JP4630124B2 - Optical fiber - Google Patents

Description

  The present invention relates to an optical fiber that suppresses the occurrence of nonlinear phenomena.

  In recent years, with the expansion of communication capacity, the amount of signal transmitted through one optical fiber has increased. In order to increase the signal capacity, it is necessary to widen the use band or narrow the wavelength interval. High-density transmission in a wide band exceeding 100 nm has already been reported, and the high-density transmission is moving toward practical use.

  As the amount of signal input to the optical fiber increases, the optical power input to the optical fiber increases, and the occurrence of nonlinear phenomena becomes noticeable. Among nonlinear phenomena, stimulated Raman amplification shifts signal energy in the vicinity of 1500 nm to the longer wavelength side by about 100 nm. Therefore, if a wide wavelength range is transmitted, transmission on the short wavelength side becomes difficult.

  Also, since the transmission loss is relatively large on the short wavelength side, it is necessary to input a larger power in order to ensure an SNR (Signal-to-Noise Ratio). However, there is a limit to the increase in power due to stimulated Brillouin scattering (SBS).

  Several definitions of acceptable SBS values, or SBS thresholds, have been proposed. FIG. 6 illustrates the definition of the SBS threshold. In the present invention, a straight line (straight line 1 in FIG. 6) indicating the linear dependence of the backscattered light intensity (Rayleigh scattered light intensity) with respect to the incident intensity at the time of low power incidence where no SBS is generated, and the backscattering after the generation of SBS. The incident light intensity at the intersection of the straight line (straight line 2 in FIG. 6) indicating the linear dependence of the light intensity (stimulated Brillouin scattered light intensity) is defined as the SBS threshold value. As is apparent from FIG. 6, the SBS light intensity is small when the input light intensity is low, and therefore the backscattered light intensity is dominated by the Rayleigh scattered light intensity from the input light. On the other hand, when the input light intensity increases, the occurrence of SBS increases, and the SBS light intensity becomes larger than the Rayleigh scattered light intensity. Therefore, SBS is dominant as the whole backscattered light intensity.

  The SBS threshold depends on the fiber length, and the longer the length, the smaller the SBS threshold. The SBS threshold value also depends on the line width of the light source, and the SBS threshold value decreases as the light source having a larger line width is used. Usually, the SBS threshold value of a single mode fiber (SMF) is about 5 mW (+7 dBm) when a light source having a wavelength of 1550 nm and a line width of 5 MHz is used at a fiber length of 20 km. Furthermore, even if the SBS threshold can be improved, nonlinear distortion is increased by high power input.

  Furthermore, SBS is caused by the acoustic wave mode in the fiber, unlike the nonlinear refractive index and Raman gain. The acoustic wave is affected by fluctuations in the longitudinal direction of the fiber and stress due to lateral pressure, and the change of the acoustic wave affects the SBS threshold. Similar to normal light waves, the characteristics of acoustic waves are determined by the refractive index profile, and the SBS threshold is increased by suppressing the gain peak of the resulting Stokes light.

A method for suppressing the generation of SBS by changing the refractive index distribution in the longitudinal direction of the optical fiber is known in Non-Patent Document 1 and the like. However, although the method of changing the refractive index distribution in the longitudinal direction can greatly suppress the generation of SBS, its production is difficult and the cost increases.
"SBS Threshold of a Fiber With a Brillouin Frequency Shift Distribution" by K. Shiraki (IEEE Journal of Lightwave Technologies, Vol. 14, No.1, January 1996, pp. 50-57)

The present invention pays attention to the fact that the optical power in a normal optical fiber is the strongest at the center, and suppresses the nonlinear phenomenon by changing this distribution. More specifically, the optical power in an optical fiber is usually the strongest at its center, and the refractive index of the optical fiber is generally the highest at the center. In order to make the refractive index of the core higher than the refractive index of the cladding, GeO ₂ is used as a dopant for ordinary optical fibers. The higher the GeO ₂ concentration, the higher the nonlinear refractive index and the nonlinear phenomenon occurs. It becomes easy. Therefore, the average nonlinear refractive index in the optical power distribution can be lowered by lowering only the refractive index near the core center without lowering the refractive index around the core. Furthermore, averaging the power distribution near the core center is also more effective for avoiding nonlinear phenomena. Similar to the nonlinear refractive index, the Raman gain increases as the core GeO ₂ concentration increases and the effective area decreases. In addition, the Raman gain coefficient of the core itself is lowered and the effective area is increased by the above method, so that the Raman gain efficiency is also reduced. Furthermore, in the present invention, by optimizing the refractive index distribution, the acoustic wave modes in the optical fiber are separated and the peak power is lowered, thereby suppressing the SBS threshold. Further, as the number of gain peaks of the Stokes light increases, the peak value becomes smaller and the SBS threshold is effectively suppressed.

1 to 3 show the refractive index distribution of the optical fiber of the present invention. The optical fiber having the refractive index profile shown in FIG. 1 includes a first core 11 having a diameter a, a second core 12 having an outer diameter b, and a clad having an inner diameter b. An optical fiber having a refractive index profile shown in FIG. 2 has a first core 21 having a diameter a, a second core 22 having an outer diameter b, a third core 23 having an outer diameter c, and an inner diameter c. Made of clad. Further, the optical fiber having a refractive index profile shown in FIG. 3 includes a central core 33 having an outer diameter a ₀ , a first core 31 having an outer diameter a, and a second core 32 having an outer diameter b, It consists of a clad having an inner diameter c.

The relative refractive index differences Δ1, Δ2, Δ3, Δ0 in FIGS. 1 to 3 are obtained by the following equations (1) to (4).
Δ1 = {(n ₁ −n _C ) / n _C } × 100 [%] (1)
Δ2 = {(n ₂ −n _C ) / n _C } × 100 [%] (2)
Δ3 = {(n ₃ −n _C ) / n _C } × 100 [%] (3)
Δ0 = {(n ₀ −n _C ) / n _C } × 100 [%] (4)
Here, n ₁ represents the minimum refractive index of the first core, n ₂ represents the maximum refractive index of the second core, and n ₃ represents the minimum refractive index of the third core when the refractive index is lower than that of the cladding. Represents the maximum refractive index of the third core when the refractive index is higher than that of the cladding. Furthermore, n ₀ represents the maximum refractive index of the central core, and n _C represents the refractive index of the cladding.

  The diameter a of the first core is a diameter at a position having a relative refractive index difference of ½ of Δ2 at the boundary between the first core and the second core. The diameter b of the second core is a diameter at a position having a relative refractive index difference of 1/10 of Δ2 at the boundary between the second core and the third core. The diameter c of the third core is a diameter at a position having a relative refractive index difference of ½ of Δ3 at the boundary between the third core and the clad. The diameter a0 of the central core is a diameter at a position having a relative refractive index difference of 1/10 of Δ0 at the boundary between the central core and the first core.

The refractive index distribution shape α ₂ is defined by the following equation (5).
n ² (r) = n ₃ ² {1-2 (Δ1 / 100) × (2r / c) ^α2 } (5)
(However, b _max <r <b / 2)
Here, r represents the position in the radial direction from the center of the optical fiber, and n (r) represents the refractive index at the position r. In addition, b _max indicates the position in the radial direction from the center of the optical fiber of the portion having the highest refractive index of the second core, and when the highest portion extends to a certain region instead of one point, Position.

  The cutoff wavelength is ITU-TG. The cable cutoff wavelength λcc specified in 650.1 is assumed. For other terms not specifically defined in this specification, see ITU-T G.C. It shall follow the definition and measurement method in 650.1.

表１のＮｏ．０は通常のＳＭＦのパラメータを示し、Ｎｏ．１乃至Ｎｏ．１３は、図１に示すプロファイルを有する本発明の光ファイバのパラメータを示す。Ｎｏ．１４及びＮｏ．１５のように第２コアの外周に第３コアを設けてもよい（図２参照）。 An optical fiber having a concave guide type refractive index profile in which the refractive index of the first core located at the central portion is reduced as compared with the second core located at the outer periphery of the first core was manufactured. Table 1 shows the parameters of the optical fiber.

No. in Table 1 0 indicates a normal SMF parameter. 1 to No. Reference numeral 13 denotes parameters of the optical fiber of the present invention having the profile shown in FIG. No. 14 and no. As shown in FIG. 15, a third core may be provided on the outer periphery of the second core (see FIG. 2).

  Compared to SMF, the SBS threshold is higher for all fibers. Of these fibers, ordinary SMF and No. The Stokes light spectra of the two fibers are shown in FIGS. 4 and 5, respectively. The SMF has one strong peak, and as the incident power is increased, one of the strongest peaks increases and reaches the SBS threshold. On the other hand, there are several peaks in FIG. 5. As the incident power is increased, the reflected light power is dispersed and increases at the same time. Therefore, it can be said that the increase in the SBS threshold is suppressed. The spectrum of FIG. The same phenomenon has been observed with other fibers.

（但し、λは光波長、ｎ₂は非線形屈折率、そしてＡ_effは有効コア断面積を示す。） In general, a nonlinear coefficient γ is used as a value indicating the degree of nonlinearity of an optical fiber. γ is a value defined by the following equation (7).

(Where λ is the optical wavelength, n ₂ is the nonlinear refractive index, and A _eff is the effective core area)

As can be understood from Equation (7), γ generally depends on the nonlinear refractive index n _{2 of the} portion having the highest refractive index of the core portion and the effective core area A _eff . Also, γ / n ₂ × A _eff = 2π / λ, which is a substantially constant value. Therefore, no. In a fiber having a unimodal profile such as 0, γ / n ₂ × A _eff becomes a substantially constant value even if the refractive index of the core is changed. That is, there is a trade-off relationship between the reduction of γ, which is effective for reducing the nonlinearity, and the expansion of A _eff , and the nonlinearity cannot be reduced even if any one is improved.

However, no. 1 to No. 9 The fiber _becomes a value smaller than the fiber having a value of γ / _{n 2} × _{A eff} since the gamma is suppressed more than the expansion of _{A eff} all single-peak profile, thus gamma / _{n 2} × _{A eff} <3.65 × 10 ⁹ is satisfied. An optical fiber satisfying γ / n ₂ × A _eff <3.65 × 10 ⁹ is No. Compared to 0 SMF, it can be said that the trade-off between the reduction of γ and the expansion of A _eff is eliminated, and the nonlinearity can be greatly reduced. This can be attributed to the fact that the concentration of power can be avoided by adopting the concave guide type refractive index profile in which the refractive index of the central part of the core is reduced from the peripheral part. No. 1 to No. In FIG. 9, both suppression of the SBS threshold and reduction of γ are realized.

No. 10 to No. 13 No. by reducing Delta] 2, to increase the alpha _2, or b 1 to No. A _eff is smaller than that of 9 fibers. However, although A _eff is smaller than that of SMF, the value of γ itself is not different from that of SMF. Thereby, γ / n ₂ × A _eff is No. 1 to No. As with 9 fibers, it is reduced.

  Generally, γ can be examined by the degree of nonlinear effect (self-phase modulation or cross-phase modulation) of the output light with respect to the input light intensity to the fiber. Γ at this time can be considered as an average value of overlapping of the refractive index profile of the fiber and the electric field distribution of light.

By the way, it is estimated that the nonlinear refractive index (n ₂ ) has a distribution in the radial direction as well as the refractive index. This is because the nonlinear refractive index also changes depending on the doping amount of GeO ₂ which is a dopant, similarly to the refractive index. Here, a nonlinear refractive index distribution (n ₂ (r)) in the radial direction is considered. Since it is difficult to actually measure n 2 a _(r), it was calculated n _{2 (r)} from the doping amount distribution of GeO ₂ in the radial direction obtained from the refractive index profile. The relationship between the doping amount of GeO ₂ and n ₂ is experimentally known. The value (n ₂ (r = 0)) at the core center obtained by calculation was compared with n _2av obtained from the actually measured values of γ and A _eff . Here, n _2av obtained from the actual measurement value is a value averaged in the radial direction.

の関係が成り立ち、波長１５５０ｎｍにおいてはγ／ｎ_２(r=0)×Ａ_ｅｆｆ＝４．０５×１０^９(ｋｍ^−１）となる。各ファイバでのγ／ｎ_２ａｖ×Ａ_ｅｆｆの実測値を見ると、通常ＳＭＦでは、４．１３×１０^９（ｋｍ^−１）であることから、ｎ_２(r=0)＞ｎ_２ａｖである。しかし、Ｎｏ．１からＮｏ．１５においてはｎ_２(r=0)＜ｎ_２ａｖの関係がある。これらはコアの中心部の屈折率を周辺部より低減させたことで、コア中心部での非線形屈折率が低減されたことに起因しているといえる。 When the relationship of n ₂ (r = 0) = n _2av is established, from the equation (7), at the center of the optical fiber,

Thus, at a wavelength of 1550 nm, γ / n ₂ (r = 0) × A _eff = 4.05 × 10 ⁹ (km ⁻¹ ). Looking at the actual measurement value of γ / n _2av × A _{eff in} each fiber, it is 4.13 × 10 ⁹ (km ⁻¹ ) in normal SMF, and therefore n ₂ (r = 0)> n _2av . . However, no. 1 to No. 15, there is a relationship of n ₂ (r = 0) <n _2av . These can be attributed to the fact that the refractive index at the central part of the core is reduced from the peripheral part, thereby reducing the nonlinear refractive index at the central part of the core.

No. 1 to No. 15, the ratio A _eff / TH ^SBS between A _eff and the SBS threshold value (TH ^SBS ) is 15 μm ² / mW or less, which is smaller than SMF. This indicates that the effect of suppressing the SBS threshold is obtained more than the effect of reducing the non-linearity due to the expansion of A _eff .

The Raman gain is no. 1 to No. 9 is reduced. This is also a characteristic change caused by reducing the refractive index of the core center part, and is effective in preventing energy transition when transmitted in a wide wavelength range. By setting the Raman gain efficiency (g _R / A _eff ) to 0.35 (1 / W / km) or less, it is possible to reduce an increase in noise due to SRS (stimulated Raman scattering) when performing broadband transmission. In contrast, no. 10 to No. The Raman gain efficiency (g _R / A _eff ) of 13 is not reduced as compared with the normal SMF. This is because A _eff is reduced compared to SMF.

When constructing a system using this fiber, the cutoff wavelength and bending loss are important characteristics as well as nonlinear phenomena. These properties are closely related to the core refractive index and refractive index shape parameter α. In order to use in the 1.55 μm band, the cutoff wavelength needs to be 1520 nm or less. In order to shift the cutoff wavelength to the short wavelength is effective to reduce the Δ2 or alpha _2. alpha ₂ is Delta] 2 Delta] 2 is smallest acceptable when satisfying the cutoff wavelength be stepwise (α _{2 =} ∞). No. In No. 4, the cut-off wavelength slightly exceeded 1530 nm. 7 realizes 1530 nm or less. It is also possible to shift the cutoff wavelength to a shorter wavelength while reducing Δ2 by sacrificing bending loss. 6 shows that when Δ2 is 0.32%, an increase in bending loss is caused even if the cutoff wavelength can be made 1520 nm or less. From this result, it can be said that the minimum value of Δ2 is 0.34%. Further, even if α ₂ is reduced, the cutoff wavelength shifts to the longer wavelength side. According to No. ₂ , if α ₂ is 6, both the cutoff wavelength and bending loss characteristics can be satisfied. 1 and no. Therefore, when α ₂ becomes 4, the cutoff wavelength and bending loss cannot be satisfied simultaneously.

  In order to use a fiber in which the generation of SBS is suppressed in the 1.3 μm band, it is necessary to further set the cutoff wavelength to 1260 nm or less. No. 10 to No. Twelve fibers are SBS suppression fibers that have a cut-off wavelength of 1260 nm or less and can be used in the entire wavelength region longer than the 1.3 μm band.

表２におけるＮｏ．１２は、パラメータの記述が異なっているが表１のＮｏ．１２と同一のファイバである。Δ１とΔ２の差を０．２％以上とすることが効果的である。 Table 2 shows the results of examining the effect of the difference between Δ1 and Δ2.

No. in Table 2 12 is different in parameter description, but No. 12 in Table 1. 12 is the same fiber. It is effective to set the difference between Δ1 and Δ2 to 0.2% or more.

  Furthermore, the SBS suppression effect is not limited to the case where the refractive index at the core center is lower than the surroundings. No. 18, no. Reference numeral 19 denotes a central core having a large refractive index provided inside Δ1 (see FIG. 3). No. 18 and No. As can be seen from FIG. 19, Δ0−Δ2 is preferably 0.1% or less from the viewpoint of the SBS suppression effect.

It is a figure which shows the profile of the optical fiber of this invention which has a 1st and 2nd core. It is a figure which shows the profile of the optical fiber of this invention which has a 1st thru | or 3rd core. It is a figure which shows the profile of the optical fiber of this invention which has a center core and the 1st and 2nd core. It is a figure showing the Stokes light spectrum of normal SMF. No. in Table 1 It is a figure showing the Stokes light spectrum of 2 optical fibers. It is a figure which shows the definition of SBS threshold value (input light intensity dependence of fiber transmitted light and backscattered light).

Explanation of symbols

11 1st core 12 2nd core 21 1st core 22 2nd core 23 3rd core 31 1st core 32 2nd core 33 Central core

Claims (6)

An optical fiber comprising a core and a clad and the core is made a first core layer in the center, and a second core layer in the outer periphery of the first core layer from including core layer, The relative refractive index difference Δ1 of the first core layer relative to the cladding is lower than the relative refractive index difference Δ2 of the second core layer relative to the cladding, and the Δ1 and Δ2 are −0.50% ≦ Δ1 ≦ 0.28. %, 0.32% ≦ Δ2 ≦ 0.55%, Δ2−Δ1 ≧ 0.2%, and the diameter a of the first core and the diameter b of the second core are 0.2 ≦ a / b ≦ 0.38, 7.1 μm ≦ b ≦ 11.1 μm ,
The SBS threshold value (TH) at a wavelength of 1550 nm at a length of 20 km when a light source having a substantially uniform refractive index distribution in the longitudinal direction and a transmission loss at a wavelength of 1550 nm of 0.3 dB / km or less and a line width of 5 MHz or less An optical fiber characterized in that ^SBS ) is 5 mW or more. An optical fiber comprising a core and a clad and the core is made a first core layer in the center, and a second core layer in the outer periphery of the first core layer from including core layer, The relative refractive index difference Δ1 of the first core layer relative to the cladding is lower than the relative refractive index difference Δ2 of the second core layer relative to the cladding, and the Δ1 and Δ2 are −0.50% ≦ Δ1 ≦ 0.28. %, 0.32% ≦ Δ2 ≦ 0.55% , Δ2−Δ1 ≧ 0.2% , and the diameter a of the first core and the diameter b of the second core are 0.2 ≦ a / b ≦ 0.38, 7.1 μm ≦ b ≦ 11.1 μm ,
A _eff (μm ² ) at a wavelength of 1550 nm and an SBS threshold value at a wavelength of 1550 nm at a length of 20 km when a light source having a transmission loss at a wavelength of 1550 nm of 0.3 dB / km or less and a line width of 5 MHz or less is used ( A ratio of A _eff / TH ^SBS to TH ^SBS ) of 15 μm ² / mW or less. 3. The optical fiber according to claim 1, further comprising a third core layer having a refractive index lower than that of the second core layer on an outer periphery of the second core layer, wherein the third core layer with respect to the clad. The relative refractive index difference Δ3 is −0.20 ≦ Δ3 ≦ 0.20, and the diameter b of the second core layer and the diameter c of the third core layer are 1.1 ≦ c / b ≦ 1. 3. An optical fiber, wherein 9.9 μm ≦ c ≦ 10.5 μm . 4. The optical fiber according to claim 1, wherein −0.50% ≦ Δ1 ≦ 0.15%, a nonlinear parameter γ (1 / W / km) at a wavelength of 1550 nm, and A _eff (Μm ² ) and non-linear refractive index n ₂ (m ² / W) have a relationship of (γ / n ₂ ) × A _eff <3.65 × 10 ⁹ (1 / km) fiber. The optical fiber according to any one of claims 1 to 4,
−0.50% ≦ Δ1 ≦ 0%, 8.0 μm ≦ b ≦ 11.1 μm ,
An optical fiber having a Raman gain efficiency of 0.35 (1 / W / km) or less by pumping light having a wavelength of 1450 nm ± 50 nm. The optical fiber according to any one of claims 1 to 5,
0.34% ≦ Δ2 ≦ 0.55%, and the refractive index distribution parameter α _{2 of} the second core layer is α ₂ > 6,
Optical fiber cable cut-off wavelength is equal to or less than 1520 nm.

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