JP2002055240A - Photonic crystal fiber and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photonic crystal fiber capable of confining light to the central axis direction. SOLUTION: In the photonic crystal fiber 1 having a multi-hole part 12 in which a large number of small holes 12a extending in the central direction of the fiber are arrayed with highest density and a core part 11 which is formed in a solid shape at the center of the multi-hole part 12, a grating 11a is formed in the direction of the central axis with respect to the core part 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photonic crystal fiber comprising a porous portion having a large number of pores extending in the central axis direction of a fiber, and a solid core formed at the center of the porous portion. About.

[0002]

2. Description of the Related Art In recent years, a photonic crystal fiber has been receiving attention as a material exhibiting a large wavelength dispersion that cannot be obtained with a normal optical fiber comprising a core and a clad. This includes a porous portion having a large number of pores extending in the direction of the central axis of the fiber, and a solid core portion formed at the center of the porous portion.

[0003]

By the way, in the photonic crystal fiber, by arranging pores around the core in a crystal form, a direction perpendicular to the center axis of the fiber (radial direction of the fiber) is obtained. , Light is confined to the core, and this has a feature that the power density in the core portion is extremely high.

In such a photonic crystal fiber, for example, if light can be confined also in the direction of the central axis, it can be considered that it can be effectively used for a fiber laser or the like.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a photonic crystal fiber capable of confining light in a central axis direction.

[0006]

In order to achieve the above object, the present invention is directed to a porous portion having a large number of fine holes extending in the central axis direction of a fiber, and a solid portion provided at the center of the porous portion. A photonic crystal fiber having a core portion formed at a specific point, wherein the core portion has a refractive index periodic structure in which the refractive index periodically changes in the central axis direction. is there.

According to the first aspect of the present invention, if a periodic structure (grating) having a relatively short pitch is formed in the core of the photonic crystal fiber, the inside of the core extends in the direction of the central axis. Among the propagating light, light having a wavelength characteristic corresponding to the pitch of the refractive index periodic structure is reflected in the central axis direction. For this reason, light is confined in the central axis direction of the fiber.

The refractive index periodic structure may be formed, for example, such that the period in which the refractive index changes varies in the direction of the central axis of the fiber. That is, a grating whose pitch is continuously changed (chirped) in the central axis direction may be used.

According to the second aspect of the present invention, the peak width of the wavelength reflected by the periodic refractive index structure increases.

In a chirped grating, the reflection of long-wavelength light can be delayed while the reflection of short-wavelength light can be accelerated, so that it can be used for pulse compression. .

Further, for example, by using a chirped grating to generate a pulse given a dispersion opposite to the wavelength dispersion in the propagation path in advance, the dispersion of the pulse propagating in the propagation path is canceled out (dispersion). Compensation) is also possible.

Here, as described in claim 3, the periodic refractive index structure portion may be doped with a laser active medium, and the laser active medium may be specifically a rare earth element.

Further, the periodic refractive index structure may be formed at a plurality of portions of the core.

According to the fourth aspect of the present invention, for example, a pair of gratings each formed at the same pitch,
If the gratings are formed apart from each other in the central axis direction of the fiber by a distance corresponding to the wavelength characteristic reflected by
A laser oscillator (fiber laser) in which each of the gratings is a mirror is configured. In this case, since the photonic crystal fiber has a very high power density in the core portion, it is possible to reduce the pump input (threshold) required for laser oscillation.

Further, for example, if a pair of chirped gratings are formed in the core portion and one of the pair of chirped gratings is expanded or contracted, the output wavelength is changed because the reflection wavelength changes. A variable and low threshold laser oscillator is constructed.
The expanding and contracting grating is not limited to a chirped grating, and may be a normal grating formed at the same pitch.

Furthermore, a plurality of laser oscillators having different wavelength outputs are provided, for example, by forming a plurality of pairs of gratings (grating pairs) formed at the same pitch on the core portion and making the pitches different for each pair of gratings. Will be. Further, for example, even if a plurality of chirped gratings are formed in the central axis direction such that the directions of the chirps are alternately formed, a grating whose pitch continuously changes in length is formed. Are provided. These can be used, for example, for a laser oscillator capable of multi-wavelength oscillation for a wavelength multiplexed signal,
A laser oscillator capable of multi-wavelength oscillation with a low threshold is configured.

When a photonic crystal fiber is used for a laser oscillator, it is preferable that a laser active medium is doped between the pair of refractive index periodic structures in the core portion. Specifically, the laser active medium may be a rare earth element.

Further, the refractive index periodic structure may be formed at a relatively short pitch as described above to be a filter for selectively reflecting light of a single wavelength, or may be formed at a relatively long pitch. May be configured as a filter that transmits light of a plurality of wavelengths.

The invention according to claim 6 relates to a method for manufacturing a photonic crystal fiber. More specifically, a porous portion having a large number of pores extending in the central axis direction of the fiber, The present invention is directed to a method for manufacturing a photonic crystal fiber having a solid core in the center.

The preform in which a plurality of holes serving as the pores and a solid portion serving as the core and having a photoinduced refractive index change provided at the center of the plurality of hole groups is formed. A preform manufacturing step to be manufactured, a drawing step of heating and stretching the preform to draw a fiber, and after the drawing step, by irradiating a predetermined position of the solid portion with writing light, And an irradiation step of forming a periodic refractive index structure in the real part.

That is, for example, quartz (Si) doped with at least germanium (Ge) and tin (Sn)
A preform in which a solid portion made of O ₂ ) and a hole around the solid portion are formed. Then, after drawing the preform into a fiber shape, a predetermined position of the solid portion serving as the core portion is irradiated with ultraviolet laser light to cause a change in the light-induced refractive index at the predetermined position, and the refractive index period is changed. A structure may be formed.

It should be noted that the solid portion may be filled with hydrogen to improve the sensitivity to the light-induced refractive index change, instead of the solid portion co-doped with Ge and Sn as described above. . In this case, the solid portion may be doped with at least Ge.

In the preform preparation step, the preform may be prepared by filling a cylindrical capillary and a rod-shaped core member into the hole of the cylindrical support tube, or the center of the cylindrical body may be formed. The preform may be manufactured by forming a solid portion that becomes a core portion after forming a fiber and forming a large number of through holes that become fine holes after forming the fiber around the solid portion. Also, a preform may be produced by bundling a cylindrical capillary around a rod-shaped core member in a close-packed state and fusing and integrating the adjacent capillaries with each other and the core member and the capillary. Good.

According to a seventh aspect of the present invention, there is provided a photonic comprising a porous portion having a large number of pores extending in the central axis direction of a fiber, and a solid core portion formed at the center of the porous portion. It is intended for the manufacturing method of crystal fiber.

A preform forming step of forming a preform in which the plurality of holes serving as the pores and the solid portion provided at the center of the plurality of holes as the core portion are formed; A drawing step of heating and stretching the preform to draw a fiber, and after the drawing step,
A doping step of forming a periodic structure of refractive index in the solid portion by ion-implanting or doping a dopant for changing the refractive index at a predetermined position of the solid portion by ion implantation or ion diffusion. It is.

Here, as the dopant for changing the refractive index, for example, as a dopant for increasing the refractive index,
Ge or aluminum (Al) may be used. vice versa,
As a dopant for lowering the refractive index, boron (B)
Alternatively, it may be fluorine (F) or the like.

The ion beam may be irradiated, for example, in the direction of the central axis or in the radial direction of the fiber.

According to any one of the sixth and seventh aspects of the present invention, a periodic refractive index structure can be easily formed in the core of the photonic crystal fiber.

[0029]

As described above, according to the photonic crystal fiber of the present invention, since the refractive index periodic structure is formed in the core, light is confined in the direction of the central axis of the fiber. As a result, it can be effectively used for, for example, a fiber laser.

Further, according to the method for manufacturing a photonic crystal fiber of the present invention, a periodic refractive index structure can be easily formed in a core portion.

[0031]

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

FIG. 1 shows a photonic crystal fiber 1 according to an embodiment of the present invention. The photonic crystal fiber 1 has a core 11 extending in the center axis direction of the fiber and having a solid shape. The porous portion 12 is provided so as to surround the outer peripheral portion of the core portion 11, and has a plurality of fine pores 12 a extending along the core portion 11, arranged in a close-packed manner.
And a support portion 13 provided to cover these
And

The core portion 11 has a periodic refractive index structure (grating) 11a in which the refractive index periodically changes in the direction of the central axis.

In the figure, only the end face of the pore 12a is shown, and other portions of the pore 12a are not shown.

Next, a method of manufacturing the photonic crystal fiber 1 will be described with reference to FIG.

First, a support tube 2 in which a hole 2a having a regular hexagonal cross section is provided along the central axis at the center of a cylindrical body made of SiO ₂ , and a plurality of cylindrical SiO ₂ tubes having the same diameter as each other. A capillary 3 and one rod-shaped SiO ₂ core member 4 having the same diameter as the capillaries 3 are prepared.

The core member 4 includes Sn or S in addition to Ge, which is a dopant for increasing the photo-induced refractive index change.
n and Al, or Sn, Al and B are doped. Such doping may be performed by various known methods, for example, by immersion.

Then, the capillary 3 is filled in the regular hexagonal hole 2a of the support tube 2 in the closest density. At this time, the first layer is formed by arranging the capillaries 3 in parallel with one surface of the inner wall of the regular hexagonal hole 2a, and a pair of adjacent capillaries 3 in the formed first layer is formed. , 3 to form a new second layer. By repeating such mounting of the capillary 3, the capillary 3 is filled in the regular hexagonal hole 2 a in the closest density, but the center position of the regular hexagonal hole 2 a is not a core 3 but a core member. 4 is arranged.

According to the above steps, the photonic crystal fiber 1 in which the capillary 3 is filled in the regular hexagonal hole 2a of the support tube 2 in the closest density and the core member 4 (solid portion) is disposed at the center position. Is completed (preform manufacturing step).

The preform 5 is subjected to a drawing process for heating and melting to reduce the diameter (to form a fiber). At the time of this drawing process, the adjacent capillaries 3, 3; the capillary 3 and the support tube 2; and the capillary 3 and the core member 4 are fused together and integrated (drawing step).

Through the above steps, as shown in FIG. 1, the core portion 11 extends in the center axis direction at the center of the fiber and is formed solid, and the outer periphery of the core portion 11 is formed along the core portion 11. A photonic crystal fiber 1 including a porous portion 12 in which a large number of extending pores 12a are arranged in a close-packed manner and a support portion 13 that covers the porous portion 12 is completed.

Then, as shown in FIG. 3, the grating 11a is formed by irradiating the photonic crystal fiber 1 with ultraviolet rays from the outside.

That is, a lattice-shaped phase mask 6 is provided on the side of the photonic crystal fiber 1, and the phase mask 6 is irradiated with an ultraviolet laser beam. As a result, the ultraviolet laser beam is applied to the phase mask 6, the support unit 13,
And the refractive index of the portion corresponding to the lattice pitch of the phase mask 6 in the core portion 11 is increased to form the grating 11a (irradiation step).

Next, the operation and effect of the above embodiment will be described.

If a grating 11a having a relatively short pitch is formed in the core portion 11 of the photonic crystal fiber 1, for example, the light propagating in the central axis direction in the core portion 11 corresponds to the pitch of the grating 11a. The light having the wavelength characteristic is reflected in the central axis direction.
Therefore, light is confined in the direction of the central axis of the photonic crystal fiber 1. Thus, the photonic crystal fiber 1 can be applied to a filter, a duplexer, a dispersion compensator, a fiber laser mirror, an EDF gain device, an oscillator, a temperature sensor, and the like.

Although not shown, the grating may be a grating whose pitch is continuously changed (chirped) in the direction of the central axis.
In this chirped grating, the peak width of the wavelength reflected by the grating is widened.

In a chirped grating, the reflection of long-wavelength light can be delayed and the reflection of short-wavelength light can be accelerated, so that it can be used for pulse compression.

Further, for example, by using a chirped grating to generate a pulse given a dispersion opposite to the wavelength dispersion in the propagation path, the dispersion of the pulse propagating in the propagation path can be canceled. .

Such a chirped grating may be formed, for example, by using a phase mask in which a phase grating is formed such that the mask pitch continuously increases.

Further, by forming the pitch of the grating 11a to be relatively long, the grating 11a may be configured as a filter that transmits light of a plurality of wavelengths.

In addition, the grating 11a or the chirped grating portion may be doped with a laser active medium. Incidentally, the laser active medium may be, for example, a rare earth element, and as the rare earth element,
For example, erbium (Er), neodymium (Nd), or ytterbium (Yb) may be used.

<First Modification> In the above embodiment, one grating 11a is formed in the core 11 of the photonic crystal fiber 1. For example, as shown in FIG. Grating 11
The laser oscillator 71 may be formed by forming the laser beams a and 11a apart from each other in the central axis direction of the fiber 1.

To construct the laser oscillator 71, the pair of gratings 11a are formed at the same pitch and the same shape, and their reflection peak wavelengths are both λ1
Set to.

The pair of gratings 11
The distance L between a and 11a is set to a distance corresponding to the wavelength λ1, that is, a distance L at which the wavelength λ1 becomes a standing wave.

By doing so, the laser oscillator 71 in which the pair of gratings 11a, 11a are mirrors is provided.
Can be configured.

The laser oscillator 71 using such a photonic crystal fiber 1 has a very high power density in the core portion 11, so that its threshold can be lowered.

The pair of gratings 11a, 11a
The core 11 between 1a is preferably doped with a laser active medium, and the laser active medium may be, for example, a rare earth element. The rare earth element may be, for example, erbium (Er), neodymium (Nd) or ytterbium (Yb).

Although not shown, for example, a pair of chirped gratings separated from each other in the central axis direction are formed on the core portion 11 and one of the chirped gratings is expanded and contracted. If it is configured to make the pitch (extend or contract the pitch), the reflection wavelength changes, so that a laser oscillator having a variable output wavelength and a low threshold value can be configured. The expanding and contracting grating is not limited to a chirped grating, and may be a normal grating formed at the same pitch.

<Second Modification> As shown in FIG. 5, the second modification is a modification of the core 1 of the photonic crystal fiber 1.
A plurality of chirped gratings 13a are formed side by side in the central axis direction of the fiber 1 so that the chirp directions are alternated.

As a result, the reflection peak wavelength becomes λ1 at the position where the pitch of the grating 13a is short, while the reflection peak wavelength becomes λ2 at the position where the pitch is long.
(Λ2 ≠ λ1), and the reflection peak wavelengths are λ1 and λ2.
Are alternately arranged in the central axis direction of the fiber 1.

This is equivalent to arranging two laser oscillators 71 and 72 having different wavelength outputs so that the laser oscillators 71 and 72 partially overlap each other. As a result, for example, a laser oscillator that can perform multi-wavelength oscillation for a wavelength multiplexed signal and has a low threshold can be configured.

In the second modified example, it is preferable that the core portion 11 is doped with a laser active medium.

Although not shown, a plurality of grating pairs, for example, a pair of gratings formed at the same pitch, may be formed in the core portion 11 and the pitch may be different for each grating pair.
Also in this case, a plurality of laser oscillators having different wavelength outputs can be provided.

<Other Embodiments> The present invention is not limited to the above-described embodiments, but includes various other embodiments. That is, in the above embodiment, the gratings 11a and 13a are formed using the phase mask 6, but the present invention is not limited to this.
As a method for forming a and 13a, for example, Ge or Al which is a dopant for increasing the refractive index or a dopant which lowers the refractive index is used for the photonic crystal fiber 1 produced through the preform production step and the drawing step. The gratings 11a and 13a may be formed by periodically changing the refractive index of the core portion 11 by ion-implanting or diffusing certain B or F into a predetermined position of the core portion 11 (doping). Process). At this time, the irradiation direction of the ion beam may be the central axis direction or the radial direction of the photonic crystal fiber 1.

When the gratings 11a and 13a are formed by doping the core 11 with a dopant that changes the refractive index, the core 1
The core member 4 serving as 1 does not have to be doped with a dopant that enhances the photoinduced refractive index change.

The method of forming the gratings 11a and 13a is not limited to the above, and the gratings 11a and 13a may be formed by other known methods.

Further, in the above embodiment, the preform 5 of the photonic crystal fiber 1 is manufactured by filling the capillaries 3 and the core member 4 into the regular hexagon 2a of the support tube 2. The preform 5
The method of manufacturing is not limited to the above. For example, the central portion of a cylindrical body made of SiO ₂ may be a solid portion that becomes the core portion 11 after being formed into a fiber, and the pore 1 may be formed around the solid portion after forming the fiber.
A preform may be manufactured by forming a large number of through holes 2a. In addition, a cylindrical capillary 3 is bundled around the rod-shaped core member 4 in a close-packed state, and the adjacent capillaries 3, 3 and the core member 4 and the capillary 3 are fused and integrated. A preform may be made.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a photonic crystal fiber.

FIG. 2 is a sectional view showing a preform of the photonic crystal fiber.

FIG. 3 is a diagram illustrating an example of a principle of manufacturing a grating.

FIG. 4 is a side view showing a photonic crystal fiber on which a pair of gratings are formed.

FIG. 5 is a view corresponding to FIG. 4 showing a photonic crystal fiber on which a plurality of chirped gratings are formed.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photonic crystal fiber 11 Core part 12 Porous part 13 Support part 12a Pores 11a, 13a Grating (periodic refractive index structure)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masatoshi Tanaka 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industry Co., Ltd. Itami Works (72) Inventor Moriyuki Fujita 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries Inside Itami Works (72) Inventor Toshikazu Gozen 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries Co., Ltd. Inside Itami Works (72) Inventor Kazuo Imamura 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd. Inside the Itami Works (72) Inventor Masataka Nakazawa 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Hirokazu Kubota 2-3-1 Otemachi, Chiyoda-ku, Tokyo Japan (72) Inventor Satoru Kawanishi 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Electric Telephone Company in the F-term (reference) 2H050 AB01Z AB05X AB07X AB09X AB18X AC82 AC84 AD00 5F072 AB08 AB09 AK06 KK30

Claims (7)

[Claims] 1. A photonic crystal fiber comprising: a porous portion having a large number of pores extending in a central axis direction of a fiber; and a solid core portion formed at the center of the porous portion. A photonic crystal fiber, wherein a refractive index periodic structure in which the refractive index periodically changes in the central axis direction is formed in the portion. 2. The photonic crystal fiber according to claim 1, wherein the refractive index periodic structure is formed such that the period in which the refractive index changes varies in the direction of the central axis of the fiber. 3. The photonic crystal fiber according to claim 1, wherein a laser active medium is doped in the refractive index periodic structure. 4. The photonic crystal fiber according to claim 1, wherein the refractive index periodic structure is formed at a plurality of portions of the core portion. 5. The photonic crystal fiber according to claim 4, wherein a laser active medium is doped between the pair of refractive index periodic structures in the core portion. 6. A method for manufacturing a photonic crystal fiber comprising: a porous portion having a large number of pores extending in a central axis direction of a fiber; and a solid core portion formed at the center of the porous portion. A preform for producing a preform in which a plurality of holes serving as the pores and a solid portion serving as the core and exhibiting a light-induced refractive index change provided at the center of the plurality of holes are formed; A manufacturing step, a drawing step of drawing the fiber by heating and stretching the preform, and, after the drawing step, by irradiating a predetermined position of the solid part with a writing light, the solid part is refracted. And an irradiation step of forming a periodic structure. 7. A method for producing a photonic crystal fiber, comprising: a porous portion having a large number of pores extending in a central axis direction of a fiber; and a solid core portion formed at the center of the porous portion. A preform producing step of producing a preform in which a plurality of holes serving as the pores and a solid portion provided at the center of the plurality of hole groups serving as the core are formed; A wire drawing step of heating and stretching to draw a fiber, and after the drawing step, a dopant for changing the refractive index is ion-implanted or ion-diffused at a predetermined position of the solid portion to thereby form the solid. And a doping step of forming a periodic refractive index structure in the portion.