JP4855429B2 - Connection method of double clad fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a connection method in which connection loss is reduced, when connecting an optical waveguide element having an optical waveguide core to a double clad fiber 10 having a core 11 doped with rare earth elements, a first clad 12 installed so as to cover the core 11, and a second clad 13 installed so as to cover the first clad 12. <P>SOLUTION: The connection method is such that one end face of the double clad fiber 10 is abutted on or brought in contact with one end face of an optical waveguide element 20, that testing light is made incident from the optical waveguide core 22 of the other end face of the optical waveguide element 20, the testing light including light of a wavelength absorbed by the rare earth elements doped in the core 11 of the double clad fiber 10, that the one end face of the double clad fiber 10 and the one end face of the optical waveguide element 20 are relatively moved in a direction perpendicular to the optical axis direction, and that, for the connection, positioning is performed at a position where the light intensity of the testing light is minimized which is emitted from the core 11 of the other end face of the double clad fiber 10. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a method for connecting an optical waveguide element having an optical waveguide core to a double clad fiber.

  In an optical apparatus such as a fiber laser or an optical amplifier, a double clad fiber is widely used as an optical amplification element.

  The double-clad fiber is provided in the center of the fiber doped with a rare earth element as an optical amplification component, a lower refractive index first clad provided around the core, and the first clad. And a second clad having a lower refractive index. In such a double-clad fiber, the excitation light incident on the first cladding propagates through a region surrounded by the second cladding while repeating reflection at the interface between the first cladding and the second cladding, and the excitation light. When passing through the core, the rare earth element doped in the core is turned into an inverted distribution state excited by the outermost electrons, and the signal light propagating through the core is amplified by the stimulated emission.

  Further, in such a double clad fiber, a method using an optical combiner is known as a method of making signal light enter the core and pump light enter the first clad.

  The optical combiner has, for example, a close-packed structure of a single single-mode optical fiber at the center and six multi-mode optical fibers surrounding the single-mode optical fiber, and has a structure in which they are fused and integrated at the tip portion (Patent Document) 1). When connecting to the double clad fiber, the portion corresponding to the signal light core of the single mode optical fiber is connected to the core of the double clad fiber, and the portion corresponding to the pump light core of the multimode optical fiber is doubled. Connection is made corresponding to the first clad of the clad fiber.

  By the way, when connecting optical waveguide elements such as two optical fibers, if the optical axes of the respective optical waveguide cores do not coincide with each other, when the light propagating through one core passes through the connecting portion, the other core Without being incident on the clad, it leaks into the cladding, resulting in connection loss. Examples of alignment methods for suppressing the occurrence of this connection loss include methods such as cladding alignment, core alignment, and power monitoring.

  As shown in FIG. 5, the clad alignment is a method of aligning optical fibers 40 and 50 using, for example, high-precision V grooves and aligning the clads 42 and 52. However, if the core of the optical fiber is eccentric, the cores 41 and 51 cannot be matched even if the clads 42 and 52 are matched. For example, since the optical combiner has a connection end face formed by melting and integrating a plurality of fibers at the connection portion, the signal light core is very easily decentered and is not suitable for clad alignment.

  As shown in FIG. 6, the core alignment is performed by directly looking at the optical fiber cores 41 and 51 from the side surfaces of the connection ends of the optical fibers 40 and 50, and by aligning the central axes of the cores 41 and 51 by image processing. It is a method of adjusting. Therefore, this method cannot be used when it is difficult to specify the position of the core by direct viewing at the connection end face as in an optical combiner.

  In the power monitor method, the end faces of the two optical fibers 40 and 50 are opposed to or in contact with each other, the inspection light is incident from one end of the one optical fiber 40, and the inspection light emitted from the other connected optical fiber 50 is emitted. In this method, the cores 41 and 51 are aligned with each other by measuring the light intensity of the core (see Patent Document 2). As shown in FIG. 7A, when the cores 41 and 51 are not coaxial, the light entering the clad 52 of the other optical fiber 50 is absorbed by the coating covering the clad 52 and measured by the detector. The light intensity of the inspection light to be weakened. On the other hand, as shown in FIG. 7B, when the respective core axes coincide with each other, the inspection light incident on the core 51 is detected as it is by the detector. For this reason, the optical fibers may be connected when the detected light intensity becomes maximum.

According to this power monitoring method, when the clad 52 is covered with a member having a lower refractive index than that of a double clad fiber, the inspection light leaking to the clad 52 leaks out of the clad and the intensity of the light is reduced. There is no weakening. Therefore, this method cannot be adjusted.
JP 2007-163650 A Japanese Patent Laid-Open No. 61-228405

  As described above, when connecting an optical waveguide element having an optical waveguide core such as an optical combiner to a double clad fiber, the conventional methods cannot efficiently align the cores, resulting in a large connection loss. There was a problem of becoming.

  An object of the present invention is to reduce connection loss when an optical waveguide element having an optical waveguide core is connected to a double clad fiber.

The present invention that achieves the above object includes a core doped with a rare earth element, a first clad provided to cover the core, and a second clad provided to cover the first clad. A method of connecting an optical waveguide element having an optical waveguide core to a double clad fiber having
The one end surface of the double clad fiber and the one end surface of the optical waveguide element are opposed to or in contact with each other,
Inspection light including light of a wavelength that is absorbed by the rare earth element doped into the core of the double clad fiber is incident from the optical waveguide core on the other end surface of the optical waveguide element,
The light intensity of the inspection light emitted from the core on the other end face of the double clad fiber by relatively moving the one end face of the double clad fiber and the one end face of the optical waveguide element in a direction perpendicular to the optical axis direction. It is characterized in that it is positioned and connected at a position where the value is minimized.

  In the connection method of the present invention, the inspection light is preferably only light having a wavelength that is absorbed by the rare earth element doped in the core of the double clad fiber.

  In the connection method of the present invention, the optical waveguide element includes a signal light core and a plurality of excitation light cores disposed so as to surround the signal light core, and the signal light core is the optical waveguide core. It is preferable that the optical combiner corresponds to

  According to the present invention, one end face of a double clad fiber and one end face of an optical waveguide element are opposed to or in contact with each other, and the rare earth element doped into the core of the double clad fiber is absorbed from the optical waveguide core on the other end face of the optical waveguide element. The inspection light containing the light of the wavelength to be incident is incident, and the one end surface of the double clad fiber and the one end surface of the optical waveguide element are moved relative to each other in the direction perpendicular to the optical axis direction, and the core on the other end surface of the double clad fiber Since the double-clad fiber is connected to the optical combiner by positioning at a position where the light intensity of the inspection light emitted from the optical fiber is minimized, the double-clad fiber core and the optical waveguide core of the optical waveguide element should be coaxial. And connection loss can be reduced.

  Hereinafter, embodiments will be described in detail with reference to the drawings.

  1 and 2 show a double clad fiber 10 and an optical combiner 20. The double clad fiber 10 is used as an optical amplifier in a laser marker optical amplifier or a welding fiber laser, for example. The optical combiner 20 is used by being connected to the double clad fiber 10, for example.

  The double clad fiber 10 includes a core 11 at the center of the fiber, a first clad 12 provided so as to cover the core 11, a second clad 13 provided so as to cover the first clad 12, and a second clad 13. And a protective layer 15 to be covered. The first cladding 12 is provided to have a lower refractive index than the core 11, and the second cladding 13 is provided to have a lower refractive index than the first cladding 12. The double clad fiber 10 has a fiber length of 3 to 40 m, for example.

  The core 11 is made of, for example, pure quartz doped with a rare earth element. Examples of rare earth elements include ytterbium (Yb), erbium (Er), neodymium (Nd), and the like. The doping amount of the rare earth element is, for example, 1000 to 20000 ppm. The rare earth element may be a single species or a plurality of species. This rare earth element is doped into quartz by a method such as a solution immersion method or a vapor deposition method. For example, the core 11 has a core diameter of 20 to 60 μm. The core 11 has a relative refractive index with respect to pure quartz of, for example, 0.03 to 0.3%.

  The first cladding 12 is made of, for example, pure quartz. For example, the first cladding 12 has a first cladding diameter of 125 to 600 μm.

  The second clad 13 may have, for example, an air clad structure in which an air hole 14 is provided in quartz glass, may be formed of a low refractive resin or the like, or may be formed of other materials. . For example, the second cladding 13 has a second cladding diameter of 150 to 650 μm. The second cladding 13 has a relative refractive index difference with respect to pure quartz of, for example, 3 to 10%.

  The protective layer 15 is made of, for example, a low refractive silicone resin, a fluororesin, or the like. The protective layer 15 has a thickness of 60 to 600 μm, for example. The protective layer 15 may be a single layer or a laminate of a plurality of layers.

  In this double clad fiber 10, the end face protective layer 15 is removed by a predetermined length using a tool, chemicals, or the like, and the connection end face cut perpendicularly and smoothly to the axis using an optical cutter or the like is connected to the optical combiner 20. .

  When the double clad fiber 10 has an air clad structure having an air hole 14, the second clad 13 having a predetermined length (for example, 5 to 20 mm) is removed in advance from the connection end face of the double clad fiber 10. It is preferable. As a result, the first clad 12 is exposed, so that it is possible to prevent a phenomenon in which the quartz melts during heating and the air holes 14 are sealed and the excitation light incident on the first clad 12 leaks.

  The optical combiner 20 includes a capillary 24, one optical fiber for signal light, and a plurality of optical fibers for excitation light. The signal light optical fiber 21 or the signal light optical fiber 21 or the plurality of excitation light optical fiber cores are peeled off from the tip of the core wire by a predetermined length. The optical fiber for pumping light 23 is exposed, and the optical combiner 20 includes the optical fiber for signal light 21 and the optical fiber for pumping light 23 arranged so as to surround the optical fiber being inserted into the capillary 24, and the tip thereof. In the part, they are formed in a melted part 25 in which they are fused and reduced in diameter.

  The capillary 24 is made of, for example, quartz glass. For example, the capillaries 24 are formed to have a total length of 3 to 10 mm (excluding the melting part 25) and an inner diameter of 380 to 400 μm.

  The signal light optical fiber 21 is made of, for example, quartz glass, and has a high refractive index signal light core 22 at the center of the fiber and a low refractive index cladding covering the periphery thereof. The signal light optical fiber 21 is generally composed of a single mode fiber. For example, the signal light optical fiber 21 has a fiber diameter of 123 to 127 μm and a core diameter of 10 to 60 μm.

  The excitation light optical fiber 23 is made of, for example, quartz glass, and has a high refractive index excitation light core at the center of the fiber and a low refractive index clad covering the periphery thereof. The pumping light optical fiber 23 is generally composed of a multimode fiber. For example, the optical fiber 23 for excitation light has a fiber diameter of 123 to 127 μm and a core diameter of 80 to 115 μm.

  In the melting part 25, a capillary 24, one optical fiber 21 for signal light, and a plurality of optical fibers 23 for pumping light are integrated, and the optical fiber 21 for signal light and the optical fiber 23 for pumping light are contained therein. Each core 11 extends in the length direction. Although it is difficult to observe by direct viewing, the signal light core 22 and a plurality of excitation light cores arranged so as to surround the end surface of the melting portion 25 are exposed.

  In this optical combiner 20, the signal light optical fiber 21 is connected to the signal light source, each of the plurality of pump light optical fibers 23 is connected to the pump light source, and the end surface of the melting part 25 is a double clad fiber. 10 is used by fusion splicing. At this time, the signal light core 22 at the end face of the melted portion 25 is connected to the core 11 of the double clad fiber 10, and the excitation light core is connected to the first clad 12 of the double clad fiber 10.

  Then, the signal light from the signal light source is incident on the core 11 of the double clad fiber 10 via the signal light optical fiber 21, and the pump light from the pump light source is transmitted through the pump light optical fiber 23 to the double clad fiber 10. When incident on the first cladding 12, the excitation light incident on the first cladding 12 repeats reflection at the interface between the first cladding 12 and the second cladding 13, while passing through a region surrounded by the second cladding 13. When propagating and passing through the core 11, the rare earth element doped in the core 11 is brought into an inverted distribution state excited by outermost electrons, and the signal light propagating through the core 11 is amplified by the stimulated emission.

  Next, the connection method of the double clad fiber 10 and the optical combiner 20 is demonstrated using FIG. This connection is performed by first aligning the central axis of the core 11 of the double clad fiber 10 with the central axis of the signal light core 22 of the optical combiner 20 and then fusing each end face. .

  First, the axial alignment of the core 11 of the double clad fiber 10 and the optical core 11 for signal light of the optical combiner 20 is performed using a fusion splicer (not shown).

  The fusion splicer includes two stages, and the connection portion 31 includes fusion means such as arc discharge.

  The two stages are provided such that the optical fibers are arranged in series with respect to the optical axis direction with the connection portion 31 therebetween. The two stages are in the optical axis direction, the horizontal direction, and perpendicular to the optical axis direction. Relative to the vertical direction and perpendicular to the vertical direction. The two stages may be relatively moved by fixing one and moving the other, or may be relatively moved by moving each of the two stages.

  In this fusion splicer, the double clad fiber 10 is fixed to one stage, the optical combiner 20 is fixed to the other stage, and each one end is fixed so as to face the connecting portion 31. At this time, in the connection part 31, the end surface of the double clad fiber 10 and the end surface of the optical combiner 20 may be arranged in a facing state with an interval of, for example, 5 to 30 μm, or may be in contact with each other, It is preferable to arrange in an opposite state.

  Subsequently, the inspection light is incident on the signal light core 22 from the other end of the optical combiner 20. This inspection light includes light having a wavelength that is absorbed by the rare earth element doped in the core 11 of the double clad fiber 10. The inspection light preferably includes only light having a wavelength that is absorbed by the rare earth element doped in the core 11, and the inspection light is preferably laser light. Specifically, for example, when the rare earth element doped in the core is Yb, the wavelength of the inspection light is preferably 870 to 990 nm. When the rare earth element is Er, the wavelengths of the inspection light are preferably 950 to 1000 nm and 1430 to 1580 nm. When the rare earth element is Nd, the wavelength of the inspection light is preferably 720 to 900 nm. As an example, FIG. 4 shows a graph of the absorption characteristics of Yb when the concentration of doping into quartz glass is 10,000 ppm. In this case, although the absorption loss is maximum when the wavelength is 977 nm, for example, inspection light having a wavelength of 960 nm is used in consideration of the limit of the detection capability of the photodetector.

  The intensity of the inspection light incident on the signal light core 22 is measured by a photodetector provided on the other end side of the double clad fiber 10.

  As shown in FIG. 3A, when the core 11 of the double clad fiber 10 and the signal light optical core 11 of the optical combiner 20 are not coaxial, the inspection light enters the first clad 12 of the double clad fiber 10. become. Since the refractive index of the second cladding 13 is lower than the refractive index of the first cladding 12, this inspection light is reflected at the interface between the first cladding 12 and the second cladding 13 and propagates inside the first cladding 12. And reaches the other end of the double clad fiber 10 and is measured by a detector.

  On the other hand, as shown in FIG. 3B, when the core 11 of the double clad fiber 10 and the signal light core 22 of the optical combiner 20 exist on the same axis, the inspection light enters the core 11 of the double clad fiber 10. It will be. The inspection light is reflected at the interface between the core 11 and the first cladding 12 and propagates through the core 11. At this time, the rare earth element doped in the core 11 is excited. That is, the inspection light entering the core 11 is absorbed by the rare earth element and gradually attenuates. Therefore, the light intensity of the inspection light measured by the detector is weaker than when the inspection light propagates through the first cladding 12 in FIG.

  Therefore, when the core 11 of the double clad fiber 10 and the signal light core 22 of the optical combiner 20 are on the same axis, the light intensity of the inspection light measured by the photodetector is minimized.

  In this fusion splicer, the one end surface of the double clad fiber 10 fixed to the two stages and the one end surface of the optical combiner 20 are relatively moved in a direction perpendicular to the optical axis direction, while the optical combiner 20 The inspection light is incident on the signal light core 22 from the other end side, and the light intensity of the inspection light emitted from the other end side of the double clad fiber 10 is measured. And it positions at the position where the light intensity of inspection light becomes the minimum, and moves to connection operation.

  After positioning the core 11 of the double clad fiber 10 and the signal light core 22 of the optical combiner 20 so as to be coaxial, the stage is finely moved in the optical axis direction so that both connection end surfaces are brought into contact with each other and arc discharge is performed. Thus, both end faces are fusion-connected. For example, the arc discharge is performed at 1500 to 2200 ° C. for 3 to 10 seconds. In addition to arc discharge, fusion may be performed by a method of heating with a flame burner or the like.

  Finally, the connection portion 31 is reinforced by a heat shrink sleeve or the like as necessary. Thus, the double clad fiber 10 can be connected to the optical combiner 20.

  According to the above method, the one end face of the double clad fiber 10 and the one end face of the optical combiner 20 face each other, and the rare earth element doped in the core 11 of the double clad fiber 10 from the signal light core 11 of the optical combiner 20 is absorbed. The inspection light containing the light having the wavelength to be incident is incident, the one end surface of the double clad fiber 10 and the one end surface of the optical combiner 20 are relatively moved in a direction perpendicular to the optical axis direction, and the other end surface of the double clad fiber 10 Since the double clad fiber 10 is connected to the optical combiner 20 by positioning at a position where the light intensity of the inspection light emitted from the core 11 is minimized, the core 11 of the double clad fiber 10 and the signal light core of the optical combiner 20 are connected. Therefore, the connection loss can be reduced.

  In the present embodiment, the method of connecting the optical combiner 20 to the double clad fiber 10 has been described. However, the present invention is not limited to this. For example, the double clad fiber 10 and a single mode optical fiber may be connected. The optical waveguide element having another optical waveguide core may be connected to the double clad fiber 10.

  In this embodiment, the double clad fiber 10 is connected by fusion splicing. However, the present invention is not limited to this. For example, the double clad fiber 10 and the optical waveguide element may be abutted and connected by an adhesive or the like. Good.

-Example-
Using the following double clad fiber 10 and optical combiner 20, the cores were aligned and connected.

(Double clad fiber)
The double clad fiber 10 includes a core 11 made of quartz glass doped with Yb, a first clad 12 made of pure quartz, an air hole layer in which air holes 14 are provided in the quartz glass, and an over clad around the air hole layer. A layer composed of a second clad 13 made of a layer and a protective layer 15 made of a first protective layer made of a low refractive index silicone resin and a second protective layer made of fluororesin (ETFE) around the first protective layer was used. The double-clad fiber 10 had a core diameter of 20 μm, a first cladding diameter of 400 μm, a second cladding diameter of 650 μm, a protective layer 15 thickness of 525 μm, and a fiber length of 3 m. The doping amount of Yb doped into the core 11 was 5000 ppm, and the difference in relative refractive index between the core 11 and the first cladding 12 was 0.070%.

  In this double clad fiber 10, the protective layer 15 of 20 mm from the connection end face of the double clad fiber 10 was previously removed, and further, the second clad 13 of 5 mm from the connection end face was removed.

(Optical combiner)
The optical combiner 20 was composed of a pure quartz glass capillary 24, one optical fiber for signal light, and six optical fibers for excitation light.

  The capillary 24 had an outer diameter of 440 μm, an inner diameter of 400 μm, and a length of 10 mm.

  The optical fiber for signal light is composed of a signal light core 22 in which quartz is doped with aluminum and a clad made of pure quartz, and a coating layer made of acrylic is provided around the clad. This optical fiber for signal light has a core diameter of 20 μm, a cladding diameter of 125 μm, a coating layer thickness of 62 μm, and a length of 5 m. The relative refractive index difference between the signal light core 22 and the clad was 0.075%.

  The optical fiber core wire for excitation light is composed of an excitation light core made of pure quartz and a clad in which quartz is doped with fluorine, and a coating layer made of acrylic is provided around the clad. The optical fiber core wire for excitation light had a core diameter of 114 μm, a cladding diameter of 125 μm, a coating layer thickness of 62 μm, and a length of 5 m.

(Connection between double clad fiber and optical combiner)
First, inspection light having a wavelength of 960 nm was incident on the signal light core 22 of the optical combiner 20, and the light emitted from the connection end side of the optical combiner 20 was measured with a detector. The light intensity measured at this time was −30 dBm.

  Next, the double clad fiber 10 and the optical combiner 20 were respectively fixed and arranged in the V-groove on the stage of the fusion splicer. This stage was capable of fine movement in the optical axis direction, the horizontal direction and the direction perpendicular to the optical axis direction, and the vertical direction by a piezo element. A detector for measuring the light intensity was connected to the output end of the double clad fiber.

  Subsequently, the double clad fiber 10 and the optical combiner 20 were brought close to each other until the distance between the connection end faces became about 10 μm.

  Subsequently, inspection light (laser light) having a wavelength of 960 nm was incident on the signal light core 22 of the optical combiner 20. Then, while operating the detector, the stage was finely moved in the direction perpendicular to the optical axis direction, and the detected light intensity was adjusted to the minimum. The light intensity detected when the light intensity was minimized was -63 dBm.

  Furthermore, the relative position between the double clad fiber 10 and the optical combiner 20 was shifted by 30 μm in the horizontal direction and in the direction perpendicular to the optical axis direction from when the light intensity became minimum. In this state, inspection light was incident on the signal light core 22 of the optical combiner 20. At this time, the light intensity measured by the detector was -33 dBm.

  Again, the positions of the double clad fiber 10 and the optical combiner 20 were adjusted so that the light intensity was minimized, and the connection end faces of the double clad fiber 10 and the optical combiner 20 were brought into contact with each other. And it connected by performing arc discharge and heat-melting both.

  According to this example, the absorption loss when the light intensity of the measured inspection light is minimum was 33 dB. This is because the core 11 of the double clad fiber 10 and the signal light core 22 of the optical combiner 20 are coaxial, so that the inspection light transmitted from the signal light core of the optical combiner enters the core 11 of the double clad fiber 10. It is considered that this is because the inspection light is incident and absorbed by Yb doped in the core 11.

  On the other hand, when the position of each core was shifted by 30 μm, the absorption loss was reduced to 3 dB. This is because the axial positions of the core 11 of the double clad fiber 10 and the signal light core 22 of the optical combiner 20 are shifted, so that the inspection light is incident on the first cladding 12 of the double clad fiber 10 and the inspection light is doubled. It is considered that this is an absorption loss caused to excite Yb doped in the core 11 when passing through the core 11 of the clad fiber 10. Actually, when excitation light having a wavelength of 915 nm is incident on the first cladding of the double-clad fiber, the light absorption coefficient resulting from exciting Yb doped in the core when the excitation light passes through the core is 0. It is known to be 34 dB / m.

  As described above, the present invention is useful for a method of connecting double clad fibers.

It is a perspective view which shows the outline of a double clad fiber. It is a perspective view which shows the outline of an optical combiner. It is explanatory drawing which shows the connection method of this embodiment. It is a graph which shows the absorption loss of ytterbium. It is explanatory drawing which shows clad alignment. It is explanatory drawing which shows core alignment. It is explanatory drawing which shows the conventional power monitor method.

Explanation of symbols

10 double clad fiber 11 core 12 first clad 13 second clad 20 combiner (optical waveguide element)
22 Signal light core

Claims (3)

An optical waveguide is provided in a double-clad fiber including a core doped with a rare earth element, a first cladding provided to cover the core, and a second cladding provided to cover the first cladding. A method of connecting an optical waveguide element having a core,
The one end surface of the double clad fiber and the one end surface of the optical waveguide element are opposed to or in contact with each other,
Inspection light including light of a wavelength that is absorbed by the rare earth element doped into the core of the double clad fiber is incident from the optical waveguide core on the other end surface of the optical waveguide element,
The light intensity of the inspection light emitted from the core on the other end face of the double clad fiber by relatively moving the one end face of the double clad fiber and the one end face of the optical waveguide element in a direction perpendicular to the optical axis direction. A connection method in which positioning is performed at a position where the minimum is possible. The connection method according to claim 1,
A connection method in which the inspection light is only light having a wavelength that is absorbed by the rare earth element doped in the core of the double clad fiber. The connection method according to claim 1 or 2,
The optical waveguide element includes a signal light core and a plurality of excitation light cores disposed so as to surround the signal light core, and the signal light core is an optical combiner corresponding to the optical waveguide core. A connection method characterized by that.

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