JP2005338435A - Multilayer optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilayer optical fiber that realizes single mode transmission and that facilitates alignment in the case of connecting the optical fiber. <P>SOLUTION: The multilayer optical fiber is designed to be a double layer optical fiber 10 composed of a first core 11, a first clad 12 surrounding the first core 11, a second core 14 surrounding the first clad 12, and a second clad 15 surrounding the second core. The first clad 12, the second core 14 and the second clad 15 are formed coaxially with the center axis of the first core 11, the effective refractive index of the first clad 12 is made lower than that of the second clad 15, the refractive index of the first core 11 is made higher than the effective index of the first clad 12, and the refractive index of the second core 14 is made higher than the effective index of the second clad 15. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a multi-layer optical fiber (for example, a double-layer optical fiber), and is optimal as, for example, a single-mode optical fiber or a photonic crystal optical fiber suitable for optical wiring.

  With increasing demand for optical communication in recent years, a large capacity of an optical transmission line and easy multiplexing are required, and various methods are being studied.

  As a method for stably carrying out large-capacity communication in both directions, there is a single star method in which an optical fiber is provided for each path. In the single star system, since different signals are transmitted through completely independent optical fibers, there is no interference between the signals and stable communication can be performed.

  In addition, in order to perform space division multiplex transmission in a single optical fiber, a multi-core optical fiber has been conventionally studied (see Patent Documents 1 and 2 below). A multi-core optical fiber has a plurality of core regions in a clad having a uniform refractive index so that a plurality of regions capable of transmitting light are provided, and stable communication can be performed.

JP-A-10-104443 JP 2001-33636 A N.A. Mortensen et al., “Modal cutoff and the V parameter in photonic crystal fibers”, Optics Letters, vol.28, No.20, published in October 2003, p.1879-1881 (p.1880, Fig.1) ITU-T, Recommendation G.652 (revised March 2003) (Table 1 / G.652, p.12)

  However, the above-described single star method has a problem that it is very inefficient in terms of cost and operation because an optical fiber must be laid for each path. In addition, although a multi-core optical fiber has a plurality of core regions in a single optical fiber, there is a problem that it is difficult to connect because all the cores must be aligned at the same time when connected.

  The present invention has been made in view of the above circumstances, and has a plurality of transmission regions (cores) capable of independently transmitting light in a single optical fiber, and the central axes of the respective transmission regions are the same. An object of the present invention is to provide a multilayer optical fiber (for example, a double layer optical fiber) that realizes single mode transmission.

The double layer optical fiber according to the present invention that solves the above problems is
A double having a first core part, a first cladding part surrounding the first core part, a second core part surrounding the first cladding part, and a second cladding part surrounding the second core part Layer optical fiber,
The first cladding part, the second core part, and the second cladding part are formed coaxially with a central axis of the first core part,
The effective refractive index of the first cladding part is lower than the effective refractive index of the second cladding part,
The refractive index of the first core part is higher than the effective refractive index of the first cladding part, and the refractive index of the second core part is higher than the effective refractive index of the second cladding part. It is a double layer optical fiber.

  The double-layer optical fiber includes a first core portion and a first cladding portion, a second core portion surrounding the first cladding portion, and a second cladding portion surrounding the second core portion. The transmission path is configured concentrically.

In the double layer optical fiber,
The double-layer optical fiber is formed of one kind of material having a uniform refractive index, and at least one of the first clad part or the second clad part is formed with a plurality of holes. It is the double layer optical fiber characterized.

In the double layer optical fiber,
Check the distance lambda ₁ between the holes, the ratio d ₁ / lambda ₁ of the diameter d ₁ of the holes formed in the first cladding part distance lambda ₁ adjacent in the holes formed in the first cladding part Is a double layer optical fiber characterized by satisfying the following formulas (1) to (3).

In the double layer optical fiber,
Check the distance lambda ₂ between holes, the diameter d ₂ of the holes formed in the second cladding part distance lambda ₂ ratio d ₂ / lambda ₂ adjacent the holes formed in the second cladding part Is a double layer optical fiber characterized by satisfying the following formulas (4) and (5).

In the double layer optical fiber,
The double-layered optical fiber is characterized in that the holes formed in the first cladding part have three or more layers.

In the double layer optical fiber,
The second core portion is a double layer optical fiber having an inner peripheral radius of 14.8 μm or more and an outer peripheral radius of 56.5 μm or less.

The multilayer optical fiber according to the present invention that solves the above problems is
A multi-layer optical fiber in which a core part and a clad part are laminated alternately and plurally on the same axis,
The effective refractive index of the cladding part is higher as the cladding part is closer to the outer periphery, and the refractive index of the core part is higher than the refractive index of the cladding part laminated on both sides of the core part. It is the multilayer optical fiber characterized.

In the multilayer optical fiber,
The multilayer optical fiber is formed of one kind of material having a uniform refractive index, and at least one of the clad portions is formed with a plurality of holes. It is an optical fiber.

As explained above, according to the present invention,
A double having a first core part, a first cladding part surrounding the first core part, a second core part surrounding the first cladding part, and a second cladding part surrounding the second core part Layer optical fiber,
The first cladding part, the second core part, and the second cladding part are formed coaxially with a central axis of the first core part,
The effective refractive index of the first cladding part is lower than the effective refractive index of the second cladding part,
The refractive index of the first core part is higher than the effective refractive index of the first cladding part, and the refractive index of the second core part is higher than the effective refractive index of the second cladding part.
An optical fiber having two regions capable of independent single-mode optical transmission can be obtained. Further, it is possible to improve the efficiency of cost and operation, and to solve the connection difficulty of the multi-core optical fiber, in which it is necessary to simultaneously align all cores at the time of connection.

Further, the effective refractive index is adjusted by forming a plurality of holes in the cladding part, or the distances Λ ₁ and Λ ₂ between the holes in each cladding part and the normalized hole diameters d ₁ / Λ ₁ and d _2. / Λ ₂ is optimized so as to satisfy the above formulas (1) to (5), three or more holes are formed in the cladding, and the inner and outer radii of the second core are predetermined values. By optimizing to the above, it is possible to reliably realize a single mode operation or to suppress the influence of signals propagating through each transmission region.

  In addition, a multi-layer optical fiber in which a core part and a clad part are alternately and plurally stacked on the same axis, and the effective refractive index of the clad part is higher as the clad part closer to the outer periphery, Since the refractive index of the core part is higher than the refractive index of the clad part laminated on both sides of the core part, it is possible to optimally transmit multiple signals.

<Double-layer optical fiber according to the first embodiment>
Hereinafter, the double layer optical fiber according to the first embodiment will be described in detail with reference to the drawings. FIG. 1 is a schematic cross-sectional structure diagram of a double-layer optical fiber according to the first embodiment (when the second cladding part is constituted by holes).

  As shown in the figure, the double-layer optical fiber 10 according to the first embodiment surrounds the first core part 11, the first cladding part 12 surrounding the first core part 11, and the first cladding part 12. The second core part 14 and the second clad part 15 surrounding the second core part 14 are configured.

The first core portion 11 is circular in cross section with a radius r ₁ , the first cladding portion 12 is circular in cross section, the second core portion 14 is circular in cross section with an outer peripheral radius r ₂ , and the second cladding portion 15 is circular in cross section. It is. The central axis of the second core part 14 and the clad parts 12 and 15 is the same as the central axis of the first core part 11 (each structure is formed coaxially).

The refractive index of the first core portion 11 is n _co1 , and the refractive index of the second core portion 14 is n _co2 . Further, the first cladding portion 12, (six in the drawing) a plurality first holes 13 of diameter d _1, it is formed at an interval lambda ₁ along the circumference, the effective refractive index n _cl1 . Further, the second cladding portion 15 is formed by a plurality of first holes 16 (six in the figure) having a diameter d ₂ , and is formed at an interval Λ ₂ along the circumference, and an effective refractive index is n. _cl2 .

At this time, the refractive index n _co1 of the first core portion 11, the refractive index n _{of co2,} the effective refractive index of the effective refractive index n _cl1 and a second cladding portion 15 of the first cladding portion 12 of the second core portion 14 n _cl2 has the relationship of the following formula (6).

<Double-layer optical fiber according to the second embodiment>
Next, a double layer optical fiber according to the second embodiment will be described in detail with reference to the drawings. FIG. 2 is a schematic cross-sectional structure diagram of a double layer optical fiber according to the second embodiment (when the refractive index of the second cladding part is made lower than the refractive index of the second core part by an additive). .

  As shown in the figure, the double-layer optical fiber 20 according to the first embodiment surrounds the first core portion 21, the first cladding portion 22 that surrounds the first core portion 21, and the first cladding portion 22. The second core part 24 and the second clad part 25 surrounding the second core part 24 are configured.

The first core portion 21 is circular in cross section with a radius r ₁ , the first cladding portion 22 is circular in cross section, the second core portion 24 is circular in cross section with an outer peripheral radius r ₂ , and the second cladding portion 25 is circular in cross section. (The outer peripheral shape may be anything). The central axis of the second core part 24 and the clad parts 22 and 25 is the same as the central axis of the first core part 21 (the structures are formed coaxially).

The refractive index of the first core portion 21 is n _co1 , and the refractive index of the second core portion 24 is n _co2 . Further, the first cladding portion 22, (six in the drawing) a plurality first holes 23 of diameter d _1, it is formed at an interval lambda ₁ along the circumference, the effective refractive index n _cl1 . The refractive index of the second cladding part 25 is adjusted by adding an additive, and the effective refractive index is n _cl2 .

At this time, the refractive index n _co1 of the first core portion 21, the refractive index n _{of co2} of the second core portion 24, the effective refractive index of the effective refractive index n _cl1 and a second cladding portion 25 of the first cladding portion 22 n _cl2 has the relationship of the above equation (6).

<Waveguide characteristics of optical fibers according to the first and second embodiments>
FIG. 3 is a schematic diagram showing the refractive index distribution of the double layer optical fiber according to the first and second embodiments. As can be seen from the figure, an optical waveguide structure is formed by the surrounding first clad parts 12 and 22 in the first core parts 11 and 21, and the first clad parts 12 and 22 and the second clad parts 15 and 25 are formed. The optical waveguide structure 4 is also formed in the second core portions 14 and 24 sandwiched between the two.

  FIG. 4 is a characteristic diagram showing the field distribution of the propagation mode of the double-layer optical fiber according to the first and second embodiments, in which the mode (a) propagates through the first core part and the second core part propagates. Mode (b) is shown.

  As can be seen from FIG. 5A, the propagation modes of the first core portions 11 and 21 are confined by the first cladding portions 12 and 22, and the first cladding portions 12 and 22 have a sufficient thickness. , Leakage to the second core portions 14 and 24 can be suppressed. Further, as can be seen from FIG. 5B, the propagation mode of the second core portions 14 and 24 penetrates into the first core portions 11 and 21 by using the refractive index distribution represented by the above formula (6). Propagation is possible without any leakage to the first core parts 11 and 21.

<Transmission of optical signal by first core unit>
The waveguide structure formed by the first core portions 11 and 21 and the first cladding portions 12 and 22 has an effective refractive index due to the plurality of first holes 13 and 23 formed in the first cladding portions 12 and 22. And the difference between the refractive indexes of the first core portions 11 and 21. This is the same structure as a conventional photonic crystal optical fiber, and the transmission characteristics can be controlled by appropriately selecting the size d ₁ of the first holes 13 and 23 and the arrangement interval Λ ₁ .

  In the optical transmission using the first core portions 11 and 21 in the double layer optical fibers 10 and 20 according to the above-described embodiment, a good connection with a conventional single mode optical fiber is possible. Therefore, the following description will be made assuming that single mode operation is possible.

FIG. 5 is a characteristic diagram showing an optimum design region of the hole interval Λ ₁ and the normalized hole diameter d ₁ / Λ ₁ in the first cladding part of the double layer optical fiber according to the first and second embodiments. It is. In the figure, the mode field diameter (hereinafter referred to as MFD) calculated in the fundamental mode propagating through the first core portion using the hole interval Λ ₁ and the normalized hole diameter d ₁ / Λ ₁ as parameters. It is shown.

  According to the said nonpatent literature 1, MFD of the conventional single mode optical fiber exists in the range of 7.9-10.2 micrometers in wavelength 1310nm. In FIG. 5, the region sandwiched between the three solid lines 31, 32, and 33 indicates a condition range in which the MFD is 7.9 to 10.2 μm.

In addition, dotted lines 34-1 and 34-2 in FIG. 5 indicate Λ ₁ and d ₁ / Λ ₁ where the MFD and the diameter of the first core portion are equal. The dotted line 37 is a straight line connecting the intersection of the dotted line 34-1 and the solid line 31 and the intersection of the dotted line 34-2 and the solid line 32. In the region to the left of the dotted line 37, the MFD is larger than the physical core diameter of the first core portion, and the mode field is significantly deviated from the circular shape, which may affect the connection characteristics with the conventional single mode optical fiber. There is sex.

Further, according to Non-Patent Document 2, by setting d ₁ / Λ ₁ to 0.43 or less, single mode operation can be realized at a wavelength of 1310 nm. That is, the single mode operation can be realized in the region left of the dotted line 38.

From the above, in order to optimally design Λ ₁ and d ₁ / Λ ₁ , it is necessary to set the values in the region indicated by the oblique lines in FIG. This region is a range surrounded by a straight line of dotted lines 35 to 38 and can be represented by the following formulas (7) to (9). The dotted line 35 is a line obtained by approximating the portion between the dotted line 37 and the dotted line 38 as a straight line in the solid line 31. Similarly, the dotted line 36 is a line obtained by approximating the portion between the dotted line 37 and the dotted line 38 as a straight line in the solid line 32.

FIG. 6 is a characteristic diagram showing the leakage amount to the second core part with respect to the number of hole layers in the first cladding part of the double layer optical fibers according to the first and second embodiments. In the figure, among Λ ₁ and d ₁ / Λ ₁ satisfying the above equations (7) to (9), Λ ₁ is 4.7 μm, d ₁ / Λ ₁ , which is the condition that the confinement loss becomes the largest. Represents the amount of leakage of propagation light having a wavelength of 1650 nm from the first core portion when the value is 0.29 (the intersection of the solid line 32 and the dotted line 34-2).

  As can be seen from the figure, by increasing the number of hole layers, the amount of leakage of propagating light from the first core portion decreases. Here, by constituting the first clad part with three or more layers of holes, the amount of propagation light leakage from the first core part is suppressed to 0.1 dB / km or less, and the propagation light of the second core part is reduced. The influence of can be suppressed. The number of pore layers refers to the number of annular layers that are laminated toward the outer periphery when the first pores 13 and 23 arranged in a ring form one layer.

<Optical transmission by the second core>
FIG. 7 is a characteristic diagram showing an optimum design region of the hole interval Λ ₂ and the normalized hole diameter d ₂ / Λ ₂ in the second cladding part of the double layer optical fiber according to the first embodiment. In the same figure, the conditions satisfying the relationship between the refractive indexes of the first cladding portion and the second cladding portion (see the above formula (6)) are shown as the optimum design region.

  In FIG. 7, a solid line 39 indicates a case where the effective refractive index of the first cladding part is equal to the effective refractive index of the second cladding part. In order to satisfy the above equation (6), the region must be above the solid line 39.

  Moreover, in FIG. 7, the dotted line 40 has shown the case where the diameter of a 2nd clad part will be 125 micrometers. In order to obtain a clad diameter equivalent to that of a standard optical fiber, the region must be below the dotted line 40.

Further, according to Non-Patent Document 2, the conditions for propagation mode of the second core portion is a single mode, d ₂ / lambda ₂ is 0.43 or less. That is, in order to realize the single mode operation, the region must be on the left side of the dotted line 41. Further, if d ₂ / Λ ₂ is less than 0.2, manufacturing difficulty and confinement loss occur, so d ₂ / Λ ₂ is set to 0.2 or more. This is because when confining light using holes, if the holes are too small, the confinement effect is weak, and it is expected that light will leak into the cladding as it propagates, limiting the transmission distance.

From the above, in order to optimally design Λ ₂ and d ₂ / Λ ₂ , it is necessary to set the values in the region indicated by the oblique lines in FIG. This region is a range surrounded by the solid line 39 and the straight lines of the dotted lines 40 and 41, and can be expressed by the following formulas (10) to (12).

The radius of the inner periphery of the second core part can be expressed by the following formula (13), where N ₁ is the number of holes in the first cladding part.

Here, as described above, when Λ ₁ is 4.7 μm or more, d ₁ / Λ ₁ is 0.29 or more, and the number of layers is three or more, the inner radius is 14.8 μm or more. Further, the cladding diameter of the standard optical fiber was 125 [mu] m (radius 62.5 .mu.m), both smaller ₂ lambda from FIG 6. Since it is O μm (when d ₂ / Λ ₂ = 0.2), the radius r ₂ of the outer periphery of the second core portion is 56.5 μm (= 62.5 μm−6.0 μm) or less. Therefore, the second core portion has an inner peripheral radius of 14.8 μm or more and an outer peripheral radius r ₂ of 56.5 μm or less.

<Other application forms>
Although the double layer optical fiber according to the embodiment has been described above, the number of holes formed in the first cladding part and the second cladding part, the material of the optical fiber, and the like are not limited to the above-described embodiment. The voids in the cladding part are not limited to vacuum, and may be filled with a gas, liquid or solid having a refractive index lower than that of the core part.

  Further, in order to obtain a refractive index profile satisfying the above formula (6), for example, a method using an additive as shown in FIG. 2 is used to form germanium in the first core portion, the first cladding portion, and the second core portion. Etc., or fluorine or the like may be added to the second cladding part, and the same effect can be obtained.

  Furthermore, in the embodiment, the double-layer optical fiber has been described. However, the number of layers may be further increased to form a multilayer optical fiber. In this case, with respect to the refractive index relationship of each component, the refractive index of each cladding portion is set to be higher as the cladding portion closer to the outer periphery, and the refractive index of each core portion is set on both sides sandwiching each core. The refractive index may be higher than that of the cladding part.

It is a schematic sectional structure figure of the double layer optical fiber (when the 2nd clad part is constituted by a hole) concerning a 1st embodiment. It is a schematic sectional structure figure of the double layer optical fiber (when the refractive index of the 2nd clad part is made lower than the refractive index of the 2nd core part by an additive) concerning a 2nd embodiment. It is the schematic which shows the refractive index distribution of the double layer optical fiber which concerns on 1st, 2nd embodiment. It is a characteristic view which shows the field distribution of the propagation mode of the double layer optical fiber which concerns on 1st, 2nd embodiment, The mode (a) which propagates a 1st core part, The mode (b) which propagates a 2nd core part Indicates. FIG. 6 is a characteristic diagram showing an optimum design region of the hole interval Λ ₁ and the normalized hole diameter d ₁ / Λ ₁ in the first cladding part of the double layer optical fiber according to the first and second embodiments. It is a characteristic view showing the amount of leakage to the 2nd core part to the number of hole layers in the 1st clad part of the double layer optical fiber concerning the 1st and 2nd embodiments. FIG. 6 is a characteristic diagram showing an optimum design region of the hole interval Λ ₂ and the normalized hole diameter d ₂ / Λ ₂ in the second cladding part of the double layer optical fiber according to the first embodiment.

Explanation of symbols

10 Double-layer optical fiber 11 First core part (first waveguide structure)
12 1st clad part 13 1st hole 14 2nd core part (2nd waveguide structure)
15 Second cladding portion 16 Second hole 20 Double layer optical fiber 21 First core portion (first waveguide structure)
22 1st clad part 23 1st hole 24 2nd core part (2nd waveguide structure)
25 Second cladding part

r ₁ Radius of the first core part d ₁ Diameter of the first hole Λ ₁ Distance between adjacent first holes

r ₂ Distance from the center of the first core part to the outer periphery of the second core part d ₂ Diameter of the second hole Λ ₂ Distance between the adjacent second holes

n _co1 refractive index n _{of co2} effective refractive index of the effective refractive index n _cl2 second cladding portion having a refractive index n _cl1 first cladding portion of the second core portion of the first core portion

Claims (8)

A double having a first core part, a first cladding part surrounding the first core part, a second core part surrounding the first cladding part, and a second cladding part surrounding the second core part Layer optical fiber,
The first cladding part, the second core part, and the second cladding part are formed coaxially with a central axis of the first core part,
The effective refractive index of the first cladding part is lower than the effective refractive index of the second cladding part,
The refractive index of the first core part is higher than the effective refractive index of the first cladding part, and the refractive index of the second core part is higher than the effective refractive index of the second cladding part. Double layer optical fiber. In the double layer optical fiber according to claim 1,
The double-layer optical fiber is formed of one kind of material having a uniform refractive index, and at least one of the first clad part or the second clad part is formed with a plurality of holes. Characteristic double layer optical fiber. In the double layer optical fiber according to claim 2,
Check the distance lambda ₁ between the holes, the ratio d ₁ / lambda ₁ of the diameter d ₁ of the holes formed in the first cladding part distance lambda ₁ adjacent in the holes formed in the first cladding part Satisfy | fills following formula (1)-(3), The double layer optical fiber characterized by the above-mentioned.

In the double layer optical fiber according to claim 2 or 3,
Check the distance lambda ₂ between holes, the diameter d ₂ of the holes formed in the second cladding part distance lambda ₂ ratio d ₂ / lambda ₂ adjacent the holes formed in the second cladding part Satisfy | fills following formula (4), (5), The double layer optical fiber characterized by the above-mentioned.

In the double layer optical fiber according to any one of claims 2 to 4,
The double-layer optical fiber is characterized in that the holes formed in the first clad portion have three or more layers. In the double layer optical fiber according to claim 5,
The second core portion has an inner peripheral radius of 14.8 μm or more and an outer peripheral radius of 56.5 μm or less. A multi-layer optical fiber in which a core part and a clad part are laminated alternately and plurally on the same axis,
The effective refractive index of the cladding part is higher as the cladding part is closer to the outer periphery, and the refractive index of the core part is higher than the refractive index of the cladding part laminated on both sides of the core part. Characteristic multilayer optical fiber. The multilayer optical fiber according to claim 7, wherein
The multilayer optical fiber is formed of one kind of material having a uniform refractive index, and at least one of the clad portions is formed with a plurality of holes. Optical fiber.

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