JP3811428B2 - Photonic crystal fiber - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a photonic crystal fiber that is attracting attention as a next-generation optical fiber in the field of information communication.
[0002]
[Prior art]
An optical fiber capable of transmitting a large amount of information is indispensable for establishing an infrastructure in the information and communication field, but it is difficult to cope with a dramatic increase in the amount of information in the future with conventional optical fibers. It has become to.
[0003]
For this reason, recently, an optical fiber called a so-called photonic crystal fiber or holey fiber has attracted attention as a new optical fiber having a possibility of breaking the limitations of conventional optical fibers.
[0004]
As shown in FIG. 8, this new optical fiber is provided with a number of holes b extending along the longitudinal direction inside a glass fiber a made of SiO ₂ glass, and its cross-sectional structure is a hole b. It has a fine structure of the optical wavelength order consisting of the inner air and the cladding layer c. When the microstructure is periodic as shown in FIG. 1A, it is a form of photonic crystal having a periodic structure of refractive index, and is called a photonic crystal fiber. Also, as shown in FIG. 5B, a holey fiber having a core layer d having a high refractive index at the center of the fiber and having a plurality of holes b regardless of whether there is periodicity around the core layer d. is called.
[0005]
[Problems to be solved by the invention]
By the way, these conventional photonic crystal fibers and holey fibers have the following problems and have not yet been put into practical use.
[0006]
(1) The loss is still too large (top data: about 0.6 dB / km) compared to the loss of a normal optical fiber (about 0.2 dB / km). One of the biggest causes of this loss is an absorption loss due to hydroxyl groups (OH ions) at a wavelength of 1.39 μm.
[0007]
(2) In particular, the absorption loss due to the OH ions tends to increase as the size of the holes b increases. That is, as a method of constructing the hole b, a preform is formed by inserting a number of smaller quartz tube glasses into the quartz glass tube, and the preform is drawn while being heated at a high temperature. This is because OH ions easily enter the glass fiber a from the holes b during the production, and the invaded OH ions easily remain in the fiber a as they are.
[0008]
(3) Even if the intrusion of the OH ions is suppressed at the time of manufacture, there is a possibility that OH ions may diffuse and infiltrate from the inner surface of the holes b due to long-term use, which causes a problem in terms of reliability. is there.
[0009]
Therefore, the present invention has been devised in order to effectively solve such problems, and the object thereof is a novel photonic crystal capable of effectively suppressing an increase in transmission loss due to the mixing of hydroxyl groups. A fiber is provided.
[0010]
In order to solve the above problems, the present invention provides a preform by bundling a number of small glass tubes around a high refractive index glass rod constituting the core portion, and drawing the wire to a predetermined diameter while melting and integrating the preforms. In the photonic crystal fiber obtained as described above, SiO ₂ In order to propagate signal light in a substantially axial center portion of the glass fiber consisting of, Ge, SiO ₂ was added one at least P, Al, and Ti And it has a core portion made of, SiO its a plurality of holes extending in the longitudinal direction around the core portion, and the fluorine on the inner surface of each of these holes, the amount of addition was added in 4 mol% or more 1 mol% ₂ Those having a Soraanamaku consisting.
[0011]
This makes it possible to control the shape of the vacancies at the time of drawing , and even if hydroxyl groups enter the vacancies during manufacture or use, the vacancies formed by adding fluorine on the inner surface of the hydroxyl groups into the fiber Diffusion penetration can be reliably prevented. As a result, it is possible to significantly reduce the absorption loss due to OH ions of the optical signal that leaks and propagates into the air holes as well as within the fiber.
[0012]
Further, as shown in claim 2, SiO ₂ in which fluorine is added around the core portion. By providing a core layer made of, it is possible to suppress the intrusion of OH ions to the core portion of high refractive index, to achieve a lower loss fiber.
[0014]
Further, as described in claim 3 , if the outer peripheral portion of the glass fiber is covered with a higher melting point SiO ₂ glass layer, the fiber can be easily produced as described later.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Next, a preferred embodiment for carrying out the present invention will be described with reference to the accompanying drawings.
[0016]
FIG. 1 is a perspective view of a photonic crystal fiber 1 according to the present invention, and FIG. 2 is an enlarged sectional view thereof.
[0017]
As illustrated, the photonic crystal fiber 1, the glass fiber 2 axial center portion in the high refractive index of the core portion 3 made of SiO ₂ glass, i.e. Ge, P, Al, a dopant to increase the refractive index such as Ti A core portion 3 made of SiO ₂ added with at least one kind is provided, and a plurality of holes 4 extending in the longitudinal direction around the core portion 3 are formed in a hexagonal lattice at the same pitch interval.
[0018]
In addition, a pore film 5 made of SiO ₂ to which fluorine is added is provided on the inner surface of each of the pores 4 so that the inner surface is not in direct contact with the outside air.
[0019]
In the photonic crystal fiber 1 of the present invention having such a structure, since the inner surface of each of the holes 4 is covered with the hole film 5, it is assumed that the inside of the holes 4 together with the air during use. Even if OH ions flow into the fiber, they are blocked by the pore film 5 and do not diffuse or penetrate into the fiber. Further, when the inventive fiber 1 is manufactured by a high temperature process, OH ions contained in the glass material react with fluorine in the pore film 5 and are released to the outside as hydrogen fluoride (HF). As a result, the content of the hydroxyl group itself originally contained in the glass material can be reduced. As a result, it is possible not only to suppress the absorption loss due to the hydroxyl group of the optical signal passing through the fiber, but also to effectively suppress the absorption loss due to the OH ion of the optical signal that leaks into the hole 4 and propagates. Transmission efficiency can be demonstrated. At the same time, the presence of the pore film 5 makes it possible to make the inner diameter of the pore 4 uniform, so that the effect of reducing the structural non-uniformity that causes optical scattering can be expected.
[0020]
Here, the addition amount of fluorine in the pore film 5 is in the range of 1 mol% to 4 mol% so as to be lower than the refractive index of SiO ₂ by about −0.5% to −1.4%. It is preferable to do. That is, when the amount added is less than 1 mol%, it is difficult to remove OH ions contained in the glass material during fiber production. This is because the shape of 4 becomes difficult to control during fiber drawing.
[0021]
The wavelength of the light guided in the fiber can be determined by the diameter dc of the holes 4, the interval sc of the holes 4, and the diameter dw of the core layer 3. The number of the holes 4 is 60 in the present embodiment, but the number can be increased or decreased as appropriate. The arrangement of the holes 4 is not limited to a hexagonal lattice if the holes 4 are located at equal intervals. Alternatively, an octagonal lattice may be used.
[0022]
3 to 7 show another embodiment of the photonic crystal fiber 1 according to the present invention.
[0023]
First, the photonic crystal fiber 1 shown in FIG. 3 and FIG. 4 includes a core having a polygonal cross section or a circular shape made of SiO ₂ to which fluorine is added around the core portion 3 in addition to the configuration of the above embodiment. A film 6 is provided. Thereby, it becomes possible to strengthen the confinement of the optical power in the core portion 3 having a high refractive index, and it is possible to realize a fiber with lower scattering loss and lower absorption loss due to OH ions. The amount of fluorine added to the core film 6 is preferably larger than the amount of fluorine added to the pore film 5. The reason is that the OH ions contained in the core part 3 are removed as much as possible during the fiber fabrication to form a low OH ion core part, thereby reducing the loss in the 1.3 μm band and 1.55 μm wavelength band. This is for the purpose of making it easier.
[0024]
Next, FIG. 5 shows an embodiment of a holey fiber that is one type of the photonic crystal fiber 1 according to the present invention. Six holes 4 are formed around the core portion 3 of the axial center. These are provided with the pore film 5 made of SiO ₂ to which fluorine is added as described above on the inner surface of each of the pores 4. Also in this case, similarly to the above embodiment, diffusion / penetration of OH ions into the fiber can be effectively suppressed, and a holey fiber with little absorption loss can be realized. Even in such a configuration, it is more effective if the periphery of the core portion 3 is covered with a core film 6 made of SiO ₂ to which fluorine is further added, as in the above embodiment. The number of surrounding holes 4 may be increased or decreased as appropriate.
[0025]
Next, FIG. 6 is intended, further comprising a glass layer 7 made of SiO ₂ on the outer circumference of the glass fiber 2, it is possible to easily create fiber by such a configuration. That is, in order to fabricate the photonic crystal fiber 1 of the present invention having the above-described configuration (see FIG. 1), it corresponds to each hole 4 around the high refractive index glass rod constituting the core portion 3. It is obtained by bundling a large number of small-diameter glass tubes with a fluorine film formed on the inner surface to form a preform with a circular cross section, and drawing it to a predetermined diameter while melting and integrating it in a high-temperature electric furnace. However, it is not easy to bundle the small-diameter glass tubes into a perfect circle, but there is an inconvenience that they are easily scattered or displaced. Therefore, it is easy to use a glass tube with a high melting point having an inner diameter substantially equal to the outer diameter of the preform in advance, and insert a glass rod into the axial center of the tube and a thin glass tube to fill the periphery. In addition, it is possible to obtain a preform having a perfectly circular cross section, and a fiber can be easily produced by drawing this together with a glass tube having a high melting point. The glass tube constituting the glass layer 7 has a high melting point, that is, its softening temperature is higher than that of a glass rod or a small-diameter glass tube, thereby suppressing fiber shape fluctuations during drawing. be able to.
[0026]
Further, in addition to the configuration of each embodiment described above, a coating material 8 made of a polymer material, a carbon material, a metal material, or the like may be coated on the outer periphery of the fiber as shown in FIG. In addition, if a plurality of fibers according to the above embodiment are bundled, it is possible to provide a high-quality bundle fiber with extremely low transmission loss. Further, the cross-sectional shape of the fiber is not limited to a perfect circle, and may be an ellipse or a polygon. Further, an appropriate amount of fluorine may be added into the glass fiber.
[0027]
【The invention's effect】
In short, according to the present invention, the following excellent effects are exhibited.
[0028]
(1) Since the inner surface of the hole is covered with a fluorine-added film, it is possible to prevent OH ions from diffusing and penetrating from the hole into the fiber during high-temperature processes, etc. Can be diffused and released. As a result, the content of OH ions causing an increase in transmission loss is greatly reduced, and excellent transmission efficiency can be exhibited.
[0029]
(2) Also, by forming a film on the inner surface of the hole, it is possible to keep the inner surface of each hole uniform and the inner diameter uniform, and optical characteristics (broadband characteristics, large structural dispersion characteristics, etc.) Can be further improved.
[0030]
(3) By configuring the core part with SiO ₂ to which at least one dopant such as Ge, P, Al or the like is added, it becomes possible to efficiently confine and propagate the optical signal in the core part, thereby further improving the transmission efficiency. Can be made.
[0031]
(4) Covering the periphery of the core portion with a film to which fluorine has been added prevents the intrusion of OH ions into the core portion, so that an increase in transmission loss in the core portion can be effectively prevented.
[0032]
(5) A fiber can be easily manufactured by providing a glass layer having a high melting point around the glass fiber.
[Brief description of the drawings]
FIG. 1 is a perspective view showing an embodiment of a photonic crystal fiber according to the present invention.
FIG. 2 is an enlarged cross-sectional view of the photonic crystal fiber of FIG.
FIG. 3 is an enlarged cross-sectional view showing an embodiment in which a core film having a polygonal cross section is provided around a core portion.
FIG. 4 is an enlarged cross-sectional view showing an embodiment in which a core film having a circular cross section is provided around a core portion.
FIG. 5 is an enlarged sectional view showing an embodiment of a holey fiber.
FIG. 6 is an enlarged cross-sectional view showing an embodiment in which a glass layer having a high melting point is provided around a glass fiber.
FIG. 7 is an enlarged cross-sectional view showing an embodiment in which a coating material is provided on the outermost surface of a glass fiber.
FIG. 8A is an enlarged cross-sectional view showing an embodiment of a conventional photonic crystal fiber.
(B) is an expanded sectional view which shows one Embodiment of the conventional holey fiber.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Photonic crystal fiber 2 Glass fiber 3 Core part 4 Hole 5 Hole film 6 Core film 7 Refractory glass layer 8 Coating material

Claims (3)

In a photonic crystal fiber obtained by bundling a large number of small-diameter glass tubes around a high-refractive-index glass rod constituting the core portion to form a preform and drawing it to a predetermined diameter while melting and integrating it, SiO 2 ₂ In order to propagate signal light in a substantially axial center portion of the glass fiber consisting of, Ge, SiO ₂ was added one at least P, Al, and Ti And it has a core portion made of, SiO its a plurality of holes extending in the longitudinal direction around the core portion, and the fluorine on the inner surface of each of these holes, the amount of addition was added in 4 mol% or more 1 mol% ₂ A photonic crystal fiber comprising a pore film made of SiO ₂ with fluorine added around the core The photonic crystal fiber according to claim 1, comprising a core film made of 3. The photonic crystal fiber according to claim 1, wherein an outer peripheral portion of the glass fiber is covered with a SiO ₂ glass layer having a melting point higher than that of the glass fiber.

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