JP3840977B2 - Dispersion compensating optical fiber - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a dispersion compensating optical fiber.
[0002]
[Prior art]
Due to the explosive increase in data communications such as the Internet in recent years, a dramatic increase in transmission capacity has been demanded. At present, a WDM (Wavelength Division Multiplexing) transmission system that simultaneously transmits a plurality of signal lights having slightly different wavelengths in one optical fiber has been put into practical use, and is used in a trunk transmission line or a submarine optical cable. Yes. Usually, such optical fiber transmission lines are provided with repeaters at regular intervals.
[0003]
A conventional repeater is a so-called regenerative repeater that converts signal light once into an electrical signal in order to compensate and amplify the signal light, and synchronously reproduces and amplifies the electrical signal to convert it into an optical signal. In WDM transmission, since reproduction / amplification must be performed slightly for each wavelength, devices corresponding to the wave number are required.
[0004]
For this reason, the increase in the wave number for increasing the capacity of WDM transmission has limitations in terms of cost and mounting space.
[0005]
However, with the development of EDFA (Erium-Doped Fiber Amplifier), signal light of all wavelengths can be amplified as a whole without converting the signal light into an electrical signal. It was. Although this EDFA has rapidly increased the transmission capacity, various problems have arisen due to an increase in wave number and an increase in the bit rate of signal light.
[0006]
For example, due to the chromatic dispersion inherent in the optical fiber, different dispersion occurs at both ends of the used wavelength band, and there is a problem that the waveform of the signal light after transmission is deteriorated or a nonlinear phenomenon occurs. The nonlinear phenomenon is a phenomenon caused by a local change in the refractive index distribution (generally depending on the optical power density) of the optical fiber constituting the transmission line, and corresponds to FWM (Four Wave Mixing) and the like.
[0007]
These dispersion and non-linear phenomena can cause the transmission quality of signal light to deteriorate. This is particularly serious when a large number of signal lights having different wavelengths are transmitted over a long distance as in WDM transmission.
[0008]
[Problems to be solved by the invention]
Here, in order to prevent the signal waveform from being deteriorated due to the chromatic dispersion of the optical fiber, it is effective to shift the so-called zero dispersion wavelength to the use wavelength so as to make the dispersion value of the use wavelength band as small as possible. For example, DSF (Dispersion Shift Fiber) having a zero-dispersion wavelength of 1550 nm is widely applied regardless of whether it is on land or on the seabed.
[0009]
However, in WDM transmission in which a plurality of signal lights are transmitted, the dispersion values generated in the signal light on the shortest wavelength side and the signal light on the longest wavelength side are different. That is, the wave number itself is limited by the presence of the dispersion slope. This is because a dispersion range (that is, a wavelength band) within which signal light after optical fiber transmission is within a discriminable limit is determined by hardware and transmission speed, and as a result, the maximum wave number is determined.
[0010]
Therefore, it is important to increase the transmission capacity to make the dispersion slope value as small as possible.
[0011]
By changing the conventional single-peak profile (refractive index profile) of an optical fiber to a complex profile such as a W-type or a triple-clad type, a low dispersion slope of 0.05 ps / nm ² / km or less is achieved. Can be achieved.
[0012]
However, on the other hand, as the wave number increases, the power density of the optical signal incident on the optical fiber increases, and the above-mentioned nonlinear phenomenon has become a big problem. For example, recent research has revealed that the above-described FWM amplifies WDM signal light in the vicinity of a zero dispersion wavelength and significantly degrades signal transmission characteristics. In order to increase the wave number of signal light while preventing this non-linear phenomenon, an optical fiber having a large effective cross-sectional area as much as possible can be used while keeping the dispersion and slope small in the entire transmission line.
[0013]
For example, by using an optical fiber with a relatively large effective area and a relatively small dispersion / slope at a portion where the light density is high immediately after amplification by an amplifier such as an EDFA, the occurrence of nonlinear phenomenon is suppressed, and the output end of the optical fiber is reduced. By connecting an optical fiber having a relatively small effective cross-sectional area and a small dispersion / slope, it is possible to suppress the dispersion to non-zero and the slope to 0.1 ps / nm ² / km or less in the entire transmission line (see FIG. 11). .)
[0014]
FIG. 11 is a diagram showing cumulative dispersion of a hybrid circuit transmission line by an effective area-enlarged optical fiber and a low dispersion slope optical fiber during wavelength division multiplexing transmission. In the figure, the horizontal axis indicates the fiber length, and the vertical axis indicates the cumulative dispersion.
[0015]
In addition, an optical fiber having an extremely large effective area and large dispersion is used immediately after amplification by an amplifier such as an EDFA (the first half of the amplification section of the rare earth-doped optical fiber), and then (the latter half of the amplification section of the rare earth-doped optical fiber) is used in the first half. A hybrid transmission line using an optical fiber that completely compensates for the accumulated dispersion and dispersion slope that has occurred has also been proposed and is about to be put into practical use (see FIG. 12).
[0016]
FIG. 12 is a diagram showing the cumulative dispersion of the hybrid transmission line by the optical fiber with an enlarged effective area and the dispersion / dispersion slope compensating optical fiber at the time of wavelength division multiplexing transmission. In the figure, the horizontal axis indicates the fiber length, and the vertical axis indicates the cumulative dispersion.
[0017]
However, SCDCF (Slope Compensation Dispersion Compensation Fiber) used in the latter half of the transmission line can realize large negative dispersion and negative dispersion slope, but has a relatively small MFD. Therefore, there has been a problem that a large connection loss occurs due to MFD mismatch when fusion splicing with a different type optical fiber such as an optical fiber having an extremely large effective area used in the first half.
[0018]
Accordingly, an object of the present invention is to provide a dispersion compensating optical fiber that solves the above-described problems and has low connection loss with different types of optical fibers.
[0019]
[Means for Solving the Problems]
To achieve the above object, a dispersion compensating optical fiber according to the present invention includes a core, a first cladding layer covering the core, a second cladding layer covering the first cladding layer, and a third cladding layer covering the second cladding layer. A dispersion compensating optical fiber including a fourth cladding layer covering the third cladding layer, wherein the relative refractive index difference Δn1 of the core and the relative refractive index difference Δn3 of the second cladding layer are determined as relative refractive index of the fourth cladding layer. larger than the rate difference .DELTA.n0, the relative refractive index difference Δn2 of the first cladding layer and the third relative refractive index difference of the cladding layer Δn4 smaller than a fourth relative refractive index difference of the cladding layer .DELTA.n0, relative refractive index of the core The difference is monotonously decreased in the radial direction from the center, the relative refractive index difference Δn1 at the center of the core is set to 1.03 to 1.07%, the radius r1 of the core is set to 2.66 to 2.72 μm, The relative refractive index difference Δn3 of the cladding layer is 0.13 to 0.15 The radius r3 of the second cladding layer is 10.7 to 11.4 μm, the radius r3 of the second cladding layer is 10.7 to 11.4 μm, and the relative refractive index difference Δn2 of the first cladding layer is −0. 285 to −0.25%, the radius r2 of the first cladding layer is 5.38 to 5.73 μm, the relative refractive index difference Δn4 of the third cladding layer is −0.15 to −0.07%, By setting the radius r4 of the three cladding layers to 13.15 μm or more and 17 μm or less , the dispersion slope at a wavelength of 1550 nm is −0.18 to −0.12 ps / nm ² / km, and the chromatic dispersion is −50 to −40 ps / km. The mode field diameter is set to 6.0 μm or more.
[0029]
According to the present invention, the outer periphery of the core is sequentially covered with the first cladding layer to the fourth cladding layer to form a quadruple cladding layer structure, and the relative refractive index difference Δn1 of the core and the relative refractive index difference Δn3 of the second cladding layer are obtained. The relative refractive index difference Δn0 of the fourth cladding layer is made larger, and the relative refractive index difference Δn2 of the first cladding layer and the relative refractive index difference Δn4 of the third cladding layer are made smaller than the relative refractive index difference Δn0 of the fourth cladding layer. As a result, the MFD can be increased, so that the connection loss that occurs when connecting to a different optical fiber can be kept low.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[0031]
FIG. 1 is a refractive index distribution diagram showing an embodiment of a dispersion compensating optical fiber of the present invention. In the figure, the horizontal axis indicates the distance from the center 0 of the core, and the vertical axis indicates the relative refractive index difference.
[0032]
The dispersion compensating optical fiber includes a first cladding layer (radius r1 to r2) 2 and a second cladding layer (radius r2) around a core (radius r1) 1 in which the relative refractive index difference monotonously decreases in the radial direction from the center. ˜r3) 3, a third cladding layer (radius r3 to r4) 4 and a fourth cladding layer (radius r4 to r5) 5 are sequentially laminated.
[0033]
In this dispersion compensating optical fiber, the relative refractive index difference Δn1 of the core 1 and the relative refractive index difference Δn3 of the second cladding layer 3 are made larger than the relative refractive index difference Δn0 of the fourth cladding layer 5, and the first cladding layer 2 The relative refractive index difference Δn2 and the relative refractive index difference Δn4 of the third cladding layer 4 are made smaller than the relative refractive index difference Δn0 of the fourth cladding layer 5, and the relative refractive index difference and the layer thickness are set as described below. Thus, the dispersion slope at a wavelength of 1550 nm is set to −0.18 to −0.12 ps / nm ² / km, the chromatic dispersion is set to −50 to −40 ps / nm / km, and the mode field diameter is set to 6.0 μm or more. Is.
[0034]
Here, the larger the MFD is, the better. However, although theoretically it does not exceed 33 m ² , a relatively large Aeff is obtained at the trial production stage. Therefore, the MFD is preferably about 40 μm ² or less.
[0035]
By setting the relative refractive index difference Δn1 at the center 0 of the core 1 to 1.03 to 1.07%, the dispersion can be set to −50 to −40 ps / nm / km (see FIG. 2).
[0036]
FIG. 2 is a diagram showing a simulation result of dispersion with respect to the relative refractive index difference Δn1 of the dispersion compensating optical fiber having the characteristics shown in FIG. In the figure, the horizontal axis represents the relative refractive index difference Δn1, and the vertical axis represents the dispersion.
[0037]
By setting the radius r1 of the core 1 to 2.66 to 2.72 μm, the dispersion can be set to −50 to −40 ps / nm / km (see FIG. 3).
[0038]
FIG. 3 is a diagram showing a simulation result of dispersion with respect to the radius r1 of the dispersion compensating optical fiber having the characteristics shown in FIG. In the figure, the horizontal axis represents the core radius r1, and the vertical axis represents the dispersion.
[0039]
By setting the relative refractive index difference Δn3 of the second cladding layer 3 to 0.13 to 0.15%, the cutoff wavelength at a reference length of 2 m is 1.7 μm or less and the dispersion is −40 ps / nm / km or less. (See FIG. 4).
[0040]
FIG. 4 is a diagram showing a simulation result of dispersion with respect to the relative refractive index difference Δn3 of the dispersion compensating optical fiber having the characteristics shown in FIG. In the figure, the horizontal axis represents the relative refractive index difference Δn3 of the second cladding layer 3, and the vertical axis represents the dispersion.
[0041]
By setting the radius r3 of the second cladding layer 3 to 10.7 to 11.4 μm, the dispersion slope is −0.12 ps / nm ² / km or less, and the cutoff wavelength at the reference length of 2 m is 1.7 m or less. (See FIG. 5).
[0042]
FIG. 5 is a diagram showing a simulation result of the cutoff wavelength with respect to the radius r3 of the dispersion compensating optical fiber having the characteristics shown in FIG. In the figure, the horizontal axis indicates the radius r3 of the second cladding layer, and the vertical axis indicates the cutoff wavelength.
[0043]
By setting the relative refractive index difference Δn2 of the first cladding layer 2 to −0.285 to −0.25%, the dispersion slope can be set to −0.18 to −0.12 ps / nm ² / km ( (See FIG. 6.)
[0044]
FIG. 6 is a diagram showing a simulation result of the demultiplexing slope with respect to the relative refractive index difference Δn2 of the dispersion compensating optical fiber having the characteristics shown in FIG. In the figure, the horizontal axis represents the relative refractive index difference Δn2 of the first cladding layer 2, and the vertical axis represents the demultiplexing slope.
[0045]
By setting the radius r2 of the first cladding layer 2 to 5.38 to 5.73 μm, the dispersion can be made −40 ps / nm / km or less and the dispersion slope can be made −0.12 ps / nm ² / km or less. (See FIG. 7).
[0046]
FIG. 7 is a diagram showing a simulation result of dispersion with respect to the radius r2 of the dispersion compensating optical fiber having the characteristics shown in FIG. In the figure, the horizontal axis indicates the radius r2 of the first cladding layer, and the vertical axis indicates dispersion.
[0047]
By setting the relative refractive index difference Δn4 of the third cladding layer 4 to −0.15 to −0.07%, the cutoff wavelength at the reference length of 2 m is 1.7 μm or less, and the dispersion slope is −0.12 ps / It can be set to nm ² / km or less (see FIG. 8).
[0048]
FIG. 8 is a diagram showing a simulation result of the dispersion slope with respect to the relative refractive index difference Δn4 of the dispersion compensating optical fiber having the characteristics shown in FIG. In the figure, the horizontal axis represents the relative refractive index difference Δn4 of the third cladding layer 4, and the vertical axis represents the dispersion slope.
[0049]
By setting the radius r4 of the third cladding layer 4 to 13.15 μm or more and 17 μm or less, the cutoff wavelength at the reference length of 2 m can be set to 1.7 μm or less.
[0050]
Here, the value of the radius r4 theoretically has little influence on the chromatic dispersion. Although the cutoff wavelength relatively changes, this cutoff wavelength decreases as the radius r4 increases, so the limit of the radius r4 has only a lower limit. However, in terms of manufacturing conditions, it is preferable that the thickness of the cladding 3 is thin, and the upper limit of the radius r is preferably 17 μm including manufacturing errors (see FIG. 9).
[0051]
FIG. 9 is a diagram showing a simulation result of the cutoff wavelength with respect to the radius r4 of the dispersion compensating optical fiber having the characteristics shown in FIG. In the figure, the horizontal axis indicates the radius r4 of the third cladding layer, and the vertical axis indicates the cutoff wavelength.
[0052]
The core 1 and the first cladding layer 2 contain 0.05 mol% or more of germanium oxide in the composition.
[0053]
By configuring the dispersion compensating optical fiber in this way, the MFD can be increased, so that the connection loss that occurs when connecting to a different optical fiber can be kept low.
[0054]
【Example】
For example, the manufacture of an optical fiber core rod called the MCVD method will be described with reference to FIG.
[0055]
FIG. 10 is a schematic view of a core rod manufacturing apparatus using the MCVD method.
[0056]
A source gas bubbled with pure oxygen from a bubbler 101 is introduced into a starting quartz tube 103 that rotates in a circumferential direction through a rotation introducing terminal 102 that is a rotary joint. Examples of the source gas include SiCl ₄ , GeCl ₄ , O ₂ , He, Cl ₂ , and C ₂ F ₆ . These raw materials are heated from the outside of the quartz tube by an oxyhydrogen burner 104 moving along the longitudinal direction of the starting quartz tube 103, and soot particles 105 are generated by a chemical reaction. A part of the soot particles adheres and accumulates on the inner surface of the starting quartz tube 103, and the rest passes through the exhaust pipe 106 and is discharged to the soot box 107. The soot particles adhering to the inner surface of the starting quartz tube 103 are heated again by the oxyhydrogen burner 104 to become transparent glass.
[0057]
The above steps are repeated as necessary to synthesize the core 1 and the cladding layers 2 to 5. After the deposition and transparent vitrification are completed, the thermal power of the oxyhydrogen burner 104 is increased to close the remaining space in the center, and solidification is performed with the surface tension of the starting quartz tube 103.
[0058]
The core rod thus obtained is externally attached with pure quartz soot by, for example, the VAD method and sintered to obtain a drawing base material. The drawing base material was drawn, for example, at a drawing speed of about 200 m / min and a tension of about 1.3 N (133 gf) to obtain an optical fiber having a length of about 100 km.
[0059]
The characteristics of the optical fiber thus manufactured were measured.
[0060]
1 has a relative refractive index difference distribution, a relative refractive index difference Δn1 = 1.05% at the center of the core 1, a radius r1 = 2.7 μm, and a relative refractive index difference Δn2 of the first cladding layer 2 = −. 0.275%, radius r2 = 5.5 μm, relative refractive index difference Δn3 = 0.145% of the second cladding layer 3, radius r3 = 11.0 μm, relative refractive index difference Δn4 = −0 of the third cladding layer 4 A dispersion-compensating optical fiber having a diameter of 0.08% and a radius of r4 = 13.45 μm was prototyped.
[0061]
As a result, the transmission loss at a wavelength of 1550 nm is 0.248 dB / km, the MFD is 6.32 μm, the dispersion is −43.8 ps / nm / km, and the dispersion slope is −0.141 ps / nm ² / km and good characteristics were obtained.
[0062]
In the above, the dispersion compensating optical fiber can increase the MFD as compared with the conventional dispersion compensating optical fiber, so that the connection loss due to the difference in MFD generated when connecting to a different type of optical fiber can be kept low. Can do.
[0063]
【The invention's effect】
In short, according to the present invention, it is possible to provide a dispersion-compensating optical fiber having a low connection loss with different types of optical fibers.
[Brief description of the drawings]
FIG. 1 is a refractive index distribution diagram showing an embodiment of a dispersion compensating optical fiber of the present invention.
FIG. 2 is a diagram showing a simulation result of dispersion with respect to a relative refractive index difference Δn1 of the dispersion compensating optical fiber having the characteristics shown in FIG.
3 is a diagram showing a simulation result of dispersion with respect to a radius r1 of a dispersion compensating optical fiber having the characteristics shown in FIG. 1. FIG.
4 is a diagram showing a simulation result of dispersion with respect to a relative refractive index difference Δn3 of the dispersion compensating optical fiber having the characteristics shown in FIG. 1. FIG.
FIG. 5 is a diagram showing a simulation result of a cutoff wavelength with respect to a radius r3 of the dispersion compensating optical fiber having the characteristics shown in FIG.
6 is a diagram showing a simulation result of a demultiplexing slope with respect to a relative refractive index difference Δn2 of the dispersion compensating optical fiber having the characteristics shown in FIG. 1. FIG.
7 is a diagram showing a simulation result of dispersion with respect to a radius r2 of the dispersion compensating optical fiber having the characteristics shown in FIG. 1. FIG.
8 is a diagram showing a simulation result of a dispersion slope with respect to a relative refractive index difference Δn4 of the dispersion compensating optical fiber having the characteristics shown in FIG.
9 is a diagram showing a simulation result of a cutoff wavelength with respect to a radius r4 of the dispersion compensating optical fiber having the characteristics shown in FIG.
FIG. 10 is a schematic view of a core rod manufacturing apparatus using an MCVD method.
FIG. 11 is a diagram showing cumulative dispersion of a hybrid circuit transmission line using an optical fiber with an enlarged effective area and a low dispersion slope optical fiber at the time of wavelength multiplexing transmission.
FIG. 12 is a diagram illustrating cumulative dispersion of a hybrid transmission line using an optical fiber with an enlarged effective area and a dispersion / dispersion slope compensating optical fiber during wavelength division multiplexing transmission;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Core 2 1st cladding layer 3 2nd cladding layer 4 3rd cladding layer 5 4th cladding layer

Claims (1)

A core; a first cladding layer covering the core; a second cladding layer covering the first cladding layer; a third cladding layer covering the second cladding layer; and a fourth cladding layer covering the third cladding layer. A dispersion compensating optical fiber, wherein the relative refractive index difference Δn1 of the core and the relative refractive index difference Δn3 of the second cladding layer are larger than the relative refractive index difference Δn0 of the fourth cladding layer, and the ratio of the first cladding layer The refractive index difference Δn2 and the relative refractive index difference Δn4 of the third cladding layer are made smaller than the relative refractive index difference Δn0 of the fourth cladding layer, and the relative refractive index difference of the core is monotonously decreased in the radial direction from the center. The relative refractive index difference Δn1 at the center of the core is 1.03 to 1.07%, the radius r1 of the core is 2.66 to 2.72 μm, and the relative refractive index difference Δn3 of the second cladding layer is 0.13 to 0.13. 0.15%, and the radius r3 of the second cladding layer is 10.7 to 1 .4 μm, the radius r3 of the second cladding layer is 10.7 to 11.4 μm, the relative refractive index difference Δn2 of the first cladding layer is −0.285 to −0.25%, and the radius of the first cladding layer r2 is 5.38 to 5.73 μm, the relative refractive index difference Δn4 of the third cladding layer is −0.15 to −0.07%, and the radius r4 of the third cladding layer is 13.15 μm to 17 μm. Thus, the dispersion slope at a wavelength of 1550 nm is set to −0.18 to −0.12 ps / nm ² / km, the chromatic dispersion is set to −50 to −40 ps / nm / km, and the mode field diameter is set to 6.0 μm or more. A dispersion-compensating optical fiber.

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