High-absorption large-mode-field ytterbium-doped multi-clad optical fiber

Abstract

The invention discloses a high-absorption large-mode-field ytterbium-doped multi-clad optical fiber, and relates to the technical field of optical fibers. The optical fiber comprises a doped core layer, a doped buffer layer, an inner cladding layer, a quartz cladding layer, an organic coating cladding layer and a protective layer which are sequentially arranged from inside to outside in a coating manner, ytterbium ions are doped in the doped core layer, and any one element of cerium and phosphorus is doped in the doped core layer. Ytterbium ions are doped in the doped buffer layer, and any one of cerium and phosphorus is doped in the doped buffer layer. According to the ytterbium ion doped fiber, a certain ytterbium ion doping concentration level can be guaranteed, cladding pumping absorption of the fiber core can be improved, the use length of the fiber can be effectively reduced, and the nonlinear effect of the fiber is reduced.

Description

Technical Field

[0001] The present application relates to the field of optical fiber technology, and in particular to a high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber. Background Art

[0002] Fiber lasers have developed rapidly due to their unique advantages such as high conversion efficiency, good heat dissipation effect, high beam quality and compact structure. They are widely used in optical communications, industrial processing, biomedicine, military defense and other related fields, becoming the dominant force in the laser industry.

[0003] Rare earth-doped optical fiber is used as the gain medium of optical fiber laser. It achieves laser output in different bands by doping different rare earth ions in the optical fiber core. The emergence of rare earth-doped optical fiber with multi-cladding structure design has greatly improved the output capacity of single-fiber laser. In recent years, the output power of fiber lasers has continued to rise, reaching an average power of kilowatts and a peak power of megawatts. However, at high power, nonlinear effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) have always restricted the demand for high peak power, high pulse energy and narrow linewidth. Increasing the ytterbium ion doping concentration to reduce the use length of the optical fiber and increasing the large mode field area are two effective ways to effectively solve the nonlinear effect.

[0004] At present, the common method is to reduce the ratio of the inner cladding and core layer of the multi-clad optical fiber to improve absorption. Although it can meet both high absorption and low core numerical aperture, it is easy to cause bending loss. Increasing the ytterbium ion doping concentration means increasing the core numerical aperture, which will lead to deterioration of beam quality. In addition, high concentration of ytterbium ion doping can easily lead to ytterbium ion aggregation and cause photon darkening. Summary of the invention

[0005] The purpose of the present application is to provide a high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber, which can improve the cladding pump absorption of the core while ensuring a certain ytterbium ion doping concentration level, effectively reduce the use length of the optical fiber, and reduce the nonlinear effect of the optical fiber.

[0006] The technical solution of this application is as follows:

[0007] The embodiment of the present application provides a high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber, which includes a doped core layer, a doped buffer layer, an inner cladding layer, a quartz cladding layer, an organic coating cladding layer, and a protective layer, which are sequentially coated from the inside to the outside;

[0008] The doped core layer is doped with ytterbium ions and any one of cerium and phosphorus;

[0009] The doped buffer layer is doped with ytterbium ions and any one of cerium and phosphorus.

[0010] Furthermore, in some embodiments of the present application, the component of the above-mentioned doped core layer is silicon dioxide; the above-mentioned doped core layer is also doped with any one or more of aluminum, fluorine, and germanium elements.

[0011] Furthermore, in some embodiments of the present application, the doped buffer layer is composed of silicon dioxide; the doped buffer layer is also doped with any one or more of aluminum, fluorine, and germanium.

[0012] Furthermore, in some embodiments of the present application, the inner cladding is composed of silicon dioxide; the inner cladding is doped with any one or more of germanium and fluorine elements.

[0013] Furthermore, in some embodiments of the present application, the component of the quartz cladding is silicon dioxide; and the refractive index of the quartz cladding is 1.4571-1.4574 under a 630nm wavelength laser test.

[0014] Furthermore, in some embodiments of the present application, the refractive index difference between the doped core layer and the quartz cladding layer is (0.3×10 -3 )~(6×10 -3 ); the refractive index difference between the doped buffer layer and the quartz cladding is (-0.2×10 -3 )~(0.2×10 -3 ); the refractive index difference between the inner cladding and the silica cladding is (-1.5×10 -3 )~(-0.2×10 -3 ); the refractive index difference between the organic coating cladding and the quartz cladding is (4.5×10 -2 )~(12.9×10 -2 ).

[0015] Furthermore, in some embodiments of the present application, the radius of the doped core layer is 5 to 50 μm, the radius of the doped buffer layer is 5 to 60 μm, the radius of the inner cladding is 10 to 100 μm, and the radius of the quartz cladding is 100 to 1200 μm.

[0016] Furthermore, in some embodiments of the present application, the ratio of the radius of the doped buffer layer to the radius of the doped core layer is 1.0 to 1.2; the ratio of the radius of the inner cladding to the radius of the doped buffer layer is 1 to 2.

[0017] The present application also provides a method for preparing the above-mentioned high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber, which comprises the following steps:

[0018] Deposition tube pretreatment: heat the deposition tube to 1700-2000℃, introduce SF6 and O2, and remove impurities on the inner surface of the deposition tube through high-temperature etching reaction;

[0019] Inner cladding deposition: the pretreated deposition tube is heated to 1600-2000° C., a first deposition material is introduced into the deposition tube, and the inner cladding is prepared by deposition on the inner surface of the deposition tube;

[0020] Doped buffer layer deposition: After the inner cladding is obtained, the deposition tube is heated to 1500-1900° C., a second deposition material is introduced into the deposition tube, and the doped buffer layer is deposited on the inner surface of the inner cladding;

[0021] Doped core layer deposition: After obtaining the doped buffer layer, the deposition tube is heated to 1500-2000° C., a third deposition material is introduced into the deposition tube, and deposited on the inner surface of the doped buffer layer to obtain a doped core layer;

[0022] Core rod preparation: after depositing the inner cladding, doped buffer layer and doped core layer on the inner surface of the deposition tube in sequence, the deposition tube is collapsed at 2100-2200° C. to obtain a solid core rod;

[0023] Preparation of optical fiber preform: The prepared core rod is sleeved with a high-purity quartz tube, and the surface is grinded and polished after the sleeve to obtain an optical fiber preform with a non-circular cross section;

[0024] Finished product preparation: The prepared optical fiber preform is drawn at 1800-2000°C, and a low-refractive-index resin material is coated on the surface to form an organic coating cladding; a high-refractive-index resin material is then coated on the surface of the organic coating cladding to form a protective layer to obtain a finished multi-cladding optical fiber.

[0025] Furthermore, in some embodiments of the present application, the first deposition material includes SiCl4, O2, SiF4 and POCl3; the second deposition material and the third deposition material both include SiCl4, O2, SiF4, Yb chelate and AlCl3.

[0026] Compared with the prior art, the embodiments of the present application have at least the following advantages or beneficial effects:

[0027] In view of the above aspects, the embodiment of the present application provides a high-absorption large-mode-field ytterbium-doped multi-clad optical fiber, which can effectively increase the absorption of the optical fiber core to the cladding pump by designing a doped buffer layer. Under the conditions of equal components and equal rare earth doping concentration, the multi-clad optical fiber provided by the present application has a higher cladding pump absorption coefficient, which shortens the length of the optical fiber in actual use and can effectively reduce the nonlinear effect of the optical fiber. At the same time, the depressed cladding design can further reduce the nonlinear effect by increasing the core mode field area, and at the same time can improve the beam quality through multi-bend mode selection. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 A schematic diagram of the cross-sectional structure of a high-absorption, large-mode-field ytterbium-doped multi-cladding optical fiber provided in an embodiment of the present application;

[0030] Figure 2 A schematic diagram of the refractive index distribution of the high-absorption, large-mode-field ytterbium-doped multi-cladding optical fiber cross section provided in Example 1 of the present application;

[0031] Figure 3 This is a schematic diagram of the refractive index distribution of the high-absorption, large-mode-field ytterbium-doped multi-cladding optical fiber cross section provided in Example 2 of the present application.

[0032] Figure numerals: 1 - doped core layer, 2 - doped buffer layer, 3 - inner cladding layer, 4 - quartz cladding layer, 5 - organic coating cladding layer, 6 - protective layer. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0034] It should be noted that relational terms such as "first", "second", "third", etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the term "comprises" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprises..." do not exclude the existence of other identical elements in the process, method, article or device including its elements.

[0035] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application may be combined with each other.

[0036] The features and performance of the present application are further described in detail below in conjunction with the embodiments.

[0037] Example 1

[0038] This embodiment provides a high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber, specifically a YbAlCeFSi-type doped optical fiber; its hierarchical structure is as follows Figure 1 As shown, it includes a doped core layer 1, a doped buffer layer 2, an inner cladding layer 3, a quartz cladding layer 4, an organic coating cladding layer 5, and a protective layer 6 which are sequentially coated from the inside to the outside; wherein, the components of the doped core layer 1 are: Yb2O3 0.23mol%, Al2O3 1.42mol%, Ce2O3 0.1mol%, SiF4 0.8mol; the components of the doped buffer layer 2 are: Yb2O3 0.2mol%, Al2O31.2mol%, Ce2O3 0.08mol%, SiF4 1.05mol%;

[0039] The radius of the doped core layer 1 is denoted as r1, which is 6.5-7.5 um, and the refractive index difference between the doped core layer 1 and the quartz cladding layer 4 is denoted as △n1, which is 17.79×10-4;

[0040] The radius of the doped buffer layer 2 is denoted as r2, which is 8.5-10.5 μm, and the refractive index difference between it and the quartz cladding 4 is denoted as △n2, which is 0.15×10-3;

[0041] The radius of the inner cladding 3 is denoted as r3, which is 14-18 μm, and the refractive index difference between it and the quartz cladding 4 is denoted as △n3, which is -0.85×10-3;

[0042] The radius of the quartz cladding 4 is denoted as r4, which is 122-130 um;

[0043] The radius of the organic coating layer is denoted as r5, which is 150-170 um, and the refractive index difference between the organic coating layer and the quartz cladding 4 is denoted as △n4, which is 8.8×10-2.

[0044] The refractive index profile of the optical fiber provided in this embodiment is as follows: Figure 2 As shown, during the preparation process, the refractive index of the doped buffer layer 2 is strictly controlled so that its refractive index matches that of the quartz cladding 4, and the △n2 refractive index difference satisfies (-0.2×10-3~0.2×10-3).

[0045] It is prepared by the following method:

[0046] Sedimentation tube pretreatment:

[0047] A high-purity quartz tube with an outer diameter of 28 mm, an inner diameter of 24 mm, and a length of 500 mm was selected as the deposition tube, and ordinary quartz tubes were welded at both ends of the deposition tube as the air inlet and outlet pipes, and sealed and connected to the lathe;

[0048] The deposition tube is heated to 1900°C, SF6 and O2 are introduced, and impurities on the inner surface of the deposition tube are removed by high-temperature etching reaction; during the high-temperature etching process, one back-and-forth movement of the heater along the deposition tube is considered to complete one layer of etching, and the number of back-and-forth movements of the heater is controlled to be 3 times;

[0049] The flow rate of SF6 introduced is 200sccm, and the flow rate of O2 is 500sccm;

[0050] Inner cladding 3 deposition:

[0051] The pretreated deposition tube is heated to 1800°C, SiCl4, O2, SiF4 and POCl3 are introduced into the deposition tube, and the inner cladding 3 is deposited on the inner surface of the deposition tube. The number of layers of the inner cladding 3 deposited is controlled to be 14 by controlling the number of round trips of the heater.

[0052] The SiCl4 flow rate is 200sccm, the O2 flow rate is 1000sccm, the SiF4 flow rate is 4sccm, and the POCl3 flow rate is 15sccm;

[0053] Doped buffer layer 2 deposition:

[0054] After the inner cladding 3 is obtained, the deposition tube is heated to 1800°C, SiCl4, O2, SiF4, Yb chelate, Ce chelate and AlCl3 are introduced into the deposition tube, and the doped buffer layer 2 is deposited on the inner surface of the inner cladding 3. The number of deposited layers of the doped buffer layer 2 is controlled to be 5 by controlling the number of round trips of the heater;

[0055] Among them, the SiCl4 flow rate is 100sccm, the O2 flow rate is 1500sccm, the SiF4 flow rate is 4.0sccm, the Yb chelate flow rate is 168sccm, the Ce chelate flow rate is 70sccm, and the AlCl3 flow rate is 160sccm;

[0056] When the materials are introduced, the Yb chelate raw material tank is heated to 200°C, the Ce chelate raw material tank is heated to 240°C, and the AlCl3 raw material tank is heated to 110°C; the Yb chelate, Ce chelate and AlCl3 raw materials are transported to the deposition tube through a high-temperature pipeline at a temperature of 280°C for deposition reaction;

[0057] Doped core layer 1 deposition:

[0058] After obtaining the doped buffer layer 2, the deposition tube is heated to 1800°C, SiCl4, O2, SiF4, Yb chelate, Ce chelate and AlCl3 are introduced into the deposition tube, and the doped core layer 1 is deposited on the inner surface of the doped buffer layer 2. By controlling the number of round trips of the heater, the number of deposited layers of the doped core layer 1 is controlled to be 8;

[0059] Among them, the SiCl4 flow rate is 100sccm, the O2 flow rate is 1500sccm, the SiF4 flow rate is 4.0sccm, the Yb chelate flow rate is 168sccm, the Ce chelate flow rate is 70sccm, and the AlCl3 flow rate is 160sccm;

[0060] When the materials are introduced, the Yb chelate raw material tank is heated to 200°C, the Ce chelate raw material tank is heated to 240°C, and the AlCl3 raw material tank is heated to 110°C; the Yb chelate, Ce chelate and AlCl3 raw materials are transported to the deposition tube through a high-temperature pipeline at a temperature of 280°C for deposition reaction;

[0061] Mandrel preparation:

[0062] After the inner cladding layer 3, the doped buffer layer 2 and the doped core layer 1 are deposited on the inner surface of the deposition tube in sequence, the deposition tube is collapsed at 2100° C. to obtain a solid core rod with a diameter of 14.5 mm and a length of 390 mm;

[0063] Optical fiber preform preparation:

[0064] The prepared core rod is sleeved with a high-purity quartz tube, and the surface is ground and polished after the sleeve, so as to obtain an optical fiber preform rod with a non-circular cross section, specifically a regular octagon;

[0065] Finished product preparation: The optical fiber preform is placed in a high-temperature drawing furnace, and is drawn at 1800-2000°C. A low-refractive-index (refractive-index is 1.369) resin material is coated on the surface of the bare fiber to form an organic coating cladding 5. A high-refractive-index (refractive-index is 1.359) resin material is then coated on the surface of the organic coating cladding 5 to form a protective layer 6, thereby obtaining a finished multi-cladding optical fiber.

[0066] Example 2

[0067] This embodiment provides a high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber, specifically a YbAlPFSi type doped optical fiber; its hierarchical structure is as follows Figure 1 As shown, it includes a doped core layer 1, a doped buffer layer 2, an inner cladding layer 3, a quartz cladding layer 4, an organic coating cladding layer 5, and a protective layer 6 which are sequentially coated from the inside to the outside; wherein, the components of the doped core layer 1 are: Yb2O3 0.18mol%, Al2O3 2.3mol%, P2O51.8mol%, SiF4 0.1mol%; the components of the doped buffer layer 2 are: Yb2O3 0.18mol%, Al2O32.1mol%, P2O5 1.8mol%, SiF4 0.3mol%;

[0068] The radius of the doped core layer 1 is denoted as r1, which is 7.75-10.25 μm, and the refractive index difference between the doped core layer 1 and the quartz cladding layer 4 is denoted as △n1, which is 1.36×10-3;

[0069] The radius of the doped buffer layer 2 is denoted as r2, which is 11.5-13.5 μm, and the refractive index difference between it and the quartz cladding 4 is denoted as △n2, which is -0.001×10×-3;

[0070] The radius of the inner cladding 3 is denoted as r3, which is 20-24 μm, and the refractive index difference between it and the quartz cladding 4 is denoted as △n3, which is -0.5×10-3;

[0071] The radius of the quartz cladding 4 is denoted as r4, which is 197 to 203 μm;

[0072] The radius of the organic coating layer is denoted as r5, which is 235-245 um, and the refractive index difference between it and the quartz cladding 4 is denoted as △n4, which is -8.8×10-2.

[0073] The optical fiber refractive index profile provided in this embodiment is as follows: Figure 3 As shown, during the preparation process, the refractive index of the doped buffer layer 2 is strictly controlled so that its refractive index matches that of the quartz cladding 4, and the △n2 refractive index difference satisfies (-0.2×10-3~0.2×10-3).

[0074] It is prepared by the following method:

[0075] Sedimentation tube pretreatment:

[0076] A high-purity quartz tube with an outer diameter of 28 mm, an inner diameter of 24 mm, and a length of 500 mm was selected as the deposition tube, and ordinary quartz tubes were welded at both ends of the deposition tube as the air inlet and outlet pipes, and sealed and connected to the lathe;

[0077] The deposition tube is heated to 1900°C, SF6 and O2 are introduced, and impurities on the inner surface of the deposition tube are removed by high-temperature etching reaction; during the high-temperature etching process, one back-and-forth movement of the heater along the deposition tube is considered to complete one layer of etching, and the number of back-and-forth movements of the heater is controlled to be 3 times;

[0078] The flow rate of SF6 introduced is 200sccm, and the flow rate of O2 is 500sccm;

[0079] Inner cladding 3 deposition:

[0080] The pretreated deposition tube is heated to 1800°C, SiCl4, O2, SiF4 and POCl3 are introduced into the deposition tube, and the inner cladding 3 is deposited on the inner surface of the deposition tube. The number of layers of the inner cladding 3 deposited is controlled to be 14 by controlling the number of round trips of the heater.

[0081] The SiCl4 flow rate is 200sccm, the O2 flow rate is 1000sccm, the SiF4 flow rate is 3sccm, and the POCl3 flow rate is 18sccm;

[0082] Doped buffer layer 2 deposition:

[0083] After the inner cladding 3 is obtained, the deposition tube is heated to 1800°C, SiCl4, O2, SiF4, P2O5, Yb chelate and AlCl3 are introduced into the deposition tube, and the doped buffer layer 2 is deposited on the inner surface of the inner cladding 3. The number of deposited layers of the doped buffer layer 2 is controlled to be 3 by controlling the number of round trips of the heater;

[0084] Among them, the SiCl4 flow rate is 100sccm, the O2 flow rate is 1500sccm, the SiF4 flow rate is 0.5sccm, the P2O5 flow rate is 65sccm, the Yb chelate flow rate is 125sccm, and the AlCl3 flow rate is 160sccm;

[0085] When the materials are introduced, the Yb chelate raw material tank is heated to 200°C, and the AlCl3 raw material tank is heated to 110°C; the Yb chelate, Ce chelate and AlCl3 raw materials are transported to the deposition tube through a high-temperature pipeline at a temperature of 280°C for deposition reaction;

[0086] Doped core layer 1 deposition:

[0087] After obtaining the doped buffer layer 2, the deposition tube is heated to 1800°C, SiCl4, O2, SiF4, P2O5, Yb chelate and AlCl3 are introduced into the deposition tube, and the doped core layer 1 is deposited on the inner surface of the doped buffer layer 2. By controlling the number of round trips of the heater, the number of deposited layers of the doped core layer 1 is controlled to be 8;

[0088] Among them, the SiCl4 flow rate is 100sccm, the O2 flow rate is 1500sccm, the SiF4 flow rate is 1.5sccm, the P2O5 flow rate is 65sccm, the Yb chelate flow rate is 125sccm, and the AlCl3 flow rate is 140sccm;

[0089] When the materials are introduced, the Yb chelate raw material tank is heated to 200°C, the Ce chelate raw material tank is heated to 240°C, and the AlCl3 raw material tank is heated to 110°C; the Yb chelate, Ce chelate and AlCl3 raw materials are transported to the deposition tube through a high-temperature pipeline at a temperature of 280°C for deposition reaction;

[0090] Mandrel preparation:

[0091] After the inner cladding layer 3, the doped buffer layer 2 and the doped core layer 1 are deposited on the inner surface of the deposition tube in sequence, the deposition tube is collapsed at 2100° C. to obtain a solid core rod with a diameter of 14.4 mm and a length of 390 mm;

[0092] Optical fiber preform preparation:

[0093] The prepared core rod is sleeved with a high-purity quartz tube, and the surface is ground and polished after the sleeve, so as to obtain an optical fiber preform rod with a non-circular cross section, specifically a regular octagon;

[0094] Finished product preparation: The optical fiber preform is placed in a high-temperature drawing furnace, and is drawn at 1800-2000°C. A low-refractive-index (refractive-index is 1.369) resin material is coated on the surface of the bare fiber to form an organic coating cladding 5; a high-refractive-index (refractive-index is 1.539) resin material is then coated on the surface of the organic coating cladding 5 to form a protective layer 6, thereby obtaining a finished multi-cladding optical fiber.

[0095] In summary, the embodiment of the present application provides a high-absorption large-mode-field ytterbium-doped multi-clad optical fiber, which can effectively increase the absorption of the optical fiber core to the cladding pump by designing the doped buffer layer 2. Under the conditions of equal components and equal rare earth doping concentration, the multi-clad optical fiber provided by the present application has a higher cladding pump absorption coefficient, which shortens the length of the optical fiber in actual use and can effectively reduce the nonlinear effect of the optical fiber. At the same time, the depressed cladding design can further reduce the nonlinear effect by increasing the core mode field area, and at the same time can improve the beam quality through multi-bend mode selection.

[0096] The embodiments described above are part of the embodiments of the present application, rather than all of the embodiments. The detailed description of the embodiments of the present application is not intended to limit the scope of the present application for protection, but merely represents the selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present application.

Claims

1. A high-absorption, large-mode-field ytterbium-doped multi-cladding optical fiber, characterized in that: It includes a doped core layer, a doped buffer layer, an inner cladding layer, a quartz cladding layer, an organic coating cladding layer, and a protective layer which are sequentially coated from the inside to the outside; The doped core layer is doped with ytterbium ions and any one of cerium and phosphorus; The doped buffer layer is doped with ytterbium ions and any one of cerium and phosphorus.

2. The high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber according to claim 1, characterized in that: The doped core layer is composed of silicon dioxide; the doped core layer is also doped with any one or more of aluminum, fluorine and germanium.

3. The high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber according to claim 1, characterized in that: The doped buffer layer is composed of silicon dioxide; the doped buffer layer is also doped with any one or more of aluminum, fluorine and germanium.

4. The high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber according to claim 1, characterized in that: The inner cladding layer is composed of silicon dioxide; the inner cladding layer is doped with any one or more of germanium and fluorine elements.

5. The high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber according to claim 1, characterized in that: The quartz cladding is composed of silicon dioxide; and the refractive index of the quartz cladding is 1.4571-1.4574 under a 630nm wavelength laser test.

6. The high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber according to claim 1, characterized in that: The refractive index difference between the doped core layer and the quartz cladding layer is (0.3×10 -3 )~(6×10 -3 ); the refractive index difference between the doped buffer layer and the quartz cladding is (-0.2×10 -3 )~(0.2×10 -3 ); the refractive index difference between the inner cladding and the silica cladding is (-1.5×10 -3 )~(-0.2×10 -3 ); the refractive index difference between the organic coating cladding and the quartz cladding is (4.5×10 -2 )~(12.9×10 -2 ).

7. The high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber according to claim 1, characterized in that: The radius of the doped core layer is 5 to 50 μm, the radius of the doped buffer layer is 5 to 60 μm, the radius of the inner cladding is 10 to 100 μm, and the radius of the quartz cladding is 100 to 1200 μm.

8. The high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber according to claim 1, characterized in that: The ratio of the radius of the doped buffer layer to the radius of the doped core layer is 1.0 to 1.2; the ratio of the radius of the inner cladding layer to the radius of the doped buffer layer is 1 to 2.

9. A method for preparing a high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber according to any one of claims 1 to 8, characterized in that: It includes the following steps: Deposition tube pretreatment: heat the deposition tube to 1700-2000℃, introduce SF6 and O2, and remove impurities on the inner surface of the deposition tube through high-temperature etching reaction; Inner cladding deposition: the pretreated deposition tube is heated to 1600-2000° C., a first deposition material is introduced into the deposition tube, and the inner cladding is prepared by deposition on the inner surface of the deposition tube; Doped buffer layer deposition: After the inner cladding is obtained, the deposition tube is heated to 1500-1900° C., a second deposition material is introduced into the deposition tube, and the doped buffer layer is deposited on the inner surface of the inner cladding; Doped core layer deposition: After obtaining the doped buffer layer, the deposition tube is heated to 1500-2000° C., a third deposition material is introduced into the deposition tube, and deposited on the inner surface of the doped buffer layer to obtain a doped core layer; Core rod preparation: after depositing the inner cladding, doped buffer layer and doped core layer on the inner surface of the deposition tube in sequence, the deposition tube is collapsed at 2100-2200° C. to obtain a solid core rod; Preparation of optical fiber preform: The prepared core rod is sleeved with a high-purity quartz tube, and the surface is grinded and polished after the sleeve to obtain an optical fiber preform with a non-circular cross section; Finished product preparation: The prepared optical fiber preform is drawn at 1800-2000°C, and a low-refractive-index resin material is coated on the surface to form an organic coating cladding; a high-refractive-index resin material is then coated on the surface of the organic coating cladding to form a protective layer to obtain a finished multi-cladding optical fiber.

10. The method for preparing a high-absorption large-mode-field ytterbium-doped multi-cladding optical fiber according to claim 9, characterized in that: The first deposition material includes SiCl4, O2, SiF4 and POCl3; the second deposition material and the third deposition material both include SiCl4, O2, SiF4, Yb chelate and AlCl3.

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