JPH08286011A - Formation of optical fiber type diffraction grating - Google Patents

Abstract

PURPOSE: To provide a method for efficiently forming an optical fiber type diffraction grating of multiwavelength reflection and optical fiber type diffraction grating having a continuous reflection wavelength region. CONSTITUTION: The optical fiber type diffraction grating 50 consisting of a first partial grating 51 and a second partial grating 52 is formed when the end faces of a first photosensitive fiber 10 and a second photosensitive fiber 20 are connected to each other to form a continuous optical fiber 100 and the point including the juncture 30 of the first photosensitive fiber 10 and the second photosensitive fiber 20 of the optical fiber 100 is irradiated with interference fringes. Since the first photosensitive fiber 10 and the second photosensitive fiber 20 have cores varying in effective refractive index from each other, the first partial grating 51 and the second partial grating 52 have the Bragg reflection wavelengths varying from each other. The diffraction grating 50 is, therefore, the diffraction grating of the multiwavelength reflection.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for forming a diffraction grating in an optical fiber by using a light-induced refractive index change.

[0002]

2. Description of the Related Art Conventionally, there is known a method of forming an optical fiber type diffraction grating by irradiating an optical fiber with interference fringes of ultraviolet light to cause a photo-induced change in refractive index in the core of the optical fiber. . For example, the publication of a patent application, Showa 62-500
Japanese Patent No. 052 describes a method of forming an optical fiber type diffraction grating (a diffraction grating with an equal grating interval) of a constant period in the core of an optical fiber by using a two-beam interferometer.

This optical fiber type diffraction grating is a region of the core of an optical fiber, in which the effective refractive index changes periodically along the axial direction, and the effective refractive index of the core and the grating period are It reflects light in a relatively narrow wavelength range centered on the Bragg reflection wavelength determined by. Such an optical fiber type diffraction grating can be used as a single wavelength filter having good wavelength selectivity.

In a multi-channel communication system or the like, a multi-wavelength reflection optical fiber type diffraction grating for reflecting a plurality of lights having different wavelengths may be required. Conventionally, an optical fiber type that performs multi-wavelength reflection as a whole by irradiating a plurality of regions of a single optical fiber with interference fringes having different periods and forming a plurality of diffraction gratings having different Bragg reflection wavelengths in series It forms a diffraction grating.

Further, when an optical fiber type diffraction grating suitable as a bandpass filter is required, in the above method,
The difference between the cycles of the interference fringes should be made sufficiently small. As a result, an optical fiber type diffraction grating having a continuous reflection wavelength range is formed.

[0006]

However, in the above method, it is necessary to form a plurality of diffraction gratings having different periods, and every time the period of the interference fringes is changed, the arrangement of the beam splitter and the reflecting mirror constituting the interferometer is arranged. Therefore, it is difficult to form a diffraction grating and it is difficult to obtain sufficient productivity. Further, since it is necessary to finely adjust the optical system, it is difficult to form a desired optical fiber type diffraction grating with good reproducibility.

The present invention has been made to solve the above problems, and efficiently and efficiently reproduces an optical fiber type diffraction grating having multiple wavelength reflections or an optical fiber type diffraction grating having a continuous reflection wavelength range. It aims at providing the method of forming with sex.

[0008]

In order to solve the above problems, the method for forming an optical fiber type diffraction grating of the present invention comprises:
A first step of forming a continuous optical fiber by connecting end faces of first and second photosensitive fibers each having a core having a different effective refractive index, and a first step of the optical fibers.
And a second step of irradiating a portion including a connecting portion between the photosensitive fiber and the second photosensitive fiber with an interference fringe.

The cores of the first and second photosensitive fibers are both made of quartz glass doped with germanium oxide, and the interference fringes may be ultraviolet light.

In this case, prior to the first step or the second step, a step of heating the first photosensitive fiber to diffuse germanium oxide may be further provided.

Further, at least one of the core and the clad of the first photosensitive fiber may be added with an additive for changing the effective refractive index of the core in addition to germanium oxide. As this additive, for example, fluorine can be used.

In this case, a step of heating the first photosensitive fiber to diffuse the additive may be further provided prior to the first step or the second step.

[0013]

In the method of forming the optical fiber type diffraction grating of the present invention, the interference fringes are irradiated onto the optical fiber, so that the refractive index changes corresponding to the interference fringes occur inside the first and second photosensitive fibers. . In this way, the refractive index change region formed from the first photosensitive fiber to the second photosensitive fiber is the optical fiber type diffraction grating formed by the present invention.

Since the effective refractive indices of the cores of the first and second photosensitive fibers are different from each other, the refractive index changing region formed in the first photosensitive fiber and the refractive index changing region formed in the second photosensitive fiber. And have different Bragg reflection wavelengths. Therefore, the optical fiber type diffraction grating formed by the present invention is a diffraction grating of multi-wavelength reflection or a diffraction grating having a continuous reflection wavelength range.

According to the present invention, an optical fiber type diffraction grating having a multi-wavelength reflection and an optical fiber type diffraction grating having a continuous reflection wavelength range are formed by irradiating the interference fringes once, which is efficient. Further, since it is not necessary to adjust the optical system for irradiating the interference fringes, the efficiency is good from this point as well, and the reproducibility when the same optical fiber type diffraction grating is repeatedly formed is excellent.

In the present invention, if an additive for changing the effective refractive index of the core is added to at least one of the core and the clad of the first photosensitive fiber, for example, the first and second photosensitive fibers are added in the first step. When splicing and the like, the additive diffuses to form an effective refractive index profile in the core of the first photosensitive fiber that changes smoothly along the axial direction. After that, when interference fringes are irradiated in the second step, a diffraction grating having a continuous reflection wavelength region is formed on the core.

When the above-mentioned additive is diffused by heating the first photosensitive fiber prior to the first or second step, the core of the first photosensitive fiber also changes smoothly along the axial direction. When the effective refractive index distribution is formed and the interference fringes are irradiated in the second step, a diffraction grating having a continuous reflection wavelength range is formed in the core.

[0018]

Embodiments of the present invention will now be described 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.
Further, the dimensional ratios in the drawings do not always match those described.

Example 1 FIG. 1 is a diagram for explaining the method of forming a diffraction grating of this example. The method of forming the diffraction grating of this embodiment will be described with reference to this drawing.

In this embodiment, first, two optical fibers 10 and 20 having cores having different effective refractive indexes are prepared. The optical fibers 10 and 20 prepared in this embodiment are both silica-based optical fibers, and are made of germanium oxide (G).
It has a core made of quartz glass (SiO ₂ ) to which eO ₂ ) is added and a clad made of substantially pure quartz glass. GeO ₂ is an additive (dopant) for increasing the refractive index of SiO ₂ , and by adding this, the refractive index of the core becomes higher than the refractive index of the clad made of pure quartz.

The optical fibers 10 and 20 differ in the amount of GeO ₂ added. As a result, the optical fibers 10 and 2 are added.
Zero cores will have different effective indices. FIG. 2 is a diagram showing the effective refractive index distribution in the cross section of the optical fibers 10 and 20, where (a) is the effective refractive index distribution of the optical fiber 10 and (b) is the effective refractive index of the optical fiber 20. The distribution is shown. The relative refractive index difference of the optical fiber 10 is 0.3%, and the relative refractive index difference of the optical fiber 20 is 1.0%. In this embodiment, the optical fiber 20
Has a larger amount of GeO ₂ added than the optical fiber 10,
Therefore, the effective refractive index of the core is also higher in the optical fiber 20.

Next, in the method of this embodiment, the optical fibers 10 and 20 are fusion-spliced by a known discharge fusion method.
Specifically, as shown in FIG. 1, the coating of the end portions of the optical fibers 10 and 20 is removed, the exposed end portions of the bare portions 11 and 21 are abutted, and then fusion-spliced. As a result, the optical fibers 10 and 20 are permanently connected to each other via the connecting portion 30, and the continuous single optical fiber 100 is used.
Becomes

Subsequently, as shown in FIG. 1, the optical fiber 100 is irradiated with ultraviolet light interference fringes having a constant period by using a known two-beam interference exposure method. As a light source of ultraviolet light, Kr
An F excimer laser light source is used. This light source is about 2
40 nm of coherent UV light at 2.2 J / cm ²
It oscillates with the strength of. This ultraviolet light is split into two light beams, a transmitted light and a reflected light, by a beam splitter (not shown), and then reflected by a reflecting mirror (not shown), respectively, and as the light beams 40 and 41 shown, the bare portions 11 and 21.
Is irradiated to a predetermined place of. The two-beam interference exposure method is described in Japanese Patent Application Publication No. 62-500052.

The light beams 40 and 41 interfere with each other to generate an interference space 45. In this interference space, interference fringes with a constant fringe spacing, that is, interference fringes with a constant period are generated. The interference fringes are applied to the part including the connection part 30 of the bare parts 11 and 21.

As is well known, the effective refractive index is increased by irradiating the quartz glass containing GeO ₂ with ultraviolet light. In this case, the wavelength of the ultraviolet light to be irradiated is usually about 150 nm
It is selected from the wavelength range of about 300 nm.

As described above, since the cores of the optical fibers 10 and 20 are made of GeO ₂ -doped silica glass, both cores of the bare portions 11 and 12 have a refractive index change corresponding to the interference fringes. Occurs. In FIG. 1, the refractive index change region generated in the bare portion 11 is denoted by reference numeral 51, and the bare portion 21
The refractive index change regions generated in the above are denoted by reference numeral 52, respectively. These refractive index change regions are regions where the effective refractive index for each propagation mode changes along the axial direction of the optical fibers 10 and 20. Since these are formed at the same time by irradiating one interference fringe, the cycles of changing the refractive index are equal to each other.

The refractive index changing region 50 which is composed of the refractive index changing regions 51 and 52 and extends from the core of the optical fiber 10 to the core of the optical fiber 20 is the optical fiber type diffraction grating formed in this embodiment. The diffraction grating 50 is a region of the core of the optical fiber 100, and is a region in which the effective refractive index for the propagation mode periodically changes along the axial direction between the minimum refractive index and the maximum refractive index. . The minimum refractive index is substantially equal to the initial effective refractive index of the core (effective refractive index before irradiation of ultraviolet light).

Refractive index changing region 5 constituting the diffraction grating 50
Also in 1 and 52, the effective refractive index with respect to the propagation mode periodically changes along the axial direction between the minimum refractive index and the maximum refractive index, and each is one diffraction grating. That is,
In the diffraction grating 50, the diffraction gratings 51 and 52 are the optical fiber 1
00 are arranged in series, and the diffraction gratings 51 and 52 form a part of the diffraction grating 50. Therefore, hereinafter, the refractive index change regions 51 and 52 will be referred to as partial gratings forming the diffraction grating 50.

The partial gratings 51 and 52 each function as a single wavelength filter that reflects light with a predetermined reflectance over a relatively narrow wavelength range centered on the Bragg wavelength λ _B. Here, the minimum refractive index n _MIN = n ₀ and the maximum refractive index n _MAX = n ₀ + Δn of the partial grating, and the period of change of the refractive index of the diffraction grating (hereinafter, simply referred to as the period of the diffraction grating) is Λ. In other words, the Bragg wavelength λ _B of the partial lattice is expressed as λ _B = 2 · n ₀ · Λ.

FIG. 3 is a diagram showing a reflection spectrum of the optical fiber type diffraction grating 50 formed by the method of this embodiment. This reflection spectrum includes partial gratings 51 and 52.
The reflection spectra of are appearing independently of each other. Since the partial gratings 51 and 52 are formed in the optical fibers 10 and 20 having different core effective refractive indexes, the Bragg reflection wavelengths λ _B are also different from each other. Specifically, the Bragg reflection wavelength of the partial grating 51 is about 1550.
and the Bragg reflection wavelength of the partial grating 52 is about 15 nm.
It is 60 nm. For this reason, the optical fiber type diffraction grating 50 formed in this embodiment is 1550 nm and 1560 n.
It is a diffraction grating of multi-wavelength reflection that reflects light of wavelength m.

As described above, according to the method of this embodiment, it is possible to form an optical fiber type diffraction grating of multi-wavelength reflection by irradiating the interference fringes once, so that the efficiency is high. Also, because it is not necessary to adjust the arrangement of the beam splitter and the reflecting mirror field that make up the interference optical system according to the Bragg reflection wavelength of each partial grating, it is possible to form a diffraction grating with excellent efficiency and reproducibility. Is.

Example 2 In this example, as in Example 1, the optical fibers 10 and 20 having cores having different effective refractive indexes are fusion-spliced to form one continuous optical fiber 100, Its connection 3
A region including 0 is irradiated with ultraviolet interference fringes to form a diffraction grating. However, the composition of the core of the optical fiber 10 is the same as in Example 1.
Is different from

FIG. 4 shows the optical fibers 10 and 2 of this embodiment.
It is a figure which shows the effective refractive index distribution in a fiber cross section about 0, (a) shows the effective refractive index distribution of the optical fiber 10,
(B) shows the effective refractive index distribution of the optical fiber 20. The refractive index distribution itself is the same as that of the first embodiment, but F (fluorine) is added to the core of the optical fiber 10 in addition to GeO ₂ , which is the difference from the first embodiment.

Contrary to GeO ₂ , F is an additive that lowers the refractive index of SiO ₂ . Optical fiber 10 of this embodiment
The core of G has an amount of G that causes a relative refractive index difference of 1.0%.
eO ₂ and an amount of F that causes a relative refractive index difference of −0.7%
Has been added. On the other hand, the composition of the core of the optical fiber 20 is the same as in Example 1, and only GeO _{2 in an} amount that causes a relative refractive index difference of 1.0% is added. Thus, in this embodiment, the optical fibers 10 and 20 have the same amount of GeO ₂ added to the core.

F added to the core of the optical fiber 10 is
Due to the heating at the time of fusion, it is diffused mainly in the radial direction of the optical fiber. This changes the effective refractive index distribution of the core. Since F is an additive that lowers the refractive index, the effective refractive index of the core increases as the diffusivity increases. GeO ₂ is F
Since the diffusion is much slower than that of, it hardly contributes to the change in the refractive index distribution in this embodiment.

FIG. 5 is a diagram showing the effective refractive index distribution in the cross section of the fiber after fusion splicing at one location of the optical fiber 10 around the connection portion 30. F in radial direction
The effective refractive index distribution before fusion due to the diffusion of (Fig. 4 (a))
Is different from.

The temperatures of the optical fibers 10 and 20 at the time of fusion bonding are higher in the portion closer to the connection portion 30 along the axial direction and lower in the portion further away. Since the diffusivity of F increases as the temperature increases, the diffusivity increases and the effective refractive index of the core also increases in the portion of the F-doped optical fiber 10 closer to the connection portion. In accordance with such a change in the diffusion degree of F along the axial direction, a smooth change in the effective refractive index along the axial direction of the optical fiber 10 occurs.

FIG. 6 is a view showing the effective refractive index distribution in the axial direction of the cores of the optical fibers 10 and 20. Most of F is diffused in the immediate vicinity of the connecting portion 30, so that the optical fiber 10 has a relative refractive index difference of 1.0% based only on GeO ₂ in this portion. Therefore, the effective refractive indices of the cores of the optical fibers 10 and 20 are substantially equal at the connecting portion 30. Further, as the distance from the connecting portion 30 increases in the axial direction, the diffusion degree of F decreases, and the effective refractive index of the core of the optical fiber 10 increases accordingly.

Also in this embodiment, similarly to the first embodiment, the portion including the connecting portion 30 is irradiated with the interference fringes of ultraviolet light. In this embodiment, the interference fringes are irradiated on the portions having the refractive index distribution as shown in FIG. 6, and the partial gratings 51 and 52 are formed on the bare portions 11 and 21, respectively.

Partial grating 52 formed on the optical fiber 20
Is the same as in Example 1, but the partial grating 51 formed in the optical fiber 10 has the Bragg reflection wavelength in the axial direction as a result of the above-described minimum refractive index changing along the axial direction as shown in FIG. It will change along with. Specifically, the relative refractive index difference of the optical fiber 10 changes from 0.3% to 1.0%, and the minimum refractive index changes accordingly, so that the optical fiber 10 is formed. The Bragg reflection wavelength of the partial grating 51 continuously changes from about 1550 nm to about 1560 nm.

FIG. 7 is a diagram showing a reflection spectrum of the optical fiber type diffraction grating 50 formed in this embodiment. Since the Bragg reflection wavelength of the partial grating 51 changes along the axial direction, the diffraction grating 50 has a continuous and wide reflection wavelength range.

As described above, according to the method of this embodiment,
Since it is possible to easily form an optical fiber type diffraction grating having a continuous reflection wavelength band by irradiating the interference fringes once, it is not necessary to form a plurality of diffraction gratings having different reflection wavelengths unlike the conventional case.
Extremely efficient. Further, since it is not necessary to adjust the arrangement of the beam splitter or the like which constitutes the interference optical system, it is possible to form the grating with high efficiency and excellent reproducibility also from this point.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments and various modifications can be made. For example, by the above method, the diffraction grating can be formed not only in the core of the optical fiber but also in the clad.

In Example 2, the diffusion of F added to the optical fiber 10 produced an effective refractive index profile that smoothly changes along the axial direction, but the optical fiber 10 containing only GeO ₂ was added. The same effective refractive index distribution can be produced by using. In this case, since the diffusion rate of GeO ₂ is low, it is difficult to generate an appropriate effective refractive index distribution only by heating during fusion. Therefore, for example, before fusion, the optical fiber 10 is subjected to heat treatment using a burner or the like to diffuse GeO ₂ and generate an effective refractive index distribution as shown in FIG. It is better to perform fusion splicing. After that, if the interference fringes are irradiated in the same manner as in this embodiment, the optical fiber type diffraction grating 50 similar to that in this embodiment can be formed. Note that the above heat treatment may be performed, for example, with a heating temperature having a gradient along the axial direction of the optical fiber 10. The higher the heating temperature, the more G
Since the diffusion of eO ₂ is also large, it is possible to cause an effective refractive index change that smoothly changes in the axial direction according to the temperature gradient. The heat treatment of the optical fiber 10 may be performed after the fusion splicing. Further, this heat treatment may be performed on the F-doped optical fiber 10 of the second embodiment.
By doing so, it becomes easy to cause a desired change in the effective refractive index, and it becomes easy to adjust the reflection wavelength range of the obtained optical fiber type diffraction grating 50. This heat treatment may be performed before fusion splicing or after fusion splicing.
By performing the heat treatment on the optical fiber, a desired effective refractive index distribution can be surely obtained, and as a result, an optical fiber type diffraction grating having a desired reflection wavelength region can be surely obtained.

Further, the photosensitive optical fiber for writing the diffraction grating does not have to include the silica glass to which GeO ₂ is added. For example, when the glass body is irradiated with light having high intensity, the glass is melted at the irradiated portion, but when the melted portion is solidified, the glass density is increased and the refractive index is also increased. By utilizing this, the diffraction fringes can be formed in the optical fiber by irradiating the glass optical fiber with the interference fringes formed by causing the light of sufficient intensity to interfere with each other. Also in the method of the present invention, by irradiating the portion including the connection portion with the interference fringes having a high light intensity, it is possible to form a multi-wavelength reflection diffraction grating or a diffraction grating having a continuous reflection wavelength range. In this case, the interference fringes to be irradiated need not be ultraviolet light. In order to achieve good glass melting by light irradiation, a light absorber that absorbs light in a predetermined wavelength range is added to the glass forming the optical waveguide, and the interference fringes formed by the light included in the absorption wavelength range are added. Should be irradiated.

[0046]

As described above in detail, according to the present invention, an optical fiber type diffraction grating having a multi-wavelength reflection or an optical fiber type diffraction grating having a continuous reflection wavelength range can be obtained by irradiating a single interference fringe. Since it is possible to form the diffraction grating and it is not necessary to adjust the optical system for irradiating the interference fringes, it is possible to realize the diffraction grating formation with extremely high efficiency and excellent reproducibility.

[0047]

[Brief description of drawings]

FIG. 1 is a diagram for explaining an optical fiber type diffraction grating forming method according to an embodiment.

2A is a diagram showing an effective refractive index distribution in the fiber cross section of the optical fiber 10 of Example 1, and FIG. 2B is a fiber cross section of the optical fiber 20 of Example 1 in the fiber cross section. It is a figure which shows the effective refractive index distribution in the inside.

3 is a diagram showing a reflection spectrum of an optical fiber type diffraction grating 50 formed by the method of Example 1. FIG.

4A is a diagram showing the effective refractive index distribution in the fiber cross section of the optical fiber 10 of Example 2, and FIG. 4B is a fiber cross section of the optical fiber 20 of Example 2 in the fiber cross section. It is a figure which shows the effective refractive index distribution in the inside.

FIG. 5 is a diagram showing an effective refractive index distribution in the fiber cross section of the optical fiber 10 after fusion bonding in Example 2.

FIG. 6 is a diagram showing an effective refractive index distribution in the axial direction in the cores of the optical fibers 10 and 20 after fusion bonding in Example 2.

7 is an optical fiber type diffraction grating 5 formed in Example 2. FIG.
It is a figure which shows the reflection spectrum of 0.

[Explanation of symbols]

10 and 20 ... Optical fiber, 11 and 21 ... Bare part, 3
0 ... Connection part, 40 and 41 ... Ultraviolet light flux, 45 ... Interference fringe, 50 ... Optical fiber type diffraction grating, 51 and 52 ... Partial grating, 100 ... Optical fiber formed by fusion-splicing optical fibers 10 and 20 .

Claims (6)

[Claims] 1. A first step of forming a continuous optical fiber by connecting first and second photosensitive fibers having cores having different effective refractive indexes to each other at their end faces, and the first step among the optical fibers. 1. A second step of irradiating a portion including a connection portion between the photosensitive fiber and the second photosensitive fiber with an interference fringe, and a method for forming an optical fiber type diffraction grating. 2. The cores of the first and second photosensitive fibers are both made of quartz glass doped with germanium oxide, and the interference fringes are ultraviolet light. Method for forming an optical fiber type diffraction grating. 3. The method according to claim 2, further comprising a step of heating the first photosensitive fiber to diffuse the germanium oxide prior to the first step or the second step. Method for forming optical fiber type diffraction grating. 4. The optical fiber according to claim 2, wherein an additive that changes the effective refractive index of the core is further added to at least one of the core and the clad of the first photosensitive fiber. Method of forming a diffraction grating. 5. The method for forming an optical fiber type diffraction grating according to claim 4, wherein the additive is fluorine. 6. The method according to claim 4, further comprising a step of heating the first photosensitive fiber to diffuse the additive prior to the first step or the second step. Method for forming an optical fiber type diffraction grating.