JP4190943B2 - Optical device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical equipment provided with a double-clad fiber.
[0002]
[Prior art]
Conventionally, a single mode core doped with an amplifying medium, a first cladding covering the periphery of the single mode core, and a second cladding covering the periphery of the first cladding have a porous structure including a large number of pores. A configured double clad fiber is known (see, for example, Patent Document 1).
[0003]
As shown in FIG. 1, the double clad fiber of Patent Document 1 includes a single mode core 11 doped with an amplifying medium, a first clad 12, and a second clad 13 including a large number of pores 13a extending in the fiber axis direction. And a support layer 14 surrounding the periphery of the second cladding 13 and a covering layer 15 covering the outer periphery of the support layer 14.
[0004]
The refractive index (effective refractive index) of the second cladding configured in the porous structure depends on the porosity, and the effective refractive index of the second cladding can be reduced by increasing the porosity. For this reason, in the double clad fiber in which the second clad has a porous structure, the relative refractive index difference between the first clad and the second clad is made larger than that of the conventional double clad fiber by adjusting the porosity. can do. As a result, the double clad fiber having a porous structure in the second clad has an advantage that the numerical aperture (NA) for the excitation light propagating inside can be increased.
[0005]
Also known is a technique for amplifying the propagation light of the single mode core by making the excitation light enter the first clad of the double clad fiber.
[0006]
A so-called side pumping system is known as a technique for making excitation light enter the first clad of a double clad fiber (see, for example, Patent Document 2).
[0007]
In Patent Document 2, the second clad of the double clad fiber is removed to expose the first clad, and a multimode optical fiber formed into a tapered shape is optically formed on the exposed side surface of the first clad. A connected optical device is disclosed. According to the invention according to Patent Document 2, it becomes possible to make the excitation light enter the double clad fiber from the connection portion by making the excitation light incident on the multimode optical fiber.
[0008]
Such optical devices are used for fiber lasers and fiber amplifiers, and propagate propagating light (signal light for fiber amplifiers and laser oscillation light for fiber lasers) through the single mode core. On the other hand, the excitation light for exciting the propagating light is incident on the first cladding. By doing so, the excitation light propagating in the first cladding excites the amplification medium doped in the core every time it intersects the core, and as a result, the propagation light propagating in the core is amplified. Become.
[0009]
However, in the double-clad fiber in which the second cladding has a porous structure, even if the coating layer is removed and the excitation light is incident from the peripheral surface of the support layer, the excitation light is not emitted because the second cladding has a porous structure. As a result, the pumping light cannot be transmitted into the first cladding. Therefore, in order to make the excitation light enter the first cladding, for example, it is conceivable to remove the support layer and the second cladding by grinding. However, grinding is extremely difficult from the viewpoint of fiber strength, and the grinding waste enters the pores of the second cladding. For this reason, it is not realistic to remove the support layer and the second cladding by grinding.
[0010]
Therefore, in the double clad fiber in which the second clad has a porous structure, the single-mode type is provided on the incident end face of the double clad fiber 1 as shown in FIG. A technique for connecting the emission end face of the optical fiber 20 has been considered. In the present technology, a multi-mode optical fiber 30 is optically connected to the peripheral surface of the single-mode optical fiber 20, and propagating light is incident on the core 21 of the single-mode optical fiber 20. Excitation light is incident on the cladding 22 of the single mode optical fiber 20 from the optical fiber 30.
[0011]
[Patent Document 1]
JP 2002-277669A [Patent Document 2]
Japanese Patent No. 3337691 [0012]
[Problems to be solved by the invention]
However, the technique of connecting the single mode optical fiber 20 to the incident end face of the double clad fiber 1 has the following problems. That is, the coupling efficiency from the multimode type optical fiber 30 to the single mode type optical fiber 20 is as low as about 0.5, and about 50% of the pumping light emitted from the multimode type optical fiber 30 is a single mode type. It was only incident on the optical fiber 20. Here, the coupling efficiency is defined as P ₂ / where P _{1 is} the amount of excitation light emitted from the multimode optical fiber, and P ₂ is the amount of excitation light incident on the single mode optical fiber. It means a value represented by P _1. As the coupling efficiency increases, the amount of pumping light incident on the single mode optical fiber 20 from the multimode optical fiber 30 increases, so that the side pumping effect can be enhanced. Therefore, in an optical device employing the side pumping method, it is necessary to improve the coupling efficiency in order to improve the amplification factor of the propagation light.
[0013]
On the other hand, it is also known that the coupling efficiency depends on the cross-sectional area ratio between the multimode optical fiber 30 and the single mode optical fiber 20 at the optical junction. That is, the cross-sectional area ratio, the cross-sectional area D ₁ of the said multi-mode optical _fiber, when the cross-sectional area of the single-mode optical fiber was D _2, refers to a value indicated by D 2 _/ D _1, wherein The larger the cross-sectional area ratio, the higher the coupling efficiency.
[0014]
Therefore, for example, as shown in FIG. 8, there has been considered a technique in which a part of the coating layer 15 of the double clad fiber 1 is removed and then heat is applied to the pores 13 a of the second clad 13. According to the present technology, when excitation light is incident from the multimode optical fiber 3 connected to the outer peripheral surface of the support layer 14, the excitation light is incident on a region where the support layer 14 and the pores 13a are crushed. be able to. The cross-sectional area of the region where the pores are crushed is considerably larger than that of a normal single mode optical fiber. Therefore, the cross-sectional area ratio can be increased, and the coupling efficiency can be improved.
[0015]
However, in this configuration, as indicated by arrows in FIG. 8, most of the incident excitation light enters the support layer 14, and the amount of excitation light propagating in the first cladding 12 is multimode type light. It was found that the amount of light incident from the fiber 3 was about 10%.
[0016]
The present invention improves the coupling efficiency of the excitation light incident on the optical device from the multimode optical fiber, and allows the incident excitation light to enter the first cladding of the double-clad fiber without fail. The purpose is to realize the device.
[0017]
[Means for Solving the Problems]
An optical device according to the present invention has a porous structure including a single mode core doped with an amplifying medium , a first cladding covering the periphery of the single mode core, and a plurality of pores covering the periphery of the first cladding. A double clad fiber having a configured second clad;
A multimode-type optical fiber whose exit end is tapered, and
With
The double clad fiber has a single mode fiber type structure having an outer diameter larger than that of the first clad formed in the middle portion by pulverizing the numerous pores of the porous structure constituting the second clad. Has a coupling area,
The coupling region is formed in a tapered shape so that the output end portion of the multimode optical fiber is optically connected to the outer peripheral surface thereof, and the output end side converges on the first cladding,
Excitation light from the multimode optical fiber is configured to be incident on the first cladding via the coupling region of the double cladding fiber .
[0018]
According to this configuration, the double-clad fiber can propagate signal light or the like to the single mode core doped with the amplification medium, and can introduce excitation light into the optical device from the coupling region. Signal light or the like can be amplified.
[0019]
That is, a multimode optical fiber that emits excitation light is optically connected to a coupling region formed at a predetermined portion of the double clad fiber. As a result, the excitation light is incident on the coupling region from the multimode optical fiber.
[0020]
Since the second cladding formed of the porous structure is not formed in the coupling region, the excitation light incident from the periphery of the coupling region is totally reflected in the coupling region without being blocked by the second cladding. Propagate repeatedly.
[0021]
In the present invention, since the outer diameter of the coupling region is formed larger than the outer diameter of the first cladding, a relatively large cross-sectional area ratio can be realized. Can be improved.
[0022]
A tapered portion is formed on the emission end side of the coupling region so as to converge to the outer diameter of the first cladding. For this reason, the excitation light incident on the coupling region is reliably incident on the first cladding of the double-clad fiber connected to the emission end side of the coupling region.
[0023]
In this way, the excitation light incident in the optical device with high coupling efficiency is surely incident and propagated in the first cladding of the double-clad fiber, so that the amplification medium doped in the single mode core has high efficiency. Excited. As a result, the propagating light propagating in the single mode core is effectively amplified.
[0024]
【The invention's effect】
As described above, according to the optical equipment of the present invention, from the multi-mode optical fiber to the coupling region of the double-clad fiber can be incident excitation light with high coupling efficiency. Moreover, since the excitation light incident on the coupling region is reliably propagated in the first cladding of the double clad fiber connected to the coupling region, the excitation light can be used with high efficiency. Thereby, the signal light propagating through the single mode core of the double clad fiber can be effectively amplified.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. The optical device according to the present embodiment amplifies the laser light emitted from a fiber amplifier device or a laser light source that amplifies the attenuated signal light that is disposed at an appropriate position when transmitting the signal light over a long distance by an optical fiber. And used as a fiber laser device for obtaining high-power laser light.
[0026]
FIG. 1 shows a double clad fiber 1 used in the optical apparatus of this embodiment. The double clad fiber 1 includes a single mode core 11 extending in the axial direction at the axial center of the fiber, a first clad 12 covering the periphery of the single mode core 11, and a first clad covering the periphery of the first clad 12. 2 cladding 13, support layer 14 covering the periphery of second cladding 13, and coating layer 15 covering the outer periphery of support layer 14.
[0027]
In the double clad fiber 1, propagating light propagates in the single mode core 11, while pumping light propagates in the first cladding 12.
[0028]
The single-mode core 11, first cladding 12, while the second cladding 13, and support layer 14 are each made of SiO _2, the coating layer 15 is made of resin. As the resin used for the coating layer 15, for example, an ultraviolet curable resin or the like is used.
[0029]
The single mode core 11 is doped with Ge or the like so as to be higher than the refractive index of the first cladding 12, and is also doped with an amplification medium. The amplification medium is excited by excitation light propagating in the first cladding 12 to form an inverted distribution trait, and emits stimulated emission therefrom. Thereby, the propagation light propagating through the single mode core 11 can be amplified. The amplification medium is appropriately selected from rare earth elements such as Er, Nd, and Yb.
[0030]
The first clad 12 is formed to extend in the fiber axis direction while surrounding the periphery of the single mode core 11 and has a circular cross section.
[0031]
In the present embodiment, the first clad 12 has a circular cross-sectional shape, but the cross-sectional shape is not limited to a circular shape, for example, a polygonal shape such as a hexagonal shape, a triangular shape, a rectangular shape, or an elliptical shape. It may be formed. Moreover, it is also possible to make it the shape which notched circular part. By making the cross-sectional shape of the first cladding 12 the non-circular shape, the skew component of the excitation light can be reduced and the excitation efficiency can be increased.
[0032]
The second cladding 13 is formed to extend in the fiber axis direction while surrounding the periphery of the first cladding 12. The second cladding 13 has a porous structure including a large number of pores 13a extending in the fiber axis direction. Each of the pores 13a is disposed substantially uniformly in the circumferential direction in the fiber cross section.
[0033]
The refractive index (effective refractive index) of the second cladding 13 having the porous structure depends on the porosity, that is, the ratio of the volume of the hole 13a to the total volume of the second cladding 13 region. The effective refractive index of the 2 clad 13 is reduced. When a support layer is formed outside the second cladding, the mechanical efficiency of the fiber can be ensured by the support layer, so that the porosity can be considerably increased. Therefore, by forming the second cladding 13 in a porous structure, the numerical aperture (NA) for the excitation light can be greatly increased as compared with the case where the second cladding 13 is configured in a solid structure. .
[0034]
The support layer 14 is formed so as to surround the second clad 13. The support layer 14 protects the porous second clad 13 and improves the mechanical strength of the double clad fiber 1. Like to do.
[0035]
As described above, the double clad fiber 1 in which the second clad has a porous structure is manufactured by heating and stretching the preform and stretching the preform into a fiber shape. Specifically, the preform is created according to the following procedure.
[0036]
First, a cylindrical support tube having a substantially circular cross section is prepared, and one core rod (solid rod), a plurality of first cladding rods and second cladding capillaries (hollow rods) are provided. Prepare each. These members are all made of quartz, and the amplification rod is doped in the core rod.
[0037]
And the said core rod is arrange | positioned in the center position in the said support pipe, and the several 1st rod for clads is regularly arrange | positioned around this core rod. At this time, it is preferable that the first clad rod is disposed almost densely around the core rod so that the molded first clad portion has a substantially circular cross section.
[0038]
Next, a large number of second cladding capillaries are regularly arranged between the first cladding capillary and the inner peripheral surface of the support tube. The order in which the core, the first cladding rod, and the second cladding capillary are arranged may be appropriately changed.
[0039]
In this way, the entire support tube is heated with each rod and capillary filled. When the heating temperature is equal to or higher than the predetermined temperature, the rods, capillaries, and support tubes are welded, the gaps between the members are densified, and the preform is completed.
[0040]
This preform is heated and drawn in a drawing furnace to form a fiber. The portion corresponding to the preform core rod is the single mode core 11 of the double clad fiber 1, and the portion corresponding to the first clad rod of the preform is the first clad 12 of the double clad fiber 1. The portion corresponding to each capillary for the second cladding forms the second cladding 13 having the pores 13a of the double cladding fiber 1, and the portion corresponding to the support tube forms the support layer 14 of the double cladding fiber 1. To do. And the coating layer 15 of the double clad fiber 1 is formed by apply | coating a coating | covering material to the outer periphery of this fiber. The coating layer 15 may be formed at the time of drawing.
[0041]
In this way, the double clad fiber 1 in which the second clad 13 has a porous structure is manufactured.
[0042]
In addition, instead of making the preform using the core rod and the first clad rod, a first preform having a core portion and a clad portion (first clad portion) is produced. You may create using.
[0043]
That is, the first preform is prepared by a known method such as a VAD method, an OVD method, or a rod-in-tube method, and this first preform is disposed at a substantially central position in the support tube. Next, a large number of second cladding capillaries are disposed between the first preform and the inner peripheral surface of the support tube. This also completes the preform having the core portion and the first and second cladding portions.
[0044]
FIG. 2 shows an optical device according to an embodiment of the present invention.
[0045]
This optical device includes the above-described double-clad fiber 1 and a multimode optical fiber 2 having a tapered emission end side.
[0046]
A coupling region 3 is formed in the middle portion of the double clad fiber 1. The covering layer 15 in the coupling region 3 is removed. The second cladding 13 is not formed in the coupling region 3 and has a single mode fiber structure. Further, the outer diameter of the coupling region is formed to be larger than the outer diameter of the first clad of the double clad fiber 1. Further, a smooth tapered portion 3a is formed on the output end side (the right end surface in FIG. 2) of the coupling region 3 so as to converge on the outer diameter of the double clad fiber. The emission end of the coupling region 3 is continuous with the first cladding 12 of the double cladding fiber 1.
[0047]
Since the coupling region 3 has such a structure, all light incident on the coupling region 3 from the multimode optical fiber 2 toward the emission end side is in the first cladding 12 of the double-clad fiber 1. It will be incident on.
[0048]
The multi-mode optical fiber 2 has a tapered portion 2a that is tapered toward the tip side. The tip portion 2 b of the multimode optical fiber 2 is optically connected to the outer peripheral surface of the coupling region 3 of the double clad fiber 1. It is preferable that the outer diameter of the distal end portion 2b is formed to be considerably smaller than the outer diameter of the coupling region 2.
[0049]
Thereby, when the cross-sectional area of the multimode optical fiber 2 is D _M and the cross-sectional area of the coupling region 3 is D _C , the cross-sectional area ratio represented by D _C / D _M can be considerably increased. Coupling efficiency from the multimode optical fiber to the coupling region can be dramatically improved.
[0050]
In the present embodiment, as shown in FIG. 2, the tip portion 2b of the multimode optical fiber 2 is optically connected to the outer peripheral surface of the coupling region 3 by heat fusion at a predetermined angle. .
[0051]
Next, a method for manufacturing the optical device according to the present embodiment will be described.
[0052]
The coupling region 3 formed at an appropriate position of the double clad fiber 1 is formed as follows. As shown in FIG. 3A, a uniform double clad fiber 1 is prepared over the entire length formed by the above method. The length of the double clad fiber 1 is not particularly limited, and is appropriately determined depending on the number of coupling regions determined according to the use of the optical device and the required performance.
[0053]
As shown in FIG. 3 (b), the pores 13a (not shown in FIG. 3) constituting the second cladding 13 are subjected to a collapse process at an appropriate position in the middle of the double-clad fiber. Smash all. Specifically, the covering layer 15 in the region where the coupling region is to be formed is peeled off by an appropriate means so as not to damage the internal fiber. Next, this region is heated by an appropriate heating means such as a burner. When this region is in a semi-molten state by heating, the pores 13a forming the second cladding 13 are crushed by the interfacial tension, and the support layer 14, the portion other than the pores 13a of the second cladding 13, and the first The clad 12 is integrated. Since the single mode core 11 is solid, it does not change at all. Thus, the region 4 (hereinafter, also referred to as “collapse region 4”) having a single mode fiber structure is formed in a part of the double clad fiber by the collapse process.
[0054]
Next, the tapered portion 3 is formed on the emission end side of the collapse region 4. In order to form the tapered portion 3, it is preferable to use an etching process. Specifically, as shown in FIG. 4, the fluororesin tube 5 is substantially on the axial center of the fluororesin tube 5, and the boundary between the collapse region 4 and the double clad fiber is the fluororesin tube. The optical fiber is fixed so as to be positioned at substantially the center in the longitudinal direction of 5. In this state, hydrogen fluoride is injected into the fluororesin tube from the notch. 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 5 as compared with both end portions thereof. When left standing in this state, the optical fiber made of quartz is dissolved 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 saturated state and stops from both ends to the center where the amount of storage is relatively small. As a result, in the collapse region, a smooth tapered portion 3a that is tapered from the incident end side (left side in FIG. 4) toward the boundary with the double clad fiber 1 is formed, and the coupling portion 3 is completed. The outer diameter of the tapered portion 3a formed here on the emission end side (the right side in FIG. 3C) needs to be equal to or smaller than the outer diameter of the first cladding 12. As described above, the outer diameter of the tapered portion 3a on the emission end side is formed to be equal to or smaller than the outer diameter of the first clad, so that the excitation light incident on the double clad fiber 1 from the coupling portion 3 is transmitted to the second clad 13. Without being leaked into the support layer 14, the light is reliably incident on the first cladding 12.
[0055]
Next, as shown in FIG. 3D, the multimode optical fiber 2 is optically connected to the outer peripheral surface of the coupling portion 3.
[0056]
As a method for connecting the multimode optical fiber 2 and the coupling region 3, for example, as shown in FIG. 5, the tapered output end 2 a of the multimode optical fiber 2 is connected to the periphery of the single mode optical fiber 3. You may make it integrate with a heat | fever in the state along the surface. Further, as shown in FIG. 6, the tapered output end portions 2 a and 2 b of the multimode optical fiber 2 may be integrated by heat in a state of being wound around the outer peripheral surface of the coupling region 3.
[0057]
Next, a method for exciting a double clad fiber using the optical device according to the present embodiment will be described.
[0058]
In the optical device shown in FIG. 2, a light source device (not shown) that emits signal light to the single mode core 11 is optically connected to the incident end side of the double clad fiber 1. A laser light source (not shown) that emits excitation light is optically connected to the incident end side of the multi-mode optical fiber 2. The wavelength of the excitation light emitted from the laser light source needs to be the absorption wavelength of the amplification medium doped in the single mode core 11.
[0059]
Signal light enters the single mode core 11 of the double clad fiber from the light source device connected to the incident end of the double clad fiber 1. The incident signal light propagates while repeating total reflection in the single mode core 11. Excitation light is continuously incident on the multimode optical fiber 2 from the laser light source. The incident excitation light propagates while repeating total reflection in the multi-mode optical fiber 2 and enters the coupling region 3 of the double clad fiber 1. The excitation light incident on the coupling region 3 propagates in the coupling region 3, is narrowed down by the tapered portion 3 a, and is reliably incident on the first cladding 12 of the double clad fiber 1. The excitation light incident on the first cladding 12 is totally reflected in the first cladding 12 and propagates while repeating crossing with the single mode core 11. When the excitation light crosses the first cladding and is transmitted, it is absorbed by the doped amplification medium to form an inverted distribution character, and the signal light can be amplified by stimulated emission therefrom.
[0060]
Thus, according to the optical device and the excitation method of the double clad fiber according to the present embodiment, the cross-sectional area ratio in the coupling region 3 can be increased. Therefore, excitation light can be incident from the multimode optical fiber 2 to the coupling region 3 of the double clad fiber 1 with high coupling efficiency.
[0061]
In addition, the excitation light incident on the coupling region 3 is reliably propagated into the first cladding 12 of the double-clad fiber connected to the coupling region by the tapered portion 3a formed on the output end side of the coupling region 3. Therefore, the excitation light can be used with high efficiency. Thereby, the signal light propagating through the single mode core 11 of the double clad fiber 1 can be amplified with high efficiency.
[0062]
Specifically, in the optical device (comparative example) having the configuration shown in FIG. 7, the ratio of the amount of excitation light incident on the first cladding 12 to the amount of excitation light incident from the multimode optical fiber 3 (excitation light). 2 was about 10%, whereas in the optical apparatus having the configuration shown in FIG. 2, the incident efficiency of excitation light was 80% or more.
[0063]
Although one embodiment of the present invention has been described, the present invention is not limited to the above embodiment.
[0064]
In the above embodiment, a signal light source is connected to the incident end of the double clad fiber 1, but a pulse light source or a laser light source may be connected instead of the signal light source. Thus, the optical device according to the present invention can be applied to a pulse generator and a fiber laser device. Furthermore, a Fabry-Perot type fiber laser can be configured by disposing mirrors having a predetermined reflection wavelength on both sides of the double clad fiber 1.
[0065]
Further, in the optical device, a plurality of the coupling regions 3 are formed, and the propagation light of the single mode core 11 of the double clad fiber 1 is incident on the pumping light from the multimode optical fiber 2 connected to each region. Can be amplified in multiple stages.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a double clad fiber according to the present invention.
FIG. 2 is a conceptual diagram showing a configuration of an optical device according to the present invention.
FIG. 3 is a conceptual diagram showing a manufacturing process of the optical device according to the present invention.
FIG. 4 is a conceptual diagram illustrating a method for processing a coupling region of an optical device according to the present invention.
FIG. 5 is a diagram illustrating an example of a connection mode between a multimode optical fiber and a coupling region.
FIG. 6 is a diagram illustrating an example of a connection form between a multimode optical fiber and a coupling region.
FIG. 7 is a conceptual diagram showing a configuration of a conventional optical device.
FIG. 8 is a conceptual diagram showing a configuration of a conventional optical device.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Double clad fiber 11 Single mode core 12 1st clad 13 2nd clad 13a Pore 14 Support layer 15 Covering layer 2 Multimode type optical fiber 2a Taper part 2b Tip part 3 Coupling area 3a Taper part 4 Collapse area 5 Made of fluororesin tube

Claims (1)

Amplification medium doped single-mode core, a first clad covering the periphery of 該Shi ring le mode core, and a second composed of a porous structure comprising a plurality of pores that covers the periphery of the first cladding and a double clad fiber having a clad,
A multi-mode optical fiber whose exit end is tapered ,
With
The double clad fiber has a single mode fiber type structure having an outer diameter larger than that of the first clad formed in the middle portion by pulverizing the numerous pores of the porous structure constituting the second clad. Has a coupling area,
The coupling region is formed in a tapered shape so that the output end portion of the multimode optical fiber is optically connected to the outer peripheral surface thereof, and the output end side converges on the first cladding,
An optical apparatus characterized in that pumping light from the multi-mode optical fiber is incident on the first cladding through the coupling region of the double-clad fiber .

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