CN100520466C - Multi-port coupler, optical amplifier, and fiber laser - Google Patents

Abstract

The present invention provides a multi-port coupler, which is formed by integrating a central signal optical fiber and a plurality of excitation optical fibers arranged around it and reducing the diameter of the head end side, and combining an excitation light source and an optical amplification package. Layer-pumped optical fiber, wherein a radiated light entrance waveguide is provided around the core of the signal optical fiber at the center, which has a larger outer diameter than the core, and has a The value of the refractive index of the core is small, and the radiated light confinement waveguide section is formed continuously from the connection section with the cladding pump fiber to the end section of the coupler section branched into a plurality of optical fibers.

Description

Multiport Couplers, Optical Amplifiers and Fiber Lasers

technical field

The present invention relates to an optical fiber amplifier used in optical communication, especially a short-distance transmission system or a multi-port coupler used in a fiber laser, and an optical amplifier and a fiber laser using the same.

This application claims priority to Japanese Patent Application No. 2006-149696 filed on May 30, 2006 and Japanese Patent Application No. 2006-260881 filed on September 26, 2006, the contents of which are incorporated herein.

Background technique

In high-output optical amplifiers, fiber lasers, and the like, a clad-pump structure is generally used. It is a device that provides a structure for propagating the excitation light necessary to amplify the light propagating through the core of the optical fiber through the cladding. At the same time that this cladding-pumped structure is widely used, a high-power laser diode (hereinafter, referred to as LD) output from a multimode fiber is used. The cladding pump fiber uses a double-clad structure in which a core doped with a rare earth element has multiple cladding layers.

In order to send the light of the high-power multimode LD to the cladding pump fiber for amplification, a multi-port coupler is used. The multi-port coupler has the function of converging the light of two or more multimode fibers and connecting it to the cladding pump fiber, and at the same time connecting to the single-mode fiber (hereinafter referred to as SM fiber) through which normal signal light passes. with the respective cores of the cladding-pumped fibers. Utilizing the function of this multi-port coupler, even if the output of one LD is several W, a larger output can be obtained by connecting several LDs to the cladding pump fiber. A configuration diagram of an optical amplifier using this multiport coupler is shown in FIG. 1 .

The optical amplifier 1 in FIG. 1 includes a multiport coupler 2 into which signal light and excitation light are injected, and a clad pump fiber 3 head-end-connected to an output end of the multiport coupler 2 . This multiport coupler 2 is formed by integrating a central signal optical fiber 5 composed of an SM optical fiber and an excitation optical fiber 4 composed of a plurality of multimode optical fibers arranged around it, and reducing the diameter at the head end side, and Signal light and excitation light can be injected into the clad pump fiber 3 for optical amplification connected to the reduced diameter output end. A signal light source (not shown) is connected to the signal optical fiber 5 of the multiport coupler 2 , and an LD 10 is connected to each of the plurality of excitation optical fibers 4 . In this optical amplifier 1, the excitation light (for example, wavelength 910-980nm) is injected into the cladding of the cladding pumping fiber 3 by means of the multiport coupler 2, and the fiber doped in the core of the cladding pumping fiber 3 Rare earth ion excitation, by injecting signal light into the core of cladding pump fiber 3 by means of multi-port coupler 2, the injected signal light is amplified, and the amplified signal light (high output signal ) can be output from the cladding pump fiber 3. In this type of optical amplifier, the level of gain may reach 20 dB or more, and the maximum output may reach 1 W to 1 kW.

However, in an optical amplifier combining a multiport coupler and a clad pump fiber conventionally used, in order to improve reliability, a longer lifetime of the LD has been proposed as a problem.

In order to realize the long life of LD, not only the high reliability design of LD itself is very important, but also the temperature control is very important as the conditions of use. However, in conventional optical amplifiers, LDs may suddenly break down due to the influence of operating conditions, which has greatly hindered the improvement of the overall reliability of optical amplifiers.

As a result of intensive research by the present inventors, it was found that the cause of the sudden failure of the excited LD is that the high-output signal light output from the optical amplifier returns to the optical amplifier from an external reflection point. The reason for the failure is that the reflected light is reversely amplified while propagating in the core of the cladding pumping fiber, and is transmitted to the cladding layer due to the connection loss generated in the connection between the cladding pumping fiber and the multi-port coupler. The leaked light reaches the excitation LD and destroys the excitation LD. In particular, when the gain of the optical amplifier is 20 dB or more, even if the reflectance is about 1%, it becomes very large light exceeding 10% after passing through the optical amplifier. When such strong reflected light returns, especially in the case of pulsed lasers, light with a power nearly 10 times the output of the excitation LD itself enters the LD, thereby destroying the LD.

Therefore, in order to solve this problem, the inventors of the present invention searched for a structure in which failure of the excitation LD does not occur even if the reflected light returns to the amplifier.

However, as a conventional technology for suppressing reflected light itself, there are optical isolators, but with optical isolators, the reflectance can only be attenuated to about -20dB, and even optical isolators corresponding to light of several W or more are very expensive. The problem.

Since the reflected light propagates inside the core in the amplification fiber, as long as the light does not leak from the core, the reflected light does not radiate to the excitation LD. Therefore, after investigating the reason why the reflected light enters the LD, it was found that the main reason why the reflected light is emitted to the excited LD is that the core of the connecting part of the multi-port coupler and the cladding pump fiber is connected to each other. connection loss between them.

Loss occurs at this connection portion because the core diameters (magnitudes of intensity distribution) of the two optical fibers are largely different. Generally, high-power cladding pump fibers have a core diameter of 20 μm or more. On the other hand, the core diameter of the SM optical fiber used for signal propagation is about 5 μm (wavelength). As a result, especially in the direction in which the reflected light propagates, the magnitude of the loss at the connecting portion becomes 5 dB or more.

The reason why the core diameter of the cladding pump fiber is large is that since the power density of light in a high-power amplifier is very large, it is affected by nonlinear optical effects in the fiber. In order to avoid this effect, generally the diameter of the core of the cladding pump fiber is increased, and as a result, the optical energy density in the fiber is designed to be as small as possible. However, in such a core with a large cross-sectional area, single-mode transmission is very difficult and bending loss increases, so it is not suitable for optical fibers for signals. Therefore, it is preferable that the optical fiber for transmission has a small core cross-sectional area, and the optical fiber for amplification has a large core cross-sectional area. Since there is a multi-port coupler in the middle of these two kinds of optical fibers, as long as the core cross-sectional area of the multi-port coupler can be set to the size in the middle of the cross-sectional area of the two optical fibers, the connection can be reduced to some extent. loss. However, in the structure of the multi-port coupler, it is obvious that rather than reducing the connection loss, it is better to increase the connection loss. This is because the outer diameter of the optical fiber is reduced because the multiport coupler generally couples a plurality of optical fibers to one optical fiber.

FIG. 2 is a diagram showing the structure of a connection portion between a conventional multiport coupler 2 and a clad pump fiber 3 . The head end of the side of the multiport coupler 2 connected to the cladding pump fiber 3 is formed in such a structure that the central signal fiber 5 is integrated with a plurality of excitation fibers 4 around it, and is directed towards the head. The diameter of the end is reduced into a tapered shape, so as to be consistent with the outer diameter of the cladding pumping fiber 3 . In this head end, the core diameter of the central signal optical fiber 5 is further reduced, and when it is connected to the clad pump optical fiber 3 with a larger core diameter, the difference in the core diameters of the two fibers is further reduced. get bigger.

Considering the above, it can be seen that it is practically difficult to reduce the connection loss generated at the connection part of the multiport coupler.

Contents of the invention

The present invention was made in view of the above circumstances, and an object of the present invention is to provide a multi-port coupling capable of eliminating the failure of the excitation light source due to reflected light in an optical amplifier or a fiber laser using a multi-port coupler and realizing a longer life of the device. device, and the optical amplifier and fiber laser that use it.

In order to achieve the above object, the present invention provides a multi-port coupler, which integrates a central signal optical fiber and a plurality of excitation optical fibers arranged around it and reduces the diameter of the head end side, and integrates the excitation light source and the plurality of excitation optical fibers. A cladding pumping fiber for optical amplification is connected, wherein a cladding is provided on the outer edge of the multi-port coupler, and a radiation light entry waveguide is provided in a concentric circle around the core of the signal optical fiber at the center. a path portion having a larger outer diameter than the core and having a refractive index value higher than that of the cladding of the multiport coupler and smaller than that of the core of the signal optical fiber, the The radiation-light confinement waveguide section is formed continuously from the connection section with the clad pump fiber to the end section of the coupler section branched into a plurality of optical fibers.

In this multi-port coupler, the radiation light confinement waveguide section may be provided concentrically around the core of the signal optical fiber. In addition, the emitted light confinement waveguide section may be designed to have a polygonal cross-section around the core of the signal optical fiber.

In addition, the present invention provides a multiport coupler in which a central signal optical fiber and a plurality of excitation optical fibers arranged around it are integrated using a capillary, the capillary, the signal optical fiber and the excitation optical fiber Both are connected to the rear end of the optical fiber for bridge, and the head end side of the optical fiber for the bridge is reduced in diameter, and the excitation light source is connected with the cladding pump optical fiber for optical amplification, wherein in the The periphery of the core of the bridging optical fiber is provided with a radiated light confinement waveguide portion having an outer diameter larger than the core and having a cladding higher than that of the bridging optical fiber but smaller than the cladding of the bridging optical fiber. The value of the refractive index of the core, and the refractive index of the capillary is lower than the refractive index of the cladding of the signal optical fiber, and the capillary has a confining effect of emitted light on the signal optical fiber.

In addition, the present invention provides a multiport coupler in which a central signal optical fiber and a plurality of excitation optical fibers arranged around it are integrated using a capillary, the capillary, the signal optical fiber and the excitation optical fiber Both are connected to the rear end of the bridging optical fiber, and the head end side of the bridging optical fiber is reduced in diameter, and the excitation light source is connected to the cladding pumping optical fiber for optical amplification, wherein the bridging optical fiber The periphery of the core is provided with a radiated light confinement waveguide section, which has an outer diameter larger than the core, and has a refractive index higher than that of the cladding of the bridging optical fiber but lower than that of the core of the bridging optical fiber value, and the refractive index of the capillary is the same as that of the cladding of the signal optical fiber, and the cladding of the excitation optical fiber is lower than the refractive index of the capillary, the cladding of the excitation optical fiber The signal optical fiber and the capillary have confinement effects of emitted light.

In this multi-port coupler, the radiation light confinement waveguide section may be provided concentrically around the core of the bridging optical fiber. In addition, the emitted light confinement waveguide section may be designed to have a polygonal cross-section around the core of the bridging optical fiber.

In the multiport coupler of the present invention, it is preferable that the outer diameter of the emitted light confinement waveguide section is smaller than the outer diameter of the signal optical fiber connected to the multiport coupler.

In the multiport coupler of the present invention, it is preferable to provide a radiation light attenuation section that winds the signal optical fiber over an appropriate distance.

In addition, the present invention provides an optical amplifier including the multiport coupler according to the present invention, a clad pump fiber for optical amplification, and an excitation light source.

In addition, the present invention provides a fiber laser including the multiport coupler of the present invention, a clad pump fiber for optical amplification, and an excitation light source.

The multi-port coupler of the present invention has a structure in which a radiated light confinement waveguide is arranged around the core of the signal optical fiber at the center, and has an outer diameter larger than the core, and has a diameter higher than the cladding and smaller than the cladding. Because of the value of the refractive index of the core, it is possible to confine the reflected light returning to the LD into the radiated light into the waveguide section, eliminate the failure of the excitation light source caused by the reflected light, and realize the long life of the device.

The optical amplifier of the present invention has a structure in which the excitation light and the signal light are coupled to the clad pump fiber for optical amplification using the multiport coupler related to the present invention, so that the reflected light returning to the LD can be enclosed in the optical amplifier. The radiated light is enclosed in the waveguide section, and the failure of the excitation light source due to the reflected light is eliminated, and the device life is extended.

Description of drawings

FIG. 1 is a configuration diagram showing an example of an optical amplifier.

Fig. 2 is a cross-sectional view showing the structure of a multiport coupler.

Fig. 3 is a cross-sectional view showing an embodiment of the multiport coupler of the present invention.

FIG. 4A is a cross-sectional view of the multiport coupler of FIG. 3 .

4B to D are cross-sectional views showing modified examples in which the radiated light-entrapment waveguide portion has a polygonal shape.

Fig. 5 is an exploded perspective view showing a second embodiment of the multiport coupler of the present invention.

Fig. 6 is a cross-sectional view along the cut plane A-A of Fig. 5 .

FIG. 7A is a cross-sectional view along the B-B cutting plane of FIG. 5 .

7B to 7D are cross-sectional views showing modified examples in which the radiated light entering waveguide portion has a polygonal shape.

Fig. 8 is an exploded perspective view showing a case where the multiport coupler of Fig. 5 is used in connection with a non-dope DCF.

FIG. 9 is a graph showing the results of Examples.

Detailed ways

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

3 is a diagram showing an embodiment of the multiport coupler of the present invention, FIG. 3A is a cross-sectional view showing the structure of the head end side of the multiport coupler 2, and FIG. 3B shows the other end side of the multiport coupler 2. Sectional view of the structure.

The multi-port coupler 2 of this embodiment is formed by integrating a central signal optical fiber 5 and a plurality of excitation optical fibers 4 arranged around it, and reducing the diameter of the head end side, and combining an excitation light source and an optical amplification package. The layer pumping optical fiber 3 is connected to each other, and it is characterized in that a radiated light closing waveguide section 7 is arranged concentrically around the core 6 of the signal optical fiber 5 located in the center, and has a larger outer diameter than the core 6. diameter, and has a refractive index value higher than that of the cladding 8 and smaller than that of the core 6, and the radiated light entry waveguide section 7 is continuously formed from the connection with the cladding pumping fiber 3 to the branch into a plurality of The end of the coupler part of the optical fiber 4,5.

The inventors of the present invention analyzed the mechanism causing LD failure, and invented a structure that does not cause LD failure even if connection loss occurs in a multiport coupler. The inventors of the present invention focused their attention on the internal structure of the multiport coupler.

As shown in FIG. 2 , the multi-port coupler 2 is manufactured by aligning a plurality of excitation optical fibers 4 around the signal optical fiber 5 as a center and integrating it. Therefore, as shown in FIG. 3, when a radiated light confinement waveguide section 7 having a refractive index slightly higher than that of the outer periphery is formed around the core 6 of the signal optical fiber 5, even if connection loss occurs at the connection section, the The light emitted to the surroundings is generated, and the emitted light can also be effectively confined in the region with a higher refractive index to prevent entry to the excitation port.

In addition, after further research, it was found that, for the relative refractive index difference between the radiated light-entrapping waveguide 7 and the cladding 8, the larger the diameter of the radiated light-entrapped waveguide 7, the smaller it is, and vice versa. The smaller the diameter of the radiated light entry waveguide 7 is, the larger it becomes.

At this time, it was found that the relationship between the diameter D of the radiation-light confinement waveguide portion 7 and the relative refractive index difference Δ of the radiation-light confinement waveguide portion 7 with respect to the cladding 8 maintains the following formula (1):

D[μm]×Δ[%]=a certain value A...(1)

When the relationship between the light and the light changes, the effect of turning off the emitted light will reach a certain level.

In this case, the constant value A may be determined according to how much leaked radiated light is trapped. In addition, A also changes with the core diameter of the cladding pump fiber 3 for optical amplification and the relative refractive index difference with the cladding. This is because the angle or intensity of emitted light is determined by the core 9 of the cladding pump fiber 3 .

The radiated light confinement waveguide section 7 needs to be provided in a region from the connection with the clad pump fiber 3 to the signal fiber 5 and the excitation fiber 4 which are discretely separated. This is because, when the emitted light cannot be shut in midway, the emitted light is coupled to the excitation optical fiber 4 from there.

In addition, the diameter of the radiated light entrance waveguide section 7 is preferably smaller than the outer diameter of the cladding of the signal optical fiber 5 . This is because, if the diameter of the radiation light confinement waveguide portion 7 is large, the radiation light confinement in the waveguide is coupled to the excitation optical fiber 4 at the end portion of the coupler.

The radiated light confined in the radiated light confinement waveguide 7 is gradually absorbed and eliminated by the resin outside the glass clad as shown in FIG. In order to actively promote absorption, it is preferable to wind the signal optical fiber 5 with a certain range of curvature over an appropriate length. For example, when φ50 mm, it is preferable to set it as about 1 m.

In addition, when it is difficult to use an optical fiber of an appropriate length, a dopant for absorbing radiated light may be added to the cladding portion to absorb radiated light. For example, in order to absorb light with a wavelength of 1064 nm, an optical fiber doped with Sm is effective.

In the multiport coupler 2 of the present embodiment, the effect of providing the radiated light confinement waveguide section 7 will be described.

For example, in a cladding pump fiber 3 with a core diameter of 20 μm and a core-cladding relative refractive index difference of 0.1%, the core diameter of the head end is 2.5 μm, and the core-cladding relative refractive index difference is 0.4%. In the connection of the multi-port coupler 2, by setting A=3[μm×%], it is possible to confine 92% of the radiated light and to make about 8% of the radiated light. In this case, the distribution example of the refractive index of the multiport coupler 2 is that the relative refractive index difference between the core and the radiated light entering waveguide is 0.4%, and the relative refractive index difference between the radiated light entering waveguide and the cladding is 0.05. %, the diameter of the radiated light entering the waveguide is 60 μm. The effect described is that if there is about 50W of reflected light returning when no countermeasures are implemented, it can be reduced to 4W.

In the past, in the case of causing failure of the excitation LD, the failure occurred at a power more than twice the output of the excitation LD itself, so the effect of reducing it to 8% was very large. Generally speaking, the excitation is about 5 to 10W.

Also, if A=1.5 [μm×%], 85% of the radiated light can be shut in in the same case. For example, in a case where the reflected light is 20W and the excitation LD fails, the reflected light will be reduced to 15% of 20W, that is, 3W, and the protection for the excitation LD is still effective.

In addition, the multiport coupler 2 of this embodiment can efficiently converge the excitation light to the core 9 of the clad pump fiber 3 by providing the emitted light confinement waveguide 7 around the signal fiber 5 . In the case of optical amplification using the cladding pump fiber 3 , it is very important to efficiently absorb the excitation light uniformly distributed in the cladding by the core 9 . As long as the excitation light can be converged toward the core 9 (since absorption occurs only in the core), the excitation light can be effectively absorbed. For example, in the case where a diameter of 50 μm and a relative refractive index difference of 0.1% is provided around the core of the signal optical fiber 5 to confine the emitted light into the waveguide 7, the core 9 of the clad pumped optical fiber 3 The absorption efficiency of excitation light will be increased by 20%.

In addition, in the case of the multi-port coupler 2 shown in FIG. 3, the core 6 of the signal optical fiber and the radiation light-entrapping waveguide 7 provided around it have concentric circular cross-sections as shown in FIG. 4A. However, the present invention is not particularly limited thereto, and the radiated light entering waveguide portion 7 may also have a hexagonal shape as shown in FIG. 4B, a quadrangular shape as shown in FIG. 4C, or a pentagonal shape as shown in FIG. 4D. Sections of polygonal shapes such as shapes. Even if the radiated light confinement waveguide section 7 has a polygonal cross-sectional shape, it can confine the reflected light into the signal optical fiber 5 and reduce the incident power to the excitation optical fiber 4 in the same way as in the case of concentric circles. Effect.

In the case where the cross-section of the radiated light confinement waveguide 7 is a polygonal shape, the outer diameter of the so-called radiated light confinement waveguide 7 is smaller than the external diameter (cladding diameter) of the signal optical fiber 5 connected to the multiport coupler 2. ), it can be understood that the diameter of the inscribed circle where the emitted light enters the waveguide section 7 is smaller than the outer diameter of the signal optical fiber 5 . This is because, in this case, since the radiation light confinement waveguide part 7 is enclosed within the cladding region of the signal optical fiber 5 on the entire cross-sectional area, the radiation light confinement waveguide part 7 is confinement. All the radiated light inside is coupled with the signal optical fiber 5, so that the same effect as that of suppressing the radiated light from coupling with the excitation optical fiber 4 at the coupling terminal can be achieved.

As mentioned above, in the waveguide structure inside the multi-port coupler, by setting a region around the fiber core that confines light, it can effectively prevent the failure of the excited LD due to reflected light, and furthermore, it can The excitation light is effectively distributed around the fiber core. In addition, by employing such a structure, the excitation light can be efficiently converged toward the periphery of the fiber core.

In addition, the multi-port coupler of the present invention may also be constructed in such a way that the signal optical fiber and the excitation optical fiber are bundled with a capillary structure, and furthermore, the head end is connected to the light-emitting waveguide section and the head end. The bridging of the reducing section is connected with an optical fiber. Hereinafter, a second embodiment of the present invention will be described with reference to FIGS. 5 to 8 .

The multi-port coupler 11 shown in FIGS. 5 and 6 integrates a central signal optical fiber 12 and a plurality of excitation optical fibers 13 arranged around it by using a capillary 14. The capillary 14 and the signal optical fiber 12 and excitation The optical fiber 13 is connected to the rear end of the bridging optical fiber 15 together, and the head end side of the bridging optical fiber 15 is reduced in diameter, and the excitation light source is connected to the multi-port coupler with the cladding pumping optical fiber 16 for optical amplification 11, which is provided with a radiation light confinement waveguide 17 around the core 15a of the bridging optical fiber 15, which has an outer diameter larger than the core 15a, and has a higher refractive index than the cladding 15b but smaller than the core 15a value.

Here, the capillary 14 is a porous capillary having a plurality of through holes (pores) into which the signal optical fiber 12 and the excitation optical fiber 13 can be inserted. The porous capillary is preferably made of silica glass or doped silica-based glass for fusion connection with the optical fiber. Fusion bonding can be performed, for example, by using a heat source such as an arc discharge, a carbon dioxide laser, or an oxyhydrogen flame. The core 12a of the signal optical fiber 12 inserted into the central aperture of the capillary 14 is coupled to the core 15a of the bridge optical fiber 15, and the core 13a of the excitation optical fiber 13 inserted into the surrounding aperture is coupled to the bridge core 15a. The cladding 15b of the optical fiber 15 is coupled. The reduced diameter portion on the head end side of the bridging optical fiber 15 can be formed, for example, by heating and stretching into a tapered shape.

In this multiport coupler 11 , there are the following (1) and (2) as configurations for preventing reflected light from entering the excitation optical fiber 13 .

(1) The refractive index of the capillary 14 is lower than the refractive index of the cladding 12 b of the signal optical fiber 12 . In this case, the capillary 14 has an effect of confining emitted light to the signal optical fiber 12 . Thus, the magnitude relationship between the refractive index of the capillary 14 and the refractive index of the cladding 13b of the excitation optical fiber 13 is arbitrary, regardless of whether the capillary 14 has a higher or lower refractive index than the cladding 13b of the excitation optical fiber 13, or Both are the same.

In this structure, since the capillary 14 plays the role of the radiated light confinement waveguide portion, when it is combined with the radiated light confinement waveguide portion of the bridging optical fiber 15, the radiated light confinement waveguide portion will become A portion formed continuously from the connection portion with the cladding pump fiber 16 to the end of the coupler portion branched into a plurality of optical fibers 12 , 13 .

(2) The refractive index of the capillary 14 is the same as that of the cladding 12 b of the signal optical fiber 12 , and the refractive index of the cladding 13 b of the excitation optical fiber 13 is lower than that of the capillary 14 .

In this case, the cladding 13 b of the excitation optical fiber 13 has an effect of confining emitted light to the signal optical fiber 12 and the capillary 14 .

In this configuration, when a connection loss occurs between the capillary 14 and the bridging optical fiber 15, the emitted light may be coupled to the capillary 14. However, since the cladding 13b of the excitation optical fiber 13 has an effect of confining the emitted light to the capillary 14 , Therefore, it is possible to suppress coupling of emitted light with the excitation optical fiber 13 .

In any of the configurations of (1) and (2), the diameter (outer diameter) of the radiated light confinement waveguide portion 17 provided in the bridging optical fiber 15 is preferably smaller than the outer diameter (cladding) of the signal optical fiber 12. path). This is because, if the diameter of the radiation light confinement waveguide portion 17 is large, the radiation light confinement in the waveguide is coupled with the excitation optical fiber 13 at the coupler terminal portion.

[59] When the radiated light entering the waveguide section 17 propagates in the opposite direction (going from right to left in FIG. The external resin is slowly absorbed and eliminated. In order to actively promote absorption, it is preferable to wind the signal optical fiber 12 with a certain range of curvature over an appropriate length. For example, when φ50 mm, it is preferable to set it as about 1 m.

[60] In addition, when it is difficult to use the signal optical fiber 12 of an appropriate length, a dopant for absorbing radiated light may be added to the cladding 12 b of the signal optical fiber 12 to absorb the radiated light. For example, in order to absorb light with a wavelength of 1064 nm, an optical fiber doped with Sm is effective.

[61] In the multiport coupler 11 of the present embodiment, the effect of providing the radiated light confinement waveguide section 17 in the bridging optical fiber 15 will be described.

[62] In the configuration (2) in which the capillary 14 and the cladding 12b of the signal optical fiber 12 have the same refractive index, the cladding 12b of the signal optical fiber 12, the capillary 14, and the core 13a of the excitation optical fiber 13 are all used Made of quartz (refractive index about 1.448), the refractive index of the cladding 13b of the excitation optical fiber 13 was set to 1.436 (the relative refractive index difference between the core 13a and the capillary 14 was 0.8%), and the thickness was set to 10 μm. The result This is because the intensity of reflected light coupled with the excitation fiber 13 can be reduced by about 20 dB (that is, about 1 percent) compared to the case where the low-refractive index portion of the cladding 13 b of the excitation fiber 13 is not provided.

[63] In the configuration (1) in which the capillary 14 has a lower refractive index than the cladding 12b of the signal optical fiber 12, both the cladding 12b of the signal optical fiber 12 and the core 13a of the excitation optical fiber 13 are made of silica (the refractive index is about 1.448), the refractive index of the cladding 13b of the excitation optical fiber 13 is set to 1.436, the thickness is set to 10 μm, and the refractive index of the capillary 14 is also set to 1.436. The results are as follows: Compared with the configuration of 2), the intensity of the reflected light coupled with the excitation optical fiber 13 is further reduced by 8 dB. Compared with the case where the low-refractive index portion is not provided, the decrease is about 28 dB.

[64] In addition, in the case of the multi-port coupler 11 shown in FIG. 5, the core 15a of the bridging optical fiber 15 and the radiated light entrance waveguide 17 arranged around it have a concentricity as shown in FIG. 7A. Structure of circular cross section. However, the present invention is not particularly limited thereto, and the radiated light entering waveguide section 17 may have a hexagonal shape as shown in FIG. 7B , a quadrangular shape as shown in FIG. 7C , a pentagonal shape as shown in FIG. 7D , etc. Cross section of polygonal shape. Even if the cross-sectional shape of the radiated light entering waveguide section 17 is a polygonal shape, it can be the same as the case of concentric circles, and the reflected light can be closed into the core 15a of the bridge optical fiber 15 to be coupled with the signal optical fiber 12 to reduce the direction of vibration. The effect of the incident power of the optical fiber 13 for excitation.

In the case where the cross-section of the radiation-light confinement waveguide 17 is polygonal, the outer diameter of the radiation-light confinement waveguide 17 is smaller than the outer diameter (cladding diameter) of the signal optical fiber 12 connected to the multiport coupler 11. ), it can be understood that the diameter of the inscribed circle where the emitted light enters the waveguide portion 17 is smaller than the outer diameter of the signal optical fiber 12 . This is because, in this case, since the radiated light confining waveguide section 17 is enclosed within the cladding region of the signal optical fiber 12 in its entire cross-sectional area, the radiated light confining waveguide section 17 All the radiated light inside is coupled to the signal optical fiber 12, so that the equivalent effect of suppressing the radiated light from being coupled to the excitation optical fiber 13 at the coupling end portion can be achieved.

Further, in the multiport coupler 11 of this embodiment, by providing the radiation light confinement waveguide section 17 around the core 15a of the bridging optical fiber 15, the excitation light from the excitation optical fiber 13 can be efficiently directed to the The core 16a of the cladding pump fiber 16 converges. In the case of optical amplification using the cladding pump fiber 16, it is very important to efficiently absorb the excitation light uniformly distributed in the cladding by the core 16a. As long as the excitation light can be converged toward the core 16a (since absorption occurs only in the core), the excitation light can be effectively absorbed.

In the example shown in FIG. 5, as the clad pump fiber 16 connected to the multiport coupler 11, a double clad fiber (DCF) in which a rare earth element is added to the core 16a is used, The core 16a has an inner cladding 16b and an outer cladding 16c around it. However, the multiport coupler 11 of the present embodiment is also effective when a clad pump fiber (non-dope DCF) 18 to which a rare earth element is not added to the core 18a is connected as shown in FIG. . In this configuration example, a DCF 16 doped with a rare earth element is connected to one end of the non-dope DCF 18, and the tip diameter reducing portion of the bridging optical fiber 15 is connected to the other end of the non-dope DCF 18.

At this time, in order not to emit emitted light to the cladding layers 18b and 18c of the non-dope DCF 18 at the junction of the non-dope DCF 18 and the DCF 16 doped with rare earth elements, it is enclosed in the core 18a, and in the fiber Between the core 18a and the claddings 18b and 18c, it is also necessary to provide a radiation light confinement waveguide section 19 having a higher refractive index than the claddings 18b and 18c and a lower refractive index than the core 18a. The cross-sectional shape of the radiated light entering waveguide 19 may be concentric or polygonal with respect to the core 18a. Even when the multiport coupler having such a structure is directly connected to the DCF doped with rare earth elements, it is possible to suppress the incidence of reflected light into the excitation optical fiber 13 at exactly the same level. .

As mentioned above, in the waveguide structure inside the multi-port coupler, by setting a region around the fiber core that confines light, it can effectively prevent the excitation LD from failing due to reflected light, and even the excitation Light is efficiently distributed around the periphery of the fiber core. In addition, by employing such a configuration, excitation light can be efficiently focused around the core.

Example

In an optical amplifier with an excitation wavelength of 915nm and a signal wavelength of 1064nm, the multiport coupler of the present invention (the multiport coupler shown in Figure 3) was applied, and the effect of LD failure suppression was studied.

A Yb-doped double-clad fiber was used as the cladding pump fiber.

The dimensions of the optical fiber at this time are as follows.

• Fiber core diameter: 20 μm, • Core-inner cladding relative refractive index difference Δ: 0.1%, • Inner cladding outer diameter: 400 μm.

In addition, 6 LDs with a wavelength of 915 nm and an output of 5 W were used for the LD used for excitation.

In addition, the excitation optical fiber connected to the excitation LD had a core diameter: 105 μm, and a core-cladding relative refractive index difference Δ: 0.55%.

At this time, a reflection point is provided at the end of the optical amplifier, and it is set so that about 5% of the light output from the optical amplifier returns to the optical amplifier again. In this kind of structure, the following two couplers are installed, and the multi-port coupler is respectively as shown in the present invention with core diameter: 2.5 μm, radiation light entering waveguide diameter: 30 μm, radiation The relative refractive index difference of the light-entry waveguide section: 0.1% was fabricated for the coupler and the coupler without the radiated light-entry waveguide section was tested.

As a result, in the case of using a coupler that does not have a radiated light confinement waveguide portion, at the same time as the test was started, 4/6 of the excitation LDs failed and became unusable.

On the other hand, in the case of using the coupler according to the present invention provided with the radiated light confinement waveguide portion, no failure of the LD occurred even if the reflected light was continuously incident for one hour or more.

This result was confirmed using a computer. As a result, as shown in FIG. ) compared to that, the emitted light is clearly shut in and the LD is protected.

Next, in the multiport coupler 11 shown in FIGS. 5 and 6 , an embodiment in which a hexagonal radiation light confining waveguide 17 is provided as shown in FIG. 7B will be described.

When the outer diameter of the cladding 15b of the bridging optical fiber 15 is 400 μm and the diameter of the core 15a is 7 μm, a radiation confinement waveguide 17 with a side of 45 μm is formed around the core 15 a. In this bridging optical fiber 15, the relative refractive index difference between the radiation-light confinement waveguide 17 and the cladding 15b is 0.1%, and the relative refractive index difference between the core 15a and the radiation-light confinement waveguide 17 is 0.18%. When the outer diameter of the waveguide section 17 for confining the radiated light is approximated by the diameter of the middle circle between the inscribed circle and the circumscribed circle of the hexagon, it is 42 μm. As the cladding pumping fiber 16, a fiber having a core diameter of 30 μm and a core-cladding relative refractive index difference of 0.12% was prepared. It was confirmed to what extent the effect of confinement of radiated light can be seen in the connection between the optical fiber 15 for the bridge and the clad pump fiber 16. Fiber core 15a.

Although preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments. Structural additions, omissions, substitutions, and other changes can be made without departing from the gist of the present invention. The invention is not limited by the description described, but only by the scope of the additional technical solution.

Claims (12)

1. A multi-port coupler, which integrates the central signal optical fiber and a plurality of excitation optical fibers arranged around it and reduces the diameter of the head end side, and combines the excitation light source and the cladding pump for optical amplification Pu optical fiber connected, where, A cladding is provided on the outer edge of the multiport coupler, Around the core of the signal optical fiber located at the center, a radiation light confinement waveguide portion having a larger outer diameter than the core and having a height higher than the cladding of the multi-port coupler is provided. The value of the refractive index is smaller than that of the core of the signal optical fiber, and the emission light confinement waveguide portion is formed continuously from the connection portion with the clad pump optical fiber to branching into a plurality of optical fibers. end of the coupler section. 2. The multiport coupler according to claim 1, wherein the radiation light confinement waveguide section is provided concentrically around the core of the signal optical fiber. 3. The multi-port coupler according to claim 1, wherein the portion for confining the radiated light into the waveguide is designed to have a polygonal cross-section around the core of the signal optical fiber. 4. A multi-port coupler that integrates a central signal optical fiber and a plurality of excitation optical fibers arranged around it by using a capillary, the capillary being connected together with the signal optical fiber and the excitation optical fiber The rear end of the bridging optical fiber is connected, and the head end side of the bridging optical fiber is reduced in diameter, and the excitation light source is connected with the cladding pumping optical fiber for optical amplification, wherein Around the core of the bridging optical fiber, a radiated light confinement waveguide is provided, which has an outer diameter larger than the core, and has a cladding higher than that of the bridging optical fiber but smaller than that of the bridging optical fiber. The value of the refractive index of the core, and the refractive index of the capillary is lower than the refractive index of the cladding of the signal optical fiber, and the capillary has a confining effect of emitted light on the signal optical fiber. 5. A multi-port coupler in which a central signal optical fiber is integrated with a plurality of excitation optical fibers arranged around it by using a capillary, the capillary being connected together with the signal optical fiber and the excitation optical fiber The rear end of the bridging optical fiber is connected, and the head end side of the bridging optical fiber is reduced in diameter, and the excitation light source is connected with the cladding pumping optical fiber for optical amplification, wherein Around the core of the bridging optical fiber, a radiated light confinement waveguide is provided, which has an outer diameter larger than the core, and has a cladding higher than that of the bridging optical fiber but smaller than that of the bridging optical fiber. The value of the refractive index of the core, and the refractive index of the capillary is the same as the refractive index of the cladding of the signal optical fiber, and the cladding of the excitation optical fiber is lower than the refractive index of the capillary, so The cladding of the excitation optical fiber has an effect of confining emitted light to the signal optical fiber and the capillary. 6. The multi-port coupler according to claim 4, wherein the radiation light confinement waveguide section is provided concentrically around the core of the bridging optical fiber. 7. The multi-port coupler according to claim 4 or 5, wherein the part for confining the radiated light into the waveguide is designed to have a polygonal cross-section around the core of the bridging optical fiber. 8. The multiport coupler according to any one of claims 1 to 6, wherein an outer diameter of the radiated light entrance waveguide portion is smaller than that of the signal optical fiber connected to the multiport coupler outside diameter. 9. The multiport coupler according to claim 7, wherein an outer diameter of the radiated light entrance waveguide portion is smaller than an outer diameter of the signal optical fiber connected to the multiport coupler. 10. The multi-port coupler according to claim 1, wherein a radiation light attenuation section is provided which winds the signal optical fiber over an appropriate distance. 11. An optical amplifier comprising the multiport coupler according to claim 1, a clad pump fiber for optical amplification, and an excitation light source. 12. A fiber laser comprising the multi-port coupler according to claim 1, a clad pump fiber for optical amplification, and an excitation light source.

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