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Dokumentenidentifikation EP0909964 04.10.2007
EP-Veröffentlichungsnummer 0000909964
Titel Dispersionsverschobene optische Faser
Anmelder Fujikura Ltd., Tokio/Tokyo, JP
Erfinder Matsuo, Shoichiro, Sakura-shi, Chiba-ken, JP;
Horikoshi, Masahiro, Sakura-shi, Chiba-ken, JP
Vertreter derzeit kein Vertreter bestellt
DE-Aktenzeichen 69838276
Vertragsstaaten DE, FR, GB, IT
Sprache des Dokument EN
EP-Anmeldetag 13.10.1998
EP-Aktenzeichen 983083049
EP-Offenlegungsdatum 21.04.1999
EP date of grant 22.08.2007
Veröffentlichungstag im Patentblatt 04.10.2007
IPC-Hauptklasse G02B 6/036(2006.01)A, F, I, 20070502, B, H, EP

Beschreibung[en]

The present invention relates in general to dispersion-shifted optical fiber of nearly zero chromatic dispersion within a 1.55 µm wavelength band, while achieving reduced non linear effects and low bending loss, and relates in particular to an optical fiber whose dispersion slope is reduced sufficiently.

Dispersion-shifted optical fiber (referred to as DS-fiber hereinbelow) is an optical fiber whose chromatic dispersion value is almost zero in a 1.55 µm wavelength band where the transmission loss is minimal for quartz group optical fiber. For example, a DS-fiber having a staircase type refractive index distribution (refractive index profile) is well known.

The DS-fiber having such a refractive index profile is characterized by having smaller bending loss compared with other types of DS-fiber, such as step-profile type or triangular-profile type fibers, and somewhat larger mode field diameter (hereinbelow referred to as MFD); however, relative to the normal single-mode fiber for 1.3 µm band, the MFD is relatively small at about 8 µm or less.

When MFD is small, transmission problems are encountered because not only splice losses are increased but, for applications requiring high power density within the fiber such as optical amplifier applications for example, non linear effects become high and transmission characteristics become seriously degraded.

A quantitative measure of non linear effects is n2/Aeff where n2 is a non linear refractive index for the fiber, and Aeff is the effective cross section area of the fiber. Because n2 is approximately constant for a given optical material, Aeff must be made large to decrease non linear effects in the fiber

On the other hand, Aeff and MFD in DS-fibers are related by the following expression: Aeff = k &pgr; / 4 MFD 2 where k is a correction factor.

Here, when the core diameter changed in DS-fibers, there are not less than two radius values for zero chromatic dispersion in the 1.55 wavelength band

Of these solution values, the smallest value is referred to as the small-diameter solution, and the next smallest value is referred to as the large-diameter solution. Generally, a DS-fiber having staircase type refractive index profile adopts the large-diameter solution.

It has been reported that the correction factor k for a DS-fiber having the normal staircase-type refractive index profile with a large-diameter solution is about 0.944 and remains unchanged regardless of processing parameters used on the fiber.

Therefore, to increase the Aeff, it is necessary to increase MFD.

However, the normal DS-fiber having the staircase type refractive index profile based on the large-diameter solution has a constant MFD value of approximately 8 µm, and therefore, it can not increase the Aeff and enable reduction in non linear effects.

To resolve such problem, the present inventors have proposed a DS-fiber having small-diameter solution in a Japanese Patent Application, First Publication No. Hei8-220362 (Application date Heisei 7, February 10).

In this invention, a small-diameter solution is adopted for DS-fiber having a staircase type refractive index profile, thereby increasing the correction factor to about 0.95~0.97, and MFD to about 7.8~8.6 µm. The result is that Aeff is increased and non linear effects have been reduced.

However, in this invention, although an advantage is gained that Aeff is increased by adopting the small-diameter solution, there remained a difficulty that bending loss and dispersion slope are increased.

Furthermore, for wavelength division multiplexing (WDM) transmission systems, which have been under active development in recent years, even greater reduction in non linear effects is demanded However, it is difficult for DS-fiber with staircase type refractive index profile to meet such a challenge, because of its limited ability to increase Aeff.

The present inventors have submitted a Japanese Patent Application, Fist Publication, No Hei10-62640 (application date Heisei 8, August 15), and disclosed a DS-fiber with emphasis on increasing the Aeff.

The DS-fiber disclosed in JPA, First Publication No. Hei10-62640 has a ring-structured refractive index profile, and is comprised by a center core section having a high refractive index, and a ring core section provided separately from the center core section having a low refractive index, cladding provided on the outer periphery of the ring core section, and an intermediate layer disposed between the center core section and the ring core section.

As disclosed in JPA, First Publication, No. Hei8-220362, in DS-fibers having the staircase refractive index profile, it was known that Aeff can be increased by adopting the small-diameter solution Therefore, in this invention, the small-diameter solution is adopted with a primary objective of increasing Aeff.

The ring-structured DS-fiber (presented in JPA, First Publication No. Hei10-62640) shows almost zero chromatic dispersion in the 1.55 µm band, and its Aeff is higher than that of DS-fiber having the staircase type refractive index profile, thereby resulting in decreased non linear effects as well as low bending loss. Therefore, this type of DS-fiber met two of the important requirements.

However, such a DS-fiber still left a problem that the dispersion slope increases because of the increase in Aeff. High dispersion slope is not desirable in wavelength multiplexing transmission systems because it causes inconsistencies in the transmission of the plural wavelengths.

Accordingly, although increasing in Aeff has been a priority topic, in the past, to reduce non linear effects in DS-fiber for use in wavelength multiplexing system, in recent years, there have been a DS-fiber with achieving sufficient reduction in dispersion slope, rather than with increasing Aeff, to meet the needs of recent system

Therefore, one of the topics of study in the present invention is to develop a DS-fiber, whose Aeff would be high enough for use in wavelength division multiplexing system to decrease non linear effects, with high priority placed on decreasing its dispersion slope

It is a distinguishing feature of the present invention to provide a DS-fiber based on large-diameter solution.

In the DS-fiber having the ring-structured refractive index profile, presented in the previous invention, small-diameter solution was used because the emphasis was primarily to increase the Aeff. For this reason, it was not possible to reduce its dispersion slope sufficiently.

In the study that led to the present invention, the emphasis was placed on reducing the dispersion slope, and it was discovered that, by using the large-diameter solution to design a DS-fiber, its dispersion slope can be made sufficiently small while increasing its Aeff more than that in DS-fiber having the staircase refractive index profile, so that the resulting DS-fiber can be used in wavelength multiplexing transmission system, by having both low non linear effects and small bending loss.

Figure 1 is a drawing showing an example of the refractive index profile of the DS-fiber of the present invention.

The DS-fiber of the present invention exhibits the following characteristics. The chromatic dispersion in the 1.55 µm wavelength band is nearly zero but not zero, the effective cross section area is 45~70 µm2, bending loss is 0.1~100 dB/m, the dispersion slope is 0.05~0.08 ps/km/nm2. It has a cutoff wavelength which answers for a single mode propagation in the 1.55 µm band

In the present invention, the operational band of 1.55 µm wavelength (the 1.55 µm wavelength band) means a range of wavelengths between 1520 to 1580 nm.

Also, chromatic dispersion nearly zero means that, within the operational band, chromatic dispersion value is in a range between -5~+5 ps/nm.km However, it is necessary that chromatic dispersion value does not actually become 0 ps/nm·km This is because, if chromatic dispersion value is 0 ps/nm.km, non linear effects such as four-optical-mixing become undesirable large.

Also, the effective cross section area Aeff is defined by the following relation. A eff = 2 &pgr; 0 r E r 2 dr 2 0 r E r 4 dr

where r is a radius of the fiber, E(r) is the electric field strength at radius r.

Bending loss is a value measured with a wavelength of 1.55 µm in a fiber bent at a bend radius (2R) of 20 mm. Cutoff wavelength is a value measured according to a method of Japanese Industrial Standards (JIS) or CCITT 2m protocol or a value measured in actual use.

Also, dispersion slope relates to wavelength-dependence of chromatic dispersion, and is obtained as a slope of a curve in a graph of wavelength (nm) on x-axis and chromatic dispersion (ps/nm·km) on y-axis.

The primary feature of DS-fiber of the present invention is that dispersion slope is in a range 0.05~0.08 ps/km/nm2 and is made small enough.

At the same time, the effective cross section area Aeff is 45~70 µm2, which is large compared with the staircase type refractive index profile, and is able to suppress non linear effects to a level sufficient for use in the WDM transmission systems.

In other words, an optical fiber having the properties described above will be a DS-fiber with sufficiently reduced non linear effects so as to be applicable to WDM transmission systems, and have small bending loss and low dispersion slope.

If Aeff is less than 45 µm2, suppression of non linear effects is insufficient If it is in excess of 70 µm2, the large-diameter solution does not exist in low bending loss region so that it is difficult to satisfy the needs for the dispersion slope

Also, bending loss in excess of 100 dB/m is not desirable, because the transmission loss becomes high even with a slight curvature in the fiber.

Also, because the large-diameter solution is adopted, it is possible to realize a sufficiently small range, 0.05~0.08 ps/km/nm2, of dispersion slope. This range is chosen, because it is difficult to produce dispersion slope of less than 0.05 ps/km/nm2, and if the dispersion slope exceeds 0.08 ps/km/nm2, such a DS-fiber does not satisfy the needs of reduced dispersion slope as defined in the present invention.

Further, DS-fiber is usually a single-mode fiber, and must always provide single-mode transmission within the operational band To do this, the cutoff wavelength must be a value to guarantee single-mode transmission.

For the present DS-fiber to possess the properties described above, the first necessary condition is that the fiber must have a ring-structured refractive index profile such as the one shown in Figure 1.

Figure 1 shows a concentric structure of DS-fiber comprised by a center core section 1, a first ring section 2 surrounding the center core section 1, a second ring section 3 surrounding the first ring section 2, and a third ring section 4 surrounding the second ring section 3, followed by a cladding surrounding the third ring section 4. The core section 1, first ring section 2, second ring section 3 and the third ring section 4 are arranged in the state of a concentric circle.

Refractive indexes n0, n1, n2, and n3, for the core section 1, first ring section 2, second ring section 3 and the third ring section 4, respectively, are related as follows: n0>n2, n2>n1, n2> n3, n3≤n4.

As will be described later, cladding 5 is made of either pure silica or F-doped silica, therefore, n4 is not limited to the refractive index of pure silica

In the refractive index profile shown in Figure 1, based on a reference of zero refractive index for cladding 5, the relative refractive index differences designated as follows &Dgr;1 for the difference between cladding 5 and center core section 1; &Dgr;2 between cladding 5 and first ring section 2; &Dgr;3 between cladding 5 and second ring section 3; and &Dgr;4 between cladding 5 and third ring section 4 The actual values are in the following ranges: +0.5~+0 8 % for &Dgr;1; -0 1~+0 1 % for &Dgr;2, +0 05~+0 5 % for &Dgr;3, and -0.2~+0.0 % for &Dgr;4.

However, a problem with such a DS-fiber is that, with increasing Aeff, there is a tendency for increase in the cutoff wavelength. Therefore, it is desirable to shift the cutoff wavelength to a lower value by selecting a low value for &Dgr;4 such as the range described above.

Therefore, a second necessary condition for the DS-fiber is to adopt the large-diameter solution for the core diameter.

To accomplish this requirement, referring to Figure 1, the outer diameters of center core section 1, first ring section 2, second ring section 3 and third ring section 4, respectively, designated as 2a, 2b, 2c, 2d, should be chosen such that b/a is not less than 1.5, and preferably between 1.5∼4.0

Also, the actual values for "a" is 2.0∼4.0 µm, and "b-a" is 1.0∼5.0 µm, "c-b" is 1.0~12 µm; and "d-c" is 0.0∼20 µm.

When b/a is less than 1.5, it is not possible to increase Aeff sufficiently, on the other hand, it is not desirable to exceed 4.0, because of the difficulty in controlling the properties of manufactured fiber.

When a is less than 2.0 µm or exceeds 4 µm, no large-diameter solution exist to satisfy the properties of the DS-fiber of the present invention.

When b-a is less than 1.0 µm, there is no benefit of providing the first ring section 2, and if it exceeds 5.0 µm, the fiber may suffer from problems of manufacture as well as some properties such as cutoff wavelength and bending loss.

When c-b is less than 1.0 µm, there is no benefit of providing the second ring section 3, and if it exceeds 12 µm, the fiber may suffer from problems of manufacture as well as some properties such as increase in the cutoff wavelength.

In overall summary, therefore, by selecting the values of the parameters (&Dgr;1, &Dgr;2, &Dgr;3, &Dgr;4, b/a, a, b-a, c-b, d-c) in the range described above, and by adopting the large-diameter solution, a DS-fiber having the present property values will be obtained.

Table 1 summarizes the properties of DS-fiber, exemplified by cutoff wavelength (&lgr;c), Aeff, bending loss (BL), Dispersion Slope (DS), and the various combinations of parameters to meet the first and second requirements.

As can be understood from Table 1, by combining parameters from a wide range of values, it is possible to obtain DS-fiber having the targeted characteristics

It is clear that such characteristics have not been attainable in conventional DS-fibers.

The DS-fiber in the prevent invention is based on adopting the large-diameter solution from the two core sizes available for zero chromatic dispersion in the 1.55 mm band, thereby attaining a low bending loss as well as a relatively high Aeff, and enabling to lower its dispersion slope to not more than 0.08 ps/km/nm2.

The present DS-fiber can be produced by normal fiber manufacturing method such as vapor-phase axial deposition (VAD) method, so that, in the present case, the center core section 1 and the second ring section 3 were made from Ge-doped silica or pure silica, and the first ring section 2, third ring section 4 and cladding 5 were made from pure silica or F-doped silica.

In a fiber having the profile shown in Figure 1, the distribution of electric field strength produced by propagating light is shaped in such a way to leave a long tail in the cladding 5 because of the presence of the second ring section 3, therefore, it is preferable that, when manufacturing mother material for the optical fiber, fair section of soot for cladding should be made at the same time as the soot for the center core.

Determination of the refractive index profile by a method such as Refracted Near-Field Profiling (RNFP) method on the DS-fiber fibers produced in this study showed that corners were found to be rounded and smooth shaped, compared with the schematic profile shown in Figure 1.

The values should be chosen such that peak values are selected for parameters such as &Dgr;1, &Dgr;2, &Dgr;3, &Dgr;4 and half value full width half peak value should be selected for a~d

Typical properties of the test fiber produced in this study are summarized in Table 2, which shows that the produced fiber meets the properties required in the present invention Table 2 Measured Items Results Aeff at 1550 nm (µm2) 53.5 MFD at 1550 nm (µm) 8.51 &lgr;c for 2 m fiber (µm) 1.7 Bending Loss at 1550 nm, 20 &PHgr; (dB/m) 1.20 Zero-dispersion wavelength (µm) 1574.00 Dispersion Value at 1550 nm (ps/km/nm) -1.32 Dispersion Slope at 1550 nm (ps/km/nm2) 0.053 Transmission Loss at 1550 nm (dB/km) 0.208 Polarization dispersion (ps/√km) 0.123


Anspruch[de]
Dispersionsverschobene optische Faser, umfassend: einen zentralen Kernabschnitt (1), einen ersten Ringabschnitt (2), der diesen zentralen Kernabschnitt umgibt, einen zweiten Ringabschnitt (3), der den ersten Ringabschnitt umgibt; einen dritten Ringabschnitt (4), der den zweiten Ringabschnitt umgibt, und einen Mantel (5), der den dritten Ringabschnitt umgibt, worin Brechungsindizes n0, n1, n2, n3 und n4 für den zentralen Kernabschnitt (1), den ersten Ringabschnitt (2), den zweiten Ringabschnitt (3), den dritten Ringabschnitt (4) und den Mantel (5) in einer Beziehung stehen, die einem Brechungsindexprofil gemäß den folgenden Beziehungen entspricht: n0 > n2, n2 > n1 > n3 und n3 < n4,

worin ein Kerndurchmesser eine Lösung mit großem Durchmesser ist, welcher der zweitkleinste Wert von Radiuswerten für eine chromatische Dispersion von Null in einem 1,55-Wellenlängenband ist,

wobei die dispersionsverschobene optische Faser folgende charakteristische Eigenschaften aufweist:

Wert der chromatischen Dispersion in einem Bereich zwischen -5 und +5 ps/nm km, aber nicht 0 ps/nm km, in einem Wellenlängenband von 1,55 µm, eine effektive Querschnittsfläche in einem Bereich von 45 bis 70 µm2, einen Biegeverlust in einem Bereich von 0,1 bis 100 dB/m und eine Dispersionssteigung in einem Bereich von 0,05 bis 0,08 ps/km/nm2,

und die eine Grenz-Wellenlänge in einem 1,55 µm Wellenlängenband erzeugt, wodurch sie immer eine Einmodenübertragung bereitstellt,

wobei die Durchmesser 2a, 2b, 2c und 2d des zentralen Kernabschnitts (1), des ersten Ringabschnitts (2), des zweiten Ringabschnitts (3) und des dritten Ringabschnitts (4) in folgender Beziehung zueinander stehen: b/a liegt in einem Bereich von 1,5 bis 4,0, wobei a in einem Bereich von 2,0 bis 4,0 µm liegt, die Differenz zwischen b und a in einem Bereich von 1,0 bis 5,0 µm liegt, die Differenz zwischen c und b in einem Bereich von 1,0 bis 12 µm liegt und die Differenz zwischen d und c größer als 0,0 µm und geringer als 20 µm ist, und wenn der Brechungsindex des Mantels (5) als Null-Referenz dient und &Dgr;1 die Brechungsindexdifferenz zwischen dem Mantel (5) und dem zentralen Kernabschnitt (1), &Dgr;2 die Brechungsindexdifferenz zwischen dem Mantel (5) und dem ersten Ringabschnitt (2), &Dgr;3 die Brechungsindexdifferenz zwischen dem Mantel (5) und dem zweiten Ringabschnitt (3) und &Dgr;4 die Brechungsindexdifferenz zwischen dem Mantel (5) und dem dritten Ringabschnitt (4) ist, dann &Dgr;1 +0,5 bis 0,8 %; &Dgr;2 -0,1 bis +0,1 %; &Dgr;3 +0,05 bis +0,5 % und &Dgr;4 -0,2 bis weniger als +0,0 % ist.
Anspruch[en]
A dispersion-shifted optical fiber comprising: a center core section (1), a first ring section (2) surrounding said center core section, a second ring section (3) surrounding said first ring section; a third ring section (4) surrounding said second ring section; and a cladding (5) surrounding said third ring section, wherein a refractive index n0, n1, n2, n3 and n4 for said center core section (1), said first ring section (2), said second ring section (3), said third ring section (4) and said cladding (5), respectively, are related in a refractive index profile according to the following relationships; n0>n2,n2>n1>n3, and n3<n4, a core diameter is a large-diameter solution which is the second smallest value of radius values for zero chromatic dispersion in a 1.55 wavelength band, the dispersion-shifted optical fiber having characterising properties of: chromatic dispersion value in a range between -5 and +5 ps/nm.km but not 0 ps/nm.km in a wavelength band of 1.55 µm, an effective cross section area in a range of 45 to 70 µm2, a bending loss in a range of 0.1 to 100 dB/m, and a dispersion slope in a range of 0.05 to 0.08 ps/km/nm2; and producing a cut-off wavelength within a 1.55 mm wavelength band so as to always provide single-mode transmission, diameters 2a, 2b, 2c and 2d, respectively for said center core section (1), said first ring section (2), said second ring section (3), and said third ring section (4) are related according to the following relationships; b/a is in the range of 1.5 to 4.0, wherein a is in a range 2.0 to 4.0 µm, the difference between b and a is in a range 1.0 to 5.0 µm, and the difference between c and b is in a range 1.0 to 12 µm, and difference between d and c is greater than 0.0µm and less than 20 µm, and when the refractive index of the said cladding (5) is a reference of zero, &Dgr;1 is the (1) refractive index difference between said cladding (5) and said center core section (1), &Dgr;2 is the refractive index difference between said cladding (5) and said first ring section (2), &Dgr;3 is the refractive index difference between said cladding (5) and said second ring section (3), and &Dgr;4 is the refractive index difference between said cladding (5) and said third ring section (4), then &Dgr;1 is +0.5 to 0.8%; &Dgr;2 is -0.1to +0.1 %; &Dgr;3 is +0.05 to +0.5%; and &Dgr;4 is -0.2 to less than +0.0%.
Anspruch[fr]
Fibre optique à dispersion décalée comprenant : une section de coeur centrale (1), une première section annulaire (2) entourant ladite section de coeur centrale, une deuxième section annulaire(3) entourant ladite première section annulaire ; une troisième section annulaire (4) entourant ladite deuxième section annulaire; et une gaine (5) entourant ladite troisième section annulaire, dans laquelle des indices de réfraction n0, n1, n2, n3 et n4 pour ladite section de coeur centrale (1), ladite première section annulaire (2), ladite deuxième section annulaire(3), ladite troisième section annulaire (4) et ladite gaine (5), respectivement, sont liés à des profils d'indice de réfraction suivant les relations suivantes : n0 > n2, n2 > n1 > n3, et n3 < n4,

un diamètre de coeur est une solution de grand diamètre qui est la seconde valeur la plus petite des valeurs de rayon pour la dispersion chromatique nulle dans une bande de longueurs d'onde de 1,55 µm,

la fibre optique à dispersion décalée ayant des propriétés de caractérisation telles que :

la valeur de dispersion chromatique est comprise dans une plage s'étendant de -5 à +5 ps/nm.km, mais non 0 ps/nm.km, dans une bande de longueurs d'onde de 1,55 µm, une surface de section efficace effective dans une plage de 45 à 70 µm2, une perte de flexion dans une plage de 0,1 à 100 dB/m, et une pente de dispersion dans une plage de 0,05 à 0,08 ps/km/nm2 ; et produisant une longueur d'onde de coupure dans une bande de longueurs d'onde de 1,55 µm de manière à toujours fournir une transmission monomode,

les diamètres 2a, 2b, 2c et 2d, respectivement, pour ladite section de coeur centrale (1), ladite première section annulaire (2), ladite deuxième section annulaire(3) et ladite troisième section annulaire (4) sont liées suivant les relations suivantes : b/a est dans la plage s'étendant de 1,5 à 4, où a est dans la plage s'étendant de 2,0 à 4,0 µm, la différence entre b et a est comprise entre 1,0 et 5,0 µm, et la différence entre c et b est comprise entre 1,0 et 12 µm, et la différence entre d et c est supérieure à 0,0 µm et inférieure à 20 µm, et

lorsque l'indice de réfraction de ladite gaine (5) est une référence de zéro, &Dgr;1 est la différence d'indice de réfraction entre ladite gaine (5) et ladite section de coeur centrale (1), &Dgr;2 est la différence d'indice de réfraction entre ladite gaine (5) et ladite première section annulaire (2), &Dgr;3 est la différence d'indice de réfraction entre ladite gaine (5) et ladite deuxième section annulaire(3), et &Dgr;4 est la différence d'indice de réfraction entre ladite gaine (5) et ladite troisième section annulaire (4), alors &Dgr;1 s'étend de +0,5 à +0,8 %, &Dgr;2 s'étend de -0,1 à +0,1 %, &Dgr;3 s'étend de +0,05 à +0,5 % et &Dgr;4 s'étend de -0,2 à moins de 0,0 %.






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