JP5531589B2 - Double clad fiber and fiber laser device - Google Patents

Description

  The present invention relates to a double clad fiber and a fiber laser device used for a laser.

  In recent years, fiber laser devices have attracted attention as light sources for lasers used in material processing.

  In particular, the use of double-clad fibers has led to widespread use in the field of material processing that requires high-power laser light that could only be achieved with a solid-state laser.

  In the case of such a fiber laser device, there is a limit to increasing the core diameter from the viewpoint of extracting high-quality laser light. On the other hand, from the viewpoint of extracting high-power laser light, high-power excitation light is required. However, high-power excitation light has a poor light collecting property, and an inner clad having a large cross-sectional area is required.

  Therefore, the double-clad fiber used in the conventional high-power fiber laser device has a waveguide that introduces pumping light and a waveguide that generates laser light in parallel as an inner cladding having a large cross-sectional area. (For example, refer to Patent Document 1 and Patent Document 2).

  In some cases, a flat portion is provided in a part of the inner cladding in order to reduce skew light (light that does not contribute to laser light generation) that is easily generated in the inner cladding having a large cross-sectional area (see, for example, Patent Document 3). .

  FIGS. 10 and 11 are cross-sectional views of a conventional double-clad fiber, in which a rare earth as a laser medium is added, a core 101 for confining the generated laser light, and an inner clad 102 for propagating excitation light to the core 101. However, the outer clad 103 and the inner clad 102 that encloses the core 101 are provided, and the outer clad 103 that confines the excitation light is provided so as to enclose the inner clad 102.

  The inner clad 102 includes an oscillation region that wraps around the core 101 and an excitation region that introduces excitation light. The cross-sectional shape of the inner clad 102 is substantially circular as shown in FIG. As shown in FIGS. 10 and 11, the oscillation region has a substantially circular shape with a flat portion in a part of the excitation region, and has the same shape over the entire length of the double clad fiber as shown in FIGS.

  The operation of the double clad fiber configured as described above will be described.

  Excitation light is injected into the inner cladding 102, and the injected excitation light propagates through the inner cladding 102.

  During propagation, the pumping light crosses the core 101 to pump the rare earth as a laser medium, generates pumping light, and is extracted as laser light by optical resonators (not shown) provided at both ends of the double clad fiber. It was.

JP 2001-144350 A (FIG. 1) Japanese Patent No. 3827819 (FIG. 3) JP 10-510104 A (FIGS. 3 and 4)

  However, since the conventional double clad fiber has the same cross-sectional shape in the fiber length direction, when skew light that does not contribute to generation of laser light is generated, there is a problem that the skew light is not reduced.

  In addition, when a flat portion is provided in a part of the excitation region of the inner clad 102 to reduce skew light, it is necessary to match the shape of the excitation light to be introduced with the shape of the inner clad 102. Since it is difficult to shape the excitation light into the provided shape and circular excitation light must be used, the excitation light of this flat portion is reflected and high efficiency excitation light introduction cannot be performed, or The excitation light has to be reduced in diameter so that the excitation light is not reflected and exposed.

  Further, in order to form the flat portion, it is necessary to mirror-polish the fiber preform in a flat shape, which is disadvantageous in terms of cost and enlargement of the preform.

  In view of these conventional problems, it is a first object of the present invention to provide a double-clad fiber that can efficiently introduce pumping light while reducing skew light, and a fiber laser that can efficiently extract laser light. A second object is to provide an apparatus.

  In order to solve the above problems, a double-clad fiber and a fiber laser device of the present invention include a core including a laser medium, an inner cladding that propagates excitation light to the core, and an outer cladding that confines the excitation light, The inner clad includes an excitation region having a substantially circular cross-sectional shape into which the excitation light is introduced, and an oscillation region having a substantially circular cross-sectional shape including the core, and a junction portion between the excitation region and the oscillation region. The width is changed along the length direction.

  As described above, since the width of the joint portion changes in the length direction of the double clad fiber, the optical path of the skew light in the inner clad changes in the length direction, and the skew light can be reduced.

  As described above, the present invention realizes high-efficiency introduction of excitation light by changing the width of the junction between the excitation region and the oscillation region of the inner cladding along the length direction, and generates skew light with a simple configuration. Can be reduced.

Sectional drawing of the double clad fiber in Embodiment 1 of this invention Sectional drawing in embodiment of the double clad fiber in Embodiment 1 of this invention 1 is a schematic configuration diagram of a fiber laser device according to a first embodiment of the present invention. Sectional drawing explaining the principle of the double clad fiber in Embodiment 1 of this invention Sectional drawing explaining the principle of the double clad fiber in Embodiment 1 of this invention Sectional drawing explaining the principle of the double clad fiber in Embodiment 1 of this invention Sectional drawing explaining the principle of the double clad fiber in Embodiment 1 of this invention Sectional drawing explaining the principle of the double clad fiber in Embodiment 1 of this invention Sectional drawing in other embodiment of the double clad fiber of this invention Cross section of conventional double clad fiber Cross section of conventional double clad fiber

  Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1)
1 is a cross-sectional view of a double-clad fiber according to Embodiment 1 of the present invention, FIG. 2 is a cross-sectional view of the embodiment of the double-clad fiber according to Embodiment 1 of the present invention, and FIG. 3 is Embodiment 1 of the present invention. FIG. 4 to FIG. 8 are sectional views for explaining the principle of the double clad fiber according to the first embodiment of the present invention.

  In FIG. 1, a core 11 for generating and transmitting laser light is made of a rare earth, which is a laser medium, preferably quartz glass to which at least ytterbium (Yb) is added, and its cross-sectional shape is substantially circular. However, the diameter of the core 11 can be selected from about 10 micrometers to about 30 micrometers, and there is an advantage that the larger the diameter of the core 11 is, the easier it is to absorb the excitation light. On the other hand, since the quality of the emitted laser beam is deteriorated, it can be designed together with the refractive index of the core so as to obtain a desired laser beam quality.

  The inner cladding 12 that propagates the excitation light to the core 11 is made of quartz glass, and includes an oscillation region 12a that includes the core 11 and has a substantially circular cross section, and an excitation region 12b that has a generally circular cross section into which the excitation light is introduced. And a joint portion 14 each of which is fused and integrated.

  In the present embodiment, the total length of the double clad fiber configured as described above is several meters to several tens of meters, and its cross-sectional shape changes along the length direction of the double clad fiber 10 as shown in FIG. I am letting.

  The cross-sectional shape of the inner clad 12 at both end faces (points 1A and 1E) of the double clad fiber 10 is a polished shape in which two circles are overlapped, each having a diameter of 400 micrometers, and its joining portion 14 Is about 20 micrometers wide.

  As shown in FIG. 2, the cross-sectional shape of the inner cladding 12 from the point 1A toward the center in the length direction of the double-clad fiber 10 (point 1C) is a junction in which a part of the excitation region 12b and the oscillation region 12a are fused and integrated. In addition, the width of the fusion-bonded joint portion 14 increases as it approaches the center in the length direction (point 1C).

  Similarly, the shape of the inner clad 12 from the point 1E to the point 1C is a shape having a joint part in which a part of the excitation region 12b and the oscillation region 12a are melted and integrated, and in addition, the melt-integrated joint The width of the portion 14 increases as it approaches the 1C point.

  The inner clad 12 includes a rare-earth-doped fiber preform having a rare-earth-doped core surrounded by a circular cross-section clad serving as the oscillation region 12a and an anhydrous synthetic quartz preform having a circular cross-section serving as the excitation region 12b. It is obtained by subtracting.

  The width of the melted and joined portion can be controlled by the spinning furnace temperature in the drawing process, the speed at which the base material is fed to the furnace, the drawing tension, and the like.

  The outer clad 13 is a low refractive index polymer used in the coating after the drawing, and protects the double clad fiber 10.

  Moreover, the refractive index is a structure which becomes low in order of the core 11, the inner clad 12, and the outer clad 13. The refractive index of the inner cladding 12 is preferably the same as that of the oscillation region 12a and the excitation region 12b.

  In the fiber laser device shown in FIG. 3, the power source 20 is connected to a semiconductor laser 21 with a fiber that generates pumping light, and drives the semiconductor laser 21 with a fiber.

  The fiber of the semiconductor laser 21 with a fiber has a core diameter of 400 μm and a cladding diameter of 440 μm, and the core is connected to the excitation region 12 b of the inner cladding 12 at the point 1A of the end surface of the double cladding fiber 10.

  Similarly, the power supply 25 is connected to a semiconductor laser 26 with a fiber that generates excitation light, and drives the semiconductor laser 26 with a fiber.

  The fiber of the fiber-attached semiconductor laser 26 has its core connected to the excitation region 12b of the inner cladding 12 at the end face 1E of the double cladding fiber 10.

  Further, at the point 1A of the end surface of the double clad fiber 10, in addition to the above connection, a highly reflective end 28 that feeds back a light of a desired wavelength of a fiber type having a core diameter of 10 micrometers and a clad diameter of 400 micrometers is provided in the oscillation region. It is connected to the core 11 of 12a.

  On the other hand, at the end face 1E point of the double clad fiber 10, the transmission fiber 29 for taking out a fiber type laser output having a core diameter of 10 micrometers and a clad diameter of 400 micrometers is connected to the core 11 of the oscillation region 12a. Connected to.

  The principle of reducing the skew light in the double clad fiber and the fiber laser device configured as described above will be described below with reference to FIGS.

  In general, skew light travels in a spiral along the propagation axis of an optical fiber, so when its optical path is projected onto a plane perpendicular to the propagation axis of the optical fiber, the skew light follows a trajectory along a certain inscribed circle. Draw.

  The diameter of the inscribed circle is slightly larger than that of the core 11, and the skew light projected on the inscribed circle having a very small diameter does not pass through the core 11 but corresponds to the skew light very close to the meridian light. To do.

  On the contrary, the diameter of the inscribed circle is approximately the same as the diameter of the excitation region 12b, and the skew light projected on the inscribed circle having a very large diameter passes through the portion of the inner cladding 12 that is farthest from the core 11. It passes through and corresponds to the skew light farthest from the meridian light.

  Hereinafter, the skew light of various optical paths is expressed by distinguishing the diameter of the inscribed circle drawn on the projection plane.

  FIG. 4 shows a case where the joint portion 14 is not fused and integrated at a point, and the cross section of the so-called excitation region 12b and the cross section of the oscillation region 12a are merely in contact at a point.

  In this case, the optical path through which the light beam in the excitation region 12b propagates to the oscillation region 12a is only the contact portion. In FIG. 4, a case is considered where skew light 54 along a certain large inscribed circle 51 propagates from the excitation region 12b to the oscillation region 12a.

  Even if the skew light 54 propagates from the excitation region 12b to the oscillation region 12a, this skew light becomes skew light along the inscribed circle 56 having the same diameter as the inscribed circle 51 in the oscillation region 12a.

  Therefore, when the skew light 58 along the inscribed circle 56 having the same diameter as the inscribed circle 51 returns from the oscillation region 12a to the excitation region 12b, the skew light again becomes along the inscribed circle 51.

  The same applies to the skew light 53 along the inscribed circle 52 having a small diameter, and becomes the skew light 57 along the inscribed circle 55 having the same diameter in the oscillation region 12a.

  Therefore, unless the diameter of the inscribed circle changes, the skew light passes through the same place and does not cross the core.

  FIG. 5 shows a case where a certain width is given to the joint portion 14 and the excitation region 12b and the oscillation region 12a are gently integrated.

  In this case, when the light beam of the excitation region 12b propagates to the oscillation region 12a, there are many possible routes for the optical path passing through the joint portion 14 other than the above-described contact portion.

  That is, in the excitation region 12b, skew light 60 and 61 along a certain inscribed circle 59 is converted into skew light along inscribed circles 63 and 64 that differ in magnitude in the oscillation region 12a by a route passing through the joint portion 14. The

  The skew light 60 that has passed through the center of the joint portion 14 becomes skew light along the inscribed circle 63 having the same diameter as the inscribed circle 59 in the oscillation region 12 a, but the oscillation region 12 a has a shallower angle than the skew light 60. In the oscillating region 12a, the optical path of the skew light 61 propagating in the direction is converted into skew light along the inscribed circle 64 having a larger diameter.

  On the other hand, the skew light 62 propagating through the joint portion 14 to the oscillation region 12a at an angle deeper than the skew light 60 is optically converted into skew light along the inscribed circle having a small diameter in the oscillation region 12a. As indicated by the chain line, it becomes skew light that can pass through the core 11 and is absorbed by the core 11 in the oscillation region 12a.

  As described above, a part of the skew light passes through the core 11 by increasing the width of the joint portion 14.

  Next, FIG. 6 shows a case where the width of the joint portion is further enlarged.

  In this case, the same phenomenon as in FIG. 5 occurs, but the optical path is changed so that the skew light 66 along the inscribed circle 65 having a larger diameter can pass through the core 11 as compared with the case of FIG. .

  As described above, of the skew light existing in the excitation region 12b, part of the skew light along the inscribed circle having a larger diameter can propagate to the oscillation region 12a and pass through the core 11. At the same time, it can be seen that the skew light along the inscribed circle with a small diameter among the skew light existing in the excitation region 12b can pass through the core 11 with a greater probability.

  Finally, FIG. 7 and FIG. 8 will be described.

  FIG. 7 shows a case where the fusion integration is further advanced than in FIG.

  In this case, all of the skew light 67, 68, 69 existing in the excitation region 12b, that is, even the skew light 69 farthest from the meridian light inscribed in the circular cross section of the excitation region 12b may pass through the core 11. become able to do.

  In order for the inscribed tangent line of the excitation region 12 b to pass through the core 11, it is necessary to increase the width 70 of the junction until the inner cladding 12 includes a tangent line that circumscribes both the core 11 and the excitation region 12 b.

  FIG. 8 shows a special case of FIG. 7 in which both the excitation region 12b and the oscillation region 12a maintain a regular circular cross section even after fusion integration.

  In this case, the contact point of the tangent line inscribed in the excitation region 12b is on the boundary between the excitation region 12b and the oscillation region 12a, and the distance between the center of each circular cross section of the excitation region 12b and the oscillation region 12a is obtained by geometric calculation. 74 can be expressed as (r + R) × √2 using the radius r of the core 11 and the radius R of the excitation region 12b. From the above, it can be seen that in order to completely eliminate the skew light component of the excitation light, it is necessary to expand the width of the joint portions 14 and 70 until the state shown in FIG. 7 or FIG. 8 is reached.

  The operation of the double clad fiber 10 configured as described above and the fiber laser device using the same will be described with reference to FIGS. 1, 2, and 3.

  First, a semiconductor laser 21 with a fiber driven by a power supply 20 generates pumping light having a wavelength for pumping rare earth, which is a laser medium added to the core 11.

  The excitation light is transmitted through a fiber having a circular cross section, and is injected into the excitation region 12b of the inner cladding 12 at the point 1A. At this time, since the cross-sectional shape of the excitation region 12b of the inner cladding 12 does not have a flat portion, the excitation light can be efficiently injected into the excitation region 12b of the inner cladding 12.

  The injected excitation light propagates through the excitation region 12b and the oscillation region 12a of the inner cladding 12. The excitation light injected at the point 1A reaches the point 1C via the point 1B while propagating through the excitation region 12b and the oscillation region 12a of the inner cladding 12. During this time, the width of the junction portion 14 between the excitation region 12b and the oscillation region 12a increases, and the skew light that could not pass through the core 11 undergoes a change in the optical path according to the principle described above, and becomes a light beam that contributes to excitation.

  Further, the excitation light is similarly injected into the opposite end face of the double clad fiber, and the skew light is reduced as described above.

  The light generated by the excitation is subjected to multiple feedback amplification in an optical resonator constituted by the high reflection end 28 and the end face of the transmission fiber 29 to become laser light, and is extracted from the end face of the transmission fiber 29.

  In this embodiment, the inner clad 12 is not provided with a flat portion, but the same effect can be obtained by providing a flat portion in the oscillation region 12a of the inner clad 12 as shown in FIG.

As described above, the double-clad fiber and the fiber laser apparatus according to the present embodiment include the core 11 including the laser medium, the inner cladding 12 that propagates the excitation light to the core 11, and the outer cladding 13 that confines the excitation light. The inner cladding 12 includes an excitation region 12b having a substantially circular cross-sectional shape into which excitation light is introduced, and an oscillation region 12a having a substantially circular cross-sectional shape including the core 11, and a junction portion between the excitation region 12b and the oscillation region 12a. width of 14 Ru der those varied along its length.
In addition, when the width of the joint portion 14 is maximized, a tangent line circumscribing both the core 11 and the excitation region 12 b is included in the inner clad 12 . Further, a flat portion having a flat cross-sectional shape is provided at a portion other than the joint portion 14 of the oscillation region 12b, and has one or more excitation light incident end faces, and the distance from the excitation light incident end face increases. The width of the joint portion 14 was increased .

  As described above, the width of the joint portion changes in the length direction of the double clad fiber, so that the optical path of the skew light in the inner clad changes in the length direction, and the skew light can be reduced. The rare earth that is the medium can be excited, and laser light can be extracted efficiently.

  The double-clad fiber and the fiber laser device according to the present invention can introduce pumping light efficiently by changing the width of the junction between the pumping region and the oscillation region along the length direction of the double-clad fiber, and at the same time, Since skew light propagating through the clad can be reduced, it is useful as a double clad fiber and a fiber laser device.

DESCRIPTION OF SYMBOLS 10 Double clad fiber 11 Core 12 Inner clad clad 12a Oscillation area | region 12b Excitation area | region 13 Outer clad 14 Junction part

Claims (5)

  A core including a laser medium, an inner cladding that propagates pumping light to the core, and an outer cladding that traps the pumping light, the inner cladding having a substantially circular cross-sectional shape into which the pumping light is introduced A double-clad fiber comprising a region and an oscillation region having a substantially circular cross-sectional shape including the core, wherein the width of the junction between the excitation region and the oscillation region is changed along the length direction.   The double clad fiber according to claim 1, wherein a tangent line circumscribing both the core and the excitation region is included in the inner clad when the width of the joint portion is maximized.   The double clad fiber according to claim 1, wherein a flat portion having a flat cross-sectional shape is provided at a portion other than the joining portion of the oscillation region. Have one or more excitation light incident end surface, double clad fiber according to any one of claims 1 to 3, it increased the width of the joint portion with increasing distance from the excitation light incident end surface . A fiber laser device using the double clad fiber according to any one of claims 1 to 4 .