JP2003294956A - Method of manufacturing optical fiber grating, and optical fiber grating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing an optical fiber grating, which easily realizes a desired grating strength and reduces thermal degradation. <P>SOLUTION: Ultraviolet light 2 is radiated with an intensity distribution after hydrogen is diffused in an optical fiber 1, so that a refractive index change is formed in accordance with the intensity distribution of the ultraviolet light. The optical fiber which has the refractive index change formed in this manner is subjected to a dehydrogeneration process and is irradiated through a phase mask 3 by the ultraviolet light 2 to perform interference exposure. In this interference exposure process, the ultraviolet light is radiated to a wide range of the optical fiber 1 with a uniform intensity to generate interference fringes in a wide area on the optical fiber 1. A refractive index change caused by interference exposure is added to the preliminarily formed refractive index change, thus forming the grating. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical fiber grating used in the field of optical communication, particularly wavelength division multiplex transmission.

[0002]

2. Description of the Related Art There is a technique called apodization as an important technique for producing an optical fiber grating. This is a technique in which, when a diffraction grating is manufactured in an optical fiber, the strength and weakness of the grating is applied in the longitudinal direction of the optical fiber so that the optical characteristics of the optical fiber grating become desired. FIGS. 7A and 7B show typical refractive index profile shapes formed by apodization. This apodization is performed by adjusting the light amount of ultraviolet light with which the optical fiber is irradiated during exposure. The grating intensity of the grating thus formed depends on the irradiation power of ultraviolet light, the irradiation time of ultraviolet light, and the interference efficiency, and the amount of ultraviolet light can be expressed as the product of the irradiation power and the irradiation time. There is a technique that enables apodization by changing the interference efficiency, but in this case, it is necessary to use a special phase mask, so this method is not widely used. The reason is that when such a special phase mask is used, the degree of freedom in design is lost, and only the same apodized shape can be obtained.

In the last few years, the demand for apodization technology has changed. In the past, apodization was a technique used to gently change the grating strength in the longitudinal direction of a grating with a length of about 10 mm. However, recently, when making moire gratings (superstructure gratings),
In an optical fiber grating having a grating length of 1 mm or less, it is required to realize abrupt grating intensity change by an apodization technique.

The following methods are available as methods for adjusting the amount of ultraviolet light to be irradiated in order to carry out apodization. The first method is to change the irradiation time.
This is to sweep a narrowed beam spot in the longitudinal direction of the optical fiber, and change the moving speed of the beam during this sweep as a function depending on the beam position.
The grating intensity is a function that roughly matches the function representing the moving speed of the beam, and apodization is possible.

The second method is to change the power of irradiation light. This is similar to the method of changing the irradiation time, in which the beam is narrowed down and swept at a uniform speed, and the intensity of the beam is changed as a function depending on the position of the beam to enable apodization. The third method is the method disclosed in Japanese Patent Laid-Open No. 9-304638. This is a method in which the beam itself is made large enough to illuminate the entire grating, and the area of the light incident on the optical fiber is changed with time using an aperture. Can be made.

[0006]

However, according to the first method, it is necessary to narrow the beam in order to apply accurate apodization, so that the power of the laser light source cannot be effectively used, which is very difficult. It becomes less efficient. Generally, since laser light has high monochromaticity, it can be easily condensed by using a lens, and laser power can be effectively used. However, when used for interference,
Since the spatial coherence is reduced by condensing the light, the light cannot be extremely condensed. As a result, the manufacturing throughput of the grating is significantly reduced. Further, when controlling the grating intensity with time, the relationship between the irradiation time and the grating intensity is not linear, or the energy distribution in the laser beam narrowed down is not uniform,
Further, it is difficult to match the function representing the moving speed of the beam with the function representing the grating intensity to a certain extent or more due to lack of time stability of the laser beam power.

Further, there is an area where interference does not always occur around the interference area between the diffracted lights under the phase mask, and therefore there is an area where the grating is not written. This state is shown in FIG. 8 (a), (b),
In (c), reference numeral 1 is an optical fiber, and the optical fiber 1 is irradiated with ultraviolet light 2 and subjected to interference exposure by the phase mask 3, whereby an optical fiber grating is manufactured. FIG. 8A shows a case where the grating length is long, and the grating length becomes shorter as it goes to FIGS. 8B and 8C. A region surrounded by a chain double-dashed line in the drawing is a region where the grating is not written because no interference occurs.

For this reason, unnecessary refractive index changes occur around the grating regardless of the length of the grating. However, since the length of the unnecessary portion is constant regardless of whether the grating length is long or short, the ratio of the unnecessary portion becomes relatively large as the grating length becomes short. Therefore, if it is attempted to fabricate a grating only in a very small area, an unnecessary change in the refractive index will always occur around it, which has been a factor of grating characteristic deterioration.

Further, according to the second method, all the problems of the first method are applied, and the relationship between the irradiation intensity of the beam and the variation of the refractive index is the relationship between the irradiation time of the beam and the variation of the refractive index. The function of irradiation intensity and the function of refractive index change do not match, because they are more complicated than the relationship of
It was difficult to accurately change the refractive index by controlling the irradiation intensity. Further, since it is more difficult to control the irradiation intensity of the beam than the control of the irradiation time, the second method is usually rarely used. Also, the third
According to this method, when the grating intensity changes continuously, there is no need to reduce the aperture size, so the problems of other methods do not occur, but when the grating becomes longer, the beam needs to be increased and the apodization When the shape is discontinuous, the problem similar to the other methods occurs when the beam size becomes small.

Further, according to any of the above-mentioned methods, the grating length of the manufactured grating is 5 mm.
It became very difficult to accurately perform apodization at the following points, and it was not possible at less than 1 mm. The reason for this is that when the beam is narrowed to a size of 0.5 mm or less, the energy distribution of the beam and the equiphase surface are disturbed by the effect of diffraction, and as a result, the phase mask does not cause interference, the efficiency of interference decreases significantly, Is stable and will not occur. On the other hand, accurate apodization cannot be performed unless the beam size is made sufficiently smaller than the length of the grating to be manufactured.

In addition, the refractive index change of a grating produced by ordinary ultraviolet irradiation is easily deteriorated by heat. Therefore, it is usually necessary to perform heat treatment before use to improve thermal stability. Therefore, a general grating manufacturing process, as shown in FIG.
After the interference exposure and the dehydrogenation process, the aging process is performed. The dehydrogenation step and the aging step shown in FIG. 9 may be performed at the same time. However, the deterioration amount of the change in refractive index due to heating in the aging process may not be constant in the grating, and therefore,
The apodized shape changes before and after aging, and the yield may be decisively deteriorated in a grating that requires a particularly delicate apodized shape. The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method of manufacturing an optical fiber grating which can easily realize a desired grating strength and is less deteriorated by heat. .

[0012]

In order to solve the above problems, the invention according to claim 1 manufactures an optical fiber grating by irradiating an optical fiber with ultraviolet light to form a high refractive index portion. In the method for manufacturing an optical fiber grating, after diffusing hydrogen into the optical fiber, the optical fiber is irradiated with ultraviolet light having an intensity distribution to generate a high refractive index portion according to the intensity distribution of the ultraviolet light, By performing interference exposure on the high refractive index portion after dehydrogenation,
A method of manufacturing an optical fiber grating, which comprises manufacturing an optical fiber grating having a desired intensity distribution. As a result, the refractive index change can be formed with good thermal stability, so that an optical fiber grating having excellent heat resistance can be manufactured. Further, since the aging process is not required, the shape of apodization does not deteriorate during aging, and a highly reliable optical fiber grating can be manufactured.

According to a second aspect of the invention, in the method of manufacturing an optical fiber grating according to the first aspect, the interference exposure is performed by irradiating the high refractive index portion of the optical fiber with ultraviolet light of uniform intensity. It is characterized by performing by. As a result, the greater the change in the refractive index due to the first irradiation of ultraviolet light, the greater the change in the refractive index due to the interference exposure, so that the desired grating intensity can be easily realized. An invention according to claim 3 is an optical fiber grating manufactured by the method for manufacturing an optical fiber grating according to claim 1 or 2.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
FIG. 1 shows an example of a method for manufacturing the optical fiber grating of the present invention. In the method of manufacturing the optical fiber grating of this example, the step of irradiating the optical fiber with ultraviolet light is divided into two steps, and after hydrogen is diffused into the optical fiber,
The desired intensity distribution is obtained by irradiating the optical fiber with an intensity distribution of ultraviolet light to generate a high-refractive-index portion corresponding to the intensity distribution of the ultraviolet light, and by performing destructive exposure on the high-refractive-index portion. Is to manufacture an optical fiber grating having. FIG. 1A shows a state of ultraviolet light irradiation performed before the formation of a grating. Reference numeral 1 is an optical fiber. After hydrogen is diffused in the optical fiber 1, an intensity distribution of ultraviolet light 2 is obtained. And irradiate. In FIG. 1A, the longer the length of the line representing the irradiated ultraviolet light, the stronger the ultraviolet light intensity. By such ultraviolet light irradiation, a change in the refractive index is formed according to the intensity distribution of the ultraviolet light, as shown in FIG.

The optical fiber having the refractive index thus changed is subjected to a dehydrogenation process, and as shown in FIG. 2 is irradiated to perform interference exposure. In this interference exposure process, ultraviolet rays are irradiated with a uniform intensity over a wide area of the optical fiber 1 to generate interference fringes in a wide area on the optical fiber 1. The interference fringes cause the grating to be written only in the region irradiated with the ultraviolet light by the first irradiation of the ultraviolet light. In this region, as shown in FIG. On the other hand, a refractive index change due to interference exposure is added to form a grating.

FIG. 2 shows another example of the method of manufacturing the optical fiber grating of the present invention. FIG. 2A shows a state in which the optical fiber 1 is irradiated with ultraviolet light 2 of uniform intensity through the phase mask 2 to form an optical fiber grating having a uniform amount of change in refractive index. , Fig. 2 (b)
Shows a state of irradiating the optical fiber grating with ultraviolet light 2 having an intensity distribution after the dehydrogenation step. In FIG. 2B, the longer the length of the line representing the ultraviolet light to be irradiated, the stronger the ultraviolet light intensity. By such ultraviolet light irradiation, as shown in FIG. 2B, a refractive index change is formed according to the intensity distribution of the ultraviolet light.

In this interference-exposed region, the intensity of the ultraviolet light used in the first irradiation of ultraviolet light is proportional to the change in the refractive index due to the subsequent interference exposure, so that the refractive index is changed by the first irradiation of ultraviolet light. The greater the change, the greater the change in refractive index due to interference exposure. Therefore, in the interference exposure step, apodization can be performed only by exposing the ultraviolet light with a uniform intensity.

Next, the principle of enabling such refractive index change formation will be described. In the method of manufacturing the optical fiber grating of the present invention, the technique of presensitization is applied. The technology of this presensitization is that in the optical fiber, the region irradiated with ultraviolet light in the presence of hydrogen changes its refractive index during the irradiation, but the region whose refractive index has changed after hydrogen desorption The refractive index is further changed by the irradiation with the ultraviolet light, and the change in the refractive index formed by this step has better thermal stability than the change in the refractive index formed by one irradiation with the ultraviolet light. For this,
Reference “Photosensitization and Photostabilization
of Laser-Induced Index Changes in Optical
Fibers ”(John Canning, Optical Fiber Technolog
y6, P275-289 (2000)).

FIG. 3A shows the above-mentioned presensitization.
Shows an example of a process for manufacturing an optical fiber grating using the technology described in (1), which irradiates ultraviolet light in the presence of hydrogen after the hydrogen diffusion process, and after the dehydrogenation process, performs interference exposure to form the optical fiber grating. Form. FIG. 4A shows how the refractive index of the optical fiber changes during this manufacturing process. First, in the ultraviolet light irradiation step after the hydrogen diffusion step, when the ultraviolet light 2 having a uniform power is applied to the optical fiber 1, a change in the refractive index occurs at the irradiated position where the change amount is constant. Next, after the dehydrogenation step, when the optical fiber 1 is irradiated with the ultraviolet light 2 through the phase mask 3 to perform interference exposure, a refractive index distribution due to interference fringes is further generated in the portion where the refractive index change has already occurred. .

FIG. 3B shows another example of the steps of the method for manufacturing an optical fiber grating. After the hydrogen diffusion step, interference exposure is performed in the presence of hydrogen, and after the dehydrogenation step, ultraviolet irradiation is performed to perform light exposure. Form a fiber grating. FIG. 4 shows how the refractive index of the optical fiber changes during this manufacturing process.
It shows in (b). First, in the ultraviolet light irradiation step after the hydrogen diffusion step, when the ultraviolet light 2 is irradiated to the optical fiber 1 through the phase mask 3 and interference exposure is performed, a refractive index distribution due to interference fringes is generated. Next, after the dehydrogenation step, when the ultraviolet light 2 having a uniform power is applied to the optical fiber 1, the refractive index change is added only to the portion where the refractive index change has already occurred due to the interference fringes.

According to the method of manufacturing an optical fiber grating of the present invention, in the manufacturing process shown in FIG. 3A, apodization is performed by giving an intensity distribution to the ultraviolet light intensity in the ultraviolet light irradiation after the hydrogen diffusion process, and performing phase masking. Is a manufacturing method for manufacturing an optical fiber grating by irradiating ultraviolet light having a uniform power at the time of interference exposure using. In the manufacturing method of this example, the size of the laser beam during irradiation with ultraviolet light could be reduced to 10 μm by using a lens having a focal length of 100 mm. Therefore, at this time, if the grating length is 1 mm, it is possible to fabricate an apodization function divided into 1 mm / 10 μm = 100.

FIG. 5 shows the result of plotting the amount of change in the refractive index caused in the subsequent interference exposure with respect to the exposure time of the ultraviolet light irradiation as the pretreatment. In FIG. 5, the horizontal axis represents the relative exposure time when the ultraviolet light irradiation time at which the amount of change in refractive index is maximum is 1. The vertical axis represents the relative refractive index change amount when the maximum value of the refractive index change is 1.
As can be seen from FIG. 5, the amount of change in the refractive index linearly increases with the exposure time until the amount of change in the refractive index reaches the maximum value, and this region can be used to form the change in the refractive index. preferable.

FIG. 6 shows how the refractive index change of the optical fiber grating manufactured by the manufacturing method of this example is deteriorated by heat. This fiber optic grating
It was prepared by irradiating with ultraviolet light for 1200 seconds in the presence of 800 atm of hydrogen and performing the dehydrogenation step at 120 ° C. for 12 hours. As a result of examining the temperature resistance of this optical fiber grating at 220 ° C. Is shown. It can be seen that the deterioration of the change in the refractive index is improved as compared with the conventional optical fiber grating.

According to the manufacturing method of the optical fiber grating of this example, after diffusing hydrogen into the optical fiber,
The desired intensity distribution is obtained by irradiating the optical fiber with an intensity distribution of ultraviolet light to generate a high-refractive-index portion corresponding to the intensity distribution of the ultraviolet light, and by performing destructive exposure on the high-refractive-index portion. It is possible to manufacture an optical fiber grating having Even if the high-refractive-index portion of the optical fiber is subjected to interference exposure by irradiating it with ultraviolet light of uniform intensity, the greater the change in the refractive index due to the first irradiation of ultraviolet light, the greater the change in refractive index due to interference exposure. Can be increased, so that a desired grating intensity can be easily realized. Further, according to the method of manufacturing the optical fiber grating of this example, since the change in the refractive index can be formed with good thermal stability, it is possible to manufacture a highly reliable optical fiber grating having excellent heat resistance. Also,
Since the aging process is not required, the shape of apodization does not deteriorate during aging, and the optical fiber grating can be manufactured with high yield.

[0025]

As described above, according to the present invention,
After diffusing hydrogen into the optical fiber, the optical fiber is irradiated with ultraviolet light having an intensity distribution to generate a high refractive index portion corresponding to the intensity distribution of the ultraviolet light, and after dehydrogenation, the high refractive index portion is By performing interference exposure, it is possible to manufacture an optical fiber grating having a desired intensity distribution. Even if the high-refractive-index portion of the optical fiber is subjected to interference exposure by irradiating it with ultraviolet light of uniform intensity, the greater the change in the refractive index due to the first irradiation of ultraviolet light, the greater the change in refractive index due to interference exposure. Can be increased, so that a desired grating intensity can be easily realized. Further, since the change in refractive index can be formed with good thermal stability, an optical fiber grating having excellent heat resistance can be manufactured. Further, since the aging process is not required, the shape of apodization does not deteriorate during aging, and a highly reliable optical fiber grating can be manufactured.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a method for manufacturing an optical fiber grating of the present invention.

FIG. 2 is a diagram showing another example of a method for manufacturing an optical fiber grating according to the present invention.

FIG. 3 is a diagram showing an example of a manufacturing process of an optical fiber grating.

FIG. 4 is a diagram showing an example of a method for manufacturing an optical fiber grating.

FIG. 5 is a diagram showing a relationship between an exposure time of pretreatment and a change in refractive index.

6A and 6B are diagrams showing, in comparison, deterioration of a change in refractive index due to temperature in an optical fiber grating of the present invention and a conventional optical fiber grating.

FIG. 7 is a diagram showing a shape of a typical refractive index distribution formed by apodization.

FIG. 8 is a diagram showing an interference region between diffracted lights under a phase mask.

FIG. 9 is a diagram showing a manufacturing process of a conventional optical fiber grating.

[Explanation of symbols]

1 ... Optical fiber, 2 ... Ultraviolet light, 3 ... Phase mask.

Claims (3)

[Claims] 1. An optical fiber grating manufacturing method for manufacturing an optical fiber grating by forming a high refractive index portion by irradiating the optical fiber with ultraviolet light, the method comprising: diffusing hydrogen into the optical fiber; By irradiating the optical fiber with ultraviolet light having a distribution to generate a high-refractive-index portion corresponding to the intensity distribution of the ultraviolet light, and by performing interference exposure on the high-refractive-index portion after dehydrogenation, a desired intensity can be obtained. A method for manufacturing an optical fiber grating, which comprises manufacturing an optical fiber grating having a distribution. 2. The method of manufacturing an optical fiber grating according to claim 1, wherein the interference exposure is performed by irradiating the high refractive index portion of the optical fiber with ultraviolet light of uniform intensity. 3. An optical fiber grating manufactured by the method for manufacturing an optical fiber grating according to claim 1.