JP2001350036A - Long-period fiber grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a long-period fiber grating, having the characteristics of being small in change of polarization state of the light propagating in a fiber, even if distortion is induced by the impression of external force. SOLUTION: This fiber grating has a core formed with a refractive index regulating structure and a clad enclosing the core. The core is formed of an optically isotropic material, and the clad is formed of an optical isotropic material which is smaller in the product of a photoelastic constant and Young's modulus than that of the core.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a fiber grating in which a diffraction grating (grating) having a periodic refractive index difference is written in a core of an optical fiber. In particular, it relates to a long-period fiber grating in which the grating period is relatively long.

[0002]

2. Description of the Related Art A long-period fiber grating is a fiber-type element having a core on which a refractive index modulation structure having a predetermined grating pitch is formed, and a cladding surrounding the core, and is selected according to the predetermined grating pitch. It has a characteristic of emitting light of a specific wavelength from the core mode to the cladding mode having a larger propagation loss than the core mode, and attenuating the intensity of the light of the specific wavelength by utilizing the loss of the emitted light of the cladding mode. ing. Utilizing this characteristic, long-period fiber gratings are used for various applications such as optical filters and gain equalizers (for example, EDF gain equalizers).

[0003]

The core and cladding of a long-period fiber grating are formed of an optically isotropic material. The optically isotropic material generates birefringence due to strain caused by the application of an external force, and exhibits optical anisotropy (photoelasticity). When the long-period fiber grating is deformed by an external force, the strain generated in the clad is larger than the strain generated in the core because the clad is outside the core. For this reason, the polarization state of the light transmitted through the clad changes more than the light transmitted through the core. Since long-period fiber gratings use the loss of cladding mode light, they are more susceptible to distortion than fiber elements that use core-mode light (eg, short-period fiber gratings). The polarization state of the propagating light tends to change (deterioration of polarization characteristics). Fiber gratings whose polarization characteristics are liable to change are 10 Gb where polarization characteristics are dominant.
It is not suitable for application to ultra-high-speed communication at or above it / s, optical fiber sensors using polarization, coherent optical communication, and the like.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a long-period fiber having a characteristic that a change in polarization state of light propagating through the fiber is small even when an external force is applied to cause a distortion. The purpose is to provide a grating.

[0005]

A long-period fiber grating according to the present invention has a core having a refractive index modulation structure formed thereon, and a clad surrounding the core, wherein the core is formed of an optically isotropic material. The cladding is made of an optically isotropic material having a smaller product of photoelastic constant and Young's modulus than the core.

The above long-period fiber grating is:
It is preferable that a coating layer surrounding the cladding is further provided, and the coating layer is formed of an optically isotropic material having a smaller product of a photoelastic constant and a Young's modulus than the cladding.

[0007] The coating layer may be formed of an optically isotropic material having a refractive index equal to or higher than that of the cladding.

Hereinafter, the operation of the present invention will be described.

In the long-period fiber grating of the present invention, the cladding formed of an optically isotropic material having a smaller product of the photoelastic constant and the Young's modulus than the core has a refractive index caused by a strain caused by the application of an external force. Since the change is small and the occurrence of optical anisotropy is suppressed, the change in the polarization state of light transmitted through the clad is small.

Further, when an external force is applied to a long-period fiber grating in which a coating layer is formed so as to surround the cladding and the fiber grating is deformed, a larger strain is generated in the coating layer than in the cladding. If the coating layer is formed using an optically isotropic material that has a smaller product of the photoelastic constant and Young's modulus than that of the coating layer, the change in the refractive index that occurs in the coating layer is small and the occurrence of optical anisotropy is suppressed. Therefore, the change in the polarization state of the light transmitted through the coating layer is small.

In a long-period fiber grating having a coating layer, if the refractive index of the coating layer is made equal to or larger than that of the cladding, the lower cladding mode of the cladding mode light can be obtained. The transmission of light into the coating increases and the contribution of lower order light to the losses increases. The low-order light has a larger incident angle to the surrounding medium of the coating layer than the light of the higher-order cladding mode, so that it is difficult to transmit to the surrounding medium, and the influence of the change of the surrounding medium (for example, temperature change). Hard to receive. Therefore, a long-period grating provided with a coating layer having a larger refractive index than the cladding,
Light loss is stabilized, and attenuation characteristics of light intensity are stabilized.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following embodiments.

FIG. 1 shows a cross section perpendicular to the axial direction of a long-period fiber grating 100 according to an embodiment of the present invention. FIG. 2 is a sectional view of the long-period fiber grating 100 taken along the line AA ′ in FIG.

As shown in FIG. 1, the long-period fiber grating 100 according to the present embodiment includes a core 10, a clad 12 surrounding the core 10, and a coating layer 14 surrounding the clad 12. As shown in FIG.
As shown in the figure, the refractive index modulation structure 16 having a predetermined pitch L
Are formed. The photoelastic constant of each part of the core 10, the clad 12, and the coating layer 14 is as follows: core 10> cladding 12> coating layer 14, as shown in FIG. In consideration of mechanical strength and the like, it is preferable to provide a coating layer, but the coating layer 14 may be omitted depending on the application.

The propagation modes of light in the long-period fiber grating 100 include a mode (core mode) in which light is confined and propagated in the core 10 by reflection, and a core mode.
And a mode propagating confined in the cladding 12 (cladding mode), and a mode originally radiated.
0, a mode that is confined in the cladding 12 and the coating layer 14 and propagates. Of the light propagating through the long-period fiber grating 100, the light of a specific wavelength selected by the grating pitch L of the refractive index modulation structure 16 formed in the core 10 is a clad having a larger propagation loss than the core mode. Mode radiated,
The intensity of the light of the specific wavelength is attenuated.

The long-period fiber grating 100 includes:
For example, it may be deformed to be mounted on an optical device or the like,
It may be deformed by external force in the usage environment. As shown in FIG. 3, the long-period fiber grating 10
When 0 is deformed, stress (strain) is generated in the core 10, the clad 12, and the coating layer 14. Since this stress increases as the distance from the central axis of the long-period fiber grating 100 increases, the coating layer 14> cladding 12> core 10
Has the relationship Since almost no stress is applied to the central axis of the fiber, the strain generated in the core 10 passing through the central axis of the fiber is much smaller than that of the coating layer 14 or the cladding 12.

The long-period grating 100 of the present embodiment
Since the core 10, the cladding 12, and the coating layer 14 have photoelastic constants of core 10> cladding 12> coating layer 14, when the fiber is deformed, the cladding 12 that generates a larger strain than the core 10. And coating layer 14
The expression of optical anisotropy in is suppressed, and the change in the polarization state of light can be reduced.

Therefore, the difference between the polarization states of the light in the core mode and the cladding mode can be reduced, and the change in the polarization state of the light propagating through the long-period fiber grating 100 can be reduced. Further, by forming the coating layer 14 which is most affected by external force from a material having a small photoelastic constant similarly to the cladding 12, the change in the polarization state of the light of the mode propagating in the coating layer 14 can be reduced, and The change in the polarization state of the generated light can be further reduced.
In particular, the coating layer 1 which is most susceptible to the application of external force
The photoelastic constant of 4 is preferably smaller than that of the cladding 12.

According to the material dynamics, the stress σ (in the direction perpendicular to the axis) generated when a cylinder having a diameter d is bent is proportional to the distance y from the central axis. Therefore, the core 10, the clad 12
If the difference in Young's modulus of the coating layer 14 is ignored, the ratio of the maximum value of the stress generated in each case is represented by the ratio of the radius (the distance from the fiber axis to the outer surface). That is, the maximum stress of the coating layer 14: the maximum stress of the cladding 12: the maximum stress of the core 10 = the radius r ₁₄ of the coating layer ₁₄ : the radius r _{12 of the} cladding ₁₂ : the radius r ₁₀ of the core 10.

Here, since the birefringence caused by photoelasticity is determined by the product of the photoelastic constant and the stress, the birefringence generated in the coating layer 14 and the clad 12 is controlled to be equal to or less than that of the core 10. Has the respective photoelastic constants as C
_Assuming that ₁₄ , C ₁₂ , and C ₁₀ , the photoelastic constant of the coating layer 14 satisfies the relationship of C ₁₄ ≦ (r ₁₀ / r ₁₄ ) · C _{10 and} the photoelastic constant of the cladding 12 from the above-described stress relationship. , C ₁₂ ≦ (r ₁₀ /
It is preferable that the relationship of r ₁₂ ) · C ₁₀ is satisfied. For example, the radius r _{10 of the} core 10 is 5 μm,
When the second radius r ₁₂ is 62.5 .mu.m, the radius r ₁₄ of the covering layer 14 is a 125 [mu] m, photoelastic constant of the coating layer 14, C ₁₄
It is preferable that the relationship of ≦ (1/25) · C _{10 and} the photoelastic constant of the cladding 12 satisfy the relationship of C ₁₂ ≦ (1 / 12.5) · C ₁₀ .

Core 10, clad 12 and coating layer 14
If the difference in Young's modulus cannot be ignored, the maximum value of the stress generated for each is not proportional to each radius,
The maximum value of the strain that occurs in each case is proportional to the respective radius (the distance from the fiber axis to the outer surface).
That is, the maximum strain of the coating layer 14: the maximum strain of the cladding 12: the maximum strain of the core 10 = the radius r of the coating layer 14.
₁₄ : radius r _{12 of the} clad 12: radius r ₁₀ of the core 10

Since the stress is obtained by the product of the Young's modulus and the strain, if the Young's modulus of the coating layer 14, the clad 12, and the core 10 are E ₁₄ , E ₁₂ , and E ₁₀ , the coating layer 1
Photoelastic constants (C ₁₄ , C ₁₂ , C ₁₀ ) for suppressing the birefringence generated in the core 4 and the cladding 12 to the same level or less as the core 10.
The above relationship for satisfies the respective photoelastic constants C ₁₄ ,
The product of K ₁₂ , C ₁₀ and Young's modulus E ₁₄ , E ₁₂ , E ₁₀ (K ₁₄ ,
And K _12, K _10. ), K ₁₄ ≦ (r ₁₀ / r ₁₄ ) · K ₁₀ for the coating layer 14 and K ₁₂ ≦ (r ₁₀ / r ₁₂ ) · K ₁₀ for the cladding 12.

For example, as described above, the radius r _{10 of the} core 10 is 5 μm, and the radius r _{12 of the} cladding 12 is 62.5 μm.
m, a radius r ₁₄ = 125 [mu] m of the coating layer 14, silica based material (photoelastic constant: about 3 × 10 ^-12 / Pa, a Young's modulus: about 7
(× 10 ¹⁰ Pa, K ₁₀ : about 0.21) In the long-period fiber grating having the core 10 formed using the core 10, the conditions for suppressing the birefringence of the cladding 12 and the cladding 14 to about 10 cores or less are as follows. , K ₁₄ ≦ about 0.0084 (0.21 / 25), K ₁₂ ≦ about 0.017
(0.21 / 12.5).

[0024] Usually, since the glass material like the cladding 12 is also the core 10 is used, the range of the Young's modulus E ₁₀ and E ₁₂ are, because it is ^{1 × 10 10 ~1 × 10 11} Pa, photoelastic cladding 12 constant C ₁₂ is a core 10 photoelastic constant C
It must be lower than ₁₀ . For example, when a low photoelastic glass containing lead described later is used, it has a Young's modulus of about 5 × 10 ¹⁰ Pa and is about 0.34 × 10 ⁻¹² / Pa
A clad having a photoelastic constant of (K ₁₂ : about 0.017) or less can be formed. Of course, the relationship of C ₁₂ <C ₁₀ may not be necessary depending on the core diameter or the clad diameter.

The coating layer 14, unlike the core 10 and the clad 12, is often formed using a resin material. The Young's modulus of a resin material, for example, an ultraviolet transmitting resin described later is 1
Since it is very low (less than two orders of magnitude) of about × 10 ^{6 to} 5 × 10 ⁸ compared to glass material, in many cases, even if the relationship of C ₁₄ <C ₁₂ is not satisfied, K ₁₄ <K ₁₂ Satisfaction with the relationship, K
₁₄ ≦ about 0.0084 (for example, E ₁₄ = about 5 × 10
A material that satisfies C ₁₄ = about 17 × 10 ⁻¹² at ⁸ is also readily available.

As described above, the long-period fiber grating 100 according to the present embodiment is a long-period fiber grating having a small change in the polarization state of light due to strain (stress) and having excellent polarization independence. It can be suitably used for ultra-high-speed communication of 10 Gbit / s or more where polarization characteristics are dominant, optical fiber sensors using polarization, and coherent optical communication.

Further, in the long-period fiber grating 100 of the present embodiment, since the change in the refractive index of the cladding 12 due to the strain is small, the refractive index between the core 10 and the refractive index modulation structure 16 written in the core 10 and the cladding 12 are small. The rate difference is hardly affected by strain (stress). Therefore, light of a specific wavelength emitted from the core mode to the cladding mode is emitted to the cladding 12 in substantially the same manner as when no distortion occurs. Therefore, the long-period fiber grating 100 of the present embodiment has stable characteristics as a grating that attenuates the intensity of light of a specific wavelength, and is particularly suitably used as a transmission filter element (such as an optical filter and a gain equalizer). be able to.

In the long-period fiber grating 100 according to the present embodiment, as shown in FIG. 1, the refractive index is such that the core 10> the clad 12, and the coating layer 14
> The cladding 12 is formed.

The fact that the refractive index of the core 10 is larger than the refractive index of the cladding 12 enables light to propagate through the core 10 while repeating total reflection. Further, when the refractive index of the coating layer 14 is larger than the refractive index of the cladding 12, transmission of low-order cladding mode light of the cladding mode light to the coating layer 14 increases, and the loss increases.
The light of the lower-order cladding mode transmitted through the coating layer 14 has a larger incident angle to the medium outside the coating layer 14 than the light of the higher-order cladding mode, and thus transmits through the medium outside the coating layer 14. Hateful. Therefore, the light in the lower-order cladding mode is hardly affected by a change in the outer medium (for example, a change in the refractive index) due to the ambient temperature or the like. Since the loss of the light of the lower-order cladding mode is large among the loss of the light of the cladding mode, the loss of the light of the cladding mode is stabilized, and the attenuation characteristic of the intensity of the light of the specific wavelength is obtained stably.

The long-period fiber grating 100 having the cladding 12 and the coating layer 14 as described above is a transmission type filter element (for example, an optical filter, an EDF gain, etc.) which behaves stably with respect to changes in the surrounding temperature and the outside medium. It is suitable as an equalizer.

Specifically, the refractive index of the core 10 is 1.4
-2.0, and preferably in the range of 1.45-1.9. The refractive index of the cladding 12 is 1.4
To 2.0, and preferably in the range of 1.4 to 1.85. The refractive index of the coating layer 14 is 1.4 to
2.0, and preferably in the range of 1.45 to 1.9. Long-period fiber grating 10 manufactured to have a refractive index within the above range.
At 0, stable core propagation can be achieved.

In the long-period fiber grating 100 according to the present embodiment, lead-silicate glass (SiO ₂ —PbO—
R ₂ O: R is an alkali metal), or Pb-doped quartz glass can be used. In particular, it is preferable to use a Pb-doped glass system.

The core 10 is preferably doped with Ge, Sn, Pb, or the like at a predetermined concentration for the purpose of steadily increasing the change in the refractive index induced by light.

The coating layer 14 has a photoelastic constant smaller than that of the cladding 12 and has a higher refractive index than that of the cladding 12 (for example, urethane acrylate resin, epoxy acrylate resin, silicon resin, (Fluorine-based resin). When the long-period grating is formed of an ultraviolet-transmissive resin, the long-period grating can be effectively written even if ultraviolet light is irradiated from above the coating layer 14 when writing the long-period grating. For removing the coating layer and re-covering after writing can be omitted. Further, there is no danger that the mechanical strength is reduced due to the above-described removal and re-coating.

The grating length is mainly 1 to 100 mm
Is formed within the range of, but is not particularly limited,
It can be set freely according to the application. The grating pitch L is freely set within a range of 50 to 1000 μm according to a desired attenuation wavelength.

In the long-period fiber grating 100 according to the present embodiment, a grating can be applied to an optical fiber manufactured by a conventional method using the above-described materials by a known method (for example, an intensity modulation mask method, a laser direct writing method, It is manufactured by writing with a width slit mask method).

[0037]

According to the present invention, it is possible to obtain a long-period fiber grating having a characteristic that a change in polarization state of light propagating through a fiber is small even when an external force is applied to cause a distortion. The long-period fiber grating of the present invention
It can be suitably used for ultra-high-speed communication of 10 Gbit / s or more, in which polarization characteristics are dominant, optical fiber sensors using polarization, and coherent optical communication.

Further, the long-period fiber grating of the present invention has a stable characteristic as a grating for attenuating the intensity of light of a specific wavelength, so that it is particularly used as a transmission type filter element (such as an optical filter and a gain equalizer). It can be suitably used.

[Brief description of the drawings]

FIG. 1 is a sectional view perpendicular to the axial direction of a long-period fiber grating according to an embodiment of the present invention, and a graph showing a photoelastic constant and a refractive index of each part.

FIG. 2 is a cross-sectional view of a long-period fiber grating according to an embodiment of the present invention.

FIG. 3 is a cross-sectional view when an external force is applied to the long-period fiber grating according to the embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 10 core 12 clad 14 coating layer 16 refractive index modulation structure 100 long-period fiber grating

Claims (3)

[Claims] 1. A core having a refractive index modulation structure formed thereon and a clad surrounding the core, wherein the core is formed of an optically isotropic material, and the clad is more light-emitting than the core. A long-period fiber grating made of an optically isotropic material having a small product of elastic constant and Young's modulus. 2. The semiconductor device according to claim 1, further comprising a coating layer surrounding the cladding, wherein the coating layer is formed of an optically isotropic material having a smaller product of photoelastic constant and Young's modulus than the cladding. 2. The long-period fiber grating according to 1. 3. The long-period fiber grating according to claim 2, wherein the coating layer is formed of an optically isotropic material having a refractive index equal to or larger than that of the cladding.