JP3802875B2 - High stress-resistant optical fiber - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical fiber having an increased effective cross-sectional area, and more particularly to a high-stress resistant optical fiber having excellent bending characteristics.
[0002]
[Prior art]
In recent years, with the rapid spread of the Internet and the like, the information capacity has increased, and the demand for an increase in capacity for information transmission media has increased. Among the technologies corresponding to the increase in capacity, the wavelength division multiplexing (hereinafter referred to as WDM) transmission method is most promising. In the WDM system, a plurality of signal lights can be transmitted with one optical fiber, so that the transmission capacity can be increased by about 100 times at a stretch. Therefore, introduction into a long-distance, large-capacity transmission line such as an optical submarine cable system connecting continents is being promoted, and the practical application stage is being reached.
[0003]
One of the technical backgrounds in which WDM technology has been rapidly established is the improvement of optical amplification technology. An erbium-doped optical fiber amplifier (EDFA), which is one of optical amplification technologies, can amplify attenuated light in the 1.55 μm band up to about 1000 times. It works to compensate for loss in the transmission line. Trans-Pacific optical submarine cable systems (TPC-5CN, China-US, etc.) using EDFA have already been put into practical use, and have achieved a large capacity transmission of 100 Gbit / s using WDM technology. In order to increase the capacity of WDM transmission, it is necessary to increase the number of wavelength multiplexing. However, since the signal power entering the optical fiber increases, the possibility of occurrence of a nonlinear phenomenon increases. For example, it has been reported that four-wave mixing causes an increase in noise and a decrease in signal light. Therefore, it is effective to use an optical fiber with a large effective cross-sectional area of the optical fiber and a reduced signal light power density in the optical fiber for a long-distance transmission line.
[0004]
Thus, in recent years, with the development of optical amplification technology and WDM technology, the power of light incident on an optical fiber has increased, and various nonlinear effect phenomena are likely to occur. For example, the nonlinear effect phenomenon When the self-phase modulation phenomenon, which is one of the above, occurs, the pulse signal waveform in the optical fiber is distorted and the transmission capacity is limited. In addition, the Brulian scattering phenomenon, which is also one of the nonlinear effect phenomena, is likely to occur, and when the Brulian scattering phenomenon occurs, the incident power of the optical fiber is saturated. Thus, when the nonlinear effect phenomenon occurs, the transmission characteristics of light propagating through the optical fiber are deteriorated.
[0005]
In addition, since the zero dispersion wavelength of the conventional single mode fiber is longer than 1.3 μm, there is no fiber having a large anomalous dispersion (positive dispersion) at 1.3 μm.
[0006]
As a novel optical fiber that solves the problem of the above-described nonlinear phenomenon, a photonic crystal optical fiber (also referred to as a PCF or photonic fiber) has recently attracted attention. PCF is an optical fiber in which a photonic crystal structure is provided in a cladding part. The photonic crystal structure is a periodic structure having a refractive index. Specifically, a space of a honeycomb structure such as a honeycomb is provided in the cladding, so that a photonic band gap (Photonic Band Gap; PBG) occurs. For example, Non-Patent Document 1 discloses a PCF using PBG as a waveguide principle. Non-Patent Document 2 discloses a hollow core PCF having a PBG structure as a guiding principle.
[0007]
Also, recently, the optical fiber having a perfect PBG structure, but the presence of pores in the cladding in the vicinity of the core of the optical fiber having a relative refractive index difference due to a difference in glass composition, makes it possible to substantially A holey fiber (HF) having characteristics that have not been obtained in the past has been reported by increasing the relative refractive index difference between the core and the cladding by lowering the refractive index. For example, Non-Patent Document 3 discloses a hole-added holey fiber in which four holes are provided in the cladding near the core of a fiber having a normal single-mode fiber structure. By enlarging, a fiber with single mode operation in the 1.2 μm band is disclosed.
[0008]
Patent Document 1 is a patent document relating to a photonic fiber.
[0009]
[Patent Document 1]
JP 2000-296440 A [Non-Patent Document 1]
“Photonic band gap guidance optical fiber” Knight et al., Science 282, 1476, 1998 [Non-patent Document 2]
“Single-Mode Photonic Band gap of Light in Air” Cregan et al., Science 285, 1537, 1999 [Non-patent Document 3]
"Novel hole-assisted lightguide fiber exhibiting large anomalous dispersion and low loss bellow 1dB / km"
Hasegawa et al., OFC 2001PD5-1, 2001
[Problems to be solved by the invention]
However, when the effective area is increased, the mode field diameter of the optical fiber also increases. Therefore, the increase in the loss of signal wavelength light that occurs when an optical fiber (for example, 170 μm ² ) having a large effective core area is wound 1 m around a cylinder having a diameter of 20 mm is as large as 80 dB / m. Currently, 1.3 μm single-mode optical fiber, which is most frequently used in optical transmission systems, is generally 2 to 10 dB / m. Therefore, an optical fiber with a large effective core area is extremely resistant to bending. There was a problem that the loss was large.
[0011]
From the above, a high stress-resistant optical fiber having high stress resistance while increasing the effective area is desired.
[0012]
Therefore, an object of the present invention is to solve the above-described problems and provide a high stress-resistant optical fiber having excellent bending characteristics.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a side core having a smaller refractive index than the center core surrounding the center core positioned at the central axis, and surrounding the side core and having a refractive index smaller than that of the center core. the W-type refractive index distribution structure is formed by arranging a greater clad than the side core, the effective cross-sectional area and from 110 [mu] m ² to 180 [mu] m ^2, 4 or more which is located axially symmetrically each equal circumferential angle about said central axis Are formed , the inner diameter of the hole is 3 μm or more and 16 μm or less, and the distance between the hole and the central axis is smaller than the mode field diameter .
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.
[0017]
As shown in the refractive index profile of FIG. 1, the high stress-resistant optical fiber according to the present invention forms a W-type refractive index profile. When viewed from this refractive index profile, the high-stress resistant optical fiber according to the present invention is provided with a side core 2 surrounding the center core 1 located at the central axis of the optical fiber, and surrounding the side core 2 with a cladding. 3 is arranged.
[0018]
Each part name and code given to the refractive index profile are directly used as a name and code representing the inner layer structure of the optical fiber.
[0019]
The clad 3 of the high-stress-resistant optical fiber according to the present invention is formed of pure silica (SiO ₂ ), and the center core 1 is a layer (GeO ₂ —SiO ₂ ) doped with germanium (Ge) that increases the refractive index of pure silica. Is formed by. Further, the side core 2 is formed by a layer (F-SiO ₂ ) in which pure silica is doped with fluorine (F) that lowers the refractive index. Since the dope composition of each layer is configured as described above, the refractive index of the side core 2 is lower than the refractive index of the center core 1, the refractive index of the cladding 3 is lower than the refractive index of the center core 1, and the refractive index of the side core 2. Higher than. Therefore, as shown in FIG. 1, a W-type refractive index profile is obtained in which the refractive index changes stepwise from high to low, highest, low, and high in order from the left.
[0020]
In the optical fiber that realizes the refractive index profile of FIG. 1, for example, in order to obtain an optical fiber having an effective area of 170 μm ² , the relative refractive index difference a (based on the cladding 3) of the center core 1 is 0.175%, The relative refractive index difference b of the side core 2 is -0.003%, the diameter c of the center core 1 is 14.8 μm, and the diameter d of the side core 2 is 61.4 μm. At this time, the mode field diameter is 14.9 μm.
[0021]
FIG. 2 shows a cross section (transverse cross section) perpendicular to the central axis (referring to the center of the center core 1) of the high stress resistant optical fiber according to the present invention. As shown in the figure, this optical fiber is composed of a core region 4 through which light passes most and a cladding region 5 through which light does not pass as compared to the core region 4. The core region 4 substantially overlaps the center core 1 and the side core 2 whose refractive index profile is shown in FIG. In the core region 4, a plurality of air holes (also referred to as pores) 6 are provided at positions radially outward from the center core 1 and radially inward from the outer periphery of the side core 2. Although not shown in the figure, these pores 6 extend in the longitudinal direction of the optical fiber. The high stress-resistant optical fiber according to the present invention is a kind of holey fiber belonging to one field of photonic fibers.
[0022]
The number of pores 6 is an even number of 4 or more. Two pores 6 are arranged at line-symmetric positions with the central axis as a symmetry axis, and the pores 6 are arranged at equal intervals in the circumferential direction. With this arrangement, the pores 6 are axisymmetric about the central axis of the core region 4 at every equal circumferential angle. Here, it is assumed that there are four pores 6. Therefore, in the cross section of the optical fiber of FIG. 2, each of these four pores 6 is located at an equal distance in the radial direction across the central axis. It is located every 90 °.
[0023]
In the above-described optical fiber having an effective area of 170 μm ² , for example, the diameter of the pores 6 is 7 μm, and the center of the pores 6 is positioned on the circumference having a radius of 12 μm from the central axis. The pores 6 are filled with air or an inert gas and have a refractive index of 1. The reason why four or more even-numbered pores 6 are arranged symmetrically around the central axis at every equal circumferential angle will be described.
[0024]
First, if the number of the pores 6 is two and the intervals are equal, the pores 6 exist only on a straight line passing through the central axis on the cross section of the optical fiber. There is a difference in the effect of reducing the refractive index of the effective cladding region 5 by the pores 6 between the straight line and the straight line having a 90 ° positional relationship. For this reason, the characteristic of the optical fiber becomes the characteristic of the pseudo polarization plane preserving fiber, the polarization dispersion characteristic is deteriorated, and this causes a problem at high speed transmission. Further, when the number of pores 6 is an odd number, no matter how the pores 6 are arranged, the effective cladding region 5 by the pores 6 with respect to two straight lines orthogonal to each other through the central axis on the cross section of the optical fiber. As a result, the refractive index distribution becomes asymmetric and the polarization dispersion characteristic deteriorates. If the pores 6 are arranged at unequal intervals, the power distribution of incident light is decentered, and anisotropic stress is applied to the core region 4 to deteriorate PMD characteristics.
[0025]
On the other hand, when four or more even-numbered pores 6 are arranged symmetrically around the central axis at equal circumferential angles, the pores are formed with respect to two straight lines orthogonal to each other through the central axis on the optical fiber cross section. Since the effective refractive index distribution of the cladding region 5 by 6 can be made symmetric, polarization dispersion characteristics can be improved. Further, since there is no anisotropy, the number of pores 6 may be six.
[0026]
In the high stress-resistant optical fiber according to the present invention, the diameter of the pores 6 is preferably 3 μm or more and 16 μm or less. The reason will be explained. A plurality of samples of the optical fiber of the present invention in which the number of pores 6 is four and the diameters of the pores 6 are different are produced, and a loss of 1.55 μm in wavelength is measured by bending each sample to φ20 mm. Each measured value was plotted in FIG. 3 to obtain an approximate line. As shown in FIG. 3, when the number of pores 6 is 4, the bending loss is 1 dB / m or less in the illustrated region where the diameter of the pores 6 (inner diameter of the pores) is 3 μm or more. This value of 1 dB / m cannot be achieved with a conventional optical fiber. Further, when considering cable formation and optical fiber laying, a practical advantage will be obtained if a bending loss characteristic of 1 dB / m or less is achieved. Therefore, the diameter of the pores 6 is preferably 3 μm or more.
[0027]
When the effective area is 180 μm ² or less, the mode field diameter is 16 μm or less. At this time, if the diameter of the pores 6 is 16 μm or more, the envelope surface that envelops the inner surface of the pores 6 on the center core 1 side in the circumferential direction becomes smaller than the diameter of the center core 1. Then, the light confinement effect is weakened, and the improvement of the bending characteristics is not remarkable. Therefore, the diameter of the pores 6 is preferably 16 μm or less.
[0028]
In the high-stress resistant optical fiber according to the present invention, the distance between the innermost surface portion of the pores 6 (the location closest to the central axis among the inner surfaces of the pores 6) and the central axis is equal to or less than the mode field diameter of the optical fiber. There should be. The reason is that in order for the effect of reducing the effective refractive index of the cladding region 5 due to the presence of the pores 6 to effectively improve the bending characteristics, the location of the pores 6 should be as close to the center core 1 as possible. This is because the existence limit of the pores 6 for improving the bending characteristic is a distance equivalent to the mode field diameter. Accordingly, when the innermost surface portion of each pore 6 is enveloped in the circumferential direction around the central axis to draw an envelope surface, the diameter of the envelope surface is larger than the diameter of the center core 1 and smaller than the mode field diameter. Determine the position. The condition that the envelope surface diameter is smaller than the mode field diameter is satisfied if the envelope surface is radially inward from the outer periphery of the side core 2.
[0029]
Further, in the high stress optical fiber according to the present invention, an effective area 110 [mu] m ² or more, preferably set to 180 [mu] m ² or less. If the effective cross-sectional area is 110 μm ² or less, sufficient bending characteristics can be obtained even with the conventional technique. If the effective area is 110 μm ² or more, the prior art becomes insufficient, and the present invention must be applied. S. C. When transmitting in the L band, it is necessary to set the cutoff wavelength to 1450 nm or less. On the other hand, if the effective area is 180 μm ² or more, the cutoff wavelength cannot be made 1450 nm or less. Therefore, the effective area is preferably 180 μm ² or less.
[0030]
Next, the manufacturing method of the high stress-resistant optical fiber of this invention is demonstrated.
[0031]
In producing the high-stress resistant optical fiber of the present invention, first, a quartz preform as a fiber preform was produced by the VAD method. Specifically, a soot preform (not shown) having a diameter of 110 mm and a length of 1 m was produced in the same manner as manufacturing an ordinary single mode optical fiber preform. Here, a germanium additive for increasing the refractive index of quartz is put in a soot region serving as a center core, as in a normal single mode optical fiber. In addition, a fluorine additive for lowering the refractive index of quartz is put in the soot region which becomes the side core.
[0032]
The soot preform was sintered in an atmosphere having a dehydrating effect such as chlorine to obtain a high-purity transparent vitrified base material (not shown) having an outer diameter of 60 mm and a length of 40 cm. Next, a hole having a diameter of 2.5 mm, which is a line-symmetrical and equiangular interval with the central axis of the core of the high-purity transparent vitrified base material as a symmetry axis, was processed by a grinding method. The base material after grinding is shown in FIG.
[0033]
As shown in FIG. 4, the vitrified base material includes a base material center core portion 7 to be the center core 1 after drawing, a base material side core portion 8 to be the side core 2, and glass regions of the base material clad portion 9 to be the cladding 3. And a pore portion 10 which becomes the pore 6.
[0034]
Next, one end of the base material of FIG. 4 was sealed, and a quartz dummy tube having an outer diameter of 60 mm and an inner diameter of 50 mm was connected to the other end to obtain a drawing preform (not shown). Further, a gas input portion for filling a gas containing chlorine into the grinding hole of the drawing preform was connected to the end face of the drawing preform.
[0035]
Next, the drawing process of the drawing preform will be described. When drawing, if the internal pressure in the drawing preform is too low, the pores 10 will be crushed, resulting in a fiber without the pores 6 after fiberization. If the internal pressure is too high, the ratio of the pores 6 in the fiber after fiber formation increases, and if the limit point of the internal pressure determined from the drawing tension and drawing speed is exceeded, the pores 10 burst during drawing to form a fiber. Is impossible. Therefore, the present inventor derived from experiments that the optimum internal pressure is about 1.5 kPa when trying to form the pores 6 having a desired diameter in the fiber from the relationship between the diameter of the pores 10 and the drawing internal pressure. It was. Drawing was performed with this optimum internal pressure set. As a result, pores 6 having a diameter of 7 μm were formed in the manufactured optical fiber of FIG.
[0036]
Thus, a high-stress resistant optical fiber 10 km of the present invention was produced. The loss of this optical fiber was 0.41 dB / km at a wavelength of 1.31 μm and 0.25 dB / km at a wavelength of 1.55 μm. When the loss factor was analyzed, the structural irregularity loss was 0.05 dB, which pushed up the whole including other losses. This structural irregularity loss is due to the processing accuracy of the preform and can be improved by improving the processing method. When the bending loss characteristic of this optical fiber was measured, the amount of increase in bending loss at a wavelength of 1.55 μm and φ20 mm was 0.6 dB / m, which is 1/100 compared to a conventional optical fiber having an effective core area of 170 μm ^2. It was the following and very small value. Further, the mode field diameter at the cut-off wavelength and the wavelength of 1.55 μm was measured and found to be 1.44 μm and 14.9 μm, respectively, and these values are in the practical range and are values with no problem. The OH absorption loss at a wavelength of 1.39 μm was 1.5 dB / km, which was the level of a normal single mode optical fiber. Moreover, the Weibull intensity was 60 to 70 N, the environmental constant n = 21, and a result equivalent to that of a normal optical fiber was obtained.
[0037]
【The invention's effect】
The present invention exhibits the following excellent effects.
[0038]
(1) A high stress-resistant optical fiber having high stress resistance while increasing the effective cross-sectional area can be obtained.
[Brief description of the drawings]
FIG. 1 is a refractive index distribution diagram in a radial direction in a core region of a high stress-resistant optical fiber showing an embodiment of the present invention. The horizontal axis represents the distance, and the vertical axis represents the relative refractive index difference with reference to the clad 5.
FIG. 2 is a cross-sectional view of a high stress resistant optical fiber showing an embodiment of the present invention.
FIG. 3 is a bending loss characteristic diagram with respect to the hole inner diameter of the high stress-resistant optical fiber of the present invention.
FIGS. 4A and 4B are structural views of a preform as a base material of the high-stress resistant optical fiber of the present invention, in which FIG. 4A is a side sectional view and FIG. 4B is a transverse sectional view.
[Explanation of symbols]
1 Center Core 2 Side Core 3 Clad 4 Core Region 5 Clad Region 6 Pore

Claims (1)

A side core having a lower refractive index than the center core is disposed around the center core positioned at the central axis, and a clad having a refractive index smaller than the center core and larger than the side core is disposed around the side core to form a W-type refraction. forming the rate distribution structure, the effective area is from 110 [mu] m ² to 180 [mu] m ^2, to form four or more even number of pores located axially symmetrically each equal circumferential angle about said central axis, said A high-stress-resistant optical fiber characterized in that the inner diameter of the hole is 3 μm or more and 16 μm or less, and the distance between the hole and the central axis is smaller than the mode field diameter .

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