JPH1138256A - Dispersion shift optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dispersion shift optical fiber that an achieve further expansion of the effective sectional area of a core and further reduction of dispersion slope. SOLUTION: The range up to a diameter 2a around an optical axis is a cavity part and the range from the inside diameter 2a to the outside diameter 2b of its circumference is a ring core part. Further, the range from the inside diameter 2b to the outside diameter 2b of its circumference is a clad part. Air or nitrogen or other gas is packed in the cavity part. The cavity part is otherwise held in a vacuum state. The refractive index n1 thereof is 1. The refractive index n2 of the ring core part is larger than the refractive index n4 of the clad part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dispersion-shifted optical fiber used in an optical transmission system.

[0002]

2. Description of the Related Art Wavelength dispersion is one of parameters indicating characteristics of an optical fiber used in an optical transmission system.
The chromatic dispersion of a single-mode optical fiber is the difference between the material dispersion (dispersion due to the wavelength dependence of the refractive index inherent to the material of the optical fiber) and the structural dispersion (dispersion due to the wavelength dependence of the group velocity of the propagation mode). Expressed as a sum. When the pulse signal light propagates through the optical fiber having the chromatic dispersion, the pulse width is widened and the signal light deteriorates, which causes a reception error.

[0003] This is because, although the wavelength of light output from a laser light source is singular, strictly, it has a certain spectral width, and the pulse signal light having this spectral width has a wavelength. Propagation through an optical fiber having dispersion is due to the fact that the propagation speed depends on the wavelength. Therefore, the optical fiber is required to have zero chromatic dispersion in the signal light wavelength band.

By the way, in the case of quartz (SiO ₂ ) which is a main material of a generally used optical fiber, the material dispersion becomes zero around a wavelength of 1.26 to 1.29 μm, and
The structural dispersion changes depending on the structure of the optical fiber. Therefore, when the structure of the optical fiber is optimally designed, the wavelength is 1.3.
In the vicinity of 1 to 1.32 μm, the material dispersion and the structural dispersion cancel each other, and the chromatic dispersion can be reduced to zero.

On the other hand, since the transmission loss of a silica-based optical fiber is the smallest in the wavelength band of 1.55 μm, it is desired to perform optical communication in this wavelength band. Therefore, the wavelength 1.55
A dispersion-shifted optical fiber having zero chromatic dispersion in the μm band has been developed and used. This dispersion-shifted optical fiber realizes zero dispersion in the wavelength band of 1.55 μm by changing the value of the structural dispersion by optimally designing the refractive index profile.

It is desired that such a dispersion-shifted optical fiber further has a large effective core area and a small dispersion slope (wavelength dependence of chromatic dispersion). That is, nonlinear phenomena that cause signal light deterioration (self-phase modulation, four-wave mixing, cross-phase modulation, nonlinear scattering, etc.)
Is larger as the light energy density per unit volume is larger. Therefore, in order to suppress or reduce the occurrence of this non-linear phenomenon, it is preferable that the effective core area is larger. Also,
In order to widen the signal light wavelength band, the smaller the chromatic dispersion over a wide wavelength range, the better, that is, the smaller the dispersion slope, the better.

Therefore, a dispersion-shifted optical fiber having a large effective core area and a small dispersion slope has been developed. For example, VABhagavatula, et al., "Dispersion
-shifted single-mode fiber for high-bit-rate and m
ultiwavelength systems ", OFC'95, ThH1 (Reference 1)
And P. Nouchi, et al., "New dispersion shifted fibe
r with effective area larger than 90μm ² ", ECOC'9
6, MoB.3.2 (Reference 2) discloses a structure in which a center depressed part having a low refractive index is provided inside a ring core part, and a dispersion-shifted optical fiber aiming at achieving both a large effective core area and a small dispersion slope. Is disclosed.

The dispersion-shifted optical fiber disclosed in Document 1 has an effective core area of 50 μm ² or more and a dispersion slope of 0.05 ps / nm ² / km or less by not adding a Ge (germanium) element to the center depressed portion. Have achieved. In addition, the dispersion-shifted optical fiber disclosed in Reference 2 adds F (fluorine) element to the center depressed portion to reduce the relative refractive index difference between the center depressed portion and the clad portion to -1%. , Effective core area 100 μm ² and dispersion slope 0.065 ps / n
m ² / km is achieved.

[0009]

However, each of the above conventional examples has the following problems. That is, in the dispersion-shifted optical fiber disclosed in Document 1, although the effective core area is increased by increasing the diameter of the central depressed portion, the dispersion slope is increased, and the effective core area is increased and the dispersion slope is increased. Reduction is in a trade-off relationship. Therefore, no matter how the structure is adjusted,
It is difficult to achieve both a further increase in the effective core area and a further reduction in the dispersion slope.

Further, in the dispersion-shifted optical fiber disclosed in Reference 2, if the refractive index of the central depressed portion can be further reduced by increasing the addition amount of F element to the central depressed portion, the effective core Further enlargement of the cross-sectional area and further reduction of the dispersion slope can be achieved. However, there is a limit to the amount of addition of the F element, and the current manufacturing technology has a limit of reducing the relative refractive index difference of the central depressed portion to about -1%. Therefore, it is difficult to further increase the effective core area and reduce the dispersion slope.

However, since the optical transmission system is required to have a longer distance, the distance between repeaters in an optical fiber network is increased while the intensity of signal light amplified and output from each repeater is increased. It is necessary to increase the effective core area in order to make the signal light reach the next-stage repeater with sufficient intensity and to suppress the occurrence of the nonlinear phenomenon. In addition, since optical transmission systems are required to have higher capacities, wavelength division multiplexing (WDM) is performed by multiplexing signal lights of a plurality of wavelengths.
In tiplex transmission, it is necessary to further reduce the dispersion slope in order to further expand the signal light wavelength band.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a dispersion-shifted optical fiber capable of further increasing the effective core area and further reducing the dispersion slope. And

[0013]

In the dispersion-shifted optical fiber according to the present invention, a cladding having a refractive index smaller than that of the ring core is provided outside the ring core in a predetermined region along the optical axis direction. A cavity is provided in a region including the optical axis inside the ring core. According to this dispersion-shifted optical fiber, the refractive index of the ring core is the largest, and the refractive index of the cavity provided inside the ring core is 1, so that the relative refractive index difference between the cladding and the cavity is 1. The absolute value of is small. Therefore, the effective core area is further increased, and the dispersion slope is further reduced.

Further, the ratio of the outer diameter of the hollow portion to the outer diameter of the ring core portion is 0.2 or more. In this case, it is particularly suitable for increasing the effective core area and reducing the dispersion slope, and can be suitably used as an optical fiber for a WDM transmission system.

Further, a depressed portion having a refractive index smaller than that of the clad portion is further provided between the ring core portion and the clad portion. In this case, the provision of the depressed portion further reduces the dispersion slope. In addition, the effective core area is 80 μm ² or more, and the dispersion slope in the wavelength band of 1.55 μm is 0.02 ps / nm ² / km or less. In this case, it is possible to further increase the capacity in the optical transmission system based on the WDM transmission system.

Further, in both or one of the end regions, instead of the hollow portion and the ring core portion,
A core portion is provided up to the optical axis. In this case, the end face of the dispersion-shifted optical fiber and the end face of another ordinary optical fiber can be connected with low loss, and moisture and foreign substances can enter the hollow portion provided in the central region. Can be prevented.

[0017]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

(First Embodiment) First, a first embodiment will be described. FIG. 1 is a refractive index profile diagram of the dispersion shifted optical fiber according to the present embodiment. This refractive index profile diagram schematically shows the distribution of the refractive index in the direction orthogonal to the optical axis of the dispersion-shifted optical fiber according to the present embodiment.

As shown in this figure, in this dispersion-shifted optical fiber, the area from the optical axis to the diameter 2a is the cavity, and the area from the inner diameter 2a to the outer diameter 2b is the ring core. In addition, a range from the inner diameter 2b to the outer diameter 2d around the area is the clad part. The cavity is filled with air, nitrogen or other gas, or is in a vacuum state, and has a refractive index n ₁ of 1. The refractive index n ₂ of the ring core is larger than the refractive index n ₄ of the clad.

With such a structure, the refractive index n ₂ of the ring core portion is the largest, and the refractive index n ₁ of the cavity portion provided inside the ring core portion is 1, so that the refractive index n ₁ with respect to the cladding portion is small. The absolute value of the relative refractive index difference of the cavity is extremely large. Therefore, as compared with the conventional product, the effective core area is further increased, and the dispersion slope is further reduced, thereby achieving further longer distance and larger capacity in the WDM optical transmission system. be able to.

For example, in the refractive index profile shown in FIG. 1, 2a = 1.71 μm and 2b = 7.44 μm.
m, 2d = 125 μm, and the relative refractive index difference Δn of the cavity (refractive index n ₁ ) with respect to the cladding (refractive index n ₄ ).
_1, and the relative refractive index difference of the ring core portion (refractive index n ₂₎ Δn ₂ respectively with respect to the cladding ^{_{portion, Δn 1 = (n 1 2}} -n 4 2) / (2n 4 2) Δn 2 = (n 2 2 −n ₄ ² ) / (2n ₄ ² ), and each value is Δn ₁ = −25.9%,
Δn ₂ = 0.7%.

Such a dispersion-shifted optical fiber has a zero dispersion wavelength of 1.551 μm, a chromatic dispersion value of -0.05 ps / nm / km at a wavelength of 1.55 μm, and an effective core area of 86.8 μm. ² , the dispersion slope is 0.048 ps / nm ² / km, the cutoff wavelength is 1.49 μm, and the bending loss at a diameter of 20 mm is 2.84 dB / m.

The above-mentioned difference in relative refractive index can be realized by making the clad portion non-added quartz and adding 10.5 wt% of GeO ₂ (germanium oxide) to the ring core portion. Further, the optical fiber having such a cavity can be manufactured by providing a cavity in a region along the optical axis in a preform stage and drawing the cavity.

FIG. 2 shows the dispersion-shifted optical fiber according to the present embodiment, where the horizontal axis represents the relative refractive index difference Δn ₂ of the ring core with respect to the cladding, and the ratio of the diameter 2a of the cavity to the outer diameter 2b of the ring core ( With 2a / 2b) as the vertical axis, the range where the effective core area Aeff is 80 μm ² or more, and the dispersion slope Slope is 0.05 ps / nm.
It is the figure which showed each range below ² / km. The area where the effective core cross section is 80 μm ² or more is a range above the line connecting the black squares in the figure, and the dispersion slope is 0.
The range below 05 ps / nm ² / km is the range above the line connecting the black triangles in the figure. The effective core area is 80 μm ² or more and the dispersion slope is 0.05 ps / nm ^2.
It is the overlapping part of both ranges that satisfies both / km or less. Therefore, in order to satisfy both conditions, the ratio of the diameter 2a of the hollow portion to the outer diameter 2b of the ring core portion (2
a / 2b) needs to be 0.2 or more.

(Second Embodiment) Next, a second embodiment will be described. FIG. 3 is a refractive index profile diagram of the dispersion shifted optical fiber according to the present embodiment. The dispersion shifted optical fiber according to the present embodiment is different from the first embodiment in that a depressed portion is further provided between the ring core and the clad.

As shown in this figure, in this dispersion-shifted optical fiber, the range from the optical axis to the diameter 2a is the cavity, and the range from the inner diameter 2a to the outer diameter 2b is the ring core. And the outer diameter 2b
The range up to c is the depressed portion, and the range from the inner diameter 2c to the outer diameter 2d around it is the clad portion. The cavity is filled with air, nitrogen or other gas, or is in a vacuum state, and has a refractive index n ₁ of 1. The refractive index n ₂ of the ring core portion is larger than the refractive index n ₄ of the cladding portion, and the refractive index n _{3 of the} depressed portion.
Is smaller than the refractive index n ₄ of the cladding.

With such a structure, the refractive index n ₂ of the ring core portion is the largest, and the refractive index n ₁ of the hollow portion provided inside the ring core portion is 1, so that the refractive index n ₁ is smaller than that of the clad portion. The absolute value of the relative refractive index difference of the cavity is extremely large. Furthermore, in the present embodiment, a depressed portion is also provided. Therefore, compared with the dispersion-shifted optical fiber according to the first embodiment, the dispersion slope is further reduced, and the capacity can be further increased in the WDM optical transmission system.

For example, in the refractive index profile shown in FIG. 3, 2a = 2.6 μm, 2b = 9.1 μm,
c = 13.0 μm, 2d = 125 μm, the relative refractive index difference Δn ₁ of the cavity (refractive index n ₁ ) with respect to the cladding (refractive index n ₄ ), and the relative refractive index difference Δn ₁ of the ring core (refractive index n ₂ ) with respect to the cladding. The values of the relative refractive index difference Δn ₂ and the relative refractive index difference Δn _{3 of the} depressed portion (refractive index n ₃ ) with respect to the cladding portion are represented by Δn ₁ = −25.9%, Δn ₂
= 0.7% and Δn ₃ = −0.4%. The relative refractive index difference Δn ₃ is defined by Δn ₃ = (n ₃ ² −n ₄ ² ) / (2n ₄ ² ).

Such a dispersion-shifted optical fiber has a zero dispersion wavelength of 1.551 μm, a chromatic dispersion value of -0.8 ps / nm / km at a wavelength of 1.55 μm,
The effective core area is 85.6 μm ² , the dispersion slope is 0.005 ps / nm ² / km, the cutoff wavelength is 1.53 μm, and the bending loss at a diameter of 20 mm is 1.82 dB / m. .

The relative refractive index difference as described above is such that the cladding is made of undoped quartz and the ring core is made of GeO ₂ .
It can be realized by adding 0.5 wt% and adding F (fluorine) element to the depressed portion at 1.48 wt%.

As described above, by providing the cavity inside the ring core and providing the depressed portion outside the ring core, the effective core area is 80 μm ² or more, and the dispersion slope is 0.02 ps /. nm ² /
km or less.

(Third Embodiment) Next, a third embodiment will be described. FIG. 4 is a cross-sectional view of the dispersion-shifted optical fiber according to the present embodiment. FIG. 4A is a cross-sectional view taken along a plane including the optical axis, and FIG. 4B is a sectional view taken along a bb plane perpendicular to the optical axis in FIG. FIG. 4C is a cross-sectional view taken along a cc plane perpendicular to the optical axis in FIG. 4A.

In the central region of the dispersion-shifted optical fiber, as shown in FIGS. 4A and 4B, the same refractive index profile as that of the dispersion-shifted optical fiber according to the first embodiment (FIG. 1). Has a rate profile. That is, the cavity 1 is provided inside the ring core 2, and the cladding 4 is provided outside the ring core 2. On the other hand, in the end region, as shown in FIGS. 4A and 4C, the hollow portion does not exist, the core portion 2A is provided in a region including the optical axis, and the cladding portion 4A is provided outside the core portion 2A. Is provided, and has a refractive index profile similar to that of a normal optical fiber.

This dispersion-shifted optical fiber can be realized by crushing the cavity in the end region of the dispersion-shifted optical fiber according to the first embodiment. When the end region of the dispersion-shifted optical fiber of each parameter value described in the first embodiment is crushed to eliminate the cavity, the ring core portion 2 having the outer diameter of 7.44 μm becomes the core having the outer diameter of 7.24 μm. Part 2A.

FIG. 5 shows a central region having a cavity of the dispersion-shifted optical fiber according to the present embodiment, an end region having no cavity of the dispersion-shifted optical fiber according to the present embodiment, and
FIG. 4 is a diagram showing an optical power distribution normalized with a peak value of 1 for each dispersion-shifted optical fiber (DSF) having a mode field diameter (MFD) of 7.8 μm. This figure also shows the refractive index profile in the central region having the cavity of the dispersion-shifted optical fiber according to the present embodiment.

As shown in this figure, the optical power distribution in the central region having the cavity of the dispersion-shifted optical fiber according to the present embodiment has a peak at the ring core and becomes minimum at the optical axis position. On the other hand, the optical power distribution in the end region of the dispersion-shifted optical fiber according to the present embodiment where there is no cavity, and the optical power distribution in the dispersion-shifted optical fiber having an MFD of 7.8 μm are both located at the position of the optical axis. Has a peak. Therefore, the connection loss between the dispersion-shifted optical fiber according to the first embodiment and the dispersion-shifted optical fiber having an MFD of 7.8 μm is 0.99 dB.
On the other hand, the connection loss between the dispersion-shifted optical fiber according to the present embodiment and the dispersion-shifted optical fiber having an MFD of 7.8 μm is as small as 0.11 dB. That is, the dispersion-shifted optical fiber according to the present embodiment is suitable because it can be connected to other ordinary optical fibers with low loss.

Further, by crushing the cavities in both end regions in this manner, there is also an effect that moisture and foreign substances can be prevented from entering the cavity 1 provided in the central region.

The present invention is not limited to the above embodiment, but can be variously modified. That is, the present invention is characterized in that a hollow portion is provided inside the ring core portion, and various modifications are possible within the scope of this purpose. For example, in the above embodiment, the refractive index changes stepwise, but the refractive index may change gradually. In addition, various aspects of the refractive index profile, in particular, with respect to a portion outside the ring core portion, are possible.

[0039]

As described above in detail, according to the present invention, in a predetermined region along the optical axis direction, a clad portion having a refractive index smaller than that of the ring core portion is provided outside the ring core portion. Since the cavity is provided in the region including the optical axis inside the ring core, the refractive index of the ring core is the largest, and the refractive index of the cavity provided inside the ring core is 1, The absolute value of the relative refractive index difference between the cavity and the cladding is small. Therefore, as compared with the conventional product, the effective core area is further increased, and the dispersion slope is further reduced.
In the optical transmission system based on the WDM system, it is possible to achieve a longer distance and a larger capacity.

When the ratio of the outer diameter of the hollow portion to the outer diameter of the ring core portion is 0.2 or more, it is particularly suitable for enlarging the effective core area and reducing the dispersion slope.
It can be suitably used as an optical fiber for a WDM transmission system.

When a depressed portion having a refractive index smaller than that of the clad portion is further provided between the ring core portion and the clad portion, the dispersion slope is further reduced. In addition, the effective core area is 80 μm ²
When the dispersion slope in the 1.55 μm band is 0.02 ps / nm ² / km or less, W
It is possible to further increase the capacity in an optical transmission system based on the DM transmission system.

In the case where a core portion is provided up to the optical axis in place of the hollow portion and the ring core portion in both or any one of the end regions, the end face of the dispersion-shifted optical fiber and another normal portion are provided. The optical fiber can be connected to the end face of the optical fiber with low loss, and it is possible to prevent moisture and foreign matter from entering the hollow portion provided in the central region.

[Brief description of the drawings]

FIG. 1 is a refractive index profile diagram of a dispersion-shifted optical fiber according to a first embodiment.

FIG. 2 is a graph showing the relative refractive index difference Δn ₂ of the ring core portion with respect to the cladding portion on the horizontal axis of the dispersion-shifted optical fiber according to the first embodiment, and the ratio (2a) of the diameter 2a of the hollow portion to the outer diameter 2b of the ring core portion; / 2b) with the vertical axis representing the range where the effective core area is 80 μm ² or more and the range where the dispersion slope is 0.05 ps / nm ² / km or less.

FIG. 3 is a refractive index profile diagram of a dispersion-shifted optical fiber according to a second embodiment.

FIG. 4 is a sectional view of a dispersion-shifted optical fiber according to a third embodiment.

FIG. 5 is a diagram illustrating a central region having a cavity of a dispersion-shifted optical fiber according to a third embodiment, an end region without a cavity of the dispersion-shifted optical fiber according to the third embodiment, and a mode field diameter (MFD). FIG. 7 is a diagram showing an optical power distribution normalized by setting a peak value to 1 for each dispersion-shifted optical fiber having 7.8 μm.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... hollow part, 2 ... ring core part, 2A ... core part, 4, 4
A: Cladding part. Attorney Yoshiki Hasegawa

Claims (5)

[Claims] In a predetermined region along an optical axis direction, a cladding portion having a refractive index smaller than a refractive index of the ring core portion is provided outside a ring core portion, and a cladding portion is provided in a region including an optical axis inside the ring core portion. A dispersion-shifted optical fiber having a cavity. 2. The dispersion-shifted optical fiber according to claim 1, wherein a ratio of an outer diameter of the hollow portion to an outer diameter of the ring core portion is 0.2 or more. 3. The dispersion according to claim 1, further comprising a depressed portion having a refractive index smaller than a refractive index of the clad portion between the ring core portion and the clad portion. Shift optical fiber. 4. An effective core area of 80 μm ² or more, and a dispersion slope in a wavelength band of 1.55 μm of 0.05 μm ² or more.
The dispersion-shifted optical fiber according to claim 3, wherein the dispersion-shifted optical fiber is equal to or less than ² ps / nm2 / km. 5. The dispersion shift according to claim 1, wherein in both or one of the end regions, a core portion is provided up to an optical axis instead of the hollow portion and the ring core portion. Optical fiber.