JP2021136242A - Fiber laser device - Google Patents

Abstract

To provide a fiber laser device capable of reducing return light which is propagated toward an excitation light source.SOLUTION: A fiber laser device 1 comprises: excitation light sources 30 and 40; an optical fiber 20 for amplification; a high reflection part 21 provided at one end side of the optical fiber 20 for amplification; a low reflection part 22 provided at the other end side of the optical fiber 20 for amplification; and a forward light adjustment part 34 disposed between the excitation light source 30 and the high reflection part 21. The forward light adjustment part 34 includes: a core portion 341 which has a refraction index n11 and to which excitation light P1 is propagated; a clad portion 342 which covers an outer periphery of the core portion 341 and has a refraction index n21 (<n11); and a high refraction index portion 343 which covers an outer periphery of the clad portion 342 and has a refraction index n31 (≥n21). By reducing a thickness C1 in a radial direction of the clad portion 342 of the forward light adjustment part 34, a light loss of return light R1 from the optical fiber 20 for amplification to the excitation light source 30 is enlarged and leaked to the high refraction index portion 343, while excitation light P1 from the excitation light source 30 to the optical fiber 20 for amplification is propagated with a small light loss.SELECTED DRAWING: Figure 4

Description

The present invention relates to a fiber laser device, and more particularly to a fiber laser device that uses excitation light to generate high-power laser light.

In such a fiber laser device, excitation light is incident on the inner clad that covers the periphery of the core of the amplification optical fiber from the excitation light source, and when the excitation light propagating through the inner clad passes through the core, the excitation light causes the core. The rare earth element ion added to the above is excited to amplify the signal light (see, for example, Patent Document 1). In such a fiber laser device, the excitation light reflected at the fusion point in the device and the signal light not coupled to the output optical fiber (hereinafter, these excitation light and signal light are collectively referred to as "return light". May) go towards the excitation light source. When such return light is incident on the excitation light source, it may cause a decrease in the reliability of the excitation light source. In particular, with the recent increase in the output of fiber lasers, the power of such return light is increasing, so that a technique for effectively removing the return light is required.

International Publication No. 2013/001734

The present invention has been made in view of such problems of the prior art, and an object of the present invention is to provide a fiber laser apparatus capable of reducing return light propagating toward an excitation light source.

According to one aspect of the present invention, there is provided a fiber laser apparatus capable of reducing the return light propagating toward the excitation light source. This fiber optic laser apparatus includes at least one first excitation light source capable of generating the first excitation light, a core to which rare earth element ions excited by the first excitation light are added, and the first excitation. An amplification optical fiber having a cladding capable of propagating light, a first reflection portion provided on one end side of the amplification optical fiber, and a second reflection portion provided on the other end side of the amplification optical fiber. And at least one first light adjusting unit arranged between the at least one first excitation light source and the first reflecting unit. The at least one first light adjusting portion has a refractive index of n ₁₁ , covers at least the first core portion through which the first excitation light propagates, and the outer periphery of the first core portion, and refracts the refraction. a first cladding portion having a refractive index lower n ₂₁ than the rate n _11, covering the outer periphery of the first cladding portion, the first high refractive index portion having a refractive index n ₃₁ above the refractive index n ₂₁ And have. When the angle formed by the light rays propagating in the optical waveguide with respect to the optical axis of the optical waveguide is defined as the propagation angle, the first core portion of the at least one first optical adjustment unit is the at least one first core portion. Of the first excitation light propagating from the excitation light source to the amplification optical fiber, the angle at which the light occupying 95% of the propagation angle distribution centered on the propagation angle of 0 degrees is incident on the first clad portion. The minimum value of is the incident angle θ _P1 , and the first core portion of the at least one first optical adjustment portion propagates from the amplification optical fiber toward the at least one first excitation light source. Of the return light, the minimum value of the angle at which light that occupies 95% of the propagation angle distribution centered on the propagation angle of 0 degrees is incident on the first clad portion _{is defined as the incident angle θ R1,} and the first excitation light is defined as the incident angle θ R1. the wavelength of the lambda _P1, the and the wavelength of the first return beam and lambda _R1, said at least one first light adjustment part of the first cladding portion radial thickness C ₁ of the following formula Meet.

When the thickness of the first clad portion of the first optical adjustment unit satisfies the above equation, the amplification optical fiber is changed to the first excitation light source in the first clad portion of the first optical adjustment unit. Since the light loss of the first return light to be directed increases, at least a part of the first return light can be leaked to the first high refractive index portion and removed. On the other hand, the optical loss of the first excitation light propagating from the first excitation light source toward the amplification optical fiber is suppressed to a low level.

The at least one first excitation light source may include a plurality of first excitation light sources. In this case, the fiber laser apparatus further includes a first optical combiner that combines the first excitation lights generated by the plurality of first excitation light sources and supplies them to the amplification optical fiber. With such a configuration, the power of the first excitation light supplied to the amplification optical fiber can be increased, so that a higher output laser light can be output from the fiber laser device.

The at least one first light adjustment unit is arranged between the first excitation light source of any one of the plurality of first excitation light sources and the first optical combiner. It may include a part. In this case, the first light adjusting unit arranged between the second excitation light source and the second optical combiner is fused to the first excitation light source of any one of the above. It is preferable that the portion is arranged closer to any one of the above-mentioned first excitation light sources than the connection portion. By arranging the first light adjusting unit at a position close to the first excitation light source in this way, the first return light toward the first excitation light source can be effectively reduced.

The fiber laser apparatus may further include at least one first excitation optical fiber extending between the at least one first excitation light source and the first optical adjustment unit. In this case, the at least one first excitation optical fiber has the refractive index n ₁₁ and at least the first fiber core through which the first excitation light propagates and the outer periphery of the first fiber core. The first fiber clad having a refractive index n ₂₃ lower than the refractive index n ₁₁ and the outer periphery of the first fiber clad having a refractive index n _{33 lower than the refractive index n 23.} It has one fiber coating.

The fiber laser apparatus may include at least one second excitation light source capable of generating a second excitation light. In this case, the rare earth element ion is excited by the first excitation light and the second excitation light. Further, the clad of the amplification optical fiber is configured to propagate the first excitation light and the second excitation light. By providing the first excitation light source and the second excitation light source in this way, the power of the excitation light supplied to the amplification optical fiber can be increased, so that the laser light having a higher output from the fiber laser device can be increased. Can be output.

The fiber laser apparatus may further include at least one second light adjusting unit arranged between the at least one second excitation light source and the second reflecting unit. The second light adjusting unit has a refractive index n ₁₂ and covers at least the second core portion through which the second excitation light propagates and the outer periphery of the second core portion, and has a refractive index n ₁₂ It has a second clad portion having a lower refractive index n ₂₂ and a second high refractive index portion that covers the outer periphery of the second clad portion and has a refractive index n _{32 having a refractive index n 22 or more.} doing. Of the second excitation light propagating from the at least one second excitation light source toward the amplification optical fiber through the second core portion of the at least one second optical adjustment unit, the propagation angle is 0. The minimum value of the angle at which light that occupies 95% of the propagation angle distribution centered on degrees is incident on the second clad portion is the incident angle θ _P2 , and the second core portion of at least one second light adjustment portion. Of the second return light propagating from the amplification optical fiber toward at least one second excitation light source, the light that occupies 95% of the propagation angle distribution centered on the propagation angle of 0 degrees is the second light. Assuming that the minimum value of the angle incident on the clad portion _{is defined as the incident angle θ R2} , the wavelength of the second excitation light is λ _P2, and the wavelength of the second return light is λ _R2 , then at least one first _{The radial thickness C 2 of} the second clad portion of the light adjusting portion 2 satisfies the following equation.

When the thickness of the second clad portion of the second optical adjustment portion satisfies the above equation, the amplification optical fiber is changed to the second excitation light source in the second clad portion of the second optical adjustment portion. Since the light loss of the second return light to be directed increases, at least a part of the second return light can be leaked to the second high refractive index portion and removed. On the other hand, the optical loss of the second excitation light propagating from the second excitation light source toward the amplification optical fiber is suppressed to a low level.

The at least one second excitation light source may include a plurality of second excitation light sources. In this case, the fiber laser apparatus further includes a second optical combiner that combines the second excitation lights generated by the plurality of second excitation light sources and supplies them to the amplification optical fiber. With such a configuration, the power of the second excitation light supplied to the amplification optical fiber can be increased, so that a higher output laser light can be output from the fiber laser apparatus.

The at least one second light adjustment unit is arranged between the second excitation light source of any one of the plurality of second excitation light sources and the second optical combiner. It may include a part. In this case, the second light adjusting unit arranged between the second excitation light source and the second optical combiner is fused closest to any of the second excitation light sources. It is preferable that the portion is arranged closer to any of the above second excitation light sources than the connection portion. By arranging the second light adjusting unit at a position close to the second excitation light source in this way, it is possible to effectively reduce the second return light toward the second excitation light source.

The fiber laser apparatus may further include at least one second excitation optical fiber extending between the at least one second excitation light source and the second optical adjustment unit. In this case, the at least one second pumping optical fiber has the refractive index n _12, a second fiber core at least the second excitation light is propagated, the outer periphery of the second fiber core It has a second fiber clad having a refractive index n ₂₂ and a second fiber coating covering the outer periphery of the second fiber clad and having a refractive index n _{42 lower than the refractive index n 22.} ..

According to the present invention, the first optical adjustment unit increases the light loss of the return light from the amplification optical fiber toward the first excitation light source and causes the light loss to leak to the high refractive index unit, while causing the first excitation light source. The excitation light from the light source to the amplification optical fiber can be propagated with a small light loss.

FIG. 1 is a block diagram schematically showing a fiber laser apparatus according to an embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing an optical fiber for amplification in the fiber laser apparatus shown in FIG. FIG. 3 is a cross-sectional view schematically showing a forward-excited optical fiber in the fiber laser apparatus shown in FIG. FIG. 4 is a cross-sectional view schematically showing a front light adjusting portion in the fiber laser apparatus shown in FIG. FIG. 5 is a graph schematically showing the propagation angle distribution of the excitation light propagating in the core portion of the forward light adjusting portion shown in FIG. FIG. 6 is a cross-sectional view schematically showing a rear-excited optical fiber in the fiber laser apparatus shown in FIG. FIG. 7 is a cross-sectional view schematically showing a rear light adjusting portion in the fiber laser apparatus shown in FIG. FIG. 8 is a graph showing the relationship between the ratio C / d of the thickness C of the clad of the optical fiber and the depth d of light exuding from the reflection interface and the light loss on the outer periphery of the clad.

Hereinafter, embodiments of the fiber laser apparatus according to the present invention will be described in detail with reference to FIGS. 1 to 8. In FIGS. 1 to 8, the same or corresponding components are designated by the same reference numerals, and duplicate description will be omitted. Further, in FIGS. 1 to 8, the scale and dimensions of each component may be exaggerated or some components may be omitted. In the following description, unless otherwise noted, terms such as "first" and "second" are only used to distinguish the components from each other and represent a particular order or order. It's not a thing.

FIG. 1 is a block diagram schematically showing a fiber laser device 1 according to an embodiment of the present invention. The fiber laser apparatus 1 in the present embodiment includes an optical resonator 2, a front excitation unit 3 that supplies excitation light to the optical resonator 2 from one end side (front) of the optical resonator 2, and the other end of the optical resonator 2. It includes a rear excitation unit 4 that supplies excitation light to the optical resonator 2 from the side (rear), and a beam delivery unit 5 that outputs laser light amplified by the optical resonator 2.

The optical resonator 2 includes an amplification optical fiber 20 having a core to which rare earth element ions are added, a high reflection unit 21 (first reflection unit) that reflects light in a predetermined wavelength band with high reflectance, and the light resonator 2. It includes a low reflection unit 22 (second reflection unit) that reflects light in a wavelength band with a lower reflectance than that of the high reflection unit 21. The high-reflection portion 21 and the low-reflection portion 22 are composed of, for example, a fiber Bragg grading or a mirror formed by periodically changing the refractive index of the optical fiber.

FIG. 2 is a cross-sectional view schematically showing the amplification optical fiber 20. As shown in FIG. 2, the amplification optical fiber 20 is composed of, for example, a double clad fiber, and rare earth element ions such as ytterbium (Yb), erbium (Er), thulium (Tr), and neodymium (Nd) are added. It has a core 201, an inner clad 202 formed around the core 201, and an outer clad 203 formed around the inner clad 202. The inner clad 202 is made of a material having a refractive index lower than that of the core 201 (for example, SiO ₂ ), and the inside of the core 201 is an optical waveguide that propagates signal light. Further, the outer clad 203 is composed of a resin having a refractive index lower than that of the inner clad 202 (for example, a low refractive index polymer), and the inside of the core 201 and the inner clad 202 is an optical waveguide that propagates the excitation light P. ing.

The forward excitation unit 3 includes a plurality of forward excitation light sources 30 (first excitation light sources) capable of generating excitation light (first excitation light) having a predetermined wavelength, and excitation lights output from the plurality of forward excitation light sources 30. A forward optical combiner 31 (first optical combiner) that is combined and introduced into the optical resonator 2, a forward excitation optical fiber 33 (first excitation optical fiber) that extends from each forward excitation light source 30, and forward excitation light. It includes a front light adjusting unit 34 (first light adjusting unit) provided in the middle of the fiber 33. As the forward excitation light source 30, for example, a high-power multimode semiconductor laser (LD) having a wavelength of 975 nm can be used. The forward optical combiner 31 has an optical fiber 32 connected to each of the front excitation optical fibers 33 and an optical fiber 35 connected to the optical resonator 2. The optical fiber 35 of the front optical combiner 31 and the optical resonator 2 are connected to each other by a fusion splicer 61.

FIG. 3 is a cross-sectional view schematically showing the forward-excited optical fiber 33. As shown in FIG. 3, the front-excited optical fiber 33 includes a fiber core 331 (first fiber core), a fiber clad 332 (first fiber clad) covering the outer circumference of the fiber core 331, and an outer circumference of the fiber clad 332. Includes a fiber coating 333 (first fiber coating) that covers. The fiber clad 332 is made of a material having a refractive index n _{23 lower than the refractive index n 13 of} the fiber core 331, and the inside of the fiber core 331 is an optical waveguide _{through which the excitation light P 1 from the forward excitation light source 30 propagates.} There is. In the present embodiment, the refractive index n ₃₃ of the fiber coating 333 is lower than _{the refractive index n 23} of the fiber clad 332. As shown in FIG. 1, the front-excited optical fiber 33 is connected to the optical fiber 32 of the front optical combiner 31 by a fusion splicer 62. Since the configuration of the optical fiber 32 of the front optical combiner 31 is the same as that of the front excitation optical fiber 33 shown in FIG. 3, detailed description thereof will be omitted.

FIG. 4 is a cross-sectional view schematically showing the front light adjusting unit 34. As shown in FIG. 4, the front light adjusting portion 34 includes a core portion 341 (first core portion), a clad portion 342 (first clad portion) that covers the outer circumference of the core portion 341, and an outer circumference of the clad portion 342. It includes a high refractive index portion 343 (first high refractive index portion) that covers the above. The clad portion 342 is made of a material having a refractive index n _{21 lower than the refractive index n 11 of} the core portion 341, and the inside of the core portion 341 is an optical waveguide _{through which the excitation light P 1 from the forward excitation light source 30 propagates.} ing. The high refractive index portion 343 is made of a material having a refractive index n ₃₁ of the clad portion 342 having a refractive index n _{21 or more.}

Since the front light adjusting unit 34 is provided in the middle of the front excitation optical fiber 33, the diameter of the core portion 341 of the front light adjustment unit 34 may be equal to the diameter of the fiber core 331 of the front excitation optical fiber 33. _{Preferably, the refractive index n 11} of the core portion 341 of the forward light adjusting portion 34 is the same as _{the refractive index n 13} of the fiber core 331 of the forward excitation optical fiber 33 _{(n 11} = n ₁₃ ). Furthermore, it is preferable that the core portion 341 of the forward light adjusting portion 34 is made of the same material as the fiber core 331 of the forward excitation optical fiber 33. _{Further, the refractive index n 21} of the clad portion 342 of the front light adjusting unit 34 may be the same as or different from _{the refractive index n 23} of the fiber clad 332 of the forward excitation optical fiber 33.

As will be described later, the forward light adjusting unit 34 changes the thickness of the clad portion 342 in the radial direction from the amplification optical fiber 20 to the forward excitation light source 30 by making it thinner than the fiber clad 332 of the front excitation optical fiber 33. While increasing the light loss of the heading return light R ₁ and causing it to leak to the high refractive index portion 343, the excitation light P ₁ from the front excitation light source 30 toward the amplification optical fiber 20 can be propagated with a small light loss. Is.

The rear excitation unit 4 includes a plurality of rear excitation light sources 40 (second excitation light sources) capable of generating excitation light (second excitation light) having a predetermined wavelength, and excitation lights output from the plurality of rear excitation light sources 40. A rear optical combiner 41 (second optical combiner) that is combined and introduced into the optical resonator 2, a rear excitation optical fiber 43 (second excitation optical fiber) extending from each rear excitation light source 40, and rear excitation light. It includes a rear light adjusting unit 44 (second light adjusting unit) provided in the middle of the fiber 43. As the rear excitation light source 40, for example, a high-power multimode semiconductor laser (LD) having a wavelength of 975 nm can be used. The rear optical combiner 41 has an optical fiber 42 connected to each rear excitation optical fiber 43 and an optical fiber 45 connected to the optical resonator 2. The optical fiber 45 of the rear optical combiner 41 and the optical resonator 2 are connected to each other by a fusion splicer 63.

_{The wavelength λ P1 of the} excitation light (first excitation light) generated by the front excitation light source 30 _{and the wavelength λ P2} of the excitation light (second excitation light) generated by the rear excitation light source 40 are the same. May be different from each other. In the present embodiment, it is assumed that the wavelength λ _{P1 of the excitation light generated by the front excitation light source 30 and the wavelength λ P2 of the} excitation light generated by the rear excitation light source 40 are the same (λ _P1 = λ _P2 ).

FIG. 6 is a cross-sectional view schematically showing the rear excitation optical fiber 43. As shown in FIG. 6, the rear-excited optical fiber 43 includes a fiber core 431 (second fiber core), a fiber clad 432 (second fiber clad) covering the outer circumference of the fiber core 431, and an outer circumference of the fiber clad 432. Includes a fiber coating 433 (second fiber coating) that covers the. The fiber clad 432 is _{made of a material having a refractive index lower than the refractive index n 14 of} the fiber core 431, and the inside of the fiber core 431 is an optical waveguide _{through which the excitation light P 2 from the rear excitation light source 40 propagates.} In the present embodiment, the refractive index n ₃₄ of the fiber coating 433 is lower than _{the refractive index n 24} of the fiber clad 432. As shown in FIG. 1, the rear excitation optical fiber 43 is connected to the optical fiber 42 of the rear optical combiner 41 by a fusion splicer 64. Since the configuration of the optical fiber 42 of the rear optical combiner 41 is the same as that of the rear excitation optical fiber 43 shown in FIG. 6, detailed description thereof will be omitted.

FIG. 7 is a cross-sectional view schematically showing the rear light adjusting unit 44. As shown in FIG. 4, the rear light adjusting portion 44 includes a core portion 441 (second core portion) and a clad portion 442 (second clad portion) that covers the outer periphery of the core portion 441. ) And a high refractive index portion 443 (second high refractive index portion) that covers the outer periphery of the clad portion 442. The clad portion 442 is made of a material having a refractive index n _{22 lower than the refractive index n 12 of} the core portion 441, and the inside of the core portion 441 is an optical waveguide _{through which the excitation light P 2 from the rear excitation light source 40 propagates.} ing. The high refractive index portion 443 is made of a material having a refractive index n ₃₂ of the clad portion 442 having a refractive index n _{22 or higher.}

Since the rear light adjusting unit 44 is provided in the middle of the rear excitation optical fiber 43, the diameter of the core portion 441 of the rear light adjustment unit 44 may be equal to the diameter of the fiber core 431 of the rear excitation optical fiber 43. _{Preferably, the refractive index n 12} of the core portion 441 of the rear light adjusting unit 44 is preferably the same as _{the refractive index n 14} of the fiber core 431 of the rear excitation optical fiber 43 _{(n 12} = n ₁₄ ). Furthermore, it is preferable that the core portion 441 of the rear light adjusting portion 44 is made of the same material as the fiber core 431 of the rear excitation optical fiber 43. _{Further, the refractive index n 22} of the clad portion 442 of the rear light adjusting portion 44 may be the same as or different from _{the refractive index n 24} of the fiber clad 432 of the rear excitation optical fiber 43.

As will be described later, the rear light adjusting unit 44 changes the thickness of the clad portion 442 in the radial direction from the amplification optical fiber 20 to the rear excitation light source 40 by making it thinner than the fiber clad 432 of the rear excitation optical fiber 43. While increasing the light loss of the returning light R ₂ and causing it to leak to the high refractive index portion 443, the excitation light P ₂ from the rear excitation light source 40 toward the amplification optical fiber 20 can be propagated with a small light loss. Is.

The beam delivery unit 5 includes a delivery fiber 50 connected to the rear light combiner 41 of the rear excitation unit 4, and a laser emission unit 51 provided at the end of the delivery fiber 50. The delivery fiber 50 includes a core and a clad that covers the periphery of the core, and the inside of the core of the delivery fiber 50 is an optical waveguide of laser light output from the optical resonator 2.

As shown in FIG. 2, the excitation light P introduced into the optical resonator 2 by the optical combiners 31 and 41 from the excitation light sources 30 and 40 propagates inside the inner clad 202 and the core 201 of the amplification optical fiber 20. When the excitation light P passes through the core 201, it is absorbed by the rare earth element ion, and the rare earth element ion is excited to generate spontaneous emission light. This naturally emitted light is recursively reflected between the high reflection unit 21 and the low reflection unit 22, and the light having a specific wavelength (for example, 1064 nm) is amplified to cause laser oscillation. The laser light amplified by the optical resonator 2 propagates inside the core 201 of the amplification optical fiber 20, and a part of the laser light passes through the low reflection portion 22. The laser light transmitted through the low reflection unit 22 propagates through the core of the delivery fiber 50 and is emitted from the laser emission unit 51 as output laser light L toward, for example, an object to be processed, as shown in FIG.

As described above, the excitation light P from the excitation light sources 30 and 40 is absorbed by the core 201 of the amplification optical fiber 20, but the residual component of the excitation light P that is not absorbed by the core 201 of the amplification optical fiber 20. A part of the light may propagate from the optical resonator 2 toward the excitation light sources 30 and 40. _{For example, a part of the residual component of the excitation light P 2} from the rear excitation light source 40 may pass through the high reflection portion 21, pass from the front light combiner 31, and pass through the front excitation optical fiber 33 toward the front excitation light source 30. _{In addition, a part of the residual component of the excitation light P 1} from the front excitation light source 30 may pass through the low reflection portion 22 and go from the rear light combiner 41 to the rear excitation light source 40 through the rear excitation optical fiber 43. By lengthening the amplification optical fiber 20, it is possible to reduce the residual component of the excitation light that remains unabsorbed by the core 201. However, if the amplification optical fiber 20 is lengthened, the nonlinear optical effect increases, so that the fiber laser This causes a decrease in the efficiency of the device 1. Further, it is conceivable that the output laser light L emitted from the laser emitting unit 51 is reflected by the object to be processed and returned to the fiber optic laser device 1 from the laser emitting unit 51, and a part of such reflected light is high. It is also conceivable to go beyond the reflecting portion 21 from the forward light combiner 31 through the forward excitation optical fiber 33 toward the forward excitation light source 30.

The forward light adjusting unit 34 in the present embodiment has a function of reducing return light such as residual components of _{excitation light P 2 directed toward the forward excitation light source 30 and reflected light.} Further, the rear light adjusting unit 44 has a function of reducing the return light such as the residual component of the _{excitation light P 1 toward the rear excitation light source 40.} Hereinafter, how the return light is reduced in the front light adjusting unit 34 and the rear light adjusting unit 44 will be described.

In the example shown in FIG. 1, _{the residual components of the excitation light P 2} from the rear excitation light source 40 reach the front excitation optical fiber 33 at six fusion junctions (coupling of the fusion junction 64 and the rear light combiner 41). It passes through the unit, the fusion splicer 63, the fusion splicer 61, the coupling portion of the front optical combiner 31, and the fusion splicer 62). _{Similarly, the residual component of the excitation light P 1} from the front excitation light source 30 reaches the rear excitation optical fiber 43 at six fusion junctions (fusion junction 62, the coupling portion of the front optical combiner 31, and fusion). It passes through the connecting portion 61, the fusion splicing portion 63, the coupling portion of the rear optical combiner 41, and the fusion splicing portion 64). Generally, when light passes through the fusion splicer, the light spread angle increases and the numerical aperture (NA) increases. For example, assuming that NA increases by 5% when light passes through one fusion junction, the NA of the residual component of the excitation light that has passed through the six fusion junctions described above was generated by the excitation light source 40. It will increase by 34% as compared to the NA of the excitation light (= 1.05 ^6). The present inventor has found that the return light toward the front excitation light source 30 and the rear excitation light source 40 can be reduced by utilizing the property that the NA of light increases when passing through such a fusion junction point.

ここで、θはクラッドに入射する光の入射角、λは光の波長、ｎ₁はコアの屈折率、ｎ₂はクラッドの屈折率である。 Strictly speaking, the light propagating through the core of the optical fiber is not reflected at the interface between the core and the clad, but is slightly exuded into the clad and reflected (evanescent light). The depth (exuding depth) d of the field where the light propagating in the core exudes from the interface between the core and the clad (reflection interface) is expressed by the following equation (1).

Here, θ is the incident angle of the light incident on the clad, λ is the wavelength of the light, n ₁ is the refractive index of the core, and n ₂ is the refractive index of the clad.

ここで、Ｉ₀は反射界面での光のパワーである。 Since θ is the angle between the perpendicular of the reflection interface and the light beam, the larger the NA of the light, the smaller the θ. Therefore, according to the above equation (1), the larger the NA of the light, the larger the depth d of the field where the light seeps out from the reflection interface. The intensity I of the exuded light at a position separated from the reflection interface by a distance z in the radial direction is expressed by the following equation (2).

Here, I ₀ is the power of light at the reflection interface.

ここで、θ_P1はクラッド部３４２に入射する励起光Ｐ₁の入射角を表しているが、本実施形態では、以下のように規定される入射角θ_P1をクラッド部３４２に入射する励起光Ｐ₁の入射角として用いる。 In the forward light adjusting unit 34 shown in FIG. 4, the exudation depth d _{P1 of the excitation light P 1} from the forward excitation light source 30 from the reflection interface 91 is represented by the following equation (3).

Here, theta _P1 is represents the incident angle of the excitation light P ₁ incident on the cladding portion 342, in this embodiment, the excitation light incident to the incident angle theta _P1 which is defined as follows in the cladding portion 342 Used as the incident angle of P _1.

If the angle formed by a light ray propagating in an optical waveguide such as the core portion 341 of the front light adjusting unit 34 with respect to the optical axis of the optical waveguide is defined as a propagation angle, the excitation propagating in the core portion 341 of the front light adjusting unit 34. The distribution of the propagation angle θ _T1 of the light P ₁ is as shown in FIG. In the present embodiment, among the excitation light P ₁ propagating through the core portion 341 of the forward light adjusting unit 34, the minimum angle at which the light, which accounts for 95% of the propagation angle distribution around the propagation angle of 0 degrees is incident to the cladding part 342 The value is used as the _{incident angle θ P1.}

ここで、λ_R1は戻り光Ｒ₁の波長である。また、θ_R1はクラッド部３４２に入射する戻り光Ｒ₁の入射角を表しているが、本実施形態では、上述した励起光Ｐ₁の入射角θ_P1と同様に、前方光調整部３４のコア部３４１を伝搬する戻り光Ｒ₁のうち、伝搬角０度を中心として伝搬角分布の９５％を占める光がクラッド部３４２に入射する角度の最小値を入射角θ_R1として用いる。 Further, the exudation depth d _R1 of the return light R _{1 from} the reflection interface 91 is expressed by the following equation (4).

Here, lambda _R1 is the wavelength of the return light R _1. Also, the theta _R1 but represents the incident angle of the return light R ₁ incident on the cladding portion 342, in this embodiment, similarly to the incident angle theta _P1 of the pumping light P ₁ described above, the forward light adjuster 34 of return light R ₁ propagating through the core portion 341, using the minimum value of the angle at which the light, which accounts for 95% of the propagation angle distribution around the propagation angle of 0 degrees is incident to the cladding part 342 as the incident angle theta _R1.

As described above, since the return light R ₁ passes through a plurality of fusion splicing portions before reaching the front light adjustment unit 34, the NA of the _{return light R 1 is increasing in the front light adjustment unit 34, and θ R1} < It becomes θ _P1. As a result, if λ _P1 = λ _R1 , then from equations (3) and (4), d _R1 > d _P1 , and the region G _{R1 where the return light R 1} exudes from the reflection interface 91 is the excitation light P. _It is larger than the area G _{P1 where 1 exudes. If the region G R1} exuded by the return light R ₁ becomes sufficiently large, as shown in FIG. 4, the return light R ₁ leaks from the clad portion 342 to the high refractive index portion 343, so that the return light R ₁ is used as the core portion. It can be removed from 341.

ここで、θ_P2はクラッド部４４２に入射する励起光Ｐ₂の入射角を表しているが、本実施形態では、上述した前方光調整部３４のコア部３４１を伝搬する励起光Ｐ₁の入射角θ_P1と同様に、後方光調整部４４のコア部４４１を伝搬する励起光Ｐ₂のうち、伝搬角０度を中心として伝搬角分布の９５％を占める光がクラッド部４４２に入射する角度の最小値を入射角θ_P2として用いる。 Further, in the rear light adjusting unit 44 shown in FIG. 7, the exudation depth d _{P2 of the excitation light P 2} from the rear excitation light source 40 from the reflection interface 92 is represented by the following equation (5).

Here, theta _P2 is represents the incident angle of the excitation light P ₂ incident on the cladding portion 442, in this embodiment, the incident excitation light P ₁ propagating through the core portion 341 of the forward light adjustment section 34 described above Similar to the angle θ _{P1 , of the excitation light P 2} propagating in the core portion 441 of the rear light adjusting portion 44, the angle at which the light occupying 95% of the propagation angle distribution centered on the propagation angle 0 degree is incident on the clad portion 442. The minimum value of is used as the _{incident angle θ P2.}

ここで、λ_R2は戻り光Ｒ₂の波長である。また、θ_R2はクラッド部４４２に入射する戻り光Ｒ₂の入射角を表しているが、本実施形態では、上述した前方光調整部３４のコア部３４１を伝搬する戻り光Ｒ₁の入射角θ_R1と同様に、後方光調整部４４のコア部４４１を伝搬する戻り光Ｒ₂のうち、伝搬角０度を中心として伝搬角分布の９５％を占める光がクラッド部４４２に入射する角度の最小値を入射角θ_R2として用いる。 Further, the exudation depth d _R2 of the return light R _{2 from} the reflection interface 92 is expressed by the following equation (6).

Here, lambda _R2 is the wavelength of the return light R _2. Further, theta _R2 is represents the incident angle of the returned light R ₂ incident on the cladding portion 442, in this embodiment, the incident angle of the return beam R ₁ propagating through the core portion 341 of the forward light adjustment section 34 described above similar to theta _R1, among the return light R ₂ propagating through the core portion 441 of the rear light adjusting unit 44, the light, which accounts for 95% of the propagation angle distribution around the propagation angle of 0 degrees of the angle incident on the cladding 442 The minimum value is used as the _{incident angle θ R2.}

As described above, since the return light R ₂ passes through a plurality of fusion splicing portions before reaching the rear light adjustment unit 44, the NA of the _{return light R 2 increases in the rear light adjustment unit 44, and θ R2} < It becomes θ _P2. As a result, if λ _P2 = λ _R2 , then from equations (5) and (6), d _R2 > d _P2 , and the region G _{R2 where the return light R 2} exudes from the reflection interface 92 is the excitation light P. _It is larger than the area G _{P2 where 2 exudes.} If the region G _R2 which return light R ₂ ooze is accustomed sufficiently large, as shown in FIG. 7, since the return light R ₂ is leaking from the cladding 442 into the high refractive index portion 443, a return beam R ₂ core part It can be removed from 441.

Based on the above findings, when the NA of 95% of the light propagating in the optical fiber is in the range of 0.16 to 0.24, the radial thickness C of the optical fiber clad and the reflection interface. When the relationship between the ratio C / d of the light exudation depth d and the light loss on the outer periphery of the clad was investigated, it was found that there was a correlation as shown in FIG.

As described above, in the forward light adjusting unit 34, the exudation depth d _{R1 of the return light R 1 from} the reflection interface 91 is larger than the exudation depth d _P1 of the excitation light P _{1 from the reflection interface 91. Therefore, from the graph of FIG. 8, if the thickness C 1} in the radial direction of the clad portion 342 of the front light adjusting portion 34 is appropriately set, _{the light loss of the excitation light P 1} is suppressed to a low level, and the light of the return light R ₁ is suppressed. It can be seen that the loss can be increased and leaked to the high refractive index portion 343.

Specifically, if the optical loss of the excitation light P ₁ is 1% / cm or less, the efficiency of laser oscillation in the optical resonator 2 does not significantly decrease. In order to reduce the light loss of the excitation light P ₁ to 1% / cm or less, C ₁ / d _P1 ≥ 7.6 must be obtained from the graph shown in FIG. Further, in order to effectively leak the return light R ₁ from the clad portion 342 to the high refractive index portion 343, it is considered necessary to increase the light loss of the _{return light R 1 to 5% / cm or more. For this purpose, C 1} / d _R1 ≤ 6.2 must be obtained from the graph shown in FIG. Therefore, the radial thickness C ₁ of the clad portion 342 needs to satisfy the following equation (7).
7.6d _P1 ≤ C ₁ ≤ _{6.2d R1} ... (7)

The above equation (7) can be expressed as the following equation (8) from the equations (3) and (4).

As described above, in the present embodiment, when the thickness C ₁ in the radial direction of the clad portion 342 of the front light adjusting portion 34 satisfies the above equation (8), the clad portion 342 of the front light adjusting portion 34 Since the optical loss of the _{return light R 1} from the amplification optical fiber 20 toward the forward excitation light source 30 _{becomes large, at least a part of the return light R 1} can be leaked to the high refractive index portion 343 and removed. _{On the other hand, the optical loss of the excitation light P 1} propagating from the forward excitation light source 30 toward the amplification optical fiber 20 is suppressed to a low level.

Similarly, in the rear light adjusting unit 44, _{in order to increase the light loss of the return light R 2} and leak it to the high refractive index unit 443 while suppressing the light loss _{of the excitation light P 2 to a low value, the following equation (9) ), The thickness C 2} in the radial direction of the clad portion 442 may be set.

As described above, in the present embodiment, when the thickness C ₂ in the radial direction of the clad portion 442 of the rear light adjusting portion 44 satisfies the above equation (9), the clad portion 442 of the rear light adjusting portion 44 Since the optical loss of the _{return light R 2} from the amplification optical fiber 20 toward the rear excitation light source 40 _{becomes large, at least a part of the return light R 2} can be leaked to the high refractive index portion 443 and removed. _{On the other hand, the optical loss of the excitation light P 2} propagating from the rear excitation light source 40 toward the amplification optical fiber 20 is suppressed to a low level.

Further, in the present embodiment, since a plurality of excitation light sources are used as the front excitation light source 30 and the rear excitation light source 40, the power of the excitation light supplied to the amplification optical fiber 20 can be increased. Therefore, a high-power laser beam can be output from the fiber laser device 1.

The position of the front light adjusting unit 34 may be anywhere as long as it is between the front excitation light source 30 and the high reflection unit 21, and for example, the front light adjusting unit 34 is formed in the middle of the optical fiber 35 of the front light combiner 31. Alternatively, the front light adjusting unit 34 may be formed between the front light combiner 31 and the high reflection unit 21. However, in order to effectively remove more return light R ₁ , the fusional connection between the forward excitation light source 30 and the forward light combiner 31, especially as in the present embodiment, is closest to the forward excitation light source 30. It is preferable to arrange the front light adjusting unit 34 at a position closer to the anterior excitation light source 30 than the unit (the fusion splicing unit 62 in FIG. 1).

Similarly, the position of the rear light adjusting unit 44 may be anywhere between the rear excitation light source 40 and the low reflection unit 22, for example, the rear light adjusting unit in the middle of the optical fiber 45 of the rear light combiner 41. 44 may be formed, or the rear light adjusting portion 44 may be formed between the rear light combiner 41 and the low reflection portion 22. However, in order to effectively remove more return light R ₂ , a fusional connection between the rear excitation light source 40 and the rear light combiner 41, particularly the closest fusion connection to the rear excitation light source 40, as in the present embodiment. It is preferable to arrange the front light adjusting unit 34 at a position closer to the rear excitation light source 40 than the unit (fusion connection unit 64 in FIG. 1).

In the above-described embodiment, the example in which the front light adjusting unit 34 is formed in a part of the front excitation optical fiber 33 has been described, but the entire length of the front excitation optical fiber 33 or the total length of the front excitation optical fiber 33 and the optical fiber 32. The above-mentioned forward light adjusting unit 34 may be formed over the entire area. Similarly, the rear light adjusting portion 44 described above may be formed over the entire length of the rear excitation optical fiber 43, or over the entire length of the rear excitation optical fiber 43 and the optical fiber 42.

In the example shown in FIG. 1, excitation portions 3 and 4 are provided on both the high reflection portion 21 side and the low reflection portion 22 side of the optical resonator 2, and the optical resonator 2 is a bidirectional excitation type fiber laser apparatus. The excitation portion may be installed only on either the high reflection portion 21 side or the low reflection portion 22 side of the optical resonator 2. Further, in the above-described embodiment, the example in which the front excitation light source 30 is the first excitation light source and the rear excitation light source 40 is the second excitation light source has been described, but the rear excitation light source 40 is the first excitation light source. Yes, the forward excitation light source 30 may be the second excitation light source. In this case, the first optical adjustment unit and the second optical adjustment unit, the first excitation optical fiber and the second excitation optical fiber, and the first reflection unit and the second reflection unit are also interchanged. ..

Further, as a fiber laser device, a MOPA fiber laser device that amplifies seed light from a seed light source by using excitation light from an excitation light source is also known, and the present invention is also applied to such a MOPA fiber laser device. can do.

Although the preferred embodiment of the present invention has been described so far, it goes without saying that the present invention is not limited to the above-described embodiment and may be implemented in various different forms within the scope of the technical idea.

1 Fiber laser device 2 Optical cavity 3 Front excitation part 4 Rear excitation part 5 Beam delivery part 20 Optical fiber for amplification 21 High reflection part (first reflection part)
22 Low reflection part (second reflection part)
30 Forward excitation light source (first excitation light source)
31 Forward optical combiner (first optical combiner)
32, 35, 42, 45 Optical fiber 33 Forward excitation optical fiber (first excitation optical fiber)
34 Front light adjustment unit (first light adjustment unit)
40 Backward excitation light source (second excitation light source)
41 Rear light combiner (second light combiner)
43 Rear excitation optical fiber (second excitation optical fiber)
44 Rear light adjustment unit (second light adjustment unit)
50 Delivery fiber 51 Laser emitting part 61-64 Fusion splicing part 91,92 Reflective interface 331 (1st) Fiber core 332 (1st) Fiber clad 333 (1st) Fiber coating 341 (1st) core Part 342 (1st) Clad part 343 (1st) High refractive index part 431 (2nd) Fiber core 432 (2nd) Fiber clad 433 (2nd) Fiber coating 441 (2nd) core Part 442 (2nd) Clad part 443 (2nd) High refractive index part P ₁ (1st) excitation light P ₂ (2nd) excitation light R ₁ (1st) return light R ₂ (1st) 2) Return light

Claims (11)

With at least one first excitation light source capable of generating the first excitation light,
An amplification optical fiber having a core to which a rare earth element ion excited by the first excitation light is added and a clad capable of propagating the first excitation light.
A first reflecting portion provided on one end side of the amplification optical fiber and
A second reflecting portion provided on the other end side of the amplification optical fiber and
It includes at least one first light adjusting unit arranged between the at least one first excitation light source and the first reflecting unit.
The at least one first light adjusting unit is
A first core portion having a refractive index of n ₁₁ and at least propagating the first excitation light,
A first clad portion that covers the outer periphery of the first core portion and has a refractive index n _{21 lower than the refractive index n 11 and}
It has a first high refractive index portion that covers the outer periphery of the first clad portion and has a refractive index n _{31 having a refractive index n 21 or more.}
When the angle formed by the light rays propagating in the optical waveguide with respect to the optical axis of the optical waveguide is defined as the propagation angle, the first core portion of the at least one first optical adjustment unit is the at least one first core portion. Of the first excitation light propagating from the excitation light source to the amplification optical fiber, the angle at which light occupying 95% of the propagation angle distribution centered on the propagation angle of 0 degrees is incident on the first clad portion. The minimum value of is the incident angle θ _P1 , and the first core portion of the at least one first optical adjustment portion propagates from the amplification optical fiber toward the at least one first excitation light source. Of the return light, the minimum value of the angle at which light that occupies 95% of the propagation angle distribution centered on the propagation angle of 0 degrees is incident on the first clad portion _{is defined as the incident angle θ R1,} and the first excitation light is defined as the incident angle θ R1. Let λ _{P1 be the wavelength of, and λ R1} be the wavelength of the first return light.
_{The radial thickness C 1} of the first clad portion of the at least one first optical adjustment portion is

A fiber laser device that meets the requirements. The at least one first excitation light source includes a plurality of first excitation light sources.
The fiber laser device further includes a first optical combiner that combines the first excitation light generated by the plurality of first excitation light sources and supplies the first excitation light to the amplification optical fiber.
The fiber laser apparatus according to claim 1. The at least one first light adjustment unit is a first light adjustment arranged between the first excitation light source of any one of the plurality of first excitation light sources and the first optical combiner. The fiber laser apparatus according to claim 2, which comprises a part. The first optical adjustment unit arranged between the first excitation light source and the first optical combiner is more than the fusion junction closest to the first excitation light source. The fiber laser apparatus according to claim 3, which is arranged at a position close to any one of the first excitation light sources. Further comprising at least one first excitation optical fiber extending between the at least one first excitation light source and the first optical conditioning unit.
The at least one first excitation optical fiber is
With the first fiber core having the refractive index n ₁₁ and at least propagating the first excitation light,
A first fiber clad that covers the outer circumference of the first fiber core and has a refractive index n _{23 lower than the refractive index n 11.}
It covers the outer periphery of the first fiber clad and has a first fiber coating having a refractive index n _{33 lower than the refractive index n 23.}
The fiber laser apparatus according to any one of claims 1 to 4. With at least one second excitation light source capable of generating a second excitation light,
The rare earth element ion is excited by the first excitation light and the second excitation light.
The clad of the amplification optical fiber is configured to propagate the first excitation light and the second excitation light.
The fiber laser apparatus according to any one of claims 1 to 5. Further comprising at least one second light adjusting unit arranged between the at least one second excitation light source and the second reflecting unit.
The second light adjusting unit is
A second core portion having a refractive index of n ₁₂ and at least propagating the second excitation light,
A second clad portion that covers the outer periphery of the second core portion and has a refractive index n _{22 lower than the refractive index n 12 and}
It has a second high refractive index portion that covers the outer periphery of the second clad portion and has a refractive index n _{32 having a refractive index n 22 or more.}
Of the second excitation light propagating from the at least one second excitation light source toward the amplification optical fiber through the second core portion of the at least one second optical adjustment unit, the propagation angle is 0. The minimum value of the angle at which light that occupies 95% of the propagation angle distribution centered on degrees is incident on the second clad portion is the incident angle θ _P2 , and the second core portion of the at least one second light adjustment portion. Of the second return light propagating from the amplification optical fiber toward the at least one second excitation light source, the light that occupies 95% of the propagation angle distribution centered on the propagation angle of 0 degrees is the second light. Assuming that the minimum value of the angle incident on the clad portion _{is defined as the incident angle θ R2} , the wavelength of the second excitation light is λ _P2 , and the wavelength of the second return light is λ _R2 .
_{The radial thickness C 2} of the second clad portion of the at least one second optical adjustment portion is

The fiber laser apparatus according to claim 6. The at least one second excitation light source includes a plurality of second excitation light sources.
The fiber laser apparatus further includes a second optical combiner that combines the second excitation light generated by the plurality of second excitation light sources and supplies the amplification optical fiber to the amplification optical fiber.
The fiber laser apparatus according to claim 7. The at least one second light adjustment unit is arranged between the second excitation light source of any one of the plurality of second excitation light sources and the second optical combiner. The fiber laser apparatus according to claim 8, further comprising a part. The second light adjustment unit arranged between the second excitation light source and the second optical combiner is closer than the fusion junction closest to the second excitation light source. The fiber laser apparatus according to claim 9, which is arranged at a position close to any of the second excitation light sources. Further comprising at least one second excitation optical fiber extending between the at least one second excitation light source and the second optical regulator.
The at least one second excitation optical fiber is
A second fiber core having a refractive index of n ₁₂ and propagating at least the second excitation light.
The second fiber clad that covers the outer circumference of the second fiber core and has the _{refractive index n 22 and}
The fiber laser apparatus according to any one of claims 7 to 10, which covers the outer periphery of the second fiber clad and has a second fiber coating having a refractive index n _{42 lower than the refractive index n 22.} ..

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