JP2005345956A - Dispersion slope compensation type dispersion compensated fiber and dispersion slope compensation type dispersion compensated fiber module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dispersion slope compensation type dispersion compensated fiber, substantially operating as a single mode fiber, even if the cut-off wavelength in a higher order mode is longer than the wavelength to be used, and to provide a dispersion slope compensation type dispersion compensated fiber module that uses the same. <P>SOLUTION: The dispersion slope compensation type dispersion compensated fiber compensates for cumulative chromatic dispersion and the cumulative chromatic dispersion slope of an optical fiber transmission path, and is provided with a loss layer 5 for selectively giving a loss only to the higher order modes inside an optical fiber, and operates substantially as a single-mode fiber, even if the cut-off wavelength in the higher order mode is longer than the wavelength to be used. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a dispersion slope compensation type dispersion compensating fiber that compensates the accumulated chromatic dispersion and accumulated chromatic dispersion slope of an optical fiber transmission line, and in particular, a dispersion that operates substantially as a single mode fiber regardless of a cutoff wavelength of a higher order mode. The present invention relates to a slope compensation type dispersion compensation fiber and a dispersion slope compensation type dispersion compensation fiber module using the same.

In recent years, the transmission capacity of optical communication systems has been increased by the wavelength division multiplexing communication system. In this optical communication system, in order to increase the transmission capacity, it is effective to increase the transmission speed per channel and expand the wavelength range used.
In this optical communication system, the higher the transmission speed, the smaller the accumulated chromatic dispersion amount allowed in the transmission path. Therefore, in order to increase the transmission speed, it is necessary to compensate for the accumulated chromatic dispersion of the transmission line. Further, in order to expand the used wavelength band, it is necessary to reduce the accumulated chromatic dispersion over the entire used wavelength band, so it is necessary to simultaneously compensate the accumulated chromatic dispersion slope of the transmission path. In order to compensate for such cumulative chromatic dispersion and cumulative chromatic dispersion slope, the fiber connected to the transmission line is called a dispersion slope compensation type dispersion compensation fiber.

Most of single-mode optical fibers for wavelength 1.3 μm transmission and non-zero dispersion shifted fibers range from S-Band (1460 nm to 1530 nm) to U-Band (1625 nm to 1675 nm), with positive chromatic dispersion and positive chromatic dispersion slope. have. Similarly, the 1.55 μm wavelength transmission dispersion shifted fiber has positive chromatic dispersion and positive chromatic dispersion slope in L-band (1565 nm to 1625 nm) and U-band.
Therefore, in order to compensate the accumulated chromatic dispersion and accumulated chromatic dispersion slope of the transmission line formed by these fibers, a dispersion slope compensation type dispersion compensating fiber having negative chromatic dispersion and negative chromatic dispersion slope is required. Become.

The dispersion slope compensation type dispersion compensating fiber is required to have a low loss and a value obtained by dividing the chromatic dispersion slope by the chromatic dispersion value in order to reduce the cumulative dispersion of the transmission line over the entire wavelength region used ( Hereinafter, RDS (RDS: Relative Dispersion Slope) must match the fiber used for the transmission path. Furthermore, in order to function as a single mode optical fiber, the cut-off wavelength of the higher order mode needs to be shorter than the wavelength used in the usage environment.
Further, the dispersion slope compensation type dispersion compensation fiber is often wound as a dispersion slope compensation type dispersion compensation fiber module by being wound around a reel having a diameter of about 30 mm to 100 mm. The characteristics required for the dispersion slope compensation type dispersion compensating fiber for the module include the following characteristics.

First, it is preferable that the dispersion compensation amount per unit loss amount (hereinafter referred to as FOM (Figure Of Merit)) is large so that the loss of the dispersion slope compensation type dispersion compensation fiber module as a whole becomes small. It is preferable to increase the absolute values of chromatic dispersion and chromatic dispersion slope while suppressing loss degradation.
Further, in order to reduce the size of the housing of the dispersion slope compensation type dispersion compensating fiber module, as described above, while keeping the RDS value constant, increasing the absolute value of the chromatic dispersion and the chromatic dispersion slope, The length of the fiber can be shortened, which is preferable. Further, if the bending loss of the dispersion slope compensation type dispersion compensating fiber is reduced, it is wound around the reel with a small radius, which is preferable for downsizing.
In addition, dispersion slope compensation type dispersion compensating fibers for some transmission fibers such as dispersion shifted fibers have a large RDS value, so that practical bending loss and chromatic dispersion are achieved while satisfying practical cutoff wavelength conditions. It is difficult to get the value. Therefore, a method that can freely design the RDS value in a wide range is required.
In order to satisfy such a requirement, a dispersion slope compensation type dispersion compensating fiber in which a second core part, a third core part, etc. having different refractive indices are provided outside the core part (central core part) of the fiber is usually used. (For example, see Patent Document 1)
JP 2003-255170 A

  However, as described above, if the second core portion, the third core portion, etc. are provided on the outer periphery of the fiber core portion, the cutoff wavelength of the higher order mode becomes longer, and the higher order mode propagates through the dispersion compensating fiber. As a result, there is a problem that the signal waveform deteriorates due to mode dispersion, and accurate transmission cannot be performed.

  DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and a dispersion slope compensation type dispersion compensation fiber that operates substantially as a single mode fiber even when a cutoff wavelength of a higher-order mode is longer than a use wavelength, and a dispersion slope compensation type using the same An object is to provide a dispersion compensating fiber module.

In order to achieve the above object, the present invention provides a dispersion slope compensation type dispersion compensating fiber that compensates for the accumulated chromatic dispersion and accumulated chromatic dispersion slope of an optical fiber transmission line, and is selective only to a higher-order mode in the optical fiber. A dispersion slope compensation type dispersion compensating fiber is provided, which is provided with a loss layer that gives loss to the fiber, and operates substantially as a single mode fiber even when the cutoff wavelength of the higher-order mode is longer than the used wavelength.
In the dispersion slope compensation type dispersion compensating fiber of the present invention, at least one of wavelength ranges of a wavelength range of 1460 nm to 1530 nm, a wavelength range of 1530 nm to 1565 nm, a wavelength range of 1565 nm to 1625 nm, and a wavelength range of 1625 nm to 1675 nm. It is preferable to have negative chromatic dispersion and a negative chromatic dispersion slope in one or more wavelength bands.
In the dispersion slope compensating dispersion compensating fiber of the present invention, the relative refractive index difference is set to 0.010 to 0.040, the outer core has a central core portion in the range of 2 to 6 μm, and the outer periphery of the central core portion. A second core portion having a refractive index difference of −0.020 to 0.005 and an outer diameter of 5 to 15 μm and an outer periphery of the second core portion, and a relative refractive index difference of 0.003 to 0 It is preferable to have a third core portion having an outer diameter of 10 to 25 μm and a clad portion provided on the outer periphery of the third core portion.
In the dispersion slope compensation type dispersion compensating fiber of the present invention, it is preferable that the difference in loss between the LP01 mode and the higher order mode is 5 dB / km or more.
In the dispersion slope compensation type dispersion compensating fiber of the present invention, it is preferable that the loss layer is provided in the clad portion.
In the dispersion slope compensation type dispersion compensating fiber of the present invention, it is preferable that the loss layer contains at least boron.
In the dispersion slope compensation type dispersion compensating fiber of the present invention, the loss layer preferably contains at least one metal dopant selected from the group consisting of cobalt, chromium, copper, iron, nickel, manganese, and vanadium.
In the wavelength band to be used, it is preferable that the loss of the loss layer is larger as the wavelength is shorter.

  The present invention also provides a dispersion slope compensation type dispersion compensation fiber module comprising the dispersion slope compensation type dispersion compensation fiber according to the present invention described above.

The dispersion slope compensating dispersion compensating fiber of the present invention is provided with a loss layer that selectively gives loss only to the higher order mode in the optical fiber, and is substantially effective even if the cutoff wavelength of the higher order mode is longer than the use wavelength. Therefore, even a dispersion slope compensation type dispersion compensating fiber having a long cutoff wavelength can be used practically.
Further, since a fiber having a large dispersion and dispersion slope can be obtained, the length of the fiber necessary for dispersion compensation can be shortened, and a small and inexpensive dispersion slope compensation type dispersion compensation fiber module can be provided.
Further, since the bending loss in the fundamental mode can be reduced, it can be wound around a small-diameter reel for use as a module, and a smaller dispersion slope compensation type dispersion compensating fiber module can be provided.
Further, practical bending loss and chromatic dispersion values can be obtained while satisfying the practical cutoff wavelength condition, and the RDS value can be freely designed in a wide range.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1 and 2 are diagrams showing an embodiment of a dispersion slope compensating dispersion compensating fiber according to the present invention. FIG. 1 is a sectional view of the dispersion slope compensating dispersion compensating fiber, and FIG. 2 is a dispersion slope compensating dispersion compensating fiber. It is a figure which shows the refractive index distribution of the cross section of a fiber. In FIG. 1, reference numeral 1 denotes a central core portion, 2 denotes a second core portion, 3 denotes a third core portion, 4 denotes a cladding portion, and 5 denotes a loss layer.

  This dispersion slope compensation type dispersion compensating fiber includes a core and a clad 4 provided on the outer periphery thereof. The core includes a central core portion 1 and a second core portion 2 provided on the outer periphery of the central core portion 1. The third core portion 3 is provided on the outer periphery of the second core portion 2. A clad portion 4 is provided on the outer periphery of the third core portion 3, and a ring-shaped loss layer 5 that selectively gives a loss only to the higher-order mode is provided on the clad portion 4. Further, the dispersion slope compensation type dispersion compensating fiber has at least one of wavelength ranges of a wavelength range of 1460 nm to 1530 nm, a wavelength range of 1530 nm to 1565 nm, a wavelength range of 1565 nm to 1625 nm, and a wavelength range of 1625 nm to 1675 nm. It is preferable to have negative chromatic dispersion and a negative chromatic dispersion slope in one or more wavelength bands.

  The central core portion 1 is made of glass such as germanium-doped quartz, has a relative refractive index difference Δ1 in the range of 0.010 to 0.030, and an outer diameter 2a in the range of 2 to 6 μm.

  The second core portion 2 is located outside the central core portion 1, has a lower refractive index than the central core portion 1, has a relative refractive index difference Δ2 in the range of −0.020 to 0.005, and has an outer diameter. 2b is in the range of 5 to 15 μm.

  The third core portion 3 is located outside the second core portion 2 and has a refractive index lower than that of the central core portion and higher than that of the second core portion 2, and a relative refractive index difference Δ3 of 0.003 to 0.00. The outer diameter 2c is in the range of 10 to 25 μm.

  The clad part 4 is located outside the third core part 3, and its refractive index is lower than that of the central core part 1 and the third core part 3 and higher than that of the second core part 2.

  A ring-shaped loss layer 5 is formed inside the dispersion slope compensation type dispersion compensating fiber. The loss layer 5 does not give a loss to the LP01 mode, but gives a loss only to the higher order mode. The loss amount of the loss layer 5 is about 20 dB to 1000000 dB / km.

  The amount of loss in each mode by the loss layer 5 can be calculated based on the following equation (1).

In Equation (1), P _l is a propagation loss of each mode, r is a distance from the core center, P _m is a material loss at a point r from the core center, R is an electromagnetic field transverse component of the LP mode, and K Okamoto, “Comparison of calculated and measured impulse responses of optical fibers”, Applied Optics, Vol. 18, pp.2199-2206, 1979. Using this equation (1), the position of the loss layer 5 and the amount of loss are determined so that only the higher-order mode receives loss.

  In such a dispersion slope compensation type dispersion compensating fiber, only the higher-order mode receives a loss due to the loss layer 5. The loss is such that the difference between the loss of the LP01 mode and the higher order mode is 5 dB / km or more, preferably 10 dB / km or more, more preferably 20 dB or more, and most preferably 30 dB / km or more.

  A typical dopant used in the loss layer 5 is boron. Boron-doped quartz glass is stable and has absorption in the 1.55 μm wavelength band, and therefore can be suitably used as a dopant material for forming the loss layer 5. However, if the loss layer 5 is large or if the boron content is too large, the difference in thermal expansion coefficient from that of quartz glass increases, causing damage to the optical fiber preform. Therefore, when boron-doped quartz glass is used as the loss layer 5, it is desirable that the loss layer 5 has a boron content of 20 mol% or less, and the loss layer 5 has a thickness of 10 μm or less.

  In addition to boron, the same effect can be obtained by doping the loss layer 5 with at least one metal dopant selected from the group consisting of cobalt, chromium, copper, iron, nickel, manganese, and vanadium. Among the metal dopants, cobalt and vanadium are preferable. If the dopant concentration of the metal dopant is too high, the viscosity of the glass increases, causing damage to the optical fiber preform. Therefore, the concentration of the metal dopant is desirably 2000 ppm or less.

  As a result, in this dispersion slope compensation type dispersion compensating fiber, propagation of higher-order modes is suppressed, and substantially functions as a single mode fiber. Therefore, when designing and manufacturing dispersion slope compensation type dispersion compensation fiber, it is not necessary to consider the cutoff wavelength of higher order mode, increasing the absolute value of chromatic dispersion and chromatic dispersion slope, and reducing bending loss. The value of RDS can be changed over a wide range, and the characteristics of the dispersion slope compensation type dispersion compensation fiber and the dispersion slope compensation type dispersion compensation fiber module configured using the fiber can be improved.

[Example 1]
Using the MCVD method, a dispersion slope compensation type dispersion compensating fiber having the structure shown in FIG. 1 and having the structural parameters shown in Example 1 in Table 1 was manufactured. This dispersion slope compensation type dispersion compensating fiber can transmit LP01, LP11, and LP02 modes at a wavelength of 1.59 μm. The optical characteristics of the LP01 mode are shown in Example 1 in Table 2.

In this dispersion slope compensation type dispersion compensating fiber, diboron trioxide is added at a concentration of 5.4 mol% to a cladding portion having a radius of 11.0 μm to 17.0 μm from the core center, and 50 dB / km at 1.59 μm. A loss layer with loss was provided. When diboron trioxide is added, the refractive index of quartz glass decreases, so germanium dioxide is co-added to the loss layer at a concentration of 1.9 mol%, and the refractive index of the loss layer is made equal to that of the cladding portion.
The loss of each mode at a wavelength of 1.59 μm was measured for the dispersion slope compensation type dispersion compensation fiber provided with the loss layer and the dispersion slope compensation type dispersion compensation fiber provided with no loss layer. As compared with the case where the loss layer was not provided, the loss of all higher-order modes could be increased by 5 dB / km or more without giving a loss to the LP01 mode. Therefore, by providing the loss layer, a dispersion slope compensation type dispersion compensating fiber that substantially functions as a single mode optical fiber was obtained.

  When a dispersion compensating fiber module was produced using this fiber, the characteristics as shown in Example 1 in Table 4 were obtained. The higher-order mode was sufficiently attenuated in the module and did not affect the transmission characteristics of the transmission line.

[Comparative Example 1]
A dispersion slope compensation type dispersion compensating fiber having the structural parameters shown in Comparative Example 1 in Table 1 was manufactured by using the MCVD method. The optical characteristics of the LP01 mode at a wavelength of 1.59 μm are shown in Comparative Example 1 in Table 2. Since this dispersion slope compensation type dispersion compensating fiber has a short cut-off wavelength, only LP01 propagates at a wavelength of 1.59 μm, and no loss layer is required. However, since the dispersion compensation amount per unit length is 80% of the first embodiment, a fiber length that is 25% longer than that of the first embodiment is required to compensate for the same accumulated dispersion.
When a dispersion slope compensation type dispersion compensation fiber module was produced using this fiber, the characteristics were as shown in Comparative Example 1 in Table 4, and the FOM was worse than the dispersion slope compensation type dispersion compensation fiber module produced in Example 1. .

[Example 2]
Using the MCVD method, a dispersion slope compensation type dispersion compensating fiber having the structure shown in FIG. 1 and having the structural parameters shown in Example 2 in Table 5 was manufactured. This dispersion slope compensation type dispersion compensating fiber can propagate LP01, LP11, and LP02 modes at a wavelength of 1.59 μm. The optical characteristics of the LP01 mode are shown in Example 2 in Table 6.

This dispersion slope compensation type dispersion compensating fiber was provided with a layer having a loss of 320,000 dB / km by adding cobalt at a concentration of 500 ppm to a clad portion of 23.0 μm to 24.0 μm from the center of the core.
The loss of each mode at a wavelength of 1.59 μm was measured for the dispersion slope compensation type dispersion compensation fiber provided with the loss layer and the dispersion slope compensation type dispersion compensation fiber provided with no loss layer. As shown in Example 2 of FIG. 7, the loss of all higher-order modes could be increased by 30 dB / km or more without giving a loss to the LP01 mode as compared with the case where the loss layer was not provided. Therefore, by providing the loss layer, a dispersion slope compensation type dispersion compensating fiber that substantially functions as a single mode optical fiber was obtained.

  When a dispersion compensating fiber module was manufactured by winding this fiber on a reel having a diameter of 30 mmφ, the bending loss was small, so that the fiber could be wound with a diameter of 30 mmφ, and the dispersion slope compensation type dispersion compensating fiber module could be reduced in size. . The higher-order mode was sufficiently attenuated in the module and did not affect the transmission characteristics of the transmission line.

[Comparative Example 2]
A dispersion slope compensation type dispersion compensating fiber having the structural parameters shown in Comparative Example 2 in Table 5 was produced by using the MCVD method. The optical characteristics of the LP01 mode at a wavelength of 1.59 μm are shown in Comparative Example 2 in Table 6. Since this dispersion slope compensation type dispersion compensating fiber has a short cut-off wavelength, only LP01 propagates at a wavelength of 1.59 μm, and no loss layer is required. However, since the bending loss is large, when the dispersion slope compensation type dispersion compensation fiber module is manufactured, it cannot be wound on a reel with a small diameter, and the dispersion slope compensation type dispersion compensation fiber module cannot be reduced in size.
Using this optical fiber, a dispersion slope compensation type dispersion compensation fiber module was produced by winding it around a reel having a diameter of 30 mm as in Example 2. As a result, a dispersion slope compensation type dispersion compensation fiber module having a large bending loss was produced. I could not.

[Example 3]
Using the MCVD method, a dispersion slope compensation type dispersion compensating fiber having the structure shown in FIG. 1 and the structural parameters shown in Comparative Example 3 in Table 8 was produced. This dispersion slope compensation type dispersion compensating fiber can propagate LP11, LP12, and LP02 modes at a wavelength of 1.55 μm. The optical characteristics of the LP01 mode are shown in Example 3 in Table 9.

In this dispersion slope compensation type dispersion compensating fiber, a layer having a loss of 290000 dB / km is provided by adding cobalt at a concentration of 500 ppm to a clad portion of 23.0 μm to 24.0 μm from the core center.
The loss of each mode at a wavelength of 1.55 μm was measured for the dispersion slope compensation type dispersion compensation fiber provided with the loss layer and the dispersion slope compensation type dispersion compensation fiber provided with no loss layer. As compared with the case where the loss layer was not provided, the loss of all higher-order modes could be increased by 30 dB / km or more without giving a loss to the LP01 mode. Therefore, by providing the loss layer, a dispersion slope compensation type dispersion compensating fiber that substantially functions as a single mode optical fiber was obtained.

  When a dispersion compensating fiber module was produced using this fiber, the characteristics shown in Example 3 in Table 11 were obtained. The higher-order mode was sufficiently attenuated in the module and did not affect the transmission characteristics of the transmission line.

[Comparative Example 3]
A dispersion slope compensation type dispersion compensating fiber having the structural parameters shown in Comparative Example 3 in Table 8 was produced by using the MCVD method. The optical characteristics of the LP01 mode are shown in Comparative Example 3 in Table 9. Since this dispersion slope compensation type dispersion compensating fiber has a short cut-off wavelength, only LP01 propagates at a wavelength of 1.55 μm, and no loss layer is required. However, since the RDS is smaller than that of the fiber of Example 3, the accumulated dispersion of the transmission line cannot be compensated at a wavelength other than 1.55 μm.
Using this optical fiber, a dispersion slope compensation type dispersion compensating fiber module was produced in the same manner as in Example 3. As shown in Comparative Example 3 in Table 11, since the RDS was small, transmission was performed at wavelengths other than 1.55 μm. The accumulated dispersion of the path could not be compensated, and the transmission characteristics of the transmission path deteriorated as compared with Example 3.

[Example 4]
A dispersion slope compensation type dispersion compensating fiber having the same structural parameters as in Example 3 was produced using the MCVD method. This dispersion slope compensation type dispersion compensation fiber is different from the dispersion slope compensation type dispersion compensation fiber of Example 3 in that vanadium is added at a concentration of 1150 ppm to a clad portion of 23.0 μm to 24.0 μm from the core center. A layer having a loss of 315,000 dB / km at 1.53 μm, 290000 dB / km at 1.55 μm, and 270000 dB / km at 1.565 μm was provided.
With respect to the dispersion slope compensation type dispersion compensation fiber provided with the loss layer thus produced and the dispersion slope compensation type dispersion compensation fiber of Example 3, the loss of the LP11 mode at wavelengths of 1.53 μm, 1.55 μm, and 1.65 μm. As shown in FIG. 3, the wavelength dependence of the higher-order mode loss is reduced. Therefore, by providing a loss layer whose absorption amount increases as the wavelength is shorter in the wavelength range used, dispersion slope compensation functions substantially as a single-mode optical fiber and the wavelength dependence of the higher-order mode absorption amount is small. Type dispersion compensating fiber was obtained.

It is sectional drawing which shows one Embodiment of the dispersion slope compensation type | mold dispersion compensation fiber of this invention. 2 is a graph showing a refractive index distribution of a cross section of the dispersion slope compensation type dispersion compensating fiber of FIG. 1. It is a graph which shows the result of the Example which concerns on this invention, and shows the wavelength characteristic of the LP11 mode loss of the dispersion slope compensation type | mold dispersion compensation fiber which provided the loss layer which doped cobalt and vanadium.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Center core part, 2 ... 2nd core part, 3 ... 3rd core part, 4 ... Cladding part, 5 ... Loss layer.

Claims (9)

  A dispersion slope compensation type dispersion compensating fiber that compensates the accumulated chromatic dispersion and accumulated chromatic dispersion slope of an optical fiber transmission line, and a loss layer that selectively gives a loss only to a higher-order mode is provided in the optical fiber. A dispersion slope compensation type dispersion compensating fiber, which operates substantially as a single mode fiber even when the cutoff wavelength of the next mode is longer than the wavelength used.   In at least one or more of the wavelength bands of the wavelength range of 1460 nm to 1530 nm, the range of wavelengths 1530 nm to 1565 nm, the range of wavelengths 1565 nm to 1625 nm, and the range of wavelengths 1625 nm to 1675 nm, negative 2. The dispersion slope compensation type dispersion compensating fiber according to claim 1, wherein the dispersion slope compensation type dispersion compensating fiber has a negative wavelength dispersion slope. A central core portion having a relative refractive index difference of 0.010 to 0.040 and an outer diameter of 2 to 6 μm;
A second core portion provided on the outer periphery of the central core portion, having a relative refractive index difference of −0.020 to 0.005 and an outer diameter of 5 to 15 μm;
A third core portion provided on the outer periphery of the second core portion, having a relative refractive index difference of 0.003 to 0.010 and an outer diameter of 10 to 25 μm;
The dispersion slope compensation type dispersion compensating fiber according to claim 1, further comprising a clad portion provided on an outer periphery of the third core portion.   The dispersion slope compensation type dispersion compensating fiber according to any one of claims 1 to 3, wherein a difference in loss between the LP01 mode and the higher order mode is 5 dB / km or more.   The dispersion slope compensation type dispersion compensating fiber according to any one of claims 1 to 4, wherein the loss layer is provided in a clad part.   6. The dispersion slope compensation type dispersion compensating fiber according to claim 1, wherein the loss layer contains at least boron.   The loss layer contains at least one kind of metal dopant selected from the group consisting of cobalt, chromium, copper, iron, nickel, manganese, and vanadium. Dispersion compensation fiber with dispersion slope compensation.   The dispersion slope compensation type dispersion compensating fiber according to any one of claims 1 to 7, wherein the loss of the loss layer is larger as the wavelength is shorter in the used wavelength band. A dispersion slope compensation type dispersion compensation fiber module comprising the dispersion slope compensation type dispersion compensation fiber according to any one of claims 1 to 8.

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