Fiber laser device

Abstract

A fiber laser device (1) is provided with: a first excitation light source (12) capable of emitting first excitation light; a first amplification optical fiber (11) to which an active element that absorbs the first excitation light is added, and which amplifies and emits the light of the first wavelength; a second amplification optical fiber (21) to which an active element that absorbs the light of the first wavelength and is the same as the active element added to the first amplification optical fiber (11) is added, and which amplifies and emits the light of the second wavelength; beam combiners (31), (32); and a light attenuation unit (60) that is provided on an optical path between the beam combiners (31, 32) and the first amplification optical fiber (11), transmits light from the first amplification optical fiber (11), and suppresses the transmission of light from the beam combiners (31, 32).

Description

Technical Field

[0001] This invention relates to fiber laser devices. Background Technology

[0002] Fiber laser devices are used in various fields such as laser processing and medicine. One example of a fiber laser device is described in Patent Document 1 below. This fiber laser device includes: a seed light source; and an optical amplifier connected to the seed light source to amplify the light from the seed light source. The optical amplifier includes: an amplifying optical fiber optically connected to the seed light source, and an excitation light source that guides excitation light into the amplifying optical fiber from the front and rear sides. The light from the seed light source incident on the amplifying optical fiber is amplified by the excitation light from the excitation light source and emitted. In Patent Document 1, a laser oscillator is used as the excitation light source.

[0003] In typical fiber laser devices, when ytterbium is added as the active element in the amplification fiber used for the optical amplifier, the wavelength of the excitation light is 915 nm-976 nm, and the wavelength of the amplified emitted light is 1060 nm-1080 nm. Therefore, there is a difference of at least 84 nm between the wavelengths of the excitation light and the amplified light. If this difference is large, transverse mode instability (TMI) caused by quantum defects can sometimes occur. If TMI occurs, connections with higher-order modes can form, leading to easy leakage of the amplified light and difficulties in thermal management when emitting high-power light. Therefore, it is necessary to suppress the generation of quantum defects to prevent beam quality degradation and emit high-power light. To meet this requirement, it is considered to make the wavelength of the excitation light close to the wavelength of the amplified light. In Patent Document 1, the wavelength of the light amplified by the optical amplifier is 1080 nm, and the wavelength of the excitation light is 1018 nm.

[0004] Existing technical documents Non-patent literature Non-profit literature 1: Ruixian Li, Hanshuo Wu, Hu Xiao, Jinyong Leng, LiangjinHuang, AND Pu Zhou, “More than 6 kW near single-mode fiber amplifier based on a bidirectional tandem pumping scheme” Vol. 61, No. 23 / 10 August 2022 / Applied Optics Summary of the Invention

[0005] (a) Technical problems to be solved In cases where the wavelength of the amplified light is close to the wavelength of the excitation light, as in the fiber laser device described in Patent Document 1, a fiber laser is considered as the excitation source. This laser directs light emitted from a semiconductor laser or similar source into an amplifying fiber containing an active element identical to the active element added to the amplifying fiber of the optical amplifier. By configuring the excitation source in this way, the wavelength of light from the semiconductor laser can be brought close to that of the light amplified by the optical amplifier within the excitation source.

[0006] However, in fiber laser devices, light of a wavelength amplified by the amplifying fiber sometimes enters the excitation source. In a rearward excitation source that guides the excitation light from the amplifying fiber towards the rearward side of the laser emission direction, a combiner is provided in the amplifying fiber. In this case, the aforementioned light may enter the excitation source as described below. That is, when light from the source is amplified, due to reflections in the combiner, a portion of the amplified light may enter the excitation source. Furthermore, in a forward excitation source that guides the excitation light from the rearward side of the amplifying fiber, a combiner may also enter the excitation source as described below. That is, when light amplified by the amplifying fiber exits the fiber laser device, it may be reflected by the emission end or the workpiece, and as return light, enter the amplifying fiber. Sometimes, a portion of this return light may enter the excitation source from the combiner. As mentioned above, when the active element added to the amplification fiber of the excitation source is the same as the active element added to the amplification fiber of the optical amplifier, as in these examples, if a portion of the wavelength of light amplified by the amplification fiber is incident on the excitation source, the amplification efficiency of the excitation light in the excitation source may sometimes deteriorate. As a result, the power of the excitation light decreases, and the power of the light emitted from the fiber laser device may decrease.

[0007] Therefore, the object of the present invention is to provide a fiber laser device capable of emitting high-power light.

[0008] (II) Technical Solution To address the aforementioned issues, Embodiment 1 of the present invention provides a fiber laser device, characterized by comprising: a first excitation light source capable of emitting a first excitation light; a first amplification fiber having an active element added thereto absorb the first excitation light and amplify and emit light of a first wavelength; a second amplification fiber having an active element added thereto, the same active element added to the first amplification fiber, absorbing the first wavelength light as excitation light and amplifying and emitting light of a second wavelength; a beam combiner capable of directing the first wavelength light into the second amplification fiber; and an optical attenuation section disposed in the optical path between the beam combiner and the first amplification fiber, allowing light from the first amplification fiber to pass through and suppressing the transmission of light from the beam combiner.

[0009] In this way, the active element added to the first amplifying fiber is the same as the active element added to the second amplifying fiber. The first excitation light is converted into light of the first wavelength using the first amplifying fiber, thus making the first wavelength closer to the second wavelength than the wavelength of the first excitation light. Therefore, according to this fiber laser device, compared to the case where the first excitation light is incident on the second amplifying fiber, the generation of quantum defects in the second amplifying fiber can be suppressed, and the reduction in the beam quality of the second wavelength light emitted from the second amplifying fiber can be suppressed. Furthermore, in this fiber laser device, even when the second wavelength light amplified by the second amplifying fiber travels towards the first amplifying fiber via a combiner as described above, the light from the combiner can be attenuated by the optical attenuation section. Therefore, the propagation of the second wavelength light into the first core of the first amplifying fiber can be suppressed, and the reduction in the amplification efficiency of the first wavelength light in the first amplifying fiber can be suppressed. Therefore, the reduction in the power of the first wavelength light, which is the excitation light incident on the second amplifying fiber, can be suppressed, and the reduction in the amplification efficiency of the second wavelength light in the second amplifying fiber can be suppressed. Therefore, according to embodiment 1, the fiber laser device can emit high-power light.

[0010] In embodiment 2 of the present invention, the fiber laser device of embodiment 1 is characterized in that the optical attenuation section is a filter that allows light of the first wavelength to pass through and suppresses the transmission of light of the second wavelength.

[0011] According to the structure of Method 2, the light of the second wavelength from the combiner toward the first amplifying fiber can be reduced more appropriately by utilizing the optical attenuation section.

[0012] In embodiment 3 of the present invention, the fiber laser device of embodiment 2 is characterized in that the filter is a reflector that reflects light of the second wavelength.

[0013] According to the structure of Method 3, when light of the second wavelength travels towards the first amplifying fiber via the combiner, the second wavelength light can be reflected in a desired direction. For example, by setting the reflector in such a way that it reflects the second wavelength light traveling towards the first amplifying fiber toward the combiner, the second wavelength light can be re-intruded into the second core of the second amplifying fiber and amplified by the second core. In this case, the reduction in amplification efficiency in the second amplifying fiber can be further suppressed.

[0014] In embodiment 4 of the present invention, the fiber laser device of embodiment 3 is characterized in that the reflector is an FBG (Fiber Bragg Grating).

[0015] In this case, the second wavelength of light traveling towards the first amplifying fiber can be reflected towards the combiner. Therefore, the second wavelength of light can be re-injected into the second core of the second amplifying fiber and amplified by the second core. Thus, the reduction in amplification efficiency in the second amplifying fiber can be further suppressed.

[0016] In embodiment 5 of the present invention, the fiber laser device of embodiment 4 is characterized in that the FBG is tilted.

[0017] In embodiment 6 of the present invention, the fiber laser device of embodiment 2 is characterized in that the filter is a photonic bandgap fiber.

[0018] Photonic bandgap fiber exhibits excellent filtering performance, allowing light of the desired wavelength to pass through while blocking light of other wavelengths. Therefore, it can further suppress the propagation of a second wavelength of light into the first amplification fiber.

[0019] In embodiment 7 of the present invention, the fiber laser device of embodiment 5 or 6 is characterized in that the filter and the first amplifying fiber are connected by an optical fiber comprising a core and a cladding, and a cladding mode canceller is provided in the middle of the fiber.

[0020] When using a tilted FBG (fiber-free bandgap) or photonic bandgap fiber as a filter, light of a second wavelength may sometimes be incident on the fiber cladding. Even in this case, a cladding mode canceller can prevent light propagating in the cladding from leaking out of the fiber cladding. Therefore, it is possible to further suppress light propagating in the cladding from being incident on the first amplification fiber.

[0021] Furthermore, the optical attenuation section can also be a structure other than a filter. In this case, for example, the fiber laser device of embodiment 1 is characterized in that the optical path between the first amplifying fiber and the combiner includes: a first fiber optically connected to the first amplifying fiber; and a second fiber optically connected to the first fiber and optically connected to the combiner, wherein the optical attenuation section includes a connection between the first fiber and the second fiber, and the core diameter of the second fiber is larger than the core diameter of the first fiber.

[0022] In this method, an optical attenuation section is formed by setting a difference in the diameter of the cores of the two optical fibers, thus making it easy to construct an optical attenuation section.

[0023] In embodiment 8 of the present invention, any one of the fiber laser devices in embodiments 1-7 is characterized in that it further comprises: a second excitation light source capable of emitting a second excitation light; and a third amplifying fiber, which has an active element added to it that is the same as the active element added to the second amplifying fiber, absorbs the second excitation light, and is capable of amplifying and emitting light of the second wavelength, wherein the light of the second wavelength emitted from the third amplifying fiber is incident on the second amplifying fiber from one end.

[0024] Alternatively, in embodiment 9 of the present invention, any one of embodiments 1-7 of the fiber laser device is characterized by further comprising: a first reflector optically connected to the second amplifying fiber at one end of the second amplifying fiber to reflect light of the second wavelength; and a second reflector optically connected to the second amplifying fiber at the other end of the second amplifying fiber to reflect light of the second wavelength with a lower reflectivity than the first reflector.

[0025] (III) Beneficial Effects As described above, according to the present invention, a fiber laser device capable of emitting high-power light is provided. Attached Figure Description

[0026] Figure 1 This is a diagram illustrating a fiber laser device according to a first embodiment of the present invention.

[0027] Figure 2 This is a diagram representing the excitation source.

[0028] Figure 3 This is a diagram showing the optical fiber used for amplification.

[0029] Figure 4 It is a diagram representing a type of light source.

[0030] Figure 5 This is a diagram illustrating a fiber laser device according to a second embodiment of the present invention.

[0031] Figure 6 This is a diagram showing a first modified example of the optical attenuation section.

[0032] Figure 7 This is a diagram showing a second modified example of the optical attenuation section.

[0033] Figure 8 This is a diagram representing a photonic bandgap fiber.

[0034] Figure 9 This is a diagram showing a third modified example of the optical attenuation section. Detailed Implementation

[0035] Hereinafter, preferred embodiments of the fiber laser device of the present invention will be described in detail with reference to the accompanying drawings. The embodiments illustrated below are for ease of understanding of the present invention and are not intended to limit or interpret the present invention. The present invention can be modified and improved upon without departing from its spirit within the scope of the claims. Furthermore, for ease of understanding, the scale of the various figures may sometimes differ from the scale described in the following description.

[0036] Figure 1 This is a diagram illustrating the laser device of this embodiment. (As shown) Figure 1 As shown, the fiber laser device 1 of this embodiment has a seed light source MO and an optical amplifier PA as its main structures. That is, the fiber laser device 1 of this embodiment is an MO-PA (Master Oscillator-Power Amplifier) ​​type fiber laser device.

[0037] First, let's explain the optical amplifier PA. The optical amplifier PA has multiple excitation light sources 10, a second amplification fiber 21, a combiner 31 and 32, and multiple optical attenuation sections 60 as its main structures.

[0038] Figure 2 This is a diagram representing the excitation source 10. (For example...) Figure 2 As shown, the excitation source 10 has a first amplification fiber 11, a first excitation source 12, an HR-FBG 16, and an OC-FBG 17 as its main structures.

[0039] The first excitation source 12 is composed of multiple laser diodes 12D, emitting a first excitation light of a wavelength that excites the active element added to the first amplification fiber 11. In the case where the active element added to the first amplification fiber 11 is ytterbium (Yb), as described later, the wavelength of the first excitation light is, for example, 976 nm. Furthermore, the first excitation source 12 is connected to a predetermined power supply, emitting first excitation light with a power corresponding to the current from that power supply. Preferably, this power supply allows for variable output current. Each laser diode 12D of the first excitation source 12 is connected to one end of an optical fiber 121. The other end of each optical fiber 121 is connected to a combiner 13.

[0040] One end of the fiber 14 (described later) is connected to the combiner 13, and the other end of the fiber 14 is connected to one end of the first amplification fiber 11.

[0041] Figure 3 It means Figure 2 A diagram showing the cross-section of the first enlarged optical fiber 11. (See diagram below.) Figure 2As shown, the first amplifying optical fiber 11 has the following main components as its structure: a first core 111; a first cladding 112 as the inner cladding, which surrounds the outer peripheral surface of the first core 111 without gaps; an outer cladding 113, which surrounds the outer peripheral surface of the first cladding 112; and a cover layer 114, which covers the outer cladding 113. That is, from a structural point of view, the first amplifying optical fiber 11 is a double-clad optical fiber. The refractive index of the first cladding 112 is lower than that of the first core 111, and the refractive index of the outer cladding 113 is lower than that of the first cladding 112. The diameter of the first core 111 is, for example, 28 μm, and the outer diameter of the first cladding 112 is, for example, 400 μm. In this embodiment, from an optical point of view, the first amplifying optical fiber 11 is a multimode optical fiber, for example, 2LP mode light propagates in the first core 111. In addition, as described later, a first excitation light is incident on the first cladding 112, and this first excitation light mainly propagates in the first cladding 112. The length of the first amplification fiber 11 is, for example, 20m.

[0042] The first core 111 contains an active element that is excited by the first excitation light emitted from the first excitation light source 12. In this embodiment, ytterbium is added as the active element. Furthermore, to improve resistance to photodarkening, aluminum and phosphorus are preferably added to the first core 111. Additionally, to adjust the refractive index, dopants such as fluorine (F) and boron (B) may be added to at least a portion of the first core 111. However, unlike this embodiment, the active element added to the first core 111 may also be an active element other than ytterbium. Examples of such active elements, besides ytterbium, include thulium (Tm), cerium (Ce), neodymium (Nd), europium (Eu), and erbium (Er). Furthermore, examples of active elements, besides rare earth elements, include bismuth (Bi).

[0043] Furthermore, the first cladding layer 112 may be composed, for example, pure quartz without any dopants, or quartz with dopants such as fluorine added to lower the refractive index. The outer cladding layer 113 may be composed, for example, a resin with a lower refractive index than the first cladding layer 112, or quartz with elements such as fluorine added to lower the refractive index. The capping layer 114 may be composed, for example, a thermosetting or UV-curing resin; if the outer cladding layer 113 is composed of resin, it may be composed of a resin different from the resin in the outer cladding layer 113. Furthermore, the first cladding layer 112 may also be composed of multiple layers with different refractive indices and additives.

[0044] Except for the absence of active elements in the fiber core, the optical fiber 14 connected to one end of the first amplifying optical fiber 11 has the same structure as the first amplifying optical fiber 11. Therefore, the optical fiber 14 has a fiber core, an inner cladding surrounding the fiber core, an outer cladding surrounding the inner cladding, and a capping layer covering the outer cladding. The first fiber core 111 of the first amplifying optical fiber 11 is connected to the fiber core of the optical fiber 14, and the first cladding 112 of the first amplifying optical fiber 11 is connected to the inner cladding of the optical fiber 14.

[0045] An HR-FBG16 is disposed in the core of optical fiber 14. Thus, the HR-FBG16 is disposed at one end of the first amplifying optical fiber 11 and optically connected to the first core 111. The FBG is constructed by repeatedly forming high-refractive-index portions and lower-refractive-index portions along the long side of the core at a fixed period, enabling it to reflect light of a predetermined wavelength while allowing light of other wavelengths to pass through. By adjusting this period, the HR-FBG16 reflects at least a first wavelength of light emitted by the active element of the first amplifying optical fiber 11 in the excited state. The first wavelength is longer than the wavelength of the first excitation light. As described above, when the active element added to the first amplifying optical fiber 11 is ytterbium, the first wavelength is, for example, 1020 nm, and the HR-FBG16 reflects the first wavelength of light with a reflectivity of, for example, 99% or more.

[0046] In the combiner 13, the core of each fiber 121 is optically connected to the inner cladding of the fiber 14. Therefore, the first excitation light emitted from the first excitation source 12 can propagate through the fiber 121, the combiner 13, and the inner cladding of the fiber 14 to the first cladding 112 of the first amplifying fiber 11.

[0047] One end of the first amplifying optical fiber 11 is connected to one end of the optical fiber 15. The optical fiber 15 is an optical fiber with a core and a cladding. The first core 111 of the first amplifying optical fiber 11 is connected to the core of the optical fiber 15, and the first cladding 112 of the first amplifying optical fiber 11 is connected to the cladding of the optical fiber 15.

[0048] An OC-FBG17 is disposed in the core of optical fiber 15. Thus, the OC-FBG17 is disposed at the other end of the first amplifying optical fiber 11 and optically connected to the first core 111. The OC-FBG17 is configured to reflect light of the first wavelength reflected by HR-FBG16 with a lower reflectivity than HR-FBG16. For example, the OC-FBG17 is configured to reflect light of the first wavelength with a reflectivity of, for example, 10%. Thus, HR-FBG16, the first amplifying optical fiber 11, and the OC-FBG17 form a resonator that resonates with light of the first wavelength. A portion of the light of the first wavelength resonating through this resonator passes through the OC-FBG17.

[0049] The excitation light source 10 with the above structure emits excitation light as the first wavelength.

[0050] The other end of optical fiber 15 is connected to one end of optical fiber 19, which has the same structure as optical fiber 15. Alternatively, optical fiber 15 can be extended to become optical fiber 19. The other end of optical fiber 19 is connected to combiner 31 or combiner 32.

[0051] An optical attenuation section 60 is provided in the optical path between the combiners 31 and 32 and the first amplification optical fiber 11 at the midpoint of the optical fiber 19. In this embodiment, the optical attenuation section 60 is a filter that allows light of a first wavelength to pass through while suppressing the transmission of light of a second wavelength longer than the first wavelength. In this embodiment, the filter is composed of an FBG 61. The FBG 61 constituting the optical attenuation section 60 reflects light of the second wavelength while allowing light of the first wavelength to pass through. In the FBG 61 of the optical attenuation section 60 of this embodiment, the interface between the high-refractive-index portion and the low-refractive-index portion, which are repeatedly formed at a fixed period, is approximately perpendicular to the long side direction of the fiber core of the optical fiber 19. Therefore, the optical attenuation section 60 of this embodiment reflects the second-wavelength light incident on the optical attenuation section 60 in a direction opposite to the direction of travel of the second-wavelength light incident on the optical attenuation section 60. As will be described later, when the active element added to the second amplification optical fiber 21 is ytterbium, the second wavelength is, for example, 1064 nm. The FBG61 constituting the light attenuation section 60 reflects light of the second wavelength with a reflectivity of, for example, 99% or more.

[0052] Furthermore, in this embodiment, cladding mode eliminators 70 are provided between the light attenuation section 60 (which serves as a filter) and the combiner 31, and between the light attenuation section 60 and the combiner 32. The cladding mode eliminator 70 is a component that prevents light propagating within the cladding of the optical fiber 19 from leaking to the outside of the cladding. The structure of the cladding mode eliminator 70 is not particularly limited; for example, a component with a refractive index higher than the refractive index of the cladding of the optical fiber 19 can be provided in a manner that contacts the cladding of the optical fiber 19. Alternatively, the cladding mode eliminator 70 can be provided at other locations as long as it removes light propagating within the cladding of the optical fiber 19. By providing such a cladding mode eliminator 70, it is possible to suppress light propagating from the combiners 31 and 32 within the cladding of the optical fiber 19 to the first amplification optical fiber 11. Therefore, it is possible to suppress the reduction in the amplification efficiency of the excitation light source 10 and the damage to the first excitation light source 12 in the excitation light source 10.

[0053] The second amplifying fiber 21 in the optical amplifier PA is an amplifying fiber used to amplify and emit light emitted from the seed light source MO. Therefore, the seed light source MO side of the second amplifying fiber 21 is the rear side, and the side of the second amplifying fiber 21 opposite to the seed light source MO side is the front side. The direction from the rear side to the front side is the direction of light propagation emitted from the fiber laser device 1, and therefore this direction is positive.

[0054] The second amplifying optical fiber 21, not specifically illustrated, has the same structure as the first amplifying optical fiber 11. Specifically, the second amplifying optical fiber 21 comprises the following main components: a second core, which has the same structure as the first core 111; a second cladding, which has the same structure as the first cladding 112 and surrounds the outer periphery of the second core without gaps; an outer cladding, which surrounds the outer periphery of the second cladding with the same structure as the outer cladding 113; and a capping layer covering the outer cladding. The second core contains the same active element as the active element added to the first core.

[0055] A combiner 31 is disposed at one end of the second amplifying optical fiber 21 on the seed light source MO side, and a combiner 32 is disposed at the other end of the second amplifying optical fiber 21. Combiners 31 and 32 are, for example, constructed of optical fibers having a core and a cladding surrounding the core. In this case, the core of combiners 31 and 32 is connected to the second core of the second amplifying optical fiber 21, and the cladding of combiners 31 and 32 is connected to the second cladding of the second amplifying optical fiber 21. Furthermore, when combiners 31 and 32 have this structure, they can also be tapered optical fibers with a smaller diameter on the side opposite to the second amplifying optical fiber 21 and a larger diameter on the side opposite to the second amplifying optical fiber 21. As described above, each optical fiber 19 is connected to combiner 31 or combiner 32. Specifically, the core of a portion of the optical fibers 19 is optically connected to the second cladding of the second amplifying optical fiber 21 in the combiner 31, while the core of another portion of the optical fibers 19 is optically connected to the second cladding of the second amplifying optical fiber 21 in the combiner 32. When the combiners 31 and 32 are made of optical fibers as described above, the core of a portion of the optical fibers 19 is connected to the cladding of the combiner 31, and the core of another portion of the optical fibers 19 is connected to the cladding of the combiner 32. From the viewpoint of enabling higher power excitation light to be incident on the second amplifying optical fiber 21, it is preferable that the number of excitation sources 10 optically connected to the second cladding via the optical fibers 19 and combiner 31 is equal to the number of excitation sources 10 optically connected to the second cladding via the optical fibers 19 and combiner 32. Furthermore, the excitation source 10, which is optically connected to the second cladding via the combiner 31, is a forward excitation source that introduces the first light as excitation light from the rear side to the front side of the second amplifying optical fiber 21. The excitation source 10, which is optically connected to the second cladding via the combiner 32, is a backward excitation source that introduces the first light as excitation light from the front side to the rear side of the second amplifying optical fiber 21. Moreover, the structures of the combiners 31 and 32 are not limited to the structures described above. For example, the core of the optical fiber 19 may be directly connected to the second cladding of the second amplifying optical fiber 21, and this connection may be the combiner 31 or 32.

[0056] Additionally, one end of the transmission optical fiber 40, which has a core and a cladding, is connected to the combiner 32. The core of the transmission optical fiber 40 is optically connected to the second core of the second amplification optical fiber 21 in the combiner 32.

[0057] An exit section 41 is connected to the other end of the transmission optical fiber 40. The exit section 41 is, for example, made of a glass rod with a diameter larger than the core of the transmission optical fiber 40. Therefore, the diameter of the light incident from the core of the transmission optical fiber 40 to the exit section 41 is enlarged and it exits from the exit section 41.

[0058] Next, the seed light source MO will be explained.

[0059] Figure 4 This is a diagram illustrating an example of a seed light source MO. For example... Figure 4 As shown, the seed light source MO has a main structure consisting of a third amplification fiber 51, a second excitation light source 52, an HR-FBG 56, and an OC-FBG 57. The seed light source MO has a structure that is largely the same as that of the excitation light source 10.

[0060] The third amplifying optical fiber 51, not specifically illustrated, has the same structure as the second amplifying optical fiber 21. Specifically, the third amplifying optical fiber 51 has the following main components: a third core, which has the same structure as the second core; a third cladding, which has the same structure as the second cladding and surrounds the outer periphery of the third core without gaps; an outer cladding, which surrounds the outer periphery of the third cladding with the same structure as the outer cladding of the second amplifying optical fiber 21; and a cover layer, which covers the outer cladding. The third core contains the same active element as that added to the second core. In other words, in this embodiment, the same active element is added to the first core 111 of the first amplifying optical fiber 11, the second core of the second amplifying optical fiber 21, and the third core of the third amplifying optical fiber 51. From a structural point of view, the third amplifying optical fiber 51 is a double-clad optical fiber. Furthermore, the third core of the third amplifying optical fiber 51, for example, propagates light in a 2LP mode or lower.

[0061] An optical fiber 54 is connected to one end of the third amplification optical fiber 51. The optical fiber 54 has the same structure as the third amplification optical fiber 51, except that no active element is added to its core. Therefore, the optical fiber 54 has a core, an inner cladding surrounding the core, an outer cladding surrounding the inner cladding, and a capping layer covering the outer cladding. The third core of the third amplification optical fiber 51 is connected to the core of the optical fiber 54, and the third cladding of the third amplification optical fiber 51 is connected to the inner cladding of the optical fiber 54.

[0062] An HR-FBG56 is disposed in the core of optical fiber 54. Thus, the HR-FBG56 is disposed at one end of the third amplification optical fiber 51 and optically connected to the third core of the third amplification optical fiber 51. The HR-FBG56 reflects the aforementioned second wavelength of light. As described above, the second wavelength is, for example, 1064 nm, and the HR-FBG56 reflects the second wavelength of light with a reflectivity of, for example, 99% or higher.

[0063] The second excitation source 52 is composed of multiple laser diodes 52D identical to those in the first excitation source 12, emitting a second excitation light at a wavelength that excites the active element added to the third amplification fiber 51. The power of the second excitation light is, for example, the same as the power of the first excitation light emitted from the first excitation source 12, and the wavelength of the second excitation light is, for example, the same as the wavelength of the first excitation light emitted from the first excitation source 12. Furthermore, the second excitation source 52 is connected to a predetermined power supply, emitting second excitation light with a power corresponding to the current from that power supply. Preferably, the power supply allows for variable output current. Each laser diode 52D of the second excitation source 52 is connected to one end of an optical fiber 521. The optical fiber 521 has the same structure as the optical fiber 121. The other end of each optical fiber 521 is connected to a combiner 53. Additionally, the power of the second excitation light may not be the same as the power of the first excitation light emitted from the first excitation source 12, and the wavelength of the second excitation light may not be the same as the wavelength of the first excitation light emitted from the first excitation source 12.

[0064] The fiber 54 is connected to the end of the combiner 53 on the side opposite to the side connected to the third amplifying fiber 51. In the combiner 53, the core of each fiber 521 is optically connected to the inner cladding of the fiber 54. Therefore, the excitation light emitted from the second excitation source 52 can be transmitted through the core of the fiber 521 and the inner cladding of the fiber 54 to the third cladding of the third amplifying fiber 51. Thus, the second excitation source 52 is a source that introduces the second excitation light into the third cladding of the third amplifying fiber 51 from the HR-FBG56 side.

[0065] One end of fiber 55 is connected to the other end of the third amplification fiber 51. Fiber 55 has the same structure as fiber 54, and an OC-FBG57 is disposed in the core of fiber 54. Thus, the OC-FBG57 is disposed on the other end of the third amplification fiber 51 and optically connected to the third core of the third amplification fiber 51. The OC-FBG57 reflects light of the second wavelength with a lower reflectivity than the HR-FBG56, for example, 10%. In this way, a resonator is formed by the HR-FBG56, the third amplification fiber 51, and the OC-FBG57. A portion of the second wavelength light resonating through this resonator passes through the OC-FBG57 and is emitted from the seed light source MO.

[0066] One end of an optical fiber 55 with the same structure as optical fiber 55 is connected to the optical fiber 55 of the seed light source MO. Furthermore, optical fiber 55 can be extended to become optical fiber 35. Optical fiber 35 propagates light of the second wavelength emitted from the seed light source MO. The other end of optical fiber 35 is optically connected to a second amplifying optical fiber 21 in a combiner 31, and the core of optical fiber 35 is optically connected to the second core of the second amplifying optical fiber 21. Therefore, light of the second wavelength emitted from the seed light source MO can be incident on the second core of the second amplifying optical fiber 21 via the core of optical fiber 35.

[0067] Next, the operation of the fiber laser device 1 will be explained.

[0068] First, in the seed light source MO, a second excitation light is emitted from each laser diode 52D of the second excitation light source 52. As described above, the wavelength of this excitation light is, for example, 976 nm. The second excitation light emitted from the second excitation light source 52 is incident from one end of the third amplifying fiber 51 via the fiber 521, the combiner 53, and the fiber 54 into the third cladding, which serves as the inner cladding of the third amplifying fiber 51, and propagates primarily within the third cladding. At least a portion of the second excitation light propagating within the third cladding is absorbed by an active element added to the third fiber core, which is surrounded by the third cladding. Therefore, the active element is excited. Through the light emitted by the active element in the excited state, the second wavelength light resonates between HR-FBG56 and OC-FBG57 and is amplified. A portion of the second wavelength light passes through OC-FBG57 and is emitted from the seed light source MO as seed light. Thus, seed light of the second wavelength is emitted from the seed light source MO. As described above, the second wavelength is longer than the first wavelength, for example, 1064 nm.

[0069] Furthermore, in each excitation source 10, a first excitation light is emitted from each laser diode 12D of the first excitation source 12. As described above, the wavelength of this excitation light is, for example, 976 nm. The first excitation light emitted from the first excitation source 12 is incident from one end of the first amplifying fiber 11 via the fiber 121, the combiner 13, and the fiber 14, onto the first cladding 112, which serves as the inner cladding of the first amplifying fiber 11, and propagates primarily within the first cladding 112. At least a portion of the first excitation light propagating in the first cladding 112 is absorbed by an active element added to the first fiber core 111, which is surrounded by the first cladding 112, and this active element is excited. Through the light emitted by the active element in the excited state, the first wavelength light resonates between HR-FBG16 and OC-FBG17 and is amplified. A portion of the first wavelength light passes through OC-FBG17 and is emitted from the excitation source 10 as excitation light. Thus, excitation light of the first wavelength is emitted from the excitation source 10. As mentioned above, the first wavelength is longer than the wavelength of the first excitation light, for example, 1020 nm.

[0070] The second-wavelength seed light emitted from the seed source MO is incident via fiber 35 through combiner 31 into the second core of the second amplifying fiber 21, where it propagates. Meanwhile, the first-wavelength excitation light emitted from each excitation source 10 is incident via fiber 19 and optical attenuation section 60 through combiners 31 and 32 into the second cladding of the second amplifying fiber 21, where it propagates. The excitation light propagating in the second cladding is absorbed by an active element added to the second core as it passes through the second core surrounded by the second cladding. Therefore, the active element is excited. The excited active element induces stimulated emission (SEE) through the seed light propagating in the second core, amplifying the seed light. In other words, the second-wavelength light is amplified.

[0071] The amplified second wavelength light is emitted from the combiner 32 via the transmission fiber 40 from the emission section 41. In this way, the second wavelength light is emitted from the fiber laser device 1.

[0072] Furthermore, sometimes a portion of the second wavelength light amplified by the second amplification fiber 21 propagates to the fiber 19 in the combiner 32 via reflection. Additionally, there are cases where the second wavelength light is reflected when emitted from the emission section 41, or reflected from the workpiece after emission from the emission section 41, thus the second wavelength light, as reflected light, enters the second amplification fiber 21 and propagates towards the rear. In this case, sometimes the second wavelength light propagating towards the rear propagating direction propagates from the combiner 31 to the fiber 19. However, an optical attenuation section 60 is provided in the optical path of each fiber 19. The optical attenuation section 60 suppresses the transmission of the second wavelength light. Therefore, the propagation of the second wavelength light towards the first amplification fiber 11 of the excitation light source 10 is suppressed. Furthermore, in this embodiment, the optical attenuation section 60 is composed of an FBG 61. As described above, the FBG 61 reflects the second wavelength light. Therefore, the second wavelength light propagating from the combiners 31 and 32 towards the excitation light source 10 is reflected towards the combiners 31 and 32 in the optical attenuation section 60.

[0073] Furthermore, even if the second wavelength of light reflected by the light attenuation section 60 leaks into the cladding of the optical fiber 19, at least a portion of the light also leaks to the outside of the optical fiber 19 and is removed from the optical fiber 19 by the cladding mode canceller 70.

[0074] As described above, the fiber laser device 1 of this embodiment includes: a first excitation light source 12 that emits first excitation light; a first amplification fiber 11 that includes a first core 111 to which an active element absorbing the first excitation light is added, and a first cladding 112 surrounding the first core 111 and allowing the first excitation light to be incident, wherein the first wavelength light is amplified and emitted in the first core 111; a second amplification fiber 21 that includes a second core to which an active element absorbing the first wavelength light and the same active element added to the first amplification fiber 11 is added, and a second cladding surrounding the second core, wherein the second wavelength light is amplified and emitted in the second core; combiners 31 and 32 that optically connect the second cladding to the first core 111, allowing the first wavelength light to be incident on the second cladding; and an optical attenuation section 60 that is disposed in the optical path between the combiners 31 and 32 and the first amplification fiber 11, allowing light from the first amplification fiber 11 to pass through and suppressing the transmission of light from the combiners 31 and 32.

[0075] In this way, the active element added to the first amplifying fiber 11 is the same as the active element added to the second amplifying fiber 21. The first excitation light is converted into light of the first wavelength using the first amplifying fiber 11, thus making the first wavelength closer to the second wavelength than the wavelength of the first excitation light. Therefore, according to the fiber laser device 1 of this embodiment, compared to the case where the first excitation light is incident on the second amplifying fiber 21, the generation of quantum defects in the second amplifying fiber 21 can be suppressed, and the reduction in the beam quality of the second wavelength light emitted from the second amplifying fiber 21 can be suppressed. Furthermore, in this fiber laser device, as described above, even when the second wavelength light amplified by the second amplifying fiber 21 travels towards the first amplifying fiber 11 via the combiners 31 and 32, the light from the combiners 31 and 32 can be attenuated by the light attenuation section 60. Therefore, the propagation of the second wavelength light to the first core 111 of the first amplifying fiber 11 can be suppressed, and the reduction in the amplification efficiency of the first wavelength light in the first amplifying fiber 11 can be suppressed. Therefore, it is possible to suppress the reduction in power of the first wavelength light incident as excitation light into the second amplification fiber 21, and to suppress the reduction in amplification efficiency of the second wavelength light in the second amplification fiber 21. Therefore, the fiber laser device 1 according to this embodiment can emit high-power light.

[0076] Furthermore, from the viewpoint of suppressing quantum depletion, the difference between the first wavelength and the second wavelength is preferably 50 nm or less, and more preferably 45 nm or less. Additionally, from the viewpoint of allowing light of the first wavelength to pass through and suppressing the transmission of light of the second wavelength in the light attenuation section 60 of the FBG61, the difference between the first wavelength and the second wavelength is preferably 5 nm or more, and more preferably 10 nm or more.

[0077] (Second Implementation) Next, the second embodiment of the present invention will be described in detail. Furthermore, unless specifically stated otherwise, the same reference numerals will be used for components that are identical or equivalent to those in the first embodiment, and repeated descriptions will be omitted.

[0078] Figure 5 This is a diagram illustrating the fiber laser device of this embodiment. (As shown) Figure 5 As shown, the main difference between the fiber laser device 1 of this embodiment and the fiber laser device 1 of the first embodiment is that it does not have a seed light source MO, but has HR-FBG26 and OC-FBG27. That is, the fiber laser device 1 of this embodiment is a resonator-type fiber laser device.

[0079] In this embodiment, an optical fiber 24 is connected to one end of the second amplifying optical fiber 21, and the end of the optical fiber 24 opposite to the side of the second amplifying optical fiber 21 is connected to the combiner 31. The optical fiber 24 has the same structure as the second amplifying optical fiber 21, except that no active element is added to its core. Therefore, the optical fiber 24 has a core, an inner cladding surrounding the core, an outer cladding surrounding the inner cladding, and a cover layer covering the outer cladding. The second core of the second amplifying optical fiber 21 is connected to the core of the optical fiber 24, and the second cladding of the second amplifying optical fiber 21 is connected to the inner cladding of the optical fiber 24. Furthermore, in this embodiment, in the combiner 31, a portion of the core of the optical fiber 19 is optically connected to the inner cladding of the optical fiber 25.

[0080] An HR-FBG26, serving as a first reflector, is disposed in the core of optical fiber 24. Thus, the HR-FBG26 is positioned at one end of the second amplifying optical fiber 21 and optically connected to the second core of the second amplifying optical fiber 21. The HR-FBG26 reflects light of the second wavelength described above. As mentioned above, the second wavelength is, for example, 1064 nm, and the HR-FBG26 reflects the second wavelength light with a reflectivity of, for example, 50%-99%.

[0081] An optical fiber 25 is connected to the other end of the second amplifying optical fiber 21. The end of the optical fiber 25 opposite to the side of the second amplifying optical fiber 21 is connected to the combiner 32. The optical fiber 25 has the same structure as the optical fiber 24. The second core of the second amplifying optical fiber 21 is connected to the core of the optical fiber 25, and the second cladding of the second amplifying optical fiber 21 is connected to the inner cladding of the optical fiber 25. In addition, in this embodiment, in the combiner 32, the core of another portion of the optical fiber 19 is optically connected to the inner cladding of the optical fiber 25.

[0082] An OC-FBG27, serving as a second reflector, is disposed in the core of optical fiber 25. Thus, the OC-FBG27 is positioned at the other end of the second amplifying optical fiber 21 and optically connected to the second core of the second amplifying optical fiber 21. The OC-FBG27 reflects light of the second wavelength with a lower reflectivity than the HR-FBG26, for example, 10%. In this way, a resonator is formed by the HR-FBG26, the second amplifying optical fiber 21, and the OC-FBG27.

[0083] Next, the operation of the fiber laser device 1 in this embodiment will be explained.

[0084] Similar to the fiber laser device 1 of the first embodiment, excitation light of a first wavelength is emitted from each excitation source 10. The excitation light emitted from a portion of the excitation sources 10 is incident on the second cladding of the second amplification fiber 21 via the combiner 31 and the inner cladding of the fiber 24. In addition, the excitation light emitted from another portion of the excitation sources 10 is incident on the second cladding of the second amplification fiber 21 via the combiner 32 and the inner cladding of the fiber 25.

[0085] At least a portion of the excitation light of the first wavelength propagating in the second cladding of the second amplifying fiber 21 is absorbed by an active element added to the second fiber core, which is surrounded by the second cladding. Therefore, the active element is excited, and the second wavelength light resonates and is amplified between HR-FBG26 and OC-FBG27 through the light emitted by the excited active element. A portion of the second wavelength light passes through OC-FBG27. The second wavelength light passing through OC-FBG27 is emitted from the combiner 32 via the transmission fiber 40 from the emission section 41. Thus, the second wavelength light is emitted from the fiber laser device 1.

[0086] Furthermore, similar to the first embodiment, in this embodiment, the second wavelength light also propagates from the combiners 31 and 32 to the optical fiber 19. However, in this embodiment, since an optical attenuation section 60 composed of FBG61 is provided in the optical path of each optical fiber 19, the optical attenuation section 60 suppresses the transmission of the second wavelength light, just like in the first embodiment. Therefore, in this embodiment, the propagation of the second wavelength light to the first amplification optical fiber 11 of the excitation light source 10 is also suppressed.

[0087] As explained above, the fiber laser device 1 of this embodiment can also emit high-power light in the same way as the fiber laser device 1 of the first embodiment.

[0088] (Modified example) Next, a modified example of the light attenuation section 60 will be described. Furthermore, for components that are the same as or equivalent to those in the above embodiment, unless otherwise specified, the same reference numerals will be used and repeated descriptions will be omitted. In the following modified examples, only one optical path from the combiner 32 to the excitation light source 10 will be described as an example; however, other optical paths from the combiner 32 to the excitation light source 10, and optical paths from the combiner 31 to the excitation light source 10, are also constructed in the same way.

[0089] (First variation) Figure 6 This is a diagram illustrating this variation. For example... Figure 6 As shown, the optical attenuation section 60 in this modified example, like the embodiment described above, is a filter that transmits light of a first wavelength and suppresses the transmission of light of a second wavelength longer than the first wavelength. This filter is an FBG, but it differs from the embodiment above in that the FBG is tilted. In the tilted FBG 62, the interface between the high-refractive-index portion and the lower-refractive-index portion, which are repeatedly formed at a fixed period, is tilted relative to the long side direction of the fiber core of the optical fiber 19. Therefore, the optical attenuation section 60 in this modified example reflects the second-wavelength light incident on the optical attenuation section 60 in a direction opposite to the direction of travel of the second-wavelength light incident on the optical attenuation section 60.

[0090] Furthermore, in this modified example, a cladding mode canceller 70 is provided midway along the optical fiber 19 between the optical attenuation section 60 (which serves as a filter) and the excitation light source 10. In this modified example, by tilting the FBG 62, the second wavelength light incident on the optical attenuation section 60 can be reflected in a desired direction. Additionally, when using the tilted FBG 62 as a filter, the second wavelength light sometimes incident on the cladding of the optical fiber 19. However, in this modified example, the cladding mode canceller 70 allows light propagating within the cladding of the optical fiber 19 to leak outside the cladding. Therefore, it is possible to further suppress light propagating within the cladding from incident on the first amplifying optical fiber 11.

[0091] The angle of the aforementioned interface in the tilted FBG62 can also be adjusted so that when light of the second wavelength is incident on the optical attenuation section 60, the light of the second wavelength reflected by the tilted FBG62 leaks out from the cladding of the optical fiber 19. For example, by making the angle 45 degrees relative to the long side direction of the optical fiber 19, even when light of the second wavelength is incident on the optical attenuation section 60, the light can be reflected in a direction of 90 degrees relative to the long side direction of the optical fiber 19, and the light of the second wavelength can leak out to the outside of the optical fiber 19 in the optical attenuation section 60.

[0092] (Second variation) Figure 7 This is a diagram illustrating this variation. For example... Figure 7As shown, the optical attenuation section 60 in this modified example is similar to that in the above embodiment, a filter that allows light of the first wavelength to pass through and suppresses the transmission of light of the second wavelength, which has a longer wavelength than the first wavelength. However, this filter is composed of a photonic bandgap fiber 63. Therefore, in this modified example, the fiber 19 is divided into a first fiber 19a connected to the excitation light source 10 and a second fiber 19b connected to the combiner 32, and an optical attenuation section 60 composed of a photonic bandgap fiber 63 is connected between the first fiber 19a and the second fiber 19b.

[0093] Figure 8 This diagram illustrates a photonic bandgap fiber 63. The photonic bandgap fiber 63 has a core 631, a plurality of high-refractive-index components 633 arranged to surround the core, and a cladding 632 that surrounds the core 631 and each of the high-refractive-index components 633 without gaps. The photonic bandgap fiber 63 exhibits excellent filtering performance, allowing light of a desired wavelength to pass through while preventing light of other wavelengths from passing through. In this modified example, when light of a first wavelength is incident on the core 631 of the photonic bandgap fiber 63, the first wavelength propagates within the core 631; when light of a second wavelength is incident on the core 631, the second wavelength leaks outside the core 631. Therefore, according to this modified example, the propagation of second wavelength light into the first amplification fiber 11 can be further suppressed.

[0094] When light of the second wavelength is incident on the optical attenuation section 60, the second wavelength light may sometimes leak from the photonic bandgap fiber 63 into the cladding of the first fiber 19a. However, even in this case, the cladding mode canceller 70 can prevent the light propagating in the cladding of the first fiber 19a, which is part of the fiber 19, from leaking out of the cladding. Therefore, it is possible to further suppress the light propagating in the cladding from being incident on the first amplifying fiber 11.

[0095] As in this modified example, when using a photonic bandgap fiber 63 as a filter, from the viewpoint of allowing light of the first wavelength to pass through and suppressing the transmission of light of the second wavelength in the light attenuation section 60, the difference between the first wavelength and the second wavelength is preferably 5 nm or more, and more preferably 10 nm or more.

[0096] (Third variation) Figure 9 This is a diagram illustrating this variation. (As shown...) Figure 9As shown, in this modified example, the difference from the second modified example described above is that the first optical fiber 19a connected to the excitation light source 10 and the second optical fiber 19b connected to the combiner 32 are directly connected at the connection portion 64. The first optical fiber 19a has a core 191a and a cladding 192a, and the second optical fiber 19b has a core 191b and a cladding 192b. Furthermore, in this modified example, the diameter of the core 191a of the first optical fiber 19a is larger than the diameter of the core 191b of the second optical fiber 19b. That is, in this modified example, the optical path between the excitation light source 10 and the combiner 32 includes an optical fiber 19 composed of the first optical fiber 19a and the second optical fiber 19b, and the optical attenuation portion 60 includes a connection portion 64 between the first optical fiber 19a and the second optical fiber 19b, wherein the diameter of the core 191b of the second optical fiber 19b is larger than the diameter of the core 191a of the first optical fiber 19a.

[0097] In this modified example, with the above structure, a portion of the second wavelength light propagating from the combiner 32 can leak from the core 191b of the second optical fiber 19b to the outside of the core 191a of the first optical fiber 19a in the connection portion 64. Furthermore, in cases where at least a portion of the leaked second wavelength light is incident on the cladding 192a of the first optical fiber 19a, this portion of light can also be removed by the cladding mode canceller 70. According to this modified example, since the optical attenuation portion 60 is formed by setting a difference in the diameter of the cores of the two optical fibers, the structure of the optical attenuation portion 60 can be simplified.

[0098] The present invention has been described above with reference to specific embodiments. However, the present invention is not limited to the above embodiments, and the structure can be appropriately modified within the scope of achieving the purpose of the present invention.

[0099] For example, a resonator-type fiber laser is used as the excitation source 10, but the excitation source 10 can also be other structures as long as it uses the first excitation source 12 and the first amplification fiber 11.

[0100] Alternatively, the optical attenuation section 60 can be any other structure, such as an isolator, as long as it allows light from the first amplification optical fiber to pass through and suppresses light from the combiner to pass through.

[0101] Furthermore, the cladding mode eliminater 70 is not a necessary structure. However, when the fiber laser device 1 is equipped with the cladding mode eliminater 70, unwanted second wavelength light can be removed from the fiber 19, so it is preferred.

[0102] Furthermore, the fiber laser device 1 described in the above embodiment is a bidirectional excitation type fiber laser device having an excitation source 10 optically connected to the second amplification fiber 21 via combiners 31 and 32. However, the fiber laser device 1 of the present invention may be a backward excitation type fiber laser device having an excitation source 10 optically connected to the second amplification fiber 21 via combiner 32 but not the excitation source 10 optically connected to the second amplification fiber 21 via combiner 31, or it may be a forward excitation type fiber laser device having an excitation source 10 optically connected to the second amplification fiber 21 via combiner 32 but not the excitation source 10 optically connected to the second amplification fiber 21 via combiner 32.

[0103] As explained above, according to the present invention, a fiber laser device capable of emitting high-power light is provided, and its use in laser devices for processing and the like is anticipated.

Claims

1. A fiber laser device, characterized in that, have: The first excitation source is capable of emitting the first excitation light; The first amplification optical fiber contains active elements that absorb the first excitation light and can amplify and emit light of the first wavelength. The second amplifying optical fiber has the same active element added to the first amplifying optical fiber. It absorbs the first wavelength of light as excitation light and can amplify and emit the second wavelength of light. A beam combiner, capable of directing light of the first wavelength into the second amplifying optical fiber; and An optical attenuation section is disposed in the optical path between the combiner and the first amplifying optical fiber, allowing light from the first amplifying optical fiber to pass through while suppressing the transmission of light from the combiner.

2. The fiber laser device according to claim 1, characterized in that, The light attenuation section is a filter that allows light of the first wavelength to pass through while suppressing the transmission of light of the second wavelength.

3. The fiber laser device according to claim 2, characterized in that, The filter is a reflector that reflects light of the second wavelength.

4. The fiber laser device according to claim 3, characterized in that, The reflector is an FBG, or fiber Bragg grating.

5. The fiber laser device according to claim 4, characterized in that, The FBG is slanted.

6. The fiber laser device according to claim 2, characterized in that, The filter is a photonic bandgap fiber.

7. The fiber laser device according to claim 5 or 6, characterized in that, The filter and the first amplifying optical fiber are connected by an optical fiber containing a core and a cladding, and a cladding mode canceller is provided in the middle of the optical fiber.

8. The fiber laser device according to any one of claims 1-7, characterized in that, It also has: The second excitation source is capable of emitting a second excitation light; and The third amplifying optical fiber contains the same active element as that added to the second amplifying optical fiber. It absorbs the second excitation light and amplifies and emits the light of the second wavelength. The second wavelength light emitted from the third amplifying optical fiber is incident on the second amplifying optical fiber from one end.

9. The fiber laser device according to any one of claims 1-7, characterized in that, It also has: A first reflecting mirror is optically connected to the second amplifying fiber at one end, and reflects light of the second wavelength. as well as The second reflector is optically connected to the second amplifying fiber at the other end of the second amplifying fiber, and reflects the light of the second wavelength with a lower reflectivity than the first reflector.

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