JP6048890B2 - Optical fiber - Google Patents

Description

  The present invention relates to a communication optical fiber that transmits a large-capacity signal.

  In optical fiber communication systems, non-linear effects and fiber fuses that occur in optical fibers are problematic, and transmission capacity is limited. In order to alleviate these restrictions, it is necessary to reduce the density of light guided to the optical fiber. As shown in Non-Patent Documents 1 and 2, a large core fiber has been studied.

  However, bending loss reduction, single mode operation area expansion, and effective area expansion are in a trade-off relationship with each other, and there is a problem that the amount of effective area expansion under certain conditions is limited. Therefore, as shown in Non-Patent Document 3, a mode multiplex transmission in which a multimode is used for a transmission optical fiber and signals are transmitted in parallel using a plurality of propagation modes is being studied in order to increase the transmission capacity.

  In a mode multiplex transmission system, each transmitted signal propagates through a different propagation mode, so all modes must achieve the desired bending loss. For example, ITU-T G.I. The recommended bending loss at 656 is 0.5 dB / 100 turn or less at a bending radius of 30 mm.

  Further, in order to realize mode multiplexing transmission using N signals, the optical fiber needs to have at least N or more propagation modes.

  In addition, the effective area of each mode should be large in order to reduce the nonlinear effect generated in the optical fiber.

  In a mode multiplex transmission system, it is possible to perform parallel transmission by the number of modes, and an increase in the number of modes leads to an increase in transmission capacity. Therefore, it is effective to increase the number of modes of the optical fiber.

T. T. Matsui, et al. , "Applicability of Photonic Crystal Fiber With Uniform Air-Hole Structure to High-Speed and Wide-Band Transmission OverTownConventionmm." Lightwave Technol. 27, pp. 5410-5416, 2009. K. Mukasa, K .; Imamura, R.A. Sugizaki and T.A. Yagi, "Comparisons of merits on wide-band transmission systems between using extremely improved solid SMFs with Aeff of 160μm2 and loss of 0.175dB / km and using large-Aeff holey fibers enabling transmission over 600 nm bandwidth", the Proceedings of OFC2008, OthR1, Feb. 2008. N. Hanzawa, Kunimasa Saitoh, Taiji Sakamoto, Takashi Matsui, Shigeru Tomita, Masanori Koshiba, "Demonstration of mode-division multiplexing transmission over 10 km two-mode fiber with mode coupler", OFC2011, paper OWA4 (2011) Okamoto “Basics of Optical Waveguide” Corona T. T. Sakamoto et al. , “Differential Mode Delay Managed Transmission Line for WDM-MIMO System Using Multi-Step Index Fiber”, J. et al. Lightwave Technol. vol. 30, pp. 2783-2787 (2012). Y. Katsyuyama, M .; Tokuda, N.A. Uchida, and M.M. Nakahara, “New method for measuring the V-value of a single-mode optical fiber”, Electron. Lett. , Vol. 12, pp. 669-670, Dec. 1976. M.M. B-Astruc et al. , “Design and fabrication of weakly-coupled two-modes fibers”, IEEE summer topicals, paper TuC1.1 (2012)

  However, in an optical fiber having four propagation modes, there is a small difference in effective refractive index between the third mode (LP21 mode) and the fourth mode (LP02 mode), and therefore, inter-mode crosstalk is likely to occur in the fiber. For this reason, an optical fiber having four propagation modes requires a MIMO equalizer for compensating for crosstalk at the receiving end, and there is a problem that the structure of the system becomes complicated.

  Further, in an optical fiber having four propagation modes, the electric field overlap integral between the LP01, LP11, and LP21 modes is 0, and coupling between these modes hardly occurs at the connection point. However, there is another problem that mode conversion is likely to occur at the connection point because the electric fields of these have only in-phase components.

  In order to solve the above problem, an optical fiber that does not propagate the fourth LP02 mode is expected. However, since the cut-off v values of the third mode (LP21 mode) and the fourth mode (LP02 mode) are almost the same in the three-mode region of the step-type optical fiber (see, for example, Non-Patent Documents 4 and 5). However, the step type optical fiber has a problem that the propagation mode cannot be limited to 3.

  Accordingly, an object of the present invention is to provide an optical fiber in which the LP02 mode is blocked and the number of propagation modes is 3, in order to solve the above-described problems.

  In order to achieve the above object, according to the present invention, the core is formed in a ring shape or holes are arranged around the core.

  Specifically, the optical fiber according to the present invention has a center core, a ring core that surrounds the center core, and a cladding that surrounds the ring core, and the refractive index of the center core is lower than the refractive index of the ring core. It is characterized by. The LP02 mode can be cut off by adjusting the refractive indexes of the center core and the ring core. Therefore, the present invention can provide an optical fiber in which the LP02 mode is blocked and the number of propagation modes is three.

  In the optical fiber according to the present invention, the refractive index of the center core is preferably lower than the refractive index of the cladding.

  For this purpose, the center core is made of quartz or holes added with fluorine.

  In addition, when the diameter of the center core of the optical fiber according to the present invention is d and the radius from the center of the center core to the ring core is a, d / (2a) ≧ 0.3 may be satisfied.

  On the other hand, an optical fiber according to the present invention having another structure includes a core, a clad surrounding the core, and a discrete within a cross section within the clad that is continuous in the light propagation direction and perpendicular to the light propagation direction. And a hole portion provided in a conventional manner. The LP02 mode can be blocked by arranging holes around the core. Therefore, the present invention can provide an optical fiber in which the LP02 mode is blocked and the number of propagation modes is three.

  The present invention can provide an optical fiber in which the LP02 mode is blocked and the number of propagation modes is three.

It is a figure explaining the cross section of the optical fiber which concerns on this invention. In the optical fiber which concerns on this invention, it is a figure explaining the change of the effective refractive index of each propagation mode when the diameter d of a center core is changed. In the optical fiber according to the present invention, the propagation mode is set to 3, and the regions a and Δ for satisfying a desired bending loss and the effective cross-sectional area of the LP01 mode are described. In the optical fiber which concerns on this invention, it is a figure explaining the effective cross-sectional area of the area | region of a and (DELTA) and LP11 mode for setting a propagation mode to 3 and satisfy | filling a desired bending loss. In the optical fiber which concerns on this invention, it is a figure explaining the effective cross-sectional area of the area | region of a and (DELTA) and LP21 mode to make propagation mode 3 and satisfy | fill a desired bending loss. In the optical fiber which concerns on this invention, it is a figure explaining the effective refractive-index difference between the area | region of a and (DELTA) and LP01-LP11 mode for setting propagation mode to 3 and satisfy | filling a desired bending loss. In the optical fiber which concerns on this invention, it is a figure explaining the effective refractive index difference between the area | region of a and (DELTA) and LP11-LP21 mode for setting propagation mode to 3 and satisfy | filling a desired bending loss. It is a figure explaining the refractive index distribution of a W-type refractive index distribution optical fiber. It is a figure explaining the refractive index distribution of the optical fiber which concerns on this invention. It is a figure explaining the refractive index distribution of the optical fiber which concerns on this invention. In a W-type gradient index optical fiber, when b / a = 2.25 and Δ ₂ = −0.5%, the propagation mode is 3, and the region of a and Δ for satisfying a desired bending loss, LP01 The effective area of mode and the effective refractive index difference between LP01-LP11 modes are shown. It is a figure explaining the cross section of the optical fiber which concerns on this invention. In the optical fiber which concerns on this invention, when (DELTA) of a core is 0.8%, it is a figure explaining the design range for setting the bending loss to 0.5 dB / 100turn and the number of propagation modes to three. In the optical fiber which concerns on this invention, when (DELTA) of a core is 0.9%, it is a figure explaining the design range for making bending loss 0.5dB / 100turn and propagation mode number three. It is a figure explaining the optical fiber transmission system relevant to this invention.

  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.

(Embodiment 1)
FIG. 1 is a cross-sectional view of the optical fiber of the present embodiment. The optical fiber of this embodiment has a center core 10, a ring core 20 surrounding the center core 10, and a clad 30 surrounding the ring core 20, and the refractive index of the center core 10 is lower than the refractive index of the ring core 20. And

FIG. 9 is a diagram illustrating the refractive index distribution of the optical fiber according to the present embodiment. The present optical fiber has a ring-shaped ring core 20. The ring core 20 has a core radius a, and the relative refractive index difference Δ of the ring core 20 with respect to the refractive index of the cladding is greater than zero. The center core 10 has a diameter d, and the relative refractive index difference Δ ₂ of the center core 10 with respect to the refractive index of the cladding is less than Δ.

なお、リングコア２０の屈折率、センタコア１０の屈折率、クラッドの屈折率をそれぞれｎ_ｃｏｒｅ，ｎ_{ｃｅｎｔｅｒ}，ｎ_ｃｌａｄとしている。 Δ and Δ ₂ are expressed by the following equations.

Note that the refractive index of the ring core 20, the refractive index of the center core 10, and the refractive index of the cladding are n _core , n _center , and n _clad , respectively.

FIG. 2 is a diagram for explaining the change in effective refractive index of each mode with respect to d when a = 7.0 μm, Δ = 0.9%, and Δ ₂ = 0 in the present optical fiber. It can be seen that as d increases, the LP01 and LP02 modes having an electric field peak at the center are greatly affected and decreased. Also, by setting d / (2a) to 0.3 or more, the effective refractive index of the LP02 mode can be made LP31 or less, the LP02 mode can be efficiently emitted, and the three modes LP01, LP11, and LP21 can be achieved. Can be propagated.

  FIGS. 3 to 5 are diagrams showing the results of calculating the range of Δ and a of an optical fiber in which the used wavelength band is 1530 to 1565 nm, the bending loss is 0.5 dB / 100 turn or less, and the number of propagation modes is 3. . In FIG. 3 to FIG. 5, thin curved dotted lines represent the effective cross-sectional areas of the LP01, LP11, and LP21 modes. The thick curved line, thick curved dotted line, and thick curved broken line are the ranges of Δ and a of the optical fiber in which the cutoff wavelength of LP02 and LP31 is 1530 nm, respectively, and the bending loss of LP21 mode is 0.5 dB / 100 turn at a bending radius of 30 mm. The range of Δ and a of the optical fiber is shown.

  Here, the condition that the fourth mode does not propagate is that the bending loss at the bending radius of 140 mm in LP02 or LP31 mode, which is the fourth propagation mode in the used wavelength band, is 1 dB / m or more. This condition affects the signal transmission when a bend radius of 140 mm is used to measure the cut-off wavelength (see Non-Patent Document 6), and when the loss of unnecessary higher-order modes is 1 dB / m or more. This is based on the assumption that there is no (see Non-Patent Document 1). As for bending loss, fiber recommendation G. ITU-T. The upper limit of the bending loss described in 656 is 0.5 dB / 100 turn.

  As a result of the above calculation, if the design is in the shaded region of FIGS. 3 to 5, the propagation mode is 3, and the bending loss of these modes is 0.5 dB / 100 turn or less at a bending radius of 30 mm. Can be realized.

6 and 7 are obtained by changing the thin dotted line in FIGS. 3 to 5 to the effective refractive index difference between the LP01-LP11 modes or between the LP11-LP21 modes. The effective refractive index difference necessary for suppressing the mode coupling was set to 0.5 × 10 ⁻³ (see Non-Patent Document 7). In the design of this optical fiber, since the effective refractive index difference is not less than the above value, it can be understood that the crosstalk between the three modes can be suppressed and the three-mode multiplex transmission can be realized.

As the refractive index distribution of the optical fiber in FIG. 10, the lower the refractive index of the center core 10, the higher the effect of lowering the effective refractive index of the LP02 mode. Specifically, by providing quartz or fluorine with fluorine added to the center core 10, Δ ₂ <0, and a three-mode fiber can be effectively realized.

(Comparative example)
A three-mode optical fiber can also be realized with the W-type refractive index profile shown in FIG. The W-type refractive index distribution optical fiber has a core having a core radius of a μm and a relative refractive index difference Δ ₁ > 0, and a low refractive region (trench) having a relative refractive index difference Δ ₂ <0. Let b be the radius to the boundary between the trench and the cladding.

1530~1565nm description as well as the wavelength band used in FIGS. 3-5, the bending loss 0.5dB / 100turn below were calculated delta ₁ and the range of a optical fiber count propagation mode is 3. The calculation results are shown in FIG. In the calculation, b / a = 2.25 and Δ ₂ = −0.5%. Comparing delta ₁ and a range of delta and a range (FIGS. 3 to 5) and W-shaped refractive index profile optical fiber of the optical fiber of this embodiment (FIG. 11), the shaded area in FIG. 11 ( The area where the bending loss satisfies the above conditions) is narrow, and may be out of the design range due to manufacturing errors. That is, by adopting the structure as shown in FIG. 1, it can be seen that a three-mode optical fiber can be manufactured more stably with a wide manufacturing margin.

FIG. 11 shows the effective cross-sectional area of the LP01 mode of the W-type gradient index optical fiber and the effective refractive index difference between the LP01 mode and the LP11 mode. The effective refractive index difference is a value sufficient to suppress mode coupling, while the effective area is 80 μm ² or less. That is, by adopting the structure as shown in FIG. 1, the effective area can be increased, and a low nonlinear optical fiber can be realized.

The effective area of the conventional single mode optical fiber is 80 μm ² . The three-mode optical fiber of the present embodiment can have a value of 80 μm ² or more that the effective area of the propagation mode cannot be realized with a W-type gradient index optical fiber. In other words, if the three-mode optical fiber of this embodiment is used, the same system design such as repeater spacing and transmission power as the system using the existing single-mode optical fiber can be adopted, which is advantageous in terms of cost. A multiplex transmission system can be constructed.

(Embodiment 2)
FIG. 12 is a cross-sectional view of the optical fiber of the present embodiment. The optical fiber of the present embodiment includes a core 40, a clad 50 that surrounds the core 40, and an empty space that is discretely provided in the clad 50 within a cross section that is continuous in the light propagation direction and orthogonal to the light propagation direction. And a hole 60.

In the present optical fiber, a core 40 having a refractive index n ₁ , a clad 50 being n ₂ , and a plurality of holes 60 are arranged in a regular hexagonal shape in the clad, and n ₁ > n ₂ . In addition, although the hole 60 surrounding the core 40 is described as a case of one layer, two or more layers may be used. In FIG. 12, the holes 60 are arranged in a regular hexagonal shape, but may be another regular polygonal shape or an annular shape. The radius of the core 40 is a, the diameter of the hole 60 is d ₁ , and the radius of the inscribed circle of the hole 60 is R.

In order to satisfy the condition of n ₁ > n ₂ in the structure of this optical fiber, the material of the core 40 is quartz doped with an impurity that increases the refractive index such as germanium (Ge) or aluminum (Al), and the material of the cladding 50 is Pure quartz is used. The material of the core 40 may be pure quartz, and the material of the clad 50 may be quartz to which an impurity such as fluorine (F) or boron (B) is added to reduce the refractive index. Further, the core 40 may be made of quartz with an increased refractive index, and the clad 50 may be made of quartz with a lowered refractive index.

  FIG. 13 shows that when the relative refractive index difference Δ of the core 40 with respect to the clad 50 is 0.8%, the used wavelength band is 1530 to 1565 nm, the bending loss is 0.5 dB / 100 turn or less, and the number of propagation modes is 3. The design range is calculated.

  The + or x lines with markers indicate structures in which the cutoff wavelength of LP02 is 1530 nm and the bending loss of the LP21 mode is 0.5 dB / 100 turn at a bending radius of 30 mm.

  Here, the condition that the fourth mode does not propagate is that the bending loss at the bending radius of 140 mm of the LP02 mode that is the fourth propagation mode in the used wavelength band is 1 dB / m or more. This condition affects the signal transmission when a bend radius of 140 mm is used to measure the cut-off wavelength (see Non-Patent Document 6), and when the loss of unnecessary higher-order modes is 1 dB / m or more. This is based on the assumption that there is no (see Non-Patent Document 1).

  FIG. 14 shows that when the relative refractive index difference Δ of the core 40 with respect to the clad 50 is 0.9%, the used wavelength band is 1530 to 1565 nm, the bending loss is 0.5 dB / 100 turn or less, and the number of propagation modes is 3. The design range is calculated. Similar to the results of FIG. 13, it can be seen that by controlling d1 and R, the desired bending loss characteristic can be realized while limiting the number of propagation modes to three.

For example, when the relative refractive index difference of the core is 0.8%, a = 6.3 μm, d ₁ /(2a)=0.35, and R / a = 1.07. The area becomes 80 μm ² , which is equivalent to the effective cross-sectional area of the conventional single mode fiber.

In addition, when the relative refractive index difference of the core is 0.9%, a = 5.8 μm, d ₁ /(2a)=0.35, and R / a = 1.12. The area is 71 μm ² , and a value equivalent to the effective area of the conventional single mode fiber can be realized.

(Embodiment 3)
FIG. 15 is a diagram illustrating an optical fiber transmission system 300 according to the present embodiment.

  Signals emitted from the N transmitters 100 are converted into different modes by the mode multiplexer 140, and after being combined, are incident on the optical fiber 150. At the output end of the optical fiber 150, the mode demultiplexer 160 similarly demultiplexes the signals into N ports, which are respectively received by the receiver 200. The optical fiber 150 is an optical fiber having the propagation mode number 3 described with reference to FIG.

  The optical fiber 150 has three LP modes, and each LP mode has a plurality of modes having equivalent characteristics called degenerate modes. The number of these degenerate modes (including orthogonal polarization modes) is 8 in the optical fiber of FIG. However, the number of the above-mentioned transmitters / receivers is for a single wavelength, and when a wavelength division multiplexing transmission system is constructed, the transmitter 100 and the receiver 200 having a maximum number of 8 × wavelength multiplexing are required.

[Appendix]
The following describes the optical fiber according to the present invention.
(1): Consists of a ring-shaped core portion, a low refractive index region existing inside the ring-shaped core region, and a clad portion surrounding them, blocking LP02 mode propagation and reducing the number of propagation modes. 3. An optical fiber characterized by being set to 3.
(2): The light as described in (1) above, wherein the low refractive index region present in the core is quartz, quartz to which a substance that lowers the refractive index is added, or a hole. fiber.
(3) The optical fiber according to (1) or (2) above, wherein d / 2a> 0.3, where d is a diameter of the low refractive index region and a is a core radius.
(4): A core portion, a clad portion surrounding the core portion, and vacant spaces provided discretely within a cross section perpendicular to the optical fiber axial direction and continuously in the optical fiber axial direction. An optical fiber, wherein the core portion is made of a material having a refractive index higher than that of the material of the cladding portion, the propagation of the LP02 mode is blocked, and the number of propagation modes is set to 3. .

  The present invention is a three-mode optical fiber capable of constructing a mode multiplex transmission system in which LP02 mode does not propagate, crosstalk between modes in the fiber and at connection points can be suppressed, and a MIMO equalizer is unnecessary at the receiving end. Is to provide.

  According to the present invention, there is no need for a MIMO equalizer at the receiving end in the three-mode multiplex transmission system, and it is possible to construct a low-cost system.

Furthermore, according to the present invention, the transmission capacity can be increased as compared with the two-mode multiplex transmission system. The design area for realizing the three-mode fiber is widened, and the requirement for manufacturing error is relaxed.

  Furthermore, according to the present invention, it is possible to realize a three-mode optical fiber having an effective area larger than that of a conventional single mode optical fiber, and it is possible to suppress a nonlinear phenomenon occurring in the fiber.

  The present invention can realize large capacity / long distance communication by suppressing nonlinear phenomenon in a fiber or using a mode.

10: Center core 20: Ring core 30: Clad 40: Core 50: Clad 60: Hole 100: Transmitter 140: Mode multiplexer 150: Optical fiber 160: Mode duplexer 200: Receiver 300: Optical fiber transmission system

Claims (1)

The center core,
A ring core surrounding the center core;
A clad surrounding the ring core,
The radius a from the center of the center core to the outer periphery of the ring core is 7.0 μm, the relative refractive index difference Δ of the ring core with respect to the refractive index of the cladding is 0.9%, and the relative refractive index of the center core with respect to the refractive index of the cladding the difference delta ₂ to 0, and the relationship between the radius a and the diameter d of the center core by a d / (2a) ≧ 0.3, and 3 the number of propagation modes to interrupt the LP02 mode <br / > Optical fiber characterized by that.

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