JP2004341345A - Multimode isolator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the instability of an exciting light source by releasing the returning light radiated to the exciting light source into air. <P>SOLUTION: The multimode isolator is provided with a separation region 1a having: an exciting light incident core 22 which propagates the exciting light; an air clad 23 which is composed in a porous structure including a lot of fine holes and covers the periphery of the exciting light incident core 22; and a support layer 24 which covers the periphery of the air clad 23, and a solid taper part 1b which is connected to the support layer 24 at one end of the separation region 1a with a same diameter and formed in a taper state. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a multi-mode isolator used in an optical device to which an amplifying action by excitation light is applied, such as a fiber laser device, an ASE light source device, or an optical amplifier device.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in the optical communication field and the optical measurement field, an optical device using an optical amplification function, such as a fiber laser device, an ASE light source device, or an optical amplifier device, has been developed. In these devices, an excitation light is incident on an optical fiber having a core doped with an amplification medium, thereby forming a population inversion characteristic and utilizing stimulated emission therefrom.
[0003]
In recent years, use of a double-clad fiber has been put to practical use as an optical fiber having the above-mentioned optical amplification action. The double-clad fiber has a single mode core doped with an amplification medium, a first clad covering the periphery of the single mode core, and a second clad covering the periphery of the first clad. As one of the double clad fibers, there is also known one in which the second clad is configured to have a porous structure including a large number of pores (for example, see Patent Document 1).
[0004]
In the double clad fiber disclosed in Patent Literature 1, the refractive index (effective refractive index) of the second clad having a porous structure depends on the porosity, and if the porosity is increased, the effective refractive index of the second clad is increased. Can be smaller. For this reason, in the double clad fiber in which the second clad has a porous structure, by adjusting the porosity, the relative refractive index difference between the first clad and the second clad is larger than that of the conventional double clad fiber. can do. As a result, the double clad fiber having the porous structure in the second clad has an advantage that the numerical aperture (NA) of the pump light propagating inside can be increased.
[0005]
When the pumping light from the pumping light source enters the first cladding of such a double-clad fiber, the single-mode core amplification medium forms a population inversion characteristic, and guides light having a wavelength different from the pumping light therefrom. discharge. By appropriately using the emitted light, each of the above optical devices having a desired function can be configured.
[0006]
[Patent Document 1]
JP-A-2002-277669 (Claim 6)
[0007]
[Problems to be solved by the invention]
However, the optical device using the double clad fiber has the following problems. That is, in the above optical device, there is a problem that the operation of the light source is destabilized due to the influence of so-called “return light” or the light source component is damaged. Here, the return light refers to light emitted from the amplification medium doped in the single mode core of the double clad fiber to the pump light source instead of the output end side.
[0008]
Return light is generated based on the following principle. As shown in FIG. 4, the pumping light 104 incident on the first clad 102 of the double clad fiber 100 propagates while repeating total reflection at the interface with the second clad 103. When the pump light 104 passes through the single mode core 101, spontaneous emission light having a wavelength different from that of the pump light 104 is generated from the amplification medium doped in the single mode core 101.
[0009]
Since the spontaneous emission light propagates in the single mode in both directions in the single mode core regardless of the incident angle of the excitation light 104, a part thereof propagates as return light 7 toward the excitation light source of the single mode core 101. . A similar phenomenon occurs when stimulated emission is performed. That is, when the return light 7 of the spontaneous emission light is incident on an excitation light source such as a multi-mode laser diode (hereinafter, referred to as “MMLD”), the operation of the light source is destabilized, and in the worst case, This causes problems such as damaging the end face of the MMLD chip.
[0010]
Therefore, in order to prevent the return light from being incident on the excitation light source, an excitation light transmission / return light reflection filter is interposed between the light source and the double clad fiber to reflect all the return light toward the double clad fiber side. Had been
[0011]
However, even when the above-mentioned excitation light transmission / return light reflection filter is used, there are the following problems. In this case, in order to insert the excitation light transmission / return light reflection filter, a multimode fiber end face extending from the light source and the filter surface, and a contact portion between the double clad fiber end face and the filter end face are provided. In addition, since the pump light propagates in space, excess loss of the pump light occurs. For this reason, there is a problem that the utilization efficiency of the excitation light is reduced.
[0012]
Further, when the excitation light having a high energy density is transmitted through the excitation light transmission / return light reflection filter, if there is a portion where the film formation accuracy of the filter is poor or there is a portion where foreign matter adheres to the filter surface, the portion is removed. There was also a problem that the filter was burned and damaged.
[0013]
Further, since the multi-mode fiber and the double clad fiber are not directly fusion-spliced, there is a possibility that axial misalignment may occur during long-term use, and there is a problem in long-term reliability. In addition, there is a problem that it is difficult to save space by using the excitation light transmission / return light reflection filter.
[0014]
Thus, there is a limit to the elimination of the return light by the conventional filter. Therefore, the present invention provides a novel optical isolator that replaces the above-mentioned filter, and aims to solve the above-mentioned problem.
[0015]
[Means for Solving the Problems]
The multi-mode isolator according to the first aspect includes an isolation region and a tapered portion. The separation region is configured to have an excitation light incident core that is connected to a multi-mode optical fiber extended from an external laser light source and propagates the excitation light, and a porous structure including a large number of pores. And a support layer that covers the periphery of the air clad. The tapered portion is connected at one end of the separation region to the same diameter as the support layer, and is tapered at the other end so as to converge to the core diameter of the optical fiber to be connected. The tapered portion has no air cladding and is formed solid.
[0016]
According to this configuration, the excitation light incident on the excitation light incident core in the separation region by the multimode fiber extended from the external laser light source repeats total reflection at the interface between the core and the air clad. The light propagates and enters the tapered portion connected to one end of the separation region. The excitation light incident on the tapered portion is narrowed down while repeating total reflection in the tapered portion, and is incident on the core of the optical fiber connected to the other end of the tapered portion.
[0017]
Usually, an optical device having an optical fiber doped with an amplification medium is connected to the other end of the tapered portion. A typical optical device is an ASE light source device. In the multimode isolator according to the present invention, the excitation light is incident on the first clad of the double clad fiber provided in the ASE light source device from the other end of the tapered portion. The excitation light incident on the first cladding propagates through the first cladding while intersecting with the core doped with the amplification medium. When the excitation light crosses the core, the amplification medium generates spontaneous emission light in the core. Part of this spontaneous emission light propagates in the core toward the light source as "return light".
[0018]
The return light emitted from the end of the core of the double clad fiber enters the tapered portion of the multimode isolator according to the present invention. The return light incident on the tapered portion propagates toward the separation region while being repeatedly reflected in the tapered portion.
[0019]
Since the tapered portion has a tapered shape from the return light incident side toward the separation region, the reflection angle becomes shallower each time the propagating return light is reflected. As a result, the return light that has reached the separation region is not incident on the core of the separation region but is incident on the support layer that covers the periphery of the air clad.
[0020]
Since the return light incident on the support layer cannot pass through the air clad, the return light propagates while repeating total reflection in the support layer in the separation region. The return light propagating in the support layer is emitted into the air at the end of the separation region, and does not enter the excitation light source.
[0021]
The multi-mode isolator according to the second aspect includes a separation region, a tapered portion, and a connection end region. The separation region is configured to have an excitation light incident core that is connected to a multi-mode optical fiber extended from an external laser light source and propagates the excitation light, and a porous structure including a large number of pores. And a support layer that covers the periphery of the air clad. The tapered portion is connected at one end of the separation region to the same diameter as the support layer and is tapered. The tapered portion has no air cladding and is formed solid. The connection end region has a connection end core connected to the same diameter as the end of the tapered portion formed in the tapered shape, a connection end air clad covering the periphery of the connection end core, and a periphery of the connection end air clad. And a connection end support layer that covers the connection end.
[0022]
According to this configuration, similarly to the multi-mode isolator according to the first aspect, it is possible to prevent return light from being incident on the excitation light source. Further, the multi-mode isolator according to the present invention can be easily manufactured from a normal multi-mode fiber having an air clad using a collapse process and an etching process.
[0023]
The multi-mode isolator according to claim 3 is the multi-mode isolator according to claim 1 or 2, wherein a material having a higher refractive index around the support layer in the separation region than the material of the support layer. Is coated.
[0024]
According to this configuration, the return light incident on the support layer in the separation region passes through the coating layer and is emitted into the air without being reflected at the interface between the support layer and the coating layer.
[0025]
【The invention's effect】
As described above, the multi-mode isolator according to the present invention is an optical device that uses an excitation light source, and when used in an optical device that may generate so-called return light, the multi-mode isolator is connected to the excitation light source. Of the return light can be prevented.
[0026]
The multimode isolator according to the present invention, unlike the case where a filter is conventionally used, can prevent excessive attenuation of the excitation light because there is no spatial connection.
[0027]
Further, the multi-mode isolator according to the present invention does not have a problem of variation in film forming accuracy and sticking due to foreign matter adhesion, which have conventionally been problems in using the excitation light transmission / return light reflection filter. Therefore, the present multi-mode isolator can be directly fusion-spliced to a multi-mode fiber extended from the excitation light source, so that there is no possibility that axial misalignment will occur. Therefore, the long-term reliability of the optical device can be dramatically improved.
[0028]
Further, the multi-mode isolator according to the present invention can be composed of only an optical fiber having an air clad, and thus has excellent space saving.
[0029]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. The multimode isolator 1 according to the present embodiment is used for a fiber laser device, an ASE light source device, and the like.
[0030]
<First embodiment>
FIG. 1 is a configuration diagram of an air clad fiber 21 that forms an isolation region 1a of the multimode isolator 1 according to the present embodiment. The air-clad fiber 21 includes an excitation light incident core 22 extending in the axial direction at the axis of the fiber, an air clad 23 covering the periphery of the excitation light incident core 22, and a support covering the periphery of the air clad 23. It has a layer 24 and a coating layer 25 covering the outer periphery of the support layer 24.
[0031]
The excitation light incident core 22, the air clad 23, and the support layer 24 are each made of SiO. ₂ And the coating layer 25 is made of SiO. ₂ It is formed of a material having a higher refractive index. As a resin used for the coating layer 25, for example, an ultraviolet curable acrylate resin or the like is used.
[0032]
The air clad 23 is formed to extend in the fiber axis direction while surrounding the periphery of the excitation light incident core 22. The air clad 23 of the present invention has a porous structure including a large number of pores 23a extending in the fiber axis direction. The pores 23a are arranged substantially uniformly in the circumferential direction in the cross section of the fiber.
[0033]
The refractive index (effective refractive index) of the air clad 23 having the porous structure depends on the porosity, that is, the ratio of the volume of the pores 23a to the total volume of the air clad 23 region. 23 has a smaller effective refractive index. When the support layer 24 is formed outside the air clad 23, the mechanical strength of the fiber can be secured by the support layer 24, so that the porosity can be considerably increased. Therefore, by making the air clad 23 have a porous structure, the numerical aperture (NA) for the excitation light can be greatly increased as compared with the case where the air clad 23 is formed of a solid clad.
[0034]
The support layer 24 is formed so as to surround the periphery of the air clad 23. The support layer 24 protects the porous air clad 23 and improves the mechanical strength of the air clad fiber 21. I have to.
[0035]
As described above, the air clad fiber 21 in which the air clad 23 has a porous structure is manufactured by heating and stretching a preform and stretching it into a fiber shape. Specifically, a preform is created according to the following procedure.
[0036]
First, one support tube having a circular cross section is prepared, one core rod (solid bar), and a number of air cladding capillaries (hollow bars). All these members are made of SiO ₂ It is formed with. Since the air clad fiber 21 according to the present invention enables multi-mode propagation, the core rod is selected to be thicker than the air clad capillary.
[0037]
Then, the core rod is arranged at a center position in the support tube, and a plurality of air-clad capillaries are regularly arranged around the core rod. At this time, it is preferable that the capillary for the air clad fiber is disposed almost densely around the core rod so that the formed air clad has a substantially annular cross section.
[0038]
With the support tube filled with the rods and capillaries, the whole is heated. When the heating temperature is equal to or higher than a predetermined temperature, the rods, capillaries, and support tubes are welded, the gaps between the members are densified, and the preform is completed.
[0039]
The preform is heated and drawn in a drawing furnace to form a fiber. The part corresponding to the rod for the core of the preform is the excitation light incident core of the air clad fiber, the part corresponding to the capillary for the air clad of the preform is the air clad of the air clad fiber, and the part corresponding to the support tube. Then, a support layer of the air clad fiber is formed. Then, a coating material is applied to the outer periphery of the fiber to form a coating layer of the air clad fiber. The formation of the coating layer may be performed at the time of drawing.
[0040]
Thus, the air clad fiber 21 in which the air clad 23 has a porous structure is manufactured.
[0041]
FIG. 2 shows a multi-mode isolator 1 according to the embodiment.
[0042]
The multi-mode isolator 1 includes an isolation region 1a, a tapered portion 1b, and a connection end region 1c.
[0043]
The separation region 1a has a porous structure including an excitation light incidence core 22 connected to a multi-mode fiber extended from an external MMLD light source at the incidence end 26 to propagate excitation light, and a large number of pores 23a. An air clad 23 covering the periphery of the excitation light incident core 22, a support layer 24 covering the periphery of the air clad 23, and a coating layer 25 covering the periphery of the support layer 24.
[0044]
Since the isolator 1 according to the present invention is a multimode isolator having a plurality of propagation modes, the core diameter of the excitation light incidence core 22 is designed to be large, and the porosity of the air clad 23 is increased to make the excitation light incidence. The relative refractive index difference between the core 22 and the air clad 23 is also set large. The core diameter of the excitation light incident core 22 is not particularly limited, but is preferably about 75 to 200 μm.
[0045]
The coating layer 25 may be made of either an inorganic material or an organic material. ₂ It is formed of a material having a higher refractive index than that of the material. For example, the coating layer 25 is formed by appropriately selecting a material such as an ultraviolet-curable acrylate resin.
[0046]
The tapered portion 1b is connected to one end of the separation region 1a to have the same diameter. The tapered portion 1b is made of SiO ₂ It is formed solidly. The tapered portion 1b is formed in a smoothly tapered shape tapering toward the other end. If the inclination of the tapered portion 1b is too large, the incident excitation light may exceed the critical angle and may be emitted into the air. Accordingly, the inclination is not particularly limited, but is preferably 10 ° or less with respect to the axis. The tapered portion 1b is connected at the other end formed in a tapered shape to the same diameter as the connection end core 27 constituting the connection end region 1c.
[0047]
The tapered portion 1b can be formed separately from the separation region 1a and connected thereto, but is preferably formed integrally with the separation region 1a and connected by a manufacturing method described later.
[0048]
The connection end region 1c includes a connection end core 27 connected to the same diameter as the end of the tapered portion 1b formed in a tapered shape, a connection end air clad 28 covering the periphery of the connection end core 27, and a connection end. A connection end support layer 29 covering the periphery of the air clad 28; The connection end region 1c has the same air-clad fiber structure as the separation region 1a.
[0049]
That is, the connection end core 27 is designed to be capable of multi-mode propagation of the pump light, similarly to the pump light incident core 22. Like the air clad 23, the connection end air clad 28 has a porous structure including many pores. The connection end support layer 29 is also formed so as to cover and protect the periphery of the connection end air clad similarly to the support layer of the separation region 1a. It is preferable to form the coating layer 30 so as to cover the periphery of the connection end support layer 29.
[0050]
The connection end 31 of the connection end region 1c is connected to an incident end of a fiber laser device or an ASE light source device.
[0051]
Although the connection end region 1c can be formed separately from the tapered portion 1b and connected thereto, it is preferable that the connection end region 1c be formed integrally with the tapered portion 1b and connected by a manufacturing method described later.
[0052]
A method for manufacturing the multi-mode isolator 1 according to the embodiment will be described.
[0053]
As shown in FIG. 3A, a uniform air-clad fiber 21 is prepared over the entire length formed by the above method.
[0054]
As shown in FIG. 3B, the coating layer 25 is removed in an appropriate length range in the middle of the air clad fiber 21. The method of removing is not particularly limited, but an appropriate method is selected so as not to damage the internal fiber.
[0055]
As shown in FIG. 3C, the region from which the coating layer 25 has been removed is subjected to a collapse treatment, thereby crushing all the pores 23a constituting the air clad 23. Specifically, this region is heated by a suitable heating means such as a burner. When the air-clad fiber 21 is brought into a semi-molten state by heating, the pores 23a are crushed by interfacial tension, and the support layer 24, the portion other than the pores 23a of the air clad 23, and the excitation light incident core 22 are integrated.
[0056]
As described above, the region 4 without the air clad 23 (hereinafter, also referred to as “collapse region 4”) is formed in a part of the air clad fiber 21 by the collapse process. The collapsed region 4 has a slightly tapered region at both ends, but is formed to have substantially the same diameter over the entire region.
[0057]
Next, as shown in FIG. 3D, the collapse region 4 is tapered. The taper processing is preferably performed by using an etching process using hydrogen fluoride (HF) or the like.
[0058]
More specifically, as shown in FIG. 7, the portion is substantially on the axial center of the fluororesin tube 60 partially cut away in the longitudinal direction and includes the entire collapse region 4, and the collapse region 4 and the air clad 23 are The fiber is fixed such that the boundary with the remaining region is located substantially at the center in the longitudinal direction of the fluororesin tube.
[0059]
In this state, hydrogen fluoride is injected into the fluororesin tube from the notch 61. Due to the effect of the surface tension, a relatively large amount of hydrogen fluoride is stored in the central portion of the fluororesin tube as compared with both end portions thereof. When left standing in this state, the quartz optical fiber melts from the outer peripheral surface toward the axis. In a state where there is no movement of the optical fiber and no convection of hydrogen fluoride, the reaction between hydrogen fluoride and quartz sequentially reaches a saturation state and stops from both ends where the storage amount is relatively small to the center.
[0060]
As a result, in the collapsed region 4, a smooth tapered portion 1b tapered from the incident end side toward the boundary with the region where the air clad 2 remains is formed. It is necessary that the outer diameter of the tapered portion 1b formed here on the emission end side is equal to or smaller than the outer diameter of the connection end core 27 in the connection end region.
[0061]
By performing the collapse process and the etching process on the air-clad fiber in this manner, a multi-mode isolator in which the separation region 1a, the tapered portion 1b, and the connection end region 1c are integrally formed can be formed.
[0062]
<Embodiment 2>
As shown in FIG. 6, the multi-mode isolator 1 according to the present embodiment includes an isolation region 1a and a tapered portion 1b.
[0063]
The separation region 1a is connected to a multi-mode fiber extended from an external laser light source and configured to have an excitation light incident core 22 for transmitting excitation light, and a porous structure including a large number of pores. An air clad 23 covers the periphery of the core 22, a support layer 24 covers the periphery of the air clad 23, and a coating layer 25 covers the periphery of the support layer 24.
[0064]
The tapered portion 1b is connected to the separation region 1a at one end to have the same diameter, and is tapered so as to converge to the core diameter of the optical fiber connected at the other end. The air clad 23 is not formed on the tapered portion 1b, but is formed solid.
[0065]
In this embodiment, the functions of the separation region 1a and the tapered portion 1b are the same as those of the separation region 1a and the tapered portion 1b according to the first embodiment.
[0066]
The multimode isolator 1 according to the present embodiment is also used for a fiber laser device, an ASE light source device, an optical amplifier device, or the like, like the multimode isolator of the first embodiment. It is used by being directly connected to the core of an optical fiber doped with an amplification medium.
[0067]
【Example】
Hereinafter, specific examples of the optical device according to the present invention will be described with reference to the drawings.
[0068]
FIG. 5 is a configuration diagram of an optical device using the multi-mode isolator 1 according to the first embodiment as the ASE light source device 50.
[0069]
The optical device according to the present embodiment includes a multimode isolator 1, an ASE light source device 50, a multimode laser diode pumping light source (hereinafter, referred to as “MMLD pumping light source”) 51, and a light source multimode fiber (hereinafter, referred to as “MMLD pumping light source”). , "Light source MMF") 52, a single mode fiber 53, a single mode isolator 54, and the like.
[0070]
The MMLD excitation light source 51 supplies excitation light to the optical device 1 via the light source MMF52. The light source MMF52 is a large-diameter multimode fiber having a larger core diameter than a normal multimode fiber, and the core diameter is about 200 μm. By using a large-diameter multimode fiber for the light source MMF52, the output of the MMLD 51 can be increased.
[0071]
The emission end of the light source MMF52 is directly fusion-spliced to the excitation light incidence core 22 of the multimode isolator 1 according to the present invention. The excitation light that has entered the excitation light incident core 22 passes through the tapered portion 1b and the connection end region 1c, and enters the first clad 50b of the double clad fiber 50e of the ASE light source device 50. A core 50a doped with an amplification medium is arranged in the first cladding 50b. In the core 50a, the spontaneous emission light is generated every time the excitation light propagating while reflecting in the first cladding 50b crosses the core 50a. Of the generated spontaneous emission light, the light emitted toward the emission end 55 enters the single mode fiber 53, is extracted from the emission end 55 via the isolator 54, and is used.
[0072]
On the other hand, of the generated spontaneous emission light, the light emitted toward the MMLD excitation light source 51 is incident on the connection end core 27 of the multimode isolator 1 as return light 7. The return light 7 incident on the connection end core 27 is incident on the tapered portion 1b of the isolator 1. The reflection angle of the return light 7 propagating while being reflected in the tapered portion 1b becomes shallower each time it is reflected.
[0073]
The return light 7 that has reached the separation region 1a is not incident on the excitation light incident core 22 of the separation region 1a, but is incident on the support layer 24 formed around the air clad 23. When the return light 7 incident on the support layer 24 enters the coating layer 25 formed of a material having a higher refractive index than the support layer, the return light 7 is emitted into the air without being reflected. Also, the return light that reaches the incident end 26 without being radiated into the air is radiated from the incident end 26 into the air from the incident end 26 without being incident on the light source MMF52.
[0074]
As described above, in the ASE light source device 50 according to the present embodiment, the return light 7 generated in the optical fiber doped with the amplification medium is efficiently removed by the multimode isolator 1 and is incident on the MMLD pump light source 51. Never be. Therefore, it is possible to prevent the return light from destabilizing or damaging the operation of the MMLD excitation light source 51. According to the multi-mode isolator according to the present example, it was confirmed that about 90% or more of the return light could be emitted into the air.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an air-clad fiber according to the present invention.
FIG. 2 is a conceptual diagram illustrating a configuration of a multi-mode isolator according to an embodiment of the present invention.
FIG. 3 is a conceptual diagram illustrating a manufacturing process of the multi-mode isolator according to the present invention.
FIG. 4 is a conceptual diagram illustrating the principle of generation of return light.
FIG. 5 is a configuration diagram of an ASE light source device according to an embodiment of the present invention.
FIG. 6 is a conceptual diagram showing a configuration of a multi-mode isolator according to another embodiment of the present invention.
FIG. 7 is a conceptual diagram showing an outline of an etching process.
[Explanation of symbols]
1 Multi-mode isolator
1a Isolation area
1b Taper
1c Connection end area
21 Air-clad fiber
22 Excitation light injection core
23 Air Clad
24 Support layer
25 Coating layer

Claims (3)

An excitation light incident core that propagates excitation light by being connected to a multimode optical fiber extended from an external laser light source, and a porous structure including a large number of pores, covering the periphery of the excitation light incident core. Air clad, and a separation region having a support layer covering the periphery of the air clad,
A multimode isolator having a solid tapered portion connected at one end of the separation region to have the same diameter as the support layer and tapered at the other end so as to converge to a core diameter of the optical fiber to be connected. An excitation light incident core that propagates excitation light by being connected to a multimode optical fiber extended from an external laser light source, and a porous structure including a large number of pores, covering the periphery of the excitation light incident core. Air clad, and a separation region having a support layer covering the periphery of the air clad,
At one end of the separation region, connected to the same diameter as the support layer, a solid tapered portion formed in a tapered shape,
A connection end core connected to the same diameter as the end of the tapered portion formed in the tapered shape; a connection end air clad covering the periphery of the connection end core; and a connection end support covering the periphery of the connection end air clad. A connection end region having a layer;
Multi-mode isolator with. 3. The multimode isolator according to claim 1, wherein a coating layer made of a material having a higher refractive index than the material of the support layer is coated around the support layer in the separation region. 4.

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