JP2018055043A - Optical fiber combiner and laser device - Google Patents

Abstract

An object of the present invention is to suppress the increase in heat of a combiner and reduce the possibility of damage to a fiber and a combiner. An optical fiber combiner 39 having a plurality of optical fibers 37a to 37c, each optical fiber having a core 57, an inner cladding 59 surrounding the core, and an intermediate cladding 62 surrounding the inner cladding 59. The refractive index of the intermediate cladding 62 is smaller than the refractive index of the inner cladding 59. The light that is going to travel from the inner cladding 59 to the intermediate cladding 62 is reflected at the interface between the inner cladding 59 and the intermediate cladding 62 when the incident angle becomes larger than the critical angle (when the intermediate cladding is viewed from the inner cladding). . [Selection] Figure 8

Description

  The present invention relates to an optical fiber combiner that couples a plurality of optical fibers to another optical fiber, an optical fiber used in the combiner, and a laser apparatus using the optical combiner.

  FIG. 1 is an explanatory diagram of a general optical fiber combiner (combiner).

  As shown in the figure, the optical fiber combiner 100 includes an input fiber 101 and a capillary tube (capillary) 103. The capillary tube (capillary) 103 includes a region 104 that accommodates the distal end portion of the input fiber 101, is tapered in the region 102, and holds the input fiber 101 that is capillaryized. The tip of the input fiber 101 and the capillary 103 are melted in the region 104. The melted combiner is cut at a position 105 and joined to the cut end of the output fiber 106 at the cutting position 105.

  Such a tapered optical fiber combiner is manufactured as follows, for example.

  First, a capillary 103 having a tapered portion 102 is created.

  Next, as will be described in detail later, the thickness of the clad material at the tips of the plurality of input fibers 101 is removed by etching and then thinned to align them with a bundle, and this bundle is inserted into the capillary 103.

  Thereafter, the tip portion is melted and further tapered, and then the tip portion 105 of the waist portion 104 is cut. Then, the cutting part 105 is bonded to the output fiber 106.

  Of these processes, tapering is the most important process. That is, in a combiner that bundles low-order mode (LM) and multi-mode (MM) input fibers, the usual method of bundling a plurality of fibers is to form a capillary 103 having a low refractive index by doping impurities. Inserting them in parallel (FIG. 1). Such a capillary can be a simple tube form or a complicated form such as a preform having a plurality of holes as described in Patent Document 1.

  The reason why the thickness of the clad material at the tip of the input fiber is removed by etching is as follows.

  That is, the input fiber of the combiner is substantially the same as the output fiber of, for example, a fiber laser (fiber amplifier). By the way, typical output fibers of high-power fiber lasers are double clad (double clad (DC)) fibers, which have a core diameter of about 20 to 30 μm and an inner clad diameter of 400 to 400 μm. It has a size of about 500 μm. This results in a large spacing between the fiber cores. Therefore, the taper ratio increases, and the beam numerical aperture NA at the combiner input end becomes very large. As a result, the propagation efficiency is lowered and the beam quality is lowered. Furthermore, as the taper ratio increases, the dimensions of the device increase and packaging becomes difficult.

  Therefore, the diameter of the clad at the tip of the fiber is partially removed by etching.

  FIGS. 2A and 2B are explanatory views showing etching of a clad in a double-clad fiber. FIG. 2A shows a fiber before etching, and FIG. 2B shows a fiber after etching.

  As shown in the figure, the clad 109 is etched at the boundary 108 at the fiber tip to reduce the diameter. For example, in FIG. 2B, the surface 111 has a diameter of about 100 μm. In FIG. 2A, the outermost layer 110 is an outer cladding made of a double-layer fluoroacrylate polymer.

US Patent Publication No. 2009 / 0154881A1

  However, the conventional combiner has the following problems. That is, in the portion where the bundle of fibers is tapered, an air gap exists between the outer surface of the fiber and the inner surface of the capillary. Even after the fiber and the capillary are melted into a single object, an air gap exists in the part. Because of this air gap, there may be microscopic contaminants on the outer surface of the fiber.

  On the other hand, in the optical fiber portion where the taper is formed, the power density on the side surface of the fiber becomes extremely large. For example, in a tapered combiner, a double clad fiber (FIG. 2) of an etched input fiber has a core of 30 μm (NA 0.075) and a quartz clad of 100 μm or less to 10 MM with a taper ratio of 4.35. It is assumed that the taper is formed over the entire area. This taper is set to couple the fiber to a 50 μm output fiber. Consider a case where SM or MM rays are input to this combiner. For example, the SM wave is LP01, and the MM wave means seven equally stimulated or exited modes (LP01, LP11, LP21, LP02, LP31, LP12, LP41). Then, the power load on the side surface of the fiber with the MM input wave becomes much larger than that with the SM input wave as it approaches the taper waist (for example, when the taper ratio is greater than 3.75, the former is 20 times or more of the latter). ).

  Therefore, a local high heating spot may be formed by the high-intensity laser beam even on the side surface of the input fiber, even with microscopic contaminants. Heat from such spots cannot be easily removed. Therefore, defects and the like are formed on the side surface of the fiber, and the formed defect grows and the fiber and the combiner may be damaged.

  This invention is providing the combiner which solves the said subject, the optical fiber used for this combiner, and the laser apparatus using this combiner.

  The optical fiber combiner of the present invention is an optical combiner having a plurality of input optical fibers, and each input optical fiber has a core, an inner cladding, and an intermediate cladding formed outside the inner cladding. . Here, the refractive index of the intermediate cladding is smaller than the refractive index of the inner cladding.

  According to the optical fiber combiner, contaminants existing inside the capillary tube only contact the intermediate cladding and do not contact the inner cladding. On the other hand, most of the laser light propagating through the inner cladding is totally reflected at the boundary surface between the inner cladding and the intermediate cladding. Therefore, most of the laser light propagating through the inner cladding does not come into contact with contaminants attached to the outside of the intermediate cladding. Therefore, the intensity of the laser beam for heating the combiner can be reduced.

In the combiner, it is desirable that the numerical aperture NA ₂ of the intermediate cladding is larger than the numerical aperture NA _Lxy based on the multimode modal index and smaller than the numerical aperture NAo of the output fiber.

  According to the combiner, the laser light emitted from the combiner is incident on the output fiber at an angle at which no leakage occurs in the output fiber.

  The thickness of the intermediate clad is preferably 10 times or more the wavelength of the laser beam.

  With the above configuration, the light intensity of the laser light reaching the outer side surface of the intermediate cladding can be sufficiently reduced. Here, “sufficient” means that the light intensity is reduced to such an extent that the intermediate cladding is not damaged.

  According to the present invention, it is possible to suppress an increase in heat of the combiner, and to reduce the possibility of damage to the fiber and the combiner.

It is explanatory drawing of a general fiber combiner. It is sectional drawing of the conventional double clad fiber. The inner cladding portion is etched at the boundary and the outside is removed. It is explanatory drawing which shows the laser apparatus in one Embodiment of this invention. It is explanatory drawing which shows the manufacturing method of the combiner of one Embodiment of this invention. FIG. 5A is a cross-sectional view of the input fiber before the etching of the combiner according to the embodiment of the present invention, and FIG. 5B is a view of the fiber after the outer cladding portion is removed by etching in the fiber. A cross-sectional view is shown. FIG. 2B shows an example of refractive indexes of the core, the inner cladding, and the intermediate cladding. FIG. 6A is a cross-sectional view of the tip portion of a conventional combiner, and FIG. 6B is a cross-sectional view of the tip portion of the combiner according to the embodiment of the present invention. However, both are schematic diagrams. FIG. 7 is a cross-sectional view of the tip of the combiner in a state where the capillary and the input fiber tip are integrated. FIG. 8A is an explanatory diagram showing light propagation in the input fiber of the conventional optical combiner, and FIG. 8B is an explanatory diagram showing light propagation in the input fiber according to the embodiment of the present invention. . FIG. 8C is an explanatory diagram showing the positions of AA, CC, A′-A ′, and C′-C ′ in the entire combiner. Explanatory diagram showing the relationship between the core refractive index of the input fiber, the inner cladding refractive index (quartz refractive index), the intermediate cladding refractive index, and the modal index (modal effective refractive index) of light propagating through the tapered input fiber. It is. FIGS. 10A to 10C are explanatory diagrams showing the light intensity distribution in the radial direction when the SM wave is input to the input fiber according to the embodiment of the present invention. FIGS. ) Is an explanatory diagram showing a light intensity distribution in the radial direction when an MM wave is input to the input fiber according to the embodiment of the present invention. It is explanatory drawing which shows the relationship between the thickness of an intermediate | middle clad, and the optical load in the fiber side surface in a taper waist.

  Hereinafter, embodiments of the present invention will be described with reference to FIGS. 3 to 11.

  FIG. 3 is an explanatory diagram showing an outline of a laser device 31 according to an embodiment of the present invention.

  As shown in the figure, the laser device 31 includes fiber lasers 33a, 33b, and 33c, module fibers 35a, 35b, and 35c, input fibers 37a, 37b, and 37c (for combiners), a combiner 39, and an output fiber. (Feed fiber) 41.

  The module fibers 35a, 35b, and 35c have a core with a diameter of 30 μm, for example, and the NA is about 0.08, for example.

  The input fibers 37a, 37b, and 37c have, for example, a core having a diameter of 30 μm and an NA of about 0.08.

  The output fiber 41 has a core with a diameter of about 50 μm, for example, and the NA is, for example, about 0.20 to 0.22.

  In the combiner 39, the tips of the plurality of input fibers 37a, 37b, 37c (as will be described later, the inner cladding of the tips are removed by etching) inside a tapered capillary similar to the fiber of FIG. ) Is inserted, and the tip of the combiner is joined (spliced) to the output fiber 41.

  4 to 7 are explanatory views of a method for manufacturing the combiner 39. FIG.

  In step 1, as shown in FIG. 4A, a capillary 53 having a tapered portion 53a and a waist portion 53b is produced.

  Such a tapered capillary heats and melts the central part of parallel capillary tubes, and then stretches both ends of the tube to form a waist part 53b at the central part, and the capillary part is cut at the waist part 53b. (For example, an example is described in FIG. 3 of Patent Document 1. In Patent Document 1, a multi-hole capillary tube is described. In this embodiment, a single-hole capillary tube is used. To do.)

In step 2, as the input fibers 37a, 37b, and 37c, a triple clad fiber shown in FIG. 5A is prepared. As shown in the figure, the triple-clad fiber includes a core 57 (SiO ₂ —Al ₂ O ₃ ; NA = 0.08), an inner clad 59 made of quartz (SiO 2), and a double-layer fluoracrylate polymer ( and an outer clad 60 made of a dual-layer fluoroacrylate polymer. A silica layer doped with fluorine is introduced into the inner cladding 59 as the intermediate cladding 62. Here, the refractive index n2 of the intermediate cladding 62 is lower than the refractive index n1 of the inner cladding 59 (see FIG. 5B).

  In step 3, as shown in FIG. 4B, the tip of the triple clad fiber (the portion inserted into the tapered portion 53a and the waist portion 53b of the capillary 53) 261 (FIG. 4B) is etched. Remove with.

  5A and 5B are explanatory views showing details of the etching.

  FIG. 5A is a cross-sectional view of the input fiber before etching, and the boundary 58 of the clad to be etched is located on the outer surface of the intermediate clad 62 (strictly, slightly inside it).

  Then, by removing the inner cladding 59 and the outer cladding 60 outside the boundary 58 by etching, as shown in FIG. 5B, the core 57, the inner cladding 59 surrounding the core 57, and the inner cladding 59 are surrounded. A fiber tip portion comprising the intermediate cladding 62 is formed. In other words, the fiber tip has an outer periphery 61 composed of the intermediate cladding 62.

  As shown in the lower part of FIG. 5B, the core 57 has a refractive index n0, the inner cladding 59 has n1 smaller than n0, and the intermediate cladding 62 has n2 smaller than n1 ( In the illustrated example, the difference between n1 and n2 is about four times the difference between n0 and n1.)

  In step 4, the distal ends of the input fibers 37a, 37b, and 37c are inserted into the tapered portion 53a and the waist portion 53b (FIG. 4A) of the capillary 53.

  FIG. 6B shows a sectional view of the tip of the combiner in this state.

  As shown in the figure, an air gap 63 is formed inside the capillary 53. The air gap 63 may contain contaminants as described above.

  FIG. 6A shows the fiber arrangement at the tip of the capillary 103 in the conventional combiner. In the conventional combiner, an air gap 113 is formed between the capillary and the clad.

  In step 5, the capillary is again heated, melted and stretched with the inserted input fiber. Thereby, the input fiber and the capillary are further tapered and integrated with each other.

  FIG. 7 is a cross-sectional view of the tip of the combiner 39 in a state where the tip of the capillary 53 and the tips of the input fibers 37a, 37b, and 37c are integrated.

  An air gap 63 exists between the capillary and the input fiber.

  In Step 6, the tip of the combiner is cut at a predetermined position 53c (FIG. 8C) and joined to the output fiber 41. As described above, at this time, the (etched) tip portions of the input fibers 37a, 37b, and 37c and the capillaries are integrated by being melted and tapered. The front end of the integrated melting portion is cut and joined to the output fiber 41 (FIG. 3).

  As understood from the above manufacturing method, in the combiner 39, together with the capillary 53 (inserted into the capillary 53), the tips of the input fibers 37a, 37b, and 37c (and the core 57, the inner cladding 59, and the intermediate cladding 62 inside the fiber). ) Is also tapered by melting and stretching in step 5 above.

  FIG. 8B is an explanatory diagram showing the structure and operation of the tapered input fiber tip.

  As shown in the figure, the input fiber tip has a tapered core 57, a tapered inner cladding 59, and a tapered intermediate cladding 62. Here, the capillary 53 is omitted for simplicity.

  As described above, the tip end of the input fiber is a part manufactured by melting and stretching the capillary 53 and the input fibers 37a, 37b, and 37c. Therefore, the tip has an air gap 63 between the capillary and the input fiber, and the air gap 63 may contain contaminants 66.

  FIG. 8A is an explanatory view showing the structure of an input fiber and its operation in a conventional combiner manufactured by the same method as described above. As shown in the figure, the tip of this combiner also has an air gap 113 between the capillary and the input fiber, and this air gap 113 may contain contaminants 16.

  In the figure, a point 114 indicates a position where the inner clad-air (quartz-air) interface is a critical angle.

  As shown in FIG. 8 (a), in the input fiber of the conventional combiner, when multimode light is incident, the light enters the cladding 109 from the core 107, and is entirely at the boundary between the cladding 109 and the air 113. Reflected back to the center. In this case, if the contaminant 116 is attached to the boundary, the contaminant is heated by the laser beam at the attached portion. In particular, since laser light from a fiber laser has an extremely high intensity, extremely large heat is generated at the site. This heat accumulates and may damage the fiber.

  On the other hand, in FIG. 8B, a point 64 indicates a position at which the intermediate clad-air (fluorine-doped quartz-air) interface becomes a critical angle, and a point 65 indicates an inner clad-intermediate clad (quartz-fluorine-doped quartz). ) Indicates the position where the boundary surface becomes the critical angle.

  Compared with the input fiber of the conventional combiner, in the input fiber of the combiner according to the embodiment of the present invention, as shown in FIG. When receiving total reflection, the laser beam is totally reflected at the boundary 301 between the inner cladding 59 and the intermediate cladding 62 and does not reach the outer surface of the intermediate cladding 62 where the contaminant 66 is present.

  Therefore, such a laser beam does not heat the tapered portion of the fiber via the contaminant 66. In other words, according to the combiner including the input fiber of the present embodiment, the light intensity reaching the fiber surface where the contaminant 66 is present can be reduced as compared with the conventional input fiber.

    In this case, the refractive index of the intermediate cladding is preferably smaller than the refractive index of the inner cladding as already described. Further, when the core refractive index is n0, the inner cladding refractive index is n1, and the intermediate cladding refractive index is n2, (n1-n2) / (n0-n1) is preferably larger than 1, and larger than 2. More preferably, it is more preferably greater than 3.

  FIG. 9 is an explanatory diagram for explaining a method for determining the refractive index of the intermediate cladding in more detail.

  In the figure, the horizontal axis indicates the radial position of the input fiber, and the vertical axis indicates the refractive index. Specifically, the solid line indicates the radial distribution of the refractive index in the input fiber, n0 indicates the refractive index of the core 57, and n1 indicates the refractive index of the inner cladding 59.

On the other hand, the broken line indicates the effective refractive index (modal index (MODAL INDEX)) n _eff of the propagation mode propagating in the input fiber. Here, the effective refractive index neff is the wave number of light propagating along a wave guide (that is, an optical fiber) having a finite width in the lateral direction and having a refractive index larger than 1, the wave number of light propagating in a vacuum. It represents an increase and depends on the mode and the like.

That is, for the amplitude A (z) of the wave propagating in the z direction, neff is
A (z) = A (0) * exp (γz)
γ = i β
β = neff * 2π / λ
For a plane wave that extends infinitely in the lateral direction, neff is nothing but the refractive index of the medium, but neff inside the wave guide depends on the mode and frequency of the propagating wave.

  The effective refractive index is theoretically obtained assuming that the Helmholtz equation is established for each layer (core, inner cladding) of the optical fiber, and that the amplitude and its one-time derivative are continuous as boundary conditions of each layer. In practice, it is calculated by the mode solver software. Of course, the calculated effective refractive index value depends on the refractive index distribution of the wave guide.

  Then, for example, assuming that only the core and the inner cladding exist, by solving the above equation, as shown by the broken line in FIG. 9, for example, for LP01, neff (nLP01) slightly lower than the core refractive index n0 is obtained.

  For mode LP11, a lower effective refractive index nLP11 is obtained, and for mode nLP41, an effective refractive index (nLP41) lower than that of the cladding is obtained. As is well known, when the effective refractive index is smaller than the inner cladding refractive index n1, these modes pass through the inner cladding and diffuse. Therefore, in the embodiment of the present invention, an intermediate cladding is provided to prevent this diffusion. Here, for example, in order to prevent light diffusion in the mode LP41 (in other words, confine the light), it is preferable that the refractive index n2 of the intermediate cladding is n2 <LP41.

This relationship is easier to understand if expressed by the numerical aperture NA of the object, and will be described as follows. That is, in general, the NA _x of the object _x is

NA _x = ( _ns ² _−nx ² ) ^1/2

Means. Here _{n s} denotes a refractive index of silica _{(S i O 2), n} x means a refractive index of the object x. For example, if the refractive index of the intermediate cladding is n ₂ , the NA ₂ of the intermediate cladding is

NA ₂ = ( _ns ² −n ₂ ² ) ^1/2

It becomes.

  Therefore, when the confinement condition of the mode nLP41 in the intermediate cladding is expressed by the numerical aperture NA, NA2> NALP41. Therefore, for example, when NALP41 is 0.15 to 0.16, NA2 is preferably larger than these values.

  On the other hand, NA2 must be smaller than the NA of the output fiber in order to prevent light emitted from the combiner from the side of the output fiber. Therefore, when the NA of the output fiber is about 0.2 to 0.22, the NA of the intermediate cladding is preferably set to about 0.18.

  Next, a method for determining the thickness of the intermediate cladding will be described. The thickness of the intermediate cladding is determined from the relationship between the thickness and the average light intensity of the multimode on the side surface of the fiber.

  10 (a) to 10 (f) show a single mode or a multimode (LP01, LP11, LP21, LP02, LP31, LP12, LP41) in the input fiber used for the determination. ) Shows the calculation result of the light intensity distribution. The horizontal axis represents the radial distance from the fiber center, and the vertical axis represents the light intensity at each position. In the figure, the thin line represents the light intensity without the intermediate cladding, and the thick line represents the light intensity with the intermediate cladding. In this calculation, the intermediate cladding has a thickness of 15 μm before taper.

  More specifically, FIGS. 10A to 10C show the AA, BB, CC plane and A′-A ′, B′-B ′, FIG. FIG. 10 (d)-(f) shows the light intensity distribution on the C′-C ′ plane, and FIGS. 10 (d)-(f) show the AA, BB, CC plane and A of FIG. It shows the light intensity distribution on the '-A', B'-B ', and C'-C' planes.

  FIG. 11 shows the calculation of the light intensity of the multimode shown in FIGS. 10D to 10F with respect to the thickness of the intermediate cladding of 0 μm, 5 μm, 10 μm, 15 μm, and 20 μm. The axial average light intensity is calculated and summarized. Here, the NA of the intermediate cladding is 0.18, and the taper ratio is 4.35. The taper ratio represents the ratio between the diameter of the taper base end and the diameter of the taper tip. In the calculation, seven multimodes were stimulated or excited with the same intensity.

  As understood from the figure, it is understood that the optical load on the side surface of the fiber is reduced at the tapered waist according to the thickness of the intermediate cladding.

  More specifically, when the thickness of the intermediate clad is 5, 10, 15, 20 μm (value before taper), the power load on the side surface of the fiber at the taper waist is about 5, 15, 23, 26 times. Each decreases. If the thickness exceeds 15 μm, the reduction efficiency decreases.

  According to this, the optical load of about 10% or more can be reduced by setting the thickness of the intermediate cladding to about 10 to 15 μm, for example.

  Further, by setting the thickness to about 15 μm, the optical load on the fiber surface due to the multimode wave at the taper waist can be reduced to the optical load of the single mode wave.

  As mentioned above, according to the combiner of embodiment of this invention, the heating in a waist part can be reduced significantly. As described above, this is achieved by separating the inside of the carrier where contaminants are present in the waist portion and the side surface of the inner cladding by the intermediate cladding.

Claims (13)

An optical fiber combiner that couples multiple input fibers to an output fiber,
Multiple input fibers,
A capillary that covers the input fibers on the outer periphery of a plurality of input fibers;
Have
Each fiber in the plurality of input fibers has a taper portion including a core, an inner clad surrounding the core, and an intermediate clad surrounding the inner clad,
Each fiber has a taper portion with a degree of taper of 4 ≦ Tr ≦ 5,
The refractive index of the intermediate cladding is smaller than the refractive index of the inner cladding,
The numerical aperture NA ₂ of the intermediate cladding is smaller than the numerical aperture NAo of the output fiber,
The intermediate clad is an optical fiber combiner including fluorinated doped silica and having a thickness of 5 μm or more. An optical fiber combiner having a plurality of optical fibers,
Each optical fiber has a taper portion including a core, an inner cladding surrounding the core, and an intermediate cladding surrounding the inner cladding,
An optical fiber combiner in which the refractive index of the intermediate cladding is smaller than the refractive index of the inner cladding.   The combiner according to claim 2, wherein the degree of taper Tr is 4≤Tr≤5.   The combiner according to any one of claims 2 to 3, further comprising a capillary that covers the optical fibers on an outer periphery of the plurality of optical fibers. 5. The combiner according to claim 2, wherein the combiner is connected to an output fiber, and the numerical aperture NA ₂ of the intermediate cladding is smaller than the numerical aperture NAo of the output fiber.   The combiner according to any one of claims 2 to 5, wherein the thickness of the intermediate clad is 10 times or more the wavelength of the laser beam.   The combiner according to any one of claims 2 to 6, wherein the intermediate cladding includes fluorinated doped silica.   A laser apparatus comprising: a plurality of fiber lasers; an output fiber; and the combiner according to claim 1 for coupling the fiber laser to the output fiber. The refractive index n ₂ of the intermediate cladding has a numerical aperture NA ₂ larger than the numerical aperture NA _Lxy of the modal refractive index n _Lxy of the multimode Lx, y and smaller than the numerical aperture NAo of the output fiber. The combiner according to any one of claims 1 to 7, wherein (x + y) is greater than two.   The combiner according to any one of claims 1 to 7, wherein the thickness of the intermediate clad is not less than 10 times the wavelength of the laser beam and within 20% of the inner clad diameter before taper.   The combiner according to any one of claims 1 to 7, wherein the numerical aperture of the intermediate cladding is 0.18-0.20 or more and does not exceed the numerical aperture (0.20-0.22) of the output fiber. An optical fiber used in the optical fiber combiner of claim 1,
A core, a first inner cladding surrounding the core, an intermediate cladding surrounding the first inner cladding, and a second inner cladding surrounding the intermediate cladding;
An optical fiber in which the refractive index of the intermediate cladding is lower than the refractive index of the inner cladding.   The combiner according to any one of claims 1 to 7, wherein (n0-n2) / (n0-n1) is 1 when the refractive indexes of the core, the inner cladding, and the intermediate cladding are n0, n1, and n2, respectively. Big combiner.

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