JP2976959B2 - Dispersion shift fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single mode optical fiber used as a transmission line in optical communication and the like, and more particularly to a wavelength division multiplexing (WDM).
ltiplexing) and a dispersion-shifted fiber suitable for transmission.

[0002]

2. Description of the Related Art Conventionally, in an optical communication system to which a single mode optical fiber is applied as a transmission line, light in a 1.3 μm wavelength band or 1.55 μm wavelength band has often been used as communication signal light. . However, in recent years, the use of light in a 1.55 μm wavelength band has been increasing from the viewpoint of reducing transmission loss in a transmission path. 1.55 μm
In a single-mode optical fiber (hereinafter, referred to as a single-mode optical fiber for 1.55 μm) applied to a transmission path of light in a wavelength band, the wavelength dispersion of light in the 1.55 μm wavelength band (the propagation speed of light depends on wavelength) It is designed so that the phenomenon that the pulse wave spreads due to the difference is zero (dispersion shift fiber having a zero dispersion wavelength of 1.55 μm).
As such a dispersion-shifted fiber, for example,
Japanese Patent Publication No. -18161 proposes a dispersion-shifted fiber having a dual-shape core structure with a refractive index profile in which a core region includes an inner core and an outer core having a refractive index lower than that of the inner core. Have been. Further, Japanese Patent Application Laid-Open No. 63-43107 and Japanese Patent Application Laid-Open
No. 141704 discloses a dispersion-shifted fiber having a refractive index profile of a depressed clad dual-shape core structure in which a cladding region is composed of an inner cladding and an outer cladding having a larger refractive index than the inner cladding. , "VABhagavatula et a
l., OFC'95 Technical Digest, Paper ThH1, 1995, and P. Nouchi et al., ECOC'96, Paper MoB.3.2, 1996, describe dispersion-shifted fibers with a refractive index profile with a ring-shaped core structure. Proposed.

In recent years, with the advent of wavelength division multiplexing (WDM) transmission and optical amplifiers, long-distance transmission has become possible. In order to avoid nonlinear optical effects, the above-mentioned dual-shape core structure and depressed clad / dual-shape core have been proposed. A dispersion-shifted fiber in which the refractive index profile of the structure is adopted and the zero-dispersion wavelength is shifted to a shorter wavelength or a longer wavelength than the center wavelength of the signal light has also been proposed (Japanese Patent Application Laid-Open No. 168046/1995, US Pat. Patent number 54
No. 83612). Note that the nonlinear optical effect refers to light caused by nonlinear phenomena such as four-wave mixing (FWM), self-phase modulation (SPM), and cross-phase modulation (XPM). This is a phenomenon in which the signal light pulse is distorted in proportion to the intensity density or the like, and becomes a limiting factor for the transmission speed and the relay interval in the relay transmission system.

On the other hand, Japanese Patent Application Laid-Open No. Hei 8-248251 discloses that the above-mentioned nonlinear optical phenomena generated when light of high power enters an optical fiber is suppressed, and an optical signal caused by these nonlinear optical phenomena is suppressed. There has been proposed an optical fiber having a structure for reducing distortion of the optical fiber. Such optical fibers have a refractive index profile designed to have an effective core area A _eff greater than about 70 μm ² .

The effective core area A _eff is _disclosed in
As described in JP-A-248251, the following (1)
Given by the formula.

[0006]

(Equation 1) Here, E is the electric field associated with the propagating light, and r is the radial distance from the center of the core.

[0007] The dispersion slope is defined by the slope of a graph showing dispersion characteristics in a predetermined wavelength band.

[0008]

Generally, as the effective core area A _eff increases, the dispersion slope also increases.
No consideration has been given to optimizing the dispersion slope value related to the occurrence of distortion of the signal light waveform due to dispersion and the occurrence of distortion of the signal light waveform due to the nonlinear optical effect from the viewpoint of reducing distortion in the entire waveform.

Therefore, in view of the progress of wavelength multiplexing in the future along with the advancement of communication, it is expected that it will be difficult to maintain transmission quality by simply applying the conventional dispersion-shifted fiber.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and has a structure for effectively suppressing the occurrence of nonlinear optical phenomena, and is suitable for WDM transmission suitable for long-distance submarine cables and the like. It is intended to provide a shift fiber.

[0011]

SUMMARY OF THE INVENTION A dispersion-shifted fiber according to the present invention comprises a core region extending along a predetermined reference axis and a cladding region provided on the outer periphery of the core region. Band signal light (the center wavelength of which is 1.
This is a single mode optical fiber for transmitting signal light within a wavelength band of 1500 μm to 1600 nm, which is a 55 μm wavelength band. The dispersion-shifted fiber has its zero-dispersion wavelength shifted from the center wavelength (1550 nm) of the 1.55 μm wavelength band to the short wavelength side or the long wavelength side.

In particular, the dispersion-shifted fiber according to the present invention has at least an absolute value of at least 1.0 to 1.55 μm in the center wavelength (1550 nm) of the 1.55 μm wavelength band.
A dispersion of 4.5 ps / nm / km, and 0.05 to 0.
It has a dispersion slope of 09 ps / nm ² / km, an effective core area of 70 μm ² or more, and a cutoff wavelength of 1300 nm or more at a fiber length of 2 m.

In general, when wavelength division multiplexing transmission is performed, if the dispersion slope is small, four-wave mixing is likely to occur, so that the waveform of the signal light is greatly distorted. Conversely, even when the dispersion slope is large, the waveform of the signal light is greatly distorted due to the synergistic effect of accumulation of dispersion and self-phase modulation.

According to the findings obtained by the inventors as a result of the research, at a wavelength of 1550 nm, the absolute value of the dispersion is 1.
When the effective core area is 70 μm ² or more, the dispersion slope is 0.05 to 0.5 ps / nm / km.
It has been found that when the transmission speed is 09 ps / nm ² / km, the total distortion of the signal light waveform can be reduced in long-distance transmission. The total amount of distortion of the signal light waveform is the sum of the distortion of the signal light waveform due to four-wave mixing and the distortion of the signal light waveform due to the synergistic effect of accumulation of dispersion and self-phase modulation. That is, according to the dispersion-shifted fiber according to the present invention having the above-described characteristics, generation of distortion due to the nonlinear optical effect is suppressed, and high-quality signal transmission can be realized.

Further, in the dispersion-shifted fiber according to the present invention, the core region is provided on an inner core having a first refractive index and an outer periphery of the inner core, and the second region is higher than the first refractive index. An outer core having a refractive index is provided, and a cladding region having a lower refractive index than the second refractive index is provided around the outer core. This means that the dispersion-shifted fiber can be suitably realized by a single-mode optical fiber having a refractive index profile of a ring-shaped core structure.

It is known that a dispersion-shifted fiber having a dual-shape core structure or a depressed clad / dual-shape core structure has a large effective core area A _eff. The diameter (MFD) also increases. This is because, for example, as disclosed in JP-A-8-220362, the effective core area A _eff and M
A _eff = k · π · (MFD / 2) ² (2) between the FD and the FD. The effective core area A
_eff is given by the above equation (1).

In a dispersion-shifted fiber having a refractive index profile of a ring-shaped core structure, the core diameter (the outer diameter of the outer core) is changed while keeping the ratio of the outer diameter of the inner core to the outer diameter of the outer core constant. At times, the inventors have obtained the following findings. That is, in the range where the core diameter is small, the effective core area A _eff decreases as the core diameter increases.
On the other hand, in a range where the core diameter is somewhat large, the effective core area A _eff increases as the core diameter increases. This means that in a dispersion-shifted fiber having a refractive index profile of a ring-shaped core structure, there are two core diameters having the same effective core area A _eff .

Similarly, in a dispersion-shifted fiber having a refractive index profile of a ring-shaped core structure, the core diameter (outer diameter of the outer core) is maintained while keeping the ratio of the outer diameter of the inner core to the outer diameter of the outer core constant. When changed, the dispersion slope also changes. That is, in the range where the core diameter is small, the effective core area A _eff decreases as the core diameter increases, and the dispersion slope also decreases. On the other hand, in a range where the core diameter is somewhat large, the effective core area A _eff increases with an increase in the core diameter, but the dispersion slope decreases. This means that, in a dispersion-shifted fiber having a refractive index profile having a ring-shaped core structure, there is a core diameter region where the dispersion slope decreases as the effective core area A _eff increases.

From the above, it is possible to obtain a dispersion-shifted fiber having both an effective core area A _eff controlled to be larger and a dispersion slope controlled to be smaller.

As a result, in setting the effective core area A _eff to a predetermined value, it is possible to appropriately select a desired dispersion slope value from two different dispersion slope values. The fiber can be easily realized.

Furthermore, in the dispersion-shifted fiber having a refractive index profile of a ring-shaped core structure, in the range core diameter is small, both effective core area A _{ef f} and MFD decreases with increasing core diameter. On the other hand, in the range where the core diameter is somewhat large, the MFD increases with the increase of the core diameter.
Decreases while the effective core area A _eff increases. The core diameter (the outer diameter of the outer core) is changed while the ratio of the outer diameter of the inner core to the outer diameter of the outer core is constant. In general, the smaller the MFD and the longer the cut-off wavelength, the smaller the bending loss. Therefore, from this viewpoint, it is advantageous to increase the core diameter.

According to the findings of the present inventors, in order to realize a dispersion-shifted fiber having the above-mentioned characteristics, the outer diameter of the inner core is set to 2a and the outer diameter of the outer core is set to 2b.
It is necessary to satisfy the following relationship: 0.4 ≦ Ra (= 2a / 2b) ≦ 0.85 μm ≦ 2b ≦ 14 μm

In the dispersion-shifted fiber, the relative refractive index difference between the outer core and the cladding region is Δn _1, and the relative refractive index difference between the inner core and the cladding region is Δn.
_{When 2} , the relationship of Δn ₁ −Δn ₂ ≧ 1% is satisfied. That is, the dispersion value of the dispersion-shifted fiber depends on the concave amount (Δn ₁ −Δn ₂ ) of the concave region corresponding to the core central region in the radial refractive index profile in the core region. This is because, in order to obtain, at least the dent amount needs to be 1.0% or more. Note that the relationship between the outer diameter 2a of the inner core and the outer diameter 2b of the outer core described above does not depend on the values of the relative refractive index differences Δn ₁ and Δn ₂ .

Further, reducing the relative refractive index difference Δn ₂ of the inner core with respect to the cladding region (increase the negative absolute value) has the effect of shortening the cutoff wavelength. Therefore, in consideration of short-distance optical transmission, Δn ₂ needs to be −0.4% or less in order to make the cutoff wavelength at 1550 nm or less at a fiber length of 2 m.

Here, the cladding region may have a single structure (hereinafter, referred to as a matched cladding structure), but is provided around the outer core and has a second refractive index (the refractive index of the outer core). ), And an outer cladding provided on the outer periphery of the inner cladding and having a fourth refractive index higher than the third refractive index. That is, the refractive index profile of the dispersion-shifted fiber is depressed cladding
It is also possible to have a refractive index profile of a ring-shaped core structure (double structure).

At this time, the dispersion-shifted fiber having the refractive index profile of the depressed clad structure reduces unnecessary second-order modes as compared with the dispersion-shifted fiber having the refractive index profile of the matched clad structure without depression. Since there is an effect, such a depressed cladding structure is effective when it is desired to shorten the cutoff wavelength of the second mode. However, in the refractive index profile of the depressed cladding structure, when the width of the recess provided (corresponding to the radial thickness (c−b) of the inner cladding) is too narrow (when 2c / 2b approaches 1) or wide. When too large (when 2c / 2b becomes too large)
Does not provide the effect of the depressed cladding structure on the matched cladding structure. Therefore, the inner cladding needs to have an appropriate outer diameter with respect to the outer core. For a dispersion-shifted fiber having a refractive index profile of a depressed clad ring-shaped core structure, the outer diameter of the outer core is 2b, and the inner cladding is When the outer diameter of is 2c, it is preferable to satisfy the following relationship: 1.2 ≦ (2c / 2b) ≦ 2.2.

In a dispersion-shifted fiber having a refractive index profile of a depressed clad ring-shaped core structure, Δn ₁ is the relative refractive index difference of the outer core with respect to the outer clad, and Δn ₂ is the inner refractive index with respect to the outer clad. This is the relative refractive index difference of the core. In this case, even if the relative refractive index differences Δn ₁ and Δn ₂ have the same value, the dispersion shift having the refractive index profile of a simple ring-shaped core structure (matched clad ring-shaped core structure) having no depressed clad structure. Its cutoff wavelength is shorter compared to fiber.

Next, in the optical power distribution (or electromagnetic field distribution) of the conventional optical fiber disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 8-248251, the core center of the optical fiber is maximum. In order to increase the effective core area A _eff while maintaining the shape of the optical power distribution having such characteristics, it is necessary to widen the skirt portion of the optical power distribution. In order to widen the skirt portion in the optical power distribution in this manner, the conventional optical fiber is provided with another segment (outer core) outside the central segment (inner core).

However, as is apparent from the above equation (2), in the dispersion-shifted fiber having the refractive index profile of the dual shape core structure or the depressed clad dual shape core structure, the effective core area A _eff is reduced. Increasing the size also increases the mode field diameter (MFD).

For the above reasons, the effective core area A _eff
JP-A-8-248251 designed to enlarge
In the optical fiber disclosed in Japanese Unexamined Patent Application Publication No.
There is a problem that the loss due to microbend or macrobend (hereinafter referred to as bending loss) increases as _eff increases.

Therefore, the dispersion-shifted fiber according to the present invention has at least a refractive index profile of a ring-shaped core structure in order to effectively suppress the occurrence of a nonlinear optical phenomenon while keeping the value of MFD small. By
In a cross section perpendicular to the signal light guiding direction, a portion where the optical power distribution of the fundamental mode of the signal light or the electromagnetic field distribution accompanying it is maximized is separated from the center of the core region by a predetermined distance in the radial direction. It is characterized by:

Incidentally, even in the case of an optical fiber having a refractive index profile of a ring-shaped core structure, if the outer diameter of the inner core is small, the propagation of the optical fiber is smaller than that of an optical fiber having a refractive index profile not having the ring-shaped core structure. There is no large difference in the light power distribution of light or the electromagnetic field distribution accompanying it. That is, even in an optical fiber having a refractive index profile of a ring-shaped core structure, if the outer diameter of the inner core is small, the portion where the optical power distribution or the electromagnetic field distribution of the fundamental mode of the signal light is the largest in the core region. Almost overlap with the center of In such a state, the characteristics of the refractive index profile of the ring-shaped core structure are not sufficiently exhibited.

Specifically, in the dispersion-shifted fiber according to the present invention, in a cross section perpendicular to the signal light guiding direction, a portion where the optical power distribution of the fundamental mode of the signal light or the electromagnetic field distribution associated therewith is maximum is: It is spaced apart from the center of the core region by about 0.5 μm to 5 μm in the radial direction.

In this case, it is particularly preferable to satisfy the condition of the following expression (3) in order to sufficiently exert the effect of the refractive index profile of the ring-shaped core structure. That is,
In the dispersion-shifted fiber according to the present invention, the optical power of the fundamental mode of the signal light at the center of the core region or the intensity of the electromagnetic field associated therewith is defined as P _0, and the fundamental mode of the signal light in the radial direction from the center of the core region. when the maximum value of optical power distribution or the electromagnetic field distribution accompanying was _{P 1, P 1> 1.2 ×} P 0 ... (3) the relationship is satisfied.

Thus, the M of the dispersion-shifted fiber is
Since it is possible to increase the effective core area A _eff while keeping the FD at a small value, it is possible to reduce the nonlinear optical phenomenon without increasing the bending loss.

The dispersion-shifted fiber according to the present invention has a center wavelength (155) in the 1.55 μm wavelength band.
This is a dispersion-shifted fiber whose zero dispersion wavelength is shifted by a predetermined amount from 0 nm). As described above, by using both the enlargement of the effective core area A _eff and the shift of the zero-dispersion wavelength, it is possible to more effectively suppress signal deterioration due to four-wave mixing.

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the dispersion-shifted fiber according to the present invention will be described below with reference to FIGS. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

(First Embodiment) FIG. 1 is a diagram showing a sectional structure and a radial refractive index profile of a dispersion-shifted fiber according to a first embodiment of the present invention. As shown in FIG. 1, the dispersion-shifted fiber 100 has 1.55
μm wavelength band quartz glass for propagating signal light (1500 to 1600 nm) with a single-mode optical fiber mainly composed, an inner core 111 having an outer diameter with 2a, the refractive index n _1, the inner core 111 is provided on the outer periphery,
The outer diameter is 2b (that is, the diameter of the core region 110 = 2
b) an outer core 112 having a refractive index n ₂ (> n ₁ );
And it is provided on the outer periphery of the outer core 112 and has a refractive index n
₃ (<n ₂ ). The core region 110 includes the inner core 111 and the outer core 112. The refractive index profile 101 indicates the refractive index at each part on the line L1 in the figure.

Then, the dispersion-shifted fiber 100
As various characteristics at a wavelength of 1550 nm, the absolute value of the dispersion is in the range of 1.0 to 4.5 ps / nm / km, the dispersion slope is in the range of 0.05 to 0.09 ps / nm ² / km, The effective core area is 70 μm ² or more, and the cutoff wavelength at a fiber length of 2 m is 130 μm.
Each is set to 0 nm or more.

As a preferred example, the refractive index n ₁ = refractive index n ₃
And (inside relative refractive index difference [Delta] n ₂ = 0 of the core 111 with respect to the cladding region _{210), Δn 1 = (n} 2 2 -n 3 2) / (2n 3 2) ... are defined in (4), the cladding region 210 Outer core 1 for
12, the relative refractive index difference Δn ₁ is 1.5%, the core diameter (the outer diameter 2b of the outer core 112) is 9 μm, and the outer diameter 2 of the inner core 111 is
a and the ratio Ra (= a / b) of the diameter 2b of the core 110 to 0.
Assuming that the wavelength is 6, at a wavelength of 1550 nm, dispersion (ps / nm / km): -2.28 dispersion slope (ps / nm ² / km): 0.082 effective core area (μm ² ): 78.2 MFD (μm) ): 6.2
Becomes The cutoff wavelength at a length of 2 m is 251.
0 nm. In this specification, the relative refractive index difference is expressed in percentage.

In this preferred example, the cutoff wavelength is 2510 nm, and for a fiber length of 2 m, the cutoff wavelength is 1.10 nm.
Although single-mode propagation cannot be guaranteed for signal light in the 55 μm band, the cutoff wavelength becomes shorter as the transmission distance increases, and in the case of long-distance transmission such as about 1000 km, the cutoff wavelength becomes smaller. There is no practical problem since the wavelength is shorter than the wavelength of (the single mode propagation can be guaranteed).

FIG. 2 is a graph showing changes in the effective core area _Aeff and the mode field diameter (MFD) when the core diameter 2b (= the outer diameter of the outer core 112) is changed. In FIG. 2, the relative refractive index difference Δn ₁ = 1.5
%, And Ra (= a / b) = 0.6.

From FIG. 2, it can be seen that in the range where the core diameter 2b is 4 μm or less, the effective core area A
Both _eff and MFD decrease. On the other hand, when the core diameter 2b is 4
In the range larger than μm, it can be confirmed that as the core diameter 2b increases, the MFD decreases but the effective core area A _eff increases. That is, it can be seen that there are two core diameters 2b having the same effective core area A _eff .

FIG. 3 shows the case where the core diameter 2b is changed.
5 is a graph showing changes in cutoff wavelength λ _C and dispersion slope at a fiber length of m. In FIG. 3,
Relative index difference Δn ₁ = 1.5%, Ra (= a / b) = 0.
It is 6.

From FIG. 3, it can be confirmed that the cutoff wavelength λ _C increases as the core diameter 2b increases. On the other hand, the dispersion slope increases as the core diameter 2b increases.
In the range of b ≦ 4 μm, it decreases, and 4 μm <core diameter 2b ≦ 7
It can be seen that it increases in the range of μm and decreases in the range of core diameter 2b> 7 μm.

Therefore, from FIGS. 2 and 3, generally,
When setting the effective core area A _eff to a predetermined value, 2
It can be seen that a desired dispersion slope value can be selected from three different dispersion slope values.

FIG. 4 is a graph showing a change in 5G with a change in dispersion slope.
FIG. 4 is a diagram for explaining a change in a bit error rate (BER) at bps. When measuring, 1
In transmission of six signal lights (center wavelength = 1557.2 nm, wavelength interval = 0.55 nm), dispersion value at center wavelength = −2 ps / nm / km, effective core area = 70 μm ²
And the dispersion slope (ps / nm ² / km) is 0.03
(See FIG. 4 (a)), 0.05 (see FIG. 4 (b)),
0.09 (see FIG. 4 (c)) and 0.12 (see FIG. 4 (d)). Also, the transmission distance of the dispersion-shifted fiber to be measured is 9000 km, and an optical fiber for dispersion compensation every 400 km (actually, the dispersion value at the center wavelength is 18p) so as to make the accumulated dispersion at the center wavelength zero.
A single-mode optical fiber for 1.3 μm band of s / nm / km is inserted. In FIG. 4, the vertical axis uses a Q value that is an amount equivalent to the bit error rate.

FIG. 5 is a diagram for explaining the Q value. As shown in the figure, in the eye pattern of the signal light waveform at the receiving end, the Q value is the average value ν ₀ of the light intensity at the logical “0” level, and the standard deviation of the light intensity at the logical “0” level. Q (dB) = 10 × log ((ν ₁ −ν ₀ ) /, where σ ₀ , the average value ν ₁ of the light intensity of the logical “1” level, and the standard deviation σ ₁ of the light intensity of the logical “1” level (Σ ₁ + σ ₀ ))... (5)

In optical transmission, if the BER is 10 ^-9 or less at the above transmission distance, sufficient performance is obtained.
An R of 10 ^-9 or less corresponds to a Q value of 15.7 dB or more.

As can be seen from FIG. 4, when the value of the dispersion slope is small, the deterioration of the Q value is large in the center channel.
When the value of the dispersion slope is large, the deterioration of the Q value is large in the long wavelength region. This is because if the value of the dispersion slope is small, four-wave mixing is likely to occur at a wavelength near the center channel, and if the value of the dispersion slope is large, the dispersion value in a long wavelength region such as channel numbers 14 to 16 is obtained. Is considered to be due to a synergistic effect of the increase of the cumulative dispersion and the self-phase modulation due to the increase of.

Therefore, the BER in the wavelength band of the signal light
It can be seen that there is an appropriate dispersion slope value in order to reduce the, that is, to improve the Q value in the wavelength band. More specifically, in FIG.
The value of the dispersion slope at which R is 10 ^-9 or less is 0.05 to
It can be seen that it is 0.09 ps / nm ² / km.

FIG. 6 is a diagram for explaining the optical power distribution of the fundamental mode of the signal light in the above preferred example. As shown in FIG. 6, in a cross section perpendicular to the signal light guiding direction, a portion where the optical power distribution (equivalent to the electromagnetic field distribution accompanying the propagation of the signal light) of the fundamental mode of the signal light is: It is set at a position radially away from the center of the core region 110 by a predetermined distance.

If the optical power of the fundamental mode of the signal light at the center of the core region 110 is P ₀ and the maximum value of the optical power distribution is P ₁ , P ₁ ≒ 5.5 × P ₀ > 1 2 × P ₀ (6), which satisfies the condition of the above expression (3).

FIG. 7 shows the relative refractive index difference Δn ₁ = 1.5% (n _{1
= N 3), Ra (=} a / b) = 0.6 for an optical fiber, comparative example of an optical fiber having a dispersion same effective core and shifted fiber cross-sectional area A _{ef f} in FIG. 6 (= 78μm ²⁾ FIG. 5 is a diagram for explaining the optical power distribution of the fundamental mode of the signal light of FIG.

As shown in FIG. 7, the optical fiber of the comparative example has an inner core 191 having an outer diameter (2a) of 1.36 μm and a refractive index n _1, and is provided on the outer periphery of the inner core 191. An outer core 192 having an outer diameter (2b) of 2.26 μm (that is, the diameter of the core region 190 = 2.26 μm) and a refractive index n ₂ (> n ₁ ), and is provided on the outer periphery of the outer core 192. , And a cladding region 290 having a refractive index n ₃ (<n ₂ ). Here, the core region 190 includes the inner core 191 and the outer core 192.

As can be seen from FIG. 7, in the optical fiber of the comparative example, the optical power distribution of the fundamental mode of the signal light becomes maximum at the center of the core region 190 in a cross section perpendicular to the signal light guiding direction. ing.

When the optical power of the fundamental mode of the signal light at the center of the core region 190 is P ₀ and the maximum value of the optical power distribution of the fundamental mode of the signal light is P ₁ , P ₁ = P ₀ <1.2 × P ₀ (7), which does not satisfy the condition of the above equation (3).

The dispersion slope in this optical fiber is:
0.126 ps / nm ² / km, and the above-mentioned preferable BER cannot be achieved with this optical fiber.

Therefore, by actively utilizing the refractive index profile such that the optical power distribution in the radial direction of the fundamental mode has a concave portion at the center, the effective core area A _eff is large and the dispersion slope is small. A small dispersion-shifted fiber can be realized.

In the above description of the first embodiment, the refractive index n ₁ of the inner core 111 and the refractive index n _{3 of the} cladding 210 are described.
Has been described with reference to the same refractive index profile. However, as shown in FIG. 8A, the refractive index profile may be such that refractive index n ₁ > refractive index n ₃ . Further, the refractive index profile may be such that refractive index n ₁ <refractive index n ₃ as shown in FIG. 8B.

[0061] For example, in the dispersion-shifted fiber of Fig. _{8 (b), Δn 1 =} (n 2 2 -n 3 2) / (2n 3 2) ... are defined by (8), of the outer core 112 with respect to the cladding region the relative refractive index difference _{Δn 1 1.1%, Δn 2 =} (n 1 2 -n 3 2) / (2n 3 2) is defined by (9), the relative refractive index difference of the inner core 111 with respect to the cladding region Δn ₂ -0.6%, core diameter 2b (outer diameter of outer core) = 6.8 μm, ratio Ra (= a / b) of outer diameter 2a of inner core 111 to diameter 2b of core 110 = 0. Assuming that at 6, at 1550 nm, the dispersion value (ps / nm / km): -2.18 Dispersion slope (ps / nm ² / km): 0.086 Effective core area (μm ² ): 81.4 MFD (μm) ): 7.6. In addition, the cutoff wavelength at 2 m length is 150
0 nm.

In this example, since the cutoff wavelength at a fiber length of 2 m is 1500 nm, which is shorter than the wavelength of the signal light in the 1.55 μm band, it can be used for short-distance transmission.

(Second Embodiment) FIG. 9 is a view showing a sectional structure and a radial refractive index profile of a dispersion-shifted fiber according to a second embodiment of the present invention. As shown in FIG. 9, the dispersion-shifted fiber 200 has 1.55
This is a single mode optical fiber for transmitting signal light in the μm wavelength band, and particularly has a refractive index profile 201 of a depressed clad ring-shaped core structure. That is, the dispersion-dispersion shift fiber 20 of the second embodiment
0 is an inner core 121 having an outer diameter 2a and a refractive index n ₁
And an outer diameter 2b (that is, the diameter of the core region 120 = 2b), a refractive index n ₂ (>
n ₁ ), an inner cladding 221 provided on the outer periphery of the outer core 122 and having an outer diameter 2c and a refractive index n ₃ (<n ₂ ), and an inner cladding 22
1 and an outer cladding 222 having a refractive index n ₄ (> n ₃ ). Here, the core region 12
Numeral 0 is constituted by the inner core 121 and the outer core 122, and the cladding region 220 is constituted by the inner clad 221 and the outer clad 222. The refractive index profile 201 is represented by a line L2 in the drawing.
The refractive index at each of the upper portions is shown.

Then, the dispersion-shifted fiber 200
Is an absolute value of dispersion within a range of 1.0 to 4.5 ps / nm / km as various characteristics at a wavelength of 1550 nm.
Dispersion slope 0.05~0.09ps / nm ² / km
The effective core area is 70 μm ² or more,
The cutoff wavelength at a fiber length of 2 m is 1
It is set to 300 nm or more.

As a preferred example, the refractive index n ₁ = refractive index n ₃
Δn ₁ = (n ₂ ² −n ₄ ² ) / (2n ₄ ² ) (10)
The relative refractive index difference [Delta] n ₁ of _{22 1.1%, Δn 2 = (} n 1 2 -n 4 2) / (2n 4 2) is defined by (11), the inner core with respect to the outer cladding 222 1
21, the relative refractive index difference Δn ₂ is −0.4%, the core diameter 2b (the outer diameter of the outer core 122) is 9 μm, and the ratio Ra (= a) between the outer diameter 2 a of the inner core 121 and the diameter 2 b of the core region 120. / B)
Is 0.6 and the outer diameter 2c of the inner cladding 221 is 18 μm. At a wavelength of 1550 nm, the dispersion value (ps / nm / km): -2.62 Dispersion slope (ps / nm ² / km): 0.076 Effective Core sectional area (μm ² ): 80.1 MFD (μm): 6.3 In addition, the cutoff wavelength at 2 m length is 170
2 nm.

In this preferred example, the cutoff wavelength is 1702 nm, and for a fiber length of 2 m, the cutoff wavelength is 1.70 nm.
Although single-mode propagation is not guaranteed for signal light in the 55 μm band, the cutoff wavelength becomes shorter as the transmission distance becomes longer. When transmitting over a distance of several hundred meters or more, the cutoff wavelength becomes shorter. Is shorter than the wavelength of the signal light, so that there is no practical problem.

In the dispersion-shifted fiber having the refractive index profile of the depressed clad ring-shaped core structure as in the second embodiment, changes in the effective core area A _eff and the MFD when the core diameter 2b is changed. Is
It shows the same tendency as the first embodiment, that is, the same tendency as the change shown in FIG. Therefore, also in the case of such a dispersion-shifted fiber, there are two core diameters 2b having the same effective core area A _eff .

In the dispersion-shifted fiber having the refractive index profile of the depressed clad ring-shaped core structure as in the second embodiment, when the core diameter 2b is changed, the cutoff wavelength λ at a length of 2 m is changed. The change in _C and the dispersion slope shows the same change as in the first embodiment described above, that is, the same tendency as the change shown in FIG. Accordingly, in the case of such a dispersion-shifted fiber, in general, a desired dispersion slope value can be selected from two different dispersion slope values when the effective core area A _eff is set to a predetermined value.

Further, in the dispersion-shifted fiber having the refractive index profile of the depressed clad ring-shaped core structure as in the second embodiment, the change in BER at 5 Gbps due to the change in dispersion slope is the first.
A change similar to the embodiment, that is, a tendency similar to the change illustrated in FIG. 4 is shown. Therefore, even in the case of such a dispersion-shifted fiber, the dispersion slope value at which the BER is 10 ⁻⁹ or less is 0.05 to 0.09 ps / nm ² / km.

FIG. 10 is a diagram for explaining the optical power distribution of the fundamental mode of signal light in a preferred example of the second embodiment. As shown in FIG. 10, as in FIG. 6 in the first embodiment, in the cross section perpendicular to the signal light guiding direction,
The portion where the optical power distribution of the fundamental mode of the signal light is maximum is set at a position separated from the center of the core region 120 by a predetermined distance in the radial direction.

When the optical power of the fundamental mode of the signal light at the center of the core region 120 is P ₀ and the maximum value of the optical power distribution is P ₁ , P ₁ ≒ 5.3 × P ₀ > 1 2 × P ₀ (12), which satisfies the condition of the above equation (3).

In the dispersion-shifted fiber having the refractive index profile of the depressed clad ring-shaped core structure as in the second embodiment, the relative refractive index difference Δn ₁
= 1.1%, relative refractive index difference Δn ₂ = −0.4%, Ra (=
a / b) = 0.6, the same effective core area A _eff (= 80.
In the optical fiber of the comparative example having 1 μm ² ), in a cross section perpendicular to the signal light guiding direction, the optical power distribution of the fundamental mode of the signal light becomes maximum near the center of the core region. The preferred BER described above cannot be achieved.

Therefore, by actively utilizing the refractive index profile in which the optical power distribution in the radial direction of the fundamental mode has a concave portion at the center, the effective core area A _eff is large and the dispersion slope is small. A small dispersion-shifted fiber can be realized.

[0074] In the above description of the second embodiment, the refractive index n ₃ of the refractive index n ₁ and the inner cladding 221 of the inner core 121 has been described for the same refractive index profile, the refractive index profile, As shown in FIG. 11A, the refractive index n ₁ > the refractive index n ₃ can be satisfied. Further, as shown in FIG. 11 (b), the refractive index profile may be such that refractive index n ₁ <refractive index n ₃ .

However, in the refractive index profile of the depressed cladding structure as shown in FIG. 11, the width of the recess provided in the refractive index profile (corresponding to the radial thickness (c-b) of the inner cladding) and The depths (corresponding to the difference in the relative refractive index of the inner cladding with respect to the outer cladding) must be set in appropriate ranges.

That is, the dispersion-shifted fiber having the refractive index profile of the depressed clad structure has an effect of reducing unnecessary second-order modes as compared with the dispersion-shifted fiber having the refractive index profile of the matched clad structure without depression. Therefore, such a depressed cladding structure is effective when it is desired to shorten the cutoff wavelength of the secondary mode. However, in the refractive index profile of the depressed cladding structure, when the width of the recess provided (corresponding to the radial thickness (c−b) of the inner cladding) is too narrow (when 2c / 2b approaches 1) or wide. When too large (when 2c / 2b becomes too large)
Does not provide the effect of the depressed cladding structure on the matched cladding structure. Therefore, the inner cladding needs to have an appropriate outer diameter with respect to the outer core. For a dispersion-shifted fiber having a refractive index profile of a depressed clad ring-shaped core structure, the outer diameter of the outer core is 2b, and the inner cladding is When the outer diameter of is 2c, it is preferable to satisfy the following relationship: 1.2 ≦ (2c / 2b) ≦ 2.2.

In addition, the dispersion-shifted fiber according to the present invention has a relative refractive index difference Δn ₁ of the outer core with respect to the cladding region (or the outer cladding) and a relative refractive index difference of the inner core with respect to the cladding region (or the outer cladding). Δ
It is preferable to satisfy the relationship of Δn ₁ −Δn ₂ ≧ 1% with n ₂ .

That is, the dispersion value of the dispersion-shifted fiber is determined by the amount of depression (Δn ₁₎ in the concave region corresponding to the core central region in the radial refractive index profile in the core region.
−Δn ₂ ), and therefore, in order to obtain a sufficient dispersion value, at least the dent amount needs to be 1.0% or more.

[0079] For example, in the dispersion-shifted fiber having a refractive index profile shown in FIG. 11 (b), it is defined by ^{_{Δn 1 = (n 2 2 -n}} 4 2) / (2n 4 2) ... (13) Outer core 1 for outer cladding 222
The relative refractive index difference [Delta] n ₁ of _{22 0.97%, Δn 2 = (} n 1 2 -n 4 2) / (2n 4 2) is defined by (14), the inner core with respect to the outer cladding 222 1
The relative refractive index difference Δn ₂ of −21 is −0.45%, Δn ₃ = (n ₃ ² −n ₄ ² ) / (2n ₄ ² ) (15) The relative refractive index difference Δn ₂ is −0.20%, and the core diameter 2
b (the outer diameter of the outer core 122) is 8 μm,
Ratio Ra (= a) between the outer diameter 2a of the core region 120 and the diameter 2b of the core region 120.
/ B) is 0.6 and the ratio (2c / 2b) of the outer diameter 2b of the core region 120 to the outer diameter 2c of the inner cladding 221 is 2.0, the dispersion value (ps / nm / km) at a wavelength of 1550 nm. ): −1.88 Dispersion slope (ps / nm ² / km): 0.077 Effective core area (μm ² ): 83.0 MFD (μm): 7.2 The cutoff wavelength at a length of 2 m is 147.
3 nm.

When the optical power of the fundamental mode of the signal light at the center of the core region 120 is P ₀ and the maximum value of the optical power distribution is P ₁ , P ₁ ≒ 3.6 × P ₀ > 1 2 × P ₀ (16), which satisfies the condition of the above equation (3).

[0081] ^{_{Further, Δn 1 = (n 2 2}} -n 4 2) / (2n 4 2) is defined by (17), the outer core with respect to the outer cladding 222 1
The relative refractive index difference [Delta] n ₁ of _{22 0.90%, Δn 2 = (} n 1 2 -n 4 2) / (2n 4 2) is defined by (18), the inner core with respect to the outer cladding 222 1
The relative refractive index difference Δn ₂ of −21 is −0.45%, Δn ₃ = (n ₃ ² −n ₄ ² ) / (2n ₄ ² ) (19) The relative refractive index difference Δn ₂ is −0.45%, and the core diameter is 2
b (outer diameter of outer core 122) is 8.3 μm, inner core 1
21 of the outer diameter 2a of the core region 120 and the diameter 2b of the core region 120.
Assuming that (= a / b) is 0.6 and the ratio (2c / 2b) of the outer diameter 2b of the core region 120 to the outer diameter 2c of the inner cladding 221 is 1.5, the dispersion value (ps / ps) at a wavelength of 1550 nm is obtained. nm / km): -2.19 Dispersion slope (ps / nm ² / km): 0.069 Effective core area (μm ² ): 77.2 MFD (μm): 7.0 The cutoff wavelength at a length of 2 m is 147.
6 nm.

When the optical power of the fundamental mode of the signal light at the center of the core region 120 is P ₀ and the maximum value of the optical power distribution is P ₁ , P ₁ ≒ 3.2 × P ₀ > 1. 2 × P ₀ (20), which satisfies the condition of the above equation (3).

Further, reducing the relative refractive index difference Δn ₂ of the inner core to the cladding region (or the outer cladding) (increase the negative absolute value) has the effect of shortening the cutoff wavelength. Therefore, in consideration of short-distance optical transmission, Δn ₂ needs to be −0.4% or less in order to make the cutoff wavelength at 1550 nm or less at a fiber length of 2 m.

Next, as the various characteristics that can be obtained for each of the above-described dispersion-shifted fibers, Δn ₁ , Δn ₂ , Ra (= 2a /
FIG. 12 shows the allowable ranges of 2b) and 2b. In the case of a dispersion-shifted fiber having a refractive index profile of a ring-shaped core structure, Δn ₁ is a relative refractive index difference of the outer core with respect to the cladding region, and Δn ₂ is a relative refractive index difference of the inner core with respect to the cladding region. In the case of a dispersion-shifted fiber having a refractive index profile of a depressed clad ring-shaped core structure, Δn ₁ is the relative refractive index difference of the outer core to the outer clad, and Δn ₂ is the relative refractive index of the inner core to the outer clad. Is the difference.

As can be seen from the table of FIG. 12A, the outer diameter 2a of the inner core and the outer diameter 2b of the outer core (= core diameter) for obtaining the above-described characteristics have the following relationship. Need to meet.

0.4 ≦ Ra (= 2a / 2b) ≦ 0.85 μm ≦ 2b ≦ 14 μm The above relationship does not depend on the relative refractive index differences Δn ₁ and Δn ₂ . If the above relationship is not satisfied, the above-described various characteristics of the dispersion-shifted fiber cannot be obtained. That is, in the graph shown in FIG. 12B, the hatched area indicates the outer diameter 2a of the inner core and the outer diameter 2b of the outer core.
(= Core diameter) is an area that satisfies the above relationship,
For example, when the values of Ra (= 2a / 2b) and 2b are in the area indicated by A in (a) of FIG. 12B, the value of the dispersion slope becomes too large, and the value of (b) in FIG. In the region indicated by B in b), the effective core area A _eff becomes too large and the bending loss increases, and (c) FIG.
In the region indicated by C in (b), the dispersion value becomes too large, and (d) in the region indicated by D in FIG. 12B, the effective core area A _eff is small. It becomes too much.

In addition, in the dispersion-shifted fiber according to the present invention having the above-described structure, the position where the optical power distribution of the fundamental mode of the signal light is maximum is separated from the center of the core region by a predetermined distance in the radial direction. Thus, the refractive index profile is adjusted. Specifically, the inventors consider that the shape of the electromagnetic field distribution (corresponding to the optical power distribution) can be approximately regarded as a Gaussian distribution, and the position where the electromagnetic field distribution becomes maximum is separated from the center of the core region by a predetermined distance. We have prototyped several optical fibers. FIG. 13 is a typical example (graph) of an electromagnetic field distribution in a plurality of prototype optical fibers. In this graph, the electric field amplitude on the vertical axis is standardized with the maximum value being 1.

The inventors further selected optical fibers having an effective core area A _eff of about 80 μm ² from the prototyped optical fibers, and determined the maximum position of the electromagnetic field distribution and the core area of the selected optical fibers. The relationship between the distance from the center and the mode field diameter (MFD) was investigated. In addition,
FIG. 14 is a graph showing the distance dependence of the MFD, and FIG. 15 is a graph showing the distance dependence of the loss due to microbending. As shown in FIG. 14, the distance between the maximum position of the electromagnetic field distribution and the center of the core region is 0.5 μm to 2.5 μm.
Up to about μm, the MFD decreases as the distance increases, and the dispersion-shifted fiber according to the present invention maintains the same effective core area A _eff as the conventional optical fiber while maintaining the MFD.
It was confirmed that can be reduced.

On the other hand, as shown in FIG. 15, the loss due to microbending decreases as the distance from the center of the core region to the position where the electromagnetic field distribution is maximized increases. Therefore, considering the loss due to microbending, it is preferable that the distance from the center of the core region to the position where the electromagnetic field distribution is maximized is larger. However, if the distance exceeds 5 μm, the cutoff wavelength becomes extremely large. Therefore, the optical transmission in the single mode cannot be ensured.

In consideration of the above, in the dispersion-shifted fiber according to the present invention, the position where the optical power distribution of the fundamental mode of the signal light is maximum is 0. 0 in the radial direction from the center of the core region.
It is designed to be separated by 5 μm to 5 μm.

Next, FIG. 16 shows 1.55 for two samples having a single ring structure refractive index profile.
The results of measuring various characteristics with respect to light of μm (1550 nm) are shown in FIG. In this table, Δn ⁻ and Δ
n ^{+ is} a relative refractive index difference between the inner core and the outer core with respect to the cladding region, respectively, and is defined as follows.

[0092] _{^{Δn - = (n 1 2 -n}} cld 2) / (2n cld 2) ... (21) Δn + = (n 2 2 -n cld 2) / (2n cld 2) ... (22) Here, n ₁ is the refractive index of the inner core, n ₂ is the refractive index of the outer core provided on the outer periphery of the inner core, n _cld is the refractive index of the cladding region provided on the outer periphery of the outer core, and each value is a percentage. Is displayed. Further, since the refractive indices in the respective formulas are in no particular order, if the relative refractive index difference takes a negative value, it means that the refractive index of the glass region is lower than the refractive index of the cladding region. In the case of the refractive index profile of the depressed cladding structure, the reference refractive index n _cld is the refractive index of the outer cladding of the cladding region.

In addition, the non-linear refractive index N2 in the table is defined as follows. That is, the refractive index <N> of the medium under strong light changes depending on the light power.
Therefore, the lowest order effect on the refractive index <N> is: <N> = <N ₀ > + <N ₂ > · E ² (23) where <N ₀ >: refractive index <N for linear polarization ₂ >: Non-linear refractive index for third-order non-linear polarization E: Expressed by photoelectric field amplitude Under intense light, the refractive index <N> of the medium is given by the sum of the normal value <N ₀ > and an increase proportional to the square of the electric field amplitude E. In particular, the proportional constant of the second term <N ₂ >
(Unit: m ² / V ² ) is called a nonlinear refractive index.

FIG. 17 is a graph showing the relationship between the refractive index profile of Sample 1 shown in the table of FIG. 16 and the optical power along the radial direction from the center. Note that the numerical value of the optical power is standardized with the maximum value being 1, and the optical power is based on the fundamental mode (LP01) and the secondary mode (LP01) of the signal light.
11) is shown. Also, in both samples 1 and 2,
A single ring structure is provided. In the vicinity of the glass part (outer core) having a maximum refractive index outside the center of the core (inner core), germanium oxide is added to pure quartz glass to increase the refractive index. ing. On the other hand, near the core center (inner core), fluorine is added to pure quartz glass, and the refractive index is lowered.

In the optical fiber of sample 1, 1.55 μm
m (center wavelength of 1.55 μm wavelength band: 1550 nm), the effective core area A _eff is 87 μm ² , M
FD is 11.2 μm, equivalent effective core area A
The MFD of the conventional optical fiber having _eff can be reduced by about 20% as compared with about 13 μm.

Further, to evaluate the loss due to microbending, the inventors wound the dispersion-shifted fiber of Sample 1 around a 280 mm diameter reel with sandpaper attached to the surface, and measured the loss fluctuation before and after winding. . As a result, while the increase in loss of Sample 1 with respect to light having a wavelength of 1.55 μm was 0.19 dB / km, the conventional optical fiber had a loss increase of 1.12 dB / km, and the dispersion of Sample 1 was increased. It has been confirmed that the shift fiber greatly reduces the increase in loss due to microbending.

Next, in the dispersion-shifted fiber of the sample 2, the maximum position of the optical power for the light having the wavelength of 1.55 μm is about 3 μm away from the center of the core. 1.55μ wavelength
m 2 effective core area A _eff of this sample 2 for light of m
Is 86 μm ² and the MFD is 11.0 μm. In both samples 1 and 2, the cutoff wavelength λ _C at a fiber length of 2 m is 1.7 μm, which is longer than the operating wavelength (1.55 μm wavelength band), but the wavelength is 1.55 μm at a fiber length of 100 m or more. Since a single mode is ensured for the light, the single mode is effectively performed in a normal use state.

[0098] With respect to the dispersion-shifted fiber of Sample 2, similarly, the increase in loss due to microbending was evaluated. As a result, it was confirmed that the increase in loss was as small as 0.11 dB / km. The chromatic dispersion value of Sample 2 for light having a wavelength of 1.55 μm is -2.6 ps / mm /
km and the dispersion value at the used wavelength is not zero (the zero dispersion wavelength λ ₀ is shifted to a longer wavelength side than the used wavelength 1.55 μm). For this reason, the generation efficiency of four-wave mixing that leads to signal degradation in wavelength division multiplexing transmission is reduced.

The above samples 1 and 2 correspond to FIG.
As shown in (a), the refractive index of the inner core had a refractive index profile of a single ring structure lower than the refractive index of the cladding region. However, the present invention is not limited to this refractive index profile. . That is, as the refractive index profile applicable to the dispersion-shifted fiber according to the present invention,
For example, as shown in FIG. 18 (b), the shape may be such that the refractive index of the inner core and the refractive index of the cladding region match, or as shown in FIG. A ring structure or a double ring structure as shown in FIG.

[0100]

As described above, according to the dispersion-shifted fiber according to the present invention, the absolute value of the dispersion is 1.0 to 4.5 ps / nm / k as the various characteristics at the wavelength of 1550 nm.
m, the dispersion slope is 0.05 to 0.09 ps /
Within the range of nm ² / km, the effective core area A _eff is set to 70 μm ² or more. Further, the cutoff wavelength at a fiber length of 2 m is set to 1300 nm or more. Thus, the dispersion-shifted fiber according to the present invention realizes high-quality long-distance optical transmission while effectively suppressing the development of the nonlinear optical effect.

The dispersion-shifted fiber according to the present invention has a refractive index profile of a ring-shaped core structure (including a refractive index profile having a depressed clad ring-shaped core structure). A fiber is preferably realized.

Further, according to the dispersion-shifted fiber according to the present invention, the position where the optical power distribution of the fundamental mode of the signal light becomes maximum is separated from the center of the core region by a predetermined distance in the radial direction, and When the optical power of the fundamental mode of the signal light at the center is P ₀ and the maximum value in the optical power distribution of the fundamental mode is P ₁ , P ₁ > 1.2 × P
₀ , the effective core area A is maintained with the mode field diameter (MFD) kept small.
There is an effect that a structure capable of increasing _eff can be realized.

More specifically, the position where the maximum value of the optical power is obtained is separated from the center of the core region by 0.5 to 5 μm, so that the effective core area A _eff can be effectively expanded. This has the effect. Since the refractive index profile is designed so as to obtain desired values as various characteristics with respect to light having a center wavelength (1550 nm) in the 1.55 μm wavelength band, it is possible to effectively suppress the occurrence of nonlinear optical phenomena. There is also an effect that can be done.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cross-sectional structure and a radial refractive index profile of a dispersion-shifted fiber according to a first embodiment of the present invention.

FIG. 2 is a graph showing changes in the effective core area A _eff and the MFD when the core diameter (outer diameter of the outer core) 2b is changed in a dispersion-shifted fiber having a refractive index profile of a ring-shaped core structure. .

FIG. 3 is a graph showing the relationship between the cutoff wavelength λ _C and the dispersion slope at a fiber length of 2 m when the core diameter (outer diameter of the outer core) 2b is changed in a dispersion-shifted fiber having a refractive index profile of a ring-shaped core structure. It is a graph which shows a change.

FIG. 4 shows a bit error rate (B) due to a change in dispersion slope.
5A is a graph for explaining a change in ER), and FIG. 6A shows B when the dispersion slope is 0.03 ps / nm ² / km.
ER change, (b) dispersion slope 0.05 ps / n
BER change in time of the m ² / km, (c) the change in BER when the dispersion slope of 0.09ps / nm ² / km, and, (d) is the dispersion slope 0.12 ps / n
the change in BER when the m ² / km, respectively show.

FIG. 5 is a diagram for explaining a Q value.

FIG. 6 is a diagram (including a sectional structure and a radial refractive index profile) for explaining the optical power distribution of the fundamental mode of the signal light in the dispersion-shifted fiber of FIG. 1;

FIG. 7 is a diagram (including a sectional structure and a radial refractive index profile) for explaining the optical power distribution of the fundamental mode of the signal light in the dispersion-shifted fiber according to the comparative example of the first embodiment.

FIG. 8 is a diagram showing a cross-sectional structure and a radial refractive index profile of a dispersion-shifted fiber according to a modification of the first embodiment.

FIG. 9 is a diagram showing a cross-sectional structure and a radial refractive index profile of a second embodiment of the dispersion-shifted fiber according to the present invention.

10 is a diagram (including a sectional structure and a radial refractive index profile) for explaining the optical power distribution of the fundamental mode of signal light in the dispersion-shifted fiber of FIG. 9;

FIG. 11 is a diagram illustrating a cross-sectional structure and a radial refractive index profile of a dispersion-shifted fiber according to a modification of the second embodiment.

FIG. 12 is a table for explaining an allowable range of structural parameters for realizing various characteristics of the dispersion-shifted fiber according to the present invention, and a graph showing a relationship between an outer diameter of an inner core and an outer diameter of an outer core. It is.

FIG. 13 shows an electromagnetic field distribution (corresponding to an optical power distribution) along a radial direction in the dispersion-shifted fiber according to the present invention.
FIG.

FIG. 14 shows an electromagnetic field value (corresponding to optical power) from the center of the core region in the dispersion-shifted fiber according to the present invention.
And the MFD (μm)
7 is a graph showing the relationship with m).

FIG. 15 shows an electromagnetic field value (corresponding to optical power) from the center of the core region in the dispersion-shifted fiber according to the present invention.
7 is a graph showing a relationship between a distance (μm) to a position where the maximum value is obtained and an increase in loss due to microbending (dB / km).

FIG. 16 is a table showing various characteristics of specific prototypes of the dispersion-shifted fiber according to the present invention.

FIG. 17 is a graph showing an example of a refractive index profile and its optical power distribution along a radial direction in the dispersion-shifted fiber according to the present invention.

FIG. 18 is a diagram showing an example of a refractive index profile of a core structure applicable to the dispersion-shifted fiber according to the present invention.

[Explanation of symbols]

100, 200: dispersion shift fiber, 110, 120
... core region, 111, 121 ... inner layer core, 112, 12
2 ... outer layer core, 210, 220 ... cladding region, 221
... inner layer cladding, 222 ... outer layer cladding.

Continuation of the front page (56) References JP-A-9-33744 (JP, A) JP-A-8-248251 (JP, A) JP-A-7-168046 (JP, A) JP-A-51-133041 (JP) , A) JP-A-10-62640 (JP, A) JP-A-10-239550 (JP, A) JP-A-10-293225 (JP, A) JP-A-11-38256 (JP, A) Patent 2668677 ( JP, B2) International Publication 96/7942 (WO, A1) JOURNAL OF LIGHTWAVE AVE TECHNOLOGY, VOL. 11, NO. 11 (NOVEMBER 1993), pp. 139-157. 1717-1720; Mahmood et al. : "Transmission Characteristics of a Coaxial Optical Fiber Line" International Wire & Cable Symposium Proceedings 1996, pp. 939-945; Nouchi et al. : "NOVEL INDEX PROFILE FOR IMPROV VED LARGE EFFECTIVE AREA FIBER" (58) Fields investigated (Int. Cl. ⁶ , DB name) G02B ⁶ /00-6/54

Claims (4)

(57) [Claims] 1. A dispersion-shifted fiber comprising a core region extending along a predetermined reference axis and a cladding region provided on an outer periphery of the core region, wherein the core region has a first refractive index. An inner core having an outer core provided on the outer periphery of the inner core and having a second refractive index higher than the first refractive index, and the cladding region is provided on an outer periphery of the outer core. And an inner cladding having a third refractive index lower than the second refractive index; and an outer cladding provided on an outer periphery of the inner cladding;
An outer clad having a fourth refractive index higher than the refractive index of the inner core, the outer diameter of the inner core is 2a, the outer diameter of the outer core is 2b, and the outer diameter of the inner clad is 2c. 0.4 ≦ Ra (= 2a / 2b) ≦ 0.8 5 μm ≦ 2b ≦ 14 μm 1.2 ≦ (2c / 2b) ≦ 2.2 and the absolute value for the signal light having a wavelength of 1550 nm Is 1.0-
A dispersion of 4.5 ps / nm / km and a wavelength of 1550 n
m signal light, and an effective core area of 70 μm ² or more, and 0.05 to
A dispersion shifted fiber having a dispersion slope of 0.09 ps / nm ² / km and a cutoff wavelength of 1300 nm or more at a fiber length of 2 m. 2. When the relative refractive index difference of the outer core with respect to the outer cladding is Δn ₁ and the relative refractive index difference of the inner core with respect to the outer cladding is Δn ₂ , Δn ₁ −Δn ₂ ≧ 1%. 2. The dispersion-shifted fiber according to claim 1, wherein the relationship is satisfied. 3. The dispersion-shifted fiber according to claim 1, wherein a relative refractive index difference Δn ₂ of the inner core with respect to the outer cladding is −0.4% or less. 4. At a wavelength of 1550 nm, when the optical power of the fundamental mode at the center of the core region is P ₀ and the maximum value of the fundamental mode is P ₁ , the relationship of P ₁ > 1.2 × P ₀ is satisfied. 2. The dispersion shift according to claim 1, wherein the position where the optical power distribution of the fundamental mode of the signal light becomes maximum is separated from the center of the core region by 0.5 μm to 5 μm in the radial direction. fiber.