JP5173137B2 - Double clad fiber, coupling structure, fiber amplifier and fiber laser - Google Patents

Description

The present invention, in a fiber amplifier or a fiber laser, binding structure capable of guiding the signal light and the pumping light with high efficiency pumping medium, such as a rare earth-doped double-clad fiber, to the fiber amplifier and fiber laser.

  In recent years, in high-power fiber amplifiers and fiber lasers, in order to avoid damage to the core due to pumping light, the use of clad pumping is increasing in the amplifying section. In clad pumping, signal light propagates through the core and excitation light propagates through the clad. One factor that determines the efficiency of amplification by clad pumping is the coupling efficiency of pumping light from the pumping light source to the rare-earth doped fiber that is the amplifying unit. Therefore, the structure of the connecting portion is a very important mechanism for these products.

As a structure of this coupling portion, a tilted fiber bundle disclosed in Patent Document 1 has been proposed. The inclined fiber bundle includes a clad pumping fiber having a certain length and a plurality of multimode fibers bundled together and inclined to a reduced cross-sectional area region, and the reduced cross-sectional area includes the clad pumping fiber. It has a structure that is fused and connected.
Japanese Patent No. 3415449

  The tilted fiber bundle described in Patent Document 1 is capable of optically coupling optically. On the other hand, when the pump port shape is a circular shape that can be coupled to a rare earth-doped fiber most efficiently in terms of bundling a plurality of fibers, 7 (6 if a single-mode optical fiber for signal light guide is present) or 19 (18 if a single-mode optical fiber for signal light guide is present) ) Etc., there is a drawback that only the number of arrangements determined structurally can be taken. The fact that only a structurally determined number of arrangements can be taken means that the number of pumping light sources is limited or an unused port is created, resulting in a lower pumping light density. Here, the excitation light density is obtained by dividing the excitation light amount by the cross-sectional area through which light passes, and this also affects the amplification efficiency.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a fiber that can introduce signal light and pump light into a pumping medium with high efficiency, a coupling structure using the fiber, a fiber amplifier, and a fiber laser.

In order to achieve the above object, the present invention connects a signal light guide fiber and an excitation light guide fiber to one end A of a double clad fiber, and connects a pumping medium to the other end B of the double clad fiber. The coupling structure is configured such that signal light and pump light can be introduced into a pumping medium, and the double clad fiber has a core and an inner clad surrounding the outer periphery of the core. The light is guided and extends from the end A to the end B of the double clad fiber in the longitudinal direction. The inner clad diameter gradually decreases and the core diameter gradually decreases. The mode field diameter gradually decreases and then increases gradually. Provided is a coupling structure characterized in that <B and the inner cladding diameter is A> B.

In the coupling structure of the present invention, the ratio B / A of the mode field diameter to the signal light at the two ends A and B is 1.7 or more, and the ratio B / A of the inner cladding diameter of the two ends A and B is It is preferable that it is 0.67 or less.

In the coupling structure of the present invention, it is preferable that the end of the mode field diameter that is smaller has a mode field diameter of 6 μm or less at the wavelength used.

In the coupling structure of the present invention, it is preferable that the end of the side having the larger mode field diameter has a mode field diameter of 12 μm or more at the used wavelength.

In the bonded structure of the present invention, it is preferable that the change rate of the core diameter is 100% or less per mm over the longitudinal direction.

In the coupling structure of the present invention, it is preferable that the inner cladding diameter is 400 μm or less at the end on the smaller inner cladding diameter side.

In the coupling structure of the present invention, it is preferable that the numerical aperture at the end having the smaller inner cladding diameter is 0.70 or less.

In the coupling structure of the present invention, the numerical aperture at the end on the smaller inner cladding diameter side is preferably 0.50 or less.

In the coupling structure of the present invention, the outer cladding surrounding the outer periphery of the inner cladding is preferably glass, polymer, air, or a hole formed in the cladding, which has a lower refractive index than the inner cladding.

In the coupling structure of the present invention, a small side end of the mode field diameter of the double clad fiber, and a signal light guiding fibers and the excitation light guide fiber, it is preferable that connects via a multicore fiber.

In the coupling structure of the present invention, the pumping medium is preferably a rare earth-doped double-clad fiber.

The present invention provides a fiber amplifier which is characterized by having a coupling structure according to the present invention.

The present invention also provides a fiber laser comprising the coupling structure according to the present invention.

In the double clad fiber of the present invention, the two ends A and B are configured such that the mode field diameter with respect to the signal light is A <B and the inner clad diameter is A> B, so that clad pumping is performed. In such a fiber amplifier or fiber laser, when the fiber is used in a coupling portion between a signal light guiding fiber and a pumping light guiding fiber and a pumping medium such as a rare earth-added double clad fiber, high efficiency coupling of signal light and pumping light is achieved. As appropriate, the mode field diameter and inner cladding diameter of the two ends A and B of the fiber can be set, so that the signal light and the pump light can be introduced into the pumping medium with high efficiency. it can. Further, since the diameter of the end of the incident side of the fiber can be set so that the best excitation light density can be obtained according to the number of excitation light sources, the limitation on the number of excitation light sources can be eliminated.
In the coupling structure of the present invention, the signal light guiding fiber and the excitation light guiding fiber are connected to one end of the double clad fiber according to the present invention having a small mode field diameter, and the other end of the double clad fiber is connected to the other end of the double cladding fiber. Since the pumping medium is connected, signal light and excitation light can be introduced into the pumping medium with high efficiency. Further, since the diameter of the end of the incident side of the fiber can be set so that the best excitation light density can be obtained according to the number of excitation light sources, the limitation on the number of excitation light sources can be eliminated. Furthermore, even when the apparatus is configured using a normal optical fiber or a commercially available pumping medium, the signal light and the excitation light can be introduced into the pumping medium with high efficiency.
Since the fiber amplifier and fiber laser of the present invention have the structure having the coupling structure according to the present invention, signal light and pump light can be introduced into the pumping medium with high efficiency, and amplification efficiency can be improved. . In addition, since the diameter of the end of the incident side of the fiber can be set so that the best excitation light density can be obtained according to the number of excitation light sources, the number of excitation light sources can be eliminated, and the design The degree of freedom can be expanded. Furthermore, even when a device is configured using a normal optical fiber or a commercially available pumping medium, signal light and pumping light can be introduced into the pumping medium with high efficiency. This can be used to reduce the cost.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1 and 2 are views showing a first embodiment of a double clad fiber according to the present invention. FIG. 1 is a radial sectional view of the double clad fiber, and FIG. 2 is a longitudinal sectional view of the same double clad fiber. .

The double clad fiber 10 of the present embodiment is composed of a central core 11 made of quartz glass having a refractive index increased by adding a dopant such as GeO ₂ , and a quartz glass having a refractive index lower than that of the core 11. Inner cladding 12 surrounding the outer periphery, a transparent material having a lower refractive index than the inner cladding, an outer cladding 13 surrounding the outer periphery of the inner cladding 12, and a resin protection provided on the outer periphery of the outer cladding 13 as necessary The two ends A and B of the double clad fiber 10 are in a relationship such that the mode field diameter for signal light is A <B and the inner clad diameter is A> B.

  In the double clad fiber 10 of the present embodiment, the two ends A and B are configured such that the mode field diameter with respect to the signal light is A <B and the inner clad diameter is A> B. In such fiber amplifiers and fiber lasers as used, the double clad is connected to the coupling portion between the signal light guiding fiber and the pumping light guiding fiber and a pumping medium such as a rare earth doped double clad fiber (hereinafter referred to as a rare earth doped double clad fiber) When the fiber 10 is used, the mode field diameter and the inner cladding diameter of the two ends A and B of the double clad fiber 10 can be set so as to be suitable for high-efficiency coupling of signal light and pump light. As a result, the signal light and the pump light are efficiently transmitted to the rare earth-doped double clad fiber. It is possible to enter. Further, since the diameter of the end of the incident side of the fiber can be set so that the best excitation light density can be obtained according to the number of excitation light sources, the limitation on the number of excitation light sources can be eliminated.

  In the double clad fiber 10 of this embodiment, the ratio B / A of the mode field diameter to the signal light at the two ends A and B is 1.7 or more, and the ratio B of the inner cladding diameter at the two ends A and B / A is preferably 0.67 or less. This makes it possible to more efficiently couple the pump light guide fiber on the incident side, the signal light guide fiber, and the rare earth-doped double clad fiber on the output side, which are generally used at present, so that the signal light and the pump light are more efficiently combined. In particular, it can be introduced into a rare earth-doped double clad fiber.

  In general, there is a large difference between the mode field diameter of the signal light guiding fiber on the incident side and the mode field diameter of the rare earth-doped double clad fiber on the output side, and the difference in mode field diameter between both ends of the double clad fiber 10 is 1.7. When the ratio is less than double, a large connection loss occurs in the signal light at the connection at the incident end or the output end, and the signal light cannot be efficiently introduced into the rare earth-doped double clad fiber. Furthermore, the cladding outer diameter of the incident-side excitation light guiding fiber and the rare-earth-doped double-clad fiber on the emission side are also greatly different. Similarly, if the difference in the cladding outer diameters at both ends of the double-clad fiber 10 is small, A large connection loss occurs in the pumping light at the connection at the part or the emitting end (usually the emitting end), and the pumping light cannot be efficiently introduced into the rare earth-doped double clad fiber.

  In the double clad fiber 10 of the present embodiment, the end A on the side with the smaller mode field diameter preferably has a mode field diameter of 6 μm or less at the used wavelength. As a result, the connection loss between the signal light guiding fiber generally used for guiding the signal light and the double clad fiber 10 of the present embodiment can be reduced, so that the signal light can be more efficiently introduced into the rare earth-doped double clad fiber.

  In general, the mode field diameter of the signal light guiding fiber used for guiding the signal light is about 4 to 6 μm. Therefore, if the mode field diameter at the end A on the incident side of the double clad fiber 10 exceeds 6 μm, the mode The connection loss of the signal light due to the field difference is increased, and the signal light cannot be efficiently introduced into the rare earth-doped double clad fiber.

  In the double clad fiber 10 of the present embodiment, it is preferable that the end B on the side with the larger mode field diameter has a mode field diameter of 12 μm or more at the used wavelength. As a result, the connection loss between the rare earth doped double clad fiber, which is a general pumping medium used for clad pumping, and the double clad fiber 10 can be reduced, so that signal light can be more efficiently introduced into the rare earth doped double clad fiber. .

  Since the mode field diameter of a general rare earth-doped double clad fiber is about 14 to 20 μm, the mode field diameter of the end B on the side connected to the rare earth-doped double clad fiber of the double clad fiber 10 is less than 12 μm. The connection loss of the signal light due to the mode field difference is increased, and it is not preferable because the signal light cannot be efficiently introduced into the rare earth-doped double clad fiber.

  In the double clad fiber 10 of the present embodiment, it is preferable that the change rate of the core diameter is 100% or less per 1 mm. Thereby, since the change of the core diameter becomes gradual, the leakage of the signal light in the double clad fiber 10 can be avoided, the loss of the signal light in the double clad fiber 10 is suppressed, and the signal is more efficiently signaled. Light can be introduced into a rare earth-doped double clad fiber. If the rate of change of the core diameter exceeds 100% per mm, signal light leakage occurs there, and the light guide loss increases, which is not preferable.

  In the double clad fiber 10 of the present embodiment, the inner clad diameter of the end B on the side having the smaller inner clad diameter is preferably 400 μm or less. Thereby, the optical density of the pumping light introduced into the rare earth-doped double clad fiber is increased, and the pumping efficiency in the rare earth-doped double clad fiber can be further increased. Since the clad outer diameter of a general rare earth-doped double clad fiber is 400 μm or smaller, if the inner clad diameter at the output side end B of the double clad fiber 10 connected to the rare earth-doped double clad fiber is larger than 400 μm, The connection loss of the pumping light due to the area difference is increased, which is not preferable because the pumping light cannot be efficiently introduced into the rare earth-doped double clad fiber.

  In the double clad fiber 10 of the present embodiment, it is preferable that the numerical aperture of the inner cladding 12 is 0.70 or less at the end B on the side having the smaller inner cladding diameter. As a result, the numerical aperture of any rare earth-doped double clad fiber is generally smaller than expected, so the connection loss of the pumping light due to the numerical aperture difference can be suppressed, and the pumping light can be efficiently distributed to the rare earth-doped double clad. Can be introduced into fiber.

  In the double clad fiber 10 of the present embodiment, it is preferable that the numerical aperture of the inner cladding 12 is 0.50 or less at the end B on the side with the smaller inner cladding diameter. Since this numerical aperture is smaller than the numerical aperture of the most common rare earth doped double clad fiber, the connection loss of the pumping light due to the numerical aperture difference can be suppressed, and the pumping light can be added more efficiently. Can be introduced into double clad fiber. Therefore, even when fiber amplifiers and fiber lasers are constructed using rare-earth-doped double clad fibers that are commercially available at low cost, the connection loss of pumping light due to the numerical aperture difference can be suppressed, so efficient pumping is possible. A fiber amplifier or a fiber laser capable of coupling light to a rare earth-doped double clad fiber can be provided at low cost.

  The double clad fiber 10 of the present embodiment is prepared by first producing a fiber preform having a core and a double clad structure surrounding the core in the same manner as in the production of a conventionally known quartz glass double clad fiber. It can be manufactured by a method in which a fiber is spun to have a diameter at the end on the thick side, and then a part of the obtained fiber is melt-drawn to produce a double clad fiber having a different end diameter. When melt-drawing one side of the fiber, the desired properties can be obtained by drawing while measuring the outer diameter during drawing or by drawing under predetermined drawing conditions in order to obtain the desired inner cladding diameter and core diameter. Can be obtained.

FIG. 6 is a diagram showing a second embodiment of the double clad fiber of the present invention. The double clad fiber 30 of the present embodiment is composed of a central core 31 made of quartz glass having a refractive index increased by adding a dopant such as GeO ₂ , and a quartz glass having a lower refractive index than the core 31. The inner cladding 32 surrounds the outer periphery, and the outer cladding 33 is made of a polymer having a lower refractive index than the inner cladding 32 and surrounds the outer periphery of the inner cladding 32. Although not shown in the drawing, the two ends A and B of the double clad fiber 30 have the relationship that the mode field diameter for the signal light is A <B and the inner clad diameter is A> B, as in the first embodiment. It has become.

FIG. 7 is a view showing a third embodiment of the double clad fiber of the present invention. The double clad fiber 40 of the present embodiment is composed of a central core 41 made of quartz glass having a refractive index increased by adding a dopant such as GeO ₂ , and a quartz glass having a lower refractive index than the core 41. The inner clad 42 surrounds the outer periphery. In the double clad fiber 40 of this embodiment, the atmosphere 43 (air) around the inner clad 42 functions as a clad, and functions as a double clad structure. Although not shown, the two ends A and B of the double-clad fiber 40 have a relationship in which the mode field diameter for the signal light is A <B and the inner cladding diameter is A> B, as in the first embodiment. It has become.

FIG. 8 is a view showing a fourth embodiment of the double clad fiber of the present invention. The double clad fiber 50 of the present embodiment is composed of a central core 51 made of quartz glass having a refractive index increased by adding a dopant such as GeO ₂ , and a quartz glass having a lower refractive index than the core 51. An inner cladding 52 that surrounds the outer periphery and a hole 53 that functions as an outer cladding that surrounds the inner cladding 52 are provided. Although not shown in the drawing, the two ends A and B of the double clad fiber 30 have the relationship that the mode field diameter for the signal light is A <B and the inner clad diameter is A> B, as in the first embodiment. It has become.

Next, an embodiment of a coupling structure according to the present invention will be described.
FIG. 4 is a diagram showing a first embodiment of a coupling structure according to the present invention. The coupling structure 20 of the present embodiment includes a signal light source 21, a signal light guide fiber 22 that guides the signal light from the signal light source 21, a large number of excitation light sources 23, and each excitation light source 23. As a pumping medium having a structure in which an excitation light guiding fiber 24 for guiding excitation light and a core made of quartz glass to which rare earth elements such as Yb, Er, Tm, Gd, and Eu are added are surrounded by a clad of two layers. The signal light guide fiber 22 and the multiple excitation light guide fibers 24 are connected to the rare earth-doped double clad fiber 25 and the end A of the two ends having the larger inner cladding diameter, and the inner cladding diameter is small, A double-clad fiber 10 of the present invention provided by connecting one end of the rare earth-doped double-clad fiber 25 to an end B on the side having a larger field diameter. It is configured.

  The signal light guide fiber 22 is fusion-bonded so that signal light can be introduced into the core 11 at the end A of the double clad fiber 10, and a number of pump light guide fibers 24 are connected to the inner cladding 12 at the end A of the double clad fiber 10. It is fusion-bonded so that excitation light can be introduced into it. The rare earth-doped double clad fiber 25 and the end B of the double clad fiber 10 are fusion-bonded so that the respective cores are coaxial and the respective inner clads coincide. The types of the signal light guide fiber 22 and the excitation light guide fiber 24 are not limited. For example, the signal light guide fiber 22 may be a single mode optical fiber, and the excitation light guide fiber 24 may be a multimode optical fiber. Further, the signal light source 21 and the excitation light source 23 are not particularly limited, and a laser diode (LD) or a fiber amplifier can be used. The signal light may be pulsed light or continuous light.

  The coupling structure 20 configured as described above drives the signal light source 21 and the excitation light source 23 so that the signal light guided through the signal light guide fiber 22 is introduced into the core 11 of the double clad fiber 10. The light is guided through the core 11 and introduced into the core of the rare earth-doped double clad fiber 25 connected to the end B. The pumping light guided through the pumping light guide fiber 24 is introduced into the inner cladding 12 of the double cladding fiber 10, guided in the inner cladding 12, and the inner cladding of the rare earth-doped double cladding fiber 25 connected to the end B. To be introduced. The excitation light introduced into the inner cladding of the rare earth-doped double clad fiber 25 excites the rare earth ions added to the core, amplifies the signal light guided through the core, and the high-power amplified light becomes the rare earth-doped double clad fiber. 25 is output from the other end.

  The coupling structure 20 of the present invention has a coupling portion for inclining the diameter for introducing excitation light into a pumping medium, a core that guides signal light instead of a fiber bundle, and an inner cladding that guides excitation light. By adopting a double clad fiber structure having a structure in which the diameter of the double clad fiber 10 changes along the longitudinal direction, the drawbacks of Patent Document 1 are solved. That is, by separating the pumping light guide fiber and the portion responsible for the diameter change, it is possible to obtain an effect that a number of pumping light sources other than the structurally determined number can be freely connected.

  Further, the double clad fiber 10 used in the coupling structure 20 of the present invention includes a core 11 that guides signal light, and the mode field diameter of the core 11 is small at the end A where the fiber diameter is large. The diameter is large at the small end B. In order to explain the effect of this, first, the performance required for the optical fiber that enters the signal light into the double clad fiber 10 and the rare earth-doped double clad fiber 25 having the clad pumping function coupled by the double clad fiber 10 will be explained. .

  The signal light guide fiber 22 that enters the signal light into the double clad fiber 10 guides the signal light with relatively weak power. Also, optical components such as modulators are often connected before amplification. Therefore, a fiber having a relatively small confinement in the core and strong in bending and having a small mode field diameter is used. An example of such a fiber is Corning Incorporated's HI 1060 fiber. The mode field diameter of this HI1060 fiber is about 5.4 μm (illustrative and not limiting of the invention). On the other hand, the rare earth-doped double clad fiber 25 having the clad pumping function is amplified there, so that it is necessary to guide light with very strong power. An example of such a fiber is Nufern's 20/400 LMA fiber. The mode field diameter of the 20/400 LMA fiber is approximately 20 μm (illustration is not intended to limit the invention).

  As described above, the double clad fiber 10 needs to be connected to each end with fibers having different mode field diameters and clad outer diameters. In both signal light and pumping light, it is necessary to guide light efficiently to the rare earth-doped double clad fiber 25, so it is desirable to reduce the connection loss at these ends as much as possible. Regarding the connection loss of excitation light, the relative relationship (size, overlap, etc.) of the light guide cross-sectional area, the numerical aperture, the connection angle, and the like are related. If the incident side diameter is the same as or larger than the receiving side diameter, the numerical aperture is the same or smaller on the incident side, and the axes of the incident side core center and the receiver side core center match as much as possible, the loss is reduced. Can connect. Regarding the signal light, the relative relationship (size, overlap, shape, etc.) of the mode field diameter and the connection angle are related. If fibers with close field distribution are connected with their axes aligned as much as possible, the loss will be low. These are described in, for example, “Fiber / Optical Communication (J. C. Palais original work, Hirahachi Sato translation, Morikita Publishing)”.

  In this coupling structure 20, since the cladding outer diameter and the mode field diameter on the incident side and the emission side are different from each other, in order to couple efficiently with these fibers (that is, to reduce the connection loss), the coupling structure 20 is related to the present invention. It is necessary to make the mode field diameter of the double clad fiber 10 close to the mode field diameter of the connection target at each end A and B. That is, it is better that the mode field diameter is smaller on the incident side and the mode field diameter is larger on the outgoing side. In the coupling structure of the present invention, in consideration of these points, the double clad fiber 10 is an effective structure for efficiently introducing signal light and pump light into the rare earth-doped double clad fiber 25. Used.

Further, as illustrated above, assuming a specific connection object, the double clad fiber 10 used in the connection structure 20 has a difference between the mode field diameter on the incident side and the mode field diameter on the output side of 1.7 times. It is desirable that there be more.
Further, it is desirable that the inner cladding diameter is 2/3 or less on the exit side compared to the entrance side. Here, by making the inner cladding diameter sufficiently small, it is possible to increase the power density of the pumping light, and therefore it is possible to increase the amplification efficiency in the rare earth-doped double cladding fiber 25 that is a pumping medium. At this time, it is more effective if the inner cladding diameter of the rare earth-doped double cladding fiber 25 to be connected is approximately the same as or slightly larger than the inner cladding diameter of the end B of the double cladding fiber 10. Moreover, it is not preferable that the inner cladding diameter is too large from the viewpoint of excitation light density. The inner clad diameter of the end B of the double clad fiber 10 is desirably 400 μm or less, and the rare earth-doped double clad fiber that is actually commercially available has an inner clad diameter of 400 μm. By doing so, since a commercially available rare earth-doped double clad fiber can be used, an inexpensive product can be provided.

  Usually, as the cladding diameter is reduced, the mode field diameter of the core is reduced. However, the inventors have intensively studied the structure of the core, and devised the mode field diameter of the core to increase when the cladding diameter is reduced, leading to the realization of the present invention. . As an example, FIG. 3 shows a calculation result of a change in the cladding diameter and a change in the mode field diameter of the core (illustration is not intended to limit the invention).

Further, limiting the mode field diameter on the incident side (end A) of the double clad fiber 10 to 6 μm or less reduces the connection loss with the optical fiber (signal light guide fiber 22) on the incident side as described above. Is important above.
Similarly, limiting the mode field diameter on the emission side (end B) of the double-clad fiber 10 to 12 μm or more reduces the connection loss with the rare-earth-doped double-clad fiber 25 on the emission side, as described above. is important.

  In the present invention, attention was also paid to the reduction ratio of the diameter of the double clad fiber 10 used in the coupling structure 20. If the rate of change of the mode field diameter in the longitudinal direction of the core 11 that guides the signal light is large, the signal light may leak out from the core 11 during the light guide and the transmission loss may increase. According to a study by the present inventors, if the rate of change in the mode field diameter in the longitudinal direction of the core 11 is not less than 100% per mm (that is, the change in mode field diameter per mm is less than twice), the signal light It turns out that leakage is a problem. Since it varies depending on the refractive index distribution structure of the core, it cannot be generally stated, but the refractive index distribution of the core 11 investigated by the present inventors has a reduction rate of 80% / mm and a loss of about 0.1 dB. On the other hand, at a reduction rate of 135% / mm, the loss increased to about 1.2 dB, and the loss increased around 100% / mm (illustration is not intended to limit the invention). . It has also been found that even if the refractive index distribution is changed, there is a point that the loss changes abruptly around 100% / mm (however, the absolute value of the loss varies depending on the refractive index distribution). Therefore, in order to reduce the loss in the double clad fiber 10 of the present invention, it is desirable that the rate of change of the core diameter is 100% or less per mm in the longitudinal direction.

  What must be considered regarding the clad of the double clad fiber 10 is the power density of the pumping light and the coupling loss of the pumping light. As described above, a higher power density is desirable. Therefore, the inner cladding diameter is desirably 400 μm or less. Factors affecting the coupling loss of pumping light are the numerical aperture (NA) and the inner cladding diameter. The NA has a small connection loss when connecting from a fiber having a small NA to a fiber having a large NA. The inner clad NA of the rare earth-doped double clad fiber for clad pumping currently commercially available or published by academic societies is about 0.70 or less even at the largest, so it is connected to such a rare earth doped double clad fiber. The NA of the double clad fiber 10 of the present invention is desirably 0.70 or less. In the case of a commercially available rare earth-doped double clad fiber, NA is about 0.50 or less, and in order to cope with this, the inside of the emission side (end B) of the double clad fiber 10 used in the connection structure 20 of the present invention is used. The NA of the clad is more preferably 0.50 or less. If the inner cladding diameter is 400 μm or less, the inner cladding diameter is smaller than the inner cladding diameter of the rare-earth-doped double-clad fiber 25 to be connected.

  FIG. 5 is a diagram showing a second embodiment of the coupling structure according to the present invention. The coupling structure 26 of the present embodiment includes the same components as those of the coupling structure 20 of the first embodiment shown in FIG. 4 described above. Further, the end A of the double-clad fiber 10 on the smaller mode field diameter side, and the signal light guide The fiber 22 and the pumping light guide fiber 24 are connected via a multi-core fiber 27. The coupling structure 26 of the present embodiment can obtain the same effects as the coupling structure 20 of the first embodiment shown in FIG. 4 described above.

  As described so far, in the coupling structure of the present invention, in order to use the double clad fiber 10 effectively, the signal light and the pump light are incident from the side of the double clad fiber 10 having a small mode field diameter. It is desirable to connect and use as described above.

  Furthermore, configuring a fiber amplifier or fiber laser using the coupling structures 20 and 26 described above has an advantage that the amount of incident excitation light and signal light can be reduced because loss of excitation light and signal light is small. . As a result, it is possible to provide a fiber amplifier or fiber laser that is advantageous in terms of safety and cost.

(Examples 1 to 10)
A double clad fiber having the structure shown in FIG. 6 was produced.
First, a fiber preform made of quartz glass for producing a core and an inner clad surrounding the core was produced. Of the fiber preforms of each Example, the preforms of Examples 4 and 7 were produced near the core by the VAD method, and the fiber preforms of the other examples were produced near the core by the MCVD method. Then, using an external method, quartz glass was deposited until the required core / inner clad ratio was obtained, and placed in a sintering furnace to form a transparent glass to prepare a fiber preform.
Next, the obtained fiber preform was set in a spinning device, and was spun so as to have an inner cladding diameter at an end A shown in Table 1 to produce a fiber.
Next, a part of the obtained fiber was melt-drawn to form an end having a narrow inner cladding diameter. Next, a polymer solution having a low refractive index was applied to the outer periphery of the inner cladding and cured to form an outer cladding, and double-clad fibers of Examples 1 to 10 were manufactured.
Table 1 shows the specifications of the manufactured double-clad fibers of Examples 1 to 10 and the evaluation results of the insertion loss and the like of the incident fiber and the outgoing fiber.

  In producing the double clad fibers of Examples 1 to 10, the inner clad diameter and the core diameter were drawn while measuring the outer diameter at the time of drawing in order to obtain desired values.

(Comparative Examples 1-3)
As shown in Table 1, double clad fibers of Comparative Examples 1 to 3 in which the relationship between the end A and the end B was such that the mode field diameter with respect to the signal light was not A <B were manufactured. Table 1 shows the specifications of the manufactured double-clad fibers of Comparative Examples 1 to 3 and the evaluation results of the insertion loss and the like of the incident fiber and the outgoing fiber.

  From the results of Table 1, by using the double clad fiber according to the present invention and forming the coupling structure shown in FIG. 4 for introducing the signal light and the pump light into the rare earth-doped double clad fiber, the signal light and the pump light are combined. It can be seen that it can be introduced into a rare earth-doped double clad fiber with low loss, and amplification can be performed with high efficiency.

It is a figure which shows sectional drawing and refractive index distribution of the radial direction which show 1st Embodiment of the double clad fiber of this invention. It is sectional drawing of the longitudinal direction which shows 1st Embodiment of the double clad fiber of this invention. It is a graph which illustrates the relationship between the end B / end A inner clad diameter ratio of a double clad fiber, and a mode field diameter (MFD). It is sectional drawing of the longitudinal direction which shows 1st Embodiment of the coupling structure of this invention. It is sectional drawing of the longitudinal direction which shows 2nd Embodiment of the coupling structure of this invention. It is sectional drawing of the radial direction which shows 2nd Embodiment of the double clad fiber of this invention. It is sectional drawing of the radial direction which shows 3rd Embodiment of the double clad fiber of this invention. It is sectional drawing of the radial direction which shows 4th Embodiment of the double clad fiber of this invention.

Explanation of symbols

10, 30, 40, 50 ... double clad fiber, 11.31, 41, 51 ... core, 12, 32, 42, 52 ... inner clad, 13, 33 ... outer clad, 14 ... protective layer, 20, 26 ... coupling Structure 21 ... Signal light source 22 ... Signal light guide fiber 23 ... Pump light source 24 ... Pump light guide fiber 25 ... Rare earth doped double clad fiber 27 ... Multi-core fiber 43 ... Air (outer clad) 53 ... holes (outer cladding).

Claims (13)

A signal light guiding fiber and an excitation light guiding fiber are connected to one end A of the double clad fiber, and a pumping medium is connected to the other end B of the double clad fiber to introduce the signal light and the pumping light into the pumping medium. A coupling structure configured to be possible,
The double clad fiber has a core, an inner cladding surrounding the outer periphery of the core, and guide the signal light and the pumping light,
As the inner cladding diameter gradually decreases from the end A to the end B of the double-clad fiber, the core diameter gradually decreases, the mode field diameter gradually decreases and then increases, and the mode field diameter for the signal light is A <B, and A coupling structure characterized in that the inner cladding diameter has a relationship of A> B. The ratio B / A of the mode field diameter to the signal light at the two ends A and B is 1.7 or more, and the ratio B / A of the inner cladding diameter at the two ends A and B is 0.67 or less. The coupling structure according to claim 1. 3. The coupling structure according to claim 1, wherein a mode field diameter at an operating wavelength is 6 μm or less at an end on a side having a smaller mode field diameter. The coupling structure according to any one of claims 1 to 3, wherein a mode field diameter at an operating wavelength is 12 µm or more at an end on a larger mode field diameter side. The bonded structure according to any one of claims 1 to 4, wherein a change rate of the core diameter is 100% or less per mm in the longitudinal direction. The coupling structure according to any one of claims 1 to 5, wherein an end of the inner cladding diameter on the smaller side has an inner cladding diameter of 400 µm or less. The coupling structure according to any one of claims 1 to 6, wherein a numerical aperture is 0.70 or less at an end having a smaller inner cladding diameter. 8. The coupling structure according to claim 7, wherein a numerical aperture is 0.50 or less at an end having a smaller inner cladding diameter. The coupling according to any one of claims 1 to 8, wherein the outer cladding surrounding the outer periphery of the inner cladding is glass, polymer, air, or a hole formed in the cladding having a refractive index lower than that of the inner cladding. Structure . A double clad less side end of the mode field diameter of the fiber, according to the signal-light guiding fibers and the excitation light guide fiber, in any one of claims 1 to 9, characterized in that connected through a multicore fiber Bond structure. Pumping medium, the coupling structure according to claim 1, characterized in that the rare earth-doped double-clad fiber. Fiber amplifier characterized by having a coupling structure according to any one of 請 Motomeko 1-11. A fiber laser comprising the coupling structure according to claim 1 .

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