JP2004037745A - Manufacturing method for optical waveguide grating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method for manufacturing an optical waveguide grating with an excellent performance by obtaining a high refractive index change with a smaller amount of ultraviolet ray irradiation than before. <P>SOLUTION: After diffusing hydrogen to an optical waveguide 1, a grating part 5 having a high refractive index part 4 is formed by performing interference exposure of ultraviolet rays and the entire grating part 5 is irradiated with the ultraviolet rays after dehydrogenation. Thus, since a refractive index modulation width is improved, the grating part 5 is easily and surely formed and the optical waveguide grating with excellent characteristics is manufactured. Further, it is preferable to heat the optical waveguide 1 and apply aging treatment thereto before irradiating the entire grating part 5 with the ultraviolet rays after the interference exposure. Thus, the optical waveguide grating of higher heat stability is manufactured. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing an optical waveguide grating that forms a grating portion having a high refractive index portion by irradiating ultraviolet light.
[0002]
[Prior art]
Optical waveguide gratings such as optical fiber Bragg gratings (FBG) are widely used as wavelength selection devices and the like. Conventionally, this type of optical waveguide grating diffuses hydrogen in the optical waveguide, performs interference exposure using ultraviolet light, and periodically induces a refractive index change in a part of the core of the optical waveguide, It is manufactured by forming a grating portion having a high refractive index portion.
[0003]
It is known that the high refractive index portion formed by ultraviolet light irradiation contains an unstable component whose refractive index is easily degraded by heat and a stable component that is hardly degraded by heat, This content is T. J. Eldogan et al., "Delay of Ultraviolet-Induced Fiber Bragg Gratings", J. Biol. Appl. Phys. 76 (1), July 1994.
[0004]
Since the thermally unstable component deteriorates with time according to the environmental temperature, the performance of the optical waveguide grating may change over time, which may hinder the design of a communication system operated for a long time. . Therefore, in order to reduce this effect, a thermally unstable component is removed by heating the optical waveguide grating. This is called aging processing.
FIG. 5 shows a manufacturing process of a general optical waveguide grating. This manufacturing process includes a hydrogen diffusion process, an interference exposure process, a dehydrogenation process, and an aging process. Note that the dehydrogenation step and the aging step shown in FIG. 5 may be performed simultaneously.
[0005]
[Problems to be solved by the invention]
By the way, when the aging step is performed, a thermally unstable component in the high refractive index portion is removed, so that the refractive index of the high refractive index portion decreases. In general, the amount of decrease in the refractive index due to the aging process differs depending on the subtle differences in the conditions when forming the grating portion, and it is difficult to predict from the characteristics of the grating before the aging process.
[0006]
For example, as shown in FIG. 6A, the case where the refractive index of the low refractive index portion 6 between the high refractive index portions 4 is substantially equal to the refractive index of the core 2 and FIG. As shown in the figure, the amount of change in the refractive index from the core 2 of the high refractive index portion 4 differs between the case where the low refractive index portion 6 is also exposed to ultraviolet rays and the refractive index changes. The rate of decrease is different. For this reason, even if the refractive index modulation width Δn (the refractive index difference between the high refractive index part 4 and the low refractive index part 6) is equal before the aging processing, a difference occurs in the refractive index modulation width Δn after the aging processing, Grating characteristics such as the center wavelength, bandwidth, and reflectivity of the reflection band may change.
[0007]
For example, the reflectance R is given by the following equation (1), where L is the length of the grating portion, Δn is the refractive index modulation width, and λ is the wavelength.
[0008]
R = tanh ² (πLΔn / λ) (1)
[0009]
According to equation (1), the reflectance R decreases as Δn decreases. In addition, when Δn decreases, the center wavelength and bandwidth of the reflection band also decrease, and the wavelength selection characteristics deteriorate.
[0010]
If the amount of decrease in the refractive index due to the aging treatment is larger than expected, if the amount of change in the refractive index after the aging treatment is insufficient, the characteristics of the optical waveguide grating after the aging treatment fall below the design value, which may reduce the yield. is there. Therefore, conventionally, in anticipation of a decrease in the refractive index due to the aging process, the amount of increase in the refractive index of the high refractive index portion 4 is made larger than the amount required to obtain the desired grating characteristics.
However, as shown in FIG. 7, since the increase in the refractive index per ultraviolet light irradiation time decreases as the ultraviolet light irradiation time becomes longer, if the refractive index increase is made larger than necessary, the ultraviolet light irradiation time becomes very long. It is long and disadvantageous in terms of productivity.
In addition, it is known that an excessive amount of ultraviolet light irradiation tends to cause a significant increase in loss due to absorption of a hydroxyl group (OH group) in the 1.38 μm band, which deteriorates the performance of the grating. is there.
[0011]
The present invention has been made in view of the above circumstances, and provides a manufacturing method capable of obtaining a high refractive index change with a smaller amount of irradiation of ultraviolet light than before, and manufacturing an optical waveguide grating having excellent performance. The task is to
[0012]
[Means for Solving the Problems]
The object is to form a grating portion having a high refractive index portion by irradiating an optical waveguide with ultraviolet light to form an optical waveguide grating.In the method of manufacturing an optical waveguide grating, hydrogen is diffused into the optical waveguide. After that, the problem is solved by irradiating ultraviolet rays to form a grating portion having a high refractive index portion, and after dehydrogenation, irradiating the entire grating portion with ultraviolet rays.
Furthermore, after forming the grating portion and before irradiating the entire grating portion with ultraviolet rays, it is preferable to perform aging treatment by heating the optical waveguide. Thereby, an optical waveguide grating having higher thermal stability can be manufactured.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments.
FIG. 1 shows a schematic configuration of an optical waveguide grating. In the figure, reference numeral 1 denotes an optical waveguide. The optical waveguide 1 includes a core 2 containing germanium and a clad 3 having a lower refractive index than the core 2. The type of the optical waveguide 1 may be an optical fiber or a substrate type optical waveguide.
A high refractive index portion 4 is periodically formed in a part of the core 2 to form a grating portion 5.
[0014]
FIG. 2 shows a process chart for manufacturing the optical waveguide grating. In the present embodiment, after the hydrogen diffusion step, in the state where hydrogen is diffused in the optical waveguide 1, interference exposure is performed using ultraviolet light to write a grating, and after the dehydrogenation step, the optical waveguide 1 is heated. After the treatment (aging treatment), the entire grating section 5 is irradiated with ultraviolet rays.
[0015]
In the hydrogen diffusion step, hydrogen is diffused into the optical waveguide 1 at least under a hydrogen pressure of 10 MPa or more, more preferably 50 MPa or more. At this time, the higher the pressure of the pressurized hydrogen gas, the greater the change in the refractive index due to the irradiation of ultraviolet light. The higher the concentration of germanium added to the core 2, the greater the change in the refractive index due to the irradiation of ultraviolet light.
[0016]
Next, interference exposure is performed. In the interference exposure step, when irradiating the core 2 of the optical waveguide 1 with ultraviolet light, the ultraviolet light interferes to provide a periodic intensity to the irradiation intensity, thereby causing a refractive index-induced change in a portion where the irradiation intensity is increased. Thereby, a periodic high refractive index portion 4 is formed in a part of the core 2. At this time, the larger the amount of ultraviolet light irradiation, the larger the amount of change in the refractive index. The irradiation amount of ultraviolet light is at least 0.5 mJ / mm ² or more, more preferably 1.0 mJ / mm ² or more, for 30 seconds or more.
[0017]
Examples of the method of causing the ultraviolet light to interfere include a method using an intensity mask or a phase mask, a two-beam interference method, a step-by-step method, and the like.
Among them, the intensity mask is formed by forming a part of the transparent body through which a light cannot be transmitted in a slit shape, and the ultraviolet light is applied to the optical waveguide 1 through the intensity mask to form a light guide. A periodic high refractive index portion 4 is formed in the waveguide 1.
[0018]
The phase mask is generally a transmission type diffraction grating formed of a transparent body, and generates interference between + 1st-order or -1st-order diffracted light, so that the phase mask has equal intervals or irregular intervals such as chirp. This causes spatial modulation of light intensity. By irradiating the thus-modulated ultraviolet light to the optical waveguide 1, a change in the refractive index of the optical waveguide 1 at equal or unequal intervals is caused, and the high refractive index portion 4 can be formed.
[0019]
The two-beam interference method is a method in which ultraviolet light is divided into two light beams by a beam splitter, and a grating having a desired period is formed by controlling an angle formed by these light beams. In the longitudinal direction of the optical waveguide 1 for exposure.
Which of these methods is used depends on the use of the optical waveguide grating to be manufactured.
[0020]
After the high-refractive-index portions 4 are periodically formed in a part of the core 2 in this manner, the hydrogen diffused into the optical waveguide 1 is eliminated. In the dehydrogenation step, in order to release the hydrogen added into the optical waveguide 1 in the hydrogen diffusion step, it is left at normal temperature or subjected to a high-temperature heat treatment. When left at room temperature, it is left for at least 3 weeks, more preferably 4 weeks or more. In the case of high-temperature treatment, for example, if the temperature is 120 ° C. or more, it is left for at least 12 hours or more, if it is 220 ° C., it is left for at least 80 minutes or more, more preferably 120 minutes or more.
[0021]
Next, the dehydrogenated optical waveguide 1 is irradiated with ultraviolet light to the entire grating section 5. Specifically, by irradiating ultraviolet light having an energy density of 0.5 mJ / mm ² or more for 60 seconds or more, the refractive index of the high refractive index portion 4 formed by interference exposure increases.
[0022]
Next, the operation of the present invention will be described. The present invention is an application of a presensitization effect. This presensitization effect means that, in the optical waveguide 1, a region irradiated with ultraviolet light in the presence of hydrogen changes its refractive index upon irradiation with ultraviolet light. The refractive index changes even after irradiation with ultraviolet light after dehydrogenation, and thus the change in refractive index formed by two irradiations of ultraviolet light is more stable than the change in refractive index formed by one irradiation of ultraviolet light. It is that the sex becomes better. This effect is described in the article by John Canning, "Photosensitization and Photostabilization of Laser-induced Index Changes in Optical Fiberbooks, 752," Photosensitization and Photostabilization of Laser-induced Index Changes in Optical Fiberbooks, 2nd ed. -289 (2000).
[0023]
FIG. 3 shows a change in the refractive index of the high refractive index portion 4 in the manufacturing process shown in FIG. As shown in FIG. 3, the change in the refractive index of the grating portion 5 formed by the interference exposure is reduced by the dehydrogenation step. However, by the second irradiation of the ultraviolet light, the refractive index of the high refractive index portion 4 can be increased again. Therefore, the influence of a decrease in the refractive index due to the dehydrogenation step can be reduced.
[0024]
In addition, the increase in the refractive index due to the second irradiation of the ultraviolet light after the dehydrogenation occurs only in the portion where the refractive index change is induced by the interference exposure due to the presensitization effect. For this reason, in the second ultraviolet light irradiation, the ultraviolet rays can be applied to the entire grating section 5, so that the implementation is easy.
The increase in the refractive index change due to the presensitization effect is a change in the direction in which the refractive index decrease is canceled (the direction in which the refractive index is increased) with respect to the refractive index decrease due to the dehydrogenation step. The characteristics can be improved.
[0025]
Furthermore, since the refractive index modulation width Δn can be improved by the second ultraviolet irradiation, the amount of ultraviolet light irradiation in interference exposure can be reduced as compared with the related art. As a result, it is possible to reduce the loss due to the generation of OH groups in the 1.38 μm band due to excessive ultraviolet light irradiation.
[0026]
Next, a second embodiment of the present invention will be described. FIG. 4 is a process chart showing a second embodiment of the present invention. In the present embodiment, after forming the grating portion 5 by interference exposure, before irradiating the entire grating portion 5 with ultraviolet rays, the optical waveguide 1 is heated to perform an aging process. The aging step is performed by heating the optical waveguide 1 after dehydrogenation at, for example, 200 to 300 ° C. for 1/10 to 1/2 hour. This makes it possible to efficiently remove thermally unstable components, thereby further improving the thermal stability of the optical waveguide grating.
[0027]
【Example】
<Example 1>
A single mode optical fiber having a core diameter of 9 μm and a cladding diameter of 125 μm is used as the optical waveguide 1, and the optical fiber is irradiated with ultraviolet light having an energy density of 1.8 mJ / mm ² through a phase mask for 50 seconds. In 2, a grating portion 5 having a high refractive index portion 4 periodically was formed. The length of the grating section 5 was 0.9 mm, the period was 1.058 μm, and the refractive index modulation width Δn immediately after interference exposure was 1 × 10 ⁻³ .
Next, the optical waveguide 1 was heated at 120 ° C. for 12 hours to perform dehydrogenation treatment, and the entire grating section 5 was irradiated with ultraviolet light having an energy density of 1.8 mJ / mm ² for 1000 seconds. When the characteristics of the optical fiber grating thus obtained were measured, the refractive index modulation width Δn was 2.5 × 10 ⁻³ , the reflectance R at a wavelength of 1533.5 nm was 98.2%, and an excellent grating was obtained. The characteristics were shown.
[0028]
<Comparative Example 1>
The same optical fiber as that used in Example 1 was used as the optical waveguide 1, and interference exposure was performed on the same optical fiber as in Example 1 to form a grating portion 5 in the core 2. The refractive index modulation width Δn immediately after the interference exposure was 2.0 × 10 ⁻³ .
After the interference exposure, the optical waveguide 1 was heated at 120 ° C. for 12 hours to perform a dehydrogenation treatment, and then heated at 250 ° C. for １ ／ hour to perform an aging treatment. When the characteristics of the optical fiber grating thus obtained were measured, the refractive index modulation width Δn was 1.3 × 10 ⁻³ , the reflectance R at a wavelength of 1533.5 nm was 88%, and the grating characteristics were insufficient. It was something.
[0029]
【The invention's effect】
As described above, the method of manufacturing an optical waveguide grating according to the present invention includes forming an optical waveguide in which hydrogen has been diffused by interference exposure to form a grating having a high refractive index portion, dehydrogenating, and then applying ultraviolet light to the entire grating portion. Therefore, the refractive index of the high refractive index portion reduced in the dehydrogenation step can be increased again by the presensitization effect. Thereby, it is possible to suppress the deterioration of the grating characteristics due to the decrease in the refractive index of the high refractive index portion, and to improve the yield.
In addition, the amount of change in the refractive index due to the interference exposure step does not need to be much larger than the amount of change in the refractive index required to obtain the desired grating characteristics. Therefore, productivity can be improved, manufacturing costs can be reduced, and an increase in loss due to excessive ultraviolet light irradiation can be reduced.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram illustrating an example of an optical waveguide grating.
FIG. 2 is a process chart showing a first embodiment of the present invention.
FIG. 3 is a diagram illustrating an example of a change in a refractive index according to the first embodiment of the present invention.
FIG. 4 is a process chart showing a second embodiment of the present invention.
FIG. 5 is a process chart showing an example of a conventional method for manufacturing an optical waveguide grating.
FIG. 6 is a diagram illustrating variations in characteristics of the optical waveguide grating due to an aging process.
FIG. 7 is a graph showing an example of a relationship between an ultraviolet light irradiation time and a refractive index increase amount.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Optical waveguide, 2 ... Core, 3 ... Cladding, 4 ... High refractive index part, 5 ... Grating part.

Claims (2)

In a method of manufacturing an optical waveguide grating for manufacturing an optical waveguide grating by forming a grating portion having a high refractive index portion by irradiating the optical waveguide with ultraviolet light,
After diffusing hydrogen into the optical waveguide, irradiating ultraviolet rays to form a grating portion having a high refractive index portion, and after dehydrogenation, irradiating the entire grating portion with ultraviolet rays, an optical waveguide grating. Manufacturing method. The method for manufacturing an optical waveguide grating according to claim 1, wherein after forming the grating portion and before irradiating the entire grating portion with ultraviolet rays, the optical waveguide is heated and subjected to an aging treatment.

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