JP2007025087A - Photonic crystal fiber and optical transmission system using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photonic crystal fiber having a simple structure in which a wavelength dispersion is easily controlled in a wide wavelength band and to provide an optical transmission system using the photonic crystal fiber. <P>SOLUTION: The photonic crystal fiber 10 has a core part 11 and a clad part 12 surrounding the core part 11, the core part 11 has a larger refractive index than that of the clad part 12, and a plurality of holes 11a having a diameter d are formed in the core part 11 at intervals of a pitch Λ. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a photonic crystal fiber and an optical transmission system using the photonic crystal fiber.

  In high-speed optical transmission systems, distortion of optical signals due to cumulative dispersion becomes a problem, and various types of optical fibers with reduced chromatic dispersion in the communication wavelength band have been developed so far. For example, by having a configuration composed of three or more layers having different refractive indexes, an optical fiber having characteristics suitable as a dispersion flat fiber or a dispersion compensation fiber, or as described in Patent Document 1 below, Using a photonic crystal fiber structure having a large number of holes, there are a dispersion flat fiber, a dispersion compensation fiber, and the like having a high degree of freedom in designing dispersion characteristics.

JP 2003-255152 A

  However, in the conventional optical fiber as described above, when the chromatic dispersion is controlled in a wide wavelength band using the refractive index distribution, there is a problem that the refractive index distribution becomes very complicated. When chromatic dispersion is controlled by using a fiber structure, there are problems that a very large number of holes are required and a method for arranging holes is complicated.

  Accordingly, an object of the present invention is to provide a photonic crystal fiber capable of easily controlling chromatic dispersion in a wide wavelength band and an optical transmission system using the same, although having a simple structure. And

  A photonic crystal fiber according to a first aspect of the present invention for solving the above-described problem has a core part and a clad part surrounding the core part, and the core part is higher in refraction than the clad part. And a plurality of holes are formed inside the core portion.

  The photonic crystal fiber according to a second aspect is characterized in that, in the first aspect, the core portion has an arbitrary refractive index distribution.

  The photonic crystal fiber according to a third aspect is characterized in that, in the first or second aspect, the diameter of the hole of the core portion is 2.0 μm or less.

  The photonic crystal fiber according to a fourth invention is characterized in that, in the first or second invention, the interval between the adjacent holes is 2.0 μm or less.

  A photonic crystal fiber according to a fifth invention is the photonic crystal fiber according to any one of the first to fourth inventions, wherein a relative refractive index difference between the core part and the clad part is 1.0 to 2.5%. It is characterized by being.

  The photonic crystal fiber according to a sixth aspect of the present invention is the photonic crystal fiber according to any one of the first to fifth aspects, wherein the length is between the radius of the core portion and the outer periphery of the plurality of hole layers formed. Is 5.0 μm or less.

  A photonic crystal fiber according to a seventh invention is the photonic crystal fiber according to any one of the first to sixth inventions, wherein the holes are periodically arranged in an annular shape or a regular polygon shape in the core portion. It is characterized by.

  The photonic crystal fiber according to an eighth invention is the photonic crystal fiber according to any one of the first to seventh inventions, wherein a medium having a refractive index lower than the refractive index of the core portion is filled in the holes. It is characterized by.

  The photonic crystal fiber according to the ninth invention is the photonic crystal fiber according to any one of the first to eighth inventions, wherein the chromatic dispersion at a wavelength of 1460 to 1625 nm is −12.5 to 12.5 ps / nm · km. It is characterized by.

  A photonic crystal fiber according to a tenth invention is characterized in that, in any of the first to eighth inventions, the chromatic dispersion at a wavelength of 1310 nm is 30 ps / nm · km or more.

  A photonic crystal fiber according to a tenth invention is characterized in that, in any of the first to eighth inventions, the chromatic dispersion at a wavelength of 1550 nm is −30 ps / nm · km or less.

  An optical transmission system according to the tenth aspect of the invention for solving the above-described problem is obtained by using the photonic crystal fiber according to any of the first to ninth aspects of an optical transmission line. It is characterized by being.

  An optical transmission system according to a twelfth invention uses the photonic crystal fiber according to the tenth invention as a dispersion compensating fiber in at least one of a front stage and a rear stage of an optical transmission line having a negative dispersion at a wavelength of 1310 nm. It is characterized by being.

  An optical transmission system according to a thirteenth invention uses the photonic crystal fiber according to the tenth invention as a dispersion compensating fiber in at least one of a front stage and a rear stage of an optical transmission line having a positive dispersion at a wavelength of 1550 nm. It is characterized by.

  The photonic crystal fiber according to the present invention has a structure having a core part and a clad part, and a plurality of holes arranged in the core part. Chromatic dispersion can be easily controlled.

  Moreover, according to the optical transmission system using the photonic crystal fiber according to the present invention, it is possible to improve the transmission characteristics at the time of wavelength multiplexing transmission.

  In particular, when the photonic crystal fiber according to the present invention is used as a dispersion compensating fiber for an optical transmission line having a positive dispersion at a wavelength of 1550 nm or a dispersion compensating fiber for an optical transmission line having a negative dispersion at a wavelength of 1310 nm, The transmission characteristics of the optical transmission line in the band can be greatly improved.

  Embodiments of a photonic crystal fiber and an optical transmission system using the same according to the present invention will be described below with reference to the drawings.

  FIG. 1 shows a schematic structure of a photonic crystal fiber according to the present embodiment and its refractive index distribution. As shown in FIG. 1, the photonic crystal fiber 10 according to the present embodiment includes a core portion 11 and a cladding portion 12 that surrounds the core portion 11, and the core portion 11 is more than the cladding portion 12. In addition to having a high refractive index, a plurality of holes 11 a having a diameter d are formed in the core portion 11 at intervals (pitch) Λ.

Here, the refractive index of the core part 11 is n ₁ , the refractive index of the cladding part 12 is n ₂ , and the relative refractive index difference Δ between the core part 11 and the cladding part 12 is defined by the following equation.

Δ≡ (n ₁ ² −n ₂ ² ) / (2n ₁ ² )

  Since the photonic crystal fiber 10 according to the present embodiment exhibits high dispersibility due to the holes 11a inside the core portion 11, the wavelength dispersion can be reduced by changing the arrangement method and size of the holes 11a. It can be easily controlled with a belt. As shown in FIG. 1, the photonic crystal fiber 10 according to the present embodiment is not limited to the uniform refractive index distribution of the core portion 11, and the refractive index distribution of the core portion 11 is any other desired one. Even if it exists, the same effect can be acquired.

  In the photonic crystal fiber 10 according to the present embodiment, the diameter d of the hole 11a is 2 μm or less, or the length δ (= Λ−d) between the adjacent holes 11a is 2 μm or less. . Propagating light is subjected to very large structural dispersion in the communication wavelength band due to the presence of a plurality of holes 11a having a size equal to or smaller than the wavelength of the communication band in the light propagation region. Similarly, since the air holes 11a are arranged at intervals equal to or less than the wavelength of the communication band, the propagation light is subjected to very large structural dispersion in the communication wavelength band. Therefore, in the photonic crystal fiber 10 according to the present embodiment, the chromatic dispersion can be easily controlled by appropriately designing the arrangement and size of the air holes 11a formed in the core portion 11.

  FIG. 2 shows the structure-dependent characteristics of wavelength dispersion with respect to the length (difference) r between the radius a of the core portion 11 and the outer periphery of the plurality of holes 11a formed in the photonic crystal fiber 10 according to the present embodiment. Indicates. The interval Λ was 2.6 μm, the diameter d was 0.52 μm, the relative refractive index difference Δ was 1.0%, and the wavelength λ was 1550 nm. As can be seen from FIG. 2, when the difference r increases, the chromatic dispersion approaches the material dispersion, and the structural dispersion due to the holes 11a decreases. It can be seen that the difference r is preferably 5 μm or less in order to increase the structural dispersion caused by the holes 11a.

FIG. 3 shows the confinement loss characteristic with respect to the relative refractive index difference Δ in the photonic crystal fiber 10 according to the present embodiment. The interval Λ was 2.6 μm, the diameter d was 0.52 μm, the radius a was 6.8 μm, and the wavelength λ was 1550 nm. As can be seen from FIG. 3, the loss increases as the relative refractive index difference Δ decreases. In the photonic crystal fiber 10 according to the present embodiment, since the holes 11a effectively lower the refractive index n ₁ of the core portion 11, when the relative refractive index difference Δ is low, the effective relative refractive index difference Δ is It becomes small and the leakage to the clad part 12 increases. When the relative refractive index difference Δ is 1.0% or more, the confinement loss can be 0.1 dB / km or less, and the leakage to the clad portion 12 can be reduced. In view of manufacturing feasibility, the relative refractive index difference Δ is preferably 2.5% or less.

<First application example>
FIG. 4 shows chromatic dispersion characteristics of the first application example of the photonic crystal fiber 10 according to the present embodiment. FIG. 4 shows wavelength dispersion when the distance Λ is 2.6 μm, the diameter d is 0.52 μm, the radius a is 6.25 μm, and the relative refractive index difference Δ is 1.0%. It represents the characteristics.

  As can be seen from FIG. 4, in the case of this application example, the average value of the chromatic dispersion in the communication wavelength band (wavelength 1460 to 1625 nm) in optical communication is −0.39 ps / nm · km, and the fluctuation amount is 1.0 ps / It can be said that the characteristic suitable as a dispersion | distribution flat fiber can be expressed since it becomes nm * km and shows both small values. When optical transmission is performed at 10 Gbit / s in the wavelength band, if the accumulated dispersion exceeds 1000 ps / nm · km, a signal error due to transmission distortion occurs. Considering long-distance transmission, the relay distance is about 80 km, and the absolute value of chromatic dispersion per unit is preferably 12.5 ps / nm · km or less in the wavelength band.

  FIG. 5 shows a schematic configuration when the first application example of the photonic crystal fiber 10 according to the present embodiment is used in the optical transmission system 110 as the optical transmission path 111. Since the optical transmission system 100 according to the present embodiment can obtain low chromatic dispersion characteristics over the entire communication wavelength band, distortion of an optical signal due to chromatic dispersion can be reduced in WDM transmission in the wavelength band. In FIG. 5, 112 is a transmitter and 113 is a receiver.

<Second application example>
FIG. 6 shows chromatic dispersion characteristics of the second application example of the photonic crystal fiber 10 according to the present embodiment. FIG. 6 shows wavelength dispersion when the distance Λ is 2.5 μm, the diameter d is 1.0 μm, the radius a is 6.25 μm, and the relative refractive index difference Δ is 1.5%. It represents the characteristics.

  As can be seen from FIG. 6, in the case of this application example, the chromatic dispersion at a wavelength of 1310 nm is 38.2 ps / nm · km, which is suitable as a dispersion compensating fiber for an optical transmission line having negative chromatic dispersion in the wavelength band. It can be said that it has characteristics.

  FIG. 7 shows a schematic configuration when the second application example of the photonic crystal fiber 10 according to the present embodiment is used in the optical transmission system 120 as the dispersion compensating fiber 121 at a wavelength of 1310 nm. Since the optical transmission system 120 according to the present embodiment can obtain a large positive chromatic dispersion characteristic at the wavelength of 1310 nm, the accumulated dispersion of the optical transmission line 124 having negative chromatic dispersion in the wavelength band can be canceled. . Therefore, it is possible to improve the transmission characteristics of the optical transmission line 124 having negative wavelength dispersion at the wavelength of 1310 nm.

  7 shows the case where the second applied example of the photonic crystal fiber 10 according to the present embodiment is arranged as the dispersion compensating fiber 121 in the subsequent stage of the optical transmission line 124. However, the present invention is not limited to this. The same effect as described above can also be obtained by arranging in the front stage of the transmission path 124 or in both the front stage and the rear stage of the optical transmission path 124. In addition, the second application example of the photonic crystal fiber 10 according to the present embodiment can be applied to constituent members of the transmitter 112 and the receiver 113, and can also be applied to constituent members such as a repeater. .

<Third application example>
Further, as shown in FIG. 1, the photonic crystal fiber according to the present embodiment has a first core region 11A in a range where the core portion 11 is surrounded by the layer of the holes 11a and the outer side of the layer of the holes 11a. And the second core region 11 </ b> B between the cladding portion 12 and the cladding portion 12.

  As described above, it is known that the structure having the two core regions 11A and 11B concentrically causes a super mode phenomenon exhibiting a large negative dispersion at a specific wavelength. In addition, since there is a large structural dispersion due to the holes 11a, a combination of the super mode phenomenon and the structural dispersion due to the holes 11a provides a large negative dispersion at a desired wavelength, and a chromatic dispersion in a wide wavelength band. Can be controlled. In the first application example described above, the wavelength at which the super mode phenomenon occurs is set outside the communication wavelength band, and the first core region 11A is designed to be dominant.

  FIG. 8 shows the chromatic dispersion characteristics of the third application example of the photonic crystal fiber 10 according to the present embodiment. FIG. 8 shows wavelength dispersion when the interval Λ is 2.5 μm, the diameter d is 0.7 μm, the radius a is 6.25 μm, and the relative refractive index difference Δ is 1.0%. It represents the characteristics.

  As can be seen from FIG. 8, in the case of this application example, the chromatic dispersion at a wavelength of 1550 nm is −104.9 ps / nm · km, which is suitable as a dispersion compensating fiber for an optical transmission line having positive chromatic dispersion in the wavelength band. It can be said that it has special characteristics.

  FIG. 9 shows a schematic configuration when the third application example of the photonic crystal fiber 10 according to the present embodiment is used in the optical transmission system 130 as the dispersion compensating fiber 131 at a wavelength of 1550 nm. Since the optical transmission system 130 according to the present embodiment can obtain a large negative chromatic dispersion characteristic at a wavelength of 1550 nm, the accumulated dispersion of the optical transmission line 134 having positive chromatic dispersion in the wavelength band can be offset. . Therefore, it is possible to improve the transmission characteristics of the optical transmission line 134 having positive wavelength dispersion at a wavelength of 1550 nm.

  9 shows a case where the third applied example of the photonic crystal fiber 10 according to the present embodiment is arranged as the dispersion compensating fiber 131 in the subsequent stage of the optical transmission line 134. However, the present invention is not limited to this. The same effect as described above can also be obtained by arranging in the front stage of the transmission path 134 or in both the front stage and the rear stage of the optical transmission path 134. In addition, the third application example of the photonic crystal fiber 10 according to the present embodiment can be applied to constituent members of the transmitter 112 and the receiver 113, and can also be applied to constituent members such as a repeater. .

<Other embodiments>
The above-described embodiments of the photonic crystal fiber and the optical transmission system using the same according to the present invention are examples, and the present invention is not limited to the above-described embodiments.

  For example, in the above-described embodiment, as shown in FIG. 1, the number of layers of the holes 11a in the core part 11 is two, but the number of layers of the holes 11a in the core part 11 is one or three. The above is also possible. In the above-described embodiment, as shown in FIG. 1, the holes 11a of the core portion 11 are arranged in a regular hexagonal shape. However, they can be arranged in an annular shape or other regular polygonal shapes. is there. Further, it is possible to fill a medium having a refractive index lower than the refractive index of the material constituting the core portion 11 in the air holes 11 a of the core portion 11.

  Since the photonic crystal fiber and the optical transmission system using the photonic crystal fiber according to the present invention can easily control the chromatic dispersion in a wide wavelength band, they can be used extremely beneficially industrially.

It is a figure which shows the general | schematic structure of embodiment of the photonic crystal fiber based on this invention, and its refractive index distribution. 6 is a graph showing the structure-dependent characteristics of chromatic dispersion with respect to the difference r between the radius of the core part and the outer periphery of a plurality of hole layers formed in the embodiment of the photonic crystal fiber according to the present invention. It is a graph which shows the confinement loss characteristic with respect to relative refractive index difference (DELTA) in embodiment of the photonic crystal fiber which concerns on this invention. It is a graph which shows the wavelength dispersion characteristic of the 1st application example of embodiment of the photonic crystal fiber which concerns on this invention. It is a schematic block diagram at the time of utilizing for the optical transmission system the 1st application example of embodiment of the photonic crystal fiber which concerns on this invention as an optical transmission line. It is a graph which shows the wavelength dispersion characteristic of the 2nd application example of embodiment of the photonic crystal fiber which concerns on this invention. It is a schematic structure at the time of utilizing for the optical transmission system the 2nd application example of embodiment of the photonic crystal fiber based on this invention as a dispersion compensation fiber in wavelength 1310nm. It is a graph which shows the wavelength dispersion characteristic of the 3rd application example of embodiment of the photonic crystal fiber which concerns on this invention. It is a schematic block diagram at the time of utilizing for the optical transmission system the 3rd application example of embodiment of the photonic crystal fiber based on this invention as a dispersion compensation fiber in wavelength 1550nm.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Photonic crystal fiber 11 Core part 11a Hole 11A 1st core area | region 11B 2nd core area | region 12 Cladding part 110 Optical transmission system 111 Optical transmission path 112 Transmitter 113 Receiver 120 Optical transmission system 121 Dispersion compensation fiber 124 Light Transmission path 130 Optical transmission system 131 Dispersion compensation fiber 134 Optical transmission path a Radius of core part Λ Space between air holes (pitch)
d Hole diameter δ Length between adjacent holes r Length (difference) between the radius of the core portion and the outer periphery of a plurality of hole layers formed
n ₁ refractive index of core n ₂ refractive index of cladding n ₀ refractive index of hole

Claims (14)

The core,
A clad portion surrounding the core portion,
The core portion has a higher refractive index than the cladding portion,
A plurality of holes are formed inside the core part. A photonic crystal fiber, wherein: In claim 1,
The photonic crystal fiber, wherein the core portion has an arbitrary refractive index distribution. In claim 1 or claim 2,
The photonic crystal fiber, wherein a diameter of the hole in the core portion is 2.0 μm or less. In claim 1 or claim 2,
A photonic crystal fiber, wherein a length between adjacent holes is 2.0 μm or less. In any one of Claims 1-4,
The photonic crystal fiber, wherein a relative refractive index difference between the core portion and the clad portion is 1.0 to 2.5%. In any one of Claims 1-5,
The length between the radius of the core part and the outer periphery of the plurality of hole layers formed is 5.0 μm or less. A photonic crystal fiber, wherein: In any one of Claims 1-6,
The photonic crystal fiber, wherein the holes are periodically arranged in an annular shape or a regular polygonal shape in the core portion. In any one of Claims 1-7,
The photonic crystal fiber, wherein a medium having a refractive index lower than that of the core portion is filled in the holes. In any one of Claims 1-8,
The photonic crystal fiber, wherein chromatic dispersion at a wavelength of 1460 to 1625 nm is −12.5 to 12.5 ps / nm · km. In any one of Claims 1-8,
A chromatic dispersion at a wavelength of 1310 nm is 30 ps / nm · km or more. In any one of Claims 1-8,
The photonic crystal fiber, wherein chromatic dispersion at a wavelength of 1550 nm is −30 ps / nm · km or less. An optical transmission system, wherein the photonic crystal fiber according to any one of claims 1 to 9 is used in an optical transmission line. An optical transmission system using the photonic crystal fiber of claim 10 as a dispersion compensating fiber in at least one of a front stage and a rear stage of an optical transmission line having a negative dispersion at a wavelength of 1310 nm. An optical transmission system using the photonic crystal fiber of claim 11 as a dispersion compensating fiber in at least one of a front stage and a rear stage of an optical transmission line having a positive dispersion at a wavelength of 1550 nm.

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