JP2002062442A - Method for forming grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a grating by which a clad mode combining loss is reduced. SOLUTION: The grating having small clad mode coupling loss is realized by coating a part in which the grating 2 is formed with a coating layer 6. That is, an optical fiber 1 which is currently in use is used as it is to form the grating 2 without changing a manufacturing condition of the optical fiber 1 or the like, and only the grating 2 is coated again with the coating layer 6. Thus, the clad mode coupling loss is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for forming a grating.

[0002]

2. Description of the Related Art In recent years, with the development of wavelength multiplexing transmission technology,
The optical waveguide grating technology has become important from the aspect of application to a narrow band reflection element for wavelength multiplexing / demultiplexing, a device for dispersion compensation, and the like.

An optical waveguide grating uses a photosensitivity in which a refractive index is permanently changed by light irradiation, and has a constant periodic change in the length direction of an optical fiber or a core portion of a planar optical waveguide circuit. A grating of a refractive index modulation type is formed by forming a periodic change in the core refractive index.

[0004] A narrow-band reflecting element must have high performance in terms of high reflectivity, narrow spectral width, characteristics of blocking light having a wavelength other than the transmission wavelength, and the like. As a method of forming a grating by causing a periodic change in the core refractive index in an optical waveguide, when a quartz glass to which GeO ₂ is added is irradiated with strong ultraviolet light, the refractive index increases according to the irradiation amount. There is a known method for utilizing the information. For example, a method is known in which a quartz optical fiber having a core to which GeO ₂ is added is subjected to a hydrogenation treatment in a hydrogen pressurized container (100 atm), and then the quartz optical fiber is irradiated with ultraviolet light.

FIG. 3 is a conceptual view of a conventional grating forming method.

The ultraviolet light 21 having a wavelength of 248 nm emitted from a light source (excimer laser, not shown) is
Is reflected by a mirror 22 made of a material that totally reflects the light, and the ultraviolet light 21 is irradiated vertically through the phase mask 23 onto the portion (outer diameter 125 μm) of the optical fiber (coated outer diameter 250 μm) from which the coating layer has been removed. Has become.

In order to manufacture an optical fiber grating using the apparatus having such a configuration, ultraviolet light 21 emitted from a light source is transmitted through a phase mask 23 to an optical fiber grating 24.
May be irradiated. Such irradiation increases the refractive index of the core only in the portion of the optical fiber 24 where the ultraviolet light is irradiated, so that the grating 25 whose core refractive index changes periodically is formed. Furthermore, a method of irradiating the ultraviolet light 21 while moving the mirror 22 in the longitudinal direction (the direction of the arrow) of the optical fiber 24, and controlling the refractive index modulation by changing the speed of moving the ultraviolet light 21 and the irradiation time at each irradiation position. Is used to form a grating.

The grating formed by the above-described method is incorporated into a temperature compensation package in which the center wavelength does not change even when the use environment temperature changes, and is commercialized.

FIG. 4 is a conceptual diagram of a fiber grating incorporated in a temperature compensation package.

The optical fiber 11 having the grating 12 formed on the portion where the coating layer has been removed is fixed to a package 14 made of a material having a different coefficient of linear expansion using an adhesive 13 and housed in a housing 15. ing.

FIG. 5A is a characteristic diagram showing transmission and reflection spectrum characteristics of the grating formed by the method described with reference to FIGS. 3 and 4, and FIG.
It is the elements on larger scale of area | region A. 5A and 5B, the horizontal axis is the wavelength axis, and the vertical axis is the loss axis.

From FIG. 5A, it is possible to form a grating having a transmission cutoff ratio of about 30 dB and having no side lobe in the reflection spectrum.

FIG. 6 shows the temperature characteristics of the grating incorporated in the temperature compensation package. The horizontal axis indicates the temperature axis, and the vertical axis indicates the center wavelength shift axis.

It can be seen from FIG. 1 that a grating having a central wavelength controllability of about 0.03 nm or less in a temperature range of -20.degree. C. to 80.degree.

[0015]

By the way, in the above-mentioned prior art, it is possible to form a grating by controlling the type of refractive index modulation by irradiating ultraviolet light while moving it in the longitudinal direction of the optical fiber. It is.

However, it has been found that forming a grating by such a method has the following problems.

In FIG. 5A, a ripple occurs on the short wavelength side from the center wavelength of the transmission spectrum. This is due to the coupling of the guided mode to the cladding mode. This cladding mode coupling loss needs to be suppressed as much as possible to give a loss to other wavelengths within the wavelength band used by the WDM system. As a method for suppressing the cladding mode coupling loss, a method of forming a grating in a fiber doped with GeO ₂ also in the cladding of the optical fiber is used. A grating is also formed on the clad by such a method, and the occurrence of a clad mode can be suppressed. FIG. 5A shows an example in which the amount of GeO ₂ added to the core relative to the core is set to 0.6, but a coupling loss of about 0.5 dB occurs in the cladding mode. These ripples and cladding mode coupling loss pose problems when applied to WDM systems.

In order to further suppress the cladding mode coupling loss, it is necessary to further increase the amount of GeO ₂ added to the cladding.
However, if the added amount of O ₂ becomes equal (the added amount of GeO ₂ of the clad to the core may be set to 1), however,
It is very difficult to control the amount of GeO ₂ added to the cladding, and there is variation among the fibers produced, and reducing the cladding mode coupling loss only by the fiber production conditions requires a very long time and labor. was there.

An object of the present invention is to solve the above-mentioned problems and to provide a method of forming a grating with reduced cladding mode coupling loss.

[0020]

In order to achieve the above object, a method of forming a grating according to the present invention comprises irradiating an optical fiber with ultraviolet light to a portion from which a coating layer is removed, and periodically irradiating the optical fiber in a longitudinal direction. In the method of forming a grating for forming a grating composed of a region having a changed refractive index, only the region where the grating has been formed after the irradiation of ultraviolet light is again covered with a coating layer.

In the method of forming a grating according to the present invention, in addition to the above structure, it is preferable that the length of the coating layer is at least equal to or longer than the length of the grating.

In the method of forming a grating according to the present invention, in addition to the above configuration, it is preferable that the thickness of the coating layer is at least the same as that of the optical fiber before the coating layer is removed.

According to the present invention, by covering the portion where the grating is formed with the coating layer, it is possible to realize a grating having small cladding mode coupling loss. That is, the cladding mode coupling loss can be reduced by forming a grating using the currently used optical fiber as it is without changing the fiber manufacturing conditions and the like, and covering only the grating again with the coating layer. it can.

[0024]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a conceptual diagram showing an embodiment of a grating forming method according to the present invention.

The method of forming the grating is to irradiate the portion of the optical fiber 1 from which the coating layer has been removed with ultraviolet light, thereby forming a grating comprising a region whose refractive index periodically changes in the longitudinal direction on the optical fiber 1. In which only the region where the grating is formed after the irradiation of the ultraviolet light is again covered with the coating layer.

According to the present forming method, a grating having a small cladding mode coupling loss can be realized by covering the portion where the grating is formed with the coating layer.
That is, the cladding mode coupling loss can be reduced by forming a grating using the currently used optical fiber as it is without changing the fiber manufacturing conditions and the like, and covering only the grating again with the coating layer. it can.

[0028]

Next, the present invention will be described with reference to specific numerical values, but the present invention is not limited thereto.

The optical fiber 1 used in the present forming method is
It comprises a core and a clad having a lower refractive index than the core, and the relative refractive index difference is set to about 0.3 to 6%. The outer diameter of the coating of the optical fiber 1 is 250 μm. At least GeO ₂ is added to the core of the optical fiber 1, and when ultraviolet light is irradiated, the refractive index changes according to the intensity of the ultraviolet light and the irradiation time. Also, at least GeO ₂ is added to the cladding of the optical fiber 1 in an amount of 0.1% of the core.
It is added about 6 times, and has a structure in which the refractive index is lower than that of the core by adding F or the like. In addition to GeO ₂ , a group III element such as Al, a rare earth element such as Er, and a group IV element such as Ti may be appropriately added to the core of the optical fiber 1.

The wavelength of the ultraviolet light used in the grating forming method of the present invention is preferably about 200 to 300 nm, and the light source is, for example, a KrF excimer laser (wavelength 2).
48 nm) is used. A grating 2 having a length of 10 mm is formed on an optical fiber 1 in the same manner as in the prior art. In order to form the grating 2, a hydrogenation treatment is performed prior to the irradiation of the optical fiber 1 with ultraviolet light. This hydrogenation treatment is performed in order to sufficiently obtain a change in the refractive index of the core due to irradiation with ultraviolet light.
This can be achieved by holding in a hydrogen pressurized container adjusted to about 0 atm and about 50 ° C. for about one week.

Next, the coating layer of the optical fiber 1 is removed over a length of about 30 mm by a mechanical method using a tool such as a wire stripper or a chemical method using a chemical, and a phase mask is applied to the optical fiber core (outer diameter 125 μm). To form a grating 2.

Optical fiber 1 on which grating 2 is formed
Performs the following processing before fixing to the temperature compensation package 4 made of materials having different linear expansion coefficients.

The coating layer 6 is formed again on the portion (length 10 mm) where the grating 2 is formed. As the coating layer 6, a UV curable resin was used as in the case of the original optical fiber 1. At this time, the length of the coating layer 6 depends on the fixed portion 3 of the temperature compensation package 4.
Shorter than the length of the grating 2 and longer than the length of the grating 2.

In this embodiment, since the length of the fixed portion was 25 mm, the length of the coating layer was 15 mm. The thickness of the coating layer 6 was adjusted so that the outer diameter of the optical fiber 1 after being covered with the coating layer 6 was 250 μm, which is the same as the original optical fiber 1. Further, in addition to the coating material described in this embodiment mode, a coating material such as polyimide may be used. The optical fiber 1 in which the grating 2 is covered with the coating layer is fixed to the fixing part 3 of the temperature compensation package 4 with an adhesive, and is housed in the housing 5 to be commercialized.

FIG. 2A is a characteristic diagram showing transmission and reflection spectrum characteristics of the grating formed by the grating forming method of the present invention, and FIG.
It is the elements on larger scale of area | region B of (a). FIG. 2 (a),
In (b), the horizontal axis indicates the wavelength axis, and the vertical axis indicates the loss axis.

According to the present forming method, as shown in FIG.
B could be reduced to about B. According to the present forming method, a grating having a small cladding mode coupling loss can be formed without changing the manufacturing conditions of an optical fiber that is difficult to control.

As described above, according to the present invention, the cladding mode coupling loss of the grating can be easily controlled by covering only the portion where the grating is formed with the covering layer. Also, by eliminating the troublesome task of controlling the manufacturing conditions of the currently used optical fiber, the cladding mode coupling can be performed by simply applying a coating layer to the grating part of the currently used optical fiber. A grating with reduced loss can be realized.

[0038]

In summary, according to the present invention, the following excellent effects are exhibited.

It is possible to provide a method of forming a grating with reduced cladding mode coupling loss.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an embodiment of a grating forming method according to the present invention.

2A is a characteristic diagram showing transmission and reflection spectrum characteristics of a grating formed by the grating forming method of the present invention, and FIG. 2B is a partially enlarged view of a region B of FIG.

FIG. 3 is a conceptual diagram of a conventional grating forming method.

FIG. 4 is a conceptual diagram of a fiber grating incorporated in a temperature compensation package.

5A is a characteristic diagram showing transmission and reflection spectrum characteristics of a grating formed by the method described with reference to FIGS. 3 and 4, and FIG. 5B is a partial enlarged view of a region A in FIG. FIG.

FIG. 6 shows a temperature characteristic of a grating incorporated in a temperature compensation package.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber 2 Grating 3 Optical fiber fixing part 4 Temperature compensation package 5 Housing 6 Coating layer

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Aki Hongo 5-1-1, Hidaka-cho, Hitachi-shi, Ibaraki F-term in Opto-Systems Research Laboratory, Hitachi Cable, Ltd. 2H049 AA02 AA33 AA44 AA45 AA51 AA59 AA62 2H050 AA07 AC82 BA00

Claims (3)

[Claims] 1. A method for forming a grating, comprising: irradiating an ultraviolet light to a portion of an optical fiber from which a coating layer has been removed, and forming a grating comprising a region in which a refractive index periodically changes in a longitudinal direction on the optical fiber. A method for forming a grating, wherein only a region where a grating is formed after irradiation with ultraviolet light is covered again with a coating layer. 2. The method of forming a grating according to claim 1, wherein the length of the coating layer is at least equal to or longer than the length of the grating. 3. The method of forming a grating according to claim 1, wherein the thickness of the coating layer is at least the same as that of the optical fiber before removing the coating layer.