JP2010008900A - Long-period fiber grating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a long-period fiber grating of a small size. <P>SOLUTION: The long-period fiber grating device includes an optical fiber 11, a grooved support 12 which has grooves of a periodic structure formed therein and makes the grooves of the periodic structure abut along the axial direction of the optical fiber, a grooveless support 13 which has a flat external surface and makes the flat external surface abut on the optical fiber along its axial direction. and a tubular fiber pressure contact member 14 which holds the optical fiber pinched by the grooved support and the grooveless support, and fixes the optical fiber integrally in a pressure contact state. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to a fiber grating device in which a diffraction grating (grating) based on a refractive index profile of a periodic structure is formed in a core portion of an optical fiber, and more specifically, a long-period fiber grating (LPFG) using a photoelastic effect. The present invention relates to a deviceized long-period fiber grating device.
Here, the “long-period fiber grating” specifically refers to a fiber grating having a refractive index distribution period of about 0.1 mm to 1 mm.

The long-period fiber grating is a fiber-type element including a core whose refractive index periodically changes at a pitch of 0.1 mm to 1 mm and a clad surrounding the core.
In a long-period fiber grating whose refractive index changes periodically with a pitch in the above-mentioned range, the core mode that guides the core in the positive direction and the cladding mode that guides the cladding in the positive direction are optically coupled. Therefore, it has the property that the light of a specific wavelength can be lost.

In general, the wavelength lost due to the coupling between the core mode and the cladding mode of the long-period fiber grating is given by the following equation (1).
^{_{λ = Λ (n e-co}} -n m e-cl) ··· (1)
here,
λ: center wavelength of loss peak Λ: period of grating (refractive index) (0.1 mm to 1 mm)
n _e-co : equivalent refractive index of core mode n ^m _e-cl : equivalent refractive index of clad mode (m is the order of the clad mode)

According to the above formula (1), the loss peak is formed by a combination of the core mode and the cladding mode of each order, and a loss characteristic as shown in FIG. 6, for example, is obtained.
Long-period fiber gratings can be used as devices such as optical filters (band-rejection filters, band-pass filters, etc.) and gain flatteners for optical amplifiers using such loss characteristics, or erbium-doped fiber amplifiers ( It is used to suppress spontaneous emission light (ASE) of (EDFA).

Such a long-period fiber grating has been manufactured by several methods so far.
One of them is disclosed that a perturbation of the refractive index is formed in the core by irradiating the core to which Ge is added with ultraviolet rays through a mask in which a mask pattern having a predetermined interval is formed (patent). Reference 1).
As another method, it is disclosed that a fiber grating is formed by forming a periodic refractive index variation region using arc discharge (see Patent Document 2).

As another method, as shown in FIG. 7, a long-period fiber grating 50 using the photoelastic effect is configured by sandwiching an optical fiber 51 between a grating plate 52 and a flat plate 53 and applying a load with a weight 54. Is disclosed (see Patent Document 3).
Japanese Patent Laid-Open No. 10-170736 Japanese Patent Laid-Open No. 10-142212 US Pat. No. 5,260,823

The long-period fiber grating changes the grating period (Λ) based on the equation (1), or changes the core mode equivalent refractive index (n _e-co ) and (m-order) clad mode equivalent refractive index (n ^m _{e− The} absorption characteristic (loss characteristic) can be changed by changing the difference from _cl ).
Therefore, by utilizing this property aggressively, the grating period (lambda) and the equivalent refractive index difference _{(n e-co -n m e} -cl) was adjusted to a desired value, or variable, absorption A device having a (loss) wavelength at a desired value can be made, or a tunable device can be made.

  In particular, long-period fiber gratings, which are formed by contacting the grating plate with a desired pitch on the optical fiber using the photoelastic effect, can change the pitch of the periodic structure of the grating plate and change the grating period with high accuracy. Since (Λ) can be easily given to an optical fiber, it has excellent loss characteristic controllability.

On the other hand, in the long-period fiber grating using the photoelastic effect, it has been difficult to reduce the size and make the device because the load due to the weight has been applied to the grating plate until now.
That is, the diameter of a general fiber grating composed of a core and a clad is about 125 μm, and is about 250 μm including the outer coating. On the other hand, in order to stably press-contact with a planar grating plate with a weight, it is necessary to mount the weight using a grating plate having a certain width.
Furthermore, in order to commercialize a device with a long period fiber grating, it is necessary to protect the optical fiber from damage.

  Accordingly, an object of the present invention is to provide a long-period fiber grating device, which is a fiber type device, as a device having a structure that is downsized and hardly damaged.

The long-period fiber grating device of the present invention, which has been made to solve the above problems, includes an optical fiber having a core and a clad surrounding the core, and grooves having a periodic structure with a pitch of 0.1 mm to 1 mm formed on the outer surface, A grooved support that abuts the groove of the periodic structure along the axial direction of the optical fiber, and a flat support that has a flat outer surface and abuts the flat outer surface along the axial direction of the optical fiber. And a tube-like fiber pressure contact member that is integrally fixed in a pressure contact state so that the optical fiber is sandwiched between the grooved support body and the grooveless support body.
Here, the optical fiber is not particularly limited as long as a core mode and a clad mode are generated. Moreover, you may provide a coating layer in the outer side of a clad.
The grooved support is preferably a metal material that can be easily grooved with a periodic structure, but may be other than metal as long as it is a material that can be grooved and can be pressed against the optical fiber. The support without grooves may be any material that can be pressed against the optical fiber.
Further, as the fiber pressure contact member, a tube-shaped member that can pressurize these supports from the outside of the support with grooves and the support without grooves is used. For example, an elastic material such as a rubber tube can be used.

  According to the present invention, the optical fiber is sandwiched between the grooved support body and the grooveless support body, and the grooved support body, the grooveless support body, and the optical fiber are integrated into a tube-like fiber pressure contact member. The cover is fixed so that the optical fiber is periodically pressurized by the grooved support.

  According to the present invention, since the grooved support body and the optical fiber can be press-contacted without using a weight, a small and compact long-period fiber grating device can be formed. In addition, since the optical fiber is sandwiched between the grooved support and the grooveless support, the optical fiber is protected by the support, and mechanical damage can be prevented.

(Means and effects for solving other problems)
The fiber pressure contact member may be made of a heat shrinkable tube. Thereby, while being able to process easily, it can press-contact with the contraction force of a heat contraction tube.

  The grooved support body and the grooveless support body may be formed of a linear bar. Thereby, a linear long-period fiber grating device can be formed, and can be provided as a long-period fiber grating device having a thin shape.

In this case, the optical fiber may be integrally fixed in a pressure contact state so as to be sandwiched between one grooved support and a plurality of grooveless supports. Although the optical fiber is surrounded by a total of three or more supports and the thickness increases, it can be provided as a device having a stable shape.
In this case, the optical fiber may be covered with a tubular heat shrink member. If the diameter of the optical fiber is small, using metal rods that are sufficiently thicker than the optical fiber will cause the metal rods to come into contact with each other. It becomes difficult to press. Therefore, by covering the optical fiber with a tube-shaped heat-shrinkable member and making the diameter of the optical fiber sufficiently thick, the heat-shrinkable member can be pressed with a metal rod so that the optical fiber can be pressurized. become.
In addition, there is an advantage that loss characteristics (absorption characteristics) can be greatly changed by changing the refractive index of the heat shrinkable tube material in various ways by directly covering the optical fiber with the heat shrinkable tube. can get.

  The grooved support may be made of metal. In addition to being used as a support, the pitch of the periodic structure can be changed by giving a temperature change to the metal, so that the loss wavelength can be made variable.

  The grooved support may be made of a piezo element. The piezoelectric element can change its axial length and thickness by applying a voltage, so the pitch of the periodic structure can be changed and the pressure state can be changed. And loss characteristics can be made variable.

The grooved support body is formed of a cylindrical or columnar shaft core member, and a groove having a periodic structure that is uneven in the circumferential direction is formed on the outer peripheral surface of the shaft core member. The outer peripheral surface of the member is spirally wound, and the non-grooved support is in contact with the outer side of the spiral optical fiber and a notch is formed along the axial direction to change the diameter. The optical fiber may be press-contacted between the shaft core member and the cylindrical member so as to be formed of a cylindrical member that can be formed, and the fiber pressing member is covered from the outside of the cylindrical member.
As a result, a long-period fiber grating device can be made for an optical fiber that is longer than an optical fiber simply extended in a straight line, so that a deep absorption device can be formed for the loss wavelength.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Here, an example in which a long-period fiber grating device is used as an optical filter will be described.

  FIG. 1 is a view showing a long-period fiber grating device 10 according to an embodiment of the present invention, FIG. 1 (a) is an external view thereof (partially cut away), and FIG. 1 (b) is an optical fiber. The enlarged view of a part and FIG.1 (c) are sectional drawings.

  The long-period fiber grating device 10 mainly includes an optical fiber 11, a metal rod 12 having a groove having a periodic structure formed on the outer peripheral surface, a metal rod 13 having a flat outer peripheral surface, and a heat-shrinkable tube 14.

The optical fiber 11 includes a core layer 11a, a clad layer 11b, and a covering layer 11c that protects them.
The metal rod 12 has a cylindrical shape, and spiral grooves are formed on the outer peripheral surface at a pitch in the range of 0.1 mm to 1 mm. The grooves are in contact with the optical fiber 11. The reason why the rod shape is cylindrical is that it is easy to process the spiral groove, but if the ease of processing is not taken into account, instead of this, for example, a periodic groove was formed on a rod having a square cross section. It may be a thing.

The metal rod 13 has a cylindrical shape like the metal rod 12, and its outer peripheral surface is flat.
The diameters of the metal rods 12 and 13 are not particularly limited as long as they can contact and pressurize the optical fiber 11, but are preferably the same diameter or larger than the optical fiber.
As the material of the metal rods 12 and 13, aluminum, copper, etc. are easy to use. However, when it is desired to suppress the variation of the loss characteristics by suppressing the influence of temperature change, the thermal conductivity is small and the thermal expansion coefficient is relatively small. It is preferable to use a metal. When the loss characteristic of the superperiod grating device 10 is desired to be variable by changing the temperature, the metal rods 12 and 13 are made of a material with good thermal conductivity, or the metal rods 12 and 13 themselves have a thermal expansion coefficient. The pitch of the periodic structure may be changed by using a high material.

As the heat-shrinkable tube 14, for example, it is preferable to use a heat-shrinkable tube (trade name: “reinforcing heat sleeve”, for example, manufactured by Nitto Kogyo Co., Ltd.) used to reinforce the connection portion after connecting an optical fiber. However, it is not limited to this. The heat-shrinkable tube is required to have a certain degree of contractibility, and the metal rod 12 is pressed against the optical fiber 11 by this contraction force so that a photoelastic effect is generated.
In addition, in order to make the entire cross-sectional shape close to a circle, a molding buffer layer 15 may be further provided. The buffer layer 15 can be filled with a deformable powder material, for example. Further, in order to fix the optical fiber 11 and the metal rod, a buffer layer 15 in which UV resin or the like is solidified may be used.
If the buffer layer 15 is not provided, the shape of the side surface is uneven, so that it does not look good, but that may be practical.

  In the long-period fiber grating 10 having the above-described configuration, the optical fiber 11 is sandwiched between the metal rod 12 and the metal rod 13, and the heat shrinkable tube 14 pressurizes the optical fiber 11 to such an extent that it is not damaged. As a result, a force of a periodic structure is applied to the optical fiber 11 to function as a long-period fiber grating due to the photoelastic effect.

2 and 3 are sectional views (corresponding to FIG. 1c) showing another embodiment of the long-period fiber grating of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted.
In FIG. 2, one metal rod 12 having a periodic groove formed on the outer peripheral surface and two metal rods 13 having a flat outer peripheral surface are placed inside the heat shrinkable tube 14. The optical fiber 11 is pressurized.
In FIG. 3, one metal rod having a periodic groove and three metal rods 13 having a flat outer peripheral surface are placed inside the heat shrinkable tube 14, and the optical fiber 11 is formed by these four metal rods. Is pressurized.
In the example of FIGS. 2 and 3, the overall diameter is slightly thick, but the optical fiber 11 is surrounded by a metal rod, and the optical fiber 11 is protected by these, so that the structure is not easily damaged.

Further, when the diameter of the optical fiber 11 is thin (for example, in the case of an optical fiber composed of a core and a clad without a coating layer, the diameter is about 0.125 mm), the metal rod 12 having a sufficiently thicker diameter than the optical fiber 11, When 13 (for example, a diameter of about 1 mm) is used, the metal rods 12 and 13 come into contact with each other, and the optical fiber 11 moves around in the central space between the metal rods 12 and 13 and it is difficult to pressurize. Become. Therefore, as shown in FIG. 4, by covering the outer periphery of the optical fiber 11 with a tubular heat-shrinkable member 14 a, the heat-shrinkable member 14 a is made of metal rods 12, 13 by sufficiently increasing the overall diameter. So that the optical fiber 11 can be pressurized via the heat shrinkable member 14a.
Further, by making the structure in which the heat shrinkable tube is directly covered on the optical fiber 11, the loss characteristic (absorption characteristic) can be greatly changed by changing the refractive index by changing the heat shrinkable tube material. There are also benefits.

  In the above embodiment, the metal rod 12 is used as a support member that is pressed against the optical fiber 11. May be. In this case, since the pressure state can be changed by the piezo element, the loss characteristic can be made variable.

FIG. 5 is a long-period fiber grating 20 showing another embodiment of the present invention, FIG. 5 (a) is an external view, and FIG. 5 (b) is a cross-sectional view.
In this embodiment, a long-period fiber grating is formed by spirally winding an optical fiber over a relatively long distance. When the length of the fiber grating is increased, the absorption at the absorption wavelength (loss wavelength) can be further increased.

The long-period fiber grating 20 includes an optical fiber 21, an axial core member 22, a cylindrical member 23, and a heat shrinkable tube 24.
The shaft core member 22 is made of a cylindrical (or columnar) metal, and a groove 22a and a convex portion 22b extending in the axial direction are formed on the outer peripheral surface thereof. The grooves 22a and the protrusions 22b are periodically formed at a pitch of 0.1 mm to 1 mm in the circumferential direction. When the optical fiber 21 is wound around the outer peripheral surface, the optical fiber 21 comes into periodic contact with the shaft core member 22 at regular intervals.

  A metal cylindrical member 23 having a flat inner surface is formed outside the optical fiber 21. The cylindrical member 23 is formed with a notch 25 along the axial direction and can be deformed by being pressed from the outside.

  A heat-shrinkable tube 24 is wound around the outer side of the cylindrical member 23 so that the cylindrical member 23 is in pressure contact. As a result, the optical fiber 21 comes into pressure contact with the cylindrical member 23 and the shaft core member 22, and the long-period fiber grating 20 utilizing the photoelastic effect is configured.

  With the above configuration, it is possible to form a long-period fiber grating in which an optical fiber is elongated to some extent, and it is possible to provide a device having a large absorption at an absorption wavelength (loss wavelength).

  The present invention can be used to manufacture a small and compact long-period fiber grating.

The figure which shows the structure of the long-period fiber grating device which is one Embodiment of this invention. The figure which shows the structure of the long-period fiber grating device which is other one Embodiment of this invention (only sectional drawing). The figure which shows the structure of the long-period fiber grating device which is other one Embodiment of this invention (only sectional drawing). The figure which shows the structure of the long-period fiber grating device which is other one Embodiment of this invention (only sectional drawing). The figure which shows the structure of the long-period fiber grating device which is other one Embodiment of this invention. The figure which shows an example of the loss characteristic of a long period fiber grating device. The figure which shows the specific example of the conventional long period fiber grating device.

Explanation of symbols

10: Long period fiber grating device 11: Optical fiber 12: Metal rod (support with groove)
13: Metal rod (no groove support)
14: Heat-shrinkable tube 20: Long-period fiber grating device 21: Optical fiber 22: Shaft core member (grooved support)
23: Cylindrical member (support without groove)
24: Heat shrinkable tube

Claims (8)

An optical fiber with a core and a cladding surrounding the core;
Grooves having a periodic structure with a pitch of 0.1 mm to 1 mm are formed on the outer surface, and a grooved support that abuts the grooves with the periodic structure along the axial direction of the optical fiber;
A grooveless support that has a flat outer surface and abuts the flat outer surface along the axial direction of the optical fiber;
A long-period fiber grating device comprising: a tube-shaped fiber pressure contact member that is integrally fixed in a pressure contact state so that the optical fiber is sandwiched between the grooved support body and the grooveless support body. The long-period fiber grating device according to claim 1, wherein the fiber pressure welding member is formed of a heat shrinkable tube. The long-period fiber grating device according to claim 1 or 2, wherein the grooved support body and the grooveless support body are formed of linear rod bodies.   4. The long-period fiber grating device according to claim 3, wherein the optical fiber is fixed integrally in a pressure contact state so as to be sandwiched between one grooved support and a plurality of grooveless supports.   The long-period fiber grating device according to claim 4, wherein the optical fiber is covered with a tubular heat-shrinkable member.   The long-period fiber grating device according to claim 1, wherein the grooveless support is made of metal.   The long-period fiber grating device according to claim 1, wherein the non-grooved support is made of a piezo element. The grooved support is formed of a cylindrical or columnar shaft core member, and a groove having a periodic structure that is uneven in the circumferential direction is formed on the outer peripheral surface of the shaft core member.
The optical fiber is spirally wound around the outer peripheral surface of the shaft core member,
The non-grooved support is formed by a cylindrical member that abuts so as to cover the outer side of the spiral optical fiber and has a notch formed in the axial direction so that the diameter can be deformed.
2. The long-period fiber grating device according to claim 1, wherein the optical fiber is pressure-welded between the shaft core member and the cylindrical member so that the fiber pressure-welding member covers from the outside of the cylindrical member.

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