JP2015045704A - Optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber having small leakage loss.SOLUTION: An optical fiber comprises: a core part 11; a cladding part 12 surrounding the core part 11 and whose refractive index is smaller than a refractive index of the core part 11; and a covering part 13 surrounding an outer circumferential edge of the cladding part 12 and whose refractive index is larger than the refractive index of the cladding part 12 and smaller than an effective refractive index of a waveguide mode of the core part 11.

Description

  The present invention relates to an optical fiber with small leakage loss.

  In order to reduce optical loss, optical fibers with low leakage loss have been studied (for example, see Non-Patent Documents 1 and 2).

  In addition, in order to expand transmission capacity, optical fiber communication systems in which multiple wavelengths of light are wavelength-multiplexed have problems with nonlinear effects and fiber fuses that occur in the optical fiber, limiting transmission capacity. Yes. In order to alleviate these restrictions, a multi-core fiber having a plurality of cores in one optical fiber has been studied (for example, see Non-Patent Documents 3 to 7).

Y. Katsyuyama, M .; Tokuda, N.A. Uchida, and M.M. Nakahara, “New method for measuring V-value of a single-mode optical fiber”, Electron. Lett. , Vol. 12, pp. 669-670, Dec. 1976. "Waveguide optics" (Chapter 12), Junichi Sakai, Kyoritsu Shuppan, 2004 H. Takara et al. , “1.01-Pb / s (12 SDM / 222 WDM / 456 Gb / s) Crosstalk-managed Transmission with 91.4-b / s / Hz Aggregate Spectral Efficiency”, in ECOC2012, h. 3. C. 1 (2012) J. et al. Sakaguchi et al. "305 Tb / s Space Division Multiplexed Transmission Using Homegeneous 19-Core Fiber", J., et al. Lightwave Technol. vol. 31, pp. 554-562 (2013). T. T. et al. Hayashi et al. "Design and fabrication of ultra-low cross and low-loss multi-core fiber", Opt. Express vol.19, pp. 16576-16592 (2011). K. Imamura et al. , “Multi Core Fiber with Large Aeff of 140 μm2 and Low Crosstalk”, in ECOC2012, paper Mo. 1. F. 2 T. T. et al. Hayashi et al. "Uncoupled multi-core fiber enhancing signal-to-noise ratio", Opt. Express vol. 20, pp. B94-B103 (2012).

  If the cladding thickness from the center of the core part to the outer peripheral end of the cladding part is increased in order to reduce leakage loss, there is a problem that the outer diameter of the cladding part increases. When the outer diameter of the clad portion is increased, the mechanical reliability against bending of the optical fiber is deteriorated.

  Even in a multi-core optical fiber, in order to reduce leakage loss, increasing the cladding thickness from the center of the core closest to the outer periphery of the cladding to the outer periphery of the cladding increases the outer diameter of the cladding. There is a problem. When the outer diameter of the clad portion increases, the number of core portions existing per unit area of the cross section of the clad portion decreases, and space utilization efficiency decreases. Further, not only the mechanical reliability against bending of the optical fiber is deteriorated, but also when the multi-core optical fibers are connected to each other, an allowable angular deviation is reduced in order to keep the connection loss below a certain level.

  Therefore, in order to solve the above-described problems, an object of the present invention is to provide an optical fiber having a small leakage loss without increasing the outer diameter of the cladding portion.

  In order to achieve the above object, the refractive index of the covering portion surrounding the outer peripheral end of the cladding portion is an optical fiber within a predetermined range.

Specifically, the present invention provides:
The core,
A cladding that surrounds the core and has a refractive index less than the refractive index of the core;
Surrounding the outer peripheral edge of the cladding part, a coating part having a refractive index larger than the refractive index of the cladding part and smaller than the effective refractive index of the waveguide mode of the core part;
An optical fiber comprising:
According to this configuration, it is possible to provide an optical fiber with a small leakage loss without increasing the outer diameter of the cladding portion.

In the optical fiber of the present invention, the cladding thickness from the center of the core part to the outer peripheral end of the cladding part may be 35 μm or less.
According to this configuration, it is possible to provide an optical fiber having a small leakage loss and a thin cladding thickness.

The optical fiber of the present invention may have a plurality of the core portions.
According to this configuration, it is possible to provide a multi-core optical fiber with a small leakage loss without increasing the outer diameter of the cladding portion.

The optical fiber of the present invention may have a cladding thickness of 35 μm or less from the center of the core portion closest to the outer peripheral end of the cladding portion to the outer peripheral end of the cladding portion among the plurality of core portions. .
According to this configuration, it is possible to provide a multi-core optical fiber having a small leakage loss and a thin cladding thickness.

  The optical fiber of the present invention can reduce the leakage loss without increasing the outer diameter of the cladding.

It is the schematic which shows the cross-section of the optical fiber of this invention. It is the schematic which shows the cross-section of the multi-core optical fiber of this invention. It is the figure which showed the change of the leakage loss with respect to the refractive index of the coating | coated part in wavelength 1550nm. It is a conceptual diagram regarding the change of the transition point in the case of (DELTA) coat> (DELTA) neff. It is a conceptual diagram regarding the change of the transition point in the case of (DELTA) coat <(DELTA) neff. It is the figure which showed the change of the leakage loss with respect to the refractive index of the coating | coated part in wavelength 1625nm.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

FIG. 1 schematically shows the cross-sectional structure of the optical fiber of the present invention. In FIG. 1, a cladding portion 12 is disposed around a core portion 11, and the periphery of the cladding portion 12 is covered with a covering portion 13. Refractive index of the core portion 11, the clad portion 12 and the covering portion 13, _{respectively,} is n 1, _{n 2} and _{n 3.} The clad thickness is defined by a distance CT from the center of the core part 11 to the outer peripheral end of the clad part 12.

Moreover, the outline which shows the cross-section of the multi-core optical fiber of this invention is shown in FIG. In FIG. 2, a clad part 12 is disposed around a plurality of core parts 11, and the clad part 12 is covered with a covering part 13. Refractive index of the core portion 11, the clad portion 12 and the covering portion 13, _{respectively,} is n 1, _{n 2} and _{n 3.} The clad thickness is defined by a distance CT from the center of the core portion 11 located closest to the outer peripheral end of the clad portion 12 to the outer peripheral end of the clad portion 12 in the core portion 11.

  The following description is common to the optical fiber shown in FIG. 1 and the multi-core optical fiber shown in FIG.

Here, the relative refractive index difference Δcore of the core portion 11 with respect to the cladding portion 12 and the relative refractive index difference Δcoat of the covering portion 13 with respect to the cladding portion 12 are expressed by the following equations.
Δ _core = (n ₁ ² −n ₂ ² ) / (2n ₁ ² ) (1)
Δ _coat = (n ₃ ² −n ₂ ² ) / (2n ₃ ² ) (2)
In the above formulas (1) and (2), if n ₁ > n ₂ and the refractive index of the covering portion 13 is about 1.5, the relative refractive index difference of the core portion 11 is at most about 1%. Therefore, the relationship of n ₃ > n ₁ > n ₂ can be realized.

The condition of n ₁ > n ₂ is that the material of the core portion 11 and the cladding portion 12 is pure quartz glass, or impurities such as germanium (Ge), aluminum (Al), phosphorus (P), or the like that increase the refractive index, This can be realized by using quartz glass to which an impurity such as fluorine (F) or boron (B) is added to reduce the refractive index.

  FIG. 3 shows a calculation of the relationship between the leakage loss and the relative refractive index difference of the covering portion 13. In FIG. 3, the cladding thickness CT, which is the distance from the center of the core portion 11 to the outer peripheral end of the cladding portion 12, is changed in the range of 20 to 40 μm. The wavelength is 1550 nm, the radius of the core 11 is 4.5 μm, Δcore is 0.35%, and the leakage loss is the loss when the bending radius of the optical fiber is 140 mm. In this optical fiber structure, the effective refractive index neff of the waveguide mode of the core portion 11 is such that the relative refractive index difference Δneff of the waveguide mode with respect to the cladding portion 12 is 0.169%.

  Here, the reason for setting the bending radius to 140 mm is that, as described in Non-Patent Document 1, the bending radius of 140 mm is used for measuring the cutoff wavelength, and the effective radius of bending that the optical fiber receives when laying the optical cable is 140 mm. Because.

From the calculation results shown in FIG. 3, the leakage loss hardly changes in the region where the refractive index of the covering portion 13 is large, and the leakage loss decreases with a certain value as a threshold value. In the core portion 11, the effective refractive index of the waveguide mode is neff, and the relative refractive index difference of the waveguide mode with respect to the cladding portion 12 is expressed by the following equation.
Δ _neff = (n _neff ² −n ₂ ² ) / (2n _neff ² ) (3)
As described above, the relative refractive index difference Δneff was 0.169%. As a result of calculation, it was found that the threshold value in FIG. 3 corresponds to this Δneff. That is, the refractive index n ₃ of the cover portion 13 is leakage loss in the effective refractive index neff larger area of the guided mode of the core portion 11 does not change, the effective refractive index refractive index n ₃ is guided modes of the cover portion 13 It has been found that leakage loss is reduced when it is smaller than neff.

As shown in FIG. 4, when Δcoat> Δneff, the transition point r _c (the position where the refractive index of the coating exceeds neff) does not depend on Δcoat. Therefore, the bending loss value should be constant for the cladding thickness CT. If does not depend on Derutacoat, when the Δcoat <Δneff as shown in FIG. 5, as the transition point _{r c} Δcoat decreases increases, if the cladding thickness CT is constant, the value of bending loss as the value of Derutacoat small This is because becomes smaller. As described in Non-Patent Document 2, an electric field at a position exceeding the transition point shows vibration characteristics and becomes a radiation component. This is because when the transition point moves from the center of the fiber to the clad portion 12 side, the electric field strength existing after the transition point decreases, and the amount of electric field power to be radiated decreases.

When the refractive index difference n ₃ of the covering portion 13 is smaller than the refractive index n ₂ of the cladding portion 12, a large number of propagation modes are generated with the cladding portion 12 as a core and the covering portion 13 as a cladding. That is not preferable.

By expressing 0% <Δcoat <Δneff, that is, refractive index, and n _{2 <} n _{3 <} neff, an increase in leakage loss due to a decrease in cladding thickness CT can be suppressed. The cladding thickness CT can be reduced. In the case of a multi-core optical fiber, a large number of core portions 11 can be arranged in the cladding portion 12 having a small outer diameter, and space utilization efficiency can be improved.

  Similarly, FIG. 6 shows a calculation of the relationship of leakage loss to the relative refractive index difference of the coating at a wavelength of 1625 nm. Conditions other than the wavelength are the same as in FIG. In this optical fiber structure, the effective refractive index neff of the waveguide mode of the core portion 11 is such that the relative refractive index difference Δneff of the waveguide mode with respect to the cladding portion 12 is 0.159%. Generally, since the leakage loss increases as the wavelength becomes longer, the leakage loss is larger than the calculation result at 1550 nm, but the tendency of the leakage loss to decrease at Δneff is the same.

ITU-T Fiber Recommendation G. In 652, the upper limit of optical loss is defined as 0.4 dB / km, and if the wavelength is 16 dB nm or less at 0.4 dB / km, the leakage loss is reduced to 0 in the communication wavelength band of 1625 nm or less that is generally used. .4 dB / km or less. When the refractive index of the covering portion 13 is not limited, that is, when n ₃ > n ₁ > n ₂ , CT> 35 μm is required, but the refractive index of the covering portion 13 should be 0% <Δcoat <Δneff. Thus, the leakage loss can be 0.4 dB / km or less even in the region of CT <35 μm.

  Since the present invention can be applied as a transmission medium of an optical communication system, it can be used for an optical communication system.

11: Core part 12: Cladding part 13: Covering part

Claims (4)

The core,
A cladding that surrounds the core and has a refractive index less than the refractive index of the core;
Surrounding the outer peripheral edge of the cladding part, a coating part having a refractive index larger than the refractive index of the cladding part and smaller than the effective refractive index of the waveguide mode of the core part;
An optical fiber comprising:   The optical fiber according to claim 1, wherein a clad thickness from the center of the core part to an outer peripheral end of the clad part is 35 μm or less.   The optical fiber according to claim 1, comprising a plurality of the core portions.   4. The clad thickness from the center of the core portion closest to the outer peripheral end of the clad portion to the outer peripheral end of the clad portion among the plurality of core portions is 35 μm or less. Optical fiber.

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