JP2005309026A - Optical fiber and light signal processing apparatus using optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To easily provide an optical fiber having both high nonlinearity and low dispersion and to provide a light signal processing apparatus using the optical fiber, more concretely, an optical wavelength transducer and a pulse compressor. <P>SOLUTION: The optical fiber has the absolute value of dispersion of not more than 20 ps/nm/km at the wavelength of 1,550 nm, the effective core sectional area not more than 15 μm<SP>2</SP>at the wavelength of 1,550 nm and the nonlinear constant equal to or more than 25×10<SP>-10</SP>/W at the wavelength of 1,550 nm by using a germanium doping technique to a clad 4. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical fiber excellent in non-linearity and an optical signal processing device using the optical fiber.

In recent years, there has been an increasing demand for higher speeds, larger capacities, and longer distances in optical signal transmission. For this reason, signal processing techniques for increasing the processing speed of optical signals and achieving long-distance transmission have been developed. It is sought after.
As one of optical signal processing techniques, there is a method of converting an optical signal into an electric signal, processing the converted electric 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 signal processing is possible.

By the way, all-optical signal processing techniques include 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 nonlinear optical phenomenon that occurs 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-wave mixing and self-phase modulation in an optical fiber, the nonlinear phenomenon can occur greatly as an optical fiber. An optical fiber, that is, an optical fiber having high nonlinearity is required.

There is a nonlinear constant as an index representing the nonlinearity of the optical fiber. The nonlinear constant is represented by the following formula (1).
Nonlinear constant = n ₂ / Aeff (1)
In equation (1), n ₂ represents the nonlinear refractive index of the optical fiber, and Aeff represents the effective core area of the optical fiber.

From the above formula (1), 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 core area Aeff (hereinafter referred to as Aeff). Understand.
In order to achieve this, for example, a first core located in the center of the optical fiber is doped with a large amount of germanium, for example, to increase the nonlinear refractive index n ₂ , or the relative refractive index difference between the first core and the cladding. There is a method of increasing Aeff and decreasing Aeff. However, when the relative refractive index difference between the first core and the clad is increased, the dispersion slope is increased, and the cutoff wavelength is shifted to the long wavelength side.
Therefore, by using a so-called W-type refractive index profile in which the second core having a lower refractive index than the cladding is provided on the outer periphery of the first core, the dispersion slope can be reduced and the cutoff wavelength can be shifted to the short wavelength side. It is known that it can be done.

As an optical fiber that realizes this, there is one proposed in Patent Document 1, for example. In this optical fiber, the refractive index profile of the optical fiber is changed to a W-type refractive index profile, so that the concentration of germanium doped in the first core is increased to increase the nonlinear refractive index n _2. Even when the relative refractive index difference with the cladding is increased to reduce Aeff, an optical fiber having a sufficiently short cutoff wavelength is realized.

JP 2002-207136 A

Here, in order to change the refractive index profile of the optical fiber to the W-type refractive index profile, for example, there is a method of doping the second core with fluorine as means for lowering the second core refractive index located on the outer periphery of the first core.
However, in order to obtain good characteristics as an optical fiber having high nonlinearity, it is necessary to increase the refractive index of the second core to the negative side, and in order to realize this, the second core has a high concentration of fluorine. It is necessary to dope.
However, when doping fluorine under atmospheric pressure, the limit is to lower the refractive index to about -0.7% due to the relative refractive index difference with respect to pure silica, in order to make the refractive index of the second core smaller than this. For example, a technique of doping fluorine in a high-pressure atmosphere is required. (Here, pure silica means pure silica glass that is not doped with a dopant that changes the refractive index.) However, the technology of doping fluorine under a high-pressure atmosphere requires a very advanced technology and is manufactured. There is a problem that not only the equipment becomes complicated, but also the production yield deteriorates.

In order to solve the above problems, an optical fiber according to the present invention includes a first core located in the center, a second core provided on an outer periphery of the first core, and having a refractive index lower than that of the first core; An optical fiber provided outside the second core and having a cladding having a refractive index lower than that of the first core and higher than that of the second core, and having an absolute value of dispersion at a wavelength of 1550 nm 20 ps / nm / km or less, effective core area Aeff at a wavelength of 1550 nm is 15 μm ² or less, and nonlinear constant at a wavelength of 1550 nm is 25 × 10 ⁻¹⁰ / W or more, more preferably 40 × 10 ⁻¹⁰ / W In addition, the cladding is doped with germanium.

  According to the optical fiber according to the present invention thus configured, the relative refractive index difference with respect to the cladding of the second core can be easily increased to the negative side by using the technique of doping germanium into the cladding, An optical fiber having both high nonlinearity and low dispersion can be easily provided. 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.

Embodiments of an optical fiber of the present invention and an optical wavelength converter and a pulse compressor using the optical fiber will be described in detail with reference to FIGS.

FIG. 1 shows an example of an optical fiber according to the present invention. FIG. 1A shows the refractive index distribution of this optical fiber, and FIG. 1B shows a part of the cross section thereof. The outer line of the clad 4 is omitted.
As shown in FIG. 1 (a), this optical fiber is located at the center, and has a first core 1 having a refractive index profile of α power expressed by the following formula (2), and provided outside the first core 1. A second core 2 having a lower refractive index than the first core 1, and a clad 4 provided outside the second core 2 and having a higher refractive index than the second core 2. It has a so-called W-type refractive index profile. Moreover, the refractive index of the clad 4 is higher than the refractive index of pure silica shown by the dotted line in the figure.

Here, α representing the shape of the refractive index profile of the first core is defined by the following formula (2).
n ² (r) = n _c1 ² {1-2 · (Δ1 / 100) · (2r / D1) ^α } (2)
However, 0 <r <D1 / 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. Further, _nc1 is the maximum refractive index of the first core 1, and D1 is the diameter of the first core 1.

The relative refractive index difference Δ1 of the first core 1 with respect to the cladding 4, the relative refractive index difference Δ2 of the second core 2 with respect to the cladding 4, and the refractive index difference ΔC of the cladding with respect to the pure silica are expressed by the following equations (3) to (5). ).
Δ1 = {(n _c1 −n _c ) / n _c1 } · 100 (3)
Δ2 = {(n _c2 −n _c ) / n _c2 } · 100 (4)
ΔC = {(n _c −n _s ) / n _c } · 100 (5)
Wherein the in each formula, n _c1 is the maximum refractive index of the first core 1, n _c2 is the minimum refractive index of the second core 2, n _c is the refractive index of the cladding 4, n _s is the refractive index of pure silica .

  In the present specification, the diameter D1 of the first core 1 is a diameter at a position where the first core 1 has a refractive index equal to that of the cladding 4, and the diameter D2 of the second core 2 is the same as that of the second core 2 and the cladding. In the boundary area with 4, the diameter at a position where the refractive index is 1/2 of Δ2.

FIG. 2 shows another example of the optical fiber according to the present invention. As in FIG. 1, FIG. 2 (a) shows the refractive index distribution of this optical fiber, and FIG. 2 (b) shows a part of its cross section. The outer line of the clad 4 is omitted.
As shown in FIG. 2 (a), this optical fiber is located at the center, and is provided outside the first core 1 having a first core 1 having a refractive index profile of α power as shown in the equation (2). A second core 2 having a refractive index lower than that of the first core 1 and a refractive index lower than that of the first core 1 and higher than that of the second core 2 provided outside the second core 2. A so-called third core 3 having a refractive index, and a cladding 4 provided outside the third core 3 and having a refractive index lower than that of the third core 3 and higher than that of the second core 2. It has a W segment type refractive index profile. Further, the refractive index of the cladding 4 is higher than the refractive index of pure silica, which is indicated by a dotted line in the figure.

Here, the refractive index difference Δ3 of the third core 3 with respect to the clad 4 refers to that represented by the following formula (6). Further, α representing the shape of the refractive index profile of the first core, the refractive index difference Δ1 of the first core 1 with respect to the cladding 4, the relative refractive index difference Δ2 of the second core 2 with respect to the cladding 4, and the refractive index of the cladding with respect to the pure silica. The difference ΔC is the same as the above-described formulas (2) to (5).
Δ3 = {(n _c3 −n _c ) / n _c3 } · 100 (6)
Here, in each of the above formulas, _nc3 is the maximum refractive index of the third core 3.

  In the present specification, the diameter D3 of the third core 3 is a diameter at a position where the refractive index is 1/10 of Δ3 in the boundary region between the third core 3 and the clad 4. The diameter D1 of the first core 1 and the diameter D2 of the second core 2 are the same as those defined above.

In the optical fiber according to the present invention, the relative refractive index difference with respect to the cladding of the second core can be easily increased to the negative side by using the technique of doping germanium in the cladding, and high nonlinearity and low dispersion can be achieved. It is possible to easily provide an optical fiber having both.
The cladding can be doped with germanium in the optical fiber soot base material manufacturing process, for example, OVD (outside vapor deposition) process, etc., and adjusting the doping amount adjusts the refractive index difference ΔC of the cladding with respect to pure silica. It is possible.
For example, the second core is doped with fluorine so that the relative refractive index difference with respect to the pure silica of the second core is −0.7%, and germanium is added to the cladding so that the refractive index difference ΔC of the clad with respect to the pure silica is 0.5%. In the case of doping, if the refractive index of pure silica is S, the refractive index of the clad is 1.005 · S, and the refractive index of the second core is 0.993 · S. Here, the second core refractive index difference Delta] 2 of the refractive index of the cladding and the cladding when the S _c is a Δ2 = (0.993 · S-S c) / S c · 100 = -1.194%. As described above, by using the technique of doping the cladding with germanium, it is possible to easily increase the refractive index difference with respect to the cladding of the second core.

Hereinafter, characteristics that are favorable as an optical fiber according to the present invention and structural parameters of a refractive index profile for obtaining the characteristics will be described in detail.
(1) Effective core area Aeff
As described above, it is effective to reduce Aeff in order to increase the nonlinear constant of the optical fiber. In the present invention, Aeff at a wavelength of 1550 nm is set to 15 μm ² or less, more preferably 12 μm ² or less. Thus, by reducing Aeff, an optical fiber having a large nonlinear constant of 25 × 10 ⁻¹⁰ / W or more, more preferably 40 × 10 ⁻¹⁰ / W or more at a wavelength of 1550 nm 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 in order to derive an appropriate relative refractive index difference Δ1. Table 1 shows structural parameters other than Δ1 of the optical fiber used in the simulation.
The definitions of Δ, α and the like are the same as described above, and follow the above formulas (2) to (6). In addition, Example 1-1 is a W segment type refractive index profile shown in FIG. 2, and Examples 1-2 and 1-3 are W type refractive index profiles shown in FIG.

FIG. 3 shows an example of the relationship between the relative refractive index difference Δ1 and Aeff of an optical fiber having the structural parameters shown in Table 1. As shown in FIG. 3, Aeff decreases as the relative refractive index difference Δ1 increases. In order to satisfy the requirement that Aeff is 15 μm ² or less, the relative refractive index difference Δ1 needs to be 1.0% or more as shown in FIG.

  Further, when the relative refractive index difference Δ1 is increased, the cutoff wavelength is shifted to the longer wavelength side, and when Δ1 exceeds 5.0%, it becomes difficult to form a single mode optical fiber. When Δ1 exceeds 5.0%, the dispersion slope at the wavelength of 1550 nm increases, and the difference in dispersion value between wavelengths increases when performing optical signal processing. Furthermore, it is very difficult to manufacture a core having a relative refractive index difference Δ1 exceeding 5.0%. Therefore, the relative refractive index difference Δ1 is preferably 1.0% or more and 5.0% or less.

Further, when the relative refractive index difference Δ2 is increased to the negative side, the absolute value of the dispersion slope can be decreased while decreasing the absolute value of dispersion at 1550 nm, and the cutoff wavelength can be further shifted to the short wavelength side. As described above, when the relative refractive index difference Δ1 is 1.0% or more and 5.0% or less, if the relative refractive index difference Δ2 is −0.2% or less, the absolute value of the dispersion slope is 0.03 ps / nm ² / km or less. The cutoff wavelength can also be made 1500 nm or less. On the other hand, when the relative refractive index difference Δ2 is set to −2.4% or less, for example, it is necessary to dope the second core with a large amount of fluorine or the cladding with a large amount of germanium, which makes the production very difficult. Therefore, the relative refractive index difference Δ2 is preferably −2.4% or more and −0.2% or less.

On the other hand, when the relative refractive index difference Δ1 is 2.0% or more and the relative refractive index difference Δ2 is −2.0% or more and −0.6% or less, Aeff can be made 12 μm ² or less as shown in FIG. The non-linear constant can be 40 × 10 ⁻¹⁰ / W or more. Also, by providing a third core having a higher refractive index than the cladding, Aeff is somewhat larger, but Aeff is still 12 μm ² or less, as shown in Example 1-1 of FIG. 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 2.0% to 4.0% and the relative refractive index difference Δ2 is −2.0% to −0.6%, the absolute value of the dispersion slope is 0.01 ps / nm ² / km or less. A sufficiently high nonlinearity and a dispersion slope with a small absolute value can be obtained, and an optical fiber having a cutoff wavelength of 1500 nm or less can be realized. 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 −2.0% to −0.6%.

(3) Dispersion and Dispersion Slope The optical fiber according to the present invention is an 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. Therefore, in the optical fiber according to the present invention, the absolute value of the dispersion value at the wavelength of 1550 nm is desirably 10 ps / nm / km or less, and more desirably 5 ps / nm / km or less.

Furthermore, the optical fiber according to the present invention is also required to have a small difference in dispersion between wavelengths at the used wavelengths. Therefore, the absolute value of the dispersion slope at 1550 nm of the optical fiber according to the present invention is 0.03 ps / nm ² / km or less, more preferably 0.01 ps / nm ² / km or less.
As a result, it is possible to provide an optical fiber having a small dispersion difference between wavelengths and a small absolute value of dispersion in a wide use wavelength region near the wavelength of 1550 nm. By using the optical fiber according to the present invention, it is possible to perform good optical signal processing utilizing nonlinear phenomena in a wide wavelength region.

  Even when the optical fiber according to the present invention is used with a length of 1 km to several km, it is ensured that the difference in dispersion is small over the entire length of the optical fiber. In the optical fiber according to the present invention, the fluctuation range of dispersion in the longitudinal direction of the optical fiber at any wavelength of 1510 to 1590 nm is 1 ps / nm / km or less, more preferably 0.2 ps / nm / km or less. As described above, since the difference between the maximum value and the minimum value of dispersion in the longitudinal direction is small, an optical signal processing device such as a wavelength converter or a pulse compressor using a high-quality nonlinear phenomenon can be configured.

  Note that the dispersion fluctuation range described above is the dispersion fluctuation range (difference between the maximum and minimum dispersion values) measured by a dispersion distribution measuring device over the entire length of one optical fiber constituting an optical signal processing apparatus or the like. ). The dispersion distribution of the optical fiber can be measured by, for example, a dispersion distribution measuring device using a method studied by Mollenauer.

  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 synthesizing the optical fiber soot base material by the OVD method or VAD (vapour-phase axial deposition) method, the deposition conditions during synthesis are controlled so that the thickness of the core and the clad is uniform. Therefore, when this optical fiber preform is drawn to a desired outer diameter, it is required to perform drawing with high accuracy so that the fluctuation of the outer diameter is within ± 0.2% of the average outer diameter. Also, when drawing an optical fiber from an optical fiber preform, it is necessary to manage the outer diameter of the optical fiber to be substantially constant (for example, the outer diameter fluctuation is within ± 0.2% of the average outer diameter of the fiber).

(4) Cut-off wavelength In the single mode optical fiber, the cut-off wavelength λc is required to be smaller than the used wavelength. Therefore, it is desirable that the cutoff wavelength λc is 1500 nm or less, more preferably 1200 nm or less. By setting the cutoff wavelength λc to 1500 nm or less, single mode operation is ensured in a wide wavelength region of 1500 nm or more. Furthermore, when the cut-off wavelength λc is 1200 nm or less, single mode operation can be ensured in a wavelength region of 1200 nm or more, and it can be used in a wide wavelength region including the 1.3 μm band.

  Here, the cutoff wavelength λc is ITU-T (International Telecommunication Union) G.I. The 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 and the relative refractive index difference ΔC of the cladding
By adjusting the ratio D1 / D2 between the outer diameter D1 of the first core and the outer diameter D2 of the second core, an optical fiber having a small Aeff, a small cutoff wavelength λc, and a small absolute value of the dispersion slope is obtained. be able to.
Therefore, a change in the dispersion slope by adjusting the ratio D1 / D2 between the outer diameter D1 of the first core and the outer diameter D2 of the second core will be described using a simulation example.

  Table 2 shows structural parameters other than D1 / D2 of the optical fiber used in the simulation. The refractive index profiles were all W-type refractive index profiles shown in FIG. As shown in Table 2, using the same core rod, the characteristics were changed by changing ΔC. Δ, α and the like are the same as described above, and follow the above formulas (2) to (6).

FIG. 4 shows the relationship between D1 / D2 in an optical fiber having the structural parameters shown in Table 2 and the dispersion slope when the dispersion at a wavelength of 1550 nm is 0 ps / nm / km. As shown in FIG. 4, in common with the optical fiber examples 2-1, 2-2, and 2-3, the ratio of D1 / D2 approaches 0.1, and as the value approaches 1, the dispersion slope increases in the positive direction. I understand that For this reason, in order to reduce the dispersion slope, it is necessary to set D1 / D2 to around 0.5. It can also be seen that the dispersion slope can be reduced as ΔC is increased to the negative side. In particular, the dispersion slope can be greatly reduced when D1 / D2 is 0.8 or less. For example, when the ratio of D1 / D2 is 0.8, the dispersion slope is 0.007 ps / in Example 2-2 compared to Example 2-1. Compared with Example 2-1, nm ² / km, Example 2-3 shows a reduction of 0.014 ps / nm ² / km. On the other hand, when the ratio of D1 / D2 is 0.9, the dispersion slope is 0.003 ps / nm ² / km in Example 2-2 compared to Example 2-1 and 0.005 in Example 2-3 compared to Example 2-1. Only ps / nm ² / km can be reduced.

In addition, paying attention to Example 2-2 in FIG. 4, the cladding is doped with germanium so that ΔC = 0.4%, and the absolute value of the dispersion slope is set to 0.015 ps by setting the D1 / D2 range from 0.4 to 0.7. / Nm ² / km or less. Therefore, the ratio D1 / D2 between the outer diameter of the first core and the outer diameter of the second core is preferably 0.01 or more and 0.8 or less, more preferably 0.4 or more and 0.7 or less.

Furthermore, FIG. 5 shows the relationship between D1 / D2 in an optical fiber having the structural parameters shown in Table 2 and Aeff when the dispersion at a wavelength of 1550 nm is 0 ps / nm / km.
As shown in FIG. 5, when the ratio of D1 / D2 approaches 0.1, Aeff decreases. Further, Aeff increases rapidly as the ratio of D1 / D2 approaches 1, and the increase becomes more remarkable as Δ2 is larger on the negative side. Therefore, D1 / D2 is preferably 0.1 or more and 0.8 or less, more preferably 0.4 or more and 0.7 or less.

(6) Specific refractive index difference Δ3 and core outer diameter D3 of the third core
By using the W segment type refractive index profile shown in FIG. 2, the dispersion slope can be further reduced.
At this time, it is preferable that a 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. Further, in order to increase the relative refractive index difference Δ3 of the third core with respect to pure silica, it is necessary to dope the third core with a large amount of germanium, but high technical skill is required to dope germanium stably. Therefore, in consideration of manufacturability, Δ3 is desirably 0.1% or more and 0.9% or less.

(7) Refractive index distribution shape of the first core The refractive index distribution shape of the first core is an α-power profile. By increasing α, the dispersion slope can be reduced and Aeff can also be reduced. Therefore, it is desirable that the refractive index distribution shape of the first core is an α power profile, and α is 3 or more, more preferably α is 6 or more. Here, α follows the formula (2).
Here, the fact that it is superior that α is large will be described using a simulation example of the optical fiber structure described in the present invention. FIG. 6 shows the relationship between α and dispersion slope of an optical fiber having the structural parameters shown in Table 3, and FIG. 7 shows the relationship between α and Aeff. In Table 3, Δ3 is the maximum relative refractive index difference of the third core with respect to the cladding, and ΔC is the relative refractive index difference of the cladding with respect to pure silica.

As shown in FIG. 6, the dispersion slope can be reduced by increasing the value of α. Especially by the α from 2 3, about 0.0092 ps / nm ² / km less Example 3-1, and it is possible to reduce about 0.008ps / nm ² / km in example 3-2. Increasing α in this way is very effective for reducing the dispersion slope.
Further, as shown in FIG. 7, Aeff can be reduced by increasing the value of α. In particular, by reducing α from 2 to 3, Aeff can be reduced by about 5% in both Example 3-1 and Example 3-2.

In order to increase the α of the first core, when the core base material is manufactured by the VAD method or the modified chemical vapor deposition (MCVD) method, a core base material having a large α is prepared in advance, or these methods are used. The surface of the produced core base material may be cut off by etching with HF or mechanical cutting, and only the portion having a large α at the core center may be used.
When using the above method, it is relatively easy to make α 3 or more from the viewpoint of manufacturing.

Table 4 shows the structural parameters of the optical fibers shown in Comparative Example 1 and Examples 1 to 10 of the present invention and characteristic values obtained by simulation. Each characteristic value is a value when dispersion at a wavelength of 1550 nm is set to 0 ps / nm / km, and MFD means a mode field diameter. The refractive index profile was the W-type refractive index profile shown in FIG.
Examples 1 to 10 use the same core, change the doping amount of germanium into the cladding, and change the relative refractive index difference ΔC between the cladding and pure silica. Comparative Example 1 The relative refractive index difference ΔC between the clad and pure silica is 0, that is, the clad is made of pure silica.

In any of the optical fibers of Examples 1 to 10, 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 Aeff is 12 μm ² or less.
In addition, as shown in Table 4, as the relative refractive index difference ΔC of the cladding with respect to pure silica is increased, the absolute value of the dispersion slope is monotonously reduced to 0.01 ps / nm ² / km or less, and the cutoff wavelength λc is also Shifts to the short wavelength side monotonously. Aeff is also small but small.
Therefore, regarding the refractive index profiles of Examples 1 to 10, it can be said that increasing the relative refractive index difference ΔC of the cladding with respect to pure silica is very effective for obtaining an optical fiber having excellent nonlinearity. However, since it is difficult to uniformly dope a large amount of germanium, ΔC is preferably 0.1% or more and 1.0% or less. Moreover, when the outer diameter D1 of the first core is 2 to 5 μm, good characteristics can be obtained.

  Table 5 shows structural parameters and characteristic values of optical fibers of Examples 11 to 15 and Comparative Example 2 of the present invention, and Table 6 shows Examples 16 to 19 and Comparative Example 3 of the present invention. Similarly to Table 4, Examples 11 to 15 and Examples 16 to 19 each use the same core and change only the relative refractive index difference ΔC of the clad with respect to pure silica. In Comparative Examples 2 and 3, the relative refractive index difference ΔC of the clad with respect to pure silica is 0, that is, the clad is made of pure silica. For easy comparison, the dispersion at a wavelength of 1550 nm was set to 0 ps / nm / km. The refractive index profile was the W-type refractive index profile shown in FIG.

In all of the optical fibers of Examples 11 to 15 and Examples 16 to 19, 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 Aeff Is 12 μm ² or less.
In addition, as shown in Tables 5 and 6, as the relative refractive index difference ΔC of the cladding with respect to pure silica is increased, the dispersion slope once decreases monotonously, and the absolute value of the dispersion slope becomes 0.01 ps / nm ² / km or less. Become. However, as shown in Example 14, Example 15, and Example 19, if ΔC is increased too much, the dispersion slope increases to the negative side, and the absolute value of the dispersion slope becomes larger than 0.01 ps / nm ² / km again. . Further, as ΔC increases, the cutoff wavelength λc monotonously shifts to the short wavelength side, and Aeff increases.
Therefore, regarding the optical fibers of Examples 11 to 15 and Examples 16 to 19, increasing the relative refractive index difference ΔC of the clad with respect to pure silica reduces the dispersion slope in the range of ΔC = 0.6% or less. This is very effective for shifting the cutoff wavelength to the short wavelength side.

  Table 7 shows structural parameters and characteristic values of optical fibers of Examples 20 to 22 and Comparative Example 4 of the present invention, and Table 8 shows Examples 23 to 24 and Comparative Example 5 of the present invention. Similarly to Table 4, Examples 20 to 22, and Examples 23 to 24 each use the same core and change only the relative refractive index difference ΔC of the clad with respect to pure silica. In Comparative Examples 4 and 5, the relative refractive index difference ΔC of the clad with respect to pure silica is 0, that is, the clad is made of pure silica. For easy comparison, the dispersion at a wavelength of 1550 nm was set to 0 ps / nm / km. The refractive index profile was the W segment type refractive index profile shown in FIG.

  Here, the behavior of the dispersion slope, cutoff wavelength λc, and Aeff by increasing ΔC is the same as that having the W-type refractive index profile shown in Tables 5 and 6, and the range of ΔC = 0.6% or less is the dispersion. This is effective for reducing the slope and shifting the cutoff wavelength to the short wavelength side.

  Here, as shown in Tables 7 and 8, when the third core is provided, the cutoff wavelength tends to shift to the longer wavelength side, but the cladding is doped with germanium, and the relative refractive index difference ΔC of the cladding with respect to pure silica. By increasing the value, the cutoff wavelength can be shifted to the short wavelength side. Thus, even in a refractive index profile where the cutoff wavelength tends to shift to the long wavelength side, the cutoff wavelength can be shifted to the short wavelength side by using the technique of doping germanium in the cladding, which is very effective. is there.

  An optical fiber was actually manufactured using the simulation results described above. The results are shown in Table 9 and Table 10. Table 9 shows structural parameters and characteristic values of optical fibers of Example 25 and Comparative Example 6 of the present invention, and Table 10 of Examples 26 and Comparative Example 7. In each of these comparative examples and examples, the same core was used, only the amount of germanium doped in the cladding was changed, and the relative refractive index difference ΔC of the cladding with respect to pure silica was changed. In Examples 25 and 26, germanium was doped so that the relative refractive index difference ΔC was 0.6%. Here, Comparative Example 6 and Example 25, and Comparative Example 7 and Example 26 were manufactured so that the dispersions were substantially the same for easy comparison.

In Example 25 and Example 26, the absolute value of dispersion at a wavelength of 1550 nm is 20 ps / nm / km or less, the effective core area Aeff at a wavelength of 1550 nm is 15 μm ² or less, and the nonlinear constant at a wavelength of 1550 nm is 25. × 10 ⁻¹⁰ / W or more is satisfied, and almost the same characteristics as the results obtained by the simulation are obtained.
Here, comparing Comparative Example 6 and Example 25, the dispersion slope of Example 25 is 0.009 ps / nm ² / km smaller, and the cutoff wavelength is also shifted to the shorter wavelength side by about 200 nm. Further, since D1 / D2 = 0.25 and the second core region is wide, Aeff is also slight, but Example 25 is smaller than Comparative Example 6, and as a result, Example 25 has a nonlinear constant n2. / Aeff has increased. A similar tendency is seen when comparing Comparative Example 6 and Example 26, and it can be said that it is very effective to increase the relative refractive index difference ΔC of the clad with respect to pure silica.

  By using the optical fiber of the present invention for an optical signal processing apparatus, it is possible to perform optical signal processing with stable performance in a wide wavelength range.

FIG. 8 shows an example of an optical wavelength converter as an example of an optical signal processing apparatus using the optical fiber of the present invention. According to this optical wavelength converter, the wavelengths of the signal light can be collectively converted to other wavelengths.
Here, FIG. 8 will be briefly described. Note that the wavelength at which the dispersion value of the optical fiber 17 of the present invention is zero is examined in advance.
First, excitation light (wavelength λs) in the vicinity of the wavelength where the dispersion value becomes zero is emitted from the light source 11 and coupled with the signal light 12 (wavelength λp). And it injects into the optical fiber 17 of this invention. At this time, 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)

  8, reference numeral 13 denotes a polarization controller for aligning polarization, reference numeral 14 denotes an EDFA, that is, an erbium-doped fiber amplifier (optical amplifier), and reference numeral 15 denotes a pump light (wavelength λs) from the light source and the signal light 12. And 16 represents a polarizer.

  FIG. 9 shows an example of a pulse compressor using the optical fiber of the present invention. 9, reference numerals 21 and 22 indicate light sources having different wavelengths, reference numeral 23 indicates a polarization controller, and reference numeral 24 indicates a coupler. Reference numeral 25 denotes a polarizer, and reference numeral 26 denotes an EDFA. The optical fibers connecting the light sources 21 and 22 to the EDFA 26 and the optical fiber indicated by reference numeral 28 are general single mode optical fibers, and reference numeral 27 is the optical fiber of the present invention. In this way, the optical fiber 27 of the present invention and the general single mode optical fiber 28 are alternately connected every predetermined length to constitute a pulse compressor.

  8 and 9 show only an optical wavelength converter and a pulse compressor as an optical signal processing apparatus using the optical fiber of the present invention. It goes without saying that the optical fiber of the invention can be applied.

1 shows an embodiment of an optical fiber according to the present invention. FIG. 1 (a) shows a refractive index distribution, and FIG. 1 (b) is a cross-sectional view showing a part of the cross-section. FIGS. 2A and 2B show another embodiment of the optical fiber according to the present invention. FIG. 2A shows a refractive index distribution, and FIG. 2B is a cross-sectional view showing a part of the cross-section. 3 is a graph showing the relationship between Δ1 and Aeff of an optical fiber having the structure of Table 1. 3 is a graph showing the relationship between D1 / D2 and dispersion slope of an optical fiber having the structure of Table 2. 3 is a graph showing the relationship between D1 / D2 and Aeff of an optical fiber having the structure of Table 2. 4 is a graph showing the relationship between α and dispersion slope of an optical fiber having the structure of Table 3. 4 is a graph showing the relationship between α and Aeff of an optical fiber having the structure of Table 3. It is a schematic diagram which shows one Example using the optical fiber which concerns on this invention for a wavelength converter. It is a schematic diagram which shows one Example using the optical fiber which concerns on this invention for a pulse compressor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st core 2 2nd core 3 3rd core 4 Clad 11, 21, 22 Light source 12 Signal light 13, 23 Polarization controller 14, 26 EDFA
15, 24 Coupler 16, 25 Polarizer 17, 27 Optical fiber

Claims (23)

A first core located in the center; a second core provided on an outer periphery of the first core and having a refractive index lower than that of the first core; and provided on an outer side of the second core. An optical fiber having a lower refractive index than the second core and a higher refractive index than the second core, the absolute value of dispersion at a wavelength of 1550 nm being 20 ps / nm / km or less, and an effective core at a wavelength of 1550 nm An optical fiber characterized in that the cross-sectional area Aeff is 15 μm ² or less, the nonlinear constant at a wavelength of 1550 nm is 25 × 10 ⁻¹⁰ / W or more, and the cladding is doped with germanium. ^2. The optical fiber according to claim 1, wherein an effective core area Aeff at a wavelength of 1550 nm is 12 μm ² or less, and a nonlinear constant at a wavelength of 1550 nm is 40 × 10 ⁻¹⁰ / W or more.   3. The optical fiber according to claim 1, wherein a relative refractive index difference of the clad with respect to pure silica is 0.1% to 1.0%.   The relative refractive index difference Δ1 of the first core with respect to the cladding is 1.0% to 5.0%, and the relative refractive index difference Δ2 of the second core with respect to the cladding is −2.4% to −0.2%. The optical fiber in any one of Claims 1-3.   The relative refractive index difference Δ1 of the first core with respect to the cladding is 2.0% to 4.0%, and the relative refractive index difference Δ2 of the second core with respect to the cladding is −2.0% to −0.6%. The optical fiber according to claim 1.   6. The optical fiber according to claim 1, wherein the absolute value of dispersion at a wavelength of 1550 nm is 10 ps / nm / km or less.   6. The optical fiber according to claim 1, wherein an absolute value of dispersion at a wavelength of 1550 nm is 5 ps / nm / km or less. The optical fiber according to any one of claims 1 to 7, wherein an absolute value of a dispersion slope at a wavelength of 1550 nm is 0.03 ps / nm ² / km or less. The optical fiber according to any one of claims 1 to 7, wherein an absolute value of a dispersion slope at a wavelength of 1550 nm is 0.01 ps / nm ² / km or less.   The difference between the maximum value and the minimum value of dispersion in the longitudinal direction of the optical fiber at any wavelength of 1510 to 1590 nm is 1 ps / nm / km or less. Optical fiber.   The difference between the maximum value and the minimum value of dispersion in the longitudinal direction of the optical fiber at any wavelength of 1510 to 1590 nm is 0.2 ps / nm / km or less. The optical fiber described.   The optical fiber according to any one of claims 1 to 11, wherein a cutoff wavelength λc is 1500 nm or less.   The optical fiber according to any one of claims 1 to 11, wherein a cutoff wavelength λc is 1200 nm or less. The light according to any one of claims 1 to 13, wherein a ratio D1 / D2 between an outer diameter D1 of the first core and an outer diameter D2 of the second core is 0.01 or more and 0.8 or less. fiber.   The light according to any one of claims 1 to 13, wherein a ratio D1 / D2 between the outer diameter D1 of the first core and the outer diameter D2 of the second core is 0.4 or more and 0.7 or less. fiber.   16. The third core according to claim 1, further comprising a third core having a refractive index lower than that of the first core and higher than that of the cladding portion on an outer periphery of the second core. Optical fiber. The optical fiber according to claim 16, wherein a relative refractive index difference Δ3 of the third core with respect to the cladding is 0.1% to 0.9%. The light according to claim 16 or 17, wherein a 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. fiber.   The optical fiber according to any one of claims 1 to 18, wherein a refractive index distribution shape of the first core is an α power profile, and the α is 3.0 or more.   The optical fiber according to any one of claims 1 to 18, wherein a refractive index profile shape of the first core is an α power profile, and the α is 6.0 or more.   21. An optical signal processing apparatus using the optical fiber according to any one of claims 1 to 20.   The optical signal processing device according to claim 21, wherein the optical signal processing device is an optical wavelength converter.   The optical signal processing apparatus according to claim 21, wherein the optical signal processing apparatus is a pulse compressor.

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