JP2010141086A - Optical fiber coupler, optical amplifying device, and fiber laser - Google Patents

Abstract

In an optical fiber coupler, signal light leaking from a core connection part of an optical fiber is not allowed to travel to a pumping light source, and unstable operation and destruction of the pumping light source are prevented.
One end of a plurality of pumping light input fibers is bundled and integrated with a clad pump fiber, and the tip is gently reduced in diameter, and the reduced tip is a multimode fiber for pumping light output. And the pumping light propagation part coupled between the end faces and the signal light propagation part formed by arranging the signal light propagation fibers along the pumping light propagation part, and the inner part of the pumping light propagation part and the cladding pump fiber The connecting portion with the cladding region is set at a position exceeding a certain distance with the axis of the signal light core of the cladding pump fiber as the center.
[Selection] Figure 1

Description

  The present invention relates to an optical fiber coupler capable of simultaneously coupling pumping light and signal light, a highly reliable optical amplifying device including the optical fiber coupler, and a fiber laser.

  A clad pump fiber is a light that has a core that propagates signal light, an inner cladding that has a lower refractive index than the core around the core, and an outer cladding that has a lower refractive index than the inner cladding around the inner cladding. An optical amplification medium using a fiber.

  The excitation light incident on the inner cladding propagates through the inner cladding while repeating total reflection at the interface between the inner cladding and the outer cladding. A rare earth element such as ytterbium is added to the core, and when the excitation light propagating through the inner cladding passes through the core, the excitation light is absorbed by the inversion distribution and the signal light is induced and emitted. The signal light amplified by the core propagates through the core while repeating total reflection between the core and the inner cladding, and is emitted from the end face of the fiber.

  A clad pump fiber is a structure that allows a large amount of pump light to be incident on an inner clad having a large cross-sectional area. Compared with a pump fiber that only enters a pump light such as a single clad structure, the clad pump fiber has a significantly higher output. It becomes possible. Hereinafter, “signal light” means laser light stimulated and emitted from a clad pump fiber.

  The optical fiber coupler is responsible for passing light between the pumping light source and the clad pump fiber, and between the signal light input / output object and the clad pump fiber. Specifically, pumping light emitted from a plurality of multimode semiconductor lasers capable of high-power oscillation is condensed and densified by an optical fiber coupler and then transmitted to a cladding pump fiber.

  The signal light generated by stimulated emission at the core of the cladding pump fiber is emitted to the outside through the optical fiber coupler. Alternatively, it has a function of making the signal light to be amplified enter the clad pump fiber. Optical fiber couplers do not require fine adjustment of the optical axis, which is often required for spatial couplers represented by lenses, etc., and have a structure in which transmission media are firmly connected. Can be configured.

  The optical amplification system using a clad pump fiber propagates a high-power light through a minute core and is in an extremely high energy density state. Therefore, the optical fiber itself is melted, resin is ignited, and damage is caused at the exit end face. Countermeasures are necessary. As the optical amplification system becomes higher in output, it is required to further take measures against damage to the optical fiber transmission line.

  Conventionally, Patent Documents 1 and 2 disclose the structure of an optical fiber coupler having a function of condensing and densifying the excitation light and coupling it to a clad pump fiber and simultaneously coupling signal light generated in the clad pump fiber. It is disclosed.

In Patent Document 1, as shown in FIG. 6, a plurality of multimode fibers 902 are arranged around and bundled around a single signal light propagation fiber 901, and a tip is collected to collect excitation light. The diameter is reduced, and the reduced end face and the end face of the cladding pump fiber 903 are connected. In Patent Document 2, the refractive index of the signal light propagation fiber 901 at the center of the fiber bundle is lower than that of the core and higher than that of the cladding so that the signal light leaking from the core does not enter the pumping light source. An optical fiber coupler having a structure including a “radiated light confining waveguide” is disclosed.
US Pat. No. 5,864,644 JP 2008-10804 A

  However, in the conventional optical fiber coupler, as shown in FIG. 6, a signal light propagation optical fiber 901 that is a signal light propagation region is arranged inside a bundle of multimode fibers 902 that is a pump light propagation region. Yes. For this reason, due to the mismatch of the signal light core at the connection portion 904 between the fiber bundle tip and the clad pump fiber 903, a part of the high output signal light leaks from the core to the clad at the connection portion 904, and leaks out. A semiconductor in which signal light (hereinafter, signal light leaking from the signal light core is referred to as radiation signal light 905) propagates in the opposite direction in the fiber bundle for condensing excitation light and is connected to the input end 906. It enters the laser. As a result, a phenomenon that the semiconductor laser operates unstable occurs, and there is a problem that the semiconductor laser is destroyed in the worst case.

  The semiconductor laser is the most expensive component that accounts for the majority of the cost breakdown among optical amplification systems using optical fibers, and the damage when the semiconductor laser is destroyed is significant. When the output of an optical amplification system is increased, radiation signal light is also output with high power, and semiconductor lasers are liable to have an adverse effect.In addition, the number of semiconductor lasers is increased to increase the output of signal light. The number of semiconductor lasers that are subjected to this will inevitably increase.

  If there is no problem in the structural design of the optical fiber itself, the cause of signal light leakage from the core is that leakage of signal light from the core due to a small radius of curvature or the core connection of each optical fiber The cause of this is the leakage of signal light.

  Radiated signal light due to bending can be prevented by making the radius of assembly appropriate when assembling, and by taking care not to apply excessive force at the fixed portion of the optical fiber. The misalignment of the cores at the connecting portion may be caused by differences in cross-sectional shape, mode field diameter, numerical aperture, connection angle, and the like. There is no problem if the connection can be made without any difference in the characteristics of each optical fiber to be connected. However, the manufacturing tolerance of the optical fiber, the accidental misalignment at the time of connection, and the core diameter and numerical aperture are intentionally set. The radiation signal light can be suppressed as much as possible, but it is difficult to suppress it completely. As a countermeasure for extinguishing the generated radiated signal light, a method is generally used in which it is quickly absorbed by a coating resin around the signal light propagation fiber and converted into thermal energy.

  In the prior art disclosed in Patent Document 2, the radiation signal light 905 does not easily travel to the excitation light propagation multimode fiber 902 due to the presence of the radiation light confining waveguide around the core of the signal light propagation fiber 901. is there.

  However, when the radiated signal light 905 is generated at a scattering angle larger than the total reflection critical angle of the radiated light confining waveguide, there is no effect of the radiated light confined waveguide, and the semiconductor propagates through the multimode fiber 902 for propagating light propagation. There is a risk of proceeding to the laser.

  Further, it is desirable that the radiated signal light 905 is promptly absorbed by the coating oil layer around the cladding. However, since there is a radiated light confining waveguide, it is captured again by the signal light core, and in pulse oscillation, the signal is detected as noise. There is also the possibility that the system will propagate through the optical core and adversely affect the system.

  In addition, since this structure is a state in which the refractive index mismatch at the connection portion 904 is increased by the increase in the refractive index of the synchrotron radiation waveguide, signal light inevitably leaks compared to the case without the synchrotron radiation waveguide. Easy to use.

  The present invention relates to an optical fiber coupler that effectively and easily prevents unstable operation and destruction of a semiconductor laser due to radiation signal light, and a highly reliable optical amplifying device and fiber laser using the optical fiber coupler. The purpose is to provide.

  In order to solve the above-described problems, the present invention provides a signal light propagation unit in a state in which the signal light propagation portion does not exist inside the excitation light collecting fiber bundle, that is, along the outside of the excitation light collecting fiber bundle. An array structure with a propagating fiber is provided.

  Specifically, the pumping light propagation unit bundles and integrates one end of a plurality of pumping light input fibers, and gradually reduces the diameter of the tip part in order to efficiently collect the pumping light. The tip part is coupled to the multimode fiber for pumping light output at the end faces. The signal light propagation part arranges the signal light propagation fibers along the excitation light propagation part.

  The portion of the signal light propagation portion that directly contacts the cladding side surface is covered with a resin having a refractive index higher than that of the cladding so that the radiated signal light disappears quickly, and from the reduced diameter portion of the excitation light propagation portion to the emission end. In order to propagate the pumping light, the core and the side of the cladding that is in direct contact with the side of the cladding are coated with a resin having a refractive index lower than that of the core and the cladding, and the signal light propagation part and the excitation light propagation part are optically coupled. I won't let you.

  The connection with the clad pump fiber is made by combining the connection portion between the signal light propagation portion and the core region of the clad pump fiber and the connection portion between the excitation light propagation portion and the inner clad region of the clad pump fiber at different positions. The connection position is characterized in that there is no connection portion with the pumping light propagation portion in a radius region five times the core radius centering on the axis of the signal light core of the clad pump fiber.

  According to this configuration, the signal light waveguide region and the pumping light waveguide region of the optical fiber coupler can be separated so as not to interfere with each other, and the signal light is affected at the signal light core connection portion with the clad pump fiber. Since no excitation light propagation region is provided in the cladding region around the core connection portion, the radiated signal light generated from the signal light core connection portion does not travel to the excitation light collecting fiber bundle.

  Also, the semiconductor laser in which the radiated signal light generated from the signal light core connecting portion is connected to the fiber bundle for condensing the excitation light by providing the optical amplifier of the present invention in the optical amplification device and the fiber laser. Is not incident on.

  The optical fiber coupler of the present invention, the optical amplifying device equipped with the optical fiber coupler, and the fiber laser have a structure in which the signal light propagation region and the pumping light propagation region are separated separately, and are pumped in the traveling direction of the radiated signal light. Since there is no light propagation region, the radiated signal light generated at the signal light core connecting portion does not travel to the excitation light collecting fiber bundle. For this reason, even if radiation signal light is generated, it does not enter the semiconductor laser, and unstable operation and destruction of the semiconductor laser are eliminated, and the reliability of the optical amplification system is improved. Moreover, higher output can be expected.

  Embodiments of an optical fiber coupler, an optical amplification device, and a fiber laser of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the optical fiber coupler 10 has a tapered reduced diameter portion 12 in which a plurality of excitation light input multimode fibers 11 are bundled and integrated at one end, and the reduced diameter tip portion and the excitation portion are pumped. The optical output multimode fiber 13 is coupled at the end face to have a connection portion 14, and the signal light propagation fiber 15 is disposed along the fiber bundle for collecting the excitation light.

  The pumping light output multimode fiber 13 is coupled at the connection portion 17 between the pumping light output multimode fiber and the cladding pump fiber so as not to overlap the signal light emitting region of the cladding pump fiber 16, while the signal light The signal light core 18 of the propagation fiber 15 and the signal light core 19 of the clad pump fiber 16 are coupled by a connection portion 20. The signal light propagation fiber 15 is not arranged inside the fiber bundle for collecting the excitation light, but is arranged along the outside of the fiber bundle so that the excitation light and the signal light are divided and coupled to the clad pump fiber. Is possible.

  FIG. 2A shows a cross-sectional view of an example of the pumping light input multimode fiber 11. The core 31 through which the excitation light propagates is preferably quartz glass having heat resistance. The clad 32 is preferably made of quartz glass doped with fluorine, boron or the like so as to have a refractive index lower than that of the core 31. The numerical aperture of the core is generally 0.22.

  FIG. 2B shows a sectional view of an example of the pumping light output multimode fiber 13. The core 31 through which the excitation light propagates is preferably quartz glass having heat resistance. The clad 33 is preferably made of a low refractive index coating resin so that the refractive index is lower than that of the core 31 and the numerical aperture is increased. The clad of the low refractive index coating resin can make the numerical aperture of the core larger than that of the quartz glass, and the numerical aperture of the core is generally 0.46.

  FIG. 2C shows a cross-sectional view of an example of the signal light propagation fiber 15. The core 18 through which signal light propagates is preferably quartz glass doped with germanium or the like to increase the refractive index, and the clad 34 is preferably quartz glass. When the refractive index of the core 18 is ncore, the refractive index of the cladding 34 is n1, and the refractive index of the coating resin 35 is n ′, the relationship of ncore> n1 <n ′ is satisfied, and light is propagated only to the core. It has become. Light incident on the coating resin 35 is absorbed and converted into thermal energy.

  In the manufacturing method, first, after covering a necessary region of the plurality of excitation light input multimode fibers 11, they are arranged so as to have a close-packed structure if possible. After the close-packing arrangement, the fiber bundle of the portion from which the coating has been removed is heated and integrated with a heating source such as a heater, and the softened portion is stretched. By stretching, each of the excitation light input multimode fibers 11 is reduced in outer diameter, and becomes a fiber bundle having a gradually reduced diameter portion 12 in the longitudinal direction. This method is the same as the method for producing a normal optical fiber star coupler. By reducing the diameter to the same size as the core diameter of the pumping light output multimode fiber 13 so as to minimize the connection loss, scratching the constricted central portion of the fiber bundle, and adding a tension to divide it, one end is A fiber bundle having a reduced diameter and an integrated structure can be produced.

  Then, the tip of the reduced diameter portion 12 is connected to the end face of the excitation light output multimode fiber 13. In consideration of mechanical reliability, it is desirable that the connection portion 14 be fusion-bonded. The coating for protecting this portion uses a low refractive index coating resin 36 having the same refractive index as that of the cladding 33 of the pumping light output multimode fiber 13 so as not to leak the pumping light.

  In a fiber bundle that is gradually reduced in diameter in order to efficiently collect pumping light, the core diameter of the pumping light input multimode fiber 11 at the input end 37 is Din, and the numerical aperture of incident propagating light is NAin. When the core diameter at the diameter-reduced end face of the pumping light input multimode fiber 11 is Dout and the numerical aperture of the output propagation light is NAout, Expression (1) is established.

  From this equation, since NAout is increased by the reduced core diameter ratio, the output multimode fiber 13 preferably has a structure in which the core numerical aperture can be increased by the clad 33 of the resin material as shown in FIG. In the clad 32 using a glass material, light leakage is large and propagation efficiency is deteriorated.

  FIG. 3 is a cross-sectional view showing an example of the clad pump fiber 16. A rare earth element is added to the core 19 through which the signal light propagates, and a single layer or a plurality of layers of inner clad 41 surrounded by quartz glass, and an outer clad 42 surrounded by a low-refractive index coating resin. It is the structure which consists of.

  When the absolute refractive index of the core 19 is ncore, the absolute refractive index of the inner cladding 41 is n1, and the absolute refractive index of the outer cladding 42 is n2, the relationship ncore> n1> n2 is satisfied.

Light that has entered the inner cladding 41 with a numerical aperture satisfying the above condition propagates through the optical fiber while repeating total reflection at the boundary between the inner cladding 41 and the outer cladding 42. The inner clad 41 is preferably an inner clad 41 having a rectangular cross section for the purpose of eliminating the skew light and efficiently guiding the excitation light to the core 19.

  In general, the numerical aperture of the inner cladding 41 is 0.46, and the numerical aperture of the core 19 is 0.06 to 0.12 in order to achieve high beam quality. In high-power oscillation, it is preferable to use a fiber doped with ytterbium as a rare earth element. In the case of ytterbium, a clad pump fiber 16 having a light-light conversion efficiency of 70% or more can be manufactured.

  Next, the signal light cores 18 and 19 are connected. In consideration of mechanical reliability, it is desirable that the connecting portion 20 of the signal light core is fusion-bonded. The coating for protecting the connecting portion 20 of the signal light core uses a coating resin 35 having a refractive index higher than that of the clad 34 like the signal light propagation fiber, and the signal light propagates only to the signal light core 18. Leave it in a state.

  When the signal light cores 18 and 19 are connected, it is premised that the optimum core diameter and refractive index are calculated in advance so that the connection loss is reduced. There is a risk that the connection loss will increase due to a deviation. For this reason, it is desirable to measure the transmission state before and after connection while passing light having a wavelength unrelated to excitation through the signal light cores 18 and 19.

  In the unlikely event that it is found that the connection has been made with a large connection loss, the high-power signal light is likely to leak, so the two connected optical fibers will be split and the end faces will be processed smoothly. And have to reconnect. In the structure where the signal propagation part is provided in the fiber bundle for condensing the excitation light, the fiber diameter will inevitably increase due to the smoothing of the end face when reconnecting. This will increase the manufacturing cost.

  After the signal light core is connected, the pumping light output multimode fiber 13 and the cladding pump fiber 16 are connected. In consideration of mechanical reliability, it is desirable that the connection portion 17 be fusion-bonded. The coating for protecting the connection portion 17 between the pumping light output multimode fiber and the cladding pump fiber has a low refractive index coating that has the same refractive index as the outer cladding 42 of the double cladding fiber 16 so as not to leak the pumping light. Resin 36 is used.

  Here, as shown in FIGS. 4A and 4B, the connection portion 17 of the pumping light output multimode fiber 13 is in a region that does not affect the signal light guided through the core 19 of the cladding pump fiber 16. It is important to connect. In general, in the case of single mode light having a numerical aperture of 0.12, the signal light propagates while the light oozes out into the clad about 5 times the core diameter.

  For example, if the core diameter is φ6 μm, if a part of the pumping light output multimode fiber 13 exists in a region within a distance of 6 × 5 ÷ 2 = 15 μm from the core center, the signal light is affected. There is a possibility that the propagation efficiency deteriorates and the signal light enters the multimode fiber 13 for pumping light output. Considering the fusion splicing with the fiber, the fiber diameter of a commercially available optical fiber is φ125 μm, so the distance between the core center and the surface of the output multimode fiber 13 should be 125 μm / 2 = 62.5 μm or more. No problem.

  The connection form is not particularly limited to this, and the connection form between the pumping light output multimode fiber 13 and the clad pump fiber 16 is such that each resin is removed as shown in FIG. There is also a way to connect the sides. In this case, it is preferable that the distal end portion of the pumping light output multimode fiber 13 has a smooth taper shape so that the pumping light can be efficiently transferred to the cladding pump fiber 16. As a method for producing a tapered shape, there are a method in which the fiber bundle is melt-drawn in the same manner as that in which the fiber bundle is produced, or a method in which the tip of the fiber is dissolved in a tapered shape with hydrofluoric acid.

  Finally, a semiconductor laser (not shown) with a fiber pigtail is connected to the input end 37 of the pumping light input multimode fiber 11. The excitation wavelength uses a specific wavelength that efficiently inverts and distributes rare earth elements contained in the core 19 of the cladding pump fiber 16. When ytterbium is contained in the core, the excitation wavelengths are 975 nm and 915 nm as typical values, and when neodymium is contained in the core 19, the excitation wavelength is 807 nm as a typical value.

  Again, in consideration of mechanical reliability, it is desirable that the connection at the input end 21 is a fusion connection. As the protective coating, a resin having a refractive index lower than that of the core is used when the glass cladding 32 is not present, and the refractive index may not be considered when the glass cladding 32 is present.

  Since the optical fiber coupler 10 using the above-described connection structure does not propagate to the pumping light output multimode fiber 13 even if the radiated signal light 5 is generated at the signal light core connection portion 20, Unstable operation and destruction of the semiconductor laser can be prevented.

  Further, since the signal light core 18 does not have a radiated light confining waveguide, the radiated signal light 5 is quickly absorbed by the coating resin 35 around the cladding without being captured by the signal light core 18 again. This does not cause noise in the optical amplification system.

  In consideration of safety, it is necessary to disperse the radiated signal light 5 in a specific place of the coating resin 35 without being dispersed, and to be absorbed by the coating resin 35. It is desirable to increase the radius. If the radius of curvature is reduced and bent, a portion that concentrates and absorbs the radiated signal light 5 is formed, and the coating resin 35 may burn and destroy the entire optical amplification system.

  The present invention will be described in detail by the following examples. However, the following examples are merely illustrative of the present invention and do not limit the present invention.

  The optical fiber coupler having the configuration shown in FIG. 1 was manufactured by the following procedure. The following fibers were used. The manufacturing method is the same as the best mode described above.

  The optical fiber coupler 10 is composed of members having the following specifications.

  The specifications of the pumping light input multimode fiber 11 are a core diameter of 105 μm, a cladding diameter of 125 μm, and a core numerical aperture of 0.22. The specifications of the pumping light output multimode fiber 13 are a core diameter of 125 μm, a resin cladding diameter of 200 μm, and a core numerical aperture of 0.46. The specifications of the signal light propagation fiber 15 are a core diameter of 6 μm, a cladding diameter of 125 μm, and a core numerical aperture of 0.12.

  The specifications of the cladding pump fiber 16 are an ytterbium-doped core, a core diameter of 6 μm, an inner cladding dimension of 400 μm, a core numerical aperture of 0.12, and an inner cladding numerical aperture of 0.46.

  First, the pumping light input multimode fibers 11 are bundled so as to form a close-packed array, melted and integrated with a heater, and then drawn to disconnect the pumping light output multimode fiber 13 from the waveguide. After the areas were made the same, both were fusion spliced to form the connecting portion 14.

  Next, seven semiconductor lasers 51 were fused and connected to the input end of the optical fiber coupler 10. The specifications of the semiconductor laser 51 were an oscillation wavelength of 915 nm and a low-grade output of 10 [W]. As a result of installing a light receiver at the output end and reading the output value, the excitation light propagation region has a high light collection efficiency of 90% or more.

  The connection with the clad pump fiber 16 is made by fusion splicing so that the central axis of the pumping light output multimode fiber 13 is located at a distance of 125 μm or more from the core central axis of the clad pump fiber 16 to form the connection portion 17. In addition, the connecting portions 20 were formed by fusion-connecting the signal light cores. Finally, the optical fiber coupler 10 was completed by coating with a low refractive index coating resin 36.

  In order to confirm the influence on the semiconductor laser, an optical amplifier as shown in FIG. 5 was constructed. A signal light source was optically coupled to the core at one end of the clad pump fiber 16, and an optical fiber coupler 10 was connected to the signal light output side. A quartz glass end cap 53 was fused and connected to the output end of the signal light propagation portion to prevent breakage. The end cap 53 has an anti-reflection coating.

  Although laser oscillation was continued for 1 hour or longer, all the semiconductor lasers 51 connected to the input terminal 37 operated without fluctuation in output and were not destroyed. In an experiment in an optical fiber coupler in which a signal light core is included in the center of a fiber bundle for condensing excitation light as shown in FIG.

  Since the optical fiber coupler according to the present invention can prevent unstable operation and destruction of the semiconductor laser, it contributes to the high reliability of the optical amplifying device and the fiber laser and can be expected to increase the output.

(A) A side view of an example of the optical fiber coupler of the present invention, (b) a sectional view of an example of the vicinity of the output end of the optical fiber coupler of the present invention, (c) a bundled array of optical fiber couplers of the present invention Sectional view of an example (A) A cross-sectional view of an example of a pumping light input multimode fiber, (b) a cross-sectional view of an example of a pumping light output multimode fiber, and (c) a cross-sectional view of an example of a signal light propagation fiber. Cross section of an example of a clad pump fiber Schematic showing multiple examples of connection form with clad pump fiber 1 is a configuration diagram illustrating an example of an optical amplifying device including an optical fiber coupler according to the present invention. Side view of a prior art optical fiber coupler

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical fiber coupler 11 Multimode fiber for pumping light input 12 Diameter reduction part 13 Multimode fiber for pumping light output 14 Connection part 15 Signal light propagation fiber 16 Cladding pump fiber 17 Connection part 18 Signal light core 19 For signal light Core 20 Signal light core connecting portion 31 Core 32 Cladding 33 Cladding (resin)
34 Cladding 35 Coating resin 36 Low refractive index coating resin 37 Input end 41 Inner cladding 42 Outer cladding 51 Pumping light source (semiconductor laser)
52 Signal light source 53 End cap

Claims (5)

A core in which signal light propagates, an inner clad formed of a material having a lower refractive index than the core around the core, and an outer clad formed of a material having a lower refractive index than the inner clad around the inner clad For clad pump fiber with clad,
One end of a plurality of pumping light input fibers is bundled and integrated and the tip is gently reduced in diameter, and the reduced tip is coupled to the pumping light output multimode fiber by the end faces. When,
An optical fiber coupler that couples a signal light propagation fiber formed by arranging a signal light propagation fiber along the excitation light propagation part,
The connection part between the pumping light propagation part and the inner cladding region of the cladding pump fiber is coupled at a different position to the connection part between the signal light propagation part and the core region of the cladding pump fiber, and the connection An optical fiber coupler characterized in that the position is set to a position exceeding a certain distance centering on the axis of the signal light core of the clad pump fiber. 2. The optical fiber coupler according to claim 1, wherein the predetermined distance is at least five times the core radius. The portion directly in contact with the clad side surface of the signal light propagation portion is covered with a resin having a higher refractive index than the clad, and the core from the reduced diameter portion to the emission end of the excitation light propagation portion and the portion in direct contact with the clad side surface The optical fiber coupler according to claim 1 or 2, wherein the optical fiber coupler is coated with a resin having a lower refractive index than the core and the clad. An optical amplifying device comprising the optical fiber coupler according to claim 1. A fiber laser device comprising the optical fiber coupler according to claim 1.

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