JP4219825B2 - Nonlinear dispersion-shifted optical fiber - Google Patents

Description

  The present invention relates to an optical fiber having excellent nonlinearity and an optical signal processing apparatus using the optical fiber.

  In recent years, there has been an increasing demand for higher speed, higher capacity, and longer distance transmission in optical signal transmission. For this reason, signal processing technology for achieving higher speed of optical signal processing and longer distance transmission is desired. Yes. As one of optical signal processing techniques, there is a method of converting an optical signal into an electrical signal, processing the converted electrical signal, and returning it to an optical signal again. However, this method is not suitable for high-speed signal processing because it involves processing to change an optical signal into an electrical signal and return it to an optical signal.

  On the other hand, there is an all-optical signal processing technique for processing an optical signal as it is. Since this processing technique handles an optical signal directly as an optical signal without changing the optical signal into an electrical signal, high-speed optical signal processing becomes possible.

  By the way, the all-optical signal processing technology includes a method using a nonlinear optical phenomenon that occurs in an optical fiber that transmits an optical signal, or a method that uses a nonlinear phenomenon that occurs in an optical waveguide made of a highly nonlinear material. . The all-optical signal processing technique using the non-linear optical phenomenon generated in the former optical fiber has attracted particular attention in recent years because it can perform high-speed processing and reduce transmission loss. Nonlinear phenomena that occur in the optical fiber include four-wave mixing, self-phase modulation, cross-phase modulation, and Brillouin scattering. Among these, optical signal processing techniques such as wavelength conversion using four-wave mixing, pulse compression using self-phase modulation, and waveform shaping have already been reported.

  Four-wave mixing is a phenomenon in which when two or more wavelengths of light are introduced into an optical fiber, light of a new wavelength is generated with a specific rule due to a nonlinear phenomenon. In the optical signal processing technique described above, the phenomenon in which light having a new wavelength is generated is to be used for wavelength conversion. Further, the wavelength conversion using the four-wave mixing has an advantage that a large number of signal wavelengths can be converted at once. In addition, by using self-phase modulation or cross-phase modulation, it is possible to reshape a waveform deteriorated during transmission and perform all-optical signal processing that enables long-distance transmission.

  By the way, in order to apply optical signal processing techniques such as wavelength conversion and waveform shaping using nonlinear phenomena such as four-light mixing and self-phase modulation in an optical fiber, nonlinear phenomena can be caused greatly as an optical fiber. An optical fiber, that is, an optical fiber having high nonlinearity is required. An optical fiber having high nonlinearity is proposed by Patent Document 1. Various characteristics of this optical fiber at a wavelength of 1550 nm are shown in detail in FIGS.

JP 2002-207136 A

However, in the optical fiber disclosed in Patent Document 1, the dispersion slope value at the wavelength of 1550 nm is −0.267 ps / nm ² / km to +0.047 ps / nm ² / km, and the dispersion is large. The absolute value of the lower limit value is an extremely large value of 103.2 ps / nm / km, such as 103.2 ps / nm / km to +3.3 ps / nm / km. That is, it is impossible to provide an optical fiber having a small absolute value of dispersion and dispersion slope at a wavelength of 1550 nm, and therefore it is not possible to provide an optical fiber having a low dispersion in a wide wavelength region near the wavelength of 1550 nm. It was.

  In order to achieve the above object, a nonlinear dispersion-shifted optical fiber according to the present invention includes a first core located at the center, and a second core provided on the outer periphery of the first core and having a lower refractive index than the first core. A third core provided on the outer periphery of the second core and having a refractive index lower than that of the first core and higher than that of the second core; and a refractive index lower than that of the third core on the outer periphery of the third core; A clad having a refractive index higher than that of the core is provided.

  According to the present invention, it is possible to provide an optical fiber having a stable and low dispersion in a wide wavelength region including a wavelength of 1550 nm (for example, 1460 nm to 1620 nm). Further, it is possible to provide an optical signal processing apparatus using this optical fiber, specifically, an optical wavelength converter or a pulse compressor having excellent performance.

  FIG. 1 shows an example of a typical refractive index distribution of a nonlinear dispersion-shifted optical fiber according to the present invention. As shown in FIG. 1, the optical fiber of the present invention is located at the center, the first core whose refractive index profile is an α-power profile, and the outer periphery of the first core, which is lower than the first core. A second core having a refractive index, provided on an outer periphery of the second core, provided on an outer periphery of the third core having a refractive index lower than that of the first core and higher than that of the second core; The clad has a refractive index lower than that of the three cores and higher than that of the second core. D1 is the outer diameter of the first core located at the center, D2 is the outer diameter of the second core provided on the outer periphery of the first core, and D3 is the third core provided on the outer periphery of the second core. Indicates the outer diameter. Here, the outer diameter D <b> 1 of the first core 1 is the length of a line connecting positions where the first core 1 has a refractive index equal to that of the cladding 4. The outer diameter D2 of the second core 2 is the length of a line connecting positions where the refractive index difference is ½ of the relative refractive index difference Δ2 in the boundary region between the second core 2 and the third core 3, The outer diameter D3 of the three cores is the length of a line connecting positions where the refractive index difference is 1/10 of the relative refractive index difference Δ3 in the boundary region between the third core 3 and the cladding. The outer diameter D1 of the first core is 2 to 5 μm. The shape of the third core may be a step type or an α power profile, and may further include a fourth core, a fifth core, and the like on the outer periphery of the third core.

Here, in the present specification, Δ1 is the relative refractive index difference of the first core relative to the cladding, Δ2 is the relative refractive index difference of the second core relative to the cladding, and Δ3 is the relative refractive index difference of the third core relative to the cladding. Δ1 to Δ3 have the following relations (1) to (3).
Δ1 = {(n _c1 −n _c ) / n _c1 } · 100 (1)
Δ2 = {(n _c2 −n _c ) / n _c2 } · 100 (2)
Δ3 = {(n _c3 −n _c ) / n _c3 } · 100 (3)
Further, α representing the refractive index distribution shape of the first core is expressed by Equation (4) when the refractive index of the first core at a distance r (0 ≦ r ≦ D1 / 2) from the center is n ² (r). Defined.
n ² (r) = n _c1 ² {1-2 · Δ1 _n · (2r / D1) ^α } (4)
In Expressions (1) to (4), n _c1 represents the maximum refractive index of the first core, n _c2 represents the minimum refractive index of the second core, and n _c3 represents the maximum refractive index of the third core. Note that Δ1 _n in Equation (4) does not represent Δ1 as a percentage, and has a relationship of Δ1 _n = {(n _c1 −n _c ) / n _c1 }.

  In general, in an optical fiber composed of a first core located in the center, a second core provided on the outer periphery of the first core, and a cladding provided on the outer periphery of the second core, the relative refraction of the second core with respect to the cladding. When the rate difference Δ2 (see formula (2)) is increased to the negative side, it is possible to reduce the dispersion slope while reducing the absolute value of dispersion at the wavelength of 1550 nm. However, in order to further reduce the dispersion slope, a third core that is provided on the outer periphery of the second core and has a higher refractive index than the cladding is required. Hereinafter, it will be described that the dispersion srobe can be reduced by having the third core.

  In the optical fiber having the refractive index distribution shown in FIG. 1, Δ3 was changed, and changes in dispersion slope, Aeff, and cutoff wavelength were obtained by simulation. Table 1 shows structural parameters other than Δ3 of the optical fiber used in the simulation.

表１ シミュレーションで用いた光ファイバの構造パラメータ（１）

Table 1 Structural parameters of optical fiber used in the simulation (1)

  FIG. 2 shows the relationship between the relative refractive index difference Δ3 and the dispersion slope of an optical fiber having the structural parameters shown in Table 1 at a wavelength of 1550 nm. As shown in FIG. 2, when the relative refractive index difference Δ3 is increased, the dispersion slope is reduced. However, when the relative refractive index difference Δ3 is increased, the effective core area Aeff is increased as shown in FIG. 3, and the obtained nonlinearity is relatively decreased. Furthermore, as shown in FIG. 4, the cutoff wavelength is shifted to the long wavelength side. Therefore, it is necessary to adjust the relative refractive index difference Δ3 and the core diameter of the third core so that Aeff does not become too large and the cutoff wavelength does not exceed 1500 nm. Specific adjustment will be described below.

(1) Effective core area Aeff
The nonlinear constant is represented by the following equation (5).
Nonlinear constant = (n ₂ / Aeff) (5)
Where λ is the measurement wavelength, n ₂ is the nonlinear refractive index in the optical fiber, and Aeff is the effective area of the optical fiber.
From equation (5), it can be understood that in order to increase the nonlinear constant of the optical fiber, it is necessary to increase the nonlinear refractive index n ₂ or decrease the effective area Aeff. However, since n ₂ is a value determined by the material, it cannot be easily increased. Therefore, it is practical to make the value of the effective area Aeff of the optical fiber as small as possible. Therefore, in the present invention, the effective area Aeff of the optical fiber is set to 15 μm ² or less, preferably 12 μm ² or less. As a result, an optical fiber having a larger nonlinear constant such that the nonlinear constant at a wavelength of 1550 nm is 25 × 10 ⁻¹⁰ / W or more, and further 40 × 10 ⁻¹⁰ / W or more can be obtained.

(2) Specific refractive index differences Δ1 and Δ2
In order to reduce Aeff, it is most effective to increase the relative refractive index difference Δ1. Therefore, a simulation was performed to derive an appropriate relative refractive index difference Δ1. Table 2 shows structural parameters other than Δ1 of the optical fiber used in the simulation. Δ2, Δ3, etc. are the same as described above, and follow the above formulas (1) to (4). Note that Example 2-2 and Example 2-3 do not have the third core.

表２ シミュレーションで用いた光ファイバの構造パラメータ（２）

Table 2 Optical fiber structural parameters used in the simulation (2)

FIG. 5 shows the relationship between the relative refractive index difference Δ1 of the optical fiber having the structural parameters shown in Table 2 and the effective core area Aeff. As shown in FIG. 5, when the relative refractive index difference Δ1 increases, the effective core area Aeff decreases. Further, comparing Example 2-1 and Example 2-2, which have the same structural parameters except for the presence or absence of the third core, Aeff can be somewhat expanded by providing the third core having a higher refractive index than the cladding. I understand. If the relative refractive index difference Δ1 is less than 1.5%, Aeff is large and nonlinearity is relatively small. Therefore, in order to satisfy the requirement that Aeff is 15 μm ² or less in the optical fiber having the third core, it is necessary that at least the relative refractive index difference Δ1 is 1.5% or more.

  On the other hand, when the relative refractive index difference Δ1 is increased, the cutoff wavelength is shifted to the long wavelength side. Therefore, when the relative refractive index difference Δ1 exceeds 5.0%, consideration for the cutoff wavelength for the single-mode operation of the optical fiber is increased, and the productivity is deteriorated. In addition, it is very difficult to manufacture a core having a relative refractive index difference Δ1 exceeding 5.0%. Further, when the relative refractive index difference Δ1 exceeds 5.0%, the value of the dispersion slope at 1550 nm increases, and when optical signal processing is performed, variation in dispersion increases for different wavelengths near the wavelength of 1550 nm. Therefore, the relative refractive index difference Δ1 is preferably 1.5 to 5.0%.

Further, if the relative refractive index difference Δ2 is increased to the negative side, it is possible to reduce the dispersion slope while reducing the absolute value of dispersion at 1550 nm, and the cutoff wavelength is shifted to the short wavelength side. As described above, when the relative refractive index difference Δ1 is 1.5 to 5.0%, when the relative refractive index difference Δ2 is −0.1% or less, the absolute value of the dispersion slope is 0.03 ps / nm ² / It is possible to set the absolute value of the dispersion slope to 0.01 ps / nm ² / km or less when the relative refractive index difference Δ2 is −0.7% or less. Also, the cutoff wavelength can be made 1500 nm or less. On the other hand, if the relative refractive index difference Δ2 is set to −1.4% or less, for example, it is necessary to dope a large amount of fluorine, which makes manufacturing difficult. Accordingly, the relative refractive index difference Δ2 is preferably −1.4 to −0.1%, and more preferably −1.4 to −0.7%.

On the other hand, when the relative refractive index difference Δ1 is 2.4% or more and the relative refractive index difference Δ2 is −1.4 to −0.7%, as shown in Example 2-1 and Example 2-2 in FIG. Further, the effective area Aeff can be made 11 μm ² or less, and a nonlinear constant n ₂ / Aeff value of 40 × 10 ⁻¹⁰ / W or more can be obtained. Further, Aeff is somewhat broadened by providing a third core having a higher refractive index than the cladding, but Aeff is still 12 μm ² or less. In this case, a nonlinear constant n ₂ / Aeff value of 35 × 10 ⁻¹⁰ / W or more is obtained. When the relative refractive index difference Δ1 is 4.0% or less and the relative refractive index difference Δ2 is −1.4 to −0.7%, sufficiently high non-linearity and low dispersion slope are obtained, An optical fiber having a cutoff wavelength of 1500 nm or less can be realized. Furthermore, since the degree of freedom of the refractive index profile is increased by providing the third core, the yield is improved, and high productivity and stability can be obtained. Therefore, it is more preferable that the relative refractive index difference Δ1 is 2.4 to 4.0% and the relative refractive index difference Δ2 is −1.4 to −0.7%.

(3) Dispersion and dispersion slope The nonlinear dispersion-shifted optical fiber according to the present invention is a nonlinear dispersion-shifted optical fiber that can be used in a wide wavelength region including 1550 nm, and it is necessary that the absolute value of dispersion at the wavelength used is small. It is. Therefore, in the nonlinear dispersion-shifted optical fiber of the present invention, the absolute value of dispersion at a wavelength of 1550 nm is preferably 10 ps / nm / km or less, and more preferably 5 ps / nm / km or less.

Furthermore, it is necessary that the difference in dispersion between the wavelengths used is small. Therefore, the absolute value of the dispersion slope at the wavelength of 1550 nm of the nonlinear dispersion-shifted optical fiber in the present invention is 0.03 ps / nm ² / km or less, preferably 0.01 ps / nm ² / km or less. As a result, the dispersion difference between the operating wavelengths is small in a wide operating wavelength range, and optical signal processing at various wavelengths can be performed with one optical fiber. Signal processing is possible.

  The nonlinear dispersion-shifted optical fiber of the present invention ensures that the difference in dispersion is small over the entire length of the optical fiber, even when it is used in a long length of 1 km to several km. As a result, it is effective for use in an optical signal processing apparatus such as a wavelength converter or a pulse compressor utilizing the nonlinear phenomenon of light. Therefore, in the nonlinear dispersion-shifted optical fiber of the present invention, the difference (variation width) between the maximum value and the minimum value in the longitudinal direction of the optical fiber at any wavelength of 1510 nm to 1590 nm is 1 ps / nm / km or less, preferably It is 0.2 ps / nm / km or less. Thus, since the fluctuation range of dispersion is small, it is very effective when used for an optical signal processing apparatus such as a wavelength converter or a pulse compressor using a nonlinear phenomenon of light. The dispersion fluctuation range described above means a dispersion fluctuation range measured by a dispersion distribution measuring device over the entire length of the optical fiber having a practical length. The distribution distribution of the optical fiber can be measured by a dispersion distribution measuring device using a method studied by Mollenauer, for example.

  Actually, in order to suppress fluctuations in dispersion in the longitudinal direction of the optical fiber, it is required that the thickness of the core and the clad be uniform at the stage of the optical fiber preform. Specifically, for example, at the stage of soot synthesis by the OVD method or the VAD method, it is necessary to manage the deposited raw material to be uniform, and when the optical fiber preform is stretched to a desired outer diameter. Therefore, highly accurate stretching is required so that the difference in outer diameter fluctuation is 0.2% or less. Even when the optical fiber is drawn from the optical fiber preform, it is necessary to manage the optical fiber at a substantially constant diameter so that the fluctuation of the outer diameter of the optical fiber is 0.2% or less.

(4) Cutoff wavelength In the single mode optical fiber, the cutoff wavelength λc is required to be smaller than the operating wavelength. Therefore, it is desirable that the cutoff wavelength λc is 1500 nm or less, more preferably 1460 nm or less. When the cutoff wavelength λc is 1500 nm or less, it can be used for a wide wavelength range of 1500 nm or more, and when the cutoff wavelength λc is 1460 nm or less, the S band (1460 nm to 1530 nm) and the C band (1530 nm to 1565 nm) and a wide wavelength region including the L band (1565 nm to 1625 nm) can be used.

  Here, the cutoff wavelength λc is ITU-T (International Telecommunication Union) G.I. An optical fiber cutoff wavelength λc defined by 650 is referred to. For other terms not specifically defined in this specification, see ITU-T G.C. The definition and measurement method in 650 shall be followed.

(5) Ratio D1 / D2 between the outer diameter of the first core and the outer diameter of the second core
By adjusting the ratio D1 / D2 between the outer diameter D1 of the first core and the outer diameter D2 of the second core, the light having a small effective core area Aeff, a low cutoff wavelength λc, and a small dispersion slope value. A fiber can be obtained. A change in the value of the dispersion slope by adjusting D1 / D2 will be described using a simulation example.

  Table 3 shows structural parameters other than D1 / D2 of the optical fiber used in the simulation. Δ1, Δ2, etc. are the same as described above, and follow the above formulas (1) to (4).

表３ シミュレーションで用いた光ファイバの構造パラメータ（３）

Table 3 Optical fiber structural parameters used in the simulation (3)

  FIG. 6 shows the relationship between D1 / D2 and the dispersion slope value when the dispersion at a wavelength of 1550 nm is 0 ps / nm / km in an optical fiber having the structural parameters shown in Table 3. FIG. The relationship between D1 / D2 and the cut-off wavelength when the dispersion at a wavelength of 1550 nm is 0 ps / nm / km in an optical fiber having the following characteristics is shown.

  As shown in FIG. 6, it can be seen that the dispersion slope increases when D1 / D2 approaches 0 or 1. Therefore, in order to reduce the dispersion slope, it is necessary to set D1 / D2 in the vicinity of 0.5 between 0 and 1. Further, as shown in FIG. 7, when D1 / D2 becomes smaller than 0.3, the cutoff wavelength is rapidly shifted to the long wavelength side and exceeds 1500 nm. On the other hand, when D1 / D2 is larger than 0.8, the cut-off wavelength exceeds 1500 nm although it is moderate. Therefore, an appropriate D1 / D2 is 0.3 or more and 0.8 or less.

Further, by setting the range of D1 / D2 to 0.4 or more and 0.7 or less, the absolute value of the dispersion slope can be made 0.01 ps / nm ² / km or less. Therefore, D1 / D2 is preferably set to 0.4 or more and 0.7 or less.

(6) Specific refractive index difference Δ3 and core outer diameter D3 of the third core
In order to prevent the cutoff wavelength from exceeding 1500 nm, it is necessary to adjust the relative refractive index difference Δ3 and the outer diameter D3 of the third core. Therefore, the relative refractive index difference Δ1 obtained above is in the range of 1.5 to 5.0%, the relative refractive index difference Δ2 is in the range of −1.4 to −0.1%, and the outer diameter D1 of the first core. These structural parameters are changed so that the ratio D1 / D2 between the outer diameter D2 and the outer diameter D2 of the second core is in the range of 0.3 to 0.8. Under this condition, the relative refractive index difference Δ3 was changed, and the range of D2 / D3 in which the cutoff wavelength in dispersion of −10 to 10 ps / nm / km was 1500 nm or less was obtained by simulation. The simulation result is shown in FIG.

FIG. 8 shows that when the relative refractive index difference Δ3 is 0.1 to 1.0%, D2 / D3 is decreased (that is, the core width of the third core is increased), and −10 to 10 ps / nm / It is the figure which plotted D2 / D3 when the cutoff wavelength in dispersion of km exceeds 1500 nm. Therefore, for each relative refractive index difference Δ3, a cutoff wavelength of 1500 nm or less is assured in a range where D2 / D3 is larger than the plotted line of FIG. 8 (the core width of the third core is narrow). From FIG. 8, the range of D2 / D3 is as shown in Equation (6).
D2 / D3> Δ3 + 0.25 (0.1% ≦ Δ3 ≦ 0.2%)
D2 / D3> (1/2) .Δ3 + 0.35 (0.2% ≦ Δ3 ≦ 0.6%) (6)
D2 / D3> (1/4) .Δ3 + 0.5 (0.6% ≦ Δ3 ≦ 1.0%)

(7) Refractive index profile of the first core The refractive index profile of the first core is an α power profile. By increasing α, it is possible to reduce the dispersion slope and Aeff. Here, it will be explained using a simulation example that α is superior.

  Table 4 shows structural parameters other than α of the optical fiber used in the simulation for explaining that a large α is superior. Δ1, Δ2, etc. are the same as described above, and follow the above formulas (1) to (4).

表４ シミュレーションで用いた光ファイバの構造パラメータ（４）

Table 4 Optical fiber structural parameters used in the simulation (4)

9 shows the relationship between α and dispersion slope of the first core of the optical fiber having the structural parameters shown in Table 4, and FIG. 10 shows the relationship between α and Aeff of the optical fiber having the structural parameters shown in Table 4. As shown in FIG. 9, the dispersion slope can be reduced by increasing the value of α. In particular, when α is increased from 2 to 3, the optical fiber C can be reduced by about 0.0095 ps / nm ² / km, and the optical fiber D can be reduced by about 0.012 ps / nm ² / km. . Thus, increasing α is very effective in reducing the dispersion slope. Further, as shown in FIG. 10, Aeff can be reduced by increasing the value of α. In particular, when α is increased from 2 to 3, Aeff can be reduced by about 10% in both the optical fiber C and the optical fiber D.

  In order to increase α of the first core, when the core base material is manufactured by the VAD method or the MCVD method, a core base material having a large α power profile of the refractive index distribution shape is prepared in advance. Alternatively, the surface of the core base material produced by these methods can be cut off by etching with HF or mechanical cutting. When α is increased by these methods, it is relatively easy to make α 3 or more from the viewpoint of manufacturing. Further, as shown in FIG. 9, the dispersion slope can be further reduced by further increasing the value of α to 6 or more. Furthermore, as shown in FIG. 10, when A is increased, Aeff can be decreased. In the region where α is 6 or more, the decreasing trend of the dispersion slope continues as shown in FIG. 9, but the decrease of Aeff becomes almost saturated as shown in FIG. Therefore, at least α is preferably 6 or more.

(8) Examples of Nonlinear Dispersion Shift Optical Fibers of the Present Invention Table 5 shows structural parameter values and characteristic values obtained by simulation of optical fibers of Examples 1 to 8 and Comparative Examples 1 and 2 of the present invention. Indicates. In Table 5, MFD means mode field diameter. In this simulation, the refractive index of the clad was set to be substantially the same as that of pure silica. In all of Examples 1 to 8, the absolute value of dispersion at a wavelength of 1550 nm is 10 ps / nm / km or less, and the absolute value of the dispersion slope is 0.03 ps / nm ² / km or less. The cutoff wavelength λc is 1500 nm or less, and the effective area Aeff is 15 μm ² or less. For ease of comparison, the dispersion values at the wavelength of 1550 nm in Examples 1 to 8 and Comparative Examples 1 and 2 were made substantially the same.

  Comparing Example 1 and Example 2, in Example 1, the refractive index distribution shape of the first core is an α-power profile, and α is 3. On the other hand, in Example 2, α is 6. Comparing the characteristic values of the obtained optical fibers, the dispersion slope at the wavelength of 1550 nm is smaller in Example 2 than in Example 1, and the effective area Aeff is also smaller. The same can be said for the relationship between Example 5 and Example 6. From this viewpoint, it is suggested that α is preferably 6.0 or more than that having 3.0 or more.

  The relative refractive index difference Δ3 of the optical fiber of Example 1 is 0.3%, and in Example 3, the relative refractive index difference Δ3 is 0.5%. Comparing the characteristic values of the obtained optical fibers, the absolute value of the dispersion slope at the wavelength of 1550 nm is smaller in Example 3 than in Example 1, but the effective area Aeff is larger. The relationship between the sixth embodiment and the seventh embodiment is the same. However, since D1 / D2 is large (that is, the distance between the first core and the third core is short), the third relationship is greater than the relationship between the first and third embodiments. It can be seen that the characteristics are sensitive to changes in the relative refractive index difference Δ3 of the core.

  In the optical fiber of Example 1, the ratio D1 / D2 between the outer diameter D1 of the first core 1 and the outer diameter D2 of the second core 2 is 0.375, and in Example 5, D1 / D2 is 0.00. 5 Comparing the characteristic values obtained in both optical fibers, the effective area Aeff of Example 5 is larger than that of Example 1 and the cutoff wavelength λc is on the longer wavelength side, but the dispersion slope at the wavelength of 1550 nm is larger. The absolute value shows a considerably small value. That is, from the viewpoint of the dispersion slope, it is suggested that D1 / D2 is preferably 0.4 or more and 0.7 or less than 0.3 or 0.8 or less.

  Further, Comparative Example 1 has a structure without the third core part in Example 1, and Comparative Example 2 has a structure without the third core part in Example 5. Comparing the characteristic values obtained in both optical fibers, Example 1 has a larger effective area Aeff than Comparative Example 1, and the absolute value of the dispersion slope at a wavelength of 1550 nm is smaller. Further, Example 5 also has an effective area Aeff larger than that of Comparative Example 2, and the absolute value of the dispersion slope at the wavelength of 1550 nm is a small value. That is, from the viewpoint of reducing the dispersion slope, it is effective to have the third core as in the present invention. Further, since the third core is provided, the degree of freedom of the refractive index profile is increased, so that there is an advantage that the yield is improved and high productivity and stability can be obtained.

Further, Table 6 shows light when various structural parameters are used for the reference refractive index difference Δ1 of the first core and the relative refractive index difference Δ2 of the second core in Reference Examples 9 and 10 and Examples 11 to 18 of the present invention. The values of the structural parameters of the fiber and their characteristic values are shown. Here, the characteristic values shown in Reference Examples 9 and 10 and Examples 11 to 16 are the results obtained by simulation, and Examples 17 and 18 are the characteristics obtained by actually producing and evaluating an optical fiber. Value. The characteristic values of the actually produced optical fiber are almost the same as those in the simulation. In Reference Examples 9 and 10 and Examples 11 to 18, the absolute value of dispersion at a wavelength of 1550 nm is 10 ps / nm / km or less, and the absolute value of dispersion slope is 0.03 ps / nm ² / km or less. The cutoff wavelength λc is 1500 nm or less, and the effective area Aeff is 15 μm ² or less.

  Further, dispersion fluctuations in the longitudinal direction of the optical fibers shown in Example 17 and Example 18 were measured. As a result, in Example 17, the difference between the maximum value and the minimum value of dispersion in the longitudinal direction in the optical fiber having a length of 1 km was 0.8 ps / nm / km at a wavelength of 1552 nm. For the optical fiber shown in Example 18, the difference between the maximum value and the minimum value of dispersion in the longitudinal direction in the optical fiber having a length of 1 km was 0.2 ps / nm / km at a wavelength of 1554 nm. The non-linear dispersion-shifted optical fiber as in the present application is most commonly used at a length of about 400 m to 1 km, and the difference between the maximum value and the minimum value of the dispersion in the longitudinal direction at the length of the used optical fiber is It is within the allowable range.

  By using the nonlinear dispersion-shifted optical fiber of the present application in an optical signal processing device, it is possible to perform optical signal processing with stable performance in a wide wavelength range.

  FIG. 11 shows an optical wavelength converter that collectively converts the wavelengths of signal light into other wavelengths as an example of an optical signal processing apparatus using the optical fiber of the present invention. The optical wavelength converter of FIG. 11 includes a polarization controller 13 for aligning polarization, an erbium-doped optical fiber amplifier (EDFA) 14, a coupler 15 for coupling pumping light (wavelength λs) and signal light 12 from a light source, and a polarizer 16 It has. The optical wavelength converter of FIG. 11 will be briefly described below.

The wavelength at which the dispersion of the optical fiber 7 of the present invention becomes zero is examined in advance, and excitation light (wavelength λs) near the wavelength at which this dispersion becomes zero is generated from the light source 11 and coupled with the signal light 12 (wavelength λp). After that, it is introduced into the optical fiber 17 of the present invention. At this time, a large nonlinear phenomenon called four-wave mixing occurs in the optical fiber 17, and the signal light 12 is converted to the wavelength λ in the following equation (7). As a result, optical wavelength conversion is performed collectively.
λ = (λp−λs) + λp (7)

  FIG. 12 shows an example of a pulse compressor using the optical fiber of the present invention. The pulse compressor shown in FIG. 12 includes light sources 21 and 22 having different wavelengths, a polarization controller 23, a coupler 24, a polarizer 25, an EDFA 26, a general single mode optical fiber 28, and an optical fiber 27 of the present invention. In the pulse compressor of FIG. 13, the optical fiber 27 of the present invention and a general single mode optical fiber 28 are alternately connected for each predetermined length.

  11 and 12 show only an optical wavelength converter and a pulse compressor as an optical signal processing apparatus using the optical fiber of the present invention, but other than this, for example, a waveform shaper or the like. Needless to say, the optical fiber of the present invention can be applied.

It is a figure which shows the example of the typical refractive index distribution of the nonlinear dispersion shift optical fiber which concerns on this invention. It is a figure which shows the relationship between the relative refractive index difference (DELTA) 3 of the 3rd core of an optical fiber which has the structural parameter of Table 1, and a dispersion slope. It is a figure which shows the relationship between the relative refractive index difference (DELTA) 3 of the 3rd core of an optical fiber which has the structural parameter of Table 1, and effective area Aeff. It is a figure which shows the relationship between the relative refractive index difference (DELTA) 3 of the 3rd core of an optical fiber which has the structural parameter of Table 1, and a cutoff wavelength. It is a figure which shows the relationship between relative refractive index difference (DELTA) 1 of the 1st core of an optical fiber which has a structural parameter of Table 2, and effective area Aeff. It is a figure which shows the relationship between ratio D1 / D2 of the outer diameter D1 of the 1st core of the optical fiber which has a structural parameter of Table 3, and the outer diameter D2 of a 2nd core, and a dispersion | distribution srobe. It is a figure which shows the relationship between ratio D1 / D2 of the outer diameter D1 of the 1st core of the optical fiber which has a structural parameter of Table 3, and the outer diameter D2 of a 2nd core, and a cutoff wavelength. The relative refractive index difference Δ3 and D2 / D3 of the third core were changed at a predetermined first core relative refractive index difference Δ1, a predetermined second core relative refractive index difference Δ2, and a predetermined D1 / D2. It is a figure which shows the range from which the cutoff wavelength in dispersion | distribution of -10 thru | or 10ps / nm / km becomes 1500 nm or less. It is a figure which shows the relationship between (alpha) showing the refractive index profile shape of the 1st core of the optical fiber which has a structural parameter of Table 4, and a dispersion slope. It is a figure which shows the relationship between (alpha) showing the refractive index profile shape of the 1st core of the optical fiber which has a structural parameter of Table 4, and effective area Aeff. It is a figure which shows an example of the optical wavelength converter using the nonlinear dispersion shift optical fiber which concerns on this invention. It is a figure which shows an example of the pulse compressor using the nonlinear dispersion shift optical fiber which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st core 2 2nd core 4 Clad 11, 21, 22 Light source 12 Signal light 13, 23 Polarization controller 14, 26 EDFA
15, 24 Couplers 16, 25 Polarizers 17, 27 Optical fiber of the present invention 28 Single mode optical fiber

Claims (12)

A first core located in the center;
A second core provided on an outer periphery of the first core and having a lower refractive index than the first core;
A third core provided on an outer periphery of the second core, having a refractive index lower than that of the first core and higher than that of the second core;
A non-linear dispersion-shifted optical fiber provided on the outer periphery of the third core, comprising a cladding having a refractive index lower than that of the third core and higher than that of the second core;
The relative refractive index difference Δ1 of the first core with respect to the cladding is 2.5% to 5.0%,
The relative refractive index difference Δ2 of the second core with respect to the cladding is −1.4% to −0.1%,
The relative refractive index difference Δ3 of the third core with respect to the cladding is 0.1% to 1.0%,
The outer diameter D1 of the first core is 2 μm to 5 μm,
A ratio D1 / D2 between the outer diameter D1 of the first core and the outer diameter D2 of the second core is 0.3 or more and 0.8 or less,
The ratio D2 / D3 of the outer diameter D2 of the second core and the outer diameter D3 of the third core is 0.35 or more and 0.99 or less,
The dispersion value at a wavelength of 1550 nm is −3.9 ps / nm / km to +3.8 ps / nm / km ,
The absolute value of the dispersion slope at a wavelength of 1550 nm is 0.03 ps / nm ² / km or less,
A nonlinear dispersion-shifted optical fiber, wherein an effective core area at a wavelength of 1550 nm is 12 μm ² or less, and a nonlinear constant n ₂ / Aeff at a wavelength of 1550 nm is 40 × 10 ⁻¹⁰ / W or more. The relative refractive index difference Δ1 of the first core is 4.0% or less,
2. The nonlinear dispersion-shifted optical fiber according to claim 1, wherein a relative refractive index difference Δ <b> 2 of the second core is −1.4% to −0.7%. 3. The nonlinear dispersion-shifted optical fiber according to claim 1, wherein an absolute value of a dispersion slope at a wavelength of 1550 nm is 0.01 ps / nm ² / km or less.   4. The nonlinear dispersion shift according to claim 1, wherein the difference between the maximum value and the minimum value of dispersion in the longitudinal direction of the optical fiber at a wavelength of 1510 nm to 1590 nm is 1 ps / nm / km or less. Optical fiber.   4. The nonlinearity according to claim 1, wherein a difference between a maximum value and a minimum value of dispersion in the longitudinal direction of the optical fiber at any wavelength of 1510 nm to 1590 nm is 0.2 ps / nm / km or less. Dispersion shifted optical fiber.   The nonlinear dispersion-shifted optical fiber according to any one of claims 1 to 5, wherein a cutoff wavelength is 1500 nm or less.   The nonlinear dispersion shifted optical fiber according to any one of claims 1 to 6, wherein the D1 / D2 is 0.4 or more and 0.7 or less. Said D2 / D3 is
D2 / D3> Δ3 + 0.25 (0.1% ≦ Δ3 ≦ 0.2%)
D2 / D3> (1/2) · Δ3 + 0.35 (0.2% ≦ Δ3 ≦ 0.6%)
D2 / D3> (1/4) .Δ3 + 0.5 (0.6% ≦ Δ3 ≦ 1.0%)
The nonlinear dispersion-shifted fiber according to claim 1, wherein the following relationship is satisfied.   9. The nonlinear dispersion-shifted optical fiber according to claim 1, wherein the refractive index profile of the first core is an α power profile, and α is 3.0 or more.   9. The optical fiber according to claim 1, wherein the refractive index profile of the first core is an α power profile, and α is 6.0 or more.   An optical wavelength converter using the optical fiber according to any one of claims 1 to 10.   A pulse compressor using the optical fiber according to claim 1.

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