Optical filter and laser device equipped with same

Abstract

Provided is an optical filter capable of suppressing damage to an optical component caused by Raman scattered light. This optical filter (1) is provided with a first optical fiber (10) and a second optical fiber (20) connected to the first optical fiber (10). The first optical fiber (10) is composed of a photonic bandgap fiber including a core region (11) and a plurality of high refractive index portions (13) which have a refractive index higher than the refractive index of the core region (11) and which are arranged periodically around the core region (11) so as to suppress the propagation of Raman scattered light within the core region (11). The second optical fiber (20) includes a core (21) and cladding (22) which has a refractive index lower than the refractive index of the core (21) and which covers the outer periphery of the core (21). The optical filter (1) is provided with: a high refractive index resin (50) that removes Raman scattered light that has passed through the high refractive index portions (13) of the first optical fiber (10) and is incident on the cladding (22) of the second optical fiber (20); and a fiber accommodating portion (30) that accommodates the optical fibers (10), (20) and protects the connected portions of the optical fibers (10), (20).

Description

Optical filter and laser device equipped with same

[0001] The present invention relates to an optical filter and a laser device including the same.

[0002] In a laser device such as a fiber laser, as the output power of the emitted laser beam increases, the power density of the laser beam propagating inside the core of the optical fiber increases, causing nonlinear phenomena such as stimulated Raman scattering. In recent years, the use of a photonic bandgap fiber (PBGF) has been proposed to suppress such stimulated Raman scattering (see, for example, Patent Document 1).

[0003] This PBGF includes a core region and a number of high refractive index sections periodically arranged around the core region, allowing light of a specific wavelength band to propagate within the core region while suppressing the propagation of light of other wavelength bands within the core region. By matching the wavelength band in which propagation within the core is suppressed to the wavelength band of Raman scattered light, it is possible to remove Raman scattered light from the light propagating through the core. The Raman scattered light removed from the light propagating through the core propagates through the high refractive index sections of the PBGF, but this Raman scattered light may enter the cladding of an optical fiber connected to the PBGF and propagate within the cladding, potentially causing damage to various optical components.

[0004] Japanese Patent Application Laid-Open No. 2020-20885

[0005] The present invention has been made in view of the above problems of the prior art, and has as its object to provide an optical filter that can suppress damage to optical components caused by Raman scattered light.

[0006] According to one aspect of the present invention, there is provided an optical filter capable of suppressing damage to optical components due to Raman scattered light. The optical filter includes: a first optical fiber configured as a photonic bandgap fiber including a core region and a plurality of high-refractive-index portions having a refractive index higher than that of the core region and periodically arranged around the core region so as to suppress propagation of Raman scattered light within the core region; and a second optical fiber connected to the first optical fiber. The second optical fiber includes a core optically coupled to the core region of the first optical fiber and a cladding having a refractive index lower than that of the core and covering the outer periphery of the core. The cladding is optically coupled to the plurality of high-refractive-index portions of the first optical fiber. The optical filter includes: a first light removing unit that removes the Raman scattered light that passes through the plurality of high-refractive-index portions of the first optical fiber and enters the cladding of the second optical fiber; and a fiber accommodating unit that accommodates the first optical fiber and the second optical fiber and protects the spliced ​​portion between the first optical fiber and the second optical fiber.

[0007] FIG. 1 is a plan view schematically showing an optical filter according to a first embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1. FIG. 3 is a cross-sectional view taken along line B-B of the first optical fiber shown in FIG. 1. FIG. 4 is a cross-sectional view taken along line C-C of the second optical fiber shown in FIG. 1. FIG. 5 is a cross-sectional view schematically showing an optical filter according to a second embodiment of the present invention. FIG. 6 is a cross-sectional view schematically showing an optical filter according to a third embodiment of the present invention. FIG. 7 is a cross-sectional view schematically showing an optical filter according to a fourth embodiment of the present invention. FIG. 8 is a cross-sectional view schematically showing an optical filter system according to an embodiment of the present invention. FIG. 9 is a graph showing the ratio of Raman scattered light measured downstream when the length of the first optical fiber of the optical filter according to the first embodiment of the present invention is changed. FIG. 10 is a block diagram schematically showing a laser device according to an embodiment of the present invention. FIG. 11 is a block diagram schematically showing a laser device according to another embodiment of the present invention.

[0008] Hereinafter, an embodiment of an optical filter and a laser device including the same according to the present invention will be described in detail with reference to FIGS. 1 to 11. In FIGS. 1 to 11, identical or corresponding components are denoted by the same reference numerals, and redundant description will be omitted. In addition, in FIGS. 1 to 11, the scale and dimensions of each component may be exaggerated, and some components may be omitted. In the following description, unless otherwise specified, terms such as "first" and "second" are used only to distinguish components from each other, and do not represent a specific order or ranking.

[0009] FIG. 1 is a plan view schematically illustrating an optical filter 1 according to a first embodiment of the present invention, and FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1. As shown in FIGS. 1 and 2, the optical filter 1 includes a first optical fiber 10, a second optical fiber 20 fusion-spliced ​​to the first optical fiber 10, and a fiber accommodating section 30 that accommodates the optical fibers 10 and 20. The fiber accommodating section 30 has a fiber accommodating groove 31 extending along its longitudinal direction, and the optical fibers 10 and 20 are disposed in the fiber accommodating groove 31. The first optical fiber 10 is fixed in the fiber accommodating groove 31 by a fixing resin 32 filled at one longitudinal end of the fiber accommodating groove 31, and the second optical fiber 20 is fixed in the fiber accommodating groove 31 by a fixing resin 33 filled at the other longitudinal end of the fiber accommodating groove 31. The fiber accommodating section 30 protects the optical fibers 10 and 20 and the fusion-spliced ​​section 40 from external forces, impacts, and vibrations.

[0010] Fig. 3 is a cross-sectional view of the first optical fiber 10 taken along line B-B in Fig. 1. The first optical fiber 10 is made of a solid photonic bandgap fiber, and as shown in Fig. 3, includes a core region 11, a cladding region 12 located outside the core region 11, and a coating 18 covering the outer periphery of the cladding region 12. The coating 18 is formed from, for example, an ultraviolet-curable resin, and may include multiple layers.

[0011] The cladding region 12 includes a large number of high-refractive-index portions 13 periodically arranged around the core region 11, and low-refractive-index portions 14 that fill the spaces between the high-refractive-index portions 13 and cover the outside of the high-refractive-index portions 13. The high-refractive-index portions 13 have a refractive index higher than that of the core region 11 and are arranged around the core region 11 in, for example, a triangular lattice pattern. The low-refractive-index portions 14 have a refractive index lower than that of the high-refractive-index portions 13. In this embodiment, the refractive index of the core region 11 and the low-refractive-index portions 14 are the same, but the refractive index of the core region 11 and the low-refractive-index portions 14 may be different. The coating 18 has a refractive index lower than that of the low-refractive-index portions 14.

[0012] For example, the core region 11 and the low refractive index portion 14 may be formed from pure silica glass (SiO), and the high refractive index portion 13 may be formed by adding a dopant (e.g., germanium (Ge)) that has the property of increasing the refractive index to the silica glass (SiO). Alternatively, the high refractive index portion 13 may be formed from pure silica glass (SiO), and the core region 11 and the low refractive index portion 14 may be formed by adding a dopant (e.g., fluorine (F) or boron (B)) that has the property of decreasing the refractive index to the silica glass (SiO). In this embodiment, no active element is added to the core region 11, and the first optical fiber 10 is configured as a passive fiber that does not exhibit optical amplification.

[0013] Fig. 4 is a cross-sectional view of the second optical fiber 20 taken along line CC in Fig. 1. As shown in Fig. 4, the second optical fiber 20 includes a core 21 optically coupled to the core region 11 of the first optical fiber 10, a cladding 22 covering the outer periphery of the core 21, and a coating 28 covering the outer periphery of the cladding 22. The cladding 22 is optically coupled to the high refractive index portion 13 of the first optical fiber 10. The coating 28 is formed of, for example, an ultraviolet-curable resin, and may include multiple layers.

[0014] The cladding 22 has a refractive index lower than that of the core 21, and the coating 28 has a refractive index lower than that of the cladding 22. For example, the core 21 may be formed from pure silica glass (SiO2), and the cladding 22 may be formed by adding a dopant (e.g., fluorine (F) or boron (B)) that has the property of lowering the refractive index to the silica glass (SiO2). Alternatively, the cladding 22 may be formed from pure silica glass (SiO2), and the core 21 may be formed by adding a dopant (e.g., germanium (Ge)) that has the property of raising the refractive index to the silica glass (SiO2).

[0015] 1 and 2 , the coating 18 of the first optical fiber 10 is stripped away over a certain distance from the end, forming an exposed cladding portion 12A where the cladding region 12 is exposed from the coating 18. Also, the coating 28 of the second optical fiber 20 is stripped away over a certain distance from the end, forming an exposed cladding portion 22A where the cladding 22 is exposed from the coating 28. The exposed cladding portion 12A of the first optical fiber 10 and the exposed cladding portion 22A of the second optical fiber 20 are fusion-spliced ​​to each other at a fusion splice portion 40.

[0016] The first optical fiber 10 is connected to a laser light source such as an optical fiber amplifier, as will be described later, and laser light generated by the laser light source propagates through a core region 11 of the first optical fiber 10 and then through a core 21 of the second optical fiber 20. In the following description, the direction in which the laser light propagates from the laser light source through the first optical fiber 10 toward the second optical fiber 20 will be referred to as the "downstream side," and the opposite direction will be referred to as the "upstream side."

[0017] The high-refractive-index portions 13 in the cladding region 12 of the first optical fiber 10 are periodically arranged around the core region 11 so as to suppress propagation of light in the wavelength band of Raman scattered light within the core region 11. Therefore, when Raman scattered light generated in a previous stage of the first optical fiber 10 enters the core region 11 of the first optical fiber 10, propagation of the Raman scattered light within the core region 11 is suppressed, and the Raman scattered light leaks from the core region 11 into the high-refractive-index portions 13 as shown by the arrows in Figure 2 and propagates within the high-refractive-index portions 13. The Raman scattered light propagating through the high-refractive-index portions 13 then enters the cladding 22 of the second optical fiber 20.

[0018] As described above, the high refractive index portion 13 of the first optical fiber 10 can be formed by adding a dopant such as germanium to silica glass, and the cladding 22 of the second optical fiber 20 can be formed from silica glass. In this case, the refractive index of the high refractive index portion 13 of the first optical fiber 10 can be equal to or higher than the refractive index of the cladding 22 of the second optical fiber 20.

[0019] When the refractive index of the core region 11 of the first optical fiber 10 is n1 and the refractive index of the core 21 of the second optical fiber 20 is n2, the refractive index difference between the core region 11 and the core 21 is Δ=(n1 2 -n2 2 ) / 2n1 2 is within ±0.2%, the reflectance R at the fusion spliced ​​portion 40 is R=((n1-n2) / (n1+n2)) 2 can be suppressed to -60 dB or less.

[0020] The optical filter 1 of this embodiment includes a high-refractive index resin 50 (first resin) as a first light removal section that removes Raman scattered light incident on the clad 22 of the second optical fiber 20. The high-refractive index resin 50 is a resin having a refractive index equal to or higher than that of the clad 22 of the second optical fiber 20, and may cover the entire circumference (outer periphery) of the exposed clad portion 22A of the second optical fiber 20 over at least a portion of the longitudinal direction. For example, the high-refractive index resin 50 is formed from a material that is transparent to light having a wavelength corresponding to the Raman scattered light incident on the clad 22. An example of a transparent material is silicone resin. In the example shown in FIGS. 1 and 2 , the high-refractive index resin 50 covers the end of the coating 28 of the second optical fiber 20, the entire length of the exposed clad portion 22A, and a portion of the exposed clad portion 12A of the first optical fiber 10.

[0021] 1 and 2 , when the high-refractive-index resin 50 covers the end of the coating 28 of the second optical fiber 20, it is preferable that the refractive index of the coating 28 of the second optical fiber 20 be equal to or lower than the refractive index of the high-refractive-index resin 50. In particular, when the coating 28 is composed of multiple layers, it is preferable that the refractive index of the innermost layer (primary layer) of the coating 28 that comes into contact with the cladding 22 be equal to or lower than the refractive index of the high-refractive-index resin 50.

[0022] Since the outer periphery of the cladding exposed portion 22A is covered with the high-refractive-index resin 50 in this manner, the Raman scattered light incident on the cladding 22 of the second optical fiber 20 as described above is likely to leak from the cladding 22 to the high-refractive-index resin 50. This causes the Raman scattered light to be emitted to the outside from the high-refractive-index resin 50, making it possible to effectively remove the Raman scattered light from the cladding 22 of the second optical fiber 20. Therefore, damage to optical components located downstream of the second optical fiber 20 can be suppressed.

[0023] Here, the diameter D2 (see FIG. 4) of the core 21 of the second optical fiber 20 is preferably equal to or smaller than the diameter D1 (see FIG. 3) of the smallest circle C tangent to the innermost high refractive index portion 13A of the multiple high refractive index portions 13 of the first optical fiber 10. By making the diameter D2 of the core 21 of the second optical fiber 20 equal to or smaller than the diameter D1 of the smallest circle C tangent to the high refractive index portion 13A, it is possible to prevent Raman scattered light propagating through the high refractive index portion 13 from being coupled from the high refractive index portion 13 to the core 21 of the second optical fiber 20 and propagating downstream.

[0024] 5 is a cross-sectional view schematically illustrating an optical filter 101 according to a second embodiment of the present invention. In this embodiment, the high-refractive-index resin 50 described above is not provided, and the outer peripheral surface of the cladding exposed portion 22A is roughened over at least a portion of the longitudinal direction by, for example, etching. By forming such a rough surface 150 on the cladding 22, Raman scattered light incident on the cladding 22 of the second optical fiber 20 is easily scattered by the rough surface 150 and emitted to the outside, thereby effectively removing the Raman scattered light from the cladding 22 of the second optical fiber 20. Thus, the rough surface 150 of the cladding exposed portion 22A in this embodiment functions as a first light removal portion that removes Raman scattered light incident on the cladding 22 of the second optical fiber 20.

[0025] 6 is a cross-sectional view schematically showing an optical filter 201 according to a third embodiment of the present invention. A portion of the Raman scattered light propagating through the high refractive index portion 13 of the first optical fiber 10 may leak into the low refractive index portion 14 outside the high refractive index portion 13. The optical filter 201 in this embodiment has a high refractive index resin 250 (second resin) as a second light removal section that removes such Raman scattered light that has leaked into the low refractive index portion 14 of the first optical fiber 10.

[0026] The high-refractive-index resin 250 is a resin having a refractive index equal to or higher than the refractive index of the low-refractive-index portion 14 of the first optical fiber 10, and may cover the entire circumference (outer periphery) of the exposed cladding portion 12A of the first optical fiber 10 over at least a portion of the longitudinal direction. For example, the high-refractive-index resin 250 is formed from a material that is transparent to light of the wavelength of Raman scattered light leaking into the low-refractive-index portion 14. An example of a transparent material is silicone resin. In the example shown in FIG. 6 , the high-refractive-index resin 250 covers the end of the coating 18 of the first optical fiber 10 and a portion of the exposed cladding portion 12A.

[0027] 6 , when the high-refractive-index resin 250 covers the end of the coating 18 of the first optical fiber 10, it is preferable that the refractive index of the coating 18 of the first optical fiber 10 is equal to or lower than the refractive index of the high-refractive-index resin 250. In particular, when the coating 18 is composed of multiple layers, it is preferable that the refractive index of the innermost layer (primary layer) of the coating 18 that contacts the cladding 22 is equal to or lower than the refractive index of the high-refractive-index resin 250.

[0028] Since the outer periphery of the exposed cladding portion 12A is covered with the high-refractive-index resin 250 in this manner, the Raman scattered light that has leaked into the low-refractive-index portion 14 of the first optical fiber 10 as described above is more likely to leak from the low-refractive-index portion 14 to the high-refractive-index resin 250. This causes the Raman scattered light to be emitted to the outside from the high-refractive-index resin 250, making it possible to effectively remove the Raman scattered light from the cladding region 12 of the first optical fiber 10.

[0029] In this embodiment, a high refractive index resin 250 is used as the second light removal portion. However, instead of this high refractive index resin 250, as in the second embodiment, the outer periphery of the cladding exposed portion 12A of the first optical fiber 10 may be roughened over at least a portion of the longitudinal direction, and the second light removal portion may be constituted by the roughened surface formed on the outer periphery of the cladding exposed portion 12A.

[0030] 7 is a cross-sectional view schematically showing an optical filter 301 according to a fourth embodiment of the present invention. In the third embodiment described above, the first light-removing unit that removes Raman scattered light incident on the cladding 22 of the second optical fiber 20 and the second light-removing unit that removes Raman scattered light that has leaked to the low refractive index portion 14 of the first optical fiber 10 are made of separate high refractive index resins 50 and 250. However, in this embodiment, as shown in FIG. 7, the first light-removing unit and the second light-removing unit are made of a single high refractive index resin 350 (third resin).

[0031] The high-refractive-index resin 350 has a refractive index equal to or greater than the refractive index of the cladding 22 of the second optical fiber 20 and equal to or greater than the refractive index of the low-refractive-index portion 14 of the first optical fiber 10. The high-refractive-index resin 350 covers the entire circumference (outer periphery) of the exposed cladding portion 22A of the second optical fiber 20 over at least a portion of the longitudinal direction, and also covers the entire circumference (outer periphery) of the exposed cladding portion 12A of the first optical fiber 10 over at least a portion of the longitudinal direction. For example, the high-refractive-index resin 350 is formed from a material that is transparent to light of the wavelength of the Raman scattered light leaking into the low-refractive-index portion 14. An example of a transparent material is silicone resin. In the example shown in FIG. 7 , the high-refractive-index resin 350 covers the end of the coating 28 of the second optical fiber 20, the entire length of the exposed cladding portion 22A, the entire length of the exposed cladding portion 12A of the first optical fiber 10, and the end of the coating 18. In this way, Raman scattered light can be efficiently removed with a small number of components by configuring the first light-removing unit and the second light-removing unit from a single high-refractive-index resin 350. Furthermore, by covering the entire length of the exposed cladding portion 22A of the second optical fiber 20 and the entire length of the exposed cladding portion 12A of the first optical fiber 10 with the high-refractive-index resin 350 as in this embodiment, the total amount of Raman scattered light removed by the high-refractive-index resin 350 (the first light-removing unit and the second light-removing unit) can be increased.

[0032] In this case, it is considered that the amount of Raman scattered light propagating through the cladding 22 of the second optical fiber 20 will be greater than the amount of Raman scattered light leaking into the low-refractive-index portion 14 of the first optical fiber 10. Therefore, in order to more efficiently remove the Raman scattered light, it is preferable that the length L2 over which the high-refractive-index resin 350 covers the cladding 22 along the longitudinal direction of the second optical fiber 20 is longer than the length L1 over which the high-refractive-index resin 350 covers the exposed cladding portion 12A (low-refractive-index portion 14) along the longitudinal direction of the first optical fiber 10, as shown in FIG.

[0033] 8 is a cross-sectional view schematically illustrating an optical filter system 401 according to an embodiment of the present invention. As shown in FIG. 8, the optical filter system 401 according to this embodiment includes two optical filters 301 according to the fourth embodiment, which are connected in series. That is, the optical filter system 401 includes a first optical processing section 301A including the optical filter 301 according to the fourth embodiment and a second optical processing section 301B including the optical filter 301 according to the fourth embodiment.

[0034] The core 21 of the second optical fiber 20 of the first optical processing section 301A is optically coupled to the core region 11 of the first optical fiber 10 of the second optical processing section 301B, and the cladding 22 of the second optical fiber 20 of the first optical processing section 301A is optically coupled to the plurality of high refractive index portions 13 of the first optical fiber 10 of the second optical processing section 301B. The optical filter system 401 configured in this manner can remove more Raman scattered light than the optical filter 301 of the fourth embodiment. Similarly, an optical filter system may be configured by connecting a plurality of optical filters 1, 101, 201 of the first to third embodiments in series, or by combining the optical filters 1, 101, 201, 301 of different embodiments and connecting a plurality of optical filters in series.

[0035] In order to allow a large amount of Raman scattered light to leak from the core region 11 of the first optical fiber 10 to the high refractive index portion 13 and then remove this light by the light removing unit, at least a portion of the first optical fiber 10 may be bent into an arc or circle upstream of the fixing resin 32. In this case, the bending diameter of the first optical fiber 10 is, for example, 100 mm.

[0036] Here, if the length along the longitudinal direction of the first optical fiber 10 (photonic bandgap fiber) is short, it is considered that the Raman scattered light will not leak sufficiently from the core region 11 to the high refractive index portion 13, and the Raman scattered light will not be able to be effectively removed in the above-mentioned first light removing unit and second light removing unit. From this viewpoint, the inventors have determined that, in the configuration of the first embodiment, the length L along the longitudinal direction of the first optical fiber 10 located on the opposite side to the direction in which the Raman scattered light propagates with respect to the high refractive index resin 50 (first light removing unit), i.e., on the upstream side, is P We investigated how the amount of Raman scattered light measured downstream changes when the temperature is changed.

[0037] The results are shown in a graph in Fig. 9. In Fig. 9, the horizontal axis represents the length L along the longitudinal direction of the first optical fiber 10 (photonic bandgap fiber) located upstream of the high refractive index resin 50. P 9 (see FIG. 2), and the vertical axis indicates the amount of Raman scattered light measured by a spectrum analyzer installed downstream. This amount of Raman scattered light is shown as a ratio to the amount of Raman scattered light measured downstream when the first optical fiber 10 is composed of a double-clad fiber similar to the second optical fiber 20 and the second optical fiber 20 does not have the high-refractive-index resin 50. From FIG. 9, it can be seen that the length L of the first optical fiber 10 located upstream of the high-refractive-index resin 50 P It can be seen that when the length L of the first optical fiber 10 located upstream of the high refractive index resin 50 is shorter than 1 m, the Raman scattered light is not sufficiently removed (the Raman scattered light does not leak sufficiently from the core region 11 to the high refractive index portion 13). P6, the length L′ along the longitudinal direction of the first optical fiber 10 located on the opposite side to the direction in which the Raman scattered light propagates with respect to the high refractive index resin 250 (second light removing portion), i.e., on the upstream side, is P is preferably 1 m or more.

[0038] 10 is a block diagram schematically illustrating a laser device 501 according to one embodiment of the present invention. This laser device 501 includes a first optical fiber amplifier 510 as a laser light source for generating laser light, the optical filter 301 described in the fourth embodiment, a second optical fiber amplifier 530 for amplifying the laser light generated by the first optical fiber amplifier 510, and a laser emission unit 540 for emitting the laser light amplified by the second optical fiber amplifier 530. Hereinafter, an example using the optical filter 301 according to the fourth embodiment will be described, but it goes without saying that the optical filters according to the other embodiments may also be used. Hereinafter, the direction in which the laser light propagates from the first optical fiber amplifier 510 toward the laser emission unit 540 will be referred to as the "downstream side," and the opposite direction will be referred to as the "upstream side."

[0039] The first optical fiber amplifier 510 is configured as a fiber laser and includes an optical resonator 511, a plurality of pumping light sources 512A that introduce pumping light into the optical resonator 511 from the upstream side of the optical resonator 511, an optical combiner 513A to which these pumping light sources 512A are connected, a plurality of pumping light sources 512B that introduce pumping light into the optical resonator 511 from the downstream side of the optical resonator 511, and an optical combiner 513B to which these pumping light sources 512B are connected. The optical resonator 511 includes an amplification optical fiber 514 having a core doped with rare earth ions such as ytterbium (Yb), erbium (Er), thulium (Tm), or neodymium (Nd), a high-reflection portion 515 that reflects light in a predetermined wavelength band (e.g., 1060 nm to 1100 nm) with a high reflectivity, and a low-reflection portion 516 that reflects light in this wavelength band with a reflectivity lower than that of the high-reflection portion 515. For example, the amplification optical fiber 514 is configured by a double-clad fiber having an inner clad formed around the core and an outer clad formed around the inner clad.

[0040] For example, high-power multimode semiconductor lasers (LDs) with a wavelength of 915 nm can be used as the pumping light sources 512A and 512B. The optical combiner 513A combines the pumping light output from the pumping light source 512A and introduces it into the inner cladding of the amplification optical fiber 514, and the optical combiner 513B combines the pumping light output from the pumping light source 512B and introduces it into the inner cladding of the amplification optical fiber 514. This allows the pumping light to propagate inside the inner cladding of the amplification optical fiber 514.

[0041] The high-reflection portion 515 is connected to the amplification optical fiber 514 and the optical combiner 513A, and the low-reflection portion 516 is connected to the amplification optical fiber 514 and the optical combiner 513B. The high-reflection portion 515 and the low-reflection portion 516 are configured, for example, by a fiber Bragg grating (FBG) or a mirror formed by periodically changing the refractive index of the optical fiber along the propagation direction of the light. In the example shown in Figure 10, the high-reflection portion 515 and the low-reflection portion 516 are configured by a fiber Bragg grating. The high-reflection portion 515, the amplification optical fiber 514, and the low-reflection portion 516 configure an optical resonator 511 that recursively amplifies light in a specific wavelength band between the high-reflection portion 515 and the low-reflection portion 516 to generate laser oscillation.

[0042] A delivery fiber 518 extends from the optical combiner 513B, and the optical filter 301 is fusion-spliced ​​to the delivery fiber 518 at a fusion splice 519. The core of the delivery fiber 518 is coupled to the core region 11 of the first optical fiber 10 of the optical filter 301.

[0043] The second optical fiber amplifier 530 is configured as a fiber laser and includes an amplification optical fiber 531, multiple pumping light sources 532 that generate pumping light, an optical combiner 533 to which the pumping light sources 532 are connected, and a delivery fiber 534 extending from the optical combiner 533 to a laser emission unit 540. The amplification optical fiber 531 is configured as a double-clad fiber having a core doped with rare-earth ions such as ytterbium (Yb), erbium (Er), thulium (Tm), or neodymium (Nd), an inner cladding formed around the core, and an outer cladding formed around the inner cladding. The refractive index of the inner cladding is lower than that of the core, and the core serves as an optical waveguide through which signal light propagates. The refractive index of the outer cladding is lower than that of the inner cladding, and the inner cladding and core serve as an optical waveguide through which pumping light propagates.

[0044] The optical filter 301 is fusion-spliced ​​to the amplification optical fiber 531 at a fusion splice 535. The core of the amplification optical fiber 531 is coupled to the core 21 of the second optical fiber 20 of the optical filter 301.

[0045] In the laser device 501 configured as described above, signal light is generated in a first optical fiber amplifier 510 serving as an MO (master oscillator). Specifically, pumping light introduced from a pumping light source 512A via an optical combiner 513A into an optical resonator 511 and pumping light introduced from a pumping light source 512B via an optical combiner 513B into the optical resonator 511 propagate through the inner cladding and core of an amplification optical fiber 514. As this pumping light passes through the core, it is absorbed by rare-earth element ions doped in the core, exciting the rare-earth element ions and generating spontaneous emission light. This spontaneous emission light is retroreflected between a high-reflection portion 515 and a low-reflection portion 516, amplifying light of a specific wavelength (e.g., 1070 nm) and generating laser oscillation. The laser light thus amplified by the optical resonator 511 propagates through the core of the amplification optical fiber 514, and a portion of it passes through the low-reflection portion 516. The laser light transmitted through the low-reflection portion 516 propagates through the core of the delivery fiber 518, passes through the optical filter 301, and is introduced into a second optical fiber amplifier 530 serving as a PA (Power Amplifier).

[0046] In the second optical fiber amplifier 530, the signal light from the first optical fiber amplifier 510 propagates inside the core of the amplification optical fiber 531, and the pumping light from the pumping light source 532 propagates inside the inner cladding and core of the amplification optical fiber 531. As the pumping light propagates through the core, the rare earth element ions doped in the core absorb the pumping light and are excited, and the signal light propagating through the core is amplified by stimulated emission. The amplified signal light is output from the laser emission unit 540 via the delivery fiber 534.

[0047] The Raman scattered light generated in the first optical fiber amplifier 510 is removed by the first light removing unit (or the first and second light removing units if a second light removing unit is provided) when passing through the optical filter 301. Therefore, it is possible to prevent optical components in the laser device 501 from being damaged by the Raman scattered light.

[0048] 10 , the first optical fiber amplifier 510 is provided with pumping light sources 512A and 512B and optical combiners 513A and 513B on both the high-reflection portion 515 side and the low-reflection portion 516 side, forming a bidirectionally pumped fiber laser, but the pumping light source and the optical combiner may be provided only on either the high-reflection portion 515 side or the low-reflection portion 516 side. Also, the second optical fiber amplifier 530 is provided with a pumping light source 532 only on the downstream side of the amplification optical fiber 531, but a pumping light source may be provided upstream of the amplification optical fiber 531, or pumping light sources may be provided both upstream and downstream of the amplification optical fiber 531.

[0049] 10 , the optical filter 301 is connected between the first optical fiber amplifier 510 and the second optical fiber amplifier 530. However, as shown in FIG. 11 , in addition to the optical filter 301, an optical filter 301 may also be connected between the second optical fiber amplifier 530 and the laser emission unit 540. In the example shown in FIG. 11 , the optical filter 301 disposed between the second optical fiber amplifier 530 and the laser emission unit 540 is fusion-spliced ​​to the delivery fiber 534 at fusion splices 551 and 552. When the optical filter 301 is connected between the second optical fiber amplifier 530 and the laser emission unit 540 in this manner, the optical filter 301 between the first optical fiber amplifier 510 and the second optical fiber amplifier 530 may be omitted. Furthermore, the optical filter connected between the second optical fiber amplifier 530 and the laser emission unit 540 is not limited to the optical filter 301 of the fourth embodiment, and may be an optical fiber of another embodiment.

[0050] 8 may be disposed between the first optical fiber amplifier 510 and the second optical fiber amplifier 530, or an optical filter system including a plurality of optical filters as shown in Fig. 8 may be disposed between the second optical fiber amplifier 530 and the laser emission unit 540. In either optical filter, in order to efficiently remove Raman scattered light, it is preferable that the length along the longitudinal direction of the first optical fiber 10 located upstream of the first light removing unit (or the second light removing unit if a second light removing unit is provided) is 1 m or more.

[0051] As described above, the present invention includes the following aspects.

[0052] A first aspect of the present invention is an optical filter comprising: a first optical fiber constituted by a photonic bandgap fiber including a core region and a plurality of high refractive index portions having a refractive index higher than that of the core region and periodically arranged around the core region so as to suppress propagation of Raman scattered light within the core region; a second optical fiber connected to the first optical fiber, the second optical fiber including: a core optically coupled to the core region of the first optical fiber; and a cladding having a refractive index lower than that of the core and covering an outer periphery of the core, the cladding being optically coupled to the plurality of high refractive index portions of the first optical fiber; a first light removing unit that removes the Raman scattered light that passes through the plurality of high refractive index portions of the first optical fiber and enters the cladding of the second optical fiber; and a fiber accommodating unit that accommodates the first optical fiber and the second optical fiber and protects a connection portion between the first optical fiber and the second optical fiber.

[0053] A second aspect of the present invention is an optical filter according to the first aspect, wherein the diameter of the core of the second optical fiber is equal to or smaller than the diameter of the smallest circle tangent to the innermost high refractive index portion of the plurality of high refractive index portions of the first optical fiber.

[0054] Aspect 3 of the present invention is an optical filter according to Aspect 1 or 2, wherein the first light-removing portion is made of a resin that covers the outer periphery of the cladding of the second optical fiber over at least a portion of the longitudinal direction of the second optical fiber, and has a refractive index equal to or higher than the refractive index of the cladding.

[0055] A fourth aspect of the present invention is the optical filter of the first or second aspect, wherein the first light-removing portion is configured by a rough surface formed on an outer periphery of the cladding of the second optical fiber over at least a portion of the longitudinal direction of the second optical fiber.

[0056] A fifth aspect of the present invention is the optical filter of any one of the first to fourth aspects, wherein the length along the longitudinal direction of the photonic bandgap fiber located on the opposite side of the first light removing unit from the direction in which the Raman scattered light propagates is 1 m or more.

[0057] A sixth aspect of the present invention is an optical filter according to any one of the first to fifth aspects, wherein the first optical fiber includes a low refractive index portion having a refractive index lower than that of the plurality of high refractive index portions and covering the outside of the plurality of high refractive index portions, and the optical filter further includes a second light removal portion that removes the Raman scattered light that has leaked into the low refractive index portion of the first optical fiber.

[0058] A seventh aspect of the present invention is an optical filter according to the sixth aspect, wherein the second light-removing portion is a second resin covering the outer periphery of the low refractive index portion of the first optical fiber over at least a portion of the longitudinal direction of the first optical fiber, the second resin having a refractive index equal to or higher than the refractive index of the low refractive index portion.

[0059] Aspect 8 of the present invention is an optical filter according to aspect 7, wherein the first light-removing portion and the second light-removing portion are made of a single third resin that covers the outer periphery of the cladding of the second optical fiber and the outer periphery of the low refractive index portion of the first optical fiber over at least a portion of the longitudinal direction of the second optical fiber and at least a portion of the longitudinal direction of the first optical fiber, and has a refractive index that is equal to or greater than the refractive index of the low refractive index portion and the refractive index of the cladding.

[0060] A ninth aspect of the present invention is an optical filter according to the eighth aspect, wherein the length over which the third resin covers the cladding along the longitudinal direction of the second optical fiber is longer than the length over which the third resin covers the low refractive index portion along the longitudinal direction of the first optical fiber.

[0061] A tenth aspect of the present invention is the optical filter of the sixth aspect, wherein the second light-removing portion is configured by a rough surface formed on the outer periphery of the low refractive index portion of the first optical fiber over at least a portion of the longitudinal direction of the first optical fiber.

[0062] Aspect 11 of the present invention is the optical filter of any one of Aspects 6 to 10, wherein the length along the longitudinal direction of the photonic bandgap fiber located on the opposite side of the second light removing unit from the direction in which the Raman scattered light propagates is 1 m or more.

[0063] A twelfth aspect of the present invention is the optical filter of any one of the first to eleventh aspects, wherein at least a portion of the first optical fiber is bent in an arc or circle.

[0064] A thirteenth aspect of the present invention is an optical filter system comprising: a first optical processing section including an optical filter according to any one of Aspects 1 to 12; and a second optical processing section including an optical filter according to any one of Aspects 1 to 12, wherein the core of the second optical fiber of the first optical processing section is optically coupled to the core region of the first optical fiber of the second optical processing section, and the clad of the second optical fiber of the first optical processing section is optically coupled to the multiple high refractive index sections of the first optical fiber of the second optical processing section.

[0065] A fourteenth aspect of the present invention is a laser device comprising: an optical filter according to any one of the first to twelfth aspects; and an amplification optical fiber connected to the first optical fiber of the optical filter.

[0066] Although the preferred embodiments of the present invention have been described above, it goes without saying that the present invention is not limited to the above-described embodiments and may be embodied in various different forms within the scope of the technical concept thereof.

[0067] This application is based on and claims priority from Japanese Patent Application No. 2024-046385, filed on March 22, 2024, the disclosure of which is incorporated herein by reference in its entirety.

[0068] The present invention is suitably used in an optical filter and a laser device including the same.

[0069] 1, 201, 301, 401 Optical filter 10 First optical fiber 11 Core region 12 Cladding region 13 High refractive index portion 14 Low refractive index portion 18 Coating 20 Second optical fiber 21 Core 22 Cladding 28 Coating 30 Fiber accommodating portion 31 Fiber accommodating groove 50 High refractive index resin (first light removing portion, first resin) 150 Rough surface (first light removing portion) 250 High refractive index resin (second light removing portion, second resin) 301A First light processing portion 301B Second light processing portion 350 High refractive index resin (first light removing portion and second light removing portion, third resin) 501 Laser device

Claims

1. An optical filter comprising: a first optical fiber constituted by a photonic bandgap fiber including a core region and a plurality of high refractive index sections having a refractive index higher than that of the core region and periodically arranged around the core region so as to suppress propagation of Raman scattered light within the core region; a second optical fiber connected to the first optical fiber, the second optical fiber including: a core optically coupled to the core region of the first optical fiber; and a cladding having a refractive index lower than that of the core and covering the outer periphery of the core, the cladding optically coupled to the plurality of high refractive index sections of the first optical fiber; a first light removing section that removes the Raman scattered light that passes through the plurality of high refractive index sections of the first optical fiber and enters the cladding of the second optical fiber; and a fiber accommodating section that accommodates the first optical fiber and the second optical fiber and protects the connection between the first optical fiber and the second optical fiber.

2. An optical filter according to claim 1, wherein the diameter of the core of the second optical fiber is equal to or smaller than the diameter of the smallest circle tangent to the innermost high refractive index portion of the plurality of high refractive index portions of the first optical fiber.

3. An optical filter as described in claim 1 or 2, wherein the first light-removing section is made of a resin that covers the outer periphery of the cladding of the second optical fiber over at least a portion of the longitudinal direction of the second optical fiber and has a refractive index equal to or greater than the refractive index of the cladding.

4. An optical filter as claimed in claim 1 or 2, wherein the first light-removing section is constituted by a roughened surface formed on the outer periphery of the cladding of the second optical fiber over at least a portion of the longitudinal direction of the second optical fiber.

5. An optical filter according to any one of claims 1 to 4, wherein the length along the longitudinal direction of the photonic bandgap fiber located on the opposite side of the first light removal section from the direction in which the Raman scattered light propagates is 1 m or more.

6. An optical filter according to any one of claims 1 to 5, wherein the first optical fiber includes a low refractive index section that has a refractive index lower than the refractive index of the plurality of high refractive index sections and covers the outside of the plurality of high refractive index sections, and the optical filter further includes a second light removal section that removes the Raman scattered light that has leaked into the low refractive index section of the first optical fiber.

7. An optical filter as described in claim 6, wherein the second light-removing section is composed of a second resin that covers the outer periphery of the low refractive index section of the first optical fiber over at least a portion of the longitudinal direction of the first optical fiber, and that has a refractive index equal to or higher than the refractive index of the low refractive index section.

8. An optical filter as described in claim 7, wherein the first light-removing section and the second light-removing section are made of a single third resin that covers the outer periphery of the cladding of the second optical fiber and the outer periphery of the low refractive index section of the first optical fiber over at least a portion of the longitudinal direction of the second optical fiber and at least a portion of the longitudinal direction of the first optical fiber, and has a refractive index that is equal to or greater than the refractive index of the low refractive index section and the refractive index of the cladding.

9. An optical filter as described in claim 8, wherein the length over which the third resin covers the cladding along the longitudinal direction of the second optical fiber is longer than the length over which the third resin covers the low refractive index portion along the longitudinal direction of the first optical fiber.

10. An optical filter as described in claim 6, wherein the second light-removing section is constituted by a roughened surface formed on the outer periphery of the low refractive index section of the first optical fiber over at least a portion of the longitudinal direction of the first optical fiber.

11. An optical filter according to any one of claims 6 to 10, wherein the length along the longitudinal direction of the photonic bandgap fiber located on the opposite side of the second light removal section from the direction in which the Raman scattered light propagates is 1 m or more.

12. An optical filter according to any one of claims 1 to 11, wherein at least a portion of said first optical fiber is bent in an arc or circle.

13. An optical filter system comprising: a first optical processing section including an optical filter according to any one of claims 1 to 12; and a second optical processing section including an optical filter according to any one of claims 1 to 12, wherein the core of the second optical fiber of the first optical processing section is optically coupled to the core region of the first optical fiber of the second optical processing section, and the clad of the second optical fiber of the first optical processing section is optically coupled to the plurality of high refractive index sections of the first optical fiber of the second optical processing section.

14. A laser device comprising: an optical filter according to any one of claims 1 to 12; and an amplification optical fiber connected to the first optical fiber of the optical filter.

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