JP2003202433A - Apparatus and method for forming fiber bragg grating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus and method for forming a fiber Bragg grating which can form a fiber Bragg grating having high-precision reflection characteristics and transmission characteristics. <P>SOLUTION: The apparatus for forming the fiber Bragg grating FBG in the core C of an optical fiber F comprises a laser light source 15 which emits laser light UV in the ultraviolet region, a diffraction grating 30, and an incidence means 20 which makes the ultraviolet laser light UV incident on the diffraction grating 30. The incidence means 20 is equipped with a luminous flux size converter 21 which converges the ultraviolet laser light UV and makes it incident on the diffraction grating 30 while the external surface of the luminous flux of the ultraviolet laser light UV along the axis of the optical fiber F is so converged as to slant to the axis of the luminous flux. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an apparatus and method for forming a fiber Bragg grating.
A fiber Bragg grating is a refractive index of the core of an optical fiber that is periodically changed along the axial direction of the core, forming a part where the refractive index changes in a striped pattern along the axial direction of the core. Has been done. That is, the fiber Bragg grating is a diffraction grating formed in the core of the optical fiber. The portion where the fiber Bragg grating is formed has a property of reflecting or transmitting light having a wavelength corresponding to the interval between the stripes whose refractive index changes. Since this fiber Bragg grating can set the reflection wavelength with high precision, it is used in a wide range of applications such as filters, demultiplexers, laser diode oscillation wavelength stabilizers, and various sensors. The present invention relates to a forming apparatus and a forming method for forming such a fiber Bragg grating in a core of an optical fiber.

[0002]

2. Description of the Related Art Conventionally, a fiber Bragg grating has been made by utilizing the fact that germanium (Ge) doped in the core of an optical fiber responds to ultraviolet light to increase its refractive index, which makes it possible to increase or decrease the ultraviolet light intensity in the longitudinal direction of the optical fiber. It is formed by irradiating light. As described above, as a method for forming a fiber Bragg grating by irradiating an ultraviolet light having a different intensity, a phase mask method (for example, US Patent No. 5,367,588) and 2 have been conventionally used.
Luminous flux interferometry (for example, US Patent No. 4,725,110, U.
S. Patent No. 4,807,950) is known, but the phase mask method, which is more resistant to vibration and has a simple structure, is often used than the two-beam interference method.

In the phase mask method, a phase mask in which a diffraction grating with a grating interval Λ is formed on a quartz substrate is used, and an ultraviolet light beam whose optical axis is perpendicular to the surface of the phase mask and is completely parallel is incident on the phase mask. To do. Then,
The + 1st-order and -1st-order diffracted light of the incident ultraviolet light can be emitted, and if the emitted + 1st-order diffracted light and the -1st-order diffracted light interfere with each other, an interference fringe with an interval Λ / 2 can be generated. it can. Therefore, a fiber Bragg grating can be formed by transferring this interference fringe to the core of the optical fiber.

[0004]

In optical communication, a plurality of channels having different wavelengths are simultaneously propagated in one optical fiber. However, as the number of channels simultaneously propagated increases with the spread of multi-wavelength technology on the Internet and the like, the wavelength interval between adjacent channels shortens, so that there is a high possibility that so-called crosstalk will occur in which information leaks to adjacent channels. . In order to prevent this crosstalk, it is possible to extract only the channel of the desired wavelength from multiple channels that are simultaneously propagated, or to supply the signal only to the channel of the desired wavelength without picking up the signal of the adjacent channel. There is a need for an optical multiplexer / demultiplexer equipped with a high-precision filter that can achieve the above. Further, even if only the channel of the desired wavelength can be extracted, if the strength of the extracted signal decreases, the ratio of noise to the signal strength increases,
The signal quality is reduced. In order to prevent the deterioration of the signal quality, the wavelength band having effective signal intensity with respect to the center wavelength of the reflected signal, that is, the attenuation amount of the signal intensity in the bandwidth of each channel is reduced and the bandwidth is reduced. It is necessary to reduce the signal strength in the outer wavelength range. Specifically, the signal strength is -3 with respect to the signal strength of the central wavelength.
The wavelength range of dB (hereinafter referred to as -3 dB bandwidth) is
It should be close to the bandwidth of that channel.

In view of such circumstances, it is an object of the present invention to provide a fiber Bragg grating forming apparatus and method capable of forming a fiber Bragg grating having highly accurate reflection characteristics and transmission characteristics.

According to a first aspect of the present invention, there is provided a fiber Bragg grating forming apparatus for forming a Bragg grating in an optical fiber core, wherein the forming apparatus emits coherent ultraviolet laser light. A light source, a diffraction grating having a radiation surface on which a plurality of interference grooves are formed, the radiation surface being disposed so as to face the optical fiber; and an ultraviolet laser light emitted from the laser light source, The diffraction grating is formed on the radiation surface of the diffraction grating such that the incidence surface of the diffraction grating is parallel to the radiation surface of the diffraction grating. The plurality of interference grooves formed are arranged so as to be orthogonal to the axial direction of the optical fiber, and the plurality of interference grooves of the diffraction grating are the incident surfaces of the diffraction grating. Incident ultraviolet range laser beam +
The first-order diffraction light and the -1st-order diffraction light are formed so that the + 1st-order diffraction light and the -1st-order diffraction light are emitted from the emission surface. It is characterized in that it is provided with a condensing section for making the outer surface in the axial direction of the optical fiber in the light beam of the region laser light narrowed so as to be inclined with respect to the axis of the light beam and making it enter the diffraction grating. The apparatus for forming a fiber Bragg grating according to claim 2 is the device according to claim 1.
In the invention described above, the angle formed by the outer surface in the axial direction of the optical fiber in the light flux of the ultraviolet laser light incident on the incident surface of the diffraction grating with respect to the axis of the light flux is 0.4 to 2.5. It is characterized in that it is in milliradians. According to a third aspect of the present invention, in the fiber bragg grating forming device of the first aspect, the incident angle of the laser light in the ultraviolet region is made incident such that the axis of the light beam is perpendicular to the incident surface of the diffraction grating. It is characterized by having an adjusting unit. A method for forming a fiber Bragg grating according to claim 4 is a method for forming a Bragg grating in a core of an optical fiber, wherein an ultraviolet laser light is emitted from a laser light source,
The ultraviolet region laser light is condensed by the condensing unit such that the outer surface of the optical flux in the axial direction of the light flux is inclined with respect to the axis of the luminous flux, and the condensed ultraviolet laser light is incident on the diffraction grating. The ultraviolet laser light is diffracted by the + 1st-order diffraction light and the -1st-order diffraction light by a plurality of interference grooves formed on the radiation surfaces parallel to the incidence surface of the diffraction grating and orthogonal to the axial direction of the optical fiber. ,
The + 1st-order diffracted light and the -1st-order diffracted light of the ultraviolet laser light are emitted toward the optical fiber from the emission surface of the diffraction grating. According to a fifth aspect of the present invention, in the method of forming the fiber Bragg grating according to the fourth aspect, an outer surface in the axial direction of the optical fiber in the light flux of the ultraviolet laser light incident on the incident surface of the diffraction grating is The angle made to the axis is 0.
It is characterized by being 4 to 2.5 milliradians.

According to the invention of claim 1, as compared with the fiber Bragg grating formed by incident parallel light, the reflection characteristic of the formed fiber Bragg grating for a specific wavelength can be improved, and the specific wavelength can be improved. It is possible to improve the transmission characteristics of wavelengths other than the above. Therefore, if the fiber Bragg grating formed by this forming apparatus is used as a filter of an optical multiplexer / demultiplexer in optical fiber communication, crosstalk can be greatly reduced. According to the invention of claim 2, by using the fiber Bragg grating formed by the present forming apparatus, it is possible to significantly reduce the attenuation of the signal intensity within the -3 dB bandwidth in the reflected signal, and to reduce the -3 dB band. It is possible to significantly reduce the intensity of signals of wavelengths outside the width. Moreover, the −3 dB bandwidth can be matched to the bandwidth of the desired channel. Therefore, if the fiber Bragg grating formed by this forming apparatus is used as a filter of an optical multiplexer / demultiplexer in optical fiber communication, crosstalk can be greatly reduced. According to the third aspect of the invention, since the axis of the light flux of the ultraviolet laser light is made to be perpendicular to the incident surface of the diffraction grating, the fiber Bragg grating can be accurately formed. According to the invention of claim 4, the reflection characteristic of the fiber Bragg grating for a specific wavelength can be improved and the transmission characteristic of a wavelength other than the specific wavelength can be improved as compared with the case where parallel light is incident. . Therefore, the fiber Bragg grating formed by this forming method is used for optical multiplexing / combining in optical fiber communication.
If it is used as a filter of a duplexer, crosstalk can be greatly reduced. According to the invention of claim 5, by using the fiber Bragg grating formed by this forming apparatus, it is possible to significantly reduce the attenuation of the signal intensity within the −3 dB bandwidth of the reflected signal, and −3
The strength of signals at wavelengths outside the dB bandwidth can be significantly reduced. Moreover, the −3 dB bandwidth can be matched to the bandwidth of the desired channel. Therefore, if the fiber Bragg grating formed by this forming apparatus is used as a filter of an optical multiplexer / demultiplexer in optical fiber communication, crosstalk can be greatly reduced.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Next, an embodiment of the present invention will be described with reference to the drawings. In the drawings shown below, the relative scale is changed for easier understanding of the respective components, but the diameter of the optical fiber is usually 1/8 m.
m, the thickness of the diffraction grating is about 1 mm, the grating interval is about 1 micron, and the optical components such as the reflecting mirror and the light beam size converter are several cm to several tens cm.

First, before describing the fiber Bragg grating forming apparatus 10 of the present invention, a fiber Bragg grating (hereinafter referred to as FBG) will be briefly described. FIG. 3A is a schematic explanatory diagram of an optical fiber F in which an FBG is formed, and FIG. 3B is a spectrum of incident light LB0 incident on the FBG, a spectrum of reflected light LB1 reflected by the FBG, and a spectrum reflected by the FBG. It is the figure which showed the spectrum of transmitted light LB2. As shown in FIG. 3 (A), the FBG is a refractive index of the core C of the optical fiber F which is periodically changed along the axial direction thereof, and a portion CB of the core C having a high refractive index.
And a portion CA having a low refractive index of the core C with a constant fringe spacing Λ.
It is formed alternately for each. Therefore, in FIG.
As shown in (B), the optical fiber F on which the FBG is formed reflects the light of a constant wavelength in the incident light LB0 as the reflected light L.
B1 is reflected and light of other wavelengths is transmitted as LB2.
Can be transmitted as. The center wavelength of the spectrum of the reflected light LB1 at this time is 2nΛ.
Here, n is the effective refractive index of the optical fiber F. Further, as shown in FIG. 3B, the reflected light LB1 reflected by the FBG shows a substantially trapezoidal spectrum centered on the center wavelength thereof. Then, the reflection characteristic of the FBG is evaluated by the wavelength range W (hereinafter, referred to as -3 dB bandwidth W) in which the reduction rate of the signal intensity with respect to the signal intensity at the central wavelength of the reflected light LB1 is suppressed to -3 dB or less. Specifically, this −3 dB bandwidth W is almost IT.
It matches the bandwidth of the UT standard (for example, 0.4 nm or 0.8 nm), and can significantly reduce the attenuation of the signal strength within the -3 dB bandwidth in the reflected signal, and outside the -3 dB bandwidth. The FBG, which can significantly reduce the intensity of the signal having the wavelength of, is evaluated as a high quality filter.
By using such an FBG, crosstalk between adjacent channels can be significantly reduced, and a signal having a desired wavelength can be accurately extracted.

The fiber Bragg grating forming apparatus 10 of this embodiment will be described. FIG. 1 is a schematic explanatory view of a fiber Bragg grating forming device 10 of this embodiment. FIG. 2 is an enlarged sectional view of the diffraction grating 30. As shown in FIGS. 1 and 2, the fiber Bragg grating forming apparatus of the present embodiment diffracts the ultraviolet laser light UV by a diffraction grating 30 (generally called a phase mask), and the diffracted lights interfere with each other. Is an apparatus for forming an FBG on the core C of the optical fiber F by applying interference fringes generated by the optical fiber F to the core C of the optical fiber F. The outer surface UV1 (hereinafter referred to as the diaphragm surface UV1) is characterized by being narrowed so as to be inclined with respect to the axis BC of the light beam.

In FIG. 1, reference numeral 15 is capable of emitting ultraviolet laser light UV having a wavelength of 248 nm.
A laser light source such as a KrF excimer laser is shown.
Further, the symbol F indicates an optical fiber in which the FBG is formed. The optical fiber F has a core containing a doping element such as germanium (Ge), phosphorus (P), or boron (B).

Between the laser light source 15 and the optical fiber F, an incident means 20 and a diffraction grating 30 are arranged in order from the laser light source 15 side. This diffraction grating 30 is
The surface on the incident means 20 side is the incident surface 31, and the surface on the optical fiber F side is the emitting surface 32. The incident surface 31 and the radiation surface 32 are parallel to each other and are both formed as flat surfaces. On the surface of the radiation surface 32, a plurality of interference grooves 33 parallel to each other are formed.
The interference groove 33 diffracts the ultraviolet laser light UV incident from the incident surface 31 into the + 1st-order diffraction and the -1st-order diffraction,
The + 1st order light and the −1st order light are emitted from the emitting surface 32, and the 0th order light emitted from the emitting surface 32 can be suppressed. Moreover, the distance between the adjacent interference grooves 33 is equal to the FBG formed in the optical fiber F.
Is formed so as to have a length twice as long as the distance Λ in the above. The diffraction grating 30 is arranged such that the groove direction of the interference groove 33 is orthogonal to the axial direction of the optical fiber F. The diffraction grating 30 needs to be arranged such that the distance L between the radiation surface 32 and the optical fiber F is 1 mm or less, and the closer the two are, the more the FBG is.
Formation efficiency is high. And, the most preferable state is a state where the radiation surface 32 of the diffraction grating 30 is in light contact with the optical fiber F.

The diffraction grating 30 is attached to an incident angle adjusting section (not shown). The incident angle adjusting section can incline the incident surface 31 of the diffraction grating 30 with respect to the axial direction of the optical fiber F. Therefore, the axis BC of the light flux of the ultraviolet laser light UV can be finely adjusted so as to be completely perpendicular to the incident surface 31 of the diffraction grating 30.

As shown in FIG. 1, the incident means 20 is for making the laser light UV emitted from the laser light source 15 incident on the diffraction grating 30, and includes a light flux size converter 21 and a reflecting mirror 22. And a cylindrical lens 23.

The light flux size converter 21 includes a laser light source 1
Aperture surface UV1 of the ultraviolet laser light UV emitted from 5
Is emitted in a state of being narrowed so as to be inclined with respect to the axis BC of the light flux, and the light flux size converter 21 is the light condensing unit in the claims. The light flux size converter 21 is, for example, a beam expander having a diaphragm adjusting function.

Normally, the ultraviolet laser light UV emitted from the laser light source 15 itself has a slight spread, but the conventional fiber Bragg grating forming apparatus does not correct this spread and the ultraviolet laser is emitted. The light UV is incident on the diffraction grating 30. On the other hand, in the fiber Bragg grating forming device 10 of the present embodiment, the luminous flux of the ultraviolet laser light UV emitted from the luminous flux size converter 21 is reduced by 0.6 to 2.7 milliradian, and the diaphragm surface UV1 and the luminous flux The angle θ with the axis BC is narrowed to 0.4 to 2.5 milliradians.

For example, in the laser light source used in the examples described later, since the emitted laser light UV in the ultraviolet region has a spread of about -0.2 milliradian,
By narrowing down by about 2 milliradian, the laser light UV in the ultraviolet region can be made into a parallel light having a spread of about 0 milliradian as described above. However, in the fiber Bragg grating forming apparatus 10 of the present embodiment, the laser light UV in the ultraviolet region is 0. The diaphragm beam is made larger than 0.2 milliradian, and the ultraviolet laser light UV is used as the diaphragm beam.

The reflecting mirror 22 is for refracting the ultraviolet laser light UV emitted from the light beam size converter 21 toward the diffraction grating 30. The ultraviolet light emitted from the laser light source 15 is also included in the reflecting mirror 22. If the tip of the laser UV is substantially perpendicular to the diffraction grating 30, it may not be provided.

The cylindrical lens 23 is a reflecting mirror 22.
It is for condensing and constricting the ultraviolet laser light UV reflected by the light flux so that the outer surface of the light flux in the direction perpendicular to the axial direction of the optical fiber F is inclined with respect to the optical axis. Then, the energy density of the ultraviolet laser light UV increases, and the light with which the optical fiber F is irradiated can be strengthened.

The luminous flux size converter 21 is not limited to the beam expander, and the diaphragm surface UV1 of the ultraviolet laser light UV emitted from the laser light source 15 is narrowed so as to be inclined with respect to the axis BC of the luminous flux. A combination of a convex lens and a concave lens may be used as long as the light can be emitted in a state, and there is no particular limitation.

Next, the operation and effect of the fiber Bragg grating forming apparatus 10 of this embodiment will be described. First, in the optical fiber F forming the fiber Bragg grating, the core C is preliminarily impregnated with hydrogen in a high-pressure hydrogen gas atmosphere. Then,
Ultraviolet laser light U of germanium contained in core C
The photosensitivity to V can be improved.

When the laser light source 15 emits an ultraviolet laser light UV, the ultraviolet laser light UV is emitted.
Enters the light flux size converter 21. Then, the light beam size converter 21 causes the laser light UV in the ultraviolet range to have an angle θ formed by the diaphragm surface UV1 with respect to the axis BC of the light beam.
Is squeezed to be 0.4 to 2.5 milliradians and then radiated toward the reflecting mirror 22. Then, the ultraviolet laser light UV is reflected by the reflecting mirror 22 to the diffraction grating 30.
The light is refracted toward the incident surface 31 of.

The ultraviolet laser light UV reflected by the reflecting mirror 22 enters the diffraction grating 30 through the cylindrical lens 23. At this time, the ultraviolet laser light UV is focused and focused so that the outer surface of the light flux in the direction perpendicular to the axial direction of the optical fiber F is inclined with respect to the optical axis. The angle θ with respect to the axis BC is incident on the incident surface 31 of the diffraction grating 30 while being kept at 0.4 to 2.5 milliradians.

Then, the ultraviolet laser light UV is diffracted by the interference groove 33 of the diffraction grating 30, and its radiation surface 32.
The + 1st-order light and the -1st-order light emitted from the two interfere with each other to form an interference fringe having a fringe spacing of Λ in the central axis direction of the optical fiber F, and the interference fringe is applied to the core C of the optical fiber F.

Then, the germanium contained in the core C is exposed to the laser light UV in the ultraviolet region, and a change in the refractive index corresponding to the distribution of the energy intensity of the light of the interference fringes occurs in the core C. It can be formed on the core C of the optical fiber F.

As described above, according to the fiber Bragg grating forming apparatus 10 of this embodiment, the FBG can be formed in the core C of the optical fiber F. This FBG can clarify the contrast of the striped pattern formed on the core C of the optical fiber, as compared with the FBG formed by entering the ultraviolet laser light UV that has become parallel light into the diffraction grating 30. Therefore, it is possible to improve the reflection characteristics of the FBG with respect to the specific wavelength and also improve the transmission characteristics of wavelengths other than the specific wavelength. Therefore, if the FBG formed by the fiber Bragg grating forming device 10 of the present embodiment is used as a filter of an optical multiplexer / demultiplexer in optical fiber communication or the like, crosstalk can be greatly reduced.

The angle θ formed by the diaphragm surface UV1 of the ultraviolet laser light UV incident on the diffraction grating 30 with the axis BC of the light beam is 0.4 to 2.5 milliradians. Therefore, the FBG formed by the fiber Bragg grating forming apparatus 10 according to the present embodiment is more attenuated than the FBG formed by the conventional fiber Bragg grating forming apparatus in the intensity of the signal within the −3 dB bandwidth of the reflected signal. Can be significantly reduced, and the strength of signals at wavelengths outside the −3 dB bandwidth can be significantly reduced.

[0028]

[Embodiment] Next, Embodiment 1 will be described. The FBG formed by the fiber Bragg grating forming apparatus 10 of the present invention, that is, the FBG formed by irradiating the diaphragm light (diaphragm angle of 1.1 milliradian) (Example 1), and the ordinary fiber Bragg grating forming apparatus. FBG formed by irradiating parallel light (diaphragm angle of 0 milliradian) (comparative example 1) and divergent light (diaphragm angle of -3.3 milliradian) (comparative example) In 2), the reflection characteristics for specific wavelengths were compared. The stop angle means that the outer surface in the axial direction of the optical fiber F in the light flux of the ultraviolet laser light UV incident on the diffraction grating 30 is the axis BC of the light flux.
The angle θ is defined as follows. When +, the light beam is condensed as it goes from the light beam size converter 21 to the diffraction grating 30, and when −, it goes from the light beam size converter 21 to the diffraction grating 30. Accordingly, the luminous flux spreads.

An irradiation excimer laser device (Compex 102MJ manufactured by Lambda Physics Co., Ltd.) is used as the laser light source 15, and excimer laser light having a wavelength of 248 nm is emitted at an energy of 170 mJ per pulse and a repetition frequency of 20 Hz. The optical fiber was illuminated for 4 minutes.
In the ultraviolet laser light UV emitted from the laser light source 15, the angle θ between the outer surface of the light beam and the axis BC of the light beam is −0.2 milliradian. As the light flux size converter 21 of the incident means 20, a beam expander for excimer laser light band (manufactured by CVC) that can emit incident light as diaphragm light is used. The optical fiber F forming the FBG is a standard single mode SMF28 fiber, which was used after hydrogen loading was performed in a hydrogen atmosphere of 100 atm. In addition, the radiation surface 3 of the diffraction grating 30
The 0th-order light and the 2nd-order diffracted light emitted from 2 are 2
%, Which is suppressed to such an extent that it does not affect the formation of interference fringes by the + 1st-order diffracted light and the -1st-order diffracted light.

FIG. 4A is a diagram comparing the transmission characteristics of FBGs, and FIG. 4B is a diagram comparing the reflection characteristics of FBGs. In FIG. 4, (a) shows the reflection signal spectrum of Example 1, (b) shows the reflection signal spectrum of Comparative Example 1, and (c) shows the reflection signal spectrum of Comparative Example 2. As shown in FIG. 4 (B), in Comparative Example 2, generation of uneven spectrum (hereinafter referred to as side lobe) is observed on both the long wavelength side and the short wavelength side. This side lobe is slightly smaller in Comparative Example 1, but is not significantly improved. However, in the spectrum of Example 1, it can be confirmed that the side lobes are almost eliminated and the spectrum changes smoothly. Since this side lobe causes noise in the reflected signal, it can be confirmed that the quality of the reflected signal is improved in Example 1 as compared with Comparative Examples 1 and 2. Further, as shown in FIG. 4A, in the spectrum of the transmission signal of Example 1, the width of the recess near the center wavelength is wider than that of the spectra of the transmission signals of Comparative Example 1 and Comparative Example 2. From this, it can be confirmed that the quality of the reflected signal is improved in Example 1. Therefore, the diaphragm surface UV of the ultraviolet laser light UV
It can be confirmed that the transmission characteristics of the formed FBG can be improved by narrowing 1 so as to incline with respect to the axis BC of the light flux and then making it enter the diffraction grating 30.

Next, a second embodiment will be described. In the FBG formed by the fiber Bragg grating forming apparatus 10 of the present embodiment, the reflection characteristics when the aperture angle of the irradiation light was changed were compared. For comparison, the reflection characteristic and the transmission characteristic for a specific wavelength were compared when the aperture angle was 1.1 milliradian (Example 2) and when the aperture angle was 3.3 milliradian (Comparative Example 3).

An irradiation excimer laser device (Compex 102MJ manufactured by Lambda Physics Co., Ltd.) is used as the laser light source 15, and excimer laser light having a wavelength of 248 nm is emitted at an energy per pulse of 170 mJ and a repetition frequency of 20 Hz. The optical fiber was illuminated for 4 minutes.
In the ultraviolet laser light UV emitted from the laser light source 15, the angle θ between the outer surface of the light beam and the axis BC of the light beam is −0.2 milliradian. As the light flux size converter 21 of the incident means 20, a beam expander for excimer laser light band (manufactured by CVC) that can emit incident light as diaphragm light is used. The optical fiber F forming the FBG is a standard single mode SMF28 fiber, which was used after hydrogen loading was performed in a hydrogen atmosphere of 100 atm. In addition, the radiation surface 3 of the diffraction grating 30
The 0th-order light and the 2nd-order diffracted light emitted from 2 are 2
%, Which is suppressed to such an extent that it does not affect the formation of interference fringes by the + 1st-order diffracted light and the -1st-order diffracted light.

FIG. 5A is a diagram comparing the transmission characteristics of the FBGs, and FIG. 5B is a diagram comparing the reflection characteristics of the FBGs. In FIG. 5, (a) shows the reflection signal spectrum of Example 2 and (b) shows the reflection signal spectrum of Comparative Example 3. Figure 5
As shown in (B), in Comparative Example 3, the occurrence of side lobes, which was not observed in Example 2, can be seen. Also, FIG.
As shown in (A), in the spectrum of the transmission signal of Comparative Example 3, the recess near the center wavelength is shallower than in the spectrum of the transmission signal of Example 2. That is, it can be confirmed that the number of signals transmitted without being reflected by the FBG increases. In other words, the laser light UV in the ultraviolet region is used for diffraction grating
If the angle of inclination with respect to the diaphragm surface UV1 and the axis BC of the light flux when the light is incident on 0 is too large, that is, if the ultraviolet laser light UV is too narrow, the FB formed
It can be confirmed that the transmission characteristic of G deteriorates.

Next, a third embodiment will be described. In the FBG formed by the fiber Bragg grating forming device 10 of the present embodiment, a change in −3 dB bandwidth with respect to a change in the aperture angle of the irradiation light was confirmed. In the experiment, the aperture angle of the irradiation light was -3.3 milliradians and the aperture angle was 3.3.
I changed to milliradians.

An irradiation excimer laser device (COMPex 102MJ manufactured by Lambda Physics, Inc.) is used as the laser light source 15, and excimer laser light having a wavelength of 248 nm is emitted at an energy per pulse of 170 mJ and a repetition frequency of 20 Hz. The optical fiber was illuminated for 4 minutes.
In the ultraviolet laser light UV emitted from the laser light source 15, the angle θ between the outer surface of the light beam and the axis BC of the light beam is −0.2 milliradian. As the light flux size converter 21 of the incident means 20, a beam expander for excimer laser light band (manufactured by CVC) that can emit incident light as diaphragm light is used. The optical fiber F forming the FBG is a standard single mode SMF28 fiber, which was used after hydrogen loading was performed in a hydrogen atmosphere of 100 atm. In addition, the radiation surface 3 of the diffraction grating 30
The 0th-order light and the 2nd-order diffracted light emitted from 2 are 2
%, Which is suppressed to such an extent that it does not affect the formation of interference fringes by the + 1st-order diffracted light and the -1st-order diffracted light.

FIG. 6 is a diagram showing a change in -3 dB bandwidth with respect to a change in aperture angle. As shown in FIG. 6, when the aperture angle is increased from parallel light, that is, when the angle at which the aperture surface UV1 of the ultraviolet laser light UV is inclined with respect to the axis BC of the light beam is increased, the -3 dB bandwidth becomes wider and the ITU- It is almost the same as the T standard bandwidth of 0.4 nm. And -3d
The B bandwidth is almost constant between 0.4 and 2.5 milliradians. And, when it exceeds 2.5 milliradian, -3
The dB bandwidth sharply narrows, and if the aperture angle is increased more than that, the -3 dB bandwidth gradually narrows. On the contrary, when the aperture angle is smaller than 0.4 milliradian, the -3 dB bandwidth becomes narrow rapidly, and when the aperture angle is made smaller than that, the -3 dB bandwidth becomes gradually narrower. It can be confirmed that the -3 dB bandwidth continues to be gradually narrowed even when the light is expanded from parallel light. Therefore, when the ultraviolet laser light UV is narrowed down and incident on the diffraction grating 30, the diaphragm angle has an optimum range, and in this embodiment,
It can be confirmed that the range is 0.4 to 2.5 milliradians.

[0037]

According to the invention of claim 1, as compared with the fiber Bragg grating formed by incident parallel light, the reflection characteristic of the formed fiber Bragg grating for a specific wavelength can be improved, and The transmission characteristics of wavelengths other than the specific wavelength can be improved. Therefore, if the fiber Bragg grating formed by this forming apparatus is used as a filter of an optical multiplexer / demultiplexer in optical fiber communication, crosstalk can be greatly reduced. According to the invention of claim 2, by using the fiber Bragg grating formed by this forming apparatus, it is possible to significantly reduce the attenuation of the signal intensity within the −3 dB bandwidth in the reflected signal,
In addition, it is possible to significantly reduce the intensity of signals having wavelengths outside the -3 dB bandwidth. Moreover, the -3 dB bandwidth is
It can be tailored to the bandwidth of the desired channel. Therefore, the fiber Bragg grating formed by this forming device is used for optical multiplexing / combining in optical fiber communication.
If it is used as a filter of a duplexer, crosstalk can be greatly reduced. According to the invention of claim 3, since the axis of the light flux of the ultraviolet laser light is made incident so as to be perpendicular to the axial direction of the optical fiber and the emission surface of the diffraction grating, the fiber Bragg grating can be accurately measured. It can be formed well. According to the invention of claim 4, the reflection characteristic of the fiber Bragg grating for a specific wavelength can be improved and the transmission characteristic of a wavelength other than the specific wavelength can be improved as compared with the case where parallel light is incident. . Therefore, if the fiber Bragg grating formed by this forming method is used as a filter of an optical multiplexer / demultiplexer in optical fiber communication, crosstalk can be greatly reduced. According to the invention of claim 5, by using the fiber Bragg grating formed by the present forming apparatus, it is possible to significantly reduce the attenuation of the signal intensity within the -3 dB bandwidth in the reflected signal, and the -3 dB band. It is possible to significantly reduce the intensity of signals of wavelengths outside the width. Moreover, the −3 dB bandwidth can be matched to the bandwidth of the desired channel. Therefore, the fiber Bragg grating formed by this forming apparatus is
When used as a filter for an optical multiplexer / demultiplexer in optical fiber communication, crosstalk can be greatly reduced.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of a fiber Bragg grating forming device 10 of the present embodiment.

FIG. 2 is an enlarged sectional view of a diffraction grating 30.

3A is a schematic explanatory view of an optical fiber F on which an FBG is formed, and FIG. 3B is a spectrum of incident light LB0 incident on the FBG, a spectrum of reflected light LB1 reflected by the FBG, and an FBG. It is the figure which showed the spectrum of the reflected transmitted light LB2.

FIG. 4A is a diagram comparing the transmission characteristics of FBGs, and FIG. 4B is a diagram comparing the reflection characteristics of FBGs.

5A is a diagram comparing the transmission characteristics of FBGs, and FIG. 5B is a diagram comparing the reflection characteristics of FBGs.

FIG. 6 is a diagram showing a change in -3db bandwidth with respect to a change in aperture angle.

[Explanation of symbols] 10 Fiber Bragg grating forming device 15 Laser light source 20 means of incidence 30 diffraction grating 31 incident surface 32 Radiating surface 33 interference groove UV UV laser light F optical fiber C core

Continued front page    (72) Inventor Susumu Kimura             1298 Shido, Shido-machi, Okawa-gun, Kagawa             Shinko Electric Cable Co., Ltd. (72) Inventor Jun Fukuyama             1298 Shido, Shido-machi, Okawa-gun, Kagawa             Shinko Electric Cable Co., Ltd. (72) Inventor Yasuo Mizutani             1298 Shido, Shido-machi, Okawa-gun, Kagawa             Shinko Electric Cable Co., Ltd. (72) Inventor Yutaka Takemura             1298 Shido, Shido-machi, Okawa-gun, Kagawa             Shinko Electric Cable Co., Ltd. F-term (reference) 2H049 AA02 AA06 AA34 AA52 AA55                       AA58 AA59 AA62                 2H050 AB05X AC82 AC84 AD00

Claims (5)

[Claims] 1. A forming apparatus for forming a Bragg grating in a core of an optical fiber, wherein the forming apparatus is provided with a laser light source for emitting coherent ultraviolet laser light and a plurality of interference grooves. Diffractive grating having a radiating surface, the radiating surface facing the optical fiber, and an incident means for allowing the ultraviolet laser light emitted from the laser light source to be incident on the incident surface of the diffractive grating. And the plurality of interference grooves formed on the radiation surface of the diffraction grating are formed so that the incident surface of the diffraction grating is parallel to the radiation surface of the diffraction grating. Are arranged so as to be orthogonal to the axial direction of the diffraction grating, and the plurality of interference grooves of the diffraction grating add +1 to the ultraviolet laser light incident on the incident surface of the diffraction grating.
The 1st and -1st orders are diffracted, and the + 1st order diffracted light and-
It is formed so as to radiate the first-order diffracted light from the radiation surface, and the incidence means collects the ultraviolet region laser light to form an outer surface in the axial direction of the optical fiber in the light flux of the ultraviolet region laser light. An apparatus for forming a fiber Bragg grating, comprising: a condensing unit that is made to enter the diffraction grating in a state of being narrowed so as to be inclined with respect to the axis of a light beam. 2. The angle formed by the outer surface in the axial direction of the optical fiber in the light flux of the ultraviolet laser light incident on the incident surface of the diffraction grating with respect to the axis of the light flux is 0.
The fiber Bragg grating forming apparatus according to claim 1, wherein the fiber Bragg grating has a diameter of 4 to 2.5 milliradians. 3. An incident angle adjusting unit for causing the laser light in the ultraviolet region to enter such that the axis of the light flux is perpendicular to the incident surface of the diffraction grating.
Apparatus for forming the fiber Bragg grating described. 4. A method for forming a Bragg grating in a core of an optical fiber, wherein a laser light source emits an ultraviolet region laser beam, and the ultraviolet region laser beam is directed to an outer surface in an axial direction of the optical fiber. The ultraviolet rays are condensed by the condensing unit so as to be inclined with respect to the axis of the light flux, and the condensed laser light in the ultraviolet region is incident on the incident surface of the diffraction grating, and the emission surface parallel to the incident surface of the diffraction grating. Formed by, by a plurality of interference grooves orthogonal to the axial direction of the optical fiber, the ultraviolet region laser light is + 1st order diffraction and -1st order diffraction, + 1st order diffraction light and -1st order diffraction light of the ultraviolet range laser light, A method of forming a fiber Bragg grating, characterized in that the light is emitted from an emission surface of the diffraction grating toward an optical fiber. 5. The angle formed by the outer surface in the axial direction of the optical fiber of the light flux of the ultraviolet laser light incident on the incident surface of the diffraction grating with respect to the axis of the light flux is 0.
The method for forming a fiber Bragg grating according to claim 4, wherein the fiber Bragg grating has a diameter of 4 to 2.5 milliradians.