JPH1138264A - Optical fiber crating and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate the production and to obviate the appearance of a side peak in transmission attenuation characteristic, by forming a grating having periodic refractive indices and satisfying single mode conditions with respect to the use wavelength at the average refractive index in the grating. SOLUTION: The optical fiber grating is formed in the core part 11A of a grating forming region 13, which is a specified region along the optical axis of an optical fiber 10. The refractive index of the hatched part in Fig. in the core part 11A in the grating forming region 13 is higher that the refractive index of the un-hatched part. The respective diameters of the core part 11A and clad part 12A in the grating forming region 13 are smaller than the respective diameters of the core part 11 and clad part 12 intrinsic to the optical fiber. The single mode conditions for the use wavelength at the average refractive index of the grating forming region 13 formed to the reduced diameter are satisfied in this region.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber grating used as an optical filter, an optical multiplexer / demultiplexer, a dispersion compensator, and the like in an optical transmission system, and a method of manufacturing the same.

[0002]

2. Description of the Related Art An optical fiber grating has a structure in which at least a core portion of an optical fiber has a periodic refractive index change in an optical axis direction, and transmits or propagates the propagating light according to the wavelength of the propagating light. The light is reflected, and various applications such as an optical filter, an optical multiplexer / demultiplexer, and a dispersion compensator have been made. This optical fiber grating has a G component as a photosensitive component in advance at least in a core portion of a silica-based optical fiber.
It is manufactured by adding eO ₂ (germanium oxide), irradiating it with light of a predetermined wavelength to form interference fringes, and causing a change in the refractive index according to the light energy intensity distribution due to the interference fringes. Is common. The mechanism of the refractive index change is considered to be that a Ge defect is generated in the optical fiber by irradiating light of a predetermined wavelength, and the Ge defect causes a change in the refractive index.

[0003] The optical fiber grating manufactured in this manner is based on the effective refractive index n _eff of the optical fiber and the grating period Λ, and has a Bragg expressed by λ _B = 2 · n _eff · Λ (1) It has a wavelength λ _B and generally reflects propagating light at its Bragg wavelength and wavelengths very close to it, and transmits propagating light at other wavelengths. The effective refractive index n _eff of the optical fiber is determined by the refractive index of each of the core portion and the clad portion of the optical fiber and the diameter of the core portion.

[0004]

However, when the wavelength dependence of transmission attenuation of propagating light is actually examined using an optical fiber grating in which a grating is formed on a single mode optical fiber, it is found that the transmission attenuation at the position of the Bragg wavelength. Not only is there a main peak of attenuation, but also a sub-peak of transmission attenuation periodically exists at a position on the shorter wavelength side than the main peak position. This phenomenon occurs because the coupling between the fundamental mode and the higher-order mode occurs in the high-refractive-index portion in the grating forming region, and the coupling between the higher-order mode and the cladding mode occurs in the low-refractive-index portion. It is considered that transmission attenuation is caused by the occurrence of.

When an optical fiber grating having a sub-peak in addition to a main peak of transmission attenuation as described above is used as an optical filter, an optical multiplexer / demultiplexer, a dispersion compensator, and the like in an optical transmission system, it is designed as designed. Is inconvenient because it may not operate and may operate unexpectedly.

Japanese Patent Laid-Open Publication No. Hei 6-230207 discloses a method of manufacturing an optical fiber grating in which the sub peak is reduced. However, this manufacturing method is a method of manufacturing by spatially changing the amplitude of the refractive index change or the grating period in the grating in the optical axis direction, and is not easy.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and provides an optical fiber grating which can be easily manufactured and has no subpeak in transmission attenuation characteristics, and a method of manufacturing the same. The purpose is to:

[0008]

In the optical fiber grating according to the present invention, a grating having a periodic refractive index change is formed in a predetermined region along the optical axis direction of the core portion of the optical fiber, and the average in the grating is obtained. It is characterized in that a single mode condition is satisfied with respect to a wavelength to be used by a refractive index. According to this optical fiber grating,
At the used wavelength, only the fundamental mode can propagate as the core mode in the predetermined region, and therefore, coupling with the cladding mode cannot occur. as a result,
The transmission attenuation characteristics are excellent because no sub-peak appears near the main peak. Here, the operating wavelength is the wavelength of the light input when the optical fiber grating is used, and the single mode condition is that the light of the operating wavelength propagates as the core mode only in the fundamental mode. It is a condition for being.

Further, the invention is characterized in that the outer diameter is substantially constant in a predetermined region of the optical fiber. In this case, the optical fiber grating is easy to manufacture and has a large transmission attenuation at the Bragg wavelength.

[0010] Furthermore, in a predetermined region of the optical fiber, the cutoff wavelength before forming the grating is approximately 2/3 or less of the used wavelength. In this case, even after formation of the grating, the average refractive index of the grating can satisfy the single mode condition with respect to the wavelength used.

[0011] Furthermore, in a cross-sectional area of a predetermined region of the optical fiber, an area of a portion containing a photosensitive component which can contribute to formation of a grating is approximately 10% or more. In this case, the optical fiber grating has a large transmission attenuation rate at the Bragg wavelength.

An optical fiber grating manufacturing method according to the present invention is directed to an optical fiber grating for manufacturing an optical fiber grating in which a grating having a periodic refractive index change is formed in a predetermined region along a direction of an optical axis of a core portion of the optical fiber. A grating manufacturing method, (1) an optical fiber manufacturing step of manufacturing an optical fiber to which a photosensitive component capable of contributing to the formation of the grating in the predetermined region is added,
(2) a step of reducing the diameter of a predetermined region until a single mode condition is satisfied with respect to the wavelength used at the average refractive index when the grating is formed later; and (3) a step of reducing the diameter of the optical fiber. Irradiating a predetermined region with light of a predetermined wavelength having a periodic intensity distribution in the optical axis direction, and forming a grating by generating a periodic refractive index distribution in the region to which the photosensitive component is added, and forming a grating. , Is provided.

According to this optical fiber grating manufacturing method, the optical fiber added with the photosensitive component manufactured in the optical fiber manufacturing process has an average refractive index when the grating is formed later in the diameter reducing process. The predetermined area is narrowed until the single mode condition is satisfied with respect to the wavelength used, and a predetermined wavelength having a periodic intensity distribution in the optical axis direction in the narrowed predetermined area of the optical fiber by the grating forming step. Is irradiated, a periodic refractive index distribution is generated in a region to which the photosensitive component is added, and a grating is formed. The optical fiber grating manufactured in this way is the above-described optical fiber grating according to the present invention.

Further, the method further comprises a substrate fixing step of fixing, on a substrate, two points of the optical fiber whose diameter has been reduced by the diameter reducing step, wherein the grating forming step includes a predetermined step between the two points of the optical fiber. Forming a grating in the region. In this case, the optical fiber whose diameter has been reduced by the diameter reducing step is fixed at the two points on the substrate by the substrate fixing step, and the subsequent handling is facilitated. A grating is formed in a predetermined area between two points of the optical fiber.

Further, the method further comprises a hydrogen impregnating step of impregnating the optical fiber reduced in diameter by the diameter reducing step with hydrogen, and the grating forming step includes forming a grating in a predetermined region of the optical fiber impregnated with hydrogen. The step of performing In this case, the optical fiber reduced in diameter by the diameter reduction step is subjected to the hydrogen impregnation step.
Hydrogen is impregnated in a short time, and a grating is formed in a predetermined region of the optical fiber impregnated with the hydrogen by the grating forming step.

Further, the optical fiber manufacturing step is a step of manufacturing an optical fiber in which the area of the portion containing the photosensitive component in the cross-sectional area in the predetermined region is about 10% or more. I do. In this case, the manufactured optical fiber grating has a large transmission attenuation rate at the Bragg wavelength.

Further, the diameter reducing step is characterized in that a predetermined area of the optical fiber is heated and drawn to reduce the diameter. In this case, the sensitivity of the photosensitive component to irradiation with light of a predetermined wavelength in the grating forming step increases, and thus the manufactured optical fiber grating has a large transmission attenuation at the Bragg wavelength.

Further, the step of reducing the diameter is a step of making the outer diameter substantially constant in a predetermined region of the optical fiber. In this case, the optical fiber grating is easy to manufacture and has a large transmission attenuation at the Bragg wavelength.

Further, in the step of reducing the diameter, the cut-off wavelength in a predetermined region of the optical fiber is approximately 2/1/2 of the used wavelength.
The process is characterized in that the diameter is reduced to 3 or less. In this case, even after forming the grating,
The average refractive index of the grating can satisfy the single mode condition for the wavelength used.

Further, the grating forming step is characterized in that the grating is formed while applying tension to a predetermined region of the optical fiber. In this case, the predetermined area of the optical fiber is fixed in position without vibrating, so that a good transmission attenuation characteristic can be obtained.
By appropriately setting the tension after forming the grating,
Since the grating period is adjusted, the Bragg wavelength is set appropriately.

[0021]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

First, the configuration of the optical fiber grating according to the present embodiment will be described. FIG. 1 is a configuration diagram of an optical fiber grating according to the present embodiment. Here, the optical fiber 10 constituting the optical fiber grating has a core 11 including an optical axis, and the core 1
It is assumed that a clad portion 12 having a refractive index lower than that of the core portion 11 is provided around 1 and the simplest configuration will be described. The optical fiber grating has its optical fiber 1
It is formed in the grating forming region 13 which is a fixed region along the optical axis of 0. In this optical fiber grating, a grating is formed in at least the core portion 11 </ b> A in the grating forming region 13. In this figure, the core portion 11A in the grating forming region 13 is shown.
In the graph, the hatched portion has a larger refractive index than the unhatched portion.

The optical fiber 10 generally has an outer diameter of 125.
In the grating forming region 13, the diameter is reduced by heating and stretching. That is, the diameter of each of the core portion 11A and the clad portion 12A in the grating forming region 13 is smaller than the diameter of each of the core portion 11 and the clad portion 12 which are the original optical fibers 10. In the narrowed grating forming region 13, the average refractive index satisfies the single mode condition with respect to the used wavelength.

Here, the used wavelength is the wavelength of light input when the optical fiber grating is used. For example, the optical fiber grating has a wavelength of 1.55 μm.
When used in band WDM transmission, the used wavelength is a wavelength in a band from about 1.52 μm to 1.57 μm. The single mode condition is a condition for allowing light of the used wavelength to propagate only in the fundamental mode as a core mode. The radius a (μm) of the core portion, the refractive index n ₁ of the core portion, and the refraction of the cladding portion. This is determined based on the ratio n ₂ and the wavelength λ (μm) used. That is, defined in _{V = 2π · a · n 1} · (2 · Δn) 1/2 / λ ... (2) Δn = (n 1 2 -n 2 2) / (2 · n 1 2) ... (3) If the obtained V value is less than 2.405, the single mode condition is satisfied for the wavelength λ used, and only the fundamental mode can propagate as the core mode. On the other hand, when the V value is 2.
If it is 405 or more, higher-order modes can also propagate. Note that Δ
n is a relative refractive index difference.

As can be seen from the definition of the V value represented by the equation (2), the V value increases as the refractive index n ₁ or the relative refractive index difference Δn of the core increases, and the V value increases. V
When the value is equal to or greater than 2.405, the single mode condition is not satisfied, and higher order modes can be propagated. Therefore, coupling of the fundamental mode and higher order mode may occur for light having the used wavelength λ. In the optical fiber grating according to the present embodiment, although a grating is formed in the core portion 11A in the grating forming region 13 and a portion having a periodically high refractive index is present, the core portion 11A has a large refractive index.
Even in the portion where the refractive index of A is large, the V value is less than 2.405 with respect to the used wavelength, and only the fundamental mode can propagate as the core mode. Therefore, in this optical fiber grating, the coupling between the fundamental mode and the higher-order mode cannot occur at the used wavelength, and no subpeak occurs in the transmission attenuation characteristic.

As described above, in order to satisfy the single mode condition with respect to the used wavelength with the average refractive index in the grating forming region 13, the cutoff wavelength before forming the grating in the grating forming region 13 of the optical fiber 10 is approximately equal to the used wavelength. It is preferable that it is set to 2/3 or less. In this case, a grating is formed in the core portion 11A, causing a periodic change in the refractive index, and the relative refractive index difference of the average refractive index in the grating is 0.1% to 0.2%.
% (Normal rise in relative refractive index difference), the V value is less than 2.405 with respect to the wavelength used.
Single mode condition can be satisfied.

Here, the cutoff wavelength is determined by the grating forming region 1 of the optical fiber 10 before the grating is formed.
In No. 3, this is the minimum value of the used wavelength at which only the fundamental mode can propagate as the core mode, that is, the minimum value of the used wavelength λ at which the V value represented by the above equation (2) can be less than 2.405. If the refractive index of each of the core portion 11A and the cladding portion 12A is constant, the smaller the diameter of the core portion 11A, the smaller the cutoff wavelength in proportion thereto. Therefore, as shown in FIG. 1, by reducing the diameter of the grating forming region 13 and reducing the core diameter,
The cut-off wavelength can be set to approximately ／ or less of the used wavelength.

It is preferable that the outer diameter of the grating forming region 13 of the optical fiber 10 is constant. If the outer diameter is constant in the grating forming region 13, this optical fiber grating is easy to manufacture,
The transmission attenuation at the Bragg wavelength is large. Further, in the cross-sectional area when the grating forming region 13 of the optical fiber 10 is cut along a plane perpendicular to the optical axis direction, a photosensitive component (for example, G
The area of the portion containing eO ₂ ) is preferably about 10% or more. In this case, the optical fiber grating has a large transmission attenuation rate at the Bragg wavelength.

Next, a method of manufacturing the optical fiber grating will be described. FIG. 2 is a flowchart of the optical fiber grating manufacturing method according to the present embodiment, and FIGS. 3 to 5 are explanatory diagrams of each step of the optical fiber grating manufacturing method according to the present embodiment.

First, an optical fiber 10 in which GeO ₂ as a photosensitive component is added to at least the core portion 11 is prepared. Such an optical fiber 10 can be manufactured by a normal manufacturing method. At this time, the optical fiber 1
0 is cut along a plane perpendicular to the optical axis direction, the area of the portion containing GeO _{2 in the} cross-sectional area is approximately 10
% Or more is preferable because the manufactured optical fiber grating has a large transmission attenuation factor at the Bragg wavelength.

Subsequently, in the step of reducing the diameter, the optical fiber 1
The predetermined area of 0 is reduced in diameter. As shown in FIG. 3, in this diameter reduction step, a fixing table 22 placed on a stage 21 is used.
The holding members 24 and 25 of the moving table 23 respectively hold two predetermined points of the optical fiber 10. Then, the space between the two held optical fibers 10 is heated by the flame from the burner 26 and the burner 26
Is moved in the direction of the optical axis of the optical fiber 10, and the moving table 23 is moved on the stage 21 so as to move the moving table 23 away from the fixed table 22, thereby extending the optical fiber 10. Thus, the grating forming region 1 of the optical fiber 10
3 is heated and stretched in a region to be 3 (diameter reduction region 13A);
When the grating is formed later, the diameter is reduced until the single mode condition is satisfied with respect to the wavelength used at the average refractive index.

At this time, it is preferable to reduce the diameter until the cut-off wavelength in the reduced-diameter region 13A becomes approximately ／ or less of the used wavelength. That is, for example, the optical fiber 10 is heated and stretched while monitoring the outer diameter of the reduced-diameter region 13A by an optical method, and the reduced-diameter region 13A is reduced in diameter until the outer diameter satisfies this condition. Even after formation, the single mode condition can be satisfied for the wavelength used by the average refractive index of the grating.

It is preferable that the outer diameter of the reduced diameter region 13A is constant. If the outer diameter is constant in the diameter-reduced region 13A, the diameter-reduced region 13 will be
Hydrogen can be uniformly impregnated in the optical axis direction of A, and a uniform grating is formed by forming interference fringes with a constant period in the grating forming step, so that the manufactured optical fiber grating is narrow. It has band characteristics.

Subsequently, in the substrate fixing step, the optical fiber 10
Is fixed on the glass substrate 30. As shown in FIG. 4, in this substrate fixing step, the diameter-reduced region 13 of the optical fiber 10 is used.
The region containing A is placed on the glass substrate 30 and the optical fiber 1
The adhesive fixing portions 31 and 32, which are predetermined two points which are not stretched out of 0, are respectively bonded to the glass substrate 3 using an adhesive.
And adhesively fixed on top. By adhering and fixing on the glass substrate 30 in this manner, the diameter-reduced region 13A of the optical fiber 10 becomes easier to handle in spite of the reduced diameter and the easier breakage thereafter. . Subsequently, in a hydrogen impregnation step, the diameter-reduced region 13A of the optical fiber 10 is impregnated with hydrogen.

Then, in the grating forming step, a grating is formed on the reduced diameter region 13A of the reduced optical fiber 10 by a phase grating method or the like. As shown in FIG. 5, in this grating forming step, the optical fiber 1
A coherent light of a predetermined wavelength (for example, an ultraviolet laser light having a wavelength of about 240 nm) is irradiated through a phase mask 40 having periodic irregularities formed on the surface in a direction perpendicular to the optical axis direction of the glass substrate. An interference fringe is formed on the diameter-reduced region 13A of the optical fiber 10 fixed on the optical fiber 30. By doing so, a change in the refractive index according to the brightness of the interference fringes is induced in the diameter-reduced region 13A of the optical fiber 10. In other words, the diameter-reduced region 13A becomes the grating-formed region 13 in which the grating is formed. Thus, the optical fiber grating shown in FIG. 1 is manufactured.

In this grating forming step, it is preferable to apply a tension to the reduced diameter region 13A of the optical fiber 10. In order to apply the tension to the optical fiber 10 in this manner, in the substrate fixing step, both or one of the adhesive fixing portions 31 and 32 is adhesively fixed with an adhesive such as a low Young's modulus silicone resin. By doing so, the diameter reduction region 13A of the optical fiber 10 is formed.
Is fixed without vibration, so that good transmission attenuation characteristics can be obtained. Also, after forming the grating, the Bragg wavelength is appropriately set by adjusting the tension, that is, the grating period, while monitoring the transmission attenuation characteristics, and then both the adhesive fixing portions 31 and 32 are adhesively fixed with an adhesive having a high Young's modulus. Then, an optical fiber grating having desired transmission attenuation characteristics can be manufactured.

Next, a specific example of the optical fiber grating manufacturing method according to the present embodiment and transmission attenuation characteristics of the optical fiber grating manufactured by the specific manufacturing method will be described.

In the optical fiber manufacturing process, an optical fiber having a refractive index profile shown in FIG. 6 was manufactured. The refractive index profile shown in this figure shows the distribution of the refractive index in a direction perpendicular to the optical axis of the optical fiber. This optical fiber is a quartz (SiO ₂ ) based optical fiber in which a depressed portion having a refractive index smaller than that of the clad portion is provided between the core portion and the clad portion, 5.5 wt% of GeO ₂ is added to the core part, and 2.5 wt% of G is added to the depressed part.
eO ₂ and 1.0 wt% of F (fluorine) element are added. This optical fiber has an outer diameter of 125 μm.
The core diameter is 8.5 μm, the relative refractive index difference Δn is 0.35%, and the cutoff wavelength is 1.34 μm. Hereinafter, the optical fiber before the diameter reduction is referred to as an optical fiber A.

In the diameter reducing step, the optical fiber A was reduced in diameter, and two types of optical fibers B and C having different diameters in the diameter reducing region 13A were formed. The optical fiber B has an outer diameter of 85 μm, a core diameter of 5.8 μm, and a cutoff wavelength of 0.5 μm in the diameter-reduced region 13A.
It is 93 μm. On the other hand, the optical fiber C has a reduced diameter region 1.
At 3A, the outer diameter is 62.5 μm, the core diameter is 4.3 μm, and the cutoff wavelength is 0.68 μm. FIG. 7 is a chart showing the outer diameter, core diameter, and cutoff wavelength of each of the optical fibers A, B, and C.

In the substrate fixing step, a silicon resin was used as an adhesive to fix the substrate. In the hydrogen impregnation step, the processing temperature was 20 ° C., the pressure of hydrogen was 200 atm, and the processing time was 14 days. In these steps, the same conditions were set for each of the optical fibers A, B and C.

In the grating forming step, the laser light output from the KrF excimer laser light source is
Irradiation was performed on the thinned region 13A of the optical fiber through the line 0 to form an interference fringe, and a grating having a Bragg wavelength of 1.556 μm was formed. At this time, the tension applied to each optical fiber together with the laser beam irradiation is 100
g, 45 g for the optical fiber B, and 25 g for the optical fiber C, and the tension per unit cross-sectional area, that is, the elongation percentage, in each of the diameter-reduced regions 13A (that is, the grating forming regions 13) was made substantially the same.

FIG. 8 is a characteristic diagram of transmission attenuation of the optical fiber grating manufactured under the above conditions. FIG.
In each of (a) and (b), the horizontal axis represents the wavelength of the propagating light, the vertical axis represents the transmission attenuation rate, and FIG.
FIG. 8A is an enlarged view of the vertical axis. As shown in this figure, each of the gratings formed on each of the optical fibers A, B and C has a Bragg wavelength of 1.5.
A main peak of transmission attenuation of 45 dB or more appears near 56 μm. Further, in the case of the grating formed on the optical fiber A, the wavelength 1.times.
A secondary peak of transmission attenuation appears around 5543 μm,
The transmission attenuation at the sub-peak is 0.8 dB. On the other hand, in the case of the grating formed on each of the optical fibers B and C, no sub-peak of the transmission attenuation is observed.

By the way, the cut-off wavelength of the optical fibers B and C before forming the grating in the grating forming region 13 is 0.93 μm and 0.68 μm, respectively.
μm, which are all used wavelengths (in this case, the lower limit of the wavelength band shown in the horizontal axis of FIG.
50 μm) or less. Therefore, even after the formation of the grating, at the working wavelength, only the fundamental mode can propagate as the core mode in the grating forming region 13, and therefore, coupling with the cladding mode cannot occur. As a result, the transmission attenuation characteristic has no sub-peak near the main peak,
It has good characteristics. That is, these optical fiber gratings are related to the present invention.

Next, the relationship between the hydrogen impregnation time and the transmission attenuation characteristic in the optical fiber grating manufacturing method according to this embodiment will be described. The optical fibers A, B and C and the method of manufacturing the optical fiber grating used here are substantially the same as those described above except for the hydrogen impregnation step. However, here, the processing time in the hydrogen impregnation step is 24 hours, 48 hours, 96 hours, 168 hours,
Ultraviolet laser light was applied for 264 hours and 336 hours, respectively, and in the grating forming step until transmission attenuation at the Bragg wavelength was saturated. The saturated transmission attenuation at the Bragg wavelength for each processing time was determined. FIG. 9 is a chart showing the results, and FIG. 10 is a graph showing the results.

As shown in these diagrams and graphs, the hydrogen impregnation time required for the transmission attenuation at the Bragg wavelength to be 45 dB is 336 hours in the case of the optical fiber A, whereas the hydrogen impregnation time is 336 hours in the case of the optical fiber A. In the case of B, half of the time is 168 hours, and in the case of the optical fiber C, the time is further reduced to half. This is because, in the optical fiber grating manufacturing method according to the present embodiment, since the hydrogen impregnation step is provided after the diameter reduction step, the time required for hydrogen to diffuse into the optical fiber can be reduced.

The transmission attenuation at the Bragg wavelength is
In the case of the optical fiber A, the maximum value is 45 dB, while in the case of the optical fibers B and C, 55 dB and 53 dB, respectively, which are larger values are obtained. This is because, in the optical fiber grating manufacturing method according to the present embodiment, the optical fiber 1
This is because the sensitivity of the photosensitive component to light irradiation during the formation of the grating is increased because the region to be the zero grating forming region 13 is stretched by heating (see Japanese Patent Application Laid-Open No. 7-196334).

The present invention is not limited to the above embodiment, and various modifications are possible. For example, the area where a grating is to be formed in an optical fiber is not a process in which an optical fiber having a normal diameter is once manufactured and then reduced in diameter by heating and drawing, but rather, a step in which the optical fiber is drawn from a preform. The part region may be reduced in diameter. The refractive index profile of the optical fiber may be arbitrary. The formation of the grating is not limited to the phase grating method, but may be based on a two-beam interference method or the like.

[0048]

As described above in detail, according to the optical fiber grating according to the present invention, a grating having a periodic change in the refractive index is formed in a predetermined region along the optical axis direction of the core portion of the optical fiber. Since the single mode condition is satisfied with respect to the wavelength used by the average refractive index of the grating, only the fundamental mode can propagate as the core mode in the predetermined wavelength at the wavelength used, and therefore, the coupling with the cladding mode can be achieved. Cannot occur. As a result, the transmission attenuating characteristic does not show a sub-peak near the main peak, and is excellent.

Further, according to the optical fiber grating manufacturing method of the present invention, the optical fiber added with the photosensitive component manufactured in the optical fiber manufacturing process can be used when the grating is formed later by the diameter reducing process. A predetermined area is heated and stretched to reduce the diameter until the single-mode condition is satisfied with respect to the wavelength used at the average refractive index, and the diameter is reduced by the grating forming step. Is irradiated with light of a predetermined wavelength having a periodic intensity distribution, and a periodic refractive index distribution is generated in a region where the photosensitive component is added, thereby forming a grating. As a result, the above-described optical fiber grating having good transmission attenuation characteristics according to the present invention can be easily manufactured.

In the case where there is further provided a substrate fixing step for fixing each of two points of the optical fiber whose diameter has been reduced by the diameter reducing step to a substrate, the optical fiber whose diameter has been reduced by the diameter reducing step is In the substrate fixing step, each of the two points is fixed on the substrate, and subsequent handling is facilitated.
In addition, in the case of further including a hydrogen impregnation step of impregnating hydrogen into the optical fiber reduced in diameter by the diameter reduction step, the optical fiber reduced in diameter by the diameter reduction step can be shortened by the hydrogen impregnation step in a short time. Hydrogen is impregnated.

When the outer diameter is substantially constant in a predetermined region of the optical fiber, the optical fiber grating is easy to manufacture and has a large transmission attenuation at the Bragg wavelength. Further, in a predetermined region of the optical fiber,
When the cut-off wavelength before the formation of the grating is approximately ／ or less of the working wavelength, the single mode condition can be satisfied with respect to the working wavelength with the average refractive index of the grating even after the formation of the grating.
Further, when the area of the portion containing the photosensitive component that can contribute to the formation of the grating is approximately 10% or more in the cross-sectional area of the predetermined region of the optical fiber, the optical fiber grating has Large transmission attenuation rate.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical fiber grating according to an embodiment.

FIG. 2 is a flowchart of an optical fiber grating manufacturing method according to the embodiment.

FIG. 3 is an explanatory view of a diameter reducing step in the optical fiber grating manufacturing method according to the embodiment.

FIG. 4 is an explanatory diagram of a substrate fixing step in the optical fiber grating manufacturing method according to the embodiment.

FIG. 5 is an explanatory diagram of a grating forming step in the optical fiber grating manufacturing method according to the embodiment.

FIG. 6 is a refractive index profile diagram of an optical fiber.

FIG. 7 is a table showing the outer diameter, core diameter, and cutoff wavelength of each of three types of optical fibers.

FIG. 8 is a diagram illustrating transmission attenuation characteristics of an optical fiber grating.

FIG. 9 is a table showing transmission attenuation at a Bragg wavelength with respect to a processing time in a hydrogen impregnation step.

FIG. 10 is a graph showing the transmission attenuation at the Bragg wavelength with respect to the processing time in the hydrogen impregnation step.

[Explanation of Signs] 10 ... optical fiber, 11, 11A ... core part, 12, 12
A: clad portion, 13: grating forming region, 13
A: diameter reduction area, 21: stage, 22: fixed base, 23
… Moving table, 24, 25… holder, 26… burner, 30…
Glass substrate, 31, 32: adhesive fixing part, 40: phase mask.

Claims (12)

[Claims] 1. A grating having a periodic refractive index change is formed in a predetermined region along an optical axis direction of a core portion of an optical fiber, and a single mode condition is satisfied with respect to a wavelength used by an average refractive index in the grating. An optical fiber grating, characterized in that: 2. The optical fiber grating according to claim 1, wherein an outer diameter is substantially constant in said predetermined region of said optical fiber. 3. The optical fiber according to claim 1, wherein a cut-off wavelength before forming a grating is approximately 2/3 or less of the working wavelength in the predetermined region of the optical fiber.
The described optical fiber grating. 4. The optical fiber according to claim 1, wherein the area of a portion containing a photosensitive component that can contribute to the formation of the grating is approximately 10% or more in a cross-sectional area of the predetermined region of the optical fiber. The optical fiber grating according to claim 1. 5. An optical fiber grating manufacturing method for manufacturing an optical fiber grating in which a grating having a periodic change in refractive index is formed in a predetermined region along a direction of an optical axis of a core portion of an optical fiber, An optical fiber manufacturing step of manufacturing an optical fiber to which a photosensitive component capable of contributing to the formation of the grating is added to a region; and, when the grating is formed later, a single mode condition for an operating wavelength at an average refractive index thereof. A narrowing step of reducing the diameter of the predetermined region until the above condition is satisfied, and irradiating the predetermined region having a reduced diameter of the optical fiber with light of a predetermined wavelength having a periodic intensity distribution in an optical axis direction, A grating forming step of forming the grating by generating a periodic refractive index distribution in a region to which the photosensitive component has been added. An optical fiber grating manufacturing method comprising and. 6. A substrate fixing step for fixing, on a substrate, two points of the optical fiber whose diameter has been reduced by the diameter reducing step, wherein the grating forming step includes the step of fixing the two points of the optical fiber. The method of manufacturing an optical fiber grating according to claim 5, wherein the step of forming the grating in the predetermined region therebetween. 7. The method according to claim 1, further comprising a step of impregnating the hydrogen into the optical fiber reduced in diameter by the step of reducing the diameter, wherein the step of forming the grating includes forming the optical fiber in the predetermined region of the optical fiber impregnated with hydrogen. The method of manufacturing an optical fiber grating according to claim 5, wherein the step is a step of forming the grating. 8. The optical fiber manufacturing step is a step of manufacturing the optical fiber in which the area of a portion containing a photosensitive component in a cross-sectional area of the predetermined region is approximately 10% or more. 6. The method of manufacturing an optical fiber grating according to claim 5, wherein: 9. The method of manufacturing an optical fiber grating according to claim 5, wherein the step of reducing the diameter is a step of heating and stretching the predetermined region of the optical fiber to reduce the diameter. 10. The step of reducing the diameter is a step of making the outer diameter substantially constant in the predetermined region of the optical fiber.
6. The method of manufacturing an optical fiber grating according to claim 5, wherein: 11. The method according to claim 11, wherein the step of reducing the diameter is a step of reducing the diameter of the optical fiber until the cutoff wavelength becomes approximately ２ or less of the working wavelength in the predetermined region. Item 6. The method for manufacturing an optical fiber grating according to Item 5. 12. The method of manufacturing an optical fiber grating according to claim 5, wherein said grating forming step is a step of forming said grating while applying tension to said predetermined region of said optical fiber.