JP2005115193A - Optical fiber and optical signal processing apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber of which the dispersion value is stably low in a wide wavelength range near the wavelength of 1550nm, and to provide an optical signal processing apparatus using this optical fiber, more specifically, to provide an optical length converter and a pulse compression unit. <P>SOLUTION: In the optical fiber having a core and a clad provided outside the core, the dispersion slope is -0.01 to 0.01 ps/mm<SP>2</SP>/km at the wavelength of 1,550 nm, and the absolute value of the dispersion at the wavelength of 1,550 nm is 10ps/nm/km or lower, and the absolute value of the dispersion at the wavelength of 1,550 nm is 10 ps/nm/km or smaller, and also a non-linear constant at the wavelength of 1,550 nm is 30×10<SP>-10</SP>/W or larger. <P>COPYRIGHT: (C)2005,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 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, 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 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 the case of the optical fiber disclosed in Patent Document 1, the dispersion slope value of the wavelength 1550nm is -0.267ps / nm ² /km~+0.047ps/nm ² / km and the variation is large, yet also variance - The lower limit 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 was impossible to provide an optical fiber having a stable dispersion value and dispersion slope at a wavelength of 1550 nm. Therefore, it has not been possible to provide an optical fiber having a low dispersion value in a wide wavelength range near 1550 nm.

  Therefore, an object of the present invention is to provide an optical fiber having a stable and low dispersion value in a wide wavelength region near a wavelength of 1550 nm. Another object is to provide an optical signal processing apparatus using this optical fiber.

In order to achieve the above object, an optical fiber according to claims 1 to 3 is an optical fiber having a core and a cladding provided outside the core, and a dispersion slope at a wavelength of 1550 nm is -0.01 to 0.01 ps / nm. ² / km, preferably −0.005 to 0.005 ps / nm ² / km, the absolute value of dispersion at a wavelength of 1550 nm is 10 ps / nm / km or less, and the nonlinear constant at a wavelength of 1550 nm is 30 × 10 ⁻¹⁰ / W As described above, it is preferably 40 × 10 ⁻¹⁰ / W or more.

According to the optical fiber according to claims 1 to 3 of the present application thus configured, it is possible to provide an optical fiber having a small dispersion value variation and a small absolute dispersion value in a wide wavelength region in the vicinity of a wavelength of 1550 nm. It becomes possible.
Further, the optical signal processing at various wavelengths can be performed with one optical fiber without the dispersion value largely fluctuating with respect to the used wavelengths in a wide wavelength region. In addition, the dispersion slope is -0.01 to 0.01 ps / nm ² / km, preferably -0.005 to 0.005 ps / nm ² / km. Good optical signal processing using a nonlinear optical phenomenon can be performed in the region.
Incidentally, when the absolute value of the dispersion slope is 0.01 ps / nm ² / km or more, the dispersion value varies relatively with respect to different wavelengths in the vicinity of the wavelength of 1550 nm, and is not suitable for WDM transmission in a wide wavelength region.
Further, when the nonlinear constant is 30 × 10 ⁻¹⁰ / W or more, preferably 40 × 10 ⁻¹⁰ / W or more, an optical fiber having high nonlinearity can be obtained.

The optical fiber according to claim 4 or claim 5 is the optical fiber according to any one of claims 1 to 3, wherein the cutoff wavelength λc is 1450 nm or less, and the effective area Aeff is 12 μm ² or less, preferably It is characterized by being 10 μm ² or less.

According to the optical fiber according to claim 4 or claim 5 thus configured, the optical fiber can be operated as a single mode optical fiber. Thus, when the cutoff wavelength λc is 1450 nm or less, the optical fiber of the present invention can be used for a wide band including the S band, the C band, and the L band.
Further, by setting the effective area Aeff to 12 μm ² or less, more preferably 10 μm ² or less, a high nonlinear constant can be obtained.

Here, the nonlinear constant is represented by the following equation (1).
In the following formula (1), λ is the measurement wavelength, n ₂ is the nonlinear refractive index in the optical fiber, and Aeff is the effective cross-sectional area of the optical fiber.
Nonlinear constant = n ₂ / Aeff (1)
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 area Aeff.
Here, since n ₂ is a value determined by the material, it cannot be easily increased. Therefore, it is realistic to make the value of the effective area Aeff of the optical fiber as small as possible.
Therefore, according to the optical fiber according to claim 4 or claim 5 of the present invention, it is possible to obtain a higher nonlinear constant by setting the effective area Aeff of the optical fiber to 12 μm ² or less, preferably 10 μm ² or less. It becomes. Specifically, an optical fiber having a nonlinear constant at a wavelength of 1550 nm of 30 × 10 ⁻¹⁰ / W or more, further 40 × 10 ⁻¹⁰ / W or more can be obtained.

An optical fiber according to claim 6 of the present invention is characterized in that in the optical fiber according to any one of claims 1 to 5, the absolute value of dispersion at a wavelength of 1550 nm is 5 ps / nm / km or less. It is.
According to the optical fiber of the sixth aspect thus configured, an optical fiber exhibiting higher nonlinearity at the used wavelength can be obtained more reliably.

Furthermore, the optical fiber according to claim 7 or claim 8 is the optical fiber according to any one of claims 1 to 6, wherein the dispersion in the longitudinal direction of the optical fiber at any wavelength of 1510 to 1590 nm is changed. The width is 1 ps / nm / km or less, preferably 0.2 ps / nm / km or less per 1 km of optical fiber.
According to the optical fiber according to claim 7 or claim 8 thus configured, the fluctuation range of the dispersion value in the longitudinal direction of the optical fiber is 1 ps / nm / km or less, preferably 0.2 ps / nm / km per 1 km of the optical fiber. By the following, even when a long optical fiber is cut and used, it is guaranteed that the difference in dispersion is small in any part of the optical fiber. As a result, it is effective when used for a wavelength converter or the like.

  By the way, in the present application, the above-described variation range of the dispersion value means the variation range of the dispersion value measured by the dispersion distribution measuring device over the entire length of the practical optical fiber. The distribution measurement of the dispersion value of the optical fiber can be measured by a dispersion distribution measuring instrument using a method studied by Mollenauer, for example.

The optical fiber according to claim 9 or 10 of the present invention is the optical fiber according to claim 1, wherein the optical fiber is provided on the outer periphery of the first core having a refractive index higher than that of pure silica, and from the pure silica. A second core having a lower refractive index, and a cladding having a lower refractive index than the first core and a higher refractive index than the second core on the outer periphery of the second core, and an outer diameter D1 of the first core The 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, more preferably 0.4 or more and 0.7 or less. It is what.
Here, pure silica refers to silica that does not contain a refractive index adjusting dopant such as fluorine or germanium.

  According to the optical fiber according to claim 9 or claim 10 thus configured, 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 low dispersion slope can be obtained. That is, by making the optical fiber such a structure, an optical fiber having a small effective area Aeff, a low cutoff wavelength λc, and a small dispersion slope value can be obtained.

  Furthermore, the optical fiber according to claim 11 or 12 of the present application is the optical fiber according to any one of claim 9 or claim 10, wherein the relative refractive index difference Δ1 between the first core and the clad is 2.0. -5.0%, preferably 2.4-4.0%, and the relative refractive index difference Δ2 between the second core and the cladding is -1.4 to -0.7%, preferably -1.2 to -0.8%. Is.

  According to the optical fiber according to claim 11 or claim 12 thus configured, an optical fiber having high nonlinearity, low dispersion slope, and cutoff wavelength λc of 1450 nm or less can be stably maintained while maintaining high productivity. Can be manufactured.

  In addition, the optical fiber according to claim 13 or claim 14 is the optical fiber according to any one of claims 9 to 12, wherein the refractive index distribution shape of the first core is an α-power profile, and α Is 3.0 or more, preferably 6.0 or more.

  According to the optical fiber according to claim 13 or claim 14 thus configured, the dispersion slope can be reduced, the effective area Aeff of the optical fiber can be reduced, and an optical fiber with high nonlinearity can be obtained. .

An optical signal processing apparatus according to claim 15 of the present invention uses the optical fiber according to any one of claims 1 to 14.
According to such an optical signal processing device, it is possible to perform optical signal processing with stable performance in a wide wavelength range.

Further, claim 16 of the present application is characterized in that the optical signal processing device of claim 15 is an optical wavelength converter. According to such an optical wavelength converter, an optical wavelength converter excellent in wavelength conversion characteristics can be provided.
In addition, claim 17 of the present application is characterized in that the optical signal processing device of claim 15 is a pulse compressor. According to such a pulse compressor, a pulse compressor having excellent pulse compressibility can be provided.

  An optical fiber having a stable low dispersion value can be provided in a wide wavelength region near the wavelength of 1550 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.

  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.

An optical fiber according to the present invention is an optical fiber that generates a nonlinear phenomenon with respect to input light in the vicinity of a wavelength of 1550 nm, and has a dispersion slope of −0.01 to 0.01 ps / nm ² / km at a wavelength of 1550 nm. I am trying.
Thus, by providing a dispersion slope of −0.01 to 0.01 ps / nm ² / km, it is possible to provide an optical fiber having a small dispersion value variation and a small absolute dispersion value in a wide wavelength region near the wavelength of 1550 nm. it can.

Further, since the dispersion slope is −0.01 to 0.01 ps / nm ² / km in a wide wavelength region, the dispersion value does not fluctuate greatly in a wide wavelength region near 1550 nm. Therefore, it is possible to perform good optical signal processing using a nonlinear optical phenomenon over a wide wavelength range near 1550 nm.
By the way, when the absolute value of the dispersion slope is 0.01 ps / nm ² / km or more, the dispersion value varies relatively with respect to different wavelengths near 1550 nm. For this reason, the dispersion slope needs to be −0.01 to 0.01 ps / nm ² / km. In order to further reduce the variation of the dispersion value over a wide wavelength region near the wavelength of 1550 nm, −0.005 to 0.005 ps / nm ² / km is preferable.

It is also one of the features of the optical fiber of the present invention that the absolute value of the dispersion value is 10 ps / nm / km or less and the nonlinear constant at a wavelength of 1550 nm is 30 × 10 ⁻¹⁰ / W or more.
By setting the nonlinear constant to 30 × 10 ⁻¹⁰ / W or more, a highly nonlinear optical fiber can be obtained as described later.

In the single mode optical fiber, the cut-off wavelength λc is required to be small corresponding to the wavelength used. Therefore, the cutoff wavelength λc is desirably 1450 nm or less. When the cutoff wavelength λc is 1450 nm or less, it can be used for a wide wavelength band including the S band, the C band, and the L 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.
The effective area Aeff is preferably 12 μm ² or less. By making the effective area Aeff 12 μm ² or less, a high nonlinear constant can be obtained.

The nonlinear constant is expressed by the above-described equation (1). In order to increase the nonlinear constant, it is necessary to increase the nonlinear refractive index n ₂ of the optical fiber or to reduce the effective area Aeff as much as possible. As described above, it is necessary to make the effective area Aeff as small as possible.
Therefore, in order to obtain an optical fiber having a large nonlinearity, the optical fiber structure needs to have a small effective area Aeff. It is also necessary that the absolute value of dispersion at the wavelength used is small. Therefore, the absolute value of the dispersion value at the wavelength of 1550 nm is preferably 10 ps / nm / km or less, and more preferably 5 ps / nm / km or less.
By making the effective area Aeff 12 μm ² or less, a high nonlinear constant can be obtained. More preferably, by setting the effective area Aeff to 10 μm ² or less, a higher nonlinear constant can be obtained. As a result, an optical fiber having a nonlinear constant at a wavelength of 1550 nm of 40 × 10 ⁻¹⁰ / W or more can be obtained. Can be obtained.

  Further, as described above, the fluctuation range of the dispersion value described in the specification of the present application can be measured with a dispersion distribution measuring instrument using, for example, the method studied by Mollenauer over the entire length of the optical fiber having a practical length.

The dispersion fluctuation in the longitudinal direction of the optical fiber at any wavelength of 1510 to 1590 nm is preferably 0.001 to 1 ps / nm / km per 1 km of optical fiber.
When the fluctuation range of the dispersion value in the longitudinal direction of the optical fiber is 1 ps / nm / km or less per 1 km of the optical fiber, it is possible to perform good optical signal processing using a nonlinear phenomenon. More preferably, the fluctuation of the dispersion value in the longitudinal direction of the optical fiber at any wavelength of 1510 to 1590 nm is preferably 0.2 ps / nm / km or less per 1 km of optical fiber. Thus, even if a long optical fiber is cut and used because it is 0.2 ps / nm / km or less, the difference in dispersion value is small in any part of the optical fiber, and a better optical signal using nonlinear phenomenon Processing is possible.

  By the way, in order to actually suppress the fluctuation of the dispersion value in the longitudinal direction of the optical fiber, it is required that the thicknesses of the core and the clad are 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.

FIG. 1 shows one typical example of a nonlinear dispersion-shifted optical fiber according to the present invention. FIG. 1A shows the refractive index profile of this optical fiber, and FIG. 1B shows a part of the cross section thereof, that is, the first core 1 and the second core 2 provided outside the first core 1. The outer line of the clad 4 provided outside the second core 2 is omitted.
As shown in FIG. 1A, this optical fiber has a higher refractive index than that of pure silica, and has a first core 1 having a refractive index profile of α power represented by the following formula (2), and the first core 1. The second core 2 having a refractive index lower than that of pure silica and the cladding 4 provided outside the second core 2 are provided, and the outer diameter D1 and the second core 1 of the first core 1 are provided. The ratio D1 / D2 with the outer diameter D2 of the core 2 is 0.3 or more and 0.8 or less.

Here, in the present specification, α representing the shape of the refractive index distribution is defined by the following formula (2).
n ² (r) = n _c1 ² {1-2 · Δ1 · (2r / a) ^α } (2)
However, 0 ≦ r ≦ D1 / 2
Here, r represents a position in the optical fiber radial direction, and n (r) represents the refractive index at the position r. N _c1 is the maximum refractive index of the first core 1.

  The diameter D1 of the first core 1 is a length of a line connecting positions where the first core 1 has a refractive index equal to that of the cladding 4. The diameter D2 of the second core 2 is the length of a line connecting the positions where the refractive index is ½ of Δ2 in the boundary region between the second core 2 and the clad 4.

The refractive index difference Δ1 of the first core 1 with respect to the cladding 4 and the relative refractive index difference Δ2 of the second core 2 with respect to the cladding 4 are those represented by the following formulas (3) and (4).
Δ1 = {(n _c1 −n _c ) / n _c1 } · 100 (3)
Δ2 = {(n _c2 −n _c ) / n _c2 } · 100 (4)
Wherein in each of the formulas above, n _c1 is the maximum refractive index of the first core 1, n _c2 is the minimum refractive index of the second core 2, and n _c is the refractive index of the cladding 4.

In the structure of the optical fiber shown in FIG. 1, the value of the dispersion slope can be reduced by adjusting the ratio D1 / D2 between the outer diameter D1 of the first core 1 and the outer diameter D2 of the second core 2. .
Therefore, a change in the value of 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 in the structure of this optical fiber. .

  FIG. 2 shows the relationship between D1 / D2 at a wavelength of 1550 nm and the value of the dispersion slope when the dispersion is 0 ps / nm / km. Table 1 shows the refractive index distribution of the two types of optical fibers 1 and 2 used here.

As shown in FIG. 2, when D1 / D2 is less than 0.3 and when D1 / D2 exceeds 0.8, the dispersion slope has a dispersion slope larger than −0.01 to 0.01 ps / nm ² / km. Become.
In addition, as shown in Fig. 2, the range of D1 / D2 is further narrowed down, and by setting this value to 0.4 or more and 0.7 or less, the value of the dispersion slope can be set to the range of -0.005 to 0.005 ps / nm ² / km. It becomes. For this reason, the ratio D1 / D2 between the outer diameter D1 of the first core and the outer diameter D2 of the second core is preferably 0.4 or more and 0.7 or less.

  In general, a small effective area Aeff can be obtained by increasing the relative refractive index difference between the core and the cladding. However, merely increasing the relative refractive index difference with respect to the cladding of the core shifts the cutoff wavelength λc to the longer wavelength side, making it difficult to ensure single mode transmission in a wide band. On the other hand, for example, by taking the structure shown in FIG. 1, it is possible to achieve both a small effective area Aeff and a low cutoff wavelength λc.

The relative refractive index difference Δ1 of the first core 1 with respect to the cladding 4 is preferably 2.0 to 5.0%, and the relative refractive index difference Δ2 between the second core 2 and pure silica, that is, the cladding 4 in this example is −. It is preferable that it is 1.4 to -0.7%.
When the relative refractive index difference Δ1 with respect to the cladding 4 of the first core 1 is less than 2.0%, the effective area Aeff becomes large, and the nonlinearity of the optical fiber becomes relatively small. As the relative refractive index difference Δ1 increases, the cutoff wavelength λc shifts to the longer wavelength side. For this reason, when the relative refractive index difference Δ1 exceeds 5.0%, consideration for the cutoff wavelength λc for making the optical fiber single mode becomes too great, and as a result, the productivity of the optical fiber deteriorates.
In other words, if the relative refractive index difference Δ1 exceeds 5.0%, it becomes difficult to control the cut-off wavelength λc for making the optical fiber into a single mode. As a result, the manufacturing conditions of the optical fiber become strict and the productivity deteriorates. To do.
There is also a problem that the dispersion slope value at 1550 nm becomes large, and dispersion of the dispersion value becomes large for different wavelengths near 1550 nm when performing optical signal processing.

Further, when the relative refractive index difference Δ2 with respect to the cladding 4 of the second core 2 is increased to the negative side, it is possible to reduce the dispersion slope while reducing the absolute value of the dispersion value at the wavelength of 1550 nm.
However, when the relative refractive index difference Δ2 is increased to the negative side, the cutoff wavelength λc is shifted to the short wavelength side. Therefore, if the relative refractive index difference Δ1 is set to 2.0 to 5.0% and the relative refractive index difference Δ2 is set to −1.4% to −0.7%, the dispersion slope is set to a value of −0.01 to 0.01 ps / nm ² / km. It becomes possible. Further, the cutoff wavelength λc can be set to 1450 nm or less.
On the other hand, if the relative refractive index difference Δ2 is less than −1.4%, for example, the second core 2 needs to be doped with a large amount of fluorine, which makes it difficult to manufacture an optical fiber and deteriorates productivity.

  The relative refractive index difference Δ1 is more preferably 2.4 to 4.0%, and Δ2 is more preferably −1.2 to −0.8%. Within this range, an optical fiber having a high nonlinearity, a low dispersion slope, and a cutoff wavelength λc of 1450 nm or less can be manufactured with higher productivity, and the stability of performance is further improved.

  Furthermore, by making the refractive index profile of the first core 1 a power profile of α and increasing this α, the dispersion slope can be reduced and the effective area Aeff can also be reduced. Therefore, it is desirable that the refractive index distribution shape of the first core 1 is an α power refractive index distribution, and α is 3 or more. More preferably, α is 6 or more.

Here, the fact that increasing α is advantageous in reducing the dispersion slope, for example, will be described with reference to FIGS. 3 and 4 showing a simulation example of an example of the optical fiber of the present invention.
FIG. 3 shows the relationship between α and the dispersion slope, and FIG. 4 shows the relationship between α and the effective area Aeff. Table 2 shows the structures of the two types of optical fibers A and B used here.

As shown in FIG. 3, the dispersion slope can be reduced by increasing the value of α. Especially by the α 2 from 3, the value of the dispersion slope of about 0.009 ps / nm ² / km less in the optical fiber A, and it is understood that it is possible to reduce to about 0.01 ps / nm ² / miles optical fiber B . Increasing α in this way is very effective in reducing the dispersion slope.

As shown in FIG. 4, the effective area Aeff can be reduced by increasing the value of α. In particular, by increasing α from 2 to 3, the effective area Aeff can be reduced by about 8% in both the optical fiber A and the optical fiber B.
By the way, as one method for increasing the α of the first core 1, a core base material having a refractive index distribution shape α having a refractive index higher than that of pure silica is previously obtained by a VAD method or an MCVD method. There is a method of manufacturing. The value of α of the refractive index distribution shape can be increased by etching the surface of the core base material produced by this method with HF or by mechanical cutting.

In particular, when the above method is used, it is relatively easy to make α 3 or more from the viewpoint of manufacturing.
Further, the dispersion slope can be further reduced by further increasing the value of α as shown in FIG. 3 to 6 or more, and the effective area Aeff can also be reduced as shown in FIG.
Incidentally, in the region where α is 6 or more, as shown in FIGS. 3 and 4, the dispersion slope continues to decrease gradually as α increases, but the reduction of the effective area Aeff is almost saturated. Therefore, at least α is preferably 6 or more.

Table 3 shows parameter values and characteristic values of the optical fibers shown in Examples 1 to 10 of the present invention. In Table 3, MFD means mode field diameter.

In all of Examples 1 to 10, the absolute value of dispersion at a wavelength of 1550 nm is 10 ps / nm / km or less, and the dispersion slope is −0.01 to 0.01 ps / nm ² / km. The cutoff wavelength λc is 1450 nm or less, and the effective area Aeff is 12 μm ² or less.

Here, the characteristic values shown in Examples 1 to 7 are results obtained by simulation, and Examples 8 to 10 are characteristic values obtained by actually producing and evaluating an optical fiber.
First, attention is paid to Example 1 and Example 2. In order to facilitate the comparison, the dispersion value at the wavelength of 1550 nm was made almost the same.
In the optical fiber of the first embodiment, 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 0.35, and in the second embodiment, D1 / D2 is 0.55.
Comparing the characteristic values obtained in both optical fibers, the effective area Aeff of Example 2 is larger than that of Example 1 and the cutoff wavelength λc is on the long wavelength side, but the dispersion slope at the wavelength of 1550 nm is larger. The value shows a fairly small value. That is, from the viewpoint of the dispersion slope, it is presumed that D1 / D2 is preferably 0.4 or more and 0.7 or less than 0.3 or 0.8 or less.

  Next, Example 4 and Example 5 will be compared in the same manner as described above. In Example 4, the refractive index profile of the first core is an α power profile, and α is 4.5. On the other hand, in Example 5, α is 7. Comparing the characteristic values of the obtained optical fibers, Example 5 shows a smaller dispersion slope at the wavelength of 1550 nm than Example 4, and the effective area Aeff also shows a smaller value. From this viewpoint, it is suggested that α is preferably 6.0 or more than that having 3.0 or more.

  Further, dispersion fluctuations in the longitudinal direction of the optical fibers shown in Example 8 and Example 9 were measured. As a result, in Example 9, there was a dispersion fluctuation of 1.9 ps / nm / km over the entire length of 3 km at the measurement wavelength of 1552 nm. If this is converted per 1 km, it corresponds to a dispersion fluctuation of 0.75 ps / nm / km. Further, the optical fiber shown in Example 9 had a dispersion fluctuation of 0.15 ps / nm / km over the entire length of 15 km at the measurement wavelength of 1556 nm. Similarly, when converted per km, a dispersion fluctuation of 0.08 ps / nm / km was observed. Any variation is within an acceptable range.

Table 4 shows parameter values and characteristic values of the optical fibers shown in Comparative Examples 1 to 5.
In Table 4, MFD means mode field diameter.

  First, in the optical fiber of Comparative 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 0.25. In this optical fiber, the value of the dispersion slope becomes relatively large, and when used in a wide wavelength range, the fluctuation of the dispersion value becomes large, and good optical signal processing utilizing a nonlinear phenomenon cannot be performed.

In Comparative Example 2, the relative refractive index difference Δ2 with respect to the cladding 4 of the second core 2 is −0.5%. The value of the dispersion slope of the obtained optical fiber becomes relatively large. Similarly to the comparative example 1, when this optical fiber is used in a wide wavelength region, the dispersion value fluctuates greatly, and a nonlinear phenomenon is caused. Better optical signal processing than used cannot be performed.
In the optical fiber of Comparative Example 3, the relative refractive index difference Δ1 with respect to the cladding 4 of the first core 1 is 1.8%. In the obtained optical fiber, the effective area Aeff becomes relatively large, and a nonlinear constant γ of 30 × 10 ⁻¹⁰ / W or more cannot be obtained.

  In the optical fiber of Comparative Example 4, the refractive index profile of the first core 1 is an α power profile, and the value of α is 2.5. The value of the dispersion slope of the obtained optical fiber becomes relatively large, and the dispersion value varies greatly when used in a wide wavelength region, as in Comparative Example 1 and Comparative Example 2, resulting in a nonlinear phenomenon. This makes it impossible to perform better optical signal processing using.

  Furthermore, in Comparative Example 5, the relative refractive index difference Δ1 between the first core 1 and pure silica is 5.5%. In this optical fiber, the cutoff wavelength λc has shifted to the long wavelength side, and there is a problem when it is used at a wavelength of 1550 nm.

FIG. 5 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. 5 will be briefly described. Note that the wavelength at which the dispersion value of the optical fiber 7 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 inserts in the optical fiber 17 of this 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 (5). As a result, optical wavelength conversion is performed collectively.
λ = (λp−λs) + λp (5)

  5, 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 excitation light (wavelength λs) and signal light 12 from a light source. The coupling coupler and the reference numeral 16 denote a polarizer, respectively.

  FIG. 6 shows an example of a pulse compressor using the optical fiber of the present invention. In FIG. 6, reference numerals 21 and 22 denote light sources having different wavelengths, reference numeral 23 denotes a polarization controller, and reference numeral 24 denotes 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 at predetermined lengths to constitute a pulse compressor.

  5 and 6 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. 1A shows a refractive index distribution, and FIG. 1B is a cross-sectional view showing a part of the cross-section. It is a graph which shows the simulation result of D1 / D2 and dispersion slope of the optical fiber in FIG. It is a graph which shows the relationship between (alpha) and dispersion | distribution slope by simulation. It is a graph which shows the relationship between (alpha) and effective area Aeff by simulation. It is a schematic diagram which shows one Example which used the optical fiber of this invention for the wavelength converter. It is a schematic diagram which shows one Example which used the optical fiber of this invention for the pulse compressor.

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 Coupler 16, 25 Polarizer 17, 27 Optical fiber

Claims (17)

In an optical fiber having a core and a cladding provided outside the core, the dispersion slope at a wavelength of 1550 nm is −0.01 to 0.01 ps / nm ² / km, and the absolute value of dispersion at a wavelength of 1550 nm is 10 ps / nm / km or less An optical fiber having a nonlinear constant at a wavelength of 1550 nm of 30 × 10 ⁻¹⁰ / W or more. The optical fiber according to claim 1, wherein a dispersion slope at a wavelength of 1550 nm is −0.005 to 0.005 ps / nm ² / km. 3. The optical fiber according to claim 1, wherein a nonlinear constant at a wavelength of 1550 nm is 40 × 10 ⁻¹⁰ / W or more. The optical fiber according to any one of claims 1 to 3, wherein the cutoff wavelength λc is 1450 nm or less and the effective area Aeff is 12 µm ² or less. The optical fiber according to claim 4, wherein the effective area Aeff is 10 µm ² 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 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 per 1 km of optical fiber, according to any one of claims 1 to 6. Optical fiber.   8. The optical fiber according to claim 7, wherein the fluctuation range 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 per 1 km of optical fiber.   A first core having a higher refractive index than that of pure silica, a second core provided on the outer periphery of the first core and having a refractive index lower than that of pure silica, and an outer periphery of the second core being refracted more than the first core. A cladding having a low refractive index and a refractive index higher than that of the second core; the outer diameter D1 of the first core is 2 to 5 μm; the outer diameter D1 of the first core and the outer diameter D2 of the second core The optical fiber according to claim 1, wherein a ratio D1 / D2 of the optical fiber is 0.3 or more and 0.8 or less.   The optical fiber according to claim 9, wherein a ratio D1 / D2 between an outer diameter D1 of the first core and an outer diameter D2 of the second core is 0.4 or more and 0.7 or less.   The relative refractive index difference Δ1 between the first core and the cladding is 2.0 to 5.0%, and the relative refractive index difference Δ2 between the second core and the cladding is −1.4 to −0.7%. The optical fiber according to claim 9 or 10.   The relative refractive index difference Δ1 between the first core and the cladding is 2.4 to 4.0%, and the relative refractive index difference Δ2 between the second core and the cladding is −1.2 to −0.8%. The optical fiber according to claim 11.   The optical fiber according to any one of claims 9 to 12, wherein a refractive index distribution shape of the first core is an α-power profile, and α is 3.0 or more.   The optical fiber according to claim 13, wherein the refractive index profile of the first core is an α power profile, and α is 6.0 or more.   An optical signal processing apparatus using the optical fiber according to any one of claims 1 to 14.   The optical signal processing apparatus according to claim 15, wherein the optical signal processing apparatus is an optical wavelength converter. The optical signal processing apparatus according to claim 15, wherein the optical signal processing apparatus is a pulse compressor.

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