JP2004226541A - High anti-stress optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high stress resistant optical fiber having an excellent bending characteristic. <P>SOLUTION: The high stress resistant optical fiber has a central core section 1, a first ring section 2 disposed on the outer periphery of the central core section 1, a second ring section 3 disposed on the outer periphery of the first ring section 2, a third ring section 4 disposed on the outer periphery of the second ring section 3, and a clad 5 disposed on the outer periphery of the third ring section 4, and has a refractive index profile in which the relations among the specific refractive index difference n0 of the central core section 1, the specific refractive index difference n1 of the first ring section 2, the specific refractive index difference n2 of the second ring section 3, the specific refractive index difference n3 of the third ring section 4 and the specific refractive index difference n4 of the clad 5 are n0>n2 and n2>n1, and n3≤n4 and n1>n4. The optical fiber has an even number of pieces of ≥4 vacancies 8 existing axisymmetrically at every equal circumference angles around the central axis. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical fiber having a large effective area, and more particularly to a high stress resistant optical fiber having excellent bending characteristics.
[0002]
[Prior art]
2. Description of the Related Art In recent years, with the rapid spread of the Internet and the like, the information capacity has increased, and a demand for a large capacity of an information transmission medium has increased. A wavelength multiplexing (hereinafter, referred to as WDM) transmission method is most promising among technologies corresponding to the increase in capacity. In the WDM method, a plurality of signal lights can be transmitted by one optical fiber, so that the transmission capacity can be increased at a stretch by about 100 times. Therefore, introduction to long-distance, large-capacity transmission lines such as optical submarine cable systems connecting continents has been promoted, and the stage of practical use has been reached.
[0003]
One of the technical backgrounds in which the WDM technology has rapidly started is the improvement of the optical amplification technology. An erbium-doped optical fiber amplifier (EDFA), which is one of the optical amplification technologies, can amplify attenuated light in the 1.55 μm band up to about 1000 times. It works to compensate for the loss in the transmission line. Trans-Pacific optical submarine cable systems using EDFA (TPC-5CN, China-US, etc.) have already been put into practical use, and have realized 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 multiplexed wavelengths. However, since the signal power entering the optical fiber increases, the possibility that a non-linear phenomenon occurs 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 a fiber in which the effective area of the optical fiber is increased and the signal light power density in the optical fiber is reduced for a long-distance transmission line.
[0004]
This optical fiber is not only used for submarine cables, but is also expected to be used on land, and a system is currently being constructed. On land, optical fibers are generally taped and used.
[0005]
As described above, in recent years, with the development of the optical amplification technology and the WDM technology, the power of light incident on an optical fiber has been increased, so that various nonlinear effect phenomena are likely to occur. When the self-phase modulation phenomenon occurs, the pulse signal waveform in the optical fiber is distorted, and the transmission capacity is limited. Also, a Brillouin scattering phenomenon, which is one of the nonlinear effect phenomena, is likely to occur. When the Brillouin scattering phenomenon occurs, the incident power of the optical fiber is saturated. As described above, when the nonlinear effect phenomenon occurs, the transmission characteristics of light propagating in the optical fiber are deteriorated.
[0006]
Further, since the zero-dispersion wavelength of the conventional single mode fiber is longer than 1.3 μm, there is no optical fiber having a large anomalous dispersion (positive dispersion) at 1.3 μm.
[0007]
As a novel optical fiber that solves the above-mentioned problem of the non-linear phenomenon, a photonic crystal fiber (PCF, also referred to as a photonic fiber) has recently attracted attention. The PCF is an optical fiber in which a photonic crystal structure is provided in a clad. The photonic crystal structure is a periodic structure having a refractive index. Specifically, a photonic band gap (Photonic Band Gap; a forbidden band of light) is formed by providing a honeycomb-shaped space such as a honeycomb in a clad. 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 using a PBG structure as a waveguide principle.
[0008]
Recently, although the optical fiber is not an optical fiber having a complete PBG structure, the pores are present in the clad near the core of the optical fiber having a relative refractive index difference due to a difference in glass composition. By increasing the relative refractive index difference between the core and the clad by lowering the effective refractive index, a holey fiber (HF) having a characteristic not obtained conventionally has been reported. For example, Non-Patent Document 3 discloses a hole-added type holey fiber in which four holes are provided in a clad near a core of an optical fiber having a structure of a normal single mode fiber, and a relative refractive index difference between the core and the clad. An optical fiber having a single mode operation in the 1.2 μm band is disclosed by enlarging.
[0009]
Patent document 1 is a patent document relating to a photonic fiber.
[0010]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2000-296440 [Non-Patent Document 1]
"Photonic band gap guidance inoptic fiber" Knight et al., Science 282, 1476, 1998 [Non-Patent Document 2]
"Single-Mode Photonic band gap of guidance in Light in Air" Cregan et al., Science 285, 1537, 1999 [Non-Patent Document 3]
"Novel hole-assisted lightguide fiber exchanging large anomalous dispersion and low loss below 1 dB / km" Hasegawa et al., OFC 2001 PD5-1, 2001.
[Problems to be solved by the invention]
However, increasing the effective area also increases the mode field diameter of the fiber. Therefore, when an optical fiber (for example, 70 μm ² ) having an increased effective core cross-sectional area is wound 1 m around a cylinder having a diameter of 20 mm, an increase in signal wavelength light loss is as large as 20 dB / m. At present, in a 1.3 μm single mode optical fiber, which is most frequently used in an optical transmission system, 2 to 10 dB / m is generally used. Therefore, an optical fiber having a large effective core area is extremely resistant to bending. Large loss. Therefore, there is a problem that the loss increases when the optical fiber is taped.
[0012]
In view of the above, a high-stress-resistant optical fiber having a large effective area and a high stress resistance is desired.
[0013]
Then, an object of the present invention is to solve the above-mentioned problems and to provide a high stress-resistant optical fiber having excellent bending characteristics.
[0014]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a central core portion located on a central axis, a first ring portion provided on an outer periphery of the central core portion, and a second ring portion provided on an outer periphery of the first ring portion. A ring portion, a third ring portion provided on the outer periphery of the second ring portion, and a clad provided on the outer periphery of the third ring portion, and a relative refractive index difference n0 of the central core portion; Relationship between relative refractive index difference n1 of the first ring portion, relative refractive index difference n2 of the second ring portion, relative refractive index difference n3 of the third ring portion, and relative refractive index difference n4 of the cladding. Have a refractive index profile of n0> n2 and n2> n1 and n3 ≦ n4 and n1> n4, and four or more even-numbered vacancies positioned axially symmetrically at equal circumferential angles around the central axis. It has a hole.
[0015]
The inside diameter of the hole may be 3 μm or more.
[0016]
The holes may be present in the third ring portion.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.
[0018]
FIG. 1 shows a refractive index distribution of a high stress resistant optical fiber according to the present invention. This optical fiber has a quadruple clad type refractive index profile. When viewed from the refractive index profile of FIG. 1, the high stress resistant optical fiber according to the present invention includes a central core 1 located on the central axis of the optical fiber, and a first ring provided on the outer periphery of the central core 1. Portion 2, a second ring portion 3 provided on the outer periphery of the first ring portion 2, a third ring portion 4 provided on the outer periphery of the second ring portion 3, and an outer periphery of the third ring portion 4. And a clad 5 provided on the substrate. This refractive index profile is such that the relative refractive index difference of the central core portion is n0, the relative refractive index difference of the first ring portion is n1, and the relative refractive index difference of the second ring portion is n2 based on the refractive index of the cladding. When the relative refractive index difference of the third ring portion is n3 and the relative refractive index difference of the cladding is n4, n0> n2, n2> n1, n3 ≦ n4, and n1> n4. is there.
[0019]
The names and signs of the respective parts given to the refractive index profile are directly used as the names and signs representing the inner layer structure of the optical fiber.
[0020]
The central core part 1, the first ring part 2, and the second ring part 3 of the high stress-resistant optical fiber according to the present invention are layers (GeO ₂ —SiO ₂ ) of pure silica doped with germanium (Ge) for increasing the refractive index. Is formed by Further, the third ring portion 4 of the optical fiber according to the present invention is formed of a layer (F-SiO ₂ ) in which pure silica is doped with fluorine (F) for lowering the refractive index.
[0021]
In the optical fiber realizing the refractive index profile of FIG. 1, for example, in order to obtain an optical fiber having an effective sectional area of 70 μm ² , the relative refractive index difference n0 of the central core portion 1 is 0.54%, and the first ring portion. The relative refractive index difference n1 of the second ring portion 3 is 0.018%, the relative refractive index difference n2 of the second ring portion 3 is 0.118%, and the relative refractive index difference n3 of the third ring portion 4 is -0.046%. The diameter a of the core 1 is 7.2 μm, the diameter b of the first ring 2 is 13.6 μm, the diameter c of the second ring 3 is 33.6 μm, and the diameter d of the third ring 4 is 55.8 μm. I do.
[0022]
FIG. 2 shows a cross section (transverse cross section) perpendicular to the central axis of the high stress resistant optical fiber according to the present invention. As shown, the optical fiber includes a core region 6 through which light passes most, and a cladding region 7 through which light does not pass as compared with the core region 6. The central core portion 1, the first ring portion 2, the second ring portion 3, and the third ring portion 4 whose refractive index profiles are shown in FIG. 1 can be considered to almost overlap the core region 6 because light passes well. In the core region 6, the third ring portion 4 has a plurality of holes (also called pores) 8 so as to surround the central axis of the optical fiber (the center of the central core portion 1). Although not shown, these pores 8 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 fiber.
[0023]
The number of the pores 8 is an even number of four or more. Two pores 8 are arranged at line-symmetric positions with the central axis as a symmetry axis, and the pores 8 are arranged at equal intervals in the circumferential direction. With this arrangement, the pores 8 are axially symmetric about the central axis of the core region 6 at equal circumferential angles. Here, it is assumed that there are six pores 8. Therefore, in the optical fiber cross section of FIG. 2, two of these six pores 8 are located at equal distances in the radial direction across the central axis, and the pores 8 are located at equal intervals in the circumferential direction. .
[0024]
In the above-mentioned optical fiber having an effective area of 70 μm ² , for example, the diameter of the pore 8 is 5 μm, and the center of the pore 8 is located on a circumference having a radius of 23 μm from the central axis. The pores 8 are filled with air or an inert gas, and have a refractive index of 1.
[0025]
The reason why four or more even-numbered pores 8 are arranged axially symmetrically at equal circumferential angles around the central axis will be described.
[0026]
First, when the number of the pores 8 is two and the intervals are equal, the pores 8 exist only on a straight line passing through the center on the optical fiber cross section. There is a difference between the straight line and the straight line having a positional relationship of 90 ° in the effect of reducing the effective refractive index of the clad 5 by the pores 8. For this reason, the characteristics of the optical fiber become the characteristics of a quasi-polarization preserving fiber, and the polarization dispersion characteristics are degraded. Further, when the number of the pores 8 becomes an odd number, the effective cladding 5 formed by the pores 8 is formed on two straight lines passing through the central axis on the cross section of the optical fiber, no matter how the pores 8 are arranged. The refractive index distribution becomes asymmetric, and the polarization dispersion characteristic also deteriorates. If the pores 8 are arranged at unequal intervals, the power distribution of the incident light is decentered, and anisotropic stress is applied to the core region 6, thereby deteriorating the PMD characteristics.
[0027]
On the other hand, when four or more even-numbered pores 8 are arranged axially symmetrically at equal circumferential angles around the central axis, the pores are aligned with two straight lines passing through the central axis on the optical fiber cross section. 8, the effective refractive index distribution of the clad 5 can be made symmetrical, so that the polarization dispersion characteristics can be improved. Further, since the anisotropy is eliminated, the number of the pores 8 may be six.
[0028]
The diameter of the pore 8 is preferably 3 μm or more, and the position of the pore 8 is preferably in the third ring portion 4. The reason will be described. A plurality of samples of the optical fiber of the present invention in which the number of the pores 8 is four (arranged at every circumferential angle of 90 °), the diameter of the pores 8 is made different, and each sample is given a bend of φ20 mm to obtain a wavelength The loss that occurred in the 1.55 μm light was measured, and each measured value was plotted in FIG. 3 to obtain an approximate line. As shown in FIG. 3, when the number of the pores 8 is four, the bending loss is 4 dB / m or less in the illustrated region where the diameter of the pores 8 (the inner diameter of the pores) is 3 μm or more. This value of 4 dB / m is a value that cannot be achieved with a conventional optical fiber. In addition, in consideration of cable construction and optical fiber laying, if a bending loss characteristic of 4 dB / m or less is achieved, there is a practical advantage. Therefore, it is preferable that the diameter of the pore 8 is 3 μm or more, and the position of the pore 8 is within the third ring portion 4.
[0029]
The pores 8 are preferably provided in the third ring portion 4. The first reason is that if the pores 8 are located between the central core portion 1 and the second ring portion 3, the effective refractive index is greatly reduced, so that a low dispersion value cannot be achieved. By providing the pores 8 in the third ring portion 4, a low dispersion value can be achieved. The second reason is that the existence limit of the pores 8 for practically improving the bending characteristics is at a distance equivalent to the mode field diameter (not shown). Further, if a part of the pores 8 is applied to the second ring portion 3 or the cladding region 7, an adverse effect such as a shift of the cutoff wavelength to a longer wavelength is considered.
[0030]
Next, a method for manufacturing the high stress resistant optical fiber of the present invention will be described.
[0031]
In manufacturing the high stress-resistant optical fiber of the present invention, first, a quartz preform as a fiber preform was produced by a VAD method. Specifically, a soot preform (not shown) having a diameter of 110 mm and a length of 1 m was produced in a manner similar to that for producing a normal single-mode optical fiber preform. Here, similarly to a normal single mode optical fiber, a soot region serving as a central core portion, a first ring portion, and a second ring portion contains a germanium additive for increasing the refractive index of quartz. In addition, a fluorine additive for lowering the refractive index of quartz is added to the soot region serving as the third ring portion.
[0032]
The soot preform was sintered in an atmosphere having a dehydration 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, holes having a diameter of 2.5 mm serving as pores were formed at regular intervals with respect to the center axis of the core of the high-purity transparent vitrified base material as a symmetric axis by grinding. The base material after the grinding is shown in FIG.
[0033]
As shown in FIG. 4, the vitrified base material includes a base material center core portion 9 serving as the center core portion 1 after drawing, a base material first ring portion 10 serving as the first ring portion 2, and a second ring portion 3. The respective glass regions of the base material second ring portion 11, the base material third ring portion 12 serving as the third ring portion 4, the base material cladding portion 13 serving as the clad 5, and the pore portion 14 serving as the pore 8 (FIG. a) is omitted).
[0034]
Next, one end of the base material shown in 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 inlet for connecting a gas containing chlorine to the grinding holes of the drawing preform was connected to an end face of the drawing preform.
[0035]
Subsequently, the drawing step of the drawing preform will be described. At the time of drawing, if the internal pressure in the drawing preform is too low, the pores 14 are crushed, resulting in a fiber having no pores 8 after fiberization. On the other hand, if the internal pressure is too high, the proportion of the pores 8 in the fiber after fiberization becomes large. Becomes impossible. Therefore, the present inventor has found through experiments that the optimum internal pressure for forming the pores 8 having a desired diameter in the fiber is about 1.5 kPa from the relationship between the diameter of the pores 14 and the internal drawing pressure. Was. Drawing was performed by setting the optimum internal pressure. As a result, pores 8 of φ5 μm were formed in the manufactured optical fiber of FIG.
[0036]
As described above, 10 km of the high stress resistant optical fiber 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 0.22 dB / km at a wavelength of 1.55 μm. Analysis of the loss factors revealed that 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 characteristics of this optical fiber were measured, the increase in bending loss at φ1.5 mm at a wavelength of 1.55 μm was 2.0 dB / m, which was 1/10 that of a conventional optical fiber having an effective core area of 70 μm ^2. It was a very small value below. Also, the cutoff wavelength and the mode field diameter at a wavelength of 1.55 μm were measured. The values were 1.44 μm and 9.65 m, respectively. These values were within the practical range, and were no problem. The dispersion value at a wavelength of 1.55 μm was 8 PS / nm / km. 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. In addition, the Weibull strength was 60 to 70 N, and the environmental constant was n = 21, and a result equivalent to that of a normal optical fiber was obtained.
[0037]
When the prototype optical fiber was taped, no increase in loss was observed.
[0038]
【The invention's effect】
The present invention exhibits the following excellent effects.
[0039]
(1) A high stress resistant optical fiber having a high stress resistance while increasing the effective area is obtained.
[Brief description of the drawings]
FIG. 1 is a radial refractive index distribution diagram in a core region of a high stress resistant optical fiber according to an embodiment of the present invention. The horizontal axis represents the distance, and the vertical axis represents the relative refractive index difference based on the clad 5.
FIG. 2 is a cross-sectional view of a high stress resistant optical fiber showing one embodiment of the present invention.
FIG. 3 is a graph showing a bending loss characteristic with respect to an inner diameter of a hole in a high stress-resistant optical fiber of the present invention.
FIGS. 4A and 4B are structural views of a preform as a preform of the high stress-resistant optical fiber of the present invention, in which FIG. 4A is a side sectional view, and FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Central core part 2 First ring part 3 Second ring part 4 Third ring part 5 Cladding 6 Core region 7 Cladding region 8 Pores (holes)

Claims (3)

A central core portion located on the central axis, a first ring portion provided on the outer periphery of the central core portion, a second ring portion provided on the outer periphery of the first ring portion, and an outer periphery of the second ring portion And a cladding provided on the outer periphery of the third ring portion. The relative refractive index difference n0 of the central core portion and the relative refractive index difference n1 of the first ring portion are provided. And the relationship among the relative refractive index difference n2 of the second ring portion, the relative refractive index difference n3 of the third ring portion, and the relative refractive index difference n4 of the cladding is n0> n2 and n2> n1 and n3 ≦ A high stress resistance, having a refractive index profile of n4 and n1> n4, and having four or more even-numbered holes symmetrically positioned at equal circumferential angles around the central axis. Optical fiber. 2. The high stress resistant optical fiber according to claim 1, wherein the inner diameter of the hole is 3 μm or more. The high stress resistant optical fiber according to claim 1, wherein the hole exists in the third ring portion.

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