufacturing method described above, however, the following problems arise when the transparent glass layers 2A and 3A shown in F1G. 3 are formed by heat-treating the first and second layers 2 and 3 together shown in FIG. 2.

When a structure B1 of FIG. 2 is heat-treated, the refractive index increasing materials, dispersed in a predetermined quantity together with the glass particles which constitute the second layer 3, diffuse into the first layer 2 as the first and second layers 2 and 3 are converted into transparent glass.

In this case, the quantity of the refractive index increasing materials which diffuse from the second layer 3 into the first layer 2 gradually decreases with distance from the interface between the two layers.

Thus, in a structure B2 of FIG. 3 which includes the transparent glass layers 2A and 3A, the concentration of the refractive index increasing materials solidly dissolved in the transparent glass layer 2A shy a certain concentration gradient pattern which declines from an interface C between the transparent glass layer (core) 3A and the transparent glass layer (lower cladding) 2A toward the lower cladding 2A.

In the sectional configurations of the transparent glass layers 2A and 3A, therefore, the refractive index distribution continuously varies corresponding to the concentration gradient of the refractive index increasing materials, as shown in FIG. 8, and the distribution in the vicinity of the interface C between the layers does not display a step index profile which distinctly discontinuously changes. The larger the quantity of the refractive index increasing materials contained in the second layer 3, the more marked this trend is.

This phenomenon indicates that propagation light is liable to leak toward the transparent glass layer (lower cladding) 2A even though it is urged to be confined to the core of the manufactured optical waveguide. Inevitably, the optical waveguide is subject to increased propagation losses.

Moreover, the above-described situation raises a problem that the optical properties of the obtained optical waveguide deviate considerably from their design criteria even though the waveguide is designed so that the refractive indexes of the core and the claddings are adjusted to predetermined values.

Furthermore, the refractive index increasing materials start to evaporate if the temperature of a flame receiving surface (flame hydrolysis deposition surface), on which the fine particles of silica glass and the index increasing materials are deposited, is too high when the second layer 3 shown in FIG. 2 is formed. In such a situation, the quantity of the refractive index increasing materials solidly dissolved in the formed transparent glass layer (core) 3A is reduced, so that the refractive index of the core inevitably fails to meet its design criteria.

The above problems often arise even in case the temperature of the combustion flames is not so high as aforesaid. This is because the lower-side portions of the successively formed layers accumulate heat to reach a high temperature as the fine particles are deposited, so that the refractive index increasing materials evaporate at those portions.

Viewed from one aspect the present invention provides a manufacturing method for an optical waveguide, which comprises: a step of forming a first layer of fine glass particles to serve as a lower cladding on a substrate by a flame hydrolysis deposition process; a step of heat-treating the first layer at a temperature of 800do or more; a step of forming a second layer of fine glass particles and a refractive index increasing material on the first layer by a flame hydrolysis deposition process; a step of heat-treating the first and second layers together, thereby converting both the layers into transparent glass layers; a step of applying photolithography and dry-etching to the transparent glass of the second layer, thereby forming a core having a predetermined plane pattern; a step of burying the core and forming a third layer of fine glass particles to serve as an upper cladding by a flame hydrolysis deposition process; and a step of heattreating the third layer, thereby converting the third layer into a transparent glass layer.

Examples of methods of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 is a sectional view showing a layer of fine glass particles for a lower cladding formed by a flame hydrolysis deposition process; FIG. 2 is a sectional view showing a layer of fine glass particles for a core formed on the layer of FIG. 1 by a flame hydrolysis deposition process; FIG. 3 is a sectional view showing transparent glass layers 2A and 3A formed by heat-treating a structure B1 of FIG. 2; FIG. 4 is a sectional view showing a state in which the transparent glass layer 3A of FIG. 3 is undergoing photolithography and dry etching; FIG. 5 is a sectional view showing a core; FIG. 6 is a sectional view showing a layer of fine glass particles for an upper cladding formed by burying the core by a flame hydrolysis deposition process;; FIG. 7 is a sectional view showing a sectional configuration of a manufactured optical waveguide; FIG. 8 is a graph showing a refractive index distribution for a conventional optical waveguide; FIG. 9 is a sectional view showing a modified version of a first layer according to exemplary methods of the present invention; FIG. 10 is a sectional view showing a second layer formed on the modified layer 2' of FIG. 9 by a flame hydrolysis deposition process; FIG. 11 is a sectional view showing a state in which the second layer is being formed by an exemplary method according to the present invention; FIG. 12 is a graph showing a refractive index distribution for an optical waveguide manufactured by an exemplary method according to the present invention;; FIG. 13 is a graph showing a refractive index distribution for an optical waveguide manufactured by a method according to Comparative Example 1; and FIG. 14 is a graph showing refractive index distributions for light waveguides manufactured by methods according to Comparative Examples 2 and 3.

In manufacturing an optical waveguide by a preferred method according to the present invention, a first layer 2 of fine particles of silica-based glass with a predetermined thickness, containing no refractive index increasing materials, is formed on a substrate 1 by a flame hydrolysis deposition process, just as in the conventional case.

The first layer 2 is formed by depositing fine particles of silica-based glass of about 0.1 to 0.2 am in diameter, which may vary depending on the operating conditions for the flame hydrolysis deposition process, and the bulk density of the first layer 2 ranges from about 0.16 to 0.20 g/cm3.

In accordance with the present invention, the first layer 2 is then temporarily subjected to a heat treatment (hereinafter referred to as heat treatment 1) at a temperature of 800T or more.

When the heat treatment 1 is conducted, the first layer 2 shown in FIG. 1 is converted into a modified layer 2' (which will be mentioned later), as shown in FIG. 9.

The modified layer 2' is deemed to be in a state such that the fine particles of silica-based glass which constitute the first layer 2 shown in FIG. 1 are partially superficially fused on one another by the heat treatment 1, or are compacted as a whole, partially forming transparent glass. Thus, the bulk density of the modified layer 2', becomes higher than that of the first layer 2. The bulk density of the modified layer 2' is, for example, 1.0 g/cm3.

Since the modified layer 2' is improved in density in this manner, it is believed that refractive index increasing materials contained in a second layer (which will be mentioned later) are restrained from diffusing into the modified layer 2' when transparent glass is obtained by heat treatment after the formation of the second layer.

The heat treatment 1 produces an effect only when it is conducted at a temperature of 800"C or more.

If this temperature is too high, however, the greater part of the first layer is converted into transparent glass, and the adhesion to the second layer formed thereon is unsatisfactory. Preferably, therefore, the upper limit of this temperature is 1,380"C or thereabout. More specifically, the optimum temperature ranges from 1,000 to 1,380 C. If this temperature range is used, the treatment time, which is not restricted in particular, preferably ranges from about 60 to 100 minutes.

In accordance with the present invention, a second layer with a predetermined thickness is formed on the modified layer 2' by a flame hydrolysis deposition process, as shown in FIG. 10. The second layer is formed of a matrix of fine particles of silica-based glass and fine particles of refractive index increasing materials solidly dissolved in the matrix.

In this flame hydrolysis deposition process, a burner A is supplied with a predetermined quantity of one or more refractive index increasing materials.

including BC13, POC13, GeC14, etc., as well as oxygen, hydrogen, and SiC14.

In this process, BC13, POC13, and GeC14 are all changed into fine particles of oxides, such as B203, P205, and GeO2, respectively, which are deposited as refractive index increasing materials solidly dissolved in a matrix of SiO2, thus forming the second layer 3.

Preferably, in this process, the temperature of the surface of the developing second layer 3, that is, a flame hydrolysis deposition surface 3a on which the fine particles of the individual oxides are being deposited, as shown in FIG. 11, is controlled so as to range from 400 to 700t.

If the temperature of the flame hydrolysis deposition surface 3a is lower than 4000C, the refractive index increasing materials are separated in the form of independent crystals without being solidly dissolved in the matrix of fine particles of Si02.

Thus, a desired refractive index cannot be obtained when the materials are heat-treated into transparent glass, as mentioned later. If the temperature of the flame hydrolysis deposition surface 3a is higher than 700"C, on the other hand, the quantity of evaporation of the refractive index increasing materials produced in the process of the flame hydrolysis deposition is larger than the quantity solidly dissolved in the matrix of SiO2. Thus, the desired refractive index cannot be obtained either when the transparent glass is formed.

During the flame hydrolysis deposition, heat accumulation advances at the lower-side portion of the developing second layer, that is, the layer portion on the side nearer to the interface on the modified layer 2', so that the lower-side portion becomes higher in temperature than the upper-side portion. At the lower-side portion, therefore, evaporation of the refractive index increasing materials advances so that the materials are reduced below the predetermined quantity. Accordingly, the desired refractive index cannot be obtained either when the transparent glass is formed.

In consideration of these circumstances, according to preferred examples of the present invention, a method is employed such that the quantity of the supplied refractive index increasing materials in the vicinity of the interface between the second layer and the modified layer 2' is made to be greater than the predetermined quantity in the process of forming the second layer by successively depositing the fine particles of silica-based glass and the refractive index increasing materials on the modified layer 2'.

Also, the quantity of the supplied materials is reduced intermittently or continuously in the thickness direction of the developing second layer.

Even when the evaporation of the refractive index increasing materials advances at the lower-side portion of the developing second layer, according to this method, the quantity of the refractive index increasing meterials is retained in the lower-side portion of the developing second layer, as materials are supplied in excess. When the second layer is formed into the core of transparent glass, therefore, the desired refractive index can be also obtained on the lower side of the core (near the interface on a lower cladding).

This can be easily attained by supplying a relatively large quantity of refractive index increasing materials to the burner A at the beginning and then gradually reducing the quantity.

After the second layer is formed in this manner, the whole resulting structure is subjected to a heat treatment (hereinafter referred to as heat treatment 2), whereupon the modified layer 2' and the second layer 3 shown in FIG. 10 are converted into transparent glass layers 2A and 3A, respectively, as shown in FIG. 3.

Normally, the heat treatment 2 is conducted at a temperature of 1,300 to 1,400"C. An oxygen-rich atmosphere with an oxygen volume percentage of about 2 to 3%, for example, may be used as an atmosphere for that heat treatment 2, since it can prevent the evaporation of the refractive index increasing materials which are solidly dissolved in the matrix of SiO2 on the surface of the second layer 3.

Despite the heat treatment 2, the modified layer 2' prevents the refractive index increasing materials in the second layer 3 from diffusing into the transparent glass layer 2A. Accordingly, the refractive index distribution in the sectional configurations of the transparent glass layers 2A and 3A shown in FIG. 3 presents a step index profile.

During the flame hydrolysis deposition, moreover, the second layer 3 is formed in a manner such that the evaporation of the refractive index increasing materials is restrained or the materials are supplied so as to make up for the evaporation. Thus, the refractive index of the transparent glass layer 3A ensures the values of the design criteria.

A core 5 is formed by applying photolithography to the transparent glass layer 3A, as shown in FIG. 4, and then dry-etching to the transparent glass layer 3A, as shown in FIG. 5. Thereafter, the core 5 is buried to form a third layer 6 of fine particles of silica-based glass by a flame hydrolysis deposition process, as shown in FIG. 6. As shown in FIG. 7, moreover, the third layer 6 is heat-treated into a transparent glass layer 6A, whereupon an optical waveguide in accordance with the present invention is obtained.

Examples 1 to S As shown in FIG. 1, the first layer 2 with a thickness of 400 um was formed on the Si substrate 1 in a manner such that fine particles of Si02 were deposited by the flame hydrolysis deposition process using oxygen, hydrogen, and SiC14.

Then, the first layer 2 was converted into the modified layer 2', as shown in FIG. 9, by conducting the heat treatment 1 in air of 800"C. The modified layer 2' became a transparent glass layer.

The second layer 3 with a thickness of 60 um shown in FIG. 10 was formed on the modified layer 2' in the following manner.

First, GeCl4 (liquid) was selected as the source of a refractive index increasing material, and the flame hydrolysis deposition process was conducted by compounding GeC14 with the aforesaid combination of oxygen, hydrogen, and SiC14 so that the temperature of the flame hydrolysis deposition surface 3a shown in FIG. 11 was 400t.

The second layer 3 was formed in three stages such that three layers, lower, middle, and upper, with the same thickness of 20 am were formed in succession. In forming these individual layers, the content of the GeC14 was changed in the manner shown in Table 1.

Table 1

Lower Layer Middle Layer Upper Layer Frequency Frequency Frequency GeCl4 GeCl4 GeCl4 of Flame of Flame of Flame Hydrolysis Hydrolysis Contents Contents Contents Hydrolysis Deposition Deposition Deposition (ml) (ml) (ml) (times) (times) Example 1 3 0 3 2 5 3 2 0 2 Example 2 3 5 3 2 7. 5 3 2 0 2 Ewnple 3 3 5 3 2 7. 5 3 2 3 2 Example 4 4 0 3 3 0 3 2 5 2 Exapmle 5 5 5 2 5 0 2 4 5 2 Example 6 5 5 2 4 7. 5 2 4 0 2 Example 7 6 5 2 5 7. 5 2 5 0 2 Example 8 7 0 2 6 0 2 5 5 2 Thereafter, the heat treatment 2 was conducted so that the whole structure was kept at 1,350C for one hour, whereby the modified layer 2' and the second layer 3 were both converted into transparent glass layers, as shown in FIG. 3.After the core was then formed by photolithography and dry etching, the same flame hydrolysis deposition process for the formation of the first layer was carried out to form the third layer 6 with a thickness of 450 um, as shown in FIG.

6. Further, the resulting structure was heat-treated in a mixed atmosphere of helium and oxygen at 1,150"C for two hours, whereupon optical waveguides with the sectional configuration shown in FIG. 7were obtained.

The refractive index distribution was measured for the sectional configuration of each obtained optical waveguide by using a transmission interference microscope. Examples 1 to 8 exhibited the same distribution. FIG. 12 shows the refractive index distribution for the case of Example 1 as a representative.

As shown in FIG. 12, the refractive index distribution for each of the optical waveguides manufactured by the methods of Examples 1 to 8 presents a step index profile. As is evident from the comparison with the conventional refractive index distribution shown in FIG. 8, the step index profile indicates that no refractive index increasing material diffused into the lower cladding.

Examples 9 to 16 & Comparative Example 1 Optical waveguides were manufactured in the same manner as in Example 1 except that the temperature for the heat treatment 1 after the formation of the first layer was varied in the manner shown in Table 2.

Tale 2

Temperature ("C) Comparative Example 1 700 Example 9 900 Example 10 1000 Example 11 1100 Example 12 1200 Example 13 1250 Example 14 1300 Example 15 1350 Example 16 1380 The refractive index distribution was measured for each optical waveguide in the same manner as in Example 1. Any of Examples 9 to 16 exhibited a refractive index distribution similar to the one shown in FIG. 12. However, Comparative Example 1 displayed a refractive index distribution such as the one shown in FIG. 13, presenting a considerably deformed step index profile.This indicates that the heat treatment 1 for the first layer should be conducted at the temperature of 800T or more.

Examples 17 to 19 & Comparative Examples 2 & 3 Optical waveguides were manufactured in the same manner as in Example 15 except that the temperature of the flame hydrolysis deposition surface for the formation of the second layer was adjusted in the manner shown in Table 3.

Table 3

Temperature of Flame Hydrolysis Deposition Surface (Cc) Comparative Example 2 300 Example 17 500 Example 18 600 Example 19 700 Comparative Example 3 900 The refractive index distribution was measured for each optical waveguide in the same manner as in Example 15. Any of Examples 17 to 19 exhibited a refractive index distribution similar to the one shown in FIG. 12.

FIG. 14 shows refractive index distributions for Comparative Examples 2 and 3. In FIG. 14, broken and dashed lines represent the cases of Comparative Examples 2 and 3, respectively.

As seen from FIG. 14, the step index profile of the refractive index distribution is deformed if the temperature of the flame hydrolysis deposition surface for the formation of the second layer is too low or too high.

Example 20 Optical waveguides were manufactured in the same manner as in Example 15 except that atmospheres for the heat treatment 2 for the first and second layers to be fully converted into transparent glass layers contained 2 and He in the ratios of 1 to 3, 1 to 2, 1 to 1, 2 to 1, and 3 to 1 by volume.

The refractive index distribution was measured for each obtained optical waveguide. Thereupon, any of the manufactured waveguides exhibited a step index profile similar to the one shown in FIG. 12.

Example 21 Optical waveguides were manufactured in the same manner as in Example 1 except that the second layer was formed in a manner such that five layers, a lower layer, intermediate layers (1), (2) and (3), and an upper layer, with the same thickness of 15 am were formed in succession.

The refractive index distribution was measured for each obtained light waveguide. Thereupon, any of the manufactured waveguides exhibited the same step index profile as the one shown in FIG. 12.

Thus, according to at least preferred examples of the present invention, there is provided a method of manufacturing a low-loss optical waveguide by restraining refractive index increasing materials with which a core is doped from diffusing into a cladding and by obtaining a step index profile of a refractive index distribution in a sectional configuration; and there is provided a method in which there is secured a predetermined quantity of refractive index increasing materials in a layer of fine glass particles by preventing evaporation of the materials when the fine glass particles and the refractive index increasing material are deposited by the flame hydrolysis deposition process, or a predetermined quantity of the materials despite the evaporation, thereby maintaining the refractive index of the core in conformity to the design criteria.

Claims (6)

Claims:

1. A manufacturing method for an optical waveguide, comprising: a step of forming a first layer of fine glass particles to serve as a lower cladding on a substrate by a flame hydrolysis deposition process; a step of heat-treating said first layer at a temperature of 800 C or more; a step of forming a second layer of fine glass particles and a refractive index increasing material on said first layer by a flame hydrolysis deposition process; a step of heat-treating said first and second layers together, thereby converting both said layers into transparent glass layers; a step of applying photolithography and dryetching to the transparent glass of said second layer, thereby forming a core having a predetermined plane pattern;; a step of burying said core and forming a third layer of fine glass particles to serve as an upper cladding by a flame hydrolysis deposition process; and a step of heat-treating said third layer, thereby converting said third layer into a transparent glass layer.

2. A method according to claim 1, wherein the temperature of a flame hydrolysis deposition surface for the formation of said second layer lies in a range from 400 to 7000C.

3. A method according to claim 1 or 2, wherein said second layer is formed in a manner such that the content of said refractive index increasing material is higher at the lower-side portion of said second layer than at the upper-side portion.

4. A method according to claim 1, 2 or 3, wherein said refractive index increasing material is GeO2 or/and P2Os.

5. A method according to any preceding claim, wherein an oxygen-rich atmosphere is used in converting said first and second layers into the transparent glass layers.

6. A manufacturing method for an optical waveguide substantially as hereinbefore described with reference to any of Examples 1 to 21 and the accompanying drawings.