JP2003322734A - Optical fiber for long periodic fiber grating and fiber grating type optical component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To compensate the temperature/change with time of gain wavelength characteristic of an optical fiber amplifier such as an EDFA (erbium-doped fiber amplifier) or the like. <P>SOLUTION: The optical fiber 2 for a long periodic fiber grating is equipped with a core layer 3 which is doped with germanium (Ge) having photosensitivity to ultraviolet rays and a refractive index having positive temperature dependency, and also equipped with a clad part (first clad layer 4 and second clad layer 5) which is doped with boron (B<SB>2</SB>O<SB>3</SB>) having a refractive index having negative temperature dependency and lower than that of the core layer 3 and which covers the core layer 3. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber for a long period fiber grating and a fiber grating type optical component such as an optical filter manufactured by using this optical fiber.

[0002]

2. Description of the Related Art A fiber grating is a periodical change in the refractive index formed in the fiber axis direction of an optical fiber. In particular, the above-described refractive index change period is the wavelength of an optical signal transmitted through the fiber grating. A fiber grating having a wavelength much longer than the band, for example, 100 μm to several 100 μm is called a long-period fiber grating.

Long-period gratings can couple the guided-mode optical signal power of an optical fiber into a cladding-mode optical signal. Utilizing this characteristic, the fiber grating type optical component using the long period grating is amplified by an ASE (Amplified Spontaneous Emission) of an optical fiber amplifier such as EDFA (Erbium-doped Fiber Amplifier). WDM (Wavelength) is used as a filter device for suppressing / removing spontaneous emission and compensating for gain wavelength dependence.
It is used in various systems such as Divisional Multiplexing and optical communication systems.

In particular, recently, as a gain equalizer for compensating for a temperature change or a time-dependent change (dynamic waveform fluctuation) of a gain wavelength characteristic of an EDFA, a dynamic temperature dependence capable of compensating (canceling) the above dynamic waveform fluctuation. Gain equalizer having a transmission loss wavelength characteristic showing a so-called dynamic gain equalizer {dynamic GEQ
(Gain Equalizer)} has been developed.

That is, the temperature coefficient of the long-period fiber grating can be expressed by the product of the difference between the temperature dependence of the effective refractive index of the core layer and the temperature dependence of the effective refractive index of the cladding layer and the periodic pitch of the grating. Therefore, the gain becomes larger than that of a normal gain equalizer such as a multilayer filter.

Therefore, the temperature dependence of the transmission loss wavelength characteristic of the long-period fiber grating, which is proportional to the above temperature coefficient, also fluctuates greatly as compared with a normal gain equalizer.

Therefore, the gain wavelength characteristic of the EDFA is
It is considered to compensate for a change with time by a dynamic GEQ using a long-period fiber grating having a temperature dependence of a transmission loss wavelength characteristic based on the above temperature coefficient.

[0008]

However, a long fiber manufactured by using an ordinary single mode optical fiber (SMF; its refractive index profile is shown in FIG. 5) having a germanium-doped core layer (germanium layer). As shown in FIG. 6, the temperature dependence (temperature-dependent wavelength shift characteristic) of the center wavelength of the periodic fiber grating is about 0.05 nm / ° C. at most.

That is, in order to compensate the temperature / time-dependent change of the gain wavelength characteristic of the EDFA showing the dynamic waveform fluctuation, the temperature dependence of the center wavelength of about 0.05 nm / ° C. is not sufficient, and the long period is long. It has been necessary to increase the temperature dependence of the center wavelength of the fiber grating.

The present invention has been made in view of the above-mentioned circumstances, and an optical fiber for a long period fiber grating capable of forming a long period fiber grating capable of compensating the temperature / time-dependent change of the gain wavelength characteristic of an EDFA, and this length. It is an object of the present invention to provide a fiber grating type optical component manufactured by using an optical fiber for a periodic fiber grating.

[0011]

The present inventors have found that when the temperature dependence of the central wavelength of the fiber grating portion (temperature dependent wavelength shift characteristic), that is, the amount of wavelength shift due to temperature change is increased, the transmission loss thereof is increased. We paid attention to the fact that the waveform fluctuation width of the wavelength characteristic increases.

That is, if the wavelength shift amount based on the temperature change of the central wavelength of the grating portion is increased, the waveform fluctuation width of the transmission loss wavelength characteristic can be increased,
It is possible to deal with the dynamic waveform fluctuation of the temperature / time change of the gain wavelength characteristic in the EDFA.

In order to increase the temperature dependence of the central wavelength of the grating part, it is sufficient to increase the temperature coefficient of the grating part. Therefore, the inventors of the present invention have a concrete means for increasing the temperature coefficient. Devised.

Therefore, the optical fiber for long-period fiber grating according to the first aspect of the present invention is a core layer doped with a first material which is sensitive to ultraviolet rays and has a refractive index having a positive temperature dependence. And has a negative temperature dependence,
A second material having a refractive index lower than that of the core layer is doped, and a cladding portion covering the core layer is provided.

A fiber grating type optical component according to a second aspect of the present invention is an optical fiber for long period fiber grating according to any one of claims 1 to 4, and a core layer of the optical fiber. And a grating portion formed as a periodical change in the refractive index of the core layer at a predetermined portion along the fiber axis direction along the fiber axis direction.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view showing a fiber grating type optical component 1 according to an embodiment of the present invention.

As shown in FIG. 1, the fiber-grating type optical component 1 is provided with an optical fiber 2 for long-period fiber grating, instead of the ordinary SMF.

As shown in FIG. 1, the optical fiber 2 for long-period fiber grating is made of germanium (Ge) which has a higher refractive index than silica and is sensitive to ultraviolet rays with respect to silica glass (silica (SiO ₂ ) glass). A core layer 3 formed by doping (adding) germania (which has the same meaning as GeO ₂ ), and boron {diboron trioxide (boron; B ₂ O ₃ )} is doped in the silica layer made of the above silica. And a first cladding layer 4 that covers the core layer 3.

Further, in the optical fiber 2 for long-period fiber grating, the silica layer is formed by doping fluorine (F) having a refractive index lower than that of silica, and the first cladding layer is formed. The second clad layer 5 covering 4 is provided.

The optical fiber 2 has a mode field diameter of about 8 μm (representing the spread of the light intensity distribution within the fiber (core layer 3)), and this mode field diameter (about 8 μm) is (Dispersion shift fiber; Disper
sion shifted fiber). Further, the optical fiber 2 has a dispersion slope (wavelength dependence of dispersion characteristics) smaller than that of the DSF.

Here, silica (SiO ₂ ) as a material of each layer (core layer 3, first and second cladding layers) in the optical fiber 2, germanium (GeO ₂ ) doped in the core layer 3, and the first layer Table 1 below shows the temperature dependence of the refractive index of each of the boron (B ₂ O ₃ ) doped in the cladding layer 4 of the above.

[Table 1]

That is, the refractive index of germanium doped in the core layer 3 has a positive temperature dependency, and the refractive index of boron doped in the first cladding layer 4 is the positive temperature. It is the same value (absolute value) as the dependency,
It has a negative temperature dependency that cancels the above positive temperature dependency.

The grating type optical component 1 is provided with a long period grating portion 10 formed at a predetermined portion of the core layer 3 along the fiber axial direction (for example, a length along the axial direction is about 25 mm). There is. The length of the long-period grating portion 10 along the fiber axis direction (25 mm in this embodiment) is defined as the grating length.

The long period grating portion 10 is formed as follows. That is, high-pressure hydrogen treatment is performed on the optical fiber 2 including the core layer 3 doped with germanium, the first cladding layer 4 doped with boron, and the second cladding layer 5 doped with fluorine.
Then, with respect to a predetermined portion (grating length 25 mm) in the core layer 3 of the optical fiber 2 after the high-pressure hydrogen treatment, along the longitudinal direction corresponding to the fiber axis direction, it is formed at a constant periodic pitch. The long period grating portion 1 having a grating period (interval) of about 400 μm is obtained by irradiating ultraviolet rays with an argon (Ar) laser through a mask having a slit.
0 is formed on the core layer 3.

FIG. 2 is a diagram schematically showing the dimensions of the optical fiber 2 and the profile of the refractive index distribution (shown by the alternate long and short dash line).

The core layer 3 has a core diameter d1 of about 10 μm, and the first cladding layer 4 has a core diameter of about 60-80.
It has a cladding diameter d2 of μm (the diameter of the first cladding layer 4 including the core layer 3).

The second cladding layer 5 has a thickness of about 125 μm.
It has a cladding diameter d3 of m (the diameter of the second cladding layer 5 including the core layer 3 and the first cladding layer 4).

The refractive index of the core layer 3 and the refractive indexes of the first and second clad layers 4 and 5 change in a stepwise manner.

That is, the core layer 3 is doped with germanium which has a function of increasing the refractive index of the core layer 3 (the refractive index of the silica layer) by being exposed to ultraviolet rays, and the first cladding layer 4 and the second cladding layer 4 are doped. The clad layer 5 is doped with boron and fluorine, which have the function of lowering the refractive index of the corresponding first clad layer 4 and second clad layer 5 (the refractive index of the silica layer) when exposed to ultraviolet light. There is.

In the present embodiment, germanium has a relative refractive index difference Δ with respect to the silica layer of the core layer 3, as shown in FIG.
1 is added to the core layer 3 in an amount necessary for increasing the mode field diameter of the core layer 3 and suppressing the pulse dispersion in the wavelength band of the incident optical signal (for example, 0.35%). Has been done.

Further, as shown in FIG. 2, boron and fluorine have a relative refractive index difference Δ2 of the silica layer with respect to the first cladding layer 4 and the second cladding layer 5, for example, about 0.3.
The first clad layer 4 and the second clad layer 5 are each doped with an amount necessary to achieve the same.

Next, the operation of the fiber grating type optical component 1 and the long period fiber grating optical fiber 2 according to this embodiment will be described.

First, the wavelength cutoff action (filter action) of the fiber grating type optical component 1 will be described.

As shown in FIG. 1, the long-period grating portion 10 formed in the core layer 3 of the optical fiber 2 of the fiber grating type optical component 1 identifies, for example, the optical signal from the right to the left of the core layer in the figure. The optical signal of the propagation mode (fundamental mode) having the wavelength band of
From the first cladding layer 4 to the first cladding layer 4 and propagated through the first cladding layer 4.
1, bind to S2. As a result, the optical signal in the predetermined wavelength band coupled to the optical signals S1 and S2 in the cladding mode is lost, and the passage through the long period grating portion 10 can be cut (blocked).

Therefore, the long period grating portion 10
The fiber type optical component 1 having the above can function as a cut-off filter having a desired wavelength cutoff band (wavelength transmission loss band) or a gain equalizer.

Next, the operation peculiar to this embodiment will be described.

As described above, if the wavelength shift amount based on the temperature change of the central wavelength of the long period grating portion 10 is increased, the fluctuation width of the waveform of the transmission loss wavelength characteristic can be increased, and the gain wavelength in the EDFA is increased. It is possible to deal with the dynamic waveform fluctuation of the characteristic temperature / time change.

In order to increase the temperature dependence of the central wavelength of the long period grating portion 10, the temperature coefficient of the long period grating portion 10 may be increased.

Therefore, the core layer 3 of the long-period fiber grating optical fiber 2 in the fiber grating type optical component 1 of the present embodiment is doped with germanium having a positive temperature-dependent refractive index. The first cladding layer 4 is doped with boron having a refractive index having the same value (absolute value) as the above positive temperature dependence and negative temperature dependence.

That is, since the temperature dependence of the refractive index of the core layer 3 has a positive value and the temperature dependence of the refractive index of the first cladding layer 4 has a negative value, the core layer 3 and the first cladding layer 4 have the same temperature dependence. The difference (absolute value) in the temperature dependence of the refractive index of the cladding layer 4 increases,
As a result, the temperature gradient between the core layer 3 and the first cladding layer 4 can be increased.

Even when the core layer 3 is doped with germanium having a positive temperature-dependent refractive index and the first cladding layer 4 is doped with boron having a negative temperature-dependent refractive index, As shown in FIG. 2, it is possible to set a refractive index profile capable of propagating an optical signal in a propagation mode in the core layer 3.

In this embodiment, the optical fiber 2 for long-period fiber grating in which the difference between the temperature dependence of the refractive index of the core layer 3 and the temperature dependence of the refractive index of the first cladding layer 4 is increased. Thus, the long period grating portion 10 is formed.

As described above, the temperature coefficient of the long-period grating portion 10 has a difference between the temperature dependence of the effective refractive index of the core layer 3 and the temperature dependence of the effective refractive index of the first cladding layer 4 and the grating. Since it can be expressed by the product of the periodic pitch, the temperature coefficient of the long period grating portion 10 can be increased based on the increased temperature dependency between the core layer 3 and the first cladding layer 4.

Here, FIG. 3 shows a result of actually measuring the wavelength shift amount based on the temperature change of the central wavelength of the long period grating portion 10 in the fiber grating type optical component 1 of the present embodiment. As shown in FIG. 3, the wavelength shift amount (tail temperature dependency) based on the temperature change of the central wavelength of the long period grating 10 is about 0.25 nm / ° C.
Can be set to.

A wavelength shift amount (about 0.25 nm / based on the temperature change of the central wavelength of the long period grating 10).
C) is 5 times as much as the wavelength shift amount (about 0.05 nm / ° C.) of the long period grating manufactured by the conventional SMF, and it can be seen that the temperature greatly increases.

FIG. 4 shows the temperature characteristics of the long period grating portion 10 in the fiber grating type optical component 1 of this embodiment (temperature dependence of transmission loss wavelength characteristics;
It is a figure which shows the transmission loss wavelength characteristic of 5 degreeC, 5 degreeC, 25 degreeC, 45 degreeC, and 65 degreeC.

As shown in FIG. 4, the temperature dependence of the central wavelength in the long-period grating portion 10 (temperature-dependent wavelength shift characteristic), that is, the wavelength shift amount due to temperature change is large (about −5 ° C. → 65 ° C.). 18 nm).

Therefore, as shown in FIG. 4, the waveform fluctuation width according to the temperature change of the transmission loss wavelength characteristic of the long period grating portion 10 is set to the dynamic waveform fluctuation based on the temperature change or the time change of the gain wavelength characteristic of the EDFA. Can be set to a value sufficient to compensate for

As a result, by using the fiber grating type optical component 1 according to this embodiment, it is possible to manufacture a dynamic gain equalizer for compensating for a dynamic waveform fluctuation due to a temperature change or a temporal change of the gain wavelength characteristic of the EDFA. .

In the present embodiment, germanium was used as the dope material for the core layer 3, but the present invention is not limited to this, and rather than the refractive index of the base material (silica layer) of the core layer 3. Any material may be used as long as it has a high refractive index and a positive temperature dependency and is sensitive to ultraviolet rays.

Further, although the doping material of the first cladding layer 4 is boron, the present invention is not limited to this, and it is lower than the refractive index of the silica layer which is the base material of the cladding layer 4 and the above-mentioned. As long as the material has a refractive index having a negative temperature dependency capable of canceling the positive temperature dependency, fluorine or the like may be used. The absolute value of the positive temperature dependence of the refractive index of the doped material of the core layer 3 and the negative temperature dependence of the refractive index of the first cladding layer 4 preferably have a large difference, Any value can be used as long as it can increase the difference between the temperature dependence of the refractive index of the core layer 3 and the temperature dependence of the refractive index of the first cladding layer 4.

Further, although the doping material of the second cladding layer 5 is fluorine, the present invention is not limited to this, and any material having a refractive index lower than that of the first cladding layer 4 may be used. Good.

In particular, when the doping material of the second cladding layer 5 is a material having a refractive index lower than that of the first cladding layer 4, the cladding mode of the cladding mode propagating in the first cladding layer 4 is The optical signals S1 and S2 are totally reflected at the boundary surface with respect to the second cladding layer 5 due to the relative refractive index difference between the first and second cladding layers 4 and 5, and propagate in the first cladding layer 4. To do.

As a result, the optical signals S1 and S2 of the cladding mode propagating in the first cladding layer 4 are confined in the first cladding layer 4 without depending on the external environment of the first cladding layer 4. Therefore, it is possible to prevent the influence of the cladding mode of the external environment on the optical signal.

Furthermore, in this embodiment, boron and fluorine are separately doped to form the first cladding layer 4 and the second cladding layer 4.
The clad layer 5 was formed, but the present invention is not limited to this.

For example, a material having a refractive index lower than that of the silica layer and having a negative temperature dependence capable of canceling the positive temperature dependence (boron, fluorine, etc.) is simply doped to form a one-layer structure. It may be a clad layer.

Further, two kinds of materials (boron, fluorine, etc.) having a refractive index lower than that of the silica layer and having a negative temperature dependence capable of canceling the above positive temperature dependence are doped together to obtain 1 It may be a clad layer having a layered structure.

[0059]

As described above, according to the optical fiber for long-period fiber grating and the fiber grating type optical component according to the present invention, the first material having a refractive index having a positive temperature dependence in the core layer is used. And a clad portion covering the core layer is doped with a second material having a negative temperature-dependent refractive index.

Therefore, the temperature dependence between the core layer and the clad portion of the optical fiber for long-period fiber grating can be increased. Therefore, the temperature coefficient of the grating portion formed in the optical fiber for long-period fiber grating can be increased based on the increased temperature dependency between the core layer and the clad portion.

As a result, by using the fiber grating type optical component according to the present invention, it becomes possible to manufacture a dynamic gain equalizer or the like capable of compensating the temperature / time-dependent change of the gain wavelength characteristic of the EDFA.

[Brief description of drawings]

FIG. 1 is a perspective view showing a fiber grating type optical component according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing a profile of a size and a refractive index distribution of the optical fiber shown in FIG.

FIG. 3 is a diagram showing a result of actually measuring a wavelength shift amount based on a temperature change of a central wavelength of a long period grating portion in the fiber grating type optical component shown in FIG.

4 is a diagram showing temperature characteristics of a long-period grating portion in the fiber grating type optical component shown in FIG.

FIG. 5 is a diagram showing the temperature dependence of the center wavelength of a long-period fiber grating using SMF.

FIG. 6 is a diagram showing the temperature dependence of the center wavelength of a long-period fiber grating manufactured using SMF.

[Explanation of symbols]

1: Fiber grating type optical component 2: Optical fiber 3: Core layer 4: First clad layer 5: Second cladding layer 10: Long period grating part

Continued front page    (72) Inventor Hiroshi Kobayashi             2-6-1, Marunouchi, Chiyoda-ku, Tokyo             Kawa Electric Industry Co., Ltd. F-term (reference) 2H050 AB05X AB09Y AB10Y AC36                       AC82 AC84

Claims (5)

[Claims] 1. A core layer doped with a first material, which is sensitive to ultraviolet rays and has a positive temperature-dependent refractive index, and a negative temperature-dependent core layer, An optical fiber for a long-period fiber grating, which is doped with a second material having a low refractive index, and is provided with a cladding portion that covers the core layer. 2. The first material is germanium (Ge)
The core layer is formed by doping a silica layer made of silica (SiO ₂ ) with the germanium (Ge), and the second material is boron (B ₂ O ₃ ) and fluorine (F).
2. The length according to claim 1, wherein the clad portion is formed by doping a silica layer made of the silica with either one of the boron and the fluorine. Optical fiber for periodic fiber grating. 3. The clad portion is doped with the second material, has a negative temperature dependence with the first clad layer covering the core layer, and has a negative temperature dependence of the first clad layer. The long-period fiber grating according to claim 2, further comprising a second cladding layer that is doped with a third material having a refractive index equal to or lower than the refractive index and that covers the first cladding layer. Optical fiber. 4. The third material is the other of the boron and fluorine, and the second cladding layer is formed by doping a silica layer made of the silica with the other of the boron and fluorine. The optical fiber for long-period fiber grating according to claim 3, wherein 5. The optical fiber for long-period fiber grating according to claim 1, and a refractive index of the core layer at a predetermined portion along the fiber axis direction of the core layer of the optical fiber. And a grating portion formed as a periodical change along the fiber axis direction.