JP6018435B2 - Double clad optical fiber - Google Patents

Description

  The present invention relates to a double clad optical fiber.

  In fiber lasers and optical fiber amplifiers, double clad optical fibers are incorporated as optical amplification elements. Such a double-clad optical fiber has a core doped with an optical amplification component such as a rare earth element, a pump guide provided so as to cover the core, and a clad provided so as to further cover the pump guide. When the pump light is incident on the pump guide, the pump light propagates through the region surrounded by the clad while repeating reflection at the interface between the pump guide and the clad, and passes through the core. Is activated, and the activated light amplification component amplifies the light propagating through the core.

  By the way, if the cross-sectional shape of the outline of the pump guide is concentric with the core, the pump light contains a large amount of components that do not pass through the core, so-called skew light, and the efficiency is lowered. The outer cross-sectional shape is formed in a non-circular shape.

  Patent Document 1 discloses a double-clad optical fiber having a D-shaped cross-sectional shape of the outer periphery of the pump guide.

  Patent Document 2 discloses a double-clad optical fiber having a polygonal cross-sectional shape of the outline of a pump guide.

Japanese Patent No. 3830969 JP 2011-171619 A

  By the way, when the cross-sectional shape of the outline of the pump guide is formed in a non-circular shape, and the core is displaced from the position of the center of gravity in the cross-sectional shape of the pump guide, even if the core is arranged at the center of the fiber, When fusion splicing to the optical fiber, the position of the core moves at the end of the connection due to the effect of surface tension, resulting in misalignment and increased connection loss for light propagating through the core. There is a problem that the oscillation efficiency and the conversion efficiency of the optical fiber amplifier are lowered.

  The subject of this invention is providing the double clad optical fiber which can suppress the fall of the oscillation efficiency at the time of applying to a fiber laser apparatus, and the fall of the conversion efficiency at the time of applying to an optical fiber amplifier.

  The present invention is a double clad optical fiber comprising a core, a pump guide provided so as to cover the core, and a clad provided so as to cover the pump guide. The outer cross-sectional shape is cut out from a regular n-gonal shape where n is any of 6-9 so that the core is located at the center of gravity and the cross-sectional shape of the pump guide outer shell is rotationally symmetric. The number of non-regular polygons is n or more.

  According to the present invention, the cross-sectional shape of the outer contour of the pump guide is changed from a regular n-gonal shape where n is any one of 6 to 9, and the cross-sectional shape of the outer contour of the pump guide is rotated while the core is disposed at the center of gravity. A non-circular and non-regular polygon whose number of corners is n or more, which is partly cut out so as to be symmetrical, so that the core is arranged at the center of gravity position in the cross-sectional shape of the pump guide. Even at the time of fusion splicing with the optical fiber, the position of the core does not move at the connection end due to the influence of the surface tension, so the connection loss due to misalignment is small, resulting in a fiber laser device. A decrease in oscillation efficiency when applied and a decrease in conversion efficiency when applied to an optical fiber amplifier can be suppressed.

It is a perspective view of the double clad optical fiber core wire concerning an embodiment. (A) And (b) is a cross-sectional view of the modification of a double clad optical fiber. (A) is a figure which shows the aspect by which the corner | angular part of the cross-sectional shape of the outline of a pump guide was comprised by the intersection of a straight line and a straight line, (b) is comprised by the shape which a corner | angular part is blunted and has roundness. FIG. It is a figure which shows schematic structure of the fiber laser apparatus using the double clad optical fiber core wire which concerns on embodiment. It is a perspective view of an entrance end side optical fiber core wire and an exit end side optical fiber core wire. It is explanatory drawing of the outline of the pump guide in a double clad optical fiber. (A) And (b) is explanatory drawing of the manufacturing method of the double clad optical fiber core wire which concerns on embodiment.

  Hereinafter, embodiments will be described in detail based on the drawings.

  FIG. 1 shows a double-clad optical fiber core F according to this embodiment. The double clad optical fiber core F according to the present embodiment is an optical amplifying element that is used in a fiber laser device or an optical fiber amplifying device, for example.

  The double-clad optical fiber core F according to the present embodiment includes a double-clad optical fiber 10 and a coating layer 20 that coats the double-clad optical fiber 10. The cross-sectional shape of the double clad optical fiber core F is basically circular, and its outer diameter is, for example, 200 to 1500 μm.

  The double clad optical fiber 10 includes a core 11, a pump guide 12 provided so as to cover the core 11, and a clad 13 provided so as to cover the core. The cross-sectional shape of the double clad optical fiber 10 is also basically circular, and its outer diameter is, for example, 150 to 800 μm.

  The core 11 is disposed at the center of the fiber and is made of, for example, quartz doped with a light amplification component such as a rare earth element such as erbium (Er), ytterbium (Yb), or neodymium (Nd). The concentration of the light amplification component is, for example, 1000 to 30000 ppm. In addition, the core 11 may be doped with other dopants such as germanium (Ge). The cross-sectional shape of the outer shell of the core 11 is preferably circular, and the outer diameter thereof is, for example, 5 to 50 μm.

  The pump guide 12 is made of, for example, pure quartz not doped with a dopant, or quartz doped with a refractive index reducing component such as fluorine (F) or boron (B), and has a lower refractive index than the core 11. Rate. When the pump guide 12 is formed of quartz doped with a refractive index lowering component, the concentration of the refractive index lowering component is, for example, 5000 to 50000 ppm.

  The cross-sectional shape of the outline of the pump guide 12 is a regular n-gonal shape where n is any one of 6 to 9, and the core 11 is disposed at the center of gravity, and the cross-sectional shape of the outline of the pump guide 12 is rotationally symmetric. Thus, the shape is such that the number of non-regular polygons is n or more, with a part removed.

  Specifically, as shown in FIG. 1, the cross-sectional shape of the outer contour of the pump guide 12 was cut out from isosceles triangles at three corners so as to be 120 ° rotationally symmetric from a regular hexagon. A non-regular nine-sided nine-sided polygon. In addition, as another cross-sectional shape of the outer shape of the pump guide 12, for example, as shown in FIG. 2A, a trapezoidal portion along the four sides is cut away from the regular octagon so as to be 90 ° rotationally symmetric. An octagon that is not a regular octagon with eight corners may be used, and as shown in FIG. 2 (b), an indefinite portion (a sector portion of a corner portion) that is 180 ° rotationally symmetric from a regular hexagon. ) May be cut out and the number of corners may be eight. As long as the cross-sectional shape of the outline of the pump guide 12 is rotationally symmetric, it may be, for example, 60-degree rotational symmetry, 90-degree rotational symmetry, 120-degree rotational symmetry, 180 It may be rotationally symmetric. The outline of the pump guide 12 may be constituted only by a line segment, and may include an outwardly convex curve or an inwardly concave curve. Here, since the double-clad optical fiber 10 is manufactured by heating in a drawing process as will be described later, the corner of the outer cross-sectional shape of the pump guide 12 in this embodiment is shown in FIG. As shown in FIG. 3B, a shape that is blunted and rounded is also included in addition to the shape that is formed by the intersection of the straight line and the straight line. Further, the corners include those formed at the boundary between the straight line and the straight line, those formed at the boundary between the straight line and the curved line, and those formed at the boundary between the curved line and the curved line. The layer thickness of the pump guide 12 is, for example, 70 to 400 μm including the thin part and the thick part.

  The clad 13 is made of, for example, an ultraviolet curable resin, a thermosetting resin, or quartz, and has a lower refractive index than the pump guide 12. The outer cross-sectional shape of the cladding 13 is preferably circular, and the outer diameter is, for example, 150 to 800 μm.

  The coating layer 20 is made of, for example, a resin such as an ultraviolet curable resin, a silicon resin, or a nylon resin. The coating layer 20 may be composed of a single layer or may be composed of a plurality of layers. The layer thickness of the coating layer 20 is, for example, 25 to 300 μm.

  FIG. 4 shows a fiber laser device 30 using a double clad optical fiber core F according to this embodiment. FIG. 5 shows the incident end side optical fiber core wire 31 and the outgoing end side optical fiber core wire 32 used therein.

  This fiber laser device 30 includes an input end side optical fiber core 31 and an output end side optical fiber in which fiber gratings 31b and 32b are written in cores 31a and 32a at one end and the other end of a double clad optical fiber core F, respectively. The core wire 32 is fusion-connected.

  The incident end side optical fiber core wire 31 and the output end side optical fiber core wire 32 are preferably double clad optical fiber core wires in which the cores 31a and 32a are not doped with an optical amplification component. In that case, the cross-sectional shapes of the outer sides of the pump guides 31c and 32c of the incident end side optical fiber core wire 31 and the outgoing end side optical fiber core wire 32 do not need to reduce skew light, so that an extra manufacturing process is required. It is preferably a circular shape that can be manufactured inexpensively. The incident-side optical fiber core 31 and the double-clad optical fiber core F are more confined with pumping light as the pump guide NA is larger, and more pumping light can be confined. It is desirable to be as large as possible and often results in the same size. In this case, the outer diameter of the pump guide 31c of the incident end side optical fiber core 31 is larger than the outer diameter of the circle having the largest area in the cross-sectional shape of the outer shape of the pump guide 12 of the double clad optical fiber core F. Is larger, the pump guide 31c of the incident-side optical fiber core 31 has a portion protruding from the pump guide 12 of the double-clad optical fiber core F at the fusion spliced portion, and as a result, larger than the pump guide NA. A part of the pumping light having NA is generated, and the pumping light whose NA is larger than the pump guide NA is not input to the double-clad optical fiber core F, so that loss occurs. In the worst case, the coating layer 20 may generate heat and burn out due to excitation light that has not been input. Therefore, the outer diameter of the pump guide 31c of the incident end side optical fiber core 31 is the same as the outer diameter of the circle having the maximum area in the cross-sectional shape of the outer shape of the pump guide 12 of the double clad optical fiber core F. It is more preferable that The incident end side optical fiber core wire 31 is connected to an excitation light source 33. The excitation light source 33 is composed of, for example, a semiconductor laser, a lens, and an optical fiber. The pumping light emitted from the pumping light source 33 depends on the kind of rare earth element doped in the core 11 of the double clad optical fiber 10. For example, when the rare earth element is erbium (Er), the wavelength is about 0.98, 1.48 μm. In the case of ytterbium (Yb), the wavelengths are about 0.915 and 0.98 μm, and in the case of neodymium (Nd), the wavelength is about 0.8 μm.

  In this fiber laser device 30, when the excitation light from the excitation light source 33 is input to the incident end side optical fiber core 31 and further input to the double clad optical fiber core F, in the double clad optical fiber 10, The pumping light propagates through the core 11 and the pump guide 12 region surrounded by the clad 13 while being reflected at the interface between the pump guide 12 and the clad 13, and excites the light amplification component when passing through the core 11. The light propagating through the core 11 is amplified when the excited light amplification component shifts to a lower level state. The amplified light is input from the double-clad optical fiber core F to the core 32a of the output-end optical fiber core 32, reflected by the fiber grating 32b written therein, and again double-clad optical fiber core F. And is further amplified while propagating through the core 11. Then, the light is input from the double clad optical fiber core F to the core 31a of the incident end side optical fiber core 31, reflected by the fiber grating 31b written therein, and again of the double clad optical fiber core F. The signal is input to the core 11 and further amplified while propagating through the core 11. In this way, the light is applied to the fiber gratings 31b written in the cores 31a and 32a of the incident end side optical fiber core wire 31 and the output end side optical fiber core wire 32 connected to both ends of the double clad optical fiber core wire F. The light is reflected by 32b and repeatedly propagated while being amplified through the core 11 of the double-clad optical fiber F. That is, the laser is oscillated by the resonator constituted by the fiber gratings 31b and 32b, and a part of the laser is emitted as the output side light. Output from the fiber core wire 32.

  According to the double-clad optical fiber core F according to the present embodiment having the above-described configuration, the outer cross-sectional shape of the pump guide 12 of the double-clad optical fiber 10 is a regular n-gonal shape in which n is any one of 6-9. From the above, the core 11 is arranged at the center of gravity and a part of the outer shape of the pump guide 12 is cut away so that the cross-sectional shape is rotationally symmetric. Therefore, the core 11 is arranged at the position of the center of gravity in the cross-sectional shape of the pump guide 12, so that it can be fused with other optical fibers such as the incident end side optical fiber core wire 31 and the output end side optical fiber core wire 32. Even at the time of incoming connection, the position of the core 11 does not move due to surface tension at the connection end, so that connection loss due to misalignment is small, and as a result, the fiber laser device 30. It is possible to suppress the reduction of the definitive oscillation efficiency. Similarly, when the double clad optical fiber core F according to the present embodiment is applied to an optical fiber amplifying device, a decrease in the conversion efficiency can be suppressed.

  Moreover, when forming the cross-sectional shape of the outline of a pump guide into a polygon, it is preferable that the cross-sectional shape of the outline of a pump guide is a polygon with few corners from a viewpoint of suppressing skew light. Since the absorption coefficient of the excitation light is proportional to the area of the core / the area of the pump guide, the smaller the area of the pump guide, the more efficiently the excitation light is absorbed by the core. On the other hand, the beam quality of the emitted laser deteriorates as the core area increases, so the upper limit of the core area is determined by the desired beam quality. In addition, when the outer diameter of the circle having the largest area in the cross-sectional shape of the outer shape of the pump guide is constant, the polygon that has a small number of corners has a large area that protrudes from the circle. . Therefore, from the viewpoint of increasing the absorption coefficient, the polygon is preferably close to a circle. However, according to the double-clad optical fiber core F according to the present embodiment, the outer cross-sectional shape of the pump guide 12 of the double-clad optical fiber 10 is a regular n-gonal shape where n is any one of 6 to 9, A non-circular and non-regular polygonal polygon with n or more corners, in which the core 11 is disposed at the center of gravity and a part thereof is cut so that the cross-sectional shape of the outer shape of the pump guide 12 is rotationally symmetric. Therefore, suppression of skew light can be expected, and by increasing the area of the core 11 with respect to the area of the pump guide 12, the excitation light efficiently passes through the core 11 and the absorption coefficient of the excitation light is increased. Thus, the fiber length can be shortened, and as a result, the nonlinear effect can be reduced. The cross-sectional shape of the outer contour of the pump guide 12 is preferably a shape close to an inscribed circle of a regular n-gon. Therefore, as shown in FIG. The formed portion 12a (side) is preferably orthogonal to a line L connecting the apex S of the regular n-gon and the center of gravity G. In addition, as shown in FIG. 6, it is preferable that a portion 12 a (side) formed by cutting a part from a regular n-gon in the outer shape of the pump guide 12 is in contact with a regular n-gon inscribed circle O. The area of the core 11 relative to the area of the pump guide 12 (the area of the core 11 / the area of the pump guide 12) is preferably 0.0005 to 1, and more preferably 0.001 to 1. The fiber length is preferably 100 m or less, for example, and more preferably 50 m or less.

  When excitation light is incident on the double-clad optical fiber core F as spatial light, it is considered that the excitation light is almost circular with a certain size due to transmission so far. The effect of increasing the absorption coefficient of the excitation light is when the double-clad optical fiber core F according to the present embodiment, the incident end side optical fiber core 31 and the output end side optical fiber core 32 are fusion-connected. The present invention is not limited to this, and it can also be obtained in the case where they are spatially coupled and connected at intervals.

  Next, an example of a manufacturing method of the double clad optical fiber core F according to the present embodiment will be described.

  First, as shown in FIG. 7A, a core forming portion 41 is provided at the center axis position and covered by a known method such as the MCVD method, the VAD method, the OVD method, or the rod-in-tube method. In this way, a quartz column member 40 ′ provided with the pump guide forming part 42 is produced.

  Subsequently, the outer periphery of the cylindrical member 40 ′ is polished and ground to produce a preform 40 having the same cross-sectional shape as the pump guide shape as shown in FIG.

  The preform 40 is drawn into a fiber, and the fiber is immersed in an ultraviolet curable resin or a thermosetting resin for clad formation, and then irradiated with ultraviolet rays to form an ultraviolet curable resin. The double-clad optical fiber 10 is produced by heating or curing the thermosetting resin, and the double-clad optical fiber 10 is further coated with a resin for forming a coating layer to thereby double-clad according to the present embodiment. An optical fiber core wire F can be obtained.

  In the above embodiment, the double-clad optical fiber core F is used for the fiber laser device 30. However, the present invention is not limited to this, and the signal light is propagated to the core 11 to amplify the signal light. The structure used for the optical fiber amplifying apparatus may be used, and the reduction in the amplification efficiency due to the connection loss with other optical fiber cores can be suppressed.

  Moreover, in the said embodiment, although the preform 40 which coat | covered the core formation part 41 with the pump guide formation part 42 was drawn, it showed an example of the manufacturing method which coat | covers with the resin clad 13 what was fiber-formed, The present invention is not particularly limited to this, and the one in which the core forming part is covered with the pump guide forming part and further clad and attached to the preform is used as a preform, which is drawn into a double clad light. The fiber 10 may be manufactured.

(Double clad optical fiber core wire)
<Comparative example>
Draw a part of a regular hexagonal cross section having a core forming portion doped with ytterbium (Yb) at the central axis position so that the inscribed circle of the regular hexagonal cross section is 210 μm, and This was coated with an ultraviolet curable resin to form a double-clad optical fiber, which was further coated with a resin for forming a coating layer, thereby producing a double-clad optical fiber core wire of a comparative example. As the double-clad optical fiber core wire of this comparative example, two lots 1 and 2 were produced.

<Example>
The preform remaining in the comparative example is isosceles so that the inscribed circle is the same as the inscribed circle of the original regular hexagon at every other three corners of the six corners of the regular hexagonal cross section. The triangular portion was ground to form a nine-sided cross section (see FIG. 6 for illustration). In order to reduce the influence of the absorption coefficient of the core and the outer diameter of the core when compared with the comparative example, the preform of the nine-sided cross section is 210 μm in the inscribed circle of the nine-sided cross section as in the comparative example. Then, it is coated with an ultraviolet curable resin to form a double clad optical fiber, and further coated with a resin for forming a coating layer, thereby producing the double clad optical fiber core of the example. did. As the double-clad optical fiber core wire of this example, lots 1 and 2 corresponding to lots 1 and 2 of the comparative example were produced. The ratio of the area of the pump guide to the comparative example of the example was 96.4% when calculated from the area calculation of a regular hexagon and a hexagon.

(Absorption coefficient)
The fiber length of 60 m of each of the double-clad optical fiber cores of the example and the comparative example is wound on a bobbin having a winding diameter of 150 mm, and light from a white light source is incident on one fiber end and the other fiber end Absorption at a wavelength of 915 nm was measured for the light emitted from the light source using a spectrum analyzer. Then, while shortening the fiber length by 5 m, the absorption is measured in the same way for each fiber length up to the final fiber length of 5 m, and the inclination obtained by linearly approximating the relationship between the fiber length and the absorption is absorbed. Coefficient.

(result)
The results are shown in Table 1.

  The absorption coefficients of lot 1 in the example were 0.419 and lot 2 were 0.387, and comparative example lot 1 was 0.393 and lot 2 were 0.374. Therefore, the ratio of the absorption coefficient of the comparative example to the example was 93.8% for lot 1 and 96.6% for lot 2.

  According to the above results, both of the lots 1 and 2 have a larger absorption coefficient in the example than in the comparative example. Further, the ratio of the absorption coefficient of the comparative example to the example is approximately the same as the ratio of the area of the pump guide of the example to the comparative example. The absorption coefficient of the pump guide is larger in the embodiment than the comparative example because the area of the pump guide is smaller than in the comparative example. From these facts, it is presumed that the absorption coefficient per unit area of the pump guide is almost the same, and it can be seen that the effect of suppressing the skew light is obtained in the example as much as in the comparative example. Therefore, according to the double clad optical fiber core wire of the embodiment, it can be expected that the oscillation efficiency when applied to the fiber laser device and the reduction of the conversion efficiency when applied to the optical fiber amplifying device are expected.

  The present invention is useful for double clad optical fibers.

F Double-clad optical fiber core G Center-of-gravity L Line S connecting vertex and center of gravity S Vertex 10 Double-clad optical fiber 11, 31a, 32a Core 12, 31c, 32c Pump guide 12a Partially cut from a regular n-gonal shape Part (side)
13 Cladding 20 Coating layer 30 Fiber laser device 31 Incident end side optical fiber cores 31b and 32b Fiber grating 32 Outgoing end side optical fiber core wire 33 Excitation light source 40 Preform 40 'Column material 41 Core forming part 42 Pump guide forming part

Claims (3)

A double-clad optical fiber comprising a core doped with a rare earth element, a pump guide provided so as to cover the core, and a clad provided so as to cover the pump guide,
The cross-sectional shape of the outer contour of the pump guide is a regular hexagon, and the triangles at three corners are arranged so that the core is disposed at the center of gravity and the cross-sectional shape of the outer contour of the pump guide is 120 ° rotationally symmetric. A double-clad optical fiber having a nine-sided non-regular polygonal shape with nine corners removed . A double-clad optical fiber comprising a core doped with a rare earth element, a pump guide provided so as to cover the core, and a clad provided so as to cover the pump guide,
The cross-sectional shape of the outer periphery of the pump guide is a regular n-gonal shape where n is any one of 6 to 9, and the cross-sectional shape of the outer shape of the pump guide is rotationally symmetric while the core is disposed at the center of gravity. The shape is a part of which the number of non-regular polygons is n or more.
A double clad optical fiber in which a part formed by cutting a part of the outer periphery of the pump guide from the regular n-gon is perpendicular to a line connecting the apex and the center of gravity of the regular n-gon. A double-clad optical fiber comprising a core doped with a rare earth element, a pump guide provided so as to cover the core, and a clad provided so as to cover the pump guide,
The cross-sectional shape of the outer periphery of the pump guide is a regular n-gonal shape where n is any one of 6 to 9, and the cross-sectional shape of the outer shape of the pump guide is rotationally symmetric while the core is disposed at the center of gravity. The shape is a part of which the number of non-regular polygons is n or more.
A double clad optical fiber in which a part formed by cutting a part of the outer periphery of the pump guide from the regular n-gon is in contact with an inscribed circle of the regular n-gon.

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